Oxidation mechanism of SiC coated layer in air atmosphere at 1400°C

The SiC coating layer of HTGR fuel particles is the most critical layer in hindering the release of fission products. In this paper, SiC layer was prepared in a spout fluidized bed by chemical vapor deposition on an industrial scale. High temperature oxidation experiments were carried out at different temperatures at 1400°C. The microstructure and composition were studied using different characterization methods, including EDS and SEM analysis. High-resolution SEM results showed that the oxidation started from the surface and gradually formed a punctate distribution of the oxide. The small crystal structure (~μm) of the SiC surface was very clear at the beginning and this crystal structure was oxidized at high temperatures, and disappeared gradually to form oxides of Si on the SiC surface. There was a stacking fault stress due to the different thermal expansion coefficient and elastic modulus of SiC and Si oxides, leading to the gradual formation of cracks around the punctates, and then with the increasing temperature, the crack further increased the oxidation rate of SiC, finally the formed Si oxides layer fell off from the particles. It can be seen clearly that the layer was peeling when the temperature was increased to 1400°C. But the oxidation layer was very thin (~1 μm), so the remain SiC coated layer has the same function of hindering fission products in the TRISO particles. EDS results can also validate these conclusions.