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ZrO2-YO1.5-GdO1.5-AlO1.5 (+terns, binaries)

Gd was identified initially as one of the most promising additions, both as a co-dopant for YSZ as well as in the form of Gd2Zr2O7 as an alternate TBC material. Hence, a substantial fraction of the research effort was on modelling the ZrO2-YO3/2-GdO3/2 system. At the start of the program there was no information on the ternary, the binaries involving GdO3/2 had been experimentally studied but the reports for ZrO2-GdO3/2 were inconsistent, and the ZrO2-YO3/2 remained under some debate with only a rudimentary thermodynamic model available. All the binaries were assessed under this program and considerable experimental work was performed both on the ZrO2-GdO3/2 binary as well as the ZrO2-YO3/2-GdO3/2 ternary. The ZrO2-YO3/2 and ZrO2-GdO3/2 binaries were first assessed as part of the modelling of the respective ternary systems with AlO3/2, which is necessary to estimate the chemical compatibility between single- and co-doped zirconias and TGO.

Using the complementary binary descriptions ZrO2-AlO3/2 and GdO3/2-AlO3/2 obtained under this program as well as the literature data, thermodynamic databases for the systems ZrO2-YO3/2-AlO3/2, ZrO2-GdO3/2-AlO3/2, and YO3/2-GdO3/2-AlO3/2 were derived. Phase relations in the ZrO2-GdO3/2-AlO3/2 system were first studied experimentally under this program, while the thermodynamic description of the YO3/2-GdO3/2-AlO3/2 system was obtained by extrapolation assuming that monoclinic, perovskite and garnet structures form continuous series of solid solutions. The calculations using these databases allow estimating the limit of compatibility with TGO (in terms of the dopant content in YSZ or GdSZ) at any temperature. Furthermore, it was shown that if pyrochlore phase is in contact with AlO3/2, phase with perovskite structure forms. This makes it impossible to use the pyrochlore phase as thermal barrier coating on the top of TGO, since pyrochlore and perovskite have different thermal expansion causing cracking. However, the pyrochlore phase could be used as outer layer of TBC to avoid its direct contact with alumina.

Based on calculations of T0 (f/t) lines at different ratios of trivalent cations the iso-T0(f/t)-lines in the quasiternary systems were constructed, which restrict composition range where tetragonal phase can be obtained (stable or metastable). The assessed database for the ZrO2-YO3/2-GdO3/2 system has been used to calculate the liquidus surface, isothermal sections, isopleths at various Y:Gd ratios and traces of the T0(f/t) surface for the different temperatures and Y:Gd ratios. The latter bound the maximum combination of Y+Gd for which one can still produce a supersaturated tetragonal phase (t') at the temperature of interest, which is critical to the exploitation of toughening mechanisms that influence high temperature erosion. The liquidus surface is of critical relevance to the application of these materials by plasma spray. The thermodynamic database for the system ZrO2-YO3/2-GdO3/2-AlO3/2 was constructed by combining the thermodynamic descriptions of four quasiternary subsystems. The database is designed for Thermo-Calc software and can be used for various types of calculations, depending on the specific application. For example, the comparison of calculated vertical sections shows that increase of Gd+Y content from 7.6 to 15.2 mol.% decreases the stability field F+T and increases the stability fields F+AlO3/2 and F+YAG+AlO3/2. Increase of Y/(Gd+Y) ratio decrease the stability field of F+AlO3/2 and increases the stability field of F+YAG+AlO3/2. It should be mentioned that appearance of YAG phase is not desirable for thermal barrier coating, because the difference in thermal expansion between YAG and TBC results in crack formation during thermal cycling.

Reported by

Max-Planck-Institut fur Metallforschung-PML
Heisenbergstr. 3
70569 Stuttgart
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