Effect of heat treatment temperature on the microstructure and actuation behavior of a Ti-Ni-Cu thin film microactuator

Ti-Ni-Cu/SiO2 two layer diaphragm-type microactuators were fabricated by sputter deposition and micromachining. The influence of heat treatment temperature on the actuation behavior was investigated under quasi-static conditions. The interfacial structure of Ti-Ni-Cu/SiO2 and internal structure of the Ti-Ni-Cu layer were also investigated using transmission electron microscopy. The reaction layer formed between the Ti-Ni-Cu and SiO2 layers, and preferentially grew into the SiO2 side. The reaction layer formed at 1023 K mainly consisted of Ti4(Ni,Cu)2O. The maximum height of the diaphragm decreased with increasing heat treatment temperature. The growth of the reaction layer also affected the microstructure of the Ti-Ni-Cu layer. The density of fine platelets and Ti2Ni precipitates decreased with increasing heat treatment temperature from 873 to 923 K, and they disappeared at 973 K due to the fact that the reaction layer mainly consisted of a Ti-rich phase. The microactuator heat treated at 973 K showed the highest transformation temperature with the lowest transformation temperature hysteresis, which is attractive for high speed actuation.