Aluminide Diffusion Coatings for Ni Based Superalloys. Coating and Oxide Microstructure

Doctoral thesis, 2002

This thesis reports on investigations of high temperature oxidation resistant aluminide diffusion coatings for Ni based superalloys. Such coatings basically consist of a .BETA.-NiAl layer at the surface of the .gamma.-Ni/.gamma.'-Ni3Al based substrate alloy. The presence of .BETA.-NiAl increases the Al activity at the surface, so that, upon high temperature oxidation, a protective .alfa.-Al2O3 scale can form. However, during service, both the substrate and the .BETA.-NiAl coating degrade, due to outward Ni diffusion from the substrate and the consumption of Al by the oxide scale. Moreover, as the oxide scale grows, several different processes may reduce the adherence of the oxide scale to the metal. This results in spallation of the scale, after which catastrophic oxidation may follow. The aim of this study was to describe the development of the coating and oxide microstructures upon high temperature oxidation and to correlate them to the protective properties of the coating. It was also attempted to clarify which mechanisms that play a dominant role in the determination of the total useful life of a particular coating. Special focus was also placed on the mechanisms by which the addition of Pt improves the coatings resistance to high temperature oxidation. For this purpose, one Pt-free (PWA73) and three Pt modified (RT22, SS82A and MDC150L) coatings, applied to the same single crystalline Ni based superalloy (CMSX-4), were studied. The materials were oxidised isothermally at 1,050 ºC for times ranging from 20 to 20,000 h, and subsequently investigated by gravimetry, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). It was found that the presence of Pt in the materials did not have any significant effect on the microstructural development on the coatings. However, Pt decreased the oxide scale growth rate by the suppression of deleterious spinel phases. Besides the presence of Pt, also a low surface roughness of the coating prior to oxidation promoted a slower oxide scale growth. It was shown that a high oxide growth rate itself promotes the formation of Kirkendall voids at the coating/oxide interface, which cause scale spallation during cooling to room temperature. These factors were found to be more important for the oxidation resistance than the degradation of the coating microstructure. For example, it was observed that the transformation of .BETA.-NiAl to the less Al rich phase .gamma.'-Ni3Al was not directly correlated with the onset of oxide spallation.