Area, temperature, velocity, and edge effects in the sliding of structurally lubric 2D materials*

Graphene and many other two-dimensional (2D) materials entered the scene in the last two decades. Their strong and yet membrane-like sheets permit – actually require -- deposition, giving rise to interfaces whose tribological properties under shear stress must be understood and controlled. We carried out selected simulation case studies with variously twisted, structurally incommensurate interfaces involving graphene. There is first of all a basic difference between "small" and "large" twist angles -- only the latter are superubric and free sliding in the usual sense. The kinetic friction of a superlubrically sliding island is linear with velocity ("viscous") at large velocity or large temperature -- regimes where, dominated by the island's body, friction grows proportional to area. At low velocities and low temperatures, conversely, kinetic friction is, like static friction, dominated by the island's edges. Therefore it grows at most as the perimeter, and with a much weaker velocity dependence, as expected for stick-slip friction. Recovering and extending many results already present in literature, the overall picture obtained applies to general 2D interfaces, including current and future bilayer and multilayer systems.