  Colloidal probe Atomic Force Microscopy allows to explore sliding states of
vanishing friction, i.e. superlubricity, in mesoscopic graphite contacts.
Superlubricity is known to appear upon formation of a triboinduced transfer
layer, originated by material transfer of graphene flakes from the graphitic
substrate to the colloidal probe. Previous studies suggest that friction
vanishes due to crystalline incommensurability at the newly formed interface.
However this picture still lacks several details, such as the roles of the
tribolayer roughness and of loading conditions. Hereafter we gain deeper
insight into the tribological response of micrometric silica beads sliding on
graphite under ambient conditions. We show that the tribotransferred flakes
behave as lubricious nanoasperities with a twofold role. First, they decrease
the silica-graphite true contact area, in fact causing a breakdown of adhesion
and friction by one order of magnitude. Second, they govern mechanical
dissipation through the specific energy landscape experienced by the
topographically-highest triboinduced nanoasperity. Remarkably, such contact
junctions can undergo a load-driven atomic-scale transition from continuous
superlubric sliding to dissipative stick-slip, that agrees with the
single-asperity Prandtl-Tomlinson model. Superlubricity in mesoscopic
silica-graphite junctions may therefore arise from the load-controlled
competition between interfacial crystalline incommensurability and contact
pinning effects at one dominant nanoasperity.
