The velocity dependence of dry sliding friction at the nano-scale

We performed molecular dynamics (MD) experiments to explore dry sliding friction at the nanoscale. We used the setup comprised of a spherical particle built up of 32,000 aluminium atoms, resting on a semi-space with a free surface, modelled by a stack of merged graphene layers. We utilized LAMMPS with the COMB3 many-body potentials for the inter-atomic interactions and Langevin thermostat which kept the system at $300 K$. We varied the normal load on the particle and applied different tangential force, which caused the particle sliding. Based on the simulation data, we demonstrate that the friction force $F_{\rm fr}$ linearly depends on the sliding velocity $v$, that is, $F_{\rm fr}=-\gamma v$, where $\gamma$ is the friction coefficient. The observed dependence is in a sharp contrast with the macroscopic Amontons-Coulomb laws, which predict the velocity independence of sliding friction. We explain such a dependence by surface fluctuations of the thermal origin, which give rise to surface corrugation hindering sliding motion. This mechanism is similar to that of the viscous friction force exerted on a body moving in viscous fluid.