Scratching of Graphene-Coated Cu Substrates Leads to Hardened Cu Interfaces with Enhanced Lubricity

The decrease in friction is often observed in the initial period of sliding a fresh contact between two solid surfaces. This phenomenon is usually termed running-in. Here we report a running-in phenomenon observed in single-layer graphene grown on a Cu substrate. By repeatedly scratching the graphene surface using the tip of an atomic force microscope (AFM), it is found that the frictional force decreases within the first tens of scratching cycles and then approaches to a steady state, whereas the graphene coating stays free of wear and damage. Through specially designed experiments on changing normal load and sliding velocity as well as indentation tests by AFM, the mechanism behind this running-in process is revealed to be the hardening of the underlying Cu substrate. These observations and understandings can help future development of macroscale graphene-coated substrates with superlubricity.