Molecular dynamics simulations of (001) MgO surface contacts: effects of tip structures and surface matching

Molecular dynamics simulations were carried out to study contact behaviour of MgO pyramidal tips with atomically flat MgO surfaces in the (001) direction. Both commensurate and incommensurate contact modes over a wide range of surface interaction have been considered, from weakly interacting to highly compressed. Characteristic mechanical instability jumps effects were observed in the former case, and, to a much smaller extent. in the latter case as well. By making comparison between the rectangular MgO probes from our previous work and the pyramidal probe it was found that detailed contact and withdrawal behaviour depended very much upon the local geometrical structure of the contacting tips. Specifically, the strain distribution and the change in interplanar spacing were different during contacts. When a pyramidal probe is withdrawn, the first sign of hysteresis results in disrupted crystal structures near the contacting tip and is almost always followed by subsequent complex plastic dislocation and 'neck' formation. The probe with the 'sharpest tip' is found to undergo modification at the tip to give a truncated octahedron structure and subsequently shows a very different contact behaviour. For example, the modified probe can be withdrawn completely from the surface without any damage, a phenomenon not commonly encountered with other clean MgO contact systems in our work.