Evaluation of Magnetron sputtered protective ZR-C-AL coatings for accident tolerant zircaloy claddings

Coatings in the Zr-C-Al system, with a focus of the potential formation of Zr2AlC and Zr3AlC2 MAX phases, were deposited on Zircaloy-4 cladding substrates aiming to provide protection against high-temperature steam oxidation during accidental scenarios. A nanoscale elemental multilayer architecture with a multiple stacking sequence of zirconium, -carbon, and -aluminum layers corresponding to the stoichiometries of Zr2AlC and Zr3AlC2 were obtained. However, no MAX phases, but Zr(Al)C carbide, formed after annealing in argon at temperatures from 600°C to 1200°C. Coatings with three different Zr/Al ratios, corresponding to stoichiometries of 2Zr/C/Al, 3Zr/5C/3Al, 2Zr/5C/4Al, were deposited on polished Zircaloy-4 substrates. During oxidation at 700°C for 250 min in steam, all three coatings revealed lower oxidation rates compared to the uncoated Zircaloy-4. In particular, the two coatings with higher Al content (i.e., 3Zr/5C/3Al and 2Zr/5C/4Al) displayed better oxidation resistance. However, these two coatings demonstrated lower adhesion to the Zircaloy-4 substrates; widespread cracks and spallation of these two coatings were observed after oxidation due to the thermal expansion mismatch between the coatings and the substrate. ... mehrAll coatings demonstrated weak protection at higher temperatures oxidation (800°C and 1000°C), showing comparable or just slightly lower mass gain of coated coupons compared to uncoated ones.