Molecular dynamics simulation of single asperity contact

We present a series of molecular dynamics simulations of single asperity contact and deformation that occurs as two rough surfaces are brought together, loaded and then separated. We monitor the force between the two surfaces, the total potential energy, the Q6 order parameter, the effective minimum contact radius and the conductance (using the modified Sharvin equation). The deformation is observed using the local bond order parameter. During loading, the force–displacement relation exhibits a sawtooth form, associated with repetitive generation and motion of partial dislocations and the concomitant creation and annihilation of stacking fault pyramids. Unloading is characterized by an extended elastic deformation regime followed by plastic deformation of a type that is distinct from that in loading. The qualitative features of the force, contact area and conductance versus displacement plots are in excellent qualitative agreement with experimental observations. The simulations and experiments both show that Johnson–Kendall–Roberts theory should not be applied to describe asperity loading, but is applicable to the case of unloading, where the deformation is nearly elastic.