Microstructure of SiO₂−Al₂O₃−CaO−P₂O₅−K₂O−F⁻ Glass Ceramics. 2. Time Dependence of Apatite Crystal Growth

It is shown that fluorapatite needles formed in SiO 2 −Al 2 O 3 −CaO−P 2 O 5 −K 2 O−F - glasses upon heat treatment at 1200 °C undergo a time-dependent coarsening that fulfills the criteria of Ostwald ripening. Thermodynamic considerations imply that different growth mechanisms (normal and spiral growth) along different crystallographic directions are responsible for the strongly direction-dependent growth rates observed at 1200 °C, leading to an apatite-needle aspect ratio as large as 15:1 after 15 h at 1200 °C. The microstructure is textured by postcrystallization extrusion, facilitating a thorough investigation of the apatite needle morphology and the determination of anisotropic crystal-growth rates. During very long-term heat treatments at 1200 °C, however, the formation of isometric, blocklike apatite crystals is encountered. The latter crystals are formed at the expense of apatite needles and their occurrence proves that the needle morphology is not a result of the equilibrium shape but caused by kinetic growth effects. But although the transition from the growth habit to the equilibrium shape limits the maximum aspect ratio of fluorapatite needles (15:1), the extruded glass ceramics exhibit an extraordinary degree of texturing.