Vacuum heat treatment of LiAlO2 densified silicon nitride ceramics

The aim of the present work has been to produce high-dense Si3N4 ceramics by a cheaper pressureless sintering method and then to attain vacuum heat treatment to remove residual grain boundary glass in gaseous form. LiAlO2 was used as a sintering additive rather than using Li2O, since its grain boundary glass is not stable above 1200 °C. LiAlO2 was synthesised from 42% Li2CO3 and 58% Al2O3 powder mix reacting together at 1450 °C for 3 h in a muffle furnace. X-ray analysis showed that 95% LiAlO2 was obtained. LiAlO2 was milled and added to silicon nitride powder as a sintering additive. Hot-pressing and pressureless sintering of LiAlO2 containing Si3N4 compacts were carried out at temperatures between 1450–1750 °C. The sintered samples were vacuum heat-treated at elevated temperatures under high vacuum to remove intergranular glass and to increase refractoriness of Si3N4 ceramics. Scanning electron microscope images and weight loss results showed that Li in grain boundary glass (Li–Al–Si–O–N) was successfully volatilised, and oxidation resistance of the sintered samples was increased.