Nickel titanium and nickel titanium hafnium shape memory alloy thin films

Abstract

Shape memory alloy (SMA) coatings of NiTi and NiTiHf have been deposited onto Si substrates using pulse DC sputtering. Coatings of NiTi with compositions containing 45 to 65 at.% Ti have been fabricated by co-sputtering NiTi with Ti. NiTiHf coatings with Hf compositions ranging from 2 to 30 at.% Hf have been fabricated by co-sputtering NiTi with Hf. XRD results reveal the as-deposited coatings as amorphous. A high temperature, 1100 à °C anneal followed by a low temperature, 550 à °C anneal was employed to crystallise the coatings. The XRD then shows the coatings to be martensitic at room temperature. Two sets of samples were produced for characterisation; one set was used for indentation studies and the other set used to prepare freestanding films required for differential scanning calorimetry (DSC) studies. Using the DSC, a NiTi coating containing 52 at.% Ti shows an endothermic austenite peak phase transformation, (Ap) at around 105 à °C and an exothermic peak martensite phase transformation, (Mp) at 65 à °C, resulting in a hysteresis of 40 à °C. For a NiTi coating containing 65 at.% Ti the hysteresis remained unchanged at 40 à °C, but there was a decrease in the phase transformation enthalpies when compared with the coatings containing 52 at.% Ti. Calculated phase transformation enthalpies in the NiTi coatings ranged from 6 to 13 J/g for the austenite phase and â  8 to â  11 J/g for the martensite phase. The NiTiHf coating shows SMA behaviour for a film containing 30 at.% Hf. DSC reveals an â  Râ  phase transition in this film. It is understood that this phase is present in films that have high internal stresses and is understood to nucleate near Ti3Ni4 precipitates. Phase transformation temperatures occur at 98 à °C and 149 à °C during heating and occur at 99 à °C during cooling. Phase transformation enthalpies range between 2 and 3 J/g for the austenite phase and â  7 J/g for the martensite phase. A scratch tester equipped with a 5 mm spherical tip has been utilised with loads ranging from 1 to 5 N to determine the recovery properties of the films. The results in this study conclude that NiTi films containing 65 at.% Ti deform 3 times more than films containing 52 at.% Ti. For NiTiHf thin films, increasing the Hf composition from 2 at.% to 30 at.%, doubled the deformation measur