Lubricants doi: 10.3390/lubricants14070268

Authors: Haopeng Li Gege Liu Shumeng Wang Shaoyu Zhu Yanzhe Bi Shuyou Wang

To address the limitations of conventional magnetorheological (MR) fluid hydrostatic bearings, a variable-stiffness bearing is proposed. These limitations include insufficient load capacity, low torque margin, and poor adaptability under complex operating conditions. Taking MR fluid as the lubricating medium, the device adopts permanent magnets to provide a basic stable magnetic field. A dual-excitation configuration, consisting of stiffness-adjustment coils integrated into the bearing and shaft-mounted coils attached to the rotor, enables dynamic magnetic field regulation. This mechanism modulates the rheological behavior of the MR fluid, realizing flexible stiffness tuning and dynamic torque enhancement of the bearing. The overall structure and working principle of the device are elaborated in detail. The mathematical models of bearing stiffness-current and output torque-current are derived, and the regulation law of current on the dynamic characteristics of the bearing is clarified. ANSYS Maxwell simulation results verify the feasibility of the dual excitation decoupling control scheme. The research results can provide theoretical support and technical reference for the intelligent regulation and engineering application of MR fluid hydrostatic bearings.

Lubricants doi: 10.3390/lubricants14070267

Authors: Junjie Fei Hongfang Qi Bei Yuan Minbiao Wan Linlang Zhang

As a core load-bearing component for railway vehicles, rails are largely responsible for the safety and stability of train operation, and their service performance is inherently governed by material microstructure. In this study, rails with varied pearlite interlamellar spacing were prepared and laid on field tracks for 8 months of service testing to investigate the influence of pearlite interlamellar spacing on rail wear and rolling contact fatigue (RCF). The results indicate that decreasing pearlite interlamellar spacing facilitated tread work hardening and reduced cumulative wear loss of rails. At the early service stage, rails with coarse pearlite lamellae exhibited earlier RCF crack initiation and longer crack morphologies, while rails featuring finer pearlite lamellae exhibited the latest-occurring crack initiation. With prolonged service duration, wear loss rose continuously, and the tread hardening rate first increased sharply and then tended to gradually become stable. Obvious differences in damage evolution were observed for rails with different pearlite interlamellar spacing. Coarse-lamellar rail suffered sparse short cracks dominated by wear; fine-lamellar rail developed dense fast-growing cracks controlled by RCF; and medium-lamellar rail achieved a relatively good balance between wear and RCF. A competitive relationship exists between wear and RCF during rail service. Reasonable regulation of pearlite interlamellar spacing facilitates a balanced evolution of wear and RCF, which provides a feasible microstructural optimization strategy for improving the service performance and service life of pearlitic rails.

Lubricants doi: 10.3390/lubricants14070266

Authors: Sandra Rojas-Osorio Marco Ulises Negrete-Ríos José E. Báez María Teresa Hernández-Sierra Karla J. Moreno

Vegetable oil blends significantly improve friction and wear performance compared to individual oils. However, the oxidizing nature of these blends remains a consistent challenge. Currently, both natural and synthetic antioxidants are recommended to address this issue. This study investigated the use of ascorbyl palmitate as a natural additive in a previously evaluated mixture of castor and sesame oils, focusing on its tribological performance in an AISI 4140/AISI 52100 tribopair. The chemical composition of the biolubricants was analyzed using Fourier-transform infrared spectroscopy (FTIR), while the physical properties such as density and kinematic viscosity were measured at various temperatures. To evaluate their suitability for tribological applications, their friction and wear performance were assessed using a ball-on-disk tribometer. The friction coefficient, coefficient of lubrication efficiency (CLE), and wear behavior (volume loss, wear rate, and wear mechanism) were analyzed. This study demonstrates a notable enhancement in the tribological properties of the binary mixture with varying ascorbyl palmitate concentrations (0.25, 0.5, and 0.75 wt.%). The addition of ascorbyl palmitate reduces wear by forming protective interfacial layers, resulting in a low friction coefficient of 0.07 and a 46% reduction in volume loss. Some concentrations of ascorbyl palmitate also mitigated the severity of the wear mechanism.

Lubricants doi: 10.3390/lubricants14070265

Authors: Uma Maheshwera Reddy Paturi Mohammed Muttahir Satrio Herbirowo Nagireddy Gari Subba Reddy

Sustainable machining is gaining attention in modern manufacturing due to its cleaner operations, improved resource utilization, and reduced environmental impact. Among sustainable machining methods, minimum quantity lubrication (MQL) successfully minimizes cutting fluid consumption while maintaining adequate cooling and lubrication. This review examines recent developments and future directions in MQL-assisted machining, with particular emphasis on machine learning (ML)-based modeling and optimization techniques. A systematic review comprising literature identification, screening, scientometric analysis, and critical evaluation was employed to analyze 120 papers published mainly between 2010 and 2026. The reviewed studies employed ML models such as artificial neural networks, support vector machines, random forests, gradient boosting, and hybrid optimization approaches to predict machinability parameters, including surface roughness, tool wear, cutting force, cutting temperature, energy consumption, and chip morphology. The findings indicate that ML-assisted MQL processes improve prediction accuracy, machining efficiency, process monitoring, and sustainability performance by reducing energy consumption, minimizing cutting fluid usage, and improving machining quality. The analysis also identifies key research gaps and prospects for intelligent and sustainable machining.

Lubricants doi: 10.3390/lubricants14070264

Authors: Praveen Kumar Verma N. Jeyaprakash Hitesh Vasudev Karthik V. Shankar Jaspinder Singh

Additive manufacturing (AM) has emerged as a transformative manufacturing technology for producing complex components with unprecedented design flexibility. However, the widespread application of AM parts in tribological environments is often limited by inherent defects such as high surface roughness, porosity, residual stresses, anisotropy, and weak interlayer bonding, which adversely affect friction, wear resistance, and tribocorrosion performance. This review critically examines the tribological behavior of AM materials and components, emphasizing the influence of processing routes, material selection, secondary reinforcing phases, and microstructural evolution on tribological performance. Particular attention is given to surface engineering strategies, including thermal spray coatings, laser surface treatments, plasma electrolytic oxidation, vapor deposition technologies, and mechanical surface modification techniques for mitigating AM-induced defects and improving surface durability. Recent advances in machine learning (ML) and artificial intelligence (AI) for wear prediction, process optimization, and intelligent tribological monitoring are also discussed. The review highlights the relationships among manufacturing parameters, surface integrity, and wear mechanisms, while identifying key challenges associated with process variability, long-term reliability, and industrial implementation. Future research should focus on multifunctional surface systems, smart coatings, real-time condition monitoring, and data-driven design approaches to accelerate the deployment of tribologically optimized AM components in aerospace, biomedical, automotive, and energy applications.

Lubricants doi: 10.3390/lubricants14070263

Authors: Fulya Cemaloğlu Barış Özlü Halil Demir Fuat Kara

In this study, the milling performance of Inconel 718 alloys produced by forging (WP1), Inconel 718 produced by Selective Laser Melting (SLM) (WP2), and Inconel 718 (WP3) subjected to heat treatment after SLM, under different cooling/lubrication conditions, was evaluated using experimental and artificial intelligence-based approaches. Microstructural analysis showed a homogeneous fine-grained structure in WP1, while WP2 exhibited dendritic features and porosity. Heat treatment improved the microstructural homogeneity of WP3. The hardness values of WP1, WP2, and WP3 were 457 Hv, 303.33 Hv, and 391 Hv, respectively. Milling experiments yielded cutting forces of 336.5–1185.9 N, surface roughness values of 0.22–1.39 µm, and cutting temperatures of 168–658 °C. Compared with dry machining, MQL reduced average cutting force and cutting temperature by 15.5% and 18.65%, respectively, while improving tool wear and surface integrity. Machine learning models including LR, DTR, SVR, and GPR were developed to predict machining responses. GPR provided the highest prediction accuracy, achieving 98.72% for cutting force and 98.99% for cutting temperature. The results demonstrate that manufacturing route and cooling strategy significantly affect the machinability of Inconel 718 and that machine learning techniques can effectively support machining process optimization.

Lubricants doi: 10.3390/lubricants14070262

Authors: Yoga Palani Hao Li Deli Peng Jingyi Zhang

Structural superlubricity enables ultralow friction and wear–free sliding by cancellation of lateral forces at incommensurate, weakly interacting interfaces. However, edge–induced friction remains non–negligible. In this work, we systematically quantify edge–induced friction in atomically smooth single–crystal graphite/graphite interfaces using a controlled edge–contact architecture. By introducing holes with well–defined geometries and sizes, we systematically vary the total contact edge length while preserving the crystallinity and atomically smooth morphology of the interior graphite surface. The results reveal that friction enhancement in the patterned graphite/graphite interface is dominated by edge–mediated interactions at the hole boundary, demonstrating that total edge length, rather than real contact area, is the primary parameter governing interfacial friction. This outcome diverges from conventional contact–area–dependent friction theories, bringing to light the paramount importance of edge contributions in structurally superlubric interfaces. We show that engineering the hole perimeter provides a route to tuning friction in layered materials without changing material composition or external operating conditions.

Lubricants doi: 10.3390/lubricants14070261

Authors: Wei Jin Chao Liu Tongtong Liu Jinfeng Huang Chengshi Zhang Feng Jin Feibin Zhang Chao Zhang

Accurate quantitative diagnosis of spall sizes in rolling bearings is often hindered by the limitations of conventional dynamic models in characterizing temperature-dependent contact behavior. To address this issue, this paper presents a quantitative diagnosis method that incorporates point-contact thermo-elastohydrodynamic lubrication (TEHL) characteristics into a classical bearing dynamic framework. Specifically, rather than using prescribed or constant contact parameters, an improved equivalent stiffness–damping representation of the bearing contact interface is formulated based on TEHL-derived oil-film pressure, thickness, and temperature, while taking into account the inner–outer raceway thermal asymmetry. This localized lubricated contact representation is subsequently integrated into a classical five-degree-of-freedom (5-DOF) dynamic model to evaluate the double-impact response caused by outer-ring spalls. Comparative simulations using conventional 5-DOF, 4-DOF, and 2-DOF models, alongside experiments on a 6205-2-RS bearing with a 0.6 mm outer-ring defect, validate the proposed method. The results demonstrate that utilizing the TEHL-derived stiffness–damping representation significantly reduces spall-size estimation errors, improving both the accuracy and the physical interpretability of bearing fault quantification under thermally coupled conditions.

Lubricants doi: 10.3390/lubricants14070260

Authors: Muhammad Auni Hairunnaja Abdullah A. Alazemi Mohd Aizudin Abd Aziz

Waste cooking oil (WCO) is generated globally in large quantities, and improper disposal contributes to significant environmental problems. Recently, WCO has attracted increasing attention as a sustainable base fluid for lubricating grease due to its biodegradability, low cost, and favorable physicochemical properties after thermal degradation. This study presents a systematic review conducted with reference to the PRISMA 2020 guidelines of WCO-based grease and lubrication systems published between 2000 and 2025. Scopus was systematically searched, resulting in 22 peer-reviewed studies meeting the inclusion criteria. The review shows that thermal degradation increases WCO viscosity, polarity, and the relative proportion of saturated fatty acids, thereby enhancing boundary lubrication behavior. Tribological performance was found to depend more strongly on formulation strategy than feedstock variability, provided that appropriate pre-treatment is applied. Optimized WCO-based greases achieved coefficient of friction (COF) values as low as 0.0253 and wear scar diameters (WSD) of 467 µm, demonstrating performance comparable to conventional mineral oil greases. Non-soap thickeners exhibited thermal stability exceeding 350 °C, while additives such as molybdenum disulfide (MoS2) improved friction and wear performance. Overall, this review establishes a structure–property–performance framework linking thermal degradation chemistry, formulation design, and tribological behavior in WCO-based lubrication systems while highlighting challenges related to standardization, long-term stability, and industrial validation.

Lubricants doi: 10.3390/lubricants14070259

Authors: Xiuli Liu Changyao Zhang Lingfeng Gao Jing Liu

To address the issues of local high temperatures, thermal stress concentration, and the susceptibility to spalling of homogeneous ceramic coatings in disc brakes under high-frequency thermal–mechanical cyclic loading, this paper proposes a surface design scheme incorporating a functionally graded material (FGM) coating along the thickness direction. A three-dimensional thermal frictional contact model of a graded coated brake disc with continuously varying material properties (silicon carbide/gray cast iron) along the thickness direction is established by developing user subroutines on the Abaqus finite element platform. The effects of exponential, power-law, and trigonometric gradient distributions on the transient temperature and stress fields are systematically compared. The results indicate that the high thermal conductivity silicon carbide coating significantly reduces the disc surface temperature; however, a homogeneous coating induces interfacial thermal stress concentration due to a sudden stiffness mismatch. The graded design effectively mitigates the stress concentration through a smooth transition of material properties. Taking the power-law function (n = 1.5) as an example, this design not only significantly reduces the maximum disc surface temperature but also limits the residual equivalent stress at the end of braking to 245 MPa, which is approximately 24.8% lower than that of the homogeneous coating (325.8 MPa). The study demonstrates that the gradient function exerts a stronger regulatory effect on the stress field than on the temperature field, meaning the two cannot be simultaneously optimized. Nevertheless, exponential functions and power-law functions with small exponents can achieve a favorable balance of thermal–mechanical performance. This research reveals the mechanism by which thickness-direction gradient distributions regulate thermal–mechanical coupling behavior, providing a theoretical basis for the gradient design of thermal fatigue-resistant friction components.

Lubricants doi: 10.3390/lubricants14070258

Authors: Qun Li Xufeng Song He Zhang Yuanke Wu Liquan Yang Erbo Liu Rongrong Li

To address the complex and highly variable wheel–rail adhesion behavior on high-speed railway curves, this study establishes a numerical wheel–rail rolling contact model based on starved elastohydrodynamic lubrication (EHL) theory and Herschel–Bulkley rheological characteristics. The model validation yielded RMSE = 0.0228, MAE = 0.0217, MAPE = 11.80%, R2 = 0.828, and a 95% confidence interval of the mean residual of −0.0298 to −0.0136. The study focuses on the initial operational phase after application, systematically quantifying the fluid-dynamic regulation mechanisms of water-based friction modifiers once a thin, starved lubricating film has been formed on the rail surface under curving conditions. By analyzing rail profiles (CHN60 and CHN60N), operating parameters, and track geometry, this study shows how adhesion behavior on curved track sections is governed by the coupled effects of contact mechanics and lubrication. As the outer rail superelevation increases from 0 to 70 mm, the adhesion coefficient decreases by approximately 15–25%, mainly because the reduced normal force shifts the wheel–rail interface toward the Stribeck transition regime. Increasing axle load from 14 t to 30 t reduces the dimensionless film thickness, but the enlarged contact area contributes to a more stable adhesion level, with an increase of about 12%. Compared with the CHN60 profile, the CHN60N profile exhibits better geometric conformity, producing a lubricating film that is 10–15% thicker and leading to a lower and more stable adhesion coefficient, decreasing from approximately 0.35 to 0.1. The results also identify a critical lateral displacement of around −4 mm, beyond which the contact radius becomes stable and the adhesion coefficient reaches a minimum plateau. These findings clarify the competing effects of fluid entrainment and metallic asperity contact, and provide quantitative guidance for friction management and friction modifier application on curved track sections.

Lubricants doi: 10.3390/lubricants14070257

Authors: Changguang Zhou Chao Luo Bohao Meng Jun Xu Maocheng Jiang Hutian Feng

This paper addresses the challenges of life prediction and reliability assessment for ball screws under complex operating conditions by proposing a reliability prediction model that incorporates full-life fatigue damage. First, a full-life fatigue life prediction model encompassing the three stages of crack initiation, propagation, and fatigue cumulative spalling is developed. This model comprehensively considers the effects of material properties, geometric parameters, and loading history, enabling a systematic description of the fatigue process of ball screws from initial use to final failure. Based on this life prediction model, an enhanced adaptive Kriging–Monte Carlo simulation (E-AK-MCS) method is introduced to construct a surrogate model, which efficiently solves the high-dimensional nonlinear limit state function, thereby enabling accurate reliability assessment and parameter sensitivity analysis. Experimental results demonstrate that the proposed model achieves an average life prediction accuracy of 94.15% for the 8020 and 5005 specification ball screws, indicating its preliminary engineering applicability under the tested conditions. Reliability analysis indicates that ball diameter fracture toughness, and initial crack size are key factors influencing service reliability. This research provides systematic theoretical methods and technical support for the accurate life prediction, reliability design, and process optimization of ball screws.

Lubricants doi: 10.3390/lubricants14070256

Authors: Qingsong Wang Guoqiang Gao Jinhui Chen Tianwei Lan Pengyu Qian Bo Tang Zheng Li Hong Wang Guizao Huang Jing Hao Guangning Wu

The pantograph–catenary system (PCS) is a critical component through which electrified railway trains obtain electrical energy, and the current-carrying friction and wear behavior at the pantograph–catenary interface directly affect current collection quality and operational safety. Water environments, particularly icing conditions, may induce contact instability, arc ablation, and abnormal wear. Therefore, this paper provides a comprehensive review of research progress on the current-carrying friction and wear behavior of C/Cu contact pairs under water environments. It focuses on the interfacial evolution characteristics under three different phase states of water and analyzes their influence mechanisms on lubrication conditions, current transmission, and wear behavior. Typical protection strategies, including speed restriction, mechanical de-icing, thermal de-icing, and anti-icing measures, are summarized, and their applicability and current development status are discussed. Finally, it is suggested that future research should focus on the development of high-performance carbon strip materials, accurate monitoring of ice morphology and types, and efficient hybrid anti-/de-icing technologies, thereby ensuring the reliable operation of high-speed railways under icing conditions.

Lubricants doi: 10.3390/lubricants14070255

Authors: Tobias B. Humpf Maximilian A. Oppold Anjali K. M. DeSilva Muditha Kulatunga Wolfgang Rimkus

Accurate prediction of wear in lubricated metal-to-metal contacts remains a critical challenge, as calibration parameters derived from laboratory tests often lack transferability to finite element method (FEM) simulations. While classical linear Archard models are widely applied, they fail to capture the nonlinear load-dependent wear behavior observed under varying operating conditions. This study addresses this limitation by developing and validating a nonlinear wear formulation based on the Archard–Bayer law within a coupled experimental–numerical framework. A comprehensive Pin-on-Disc test matrix was conducted under lubricated conditions using carbide–steel contacts across varying loads and cycle counts. Wear progression was quantified and analysed using outlier-corrected weighted regression, yielding a force exponent mexp=1.58±0.34 and cycle exponent nexp= 0.41 ± 0.17. The calibrated nonlinear model was implemented in a FEM environment and systematically evaluated across multiple loading scenarios. The nonlinear formulation demonstrates improved predictive capability compared to the classical linear Archard model, particularly under higher load conditions (15 N–20 N), where deviations between simulation and experiment remain below 11%. The FEM-calibrated exponent (m = 1.35) lies within the 95% confidence interval of the experimental value, indicating that numerical adjustments required for stability are statistically non-significant. The results show that nonlinear wear models provide a more accurate representation of load-dependent wear behavior but require constrained calibration ranges for reliable application. The proposed methodology enables robust transfer of experimentally derived wear parameters into FEM simulations and provides a practical basis for tool-life prediction, parameter tuning, and model deployment in sheet metal forming processes.

Lubricants doi: 10.3390/lubricants14070254

Authors: Junyan Ma Jiafu Yang Wenwen Yang Yonggang Song Adilanmu Sitahong Duoming Pan Yong Huang

In cold hydrogenation processes, bent pipes are highly susceptible to severe localized erosion under hydrogen–silica powder gas–solid two-phase flow. However, high-fidelity numerical simulations are computationally expensive and thus inadequate for rapid assessment under multiple operating conditions. To overcome this limitation, an MI-UK-GPR-based method is proposed for predicting the erosion rate of cold hydrogenation bent pipes. Based on a validated CFD model, six input variables, namely pipe inner diameter, curvature ratio, bend angle, particle mass flow rate, particle size, and particle velocity, were selected. Latin hypercube sampling was employed to generate parameter combinations, and the corresponding maximum erosion rates were obtained through high-fidelity CFD simulations to construct an LHS-CFD sample database. The input variables were then normalized, and the maximum erosion rates were log-transformed. On this basis, an MI-UK-GPR model integrating a mechanistic trend term with a Gaussian process residual term was developed to capture both the global trend of erosion peaks and local nonlinear deviations. Model performance was assessed using leave-one-out cross-validation with MAE, RMSE, MAPE, R2, and PICP as evaluation metrics. The results show that, under leave-one-out cross-validation, the proposed MI-UK-GPR model achieved an MAE of 7.10 × 10−5, an RMSE of 1.29 × 10−4, a MAPE of 14.53%, an R2 of 0.9573, and a PICP of 88.33%, outperforming RSM, SVR, and ordinary GPR in terms of overall prediction performance. In addition, for 50 independent operating conditions, the total computational time of parameterized CFD batch simulations was 5083.51 s, whereas the trained MI-UK-GPR model required only 0.004860 s, corresponding to a speedup of approximately 1.05 × 106. Overall, the proposed method provides a physically consistent, uncertainty-aware, and computationally efficient framework for rapid erosion assessment of cold hydrogenation elbows under multiple operating conditions.

Lubricants doi: 10.3390/lubricants14070253

Authors: Paul Henke Daniel Thiele Leo Ruehrmund Annett Klinder Sven Krueger Philipp Damm Maeruan Kebbach Rainer Bader

Preclinical testing is essential for evaluating new implant designs and materials for total knee replacement (TKR). Standardized wear tests, such as ISO 14243, are widely accepted but only partially represent physiological kinematics and kinetics, as they do not account for all six degrees of freedom of the knee joint. More advanced setups, including robotic systems and joint simulators, enable complex load cases; however, the influence of lubrication conditions and testing speeds remains insufficiently standardized. This study investigated the kinematic and kinetic effects of different lubrication conditions (dry, synthetic synovial fluid, silicone oil) and speeds (static, 10%, 50%, 100% of normal gait) in a joint simulator setup using a posterior cruciate ligament-retaining TKR during level walking. Complementary pin-on-disk measurements revealed significant dependencies on both lubrication and speed. During joint simulator tests, omitting lubrication resulted in more than double the maximum flexion–extension moment, while the range of anterior–posterior femoral translation increased by approximately 73%. At 50% and 100% speed, silicone lubrication yielded results comparable to static tests, in contrast to the dry and synthetic synovial fluid conditions. These findings demonstrate that physiologically relevant lubrication and appropriate test speeds are essential for obtaining reliable results in experimental studies of TKR dynamics.

Lubricants doi: 10.3390/lubricants14070252

Authors: Juan Pablo Abdelnabe Walter Roberto Tuckart Eduardo Tomanik Wania Christinelli Germán Prieto

Industrial gear contacts operate under mixed-to-boundary lubrication where reliable antiwear protection is essential. This study assesses whether carbon nanomaterials can enhance the performance of zinc dialkyldithiophosphate (ZDDP) under severe conditions. A crossed-cylinder Reichert configuration (2 GPa, 75 °C, 1 m/s) with PAO6 was used to test ZDDP (1 wt%) and its blends with carbon nanotubes (CNT, 0.05 wt%), graphene nanoplatelets (GNP, 0.05 wt%), and few-layer graphene (FLG, 0.05 wt%) at 1, 10 and 60 min. The lubrication regime was boundary. Friction, specific wear rate (k), and tribofilm coverage were quantified. Oils containing only carbon nanoparticles could not sustain the test (seizure within minutes), confirming the necessity of ZDDP. After 60 min, average CoF remained similar across formulations and largely governed by ZDDP. By contrast, wear showed marked differences: relative to ZDDP alone (A), ZDDP + CNT (F) and ZDDP + GNP (G) reduced k by 52% and 48%, respectively, and exhibited higher tribofilm coverage (F = 68%, G = 72% vs. A = 57%). Time-resolved tests revealed that long-duration degradation was mitigated in F and G: from 10 to 60 min, k rose by 72% (F) and 58% (G) versus 159% for A; coverage decreased by only 8% (F) and 3% (G) versus 22% for A. SEM–EDS indicated no major differences in average elemental chemistry among formulations, suggesting an improvement on tribofilm coverage/stability rather than compositional change.

Lubricants doi: 10.3390/lubricants14070251

Authors: Huajie Qu Meiqin Liang Zhongpu Wen

To solve the drawbacks of conventional long-cycle wear tests for miniature standing- wave linear ultrasonic motors, an accelerated equivalent wear model and test system were proposed in this work. After primary screening of multiple pair materials, graphite and Al2O3 were adopted to modify epoxy films. The optimal friction pair is composed of 6061 hard anodic oxidation film and ECA105 composite film. The matched pair exhibits excellent driving stability and low wear loss, with fatigue wear as the main wear form. Graphite and Al2O3 exert synergistic anti-wear and load-bearing effects via forming a stable transfer film on the friction interface. Experimental results confirm that the accelerated test is equivalent to a full-life durability test. The presented method and optimized friction pair can effectively guide the development of high-performance ultrasonic motors.

Lubricants doi: 10.3390/lubricants14070250

Authors: Gaobo Lou Xiaoling Yao Yuhao Fang Yifan Chen

The influence of BN particle size on lithium grease performance was systematically compared among a base grease (Li), a micro-BN (3 µm, 0.1 wt%) modified grease (Li + 0.1% mBN), and a nano-BN (50 nm, 0.1 wt%) modified grease (Li + 0.1% nBN). SEM shows that addition nano-BN leads to a more compact soap fiber networks, whereas micro-BN tends to agglomerate and provides limited reinforcement, leaving the base grease with a loose, porous network. Consequently, Li + 0.1% nBN outperforms both Li and Li + 0.1% mBN in dropping point (199.5 °C vs. 194.9 °C and 198.6 °C), oil separation (0.39% vs. 0.64% and 0.44%), and flow point (49% vs. 45% and 47%). Its plateau modulus is significantly higher, reflecting stronger network entanglement. However, Li + 0.1% nBN shows lower structural recovery (61.0%) than Li (65.8%) and Li + 0.1% mBN (67.2%) due to rigid particle–fiber junctions. Notably, Li + 0.1% mBN exhibits a unique frequency-dependent viscoelasticity: higher tanδ at low frequencies but lower tanδ at high frequencies relative to Li. Tribologically, Li + 0.1% nBN reduces friction coefficient by 35% and wear scar diameter by 12.7% compared with Li, outperforming Li + 0.1% mBN. XPS confirms a protective hybrid tribofilm (BN + organic nitrogen species + iron oxides) on the nano-BN lubricated surface. Particle size critically governs BN–fiber interactions and the resulting rheological and tribological performance.

Lubricants doi: 10.3390/lubricants14070249

Authors: Hangyeol Jo Sung-Ho Hong Choon-Su Park Moonsuk Kim Miao Dai Sang-Woo Ban

Early fault diagnosis of ball bearings is essential for maintaining the reliability of rotating machinery and preventing unexpected downtime. This study proposes a fault diagnosis framework that combines non-contact ultrasonic visualization with a lightweight convolutional neural network (Light CNN). Seven bearing conditions, including ferrous-particle contamination and grease starvation, were investigated using ultrasonic, vibration, and acoustic emission (AE) sensors under identical experimental conditions. Saliency-map extraction and two-dimensional histogram analysis were applied to ultrasonic RGB images to generate compact feature representations, which were compressed into 20 × 20 feature maps and used as inputs to a three-layer Light CNN. The proposed method achieved an average classification accuracy of 99.98% and an F1-score of 99.98%. In addition, an average inference throughput of 11.47 IPS was obtained, representing approximately ten times higher computational efficiency than vibration- and AE-based approach-es. Stable diagnostic performance was also maintained under a low-speed operating condition of 500 rpm. These results demonstrate the effectiveness of combining ultrasonic visualization and a lightweight CNN for accurate and computationally efficient bearing fault diagnosis.

Lubricants doi: 10.3390/lubricants14070248

Authors: Enwen Zhou Jingwei Zhang Lili Fan Hui Qi Yuan Zhang Huanqing Zhang

When the magnetic field of a magnetic levitation bearing is lost, the cage-less ball bearing acts as a backup bearing to support the falling spindle. To ensure uniform distribution of the rolling elements in the cage-less ball bearing, researchers have designed local functional grooves on the outer ring raceway. However, the periodic motion of the rolling elements causes damage to these grooves, leading to discrete failure of the rolling elements and resulting in vibration during bearing operation. Therefore, this paper investigates the dynamic characteristics of the rolling elements and the factors influencing bearing vibration following damage to the local functional grooves in caged ball bearings. A vibration model for bearings with damaged functional grooves is established, and the research is conducted through theoretical analysis, numerical simulation, and experimental validation.

Lubricants doi: 10.3390/lubricants14060247

Authors: Adolfo Senatore Ibrahim Sulimieh Oleksii Nosko

Non-exhaust emissions (NEEs), particularly brake wear particles (BWPs), have become a dominant source of traffic-related particulate matter (PM), accounting for approximately 77% of PM10 and 60% of PM2.5 emissions. Accurate quantification of these emissions is essential under increasingly stringent regulations such as Euro 7. However, measurement reliability is strongly influenced by particle transport and sampling losses. This review provides a state-of-the-art analysis of laboratory-scale methodologies for investigating BWP emissions, focusing on pin-on-disc (PoD) tribometers and inertia dynamometer systems. Particular attention is given to chamber design, airflow management, sampling configurations, and the mechanisms governing particle transport efficiency. The literature indicates that PoD systems are often affected by complex and non-uniform flow fields, leading to incomplete particle capture and reduced representativeness, whereas inertia dynamometers, especially when coupled with constant volume sampling (CVS), provide more controlled and reproducible conditions. Key loss mechanisms, including inertial deposition, diffusion, gravitational settling, and non-isokinetic sampling effects, are major contributors to uncertainty. The reviewed studies highlight that aerodynamic limitations in PoD systems, particularly box-shaped chambers, promote flow recirculation and particle losses. Advanced optimization approaches that combine artificial neural networks (ANNs) with computational fluid dynamics (CFD) simulations show strong potential to improve system design and measurement reliability.

Lubricants doi: 10.3390/lubricants14060246

Authors: Huajie Wu Qiaoyi Wang Laihua Tao Xin Jiang Longwei Geng

Friction at the cold rolling interface is affected jointly by the surface roughness, lubrication state, local pressure, and relative sliding. A constant friction coefficient is therefore insufficient to describe its non-uniform distribution along the contact arc. Accordingly, this study proposes a macro–micro two-scale mixed-lubrication and dynamic friction model based on the measured roll roughness. First, the measured roll roughness profile was represented within a finite effective scale interval by a scaled and truncated Weierstrass–Mandelbrot (W–M) function. The parameters D and G were obtained as finite-scale W–M roughness parameters and were introduced into a mixed-lubrication load-sharing model to calculate the local mixed-lubrication friction coefficient. The pressure distribution along the contact arc was calculated using the Karman equation, and the local macroscopic pressure was mapped to a representative microscopic contact load. Finally, the mixed-lubrication friction coefficient was used to calibrate the dynamic friction factor separately in the forward-slip and backward-slip zones, and the friction stress distribution along the contact arc was calculated. For the selected effective scale interval and preprocessing procedure, the fitted W–M roughness parameters were D = 1.528 and G = 9.15 × 10−8 m. The W–M parameter D had a more significant influence on the mixed-lubrication friction coefficient and load-sharing behavior than the scale parameter G. Increasing the rolling speed strengthened the oil-film load-carrying effect and reduced the equivalent interfacial friction coefficient. The friction stress was positive in the backward-slip zone and negative in the forward-slip zone, with a direction reversal near the neutral point. Field forward-slip inversion showed that both the simulated and measured equivalent friction coefficients decreased with increasing rolling speed, with a difference of approximately 0.009~0.017. The proposed model can capture the main trend of cold rolling interfacial friction with variations in the rolling speed and contact state.

Lubricants doi: 10.3390/lubricants14060245

Authors: Iliyan Damyanov Durhan Saliev Iliyana Naydenova Ivaylo Peev Hristo Konakchiev Iliyan Ognyanov

Thermal overload in internal combustion engines may progressively destabilize lubricant-film integrity and promote severe tribological deterioration within highly stressed contact interfaces. This study investigates the thermo-tribological degradation sequence of a high-mileage gasoline engine subjected to prolonged idle operation under impaired cooling conditions, ultimately resulting in engine seizure. The investigated engine had accumulated 356,724 km, while the lubricant had remained in service for approximately 26,724 km prior to the experiment. The post-failure investigation combined teardown inspection, geometrical camshaft assessment, reverse gravimetric reconstruction, hydraulic tappet surface profiling, XRF surface characterization, laboratory oil analysis, and SEM/EDS evaluation of wear debris. The results demonstrated strongly localized degradation concentrated primarily within the cam–tappet interfaces. Severe non-uniform camshaft wear was accompanied by pronounced hydraulic tappet surface damage and evidence of unstable boundary-lubrication conditions. Laboratory oil analysis revealed elevated wear-metal concentrations, depletion of the alkaline reserve, increased oxidation indicators, and a final Class D oil condition assessment. SEM/EDS characterization identified Fe-bearing wear debris associated with sustained material removal and debris recirculation during the final degradation stage. The combined evidence supports a coupled thermo-tribological degradation mechanism involving lubricant deterioration, boundary-lubrication instability, adhesive wear acceleration, oxidative surface degradation, and debris-assisted surface damage preceding final engine seizure. The present case study provides experimentally documented evidence of lubrication collapse under real-engine thermal runaway conditions and highlights the critical role of lubricant condition in maintaining tribological stability under severe thermal loading.

Lubricants doi: 10.3390/lubricants14060244

Authors: Kamlesh V. Chauhan Sushant Rawal Nicky P. Patel Dattatraya Subhedar Vandan V. Vyas

The choice of brass as the substrate due to its widespread use in soft non-ferrous industrial components such as bearings and electrical connectors creates the primary basis of novelty in this study. While prior tribological studies on titanium aluminum nitride (TiAlN) coatings is primarily focused on hard substrates such as steel and WC–Co, this work addresses the research gap by presenting a systematic investigation of the combined influence of sputtering power and working pressure on TiAlN coatings deposited on brass. Application of TiAlN coatings on brass surfaces was accomplished using magnetron sputtering. Within the scope of this study, the influence of sputtering power and working pressure on the tribological and structural attributes of TiAlN films is evaluated. The analysis of surface morphology is carried out using scanning electron microscopy (SEM), while structural characteristics revealed a progressive increment in the intensity of the (103) and (107) peaks with variation in deposition parameters. An analysis was conducted to evaluate the tribological properties of the TiAlN coating using a pin-on-disk tribometer. The study involved varying the speeds, loads, and sliding lengths. The optimized condition achieved wear reduction as high as 22% compared to uncoated brass at a sliding distance of 785 m, which highlights the strong dependence of wear performance on deposition parameters. The wear rates of TiAlN-coated brass ranged between 1.03 × 10−3 and 5.87 × 10−4 mm3/Nm depending on parameters like load, sliding distance and speed. Conversely, TiAlN-coated brass pins prepared at varying power showed wear rates ranging from 1.83 × 10−4 to 5.87 × 10−4 mm3/Nm. These findings demonstrate that optimization of TiAlN coating parameters on brass can significantly enhance wear resistance, which ultimately improves the durability and performance of engineering components in tribological applications.

Lubricants doi: 10.3390/lubricants14060243

Authors: Cevher Kursat Macit Turan Gürgenç Bunyamin Aksakal Naim Aslan

Copper and its alloys are widely used in electrical, automotive, aerospace, and energy applications because of their excellent thermal and electrical conductivity. However, the low hardness and poor wear resistance of pure Cu limit its use under tribologically demanding sliding conditions. In this study, Cu matrix composites reinforced with 1 wt.% boron carbide (B4C), boron (B), chromium (Cr), cobalt (Co), alumina (Al2O3), and graphite (Gr) were fabricated by powder metallurgy and comparatively evaluated under identical processing and testing conditions. Phase constitution and microstructural characteristics were analyzed by XRD, SEM, and EDS, while mechanical and tribological behavior was assessed by Vickers hardness and dry sliding wear tests. All reinforcements improved the hardness of the Cu matrix compared with unreinforced Cu. The hardness increase followed the order Cu–B4C (68.91%) > Cu–B (66.43%) > Cu–Gr (63.97%) > Cu–Al2O3 (61.79%) > Cu–Cr (42.69%) > Cu–Co (36.04%). Dry sliding wear tests, performed under a 10 N normal load, 0.05 m s−1 sliding speed, and 1000 m sliding distance against a 316L stainless-steel ball, showed that all reinforced composites exhibited lower mass loss and more stable sliding behavior than pure Cu. Among all samples, Cu–B4C displayed the best wear performance, with a 154.8% improvement in wear resistance relative to pure Cu. SEM analysis of the worn surfaces revealed that reinforcement addition reduced severe plastic deformation, groove formation, and delamination, leading to a more stable wear regime. Graphite- and boron-containing composites benefited from interfacial lubrication and contact stabilization, whereas B4C and Al2O3 improved wear resistance through rigid-particle strengthening and enhanced load-bearing capacity. By comparing ceramic, metalloid, metallic, oxide, and solid-lubricating reinforcements at the same low addition level and under identical processing and testing conditions, this study provides a reinforcement-selection framework for Cu-based composites requiring improved hardness and dry-sliding durability.

Lubricants doi: 10.3390/lubricants14060242

Authors: Mengyao Si Xinhang Shang Li Sun Yaqing Dong Xue Jiang

Current deep learning methods for tool wear monitoring suffer from poor interpretability and struggle to reveal the intrinsic relationship between signals and wear states. To address this issue, this paper presents an interpretable tool wear monitoring method based on Sparse Identification of Nonlinear Dynamics (SINDy). Multi-domain features are extracted from cutting force and acoustic emission signals to construct a time series. The SINDy algorithm is used to identify ordinary differential equations that describe the evolution of tool wear. An iterative “predict-validate-correct” mechanism is applied to optimize model parameters. Experimental results show that the mean absolute percentage error (MAPE) between the predicted and actual values is below 6%. Moreover, the optimal model demonstrates an average MAPE as low as 0.067% in cross-condition tests. This study provides an effective solution for online tool wear monitoring that achieves high precision, strong generalization, and physical interpretability.

Lubricants doi: 10.3390/lubricants14060241

Authors: Xingang Xie Yeteng Li Zhixuan He Qian Deng Yining Zhang Tingshuo Zhang

Signals generated during tool wear are nonlinear, non-stationary, and easily affected by machining noise, which makes reliable tool condition monitoring difficult in intelligent manufacturing. To address this issue, this study proposes a tool wear degree classification framework, FCTrans-CA, that fuses time-domain and frequency-domain information through a lightweight cross-attention (CA) bridge. Fast Fourier transform (FFT) is first used to obtain frequency-domain representations. The raw time-domain signals are processed by a multi-scale one-dimensional convolutional neural network (MS-CNN) to extract temporal wear features, while the FFT-derived representations provide complementary spectral cues. These two feature streams are fused by an asymmetric CA module in which frequency-domain features guide the selection of wear-sensitive temporal features. K-means clustering is used to divide the measured flank wear (VB) trajectory of each tool into initial-, normal-, and severe-wear stages, thereby reducing subjectivity in label generation. Experiments on the PHM2010 milling dataset show that FCTrans-CA achieves 99.43% classification accuracy on 40,648 test samples. The results indicate that cross-domain feature interaction improves the separability of wear states and provides a reproducible data-driven route for tool wear monitoring.

Lubricants doi: 10.3390/lubricants14060240

Authors: Nao Hu Lili Lian Liangtao Xie Bingjie Ma Sicong Sun Jianguo Yang Guanjun Zhang Lei Hu Jun Li

Journal bearings are prone to failure due to lubrication state degradation under extreme operating conditions. To address the unclear transition mechanism and undefined state boundaries under insufficient lubrication, a coupled tribological model of engine journal bearings was established. Through parameter analysis and dynamic failure mechanism study, the effects of radial clearance, temperature, rotational speed, load, and surface roughness on the lubrication state transition were revealed. The results indicate that radial clearance, oil temperature, rotational speed, applied load and surface roughness are all decisive factors for lubrication transition, and every parameter has its unique critical threshold; once exceeding the limit, the oil film integrity is damaged and the lubrication rapidly shifts from mixed lubrication toward boundary lubrication. After crossing critical thresholds, aggravated asperity contact further triggers continuous temperature rise and viscosity reduction, which may induce closed-loop thermal deterioration and eventually accelerate bearing failure. The research findings provide a theoretical basis for robust design and operational safety monitoring of journal bearings.

Lubricants doi: 10.3390/lubricants14060239

Authors: Jiaru Zhang Tao Yu Libin Lu

Friction and wear are major factors affecting the efficiency and reliability of mechanical systems, leading to increasing interest in laser surface texturing (LST) for tribological surface engineering. This review summarizes the development of LST from conventional surface modification to multifunctional interface design and discusses the underlying process–structure–performance relationships. Different lubrication-dependent mechanisms, including micro-hydrodynamic pressure generation, wear debris entrapment, contact stress regulation, metallurgical strengthening, and wettability control, are analyzed under hydrodynamic, boundary, and dry sliding conditions. Representative processing technologies, including nanosecond, ultrafast, direct laser interference patterning (DLIP), and liquid-assisted laser processing, are compared in terms of fabrication precision, thermal effects, scalability, and tribological performance. Recent advances in hybrid surface engineering strategies integrating textures with coatings, solid lubricants, and surface hardening treatments are also reviewed. Representative applications involving bearings, cutting tools, biomedical implants, advanced ceramics, and additively manufactured materials are discussed to summarize application-oriented texture design principles. Current limitations related to thermal damage, manufacturing efficiency, coating stability, and long-term reliability are critically evaluated. Future developments are expected to focus on multifunctional surface integration, large-area manufacturing, and AI-assisted optimization for application-specific tribological interface design.

Lubricants doi: 10.3390/lubricants14060238

Authors: Honglei Zhang Tiantian Jing Jun Zhang Dong Lv Zhong Tang

This review explicitly focuses on agricultural attachments and executing components that interact directly with soil and crops, rather than the tractor vehicle itself. Operating within complex and variable farmland media environments, the key components of agricultural machinery have long been constrained by bottlenecks such as high-energy draught resistance, severe solid–liquid interfacial adhesion, and intense abrasive wear. Bionic functional surfaces, based on the coupling of micro-geometric morphology and surface-interface physical chemistry, provide a scientific approach to overcoming traditional tribological limitations by reconstructing the contact mechanics and fluid dynamics boundaries at the interface. This paper presents a comprehensive review of the latest research progress regarding bionic functional surfaces in the fields of friction reduction, wear resistance, and anti-adhesion in agricultural machinery. The article systematically categorises typical biological prototypes, such as soil-burrowing animals, aquatic organisms, and plant leaves, alongside their multidimensional feature extraction methods. It provides an in-depth analysis of core interaction mechanisms, ranging from static air cushion effects and dynamic wetting evolution to active electro-osmotic soil detachment, interfacial stress redistribution, and microscopic wear debris capture. Furthermore, it evaluates the efficacy of cross-scale coupled numerical simulation technologies in resolving interfacial interactions. At the engineering application level, this review extensively discusses the field performance of bionic structures in typical operational scenarios, including draught reduction in tillage and land preparation, blockage prevention in seed-metering channels, and low-damage harvesting in agricultural machinery. Finally, countermeasures are proposed to address the fatigue degradation of bionic surfaces under alternating field loads and the barriers to the large-scale fabrication of large-sized components. The paper further highlights the development trend towards the deep integration of bionic tribology with digital twins and intelligent wear-state perception technologies, aiming to provide systematic underlying theoretical and technical references for the research and development of the next generation of intelligent agricultural equipment characterised by low energy consumption and a prolonged service life.

Lubricants doi: 10.3390/lubricants14060237

Authors: Pavle Ljubojević Tatjana Lazović Marina Dojčinović

Cavitation in machine elements is often accompanied by surface degradation, material loss, and a reduction in functional performance and reliability. Despite extensive research on cavitation in hydraulic systems, its role in the behavior and durability of machine elements remains insufficiently addressed. This paper presents a critical review of cavitation and cavitation-induced erosion in machine elements, based on an analysis of relevant literature and standards. The study covers different types of components, including gears, plain and rolling bearings, and seals, with particular attention to the mechanisms of damage and the methods used for their investigation. The analysis shows that, although the fundamental mechanisms of cavitation are well understood and standardized testing methods are available, their application to machine elements is limited. Existing standards are not sufficiently adapted to specific components, while current numerical and experimental approaches rarely provide a direct link between cavitation phenomena and material degradation. The findings indicate the need for improved standardization, development of integrated modelling approaches, and a closer connection between cavitation mechanisms and the performance characteristics of machine elements. The presented analysis is relevant for design, reliability assessment, maintenance strategies, and the development of cavitation-resistant machine components in hydraulic and mechanical systems.

Lubricants doi: 10.3390/lubricants14060236

Authors: Xiaocui Wang Wanxin Li Quan Wang Zhiwei Wang Jiliang Mo

Train brake systems are characterized by strong friction and open-system features during the service process. Low environmental temperatures significantly affect the contact interface and the attrition characteristics of the braking frictional couple, thus intensifying friction-induced vibration and threatening operational safety. To elucidate the impact of environmental temperature on the frictional vibration characteristics of train brake systems, braking deceleration tests under different environmental temperatures were first conducted to obtain the evolution of vibration, noise, and friction coefficient with environmental temperature and brake disc rotational speed. Then, the Stribeck friction parameters under different environmental temperatures were identified using a genetic algorithm. On this basis, a brake system dynamic model was developed, incorporating disc–pad friction, wheel–rail adhesion, and the relative torsion between the brake disc and the wheelset, enabling accurate examination of the vibrational behaviour arising from friction under different environmental temperatures. And the dynamic relationship among environmental temperature, interface friction parameters, and vibration characteristics of the brake system during braking deceleration was elucidated. The findings indicate that as the environmental temperature decreases, the dynamic friction coefficient increases during the relatively high-speed braking phase, intensifying high-frequency unstable vibrations of the braking assembly. During the relatively low-speed braking phase, the friction coefficient exhibits an obvious negative-slope relationship with vehicle speed that means the friction coefficient increases as the speed decreases, and this negative slope effect is enhanced under low-temperature conditions. Consequently, it triggers intense stick–slip motion at the disc–pad interface and even severe vibrations of various components in the brake system, leading to a sudden increase in vibration intensity in the relatively low-speed range.

Lubricants doi: 10.3390/lubricants14060235

Authors: Marián Kučera Marek Svitok Tatiana Hýrošová Grzegorz Zajac

In this article, the authors have experimentally investigated the changes in four key properties of six non-edible low-impact energy carries based on rapeseed oil quality grade HM and viscosity grade VG46, which were used as a filling in the hydraulic system of a round wood sorting and transporting trolley. These oils were enriched with thermo-oxidizing, extreme-pressure additives, anti-foaming, and lubricating additives to enhance performance. Three supervised machine learning prediction algorithms were used to predict key parameters essential for optimizing their performance and RUL (remaining useful life), namely support vector regression (SVR), generalized additive model (GAM), and Gaussian process regression (GPR). The model’s performance was scored from multiple perspectives using metrics such as root mean square error (RMSE), mean absolute error (MAE), mean absolute percentage error (MAPE), and coefficient of determination (R2) to state actual values, thereby demonstrating the validity of the models in predicting lubricant lifespan. Based on the collected data, this study demonstrated that it is possible to predict the degradation of hydraulic oil factors to the limit state, integrate these parameters into a comprehensive metric for more accurate remaining useful life (RUL) estimation, and obtain actual operating trends. A negative correlation was found between the remaining useful life (RUL) and parameters such as acid number, kinematic viscosity, peroxide number, and water content. The comparison of modeling algorithms showed that all three algorithms adequately described the degradation patterns. By using these performance criteria, we defined the most accurate and reliable soft-computing model for predicting hydraulic fluid parameters, providing valuable insights into optimizing machine learning models for practical applications.

Lubricants doi: 10.3390/lubricants14060234

Authors: Oleksandr Stelmakh Yiqiao Guo Anatoliy Maystrenko Yansong Liu Ruslan Kostunik Alexsandr Vasylchuk Dmytry Kustovskyi Hao Zhang

In hybrid rolling bearings operating under extreme high-temperature and high-load conditions, steel rolling elements are prone to early failure, which has accelerated the widespread adoption of ceramic materials. To address the limitations of conventional studies, which have focused mainly on macroscopic wear parameters while neglecting subsurface failure mechanisms and the relationship among sintering process, microstructure, and fatigue performance, this work systematically compares the tribological behavior of Si3N4 ceramic balls fabricated by high-pressure electric resistance hot-pressing (REHP) and B4C ceramic balls prepared by conventional hot pressing (HP) against 52100 steel counterparts. The central innovation of this study lies in clarifying, based on Hertzian contact theory and Lundberg-Palmgren life theory, that subsurface orthogonal shear stress, rather than surface compressive stress, is the fundamental driving force for contact fatigue failure of ceramic balls. In addition, two distinct damage evolution modes are revealed: B4C exhibits early-stage brittle fracture and large-scale spalling, whereas REHP-Si3N4 is characterized by microcrack initiation and slow crack propagation. Moreover, the intrinsic mechanism by which the REHP process significantly enhances the contact fatigue life of ceramics is elucidated; namely, it refines grain size, eliminates residual porosity, and increases densification. The results show that, under the same high-load conditions, the mass loss of REHP-Si3N4 ceramic balls is only 35.7% of that of HP-B4C, while the service life is extended by 20%. This work provides a key theoretical basis for ceramic material selection and sintering process optimization in high-performance hybrid bearings.

Lubricants doi: 10.3390/lubricants14060233

Authors: Rewan Abdelrahman Mostafa El-Helaly Florian Pape Mohamed Abdelnaeem Mohamed G. A. Nassef

The recent motivation for sustainable lubrication has driven the development and advances of bio-based and potentially environmentally favorable alternatives to petroleum-based greases. Yet, their industrial adoption is largely hindered by the thickener weak network or inconsistency leading to grease unacceptable degradation under applied loads and operating temperatures in rotating machinery. This study investigates a novel grease formulated from 80% palm oil and a 20% complex thickener system from carnauba wax (CW) and glycerol monostearate (GMS). The effect of thickener composition on grease performance was investigated by testing their X-ray diffraction (XRD) spectra, Fourier transform infrared (FTIR) spectra, penetration level, oil separation percentage, viscosity, thermal properties, and tribological behavior. GMS-rich blends achieved up to 70% lower friction than lithium grease. However, they showed high wear rates and excessive oil separation ranging from 0.07% at room temperature for the 20% GMS blend to above 9% at 40 °C for softer formulations. The blend of 15% CW + 5% GMS showed only 0.113% and 3.145% oil bleed at room temperature and at 40 °C, respectively, with suitable consistency (NLGI 3) and acceptable dynamic viscosity rates. Regarding thermal behavior, CW-based samples revealed an enhanced melting point compared to GMS. For validation, investigations were conducted on rolling element bearings on a customized test setup operating at 1400 rpm under selected radial loads. The results demonstrate that CW/GMS bio-thickeners achieved lower vibration levels compared to the GMS thickener, approaching the performance of lithium grease.

Lubricants doi: 10.3390/lubricants14060232

Authors: Lirong Huang Zhaoxiang Wang Kunpeng Zhang Binbin Su

To elucidate the mechanisms responsible for high friction in micro-pillared soft tribo-contacts under wet conditions, this study investigates the liquid migration behavior across elasticity interfaces featuring bionic surface textures and examines the influence of this migration on interfacial friction properties. Micro-pillar bionic surface textures were fabricated on polydimethylsiloxane (PDMS) substrates. In situ observation of liquid migration and corresponding friction tests were systematically conducted using custom-built experimental setups on soft interfaces textured with micro-pillars of varying area densities. The results demonstrate that both geometrical shape and area density of surface textures play a critical role in regulating liquid migration behavior. Surface textures with circular and hexagonal geometries exhibit optimal migration rates, attributed to their smooth structural profiles, which reduce flow resistance within the microchannels. Liquid migration efficiency is effectively improved with increasing area density of the bionic surface texture owing to strengthened capillary forces. Correspondingly, bionic surface textures exhibiting superior liquid migration characteristics show the smallest relative reduction in friction force during transitions from dry to wet frictional states. This behavior is primarily attributed to the surface’s exceptionally rapid drainage capability, which effectively mitigates the adverse effects of interfacial liquid films on friction. Specifically, rapid liquid removal increases the effective solid–solid contact area and enhances mechanical interlocking at the interface. Consequently, these surfaces maintain outstanding frictional performance even under humid or wet conditions. These findings provide important theoretical support for the rational design of surface microstructures and the optimized regulation of friction and liquid film in wet contact conditions.

Lubricants doi: 10.3390/lubricants14060231

Authors: Linkai He Xingqiang Wu Zhenneng Chen Kaiqiang Zhang Qingnan Men Yun Tian Meilu Yu

In pursuit of a composite coating for tunnel boring machine (TBM) disc cutters that offers both high wear resistance and self-lubricating functionality, we fabricated Fe-based composite coatings reinforced with WC and MoS2 through laser cladding. Seven coating compositions with systematically tailored MoS2 contents were prepared to investigate the concentration-dependent effects of MoS2 on microstructural evolution and tribological properties, and to evaluate their performance under various rock-contact conditions. XPS results reveal that MoS2 decomposed during laser cladding, leading to the in situ formation of metal sulfides in the Fe-based matrix. These sulfides, characterized by low shear strength, readily form a continuous and stable lubricating tribofilm at the hob–rock interface. The tribofilm effectively lowers the coefficient of friction (COF), curtails friction-induced energy dissipation and surface degradation, and ultimately enhances the wear resistance of the disc cutter. Simultaneously, the rapid non-equilibrium solidification inherent in laser cladding stabilizes metastable phases, which refine the microstructure, improve densification, and bolster phase stability. Among the tested compositions, the coating containing 4 wt.% MoS2 exhibited the most favorable dry-sliding tribological performance, as evidenced by an average coefficient of friction of 0.409, a hardness of 749.5 HV1, and consistently low wear mass losses below 2.1 × 10−3 g under different rock-contact conditions. Mechanistically, XRD and SEM analyses further attributed the superior performance of the 4 wt.% MoS2 coating to concurrent strengthening mechanisms: grain refinement, dispersion strengthening from uniformly distributed second-phase particles, and increased dislocation density. Collectively, these effects substantially improve the wear resistance of the disc cutter, thereby extending its durability and service life under complex operating conditions.

Lubricants doi: 10.3390/lubricants14060230

Authors: Le Ma Yunfeng Zang Zhancheng Dou Lingyan Guo Weimin Li Qicheng Wang Xinming Li Haichao Liu

Hydrogen-fueled internal combustion engines (H2-ICEs) impose unique challenges on engine lubrication because water is an inevitable combustion product. This review summarizes the current understanding of water-induced degradation mechanisms in engine oils for H2-ICEs, with emphasis on physicochemical property variation, additive depletion, tribofilm evolution, and tribological performance. Water present in dissolved, emulsified, or free states can significantly alter lubricant viscosity, destabilize additive systems, and accelerate oxidative aging. In particular, water promotes the depletion of zinc dialkyldithiophosphate (ZDDP) through tribofilm removal and competitive adsorption at rubbing interfaces, while also inducing additive hydrolysis that transforms long-chain phosphates into shorter-chain species with inferior film-forming capability. These processes inhibit tribofilm growth and reduce the mechanical integrity of protective films, thereby deteriorating anti-wear performance. Although substantial progress has been made in understanding the role of liquid water in lubrication, the tribochemical effects of high-temperature water vapor under realistic H2-ICE operating conditions remain largely unexplored. Future research should therefore focus on water vapor-dominated lubrication environments representative of hydrogen combustion, aiming to elucidate the underlying tribochemical mechanisms and support the development of dedicated lubricants for durable and reliable H2-ICE operation.

Lubricants doi: 10.3390/lubricants14060229

Authors: Richard Heinlein Markus Grebe Christoph Herrmann

Reliable detection of lubrication conditions in sliding bearings is crucial for condition monitoring and predictive maintenance. Despite advances in tribological research, there remains a need for accurate diagnostics that indicate worsening of lubricity in mixed and boundary lubrication states. In this study, a dynamic test procedure is utilised to classify lubrication conditions with the help of a boosted tree classification algorithm. A radial journal bearing test rig is built and equipped with a high-frequency acoustic emission (AE) sensor on which experiments consisting of repeated dynamic speed and load alterations are conducted. AE signal features are extracted, compared and used to train an Extreme Gradient Boosting (XGBoost) classification model. The model achieves high accuracy (97.57%) in distinguishing adequate vs. starved lubrication conditions in mixed friction. Misclassifications are mainly observed at the lowest load or speed conditions, where residual lubrication effects make the classes less separable. The model’s generalisability is evaluated by applying it to tests with differing viscosity classes and alternative bearing materials without retraining, with the classifier retaining good performance. The model is also used to detect anomalies in a grease-lubricated system, where it successfully detects poor lubrication conditions. While it is known prior to this publication that AE is a good tool to detect anomalous behaviour in hydrodynamic journal bearings, the findings presented highlight the potential for the transferability of anomaly detection models trained in a laboratory setting and applied to different real-world applications to reduce life-cycle maintenance costs and increase uptime in industrial applications.

Lubricants doi: 10.3390/lubricants14060226

Authors: Weiqiang Zou Xigui Wang Yongmei Wang Jiafu Ruan

A multiscale contact model is developed for micro-textured gear interfaces incorporating Micro-Convex-Concave Asperity (MCCA) characteristics to elucidate the synergistic modulation between Interface Enriched Lubrication (IEL) performance and Anti-Scuffing Load-Bearing Capacity (ASLBC) of Micro-Textured Meshing Interfaces (MTMI) under transient Thermal Elastohydrodynamic Lubrication (TEHL) conditions. Homogenization theory is employed to quantify the effects of areal density and depth-to-diameter ratio on IEL characteristics. A time-resolved micro-elastohydrodynamic lubrication model, formulated through dimensionless discretization and adaptive mesh refinement, investigates the influences of autocorrelation length and MCCA amplitude on interfacial behavior. A correlation framework linking Micro-Element Texture (MET) geometric parameters to meshing ASLBC is established to identify optimal textures for simultaneous enhancement of IEL and ASLBC. Experimental observations demonstrate qualitative consistency with numerical predictions regarding the evolutionary trends of temperature fields and dynamic friction coefficients, providing preliminary physical validation for the proposed model. Univariate Sensitivity Analysis (USA) and Multiple Linear Regression (MLR) are further utilized to optimize microtexture parameters by elucidating the influences of MET sizes, area ratio, and configuration on meshing ASLBC and friction coefficients.

Lubricants doi: 10.3390/lubricants14060228

Authors: Feng Zhang Hongjian Wu Yanan Zhang Hongbin Liu Baolin Jia Xinlong Wu Kun Zhao He Liu Wenhu Zhang

To investigate the contact mechanical performance of helicopter main gearbox rotor shaft bearings under a complex load spectrum, this study focuses on the contact stress and load-carrying characteristics of bearings operating under high-speed and heavy-load conditions. Based on the rotor shaft system of a helicopter main gearbox and Hertzian contact theory, quasi-static analyses were performed on four tapered roller bearings and one cylindrical roller bearing mounted on the shaft system conducted in Romax. The results indicate that the maximum contact stresses of the bearings do not exhibit sustained high-stress states under most operating conditions. The peak-stress conditions account for only extremely small time proportions in limited cases, namely 0.003429% and 0.025%. The contact stresses on both the inner and outer raceways exhibit a non-uniform distribution along the roller length, with local peak values appearing near the highly loaded roller-raceway contact regions. This suggests that during the design process of the helicopter main gearbox rotor shaft, special attention should be given to this region. The present results provide a theoretical basis for subsequent life-index verification and offer an effective analytical method for the design and validation of such critical components.

Lubricants doi: 10.3390/lubricants14060227

Authors: Lloyd J. Augustine Wani J. Morgan Hsiao-Yeh Chu Sheng-Jye Hwang Hsin-Shu Peng

Tool wear in CNC milling increases friction and torque demand at the tool-workpiece interface, which is reflected in spindle motor current. This study develops a non-intrusive tool wear condition classification method using spindle motor current monitoring during practical CNC milling of commercial medium-carbon steel workpieces (JIS S50C/AISI SAE 1050-equivalent; as-received and non-heat-treated; nominal laboratory hardness approximately 4.3 HRC). Experiments were performed on a Tongtai MDV-508 vertical machining center at fixed cutting conditions (3000 rpm spindle speed, 2 mm axial depth of cut, 5 mm cutting width, and 300 mm/min feed rate) using eight TiAlN-coated fine-grain WC–Co solid carbide end mills (10 mm diameter, four flutes; nominal Co binder approximately 10 wt%). An oil-based HS Highstart/HS-SSHS-BH10 cutting fluid was applied through the machine external coolant nozzle in flood mode at an estimated nominal flow rate of approximately 3 L/min and near-room coolant temperature (25 ± 2 °C), and was used as supplied without dilution. A clamp-type AC current sensor was installed on one phase line supplying the spindle motor, and current was acquired using an NI-9221 module at 20 kHz. Cutting intervals were isolated by envelope-based segmentation, concatenated, and divided into 1 s windows (0.5 s overlap) for feature extraction. Three feature sets were evaluated: time-domain statistics, frequency-domain statistics, and an FFT→PCA hybrid representation. Tool states (New, Mid-life, Old) were labeled using post-process surface roughness Ra thresholds supported by microscope observation. The PCA transformation was fitted only on training data and then applied to the held-out test data. A logistic regression classifier achieved 97.44% test accuracy (152/156 windows; 95% Wilson CI: 93.59–99.00%) with the PCA-hybrid features, outperforming time-domain (89.74%) and frequency-domain (94.87%) models. The results support spindle current monitoring as a low-cost approach for quality-aligned tool condition monitoring, while the external validity remains limited to the tested machine, material, tool, coolant, and cutting-parameter combination.

Lubricants doi: 10.3390/lubricants14060225

Authors: Raimondas Kreivaitis Irenijus Rastokas Paulius Bendžiūnas Adolfo Senatore Audrius Žunda Simona Tučkutė

Lubricity is critical for maintaining the reliability of mechanical systems. Consistent monitoring of in-service lubricants and the development of new formulations require reliable test equipment and standardised procedures. The four-ball tribo-test is among the most widely used and acknowledged methods. Like other methods, its effectiveness depends on the procedures and specimens used. Therefore, in this study, we aim to emphasise the influence of ball-specimen surface parameters on lubricity results, a factor that is often insufficiently acknowledged in reported experiments. Accordingly, lubricity tests of the same motor oil were performed using balls obtained from different suppliers. The experimental procedure was identical for all ball specimens. It was found that balls from different series exhibit distinct surface properties, leading to differences in lubricity even when the same lubricating oil was used. Based on the results obtained, it was concluded that the outcome is a multiparameter solution and not the result of a single surface parameter. On the other hand, if balls from the same series are used, the results will meet the standard requirements for repeatability regardless of their surface conditions.

Lubricants doi: 10.3390/lubricants14060224

Authors: Zhaowei Yang Ying Zhang Guoli Li Kun Li Long Jiang Qingkai Fan Kang Qi

Laser cladding has attracted considerable attention for titanium alloy surface modification owing to its high energy density, rapid cooling rate, and excellent metallurgical bonding capability. To investigate the effect of Nb content on the microstructure and properties of NiTi-based coatings, composite coatings containing 10–40 wt% Nb were fabricated on a titanium alloy substrate via laser cladding. The effects of Nb content on phase constitution, microstructure evolution, mechanical properties, tribological performance, residual stress, and surface topography were systematically characterized using XRD, SEM, EDS, microhardness testing, wear testing, digital image correlation, and atomic force microscopy. The results show that increasing Nb content significantly affected the solidification behavior and phase evolution of the coatings. With increasing Nb addition, the dominant phase gradually evolved from NiTi to a coexistence structure of NbTi4 and NiTi, while Ti dilution and elemental segregation became increasingly pronounced. The crystallite size increased from 19.63 nm to 25.91 nm, accompanied by intensified dendritic segregation and surface roughening. Among all samples, the coating containing 10 wt% Nb exhibited the best overall performance, characterized by the finest microstructure, the lowest surface roughness, the lowest residual stress, and the best wear resistance. The superior performance of the low-Nb coating was mainly associated with its finer and more homogeneous microstructure, reduced elemental segregation, lower stress concentration, and enhanced grain-boundary strengthening effect. Excessive Nb addition intensified Ti dilution, grain coarsening, residual stress accumulation, and microstructural heterogeneity, thereby degrading the overall coating performance. More importantly, this study reveals that Nb-regulated Ti dilution behavior governs the synergistic evolution of elemental segregation, surface roughening, residual stress accumulation, and tribological degradation during laser cladding. This work provides new insight into the process–structure–property relationship of NiTi-based composite coatings and offers theoretical guidance for the composition optimization and engineering application of high-performance laser-clad coatings on titanium alloys.

Lubricants doi: 10.3390/lubricants14060223

Authors: Vishal Shenoy P Vijay Kini M Raghuvir Pai B Srinivas Shenoy Heckadka Raviraj Shetty Supriya J P Adithya Hegde

AISI 1040 steel is extensively used in structural and automotive applications, where surface integrity plays a significant role in service performance and coating adhesion. Furthermore, the selected cutting fluids are expected to effectively reduce surface roughness and tool wear by improving lubrication at the tool and workpiece interface. This study investigates the influence of SiO2 nanoparticle-assisted Pongamia pinnata oil on surface roughness and tool wear during the machining of AISI 1040 steel using an uncoated tungsten carbide tool by varying nanoparticle concentration (Vol.%), cutting speed (m/min), depth of cut (mm), and feed rate (mm/rev). The incorporation of 0.5 (Vol.%) SiO2 nanoparticles significantly enhances machining performance by improving surface finish and reducing tool wear. Further, a minimum surface roughness value of 1.95 microns and tool wear value of 0.047 mm were achieved at a cutting speed of 101 m/min, feed rate of 0.11 mm/rev, depth of cut of 0.25 mm and 0.5 (Vol.%) SiO2 nanoparticle concentration. ANOVA results indicate that nanoparticle concentration is the most dominant parameter affecting both surface roughness and tool wear, contributing 85.35% to the variation in surface roughness and 82.2% to the total variation in tool wear. Cutting speed is the second most influential factor, accounting for 11.63% of surface roughness variation and 11.07% of tool wear variation, while feed rate and depth of cut exhibit minimal influence in both cases. A second-order RSM model was developed to predict surface roughness and tool wear, showing excellent agreement with experimental results. The model predicted surface roughness with an average error below 2.43%, while the second-order model for tool wear exhibited an average prediction error of 4.95%, confirming its statistical significance and predictive reliability. Desirability Function Method (DFM) analysis yielded a desirability value of 1.000, confirming the optimal combination of machining parameters at 0.5354 (Vol.%) nanoparticle concentration, a cutting speed of 45 m/min, a depth of cut of 0.50 mm, and a feed rate of 0.1298 mm/rev. Overall, this study demonstrates that 0.5 (Vol.%) SiO2 nanoparticle-incorporated Pongamia pinnata oil is an effective and sustainable cutting fluid, significantly improving surface integrity and machining performance of AISI 1040 steel during machining. Under these settings, the predicted tool wear was 0.0614 mm, accompanied by a high composite desirability value of 0.92786, indicating excellent overall performance. Moreover, the close agreement between experimental, response surface model and BP-ANN-predicted tool wear and surface roughness confirms that the ANN model reliably and robustly captures the complex, nonlinear effects of machining parameters with minimal systematic error.

Lubricants doi: 10.3390/lubricants14060222

Authors: Haiyang Wang Zhongyi He Zongbin Wang Haodi Zhang Liping Xiong Xiaogang Jiang

This study investigates spherical methyl silicone resin, a potentially environmentally friendly additive free of sulfur, phosphorus, and chlorine, as a lubricant additive in polyethylene glycol 200 (PEG 200) base oils. We evaluated concentration-response characteristics and tribological performance across PEG base oils containing 0.01–0.05 wt% resin. Tribological testing was conducted with a four-ball wear tester at 98 N and 1450 rpm for 30 min. All tested concentrations demonstrated excellent friction-reduction and anti-wear performance, with an optimal efficacy observed at 0.02 wt%. Surface characterization was performed using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. This friction-reducing and anti-wear performance is attributed to the formation of silicon-oxygen species and graphene-like carbon structures, thereby effectively suppressing direct surface contact and mitigating wear. Consequently, spherical methyl silicone resin demonstrates considerable potential as a green lubricant additive for bearing steel applications.

Lubricants doi: 10.3390/lubricants14060221

Authors: Daichi Kosugi Fumiaki Aikawa Shunsuke Iwase Taisuke Maruyama Satoshi Momozono

In this study, we developed an improved electrical impedance method for measuring oil film thickness with a correction for surface roughness effects. Statistical analysis of the oil film thickness distribution revealed that rough surfaces exhibit higher capacitance values than those predicted by the ideal parallel-plate model, despite having the same mean film thickness. Consequently, a corresponding roughness correction formula was derived. The accuracy of the method was verified in ball-on-disk type apparatus using balls with a rough surface. The corrected oil film thickness agreed more closely with the Hamrock–Dowson equation and with optical interferometry measurements than did the uncorrected result. These outcomes confirm that oil film thickness can be estimated considering surface roughness. The technique is therefore expected to facilitate the optimization of lubrication conditions and enable more reliable bearing-life prediction.

Lubricants doi: 10.3390/lubricants14060220

Authors: Dan Bai Jintao Zheng Xiaohui Wang Hang Wang Yan Li Hui Cen

Capacitance-based methods are widely used to evaluate lubricant film thickness in machine elements where direct optical access is unavailable, especially in rolling bearings and other multi-contact components. This review examines the physical basis, historical development, and modern methodological routes of capacitance-based film thickness determination, with emphasis on four coupled interpretive layers: film geometry, dielectric response, electrical topology, and parasitic/background effects. The literature shows that the field has evolved from simple dielectric-gap conversion toward more strongly constrained interpretation using elastohydrodynamic lubrication priors, dielectric identification, network-aware reduction, and frequency-domain information, particularly under grease lubrication, starvation, and transient conditions. Across these studies, capacitance-derived film thickness is not a methodologically uniform quantity but an inferred result whose meaning depends on what is prescribed, what is estimated, and what ambiguity remains unresolved. The main unresolved challenges are geometry–dielectric non-uniqueness, parasitic and topology uncertainty, and limited validation under realistic operating conditions. Overall, capacitance-based film thickness determination in practical machine elements is best understood as a constrained electromechanical inference problem, and future progress will depend on stronger identifiability, more informative broadband measurements, and clearer reporting of assumptions, inference targets, and validation basis.

Lubricants doi: 10.3390/lubricants14060219

Authors: Raelene M. Cowie Danielle de Villiers Simon N. Collins Louise M. Jennings

Hip resurfacing is a bone-conserving alternative to total hip replacement particularly suited to younger or more active patients with good bone stock. Historical issues with metal-on-metal hip resurfacing devices have led to investigations of new materials. In this study, the wear of a BIOLOX delta ceramic-on-ceramic hip resurfacing under a range of activities of daily function was investigated in an experimental simulation model. Under a standard walking gait, increased frequency, and stop–dwell–start conditions, the wear of the ceramic head and cup was low and at the limit of the sensitivity of the measurement technique used (<0.1 mm3/million cycles). Only when tested under a jogging-like protocol with higher loading (maximum 4.5 kN) coupled with higher frequency (1.25 Hz), under both continuous running and stop–dwell–start conditions, was wear measurable but still low, with a mean <0.15 mm3/million cycles. At the conclusion of the study, no wear scar was visible on any components and no changes in the surface roughness of the implants was measured. This short-term, pre-clinical study showed low wear of BIOLOX delta ceramic-on-ceramic hip resurfacings when evaluated under a range of activities of daily living and that this material combination has potential for the next generation of large-diameter hip bearings.

Lubricants doi: 10.3390/lubricants14060218

Authors: Martin Ovsik Jakub Zajicek Ondrej Stalmach Michal Stanek

This work is concerned with the influence of lubricant, sheet thickness, position of production during sheet metal forming (deep drawing) on mechanical properties. The deep drawing process was carried out on a BUP-600 instrument using the Erichsen method, which uses a 20 mm diameter ball draw bar. The individual forming tests were carried out on 1.4376 plate with thicknesses of 1.5 mm and 1 mm. The drawing of the sheets was carried out with and without lubricant to assess the effect of the lubricant on the resulting properties. The actual forming results of the plates were verified by FEM analysis in AutoForm software, which pointed out the critical areas on the part. These specified locations were further subjected to mechanical property measurements. As shown in this work, the areas of sheet metal forming showed significant strengthening, which was reflected by an increase in mechanical properties at each location. The difference in mechanical properties between the unformed area of the sheet metal and the area on the sheet metal that was formed by deep drawing was up to 63% (Vickers hardness, indentation modulus). The lubricant had a significant impact on the drawing process; when applied, both the drawing distance and the force increased by approximately 25%. This can result in additional cracking of the sheet metal parts. This research has significant implications for the deep drawing of sheet metal in practice, pointing out the issue of critical points that need to be further accounted for and possibly eliminated or at least minimized. Plastic deformation occurring during deep drawing leads to an increase in material hardness and tensile strength. As a result, the formed parts achieve improved stiffness and load-bearing capability. The study contributes to a better understanding of how strain hardening can be maximized in critical regions of the drawn part without inducing fracture. Furthermore, the research describes the influence of sheet thickness variation during the drawing process, which is essential for maintaining the structural integrity of the component. The study further emphasizes the significance of lubrication and friction conditions in reducing the forces required for drawing, thereby preventing crack initiation and surface defects on the drawpiece.

Lubricants doi: 10.3390/lubricants14060217

Authors: Xinyi Liu Qilin Liu Gao Wan Yong Jin Wu Ouyang

Under long-term service conditions, vibration signals of water-lubricated stern bearings exhibit strong nonlinearity, nonstationarity, and multicomponent coupling, which makes accurate feature extraction challenging. To address this issue, this study proposes a progressive EEMD-TFMST-based analysis framework that combines spectral localization, adaptive signal decomposition, noise suppression, and high-resolution time–frequency characterization. Rotational-speed tests and long-duration wear tests were conducted using an SSB-100 test rig, and the lubrication regimes were identified based on friction coefficient variations. The results show that the dominant vibration features are strongly dependent on the lubrication regime and wear stage. With increasing rotational speed, the vibration response evolves from isolated peaks near 400 and 600 Hz under boundary lubrication to enhanced 300–400 Hz components under mixed lubrication, and further to broadband responses within 0–1000 Hz under hydrodynamic lubrication, with dominant peaks mainly concentrated in the 300–500 Hz range. With increasing rotational speed, the lubrication regime gradually changes from boundary lubrication to hydrodynamic lubrication, accompanied by a transition of vibration energy from single-IMF concentration to broadband distribution across multiple IMF components. Long-term operation induces stage-dependent changes in lubrication and vibration behavior: moderate wear improves vibration stability, whereas excessive wear deteriorates lubrication, increases the proportion of mixed lubrication, and promotes energy migration toward lower frequencies with additional high-frequency excitation. Under prolonged high-speed operation, lubrication degradation further induces broadband vibration. The proposed method enables accurate quantification of vibration features and provides a useful basis for service-performance evaluation and early fault warning of water-lubricated stern bearings.

Lubricants doi: 10.3390/lubricants14060216

Authors: Raudah Nordin Nur Aisya Affrina Mohamed Ariffin Jiahe Poy Jo-Han Ng King Jye Wong William Woei Fong Chong

Lubricant additive optimisation, such as using an Organic Friction Modifier (OFM), often relies on empirical methods because the role of interfacial energetics in boundary lubrication remains uncertain. This study explores how interfacial energy interactions affect the tribological performance of glycerol monooleate (GMO)–polyalphaolefin-4 (PAO4) blends using ball-on-disk friction and wear tests. The 7 wt% GMO blend showed the best results, with friction reduced by about 4 times and the wear scar diameter by nearly 6 times compared to neat PAO4. Film-thickness estimates indicate that all tests operated within the boundary-to-mixed lubrication regime, suggesting that friction reduction is associated with interfacial interactions rather than hydrodynamic film formation. The Owens–Wendt–Kaelble surface energy analysis reveals that increasing GMO concentration shifts the lubricant’s dispersive–polar balance, with the 7 wt% formulation exhibiting dispersive–polar characteristics closer to those of the steel substrate. Low friction persisted as sliding velocity increased, and rupture-threshold analysis is consistent with improved tribological response under increasing load and sliding conditions. These findings suggest that the favourable tribological response observed for the investigated formulations may be associated with balanced interfacial energetic characteristics, particularly between dispersive and polar interactions. The observed friction and wear behaviour under increasing sliding conditions is interpreted in terms of friction and wear responses under boundary-dominated conditions, rather than through direct structural characterisation of boundary films. These trends are interpreted in relation to changes in dispersive and polar interactions at the interface. The results provide an interpretive framework for designing OFM systems that may be relevant to high-shear environments, including applications such as hydrogen internal combustion engines.

Lubricants doi: 10.3390/lubricants14060215

Authors: Jiajun Li Ruilin Zeng Shequan Wang Ninghua Long Kongming Yan Qun Wang Chidambaram Seshadri Ramachandran

Ni-based WC composite coatings are widely used to protect hydraulic components, yet the role of WC particle morphology in binder-phase strengthening remains unclear. In this study, two Ni40-based coatings containing 55 wt.% WC were laser-cladded on 0Cr13Ni5Mo steel under identical conditions using either rough spherical WC coating (RWC) or smooth spherical WC coating (SWC). Both coatings were mainly composed of γ-Ni, residual WC, W2C, carbides, and borides. Although the rough WC particles showed about 38% lower intrinsic hardness than the smooth WC particles, the RWC exhibited a 25% higher binder-phase hardness and a 47% higher overall coating hardness. Accordingly, compared with the SWC, the RWC reduced the specific wear rate by about 33% under water-lubricated sliding. In slurry erosion, the RWC consistently showed lower erosion rates and less severe surface damage. The improved performance is attributed to the greater dissolution of rough WC during laser cladding, which strengthened the Ni-based binder and provided more stable support for the hard phases. These results demonstrate that tailoring WC particle morphology is an effective strategy for designing wear- and slurry erosion-resistant Ni-based laser-cladded coatings.

Lubricants doi: 10.3390/lubricants14060214

Authors: Bettina Suhr Mohammad-Sadegh Salehi Simon Skurka Daniel Kvarda Radovan Galas Milan Omasta Klaus Six

The wet-rail phenomenon can cause low adhesion, which negatively affects railway operation. It is believed to occur when small amounts of water mix with solid particles on wheel and rail surfaces, e.g., wear debris or iron oxides, forming a dense suspension in the wheel–rail contact, leading to sharp adhesion drops. Mini Traction Machine (MTM) tests using water-based suspensions with different particles also show adhesion drops during water evaporation, which can be linked to the wet-rail phenomenon. While the physical mechanisms underlying the adhesion drop are unclear, it is hypothesised that rapid loading raises fluid pressure in the suspension, separating wheel and rail surfaces, reducing force transfer through particle contact, thereby reducing the suspension’s shear strength. For verification, a coupled Discrete Element Method and fluid dynamics model is used to simulate a simplified MTM setting and steps towards full scale wheel–rail contact. During simulation of rapid loading, fluid pressure rises but remains negligible compared to applied contact stresses in all considered cases. Thus, it is unlikely that hydrodynamic pressure build-up within the suspension contributes significantly to the low adhesion observed. Future research should investigate additional mechanisms, such as reduced shear strength of deformed or crushed wet particles under high normal loading conditions.

Lubricants doi: 10.3390/lubricants14060213

Authors: Zhihui Cai Ziming Feng Heng Yuan Xinmin Wang

To elucidate the wear evolution and failure mechanisms of VL-type composite seals in aviation hydraulic actuators under multiple operating conditions, a two-dimensional plane-strain finite element model was developed for a VL seal consisting of a PTFE L-ring and an NBR O-ring. The model incorporated the Mooney–Rivlin hyperelastic constitutive law and the Archard wear model. The effects of O-ring compression ratio, hydraulic pressure, sliding velocity, and temperature on cumulative wear, wear rate, and contact state were systematically investigated. The results show that the compression ratio has a nonlinear influence on wear. Within 8–16%, the peak wear increases approximately linearly with compression ratio; above 16%, the peak wear reaches a plateau and a secondary wear zone appears, indicating a transition from single-contact to multi-contact sealing. Hydraulic pressure promotes wear over the range of 4–28 MPa, and at 28 MPa the opposite lip edge of the L-ring comes into contact with the cylinder wall, weakening the sealing effectiveness. Within 0.1–0.3 m/s, wear increases approximately linearly with sliding velocity. However, under high velocity and insufficient hydraulic pressure, the L-ring may undergo inversion, resulting in complete seal failure. Temperature exhibits a non-monotonic effect: material softening reduces local contact stress and wear from −55 to 80 °C, whereas excessive softening at 135 °C causes the peak wear rate to increase again. A parametric analysis scheme involving an increased L-ring height and thickness, a reduced O-ring cross-section diameter, and reserved deformation space raises the critical compression ratio for stable single-contact sealing from 16% to above 20%. These findings clarify the contact-stress/contact-area competition mechanism governing VL seal wear and provide guidance for the design of aviation hydraulic actuator seals.

Lubricants doi: 10.3390/lubricants14050212

Authors: Jinggang Zhou Xun Li Han Zhang Changrui Yu Liangbao Liu

HT250 grey cast iron is a material of significance in the manufacture of precision machine tool guideways. The performance of guideways is significantly affected by the wear resistance of the machined surface. The present paper studies comparative grinding experiments conducted on HT250 using CBN and SiC wheels. The aim was to investigate the potential benefits of CBN grinding in enhancing surface wear resistance and to illuminate the underlying mechanisms. The results of these experiments demonstrate that, compared with SiC grinding, CBN grinding produces guideway specimens’ subsurface layer with finer grains (refined by approximately 15%) and notably higher microhardness (peak value of 382 HV). These microstructural improvements directly enhance the wear resistance of the ground surface. Within the tested parameter range, the optimal wear-resistant surfaces were obtained at a grinding speed of vs = 30 m/s, a feed rate of vf = 2000 mm/min, and a depth of cut of ap = 6 μm. Under these conditions, surface roughness is better than Ra 0.4 μm, and surface microhardness achieves its maximum value. The wear tests were conducted using a ball-on-disk configuration under room temperature, oil lubrication, and applied loads ranging from 20 N to 80 N. The results show that, under the same loading and wear testing conditions, the wear depth of specimens machined with CBN wheels is reduced to 80–50% of that of specimens processed with conventional SiC wheels.

Lubricants doi: 10.3390/lubricants14050211

Authors: Hong-Jian Wang Jin Wang Yan-Xian Guo Bo Wang Dong-Lin Wu Huan-Xun Guo

Laser surface texturing (LST) was used to process circular patterns on silicon nitride (Si3N4) ceramic. The surface wettability of un-textured and textured Si3N4 ceramic was studied. It was found that all samples were hydrophilic. The hydrophilicity of the textured Si3N4 ceramic with a smaller interval was weaker. Effect of the interval on the tribological performance of Si3N4 ceramic was investigated under different reciprocating frequencies. As the reciprocating frequency increased, the coefficient of friction (COF) of Si3N4 ceramic showed an overall upward trend. And the COFs of textured Si3N4 ceramic were higher. At the reciprocating frequency of 0.5 Hz, the COFs of textured samples with different intervals were relatively close. At higher reciprocating frequency, the difference in COF gradually increased. This can be attributed to the change in lubrication state that occurs during the frictional process. Debris and plough were found at surface of all samples, and the original surface was cleaner. The surface damage of textured samples was more severe than that of un-textured samples. EDS analysis on the wear area was performed. The wear rate of the frictional pair at the interval of 250 μm was higher. It was consistent with the more significant wear marks on the surface of the Si3N4 ceramic at this interval. There were significant differences in the frictional process between un-textured and textured samples. For the un-textured surface, the debris was mainly concentrated at the edges of the friction trajectory. In contrast, textured patterns had the function of storing lubricant, while also generating micro-shear effects on debris and improving the status of lubrication, resulting in more complex frictional process.

Lubricants doi: 10.3390/lubricants14050210

Authors: Stefan Altstetter Arne Bischofberger Sascha Ott Tobias Düser

Reliable identification of the tribologically relevant braking phase in torque signals recorded on flywheel dynamometers is a prerequisite for quantitative friction analysis and data-driven modeling of dry-running friction brakes. We define brake torque plateaus as intervals with quasi-constant surface pressure and appreciable sliding velocity in which fading or drift of the coefficient of friction is explicitly admissible, while rise and decay ramps dominated by actuator dynamics are excluded. To automate this extraction across large industrial data sets, we propose a two-stage detection algorithm that sequentially narrows the search space using physics-based amplitude, gradient, and stability criteria, complemented by a Pruned Exact Linear Time (PELT)-based fallback for difficult cycles. Evaluation on 10,386 brake cycles, including 275 expert-annotated ground-truth cycles validated by a second independent expert, shows that the proposed method reaches 95% of the inter-annotator agreement ceiling on 75 held-out cycles, achieves a median Intersection-over-Union of 0.893 (11 percentage points above the strongest baseline), and a mean quality score of 9.18/10 across all cycles at under 1 ms per cycle (signals averaging 951 samples), outperforming six baseline configurations in both detection quality and runtime.

Lubricants doi: 10.3390/lubricants14050209

Authors: Yazhou Mao Linlin Guo Aoya Wang Ruiyi Ma Zixuan Wangan

This paper presents a cross-scale model for predicting the tribological behavior of textured coatings made of high-entropy alloys. The research methodology includes molecular dynamic modeling, a modified fractal surface model, and the Green’s method with fast Fourier transform. The main results demonstrate the existence of an optimal range of parameters: a fractal dimension of 2.45–2.55 and a texturing density of 15–20%, which reduces the coefficient of friction to 40% compared with untextured surfaces. The practical significance of the work lies in the creation of a theoretical basis for the integrated design and forecasting of the tribological properties of high-entropy coatings.

Lubricants doi: 10.3390/lubricants14050208

Authors: Xiaonan Pan Jianrui Zhang Jian Kang

Wear of the feedback ball head–ball seat interface changes the contact state and reduces the feedback force in electro-hydraulic servo valves, resulting in output nonlinearity and performance degradation. Existing wear models usually assume fixed surface morphology parameters, which their ability to describe time-varying wear evolution during repeated sliding. To address this issue, this study proposes a hybrid wear-prediction framework integrating fractal contact theory, Archard wear law, Gaussian process regression, and a servo-valve mechanical model. The real contact area and wear coefficient are expressed as functions of fractal parameters, while Gaussian process regression is used to predict their evolution under different loading cycles and displacement loads. Repeated loading tests and white-light interferometry measurements were performed to validate the proposed method. The results show that the fractal dimension of the ball seat increased by approximately 4.01%, whereas that of the ball head decreased by approximately 1.71%. After about 12,000 cycles, the fractal parameters tended to stabilize. The prediction error of the Gaussian process regression model was below 3%, and the wear-depth prediction error remained within approximately 1%. These results indicate that the proposed method can effectively capture the time-varying sliding wear behavior of the feedback ball head–ball seat interface.

Lubricants doi: 10.3390/lubricants14050207

Authors: Onur Gök

Wheel hubs in heavy-duty agricultural trailers operate under demanding conditions comprising rough terrain, impact loads, and highly variable load spectra. Current design practice relies predominantly on experience-based sizing rather than systematic multi-physics analysis. This study presents an integrated design methodology combining finite element analysis (FEA), density-based topology optimisation, and computational fluid dynamics (CFD) to concurrently improve the structural and tribological performance of a GGG40 spheroidal graphite cast iron agricultural trailer wheel hub. A reference commercial hub geometry was modelled and analysed under multiple load conditions with a safety factor of 5. Critical stress regions were identified, and the free design volume was optimised while preserving all functional surfaces. The optimised design achieved 35% mass reduction (14.9 to 9.6 kg), 30% lower maximum von Mises stress (235 to 165 MPa), and up to 40% stress reduction in the bearing seat region. Oil-circulation channels integrated into the bearing housing raised mean lubrication flow velocity by 28% and eliminated stagnation zones, yielding a more homogeneous oil-film distribution and directly benefiting bearing tribological performance. The proposed framework provides a manufacturable engineering methodology that concurrently addresses structural integrity and lubrication performance in agricultural wheel hub design.

Lubricants doi: 10.3390/lubricants14050206

Authors: Linlin Li Yuzhong Zhang Yuanjun Ye

Herringbone gear pairs are critical in high-speed aerospace transmissions, where windage power loss significantly impacts efficiency and thermal management. This study proposes a prediction method that decomposes the total windage loss into five components based on structural features: the tooth, end, circumferential, and relief groove surface losses for both gears, and the meshing extrusion loss. Theoretical models for each component are established to form a complete prediction method using fluid–structure interaction principles. CFD simulations analyze the velocity, pressure, and energy fields around the gear pair, with windage loss integrated via fluid torque on gear surfaces. Results indicate that windage loss escalates rapidly and becomes non-negligible when the driving gear speed exceeds 7000 rpm. The prediction model demonstrates strong agreement with CFD simulations, with a maximum relative error of 13.6%. Analysis reveals that the driving gear contributes the largest share of the total gear pair loss, with meshing extrusion accounting for 20.1–23.6%. For a single herringbone gear, the tooth surface is the primary source of loss (~83%), followed by the end surface (~8%), while relief groove and circumferential losses remain below 10%. This research provides a validated theoretical foundation for optimizing efficiency and thermal control in high-speed aerospace gear systems.

Lubricants doi: 10.3390/lubricants14050205

Authors: Longhai Li Bo Yang Wenbin Hu Hongping Qiu Xiaotong Wang Sheng Han Jincan Yan

In this study, carboxyl groups were introduced onto CNT surfaces via acid oxidation, and Ag nanoparticles were successfully deposited onto the CNTs through an in situ chemical reduction method. At an Ag-to-CNTs100 mass ratio of 3:1, the as-prepared composite achieved a thermal conductivity of 1.45 W/(m·K) in dimethyl silicone oil, representing enhancements of 187.5% and 76.9% relative to pure Ag nanoparticles and pristine CNTs100, respectively, at equivalent loadings. Concurrently, tribological tests revealed that the AgHTs-3 at a 3:1 mass ratio and 25 wt% loading exhibited a steady-state friction coefficient of 0.08–0.12, reflecting an approximately 72% reduction compared with pure dimethyl silicone oil. Electrical conductivity measurements demonstrated that CO-CNTs100 attained saturation at 30 wt% with a resistivity of 36.5 Ω·m, whereas the AgHTs-3 nanocomposite achieved a resistivity of 4.7 Ω·m at 35 wt%. The incorporation of Ag nanoparticles effectively enhanced the overall performance of the nanocomposites. Through the formation of a synergistic heterostructure with carboxyl-functionalized carbon nanotubes, the composite not only significantly improved the thermal conductivity of dimethyl silicone oil but also effectively optimized the interfacial lubricating film while substantially reducing the friction coefficient and wear volume. Moreover, the introduction of silver promoted the dispersion stability of the composites in dimethyl silicone oil, enabling higher filler loadings and thereby effectively boosting electrical conductivity.

Lubricants doi: 10.3390/lubricants14050204

Authors: Wenjing Wu Longhang Hou Wenbo Wang Guangzhou Wang Guozhen Fan Guoqing Zhang Hechun Yu

Aerostatic spindles are indispensable in the ultra-precision manufacturing field due to their high accuracy and low friction. However, rotor manufacturing errors will affect the thickness and uniformity of the air film, thereby limiting the improvement and application of the aerostatic spindle. To explore this issue, this paper presents theoretical modelling and experimental work. Rotor cylindricity errors were first evaluated based on manufacturing errors, and a calculation model of the film thickness considering rotor cylindricity errors was established. By solving the dynamic Reynolds equation considering cylindricity errors, the dynamic stiffness and damping of aerostatic spindles were obtained. The influence mechanism of rotor cylindricity errors on the dynamic stiffness and damping coefficients of the rotor–bearing system was revealed. The stiffness coefficients Kxx, Kyy, and Kxy are more sensitive to the saddle-shaped errors, and the stiffness coefficient Kyx and both damping coefficients are more closely related to bucket-shaped errors. Regarding the influence of the cylindricity errors’ extremal position, the main and cross stiffness coefficients are sensitive to saddle-shaped errors and bucket-shaped errors, respectively; the main and cross-damping coefficients are sensitive to bucket-shaped errors. Under the effect of three kinds of error shapes, when the rotor cylindricity errors value is less than 1 μm, the dynamic stiffness and damping coefficients are conducive to improving the dynamic characteristics of the rotor–bearing system. Multiple rotors were manufactured, and their cylindricity errors were measured, and then the dynamic characteristics of the assembled aerostatic spindles with these rotors were tested. It was found that the dynamic stiffness of spindles with saddle-shaped errors is larger than that of spindles with conical-shaped errors, and the greater the error values are, the worse the rotation accuracy. The experimental results are consistent with the theoretical findings, thus verifying the feasibility and validity of the established theoretical model. This study improves the error tolerance design accuracy of rotors and thereby enhances the dynamic performance of aerostatic spindles.

Lubricants doi: 10.3390/lubricants14050203

Authors: Mahdi Zare Mehrjardi Ahmad Golzar Shahri Asghar Dashti Rahmatabadi Mehrdad Rabani

The present study provides a comprehensive investigation into the hydrodynamic performance of grooved rubber journal bearings (GRJBs) employed as shaft supports in various rotating systems, with particular emphasis on marine applications. These bearings are lubricated with non-Newtonian fluids such as modern oil containing additives and viscoelastic water-based lubricant, which—owing to its complex composition including hydrocarbon chains, metal oxides, and impurity particles and contaminants such as salts, organic substances, microalgae, biopolymers, and microorganisms—deviates from the ideal Newtonian fluid model and demonstrates non-Newtonian rheological behavior. By examining various theories used in the analysis of non-Newtonian fluid behavior, the power-law model, which has a high degree of generality, has been employed in the present study. Also, to improve modeling accuracy, the elastic deformation of the rubber bush in this study is characterized using the Winkler foundation approach and analyzed via the finite element method (FEM). This advanced mechanical formulation, integrated with non-Newtonian lubrication modeling of lubricant using the power-law fluid model, and the parametric assessment of groove number and dimensions on steady-state bearing performance parameters, constitutes the core of this research. The investigation focuses on groove configurations of 4, 6, 8, and 10 channels. The findings indicate that increasing the groove count partitions the convergent pressure film zone into discrete segments, thereby reducing the maximum hydrodynamic pressure while intensifying the overall energy dissipation within the bearing. Additionally, the influences of rheological properties of the fluid—namely the power-law index (n) and the consistency index (m)—on key performance characteristics are thoroughly examined. An increase in both parameters enhances the effective viscosity and load carrying capacity; however, the exponential amplification due to the power-law index exhibits a more pronounced effect on load capacity and peak pressure compared to the consistency index.

Lubricants doi: 10.3390/lubricants14050202

Authors: Xiaolong Yang Jingxuan Han Gang Wan Fengdi Zhu Chuangji Qin Ning Xu Shuo Wang

Vision-based defect detection on bearing-pad wear surfaces is essential for quantifying damage geometry and assessing condition in hydroturbine units. Compared with 2D color images, depth images can suppress disturbances caused by complex textures, surface color variations, and specular reflections, thereby providing a more reliable basis for precise damage localization. Nevertheless, depth-based damage segmentation under a large field of view remains challenging, mainly due to fine-scale texture noise and weak defect saliency; moreover, robust defect probability estimation is often hindered by limited labeled data. To address these challenges, this paper proposes an unsupervised defect segmentation framework for hydroturbine friction components guided by local anomaly score distributions. First, a salient damage detection module is developed based on topography–texture separation, which mitigates the interference of local micro-texture noise on defect segmentation. Then, a normal reference dataset is constructed using defect-free bearing-pad depth images, and an unsupervised network is employed as the core to generate anomaly score representations of potential damage regions for coarse localization. Finally, the obtained anomaly score distribution is used as adaptive weights to fuse depth-based defect cues with morphological processing, enabling self-adaptive refinement of the damage regions. Experiments on real depth images acquired from hydroturbine bearing pads demonstrate that the proposed method achieves accurate defect extraction and reliable geometric quantification. Quantitative evaluations on the testing set yield a mean surface area error of 9.39% ± 4.25% and a volume error of 4.91% ± 2.85%, with best-case errors dropping as low as 3.67% and 1.03%, respectively. Crucially, these results demonstrate that our framework goes beyond mere visual detection; by operating entirely without pixel-level annotations, it offers a highly practical tool for diagnosing specific lubrication failure modes and driving predictive maintenance in actual hydroturbine engineering.

Lubricants doi: 10.3390/lubricants14050201

Authors: Chunlei Gao Yongzhi Liu Yongyi Du

Ice accumulation on critical infrastructure surfaces threatens operational safety in aviation, power transmission, and transportation systems. Conventional anti-icing and deicing strategies, such as chemical deicers and energy-intensive active heating, have inherent drawbacks. These include environmental pollution, high energy consumption, and low efficiency. In recent years, photothermal-responsive extremely water-repellent surfaces have attracted widespread attention. They can harvest renewable solar energy and achieve efficient anti-icing and deicing through tailored interfacial wetting properties. This review summarizes photothermal extremely water-repellent surfaces based on the “water as a lubricating layer” strategy. This strategy reduces ice adhesion strength and enables low-energy deicing. It works by forming a continuous lubricating film via photothermally induced interfacial meltwater. We discuss photothermal conversion mechanisms and strategies to enhance performance for stable lubricating film formation. We also analyze the stagewise physics of anti-icing and deicing, focusing on the interfacial tribological behavior of the water film. Key engineering challenges are addressed, including mechanical durability and all-weather applicability. Finally, we clarify future research directions for industrial translation. This review aims to provide theoretical insights and technical pathways for developing next-generation anti-icing and deicing surfaces that are efficient, eco-friendly, and sustainable.

Lubricants doi: 10.3390/lubricants14050200

Authors: Xiang Xu Xingyu Ma Chang Sun Haihua Wu Xinze Zhao Biao Liu Meiyun Zhao

Conventional solid-inlay lubrication research has focused on the overall inlay layout and generalized lubrication effects; localized inlay lubrication enables precise release of the lubricant and rapid formation of a uniform and dense lubricant film. The aim of this study is to use a combination of simulations and experimentation to establish a localized spherical friction pair containing a filled-hole structure, so that optimal lubrication can be predicted accurately. We designed seven different kinds of localized annular band inlay axial tiles, with ratios of 10, 12, 15, 18, 20, 22, and 25; carried out a transient mechanical analysis through finite element software, comparing the equivalent stress and friction coefficient of the inlay tiles with different ratios; and conducted friction wear tests through a self-developed spherical wear test bench. The friction surfaces of the tested specimens were analyzed for sliding friction coefficients and wear, as well as by SEM surface and EDS analyses. The results show that the best mechanical properties and the smallest theoretical friction coefficients are obtained when the relative ratio of the ring diameter to inlay hole diameter is 25. This study provides a theoretical basis for the design of spherical solid-inlay lubrication structures in the region of the split-belt ring design.

Lubricants doi: 10.3390/lubricants14050199

Authors: Mingyi Chang Weidong Zhang Dongzhou Jia Xiaoqiang Wu Yongqiang Fu Qi Gao

Titanium alloy is difficult to cut, with tools prone to adhesion and diffusion wear that reduces life and surface quality. Traditional coatings fail to meet precision machining demands. Based on TiAlN, Ce-doped coatings were prepared via magnetron sputtering at varying powers to investigate mechanical and tribological properties. The results show that with the increase in Ce doping amount, the hardness, elastic modulus, H/E, and H3/E2 ratios of the coating increase first and then decrease, and the friction coefficient decreases first and then increases. The performance is optimal at 50 W, the friction coefficient is 0.676, and the film-based adhesion is 113.8 N. Compared with the TiAlN coating, the hardness increased by 12%, the wear loss decreased by 24%, and the H/E and H3/E2 increased by 31% and 95%, respectively. The mechanism analysis shows that the appropriate amount of Ce doping can improve the toughness of the coating by grain refinement and solid solution strengthening and significantly inhibit adhesive wear and oxidative wear. Ce-modified tools were further prepared for titanium alloy turning experiments. Compared with uncoated and traditional TiAlN-coated tools, Ce doping can effectively reduce tool wear and improve the surface quality of the workpiece and has significant advantages under high-speed and large cutting depth conditions. This study systematically reveals the adaptive lubrication mechanism of Ce-doped TiAlN coating in the cutting process of titanium alloy and provides theoretical support and engineering guidance for the preparation of special tool coatings for difficult-to-machine materials.

Lubricants doi: 10.3390/lubricants14050198

Authors: William Skipper Anup Chalisey Roger Lewis

At present, silica sand particles are used on GB railways for traction enhancement. In this study, novel particle systems with a range of properties were assessed to see if there was potential for particles to be more widely used in friction management. The tests were carried out at representative contact pressures, using the High Pressure Torsion (HPT) approach. Particles were applied to dry, wet and leaf-contaminated interfaces. A strong relationship was found between particle hardness and traction. Particle systems were identified that could be used to lubricate the interface (friction < 0.1) or provide intermediate levels of friction (0.2–0.3), and one that could be used for traction enhancement as an alternative to silica sand (increasing friction to above 0.1 with a leaf layer present).

Lubricants doi: 10.3390/lubricants14050197

Authors: Zenghui Li Chaofan Xu Xisheng Fu Fengbin Liao Yunqi Huang Jing Hu Xiaomei Xu Hongmei Yang Yanan Zhao Xiuli Sun Yong Tang

Harsh modern industrial working conditions require high-performance lubricants, but traditional additives are limited by single functionality and poor compatibility, driving the development of multifunctional alternatives. Two novel hindered phenolic amide–esters (MADE, DAME) were synthesized and characterized. Their thermal/storage stability, antioxidant and tribological properties in synthetic oil were evaluated, with commercial 1010 and T203 as references. DFT calculations and worn surface analysis were also employed to clarify the lubrication mechanism. The results indicate that MADE exhibits better thermal/storage stability, comprehensive antioxidation and lubricating performance than DAME, with residual mass of 85.3% and 73.2% at 300 °C, respectively. A total of 1 wt.% MADE shortens the running-in period to 200 s (vs. 300 s for base oil), reduces the average. WSD by 12.1% and wear volume by 60.2%. Mechanistically, MADE adsorbs strongly on metal surfaces and forms a protective tribofilm via tribochemical reaction, exhibiting synergistic antioxidant and anti-wear effects. This work establishes a novel and sustainable paradigm for developing next-generation, multifunctional lubricant additives with high performance.

Lubricants doi: 10.3390/lubricants14050196

Authors: Xiaoxue Li Xiaoqiang Wu Zhiyong Zhang Yongqiang Fu

The performance requirements of wear-resistant and anti-corrosion coatings for marine equipment continue to increase. Diamond-like carbon (DLC) film has become a preferred protective material due to its high hardness, low friction and chemical inertia. To reveal the tribocorrosion mechanism of Si-doped DLC films in a seawater environment, a Cr-WC-WC/C transition layer and DLC-Si films with different Si contents were prepared by high-power pulsed magnetron sputtering (HiPIMS) technology on 304 stainless steel. The tribocorrosion tests were carried out under open-circuit potential and dynamic polarization conditions in seawater. The results show that Si doping improved the tribocorrosion resistance of the films. The sample with Si content of 9.26 at.% has the lowest self-corrosion current density, the smallest volume loss, complete wear scar morphology and no obvious substrate exposure. The strengthening mechanism is attributed to Si doping, which induces the formation of a SiOx passivation film and a hydrated silica gel lubrication layer. This establishes a synergistic solid-chemical lubrication system, inhibits sp2 cluster growth, prolongs the diffusion path of corrosive media, and mitigates the damaging wear–corrosion synergy. This study confirms that moderate Si doping can significantly improve the wear resistance and corrosion resistance of DLC films in a seawater environment, and provides a theoretical basis for the design and application of carbon-based protective coatings for marine equipment.

Lubricants doi: 10.3390/lubricants14050195

Authors: Xianghua Zhan Na Zhang Zhentao Li Xiaoying Li Dengke Chen Yancong Liu

Mechanical seal faces frequently operate under harsh and dynamic conditions, where maintaining stable and efficient lubrication remains a critical challenge. Surface texturing has emerged as an effective approach to improve the lubrication and tribological performance of sealing end faces. This study investigates the lubrication behavior and sealing characteristics of squamous textured mechanical sealing faces through a combination of sealing experiments and mixed lubrication modeling. The results indicate that during start-up, increasing rotational speed enhances the load-carrying capacity and reduces contact force, resulting in thicker lubricant films and lower friction coefficients. Consequently, the sealing interface gradually transitions from mixed to hydrodynamic lubrication. Moreover, with increasing medium pressure, both the critical rotational speed and duration required for this lubrication regime transition increase. Under various steady periods, smooth seal faces predominantly operate in the mixed lubrication regime, whereas textured faces maintain hydrodynamic lubrication, reducing the average friction coefficient and temperature rise by 69.5% and 51.8%, respectively. These findings provide crucial insights into the performance improvement and practical applications of squamous textures for high-efficiency, long-lifespan mechanical seals.

Lubricants doi: 10.3390/lubricants14050194

Authors: Jin Peng Zaiyu Xiang Shaohao Deng Jiakun Zhang Xiaoqin Liu

The friction-induced stick-slip vibration (FISSV) generated by intense friction between the brake disc and brake pads of high-speed trains is a critical issue affecting braking stability, the service life of foundational braking components, and ride comfort. The floating friction block structure, which effectively regulates interfacial contact characteristics through the elastic deformation of disc springs, thereby improving tribological behavior, represents an effective approach for mitigating FISSV. However, the topic of how to design the floating structure of the friction block to produce the best suppression impact on FISSV emerges, using the choice of disc spring material as an example. Thus, the purpose of this study is to look at how disc spring material affects stick-slip vibration (SSV) at the high-speed train floating brake interface. Four typical disc spring materials—304 stainless steel, Mubea-specific spring steel, 50CrVA high-alloy spring steel, and 60Si2MnA silicon-manganese spring steel—were selected. Through braking tribological tests and explicit dynamics-wear coupling simulations, the effects of material differences on interfacial friction-wear characteristics and SSV behavior were systematically studied. The findings show that the stiffness of the disc spring material greatly influences the dynamic responsiveness of the system and the contact pressure distribution at the braking interface, elasticity, and damping characteristics. 60Si2MnA spring steel, owing to its excellent elastic recovery and load equalization capability, promoted the formation of uniformly dispersed medium-to-small contact platforms on the interface, resulting in the mildest wear. Concurrently, its system vibration energy exhibited a more dispersed distribution in the frequency domain, with low SSV intensity and weak nonlinear behavior, demonstrating the best comprehensive performance. Materials with poorer compatibility, such as 304 stainless steel, tended to cause localized stress concentration, exacerbating wear and intensifying severe high-frequency SSV. The influence mechanism of disc spring material at the interface is shown by this work, providing an important basis for material optimization and vibration suppression design in floating brake pad structures.

Lubricants doi: 10.3390/lubricants14050193

Authors: Dong Sun Liqin Wang Deng Pan Tingjian Wang Le Gu Chuanwei Zhang

Boundary lubrication plays a crucial role in determining the service performance and operational lifespan of mechanical components. However, continuum mechanics models and experimental studies are unable to elucidate the dynamic evolution of intermolecular interactions at the interface under boundary lubrication from a microscopic perspective, including phenomena such as asperity contact and lubricant film rupture. In this study, a molecular dynamics simulation approach was employed to construct a boundary lubrication friction model incorporating lubricant molecules, aiming to investigate the influence of applied load and asperity height on the dynamic evolution of atomic interactions at the interface from the perspective of energy variation, under conditions both with and without asperity contact. The results indicate that van der Waals interactions dominate the frictional response, and severe asperity contact leads to a sharp increase in van der Waals energy, which in turn results in a decrease in normal force, thereby increasing the friction coefficient. When the upper and lower surfaces remain separated by the lubricant, an increase in van der Waals energy leads to higher friction force, consequently elevating the friction coefficient. In the absence of contact, the friction coefficient decreases with increasing load; however, once asperity contact occurs, higher loads accelerate lubricant film failure and intensify direct interfacial contact, leading to more pronounced stick–slip oscillations and increased wear. This study provides atomic-scale insights for the design and performance optimization of boundary lubrication interfaces.

Lubricants doi: 10.3390/lubricants14050192

Authors: Nebojša Zdravković Živana Jovanovic Pešić Dalibor Nikolić Dragan S. Džunić

This study investigates the influence of physiological body fluids on the mass stability and tribological performance of polylactic acid (PLA) samples produced by Fused Deposition Modeling (FDM) 3D printing. Body fluid exposure was simulated using Dulbecco’s Modified Eagle Medium (DMEM) under controlled conditions. Black PLA filament was printed with three infill densities (15%, 20%, and 90%) and immersed in DMEM for 7 days at 37 ± 1 °C. Mass measurements revealed that lower infill densities resulted in significantly higher mass loss, with the 15% infill samples exhibiting the greatest reduction (5.07%), while the 90% infill samples showed negligible change (0.17%). Tribological testing using a CSM nanotribometer under loads of 5 mN, 500 mN, and 1000 mN demonstrated that infill density critically affects friction and wear behavior. The 90% infill samples exhibited the lowest wear volumes and the most stable tribological response, while the 15% infill samples showed degradation-dominated contact behavior. Although the friction measurements for the 15% infill samples were consistent, their interpretation should be approached with caution due to pronounced surface deterioration and debris-mediated sliding. This behavior is attributed to structural weakening caused by immersion in DMEM, which promoted material degradation and influenced the tribological response. These findings confirm the critical role of structural density in wear resistance. To the best of our knowledge, this is the first study to systematically investigate the combined effect of hydrolytic degradation and tribological behavior of FDM-printed PLA as a function of infill density under simulated physiological conditions. These findings provide a scientific basis for optimizing infill density in the design of PLA-based surgical instrument guides, where both degradation resistance and tribological performance under body fluid exposure are essential. The findings should be interpreted within the limitations of the experimental design.

Lubricants doi: 10.3390/lubricants14050191

Authors: Guozhe Ren Rui Wang Mingzhang Wang Huan Zhao Wenfeng Xu

In order to explore the influence of baffle structure on the oil–gas two-phase flow and leakage characteristics of aero-engine bearing chamber sealing systems, based on the VOF two-phase flow model, this paper systematically carried out a transient numerical simulation of the bearing chamber sealing systems with conventional configurations and baffle configurations. The oil distribution, leakage and flow evolution of the two types of configurations under different baffle heights, sealing pressure differences and rotational speeds were compared and analyzed. The results show that the higher the height of the baffle, the more obvious the accumulation effect of the lubricating oil and the greater the leakage. The increase in sealing pressure difference helps to suppress leakage and reduce leakage fluctuation. The increase in rotational speed aggravates the centrifugal effect of the lubricating oil and makes the leakage increase significantly. This paper reveals the multi-parameter coupling mechanism of the baffle structure on the leakage control of the bearing chamber sealing system, and it provides a theoretical basis for the optimal design of the bearing chamber sealing structure of the aero-engine.

Lubricants doi: 10.3390/lubricants14050190

Authors: Alexander Fetisov Yuri Kazakov Maksim Litovchenko

This paper presents the results of numerical calculation of steady-state magnetorheological fluid flow in the gap of an eccentric seal subjected to an external radial magnetic field. A coupled problem combining magnetic field analysis and laminar viscoplastic flow with Bingham rheology is solved to obtain pressure and velocity distributions within the seal gap, from which the hydrodynamic reaction forces of the fluid film and the rotor friction torque are determined. A parametric study was conducted in the ranges of rotor angular velocity ω = 100–400 rad/s, relative eccentricity ε = 0–0.9, and magnetic flux density B0 = 0–0.5 T at the pressure differential Δp = 2 atm. Analysis of the results shows that increasing the magnetic flux density from 0 to 0.5 T leads to an increase in the seal reaction force from 12 N to 642 N and the friction torque from 0.35 N·m to 11.23 N·m. The most intensive growth of both characteristics is observed in the range B0 = 0–0.3 T, beyond which saturation occurs as the MRF yield stress reaches its plateau value. An optimal control range of B0 = 0.1–0.2 T was determined, ensuring maximum seal energetic efficiency as quantified by the load capacity-to-friction torque ratio, which is maximized at 70 N/(N·m). Based on the obtained results, the consequences of using magnetorheological seals on the performance of the rotor system are discussed, including the analysis of the sealing effect on rotor-dynamic stability. Within the proposed optimal range, it is shown that an increase in magnetic flux density leads to a sign reversal of the horizontal reaction F2, while the monotonic growth of the ratio |F2|/F1 indicates an intensification of cross-coupling and a corresponding reduction in the rotordynamic stability margin at higher values of B0.

Lubricants doi: 10.3390/lubricants14050189

Authors: Jianbin Zhang Ming Yi Leilei Li Ting Lv Yanling Wang Libang Feng Chaoyang Zhang Mohamed Kamal Ahmed Ali

Lubricant additives play a crucial role in improving the tribological performance of lubricating oils to reduce frictional energy losses and improve the durability and reliability of mechanical systems. In this study, soft metallic-based core–shell Ag@Cu microparticles were synthesized via an in-situ galvanic displacement method and incorporated into EHC-50 base oil with various concentrations. The tribological performance evaluations indicated that 0.3 wt% Ag@Cu significantly enhanced friction-reducing and anti-wear properties, achieving a stable friction coefficient of 0.12, a 45% reduction, and a wear volume reduction of 75% compared to the pristine oil. Additionally, the surface characterization techniques (SEM/EDS, XPS, XRD, and TOF-SIMS) were employed to explore the wear patterns and related lubrication mechanisms. The results indicated that the synergistic interaction between the micro-bearing effect, physical mending, and tribochemical reactions facilitated the formation of a robust tribofilm composed of metallic Ag, ternary CuFe3O2, and sulfides, which achieved higher lubrication performance. Ultimately, this research provides novel metallic micro-additives, offering a facile approach to formulating wear-resistant lubricants with significant potential for saving energy for mechanical tribosystems in industrial applications.

Lubricants doi: 10.3390/lubricants14050188

Authors: Jiashu Yu Xuexing Ding Jianping Yu

Spiral-groove dry gas seals are widely used in various rotating machinery, and their performance prediction is of great significance for structural design and operational optimization. Existing studies still face several limitations, including the limited fidelity of numerical simulations, the insufficient number of experimental samples, and the restricted generalization capability of models based on a single data source. To address these issues, this study constructed a multi-source data system integrating numerical simulation data and experimental data, and systematically compared four representative data fusion methods, namely the uncertainty-weighted fusion algorithm, TrAdaBoost, MFDNN, and CoKriging, with analysis of their applicability and predictive performance. The results show that multi-source data fusion can effectively exploit the complementary advantages of different data sources and improve the prediction accuracy of dry gas seal performance. In terms of the comparison of data fusion methods, all four methods achieved good results for the groove-depth problem; however, for the spiral-angle and groove-number problems, which exhibit stronger nonlinear characteristics, clear differences were observed among the methods. Among them, TrAdaBoost showed the best overall performance, followed by MFDNN, then CoKriging, while the uncertainty-weighted method was relatively weaker. In terms of seal performance, the influence of groove depth on seal performance was relatively direct; the spiral angle is recommended to be controlled within 10–14°, and the groove number within 12–16, so as to balance opening force and leakage rate. This study can provide a reference for the rapid performance prediction and parameter optimization of spiral-groove dry gas seals.

Lubricants doi: 10.3390/lubricants14050187

Authors: Jia Sun Zhe Jiang Songhua Li Lixiu Zhang Zhenyu Yin Shiqi Li

Under complex friction conditions, base oils usually exhibit insufficient friction-reducing and anti-wear performance, poor shear resistance of the lubricating film, and weak interfacial adsorption stability. Herein, graphene oxide (GO) was surface-modified with the silane coupling agent KH550 and compounded with Ag nanoparticles to fabricate a silver/modified graphene oxide (Ag/KGO) composite lubricant additive. The microstructure and chemical characteristics of the Ag/KGO composite were characterized by SEM, XRD, FTIR, and Raman spectroscopy. Tribological tests performed on a Si3N4/GCr15 friction pair demonstrated that the lubricant containing 0.15 wt% Ag/KGO achieved the optimal tribological performance, with the average friction coefficient decreasing to 0.053, 51.8% lower than that of the base oil, and the wear scar width and depth decreasing by 34.5% and 75.7%, respectively. Molecular dynamics simulations revealed that Ag/KGO enhanced the interfacial adsorption strength and improved the shear stability of the lubricating film. Mechanism analysis indicated that KGO facilitated the formation of a stable lubricating film at the friction interface, while Ag nanoparticles acted as nano-bearings. Their synergistic effect reduced interfacial shear resistance and alleviated wear. These findings provide theoretical support for the design and development of high-performance composite lubricant additives.

Lubricants doi: 10.3390/lubricants14050186

Authors: Parastoo Fallah Cara Hensley Charles J. Beall Rolf Wuthrich Pantcho Stoyanov

To elucidate the role of environmentally friendly oxide additives in a molybdenum disulfide (MoS2)-based solid lubricant, this study investigates the tribological behavior of a MoS2–TiO2 coating deposited via a spray-bonding process and compares it with a commercial Sb2O3-containing formulation (Everlube 620C). Interfacial characteristics and wear-related mechanisms were systematically analyzed using scanning electron microscopy (SEM), focused ion beam (FIB), Raman spectroscopy, and X-ray diffraction (XRD). The MoS2–TiO2 coating exhibited a higher steady-state coefficient of friction (0.35–0.45) and wear compared to the baseline. Its wear behavior was governed by fracture-induced three-body abrasion, driven by the hard and brittle nature of TiO2, which promotes stress concentration at particle–matrix interfaces, crack initiation, particle pull-out, and debris generation. These processes suppress the formation of a desirable MoS2-rich tribo/transfer film, leading to deformation-dominated friction. Overall, the findings indicate that the intrinsic mechanical properties and interfacial behavior of TiO2 limit its effectiveness as an additive in MoS2-based coatings, highlighting the importance of additive selection and compatibility in achieving optimal tribological performance. Notably, this study was performed at an additive volume fraction equivalent to that of Sb2O3 in Everlube 620C, serving as a foundation and indicating that further optimization of TiO2 particle size and concentration is required to achieve comparable performance.

Lubricants doi: 10.3390/lubricants14050185

Authors: Xianjin Fang Yingzi Zhang Chen Tang Zhiran Lu Zehua Hu Haiwei Chen Hunian Shan Shaohui Yang Zhilin Liu Yan Huang Chenglong Li

Single-point mooring systems are among the key systems for offshore oilfield development. The fluid swivel is a core component of such systems, enabling fluid transfer while allowing the vessel to follow the weather vane effect. The spring-energized seal is critical for ensuring reliable fluid transmission. Existing studies on spring-energized seals primarily focus on small-scale mechanisms, with limited research on large-scale seal design under complex operating conditions. This work investigates the dynamic sealing performance of the oil-transferring rotary joint in a 300,000 ton VLCC catenary single-point mooring system. A spring-energized seal is designed with a PTFE-based composite as the sealing jacket and Inconel 718 as the spring material. A finite element model of the spring-energized seal is developed in ANSYS 2022 R1, and the design is optimized to achieve lower equivalent strain, more uniform contact pressure distribution, larger contact width, and reduced friction. Fatigue life analysis of the optimized design verifies its reliability over a 10-year service period. The proposed study provides a reference for the design of dynamic seals in high-end offshore engineering equipment.

Lubricants doi: 10.3390/lubricants14050184

Authors: Andrea Mescola Federico Zanni Alberto Rota Cristina Bernini Andrea Gerbi Riccardo Carzino Luca Repetto Michał Bartkowski Silvia Giordani Renato Buzio Guido Paolicelli

Diamond-like carbon (DLC) coatings are widely used as protective and self-lubricating surfaces in metal–metal contacts. Their frictional behavior is governed by the formation and evolution of carbon-rich transfer layers (TLs), which can be tailored through functionalization with carbon nanomaterials. Recent studies have shown that graphene sheets (GSs) and nanodiamonds (NDs) act synergistically to achieve ultra-low friction in microrough (~0.2 μm) metal–DLC contacts under dry N2 at a 1 N load. Here, we probe how this lubrication mechanism evolves with increasing load from 1 to 10 N—corresponding to local contact pressures up to ~11–16 GPa—respectively, in dry N2 and humid air conditions. Ball-on-disk experiments are performed on an industrial hydrogenated DLC coating sliding against stainless-steel. In dry N2, GS–ND functionalization yields a low and stable coefficient of friction across the entire load range, reaching a minimum of about 0.05. In humid air, higher friction levels are observed across all loads (CoF ~0.10–0.15), accompanied by oxidation-driven modifications of both wear debris and the counterface contact region, with oxygen content increasing by more than a factor of three compared to dry N2. Detailed microscopy and spectroscopy analyses indicate that enhanced lubricity in dry N2 arises from TLs incorporating GSs, NDs, and nanoscroll-like structures, whereas humid air promotes interfacial amorphization and oxidation, leading to load-insensitive friction and boundary lubrication effects through physisorbed water molecules.

Lubricants doi: 10.3390/lubricants14050183

Authors: Mohammad Ashikul Alam Nihal Ahmed Md Abid Hossain Janak Paudel Bo Shen Maharshi Dey Sujan Ghosh

The poor tribological and mechanical performance of Al alloys hinders their use in practical applications where low COF and high durability are required. This study examined and evaluated a novel laser-sintered Ni-Cr coating to improve the load-carrying capacity and tribological performance of an Al alloy (Al 6061) substrate. The authors demonstrate that laser sintering cycle count is a decisive process variable governing coating dispersion, microstructural consolidation, and tribological performance in Ni-Cr coatings fabricated via Selective Laser Sintering (SLS). Increasing the laser cycle count progressively refined the surface morphology, improved coating dispersion, and strengthened interparticle bonding. As a result, the average durability after three cycles was seven times that after one laser cycle, accompanied by markedly improved COF. To further improve durability and load-carrying capacity, Ti3SiC2 was introduced into the Ni-Cr coating. The coating containing 10 wt% Ti3SiC2 exhibited a 20-fold increase in durability, extending the time to failure to approximately 70,000 s (700 m) while maintaining a low coefficient of friction (~0.48) compared with the coating containing no Ti3SiC2. The greater durability of the Ni-Cr-10wt%Ti3SiC2 coating in this novel study was attributed to improved adhesion to the substrate, better particle distribution during sintering, and greater load-carrying capacity. While further process changes do not yield feasible samples, this study showed that surface properties can be improved within the available small-process regime. Overall, laser sintering of a Ni-Cr-10wt%Ti3SiC2 coating shows promise as a means to improve the tribological and mechanical performance of Al 6061. This study should aid researchers and other stakeholders in fabricating well-adhering, durable, and tribotactic composite coatings on Al6061 and similar material systems.

Lubricants doi: 10.3390/lubricants14050182

Authors: Heng Tian Dengke Wang Gang Li

To address the problems of insufficient precision reserve, limited rotational speed, and excessive temperature rise in high-speed ultra-precision machine tool spindle bearings, the influence of frictional power loss on the thermo-mechanical behavior of the bearing system was investigated. Firstly, based on the analysis of the heat source of the bearing, the friction power consumption model of the bearing assembly is established, and the analysis of the bearing temperature field is realized by studying the heat energy transfer. Secondly, the test bench is built for experimental verification. Finally, through the study of thermal-mechanical coupling performance, the influence of different rotational speeds on bearing stress and life is analyzed. The results show that the friction power consumption generated by the spin sliding of the bearing rolling element accounts for the largest proportion, accounting for 31% of the total friction power consumption; the increase in bearing speed will increase the bearing temperature. At 55,000 r/min, the highest temperature at the rolling element is close to 75 °C, followed by the inner ring up to 68 °C, and the lowest outer ring temperature is 57 °C. The temperature has a great influence on the bearing performance. Under the same working conditions, the equivalent stress is increased by 21%, the contact pressure is increased by 25%, and the fatigue life of the bearing is reduced by 5.6%. Bearing performance is significantly affected by thermodynamic behavior.

Lubricants doi: 10.3390/lubricants14050181

Authors: Zhiming Zhang Xi Zhang Menglu Zhang Jian Zuo Yifei Zhu

The water-lubricated piston–cylinder pair is a critical tribological component in hydraulic systems, yet its performance under boundary lubrication is often limited by high friction and severe wear. Conventional epoxy coatings provide only modest improvements. In this study, graphene-modified epoxy composite coatings were developed and applied to piston substrates, then characterized via scanning electron microscopy, white light interferometry, and nanoindentation. Tribological performance was evaluated using a reciprocating tribometer under simulated pump conditions of 16 MPa and 1500 r/min. Compared to the pure epoxy coating, the graphene-modified coating reduced the friction coefficient by 33.9% and the wear rate by 77.2%, while the graphene oxide-modified coating reduced them by 16.1% and 64.5%, respectively. These results demonstrate that graphene-modified epoxy composite coatings offer an effective surface engineering solution for enhancing the durability and efficiency of water-lubricated systems, with promising potential for water hydraulic applications.

Lubricants doi: 10.3390/lubricants14040180

Authors: Xiaoyu Guo Yan Zhao Wenjing Lou Xiaobo Wang Feng Guo Haichao Liu

In this study, an elastoplastic finite element (FE) contact model was developed to evaluate the plastic deformation of a surface induction-hardened tapered roller bearing used in wind turbines, incorporating depth-dependent material properties and heat treatment-induced residual stress distribution. The validity of this model was confirmed by comparing the calculated plastic deformation with measured profiles from static compression experiments. The results show that the residual stresses generated by induction hardening have a significant influence on the elastoplastic behavior of bearings. Based on this model, a parametric analysis was performed to investigate the effects of surface hardening depth (SHD), contact pressure, and residual stress on surface plastic deformation. Empirical formulas were developed to predict surface plastic deformation and evaluate material yielding for surface-hardened tapered roller bearings, thereby preventing excessive deformation during service. This allows for the rapid estimation of the maximum plastic deformation for different hardening depths and provides an efficient approach for assessing the yielding risk.

Lubricants doi: 10.3390/lubricants14040178

Authors: Sathaporn Chuepeng Atthaphon Maneedaeng Niti Klinkaew Anupap Pumpuang Tanongsak Sukkasem Ekarong Sukjit

Although bio-hydrogenated diesel (BHD) offers drop-in compatibility and high oxidative stability, its poor lubricity remains a critical barrier to long-term engine deployment. Previous studies have primarily relied on short-term tribological assessments, leaving insufficient empirical data on sustained wear behavior under realistic conditions. This study addresses that gap through a 200 h durability evaluation of BHD–biodiesel blends in a single-cylinder diesel engine under constant load conditions per Thai Industrial Standard TIS 2618-2557. Five fuels, namely diesel, pure BHD, BHD90, BHD70, and pure biodiesel, were tested to identify the critical biodiesel threshold for optimal tribological and oxidative performance. BHD90 (90% BHD + 10% biodiesel) emerged as the optimal formulation, delivering the lowest torque reduction (11.2%) and minimal iron wear particles (101 ppm), while preserving oxidation stability. Biodiesel concentrations exceeding 10% induced accelerated lubricant oxidation through hygroscopic effects, negating the lubricity benefits. Fourier-transform infrared spectroscopy (FTIR) analysis of piston carbon deposits further revealed that higher biodiesel blends produced more oxygenated compounds, whereas pure BHD and diesel generated predominantly aliphatic hydrocarbons. These findings establish a mechanistic relationship between fuel composition, oxidation, and wear under endurance conditions, providing a practical guideline for renewable diesel formulation that balances lubrication performance, oxidation control, and long-term engine durability.

Lubricants doi: 10.3390/lubricants14040179

Authors: Ilhan Celik Abdullah Tahir Şensoy Sevki Burak Sezer

Pin–bushing bearings in heavy-duty construction machinery operating in severe industrial environments are susceptible to accelerated wear, grease degradation, and lubrication failure, yet application-specific guidance for lubricant selection and re-greasing intervals under such conditions remains limited. This study evaluates the combined effects of bushing material (hardened steel, cast bronze, and Cu–Sn alloy), grease type (three commercially used greases with viscosities of 120, 460, and 150 mm2/s at 40 °C), and lubrication interval (8, 12, and 24 h) on grease-condition indicators in a field-operating wheel loader used in slag handling, where surrounding slag temperatures may reach 700–800 °C. A Taguchi L9 orthogonal array was used to define nine experimental configurations, each applied for approximately one week under real operating conditions. Grease samples were characterised using the SKF grease analysis kit based on NLGI consistency grade, base oil release rate, and contamination particle count. All greases showed an increase in NLGI grade from 2 to 3–4 during service, indicating thickening and a possible risk of lubrication channel blockage. Oil release rates decreased by up to 60% in some configurations, indicating reduced base oil mobility during service. When the three grease-condition indicators were evaluated together by Grey Relational Analysis, the combination of steel bushing, type B grease (ISO VG 460, lithium complex with MoS2), and a 12 h lubrication interval showed the most balanced overall response. These findings provide field-based guidance for grease selection and maintenance scheduling in pin–bushing systems operating under demanding service conditions.

Lubricants doi: 10.3390/lubricants14040177

Authors: Hao Lin Yuge Han Leiming Song Xin Wei

The new-generation aeration blower, which uses a high-speed permanent-magnet synchronous motor supported by elastic-foil thrust bearings, represents the future development trend of high-end sewage treatment turbomachinery. An axial load tester was designed for the elastic-foil thrust bearings in this study. Firstly, the relationship between the axial load and the elastic-foil thrust bearing parameters was first established. An axial load tester was designed. Secondly, finite-element simulation and strain calibration of the axial load tester were performed to estimate the linear relationship between the strain and the axial load. Then, the time histories of axial load for the high-speed permanent-magnet synchronous motor were further obtained at a rotational speed of 15,000 rpm during the operation tests. Finally, the load spectrum was compiled by fitting the test data to a function. The results showed that the amplitude and frequency of the load spectrum obeyed an exponential decay function. It can be used for the life test of elastic-foil thrust bearings in the future. The method for obtaining the axial load in the direct-driven turbomachinery was proposed. The axial load tester proposed in the present study, based on operation tests, proves valuable for improving the performance of the high-speed permanent magnetic synchronous motor and the elastic-foil thrust bearing.

Lubricants doi: 10.3390/lubricants14040176

Authors: Xiang Shi Jiahao Li Yufei Liu Jian Zhang Xiaomin Chen

Graphene and its derivatives are widely recognized as effective reinforcements due to their unique mechanical, thermal and lubrication performance. Incorporation of these reinforcements into polyamide-imide (PAI) coating matrix has shown significant potential for improving the tribological performance. Here, the mechanisms underlying the tribological improvement enabled by graphene oxide (GO) are investigated via frictional experiments and molecular dynamics simulations. It was found that the coefficient of friction (COF) of PAI coating is reduced upon the addition of GO over the range of 100–400 MPa and 20–100 mm/s, with a maximum reduction of ~25% achieved at 200 MPa and 60 mm/s. Simulations reveal that the friction reduction arises from strong adhesion interactions between the embedded GO sheets and PAI molecular chains, which inhibit the shear-induced mobility of the chains during the friction process. This mechanism enables a further reduction in the COF of the GO/PAI composite coating by increasing the interfacial adhesion through the tailored modulations of surface morphology and chemistry of the GO sheets. These findings pave the way for advancing the rational design and application of graphene-based composite coatings with highly improved tribological performance.

Lubricants doi: 10.3390/lubricants14040175

Authors: Corina Birleanu Florin Popister Razvan Udroiu Horea Stefan Goia Marius Pustan Mircea Cioaza Paul Pirja Ramona-Crina Suciu

Despite the growing adoption of Ti6Al4V-ELI made by Laser Powder Bed Fusion (LPBF) in tribologically demanding applications, the influence of hybrid nanoparticle additives on its lubrication behavior under starved contact conditions remains insufficiently explored. The tribological performance of Ti6Al4V was investigated under starved boundary-to-mixed lubrication conditions using engine oil modified with Ag-doped TiO2 nanoparticles. Double-scan LPBF-fabricated discs were tested in a ball-on-disc configuration against AISI 52100 bearing steel using a TRB3 tribometer. Nanolubricants were prepared by dispersing TiO2 and Ag–TiO2 nanopowders with different Ag+/Ti4+ ratios (0.5%, 1.5%, and 2.5%) in SAE 10W-40 engine oil at a constant nanoparticle concentration of 0.05 wt%. Comprehensive physicochemical characterization of the nanopowders and nanolubricants was performed through structural, chemical, optical, morphological, rheological, and stability analyses. Tribological experiments were conducted following a full-factorial design combining three normal loads (5–15 N), three sliding speeds (0.10–0.20 m·s−1), and four lubricant formulations. The steady-state coefficient of friction ranged between 0.281 and 0.359, while the specific wear rate varied from 2.81 × 10−4 to 4.83 × 10−4 mm3·N−1·m−1. The contact temperature rise remained relatively moderate, within the interval of 1.9–9.4 °C. Among the investigated formulations, the lubricant containing 1.5% Ag–TiO2 exhibited the lowest friction coefficient, whereas the formulation with the highest Ag content showed improved stability of tribological performance across the investigated operating domain. These results indicate that Ag-modified TiO2 nanoparticles are consistent with the formation of protective tribofilms and contribute to the stabilization of friction, wear, and thermal behavior under starved lubrication conditions. ANOVA confirmed that sliding speed and the load–lubricant interaction are the dominant factors governing friction and wear, while normal load controls the thermal response. These findings support the use of Ag–TiO2 nanolubricants as a viable strategy for stabilizing interfacial behavior in LPBF-fabricated titanium components operating under starved lubrication conditions.

Lubricants doi: 10.3390/lubricants14040174

Authors: Shengdi Sun Jianqiao Zhang Wenyan Chu

The friction size effect in thin-sheet microforming constrains the attainable forming quality of microscale sheet components. In this study, T2 copper foils with thicknesses of 0.04, 0.08, 0.16, and 0.32 mm were investigated by comparative tensile testing, pin-on-disk testing, sliding-friction experiments, surface characterization, and reduced-order analysis under dry friction and three liquid-lubrication conditions. The results showed that, as the thickness decreased from 0.32 mm to 0.04 mm, elongation and tensile strength decreased by nearly 60% and 40%, respectively, whereas the direct contribution of the mechanical size effect to the friction coefficient remained limited. Under dry friction, the friction coefficient changed little with specimen size. Under soybean oil, castor oil, and Vaseline lubrication, however, the friction coefficient increased markedly as specimen size decreased and gradually approached the dry-friction value; the lowest-viscosity lubricant exhibited the greatest loss of effectiveness at small scales. This behavior was associated with the expansion of the edge non-lubricated region and the loss of closed lubricant pockets, both of which increased the real contact area. On this basis, a size-dependent friction model was established for the present material and surface conditions, and its prediction for the castor oil case was consistent with the experimental trend.

Lubricants doi: 10.3390/lubricants14040173

Authors: Lan Wu Jiangfeng Yuan Shuaijun Zhang Xiaoyan Jin Chunye Wu Yanyu Sun

Although extensive research has been conducted on peristaltic robots, early designs are often constrained by mechanical configurations and material constraints, which restrict kinematic capability, particularly steering control. In contrast, snails steer by modulating mucus secretion to redistribute ventral friction along the foot. Inspired by this strategy, we propose a friction-differential steering mechanism and develop a novel crawler that implements it. The crawler is integrated with a peristaltic robot, and three experiments are conducted to evaluate steering performance. We further establish a physical model of friction-differential steering, including cases identified from the experiments. The proposed model captures the experimentally observed trend that the steering response increases with the friction differential and provides a qualitative physical interpretation of the steering mechanism. Finally, the method is generalized by analyzing its limiting behavior, thereby clarifying the operating bounds of the proposed approach. This work provides a principled framework for steering control in peristaltic robots and offers a promising direction for improving their motion controllability.

Lubricants doi: 10.3390/lubricants14040172

Authors: Changyong Deng Dawei Dong Sipeng Wang Hongsheng Zhang Li Feng

The running reliability of rolling bearings depends on the effective lubrication state, and poor lubrication will induce abnormal vibration. Therefore, vibration-based fault diagnosis is an important means to evaluate the health of bearings through vibration characteristics. However, the lack of fault samples in actual working conditions seriously restricts the generalization ability and accuracy of an intelligent diagnosis model. A novel few-shot diagnosis method integrating multi-layer feature fusion and adaptive similarity measurement is proposed. This method adopts a meta-learning framework to simulate sample scarcity through numerous N-way K-shot diagnostic tasks. An efficient feature extractor with a cross-task feature stitching mechanism is designed to fuse features from support and query sets. To overcome the limitation of fixed-distance metrics in existing meta-learners, a learnable similarity scheduler adaptively generates optimal pseudo-distance functions. In particular, a multi-layer feature fusion strategy is introduced to compute adaptive similarities at multiple network depths, which significantly enhances feature robustness against operational variations. Experimental results demonstrate the method achieves stable diagnostic accuracy above 90% under extremely few-shot conditions and maintains over 90% accuracy when transferring from laboratory-simulated faults to natural operational faults, validating its strong potential for practical industrial applications where annotated fault data is scarce.

Lubricants doi: 10.3390/lubricants14040171

Authors: Shicai Yin Mingyang Shang Jinqiu Gao Wanshun Zang Chao Gong Yaofei Han

Accurately monitoring the wheelset–rail friction condition is crucial for ensuring the safety and operational efficiency of the traction drive system. However, the friction characteristics of wheelsets are easily influenced by factors such as ramp transitions and variable railway conditions in the complex environment. These factors significantly increase the difficulty of detecting friction anomalies and accurately locating faulty wheelsets in a timely manner. To address this issue, this paper proposes a sliding mode coordinate positioning–based friction anomaly monitoring scheme for multiple wheelsets in traction drive systems. First, a multi-sliding mode fusion-based friction characteristic observer is developed. Then, an friction coordinate analysis-based anomaly identification method is proposed. Finally, the proposed method is validated on a hardware-in-the-loop (HIL)-based experimental platform. Experimental results demonstrate that the proposed scheme can effectively detect friction anomalies and accurately locate abnormal wheelsets in multi-wheelset traction systems. Compared with traditional methods, the proposed scheme exhibits stronger robustness to varying railway conditions and does not require complex optimization mechanisms, making it suitable for practical on-board applications.

Lubricants doi: 10.3390/lubricants14040170

Authors: Francesco Pio Urbano Arne Bischofberger Sascha Ott Albert Albers

Non-exhaust particulate emissions are expected to remain a relevant source of traffic-related air pollution, including an increase in electrified vehicle fleets. Particle formation results from tribological interactions and is influenced by both operating conditions and friction material system. This study presents an extended measurement methodology under application-relevant tribological conditions for the reproducible quantification of PM10 and PM2.5 emissions from dry-running friction systems and applies it to a systematic investigation of operating parameter and friction pairing effects. A dry inertial brake test bench with an enclosed friction chamber and integrated aerosol measurement chain was used under controlled tribologically relevant conditions. Specific friction work and specific friction power were varied by adjusting sliding velocity, contact pressure, and inertial load. Six friction pairings, comprising four representative friction lining types combined with either C45 cast steel or GGG40 gray cast iron, were examined. In situ PM10 and PM2.5 measurements were complemented by gravimetric wear and microstructural analyses. The results show that specific friction work has a direct influence on PM10 and PM2.5 emissions, whereas the independent effect of contact pressure is secondary. Friction power exhibits material-dependent effects. Emissions also vary strongly with friction pairing, indicating that operating conditions and material system must be considered jointly when assessing low-emission brake systems.

Lubricants doi: 10.3390/lubricants14040169

Authors: Joss Klausner Likibi Baogang Wen Xia Zhao Zhange Zhang Jingyu Zhai

Angular misalignment of spherical roller bearings in wind turbine main shafts is a known cause of premature failure. Manufacturing and assembly tolerances introduce unavoidable variability in this misalignment—a source of uncertainty typically neglected in deterministic life models, thereby creating a gap between installation quality and system reliability. A probabilistic framework combining a Hertzian contact model, the Ioannides–Harris fatigue theory, and Monte Carlo simulation is developed to predict the fatigue life of double-row spherical roller bearings under uncertain misalignment. The sensitivity of eight geometric parameters, selected based on manufacturing tolerances, is quantified using Sobol indices for global sensitivity analysis, allowing their relative importance to be ranked. Application to a 950-series wind turbine main bearing under nominal and extreme loads shows that even with centered installation a non-negligible failure probability persists under nominal conditions. The strongly asymmetric bearing response requires asymmetrical installation tolerances to ensure high reliability. Global sensitivity analysis identifies the misalignment angle as the dominant source of uncertainty, followed by the roller contour radius. Under extreme loads, the bearing is under-dimensioned relative to the 20-year design life required for wind turbine main bearings, leading to a fatigue failure probability that approaches unity regardless of installation quality. The interaction between misalignment and radial clearance becomes pronounced under extreme overloads. Overall, the proposed framework provides a quantitative basis for reliability-based tolerance specification and emphasizes the necessity of considering the full load spectrum—including assembly variability—in bearing design.