Thermo-mechanical properties of TiNi shape memory thin film formed by flash evaporation

TiNi thin film with a nominal composition of 50 at.% Ti-50 at.% Ni and a thickness of about 6 mu m was deposited onto silicon substrates by flash evaporation, and then released from those substrates to become free-standing film. After vacuum annealing at 500 degrees C for 60 min so that an initially flat diaphragm could be formed, the thermo-mechanical properties of this thin film were evaluated using the bulge test, which causes deformation by pressurisation. Stress-strain curves revealed that it exhibited shape memory at temperatures of less than 60 degrees C and super elasticity at temperatures of more than 80 degrees C. It also showed shape recovery against pressures of up to at least 500 kPa. Under pressurised conditions, the temperature at termination of martensitic transformation, which causes deflection during cooling, increased with increasing pressure, although it was about 30 degrees C without pressure. On the other hand, the temperature at termination of reverse martensitic transformation, which causes shape recovery during heating, remained almost constant at 75 degrees C, independent of pressure. These two specific temperatures indicated that, under loading conditions, thin film could be driven easily, merely by heating and air cooling. At pressures of more than 100 kPa, slip deformation, which cannot be reversed, appeared and increased with increasing pressure. Moreover, displacement of deflection-recovery decreased during the early stages of the thermal cycle, because of work hardening due to slip deformation. After more than 50 cycles, however, displacement of deflection-recovery became constant, indicating that the thin film could give stable actuation, even under high load conditions. (C) 1999 Elsevier Science S.A. All rights reserved.