Effects of Copper and of Annealing Temperature on the Microstructural and Mechanical Properties of Sputter-Deposited NiTi Think Films

NiTi shape memory alloys are known to exhibit unusual properties such as shape memory effect and superelasticity even at the micro- and nano-scales, thus making them particularly attractive materials for micro-electro-mechanical systems (MEMS). In addition to the field of application mentioned above, being thermoelastic martensitic transformation accompanied by reversible changes in surface morphology, such a smart surfaces would be also attractive for optical and tribological applications. Furthermore, superelastic layers may be efficiently used to accommodate in a reversible manner large deformation during the mutual contact between interacting surfaces, as well as to increase the adhesion between layers of different nature. 

The use of these films to produce smart devices or protective layers for tribology requires a deep understanding of the fabrication process, which definitely affects their microstructure and consequently their functional and mechanical properties. Moreover, alloying a third element as dopant to the NiTi base composition revealed to be an effective approach to modulate their properties. Copper seems to be one of the most promising candidates when properties such as fast response to external stimuli, reduced composition sensitivity of the transformation temperatures, stability of the shape memory effect, and large recoverable strain are required. Therefore, this study is aimed to investigate the effects of Cu on the microstructural and mechanical properties of magnetron sputtered NiTi(Cu) thin films.

Ti-rich NiTi base composition has been doped with an increasing Cu content in the range 0 – 20 at.%. The as-deposited films approximately 1.4 µm thick were isothermally annealed in high vacuum at 500 and 600°C for 1 h to produce a crystalline structure. A field emission gun scanning electron microscope (FEG-SEM) equipped with Energy-dispersive X-ray spectroscopy (EDXS) was used to observe the surface and cross-sectional morphology of the thin films as well as to measure their chemical composition. The structure was evaluated by grazing incidence X-ray diffraction (GIXRD) and high resolution transmission electron microscopy (HREM), while mechanical properties were assessed by depth-sensing nanoindentation.

As-deposited films irrespective of the chemical composition exhibited an amorphous structure as confirmed by GIXRD spectra. On the other hand, with increasing annealing temperature and with different Cu contents dissimilar microstructures have been induced. In fact, GIXRD spectra revealed a change in dominant phases and precipitates, in terms of both structure and chemical composition. Such results were confirmed and further investigated by HREM analysis, which highlighted the presence of coherent and semi-coherent precipitates in the grain interior as well as of spherical precipitates at the grain boundaries. In addition, relevant effects of Cu on the grain size of NiTi thin films have been observed and investigated. Finally, the produced microstructures have been correlated to the mechanical properties, i.e. nano-hardness and reduced Young’s modulus, by multiple-load nanoindentation experiments.