Mechanism and kinetics of glass-ceramics formation in the LiO2-SiO2-CaO-P2O5-CaF2 system

Two glasses based on lithium disilicate (LS₂), with and without fluorapatite (FA), were synthesised in the Li₂O-SiO₂-CaO-P₂O₅-CaF₂ system with P₂O₅: CaO: CaF₂ ratios corresponding to fluorapatite. Glass-ceramics have then been prepared by thermal treatment. The mechanism and kinetics of crystallization as functions of grain size and rate of heating were investigated using thermal analysis methods. The smaller particles crystallize preferentially by surface crystallization, which is replaced by volume crystallization at larger particle sizes. Inclusion of FA in the LS₂ favours crystallization through the surface mechanism. The onset limit for volume crystallization replacing the surface mechanism is at about 0.3 mm for pure LS₂ glass and 0.9 mm for glass containing FA. The calculated activation energies of the glasses (₂99 ± 1 kJ mol-1 for pure LS₂ glass and ₂88 ± 7 kJ mol⁻¹ for glass containing FA according to Kissinger, or 313 ± 1 kJ mol-1 for pure LS₂ glass and 303 ± 8 kJ mol-1 for glass containing FA according to Ozawa) indicate that the tendency of the glasses to crystallize is supported by the FA presence. Bioactivity of all samples has been proved in vitro by the formation of new layers of apatite-like phases after soaking in SBF.