Effect of magnesium content on the microstructure and crystalline phases of fluoramphibole glass-ceramics

The purpose of this study was to evaluate the effect of magnesium content on the microstructure and crystalline phases of glass-ceramics in the system SiO2-MgO-CaO-Na2O-K2O-F. Four glass compositions were prepared with magnesium content varying from 12-18 wt%. The compositions were melted at 1400 degrees C for 2 h and cast into 30 x 8 mm ingots. Differential thermal analyses were performed on the powdered glasses at a heating rate of 20 degrees C/minute up to 1400 degrees C. Bars (4 x 8 x 25 mm) were cut from the ingots with a low-speed diamond saw, nucleated in the temperature range 600-650 degrees C for 1-2 h and crystallized in the temperature range 900-1000 degrees C for 4-6 h. The crystalline phases were analyzed by X-ray diffraction. The microstructure was investigated by scanning electron microscopy, The Vickers hardness was determined after indentation under a 9.81 N load. Differential thermal analyses revealed that crystallization occurred in the temperature range 700-800 degrees C. X-ray diffraction showed the presence of fluorrichterite as major crystalline phase regardless of the magnesium content in the parent glass or heat treatment temperature. The microstructure consisted of interlocked acicular crystals (5-10 micrometers), The highest magnesium content led to the coexistence of both a mica phase and fluorrichterite. This microstructure promoted crack deflections and arrest. (C) 2000 John Wiley & Sons, Inc.