Tribology Letters最新文献

Physical vapor deposited (PVD) molybdenum disulfide (MoS₂) solid lubricant coatings are an exemplar material system for machine learning methods due to small changes in process variables often causing large variations in microstructure and mechanical/tribological properties. In this work, a gradient boosted regression tree machine learning method is applied to an existing experimental data set containing process, microstructure, and property information to create deeper insights into the process-structure–property relationships for molybdenum disulfide (MoS₂) solid lubricant coatings. The optimized and cross-validated models show good predictive capabilities for density, reduced modulus, hardness, wear rate, and initial coefficients of friction. The contribution of individual deposition variables (i.e., argon pressure, deposition power, target conditioning) on coating properties is highlighted through feature importance. The process-property relationships established herein show linear and non-linear relationships and highlight the influence of uncontrolled deposition variables (i.e., target conditioning) on the tribological performance.

This study investigates the reciprocating motion of a rigid Hertzian indenter on a viscoelastic substrate with adhesion, using a finite element-based numerical model. An innovative methodology is employed to transform the sliding contact problem into an equivalent normal contact problem, enabling the accurate simulation of adhesion effects at the contact interface. The results reveal that system behaviour is governed by the interplay between viscoelasticity and adhesion, leading to notable changes in contact pressure distribution, contact area, and energy dissipation during reciprocating motion. Specifically, viscous dissipation within the substrate material dominates at intermediate sliding speeds, where the interaction between adhesion and viscoelastic relaxation processes results in pronounced hysteresis cycles. In contrast, at low and high sliding speeds (corresponding to the rubbery and glassy regions, respectively), the material behaviour is predominantly elastic, and no hysteresis is observed. Adhesion influences contact pressure distribution and contact size, particularly in the transition regime, where its effects on viscous dissipation are measurable. Moreover, the study clarifies that adhesion alone does not induce hysteresis in elastic regimes, distinguishing reciprocating contact from normal contact, where adhesive hysteresis is typically observed. New insights are also provided into how adhesion and viscoelasticity jointly impact tribological performance, offering a deeper understanding of energy dissipation mechanisms and contact mechanics during motion reversal. Interestingly, our results also show that there is a lag period after motion reversal, where friction aligns with motion direction before eventually changing direction as pressure redistribution occurs within the system. This phenomenon highlights how changes in contact mechanics affect local tribological interactions and can lead to variations in overall system response.

Graphical abstract

The application study of sliding bearings in planetary gearboxes of wind turbines has drawn much attention in recent years. However, the heavy-load and low-velocity working conditions will lead to serious wear of bearings that will affect service performance and life. CuSn12Ni2, a competitive material for such situations, is evaluated to reveal the time-varying wear behavior in this paper on a typical ball-on-disk testing rig. Furthermore, a wear model was proposed by the dimensional analysis theory. The results show that CuSn12Ni2 represents a lower friction coefficient and wear rate during the varying test duration under 90 ℃ than at room temperature, and 10 to 30 min can be considered to be the transition zone, marking the shift from a severe running-in stage to a relatively stable, rapid running-in stage. The wear surface of CuSn12Ni2 is dominated by abrasive wear and adhesive wear, accompanied by fatigue wear and tribochemical reaction at high temperature. However, the evolution model of the wear width and depth changing with wear duration is given in explicit form, and the adjusted R² is no less than 0.99. These time-varying wear data and models are meaningful for constructing the time-varying life evaluation model of sliding planet gear bearings.

Graphical abstract

The strains in α-Fe (bainite) and retained austenite (RA) exhibit an apparent discrepancy during deformation, namely strain partitioning, which can determine the tensile behavior of bainitic steels. Due to the involvement of plastic deformation and strain accumulation in the wear process, the strain partitioning phenomenon must be considered in the study of wear mechanisms. The impact wear tests results indicated that the wear behavior was strongly affected by strain partitioning between α-Fe and RA, and the initial hardness and work-hardening rate were not the main factors determining the wear resistance. The strain partitioning is closely related to the RA morphology tailored by isothermal processes, which was proved by in situ tensile test. The strain discrepancy between the continuous thin-film RA and α-Fe was shown to be less significant. A smaller strain discrepancy alleviated stress concentration and minimized the occurrence of cracks and material spalling during wear. The strain discrepancy between blocky RA and α-Fe was shown to be greater. The high-strain in blocky RA promoted the strain-induced transformation (SIT) effect and increased the work-hardening rate; however, it led to strain concentration on the worn surface and accelerated surface spalling.

The characterization of wear response in crystalline materials poses some challenges due to the presence of the size effect at small scales. In this study, we systematically conducted spherical nano-scratch simulations on (101)-oriented copper, using the mechanism-based strain gradient crystal plasticity theory, to explore the indenter size effect in the scratch hardness. The developed nano-scratch models are validated experimentally by comparing scratch depths and topographies. By examining the results obtained from conventional crystal plasticity and mechanism-based strain gradient crystal plasticity simulations, an indenter size effect in scratch hardness was identified. Furthermore, the mechanism of the indenter size effect in scratch hardness was quantitatively analyzed, by discussing the proportion of geometrically necessary dislocation lengths in the cumulative increments of dislocations.

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.