Atomistic Mechanism of Friction-Force Independence on the Normal Load and Other Friction Laws for Dynamic Structural Superlubricity
Nikolai V. Brilliantov, А. А. Цуканов, Artem K. Grebenko, Albert G. Nasibulin, Igor Ostanin
Abstract
We explore dynamic structural superlubricity for the case of a relatively large contact area, where the friction force is proportional to the area (exceeding $\ensuremath{\sim}100\text{ }\text{ }{\mathrm{nm}}^{2}$) experimentally, numerically, and theoretically. We use a setup composed of two molecular smooth incommensurate surfaces: graphene-covered tip and substrate. The experiments and molecular dynamic simulations demonstrate independence of the friction force on the normal load for a wide range of normal loads and relative surface velocities. We propose an atomistic mechanism for this phenomenon, associated with synchronic out-of-plane surface fluctuations of thermal origin, and confirm it by numerical experiments. Based on this mechanism, we develop a theory for this type of superlubricity and show that friction force increases linearly with increasing temperature and relative velocity for velocities larger than a threshold velocity. The molecular dynamic results are in a fair agreement with predictions of the theory.