Stability and Failure Mechanisms of Al2O3|Al Bilayer Coatings Exposed to 300 Bar Hydrogen at 673 K

Abstract

Hydrogen barrier coatings are important for future hydrogen economy to enable materials for applications in hydrogen tanks. In the present study, coatings consisting of amorphous Al2O3 (≈100 nm) synthesized by plasma ion-assisted deposition on top of crystalline metallic Al (≈100 nm) are exposed to 300 bar hydrogen pressure at 673 K for 6 days. This is done to mimic and accelerate conditions in hydrogen storage containers for metallic hydrides. They remain intact after such harsh conditions, although changes do occur. Blister-like features are observed consisting of a buckled oxide layer while the metallic Al layer underneath is retracted. As these features are also found for coatings annealed under 1 bar Ar atmosphere it is concluded that they are not related to the formation of gas bubbles but they form due to solid-state dewetting. This is different to literature observation where H2 bubbles are reported as a consequence of interface diffusion of H/H+ species present due to the initial precursor used for film deposition. The mechanical properties of the coatings, which are evaluated from nanoindentation load–displacement curves, change only moderately. Overall, the study shows that Al2O3|Al coatings are suitable candidates to prevent hydrogen ingress, but dewetting due to long-term exposure at elevated temperatures must be prevented.