Novel micro-thermal characterisation of thin film NiTi shape memory alloys

Abstract

Abstract A novel approach in determining the transition temperatures of thin films of NiTi shape memory alloys was investigated and compared with conventional techniques. The technique is based on micro-thermal analysis utilising a scanning thermal microscope (SThM). This method has the potential to allow the transformation temperatures of thin films to be investigated in situ. In this study, thermal micro-analysis using an SThM was investigated on thin film samples of 47.5 at.% Ni and 49.6 at.% Ni in Ti. Thin amorphous NiTi films were deposited on silicon substrates using DC plasma assisted sputtering. The thin films were recrystallised and subsequently characterised by SThM. Results were then compared with DSC, XRD and resistivity measurements. The SThM sensor variation with temperature showed clear onset and completion of transformation from martensite to austenite during a heating cycle. The small probe size of 1 μm2 allowed variations in transformation temperatures across the NiTi thin film to be investigated. A range of transition temperatures was observed, which was consistent with the difference between AS and AF as determined by resistivity and bulk DSC measurements. The linear contraction between martensite and austenite was also measured by this technique and was found to be similar to the theoretical transformation of 0.45%. This novel method will provide a route to investigate both the shape memory transformation temperatures of NiTi thin films and their spatial variation across devices such as micro-actuators.