DEVELOPMENT OF MULTILAYERED THERMAL BARRIER COATINGS FOR HYDROGEN TURBINES

Abstract

Conventional yttria-stabilized zirconia (YSZ) thermal barrier coatings face durability challenges in hydrogen-enriched combustion environments. The high concentration of oxygen vacancies in YSZ leads to elevated ionic conductivity, allowing water-derived species to penetrate the coating and accelerate the growth of the interfacial oxide layer, which ultimately triggers substrate spallation. This dissertation systematically investigates multilayer architectures incorporating alumina layers to enhance moisture resistance. Experimental results demonstrate that compared to single layer YSZ, an alumina (100 nm) /YSZ bilayer with a top-layer configuration reduces the interfacial oxide growth by a factor of over eight at flame temperatures above 1250 °C. This protection is attributed to a reduction in ionic mobility by about five orders of magnitude. Further evaluation of combustion conditions across moisture levels from 10.5% to 44.0% reveals that single-layer YSZ is highly sensitive to moisture, exhibiting a fivefold increase in oxide thickness during the initial stages of exposure. In contrast, the configuration with a 100 nm alumina top layer maintains stable moisture resistance and suppresses the oxidation reaction by up to 93% in the early exposure stages. However, this protective effectiveness diminishes after 180 minutes as oxide thicknesses converge. Investigation into alumina layer thickness (100-550 nm) and positioning reveal that top-layer alumina provides a more persistent barrier. In contrast, configurations with alumina below YSZ layer are prone to radial cracking and the formation of interconnected pathways, which compromises protection. Notably, configurations with 350 nm and 550 nm top alumina layers successfully limit the interfacial layer to below 10 nm even after 30 minutes of exposure. Finally, to address alumina’s high intrinsic thermal conductivity, a 40-period YSZ/alumina multilayer was developed. This design utilizes high-density interfaces to generate significant interfacial thermal resistance. This architecture reduces ionic mobility by five orders of magnitude while maintaining a low thermal conductivity of ~1.19 W/m·K, comparable to YSZ single layer coating. This study provides valuable insights for designing and optimizing thermal barrier coatings for hydrogen turbine applications.