Molecular dynamics investigation of the effects of tip-substrate interactions during nanoindentation

Nanoindentation in molecular dynamics (MD) simulations typically uses highly idealized indenter tip models. Such tips usually consist of either a single sphere or a collection of atoms, both of which are purely repulsive in their interactions with the substrate. It is also assumed that there is no environmental or substrate contamination, nor is there a surface oxide layer. In this work we examine the effects of these assumptions by comparing detailed MD simulations utilizing varying interaction potentials against both experimental atomic force microscopy observations and calculations using density functional theory. Specifically, we examine the effect of a tip-substrate interaction on the indenter under clean, hydrogenated, and oxidized conditions. We find that under clean or oxidized conditions (where we include oxygen on the nickel surface to mimic a passivating NiO layer) there is a substantial material transfer from the substrate to the tip. This material (Ni atoms) remains adsorbed on the tip upon retraction. However, the presence of hydrogen on the diamond tip drastically reduces, or even altogether eliminates, this material transfer, therefore having an effect much larger than that of a contaminating oxide layer.