Atomistic simulation of multicycle asperity contact

We performed molecular dynamics simulations of repetitive contact and separation of a surface containing an initially hemispherical asperity and a flat. We monitor the force between the two surfaces and the Q6 order parameter together with the evolution of the structure/morphology. Typically, the force–displacement relation exhibits a sawtooth form during loading, associated with repetitive generation and motion of partial dislocations and the creation and annihilation of stacking faults. The unloading is characterized by elastic deformation followed by plastic deformation of a type that leads to stacking faults different from those seen in loading. During repetitive contact, the system accumulates stacking faults as a result of Shockley partial glide and transforms from face-centered cubic (fcc) to hexagonal close packed (hcp). On the next unloading step, additional Shockley partial glide occurs, transforming the system back from the metastable hcp to fcc. The new fcc structure has a twin relation to the original fcc structure. The new fcc structure has a lower Schmid factor than the original fcc (or hcp) structure and consequently exhibits larger pull-off forces.