Growth and characterization of DC magnetron sputtered Ni-Ti intermetallic and near equiatomic thin films

Abstract

In recent years NiTi thin films have attracted much attention as intelligent and functional materials because of their unique properties such as shape memory effect. Shape memory alloy thin films possess several desirable properties such as high power to weight (or force to volume) ratio, and ability to recover large transformation stress and strain upon heating and cooling, pseudoelasticity (or superelasticity), high damping capacity, good chemical resistance and biocompatibility, etc. In spite of the development of advanced deposition techniques for NiTi thin films, some unresolved issues still remain, which limit precise control of composition during the deposition. In order to standardize these deposition techniques, an advanced understanding of the underlying process-property relationships are essential. This thesis mainly describes the growth and characterization of DC magnetron sputtered intermetallic, and near equiatomic NiTi alloy thin films. The correlation between deposition parameters and material properties has been established. In this thesis work, we have mainly investigated the influence of post-deposition conditions (annealing temperature and/or aging time) on NiTi thin film micro-structure, surface morphology, mechanical and surface properties. Based on XRD, it was concluded that annealed and aged intermetallic and near equiatomic NiTi thin films were crystallized, resulting in the form of various dominant phases along with intermetallic precipitates. The surface morphology of the investigated intermetallic and near equiatomic NiTi thin films from FESEM characterization was found to vary significantly with respect to annealing temperature and/or aging time. Topographical structures can be altered by varying the annealing temperature and/or aging time. The surface mean height (Ra), root mean square (RMS) and maximum peak to valley height (P-V) values have been obtained by AFM characterization. Nanoindentation tests on annealed and aged intermetallic and near equiatomic NiTi thin films were performed at room temperature. The hardness and elastic modulus values of deposited films increase probably due to formation of the intermediate metastable states by the incomplete alloying process. X-ray photo electron spectroscopy (XPS) was used to study the composition and surface chemistry of the annealed films. From HR-XPS investigations, it was observed that annealed and/or aged intermetallic and near equiatomic NiTi thin films had higher affinity to form Titanium dioxide (TiO2) layer on the film surface leaving a Nickel enriched matrix immediately behind the metal oxide layer along with carbide precipitates.