Structure, morphology and selected mechanical properties of magnetron sputtered (Mo, Ta, Nb) thin films on NiTi shape memory alloys

Abstract

In this study, pseudo-elastic Ni 50.8 at.%-Ti alloy sheets of 1000 μm thickness were coated with 10 μm thick refractory metal thin films (Mo, Ta and Nb thin films, respectively), by nonreactive d.c. magnetron sputtering. These thin films were characterized with regard to their microstructure, morphology and selected mechanical properties. Microstructural characterization of the thin films included X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) analyses. SEM images show, that Mo thin films grow in a densely packed structure with columnar grains. Pole figure analyses reveal a distinct (110) texture of the deposited Mo thin films on the NiTi substrate. The Mo thin films exhibit relatively large crystallite sizes, 290–300 nm. Ta and Nb thin films show XRD reflections of various lattice planes, i.e. polycrystalline b.c.c. structures, with preferential (110) orientation. Both of these thin films grow with much smaller crystallite size in comparison to the Mo thin films, 30–40 nm.The Young's Modulus of both the thin films and thin film/substrate composites were investigated on various scales. For this purpose, indentation experiments were performed using both nano- and microindentation techniques. A comparison of the values of the Young's Modulus of the thin film materials (in bulk, literature data) and the results obtained from nanoindentation shows a very good agreement between the measured data and bulk data. Progressive scratch tests were used to evaluate the adhesion of the thin film materials on the NiTi substrate. These scratch tests suggest that the metallic thin films undergo plastic deformation at larger normal forces. However, no thin film delamination is observed, independent of the material combinations and testing conditions applied.