Nanoindentation creep behaviors of amorphous, tetragonal, and bcc Ta films

Nanoindentation creep tests were carried out at the maximum indentation load from 500 to 9000 μN to study the indentation size effect (ISE) on the creep behavior of amorphous, nanocrystalline (NC) bcc and NC tetragonal Ta films. For NC bcc and tetragonal Ta films, the creep strain rate ε˙ decreases and stress exponent n   increases with enhanced peak loads or indent depth, and are therefore both indentation size dependent. However, an inverse ISE on ε˙ and n is found for amorphous Ta films. The difference of the ISE is attributed to the distinct creep deformation process. Several creep mechanisms including self-diffusion along the indenter/specimen interface, grain boundary diffusion and sliding, and dislocation climb have been introduced to interpret the ISE for NC Ta films. The inverse ISE on amorphous Ta films is explained by the shear transformation zone theory.