Nanomechanical Characterisation of Graded NiTi Films Fabricated Through Diffusion Modification.

Ni₄₇Ti₅₃ films of varying thickness were deposited onto Ni₅₆Ti₄₄ substrates. Annealing the films produced compositional gradients through diffusion modification. Nanoindentation measurements were used to probe the mechanical properties at various depths into the film. The films exhibited a variation in elastic modulus as a function of indentation depth that depended on the thickness of the film. A self-similarity principle permitted the mechanical properties of the graded NiTi films to be resolved further into substrate, beyond the contact depth of the tip. The observed variation of elastic modulus with indentation depth in the graded NiTi films was considered to be a combined response from changes in microstructure, substrate effects, and mechanically-induced phase transformations. This variation was predicted using an analysis that accounted for the transformation effects occurring under the tip during loading and the graded distribution of martensite volume fraction obtained from diffusion modification. [ABSTRACT FROM AUTHOR]