Thermodynamic calculations in the Y-Si-C-O-H system - Phase stability and corrosion resistance of Yttrium-Silicate EBCs on SiC in moist atmosphereThursday (29.09.2016) 10:39 - 10:42 Part of:
Economic and ecological aspects drive the need to develop turbines for electrical power generation and airplane propulsion systems with higher efficiencies and lower emissions. This can be achieved by an increase in combustion temperatures. However, the application of state-of-the-art refractory metal alloys in the hot sections of turbines is limited by their melting points and therefore requires extensive cooling systems. For next generation gas turbines, SiC-based Ceramic Matrix Composites (CMCs) are promising structural materials for the hot sections. However, 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. The resulting severe material recession necessitates the application of an environmental barrier coating (EBC). In this regard, combinations of yttrium silicates and yttrium oxide or silicon dioxide are 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 (CALculation of PHAse Diagrams) method was used to develop a thermodynamic dataset for the multi-component Y-Si-C-O-H system to be able to simulate equilibrium 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. New descriptions of the Gibbs free energies of the silicon- and yttrium-hydroxides were developed based on experimental data from the literature.
The updated thermodynamic description of the Y-Si-C-O-H system was used to calculate the thermochemical reactions between the yttrium silicate based coatings and the SiC base material as well as with the O2/H2O containing combustion atmosphere. The stabilities of yttrium silicate based coatings against erosion through formation of volatile silicon- and yttrium hydroxides was thereby evaluated.
 Cupid, D.M., Seifert, H.J., “Thermodynamic Calculations and Phase Stabilities in the Y-Si-C-O System”, J. Phase Equilib. Diffus., 28 (1), 90-100 (2007)