Dissipation Mechanisms and Superlubricity in Solid Lubrication by Wet-Transferred Solution-Processed Graphene Flakes: Implications for Micro Electromechanical Devices

Solution-processedfew-layer graphene flakes, dispensed to rotatingand sliding contacts via liquid dispersions, are gaining increasingattention as friction modifiers to achieve low friction and wear attechnologically relevant interfaces. Vanishing friction states, i.e.,superlubricity, have been documented for nearly-ideal nanoscale contactslubricated by individual graphene flakes. However, there is no clearunderstanding if superlubricity might persist for larger and morphologicallydisordered contacts, as those typically obtained by incorporatingwet-transferred solution-processed flakes into realistic microscalecontact junctions. In this study, we address the friction performanceof solution-processed graphene flakes by means of colloidal probeatomic force microscopy. We use a state-of-the-art additive-free aqueousdispersion to coat micrometric silica beads, which are then sled underambient conditions against prototypical material substrates, namely,graphite and the transition metal dichalcogenides (TMDs) MoS2 and WS2. High resolution microscopy proves that the randomassembly of the wet-transferred flakes over the silica probes resultsinto an inhomogeneous coating, formed by graphene patches that controlcontact mechanics through tens-of-nanometers tall protrusions. Atomic-scalefriction force spectroscopy reveals that dissipation proceeds viastick-slip instabilities. Load-controlled transitions fromdissipative stick-slip to superlubric continuous sliding mayoccur for the graphene-graphite homojunctions, whereas single-and multiple-slips dissipative dynamics characterizes the graphene-TMDheterojunctions. Systematic numerical simulations demonstrate thatthe thermally activated single-asperity Prandtl-Tomlinson modelcomprehensively describes friction experiments involving differentgraphene-coated colloidal probes, material substrates, and slidingregimes. Our work establishes experimental procedures and key conceptsthat enable mesoscale superlubricity by wet-transferred liquid-processedgraphene flakes. Together with the rise of scalable material printingtechniques, our findings support the use of such nanomaterials toapproach superlubricity in micro electromechanical systems.