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

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. ... mehrMn+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.