Superior performance of amorphous alumina coatings over polycrystalline alumina for fusion reactor applications

Alumina (Al₂O₃) is a promising tritium-permeation and corrosion barrier and a functional ceramic for flow-channel inserts in PbLi-cooled fusion blankets. Here we compare the helium-implantation response of a commercial polycrystalline α-Al₂O₃ bulk material and a pulsed-laser-deposited (PLD) Al₂O₃ coating on EUROFER; the coating is additionally characterised after static Pb 17Li exposure at 550 °C to assess its chemical stability. Helium implantation was carried out in a SIMS workstation on electron-transparent FIB lamellae extracted from both materials, enabling transversal irradiation across the lamella thickness and direct before/after comparison of the same specimen. Implantation doses were derived from neutronic calculations for a DCLL breeding blanket and microstructural changes were analysed by TEM/STEM. In α-Al₂O₃, bubbles and cavities preferentially decorate grain boundaries and, in some grains, the grain interior, with a strong dependence on crystallographic orientation and diffraction condition. In the PLD coating, Pb 17Li exposure produces a thin near-surface modified (Li-enriched) layer (~50 nm), while the underlying coating remains dense and nanoceramic. After He implantation, cavities are mainly confined to the modified layer, whereas only isolated cavities are observed in the underlying coating within the analysed lamellae. Orientation-dependent threshold displacement energies ( E d ) from ab-initio calculations, combined with IRAD simulations, provide a framework to interpret the heterogeneous damage in α-Al₂O₃, whereas the nanoceramic coating behaves effectively isotropically at the nanodomain scale. • PLD amorphous Al₂O₃ on EUROFER is compared with polycrystalline α-Al₂O₃ under fusion-relevant Pb 17Li and He conditions. • He implantation is performed directly on FIB TEM lamellae using a SIMS workstation as a low-energy ion implanter. • Polycrystalline α-Al₂O₃ shows grain-boundary bubble decoration and strong orientation-dependent damage, rationalized via anisotropic E d and IRAD bounds. • The PLD-derived alumina confines cavities mainly to a Li-enriched surface layer after Pb 17Li exposure, with limited damage in the underlying nanoceramic region. • Dense amorphous/nanocrystalline alumina coatings with low grain-boundary connectivity provide a more uniform damage accommodation pathway than coarse-grained alumina.