Grain size effect on structural, electrical and mechanical properties of NiTi thin films deposited by magnetron co-sputtering

In the present study NiTi films have been deposited on Si (100) substrates by dc magnetron co-sputtering in the temperature range from room temperature to 923 K. The crystallization, surface morphology and structural features were studied using X-ray diffraction (XRD), atomic force microscope (AFM), field emission scanning electron microscope (FESEM) and high resolution transmission electron microscope (HRTEM). In situ hot stage atomic force microscope was used to investigate the micro-structural changes during phase transformation in these films. Substrate temperature was found to have a great impact on the structural features and phase transformation behavior of NiTi films. The grain size and the crystallization extent increase with the increase in substrate temperature. Nanoindentation tests of these films were conducted at room temperature. Low hardness and depth recovery ratio was observed in case of the film deposited at substrate temperature of 923 K that could be due to the dominance of martensite phase at room temperature which results in more plastic deformation. The electrical properties of the films were studied using four probe resistivity method. Electrical resistance versus temperature plots show that grain size of NiTi films plays an important role in their electrical properties. NiTi based shape memory alloys exhibit a very interesting martensite to austenite phase transformation as crystal structure changes from monoclinic to cubic upon heating close to room temperature. The characteristics of this transformation are of immense technological importance due to a variety of MEMS applications.