Observation of normal-force-independent superlubricity in mesoscopic graphite contacts

We investigate the dependence of friction forces on normal load in incommensurate micrometer-size contacts between atomically smooth single-crystal graphite surfaces under ambient conditions. Our experimental results show that these contacts exhibit superlubricity (superlow friction), which is robust against the application of normal load. The measured friction coefficients are essentially zero and independent of the external normal load up to the maximum pressure of our experiment, 1.67 MPa. The observation of load-independent superlubricity in microscale contacts is a promising result for numerous practical applications.