High-temperature oxidation resistance and self-healing behavior of Cr2AlC MAX phase coating on Zircaloy-4

Abstract

Zirconium-based alloys are currently utilized as fuel cladding and structural components in commercial light water reactors due to their low thermal neutron absorption cross section, good mechanical properties and reasonable corrosion resistance during operation conditions. One undesirable feature of zirconium-based alloy cladding is their extremely fast oxidation kinetics with high-temperature steam during loss of cooling accidents (LOCA). A considerable amount of heat and hydrogen gas is produced by the reaction of zirconium and steam. The claddings undergo severe degradation and hydrogen explosion can occur, followed by subsequent release of highly-radioactive fission products to the environment like during the nuclear accidents at the Fukushima Daiichi Nuclear Power Plant in 2011. One strategy to improve the accident tolerance of the state-of-the-art zirconium-based alloy fuel claddings is to coat the outer surface with an oxidation resistant coating. This solution promises the elimination of corrosion degradation during normal operation, as well as significant reduced oxidation kinetics with steam during off-normal conditions. Mn+1AXn(MAX) phases represent a family of ternary layered carbides or nitrides which possess a unique combination of the merits of both metals and ceramics. Alumina-forming MAX phase materials, like Ti2AlC and Cr2AlC, are being considered as protective coatings with respect to their excellent oxidation resistance up to 1400°C. In this study, Cr2AlC coatings have been deposited on Zircaloy-4 substrates by magnetron sputtering using elemental nano-multilayer thin films, and subsequent thermal annealing in argon. The total thickness of the coatings is around 6.5 μm and both coatings have a 500 nm Cr layer as bonding layer and diffusion barrier. One design of coatings also deposited a 1.5μm thick Cr capping layer to migrate potential fast dissolve of Al during normal operation. Crystallization of Cr2AlC MAX phase starts from 480°C by annealing in Ar and formation of phase-pure Cr2AlC MAX phase but with surface microcracks at 550°C is confirmed. Both coatings demonstrated high adherence, excellent oxidation resistance up to at least 1200°C and self-healing capability with growth of protective Al2O3 scale or of protective Al2O3 scale beneath Cr2O3 during high-temperature oxidation.