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

Abstract

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 °C and 1700 °C are discussed. In the presence of MgO, α-Si3N4 grains with equiaxed morphology resulted in an interconnected microstructure with spherical pores. By contrast, in the absence of MgO, an α-whisker-dominant microstructure resulted in pores of various morphologies. During the post heat-treatment process, the α-whiskers gradually vanished, and grains recrystallized as very fine β-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 β phase formation, grain coarsening and linear shrinkage. The development of coarse β-rods from α-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 ≈2% linear shrinkage and porosity of ≈30 vol%. Consequently, this ceramic exhibited the highest compressive strength of ≈10 MPa.