Effect of SiO2 buffer layer on properties of sputter-deposited NiTi shape memory alloy thin films

NiTi thin films were co-sputtered at 450 °C by using a Ni50Ti50 and a Ti targets on 450 μm thick (1 0 0) silicon wafer without or with a SiO2 buffer layer (Film A/B). The phase transformation characteristics, shape recovery and mechanical property of the deposited films were investigated. Freestanding Film A shows a multi-stage transformation, while freestanding Film B has a two-step transformation during cooling. In the subsequent heating, both freestanding films show a two-step reverse transformation. Under a constant stress of 10 MPa, the shape recovery strain is approximately 1.35% for Film A and 0.25% for Film B. It is also found that the substrate-induced stress reduces the transformation hysteresis. The stress–strain curves of the two freestanding films are similar as determined by using a 6-axis micro-specimen tester.