Microstructural studies of core/rim structure of polycarbosilane-derived SiC consolidated by spark plasma sintering. Issue 9 (15th June 2019) - Lee, Ji Hwoan
Abstract: Recently, attempts have been made to use silicon carbide (SiC) ceramics as a fuel compact in nuclear reactors. However, the use of sintering additives in the conventional sintering of SiC leads to a coarse microstructure with high porosity and large grains due to the low sinterability and the large amount of grain boundaries or a second phase, deteriorating the mechanical properties, and hence limiting widespread industrial applications. In this study, polycarbosilane (PCS) and spark plasma sintering (SPS) as the SiC precursor and sintering method, respectively, were applied to overcome the low sinterability and the use of sintering additives. As the raw material, amorphous PCS was prepared by pyrolysis at 1000 °C. To decrease the particle size, pyrolyzed PCS powder was subjected to ball milling by using high-energy ball milling. The ball-milled amorphous PCS powder was sintered by SPS. Sintering was carried out at different temperatures (i.e., 1700, 1800, and 1900 °C) and pressures (i.e., 40 and 80 MPa). After SPS, the core/rim structure of the sintered pellet was formed on account of the oxygen originating from the native oxide layer and free carbon in the PCS powder as the raw materials. In the core/rim structure comprising the core, as well as the intermediate and rim regions, the microstructure and chemistry of each region exhibited marginal difference with respect to the free carbon and oxygen. The phase fraction of α-SiC increased from the rim region to core region due to the discrepancy of actual loaded pressure on account of friction with the wall surface. The existing carbon was verified to be graphitized carbon. Owing to the short reaction time, graphitization did not completely proceed, and deficient graphitization led to graphitized carbon as not graphite. The introduced oxygen changed to amorphous SiO2, which was applied to the sintering-additive-like liquid-phase sintering.