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 o n the microstructure and crystalline phases of glass-ceramics in the system SiO2-MgO-CaO-Na2O-K2O-F. Four glass compositions were prepared with magnes ium content varying from 12-18 wt%. The compositions were melted at 1400 de grees C for 2 h and cast into 30 x 8 mm ingots. Differential thermal analys es 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 d egrees C for 1-2 h and crystallized in the temperature range 900-1000 degre es C for 4-6 h. The crystalline phases were analyzed by X-ray diffraction. The microstructure was investigated by scanning electron microscopy, The Vi ckers hardness was determined after indentation under a 9.81 N load. Differ ential thermal analyses revealed that crystallization occurred in the tempe rature range 700-800 degrees C. X-ray diffraction showed the presence of fl uorrichterite as major crystalline phase regardless of the magnesium conten t in the parent glass or heat treatment temperature. The microstructure con sisted of interlocked acicular crystals (5-10 micrometers), The highest mag nesium content led to the coexistence of both a mica phase and fluorrichter ite. This microstructure promoted crack deflections and arrest. (C) 2000 Jo hn Wiley & Sons, Inc.