Complex Melting Behavior of Si-lean, Fe-Ti-CMAS and its Effects on the Recession of EB-PVD Gadolinium-Zirconate TBC

Abstract

First and second stage high-pressure turbine blades of aeroengines are typically made of Ni-based superalloys protected by thermal barrier coatings (TBC). In operation, state of the art EB-PVD ZrO2 TBC experience severe degradation by airborne inorganic particles commonly referred to as CMAS (CaO-MgO-Al2O2-SiO2). CMAS particle ingestion, deposition, and subsequent melt infiltration is a major reason for premature TBC failure. A promising way to overcome the CMAS problem is the use of highly reactive TBC/EBC materials which can trigger rapid crystallization of CMAS melts, thus mitigating melt infiltration. A variety of new TBC materials such as rare-earth zirconates having low thermal conductivity exhibit such promising resistance against simplified CMAS compositions. However, partial or incongruent melting of CMAS constituents lead to complex reaction and infiltration scenarios. Such kinds of effects were studied on EB-PVD Gd2Zr2O7 coatings and synthetic Fe, Ti rich CMAS mimicking deposits on ex-service turbine blades. It turns out that such Si-lean, fully crystalline CMAS deposits are exhibiting phase equilibria, stepwise melting, and reactions which are very sensitive to their respective bulk chemical composition, thus making it difficult to assess the corrosion resistance and operation limits of new TBC materials.