The effect of fabrication parameters on the mechanical properties of sintered reaction bonded porous Si3N4 ceramics

Porous silicon nitride ceramics were prepared via sintered reaction bonded silicon nitride at 1680 °C. The grain size of nitrided Si3N4 and diameter of post-sintered β-Si3N4 are controlled by size of raw Si. Porosity of 42.14–46.54% and flexural strength from 141 MPa to 165 MPa were obtained. During post-sintering with nano Y2O3 as sintering additive, nano Y2O3 can promote the formation of small β-Si3N4 nuclei, but the large amount of β-Si3N4 (>20%) after nitridation also works as nuclei site for precipitation, in consequence the growth of fine β-Si3N4 grains is restrained, the length is shortened, and the improvement on flexural strength is minimized. The effect of nano SiC on the refinement of the β-Si3N4 grains is notable because of the pinning effect, while the effect of nano C on the refinement of the β-Si3N4 grains is not remarkable due to the carbothermal reaction and increase in viscosity of the liquid phase.