Role of Stiffness in Friction with Graphite

When contacting solids slide against each other, friction typically increases as material stiffness decreases due to the resulting increase in real contact area. However, varying trends have been observed, influenced by wear, surface geometry, tribochemistry, and environmental factors. To disentangle these effects, we construct a microscale structural superlubric system composed of graphite slider and graphene substrate, featuring wearless, fully contacted, atomically smooth, and chemically stable interfaces. The normal stiffness is tuned using few-layer graphene with different thicknesses. Experiments show that, for the system with a clean interface, friction remains constant across substrates with varying stiffness. By contrast, a positive stiffness dependence of friction is observed in third body-rich interfaces. A comprehensive framework is provided, clarifying fundamentally how friction and its stiffness dependence originate from the configuration transition of third body molecules and symmetry of the stiffness for the two contacting surfaces, corroborated by molecular dynamics simulations.