Thermal and mechanical properties of Si3N4 ceramics obtained via two-step sintering

Si3N4 ceramics were prepared using novel two-step sintering method by mixing alpha-Si3N4 as raw material with nanoscale Y2O3-MgO via Y(NO3)(3) and Mg(NO3)(2) solutions. Si(3)N(4 )composite powders with in situ uniformly distributed Y2O3-MgO were obtained through solid-liquid (SL) mixing route. Two-step sintering method consisted of pre-deoxidization at low temperature via volatilization of in situ-formed MgSiO3 and densification at high temperature. Variations in O, Y, and Mg contents in Si3N4-Y2O3-MgO during first sintering step are dis-cussed. O and Mg contents decreased with increasing temperature because SiO2 on Si3N4 surface reacted with MgO to form low-melting-point MgSiO3 compound, which is prone to volatilize at high temperature. By contrast, Y content hardly changed due to high-temperature stability of Y-Si-O-N quaternary compound. In the second sintering step, skeleton body was densified, and the formation of Y2Si3O3N4 secondary phase occurred simultaneously. Two-step sintered Si3N4 ceramics had lower total oxygen content (1.85 wt%) than one-step sintered Si3N4 ceramics (2.51 wt%). Therefore, flexural strength (812 MPa), thermal conductivity (92.1 W/m.K), and fracture toughness (7.6 MPa.m(1/2)) of Si3N4 ceramics prepared via two-step sintering increased by 28.7%, 16.9%, and 31.6%, respectively, compared with those of one-step sintered Si3N4 ceramics.