Characterization of silicon carbide coatings on Zircaloy-4 substrates

The lifetime of light water reactor (LWR) fuels is limited by the corrosion and degradation of Zircaloy cladding in the high temperature and high pressure operating conditions. As the thickness of the oxide layer increases, stresses build up in the oxide layer due to density differences between the oxide and the zirconium metal which lead to degradation and spallation of the oxide layer. The main objective of this research is to form protective coatings on the Zircaloy clad to prevent or at least slow the oxidation which can allow higher burnup of the fuel resulting in major benefits in plant safety and economics.; Silicon carbide was identified as a candidate protective coating to reduce the corrosion and degradation of Zircaloy cladding. Silicon carbide coatings were deposited on Zircaloy substrates using plasma-assisted chemical vapor deposition (PE-CVD) and were found to be amorphous as determined by X-ray analysis.; Since the adhesion of the films to the substrate was the most important property of a coating, scratch tests were used to assess the adhesion. The effects of different parameters on the test results including residual stresses, plastic deformation and friction between the stylus and the surface are discussed. Critical loads, characterized by continuous delamination of the SiC coatings deposited on Zircaloy-4, occurred at 0.5–2.5 N. The experimental results indicated that all SiC coatings used in this project, without exception, showed an adhesive failure when tested by scratch and indentation tests. Plastic deformation of the substrate due to compressive stresses induced by the scratch stylus caused flaking of the films at the interface, which was attributed to the low interfacial toughness.; The effects of film thickness and substrate surface treatment on the quality and adhesion of SiC coatings were studied in detail. Thick films (5μm) exhibited extensive cracking. The scratch tests indicated higher adhesion with intermediate substrate surface roughness (240 grit) compared to finer (600 grit) or coarser (grit blasted) treatments. Pre-cleaning of the substrate surface provided a significant enhancement in adhesion. Heat treatment of the coatings was found to increase the adhesion due to the relief of compressive residual stresses in the film. Ion implantation of the substrate surface was found to significantly increase the hardness and strength of the Zircaloy material.; Autoclave testing was used to assess the integrity and corrosion protection under high temperature (350°C) and high pressure (3000 psi) conditions. (Abstract shortened by UMI.)...