Lubricants

27 pages, 9725 KB

Open AccessArticle

Room Temperature Production of Polyurea-Based Lubricants: Using L-Serine Derivatives, 1,5 Pentamethylene Diisocyanate, and a Planetary Ball Mill

by Lara Frentrup, Tim Stuck and Ralf Weberskirch

Lubricants 2025, 13(12), 554; https://doi.org/10.3390/lubricants13120554 - 18 Dec 2025

Cited by 1 | Viewed by 492

Abstract

In this work, we produced a new polyurea (PU)-based thickener based on serine derivatives (ethanolamine or L-Serine ethyl ester) and 1,5 pentamethylene diisocyanate (PDI), using castor oil as base oil and methylene diphenyl diisocyanate (MDI) as a reference. Polymerization was carried out in [...] Read more.

In this work, we produced a new polyurea (PU)-based thickener based on serine derivatives (ethanolamine or L-Serine ethyl ester) and 1,5 pentamethylene diisocyanate (PDI), using castor oil as base oil and methylene diphenyl diisocyanate (MDI) as a reference. Polymerization was carried out in a planetary ball mill at room temperature for 75 min. The polymerization degree of the PU thickener was examined via ¹H NMR, which ranged between 1.8 and 14.6 repeating units after the extraction of the base oil. Rheological analysis showed gel formation for ten out of twelve samples, which was strongly dependent on the polymerization degree and thickener amount. The decomposition temperature of the MDI-based PU greases was consistently roughly 20 °C higher than that of PDI-based systems. The lubricants were further evaluated through rheology experiments before and after the gels underwent an annealing process at 100 °C for 1 h (amplitude and frequency test), indicating a strong increase in the storage modulus G’, whereas the yield point γ_F remained constant or decreased. Full article

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19 pages, 3343 KB

Open AccessArticle

Influence of Alumina Abrasive Particles on Wear Behavior of Textured Surfaces Under Heavy-Load Conditions

by Dongyun Wang, Wenyao Zhang, Hongkang Dong, Xiaofeng Wei, Wei Hao and Xin Yao

Lubricants 2025, 13(12), 553; https://doi.org/10.3390/lubricants13120553 - 18 Dec 2025

Viewed by 462

Abstract

This study investigates the lubrication properties of GCr15 steel textured surfaces under the conditions of low speed, heavy load, and boundary lubrication, with varying concentrations of Al₂O₃ particles. Through pin-on-disk tests in 46# hydraulic fluid, it was found that the [...] Read more.

This study investigates the lubrication properties of GCr15 steel textured surfaces under the conditions of low speed, heavy load, and boundary lubrication, with varying concentrations of Al₂O₃ particles. Through pin-on-disk tests in 46# hydraulic fluid, it was found that the texture density had little effect on the friction in the absence of abrasive particles and that the friction increases with an increasing texture density in the presence of abrasive particles. Abrasive particle concentration significantly increases the friction on smooth surfaces, while textured surfaces can retain abrasive particles and lubricants, mitigating the increase in friction. The impact of abrasive particles can wear down the texture edges and weaken its friction-reducing effect. This study reveals the interaction between abrasive particle concentration and texture density, providing a theoretical basis for designing textured surfaces suitable for abrasive-containing lubrication environments. Full article

(This article belongs to the Topic Engineered Surfaces and Tribological Performance)

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15 pages, 1910 KB

Open AccessArticle

Experimental Analysis of Catheter Push–Pull Forces: Effects of Vascular Curvature, Motion Speed, and Lumen Material

by Jiafeng Hu, Xiaojun Chen, Xianfeng Jiang, Zhaoxian Zheng, Yongkang Fang and Chengxiong Lin

Lubricants 2025, 13(12), 552; https://doi.org/10.3390/lubricants13120552 - 17 Dec 2025

Viewed by 482

Abstract

During catheter interventional procedures, the catheter inevitably encounters the vessel wall. When the push–pull force at the catheter–vessel wall interface exceeds a certain threshold, it may cause vascular damage. The mechanical feedback at the catheter–blood interface are still areas that urgently need to [...] Read more.

During catheter interventional procedures, the catheter inevitably encounters the vessel wall. When the push–pull force at the catheter–vessel wall interface exceeds a certain threshold, it may cause vascular damage. The mechanical feedback at the catheter–blood interface are still areas that urgently need to be addressed. This study provides essential experimental data and mechanical feedback to inform and validate such simulations. Therefore, this article first analyzed the movement form and mechanical state of the catheter and blood vessel during vascular interventional surgery. Based on this, this paper analyzes the movement forms and mechanical states of the catheter and blood vessels during intervention, focusing on the contact force between the catheter and blood vessels. The results of the three-point bending test indicate that the bending deformation force of the tube increases as the radius decreases, and the overall deformation progresses from elastic deformation to the yield limit. The normal force of the tube on the lumen and the average push–pull force at the end of the tube are all positively correlated with the moving speed and bending degree of the tube. Statistical analysis revealed that the degree of lumen curvature had a significantly greater influence on the push–pull force than catheter motion speed. The above research provides guidance for friction at the medical device–vessel interface and coating design. Full article

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13 pages, 1090 KB

Open AccessArticle

Performance Prediction of Diester-Based Lubricants Using Quantitative Structure–Property Relationship and Artificial Neural Network Approaches

by Hanlu Wang, Yongkang Tang, Hui Wang, Pihui Pi, Yuxiu Zhou and Xingye Zeng

Lubricants 2025, 13(12), 551; https://doi.org/10.3390/lubricants13120551 - 17 Dec 2025

Viewed by 459

Abstract

Ester-based lubricants have been widely used owing to their excellent overall performance. In this study, the quantitative structure–property relationship (QSPR) approach was combined with molecular descriptors, a genetic algorithm (GA), and an artificial neural network (ANN) to systematically predict the key properties—kinematic viscosity [...] Read more.

Ester-based lubricants have been widely used owing to their excellent overall performance. In this study, the quantitative structure–property relationship (QSPR) approach was combined with molecular descriptors, a genetic algorithm (GA), and an artificial neural network (ANN) to systematically predict the key properties—kinematic viscosity at 40 °C and 100 °C, viscosity index, pour point, and flash point—of 64 diester-based lubricants. Quantum chemical calculations were first performed to obtain the equilibrium geometries and electronic information of the molecules. Geometry optimizations and frequency analyses were carried out using the Gaussian 16 software at the B3LYP/6-31G (d, p) level, providing a reliable foundation for molecular descriptor computation. Subsequently, topological, geometrical, and electronic descriptors were calculated using the RDKit toolkit, and the optimal feature subsets were selected by GA and used as ANN inputs for property prediction. The results showed that the ANN models exhibited good performance in predicting viscosity and flash point, with R² values of 0.9455 and 0.8835, respectively, indicating that the ANN effectively captured the nonlinear relationships between molecular structure and physicochemical properties. In contrast, the prediction accuracy for pour point was relatively lower (R² = 0.6155), suggesting that it is influenced by complex molecular packing and crystallization behaviors at low temperatures. Overall, the study demonstrates the feasibility of integrating quantum chemical calculations with the QSPR–ANN framework for lubricant property prediction, providing a theoretical basis and data-driven tool for molecular design and performance optimization of ester-based lubricants. Full article

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23 pages, 8233 KB

Open AccessArticle

Enhancement of Wear Behaviour and Optimization and Prediction of Friction Coefficient of Nitrided D2 Steel at Different Times

by Abdallah Souid, Slah Mzali, Borhen Louhichi and Mohamed Ali Terres

Lubricants 2025, 13(12), 550; https://doi.org/10.3390/lubricants13120550 - 17 Dec 2025

Viewed by 533

Abstract

The objective of this study is to evaluate the impact of thermal and thermochemical treatment, specifically gas nitriding, on the wear properties of AISI D2 cold work tool steel. The steel was austenitized at 1050 °C, then subjected to two annealing cycles at [...] Read more.

The objective of this study is to evaluate the impact of thermal and thermochemical treatment, specifically gas nitriding, on the wear properties of AISI D2 cold work tool steel. The steel was austenitized at 1050 °C, then subjected to two annealing cycles at 560 °C for two hours each. It was then gas-nitrided for 16 and 36 h. The Vickers microhardness measurements of AISI D2 steel for the three distinct conditions, non-nitrided (NN), nitride at 16 h (N16) and nitride at 36 h (N36), are 560 HV0.1, 1050 HV0.1 and 1350 HV0.1, respectively. Wear tests were conducted utilizing a ball device, under dry friction conditions at ambient temperature, with loads of 5, 10 and 15 N, over 5000, 10,000 and 15,000 cycles at a constant sliding velocity of 30 mm/s and a sliding distance of 10 mm. Furthermore, the utilization of ANFIS modeling of experimental data facilitated the prediction of the variation in the coefficient of friction as a function of nitriding conditions and specific test parameters. The results show a significant effect of nitriding, leading to a marked reduction in the coefficient of friction. In the non-nitrided condition, the average value reaches 0.80, while extended nitriding to 36 h reduces this value to around 0.49, confirming a substantial tribological improvement. This enhancement is ascribed to the formation of hard, resilient nitride layers on the steel surface, thereby increasing wear resistance and cur-tailing in industrial applications. Full article

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18 pages, 3393 KB

Open AccessArticle

Effect of Laser Power on the Microstructure and Wear and Corrosion Resistance of Ni25 Alloy Coatings

by Jingquan Wu, Jianwen Zhang, Bohao Chen, Gui Wang, Jiang Huang, Wenqing Shi, Fenju An and Xianglin Wu

Lubricants 2025, 13(12), 549; https://doi.org/10.3390/lubricants13120549 - 16 Dec 2025

Viewed by 409

Abstract

This study systematically investigates the influence of laser power (1000 W, 1400 W, 1800 W) on the microstructure and properties of Ni25 alloy coatings prepared by laser cladding to optimize process parameters for enhanced comprehensive performance. Through the analysis of multi-dimensional characterization, it [...] Read more.

This study systematically investigates the influence of laser power (1000 W, 1400 W, 1800 W) on the microstructure and properties of Ni25 alloy coatings prepared by laser cladding to optimize process parameters for enhanced comprehensive performance. Through the analysis of multi-dimensional characterization, it is found that the laser power significantly changes the thermal cycle, thus determining the evolution of microstructure. At 1000 W, a fine dendritic structure with dispersed hard phases (BNi₃, BFe₃Ni₃, CrB₂, Cr₇C₃) yielded the highest hardness (442.52 HV) but poor wear (volume loss: 0.3346 mm³) and corrosion resistance (Icorr: 2.75 × 10⁻⁴ A·cm⁻²) due to microstructural inhomogeneity. The 1400 W coating, featuring a uniform γ-Ni dendrite/eutectic network and increased B solid solubility, achieved an optimal balance with the lowest wear rate (0.0685 mm³), superior corrosion resistance (Icorr: 2.34 × 10⁻⁵; A·cm⁻²), and a stable friction coefficient (0.816), despite lower hardness (342.00 HV). At 1800 W, grain coarseness and Cr₇C₃ decomposition led to blocky hard phases, recovering hardness (415.36 HV) and reducing the friction coefficient (0.757), but resulting in intermediate wear and corrosion resistance. This study demonstrates that the uniformity and continuity of the microstructure are the key determinants governing the comprehensive service properties of the laser cladding layer, with their importance outweighing a single hardness index. 1400 W is identified as the optimal laser power, providing critical insights for fabricating high-performance Ni25 coatings in demanding service environments. Full article

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23 pages, 3564 KB

Open AccessArticle

Machine Tool Spindle Temperature Field Parametric Modeling and Thermal Error Compensation

by Geng Chen, Lin Yuan, Hui Chen, Chengliang Dou, Guangyong Ma, Shuai Li and Lai Hu

Lubricants 2025, 13(12), 548; https://doi.org/10.3390/lubricants13120548 - 16 Dec 2025

Viewed by 508

Abstract

The development of modern machining and manufacturing industry puts forward higher requirements for the machining accuracy of machine tools. The thermal error of the machine tool spindle directly affects the accuracy of the machined workpiece. To improve the accuracy of thermal error prediction, [...] Read more.

The development of modern machining and manufacturing industry puts forward higher requirements for the machining accuracy of machine tools. The thermal error of the machine tool spindle directly affects the accuracy of the machined workpiece. To improve the accuracy of thermal error prediction, this paper conducts temperature field analysis for the thermal error of the machine tool spindle and employs the Whale Optimization Algorithm (WOA) to optimize the temperature field parameters, aiming to establish a spindle temperature field model. This approach avoids the problem that traditional measurement methods cannot obtain the temperature of key rotational positions of the spindle and provides a new method for the selection of temperature-sensitive points in the thermal error measurement process. Initially, a spindle Product of Exponentials (POE) error model is constructed to map the five errors of the spindle to three-dimensional vectors in the machine tool space. Subsequently, the Whale Optimization Algorithm (WOA) is used to optimize the physical parameters of the spindle, and the optimal spindle temperature field model is determined. The calculated spindle thermal error data and temperature field model data are input into the OLGWO-SHO-CNN model for training. Finally, a case study is carried out on a machining center, and the trained model is used to perform compensation verification under constant and variable speed conditions, respectively. The experimental results show that under the constant speed condition, the compensation rates of the X-axis, Y-axis, and Z-axis are 77.2%, 73.1%, and 88.7%, respectively; under the variable speed condition, the compensation rates of the X-axis, Y-axis, and Z-axis are 74.7%, 78.2%, and 88.0%, respectively. The compensation results indicate that the established spindle temperature field model and the OLGWO-SHO-CNN model have good robustness and accuracy. Full article

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

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14 pages, 1443 KB

Open AccessArticle

The Coupling Influence of Load and Temperature on Boundary Friction of Fullerene Ball Nano-Additives

by Yu Rong, Xinran Geng, Chongyun Sun, Hailong Hu, Shuo Li, Zhichao Chen and Wenquan Lv

Lubricants 2025, 13(12), 547; https://doi.org/10.3390/lubricants13120547 - 16 Dec 2025

Viewed by 447

Abstract

This study employs molecular dynamics simulations to investigate the frictional behavior of fullerene nano-additives on Fe-C alloy surfaces under varying loads and temperatures, focusing on boundary lubrication conditions. The results show that the x-direction friction force exhibits minimal sensitivity to normal pressure [...] Read more.

This study employs molecular dynamics simulations to investigate the frictional behavior of fullerene nano-additives on Fe-C alloy surfaces under varying loads and temperatures, focusing on boundary lubrication conditions. The results show that the x-direction friction force exhibits minimal sensitivity to normal pressure due to the high rigidity of fullerene molecules, which limits variations in real contact area and atomic interactions. In contrast, temperature has a significant effect: as it rises, enhanced atomic vibrations and thermal activation lower energy barriers for sliding. The coefficient of friction (COF) consistently decreases with both increasing load and temperature, driven by the mechanism of thermally activated motion. Although partial rotational motion from sliding to rolling friction was not explicitly observed in the simulations, the study remains within the sliding-dominated regime, highlighting the importance of temperature over load in controlling friction. A linear relationship between

\ln C O F

and

1 / k_{B} T

yields an average activation energy of ~0.03 eV, supporting a thermally activated friction mechanism. By introducing a composite parameter that combines load and temperature effects, the study provides a predictive framework for modeling friction behavior under thermo-mechanical coupling. These findings enhance the understanding of the friction-reducing capabilities of fullerene additives and offer a foundation for designing advanced nano-lubricants in boundary lubrication systems. Full article

(This article belongs to the Special Issue Tribological Behavior of Nanolubricants: Do We Know Enough?)

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17 pages, 8805 KB

Open AccessArticle

Effect of Electron Beam Irradiation on Friction and Wear Properties of Carbon Fiber-Reinforced PEEK at Different Injection Temperatures

by Yi Chen, Jiahong Li, Da Bian and Yongwu Zhao

Lubricants 2025, 13(12), 546; https://doi.org/10.3390/lubricants13120546 - 16 Dec 2025

Viewed by 540

Abstract

Polyetheretherketone (PEEK) is a high-performance engineering plastic widely used in aerospace, automotive, and other industries due to its heat resistance and mechanical strength. However, its high friction coefficient and low thermal conductivity limit its use in heavy-load environments. Existing studies have extensively explored [...] Read more.

Polyetheretherketone (PEEK) is a high-performance engineering plastic widely used in aerospace, automotive, and other industries due to its heat resistance and mechanical strength. However, its high friction coefficient and low thermal conductivity limit its use in heavy-load environments. Existing studies have extensively explored the individual effects of thermal processing or irradiation on PEEK. However, the synergistic mechanism between the initial microstructure formed by mold temperature and subsequent irradiation modification remains unclear. This paper investigates the coupled effects of injection molding temperature and electron beam irradiation on the tribology of carbon fiber-reinforced PEEK composites, with the aim of identifying process conditions that improve friction and wear performance under high load by controlling the crystal morphology and cross-linking network. Carbon fiber (CF) particles were mixed with PEEK particles at a 1:2 mass ratio, and specimens were prepared at injection molding temperatures of 150 °C, 175 °C, and 200 °C. Some specimens were irradiated with an electron beam dose of 200 kGy. The friction coefficient, wear rate, surface shape, and crystallinity of the material were obtained using friction and wear tests, white-light topography, SEM, and XRD. The results show that the injection molding temperature of the material influences the friction performance. Optimal performance is obtained at 175 °C with a friction coefficient of 0.12 and wear rate of 9.722 × 10⁻⁶ mm³/(N·m). After irradiation modification, the friction coefficient decreases to 0.10. This improvement is due to the moderate melt fluidity, adequate fiber infiltration, and dense crystallization at this temperature. In addition, cross-linking of chains occurs, and surface transfer films are created at this temperature. However, irradiation leads to a slight increase in wear rate to 1.013 × 10⁻⁵ mm³/(N·m), suggesting that chain segment fracture and embrittlement effects are enhanced at this dose. At 150 °C, there is weak interfacial bonding and microcrack development. At 200 °C, excessive thermal motion reduces crystallinity and adds residual stress, increasing wear sensitivity. Overall, while irradiation reduces the friction coefficient, the wear rate is affected by the initial microstructure at molding. At non-optimal temperatures, embrittlement tends to dominate the wear mode. This study uncovers the synergistic and competitive dynamics between the injection molding process and irradiation modification, offering an operational framework and a mechanistic foundation for applying CF/PEEK under heavy-load conditions. The present approach can be extended in future work to other reinforcement systems or variable-dose irradiation schemes to further optimize overall tribological performance. Full article

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33 pages, 5511 KB

Open AccessArticle

Physics-Informed Transfer Learning for Predicting Engine Oil Degradation and RUL Across Heterogeneous Heavy-Duty Equipment Fleets

by Mohamed G. A. Nassef, Omar Wael, Youssef H. Elkady, Habiba Elshazly, Jahy Ossama, Sherwet Amin, Dina ElGayar, Florian Pape and Islam Ali

Lubricants 2025, 13(12), 545; https://doi.org/10.3390/lubricants13120545 - 16 Dec 2025

Viewed by 818

Abstract

Predicting the Remaining Useful Life (RUL) of engine oil is critical for proactive maintenance and fleet reliability. However, irregular and noisy single-point sampling presents challenges for conventional prognostic models. To address this, a hierarchical physics-informed transfer learning (TL) framework is proposed that reconstructs [...] Read more.

Predicting the Remaining Useful Life (RUL) of engine oil is critical for proactive maintenance and fleet reliability. However, irregular and noisy single-point sampling presents challenges for conventional prognostic models. To address this, a hierarchical physics-informed transfer learning (TL) framework is proposed that reconstructs nonlinear degradation trajectories directly from non-time-series data. The method uniquely integrates Arrhenius-type oxidation kinetics and thermochemical laws within a multi-level TL architecture, coupling fleet-level generalization with engine-specific adaptation. Unlike conventional approaches, this framework embeds physical priors directly into the transfer process, ensuring thermodynamically consistent predictions across different equipment. An integrated uncertainty quantification module provides calibrated confidence intervals for RUL estimation. Validation was conducted on 1760 oil samples from dump trucks, dozers, shovels, and wheel loaders operating under real mining conditions. The framework achieved an average R² of 0.979 and RMSE of 10.185. This represents a 69% reduction in prediction error and a 75% narrowing of confidence intervals for RUL estimates compared to baseline models. TL outperformed the asset-specific model, reducing RMSE by up to 3 times across all equipment. Overall, this work introduces a new direction for physics-informed transfer learning, enabling accurate and uncertainty-aware RUL prediction from uncontrolled industrial data and bridging the gap between idealized degradation studies and real-world maintenance practices. Full article

(This article belongs to the Special Issue Intelligent Algorithms for Triboinformatics)

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21 pages, 2720 KB

Open AccessStudy Protocol

Performance Study of Thermal Expansion in Magnetic Fluid Seals for Large Centrifuges

by Wenjiang Li, Weibing Zhu, Xiao Liang, Heshun Wang and Zhaoqiang Yan

Lubricants 2025, 13(12), 544; https://doi.org/10.3390/lubricants13120544 - 13 Dec 2025

Viewed by 449

Abstract

During the operation of the magnetic fluid sealing device, a large amount of heat is generated due to the viscous friction of the magnetic fluid, causing the shaft to deform and thus affecting the sealing effect. This paper explores the thermal expansion effect [...] Read more.

During the operation of the magnetic fluid sealing device, a large amount of heat is generated due to the viscous friction of the magnetic fluid, causing the shaft to deform and thus affecting the sealing effect. This paper explores the thermal expansion effect of the magnetohydrodynamic sealing device under the working conditions of an axle diameter of 1030 mm and a maximum rotational speed of 700 r/min. The temperature distribution law under the action of a magnetic field, the influence of thermal deformation caused by temperature on the sealing performance, and the influence of the selection of shaft and pole shoe materials on the magnetic fluid sealing device were studied. Research findings show that in magnetic fluid sealing, an increase in system temperature can enhance the sealing effect, but it will cause thermal expansion of the rotating shaft and change the gap. By adopting a combination of different materials for the rotating shaft and the pole shoe, the sealing performance can be optimized and improved. Full article

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18 pages, 3691 KB

Open AccessArticle

Effect of Scanning Speed on the Microstructure and Properties of Co-Cu-Ti Coatings by Laser Cladding

by Binglin Zhang, Yang Zhang, Hao Zhang, Guangliang Hu and Haicheng Yu

Lubricants 2025, 13(12), 543; https://doi.org/10.3390/lubricants13120543 - 13 Dec 2025

Viewed by 364

Abstract

Co-Cu-Ti composite coatings were fabricated on Ti6Al4V substrates by laser cladding. The Criteria Importance Through Intercriteria Correlation–Technique for Order Preference by Similarity to Ideal Solution methodology was employed to determine the optimal parameters. The effect of varying the scanning speed, a critical parameter, [...] Read more.

Co-Cu-Ti composite coatings were fabricated on Ti6Al4V substrates by laser cladding. The Criteria Importance Through Intercriteria Correlation–Technique for Order Preference by Similarity to Ideal Solution methodology was employed to determine the optimal parameters. The effect of varying the scanning speed, a critical parameter, was investigated to evaluate its influence on the coating’s microstructure and performance. The phase composition of the coatings comprises Co-Ti phases and Cu-Ti phases. The microhardness and wear resistance of the coatings initially increase as the scanning speed rises, reaching a peak before subsequently declining. The predominant wear mechanisms of the coatings are abrasive wear, with minor contributions from adhesive wear and fatigue wear. The wear resistance of the coating is superior to that of the TC4 substrate, primarily due to the synergistic enhancement from the strengthening effect of the Co-Ti phase and the lubricating effect of the Cu-Ti phase. The composite coatings fabricated at a scanning speed of 3 mm/s exhibited superior properties. Specifically, the microhardness measures 788 HV_0.2, the coefficient of friction is approximately 0.57, and the wear cross-sectional area is 3.57 × 10⁻⁹ mm². At this speed, these two effects achieve an optimal balance, making it the best process parameter for wear resistance. Full article

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17 pages, 739 KB

Open AccessReview

The Influence of Laser Alloying with Boron on the Condition and Properties of the Surface Layer of Selected Iron Alloys

by Marta Paczkowska

Lubricants 2025, 13(12), 542; https://doi.org/10.3390/lubricants13120542 - 12 Dec 2025

Viewed by 388

Abstract

This article presents the effect of laser alloying with boron on the surface layer of iron alloys: steel and grey cast iron. The general goal of this review is to specify the main differences that can be expected after this treatment of selected [...] Read more.

This article presents the effect of laser alloying with boron on the surface layer of iron alloys: steel and grey cast iron. The general goal of this review is to specify the main differences that can be expected after this treatment of selected iron-based alloys. Boron as an alloying element is first characterized. The effects of laser alloying are described in comparison to diffusion processing. The next section describes the effect of laser alloying with boron on the microstructure, hardness, and wear resistance of the surface layer of selected iron alloys. As a result of the conducted analysis, the most significant differences in the outcomes of laser alloying with boron, which may occur during the processing of various iron alloys, are as follows: the presence of graphite in the surface layer in the case of grey cast iron treatment and a clearly visible transition zone between the alloyed zone and the hardened zone during the treatment of grey cast iron as opposed to steel; variable depths of the modified surface layer and varied grain size in the alloy zone depending on the thermophysical properties of the material being treated. Full article

(This article belongs to the Special Issue Mechanical Tribology and Surface Technology, 2nd Edition)

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21 pages, 6204 KB

Open AccessArticle

Numerical Simulation of Temperature Field, Velocity Field and Solidification Microstructure Evolution of Laser Cladding AlCoCrFeNi High Entropy Alloy Coatings

by Andi Huang, Yilong Liu, Xin Li, Jingang Liu and Shiping Yang

Lubricants 2025, 13(12), 541; https://doi.org/10.3390/lubricants13120541 - 12 Dec 2025

Viewed by 714

Abstract

In this study, a multiphysics coupling numerical model was developed to investigate the thermal-fluid dynamics and microstructure evolution during the laser metal deposition of AlCoCrFeNi high-entropy alloy (HEA) coatings on 430 stainless steel substrates. The model integrated laser-powder interactions, temperature-dependent material properties, and [...] Read more.

In this study, a multiphysics coupling numerical model was developed to investigate the thermal-fluid dynamics and microstructure evolution during the laser metal deposition of AlCoCrFeNi high-entropy alloy (HEA) coatings on 430 stainless steel substrates. The model integrated laser-powder interactions, temperature-dependent material properties, and the coupled effects of buoyancy and Marangoni convection on melt pool dynamics. The simulation results were compared with experimental data to validate the model’s effectiveness. The simulations revealed a strong bidirectional coupling between temperature and flow fields in the molten pool: the temperature distribution governed surface tension gradients that drove Marangoni convection patterns, while the resulting fluid motion dominated heat redistribution and pool morphology. Initially, the Peclet number (Pe_T) remained below 5, indicating conduction-controlled heat transfer with a hemispherical melt pool. As the process progressed, Pe_T exceeded 50 at maximum flow velocities of 2.31 mm/s, transitioning the pool from a circular to an elliptical geometry with peak temperatures reaching 2850 K, where Marangoni convection became the primary heat transfer mechanism. Solidification parameter distributions (G and R) were computed and quantitatively correlated with scanning electron microscopy (SEM)-observed microstructures to elucidate the columnar-to-equiaxed transition (CET). X-ray diffraction (XRD) analysis identified body-centered cubic (BCC), face-centered cubic (FCC), and ordered B2 phases within the coating. The resulting hierarchical microstructure, transitioning from fine equiaxed surface grains to coarse columnar interfacial grains, synergistically enhanced surface properties and established robust metallurgical bonding with the substrate. Full article

(This article belongs to the Special Issue Mechanical Tribology and Surface Technology, 2nd Edition)

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19 pages, 4245 KB

Open AccessArticle

Study on the Cooling and Lubrication Mechanism and Properties of Soybean Oil-Based MQL and Different Cryogenic Media in Titanium Alloy Processing

by Zhiyong He, Dongzhou Jia, Qi Gao, Xiaoqiang Wu, Lan Wu and Yongqiang Fu

Lubricants 2025, 13(12), 540; https://doi.org/10.3390/lubricants13120540 - 11 Dec 2025

Cited by 1 | Viewed by 500

Abstract

The machining of Ti-6Al-4V thin-walled parts is characterized by high cutting temperatures, significant force fluctuations, and complex thermomechanical coupling. Cryogenic Minimum Quantity Lubrication Technology (CMQL) uses bio-lubricant as the lubrication carrier, combined with the cooling characteristics of cryogenic temperature medium, showing good cooling [...] Read more.

The machining of Ti-6Al-4V thin-walled parts is characterized by high cutting temperatures, significant force fluctuations, and complex thermomechanical coupling. Cryogenic Minimum Quantity Lubrication Technology (CMQL) uses bio-lubricant as the lubrication carrier, combined with the cooling characteristics of cryogenic temperature medium, showing good cooling and lubrication performance and environmental friendliness. However, the cooling and lubrication mechanism of different cryogenic media in synergy with bio-lubricants is still unclear. This paper establishes convective heat transfer coefficient and penetration models for cryogenic media in the cutting zone, based on the jet core theory and the continuum medium assumption. The model results show that cryogenic air has a higher heat transfer coefficient, while cryogenic CO₂ exhibits a better penetration ability in the cutting zone. Further milling experiments show that compared with cryogenic air, the average temperature rise, average cutting force and surface roughness of workpiece surface with cryogenic CO₂ as cryogenic medium are reduced by 23.6%, 32.8%, and 11.8%, respectively. It is considered that excellent permeability is the key to realize efficient cooling and lubrication in the cutting zone by Cryogenic CO₂ Minimum Quantity Lubrication Technology. This study provides a theoretical basis and technical reference for efficient precision machining of titanium alloy thin-walled parts. Full article

(This article belongs to the Special Issue Tribological Properties of Biolubricants)

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24 pages, 3158 KB

Open AccessArticle

Pressure and Temperature Dependence of the Permittivity of Mineral and PAG Oils for Hydrodynamic Journal Bearing Applications

by Björn Prase, Florian Koetz, Eckhard Kirchner and Alexander Hasse

Lubricants 2025, 13(12), 539; https://doi.org/10.3390/lubricants13120539 - 11 Dec 2025

Viewed by 541

Abstract

Electrically induced bearing failure is a reoccurring problem in modern drive train designs. To predict this damage, electrical models of bearings are required. In these models, the permittivity of lubricants is often assumed to be constant. However, the permittivity is dependent on pressure [...] Read more.

Electrically induced bearing failure is a reoccurring problem in modern drive train designs. To predict this damage, electrical models of bearings are required. In these models, the permittivity of lubricants is often assumed to be constant. However, the permittivity is dependent on pressure and temperature. For operating temperatures and pressures of journal bearings, no investigation of the permittivity of the lubricant exists. For this purpose, this study investigates the pressure and temperature dependence of lubricant permittivity using specially fabricated model bodies with layered structures of steel, ceramic insulating layers and copper in a parallel plate capacitor setup. Tests were performed applying temperatures between 20 °C and 100 °C and pressures between 1 and 250 bar. A mineral oil and a polyalkylene glycol (PAG) oil were examined. Results show a clear dependence of the permittivity on pressure and temperature. The mineral oil exhibits stronger pressure sensitivity, while the PAG oil shows more pronounced temperature dependence. Empirical equations to describe the permittivity as a function of temperature and pressure are derived. These findings provide relevant input for the selection of lubricants used in electrical environments. They also support the development of predictive models for modern electrical and tribological systems. Full article

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16 pages, 3919 KB

Open AccessArticle

Optimization of Laser-Induced Composite Micro-Textures on PEEK/CF Composites and Their Wetting–Friction Behaviors

by Yu Chen, Ping Xu, Yinghua Yu and Jiaxing Shen

Lubricants 2025, 13(12), 538; https://doi.org/10.3390/lubricants13120538 - 11 Dec 2025

Viewed by 415

Abstract

Poly(ether ether ketone)/carbon-fiber (PEEK/CF) composites possess excellent mechanical and thermal stability but exhibit inadequate friction and wear resistance for demanding tribological applications. In this study, femtosecond laser texturing was used to generate sinusoidal–circular hybrid microtextures on PEEK/CF surfaces, and the effects of laser [...] Read more.

Poly(ether ether ketone)/carbon-fiber (PEEK/CF) composites possess excellent mechanical and thermal stability but exhibit inadequate friction and wear resistance for demanding tribological applications. In this study, femtosecond laser texturing was used to generate sinusoidal–circular hybrid microtextures on PEEK/CF surfaces, and the effects of laser power and geometric parameters were systematically evaluated through a Taguchi L9 design. The optimal laser power of 0.85 W produced the highest machining quality factor (MQF = 0.968). The textures caused a hydrophilic-to-hydrophobic transition, increasing the static contact angle from 43° to 96.2°. Under boundary lubrication, all textured specimens exhibited reduced steady-state friction compared with the untreated surface. Among them, specimen L7—corresponding to the largest amplitude (A) and wavelength (B) levels in the orthogonal design—achieved the lowest average coefficient of friction (≈0.12) and generated the narrowest wear track. These results demonstrate that femtosecond-laser-fabricated hybrid microtextures effectively enhance lubricant retention and improve the tribological performance of PEEK/CF composites. Full article

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20 pages, 7436 KB

Open AccessReview

Current Status and Future Prospects of Small-Diameter Artificial Blood Vessels

by Zhaoxian Zheng, Menglin Zhou, Xiaolu Jiang, Zihan Lin, Jianhua Jin, Qi Wan, Chengxiong Lin and Li Zhang

Lubricants 2025, 13(12), 537; https://doi.org/10.3390/lubricants13120537 - 11 Dec 2025

Cited by 1 | Viewed by 1006

Abstract

Small-diameter vascular grafts (SDVGs, ≤6 mm) face significant barriers in vascular reconstruction due to poor long-term patency stemming from thrombosis, intimal hyperplasia, and mechanical mismatch. Increasing rates of cardiovascular disease and limited autologous vessel supply underscore the urgent need for functional SDVGs. This [...] Read more.

Small-diameter vascular grafts (SDVGs, ≤6 mm) face significant barriers in vascular reconstruction due to poor long-term patency stemming from thrombosis, intimal hyperplasia, and mechanical mismatch. Increasing rates of cardiovascular disease and limited autologous vessel supply underscore the urgent need for functional SDVGs. This review discusses the critical failure mechanisms of SDVGs and recent material-based advances—hydrophilic modifications, charge control, micro- and nano-engineering, antimicrobial and anti-inflammatory treatments, and controlled bioactive release (e.g., heparin, nitric oxide, t-PA). It details progress in cellular and tissue engineering for rapid endothelialization, smooth muscle regeneration, and mechanical durability. The review also highlights emerging gene engineering, the use of bioactive peptides, and molecular pathway strategies for physiological antithrombotic restoration. Finally, it outlines future directions, including smart materials, accelerated endothelialization, advanced manufacturing (3D printing, multilayer electrospinning), multifunctional composites, and clinical translation. Overall, SDVG research is shifting toward active, regenerative vascular substitutes with improved clinical prospects. Full article

(This article belongs to the Special Issue Tribology of Medical Devices)

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17 pages, 3212 KB

Open AccessArticle

Optimization of Laser Surface Texturing Parameters for Improving Friction and Wear Resistance of GCr15 Bearing Steel Sliding Pairs

by Yueyong Wang, Xuhui Wang, Fushun Hou, Risheng Long, Tianjiao Liu, Kaihong Tang and Xiumei Zhao

Lubricants 2025, 13(12), 536; https://doi.org/10.3390/lubricants13120536 - 10 Dec 2025

Cited by 1 | Viewed by 507

Abstract

GCr15 bearing steel sliding friction pairs, as key components in mechanical engineering applications, often undergo severe friction and wear under starved lubrication, which restricts their service life and reliability significantly. To solve this problem, this study investigates the effect of laser surface texturing [...] Read more.

GCr15 bearing steel sliding friction pairs, as key components in mechanical engineering applications, often undergo severe friction and wear under starved lubrication, which restricts their service life and reliability significantly. To solve this problem, this study investigates the effect of laser surface texturing on the tribological performance of GCr15 bearing steel under starved lubrication conditions. A laser marking machine is used to fabricate pit textures on GCr15 sliding surfaces, exploring the effects of processing speed, laser power, and frequency on texture integrity. Friction and wear tests under starved lubrication conditions are conducted using a vertical universal tester, and worn surfaces are characterized using a 3D surface profiler. The results show that high-integrity flat-bottom pits form at 200 mm/s, 10 W, and 80 kHz. These textures collect debris, retain lubricant, and provide secondary lubrication. At a depth-to-diameter ratio of 0.083 and area ratio of 14.9%, the friction coefficient (0.076) and wear loss (2.27 mg) decrease by 27.6% and 63.7%, respectively, compared to those of smooth samples (0.105, 6.25 mg). This study clarifies the regulatory mechanisms and provides references for improving key components’ lifespan and reliability. Full article

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17 pages, 7222 KB

Open AccessArticle

Wear and Friction Reduction on Polyethersulfone Matrix Composites Containing Polytetrafluoroethylene Coated with ZrW₂O₈ Particles at Elevated Temperatures

by Andrey I. Dmitriev, Sergei Yu. Tarasov, Dmitry G. Buslovich, Sergey V. Panin, Nikolai L. Savchenko, Lyudmila A. Kornienko, Evgeny Yu. Filatov, Evgeny N. Moskvichev and Dmitry V. Lychagin

Lubricants 2025, 13(12), 535; https://doi.org/10.3390/lubricants13120535 - 9 Dec 2025

Viewed by 428

Abstract

Polymer matrix composites (PMCs) have been prepared having a polyethersulfone (PES) matrix loaded with polytetrafluoroethylene (PTFE) particles coated with negative thermal expansion zirconium tungstate (ZT) with an aim to reduce the thermal mismatch stresses at the PES/PTFE interfaces and, thus, reduce wear rate [...] Read more.

Polymer matrix composites (PMCs) have been prepared having a polyethersulfone (PES) matrix loaded with polytetrafluoroethylene (PTFE) particles coated with negative thermal expansion zirconium tungstate (ZT) with an aim to reduce the thermal mismatch stresses at the PES/PTFE interfaces and, thus, reduce wear rate when sliding against a ball bearing AISI 52100 steel counterpart at elevated temperatures. The zirconium tungsten particles were synthesized using thermal decomposition from hydrothermally prepared precursors. The PMCs have been obtained using compression molding at 370 °C and contained, according to XRD, only the hexagonal α-ZrW₂O₈ phase. Wear testing was carried out at 25, 120, and 180 °C using a ball-on-disk scheme at 5 N and 0.3 m/s. The resulting wear tracks’ radial profiles were registered by means of profilometry, which was then used for calculating the wear rate. It was shown that both wear rate and friction reduced in testing the PES/PTFE/ZT samples at 180 °C compared to those of PES/PTFE containing only neat PTFE particles. Wear mechanism transitions have been observed from low-temperature generation of the tribological layer by the PTFE smearing to flow and abrasion wear at high temperatures. Full article

(This article belongs to the Special Issue Tribological Behaviours of Advanced Polymeric Materials)

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14 pages, 989 KB

Open AccessArticle

Comparative Study of Wheel Profile Influence on Multi-Point Wheel–Turnout Contact Using Kalker’s Theory

by Mihaela Cristina Tudorache, Razvan Andrei Oprea, Marius Adrian Spiroiu, Camil Ion Craciun and Sorin Arsene

Lubricants 2025, 13(12), 534; https://doi.org/10.3390/lubricants13120534 - 9 Dec 2025

Viewed by 387

Abstract

Turnouts represent a critical element of railway infrastructure, and are subjected to some of the highest mechanical stresses due to the discontinuity of the track geometry. Failures in this area generate high maintenance costs and may compromise traffic safety. This study investigates the [...] Read more.

Turnouts represent a critical element of railway infrastructure, and are subjected to some of the highest mechanical stresses due to the discontinuity of the track geometry. Failures in this area generate high maintenance costs and may compromise traffic safety. This study investigates the effect of wheel profile geometry on the wheel–turnout interaction in the presence of multi-point contact. Two standard wheel profiles, S78 and S1002, are compared using numerical simulations based on Kalker’s three-dimensional rolling contact theory, implemented in the CONTACT program. The methodology includes parametric analysis of the contact stresses, adhesion/slip distribution, and frictional power density for typical operational conditions. It was observed that the choice of wheel profile significantly influences the shape and load distribution of contact patches, with direct implications for wear mechanisms and guidance safety. These findings provide valuable insight for optimizing wheel–rail interface design and for reducing turnout maintenance costs. Full article

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23 pages, 4545 KB

Open AccessArticle

Waste-Derived Composite Selection for Sustainable Automotive Brake Friction Materials Using Novel MEREC-RAM Decision Framework

by Raj Kumar, Lalit Ranakoti, Akashdeep Negi, Yang Song, Gusztáv Fekete and Tej Singh

Lubricants 2025, 13(12), 533; https://doi.org/10.3390/lubricants13120533 - 8 Dec 2025

Viewed by 464

Abstract

This study aims to identify the most suitable slag waste-filled polymer composite for automotive braking applications. It employs a hybrid multi-criteria decision-making (MCDM) model that integrates the “method based on the removal effects of criteria” (MEREC) and the “root assessment method” (RAM) method. [...] Read more.

This study aims to identify the most suitable slag waste-filled polymer composite for automotive braking applications. It employs a hybrid multi-criteria decision-making (MCDM) model that integrates the “method based on the removal effects of criteria” (MEREC) and the “root assessment method” (RAM) method. Eight slag waste-filled polymer composites, evaluated using seven performance-defining criteria, were considered in the MCDM analysis. The performance evaluation criteria included the friction coefficient, wear, friction fluctuations, friction stability, fade-recovery aspects, and rise in disk temperature. The criteria were weighted through the MEREC approach, which identified fade% (0.2890) and wear (0.2829) as the most important attributes in the assessment. The RAM was employed to rank the alternatives and suggested that the composite alternative with 60 wt.% slag waste and 5 wt.% coir fiber proved to be the best composition for automotive braking applications. The results were validated using nine MCDM models and Spearman correlation coefficients, which showed that the ranking of alternatives was consistent and stable even when the normalization steps of MEREC were swapped. Statistical validation demonstrated a strong predictive accuracy (p < 0.05) with a strong correlation coefficient (>0.8) alongside a minimal mean absolute error. Furthermore, sensitivity analysis was performed by examining several weight situations to determine whether the priority weights influenced the ranking of the composite alternatives. The findings from both the correlation and sensitivity analyses confirm the proposed hybrid MEREC-RAM model’s consistency and effectiveness. Full article

(This article belongs to the Special Issue Tribology of Friction Brakes)

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18 pages, 4558 KB

Open AccessArticle

Investigation of Friction Enhancement Behavior on Textured U75V Steel Surface and Its Friction Vibration Characteristic

by Jinbo Zhou, Zhiqiang Wang, Linfeng Min, Jingyi Wang, Yongqiang Wang, Zhixiong Bai and Mingxue Shen

Lubricants 2025, 13(12), 532; https://doi.org/10.3390/lubricants13120532 - 7 Dec 2025

Viewed by 473

Abstract

The wheel–rail friction coefficient is a critical factor influencing train traction and braking performance. Low-adhesion conditions not only limit the enhancement of railway transport capacity but are also the primary cause of surface damage such as scratches, delamination, and flat spots. This study [...] Read more.

The wheel–rail friction coefficient is a critical factor influencing train traction and braking performance. Low-adhesion conditions not only limit the enhancement of railway transport capacity but are also the primary cause of surface damage such as scratches, delamination, and flat spots. This study employs femtosecond laser technology to fabricate wavy groove textures on U75V rail surfaces, systematically investigating the effects of the wavy angle and texture area ratio on friction enhancement under various medium conditions. Findings indicate that parameter-optimized textured surfaces not only significantly increase the coefficient of friction but also exhibit superior wear resistance, vibration damping, and noise reduction properties. The optimally designed wavy textured surface achieves significant friction enhancement under water conditions. Among the tested configurations, the surface with parameters θ = 150°@η = 30% demonstrated the most pronounced friction enhancement, achieving a coefficient of friction as high as 0.57—a 42.5% increase compared to the non-textured surface (NTS). This enhancement is attributed to the unique hydrophilic and anisotropic characteristics of the textured surface, where droplets tend to spread perpendicular to the sliding direction, thereby hindering the formation of a continuous lubricating film as a third body. Analysis of friction vibration signals reveals that textured surfaces exhibit lower vibration signal amplitudes and richer frequency components. Furthermore, comparison of Stribeck curves under different lubrication regimes for the θ = 150°@η = 30% specimen and NTS indicated an overall upward shift in the curve for the textured sample. The amplitude, energy, and wear extent of the textured surface consistently decreased across boundary lubrication, hydrodynamic lubrication, and mixed lubrication regimes. These findings provide crucial theoretical insights and technical guidance for addressing low-adhesion issues at the wheel–rail interface, offering significant potential to enhance wheel–rail adhesion characteristics in engineering applications. Full article

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

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41 pages, 9711 KB

Open AccessReview

Key Insights into Silver Matrix Nanocomposites Reinforced with Solid Lubricants for Electrical Contacts: A Review

by Magdalena Valentina Lungu, Alina Ruxandra Caramitu, Eduard Marius Lungulescu, Valentin Mihailov and Sergiu Ivascu

Lubricants 2025, 13(12), 531; https://doi.org/10.3390/lubricants13120531 - 6 Dec 2025

Viewed by 517

Abstract

Metal-based electrical contact materials (ECMs) are essential in switching devices and rotating electrical machines, where sliding contacts enable reliable current transmission under motion. These materials must exhibit high conductivity, low friction, and wear resistance to meet industrial demands. However, their reliability is limited [...] Read more.

Metal-based electrical contact materials (ECMs) are essential in switching devices and rotating electrical machines, where sliding contacts enable reliable current transmission under motion. These materials must exhibit high conductivity, low friction, and wear resistance to meet industrial demands. However, their reliability is limited by wear, oxidation, arcing, and other failure mechanisms that increase contact resistance and degrade performance. To address these issues, researchers have developed self-lubricating metal matrix composites (MMCs), particularly copper (Cu) and silver (Ag)-based composites reinforced with solid lubricants such as molybdenum disulfide, tungsten disulfide, graphite, carbon nanotubes, graphene, and its derivatives. While Cu and Ag provide excellent conductivity, each has trade-offs in cost, oxidation resistance, and mechanical strength. Strategies for improving reliability involve material optimization, surface treatments, lubrication, contact design modifications, and advanced manufacturing. Although MMCs are widely reviewed, self-lubricating Ag matrix nanocomposites (AgMNCs) for sliding contacts are underexplored. This review highlights recent progress in AgMNCs produced by conventional or modern powder metallurgy techniques, focusing on the role of solid lubricants, testing conditions, and microstructure on tribological performance. Wear mechanisms, research gaps, and future directions are discussed, highlighting pathways toward the development of reliable sliding contacts. Full article

(This article belongs to the Special Issue Self-Lubrication Technologies and Tribology Behavior in High-Performance Mechanical Components)

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26 pages, 7775 KB

Open AccessArticle

Influence of Thermal, Oxidative, Catalytic, and Mechanical Effects on Thickener Degradation and the Associated Lubricating Performance of Greases

by Markus Grebe, Michael Ruland, Dagmar Kuckelberg and Natalia Eurich

Lubricants 2025, 13(12), 530; https://doi.org/10.3390/lubricants13120530 - 4 Dec 2025

Viewed by 752

Abstract

Continuous advancements in application technology aimed at higher efficiency and power density place ever-increasing demands on mechanical components and construction elements—and, consequently, on the lubricating greases employed. This is particularly true for rolling bearings, where greases are exposed to high mechanical loads and [...] Read more.

Continuous advancements in application technology aimed at higher efficiency and power density place ever-increasing demands on mechanical components and construction elements—and, consequently, on the lubricating greases employed. This is particularly true for rolling bearings, where greases are exposed to high mechanical loads and wide temperature ranges. A current example can be found in the bearings of hybrid vehicle powertrains, which are subjected to extreme thermal and mechanical stress due to engine downsizing, high rotational speeds, and radiant heat from the combustion engine. A collaborative project between the Competence Center for Tribology (KTM) at Mannheim University of Applied Sciences and the OWI Science for Fuels gGmbH (OWI), affiliated with RWTH Aachen University, demonstrated that the loss of lubricating performance—which ultimately leads to bearing failure—is directly linked to changes in the thickener structure. Various degradation processes reduce yield stress and viscosity, thereby eliminating the typical grease characteristics. Mechanical, thermal, oxidative, and catalytic processes all play decisive roles. This paper presents analytical methods that enable these individual influencing factors to be investigated and evaluated independently. These approaches can significantly reduce the need for time-consuming and costly laboratory tests in grease development and qualification. Full article

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13 pages, 4169 KB

Open AccessArticle

Effects of Commensurability on Stick and Slip Conditions at Solid–Fluid Interface

by Vadym Borysiuk, Mikhail Popov and Valentin L. Popov

Lubricants 2025, 13(12), 529; https://doi.org/10.3390/lubricants13120529 - 4 Dec 2025

Viewed by 490

Abstract

We report the results of molecular dynamics simulations of the frictional behavior of a Lennard–Jones fluid confined between two solid crystalline walls. To study the effects of commensurability on friction, different ratios of interatomic distances in walls and fluid were considered. In particular, [...] Read more.

We report the results of molecular dynamics simulations of the frictional behavior of a Lennard–Jones fluid confined between two solid crystalline walls. To study the effects of commensurability on friction, different ratios of interatomic distances in walls and fluid were considered. In particular, numerical experiments with the same fluid confined between walls with five different lattice parameters were performed. System behavior was examined by analyzing calculated time dependencies of the friction force between fluid and solid walls and distributions of the velocities of fluid particles. Friction coefficients and slip length parameters were obtained as numerical characteristics of commensurability effects. Fluid behavior near the solid interface was analyzed through visualization of the atomistic configurations and calculation of radial distribution functions. In the performed simulations, a pronounced reduction in friction was observed for highly incommensurable configurations, when the ratio between fluid and wall interatomic distances is around 1.62. Full article

(This article belongs to the Special Issue Recent Advances in Lubricated Tribological Contacts)

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18 pages, 7540 KB

Open AccessArticle

Effect of Treatment Time on the Tribological Behavior of Thermally Oxidized Ti-6Al-4V Under Dry and Oil-Lubricated Conditions

by Mohammed Al-Shan, Richard Bailey and Yong Sun

Lubricants 2025, 13(12), 528; https://doi.org/10.3390/lubricants13120528 - 3 Dec 2025

Viewed by 535

Abstract

Ti-6Al-4V alloy is a popular metal in engineering, utilized in aerospace and automotive industries because of its mechanical properties. However, Ti-6Al-4V’s poor tribological characteristics cause it to be susceptible to wear due to its low surface hardness and inadequate lubricity. In this study, [...] Read more.

Ti-6Al-4V alloy is a popular metal in engineering, utilized in aerospace and automotive industries because of its mechanical properties. However, Ti-6Al-4V’s poor tribological characteristics cause it to be susceptible to wear due to its low surface hardness and inadequate lubricity. In this study, thermal oxidation (TO) was performed on Ti-6Al-4V under specific conditions of 625 °C for various oxidation durations of 0.5, 1.5, 6, 24 and 96 h and the microstructure, friction, and wear behavior of TO-treated Ti-6Al-4V under dry and oil-lubricated sliding conditions were investigated. Characterization by XRD, SEM, and EDX confirms the development of oxide layers (OL) and oxygen diffusion zones (ODZ) of varying thicknesses. Tribological tests were conducted using a ball-on-disk configuration under a 5 N load against an Al₂O₃ counterface in both dry and 10W-40 oil-lubricated environments. Under dry conditions, extended oxidation times lead to a deterioration in friction and wear performance due to the increased brittleness and decreased adhesion of the thick OL, leading to brittle failure and interfacial delamination. In contrast, under oil lubrication conditions, all oxidized samples show stable, low-friction (~0.06) and minimal wear, dominated by boundary lubrication. The best performance is achieved at short oxidation durations, where a thin OL and a stable ODZ provide strong adhesion of the OL and high surface hardness. Wear rates up to three orders of magnitude lower than untreated Ti-6Al-4V are observed for short oxidation durations, where oxygen diffusion rather than thick oxide formation dominates the surface-hardening effect. SEM and EDX analyses confirmed the lack of tribofilms or additive-derived elements on the sliding surfaces, indicating that the improved performance results from the oxygen-enrichment in the subsurface and stable boundary lubrication, rather than chemical interactions with oil additives. Overall, oxidation duration is therefore essential to balance oxide growth and OL adhesion, ensuring superior lubricated wear resistance for titanium components. Full article

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

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45 pages, 47928 KB

Open AccessArticle

A Fully Coupled Elastic–Aerodynamic Theoretical and Finite Element Model for Static Performance Analysis and Experimental Investigation of Gas Foil Bearings

by Qingsong Li, Jiaao Ning, Hang Liang and Muzhen Yang

Lubricants 2025, 13(12), 527; https://doi.org/10.3390/lubricants13120527 - 3 Dec 2025

Viewed by 712

Abstract

This paper proposes a comprehensive framework, Theory–Simulation–Experimental Verification, for the elasto-aerodynamic analysis of elastic foil gas bearings (EFGBs). In contrast to many studies that approximate the foil structure using simplified two-dimensional models, the present work adopts a macro-element beam theory model that incorporates [...] Read more.

This paper proposes a comprehensive framework, Theory–Simulation–Experimental Verification, for the elasto-aerodynamic analysis of elastic foil gas bearings (EFGBs). In contrast to many studies that approximate the foil structure using simplified two-dimensional models, the present work adopts a macro-element beam theory model that incorporates the actual 3D geometry, nonlinear elasticity, and frictional contact effects, and couples it directly with the Reynolds equation. To improve accuracy and robustness, the macro-beam results are validated against a fully coupled fluid–structure interaction (FSI) model developed in COMSOL Multiphysics. Emphasis is placed on quantifying the influence of foil thickness, clearance, and eccentricity, where the pressure distribution, foil deflection, and load capacity are obtained through the coupled solver. The results reveal that increasing foil thickness from 0.1 mm to 0.2 mm elevates the peak gas film pressure from 1.36 × 10⁵ Pa to 1.97 × 10⁵ Pa while simultaneously reducing displacement and pressure fluctuations, thereby enhancing bearing stability. Smaller clearances are shown to increase load capacity but also induce stronger oscillatory flow behavior, indicating a stiffness–stability trade-off. Additionally, prototype experiments with a 0.05 mm clearance confirm practical lift-off at 4300–7000 rpm under 10–30 N external loads, with measured torques of 0.18–0.30 N·m. By combining computational efficiency, 3D fidelity, and experimental validation, the proposed framework provides quantitative guidance for the design and optimization of EFGBs used in high-speed turbomachinery, such as aviation and compact energy systems, including turbine-based air-cycle refrigeration units and small gas-turbine rotors for unmanned aerial vehicles. Full article

(This article belongs to the Special Issue Gas Lubrication and Dry Gas Seal, 2nd Edition)

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18 pages, 2496 KB

Open AccessArticle

Experimental Study on Temperature Rise of New Energy Vehicle Drive Motor Bearings Under Grease and Driving Conditions

by Mengchen Zi, Jun Ye, Haichao Cai, Hongfan Yang, Gang Chen, Jiahao Zhang, Dengke Li and Dongliang Lu

Lubricants 2025, 13(12), 526; https://doi.org/10.3390/lubricants13120526 - 2 Dec 2025

Viewed by 601

Abstract

The temperature rise of drive motor bearings in new energy vehicles is a critical factor affecting their reliability and lifespan, with grease performance and driving conditions being the primary determinants of this rise. Addressing the lack of research on how different grease types [...] Read more.

The temperature rise of drive motor bearings in new energy vehicles is a critical factor affecting their reliability and lifespan, with grease performance and driving conditions being the primary determinants of this rise. Addressing the lack of research on how different grease types affect the temperature rise of drive motor bearings under operational conditions, this study utilizes a high-temperature, high-speed testing machine for drive motor bearings in new energy vehicles. It conducts a comparative analysis of the temperature rise in bearings lubricated with four different types of grease under three typical driving conditions: emergency start–stop, smooth driving, and high-speed driving. Results show that the temperature rise varied from 25.1 °C to 50.3 °C under rapid speed change, 22.7 °C to 40.2 °C at fixed speed, and that grease No.3 achieved the lowest temperature rise and the highest limiting speed (23,000 rpm). These results provide quantitative evidence for selecting grease and optimizing bearing design. Full article

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

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19 pages, 7854 KB

Open AccessArticle

Friction and Wear Performances of Stainless Steel Archwires After Corrosion in Oral Care Products

by Cong Zhang, Minghui Hao, Shiqi Cheng and Pengfei Wang

Lubricants 2025, 13(12), 525; https://doi.org/10.3390/lubricants13120525 - 30 Nov 2025

Viewed by 503

Abstract

To elucidate the corrosion mechanism of orthodontic archwire in fluoride-containing environments, the friction and wear behavior of archwires following corrosion in fluoride-containing oral care products was investigated. Stainless steel archwires were soaked in solutions of fluoride-free toothpaste, fluoride toothpaste, fluoride-free mouthwash, fluoride mouthwash, [...] Read more.

To elucidate the corrosion mechanism of orthodontic archwire in fluoride-containing environments, the friction and wear behavior of archwires following corrosion in fluoride-containing oral care products was investigated. Stainless steel archwires were soaked in solutions of fluoride-free toothpaste, fluoride toothpaste, fluoride-free mouthwash, fluoride mouthwash, and sodium monofluorophosphate, followed by friction testing against brackets. The average friction coefficient of the archwire–bracket tribopair increased gradually from 0.17 to 0.28 with prolonged immersion time in the fluoride-containing solution, accompanied by a progressive increase in the wear scar area on the archwire surface. In the fluoride toothpaste solution, the archwire exhibited a corrosion potential and current density of –301.8 mV and 0.348 μA/cm², respectively, indicating a higher susceptibility to corrosion. Analysis of wear debris revealed significant enrichment of fluorine and oxygen elements on the archwire surface after exposure to fluoride-containing solutions, consistent with pronounced corrosion damage. Integration of friction results and surface characterization elucidated the corrosion mechanism in fluoride-containing environments. It was proposed that fluoride ions facilitated the formation of micro-batteries, while active fluoride species accelerated the dissolution of nickel from the archwire surface and promoted oxygen accumulation, thus driving sustained electrochemical corrosion. This progressive surface degradation ultimately exacerbated the friction and wear of the archwire–bracket tribopair. Full article

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Lubricants, Volume 13, Issue 12 (December 2025) – 46 articles

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