The Effect of $P_2O_5$ and Heat Treatment on the Crystallization of $Li_2O \cdot 2SiO_2 - BaO \cdot 2SiO_2 (LS_2 - BS_2)$ Glass

In this paper, the effects of $P_2O_5$ and heat treatment on the crystalline phases and microstructure of lithium disilicate-barium disilicate glass were examined. A wider and broad peak in the differential thermal analysis curve indicates a presence of surface crystallization instead of volume crystallization despite the use of nucleating agent, $P_2O_5$. The heat treatment schedules were planned according to differential thermal analysis data. The controlled crystallization of the compositions studied was carried out using two-stage heat treatment procedure. The glass transition temperature, $T_{g}$ of the as-cast sample were used to determine the optimum nucleation temperature. The optimum nucleation temperature was determined to be 520°C. The crystallization was carried out at 720C and 880C for 15 min. Lithium disilicate and sanbornite was the major phases and moganite or coesite were also present depending on the heat treatment duration. Due to coexistence of lithium disilicate and barium disilicate phases, the change in the melting entropy prevented the formation of spherulitic morphology and recrystallization after further heat treatments. The rise in the crystallization temperature enhanced grain coarsening and formed massive microstructures.