Recent Progress in Silicon Nitride and Silicon Carbide Ceramics

We know from experience that ceramic materials are brittle and easily broken. This is one reason why ceramics have not been used as engineering materials. Fracture is the result of crack growth through the microstructure. It was Griffith who proposed that ceramics have intrinsic cracks which grow under applied stress. The concentration of the applied stress at the crack tip decreases the strength to a level of about 1% or less of the theoretical strength. If the crack starts to grow, strength decreases so sharply that a catastrophic fracture occurs.In spite of the brittle nature of ceramics, their application as engineering materials was proposed in the 1960s because ceramic materials made of silicon nitride or carbide have higher strength at high temperatures than metals and oxide ceramics. Non-oxide ceramics have lower thermal-expansion-coefficients than oxides, resulting in better thermal shock resistance, which is one of the most important requirements for engineering ceramics.