Microstructural degradation of plain and platinum aluminide coatings on superalloy CM247 during isothermal oxidation

Isothermal oxidation at 1100 degrees C of a high activity plain aluminide coating and a platinum aluminide coating, developed by the pack cementation technique, on cast nickel base superalloy CM247 has been carried out with the primary objective of systematically understanding the coating degradation process during oxidation. While the weight gains during oxidation for both plain aluminide and platinum aluminide coatings follow parabolic kinetics from the very beginning of oxidation exposure, the bare alloy was seen to exhibit a considerably long initial transient oxidation period (similar to 20 h), beyond which the parabolic law was followed. The parabolic rate constant for the platinum aluminide coating was found to be nearly two orders of magnitude lower than that for the plain aluminide coating. Alumina was identified as the only oxide phase that formed on both plain aluminide and platinum aluminide coatings during most of the oxidation exposure, although NiAl2O4 was also found in the case of the plain aluminide coating beyond similar to 200 h. The oxide layer on the bare alloy, however was found to consist of Al2O3, Cr2O3, and NiAl2O4. The microstructural degradation of both the plain aluminide and platinum aluminide coatings during oxidation tvas seen to occur in three distinct stages which, however; differed for each coating. This stagewise degradation which involves final obliteration of the interdiffusion layer in each case, is discussed in derail.