Lubricants doi: 10.3390/lubricants14040160

Authors: Huaiqing Lu Chunlin Li Haibo Liang Zhuxin Tian

Cavitation of oil film is harmful for all kinds of hydrostatic bearings, so the method to avoid cavitation in hydrostatic thrust bearings by optimizing the structure of bearings is proposed in this study. Based on the pressure distribution expressions of two kinds of hydrostatic thrust bearings (circular recess and no recess), considering the boundary conditions of pressure distribution, the threshold conditions to avoid cavitation are obtained. The reliability of these threshold conditions is illustrated by applying the threshold conditions to calculate the pressure distributions of hydrostatic thrust bearings. For hydrostatic thrust bearings with a non-dimensional recess radius larger than 0.607, an appropriate choice on the value of film thickness ratio could effectively avoid cavitation. If the non-dimensional recess radius of bearings is less than 0.607, the inertial parameter has a threshold value determined by recess radius, radius of supply hole, and film thickness ratio. For the bearing with no recess, the threshold value of inertial parameter is only determined by the radius of supply hole. And the results in this study could be applied for the design of hydrostatic thrust bearing to avoid cavitation.

Lubricants doi: 10.3390/lubricants14040159

Authors: Jie Tang Rongxue Huang Sheng Huang Yujie Qin Hao Fan

With the changing demands of society, gears, as fundamental components of mechanical devices, are evolving towards higher reliability and longer service life. To address the issue of thermal scuffing at the gear meshing interface, we propose the introduction of micro/nano-textures to improve the thermal elastohydrodynamic lubrication characteristics of the meshing surfaces, thereby enhancing the lubrication performance and anti-scuffing load capacity of the gear surfaces. First, finite element models with different microstructural features were established. Then, numerical calculations were conducted using computational fluid dynamics (CFD) software to analyze the impact of various micro-texture configurations on the lubrication performance of the tooth surface. Finally, an orthogonal experiment was performed to optimize the groove length, groove width, and areal density of the micro-textures in order to obtain the best processing parameters. The results show that, compared with the triangular, rectangular and trapezoidal micro-textures, the wedge-shaped micro-texture produces the largest pressure difference at the meshing-in and meshing-out points of the texture grooves, which causes the dynamic pressure effect to be more obvious. Compared with the triangular, rectangular and trapezoidal micro-textures, the wedge-shaped micro-texture has the largest bearing capacity and the smallest friction coefficient, so it has better bearing capacity and anti-friction and wear performance. The process parameters were optimized through orthogonal experiments, and the optimal combination of process parameters was obtained as the areal density of 50%, the depth of micro-pits of 12 µm, and the width of micro-pits of 200 µm. Under these optimal parameters, the pressure difference at the meshing-in and meshing-out points of the wedge micro-texture increased significantly by 255.6% compared to the initial model, and the oil film friction coefficient decreased by 17.857% relative to the initial model. These results demonstrate that the micro-texture with optimal parameters significantly enhances the lubrication and anti-friction/wear performance of the tooth surface.

Lubricants doi: 10.3390/lubricants14040158

Authors: Aleksandr Semenov Dmitriy Tsyrenov Nikolay Ulakhanov Irina Semenova Undrakh Mishigdorzhiyn Wen Ma Simon C. Tung George E. Totten

The design and main parameters of a plasma–chemical reactor containing two compartments are presented. One compartment houses a vacuum-arc evaporator, while the other houses a planar magnetron. The compartments are separated by a diaphragm with a dosing slot for injecting copper vapor into the TiN synthesis compartment. The conditions for the synthesis of superhard TiN-Cu composite coatings are experimentally determined. Based on established process parameters for TiN synthesis in a nitrogen-containing plasma by Ti evaporation using a vacuum-arc discharge, it is proposed to apply TiN-Cu coatings by injecting Cu vapor into the TiN synthesis area and sputtering Cu using a magnetron discharge. XRD analyses of both TiN and TiN-Cu coatings show the presence of WC, Ti2C, and TiN. EDS analysis confirms 5.57 at. % copper on the surface of the TiN-Cu coating. Real-life operating tests of TiN-Cu coatings on replaceable WC-TiC-Co (79/15/6 wt.%) alloy hexagonal inserts used for cutting 40Kh steel revealed that applying the TiN-Cu coating extends the tool life of WC-TiC-Co inserts by about 2.5 times compared with uncoated tools. Cutting force measurements on TiN-Cu-coated inserts showed no vibration or noise during cutting, driven by a reduced friction coefficient and improved heat dissipation at the contact zone between the cutting edge and the workpiece, thereby lowering the temperature in that area.

Lubricants doi: 10.3390/lubricants14040157

Authors: Xu Liu Linye Yu Ming Zhong Jin Qian Jiapeng Dai Yongan Min

Starved lubrication poses a critical challenge to hybrid ceramic bearings operating under severe conditions. This study investigates the tribological behavior of carburized 20CrMo steel sliding against Al2O3 ceramic balls and GCr15 steel balls under dry sliding, with oil-lubricated tests as a reference. Under oil lubrication, the 20CrMo/Al2O3 pair exhibits superior wear resistance, attributed to the high hardness of the ceramic counterpart. Under dry sliding, however, this pair shows a slightly lower friction coefficient but a wear rate approximately three times that of the 20CrMo/GCr15 pair. This counterintuitive behavior stems from two mechanisms: lower contact stress and friction-induced work hardening in the GCr15 pair, which together suppress wear. Further analysis reveals that secondary carbides in the carburized layer detach under repeated high shear stress, acting as hard third-body abrasives and accelerating surface damage. These findings highlight that hybrid ceramic bearings are more susceptible to lubrication failure than all-steel bearings. Under heavy loads and poor lubrication, residual compressive stress plays a key role in governing the tribological behavior of carbides on carburized surfaces.

Lubricants doi: 10.3390/lubricants14040156

Authors: Ruiran Ma Jiashuo Zhang Ming Feng Zhixin Jia Jin Wang

With the widespread use of high-speed motorized spindles in precision machining, conventional contact loading methods are no longer adequate for stiffness loading tests under high-speed operating conditions. Non-contact loading technology based on a partial-arc aerostatic radial bearing offers an effective alternative. In this study, a CFD-based hydrodynamic model was developed for the gas-film flow field in a partial-arc aerostatic radial bearing. The effects of bearing geometric parameters, such as chamber configuration, supply-orifice structure, and eccentricity, on loading characteristics were investigated. The influence of hydrodynamic effects under high-speed rotation on the loading force stability and stiffness-testing accuracy was analyzed, and an asymmetric shallow–deep composite chamber design was proposed to mitigate these effects. The results indicate that the partial-arc aerostatic radial bearing, designed based on both static characteristics and rotational performance analysis, can effectively suppress hydrodynamic effects and improve loading force stability and stiffness-testing accuracy.

Lubricants doi: 10.3390/lubricants14040155

Authors: Max Marian Stephan Tremmel

Tribology, the science of friction, wear, and lubrication, remains fundamental to modern engineering systems [...]

Lubricants doi: 10.3390/lubricants14040154

Authors: Shashivar Syla Kim Marius Brill Stefan Paulus Simon Graf Oliver Koch

A multibody simulation model for deep-groove ball bearings is presented. The model considers friction in both the raceway and cage contacts, resulting from radial and axial loads. The model is validated against experimental measurements for a 6319 bearing under oil-bath lubrication over a speed range of 500–3000 min−1 and two load ratios (C/P=10 and 6.5). Predicted friction torques show good agreement with measurements, with deviations between 5.5% and 22% at moderate speeds. In addition, electrical contact capacitances are calculated for a 6208 bearing and compared with an analytical approach, showing deviations in the range of 10–14%. Beyond friction prediction, the fully dynamic approach enables time-resolved analysis of roller kinematics and the identification of instability limits under axial excitation. The developed tool therefore enables reliable bearing loss prediction, supports efficiency-oriented drivetrain design, and provides a basis for electro-tribological and stability investigations.

Lubricants doi: 10.3390/lubricants14040153

Authors: Congrui Xu Wei Cao Yang Yan Letian Ding Yifan Wang Rongrong Hao Rui Su Niraj Khadka

The gear transmission system is the core component in industrial equipment, and its wear state directly affects the reliability and use life of equipment. The wear debris image contains important information on the mechanical wear state. By processing it and analyzing the characteristics and types of wear debris, the health status of mechanical equipment and components can be evaluated. However, wear debris images collected in real time are often affected by Gaussian noise. The improved K-SVD dictionary learning algorithm was used in this paper to remove Gaussian noise, using objective metrics to demonstrate the effectiveness of the improved K-SVD algorithm for wear debris images. Secondly, the improved marked watershed segmentation algorithm (B-FSL) was studied to segment the wear debris chains. After that, the boundary energy (BE) characteristics of the wear debris were extracted to warn about the severe wear state of equipment in advance, an EfficientNetB3 network based on transfer learning was constructed for the recognition and classification of the wear debris image, and the severity of the wear of the mechanical equipment was analyzed. Finally, an experiment was conducted to validate the above methods, proved that the BE characteristics of the wear debris can predict the failure of a planetary gearbox in advance, with the accuracy of the wear debris recognition and classification algorithm exceeding 98%.

Lubricants doi: 10.3390/lubricants14040152

Authors: Guangzhi He Xiyan Wang Yu Dai Zhan Zhu Xinhao Li Donghua Jiang Haoyuan Tan Haitao Zhu Zixiang Li

Under extreme conditions in fields including aerospace exploration, deep earth excavation, and ocean engineering, mechanical components are subjected to severe environmental challenges, such as high temperature, heavy load, fatigue fracture and corrosion, which significantly limit their service life and operational reliability. Surface engineering has emerged as a critical strategy to address these problems by modifying surface characteristics while preserving basic properties of raw materials. Among various surface modification techniques, laser-based surface modification stands out due to its precise processing, high throughput, and outstanding surface performance. However, laser-based surface modification of metallic materials for industrial applications remains limited owing to inadequate systematic understanding regarding the fabrication mechanisms. Accordingly, a comprehensive and holistic review is essential to elucidate the effect of laser-based surface modification on process optimization, system development, microstructure evolution, and performance enhancement. This review systematically expounds two fundamental strategies in laser surface modification-based material modification (exemplified by laser cladding) and structural modification (exemplified by laser shock peening) in terms of mechanism, process, performance and application. In addition, the mechanism and potential of the synergistic integration of LC (laser cladding) and LSP (laser shock peening) are emphatically discussed. Finally, perspectives regarding process optimizations, material developments, and system improvements for laser surface engineering are presented. By establishing a clear “mechanism–process–performance–application” narrative, this review aims to provide both a scientific reference and a practical guideline for the severe demands of extreme operating conditions.

Lubricants doi: 10.3390/lubricants14040151

Authors: Shuai Zu Pingsheng Hu Xi Yang Yang Li Yinhui Che Jianghong Zhang Xiaohu Yang Yi Cui

The operational reliability of Emergency Diesel Generators (EDGs) is paramount for the safety of nuclear power plants. This study investigates the fretting wear mechanism on the non-working back-face of connecting rod big-end bearings—a critical failure mode that can lead to catastrophic engine damage. A synergistic approach was employed, integrating theoretical pressure calculations, on-site strain measurement experiments, and high-fidelity non-linear finite element analysis (FEA). The results demonstrate that while the theoretical design back-face pressure ranges from 8.1 to 10.1 MPa, the actual pressure is highly sensitive to bolt preload. A 16.2% attenuation in preload (from 550 kN to 461 kN), common during maintenance cycles, causes the interfacial pressure to drop to 6.9 MPa, falling below the recommended safety threshold of 7 MPa required to inhibit fretting. Furthermore, comparative experiments reveal that used bearings exhibit significantly lower and less uniform radial pressure retention compared to new bearings, even when physical dimensions appear compliant. Dynamic FEA indicates that peak inertial loads induce an out-of-roundness (DOR) of 0.295 mm, triggering a transition from a “partial slip” to a “macro-slip” regime at the interface. The findings confirm that the coupling of preload attenuation and loss of bearing elasticity drives the fretting process, providing a theoretical basis for optimized maintenance and selective assembly strategies.

Lubricants doi: 10.3390/lubricants14040150

Authors: Haibo Yu Xiaopeng Zhang Haichao Cai Lulu Pei Yujun Xue Jing Liu

To enhance the wear resistance and load-bearing capacity of WS2 coatings, this paper employs unbalanced magnetron sputtering technology to fabricate HfO2/WS2 composite coatings by regulating the deposition pressure (0.6–1.4 Pa), leveraging the superior properties of HfO2. The microstructure, mechanical properties, and tribological behavior across a wide temperature range (room temperature to 450 °C) are systematically investigated. The results demonstrate that deposition pressure significantly modulates the coating structure and properties. At a deposition pressure of 0.6 Pa, a pronounced secondary bombardment effect leads to coarse surface particles, a thickness of only 1.525 μm, and a high hardness of 9.332 GPa, but inferior tribological performance with an average friction coefficient of 0.703. When the deposition pressure is increased to 1.4 Pa, the secondary bombardment effect weakens, resulting in an increased coating thickness of 2.125 μm, a decreased hardness of 3.88 GPa, and a significantly improved friction coefficient of 0.072. At an optimal deposition pressure of 1.0 Pa, the sputtered atoms possess moderate energy and optimal surface mobility, promoting the formation of a dense structure. The coating demonstrates a synergistic balance between mechanical load-bearing capability (hardness: 6.38 GPa) and a highly crystalline WS2 structure, yielding superior frictional behavior characterized by a mean coefficient of friction (COF) of merely 0.062. High-temperature tribological evaluations indicate that the COF displays a non-monotonic trend, declining at first before ascending as the temperature elevates. A minimum value of 0.015 is reached at 300 °C, corresponding to a wear rate of 1.127 × 10−8 mm3·N−1·m−1. At 450 °C, partial oxidation of WS2 to WO3 causes the friction coefficient to rise to 0.045, accompanied by fluctuations. Microstructural analysis confirms that HfO2 doping effectively suppresses the oxidation of WS2 at elevated temperatures and promotes the preferred growth orientation of the WS2(002) plane, thereby synergistically optimizing the wide-temperature-range lubrication performance of the coating. This study provides a novel technical approach for the design of lubricating coatings intended for high-temperature and harsh operating conditions, such as those encountered in aero-engine bearings.

Lubricants doi: 10.3390/lubricants14040148

Authors: Kanghai Chen Nini Zhen Huatang Cao Qiaoyuan Deng Feng Wen

Recently, diamond-like carbon (DLC) films have been considered for enhancing the wear resistance of rubber because rubber exhibits a high coefficient of friction and is prone to wearing out. However, the significant difference in thermal expansion coefficients between DLC films and rubber often leads to high residual stresses and poor interfacial adhesion, which limits their application in dynamic seals. In this study, Cr-C/DLC composite films were prepared using magnetron sputtering, and the effects of varying Cr contents (0.8 at.%, 1.4 at.%, 4.3 at.%, and 7.0 at.%) on interfacial adhesion and tribological properties were investigated. Scanning electron microscopy (SEM) analysis revealed no distinct demarcation lines in the composite films, indicating strong adhesion to the substrate. X-ray photoelectron spectroscopy (XPS) analysis revealed that chromium doping promoted the conversion of sp3 bonds to sp2 bonds. Adhesion and tribology tests revealed that introducing a Cr-C layer with higher Cr content within the range of 0.8 at.% to 7.0 at.% enhanced the film’s adhesion, reducing the CoF value of the composite film to 0.13–0.14. Specifically, the RF80 sample (4.3 at.% Cr) exhibited excellent interfacial adhesion and optimal tribological performance, with a CoF value reduced to 0.13 and wear rate of 3.1 × 10−4 mm3/(Nm). In summary, modulating the Cr doping content can significantly enhance the interfacial adhesion strength and tribological properties of Cr-C/DLC composite films on rubber surfaces, providing an effective solution for optimizing rubber seals.

Lubricants doi: 10.3390/lubricants14040149

Authors: Yuanbin Gui Chengtao Li Zhaoguang Zhu Sunwu Xu Bin Yang Qianwu Li Jing Wan Shugang Zhang

The fretting-corrosion behavior of a Stellite 6 cobalt-based overlay welded to 304 stainless steel was investigated in simulated high-temperature, high-pressure PWR water. Three material pairings were examined: Stellite 6/Stellite 6 (C-C), Stellite 6/304 stainless steel (C-S), and 304 stainless steel/304 stainless steel (S-S). Wear behavior was evaluated in terms of mass loss, surface morphology, surface chemistry, friction evolution, and subsurface deformation. The results show that material pairing strongly affects friction stability and damage evolution during fretting corrosion. The C-C contact exhibited a relatively stable coefficient of friction and continuous wear morphology, with damage dominated by plastic deformation. In contrast, the C-S and S-S contacts exhibited stronger wear–corrosion interaction, characterized by debris accumulation, oxide film instability, and fluctuating friction behavior. Despite the same oxide species being observed in different contact pairs, their distribution and stability varied greatly, which resulted in different modes of damage. EBSD analysis showed that fretting energy in the C-C contact was mainly accommodated by plastic strain in the near-surface region, whereas deformation in the C-S and S-S contacts was more localized and discontinuous. These results indicate that oxide film stability and subsurface strain distribution jointly control friction behavior and fretting-corrosion damage under different material pairings.

Lubricants doi: 10.3390/lubricants14040147

Authors: Fan Xue Tianqiang Yin Guoqing Wang Jingfu Song Qingjun Ding Dae-Eun Kim Gai Zhao

Surface and interface science play an important role in the tribological properties of materials. Recently, research in this field has extended from the macroscopic scale to the molecular level to elucidate energy dissipation and structural evolution mechanisms at sliding interfaces. In this work, we propose a nanolubricant strategy based on carbon nanocages (CNCs). Three types of lubricating molecules—oleylamine (amine), oleic acid (carboxyl), and stearyl alcohol (hydroxyl)—were encapsulated into a polytetrafluoroethylene (PTFE) matrix to construct a composite tribological interface model. Molecular dynamics simulations were employed to investigate the interfacial enrichment, diffusion, and interaction mechanisms of these molecules with PTFE chains and the Fe counterface. Particular emphasis was placed on how different functional groups regulate energy transfer and dissipation pathways. This study deepens the molecular–level understanding of structure–lubrication relationships and provides theoretical guidance for designing high–performance polymer–based tribological materials.

Lubricants doi: 10.3390/lubricants14040146

Authors: Xueqian Geng

With the opening of complex service routes, the importance of the service performance of brake pads under long slope braking conditions is increasing. It is necessary to analyze the slope braking adaptability of current brake pad products. This work takes the copper-based powder metallurgy brake pads of a certain in-service high-speed train as the research object and conducts friction and wear behavior tests of the brake pads based on a full-scale brake test bench. Through microscopic observation and damage analysis, the differences in friction and wear behavior of the brake pads under stop braking and slope braking conditions are compared, revealing the wear mechanism and damage evolution characteristics of the brake pads. The results show that under the impact of high speed, high braking force, and severe thermal load in the stop braking conditions, the uneven wear of brake pads is high, and the eccentric wear of friction blocks is affected by both the friction radius and friction direction. The friction surface has a large number and size of damages, and the stability of the friction interface is poor. The brake pad exhibits a composite wear mechanism dominated by abrasive wear and brittle fracture induced exfoliation. In the slope braking condition, under the action of low speed, low braking force, and long-term stable thermal load, the uneven wear of the brake pads is relatively low, the surface damage size is small, and the friction block only has eccentric wear along the friction direction. The brake pad mainly initiates cracks along the interface of the components, which propagate parallel to the friction surface, exhibiting a progressive delamination and flaking exfoliation mechanism with a low wear rate. Although the friction interface of the brake pad is relatively stable under slope braking conditions, the cumulative delamination wear of the brake pads under long-term braking action needs further attention.

Lubricants doi: 10.3390/lubricants14040144

Authors: Sören Henniger Jan Zwinge Gino Grossi Thomas Hagemann Hubert Schwarze

The application of plain bearings has become very famous in planetary gear stages in recent years. To improve load-carrying capacity, this study investigates a design method for axial profiling in which a sixth-order polynomial is iteratively derived from the curve of local minimum film thickness for each axial grid position. Two load cases with specific bearing loads of 12.0 MPa and 6.0 MPa at 0.5 m/s are considered for profile design. Calculation results and computational effort of strategies assuming rigid or elastic geometries during the optimization are compared. Results indicate that the consideration of deformation is already necessary in the design phase and a maximization of minimum film thickness leads to much higher load-carrying capacity than the minimization of the maximum film pressure. Furthermore, the impact of the lube oil pocket position on oil flow rate is experimentally and theoretically investigated to identify the optimization potential of this parameter. Results show that the oil flow varies by the factor of three if the lube oil pocket is shifted incrementally over an angular span of 180° outside the load zone. The results and possible extensions are critically discussed under the consideration of practically relevant restrictions in mechanically highly loaded planetary gearboxes.

Lubricants doi: 10.3390/lubricants14040145

Authors: Alina Corina Dumitrașcu Denis Cojocaru Vlad Cârlescu Andrei Zaharia Dumitru Olaru

In this paper, the authors have analytically and experimentally investigated the friction torque in an angular contact ball bearing (ACBB) considering the following configurations: (i) a modified ball bearing with three equidistantly positioned balls without a cage and (ii) three modified ball bearings having four, six and eight balls with a phenolic cage. The experimental tests were realised at a low axial load and a rotational speed between 100 and 500 rpm under lubricated conditions. The test results for the ACBB with three balls without a cage showed that the friction torque is generated only by the rolling contacts between the balls and the two raceways in lubricated conditions. Considering the low axial load, the influence of the hydrodynamic rolling resistance was the dominant source of the frictional torque. The cage presence added supplementary friction, causing the friction torque to increase up to 60–65% compared to the cage-less configuration. Also, an additional increase in the frictional torque by 35–40% was observed for every two additional balls. The friction torque component that is generated by the cage presence does not depend on the number of balls added but rather on the rotational speed. All the tests were performed using the spin-down methodology, and the resulting friction torques (Texp) were determined by integrating the dynamic equation during the deceleration process using Python-based software (Python version 3.10). The input and output parameters are presented for each test. Logarithmic diagrams revealing the dependence between the experimental friction torque and angular speed were obtained and fitted, and the relations have been determined for all tests. The analytical results of the friction torque were obtained based on Houpert’s model for all modified ball bearing configurations.

Lubricants doi: 10.3390/lubricants14040143

Authors: Shizhao Yang Jiaming Guo Jingpei Cao Jianqiang Hu Xin Xu Liping Tong Jingping Zhao Jun Ma Ping Qi

The effects of dust, copper particles, and iron particles on the high-temperature oxidative degradation behavior of aviation lubricating oil were systematically examined, and the high-temperature catalytic oxidation effects of single-particle and mixed-particle systems on the lubricating oil were further analyzed, respectively. Gas chromatography/mass spectrometry analysis results indicated that significant differences exist in the catalytic oxidation activity of particles toward lubricating oils, with the activity ranking in the descending order of copper particles > iron particles > dust. Notably, following oxidation by both metal and dust particles, the acid value, particle size, and viscosity of the oil sample exhibit a significant synergistic catalytic effect, even exceeding those of the oil sample oxidized by the same amount of metal particles. Specifically, relative to the pristine oil, the oil oxidized with 5 mg of copper particles and 5 mg of dust exhibits respective increases of 213.3%, 316.11%, and 661.43% in the aforementioned properties. This variation is attributed to the physical adsorption and chemical reactions between dust and antioxidants during oxidation, which deplete antioxidants and thereby exacerbate oil oxidation. Furthermore, this study further elucidates the potential synergistic oxidation mechanism induced by metal particles and dust particles.

Lubricants doi: 10.3390/lubricants14040142

Authors: Marco Brand Mareen Goßling Ion-Dragoş Uțu Gabriela Mărginean

Traditionally, repairing coated substrates requires completely removing damaged, wear-resistant layers before recoating. This process leads to high costs, extended downtime, and material waste. Flexible brazing tapes, which are composed of alloy powder and an organic binder, offer an alternative to full coating removal for targeted repairs. Despite this, the process of vacuum brazing these tapes may lead to the formation of defects, including pores caused by trapped gases or residual binder, which compromise coating durability and corrosion resistance. This study focuses on the utilization of laser remelting as a method for post-processing nickel- and iron-based mixed alloy brazing tapes, with the aim of improving the integrity of the coating. Surface quality was assessed via microscopy and microhardness testing by systematically varying laser power, scanning speed, and hatch distance. Among the parameters studied, the most suitable laser parameter combination was found to be 350 W laser power, 250 mm/s scanning speed, and a hatch distance of 0.02 mm. These parameters yielded crack- and pore-free coatings with a remelting depth of 160.3 ± 17.2 µm and a microhardness of 701 ± 23 HV1, which is an 85% increase over as-brazed samples. Wear testing revealed a reduced coefficient of friction, and electrochemical corrosion tests showed lower corrosion current density and enhanced repassivation behavior in remelted coatings. These improvements demonstrate that laser remelting significantly enhances the microstructure, hardness, wear resistance, and corrosion performance of brazed coatings, providing an effective method for localized repair while minimizing material consumption and processing duration.

Lubricants doi: 10.3390/lubricants14040141

Authors: Weichen Kong Xuan Li Gaocheng Qian Jiaqing Huang

RV reducers are vital components in industrial robots and precision equipment, where the fatigue life of the crank arm and support bearings critically influences the overall system longevity. This study presents a comprehensive performance evaluation, with a specific focus on contact mechanics and wear analysis of these critical bearings. A theoretical mathematical model for force analysis is established based on static mechanics, which is further extended to incorporate wear depth prediction based on contact pressure and sliding velocity. To validate this model and investigate bearing behavior in detail, a high-fidelity parametric simulation model is developed using MASTA software. The simulation results, encompassing contact stress, shear stress, and wear patterns, demonstrate good correlation with the predictions from the theoretical mathematical model, effectively verifying its accuracy for performance and life assessment. The systematic analysis confirms that both the investigated tapered roller and needle roller bearings meet the design requirements. This integrated approach of theoretical modeling, which includes wear analysis, and software simulation provides a reliable methodology for assessing bearing performance and fatigue life, offering significant value for the design optimization and reliability enhancement of RV reducers.

Lubricants doi: 10.3390/lubricants14040140

Authors: Ruojun Zhao Zhongjie Lu Zaixin Liu Heng Li Weiyu Yu Jianlin Cai Zhibo Geng

The start-up process of water-lubricated bearings (WLBs) exhibits strong nonlinearity and is highly sensitive to the surface topography of both the bush and the journal. However, due to machining errors and operational wear in practical manufacturing and service, the bush inevitably develops surface waviness and wear, which significantly influence its start-up behavior. This issue is particularly critical in high-speed underwater unmanned vehicles, where lightweight design precludes the use of oil lubrication systems, making WLBs a more economically favorable and reliable alternative. To address this, the present study establishes a start-up tribo-dynamic model that comprehensively incorporates both bush surface waviness and wear to systematically investigate their coupled effects on the tribo-dynamic behavior during WLB start-up. The research findings indicate that axial surface waviness reduces the hydrodynamic effect during the start-up phase. Consequently, a higher rotational speed is needed to generate the necessary hydrodynamic pressure for the WLB to start up successfully. The significance of the influence exerted by circumferential surface waviness on start-up behavior is determined by its frequency number, m. A larger m leads to greater fluctuations in both hydrodynamic pressure and contact pressure. Furthermore, when wear occurs, the amplitude of bush surface waviness mediates the influence of wear depth on start-up performance, thereby modifying the optimal wear depth previously established for smooth bearings. These findings highlight the importance of controlling circumferential waviness frequency during machining and manufacturing processes to optimize WLB start-up reliability and service life.

Lubricants doi: 10.3390/lubricants14030139

Authors: Robin Guibert Maël Thévenot Julie Lemesle Laurent Coustenoble Jean-François Brunel Philippe Dufrénoy Maxence Bigerelle

Friction braking is the most spread braking system in vehicles, where the morphologies of the disc and the braking pads are essential to ensure that friction reduces rotation speed efficiently. However, modern braking systems are submitted to a complex balance between functionalities: braking ability, resistance to wear, and limited noise emission, i.e., squealing. This article studies the evolution of the morphology of a braking pad in a pin-on-disc configuration to further understand its influence over surface functionalities. Data collected from a pin-on-disc tribometer, and topographies are coupled to perform a multiscale and multiphysics analysis of the braking pad surface. Relevancy of roughness parameters regarding braking ability, surface wear, pad temperature and noise emission is evaluated with a bootstrap-based relevancy analysis. Relevant scales of the pad morphological structures are identified for surface wear (446 µm), braking ability (19.5 µm), pad temperature (2717 and 446 µm) and squealing frequency (1720 and 15.7 µm). Correlations between test bench data and roughness parameters highlighted the role of wear plateaus on the braking pad surface. These plateaus are formed by the damaged surface peaks during braking or by compaction of the third body trapped across the braking pad surface.

Lubricants doi: 10.3390/lubricants14030138

Authors: Jing Deng Hao Yang Tianxing Li Chuang Jiang Shaoyang Li

The meshing contact performance of spiral bevel gears is critical for transmission accuracy and service life but is inevitably influenced by manufacturing deviations. Existing tooth contact analysis (TCA) and lubrication-related studies for spiral bevel gears are mostly based on ideal theoretical tooth surfaces, failing to reflect the actual meshing state of as-machined gears with inherent machining deviations, and have poor robustness for complex deviated spatial surfaces. To accurately assess the actual meshing state, this paper proposes a novel contact performance analysis method based on a high-precision digital tooth surface reconstructed from one-dimensional probe measurement data. Unlike traditional TCA methods that rely on complex principal curvature calculations, this approach eliminates the mounting distance parameter by simplifying the meshing coordinate system, and employs a variable-radius cylindrical cutting method combined with a binary search algorithm to determine the instantaneous contact ellipse, effectively reducing computational complexity and improving solution robustness for deviated tooth surfaces. Experimental validation demonstrates that the digital tooth surface achieves a reconstruction accuracy of 2.6 × 10−5 mm. Furthermore, the method accurately predicts the contact pattern location and transmission error, with a discrepancy of only 4.7% compared to theoretical design values, which is highly consistent with the no-load rolling test results. This study confirms that the proposed method effectively reflects the actual meshing condition of machined gears, providing a practical theoretical foundation for the high-quality manufacturing and control of spiral bevel gears. Meanwhile, the high-fidelity contact characteristics of as-machined tooth surfaces output by this method can provide reliable input boundaries for thermoelastohydrodynamic lubrication (TEHL) simulation, friction loss prediction and anti-scuffing design of spiral bevel gears considering machining deviations.

Lubricants doi: 10.3390/lubricants14030136

Authors: Yuting Dong Yuxi Gao Yuyuan Qiao Qi He Zhong Tang

Combine harvesters are core modern grain production equipment with high reliability, critical for food security. Yet their metal parts suffer severe grain-induced wear during operation, directly reducing efficiency, increasing grain loss, and raising maintenance costs and environmental burdens. This paper clarifies the grain-induced wear source characteristics and the dominant mechanisms and hazards for combine harvester metal surfaces, as well as summarizes the research progress of four key protection strategies: wear-resistant materials, surface engineering, structural and parameter optimization, and maintenance and remanufacturing. Based on the latest research data, the working principles, performance advantages and application scenarios of various protective technologies were analyzed. Current research faces several challenges: insufficient systematic wear data for multiple crops, unclear multi-factor coupled wear mechanisms, limited low-cost and long-lasting protective technologies, and the absence of online wear monitoring techniques. Finally, the directions for future research focus, such as the systematic research on the wear characteristics of multiple crops, the deepening of the wear mechanism of multi-factor coupling, the development of green, low-cost and long-term protection technologies, and the development of online wear monitoring and active control systems, are explored, providing theoretical support and technical reference for the transformation of wear control in combine harvesters, from passive maintenance to active protection throughout the entire life cycle. Such future work supports the high-quality development of agricultural mechanization and ensures food security.

Lubricants doi: 10.3390/lubricants14030137

Authors: Sze Wei Quek Liang Hong

This study investigates the key performance-related fuel properties of emulsifier–diesel solutions and ethanol-in-diesel microemulsions. This work begins with the in situ polymerization of long alkyl chain-substituted glycidyl methacrylate (R-GMA) in diesel and the optional presence of a second methacrylate monomer. The resulting diesel-soluble oligomer functions as a nonionic emulsifier. Controlled amounts of ethanol are subsequently incorporated into the emulsifier–diesel solution to form a stable microemulsion, referred to as E-Diesel. This study examines how the structure of the emulsifier influences key fuel properties, including (i) ethanol–diesel miscibility, (ii) gross calorific value, (iii) Ramsbottom carbon residue (% of fuel), (iv) entrapped polycyclic aromatic hydrocarbons (PAHs), and (v) fuel lubricity. Both the hydrophilic–hydrophobic balance and the structure of the emulsifier side chains are found to significantly affect these properties. Compared with neat diesel, oligomeric emulsifiers enable the substantial dispersion of ethanol in diesel (up to 18 wt.%). The resulting fuel exhibits a gross calorific value exceeding the theoretical sum of diesel and ethanol at the same composition (a synergistic effect) and achieves an enhancement in lubricity up to 49.5% relative to neat diesel at a 5% emulsifier loading. Although the presence of emulsifiers leads to an increase in the carbon residue by up to 54.7% compared to neat diesel during controlled pyrolysis, it simultaneously reduces the PAH content in the exhaust. Overall, this study establishes fundamental correlations among microemulsion stability, combustion synergy, carbon residue formulation, and fuel lubricity, which are governed by the structure of the emulsifier.

Lubricants doi: 10.3390/lubricants14030135

Authors: Recep Çağrı Orman

In this study, the effect of boron carbide (B4C), hexagonal boron nitride (hBN), holy super graphene (HSG), and hybrid (B4C + hBN + HSG) nano-additives on the tribological performance of SAE 5W-30 gasoline engine oil was investigated on Al-Si-based samples (Al 4032) prepared by cutting from a single-cylinder gasoline engine block. The addition of nano-additives regularly increased the kinematic viscosity; the 63.80 mm2/s (BO) value rose to 68.90 mm2/s at the highest level of B4C and to 70.50 mm2/s in the hybrid oil (≈10.5% increase). The lowest and most stable friction performance was found in the hybrid 0.025 g/25 mL nano-additive oil, which remained between 0.03 and 0.05 during the entire COF test. The EDS mapping and line scan results confirmed the formation of tribofilm by identifying the additive elements (B for B4C, B and N for hBN, C for HSG) in the wear scar, and the presence of increased O elements showed the restricted formation of tribo-oxidation. The results show that hybrid nano-additive oils provide the most effective friction and wear improvement, especially at low concentrations, while at high additive levels, performance does not show a consistent increase due to particle accumulation and third-body effects.

Lubricants doi: 10.3390/lubricants14030134

Authors: Yawen Bai Peipei Lu Yiming Cai Ziwen Xie

This study involves the addition of B4C particles to TC4 powder to fabricate B4C-reinforced titanium matrix composite coatings on an AISI 304L substrate using a laser cladding process. The effects of B4C addition (0–12 wt.%) on microstructure, microhardness, wear, and corrosion performance were systematically investigated. Results indicate that the composite coating with 9 wt.% B4C exhibits optimal properties, achieving a peak microhardness of 600 HV0.2 and a wear rate of 2.82 × 10−4 mm3/N·m, representing a 58.22% reduction compared to the pure TC4 coating. Electrochemical tests in 3.5 wt.% NaCl solution reveal a significant positive shift in corrosion potential and reduced corrosion current density with increasing B4C content, confirming enhanced corrosion resistance, and the improved performance is attributed to grain refinement and dispersion strengthening by B4C particles. This work provides a feasible strategy for enhancing the surface properties of 304L stainless steel under demanding wear and corrosive environments.

Lubricants doi: 10.3390/lubricants14030133

Authors: Kimika Nakamura Atsuo Watanabe Fumihiro Itoigawa Kazuhiko Kitamura

To accurately predict the shape of an automobile product, such as a crankshaft, produced by hot die forging, a preliminary simulation of the forging process indicated the significant impacts of local and varying friction coefficients between the complex-shaped die and the material. This study identified the friction coefficients through ring compression and tapered plug penetration tests, focusing on regions with high pressure or large contact areas. The results revealed variations in the friction coefficients across different regions. Consequently, the study suggests implementing locally appropriate friction coefficients on specific die surfaces exhibiting conditions akin to those observed in the friction tests. Specifically, a Coulomb’s friction coefficient of 0.14 was assigned to the product shape region of the crankshaft die. Additionally, a friction model transitioning from a Coulomb’s friction coefficient of 0.5 to a shear friction coefficient of 0.6 was applied in the flash region with significant sliding distances. By incorporating these tailored friction conditions into the simulation of hot die forging for crankshaft manufacturing, the study achieves more accurate material flow, die filling, and underfill replication.

Lubricants doi: 10.3390/lubricants14030132

Authors: Yucheng Yang Limin Gao Yafeng Wu Guojun Wu Geyang Hao

The rolling seal is a pivotal sealing technology for marine equipment such as wet-mateable connectors, ensuring operational integrity in deep-sea environments during both static and mating phases. However, its working mechanisms remain inadequately understood, and the effects of sealing parameters and seawater pressure have yet to be systematically studied. To address these issues, a refined model for rolling seals operating in deep-sea pressure-balanced conditions was developed. The model’s accuracy was enhanced by incorporating two key inputs: experimentally measured boundary lubrication friction coefficients (replacing conventional dry friction values) for finite element simulation and torque calculation, and oil pressure under pressure-balanced conditions, derived from shell theory, as a boundary load. Through systematic parametric simulations, the effects of interference fit, rotational speed, and seawater pressure on sealing performance were elucidated. An experimental torque test setup under atmospheric pressure was constructed to validate the numerical model. The results indicate that, while ensuring reliable static sealing, higher rotational speeds and smaller interference fits help reduce rotational torque. Benefiting from the pressure-balanced design, increasing water depth significantly enhances hydrodynamic performance—accounting for over 90% of the total static contact pressure at 1500 m—while leakage shows a decreasing trend. These findings provide theoretical insights for optimizing deep-sea sealing structures.

Lubricants doi: 10.3390/lubricants14030131

Authors: Yue Zhang Zhili Zhang Jiyi Jiang Tao Zhang Jiwen Li Decai Li

As a promising solution to lubrication failure in space environments where conventional oils suffer from splashing and leakage, magnetic fluids (MFs) offer significant potential. This study synthesized a perfluoropolyether (PFPE)-based MF tailored for space applications, demonstrating low-temperature fluidity at −40 °C, low saturated vapor pressure (3.37 Pa at 75 °C), and high stability (>6 months). To evaluate its lubrication effect, four magnetic thrust ball bearing structures were designed, with magnetic fields optimized via simulation. A magnetic field-controllable lubrication test bench was constructed accordingly. Comparative tests under varying friction conditions revealed that MF lubrication extended bearing service life. Specifically, the bearings lubricated with 7.5 wt.% MF exhibited the longest service life, which was doubled compared to the service life of the bearings lubricated with the carrier liquids. When compared to bearings without the application of a magnetic field, the service life of bearings lubricated with MFs of the same mass fraction increased by a factor of 3 to 4. This initial finding suggests the viability of using MFs in space lubrication applications.

Lubricants doi: 10.3390/lubricants14030130

Authors: Marek Kočiško Petr Baron Dušan Paulišin

The present study investigates the influence of low temperatures on the starting torque, viscous friction, and power intensity of a precision cycloidal reducer TwinSpin TS 140-115-E-P19-0583. Two types of plastic greases with differing viscosities were compared in the experiment: Castrol TT-1 (low-viscosity, optimised for low-temperature) and Vigo RE-0 (higher viscosity, designated for greater loads). The measurements were taken in a climate chamber in the temperature ranging from +24 °C to −20 °C in the mode accounting for no external load. The results have shown that Castrol TT-1 maintains its beneficial rheological properties at as low as −20 °C, which is manifested in a low starting torque (~0.30 Nm) and low power intensity (~0.33 kW). On the contrary, Vigo RE-0 shows a significant increase in friction: at −20 °C, the starting torque is 1.0–1.1 Nm and the power intensity of the operation increases to consume more than 1.5 kW. The correct choice of lubricant is a critical factor for reliable cold-start behaviour under no-load, internal-loss-dominated conditions. This study provides a rare experimentally verified low-temperature assessment of starting torque, viscous friction, and power intensity in fully assembled TwinSpin precision cycloidal reducers lubricated with greases of markedly different viscosity classes, addressing an important gap in the existing literature.

Lubricants doi: 10.3390/lubricants14030129

Authors: Haidong Hu Youmin Rong Hailong Cui Hanwen Zhang Yu Huang Guojun Zhang

A novel shallow oil chamber hybrid journal bearing with adjustable oil chamber depth was designed based on piezoelectric ceramics, inspired by conventional shallow oil chamber bearing structures. The computational fluid dynamics method is used to analyze the bearing characteristics of shallow oil chamber bearings, including the volume flow, the seal oil pressure, load capacity and stiffness. An experimental platform equipped with signal acquisition device and piezoelectric ceramic control device was developed. The eddy current sensors collected the displacement signal at the shaft end. The required voltage was calculated by the displacement signal. The piezoelectric ceramics elongated or shortened, causing a displacement of the same magnitude in the depth of the oil chamber, thereby controlling the radial displacement of the shaft. The adjustment effect of this bearing was verified by experiment for no-load and 500 N load at 200–1000 rpm, with a baseline initial oil chamber depth of 20 and an oil supply pressure of 2 MPa. The results showed that compared with the case without adjustment, the accuracy in Y direction has increased from 8.9 μm to 1.9 μm (max. 78.4%) after adjustment. Under the above load conditions, the displacement can be controlled below 2 μm, indicating a significant improvement in shaft vibration resistance.

Lubricants doi: 10.3390/lubricants14030128

Authors: Wei Dou Shengdi Sun Xinjie Zang Xi Kuang Zhilei Jin

The unbalanced excitation of a rotor has a significant impact on the dynamic stiffness of the bearing. Traditional unbalanced excitation force models for the calculation of bearing stiffness are usually simplified as single-directional excitation models, which cannot fully reflect the impact of unbalanced excitation of the rotor on the dynamic stiffness of the bearing. A bidirectional excitation model based on orthogonal decomposition is used in this paper and is introduced into the finite element model of the bearing based on ABAQUS. The proposed bearing mechanics model is verified through numerical software and a bearing rotor system test rig. The effects of single/bidirectional excitation models on the dynamic stiffness of bearings were compared. The variation in bearing dynamic stiffness characteristics under rotor excitation and axial load were discussed. The results show that the presented model has good consistency with experimental results (the proposed model yields a maximum stress deviation of only 2.42% compared to MESYS numerical results and a maximum dynamic stiffness difference of 9.12% against experimental data). The traditional unidirectional excitation force model can only consider the influence of excitation frequency on the dynamic stiffness of bearings. However, the unbalanced excitation force model considering bidirectional excitation can further take into account the influence of excitation amplitude on the dynamic stiffness of bearings. Under the combined effect of excitation frequency and excitation amplitude, the radial dynamic stiffness of bearings shows a quadratic nonlinear hardening trend with rotational speed. As the rotational speed increases, the contribution of axial load to the radial stiffness significantly enhances: in the low-speed zone, its influence is only approximately 8%, while in the high-speed zone, it increases to 34%. Although the modeling method formed in this paper does not take into account the thermal–fluid dynamic coupling effect of the lubricating oil film, the obtained laws can provide a basis for the dynamic design of rotor systems of actual liquid rocket engines and have certain engineering application value.

Lubricants doi: 10.3390/lubricants14030127

Authors: Pedro Henrique Pires França Gustavo Henrique Nazareno Fernandes Lucas Melo Queiroz Barbosa Márcio Bacci da Silva Paulo Sérgio Martins Álisson Rocha Machado Andre Hatem

High-speed turning of AISI 304 stainless steel is limited by rapid tool wear driven by thermal accumulation and tribological instability. This study compares five cooling/lubrication strategies (dry, flood cooling, MQL, internally cooled tools—ICT, and ICT + MQL) under a fixed severe cutting regime (Vc = 400 m/min, f = 0.1 mm/rev, ap = 0.2 mm) and develops a low-complexity tool end-of-life predictor using cutting power as the sole monitoring signal. Dry machining produced the highest cutting forces 26.7 N), whereas lubricated/cooled conditions showed statistically similar force levels (≈11 6 – 118 N). Cutting force and derived power increased monotonically with wear, supporting power as an indirect tool-state indicator. A binary XGBoost classifier trained on statistical and trend descriptors of one-second power windows achieved accuracies of 96.5% (training), 95.9% (test), and 93.3% (validation) with AUC–ROC values of 0.988, 0.993, and 0.959, respectively, despite moderate class imbalance (≈85 % healthy/15% worn). SHAP analysis identified average power and distributional descriptors (skewness and amplitude ratios) as dominant predictors, providing interpretable links between signal statistics and wear progression. The results demonstrate that reliable end-of-life detection can be achieved using a single energetic signal across heterogeneous cooling environments, supporting scalable monitoring compatible with low-fluid and closed-loop cooling strategies.

Lubricants doi: 10.3390/lubricants14030126

Authors: Félix Leaman Felipe Segura Valentina Gutiérrez

This study presents an experimental investigation of acoustic emission (AE) generated during gear tooth contact under various operating conditions. A specialized test rig was developed to measure the AE signals originating from the interaction between two large-scale involute gear teeth. The dimensions of these teeth facilitate a detailed examination of the AE waveform characteristics produced by the sliding–rolling motion inherent to this geometry. Experiments were conducted under three distinct conditions: defect-free teeth without lubrication, defect-free teeth with lubrication, and teeth with localized surface defect. Results indicate that defect-free gears exhibit stable and repeatable waveform behavior that correlates with the sliding speed between meshing teeth. Conversely, worn gear teeth produced significant changes in the AE response, characterized by increased localized amplitudes. Furthermore, the introduction of lubrication significantly altered the waveform patterns, obscuring the clear identification of the sliding–rolling motion. This research contributes to a deeper understanding of AE generation in gear transmissions through the high-resolution analysis of their characteristic waveforms.

Lubricants doi: 10.3390/lubricants14030125

Authors: Armand Tamouafo Fome Josephine Kelley Jan Torben Terwey Florian Pape Gerhard Poll Max Marian

The common view is that, in boundary lubrication, the load is transmitted solely through directly contacting asperities due to the extremely limited lubricant availability or lacking hydrodynamic force generation. The asperities may transmit force via their boundary layers or a thin liquid lubricant film in between. Hypothesizing that the latter mechanism dominates, a friction simulation model was developed for the boundary lubrication regime to investigate whether the contact shear force, and consequently the friction coefficient, are exclusively governed by the shearing of this thin lubricant film between the contacting asperities. In the very thin films at the asperity contacts, the extremely high pressures suggest that the limiting shear stress regime prevails. This means that the shear stress between two asperities sliding relative to each other is equal to the limiting shear stress corresponding to the local pressure. The model is applied to calculate the friction coefficient of a lubricated two-disc tribological contact before and after a wear experiment. It comprises a contact model, based on the Boundary Element Method (BEM), to determine the pressure distribution at the asperity level; a limiting shear stress model to evaluate the corresponding shear stress as a function of pressure; and a friction model to compute the overall coefficient of friction. Two base oils are considered in the analysis, a mineral oil and a synthetic oil, both unadditivated. The calculated coefficients of friction are compared with experimental results, the limitations of the modeling approach are discussed, and an updated model is proposed for the specific case of two contacting steel bodies lubricated with additive-free oil.

Lubricants doi: 10.3390/lubricants14030124

Authors: Yesenia Sánchez-Fuentes Rafael Carrera-Espinosa Raúl Tadeo-Rosas Cintia Proa-Coronado José A. Balderas-López Luz A. Linares-Duarte Melvyn Alvarez-Vera José G. Miranda-Hernández Enrique Hernández-Sánchez

Boriding is a thermochemical process that improves the surface properties of metallic materials, such as wear resistance, hardness, and Young’s modulus. The current work evaluated the kinetics of boride layers formed by boriding on AISI H13 steel. The AISI H13 steel samples were covered with a non-commercial powder mixture of 70% wt. SiC, 20% B4C wt. and 10% wt. KBF4. The samples were treated for 2, 4, and 6 h at 850, 875, and 900 °C, respectively. The growth kinetics of boride layers were estimated as a function of the treatment parameters, using a solution of the second Fick’s Law, as in a parabolic model. Also, the hardness of layers was assessed by Vickers microindentation. Optical examination of the samples showed a biphasic FeB/Fe2B layer at all temperatures after 6 h of treatment. In contrast, those exposed for 2 h exhibited a monophasic Fe2B layer with isolated zones of the FeB phase in all temperatures. The results suggested that the obtained layer thicknesses are highly dependent on the treatment parameters. After 2 h at 850 °C, the samples exhibited a well-defined layer with a thickness of 8.51 ± 1.01 μm, whereas after 6 h it was 24.39 ± 1.01 μm. The activation energy was estimated at 230.63 kJ/mol, with a correlation coefficient (R2) of 0.97, consistent with values reported in the literature. Additionally, the hardness values were estimated to range from 1880 to 2192 HV for the FeB phase and from 1294 to 1715 HV for the Fe2B phase, indicating that the hardness of the boride layers is highly dependent on the treatment conditions.

Lubricants doi: 10.3390/lubricants14030123

Authors: Matthew Currie Fabian Limmer Yue Huang Carl A. Gilkeson David C. Barton

The impending Euro 7 regulation will impose strict limits on brake particulate matter (PM) emissions from new light-duty vehicles, driving manufacturers to explore alternative rotor materials and/or surface treatments. This paper evaluates the friction and wear emission performance of both a laser-clad grey cast iron (GCI) rotor surface and a plasma electrolytic oxidation (PEO) treated aluminium surface compared to that of an uncoated GCI. Tests were conducted on a small-scale tribometer rig, which was specially adapted to measure airborne emissions while emulating the standard Worldwide harmonised Light vehicle Test Procedure (WLTP). The laser-clad coating was applied via extreme high-speed laser cladding to form an initial 430 L stainless steel layer, followed by a topcoat of 80/20 vol% 430L steel/TiC, both layers being c.100 micron thick. The PEO treatment applies a c.50 micron alumina coating to both a wrought and cast alloy, the latter being more suitable for the manufacture of full-size vented brake rotors. Results show that all rotor materials achieved a satisfactory coefficient of friction (CoF) against suitable low-metallic pad material, although the CoF for the wrought PEO-Al alloy was significantly higher at c.0.65 compared with c.0.50 for the other materials. The gravimetric wear of all the coated rotor surfaces after 8 WLTP cycles was almost undetectable, and pad wear was also significantly reduced. This improved wear resistance led to significant reductions in PM emissions, with the PM10 levels of the uncoated GCI reduced by around 75% for the laser-clad GCI and PEO wrought Al alloy, and by about 60% for the PEO cast Al alloy. When extrapolated to a full-sized passenger vehicle, the results indicated that both the laser-clad GCI and PEO-treated surfaces have the potential to meet the current Euro 7 emissions targets.

Lubricants doi: 10.3390/lubricants14030122

Authors: Yu Chen Qingbo Lan Hongchang Wang Xuze Wu Xinzhou Zhang Kai Wu

Collision and wear are common phenomena in revolute clearance joints, caused by the positional deviation between the journal and bearing centers. The freedom of motion and contact–impact characteristics are reflected in the mechanism’s movement. The penetration behavior of the clearance joint is described using modified elastic contact model combined with Coulomb’s friction. In addition, the dynamic model of a high-precision mechanism with a clearance joint is established using Largrange’s equation. A dynamic performance experiment is also conducted. The results prove the validity of the proposed method. The kinematic accuracy of this mechanism is then used to evaluate the stability and motion error in a case study. Furthermore, the influence of the clearance joint on the dynamic behavior of the high-precision mechanism is thoroughly analyzed. The results show that the fluctuation range of the slider’s dynamic repeated precision for slider is only 0.022 mm under high-speed conditions, meeting the design requirement.

Lubricants doi: 10.3390/lubricants14030121

Authors: Yaru Liang Jinxian Chen Renxin Liu Huamao Zhou Nianqian Kang Nanrun Zhou

Rolling bearing faults originate from complex tribodynamic interactions among rolling elements, raceways, and the cage, yielding nonlinear, non-stationary vibration signals that are highly susceptible to noise and operating-condition variations, which compromises the reliability of diagnosis. To address this issue, this paper proposes the RConvNeXt–ECGA framework. The main contributions are twofold: (1) RConvNeXt is a convolutional module based on ConvNeXt, which achieves efficient multi-scale feature extraction through grouped parallel convolutions with multiple receptive fields; (2) Efficient Content-Guided Attention (ECGA) is a novel pixel-level attention mechanism, which adaptively reweights feature maps to highlight informative regions and suppress irrelevant interference. The proposed method achieves an average accuracy of 99.8% on bearing datasets from Case Western Reserve University and Huazhong University of Science and Technology, and 94.33% under cross-operating-condition tests, demonstrating superior robustness and generalization over representative deep learning-based baseline models.

Lubricants doi: 10.3390/lubricants14030120

Authors: Stamatios Kalligeros Despina Cheilari George Veropoulos

The present study examines the physicochemical degradation and elemental contamination of marine distillate diesel fuels, which were stored in land-based tanks in operational conditions. Forty-one (41) samples, in compliance with ELOT ISO 8217:2024 were analyzed for crucial physicochemical properties. Stepwise regression identified magnesium (Mg) (positive) and chromium (Cr) (negative) as significant viscosity predictors (R2 = 0.269, p = 0.003, VIF < 2), while calcium (Ca), Phosphorus (P), zinc (Zn), copper (Cu), lead (Pb) and Ferrous (Fe) were excluded due to multicollinearity. Strong correlations (r > 0.85) between element pairs (Cu-Pb) (r = 0.996), Ca-Zn (r = 0.897), and P-Ca (r = 0.888) indicate common sources from lubricant additives (ZDDP) and brass corrosion, with individual correlations recorded for Ca (showing r = 0.679, p < 0.001), P (r = 0.722, p < 0.001), and Zn (r = 0.595, p < 0.001). The results revealed that fuels stored in carbon steel tanks under high-humidity conditions for over six (6) months recorded higher metal loads than those in stainless steel tanks with regular periodic supply. The FAME content in the studied samples ranged from 6.7 to 7.1% v/v and showed no significant correlation with degradation indicators (p > 0.05). The narrow FAME range examined precludes definitive conclusions regarding specific biodiesel effects. The threshold of 0.2 mg/kg, as set by manufacturers’ guidelines to protect injectors, was exceeded in the coastal carbon steel tank samples with eight (8) months of storage under high-humidity conditions and in the coastal carbon steel tank samples with nine (9) months of storage under high-humidity conditions examined. The current study offers a systematic correlation between viscosity and elemental contamination for marine distillate fuels under operational storage conditions regarding real-world samples.

Lubricants doi: 10.3390/lubricants14030119

Authors: Jiashu Yu Xuexing Ding Jinlin Chen Jianping Yu

Spiral-groove dry gas seals are widely used in turbomachinery. However, high-fidelity numerical simulations remain challenging because the gas film is micron-scale and features high shear and pronounced boundary-layer effects, while experimental studies are often expensive due to the large design space and tight machining tolerances. To address these issues, this study integrates a Kriging surrogate model with surrogate-based optimization (SBO) to systematically identify the key structural and operating parameters governing seal performance. The results quantify the individual effects of key geometric parameters, providing practical guidance for spiral-groove seal design and optimization. The Kriging model captures the nonlinear relationships between performance and design variables and shows good generalization, with a maximum residual standard deviation of 2.78 and all others below 1.0. Sobol analysis reveals that structural parameters dominate performance: groove depth and width exhibit total-effect indices of approximately 0.74 and 0.56, respectively, while rotational speed is the most influential operating parameter (≈0.75). Among eight structural variables, groove depth is the most critical, increasing leakage by more than 200% as it rises from 5 to 8 μm, followed by spiral angle and groove number; all remaining parameters each contribute less than 10%.

Lubricants doi: 10.3390/lubricants14030118

Authors: Bing Xue Yongbo Li Zhi Zhang Yibing Ren Ziyi Yang Ying Xu Tao Zhao Youqiang Wang

Water contamination threatens the lubrication stability of thrust bearings in hydro-generator units. This study investigates the coupling effects of inlet oil temperature, rotational speed, and water content (0–200 g/L) on lubrication performance. The results show that water content below 1 g/L has negligible effect. A critical threshold of 70 g/L is identified, where pad temperature rise rate increases sharply; at 45 °C inlet temperature, outlet zone temperature reaches 73 °C, and film thickness decreases to 12 μm. A water content of 100 g/L corresponds to the maximum friction torque of 9 N·m. Increasing rotational speed enhances hydrodynamic effects; at 25 m/s and 70 g/L, peak pad temperature reaches 72 °C. When water content exceeds 100 g/L, thermal buffering of free water mitigates temperature rise, but fluctuating oil film load-carrying capacity requires vigilance. The findings provide theoretical support for condition assessment and maintenance of thrust bearings under water-contaminated conditions.

Lubricants doi: 10.3390/lubricants14030117

Authors: Jinrui Liu Jianjun Wang Shichang Yao Huiwen Xiang Bin Zhao Meng Li Xuehua Zhang

While graphene-based lubricants are well-studied, the tribological potential of emerging carbon–nitride and carbon–boron 2D materials remains largely unexplored. Herein, by using first-principles calculations implemented in the VASP code, we systematically explored the interlayer interactions and frictional properties of bilayer homojunctions and heterostructures composed of graphene, C3N, and C3B. The DFT-D3 dispersion correction was employed to accurately capture the interlayer van der Waals forces. The results reveal that C3N/C3N, C3N/graphene (C3N/Gra), and C3B/graphene (C3B/Gra) systems exhibit significantly lower friction coefficients compared to pristine bilayer graphene (Gra/Gra). Notably, the sliding potential barrier of the C3N/Gra heterostructure is only ~0.45 meV/atom (approximately 1/10 that of the Gra/Gra system), manifesting exceptional superlubricity and considerable potential for superlubricant applications. The sliding potential barrier of the C3B/C3N heterostructure is slightly smaller than that of Gra/Gra. In contrast, the C3B/C3B homojunction exhibits high resistance to sliding; under normal loads of 1–4 nN, its potential barrier ranges from ~16 to ~115 meV/atom, which is consistently twice that of Gra/Gra. The observed frictional variations are attributed to sliding-induced interfacial charge redistribution. These findings provide fundamental insights into the tribological behavior of C3N- and C3B-based materials and establish a quantitative link between frictional properties and interfacial charge dynamics, offering a theoretical basis for the development of advanced graphene-derived lubricants.

Lubricants doi: 10.3390/lubricants14030116

Authors: Hao Xiao Yihao Zhang Xueqiang Ding Mingmin Zheng Qiuya Tu Zongde Liu Jingbin Han Xin Zhang Yuan Xu

ZnMgAl layered double hydroxides (LDHs) were synthesised via coprecipitation, and oleic acid and stearic acid were grafted onto their surfaces via dehydration condensation to obtain two nano-lubricant additives, OA-ZnMgAl LDH and SA-ZnMgAl LDH. These surface modifications significantly improved the dispersion stability of ZnMgAl LDH in lubricating oil. Tribological tests showed that, at their respective optimal concentrations for friction reduction or wear resistance, ZnMgAl LDH, OA-ZnMgAl LDH, and SA-ZnMgAl LDH reduced the coefficient of friction by 3%, 20%, and 16%, and decreased the wear scar diameter by 7%, 9%, and 14%, respectively, compared with the base oil (XMP-Mobil 320). To clarify the lubrication mechanism, the wear morphology and chemical composition were analysed using 3D optical profilometry, X-ray photoelectron spectroscopy, scanning electron microscopy, and FIB-SEM. The results indicate that LDHs react with the steel surface under load and shear to form a multilayer protective film consisting of an inner oxide layer and an outer graphite layer, preventing direct contact between friction pairs. In addition, the rolling and filling effects of partially unreacted LDHs further reduce friction and wear.

Lubricants doi: 10.3390/lubricants14030115

Authors: Jose Jaime Taha-Tijerina Dyana De Leon-Elizondo Jade Mendieta Leonardo Taha-Soto

Recent innovations with the aid of nanotechnology are more frequently seen in the industrial sectors. Lubricants are a high-end commodity resource used in many manufacturing processes; unfortunately, most of these lubricants are petroleum-based, which come with certain drawbacks, such as environmental aspects, handling issues and high costs. With the incorporation of nanostructures within fluids and lubricants, novel material alternatives are replacing conventional lubrication systems, maintaining the required thermophysical and tribological characteristics. This research provides an analysis of vegetable lubricant, castor oil (CO), and the effects of the incorporation of WS2 nanofiller at diverse filler fractions. A TEMPOS thermal analyzer device and a four-ball tribotester are used for the analysis of thermal conductivity and tribological assessments, respectively. Results showed the enhancement of thermal conductivity as the filler concentration and the evaluation temperature of the nanolubricants increased. The best thermal conductivity improvement was 27%, at 60 °C with merely 0.20 wt.% of nanofillers. For tribological performance, a decrease of 6% in the coefficient of friction (COF) and 31% in the wear scar diameter (WSD) was observed at 0.10 wt.% and 0.20 wt.%, respectively. Adhesion of the nanostructures to the steel surfaces creates a protective layer, preventing direct contact of the friction pairs. These results are an outcome of applied theoretical concepts such as Brownian motion and nano-layering of the lubricant–nanostructure interface.

Lubricants doi: 10.3390/lubricants14030114

Authors: Hanzhi Yao Yuting Zhao Bo Gao Ruizhe Li Tianxu Gao Xiang Liu Xianhao Gu Zhongnan Wang Qiuying Chang

Molybdenum disulfide (MoS2)-based composite systems are widely used as solid lubricating coatings. However, further optimization towards lower friction and higher wear resistance remains necessary to meet the extreme operating conditions and high reliability requirements of next-generation aerospace equipment. This study investigated the tribological performance of MoS2/epoxy composite coatings by comparing the effects of individual and combined additions of nano nickel (Ni) and magnesium silicate hydroxide (MSH). The coating preparation process adopted in this study is the bonding method. Experimental results showed that, under a load of 2 N and a rotational speed of 500 r/min, the coating containing 0.3 g Ni and 0.1 g MSH (labeled W03Ni01MSH) achieved a 22% reduction in wear scar width compared to the coating with only Ni, demonstrating a distinct synergistic effect. This is attributed to the complementary roles of the two additives: Ni promotes the formation of flaky wear debris, facilitating rapid formation and stabilization of a transfer film, thereby reducing friction; MSH enhances the load carrying capacity of the coating and suppresses wear propagation, thereby improving wear resistance. Furthermore, this composite coating exhibited optimal performance under the conditions of 500 r/min and 2 N. The results of this study significantly improved the friction-reducing and wear-resistant properties of the MoS2/epoxy composite coating. This provides a new strategy for the formulation design of high-performance solid lubricating coatings.

Lubricants doi: 10.3390/lubricants14030113

Authors: Yongmei Wang Xigui Wang Weiqiang Zou Jiafu Ruan

Current research lacks systematic understanding of cross-scale correlations between micro-texture geometry and macro-lubrication behavior. This study presents a multi-scale collaborative optimization methodology for gear Micro-Textured Meshing Interface (MTMI). An objective function targeting macroscopic interfacial performance is formulated, and a topology optimization strategy is employed to achieve optimal MET configuration. The homogenization analysis captures the modulating effects of MET on local flow and stress fields, while topology optimization transcends conventional parametric geometric constraints, enabling the generation of non-regular MET topological patterns tailored to complex operating conditions, thereby ensuring optimal macroscopic ASLBC. The proposed scheme is validated through numerical simulations of two representative problems capturing distinct lubrication regimes: (1) IEL, characterizing transient load-bearing dynamics governed by temporally evolving MET configurations; and (2) ASLBC, elucidating steady-state load-bearing capacity modulation via spatially heterogeneous MET distributions. A Taylor expansion-based surrogate model is developed to efficiently explore the MET configuration design space, significantly enhancing computational efficiency and solution accuracy for multi-scale optimization. While the gradient-based algorithm cannot guarantee global optimality, extensive numerical simulations and cross-validation studies demonstrate consistent convergence toward high-performance MET configurations, with sensitivity analyses of design parameters further confirming the engineering applicability of the optimized solutions.

Lubricants doi: 10.3390/lubricants14030112

Authors: Alberto Betti Gianluca Caposciutti Enrico Ciulli Paola Forte Massimo Macucci Matteo Nuti Bernardo Tellini

Tilting pad journal bearings are critical components in high-speed turbomachinery. The use of sensors within the bearing is crucial to ensure operational safety and to validate computational models. The objective of this study is to improve the experimental investigation of the performance of a tilting pad journal bearing by enhancing the selection and placement of conventional and non-conventional sensors based on the results of a thermohydrodynamic model. The multi-sensor system measures film pressure and pad temperature at multiple locations, as well as pad tilt and film thickness. Redundant measurements are also performed to evaluate the performance of new induction coils capable of detecting magnetic flux variations due to vibrations. This work contributes to the discussion of bearing instrumentation by proposing a synergic sensor system comprising a suitable number of appropriately located conventional sensors together with non-conventional, non-invasive sensors. The experimental results obtained with the refined conventional sensor system agree with the predicted results, with differences that can be attributed to manufacturing and assembly tolerances of the bearing and simplified assumptions in the model. The results of the non-conventional sensor device, although promising, need further investigation.

Lubricants doi: 10.3390/lubricants14030111

Authors: Jingtao Hu Ruiqiong Zhang Yijie Fan Gang Ji Xiangfu Meng

Condensation of water vapor into discrete droplets on the surface of transparent optical devices-commonly known as fogging-severely degrades their optical performance. To address this issue, a highly transparent, self-healing, and durable polymer-based anti-fogging coating was developed via a facile one-pot copolymerization of 2-acrylamido-2-methylpropanesulfonic acid (AMPS), acrylic acid (AA), and vinyltrimethoxysilane (VTMOS). The chemical structure and composition were thoroughly characterized. The introduction of VTMOS constructs a hydrophilic-hydrophobic microphase structure through in situ formation of a Si–O–Si network, which significantly enhances the mechanical stability and water resistance. The polymer coating can maintain high transparency (>90%) under extreme conditions (85 °C steam and −40 °C freezing), exhibits long-term anti-frosting performance for 180 days, and demonstrates rapid water-assisted self-healing within 30 s. Differential scanning calorimetry (DSC) analysis reveals that each polymer unit binds approximately seven water molecules, elucidating the mechanism behind its exceptional anti-frosting capability. This work presents a practical strategy for designing high-performance, long-lasting anti-fogging coatings suitable for extreme environment applications.

Lubricants doi: 10.3390/lubricants14030110

Authors: Tianzhao Li Bo Yu Muming Hao Fuyu Liu Yuhan Song

This study investigates the non-Newtonian effects on liquid film seal performance by considering cavitation and thermoelastic deformation—critical factors in high-pressure sealing applications such as nuclear reactor coolant pumps and aerospace systems. We developed a coupled numerical model that simultaneously solves the Reynolds equation using a power-law constitutive model to analyze hydrodynamic performance and employs the energy equation and thermal-structural analysis to determine the temperature distribution and radial taper deformation of the seal rings. The results reveal that the power-law exponent (n) critically influences sealing behavior: shear-thinning fluids (n < 1) reduce the load capacity by 12.7% due to expanded cavitation zones, whereas shear-thickening fluids (n > 1) increase the friction torque by 18.3% through thermally-induced tapered convergence effects. We established quantitative relationships between rheological properties, thermal deformation, and sealing performance, demonstrating that non-Newtonian characteristics fundamentally alter the fluid–structure interaction mechanisms in liquid-film seals. These findings provide a theoretical foundation for optimizing seal designs under extreme operating conditions where conventional Newtonian assumptions prove inadequate, particularly addressing the critical need for enhanced reliability in nuclear and aerospace sealing systems.

Lubricants doi: 10.3390/lubricants14030109

Authors: Changfu Liu Yubai Liu Xiaoning Sun Meng Wang Siqi Feng Yuelong Li Jingjing Gao

Excessive tool wear can compromise machining precision and increase costs, rendering accurate tool remaining useful life (RUL) prediction imperative in intelligent manufacturing. Traditional methods exhibit intrinsic limitations in cross-modal modeling accuracy and capturing temporal dependencies, failing to meet practical requirements. To transcend these bottlenecks, this study proposes a robust tool RUL prediction framework that combines a multi-channel CNN and a Cross-Modal Transformer. The CNN performs convolution operations to extract local features from wear signals, while the Transformer adaptively synchronizes heterogeneous features (cutting force, vibration, and acoustic emission) to capture long-term degradation trends. Empirical evaluations conducted on the PHM2010 dataset demonstrate the model’s robustness and generalization capability: under the random shuffle–split protocol, the proposed method achieves an R2 of up to 0.99, with the RMSE and MAE reaching 2.51 and 1.98, respectively. To further evaluate the framework’s extrapolation ability under domain shifts, a cross-cutter validation protocol was implemented. Under this condition, the experimental results yield an R2 of 0.961, an RMSE of 6.92, and an MAE of 6.09. Additionally, the correlation between modality-specific attention weights and their corresponding physical interpretations is systematically investigated. These results confirm the model’s potential for cross-cutter life cycle management in smart manufacturing, providing stable and physically consistent wear estimation and remaining useful life prediction in noise-intensive environments.

Lubricants doi: 10.3390/lubricants14030108

Authors: Qinghu Wu Pengge Wu Lei Zhang Shuai Zhao Miaojie Wu

Lubrication performance dominates the rating life of grease-lubricated pitch bearings. Conventionally, the life modification factor is determined using base oil viscosity, whose validity is rarely verified. This work presents an effective viscosity-based method for life evaluation of wind turbine pitch bearings. The effective viscosity of grease is measured under actual operating conditions, and a comparative study is conducted against the conventional base oil viscosity method. The rationality of the proposed approach is validated by bearing life tests. Results indicate that the life modification factor calculated from effective viscosity agrees significantly better with test data. Adopting effective viscosity can substantially improve the accuracy of bearing life prediction. The proposed method provides a reliable and practical way to assess the lubrication performance and fatigue life of pitch bearings.

Lubricants doi: 10.3390/lubricants14030107

Authors: Xu Guo Xincong Zhou Jian Huang Ziyang Yan Yun Yang Ruichen Liu Wu Ouyang Yong Jin

As key components in efforts to achieve green and sustainable development in machinery, water-lubricated stern bearings are increasingly replacing traditional oil-lubricated bearings. However, water’s inherent properties—such as low viscosity and poor film-forming ability—can induce severe friction-induced vibration and noise under specific operational conditions. These issues not only accelerate wear but also compromise the vessel’s reliability and acoustic stealth, thereby limiting their wider application. This paper provides a comprehensive review of the research progress relating to friction-induced vibration in water-lubricated bearings. It delves into the underlying mechanisms, critiques the primary methodologies used in numerical simulations, summarizes key experimental approaches, and synthesizes the prevailing vibration suppression strategies. Finally, the study clearly outlines existing challenges and proposes directions for future research.

Lubricants doi: 10.3390/lubricants14030106

Authors: Francisco J. Carrión-Vilches Ana Eva Jiménez Paloma Mostaza María-Dolores Bermúdez María-Dolores Avilés

A novel deep eutectic solvent (DES) formulated from choline hydroxide has been investigated as an additive for advanced aqueous lubrication. Comprehensive characterization of the DES enabled the determination of its viscosity, wettability, and key spectroscopic features, providing insight into its physicochemical behavior. The tribological performance of the water-based lubricants was evaluated using a pin-on-disc configuration with a stainless steel–sapphire tribopair. The resulting friction and wear data demonstrate a significant improvement in performance, particularly for the lubricant containing 10 wt.% DES, which exhibited the most favorable reduction in wear rate, achieving an 80% decrease compared to water. Electrochemical measurements, together with surface analysis by Raman microscopy, confirmed the formation of various iron oxide phases on the wear track that influence tribological performance. These oxides contribute to the development of a protective tribolayer that enhances the overall tribological response. Complementary X-ray-based analytical techniques (EDX and XPS) further substantiated the presence, composition, and stability of this tribolayer. Therefore, the study highlights the potential of the choline hydroxide-based DES as an effective component for formulating novel water-based lubricants.

Lubricants doi: 10.3390/lubricants14030105

Authors: Wenbo Wang Longhang Hou Guangzhou Wang Guoqing Zhang Hechun Yu

Rotor machining errors strongly influence the air-film pressure distribution of aerostatic spindles and fundamentally limit performance enhancement. However, existing studies rarely provide a comprehensive statistical characterization based on measured manufacturing errors. To address this gap, a multi-scale modeling framework based on harmonic analysis of form errors is developed. Measured surface topography data from a batch of rotors are decomposed to establish a harmonic statistical model, which is then incorporated into a modified Reynolds equation together with macro-scale and micro-scale error components. The static performance of the aerostatic spindle is subsequently analyzed. Results show that low-order harmonics (1st–5th) dominate cylindricity errors, with amplitudes following a log-normal distribution. The statistical bounds are described by 3σ envelopes. When the eccentricity ε exceeds 0.3, barrel-shaped errors reduce the load capacity by more than 15%, whereas waist-drum-shaped errors exhibit a self-stabilizing tendency under small deviations. Performance degradation can be partially mitigated by adjusting the supply pressure and orifice diameter. This study addresses the research gap in understanding the impact of measured manufacturing errors on aerostatic spindle performance and provides a quantitative basis for tolerance allocation and performance optimization.

Lubricants doi: 10.3390/lubricants14030104

Authors: Raj Shah Kate Marussich M. Humaun Kabir Hong Liang

Conventional tribological materials such as metals, ceramics, and synthetic polymers demand energy-intensive processing and create end-of-life waste. This motivates the search for more sustainable alternatives. Recent research demonstrates that agricultural residues, industrial by-products, post-consumer waste, and recycled polymers can be engineered into tribological systems that provide competitive wear resistance, stable friction, and multifunctional benefits, including thermal dissipation and vibration damping. This review summarizes progress across these material categories, highlighting how fillers like rice husk ash, fly ash, tire-derived carbon black, and reprocessed plastics transition from low-value waste into high-performance tribomaterials. System-level strategies such as interface engineering, hybrid reinforcement, and advanced processing are essential for overcoming material variability and achieving reliable tribological performance. In parallel, optimization approaches, including predictive modeling and smart material design, are increasingly enabling improved consistency, reproducibility, and scalability. Applications in automotive braking systems, recycled carbon black composites, acoustic damping structures, coatings, and reinforced polymers confirm the industrial viability of waste-derived materials. While challenges remain in feedstock variability, standardization, and long-term durability, these developments point to waste-based tribology as a practical pathway toward circular economy solutions that unite sustainability with engineering performance.

Lubricants doi: 10.3390/lubricants14030103

Authors: Tai Ma Jie Yang Jielin Chen Jiaqiang Dang Qinglong An Ming Chen

With the advancement of high-end manufacturing, the application of difficult-to-machine materials such as titanium alloys and superalloys is becoming increasingly widespread. Their inherent material properties pose challenges during machining, including high cutting temperatures, rapid tool wear, and difficulty in controlling surface quality. Nanofluid minimum quantity lubrication (NFMQL) technology, as an advanced lubrication and cooling method, enhances the thermal conductivity and lubricating properties of fluids by uniformly dispersing nanoparticles in the base oil. This paper reviews the preparation methods, advanced atomization techniques, and core mechanisms of NFMQL technology. It focuses on analyzing the effectiveness of this technology in four major machining processes, turning, milling, grinding, and drilling, for typical materials such as titanium alloys, steel, and superalloys. Compared to dry cutting, conventional MQL, and poured cooling, NFMQL reduces cutting forces/torque, cutting temperatures, tool wear, and surface roughness while improving material removal rates, machining accuracy, and surface integrity. This paper concludes by summarizing the technology’s advantages, current challenges, and future research directions.

Lubricants doi: 10.3390/lubricants14030102

Authors: Yuhao Ma Kang Yang Jun Tang Yanyan Tian

For optimizing the tribological behaviors of sliding bearing, an equal ratio structure of rectangular micro-grid is well constructed and then filled with the SnAgCu-CaF2 (SC) to form a surface micro/nanostructure. The reciprocating wear tests are performed at different sliding frequencies, ensuring that the tribological property at 7 Hz of a TASC-G4 is the best. During wear, the SC in a grid structure migrates to the friction surface and then spreads out to form an SC-rich lubrication film. In this film, a good wrapping in SnAgCu of CaF2 is ensured, helps a plastic enhancement of SnAgCu, an oxidation reduction and the rolling friction of CaF2. These enhance the lubrication film to resist friction damage, reduce sliding resistance, and strengthen interface lubrication, subsequently improves the tribological behaviors of the TASC-G4. The methods and conclusions are obtained to provide an important reference for improving tribological adhibition of the sliding bearings.

Lubricants doi: 10.3390/lubricants14030101

Authors: Haili Jia Wu Xiong Aimin Wang Long Wu Qianxiong Li

Thin-walled partition frame parts are key load-bearing components in aerospace structures. Machining deformation directly affects assembly accuracy and service reliability. During milling, the release of residual stress caused by material removal is one of the main factors leading to deformation of thin-walled parts. To address this problem, aluminum alloy thin-walled partition frame parts are taken as the research object. A machining accuracy control method based on stress-sensitive region analysis is proposed. Key machining accuracy is used as the constraint condition. The concepts of stress-sensitive regions and sensitive directions are introduced. A coupled analysis model of residual stress and machining deformation is established. Residual stress is applied to element meshes in a finite element analysis platform and released under a free state. The influence of residual stress in different regions on part deformation is qualitatively identified. The dominant deformation directions are also determined. Based on these results, the milling tool path is specifically optimized. Strategies are adopted to avoid highly stress-sensitive regions or to control residual stress release by region. Overall machining deformation is effectively reduced. Experimental results show that the optimized tool path significantly suppresses part deformation compared with the conventional tool path. The flatness of the bottom surface is improved by up to 25.33%. The proposed method provides a feasible approach for machining process optimization of aerospace thin-walled parts with high precision.

Lubricants doi: 10.3390/lubricants14030100

Authors: Fida Harabi Basma Ben Difallah Faten Nasri Clisia Aversa Mohamed Kharrat Massimiliano Barletta Antonio Pereira

Previous research indicates that WC-12Co contents above 60 wt.% in feedstock powders for cermet coatings impair adhesion and wear resistance. This study characterizes NiCrFeSiAlBC coatings—unreinforced or reinforced with 65 wt.% or 85 wt.% WC-12Co—applied via high-velocity oxy-fuel (HVOF) spraying onto stainless steel substrates under controlled parameters. It quantifies the influence of high carbide volume fractions within the NiCrFeSiAlBC matrix on microstructure and tribomechanical performance. Microstructural analysis revealed uniformly distributed cermet layers featuring dissolved reinforcements and WC hard phase formation, with minimal W2C crystallization. Elevated WC-12Co incorporation promoted densification and reduced porosity. Vickers microhardness tests (HV 0.3) demonstrated increased hardness upon WC-12Co addition, attributable to finer particle sizes, lower porosity, and the presence of WC phases alongside crystallographic refinements. Under dry reciprocating sliding conditions, friction coefficients and wear volumes decreased markedly. Consequently, the coating with 85 wt.% WC exhibited the best mechanical and tribological properties.

Lubricants doi: 10.3390/lubricants14030099

Authors: Jixin Liu Weihao Chang Junrong Bian Chuanqiang Li Xuxu Zheng

In order to prepare alkyl-functionalized reduced graphene oxide more simply, economically and environmentally, we adopt a two-step method of first reduction and then surface grafting. Graphite oxide (GtO) is first exfoliated to thermally-reduced graphene oxide (TRGO) and then, in a heat-induced solid-state reaction, converted to lauryl-functionalized TRGO (LTRGO). During the second step, lauryl radicals generated from the decomposition of lauroyl peroxide (LPO) open the epoxide rings on TRGO, covalently grafting the alkyl chains. The average water contact angle of LTRGO is 135.5°, and it disperses stably in base oil without surfactants or other additives. Four-ball test results show when the dosage of LTRGO is 75 mg/L, the average friction coefficient and wear scar diameter of the Formosa Plastics base oil (100 N) are decreased by 20.8% and 15.4%, respectively. The morphology and element analysis after ball-on-disk friction tests showed that the stable LTRGO physical friction adsorption film and metal oxide friction chemical reaction film could be formed between the friction pairs, thus reducing the friction wear.

Lubricants doi: 10.3390/lubricants14020098

Authors: Minti Xue Ruihua Tong Hao Lu Zhiyuan Shi Fan Jiang

A significant amount of frictional heat is generated during the braking process of mine-used monorail cranes under heavy-load and low-speed creeping (or reciprocating speed regulation) conditions, causing thermal softening and performance degradation of the brake pads. Thus, investigating the tribological evolution mechanism is necessary to ensure reliable braking in deep underground environments. In this paper, full-scale tribological testing technology is applied to the brake system, and the friction and wear characteristics of copper-based powder metallurgy (P/M) brake pads under complex hygrothermal environments are studied. A physical experimental model coupling normal load, sliding speed, and humidity is established using a custom-designed open-structure reciprocating tester, revealing the “load weakening effect” under dry conditions and the “dual regulation mechanism” of mixed lubrication and cooling flushing under high humidity. Then, a surrogate prediction model of friction coefficient and wear rate, with respect to the operating parameters, is constructed based on Central Composite Design (CCD) and Response Surface Methodology (RSM). The reliability of the model under non-linear working conditions is estimated based on Analysis of Variance (ANOVA) and blind tests. The results indicate that the model possesses high prediction accuracy (relative error < 5%), and the feasibility of utilizing the high-humidity environment to enhance wear resistance and stability is verified.

Lubricants doi: 10.3390/lubricants14020097

Authors: Ruichen Liu Hanhua Zhu Jian Huang Ao Chen Ziyang Yan Fangxu Sun Wu Ouyang Chenxing Sheng Quan Zou Hao Xie

This review provides a comprehensive evaluation of the key technologies and latest advances in cemented carbide bearings for marine environments, such as navigation equipment and deep-sea operations. Given the rigorous performance requirements imposed on bearings by the extreme conditions of marine environments, including high hydrostatic pressure, seawater corrosion and abrasive wear, this paper explores the developments within carbide material systems. It focuses on analyzing the limitations of traditional WC-Co alloys in seawater, as well as the potential and challenges of alternative binder systems such as WC-Ni and WC-high Entropy Alloys (HEAs) in enhancing corrosion resistance and comprehensive mechanical properties. Building on this foundation, the research sorts out the tribological behavior of cemented carbides under seawater lubrication, explaining the influence of the tribocorrosion mechanism on friction characteristics. Meanwhile, it also explores reliability enhancement strategies through surface modifications like coatings and texturing, and discusses the challenges associated with life prediction models. Through tribopair experiments between cemented carbides and various bearing materials, the application orientation of cemented carbides is clarified, which provides a selection framework for carbide bearing applications in different marine scenarios. Finally, the paper summarizes the current technological bottlenecks and core scientific issues, offering insights for future research and development directions in this field.

Lubricants doi: 10.3390/lubricants14020096

Authors: Qisen Cheng Zhengcheng Tang

With the development of manufacturing towards stricter precision requirements and increasingly complex geometric shapes, dimensional accuracy has become a key factor affecting precision engineering components used in many industries. Effective cooling and lubrication methods have always been a meaningful way to improve the surface quality of cutting materials. Minimum-quantity lubrication technology mixes compressed air with cutting fluid, produces a spray at ambient temperature, and guides these droplets to the cutting area under the action of high-pressure air to promote penetration into the contact area between the tool, workpiece, and chip. Minimum-quantity lubrication can be used to increase cutting speed, cool workpieces, improve workpiece quality, and significantly reduce the pollution caused by cutting fluid to the environment. However, minimum-quantity lubrication technology still cannot meet the requirements of sustainable machining in cutting processes. A test device platform for milling 6030 aluminum alloy with minimal quantity lubrication was established, and different cooling methods were used to analyze the effect on surface roughness. The spindle speed n, feed rate f, and cutting depth ap are selected as optimization variables, with surface roughness as the optimization objective. Single-factor experiments were conducted to determine the optimal range for these variables. Subsequently, a model was constructed using the response surface methodology and solved using Design-Expert software. The interaction effects of spindle speed, feed rate, and depth of cut on surface roughness were analyzed. Additionally, genetic algorithms were employed to optimize cutting process parameters for the best combination. The results demonstrated that by combining Response Surface Methodology (RSM)and genetic algorithms, when the spindle speed n was 2520 r/min, the feed rate f was 48 mm/min, and the depth of cut ap was 0.08 mm, the actual surface roughness after milling reached 0.148 µm, representing a 74.57% reduction compared to the initial surface roughness. This research method provides a theoretical foundation and technical support for optimizing minimal quantity lubrication (MQL) cutting processes.

Lubricants doi: 10.3390/lubricants14020095

Authors: Taketo Nakai Renguo Lu Hiroshi Tani Shinji Koganezawa Jinqing Wang

Condition monitoring of rolling bearings is essential for ensuring the reliability of mechanical systems operating under severe or insufficient lubrication conditions. This study proposes a fault diagnosis framework that integrates tribological interpretation of wear phenomena, acoustic emission (AE) signal analysis, and machine learning, based on bearing life tests conducted under dry conditions as an accelerated wear environment to capture damage progression within a practical experimental time. Unlike conventional studies relying on artificially introduced defects, this work focuses on AE signals obtained from bearings in which damage initiates and progresses through actual wear processes. Life tests were conducted using deep groove ball bearings under two radial load conditions. The temporal evolution of the coefficient of friction, AE signals, and surface damage was analyzed. Although the coefficient of friction was the most sensitive indicator of wear progression, its direct measurement is impractical for in-service applications. Frequency-domain analysis revealed that AE counts per second and band-specific AE energy exhibit early changes consistent with the evolution of the friction coefficient. Using these physically interpretable AE features, a fully connected neural network was developed to classify bearing conditions into normal, early-stage damage, and damage progression. The proposed model achieved an average classification accuracy of approximately 85%, demonstrating the effectiveness of AE-based machine learning for bearing fault diagnosis under real wear progression conditions rather than artificial defect scenarios.

Lubricants doi: 10.3390/lubricants14020094

Authors: Recep Çağrı Orman

In this study, boron carbide (B4C), hexagonal boron nitride (hBN), holy super graphene (HSG), and hybrid (B4C+hBN+HSG) nano-additives were added to SAE 15W-40 diesel engine oil at a range of 0.03–0.24 g per 30 mL of oil, and reciprocating tribological tests were conducted on a GG25 (EN-GJL-250) gray cast iron-based diesel piston surface in contact with an Al2O3 ball (Ø6 mm) at a load of 20 N, a sliding distance of 500 m, and a temperature of 75 °C. XRD analysis showed that the dominant phase on the piston surface was the α-Fe matrix and that no significant new phase had formed. The results obtained revealed that the nano-additive effect is strongly dependent on both the additive type and the additive level. At a low level (0.03 g/30 mL) of B4C additive, the average COF decreased by approximately 19%, while at a low level (0.03 g/30 mL) of hBN additive, this decrease amounted to approximately 54%. In the HSG additive, at the highest level (0.24 g/30 mL), the coefficient of friction (COF) decreased to ≈0.032, achieving a friction reduction of approximately 75% compared to the base oil. In the hybrid oil series, COF values remained in the range of approximately 0.082–0.087 at all additive levels and were generally 25–28% lower than those of the base oil. SEM/EDS examinations showed that a tribofilm with high carbon content formed in the HSG-additive oils, while a tribofilm layer containing C, B, and N elements together formed in the hybrid-additive oils. Overall, it was concluded that selecting the appropriate additive type and level can reduce friction and wear losses at the piston interface, thereby contributing to engine efficiency by extending the life of engine components and limiting friction-induced energy losses.

Lubricants doi: 10.3390/lubricants14020093

Authors: Bochao Wang Xingyun Jia Qiaoqiao Bao Jiang Qiu

High-power clutches operating under high-frequency engagement–disengagement cycles demand piston ring seals with exceptional leakage control and tribological reliability. Conventional architectures often experience lubrication failure and severe adhesive wear during transient pressure fluctuations. This research proposes an autonomous intelligent sealing strategy leveraging fluid pressure-induced morphological evolution. By strategically integrating periodic macroscopic structural relief features on the non-sealing surface, the sealing interface transforms into a micron-scale wavy topography in response to hydraulic loading. This structurally embedded intelligence significantly improves fluid pressure distribution, facilitating a transition toward a more favorable lubrication regime. Furthermore, a “self-healing and positional stagnation” logic is elucidated: upon pressure dissipation, the induced waviness elastically recovers to a planar state to ensure sealing integrity, while the ring maintains its axial position due to the predominant frictional resistance of the secondary seal. This synergistic mechanism effectively precludes deleterious dry friction during the clutch disengagement phase. High-fidelity numerical investigations, benchmarked against established experimental data, identify the rectangular groove configuration as the optimal geometry for maximizing waviness amplitude (≈1.5 µm). This research provides a robust framework for developing responsive, zero-wear intelligent seals in advanced power transmissions.

Lubricants doi: 10.3390/lubricants14020092

Authors: Luís Vilhena Barnabas Erhabor Tsering Wangmo Bruno Figueiredo Amílcar Ramalho

Graphene, a 2D carbon allotrope with a hexagonal atomic structure, exhibits an exceptionally low friction coefficient of approximately 0.004, making it a superior alternative to traditional lubricants. This research investigates the performance of graphene as an additive in oil-based lubricants. Experimental trials will be conducted using a block-on-ring (B-o-R) setup involving a steel rod pressed against a rotating steel ring under a fixed load. By varying the sliding velocities, the study will map the Stribeck curve across the boundary (BL), mixed (ML), and hydrodynamic (HL) lubrication regimes. Furthermore, the lubricant’s durability under extreme pressure will be assessed via Timken testing. The study identified 0.08 wt.% as the optimal concentration for PAO8, achieving a 21.25% friction reduction in the boundary regime. Furthermore, graphene as an additive mitigated wear volume by up to 90% under extreme pressure conditions (1.3 GPa), whereas epoxidized soybean oil proved to be highly effective as a base lubricant without additional nano-additives.

Lubricants doi: 10.3390/lubricants14020091

Authors: Long Wang Guanyu Deng Jun Cheng

High-temperature tribology is a multidisciplinary science that has evolved rapidly in response to the increasing performance demands of high-technology sectors, including aviation, aerospace, nuclear energy, power generation, and advanced metal forming industries [...]

Lubricants doi: 10.3390/lubricants14020090

Authors: Tianyi Shi Yanting Zhang

To enhance the load-carrying capacity and operational stability of dry gas seal gas films while reducing gas leakage, and to provide a theoretical basis for structural optimization and innovation of face seals, a numerical model of gas film lubrication in face dry gas seals considering the geometric effects of combined micro-textures is developed based on the governing equations of gas lubrication. The finite difference method is employed to numerically solve the gas film pressure distribution. With the objectives of maximizing the opening force and minimizing the leakage rate, the influences of combined micro-texture structural parameters on gas film performance under typical operating conditions are systematically investigated, and favorable parameter ranges are identified. The results show that the proposed model exhibits high accuracy and reliability, with good agreement with published data. Different combined groove textures significantly affect the gas film thickness and pressure distributions, leading to distinct bearing and stability characteristics. When opening force and leakage are jointly considered, the sealing performance ranks as triangular composite texture, semicircular composite texture, rectangular composite texture, and trapezoidal composite texture. Quantitatively, the trapezoidal texture exhibits the largest increase in the opening force–leakage ratio of approximately 0.29%, whereas the triangular texture shows the smallest increase of about 0.19%. Reasonable design of combined micro-textures can effectively improve the comprehensive gas film performance of face dry gas seals, achieving a coordinated enhancement of opening force and reduction in leakage. The present study provides theoretical guidance for the structural design and engineering application of high-performance dry gas seals.

Lubricants doi: 10.3390/lubricants14020089

Authors: Zewen Li Wei Zhao Junjie Hu Peng Zhao Liang Li Feng Kong

To enhance the sustainability of manufacturing, various clean cutting technologies have been developed, yet their sustainability assessment faces challenges in balancing multiple conflicting objectives and stakeholder interests. This paper proposes a game theory-based evaluation framework that treats environmental, technical, economic, and social dimensions as cooperative players. The Nash equilibrium model is employed to dynamically reconcile subjective weights from the analytic hierarchy process and objective weights from the entropy method, thus achieving optimal weight allocation. Experimental studies on Ti-6Al-4V titanium alloy milling compared dry milling, minimum quantity lubrication, and cryogenic minimum quantity lubrication (CMQL) under different parameters. Results demonstrate that the game-theoretic model effectively integrates preferences and achieves Nash equilibrium. CMQL showed superior performance, increasing tool life by approximately 40% and reducing surface roughness by about 25% compared to dry milling. Coated inserts reduced carbon emissions by nearly 30% versus end mills. The Nash equilibrium analysis demonstrates that dry milling with coated inserts attains the highest level of processing sustainability under high-speed conditions due to synergistic environmental and economic advantages, while simultaneously revealing practical trade-offs among competing objectives. This study confirms that the proposed framework enables scientific weight coordination and provides a quantifiable, interpretable decision-making system for sustainable process selection.

Lubricants doi: 10.3390/lubricants14020088

Authors: Zhonghao Gao Yiqiao Guo Wendi Zhu Wei Du Lanjie Huang Hao Zhang

With the significant increase in the number of motor vehicles in plateau regions, the adaptability and reliability requirements of diesel engines operating under high-altitude and cold conditions have become increasingly critical. In this study, a one-dimensional transient simulation model of the overall engine lubrication system was developed based on a physical experimental prototype. The multiphysics-coupled lubrication system was numerically modeled and analyzed, with particular emphasis on elucidating the influence mechanisms of high-altitude and cold environments on the startup performance of diesel engine lubrication systems. System responses under different ambient pressures (0.88 bar, 0.92 bar, 0.96 bar, and standard atmospheric pressure) and oil temperatures (30 °C, 55 °C, and 100 °C) were systematically investigated. In addition, variations in the opening degree of the oil pump pressure relief valve (closed, 4%, 30%, 60%, and 100%) were incorporated to reveal the governing effects of high-altitude and cold environments on lubrication system startup behavior. The results indicate that under high-altitude and cold conditions, the decrease in oil temperature is the dominant factor and exerts the most significant influence on the steady-state oil pressure and flow rate of the lubrication system. Variations in ambient pressure lead only to an equivalent shift in absolute oil pressure, with negligible effects on relative oil pressure, steady-state flow rate, response time, or filling rate. However, a reduction in atmospheric pressure leads to a decrease in the peak oil flow rate at the outlet of the oil pump. The opening degree of the pressure relief valve exhibits a nonlinear influence on the startup performance of the lubrication system, and significantly decreases the oil filling rate. This study innovatively develops a lubrication system performance prediction model under high-altitude, low-pressure, and low-temperature conditions. Calibrated using vehicle road-test data, the model quantifies for the first time the relative contributions of the three key factors to start-up lubrication performance, thereby providing a clear decision-making framework and prioritized improvement directions for the reliability-oriented design and safety threshold calibration of lubrication systems in high-altitude diesel engines.

Lubricants doi: 10.3390/lubricants14020087

Authors: Martin Ovsik Adam Cesnek Adam Pis Klara Fucikova Michal Stanek

This study examines the tribological and micro-mechanical behavior of high-density polyethylene (HDPE), which has been advanced to the class of advanced polymers through electron beam irradiation (irradiation dose of 33 kGy to 198 kGy). The tribological and mechanical behaviors were analyzed at the surface and at various depths beneath the surface to verify the extent of radiation effects across the entire cross-section of the specimen. Changes in tribological and mechanical behavior are closely related to changes in the structure of the material, mainly changes in crystallinity. As this study shows, 99 kGy appears to be the ideal radiation dose in terms of the properties examined. An increase in absorbed radiation dose leads to a deterioration of tribological and mechanical performance, which correlates with material degradation and a concomitant reduction in crystallinity. The improvement in the properties examined between unirradiated and irradiated HDPE at a dose of 99 kGy is 18% for mechanical behaviors and 8% for tribological behaviors on the surface of the sample. A maximum deviation of 39% was identified between the surface and the center of the material. There was also a change in crystallinity of up to 12%. These modifications result in enhanced surface wear resistance and increased overall stiffness, effectively shifting commodity-grade HDPE toward the performance domain of advanced polymers with only minimal cost implications.

Lubricants doi: 10.3390/lubricants14020086

Authors: Haisheng Yang Run Zhang Jiahang Zhang Zhanwang Shi Yingbin Wei

Excavators are critical equipment in mining, construction, and other fields. The four-point contact slewing bearings used in their slewing mechanisms operate under harsh conditions such as heavy loads and impacts. Furthermore, the bearing rings are prone to elliptical deformation after installation, making them susceptible to premature failure. To address this issue, this paper establishes a mechanical bearing model to investigate the load distribution among balls and the fatigue life of the bearing under elliptical deformation of the rings. It systematically analyzes the influence of key design parameters. The research finds that elliptical deformation of the rings leads to contact angle deviation and a reduction in load-bearing balls, resulting in severe degradation of bearing fatigue life; therefore, its occurrence must be strictly controlled. Designing with a groove curvature radius coefficient within the range of 0.51 to 0.52 achieves an optimal balance between fatigue life and the four-point contact geometry of the balls. There exists an “optimal clearance” that maximizes bearing fatigue life; when considering significant elliptical deformation, the clearance design should be appropriately increased. Increasing the design contact angle enhances load capacity and helps mitigate the effects of elliptical deformation. However, an excessively large contact angle can cause ellipse truncation in the raceway contact zone; thus, the contact angle should be designed based on practical conditions. Increasing the number of balls can improve the influence of ovality on load distribution and enhance the bearing’s fatigue life. This study provides a theoretical reference for the design of high-reliability slewing bearings for excavators.

Lubricants doi: 10.3390/lubricants14020085

Authors: Michał Wasilczuk Bob Shortridge Wojciech Litwin

The results of experimental tests of six various water-lubricated bearings are described. Tests were performed under conditions typical for marine stern tube bearings. The acquired low-friction coefficient values indicated that the bearings operated in the fluid friction regime over a wide range of sliding speeds and loads. Due to elastic deformations of the flexible non-metal bushings, it was not possible to measure the lubricant film thickness to confirm this phenomenon. Studying measured hydrodynamic pressure distribution profiles, and thanks to lifting force calculation, it was proven that hydrodynamic phenomena occur between strongly deformed, rough surfaces lubricated by a low-viscosity fluid.

Lubricants doi: 10.3390/lubricants14020084

Authors: Qinglin Ye Huijie Zhang Yuzhen Liu Juan Jin Kai Le Shusheng Xu Xiaoming Gao Lijun Weng

Electroless Ni–P coatings are widely used for corrosion and wear protection, yet their ability to deliver water-based superlubricity and the role of phosphorus content remain insufficiently understood. Here, electroless Ni–P coatings with four P contents (3.4, 6.4, 9.0, and 12.4 wt%) were deposited on GCr15 steel with nearly constant thickness and comparable initial roughness, and were tested against Si3N4 balls in neutral 0.5 M NaH2PO2 solution. Friction measurements, together with surface topography characterization and tribofilm analysis, were used to link P content with tribofilm chemistry and superlubricity. All coatings achieved macroscale superlubricity, exhibiting steady-state friction coefficients below 0.01, while the running-in time decreased markedly as P content increased. During sliding, the wear tracks underwent mechano-chemical polishing to Sa ≈ 11–12 nm and formed phosphate–silicate tribofilms enriched in P–O and Si–O species on both the coating and the counterface. These findings establish a composition–tribofilm–superlubricity relationship in the Ni–P/NaH2PO2 system and demonstrate that P-content optimization is an effective internal design lever to accelerate running-in, mitigate wear, and achieve robust superlubricity under neutral aqueous lubrication.

Lubricants doi: 10.3390/lubricants14020083

Authors: Leonardo Ubiola-Soto Adolfo Delgado

This paper examines the impact of pad-to-pad manufacturing variations in three-lobe journal bearings on system-level rotordynamics. Two sources of non-uniform clearance were studied: dissimilar pad clearance and preload. Both were varied independently within standard manufacturing tolerances. The results show that the conventional assumption that all pads having equal clearances at tolerance extremes does not capture worst-case conditions. Instead, specific non-uniform pad combinations caused the most significant amplification factors and the lowest stability margin. By applying a Surface Response Design of Experiments (SRDOE) method, surrogate models were developed to represent the nonlinear influence of the pads’ dissimilarity. The models identified the most critical combinations of pad and journal variables, revealing that industry-standard practice does not provide the most adverse system behavior. Worst-case conditions arise from non-uniform pad geometry: SRDOE models predict critical combinations, while uniform assumptions of industry-based standards underestimate risk. Incorporating realistic manufacturing variability in rotordynamic models provides a more reliable basis for turbomachinery design.

Lubricants doi: 10.3390/lubricants14020081

Authors: P. R. Deshmukh Dae-Hyun Cho

Triboelectric nanogenerators (TENGs) are a newly adopted technology designed to harvest freely available mechanical energy from the environment and convert it into electricity that can help to meet future demands for clean and sustainable energy. TENGs represent a promising next-generation renewable energy technology, an alternative to traditional non-renewable fossil fuel sources, with a wide range of applications, including smart sensors, wearable devices, internet of things (IoT), and portable electronics. Through contact/triboelectrification and electrostatic induction, TENGs convert mechanical energy into electrical energy. Broadly, TENGs are classified into contact–separation mode and sliding mode. In contact–separation mode, the electric output is achieved through the contact and separation of triboelectric layers, while in the sliding mode, it is generated by the sliding of one triboelectric layer over another. Sliding-mode TENGs have demonstrated better electrical output compared to the contact–separation mode; however, they suffer low durability and cannot operate for long periods due to severe wear. In addition, their electrical output performance is reduced owing to air breakdown. Lubricants have demonstrated their potential in TENGs by overcoming these limitations and improving both tribological and triboelectric performance. This review provides a discussion on the fundamental modes of TENGs, followed by a comprehensive summary of the tribological and triboelectrical performance of existing TENGs under liquid lubrication, along with a comparison of their performance under dry conditions. The effects of load, frequency, mass fraction, and volume of the liquid lubricant on both tribology and electrical output are examined. Durability is identified as a key factor for the long-term practical application of TENGs; hence, this paper also focuses on it. Furthermore, strategies for improving TENG performance and the working mechanisms under liquid lubrication are discussed. Finally, the paper summarizes demonstrations of TENG applications based on various TENG designs.

Lubricants doi: 10.3390/lubricants14020082

Authors: Dimosthenis Filon George Anastopoulos Ypatia Zannikou Dimitrios Karonis

This study investigates the synthesis and physicochemical characterization of biolubricant base oils derived from sunflower oil methyl esters (SUNOMEs) via transesterification with trimethylolpropane (TMP) using guanidine carbonate (GNDC) as a green and efficient catalyst. The transesterification process was optimized to achieve high conversion and desirable physicochemical properties suitable for lubrication applications. The synthesized esters were characterized by viscosity, density, pour point, and oxidation stability, confirming their suitability as environmentally friendly lubricants. Reaction parameters, such as catalyst concentration (3.0–5.0 wt%), were optimized under both solvent-free and vacuum-assisted conditions. The use of guanidine carbonate achieved enhanced physicochemical properties with significantly reduced reaction times (≈6 h) and eliminated soap formation. The resulting TMP triesters exhibited kinematic viscosities in ranges of 41.27–52.73 cSt (40 °C) and 8.668–10.02 cSt (100 °C), a viscosity index in the range of 180–196, and excellent oxidation stability (RSSOT: up to 54.27 min). Fourier transform infrared (FTIR) analysis confirmed the formation of complete triester structures with characteristic carbonyl and C–O stretching bands at 1735 cm−1 and 1050 cm−1, respectively. Spectra showed also distinct stretching vibrations near 1640–1670 cm−1 and 3300–3400 cm−1, which correspond to amide carbonyl and N–H characteristic groups. The tribological performance was evaluated using Four-Ball Standard Test Method, demonstrating significant improvements compared to commercial mineral oils. The results indicate that guanidine carbonate is an effective catalyst for producing sunflower-oil-derived esters with favorable lubricating properties, highlighting their potential as sustainable biolubricants for industrial applications.

Lubricants doi: 10.3390/lubricants14020080

Authors: Yikai Zheng Lun Li Yujun Xue Hanqi Wu Yipeng Ren Jiayi Zhao

The sliding shoe bearing serves as a critical rotary support component in large grinding mills. The deformation of the hollow shaft under operating conditions is a pivotal factor governing the uniformity and stability of the lubricating oil film thickness in sliding shoe bearings. To address this, a finite element model of the sliding shoe bearing system, comprising the lubricating oil film and hollow shaft, was established based on fluid–structure interaction (FSI). The model’s predictions for oil cavity pressure and hollow shaft radial displacement were validated using a custom-built test rig designed for single-shoe sliding shoe bearing oil pressure measurements. Utilizing this finite element model, the relationship between hollow shaft deformation and oil film pressure distribution was systematically investigated. The study analyzed the effects of key parameters—specifically the area ratio of the primary and secondary oil chambers, radial load, secondary oil chamber supply pressure, and primary oil chamber supply orifice diameter—on the axial and circumferential deformation of the hollow shaft. The results indicate that the oil film pressure distribution directly influences the deformation of the hollow shaft. The area ratio of the oil chambers emerges as the dominant factor affecting this deformation. Furthermore, radial load exerts a significant impact, whereas the influence of the secondary oil chamber supply pressure is relatively minor. Conversely, the inner diameter of the primary oil chamber supply orifice exhibits a negligible effect on the hollow shaft deformation.

Lubricants doi: 10.3390/lubricants14020079

Authors: Yubin Zhang Fengtao Wang Chunlan Yu Huomei Zhu Xiaoyun Zhao

Misalignment of the herringbone groove radial bearing can lead to changes in performance and system abnormalities. To investigate the effects of different misalignment modes and magnitudes on the HGJB-rotor system, a coupled dynamic model was established. Based on this model, the influences of parallel misalignment and angular misalignment on bearing performance were analyzed, and the variation law of rotor vibration was revealed. The results indicate that the rotor motion trajectory and bearing dynamic coefficients (including critical journal mass and critical whirl frequency) exhibit time-varying characteristics. Specifically, compared with the aligned condition, a parallel misalignment of δ = 8.0 × 10−6 m reduces the relative film thickness by 17.8% and increases the maximum film pressure by 1.85%. Meanwhile, an angular misalignment of θ0 = 8.0 × 10−4 rad results in a 45.9% reduction in relative film thickness and a 33.1% increase in maximum film pressure. Additionally, the increased misalignment magnitude enhances the rotor vibration amplitude significantly. For instance, the Y-direction displacement amplitude increases by 59.4% under the maximum parallel misalignment. Moreover, the misalignment also alters the axial trajectory of the rotor. Overall, different misalignment modes and magnitudes exert significant effects on the rotor vibration characteristics. The research findings provide theoretical support and technical references for the further development and engineering application of HGJBs.

Lubricants doi: 10.3390/lubricants14020078

Authors: Mohd Zaki Sharif Mohd Syafiq Abd Aziz Mohd Farid Ismail Mohd Fadzli Bin Abdollah Abdul Aziz Mohamad Redhwan Nor Azazi Ngatiman Anwar Ilmar Ramadhan

This study tests TiO2 and SiO2 nanolubricants in PAG oil using a Mini Traction Machine and an Ultra Shear Viscometer. The loads were 20 N and 40 N. The entrainment speeds ranged from 2.5 to 500 mm/s. The slide-to-roll ratio (SRR) ranged from 25 to 150%. The nanoparticle concentrations were 0.01, 0.03, and 0.05%. The ball size was 19.05 mm, and the disc was 46 mm. All tests were run at 40 °C. Only the 0.05% concentration lowered traction compared with PAG at a fixed SRR. TiO2 at 0.05% showed the largest drop, up to 4.89% at 20 N and 2.99% at 40 N. However, lower concentrations increased traction. All the nanolubricants reduced wear. TiO2 at 0.03% gave the lowest wear, with a reduction of about 35 µm at 40 N. Nanolubricant samples stayed between 40.2 and 40.5 °C, while PAG reached about 41.0 °C. TiO2 produced slightly lower temperatures than SiO2. Ultra-shear tests from 40 to 100 °C showed shear thinning. In most conditions, TiO2 at 0.05% kept the highest viscosity at 40 and 60 °C, up to 12% above PAG. SiO2 showed smaller changes. TiO2 delivered better friction, wear, temperature, and viscosity performance. Overall, both nanolubricants at 0.03% are suitable when wear reduction and thermal stability are prioritised over traction reduction, such as in refrigeration applications, while the 0.05% suits high-load or high-shear use.

Lubricants doi: 10.3390/lubricants14020077

Authors: M. Marliete F. Melo Neta Rodolpho R. C. Monteiro Paulo R. C. F. Ribeiro Filho Célio L. Cavalcante Francisco Murilo Tavares Luna

Interest in replacing petroleum-based lubricants with bio-based alternatives is driven by growing demand for lubricants, in contrast to a decreasing supply of products derived from fossil resources, coupled with environmental concerns. Biolubricants offer several advantages over conventional petroleum-based lubricants, such as biodegradability and renewability. Researchers have been seeking solutions for these challenges over the years, employing various approaches, including the use of different raw materials, chemical modifications, and different types of additives. This review evaluates a total of 504 articles published between 2010 and 2025 in the Scopus database, with the help of RStudio, using the bibliometrix package. The objective is to provide an integrated bibliometric and systematic analysis, presenting the research landscape on the tribological properties of biolubricants, which may contribute to the development of novel investigation initiatives in the field. The main thematic trends, researchers, journals, and most active countries and institutions have been evaluated. Additionally, the most cited studies, recent advances and existing gaps are presented and discussed.

Lubricants doi: 10.3390/lubricants14020076

Authors: James Layton Humberto Medina Hasna Fadhila Benjamin C. Rothwell Stephen Ambrose Katrina Farbrother Carol Eastwick

A dedicated experimental rig is presented for a half-journal bearing operating under highly loaded, well-controlled hydrodynamic lubrication conditions relevant to turbomachinery. The apparatus combines pressure measurements in the film, distributed temperature measurements in the shaft and bush, and ultrasonic film-thickness measurements that map the circumferential film-thickness profile across the lubrication region. Experiments are reported for normal loads of 5–20 kN and shaft speeds of 1000–4000 rpm with controlled oil supply conditions. The measured pressure and temperature trends are consistent with established hydrodynamic lubrication behaviour. The film thickness measurements confirm full-film operation across the tested operating envelope, while indicating increased uncertainty in regions affected by cavitation. A correlation for the temperature rise due to viscous heating is proposed as a compact representation of the data. The rig design and accompanying measurements provide a benchmark-quality data set intended for validation and development of thermal elasto-hydrodynamic lubrication (TEHL)/computational fluid dynamics (CFD) models under high load and speed conditions.

Lubricants doi: 10.3390/lubricants14020075

Authors: Tianzhao Li Muming Hao Yongfan Li Yong Song Baojie Ren Chenyin Wang Xiaozu Li

The spherical assembly mechanical end-face seal, a pivotal component of the liquid rocket engine turbopump, holds direct influence over the performance of the turbopump. This paper introduces a approach for assessing the reliability of mechanical seals. The proposed method formulates a dimensionless reliability factor, R, derived from multiple sets of monitoring data collected during the operation of the seal. The health status of the seal can be evaluated based on the value of R. The computation of R is contingent on two main elements: firstly, it relies on the threshold of evaluation parameters obtained from laboratory tests, and secondly, it incorporates the weight of parameters derived from expert experience using fuzzy set theory. And then R can be calculated by substituting real-time monitoring data of the seal. The efficacy of the proposed method was substantiated through testing and verification using two sets of real-world engineering data, and was subsequently compared with methods currently employed in engineering. The results indicate that the proposed method surpasses existing methods in terms of accuracy and sensitivity. Furthermore, the data upon which it is based can be easily monitored in an engineering context, thereby enhancing its relevance and potential for widespread application in engineering.

Lubricants doi: 10.3390/lubricants14020074

Authors: Jiazhen Chen Ashlie Martini

Wear, a critical factor governing the performance and durability of mechanical systems, is typically characterized using point-contact and line-contact test configurations. However, it remains unclear whether the wear trends observed in one test configuration would be observed in the other configuration under the same nominal conditions. In this study, ball-on-disk (ASTM G99) and block-on-ring (ASTM G77) tests were conducted under an identical maximum Hertzian contact stress and sliding speed, using the same material pair and lubricating oil, to clarify which contact configuration exhibits more wear and why. The results show that, under the same Hertzian contact stress, the line-contact configuration exhibits a specific wear rate two orders of magnitude higher than the point-contact configuration, despite exhibiting a lower and more stable coefficient of friction. The disk wear is negligible and the ball shows only mild material loss, whereas the line-contact system displays wear rates several orders of magnitude higher, with the rotating ring contributing the dominant share of the total wear. White-light interferometry and scanning electron microscopy observations reveal directional, groove-dominated surface morphologies on the ball and disk, while wear on the block is confined to edge-localized regions and the worn ring surface has smooth, polished morphology. Energy-dispersive X-ray spectroscopy confirms that a Zn- and P-rich tribofilm forms exclusively on the ring surface. Finite element analysis shows stress amplification at the finite line-contact edges, explaining the observed wear severity. These results demonstrate that matching Hertzian contact stress alone is insufficient to ensure comparable wear behavior between point and line contacts.

Lubricants doi: 10.3390/lubricants14020073

Authors: Yanan Wang Wenjun Yu Jianping Ai Xihui Tao Qingyun Guo Dongye Wang Junying Suo Xiuxu Zhao

To elucidate the contact performance and friction force variation characteristics of VL seals for aviation actuators under high-pressure conditions, this study adopted a fluid–structure interaction (FSI)-coupled finite element model to analyze the maximum contact pressure and contact width and their respective variation trends across varying oil pressures and reciprocating velocities. Subsequently, friction force tests of the seals were conducted under matching operating parameters, and the friction coefficients of polytetrafluoroethylene (PTFE) were measured and compared under different normal pressures. The results demonstrate that the friction force of the seals during both extending and retracting strokes increases with rising oil pressure, which is highly correlated with the theoretically predicted conclusion that the seal contact width expands as oil pressure increases. Further analysis confirms that reciprocating velocity exerts no significant influence on the aforementioned variation trends. This study provides a critical basis for the selection and optimal design of VL seals used in high-pressure aviation hydraulic actuators.

Lubricants doi: 10.3390/lubricants14020070

Authors: Xiangke Tian Tai Ma Jie Yang Qinglong An

Titanium alloys face challenges such as high temperatures, high forces, and tool wear during turning, milling, drilling, and grinding operations. Cryogenic minimum quantity lubrication (CMQL) technology, which combines cryogenic cooling with micro-lubrication, offers an effective solution to these challenges through its synergistic mechanism of heat suppression via cooling and friction reduction via lubrication. This paper first elucidates the cooling and lubrication principles of various CMQL technologies and their adaptability process. It then reviews CMQL applications across four titanium alloy machining processes, systematically analyzing their effects on cutting forces, temperatures, tool wear, surface integrity, and chip morphology. Research indicates that CMQL technology demonstrates universal advantages over minimum quantity lubrication (MQL) across diverse titanium alloy machining processes. Furthermore, incorporating nanofluids or integrating ultrasonic vibration to form enhanced composite processes can further improve medium permeability, reduce machining loads, and enhance surface quality. Future developments in this field will advance toward intelligent and sustainable directions, providing critical technological support for high-performance green manufacturing of titanium alloys.

Lubricants doi: 10.3390/lubricants14020072

Authors: Raj Shah Kate Marussich Vikram Mittal Andreas Rosenkranz

Artificial intelligence transforms lubricant research by linking molecular modeling, diagnostics, and industrial operations into predictive systems. In this regard, machine learning methods such as Bayesian optimization and neural-based Quantitative Structure–Property/Tribological Relationship (QSPR/QSTR) modeling help to accelerate additive design and formulation development. Moreover, deep learning and hybrid physics–AI frameworks are now capable to predict key lubricant properties such as viscosity, oxidation stability, and wear resistance directly from molecular or spectral data, reducing the need for long-duration field trials like fleet or engine endurance tests. With respect to condition monitoring, convolutional neural networks automate wear debris classification, multimodal sensor fusion enables real-time oil health tracking, and digital twins provide predictive maintenance by forecasting lubricant degradation and optimizing drain intervals. AI-assisted blending and process control platforms extend these advantages into manufacturing, reducing waste and improving reproducibility. This article sheds light on recent progress in AI-driven formulation, monitoring, and maintenance, thus identifying major barriers to adoption such as fragmented datasets, limited model transferability, and low explainability. Moreover, it discusses how standardized data infrastructures, physics-informed learning, and secure federated approaches can advance the industry toward adaptive, sustainable lubricant development under the principles of Industry 5.0.

Lubricants doi: 10.3390/lubricants14020071

Authors: Gyula Varga István Sztankovics Antal Nagy

High-feed face milling is widely adopted in industry for its productivity advantages, especially when machining medium carbon steels. However, the combined effects of lubrication regimes on both the cutting forces and surface quality remain insufficiently explored, creating a research gap in optimizing process parameters for improved performance. This study presents an experimental investigation into the effects of lubrication on cutting forces and surface topography during the high-feed face milling of C45 steel. Using a design of experiments (DOE) approach, eight distinct machining setups were developed by varying the cutting speed, depth of cut, and feed per tooth. Each setup was tested under two lubrication conditions: with flood coolant and under dry machining. Cutting forces in the X, Y, and Z directions were recorded using a dynamometer, while the post-machining surface quality was evaluated using 3D areal surface topography measurements. The results revealed that feed per tooth was the primary factor affecting both the cutting forces and surface roughness, with depth of cut having a moderate effect and cutting speed a minor influence. Flood lubrication reduced the peak forces, stabilized force fluctuations, and improved surface uniformity, particularly in the valley depths and skewness parameters. This work provides (i) a combined analysis of cutting forces and surface topography under high-feed milling, (ii) quantitative evidence of lubrication effects on force and surface consistency, and (iii) identification of dominant process parameters for optimization, offering practical guidance for enhancing productivity, surface quality, and tribological performance in high-feed milling operations.

Lubricants doi: 10.3390/lubricants14020069

Authors: Zhaoxia Luo Dingkang Zhu Wenjing Zhang Weisong Tian Yu Zhang Koucheng Zuo Lai Hu

Addressing the scientific problem that the profile modification design of tapered roller bearings primarily focuses on contact stress and fatigue life while neglecting its impact on wear evolution, this paper, based on Hertzian contact theory and the Archard wear theory, and considering centrifugal force, gyroscopic effect, and the complex contact state between rollers and raceways, constructed a comprehensive analysis framework integrating a quasi-static model for profiled rollers and a wear depth calculation model. This framework is novel in that it systematically couples roller profile modification parameters with raceway wear evolution under both pure axial and combined radial–axial loads. The validity and effectiveness of the proposed model were verified by comparing the results of the quasi-static model with load distribution data from existing literature and through measurements conducted on a specially designed bearing wear test platform. The main findings are as follows: (1) When the logarithmic modification parameter f1 increases from 0.7 μm to 3.6 μm, the maximum wear depth of the inner raceway increases by 133% under pure axial load and 144% under combined load, while that of the outer raceway increases by 142% under pure axial load and expands from 0.1–0.2 μm to 0.23–0.52 μm under combined load. (2) Combined load induces significant asymmetric wear on the outer raceway, and the difference between the two wear peaks increases from 0.13 μm to 0.35 μm as f1 rises from 0.7 μm to 3.6 μm. (3) The wear peak shifts toward the midpoint of the roller generatrix with increasing modification amount. These results provide important guidance for the wear-oriented optimization design of tapered roller bearings.

Lubricants doi: 10.3390/lubricants14020068

Authors: Meng Kong Gengyuan Gao Lei Wang Shijie Yu

To meet the demand for water-lubricated bearings (WLBs) with low vibration, low noise and high load-carrying capacity in propulsion systems, this study designed and tested a three-layer composite WLB consisting of an inner phenolic working layer, a middle rubber damping layer and a glass-fiber-reinforced composite layer. The lubrication, vibration and wear behaviors of three bearings with different groove structures, namely a non-grooved bushing, a fully straight-grooved bushing and a fully spiral-grooved bushing, were comparatively investigated under combined variations in rotational speed (20–400 r/min), specific pressure (0.18–0.8 MPa) and water flow rate (5–20 L/min). The results demonstrate that both specific pressure and flow rate strongly govern the transition from mixed lubrication to hydrodynamic lubrication and the associated vibration response. As the specific pressure and water flow rate increase, the transition speed and coefficient of friction of grooved bearings, particularly straight-grooved bearings, increase markedly. Non-grooved bearings consistently maintain the lowest levels, while spiral-grooved bearings exhibit lubrication performance intermediate between the above two types. Under low-speed and heavy-load conditions, non-grooved bearings show the smallest increase in vibration amplitude. Grooves amplify high-frequency vibrations and inject medium- and high-frequency energy as rotational speed increases. Considering lubrication, vibration control, and wear resistance simultaneously, spiral-grooved bearings exhibit the most robust overall performance under realistic operating conditions. The results provide experimental evidence and practical design guidance for groove-structure selection in multi-layer composite WLBs operating under low-speed and heavy-load conditions.

Lubricants doi: 10.3390/lubricants14020067

Authors: Nahian Siddique Yu-Sheng Li Fangxin Qian Ruichuan Yuan Bahareh Kheilnezhad Seong H. Kim Xin He

In electric vehicle (EV) drivetrains, lubricant films must not only mitigate friction and wear but also manage stray currents to safely dissipate stray charge and avoid micro-arcing. This study directly compares how a conventional antiwear additive (ZDDP) and a long-chain, ashless, sulfur-free phosphite ester (Duraphos AP240L) manage this balance under current-carrying boundary lubrication conditions. Reciprocating steel-on-steel tests were conducted at fixed load and speed with applied current densities of 0, 0.02, and 42.4 A/cm2. Friction and four-probe electrical contact resistance (ECR) were measured in situ, and impedance of tribofilms was measured over a 1–105 Hz range after friction test. In the presence of ZDDP, ECR initially increased and then decreased to a value that was as low as the initial direct contact of two solid surfaces or even lower sometimes. During the initial stage with high ECR, a well-defined impedance semicircle was observed in the Nyquist plot; after forming the tribofilm with low ECR, frequency dependence of impedance could not be measured due to the very low resistance. The decrease in ECR suggested a structural evolution of the anti-wear film on the substrate. However, post-test wear analysis indicated that the formation of this film was accompanied by tribochemical polishing of the countersurface and sometimes pitting of the substrate, which may have been due to localized electrical discharge producing trenches deeper than ~0.5 µm; in additive-free base oil, wear was dominated by ploughing with micro-cutting of the substrate. In contrast, AP240L performed better in terms of friction and wear, showing a remarkable ~30% lower coefficient of friction, while the overall cycle dependence of ECR was similar to the ZDDP case. AP240L showed negligible boundary film controlled wear producing a shallow, smooth track (depth < 0.2 µm) during the friction test, and there was no sign of electrical arc damage. These findings support long-chain, ashless, sulfur-free phosphite esters as promising candidates for EV boundary lubrication where both mechanical and electrical protection are required.

Lubricants doi: 10.3390/lubricants14020066

Authors: Hongyang Zhang Shuchong Wu Jinghua Li Yang Li

As a core component of industrial power transmission and motion control, the surface quality and dynamic performance of gears are pivotal to the transmission efficiency, durability, and reliability of mechanical equipment. Driven by extreme service conditions and the demands of high-precision applications, surface lubrication failures (such as contact fatigue and scuffing) have become a critical bottleneck limiting gear performance, making the development of advanced surface-strengthening technologies a vital direction for industrial innovation. This paper provides a systematic review of research progress in gear-related surface-strengthening technologies, with a particular focus on techniques for preparing solid lubricant layers. It elaborates on the microstructures, lubrication mechanisms, and application performance of typical solid lubricant layers (e.g., iron sulfides, nitrides, molybdenum disulfide (MoS2), diamond-like carbon (DLC) films, and graphite-like carbon (GLC) films) in gear systems. Furthermore, it offers an in-depth analysis of the synergistic mechanisms between single-surface treatments and composite-strengthening processes. Additionally, it outlines innovative applications of additive manufacturing (AM) in gear manufacturing.

Lubricants doi: 10.3390/lubricants14020065

Authors: Angel Sebastian Alvarez Lozano Armando Irvin Martínez Pérez Edgar Ernesto Vera Cárdenas Juan Manuel González Carmona Arturo Mendoza Galván

This work presents a study on the evaluation of the erosive wear behavior of laminated composites, manufactured using the vacuum-assisted resin infusion (VARI) method with a glass fiber-reinforced epoxy matrix modified with SiO2 nanoparticles (0.0, 1.5, and 3.0 wt.%). Results indicate that nanoparticle concentration and dispersion state critically influence the mechanical and tribological performance. The composite FG-1.5-SiO2 with 1.5 wt.% SiO2 exhibited optimal nanoparticle distribution, as confirmed by FTIR, GIXRD, and SEM analyses, with the lowest surface roughness (Ra = 0.215 μm), highest hardness (35.58 HV), and highest elastic modulus (19.66 GPa). These enhancements contributed to a 38% improvement in erosion rate compared to the unmodified laminated composite, with the lowest total mass loss (0.0261 mg) and erosion rate (2.3360 × 10−5 mg/g). Profilometry and SEM results revealed shallower wear depths and reduced matrix removal, indicating stronger fiber–matrix interface integrity. In contrast, the 3.0 wt.% SiO2 composite (FG-3-SiO2) suffered from nanoparticle agglomeration, which increased surface roughness, diminished mechanical properties, and reduced erosion resistance to levels comparable to the unreinforced material. The results indicate that homogeneous dispersion at an optimal concentration (1.5 wt.%) is crucial for improving erosion resistance, while agglomeration at higher concentrations negates the potential benefits of nanoparticle incorporation. These findings highlight the need to optimize nanoparticle dispersion for the development of fiberglass/epoxy composites with greater durability and erosion resistance in demanding applications.

Lubricants doi: 10.3390/lubricants14020064

Authors: Milan Omasta Tomáš Knoth Petr Šperka Michal Hajžman Ivan Křupka Pavel Polach Martin Hartl

This study investigates the influence of surface texturing on temperature distribution in lubricated sliding contacts using infrared thermography. The work addresses the broader challenge of understanding thermal effects in conformal hydrodynamic contacts, where localized heating and viscosity variations can significantly affect tribological performance. A pin-on-disc configuration was employed, featuring steel pins with laser-etched micro-dimples that slid against a sapphire disc, allowing for thermal imaging of the contact zone. A dual-bandpass filter infrared thermography technique was developed and rigorously calibrated to distinguish between the temperatures of the steel surface and the lubricant film. Friction measurements and laser-induced fluorescence were used in parallel to assess contact conditions and the behavior of the lubricant film. The results show that surface textures can alter local frictional heating and contribute to non-uniform temperature distributions, particularly in parallel contact geometries. Lubricant temperature was consistently higher than the surface temperature, highlighting the role of shear heating within the fluid film. However, within the tested parameter range, no unambiguous viscosity-wedge signature was identified beyond the dominant temperature-driven viscosity reduction captured by the in situ correction. The method provides a novel means of experimentally resolving temperature fields in sliding textured contacts, offering a valuable foundation for validating thermo-hydrodynamic models in lubricated tribological systems.

Lubricants doi: 10.3390/lubricants14020063

Authors: Minmin Zhao Jing Wei Le Huang Feng Tan Yong Wang Jinyu Hu

This study first assessed the friction and wear properties of two polytetrafluoroethylene materials sliding against electroplated chrome and high-velocity oxy-fuel-sprayed WC-10Co-4Cr coatings. Subsequently, the sealing performance of three different structure seals made from these two polytetrafluoroethylene materials was investigated on both electroplated chrome and high-velocity oxy-fuel-sprayed WC-10Co-4Cr coatings. The study results indicate the following: in terms of changes in the counter-face surface roughness, both the electroplated chrome and high-velocity oxy-fuel-sprayed WC-10Co-4Cr surfaces exhibited an increase in surface roughness after sliding, demonstrating the phenomenon of “soft material wearing hard material.” Moreover, the changes in surface roughness were greater after sliding against wollastonite mineral-filled polytetrafluoroethylene than against polyether ether ketone-filled polytetrafluoroethylene, indicating that wollastonite mineral-filled polytetrafluoroethylene was more likely to cause damage to the metal surface. Regarding the friction coefficient and wear amount, under dry friction conditions, both materials exhibited higher friction coefficients but lower wear rates on high-velocity oxy-fuel-sprayed WC-10Co-4Cr surfaces, while showing lower friction coefficients but higher wear rates on electroplated chrome surfaces. This behavior was related to the ease of transfer film formation and the stability of the transfer films formed by polytetrafluoroethylene materials on the two surfaces. In terms of the products’ sealing performance, test results showed that, for composite seals with polytetrafluoroethylene as the counter-face, sealing performance was better on high-velocity oxy-fuel-sprayed WC-10Co-4Cr surfaces than on electroplated chrome surfaces. For seals with rubber as the counter-face, there was little difference in sealing performance between high-velocity oxy-fuel-sprayed WC-10Co-4Cr and electroplated chrome surfaces.

Lubricants doi: 10.3390/lubricants14020062

Authors: Leyan Xia Gongning Li Kun Huang Yuxing Peng Yu Tang Zhou Zhou Ran Deng Xiangdong Chang

Steel wire ropes are key load-bearing components in systems such as mine hoisting, bridge cableways, elevators, and cranes, and frictional wear is among the earliest occurring and most easily accumulated form of damage. Under actual working conditions, micro-relative sliding occurs both along the internal wires of the rope and at the contact surfaces with sheaves and ropes, leading to frictional wear, crack propagation, and fatigue failure. Frictional wear, a complex phenomenon influenced by structural layout, contact load, vibration conditions, lubrication, and environmental corrosion, critically determines the service life and load-bearing capacity of steel wire ropes. Recent experimental and numerical studies have significantly clarified the fundamental mechanisms and patterns of internal and external frictional wear in steel wire ropes, offering theoretical support for the distribution of wear, fatigue evolution, and fracture behavior. Meanwhile, non-destructive testing techniques have emerged as a vital tool for the real-time monitoring of wear conditions in steel wire ropes. This review summarizes the research progress on the generation, characteristics, effects, and protection of frictional wear in steel wire ropes, and proposes future directions for tribology and service safety research of steel wire ropes.

Lubricants doi: 10.3390/lubricants14020061

Authors: Shunyao Wang

The traditional analysis of dynamic characteristics inside a ball bearing mainly focuses on the kinematic trajectory in the steady condition, while various working conditions are often neglected in investigations. A general approach for dynamic modeling and analysis of an angular contact ball bearing (ACBB) considering combined load effects is proposed in this work. In the analytical model, the characteristics of contact-impact are considered and a dissipative contact model is employed to represent energy loss effects. A dynamic performance experiment of an ACBB is performed, which illustrates that the proposed model can be satisfied with the evaluation of dynamic behavior for an ACBB. The characteristic distribution of kinematics and mechanics for an ACBB are calculated, which reveals that there is the correlation between dynamic behavior and working conditions in an ACBB. Clear contact characteristics always appear in the worst working condition. Based on the dynamic stability, the optimal design of structural parameters for an ACBB is outlined, which will prove helpful to achieve a better bearing performance.