**Metal Machining—Recent Advances, Applications and Challenges**

Editor

**Francisco J. G. Silva**

MDPI • Basel • Beijing • Wuhan • Barcelona • Belgrade • Manchester • Tokyo • Cluj • Tianjin

*Editor* Francisco J. G. Silva ISEP—School of Engineering Portugal

*Editorial Office* MDPI St. Alban-Anlage 66 4052 Basel, Switzerland

This is a reprint of articles from the Special Issue published online in the open access journal *Metals* (ISSN 2075-4701) (available at: https://www.mdpi.com/journal/metals/special issues/ metal machining advances applications challenges).

For citation purposes, cite each article independently as indicated on the article page online and as indicated below:

LastName, A.A.; LastName, B.B.; LastName, C.C. Article Title. *Journal Name* **Year**, *Volume Number*, Page Range.

**ISBN 978-3-0365-2494-8 (Hbk) ISBN 978-3-0365-2495-5 (PDF)**

© 2021 by the authors. Articles in this book are Open Access and distributed under the Creative Commons Attribution (CC BY) license, which allows users to download, copy and build upon published articles, as long as the author and publisher are properly credited, which ensures maximum dissemination and a wider impact of our publications.

The book as a whole is distributed by MDPI under the terms and conditions of the Creative Commons license CC BY-NC-ND.

## **Contents**


#### **Muhammad Asad**

### **About the Editor**

**Francisco J. G. Silva** is Associate Professor with Habilitation and has been the Director of the Master's Degree in Mechanical Engineering since November 2014. He is now member of the Technical and Scientific Council of ISEP (2020–2022), and a member of the Pedagogic Council of ISEP (2020–2022). He was Subdirector of the Mechanical Engineering Department from 2014 to 2016. He was Director of the Bachelor's in Mechanical Engineering from 2003 to 2006 at ESEIG, Vila do Conde, Portugal. He was a member of the General Council at the IPP—Polytechnic Institute of Porto— a member of the Scientific Council at ESEIG and member of the Pedagogic Council at ESEIG. He is author of the book "*Tecnologia da Soldadura - Uma Abordagem T´ecnico-Did´actica*", Publindustria, ´ Porto (2016), coeditor of the book "*Lean Manufacturing - Implementation, Opportunities and Challenges*", Nova Science, NY, U.S.A. (2019), and coauthor of the book "*Cleaner Production—Toward a better future*", Springer Nature, Switzerland, 2020. He has supervised more than 120 MSc dissertations and cosupervised two PhD theses. He is coauthor of more than 150 scientific papers in journals such as *Wear, Surface and Coatings Technology, Vacuum, Thin Solid Films, Coatings, Metals, Materials, Composites Part B - Engineering, Robotics and Computer-Integrated Manufacturing, International Journal of Advanced Manufacturing Technology, Journal of the Brazilian Society of Mechanical Sciences and Engineering, Procedia Manufacturing*, etc. He is an Editorial Board Member of the scientific journals Coatings (MDPI), *Solids* (MDPI), *Encyclopedia* (MDPI) and *Machines* (MDPI). He has conducted more than 10 Special Issues in MDPI scientific journals. He founded and was Editor-in-Chief of the journal *Coating Science Technology*, and Co-Editor-in-Chief of the journal *Research Updates in Polymer Science* (LifeScience Global). He frequently conducts reviews for journal such as: *Journal of Cleaner Production* (Elsevier), *Robotics* (Elsevier), *Composites Part A* (Elsevier), *Metals* (MDPI), *Coatings* (MDPI), *Materials* (MDPI), *MicroMachines* (MDPI), *Journal of Materials: Design and Applications* (SAGE), P*roduction and Planning Control* (Taylor and Francis), *Theoretical and Applied Fracture Mechanics* (Elsevier), *Journal of the Brazilian Society of Mechanical Sciences and Engineering* (Springer), *Surface and Coatings Technology* (Elsevier), *Wear* (Elsevier), etc. He awarded the MDPI Top Reviewer Award in 2018, and the Metals Top Reviewer in 2019. One of his papers was awarded with the Best Paper Award in Coatings (MDPI, 2019).

### *Editorial* **Metal Machining—Recent Advances, Applications, and Challenges**

**Francisco J. G. Silva**

ISEP–School of Engineering, Polytechnic Institute of Porto, 4249-015 Porto, Portugal; fgs@isep.ipp.pt; Tel.: +351-228340500

#### **1. Introduction and Scope**

Though new manufacturing processes that revolutionize the landscape regarding the rapid manufacture of parts have recently emerged, the machining process remains alive and up-to-date in this context, always presenting itself as a manufacturing process with several variants and allowing for high dimensional accuracy and high levels of surface finish [1–3]. Indeed, machining has numerous aspects that constantly need to be investigated due to the constant evolution of materials to be machined, the materials and geometry of tools, and the evolution of coatings normally applied to the tools' surfaces [4–6]. In view of this evolution, the parameters used in machining also need to be optimized, thus contributing to increased attention by researchers in this area of manufacturing [7–10]. In fact, metal alloys have significantly evolved in terms of properties, which poses additional challenges for research [11–13]. The market's demand for new alloys that need to meet increasingly demanding requirements is a constant, thus creating a greater diversity of alloys in the market and new challenges in their processing in order to achieve the characteristics required by customers. When the requirements are truly challenging, it becomes necessary to make polymeric or metallic matrix composite materials, creating even more demanding challenges in their processing that have further expanded the research field in the machining area [14–16]. Composite materials still have a huge margin of progression in terms of research, which will also allow the scientific community linked to the manufacturing processes to continue to have a lot of available topics to explore. For example, the chip that can be formed during the machining processes has been the subject of several studies because chip formation provides valuable and useful information about the way the machining process is being conducted and can provide information on the problems related to its removal from equipment and occupied space [17–19]. Dry machining has always been a great challenge [20] because lubrication causes environmental problems [21] and, in some cases, is not even allowed. Thus, aspects related to lubrication in machining have also been widely explored by using techniques that seek to minimize the use of lubrication (minimum quantity lubrication) [22,23]. On the other hand, the need to increase productivity levels has not only resorted to the science of materials and technological processes to offer the industry the necessary means to produce with the necessary quality at increasingly competitive costs but also captured the attention of the industrial engineering field [24,25]. This has led to numerous research projects aimed at the development of models and procedures that allow for the optimization of all operations involving machining processes, as well as some tools used in the process itself, such as more advanced jigs [26].

Recently, new research opportunities have opened up because machining operations are largely linked to the concepts of Industry 4.0. In fact, the operations traditionally developed between equipment can be integrated by using computer systems with greater decision power, making the whole production process much more agile [27,28]. The concepts of Industry 4.0 have also made it possible to develop other areas around machining, namely the concept of "machine learning," which allows for the creation of standard figures


**Citation:** Silva, F.J.G. Metal Machining—Recent Advances, Applications, and Challenges. *Metals* **2021**, *11*, 580. https://doi.org/ 10.3390/met11040580

Received: 29 March 2021 Accepted: 31 March 2021 Published: 2 April 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

that can be recognized by equipment and thus allowing for greater integration between CAD (Computer-Aided Design) and CAM (Computer-Aided Manufacturing) [29,30]. Similarly, the measurement operations of tools and machined parts bring new challenges that are also being scientifically explored [31]. The vibration index and cutting forces developed in tools, which naturally evolve with wear, can also be properly monitored to bring added information to the process and allow for the automation of tool change decisions or maintenance interventions for equipment [32,33].

Bearing in mind all the previously mentioned factors, it is easy to realize that there is innumerable research to be continuously developed in this field of investigation. Thus, this Special Issue intended to gather contributions from different authors in the field of machining, allowing for its easy dissemination and thus contributing to an increase of knowledge of the scientific community that works hard in this area.

#### **2. Contributions**

The contributions received for this Special Issue are high-quality and show how active the research around machining processes is. Three of the studies contained in this Special Issue are related to the chip formation and cutting behavior that are registered during the machining process. In the work "Assessment of Chip Breakability during Turning of Stainless Steels Based on Weight Distributions of Chips" developed by Du et al. [34], the breakability of the stainless-steel chip is studied in the turning of these alloys by using a new methodology: the weight distribution of chips. This methodology was shown to present very consistent results in the evaluation of the way a part is trimmed, thus allowing one to perceive the machinability of a given alloy and allowing for a comparison with similar ones. The study was developed on an AISI 316L alloy, using one without treatment and another with treatment and showing that the treatment drastically modified the breakability of the chip. Even though the chip looked very similar for both cases, the developed method showed that the obtained results were significantly different, showing how this methodology can be useful in other analyses. On the other hand, the work entitled "Predicting Continuous Chip to Segmented Chip Transition in Orthogonal Cutting of C45E Steel through Damage Modeling" performed by Devotta et al. [35] integrated dynamic strain aging in the Johnson–Cook model, which is usually used to modelling machining processes, while also using the Voyiadjis–Abed–Rusinek approach. In this way, it became possible to predict the transition from a continuous chip to a discontinuous chip regarding the widely used C45E steel, depending on the rake angle and feed rate while keeping the cutting speed constant. The main outcome of the study was to discover that chip segmentation intensity and frequency are sensitive to fracture initiation strain models. Additionally, using the finite element method, but now based on an AA2024 T351 aluminum alloy, Muhammad Asad [36] studied the influence of the tool's geometry, namely hone and chamfer, on chip segmentation and burr formation. The study demonstrated an increasing trend in the degree of chip segmentation and end burr as hone edge tool radius or chamfer tool geometry macro parameters concerning chamfer length and angle increased. With the development of this work, a model that helps in the definition of the best tool geometry and the optimization of the cutting parameters was obtained, with the aim to increase productivity, minimize the formation of burr, and avoid the formation of a continuous chip. The quality of a machined surface is also present in this Special Issue. In order to minimize the problems reported in the quality of finishing of aluminum parts, Rubio-Mateos et al. [37] studied the introduction of elastomeric systems to support parts to be machined, with a view to dampening any vibrations during the finishing process of soft materials. For this, nitrile butadiene rubber (NBR) was used. A suitable flexible vacuum fixture was also developed, allowing for the easy implementation of the system in the manufacturing process. Different sets of parameters that varied the degree of compression imposed on the flexible system were tested, verifying that it perfectly accommodated these variations. Thus, the main outcome of this work was the establishment that the milling operations of the AA2024 alloy can benefit from more flexible fixations to the

detriment of very rigid jigs. Del Sol and Rivero [38] also investigated the parameters that could give rise to skin panel and thin plate components obtained by machining, thus eliminating the need to use chemical milling in the manufacture of parts for the aeronautical industry because the rigidity presented for this type of parts is quite low. The study was essentially conducted by using the experimental pathway, measuring the cutting forces that developed during the process and keeping the surface roughness within the imposed limits. The correct selection of the cutting parameters led to a 40% reduction in the thickness variation of the components and a 20% decrease in the cutting forces, which makes the clamping process of parts easier. The study also resulted in the creation of a model capable of monitoring the quality of the process based on the measurement of equipment power consumption. The work of Berzosa et al. [39], entitled "Feasibility Study of Hole Repair and Maintenance Operations by Dry Drilling of Magnesium Alloy UNS M11917 for Aeronautical Components," also investigated the best set of parameters to be used for drilling holes in magnesium alloys, which are increasingly used in the aeronautical, aerospace, and even automotive industries. The study of the parameters allowed for an improvement of the surface quality obtained in holes made in that alloy, mainly in repair or maintenance operations. Additionally, based on the aeronautical industry, Martín Béjar et al. [40] investigated the macro-geometric deviations reported in the turning of a UNS A97075 alloy, verifying that the parts provided with a lower stiffness presented a greater sensitivity to macro dimensional deviations when adjusting parameters. It was once again verified that feed speed is the parameter with the greatest influence on the deviations recorded during the turning process. Based on the obtained results, models that allow for the prediction of macro dimensional deviations as a function of machining parameters were presented. Bañon et al. [41] carried out a quality study of the cut surface in structures composed of different materials. In that case, the abrasive waterjet cutting of a mixed structure of CFRTP (carbon fiber-reinforced thermo-plastic) with steel was studied, which presented quite different behaviors under the same cutting conditions. Two different stacking configurations were studied to investigate different sets of parameters that would lead to lower levels of roughness in waterjet cutting when using abrasives. The experimental work made it possible to draw several diagrams that enabled the correlation of the cutting parameters with the cut surfaces' quality. The sustainability related to the machining processes is also represented in this Special Issue. Indeed, sustainability can be explored in its most diverse aspects because productivity is fundamental but environmental impact—with important factors such as power consumption, the minimum use of lubricants/coolants, and social issues in which health conditions at work and ergonomics must be respected—cannot be ignored. Iqbal et al. [42] investigated the use of cryogenic coolants in the machining of the Ti6Al4V alloy, which is widely used in aeronautics. At the same time, they tried to optimize the parameters with a view to minimizing the consumption of tools by acting on the parameters of the milling process. As main outcomes, it was found that micro-lubrication was more effective than cryogenic cooling with CO2 or liquid nitrogen; it could increase tool life while also improving the surface quality of machined parts, reducing energy consumption, and reducing the overall cost of process. These authors also verified that the high levels of cutter's helix angle and cutting speed clearly contributed to an increase in process sustainability. Diaz-Álvarez et al. [43] also investigated new cutting parameters in the turning of the Haynes 282 nickel alloy while avoiding the use of lubricants/coolants. The used coated tools allowed them to optimize the cutting parameters, making it possible to obtain roughness values in the machined parts as low as those obtained using lubricants/coolants. The proper selection of parameters also kept the cutting forces as low as those obtained with lubrication, as well as extending the tools' life. In this way, the process can become more environmentally sustainable without jeopardizing product quality or economic sustainability.

In addition to the aforementioned works, this Special Issue also presents two widerevision works [5,8], one on the use of coated tools in turning and another that performs an in-depth study of the literature on TiAlN-based coatings for both the turning and

milling processes, focusing on coatings developed around that same coating, providing information on recent uses of these coatings and what elements are used in the fabrication of these types of coatings, showing their mechanical properties, and providing information on their machining performance and application. Each of the reviews is based on more than one hundred references, thus allowing for the deepening and discussing of innumerable ideas taken from a wide range of works. These works constitute a great base of work for MSc and PhD students who are starting in the area by providing (in a concentrated way) a wide range of knowledge in these areas, from the cutting performance of various coated tools in machining processes to the study of the different wear patterns and mechanisms that these tools suffer during the machining process.

#### **3. Conclusions and Outlook**

Through the research collected in this Special Issue, it can be noted that there is much work regarding the machining process, in its most diverse aspects, to be continuously carried out because there is still a huge margin of progression in almost all aspects of machining. In this Special Issue, some excellent examples of the most recent developments in this area are shown, with special emphasis given to optimizing parameters, increasing the quality of machined surfaces, and improving the sustainability of the process. Very important information is also provided regarding tool coatings, with a view to extending cutting tools' working life, which will certainly be useful for those who conduct research in this area or for young students who want to start their studies in this field of knowledge.

The continuous search for greater productivity in machining and for increases of tools' working life (always based on the improvement of the global sustainability) will lead to more and more research in this area that will continue to be collected and disseminated, thus allowing this process to continue to be competitive and capable of producing high-quality parts. Thus, the research around machining processes will surely remain challenging.

**Conflicts of Interest:** The author declares no conflict of interests.

#### **References**


### *Review* **Characteristics and Wear Mechanisms of TiAlN-Based Coatings for Machining Applications: A Comprehensive Review**

**Vitor F. C. Sousa 1, Francisco José Gomes Da Silva 1,2,\*, Gustavo Filipe Pinto 1,2, Andresa Baptista 1,2 and Ricardo Alexandre <sup>3</sup>**


**Abstract:** The machining process is still a very relevant process in today's industry, being used to produce high quality parts for multiple industry sectors. The machining processes are heavily researched, with the focus on the improvement of these processes. One of these process improvements was the creation and implementation of tool coatings in various machining operations. These coatings improved overall process productivity and tool-life, with new coatings being developed for various machining applications. TiAlN coatings are still very present in today's industry, being used due to its incredible wear behavior at high machining speeds, high mechanical properties, having a high-thermal stability and high corrosion resistance even at high machining temperatures. Novel TiAlN-based coatings doped with Ru, Mo and Ta are currently under investigation, as they show tremendous potential in terms of mechanical properties and wear behavior improvement. With the improvement of deposition technology, recent research seems to focus primarily on the study of nanolayered and nanocomposite TiAlN-based coatings, as the thinner layers improve drastically these coating's beneficial properties for machining applications. In this review, the recent developments of TiAlN-based coatings are going to be presented, analyzed and their mechanical properties and cutting behavior for the turning and milling processes are compared.

**Keywords:** machining; milling; turning; tool coating; TiAlN; TiAlN-based coatings; multilayer; nanolayer; wear mechanisms

#### **1. Introduction**

Machining remains a very important process, with the machining industry in continuous growth in recent years, and still having a considerable expected growth in the following years. The turning and milling process are the most used machining processes; however, the drilling process is quite relevant for the machining industry as well. This importance of the machining processes is based on the high demand for high-quality and complex parts for various industry applications such as, the aeronautical and the aerospace industries [1,2]. These two industries benefit specially from the machining process, as it can produce highly complex parts accurately, by employing 5-axis machining methods or even 6-axis machining methods, enabling the complete production of complex parts, from the raw material to the final part, without the need to stop [3,4]. The need for these types of parts also creates the need for process improvement, by reducing machining times, improving tool-life or by applying new machining methods. These topics are still researched recently, with many being focused on the tool used for the machining process, as the tool directly influences the machining process's productivity, with studies being made on the creation of new and improved tool designs, such as the study performed by

**Citation:** Sousa, V.F.C.; Da Silva, F.J.G.; Pinto, G.F.; Baptista, A.; Alexandre, R. Characteristics and Wear Mechanisms of TiAlN-Based Coatings for Machining Applications: A Comprehensive Review. *Metals* **2021**, *11*, 260. https://doi.org/ 10.3390/met11020260

Academic Editor: Emin Bayraktar Received: 31 December 2020 Accepted: 29 January 2021 Published: 4 February 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Siddiqui et al. [5], where the development of a self-lubricated textured tool and its employment in the dry turning of aluminum alloy Al6061- T6 is described. The textured cutting tool allowed for the use of MoS2 as a solid lubricant. It was found that this novel design significantly reduced the wear of the tool (by up to 35%), and also, the cutting temperature was reduced by up to 40% when compared to turning with the conventional tool under dry machining conditions. Still regarding the development of new tools and methods, there have also been improvements regarding the cemented carbide tools used, for example, gradient carbide tools [6] (with different layers with different values of hardness) have been developed. This enables one to tailor the base tool (uncoated substrate) to a certain machining application, conferring the tool with increased wear resistance. There have been some studies conducted on this topic, such as this study performed by Zhou et al. [7] where various gradient cemented carbides (coated and uncoated) are tested in machining tests of a titanium alloy. It was found that the gradient's layer thickness influences the cutting performance and that this thickness can be altered by changing the composition of the cemented carbide itself. Moreover, the tool with thicker layers exhibited the best cutting performance, suffering less wear damage. This is a very interesting study, as the base tool offers a better set of properties (compared to normal cemented carbide tools) that, when combined with a tool coating, will improve even further the tool's performance. Tool coatings have greatly contributed to the machining sector since they were first developed, but there is still room for improvement in this area and thus, recent studies made on machining tools also focus on the employment of coatings to machine certain materials, especially titanium alloys [8], aluminum alloys (these aluminum alloys are primarily applied to the aerospace and aeronautical industry, with some applications in the automotive industry) [9,10] and hard-to-machine materials such as Inconel [11], given that these materials are heavily used in the production of parts for the aeronautical industry. These studies are very important as they provide valuable information on what coatings are best suited for machining a certain material, or which coating to use when wanting to optimize the machining process [2,12].

Machining tools that are employed in today's machining processes are usually coated tools, either solid coated tools, machining tools with coated inserts, or just coated inserts in the case of the turning process. The use of coated tools has greatly improved the machining processes, enabling the machining of materials at higher speeds, when compared to regular uncoated tools and inserts (steel and solid carbide tools) [13–15]. These coatings have proved to be especially useful when it comes to improving tool-life and overall tool performance, when machining materials with low machinability [16,17]. This is due to the improvement of the mechanical properties of the tool by the coating, such as increased hardness, oxidation resistance, toughness, thermal stability (ability to retain microstructure at higher temperatures) and reduced friction coefficient. The employment of tool coatings also contributes for a better surface quality of the machined part [18] and reduction of the cutting forces developed during the machining process (especially due to reduction in friction coefficient), still a very important aspect in today's research of these cutting processes [19]. However, regarding tool-tip temperature and machining temperature, it has been reported that coated tools usually experience higher machining temperatures than uncoated tools [20]. Yet this fact does not seem to negatively impact the coated tool's life (in most cases), since there are other factors that occur at the tool-chip interface, such as the formation of a coating oxidation layer. Moreover, coatings can be tailored to fit a certain application, and the introduction of multiple layers influences the coating's properties [21]. These factors contribute to the tool's life, with coated tools exhibiting overall less wear when compared to uncoated tools, with numerous studies being conducted about this topic. For example, in the study performed by Thakur et al. [22], the performance of uncoated chemical vapor deposition (CVD) TiCN/Al2O3 bilayer and physical vapor deposition (PVD) TiAlN/TiN multilayer coated tools was evaluated. These tools were employed in the turning of Incoloy 825 at three different cutting speeds (51, 84 and 124 m/min). The coated tools outperformed the uncoated tools, however, this margin increased for the higher cutting speed values. Coated tools produced a better surface finish on the part

when compared to the uncoated tools, furthermore, the PVD coatings suffered overall less wear than the CVD coating, exhibiting the lowest value for friction coefficient of all the tools (coated and uncoated). Regarding tool life, coatings improved greatly in this aspect, with the uncoated tools lasting only 90 s, and the CVD and PVD tool lasting for 28 and 40 min, respectively. Studies such as these are very important for the optimization of machining processes, providing valuable information regarding coating application and material machinability. Moreover, studies such as these highlight the value that tool coatings have when employed in machining processes, especially in improving tool-life and part production quality.

Coatings are usually obtained by two different processes, either by CVD or by PVD, with some differences between the two processes [23]. The CVD deposition process was the first to be invented, being used then for the deposition of TiN and TiC coatings in the 1960s, as a response to the tool life problem, then present in the machining industry. CVD produces coatings by having a precursor pumped inside of a reactor (the flux of this precursor is regulated by valves). The precursor molecules pass by the substrate (placed inside the reactor) and are deposited on its surface, giving origin to a thin hard film that has a relatively uniform thickness throughout the substrate's surface. The working temperature of this process is quite high, reaching temperatures of up to 900 ◦C. The PVD process was developed after the CVD, having some advantages when compared to it, such as a lower deposition temperature and the ability to create different types of coating (such as the TiAlN coating, created as an improvement over the TiN coating). The first coating deposited by this technique was TiN coating, achieved in the 1970s [24]. PVD consists of various methods, such as evaporation, sputtering and molecular beam epitaxy (MBE). Regarding sputtering, the coating is achieved by placing a magnetron near the target (containing the elements that are going to be part of the coating), in a vacuum reactor chamber. An inert gas is then introduced in the chamber, then a high voltage is applied between the target and the substrate also placed inside the reactor chamber, causing the release of atomic size particles from the target. These particles are projected onto the substrate, causing the formation of a thin solid film. In the evaporation technique, however, the target itself acts as an evaporation source, while the sample's material works as a cathode, the target material is heated at a high vapor pressure, which causes particle to release and be dispersed inside de reactor. The gas that is being pumped inside the chamber clashes with these particles, causing their acceleration, which in turn creates a plasma that will be deposited onto the substrate's surface. This process, contrary to the CVD process, runs at a much lower temperature, under 500 ◦C. Thus, PVD obtained coatings can be deposited onto steel substrate and cemented carbide tools without negatively impacting the properties of these types of substrate. Furthermore, the PVD process does not involve the use of any toxic precursors, unlike the CVD process, and is more energy efficient, having a considerably lower energy consumption than the CVD process [25,26].

Choosing the right coating deposition method is very important, as seen in the previous paragraph. Different techniques confer the coatings with different properties, being increased mechanical properties, adhesion properties and even residual stresses. Both CVD and PVD methods have certain advantages and disadvantages, for example, CVD coatings are very difficult to deposit onto steel substrates, due to high deposition temperature. However, there have been studies that seek to solve this problem, by implementing an interlayer, between the coating and the substrate, that will protect the substrate during the deposition of the outer coating [27,28]. Regarding the PVD process, due to its deposition temperatures, usually, good adhesive strength of coatings can be achieved when these are deposited onto steel substrates [29]; however, this process is a line-of-sight process, which means that coating deposition on complex geometries is considerably harder when compared to CVD. Moreover, the control of the thickness throughout the substrates surface is also harder. These problems can be attenuated by using a different PVD process, such as pulsed high-power sputtering [30]. However, this can come at a cost, such as inducing excessive residual stresses in the coating or even sacrificing adhesive strength. These two

deposition processes (CVD and PVD) also influence machining performance, for example, PVD coatings are usually thinner than CVD coatings, however, there are some studies that report coatings with thicknesses up to 15 μm [31]. This coupled with the fact that PVD coatings exhibit compressive stresses, makes the cutting edge of the coated tool a very strong and resistant edge, making these types of coatings ideal for finishing operations, whereas in the case of CVD coatings, these exhibit tensile residual stresses and are usually thicker, making them more suited for roughing operations where, for example, a high material removal rate is preferred [32–34]. The control over these coatings properties makes them very versatile, moreover, they can be specifically made for a certain application, experiencing various combinations of coating's structures and compositions. Coatings are not only used on tools for metal cutting operations [35,36], their mechanical properties, high wear resistance, high temperature resistance and high corrosion resistance makes them very appealing for a wide range of applications. For example, they have seen some recent use in wood cutting processes [37], medical applications [38], mold industry [39], automotive (especially for brake pads) [40] and even being deposited in alloys used for nuclear fuel cladding [41,42].

As previously mentioned, the PVD process involves various methods for the deposition of coatings. These methods influence not only coating composition, but their properties as well. These methods are primarily divided into two groups: sputtering and evaporation. All these methods can be observed in Figure 1.

**Figure 1.** Physical vapor deposition (PVD) techniques currently being used in the production and deposition of coatings [27].

Currently, the most used method to produce PVD coatings is the direct current method (DC) for magnetron sputtering; however, there seems to be a shift in the use of these techniques to ones such as, unbalanced magnetron sputtering (UBMS) [25,26] and the high-power impulse/pulse magnetron sputtering (HiPIMS/HPPMS), the latter rising in popularity in recent years. There are several studies conducted regarding these deposition techniques, as they confer the coating with different properties, more suited to certain applications. For example, in the study carried out by Romero et al. [43], an evaluation of microstructure and tribological performance of TiAlTaN-(TiAlN/TaN) coatings has been observed. Various coatings of this type were deposited onto AISI M2 steel, the deposition consisted of a first layer of TiAlN/TaN followed by a second layer of TiAlTaN. In total, four combinations with different volume fractions for each layer were tested. Furthermore, the deposition was achieved by DC sputtering using two magnetrons and two targets, and by controlling the substrate rotation speed. The authors were able to control the coating's architecture, and thus their mechanical properties, by varying the rotation speed of the substrate during deposition. They were able to find the best values for the volume fractions of each layer, concluding that the combination of 48% TiAlTaN and 52% of TiAlN/TaN exhibited the best balance between adhesion properties, hardness (29 GPa) and friction coefficient (0.68). Recently, there have been some studies highlighting the benefit of some deposition techniques such as HiPIMS, linking this technique with an increase in mechanical properties and a betterment of adhesion properties. Zauner et al. [44] studied the influence of the HiPIMS parameter choice in the properties of TiAlN films, by using

a TiAl composite target in mixed Ar/N2 atmospheres. The parameters in question were the pulse frequency and duration, the N2 flow ratio, target composition and substrate bias voltage. The authors found that changing these parameters has a great influence on the final coating's properties, even enabling the control of the coating's structure. The authors claim to obtain hardness of the TiAlN coating of up to 36 GPa. Still regarding the influence of deposition parameters in the coating's properties, in the work performed by Zhao et al. [45] the influence of the bias voltage chosen during deposition of TiAlN coatings is evaluated. Coatings have been fabricated using a multi-arc ion plating device. Various values for this parameter were tested, the lowest value of bias voltage being −40 V and the highest value being −120 V. The lowest value produced the coating with superior toughness, being the most suited for cutting applications. Increasing the bias voltage resulted in a loss of toughness, however, there was an increase in hardness and plasticity. Choosing the right deposition technique is of great importance when fabricating a tool coating. Thus, there have been some studies whose compare some of these techniques in the deposition of tool coatings, as seen in the study presented by Tillman et al. [46] where a comparison between DCMS (Direct Current Magnetron Sputtering) and HiPIMS in the deposition of TiAlN and TiAlN/TiAlCN coatings is made. The films were deposited in heat-treated AISI H11 steel, and the samples were evaluated regarding wear resistance and residual stresses. The authors found that the coatings obtained by HiPIMS had significantly higher residual stresses than the DCMS coatings. Furthermore, the adhesion of the coatings obtained by DCMS was higher. However, the TiAlN coatings deposited by HiPIMS displayed higher wear resistance than the other coatings obtained by DCMS. The problems presented in the last study have been researched as well, with some solutions for the adhesion problems and the higher compressive stresses of HiPIMS coatings being presented, such as the use of substrate surface texturing methods using etching process, which can help increasing the coating's adhesion and relieve excessive compressive stresses [47]. There are also some very recent studies on a novel deposition technique that can produce high ionization rates like the HiPIMS method. This method is the Continuous High-Power Magnetron Sputtering (C-HPMS). Liu et al. [48] studied TiAlN coatings obtained by this technique. The authors were able to obtain a coating with a very high hardness value (34.4 GPa) and a good adhesive strength (75 N). Moreover, the deposited coating presented very few particles on its surface. The method described in this paper paves the way to obtain droplet-free coatings and good mechanical properties by employing this method, presenting benefits such as fast deposition rate and efficient ionization.

Evaporation methods are seeing some recent research as well, with DC arc evaporation being the most common technique among them. However, some attention is being given to the cathodic arc deposition method, with studies being made relating deposition method and parameters to the coating's overall properties [49], even relating rotation speed during deposition and substrate orientation to the mechanical properties of deposited coatings [50], such as the study previously presented [43]. However, this deposition technique is being recently used mostly for the deposition and synthetization of borides and borides-related coatings, which are unable to be obtained by DC arc evaporation [49,51].

Regarding coating characterization as it was previously mentioned, coatings can be designed in order to fit a certain application, by controlling its architecture and composition, and thus, its microstructure and mechanical properties. There are various different coating designs applied to substrates for a wide range of applications. These can be observed in Figure 2.

**Figure 2.** Different types of coating structure commonly applied to substrates [24].

The coatings can be identified as follows:


Different types of coating structure are chosen to deal with different kinds of problems, for example, the use of a multilayer coating can improve significantly upon the properties of the single layer coated tool. For example, a multilayered coating has significantly more crack propagation resistance than a single layered coating. The number of layers contributes to this, furthermore, an increase in the number of layers will also increase properties such as hardness [24,52,53]. A scheme of how crack propagation usually behaves depending on coating structure can be observed in Figure 3.

**Figure 3.** Crack propagation behavior for each of the common coating structure [24].

Layers can also be added to improve adhesion properties, enabling that the deposition of the outer layer, usually the "work" layer, with high adhesive strength. Usually this is done when the outer layer has problems with adhesion to the substrate. Coatings are also characterized by their chemical composition. The elements that constitute the coating confer it with different properties, as certain elements can improve properties such as, corrosion resistance, wear resistance and even thermal conductivity. For example, the first coatings (TiN) were improved by the addition of aluminum, creating the TiAlN coating. This coating proved itself to be very useful in high-speed machining application, being widely employed in many machining applications to this day, because they present an oxide layer created between the tool–workpiece interface, thus conferring this coating with high oxidation resistance [54–57]. Still regarding coating characterization, these are also characterized by their microstructure. Different coatings have different types of

microstructure, depending on the deposition method, composition and their architecture, as described in the paper developed by Du et al. [58], where the effect of interlayers of Cr and Ti on the structure of TiAlN based coatings is studied. The authors deposited four types of coatings onto cemented carbide substrates, these being Cr/(Ti,Si,Al)N; Ti/(Ti,Si,Al)N; Cr/TiAlN; and Ti/TiAlN. It was found that the presence of these interlayers influenced the microstructure of the coatings. The Cr interlayer affects the growth of TiAlN based coatings, with the structure of the coatings containing this interlayer exhibiting a mix between columnar crystal morphology and equiaxed crystal morphology. In the case of Ti interlayer, the morphology was columnar. The Cr interlayer also promoted a better adhesion of the TiAlN based coatings onto it.

In this review paper the properties of TiAlN based coatings are going to be evaluated and presented, based on the information collected from recent articles conducted on this topic. The various types of coatings are going to be presented in the subsequent sections, mentioning in more detail these coating's properties such as, structure, microstructure and composition and its influence on the coating's mechanical properties, especially hardness values and young's modulus values. Moreover, the recent applications of these TiAlN based coatings in machining are going to be analyzed, highlighting the coating's performance on the various machining process (primarily turning and milling). Still regarding coating performance in machining, the wear mechanisms that these coatings suffer are also going to be analyzed and compared between each-other, as the analysis of these wear mechanisms gives very valuable information regarding the optimization and improvement of these machining processes [39,43]. This review intends to fill an existing gap about structured information regarding TiAlN-based coatings utilized in machining tools, mainly based on the most recent developments published in this field. This information, regarding wear behavior and the mechanical properties of the coatings, is going to be presented under the form of tables in order to convey a clear and easy-to-read message. Furthermore, the various types of structure of TiAlN-based coatings were divided into sections, with a section for monolayered, multilayered and nanolayered TiAlN-based coatings being created.

#### **2. TiAlN-Based Coatings**

Since its development in the 1970s, the TiAlN coating offered a great opportunity for enhancing tool life and performance for high-speed machining applications. Due to its properties, TiAlN and TiAlN-based coatings are still among the most used coatings for machining applications today, making them a very appealing research matter, with many new coatings being tested and evaluated for a wide variety of machining applications. Furthermore, recent studies also focus on the influence of new doping elements in the properties of TiAlN-based coatings.

In this chapter, recent developments made on TiAlN-based coatings are going to be presented, namely:


In total, three coating types are going to be divided into subsections inside this section. Additionally, the information regarding these coating's mechanical properties, especially Hardness and Young's Modulus values, are going to be presented in a subsection for each of the coating types. The wear mechanisms that these coatings suffer for various machining applications are also presented, mentioning the wear behavior of these types of coatings

and presenting the obtained values for tool life based on the information provided by the various analyzed articles.

#### *2.1. Monolayered TiAlN-Based Coatings*

Regarding monolayered TiAlN-based coatings, recent research has been made on the effects on the coating properties of doping TiAlN coatings with certain elements. It has been found that the addition of certain elements to the coating can improve properties such as corrosion resistance and wear behavior. Additionally, the addition of these elements is also linked to an improvement in mechanical properties such as hardness and Young's Modulus. In the study performed by Yang et al. [59], the influence of Mo content on TiAlMoN films is presented. The authors have produced five types of TiAlMoN coatings, varying the amounts of Mo. Composition of the samples used in this work can be observed in Table 1.

**Table 1.** Chemical composition of the TiAlMoN coatings developed in the work presented by Yang et al. [59].


The authors analyzed these coatings determining the mechanical properties for each one of the produced samples. Furthermore, the microstructure of each of the TiAlMoN films was analyzed. It was noticed that the hardness and Young's Modulus increased with the addition of Mo, reaching peak levels of hardness for 12.1% Mo content (S5), concretely, and 50 GPa and 610 GPa for hardness and Young's Modulus value, respectively.

Authors registered the influence of the Mo content on the microstructure, by obtaining SEM images of the coating's cross-section. This phenomenon can be observed in the following images, starting with Figure 4, depicting the film's structure with a Mo content of 2.8 at. % (a) and 6.9 at. % (b).

**Figure 4.** SEM cross-section of the TiAlMoN films with differing Mo contents: 2.8 at. % (**a**); 6.9 at. % (**b**), presented by Yang et al. [59].

It can be observed that at low Mo contents, the structure presents some columnar grains, however, it is not yet uniform. This uniformization was registered at a Mo content of 8.3 at.%. This can be observed in Figure 5.

**Figure 5.** SEM cross-section of the TiAlMoN films with differing Mo contents: 8.3 at. % (**a**); 10.1 at. % (**b**), presented by Yang et al. [59].

There is a change in the film's structure at 10.1 at. % of Mo content, the film's structure starts becoming columnar, finally becoming fully columnar at a Mo content of 12.1 at.%. The latter structure exhibited face-centered cubic TiN-based phases with a preferred orientation. The film's structure with a 12.1 at. % Mo content is displayed in Figure 6.

**Figure 6.** SEM cross-section of the TiAlMoN films with differing Mo contents: 12.1 at. %, presented by Yang et al. [59].

The addition of Mo can also improve the wear behavior of the coating, however, the wear behavior of the TiAlMoN films did not increase with the Mo content, as seen for the mechanical properties. The coating's that exhibited the best wear behavior was the S3 coating, with a Mo content of 8.3 at. %. This is based on the toughness and mechanical properties of the film. One way to evaluate the wear performance of the film is by the analysis of the H/E (Hardness/Young's Modulus) ratio. This ratio can be related to the film's toughness; however, it provides information regarding the plastic deformation of the film (with high H/E ratio values being tied to high plastic deformation resistance). The S3 coating displaying the highest value for this ratio, among all the other TiAlMoN coatings. Still regarding the influence of Mo addition to TiAlN-based coatings, the work presented by Tomaszewski et al. [60] also evaluated the influence of the addition of this element, concluding that the addition of this element is linked to an improvement of mechanical properties. Indeed, a small addition of up to 7.7 at. % allowed to report a significant improvement on the wear behavior of the coating. The authors also registered an improvement on the corrosion resistance properties of the coating, observing that the coating exhibited an improved resistance to pitting corrosion.

The addition of Nickel to TiAlN-based coatings is also an interesting topic. Similar to Mo, the addition of Ni to coatings also has an influence on the microstructure, as reported by Yi et al. [61] in their study, where three samples of AlTiN-Ni with differing contents of Ni are analyzed and subsequently tested in the turning of Inconel 718. The analyzed AlTiN-Ni coatings had 0%, 1.5% and 3% Ni. The AlTiN-Ni coatings were obtained via PVD cathodic arc evaporation. Figure 7 shows the three cross-sectional images of the coatings.

**Figure 7.** AlTiN-Ni coatings with differing Ni contents: 0% (**a**); 1.5% (**b**); 3% (**c**), presented by Yi et al. [61].

There is an evident change in microstructure depending on the Nickel content. Observing Figure 7a, the microstructure of the AlTiN-Ni coating is columnar. However, it can be noted that the addition of Ni promotes a homogenization of the structure (nanocrystalline structure). The authors registered the hardness and Young's Modulus values for these coatings. AlTiN-Ni with 0% Ni content exhibited the highest values for these properties (26.2 GPa and 315 GPa for hardness and elastic modulus, respectively). These values decreased with the increase in Ni content. For a Ni content of 1.5%, the values registered for hardness and Young's Modulus were 24.3 GPa and 315.8 GPa, respectively, seeing another decrease to 20.9 GPa and 300.5 GPa for a Ni content of 3%. From these presented values, the decrease in mechanical properties was not very accentuated from 0% to 1.5% Ni content. Furthermore, the authors observed that the toughness of the AlTiN—Ni (1.5%) coating was the highest of all the coatings, improving tool life by 160%.

Still regarding the addition of elements to TiAlN-based coatings, in the study carried out by Liu et al. [62], the addition of Ruthenium (Ru) to TiAlN coatings is performed and evaluated. The authors compared the microstructures of base TiAlN coating and two other coatings with differing Ru contents—7% and 15%, respectively. As seen in the work presented above [61], it was noted that the Ru addition promoted a change in the coating's microstructure, and similar to the addition of Ni, it promoted a homogenization of the microstructure. By analyzing Figure 8, the microstructure changes from a columnar structure to a homogenous structure.

**Figure 8.** SEM cross-sectional image of TiAlN coatings with different Ruthenium contents: 0% (**a**), 7% (**b**), 15% (**c**), presented in the study carried out by Liu et al. [62].

An increase in mechanical properties was also registered for the coating with 7% Ru (33.15 GPa and 498.55 GPa for hardness and Young's Modulus, respectively), however, the coating with 15% Ru content showed a decrease in hardness and Young's Modulus value, even when compared to the base TiAlN coating. Other elements, such as Ytrium (Y) and Tantalum (Ta), are linked to an increase in properties for TiAlN-based coatings, as seen in the study performed by Aninat et al. [63], where the addition of Y is linked to an increase in hardness values. Ta, however, did not have a significant influence on the coating's mechanical properties, influencing the amount of residual compressive stresses of the coating, which is linked to the wear behavior of the coating. Ta coatings exhibited a better wear behavior, whereas Y doped coatings exhibited overall better mechanical properties. Regarding coating residual stress control, this factor has also been linked to coating thickness, with this being analyzed by Chandra et al. [64]. In total, three TiAlN coatings with differing thicknesses were studied. Their mechanical properties suffered low variation, exhibiting a small decrease with a thickness increase. However, the residual stresses are greatly influenced by the coating's thickness, exhibiting higher values for thinner coatings.

Another element related to a significant increase in mechanical properties is Silicon (Si), significantly increasing hardness and Young's Modulus values. TiAlSiN is an example of a TiAlN coating doped with Si. As seen in [65], TiAlSiN coatings have better mechanical properties and wear behavior when compared to TiAlN coatings.

Regarding recent improvements made on the properties and performance of TiAlNbased coatings, in the study carried out by Chaar et al. [66] two TiAlN coatings with high aluminum content and differing structures were studied. One coating had a fine-grain structure, consisting of a mixture of cubic and hexagonal phases (dual-phase coating), the other coating had a coarse-grain structure of cubic phase. It was found that, although the coatings are very similar in terms of composition, their structure greatly impacts the thermal behavior of the coatings, highlighting the advantage of controlling the coating's structure in order to obtain a desired result, especially in terms of thermal stability.

There have also been recent studies conducted on the improvement of TiAlN-based coatings' performance by applying texturing treatments to the substrates. These texturing treatments are linked with a slight increase in wear behavior and mechanical properties. Moreover, coating adhesion improves greatly in textured tools [67,68].

#### 2.1.1. Machining Applications and Coating Wear Behavior

In this subsection the recent studies on machining applications of monolayered TiAlNbased coatings are going to be presented. An analysis of recent studies was conducted on the wear behavior of these coatings. The tool wear mechanisms suffered by monolayered TiAlN-based coatings for milling and turning are going to be presented.

#### Milling Process

The milling process has a huge presence in the machining industry, with many studies being conducted about the improvement of this process, either by using new tool geometries, coatings or by employing optimization techniques, such as the Taguchi method [43,69] to optimize machining parameters. TiAlN-based coatings are also being researched, in order to try and improve the various machining processes where they are employed. As seen in the beginning of this section, using doping elements to improve the coating's mechanical properties [59–64]. Other approaches to improve the performance of these coatings involve the study of coating behavior under experimental machining conditions, or the study of the wear mechanisms sustained by these coatings during machining [39,43]. Ravi et al. [70] studied the influence of various lubrication methods on TiN and TiAlNcoated tool's performance. The authors conducted milling experiments under, dry, flooded and cryogenic (liquid nitrogen) conditions. Furthermore, the tests were conducted at 75, 100 and 125 m/min of cutting speed. The cutting temperature and cutting force variation were evaluated for all tools. It was observed that the use of the TiAlN-coated tool over the TiN caused an increase in cutting performance, exhibiting a reduction of approximately 13% in cutting force for all cutting conditions. Moreover, the cutting temperature was 18% lower for the TiAlN-coated tools when compared to TiN. Cutting temperature increased with higher cutting speed values, however, cutting force values decrease for higher cutting speeds. This was particularly evident for the cryogenic cutting conditions, where the TiAlN-coated tools benefited greatly from this, exhibiting the lower cutting force value of all tools for all cutting conditions. This force variation for all coated tools under the three lubrication conditions can be observed in Figure 9.

**Figure 9.** Cutting force variation (N) for different cutting speed values, for TiN and TiAlN-coated tools under the three different machining conditions, presented by Ravi et al. [70].

It was reported by the authors that in this case the main wear mechanism was adhesion and abrasion, for all the cutting conditions, these effects being attenuated by the employment of flooded and cryogenic cutting conditions, especially for the TiN coating. The TiN-coated tool suffered more adhesive and abrasive wear than the TiAlN-coated tool, this can be explained by the Al contained in the coating. This element can confer the coating with better thermal properties, explaining the fact that these are less affected by the temperature dependent adhesive wear. Still regarding the wear analysis of TiAlN-based coatings, Siwawut et al. [71] evaluated the cutting performance and wear characteristics of TiAlSiN and CrTiAlSiN coatings. These coatings were deposited by filtered cathodic arc onto WC (Tungsten Carbide) inserts. The authors tested the coated tools and one uncoated tool in the dry milling of cast iron turbine housings, using a range of 14–300 m/min for cutting speed. Coating's mechanical properties were evaluated, namely hardness and Young's Modulus values. The TiAlSiN coating exhibited the highest hardness value of

all tools and the highest Young's Modulus value of both the coated tools, although these values have been very similar to those of the CrTiAlSiN. This produced a higher surface finish quality when using the CrTiAlSiN-coated tool when compared to the CrTiAlSiN coated tool. The H/E ratio of these coatings was also evaluated, as this ratio is strongly correlated with wear performance. The CrTiAlSiN coating exhibited the highest value of all tools (0.112), being followed by the TiAlSiN coating (0.105). The insert wear was analyzed using SEM, whose images can be observed in Figures 10 and 11.

**Figure 10.** SEM micrographs of the TiAlSiN-coated WC insert, low magnification (**a**), and high magnification (of the zone marked with a pink square) (**b**), presented by Siwawut et al. [71].

**Figure 11.** SEM micrographs of the CrTiAlSiN-coated WC insert, low magnification (**a**) and high magnification (of the zone marked with a pink square) (**b**), presented by Siwawut et al. [71].

Both the coatings improved the wear behavior of the WC inserts, however, the Cr-TiAlSiN coating performed better in this regard, exhibiting lower values for flank wear and suffering overall less damage, when compared to the TiAlSiN-coated insert. The authors observed that TiAlSiN-coated inserts displayed primarily ploughing abrasive wear behavior, while the CrTiAlSiN-coated inserts exhibited wear related to thermal cracking and partial coating delamination.

As mentioned previously, the level of residual stresses inside the coatings impacts their properties and, thus, their machining performance [30,38,39,43]. There are some recent studies made on this topic, as the work presented by Hou et al. [72], where the influence of compressive stresses on the TiAlN-coated tool's performance when milling Ti alloy Ti-6Al-4V was investigated. Both coatings had the same composition and were deposited onto the same substrate material, one of them was unaffected by compressive stresses (Coating 1) and the other was affected (Coating 2). These coatings were characterized, determining hardness and Young's Modulus values, presented in Table 2.


**Table 2.** Hardness and Young's Modulus values determined for TiAlN coatings (Coating 1 and 2), presented by Hoy et al. [72].

The residual stresses have a clear benefit for the coating's mechanical properties, with Coating 2 having an increase in hardness and Young's Modulus values by 12% and 15%, respectively. These were then subjected to milling tests, having their wear behavior analyzed. In Figures 12 and 13 the wear mechanisms sustained by the two coated tools can be observed.

**Figure 12.** Wear patterns exhibited by Coating 1 after machining: SEM image of flank face (**a**); magnification of area b (**b**); magnification of area c (**c**); magnification of area d (**d**). Presented by Hou et al. [72].

**Figure 13.** Wear patterns exhibited by Coating 2 after machining: SEM image of flank face (**a**); magnification of area b (**b**); magnification of area c (**c**); magnification of area d (**d**). Presented by Hou et al. [72].

From Figures 12 and 13, it can be observed that the wear sustained by Coating 1 is more intense than that of Coating 2, the former presenting more cracking and spalling than the other coating. There is, however, material (titanium) adhesion on both the tool coatings. The sustained flank wear over the cutting length for both coatings was also analyzed. The authors determined that the wear was less intense on Coating 2. The wear behavior for both coatings can be observed in Figure 14.

**Figure 14.** Flank wear measurements (μm) for various cutting lengths, for both coatings, presented by Hou et al. [72].

Compressive stresses can improve the wear behavior greatly and reduce the wear, as seen from Figures 12 and 13, because the coating unaffected by stresses suffered more cracking, and subsequently, coating spalling damage, while the coating affected by compressive stresses exhibited less cracking and less wear damage for equal cutting length values. As the crack propagates deeper into the coating, high stress values can prevent this crack propagation, retarding this damage.

As seen from presented recent works, there seems to be a focus on the optimization/study of milling processes of hard-to-machine materials, as titanium alloys [72] and some other alloys with high strength applied primarily to the aeronautical industry such as aluminum, particularly from the 6000 and 7000 series [69]. Another of these alloys is Inconel, with some recent works being conducted on the study of milling cases of this alloy. TiAlN coatings have seen some application in the machining of this alloy, as seen in the study performed by Sen et al. [73], in which the wear behavior of TiAlN coated carbide tools is analyzed in the milling of Inconel 690. Similar to the study presented by Ravi et al. [70], the authors have studied the influence of different lubricating conditions on the performance of TiAlN coating, using flooding, MQL (Minimum Quantity Lubricant) with palm oil, and MQL with 0.9% alumina enriched palm oil. Due to the high temperatures developed during the machining of this material and its characteristic high hardness, the wear mechanisms that were reported were mainly abrasion and adhesion. The wear related to these mechanisms can be observed in Figure 15. This was registered for all the lubricating conditions, however, the MQL 0.9% alumina enriched condition led to less wear damage on the tool.

**Figure 15.** Abrasion marks and adhered material on TiAlN coating after milling of Inconel 690 (under the MQL with palm oil lubricating condition), presented by Sen et al. [73].

The main wear mechanisms that TiAlN-based coatings suffer in milling operations are mainly adhesion and abrasion. These mechanisms are more evident and intense in the milling experiments conducted on hard-to-machine materials, where the high machining temperature and material hardness heavily impacts tools life. High hardness values will induce heavy abrasive wear on the coatings and the high temperatures will promote adhesion of the machined material to the tool. This leads to coating delamination and, ultimately, to tool failure. These wear mechanisms are also registered for micro-milling experiments. There is a recent trend for the application of TiAlN based coatings for micromilling [74], this because of the properties that make this coating ideal for high-speed machining, whose are also suited for this process, due to high rotational speeds usually used in micro-machining. Recent studies focus on the machining of hard-to-machine alloys [75], employing TiAlN coatings and AlTiN [76] coatings in the machining of titanium alloys [75,76] and nickel-based superalloys, such as Nimonic 75 [77]. The main wear mechanism sustained by coated micro-milling tools, when machining hard-to-machine materials, are adhesion and abrasion, as seen for the milling process, with cutting speed and depth of cut being registered as the main influencer on the development of this wear.

#### Turning Process

The turning process as also seen some applications of monolayered TiAlN-based coated tools. Studies made on this topic, similarly to the ones conducted on the milling process, evaluate various coatings performances and wear behavior. The studies seem to focus on the turning of hard-to-machine alloys, however, these are focused primarily on the machining of Inconel. Zhao et al. [78] study the influence of coating thickness on the machining performance of TiAlN-coated tools. The authors studied two TiAlN coatings, TiAlN-1 having 1 μm thickness, and TiAlN-2 with 2 μm thickness. Both coatings were deposited onto a WC-Co carbide and were employed in the dry turning of Inconel 718. Turning tests were performed at the cutting speeds of 30, 60, 90 and 120 m/min. The cutting forces developed during turning were evaluated, concluding that using coating with less thickness would result in lower cutting force values. Moreover, the cutting temperature was lower when using the thinner coating. Regarding the wear mechanisms and wear behavior of these coatings, these are presented in Figures 16 and 17. These images depict the wear sustained by both coatings at the tested machining speeds.

**Figure 16.** Wear of TiAlN-1 coated tool at different cutting speeds: 30 m/min, with magnification of marked area (**a**); 60 m/min, with magnification of marked area (**b**); 90 m/min, with magnification of marked area (**c**); 120 m/min, with magnification of marked area (**d**), presented by Zhao et al. [78].

**Figure 17.** Wear of TiAlN-2-coated tool at different cutting speeds: 30 m/min, with magnification of marked area (**a**); 60 m/min, with magnification of marked area (**b**); 90 m/min, with magnification of marked area (**c**); 120 m/min, with magnification of marked area (**d**), presented by Zhao et al. [78].

The presented coatings both exhibit the same type of wear mechanisms, these being built-up edge (BUE), pitting and coating delamination. From the figures it can be noted that adhesion is also a problem, this last mechanism being responsible for BUE and coating delamination. These wear mechanisms, as in milling, are characteristic of machining hard-to-machine materials such as Inconel, due to material's mechanical properties. Still regarding TiAlN coating's performance when turning Inconel, the study performed by Kurniawan et al. [79] evaluates the machinability of modified Inconel 713C, using a TiAlNcoated WC tool. The cutting characteristics of Inconel 713C are very similar to those of Inconel 718, making it a very hard to machine material. The authors have reported abrasive wear as the main wear mechanism in the tool's flank, being followed either by tool failure or BUE (Build up Edge) formation, as can be observed in Figure 18. High machining temperatures and the ductility of Inconel 713C, caused material adhesion to the tool, which promoted abrasive wear and subsequent coating delamination.

**Figure 18.** Flank wear of TiAlN-coated WC turning tool at 100 m of cutting distance used in machining Inconel 713C, presented by Kurniawan et al. [79].

Another study performed by Zhao et al. [80], studies the cutting behavior of AlTiN coatings on the turning of Inconel 718 at cutting speeds of 40, 80 and 120 m/min. Machining temperature and cutting forces were evaluated and compared to an uncoated tool. These factors both decreased when using the AlTiN coated tool, proving that this coating is suited for the turning of these alloys. Similar to the studies presented in [78,79], the main wear mechanism suffered by the tools during machining was abrasive wear. Both the TiAlN and AlTiN coatings proved to be very useful when machining hard-to-machine materials, effectively reducing cutting force and machining temperature values. These values are tied to wear rate, with the coated tools exhibiting a significantly lower wear rate than uncoated turning tools [81]. The addition of Si to TiAlN coatings is known to significantly improve their mechanical properties [71], thus making these types of coatings ideal for the machining of materials such as titanium alloys. Lu et al. [82] compares the performance of TiAlN and TiAlSiN-coated tools in the high-speed turning of TC4 titanium alloy. The authors also studied the performance of a gradient TiAlSiN coating. The microstructures of these coatings and the distribution of Si on the gradient coating can be observed in Figures 19 and 20.

**Figure 19.** Surface morphology and microstructure of: TiAlN coating (**a**); TiAlSiN coating (**b**), presented by Lu et al. [82].

**Figure 20.** Surface morphology and microstructure (**a**), and Si distribution on TiAlSiN gradient coating (**b**), presented by Lu et al. [82].

The coating's hardness was also evaluated, the TiAlSiN coating exhibiting the highest hardness value (21 GPa) followed by the gradient TiAlSiN coating (15 GPa). The gradient TiAlSiN coating presented an increase in hardness by 47% when compared to the TiAlN coating. An improvement in surface quality and adhesion is also registered for the gradient TiAlSiN coating. All the machining experiments were conducted at 100 m/min cutting speed under flooded lubrication. The coated tools exhibited the same wear mechanism, this being mainly abrasion and BUE, with some adhesion being registered on the TiAlN coated tool. Although the wear mechanism has been the same, their intensity varied, being more intense in the TiAlN coated tool. The coated tool's wear behavior is depicted in Figure 21, where flank wear values (measured in mm) are displayed for the three coatings, for various cutting lengths.

The improved adhesion and surface quality of the gradient TiAlSiN, coupled with the increase in mechanical properties due to the addition of Si provides this gradient coating with great wear performance, suffering overall less wear and at a much later stage in the machining process. Still regarding the addition of elements to improve coating turning performance, in the study performed by Kulkarni et al. [83] the authors study the influence of Cr addition to AlTiN coatings. Here, the authors evaluate the turning performance of AlTiN, AlTiCrN and TiN/TiAlN coated tools, by analyzing cutting forces, wear mechanism and tool-life values. The coating's microstructure was evaluated, with all coatings showing a dense columnar structure, however, in terms of surface morphology, the AlTiCrN coating exhibited a smooth surface. Furthermore, the adhesion strength of this coating was the highest of all three. The coatings were employed in the dry turning of SS 304 steel at a constant feed rate and depth of cut, varying the cutting speed from 140 to 320 m/min (in 60 m/min increments). Regarding the cutting force values registered during the process, these tended to decrease as cutting speed increases, however, the AlTiCrN coating exhibited the lowest cutting force values obtained of all coatings. This can be observed in Figure 22.

**Figure 21.** Tool wear of the three coated tools tested in the turning of TC4 titanium alloy: TiAlN (**a**), TiAlSiN (**b**), Gradient TiAlSiN (**c**), presented by Lu et al. [82].

**Figure 22.** Cutting force variation for the tested cutting speeds, for all coated tools at the following parameters: depth of cut = 1 mm; feed = 0.2 mm/revolution (based on data from [83]).

For the AlTiN tool, the main wear mechanisms were abrasion, chipping and BUE. This was registered for the TiN/TiAlN coating as well, however, there was some adhesive wear. For the AlTiNCrN coating, the main wear mechanism was abrasion, albeit less intense than in the AlTiN coating. Regarding tool-life of these coated tools, it tended to decrease with an increase in cutting speed. Further, as seen for the cutting forces, the AlTiCrN coating outperformed the other coated tools, exhibiting the highest tool life values, reaching 28 min (at 200 m/min). These values can be attributed to the coating's excellent adhesion properties, smooth surface and having high hardness.

From the presented articles conducted about the turning process using TiAlN-based tools, it can be concluded that the main wear mechanisms that coated turning tools are subjected to are, abrasion and adhesion, with the formation of BUE [39]. In the following Figure 23 a clear example of BUE can be observed; in the image the built up material can be seen the coated tool's edge.

**Figure 23.** BUE observed at a gradient TiAlSiN-coated tool's tip, after some machining (roughly 125 m of cutting length [82].

#### 2.1.2. Comparison of the Coating's Mechanical Properties

In this chapter an analysis of the mechanical properties of the monolayered TiAlNbased coatings registered in the various works presented in this section is going to be presented. The best hardness and Young's Modulus values for the main coatings, that were obtained by the various authors, were collected, and are presented in Table 3.


**Table 3.** Monolayered TiAlN-based coatings mechanical properties.

The addition of elements is a very influential factor on the coating's mechanical properties. However, the coating's deposition method, microstructure and level of residual stresses also impact on the coatings' mechanical properties, with some elements having a greater influence on the hardness, such as Si and Cr. This hardness increase is well documented for these elements, with coatings such as TiAlSiN seeing great use in machining applications, due to its increased wear performance when compared to base TiAlN coatings. New doping elements are being tried, with good results coming from the addition of elements such as Mo, Ru and Y, causing a significant increase in hardness and Young's Modulus values. However, research regarding machining applications for TiAlN-based coatings doped with these elements is quite sparce, as these types of coatings are quite novel. The addition of Ta is also quite novel for monolayered, yet this element was already implemented in the architectures of multilayered TiAlN-based coatings.

Some deposition methods and even variations on the composition of TiAlN-based coatings, such as TiAlN and AlTiN, also influence their mechanical properties (as seen in Chapter 1). One common case study is the evaluation of the coating's residual stresses on their mechanical properties and wear behavior. In most cases, some amount of residual compressive stresses is preferred, as it usually increases hardness and Young's Modulus values. Furthermore, these stresses are also related with better wear performance, lowering coating wear rates and crack propagation.

Regarding these coatings wear behavior, from the works presented above, it can be noted that the H/E ratio heavily influences the coating's wear performance, with a higher value being usually preferred. This ratio is presented in Figure 24, based on the information provided by Table 3. From the analysis of the table, it can be noted that CrTiAlSiN and TiAlSiN coatings are the ones with the highest H/E value. This is especially due to the addition of elements such as Cr and Si, that confer the tool with excellent mechanical properties and wear behavior. The TiAlMoN coatings also show a high ratio value, making them a very promising coating for machining applications. Furthermore, the addition of Mo also promotes a better corrosion resistance of the coated tool. Although these values can be indicative of the coating's wear performance, this is also dependent on the machined material. For example, coatings with low hardness value will experience more abrasion (despite having a higher H/E ratio). Thermal fatigue is also a factor, for example, while the CrTiAlSiN coating is less susceptive to the wear mechanisms such as adhesion and abrasion, which revealed has suffered from thermal cracking due to the machining's high temperatures [71]. This highlights the fact that coating choice is very important for a certain machining operation.

**Figure 24.** H/E ratios for the analyzed monolayered TiAlN-based coatings.

It is also worth to note that, although some coatings have very similar H/E ratios, their composition influences greatly their wear behavior and it should be factored on the coating choice.

#### *2.2. Multilayered TiAlN-Based Coatings*

Multilayered coatings are very appealing to the machining industry, as they enable combinations of various coating's properties to best fit a machining application. Further, their multilayered structure prevents crack growth [23,53]. This versatility makes them the most used type of coatings, in terms of coating structure, in the machining industry, for both the turning and milling sectors [39,43]. In terms of multilayer coating developments in general, there seems to be a trend in optimizing layer thickness [39]. Furthermore,

there is a focus on studying the behavior of various types of coating combinations. This seems to be the case on the studies conducted around TiAlN-based coatings. Analyzing recent studies made on this regard, new combinations based on the coatings presented in Section 2.1 are being studied. There are also some studies about the improvement of already well-known TiAlN-based multilayered coatings, such as the TiAlN/TiN, Ti/TiAlN and TiN/TiAlN coatings, as seen in this work conducted by Zhang et al. [88], where the authors study the cyclic oxidation of Ti/TiAlN coatings with differing thicknesses of Ti layer, having 0.15 and 0.3 μm for coating 1 and 2, respectively. TiAlN single layer coatings with differing thicknesses were also studied. Regarding the results obtained for the multilayered coatings, it was noted that the residual stresses of the thinner coating would be relieved after the tests. This coating exhibited cracking, as the stresses could not be accommodated. However, for the coating with the thicker Ti layer, the residual compressive stresses increased, with this coating showing no cracking due to the thicker ductile Ti layers. This is a relevant study, as compressive stresses contribute for the tribomechanical properties of the coating, leading to a better cutting performance [89]. Regarding the study of TiAlN/TiN coatings, Çomakli [90] compares the mechanical and corrosion properties of TiN, TiAlN and multilayered TiAlN/TiN-coated tools. It was reported that the multilayered coating presented a smoother surface, having a smaller grain than the surfaces of the other coatings. The hardness for the multilayer coating was also higher, due to the number of layers present in the coating [39,43]. Due to the combination of coating surface and mechanical properties, the multilayered coating had a lower friction coefficient, conducing to a lower wear rate. A similar study employs the use of CrN (Chromium Nitride) on a multilayered architecture for TiAlN coatings [91]. The author compares the wear performance and mechanical properties to TiAlN monolayer coating, reaching similar conclusions to those of the previous study, significantly improving the wear behavior by reducing the friction coefficient, while increasing the coating's mechanical properties.

As mentioned in the study presented above [88], the addition of interlayers of Ti can improve the wear performance of multilayered coatings. Similar to this, the work completed by Shugurov et al. [92] studies the influence of TiAl interlayers on a TiAlN-based multilayer coating. The authors study the influence of the number of layers/interlayers and their respective thickness on coating's mechanical properties and their wear behavior. The coating's structure can be observed in Figure 25.

**Figure 25.** Multilayer structure of one of the TiAlN/TiAl coatings presented by Shugurov et al. [92].

The authors concluded that four layers of TiAlN and three interlayers of TiAl with thicknesses of 0.6 and 0.2 μm, respectively, would produce the best wear performance, exhibiting a wear rate three times inferior to that of the monolithic TiAlN coating. The results of this study provide very useful information regarding coating design, as this method can be used for a wide range of applications, especially machining.

The use of Ta is also being recently researched, as shown in the work presented in Section 1 [27], where various multilayered TiAlTaN-(TiAlN/TaN) with differing layer thicknesses were tested and subsequently characterized. In another recent study carried out by Shang et al. [93], the mechanical properties of a multilayered TiAlN/Ta coating are evaluated. The coating consists of three layers, a TiAl layer followed by a TiAlN layer and finally, a Ta layer (Figure 26). The mechanical properties of the multilayered coating were compared to a TiAlN monolithic coating. TiAlN/Ta coating exhibited a higher value of hardness and elastic modulus (31 GPa and 315 GPa, respectively), showing an increase of 29% and 47%, respectively. Ta is a ductile material and can dissipate energy through deformation, thus minimizing crack propagation. Furthermore, this element confers the coating with a better thermal stability. Studies such as these [27,93] show that Ta is very beneficial to improve TiAlN-based coating's wear behavior.

**Figure 26.** SEM cross-sectional image of the TiAlN/Ta multilayered coating, presented by Shang et al. [93].

It is known that multilayered coatings benefit from all the properties of their layers, as seen in the previous study [93], where a Ta layer promoted a lower wear rate for the coating, based on its ductility (primarily). This can be true for the same coating, yet with different mechanical properties, as seen in this study performed by Zhao et al. [94], where various multilayered TiAlSiN coatings are studied. These coatings consist of alternating layers of TiAlSiN deposited at different chamber pressures, thus causing hardness variation between the two coatings. TiAlSiN-1, obtained at 0.08 Pa chamber pressure presented 32.25 GPa of hardness, while the TiAlSiN-2, obtained at 0.2 Pa, exhibited 37.56 GPa for hardness value. Coatings were produced with 2, 5, 7 and 10 alternating layers of TiAlSiN-1 and TiAlSiN-2. It was observed that the hardness values for coatings with five or more layers were very close (of about 37 GPa). This was also the case for their elasticity modulus, however, the coating with five alternating layers showed the best plastic deformation resistance, and thus, the lowest wear rate. Though, the wear rate for coatings with five or more layers were very similar. The wear behavior of the multilayered coatings was also compared to monolayered TiAlSiN coatings, the former being significantly lower than the latter. There are also some recent coatings being developed, showing promising results, such as the TiAlCN/TiAlN coatings [30], and the TiCrAlCN/TiAlN [95]. These coatings show promising results in terms of hardness and wear behavior, however, the amount of research made around them is still quite sparse.

Although the multilayered coating is the most used in machining, recent study trends show that most of the research made about these types of coatings is about nanomultilayered coatings. This is justified by the ability to obtain very thin layers using recent technologies. These types of coatings will be the focus in Section 2.3.

In the next section, recent machining applications of the TiAlN-based multilayered coatings are going to be presented. This will be done in the same manner as in the previous Section 2.1.1.

#### 2.2.1. Machining Applications and Coating Wear Behavior

Recent machining applications for TiAlN-based coatings were analyzed. As for the development and study of multilayered coatings, recent machining applications tend to be centered on nanolayered and nanocomposite coatings. However, there is still some research being made about the use of regular multilayered coatings in machining operations, namely milling in turning.

#### Milling Process

Regarding the study of the wear behavior of multilayered coated tools, there has also been some research made on this regard. Based on its versatility, the multilayered coating is very popular in the machining industry. However, in terms of recent research, there are few papers made on the study of the milling performance of new TiAlN-based multilayered coatings.

Due to their versatility and multilayer structure, these types of coatings usually outperform regular monolayered coatings, especially due to their toughness and crack propagation resistance. With recent studies such as the one developed by An et al. [96], where the performance of CVD and PVD coatings on the face milling of Ti-6242S and Ti-555 titanium alloys is evaluated. The PVD coatings is a multilayered TiAlN+TiN coating and the CVD coating is a TiCN+Al2O3+TiN coating. The cutting forces were evaluated and during the process, these were lower for the use of the PVD coating, as seen in Figure 27.

**Figure 27.** Cutting force value over machining time, for uncoated, PVD and chemical vapor deposition (CVD)-coated tools, presented by An et al. [96].

The wear sustained by the coated tools was also evaluated, reporting for the milling of Ti-624S that the main wear mechanisms for both CVD and PVD coating was microchipping and adhesive wear. Milling tests carried out on the Ti-555 alloy produced severe chipping and adhesive wear on the CVD-coated tool. However, in the PVD-coated tool, the main wear mechanism was adhesion, being less severe than with the other tool. The TiAlN-based coating proved to be better suited for the milling of these titanium alloys, showing better wear resistance and fracture resistance than the CVD-coated tool (the wear sustained by the PVD tools can be observed in Figures 28 and 29). This is not only due to the TiAlN properties for high-speed machining, but also due to the multilayered structure and residual stresses, characteristic of PVD coatings [43]. These characteristics make these types of coating highly resistance to crack propagation. Still regarding TiAlN-based

multilayer coating applications in milling processes, the TiAlN/NbN coating is also known for its machining applications, due to their excellent mechanical properties, as shown by Varghese et al. [97], where they determine the coating's properties and employ them in the dry end milling of AISI 304 steel. The authors studied and evaluated the wear suffered by the coated inserts, reporting that abrasion was the main wear mechanism, eventually resulting in coating chipping and breakage.

**Figure 28.** SEM and EDS analysis of the PVD coated tool's wear in milling Ti-555, EDS graphics correspond to the zones A and B, providing information regarding element composition, presented by An et al. [96].

As seen from Figures 28 and 29, and the results reported by the authors, the wear mechanisms sustained by multilayered coated tools in milling are very similar to those sustained by monolayered coated tools. Indeed, the main wear mechanisms present in the milling process are adhesion and abrasion. However, the multilayer architecture contributes to an improvement of coating's properties, such as wear resistance and crack propagation resistance. This causes these mechanisms to manifest at a much later stage of the machining process, albeit, in a similar way to monolayered coatings.

**Figure 29.** SEM and EDS analysis of the PVD-coated tool's wear in milling Ti-624S, EDS graphics correspond to the zones A and B, providing information regarding element composition, presented by An et al. [96].

#### Turning Process

Similar to the case of multilayered TiAlN-based coatings for milling applications, there seems to be very few new researches being made on this topic, with the attention being shifted to nanolayered and nanostructured coatings. However, there are some recent studies that focus on the wear performance of well-known TiAlN-based multilayered coatings, such as TiAlN/TiN. Analyzing their wear mechanisms when machining certain materials, such as in the study presented by Zheng et al. [98], in which the wear mechanisms of TiAlN/TiN coated tool are analyzed, for dry turning of 300 M steel. The coated tool suffered was mainly mechanical abrasion and adhesion, leading to chipping and coating delamination. It was also reported the existence of micro-cracks in the tool flank. This is due to the high machining temperatures developed during dry turning, which also promoted adhesion. The TiAlN/TiN-coated tool's wear can be observed in Figure 30.

The use of these types of multilayer coatings in hard-to-machine materials, and the study of its wear mechanisms is still a relevant subject. As the use coatings reduces machining forces and wear rates, thus making the dry machining method viable for the machining of these alloys. The use of TiN/TiAlN coating in the dry turning of Inconel 825 is studied by Thakur et al. [99], for finishing and roughing conditions. In total, three lubrication techniques were employed; these being dry, flooded and MQL. However, only the coated tool was employed in the dry machining. For the coated tool, the machining forces that were registered were lower, the surface finish of the machined material was better. However, the machining temperature was the highest for coated tool use. Wear was also analyzed for the tested tools, registering main wear mechanisms as abrasion and BUE and some minor coating delamination for the TiN/TiAlN-coated tool.

Multilayered TiAlN coatings are also being studied for turning applications. It is known that compressive residual stresses can improve cutting behavior and wear behavior of the coated tool's, however, to much compressive stresses can be detrimental for the coating's properties. As seen in this paper by Abdoas et al. [100], where the authors deposit three 11 μm thick TiAlN multilayer coating with different levels of residual stress. The three coatings, having 1.3 GPa, 2.1 GPa and 4.5 GPa, showed different cutting performances, with the coatings with fewer residual stresses having the most tool life. However, in terms of surface roughness, these coatings produced very similar results. The increase in tool life for the coating will less stresses, can be explained by the fact that this coating had better adhesion properties than the other coatings, thus promoting a better wear behavior for the coated tool. As for the wear mechanisms, these were the same for all three coatings, albeit in different intensities. The main ones that were registered were: material adhesion and BUE, with some abrasion being registered as well. This follows a similar trend to the wear mechanisms registered in monolayered coatings, similar to the milling cases mentioned in this section.

#### 2.2.2. Comparison of the Coating's Mechanical Properties

As in Section 2.1.2, here the mechanical properties of the studied multilayered TiAlNbased coatings are going to be presented. These values are taken based on the results presented in various articles about the development/study of these types of coatings. Unfortunately, the table is missing some values that were not provided by the authors.

The various multilayered coating's properties are now going to be presented in Table 4.


**Table 4.** Multilayered TiAlN-based coatings mechanical properties.

The study of multilayered TiAlN-based coatings and their application in machining is not very abundant. However, the studies that are made about this topic focus primarily on the improvement of already existing multilayered TiAlN-based coating. There are some studies on novel coating structures such as the TiCrAlCN/TiAlN, but there is few information regarding its mechanical properties. Furthermore, machining case studies with the application of these are also sparse, these focusing on already existing multilayered coatings. Recent studies also focus on the improvement of coating's mechanical properties by introducing a novel structure, as seen in the case for TiAlSiN recently released [94]. This is also the case for recent research about TiAlN multilayer coatings, where layer thickness and residual stresses are tied to mechanical properties and wear behavior. The use of Ta is also studied for multilayered coatings, with the research made on this topic bearing better results in terms of mechanical properties and wear behavior using a first layer of Ta, it was concluded that coating's hardness and wear performance was significantly improved.

As presented in Section 2.1.2, the H/E ratio of the various analyzed coatings is going to be presented in Figure 31. Only coatings with complete information regarding this ratio will be presented.

As it was noted for monolayered coatings, the addition of certain elements such as Si, Ta and Cr significantly improve the wear behavior of the TiAlN based coatings. Recent research also shows that residual compressive stresses, microstructure and layer thickness also influence the wear behavior of the coatings, with TiAlN and TiAlSiN multilayered coatings achieving high values of hardness and Young's Modulus values, while also having a good performance in terms of wear.

#### *2.3. Nanolayered TiAlN-Based Coatings*

Recent studies made on coatings tend, in general, to be about nanostructured and nanocomposite coatings [39,43]. Nanolayered coatings, similarly to the multilayered coatings, have an increased crack propagation resistance. This is more intense on nanolayered coatings, due to the higher number of layers. Hardness values also tend to be higher on these types of coatings.

Regarding TiAlN-based coatings, the research trend is the same for other coating types, being more abundant in the area for nanolayered and nanocomposite TiAlN-based coatings. These recent papers tend to focus on the study of various novel coatings, based on the development of monolayered TiAlN-based coatings, more precisely, using elements that are tied to an improvement in mechanical properties such as Mo, Ta and Cr. In this section the various novel nanolayered/nanocomposite TiAlN-based coatings that are under development/study are going to be presented, mentioning their mechanical properties and wear behavior. Furthermore, as done in previous sections, the machining applications for these types of coatings are going to be presented for turning and milling. The various wear mechanisms that these tools are subject to are also going to be mentioned, presenting a comparison between coatings (when possible).

There is a current focus on the study of these types of coating over regular monolayered and multilayered coatings, with commonly known multilayered structures such as TiAl/TiAN [101], being attempted at a nanometric scale, with thinner layers conferring the coating with improved mechanical properties, such as high hardness, improved corrosion performance and high coating adhesion. As the layers are considerably thinner when compared to regular multilayered coatings, the number of layers is also higher in nanolayered coatings. This not only increases hardness, but also increases the crack propagation resistance of the coating. The influence of the layer thickness in nanolayered coatings is

analyzed in the study performed by Wang et al. [102], where a TiN/TiAlN coating (another well-known multilayered architecture) is characterized in terms of mechanical properties and wear behavior. By controlling the rotation speed of coating deposition, the authors were able to control the thickness of deposited TiN and TiAlN layers. They determined the layer thickness value for highest mechanical properties (hardness and Young's Modulus), this value was found to be 13 nm. It was also noted that the wear behavior of the coating was improved for lesser thick layers, as this promoted crack propagation resistance. The coatings microstructure at different deposition rates can be observed in Figure 32.

**Figure 32.** SEM cross-sectional image of TiN/TiAlN coatings for a rotation speed of 1 r.p.m. (**a**); TEM cross-sectional images for rotation of: 1 r.p.m. (**b**), 2 r.p.m. (**c**), presented by Wang et al. [102].

Another multilayered structure that has got some attention is the Al/TiAlN coating. In the study carried out by Liang et al. [103], an Al/TiAlN nanocomposite coating deposited on AZ91D magnesium alloy is analyzed. Similar to the previous study, here the authors studied the influence of different layer thickness on the mechanical properties and microstructure of the coating. In addition to the properties, the authors also analyzed the corrosion resistance of the coating. The authors produced four coatings with a thickness of 5 μm. The phases that form the nanocomposite film are TiN nanocrystal and amorphous AlN. These coatings had different interface periods, 100 nm, 200 nm, 300 nm and 400 nm. Regarding coating microstructure, it is columnar for 400 nm periods, however, it changes to multi-interfaces as the period is thinner. Hardness values reached their highest value for the lowest thickness period (100 nm), reaching 31.3 GPa. The authors also noted that the reduced thickness induced improved corrosion resistance, with the lowest thickness performing better in this regard as well. Layer thickness influence is a highly researched topic in nanolayered coatings, being related with an increase in mechanical properties (primarily hardness) and, as mentioned before, in nanolayered coatings the overall thickness of the coating improves hardness, as the high number of layers promotes a hardness increase. Additionally, this high number of layers in thick nanolayered coatings promotes a compressive stress relief on the coatings. The number of layers and their arrangement in a nanolayered coating also influences coating performance, which can be controlled by changing target arrangement (in deposition) and altering deposition time. This is highlighted in the study presented by Seidl et al. [104], which evaluated the influence of target arrangement in producing various AlCrN/TiAlTaN coatings. The control of the number of layers of AlCrN or TiAlTaN can influence greatly the coating's properties, with AlCrN (in this case) promoting better mechanical properties, while the TiAlTaN coating promoted a better oxidation resistance at higher temperatures. As seen in this study, the authors used a TiAlN coating with Ta addition, an element that is recently being researched for monolayered TiAlN-based coatings, as its addition is tied to improve mechanical properties.

The addition of Si is also very beneficial for the coating's mechanical properties, with some studies evaluating new structures such as the structure presented in [105], consisting of a first layer of TiAlN, followed by a nano multilayered TiAlN/AlCrSiN coating and finally, an outer layer of TiAlN coating. This coating's structure can be observed in Figure 33.

**Figure 33.** SEM cross-sectional image of TiAlN-(TiAlN/CrAlSiN)-TiAlN coating deposited on a cemented carbide presented by Xian et al. [105].

From the studies presented in Section 2.1, elements such as Mo and Y, when added to TiAlN coatings can significantly improve their hardness and Young's Modulus values, as well as their wear performance and corrosion resistance. In the study performed by Pshyk et al. [106], the novel nanocomposite (TiAlSiY)N and nanoscale (TiAlSiY)N/MoN multilayered coating obtained by arc-PVD are evaluated. The authors analyzed the microstructure, phase composition and mechanical properties of these coatings. Regarding the microstructure, the authors concluded that the multilayered coating had the preferred orientation, when compared to the monolayered nanocomposite coating. Moreover, the mechanical properties of the multilayered coating were better than the nanocomposite coating, showing values of 38.37 GPa and 392.5 GPa for hardness and Young's Modulus values, respectively. The (TiAlSiY)N/MoN coating also exhibited a better fracture toughness and a higher H/E ratio than the monolayered coating, thus having a better wear performance. Another study carried out by Kravchenko et al. [107] compare this novel coating with other similar structured coatings, namely, (TiAlSiY)N/CrN and (TiAlSiY)N/ZrN. These coatings were also obtained by arc-PVD and their mechanical properties were characterized. From that study, the authors concluded that the (TiAlSiY)N/MoN coating had higher values for hardness and Young's Modulus (35.9 GPa and 406.8 GPa, respectively) than the other coatings, with (TiAlSiY)N/CrN coating having 23.4 GPa and 300 GPa, and (TiAlSiY)N/ZrN having 22.1 GPa and 271 GPa, for hardness and Young's Modulus, respectively. However, the H/E and the plastic deformation indexes were very similar for all coatings, which means that these coatings (CrN and ZrN multilayer) have good tribological properties. Studies such as these [106,107] highlight the benefits of using these nano-multilayered coatings, especially for extreme tribological applications. Regarding TiAlN based nanolayered films with ZrN, Wang et al. [108] studied the influence of Zr3N4 on a nano multilayered TiAlN/Zr3N4. Additionally, the authors also studied the influence of layer Zr3N4 thickness in the coating's mechanical properties, and, as seen in the studies previously presented, thinner layers promote higher hardness and H/E ratio values. The authors also report that the Zr3N4 causes a significant increase in coating hardness (34.7 GPa) while retaining a very high toughness, conferring this coating with an excellent wear behavior. The hardness, Young's Modulus and H/E ratio variation of TiAlN/ Zr3N4 for different layer thicknesses can be observed in Figure 34.

**Figure 34.** Hardness, Young's Modulus values (**a**) and H/E ratio values (**b**) for TiAlN/ Zr3N4 nano multilayered coatings, presented by Wang et al. [108].

The addition of Cu to tool coatings has some advantages, such as the reduction of friction coefficient as it is a soft and ductile material. The addition and use of these elements in nanocomposite coating has got some promising results, as seen in the study carried out by Chen et al. [109], where nanocomposite TiAlN/Cu coatings provided with varying percentages of Cu concentration (0–1.4 at % Cu concentration) are deposited by filtered cathodic arc ion plating. The authors have evaluated the coating's microstructure (Figure 35) and mechanical properties.

**Figure 35.** SEM cross-sectional images of TiAlN/Cu coatings with different concentrations of Cu: 0% at. % Cu (**a**); 0.8 at. % Cu (**b**); 1.4 at. % Cu (**c**), presented by Chen et al. [109].

From Figure 35, it can be observed that the addition of Cu results in a reduction in grain size, decreasing from 45 nm (in TiAlN) to 30 nm for the coating containing the highest Cu concentration. TiAlN presents a distinct columnar structure that gradually fades away with the addition of Cu. Regarding the coating's mechanical properties, there is a decrease in hardness value and Young's Modulus value with the increase in Cu concentration. There is also an influence on the friction coefficient, with the lowest value being obtained for a 0.8 at. % Cu concentration. Other Cu concentrations produced a higher friction coefficient, surpassing even the TiAlN coating. The addition of this element is an interesting topic, however, the sacrifice in mechanical properties does not seem to be relevant enough to use it to decrease in friction coefficient. However, the results presented in that paper show potential for the employment of Cu in coatings, as a decrease in friction coefficient is desirable, especially for applications such as machining. Still regarding the additions of softer elements to hard coatings such as TiAlN, in a similar study performed by Mejía et al. [110], the characterization of a TiAlN (Ag,Cu) nanocomposite coating is studied. As in the study previously mentioned here [109], the influence of the addition of different concentrations of (Ag,Cu) nanoparticles on the coating's mechanical properties and microstructure is studied. The addition of this softer element causes a grain refinement in the TiAlN coating's microstructure, changing from a columnar structure to an amorphous one with a smaller grain. Equal to the addition of Cu, the addition of these softer elements causes a decrease in mechanical properties (hardness and Young's Modulus value) and a decrease in friction coefficient. Additionally, the authors noted that with an increasing concentration of (Ag,Cu) the coating's residual compressive stresses would decrease.

#### 2.3.1. Machining Applications and Coating Wear Behavior

In this section, the various studies regarding milling and turning applications of these types of coatings are going to be presented, mentioning the improvements that these types of coatings bring for machining. The wear behavior described in these studies is also going to be analyzed and described in this chapter.

#### Milling Process

Recent research on TiAlN-based coating's performance seems to be shifting to the use of these nanolayered and nanocomposite tool coatings. There have been many improvements recently on the development of new promising coatings for machining applications. However, the today's studies seem to focus on nanolayered and nanocomposite coatings containing elements such as Si and Cr, known as able to confer excellent mechanical properties and cutting performance to tool coatings. This recent paper presented by Geng et al. [111] studies the milling performance of TiSiN/AlTiN nanolayered composite film. The authors also evaluate de coating's microstructure and mechanical properties, which can be observed in Figure 36.

**Figure 36.** Surface (**a**) and SEM cross-sectional image (**b**) of TiSiN/AlTiN film, presented by Geng et al. [111].

Regarding the coating's mechanical properties, the registered peak values for hardness and Young's Modulus were 41.7 GPa and 340 GPa, respectively. These coatings were deposited onto 4-fluted end-mill with a 6-mm diameter and were subsequently employed in the dry milling of SKD 11 tool steel. The tool's wear was analyzed after the milling operations, and it was reported that the main wear mechanism was abrasion, as seen in Figure 37.

The authors noted that the TiSiN/AlTiN-coated tools showed little to no adhesion of SKD 11 to the coating's surface and the tools' edges. This resulted in a reduction of machining forces and cutting temperature, thus significantly improving the tool's life. Furthermore, there is a slight reduction in wear rate for a machining temperature of 400 ◦C. This study highlights the wear benefits that come from the employment of these coatings in machining.

As seen in the previous Section 2.3 (Nanolayered TiAlN-based coatings), multi nanolayered coatings with thinner layers exhibit an increase in mechanical properties (primarily hardness) and in wear behavior (low wear rate). This is also related to tool life, as presented by Teppernegg et al. [112]. Here, the authors studied a nano multilayer coating consisting of TiAlN and CrAlN sublayers with different thicknesses (10, 30, 100 and 300 nm), seen in Figure 38. These coatings are deposited onto inserts and their mechanical properties are evaluated; these are then employed in the milling of 42CrMo4 steel.

**Figure 37.** SEM images of the flank wear sustained bu TiSiN/AlTiN-coated tools after dry milling of SKD 11, presented by Geng et al. [111].

**Figure 38.** SEM cross-sectional image of the multilayered coatings with differing sublayer. 10 nm (**a**), 30 nm (**b**), 100 nm (**c**), 300 nm (**d**), presented by Teppernegg et al. [112].

It was noticed that an increase in Al content promoted higher hardness values being it related to cutting performance. Thus, the coating's that presented a higher Al content performed better in the milling tests. The sublayer thickness did not influence the hardness greatly, however, in terms of tool life this was not the case. The authors reported that with increased sublayer thickness the tool life would decrease. They were able to determine optimal sublayer thickness as seen in the Figure 39.

**Figure 39.** Tool life for the various layer thicknesses tested for the TiAlN/CrAlN nano multilayer coating, presented by Teppernegg et al. [112].

All the tested coatings exhibit the same type of wear. Coating failure occurs due to abrasion on the flank, causing coating erosion and delamination thus exposing the substrate. The other type of wear presented by the tools is thermal fatigue, this generating comb-cracks appearing on the coated inserts. These cracks can be seen in Figure 40.

**Figure 40.** SEM image of the coated insert's cutting edge, the black arrows mark the position of comb-cracks and the white arrows mark the position of cracks that are parallel to the cutting edge, presented by Teppernegg et al. [112].

The employment of TiAlCrSiYN/TiAlCrN has also seen some studies lately, with Chowdhury et al. [113,114] evaluating the mechanical properties of this coating for various coating architectures and differing interlayer thicknesses. The authors also employ these coatings in the dry milling of stainless steel at a machining speed of 600 m/min. As seen in previous studies, the thinner the interlayer thickness is, the higher the hardness is. Chowdhury et al. [113,114] also compare these multilayered coatings with monolithic TiAlCrSiYN and TiAlCrN coatings. It was concluded that the nanolayered coatings, with optimized interlayer thickness produced the best results in terms of cutting performance and wear behavior, indicating that these types of nanolayered coatings are an excellent choice for extreme machining applications.

#### Turning Process

Similarly, to the research presented for the use of nanolayered and nanocomposite coatings in milling, for the turning process research focuses on coatings with Cr and Si additions. These elements confer the coatings with excellent mechanical properties, and they improve the cutting performance significantly (especially when combined with TiAlN-based coatings). The use of these elements in nanolayered coatings has even more potential, increasing even more these properties and the cutting performance of coated tools, especially by extending the tool-life [115]. In the study conducted by Sui et al. [116] the performance of TiAlN/CrN coatings with different bilayer periods is evaluated in the high-speed turning of TC4 titanium alloy at 100 m/min. Bilayer periods between 12 nm and 270 nm were tested, and the influence of these periods on coating microstructure can be observed in Figure 41.

**Figure 41.** SEM cross-sectional image of the TiAlN/CrN coatings with different bilayer periods: 12 nm (**a**), 25 nm (**b**), 52 nm (**c**), 150 bm (**d**) and 270 nm (**e**), presented by Sui et al. [116].

With an increase in bilayer period, an increase in the coating's hardness was observed. This is due to a grain refinement that occurs with thinner layers. However, for the thinnest bilayer period, the hardness was also very high, such as the values obtained for the bilayer period of 270 nm. The authors also registered a higher wear rate for the thinner bilayer period (12 nm), with the thickest (270 nm) showing the best wear rate values. Regarding the wear mechanisms sustained by the tools, they were primarily abrasion and coating delamination. Although the tools experienced the same type of wear, the tool coated with the coating provided with the thickest layers exhibited less severe wear.

Still regarding the turning performance of nanolayered coatings, in the study developed by Zhang et al. [117], a comparison between AlTiN monolayered coating and AlTiN/AlCrSiN nano multilayered coatings is made. Furthermore, various nanolayered coatings were deposited with differing modulation period, that is, with differing layer thickness. The tested modulation period was between 4.2 nm and 17.8 nm. The various coatings mechanical properties were also determined. It was determined that the period of 8.3 nm (Figure 42) produced the best results in terms of hardness and Young's Modulus values for the coating (37.5 GPa and 486.9 GPa, respectively). Moreover, this coating exhibited the best values of H/E ratio and the best adhesion strength.

**Figure 42.** High-resolution TEM micrograph of AlTiN/AlCrSiN with a modulation period of 8.3 nm, presented by Zhang et al. [117].

The turning performance of these coatings was evaluated in the dry turning of SKD 11 tool steel at 250 m/min (cutting speed). The same wear mechanisms for AlTiN/AlCrSiN coated tools were reported, with abrasive and adhesive wear being reported as the main mechanisms. There was also the formation of BUE and some plastic deformation reported on the tool's rake face. The tool wear for the nanolayered coating with a modulation period of 8.3 nm can be observed in Figure 43. This coated tool exhibited the least wear rate of all coated tools, this is due to their high mechanical properties H/E ratio and high adhesion strength.

**Figure 43.** SEM image of the AlTiN/AlCrSiN (modulation period of 8.3 nm) coated tool's wear: rake face (**a**); magnified rake face (**b**), flank face (**c**), amplified flank face (**d**), presented by Zhang et al. [117].

From the studies presented, it can be concluded that abrasion is the common wear mechanisms sustained by nanolayered TiAlN-based coated tools. This is similar to the other types of coating's as well (monolayered and multilayered). However, the nanolayered coatings exhibit the highest tool-life values when compared to other types of coatings. This is due to their high count of very thin layers, conferring the coating with high hardness and high crack propagation resistance.

#### 2.3.2. Comparison of the Coating's Mechanical Properties

The research trend for TiAlN-based coatings is heavily focused on the development and characterization of novel nanolayered and nanocomposite coatings. There have been improvements to already-known coating structures that are heavily employed, such as the TiN/TiAlN and Al/TiAlN. Studies conducted about these known structures focus on the improvement of mechanical properties and wear behavior by taking advantage of the nanolayered structure's benefits, such as an increased hardness value, reduced friction coefficient, reduced wear rate and high crack propagation resistance. These benefits become nanolayered and nanocomposite coatings very appealing, and therefore there are a higher number of papers done about this topic, than about regular multilayered coatings.

In Section 2.1., the addition of Mo to TiAlN-based coatings was covered. This element promotes an increase in the coating's mechanical properties and wear behavior. This was also observed in nanolayered and nanocomposite coatings, showing satisfactory results in creating coatings with high toughness and hardness values. The mechanical properties from the coatings that were mentioned in the previous subsubsection are going to be presented in Table 5.

**Table 5.** Hardness and Young's Modulus values for the nanolayered and nanocomposite TiAlNbased coatings.


It can be seen from the number of coatings presented in Table 4 alone, that there is considerably more research being made in novel coating development for these types of coatings when compared to monolayered and multilayered TiAlN-based coatings. Once again, it can be seen that the use of Mo increases significantly the coatings mechanical properties. There is some research, however, made about the improvement of already known structures. In regard to the TiN/TiAlN, a very high value of hardness was achieved, due to the nanolayered coating's properties.

Regarding these coating's wear behavior, (TiAlSiY)N/MoN showed great potential with very high values of hardness and good Young's Modulus values, becoming the types of coating that are very appealing for extreme applications, such as where wear is very intense. Some satisfactory results also come from the use of ZrN on the coatings, especially when coupled with TiAlN, achieving incredible mechanical properties. Figure 44 shows the H/E ratio of the coating analyzed in Section 2.3.

**Figure 44.** H/E ratio for recently researched nanolayered and nanocomposite coatings.

From analyzing the H/E ratios of the various coatings the TiAlN/ZrN coating has an incredibly high ratio, due to its mechanical properties. Indeed, this coating showed an excellent wear behavior, making it very promising for machining applications. Although there is variation of these ratios, only the TiAlN/ZrN coating ratio does not vary too much, an indicator that the wear performance of the nanolayered and nanocomposite coatings is superior to that of monolayered and multilayered TiAlN based coatings.

#### **3. Machining Conditions and Tool Wear Mechanisms for TiAlN Based Coatings**

Most of the research made about the TiAlN-based coatings regarding machining applications, is usually done about hard-to-machine materials. In this section, the various papers presented in Section 2 will be analyzed, presenting the materials that are currently being used in TiAlN based coating's testing. There are two sections, one for milling and another one for turning.

#### *3.1. Milling Process*

The Table 6 summarizes the various coatings used to machine a certain material, the range of machining speeds used and the main wear mechanisms of the coating.


**Table 6.** Materials machined by TiAlN-based coatings, applied in milling.


**Table 6.** *Cont.*

As it was already concluded, the main wear mechanisms suffered while milling is abrasion and adhesion. For the milling of tool steel, abrasion is more predominant, however, for titanium alloys adhesion is more frequent. It is also important to note that multilayered and nanolayered tend to not suffer as much from adhesion problems.

#### *3.2. Turning Process*

Next, as in the previous section, the various coating used in the turning of various materials are going to be presented (Table 7), as well as the machining speeds that were used during testing.


**Table 7.** Materials machined by TiAlN-based coatings, applied in turning.

The main wear mechanisms sustained by TiAlN-based coated tools when turning is abrasion and BUE. The studies about the employment of these coatings on turning operations, similarly to the ones for milling, focus on the machining of tool steel, titanium alloys and Inconel alloys.

Regarding the machining of Inconel, there is a clear advantage in having a higher concentration of Al in the coating, as it is related to the reduction of adhesive damage when machining these alloys.

#### **4. Current Research Trends of TiAlN-Based Coatings**

An analysis of current research about TiAlN-based coatings has been made and presented in this paper. Developments regarding monolayered, multilayered and nanolayered coating are currently being made, with a clear focus on the study of the addition of certain elements to TiAlN-based coatings in order to improve them. There are also studies made about the development of new coatings, such as self-lubricating coatings. As seen in the first chapter, the use of solid lubricants can significantly improve the performance of the coated tool [5]. This nanocomposite coating proves to have great potential, especially due to its low friction coefficient (due to self-lubricating behavior) which results in reduced wear behavior.

Still regarding recent research trends of TiAlN based coatings, similarly to the other coating type, there seems to be a focus on the study and development of nanolayered and nanocomposite coatings as presented in the previous chapters. These coatings have very high hardness values, due to the high number of layers, conferring super-hardness to these types of coatings (more than 40 GPa). There is also quite a lot of research done about these super-hard coatings (that do not need to be nanolayered as seen in [59]). These TiAlN-based super-hard coatings have better cutting performance when compared to the other regular TiAlN based coatings. This is due to their high hardness, a careful architecture of the coating's microstructure [118] and addition of some elements such as Mo [59] and Ta [119].

Regarding the research made on doping elements, it seems to be a popular topic, there are quite a lot of studies about the influence of certain elements in TiAlN-based coatings, with some yielding very satisfactory results. The use of Mo in coatings is well documented, with these coatings containing Mo, such as MoSeC coatings [120]. Mo addition not only improves the coating's mechanical properties, such as hardness and Young's Modulus [59,106,107], but also improves the friction coefficient of the coating, which promotes a better tool-life (reducing wear rate) [121,122]. There are some studies conducted about the reduction of wear coefficient of coatings, such as the ones using Cu and Ag [109,110] as doping elements. As it was observed, the addition of these elements resulted in a decrease in the coating's wear coefficient, but also hindered its mechanical properties, as it caused a reduction of hardness in the coatings. Mo is quite popular as it offers an increase in the coating's mechanical properties and improves the wear behavior of the coating, as seen in [106,107]. Moreover, it can be used in multilayer architecture, to be applied under the form of nitrides, such as MoN. The development of these low-friction coatings that can perform in rough conditions is collecting some attention, as seen in the paper presented by Bondarev et al. [123], where the MoSeC coating is paired with TiAlN based coating: TiAlSiCN. This super-hard coating (with about 41 GPa of hardness) had a comb-like structure that confers a high-thermal stability [124] and can be tailored to have a high oxidation resistance [125]. When this coating is paired with the MoSeC coating, which presents low friction coefficient [120], a very hard coating with an excellent wear behavior can be achieved. The authors [123] evaluated the wear behavior of TiAlSiCN/MoSeC coatings and have found that the addition of MoSeC caused a reduction in the hardness values registered in [124]. However, this addition highly improved the wear performance of these coatings, reducing the friction coefficient and improving the wear behavior at higher temperatures (up to 300 ◦C). Studies such as these show that the employment of Mo in coatings is quite beneficial, thus making it a popular research topic. Another element that as seen some research is Si, as the use of this element is quite popular in machining applications, especially for the TiAlSiN coating, which exhibit better mechanical properties and wear behavior than the TiAlN coating [65]. Its employment is quite beneficial, being

used in some monolayered coatings, such as the CrTiAlSiN [71], and in some multilayered architectures [94], and nanolayered coatings [106,107,113,114]. As seen from the studies presented above [123–125] the TiAlSiCN coating has a very high thermal stability and excellent mechanical properties. From the study presented in [123], its employment in multilayered architectures also brings some benefits in terms of wear behavior. In the study performed by Golizadeh et al. [126], the authors evaluate the thermal stability and oxidation resistance of SiBCN/TiAlSiCN and AlOx/TiAlSiCN coatings. It was concluded that the base oxidation resistance and thermal stability of the TiAlSiCN coating were improved when using the multilayer coatings, with the AlOx/TiAlSiCN coating exhibiting the highest performance of all the coatings. Sill regarding doping elements of TiAlN-based coatings, some of these that are currently under research are Ta [63,119] and Y [63], yielding satisfactory results in terms of wear behavior and mechanical properties. There are, also, some novel coatings that use Ru [62,84] in their composition, which are also showing some promising results, however the amount of research in this matter is not as abundant as the other elements.

As previously mentioned, the nanolayered and nanocomposite coatings exhibit increased mechanical properties (presenting very high values of hardness generally) and wear behavior making them very appealing for the machining industry. New coatings have been developed, based on the research made about additive elements for monolayered coatings. Once again, the employment of Mo is under study for extreme applications, this time on nanocomposite coatings [106,107]. Still regarding the research made on nanolayered coatings, many studies are being made on the influence of layer thickness in coating's mechanical properties and cutting performance [39]. Furthermore, improvements are being made to already known coating architecture such as the TiAlN [89–100], TiAlSiN [65,94], TiAlN/TiN [90,96,98] and Al/TiAlN [103], with studies presenting increases in the coating's performances by employing methods to control layer thickness or concentration of phases, such as TiN and Al.

Regarding the machining applications that these coatings are applied to, from the analyzed research papers presented in this review, it is noticed that the coatings are mainly used in turning and milling operations. It was noticed that there are many research papers about milling operations using the TiAlN-based coated tools. However, for both machining cases, it was found that there is a large interest in the study of the wear performance of these coatings in the cutting of steels [70,71,74,81,83,97,98,100,111–114,117], titanium alloys [72,75,76,82,96,115,116] and Inconel [73,78–80,99]. This is due to these alloys being employed in the aeronautical industry, making the study of the machinability of these materials quite interesting. The applied coatings are usually novel nanocomposite coatings or already known structures (aforementioned) that have been improved in some way, being noticed that the coatings structure highly improves the wear behavior of these coatings (as previously mentioned). In the presented studies, the wear mechanisms are also evaluated, as there is a great interest in knowing how these coatings perform.

#### **5. Concluding Remarks**

In this paper, an analysis of recent research on TiAlN-based coatings applied to machining was performed. The main research topics about monolayered, multilayered and nanolayered and nanocomposite coatings were presented, mentioning the new developments made about these coating types, their benefits for the coating's mechanical properties, wear and cutting behavior. A comparison of these coating's mechanical properties was also made, as a way to link these to the coating's wear behavior. These coating's main applications were also analyzed and presented, mentioning the main wear mechanisms that these coated tools suffered when machining various types of material.

It was found that, regarding monolayered coatings, the research was primarily about the study of doping elements (as previously mentioned), with Mo additions yielding very satisfactory results. These additions promoted an increase in the coating's hardness, elastic modulus and toughness, thus promoting a slow wear rate for TiAlN-based coatings

containing Mo. This was also registered for other elements such as Ru, Zr and Ta. Regarding other type of coating architecture, the main focus of recent research appears to be nanolayered and nanocomposite coatings, as their mechanical properties exceed those of the other coating types. Some of these coatings are super-hard, presenting hardness values above 40 GPa. This is very appealing as this promotes the wear performance of the coating, protecting it from abrasive wear. Furthermore, these coatings present very high crack resistance and very high toughness.

These types of coating architecture have an influence on the coating's properties and subsequently on their wear performance and cutting behavior. Properties that are of high importance and improve the coating's wear behavior are:


Regarding the coating's wear mechanisms, it was found that the main wear mechanisms present in milling are adhesion and abrasion, however, the employment of nanolayered coatings improves the adhesive damage suffered by the coatings. Regarding the coatings employed in the turning process, these usually suffer abrasive wear and BUE, with some coating's exhibiting adhesive wear. As for milling, the use of nanolayered and nanocomposite coatings improves the cutting behavior and tool life of the coated tools, with these types of coating outperforming regular monolayered TiAlN-based coatings.

**Author Contributions:** V.F.C.S.: investigation and writing original draft; F.J.G.D.S.: conceptualization, supervision, writing—review and editing; A.B.: supervision, writing—review and editing; G.F.P.: supervision, writing—review and editing; R.A.: supervision and formal analysis. All authors have read and agreed to the published version of the manuscript.

**Funding:** The present work was done and funded under the scope of project ON-SURF (ANI| P2020| POCI-01-0247-FEDER-024521, co-funded by Portugal 2020 and FEDER, through COMPETE 2020-Operational Program for Competitiveness and Internationalization. LAETA/INEGI/CETRIB is acknowledge due to the support provided in all research activities.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest regarding this paper.

#### **References**


### *Review* **Recent Advances in Turning Processes Using Coated Tools—A Comprehensive Review**

#### **Vitor F. C. Sousa and Francisco J. G. Silva \***

ISEP—School of Engineering, Polytechnic of Porto, 4200-072 Porto, Portugal; vcris@isep.ipp.pt **\*** Correspondence: fgs@isep.ipp.pt; Tel.: +35-122-834-0500

Received: 13 December 2019; Accepted: 20 January 2020; Published: 23 January 2020

**Abstract:** Turning continues to be the largest segment of the machining industry, which highlights the continued demand for turned parts and the overall improvement of the process. The turning process has seen quite an evolution, from basic lathes using solid tools, to complex CNC (Computer Numerical Control) multi-process machines, using, for the most part, coated inserts and coated tools. These coatings have proven to be a significant step in the production of high-quality parts and a higher tool life that have captivated the industry. Continuous improvement to turning coated tools has been made, with many researches focusing on the optimization of turning processes that use coated tools. In the present paper, a presentation of various recently published papers on this subject is going to be made, mentioning the various types of coatings that have recently been used in the turning process, the turning of hard to machine materials, such as titanium alloys and Inconel, as well as the interaction of these coatings with the turned surfaces, the wear patterns that these coatings suffer during the turning of materials and relating these wear mechanisms to the coated tool's life expectancy. Some lubrication conditions present a more sustainable alternative to current methods used in the turning process; the employment of coated tool inserts under these conditions is a current popular research topic, as there is a focus on opting for more eco-friendly machining options.

**Keywords:** machining; turning process; turning tools; solid tools; cemented carbide; coated tools; coated cemented carbide; Physical Vapor Deposition (PVD); Chemical Vapor Deposition (CVD); multilayered coatings; nanolayered coatings; wear mechanism; tool life; minimum quantity Lubricant (MQL); cutting forces

#### **1. Introduction**

The machining industry has seen a significant growth in the past 5–6 years, and it is projected to be a 100 billion USD industry by 2025 [1]; this is primarily due to the high demand for higher quality products and computer numerical control machines (CNC), that enable manufacturers to develop these high quality and complex products at higher speeds [2]. Nowadays, there are 6-axis CNC machines, capable of turning feedstock bars into a complete product, which are quite appellative to the machining industry [3]. Moreover, the lathe and milling segment are still leading the market. In 2018, the lathe segment was leading the market, valued at 17.65 billion USD, followed by the milling segment [4]. The lathe market has grown significantly and is expected to keep growing in the following years [5].

The turning process can be described as the machining of a piece of material that is rotating, using a single pointed tool that is stationary, to produce a smooth and straight outside or inside radius on the piece. When turning, some considerations need to be taken into attention, such as the type of turning that is being made, for example longitudinal turning, and external or internal turning; quality demands, such as surface finish or tolerance must also be taken into consideration, because these are factors that depend on the material that is being machined, as well as the tools and the machine that is being used [6]. As previously stated, the tools used for turning are single-pointed, and can be

categorized into solid tools and insert tools; these tools can be coated to improve the process. Turning was initially carried out on a lathe, but the processes have seen significant evolution, particularly due to the development of computer numerical control (CNC) machines from the 1980s onwards, as well as turning centers, and later machining centers that could employ different processes such as turning and milling. This led to an increase in their use in the manufacturing industry [7], evolving from basic lathes, to multiple-process machining centers.

As the machines for turning evolved, so did the tools that were being used in the process, from solid tools with simple geometries, usually made out of steel, to coated cemented carbide tools, that make up to 80% of all tool inserts used in today's machining. These coated cemented carbide tools have improved the machining process significantly by enabling the machining of materials at faster rates than using conventional, uncoated tools [8,9]. These coatings are deposited on the surface of the tool to provide more wear resistance or a lower friction coefficient—in summary, these coatings provide a way to machine materials at higher speeds, while maintaining overall good surface quality and, especially, improving tool life, by reducing cutting forces, temperature and tool wear. There have been recent developments in cemented carbide tools, fabricating these tools with gradient layers, where the outer layers are, for example, harder than the substrate [10]. The fabrication of these gradient composite tools will provide tools with more versatility, as the desired properties can be applied on the base tool and improved on the surface, increasing their performance. Studies have been made to analyze the way the thickness of these gradient layers affect the properties of these tools [11]. A study carried out by Zhou et al. [12], tested different gradient cemented carbides, with layers of differing thicknesses, and has tested these with different coatings in the high speed cutting of a titanium alloy. Thus, Zhou et al. found that the thickness of the gradient layer's influences cutting performance, and that this thickness can be controlled by altering the contents of the cemented carbide constituents. Additionally, the authors concluded that the carbide with the thickest layer had the best cutting performance, making the control of these thicknesses very appealing when dealing with this type of substrate.

Coatings can be obtained using two different processes, either by Chemical Vapor Deposition (CVD), or by Physical Vapor Deposition (PVD). CVD films are achieved by having a precursor pumped inside a reactor; this precursor is regulated by control valves. The precursor molecules pass by the substrate and are deposited on its surface, achieving a thin, hard coating. This process runs at high temperatures, reaching temperatures of up to 900 ◦C and, additionally, the film thickness is usually uniform throughout the substrate surface. PVD consists of different methods, such as evaporation, sputtering and molecular beam epitaxy (MBE). In the sputtering technique, the applied coating is achieved by placing a magnetron near the target, in a vacuum. Then, an inert gas is introduced, then a high voltage is applied between the target and the substrate, releasing atomic size particles from the target. These particles are projected onto the substrate and they start to form a solid film. In the evaporation technique, the target acts as an evaporation source, having the material to be deposited, which works as a cathode. The material is heated at a high vapor pressure, which causes the release of the particles. The pumped gas, inserted in the reactor, clashes with the nano particles, which causes the acceleration of these particles, which in turn creates a plasma. This plasma proceeds trough the deposition chamber, thus depositing the coating's layers onto the substrate. Usually, PVD processes, when compared to CVD, run at a lower temperature (under 500 ◦C), and are more environmentally safe due to the type of precursors used in the CVD process, which are toxic. Additionally, the energy consumption of the PVD process is considerably lower than that of the CVD process [13–16].

Regarding the PVD process, there are various methods that can be used to obtain different coatings, either in composition or with different properties. Some of these methods are quite novel, or are seeing a new use in order to obtain certain coatings. As previously mentioned, the various methods that are applied in the coating industry are either sputtering or evaporation methods. Magnetron sputtering and arc evaporation methods have seen recent use in the deposition of coatings. Figure 1 shows the various PVD methods.

**Figure 1.** Different physical vapor deposition (PVD) techniques that are used currently in the production of advanced coatings [16].

The most used method, for magnetron sputtering PVD coating, is the direct current method (DC), however there are many more (as seen in the previous image), such as unbalanced magnetron sputtering (UBMS) and the novel high-power impulse/pulse magnetron sputtering (HiPIMS/HPPMS). Regarding these sputtering methods, there is a recent paper, that presents a method on how to control the boron-to-titanium ratios of TiBx thin films. The authors show, in this paper, that the addition of an external magnetic field during the strongly magnetically unbalanced magnetron sputtering of a TiB2 target in Ar, enables the ability to control the ratio of B/Ti. This research paves the way for synthesizing stoichiometric single-crystal transition-metal diborides [17]. Still regarding magnetron sputtering, a paper by Romero et al. [18] studies the properties of TiAlN/TaN nanostructured coatings deposited by DC magnetron sputtering. These coatings were deposited at different substrate rotation speeds. These different rotation speeds enable the control of the coating's architecture and mechanical properties. Recent studies on the HiPIMS reveal interesting results, such as having better coating adhesion or even having better coating mechanical properties. This paper by Zauner et al. [19], studies the influence of HiPIMS parameters on the properties of Ti–Al–N thin films, by using a Ti–Al composite target in mixed Ar/N2- atmospheres. The parameters that were studied were both the pulse frequency and duration, and the N2 flow ratio, substrate bias voltage and target composition. The optimal parameters for obtaining good values of hardness and moderate compressive stresses were determined, and it was found that the regulation of these parameters enables the control of the coating's structure. These variations can promote the formation of a highly preferred cubic phase, though altered gas-to-metal ratios arriving at the film surface.

Regarding evaporation methods, the arc evaporation has seen some use in recent research, especially the cathodic arc deposition method. In this paper, coatings obtained by this method will be mentioned. In a recent paper by Zhirkov et al. [20], a stable and reproductible arc plasma generation from a TiB2 cathode is presented. The authors show that the use of a Mo (molybdenum) cylinder around the boride cathode limits the movement of the arc spots to within the rim. This, coupled with a TiB2 cathode containing 1wt% of carbon, generated a stable arc with high reproducibility. These borides are not usually synthesized using DC arc evaporation, although the authors show with these results that the cathodic arc is an efficient method for the synthesis of these metal borides

When selecting the type of coating desired for a tool, the machining process being implemented must be taken into consideration, as there are some advantages and disadvantages to coatings that are applied to cutting tools. For example, in the paper presented by Hovsepian and Ehiasarian [21], the production of coatings using different PVD techniques is explored, namely the conventional DC magnetron sputtering and the HiPMS technique. The produced coatings were tailored for the applications that were chosen, while investigating the properties of the produced coatings. Studies such as these highlight the necessity of good planning when choosing the right coating for the right application. Furthermore, some parameters, such as coating properties (i.e., chemical composition, hardness or coating design), influence tool performance when applied to different machining processes (for example, roughing or finishing), but the deposition method also influences the cutting behavior of these coatings. CVD coatings are usually applied to cemented carbide cutting tools due to the good behavior of these materials under elevated temperatures, whereas, due to the overall low temperature of the PVD process, this also means that this method can be used to coat tool steel, with some studies having been done on the preparation and evaluation of these coatings [22]. However, there are some studies that propose an approach to solve the problem of diamond deposition on steel substrates using the CVD process. The authors propose a Ni/Cu/Ti interlayer for the diamond coating to adhere. The coating showed good adhesion to the multi-metal interlayer, and good wear resistance [23,24]. The study by Silva et al. [25] evaluates the wear resistance of Ti–Al–Si–N coatings deposited by PVD on a steel substrate; the coating was evaluated, including the adhesion of the coating, and the authors reported that a good adhesion of the coating to the tool steel was achieved. PVD, being a line-of-sight process, has some disadvantages, for example, it is harder to apply PVD coatings on complex geometries, although this can be achieved by using, for example, pulsed high-power sputtering [26]. Another disadvantage of PVD is that the coating thickness is harder to control throughout the substrate surface, however, using the mentioned pulsed high-power sputtering, the thickness distribution of the PVD films is more homogenous [26]. PVD coatings are usually thinner than CVD coatings. A thin PVD coating is more suited for finishing operations, as the thinner PVD coatings, confer the tool with a sharp cutting edge (when compared to thicker CVD coatings), also, the compressive stresses exhibited by PVD coatings (CVD coatings exhibit tensile stresses, contrary to the PVD coatings) are favorable; these stresses, coupled with the small layer thickness, make for a stronger and sharper cutting edge, making these coatings ideal for finishing operations [27–29]. Still regarding coating thickness, although generally PVD coatings are thinner than CVD coatings, there is a novel method that enables the deposition of thick PVD coatings, using a state of the art arc-evaporated PVD technique, that can grow coatings up to 24.5 μm [30].

As previously stated, coatings provide tools with properties that best fit machining applications, and, as in the case of gradient cemented carbide tools, that have different layers with different properties; coatings can use the same principle as these tools. This means that a coated tool may have a multi-layered coating, in which, for example, the outer layer has elevated wear resistance and the subsequent layer has a main thermal dissipation function. This versatility makes coated tools very appealing.

Tool coatings are classified by their architecture type (number of layers/layer arrangement) and by their chemical composition (layer composition). They are also characterized by their mechanical properties, such as hardness, indentation modulus and their stress state (stresses induced during the deposition of these coatings). Additionally, the microstructures of these coatings are often analyzed, as these are linked to the mechanical properties of the coatings. There is recent research that studies the altering of the microstructures of some coatings, in order to improve their performance.

Coatings can have various designs, from single layer to multi-layered coating. The types of coating architecture are as follows:


Different coatings with different designs have different types of architecture; the coating can have, for example, a multilayered architecture. The architecture of the coating is linked to coating design, meaning different architectures are chosen to deal with different problems.

Multilayer coating is a type of coating that shows more application in the industry by combining more than one appealing characteristic of each layer of coating. The higher number of layers also contributes to a hardness increase and to a higher crack propagation resistance. In this way, the tool performance can be enhanced, making this type of coating very attractive [14,18,30,31]. Additionally, in a study carried out by Kainz [32], multilayered CVD coating (TiN/TiBN) is compared to single-layered TiN and TiBN, concluding that performance is better with the multilayered coating.

Figure 2 shows, in a scheme, how the different types of coating look when applied on the substrate.

**Figure 2.** Different designs of hard coatings. Reproduced from [14], with permission from Elsevier, 2017.

Coatings are also characterized by their chemical composition, for example, different types of coatings with different chemical compositions are obtained through different deposition processes. The most common coatings obtained by PVD processes are TiN, Ti(C, N), (Ti, Al)N, while the most common coatings obtained by CVD are Ti(C, N), Al2O3, and TiN. Notice that these coatings are employed in different types of architecture, in order to obtain a better-suited cutting tool for a certain machining operation [27]. It is important to note that the chemical composition is important, as different chemical compositions have different hardnesses, friction coefficients and different thermal conductivities, which directly influences coating wear patterns. Regarding coating microstructure, it varies with the coating's chemical composition. Images will be presented regarding the microstructure of various coatings, especially those that have recently been researched. There are various studies regarding coating microstructure, either analyzing coatings obtained by a novel process or analyzing microstructural changes that might occur after machining. In a study by Longt et al. [33], the cutting performance of TiAlN- and CrAlN-coated silicon nitride inserts is analyzed in the dry turning of cast iron (Figure 3). The microstructure of these coatings is also analyzed and is displayed in the following images.

The mentioned coatings were obtained by PVD cathodic arc evaporation, and their thickness were about 4 μm for the TiAlN coating and about 2 μm for the CrAlN coating.

**Figure 3.** (**a**) TiAlN coating microstructure (**b**) CrAlN coating microstructure. Reproduced from [33], with permission from Elsevier, 2014.

The authors reported that the TiAlN coating had a dense and smooth structure with small grains, while the CrAlN coating's structure had formed columnar crystallites.

Still regarding coating microstructure, another study [34] analyzes the influence of Ni on the microstructure of a PVD cathodic arc evaporation obtained using AlTiN coating (Figure 4). The cutting performance is also analyzed in this study. Three samples were analyzed, one coating composed of AlTiN, another with 1.5% Ni and the third one with 3% Ni content. The following images are of the microstructures of these coatings.

**Figure 4.** (**a**) AlTiN coating microstructure with 0% Ni; (**b**) AlTiN coating microstructure with 1,5% Ni (**c**) AlTiN coating microstructure with 3% Ni. Reproduced from [34], with permission from Elsevier, 2019.

Notice the microstructural change that is occurring: while the coating with 0% Ni (a) exhibits a columnar microstructure, the addition of Ni promotes a more compact nanocrystalline structure. Although the nanohardness and elastic modulus for the AlTiN coatings had the highest values (26.2 GPa and 315.8 GPa respectively), the authors report that the coating with 1.5% Ni (b) has the best cutting performance. The lowest hardness values and elastic modulus came from the coating with 3% Ni (c) (20.9 GPa and 300.5 GPa respectively). It was also reported that the coating adhesion was worst for the 3% Ni (c) coating, while the 1.5% Ni (b) coating was practically tied with the AlTiN coating in terms of adhesive strength. However, the 1.5% Ni (b) coating was the coating with the greatest toughness, improving tool life by 160%.

Regarding multilayered coatings, these are very appellative to the machining industry, as they confer tools with various properties that can be combined in order to achieve a satisfactory machining process. The trend seems to be reducing layers' thickness, in order to achieve a greater combination of various properties, such as high hardness, a low friction coefficient, thermal conductivity, diffusion barrier and corrosion resistance [35]. Figure 5 shows the overall structure of multilayered coatings.

**Figure 5.** Example of a complex CVD-obtained multilayer coating microstructure. Reproduced from [35], with permission from John Wiley and Sons, 1969.

The nanolayered coatings are a type of multilayered coating, the difference being the layer thickness, which is in the nanometer range. Figure 6 shows the structure of a nanolayer TiB2 coating.

**Figure 6.** TiB2 nanolayer coating structure. Reproduced from [35], with permission from John Wiley and Sons, 1969.

The coating's structure influences the overall coating strength and adhesion to the substrate. It is an important aspect of fabricating new coatings or finding the correct application to a certain coating.

As previously mentioned, the coatings are also characterized by their mechanical properties, such as hardness, indentation modulus and the stress state of the coated tool (residual stresses that were induced during the deposition process). These mechanical properties can be altered by changing the coating's microstructure, as seen in the previous study [34], or by changing the coating's architecture or structure. As previously mentioned, a multilayered coating has more compressive stresses (e.g., a thin PVD coating); this will make for a stronger and more tough cutting edge. Additionally, their high number of interfaces confers coating strength. However, these residual stresses may provoke coating adhesion problems. In a study by Dai et al. [36], the properties of TiB2/Cr multilayered coatings with double periodical structures are studied. Various multilayered architectural structures were studied, changing the thickness of the Cr layer in order to see the changes to the microstructure and mechanical properties. It was reported that this double periodical multilayer structure can refine the growth structure of the TiB2 grains, resulting in a betterment of the coating's mechanical properties. The authors also reported that the residual stresses can be decreased through the deformation of the metallic Cr layers, which, in turn, causes a dramatical improvement to the coating adhesion. Additionally, keeping these residual stresses low can also boost the peel resistance of the coatings [37].

Coatings are applied depending on the machining process and based on the process's pre-requisites, since their performance is heavily tied to the coating's properties (chemical composition, architecture, microstructure and mechanical properties). Because of this, coating design is very important [21,38]. To know which coating to apply and where to apply it, studies have been conducted to evaluate cutting tool behavior. There are many parameters able to affect the cutting performance of a tool, such as cutting speed, feed, depth of cut and lubrication regimen, and tool performance issues which affect the tool's life and the overall finish quality of the workpiece. Cutting tool behavior knowledge has proven to be key in optimizing the turning process. By changing these parameters accordingly, a process can be optimized to meet the expected results [39], such as in the study performed by Krishnan [40], which used the Taguchi method to predict the best parameters to attain the lowest surface roughness when turning IS2062 E250 Steel. The parameters that were varied (input parameters) were the cutting speed, depth of cut and feed rate. The Taguchi loss function was used to compare the experimental values to the desired ones and to predict the output results (surface roughness and material removal rate (MRR)). An analysis of variance (ANOVA) method was used to determine the parameters that most influenced the desired result or the output parameters, in this case the surface roughness and MRR. The process could then be optimized to achieve the good surface roughness that was desired. Similarly, a study conducted by Durga [41], used the Taguchi method to predict the best machining parameters for turning AISI 304 stainless steel with a TiAlN nano-coated tool. The variable parameters of this study were the cutting speed, depth of cut and feed rate, then the authors developed regression equations based on the results for the surface roughness and material removal rate obtained from the empirical tests to develop equations to determine the surface roughness and material removal rate when using coated or uncoated tools for this type of experiment. Recently, studies on the parameters that influence tool performance are focusing on the minimum quantity lubricant (MQL), studying the effect of using this method in the machining of various materials using different tools. The study performed by Khan and Maity [42], which employs MQL using vegetable oil and compares it to dry cutting and flood cooling when turning commercially pure titanium, obtained satisfactory results, reducing cutting forces and cutting temperature when using this approach. Tool geometry also affects the cutting performance, because, by analyzing the cutting tool behavior more adequately, geometries can be created to achieve the desired results. Regarding the study carried out by Harisha et al. [43], cutting tool geometry is analyzed in order to minimize the cutting force when turning hardened steels, this is because tool geometry, when incorrect, leads to energy loss which results in loss of productivity.

In this paper, the recent advances in coated turning tool behavior are analyzed, looking at the different coatings that have been used in recent years. The different wear mechanisms which the coated turning tools are subjected are also going to be analyzed and presented, relating the different wear mechanisms to the coated tool life. There is also a chapter in this paper that will be reserved to the behavior of coated tools under advanced lubrication or cutting conditions, such as, turning using MQL conditions or using cryogenic conditions. Finally, the paper will conclude with a summary of each chapter and there will be mention of what are the recent trends in the turning process using coated tools.

#### **2. Coatings for Turning Tools**

In coatings applied to tools, or hard coatings, generally, nitrides, carbides, borides and oxides of transition metals are used. The nitrides used as coatings for cutting tools are TiN, TiAlN, CrN, ZrN, TiSiN, TiAlSiN, CrAlN, TiAlCrN, and cBN [8,9,14,41]; carbides for coatings are TiC, CrC, and WC. For boride coatings, TiB2 is used due to its chemical inertness, high hardness and good wear resistance. Additionally, they can be deposited in tool steel with good adhesive capabilities [44,45]. One of the most widely used oxide coatings is the Al2O3. Other somewhat common coatings for cutting tools are DLC, MoS2 and WC-C [13,15].

Due to the drastic tool life reduction, and overall unsatisfactory surface roughness values of the workpiece when using high speed steel (HSS) as a tool material, the industry has been using tool coatings to overcome the issues arising from the use of HSS. In a study by Gupta et al. [46], the cutting characteristics of PVD-coated turning tools were analyzed. The test involved the turning of C45 steel using solid tools coated with TiN, AlCrN and TiAlN. Cutting forces were measured, while cutting speed and feed rate were varied throughout the testing process. The TiAlN coating proved to be the most efficient, relative to tool life, due to its hardness and self-lubricating ability, almost avoiding the adhesion of the workpiece material to the tool surface, and therefore increasing tool life. Research like this continues to be important, as the choice of the right coating can prove to be a profitable choice for manufacturers, a fact that explains the vast amount of studies performed in this area, from finding optimal conditions to machine a certain material, to comparing various types of coatings with different structures or coating methods. In the following paragraphs, recent studies made on the comparison of various coatings, and studies that use coated tools with novel/complex geometries in the turning process, are going to be presented.

The work developed by Koyilada [47], testing the machinability characteristics of Nimonic C-263, used coated cemented carbide for turning that material under dry machining conditions. Carbides were coated with commercially available CVD bilayer coating (TiCN (bottom layer)/Al2O3 (top layer)), and PVD multilayer coating (TiAlN/TiN), consisting of alternating layers of TiN and TiAlN on the substrate. Both these coating types were compared. The results show that the PVD coating presented a remarkable improvement in the surface finish of the workpiece when compared to the CVD-coated tool. Cutting forces measured in the tests were also lower when the PVD coating was in use. Additionally, the PVD coating outperformed the CVD bilayer coating in terms of tool life. This is due to the PVD-coated tool presenting a superior compressive strength when compared to the CVD-coated tool (due to deposition technique and multilayer configuration), making it more suitable to work under a fluctuating load. Due to the microstructure of Nimonic C-263, the tool is subjected to dynamic fatigue, even in continuous machining, which explains the underperformance of the CVD coating in this case. Although the PVD multilayer coating is preferred for higher cutting speeds, ranging from 50–90 m/min (further cutting speed augmentation would require the use of a cutting fluid paired with the coated cutting tools), the CVD bilayer coating should be used to turn this material at speeds below 60 m/min, because the top layer of Al2O3 has a low thermal conductivity, which results in a higher machining temperature at higher cutting speeds. This eventually leads to wear problems, especially material adhesion and coating disaggregation.

Another study that compares the characteristics of CVD and PVD-coated carbide tools, is a work presented by Ginting [48] which studies the productivity of AISI 4340 hard turning using multilayered CVD (TiN (top layer)/Al2O3/TiCN (bottom layer)) and monolayered PVD coatings (TiCN). Productivity was characterized by the material removal rate (MRR) and volume of material removal (VMR). Otherwise, the variables analyzed in the tests were cutting speed, feed rate and depth of cut. The upper limits set to the coated tools reveal that the CVD multilayer-coated carbide can achieve a slightly higher feed rate and depth of cut value. In terms of tool life, the PVD monolayer coated tool endured for longer than the CVD-coated tool. In terms of surface roughness, the PVD coating was more effective as well. As mentioned above, PVD coatings have more sharp edges and are thinner, providing a better quality surface finish, however, in terms of productivity the CVD-coated carbide achieved values higher by about 78–125% than the monolayer PVD-coated tool. This highlights the fact that choosing the right coating method and type of coating can prove an advantage. As seen in that study, the PVD-coated tool performs better regarding tool life and overall surface finish quality, but in terms of MRR the CVD-coated tool is more effective. This is due to the coating properties: while the PVD coating (TiCN) is harder than the outer layer of the CVD coating (TiN), its friction coefficient

is also lower when compared to the TiN. This, coupled with the fact that thin PVD coatings confer the tool with sharper cutting edges and have more compressive stresses (caused during the PVD process [29]), give the PVD coating superior finishing capabilities when compared to this CVD coating.

In the work presented by Kumar et al. [49], the performances of PVD coated carbides using TiAlN, AlCrN and TiAlN (top layer)/AlCrN (bottom layer) were tested in the turning of Inconel 825, as well as uncoated carbide tools. The performance of each tool was evaluated, taking into consideration the flank wear of the tool, work-piece surface roughness, cutting force generated during the cutting process and chip formation. The optimal machining parameters were analyzed using grey relational analysis under multiple response optimization, and the results showed that the bilayer coating (TiAlN/AlCrN) outperformed the single layered coatings, TiAlN and AlCrN, in the machining of this alloy.

Still regarding the comparison between CVD- and PVD-coated tools, the study performed by Koseki et al. [50] compares TiN-coated tools obtained by different deposition methods in the continuous turning of Ni-based super-alloys. These high-strength, low-conducting alloys require higher cutting forces and temperatures than other materials during the machining process. The damage suffered during the tests was investigated, and the CVD coating proved to be more efficient in the machining of these alloys, suffering almost no change in coating hardness and overall less plastic deformation in the process.

A method that has seen some use is the coating of a textured tool in order to promote better chip removal, better adhesion of the coating to the tool and even an improved tool life. In the work developed by Mishra [51], the machining performance of laser-textured chevron shaped tools, and untextured tools was evaluated. These tools were coated with AlTiN and AlCrN using PVD. Cutting forces and tool wear were analyzed for textured and untextured cutting tools. Coating growth on textured tools was better, presenting a reduced number of microcavities and macroparticles for both coatings. The value of the cutting forces was lower for the textured tools, resulting in less tool wear, and the texture on the tools improved the tool-coating adhesion. The chips formed by the textured tools were thinner when compared to untextured tools, however, chip fragments were embedded in the textures. Thus, machining parameters need to be adjusted in order to find a balance between these two phenomena in order to produce favorable machining conditions.

As previously stated in Chapter 1, discussing developments in gradient cemented carbide tools, studies on the influence of this gradient and on how to control the grain size of the cemented carbide, have been conducted [11,12]. These gradient cemented carbides already represent a significant improvement when using uncoated cemented carbide tools. Additionally, these gradient cemented carbides may improve the quality of the coating, making it more adherent to the substrate, and even improve tool performance. A paper presented a study regarding CVD coating application on gradient cemented carbide tools of different grain sizes, where it was observed that the coating thickness is related to the carbide grain size, coating thickness increases with smaller grain sizes [52]. It was also concluded that the adhesion strength of the coating is overall better on gradient cemented carbides when compared to regular cemented carbides, however, the adhesion strength drops when the grains of the cemented carbide are finer. A thicker coating may not be ideal for finishing operations, however, for roughing operations, having a thicker coating may be beneficial.

Cutting tool coatings may also provide a sustainable eco-friendlier alternative when machining certain materials, as some of these materials, for example, nickel-based super-alloys, already mentioned in this chapter, require higher cutting speeds, which results in higher cutting forces and a higher temperature in the cutting area. To counteract these problems, lubrication is employed, sometimes by flooding the cutting area. Practices like these have proven to be unsustainable and damaging to the environment.

A recent tendency is to employ methods able to reduce the high usage of these lubricants and make the overall machining process more eco-oriented. These methods (Figure 7) sometimes use biodegradable oils, such as vegetable oils, as lubricants. Dry machining is also a sustainable option, as it removes the use of cooling fluid altogether, having as its alternative the minimum quantity lubrication (MQL) method, in which small amounts of lubricant are employed. MQL presents itself as being a viable alternative to dry machining when having problems with cutting temperature or surface finish quality. Other sustainable methods, such as cryogenic cooling, or the use of a high-pressure coolant, in which the coolant is applied to a select area of the tool, are employed as an alternative to the conventional flood cooling method [53].

**Figure 7.** Wheel of sustainable machining. Reproduced from [53], with permission from Elsevier, 2019.

The work produced by Thakur and Gangopadhyay [54] proposes a sustainable alternative to the machining of nickel-based super alloy, by employing TiN/TiAlN PVD-coated tools and dry turning of the Incoloy 825. Tests were conducted employing different lubrication methods, such as dry machining, flood machining and MQL. Cutting forces, temperature of the cutting area, tool wear and surface integrity were evaluated. While the temperature in the cutting area was higher for dry machining when using a PVD-coated tool, the overall surface finish was of better quality when compared to flood cooling and MQL. Moreover, cutting forces also presented lower values when using a PVD coating, even in dry machining. This means that the cutting force necessary would be lower, therefore promoting a more environment-friendly alternative (dry machining); an overall better surface finish was obtained using the PVD-coated tool under an MQL environment.

Regarding nanolayered coatings, they are quite appealing, mainly due to their high hardness value, for example, research was done on nanolayer TiN/VN coatings, and results found that there was a very high hardness increment [55]. This hardness increase is related to the nanolayer thickness, as see in Figure 8.

**Figure 8.** Hardness increase of a TiN/VN coating, over the thickness of the bilayer. Reproduced from [14], with permission from Elsevier, 2017.

The reason for this hardness increase was attributed to the large number of interfaces between layers, characteristic of nanolayered coatings. In Figure 9, the structure of a CrN/TiAlN nanolayered coating deposited on steel is shown [14].

**Figure 9.** CrN/TiAlN nanolayered coating structure. Reproduced from [14], with permission from Elsevier, 2017.

The structure shown in Figure 9, characteristic of nanolayered coatings, confers the coatings a very high crack propagation resistance, as the cracks tend to not propagate as deep as in a monolayered coating, thus risking damage to the substrate. These coatings are very similar to the multilayered coatings, the main difference being the thickness of each layer and that the hardness value for a nanolayered coating is not equal to the average hardness of its constituents—however, this is the case for regular multilayered coatings [14]. A recent study about the influence of the thickness of these nanolayers in coatings found that, for hardness values at room temperature, there were no significant changes between the coatings with differing thickness layers. However, the cutting properties of the coatings were different, with the coating with the thinnest nanolayers exhibiting a higher tool life than the coating with thicker nanolayers [56].

As mentioned previously, understanding cutting tool behavior is the key to correctly optimizing a machining process. There are many studies on coatings' behavior while cutting, for example, in the study presented by Gassner et al. [57], the thermal crack network on CVD TiCN/Al2O3-coated cemented carbide cutting tools is analyzed. The theme of coating tools and coating performance is heavily researched, comparing coating methods to achieve overall better machining results, or even finding an eco-friendlier alternative to machining certain materials.

#### **3. Coating Influence on Turned Surface Quality**

There are many factors that influence surface quality in the turning process, as seen in Chapter 1. A turning process can be optimized in order to have the best possible surface quality, by changing certain parameters, such as rotation speed, feed rate and depth of cut. These parameters are mainly dependent on the machine, as the machine also influences cutting performance and the overall surface finish of the machined piece [58]. There have been studies that relate the chip formation thickness to the overall surface roughness of the machined piece [59]. Using a prediction method to determine chip formation thickness can serve as a monitor of the surface roughness of certain materials. However, there are more factors that influence the surface roughness of turned pieces, such as lubrication method and tool geometry, and the coating that is being employed also affects the overall surface quality of the workpiece. As previously mentioned, thin PVD coatings are very well suited to surface finish operations, and provide an overall better surface quality than thicker coatings. This is mainly due to the sharp edges that are conferred to the substrate, and compressive stresses that confer the tool edge strength [27–29].

In this chapter, the influence of coated tools on the surface quality of various materials are presented. The materials selected are mainly titanium alloys and nickel-based super-alloys, as these are very appealing for structural and engineering applications, especially due to their strength-to-weight ratio. Although these alloys have some processing problems associated with them, for example low machinability rating, their poor machinability may be attributed to material properties, such as high hardness at high temperatures, low thermal conductivity and high chemical reactivity [60]. There have been some studies conducted on the turning of hardened steels, which will also be presented in this chapter, highlighting the coating's influence on turned surface quality.

In the aforementioned study [40], a comparison of the machining performance of coated tools, using a monolayer PVD coating TiCN and a multilayer CVD coating TiN/Al2O3 in the hard turning of AISI 4340 steel. The authors found that, in this case (and as elaborated above), the PVD coating would be best suited for finishing operations, especially due to the hardness values of TiCN and the lower friction coefficient (when compared to the TiN top layer of the CVD coating). The surface roughness values obtained for the PVD-coated carbide and for the CVD-coated tool, are (0.8–1.6) micron and (1.6–3.2) micron, respectively. However, for material removal rate the CVD coating is preferred, although the turned surface quality is poorer than those using PVD coatings.

The study by Fernández-Abia et al. [28], presents a comparison of four coatings (and an uncoated tool) in the turning of austenitic stainless steel, AISI 304L. The cutting behavior of these coated tools were analyzed. The authors mention that these types of PVD coatings are best suited for achieving low values of surface roughness, especially due to the sharp edges conferred by the PVD process. The coatings used were, AlTiN; AlTiSiN; AlCrSiN; and, finally, TiAlCrN. The first three coatings are nano-structured coatings. The graph of Figure 10 shows the results obtained from this study, regarding the surface roughness of the machined material.

**Figure 10.** Graphic of surface roughness value (Ra) for different PVD coatings [28].

The best coatings for the machining of this material are the AlTiN and AlTiSiN coatings; this is due to their nano-crystalline structure. Although AlCrSiN also has this beneficial nanostructure, the presence of chromium in its chemical composition favors the creation of an oxide protective layer that is inferior to the AlTiN and AlTiSiN coatings.

Regarding the turning of nickel-based super-alloys, in this study [54], the influence of coating and lubrication/cooling method was observed in the turning performance of Incoloy 825, using a TiN/TiAlN multilayer coating. The results regarding surface roughness obtained from these tests can be interpreted from the graphs in Figure 11.

**Figure 11.** Different values of surface roughness for the machining duration. (**a**) and (**b**) have different cutting parameters. Reproduced from [54], with permission from Elsevier, 2016.

The coated tool provides the better surface roughness quality from the tests that were carried out. The fact that dry machining with coated tools provides a better surface quality is quite an interesting finding, as this type of machining is sustainable and eco-friendly, and these coatings enable the machining of high-quality parts at a lower price/environmental impact. The authors also added that the cutting temperature was lower for the dry machining using PVD TiN/TiAlN-coated tools. A study was also mentioned before [49], in which a PVD and CVD coating were used in the machining of Nimonic C-263; the findings of these authors report that the PVD TiN/TiAlN multilayered coated tool is better for the surface finish. Due to the sharper edges, this coating provided a 14.3% reduction

in surface roughness value when compared to the CVD TiCN/Al2O3 counterpart, in which a higher edge radius contributed to the lower turned surface quality. As stated previously, the PVD coating is preferred for higher machining speeds than the CVD coating.

Without a doubt, coatings improve the overall surface finish quality of turned parts, however, tools with the correct geometry can rival the low roughness values obtained, especially those with thinner PVD coatings. By analyzing the literature, a trend can be seen, with thin PVD coatings usually being employed in finishing operations, explained by the residual stresses that thin PVD coatings have (compressive stresses), and by the sharp cutting edge that these coatings confer to tools. Their chemical composition is also a contributing factor, as, depending on the coating chemical composition, these will react differently with the material that is being turned, sometimes even forming hard protective layers that lower the friction coefficient, thus improving overall coated tool performance [25].

#### **4. Tool Wear Mechanisms**

Coated cutting tools significantly improved the tool life of conventional tools, as coated tools suffer overall less wear in the same lifetime as an uncoated one, particularly at high machining speeds [61], however, coated tools eventually give out, due to the fact that a lot of these coated tools are used in dry machining conditions, which means that the machining temperature is overall higher. A coated tool has different wear mechanisms, such as abrasive wear, thermal cracks, adhesive wear, build up edge (BUE), or coating structure failure, resulting in spalling or cracks appearing on the coating. Understanding the different wear mechanisms for each coating type helps one make a better coating or machining parameters choice to achieve the desired results. These wear mechanisms are related to some parameters; for example, when there is an increase in the cutting force, it can be assumed that the coating is suffering abrasive wear, or that there is a problem with the coated tool's edge. Wear is also related to the coating properties, different coatings (or coating layers) have different hardnesses, friction coefficients and thermal conductivities, all factors that also contribute to the wear patterns of these coated tools. Coating microstructures can influence factors, such as coating adhesion, that might cause fracture wear later. Mechanical properties, such as hardness, indentation modulus and the stress state of the coating itself, affect the wear patterns that these will suffer. In some cases, high residual stresses can cause problems with coating adhesion, that is, the coating's adhesive strength is lower, and the coating is more likely to suffer spalling or delamination, although, as mentioned before, different coatings obtained by different deposition methods have different stress states. Hardness is primarily tied with abrasive wear, as well as the coefficient of friction (COF); the latter being related to coating design (layer thickness or layer chemical composition), as seen in the study by Dai et al. [36], where it is reported that the increase in the thickness of a Cr layer, in a multilayered TiB2/Cr coating with double periodical structures, would result in a decrease in coating hardness and an increase in coefficient of friction, which will affect the coated tool's wear rate.

In this chapter, some studies regarding different wear mechanisms and coating degradation will be addressed, presenting images for some types of wear mechanisms, as well as a summary of the findings of these studies.

In a work mentioned in the previous chapter [50], there is a study on the wear of TiN coatings obtained by different deposition methods. In addition to abrasive wear and fracture wear, a common wear mechanism is adhesive wear, where the material adheres to the coated tool. In Figure 12, the wear mechanisms for the TiN coating, obtained by different deposition methods, can be seen on the tool's cutting edge.

**Figure 12.** Wear mechanisms on TiN coating, in the continuous turning of a nickel-based super-alloy using different coating techniques: (**a**) PVD-arc, (**b**) PVD-SP, (**c**) PVD-HCD and (**d**) CVD. Reproduced from [50], with permission from Elsevier, 2015.

In Figure 12, in addition to noticing the adhesive material on the cutting edge, other wear mechanisms can be seen, such as fracture wear, where the cemented carbide substrate is exposed. There is also micro-abrasion wear that can be noted on all the samples.

Still regarding the same study [60], some plastic deformation can be observed in the coating (Figure 13). In this image, a broken coating area and adhesive material can also be seen.

**Figure 13.** High resolution TEM image regarding a TiN coating obtained by PVD-Arc, noticing wear on the surface of the tool. Reproduced from [50], with permission from Elsevier, 2015.

Also due to temperature, thermal cracking may occur on the coating. Another study regarding thermal cracking of CVD TiCN/Al2O3-coated cemented carbides [57] analyzes various methods to close the cracks that occur due to excessive cutting temperature, such as wet blasting or filling the cracks with TiO2. This wear mechanism can be observed in Figure 14, taken from the same work.

**Figure 14.** (**a**) SEM image of rake face of a coated CVD insert after face turning (**b**) highlighted section of the rake face. Reproduced from [57], with permission from Elsevier, 2019.

Regarding crack defects on cutting tools, the study performed by Vereschaka et al. [56] highlights the influence of the PVD Ti-TiN-(Ti,Al,Cr,Si,)N nanolayer coating thickness, which was tested in the turning of AISI 321 steel. The coatings differed in the number of nanolayers and nanolayer thickness; the coating with the ticker nanolayers had a total of 33 nanolayers (Coating A), and the other coating, with the thinner nanolayers, had a total of 57 of these layers (Coating B). These coatings were also tested against monolayered Ti-(Ti,Al)N-coated and uncoated tools, and both the nanolayered coatings presented a higher tool life than these last two. However, Coating A has less wear resistance and, with the increase in cutting speed, the cutting temperature also increased, inducing thermal stresses in the superficial layers of the tool. This can be observed in Figure 15, where (a) is the image regarding the thickest nanolayered coating and (b) is the thinnest one. The thinner layers provide the coating with a higher wear resistance, which means that thinner layers better resist thermal stress crack formations.

**Figure 15.** Crack formation on PVD nanolayered coating surface, area directly adjacent to the cutting edge; (**a**) coating with nanolayers of 80 nm and (**b**) coating with 40 nm nanolayer thickness [56].

Coatings, when deposited, mirror the substrate surface, resulting sometimes in an uneven coating surface, with imperfections such as cracks, and even residual stresses that may have resulted from the coating process. These superficial imperfections may cause material transfer and have a negative impact on the coated tool performance, by promoting tool wear or by not conferring the desired surface finish. Some methods have been proposed to minimize these defects, such as a post-deposition polishing of the coated tool, in order to lower its overall surface roughness and minimize the potential for material transfer [62]. Another method proposed to deal with cracks is shown in the paper presented by Faksa et al. [63], where the authors study the effect of shot peening on residual stresses and crack closure in CVD-coated hard metal cutting inserts. Due to the stresses seen on the deposition process of the CVD coating on hard metal, the surface layer of the coating exhibits cracks. The authors conclude that well placed and calculated shots can close these cracks and prevent crack nucleation and growth in CVD coatings. These residual stresses also have an influence on PVD films, as shown by Skordaris et al. [64], who studied the effects of PVD films' residual stresses on their mechanical properties, brittleness, adhesion and cutting performance. The coatings used are PVD TiAlN, and different coatings were used, these having different levels of residual stress, obtained by heat treatment. The authors concluded that there is a significant contribution of the film's compressive stresses to increasing the mechanical properties of the coating and adhesion, consequently improving tool life.

The wear mechanisms of a MTCVD–TiCN–Al2O3-coated cemented tool was also analyzed in another study [65], where wear patterns were observed after 142 min of turning 300 M steel, at a cutting speed of 300 m/min. Crater spalling could be observed, as well as evidence of molten metal particles, which means that the cutting temperature was very high. Signs of adhesion, matrix exposure and build up layer (BUL) were also observed. These wear patterns can be observed in Figure 16.

**Figure 16.** Different wear patterns on the MTCVD coating after 142 min of turning 300 M steel. The rake face (**a**) was analyzed in two regions A (depicted in (**b**) presenting signs of cooling molten metal), and region B (depicted in subfigure (**c**), where signs of BUL and micro-cracks can be noticed). In subfigure (**d**) the flank face is displayed, and zones **C** and **D** are analyzed. Zone **C** is displayed in subfigure (**e**), where adhesion damage can be noted, and some grooves. Lastly, zone **D** is displayed in (**f**), where adhesion damage is the predominant type of wear mechanism. Reproduced from [65], with permission from Elsevier, 2017.

An analogous study [66] using similar conditions (maintaining the same coating and machined material) was conducted in order to analyze the main wear mechanisms of the coated tool. The wear patterns are like those observed in the previous study [65]: adhesive wear, build-up layer (BUL), molten material and crack wear patterns were detected in the coated tool. The authors conclude that the main wear mechanisms are adhesive wear, abrasive wear, oxidation wear and diffusion wear. During the tests, cutting parameters were varied, such as cutting speed and feed rate. Cutting forces were also analyzed, because this is an important step when optimizing a process.

In another work [67], the wear mechanism of PVD-, CrAlN- and TiAlN-coated Si3N4 ceramic cutting tools was studied. The conducted tests consisted of the turning of GT250 gray cast iron using these coated tools and characterizing the wear mechanisms. It was found that the adhesive strength of the TiAlN was stronger than the CrAlN coating; during the turning of the material, the CrAlN coating suffered spalling at a cutting speed of 400 m/min, due to low adhesive strength. During the dry turning of the material, abrasive wear and minor adhesive wear were found to be the main wear mechanisms. It was possible to observe that coated tools suffered more adhesive wear than the uncoated inserts. Still regarding the coated Si3N4 ceramic cutting tools, studies made on the wear mechanism of these tools coated with diamond were conducted [68,69], the authors observed the machining performance of coated and uncoated tools; it was found that the cutting force was higher during machining with diamond-coated tools, due to the surface roughness of the rake face. Additionally, the parameter that influenced these cutting forces the most was the feed rate, contributing more to the increase in cutting force than cutting speed. Regarding wear mechanisms, it was observed that the high machining temperature promoted the graphitization of the diamond coating, which resulted in its removal from the tool; however, there was no delamination observed in the coating after machining.

As previously stated, a recent shift to dry turning as an alternative to some machining methods has been observed, due to economic and environmental reasons. Naskar et al. [70] in their investigation, compared the flank wear mechanism of CVD and PVD hard coatings in the high-speed dry turning of low- and high-carbon steel. The steels in question are C20 and C80, and they were turned at a cutting speed of 300 m/min and 600 m/min with CVD Al2O3 (top layer)/TiC (bottom layer), TiCN (top layer)/TiC (bottom layer) bilayer-coated, and PVD TiAlN single-layer-coated, inserts. The authors found that the main wear mechanisms were abrasive wear, however, plastic deformation-induced necking and dissolution-diffusion were also contributing to the acceleration of tool wear. They concluded that when designing a coating material for high speed machining, the solubility of coating materials has to be taken into account.

Figure 17, taken from the paper presented by Naskar et al. [70], exhibits the wear of coated tools (coatings presented in the above paragraph), in the machining of C80 steel.

**Figure 17.** Wear patterns for necking (caused by plastic deformation) (**a**) and abrasion marks (**c**). In image (**b**) the coating's surface is smooth after wear, with no evident abrasion marks or plastic deformation induced defects. Reproduced from [70], with permission from Elsevier, 2018.

Studies like these are very important to understanding and finding ways to improve tool life, by understanding how the wear patterns are displayed. Moreover, machining parameters can be found/calculated in order to optimize the process and improve tool life. Additionally, by observing these wear patterns, new ways of fabricating novel coatings can be achieved. The development of nanostructured composite coatings is still quite novel, and studies such as these help to gain a better understanding of how they behave in certain conditions, such as, for example, in the study carried out by Vereschaka et al. [71], in which the behavior of a nanostructure multilayered composite coating is tested under the high speed turning of steel. It was found that there was adhesive wear from the steel that was being turned. As a result of the turning process, a top layer was "destroyed", no longer exhibiting a nanostructure.

#### **5. Tool Life**

Improving tool life has been a strong focus of the machining industry, as having cutting tools last longer and not underperform is quite appealing. Because of this, many studies have been conducted to increase understanding how to improve cutting tool life. As seen in the previous chapter, the knowledge of the cutting tool wear mechanisms is crucial when wanting to improve tool life [70]. There are many factors that directly influence tool life, such as the coating's properties, i.e., mechanical properties, coating architecture and microstructure, and chemical composition. These factors influence the coated tool's wear patterns, thus influencing tool life. Machining parameters also influence the tool's life, such as cutting speed, feed rate and even tool geometry, as there are some papers that study the influence of the micro-textures of cutting tools, relating their surface geometry with tool life [72]. Regarding the influence of machining parameters on cutting tool life, the study carried out by Asha et al. [73], analyzes the effect of these parameters on cutting temperature and tool life while turning EN24 and HCHCr Grade alloy steel. The authors carried out tests where the cutting speed and feed rate were varied, and the depth of the cut was kept constant. The coatings used in the carbide inserts were an M15 grade multilayer coating and an M20 grade. It was determined from the results of the turning tests, that the cutting temperature was higher when machining the HCHCr grade alloy steel, when compared to the EN24; the cutting speed increase applied during tests caused tool life to decrease, however, feed rate did not have a high impact on the tool life. It was also found that the tool life was lower when machining the HCHCr alloy steel; this was possibly due to the presence of high alloying elements in the steel and the hardness of the HCHCr steel.

In this chapter, some studies focused on improving tool life are presented, and the various methods for determining tool life for different tools are also mentioned.

In the study carried out by Boing et al. [74], the tool life of PVD- and CVD-coated tools is evaluated when turning AISI 4340-, 52-, 100- and D2-hardened steels. The authors found that the TiAlN PVD-coated tool promoted better results when turning AISI 4340 steel. Otherwise, the MTCVD TiCN/Al2O3//TiN proved to be better at turning the other steels. The authors also found that the hardness and microstructure of these steels were the limiting factor, meaning that the carbide fraction that is present in the steel microstructure limits tool life, due to the impact on the cutting edge, similar to the study presented in the beginning of the chapter by Asha et al. [73].

Another paper presented by Vereschaka et al. [75] relates the coating thickness of a composite nanostructured coating Ti-TiN-(Ti,Al,Cr)N to its tool life, similar to the results from another work [56]. The coating with thinner layers presents overall better mechanical properties and wear resistance properties, having a longer tool life than its thicker competitor.

Another method to improve tool life involves ANOVA analysis, determining the best machining parameters in order to obtain the desired effects, from a better surface finish to a longer tool life. This method is presented in the work by Ranjan Das et al. [76], in which a process that involves the hard turning of AISI 4340 using a CVD TiN/TiCN/Al2O3//TiN coating is optimized.

There are some methods proposed to predict tool life based on certain parameters, such as the study that proposes a new model for the prediction of a time-varying heat partition coefficient at the coated tool–chip interface in continuous turning [77]. The heat partition coefficient at the tool-chip interface is important, as it helps to accurately estimate the distribution of heat flux and temperature while machining, therefore this is important, as it gives insight on what influences the heat distribution on the tool-chip; as, for example, coating thickness and substrate material influence the heat partition coefficient, the type of coating and cutting parameters also influences this coefficient, for example, increased cutting speed resulted in higher temperatures, and all of this has an influence on tool life. With this proposed method, there is a new way to design better and more optimized coated tools, and even help the selection of these tools for machining applications.

The study by Zhang et al. [78], proposes the prediction of tool wear, using a 2D Fractal analysis of the cutting force and surface profile, in the turning of an Iron-based super-alloy. Coated carbide tools and cermet tools were used to turn the material, and a dynamometer was used to measure the cutting force. MATLAB (MathWorks, Natick, MA, USA.) was used to calculate the fractal dimension. The results from these tests showed that the cutting force curve and the machined surface profile had fractal characteristics; the authors determined that by using this method, tool wear and machined surface finish quality could be predicted. This study yielded additional results, such as the coated carbide tools having a higher tool life than the cermet tools, and demonstrated that by increasing cutting speed the surface finish quality would also increase, however, with the increase in tool wear the surface quality would deteriorate. Having a prediction method such as this for tool life is very appealing for the industry.

There are also some studies on tool life that focus on the substrate. For example, in the study carried out by Uhlmann et al. [79], the substitution of commercially coated tungsten carbide tools in the dry cylindrical turning process with HiPIMS-coated niobium carbide cutting inserts is proposed. These niobium coatings have shown potential in the machining of iron-based materials. The authors found that, although the cutting performance of the HiPIMS-coated niobium inserts was higher than that of the uncoated inserts, when compared to the commercially coated tungsten carbide tool performance was not improved noticeably. However, it was found that the adhesion of this coating to the substrate was good, providing an alternative to regular coated tungsten carbide tools. Research like this is

important in finding new ways to optimize the machining of certain materials, as these new coatings may be a reliable option when wanting to improve cutting tool life.

#### **6. Tool Coatings Under Advanced Cutting and Lubrication Conditions**

Since using coated cutting tools for turning has its limitations, especially regarding the lubrication method, there have been some studies on employing some alternative lubrication methods in order to achieve better results in the finished product, and even optimizing the process, making it cheaper by improving tool life or even reducing power usage and, additionally, making the overall process safer for the environment.

In this chapter, recent studies regarding alternative lubrication methods are presented, drawing attention to the minimum quantity lubricant (MQL) method and cryogenic lubrication methods, paying attention to the overall process efficiency and tool behavior while turning. For example, in some cases MQL regimens can have a good impact on surface finish quality when compared, for example, to dry turning. Additionally, the dry turning of some alloys may cause a high temperature in the cutting area, provoking more work hardening when compared to MQL regimen [80]. As mentioned in Chapter One, there are papers that study the influence of extreme pressure anti-wear additives (EP/AW), in the MQL regimen, obtaining good results when compared to other MQL (without additives) regimen and dry turning. These types of lubrication method not only affect the overall finished quality of the product and tool life/performance, but they affect the microstructure as well. Studies show that MQL is quite advantageous when the best surface finish is one of the goals [81,82].

In the work presented by Marques et al. [83], the turning of Inconel 718, applying a vegetable-base cutting fluid mixed with solid lubricants by MQL, is proposed. The authors studied the turning process of this super-alloy under dry machining conditions and under MQL while using graphite solid lubricant. The authors found that under MQL conditions the tool life was improved, because the addition of solid lubricants reduced the cutting forces during the process, making it a good option when intending to extend tool life when turning Inconel 718.

Regarding the use of vegetable-based coolants in the turning process, in the study carried out by Elmunafi et al. [84] the tool life of a tool coated with TiAlN is analyzed under MQL using castor oil. The authors achieve satisfactory results by reducing the overall cutting temperature and cutting forces, suggesting that MQL method would prove to be useful in the turning of hard stainless steels. It was also found that the tool life is inversely proportional to both cutting speed and feed, with the effect of the first being more significant. Additionally, the use of castor oil is more environmentally friendly when compared to other coolants.

In the work presented by Chetan et al. [85], the wear behavior of PVD TiN-coated carbide inserts during the machining of Nimonic 90 and Ti6Al4V super-alloys under dry and MQL conditions is studied. The authors determined that the main mechanism for the wear of the coating during the machining of Nimonic 90 alloy was the abrasive wear and nose fracture, which caused the catastrophic failure of the tool. However, due to the wettability of Ti6Al4V under MQL mode, this provided less intense flank wear at high cutting speeds. Studies such as these help to understand when to apply certain lubrication regimens, depending not only on tool but the workpiece material as well.

There have been some recent studies on cryogenic pre-treated coated tool performance, such as the study performed by Kumar and Senthil [86]. In this work, PVD-coated TiN/AlTiN tungsten carbide inserts were used in the dry turning of a Ti6Al4V titanium alloy. The authors concluded that this treatment increased the hardness of the inserts. Additionally, the cutting forces obtained were lower when using cryogenic treated tools; overall better surface finish was reported, as well as less significant tool wear on the cryogenically treated tools.

Cryogenic cooling methods are a recent focus of attention as well, concerning coated tool behavior. In the study carried out by Dhananchezian et al. [87], the effects of cryogenic cooling on the turning of 2205 duplex stainless steel, using a PVD-coated nano multilayered TiAlN cutting tool, were analyzed and compared to dry turning. From this study, it was found that cryogenic cooling reduced cutting

temperature by more than 50% when compared to dry turning and decreased cutting forces by up to 40%. An improvement on roughness was also registered, of about 20%. These results contribute to finding better alternatives, especially when machining hard materials such as duplex steel. Regarding machining under cryogenic conditions, the Taguchi Method can also be used to optimize the machining parameters for these conditions, as shown in the study by Khare et al. [88]. The optimal parameters were chosen when machining AISI 4340. These parameters were: cutting speed, depth of cut and feed rate, while under cryogenic conditions. By optimizing a process such as cryogenic turning, it makes the process more viable, from a financial standpoint, making it more likely to be used in the industry. A similar method, the Taguchi incorporated Gray relational analysis (TGRA), is shown to be implemented with success in another study, this one regarding the cryogenic machining of 17-4 PH stainless steel. As in the previous study, the optimal parameters were predicted, these parameters being the cutting speed, feed rate and depth of cut [89].

There are some additives that can be used in lubricants to improve machining performance, reduce tool wear and even improve tool life. In this study, performed by Gutnichenko et al. [90], a study of the influence of adding graphite nanoplatelets (GnP) to vegetable oil on the MQL-assisted turning of Alloy 718 was performed. From the turning tests, the additives impact the machining performance in a positive manner; by adding the GnP to the vegetable oil, an increase in terms of tool life, surface finish and overall process stability was noted. In addition, adding GnP particles to the oil results in a significant reduction in friction in the cutting area. It was also noted that these particles influence the chip formation process, whereas a pure oil lubrification would act as a coolant. The authors also reported that using the vegetable oil without the nanoparticles would improve the machining process, however, these nanoparticles would contribute heavily to overall process stability.

#### **7. Concluding Remarks**

As the machining industry grows, the interest in making it more profitable grows as well. The turning segment still has an important presence in the machining industry and, because of this, is an important focus of research in order to optimize the process to achieve more and more satisfactory results. Tool behavior knowledge is necessary to understand the process, with a vast amount of literature existing in this field. In this paper, the recent studies on and advances in coated tool technology were presented.

Regarding the development of new coatings, the focus seems to be the development of nanolayered and nanocomposite coatings, however, there are many studies on the behavior of the most common CVD and PVD coatings. Many of these studies focus on the comparison of PVD- and CVD-coated tool performance, evaluating the influence of these coatings on the machining of certain materials. The coating's chemical composition, architecture and deposition method are all factors that contribute to the cutting performance of coated tools. Researches show that CVD- and PVD-obtained coatings are used for different types of operations, for example, thin PVD coatings provide a suitable option when the finishing quality is the most desired parameter, and, in turn, CVD coatings prove to be useful in most materials for roughing operations. Of course, this depends heavily on the material that is being machined and the coating properties.

Wear mechanisms were also analyzed, showing how different wear patterns present themselves on the coated cutting tool. There are many comparative studies as well in this field, as understanding how different coatings develop their wear mechanisms and how they develop is key when wanting to improve tool life. These wear patterns usually occur due to abrasion failure, adhesive wear or coating destruction. Properties, such as coating hardness, residual stress and chemical composition, directly influence these wear patterns.

One focus that remains the same is wanting to improve tool life. If a tool can function normally for longer, the machining process would be cheaper, as cutting tools wear out considerably quickly and are quite expensive. Attempts to lower these costs comprises a field with a large amount of research. In addition to tool life improvement, the recent trend is to increase eco-friendliness, with studies that

employ alternative lubrication methods in order to reduce the amount used in some current turning processes. This, coupled with the fact that some of these methods reduce the cutting forces of the process, means that the overall power usage will also be lower, lowering the price of the process, albeit slightly, thus making it more environmentally friendly.

**Author Contributions:** Conceptualization, F.J.G.S. and V.F.C.S.; methodology, V.F.C.S.; investigation, V.F.C.S.; resources, F.J.G.S.; writing—original draft preparation, V.F.C.S.; writing—review and editing, F.J.G.S.; project administration, F.J.G.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

#### **Sustainable Milling of Ti-6Al-4V: Investigating the E**ff**ects of Milling Orientation, Cutter**- **s Helix Angle, and Type of Cryogenic Coolant**

**Asif Iqbal 1,\*, Hazwani Suhaimi 1, Wei Zhao 2, Muhammad Jamil 2, Malik M Nauman 1, Ning He <sup>2</sup> and Juliana Zaini <sup>1</sup>**


Received: 9 December 2019; Accepted: 21 January 2020; Published: 17 February 2020

**Abstract:** Ti-6Al-4V, the most commonly used alloy of titanium, possesses excellent mechanical properties and corrosion resistance, which is the prime reason for the continual rise in its industrial demand worldwide. The extraordinary mechanical properties of the alloy are viewed as a hindrance when it comes to its shaping processes, and the process of milling is no exception to it. The generation of intense heat flux around the cutting zones is an established reason of poor machinability of the alloy and unacceptably low sustainability of its machining. The work presented in this paper attempts to enhance sustainability of milling Ti-6Al-4V by investigating the effects of milling orientation, cutter's helix angle, cutting speed, and the type of cryogenic coolant and lubricant on the sustainability measures, such as tool damage, cutting energy consumption, process cost, milling forces, and work surface roughness. It was found that micro-lubrication is more effective than the two commonly used cryogenic coolants (carbon dioxide snow and liquid nitrogen) in reducing tool wear, work surface roughness, process cost, and energy consumption. Furthermore, down-milling enormously outperformed up-milling with respect to tool wear, work surface quality, and process cost. Likewise, the high levels of cutter's helix angle and cutting speed also proved to be beneficial for milling sustainability.

**Keywords:** cutting energy; tool damage; machining; liquid nitrogen; carbon dioxide snow

#### **1. Introduction**

Titanium alloys gained an unprecedented rise in their demand from various engineering sectors due to their excellent mechanical properties and corrosion resistance. The same properties considered as excellent during the "use" phase are termed as unfavorable during the "manufacturing" phase of their life cycle. With regard to the machining domain, the same unfavorable properties are responsible for their low machinability, which render the cutting process unsustainable. The low machinability is attributed mainly to high strength, chemical affinity with the tool materials, and a short chip-rake contact length [1]. A titanium work is, thus, machined with formation of an intense heat flux around the cutting edge and consumption of exceedingly high cutting energy, leading to acceleration of the temperature-dependent modes of tool wear [2]. The high tool wear rates, resulting in frequent changes of cutting tools or edges, leave the machining process highly unsustainable, economically as well as environmentally [3]. The problem is generally negotiated either by lowering the material removal rates or by applying emulsion-based coolants, neither of which actually offers a sustainable solution.

With regard to quashing the intense heat flux around the cutting areas, the application of cryogenic fluids, especially liquid nitrogen (LN2), fares very well. The fluids offer a viable solution because of extremely low operational temperatures, no waste generation, and controllable flow rates. Additionally, application of micro-lubrication, also known as minimum quantity of lubrication (MQL), also proved to be very beneficial in enhancing tool life and improving surface quality in machining of titanium alloys conducted at medium cutting speeds [4,5]. MQL is a near-dry machining method in which a miniscule quantity of lubricating oil is pulverized into a stream of air, and the resulting aerosol is applied onto the cutting areas [6].

Although steady progress is being made regarding quantification of the effects of cryogenic coolants with regard to the continuous machining processes, not much effort is being put up concerning interrupted machining processes, such as milling. Cryogenic milling is distinct from continuous machining processes performed under cryogenic environment in the sense that the cutting teeth of a milling tool periodically and rapidly engage and disengage with the work material, leading to cyclic heating (caused by thermal energy released by work material's plastic deformation) and cooling (caused by interaction with the incoming cryogenic fluid) of the cutting edges. Such a course is expected to induce thermal shocks in the teeth, leading to cracking, chipping, and more catastrophic forms of tool damage. Furthermore, a comparative analysis regarding the cooling effects of an evaporative cryogenic fluid, such as LN2, and a throttling-based cryogenic fluid, such as CO2 snow, is also required. With regard to the milling process, two distinct parameters, milling orientation and cutter's helix angle, are also expected to have effects on the process's sustainability measures, such as tool damage rate, cutting energy consumption, work surface roughness, process cost, and cutting forces.

#### *Literature Review*

This sub-section presents a review of the published work concerning the issues of milling titanium alloys, application of cryogenic coolants in machining (especially milling) of titanium, potentials of using micro-lubrication in milling, and machining sustainability measures.

It is reported that an increase in the flow rate of liquid nitrogen can prolong the tool life in machining of titanium alloys [7]. Furthermore, the surface integrity would be greatly improved when the pressure and flow rate of the coolant are increased. Milling of Ti-6Al-4V with liquid carbon dioxide can greatly reduce the lateral crack propagation and chipping [8]. Therefore, it can be used to prolong tool life as compared to emulsion-based cooling. The effects of tool life criterion, work material's temper state, cutting parameters, and micro-lubrication on the sustainability measures of a milling process were studied [9]. It was found that the material's temper state and the option of using MQL were the most influential parameters with respect to the sustainability measures, such as specific cutting energy, tool life, and process cost. Sartori et al. reported that an MQL system amalgamated with a CO2 and LN2 distribution system could optimize the lubrication and cooling effect, leading to a significant reduction in crater wear [10]. In another study, it was reported that machining under flood coolant does not reduce surface roughness [11]. The authors also reported that LN2 hybridized with oil-based MQL can yield the lowest cutting forces of all the tested coolants. Isakson et al. reported that the cooling methods utilizing LN2 and an emulsion-based coolant yielded similar effects on surface integrity [8]. Furthermore, the authors also managed to reduce consumption of the cryogenic coolant to provide a good surface quality without conceding any negative environmental or economic impact. It was found that milling of Ti-6Al-4V with liquid CO2 could greatly reduce chipping and lateral crack propagation; therefore, it can be used to significantly prolong tool life in comparison with emulsion-based cooling [12]. The effects of using CO2 snow as a coolant and its merger with MQL were investigated in continuous machining of a high-strength β-titanium alloy [13]. It was found that the usage of CO2 snow and the location of its application was highly influential with respect to the sustainability measures. In another work, it was reported that the cryogenic cooling with LN2 could considerably reduce tool wear and, thus, lead to an increase in material removal rate [14]. It was concluded that cryogenic machining operating at a given cutting speed can cause much lower energy consumption than machining with a flood coolant. Mia et al. reported that the use of dual jets of LN2 is an excellent way to reduce energy consumption and working temperature, as well as to improve work surface quality [15]. In another experimental study, it was reported that the use of liquid CO2 at a temperature of −79.5 ◦C in cutting of a nickel-chromium alloy could reduce average surface roughness by 42–47%, 24–27%, and 16–21% over dry, wet, and MQL cutting, respectively [16]. Furthermore, the cryogenic cooling was also found to increase the compressive stresses on the surface and decrease the flank wear. Li et al. presented optimization of milling Ti-6Al-4V alloy with a graphene-dispersed vegetable-oil-based cutting fluid [17]. The results showed significant improvements in the milling performance measures including milling force, temperature, surface micro-hardness, and work surface roughness. Dry and MQL-based milling processes were compared for machining of Inconel 718 [18]. MQL was found to improve the tool life, as well as the work surface finish. An experimental study focused on modeling the effects of tool wear rate on economic sustainability of milling a titanium alloy [19]. In total, 47.5% and 47.59% less electricity consumption cost and machine operational cost, respectively, were achieved for the cryogenic cooling approach in comparison with dry machining. In another experimental study concerning end-milling of Ti–6Al–4V titanium alloy, the effects of cryogenic cooling on work's surface integrity were compared with those under dry and flood cooling environments [20]. The authors reported that cryogenic cooling resulted in up to 31% and 39% lower surface roughness when compared to flood cooling and dry approaches, respectively. A significant reduction in microscopic surface defects under the cryogenic environment was also reported. Dawood et al. studied the effects of the three cutting parameters on machining performance under flood cooling and sustainable dry environments [21]. Dry machining was found to yield better surface finish, but it also sustained more severe adhesion wear, crater wear, and formation of built-up edge. An experimental study evaluated the effects of applying cryogenic cooling with MQL lubrication in contour milling of Inconel 718 [22]. The authors claimed superiority of the proposed CroMQL method over the other lubri-cooling techniques. Pusavec et al. presented an experimental study on sustainable machining of Inconel 718 under various lubri-cooling environments such as dry, MQL, cryogenic, and cryo-lubrication [23]. Based on the statistical analyses of the results, the authors concluded that the cooling/lubrication condition had significant effects on the sustainability measures including tool life, cutting forces, and power consumption.

Milling orientation (up- and down-milling) and cutter's helix angle are amongst the milling parameters, which do not receive much attention with respect to their influence on the process's sustainability measures. It was reported that both the parameters possessed significant effects on tool life and work surface roughness in high-speed milling of hardened steels using carbide cutters [24]. Milling cutters with 45◦ helix angle yielded significantly longer tool life and marginally better surface finish than the 30◦ cutters. Moreover, down-milling was found to provide much better surface finish, but equal tool life as compared to up-milling. A tool orientation optimization model was presented that includes the effect of deflection error caused by milling forces to achieve better machining precision controlling in five-axis surface milling [25]. The effects of up- and down-milling were compared in peripheral milling of a high-alloy steel [26]. The up-milling approach was found to generate compressive residual stresses in the work surface, but with a poor surface finish in comparison with down-milling. An experimental study was performed to compare the effects of up- and down-milling in end-milling of Inconel 718 [27]. It was found that the down-milling approach yielded better results in terms of tool wear as compared to up-milling. Furthermore, the chips formed in up-milling were segmented and continuous as compared to discontinuous ones produced in down-milling. Wan et al. presented an analytical model to quantify the influence of tool s helix angle on peak cutting force [28]. The authors found that the peak value of cutting forces decreased with an increase in helix angle for a single engaged cutting edge and that the optimal helix angle corresponding to the minimum peak cutting force was a function of the number of flutes, axial depth of cut, and cutter diameter. Another work focused on quantifying the effect of helix angle on performance of coated carbide end mills for dry side-milling of 304L stainless steel [29]. It was found that the number of axis contact points and

effective cutting length increased with increasing helix angle, leading to reduced tool wear and thinner chips, but with higher cutting temperature. It was concluded that the TiAlN-coated end mill with a high helix angle of 60◦ yielded the best surface finish with an acceptably long tool life. Another study presented a mathematical model for predicting surface topography and various surface roughness metrics by considering the effects of cutter's helix angle, feed rate, and tool's eccentricity [30].

The literature review reveals the following gap between the state of the art and the objectives of this work: (1) a very limited amount of work is done so far regarding application of cryogenic fluids to milling of titanium alloys as most of the published investigations have focused on turning process only; (2) no searchable work was found regarding comparison of cooling effectiveness between throttling and evaporation based cryogenic coolants applied to intermittent cutting processes; (3) a limited amount of investigative work is available that quantifies the effects of milling orientation and tool's helix angle on the sustainability measures of titanium machining.

In perspective of the abovementioned research gap, the presented work aims to quantify and improve the sustainability measures in respect of side- and end-milling of an α + β titanium alloy (Ti-6Al-4V) while employing three kinds of cutting fluids (an evaporative cryogenic coolant, a throttling-based cryogenic coolant, and micro-lubrication), two modes of milling orientation, and two levels each of cutting speed and milling cutter's helix angle. The sustainability measures of the milling process to be evaluated are cutting energy consumption, tool damage, work surface roughness, machining forces, and process cost.

#### **2. Experimental Work**

This section presents the details regarding work material and tooling, predictor variables, responses (sustainability measures), design of experiments, fixed parameters, equipment, and measuring instruments.

#### *2.1. Work Material and Tooling*

The work material used in the study is Ti-6Al-4V, a commonly used α + β alloy of titanium. The annealed form of the material is used in the form of a plate having dimensions 75 mm × 200 mm × 19 mm. The heat treatment was done by soaking the work pieces at a temperature between 778 and 782 ◦C for about 70 min, followed by air cooling. The work material, after carrying out the heat treatment process, possessed ultimate tensile strength, yield strength (0.2% proof stress), and elongation of 1003.5 MPa, 927.3 MPa, and 15%, respectively.

The milling cutters used in this study were FIRE-coated cemented tungsten carbide flat end mills from Guhring Inc., Berlin, Germany, having diameter of 8 mm and number of cutting flutes equal to four. FIRE is a multi-layer TiN + TiAlN ceramic coating system that provides extreme wear- and heat-resistant properties to the tool. The hardness of the coating was 3300 HV, and the coefficient of friction was 0.6. The cutters with a helix angle of 30◦ had total and cutting lengths of 68 mm and 22 mm, respectively, while those with a helix angle of 42◦ had 63 mm and 19 mm, respectively. A new end mill cutter was used for each experimental run. Figure 1 presents the two kinds of milling cutters used in the experiments.

**Figure 1.** Ceramic-coated tungsten carbide side- and end-mill cutters having helix angles of (**a**) 42◦ and (**b**) 30◦.

*2.2. Predictors, Responses, and Design of Experiments*

The following four predictor variables were controlled in the milling experiments:


The aforementioned levels of the four predictor variables yielded a total of 24 (= 3 × 2 × 2 × 2) experimental runs for the sake of executing a full-factorial design of experiments. Table 1 presents the details regarding the levels of the four predictors controlled in the experiments. It is to be noted that the first and the third predictors were categorical while the other two were numerical. The two levels of the cutting speed (100 and 175 m/min) were selected on the basis of the preliminary tests. Cutting speeds in excess of 200 m/min at the given feed rate and radial depth of cut resulted in the tools getting red-hot toward the end of the cuts and the cutting edges getting covered with thick adhesions. This observation led to fixation of the upper level of the cutting speed equal to 175 m/min. The lower level, thereupon, was decided as a value between 50% and 60% of the upper one.

**Table 1.** Levels of the four predictor variables controlled in the experiments. MQL—minimum quantity of lubrication.


Each experimental run involved removing 600 mm<sup>3</sup> of volume of the work material under the following dimensions: 0.5 mm (radial depth of cut) × 8 mm (axial depth of cut) × 150 mm (length of cut). The 150-mm length of cut was completed in two passes of equal length. Figure 2 presents the pictorial description regarding the length of cut and the two depths of cut. A new side- and end-mill cutter was used for each experimental run. Up-milling is a cutting approach in which a cutting tooth of the milling cutter enters the work surface with zero chip thickness and exits with a maximum. On the contrary, the tooth enters and exits the work surface with a maximum and zero chip thickness in the down-milling approach.

**Figure 2.** Cutting schematic for each experimental run showing length of cut = 2 <sup>×</sup> 75 mm<sup>2</sup> (not to scale), axial depth of cut = 8 mm, and radial depth of cut = 0.5 mm. The numbers 1 and 2 show the order of the slice removal. The total volume (600 mm3) of material to be removed in each run is shaded gray.

The following responses were evaluated for each of the 24 experimental runs:


Additionally, the other sustainability measures of the milling process, such as waste generation, operator's safety, and health are discussed in a qualitative way.

#### *2.3. Experimental Set-Up and Measurements*

All experiments were performed on Mikron UCP 710, a five-axis, vertical machining center (Mikron Holding, Biel, Switzerland) having maximum rotational speed, feed rate, and power of 18,000 rpm, 20 m/min, and 16 kW, respectively. Milling was performed in a straight line, cutting through the 75-mm side twice during each run. Figure 3 presents the experimental set-up.

The throttling-based cryogenic cooling equipment consisted of a storage bottle containing CO2 gas compressed at a pressure of 5.5 MPa. The compressed gas was transported from the bottle to the exit nozzle through a copper tube. The mass flow rate of the CO2 gas at the exit of the 2-mm-diameter nozzle was measured to be 0.5 kg/min against the storage pressure of 5.5 MPa. The exit of the nozzle was located very close to the machining area such that the CO2 gas impacted directly on to the cutter's teeth. The gas on exit expanded and absorbed heat from its surroundings due to the Joule-Thomson effect [31]. The throttling gas, consequently, cooled down to a temperature of about −72 ◦C, which caused it to convert to a semi-solid state (CO2 snow) adhering to the tooling system and the work's surface. The evaporation-based cryogenic cooling equipment consisted of a storage dewar containing nitrogen, which was cooled down to a liquid state. The jet of LN2 impinged on to the milling cutter under a flow rate and pressure of 0.5 L/min and 0.1 MPa, respectively, through a 6-mm-diameter nozzle. The temperature of the LN2 jet, measured at the nozzle's exit, was −197 ◦C. The direction of the nozzle was adjusted such that the maximum mass of the fluid directly impacted that portion of the cutting teeth which periodically engaged and disengaged with the work material. The micro-lubrication, in the form of minimum quantity of lubrication, was supplied by mixing a vegetable-based oil at a rate

*Metals* **2020**, *10*, 258

of 25 mL/h in the flow of air compressed to a pressure of 0.6 MPa. The resulting aerosol was applied to the milling cutter at the region adjacent to the work surface.

**Figure 3.** Experimental set-up: (**a**) Ti-6Al-4V work, milling cutter, and MQL duct; (**b**) application of CO2 snow; (**c**) CO2 gas storage bottle; (**d**) force data acquisition system; (**e**) application of LN2; (**f**) storage dewar for LN2.

(**d**) (**e**) (**f**)

Flank wear land of the used cutters' teeth was measured using a camera-fitted optical microscope ARTCAM 130-MT-WOM (Tokyo, Japan). The captured images of the flank faces were processed according to the scale to determine average width of flank wear land on each of the four cutting teeth. *VB* was then calculated out by taking average of the flank wears of the four teeth. The roughness of the side surface, after finishing two passes, was measured using Mahr MarSurf M 300 C (Mahr GmbH, Göttingen, Germany) a mobile roughness measuring instrument. The instrument used a 2-μm contact stylus to find the arithmetical mean height (*R*a) of the milled surface according to ISO 11562 standard. The sampling length for each measurement was 4 mm, and Gaussian Profile Filter was used to get the roughness values. Four measurements were taken for each experimental run at the distances of 15, 30, 45, and 60 mm from the starting edge of the milled side surface. The *R*a was then obtained by taking the average of the four measurements. Cutting forces were measured using a Kistler piezoelectric dynamometer 9265B (Kistler AG, Bern, Switzerland), utilizing a force plate 9443B. The dynamometer possessed a measuring range of 0–15 kN in the *x*- and *y*-directions and 0–30 kN in the *z*-direction.

Process cost, *PC,* comprised five components. Firstly, tooling cost was quantified by multiplying *VB(*mm*)*/(0.3 mm <sup>×</sup> 600 mm3) by the current market price (BND) of the milling cutter, where *VB* is the average flank wear of a milling cutter measured after removing 600 mm<sup>3</sup> of the work material's volume. The number "0.3" in mm represents the commonly adopted tool life criterion for the machining tools. As the used tools can be resold after grinding and recoating, the average resale price of the cutter was subtracted from the purchase cost of the new tools. Clearly, a larger *VB* results in a higher tooling cost. Secondly, the direct electricity consumption cost, for each experimental run, was equal to the product of the commercial electricity tariff (BND/kWh) and the specific energy (kWh/mm3) taken by the CNC machine tool and other associated equipment (LN2/MQL) during the actual cutting process. Thirdly, the overhead cost included wage cost of one skilled operator (BND/h) and costs of using lighting and heating, ventilation, and air conditioning (HVAC) in a small room containing the milling machine. The latter was obtained as a product of the electricity tariff and the total wattage of the lights and the HVAC system (BND/kWh × kW = BND/h). The overhead cost was calculated for the actual duration of the cutting process. The cost in BND/h was divided by the respective *MRR* to obtain the overhead cost in BND/mm3. Fourthly, the equipment's depreciation cost was calculated using the actual purchase cost of each equipment (CNC milling machine, MQL, LN2 equipment, and CO2 storage bottle), proportioned for the actual time spent removing 1 mm<sup>3</sup> of work material's volume. The useful life and the salvage value of each of the four equipment were taken as 10 years and zero, respectively. Machining times required for removing 600 mm<sup>3</sup> of work material with respect to the runs employing the cutting speeds of 100 and 175 m/min were 5.65 s and 3.23 s, respectively. The numbers of working days in a year and working hours in a day were taken as 250 and eight, respectively. The straight-line depreciation model was used for calculating the depreciation cost. Lastly, the cutting fluids' consumption cost was obtained by multiplying the mass flow rate (kg/min) of the fluid (LN2, vegetable oil, or CO2) by its per mass unit purchase cost (BND/kg) and dividing by the material removal rate (*MRR*) (mm3/s) of the run. Fine details of the employed costing approach can be read in Reference [9]. Consequently, the *PC*, for each run, was obtained in BND/mm3 of the work material removed. It was quantified using the following equation:

$$PC\left(\text{BNID}/\text{mm}^3\right) = \frac{VB \times (A\_1 - A\_2)}{180} + \frac{SE \times B}{3.6c6} + \frac{(C + D \times B)}{MRR} + \frac{F \times t\_m}{10 \times 7.2c6 \times 600} + \frac{\dot{m} \times G}{MRR}, \quad (1)$$

where *A*<sup>1</sup> = purchase cost of a new end mill cutter (BND), *A*<sup>2</sup> = average resale price of the cutter (BND), *SE* = specific energy taken in by the machine tool and the associated cooling/lubricating equipment (J/mm3), *B* = commercial electricity tariff (BND/kWh), *C* = hourly wage of a skilled machine operator (BND/h), *D* = total wattage of the lights and HVAC in kW, *F* = procurement cost of all the relevant equipment; *G* = purchase cost of the cutting fluid (BND/kg), *t*<sup>m</sup> = actual machining time (5.65 s and 3.23 s), and *m˙* = mass flow rate of the cutting fluid. The numbers 3.6e6 and 7.2e6 represent the factors for converting kWh to Joules and years to seconds, respectively.

Considering the ever-present inflation, it is more meaningful to present the process cost in terms of a 100-scale comparative cost structure. In such an arrangement, the costliest result is presented as 100, the most economical as 0, and all others as proportionately determined numbers lying between 0 and 100.

The consumption of specific cutting energy for each experimental run was determined as follows:

I. Three current clamp meters, Hantek CC 65, were applied to the three phases of the AC supply of the CNC machine tool to measure the electric power drawn during the machining process (*P*total). The input voltage (*V*) and the power factor (*PF*) were measured as 220 V and 0.85, respectively. The total power was calculated using the following formula:

$$P\_{\text{total}} = \frac{\sqrt{3}.PF.V(I\_1 + I\_2 + I\_3)}{3},\tag{2}$$

where I1, I2, and I3 represent the current, in amperes, as measured by the three clamp meters.

II. The non-cutting power (*P*non-cut) was determined by rotating and linearly moving the milling cutter in the direction of feed at the given combination of rotational speed and feed speed. The cutter was moved linearly at the feed speeds of 1592 and 2785 mm/min for the runs employing the cutting speeds of 100 and 175 m/min, respectively.


Analysis of variance (ANOVA) was also performed on the experimental data in order to have a better understanding of the individual effects of the controlled predictors, as well as their interactive effects on the measured responses. ANOVA not only helps to isolate the effect of each individual predictor; it also reveals the strength of the effect.

#### *2.4. Fixed Parameters*

The values of feed per tooth, axial depth of cut, and radial depth of cut were fixed to 0.1 mm/z, 8 mm, and 0.5 mm, respectively, for all the experimental runs. Each cutter was held in a collet at a distance of 28 mm from the cutting end. All the runs were performed by carrying out the milling cuts twice in a straight line. The supply of the relevant cutting fluid (LN2/CO2/MQL) was started 20 s prior to first engagement of the tool with the work and was not stopped during the transition between the two passes. The supply was shut down immediately after completion of the second pass.

#### **3. Experimental Results**

The section provides details on the experimental results regarding tool wear, surface roughness, specific cutting energy consumption, cutting forces, and process cost.

#### *3.1. Tool Wear*

Figure 4 presents the experimental results of the 24 runs regarding *VB* obtained after removing 600 mm3 of the work material. The results are grouped into three plots based on the type of cutting fluid used. A striking result is evident that, in general, micro-lubrication yielded lower levels of tool wear as compared to the two cryogenic cooling options. Of the two cryogenic cooling options, it is evident that the evaporation-based coolant (LN2) fared better. Furthermore, the smallest *VB* (0.04 mm) of all the runs was also yielded by LN2. Furthermore, it is clearly observable that down-milling generated remarkably smaller wear than up-milling when the applied cutting fluid was LN2.

With regard to cutter's helix angle, the larger angle (42◦) performed better than the smaller angle (30◦) with respect to tool wear. Unfortunately, the dependence of helix angle on any other predictor with respect to *VB* was not clear from the plots. Although a higher helix angle favored higher material removal rates (realized by high levels of speeds and feeds), the resulting smaller flute spacing caused a problem in chip evacuation at these rates, especially for a sticky material such as Ti-6Al-4V. This is why no interactive effect between helix angle and cutting fluid was visible on tool wear. Down-milling (also known as climb milling) generally yielded lower tool wear than up-milling (conventional milling). This effect was especially prominent in the milling conducted under LN2. The gradual decrease in chip thickness, as the cut proceeded in down-milling, reaching zero at the end, prevented the cutting edge and adjacent flank face from rubbing and burnishing against the work surface. The lower levels of flank wear observed in down-milling were attributed to this mode of frottage-free cutting. With respect to cutting speed, its higher level was generally found to yield higher levels of tool wear, although the effect was quite insignificant. A high level of cutting speed instigated or accelerated the temperature-dependent modes of tool damage because of an enhancement in the rate of heat generation and led to intensification of tool wear. As a result, the cutting edge and the adjacent faces incurred higher magnitudes of wear per unit volume of work material removed.

**Figure 4.** Bar graphs present the measurements of the average width of flank wear land (*VB*) for the 24 experimental runs.

ANOVA was performed on the data shown in Figure 4. The analysis revealed milling orientation as the most influential factor regarding *VB*, followed by the choice of cutting fluid. The interaction between milling orientation and cutting fluid stood third in terms of statistical significance, followed by the solitary effects of cutting speed and cutter's helix angle. The strong interactive effect between milling orientation and cutting fluid on tool wear is evident from the three plots of Figure 4 in the manifestation that down-milling yielded vastly lower tool wear than up-milling only when the applied cutting fluid was liquid nitrogen. Shokrani et al. claimed that the coated carbide cutter used for cryogenic (LN2) milling of Ti-6Al-4V, operated at 200 m/min cutting speed and 0.03 mm/tooth feed rate, showed the minimum level of flank wear [14]. The current work, on the other hand, found MQL to be

a better cutting fluid than the cryogenic fluids with respect to tool damage. The finding was based on milling performed at a slightly lower cutting speed but an enormously larger feed rate.

Figure 5 presents the microscopic images of the selected used cutters. The six cutters were selected so as to ensure a maximum level of diversity regarding the predictor variables. The microscopic visual analysis revealed occurrence of progressive mechanical wear and adhesion of work material as the major modes of tool damage, while chipping of cutting edge was also observed in a few tools. As can be seen from Figure 5a,d, all the cutters used in the runs involving up-milling and either of the two cryogenic fluids experienced thick adhesion of work material at their cutting edges. The adhered material, in all these runs, was in the form of minute flakes. The observation tallies with the findings of the previous works. It was reported that adhesion wear was the main tool damage mechanism in LN2-assisted machining of Ti-6Al-4V [7]. In addition, up-milling was also reported to instigate more severe tool wear in the peripheral milling of a hardened steel [24].

**Figure 5.** Micrographs showing the modes of damage incurred by the teeth of the milling cutters used in the following runs: (**a**) CO2, 42◦, up, 175 m/min; (**b**) CO2, 42◦, down, 175 m/min; (**c**) LN2, 42◦, down, 175 m/min; (**d**) LN2, 30◦, up, 100 m/min; (**e**) MQL, 42◦, up, 100 m/min; (**f**) MQL, 30◦, down, 100 m/min.

As a cutting edge, in up-milling, is known to enter the work with a minimum thickness and exit with a maximum, it sees a tendency of the removed chip to remain adhered to it under the influence of high temperature. Rapid cooling offered by the incoming cryogenic fluid strengthens the attachment of the chip, or part of it, to the edge and the adjacent faces. On the other hand, as is visible from Figure 5e,f, all the runs involving micro-lubrication yielded adhesion of micro-chips on the tool surface located inside of the flutes and away from the cutting edges. Micro-sized particles of the removed material were vulnerable to be caught up by the oily surface of the flutes and remained clung even against the high centrifugal forces of the rotating tool. Progressive mechanical wear (abrasion), in the form of a bright line existing close to the cutting edge, is evident in all the six images. Not much information can be deduced from the images regarding dependence of its severity on the various predictors controlled in the experiments. It was reported that abrasion was not the wear-determining mechanism in cryogenic or wet milling of Ti-6Al-4V using coated and uncoated tools irrespective of the coolant's choice [12]. Chipping was observed in no more than three milling cutters. One of them is shown in Figure 5c. All of them were involved in milling at the high level of cutting speed (175 m/min). Furthermore, the chipping, in all three instances, occurred at the corner of the relevant tooth. In a previous work, the application of CO2 was reported to slow down the chipping process [12]. Thermal cracking of the cutting teeth, normally caused by rapid heating and cooling cycles of cryogenic milling, was not evident, probably, due to a considerably short length of cut employed in the experimental runs.

#### *3.2. Work Surface Roughness*

Figure 6 presents the measurements of *R*a for the 24 runs, categorized by the type of cutting fluid used. The error bars present the standard deviation of the measured data for each experimental run. Quite a few conclusions can be drawn directly from the plots. As far as the type of cutting fluid is concerned, it is evident that micro-lubrication yielded better surface finish than the cryogenic fluids. Furthermore, down-milling fared better than up-milling. The effects of other two predictors were not clear from the graphs. ANOVA was performed on the *R*a data to get further insights. The analysis revealed milling orientation to be the most influential predictor, followed by cutting speed and cutting fluid. The effect of cutter s helix angle was found to be statistically insignificant. With regard to milling of a hardened tool steel, the cutters with a 45◦ helix angle were reported to yield significantly lower surface roughness than the 30◦ cutters [24]. Up-milling is clearly not the better choice for the sake of good surface finish. As described in the previous sub-section, the cutting teeth, while following the up-milling approach, remove the work material with a lot of work material adhesion. The adhesion is believed to compromise the true geometry and sharpness of the cutting edge, thus leading to generation of a rougher surface. The superiority of down-milling regarding work surface finish was also reported for the milling of a tool steel [26].

It is quite surprising to see micro-lubrication yielding better surface finish than the two cryogenic coolants. As far as a continuous machining process is concerned, many papers reported strikingly improved work surface finish caused by the use of a cryogenic coolant. On the contrary, cryogenic cooling, in this study, yielded a poor work surface quality regarding the milling process. A plausible explanation for this observation is that milling is an interrupted machining process in which cutting teeth periodically engage and disengage with the work. Each tooth engagement causes an increase in temperature of the tooth, as well as the newly generated work surface. The surface tends to expand due to the resulting increase in temperature but immediately gets impinged upon with the flow of a cryogenic fluid as the cutter clears the area following its feed. The impact of the super-cool fluid immediately lowers its temperature, pushing the surface into the contraction mode. Such an intense thermal effect instigated upon the newly generated surface, sending it into abrupt modes of expansion and contraction, causes deterioration of the surface quality. A 25% reduction in work surface roughness was reported when graphene-dispersed vegetable-oil-based micro-lubrication was applied in the milling of Ti-6Al-4V [17]. An increase in cutting speed was found to reduce the surface roughness when the milling orientation was down-milling. Its influence on *R*a with up-milling is not clear. An increase

in cutting speed was also reported to reduce the work surface roughness in milling of a hardened steel (AISI D2) [9].

**Figure 6.** Bar graphs present the experimental results regarding arithmetic average surface roughness for the 24 runs.

Figure 7 presents the textures of the four milled surfaces obtained after completion of the selected experimental runs. Clearly, the surfaces produced by up-milling (Figure 7a,b) were marred by the adhesion of micro-chips. The adhesion caused serious deterioration of the work surface. Figure 7a presents the roughest of all the surfaces generated in the 24 runs. All the runs employing the combination of CO2 snow and up-milling yielded severe adhesion of micro-chips on the milled surfaces. Figure 7c,d do not show any sign of adhesion, as both of them were associated with the runs employing the down-milling orientation. It is to be noted that the pattern of vertical lines on the texture was the same in all the images. This is because all the runs were carried out at a fixed value of feed per tooth. Figure 7b shows a white horizontal line running through the generated surface. The line was generated by the chipped portions of the four flutes of the cutter, which might have encountered a hard phase during the first pass of the run.

**Figure 7.** Work surface texture obtained after completion of the following runs: (**a**) CO2, 42◦, up, 175 m/min; (**b**) LN2, 30◦, up, 175 m/min; (**c**) LN2, 42◦, down, 100 m/min; (**d**) MQL, 30◦, down, 100 m/min.

#### *3.3. Specific Cutting Energy and Machining Forces*

Table 2 presents the data regarding measurement of total electric power and cutting and non-cutting powers and evaluation of the *SCE*s for the 24 experiments. The procedure for measuring the electric powers was already detailed in sub-Section 2.3. Figure 8 presents the experimental results of the 24 runs regarding consumption of specific cutting energy. Two conclusions can be drawn directly from the plots. Firstly, the cutters with a 42◦ helix angle consumed less energy than those with a 30◦ helix angle. Secondly, the higher level of cutting speed caused a decline in energy consumption. ANOVA applied to the *SCE* data revealed extreme statistical influences of cutting speed and cutter's helix angle. The effect of cutting fluid was found to be marginally significant, and that of milling orientation was found to be insignificant.

The resulting diminution in specific cutting energy caused by cutting speed was attributed to an increase in material removal rate to a larger extent and an increase in cutting power consumption to a smaller extent. As *SCE* is the ratio of cutting power to material removal rate, an increase in cutting speed resulted in lessening of its magnitude. The cutting power (and energy) is required at the cutting edge to plastically deform the work material in shear and convert it into a chip. The formation of the chip is realized along with the generation of process heat. A higher cutting speed results in a higher heat generation rate. The process heat was reported to reduce the flow stress of the work material causing reduction in the cutting energy required to remove the same volume of material [32]. This phenomenon is termed thermal softening, and it was the reason for a lesser than expected increase in cutting power with increase in cutting speed. Hence, the specific cutting energy was found to decline with a step up in cutting speed. An increase in cutting speed, in side- and end-milling of hardened tool steel, was also reported to significantly reduce the specific energy consumption [9].


**Table 2.** Measurements and calculations regarding total power consumed by the machine tool, non-cutting power, cutting power, and specific cutting energy. MRR—material removal rate.

A cutter with a larger helix angle cuts the work material with a larger ratio of the axial force component to the radial. The radial component of the machining force is responsible for causing vibrations in the tool and stirring instability in the milling process. Such a phenomenon becomes more prominent in machining a difficult-to-cut material, such as Ti-6Al-4V. Stronger vibrations and higher instability means a higher consumption of energy for removing the same volume of material. As such, a larger helix angle causes a reduction in the proportion of the radial force component, thus stabilizing the milling process and causing a diminution in cutting energy.

Among the three cutting fluids, the throttling-based cryogenic coolant (CO2 snow) produced the best results regarding specific cutting energy, although not by a momentous difference, followed by the evaporative cryogenic coolant (LN2). Both the cryogenic coolants reduced cutting energy consumption by curbing the tool wear progress. The super-cool fluid put a check on the temperature-dependent tool wear modes, which helped to maintain the cutting-edge geometry for a longer period of time. Consequently, the maintained sharpness of the cutting edge needed a smaller cutting force and lesser energy to cut the given volume of work material. In another work, cryogenic milling of Ti-6Al-4V was also found to be hugely energy conservative in comparison with wet milling due to avoidance of additional power drawn in by the coolant pump [14].

**Figure 8.** Bar graphs present the experimental results regarding the *SCE* for the 24 runs.

From the perspective of machining sustainability, it is often urged to include the relevant energies required to liquefy, compress, and deliver per unit mass of the cutting fluids in the *SCE* calculations. The authors would like to present some quantifications in this regard. For this study, the measured volumetric flow rate of LN2 as a coolant was 0.5 L/min. Furthermore, the density of nitrogen in the liquid state was equal to 804 kg/m3 at a temperature of <sup>−</sup>195.8 ◦C. European Industrial Gases Association (EIGA) reported that separating nitrogen at 0.1013 MPa and liquefying it from a temperature of 285 K requires consumption of electrical energy of about 549 kWh per ton of the gas [33]. Converting this figure into the power requirement against the mass flow rate of 0.402 kg/min gives 13.24 kW. Moreover,

the storage dewar used in the experimental work consumes additional 50 watts of power to pump the LN2 at the given flow rate. It was reported that capture and compression of CO2 gas consumes electrical energy in the range of 250–300 kWh/ton of the gas [34]. For the average value of the reported range, 275 Wh/kg, and the mass flow rate of 0.5 kg/min, the average electric power required to supply and maintain the given flow rate of the CO2 gas is 8.25 kW. Lastly, the average power required to drive the employed MQL system at the air pressure, oil mixing rate, and air flow rate of 0.6 MPa, 30 mL/h, and 0.4 L/s, respectively, is 418 Watts. The details regarding the calculations can be seen in Reference [35]. By adding the above calculated values of power requirements, the *SCE* values ranged from 57.6 to 82 J/mm3, 74.5 to 131 J/mm3, and 5.2 to 8.3 J/mm3, respectively, for CO2 snow, LN2, and micro-lubrication. Clearly, the modified values of *SCE* rendered the comparisons among the three cutting fluids trivial, with LN2 topping the list of the tested fluids by an unprecedented margin.

Figure 9 presents the experimental data regarding the three components of the machining forces obtained from the 24 experimental runs. As described before, *F*<sup>x</sup> and *F*<sup>z</sup> are aligned with the feed direction and the cutter's axis, respectively, while *F*y is perpendicular to the side surface of the work. Thus, it can be said that the axial component of the milling force is *F*z, while the radial component is the resultant of the other two.

A few inferences can be drawn directly from the three plots. Firstly, the cutter with the larger helix angle experienced lower magnitudes of all the force components. Secondly, *F*x rose unusually for all the three cutting fluids when a 30◦ helix cutter was used to mill at the high level of cutting speed using the down-milling orientation. Thirdly, with respect to the choice of cutting fluid, LN2 yielded the lowest magnitudes of the milling force components, followed by CO2 snow. ANOVA was applied to get in-depth results. For all the three force components, cutter's helix angle was found to be a highly and the most influential predictor. The effects of milling orientation and cutting fluid were found to be marginally significant, whereas that of cutting speed was found to be insignificant. Furthermore, an interactive effect between cutting fluid and milling orientation was found to be significant on the radial components only.

A milling cutter with a large helix angle is ideal for dynamic stability in milling, machining with small depths of cut, and gaining a good work surface quality. On the other hand, a cutter with a small helix angle is good for rough machining and removing work material at high removal rates. Dynamic stability of milling is reflected by small magnitudes of machining forces. The results in this study show that a 42◦ helix angle yielded better dynamic stability than a 30◦ angle, as reflected by the results regarding the milling force components. The observed effectiveness of LN2, in curbing machining forces, can be attributed to its more effective heat dissipation capability. An extremely low operational temperature of the fluid is believed to equip it with such a capability. Better heat dissipation mechanism helps keeping a check on tool wear progress, which, in turn, helps maintaining cutting edge's geometry and suppressing the machining forces. In other works, hybrid cryogenic cooling/lubrication was reported to yield the lowest machining forces compared to both emulsion and cryogenic conditions in machining of titanium alloys [11,13]. The effect of milling orientation was neither very significant nor clear. Down-milling yields lower magnitudes of force components for CO2 snow and MQL and higher magnitudes for the third fluid. Moreover, up-milling was found to be the better orientation to be used at higher cutting speeds. Li et al. reported an 18% reduction in milling forces when graphene-dispersed vegetable-oil-based MQL was applied in the milling of Ti-6Al-4V [17].

**Figure 9.** Bar graphs present the experimental results regarding the three components of the machining forces (*F*x, *F*y, and *F*z) for the 24 runs.

Figure 10 presents the progress of the three force components with time for the selected experimental runs. Two concentration zones are visible in each of the three graphs representing the two cutting passes per experimental run. The two zones in Figure 10a are thinner than those in the others because they represent the cutting passes conducted at the higher speed level. Based on the feed directions, the radial force components (*F*<sup>x</sup> and *F*y) were found in the positive and negative regions for down- and up-milling, respectively. The axial component (*F*z) was located in the negative region irrespective of the milling orientation.

**Figure 10.** Progress of the milling force components with time for the following experimental runs: (**a**) CO2, 42◦, down, 175 m/min; (**b**) LN2, 30◦, up, 100 m/min; (**c**) MQL, 30◦, down, 100 m/min.

The graphs highlight a phenomenon that sets apart the cryogenic fluids from micro-lubrication. It can be seen from Figure 10a,b that the instantaneous values of the force components sharply decreased as the cutting pass progressed. The magnitudes remained fairly constant in the third graph. This is attributed to the temperature dependent variation in yield strength/flow stress of the work material. As described before, the supply of a cryogenic fluid was opened 20 s prior to the start of the first milling pass. As the cryogenic fluid was also received by the work, in addition to the tool, the surface temperature fell sharply, raising the material's yield strength. Thereafter, as the cutting process started, the process heat raised the temperature of the work material, thereby reducing its flow stress. The fall in the work material's flow stress is reflected by the diminution in the force components as the cutting process proceeded.

The cutting power required at the cutting edge for plastically deforming the work material into a chip is presented as the product of cutting force and cutting speed (*P*cut = *v*<sup>c</sup> × *F*c), where *F*c, in peripheral milling, is a function of the three orthogonal force components, tool geometry, and shear plane angle [36,37]. This means that the cutting power should have positive correlations with the measured orthogonal machining force components. The correlation coefficient between *P*cut and the resultant of the two force components acting perpendicular to the cutter's axis (= [*F*<sup>x</sup> <sup>2</sup> + *F*<sup>y</sup> 2] <sup>1</sup>/2) was found to possess a value of 0.66. Likewise, the correlation coefficient between *P*cut and the resultant of all the three force components (= [*F*<sup>x</sup> <sup>2</sup> + *F*<sup>y</sup> <sup>2</sup> + *F*<sup>z</sup> 2] <sup>1</sup>/2) was 0.68. The two correlations suggest that strong uphill relationships exist between the cutting power and the machining forces. It can, thus, be safely stated that machining force data can estimate cutting energy consumption fairly accurately.

#### *3.4. Process Cost*

While evaluating *PC* for the 24 experimental runs, it was found that tooling cost was the most weighted contributing factor, followed, in descending order of weightage, by acquisition cost of cutting fluids, overhead cost, direct energy consumption cost, and equipment depreciation cost. The process cost's estimation is shown in Figure 11. The figure clearly shows that, of the three cutting fluids tested, micro-lubrication yielded the most economical results, which is attributed to the low levels of *VB* and low unit cost and consumption rate of the associated cutting fluid. Of the two cryogenic coolants, CO2 snow yielded better results when milling orientation was up, while LN2 was clearly more economical for down-milling. Furthermore, a high helix angle produced better results, especially under the cryogenic cooling environments, due to the associated low levels of tool damage incurred. Lastly, the effect of cutting speed on process cost was not very clear, as this predictor asserted its effects on all five constituents of *PC* in different ways. With regard to a continuous cutting process, it was reported that high-speed machining of Ti-6Al-4V yielded better surface finish and caused lower consumption of specific energy than conventional machining, but fared poorly with respect to the sustainability measures of tool life and process cost [35].

**Figure 11.** *Cont.*

**Figure 11.** Bar graphs present the process costs for the 24 experimental runs.

#### **4. Discussion on Milling Sustainability**

The previous section presented comprehensive quantifications and analyses with respect to the vital sustainability metrics regarding milling of the most commonly used titanium alloy. The current section discusses the implications of the experimental results in the perspective of the three pillars of sustainability: economic, social, and environmental.

Process cost is the most important performance measure related to the economic aspect of sustainability. As detailed in the previous section, tooling cost and cutting fluid s acquisition cost are the most important contributors. Tooling cost is governed by tool wear (*VB*); a larger value results in a higher cost. On the other hand, *VB* is also influenced in a positive way by the application of cutting fluids. The experimental results regarding *PC* suggest that micro-lubrication, combined with the down-milling approach, ensured the most economical milling of Ti-6Al-4V, realized in the form of slightest tool wear. Work surface roughness also possesses significance with respect to the economic aspect. An acceptable level of surface finish avoids financial losses due to the following factors: (1) additional processing cost caused by rework, and (2) additional material and processing cost incurred by rejection of the work. In this regard, once again, micro-lubrication and down-milling arose as the ideal combination, as explained in Section 3.2.

The social dimension of machining sustainability is highly qualitative. The factors covered under this pillar are workers safety, health protection, ergonomics, payment equity, and work pressure and intensification. Cutting speed is expected to affect ergonomics and work pressure as its low levels may increase working hours of the machine operators and build-up on production slack, leading to enhancement of work pressure. Choice of cutting fluid is another influential parameter with respect to the social pillar. All three levels of cutting fluid tested in this work demand extra space and workload for setting up and operating the fluid supply system. A minor cleaning effort is also required after the milling process utilizing micro-lubrication and cryogenic fluids is over. The oil delivered to the machining area in micro-lubrication is of minute quantity, which requires an easy cleaning effort. Moreover, the commonly used oils are vegetable-based and are, thus, not hazardous. Application of LN2 to the milling area immediately turns the fluid into its gaseous form, which simply escapes into the environment without affecting the operators. Likewise, compressed CO2 gas, after throttling, converts into snow, which gradually sublimates to a gaseous form and escapes. A small flow rate (0.5 kg/min) of the gas does not pose any asphyxiation risk.

The environmental pillar of milling sustainability is estimated by specific cutting energy, tool damage, and waste generation. The experimental results regarding *SCE* suggest that Ti-6Al-4V should be milled at a high level of cutting speed using a cutter of a medium-to-high helix angle. Moreover, the application of micro-lubrication is much more energy-conservative than the cryogenic fluids. A high *VB* causes more frequent tool replacements, leading to a higher number of tools required to remove a given volume of work material. As a result, more energy is consumed for remanufacturing/recycling the worn-out tools or, in a worse case, more landfilling of the end-of-life tooling and extraction of the raw material for making new tools is required. Such a situation quickens depletion of natural resources and causes a harmful effect on the environment. The analysis carried out on tool wear data suggests that the titanium alloy should be milled with the down-milling orientation and under the effect of micro-lubrication. Regarding the issue of machining waste generation, the tested levels of milling orientation, cutter's helix angle, and cutting speed did not make any difference. The total mass of chips generated was the same for any combination of these three predictors. On the other hand, all three options of cutting fluid tested in the work were better than the conventional approach of emulsion-based flood cooling. Emulsion coolant is known to create swarf, leading to the need for cleaning the chips before the commencement of their recycling process. Furthermore, the emulsion fluid requires filtration and pumping for its repeated use until it becomes toxic due to contamination from swarf, sump, and exposure to air. Milling under cryogenic and MQL environments does not create any swarf; thus, the chips remain clean. Furthermore, no greenhouse gases are emitted and recycling of the used fluid is not required. Regarding the option of LN2, nitrogen gas is harmless to human health and is also environmentally benign. Moreover, the use of CO2 gas, as a cryogenic coolant, does not increase carbon footprint because a minute quantity is used for cooling. A viable cutting speed of up to 200 m/min was claimed in machining titanium alloys using a natural diamond tool and under the effect of flood coolant [2]. On the other hand, the current work presented sustainable machining of the titanium alloy up to a speed of 175 m/min, but with more economical tooling and environmentally friendly coolants.

In the context of the discussion provided above, the authors would conclude that, with respect to side- and end-milling of Ti-6Al-4V, the application of micro-lubrication is more favorable than the two cryogenic fluids tested. It fared significantly better in terms of all the sustainability measures, namely, tool damage, process cost, work surface quality, and energy consumption (with inclusion of the fluids' energies). Not all the previously published reports claimed superiority of the cryogenic fluids over conventional approaches. Isakson et al. reported development of tool wear at a faster rate under LN2 than under emulsion-based coolant in the machining of titanium alloy, although the limited supply of coolant was blamed for its below-par performance [8]. Additionally, in the current work, the down-milling orientation clearly outperformed up-milling with respect to tool damage, process cost, and work surface quality. With regard to the other two predictors, the high levels of cutter's helix angle and cutting speed generally yielded better results, but the observed effects were not emphatic.

#### **5. Conclusions**

The presented work aimed to quantify and enhance sustainability of milling a commonly used titanium alloy under the environments of micro-lubrication and cryogenic cooling. The cryogenic cooling environment was set up by the application of two kinds of coolants, throttle and evaporative, realized by expansion of compressed CO2 gas and evaporation of liquid nitrogen, respectively. Additionally, the effects of employing two milling orientations and two levels each of tool's helix angle and cutting speed on the sustainability measures were also quantified.

The most prominent finding of the work is that application of micro-lubrication in milling of Ti-6Al-4V is more sustainable than that of a cryogenic coolant. Minimum quantity of lubrication outperformed both the cryogenic coolants in terms of tool wear, work surface quality, process cost, and energy consumption. Dissipation of process heat using a cryogenic coolant is not as viable in milling as it is in a continuous machining process such as turning. The interrupted nature of cutting caused by periodic engagement and disengagement of cutting teeth in the work under the action of a super-cool fluid renders the cryogenic coolant far less effective. Among the other predictors, the effects of milling orientation on the sustainability measures in milling of the titanium alloy were highly significant. Down-milling was found to be enormously better than up-milling with respect to tool wear, work surface quality, and process cost. Thus, it is highly recommended to use the combination of micro-lubrication and down-milling for sustainable milling of Ti-6Al-4V.

The effects of tool s helix angle and cutting speed were different for different measures of sustainability, and they were not highly assertive. The high level of helix angle was excellent for reducing specific cutting energy and also performed well for the sake of reduction in process cost, tool wear, and milling forces. Likewise, high cutting speed was good for reducing work surface roughness and specific cutting energy consumption.

**Author Contributions:** A.I. conceived and designed the experiments, analyzed the results, and partially wrote the paper; H.S. arranged the tooling and work material; W.Z. and M.J. performed the experiments; M.M.N. determined the responses from the measurements; N.H. arranged the measuring instruments and performed the sustainability analysis; and J.Z. developed the graphs and partially wrote the paper. All authors have read and agreed to the published version of the manuscript.

**Funding:** The work presented in the article is financially supported by Universiti Brunei Darussalam, Brunei through its University Research Grant scheme (grant number: UBD/RSCH/URC/RG(b)/2018/003).

**Acknowledgments:** The authors of the article are grateful to the management of the Laboratory of Advanced Cutting Technology at Nanjing University of Aeronautics and Astronautics, China, for providing access to the CNC milling machine, cutting fluid equipment, and measuring instruments.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
