  Colloidal probe Atomic Force Microscopy (AFM) allows to explore sliding
friction phenomena in graphite contacts of nominal lateral size up to hundreds
of nanometers. It is known that contact formation involves tribo-induced
material transfer of graphite flakes from the graphitic substrate to the
colloidal probe. In this context, sliding states with nearly-vanishing
friction, i.e. superlubricity, may set in. A comprehensive investigation of the
transfer layer properties is mandatory to ascertain the origin of
superlubricity. Here we explore the friction response of micrometric beads, of
different size and pristine surface roughness, sliding on graphite under
ambient conditions. We show that such tribosystems undergo a robust transition
towards a low-adhesion, low-friction state dominated by mechanical interactions
at one dominant tribo-induced nanocontact. Friction force spectroscopy reveals
that the nanocontact can be superlubric or dissipative, in fact undergoing a
load-driven transition from dissipative stick-slip to continuous superlubric
sliding. This behavior is excellently described by the thermally-activated,
single-asperity Prandtl-Tomlinson model. Our results indicate that upon
formation of the transfer layer, friction depends on the energy landscape
experienced by the topographically-highest tribo-induced nanoasperity.
Consistently we find larger dissipation when the tribo-induced nanoasperity is
sled against surfaces with higher atomic corrugation than graphite, like MoS2
and WS2, in prototypical Van der Waals layered hetero-junctions.
