SGTE Thermal Barrier Coating Database
The database covers the ZrO2-Gd2O3-Y2O3-Al2O3
system and is suitable
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.
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].
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.
For further information please click here
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Zinkevich M., Levi C.G., Aldinger F., The thermodynamic database for
the ZrO2-GdO1.5-YO1.5-AlO1.5 system: application for thermal barrier
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