Atomistic mechanism of friction force independence on the normal load and other friction laws for dynamic structural superlubricity

We explore dynamic structural superlubricity for the case of a relatively large contact area, where the friction force is proportional to the area (exceeding $\sim 100\,nm^2$) experimentally, numerically, and theoretically. We use a setup comprised of two molecular smooth incommensurate surfaces -- graphene-covered tip and substrate. The experiments and MD 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 of 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 MD results are in a fair agreement with predictions of the theory.
Comment: Accepted to Physical Review Letters on November 14, 2023