Abstract
Si3N4 ceramics with additive system (3mass% SiO2 + 3mass% MgO + 1-5mass% Y2O3) could be sintered to almost full density at relatively low temperature as 1650oC for 2 h under an ambient gas pressure of N2 without using a packing powder. The resulting materials have high bending strength as 980 MPa with low thermal conductivity of 30 W/m.K when 5mass% of Y2O3 was added. Based on the creation of low-temperature pressureless sintering without a packing powder, a novel two-step sintering (once firing) was proposed. Two-step sintering conducted by sintering at 1650oC under 0.1 MPa-N2 for 2 h for densification in the first-step. Followed by heated up to and kept at 1950oC for 8 h under 1.0 MPa-N2 in the second step. Glassy phase, needed for the densification in the first-step, was removed from the dense ceramic by evaporation of SiO, Mg and N2 gas in the second-step. The Si3N4 ceramics could be fabricated with relatively high thermal conductivity of 90 W/m.K. However, the decreasing in bending strength of 620 MPa after the two-step sintering was observed due to the effect of grain growth. The effects on mass loss, oxygen content and chemical composition were also observed.
