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F. Seifried, H. Riedl, S. Baumgärtner, H. Leiste, R. Schwaiger, S. Ulrich, H. Seifert, P.H. Mayrhofer, M. Stüber:
"Magnetron Sputtering of Refractory Metal Thin Films on NiTi Shape Memory Alloy Sheets";
Talk: 45th International Conference on Metallurgical Coatings and Thin Films 2018, San Diego, CA; 2018-04-23 - 2018-04-27; in: "Book of Abstracts 45th ICMCTF 2018", (2018), 12.

Pseudo-elastic Ni 50.8 at.%-Ti alloy sheets of 1000 microns thickness were coated with 10 microns thick refractory metal thin films (e.g. Mo, Ta and Nb thin films), by non-reactive d.c. magnetron sputtering. These thin films were characterized with regard to their microstructure and selected mechanical properties. Microstructural characterization of the thin films included X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) analyses. Mo thin films grow in a densely packed, (110) textured b.c.c. structure with columnar grains and larger crystallite size (280-300 nm) on the NiTi substrate. Ta and Nb thin films grow as well in a dense columnar structure; however they show X-Ray diffraction peaks of various lattice planes of the b.c.c. structures (i.e. no texture) and exhibit much smaller crystallite sizes (30-40 nm). Considering the specific thin film/substrate thickness ratio (1:100) of the samples, the mechanical properties of both the thin films and thin film/substrate composites were investigated on different length scales, using nano- and microindentation techniques. Nanoindentation results suggest that the values of the Young´s modulus of all thin film materials are identical with the values of the related bulk metals. Microindentation was used to evaluate the thin film/NiTi composites. It revealed that the Young`s modulus of the Nb/NiTi composite is closest to that of the pure NiTi substrate. Progressive scratch tests indicate excellent adhesion of all metal films on NiTi. These scratch tests suggest that the metal thin films undergo plastic deformation at larger normal forces. While the Mo films did not show any buckling or chipping during the scratch tests, Ta and Nb showed partially chipping. However, neither thin film delamination nor thin film cracking was observed, independent of the material combinations and testing conditions applied. To further evaluate the elasticity of the metal films and the integrity of the composites during elastic deformation tensile tests were performed on the thin film/NiTi composites with a maximum strain of 6%. These pre-strained samples were subsequently used for cycle fatigue testing. The resulting stress-strain fatigue curves were recorded under conditions of 2% mean strain and strain amplitude of 0.25%. To evaluate the potential impact of the surface coating and the deposition process on the phase transformation behaviour of the NiTi shape memory alloy, differential scanning calorimetry (DSC) analyses according to ASTM F2004-05 were done. We will discuss in detail the correlation of microstructure and static and dynamic mechanical properties of metal coated NiTi shape memory alloys. Conclusions and recommendations will be given for a material selection of refractory thin films design as radiopaque coatings on NiTi substrates for medical applications.