Addressing the challenges of inadequate lubrication performance and limited thermal stability of variable hyperbolic circular-arc-tooth-trace (VH-CATT) cylindrical gear pairs under conditions of high speed and heavy load, this study presents a multi-field coupled thermo-elastohydrodynamic lubrication (TEHL) model. The model integrates cutter tilt-induced lead modification, tooth contact analysis (TCA), finite element-based contact mechanics and thermal-elastohydrodynamic lubrication simulations. By selecting representative meshing instances, the research systematically examines the combined influence of entrainment angle (theta e) and lead modification angle (gamma) on key parameters, including oil film thickness, pressure distribution and temperature elevation. The findings indicate that when theta e deviates from 0 degrees to either less than 0 degrees or greater than 0 degrees, the minimum oil film thickness decreases by approximately 5.03%. Concurrently, the maximum oil film pressure exhibits an increase of 22.33% and a decrease of 21.14%, respectively, while the temperature rise diminishes by 63.81% and 34.12%, respectively. Upon the introduction of lead modification with gamma set at 5 degrees, variations in theta e below and above 0 degrees result in reductions in minimum oil film thickness by 4.29% and 4.81%, respectively; decreases in maximum pressure by 8.89% and 3.97%, respectively; and changes in temperature rise characterised by a decrease of 83.75% and an increase of 72.94%, respectively. A comprehensive evaluation suggests that maintaining theta e within the range of 0 degrees-5 degrees, alongside gamma fixed at 5 degrees, facilitates the synergistic optimisation of lubrication integrity, load-bearing capacity and thermal management performance. This insight offers a theoretical framework and practical foundation for enhancing lubrication efficacy and structural design of VH-CATT cylindrical gear pairs under complex thermal loading conditions.
This study uses a parametric optimization and characteristic analysis of water-lubricated bearings (WLB) under various design configurations and operating conditions. To characterise the performance of WLB, 31 simulation cases were designed using the Taguchi method. The study examines the impact of key design and operating parameters-specifically, attitude angle, eccentricity ratio, length-to-diameter (L/D) ratio and rotational speed (RPM)-on the resulting temperature distribution, structural deformation and stress within the bearing. The results show that increasing the eccentricity ratio improves the load-bearing capacity, with the peak pressure rising from 500 000 Pa (e/c = 0.5) to 4 060 000 Pa (e/c = 0.9). Similarly, attitude angle has a significant effect on hydrodynamic pressure. The result shows that when the attitude angle goes from 20 degrees to 60 degrees, the maximum pressure goes from 730 000 to 1 960 000 Pa. The impact of RPM fluctuations on the temperature of the water film is apparent, with Case 21 (450 RPM) measuring 305.88 K, while Case 14 (525 RPM) attains 314.32 K, illustrating the influence of rotational speed on thermal dissipation. Furthermore, ANN-based optimization was employed to determine the optimal design and operating parameters for achieving maximum load-bearing capacity and minimising heat dissipation. This led to an ideal bearing pressure of 35589.14 Pa and a water film temperature of 314.32 K. This study provides valuable insights into optimising the performance of water-lubricated journal bearings, enhancing their longevity and reliability in industrial, maritime and hydropower applications.
Inspired by the structure of shield scales on the surface of a shark skin, a surface texture of the shield scale with asymmetric, converging-diverging depth profiles is designed and its tribological performance is investigated through a combination of numerical modelling, optimisation and experimental validation. The primary objective is to explore how complex depth-direction geometries and non-Gaussian surface roughness influence lubrication and friction behaviour under mixed lubrication conditions. To this end, a comprehensive numerical model is developed, which incorporates a modified Reynolds equation with mass-conserving cavitation (JFO) boundary conditions, coupled with a Kogut-Etsion contact model and a statistical description of non-Gaussian surface roughness using skewness and kurtosis. Using the developed numerical model, the lubrication performance of bionic shield scale texture with different geometric parameters has been calculated. Then a hybrid method combining artificial neural networks (ANN) and genetic algorithms (GA) is employed to optimise the geometric parameters of bionic shield scale texture and the optimal combination of area ratio, depth gradient and aspect ratio has been found. Representative samples with circle, hexagon and shield scale textures are fabricated by laser surface texturing technique and comparison tribology experiments are carried out. The better performance of the shield scale texture with optimised parameters is observed, which is consistent with numerical predictions.
Solid lubricants have long been used to combat friction and wear, especially under harsh operating conditions of numerous moving mechanical systems. In this study, we explored the effect of another harsh condition, i.e., electrical current, on the friction and wear performance of two solid lubricants: one with very high electrical conductivity (i.e., graphite) and one with very low electrical conductivity (i.e., boric acid (H 3 BO 3 )). They were applied as solution-processed powder coatings on AISI 52100 steel discs and tested against uncoated 52100 steel balls with and without contact electrification. The main goal was to determine whether electrical conductivity plays a critical role in the tribological performance of these solid lubricants. Results showed that graphite was very effective in reducing friction and wear under non-electrified conditions. Specifically, it reduced the coefficients of friction (COFs) of steel test pairs by about 75% and the wear volume of the balls by a factor of more than 50 compared to the uncoated unelectrified condition. However, under contact electrification, the reduction in COF was about 60%, and the wear volume of the balls increased significantly, but graphite was still able to reduce the wear volume by fourfold compared to uncoated electrified steel. On the other hand, the COF and wear volume of the balls tested against the boric acid coated disc were about 75% and 1227 times lower, respectively, than that of the ball tested against uncoated and unelectrified disc. However, under electrification, the friction coefficient did not change much, while the wear volume of the balls went up but was still 20 times lower than that of the balls slid against uncoated electrified disc. Post-test microscopic and chemical analyses showed that boric acid remained structurally and chemically intact under electrified and non-electrified conditions and thus effectively protected the sliding surfaces against wear. Conversely, graphite was partially worn out or removed from the sliding surfaces when tested under electrified conditions and thus provided marginal benefits. The superior friction and wear performance of boric acid is attributed to its low shear strength and highly insulating nature, demonstrating its potential applications in electric vehicle drivetrains.
This study prepared graphene/molybdenum disulphide (RGO/MoS 2 ) composite additives via freeze-drying to enhance their dispersion stability and oxidation resistance. The tribological properties and lubrication mechanisms of these nano-additives in lubricating oils were systematically investigated through experimental characterisation and molecular dynamics simulations. Results demonstrated that MoS 2 's interlayer sliding intensifies with increasing load/speed, though excessive mechanical stress induces structural degradation; graphene provides stable support for MoS 2 , reducing atomic slippage and improving synergistic lubrication. A comparison between the experimental and molecular dynamics simulation results leads to the following conclusions: MoS 2 demonstrates superior friction performance under high-speed and light-load conditions compared to RGO/MoS 2 . Conversely, RGO/MoS 2 exhibits better friction performance under high-load and low-speed conditions than MoS 2 alone. Importantly, across various testing conditions, the trends in the friction coefficient curves obtained from molecular dynamics simulations align well with the experimental data.
This article investigates the synergistic effect of surface texturing and Magneto-rheological (MR) lubricants on the steady-state and dynamic performance indices of two-lobe journal bearings. MR lubricants, composed of magnetic particles in mineral oil, offer tunable viscosity through applied magnetic fields. The study employs a generalised Reynolds equation, incorporating a continuous Bingham fluid model to capture the lubricant's non-linear behaviour. The Dave model has been used to describe the shear stress of commercially available MRF122EG lubricant, as a function of volume fraction of magnetic particles and magnetic field. Surface textures, comprising arrays of spherical and conical cap dimples in various circumferential configurations, are analysed. The Reynolds equation is solved using a finite element approach coupled with the Newton-Raphson method, and a mass-conserving algorithm addresses gaseous cavitation. Optimal surface texture configurations for maximising direct stiffness parameters are determined, revealing that partial texturing in the first half of the bearing is most effective. The study further examines the influence of dimple shape, texture configuration and MR lubricant on the performance of circular and two-lobe journal bearings. Journal center trajectories are predicted by solving the equation of motion using the fourth-order Runge-Kutta method. The key findings indicate that the combined application of MR lubricant and partial surface texturing in the first half of two-lobe bearings enhances direct stiffness parameters by up to 781.9%. Threshold speed, a critical design parameter, is significantly improved (89.3%) in two-lobe bearings through surface texturing and MR effects. Two-lobe journal bearings with MR lubricants exhibit smaller and more stable journal center trajectories and limit cycles, indicating enhanced dynamic stability. Consequently, for applications requiring enhanced stability and performance, first half partially textured surface two-lobe journal bearings with MR lubricants should be preferred.
To cope with lubrication failure of mechanical parts caused by leakage of water-based lubricants, we designed a novel and facile hydrophilic modification method for MoO3 nanoparticles to generate oleic acid diethanolamide (ODEA)-functionalized MoO3 nanoparticles (ODEA-MONP). The as-prepared ODEA-MONP nanohybrids were systematically characterised using technologies such as XRD, TEM, XPS, and FTIR. The tribological properties of ODEA-MONP and water-based sulfur-containing additive (W4770) were evaluated with UMT-2 and SRV5 tribometers; concurrently, the collaborative lubrication process involving ODEA-MONP and W4770 was investigated. It was found that water plays a decisive role in the initial stage by promoting the formation of a uniform and stable lubricating film structure (SEM shows homogeneous Mo/S distribution), which critically determines the lubrication performance in the subsequent water-free state. XPS analysis confirmed that after high-temperature water evaporation in the aqueous system, a composite tribofilm consisting of MoS2, MoO3, MoOx, iron oxides, and organic compounds formed. The present work not only provides new insights into the in situ vulcanization of molybdenum-containing materials in water-lubricated systems but also broadens the application potential of water-based additives under water-depleted conditions.
The present study investigates the dynamic characteristics of journal bearings with rectangular macro-grooves, focusing on parameters such as stiffness, damping coefficients and stability threshold speed. Using the finite element method to solve the governing Reynolds equation for lubricant flow, the analysis explores the influence of varying macro-groove texture numbers, groove depth and area density across different bearing regions such as the full region, first half, second half and pressure-increasing region to identify the optimal performance parameters in each region. Findings reveal that both stiffness and dynamic coefficients of the journal bearing improve at higher and lower eccentricity ratios. Among the various investigated macro-grooved regions, at an eccentricity ratio of 0.2, the maximum enhancement of stability threshold speed of 84.23% is found for full grooved macro rectangular-textured surface with two macro-grooves at non-dimensional groove depth of 0.9 with an area density of 54.01%. Additionally, incorporating 0.5% weight fractions of copper oxide and cerium oxide nanoparticles into the base lubricating oil at 90 degrees C significantly enhances stiffness, damping coefficients and stability threshold speed.
In this study, a dynamic friction testing apparatus was independently developed to investigate the frictional properties of tire-asphalt pavements under varying road conditions, tailored to complex working environments. This device underwent rigorous reliability analysis to ensure compliance with the experimental standards. The calibration of asphalt pavements with varying heating durations established regions of uniform temperature. Experiments were conducted using custom-designed equipment under conditions involving changes in particle size, temperature, and surface roughness. Results showed that both the maximum static friction and average friction forces increased with increasing temperature. Smaller particle sizes intensified stick-slip behaviour but reduced frictional forces, whereas greater tire surface roughness enhanced friction and accentuated stick-slip phenomena. An improved spring-slider model was proposed to simulate the frictional behaviour of asphalt pavements, yielding results consistent with the experimental data. Adjusting the roughness parameter in the model demonstrated that friction gradually decreases with reduced roughness, while stick-slip effects diminish, aligning qualitatively with the experimental observations.
The surface of copper-based electronic devices typically exhibits nanoscale roughness rather than being an ideal smooth plane. To accurately assess the authentic tribological performance of graphene (Gr) and hexagonal boron nitride (h-BN) coated on rough substrates, a molecular dynamics model is constructed to simulate the interaction between a diamond tip and a coated copper substrate. The sliding friction simulation is accomplished on both smooth and rough substrates. The friction, wear and subsurface damage on substrates coated with Gr and h-BN are compared. The results show that the substrate coated with h-BN exhibits enhanced friction, increased wear and fewer dislocations compared to the one with Gr for the same rough topography. Furthermore, the transition of the substrate topography from smooth to rough leads to higher friction, increased wear and reduced subsurface damage within the same coating. This effect is more pronounced for h-BN than for Gr, due to the strong adhesion of h-BN and the high compressive strength of Gr, which results in distinct differences in coating buckling behaviour on the rough substrate. This study offers valuable guidance for the development of durable coatings in microelectronic devices.
Due to the incomplete understanding of the mixed lubrication mechanisms of novel multi-layered composite water-lubricated stern bearings under complex operational conditions, this paper addresses the cantilevered offset loading conditions and the multifactorial coupling characteristics that these bearings frequently encounter in such complex scenarios. Firstly, a mathematical and physical model for mixed lubrication within the multi-layered composite water-lubricated bearing-flexible rotor system was established. Secondly, numerical simulations were utilised to analyse the impact of coupled factors such as rotational speed, load, water supply pressure and radial clearance on the mixed lubrication performance of the bearings. Finally, a water-lubricated bearing test rig was constructed to conduct multi-condition and multi-section lubrication performance tests on the bearings. The research findings indicate that under single-sided loading conditions with the same velocity increment, the water film pressure decay rate accelerates from measurement points P1 to P5, with a pronounced decay observed at section P5, with a decrease of 48%. As the rotational speed increases, the squeezing effect diminishes for sections further from the cantilever end, leading to a reduction in water film pressure and alleviation of pressure concentration. Under double-sided loading conditions, the water film pressure in the cross-section at measurement point P2 decreases by 10%, and this trend moderates as the load increases. In contrast, the water film pressure in the cross-section at measurement point P5 increases to 25 kPa, and the circumferential distribution of the water film broadens. Moreover, as the rotational speed increases, the water film pressure decreases and the circumferential distribution of the water film narrows.
The interfacial bonding between a matrix and a lubricating phase has a considerable effect on the mechanical properties of lead-free FeS/Cu-Bi self-lubricating composites, but the effect of the load-induced interfacial bonding behaviour of heterophases on the composites' tribological properties is unclear. In this study, FeS/Cu-Bi self-lubricating composites with weak and strong FeS/Cu interface bonding were prepared, and the effects of sliding load on the precipitation characteristics of FeS particles and tribological properties of FeS/Cu-Bi composites were studied through experiments and numerical simulation. Results showed that the interfacial failure of FeS/Cu heterophases and precipitation morphology of FeS particles varied because of the varied strength of FeS/Cu bonds. In FeS/Cu-Bi composites with weak interfacial bonding between FeS and Cu, FeS is prone to detachment and expulsion from the friction interface in the form of wear debris during sliding. Under low-load conditions, the effect of FeS discharged as wear debris is weak, and the material exhibits a relatively low friction coefficient (0.31) and wear rate (5.7 x 10-5 mm3N-1m-1). The friction coefficient and wear rate of the material gradually increased with load. Strengthening the interfacial bonding between FeS and Cu impedes the detachment of FeS as wear debris during sliding, promoting its precipitation and deposition on the worn surface and thereby mitigating adhesive wear. The friction coefficient and wear rate of the material decreased initially and then increased with increasing load. The material with strong FeS/Cu interfacial bonding effectively induced the precipitation of FeS towards the worn surface under a moderate-load condition and formed a complete and continuous lubricating film. The minimum friction coefficient and wear rate were 0.22 and 1.3 x 10-5 mm3N-1m-1, respectively.
In this study, lithium stearate and fumed silica co-thickened grease with po-ly(sodium-4-styrenesulfonate) (PSSNa) additive was synthesised and characterised for application as an automotive lubricant, including for the potential use in an electric vehicle (EV). Oscillatory shear and steady shear rheological tests confirmed that the sample exhibited appropriate shear thinning required in greases. Fumed silica was found to increase thermal stability. Additionally, the conductivity of the synthesised grease was higher than the threshold conductivity required to avoid electrical arcing and static charge buildup (4 x 10-12 S/cm). The tribo-pair lubricated with LSFSPNa grease demonstrated a significant improvement in tribological performance, with the coefficient of friction reduced by approximately 65% compared to commercial grease, decreasing from 0.31 to 0.11. The wear volume showed a tenfold reduction, accompanied by a substantial decrease in surface roughness (Ra), which dropped from 0.81 mu m with commercial grease to 0.17 mu m with LSFSPNa grease. At the same time, the synthesised grease exhibited better copper corrosion resistance. Overall, the synthesised grease was found to be compatible for EV applications in terms of rheology, friction reduction, copper corrosion resistance, conductivity, and thermal stability.
Rolling contact fatigue (RCF) is the default mode of failure observed in Hertzian rolling contact elements, such as bearings. Experimentally, the current study examines the influence of the existence of solid lubricant dispersed in liquid base lubricant on RCF life. Polytetrafluoroethylene (PTFE), a solid lubricant additive, is selected for this purpose, and the RCF performance of varying percentages of PTFE (0.1-2.5 wt%) on the base lubricant has been examined. RCF tests were carried out on a two-disc on-cylinder test rig under pure rolling conditions within the mixed lubrication regime. The RCF life was enhanced for every concentration of PTFE-added lubricant composition. The base lubricant containing 2 wt% PTFE concentration exhibits the highest improvement in mean and L10 RCF life, yielding a 2.3-fold increase in improvement over the base lubricant. The excellent RCF performance is attributed to the decreased wear rate, PTFE particle adsorption and tribo-chemical film formation, and there is a decreased likelihood of metal-to-metal contact and a delayed onset of micropitting.
This study aims to explore the potential application of LaF3 nanoparticles as high-performance gear oil additives. Lanthanum trifluoride nanoparticles modified by di(2-ethylhexyl) phosphoric acid (D2EHPA) [denoted as LaF3-D2EHPA], which exhibited excellent dispersion stability, were prepared using a liquid-phase chemical method. The tribological properties of LaF3-D2EHPA nanoparticles in weakly polar mineral base oil (150 N), polyalphaolefin (PAO6), and strongly polar synthetic ester oil diisooctyl sebacate (DIOS) were investigated using a four-ball friction and wear tester. In combination with the adsorption experiments using a dissipative quartz crystal microbalance (QCM-D), the compatibility and action mechanisms of LaF3-D2EHPA nanoparticles with extreme-pressure and anti-wear additives for typical gear oils were further investigated. The results indicate that when the mass fraction of LaF3-D2EHPA nanoparticles is 0.2%, the tribological properties of all three base oils are improved to the greatest extent. When LaF3-D2EHPA nanoparticles are mixed with gear oil additives T321 (sulfurized isobutylene) and T307 (ammonium thiophosphate), they exhibit high film-forming rates and anti-wear properties, forming a boundary lubricating film on the worn surface. This film consists of a tribochemical reaction film containing sulfur and phosphorus, as well as LaF3-D2EHPA nanoparticles that have been deposited.
Lubricant composition plays a crucial role in reducing wear and friction between moving components, thereby ensuring the smooth operation and protecting critical components of industrial machinery. Friction modifiers, as key additives, play a significant role in lowering the coefficient of friction and improving the energy efficiency of lubricants. In this study, we investigated the influence of friction modifier chemistry and concentration on the physico-chemical and tribological properties of industrial lubricating oil. Results demonstrated that increasing the friction modifier concentration effectively reduces the friction coefficient in tribological tests and enhances the elastohydrodynamic film thickness. Furthermore, the study highlights the significant role of surface temperature in activating the friction modifier. These findings provide valuable insights for the design of energy-efficient lubricants with optimised performance characteristics.
Here we provide atomistic insights into the intrinsic origin of friction from interfacial charge fluctuation by density functional theory (DFT) calculations of typical 2D vdWs systems, including graphene, h-BN and transition metal chalcogenides. We proposed a quantitative method to discriminate the redistribution of interfacial charge density in the sliding process. It is thus discovered that the synchronous evolution between sliding charge fluctuation and potential energy surfaces corrugation is necessary to surmount frictional resistance, giving a linear relation coefficient k for charge transfer and potential energy acting on the frictional sliding under different normal stress. This study implies that the inherent friction characteristic of the sliding systems could be traced back to the evolution of charge transfer, enabling prediction and design of tribological response.
Among atmospheric contaminants, soot is known to cause severe wear of mechanical components and degrade lubricating oils; however, its impact on lubricity grease remains largely unexplored. This study investigates the interactions between carbon black (as a soot surrogate) and other lubricant additives, including molybdenum disulfide (MoS2) and anti-wear additive zinc dialkyldithiophosphate (ZDDP). Additionally, the effects of individual and combined additions of MoS2 and ZDDP on the tribological properties of soot-contaminated lithium grease (LG) were compared. The lubricity of LG decreased progressively with increasing soot concentration. Conversely, incorporating MoS2 improved the lubricity of LG, with optimal performance observed at 0.5 wt%. The individual addition of MoS2 to LG minimally mitigated the abrasive wear caused by soot; however, the inclusion of ZDDP T202 enhances the effectiveness of MoS2 in reducing the friction and wear. MoS2 and ZDDP participate in tribochemical reactions to form protective films during sliding. MoS2 contributes by reducing friction, while ZDDP reacts to form protective films that enhance wear resistance. However, the combined addition of ZDDP and MoS2 demonstrates limited effectiveness in fully mitigating the adverse effects of soot on grease lubrication. Consequently, exploring more efficient methods to prevent lubrication failure caused by soot.
The contact fatigue of carbon steel gear tooth surfaces under different lubrication conditions has been investigated and a model of the effect of lubrication conditions on contact fatigue has been established. Micro damage less than Hertzian contact is more likely to expand into macro pitting. The pressure around the micro damage increases when the micro damage partially coincides with the contact width, so the tooth surface around the pit is more likely to produce new pits, and then forms the superposition and aggregation. The larger the lubricating oil supply, the larger the contact width, and the larger the width of the micro damage that can be activated, the improvement of lubrication can promote the propagation of micro damage.
According to the Environmental Protection Agency, transportation is the largest contributor of greenhouse gas emissions (28% of total emissions). Electric vehicles have remarkably grown in popularity and represent a greener future for the automotive industry. This growth has prompted lubricant and grease technology to adapt to an entirely new environment, which is exposed to new factors including external electric currents/fields and extreme temperatures and pressures originating from electric motors and power electronics. Consequently, novel lubricants and greases need to be developed and explored to ultimately improve fuel efficiency and performance. Nano‐additives have offered exceptional opportunities to enhance electrical, thermal and tribological properties of the lubricants and greases used. It is thus vital to fully explore and understand the effects of nanoparticles' addition to lubricants and greases, as well as the mechanisms by which improvements are obtained. This perspective summarises the recent trends of developing lubricants and greases for electric vehicles.
