Oxidation Resistance Investigation Of Aluminide Coatings Prepared By In-situ CVD Process

Nickel-base superallloy has been widely used in?aeronautic and astronautic field due to its excellent high-temperature oxidation resistance which can be enhanced further through the overlay coating method. Pack cementation process is a versatile and economical process usually used to deposit aluminide diffusion coatings on nickel-base superalloys. But most of the processes are carried out at temperature higher than 900℃and last a long time and long time thermal treatment at these temperatures can fatally degrade the alloy's mechanical properties. Base on the discussion above, the study of the low-temperature pack cementation process is important.The pack cementation process is essentially an in-situ chemical vapour deposition process (CVD) and thermal diffusion process. In this study, the low temperature formation of Al-pack cementation coatings on the nickel-base superalloy had been investigated. The pack cementation coatings exhibiting high density and homogeneity possessed a two-layer structure. The top layer mainly consisted of Al3Ni2 and Al3Ni, while the bottom layer was composed of Al3Ni2. The effects of the experimental parameters on the microstructure and the composition distribution of the coatings had been studied. The results showed that all the parameters except for the pack activator (NH4Cl) content affected the coating thickness, but not the microstructure and the composition distribution. The relationship of coating thickness with treatment duration followed a parabolic relation, suggesting that the process was diffusion controlled. Furthermore, a linear relationship between the coating thickness and the reciprocal deposition temperature had been proved.The high-temperature oxidation resistance of the Al-pack cementation coatings had been investigated. The specimens with aluminide coatings exhibited better high-temperature oxidation resistance than the uncoated specimens. At the beginning of the oxidation tests, the major forming oxide was unstableθ-Al2O3 which was transformed into stableα-Al2O3 with the increase of the oxidation duration. As a result, the weight gain was more obvious at the beginning. The co-deposition of Al and Si on the nickel-base superalloy by traditional pack cementation process had been studied. The Al-Si coating processed a two-layer structure. The top layer of the Al-Si coating mainly consisted of Ni2Si, while the bottom layer was composed of Ni2Si and Ni0.9Al1.1. A transitional layer was observed between the coating and substrate. Then the co-deposition of Al and Si by in-situ chemical vapor deposition process on the internal surface of a hollow cylinder workpiece prepared wirh nickel base superalloys had been studied. The prepared coating exhibited the same structure and composition with the coating discussed above.