Formation and cyclic oxidation resistance of Hf–Co-modified aluminide coatings on nickel base superalloys

• A suitable condition for codeposition of Co, Al and Hf was determined.
• A Co–Al–Hf coating with high Co and minute Hf content was obtained.
• The Co–Al–Hf coating had excellent cyclic oxidation resistance due to improved adhesion.

The codeposition of Co, Al and Hf on nickel base superalloys by pack cementation in a single-step process was investigated in this work. Thermochemical analyses were applied to search for suitable conditions including pack composition and deposition temperature. Co, Al, Hf, NH4Cl, NH4I and Al2O3 made up the pack powder mixture. According to a series of thermochemical calculations, the pack powder mixture of 20Co–10Al–2Hf–4NH4Cl–4NH4I–60Al2O3 (wt.%) was adopted. Further experimental results demonstrated that the codeposition of Co, Al and Hf could be achieved practically. The coating consisted of a diffusion zone and an outer layer. The outer layer was mainly composed of Al0.9Ni1.1 where a part of Ni was replaced by Co or Hf. The trace element Hf was enriched in the interface between the outer layer and the diffusion zone. The Co–Al–Hf coating exhibited excellent cyclic oxidation resistance due to improvement in adhesion between the oxide scale and the coating.