Nanoscale YSZ/alumina multilayered thermal barrier coatings for dual control of thermal and ionic transport

Conventional yttria-stabilized zirconia (YSZ)–based thermal barrier coatings degrade in high-moisture environments generated by hydrogen-enriched fuel combustion, largely due to the high ionic conductivity of YSZ. In this study, a nanoscale YSZ (2.5 nm)/alumina (2.5 nm) multilayer coating was developed to enhance resistance to oxygen diffusion while retaining thermal insulating performance comparable to state-of-the-art YSZ coatings. In this architecture, ionically insulating alumina sublayers hinder ionic transport, while the high density of interfaces suppresses heat transport, thereby compensating for the higher intrinsic thermal conductivity of alumina. The multilayer coating was fabricated by alternating radio frequency sputter deposition of YSZ and alumina onto Si substrates. Transmission electron microscopy confirmed the formation of a uniform, well-defined nanoscale periodic structure. Electrochemical impedance spectroscopy revealed an ionic conductivity of approximately 3.73×10−10 Ω−1 cm−1 at 340 °C, representing a reduction of about five orders of magnitude compared with single-layer YSZ, accompanied by an increase in activation energy from 0.825 to 1.01 eV. This pronounced suppression of ionic transport is attributed to the intrinsically low ionic conductivity of the alumina sublayers and the disruption of continuous ion-conduction pathways by the multilayer architecture. Thermal conductivity measurements using the 3ω method showed that the multilayer retained a low thermal conductivity of ∼1.19 W m−1 K−1 at 70 °C, comparable to that of single-layer YSZ. Collectively, these results demonstrate that nanoscale multilayer designs can effectively reconcile the trade-off between ionic blocking and thermal insulation in advanced thermal barrier coatings.