Environmental Barrier Coatings in Gas Turbines: Thermodynamics and Recession Modeling
Nathan S. Jacobson
NASA Glenn Research Center
Cleveland, OH 44135
Abstract
Silicon-based Ceramics and Composites show great promise for hot-stage parts of gas turbine
engines. It is well-known that these materials are subject to attack by the water vapor component
of the combustion gases:
SiC + 3/2 O2(g) = SiO2 + CO(g)
SiO2 + 2 H2O(g) = Si(OH)4(g)
In order to limit this attack, refractory oxide coatings with lower thermodynamic activities of
silica have been developed. The current leading candidates are rare earth silicates. A simple
boundary layer expression accurately models the degradation process. The inputs to this model
are the fluid parameters and also the thermodynamics of hydroxide formation and the
thermodynamics of rare earth silicate. Hydroxide formation thermodynamics are measured with
a transpiration technique. Thermodynamics of the rare earth silicates are measured with
Knudsen Effusion Mass Spectrometry (KEMS). The extraction of the necessary data on
Si(OH)4(g) and Y2O3.n(SiO2) and Yb2O3.n(SiO2) are discussed via these methods. These data
are put into the model and compared to literature data.
Nathan Jacobson did his graduate work at the University of California in Berkeley and did a
post-doc at the University of Pennsylvania in Philadelphia. He has been at NASA Glenn since
the 1980s working on a variety of problems involving high temperature gas/solid reactions,
vaporization, phase equilibria, and re-entry materials.