Pack codeposition of Al and Cr to form diffusion coatings resistant to high temperature oxidation and corrosion for γ-TiAl

Abstract

Thermochemical analyses were carried out for a series of pack powder mixtures formulated for codepositing Al with Cr to form diffusion coatings on γ-TiAl resistant to high temperature oxidation by the pack cementation process. Based on the results obtained, experimental studies were undertaken to identify optimum pack powder mixtures for codepositing Al with Cr to form diffusion coatings with an adherent and coherent coating structure. The results of the thermochemical calculations performed indicated that codeposition of Al and Cr is possible with CrCl₃.6H₂O and AlCl₃ activated pack powders containing elemental Al and Cr as depositing sources. However, experimental results obtained at 1100°C revealed that CrCl₃.6H₂O is not suitable for use as an activator for codepositing Al with Cr on γ-TiAl. It caused a significant degree of degradation indicated by weight losses instead of coating deposition to the substrate. However, adherent coatings with excellent structural integrity consisting of an outer Cr doped TiAl₃ layer containing Al₆₇Cr₈Ti₂₅ phase and an inner layer containing TiAl₃ and TiAl₂ phases were successfully codeposited at 1100°C using pack powder mixtures activated by AlCl₃. It is suggested that such coatings were formed via a sequential deposition mechanism through inward diffusion of aluminium and chromium. Conditions that affect the pack codeposition process, and hence need to be carefully controlled, are discussed in relation to the mechanism of the formation of diffusion coatings with an integral structure free from microcracking on γ-TiAl.