SYNTHESIS OF GLASS-CRYSTAL MATERIALS IN THE SYSTEM OF CaO.MgO.SiO2-Al2O3-SiO2 WITH HIGH VALUES OF MICROHARDNESS

Abstract

The glass-forming region of the CaO • MgO • SiO₂-Al₂O₃-SiO₂ system and the fusibility diagram of the pseudo-binary section of CaO • MgO • 2SiO₂-3Al₂O₃ • 2SiO₂ with a metastable liquation region have been studied. CaF₂ has been additionally introduced into the system resulting in reducing the temperature of liquidus system and expanding the range of metastable liquation. It has been revealed that the process of glasses crystallization in the indicated section is accompanied by the formation of silicates belonging to melilite group: gelenite Ca₂Al (AlSiO₇) and ackermanite Ca₂Mg (Si₂O₇) with twintetrahedrals forming a continuous series of solid solutions and differing in various values of density and thermal expansion.

Electron spectroscopy study of the tempered glasses microstructure has discovered that within the region of metastable liquation, the concentrations of elements at different points of two-phase glass are significantly variable: in at.% Si –2.62; Al-1.24.

It was established that during a low-temperature heat treatment of glasses containing

5 mol% CaF₂ and MgF₂, a liquation inhomogeneous structure characterized by the periodicity of the second vitriform phase locations (the size of microphase sections is up to 80 ... 120 μm) with the phase boundaries of different sizes from 5 to 10 microns is formed. It is observable that an increase in the content of 3Al₂O₃ • 2SiO₂ in the initial glass with metastable liquation leads to a sharp increase in density and a continuous increase in optical density during their heat treatment in the precrystallization period.

It has been revealed that an increase in the temperature coefficient of linear expansion (TCLE) and microhardness (Hv) of glass metals is conditioned by the increasing of the molar volume of the residual vitriform matrix and the amounts of less strong Si - O - Me bonds. It has been shown that glassceramics having high value of thermal expansion (up to 90·10^-71/⁰C) and micro-hardness (750…780 kg/mm²) can be produced by a high temperature heat-treatment of glasses with liquation structure.