Effects of CaF2 on the sintering and crystallisation of CaO-MgO-Al2O3-SiO2 glass-ceramics

The effects of CaF2 (1-5 wt.%) on the glass structure, crystallisation behaviour, sinterability, microstructure, and properties of a CaO-MgO-Al2O3-SiO2 glass-ceramic system were investigated. The results show that the glass transition temperature, crystallisation peak temperature and the activation energy for crystallisation decreased with the addition of CaF2. The crystallisation of diopside was initiated at the particle boundaries and followed with progressive advancing towards the centre. The enhanced crystallisation for CaF2-containing samples was observed and is likely due to the depolymerisation of the glass structure. However, the enrichment of Ca-2F complexes resulted in fluorite phase precipitation at the crystallisation front of the diopside phase when the CaF2 addition exceeded 3 wt.%. The hindered viscous flow sintering from a lower crystallisation temperature led to a decline in the sintering shrinkage, the extent of which was proportional to the amount of added CaF2. Plastic deformation was observed along with the newly-formed amorphous phase as the sintering temperature approached the glass melting point, which benefits densification. The performance analysis reveals that the fabricated glass-ceramics with the proper addition of CaF2 exhibited significant improvements in its density and bending strength. Whereas, the Vickers hardness, water absorption and chemical resistance all decreased.