Atomistic modelling of ploughing friction in silver, iron and silicon

Abstract

Molecular dynamics (MD) simulations of atomic-scale stick–slip have been performed for a diamond tip in contact with the (100) surface of fcc Ag, bcc Fe, Si and H-terminated Si, at a temperature of 300 K. Simulations were carried out at different support displacements between 5 and 15 Å. The simulations illustrate the important mechanisms that take place during stick–slip. In particular, for the case of the metals they show a direct link between tip slip events and the emission of dislocations from the point of contact of the tip with the substrate. This occurs both during indentation and scratching. For the case of silicon, no slip events were observed and no subsurface dislocations were generated underneath the scratch groove. At the deeper support displacement of 15 Å the silicon atoms undergo some local phase transformations and the atom coordination number varies between 5 and 8, with the majority being five-fold or six-fold coordinated. Both the dynamic and the static friction coefficients were found to be higher for Si compared to the corresponding values for H-terminated Si. Comparisons were made between the MD simulations and experimental measurements for indentation on the (100) surface of Si and Al. A good qualitative agreement was observed between the experimental and theoretical results. However, in both the cases of Si and metals the MD simulations give a contact pressure under load that is depth dependent and values that are higher than experimental nanohardness values.