Hardness evolution of NiTi and NiTiAl thin films under various annealing temperatures

Ni50.5Ti49.5 and Ni45.6Ti49.3Al5.1 thin films were fabricated by direct current (DC) magnetron sputtering and post-annealed at 773, 798, 823, 873 and 923 K. The crystallization kinetics of as-deposited nickel-titanium-aluminum films was investigated by differential scanning calorimetry (DSC). Hardness and grain size of nickel-titanium-aluminum films annealed at different temperature were evaluated by nanoindentation test and X-ray diffactometer (XRD), respectively. The peak hardness of Ni45.6Ti49.3Al5.1 film was 12.9 GPa at 823 K, which was significantly improved as compared to Ni50.5Ti49.5 film with only 8.8 GPa. Hardness of both as-deposited and heat treated Ni45.6Ti49.3Al5.1 film increased with the amount of aluminum due to the effect of solid solution in the nickel-titanium system. The appropriate annealing temperature for specific Ni45.6Ti49.3Al5.1 film was selected at 823 K on the basis of the hardness consideration.