Analysis of grain boundaries for reaction-bonded silicon nitride with yttria addition

Abstract

Silicon nitride is often the developmental ceramic of choice for high-temperature engine components. Sintering this ceramic, however, is very difficult due to the covalent nature of the bonding. Complicating the matter, Si3N 4 tends to dissociate at temperatures above 1700 ° C. Dissociation can be impeded by application of a nitrogen overpressure, while the general sintering problem is overcome by the addition of small amounts of various oxides. These sintering aids react with the silicon nitride and each other to form liquid phases, allowing sintering at temperatures between 1600 and 1900°C. Common additives include magnesia, yttria, alumina, and their various combinations. Normal additive content is in the range of 1 to 10% by weight. Although these sintering aids produce highly dense structures, they also lead to the formation of secondary phases along grain boundaries. The resulting materials are very strong at room temperature, but they rapidly lose their strength at high temperature (> 1000°C) because high-temperature mechanical properties such as creep are controlled by the grain boundary phases. The grain-boundary phases, in turn, are determined by the quantity and combination of sintering aids, by the firing cycle, and by the densification process (e.g. hot-pressed or reaction-bonded). Thus, by correlating the microstructure with the physical properties and processing/composition with microstructure, the effect of processing and compositional factors upon the physical properties can be determined and controlled. Although MgO was initially used as a sintering aid in the past, Y203 and YaO3-A1203 yield greater hightemperature strength [1, 2]. Yttria is an attractive additive because at low temperatures it reacts with the SiO2 (and some Si3N4) on the surface of the Si3N4 to form a liquid, greatly enhancing the sintering rate [3]. At higher temperatures when densification is complete, the liquid reacts with more Si 3 N 4 to give a highly refractory bonding phase along the grain boundaries. Tsuge et al. [4] showed that crystallization of the grain-boundary phase could improve high-temperature strength. Investigations of Si3N4-Y203 and Si3Na-Y203SiO2 phase diagrams [3, 5, 6] have been used to predict and explain the various grain-boundary constitutents and resulting properties. Using hot-pressed materials, Lange and co-workers [5, 7] found that the phases within the Si3N4 Si2N20 Y2Si207 compatibility triangle (in the Si3N4 Y203 SiO2 phase diagram) were extremely oxidation resistant, while Si3Y203N4, YSiOzN (K-phase), YxoSiyO23N4 (H-phase) and Y48i207N2 (J-phase) readily oxidize at 1000 ° C. Richer-