The effect of carbon on surface quality of solid-state-sintered silicon carbide as optical materials

The microstructure and the distribution of carbon (C) in silicon carbide (SiC) ceramics were investigated by scanning electron microscopy and transmission electron microscopy. The results show that C can restrain the growth of SiC grains and densify SiC ceramics with the increase of the C content, but residual C introduces a new phase-C to SiC ceramics. The hardness of C is less than that of SiC, so it's difficult to be polished as optical materials. The existence of C phase doesn't lead to the increase of surface roughness on SiC optical materials, but it leads to the decrease of the reflectance of SiC as the optical materials because the optical absorption of C in visible light is stronger than that of SiC. It indicates that C content is very important to the surface properties of SiC, which will affect the coating of chemical vapor deposition SiC or Si on the surface of SiC ceramics because of the different physical and chemical properties between C and SiC.