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

The vapour pressure of halide species generated at high temperatures in pack powder mixtures containing Al, Si, halide salt and Al2O3 were analysed by means of thermochemical calculations in an attempt to identify suitable halide activators and pack compositions for codepositing Al and Si to form diffusion coatings on nickel-base superalloys by the pack cementation process. The halide salts investigated were AlF3, AlCl3, NH4Cl and CrCl3·6H2O. The results of thermochemical calculations suggested that compositions of pack powder mixtures activated by NH4Cl and CrCl3·6H2O may be adjusted to create deposition conditions favourable for codepositing Al and Si, but the pack powder mixtures activated by AlF3 or AlCl3 may only deposit Al. Guided by the thermochemical calculations, a series of pack powder mixtures activated by CrCl3·6H2O were formulated and coating deposition experiments were carried out at 1000 and 1100 °C. With adequate control of pack compositions and deposition conditions, it was found that codeposition of Al and Si could indeed be achieved at these temperatures. A mixture of elemental Al and Si powders may be used as a depositing source instead of using Al–Si master alloy powders as conventionally recommended. Depending on the deposition temperature used, the coatings could be formed either through the inward diffusion of Al and Si or through the outward diffusion of Ni together with other substrate elements such as Cr and Co with the respective coating structures associated with the two coating formation mechanisms. The pack compositions and deposition conditions may be adjusted to control the microstructure of the coatings formed by the codeposition process.