  The sliding motion of gold slabs adsorbed on a graphite substrate is
simulated using molecular-dynamics. The central quantity of interest is the
mean lateral force, i.e., the kinetic friction rather than the maximum lateral
forces, which correlate with the static friction. For most set-ups, we find
Stokesian damping to resist sliding. However, velocity-insensitive (Coulomb)
friction is observed for finite-width slabs sliding parallel to the armchair
direction if the bottommost layer of the three graphite layers is kept at zero
stress rather than at zero displacement. Although the resulting kinetic
friction remain much below the noise produced by the erratic fluctuations of
(conservative) forces typical for structurally lubric contacts, the nature of
the instabilities leading to Coulomb friction could be characterized as
quasi-discontinuous dynamics of the Moir\'e patterns formed by the normal
displacements near a propagating contact line. It appears that the interaction
of the graphite with the second gold layer is responsible for the symmetry
breaking occurring in the interface when a contact line moves parallel to the
armchair rather than to the zigzag direction.
