SOME STUDIES ON NUCLEATION, CRYSTALLISATION, MICROSTRUCTURE AND MECHANICAL PROPERTIES OF MICA GLASS-CERAMICS IN THE SYSTEM 0.2BaO.0.8K2O.4MgO.Al2O3.6SiO2.2MgF2

The crystallisation, microstructure, microhardness and theoretical machinability of the glass-ceramics were investigated by DTA, XRD, SEM and microhardness indentor. Two distinct crystallisation exotherms in the DTA curve were observed and resolved. The first peak corresponded to the initial formation of potassium fluorophlogopite and the second was due to the formation of barium fluorophlogopite. The activation energy for precipitation of each crystalline phase was evaluated and the crystallisation mechanism was studied. DTA analyses were conducted at different heating rates and the activation energy was determined graphically from Kissinger and Ozawa equations. The average activation energy was calculated as 276 KJ/mol for the first and 366 KJ/mol for the second crystallisation peak. The Avrami exponent for first and second crystallisation peak temperatures, determined by the Augis and Bennett method, were 3 and 3.9, respectively. The results indicated that the growth of mica was a two and three dimensional process, controlled by the crystal-glass interfacial reaction. The Vickers hardness decreased steadily at intermediate heat treatment temperature with the formation of barium and potassium fluorophlogopite phase, but the decrease in hardness was more rapid at higher temperatures with the development of an interconnected "house of cards" microstructure.