Influence of LiF on the Sintering of Si3N4 Ceramics Containing SiO2-CaO-Al2O3: Densification, Microstructure, and Mechanical Properties

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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 1650-1815ºC for 1 hour. The ceramics were characterized in terms of density, microstructure, and hardness. At 1650ºC, the β/(α+β)-Si3N4 ratio rose from 28.4% (0 wt. % LiF) to 63.7% (2 wt. % LiF), exceeded 98% at 1700ºC, and reached 100% at ≥1770ºC. 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 1650ºC. At higher temperatures, LiF decreased both properties, likely due to its tendency to evaporate.

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