Molecular dynamics simulations of nanofriction and wear

This thesis presents simulations of nanometre-scale ploughing friction and wear behaviour when a pyramidal diamond indenter is ploughed through the surface of bcc and fcc metals and semiconductors. Parallel molecular dynamics (MD) simulations of nanoindentation followed by nanoscratching using Newtonian mechanics have been employed to investigate the different friction mechanisms occurring at the atomic scale. Three models have been developed to carry out our investigations on nanofriction, namely the steady-state model, the spring model and the finite temperature model. Each model allows the study of distinctive aspects of atomic-scale friction. For instance, the steady-state model was employed to study the behaviour of the friction coefficient, contact pressure and scratch hardness of a silver surface as a function of depth. The effect of indenter orientation has also been investigated with results showing a diverse range of pile-up behaviour. The work material undergoes both elastic and plastic deformation during the scratching and we have studied these to analyse the origins of friction. The spring model and the finite temperature model have been employed to investigate the stick-slip phenomenon at a low temperature of 0K and at room temperature (i.e. 300 K), respectively. The dynamics of the indenter and the substrate, including the behaviour of the different forces in action and the coefficient of friction, at particular stick and slip events have been studied. The variation of the sliding speed and indentation depth and their effects on the occurrence of the stick-slip events has also been investigated. Some qualitative comparisons have been made between the results from the simulations and experiments where possible. Due to available computer power, feasible indentation depths and scratch lengths were an order of magnitude smaller than experiment, while simulation times were several orders of magnitude shorter. The MD simulations, however, gave a good description of nanoindentation and nanoscratching and correlated well with the experiments.