Reaction sequences occurring in dense Li-doped sialon ceramics: influence of temperature and holding time

摘要

Spark Plasma Sintering (SPS) has been used to consolidate a lithium-doped duplex alpha-beta sialon with the overall composition Li0.5Si9.5Al2.5O2.0N14. The kinetics of densification has been studied, and the phase transformation, reactions and grain growth occurring in the dense compacts upon further heat treatment have been investigated. Two sources of Si3N4 powders were used, namely alpha- and beta-Si3N4. Green bodies heated at a rate of 100 degreesC min(-1) yielded fully dense compacts at 1450 degreesC (alpha-Si3N4) and 1500 degreesC (beta-Si3N4) without holding, and these compacts consisted mainly of a locally formed liquid and precursor Si3N4 particles. Upon further heating it was observed that alpha-sialon is formed initially, irrespectively of whether alpha-Si3N4 or beta-Si3N4 powder is used as Si3N4 source; and when alpha-Si3N4 is used as starting powder, almost monophasic alpha-sialon compacts are formed before any transformation to beta-sialon takes place on further heating. When beta-Si3N4 is used as starting powder the formation of beta-sialon is kinetically promoted, and compacts containing alpha-sialon, beta-sialon and beta-Si3N4 are obtained before the equilibrium phase assemblage is reached, i.e. a lithium-doped duplex alpha-beta sialon ceramic. These observations can be interpreted in terms of the Ostwald step rule. Grain growth does not occur until the equilibrium phase assemblage has been established. The separation of grain growth from densification and phase transformation has implications for preparing Si3N4-based nano-ceramics and provides possibility for further studies of the kinetics of grain growth in Si3N4-based ceramics. [References: 13]