Chemical and physical origins of friction on surfaces with atomic steps.

Friction occurs through a complex set of processes that act together to resist relative motion. However, despite this complexity, friction is typically described using a simple phenomenological expression that relates normal and lateral forces via a coefficient, the friction coefficient. This one parameter encompasses multiple, sometimes competing, effects. To better understand the origins of friction, here, we study a chemically and topographically well-defined interface between silica and graphite with a single-layer graphene step edge. We identify the separate contributions of physical and chemical processes to friction and show that a single friction coefficient can be separated into two terms corresponding to these effects. The findings provide insight into the chemical and topographic origins of friction and suggest means of tuning surfaces by leveraging competing frictional processes.