Oxidation Resistant Cr-Nb-N PVD Coatings for Cladding Protection in BWR Conditions

Paper in proceeding, 2025

For there to be a benefit of coating fuel cladding, the coatings must be able to withstand the reactor environment without significantly degrading. In this work a summary of findings on cladding coatings intended for BWR environment is reported. Two oxidation resistant Cr-Nb-N coatings deposited onto Zr-based tubes by direct current magnetron sputtering are presented as successful examples. Analytical electron microscopy and atom probe tomography reveal the coatings to be roughly 8 µm thick, with a multilayered structure consisting of Nb-rich and Cr-rich layers and a global composition close to (Cr2Nb)N stoichiometry. The two coating designs differ somewhat in have terms of constituent element distribution, crystal structure and morphology. Despite their differences, both designs exhibit excellent oxidation resistance in BWR water conditions, maintaining their integrity after 60 days of autoclave exposure. This is contrasted with the behavior of cathodic arc deposited TiN and CrN coatings that form a non-passivating oxide and dissolve, respectively, under the same conditions. Following the successful testing reported in the present work, the (Cr2Nb)N coatings are destined for exposure under in-reactor conditions to further investigate their stability and potential for application in commercial reactors. Suggestions for further studies to be carried out to deepen the mechanistic understanding of coating degradation in autoclave environments are also included in this work.