Ceramic coatings for small modular reactors

In the nuclear sector, ceramic coatings can be applied to reactor components such as bearings and bolts to improve durability. Historically, electrodeposited hard chromium plate (HCP) has been used to provide wear and corrosion resistance in reactor environments. As a result of regulation changes and adverse health effects associated with Cr(VI), the use of HCP will soon be heavily restricted. Therefore, candidate ceramic materials have been identified which could match or improve upon the performance of HCP as the Rolls-Royce small modular reactor is developed. After a detailed materials selection process, ceramic coatings of Cr2O3, WC-(W,Cr)2C-Ni, Cr3C2-NiCr, CrN and CrAlN thermally sprayed or electron beam physical vapour deposited on to substrates of Inconel 625 were acquired. HCP, also deposited on Inconel 625, using electrodeposition was obtained for comparison. Tests to emulate nuclear reactor conditions including irradiation, corrosion and sliding wear were devised and undertaken to provide an initial screening programme for the candidate materials. A preliminary study into the irradiation performance used implanted ions, as surrogates for neutrons, with the HCP and Cr3C2-NiCr coatings showing no discernible damage in terms of peak shifts or broadening. Corrosion performance was also initially assessed, using a high temperature, high pressure autoclave, filled with simulated nuclear reactor coolant water. The Cr3C2-NiCr and CrN coatings were the least affected by this corrosive environment, as assessed by mass loss. The sliding wear performance of the coatings was examined in greater detail using an Inconel 625 ball undergoing reciprocating motion on a flat coating counterface under dry conditions, as well as in deionised water and simulated nuclear reactor coolant water environments, all at room temperature. Two wear rate analysis methods (mass loss and optical profilometry) were compared and were found to provide complementary information. Of the coatings, Cr2O3 was found to have the highest wear resistance, with no particle pull-out, likely due to its homogenous microstructure. Overall, the results from this initial screening programme indicated that the candidate coatings were unable to match the performance of HCP across all three testing regimes of irradiation, corrosion and wear. The Cr2O3 coating had the best overall performance in irradiation and wear testing but there were potential concerns over its resistance to pitting corrosion. The Cr3C2-NiCr coating was shown to offer comparable irradiation and corrosion performance to HCP and could be considered for use in applications where wear performance is not as critical. In addition to the outcomes for specific coatings, the study highlighted some key factors for further testing, such as the importance of using borated water in wear testing and the benefit of X-ray photon electron spectroscopy in providing additional information regarding wear mechanisms.