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.

Lubricants doi: 10.3390/lubricants14080286

Authors: Ibrohim Madatov Qiang Li Valentin L. Popov

The superposition of normal and tangential oscillations in frictional contacts can fundamentally alter the macroscopic friction law and generate directed motion and force. In this work, we investigate the transition between friction reduction, dynamic ratcheting, and vibrational actuation within a unified numerical framework based on a compliant Coulomb friction contact. The system is subjected to simultaneous harmonic oscillations in the normal and tangential directions with an arbitrary phase shift. First, the limiting cases of purely normal and purely tangential oscillations are revisited, demonstrating that both produce equivalent friction–reduction behavior when expressed in terms of appropriate dimensionless parameters. For sufficiently large tangential oscillation amplitudes, a transition to a bidirectional stick-slip regime is identified, characterized by alternating forward and backward motion within a single oscillation cycle. The general case of dual-mode excitation is then analyzed over a broad parameter range. Numerical simulations show that the macroscopic friction coefficient is governed by four dimensionless parameters: the normalized sliding velocity, the normal oscillation ratio, the tangential oscillation parameter, and the phase shift between the oscillation modes. The combined oscillations break the symmetry of the friction law with respect to the direction of motion, resulting in different critical velocities and friction coefficients for positive and negative sliding directions. Depending on the parameter combination, the system exhibits three distinct operational regimes: active friction control, dynamic ratcheting, and vibrational actuation. In the latter regime, the effective friction coefficient becomes negative, indicating a conversion of oscillatory energy into directed mechanical work. The results provide a unified physical interpretation of oscillation-induced transport and force generation in frictional contacts and establish general design principles for vibration-assisted friction-control systems, dynamic ratchets, and oscillatory actuators.

Lubricants doi: 10.3390/lubricants14080285

Authors: Jiabao Li Zhonggang Xiong Zhong Liu Xintao Liu Sibo Liu Cong Guo Xingyu Zhou Wenqiang Hu

In axial piston pumps under variable loading, the system-level excitation and local tribological responses of the slipper pair can become temporally and spatially decoupled. The scientific contribution of this paper is a phase-consistent chain that achieves three things: it resolves the central-pocket pressure boundary with a dual-orifice and dual-control-volume model instead of directly imposing piston chamber pressure, propagates this boundary through posture-dependent clearance to three-dimensional flow–thermal and single-slipper structural responses within the same local cycle, and screens candidate high-risk regions from the spatial proximity and phase relationship of multi-field cycle envelopes rather than from a single peak. The results show that the central pocket pressure exhibits peak attenuation, peak-time difference, and pressure-rate weakening relative to the piston chamber pressure. The steady peak attenuation ratio is 2.83–3.33%, while pressure-rate weakening under variable loading is 6.29–7.14%; the high-to-low unloading case gives the largest attenuation of 4.81%. Increasing steady load reduces the tilt amplitude and raises the minimum film thickness from about 13.024 to 13.452μm, but the maximum temperature rise increases from 34.12 to 65.01K. A 10% cycle-envelope projection shows no common overlap among the film-thinning, oil-film temperature-rise, and structural-stress core high-response regions with pairwise overlap ratios of 0–3.27%. This traceable chain supports comparative lubrication-safety screening; the identified zones remain numerical candidates rather than experimentally confirmed wear or failure regions.

Lubricants doi: 10.3390/lubricants14080284

Authors: Jinhui Chen Guoqiang Gao Rong Fu Qingsong Wang Tianwei Lan Pengyu Qian Guizao Huang Hong Wang Guangning Wu

The pantograph–catenary system is the critical pathway for energy collection for high-speed trains, and its interfacial state directly affects current-collection quality and operational safety. Due to the coupled effects of multiple factors, the friction coefficient at the interface exhibits significant nonlinearity, time variability, and stochastic fluctuations, posing substantial challenges for friction-behavior prediction. To improve the prediction accuracy and generalization capability of friction-coefficient models under complex current-carrying conditions, a CNN-LSTM model optimized by a physics-informed Sparrow Search Algorithm, namely PISSA-CNN-LSTM, is proposed in this study. Based on current-carrying friction tests, the effects of current, contact load, and sliding speed on the dynamic evolution of the friction coefficient are analyzed. The physics-based regularities associated with operating conditions are further incorporated into the SSA-based hyperparameter optimization process, enabling directed optimization under physical constraints. The results show that PISSA-CNN-LSTM outperforms CNN-LSTM and SSA-CNN-LSTM in prediction accuracy, convergence speed, and optimization efficiency. The test-set R2 reaches 0.9904, and the optimization time is reduced by 49.38% compared with SSA-CNN-LSTM. This work provides a more accurate, robust, and interpretable modeling approach for predicting pantograph–catenary interfacial friction behavior under complex current-carrying conditions.

Lubricants doi: 10.3390/lubricants14080283

Authors: Aaron Mora Luis Chandía Nicolás Landero Christopher Salvo Nicolás Araya Claudio Aguilar Guilherme Oliveira Neves

Self-lubricating iron-based composites produced by powder metallurgy are promising materials for reducing friction and improving component durability under dry-sliding conditions. In this study, three composites reinforced with graphite and/or graphene oxide (Fe10%Gr, Fe10%GO, and Fe5%Gr5%GO) were fabricated to evaluate the influence of solid lubricants on densification, hardness, and tribological behavior. Powders were uniaxially compacted at 500 MPa and low-temperature-sintered at 880 °C under an argon atmosphere. The tribological performance was assessed by pin-on-disk tests against an AISI 52100 steel ball. Pure Fe exhibited the highest hardness (approximately 97 HV) and a final porosity of 17.5%, whereas the lubricant-containing composites showed porosities ranging from 17.3% to 22.8% and lower hardness values of 45–60 HV. Despite the reduction in hardness, all lubricant-containing compositions decreased the coefficient of friction relative to pure Fe (~0.33). Fe10%Gr and Fe5%Gr5%GO exhibited the lowest friction coefficients, reaching ~0.08 and ~0.05, respectively, while Fe10%GO showed the highest wear rate (~1.1 × 10−2 mm3/Nm). Among the compositions studied, Fe5%Gr5%GO showed the best friction–wear balance among the self-lubricating formulations, combining low friction and moderate wear due to a surface synergy where graphite promotes continuous low-shear lubrication while GO promotes the formation of a more compact and resilient protective tribolayer.

Lubricants doi: 10.3390/lubricants14070282

Authors: Qianzhi Wang Haofeng Yin Zhifeng Zhou

The positive effect of paraffin on lowering the temperature, friction coefficient, and wear mass of a circular-textured surface has been proved. On this basis, this study is meant to reveal the influence of surface density and diameter of circular texture on the lubricating effect. It was found that the friction coefficient and wear kept decreasing as surface density increased from 6.8% to 24.2% due to increasing dynamic pressure up to 285 Pa. However, as surface density further increased to 40.1%, the tribological properties were weakened due to high contact stress from a smaller contact area. On the basis of the optimized surface density of 24.2%, the friction coefficient and wear kept decreasing due to increasing dynamic pressure up to 326 Pa as the diameter of the circular texture increased from 400 μm to 550 μm. When the diameter of the circular texture increased to 650 μm, the tribological properties of the textured surface were weakened. Compared with a depth of 500 μm, a depth of 1000 μm could store more paraffin, providing better temperature-reducing and lubricating effects. Ultimately, the minimum temperature of 40.78 °C, minimum friction coefficient of 0.063 ± 0.003, and minimum wear loss of 0.70 ± 0.10 mg were obtained under the optimal texture surface density, diameter, and depth of 24.2%, 550 μm, and 1000 μm, respectively.

Lubricants doi: 10.3390/lubricants14070281

Authors: Filimonas Kaliafetis Daniele Dini James P. Ewen Suhaib Ardah

A unified transient deterministic lubrication model is developed for the analysis of rough, starved, and coated contacts within a single, fully-coupled numerical framework capable of resolving boundary, mixed, and full-film lubrication regimes. The model is formulated with the finite volume method on a curvilinear grid and extends conventional full-film formulations through the introduction of a semi-system methodology, enabling robust treatment of complex multi-regime conditions. A key distinguishing feature of the framework is the direct resolution of thermal effects within both the lubricant and solid domains through solution of the energy equation. Unlike many existing mixed lubrication models that rely on analytical temperature approximations, the present approach captures transient, asperity-scale temperature evolution explicitly, allowing accurate representation of local thermo-mechanical interactions. Two case studies are presented to demonstrate the capabilities of the model. The first examines transient starvation in rough contacts with isotropic sinusoidal topographies of varying wavelength, as well as random machined surfaces, revealing a strong dependence of lubricant entrainment, asperity interaction, and localised heating on surface morphology. The second study investigates the role of coating thermal properties under transient starved conditions, demonstrating strong coupling between heat transport, viscosity variations, and frictional response. Overall, the proposed framework provides a robust and physically consistent platform for the simulation of transient lubrication phenomena under realistic operating conditions, enabling detailed insight into roughness, starvation, and thermal effects across regimes using a fully-coupled approach.

Lubricants doi: 10.3390/lubricants14070280

Authors: Elena Mihalcea Jorge Chávez Omar Jiménez Martín Flores Francisco Alvarado-Hernández Juan Pablo Camarillo-García Horacio Flores-Zúñiga Marco Aurelio González-Albarrán Luis Olmos

Tantalum (Ta) is an excellent alternative for structural orthopedic implants because of its high biocompatibility and chemical stability, but its melting point of 3020 °C severely complicates conventional casting. To address this, the study evaluates spark plasma sintering (SPS) to consolidate pure Ta powders at 1450 °C and 50 MPa and analyzes the impact of heating rates (50–200 °C/min) on microstructural, mechanical, wear, and corrosion properties. Results indicate that heating rate dictates final densification: a 50 °C/min rate achieved 98.59% relative density, whereas 200 °C/min yielded only 82.28% due to reduced thermal exposure. Sintering involved dislocation creep and viscous flow mechanisms, with X-ray diffraction confirming a stable α-Ta matrix across all samples. Mechanically, the 50 °C/min samples achieved a maximum microhardness of 285 HV, whereas higher porosity at 200 °C/min reduced hardness by 27.8%. Wear testing showed a two-stage friction evolution: an initial Ta2O5 solid-lubricating effect, followed by predominant abrasion and adhesion, with stable wear rates (3.2 to 3.6 × 10−3 mm3/N·m) for dense specimens. Finally, tests in simulated body fluid confirmed spontaneous self-passivation. However, the corrosion rate increased with heating rates, indicating that the resulting porosity adversely affects the material’s surface response.

Lubricants doi: 10.3390/lubricants14070279

Authors: Tianyi Yu Zhenming Liu Zhifei Dang Guifeng Liu Baiqi Huo Mei Li Jingbin Liu

When predicting cavitation erosion risk in practical engineering bearings, traditional vapor cavitation models neglect the effect of dissolved air in the lubricant and fail to account for its precipitation and dynamic evolution in local low-pressure regions, leading to deviations in the prediction of cavitation extent, intensity, and distribution. Taking the main bearing of a certain type of diesel engine as the research object, a coupled cavitation model combining the Schnerr–Sauer vapor cavitation model and a Henry’s law-based gas dissolution model is established. Together with large eddy simulation (LES), the accuracy of the numerical model is verified by constructing a visualization experimental platform. Numerical analysis is then carried out to investigate the effect of dissolved air in the lubricant on the cavitating flow field of the bearing oil film under initial conditions ranging from undersaturated to saturated states (air mass concentration of 0–0.4 g/L). The results show that as the mass concentration increases, the amount of air precipitation in the near-wall region gradually increases and extends downstream. When the mass concentration reaches 0.1 g/L, the precipitated air mass reaches 50% of the vapor cavitation mass, significantly expanding the cavitation range. In the critical mass concentration range of 0.24–0.28 g/L, intense air precipitation occurs inside the oil hole and covers the entire region. Through the synergistic effects of inhibiting high-speed jets, altering the local pressure field, and competing for cavitation nuclei, the precipitated air significantly suppresses the development of near-wall vapor cavitation, and the vapor cavitation mass decreases by 83.1% at saturation. In summary, within the practical mass concentration range, the presence of dissolved air significantly enhances the gas phase intensity near the bearing bush surface and expands the cavitation range, thereby exacerbating the risk of cavitation erosion damage in this region, while having little effect on the oil supply performance of the oil hole.

Lubricants doi: 10.3390/lubricants14070277

Authors: Saša Milojević Slavica Miladinović Sandra Gajević Stefan Čukić Blaža Stojanović

The use of aluminium alloys in internal combustion engines is an effective strategy for increasing energy efficiency, reducing component mass, and lowering harmful gas emissions. This paper analyses different types of Al–Si alloys (hypoeutectic, eutectic, and hypereutectic) in the context of their use in the production of engine blocks, pistons, and cylinder liners. Special attention is given to the tribological challenges of using aluminium, such as increased wear and lower strength compared to traditional materials such as cast iron. Modern methods for improving wear resistance are discussed, including alloying, the application of solid lubricants and protective coatings (Ni–SiC, Al2O3, MoS2, etc.), and advanced surface engineering technologies such as PTWA, HVOF, and laser alloying techniques. This paper also presents practical examples of industrial applications, highlighting their essential role in the development of lightweight, high–performance internal combustion engines. This review highlights that the combination of optimized alloy composition and advanced surface engineering technologies represents the most effective approach for enhancing the durability and tribological performance of lightweight engine components.

Lubricants doi: 10.3390/lubricants14070278

Authors: Ahmed Saleh Georg Jacobs Wenxi Chen Mattheüs Lucassen Benjamin Lehmann

This study proposes a model-based method for real-time monitoring of the lubrication regimes in dynamically loaded radial sliding bearings using Physics-Informed Neural Networks (PINN). The proposed method replaces computationally intensive elastohydrodynamic lubrication (EHD) simulations with a PINN-based surrogate model. The model predicts hydrodynamic pressure and lubricant film-thickness distributions with comparable accuracy under dynamically varying operating conditions, enabling reliable assessment of lubrication regimes. The proposed model advances the state of the art in physics-informed modelling of mixed lubrication by extending existing approaches to simultaneously account for mixed-friction regimes through the Greenwood–Tripp contact model, transient operating conditions, and bearing surface deformation. Using only the bearing load and shaft rotational speed as inputs, the resulting hydrodynamic pressure field and corresponding lubricant film thickness can be monitored, enabling the direct assessment of the lubrication regime and potential wear risk. The proposed method is applied to a validated EHD model of a 30 mm sliding bearing test rig, where EHD simulation results are used to train, validate, and evaluate the model. The proposed framework achieved an average lubricant film-thickness prediction error of 2.34% and lubrication-regime classification errors of 7.8% and 8.2% for the static and dynamic validation cases, respectively. Furthermore, the computation time for the complete 18-time-step load case was reduced from approximately 35 h to 61.2 ms.

Lubricants doi: 10.3390/lubricants14070276

Authors: Yanan Zhang Baolin Jia Hongjian Wu Xinlong Wu Yonggang Meng Chuke Ouyang

The effect of different concentrations of oleic acid additive on the yield behavior of a magnetorheological fluid (MRF) was investigated from the perspective of tribology. Different concentrations of oleic acid (OA) influence the magnetorheological effect by altering the lubricating properties of the base carrier liquid. Good lubricity of the carrier fluids led to lower shear stresses. The friction between the particles and that between the particles and plates were a part of the shear stress. The structural evolution was promoted by both lubrication and shearing. The shearing thinning of the MRF was delayed under good lubricity, and the particles were easier to roll. The friction forces, along with Brownian forces, Stokes forces, and magnetic attraction, affect the structural evolution of MRF. This study provides a new theory for the formulation design of magnetorheological fluids, reducing design costs and improving production efficiency.

Lubricants doi: 10.3390/lubricants14070275

Authors: Jianfeng Li Meng He Fei Wang Ziang Ge

The accessory gearbox of an aero-engine operates under high-speed and heavy-load conditions, where insufficient lubrication may lead to oil-film failure, increased frictional losses, and reduced transmission reliability. Therefore, understanding oil-jet injection and tooth surface oil-film spreading characteristics is essential for improving lubrication performance. In this study, a three-dimensional geometric model incorporating the meshing region and oil nozzles was established based on a typical accessory gear pair. The model employs the VOF multiphase flow approach and the standard k-ε turbulence model, coupled with dynamic mesh techniques to accurately capture the transient interactions between gear rotation and oil–air two-phase flow. Numerical simulations reveal the dynamic evolution of oil injection, impingement on the tooth surface, oil-film spreading, and transport into the meshing zone, while the effects of injection velocity and nozzle length on lubrication performance are quantitatively analyzed. Results indicate that an injection velocity of 45–55 m/s yields optimal oil-film coverage and uniformity, and a nozzle length of h = 30 mm minimizes jet energy decay and airflow interference, achieving uniform oil filling in the meshing zone. The optimal lubrication performance for accessory gears is obtained at an injection velocity of 45–55 m/s and a nozzle length of 30 mm. This study provides a reference for the design optimization of accessory gear lubrication systems.

Lubricants doi: 10.3390/lubricants14070274

Authors: Yijin Sui Pengfei Xing Guobin Li Hongpeng Zhang

Electrical slip rings are key components for power and signal transmission in rotating equipment, and degradation of sliding electrical contact is a major factor limiting their reliability. To analyze the sliding electrical contact behavior of slip rings, an elastoplastic contact framework incorporating thermal–mechanical–electrical coupling is developed. The semi-analytical method combined with discrete convolution-fast Fourier transform is employed to efficiently solve the coupled contact problem, while J2 flow theory and radial return algorithm are adopted to determine plastic deformation. Based on the proposed model, the elastoplastic sliding electrical contact behaviors of smooth and sinusoidal surfaces are systematically investigated. The results show that plastic deformation increases the contact area, thereby reducing the electrical contact resistance, current density at the contact edge, and maximum temperature rise, although it may induce the residual stress. Reducing the asperity height of sinusoidal surfaces while maintaining multiple discrete micro contact spots can effectively lower the electrical contact resistance and interfacial temperature rise. The proposed model provides a useful theoretical tool for evaluating the thermal–mechanical–electrical performance of sliding electrical contact in slip rings.

Lubricants doi: 10.3390/lubricants14070273

Authors: Lai Hu Jun Wang Chen Yin

High-quality equipment in aerospace, energy, transportation, and advanced manufacturing sectors is increasingly required to operate with higher speeds, loads, precision, and autonomy [...]

Lubricants doi: 10.3390/lubricants14070272

Authors: Min Wang Guo-Jun Qin Ming Liu

Oxidative degradation stands as the principal cause of gear oil failure and transmission system malfunctions in wind turbines. Electrochemical impedance spectroscopy (EIS) offers a novel technical avenue for the condition monitoring of gear oil. This research centers on the evolution mechanism of electrochemical properties during the oxidative degradation process, utilizing high-viscosity gear oil commonly employed in wind turbines as the research subject. Through a combination of accelerated oxidation tests, broadband EIS measurements, and equivalent circuit fitting, the study examines the variations in the electrochemical response of gear oil with respect to oxidation temperature and duration. The findings reveal that oxidative degradation does not modify the single-relaxation dielectric characteristics of the gear oil; however, various electrochemical parameters undergo systematic evolution. Following oxidation at temperatures ranging from 90 to 120 °C, the charge transfer resistance escalates by approximately 5.9-fold; the base resistance diminishes by 10% to 20%; both the admittance constant and dispersion index of the constant phase element (CPE) exhibit changes of less than 5%, indicating that the system retains its capacitive properties. During constant-temperature oxidation at 90 °C for durations spanning 50 to 175 h, the charge transfer resistance increases in an approximately linear fashion with oxidation time, while the base resistance continues to decline, and the CPE parameters remain largely stable. Various electrochemical parameters evolve monotonically with the extent of oxidation, with charge transfer resistance demonstrating the highest sensitivity to thermal oxidation and thus serving as a pivotal indicator for evaluating the degree of thermal oxidative degradation in gear oil. This study lays an experimental foundation for the application of EIS technology in the realm of online gear oil monitoring.

Lubricants doi: 10.3390/lubricants14070271

Authors: Xuze Wu Qingyun Ye Guo Li Chunyuan Shi Wen Liu Hang Wang Yu Chen

Joint clearances inevitably deteriorate kinematic accuracy and operational stability of heavy-load mechanical presses. Most existing studies focus on single-type clearances or uniform contact states, failing to reveal the coupled dynamic behavior of multi-type clearances with differentiated lubrication conditions. This work develops a five-degree-of-freedom coupled dynamic model for a double-crank mechanical press, integrating hydrodynamic lubricated revolute joints and dry contact–impact translational joints. Nonlinear dynamic responses under varying clearances, driving speeds and contact regimes are systematically analyzed. Results show that moderately enlarged translational clearance improves positioning accuracy by suppressing oil film whirl-induced chaos; lubricated revolute joints effectively isolate high-frequency impact energy via squeeze-film damping, and the system exhibits non-monotonic dynamic characteristics with speed. This study provides a theoretical basis for clearance matching and operating optimization of mechanical presses.

Lubricants doi: 10.3390/lubricants14070270

Authors: Nikolay Ovcharenko

Thermal kinematics in two-layer systems, analogous to those employed in plain bearings, are investigated. The systems are based on a PEEK/CF30 composite, considered an established alternative to traditional babbitt alloys. Experimental modelling of transient and quasi-steady-state heat transfer regimes was performed on a test bench implementing a one-dimensional heat conduction model within multi-layered media. Samples included babbitt alloy and polymer coatings with thicknesses of 0.40, 0.46, and 1.92 mm. A phenomenological model of heat transfer regimes is proposed, encompassing five sequential phases that represent a complete operating cycle of plain bearing temperature traces. The observed constants of temperature deviations and phase lags are discussed, including their application as instrumental invariants. It was established that reducing the antifriction layer thickness to 0.4 mm significantly lowers its thermal resistance and decreases the time lag by a factor of 5–8. This brings the system response time close to values characteristic of classical babbitt alloys. It is demonstrated that in steady-state hydrodynamic friction regimes, the temperature deviation for thin layers is less than 1 °C, obviating the necessity for algorithmic data compensation. The findings confirm the safety of using polymer materials as the working layer in plain bearings. Despite their lower thermal conductivity and increased time lag, the composites’ high thermal stability margin comfortably compensates for potential temperature deviations, ensuring equipment reliability and safety.

Lubricants doi: 10.3390/lubricants14070269

Authors: Fucong Liu Faqiang Wen Baokaidi Tian Lei Yu Tianxiang Yu Min Li Yixin Geng Sai Lou

Accurate tool wear prediction is essential for predictive maintenance in complex machining processes, but non-stationary sensor signals make it difficult for a single model to capture both long-term degradation trends and local transient disturbances. This study introduces a residual-compensated Hybrid CNN-Informer + LightGBM framework for tool wear prediction. The workflow first preprocesses multi-source sensor signals and selects wear-sensitive statistical descriptors to guide FiLM-based deep feature modulation. A CNN-Informer backbone then estimates the main wear trend by combining local feature extraction with long-range temporal modeling, and a LightGBM module performs secondary compensation on the remaining prediction residuals. On the PHM 2010 milling benchmark, the proposed framework achieved an RMSE of 4.0016, MAE of 2.8271, and R2 of 0.9870, reducing RMSE and MAE by 47.9% and 48.5% compared with a standard Transformer. Ablation results showed that both the CNN branch and residual compensation contributed to the final accuracy. External validation on the HMoTP dataset using an independent held-out tool further yielded an RMSE of 9.1111, MAE of 7.3522, and R2 of 0.9729. These results indicate that separating main-trend learning from residual correction provides a practical strategy for robust tool wear prediction under the tested machining conditions.

Lubricants doi: 10.3390/lubricants14070268

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

To address the limitations of conventional magnetorheological (MR) fluid hydrostatic bearings, a variable-stiffness bearing is proposed. These limitations include insufficient load capacity, low torque margin, and poor adaptability under complex operating conditions. Taking MR fluid as the lubricating medium, the device adopts permanent magnets to provide a basic stable magnetic field. A dual-excitation configuration, consisting of stiffness-adjustment coils integrated into the bearing and shaft-mounted coils attached to the rotor, enables dynamic magnetic field regulation. This mechanism modulates the rheological behavior of the MR fluid, realizing flexible stiffness tuning and dynamic torque enhancement of the bearing. The overall structure and working principle of the device are elaborated in detail. The mathematical models of bearing stiffness-current and output torque-current are derived, and the regulation law of current on the dynamic characteristics of the bearing is clarified. Ansys Maxwell 2022 R1 simulation results verify the feasibility of the dual excitation decoupling control scheme. The research results can provide theoretical support and technical reference for the intelligent regulation and engineering application of MR fluid hydrostatic bearings.

Lubricants doi: 10.3390/lubricants14070267

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

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

Lubricants doi: 10.3390/lubricants14070266

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

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

Lubricants doi: 10.3390/lubricants14070265

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

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

Lubricants doi: 10.3390/lubricants14070264

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

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

Lubricants doi: 10.3390/lubricants14070263

Authors: Fulya Cemaloğlu Barış Özlü Halil Demir Fuat Kara

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

Lubricants doi: 10.3390/lubricants14070262

Authors: Yoga Palani Hao Li Deli Peng Jingyi Zhang

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

Lubricants doi: 10.3390/lubricants14070261

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

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

Lubricants doi: 10.3390/lubricants14070260

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

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

Lubricants doi: 10.3390/lubricants14070259

Authors: Xiuli Liu Changyao Zhang Lingfeng Gao Jing Liu

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

Lubricants doi: 10.3390/lubricants14070258

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

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

Lubricants doi: 10.3390/lubricants14070257

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

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

Lubricants doi: 10.3390/lubricants14070256

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

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

Lubricants doi: 10.3390/lubricants14070255

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

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

Lubricants doi: 10.3390/lubricants14070254

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

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

Lubricants doi: 10.3390/lubricants14070253

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

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

Lubricants doi: 10.3390/lubricants14070252

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

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

Lubricants doi: 10.3390/lubricants14070251

Authors: Huajie Qu Meiqin Liang Zhongpu Wen

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

Lubricants doi: 10.3390/lubricants14070250

Authors: Gaobo Lou Xiaoling Yao Yuhao Fang Yifan Chen

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

Lubricants doi: 10.3390/lubricants14070249

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

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