Microstructure and oxidation behavior of Al and Hf co-deposition coatings on nickel-based superalloys

This study focuses on investigating the microstructure and oxidation behavior of Al and Hf co-deposition diffusion coatings deposited by Pt electroplating followed by combined aluminizing and hafnizing using the pack cementation technique. The composition (in wt.%) of the packs was 2NiAl–2NH4Cl–xHf–(96 − x)Al2O3 with different Hf levels (x = 2, 6, 12 and 16), and the corresponding microstructure of the coatings was γ-Ni + γ′-Ni3Al, NiAl + Al16Hf6Ni7, Al16Hf6Ni7 and Al1.65HfNi0.35, respectively. In the post heat-treatment process, with Hf and Al diffusing towards the superalloy substrate and Ni diffusing in the opposite direction, Al16Hf6Ni7 and Al1.65HfNi0.35 were transformed into Ni3Al + Al3HfNi12 and NiAl + Al3HfNi12, respectively. The isothermal oxidation tests at 1000 °C in air indicated that the coating formed in the packs containing 2 wt.% Hf had the lowest weight gain, while the weight gain of the coating formed in the packs having 6 wt.% Hf was the largest. The mechanisms of Hf-rich phase formation and their effects on oxidation resistance of the coatings were discussed.