  We calculate the friction of fully mobile graphene flakes sliding on
graphite. For incommensurately stacked flakes, we find a sudden and reversible
increase in friction with load, in agreement with experimental observations.
The transition from smooth sliding to stick-slip and the corresponding increase
in friction is neither due to rotations to commensurate contact nor to
dislocations but to a pinning caused by vertical distortions of edge atoms also
when they are saturated by Hydrogen. This behavior should apply to all layered
materials with strong in-plane bonding.
