Co-dopping of traditional YSZ by rare-earth oxides enhances an insulating efficiency of thermal barrier system. Matreials based on co-doping of zirconia with Y and Gd are of interest as possible new thermal barrier coating (TBC). The pyrochlore structure formed in ZrO2-Gd2O3 system is also of interest as alternative material for TBC due to it low thermal conductivity. The phase relations in the ZrO2-Gd2O3-Al2O3 system were experimentally studied at 1473 K by [02Lec] and it was shown that pyrochlore interacts with Al2O3 forming GAP phase. This makes it impossible to use pyrochlore as a thermal barrier coating if it is in contact with Al2O3, which is used as thermally grown oxide (TGO). The multilayered coating consisting from YSZ layer on TGO and pyrochlore as outer layer could have better properties than traditional TBC. The phase relations in the ZrO2-Y2O3-Gd2O3-Al2O3 system are important to understand interaction between TBC and TGO. The aim of this study is to derive thermodynamic database for ZrO2-Gd2O3-Al2O3, ZrO2-Gd2O3-Y2O3 and Gd2O3-Y2O3-Al2O3 systems and to calculate phase diagrams.
The thermodynamic databases for the ternary systems ZrO2-Gd2O3-Al2O3, ZrO2-Gd2O3-Y2O3 and Gd2O3-Y2O3-Al2O3 have been derived by combining descriptions of binary systems. Compound enregy formalism has been used to describe solid phases; liquid phase has been described by two-sublattice ionic liquid model. The thremodynamic parameters for binary systems have been assessed using phase equilibrium and calorimetric data. The calculated isothermal section for ZrO2-Gd2O3-Al2O3 system at 1473 K is in reasonable agreement with experimental data of [02Lec]. The isothremal sections have been also calculated at 1673 and 1923 K, where experiments have not been performed yet. The liquidus surface for the ZrO2-Gd2O3-Al2O3 system has been calculated in this study. It is in a good agreement with experimental data of [03Lak]. The temperatures of invariant reactions are in a good agreement with experimental data except for the temperature of Liq=Al2O3+Flu+GAP equilibrium. Taking into account ternary interactions in liquid phase could improve the consistency with experimental data. The reasonable reproducing of experimental data by the description based on using of only binary parameters in ternary system show that this approach could be applied for planning of new experiments in similar systems. The systems Y2O3-Gd2O3-Al2O3 and ZrO2-Gd2O3-Y2O3 are the parts of quaternary system ZrO2-Gd2O3-Y2O3-Al2O3 and they have not been experimentally studied so far. Isothermal sections for the Y2O3-Gd2O3-Al2O3 and ZrO2-Gd2O3-Y2O3 systems have been calculated at 1473, 1673 and 1923 K. The obtained data are used for planning of new experiments, which will be used to improve thermodynamic decription of these systems.
References
[02Lec] R. Leckie, C.G. Levi: personal communication
[03Lak] S. N. Lakiza, L.M. Lopato: personal communication