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Brake Wear Particle Emissions from Dry-Running Friction Systems: Influence of Operating Parameters and Friction Pairing Based on an Application-Oriented Extended Measurement Methodology
Influence of TiO₂ Additive on the Tribological Performance of Bonded MoS₂ Solid Lubricants
Low Adhesion Due to the Wet-Rail Phenomenon: Influence of Particle–Fluid Interaction in Wheel–Rail Contact
Automated Detection of Tribologically Relevant Brake Torque Plateaus: A Two-Stage Approach for Flywheel Dynamometer Testing
Visualization of Lubrication Conditions Using the Electrical Impedance Method Considering Surface Roughness

Journal Description

Lubricants

Lubricants is an international, peer-reviewed, open access journal on tribology, published monthly online by MDPI.

Open Access— free for readers, with article processing charges (APC) paid by authors or their institutions.
High Visibility: indexed within Scopus, SCIE (Web of Science), Ei Compendex, Inspec, CAPlus / SciFinder, and other databases.
Journal Rank: JCR - Q2 (Engineering, Mechanical) / CiteScore - Q2 (Mechanical Engineering)
Rapid Publication: manuscripts are peer-reviewed and a first decision is provided to authors approximately 13 days after submission; acceptance to publication is undertaken in 2.6 days (median values for papers published in this journal in the first half of 2026).
Recognition of Reviewers: reviewers who provide timely, thorough peer-review reports receive vouchers entitling them to a discount on the APC of their next publication in any MDPI journal, in appreciation of the work done.

Impact Factor: 3.6 (2025); 5-Year Impact Factor: 3.7 (2025)

Imprint Information Journal Flyer Open Access ISSN: 2075-4442

Latest Articles

17 pages, 8404 KB

Open AccessArticle

Tribological Properties of Additively Manufactured Aluminum Alloy Subjected to Surface Mechanical Attrition Treatment

by Yong Sun, Congcong Zhao, Huaping Tang and Yunjie Bi

Lubricants 2026, 14(9), 342; https://doi.org/10.3390/lubricants14090342 - 4 Sep 2026

To improve the surface quality of an aluminum alloy manufactured by additive manufacturing (AM), mechanical attrition treatment (SMAT) has been applied to the as-fabricated surface of the alloy produced by selective laser melting (SLM). The morphology and properties of the SMAT-treated surfaces have [...] Read more.

To improve the surface quality of an aluminum alloy manufactured by additive manufacturing (AM), mechanical attrition treatment (SMAT) has been applied to the as-fabricated surface of the alloy produced by selective laser melting (SLM). The morphology and properties of the SMAT-treated surfaces have been investigated and detailed friction and wear tests have been conducted to evaluate the tribological behavior of the SMAT specimens under both dry and oil-lubricated conditions. The results demonstrate that SMAT is effective in improving the surface finish of the alloy by up to 87%, which also results in an increase in surface hardness of 23% to 29%. Ball-on-disc reciprocating wear tests show that under dry sliding conditions, SMAT for 10 min is effective in improving the wear resistance of the alloy by 25% to 65%, while increasing the SMAT time to 20 min and 30 min results in deteriorated wear resistance as compared to the as-SLM surface. However, under oil-lubricated conditions, SMAT for various times from 10 min to 30 min is effective in improving the wear resistance of the alloy by a factor of 2 to 3.5, depending on the SMAT time and contact loads. The results are discussed considering surface finish enhancement, surface and subsurface hardening effects and surface damage caused by SMAT. Full article

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15 pages, 2726 KB

Open AccessArticle

Theoretical Study on the Rheology-Driven Lubrication Synergy of Gel on Textured High-Entropy Alloy Coatings

by Yazhou Mao, Linlin Guo, Anzixuan Wang, Runyi Ma, Pengfei Gao and Aoya Wang

Lubricants 2026, 14(9), 341; https://doi.org/10.3390/lubricants14090341 - 2 Sep 2026

This work is a theoretical study aimed to address the lubrication failure of Tom-Pac TP-2557 gel lubricant on the textured high-entropy alloy coatings (THEACs) under wide-temperature-range operating conditions; the rheological lubrication properties of the surface are investigated in this work. Based on lubrication [...] Read more.

This work is a theoretical study aimed to address the lubrication failure of Tom-Pac TP-2557 gel lubricant on the textured high-entropy alloy coatings (THEACs) under wide-temperature-range operating conditions; the rheological lubrication properties of the surface are investigated in this work. Based on lubrication theory and non-Newtonian fluid mechanics, a gel lubrication viscosity model considering temperature dependence and a thermo-mechanical coupled constitutive relationship for the THEACs are established. The results show a significant shear-thinning behavior of the gel within a moderate low-to-medium temperature range, and the onset temperature of thermal degradation is identified. Optimal geometrical and distributional parameters of the surface textures, along with a favorable surface energy range, are determined to achieve desirable interfacial shear strength. Moreover, an anchoring-slip synergistic mode arising from surface energy heterogeneity is found to further enhance lubricating film stability. This research provides a theoretical basis for the gel lubrication design of the THEACs. Full article

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32 pages, 9540 KB

Open AccessArticle

Study on the Seepage Behavior of the Porous Oil-Containing Polyimide Cages in Bearings for Space-Craft Attitude Control Systems

by Wenhu Zhang, Shuanglin Wang, Chunwei Li, Lingzhi Chai and Wanjia Li

Lubricants 2026, 14(9), 340; https://doi.org/10.3390/lubricants14090340 - 1 Sep 2026

Porous oil-containing polyimide cages are critical for ensuring long-life lubrication in momentum wheel bearings of spacecraft attitude control systems. However, the underlying microscopic seepage mechanisms governing lubricant release under operational conditions remain inadequately understood, posing challenges for precision lubrication design. This study establishes [...] Read more.

Porous oil-containing polyimide cages are critical for ensuring long-life lubrication in momentum wheel bearings of spacecraft attitude control systems. However, the underlying microscopic seepage mechanisms governing lubricant release under operational conditions remain inadequately understood, posing challenges for precision lubrication design. This study establishes a three-dimensional stochastic pore structure model of the cage using the Quartet Structure Generation Set (QSGS) method, from which a Representative Elementary Volume (REV) is extracted. A thermo-hydro-mechanical coupled simulation model is developed within the COMSOL 6.3 Multiphysics platform to investigate the seepage behaviorp. The effects of key operational parameters—rotational speed and thermal gradients—combined with the structural parameter of porosity on the lubricant transport characteristics are systematically elucidated. Based on the apparent outflow rate calculated from the REV model, a simplified formulation for estimating the oil throw-off rate of the porous cage is proposed and experimentally validated. The findings provide a fundamental theoretical framework and a practical design tool for optimizing the lubrication performance of porous cages in high-precision aerospace bearings. Full article

16 pages, 16741 KB

Open AccessArticle

Ultralow Friction in Graphene Multilayers with Amorphous Carbon Interlayers

by Liping Liao, Ting Liu, Daiying Yuan and Zhongnan Wang

Lubricants 2026, 14(9), 339; https://doi.org/10.3390/lubricants14090339 - 1 Sep 2026

Amorphous carbon layers (ACLs) have been experimentally realized as a two-dimensional carbon material, but their influence on sliding in graphene multilayers is still unclear. In this study, molecular dynamics simulations were carried out for pristine graphene (PG) multilayers and graphene/amorphous-carbon (AG) multilayers under [...] Read more.

Amorphous carbon layers (ACLs) have been experimentally realized as a two-dimensional carbon material, but their influence on sliding in graphene multilayers is still unclear. In this study, molecular dynamics simulations were carried out for pristine graphene (PG) multilayers and graphene/amorphous-carbon (AG) multilayers under normal pressures of 0.1, 1, and 10 GPa. Both models show ultralow friction. The coefficient of friction (COF) decreases with increasing pressure because the friction force increases much more slowly than the normal force. AG has lower COF than PG at all pressures, with reductions of about 60%, 54%, and 26%, respectively. Layer displacement and interlayer sliding analyses show that shear in PG is transferred through several graphene/graphene interfaces, whereas relative sliding in AG is mainly located near the ACLs. Under high pressure, the ACLs become flatter, which may help stabilize local sliding. These results suggest that amorphous carbon interlayers may modify shear transfer and facilitate ultralow-friction sliding in graphene-based multilayers under the present simulation conditions. Full article

(This article belongs to the Special Issue Superlubricity Mechanisms and Applications)

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24 pages, 13385 KB

Open AccessArticle

Film Thickness Evolution Behavior and Mechanism of Lubricating Grease at High Speeds

by Lingtong Sun, Wenzhong Wang, Zhi Chen, Jianlong Liu and He Liang

Lubricants 2026, 14(9), 338; https://doi.org/10.3390/lubricants14090338 - 1 Sep 2026

In most cases, lubricating grease serves as the primary lubricating medium for rolling bearings, ensuring normal operation by facilitating lubrication between bearing components. Current experimental research on grease lubrication predominantly focuses on low-speed conditions (below 2 m/s), which no longer aligns with the [...] Read more.

In most cases, lubricating grease serves as the primary lubricating medium for rolling bearings, ensuring normal operation by facilitating lubrication between bearing components. Current experimental research on grease lubrication predominantly focuses on low-speed conditions (below 2 m/s), which no longer aligns with the increasingly high operational speeds of rolling bearings. To address this gap, we conduct the high-speed lubrication experiments with a maximum speed up to 10.68 m/s using a ball-on-ring test rig under a certain amount of grease supply. The results reveal a clear distinction in film thickness evolution between high-speed and low-speed stages: at low speed, the film thickness gradually decreases with the number of ring revolutions until it stabilizes after the grease reservoir reformation; however, at high speed, a significant recovery in film thickness occurs following reservoir reformation. Observations of grease distribution further indicate that, under shear forces, the thickener on both sides of the track undergoes shear-induced breakdown and becomes uniformly distributed. This intensified shear promotes the formation of larger grease reservoirs along the contact sides and leads to the recovery of film thickness under high-speed conditions. The amount of effectively sheared grease that ultimately participates in lubrication can be characterized by the width of the grease ridge. Full article

(This article belongs to the Special Issue New Advances in Lubrication Film Detection)

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21 pages, 15572 KB

Open AccessArticle

Load-Dependent Transition in Friction-Induced Vibration Responses of Water-Lubricated Bearings Under Low-Speed Conditions

by Gengyuan Gao, Meng Kong, Shijie Yu and Xiuli Zhang

Lubricants 2026, 14(9), 337; https://doi.org/10.3390/lubricants14090337 - 29 Aug 2026

Water-lubricated bearings (WLBs) may exhibit marked friction-induced vibration during low-speed and heavy-load operation as hydrodynamic lubrication becomes insufficient. However, the load dependence of the low-speed operating limit and associated vibration characteristics remains insufficiently understood. In this study, a WLB was tested under specific [...] Read more.

Water-lubricated bearings (WLBs) may exhibit marked friction-induced vibration during low-speed and heavy-load operation as hydrodynamic lubrication becomes insufficient. However, the load dependence of the low-speed operating limit and associated vibration characteristics remains insufficiently understood. In this study, a WLB was tested under specific pressures of 0.28, 0.42, 0.56, and 0.84 MPa during stepwise deceleration from 20 to 6 r/min. A joint three-standard-deviation criterion based on root mean square and peak-to-peak acceleration was used to identify the first measured speed point with marked vibration amplification. The coefficient of friction, time-domain features, spectral energy distribution, envelope characteristics, and FSI-based limiting hydrodynamic capacity were analyzed. The first vibration amplification points occurred at 6, 8, 10, and 10 r/min, respectively, accompanied by audible abnormal sound used only as qualitative corroboration. The onset responses varied from isolated or repeated bursts to pronounced medium-high-frequency impulsive excitation and quasi-periodic low-frequency amplitude modulation. The limiting hydrodynamic capacity calculated at a prescribed eccentricity ratio decreased with increasing load and was lower at the onset condition than at the adjacent pre-onset condition. This vibration-based framework provides an operational method for identifying low-speed operating boundaries related to loads and may support operating-condition selection and early warning of abnormal vibration in WLB systems. Full article

(This article belongs to the Special Issue Green Water-Lubricated Bearings)

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36 pages, 29727 KB

Open AccessArticle

A Unified Transient EHL-Based Framework for Determining Contact Stiffness and Damping for Application in Spur Gear Dynamics

by Tom Caston, Nader Dolatabadi and Ramin Rahmani

Lubricants 2026, 14(9), 336; https://doi.org/10.3390/lubricants14090336 - 29 Aug 2026

Time-varying mesh stiffness (TVMS) is a primary contributor to gear noise, vibration and harshness (NVH). Accurate implementation of the lubrication effect on the TVMS is often neglected. This work presents a numerical lubricated contact stiffness extraction method by combining the lubricant film and [...] Read more.

Time-varying mesh stiffness (TVMS) is a primary contributor to gear noise, vibration and harshness (NVH). Accurate implementation of the lubrication effect on the TVMS is often neglected. This work presents a numerical lubricated contact stiffness extraction method by combining the lubricant film and elastic compliances through rigid-body separation for spur gears. A deformation datum for infinite line contacts is defined using Weber’s local elastic deformation of gear teeth, resolving inconsistent treatments in the literature. A novel apparent transient damping coefficient is extracted using transient multigrid elastohydrodynamic (EHL) simulations. At high load and low speed, the EHL contact stiffness approaches the analytical local elastic stiffness, whereas at low load and high speed, it deviates by up to an order of magnitude. The stiffness linearisation is compared directly against fixed-separation EHL solutions, and force reconstruction confirms recovery of the full transient contact force. The results show that lubricant film compliance can significantly alter the EHL contact stiffness under low-load/high-speed conditions, which is relevant to dynamic mesh force fluctuations in high-speed transmissions. Thus, accurate lubricant effects must be included for reliable NVH predictions in such cases. Full article

(This article belongs to the Special Issue Modeling and Simulation of Elastohydrodynamic Lubrication)

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23 pages, 21469 KB

Open AccessArticle

Investigation of Tribological Performance in Multi-Stage Cold Forging Using Combined Upsetting—Extrusion Type Tribotests

by Kosuke Furukawa, Yoshihiro Kubota, Yuki Shimomura and Kunio Hayakawa

Lubricants 2026, 14(9), 335; https://doi.org/10.3390/lubricants14090335 - 28 Aug 2026

Multi-stage cold forging processes are widely adopted in actual industrial manufacturing of complex components. However, the influence of deformation history on lubrication behavior during such processes has not been fully clarified. This study investigates the friction coefficient at the die–workpiece interface in a [...] Read more.

Multi-stage cold forging processes are widely adopted in actual industrial manufacturing of complex components. However, the influence of deformation history on lubrication behavior during such processes has not been fully clarified. This study investigates the friction coefficient at the die–workpiece interface in a multi-stage cold forging process consisting of upsetting followed by forward extrusion, with a focus on the lubrication performance of a zinc phosphate coating. Two types of multi-stage tribotests were conducted: one in which upsetting was followed by a forward rod–backward can extrusion type tribotest, and another in which upsetting was followed by a forward and backward can extrusion type tribotest. As a result, the evaluated friction coefficients were comparable, and their 99% confidence intervals overlapped. These values were higher than those obtained from the corresponding single-stage forward rod–backward can extrusion tribotest, indicating that prior upsetting affects the lubrication state during subsequent extrusion. Although localized galling and coating damage were observed, no extensive seizure occurred, suggesting that the lubricant coating retained a certain degree of effectiveness after upsetting. Furthermore, a comparison of the tribological conditions in each tribotest reveals that, although the contact pressure is at a comparable level, differences are observed in workpiece temperature, surface expansion ratio, and relative sliding velocity. Therefore, under the conditions of the present study, it is suggested that the effects of workpiece temperature, surface expansion ratio, and relative sliding velocity on the evaluated friction coefficient are limited. Full article

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26 pages, 4777 KB

Open AccessReview

A Review of Gear Wear and Transmission System Dynamics: Coupling Mechanisms, Influencing Factors, and Profile Modification Strategies

by Weichao Liu, Huijun Yue, Yaoting Wu and Jiachun Lin

Lubricants 2026, 14(9), 334; https://doi.org/10.3390/lubricants14090334 - 27 Aug 2026

Gear wear is a tribological surface damage process caused by contact loading and relative motion between meshing tooth flanks. It involves progressive material removal or transfer, changes tooth-flank topography, and consequently affects contact conditions, mesh stiffness, transmission error, and dynamic response. This paper [...] Read more.

Gear wear is a tribological surface damage process caused by contact loading and relative motion between meshing tooth flanks. It involves progressive material removal or transfer, changes tooth-flank topography, and consequently affects contact conditions, mesh stiffness, transmission error, and dynamic response. This paper reviews recent advances in gear wear research, including typical tooth-surface damage, wear prediction under various lubrication conditions, tribodynamic behavior, dynamic effects of wear, and the influence of assembly errors, parameter uncertainties, and gear modification on meshing characteristics. Existing studies have progressed from isolated descriptions of wear to integrated analyses involving lubrication, surface condition, and dynamics. Nevertheless, the long-term bidirectional coupling between wear evolution and tribodynamics remains insufficiently understood, while the effects of assembly errors and multi-source uncertainties have received limited attention. Gear modification studies have also focused mainly on initial transmission performance rather than its sustained role during wear degradation. Future research should therefore establish coupled wear–friction dynamic models and develop wear analysis methods that account for actual assembly conditions and parameter uncertainties. Full article

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18 pages, 6442 KB

Open AccessArticle

Unveiling the Lubrication Mechanism of Graphene Coating on PEEK: A Molecular Dynamics Study Under Dry Friction and Seawater

by Xiang Jiao, Guochen Huang, Yiqin Wang, Chenchen Peng and Guoqing Wang

Lubricants 2026, 14(9), 333; https://doi.org/10.3390/lubricants14090333 - 27 Aug 2026

Graphene coatings have considerable potential to improve the tribological performance of polyetheretherketone (PEEK) under dry friction conditions. For PEEK components operating in marine environments, direct exposure to seawater introduces water molecules and dissolved ions into the sliding interface, which may alter interfacial interactions [...] Read more.

Graphene coatings have considerable potential to improve the tribological performance of polyetheretherketone (PEEK) under dry friction conditions. For PEEK components operating in marine environments, direct exposure to seawater introduces water molecules and dissolved ions into the sliding interface, which may alter interfacial interactions and lubrication behavior. However, how seawater alters the lubrication mechanism and protective effect of graphene at the PEEK interface remains poorly understood. To address this issue, molecular dynamics simulations were performed to compare PEEK with and without graphene under dry and seawater conditions, thereby revealing the interfacial deformation and molecular response. The results show that graphene preserved the structural integrity of PEEK, reduced its tangential displacement by more than 99%, and decreased the shear stress transmitted into the polymer matrix by approximately 85% to 90%. In the absence of graphene, seawater reduced the tangential mean square displacement of PEEK by approximately 20%, while increasing the normal mean square displacement by about 17%, indicating that seawater redistributed part of the polymer motion from tangential dragging toward normal and local molecular rearrangement. The combined analysis shows that graphene mainly limits the penetration of sliding loads into PEEK, whereas seawater regulates the direction and spatial distribution of the remaining deformation. This work provides a molecular basis for the design of graphene-protected PEEK interfaces in marine friction applications. Full article

(This article belongs to the Special Issue Molecular Dynamics Simulations in Tribology)

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19 pages, 31917 KB

Open AccessArticle

Experimental Study on the Dynamic Characteristics of Needle Roller Bearings Under Periodic Impact Load

by Baogang Wen, Libin Xuan, Zhihao Zan, Xu Zhang and Jingyu Zhai

Lubricants 2026, 14(9), 332; https://doi.org/10.3390/lubricants14090332 - 26 Aug 2026

Needle roller bearings are characterized by rolling elements with relatively high length-to-diameter ratios and are widely used in mechanical systems with limited radial installation space. In gear transmission systems, periodic impact loads induced by gear meshing may be superimposed on steady radial loads, [...] Read more.

Needle roller bearings are characterized by rolling elements with relatively high length-to-diameter ratios and are widely used in mechanical systems with limited radial installation space. In gear transmission systems, periodic impact loads induced by gear meshing may be superimposed on steady radial loads, thereby altering the dynamic response of the bearing. However, the effects of the amplitude and frequency of periodic impact loading on the dynamic characteristics of needle roller bearings remain insufficiently understood. In this study, a needle roller bearing test rig capable of applying periodic impact loading was developed, and a multi-sensor measurement system was configured to measure outer-ring vibration, inner-ring motion, cage motion, and the friction torque of the bearing system. Dynamic tests were conducted under different amplitudes and frequencies of periodic impact loading. A reduction in bearing motion stability was observed under periodic impact loading, as evidenced by increased outer-ring vibration and enlarged cage motion in both the horizontal and vertical directions. As the loading amplitude increased, the RMS values of outer-ring acceleration, inner-ring displacement, and cage displacement increased, while an increase in the mean friction torque was also observed. The inner-ring trajectory expanded along the loading direction, and the whirling range of the cage trajectory increased. As the loading frequency increased, the RMS values of outer-ring acceleration, inner-ring displacement, and cage displacement increased, and the mean friction torque increased. In contrast, the whirling range of the cage trajectory decreased. These findings clarify the distinct effects of periodic impact-loading amplitude and frequency on bearing vibration, internal motion, and friction-torque characteristics and provide experimental support for the dynamic performance evaluation of needle roller bearings under impact conditions. Full article

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11 pages, 8731 KB

Open AccessArticle

Ion Correlation Enhances Macroscale Boundary Lubrication

by Renshan Xia, Zhi Xu, Jiaoyan Ma, Xiaoming Zong, Shangchu Yang, Yanyan Wang, Han Li and Ming Ma

Lubricants 2026, 14(9), 331; https://doi.org/10.3390/lubricants14090331 - 24 Aug 2026

While ion correlation is known to enhance molecular-scale solvation forces, its capacity to improve macroscale boundary lubrication on engineering surfaces remains unverified. This study demonstrates that multivalent electrolyte-induced ion correlation significantly reduces macroscopic boundary friction, achieving up to a 67% reduction on alumina [...] Read more.

While ion correlation is known to enhance molecular-scale solvation forces, its capacity to improve macroscale boundary lubrication on engineering surfaces remains unverified. This study demonstrates that multivalent electrolyte-induced ion correlation significantly reduces macroscopic boundary friction, achieving up to a 67% reduction on alumina surfaces. This macroscopic enhancement is driven by interfacial electrochemical properties, where highly charged polar oxide interfaces trigger strong electrostatic correlation to restructure confined solvents into a rigid, load-bearing barrier. Ultimately, this work proves that interfacial ion correlation directly dictates and enhances macroscale boundary lubrication, bridging molecular-level force control with practical tribological applications. Full article

(This article belongs to the Special Issue Superlubricity Mechanisms and Applications)

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27 pages, 7057 KB

Open AccessArticle

An Adhesive Wear Model for Gears in Mixed Elastohydrodynamic Lubrication

by Hongbing Wang, Wei Shi, Yuping Wu, Xingming Chen, Jie Su, Lairong Yin and Bo Hu

Lubricants 2026, 14(9), 330; https://doi.org/10.3390/lubricants14090330 - 23 Aug 2026

In this study, an adhesive wear model for a gear drive in mixed elastohydrodynamic lubrication (EHL) is proposed. The mixed-EHL model combines the average Reynolds equation with the ZMC rough-surface contact model to determine the asperity contact pressure. By incorporating the fractional film [...] Read more.

In this study, an adhesive wear model for a gear drive in mixed elastohydrodynamic lubrication (EHL) is proposed. The mixed-EHL model combines the average Reynolds equation with the ZMC rough-surface contact model to determine the asperity contact pressure. By incorporating the fractional film defect into the Archard wear equation, a wear rate model under mixed EHL is developed and verified against published experimental data. This wear-rate model is subsequently coupled with a transient line-contact mixed-EHL model for gears to establish a tooth-surface wear prediction model that iteratively updates the tooth geometry and contact pressure. The evolution of tooth-surface wear under mixed EHL is investigated, and the resulting wear characteristics are compared with those under dry-contact conditions. The influence of tooth-surface roughness is also systematically evaluated. The results indicate that tooth-surface wear is substantially reduced under mixed EHL and that the maximum wear occurs between the lowest point of single-tooth contact and the pitch point. Increasing surface roughness intensifies wear and shifts the maximum-wear location toward the tooth root. These findings demonstrate that appropriate lubricant selection and effective control of tooth-surface roughness are important for improving the wear resistance of gear drives. Full article

(This article belongs to the Special Issue High-Performance Lubrication and Surface/Interface Technologies for Advanced Mechanical Transmission Systems)

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16 pages, 3257 KB

Open AccessArticle

Effect of Cu/Cu₂O/CuO Nanopowder Additives on the Antiwear Performance of Industrial Mineral Oil

by Shunqi Mei, Andrey Nomoev, Erzhena Khartaeva, Undrakh Mishigdorzhiyn, Sergei Nomoev, Sayan Badmaev and Bair Garmaev

Lubricants 2026, 14(9), 329; https://doi.org/10.3390/lubricants14090329 - 22 Aug 2026

This study evaluates the effect of copper-containing composite nanopowders on the antiwear properties of additive-free I-20A mineral oil. The powders were produced by the same electron-beam evaporation route in argon under different current–time regimes and differed in phase composition and particle characteristics. N1 [...] Read more.

This study evaluates the effect of copper-containing composite nanopowders on the antiwear properties of additive-free I-20A mineral oil. The powders were produced by the same electron-beam evaporation route in argon under different current–time regimes and differed in phase composition and particle characteristics. N1 was synthesized at 1.4 MeV and 20 mA for 15 min and contained 91 wt.% Cu, 2 wt.% Cu₂O, and 7 wt.% CuO (mean particle size 140 nm), whereas N2 was synthesized at 1.4 MeV and 15 mA for 25 min and contained 38 wt.% Cu, 48 wt.% Cu₂O, and 14 wt.% CuO (187 nm). Six separate oil suspensions containing 0.01, 0.1, or 1 wt.% N1 or N2 were evaluated in block-on-ring tests; three independent tests were performed for each lubricant condition (21 individual measurements in total). The lowest mean mass loss was obtained with 1 wt.% N2 (0.00290 ± 0.00015 g), representing a 43.1% decrease relative to pure I-20A oil (0.00510 ± 0.00026 g; Holm-adjusted p = 0.0045). The 0.01 and 0.1 wt.% N2 formulations did not differ significantly from pure oil, whereas 0.01 and 1 wt.% N1 significantly increased mass loss. In selected SEM/EDS regions, the wear scar produced with 1 wt.% N1 showed deeper longitudinal grooves and no detectable Cu, whereas the scar produced with 1 wt.% N2 showed a smoother local morphology and 0.58 wt.% Cu. These local observations are consistent with different particle–surface interactions and greater local retention of Cu-containing material for N2, but they do not identify the copper oxidation state or prove formation of a continuous tribofilm. Overall, the powders exhibited formulation- and concentration-dependent antiwear behavior: 1 wt.% N2 was beneficial, whereas 1 wt.% N1 was strongly detrimental. Because the synthesis regime, phase composition, particle size, and morphology varied together, the observed difference cannot be attributed exclusively to the Cu/Cu₂O/CuO ratio. Full article

(This article belongs to the Special Issue Preparation, Tribological Behavior, and Applications of Lubricant Additives, 2nd Edition)

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28 pages, 3482 KB

Open AccessArticle

Wear Prediction of Cylindrical Gears Based on Deep Neural Networks

by Jiachun Lin, Xudong Zhao, Huijun Yue, Yunjin Xiang, Peng Wang, Minghui Tu and Ulf Olofsson

Lubricants 2026, 14(8), 328; https://doi.org/10.3390/lubricants14080328 - 21 Aug 2026

Gears serve as core transmission components, and their wear evolution directly affects equipment stability and service life under long-duration complex loading. Especially under complex loading and long-term service conditions, the tooth surface topography undergoes continuous evolution. However, traditional wear prediction methods based on [...] Read more.

Gears serve as core transmission components, and their wear evolution directly affects equipment stability and service life under long-duration complex loading. Especially under complex loading and long-term service conditions, the tooth surface topography undergoes continuous evolution. However, traditional wear prediction methods based on physical models or empirical formulas have significant limitations in addressing nonlinear problems involving multiple coupled variables. This study proposes a deep neural network (DNN)-based method for gear wear prediction. Geometric parameters, loading conditions, and surface topography characteristics are integrated as model inputs to enable point-by-point prediction of tooth-profile wear. Experimental results demonstrate that the proposed model achieves excellent predictive performance in the mild-wear regime, with a mean absolute error (MAE) below 2.5 × 10⁻⁴ mm, a root mean square error (RMSE) below 5.0 × 10⁻⁴ mm, and R² values ranging from 0.92 to 0.99. The model also achieves satisfactory prediction accuracy at previously unseen measurement positions and for previously unseen superfinished gear samples. The proposed DNN effectively learns implicit wear-evolution patterns from experimental data and exhibits strong generalization capability, providing a practical approach for gear health monitoring and predictive maintenance. Full article

(This article belongs to the Special Issue Advanced Gear Tribology)

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58 pages, 6331 KB

Open AccessReview

Eco-Friendly Production of Sustainable Bio-Based Lubricants with Green-Synthesized Nanoparticles

by Raj Shah, Brandon Juran and Stefanos Nitodas

Lubricants 2026, 14(8), 327; https://doi.org/10.3390/lubricants14080327 - 21 Aug 2026

Eco-friendly lubricant manufacturing focuses on using biodegradable, renewable base oils, such as waste oil, and green-synthesized nanomaterials, such as agricultural residue-based nanoparticles, in order to replace conventional toxic lubricants with biodegradable alternatives and reduce environmental impact. These nanolubricants are produced through low-energy methods [...] Read more.

Eco-friendly lubricant manufacturing focuses on using biodegradable, renewable base oils, such as waste oil, and green-synthesized nanomaterials, such as agricultural residue-based nanoparticles, in order to replace conventional toxic lubricants with biodegradable alternatives and reduce environmental impact. These nanolubricants are produced through low-energy methods like mechanical, microwave, or chemical-free synthesis that can result in improvement in their performance. The produced lubricants exhibit enhanced tribological properties, including reduced friction and wear. Several formulations also raise the thermal degradation onset above that of their mineral benchmarks, although oxidative stability remains the weakest property of bio-based systems. This approach addresses industrial demands for sustainable, cost-effective, and environmentally compliant lubrication technologies. Our study reviews sustainable, eco-friendly synthesis methods for producing high-performance nanolubricants from different waste oils, including cooking and mineral oils. In addition to the incorporation of residue-based nanoparticles (e.g., eggshell) in waste oils, the performance and properties of bio-based lubricants with engineered nanoparticles, such as metal nano-oxides and carbon-based nanomaterials, are also reviewed for comparison purposes. Within bio-based fluids at moderate contact severity, residue-derived additives are found to match their engineered counterparts in terms of friction and wear, while engineered two-dimensional lamellar additives retain an advantage under extreme-pressure conditions where residue-derived particles have not yet been evaluated. Neither class has been assessed by life cycle or biodegradation testing as a finished formulation. Full article

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26 pages, 8865 KB

Open AccessArticle

Research on RFID Detection Method for Lubricating Oil Moisture-Based on Phase-RSSI Orthogonal Fusion

by Na Wu and Qian Song

Lubricants 2026, 14(8), 326; https://doi.org/10.3390/lubricants14080326 - 21 Aug 2026

Moisture significantly reduces the load-carrying capacity of lubricating oil films, accelerates oxidative degradation, and induces equipment corrosion, making it a critical hazard factor affecting lubrication reliability. To overcome the limitations of existing methods for determining water content—such as complex operation, poor real-time performance, [...] Read more.

Moisture significantly reduces the load-carrying capacity of lubricating oil films, accelerates oxidative degradation, and induces equipment corrosion, making it a critical hazard factor affecting lubrication reliability. To overcome the limitations of existing methods for determining water content—such as complex operation, poor real-time performance, and high cost—this paper proposes a radio frequency identification (RFID)-based method for lubricating oil water content detection via the orthogonal fusion of phase and received signal strength indicator (RSSI) as an off-line analytical tool. The method exploits the signal variation characteristics when RF signals penetrate media with different dielectric properties; by analyzing the phase and RSSI of backscattered RFID signals, non-contact moisture sensing is achieved. First, a theoretical model integrating phase and RSSI for water content detection is established to reveal the differential response mechanisms of the two parameters to water content. Second, a detection method based on phase-RSSI orthogonal fusion is proposed, and performance evaluation metrics are constructed. Finally, comparative experiments with different detection approaches are conducted. It is found that as water content increases, the mean phase continuously rises with significantly increased fluctuation, while RSSI exhibits a linear decreasing trend, demonstrating clear complementary response characteristics. Compared with single-phase or single-RSSI methods, the proposed fusion method achieves a coefficient of determination (R²) of 0.95 over the 0–2.0% water content range, with reliable detection verified at concentrations as low as 0.1%, and exhibits superior detection sensitivity in the low-water-content range. Furthermore, it possesses a type-discrimination capability absent in single-parameter methods—that is, it can effectively distinguish whether response variations originate from moisture contamination or non-moisture interference. The method offers stable response and high detection efficiency, providing a new approach for accurate determination of water content in lubricating oil. Full article

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18 pages, 16431 KB

Open AccessArticle

Film Thickness Prediction from Dichromatic Interference Images Based on Dual-Wavelength Physics-Guided Graph Neural Network

by Peng Yue, Jiaqing Wang, Zhimin Shi, Chen He, Xiaoran Zhu and Yujuan Zhang

Lubricants 2026, 14(8), 325; https://doi.org/10.3390/lubricants14080325 - 21 Aug 2026

Dichromatic optical interferometry provides rich optical information for lubricant film-thickness measurement. However, experimental data are typically limited to a small number of discrete operating conditions, making it difficult to learn the nonlinear relationship among entrainment speed, dichromatic interference images, and lubricant film thickness. [...] Read more.

Dichromatic optical interferometry provides rich optical information for lubricant film-thickness measurement. However, experimental data are typically limited to a small number of discrete operating conditions, making it difficult to learn the nonlinear relationship among entrainment speed, dichromatic interference images, and lubricant film thickness. To address this limitation, a Dual-Wavelength Physics-Guided Graph Neural Network (DW-PG-GNN) is proposed for intermediate lubrication-state characterization from sparse experimental observations. Rather than directly regressing film thickness from interference images, the proposed framework learns the nonlinear relationship among entrainment speed, dichromatic interference images, and lubricant film thickness through a unified physics-guided learning framework. Specifically, graph representations capture the structural continuity and cross-wavelength coupling characteristics of dichromatic interference fringes, while a physics-guided residual learning strategy embeds analytical film-thickness priors from classical elastohydrodynamic lubrication (EHL) theory into an implicit neural representation for intermediate state prediction. A differentiable interference renderer further constrains the consistency between predicted film thickness and reconstructed dichromatic interference images, ensuring optical and physical consistency throughout the learning process. Validation based on sparse experimental datasets acquired from a ball-on-disc EHL rig demonstrates reliable average film-thickness prediction and interference-image reconstruction under the investigated operating conditions. The prediction errors remain within 2.2% for trained conditions and 8.67% for unseen entrainment-speed interpolation conditions. Full article

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17 pages, 3107 KB

Open AccessArticle

Friction–Adhesion Coupling Behavior of SBS-Modified Asphalt Pavement with Discontinuous Surface Texture

by Gang Li, Jiangang Li, Zhane Li, Xin Lu, Yingling Li, Yun Lin, Xingnan Hu and Wei Kang

Lubricants 2026, 14(8), 324; https://doi.org/10.3390/lubricants14080324 - 21 Aug 2026

Pitting distress introduces discontinuous surface texture, thereby modifying the friction–adhesion coupling at the rubber–asphalt interface. This study systematically investigates the interfacial friction response of SBS-modified asphalt pavements with three pitting distribution patterns (sparse, medium, and dense) under varying sliding rates, temperatures, and cyclic [...] Read more.

Pitting distress introduces discontinuous surface texture, thereby modifying the friction–adhesion coupling at the rubber–asphalt interface. This study systematically investigates the interfacial friction response of SBS-modified asphalt pavements with three pitting distribution patterns (sparse, medium, and dense) under varying sliding rates, temperatures, and cyclic loading using a custom-developed friction testing apparatus. The results reveal that adhesion-dominated friction is strongly dependent on both texture distribution and operating conditions. With increasing temperature, the dense pattern exhibits the most pronounced adhesion enhancement, with an average friction increase of 55.0%. The sparse pattern shows a continuous decreasing trend in the low-to-medium speed range, but exhibits a strengthening rebound at high speeds, demonstrating the most complex rate sensitivity. Under cyclic loading, among the three pitting patterns tested, the dense pattern exhibits the best friction retention, with an average friction reduction of only 6.2% after 30 cycles, compared to 9.5% for the sparse pattern and 10.8% for the medium pattern. Three-dimensional topography indicates that the medium pattern exhibits a continuous and relatively uniform wavy roughness, while the dense pattern transforms into a high-frequency serrated morphology. However, the wear in both patterns is dominated by homogenized micro-grooves, without the formation of local deep pits or sharp undulations, which favors the maintenance of stable skid resistance under the laboratory cyclic loading conditions tested in this study. Furthermore, a unified regression model with distribution dummy variables and interaction terms (R² = 0.826; cross-validated Q² = 0.646) quantifies the synergistic effect of pitting density and temperature, with the temperature sensitivity increasing six-fold from the sparse to the dense pattern (0.011 to 0.071 N/°C). Full article

(This article belongs to the Special Issue Tire/Road Interface and Road Surface Textures, 2nd Edition)

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16 pages, 3467 KB

Open AccessArticle

A Method for Predicting Motion Error of Internal Feedback Hydrostatic Turntable Under Eccentric Load

by Honglie Ma, Qingkai Shen, Xiaolei Deng, Qiang Cheng and Mingyue Zhang

Lubricants 2026, 14(8), 323; https://doi.org/10.3390/lubricants14080323 - 21 Aug 2026

This paper proposes a method to analyze motion errors in a five-degree-of-freedom hydrostatic turntable with internal feedback under eccentric load. The motion error models of thrust and journal bearings are derived separately, revealing the mechanism of the influence of manufacturing errors of thrust [...] Read more.

This paper proposes a method to analyze motion errors in a five-degree-of-freedom hydrostatic turntable with internal feedback under eccentric load. The motion error models of thrust and journal bearings are derived separately, revealing the mechanism of the influence of manufacturing errors of thrust plate and shaft on motion errors. The results demonstrate that the hydrostatic oil film exhibits an error averaging effect. When the amplitude of the mating surface error reaches 15 μm, the corresponding linear deviation of the turntable remains below 0.3 μm, indicating that the oil film can effectively suppress the transmission of manufacturing errors. However, the pressure oil film cannot completely balance the errors on the film binding surface, especially when the amplitude of the binding surface error is larger, resulting in a weaker ability of the oil film to balance. Full article

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