The role of carbon and SiO2 in solid-state sintering of SiC

The role of carbon in reducing SiO2 during solid-state sintering of SiC was directly shown using a model experiment, where transmission electron microscopy was used to follow oxidation and reduction of the surface of SiC single crystals. These results corroborated the microstructural characterization of pressureless sintered polycrystalline SiC, undoped or doped with carbon (3 wt.%) and boron (0.5 wt.%). While samples doped with carbon and boron reached a density of 96% after 4 h of sintering at 2100 °C, undoped samples did not sinter, and the microstructure was characterized by particle coarsening and a film of SiO2. The lack of densification for undoped samples is explained by the low surface energy of SiO2 compared to the grain boundary energy of SiC, which reduces the driving force for densification.