Tribology Letters最新文献

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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This study has developed the sliding contact resonance (SCR) method, which measures three timescales, in no contact, stationary contact, and sliding contact, to investigate the mechanism of abrasion pattern (AP) formation engraved on rubber surfaces. The SCR method employs a unique homemade apparatus of a single-degree-of-freedom forced oscillation system utilizing a macroscale sliding contact between a rubber roller and a rigid surface. This paper focuses on the timescales, based on the hypothesis that the product of the drive speed and an intrinsic time determines the AP spacing. As a result, we find that it is not the mechanical or material timescale, but rather the timescale of sliding contact, that determines the limiting AP spacing. Their strong correlation suggests that the intrinsic time of the rubber surface, required for deformation and recovery in sliding contact, determines the periodic spacing engraved on the surface.

Nearly one third of workplace injuries results from slip- and trip-induced falls. Solid particles are among the most common floor contaminants in both occupational and outdoor environments, reducing shoe–floor friction and increasing slip risk. This study investigates how rubber hardness and surface roughness affect the frictional behaviour of shoe soles on smooth, particle-contaminated floors. Coefficient of friction (COF) measurements and post-test surface wear analyses were conducted using nitrile rubbers with hardness between 57.9 and 84.0 ShA and varied surface roughness. Samples were slid against smooth epoxy flooring in a pin-on-plate test simulating the heel-strike phase of walking. The floor was either clean or uniformly covered with corundum particles (40–50 µm, 120–140 µm, or 280–315 µm). On clean floors, increasing rubber hardness and roughness significantly decreased COF (p < 0.0001) due to reduced real contact area. Under contaminated conditions, softer and rougher rubbers yielded higher COF values (p < 0.0001). Higher COF correlated with greater floor wear, showing long scratches and grooves, suggesting slip occurs mainly at the particle–floor interface. Rubber hardness and surface roughness primarily influence the strength of the particle–elastomer interface; greater particle–elastomer strength suppresses particle rolling and thereby leads to an increase in COF. These findings indicate that, on particle-contaminated smooth floors, slip resistance is governed more by particle–floor interactions than by rubber adhesion. Increasing outsole roughness and reducing hardness can help mitigate the adverse effects of particle rolling within the contact area and improve the frictional performance of the outsole.

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Climbing robots have vital uses in uncharted terrain exploration, military intelligence collecting, and other areas. To address the drawbacks of classic wall-climbing robots, this study introduced a novel soft climbing robot constructed of the smart wood with switchable adhesion force. The experimental findings indicated that the soft robot implemented in this research could effectively perform climbing movements on diverse walls and sloping pavements, enabled by temperature control through cold/hot water circulation and pneumatic actuation. Further research revealed that the reversible phase transition of PNIPAM at different temperatures was the main reason for the variable adhesion force of the smart wood. Moreover, the adhesion force model developed in this work indicated that the adhesion force of the smart wood surface was mostly composed of the contact mechanics force and the capillary force. Finally, this study will offer novel insights for the design of climbing robots and advance their potential applications.

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Tribocorrosion studies have primarily focused on hard-on-hard articulations, with limited research on metal-on-polymer (MoP) configurations despite their clinical relevance. This study investigates the tribocorrosion behaviour of cobalt-chromium (CoCr) alloy against 3D-printed polyetheretherketone (PEEK), conventionally manufactured PEEK, and ultrahigh molecular weight polyethylene (UHMWPE), with UHMWPE as the reference material. Tests were conducted using a reciprocating tribometer under open circuit and potentiostatic conditions in phosphate buffered saline (PBS) and bovine calf serum, with in-situ electrochemical monitoring. The results demonstrated that the presence of serum significantly decreased the charge transfer of the CoCr surface, hence decreasing electrochemical degradation when compared to PBS lubrication. The manufacturing methods for PEEK resulted in different surface characteristics, leading to variations in tribocorrosion behaviour; however, polishing to achieve homogeneous roughness minimized these differences. Furthermore, CoCr exhibited significantly higher charge transfer when slid against all tested PEEKs compared with UHMWPE (at least two-fold higher), suggesting that a PEEK-CoCr tribocouple results in an increase of tribocorrosion and a greater potential for metal ion release than a UHMWPE-CoCr tribocouple.

Selecting cost-effective materials for high-wear applications requires the exploration of alternative materials such as high-chromium cast irons regarding the resulting wear resistance and energy efficiency, justifying potential cost reductions. Our study investigates the tribological performance of high-chromium cast irons depending on the adjusted chromium content (11, 15, and 30 wt.-%) and heat treatment. In this regard, the resulting microstructure, mechanical properties, and wear resistance were analyzed, comparing the performance of high-chromium cast irons with benchmarking high-carbon steel. Complementary materials characterization combined with nanoindentation revealed that an increasing chromium content induced a higher volume fraction of eutectic carbides (M₇C₃), thus improving the wear resistance. The sample containing 30 wt.-% of Cr exhibited the lowest wear rate due to its dense carbide network, which acted as a physical barrier against abrasion. While hardness remained stable, the elastic modulus increased with carbide content, indicating a greater material stiffness. Our findings underscore the importance of optimizing the alloy composition and heat treatment to improve the durability and efficiency of materials used in abrasive environments thus providing valuable insights to develop advanced tribological solutions, contributing to energy savings and reduced CO₂ emissions.

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