Dynamic friction weakening drives both earthquakes and the slip behavior of tribological systems across scales, yet the dependence of unstable slip events on frictional evolution remains to be clarified. Here, we show the critical role of the peak friction weakening rate in determining the magnitude of unstable slip events. Direct experimental measurements distinguishing frictional from shear forces reveal that peak slip velocity scales linearly with peak friction weakening rate in unstable slip. Friction weakening rate plays an excitation for the dynamic slip. The lag of peak slip velocity behind the peak friction weakening rate is associated with the self-limitation of slip velocity and the self-arrest of slip. We identify three characteristic modes of friction weakening during unstable slip. The average friction weakening rate during the rising period of slip acceleration is significantly correlated with the rate of increase of slip acceleration. Our findings suggest that the peak friction weakening rate serves as the fundamental scaling parameter for unstable slip events, thereby suggesting it as the underlying mechanism through which external factors modulate event magnitude.
Nickel-based superalloy Inconel 718 is widely used in high-temperature applications in the aerospace and power generation industries, but its tribological performance at elevated temperatures is of major concern, especially for additively manufactured components. Although a temperature range of about 750 °C is important for practical applications, systematic investigations in this range are rare, even though it is the subject of many studies conducted at room temperature (R.T.) and at selected high temperatures. In this work, the wear characteristics of additively manufactured Inconel 718 were studied, focusing on microstructural changes and associated wear mechanisms, using scanning electron microscopy (SEM). Dry sliding wear tests were performed under severe high-temperature conditions, and the worn surfaces were characterized to understand the predominant wear mechanisms and the degradation pathways of the microstructures. The SEM observations indicated that there are significant relationships between the microstructural features, such as the grain morphology, the distribution of porosity, the sites of crack initiation, and the occurrence of oxide formation, with the wear behaviour. As the temperatures approached 750 °C, the predominant wear mechanism shifted from abrasive and adhesive wear to a combination of oxidative wear, plastic deformation, and surface delamination. Under these conditions, the formation and instability of oxide layers and the propagation of cracks in the subsurface were important in the removal of material. The coefficient of friction drops significantly from 0.75 (Ambient Temperature) to 0.34 (750 °C). This document offers important insights into the wear mechanisms and is beneficial for predicting the performance of components in intermediate- to high-temperature service applications involving Inconel 718.
Thermal elastohydrodynamic lubrication (TEHL) simulations of line contacts lead to strongly coupled, high-dimensional nonlinear systems. Accurate prediction of pressure, film thickness, temperature rise, shear stress, and friction generally requires full-system finite element (FE) models, whose computational cost limits repeated parametric studies. Previous reduction efforts for this class of TEHL models have mainly targeted the linear elastic contribution, for example through Static Condensation with Splitting (SCS). However, after elastic reduction, the nonlinear thermal subsystem remains the dominant bottleneck, as it contains the largest number of degrees of freedom (DOFs) and still requires repeated Jacobian assembly and projection during the Newton iterations. This work extends the reduction of full-system FE TEHL line contact simulations to the nonlinear thermal contribution. The proposed formulation retains SCS for the elastic reduction, uses Proper Orthogonal Decomposition (POD) for the thermal unknowns, and applies the Matrix Discrete Empirical Interpolation Method (MDEIM) to hyper-reduce the nonlinear thermal Jacobian block. The method is validated on out-of-training cases spanning low, moderate, and high loads. The coupled algebraic system is reduced from 23,880 to 1,533 DOFs, and only 1000 selected nonzero thermal-Jacobian entries are evaluated out of 161,249 during the online stage. The reduced-order model (ROM) achieves an average speedup of 38
Surface topography plays a critical role in the tribological behavior of interacting surfaces. In experimental studies, a controlled variable strategy commonly relies on a limited number of parameters to characterize the state of a surface. However, this approach is intrinsically limited because it reduces multiscale surface morphology to a few descriptors and assumes that the remaining topographic features can be kept comparable. In practice, preparing surfaces that are similar across all length scales is highly challenging. In this study, we established a customized radial grinding strategy to produce circumferential tracks with closely comparable surface topographies at different radial positions on the same 100Cr6 steel disk. The topographic comparability of these tracks was verified across multiple length scales with both roughness parameters and power spectral density (PSD). Lubricated ball-on-disk tests were then performed using 10 mm 100Cr6 steel balls, FVA 1 base oil, a normal load of 2 N, and a multistep sliding speed protocol ranging from 50 mm/s to 600 mm/s. Angle-resolved friction analysis showed that the main spatial distribution of friction was largely preserved among the different radii, although the mean friction level between radii varied systematically with sliding speed. Despite the verified topographic comparability and the similar spatial friction distribution, the resulting Stribeck type curves did not collapse under identical linear sliding speeds. This deviation may arise from differences in angular velocity and/or from multiscale effects of surface topography that are not fully captured by conventional statistical parameters.
In atomic force microscopy (AFM) friction testing, flexible two-dimensional (2D) solid lubricants typically rely on a substrate for support to suppress the out-of-plane bending and maintain their atomic-level flatness. However, the impacts of interface charge transfer caused by contact with the substrate on their surface friction behavior is commonly underestimated. Here, we employed density functional theory to simulate the single-asperity sliding of a diatomic Au tip on a graphene (Gr) surface supported by various metal substrates, and to track changes in the charge distribution at contact interface. It is found that the contact between metal substrate and Gr surface causes charge transfer between them, resulting in contact charge doping. This, in turn, alters the charge density fluctuation at the tip-Gr contact region during sliding, thereby modulating the energy barrier (i.e., friction energy dissipation) that the tip must overcome to slide across Gr surface. This effort elucidates the electronic mechanism by which the contact doping induced by metal substrate influences the single-asperity sliding of Au tip on Gr surface, providing an electron-scale insight into the influence of substrate support on the surface friction properties of 2D solid lubricants in AFM friction experiments.
