Effect of deposition temperature on microstructure and hardness of ZrC coating layers of TRISO-coated particles fabricated by the FBCVD method

Abstract

Tristructural-isotropic (TRISO)-coated particles were fabricated by a fluidized-bed chemical vapor deposition (FBCVD) method for use in a very high temperature gas-cooled reactor (VHTR). ZrC as a constituent layer of TRISO coating layers was deposited by a chloride process using ZrCl4 and CH4 source gases in a temperature range of 1400°C and 1550°C. The change in the microstructure of ZrC depending on the deposition temperature and its effect on the hardness were evaluated. As the deposition temperature increased to 1500°C, the grain size of the ZrC increased and the hardness of the ZrC decreased according to the Hall-Petch relationship. However, at 1550°C, the ZrC layer was highly non-stoichiometric and carbon-rich and did not obey the Hall-Petch relationship in spite of the decrease of the grain size. A considerable amount of pyrolytic carbon at the grain boundaries of the ZrC as well as coarse granular pyrolytic carbon were locally distributed in the ZrC layer deposited at 1550°C. Therefore, the hardness decreased largely due to the formation of a large amount of pyrolytic carbon in the ZrC layer.