  Solution-processed few-layers graphene flakes, dispensed to rotating and
sliding contacts via liquid dispersions, are gaining increasing attention as
friction modifiers to achieve low friction and wear at technologically-relevant
interfaces. Vanishing friction states, i.e. superlubricity, have been
documented for nearly-ideal nanoscale contacts lubricated by individual
graphene flakes; there is however no clear understanding if superlubricity
might persist for larger and morphologically-disordered contacts, as those
typically obtained by graphene wet transfer from a liquid dispersion. In this
study we address the friction performance of solution-processed graphene flakes
by means of colloidal probe Atomic Force Microscopy. We use an additive-free
aqueous dispersion to coat micrometric silica beads, which are then sled under
ambient conditions against prototypical material substrates, namely graphite
and the transition metal dichalcogenides (TMDs) MoS2 and WS2. High resolution
microscopy proves that the random assembly of the wet-transferred flakes over
the silica probes results into an inhomogeneous coating, formed by graphene
patches that control contact mechanics through tens-of-nanometers tall
protrusions. Atomic-scale friction force spectroscopy reveals that dissipation
proceeds via stick-slip instabilities. Load-controlled transitions from
dissipative stick-slip to superlubric continuous sliding may occur for the
graphene-graphite homojunctions, whereas single- and multiple-slips dissipative
dynamics characterizes the graphene-TMD heterojunctions. Systematic numerical
simulations demonstrate that the thermally-activated single-asperity
Prandtl-Tomlinson model comprehensively describes friction experiments
involving different graphene-coated colloidal probes, material substrates and
sliding regimes.
