Tribology Letters最新文献

This study investigates the effects of graphene oxide (GO) reinforcement on the microstructural, mechanical, and tribological properties of copper (Cu) matrix composites produced via powder metallurgy and hot pressing. GO nanoparticles synthesized by the Hummers method were incorporated into Cu at different weight ratios (0.5, 0.75, and 1 wt %), and the composites were characterized using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX). The results revealed that GO was homogeneously distributed within the Cu matrix without forming new detectable crystalline phases, and its presence led to grain refinement and improved interfacial bonding. Hardness measurements showed that the composite containing 1 wt % GO exhibited a 109.97% increase compared to pure Cu, while tribological tests under dry sliding conditions demonstrated significant enhancements in wear resistance and reductions in the coefficient of friction. Notably, the 1 wt % GO composite achieved an 82.88% reduction in wear loss and a 51.33% decrease in friction coefficient. Post-wear SEM analysis confirmed the formation of a protective tribofilm, reduced microcrack formation, and minimized wear-induced damage. These findings highlight the effectiveness of GO as a multifunctional reinforcement for the development of high-performance Cu-based composites with superior wear resistance, making them suitable candidates for applications in electrical contact systems and other tribologically demanding environments.

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This study proposes a physics-based model for characterizing the contact behavior and state of joint surfaces. Based on the microcontact mechanics model, surface topography, material properties, loads, and asperity interactions were combined to construct a normal contact model. Subsequently, the sticking and sliding behaviors of a single asperity were characterized with the Jenkins element from the Iwan model. Conversely, a parallel arrangement of multiple Jenkins elements represented the overall stick–slip behavior. A distribution function incorporating non-Gaussian distribution and asperity interactions was derived to describe the tangential yield force distribution, thereby constructing a physically explicit tangential contact model. Finally, the results of the predictive model were validated against the experimental data. A slip ratio was introduced to quantifiably evaluate the surface contact states—full stick, stick–slip, and gross slip micromotion, and the factors influencing the contact characteristics of the joint surface were investigated.

Understanding the relationship between sliding velocity, temperature, friction, and wear is of fundamental importance in materials science and engineering. Here, we explore sliding friction and wear of a silicon carbide nano-asperity sliding over a silicon carbide substrate for a broad range of temperatures and velocities using molecular dynamics simulations. Our study reveals three distinct friction regimes over four velocity decades: velocity-weakening at moderate velocities, velocity-strengthening at high velocities, and an additional velocity-strengthening behavior at very low velocities and elevated temperatures. We theoretically describe these findings with physics-based friction models. For the low-velocity regime, we refine the Prandtl-Tomlinson model by incorporating a logarithmic aging mechanism that accounts for surface diffusion-driven contact evolution. For the high-velocity regime, we introduce a linear viscous friction model with an Arrhenius temperature dependence. These models demonstrate strong agreement with the molecular dynamics simulation results in their respective velocity regimes. We then explore wear mechanisms, distinguishing between atomic attrition at low velocities and collective material removal at high velocities, thus providing a comprehensive framework for understanding the velocity and temperature dependence of nanoscale friction and wear of silicon carbide.

The progressive escalation of operational velocities in high-speed railways has intensified cyclic contact stresses at wheel–rail interfaces, leading to accelerated fatigue failure. Hence, it is essential to create new wheel–rail materials with superior performance to align with the requirements of high-speed railway systems. This paper introduces a newly developed high-speed wheel steel (CLD400) that demonstrates enhanced rolling contact fatigue (RCF) performance and the influence of running speed on RCF damage in wheel steel. The results reveal that the CLD400 wheel steel exhibits excellent RCF life, which is 5.9 times higher than that of ER8 wheel steel. The performance of wheel steel can be effectively enhanced by decreasing the size of the austenite grain, pearlite colony, and interlamellar spacing, while increasing the pearlite content. In situ observations indicate that under oil-lubricated conditions, cracks gradually develop on the contact surface of the wheel steel and eventually expand into noticeable peeling pits, leading to material failure. As the cycles increases, the area and perimeter of defects on the wheel steel contact surface gradually increase, whereas the shape factor gradually decreases. As running speed increases, the deterioration of the wheel's steel material begins sooner, although with a reduced level of severity. The force driving crack growth decreases with speed increases, resulting in smaller RCF crack sizes. These findings enable the targeted design of wheel materials for 400 km/h high-speed trains.

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Neutron reflectometry is a technique for measuring structure near planar interfaces that has been previously used to non-destructively characterize the polymer density of hydrated, dilute, and soft materials. Previous investigations have conducted neutron reflectometry measurements of liquids, gels, emulsion, and polymer solutions at rest, in compression, and subject to shear stress. However, correlating structure with tribological properties of soft materials presents significant experimental challenges for prior instruments due to wall slip, sample thickness, and structural heterogeneity (e.g., depth-wise gradients). A linear reciprocating tribometer offers several advantages for in situ neutron reflectometry studies, including uniform velocity profiles, constant shear stress over large regions of interest, and independent control of normal force and sliding velocity during measurements. This work outlines basic considerations for the design of a custom linear reciprocating tribometer that operates in a neutron beamline and includes commissioning measurements. The tribometer is designed to compress soft and hydrated materials against linearly reciprocating silicon disks. The three key design considerations for this tribometer are (1) safety, (2) neutron transmission, and (3) sample positioning. This instrument design will enable in situ studies of soft matter and illuminate the role of interfacial structure on tribological phenomena.

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Due to their exceptional tribological properties, specifically under vacuum conditions, MoS₂ solid lubricants have been extensively used in several industries. Since the effectiveness of pure MoS₂ tends to deteriorate in humid and oxygen-containing environments, it is co-deposited by conventional Pb-based compounds to improve its oxidation resistance and tribological performance. However, lead-based compounds are required to be replaced with environmentally-friendly alternatives such as hexagonal boron nitride (hBN) due to their toxicity. Additionally, as extreme temperatures can affect the tribological performance of the coatings, understanding the interfacial phenomenon under realistic service conditions is necessary to mimic the operating conditions of aerospace applications. In this study, MoS₂ coatings with various hBN contents (9.5, 11.5, 13.5, 15.5, and 17.5 wt%) were developed using spray bonding process. The friction behavior was evaluated using a ball-on-flat tribometer at low temperature (i.e., − 50 °C). Subsequently, the coatings were characterized by ex-situ analysis techniques such as scanning electron microscopy (SEM), focused ion beam (FIB), Raman spectroscopy, and atomic force microscopy (AFM). The results demonstrated that all coatings exhibited significantly lower steady-state friction at − 50 °C compared to room temperature. A clear distinction was observed between the mechanisms governing the run-in and steady-state stages. The run-in stage was likely influenced by surface morphology and the intrinsic properties of hBN. Increasing hBN content beyond the optimal level led to a prolonged run-in phase and intensified abrasive wear. Conversely, the steady-state performance seemed to be influenced by the formation of a lubricating interfacial ice layer, facilitating low-friction sliding regardless of composition.

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Surface performance is critically influenced by topography in virtually all real-world applications. The current standard practice is to describe topography using one of a few industry-standard parameters. The most commonly reported number is (R)a, the average absolute deviation of the height from the mean line (at some, not necessarily known or specified, lateral length scale). However, other parameters, particularly those that are scale-dependent, influence surface and interfacial properties; for example the local surface slope is critical for visual appearance, friction, and wear. The present Surface-Topography Challenge was launched to raise awareness for the need of a multi-scale description, but also to assess the reliability of different metrology techniques. In the resulting international collaborative effort, 153 scientists and engineers from 64 research groups and companies across 20 countries characterized statistically equivalent samples from two different surfaces: a “rough” and a “smooth” surface. The results of the 2088 measurements constitute the most comprehensive surface description ever compiled. We find wide disagreement across measurements and techniques when the lateral scale of the measurement is ignored. Consensus is established through scale-dependent parameters while removing data that violates an established resolution criterion and deviates from the majority measurements at each length scale. Our findings suggest best practices for characterizing and specifying topography. The public release of the accumulated data and presented analyses enables global reuse for further scientific investigation and benchmarking.

We present experimental wear data for polymethyl methacrylate (PMMA) sliding on tile, sandpaper, and polished steel surfaces, as well as for soda-lime, borosilicate, and quartz glass sliding on sandpaper. The results are compared with a recently developed theory [1] of sliding wear based on crack propagation (fatigue), originally formulated for elastic contact and here extended to include plasticity. The elastoplastic wear model predicts wear rates that agree reasonably well with the experimental results for PMMA and soda-lime glass. However, deviations observed for quartz suggest that material-specific deformation mechanisms, particularly the differences between crystalline and amorphous structures, may need to be considered for accurate wear predictions across different materials. In addition, the model reveals a non-monotonic dependence of the wear rate on the penetration hardness (sigma _{textrm{P}}). Thus, for plastically soft material, the wear rate increases with increasing (sigma _{textrm{P}}), while for hard materials, it decreases. This contrasts with Archard’s wear law, where the wear rate decreases monotonically with increasing (sigma _{textrm{P}}).

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The viscosity of fluids and their dependence on shear rate, known as shear thinning, plays a critical role in applications ranging from lubricants and coatings to biomedical and food-processing industries. Traditional models such as the Carreau and Eyring theories offer competing explanations for shear-thinning behavior. The Carreau model attributes viscosity reduction to molecular distortions, while the Eyring model describes shear thinning as a stress-induced transition over an activation energy barrier. This work proposes an extended-Eyring model that incorporates stress-dependent activation volumes, bridging key aspects of both theories. In modifying transition-state theory by using an Evans-Polanyi perturbation analysis, we derive a generalized viscosity equation that accounts for the molecular-scale rearrangements governing fluid flow. The model is validated against computational and experimental data, including shear-thinning behavior of pure squalane and polyethylene oxide (PEO) aqueous solutions. Comparative analysis with Carreau-Yasuda and conventional Eyring models demonstrates excellent accuracy in predicting viscosity trends over a wide range of shear rates. The introduction of stress-dependent activation volumes provides a description of molecular exchange kinetics accounting for structural reorganization under shear. These findings offer a unified framework for modeling shear thinning and have broad implications for designing advanced lubricants, polymer solutions, and complex fluids with tailored flow properties.

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