Tip size effects on atomic force microscopy nanoindentation of a gold single crystal

The effect of tip radius on atomic force microscopy (AFM) nanoindentation is investigated through indentation on the (111) face of a gold single crystal. The hardness is derived using two different methods: by measuring directly the projected area of the residual indent with AFM images and by measuring the cross-sectional area of the indenter before and after each nanoindentation test. The hardness values obtained from the cross-sectional area of the indenter are comparable with those obtained from images of the residual indent scanned with a sharp tip. Two AFM tips of average radii of 70±12 and 112±26 nm are used to indent the sample to various depths ranging from 4 to 50 nm. For depths above 30 nm, hardness values remain constant around 500 MPa for both indenters. For depths below 30 nm, the hardness increases as the indent depth decreases for the sharp and blunt indenters, and the indent depth dependence is observed over a wider depth range for the sharp indenter. For depths below 30 nm, the hardness values obtained with the sharp indenter are also consistently higher than those obtained with the blunt indenter. The results confirm a size scale effect during nanometer scale indentation for both varying penetration depth and tip radius, both of which influence the volume of material sampled during deformation.