Tribology Letters最新文献

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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The development of environmentally acceptable lubricants and lubricant additives has become a focal point within tribology due to increasing regulatory and sustainability demands. In this context, low-viscosity lubricants are gaining attention for their potential to reduce energy losses. However, their performance under a boundary lubrication regime, where thinner oil film build-up is present, requires more efficient boundary additives. This work evaluates a polyoxometalate-ionic liquid (POM-IL) as a multifunctional boundary additive in a low-viscosity polyalphaolefin-based lubricant, comparing its performance to zinc dialkyldithiophosphate (ZDDP) and a halogen-containing ionic liquid (IL). Tribological tests on AISI 316L stainless steel and AISI 52100 bearing steel revealed that while ZDDP showed substrate-independent adsorption and tribological performance, the IL-based additives had substrate-dependent behaviour. Strong chemisorption was consistent for both IL-based additives, yet their anti-wear and friction-reducing properties differed, showing evidence for the presence of a combined mechanism that includes both strong adsorption and tribochemical reactions. Additionally, the interaction between POM-ILs’ negatively charged surfaces, W atoms, and Cr(III) in 316L was identified as a key factor in their performance. Notably, significant work-hardening was observed in 316L lubricated with POM-IL-containing blends, further enhancing its anti-wear properties. These findings emphasize the role of substrate chemistry in boundary lubricant additive performance in low-viscosity lubricants, offering insights for the development of more efficient multifunctional boundary lubrication solutions.

Pressure boundary conditions are required to solve the Reynolds equation for hydrodynamic lubrication. Several boundary conditions have been proposed for the outlet end of the pressure profile while a limited choice is available for the inlet zone. In order not to impose a priori conditions, here we apply the smoothed particle hydrodynamics method (SPH) to the hydrodynamic lubrication problem solved with the Navier–Stokes equations. First, surface tension calculation which is based on the CSF method is developed to consider the deformation of the liquid–air interface. The action of surface tension is verified by comparing the theoretical values of the Laplace pressure and the period of the surface tension oscillation of a circular droplet. The SPH analysis is then used to simulate the hydrodynamic lubrication problem with a limited amount of fluid. The pressure profiles obtained by the SPH analysis show a good agreement with reported FEM and experimental results. Especially in the outlet zone, the minimum pressure and the location of the outlet meniscus boundary agree with the experimental results over a wide range of capillary numbers. Film profiles in the inlet zone are affected by the direction of the gravitational force. In addition, the approach developed here allows the visualization of a vortex flow in the inlet zone and shows that only a limited part of the bottom flow driven by the moving surface is passing through the minimum gap toward the outlet side. This approach opens the way to simulate accurately and without a priori assumptions the hydrodynamic lubrication problem with a free surface flow as found in all starved lubricated contacts.

Polytetrafluoroethylene (PTFE) improves friction and wear performance by forming a transfer film on counterfaces. However, its high wear rates reduce the fitting accuracy and limit its service life. Adding fillers to PTFE can significantly lower its wear rate. Early theory suggests that fillers reduce wear by providing preferential load support and preventing the development and propagation of subsurface cracks. However, this theory cannot explain the ultra-low wear behavior of some nano fillers and lamellar fillers, and it is also found that the model ignored the effect of the transfer film. With the in-depth study of the wear reduction mechanism, it has been revealed that the ultra-low wear behavior of these fillers is closely related to tribochemistry and the formation of the transfer film. Moreover, research has shown that the type of filler affects the tribological properties of composites. Therefore, it is essential to investigate the wear-reduction mechanisms of different fillers. In this study, we investigated the wear reduction mechanisms of four representative filler-filled PTFE composites, which included carbon-based materials, metals, polar polymers, and non-polar polymers. The results show that (1) the accumulation and preferential load support of fillers on the polymer surface determine its wear resistance, and (2) filler-induced PTFE chain breakage promotes tribochemistry and facilitates the formation of adherent transfer films. Based on these findings, recommendations are provided for the design of low-wear PTFE friction systems.

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Nano-sized calcium carbonate and micro-sized graphite are commonly used additives in calcium sulfonate grease. It is worth studying how the two additives together affect the tribological properties in the complex oil-soap structure of calcium sulfonate grease. Regarding this issue, nano-sized CaCO₃ and micro-sized graphite blends in different proportions were selected to investigate the lubrication performance. A combination of 1 wt% micro-sized graphite and 5 wt% nano-sized CaCO₃ contributed the excellence friction-reducing and anti-wear compared to the calcium sulfonate grease without additives or the calcium sulfonate grease with above additions alone. The additive blends could be synergistic to take part in chemical reactions on the Hertz contact area and form films to maintain a low friction state during whole friction process.

Additives made of nitrogen-functionalized copolymers were paired with diamond-like carbon (DLC) coatings doped with different amounts of oxygen and silver to form systems capable of improving tribological performance against undoped-DLC/additive systems. Initial characterisation indicated that silver doping reduced hardness and wettability in the surface, contrary to oxygen doping. Adhesion improved with higher doping levels. Tribological testing was done in boundary conditions, with silver-doped DLC coatings achieving a reduction in wear, but not friction. Oxygen-doped DLC coatings showed similar behaviour. Micrographs identified the wear mechanism as pure polishing and proved the protective effect of doped-DLC/additive systems. The findings suggest an across-scales effect of properties in the performance of the system and promising use in applications requiring wear resistance and friction reduction.

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