Impact of Moisture Content and Exposure Duration on the Performance of ALD-Alumina/YSZ Thermal Barrier Coatings

Abstract This study systematically investigates the influence of moisture content (10.5%–44.0%) and exposure duration (up to 180 minutes) on the performance of monolithic yttria-stabilized zirconia (YSZ) and bilayer alumina/YSZ thermal barrier coatings. A dense 100-nm alumina top layer, deposited via atomic layer deposition (ALD), was integrated as a potent diffusion barrier to suppress interfacial oxidation. Both the monolithic YSZ baseline and the bilayer alumina/YSZ samples were subjected to 1500 K in a custom-designed tube furnace replicating hydrogen-enriched combustion conditions. Experimental results demonstrated that the ALD-alumina layer significantly inhibited interfacial oxide growth by restricting ion transport. Notably, at 10.5% moisture content, the bilayer alumina/YSZ system limited interfacial oxide growth to just 35 nm after 45 minutes of exposure, whereas the monolithic baseline reached 320 nm. Furthermore, the bilayer alumina/YSZ system exhibited negligible interfacial growth during initial exposure (<30 minutes), even as moisture contents increased to 36.1%. However, this protective efficacy is time- and moisture-dependent. After 180 minutes of exposure, the interfacial layer growth for both configurations converged to approximately 1.05 μm, regardless of moisture content. This degradation is likely driven by alumina volatilization as aluminum hydroxide when exposed to high-temperature water vapor or its diffusion into the YSZ layer, which increases the system's overall ionic conductivity. This work provides critical insights into moisture-driven degradation kinetics and identifies key challenges in designing robust thermal barrier coatings for extreme, high-moisture environments.