Abstract
The aim of the present work has been to produce high-dense Si 3N 4 ceramics by a cheaper pressureless sintering method and then to attain vacuum heat treatment to remove residual grain boundary glass in gaseous form. LiAlO 2 was used as a sintering additive rather than using Li 2O, since its grain boundary glass is not stable above 1200 °C. LiAlO 2 was synthesised from 42% Li 2CO 3 and 58% Al 2O 3 powder mix reacting together at 1450 °C for 3 h in a muffle furnace. X-ray analysis showed that 95% LiAlO 2 was obtained. LiAlO 2 was milled and added to silicon nitride powder as a sintering additive. Hot-pressing and pressureless sintering of LiAlO 2 containing Si 3N 4 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 Si 3N 4 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.
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