Thermodynamic calculations in the Y-Si-C-O-H system for yttrium silicate based EBCs

Si-based Ceramic Matrix Composites (CMCs) are promising structural
materials for the hot sections of next generation gas turbines.
Since the presence of water vapor in the combustion gas may lead to
the formation of gaseous hydroxides which cause the volatilization
of the protective SiO2 scale, environmental barrier coatings (EBC)
are required. Combinations of yttrium silicates with Y2O3 or SiO2
are one of the most promising EBC materials, and it is necessary
to understand their behavior at high-temperature and in O2/H2O
containing combustion atmospheres. In this work, the CALPHAD
method was used to develop a thermodynamic dataset of the
multi-component Y-Si-C-O-H system to be able to simulate heterogeneous
reactions between the EBC and various gas atmospheres.
An existing thermodynamic description of the Y-Si-C-O system
was therefore refined by updating the description of the Y2O3-SiO2
pseudo-binary system and including descriptions of the Gibbs free
energies of the Si-O-H and Y-O-H gas species. The newly developed
dataset was used to calculate the thermochemical reactions between
the yttrium silicate coating and the SiC base material as well as with
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the O2/H2O containing combustion atmosphere. The stabilities of
yttrium mono- and disilicate against erosion was also evaluated as a
function of coating and gas composition, temperature and pressure.