Directional anisotropy of friction in microscale superlubric graphite/ h BN heterojunctions

Structural superlubricity is one of the most fascinating tribological concepts, describing the state of almost vanishing friction, which arises from the cancellation of the lateral interactions between incommensurate rigid surfaces. In this paper, the dependence of friction on sliding directions is experimentally studied under ambient conditions using state of the art friction force microscopy of misaligned microscale heterogeneous contacts between the single crystalline $h\mathrm{BN}$ and graphite. The observed minor frictional anisotropy (\ensuremath{\le}20%) demonstrates that the microscale superlubric translational motion is maintained along all sliding directions. With atomistic simulations, we further show that the frictional anisotropy results from a change in the corrugation of the sliding potential energy surface with changing the sliding direction. Our results validate the robustness of the structural superlubricity against the sliding direction, and provide an effective approach to precise friction control.