Identifying Physical and Chemical Contributions to Friction: A Comparative Study of Chemically Inert and Active Graphene Step Edges

Friction has both physical and chemical origins. To differentiate these origins and understand their combined effects, we study friction at graphene step edges with the same height and different terminating chemical moieties using atomic force microscopy (AFM) and reactive molecular dynamics (MD) simulations. A step edge produced by physical exfoliation of graphite layers in ambient air is terminated with hydroxyl (OH) groups. Measurements with a silica countersurface at this exposed step edge in dry nitrogen provide a reference where both physical topography effects and chemical hydrogen-bonding (H-bonding) interactions are significant. H-bonding is then suppressed in AFM experiments performed in alcohol vapor environments, where the OH groups at the step edge are covered with physisorbed alcohol molecules. Finally, a step edge buried under another graphene layer provides a chemically inert topographic feature with the same height. These systems are modeled by reactive MD simulations of sliding on an OH-terminated step edge, a step edge with alkoxide group termination, or a buried step edge. Results from AFM experiments and MD simulations demonstrate hysteresis in friction measured during the step-up versus step-down processes in all cases except the buried step edge. The origin of this hysteresis is shown to be the anisotropic deflection of terminal groups at the exposed step edge, which varies depending on their chemical functionality. The findings explain why friction is high on atomically corrugated and chemically active surfaces, which provides the insight needed to achieve superlubricity more broadly.