Enhancement of hardness and thermal stability of W-doped Ni₃Al thin films at elevated temperature

Nanocrystalline (NC) Ni₃Al thin films have poor mechanical properties at a high temperature and in order to improve the thermal performance, it is necessary to develop novel micro- or nanostructures that endow the coatings with both high thermal stability and hardness. In this work, Ni₃Al thin films with different W concentrations up to 12.5 at% are deposited on Si/SiO₂ substrates by magnetron sputtering and then vacuum-annealed at 500 °C and 700 °C. Addition of W to the nanostructured Ni₃Al thin films leads to grain refinement and high thermal stability. The hardness (H) of the film with 12.5 at% W annealed at 700 °C increases by 355% compared to pure Ni₃Al. X-ray diffraction and transmission electron microscopy reveal non-equilibrium phases such as the amorphous phase and supersaturation solid solution in the films annealed at 500 °C. After annealing at 700 °C, the α-W phase precipitated in the fine nanocrystalline state plays a crucial role in the high thermal stability of the nano-grains as well as enhanced hardness. The results suggest that these durable Ni-based coatings are suitable for applications at high temperature.