Oxidation of simple and Pt-modified aluminide diffusion coatings on Ni-base superalloys - I. Oxide scale microstructure

The isothermal oxidation at 1050degrees C of one simple (PWA73) and three Pt-modified (RT22, SS82A and MDC150L) aluminide diffusion coatings, deposited on the same single crystalline Ni-base superalloy, CMSX-4, was investigated. The oxidation was studied by gravimetry, scanning-electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectrometry (EDS) and grazing angle X-ray diffraction (XRD). TEM samples were prepared by focused ion beam (FIB) milling. It was found that the oxide on the simple-aluminide coating grew much faster and started to spall much earlier than those on the Pt-modified coatings. This was related to the higher amount of other phases than alpha-Al2O3 in the oxide scale on the simple-aluminide coating. It was shown that the presence of Pt in the coating suppressed the formation of deleterious phases such as spinels in the oxide scale, but also that the surface morphology of the coating prior to oxidation plays an important role.