Understanding the mechanisms of adhesive wear for heterogeneous materials through atomistic simulations

When two rough surfaces slide against each other, two main behaviors can be observed at the asperity level: A ductile behavior where the asperities tend to smooth out or a brittle behavior where the asperities detach and form debris. Recently, a critical length scale controlling the transition between both behaviors was identified. Its formulation depends only on material properties and a geometrical factor (of the order of unity). This finding was achieved thanks to molecular dynamics (MD) simulations with hardness-tunable interatomic potentials. However, the materials studied so far have been homogeneous. Here, the work is extended to a class of simple heterogeneous materials with random fluctuations of their properties. A comparative study of the heterogeneous materials against an equivalent homogeneous material is conducted to study the influence of the local variations on the global behavior. It is found that the critical junction size formula works well for hard materials up until a certain ductility threshold after which a third behavior, mixing shear localization and mode II crack opening, appears. There is a transition zone, larger for the heterogeneous material, from the ductile to the brittle behaviors due to local fluctuations and non-deterministic nature of finite temperature atomistic simulations. Local fluctuations render the heterogeneous material weaker than the equivalent homogeneous material and thus, following the critical junction size formula, should lower the probability of forming debris. While this effect can be observed in the single asperity simulations, its magnitude is lower than expected, hinting at the dichotomic effect of local variations: while they increase the overall ductility, they also create particularly brittle areas prone to crack nucleation.