Lubricants

15 pages, 2805 KB

Open AccessArticle

Mechanism of Inner Rail Corrugation on Large-Radius Curves in Metro Systems

by Qifeng Song, Yan Hu, Feng Wen, Hutang Sang, Xi Kang and Dapeng Zhang

Lubricants 2026, 14(1), 19; https://doi.org/10.3390/lubricants14010019 (registering DOI) - 1 Jan 2026

This paper investigates the underlying cause of inner rail corrugation on large-radius curved tracks in metro systems. A dynamic model of the vehicle–track system (VTS) was developed to analyze the creep characteristics between the guiding wheelset and the rails when the vehicle negotiates [...] Read more.

This paper investigates the underlying cause of inner rail corrugation on large-radius curved tracks in metro systems. A dynamic model of the vehicle–track system (VTS) was developed to analyze the creep characteristics between the guiding wheelset and the rails when the vehicle negotiates large-radius curves under coasting, traction, and braking conditions. A finite element-based complex eigenvalue analysis was conducted to evaluate the stability of the wheel–rail frictional system. The results reveal that under coasting conditions, the wheel–rail creep forces on large-radius curves remain unsaturated, substantially reducing the likelihood of corrugation formation. In contrast, during braking, the creep force may approach saturation on the guiding inner wheel, increasing the possibility of wheel–rail sliding. This braking-induced sliding can trigger friction-induced self-excited vibrations at the wheel–rail interface, leading to the development of inner rail corrugation on large-radius curves. Full article

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

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33 pages, 3071 KB

Open AccessSystematic Review

Friction and Cartilage Wear in Hemiarthroplasty: A Systematic Review of Key Influencing Factors

by Victoria P. Marino, Francesca De Vecchi, Dominik J. Federl, Landon M. Begin, Afton K. Limberg, Douglas C. Moore, Joseph J. Crisco, Douglas W. Van Citters and Markus A. Wimmer

Lubricants 2026, 14(1), 18; https://doi.org/10.3390/lubricants14010018 - 31 Dec 2025

Abstract

Hemiarthroplasty addresses joint damage confined to one side, preserving native cartilage and bone, but accelerated degeneration of the opposing cartilage can compromise outcomes. This systematic review should clarify whether coefficient of friction (COF) reliably predicts cartilage wear when evaluating hemiarthroplasty bearing materials (HBMs). [...] Read more.

Hemiarthroplasty addresses joint damage confined to one side, preserving native cartilage and bone, but accelerated degeneration of the opposing cartilage can compromise outcomes. This systematic review should clarify whether coefficient of friction (COF) reliably predicts cartilage wear when evaluating hemiarthroplasty bearing materials (HBMs). Thirty in vitro studies reporting both outcomes were identified. Data were extracted on COF, wear, and testing parameters, and wear was standardized using a 0–4 rubric to enable cross-study comparison. Three analytical approaches were applied: linear model fits, Pearson’s correlations, and predictive modeling. Reported COFs increased significantly with testing time, while contact stress and sliding velocity showed variable associations with COF. Predictive models for cobalt–chromium (CoCr), the most studied HBM, showed moderate fit, suggesting that mechanical parameters explain only part of COF variability. For wear, linear models showed poor fit with COF, but correlations indicated positive associations with contact stress. Inconsistent effects of velocity and distance were found. Predictive models explained little variability. Together, these findings suggest that outcomes are strongly influenced by testing conditions, lubricants, and HBM selection, and COF alone is an unreliable predictor of cartilage wear in an experimental setting. Full article

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

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25 pages, 6890 KB

Open AccessArticle

Development of Oleic Acid-Assisted Nanolubricants from Palm Kernel Oil for Boundary Lubrication Performance Under Extreme Pressure

by Aiman Yahaya, Syahrullail Samion, Zulhanafi Paiman, Nurul Farhanah Azman and Shunpei Kamitani

Lubricants 2026, 14(1), 17; https://doi.org/10.3390/lubricants14010017 - 30 Dec 2025

Abstract

The stability of nanolubricants is critical for ensuring effective performance under extreme pressure (EP) conditions, where severe boundary lubrication governs friction and wear behaviour. This study examines palm kernel oil (PKO)-based nanolubricants enhanced with carbon graphene (CG), hexagonal boron nitride (hBN), and molybdenum [...] Read more.

The stability of nanolubricants is critical for ensuring effective performance under extreme pressure (EP) conditions, where severe boundary lubrication governs friction and wear behaviour. This study examines palm kernel oil (PKO)-based nanolubricants enhanced with carbon graphene (CG), hexagonal boron nitride (hBN), and molybdenum disulfide (MoS₂), with and without oleic acid (OA) as a surfactant. OA incorporation improved CG dispersion stability, reducing agglomerate size by 30.4% (17.61 μm to 12.23 μm) and increasing the viscosity index from ~176 to 188, compared to 152 for the commercial hydrogen engine oil baseline. Under EP conditions, PKO + CG + OA achieved a 51.7% reduction in the coefficient of friction (0.58 to 0.28) and 18.2% improvement in weld load resistance, while wear scar diameter decreased by 13.4%. Surface and elemental analyses indicated the formation of a composite tribofilm containing oxide species, graphene platelets, and carboxylate-derived compounds from OA, consistent with iron–oleate-like chemistry that enhances load-carrying capacity and wear protection. These findings demonstrate the potential of OA-assisted PKO nanolubricants as sustainable, high-performance formulations for extreme pressure boundary lubrication, contributing to the advancement of green tribology. Full article

(This article belongs to the Special Issue Tribological Impacts of Sustainable Fuels in Mobility Systems)

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19 pages, 2106 KB

Open AccessArticle

Numerical and Experimental Investigation of Different Oil Levels and Operation Conditions on the Individual Hydraulic Losses of Spherical Rolling Bearings

by Thomas Christoph Petrzik, Kim Marius Brill, Georg Jacobs, Oliver Koch, Benjamin Lehmann, Peter Rößler and Amirreza Niazmehr

Lubricants 2026, 14(1), 16; https://doi.org/10.3390/lubricants14010016 - 30 Dec 2025

Abstract

Improving the energy efficiency of rolling bearings requires a component-resolved understanding of loss mechanisms. While analytical models capture load-dependent losses, load-independent hydraulic losses demand a physics-based approach. This paper presents a computational fluid dynamics (CFD) methodology for the qualification of individual hydraulic loss [...] Read more.

Improving the energy efficiency of rolling bearings requires a component-resolved understanding of loss mechanisms. While analytical models capture load-dependent losses, load-independent hydraulic losses demand a physics-based approach. This paper presents a computational fluid dynamics (CFD) methodology for the qualification of individual hydraulic loss contributions and to assess their sensitivity to operating conditions. The approach decomposes the total hydraulic loss of the spherical roller bearing 22320 into component-level shares and is benchmarked against dedicated experiments. The simulated results show good agreement with experimental measurements, supporting the validity of the methodology. The discrepancy between the measured and simulated friction torque values averaged at 2–7%, with a single outlier. Furthermore, CFD methods have been demonstrated to be capable of predicting trends in hydraulic losses resulting from variations in speed and temperature. A consistent finding across all investigated conditions is that the rolling elements dominate the hydraulic losses. Churning-induced losses of the rolling elements contribute for more than 50% of the hydraulic losses of the hole bearing in every test. The proposed methodology offers a reproducible way to assign losses individually, compare operating scenarios and guide targeted design measures for loss reduction in rolling bearings. Furthermore, dynamic kinematic simulations of rolling bearings can be equipped with component-resolved hydraulic losses. This is enabling more accurate predictive modelling of the bearing kinematics and detecting effects such as slippage. Full article

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

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119 pages, 50914 KB

Open AccessArticle

Contact Roughness Characterization Parameters for Abrasion-Resistant Epoxy-Coated Surfaces Enhanced with Micro and Nanoparticles

by Maria A. Sáenz-Nuño and Cristina Puente

Lubricants 2026, 14(1), 15; https://doi.org/10.3390/lubricants14010015 - 29 Dec 2025

Abstract

In recent years, numerous studies have been conducted to characterize the physical parameters that define the behavior of surfaces coated with epoxy resins, particularly in terms of hardness and resistance. Many of these surfaces have been doped with micro- and nanoparticles. In this [...] Read more.

In recent years, numerous studies have been conducted to characterize the physical parameters that define the behavior of surfaces coated with epoxy resins, particularly in terms of hardness and resistance. Many of these surfaces have been doped with micro- and nanoparticles. In this work, we present the internationally defined roughness parameters that are typically of interest for the use of these materials in industrial parts. We analyze the information these parameters provide about the coatings when measured by contact methods, not just optically before and after physical abrasion. The contact profile roughness parameters (R) are highlighted, as they can offer more reliable information regarding the physical wear of these surfaces due to abrasion (typically Ra, Rq, Rz, Rsk, Rku and Rmr). The main advantage is that this approach allows for discerning parameters that, when linked with other functionalities of the parts, provide more comprehensive information without being limited to purely optical or non-contact SEM analysis. The characterization of nanometric particle-doped surfaces with Ra, Rq, and Rz, and of micrometric particle-doped surfaces with Rmr (10–20%) is proposed, in order to clearly characterize the final behavior of the surface before and after wear. Full article

(This article belongs to the Special Issue Experimental Modelling of Tribosystems)

34 pages, 1550 KB

Open AccessReview

A Comprehensive Review of Lubricant Behavior in Internal Combustion, Hybrid, and Electric Vehicles: Thermal Demands, Electrical Constraints, and Material Effects

by Subin Antony Jose, Erick Perez-Perez, Terrence D. Silva, Kaden Syme, Zane Westom, Aidan Willis and Pradeep L. Menezes

Lubricants 2026, 14(1), 14; https://doi.org/10.3390/lubricants14010014 - 28 Dec 2025

Abstract

The global transition from internal combustion engines (ICEs) to hybrid (HEVs) and electric vehicles (EVs) is fundamentally reshaping lubricant design requirements, driven by evolving thermal demands, electrical constraints, and material compatibility challenges. Conventional ICE lubricants are primarily formulated to withstand high operating temperatures, [...] Read more.

The global transition from internal combustion engines (ICEs) to hybrid (HEVs) and electric vehicles (EVs) is fundamentally reshaping lubricant design requirements, driven by evolving thermal demands, electrical constraints, and material compatibility challenges. Conventional ICE lubricants are primarily formulated to withstand high operating temperatures, mechanical stresses, and combustion-derived contaminants through established additive chemistries such as zinc dialkyldithiophosphate (ZDDP), with thermal stability and wear protection as dominant considerations. In contrast, HEV lubricants must accommodate frequent start–stop operation, pronounced thermal cycling, and fuel dilution while maintaining performance across coupled mechanical and electrical subsystems. EV lubricants represent a paradigm shift, where requirements extend beyond tribological protection to include electrical insulation and conductivity control, thermal management of electric motors and battery systems, and compatibility with copper windings, polymers, elastomers, and advanced coatings, alongside mitigation of noise, vibration, and harshness (NVH). This review critically examines lubricant behavior, formulation strategies, and performance requirements across ICE, HEV, and EV powertrains, with specific emphasis on heat transfer, electrical performance, and lubricant–material interactions, covering mineral, synthetic, and bio-based fluids. Additionally, regulatory drivers, sustainability considerations, and emerging innovations such as nano-additives, multifunctional and smart lubricants, and AI-assisted formulation are discussed. By integrating recent research into industrial practice, this work highlights the increasingly interdisciplinary role of tribology in enabling efficient, durable, and sustainable mobility for next-generation automotive systems. Full article

(This article belongs to the Special Issue Tribology in Vehicles, 2nd Edition)

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10 pages, 1544 KB

Open AccessBrief Report

Efficient Characterization of Extreme Pressure Properties of Lubricants Using Advanced Four-Ball Test Methodology

by Krishnamurti Singh, Tushar Khosla and Mathias Woydt

Lubricants 2026, 14(1), 13; https://doi.org/10.3390/lubricants14010013 - 28 Dec 2025

Abstract

The classic four-ball test was developed in the 1930s and has remained unchanged to this day. This versatile and widely used methodology would benefit from further development with modern tools and techniques. The classic four-ball test is traditionally performed with load steps, which [...] Read more.

The classic four-ball test was developed in the 1930s and has remained unchanged to this day. This versatile and widely used methodology would benefit from further development with modern tools and techniques. The classic four-ball test is traditionally performed with load steps, which makes it slow, and determining the exact last non-seizure load is quite challenging. To overcome this situation, a new methodology for testing the high-pressure properties (EP) of greases and oils is presented, using a continuous, constant-load ramp rate. The peak in the evolution of the coefficient of friction represents the occurrence of the last non-seizure sliding. The load at which the final non-seizure sliding occurs is defined as the last non-seizure load (LNSL). The obtained results are consistent with historical experience with “classic” four-ball EP tests, and the test procedure is fast and highly repeatable. Full article

(This article belongs to the Special Issue Advances in Tribology and Lubrication for Bearing Systems)

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23 pages, 6077 KB

Open AccessArticle

Comparative Analysis of CNN and LSTM for Bearing Fault Mode Classification and Causality Through Representation Analysis

by Jung-Woo Kim, Jong-Hak Lee, Dong-Hun Son, Sung-Hyun Choi and Kyoung-Su Park

Lubricants 2026, 14(1), 12; https://doi.org/10.3390/lubricants14010012 - 28 Dec 2025

Abstract

This study investigates how the clarity of frequency-domain characteristics in vibration signals affects the performance of deep learning models for bearing fault classification. Two datasets were used; these were the CWRU benchmark dataset, which exhibits distinct and easily separable spectral signatures across fault [...] Read more.

This study investigates how the clarity of frequency-domain characteristics in vibration signals affects the performance of deep learning models for bearing fault classification. Two datasets were used; these were the CWRU benchmark dataset, which exhibits distinct and easily separable spectral signatures across fault modes, and a custom low-speed bearing dataset in which small defects do not significantly alter the frequency spectrum. To enable a clear and interpretable comparison, simplified CNN and LSTM architectures with a single core layer were deliberately employed. This design choice allows performance differences to be attributed directly to the inherent learning mechanisms of each architecture rather than to model complexity. Representation analysis shows that LSTM-F achieves the highest accuracy when the dataset contains clearly distinguishable spectral patterns, as in the CWRU case. In contrast, CNN-S outperforms both LSTM models in the experimental dataset, where fault-induced frequency characteristics are weak or ambiguous. Additional representation analyses further reveal that LSTM-F relies on consistent frequency-indexed patterns, whereas CNN-S captures more complex time–frequency interactions, making it more robust under low-separability conditions. These findings demonstrate that the optimal deep learning architecture for bearing fault classification depends on the degree of frequency separability in the data. LSTM-F is preferable for severe faults with distinct spectral features, while CNN-S is more effective for minor defects or systems exhibiting complex, weakly discriminative frequency behavior. Full article

(This article belongs to the Special Issue Advances in Wear Life Prediction of Bearings)

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24 pages, 2190 KB

Open AccessArticle

Improving Coating Stability Using Slip Conditions: An Analytical Approach to Curtain Coating

by Laraib Mehboob, Khadija Maqbool, Abdul Majeed Siddiqui and Zaheer Abbas

Lubricants 2026, 14(1), 11; https://doi.org/10.3390/lubricants14010011 - 27 Dec 2025

Abstract

Curtain deflector coating is a widely employed technique for producing thin, uniform films in numerous industrial applications. The flow dynamics in curtain coating become complex near the corner region due to the interaction of the moving substrate and the falling liquid curtain. In [...] Read more.

Curtain deflector coating is a widely employed technique for producing thin, uniform films in numerous industrial applications. The flow dynamics in curtain coating become complex near the corner region due to the interaction of the moving substrate and the falling liquid curtain. In this study, an analytical investigation is conducted for the steady, in-compressible, and creeping flow of a Maxwell fluid, under the Navier slip condition at the substrate. The mathematical model is derived from the conservation of mass and momentum representing the nonlinear system which is solved using the Langlois recursive technique in combination with the inverse method. The inclusion of the Navier slip boundary condition in this research makes it novel and remove the singularity which produce the unstable stresses at a sharp corner due to no slip, but the Navier slip gives a stable solution for the stresses at a sharp corner. The analysis demonstrates that substrate slip significantly reduces tangential stresses and enhances the stability of the coating flow. Residual error analysis is also performed to verify the accuracy and convergence of the analytical solutions. The results provide a deeper understanding of how slip effects can be utilized to improve coating uniformity and optimize the operational performance of curtain deflector coating systems. Full article

(This article belongs to the Special Issue Wear-Resistant Coatings and Film Materials, 2nd Edition)

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24 pages, 3769 KB

Open AccessArticle

Study on Transient Thermal Characteristics of Aviation Wet Clutches with Conical Separate Discs for Helicopters in Successive Shifting

by Xiaokang Li, Dahuan Wei, Yixiong Yan, Hongzhi Yan, Mei Yin and Yexin Xiao

Lubricants 2026, 14(1), 10; https://doi.org/10.3390/lubricants14010010 - 25 Dec 2025

Abstract

Thermal gradients induced by friction frequently trigger buckling deformation of the friction elements, especially in heavy-duty helicopters. Nevertheless, the subsequent influence of such post-buckling deformation on transient thermal characteristics during helicopter successive shifting remains insufficiently addressed in existing research. In the present work, [...] Read more.

Thermal gradients induced by friction frequently trigger buckling deformation of the friction elements, especially in heavy-duty helicopters. Nevertheless, the subsequent influence of such post-buckling deformation on transient thermal characteristics during helicopter successive shifting remains insufficiently addressed in existing research. In the present work, a gap model for friction pairs with conical separate discs is first proposed. Subsequently, a comprehensive thermal-fluid-dynamic model incorporating spline friction, split springs, and time-varying thermal parameters is developed to investigate the transient thermal characteristics of wet clutches with conical separate discs in successive shifting. A corresponding qualitative analysis is performed to explore the transient thermal response and influence mechanisms of operating parameters, including shifting interval, rotation speed and control oil pressure. The results indicate that a rise in the control oil pressure from 1.5 MPa to 1.9 MPa facilitates a 42.65% increase in the maximum radial temperature gradient and augments the maximum axial temperature gradient by 24.35%. Meanwhile, an increase in rotation speed accelerates heat dissipation but compromises the uniformity of the temperature field. Additionally, extended shifting intervals under inadequate heat dissipation exacerbates thermal buildup, driving a persistent and significant escalation in the temperature of friction elements. The conclusions can provide a theoretical basis for the optimal design, condition monitoring, and fault diagnosis of aviation clutches. Full article

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29 pages, 5968 KB

Open AccessArticle

Lubrication Characteristics and Thermal Behavior of Non-Orthogonal Face Gear Pairs in Point Contact

by Xiaomeng Chu, Faqiang Chen and Jiangjun Wang

Lubricants 2026, 14(1), 9; https://doi.org/10.3390/lubricants14010009 - 24 Dec 2025

Abstract

The thermal elastohydrodynamic lubrication problem of non-orthogonal surface gear pairs under point contact conditions was studied. Firstly, a mathematical model is established through a comprehensive contact and lubrication analysis. On this basis, the Reynolds equation, energy equation, film thickness equation, viscosity-pressure–viscosity-temperature relationship, and [...] Read more.

The thermal elastohydrodynamic lubrication problem of non-orthogonal surface gear pairs under point contact conditions was studied. Firstly, a mathematical model is established through a comprehensive contact and lubrication analysis. On this basis, the Reynolds equation, energy equation, film thickness equation, viscosity-pressure–viscosity-temperature relationship, and density equation are coupled and solved, and the finite difference method is adopted for numerical solution. The distribution characteristics of oil film pressure, oil film thickness, and temperature rise at different meshing points were systematically analyzed. The results show that during the meshing cycle, the maximum pressure of the oil film at the meshing point first increases and then decreases, while the minimum oil film thickness is the opposite. Near the point S3, the meshing state is close to pure rolling, with the lowest friction coefficient and temperature rise. Furthermore, the heat absorbed by the lubricating oil film through forced convection accounts for only approximately 1% of the total frictional heat, indicating that at the moment of gear meshing contact and in the microscopic region, frictional heat is mainly dissipated through heat conduction. Full article

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25 pages, 8136 KB

Open AccessArticle

Numerical Analysis of Lubrication and Oil Churning Power Loss of High Contact Ratio Internal Gear Pair

by Xiaomeng Chu, Zhijun Gao and Jia Shen

Lubricants 2026, 14(1), 8; https://doi.org/10.3390/lubricants14010008 - 24 Dec 2025

Abstract

Planetary gear is the mainstream deceleration transmission device, and its derivative form of high contact ratio internal gear adopts the structure of full internal meshing. While improving the compactness and efficiency of the transmission, it is necessary to focus on its lubrication characteristics [...] Read more.

Planetary gear is the mainstream deceleration transmission device, and its derivative form of high contact ratio internal gear adopts the structure of full internal meshing. While improving the compactness and efficiency of the transmission, it is necessary to focus on its lubrication characteristics and churning power consumption. In this paper, based on the actual meshing state of high contact ratio internal gear, combined with its geometric parameters, motion speed, and pressure bearing state, the Computational Fluid Dynamics (CFD) model is used to analyze the oil distribution during gear motion. According to the oil state, the oil pressure and viscous force on the gear surface are extracted, the churning loss of the gear is calculated, and the influence of different parameters on the churning loss is analyzed. Finally, based on the influence of the oil churning parameters on the lubrication performance, the representative oil churning parameters are selected for the test. The test results are consistent with the results obtained by the simulation analysis, which provides data support for the study of the lubrication of high contact ratio internal gears. Full article

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15 pages, 8607 KB

Open AccessArticle

Identification and Evaluation of Tool Tip Contact and Cutting State Using AE Sensing in Ultra-Precision Micro Lathes

by Alan Hase

Lubricants 2026, 14(1), 7; https://doi.org/10.3390/lubricants14010007 - 23 Dec 2025

Abstract

The growing demand for miniature mechanical components has increased the importance of ultra-precision micro machine tools and real-time monitoring. This study examines acoustic emission (AE) sensing for the intelligent control of an ultra-precision micro lathe. AE signals were measured while brass and aluminum [...] Read more.

The growing demand for miniature mechanical components has increased the importance of ultra-precision micro machine tools and real-time monitoring. This study examines acoustic emission (AE) sensing for the intelligent control of an ultra-precision micro lathe. AE signals were measured while brass and aluminum alloys were turned with cermet and diamond tools at different spindle speeds and cutting depths. Finite element simulations were performed to clarify the AE generation mechanisms. The AE waveform amplitude changed stepwise corresponding to tool–workpiece contact, elastoplastic deformation, and chip formation, enabling precise contact detection at the 0.1 μm level. The AE amplitude increased with increasing spindle speed and increasing depth of cut except during abnormal conditions (e.g., workpiece adhesion). Frequency analysis revealed a dominant peak near 0.2 MHz during normal cutting, as well as high-frequency (>1 MHz) components linked to built-up edge formation. Simulations confirmed that these AE features reflect variations in the strain rate in the shear zone and on the rake face. They also confirmed that cutting force spectra under high friction reproduce the experimentally observed high-frequency peaks. These findings demonstrate the feasibility of using AE sensing to identify the cutting state and support the development of self-optimizing micro machine tools. Full article

(This article belongs to the Special Issue Analysis, Evaluation and Diagnosis of Tribological Phenomena in Machining Processes)

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Graphical abstract

12 pages, 9132 KB

Open AccessArticle

Properties of Galba, Avocado and Moringa Oils in Lubricant Formulations

by Emeline Sophie, Ashaina Blathase, Philippe Thomas, Yves Bercion, Philippe Bilas and Nadiège Nomède-Martyr

Lubricants 2026, 14(1), 6; https://doi.org/10.3390/lubricants14010006 - 23 Dec 2025

Abstract

Due to growing environmental challenges, many studies are focusing on vegetable-based lubricants. Industrial lubricants pose a significant risk to the environment and human health. The tribological performances of calophyllum calaba (galba) and avocado when used as a base oil and as a liquid [...] Read more.

Due to growing environmental challenges, many studies are focusing on vegetable-based lubricants. Industrial lubricants pose a significant risk to the environment and human health. The tribological performances of calophyllum calaba (galba) and avocado when used as a base oil and as a liquid additive were compared to those of moringa oil. The different lubricant formulations were investigated under an ambient atmosphere, using a reciprocating ball-on-plane tribometer in a boundary lubrication regime. Graphite particles were used as solid lubricant additives due to their excellent friction performances in these formulations. Dodecane was the mineral oil used as a reference and base oil in some lubricant formulations. It was found that the percentage used and the fatty acid molecule composition of vegetable oils have an important influence on the tribological performances of the different formulations. The presence of oleic acid molecules shows a positive effect but is not sufficient to explain the friction reduction obtained with moringa oil. The triglyceride shape of an oleic acid molecule is the key to an important friction reduction, despite the small amount (2 wt% as liquid additive) in lubricant formulations. Full article

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15 pages, 2447 KB

Open AccessArticle

Investigation on Microstructure, Thermal Fatigue Resistance, and Tribological Behavior of Mo₂FeB₂-Based Cermet Coating on GCr15 Steel Substrate

by Hao Zhang, Yang Zhang, Lufan Jin, Binglin Zhang and Yu Zhang

Lubricants 2026, 14(1), 5; https://doi.org/10.3390/lubricants14010005 - 23 Dec 2025

Abstract

In this study, a boride cladding layer with Mo₂FeB₂ hard phase was prepared on the GCr15 steel via plasma cladding. The phase composition, microstructure, thermal fatigue resistance, microhardness, and wear resistance of the boride cladding layer were investigated. The results [...] Read more.

In this study, a boride cladding layer with Mo₂FeB₂ hard phase was prepared on the GCr15 steel via plasma cladding. The phase composition, microstructure, thermal fatigue resistance, microhardness, and wear resistance of the boride cladding layer were investigated. The results revealed that the hard phases in the boride cladding layer were Mo₂FeB₂ and (Cr,Fe)₂₃(C,B)₆, while the binder phase consisted of α-Fe martensite. When the thermal fatigue times increased, the indentation crack length extended in a quadratic pattern, and the crack propagation rate increased. Crack propagation in the cladding layer occurred via both transgranular and intergranular modes. When the thermal fatigue temperature was below 600 °C, the cladding layer exhibited good thermal stability, and a reliable metallurgical bond was formed between the cladding layer and the GCr15 steel substrate. The microhardness of the cladding layer reached 1022.1 HV_0.5, approximately 2.6 times that of the GCr15 steel. The mass loss of the cladding layer increased with the increase in wear load and wear time. The wear of the cladding layer was mainly three-body abrasion wear, resulting from brittle spalling of the hard phase on the worn surface. This study demonstrates the potential of Mo₂FeB₂-based cladding layers for extending the service life of high-value industrial components. Full article

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16 pages, 1528 KB

Open AccessArticle

Structure–Performance Relationship Study of PMA Viscosity Index Improver in New Energy Vehicle Transmission Fluid

by Jinglin Yin, Xiao Shi, Ling Lei, Jingsi Cao, Qianhui Zhao and Haipeng Zhao

Lubricants 2026, 14(1), 4; https://doi.org/10.3390/lubricants14010004 - 23 Dec 2025

Abstract

This study systematically investigates the structure–performance relationship of PMA (PolyMethacrylate) viscosity index improvers in new energy vehicle (NEV) transmission fluids. We developed an integrated analytical framework combining spectroscopic and chromatographic techniques to simultaneously characterize its side chain length distribution, molecular weight polydispersity, and [...] Read more.

This study systematically investigates the structure–performance relationship of PMA (PolyMethacrylate) viscosity index improvers in new energy vehicle (NEV) transmission fluids. We developed an integrated analytical framework combining spectroscopic and chromatographic techniques to simultaneously characterize its side chain length distribution, molecular weight polydispersity, and branching architecture. Key findings reveal that the kinematic viscosity of formulated oils positively correlates with PMA molecular weight, low-temperature performance is governed by side-chain length (≥C14 fatty alcohols), shear stability is predominantly determined by molecular weight, and nitrogen-modified PMA enhances oxidation resistance by mitigating kinematic viscosity increase. These insights provide actionable guidance for the molecular design of viscosity index improvers and the formulation optimization of advanced lubricants to meet the stringent demands of electric vehicle transmission systems. Full article

(This article belongs to the Special Issue Novel Lubricant Additives in 2025)

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24 pages, 5148 KB

Open AccessArticle

Improving Tribological Properties of Oil-in-Water Lubricating Fluid Using Hybrid Protic Ionic Liquid and Nanoparticle Additives

by Raimondas Kreivaitis, Jolanta Treinytė, Artūras Kupčinskas, Milda Gumbytė and Ieva Gaidė

Lubricants 2026, 14(1), 3; https://doi.org/10.3390/lubricants14010003 - 22 Dec 2025

Abstract

Water is attractive as a base fluid due to its availability and environmental friendliness. To enhance its lubricity, environmentally friendly additives must be applied. This study combined protic ionic liquid and several nanoparticles to form hybrid additives for an oil-in-water lubricant. The performance [...] Read more.

Water is attractive as a base fluid due to its availability and environmental friendliness. To enhance its lubricity, environmentally friendly additives must be applied. This study combined protic ionic liquid and several nanoparticles to form hybrid additives for an oil-in-water lubricant. The performance of these additives was evaluated using wettability, tribo-testing, and worn-surface analysis. The tribo-test employed a ball-on-plate reciprocating tribometer that used bearing steel/bearing steel and WC/bearing steel friction pairs. The results were compared with those obtained using two commercial additives. It was found that the investigated additives are promising candidates for water-based lubricants, as they exhibit comparable wettability. Moreover, they outperform the reference samples in terms of lubricity. According to the results, the suggested lubrication mechanism includes enhanced wettability, composite tribo-film formation, surface polishing, and mending. Full article

(This article belongs to the Special Issue Lubrication Mechanism of Ionic Liquids)

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24 pages, 10537 KB

Open AccessArticle

Study on Ultrasonic Rolling Enhancement of TC4 and Its Tribological Characteristics Under Seawater Lubrication

by Shuaihui Wang, Xianshuai Ma, Donglin Li, Yong Tang, Feng Zhao, Yan Lu and Xiaoqiang Wang

Lubricants 2026, 14(1), 2; https://doi.org/10.3390/lubricants14010002 - 20 Dec 2025

Abstract

To enhance the abrasion resistance of TC4 titanium alloy and meet the demand for wear-resistant and corrosion-resistant friction pair materials for water-hydraulic components of marine equipment, the tribological properties of the material subsequent to ultrasonic rolling extrusion surface strengthening under seawater-lubricated conditions were [...] Read more.

To enhance the abrasion resistance of TC4 titanium alloy and meet the demand for wear-resistant and corrosion-resistant friction pair materials for water-hydraulic components of marine equipment, the tribological properties of the material subsequent to ultrasonic rolling extrusion surface strengthening under seawater-lubricated conditions were investigated. The process of ultrasonic rolling machining was simulated and analyzed by the finite element method. The influence of process parameters on surface residual stress and surface roughness of TC4 was studied, and the appropriate range of process parameters was determined. The effects of key process parameters such as rolling times, static pressure, amplitude, and rotational speed on the surface properties of TC4 were investigated by the single-factor test method. Based on the response surface methodology, a prediction model of surface hardness and roughness of TC4 was constructed, and the process parameters were optimized and analyzed. The friction coefficient, wear amount, and wear rate of TC4 and CFRPEEK under seawater lubrication before and after strengthening were studied by wear tests. The wear morphologies of the specimens prior to and subsequent to strengthening were analyzed, and the friction and wear mechanisms were explored in depth. The results indicate that ultrasonic rolling extrusion surface strengthening process facilitates grain refinement in the surface layer of TC4, enhances surface hardness, and optimizes surface roughness, thereby improving its wear resistance. This is of guiding significance to the design and use of hydraulic components in seawater and has a promoting effect on the development of marine equipment. Full article

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

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17 pages, 1274 KB

Open AccessArticle

Integrating Pavement Friction and Macrotexture into a Speed-Dependent Pavement Safety Metric for Safety Performance Modeling

by Behrokh Bazmara, Edgar de León Izeppi, Samer W. Katicha, Ross McCarthy and Gerardo W. Flintsch

Lubricants 2026, 14(1), 1; https://doi.org/10.3390/lubricants14010001 - 20 Dec 2025

Abstract

The paper proposes a pavement safety index, the estimated available friction at the expected travel speed, FRS(v), to model the composed effect of low-slip speed friction and macrotexture on roadway crashes. This index seems to capture the relative contributions of microtexture and macrotexture [...] Read more.

The paper proposes a pavement safety index, the estimated available friction at the expected travel speed, FRS(v), to model the composed effect of low-slip speed friction and macrotexture on roadway crashes. This index seems to capture the relative contributions of microtexture and macrotexture across different operating speeds. Speed-dependent available friction at 40, 55, and 70 mph was estimated using the speed-correction procedure in ASTM E1960-07 and integrated into Safety Performance Function (SPF) development. Comparison of the resulting SPF models suggests that FRS values corresponding to typical operating speeds can capture the combined influence of SFN (40) and macrotexture on expected crashes for freeways and rural two-lane, two-way highways. For freeways, the estimated available friction at 70 mph (FRS113) produced the most appropriate SPF, evidenced by the lowest AIC. For rural two-lane, two-way highways, the estimated available friction at 40 mph (FRS65) resulted in the lowest AIC value, consistent with the typical operating speeds on these facilities. In contrast, none of the speed-specific friction estimates produced satisfactory model performance for urban and suburban arterials, likely due to the wide variation in traveling speeds and geometric characteristics on these facilities. The applicability of the proposed metric was demonstrated through the development of illustrative investigatory friction levels based on observed crash data, and the identification of candidate roadway segments for friction improvement interventions, and the estimation of the corresponding return on investment for these interventions. Full article

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

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