Tribology Letters最新文献

Scuffing is a severe wear mode that occurs under boundary lubrication. This wear mode remains challenging to predict, and no objective criteria have been established in the literature to characterize the evolution of the phenomenon across the distinct phases of frictional behavior. This study proposes a quantitative method to characterize the three typical scuffing phases (stable, incipient, and severe) based on friction force evolution. Block-on-ring tests were conducted using SAE 52100 steel and two biolubricants: pure castor oil (CO) and aminolyzed castor oil (ACO). A mathematical approach was developed by analyzing the variation of friction force (delta friction) across discretized time intervals. Real-time imaging enabled direct correlation between friction behavior and surface damage. SEM/EDS analyses revealed that the material accumulated during severe scuffing consisted of oxidized oil and steel particles. The dynamic of formation and destruction of these accumulated material explains the observed friction instabilities. The proposed method successfully identified all scuffing phases across tests and was validated by consistent morphological and chemical evidence. ACO significantly increased the time to reach severe scuffing compared to CO, with results confirmed by Weibull statistical analysis. The approach presented herein provides a robust and replicable framework for scuffing characterization and supports the application of chemically modified biolubricants as sustainable alternatives with enhanced tribological performance.

This study develops an elastoplastic fractal contact model for rough hard-coated surfaces along, with a corresponding contact stiffness model based, on a coated asperity contact model and a statistical rough surface contact model. To establish the contact stiffness model for the coated surface asperities, seven fundamental postulates were developed. Numerical simulations were conducted to systematically analyze the influence of the ratio between the coating thickness and the asperity radius, the material properties of the coating and the substrate, and the substrate surface roughness on the contact behavior and stiffness of the coated surfaces. The results demonstrate that under a given load, hard-coated rough surfaces exhibit a smaller real contact area but higher stiffness compared to uncoated surfaces. Thicker and stiffer coatings further reduce the contact area while increasing stiffness. Smoother substrate surfaces lead to a larger contact area and higher stiffness in the coated systems. These findings align with the existing statistical contact models for hard-coated rough surfaces, and the predicted contact area closely matches the prior simulation results. To validate the model, experimental tests were conducted on the TiN-coated specimens. The theoretically predicted contact stiffness showed strong agreement with the experimental measurements, confirming the accuracy and applicability of the proposed fractal-based stiffness model.

We present a study of sliding friction for rigid triangular steel sliders on soft rubber substrates under both lubricated and dry conditions. For rubber surfaces lubricated with a thin film of silicone oil, the measured sliding friction at room temperature agrees well with theoretical predictions obtained from a viscoelastic model originally developed for rolling friction. On the lubricated surface, the sliding friction is primarily due to bulk viscoelastic energy dissipation in the rubber. The model, which includes strain-dependent softening of the rubber modulus, accurately predicts the experimental friction curves. At lower temperatures ((T = -20^circ textrm{C}) and (-40^circ textrm{C})), the measured friction exceeds the theoretical prediction. We attribute this increase to penetration of the lubricant film by surface asperities, leading to a larger adhesive contribution. For dry surfaces, the adhesive contribution becomes dominant. By subtracting the viscoelastic component inferred from the lubricated case, we estimate the interfacial frictional shear stress. This shear stress increases approximately linearly with the logarithm of the sliding speed, consistent with stress-augmented thermal activation mechanisms.

Reducing alloy friction to achieve ultra-low friction is a valuable approach to save energy and reduce pollution from oil use, which is a major challenge for researchers. This study introduces a successful method to achieve ultra-low friction in Ti-6Al-4 V using a hydrated lubricant composed of hydroxyethyl cellulose (HEC). And the effects of speed and concentration on lubricating were investigated. It was found that excessive sliding speeds may lead to lubricant detachment and consequent friction increase, indicating that the adsorption ability of HEC needs to be enhanced in future studies. In addition, when the concentration exceeds 5 wt%, wear loss tends to stabilize across tests with different concentrations, while the friction force increases with rising concentrations. Based on these findings, microscopic studies were conducted to investigate the mechanism of friction reduction. Notably, distinct topographic features resembling ‘valleys’ and ‘plateaus’ were identified on the wear scars in a nanoscale scope. The movement of the surfaces induces the hydrated HEC lubricant to flow from the lower valleys to the higher plateaus, suggesting elastohydrodynamic lubrication mechanisms to form robust films. The valleys serve as lubricant reservoirs, while the plateau tops support the lubricant films to prevent contacts between Ti-6Al-4 V and Si₃N₄. The schematic illustrations depict the microscopic mechanisms for achieving of ultra-low friction on Ti-6Al-4 V alloy.

The surface texturing of water-lubricated ultra-high molecular weight polyethylene (UHMWPE) bearing has the advantages of enhancing the hydrodynamic effect, capturing wear debris, reducing secondary friction, and storing water, and is one of the research hotspots in tribology. However, the influence of the surface texture of water-lubricated UHMWPE bearing on the friction performance of bearing is still unclear. Based on the laser induced fluorescence (LIF) method, this study developed an in situ-visualization observation system for water film on the friction interface of non-metallic bearing with the help of transparent glass friction pair. The typical textures with different area ratios, aspect ratios and distribution angles were used as the research objects. The water film thickness, friction coefficient and surface morphology of the friction interface of UHMWPE bearing with different textures under low-speed and heavy-load conditions were analyzed. The results show that elongated textures with a reasonable area ratio and length-to-width ratio, arranged along the direction of friction, are more likely to form hydrodynamic effects and create strong support points at the friction interface. Additionally, the divergent and convergent spaces formed at the inlet and outlet of the textured units cause the lubrication pressure at the inlet to be lower than that at the outlet, resulting in severer wear at the inlet compared to the outlet. The overall trends and the mechanism discussed in this research may be considered as a guideline for the design and optimization of surface texture of water-lubricated UHMWPE bearing.

Due to the high surface-to-volume ratio, micro/nano-electromechanical systems (MEMS/NEMS) undergo severe wear during the relative sliding. Graphene, possessing excellent mechanical, physical, and chemical properties, can achieve an ultralow friction and wear state, making it highly promising for significantly minimizing friction and wear in MEMS/NEMS. However, graphene films used in MEMS/NEMS are typically subjected to thermal annealing pretreatment during the fabrication process. To maintain optimal performance, it is particularly necessary to investigate the evolution of graphene tribological properties after high-temperature annealing. In this article, by performing nanoscale atomic force microscopy (AFM) measurements on mechanically exfoliated graphene, we reveal that the friction force on graphene decreases slightly upon annealing to approximately 200 °C, then gradually increases before rising rapidly once the annealing temperature exceeds 500 °C. Raman spectroscopy identifies that the changes in friction result from the annealing-induced compressive stress accumulation and defects creation. Our results provide deep insights for the application of graphene in sliding MEMS/NEMS.

Graphical Abstract

In the context of low-viscosity aqueous lubrication, this study systematically investigates the lubrication mechanisms of polyacrylic acid (PAA) solutions. It combines experimental techniques with molecular dynamics simulations to elucidate the effects of concentration-dependent molecular structures and temperature-dependent adsorption stability. Results show that with increasing PAA concentration, the friction coefficient first decreases and then increases, with optimal lubrication observed at 0.2%. Higher concentrations lead to increased PAA adsorption, elevated acidity, and molecular aggregation, resulting in surface corrosion and intensified wear. At high temperatures, while adsorption energy remains stable, enhanced chain flexibility and entanglement raise viscosity, reduce film fluidity, and accelerate wear. These findings provide valuable insights for optimizing water-based lubrication systems using PAA additives.

Graphical Abstract

Accurate simulation of the motion of a bowed violin string requires a reliable tribological model for rosin, with which the bow-hairs are coated. None of the models proposed in the past have given entirely satisfactory results. An enhanced model is proposed here, which combines influences from the contact temperature and the sliding speed. Transient vibration predictions using this model are compared with earlier experimental measurements in the “Guettler diagram”, demonstrating better agreement than with earlier models.

In recent interesting experiments, Peng et al. ((2015) PRL, 134, 176202) have shown that the static friction coefficient in a spherical contact drops of a factor close to 2 over 3 decades of increase of normal load, converging to a dynamic friction coefficient. The difference is larger than what commonly attributed in dry metals. They have interpreted this with a numerical boundary integral contact calculations involving many asperities using two input parameters (a static and a dynamic friction coefficient from AFM experiments at nanoscale). However, we show that similar drop with normal load is also expected from the theory of "Griffith" or "JKR" friction (Ciavarella (2015) J Mech Phys Solids 84: 313–324) which has the advantage of being a simple analytical theory and also of being closely connected with friction laws used commonly today in geophysics. Further, it also uses two input parameters, and requires no numerical solution of the rough contact problem for the sphere.

This study builds upon the work published by Hansen et al. (Tribol Lett 69:1–17, 2021), which introduced a revised film parameter, ({Lambda }^{*}={({h}_{text{m}}+h}_{text{c}}{f}_{text{q}})/Spk), for evaluating rough surface contacts in the microelastohydrodynamic (micro-EHL) and mixed lubrication (ML) regimes. The parameter incorporates a micro-EHL correction term (({f}_{text{q}})) that accounts for different roughness lays (or pattern), the reduced peak height parameter ((Spk)) for more relevant roughness representation, and an updated criterion for the EHL-to-ML transition (({Lambda }^{*}=1)). These advancements address the limitations of the traditional (Lambda)-ratio by offering improved sensitivity to running-in wear, resilience to measurement artefacts, and more realistic predictions of lubrication quality. In the present study, we investigate how measurement and calculation methods influence the robustness and sensitivity of the ({Lambda }^{*})-ratio. Key considerations include the impact of spatial resolution from surface roughness measurements, the sensitivity of asperity summit radius calculation methods, and the use of amplitude reduction theory (ART) as an alternative approach to compute ({Lambda }^{*}) and benchmark its performance. Within the given scope, the results show that spatial biases can be mitigated with appropriate filter sizes and that ({Lambda }^{*}) consistently predicts the EHL–ML transition more accurately than the traditional Λ-ratio, regardless of the asperity radius method used. Furthermore, we found that while ART can be used to compute ({Lambda }^{*}), the original approach using the ({f}_{text{q}})-term offers both overall improved accuracy and simplicity. By critically assessing key uncertainties with ({Lambda }^{*}), this study strengthens the parameter's robustness and enhances its applicability as a reliable tool for analysing and designing tribological systems.