Transient stage oxidation of MCrAlY bond coat alloys in high temperature, high water vapor content environments

Abstract

In order to evaluate the effect of water vapor on transient stage oxidation of MCrAlY (MNi and Co) bond coats, short-term exposures were carried out on thermally sprayed and cast alloy specimens at 1125°C in a range of N2–O2–H2O environments, and resultant thermally grown oxide (TGO) development was evaluated. On cast metal specimens, for which the three-phase alloy dispersion is coarse, growth of unwanted (Ni,Co)(Al,Cr)2O4 spinel was primarily correlated spatially to the Al-poor phases, namely γ, while the preferred TGO product, α-alumina, correlated to the Al-rich β phase. At higher water vapor contents, spinel nucleated and grew out past the γ boundaries to develop above β, too. By thresholding plan view BSE images of TGO surfaces, the amount of spinel vs. alumina surface area coverage was quantified. Spinel coverage was correlated to increasing PH2O, increasing PO2 and, for the highest surface coverage observed, a combination of high PH2O and low PO2. When a worst case spinel-creating environment of 30vol.% H2O and 10vol.% O2 was presented to the more commercially relevant (i.e. sprayed) version of CoNiCrAlY, complete spinel coverage was achieved, indicating that what is often thought of as a long-term spinel growth problem related to Al-depletion of the bond coat, can be created in several hours. Water vapor may enhance transient spinel growth by extending the lifetime of the metastable γ- and δ-alumina phases, the defect spinel structure of which promotes the diffusion of Cr, Co and Ni cations to the TGO surface, whereupon they participate in the development of non-ideal oxides such as spinel. All exposures were carried out without a yttria-stabilized zirconia (YSZ) top coat present.