Abstract This study aims at the characterisation of physicochemical properties and tribological behaviour of quaternary phosphonium based ionic liquids (QPILs) P44414Sac and P44414Ace at ambient temperature and elevated temperature. The kinematic viscosity, viscosity index (VI), thermal stability, corrosivity and tribological properties of the ionic liquids (ILs) were systematically investigated. The copper strip corrosion test results revealed that the two green ILs are non‐corrosive because of their halogen‐free characteristic. Tribological data demonstrated that the present ILs exhibited extremely superior friction‐reducing and anti‐wear properties to conventional IL lubricant 1‐Butyl‐3‐methylimidazolium hexafluorophosphate (L‐P104), 1‐n‐Octyl‐3‐methyl imidazolium hexafluorophosphate (L‐P108) and commercially available synthetic oil PAO10. Electrical contact resistance (ECR) and surface analysis of the worn‐out steel disc surfaces revealed that the synergistic effect of a polarity induced physical adsorption layer and a tribo‐chemical reaction film formed a boundary film to protect steel‐steel contact between rubbing surfaces.

Abstract Good dispersion stability of lubricating oil blends added with graphene is the premise of playing its superior tribology property. In this paper, the dispersion stability of modified graphene in lubricating oil and the wear property of nanolubricating oil on the key friction pair for a diesel engine was studied by simulation and experiment. The results showed that the dispersion of modified graphene in lubricating oil was improved at ambient temperature and pressure and at high temperature and pressure. However, the dispersion effect of graphene at high temperature was slightly worse than that at ambient temperature. The wear property of lubricating oil blends added with modified graphene with a concentration of no more than 100 ppm was improved. With the increase of graphene addition, the wear reduction effect increased first and then decreased. The lubricating oil blend added with 25 ppm modified graphene is the best for wear reduction.

Abstract Tungsten‐doped diamond‐like carbon (W‐DLC) films were prepared by magnetron sputtering, followed by annealing in the temperature range of 100–500°C. The effects of the annealing treatment on the chemical compositions, mechanical properties and tribological behaviours of annealed W‐DLC films with a base oil were tested by scanning electron microscopy, atomic force microscopy, Raman spectroscopy, x‐ray diffraction and nanoindenter tests. The results indicated that increasing the annealing temperature led to a decrease in the thicknesses and hardness values of the W‐DLC films due to the graphitization and oxidation of the film surfaces. However, the structures of the W‐DLC films annealed at 500°C were destroyed. Under poly‐alpha olefin lubrication, when the annealing temperature was lower than 300°C, the tribological properties of the W‐DLC films were stable. This occurred because the graphitization of the sample surface was conducive to the removal of abrasive particles.

Abstract In this study, an interfacial slip model including the limiting shear stress is proposed and applied to the thermal elastohydrodynamic lubrication (EHL) analysis of a helical gear pair. The main difference between the proposed model and the classical EHL model is that, the term of entrainment velocity in Reynolds equation is modified. The influences of interfacial slip, thermal effect, initial limiting shear stress and operating conditions on the tribological properties are evaluated. Due to the interfacial slip, the pressure distribution moves towards the inlet region, and the fluctuation distributions of entrainment velocity and film thickness are similar to the trigonometric function. The influence of thermal effect on interfacial slip cannot be ignored, especially in the case of high speed and heavy load. As the input torque and input rotational speed increase, the interfacial slip gradually extends to the whole meshing process.

Abstract The lamellar structure and weak interlayer shear force of graphene endow itself potential application value in lubrication fields, while its lubrication performance has been vastly investigated in various fields. However, there are few studies on the composite tribological performance of graphene and ceramic films. In this study, a solid–liquid lubricating system which TiN film slides in graphene aqueous dispersion is developed, aiming to achieve low friction of the TiN ceramic film in macroscale contact. Two kinds of TiN films were obtained by multi‐arc ion plating and magnetron sputtering respectively. The tribological test show that the friction coefficient of the TiN film drops when adding graphene aqueous dispersion and varies with the concentrations of graphene. The results indicate that the graphene might act in two ways to achieve low friction of TiN film, which are liquid lubricating effect of solvents and the formation of smooth sliding interface promoted by graphene.

Abstract This study proposes a tribo‐dynamic model for point contacts under lubrication, focusing on the excitations caused by surface movement, defect and load impact. Surface movement and load impact can cause changes in contact force, resulting in acceleration and dynamic displacement, which may affect the flow of the lubricant. The traditional lubrication model is usually steady or transient, and load balance is used to correct the normal height, ignoring a series of influences such as the mass, stiffness and damping. It requires repeated corrections to obtain a balanced position, which is very time‐consuming. By combining tribology and dynamics, dynamic excitation is introduced in the contact process in this study. After determining the initial value, the normal height can be directly obtained from the dynamic equation. Besides, a comparison between the tribo‐dynamic model and the steady‐state model is presented. It is found that the film thickness and pressure distribution calculated by the two models are almost the same at medium and low speeds, while the calculation results of the two models are different at high speed or under load impact, which emphasises the importance of considering the dynamic behaviour in the study of contacts.

Abstract In this study, a vapour‐assisted drag reduction mechanism of texture surface with elevated temperature is explored. The computational fluid dynamics simulation is performed to understand the vapour membrane generated by the coupled effect of elevated temperature and surface texture. Simulation results indicated that elevated temperature may help generate the vapour membrane which can isolate the liquid water from the solid surface, thereby reducing friction. Meanwhile the introduction of surface micro‐texture has dramatic positive effect in vapour volume fraction, vapour thickness and the average velocity of outflow. It was found that introducing the micro‐texture to the elevated surface has dramatic positive effect in reducing frictional drag, the groove formed by the micro‐texture can store vapour and form a vapour vortex inside, which may have a certain driving effect on the fluid.

Abstract Many contributions have focused on the pitting analysis in gear transmissions. However, studies on pitting fatigue in spiral bevel gears are limited due to the complexity of tooth contact and lubrication analysis. This paper is to present a reasonable methodology to simulate the pitting crack nucleation of spiral bevel gears considering the real three‐dimensional (3D) rough surface contact where the intermittent asperity stress concentrations is prominent. The proposed model includes a loaded tooth contact analysis model, a mixed elastohydrodynamic lubrication model proposed previously, 3D stress filed formulation, and multi‐axial fatigue criterions. In order to determine an appropriate multi‐axial fatigue criterion for spiral bevel gear, a characteristic plane approach and a conventional critical plane approach are employed in this study. The predicted micro‐pitting lives are compared to published micro‐pitting investigations to assess the reliability of two fatigue criterions in simulation of the micro‐pitting failure. It is observed that the characteristic plane approach is better to reveal the failure mechanism in spiral bevel gears.

Abstract As the progress of the high‐speed performance requirement for porous gas journal bearings continuing, the aerodynamic effect, which plays a key role in the state of lubrication, causes the porous gas journal bearings being gradually to be used as hybrid bearings. This paper firstly presents a theoretical analysis on the lubrication mechanism of the hybrid bearings. The gas flow in the porous and clearance is modelled by using the mass continuity equation. Two experimental test rigs are designed to systematically test the static and dynamic characteristics of the hybrid bearings. The numerical results are corroborated by the experimental data. Results show that there is optimal clearance for hybrid bearings to obtain maximum load capacity. The load capacity of the hybrid bearings is increased by the aerodynamic effect to be higher than that of pure aerostatic bearings especially at higher rotational speed and larger eccentric ratio. However, due to the occurrence of the aerodynamic effect, the hybrid bearings show clear cross‐coupling dynamic coefficients, which may lead to bearing instability. It is interesting to note that the direct dynamic stiffness and damping coefficients are both influenced by the aerodynamic and aerostatic effects. Higher rotational speed and external supply pressure will result in higher direct dynamic coefficients.