Crystallisation in apatite-mullite glass-ceramics as a function of fluorine content

Apatite-mullite glass-ceramics are materials prepared by the controlled heat-induced devitrification of glasses of suitable composition and are under investigation for applications in dentistry and orthopaedics. The glasses used here are based on a system with the composition 1.5(5-x)SiO₂· (5-x)Al₂O₃·1.5P₂O₅·(5- x)CaO·xCaF₂. The amount of fluorine in the glasses was varied to investigate the crystallisation behaviour as a function of both fluorine content and temperature. The resultant crystalline phases are fluorapatite [Ca₁₀(PO₄)₆F₂], mullite [Al ₆Si₂O₁₃] and in some cases, anorthite [CaAl₂Si₂O₈]. Crystal phases were identified using X-ray diffraction (XRD) from both the surface and the bulk of heat-treated monolithic samples and scanning electron microscopy (SEM) was used to image the crystal phase morphologies. Crystallisation characteristics varied widely in terms of apparent nucleation mechanism, crystal phases formed and microstructure. In general, glasses with higher fluorine content devitrified more readily to fluorapatite (FAp) with a higher nucleation density and for glasses with an intermediate to low fluorine content there tended to be an interdependence between FAp and mullite crystallisation. A greater tendency towards anorthite formation, especially at surfaces, was observed for glasses with lower fluorine contents. Furthermore, on decreasing the fluorine content, glasses tended to crystallise by formation of FAp spherulites with increasing diameter and with greater crystal aspect ratio.