Spherical Load Indentation in Submicron NiTiCu Shape Memory Thin Films

Abstract

Nanoindentation with spherical tipped indenters provides a powerful technique to explore surface and thin film mechanical properties through the application of Hertzian contact mechanics. The full range of mechanical response can be obtained from elastic, through the yield point, to permanent deformation. In this study spherical indentation has been used for probing MBE-grown NiTiCu alloy thin films into superelasticity or stress-induced martensitic transformation. By this way, obstacles typically occurring related to the fabrication of freestanding films (film thickness < 1 µm) are avoided. The indentation measurements were performed starting from the parent austenite state. Notably, for loads as small as 0.5 mN, deformation appears to be completely reversible. As loading is increased (up to 5 mN) the indent becomes irreversible following local plastic deformation within the tip-specimen contact area. Using finite-element simulations the indentation data were converted into a stress-strain diagram aimed at simulating uniaxial tension load. Therefrom, the superelastic strain is estimated to be around 3%.