Corrosion of SiC layers on coated zirconia particles in wet atmosphere

Abstract Properties of SiC coating layer in the postulated accident condition of air and water ingress is crucial to the in-pile safety of tristructural-isotropic (TRISO) coated fuel particles of high temperature gas-cooled reactors (HTR). Particles of ZrO 2 kernels coated with low-density pyrolytic carbon buffer layer, high-density pyrolytic carbon layer and SiC layer fabricated by fluidized-bed chemical vapour deposition (FBCVD) were used to simulate real fuel particles in this research. The coated particles were tested at 1673 K in a certain flowing air with different water content between 0 and 100 g/m 3 for 10 h. Analysis results show that the corrosion reaction took place mainly on the outer surface of SiC layer with changing infiltration depth according to the flowing gas compositions. Steam content was controlled according to the relationship between saturated water vapour pressure and environment temperature. SiO 2 layer was the main product formed on the surface with thickness increase from 1.5 µm to 3 µm along with the content of water vapour in air. Formation of two types of micro-cracks after oxidation are observed: micro-cracks on spherulitic SiO 2 and penetrating micro-cracks on flat SiO 2 . Micro-stress generation mechanism is illustrated. Three are also some bubbles are observed either in SiO 2 layer or between SiO 2 and SiC layers. Results showed that no fracture occurred in the SiC layer after wet oxidation.