Scratching a soft layer above a hard substrate

Soft layers may develop on a harder substrate during tribological processes. We perform molecular dynamics simulations of a model system, in which a hard substrate covered by a 4-nm thick soft layer is indented and scratched with a spherical tip along the surface. When scratching at a depth of only d = 2 nm – i.e. fully in the soft layer – the friction coefficient μ is the same as in a homogeneous (purely soft or hard) material; this is expected since in the ploughing regime, the friction coefficient of a sphere depends only on the indentation depth but not on the material. When scratching at d = 4 nm – i.e. at the interface between soft and hard material – μ of the bilayer material is strongly reduced compared to both the soft and the hard material. This reduction is caused by the fact that the scratching force is similar to that in a purely soft material, while the normal force is increased towards its value in the hard material. This effect also persists when scratching at d = 6 nm.