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

Si 3 N 4 ceramics were prepared using novel two-step sintering method by mixing α -Si 3 N 4 as raw material with nanoscale Y 2 O 3 –MgO via Y(NO 3 ) 3 and Mg(NO 3 ) 2 solutions. Si 3 N 4 composite powders with in situ uniformly distributed Y 2 O 3 –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 MgSiO 3 and densification at high temperature. Variations in O, Y, and Mg contents in Si 3 N 4 –Y 2 O 3 –MgO during first sintering step are discussed. O and Mg contents decreased with increasing temperature because SiO 2 on Si 3 N 4 surface reacted with MgO to form low-melting-point MgSiO 3 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 Y 2 Si 3 O 3 N 4 secondary phase occurred simultaneously. Two-step sintered Si 3 N 4 ceramics had lower total oxygen content (1.85 wt%) than one-step sintered Si 3 N 4 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 Si 3 N 4 ceramics prepared via two-step sintering increased by 28.7%, 16.9%, and 31.6%, respectively, compared with those of one-step sintered Si 3 N 4 ceramics.