Synthesis of fibrous (beta)-Si3N4 structured porous ceramics using carbothermal nitridation of silica

Porous Si3N4 ceramics with a fibrous interlocking microstructure were synthesized directly by carbothermal nitridation (CT/N) of SiO2. Carbon black was used as the carbon source and (alpha)-Si3N4 was used as seed. The cold-pressed samples of powder mixtures were heated at nitrogen pressures above 0.6 MPa and temperatures exceeding 1600 °C. The addition of the (alpha)-Si3N4 to the initial powder mixtures had important effects on the microstructure and mechanical properties of the porous Si3N4. Fine elongated fibrous (beta)-Si3N4 grains were developed in the seeded samples with stoichiometric C-SiO2 ratio and Y2O3 as the sintering additive, when sintered at 1700-1750 °C. A sample of outstanding strength resulted, five times stronger than the seed-free samples. Such a technique offers the possibility of synthesizing highly porous and strong Si3N4 materials at considerably lower cost than at present.