Mitigating CMAS Attack in Model YAlO3 Environmental Barrier Coatings: Effect of YAlO3 Crystal Orientation on Apatite Nucleation

Environmental barrier coatings (EBCs) are used to protect ceramic-matrix composites 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 attack at 1350 °C of four different single crystal orientations of yttria aluminate perovskite (YAP), a promising EBC candidate. The EBC/CMAS interfacial energy and, thus, reaction behavior varies with YAP orientation. In regions with low CMAS loading, rapid apatite growth is seen on YAP substrates with orientations associated with high EBC/CMAS interfacial energy. However, CMAS penetration is most significant in these samples because the apatite growth is facilitated by recession of the YAP substrate nearby. Such behavior is not observed in regions with high CMAS loading where small apatite crystals form on top of an yttrium aluminate garnet (Y3Al5O12, YAG) phase. This study shows that strategies that control the nucleation and growth of apatite will provide better protection against CMAS.