Variation in phase transformation paths of NiTi films as a function of film thickness

NiTi shape memory alloy thin films of different thickness have been grown on silicon (1 0 0) substrates using dc magnetron co-sputtering. Differences in microstructure such as crystallinity, grain size, grain size distribution and surface roughness of these films were studied using X-ray diffractrometer (XRD) and atomic force microscope (AFM). The influence of film thickness on phase transformation behavior was studied using four probe resistivity method. NiTi films exhibited mainly three kinds of transformation behaviors i.e. incomplete, constrained and complete austenite to martensite phase transformation for the films having lower (≤300 nm), intermediate (0.6-1.1 μm) and higher (2.3 μm) thickness, respectively. It is proposed that the constrained or incomplete transformation could be due to the special constraints (resistance force) introduced by the inter-diffusion of film, higher number of grain boundaries and increased level of the intrinsic defects. The level of actuation force can be tuned by choosing the different transformation paths that can have immense technological importance in the fabrication of various types of MEMS devices.