Effects of atomic scale geometry on contact between rough solids
Microscopic roughness on real surfaces is important in processes such as friction, wear, and contact. When the roughness extends down to the atomic scale, where the discrete size of atoms matters, the effects of roughness become unclear, and a better understanding may affect industrial applications and nanosystem design. Here we approximate rough surfaces as statistically self-affine fractals and analyze contact using molecular dynamics simulations. We simulate up to 27 million atoms near the surfaces and use a Green’s function method for the elastic response of the bulk, allowing us to reach cubic systems 1024 atoms on a side. We find that “atomic steps,” created by atoms’ preference to form a lattice, lead to new patterns of contact, as is apparent in the contact correlation function. These steps also lead to increased plasticity and produce changes in the distribution of pressures at the surface. Correspondingly, more contact area is obtained at a given load due to the steps, though this effect is diminished by increased anharmonicity and becomes less relevant in large system sizes.