Nanoporous alumina-silica nanolaminates grown by mixed thin films processes for selective environmental barrier layers on carbon surfaces

• 1 nm thick alumina ALD layering efficently templated silica CatALD growth. • Frequency and thickness of ALD interspersion determined intra-diffusion kinetics. • Air annealing induced a film expansion and migration of alumina towards the surface. • Film nanoporosity analysis showed potential for use as selective diffusion barriers. Alumina-silica nanolaminates are promising candidates for diffusion barriers on carbon and in harsh environments, providing that their conformal deposition in mild oxidative conditions can be achieved. Recently, an adaptation of rapid atomic layer deposition (Catalytic ALD), has been developed to deposit alumina-silica thin films directly on carbon surfaces, but suffers from limitations in silica chemistry control. Intermixing alumina-silica CatALD with amorphous alumina ALD diffusion barrier layers enabled to resolve issues of interfacial diffusion of silanol precursor and to maintain a regular silica growth independently of the number of deposition cycles. In this study, the generated alumina-silica nanolaminates by process combination were varied in terms of Al content, ALD layer thickness and interspersion frequency. Four alumina-silica ALD/CatALD nanolaminates benchmarked against an alumina-silica CatALD film were further investigated with regards to the formation of alumino-silicate clusters and alumina in silica migration upon annealing following different thermal routes. Thin film open pore and defect analysis by means of positron annihilation lifetime spectroscopy (PALS) showcased the role of ALD in the tuning of film nanoporosity. PALS further highlighted the relationship between the propensity in internal defects and the annealing temperature, related to concomitant events of carbide bond cleavage, silica densification, and alumina dehydration.