INFLUENCE OF SHEAR RATE AND TEMPERATURE ON THE CRYSTALLIZATION OF SPONTANEOUS CRYSTALLIZING GLASS-CERAMIC

The influence of shear rate and temperature on the viscosity and microstructure of phlogopit glass-ceramic during cooling is investigated. Viscosity was measured in the crystallization temperature range. These experiments were carried out using a high-temperature Couette viscometer specially developed for this investigation. All samples were sectioned, polished, and examined by light microscopy after cooling to room temperature. It is shown that the viscosity of the melt decreases with increasing shear rate because the initial platelets (glimmer crystals) were fragmented by shearing. The resulting glass-ceramic has a high density of broken short crystals. Such a structure has not previously been recorded for this material. A mathematical model to calculate the viscosity of crystallizing glass-ceramic depending on shear rate, temperature and other parameters is presented. This model is based on two modified first-order ordinary differential equations to describe crystal nucleation and crystal growth, and an equation to calculate the viscosity of the resulting suspension. The model also includes a description of crystal fragmentation as a first trial. The equations are coupled by melt viscosity. Numerical solutions are discussed.