Indentation size effects in face centered cubic single crystal thin films

This paper presents the results of an experimental study of indentation size effects in Au, Ag and Ni single crystals with (0 0 1), (0 1 1) and (1 1 1) orientations. The studies show clearly that the hardness increases with decreasing indentation size, for indents between ∼30 and 600 nm in depth. The observed size dependence is rationalized using the mechanism-based strain gradient plasticity theory by Nix and Gao. The shape and extent of material pile-up is shown to depend strongly on the material hardness and crystal orientation. In the case of (0 0 1) oriented crystals, the extent of material pile-up is low in Ag, compared to that in the Au and Ni crystals. The material pile-up profiles are conventional three-lobes shapes. However, for indents in (0 1 1) crystals, butterfly shaped pile-up profiles are observed in Ni, compared to conventional pile-up profiles in Au and Ag. In contrast, flower shaped pile-up profiles are observed in (1 1 1) Ni crystals, compared to conventional pile-up profiles in Au and Ag. Displacement bursts are also shown to occur in (0 1 1) and (1 1 1) oriented Ni at indentation load levels of ∼80-100 μN. These are attributed to the initial onset of dislocation slip activity, when the shear stress exceeds the estimated theoretical shear strengths of the materials.