Cyclic fatigue crack growth behaviour of silicon nitride at 1400°C

Subcritical crack growth under sustained and cyclic loads at 1400°C has been investigated in a high strength Si3N4. Two different crack growth behaviours were found in this material depending on the high temperature testing history. During short-time testing, fatigue crack growth obeyed the Paris power-law expression, and under these conditions high frequency loading markedly reduced the crack propagation rate coMPared with that under sustained load or lower frequencies. Scanning electron microscopy observation revealed that this beneficial effect of cyclic loading was associated with viscous bridging by the glass phase at the crack wake, which effectively shielded the crack tip. As the fatigue test proceeded, however, crack growth rates reduced, eventually resulting in a lack of correlation between crack growth rate and stress intensity factor, and producing no discernible effect of loading frequency. Microstructural observations revealed that during this phase cavity nucleation and growth occurred and accumulated ahead of the crack tip owing to the combined action of three time-dependent events: creep, oxidation and crystallisation of the intergranular glass phase. The observed crack growth behaviour during this stage can be explained by crack tip blunting due to cavitation and glass flow and by a reducing bonding force of the glass phase across the crack surfaces due to the combined effects of oxidation, creep and crystallisation of the intergranular glass phase.