Theoretical Evaluation of YSZ and Alumina-YSZ Thermal Barrier Coatings in a Hydrogen Enriched Combustion Environment

Yttria stabilized zirconia (YSZ) ((ZrO2)0.93(Y2O3)0.07) and alumina-yttria stabilized zirconia ((Al2O3)0.853 + (ZrO2)0.93(Y2O3)0.07) thermal barrier coatings (TBCs) were modeled in the presence of hydrogen enriched combustion product gases to evaluate phase composition and thermal expansivity (coefficient of thermal expansion). Thermal equilibrium simulations for various equivalence ratios (0.5–0.75) and hydrogen enrichment percentages (0%–50%) were conducted to determine the product gas composition for various combustor operating conditions. The obtained product gases were then used in a second thermal equilibrium simulation to demonstrate their effect on the defined thermal barrier coatings. The modeling predictions showed that hydrogen enrichment percentage and equivalence ratio were positively correlated to thermal expansivity for both the thermal barrier coatings examined. The alumina-YSZ composite coating exhibited a higher coefficient of thermal expansion (CTE), more closely matching the CTE of a metallic bond coat, for the studied conditions. This closer match of thermal expansivity results in less significant thermal stresses than the YSZ thermal barrier coating. Increase in hydrogen enrichment percentage and equivalence ratio yielded increased percentages of phase transitions from tetragonal zirconia (t-ZrO2) to cubic zirconia (c-ZrO2). The YSZ thermal barrier coating had a larger percentage of phase transitions throughout the operating range examined, which renders concerns for potential failure from thermal cycling and creep. Theoretical examination of the phase composition and thermal expansivity provided further insights on the fate and behavior of the thermal barrier coatings.