Nano-crystalline NiTi alloy thin films fabricated using magnetron co-sputtering from elemental targets: Effect of substrate conditions

The primary goal of this work is to examine the effect of in situ substrate conditions viz., substrate bias voltage and substrate temperature on the structural, morphological, compositional, and mechanical properties of co-sputtered NiTi alloy thin films. Application of substrate bias voltage during deposition creates defect sites which facilitate the growth of nano-crystalline NiTi films by imposing restriction on the coalescence of neighboring grains which has lead to interesting properties. Glancing Incidence X-ray diffraction studies revealed the co-existence of B2 and B19′ crystal structures and thus possess potential for martensitic phase transformation, which is the prerequisite for shape memory behavior. Thermally-induced martensitic twin bands were observed within the nano-grains with high-resolution transmission electron microscopy. Surface elemental composition was investigated by X-ray photoelectron spectroscopy to understand the chemical state of NiTi thin films. Differential scanning calorimetric measurements confirms the martensitic transformation below room temperatures. Nanoidentation studies have shown higher hardness values for NiTi films processed at high substrate temperature when compared to the film deposited under the influence of bias voltage due to the deformation mechanism governed by grain boundary absorption of the localized strain in nanometer scale.