Tribology Letters最新文献

This study presents the development of an adaptive and interactive algorithm for the evaluation and statistical analysis of the static, dynamic, and integral coefficients of friction (CoF) from high-resolution friction signals. The algorithm is grounded in extensive data collected from a variety of reciprocating friction test configurations, with two representative cases analyzed in detail. Its performance was validated against previously developed semi-automatic evaluation methods. The validation confirmed the algorithm’s robustness and adaptability to different test configurations and varying parameters throughout the testing process. Overall, the algorithm demonstrates high reliability and efficiency, establishing a foundation for a systematic and unified approach to CoF evaluation and addressing a significant gap in tribological data analysis.

Graphical Abstract

Cattaneo and Mindlin showed that if a Hertzian contact is loaded first by a normal force and then by a monotonically increasing tangential force, the resulting shear tractions can be expressed as a simple superpositon, provided we assume that the frictional tractions in the slip zone are everywhere aligned with the applied force—a result which is strictly true only in the special case where Poisson’s ratio is zero. Ciavarella later showed that with this assumption, a similar superposition applies to any uncoupled three-dimensional contact problem for the half space. Here we relax this assumption and develop a general solution for the case where the tangential force is relatively small, so that the slip annulus is thin compared with other dimensions of the contact area. The local conditions are then characterized by the mode II and III stress-intensity factors in the corresponding adhesive problem. We show that the slip direction depends only on a dimensionless coordinate, so that all points in the slip zone pass through scaled versions of the same expressions as the applied shear force increases. The results show a surprisingly large deviation in slip direction, particularly for incompressible materials.

Hydrogels are ideal substitute materials for articular cartilage. The fretting wear behavior of polyvinyl alcohol (PVA) hydrogel and three types of PVA-based composite hydrogels under dry friction condition is investigated experimentally. Polyvinyl pyrrolidone (PVP), graphene oxide (GO), and multi-walled carbon nanotubes (MWCNTs) are mixed into PVA hydrogels as additives to prepare three composite hydrogels. A sphere-on-flat contact model is established between the PVA-based hydrogel coating and a stainless steel ball. The reciprocating fretting wear equipment is used to conduct fretting tests. Surface wear morphology and wear depth are characterized using three-dimensional (3D) ultra-depth of field microscopy and 3D white-light interference profilometer. The impacts of displacement amplitude, normal force, exposure time, and the number of freeze–thaw cycles on fretting wear behavior of four PVA-based hydrogels are investigated. The results show that the PVA/MWCNTs composite hydrogel exhibits the highest friction coefficient and relatively good wear resistance.

Graphical Abstract

Pursuing durable superlow friction tribological systems is essential for addressing global energy challenges and achieving carbon neutrality by 2050. Inspired by the exceptional frictional performance of human knee joints, concentrated polymer brushes (CPBs) have emerged as a promising solution for artificial tribological applications. However, a major challenge remains in the reduced durability of CPBs compared to traditional hard materials, particularly in maintaining superlow friction over extended use. This study focuses on understanding the mechanisms behind the vanishing superlow friction and wear of CPBs lubricated with ionic liquids (ILs). To investigate these phenomena, we developed a real-time and spatially resolved “in-operando Raman tribometer” for real-time monitoring of molecular dynamics at friction interfaces. Time and spatially resolved Raman measurements revealed critical insights into the polymer chain behavior, IL film drainage, and internal stress changes that influence friction and wear mechanisms. Our results suggest that the “probe vertical lift” phenomenon and distribution of the solid–liquid interface structure play key roles in the transition from superlow to high friction and accelerated wear. These findings provide a foundation for developing durable, energy-efficient tribological systems in various industrial applications.

Experimental studies on wear often employ incomplete contact configurations which cause variations in uncontrolled variables as wear progresses, ultimately leading to coupled results. This study observes fundamental wear mechanisms under dry gross sliding conditions. Reciprocating wear experiments are conducted at constant normal load using annular samples having flat-on-flat common edge contact configuration that are made of Al 6082 T6. This configuration ensures constant nominal contact area and pressure, nominally uniform contact pressure distribution, an edge-free contact region along the sliding direction and uniform air exposure of the contact area. A comprehensive investigation of fundamental wear behaviour is conducted through measurements of wear mass, tangential and vertical displacement, hysteresis loop area, coefficient of friction and tangential contact stiffness. In contrast to some previous studies on annular contacts, the results show increasing wear mass with normal load. The findings also indicate that a reliable displacement measurement technique, here digital image correlation, is essential for accurate tracking of relative surface displacements. This enables correction for machine compliance and hence accurate tangential contact stiffness measurements, and accurate observation of dilatant motion. In contrast, the test machine’s nominal measurements are shown to be sufficient for work input calculations. The evolution of both the vertical displacement and the hysteresis loop area implies the presence of two distinct processes during the initial transient stage: surface roughening and debris bed formation. Competition between these processes is shown to govern transient wear.

Graphical Abstract

Understanding and controlling friction is crucial for advancements in nanoscale systems and microelectromechanical devices. Moiré superlattices formed on graphene-covered metallic substrates present a unique opportunity to modulate frictional behavior. However, the role of graphene/substrate interaction in lubrication within these heterostructures remains inadequately explored. In this study, molecular dynamics simulations combined with theoretical modeling are employed to investigate the impact of graphene-substrate interactions on friction in moiré heterostructures. The results reveal a nonmonotonic dependence of lateral forces on interaction strength, governed by the competition between graphene wrinkling and elastic energy dynamics. In the weak interaction regime, tip/graphene-adhesion-induced graphene wrinkling enhances friction by increasing pinning atoms. In contrast, strong interactions lead to elastic energy accumulation and relaxation, elevating friction during stick–slip motion. Additional factors, such as normal load, tip size, tip geometry, and tip/graphene interaction, significantly influence frictional behavior. An improved Prandtl-Tomlinson model is developed to validate the findings, demonstrating excellent agreement with simulation results. This work elucidates the mechanisms underlying graphene-substrate-interaction-dependent frictional behavior and offers a framework for tuning tribological properties in two-dimensional materials, enabling the design of advanced MEMS and NEMS with controllable lubrication.

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Cross-country skiers employ various techniques, where the ski is exposed to different forces during the motion. This study utilized a novel sled tribometer to investigate the combined effects of load and positioning of the skier on the coefficient of friction (COF) between the skis and snow. Three different loads (40 kg, 80 kg and 120 kg) were applied to the sled, and the center of mass was systematically varied between three positions behind the binding position: 70mm (leaning forward), 140mm (centered) and 210mm (backward). A variety of skis were used, including different models of skate skis and one classic-style ski with grip wax. The results consistently demonstrated that increasing the load on the sled reduced the COF by up to 15% (from the lowest to highest load), regardless of the position of the center of mass. The position of the center of mass had a minimal effect on COF in most tests. An exception was observed when using grip wax, where a forward-leaning position combined with a heavy load significantly increased the COF (~ 8%) compared to what is expected without grip wax. This load-dependent reduction in the COF was observed across different skis and test sessions. The ski camber profile was measured for all skis in all configurations. In general, increasing the load increases the glide zone length but at the same time increasing the average pressure. The position of the center of mass has little to no effect on the rear glide zone but slightly alters the length and position of the front glide zone. While the mechanisms of friction are discussed, a complete understanding of these mechanisms has not yet been reached.

Titanium-zirconium-molybdenum (TZM) alloy, one of the most widely utilized molybdenum alloys, usually experiences friction during service, which leads to tribological failure. However, its friction performances greatly vary under different loads, and the underlying mechanisms remain unclear. In this study, we developed reciprocating friction tests of TZM alloy in aerobic conditions, and evaluated its tribological properties and mechanism under the normal force ranging from 1 to 10 N. The wear process is primarily affected by oxidation, adhesion, and fatigue wear, and adhesive wear intensifies with increasing load, which mainly results in the gradual decline in coefficient of friction from 1.022 to 0.510, as well as variations of friction performances and wear track morphology, thereby elucidating the wear mechanism. There is a long running-in period at a low load of 1 N, with a significant accumulation of mixed oxides of MoO₃ and Mo₄O₁₁ in the wear tracks, leading to a minimal wear rate of 1.68 × 10^–5 mm³·N⁻¹·m⁻¹ after 9000 cycles. At 2 N and 4 N, these oxides undergo repeated spalling, crushing, and re-forming processes. As the load increases to 7 N and 10 N, the wear rate surges to above 1.31 × 10^–4 mm³·N⁻¹·m⁻¹ due to intensified adhesion and debris discharge.

Graphical Abstract

Developing and utilizing high-extreme pressure (EP) performance solid lubricants has become imperative to meet the lubrication requirements for harsh operating conditions. Based on the action mechanism of rare earth on alloys, the influence of material corrugated layered crystal structure, and sulfur content on lubrication properties at the atomic and molecular levels, layered rare earth sulfates have become a new type of high EP performance solid lubricants. Herein, we prepared layered rare earth sulfate materials (RE₂(OH)₄SO₄·nH₂O, RE = La/Sm/Ce, i.e., RES, LaS, SmS, CeS, respectively) using a mild hydrothermal method for controlled particle size and morphology. To evaluate its applicability, we tested the new solid lubricants against high load, high temperature, long-term, different friction pair materials, and fretting. Overall, the RES samples exhibited excellent lubricating behavior when added to polyurea grease, better than MoS₂. By adding layered lanthanum sulfate, samarium sulfate, and cerium sulfate, the seizure loads can reach up to 1400, 1300, and 1300 N, respectively. Microscopic analysis of the friction pairs confirmed that the lubricating films formed on the worn surface during friction. Excitingly, the RES materials exhibited excellent synergistic effects on EP performance and corrosion resistance. Thus, these materials have broad application prospects as lubricants.

Graphical Abstract

In this study, Fe₃O₄/WS₂ and CuO/WS₂ composites were synthesized via a one-step hydrothermal method, and conductive lubricating greases were formulated using polyurea grease (PUG) as the base. The tribological performance and conductivity of the prepared greases were systematically investigated. The experimental results demonstrate that compared to WS₂ grease, Fe₃O₄/WS₂ and CuO/WS₂ greases exhibit superior tribological performance and enhanced conductivity. Fe₃O₄/WS₂ and CuO/WS₂ composites can form physically adsorbed films and tribochemical reaction films on frictional surfaces, reducing the COF and wear. Furthermore, they enhance the carrier concentration at the friction interface, thereby improving conductivity and significantly mitigating triboelectric effects. These properties underscore their promising potential for applications in conductive lubrication and electrostatic elimination.