Solid-Gas Reaction during Sintering of Si3N4 Ceramics. (Part 1). Stability of SiO2, Y2O3 and Al2O3 at High Temperature.

Abstract

The stability during sintering of SiO2, Y2O3 and Al2O3, well known as sintering aids of Si3N4 ceramics, was examined from both thermodynamical and experimental aspects. Both the heat treatment of SiO2-Y2O3-Al2O3 glass and the sintering of Si3N4 ceramics were carried out. In the thermodynamical considerations, the stability of the glass composition was discussed on the basis of known volatility diagrams for SiO2, Al2O3 and MgO and the thermodynamic data of Y2O3 available in the literature. The results suggest that Y2O3 is the most stable among all materials in the sintering temperature range usually selected for Si3N4 ceramics. SiO2-Y2O3-Al2O3 (equi-mass%) glass was heat-treated in a BN sagger at 2023K for 2h in N2 gas of 0.1 MPa. Si3N4 with addition of 5 mass% Y2O3 and 3 mass% Al2O3 was sintered in the BN sagger at 2023K for 0 to 16h in N2 gas of 0.1MPa. The elemental contents in the glass and ceramics before and after heat treatment and sintering were determined by chemical analysis. As a result, it was found that only SiO2 in the glass and SiO2 and Si3N4 in the Si3N4 ceramics vaporized. On the other hand, the Y2O3 and Al2O3 contents in both the glass and Si3N4 ceramics did not decrease under the selected temperature.