Generalized Scaling Law of Structural Superlubricity

Structural superlubricity, which promises an ultralow sliding friction due to the cancellation of the lateral force between two incommensurate interfaces, is a fundamental phenomenon in modern tribology. Achieving macroscale superlubricity is critical to its practical application, and the key is understanding how friction scales with real contact area, that is, the scaling law, especially for kinetic friction which accounts for most of the energy dissipation during sliding. Here, inspired by extensive molecular dynamics simulations we introduce an analytical general theory for the scaling law of structural superlubricity, which could well explain existing experimental measurements on the nanoscale. On the microscale, the scaling law is validated by measuring the friction of several microscale superlubric graphite/hexagonal boron nitride heterojunctions. The proposed theory predicts a characteristic size D = O(100 nm) above which the scaling transits from sublinear to linear. Our results provide insights in the origin of friction for structural superlubricity and benefit its application on macroscale.