Phase equilibria in the zirconia–yttria/gadolinia–silica systems

Phase equilibria in ZrO₂-YO_1.5-SiO₂ (ZYS) and ZrO₂-GdO_1.5-SiO₂ (ZGS) were experimentally assessed at 1400 °C and 1600 °C as they can offer insight on reactions between thermal barrier coatings (TBCs) based on ZrO₂-YO_1.5/GdO_1.5 and molten silicate deposits in gas turbine engines. Features shared in both systems include the absence of ternary compounds and no ternary solubility in the binary phases. In ZYS however, a quaternary invariant reaction was observed that eliminates the zircon-disilicate equilibrium at higher temperatures. The results suggest no appreciable difference in the reactions between silica and thermal barrier oxides based on ZrO₂-YO_1.5 or ZrO₂-GdO_1.5, or environmental barrier coatings based on the corresponding Y/Gd silicates. The phase diagrams derived from these experiments are part of a broader effort to develop thermodynamic databases that can help guide the design of next-generation TBCs.