MICROSTRUCTURAL EVOLUTION IN ANNEALED AND CREPT SILICON-NITRIDE

The microstructure of a commercial sintered silicon nitride has been e xamined in the as-sintered condition, after annealing to a stable micr ostructure in either air or argon, and after creep deformation. Both a s-sintered material and material annealed to a stable microstructure w ere crept. Extensive analytical electron microscopy has been used to d etermine the composition of the intergranular material (amorphous and crystalline). The as-sintered material contains an amorphous intergran ular phase which partially devitrifies upon exposure to high temperatu res. The residual intergranular glass is homogeneous throughout the sa mples (even though different crystalline products form near the surfac e and in the center of samples annealed in air) and stable. Creep defo rmation does not affect the microstructural evolution. It is, however, responsible for internal strain and some cavitation in the material.