Microstructural changes due to heat-treatment of annealing and their effect on the creep behavior of self-reinforced silicon nitride ceramics

In order to understand the improvement of creep resistance by furnace and microwave annealing, we have investigated the effect of heat-treatment on the microstructural characteristics of the crept self-reinforced silicon nitride (Si3N4) ceramic GS44. X-ray diffraction was performed on the as-sintered and heat-treated samples to study the phase changes due to annealing treatment. Optical microscopy and scanning electron microscopy (SEM) were used to study the fracture surface and to identify the creep mechanism. High resolution and analytical transmission electron microscopy (TEM) were employed to analyze the microstructures of the crept samples with as-received and heat-treated conditions. It has been reported that both conventional furnace and microwave annealing enhance the creep resistance of the material, and microwave annealing had the most significant effect. Fractography showed that the microwave annealed samples exhibits least creep damage. Furnace annealing also reduces the creep damage, but the effect is far less as compared to microwave annealing. Scanning electron microscopy (SEM) analysis showed significant amount of multiple-junction cavitation in the creep-tested samples. TEM observations showed significant devitrification of the amorphous phases in the microwave annealed specimens, as verified by micro-diffraction studies of the junction phases. This is also confirmed by X-ray diffraction and high-resolution lattice image of the triple junction phases. The microstructural observations were combined with a recent model of the effect of amorphous residues in ceramics on the creep behavior to explain the improvement in the creep resistance due to annealing.