Grown Crystalline Ni-Ti Shape Memory Alloy Thin Films

摘要

Crystallization of sputter deposited Ni-Ti thin film is commonly achieved with high temperature annealing to induce the shape memory effect. High temperature annealing has several disadvantages such as formation of precipitates, exclusion of unstable substrates and increase of residual stress. An attempt has been made to obtain as grown crystallized film by using hot target as a process parameter so that those disadvantages can be overcome. In this paper it will be shown that the transformation properties of sputter deposited as grown NiTi thin films from a hot Ti-rich target on single crystal Si substrate is crystalline in nature and shape memory above room temperature. This is true even though the materials did not undergo crystallization process. X-ray diffraction reveals that as grown films are crystalline and shows a mixture of martensite and rhombohedral phases. Transformation temperatures of the sputtered films are determined by using both differential scanning calorimetry and four point probe technique. Film microstructure has been studied by using transmission electron microscopy. The as deposited films have large sized grains with well defined twinned structure. We believe that the film is crystalline because the composition of the target and the high kinetic energy of the sputtered species create a favorable condition to form crystalline film.