Anisotropic Attack of YAlO3 Model Environmental Barrier Coatings by Molten CaO-MgO-Al2O3-SiO2 Deposits

 Environmental barrier coatings (EBCs) are used to protect ceramic-matrixcomposites from undesirable reactions with steam and calcia–magnesia–alumina silicate (CMAS) particulates found in gas-turbine engine environments. Effective EBCs contain yttria or rare earth ions that will react with molten CMAS to form a protective apatite layer that prevents further attack. Methods to improve the EBCs’ CMAS mitigation capabilities focus on improving the apatite yield but neglect optimizing the apatite formation behavior. This study investigates the effect of apatite nucleation behavior on CMAS penetration by comparing the CMAS wettability and attack in the range of 1200ºC to 1350ºC of four different single crystal orientations of yttrium aluminate perovskite (YAlO3, YAP), a promising EBC candidate. The EBC/CMAS interfacial energy and, thus, reaction behavior varies with YAP orientation though no wetting anisotropy could be observed at times greater than 10 min. In regions with low CMAS loading, rapid apatite growth is seen on YAP substrates with orientations associated with high EBC/CMAS interfacial energy. CMAS attack is significant in these samples because the coarse apatite morphology does not form a dense layer. Such behavior is not observed in regions with high CMAS loading where small apatite crystals do form a layer. These layers form on top of an yttrium aluminate garnet (Y3Al5O12, YAG) layer. Garnet influence on apatite layer density has been observed, but the mechanisms are not well understood and are the subject of ongoing work. S/TEM results have found that YAG inclusions can have an orientation relationship with the surrounding apatite crystal. Ongoing work aims to determine if the YAG layers are textured to confirm that it is mediating the apatite nucleation and growth. The present study shows that an environmental barrier coating’s CMAS-mitigating properties can be improved by controlling the nucleation and growth of apatite via secondary phases like garnet and making use of interfacial energy anisotropy.