Tribology Letters最新文献

Autoparametric resonance in a combined contact (tip apex)–driving-spring (cantilever) system, that is responsible for the appearance of multiple peaks in friction as a function of scanning velocity, is investigated in a wide range of the possible system damping. The role of cantilever damping, being practically inessential in conventional stick–slip regime at lower velocities, is shown to be crucial for the appearance of friction force peaks at the resonant and quasi-resonant velocities. With changing damping, the evolution of different peaks turns out to be nontrivial, that is related with an unusual manifestation of double slips of the tip and memory effects. Relative value of the main force peaks as functions of damping factor are non-monotonous with maximum, which can reach several tens percent, depending on the system parameters and temperature. Such a strong resonance enhancement of energy dissipation is likely to occur in practical systems, where damping of driving spring can be significant, in contrast to nearly ideal cantilevers used in AFM experiments.

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The friction behavior between soft and hard materials has long been a crucial research subject in diverse fields, such as artificial joints, human skin contact, and robotic grasping. This study combines theoretical analysis with experimental exploration. By analyzing the variation in friction force and the characteristics of the sliding traces of silicone rubber materials during the entire friction process, it aims to delve into the frictional characteristics of soft–hard material interfaces. The results show that the sliding behavior at the soft–hard interface occurs before the peak of static friction. Through the approximation of a frictional theoretical model, the relationship between the friction force and the relative sliding distance at the interface is verified. Moreover, a method for predicting the interface frictional contact state and the relative sliding distance at the interface based on the friction force–tangential displacement curve is proposed. This research enhances our understanding of the friction mechanism at soft–hard interfaces and offers both theoretical support and practical guidance for the development of fine tactile feedback and dexterous manipulation systems in robotic grasping.

A novel methodology using atomic force microscopy (AFM) has been developed for assessing the effect of surface oxidation on the adsorption and frictional properties of oiliness additives. This study focused on comparing the tribological behaviors of ester-based oiliness additives on oxidized versus pure Ti surfaces. The substrates were patterned to facilitate precise AFM-based friction testing. The oxide layer thickness was characterized by X-ray photoelectron spectroscopy and subsequently abraded using a diamond-coated AFM probe to expose the pure Ti surface. Tribological performance under atmospheric conditions was evaluated by comparing frictional properties using ester-based additives of varying ester functionalities, ranging from monoester to tetraester. Adsorption properties were characterized through neutron reflectometry and contact angle measurements. The results demonstrated a clear correlation between adsorption density and friction reduction, with tetraester showing superior performance, especially on pure Ti surfaces. These findings highlight the critical effect of oxidation states and additive molecular structures on frictional properties and adsorption behaviors at nanoscale resolution, providing valuable insights into boundary lubrication mechanisms and lubricant optimization.

This study examined and compared the tribological properties of a cold-sprayed CrMnCoFeNi high entropy alloy (Cantor alloy) coating under ambient and dry air conditions. Tribological testing was conducted using an in situ tribometer equipped with video microscopy, allowing real-time monitoring of the evolution of the sliding interfaces through a transparent sapphire counterface. This experimental setup provided the opportunity to observe phenomena that would otherwise remain concealed between the contacting bodies. The wear rate was 1.8 ± 0.5 × 10⁻⁴ mm³/Nm in ambient air and 7.5 ± 0.7 × 10⁻⁴ mm³/Nm in dry air. In dry air, the velocity accommodation mode was characterized by interfacial sliding of a static transfer film against the wear track, resulting in a stable steady-state coefficient of friction (CoF) of 0.5. In contrast, ambient air conditions led to an average CoF of 0.8, with fluctuations attributed to plastic shearing of the transfer film observed in situ. The higher humidity in ambient air inhibited cold welding of wear particles, resulting in a less stable transfer film that underwent removal or extrusion events, which were associated with sudden drops in CoF. Additionally, a “metal debris” oxide formation mechanism was observed in ambient air, contributing to the formation of a protective tribofilm and a reduction in the wear rate. In dry air, the “oxidation-scrape-reoxidation” mechanism dominated, facilitated by the absence of adsorbed water droplets. This resulted in an increased wear rate under dry conditions.

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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.

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