Pressureless sintered silicon carbide is considered to be the most promising sintered silicon carbide, and complex shapes and large sizes of silicon carbide ceramics can be prepared by the pressureless sintering process. Depending on the sintering mechanism, this kind of sintered silicon carbide can be further divided into solid-phase sintering and liquid-phase sintering. β-SiC containing trace amounts of SiO can be sintered at atmospheric pressure by adding B and C. This method significantly improves the sintering kinetics of silicon carbide. Doped with an appropriate amount of B, B is on the SiC grain boundaries during sintering and partially forms a solid solution with SiC, thus reducing the grain boundary energy of SiC. The doping of moderate amount of free C is beneficial for solid-phase sintering because the SiC surface is usually oxidized with a small amount of SiO generation, and the addition of moderate amount of C helps to make the SiO film on the surface of SiC reduced and removed, thus increasing the surface energy. However, the liquid-phase sintering will have a negative effect, because C will react with the oxide additives to generate gas, the formation of a large number of openings in the ceramic sintering body, affecting the densification process. The purity, fineness, and phase composition of the raw material are very important in the sintering process of silicon carbide.S.Proehazka sintered sintered silicon carbide with a density higher than 98% at 2020°C under atmospheric pressure by adding appropriate amounts of B and C simultaneously to ultrafine β-SiC powders (containing less than 2% oxygen). However, SiC-B-C system belongs to the category of solid-phase sintering, which requires a high sintering temperature, and low fracture toughness, fracture mode is a typical through-crystal fracture, coarse grains and poor uniformity. The focus of foreign research on SiC is mainly concentrated in the liquid phase sintering, that is, a certain number of sintering additives, at a lower temperature to achieve SiC densification. Liquid-phase sintering of SiC not only reduces the sintering temperature relative to solid-phase sintering, but also improves the microstructure, and thus the properties of the sintered body are improved compared with those of the solid-phase sintered body.
M. Omori et al. used rare earth oxides mixed with AlO or borides to sinter SiC densely. Suzuki, on the other hand, sintered SiC with only AlO as an additive at about 2000°C. A. Mulla et al. sintered 0.5 μm β-SiC (with a small amount of SiO on the surface of the particles) with AlO and YO as additives at ,1850-1950°C, and obtained a relative density of SiC ceramics that was greater than 95% of the theoretical density, and the grains were fine, with an average size of 1.5 μ m.
The microstructure of the silicon carbide ceramics was found to have coarse grains and a rod-like structure with good fracture toughness. The rod-like grains increase the fracture toughness while decreasing the strength of the silicon carbide ceramics. In order to obtain better strength and toughness while lowering the sintering temperature, many attempts have been made to improve the properties of this sintered silicon carbide by adjusting the glass phase composition with different additives. During the sintering process, the introduction of liquid phase at the grain boundary and the unique interfacial structure led to the weakening of the interfacial structure and the fracture of the material changed to a complete along-crystal fracture mode, which resulted in a significant increase in the strength and toughness of the material. However, considering that the use of AlO additive generates a glassy phase with low melting point and high volatility, which will undergo strong volatilization at higher temperatures, causing weight loss of the material and adversely affecting the densification of the material, the mass fraction of AlO in the additive should be increased appropriately.