An experimental method for characterizing crack propagation in dense silicon nitride

In dense silicon nitride the effect of grain elongation on crack propagation was studied using scanning electron microscopy. The crack front proflle could be interpreted by both intergranular and transgranular crack deflection. This crack front deflection was characterized by means of a median deflection angle, which was computed from the surface trajectories of stable cracks. This method was applied to the evaluation of cracks in a set of hot-pressed silicon nitride materials, in which the grain elongation was varied in a controlled way by changing the additive concentration. It was found that the crack front deflection at the start of the critical crack propagation increased with grain elongation. At the same time, it was observed that room temperature fracture toughness increased with the median deflection angle. This indicates that crack front deflection contributes to the correlation between fracture toughness and grain elongation