*4.3. Types of Tool Wear*

The abrasive nature of multi-materials, with consequences for the substrate in the drilling process, such as fiber pull out, particle fracture, delamination and debonding at the fiber or particle and matrix interface, causes severe tool wear [55]. This is one of the reasons a tool coating is employed several times, so it can be possible to extend the tool life as much as possible, thus improving the process and reducing the associated costs [55].

It has been found that the tool wear increases the cutting force values [101]. This can be explained due to the loss of the tool's capacity to perform the intended job, as the contact surface with the substrate is reduced and its roughness increased, which is translated into higher cutting forces and less cutting quality. The types of wear suffered by tools during machining processes include adhesive, abrasive, fatigue and corrosive wear [57].

The adhesive wear, represented in Figure 15, can be produced by two different ways: directly, with high-speed chip particles impacting the tool, creating a micro blasting effect that reduces the cutting angle and the rigidity of the tool, and indirectly, by the incorporation of fragments of the substrate material to the tool [57]. When these fragments are removed, they can drag out tool particles, which leads to its wear by the loss of tool material, but also by the abrasion process caused by the friction of those particles with the tool rake face when they are dragged by the chip [84,102]. This abrasion action acts mainly on the flanks and outer corners of the drill bit in an irregular pattern, rounding its cutting edges and possessing various forms, such as crater wear, chisel edge wear and chipping [103–105]. Fatigue wear is the result of an excessive number of cycles performed by the tool, and the corrosive ones occur by a chemical reaction, degrading the tool material and thus keeping it from performing a proper cut.

**Figure 15.** Adhesive wear in a drilling tool: (**a**) edge chipping in the flank face, (**b**) chipped zone in the rake face, (**c**) chipped zone in the flank face and (**d**) flank wear and edge rounding [105].

Sometimes, a phenomenon that can occur is the clogging of the drill flutes with chips coming from the substrate, which prevents the drill from working correctly and therefore threatens the final quality of the product [57]. In this case, as well as with certain stages of adhesive wear, cleaning the drill with sodium hydroxide is required, with the aim to eliminate these negative aggregations and accordingly return the drill to its normal shape [106]. For the milling process of multi-materials, tools with diamond inserts possess good results, due to their high hardness [101]. The same method is applied in the drilling of FRPs, with diamond-mounted points to reduce the wear at the cutting edges, due to their high abrasive wear resistance [107].

## **5. Process Sustainability**

Progress towards sustainable development is increasing in all industrial sectors. Within the integrated life cycle management strategy, the manufacturing phase is one of the key performance phases due to resource consumption, emissions, and other negative impacts. To minimize environmental, ecological, and social damage, sustainable manufacturing design must take into account the three dimensions of sustainability: environmental, economic and social [108].

The machining process is an important part of the manufacturing process; therefore, it is expected to have a great impact by improving sustainability performance. Among the elements of machines, conventional coolant application has been considered as a critical limiting factor to achieving better sustainability performance [109].

The machining process of high-tech materials requires oil-based cooling/lubrication, which is one of the main non-sustainable elements, leading the R&D process in the search for alternative cooling and/or lubrication mechanisms [110]. To achieve better sustainability performance, alternative approaches to conventional flood cooling such as dry machining, minimum quantity lubrication (MQL) and cryogenic machining have been applied. Comparing MQL and dry machining, a similar result is observed; MQL does not remove the heat generated in the cutting zone sufficiently. Cryogenic machining not only reduces the disposal of large quantities of lubricating oil used in machining, but also increases tool life by 60% [109].

Some authors have studied these three cooling methods and compared their findings. Nagaraj et al. [111] compared MQL and cryogenic cooling methods to drill a CFRP composite. They concluded that cryogenic drilling gives better results than dry drilling in terms of drilled hole diameter and roundness, and sequentially they considered the MQL method as the next best alternative to dry drilling. Sharma et al. [112] presented a review to understand the influence of different types of coolants, mineral oils, vegetable oils, and nanofluid-based cutting fluids with the help of the MQL technique. They concluded that MQL reduced cutting temperature and produced metallic color chips, showed a reduction in dimensional deviation, improved tool life, produced better surface quality and showed an approximately 40% increase in material removal rate compared to aqueous flood coolant. To facilitate the analysis of the sustainability of the machining process, Hegab et al. [113] developed a detailed and general assessment model for machining processes. The four life cycle stages (pre-manufacturing, manufacturing, use and post-use) are included in the proposed algorithm. Energy consumption, machining costs, waste management, environmental impact, and personal health and safety are used to express the overall sustainability assessment index. They applied the model to three literature case studies and found that it was able to predict optimal parameters in close agreement with the experimentally measured results. Lv et al. [114] applied a model of the energy efficiency, carbon efficiency, and green degree in five types of machine tools selected for the typical machining processes (turning, milling, planning, grinding and drilling). After a careful analysis, they observed the increase in the back engagement will increase the material removal rate, thus, its impact degree is small, and increasing spindle speed and feed rate will reduce processing time, energy consumption, and emission, thus presenting a positive correlation, reaching an energy efficiency of 30%.

To achieve a sustainable machining process and estimate earnings, it is necessary to establish new models that reduce this complexity in the design and management stage, allowing the development of task manufacturing to promote sustainability in the processes involved while maintaining technical, economic and quality feasibility.
