Tribology Letters最新文献

Analytical contact mechanics theories depend on surface roughness through the surface roughness power spectrum. In the present study, we evaluated the usability of various experimental methods for studying surface roughness. Our findings indicated that height data obtained from optical methods often lack accuracy and should not be utilized for calculating surface roughness power spectra. Conversely, engineering stylus instruments and atomic force microscopy (AFM) typically yield reliable results that are consistent across the overlapping roughness length scale region. For surfaces with isotropic roughness, the two-dimensional (2D) power spectrum can be derived from the one-dimensional (1D) power spectrum using several approaches, which we explored in this paper.

Graphical Abstract

Thread seizure is a common failure mode for bolted joints during the process of tightening, significantly influencing their reliability and detachability. Research results have demonstrated that the accumulation and blocking of wear debris are the main reasons for thread seizure. This study proposed a theoretical model to predict the wear volume on thread surface in the tightening process for the first time. First, many sub-regions on the thread surface were divided. The real contact force and area on each region were calculated considering the nonuniform axial load distribution in a bolted joint. Second, for each sub-region, the micro morphology was characterized by fractal function. Based on the fractal contact theory, the contact model of single asperity was built, and the contact force and area of single asperity were calculated in the stages of elastic, elastoplastic, and plastic deformations. Subsequently, the contact force and contact area of each sub-region were obtained by integral on single asperity. The former was compared with the contact force of sub-region calculated by nonuniform axial load distribution to determine the termination condition of iteration. The latter was brought into the wear prediction model based on Archard wear theory. According to the theoretical model of predicting the wear volume on thread surface, the effects of axial load distribution coefficient, preload, fractal parameters, friction coefficient, and thread pitch on the wear volume of thread surface were analyzed and discussed. Finally, experiments were conducted to validate the reliability of the proposed theoretical prediction model for wear volume.

With the increasing attention to environmental issues and the improvement of environmental regulations, traditional lubricant additives are facing huge challenges, while biodegradable green lubricant additives are facing new opportunities. Organic borate esters, as lubricating additives, have core competitiveness over traditional lubricating additives containing S, P, and Cl. Borate esters not only overcome the poor corrosion resistance of traditional additives, but also have excellent load-bearing capacity and extreme pressure performance. In addition, introducing fatty acid diethanolamide containing active groups such as hydroxyl and amide bonds into borate esters enhances the adsorption capacity. In this article, three borate esters with different carbon chain length were prepared, namely stearic acid diethanolamide borate ester with a chain length of 18, tetradecanoic acid diethanolamide borate ester with a chain length of 14, and octanoic acid diethanolamide borate ester with a chain length of 8. The as-prepared borate ester, especially C₁₈ONB, exhibits excellent tribological properties as lubricant additives in poly (a-olefin) (PAO6), significantly improving the friction reducing and antiwear properties of the base oil. This is due to the weak polarity of C₁₈ONB with long carbon chains, which exhibits good solubility in PAO6 with weaker polarity and forms thick multilayer viscoelastic adsorption film on the friction surface. In addition, the adsorption film undergoes tribochemical reactions during the rubbing process, generating a tribofilm containing excellent lubricants such as B₂O₃ and BN, which further plays a good role in reducing friction and antiwear.

Adhesive friction considering electrostatic interaction is an important problem in practical engineering. However, the friction mechanism considering electrostatic interaction and adhesion of elastoplastic materials on the contact interfaces remains poorly understood. A sliding friction model considering the van der Waals attraction, repulsive and electrostatic interactions is established in this work. The effect of charge density on the normal force, friction force, contact area and stress distribution is thoroughly investigated. And the repeated sliding friction process is also quantitatively analyzed. It is shown that adhesion enhances with the increase of surface charge density. Moreover, the contribution of electrostatic interaction to adhesion in the contact process is always greater than that to friction in the sliding friction process under the conditions studied. In the initial stage of friction, the friction will gradually increase and the normal force will gradually decrease. It is also found that a higher charge density results in a lower normal force in the friction process. Furthermore, the increase of charge density leads to a bigger contact diameter and the increased asymmetry of stress field, resulting in the increase of friction force, equivalent plastic strain and friction coefficient. In addition, both normal force and friction force arrive at a steady state after several repeated friction circles for elastoplastic materials, which is due to the fact that the contact diameter tends to be constant because of the accumulation of plastic deformation. This work reveals the contribution of electrostatic interaction to friction during adhesive sliding process and provides some insights into the adhesive friction problem considering electrostatic interaction.

Aiming at the phenomenon that nanosilver lubricants are prone to agglomeration and sedimentation, the influence of different anions and cations in ionic liquids on the adsorption capacity of nanosilver particles is studied, and three ionic liquids with different structures are selected to improve the agglomeration of nanosilver particles in lubricants and to improve the anti-wear performance of lubricants; the dispersive performance of different ionic liquids is studied using quantum chemical simulation methods to study the adsorption and dispersion mechanism of nanosilver. The tribological properties of the three different ionic liquids in combination with the nanosilver lubricant were investigated using a multifunctional friction and wear tester and a field emission scanning electron microscope. The results show that hexyltributylphosphine hexafluorophosphate ([P4446][PF6]) can stabilise the dispersion of nanosilver lubricant and solve the problem of agglomeration and sedimentation due to its strong adsorption cation, which is attributed to its high adsorption and the synergistic effect of friction chemistry, and its anionic intermixing is good, and the ionic liquids have excellent anti-friction and wear reduction properties. It is shown that the design and utilisation of functional ionic liquids with the compound effect of nanosilver particles can effectively improve the dispersion of nanosilver as a lubricant additive.

Graphic Abstract

Layered two-dimensional nanomaterials such as graphene and WS₂, possess superlubricity properties and thus offer a promising solution to mitigate friction and wear in micro-electromechanical systems. In this study, the atomic friction properties of graphene/graphene, WS₂/WS₂, and graphene/WS₂ bilayer heterostructure systems were examined through density functional theory simulations. Results indicated that the friction strength of the graphene/WS₂ bilayer heterostructure system was lower than that of the graphene/graphene and WS₂/WS₂ systems. Specifically, the graphene/WS₂ bilayer heterostructure system demonstrated ultra-low friction coefficients ranging from 0.0006 to 0.0096, resulting in friction strengths in the range of 10^⁻³ nN. Furthermore, the heightened electrostatic repulsion and smooth potential energy fluctuation helped reduce friction, validating the superlubricity performance of the graphene/WS₂ heterostructure system.

A vehicle start–stop system automatically shuts down and restarts the internal combustion engine to reduce the time the engine spends idling, thereby reducing fuel consumption and emissions. For the start–stop system to work, the engine must be at a certain temperature and conditions. If the engine is too hot, the system may not activate. This study explores the tribological characteristics of the start–stop system by applying principles of Continuum Damage Mechanics (CDM) to predict both the lifespan and wear volume subsequent to the start–stop cycles. A series of pin-on-disk tests were conducted to evaluate the efficacy of the modeling and predictions. The results from these tests were compared to the CDM predictions, demonstrating satisfactory accuracy. Additionally, a Finite Element Method (FEM) analysis was employed to model temperature variations during the start–stop cycles. Findings indicate that an increase in consecutive start–stop cycles impedes the system’s ability to sufficiently cool, thereby increasing wear. Conversely, extending the duration of the stop phase reduces wear and enhances the system’s lifespan.

Polyamide 66 (PA66) is one of the commonly used polymer gear materials. This paper focuses on the tribological properties of glass fiber reinforced PA66 composites in self-mated contact using a pin-on-disc tribometer. The effects of glass fiber content, PV (the product of the contact pressure and sliding speed), and lubrication on the tribological properties of the specimens are also investigated. The results show that the glass fiber reinforced PA66 exhibit higher coefficients of friction and specific wear rates than PA66 under dry sliding conditions. This is probably due to the peeled glass fibers during the sliding process acting as abrasive particles which have an aggressive effect on the surface. Under grease lubricated conditions, PA66 + 33% GF has the lowest coefficient of friction and specific wear rate due to its higher strength. Under dry sliding conditions, all specimens show the highest friction coefficient and specific wear rate at 30 MPa·m/s with the change of PV value. Under grease lubricated conditions, all specimens show the highest friction coefficient and specific wear rate at 4 MPa·m/s with the change of PV value. The addition of grease improves friction and wear of PA66 composites under most of the experimental conditions. However, the specific wear rates of PA66 and PA66 + 13% GF under grease lubrication are higher than those under dry sliding conditions at low PV values. This may be due to the fact that greases can reduce the surface mechanical strength of PA66 and PA66 + 13% GF.

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Wear problem has become an important issue limiting the functionality and lifetime of sliding electrical contact components. Adding conductive solid lubricants is a potential means of improving the tribological performance of these devices. Graphene, a two-dimensional material with excellent electrical conductivity and lubrication property, has been proposed to be a promising candidate for such applications. However, the tribological performance graphene has been demonstrated to be very susceptible to humidity even under non-current-carrying conditions. In this work, we study the effect of humidity on the wear behavior of graphene in the sliding electrical contact interfaces. The tribological behaviors of graphene under 10%, 30%, 60%, and 90% relative humidity conditions and 1 A current are investigated. The results show that the humidity can effectively slow down the wear of graphene in the sliding electrical contact interface by two key mechanisms. Firstly, as revealed by the infrared temperature measurements, higher humidity can significantly reduce the Joule heating. Secondly, X-ray photoelectron spectroscopy shows that with the existence of the electric current, at high humidity water molecules can passivate the graphene carbon dangling bonds more readily thereby reducing oxidation and slowing down the wear process. At low humidity, Joule heating not only caused graphene to oxidize but also accelerated the evaporation of water molecules, which was not conducive to its passivation, resulting in severe wear of the graphene.