Lubricants

21 pages, 4542 KiB

Open AccessArticle

Tribo-Dynamics and Fretting Behavior of Connecting Rod Big-End Bearings in Internal Combustion Engines

by Yinhui Che, Meng Zhang, Qiang Chen, Hebin Ren, Nan Li, Shuo Liu and Yi Cui

Lubricants 2025, 13(9), 376; https://doi.org/10.3390/lubricants13090376 (registering DOI) - 23 Aug 2025

With the increased power density of internal combustion engines (ICE) and growing demands for lightweight design, the connecting rod big-end bearings are subjected to significant alternating loads. Consequently, the interference–fit interfaces become susceptible to fretting damage, which can markedly shorten engine service life [...] Read more.

With the increased power density of internal combustion engines (ICE) and growing demands for lightweight design, the connecting rod big-end bearings are subjected to significant alternating loads. Consequently, the interference–fit interfaces become susceptible to fretting damage, which can markedly shorten engine service life and impair reliability. In the present study, the effects of the big end manufacturing process, bolt preload, and bearing bush interference fit are considered to develop a coupled lubrication–dynamic model of the connecting rod big-end bearing. This model investigates the fretting damage issue in the bearing bush of a marine diesel engine’s connecting rod big end. The results indicate that the relatively low stiffness of the big end is the primary cause of bearing bush fretting damage. Interference fit markedly affects fretting wear on the bush back, whereas the influence of bolt preload is secondary; nevertheless, a decrease in either parameter enlarges the fretting distance. Based on these findings, an optimized design scheme is proposed. Full article

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25 pages, 7172 KiB

Open AccessArticle

Evaluation of Long-Term Skid Resistance in Granite Manufactured Sand Concrete

by Hongjie Li, Biao Shu, Chenglin Du, Yingming Zhuo, Zongxi Chen, Wentao Zhang, Xiaolong Yang, Yuanfeng Chen and Minqiang Pan

Lubricants 2025, 13(9), 375; https://doi.org/10.3390/lubricants13090375 (registering DOI) - 23 Aug 2025

Abstract

The widespread application of granite manufactured sand (GS) concrete in pavement engineering is limited by issues such as suboptimal particle size distribution and an unclear optimal rock powder content. Furthermore, research on the long-term evolution of the skid resistance characteristics of GS concrete [...] Read more.

The widespread application of granite manufactured sand (GS) concrete in pavement engineering is limited by issues such as suboptimal particle size distribution and an unclear optimal rock powder content. Furthermore, research on the long-term evolution of the skid resistance characteristics of GS concrete remains relatively scarce. This knowledge gap makes it difficult to accurately assess the skid resistance performance of GS concrete in practical engineering applications, thereby compromising traffic safety. To address this research gap, this study utilized a self-developed indoor abrasion tester for pavement concrete to assess the skid resistance of GS concrete. Three-dimensional laser scanning was employed to acquire the concrete’s surface texture parameters. Using the friction coefficient and texture parameters as skid resistance evaluation indicators, and combining these with changes in the concrete’s surface morphology, the study explores how effective sand content, stone powder content, and fine aggregate lithology affect the long-term skid resistance of GS concrete pavements and reveals the evolution trends of their long-term skid resistance. Research results show that as the number of wear cycles increases, low and high effective sand content affect the surface friction coefficient of specimens in opposite ways. Specimens with 95% effective sand content exhibit superior skid resistance. Stone powder content influences the friction coefficient in three distinct variation patterns, showing no clear overall trend. Nevertheless, specimens with 5% stone powder content demonstrate better skid resistance. Among different fine aggregate lithologies, GS yields a higher friction coefficient than river sand (RS), while limestone manufactured sand (LS) shows significant friction coefficient fluctuations across different wear cycles. Adding stone powder substantially enhances mortar strength and delays groove collapse edge formation. Moreover, higher effective sand content and proper stone powder content mitigate bleeding, thereby improving mortar performance. Full article

(This article belongs to the Special Issue Tire/Road Interface and Road Surface Textures)

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16 pages, 4428 KiB

Open AccessArticle

Toward Coarse and Fine Bimodal Structures for Improving the Plasma Resistance of Al₂O₃

by Jeong Hyeon Kwon, I Putu Widiantara, Siti Fatimah, Warda Bahanan, Jee-Hyun Kang and Young Gun Ko

Lubricants 2025, 13(9), 374; https://doi.org/10.3390/lubricants13090374 - 22 Aug 2025

Abstract

In the quest to produce high-purity alumina, bottom-up engineering via architecting the interior of ceramic with bimodal structures of alumina powders in the absence of any additives has gained considerable attention owing to the simplicity offered. The present work investigated the influence of [...] Read more.

In the quest to produce high-purity alumina, bottom-up engineering via architecting the interior of ceramic with bimodal structures of alumina powders in the absence of any additives has gained considerable attention owing to the simplicity offered. The present work investigated the influence of bimodal structures containing micron (~35 μm) and submicron (~600 nm) Al₂O₃ powders on the formation of dense Al₂O₃ ceramic. To this end, ball-milling was conducted to prepare the desired sizes of powders, followed by two-step sintering in a vacuum at 1450 °C and 1650 °C with 6 h and 4 h holding times, consecutively. The bimodal structures induced the formation of Al₂O₃ ceramic with nearly full densification (>99%; ρ 3.95 g/cm³). Both the coarse and fine-grained moieties synergistically balanced the densification kinetics whilst suppressing abnormal grain growth. The uniform and homogeneous grain size minimized the plasma porosity down to <6.0%, limiting the penetration of plasma during the etching process. Full article

(This article belongs to the Special Issue Tribology in Ball Milling: Theory and Applications)

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29 pages, 5483 KiB

Open AccessReview

Advances on Hydrogel Lubrication Modification Under Diverse Design Strategies

by Ying Xu, Youqiang Wang, Kai An, Chenbing Ni, Haiyang Zhang, Yibing Ren and Ziyi Yang

Lubricants 2025, 13(9), 373; https://doi.org/10.3390/lubricants13090373 - 22 Aug 2025

Abstract

Hydrogel is a new type of lubricating material, and its lubrication performance is influenced by factors such as water content, chemical structure, cross-linking density, and friction pair materials. Currently, research on the lubrication performance of hydrogels has not formed a unified standard and [...] Read more.

Hydrogel is a new type of lubricating material, and its lubrication performance is influenced by factors such as water content, chemical structure, cross-linking density, and friction pair materials. Currently, research on the lubrication performance of hydrogels has not formed a unified standard and system. Given that the lubricity of hydrogels mainly depends on their components and structure, as well as the chemical interactions between the polymer chains of the hydrogel network and the friction interface, this review systematically discusses the diverse design strategies of hydrogel lubricants based on the hydration and boundary lubrication mechanisms of gels, while elucidating the underlying enhancement mechanisms, as well as the corresponding tribological behavior and applications for different strategies. Finally, the main challenges and future research directions are underlined, aiming to provide theoretical and technical support for the design optimization and practical application of advanced hydrogel lubrication materials. Full article

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22 pages, 2805 KiB

Open AccessReview

Recent Developments in Self-Lubricating Thin-Film Coatings Deposited by a Sputtering Technique: A Critical Review of Their Synthesis, Properties, and Applications

by Sunil Kumar Tiwari, Turali Narayana, Rashi Tyagi, Gaurav Pant and Piyush Chandra Verma

Lubricants 2025, 13(8), 372; https://doi.org/10.3390/lubricants13080372 - 21 Aug 2025

Abstract

In response to the demand for advanced materials in extreme environments, researchers have developed a variety of bulk and thin-film materials. One of the best-known processes for altering the mechanical and tribological properties of materials is surface engineering techniques. These involve various approaches [...] Read more.

In response to the demand for advanced materials in extreme environments, researchers have developed a variety of bulk and thin-film materials. One of the best-known processes for altering the mechanical and tribological properties of materials is surface engineering techniques. These involve various approaches to synthesize thin-film coatings, along with post-deposition treatments. The need for self-lubricating materials in extreme situations such as high-temperature applications, cryogenic temperatures, and vacuum systems has attracted the attention of researchers. They have fabricated several types of thin films using CVD and PVD techniques to meet this demand. Among the various techniques used for fabricating self-lubricating coatings, sputtering stands out as a special one. It contributes to developing smooth, homogeneous, and crack-free dense microstructures, which further enhance the coatings’ properties. This review explains the need for self-lubricating materials and the different techniques used to synthesize them. It discusses and summarizes the concept of synthesizing various types of self-lubricating films. It shows the different types of self-lubricating material systems, like transition metal-based nitrides and carbides, diamond-like carbon-based materials, and so on. This work also reflects the governing factors like the deposition temperature, doping elements, thickness of the film, deposition pressure, gas flow rate, etc., that influence the deposition results and, consequently, the properties of the film, as well as their advanced applications in different areas. This work reflects the self-lubricating properties of different kinds of films exposed to various environments in terms of their coefficient of friction and wear rate, emphasizing how the friction coefficient affects the wear rate. Full article

(This article belongs to the Special Issue Advanced Surface Treatments and Coatings for Friction and Wear Reduction)

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18 pages, 8907 KiB

Open AccessArticle

Using the Principle of Newton’s Rings to Monitor Oil Film Thickness in CNC Machine Tool Feed Systems

by Shao-Hsien Chen and Li-Yu Haung

Lubricants 2025, 13(8), 371; https://doi.org/10.3390/lubricants13080371 - 21 Aug 2025

Abstract

The lubrication state of the feed system of a CNC machine tool will affect its positioning accuracy, repetition accuracy, and minimum movement amount. Insufficient or excessive lubrication will affect the accuracy. The primary objective of this study is to resolve issues related to [...] Read more.

The lubrication state of the feed system of a CNC machine tool will affect its positioning accuracy, repetition accuracy, and minimum movement amount. Insufficient or excessive lubrication will affect the accuracy. The primary objective of this study is to resolve issues related to the lubrication condition of the feed system, aiming to enhance its operational stability and accuracy. In this study, a measurement system based on images of Newton’s rings was developed. The relationship between the pattern of Newton’s rings and the oil film thickness was established based on the theoretical principle of Newton’s rings. Furthermore, fuzzy logic theory was applied to predict the oil film thickness. In the oil film thickness prediction model based on the radius of Newton’s rings, the average error is 6.5%. When the average feed rate increases by 2 m/min, the oil film thickness value decreases by 43%. Finally, the prediction model is compared with the results of an actual verification experiment. The trends in oil supply timing are consistent between the predicted and experimental results, and the relative error values are less than 10%. Therefore, this study solves the problem of insufficient or excessive oil supply in the feed system guideway, increasing the accuracy of CNC machine tools and contributing to green energy technology. Full article

(This article belongs to the Special Issue Recent Advances in Tribological Properties of Machine Tools)

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20 pages, 6506 KiB

Open AccessReview

Theoretical Modeling and Numerical Simulation of Current-Carrying Friction and Wear: State of the Art and Challenges

by Yijin Sui, Pengfei Xing, Guobin Li, Hongpeng Zhang, Wenzhong Wang and Haibo Zhang

Lubricants 2025, 13(8), 370; https://doi.org/10.3390/lubricants13080370 - 21 Aug 2025

Abstract

Current-carrying friction and wear in contact components are key issues in modern electromechanical systems such as slip rings, electrical connectors, motors, and pantographs, directly influencing their efficiency, reliability, and lifespan. Due to the limitations of experimental methods under some extreme conditions, computational simulations [...] Read more.

Current-carrying friction and wear in contact components are key issues in modern electromechanical systems such as slip rings, electrical connectors, motors, and pantographs, directly influencing their efficiency, reliability, and lifespan. Due to the limitations of experimental methods under some extreme conditions, computational simulations have become essential for studying current-carrying friction and wear in such scenarios. This paper presents a comprehensive review of theoretical modeling and numerical simulation methods for current-carrying friction and wear. It begins with discussions of approaches to solve the electrical contact resistance (ECR), a critical parameter that governs current-carrying friction and wear behaviors. Then, it delves into various modeling strategies for current-carrying friction, with an emphasis on the coupled effects of thermal, mechanical, electrical, and magnetic fields. Finally, the review addresses modeling techniques for current-carrying wear, encompassing mechanical wear and arc erosion. By summarizing existing research, this paper identifies key advancements, highlights existing challenges, and outlines future directions, advocating for the development of efficient, universal, and industry-oriented tools that can seamlessly bridge the gap between theoretical modeling and practical applications. Full article

(This article belongs to the Special Issue Advances in Dry and Lubricated Electrical Contacts)

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19 pages, 3234 KiB

Open AccessArticle

Effects of Additives Adsorbed to Wet Clutch Disks on Their Tribological Performance Found in a Comparative Investigation

by Charlotte Besser, Christian Tomastik, Astrid Lebel, Mirjam Bäse, Johannes Wirkner, Patrick Strobl, Katharina Voelkel and Karsten Stahl

Lubricants 2025, 13(8), 369; https://doi.org/10.3390/lubricants13080369 - 20 Aug 2025

Abstract

Tribolayer build-up was investigated on wet clutch steel disks with experiments in three different test levels of the tribological verification chain via X-ray photoelectron spectroscopy. A new characteristic value of the tribolayers was developed, i.e., the lateral elemental distribution uniformity. This value together [...] Read more.

Tribolayer build-up was investigated on wet clutch steel disks with experiments in three different test levels of the tribological verification chain via X-ray photoelectron spectroscopy. A new characteristic value of the tribolayers was developed, i.e., the lateral elemental distribution uniformity. This value together with the parameters of additive elemental concentration as well as the thickness and the vertical composition of the layers were the basis for the subsequent correlation of the tribolayer properties with the tribological performance of clutch systems from different tests. One important finding was a non-zero threshold concentration of the additive elements calcium and phosphorous below which the clutch performance was unsatisfactory, even with a uniformly covered disk, concluding that the presence of some slightly higher additive element concentration seemed to stabilize the clutch performance. The importance of representative investigation area selection was demonstrated. Full article

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14 pages, 3369 KiB

Open AccessArticle

Influence of Machining Environments on the Burnishing Performance of Aluminum Alloy EN AW-2007

by Irina Beșliu-Băncescu and Laurențiu Slătineanu

Lubricants 2025, 13(8), 368; https://doi.org/10.3390/lubricants13080368 - 19 Aug 2025

Abstract

The presence of a minimum quantity lubrication (MQL) under the conditions of a burnishing process can contribute to an improvement in the process performance by reducing the heights of the resulting surface asperities, by decreasing the temperature values, and by diminishing the size [...] Read more.

The presence of a minimum quantity lubrication (MQL) under the conditions of a burnishing process can contribute to an improvement in the process performance by reducing the heights of the resulting surface asperities, by decreasing the temperature values, and by diminishing the size of the burnishing force components. On the other hand, there are situations in which it is possible to increase the service life of the parts made of EN AW-2007 aluminum alloy by applying a burnishing process. To verify how the results of applying a burnishing process applied to cylindrical specimens in the aluminum alloy when using and not using a minimum quantity lubrication, an experimental research based on a planned variation between certain limits of the values of the peripheral speed and the feed rate has been conceived and materialized. The experimental results were processed mathematically. It has been found that by using the minimum quantity of mineral oil type Valona MS7023 HC, it was possible to reduce the value of the Sa roughness parameter by up to 18%, a decrease in temperature by about 20 °C, and the size of the burnishing force by up to 45%. Full article

(This article belongs to the Special Issue High Performance Machining and Surface Tribology)

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17 pages, 4304 KiB

Open AccessArticle

Environmentally Friendly Phosphating Treatment for Wear-Resistant and Anti-Corrosion Coating on Steel Substrate

by Tengfeng Yan, Ling Li, Lin Zhang, Dan Bai, Guoxin Xie, Bin Wei, Yang Xiao, Chenyang Pan and Guoxing Chen

Lubricants 2025, 13(8), 367; https://doi.org/10.3390/lubricants13080367 - 18 Aug 2025

Abstract

An environmentally friendly phosphating process was proposed, which used the synergistic action of citric acid and sodium citrate to form a uniform and dense phosphating film. Compared to the phosphate coating without sodium citrate, the |Z_0.01 Hz| of the coating with 0.8 [...] Read more.

An environmentally friendly phosphating process was proposed, which used the synergistic action of citric acid and sodium citrate to form a uniform and dense phosphating film. Compared to the phosphate coating without sodium citrate, the |Z_0.01 Hz| of the coating with 0.8 g/L sodium citrate was approximately double. The friction coefficient and wear rate decreased by 29.25% and 94.8%, respectively. The phosphating treatment method reported in this study is expected to become an important way for the anti-corrosion field to environmental protection and economic benefits development. Full article

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26 pages, 7806 KiB

Open AccessArticle

Ultrasonic-Assisted Hot-Press Sintering: A Novel Method to Reduce the Densification Temperature and Enhance the Mechanical and Tribological Properties of Cu–Graphite Composites

by Shijia Zhou, Shuang Zhang, Huimin Xiang, Wei Xu, Kuang Sun, Cheng Fang, Wei Xie, Hailong Wang and Yanchun Zhou

Lubricants 2025, 13(8), 366; https://doi.org/10.3390/lubricants13080366 - 18 Aug 2025

Abstract

Cu–graphite composites are widely used in pantograph sliders and crane brushes. Conventional sintering protocols, however, mandate processing temperatures above 860 °C with prolonged holding periods, which inevitably introduce defects within copper matrices. Drawing inspiration from ultrasonic machining, this study presents an innovative ultrasonic-assisted [...] Read more.

Cu–graphite composites are widely used in pantograph sliders and crane brushes. Conventional sintering protocols, however, mandate processing temperatures above 860 °C with prolonged holding periods, which inevitably introduce defects within copper matrices. Drawing inspiration from ultrasonic machining, this study presents an innovative ultrasonic-assisted hot-press sintering (UAHP) technique. Using this novel method, Cu–graphite composites with graphite contents ranging from 0 to 30 vol.% were fabricated at a reduced temperature of 700 °C and a short holding time of 10 min, achieving a ca. 160 °C reduction in densification temperature. Aside from the high densification efficiency, enhanced mechanical properties were also obtained for UAHP-sintered Cu–graphite composites. The Cu–15 vol.% graphite composite exhibited a yield strength of 232 MPa, showing only a 7.2% decrease compared to pure Cu. The friction coefficient decreased with increasing graphite content, reaching a plateau value of 0.140 at 15 vol.% graphite, representing a 77.16% reduction compared to pure Cu. The wear rate initially decreased but gradually increased with the increase in graphite content. The superior tribological performance is attributed to the insitu-formed graphite lubricating film, while the primary wear mechanisms are deemed to be oxidative and abrasive wear. The ultrasonic vibration provides additional sintering driving force and enhances elemental diffusion, establishing UAHP as an innovative technical route for fabricating composites with an optimized microstructure and superior comprehensive properties. Full article

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19 pages, 944 KiB

Open AccessArticle

A Skid Resistance Predicting Model for Single Carriageways

by Miren Isasa, Ángela Alonso-Solórzano, Itziar Gurrutxaga and Heriberto Pérez-Acebo

Lubricants 2025, 13(8), 365; https://doi.org/10.3390/lubricants13080365 - 16 Aug 2025

Abstract

Skid resistance, or friction, on a road surface is a critical parameter in functional highway assessments, given its direct relationships with safety and accident frequency. Therefore, road administrations must collect friction data across their road networks to ensure safe roads for users. In [...] Read more.

Skid resistance, or friction, on a road surface is a critical parameter in functional highway assessments, given its direct relationships with safety and accident frequency. Therefore, road administrations must collect friction data across their road networks to ensure safe roads for users. In addition, having a predictive model of skid resistance for each road section is essential for an efficient pavement management system (PMS). Traditionally, road authorities disregard rural roads, since they are more focused on freeways and traffic-intense roads. This study develops a model for predicting minimum-available skid resistance, which occurs in summer, measured using the Sideway-force Coefficient Routine Investigation Machine (SCRIM), on bituminous pavements in the single-carriageway road network of the Province of Gipuzkoa, Spain. To this end, traffic volume data available in the PMS of the Provincial Council of Gipuzkoa, such as the annual average daily traffic (AADT) and the AADT of heavy vehicles (AADT.HV), were uniquely used to forecast skid-resistance values collected in summer. Additionally, a methodology for eliminating outliers is proposed. Despite the simplicity of the model, which does not include information about the materials at the surface layer, a coefficient of determination (R²) of 0.439 was achieved. This model can help road authorities identify the roads for which lower skid-resistance values are most likely to occur, allowing them to focus their attention and efforts on these roads, which are key infrastructure in rural areas. Full article

(This article belongs to the Special Issue Tire/Road Interface and Road Surface Textures)

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24 pages, 6917 KiB

Open AccessArticle

Multi-Sensor Fusion and Deep Learning for Predictive Lubricant Health Assessment

by Yongxu Chen, Jie Shen, Fanhao Zhou, Huaqing Li, Kun Yang and Ling Wang

Lubricants 2025, 13(8), 364; https://doi.org/10.3390/lubricants13080364 - 16 Aug 2025

Abstract

Lubricating oil degradation directly impacts friction coefficient, wear rate, and lubrication regime transitions, making precise health quantification essential for predictive tribological maintenance. However, conventional evaluation methods fail to capture subtle tribological changes preceding lubrication failure, often oversimplifying complex multi-parameter relationships critical to friction [...] Read more.

Lubricating oil degradation directly impacts friction coefficient, wear rate, and lubrication regime transitions, making precise health quantification essential for predictive tribological maintenance. However, conventional evaluation methods fail to capture subtle tribological changes preceding lubrication failure, often oversimplifying complex multi-parameter relationships critical to friction and wear performance. To address this challenge, this study proposes Seasonal–Trend decomposition using Loess, a Factor Attention Network, a Temporal Convolutional Network, and an Informer with Long Short-Term Memory Variational Autoencoder (SFTI-LVAE) framework for continuous tribological health assessment of diesel engine lubricants. The approach integrates Seasonal–Trend decomposition using Loess (STL) for trend–seasonal separation, a Factor Attention Network (FAN) for multidimensional feature fusion, and a Temporal Convolutional Network (TCN)-enhanced Informer for capturing long-term tribological dependencies. By combining Long Short-Term Memory (LSTM) temporal modeling with Variational Autoencoder (VAE) reconstruction, the method quantifies lubricant health through reconstruction error, establishing a direct correlation between data deviation and tribological performance degradation. Additionally, permutation importance-based feature evaluation and parameter contribution quantification techniques enable deep mechanistic analysis and fault source tracing of lubricant health degradation. Experimental validation using multi-sensor monitoring data demonstrates that SFTI-LVAE achieves a 96.67% fault detection accuracy with zero false alarms, providing early warning 6.47 h before lubrication failure. Unlike traditional anomaly detection methods that only classify conditions as abnormal or normal, the proposed continuous health index reveals gradual tribological degradation processes, capturing subtle viscosity–temperature relationships and wear particle evolution indicating early lubrication regime transitions. The health index correlates strongly with tribological performance indicators, enabling a transition from reactive maintenance to predictive tribological management, providing an innovative solution for equipment health evaluation in the digital tribology era. Full article

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20 pages, 4979 KiB

Open AccessArticle

Investigation of the Effects of Cutting Tool Coatings and Machining Conditions on Cutting Force, Specific Energy Consumption, Surface Roughness, Cutting Temperature, and Tool Wear in the Milling of Ti6Al4V Alloy

by Barış Özlü, Hasan Basri Ulaş and Fuat Kara

Lubricants 2025, 13(8), 363; https://doi.org/10.3390/lubricants13080363 - 15 Aug 2025

Abstract

The present study aims to investigate the effects of cutting parameters (cutting speed, Vc: 60–90–120 m/min; feed rate, f: 0.055–0.085–0.115 mm/rev), cutting tool coatings (CVD: TiN/TiCN/Al₂O₃ and PVD: TiAlN), and machining conditions (dry, air, and MQL) on cutting force (Fc), [...] Read more.

The present study aims to investigate the effects of cutting parameters (cutting speed, Vc: 60–90–120 m/min; feed rate, f: 0.055–0.085–0.115 mm/rev), cutting tool coatings (CVD: TiN/TiCN/Al₂O₃ and PVD: TiAlN), and machining conditions (dry, air, and MQL) on cutting force (Fc), specific energy consumption (SEC), surface roughness (Ra), cutting temperature (T), and tool wear (Vb) during the milling of Ti6Al4V alloy. As a result, it was observed that all machining tests conducted with the Al₂O₃-coated cutting tool showed improvements of 4.7%, 10.75%, 3.8%, and 6.3% in Fc, SEC, Ra, and T, respectively, compared to the tests performed with the TiAlN-coated cutting tool. Under dry machining conditions, the average Fc, SEC, Ra, and T values were 302.82 N, 4.88 j/mm³, 0.653 µm, and 241.06 °C, respectively. Compared to dry machining conditions, the air and MQL machining conditions demonstrated improvements in the average Fc by 5.15% and 6.3%, SEC by 10.27% and 17.79%, Ra by 6.23% and 11.17%, and T by 8.9% and 19.68%, respectively. The lowest Fc and Ra values for the Al₂O₃-coated cutting tool were measured at 228.33 N and 0.402 µm, respectively, under the MQL machining condition, at a cutting speed of 120 m/min and a feed rate of 0.055 mm/rev. The lowest SEC value (2.694 J/mm³) was also obtained using the Al₂O₃-coated tool under MQL conditions at a cutting speed of 120 m/min and a feed rate of 0.115 mm/rev. Similarly, the lowest cutting temperature (129 °C) was achieved with the Al₂O₃-coated tool under MQL conditions at a cutting speed of 60 m/min and a feed rate of 0.055 mm/rev. The wear performance of the Al₂O₃-coated cutting tool was observed to be superior to that of the TiAlN-coated tool. Full article

(This article belongs to the Special Issue High Performance Machining and Surface Tribology)

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34 pages, 7404 KiB

Open AccessArticle

Degradation Law Analysis and Life Estimation of Transmission Accuracy of RV Reducer Based on Tooth Surface and Bearing Wear

by Chang Liu, Wankai Shi, He Yu and Kun Liu

Lubricants 2025, 13(8), 362; https://doi.org/10.3390/lubricants13080362 - 15 Aug 2025

Abstract

As a core component of industrial robots, the transmission accuracy life (TAL) of rotary vector (RV) reducers constitutes a primary factor determining the high-precision operation of robotic systems. However, current life evaluation methods for RV reducers predominantly rely on conventional bearing strength life [...] Read more.

As a core component of industrial robots, the transmission accuracy life (TAL) of rotary vector (RV) reducers constitutes a primary factor determining the high-precision operation of robotic systems. However, current life evaluation methods for RV reducers predominantly rely on conventional bearing strength life calculations, while neglecting its transmission accuracy degradation during operation. To address this limitation, a static analysis model of RV reducers is established, through which a calculation method for transmission accuracy and TAL is presented. Simultaneously, tooth surface and bearing wear models are developed based on Archard’s wear theory. Through coupled analysis of the aforementioned models, the transmission accuracy degradation law of RV reducers is revealed. The results show that during the operation of the RV reducer, the transmission error (TE) maintains relative stability over time, whereas the lost motion (LM) exhibits a continuous increase. Based on this observation, LM is defined as the evaluation metric for TAL, and a novel TAL estimation model is proposed. The feasibility of the developed TAL estimation model is ultimately validated through accelerated transmission accuracy degradation tests on RV reducers. The error between the predicted and experimental results is 11.06%. The proposed TAL estimation model refines the life evaluation methodology for RV reducers, establishing a solid foundation for real-time transmission accuracy compensation in reducer operation. Full article

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14 pages, 4908 KiB

Open AccessArticle

The Synergistic Anti-Friction and Anti-Wear Mechanisms of Betaine-Functionalized Montmorillonite Nano-Lubricants

by Qiang Wang, Zhengkun Yao, Diange Guo, Shuai-Shuai Li and Xia Zhang

Lubricants 2025, 13(8), 361; https://doi.org/10.3390/lubricants13080361 - 14 Aug 2025

Abstract

To address the challenges of friction and wear in mechanical systems, two functionalized montmorillonite (MMT) nanolubricants were developed through mechanochemistry, namely 3-sulfotetradecyldimethyl betaine-modified MMT (BS-MMT) and coconut amide propyl betaine-modified MMT (CAB-MMT) lubricants. The modification significantly expanded MMT’s interlayer spacing, with CAB-MMT exhibiting [...] Read more.

To address the challenges of friction and wear in mechanical systems, two functionalized montmorillonite (MMT) nanolubricants were developed through mechanochemistry, namely 3-sulfotetradecyldimethyl betaine-modified MMT (BS-MMT) and coconut amide propyl betaine-modified MMT (CAB-MMT) lubricants. The modification significantly expanded MMT’s interlayer spacing, with CAB-MMT exhibiting superior delamination and dispersion stability due to its coconut fatty amide groups. Tribological tests demonstrated that 0.5% CAB-MMT reduced the friction coefficient by 71.4% (to 0.08) and wear scar diameter by 58.8%, while maintaining stable performance under high loads (392 N) and speeds (1450 rpm). The exceptional performance stems from a synergistic mechanism involving the physical adsorption of MMT nanosheets, chemical adhesion via Fe-N/C-N⁺ bonds, and dynamic repair by friction-induced oxides. This work presents an eco-friendly, high-performance water-based nano-lubricant with broad industrial application potential. Full article

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27 pages, 8160 KiB

Open AccessArticle

Real-Time Prediction of Pressure and Film Height Distribution in Plain Bearings Using Physics-Informed Neural Networks (PINNs)

by Ahmed Saleh, Georg Jacobs, Dhawal Katre, Benjamin Lehmann and Mattheüs Lucassen

Lubricants 2025, 13(8), 360; https://doi.org/10.3390/lubricants13080360 - 14 Aug 2025

Abstract

The increasing application of plain bearings in various industries, especially under challenging conditions like thin lubricating films and high temperatures, necessitates effective monitoring to prevent failures and ensure reliable performance. While sensor-based monitoring incurs significant costs and complex installation due to physical sensors [...] Read more.

The increasing application of plain bearings in various industries, especially under challenging conditions like thin lubricating films and high temperatures, necessitates effective monitoring to prevent failures and ensure reliable performance. While sensor-based monitoring incurs significant costs and complex installation due to physical sensors and data acquisition systems, model-based tracking offers a more cost-effective alternative. Model-based monitoring relies on mathematical or physics-based models to estimate system behaviour, reducing the need for extensive sensor data. However, reliable results depend on real-time capable and precise simulation models. Conventional real-time modelling techniques, including analytical calculations, empirical formulas, and data-driven methods, exhibit significant limitations in real-world applications. Analytical methods often have a restricted range of applicability and do not match the accuracy of numerical methods. Meanwhile, data-driven approaches rely heavily on the quality and quantity of training data and are inherently constrained to their training domain. Recently, Physics-Informed Neural Networks (PINNs) have emerged as a promising solution for model-based monitoring to capture complex system behaviour. This approach combines physical modelling with data-driven learning, allowing for better generalisation beyond the training domain while reducing reliance on extensive data. Thus, this study presents an approach for load monitoring in radial plain bearings using PINNs. It extends the application of PINNs by relying solely on simple sensor inputs, such as radial load and rotational speed, to predict the hydrodynamic pressure and oil film thickness distribution under varying stationary conditions. The real-time model is trained, validated, and evaluated within and beyond the training domain using elastohydrodynamic simulation results. The developed real-time model enables load monitoring in plain bearings by identifying critical hydrodynamic pressure and oil film thickness values using readily available speed and load sensor data under varying stationary conditions. Full article

(This article belongs to the Special Issue New Horizons in Machine Learning Applications for Tribology)

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14 pages, 7694 KiB

Open AccessArticle

Effect of Laser Scanning Speed on Microstructure and Wear Resistance of TiC-TiB2-Reinforced 316L Laser-Clad Coatings

by Dongdong Zhang, Jingyu Jiang, Yu Liu, Haozhe Li and Zhanhui Zhang

Lubricants 2025, 13(8), 359; https://doi.org/10.3390/lubricants13080359 - 13 Aug 2025

Abstract

To enhance the wear resistance of laser-clad coatings, this study investigates the underlying modulation mechanisms of scanning speed on the microstructure and properties of TiC-TiB₂-reinforced 316L stainless steel composite coatings. TiC/TiB₂ particle-reinforced 316L stainless steel composite coatings were fabricated on [...] Read more.

To enhance the wear resistance of laser-clad coatings, this study investigates the underlying modulation mechanisms of scanning speed on the microstructure and properties of TiC-TiB₂-reinforced 316L stainless steel composite coatings. TiC/TiB₂ particle-reinforced 316L stainless steel composite coatings were fabricated on 45# steel substrates via laser cladding. Our analysis reveals that scanning speed critically governs the thermal cycle of the melt pool, thereby modulating the coating’s microstructure and properties: Lower scanning speeds prolong melt pool duration, consequently intensifying ceramic particle dissolution, coarsening, and tendencies toward agglomeration and settling. Conversely, higher scanning speeds promote rapid solidification, which both preserves ceramic particles and refines the matrix grains. With increasing scanning speed, accelerated melt pool cooling rates drive a microstructural transition from coarse dendrites to refined equiaxed grains, accompanied by dramatically enhanced uniformity in ceramic particle distribution. Coatings deposited at higher scanning speeds exhibit a 22% increase in hardness compared to those at lower speeds. Wear resistance evolution parallels this hardness trend: at 480 mm/min scanning speed, wear reduction can be expected, with the wear volume decreasing by 58.60% and the friction coefficient reducing by 42.1% relative to 120 mm/min. Full article

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23 pages, 2358 KiB

Open AccessArticle

Synergistic Enhancement of Tribological Behavior and Colloidal Stability in CuO Nanolubricants via Ligand Tuning

by Sherif Elsoudy, Sayed Akl, Ahmed A. Abdel-Rehim, Esme Lane, Abas Hadawey and Philip D. Howes

Lubricants 2025, 13(8), 358; https://doi.org/10.3390/lubricants13080358 - 12 Aug 2025

Abstract

Nanoparticle-based lubricants, or nanolubricants, can exhibit superior tribological properties compared to unmodified base oils. However, these performance gains are highly dependent on the nanoparticle surface chemistry, particularly in maintaining stable colloidal dispersions. This study explores the influence of oleic acid (OA) and oleylamine [...] Read more.

Nanoparticle-based lubricants, or nanolubricants, can exhibit superior tribological properties compared to unmodified base oils. However, these performance gains are highly dependent on the nanoparticle surface chemistry, particularly in maintaining stable colloidal dispersions. This study explores the influence of oleic acid (OA) and oleylamine (OAm) functionalization on the tribological and colloidal properties of CuO nanoparticles dispersed in an SAE 20W50 base oil. We present a hybrid optimization framework combining Response Surface Methodology (RSM) with Bayesian Optimization (BO) to identify the optimal OA to OAm ratio (OA–OAm) for CuO nanolubricants. Unlike prior studies that employed either RSM alone or trial-and-error approaches, this integrated method enables precise tuning of ligand ratios, achieving balanced tribological performance and colloidal stability. Characterization techniques, including UV–vis spectroscopy, FTIR, Raman spectroscopy, and TGA, were employed to investigate dispersion stability. Results demonstrate that OA/OAm-functionalized CuO nanoparticles exhibit improved dispersion stability and reduced sedimentation compared to non-functionalized counterparts. Tribological evaluations using the four-ball test revealed that the ligand-tuned CuO nanolubricants maintained their tribological enhancements under a variety of additive loadings and ligand combinations, with an improvement ranging from 44.9% to 60.6% in the coefficient of friction (COF) and from 29.2% to 63.9% in the specific wear rate (SWR). For the colloidal stability, OA/OAm-functionalized CuO nanoparticles exhibited a 75% reduction in sedimentation rate (k = 0.003 day⁻¹) compared to unfunctionalized CuO (k = 0.012 day⁻¹). Finally, the high thermal stability of the functionalized nanoparticles ensures their suitability for high-performance applications. Overall, this work represents a crucial step towards commercial applications of CuO-enhanced lubricants. Full article

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