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SGTE Thermal Barrier Coating Database

The database covers the ZrO2-Gd2O3-Y2O3-Al2O3 system and is suitable for calculations on applications associated with Thermal Barrier Coatings. The database was developed at the Max-Planck-Institute Institute for Metal Research in Stuttgart, Germany.

Application of the TBC database

The system ZrO2-Gd2O3-Y2O3-Al2O3 is of interest for various diverse fields of technology. The yttria stabilised zirconia (YSZ) has a number of industrial applications [04Che, 04Fab]. For example, the phase with fluorite structure is used as a solid electrolyte [93Min]. The tetragonal phase with 6-8 wt. % Y2O3 is used as a thermal barrier coating (TBC) on metal substrates [04Lev]. Co-doping of this traditional YSZ with Gd enhances an insulating efficiency of the thermal barrier system. Rebollo et al. [03Reb] found that metastable tetragonal phase t' stabilized by Gd alone is less resistant to partitioning at high temperature than its Y counterpart with the same amount of stabilizer. However, modest substitution of Gd for Y does not degrade the stability and may improve it in some cases. Materials, based on co-doping of zirconia with Y and Gd, are of interest as possible new thermal barrier coatings [02Nic, 04Por]. The pyrochlore structure formed in the ZrO2-Gd2O3 system is also of interest as an alternative material for thermal barrier coatings [02Wu, 04Lev]. However, the Gd2Zr2O7 pyrochlore is prone to interact with the thermally grown oxide (TGO) forming perovskite structure which results in TBC failure [04Lev]. New TBC based on double-layer systems with a first layer of YSZ and a top layer made of pyrochlore materials shows better performance at high temperature than a single layer YSZ [04Lev, 04Vas]. The multilayer coating Y3Al5O12 (YAG)/YSZ has been suggested to enhance bond coat oxidation resistance. A thin layer of alphas-Al2O3 (thermally grown oxide, TGO) forms between metallic bond coat (BC) and TBC in the process of thermal cycling. Phase relations in the ZrO2-Gd2O3-Y2O3-Al2O3 system are therefore important in order to understand the interactions between the TBC and the TGO, stability issues of TBC materials and interactions within multilayer TBCs.

The thermodynamic database for the ZrO2-Gd2O3-Y2O3-Al2O3 system has been developed to calculate phase equilibria within the temperature range 298-3000 K. The thermodynamic parameters were assessed using experimental data on phase equilibria between 1100 and 3000 C along with different kind of calorimetric measurements and vapour pressure data. Since high temperature data were used to develop this database the results of calculations are thought to be reliable between 1100 and 3000 C, while extrapolation to lower temperature could result in uncertainty. The T0-lines for diffusionless transformations e.g., Fluorite=Tetragonal can also be calculated using this database. Driving forces for partitioning of non-equilibrium phases to the equilibrium assemblage can also be calculated. Users should be cautious when calculating phase relations in the Gd2O3-Y2O3-Al2O3 system, because experimental data are not available for this ternary system. The database should not be used to calculate equilibria in metallic, metal-oxygen systems and those involving gas phase.

Phase modelling

Most of the phases stable in the system are solid solutions and they have been described using the compound energy formalism. The liquid phase has been described using the two-sublattice partially ionic liquid model. Two phases, delta-Zr3Y4O12 and corundum Al2O3, were assumed to be stoichiometric compounds.

State of validation

Oxides (Al2O3, ZrO2, Y2O3, Gd2O3) The thermodynamic data for pure oxides were accepted for Al2O3 from [93Hal], ZrO2 and Y2O3 from [04Fab] and Gd2O3 from [05Lak1]. A small deviation from stoichiometry in oxide phases in the system of Zr-O [04Wan] and Y-O [98Swa] was not taken into account.

Binary systems (ZrO2-Y2O3, ZrO2-Gd2O3, ZrO2-Al2O3, Gd2O3-Al2O3, Y2O3-Al2O3, Gd2O3-Y2O3) The assessment of thermodynamic parameters in the ZrO2-Y2O3 system was based on phase equilibrium data, calorimetric measurements and vapour pressure data as described in [05Fab1]. The ZrO2-Gd2O3 and Gd2O3-Al2O3 thermodynamic descriptions [05Lak1] were based on phase equilibrium data and calorimetric measurements. The descriptions of ZrO2-Al2O3 and Gd2O3-Y2O3 systems ([05Lak1] and [05Fab1] respectively) were based on phase equilibrium data only and therefore they are less reliable. The thermodynamic parameters of system were re-assessed [04Fab] using phase equilibrium data and calorimetric measurements. It should be mentioned that the descriptions of the ZrO2-Y2O3 and ZrO2-Gd2O3 systems were checked for consistency with tie-lines in ternary systems ZrO2-Y2O3-Al2O3 and ZrO2-Gd2O3-Al2O3.

Ternary systems (ZrO2-Y2O3-Al2O3, ZrO2-Gd2O3-Al2O3, ZrO2-Gd2O3-Y2O3, Gd2O3-Y2O3-Al2O3)

The thermodynamic data for the ZrO2-Gd2O3-Al2O3 system were derived by combining binary descriptions. The ternary parameter for liquid phase was assessed to fit experimental liquidus surface [05Lak1]. The calculated isothermal sections are in reasonable agreement with experimental data [05Lak1, 05Lec]. The thermodynamic database for the ZrO2-Y2O3-Al2O3 system was derived by combining binary descriptions [05Lak2]. The ternary parameter for liquid phase was assessed to fit experimental liquidus surface [97Lak]. The calculated isothermal sections are in reasonable agreement with experimental data [05Lak2]. The calculated isoplethal sections are in reasonable agreement with experimental data of [97Lak] except for high temperature liquidus data. The thermodynamic data for the ZrO2-Gd2O3-Y2O3 system were derived by combining binary descriptions in [05Fab1]. The calculated isothermal sections in the range 1473-1873 K were checked experimentally [05Fab1]. According to calculations the liquidus surface of this system contains only one invariant point at 2589 K. Experimental data for the liquidus surface are not available and ternary interactions in liquid are assumed to be zero. The thermodynamic data for the Gd2O3-Y2O3-Al2O3 system were derived by combining binary descriptions in [05Fab2]. It is assumed that YAM and GAM and YAP and GAP form a complete series of solid solutions LnAM and LnAP respectively, while YAG has limited solubility of Gd3Al5O12 forming LnAG solid solution. The experimental data for the Gd2O3-Y2O3-Al2O3 system is not available so far. The calculated liquidus surface is in agreement with prediction of Lakiza [05Fab2].

Quaternary system

The thermodynamic data for the ZrO2-Gd2O3-Y2O3-Al2O3 system was derived by combining ternary descriptions in [05Lak1, 05Lak2, 05Fab1, 05Fab2]. Experimental data are not available for this system. However, if binary extrapolations give good agreement with experimental data in ternary system [05Lak1, 05Lak2, 05Fab1] it is assumed that extrapolations to quaternary system will give realistic results too.

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References

[93Min] N.Q. Minh, Ceramic fuel cells. J. Am. Ceram. Soc. 76 (1993) 563-588.

[93Hal] Hallstedt, B., Thermodynamic calculation of some subsystems in the Al-Ca-Mg-Si-O system. J. Phase Equilibria, 1993, 14, 662-675.

[97Lak] Lakiza S.M., Lopato L.M., Stable and metastable phase relations in the system Alumina -Zirconia-Yttria. J. Amer. Ceram. Soc., 1997, 80, 893-902.

[98Swa] Swamy V., Seifert H.J., Aldinger F., Thermodynamic properties of Y2O3 phases and the yttrium-oxygen phase diagram. J. Alloys Comp. 1998, 269, 201-207.

[02Nic] Nicholls J.R., Lawson K.J., Johnstone A., and Rickerby D.S., Methods to reduce the thermal conductivity of EB-PVD TBCs. Surface and Coatings Technology, 2002, 151-152, 383-391.

[02Wu] Wu, J., Wei, X., Padture, N.P., Klemens, P.G., Gell, M., Garcia, E., Miranzo, P., and Osendi M.I., Low-Thermal-Conductivity Rare-Earth Zirconates for Potential Thermal-Barrier-Coating Application. J. Am. Ceram.Soc., 2002, 85, 3031-3035.

[03Reb] Rebollo, N.R., Fabrichnaya, O., and Levi, C.G., Phase stability of Y+Gd co-doped zirconia. Z. Metallkd., 2003, 95, 163-170.

[04Che] M. Chen, B. Hallstedt, and L.J. Gauckler, Thermodynamic modelling of the ZrO2-YO1.5 system. Solid State Ionics 170 (2004) 255-274.

[04Fab] Fabrichnaya, O., Aldinger F., Assessment of thermodynamic parameters in the system ZrO2-Y2O3-Al2O3. Z. Metallkd., 2004, 95, 27-39.

[04Por] Portinha A., Teixeira V., Carneiro J., Costa M.F., Barradas N.P., and Sequeira A.D. Stabilization of ZrO2 coatings with Gd2O3. Surf. Coat. Techn. 2004, 188-189, 107-115.

[04Lev] Levi, C.G, Emerging materials and processes for thermal barrier system. Current Opinion in Solid State and Materials Science, 2004, 8, 77-91.

[04Vas] Weissen R., Traeger F., Stoever D., New thermal barrier coatings based on pyrochlore/YSZ double-layer systems. Int. J. Appl. Ceram. Technol. 2004, 1, 351-361.

[04Wan] Wang Ch., Zinkevich M., Aldinger F. On the thermodynamic modeling of the Zr-O system. Calphad 2004, 28, 281-292.

[05Lak1] Lakiza S., Fabrichnaya O., Wang Ch, Zinkevich M., and Aldinger F., Phase diagram of the ZrO2-Gd2O3-Al2O3 system. J. Eur. Ceram. Soc. (2005) in press.

[05Fab1] Fabrichnaya O., Wang Ch., Zinkevich M., Levi C.G., Aldinger F., Phase equilibria and thermodynamic propreties of the ZrO2-GdO1.5-YO1.5 system. J. Phase Equilibria and Diffusion (2005) (submitted).

[05Lec] R.M. Leckie, S. Kraemer, M. Ruhle, and C.G. Levi, Thermochemical compatibility between Alumina and ZrO2-GdO3/2 thermal barrier coatings, Acta Materialia, 2005 (in press).

[05Lak2] Lakiza S., Fabrichnaya O., Zinkevich M., and Aldinger F., Phase relations in the ZrO2-YO1.5-AlO1.5 system. Calphad (submitted).

[05Fab2] Fabrichnaya O., Lakiza S., Wang Ch., Zinkevich M., Levi C.G., Aldinger F., The thermodynamic database for the ZrO2-GdO1.5-YO1.5-AlO1.5 system: application for thermal barrier coating. J. Eur. Ceram. Soc. (submitted).

 

Updated 20 April 2010