Excellent Properties and Crystal Forms of Silicon Carbide Ceramic

Silicon carbide (SiC) ceramic materials have high strength, strong high temperature oxidation resistance, good wear resistance performance, great thermal stability, small thermal expansion coefficient, thermal conductivity, high hardness, thermal shock resistance, chemical corrosion resistance and other excellent features. It is widely used in automobile, machinery, chemical industry, environmental protection, space technology, information electronics, energy and other fields, and has become structural ceramics that other performance materials can not replace in other fields.

The excellent properties of SiC ceramic is closely related with its unique structure. SiC is a kind of compound whose covalent bond is very strong. The ionicity of Si-C key in SiC is only about 12%. Therefore, SiC has high strength, big modulus of elasticity and excellent wear resistance. Pure SiC will not be eroded by HCl, HNO3, H2SO4, HF and other acid solution or alkali solution, such as NaOH.

It is easy to get oxidation in air heating, but the surface SiO2 formed in oxidation will inhibit further diffusion of oxygen, so the oxidation rate is not high. In terms of performance, SiC has semiconducting property, and the introduction of a small amount of impurities will show good electrical conductivity. In addition, SiC has good thermal conductivity.

There are two crystal types: α and β. The crystal structure of β-SiC is cubic crystal system. Si and C compose face-centered cubic lattice respectively; α-SiC has more than 100 kinds of polytypes, such as 4H, 6H and 15R. Among them, 6H polytype is most common used in the industrial applications.

There is a certain relationship of thermal stability between a variety of polytypes of SiC. When the temperature is below 1600 ℃, SiC exists in β-SiC form. When it is higher than 1600 ℃, β-SiC will slowly change into the polytypes of α-SiC. 4H-SiC is easier to be generated at about 2000 ℃; 15R and 6H polytypes need to be above 2100 ℃; For 6H-SiC, even if the temperature is over 2200 ℃, it is still very stable. The free energy difference between all kinds of polytypes in SiC is very small. Therefore, solid solution in trace impurities can also cause some changes about thermal stability between polytypes.