  We report microscale friction experiments for diamond/metal and
diamond/silica contacts under gigapascal contact pressures. Using a new
nanoprobe technique which has sufficient dynamic range of force and stiffness,
we demonstrate the processes involved in the transition from purely interface
sliding at the nanoscale to the situation where at least one of the sliding
bodies undergoes some plastic deformation. For sliding of micrometer-scale
diamond spherical tips on metallic substrates, additional local plastic
yielding of the substrate resulting from tangential tractions causes the tip to
sink into the surface, increasing the contact area in the direction of loading
and resulting in a static friction coefficient higher than the kinetic during
ploughing. This sink-in is largely absent in fused silica, and no friction drop
is observed, along with lower friction in general. The transition from sinking
in within the static friction regime to ploughing in the sliding friction
regime is mediated by failure of the contact interface, indicated by a sharp
increase in energy dissipation. At lower contact pressures, the elastic
interfacial sliding behaviour characteristic of scanning probe or surface force
apparatus experiments is recovered, bridging the gap between the exotic realm
of nanotribology and plasticity dominated macroscale friction. We delineate the
material and geometric factors which determine the transition. We also find
that some unexpected light is cast on the origins of the difference between
static and dynamic friction.
