Atomistic-scale sliding friction of fault gouge: Insight from a quartz-kaolinite-quartz system

Fault gouges are ubiquitous, which significantly reduces fault strength and affects earthquake rupture. However, the microscopic frictional behavior of the fault system, when considering the host rock and fault gouge as a coupled dual-interface structure, remains insufficiently understood. This study investigates the atomistic-scale frictional behavior of a quartz-kaolinite-quartz system using molecular dynamics. The interfacial forces during sliding are analyzed, and the effects of normal stress, sliding velocity and quartz crystallographic orientation on nanoscale friction are discussed. Results show that the friction force is positively correlated with the normal stress in the multi-friction surface system, consistent with previous findings of single-interface systems. Normal stress directly affects the slip behavior and the structural integrity of the kaolinite layer. As long as the normal stress does not reach the compressive strength limit of kaolinite, the kaolinite’s morphology always maintains the structural integrity. At lower normal stress, the Si-O plane of kaolinite serves as the primary slip surface. With increasing normal stress, the dominant slip surface gradually shifts to the Al-OH plane until the structural failure of kaolinite occurs. The friction of the quartz-kaolinite-quartz system increases with sliding velocity, showing a logarithmic velocity dependence due to thermal activation effects. Throughout the simulated velocities, the system consistently exhibits velocity-strengthening behavior. The crystallographic orientation of the quartz substrate significantly affects the friction force. This is attributed to differences in the distribution and amplitude of the potential energy ripples on the quartz surface, as well as variations in the dominant slip surfaces with different orientations. These findings could offer insights into the nanoscale mechanisms governing slip of fault gouges.