Molecular dynamics simulation of atomic-scale friction

Abstract

Molecular dynamics simulations of nanoindentation followed by nanoscratching were conducted on single crystal aluminum (with the crystal set up in the (001) [100] orientation and scratching performed in the [100] direction) at extremely fine scratch depths (from 0.8 nm to almost zero) to investigate the atomic-scale friction. The friction coefficients at these depths were found to be rather high (\ensuremath{\sim}0.6), nearly constant, and independent of scratch depth except for zero depth when the magnitudes of the forces were extremely small. The high values of the friction coefficient even at these fine scratch depths are attributed to the finite value of the scratch force involved in breaking and reforming of the atomic bonds, the high negative rake angle generally presented by the indenter (in the present case -45\ifmmode^\circ\else\textdegree\fi{}) at fine scratch depths, which results in higher normal force (about twice the scratch force), and the absence of any lubricating film or contaminant between the sliding surfaces. The friction coefficient was also found to be close to the mean grinding coefficient, which is the ratio of the cutting to the thrust force with a high negative rake tool. Consequently, it appears that whenever material removal is involved in atomic-scale friction even at extremely fine scratch depths, the magnitude of the friction coefficient can be high, dependent on the rake angle presented by the tool, and independent of the normal force. This is because the magnitude of both normal and scratch forces increases with an increase in scratch depth and negative rake angle. Both the scratch hardness and indentation hardness were found to increase with decreasing scratch/indentation depth, strongly suggesting a size effect at fine scratch depths.