Investigation Of Microstructural Changes In Sputtered NiTi Thin Films Under Fatigue

During the last two decades binary NiTi alloys found their use in a variety of applications, in particular as a functional material for medical devices. Recently, it was shown that magnetron sputtered NiTi films structured by microsystems technology exhibit excellent mechanical fatigue properties with high fatigue endurance limit of ±1.5% alternating strain [Siekmeyer et al., J. Mater. Eng. Perform. 23(7) 2014].

In this study the microstructure of magnetron sputtered micropatterned binary NiTi films is investigated by different microstructure analysis methods. For this purpose, Ti-rich as well as Ni-rich films with thickness in the range of 20 – 30 µm were fabricated. Depending on their composition, films were fatigued thermo-mechanically or mechanically in order to examine actuator and superelastic behavior, respectively. The cyclic degradation of their performance is investigated and linked to microstructural and surface property changes. Transmission electron microscope (TEM) sample preparation of the uncycled and the cycled films were carried out by focused ion beam (FIB) cross sectioning and milling.