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.