The influence of glass composition on nucleation crystallisation microstructure and properties of apatite-mullite glass-ceramics

Glasses from the system SiO2-Al2O3-P2O5-CaO-CaF2 can crystallise to fluorap atite(Ca-5(PO4)(3)F) and mullite (2SiO(2)3Al(2)O(3)). The present paper inv estigates the influence of fluorine content and calcium to phosphate (Ca:P) ratio on the nucleation and crystallisation behaviour, of apatite glass-ce ramics. Fluorine is critical to apatite formation and in the absence of flu orine P 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 fluorapatit e is favoured in glass compositions with a Ca:P ratio of 1.67 and by high f luorine contents. Glasses with Ca:P ratios lower or higher than 1.67 tend t o surface nucleate. However such glasses can be made to bulk nucleate by ho lding for one hour above Tg. Bulk nucleation occurs via prior amorphous pha se separation (APS). Mullite forms following crystallisation of apatite, but anorthite and an al uminium phosphate phase may also form depending on the glass composition. Compositions either side of the apatite stoichiometry give rise to less cry stal nucleation than compositions at the apatite stoichiometry and give ris e to apatite crystals that coarsen more readily on heat treatment. Biaxial flexural strength and fracture toughness depend on the apatite volume fract ion and size.