Thermochemical interactions between CMAS and Ca2Y8(SiO4)6O2 apatite environmental barrier coating material

Thermochemical interactions between Ca2Y8(SiO4)(6)O-2 apatite, a potential environmental barrier coating (EBC) material, and a synthetic CMAS having the composition 23.3 CaO - 6.4 MgO - 3.1 Al2O3 - 62.5 SiO2 - 4.1 Na2O - 0.5 K2O - 0.04 Fe2O3 mole % were investigated. Pellets of apatite + CMAS powder and hot-pressed apatite discCMAS couples were annealed at 1200-1500 degrees C for 1-50 hours in air. Powder X-ray diffraction (XRD) was used to identify the phases present. Polished cross-sections of the heat treated pellets and diffusion couples were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), high angle annular dark field (HAADF) imaging, selected area electron diffraction (SAED), and energy dispersive X-ray spectroscopy (EDS). Ca3Y2(Si3O9)(2) cyclosilicate, apatite, and amorphous phases were present in the samples heat treated at 1200 and 1300 degrees C, whereas no cyclosilicate was detected in samples annealed at 1400 and 1500 degrees C. A distinct cyclosilicate layer was observed at the apatite-CMAS interface in the diffusion couples heat treated at 1200 and 1300 degrees C. However, at 1400 and 1500 C, due to its much lower viscosity, CMAS quickly infiltrated the apatite substrate through pores and along the grain boundaries and no cyclosilicate was observed; the apatite grains dissolved in molten CMAS followed by re-precipitation of apatite needles within an amorphous phase on cooling.