Pore structure, porosity and compressive strength of highly porous reaction-bonded silicon nitride ceramics with various grain morphologies

Complex characteristics of the pores and properties of porous reaction-bonded Si3N4 have been investigated and correlated with the microstructure of Si3N4 grains. Porous ceramics with porosities of ≤ 75 vol% and α-Si3N4 matte grains (α/β phase ratio of 1.5) or α-Si3N4 whiskers (α/β phase ratio of 0.36) were prepared by in situ nitridation of silicon powder. To obtain various microstructures by α → β-phase transformation and grain morphology modification, samples were heat-treated at 1700 °C while embedded in a Si3N4 powder bed containing MgO. By the growth of α-matte or β-Si3N4 grains on the pore walls, highly interconnected structures with spherical cavities and unimodal pore size distributions resulted with d50 ≈ 8.8 µm and ≈ 6.5 µm, respectively. In contrast, α-whiskers grew inside the pore cavities; thus, complex and irregular inter-particle pores appeared which generated an extra peak near d50 ≈ 1 µm forming a bimodal pore size distribution. Compared to the α-matte grains, α-whiskers densified upon heat treatment and produced a large drop in porosity, which resulted in a structure with less interconnectivity. As a consequence of growth of fine β-rods, pore walls became relatively smooth and whisker free; thus, inter-cluster channels were modified to spherical cavities with d50 ≈ 3.7 µm. Samples exhibiting networked whiskers and fine pores or low porosity demonstrated higher compressive strength than the interconnected structures with spherical cavities.