NiTi shape memory alloy (SAM) thin films have attracted much attention because of their unique properties such as the ability to recover large stresses and strains, good corrosion resistance and biocompatibility. The reversible phase transformation between the austenite (B2) and martensite (B19?) phases, which can be either stress-induced (pseudo-elastic effect) or temperature driven (shape memory effect), is accompanied by changes in the mechanical properties 1,2.
NiTi Thin films sputtered by magnetron sputtering, depending on the deposition temperature, exhibited either amorphous or crystalline. The thin films deposited at room temperature are amorphous and a post-sputter annealing of the amorphous NiTi will crystallize it 3, 4, while thin films deposited at high temperature (above 450 oC), have crystalline structure 5, 6.
The mechanical behavior of the NiTi thin films is essential for effective use in the micro-electro-mechanical systems (MEMS), such as micro-valves, micro-actuators, and micro-pumps 7-12.
The most common methods for investigation of the mechanical properties of thin films are based on advanced methods such as interferometry 13, nanoindentation 14,15, X-ray diffraction 16 and MEMS-based experiment 17. These methods often necessitate complex and advanced tools and aren’t well-matched with the conventional explanation of mechanical properties that are defined in terms of conventional tensile testing. The direct tension test is an effective way to evaluate the mechanical properties as it produces reliable and simply interpretable outcomes. However, tensile testing is not simple to be executed for nano- or micro-scale materials such as thin films because of the minor dimensions of the samples. So that to overcome this limitation, another method is to deform thin films that are attached to a thicker substrate 18-20. Mechanical properties of the film can be extracted using the rule of the mixture from the stress-strain curves of thin film/substrate composite. Hou et al. were the primary researchers who attempted to sputter NiTi thin films on Polyimide film at elevated temperature 21. In their work, before deposition of the NiTi, the Polyimide film was baked at 350 oC for 1h. In addition, Kotnur et al. studied the structure and phase transformation of NiTi thin films deposited at an elevated temperature on Polyimide 22, 23.
In the present work, a bi-layer structure was obtained by depositing Ni-rich NiTi thin films on Kapton, which can be used as inflexible deformable tools, flexible actuators and for miniaturized bio-mechanical systems. Furthermore, in order to be able to control the microstructure and evaluation of its effect on the mechanical properties of the thin films, we have investigated the microstructural features of the NiTi thin films as a function of sputtering conditions such as the working gas pressure. If the sputtering parameters were changed, the microstructure and properties of sputtered thin films were observed to change significantly. In addition, the mechanical properties of crystalized Ni-rich thin film on Kapton were evaluated using a universal testing machine with specific clamps.