The excellent performance of SiC ceramics is closely related to its unique structure. SiC is a compound with strong covalent bonds. The ionicity of Si-C bonds in SiC is only about 12%. Therefore, SiC has high strength, large elastic modulus, and excellent wear resistance. Pure SiC will not be attacked by acid solutions such as HCl, HNO3, H2SO4 and HF, and alkali solutions such as NaOH. Oxidation is prone to occur when heated in the air, but SiO2 formed on the surface during oxidation inhibits further diffusion of oxygen, so the oxidation rate is not high. In terms of electrical properties, SiC is semi-conductive, and introduction of a small amount of impurities shows good conductivity. In addition, SiC has excellent thermal conductivity. The
SiC has two crystal forms of α and β. The crystal structure of β-SiC is cubic, and Si and C respectively constitute the face-centered cubic lattice. There are more than 100 polytypes such as 4H, 15R, and 6H in α-SiC, among which the 6H polytype is industrially applicable. The most common one. There is a certain thermal stability relationship between the various types of SiC. At temperatures below 1600°C, SiC exists as β-SiC. Above 1600°C, β-SiC slowly transforms into various polytypes of α-SiC. 4H-SiC is easily generated at about 2000°C; 15R and 6H polytypes are required to be generated at temperatures higher than 2100°C. For 6H-SiC, even at temperatures exceeding 2200°C, they are very stable. The free energy difference between various polytypes in SiC is very small, so the solid solution of trace impurities can also cause changes in the thermal stability of polytypes.
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