Strengthening low-density silicon nitride beads via an inorganic polymer route

Low-density silicon nitride-containing spheroidal beads 3–4 mm in diameter were made by an alginate route from Si3N4 powder. After pyrolysis of organics and volatiles at 500°C, the resulting porous Si3N4 beads were vacuum infiltrated with an inorganic polyureasilazane polymer/catalyst solution. Thermal curing of the incorporated polymer followed by pyrolysis of the beads in NH3 and sintering in N2 converted the polymer to Si3N4. Diametral compression strength tests showed the resulting Si3N4 beads to be significantly stronger than sintered, alginate-derived Si3N4 beads without polymer infiltration.