Eva Kuzielová

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.

Potential Applications of Glass and Glass-Ceramics Based on Lithium Disilicate as Biomaterials

Lithium disilicate glass and glass with addition of P2O5, CaO and CaF2 (in relative ratios corresponding to 10 wt. % of „apparent fluoroapatite”) were prepared by quenching their melts obtained at 1400 °C. Then, the parent glasses were thermally treated at 550°C and 750°C to obtain glass ceramics. The bioactivity test in vitro and the Wicker hardness in relation to „apparent fluoroapatite“ presence and heat treatment were investigated. The presence of fluoroapatite in samples promotes mineralization of new phase apatite-like on the surface of glass and glass ceramics after 6-week immersion in SBF as proved by SEM and EPMA. The bioactivity decreases with thermal treatment of parent glass. However, whole surface of glass-ceramics was covered with apatite phase after long-term immersion in SBF. The Vickers hardness of samples increases with increasing temperature treatment and with the presence of fluoroapatite.