Random occurrence of macroscale superlubricity of graphite enabled by tribo-transfer of multilayer graphene nanoflakes

Superlubricity of layered materials, such as graphite, boron nitride, and molybdenum disulfide, is easy to achieve at the nano- or microscale by the formation of ideal incommensurate contact, but it has never been observed at the macroscale due to the size limitations of the contact zone. In the present study, the instantaneous superlubricity of graphite against steel was achieved at the macroscale, through the formation of many tribo-transferred multilayer graphene nanoflakes (MGNFs) on the steel contact zone after the initial sliding. The friction coefficient could reduce to a minimum of 0.001, which randomly appeared as the test progressed, with a maximal sliding distance of 131 μm. The macroscale superlubricity was derived from the statistical frictional forces of multiple transferred MGNFs (in the contact zone) sliding on the graphite with atomic steps. This finding provides a possible approach to achieving the macroscale superlubricity of layered materials by the discretization of a large contact area into multiple, dispersed nanoflake contacts.