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 m icrowave annealing, we have investigated the effect of heat-treatment on th e microstructural characteristics of the crept self-reinforced silicon nitr ide (Si3N4) ceramic GS44. X-ray diffraction was performed on the as-sintere d and heat-treated samples to study the phase changes due to annealing trea tment. Optical microscopy and scanning electron microscopy (SEM) were used to study the fracture surface and to identify the creep mechanism. High res olution and analytical transmission electron microscopy (TEM) were employed to analyze the microstructures of the crept samples with as-received and h eat-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 th at the microwave annealed samples exhibits least creep damage. Furnace anne aling 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 sam ples. TEM observations showed significant devitrification of the amorphous phases in the microwave annealed specimens, as verified by micro-diffractio n studies of the junction phases. This is also confirmed by X-ray diffracti on and high-resolution lattice image of the triple junction phases. The mic rostructural observations were combined with a recent model of the effect o f amorphous residues in ceramics on the creep behavior to explain the impro vement in the creep resistance due to annealing. (C) 2001 Elsevier Science B.V. All rights reserved.