Nanoindentation studies of graded shape memory alloy thin films processed using diffusion modification

Graded shape memory alloy thin films were fabricated and characterized to study the effects of compositional gradients on elastic modulus and hardness measurements. Ni47Ti53(at.%) films of varying thickness were deposited onto Ni56Ti44 substrates using dc magnetron sputtering. The films were annealed, producing a compositional gradient across the film-substrate interface through diffusion modification. Nanoindentation measurements were used to characterize the mechanical properties at various depths relative to the film-substrate interface. Before annealing, the dependence of elastic modulus and hardness on relative indentation depth was self-similar for films of different thicknesses due to substrate effects of a two layer system. Annealing allows the diffusion across the film-substrate interface to fully or partially penetrate the film, depending on the thickness of the sputtered layer. The fully penetrated thinner annealed films also display the self-similar behavior of a two layer system. This self-similarity deviates for annealed films thicker than approximately 500nm where partial penetration has effectively created a three layer system. This characteristic length is used to estimate a diffusion coefficient on the order of 10−16m2s−1, which is consistent with previously reported values. The proposed technique has the potential to characterize general graded film systems.