  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 (super-low 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 a pressure of 1.67 MPa. The observation of load-independent
superlubricity in micro-scale contacts is a promising result for numerous
practical applications.
