Enhanced fracture toughness of silicon nitride ceramics at cryogenic temperatures

We reported, for the first time, the fracture toughness of Si ₃N ₄ ceramics at cryogenic temperatures. The fracture toughness increased obviously from 4.77 ± 0.39 to 6.78 ± 0.50 MPa m ^1/2 with decreasing temperature from 293 to 77 K. A higher fraction of intergranular fracture calculated from the observations of crack path microstructures is responsible for the improvement in toughness at 77 K. The effect of residual stresses on the fracture mode and thus the toughness is discussed on the basis of theoretical calculation and analysis.