Thermodynamic modeling of ZrO2–Y2O3–SiO2 and ZrO2-Gd2O3–SiO2 systems

Abstract Thermodynamic modeling in ZrO2–Y2O3–SiO2 and ZrO2-Gd2O3–SiO2 is part of a broader effort to develop thermodynamic databases that can help offer insight on reactions between thermal barrier coatings (TBCs) based on ZrO2–Y2O3/Gd2O3 and molten silicate deposits in gas turbine engines. The main purpose of the present work is devoted to providing a set of self-consistent thermodynamic parameters for ZrO2–Y2O3–SiO2 and ZrO2-Gd2O3–SiO2 systems by means of the CALPHAD (CALculation of PHAse Diagram) method. Firstly, the ZrO2–SiO2 and Gd2O3–SiO2 sub-systems are assessed. By combining with thermodynamic parameters of ZrO2–Y2O3, Y2O3–SiO2 and ZrO2-Gd2O3 in the literatures, phase equilibria in these two ternary systems are extrapolated and calculated. However, comparing the experimental data and the calculations reveals major inconsistencies, which contains three-phase equlibrium and a quaternary reaction. The bad predictions are mainly due to overestimating or underestimating phase stabilities of compounds in the binary sub-systems. Accordingly, Y2O3–SiO2 and ZrO2-Gd2O3 systems have been re-assessed based on new experiments. Finally, the experimental data for ternary systems are well reproduced by the improved thermodynamic descriptions. Our work exhibits that the accurate thermodynamic parameters of the sub-systems are significant to provide a reliable basis for thermodynamic extrapolations and calculations in higher-order systems.