Phase equilibria and crystal chemistry in the calcia-silica-yttria system

The phase equilibria in the CaO-SiO2-Y2O3 system have been assessed at 1400 °C and 1600 °C using powders prepared by precipitation from precursors. Four ternary phases were identified: Ca/Y-cyclosilicate (Ca3Y2Si6O26), silico-carnotite (Ca3Y2Si3O12), cuspidine (nominally Ca2Y2Si2O9), and apatite (solid solution based on Ca2Y8Si6O26). The melting temperature of each ternary phase was measured and the shape of the liquid field at 1600 °C was assessed. The study emphasizes understanding the extent and crystallographic origin of the solid solution homogeneity range for apatite between Ca0.5Y9(SiO4)6O2 and Ca2.5Y7.5(SiO4)6O1.75, deviations from the nominal cuspidine stoichiometry, and solubility of Y2O3 in the high-temperature α-Ca2SiO4 polymorph. The results are discussed in the context of molten silicate (CMAS) degradation of thermal and environmental barrier coatings (T/EBCs).