  We study the sliding of elastic solids in adhesive contact with flat and
rough interfaces. We consider the dependence of the sliding friction on the
elastic modulus of the solids. For elastically hard solids with planar surfaces
with incommensurate surface structures we observe extremely low friction
(superlubricity), which very abruptly increases as the elastic modulus
decreases. We show that even a relatively small surface roughness may
completely kill the superlubricity state.
