Fracture Toughness and Subcritical Crack Growth of Small Crack in Silicon Nitride at Room Temperature

The effect of microstructure in silicon nitride on fracture toughness and Subcritical crack growth (SCG) behavior at room temperature was investigated. Three kinds of silicon nitride materials containing 5 mass% of Y2O3 and Al2O3 were sintered under different conditions, and they showed different microstructures. Fracture toughness of the materials was obtained by single edge precracked beam (SEPB) method and controlled surface flaw (CSF) method with Knoop indentation-crack in order to investigate the dependence of the fracture toughness on microstructure. The fracture toughness data measured by the CSF method increased with increasing crack size for all kinds of the materials. The increment of the fracture toughness per crack length was more considerable for coarser grain sizes. It can be due to stress shielding at crack tip by such toughening mechanism as crack bridging. The static fatigue behavior of the materials was investigated by the dynamic fatigue technique at room temperature. The fatigue parameter n in SCG model (da/dt=AKIn) increased with increasing as fracture toughness and grain size increased. Based on stress shielding model at crack tip, the fatigue crack growth parameters in SCG model were calculated. The predicted value agreed to the experimental results.