Ceramics-Silikáty 45 (3) 89-96 (2001)

CRYSTALLIZATION BEHAVIOUR OF FLUORPHLOGOPITE GLASS CERAMICS

E. M. A. Hamzawy 

Transparent fluorphlogopite based glasses were prepared using talc, glass sand and phosphate raw material. The effect of TiO₂, ZrO₂, phosphate or combinations of either B₂O₃-phosphate or TiO₂-ZrO₂ on the crystallization of fluorphlogopite base glass was examined. X-ray diffraction analysis showed formation of fluorphlogopite at 765 °C in the base glass. Addition of ZrO₂ or 5 wt.% phosphate did not affect the monophase crystallization. However, when the amount of phosphate was increased to 10 wt.%, fluorapatite was crystallized together with fluorphlogopite. TiO₂ and TiO₂-ZrO₂ addition enhance the formation of enstatite together with fluorphlogopite. Thermal treatment of ZrO₂-containing glasses was found to induce crystallization of fluorphlogopite as a main phase. The addition of TiO₂ alone or mixed with ZrO₂ catalyzed crystallization of enstatite and fluorphlogopite above 1000 °C. SEM analysis showed that glasses containing fluorphlogopite acquired fine-grained microstructure at 650 - 765 °C. At higher temperature, 1000 °C, the crystal structure of fluorphlogopite was modified into interlocked flakes. Vicker’s hardness values of the glass-ceramics increased in presence of enstatite (7358 MPa) and decreased in the presence of fluorapatite (4905 MPa). Addition of B2O3 decreased the density of glass to 2.634 g/cm³ and glass-ceramic to 2.780 g/cm³ . Phosphate or ZrO₂ additions showed a reverse effect (2.864 g/cm³). Thermal expansion coefficients of fluorphlogopite glass-ceramic samples in temperature range 20 - 500 °C changed from 7.8x10^-6 °C^-1 in presence of fluorapatite to 9.2x10^-6 °C^-1 in case of the dominance of enstatite.

Keywords: Glass-ceramics, Fluorphlogopite, Eustantite, Fluorsilicate glasses