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.

This work executes a novel multi -objective optimization for a multi -stage Wind Turbine Gearbox (WTG), using a Non -Dominated Sorting Genetic Algorithm (NSGA-II) optimization technique with scuffing constraint for three different involute gear profiles: unmodified (U), smooth meshing (SM), and high load capacity (HLC). Further, ISO VG PAO synthetic -based oils are used for the WTG at a given rated speed. Two objective functions, namely, weight and power loss minimization are constructed with various traditional design constraints related to mechanical and scuffing wear. The result of one spur gear pair is validated with commercial gear software KISSsoft using ISO VG 460 mineral oil and obtained 0.605% efficiency improvement with scuffing constraint. Further, results with and without scuffing clearly shows that PAO 680 is the best -performing oil among the other oils. Based on the performance and Pareto front of the PAO 680, obtained minimum weight and power loss with scuffing is 7215.44 kg and 17.61 kW for high load capacity gear tooth profile.

The lubrication regime of a hip implant is influenced by various parameters, including material, geometry, roughness, relative angular movement, and loading circumstances. These factors impact friction and wear performance, ultimately limiting the implant's longevity. In this study, a ball-on-plane model is considered to predict the lubrication regime across the entire gait cycle for the metal-on-metal and ceramic-on-ceramic hip implants. According to ISO 14242-1, a normal walking gait cycle is considered for the loads and rotations in the hip implant for this study. A comprehensive analysis is done to estimate the lubrication regime by considering different material combinations, body weights, femoral head sizes, clearances, and roughness across the entire gait cycle. The correlation coefficients show that the geometrical parameters are dominant in affecting the lubrication film thickness compared to the operational and material parameters. The femoral head size and body weight are found to be the most dominant and least dominant parameter respectively. Among the hard-on-hard tribo-pairs, ceramics operate predominantly in the full-film lubrication regime compared to metals due to its fine surface finish. The present research suggests that in order to maximise the longevity of a hip implant, an orthopaedic surgeon should choose one with a larger femoral head diameter, less clearance, and an ultra-fine surface finish, regardless of any material tribo-pair.

Purpose This paper aims to investigate the tribological benefits of a biomimetic teardrop surface texture inspired by snakeskin compared to conventional surface textures with the help of geometrical and flow parameters using computational fluid dynamics techniques. Design/methodology/approach The lubricant is assumed to be Newtonian, and the flow is laminar with constant viscosity and isothermal property. The governing equations, continuity and Navier–Stokes equation, are discretised by the finite volume method, and cavitation modelling is included. The discretisation for the momentum equations is carried out using the second-order difference method for the SIMPLEC algorithm of pressure–velocity coupling. Findings The results indicate that biomimetic teardrop surface texturing performs better than conventional shapes surface textures in improving tribological performance. Furthermore, the parallel texture orientation along with the flow generates a high-pressure distribution relative to other orientations. Surface texture area density also highly influences the load-carrying capacity, which is optimum at 29%. Zigzag pattern arrangement performs better compared to linear pattern arrangement of texturing. Originality/value The paper proposes that this unique biomimetic teardrop shape can give better tribological performance than conventional shapes. Peer review The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-02-2024-0053/

This article discusses the effects of various steels and lubricants on subsurface microstructural changes during a high-frequency dynamic pin-on-disc test under pure sliding and boundary lubrication conditions. Steel materials chosen for this study are 55SiMo8 steel, 100Cr6 steel, and 8%Cr steel. Fully formulated polyalkylene glycols-based oil, synthetic hydrocarbon oil, and per-fluoro polyether base oil were selected as the lubricants. In 8%Cr, the average roundness of large carbides was reduced by 33% due to the deformation of carbides that occurred during severe cyclic loading. The outcomes showed that the material effect has a greater impact on the generation of white etching areas than the effect of lubricants under high-frequency dynamic loading.

A novel multi-objective wind turbine gearbox (WTG) optimization problem is carried out using Non-Dominated Sorting Genetic Algorithm (NSGA-II). Weight of the gearbox and power loss are the two conflicting objective functions set to be minimized. Various design constraints that are related to the mechanical as well as tribological are considered in this study. Three different types of gear tooth involute profiles are considered, namely unmodified, smooth meshing, and high load capacity. These three profiles are tested with different synthetic-based ISO VG PAO (Polyalphaolefin) oils at the rated speed of 20 rpm. The results of WTG are compared with and without tribological constraint by using ISO VG PAO 320, 680, and 1000 oils. Also, the result of one helical gear pair is validated with commercial gear software KISSsoft using ISO VG 680 mineral oil. Pareto fronts obtained from various ISO VG PAO synthetic-based lubricants are compared for all three gear profiles. From the results, PAO 320 oil outperforms than other two grades (PAO 680 and 1000). Further for the chosen model, on comparing with and without tribological constraint, power loss is notably reduced with tribological constraint.

White etching areas (WEAs) are premature failure witnessed in wind turbine gearbox bearings, due to which it fails within 10 % of design life causing huge economic losses. Among all the drivers for WEAs formation, hydrogen is known to increase the plasticity of the region by reducing the critical stress for dislocation to flow, thereby accelerating the degradation. Retained austenite is found to have good intraphase trapping sites which reduces the mobility of hydrogen. In this research work, the role of retained austenite in influencing hydrogen diffusivity and WEAs formation is evaluated against dynamic loading. The outcome of the study showed that stabilizing microstructure with retained austenite was beneficial in mitigating WEAs formation in bearing steel under a high hydrogen environment.

White etching areas (WEAs) and white etching cracks (WECs) are the catastrophic premature failure modes associated with wind turbine gearbox bearings. Several drivers, including lubricant chemistry, influence the evolution of WEAs and WECs. This study investigated the performance of base oils with various lubricant chemistry against microstructural decay in DLC-coated AISI 52100 bearing steel. Tests were performed in sliding-dynamic loading conditions under severe boundary lubrication. The effect of base oils against WEAs formation in DLC-coated steel is explored by analysing the condition of oils, wear behaviour, and subsurface microstructural change. The study outcomes revealed that the selection of base oils plays a vital role against the WEAs formation in DLC-coated bearing steel subsurface.

Contact mechanics models that can accurately estimate the contact conditions and, consequently predict wear must be developed for hip implants. This study analyses and verifies the existing Hertz and Fang analytical models applicable to hard-on-hard hip implants. The contact parameters, such as the maximum contact pressure, contact radius and the maximum deformation, are considered for the validation with FEM. Both analytical models fail to predict the contact conditions throughout a gait cycle. A novel data-driven ANN model is developed to comprehensively predict the contact conditions considering different input parameters. The analysis shows that cup thickness significantly affects the output contact conditions. Therefore, it is recommended to consider cup thickness in the analytical model interpretation for hard-on-hard hip implants.

Hard-on-Hard hip implants, specifically ceramic tribo-pair, have produced the highest in-vivo wear resistance, biocompatibility, superior corrosion resistance, and high fracture toughness. However, this ceramic tribo-pair suffers from edge loading, sharply increasing wear and accelerating early implant failures due to micro-separation. Even though in-vitro studies have tested the occurrence of wear due to dynamic edge loading, the Finite Element Method (FEM) gives the advantage of accurately estimating the wear, minimizing the experimental time and cost. A new fundamental FEM model is developed to predict wear for ceramic hip replacement bearings under dynamic edge loading conditions for a fixed separation and fixed inclination angle. The model is directly validated with the existing hip simulator data up to 3 million cycles in terms of wear depth, wear scar and volumetric wear rate. The results from the model show that the accuracy in wear prediction was more than 98% for the wear depth and volumetric wear rate for the dynamic edge loading condition. A stripe wear scar is captured, depicting the edge loading conditions. The developed model from this study can predict wear under pure standard and dynamic edge loading conditions.

The tribological performance of piston ring-cylinder liner conjunction (PRCLC) notably depends on the surface finish of cylinder liner. This study investigates the mixed lubrication condition near dead centres considering non-Gaussian roughness and cavitation. The non-Gaussian flow factors are incorporated in the average Reynold's equation to include the effect of non-Gaussian roughness. Whereas, the Weibull probability distribution function is employed to model the asymmetry in asperity heights. It is found that surfaces with more negative skewness exhibit lower engine total frictional force in the vicinity of the dead centres. No substantial effect of the skewness and surface roughness is observed in the mid-stroke of the piston. It is shown that PRCLC operates in mixed-EHL regime near the dead centres. The lambda ratio is found to increase with an increase in skewness of the rough surface.

The premature failures in wind turbine gearbox bearings are primarily caused by the decay of subsurface microstructure called white etching areas and white etching cracks. Severe operating conditions of wind turbine gearbox bearings necessitated for tribological coatings to enhance the system performance. The current study investigated the performance of hydrogenated diamond-like‑carbon (DLC) coating against white etching areas formation under sliding-dynamic conditions. The dynamic loading experiments were carried out in the dynamic pin-on-disc tribometer with Polyalphaolefin base stock oil under a boundary lubrication regime. Prior to the dynamic load tests, detailed tribological characterisation of the deposited DLC coating was carried out. The post-test analysis results were compared with the uncoated AISI 52100 bearing steel tribopair to evaluate the performance of DLC-coated bearing steel against white etching areas formation under given conditions. The results obtained show the capability of the proposed coating to delay the formation of white etching areas in the bearing steel.

The main objective of the work is to investigate the friction and wear behavior of sintered copper-based brake composite friction material with a change in the volume percentage of soft reinforcement particles namely MoS2 by pin-on-disc tribometer for medium-duty automotive applications. The composite brake friction material contains copper (Cu) as a matrix, tin (Sn) as an additive, silicon carbide (SiC) and molybdenum disulfide (MoS2) as hard and soft reinforcement particles and barium sulfate (BaSO4) as filler. These hybrids copper-based brake composite friction (pin) samples are successfully prepared by a change in compositions of MoS2 from 0 to 5 vol. % in the step of 1 vol. % and the characterizations of friction samples are studied to understand the physical and mechanical properties such as density, hardness, and compressive strength. Finally, the dry sliding friction and wear test is conducted against grey cast iron material (disc) at constant load and sliding speed of 50 N and 5 m/s respectively using pin-on-disc equipment under room atmosphere. Based on the analysis of the result, the developed copper-based brake composite friction sample with 2 vol. % of MoS2 has shown better mechanical and tribological properties among other compositions. Further, post-test analysis on the worn-out sample surfaces using a field emission scanning electron microscope (FESEM) with energy dispersive spectroscopy (EDS) revealed that change in wear mechanisms from abrasion to adhesion as an increase in the volume percentage of MoS2.

Premature bearing failures commonly occur in applications that operate under extreme boundary conditions. The lubrication engineers and tribologists are confronting a key challenge in these bearing failures associated with microstructure decay, such as White Etching Cracks (WECs) and White Etching Areas (WEAs). Lubricant degradation is one of the critical factors for the subsurface decay in the bearing steel. This work evaluates the performance of mineral oil (heavy paraffin oil) and synthetic poly alkyl glycol (PAG) against WEAs formation under pure sliding with cyclic compressive loading. The performance of lubricants was evaluated using Infrared Fourier Transform (FTIR) and Electron Spin Resonance (ESR) spectroscopy. The outcomes reveal that the free radical formation rate is higher for paraffin than PAG. This study found that WEAs formation in the bearing steel is delayed in the PAG tested samples compared to paraffin.

Despite of more than two decades of research in white etching areas (WEAs), this phenomenon still persists in wind turbine gearbox bearings leading to a reduction of L10 life by 90 percent. There are various drivers for WEAs formation among which hydrogen is considered one of the root causes which accelerates the microstructural degradation. Diffusible atomic hydrogen reduces the threshold for dislocation motion leading to localized strain accumulation. The tendency to form WEAs in bearing steel depends on the stability of carbide precipitates during plastic deformation and resistance to diffusible hydrogen. In this research work, the role of size and volume fraction of carbides on their relative stability against decomposition and hydrogen embrittlement is studied. The performance of bearing ball samples with different carbide sizes and distribution is tested in the presence of two different lubricants in the dynamic load pin-on-disc (PoD) test rig. The results from the study show that reducing the size of carbide precipitates improved their stability against plastic deformation leading to the stagnation of WEAs formation.

Ti6Al4V alloy is widely used for orthopaedic applications due to its excellent osteointegration, good biocompatibility, great strength-to-weight ratio, and comparable elasticity with bone. However, when this alloy is exposed for long-term in the body fluid, it releases the corrosion products of metals and metal oxides. The released particles react with the surrounding tissue and cause an adverse local tissue reaction (ALTR) which leads to pseudotumor formation, and osteolysis (bone degradation). Thus, corrosion is a critical parameter for bio-implants and in this study to minimize corrosion, thermal oxidation was performed on micro-blasted Ti6Al4V surfaces for 24 and 48 h. Surface morphology, phase analysis, surface roughness, and wettability of all types of samples were evaluated using FESEM, EDS, XRD, AFM, and Goniometer. The electrochemical performance was studied in 0.9 wt% NaCl. The combined mechanical and thermal process increases surface roughness and improves the hydrophilic nature of the Ti6Al4V surface by 67 %. The formation of metastable Ti2O3 during micro-blasting and further transformation of anatase and rutile TiO2 during thermal oxidation increases the intensity of anatase and rutile TiO2 formed on the surface and enhances corrosion resistance. Overall, micro-blasting with 48 h thermal oxidation showed 2 times higher corrosion resistance and reduced corrosion rate by almost 50 % as compared with the untreated Ti6Al4V.

The combination of laser surface texturing and heat treatment process was barely investigated in the past, particularly for improved surface characteristics of Ti6Al4V. Hereby, the important role of laser scanning and post heat treatment in improving the oxygen diffusion depth and overall surface characteristics was analyzed. Polished Ti6Al4V surface was scanned using Nd:YAG nanosecond laser and analyzed for the oxygen pickup, phase change and hardness variation. The laser textured surface was heat treated to grow oxide coating over textured surface. In comparison with heat treated surface, the synergic effect of laser scanning and heat treatment improved the surface hardness by 15% and raised the TiO2 rutile phase fraction from 0.36 to 0.73. The dual engineered Ti6Al4V surface was found to have superior HF1 quality adhesion strength along with the presence of low magnitude tensile residual stress. The increase in oxygen diffusion depth, diffusion coefficient and TiO(2 )rutile phase fraction due to synergic effect of laser scanning and heat treatment were associated to the Ti2O(3) and C doping assisted anatase to rutile transformation mechanism.

Water-lubricated journal bearings (WLJB) are one of the important components of the stern tube shaft. Surface topography of WLJB is an important aspect in the design of stern tube shaft, and significantly affects the lubrication characteristics of WLJB. This work aims to investigate the effect of non-Gaussian surface topography on tribological performance of water-lubricated journal bearing (WLJB) under mixed-elastohydrodynamic lubrication (mixed-EHL) regime. The non-Gaussian flow factors, and a normalized Weibull probability density function is used to tackle the non-Gaussian roughness in hydrodynamic and asperity contact models respectively. From results, it is observed that skewness ( S sk ) significantly affects both asperity contact pressure and hydrodynamic pressure along circumferential direction of bearing. It is concluded that for skewness, S sk = −1 to 1, lambda ratio falls between 1 to 3. The relation between altitude angle and skewness is established. The effect of bearing length to diameter (L/D) ratio on lubrication performance of WLJB is also discussed in detail.

This study combines the laser surface texturing technology and heat treatment process to fabricate a dual surface engineered Ti6Al4V consisting of micro-groove crosshatch pattern texture covered with hard TiO 2 oxide coating to reduced friction and improve the wear resistance at the bio-lubricated interface. Crosshatch texture with 25 μm width, 5 μm depth at 25% area density were fabricated using nanosecond Nd:YAG laser over Ti6Al4V surface and then heat treated at 600 °C for 48 hours. XRD result showed rutile TiO 2 phase formation along with the presence of anatase TiO 2 , Al 2 O 3 and Ti 3 O minor phases whereas, the surface hardness was increased to 1538±41 HV. Bio-tribology experiments were carried out for 45 and 90 o oriented micro-groove crosshatch textures, with and without heat treatment, under partially replicating hip implant articulation. Results demonstrated 60% friction reduction corresponding to the 45 o oriented crosshatch texture with heat treatment. Further, the worn-out surface morphology showed reduced wear damage and good wear debris entrapment inside the micro-grooves.

Bearings fail prematurely in transient operating conditions, as a result of flaking that causes changes in subsurface microstructure. In this type of failure, the formation of White Etching Cracks (WECs) is preceded by the formation of White Etching Areas (WEAs). Typically the WEAs are formed due to the dissolution of cementite into the matrix in AISI 52100 bearing steel. This feasibility work investigates the performance of AISI 440C steel against WEAs formation using dynamic load Pin-on-Disc tribometer. Experiments were performed at 4.5 Hz loading frequency, Hertzian pressure 2.02 GPa and sliding velocity of 0.2 ms-1 under the boundary lubrication using Poly Alpha Olefins (PAO). From post-test analysis, it is found that AISI 440C steel trapped more hydrogen than AISI 52100 bearing steel. The comprehensive analysis of materials and lubricants revealed that the performance of AISI 440C stainless steel against WEAs formation is better than that of AISI 52100 bearing steel.