The Influence Of Glass Composition Of Nucleation Crystallization Microstructure And Properties Of Apatite-Mullite Glass Ceramics

Glasses from the system SiO2-Al2O3-P2O5-CaO-CaF2 can crystallise to fluorapatite (Ca5(PO4)3F) and mullite (2SiO23Al2O3). The present paper investigates the influence of fluorine content and calcium to phosphate (Ca:P) ratio on the nucleation and crystallisation behaviour, of apatite glass-ceramics. Fluorine is critical to apatite formation and in the absence of fluorine B tricalcium phosphate is formed rather than fluorapatite. Increasing fluorine content results in a decrease in glass transition temperature (Tg) and peak crystallisation temperatures(Tc)s. Bulk nucleation of fluorapatite is favoured in glass compositions with a Ca:P ration of 1.67 and by high fluorine contents. Glasses with Ca:P ratios lower or higher than 1.67 tend to surface nucleate. However such glasses can be made to bulk nucleate by holding for one hour above Tg. Bulk nucleation occurs via prior amorphous phase separation (APS). Mullite forms following crystallisation of apatite, but anorthite and an aluminium phosphate phase may also form depending on the glass composition. Compositions either side of the apatite stoichiometry and give rise to apatite crystals that coarsen more readily on heat treatment. Biaxial flexural strength and fracture toughness depend on the apatite volume fraction and size.