Environmental-barrier coating ceramics for resistance against attack by molten calcia-magnesia-aluminosilicate (CMAS) glass: Part I, YAlO3 and gamma-Y2Si2O7

The high-temperature (1500 degrees C) interactions of two promising dense, polycrystalline EEC ceramics, YAIO(3) (YAP) and gamma-Y2Si2O7, with a calcia-magnesia-aluminosilicate (CMAS) glass have been explored as part of a model study. Despite the fact that the optical basicities of both the EBC ceramics and the CMAS are similar, they both react with the CMAS. In the case of the Si-free YAIO(3), the reaction zone is small and it comprises three regions of reaction-crystallization products, including Y-Ca-Si apatite solid -solution (ss) and Y3Al5O12 (YAG)(ss). In contrast, only Y-Ca-Si apatite(ss) forms in the case of Si-containing gamma-Y2Si2O7, and the reaction zone is an order-of magnitude thicker. These CMAS interactions are analyzed in detail, and are found to be strikingly different than those observed in Y-free EBC ceramics (beta-Yb2Si2O7 and beta-Sc2Si2O7) in the accompanying Part II paper. This is attributed to the presence of the Y in the YAIO(3) and gamma-Y2Si2O7 EBC ceramics.