Granite sliding on granite: friction, wear rates, surface topography, and the scale-dependence of rate-state effects

TL;DR

This study investigates granite–granite friction as a model for tectonic faulting by integrating controlled experiments, molecular dynamics simulations, and Persson-based contact mechanics to connect microscale processes with macroscopic behavior. It shows that adhesion-driven cold welding at plastically deformed asperity junctions largely controls friction, while gouge formation suppresses wear, and that surface topography evolves toward multi-scale isotropy below ~10 μm, consistent with natural faults. Water dramatically reduces wear but only modestly reduces friction, and MD reveals stress-induced phase transformations and flash heating consistent with observed frictional behavior, including a notable drop in shear strength above 600°C. The results imply that rate-state aging effects are negligible at macroscopic fault scales due to scale-dependent pre-slip, offering a cohesive framework that links atomistic mechanisms, intermediate-scale wear, and large-scale fault dynamics.

Abstract

We study tribological granite-granite contacts as a model for tectonic faulting, combining experiments, theory, and molecular dynamics simulations. The high friction in this system is not dominated by particulate wear or plowing, as frequently assumed, but by cold welding within plastically deformed asperity junctions. We base this conclusion on the observation that wear is repeatedly high after cleaning contacts but decreases as gouge accumulates, while friction shows the opposite trend. Moreover, adding water reduces wear by a factor of ten but barely decreases friction. Thermal and rate-dependent effects - central to most earthquake models-are negligible: friction remains unchanged between -40°C and 20°C, across abrupt velocity steps, and after hours of stationary contact. The absence of rate-state effects in our macroscopic samples is rationalized by the scale-dependence of pre-slip. The evolution of surface topography shows that quartz grains become locally smooth, with height spectra isotropic for wavelength below 10 microns but anisotropic at longer wavelengths, similar to natural faults. The resulting gouge particles have the usual characteristic sizes near 100 nm. Molecular dynamics simulations of a rigid, amorphous silica tip sliding on α-quartz reproduce not only similar friction coefficients near unity but also other experimentally observed features, including stress-introduced transitions to phases observed in post-mortem faults, as well as theoretical estimates of local flash temperatures. Additionally, they reveal a marked decrease of interfacial shear strength above 600°C. The overall correspondence between experiments, simulations, theory, and field observations indicates that our model system captures essential aspects of rock friction.

Figures (32)

• Figure 1: Low-temperature linear friction slider used for the experiments. The instrument allows precise control of temperature ($-40^\circ{\rm C}$ to $+20^\circ{\rm C}$), sliding speed ($1~{\rm \upmu m/s}$ to $1~{\rm cm/s}$), and normal load ($250$–$1000~{\rm N}$).
• Figure 2: The surface height topography of the corundum abrasive paper and fabric.
• Figure 3: Granite consist of several minerals but the largest fractions are quartz and feldspar. Our granite looks visually similar to granite whose composition has been studied in detail, with $\approx 72\%$ quartz and $\approx 15\%$ feldspar.
• Figure 4: Schematics of a granite--granite contact with (a) few versus (b) many gouge particles. (c) SEM image of wear-particle clusters.
• Figure 5: Cumulative wear mass as a function of sliding distance for a normal load of $F_\mathrm{N} = 250\,\mathrm{N}$ and a sliding velocity of $v = 3\,\mathrm{mm/s}$. The symbols correspond to 1-run measurements, each consisting of 0.2 m forward and 0.2 m backward motion, after which the accumulated wear is removed. Green lines connecting the symbols show the mean wear obtained from 100-run measurements, in which wear accumulates over a total sliding distance of 40 m.