Nonlinear Area Dependence of Graphene Friction at the Nanoscale

How friction changes with contact area remains a fundamental topic of friction. Classical laws suggested that for macroscopic friction the friction force between two solids is independent of the nominal contact area, and then the linear relation between friction force and the real contact area is established. In this work, we show with molecular dynamics that at the nanoscale, however, there exists a more complicated relation between friction and contact area. We systematically simulated how the friction between a graphene substrate and a sliding monolayer square graphene tip varies with the contact area (from a single atom to 1000 nm2). The results show that in commensurate contact mode the friction stress oscillates violently with the tip area and the oscillatory behavior gradually disappears until the area is larger than 100 nm2. Detailed analysis indicates that the friction stress is sensitive to the edge structure of tips: the ultralow friction occurs for the tips in bearded edge configuration, and friction increases abruptly when all the edge configurations are either zigzag or armchair. Characterization of the potential energy surface and the tip motion pathway demonstrate that the presence of the bearded edge configuration distorts the potential energy surface sensed by the tip, which modifies the tip motion paths and causes a dramatic drop in friction.