Pack Deposition of Oxidation Resistant Silicide and Aluminide Coatings on γ-TiAl

摘要

Thermochemical analyses were undertaken for a series of pack powder mixtures for co-depositing Al and Si to form silicide and aluminide diffusion coatings on γ-TiAl by the pack cementation process. Based on the results obtained, experimental studies were carried out to identify optimum pack powder mixtures and conditions for the co-deposition process. The thermochemical calculation results suggested that co-deposition of Al and Si is possible with CrCl_3 · 6H_2O and AlCl_3 activated packs containing elemental Al and Si as depositing sources. But, experimental results obtained at 1100℃ revealed that CrCl_3 · 6H_2O is not a suitable activator for the co-depositing process. It caused a significant degree of degradation to the substrate indicated by the specimen weight losses. However, pack powder mixtures activated by AlCl_3 were successfully used to co-deposit Al and Si to form diffusion coatings on γ-TiAl at 1100℃. The coatings produced had a multi-layered structure consisting of an outer silicide layer, an inner aluminide layer of TiAl_3 and a transitional layer at the boundary between the deposited coating and the substrate. It is suggested that such coatings were formed via a sequential deposition mechanism through inward diffusion of aluminium and silicon. A discussion is presented on the parameters that need to be effectively controlled to ensure the co-deposition of Si and Al to form silicide and aluminide diffusion coatings on γ-TiAl with a coherent structure free from microcracking and spallation by the pack cementation process.