Friction and Plasticity in Contacts Between Amorphous Solids

Friction pervades nearly every aspect of our daily lives and has played a central role in technology ever since man created fire by rubbing sticks together. Despite its fundamental importance, a microscopic understanding of friction has remained elusive. Friction ultimately results from atomic-scale interactions between contacting bodies, but elastic and plastic deformations at larger scales and material transport control the area and composition of the contacting regions. Such atomic-scale interactions and geometry of contacting surface is difficult to characterize in experiments with sufficient accuracy. Here, we show the large-scale molecular dynamics simulations of contact and friction between two amorphous solids with self-affine fractal surfaces. We found that the plastic deformation on solid surfaces plays an important role in the interfacial friction that is much higher than expected for elastically deforming surfaces. Indeed, friction forces for different surface roughness collapse when plotted against the number of plastic rearrangements per unit sliding distance. Our results indicate that the microscopic friction origins from the overcome of finite energy barriers between plastically deformed local atomic structures.