Codeposition of Co and Al on nickel base superalloys by pack cementation process

To seek favorable conditions for the codeposition of Co and Al to form diffusion coatings on nickel base superalloys, the thermochemical analyses were adopted for a series of pack powder mixtures containing Co, Al, halide salts and Al ₂O ₃. The halide salts studied were AlF ₃ and NH ₄Cl. On the basis of the thermochemical calculations, the pack powder mixture of 7.6Al-20Co-4NH ₄Cl-68.4Al ₂O ₃ (wt.%) activated by NH ₄Cl was formulated. The coatings had a two-layer structure, of which the outer layer was mainly composed of Al _0.9Ni _1.1 where a part of Ni was replaced by Co. Once this was achieved, a series of further experiments was undertaken to investigate the effects of deposition duration and temperature on the kinetics of coating growth process. It was observed that these parameters only affected the coating thickness. The parabolic relationship between the coating thickness and the deposition duration confirms that the process was mainly diffusion-controlled. The experimental results of the oxidation showed that the coating can prevent efficiently substrate from oxidizing. The excellent oxidation resistance was attributed to a continuous and compact α-Al ₂O ₃ film formed on the surface of sample.