Characterization of Ni-Ti (Shape memory alloy) thin film by in-situ XRD and complementary ex-situ techniques

Abstract

Ni-Ti SMA are smart materials undergoing first order martensitic transformations driven by temperature and/or stress. In the form of film they are very attractive candidates for microelectro- mechanical system (MEMS) applications. Future directions include the production of functionally graded films by changing deliberately the ratio Ti/Ni across their thickness. However, for the successful development of this type of films, it is important to characterize, model and control the variations in composition, crystalline structure and transformation temperatures. Our approach is in-situ XRD study of the actual growth of the films of varying composition along the thickness carried out using a deposition chamber installed at a synchrotron radiation beamline. These studies were complemented with ex-situ analysis techniques. The results achieved on a Ni-Ti film co-sputtered from Ni-Ti and Ti targets on a TiN buffer layer are presented in this paper. The deposition started by using optimised parameters for a near equiatomic composition. After 1 h (≈330 nm thick film), the Ti power was increased from 20 to 25W, leading to the precipitation of Ti2Ni. The evolution of the lattice parameter values of the B2 phase, calculated from the corresponding XRD data, is clearly linked with the increase of the Ti power. The depth profile of the atomic concentrations determined by Auger Electron Spectroscopy (AES) is in agreement with the in situ XRD results. The temperature dependence of the electrical resistivity was used to monitor phase transformations, Scanning Electron Microscopy (SEM) has shown the presence of twinned martensite on the film’s surface at room temperature.