Tribology Letters最新文献

The superlubricity of sputtered MoS₂ film under dry air environment was achieved by low-energy proton irradiation of 25 keV for the first time. We found that proton (H⁺) irradiation is able to break the Mo-S covalent bonding of as-deposited MoS₂ film and leads to the formation of MoS₂ nanocrystalline domains. The dangling bonds at edge planes or newly exposed edges could be passivated with hydrogen ions by bonding interaction under proton irradiation, forming hydric MoS₂ nanocrystalline domains with stable S–H bind, resulting in superior antioxidant capacity of proton-irradiated MoS₂ film compared to its non-irradiated counterpart. Importantly, proton irradiation can penetrate the interior of sputtered MoS₂ film with thickness over 1 µm and restructure the as-deposited MoS₂ film into nanocrystalline MoS₂ domains to achieve superlubricity and life extension under dry air conditions.

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Surface temperature rise during the sliding process significantly affects friction and wear, which is crucial for the performance of mechanical systems. In this work, the finite element method is adopted to simulate the frictional contact between a two-dimensional cylinder and an elastic–plastic substrate with a self-affine fractal surface. The influences of surface profile and external load on the maximum temperature rise are examined. Either the increase of roughness or the decrease of Hurst index would result in a reduction in contact area, and are more likely to produce higher maximum temperature and surface damage. Additionally, higher sliding velocity increases the maximum temperature, but the uplift of external load tends to eliminate the effects of rough profile on temperature and contact pressure. A general relation between maximum surface temperature rise and contact area for rough surfaces is proposed to predict the occurrence of the high-temperature hotspots during the sliding process. This study provides insights and novel perspectives for the understanding of the frictional and thermodynamic behavior of contact in mechanical structures.

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Ti-6Al-4V (TC4) alloy with ultra-smooth surfaces has found extensive application in biomedical fields. Chemical mechanical polishing is a crucial method for achieving ultra-smooth surfaces, but its efficiency in polishing TC4 alloy surfaces is low. This study proposes a new approach to enhance the polishing efficiency by tuning counterions, which significantly influence both chemical corrosion and microscopic interaction forces. The mechanism involves Li⁺/Na⁺/K⁺ regulating the action intensity at the tribological interface by altering the thickness of the electrical double layer and dynamic ionic radius. On the one hand, reducing the thickness of the electrical double layer from 1.41 to 0.46 nm can enhance the intensity of chemical reactions, and the smaller the dynamic ionic radius of the counterion, the more pronounced the chemical corrosion caused by H₂O₂ becomes. Combining the two, the reaction products of H₂O₂ (HO₂⁻ and OOH⁻) can more readily react with Ti to form fragile reaction products with the help of K⁺. On the other hand, as the electrostatic repulsion force weakens, the SiO₂ particles exert a stronger mechanical force, allowing for quicker removal of the fragile reaction products. Thus, in the presence of 10 wt%H₂O₂ and 200 mM K₂SO₄, a polishing efficiency of 1197 nm/min is achieved, with the S_a of 2.7 nm over a scanning area of 195.8 × 195.8 μm², and without polishing damage layer on the substrate. The findings provide mechanistic insight for further exploring the limits of polishing performance in CMP of titanium alloys.

From the earliest theoretical studies on elastohydrodynamic lubrication, it was believed that film build-up is governed by lubricant rheology in the low-pressure contact inlet. Recently, it was discovered that this is only true for the theoretical line contact case, where lubricant out-of-contact lateral flow is absent. In actual contacts, though central film thickness is indeed governed by low-pressure lubricant rheology, minimum film thickness is additionally influenced by the high-pressure response. Thus, a proper prediction of minimum film thickness (either by analytical formulae, or machine-learning frameworks) would require input parameters that define the high-pressure viscous response of the lubricant. The current work identifies and formulates these parameters with the help of machine-learning regression tools. These are fed with minimum film thickness results from finite element simulations of smooth steady-state isothermal Newtonian circular contacts, lubricated with sets of hypothetical fluids having the same pressure-viscosity response at low pressure, but different high-pressure ones. It is found that conventional dimensionless groups are not sufficient to describe minimum film thickness formation, and that an additional pressure-viscosity coefficient—evaluated at half the Hertzian contact pressure—is required.

The abrasion testing process of topographically modified surfaces is investigated and their mechanical durability and wear characteristics are presented. The primary aim of the study is to demonstrate that a simple abrasion testing process carries a number of subtle complexities which are crucial for getting comparable results—i.e., sample area, abrasion duration, and presence of a microstructure. All of these factors can significantly alter the results of the testing process and have to be considered during comparison with other durability results. This study also demonstrates how topographically modified aluminum structures tend to restore their hydrophobicity after noticeable mechanical damage due to the natural oxidation process, whereas control samples stay in a hydrophilic state. The findings could be applied to improving the performance of wind or steam turbine blades.

Graphical Abstract

Many properties of soft contact interfaces are controlled by the contact area (e.g. friction, contact stiffness and surface charge generation). The contact area increases with the contact age at rest. In contrast, it usually reduces under unidirectional shear loading. Although the physical origin of such a reduction is still debated, it always happens in an anisotropic way because the reduction mainly occurs along the shearing direction. Whether such anisotropy is a necessary condition for shear-induced area reduction remains an open question. Here, we investigate the contact area evolution of elastomer-based sphere-plane contacts under an isotropic shear loading, i.e. torsional loading. We find that, when macroscopic sliding is reached, the contact area has undergone a net area reduction. However, the area evolves non-monotonically as the twisting angle increases, with an initial rise up to a maximum before dropping to the value during macroscopic sliding. The ratio of maximum to initial contact area is found weakly dependent on the normal load, angular velocity and dwell time (time interval between the instants when the normal load and twist motion are first applied) within the investigated ranges. We show that non-monotonic area evolution can also be found under unidirectional shear loading conditions under large normal force. These observations challenge the current descriptions of shear-induced contact area evolution and are expected to serve as a benchmark for future modelling attempts in the field.

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The characteristic parameters, such as curvature radius of asperity, height distribution, and asperity density play a decisive role when studying the contact characteristics of rough surfaces. A new method of asperity definition based on curve fitting and peak refit, named the deterministic method, is proposed in this paper. The real topography of the rough surface is described by the moving least-squares method. And the local maximum of the curve is defined as the asperity, and the local minimum is defined as the valley. To improve the stability of characteristic parameters of the rough surfaces, this method regenerates a new asperity when the asperities are gathered too closely. Both the characteristic parameters obtained by the deterministic method and the spectral moment method are used in two typical elastic–elastoplastic–plastic contact models, to analyze the contact characteristics of rough surfaces. Numerical calculation results show that, compared to the spectral moment method, the deterministic method demonstrates greater consistency across different sampling intervals, indicating lower sensitivity to sampling interval variations. This improves the accuracy and stability of contact performance parameters, validating the effectiveness of the proposed method, which can serve as a feasible approach for analyzing fine contact on rough surfaces.

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Ultralow-wear polyethylene (ULWPE) was proposed to replace conventional UHMWPE as an artificial joint material. Different molecular weights of ULWPE, ULWPE-200, ULWPE-300, and ULWPE-700 were examined against CrCoMo compared to conventional UHMWPE in multidirectional motion. The wear mechanism was elucidated from the perspective of macroscopic wear behavior and microscopic wear debris characterization. It was found that the morphologies of the ULWPE worn surface were similar to that of UHMWPE, with scratches, burnishing, and protuberances. ULWPE-700 possessed the lowest wear loss at all loading conditions, and the wear loss was 40.3% lower than that of UHMWPE at 3 MPa. Furthermore, wear debris was consistent in morphology and size range but showed differences in quantity, size distribution, and shape distribution. Combined with the wear surface morphology and wear debris analysis, it showed that plastic deformation was the main cause of wear debris formation and the wear mechanisms were adhesive wear and abrasive wear. Moreover, the FBA of ULWPE-700 was 64% lower than that of UHMWPE at 3 MPa, suggesting that ULWPE-700 wear debris had the lowest potential osteolysis. This study provides deeper insight into the bio-tribological behavior and the potential biological activity of ULWPE as an artificial joint material.

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The recent demand for more efficient gas turbine engines has led to a growing need for new high performance materials and engineered surfaces. Consequently, there has been recent development of thermally sprayed coatings capable of withstanding harsh environments to advance these engines. For instance, oxide-based coatings exhibit high temperature stability, making them potential coating candidates for applications at elevated temperatures, thereby further improving gas turbine engines' efficiency. In particular, cobalt- and chromium- based oxides have previously been shown to be beneficial in terms of reducing friction and wear in high temperature environments. However, limited work has been performed on the deposition of such coatings by means of thermal spray processes. Therefore, the main purpose of this study is to develop and critically evaluate thermally sprayed cobalt- and chromium-based coatings for extreme environments. More specifically, the coatings were deposited by means of suspension plasma spray (SPS) and characterized before and after ball-on-flat tests at different temperatures. The coatings developed in this study have demonstrated high resistance to wear when tested against IN718. In all cases, most of the wear was observed on the counterballs. The CoO coating exhibited the lowest combined wear when compared to the other coatings. Ex-situ Raman analysis revealed the formation of Co₃O₄ for the worn cobalt oxide-based coatings tested at 450 °C, which correlates well with the lower wear rates.

Using a powerful synchrotron radiation X-ray source, we developed a cell that can perform Small Angle X-ray Scattering (SAXS) measurements under high shear (~ 10⁵ s⁻¹). We successfully and quantitatively visualized the deformation of polymer chains as polymer additives in oil under high shear. We found that poly(alkylmethacrylate) (PMA) with the lowest molecular weight was not deformed by the shear flow and did not show the shear thinning behavior. On the other hand, the other PMAs were deformed and exhibited shear-thinning behaviors. We compared the experimental results with the simulation by Ryder et al.( The Journal of Chemical Physics 45: 194906 (2006)) and found the shrink perpendicular to the flow direction in the experiment is stronger than that in the simulation, indicating that the rigidity of the polymer chains enhanced by long side groups induced the alignment of the chain along the flow direction. The decrease in viscosity was less than that estimated from the rate of deformation estimated by SAXS due to the effects of polydispersity of PMA polymers.

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