Performance evaluation of plasma sprayed yttria coatings on high density graphite for cathode processor applications

Cathode processing in the pyrochemical reprocessing of spent metallic nuclear fuels encompasses melting and consolidation of uranium deposit obtained from the electro-refining step. Ceramic coated High density graphite (HDG) is considered for process crucibles in the cathode process. Thermal cycling behaviour of plasma sprayed yttria coated HDG discs with NiCrAlY bond coat was investigated at 1000 and 1100 °C in vacuum. Each thermal cycle comprised soaking for 1 h at the isothermal temperature followed by furnace cooling to ambient temperature. The microstructure and elemental composition of the as-received and thermal cycled yttria coated samples were analysed by scanning electron microscopy coupled with energy dispersive spectroscopy. Phase identification of the coating, performed by X-ray diffraction and laser Raman spectroscopic techniques confirmed the presence of only cubic phase in the as-received as well as thermal cycled samples. Yttria coating failed after 35 thermal cycles at 1000 °C and 19 cycles at 1100 °C. Micro-cracks developed after three uranium melting experiments at 1300 °C could be owing to phase change in the bond coat.