Influence of LiF on the Sintering of Si3N4 Ceramics Containing SiO2–CaO–Al2O3

Abstract

This study investigates the influence of LiF on the sintering behavior of Si3N4 ceramics with SiO2–CaO–Al2O3 at the eutectic composition. Compositions containing 90 wt.% Si3N4 were sintered at 1650–1815 °C for 1 hour. The ceramics were characterized in terms of density, microstructure, and hardness. At 1650 °C, the β/(α+β)-Si3N4 ratio rose from 28.4 % (0 wt.% LiF) to 63.7% (2 wt.% LiF), exceeded 98 % at 1700 °C, and reached 100 % at ≥ 1770 °C. In LiF-free samples, higher sintering temperatures resulted in a higher relative density (95.6 ± 0.4%) and hardness (12.99 ± 0.42 GPa). LiF improved densification and hardness at low content and sintering temperature, yielding 89.4 ± 0.3 % relative density and 9.71± 0.42 GPa hardness (1 wt.% LiF) compared to 84.1 ± 0.5 % and 6.78 ± 0.29 GPa (0 wt.% LiF), both at 1650 °C. At higher temperatures, LiF decreased both properties, likely due to its tendency to evaporate.