Formation of Hf- and W-modified aluminide coatings on nickel–base superalloys by the pack cementation process

Abstract

Thermochemical analyses were undertaken for a series of pack powder mixtures containing elemental Al and Hf or W powders as depositing elements and CrCl3·6H2O or AlF3 or CrF3 as an activator and Al2O3 as the inert filler for codepositing Al with Hf or W to form diffusion coatings on nickel–base superalloys by the pack cementation process. The results indicated that Al could be codeposited with Hf, but not with W, from the vapour phase. Compared with both AlF3 and CrF3, CrCl3·6H2O has been shown to be a more suitable activator for codepositing Al and Hf. The optimum coating temperature has been identified to be in the range of 1050–1150°C. Based on the thermochemical analysis, a series of coating deposition studies were carried out, which confirmed that codeposition of Al and Hf could be achieved at a coating temperature of 1100°C in the CrCl3·6H2O activated packs containing either only Hf or both Al and Hf. The coating structure produced depended on the pack composition. It is suggested that compositions suitable for codepositing Al and Hf could be effectively identified by comparing the vapour pressures of HfCl4 and HfCl3 with that of AlCl in the packs activated by chloride salts. It has also been experimentally demonstrated that, although W could not be deposited from the vapour phase, a high volume of fine W particles can be entrapped into the NiAl coating layer formed through the outward Ni diffusion using a modified pack cementation process, resulting in the formation of a composite coating layer with W particles evenly distributed in a NiAl matrix. It is suggested that this modified pack process could be similarly applied to produce nickel aluminide coatings containing uniformly distributed fine particles of other refractory metals that may not be codeposited with Al from the vapour phase.