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

TL;DR

This study combines experiments, theory, and simulations to understand granite friction, revealing cold welding as the main cause of high friction, and shows scale-dependent effects and minimal thermal or rate effects relevant to earthquake modeling.

Contribution

It provides new insights into the mechanisms of granite friction, emphasizing cold welding and scale-dependent rate-state effects, supported by molecular dynamics simulations and surface topography analysis.

Findings

High friction is due to cold welding, not particulate wear.

Water reduces wear significantly but barely affects friction.

Friction remains stable across temperature and velocity changes.

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{\deg}C and 20{\deg}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 {\alpha}-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{\deg}C.