Lubricants doi: 10.3390/lubricants14100386

Authors: Keiichi Narita Takashi Yanagihara Daisuke Takekawa Hiroyuki Tatsumi

Enhancing electric-transaxle (e-axle) efficiency is essential for extending the driving range of battery electric vehicles. This study investigated how base-oil properties and additive-controlled boundary friction affect e-axle efficiency and load-dependent gear friction loss. Four lubricants were evaluated using a production oil-cooled e-axle, and their gear-mesh friction characteristics were further investigated using a high-speed back-to back gear test rig. At a constant kinematic viscosity of 11.8 mm2/s at 40 °C, varying the base-oil type produced a 0.72-percentage-point difference in WLTC-weighted efficiency. The API Group IV-based lubricant exhibited higher thermal conductivity and a 60% lower traction coefficient than the Group I-based lubricant, consistent with improved motor cooling and reduced fluid-film shear losses. Compared with the Group II-based lubricant, it reduced gear friction loss by an average of 15% over the investigated speed range. Reducing the block-on-ring friction coefficient from 0.093 to 0.022 increased WLTC-weighted efficiency by approximately 0.07 percentage points; however, this difference was comparable to the observed repeat-to-repeat variation. Nevertheless, the low-friction formulation improved e-axle efficiency under low-speed, high-torque conditions and substantially reduced gear friction loss at pitch-line velocities below 5 m/s. Gear friction loss decreased rapidly up to approximately 10 m/s and then approached a plateau. The increasing difference between the additive formulations at λ ≤ 1 indicated a growing contribution of boundary lubrication. These results demonstrate that jointly optimizing thermal conductivity, fluid-film traction, and additive-derived boundary friction is essential for maximizing e-axle efficiency across practical operating conditions.

Lubricants doi: 10.3390/lubricants14100385

Authors: Wei-Min Shao Yun-Zhuo Zhang Shu-Qi Wang Jia-Hao Li Jia-Jun Zhao Ya-Ming Wang Yong-Chun Zou Jia-Hu Ouyang

Additive manufacturing enables ceramic–metal interpenetrating phase composites (IPCs) with designed load paths, but their tribological performance is constrained by coupled infiltration, interfacial damage, and lubrication processes. This review critically evaluates these constraints through an architecture–infiltration–interface–performance framework. Unlike conventional particle-reinforced metal matrix composites, IPCs require both phases to form three-dimensionally continuous networks; successful infiltration additionally requires preservation of the ceramic skeleton, adequate filling of the intended metal channels, and interfaces capable of transferring load. The central mechanical trade-off is between ceramic load support and accommodation of incompatible phase deformation. Thermal-expansion mismatch (Δα) generates residual stresses during post-infiltration cooling, while cyclic frictional heating superimposes spatially nonuniform thermal stresses that can promote interfacial microcracking and delamination. Increasing ceramic content or refining load paths must therefore be assessed against the remaining metal-ligament capacity for plastic accommodation and crack bridging, rather than hardness alone. Lubrication introduces a second trade-off: sufficient lubricant delivery can shift sliding from direct asperity contact toward tribofilm-mediated shear, whereas excessive lubricant content or reservoir porosity can weaken structural continuity and increase wear despite low friction. Sustained protection requires lubricant supply and film formation to offset depletion and removal. Current evidence does not establish universal optimal phase fractions or topology rankings, because direct, matched tribological studies of printed and subsequently infiltrated IPCs remain scarce. Quantitative design therefore requires joint characterization of phase connectivity, infiltration defects, residual and cyclic thermal stresses, interfacial fracture resistance, and tribofilm persistence under specified contact conditions.

Lubricants doi: 10.3390/lubricants14100384

Authors: Mihaela Toderas

This research presents an integrated, multidisciplinary framework for transitioning from traditional preventive maintenance to predictive (PdM) and risk-based maintenance (RBM) strategies applied to heavy excavation machinery (Liebherr R9250 and R9350 fleets). The study merge’s reliability engineering principles with advanced Industry 4.0 technologies—including Digital Twin architectures, Stacking Ensemble Machine Learning algorithms, and IoT sensor networks—to maximize Mechanical Availability (MA ≥ 85%) and Overall Equipment Effectiveness (OEE ≥ 75%). The study evaluates these performance targets as standard benchmarks synthesized from state-of-the-art literature for optimized fleets, rather than as values derived from a single isolated field experiment. Special emphasis is placed on the direct correlation between mechanical degradation (wear of Ground-Engaging Tools—G.E.T. and hydraulic systems) and ergo-physical impacts on operators (structural vibrations and elevated acoustic emissions), evaluated using the HFACS framework and biometric data fusion. From a financial perspective, mathematical modeling of cumulative cost functions (Total Cost of Ownership—TCO) demonstrates a break-even point at t = 1.428,57 operating hours and a net maintenance cost reduction of 18–22% over a 5000 h operational cycle. Furthermore, the implementation of Fault Tree Analysis (FTA) and 5 × 5 risk matrices confirms up to a 66% risk score reduction for critical failure modes. The synthesized outcomes validate the practical execution framework designed to support the ‘Triple Zero’ strategic paradigm (zero accidents, zero unplanned downtime, zero environmental compromise) as a long-term management vision and progressive operational objective, offering a sustainable decision-making model for surface mining operations.

Lubricants doi: 10.3390/lubricants14100383

Authors: Shilong Wei

High-speed train brake pads are safety-critical consumable components whose wear condition affects braking safety, maintenance scheduling, and component life utilization. This study assesses the predictive potential and limitations of sparse life-cycle maintenance records for brake-pad wear-rate estimation when continuous degradation trajectories are unavailable. A dataset of 54 valid brake-pad life-cycle records was constructed, including replacement date, accumulated mileage, wear per 10,000 km, installation position, motor/trailer classification, and train identifier. Linear regression, ridge regression, random forest, Gaussian process regression, and quantile regression were evaluated under leave-one-out cross-validation. Random forest produced the numerically best aggregate LOOCV point-prediction metrics, with a mean absolute error of 0.0823 mm/10,000 km, a root mean square error of 0.1030 mm/10,000 km, and an R2 of 0.6008; relative to the global-mean baseline, MAE and RMSE were reduced by 39.0% and 36.8%, respectively. The P10–P90 quantile interval achieved an empirical coverage of 62.96% and an average width of 0.2165 mm/10,000 km, with 11 observations exceeding the P90 upper bound, indicating undercoverage in the upper tail. Feature-importance analysis showed pronounced temporal stratification. Removing replacement year/month reduced random forest LOOCV R2 from 0.6008 to 0.2440, whereas leave-one-train-ID-out grouped validation yielded R2 = 0.5687; expanding-window chronological validation was materially weaker (R2 = −0.0590 for the full retrospective feature set). These results indicate that the temporal descriptors capture dataset-specific temporal structure and may partly act as statistical proxies for unrecorded time-varying conditions; they should not be interpreted as physical determinants, and prospective temporal transferability remains unverified. As a secondary application of the prediction outputs, the point prediction and upper conditional quantile were linked with current pad thickness, the allowable thickness limit, and planned operating mileage, to illustrate a preliminary maintenance-screening interface. This interface is intended as a preliminary maintenance-screening procedure rather than a validated replacement policy. The results show that sparse maintenance records contain record-level predictive information for brake-pad wear-rate estimation, while temporal dependence, upper-tail undercoverage, and the absence of field decision-outcome validation currently limit direct operational use.

Lubricants doi: 10.3390/lubricants14100382

Authors: Zhiqiang Luan Ruochong Zhang Xiaohong Tan Jinlian Wang Min Wang Xueliang Zhang Xuefeng Xu

To address the inferior lubricity of synthetic cutting fluids compared with oil-based cutting fluids, a strategy is proposed to improve the cutting performance by exploiting the electroosmotic effect to enhance penetrability at the friction interfaces in the cutting zone. The triboelectrification potential and tribo-emission of charged particles were measured at the accessible major flank face–workpiece interface to characterize the electrical behavior of an alumina ceramic–AISI 52100 steel friction pair. Taking the rake face–chip interface as a representative case, the distribution characteristics of the tribo-induced electric field (TIEF) and the electroosmotic flow field of the cutting fluid within an interfacial capillary were analyzed using COMSOL Multiphysics 5.5 simulations, based on the measured electrical boundary conditions and zeta potentials. Turning tests on AISI 52100 steel with alumina ceramic tools were also conducted under the lubrication of cutting fluids with distinct electroosmotic properties. The simulations indicate that the TIEF can drive electroosmotic flow toward the interior of the interface. The experiments show that the electroosmotic performance of the cutting fluid is positively correlated with its cutting performance. The simulated TIEF intensity increases with increasing cutting speed and depth, and the electroosmotic regulation of cutting performance is correspondingly enhanced.

Lubricants doi: 10.3390/lubricants14100381

Authors: Tai Zhang Feilong Huang Shengjie Du Xiao Yang Yang Wang Ning Zhang Lei Ma Yuying Cao

Three two-dimensional Tp-based covalent organic frameworks, namely TpPa-1, TpPa-2, and TpBD, were synthesized and incorporated into Shell Helix H6 lubricating oil (SO) to investigate their potential as lubricant additives in commercial lubricating oils. Tribological tests demonstrated that all three Tp-based frameworks enhanced lubrication performance, with TpPa-1 exhibiting the best overall performance. Under a 5 N load, the coefficient of friction (μ) and wear rate decreased by 35.5% (to 0.091) and 55.9% (to 4.1 × 10−6 mm3/(N·m)), respectively. Under a 10 N load, μ and wear rate decreased by 38.3% (to 0.095) and 60.9% (to 4.8 × 10−6 mm3/(N·m)), respectively. The lubrication mechanism was attributed to interlayer sliding of the Tp-based framework nanosheets and the formation of a protective tribofilm containing iron oxides and deposited Tp-based frameworks, which filled wear pits and reduced wear volume.

Lubricants doi: 10.3390/lubricants14100380

Authors: Pawan Panwar Paul Michael Ricardo Gomes Thorsten Bartels Frank-Olaf Mähling

The ASTM D7721 standard practice for determining the effect of fluid selection on hydraulic system performance was applied to the study of two excavators and a hydraulic dynamometer. Six fully formulated fluids spanning two viscosity grades and three viscosity index (VI) levels were evaluated using the dynamometer. Four of these fluids were also evaluated in the excavators. A common monograde ISO VG 46 hydraulic oil served as the reference fluid in both studies. In dynamometer testing, high-VI, low-traction fluids reduced hydraulic motor torque losses by 9 to 35 N·m. In excavator trials, multigrade hydraulic fluids shortened the average bucket filling time by 5.1% and increased the average bucket payload by 2.8%. Based on the physical properties of the test fluids and fundamental principles of fluid mechanics, the observed performance gains are believed to be the result of reduced viscous shear, boundary friction and traction losses. The results presented explore correlations between fluid properties and hydraulic system performance, enabling formulation strategies that can help reduce parasitic losses and improve machine productivity.

Lubricants doi: 10.3390/lubricants14100379

Authors: Qun Li Xufeng Song Xuejun Shi Yuanke Wu Liquan Yang Erbo Liu Rongrong Li

Top-of-rail friction modifiers (TORFMs) act as transient interfacial films that regulate wheel–rail adhesion under rolling–sliding contact. Unlike permanent coatings, these films undergo continuous formation, depletion, and regeneration during repeated wheel passages. This study investigates TORFM-treated interfaces by combining laboratory rolling-contact tests with a starved non-Newtonian elastohydrodynamic lubrication (EHL) model. Experimentally, application dosage and viscosity were varied on a CL60 wheel–U71Mn rail pair, and the adhesion coefficient was measured as the validation quantity. Numerically, a model-based sensitivity analysis was used to examine how dosage-derived effective supply thickness, viscosity, and rheological index influence film formation and load sharing. The experiments show that increasing dosage alleviates inlet starvation and lowers initial adhesion, which then recovers toward dry-contact levels during repeated rolling. Increasing viscosity from 0.002 to 0.022 Pa·s reduced the minimum adhesion coefficient from 0.21 to 0.086, although excessive viscosity may prolong the low-adhesion stage. The rheological-index calculations (n = 0.7–1.0 around the measured formulation value n = 0.82) indicate that shear-responsive flowability can affect contact-zone supply and the film-supported load fraction; these rheological-index trends are model-based sensitivities rather than independently validated experimental effects. Overall, the results support a transient interfacial-film interpretation of TORFM-controlled adhesion and suggest that practical design should target a controllable medium-adhesion window (approximately 0.15–0.30 under the present laboratory conditions) rather than minimum friction alone.

Lubricants doi: 10.3390/lubricants14100378

Authors: Zongyi Mu Hongwei Wang Fajuan Xiao Genbao Zhang

As a core component enabling multi-axis precision machining in CNC machine tools, the CNC rotary table’s precision state directly affects machining quality and production efficiency. Real-time and accurate prediction of its precision remaining useful life (PRUL) is of great significance for optimizing maintenance costs, ensuring production safety, and extending equipment service life. To this end, a digital twin framework for PRUL prediction of CNC rotary tables was first established, defining a five-dimensional digital twin model. Then, based on the Meta-action theory, an initial precision model of the CNC rotary table was constructed. Time-varying errors were quantitatively characterized using thermal error simulation and the wear model, and combined with the Wiener process model, theoretical PRUL prediction within the virtual mirror was realized. Finally, the virtual mirror data and physical entity data were fused to generate corrected model parameters, yielding a real-time PRUL prediction model for the CNC rotary table driven by digital twin data. The case study results demonstrate that the proposed method, which integrates the mechanistic model with real-time data, achieves an improvement of at least 16% in both prediction accuracy and stability compared to methods based solely on either virtual mirror data or monitoring data.

Lubricants doi: 10.3390/lubricants14100377

Authors: Dawei Li Jianhui Shi Jiren Gu Zhaoyu Zhang Yuguan Wu Jie Wang Zhen Guo Fengming Du

To predict the mass loss of grooved journal-bearing shells under varying operating conditions while reducing experimental effort, accelerated wear tests were conducted on a component-level journal-bearing test rig. The effects of load, rotational speed, and oil temperature were investigated, and the worn surfaces were characterized microscopically to elucidate the wear mechanism. A backpropagation (BP) neural network was trained on 42 mass-loss samples and evaluated using 18 additional samples. Candidate numbers of hidden-layer neurons were identified using an empirical relation and compared based on prediction performance. A genetic algorithm (GA) and the sparrow search algorithm (SSA) were then used to optimize the initial weights and biases of the BP network. Mass loss increased with load and oil temperature but decreased with rotational speed; all responses were nonlinear. After testing, the groove structure remained visible, while the lead-based overlay became thinner and underwent plastic flow. Pb and Sn were detected on the aluminum-alloy surface, suggesting transfer of overlay material during sliding. The mean absolute percentage error (MAPE) and coefficient of determination (R2) were 26.986% and 0.54552, respectively, for the conventional BP model; 8.436% and 0.9862 for the GA-BP model; and 2.6296% and 0.99714 for the SSA-BP model. For the present dataset, the SSA-BP model achieved lower prediction errors than the BP and GA-BP models and reproduced the variation in mass loss across the investigated operating conditions.

Lubricants doi: 10.3390/lubricants14100376

Authors: Xiaoming Zong Zehao Li Mingyi Tang Renshan Xia Jiaoyan Ma Lei Zhang Xu Yang Han Li Zhi Xu Ming Ma

This study presents a numerical framework for analyzing mixed elastohydrodynamic lubrication (EHL) of rough contact surfaces under grease lubrication, motivated by the need for reliable lubrication design in tribological components such as mechanical face seals and rolling bearings operating in space environments. Considering surface roughness effects, the proposed method integrates an EHL model derived from the Ostwald constitutive equation with the Kogut–Etsion (KE) elastic–plastic asperity contact model. The methodology is demonstrated through a case study of a vacuum grease-lubricated double-row angular contact ball bearing employed in a spacecraft antenna rotation mechanism under low-speed and heavy-load conditions. The governing equations were non-dimensionalized and solved numerically to obtain the lubricant film thickness and pressure distributions under various rotational speeds and axial preloads. The friction torque generated by viscous shear of the lubricant and asperity contact, and the asperity load ratio, were also determined. The novelty of this work lies in two aspects: (i) the integration of the Ostwald grease rheology model and the KE elastic–plastic asperity contact model into a unified mixed EHL framework; and (ii) a systematic investigation of grease-lubricated bearing behavior at low rotational speeds (11.5–55.2 rpm) with explicit consideration of surface roughness. The results indicate that rotational speed and axial preload exert limited influence on film thickness. The film pressure along the rolling direction increases with speed, whereas the asperity contact pressure decreases with speed and increases with preload; the asperity load ratio follows the same trends. The fluid pressure exhibits a single peak on the inlet side of the contact, and no outlet film constriction is observed in the thickness profile. The friction torque decreases with increasing speed, and the asperity load ratio follows the same trend. These findings demonstrate that appropriate adjustment of preload and an increase in rotational speed can reduce both friction torque and the asperity load ratio, thereby improving lubrication conditions and extending bearing service life.

Lubricants doi: 10.3390/lubricants14100375

Authors: Ivana Ban Jacopo Bonari Marco Paggi

Two-dimensional profiles extracted from three-dimensional pavement surface textures acquired by close-range photogrammetry are employed as geometrical input for modeling complex interfaces in finite element contact simulations of viscoelastic materials. The methodology allows to establish a robust computational framework for the systematic investigation of possible correlations between the simulated viscoelastic friction coefficients of profiles, the profile roughness descriptors, and the measured surface frictional performance assessed through the static skid resistance tester (SRT) parameter. The study shows that the proposed methodology successfully recovers consistent values of the friction coefficient in the range of velocities typical of the equivalent experimental tests. The methodology is suitable for large-scale statistical analyses and uncertainty quantification, which emerge as key aspects for future research.

Lubricants doi: 10.3390/lubricants14100374

Authors: Subrahmanya Ranga Viswanath Mantha Rakesh Prasad Zuraida Abal Abas Veeresh Kumar Gonal Basavaraja Pramod Ramakrishna Shashi Kumar M E Santosh Kumar Sahu Mohammed Aman

This study investigated the dry sliding wear behaviour of Al2O3 particulates (2–6 wt.%) in Al6061 metal matrix composites produced by ultrasonic stir casting, examining mechanical and tribological behaviour. We modelled wear rate using machine learning. Optical studies confirmed uniform Al2O3 particle dispersion, minimal agglomeration, and strong interfacial bonding between the Al6061 and Al2O3 phases. With an increase in Al2O3 content, density and hardness increased by 1.3% and 33%, respectively. The Al6061–6 wt.% Al2O3 composite exhibited 40% higher wear resistance than the base alloy in dry-sliding conditions, with sliding distance varying between 0 and 10,000 m and load varying between 0 and 50 N. At lower loads and shorter sliding distances, abrasive wear dominated; as load and sliding distance increased, the dominant wear mechanism shifted to delamination and adhesion wear. Moreover, tribological testing at 6 wt.% reinforcement showed a 40% improvement in dry sliding wear resistance, with applied normal load and sliding distance varying between 10 and 50 N, and 1000 and 10,000 m, respectively. The specific wear rate was subsequently modelled and predicted using K-Nearest Neighbours (KNN), Support Vector Regression (SVR), Artificial Neural Networks (ANNs), Random Forests (RFs), and Gradient Boosting Machines (GBMs). Among these, the RF model achieved the highest accuracy (R2 = 0.946). Based on feature importance analysis, applied normal load and sliding distance are the most influential factors in wear. As a result, in dry-sliding conditions, Al6061-Al2O3 MMCs show a stable wear response, thereby improving dataset homogeneity and model performance. Overall, this study used experimental insights and predictive analytics to predict MMC wear behaviour. By employing ML models, composite design and wear can be optimised. Additionally, feature importance analysis showed that applied normal load and sliding distance best predicted wear rate.

Lubricants doi: 10.3390/lubricants14100373

Authors: Mohd Hafis Sulaiman Muhammad Shuhaimi Ibrahim Mohd Idris Shah Ismail Shahira Liza Muhammad Hasnulhadi Mohammad Jaafar

Adhesion strength, toughness, and resistance to frictional heating are critical characteristics for suppressing premature coating delamination and protecting the substrate from plastic deformation. This study investigates the synergistic role of intermediate adhesion-promoting layers on the mechanical and tribological performance of diamond-like carbon (DLC) coatings deposited on tool steel substrates via high-power impulse magnetron sputtering (HiPIMS). Nanoindentation and micro-scratch testing were conducted to evaluate single-layer TiAlN, single-layer DLC, and multilayer DLC/TiAlN coating architectures. Owing to the TiAlN interlayer beneath the DLC topcoat, the multilayer DLC/TiAlN system demonstrated superior composite hardness and interfacial adhesion strength. Microstructural characterization via SEM-EDS and Raman spectroscopy confirmed that a dense, columnar TiAlN interlayer with a high aluminum concentration provided vital load-bearing capacity for the overlying DLC film. During dry CNC turning tests on aluminum and steel alloys, the multilayer DLC/TiAlN coating exhibited the lowest cutting forces, cutting temperatures, and surface roughness. The architecture effectively suppressed flank and crater wear, which reduced the chip compression ratio and mitigated plastic shear strain at the tool–chip interface. This synergistic interaction within the DLC/TiAlN coating system minimizes tribological degradation, demonstrating high potential for demanding machining applications.

Lubricants doi: 10.3390/lubricants14100372

Authors: Xuemei Shang Xinchao Xi Jiahao Gao Qichen Shang Xiaopeng Wang Qingyu Li Qunfeng Zeng Qian Jia

This study investigates the influence of thermoelastic deformation on the lubrication performance of a superconducting magnetically assisted turbopump mechanical seal, with particular emphasis on film thickness and leakage. A spiral-groove hydrodynamic lubrication model based on the Reynolds equation and film energy equation is used to calculate the liquid-film load capacity and leakage, while a superconducting magnetic-force model based on the Bean model and electromagnetic finite-element analysis is employed to determine the magnetic-force contribution. The calculated mechanical loads and thermal boundary conditions are subsequently introduced into a thermoelastic finite-element model to obtain the deformation of the rotating and stationary seal rings. The results show that increasing the sealing clearance causes both the superconducting magnetic force and hydrodynamic liquid-film force to decrease nonlinearly. At a fixed clearance, the magnetic force is nearly independent of rotational speed, whereas the liquid-film force increases with increasing speed. The rotating ring exhibits greater deformation than the stationary ring, resulting in a non-parallel, approximately conical seal face. The deformation-induced change in the seal-face geometry is incorporated into the film-thickness model using the minimum and maximum film thicknesses, hmin and hmax. The results show that considering seal-face deformation leads to a smaller predicted minimum film thickness and a larger leakage rate than those obtained from the undeformed model. These findings indicate that thermoelastic deformation should be considered in the lubrication-performance evaluation of superconducting magnetically assisted mechanical seals, particularly under low-speed operating conditions.

Lubricants doi: 10.3390/lubricants14100371

Authors: Jun Young Park Kwang Jo Lee

Procedures that predict or test wear in total hip replacements use the contact force waveform as their principal input. This approach rests on the assumption that activities producing large contact forces also impose a large frictional burden. Because friction is governed by sliding at the articulating surfaces and by the state of the lubricating film, the correspondence between the two quantities requires verification. The present study reanalysed publicly available measurements from an instrumented hip implant to test whether the ranking of peak contact force across activities agrees with the ranking of peak friction moment, and to establish how that result depends on how the metric is defined. Hip Joint III data from the OrthoLoad database were used. Of the activities performed by ten subjects, the eight with a cyclic structure were analysed, comprising 72 individual trials; one-legged stance was treated separately because the timing of its peaks is not defined. The accuracy of the data import was confirmed by comparing the imported HIGH100 standardised loads with the originally published values. At the activity level, an exhaustive permutation test over all 8! = 40,320 orderings was applied; at the trial level, a linear mixed model including the activity-by-metric interaction was fitted. The rankings of contact force and friction moment diverged at the peak (Spearman rho = 0.67; mean rank shift 1.25). In the trial-level model, the activity-by-metric interaction was significant (Wald chi-square = 40.3, df = 7, p = 1 × 10−6) and was largest for jogging (−1.74 ± 0.43 standard deviations, p = 0.0001). When the same data were expressed as cycle impulse, the rankings agreed almost completely (rho = 0.98; mean rank shift 0.25) and the interaction was not significant (chi-square = 1.7, p = 0.975). The instant at which the friction moment peaked differed by activity: in walking, stair ascent and descent, and jogging it occurred 29 to 37% of the cycle after the contact force peak, whereas in postural transitions and cycling the two instants coincided. The discordance between contact force and friction moment is confined to peak values and does not appear in cumulative quantities. The delayed friction moment peak is consistent with a within-cycle change in lubrication regime, in which the film thins under sustained load and the greatest interfacial shear arrives after the greatest load. Given that polyethylene wear depends on the shear at the articulating surfaces, a load waveform alone may not reproduce the phase of the cycle in which wear actually occurs.

Lubricants doi: 10.3390/lubricants14100370

Authors: Yanjiao Nie Jinhao Shao Li Xiao Wenhao Dai Dehui Ji Limei Kang

The pantograph–catenary system serves as the core power supply device, ensuring stable operation of high-speed trains, and its contact interface represents a typical current-carrying friction system, where the introduction of electric current significantly complicates the wear mechanisms. The coupling effect of mechanical wear and electrical wear exacerbates damage to and degradation of carbon strip materials, while electric arcs induced by unstable contact states directly regulate the tribological characteristics and electrical conductivity of the friction pair. To investigate the current-carrying friction and wear behavior of carbon strips under the dominant effect of electric arcs, this study simulated the irregularity conditions of contact wires, observed the evolution characteristics of electric arcs, and systematically analyzed the effects of test current and contact load on the damage characteristics and current-carrying performance of the contact pair. The results indicate that arc morphology exhibits differentiated characteristics across friction cycles, with significant fluctuations in arc intensity. Regarding the friction coefficient, under a contact load of 25 N, the friction coefficients corresponding to currents of 0 A, 40 A, and 80 A are 0.378, 0.242, and 0.335, respectively; under a contact load of 50 N, the corresponding values are 0.432, 0.180, and 0.209, respectively. The friction coefficients under current-free conditions are consistently higher than those under current-carrying conditions. In terms of wear rate, under a contact load of 25 N, the wear rates at 0 A, 40 A, and 80 A are 0.0069, 0.299, and 2.70 mm3/(N·m), respectively; under a contact load of 50 N, they are 0.010, 0.133, and 1.133 mm3/(N·m), respectively. No positive correlation is observed between contact load and the wear rate of the carbon-based friction pair, whereas increasing test current significantly exacerbates material damage. Regarding contact resistance, under a contact load of 25 N, the contact resistances at 40 A and 80 A are 34.7 Ω and 46.6 Ω, respectively; under a contact load of 50 N, they are 27.7 Ω and 37.9 Ω, respectively, indicating that lower contact loads correspond to higher contact resistances, and higher test currents also result in higher contact resistances. In summary, both increasing current and decreasing contact load aggravate the electrical damage to the contact pair materials, while increasing contact load exerts a significant inhibitory effect on electrical damage. This study provides theoretical insights into the damage mechanism and arc morphology evolution laws induced by electric arcs under contact wire irregularity conditions.

Lubricants doi: 10.3390/lubricants14100369

Authors: Lanxia Zhang Hongjun Ma Jinjun Cao Zhanyang Chen Jinping Ren Shuai Yang

To address the stringent sealing requirements of specialized equipment in extreme deep-sea environments, where traditional unidirectional sealing structures often fail to accommodate complex bidirectional pressure fluctuations, this study focuses on bidirectional T-groove dry gas seals. The primary objective is to elucidate the influence of groove depth evolution on gas film lubrication characteristics. Based on gas film lubrication theory, a hydrodynamic lubrication governing equation was established. Furthermore, a compressible gas film thickness equation was proposed to characterize the stepwise evolution of the groove geometry, accurately describing the evolution of the gas film thickness with varying groove profiles. Through numerical simulations, key performance parameters—including opening force, leakage, gas film stiffness, and stiffness-to-leakage ratio—were systematically evaluated for six T-groove structures with varying depths. The results reveal a competitive mechanism between operating conditions and sealing performance: while elevated pressure enhances the hydrodynamic effect and gas film stiffness, it simultaneously increases the leakage rate. Similarly, higher rotational speeds improve load-carrying capacity but exacerbate gas flow and leakage. A comparative analysis of the stiffness-to-leakage ratio across the six models indicates that Type 4 and Type 3 achieve the optimal balance between gas film stiffness and leakage control. This study confirms that rational groove design can effectively balance the opening force and leakage rate, thereby enhancing sealing stability. Ultimately, this research uncovers the specific mechanisms by which bidirectional T-groove structures influence gas film lubrication performance, providing a theoretical foundation for optimizing the sealing structures of high-end deep-sea equipment.

Lubricants doi: 10.3390/lubricants14100368

Authors: Junjie Meng Guorui Yu Gaofeng Ai Qing Zhao Xin Zhao Qiang Yu Jinshan Dong Kun Niu Xing Du

The ball screw mechanism (BSM) is extensively employed in mechanical systems to convert rotational motion into linear motion. Most of the existing studies primarily concentrate on developing time-invariant calculation methods for the BSM, leading to an incomplete understanding of its time-varying contact characteristics. This study establishes a time-varying contact force model that incorporates decomposed circumferential and axial surface waviness of the thread raceways and the ball, aiming to analyze their dynamic effects on load distribution within the BSM. The results reveal that the influence of surface waviness order on load distribution follows an irregular pattern, whereas the influence of waviness magnitude exhibits a systematic correlation. Specifically, parametrically increasing the circumferential and axial waviness amplitudes from 0.5 μm to 2 μm induces a progressive amplification of peak contact forces, accompanied by a synchronized attenuation of valley contact forces over different time instants. This investigation offers fundamental insights into time-variant contact forces arising from machining surface errors, providing valuable guidance for the design and manufacturing of the BSM.

Lubricants doi: 10.3390/lubricants14100367

Authors: Wenjun Yu Yanan Wang Weiwei Xu Qingyun Guo Jianping Ai Shuang Zhang Junying Suo Xiuxu Zhao Xiang Shi

The reciprocating dynamic sealing system is crucial for the operational reliability of aircraft high-pressure hydraulic actuators, particularly for civil aircraft with 21 MPa (e.g., B737, A320) and 35 MPa (e.g., B787, A380) pressure systems, where real-time monitoring and prediction of friction and oil-film thickness are essential for health assessment. This study proposes a novel data-driven predictive framework for dynamic seal performance utilizing a long short-term memory (LSTM) network. A specialized reciprocating seal test rig was developed to acquire high-fidelity multimodal data. By integrating ultrasonic detection, fiber Bragg grating (FBG) sensors, and load cells, the simultaneous measurement of oil-film thickness, contact stress, and friction force under high-pressure conditions up to 35 MPa was achieved. The measured oil-film thickness, determined by the sealing gap, provides the physical basis for leakage characterization. An LSTM-based model was then trained on this time-series dataset to predict oil-film thickness and friction force. The results show that the LSTM model achieves a coefficient of determination (R2) above 0.94 and a symmetric mean absolute percentage error (sMAPE) below 2% for both predictions on the test set. Ultimately, this research provides theoretical support for the condition assessment and leakage risk early warning of dynamic seals, advancing the application of artificial intelligence and data-driven methodologies within aviation sealing technologies.

Lubricants doi: 10.3390/lubricants14100366

Authors: Jiandong Li Pengna Wei Jie Yang Wei Kang Shihao Zhang Qunfang Wang Wansheng Chang

The axial nonlinear stiffness of machine tool spindles critically governs machining stability, yet isolating its thermal-driven evolution from intertwined, antagonistic rotational dynamic interferences remains a long-standing challenge. The main contribution of this study is the “Double Decoupling” framework, which isolates the thermal-driven evolution of spindle axial stiffness from dynamic interferences and distinguishes internal preload relaxation from localized component expansion. Experimentally, a “thermal-equilibrium and rapid-shutdown” methodology is developed to physically isolate quasi-static thermal boundaries, successfully capturing pristine intrinsic stiffness profiles within a critical 5 s post-shutdown window. Analytically, a thermomechanical model coupling micro-geometric thermal distortions with dynamic preload evolution is established. Our findings quantitatively elucidate that temperature escalation (26 °C to 43 °C) prompts a monotonic global stiffness degradation exceeding 12% in the central stiffness plateau. Thermomechanical inversions reveal that localized bearing thermal expansion plays merely a secondary role; instead, the non-uniform internal temperature field drives a macroscopic axial thermal mismatch between the spacers, triggering “thermal preload relaxation” as the primary contributor to global thermal stiffness loss. Consequently, practical thermal stiffness regulation should preferentially target macro-preload and spacer management rather than complex component dimensional compensation.

Lubricants doi: 10.3390/lubricants14100365

Authors: Weihao Liu Xinqing Wang Hengrui Du Jianguo Zhang Chang Shu Tinyau Wan Bingli Lan Yutong Lan Jing Wang

Under intermittent source-side excitation, electric-drive systems can maintain mechanical operation, so nominal energization parameters do not directly represent the interfacial electrical state. GCr15 ball-on-disk tests compared dry and polyalphaolefin (PAO 40)-lubricated conditions at 6 N under 5 V, 1 Hz switching. Tests covered nominal duty cycles from D = 0 to 1.00, where D is the ratio of the voltage on-time to the voltage period. Equal-on-time friction tests compared periodic D = 0.50, on→off and off→on. Wear was compared among periodic D = 0.50, off→on, and continuous direct current (DC). Friction increased overall with D. Longer on-time prolonged concurrent microcontact Joule heating and mechanical friction. Dry-sliding wear volume varied non-monotonically, peaking at D = 0.50 with changes in surface-layer removal and O-containing debris retention. Under PAO 40 boundary/mixed lubrication, wear-track width and volume increased monotonically with electrical-heating-related changes in oil film support and separation, microcontact, plowing, and plastic deformation. Equal-on-time friction histories differed. Periodic D = 0.50 and off→on produced similar wear volumes but different morphologies. Dry off→on and continuous DC wear volumes were close. Wear depended on timing and interface state at energization. Interface evolution altered contact resistance and voltage division, feeding back on wear. These findings support energization-parameter and bearing wear risk assessment under intermittent source-side excitation.

Lubricants doi: 10.3390/lubricants14100364

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

Most existing friction torque models for ball screws mainly address two-point contact and are difficult to apply to four-point contact configurations preloaded by increasing ball diameter. This study develops a differentiated model based on the distinct kinematics of the two contact pairs in four-point contact: one is rolling-dominated with slight sliding, whereas the other is additionally generated by the enlarged ball diameter and is sliding-dominated. First, Hertz theory is used to determine the normal load at each contact point. Then, for the rolling-dominated pair, elastic hysteresis rolling friction, differential microslip friction, and elastohydrodynamic oil-film shear friction are considered; for the sliding-dominated pair, sliding friction and oil-film shear friction are included. By combining the Hamrock–Dowson film-thickness formula, Roelands pressure-viscosity relationship, and Eyring shear model, friction torque models for the two contact pairs under mixed lubrication are established and superposed to obtain the total torque, thereby relating ball diameter increment, rotational speed, and friction torque. Friction torque tests conducted on the 4010 ball screw showed that the MAPE between theoretical calculations and experimental results on the internal validation set was 8.55%. The model also reflects the characteristics of a significant increase in friction torque with rotational speed under low preload and its tendency to stabilize under high preload.

Lubricants doi: 10.3390/lubricants14100363

Authors: Runchao Zhao Denis Shutin Thai Ha Nguyen Shengbo Li Zhaobo Chen

Using conical bearings instead of a combination of thrust and radial bearings offers advantages, primarily in terms of improved weight and dimensional characteristics. On the other hand, conical fluid film bearings also present a number of limitations, largely related to stability issues related to their dynamic characteristics. Combining conical and cylindrical parts in a single bearing is considered in this article as a possible way to comprehensively improve fluid film bearings with conical component without requiring excessive complications in the machine design common to some other approaches. The numerical study discovers load capacity, lubricant flow rate, power losses due to viscous friction along with the dynamic characteristics of conical–cylindrical bearings to explore the prospects for their application in advanced rotary machines. The results, in particular, demonstrate the potential for such solutions to provide greater stability compared to pure conical hydrodynamic bearings. The results of a comprehensive comparison of conical and conical–cylindrical bearings with each other and with more conventional combination of journal and thrust hydrodynamic bearings also made it possible to highlight the features and nuances of each of these solutions that should be taken into account when designing rotary machines.

Lubricants doi: 10.3390/lubricants14100362

Authors: Ming Zhang Xiaoling Liu Qiangjun Ding Wenhan Cao

To address the challenge where early fault signals of rolling bearings are easily submerged by strong noise, and fault features are difficult to extract under harsh working conditions, this paper proposes an adaptive fault diagnosis method termed HCF-IAPO-VME. Firstly, a novel harmonic coherence factor (HCF) is constructed to simultaneously evaluate the intensity of periodic impulse features and the consistency of harmonic structures without relying on prior fault frequency information. Secondly, the Improved Arctic Puffin Optimization (IAPO) algorithm is adopted, and HCF is taken as the fitness function to adaptively determine the optimal parameters of variational mode extraction (VME). Different from conventional VME and optimization-based methods that depend on prior fault frequencies and single-feature metrics, the proposed method achieves adaptive parameter optimization and reliable weak fault extraction under heavy-noise conditions. Subsequently, the optimized VME is used to extract fault-related modes, and envelope demodulation is applied to identify fault characteristic frequencies. Simulation signals and two public experimental datasets are used to verify the performance of the proposed approach. Quantitative and qualitative comparisons with VMD, FK and AVME are carried out using the fault feature coefficient (FFC), kurtosis, signal-to-noise ratio (SNR) and envelope entropy (EE). The results show that HCF-IAPO-VME can effectively suppress strong noise interference and accurately extract weak early fault features in rolling bearings.

Lubricants doi: 10.3390/lubricants14100361

Authors: Yiran Wang Yan Zhao

Cu-Ni-graphite self-lubricating composites were developed for railway switch slide baseplate applications, which require a low dry friction coefficient and adequate load-bearing capacity under low-speed sliding. The powder-metallurgy route—comprising planetary ball milling, cold compaction, and vacuum sintering—was systematically evaluated. The selected processing window consisted of a ball-to-powder ratio of 1:10, milling at 200 rpm for 15 h, compaction at 700 MPa, sintering at 850 °C, and a holding time of 2 h. Under these conditions, the Cu-8Ni-8graphite composite achieved a Vickers hardness of approximately 83 HV and a flexural strength of about 348 MPa. Under dry sliding against U75V rail steel at 20 N and 0.065 m/s, the composite exhibited a steady-state friction coefficient of 0.212 ± 0.012 and a specific mass wear rate of (4.20 ± 0.30) × 10−5 g/(N·m)—reductions of 76% and 49%, respectively (p < 0.05, t-test, n = 5), relative to the graphite-free Cu-8Ni alloy. The improved tribological response was attributed to the formation of a compact mechanically mixed tribofilm comprising the Cu-Ni solid-solution phase, fragmented graphite, and minor oxides. This film reduced direct metal-to-metal contact, adhesive transfer, and micro-cutting. These results establish a process–microstructure–property relationship for Cu-Ni-graphite composites and provide a basis for further component-level and long-duration validation.

Lubricants doi: 10.3390/lubricants14100360

Authors: Lei Zhang Shiyuan Pei

The lubrication response of a double-flow ring seal in a large hydrogen-cooled generator is investigated under two altered boundaries: an equivalent hydrogen-side outlet restriction representing an oil-retaining ring and the interruption of active hydrogen-side oil supply. The steady model couples Reynolds-film pressure, statistical asperity contact, temperature-dependent oil properties and linear thermoelastic deformation with axial positioning at a prescribed radial eccentricity ratio of 0.30. In the reference case and the field-flow-calibrated equivalent-outlet case, single-end hydrogen-side flow decreases from 387 to 149 L/min and total flow from 426 to 188 L/min. The minimum radial gap remains 0.091 mm, while the maximum radial-surface temperature increases from 91.8 to 95.5 °C. A representative supply-interruption case at hydrogen pressure 0.50 MPa and total thrust clearance 0.41 mm gives 44.6 L/min total flow and a maximum radial-surface temperature of 113 °C. Auxiliary pressure-sweep records show a marked redistribution of flow and thrust clearance among the sampled states around 0.5 MPa. Together, these results illustrate how altered boundaries redistribute pressure forces and axial clearances, and why flow reduction alone is insufficient to assess lubrication performance. The reported magnitudes are case-specific model results, subject to the stated input and validation limitations, rather than general design or safety limits.

Lubricants doi: 10.3390/lubricants14090359

Authors: Xiuxu Zhao Shuo Zhang

During practical service, the interfaces of hydraulic reciprocating seals experience complex coupling interactions of lubrication, contact stress and thermal effects. Reliance solely on standard indicators including leakage rate and cycle count cannot capture latent interfacial risks originating from lubrication failure, contact overload and thermal accumulation under designated operating conditions, which greatly limits the evaluation and optimal selection of hydraulic reciprocating seal configurations for targeted service scenarios. This work presents a multi-state monitoring approach based on ultrasonic oil-film measurement and fiber Bragg grating contact-stress sensing, and constructs a multi-dimensional evaluation indicator system for seal-interface latent risks. Combined with an interface–risk-coupled weighting strategy, indicators such as oil-film thickness, contact stress and interface temperature are utilized to quantify seal-interface risks and enable comprehensive performance assessment and comparison among different seal configurations. Long-cycle comparative experiments were conducted on three VL combined-seal configurations at 2 MPa, 21 MPa and 35 MPa via a high-speed hydraulic reciprocating seal performance-test rig. The results show clear pressure-dependent adaptability: the 2.2 mm configuration achieved the highest closeness coefficient at 2 MPa (Ci=0.8176), whereas the 2.1 mm configuration ranked first at 21 and 35 MPa, with Ci=0.6992  and 0.9252, respectively. These results provide a quantitative basis for structural optimization and operating-condition-matched configuration selection.

Lubricants doi: 10.3390/lubricants14090358

Authors: Fan Zhang Hongjie Xu Haifeng Zhang Jingxu Wang Dong Tian Shuai Zu

To address oil-film electrical breakdown in sliding bearings during operation, an electro-thermal coupled transient model that takes into account the electric field, oil film characteristics, heat conduction, and continuous damage was developed in this paper. The coupled effects of applied voltage U, initial oil-film temperature T0, and oil-film thickness h on the current density, heat generation rate, and temperature are investigated in the spatial film field and at the characteristic points. The transformation of the oil film from an insulating state to a highly conductive state is revealed. The influencing parameters and rules for the oil-film breakdown time tb and the characteristic point damage time tc are also obtained. The results show that increasing the applied voltage or decreasing the equivalent local oil-film thickness shortens the overall breakdown time tb, while increasing the current density and heat generation rate. In contrast, the initial oil-film temperature primarily affects the overall breakdown time tb. During oil-film breakdown, two distinct transition points, tc and tb, appear in the responses at the characteristic points. At these transition points, the current density and heat generation rate exhibit pronounced transient changes, whereas the oil-film temperature either remains stable or continues to increase.

Lubricants doi: 10.3390/lubricants14090357

Authors: Jianlei Wang Qichen Shang Xinchao Xi Jianke Li Jiahao Gao Yanchao Zhang Jing Zhang Yuhang Wang Qian Jia

To address the insufficient liquid film load capacity, high leakage rate, and severe friction and wear under low-speed operating conditions of mechanical seals in high-speed and large-pressure-difference pump turbines, a stepped-groove face seal for liquid rocket engine turbopumps is proposed in this study. Different from conventional linear and spiral grooves, this novel structure exhibits favorable friction and wear performance under low-speed steady-state conditions and exhibits favorable friction and wear performance under low-speed steady-state conditions. Fluent numerical simulations and friction-wear tests are conducted to compare the flow field characteristics and sealing performance of the three groove types, and the optimal structural parameter range of the stepped groove is determined. The results demonstrate that the stepped groove outperforms spiral and linear grooves in overall sealing performance. Within a groove depth of 6–14 μm, its opening force is 2.25% and 59.27% higher, while the leakage rate is 3.35% and 10.95% lower, respectively. Within the investigated helix-angle range of 13–17°, the stepped groove consistently exhibits better sealing performance than the spiral groove. The maximum opening force of 256.867 N is obtained at a helix angle of 15°, which is 2.95% higher than that of the spiral groove, whereas the minimum leakage rate of 0.52071 g/s occurs at 13°, approximately 3.95% lower than that of the spiral groove. Particularly under high-speed conditions, the stepped groove exhibits further enhanced advantages, with opening force increased by 18.23% and 62.43% and leakage rate reduced by 6.40% and 17.58% compared with spiral and linear grooves. The stepped-groove structure exhibits favorable sealing and tribological performance under the tested conditions, providing a useful design reference for high-performance mechanical seal optimization.

Lubricants doi: 10.3390/lubricants14090356

Authors: Jie Liu Wenjing Zhao Xiangkai Meng Kun Li Xiang Li Siyun Ding Xudong Peng

A three-dimensional thermo-elasto-hydrodynamic (TEHD) lubrication model is presented for the high-pressure deep-grooved mechanical seal employed. The thermo-elasto deformation behaviors of the seal rings are investigated using the finite element method (FEM). The parametric studies are conducted to explore the deformation characteristics of the seal face and sealing performance under different operating conditions. The results reveal that the deep-grooved face seal induces circumferential waviness deformation of the seal face, generating the hydrodynamic wedge effect dominated by the axial mechanical deformation along the circumferential direction. However, a significant hydrostatic effect produced by the deformation of the seal face along the radial direction is dominant in the load-carrying capacity of the fluid film. The thermo-mechanical coupling deformation of the seal face decreases with increasing fluid pressure and increases as the spring force and rotational speed increase. The leakage rate increases with higher fluid pressure, while it decreases with increased spring force and rotational speed. The axial stiffness of the fluid film increases with greater spring force and rotational speed but decreases with increasing fluid pressure. These findings can serve as theoretical guidance for developing high-pressure deep-grooved mechanical seals with enhanced reliability and stability.

Lubricants doi: 10.3390/lubricants14090355

Authors: Duval A. Johnson Azhar Vellore Ashlie Martini

Molybdenum disulfide (MoS2) dry film lubricants (DFLs) are widely used in aerospace mechanisms when conventional liquid lubricants are impractical. While the effects of environment on MoS2 are well established, the influence of substrate characteristics and operating conditions remains less understood. In this work, three commercial MoS2-based DFLs (Lube-Lok 4396, Everlube 620C, and Everlube 9002) were applied to five aerospace-relevant substrate materials and characterized in terms of surface roughness, flatness, coating thickness, friction, wear life, and load-carrying capacity. Vendor surface preparation and DFL deposition increased roughness, reduced flatness, and shifted the surface toward a more peak-dominated morphology, with the magnitude of these changes depending on both the substrate and coating. Tribological performance was evaluated for a subset of the samples which showed that friction was substantially lower in dry nitrogen than in ambient air, increasing contact pressure reduced friction, and sliding speed had little effect. Lube-Lok 4396 exhibited the longest wear life and highest load-carrying capacity, while Everlube 620C exhibited the lowest friction. Measured friction, wear life, and load-carrying capacity also differed from vendor specifications. These results highlight the effects of substrate material, surface preparation, and operating conditions on the surface characteristics and tribological performance of commercial MoS2 DFLs for aerospace applications.

Lubricants doi: 10.3390/lubricants14090354

Authors: Jae-Hyung Kim Syed Muntazir Mehdi Young Cheol Kim

The dynamic behavior of brush-seal bristles governs seal performance and life. Brush-seal hysteresis and the contact and friction of the bristles have been studied extensively by experiments, by three-dimensional finite-element analysis, and more recently by models of inter-bristle friction. What remains is to combine these consistently within a reduced-order multibody analysis and to fix, on physical grounds, the parameters that such a combination requires. This work implemented, in C++ and Python within the open-source multibody system Exudyn, an incremental corotational beam with an analytic tangent stiffness and analyzed the three-dimensional large-deformation dynamics of a bristle. A stick–slip friction model treats the stick–slip transition continuously, and its two friction parameters—left to user input in commercial codes—are derived from the elastic energy of the contact point. Contact is formulated with a Lagrangian constraint at the contact point, and inter-bristle friction damping is represented by Rayleigh damping based on a measured loss factor. The implemented beam agrees with the built-in Exudyn beam to within 0.02% (tip displacement) and 0.03% (shape) and is cross-validated against a prior study at a friction coefficient of 0.3. Fixing the contact point reduces the numerical oscillation of the reaction force by about 9 times relative to re-searching it at every step, and the inter-bristle friction damping yields a dynamic response distinct from material damping alone. The friction force recorded during the analysis is a single-valued function of the elastic state of the contact, reaching the limiting friction exactly when the elastic slip reaches the derived limit, and it reverses sign between incursion and retraction.

Lubricants doi: 10.3390/lubricants14090353

Authors: Bahram Turapov Róbert Keresztes György Barkó Gábor Kalácska

Despite extensive research on the abrasive properties of lunar regolith, the use of in-process tribological signals for wear assessment remains insufficiently studied. The aim of this work is to evaluate the relationship between torque response and abrasive wear severity in rotating shaft–seal systems exposed to lunar regolith simulants. Rotating EN 1.4404 stainless-steel shafts and spring-loaded natural polytetrafluoroethylene (PTFE) lip seals were tested under three-body abrasive wear conditions. Five particle-size fractions of LX-M100 Lunar Mare and LX-TH100 simulants were investigated at test durations of 15 min, 30 min and 1440 min. Frictional torque was continuously recorded, while post-test shaft surface roughness was used to characterize wear severity and Scanning Electron Microscope (SEM) analysis of the PTFE counterface was used to identify wear mechanisms. The results showed that torque response systematically depended on particle-size fraction and simulant type. Furthermore, the torque-response descriptors provide in-process information related to abrasive interaction severity, with peak torque events (Tmax) showing the strongest relationship with abrasive surface modification. The statistical analysis was based on 27 complete observations, and considering the prematurely terminated test runs and the resulting limitations of the dataset, the obtained relationships are interpreted as exploratory. These findings provide a physical basis for torque-based wear monitoring in sealed rotating mechanisms for future lunar applications.

Lubricants doi: 10.3390/lubricants14090352

Authors: Shpresa Caslli Ilirjan Braha Matilda Ruvina Ervin Kalemaj

The premature degradation of aluminium extrusion dies remains one of the major challenges affecting process efficiency, product quality, and tooling costs. Although numerous studies have investigated wear mechanisms and proposed solutions such as surface treatments, coatings and lubrication, the absence of a generic and reproducible laboratory methodology for evaluating tribological performance under representative extrusion die contact conditions limits the objective and systematic comparison of alternative tribological solutions. This study proposes and experimentally validates a generic tribological methodology for laboratory simulation of aluminium extrusion die contacts. Rather than reproducing the complete extrusion process, the methodology isolates the dominant physical mechanisms governing die degradation and reproduces their essential characteristics under controlled laboratory conditions, providing a representative platform for systematic tribological investigations. The methodology was developed through the selection and scaling of representative contact parameters, including contact geometry, normal load, sliding velocity and operating temperature. The experimental programme incorporated physical similarity principles, a controlled run-in procedure and repeated use of the same hardened steel counterface to reproduce cumulative die exposure under successive aluminium contacts. Two aluminium alloys (AA6063 and AA6082) were evaluated using a small ring-on-disc configuration against a hardened GCr15 steel counterface. Experimental validation was carried out using two extrusion lubricant systems, complemented by three additional commercial lubricants to assess the robustness and general applicability of the proposed methodology. The experimental results demonstrate that the proposed methodology provides repeatable and sufficiently sensitive measurements of friction and wear, allowing clear differentiation between lubricant systems and aluminium alloy–lubricant combinations while maintaining representative contact conditions. The study also demonstrates that steady-state friction should be identified from the actual friction evolution rather than by applying a fixed averaging interval. Although lubricant evaluation is employed here as the experimental validation case, the proposed methodology is intended as a generic experimental framework applicable to the assessment of surface treatments, coatings, tool materials, lubrication systems, and other tribological strategies aimed at extending extrusion die service life.

Lubricants doi: 10.3390/lubricants14090351

Authors: Jianfeng Zhong Ruiqi Tang Juan Liu Caihua Yang Yu Dai

Under-race lubrication is an effective oil supply method for high-speed angular contact ball bearings, but the relationship between internal oil–air two-phase flow distribution and end oil return behavior remains insufficiently understood. In this study, a Volume of Fluid (VOF)-based oil–air two-phase flow model was developed for an under-race lubricated angular contact ball bearing and validated experimentally. The effects of oil flow rate, rotational speed, inlet-hole axial position, diameter, and number were investigated. The maximum relative error between the numerical and experimental oil return ratios was 3.17%. At 3000 rpm, increasing the oil flow rate from 65 to 140 L/h increased the average oil volume fraction on the rolling element surfaces from approximately 0.037 to 0.089. In contrast, increasing rotational speed reduced oil retention on bearing component surfaces and the oil return ratio at the large-clearance end. At 140 L/h, the oil return ratio decreased from approximately 97% at 3000 rpm to 86.5% at 6000 rpm. Shifting the inlet holes toward the small-clearance end enhanced cage wetting but reduced oil retention on the rolling elements and large-clearance-end oil return. Within the investigated range, a mid-plane inlet position, smaller inlet-hole diameter, and larger number of inlet holes were more favorable for oil delivery to the rolling elements while maintaining a relatively high oil return ratio.

Lubricants doi: 10.3390/lubricants14090350

Authors: Raj Shah Mathew Stephen Roshan Sunghan Kim Amit Sutradhar Hong Liang

Tribology, the science of friction, wear, and lubrication, governs the reliability of nearly every mechanical system. Tribological contacts account for approximately 23% of global energy consumption, including 20% used to overcome friction and 3% associated with remanufacturing worn components. Nevertheless, the field has remained constrained by low experimental throughput, operator variability, and a scarcity of standardized, reusable datasets. This review surveys two converging trends positioned to address these limitations: the development of robotic and automated platforms for tribological experimentation, and the growing application of artificial intelligence to tribological analysis. High-throughput tribometer architectures, robotic specimen preparation, and multi-modal in situ sensing are examined as components of an emerging automated tribometry infrastructure. Supervised learning, physics-informed neural networks, and Bayesian optimization are reviewed as AI methods organized by the data regime in which they operate. The convergence of these trends in closed-loop autonomous tribological experimentation is assessed, including system architecture, optimization target specification, current partial implementations, and tribology-specific integration barriers that distinguish this domain from adjacent self-driving laboratory applications. Application domains spanning industrial machinery, biomedical implants, and aerospace and automotive drivetrains are discussed. Key challenges including dataset standardization, model transferability, and hardware-software integration complexity are identified. Prospects for fully autonomous tribological discovery pipelines are outlined, with emphasis on open-access data infrastructure and physics-constrained learning as the enabling conditions for the field.

Lubricants doi: 10.3390/lubricants14090349

Authors: Yunliang Wang Kang Yang Jun Tang

To optimize the tribological properties of a mechanical transmission skateboard, rectangular micro/nanogrid structures of different sizes are studied. The reciprocating sliding tests are carried out under friction and wear. The results show that with an increase in the lengths from 330 to 484 μm and widths from 247 to 363 μm of the grid structures, the friction coefficients and wear rates first decrease and then increase. This results in the smallest friction coefficient and wear rate of the TCSC-G-4 with 440 μm length and 330 μm width. This is because the micro/nanogrid structures provide the lubricant SnAgCu-CaF2 (SC), which is sufficient for lubrication film formation. During wear, SnAgCu deformation and CaF2 rolling lead to mixed lubrication that is dominated by the rolling, sliding and roller–sliding, enhancing the plastic flow of the lubrication film for surface repair. This is of great significance for guiding the bionic tribological designs of the skateboard.

Lubricants doi: 10.3390/lubricants14090348

Authors: Weitao He Xiaoping Xiao Yimin Yang Yangzhi Chen

To mitigate the deterioration of the tribological performance of end-face friction pairs under low-speed conditions, this study investigated a water-lubricated CF/PEEK–316L stainless steel end-face friction pair. Surface micro-textures with different depth-to-diameter ratios and texture area ratios were fabricated on the 316L stainless steel surface to evaluate the effects of texture parameters and sliding speed on tribological performance. The results showed that appropriately designed micro-textures effectively reduced the coefficient of friction and improved the wear characteristics of the friction pair. Among the tested conditions, the CF/PEEK–316L stainless-steel tribo-pair with a 316L counterface textured at ε = 0.05 and s = 10% exhibited the lowest average friction coefficient. At 200 rpm, this textured tribo-pair exhibited an average friction coefficient approximately 34.8% lower than that of the untextured tribo-pair. Furthermore, laser micro-textures suppressed interfacial material transfer and mitigated adhesive wear and three-body abrasive wear by capturing wear debris. Gear-pump prototype tests further showed that the textured surface improved flow-delivery and volumetric-efficiency performance, while promoting more uniform end-face wear morphology, indicating its potential application in practical gear-pump components. The findings provide guidance for surface-texture design and tribological-performance optimization of polymer/metal end-face friction pairs.

Lubricants doi: 10.3390/lubricants14090346

Authors: Xiaofei Liu Xue Liu Keyi Zhou

Gearbox remaining useful life (RUL) prediction under variable operating conditions remains challenging. The fundamental difficulty lies in the fact that the signal responses are jointly influenced by progressive degradation and variations in load and speed. To reduce the influence of condition variations on RUL prediction, this paper proposes a Condition-Calibrated Liquid Neural Network (CC-LNN) for gearbox prognostics. It utilizes the run-to-failure degradation data of constant operating conditions to train the model, which is subsequently applied to prediction tasks under variable operating conditions. Firstly, multi-domain degradation features are extracted from full-life vibration signals. Then, a condition-calibration and weak-gating mechanism is proposed to mitigate torque and speed-induced feature variations while preserving residual condition–degradation coupling. The calibrated features and their first-order differences form the sequential inputs, while condition descriptors regulate liquid-state updates for degradation evolution. Finally, smoothness and monotonicity terms are incorporated into the training objective to suppress condition-induced prediction fluctuations. The proposed method was validated through the run-to-failure experiments on gearboxes. Experimental results demonstrate that CC-LNN provides accurate and stable RUL estimates, supporting its effectiveness for cross-condition gearbox prognostics.

Lubricants doi: 10.3390/lubricants14090347

Authors: Baozun Zhai Chen He Zhimin Shi Jiaqing Wang Shuyang Liu Xiaoran Zhu Bing Xue Ren Sheng

Acoustic emission (AE) monitoring has been widely employed for condition assessment of mechanical seals. However, the physical origin of high-frequency AE activity under elastohydrodynamic lubrication (EHL) conditions remains insufficiently understood. To address this, a physics-based forward-modelling framework is developed by integrating transient EHL analysis, dynamic interfacial load mapping, and piezoelectric signal transduction. The simulated AE responses are compared with experimental measurements acquired from a GM150 sensor under both normal and defect operating conditions. The results indicate that dynamic oil-film pressure fluctuations provide a physically plausible excitation mechanism for high-frequency AE activity under EHL conditions. The present study provides a mechanistic relationship between lubrication behavior and AE responses, providing a physics-based foundation for the interpretation of AE signals and condition monitoring of lubricated mechanical seal systems.

Lubricants doi: 10.3390/lubricants14090345

Authors: Li Xiao Xitian Ding Min Zhang Naifeng Zhang Long Zhang Kunzhi Zhang Hantai Zhang

Forced spray on transmission gears performs the triple functions of lubrication, heat dissipation, and tooth surface cleaning under high-speed and heavy-duty conditions, serving as a key technical means to prevent scuffing, pitting, or even tooth breakage failure and ensure highly reliable operation of the transmission system. This study conducted numerical simulations using CFD to investigate the lubrication performance of high-speed spur gear pairs with respect to nozzle parameters including jet velocity, nozzle position, included angle, length, and number of nozzles. The results show that the medium jet velocity of 40–60 m/s achieves an optimal balance between penetration depth and spray dispersion. The results indicate that a medium jet velocity of 40–60 m/s optimally balances penetration depth and spray dispersion. Dual-nozzle oil injection significantly improves spatial uniformity and establishes a stable circular recirculation structure, increasing the oil volume fraction in the meshing zone by approximately 40% compared to the single-nozzle configuration, and reducing the area of dry patches by over 60%. A nozzle inclination angle of 60–90° combined with a length of 30 mm yields the best combination of oil delivery and coverage. Furthermore, upgrading from a 2 + 1 to a 3 + 1 nozzle layout enhances oil film continuity and suppresses abrupt negative-pressure fluctuations during meshing, thereby stabilizing the hydrodynamic lubrication effect. These findings provide quantitative guidance for optimizing nozzle geometry and layout in high-speed gearbox lubrication systems, contributing to improved reliability and reduced energy loss.

Lubricants doi: 10.3390/lubricants14090344

Authors: Akira Yanagida Yuya Hayashi Kou Takahashi

Despite the extensive research that has been carried out on alternative liquid lubricants for extrusion processes, it remains challenging to apply liquid lubricants to forms involving long components, such as rotor shafts, which undergo substantial surface expansion. In this study, a continuous lubrication method is proposed, in which lubricant is sealed into the extrusion die using a counterpunch. The negative pressure generated by the punch pulse motion during processing is utilised, and this is applied to the forward extrusion of a two-step shaft profile. The clearance between the container and the billet has a significant impact on the pooling of the lubricant within the extrusion die. It was established that, at a die half-angle of 15°, the maximum load was reduced by 15% at a billet diameter of 9.85 mm for a container diameter of 10 mm. A comparison of load–stroke diagrams from finite element method (FEM) simulations shows that the coefficient of friction (μ) decreased to approximately 0.05–0.1 during reloading and increased to a maximum of approximately 0.12 during loading. It was determined that the die half-angle of 30° resulted in a shorter sliding distance within the die than the 15° angle. This finding enables the suppression of the increase in friction. The findings of the FEM analysis substantiate the notion that material influx into the undercut area exerts an influence on relubrication.

Lubricants doi: 10.3390/lubricants14090343

Authors: Liang Ye Keyang Xue Yanwei Zhang Beile Liang Wenhu Zhang Xianghui Zhu Wenchao Li Rongjun Niu

Raceway topography and lubrication jointly govern rolling-contact conditions and vibration transmission in angular-contact ball bearings. This study examined associations between eight inner- and outer-raceway descriptors—roundness, waviness, groove-profile deviation, and roughness—and four vibration responses of thirty production 7208 bearings from one batch. All bearings were tested as received with factory grease at 1800 r min−1 and a radial load of 150 N. The responses comprised vibration-acceleration levels and vibration velocities in the 50–300, 300–1800, and 1800–10,000 Hz bands. Four grey relational schemes—initial-value-normalized, mean-value-normalized, relative, and absolute—were applied. A descriptor was retained when it ranked among the top three under at least two schemes. Inner-raceway roundness was retained for all four responses, while inner-raceway waviness was retained for acceleration and the low- and medium-frequency velocity responses. Inner-raceway roughness, outer-raceway groove-profile deviation, and outer-raceway roughness were each retained for two responses, whereas the remaining descriptors showed response-specific associations. Rankings varied with preprocessing and relational-degree formulation. The findings provide screening evidence for prioritizing raceway metrology and subsequent controlled experiments.

Lubricants doi: 10.3390/lubricants14090342

Authors: Yong Sun Congcong Zhao Huaping Tang Yunjie Bi

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

Lubricants doi: 10.3390/lubricants14090341

Authors: Yazhou Mao Linlin Guo Anzixuan Wang Runyi Ma Pengfei Gao Aoya Wang

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

Lubricants doi: 10.3390/lubricants14090340

Authors: Wenhu Zhang Shuanglin Wang Chunwei Li Lingzhi Chai Wanjia Li

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

Lubricants doi: 10.3390/lubricants14090339

Authors: Liping Liao Ting Liu Daiying Yuan Zhongnan Wang

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

Lubricants doi: 10.3390/lubricants14090338

Authors: Lingtong Sun Wenzhong Wang Zhi Chen Jianlong Liu He Liang

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

Lubricants doi: 10.3390/lubricants14090337

Authors: Gengyuan Gao Meng Kong Shijie Yu Xiuli Zhang

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

Lubricants doi: 10.3390/lubricants14090336

Authors: Tom Caston Nader Dolatabadi Ramin Rahmani

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

Lubricants doi: 10.3390/lubricants14090335

Authors: Kosuke Furukawa Yoshihiro Kubota Yuki Shimomura Kunio Hayakawa

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

Lubricants doi: 10.3390/lubricants14090334

Authors: Weichao Liu Huijun Yue Yaoting Wu Jiachun Lin

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

Lubricants doi: 10.3390/lubricants14090333

Authors: Xiang Jiao Guochen Huang Yiqin Wang Chenchen Peng Guoqing Wang

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

Lubricants doi: 10.3390/lubricants14090332

Authors: Baogang Wen Libin Xuan Zhihao Zan Xu Zhang Jingyu Zhai

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

Lubricants doi: 10.3390/lubricants14090331

Authors: Renshan Xia Zhi Xu Jiaoyan Ma Xiaoming Zong Shangchu Yang Yanyan Wang Han Li Ming Ma

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

Lubricants doi: 10.3390/lubricants14090330

Authors: Hongbing Wang Wei Shi Yuping Wu Xingming Chen Jie Su Lairong Yin Bo Hu

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

Lubricants doi: 10.3390/lubricants14090329

Authors: Shunqi Mei Andrey Nomoev Erzhena Khartaeva Undrakh Mishigdorzhiyn Sergei Nomoev Sayan Badmaev Bair Garmaev

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

Lubricants doi: 10.3390/lubricants14080328

Authors: Jiachun Lin Xudong Zhao Huijun Yue Yunjin Xiang Peng Wang Minghui Tu Ulf Olofsson

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

Lubricants doi: 10.3390/lubricants14080327

Authors: Raj Shah Brandon Juran Stefanos Nitodas

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

Lubricants doi: 10.3390/lubricants14080326

Authors: Na Wu Qian Song

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

Lubricants doi: 10.3390/lubricants14080325

Authors: Peng Yue Jiaqing Wang Zhimin Shi Chen He Xiaoran Zhu Yujuan Zhang

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

Lubricants doi: 10.3390/lubricants14080324

Authors: Gang Li Jiangang Li Zhane Li Xin Lu Yingling Li Yun Lin Xingnan Hu Wei Kang

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

Lubricants doi: 10.3390/lubricants14080323

Authors: Honglie Ma Qingkai Shen Xiaolei Deng Qiang Cheng Mingyue Zhang

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

Lubricants doi: 10.3390/lubricants14080322

Authors: Changzheng Li Guoliang Zhang Weichao Liu Changyi Chen Change Wu Jia Xiao Rui Shu Feng Wang Shaohong Zhang Xiang Liu

This study investigates the wear behavior of PWR control rod cladding under three representative loading modes: pure impact, circumferential sliding, and impact–sliding coupling, in high-temperature (300 °C) and high-pressure (15.5 MPa) water. Pre-oxidized and non-oxidized specimens were compared using white-light interferometry, net mass change, SEM, and EDS. Maximum wear depth and wear volume were used as the primary quantitative indicators of wear because net mass change may also reflect oxidation, transferred material, and retained debris. The results show that pre-oxidation does not uniformly reduce wear but changes its distribution between the contacting bodies. Under circumferential sliding, for example, pre-oxidation increased the cladding wear depth from 12.85 to 20.05 μm while reducing the guide-card depth from 24.99 to 11.93 μm. Impact–sliding coupling produced a distinct edge-localized wear morphology, although it did not yield the largest value for every wear metric. Surface observations revealed oxide-layer cracking and spallation in pre-oxidized specimens and stronger adhesion-related transfer features in non-oxidized specimens. The results show that control-rod wear assessment should consider not only the total wear magnitude but also its distribution between the cladding and guide card.

Lubricants doi: 10.3390/lubricants14080321

Authors: Syed Muntazir Mehdi Jae-Hyung Kim Young Cheol Kim

Brush seals using compliant bristle packs can reduce turbomachinery leakage more effectively than conventional labyrinth seals, but their coupled structural and flow behavior makes design difficult. Under large pressure loading, bristles can deflect, lose contact with the rotor, and generate clearance, causing the sharp leakage increase known as blow-out. This study develops a model linking nonlinear bristle deflection, rotor–bristle contact loss, and leakage response. The bristle is treated as an inextensible nonlinear elastic member subjected to distributed pressure loading, backing-plate support, and frictional rotor contact. Contact and separated states are solved iteratively using boundary-value and initial-value solvers. Leakage through the bristle pack is calculated using a random bristle-bed formulation, and leakage through generated clearance is evaluated with an orifice-flow model. The model agrees well with published bristle-deflection predictions. Increasing pressure load reduces normal contact force until lift-off occurs, producing clearance and a sharp rise in leakage. Increasing front-plate free height shifted lift-off from pressure ratio ≈4 to ≈2, while clearance flow contributed up to 36.5% after lift-off. Brush-seal blowout is therefore governed by the transition from rotor–bristle contact to separation. Lower back-plate height can delay blow-out, but hysteresis and durability tradeoffs must be considered.

Lubricants doi: 10.3390/lubricants14080320

Authors: Bo Yang Chaojun Deng Linyuan Kuang Zeyuan Yu Ying Luo

The first step in the analysis of the contact mechanics and leakage prediction of metallic seals applied to nuclear reactor pressure vessels is the proper characterization of the surface topography. At present, two approaches are used for this characterization. On the one hand, there are non-parametric techniques such as HPD and PSD, which retain all the characteristics of the surfaces that have been measured, but the results are high dimensional; hence, they cannot be analyzed analytically. The other type is parametric fractal methods, where the parameters used are fractal dimension D and characteristic scale G, where the analytical derivations can be made; however, this leads to systematic deviations in the mechanical response due to some idealized assumptions, like isotropy, Gaussian distribution, and infinite self-similarity. In this article, we propose an equivalent fractal parameter inversion model (EFPIM) that does not rely on geometric fitting; instead, it fits the mechanical contact behavior of a physical surface. This inversion procedure reduces three errors simultaneously. Thus, the EFPIM does not use D and G as the geometrical fitting variables but rather redefines them as mechanically equivalent ones, the purpose of which is to minimize the difference between the W-M fractal surface and the real measured surface. To address the problem of constrained inversion, we adopt a genetic algorithm with BFGS. To prove its effectiveness, we carried out experiments on C-ring seal surfaces and found that the deviation in the contact area was reduced by an order of magnitude in comparison to traditional structure-function extraction, and the deviation in the approach and the maximum pressure were less than 2%. Moreover, the equivalent parameters shift systematically away from their geometric counterparts in the direction that compensates for the dominant non-ideal deficit of the W-M surface; when both parameters are free, the equivalent fractal dimension decreases, while the equivalent characteristic scale increases, compensating for the absent non-Gaussian deep valleys of ideal W-M surfaces. Existing models of analytical contact and leakage may be directly implemented using equivalent parameters and with accuracy comparable to that of FFT-based simulations, with the modest cost of the offline computations.

Lubricants doi: 10.3390/lubricants14080319

Authors: Sibo Liu Hongwang Zhao Jiabao Li Dandan Wu Hao Li Zhong Liu

Existing piston–cylinder lubrication studies often simplify the pressure boundary as a constant load or a single field, making it difficult to capture pump-level pressure excitation, local oil-film response, and non-concentric posture under variable loading. This paper establishes a thermo–fluid–solid coupling framework integrating an AMESim full-pump model, a Fluent transient oil-film model, and a Transient Structural model; UDF transfer of periodic pressure, dynamic meshes, and a calibrated Roelands law were used to analyze parallel-offset and center-tilted postures. As the load pressure increased from 10 to 30 MPa, the maximum discharge–half-cycle temperature rose from 28.39 to 36.95 °C, and the average positive leakage during the third-cycle high-pressure stage increased from 0.0201 to 0.1026 L/min; increasing speed from 1000 to 3000 r/min reduced cycle-averaged leakage by 8.93%. At 500 r/min and 30 MPa, the parallel-offset case reached 46.34 °C, 41 kPa, and 0.0990 L/min in maximum temperature, maximum shear stress, and average leakage, whereas the center-tilted case produced a peak resultant force of 3537.12 N, a cylinder inner-wall high-stress band of 76.96 MPa, and a maximum piston deformation and equivalent stress of 4.31 μm and 83.16 MPa. These results clarify the distinct lubrication behavior and potential uneven-wear risk associated with the two representative non-concentric postures, and provide a basis for clearance design and posture-sensitive condition assessment of axial piston pumps.

Lubricants doi: 10.3390/lubricants14080318

Authors: Ján Kosiba Zdenko Tkáč Daniel Skladaný Martin Nagy Ladislav Tóth Siniša Bikić Samuel Danis Martin Olejár

This paper presents an experimental investigation into the flow characteristics and volumetric efficiency (ηvol) of a fixed-displacement external gear pump (GHD 17R) operating under coupled hydraulic parameters using an eco-friendly synthetic ester-based hydraulic fluid (48 mm2·s−1 at 40 °C). Measurements were performed on a laboratory single-circuit hydraulic test rig across a rotational speed range of 500–2500 min−1, operating pressure range of 2–10 MPa, and fluid temperature range of 30–60 °C. To eliminate flow fluctuations caused by structural vibrations at 1250 and 1750 min−1, a 15% trimmed mean statistical filter was successfully implemented. A comparative sensitivity analysis—evaluating absolute, normalized, and relative significance—was developed and compared against a three-way analysis of variance (ANOVA) effect size model (η2 and partial η2). The relative sensitivity approach identified rotational speed as the dominant parameter for direct hydraulic flow, accounting for 95.80% of total variation. Conversely, when evaluating volumetric efficiency, the proportional impact of speed was removed, revealing a balanced distribution of internal losses: rotational speed contributed 54.73%, fluid temperature 26.08%, and pressure 19.19%. The three-way ANOVA confirmed that all primary parameters and their cross-interactions had a statistically significant effect (p < 0.05). The findings scientifically demonstrate that temperature-induced viscosity collapse exhibits a stronger relative dynamic sensitivity on volumetric losses than pressure fluctuations within standard operating envelopes. The constructed multi-dimensional flow and efficiency maps provide practical input for advanced diagnostic tools, real-time thermal condition monitoring, predictive maintenance, and energy-optimized control schemes in modern fluid power systems using eco-friendly lubricants.

Lubricants doi: 10.3390/lubricants14080317

Authors: Xuan Yin Qiang Hao Haosheng Pang Dameng Liu

The surface mechanical behavior of metallic bionic foot exerts a decisive influence on their service life. A face-to-face friction test system was employed to simulate the surface tribological behavior of bionic foot during actuation. Cellulose coatings were in situ formed on the contact surfaces of three types of bionic foot to replicate practical operating conditions, followed by tribological testing and characterization analysis. This study investigated the effect of cellulose coating in retarding the wear of the three bionic feet and elucidated their surface lubrication properties. The results demonstrate that surface deformation of the bionic foot is primarily concentrated at the toe tips, as well as the edges and corners of the feet. Tribological test results indicate that after modification with cellulose coatings, the friction coefficient first decreases and then increases with increasing load; when the load reaches 20 N, the friction coefficient of the bionic foot surface drops to as low as 0.043, and the wear scar depth also reaches its minimum value. The underlying mechanism lies in the fact that friction-induced effects promote the formation of adsorbed tribofilms of cellulose on the surface of the bionic foot. These tribofilms effectively isolate the direct contact between the upper and lower friction pairs, thereby enhancing lubrication efficiency while significantly mitigating adhesive wear and abrasive wear. This study establishes mechanical models of the bionic foot to provide experimental validation and design guidelines aimed at enhancing its service life.

Lubricants doi: 10.3390/lubricants14080316

Authors: Dávid Medveď Jana Andrejovská Viktor Puchý Róbert Džunda Ondrej Petruš

This study investigates the mechanical and tribological behavior of three composites with a ZrO2 matrix stabilized with 1.75 mol% Y2O3 and containing 1 wt.% Al2O3, reinforced with TiC (A), TiC + ZrC (B), and TiC + WC + Mo2C (C). The matrix powder was synthesized by chemical coprecipitation, and the composites were consolidated by spark plasma sintering at 1350 °C. Dry reciprocating sliding tests against 100Cr6 steel were performed at 10 and 25 N. Composite A exhibited the highest HV10 hardness (1268), while the indentation fracture toughness values were similar (6.41–6.57 MPa·m1/2). Wear resistance did not follow the hardness ranking. At 25 N, composite A exhibited surface fragmentation and a specific wear rate of 2.52 × 10−6 mm3·N−1·m−1, while composite B showed extensive and heterogeneous transfer of steel-derived material. Composite C exhibited the lowest coefficient of friction and specific wear rate, reaching 0.392 and 7.04 × 10−8 mm3·N−1·m−1, respectively, at 25 N. EDS mapping revealed an area-integrated Fe content of 0.7 at.% for C, compared with 7.2 at.% for A and B. The superior wear resistance of C was associated with substantially lower steel-derived material transfer and a relatively smooth wear-track surface rather than with the highest bulk hardness.

Lubricants doi: 10.3390/lubricants14080315

Authors: Yanan Zhang Xinlong Wu Jinyu Liu Hongjian Wu Yonggang Meng Chuke Ouyang

To investigate the regulation mechanism of poly (alkyl methacrylate) (PMA) additives on the viscosity–temperature characteristics and tribological performance of different types of base oils, Group III mineral base oils and Group IV PAO synthetic base oils were selected as the research objects. Rheological and boundary-lubrication tests were systematically conducted at different PMA addition levels, with emphasis on comparatively analyzing the polymer conformational evolution, interfacial adsorption behavior, and lubrication-performance response induced by differences in the solvent polarity of the base oils. The results showed that the modification effect of PMA on base oils exhibited pronounced matrix dependence and non-monotonic concentration characteristics, and its lubrication-regulating behavior was dominated by the coupled trade-off among polymer solubility, molecular conformational stability, and interfacial competitive adsorption ability. In the mineral-oil system, where the base oil acts as a good solvent, the solubility parameters of PMA and the base oil are well matched, allowing the polymer molecular chains to sufficiently swell and extend and providing excellent adsorption and film-forming ability. With increasing PMA concentration, the viscous-flow activation energy of the oil continuously decreased, while the viscosity–temperature performance and boundary-lubrication stability were simultaneously improved, resulting in stable and reliable modification effects. In contrast, in the PAO synthetic-oil system, where the base oil acts as a poor solvent, the PMA molecular chains tend to adopt coiled conformations, with their conformations being highly sensitive to temperature and shear rate, while their interfacial adsorption ability is weaker than that of the base-oil molecules. An optimum critical PMA concentration of 1.0 wt% was observed in this system. Above this concentration, intramolecular friction increased, resulting in deterioration of both viscosity–temperature characteristics and friction performance. This study clarifies the differentiated modification mechanisms of PMA in base oils with different polarities and reveals the dominant role of solvent effects in polymer rheological and tribological behaviors, thereby addressing the insufficient understanding in existing studies of the non-monotonic modification behavior of PMA and its multi-factor coupled mechanism. The findings provide a theoretical basis for PMA structural selection and precise concentration formulation in lubricating oils under different operating conditions and have important engineering application value for optimizing viscosity–temperature performance over a wide temperature range, improving service stability under boundary lubrication, and balancing lubrication reliability with formulation economy.

Lubricants doi: 10.3390/lubricants14080314

Authors: Hanqian Zhang Mingjin Lan Qun Lei Zhentao Cheng Jianjun Du

Aerostatic journal bearings with small-hole restrictors are widely adopted due to their superior stiffness, making them effective solutions to meet the growing demands for improved stability and higher rotational speeds in precision machinery. In this study, the nonlinear dynamic behaviors of a rigid rotor supported by aerostatic bearings with micro-hole and small-hole restrictors were systematically investigated and numerically compared. Experimental results indicate that aerostatic journal bearings with integrated micro-holes exhibit superior dynamic stability to their small-hole counterparts, with the threshold speed of half-frequency whirl increased by 10.5%. This research provides critical insights for optimizing the performance of aerostatic journal bearings in high-speed applications.

Lubricants doi: 10.3390/lubricants14080313

Authors: Risheng Long Xiaoqing Wang Siwei Wang Fangfeng Gao Peilin Song Yonglin Wang Lin Zong

Biomimetic surface texturing provides a promising strategy for regulating the vibration behavior of rolling bearings under starved lubrication. In this study, vein-like, elliptical, semi-elliptical, and composite textures inspired by Monstera deliciosa leaves were fabricated on the shaft-washer raceways of thrust cylindrical roller bearings at depths of 4 μm, 8 μm, and 12 μm. Tangential and normal vibration signals were analyzed using time-domain parameters, frequency spectra, power spectral density, and time–frequency maps. The results showed that both texture morphology and depth strongly affected vibration stability. Most textured bearings exhibited lower vibration responses than the smooth bearing after prolonged operation. Among the tested depths, 8 μm produced the most stable response, characterized by lower peak values, smoother root mean square curves, reduced power spectral density levels, and more uniform time–frequency energy distributions. The 8 μm semi-elliptical texture exhibited the best overall performance by suppressing transient impacts and high-frequency energy concentration. These findings indicate that vibration regulation in textured rolling bearings depends primarily on the synergistic matching between texture morphology and depth rather than texture complexity alone.

Lubricants doi: 10.3390/lubricants14080312

Authors: Junyou Zhang Yu Zhang Jian Zhang Jinghui Xia

Disc-cutter wear in abrasive strata is spatially non-uniform, yet mean wear depth cannot indicate where it concentrates. This retrospective feasibility study develops a machine-learning-based condition-monitoring framework that predicts the relative severity of eccentric wear from engineering data available before inspection, supervised by 3D-scan morphology labels. An Eccentric-Wear Morphology Index (EWI) is constructed from post-replacement 3D morphology and used solely as a relative-severity label; its tertile-based grades are cohort-relative rather than universal engineering thresholds. The analysis cohort comprised 244 quality-controlled 19-inch cutter rings, and an engineering-prioritized redundancy review condensed 58 candidate variables into a frozen 22-variable set. In five-fold out-of-fold evaluation, the Random Forest achieved 0.779 accuracy, 0.775 macro F1, and 0.939 high-severity recall. With nested threshold selection, in which the operating threshold was chosen only within the training folds, the pooled held-out screening result reached 0.988 recall and 0.946 F2 while including 41.0% of the samples in the review pool, and this operating point was insensitive to false-negative-to-false-positive cost ratios between 5:1 and 15:1. Grouping both rings of each twin cutter into the same fold left the screening operating points essentially unchanged. The framework shows potential to support within-project inspection prioritization; external validation and calibration remain necessary because the screening signal is strongly associated with service exposure and the project-specific cutter-change schedule.

Lubricants doi: 10.3390/lubricants14080311

Authors: Varun Singhal Daksh Shelly Gurpreet Singh Matharou Anil Prakash Singh

Aluminum matrix composites (AMCs) reinforced with natural mineral ilmenite offer a cost-effective and thermally stable alternative to conventional cast iron brake drum materials. This study investigates the synergistic effect of fine (32–50 µm) and coarse (75–106 µm) ilmenite particles at four fine-to-coarse weight ratios (1:4, 2:3, 3:2, and 4:1) and three reinforcement contents (5, 10, and 15 wt.%) on the thermal stability and dry sliding wear behavior of stir-cast LM30 Al composites. Ilmenite reinforcement progressively reduces the coefficient of thermal expansion of the LM30 matrix, with the 15 wt.% 4:1 fine-to-coarse ratio composite (15DRP41) exhibiting the lowest coefficient of thermal expansion of ~16.54 × 10−6/°C, a ~33.3% reduction relative to the unreinforced alloy (~24.8 × 10−6/°C). The 15DRP41 composite demonstrates the lowest wear rate of all the fabricated composites, 1.82 × 10−3 mm3/m at 9.81 N and 9.56 × 10−3 mm3/m at 68.67 N at 200 °C. Under the most severe load condition (68.67 N, 200 °C), the coefficient of friction of 15DRP41 is reduced by up to 44% compared with the LM30 alloy. A comparative test against commercial grey cast iron shows that 15DRP41 has a similar wear rate up to 200 °C, while its density (~2.9 g/cm3) is significantly lower and it has excellent dimensional stability. Scanning electron microscopy and energy-dispersive X-ray spectroscopy of worn surfaces and debris confirm a progressive change from oxidative and mild abrasive wear at low loads and temperatures to severe wear by delamination at 68.67 N and 300 °C, as evidenced by the presence of a multi-component mechanically mixed layer. The results have confirmed that the optimum fine-to-coarse ratio for the reinforcement was 4:1, which led to the maximum wear resistance and thermal stability in ilmenite-reinforced LM30 composites for lightweight automotive brake drum applications, and that the optimum weight percentage for the reinforcement was 15 wt.%.

Lubricants doi: 10.3390/lubricants14080310

Authors: Adam Agocs Georg Vorlaufer Marcella Frauscher Charlotte Besser

Wear metal monitoring is an important component of lubricant condition monitoring but commonly relies on elemental techniques such as inductively coupled plasma optical emission spectroscopy (ICP-OES), which require dedicated laboratory infrastructure and sample preparation. This study evaluates whether Fourier-transform infrared (FT-IR) spectra of used engine oils can be combined with partial least squares (PLS) regression to provide a rapid screening estimate of iron (Fe) concentration. Used petrol and diesel engine oil samples were analyzed by FT-IR spectroscopy and ICP-OES. PLS models were developed using processed FT-IR spectra as predictor variables and ICP-OES-derived Fe concentrations as response variables. For petrol used oil samples, the optimized model employing 18 latent variables achieved a root mean squared error of 5.02 ppm and a coefficient of determination of 0.97 between measured and predicted Fe concentrations. Model loadings indicated contributions from spectral features associated with soot, oxidation, nitration, antioxidant (AO) depletion, and zinc dialkyldithiophosphate depletion. Combining petrol and diesel samples in a single model reduced predictive performance and increased uncertainty, indicating that their differing degradation pathways cannot be adequately represented by one common latent variable model. The approach does not directly measure Fe and is not intended to replace elemental analysis. Instead, it provides a rapid, low-cost screening tool for identifying samples with potentially elevated wear metal concentrations and prioritizing them for confirmatory analysis.

Lubricants doi: 10.3390/lubricants14080309

Authors: Abdul Majeed Siddiqui Mehwish Ahmed Muhammad Israr Siddiqui Khadija Maqbool

Ciliary-driven flow refers to the movement of fluid by the rhythmic and coordinated beating cilia and finds applications in the respiratory tract, fallopian tube, embryonic node, brain ventricles, paranasal sinuses, and understanding flows in the auditory tube. Previous research on cilia-driven flow has demonstrated forced convective flow with no-slip boundary conditions, which is crucial in mucus clearance and is not firmly stuck to the periciliary layer. This paper develops the mixed convective flow of Ellis fluid near the periciliary layer with a lubricated surface. The partial slip boundary condition provides reduced friction near the periciliary layer for the Ellis fluid flow. The momentum and energy equations are simplified by the lubrication approach, and the resulting problem is solved analytically. This research achieves the exact solutions for the temperature and velocity profiles for the consistency index 3. The findings show that the mucus flow along the lubricated surface is enhanced by the slip parameter and viscosity (shear-thinning fluid) parameter beta, but the flow across the trachea decays due to the slip and viscosity parameters. The mucus temperature rises due to the radiation and Prandtl number, which also help to reduce the frictional forces near the periciliary layer and facilitate faster mucociliary clearance.

Lubricants doi: 10.3390/lubricants14080308

Authors: Kazuhito Asai Kazuhiko Kitamura Takumi Nishi

Ironing is an effective process for precisely finishing a formed product at the final stage of a series of forming processes. Generally, a high-performance lubricant oil with high viscosity is used to prevent galling under severe forming conditions, but high-viscosity oil is difficult to remove from the workpiece after the forming process. Residual oil can interfere with subsequent processes such as welding, heat treatment, and painting. Therefore, low-viscosity oils with excellent cleanability are desirable under severe ironing conditions. In this study, differential lubrication was applied to ironing to enable the use of low-viscosity lubricant oil. The results showed that severe galling occurred on the inner surface of the workpiece when a low-viscosity oil was used. However, no galling was observed when low- and high-viscosity oils were applied to the inner and outer surfaces of the workpiece, respectively. These results indicate that differential lubrication enables the use of a low-viscosity lubricant oil without galling while maintaining high surface quality.

Lubricants doi: 10.3390/lubricants14080307

Authors: Yanghuan Li Haonan Zhang Xiang Li Dongzhou Jia Yongqiang Fu

In the field of flexible electronics, traditional composite copper foils generally suffer from weak interfacial adhesion between the copper layer and polymer substrate, poor corrosion resistance, insufficient surface uniformity, and limited functional adaptability. To address these issues, Cu/Cu-CeO2 composite coatings were deposited on polyimide (PI) substrates via PVD magnetron sputtering using argon as the working gas, aiming to enhance the comprehensive properties of composite copper foils, including interfacial bonding strength and corrosion resistance. Initially, pure Cu coatings were deposited on polyimide (PI), polyethylene terephthalate (PET), and polypropylene (PP) substrates. The deposition parameters were optimized through orthogonal and single-factor experiments, and the optimal process combination was determined as follows: PI substrate, sputtering time of 20 min, sputtering power of 60 W, and argon flow rate of 90 sccm, which achieved a balance between mechanical and electrical properties. Subsequently, comparative studies of Ar plasma treatment (100 s, 200 s, 300 s, and 400 s) and NaOH chemical etching (0 mol/L, 1 mol/L, 2 mol/L, and 3 mol/L) were conducted on the three polymer substrates. Comprehensive analyses of water contact angle, surface energy, bonding strength, and surface roughness demonstrated that the PI substrate treated with Ar plasma for 300 s exhibited superior overall performance, with a water contact angle of 48.5°, surface energy of 61.78 × 10−3 J/m2, bonding strength of 4.56 N, and surface roughness of 0.89 μm. On this basis, the performance of pure Cu coatings and Cu/Cu-CeO2 composite coatings prepared under different CeO2 sputtering powers (20 W, 30 W, 40 W, and 50 W) was further investigated. Combined analyses of SEM, EDS, and XPS characterizations, together with bonding strength, resistivity, electrochemical impedance spectroscopy, polarization curves, and corrosion morphology tests, revealed that the Cu/Cu-CeO2 composite coating prepared at a sputtering power of 50 W exhibited superior overall performance in terms of interfacial bonding strength and corrosion resistance.

Lubricants doi: 10.3390/lubricants14080306

Authors: Junjie Yuan Zhichao Wang Gang Zou Rui Sun Donghui Li Guoliang Liu

TiAl alloy exhibits excellent strength, oxidation resistance and creep resistance, making it a preferred candidate material to replace high-temperature alloys. Currently, TiAl alloy has been widely applied in aerospace, the marine industry and other fields involving high-stress contact or highly corrosive environments. Selective laser melting (SLM) technology provides a brand-new approach for the fabrication of TiAl alloys, which enables direct forming of workpieces with complex structures and significantly reduces manufacturing cycles. However, the quality and performance of SLM fabricated TiAl alloys are highly dependent on laser energy input. Therefore, this study fabricated TiAl4822 alloy under different SLM process parameters, and systematically conducted investigations on its tribological properties and electrochemical corrosion behavior. The experimental results show that the SLM process did not alter the basic phase composition of TiAl4822 alloy, with Ti0.6Al0.4 as the dominant phase. TiAl4822 alloys fabricated under the parameter combinations of 1000 mm/s + 140 W exhibited outstanding wear resistance, and the wear mechanism transformed from severe adhesion and abrasive wear to mild oxidative wear. When the laser power was 100 W and the scanning speed was 1200 mm/s, the alloy achieved the highest corrosion resistance, with the corrosion potential reaching the maximum value of −390.065 mV and the corrosion current density decreasing to the minimum value of 8.73 × 10−6 A/cm2. Thus, different parameter combinations can realize the optimization of tribological properties and electrochemical corrosion performance respectively. This study lays a theoretical foundation for promoting the high-performance engineering application of this alloy in harsh wear-resistant and corrosion-resistant environments.

Lubricants doi: 10.3390/lubricants14080305

Authors: Ning Li Jingyu Zhai Jingqi Zhang Shihai Cui

To address the challenges in understanding the raceway failure mechanisms of bearings under dynamic radial excitations, this study proposes a vibration evolution analysis method based on multi-source data fusion and a Granger causality test. Firstly, a vertical bearing vibration test bench that can simulate the dynamic excitation in engineering practice is built, and the bearing acceleration, inner ring displacement and cage data are collected at the same time. Subsequently, the evolution law and correlation relationship of bearing vibration signals during the expansion process of bearing raceway damage were studied. Based on this, a multi-source vibration data fusion method was proposed, and the effectiveness of different data fusion schemes in characterizing raceway damage expansion was compared. Finally, the Granger causality test was applied to analyze the causal relationship between the evolution of various vibration behaviors during the damage propagation process. Research results demonstrate that under complex loading conditions during sustained operation, the “False Brinelling” indentation gradually develops into raceway surface damage. The vibration behavior of bearings exhibits distinct stage-specific characteristics under dynamic radial excitations. Notably, variations in vibration behavior amplitude and transition timing between different operational phases demonstrate significant discrepancies. Significant alterations in causal relationships between vibration behaviors were observed throughout different degradation phases. The combined approach proposed in this paper, encompassing complex load simulation, multi-source data fusion, and causal analysis, offers a new understanding of the raceway failure mechanism of bearings under real-world operating conditions.

Lubricants doi: 10.3390/lubricants14080304

Authors: Rui Qiu Yang Guo Runqi Yu Siyi Dong Yu Chen

Generally, a clearance joint can cause contact–impact characteristics and nonlinear dynamic behavior of a mechanism, and the introduction of an external load would improve motion instability. The main concern in mechanism design is to obtain the dynamic response of a mechanism with clearance joints. In this study, a dynamic model of a high-precision mechanism is established. The contact characteristics of the clearance joints are described using the dissipative contact model and the Coulomb friction law. Meanwhile, an experiment platform for impact load and dynamic behavior for this mechanism is designed. And the impact load value is introduced into the dynamic model. Eventually, the influence of clearance characteristics and design parameters on the nonlinear response of the high-precision mechanism is analyzed using a case study.

Lubricants doi: 10.3390/lubricants14080303

Authors: Xiangyu Du Shaowei Liu Xiaoquan Lu Tianyou Zheng

Regarding heat conduction in sliding contact pairs, this paper investigates the interfacial heat partition problem with conduction heat flux taken into account to address the issue of the heat partition coefficient falling outside its physically reasonable range. The main contributions of this study are as follows. First, conduction heat flux is explicitly introduced, and the governing equation for the heat partition coefficient incorporating conduction heat flux is derived via Green’s function method. Subsequently, to tackle the nonlinearity caused by the time-varying velocity and heat source of the contact pair, least-squares estimation is adopted to solve for the heat partition coefficient and conduction heat flux. The results indicate that under extreme operating conditions with drastic variations in heat source and velocity, traditional heat partition models yield unphysical results where the heat partition coefficient is less than 0 or greater than 1, whereas the modified model effectively resolves this issue. Furthermore, this paper analyzes the effects of material parameters, motion characteristics, and thermal loads on heat partition. The findings of this work provide a reference for interfacial thermal design and thermal management of various sliding contact pairs.

Lubricants doi: 10.3390/lubricants14080302

Authors: Yongquan Gan Lanlan Pan Hanbing Zhang Haixuan Sun Chunliang Niu Jiakun Wu

Lubrication performance is a critical index determining the service performance of self-lubricating ceramic cutting tools, but the coupling between mechanical properties, lubrication, and wear makes the lubrication effect challenging to predict. In this paper, WC-cBN-MoS2 self-lubricating ceramics with 15 vol% MoS2 and a range of mechanical properties (Vickers hardness: 10.33–20.22 GPa; fracture toughness: 1.8–5.95 MPa·m1/2) were fabricated by high-pressure sintering. A lubricating particle release model is established to analyze the contributions of matrix deformation and surface wear to lubricant release. It is found that matrix deformation alone cannot extrude lubricating particles to the surface; instead, wear is the primary mechanism supplying particles to the friction interface. The lubricating film, containing both ceramic matrix debris and lubricating particles, exhibits characteristics of multi-media powder lubrication, with film thickness increasing linearly with applied load and sliding speed. The friction coefficient shows a non-monotonic relationship with load and speed, attributed to the coexistence of powder lubrication within the film and quasi-boundary lubrication at the upper interface. A predictive model is established based on the superposition of these two mechanisms, and its predictions agree well with experimental measurements. This work clarifies the multiple lubrication mechanisms of self-lubricating ceramics and provides a quantitative framework for predicting their tribological performance.

Lubricants doi: 10.3390/lubricants14080301

Authors: Yubin Zhang Junan Qian Fengtao Wang Chunlan Yu Bolan Kong Xiaoyun Zhao

To address lubricant film oscillation and rotor whirl instability caused by unreasonable bearing configurations in X-ray tubes, this study systematically investigated the dynamic performance of asymmetrically distributed herringbone-groove journal bearings lubricated with gallium-based liquid metal. On the basis of hydrodynamic lubrication theory and turbulence effects, an unsteady dynamic Reynolds equation and a perturbation pressure differential equation are established. The physical definitions and coordinate transformation relationships of the lubricant film stiffness and damping coefficients are clarified. Comparative analyses of symmetric and asymmetric bearing structures are conducted on the COMSOL Multiphysics platform under varying eccentricities, rotational speeds, bearing clearances, and groove depths. Compared with the symmetric design, the asymmetric structure generates a significantly higher damping peak in the medium-to-high eccentricity range, achieving an optimal combination of high stiffness and moderate damping. A stable, directional, high-pressure zone can form at zero eccentricity, which actively guides the lubricant to establish a steady hydrodynamic film under misaligned operating conditions. This study provides theoretical support for the optimal design of high-speed bearing systems.

Lubricants doi: 10.3390/lubricants14080300

Authors: Mohsen Soori

Cutting tool wear significantly influences machining performance, surface quality, and manufacturing cost. Proper minimization of cutting tool wear will result in enhanced life of the cutting tool, surface integrity, precision, and sustainability of the machining process. There are various methods for minimizing cutting tool wear in machining operations. These include the optimization of parameters such as reducing the feed and speed, use of proper coating such as TiN and Al2O3, lubrication/cooling, and proper material for the cutting tool like carbide and ceramic materials. The application of chip breakers and high machine rigidity can minimize wear by lowering heat and friction, which are the major causes of wear. Reduction in wear will ensure a better surface finish, enhanced tool life, and economic efficiency of the machining process. The main objective of this research paper is to conduct an extensive study on wear of cutting tools in machining operations. As a result, the study discusses several advanced methods of tool wear detection in cutting tools, including sensor-based methods, machine vision, and AI/ML-assisted predictive maintenance. Additionally, a critical assessment in tool wear minimization is conducted to apply new material to the cutting tool, the coating process, cutting parameter and path optimization, cooling and lubrication systems such as minimum amount lubrication and cryogenic cooling. Moreover, various challenges with intelligent and autonomous manufacturing systems that arise in tool wear prediction with regard to availability of data and reliability of prediction models are discussed in the study. Finally, potential future research directions are provided, with an emphasis on the importance of using digital twin technologies and sustainable manufacturing approaches in tool wear management.

Lubricants doi: 10.3390/lubricants14080299

Authors: Lexia Wei Haowen Qin Xuan Chen Chenxi Wu Shiyu Liu Chaohua Wu Xiaoliang Shi

Surface-dominated failures such as micropitting, scuffing, and rolling contact fatigue remain critical challenges for 20CrMnTi gear steel under high-stress and poor lubrication conditions. Inspired by the composite bionic “checkerboard + dot-matrix dimple” architecture found in natural surfaces, this study aims to develop an optimized biomimetic texture combined with a graphene solid lubricant coating to enhance the tribological performance of 20CrMnTi alloy. Laser surface texturing was employed to fabricate the bionic pattern on 20CrMnTi substrates, followed by deposition of an oxysilane-graphene coating. Reciprocating ball-on-disc tribological tests were conducted under both dry friction and oil-lubricated conditions. The results showed that under dry friction, the optimized textured and coated specimen achieved an average friction coefficient of ~0.18, representing a ~62% reduction compared to the untextured surface (~0.47). Under oil lubrication, the friction coefficient was further reduced to ~0.10–0.11, demonstrating a synergistic effect among the texture, graphene coating, and lubricant. Wear volume decreased by over 70% under dry conditions. The dominant wear mechanism shifted from severe adhesive–abrasive–oxidative wear to mild adhesive and abrasive wear. These findings suggest that the proposed composite bionic texture combined with a graphene coating offers an effective strategy for improving the tribological durability of 20CrMnTi gear.

Lubricants doi: 10.3390/lubricants14080298

Authors: Kang Yang Fang Qian Xue Yin Jun Tang Yulong Shi

To enhance the service life and operational precision of Ti-based aerospace gears, sinusoidal closed-cycle structures containing S (SnCuAg), G (Graphene), A (Al2O3), SG, SA and SGA are fabricated using laser additive manufacturing and high-temperature infiltration. The results reveal that SGA lubricants ensure superior synergistic lubrication. During the wear process, a substantial amount of SGA lubricants continuously exude from the sinusoidal closed structures and migrate to the contact wear interfaces, subsequently accumulating into a well-distributed lubrication film. The lubricants, S, G, A, undergo good plastic deformation, interlayer delamination and rolling friction, respectively. Hence, synergistic lubrication emerges among SG, SA, and SGA. This synergistic effect protects the wear surface and suppresses sliding-induced damage, markedly reducing the sliding resistance of mating pairs, thereby improving the anti-friction and anti-wear abilities of the film. Ultimately, Ti-SGA shows outstanding tribological behavior, achieving a friction coefficient of approximately 0.26 and a wear rate of approximately 2.73 × 10−4 mm3N−1m−1.

Lubricants doi: 10.3390/lubricants14080297

Authors: Abdullah A. Alazemi Abdullah F. Alajmi Sultan M. Al-Salem

The escalating global accumulation of plastic waste (PW) poses a critical obstacle to environmental sustainability, necessitating the development of viable valorization strategies. Pyrolysis process offers a thermo-chemical conversion pathway capable of transforming waste plastics into a potentially functional liquid product. This work presents a systematic and comprehensive investigation of the physicochemical, wettability, thermal stability, rheological, and tribological properties of pyrolysis oil (i.e., pyro-oil) derived from virgin linear low-density polyethylene (LLDPE) pellets to mimic the behavior of common polymers in such thermo-chemical conversion processes. Pyro-oil was produced under controlled slow pyrolysis conditions in a fixed-bed reactor at 700 °C, and its properties were benchmarked against tire-derived pyro-oil and a commercial engine oil. Chemical and structural characterization was conducted via Fourier transform infrared spectroscopy (FTIR), wavelength dispersive X-ray fluorescence (WDXRF), and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS). Wettability was evaluated through temperature-dependent contact angle measurements, and thermal stability was assessed by thermogravimetric analysis (TGA). Rheological profiling of the plastic pyro-oil was investigated using a rheometer apparatus at different temperatures. Lastly, the tribological properties of plastic pyro-oils at various temperatures were examined using a tribometer instrument in a ball-on-disk configuration. Results demonstrate that plastic-derived pyro-oil exhibits a hydrocarbon-dominant chemical composition similar to that of tire pyro-oil and engine oil. Its absolute viscosity is nearly similar to that of the tire pyro-oil and is lower than that of engine oil across different temperatures. Tribological testing revealed that the plastic pyro-oil results in lower friction at elevated temperatures compared to both tire pyro-oil and commercial engine oil. However, the thermal examinations showed that plastic pyro-oil has lower thermal stability than tire pyro-oil and engine oil. Overall, these findings indicate that plastic pyro-oil holds promise as a functional lubricant for low-load and moderate-temperature industrial applications, positioning PW pyrolysis as a viable contributor to circular economy strategies in the lubricants sector.

Lubricants doi: 10.3390/lubricants14080296

Authors: Junze Qian Bowen Zhang Yuhang Dai Yuhang Guo Shijun Zhao Xiang Li Qingda Zhu Meng Zhao Zhenpeng He

As an advanced non-contact dynamic sealing technology, compliant foil seals offer notable advantages, including a simple structure, light weight, ease of installation, and strong self-adaptability. However, in most designs, the foil stiffness is uniform, resulting in a substantial increase in gas leakage under high inlet pressures. Considering the distinct pressure conditions in compliant foil seals, a variable foil thickness model (VTM) along the axial direction is designed to match the pressure gradient from the inlet to the outlet. By aligning the foil thickness variation with the pressure gradient, the foil deformation is more uniformly distributed axially, thereby maintaining low leakage under high-pressure differentials. In this study, the gas film thickness equation and the Reynolds equation for the compliant foil seal are established and solved using the finite difference method combined with a point-wise iterative approach. First, the static characteristics of a traditional uniform-stiffness compliant foil seal under different rotational speeds and inlet pressures are analyzed. The results indicate that leakage increases substantially under high inlet pressure. The performance of the VTM under different operating conditions—including rotational speed and inlet pressure—is investigated. The results show that an appropriately designed VTM can maintain very low leakage under high-parameter conditions, albeit with some sacrifice in gas film pressure and an increase in viscous friction. Furthermore, surface micro-textures are integrated with the VTM. The study finds that the two approaches exhibit complementary effects: micro-textures enhance the dynamic pressure effect of VTM, while the variable-thickness design maintains extremely low leakage. The combined model demonstrates excellent performance across different speeds and inlet pressures. For instance, at a rotational speed of 30,000 r/min, the gas leakage is reduced by 50.04%, and the maximum gas film pressure is increased by 70%.

Lubricants doi: 10.3390/lubricants14080295

Authors: Anuj Khare Georg Jacobs Thao Baszenski Marius Bürger Mattheüs Lucassen Benjamin Lehmann Atharv Deore

Wear occurs in sliding bearings as they often operate under mixed friction conditions in heavy-duty applications. Over the years, this has led to the development of wear simulations to predict wear evolution in sliding bearings. As part of wear evolution, bearing surface topography changes, and the bearing clearance varies due to temperature rise. For realistic wear predictions, both the surface topography changes and thermal effects must be considered. However, existing wear simulations do not accurately account for thermal effects during mixed friction. Effects such as local and transient temperature rise and thermal expansion alter bearing contact conditions (clearance) and thus influence wear evolution. Since these effects are not accurately modelled, realistic wear prediction in sliding bearings remains challenging with current approaches. Therefore, this paper introduces the extension of wear simulations by local and transient modelling of bearing temperature and clearance changes. The novelty lies in enabling the simultaneous consideration of transient thermal changes and surface topography changes during wear evolution. This enables a more realistic prediction of wear development in sliding bearings which can be applied to evaluate wear safety in bearing designs. The thermal wear simulation is validated using experimental measurement of friction torque, bearing temperature, and wear volume obtained from a radial sliding bearing test bench.

Lubricants doi: 10.3390/lubricants14080294

Authors: Zhenhua Liu Xianzheng Liu Haotian Wu Rongji Tang Dongpo Wei

This work characterizes oil film evolution and predicts the hydrostatic-to-hydrodynamic dominance transition in low-speed heavy-duty journal bearings under starved lubrication induced by insufficient inlet pressure, accounting for the thermo-viscous coupling effect. A thermo-viscous coupling model (TVCM) based on the Vogel equation is established and compared with the conventional constant viscosity model (CVM). Analyses are conducted with VG460, VG680, and VG1000 lubricants at rotational speeds of 10–50 rpm. Results show that the CVM systematically overestimates temperature rise and effective viscosity by neglecting the negative feedback among temperature rise, viscosity attenuation, and reduced heat generation, with deviations increasing with rotational speed and lubricant viscosity. Under insufficient oil supply, load-carrying capacity rises rapidly then stabilizes, reflecting the hydrostatic-to-hydrodynamic dominance transition. In the hydrodynamic-dominated stage, a dominance shift between hydrodynamic enhancement and thermal softening is identified: the peak load point marks the switching of dominant factors, and the corresponding critical speed decreases with rising lubricant viscosity. This transition is accompanied by a failure mode shift from global oil film breakdown to localized high-temperature adhesive wear and fatigue spalling. These findings provide a theoretical basis for formulating emergency speed limits and safe operation strategies for journal bearings under insufficient oil supply.

Lubricants doi: 10.3390/lubricants14080293

Authors: Peichen Chu Honglei Zhang Zhao Ding Meng Fang Zhan Su Zhong Tang

Agricultural soil contact components, including ploughshares, rotary blades, and furrow openers, form the active interface between machinery and complex field media. Operating in multiphase environments, these tools face severe abrasive wear, impact fatigue, and interfacial adhesion. These destructive forces irreversibly alter edge geometry and drastically degrade macroscopic operation quality. This review integrates tillage and precision seeding components into a unified tribological framework. It highlights the nonlinear relationship between microscopic material removal and geometric edge retention. Profile degradation is heavily dictated by soil texture, where sandy soils cause micro-cutting, clay soils induce severe adhesion, and gravelly soils produce impact fracture. To predict these complex wear behaviours accurately, coupled multiphysics numerical simulation using the discrete element method for particle flow dynamics and finite element analysis for transient contact stress provides a highly robust methodology. Mitigating these failures requires a functionally zoned surface engineering approach. Carbide hardfacing offers localized abrasion resistance, while polymer composite layers and bionic nonsmooth structures effectively interrupt continuous liquid films in wet cohesive soils. Ultimately, integrating online multidimensional sensing with full life cycle digital-twin models represents the future trajectory for developing adaptive and highly durable agricultural equipment.

Lubricants doi: 10.3390/lubricants14080292

Authors: Xinmeng Song Linqing Bai Yanqiang Hu Ling Ma Hui Cao

Rolling bearings serve as core rotating components in high-end equipment such as aerospace systems, wind turbines, and high-speed electric multiple units, and their service life directly affects the operational reliability and service life of the host machinery. To clarify the research landscape of rolling bearing life prediction, summarize existing prediction techniques, and identify future development trends, this paper systematically reviews the major research advances in this field. The review first traces the evolution of bearing life models, with particular emphasis on the roles of key influencing factors, including stress thresholds, material defects, and lubrication conditions, in their development. Second, it presents a comparative analysis between conventional life calculation methods and those that account for dynamic variations in lubrication conditions, thereby revealing the influence patterns and underlying mechanisms through which surface topography and oil-film characteristics affect fatigue life. Third, it discusses methods for assessing bearing system life, with special attention given to accelerated life testing techniques and bearing condition monitoring approaches. Finally, it summarizes the state of the art in data-driven bearing life prediction and identifies online sensing of lubrication states, system-level reliability design, and improvements in the interpretability and robustness of AI-based prediction models as important future research directions in rolling bearing life prediction.

Lubricants doi: 10.3390/lubricants14080291

Authors: Taili Tian Bo Zhao Chen Wang Xiaotian Zhang Yuyan Fan Peng Xiao

Contact electrification (CE), also referred to as triboelectrification, describes electron transfer occurring at the interface of dissimilar materials. Its microscopic mechanism remains unclarified due to the complex coupling of multiple physical fields, yet the rapid development of triboelectric nanogenerators (TENGs) has rendered CE a prominent research hotspot in tribology on account of its promising application prospects. Metal/polymer combinations have been widely employed for CE research due to their significant differences in electron gain and loss. Nevertheless, most existing studies focus solely on saturated polymers, and systematic comparative analyses between saturated and unsaturated molecular structures are rarely reported. Accordingly, the intrinsic microscopic origin of enhanced interfacial electrification performance induced by unsaturated groups has not been fully understood. In this work, first-principles calculations based on density functional theory (DFT) are implemented to establish interfacial models consisting of an Al substrate and three types of PTFE single chains: fully saturated-PTFE, PTFE with unsaturated bonds at the chain terminus, and PTFE with unsaturated bonds in the middle of the chain. The inherent mechanism governing the modulation of CE behaviors by unsaturated structures are comprehensively revealed from multiple perspectives, including charge transfer, electrostatic potential, and frontier orbital distribution. Computational results demonstrate that unsaturated groups drastically elevate local electrostatic potential and strengthen the electron-trapping capability of molecular chains, thereby substantially boosting CE performance. Moreover, this modulation effect exhibits remarkable position dependence, where unsaturated structures located in the middle of molecular chains deliver better performance improvement than terminal unsaturated moieties. The electron-donating and electron-accepting properties of materials are dominated by the energy level characteristics of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), respectively. This study elucidates the microscopic mechanism of CE at unsaturated polymer/metal interfaces at the molecular scale, and provides theoretical support for optimizing the output performance of TENGs through surface modification strategies.

Lubricants doi: 10.3390/lubricants14080290

Authors: Shengyao Wang Meng Li Yu Cao Yuan Ma Jian Ren

Identifying rolling bearing faults under few-shot conditions remains difficult because fault samples are scarce, class-space distributions are unstable, and inter-class boundaries may become ambiguous. This paper proposes a fault diagnosis model based on the Multi-Aspect Geometric Structure Learning and Discrimination Framework (MAGS-LDF). First, one-dimensional vibration signals are mapped into three two-dimensional representations, namely angle field (AF), band-energy (BE), and time-frequency (TF) images, to describe fault information from temporal correlation, frequency–band energy distribution, and time-frequency response perspectives. Second, a dual-branch feature extraction network is designed to extract fusion features and channel features. For each fault class, channel features are aggregated into channel centers, which are further fused to obtain a public center. Moreover, regular polytope anchor centers aligned with the public-center distribution are introduced to impose geometric constraints, encouraging intra-class compactness and inter-class separation. Finally, channel distances and public distances are jointly used to construct a multi-scale distance discrimination mechanism for few-shot fault classification. Experiments on CWRU and SEU show that MAGS-LDF outperforms the best comparison method by 2.34%, 5.37%, and 5.34% on CWRU and by 3.55%, 5.10%, and 3.46% on SEU under the three-shot, five-shot, and 10-shot settings, respectively.

Lubricants doi: 10.3390/lubricants14080289

Authors: Kirill A. Bashmur Alexander V. Zagulyaev

Normal contact stiffness of textured interfaces is controlled by the load-bearing contribution of deterministic texture and by the nonlinear response of rough load-bearing regions. This study formulates a hybrid deterministic–microlevel model that couples regular relief patterns—including dimples, grooves, periodic ribs and scraped high points—with a micromechanical representation of plateau roughness. Depending on texture topology and scale hierarchy, the microlevel response is represented either by a Greenwood–Williamson (GW) statistical contact model with a smooth elastic–plastic (EP) transition or by a fractal contact model. The deterministic level accounts for open-area fraction, texture depth and load redistribution, and it includes a finite-gauge spectral correction for periodic ribs and grooves to account for the finite measurement window. In a metallic dimple benchmark, the hybrid deterministic-texture/GW–EP formulation yields a mean relative error of 16.3% across all data points in the two selected textured series. In a saturated square-wave benchmark, the finite-gauge spectral correction yields a mean relative error of 10.37% for the independent patterned points. A compliance-based topology criterion is then established to determine, from open-area fraction, element depth, applied load and the ratio between texture period and plateau-roughness spacing, whether stiffness is governed primarily by deterministic texture, by micro-roughness or by their coupled response. The resulting formulation supports early-stage design exploration without requiring a full three-dimensional contact calculation at every parameter point. Independent periodic three-dimensional checks for circular-dimple cells showed that the finest FE solution agreed with the spectral prediction of the deterministic normal approach within 1.2%; the correction estimated by direct unilateral BEM changed total stiffness by no more than approximately 5% in the tested texture-influenced case.

Lubricants doi: 10.3390/lubricants14080288

Authors: Yuki Yoshimura Shodai Sakabe Yuki Kawamoto Akihiko Azetsu Masayuki Ochiai

Improving motor efficiency by reducing frictional losses in the journal bearings used in many rotating machines is critical for advancing sustainable mechanical systems, especially automobiles. This study investigates the application of microbubbles, which are known to reduce frictional drag in fluids. The incorporation of microbubbles into lubricants offers an environmentally friendly friction-reduction method that avoids chemical additives. In this work, microbubbles were generated within lubricating oil and applied to a journal bearing. Experimental measurements of friction torque during shaft rotation demonstrated that lubricating oil containing microbubbles yielded lower torque than oil without microbubbles. These results indicate that microbubbles effectively reduce friction in journal bearings. Furthermore, a transparent quartz glass bearing and acrylic shaft were used to directly observe the dynamics of microbubbles flowing within the journal bearing clearance to elucidate the underlying frictional torque reduction mechanism.

Lubricants doi: 10.3390/lubricants14080287

Authors: Varatharajulu Muthukrishnan Muthukannan Duraiselvam

Tribology is a growing field concerning reductions in the environmental footprint of tribological systems while increasing their operational efficiency by minimizing friction, wear and lubrication in an environmentally conscious manner. This review aims to discuss in detail sustainable lubrication strategies, with particular emphasis on surface modification and coating-based strategies for advanced tribological applications. The surface engineering techniques covered in this study are critically reviewed with regard to their ability to improve wear resistance, reduce friction and increase the durability of components, such as laser surface texturing (LST), nitriding, plasma treatment and advanced coating technologies such as DLC-, TiN-, CrN- and PVD-based coatings. Another key focus is on the synergy of sustainable lubricants and engineered surface coatings. The tribochemical compatibility of eco-friendly lubricants, bio-lubricants, ionic liquids and advanced coated surfaces is explained in detail, because they are essential for the formation of stable tribofilms, lubricant retention, reductions in surface degradation, and the minimization of tribological losses. They are an important component of environmental footprint, energy consumption and the thermal stability and service life of tribological components. Recent developments in coating-assisted green tribology, sustainable tribochemistry and circular sustainability concepts of the design of tribological systems are also included in the review. In addition, the review briefly outlines the use of Life Cycle Assessment (LCA) as a potential tool for the future evaluation of the environmental sustainability of tribological systems. The proposed expanded set of Green Tribology principles offers useful guidance for the implementation of tribology-based solutions toward more sustainable engineering systems and responsible resource utilization.