Study of CMAS infiltration and evaporation behaviour under water vapour/sulphur oxide conditions in EB-PVD 7YSZ

The infiltration behaviour of CaO-MgO-Al2O3-SiO2 (CMAS) in Electron Beam Physical Vapour Deposited (EBPVD) 7 wt% yttria stabilised zirconia (7YSZ) under controlled atmospheres (dry, wet and sulphurizing) was studied in detail. The microstructure of the CMAS-infiltrated 7YSZ and the CMAS residue on top were analysed to interpret the corrosion mechanisms. One of the major goals was to investigate the stability and the infiltration of CaSO4 into the 7YSZ columns. At the testing temperature of 1250 °C, CaSO4 was stabilised only under the SO2 atmosphere within the TBC coating and only in CaSO4-containing CMAS. Hydroxylation of CMAS elements was found under wet conditions, which changed the local CMAS composition and infiltration behaviour. These chemistry changes have provoked acidity changes in the CMAS which directly affected their aggressivity and viscosity. Thermodynamic equilibrium predictions on the volatility of CMAS components and the anhydrite stability under various atmospheres were used to understand the experimental findings and the underlying mechanisms.