The Fluorine Effect on the Grain-Growth of -SiAlON Ceramics

SiAlON ceramics are of major interest for industrial applications due to their excellent mechanical properties. The starting Si₃N₄ powder (α or β), the type of sintering additive and sintering conditions govern the microstructural evolution of SiAlON ceramics. In this study, α^ı:β^ı SiAlON ceramics with an equal mole intergranular phase content, different type cation systems (Y:Sm:Ca and Ca:Ce) with different molar ratio (30:64:6 and 33:67) were produced by using coarse (d₅₀=1 µm) β-Si₃N₄ powder. CaF₂ and CaCO₃ were utilized as Ca sources. An increase in α^ı-SiAlON amount in the case of employing CaF₂ as opposed to CaCO₃ in Y:Sm:Ca cation system was observed. Despite the fact that
α^ı-SiAlON phase cannot be stabilized in Ca:Ce (CaCO₃ as a Ca source) system,
α^ı-SiAlON phase stability and unpredictable α^ı-SiAlON grain-growth in both a and c directions was easily achieved when use of CaF₂. Furthermore, using CaF₂ instead of CaCO₃ gives rise to an increase in densities. Therefore, the impressive effect of CaF₂ on abnormal grain growth and α^ı-SiAlON phase stabilization was discussed.