Deposition of silicon modified aluminide coatings on nickel base superalloys by pack cementation process

Abstract

The equilibrium partial pressures of vapour species generated in halide activated pack powder mixtures at high temperatures were calculated for a series of compositions using thermochemical analysis tools. The results obtained were applied to identify suitable activators and pack compositions for codepositing Al and Si to form diffusion coatings on nickel base superalloys by the pack cementation process. The calculation results suggested that compositions of the packpowder mixtures activated by CrCl₃.6H₂O may be adjusted to create deposition conditions favourable for codepositing Al and Si, but, those activated byAlF₃or AlCl₃ may only deposit Al.A series of coating deposition experiments were also carried out at 1000 ° C and 1100 ° C and the results obtained confirmed that, with adequate control of pack compositions and deposition conditions, codeposition of Al and Si can be achieved with CrCl₃.6H₂O activated pack powder mixtures. 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. The coatings could be formed either through the inward diffusions of Al and Si or through the outward diffusion of Ni together with other substrate elements such as Cr and Co, depending on the deposition temperature used. Prolonged deposition at 1100 ° C ledtothe formationofa coatingwith amultilayeredstructure consistingofanouter nickelsilicide layerand a middle Simodified NiAl layer followed by a diffusion zone. The pack compositions and deposition conditions may be adjusted to control the microstructure of the coatings formed by the codeposition process.