Lubricants

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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35 pages, 7315 KiB

Open AccessReview

A Comprehensive Review of Safety Tire Research

by Liguo Zang, Jing Sun, Xinlei Peng, Fen Lin, Yaoji Deng and Yuxing Bai

Lubricants 2025, 13(8), 357; https://doi.org/10.3390/lubricants13080357 - 12 Aug 2025

Abstract

In frequent traffic accidents, the issue of vehicle losing control due to tire blowouts is particularly critical. Once it occurs, it severely threatens the safety of life and property. Therefore, developing safety tires that meet public needs is of significant practical importance. This [...] Read more.

In frequent traffic accidents, the issue of vehicle losing control due to tire blowouts is particularly critical. Once it occurs, it severely threatens the safety of life and property. Therefore, developing safety tires that meet public needs is of significant practical importance. This paper first reviews the evolutionary history of safety tires, classifying them by structural design and detailing the characteristics of run-flat tires (RFTs) and non-pneumatic tires (NPTs). It summarizes the key experimental characterization, numerical simulation and theoretical modeling methods that support their development, while also emphasizing the insufficiency in experimental verification. Next, the research advancements in materials and manufacturing processes for key components across these two tire categories are summarized. Subsequently, a review is provided on the mechanical characteristics of RFTs and NPTs. Finally, this paper discusses existing research gaps and future development directions for safety tires, emphasizing the importance of multi-disciplinary integration and sustainable design. Full article

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23 pages, 11067 KiB

Open AccessArticle

The Influence of Selected Process Parameters on Wire Wear and Surface Quality of Nickel, Titanium and Steel Alloy Parts in WEDM

by Jarosław Buk, Anna Bazan and Paweł Sułkowicz

Lubricants 2025, 13(8), 356; https://doi.org/10.3390/lubricants13080356 - 12 Aug 2025

Abstract

Research on the WEDM process has traditionally focused on analyzing discharge initiation, material removal mechanisms and surface formation from the perspective of the machined part. However, the same phenomena also affect the tool, namely the wire electrode. A comprehensive understanding of the process [...] Read more.

Research on the WEDM process has traditionally focused on analyzing discharge initiation, material removal mechanisms and surface formation from the perspective of the machined part. However, the same phenomena also affect the tool, namely the wire electrode. A comprehensive understanding of the process requires to examine how these effects impact the electrode itself, particularly in terms of wear. Despite its significance, electrode wear in WEDM is not a topic frequently addressed in the literature. The most common method for evaluating wear involves determining the wire wear ratio (WWR), based on the electrode’s weight before and after machining. However, this approach does not provide insight into changes in the microstructure of the electrode surface. This study presents an alternative approach to interpreting wire electrode wear, using surface roughness parameters in relation to the surface texture of the machined workpiece. Measurements were conducted using an optical focus variation microscope. The influence of selected process parameters—including discharge current I_p, pulse-off time t_off and workpiece height h—on selected surface roughness parameters was investigated. The experimental tests were carried out for three alloys representing distinct material groups: 42CrMo4 steel, Inconel 718 nickel alloy, and Ti6Al4V titanium alloy. The results were compared with the roughness parameters of the corresponding machined surfaces. The presented interpretation of the key factors affecting the electrode surface condition after WEDM serves as an initial step in a broader research initiative. It lays the foundation for further studies on wire electrode wear and the development of new wear assessment parameters such as the electrode wear index based on surface texture parameters. Full article

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

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13 pages, 1633 KiB

Open AccessArticle

An Optimal Preprocessing Method for Predicting the Acid Number of Lubricating Oil Based on PLSR and Infrared Spectroscopy

by Fanhao Zhou, Jie Shen, Xiaojun Li, Kun Yang and Ling Wang

Lubricants 2025, 13(8), 355; https://doi.org/10.3390/lubricants13080355 - 10 Aug 2025

Abstract

The acid number evaluates the degree of deterioration of lubricating oil. Existing methods for evaluating the performance degradation of lubricating oils are mostly based on the detection of traditional physical and chemical indicators, which often only reflect a single dimension of the degradation [...] Read more.

The acid number evaluates the degree of deterioration of lubricating oil. Existing methods for evaluating the performance degradation of lubricating oils are mostly based on the detection of traditional physical and chemical indicators, which often only reflect a single dimension of the degradation process, thus affecting the accuracy and repeatability of the results. Integrating multi-dimensional information can more comprehensively reflect the essence of degradation, which can improve the accuracy and reliability of the evaluation results. Mid-infrared spectroscopy is an effective means of monitoring the acid number. In this study, a combination of infrared spectroscopy quantitative analysis and chemometrics was used. The oil sample data was divided into training set and validation set by the Kennard–Stone method. In the experiment, a Fourier transform infrared spectrometer equipped with an attenuated total reflection accessory (ATR-FTIR) was used to collect spectral data of the samples in the wavenumber range of 1750–1700 cm⁻¹ (this range corresponds to the characteristic absorption of carboxyl groups and is directly related to the acid number). Meanwhile, a G20S automatic potentiometric titrator was used to determine the acid number as a reference value in accordance with GB/T 7304. The study compared various preprocessing methods. A regression prediction model between the spectra and acid number was established using partial least squares regression (PLSR) within the selected wavenumber range, with the root mean square error of cross-validation (RMSECV), root mean square error of prediction (RMSEP), and coefficient of determination (R) as evaluation indicators. The experimental results showed that the PLSR model established after preprocessing with second derivative combined with seven-point smoothing exhibited the optimal performance, with an RMSECV of 0.00505, an RMSEP of 0.14%, and an R of 0.9820. Compared with the traditional titration method, this prediction method is more suitable for real-time monitoring of production lines or rapid on-site screening of equipment. It can in a timely manner warn of the deterioration trend of lubricating oil, reduce the risk of equipment wear caused by oil failure, and provide efficient technical support for lubricating oil life management. Full article

(This article belongs to the Special Issue Future of Digital Tribology: Prediction of Tribological Performance Using Sensors, Signal Processing and Machine Learning)

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32 pages, 6746 KiB

Open AccessArticle

Tribo-Electric Performance of Nano-Enhanced Palm Oil-Based Glycerol Grease for Electric Vehicle Bearings

by Amany A. Abozeid, May M. Youssef, Tamer F. Megahed, Mostafa El-Helaly, Florian Pape and Mohamed G. A. Nassef

Lubricants 2025, 13(8), 354; https://doi.org/10.3390/lubricants13080354 - 8 Aug 2025

Abstract

Rolling Bearings are crucial components for induction motors and generators in electric vehicles (EVs), as their performance considerably influences the system’s operational reliability and safety. However, the commercial greases used for bearing lubrication in EV motors pose a detrimental impact on the environment. [...] Read more.

Rolling Bearings are crucial components for induction motors and generators in electric vehicles (EVs), as their performance considerably influences the system’s operational reliability and safety. However, the commercial greases used for bearing lubrication in EV motors pose a detrimental impact on the environment. In addition, they are ineffective in mitigating the effect of electric discharges on rolling surfaces leading to premature bearing failures. This study investigates the viability of a developed eco-friendly grease from palm olein as the base oil and glycerol monostearate as the thickener, enhanced with conductive multi-walled carbon nanotubes (MWCNTs) for EV motor bearings prone to electrical currents. Chemical–physical, tribological, and electrical tests were conducted on the developed grease samples without and with MWCNTs at 1 wt.%, 2 wt.%. and 3 wt.% concentrations and results were compared to lithium and sodium greases. Palm grease samples demonstrated a lower EDM voltage range reaching 1.0–2.2 V in case of 3 wt.% MWCNTs blends, indicating better electrical conductivity and protecting the bearing surfaces from electric-related faults. These findings were further confirmed using vibrations measurement and SEM-EDX analysis of the electrically worn bearings. Bearings lubricated with palm grease blends exhibited lower vibration levels. Palm grease with 2 wt.% MWCNTs reduced vibration amplitudes by 28.4% (vertical) and 32.3% (horizontal). Analysis of bearing damaged surfaces revealed enhanced damaged surface morphology for MWCNT-enhanced palm grease as compared to surface lubricated by commercial greases. The results of this work indicate that the proposed bio-grease is a promising candidate for future application in the field of next-generation electric mobility systems. Full article

(This article belongs to the Special Issue Tribology in Vehicles)

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19 pages, 440 KiB

Open AccessArticle

Reynolds Equation for a Micro-Scale Lubrication of a Gas Between Eccentric Circular Cylinders with an Arbitrary Temperature Difference Based on Slip-Flow Theory

by Toshiyuki Doi

Lubricants 2025, 13(8), 353; https://doi.org/10.3390/lubricants13080353 - 7 Aug 2025

Abstract

Micro-scale lubrication flow of a gas between eccentric circular cylinders with an arbitrary temperature difference is studied on the basis of the Navier–Stokes set of equations and the velocity slip and temperature jump boundary conditions. The dimensionless curvature, which is defined as the [...] Read more.

Micro-scale lubrication flow of a gas between eccentric circular cylinders with an arbitrary temperature difference is studied on the basis of the Navier–Stokes set of equations and the velocity slip and temperature jump boundary conditions. The dimensionless curvature, which is defined as the mean clearance divided by the radius of the inner cylinder, is small, the Knudsen number and the Reynolds number based on the mean clearance are small, and the temperature ratio is arbitrary. The Reynolds-type lubrication equation is derived analytically. For a verification of the equation, an assessment is conducted against the solution of the direct numerical analysis of the Bhatnagar–Gross–Krook–Welander (BGKW) model of the Boltzmann equation in the author’s previous work [Doi, T. Phys. Fluids 2024, 36, 042016]. The solution of the lubrication equation agrees with that of the Boltzmann equation satisfactorily well over the slip flow regime, not only in the eccentric force and the torque but also in the local distribution of the temperature, flow velocity, and the normal stress. A superiority of the lubrication equation over the lubrication model proposed in the author’s previous work is also discussed. Full article

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

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26 pages, 8019 KiB

Open AccessArticle

Tribo-Dynamic Investigation of Cryogenic Ball Bearings Considering Varying Traction Parameters

by Shijie Zhang, Shuangshuang Jia, Yuhao Zhao, Jing Wei and Yanyang Zi

Lubricants 2025, 13(8), 352; https://doi.org/10.3390/lubricants13080352 - 5 Aug 2025

Abstract

The traction behavior in cryogenic solid-lubricated ball bearings (CSLBBs) used in liquid rocket engines (LREs) affects not only the dynamic response of the bearing but also the lubricity and wear characteristics of the solid lubrication coating. The traction coefficient between the ball and [...] Read more.

The traction behavior in cryogenic solid-lubricated ball bearings (CSLBBs) used in liquid rocket engines (LREs) affects not only the dynamic response of the bearing but also the lubricity and wear characteristics of the solid lubrication coating. The traction coefficient between the ball and raceway depends on factors such as contact material, relative sliding velocity, and contact pressure. However, existing traction curve models for CSLBBs typically consider only one or two of these factors, limiting the accuracy and applicability of theoretical predictions. In this study, a novel traction model for CSLBBs is proposed, which incorporates the combined effects of contact material, relative sliding velocity, and contact pressure. Based on this model, a tribo-dynamic framework is developed to investigate the tribological and dynamic behavior of CSLBBs. The model is validated through both theoretical analysis and experimental data. Results show that the inclusion of solid lubricant effects significantly alters the relative sliding and frictional forces between the rolling elements and the raceway. These changes in turn influence the impact dynamics between the rolling elements and the cage, leading to notable variations in the bearing’s vibrational response. The findings may offer valuable insights for the wear resistance and vibration reduction design of CSLBBs. Full article

(This article belongs to the Special Issue Tribological Characteristics of Bearing System, 3rd Edition)

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18 pages, 1899 KiB

Open AccessArticle

Performance Analysis of New Deuterium Tracer for Online Oil Consumption Measurements

by Francesco Marzemin, Martin Vareka, Kevin Gschiel, Bernhard Rossegger, Peter Grabner, Michael Engelmayer and Nicole Wermuth

Lubricants 2025, 13(8), 351; https://doi.org/10.3390/lubricants13080351 - 5 Aug 2025

Abstract

The accurate and precise measurement of lubricating oil consumption is critical for developing environmentally friendly internal combustion engines, particularly hydrogen-fueled internal combustion engines. The deuterium tracer method is based on the addition of poly-deuterated base oil tracers to fully formulated oils for precise, [...] Read more.

The accurate and precise measurement of lubricating oil consumption is critical for developing environmentally friendly internal combustion engines, particularly hydrogen-fueled internal combustion engines. The deuterium tracer method is based on the addition of poly-deuterated base oil tracers to fully formulated oils for precise, accurate, and fast lubricating oil consumption measurements. Previously performed measurements have shown that the use of poly-deuterated poly-alpha olefins has minimal impact on lubricating oil properties, except for a slight drop in oil viscosity. To further reduce the impact on lubricating oil characteristics, a new base oil for the synthesis of a poly-deuterated tracer is introduced, and its influence on the lubricating oil’s chemical, tribological, and rheological properties is analyzed. Furthermore, the influence of the tracer addition on the preignition tendencies of the fully formulated oil is also examined. Based on the analyses, no relevant changes in the lubricating oil properties, such as viscosity, density, and thermal degradation behavior, can be observed. Additionally, the deuterium tracer does not negatively influence combustion anomalies, thus reducing preignition tendencies. These results establish the method’s compatibility with new-generation engines, especially hydrogen-fueled internal combustion engines. Full article

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24 pages, 13175 KiB

Open AccessArticle

Fault Diagnosis for CNC Machine Tool Feed Systems Based on Enhanced Multi-Scale Feature Network

by Peng Zhang, Min Huang and Weiwei Sun

Lubricants 2025, 13(8), 350; https://doi.org/10.3390/lubricants13080350 - 5 Aug 2025

Abstract

Despite advances in Convolutional Neural Networks (CNNs) for intelligent fault diagnosis in CNC machine tools, bearing fault diagnosis in CNC feed systems remains challenging, particularly in multi-scale feature extraction and generalization across operating conditions. This study introduces an enhanced multi-scale feature network (MSFN) [...] Read more.

Despite advances in Convolutional Neural Networks (CNNs) for intelligent fault diagnosis in CNC machine tools, bearing fault diagnosis in CNC feed systems remains challenging, particularly in multi-scale feature extraction and generalization across operating conditions. This study introduces an enhanced multi-scale feature network (MSFN) that addresses these limitations through three integrated modules designed to extract critical fault features from vibration signals. First, a Soft-Scale Denoising (S2D) module forms the backbone of the MSFN, capturing multi-scale fault features from input signals. Second, a Multi-Scale Adaptive Feature Enhancement (MS-AFE) module based on long-range weighting mechanisms is developed to enhance the extraction of periodic fault features. Third, a Dynamic Sequence–Channel Attention (DSCA) module is incorporated to improve feature representation across channel and sequence dimensions. Experimental results on two datasets demonstrate that the proposed MSFN achieves high diagnostic accuracy and exhibits robust generalization across diverse operating conditions. Moreover, ablation studies validate the effectiveness and contributions of each module. Full article

(This article belongs to the Special Issue Advances in Tool Wear Monitoring 2025)

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29 pages, 2636 KiB

Open AccessReview

Review on Tribological and Vibration Aspects in Mechanical Bearings of Electric Vehicles: Effect of Bearing Current, Shaft Voltage, and Electric Discharge Material Spalling Current

by Rohan Lokhande, Sitesh Kumar Mishra, Deepak Ronanki, Piyush Shakya, Vimal Edachery and Lijesh Koottaparambil

Lubricants 2025, 13(8), 349; https://doi.org/10.3390/lubricants13080349 - 5 Aug 2025

Abstract

Electric motors play a decisive role in electric vehicles by converting electrical energy into mechanical motion across various drivetrain components. However, failures in these motors can interrupt the motor function, with approximately 40% of these failures stemming from bearing issues. Key contributors to [...] Read more.

Electric motors play a decisive role in electric vehicles by converting electrical energy into mechanical motion across various drivetrain components. However, failures in these motors can interrupt the motor function, with approximately 40% of these failures stemming from bearing issues. Key contributors to bearing degradation include shaft voltage, bearing current, and electric discharge material spalling current, especially in motors powered by inverters or variable frequency drives. This review explores the tribological and vibrational aspects of bearing currents, analyzing their mechanisms and influence on electric motor performance. It addresses the challenges faced by electric vehicles, such as high-speed operation, elevated temperatures, electrical conductivity, and energy efficiency. This study investigates the origins of bearing currents, damage linked to shaft voltage and electric discharge material spalling current, and the effects of lubricant properties on bearing functionality. Moreover, it covers various methods for measuring shaft voltage and bearing current, as well as strategies to alleviate the adverse impacts of bearing currents. This comprehensive analysis aims to shed light on the detrimental effects of bearing currents on the performance and lifespan of electric motors in electric vehicles, emphasizing the importance of tribological considerations for reliable operation and durability. The aim of this study is to address the engineering problem of bearing failure in inverter-fed EV motors by integrating electrical, tribological, and lubrication perspectives. The novelty lies in proposing a conceptual link between lubricant breakdown and damage morphology to guide mitigation strategies. The study tasks include literature review, analysis of bearing current mechanisms and diagnostics, and identification of technological trends. The findings provide insights into lubricant properties and diagnostic approaches that can support industrial solutions. Full article

(This article belongs to the Special Issue Tribology of Electric Vehicles)

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32 pages, 14398 KiB

Open AccessArticle

Crushing Removal Conditions and Experimental Research on Abrasive Water Jets Impacting Rock

by Hongqi Wang, Ruifu Yuan, Xinmin Zhang, Penghui Zai, Junkai Fan and Junhao Deng

Lubricants 2025, 13(8), 348; https://doi.org/10.3390/lubricants13080348 - 4 Aug 2025

Abstract

This paper describes the complex process of rock crushing removal by AWJ impact from the microscopic perspective. The acceleration and deceleration mechanism of abrasive particles throughout the whole process of single abrasive particles impacting rocks, the spherical cavity expansion mechanism of the abrasive [...] Read more.

This paper describes the complex process of rock crushing removal by AWJ impact from the microscopic perspective. The acceleration and deceleration mechanism of abrasive particles throughout the whole process of single abrasive particles impacting rocks, the spherical cavity expansion mechanism of the abrasive particles’ impact on the rock, and the elastic contact force of the collision between the abrasive particles and rock were investigated; a mathematical model of AWJ’s impact on the rock crushing removal conditions was established; and the threshold values of the jet impact parameters were obtained. The mathematical model of the rock crushing removal conditions was verified through numerical simulation and jet impact experiments. The research results show that the theoretical value of the jet impact velocity that meets the conditions for limestone crushing removal is greater than or equal to 36 m/s, and the theoretical value of the pressure is greater than or equal to 2.7 MPa. Numerical simulation was used to obtain the displacement of marked points, stress, and strain variation in marked elements of rock under different impact velocities. The effect of impact rock breaking obtained through the experiment demonstrates the correspondence between the test pressure and the theoretical pressure, which verifies the accuracy of the mathematical model of the rock crushing removal conditions. Full article

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13 pages, 4747 KiB

Open AccessArticle

Biodegradable Polyalphaolefins for Gear Lubrication in Electrical Drives: Aging and Wetting

by Kevin Holderied, Joachim Albrecht, Elisabeth Distler, Katharina Weber and Nahed El Mahallawy

Lubricants 2025, 13(8), 347; https://doi.org/10.3390/lubricants13080347 - 4 Aug 2025

Abstract

Electric propulsion requires engines and transmission systems that run at higher speeds compared to combustion engines. For improving sustainability and environmental protection, biodegradable oils are suggested for the lubrication of high-speed gears that require particularly quick wetting of the steel surfaces. Newly developed [...] Read more.

Electric propulsion requires engines and transmission systems that run at higher speeds compared to combustion engines. For improving sustainability and environmental protection, biodegradable oils are suggested for the lubrication of high-speed gears that require particularly quick wetting of the steel surfaces. Newly developed promising candidates include short-chained polyalphaolefins. In the present work, a study on the applicability of such oil is presented and discussed with respect to different aging levels based on biodegradable properties. It focuses on the wettability of metallic surfaces investigated through time-resolved contact angle measurements. Carbon steels with different carbon contents and microstructures are selected as the most commonly used materials for gears. Effects of steel composition, surface roughness and oil oxidation are studied. The results show that in most cases, the application of biodegradable polyalphaolefins is not critical; however, a combination of steels with inhomogeneous microstructure, high surface roughness and aged oil can be critical because of limited wetting. Full article

(This article belongs to the Special Issue Tribology of Electric Vehicles)

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17 pages, 3469 KiB

Open AccessArticle

Performance Characteristics of a New Aerostatic Thrust Bearing with Poro-Elastic Restrictor

by Alin Mărgineanu, Alice Marinescu, Aurelian Fatu, Traian Cicone and Yann Henry

Lubricants 2025, 13(8), 346; https://doi.org/10.3390/lubricants13080346 - 3 Aug 2025

Abstract

Aerostatic bearings were proven to be an optimal choice in situations where low friction, cleanliness, and high motion accuracy are required. Their functionality relies heavily on flow restrictors, which are responsible for regulating and controlling the supply flow, and consequently, the thickness and [...] Read more.

Aerostatic bearings were proven to be an optimal choice in situations where low friction, cleanliness, and high motion accuracy are required. Their functionality relies heavily on flow restrictors, which are responsible for regulating and controlling the supply flow, and consequently, the thickness and stiffness of the fluid film. A diverse range of restrictors with varying characteristics is used, among which are the porous restrictors. The current work introduces a novel solution involving a porous, highly compressible restrictor, whose element of novelty compared to its predecessors consists of its variable thickness and corresponding permeability, regulated by the load on the bearing. The gas is supplied through an annular, elastic, deformable, porous disc, which is compressed by a metal plate, subjected to compression by the recess pressure on one side and by the supply pressure on the other side. One or more springs are used in parallel with the porous disc to obtain the optimum elastic response. The objective of this study is to evaluate the performance characteristics and compare them to a conventional restrictor. A parametric analysis is performed to define the size and properties of the porous restrictor. Full article

(This article belongs to the Special Issue Advances in Lubricated Bearings, 2nd Edition)

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15 pages, 1258 KiB

Open AccessArticle

Synthesis and Evaluation of Sunflower-Oil-Based Esters as Biolubricant Base Oils Using Ca/TEA Alkoxide Catalyst

by Dimosthenis Filon, George Anastopoulos and Dimitrios Karonis

Lubricants 2025, 13(8), 345; https://doi.org/10.3390/lubricants13080345 - 2 Aug 2025

Abstract

This study evaluates the production of base oils for biolubricants using fatty acid methyl esters (FAMEs) derived from sunflower oil as the raw material. The production process involved the synthesis of oleochemical esters through a single-step alkaline transesterification reaction with a high-molecular-weight polyol, [...] Read more.

This study evaluates the production of base oils for biolubricants using fatty acid methyl esters (FAMEs) derived from sunflower oil as the raw material. The production process involved the synthesis of oleochemical esters through a single-step alkaline transesterification reaction with a high-molecular-weight polyol, such as trimethylolpropane (TMP). To assess the effectiveness of the developed catalytic system in conducting the transesterification reactions and its impact on the properties of the final product, two types of alkaline catalysts were used. Specifically, the reactions were carried out using either Ca/TEA alkoxide or sodium methoxide as catalysts in various configurations and concentrations to determine the optimal catalyst concentration and reaction conditions. Sodium methoxide served as the commercial benchmark catalyst, while the Ca/TEA alkoxide was prepared in the laboratory. The optimal concentration of Ca/TEA was determined to be 3.0% wt. in the presence of iso-octane and 3.5% wt. under vacuum, while the corresponding concentrations of CH₃ONa for both cases were determined to be 2.0% wt. The synthesized biolubricant esters exhibit remarkable performance characteristics, such as high kinematic viscosities and low pour points—ranging from 33–48 cSt at 40 °C, 7.68–10.03 cSt at 100 °C, to −14 to −7 °C, respectively—which are comparable to or improved over those of mineral oils such as SN-150 or SN-500, with the Ca/TEA alkoxide-catalyzed systems showing superior oxidation stability and reduced byproduct formation. Full article

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

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22 pages, 24500 KiB

Open AccessArticle

Ambient to Elevated Temperature: Ecotribology of Water-Based Lubricants Incorporating hBN/TiO₂ Nanoadditives

by Afshana Morshed, Fei Lin, Hui Wu, Zhao Xing, Sihai Jiao and Zhengyi Jiang

Lubricants 2025, 13(8), 344; https://doi.org/10.3390/lubricants13080344 - 1 Aug 2025

Abstract

Ecotribology focuses on both saving energy resources and reducing environmental pollution. Considering environmental concerns, water-based nanolubricants have gained significant attention over conventional oil-based ones. Non-ecotoxic and highly environmentally friendly nanoadditives were chosen for nanolubricant synthesis, especially considering their use at elevated temperatures. In [...] Read more.

Ecotribology focuses on both saving energy resources and reducing environmental pollution. Considering environmental concerns, water-based nanolubricants have gained significant attention over conventional oil-based ones. Non-ecotoxic and highly environmentally friendly nanoadditives were chosen for nanolubricant synthesis, especially considering their use at elevated temperatures. In this study, hexagonal boron nitride nanosheets (hBNNSs) and titanium dioxide nanoparticles (TiO₂ NPs) were used to prepare water-based lubricants with glycerol and surfactant sodium dodecyl benzene sulfonate (SDBS) in water under ultrasonication. An Rtec ball-on-disk tribometer was used to investigate the tribological performance of the synthesised water-based lubricants containing different nano-hBN/TiO₂ concentrations, with dry and water conditions used as benchmarks. The results indicated that the water-based nanolubricant containing 0.5 wt% hBN and 0.5 wt% TiO₂ exhibited the best tribological performance at both ambient (25 °C) and elevated (500 °C) temperatures. This optimal concentration leads to a reduction in the coefficient of friction (COF) by 72.9% and 37.5%, wear of disk by 62.5% and 49%, and wear of ball by 74% and 69% at ambient and elevated temperatures, respectively, compared to that of distilled water. Lubrication mechanisms were attributed to the rolling, mending, tribofilm, solid layer formation, and synergistic effects of hBNNSs and TiO₂ NPs. Full article

(This article belongs to the Special Issue Tribology in Manufacturing Engineering)

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19 pages, 4045 KiB

Open AccessArticle

Response Surface Optimization Design for High-Speed Ball Bearing Double-Lip Seals Considering Wear Characteristics

by Hengdi Wang, Yulu Yue, Yongcun Cui, Lina Lou and Chang Li

Lubricants 2025, 13(8), 343; https://doi.org/10.3390/lubricants13080343 - 1 Aug 2025

Abstract

This paper focuses on the sealing failure problem of double-lip seal rings for high-speed ball bearings used in unmanned aerial vehicles. By using ANSYS 2023R1 software, a thermal–stress–wear coupled finite element model was established. Taking the contact pressure and volume loss due to [...] Read more.

This paper focuses on the sealing failure problem of double-lip seal rings for high-speed ball bearings used in unmanned aerial vehicles. By using ANSYS 2023R1 software, a thermal–stress–wear coupled finite element model was established. Taking the contact pressure and volume loss due to wear as indicators to evaluate sealing performance, this study analyzed the influence of lip seal structural parameters on sealing performance, performed response surface optimization of the seal structure parameters and conducted a comparative test on lip seals before and after optimization. The research results show that the contact pressure at the main lip of the lip seal was the greatest, which was 0.79 MPa, and the volume loss due to wear lip seal was 7.94 × 10⁻⁷ mm³. Optimal sealing performance is achieved when the seal lip inclination angle is 41.68°, the middle width of the lip seal is 0.153 mm, the main lip height is 0.179 mm, the spring center distance is 0.37 mm and the radial interference is 0.0034 mm. After optimization, the grease leakage rate of the sealing ring decreased by 48% compared to before optimization. Full article

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25 pages, 5020 KiB

Open AccessReview

Research Progress on Tribological Properties of High-Entropy Alloys

by Shuai Zhang, Zhaofeng Wang, Wenqing Lin and Haoyu Guo

Lubricants 2025, 13(8), 342; https://doi.org/10.3390/lubricants13080342 - 1 Aug 2025

Abstract

As a new type of alloy system composed of five or more principal components, high-entropy alloys demonstrate outstanding comprehensive performance in the field of friction and wear through the synergistic effects of the high-entropy effect, lattice distortion effect, hysteresis diffusion effect and cocktail [...] Read more.

As a new type of alloy system composed of five or more principal components, high-entropy alloys demonstrate outstanding comprehensive performance in the field of friction and wear through the synergistic effects of the high-entropy effect, lattice distortion effect, hysteresis diffusion effect and cocktail effect. This paper systematically reviews the research progress on the friction and wear properties of high-entropy alloys. The mechanisms of metal elements such as Al, Ti, Cu and Nb through solid solution strengthening, second-phase precipitation and oxide film formation were analyzed emphatically. And non-metallic elements such as C, Si, and B form and strengthen the regulation laws of their tribological properties. The influence of working conditions, such as high temperature, ocean, and hydrogen peroxide on the friction and wear behavior of high-entropy alloys by altering the wear mechanism, was discussed. The influence of test conditions such as load, sliding velocity and friction pair matching on its friction coefficient and wear rate was expounded. It is pointed out that high-entropy alloys have significant application potential in key friction components, providing reference and guidance for the further development and application of high-entropy alloys. Full article

(This article belongs to the Special Issue Tribological Performance of High-Entropy Alloys)

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