Interface-mediated hydrogen permeation resistance in Al₂O₃/TiO₂ nano-multilayer coatings: The roles of period thickness and defect segregation

Hydrogen embrittlement remains a critical bottleneck limiting the reliability of structural materials in hydrogen energy systems, thus fueling the imperative demand for high-performance hydrogen permeation barriers (HPBs). Ceramic nano-multilayer coatings offer unique advantages over single-layer counterparts by leveraging interface-driven hydrogen trapping and blocking. Herein, Al₂O₃/TiO₂ nano-multilayer coatings with tailorable period thicknesses (2–50 nm) were fabricated on T91 steel via atomic layer deposition. Their microstructure, electrochemical behavior, and hydrogen permeation resistance were systematically characterized. The results demonstrate that the as-prepared coatings exhibit a dense, amorphous microstructure, accompanied by superior corrosion resistance compared to single-layer Al₂O₃, TiO₂, and the bare T91 substrate. Notably, a non-linear correlation between hydrogen permeation resistance and period thickness was unveiled, a discovery that fills a critical knowledge gap in the field of HPBs design. Specifically, hydrogen permeation resistance was found to rise with increasing interface number up to a critical value, beyond which insufficient spatial isolation of hydrogen-induced defects gives rise to interconnected diffusion pathways and degraded barrier efficiency. The optimal Al₂O₃/TiO₂ nano-multilayer coating achieves an order-of-magnitude reduction in the hydrogen diffusion coefficient relative to the T91 substrate, arising from the synergistic effect of interface-mediated hydrogen trapping and effective segregation of hydrogen-induced defects. This work elucidates the fundamental structure-performance relationship governing Al₂O₃/TiO₂ nano-multilayer coatings and provides key insights into interface engineering and period thickness optimization for next-generation HPBs in hydrogen energy applications.