UNDERSTANDING THE PRESENCE OF CASO4 WITHIN CMAS AND ITS EFFECT ON THE INFILTRATION BEHAVIOUR IN EB-PVD 7YSZ

Abstract

Infiltration of CaO-MgO-Al2O3-SiO2 (CMAS) in TBCs is an issue of concern for the aeronautical industry. From the existing research it is known that as soon as CMAS melts it infiltrates the TBC structure, chemically attacks the TBC, changes its phase composition and ultimately leads to the spallation of the TBC. Few investigations have shown the presence of anhydrite (CaSO4) within CMAS deposits. These investigations could not provide a complete understanding of the presence and the effect of CaSO4 on CMAS induced TBCs damage in this study. A systematic approach in understanding the effect of CaSO4 in CMAS has been taken. Five different CMAS compositions with and without CaSO4 were synthesized in the laboratory and their infiltration behaviours were investigated by depositing them on EB-PVD 7YSZ samples and subsequent heating at 1225 and1250°C. In addition, mass spectroscopy was applied on CaSO4 containing CMAS and the vaporization behaviour of sulphur was studied at high temperature. XRD was applied on the molten CMAS compositions and differences in phase formation due to the presence of CaSO4 were analysed. Based on this information, it is proved that sulphur which was present in the form of anhydrite evaporates as SO3 during the high temperature heating and extra CaO adds to the CMAS composition. This CaO-enriched CMAS is found to be more destructive than the initial CMAS. Hence it can be emphasised that CaSO4 presence in CMAS has no direct effect on the infiltration, but rather an indirect effect in changing the CMAS composition.