Ceramics and Composite Materials

Biography

Biography: Hans J Seifert

Abstract

For next generation power plants and gas turbines, Si-based Ceramic Matrix Composites (CMCs) are promising structural materials for the hot sections. However, the presence of water vapor in e.g. combustion gases may lead to the formation of gaseous hydroxides which cause the volatilization of protective SiO2 scale. The resulting severe material recession necessitates the application of environmental barrier coatings (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 in combination with key experiments was used to develop a thermodynamic dataset for the multi-component Y-Si-C-O-H system. By this, the equilibrium heterogeneous reactions between EBC and various gas atmospheres can be assessed. An existing thermodynamic description of the Y-Si-C-O system was refined by updating the description of the Y2O3-SiO2 pseudo-binary system and 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.