A Theoretical and Computational Study of Superlubricity and the Role of the Roughness Exponent

Superlubricity can only be achieved if the intra-bulk elastic interactions dominate interfacial shear forces at every single length-scale in a contact. Otherwise, there will be some type of plucking motion which will lead to friction-velocity relationships akin of Coulomb’s friction law. For nominally flat surfaces, it has been predicted theoretically and demonstrated experimentally that plucking motion and hence kinetic friction can be avoided under certain circumstances. We present theoretical arguments, why these findings may extend to fractal surfaces. The theories are checked by molecular dynamics simulations. It turns out that the roughness exponent and the absolute magnitude of the roughness both play a crucial role in determining whether there can be superlubricity.