Microstructural evolution mechanism of porous reaction bonded silicon nitride ceramics heat-treated in two powder beds

Porous Si3N4 with various grain morphologies was prepared by direct nitriding of silicon powder and subjected to heat-treatment while embedded in a Si3N4 powder bed. The influence of MgO, added either to the starting silicon or to the powder bed, on the microstructural transformation and morphology of pores at temperatures between 1425 degrees C and 1700 degrees C are discussed. In the presence of MgO, alpha-Si3N4 grains with equiaxed morphology resulted in an interconnected microstructure with spherical pores. By contrast, in the absence of MgO, an alpha-whisker-dominant microstructure resulted in pores of various morphologies. During the post heat-treatment process, the alpha-whiskers gradually vanished, and grains recrystallized as very fine beta-Si3N4 rods. Consequently, pores became spherical, large and whisker-free. In agreement with the results of SEM-EDX along with XRD analysis, the observed morphology transition and full phase transformation occurred by vapor phase transport of MgO from the powder bed and a subsequent solution-precipitation mechanism. The presence of volatile MgO in the powder bed caused a substantial decrease in weight losses, while enhancing beta phase formation, grain coarsening and linear shrinkage. The development of coarse beta-rods from alpha-matte grains in a Si-Mg-O-N glassy phase was related to the presence of substantial liquid phase during the growth mechanism. Compared to the granular morphology, whiskers with high aspect ratio gave rise to a high sintering driving force and led to a maximum value of approximate to 2% linear shrinkage and porosity of approximate to 30 vol%. Consequently, this ceramic exhibited the highest compressive strength of approximate to 10 MPa.