Effect of Oxide Composition on Glass Viscosity and Its Relation to High Temperature Strength of Si3N4

Abstract

Tests were performed on Al2O3-Y2O3 based compounds, which are the most widely used sintering additive for Si3N4, with the objectives of evaluating the high-temperature viscosity characteristics of Al-Y-Si-O based glass and clarifying the relationship between high-temperature viscosity and the high-temperature strength of Si3N4. The viscosity of the glass at 1600°C was found to be about 8-50P, while the activation energy for high-temperature viscosity was about 9-37 kJ/mol. Properties of glass such as viscosity are governed by the composition of the glass; for compositions within the glass forming region, it is thought that the network structure, hardens and viscosity becomes high in compositions having a substantial amount of SiO2, or in compositions where the ratio of Y2O3/Al2O3 (close to the eutectic composition) is 1.0. For compositions outside the glass forming region, the influence of the precipitation of the crystal phase is considered to become more prominent. The room temperature strength of Si3N4 is not governed by the properties of the glass phase, but by micro structural factors such as the residual amount of pores. However, the properties of the glass phase in the sintering process are considered to determine structural factors such as the density of sintered bodies and the morphology of Si3N4 grains.The high-temperature strength of Si3N4 is dependent on the high temperature viscosity characteristics of the glass phase, and this was seen to impart high high-temperature strength to Si3N4 for which glass with high high-temperature viscosity would exist at the grain boundaries. For Si3N4 with similar viscosity characteristics at high temperatures, high-temperature strength is dependent on the size of the Si3N4 grains, seen to result in superior high-temperature strength for Si3N4 with large Si3N4 grains. Correlation between the rate of change in 1300°C strength and the activation energy of viscosity for Si3N4 was observed when evaluated in high temperature strength tests conducted at slow cross-head speeds of 0.1mm/min to 0.5 mm/min, and it was found that the rate of change in the 1300°C strength tended to become smaller for Si3N4 compositions with lower activation energy.