Creep Damage in an Advanced Self-Reinforced Silicon Nitride: Part I, Cavitation in the Amorphous Boundary Phase

Microstructural defects in a commercial self-reinforced silicon nitride (SN 88M), after tensile creep at 1200°-1400°C under stresses that correspond to lifetimes that range from several hours to 10000 h, were investigated by using electron microscopy methods to reveal the type of cavities and mechanisms that control creep deformation. Creep damage is classified in terms of intergranular cavitation in the amorphous boundary phase and intragranular defects that are connected to the Si3N4 grains. Intergranular defects include irregular multigrain-junction cavities, oblate two-grain-junction cavities, microcracks, and creep cracks. Intragranular defects involve broken large Si3N4 grains, small symmetrical cavities of lenticular shape that are formed between two matrix grains, and small asymmetrical cavities and large cracklike cavities that both penetrate into the large Si3N4 grains. Very similar intergranular cavities and broken grains have been observed after both short-term and long-term tests, whereas intragranular cavities have been observed only in long-term tests. Cavitation in multigrain junctions is thought to be controlled by dilatant hydrostatic tensile stresses that develop in dense microstructures after any grain-boundary sliding (GBS). Different sliding rates between large reinforcing grains and small matrix grains have been proposed to generate local tensile stresses that result in cavitation on the facets of large grains. The local tensile stresses that occur at irregularities of two sliding interfaces seem to control the formation of two-grain-junction cavities. Although the controlling stresses differ, GBS and viscous flow are the common mechanisms for all types of intergranular cavities. Intragranular cavitation is fundamentally different and requires local dissolution of Si3N4 in the surrounding glassy phase. Comparison of the apparent density and dimensions of different types of cavities, even without direct measurement of cavitation kinetics, suggests that the multigrain-junction cavities provide the main contribution to the total volume fraction of cavities, whereas two-grain-junction cavities and the lenticular-type cavities have minor importance. The role of intragranular cavities in the large grains in cavitational strain is negligible. Full-facet-size cavities and multigrain-junction cavities can form within a period of <1 h. Numerous very small cavities, which coexist with the large cavities of both types that are observed after very long creep tests, indicate a continuous cavitation during the entire creep deformation. By using a dilatational model, tensile strain has been proposed to result from the continuous addition of multigrain-junction cavities of approximately uniform size, which explains the linear dependence of cavitational strain on the tensile strain that has been reported for this material. GBS and viscous flow, which are considered as being the mechanisms for intergranular cavitation, are controlled by the amount and viscosity of the residual amorphous phase. Therefore, improvement of the creep resistance in silicon nitride ceramics can be expected from the engineering of the residual glass.