Preparation, Reactive Crystallization Mechanism And Properties Of Machinable Fluoamphibole Glass-ceramics

A novel route-reactive crystallization technology, directly mixing fluormica crystals with soda-lime glass powder and sintering, was proposed to fabricate machinable fluoramphibole glass-ceramics. The new technique is different from the traditional routes: melting and crystallizing and sintering crystallization, fluoramphibole crystals in the glass-ceramics were formed via a reaction between fluormica and glass powder during sintering process instead of a precipitation from parent glass. Therefore, not only the procedure of glass-ceramic preparation was simplified and the production cost was reduced, but also the recycled glass was utilized too, the new technique is a promising and potential route for fabricating fluoramphibole glass-ceramics.The mechanism of reactive crystallizing, the effects on the behaviors of reactive crystallizing, the mechanism of sintering, the correlation between microstructures and properties of glass-ceramics and the effects of processing parameters on the properties of glass-ceramics were systematically investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX).During sintering process, the formation of fluoramphibole crystals consists of two steps: precipitation by the reactive crystallizing and growth. The types of fluormica crystals have a considerable effect on the reactive crystallization of fluoramphibole, and the types of glasses have no effect on it. The content of fluoramphibole crystal by the reactive crystallizing was double one of fluormica addition, the reactive crystallizing formula was as follow: KMg₃AlSi₃O₁₀F₂ + CaO + Na₂O + 5SiO₂→KNa₂CaAlMg₃Si₈O₂₂F₂A crystallography model describing the transformation from fluormica into fluoramphibole was proposed. During the precipitation of fluoramphibole crystals by the reactive crystallizing, the diffusion of Na2O and CaO from glass into fluormica results in the break of [SiO4]4- tetrahedron sheet structure of fluormica crystal, this makes the sheet structure change into a double chain [SiO4]4- tetrahedron structure and fluormica crystal into fluoramphibole crystal. At the same time, the diffusion of K, Mg, F from fluormica crystal to glass also makes the silicate network of the glass be broken, this facilitates [SiO4]4- tetrahedron in glass around fluormica crystal to be precipitated as a chain structure on the nuclei of fluoramphibole. During the growth of fluoramphibole crystal, the precipitation of fluoramphibole crystals stops, the content of fluoramphibole crystal remains unchangeable, the number of grain decreases continuously, and the diameter of grain is directly proportional to the cube root of isothermal time. Fluoramphibole grains grow through the dissolution of smaller grains, and then precipitation on larger grains, the mechanism of grain growth obeys the Ostwald ripening.The fluormica addition had a great effect on the reactive crystallization final phases, while the addition of fluormica was less than 20 wt%, diopside was formed via reactive crystallization; 40 wt% fluormica was added, fluoramphibole was formed; fluormica and fluoramphibole coexisted in compact with 60 wt% fluormica addition; a large amount of fluormica remained unchangeable and only a few of fluoramphibole was formed while 80 wt% fluormica addition. The reactive crystalline final phases are controlled by the O/Si mole ratio in crystals, the fluormica addition has an effect on the glassy phase content in the glass-ceramics, and which affects in turn the oxygen content migrated from glass into fluormica crystal, and finally this results in the change of the O/Si mole ratio in crystals. The densification of fluoramphibole glass-ceramics is mainly achieved by the viscous flow of the glassy phase during the sintering process. The addition of fluormica inhibited the viscous flow of the glassy phase, the relative densities of glass-ceramics decreased with increasing the fluormica content. Raising sintering temperature could improve the densification of glass-ceramics, however, a full densification couldn't be achieved in the glass-ceramics containing fluormica crystals. A suitable sintering temperature range existed at which the higher densities of glass-ceramics can be obtained. The temperature range for each of the glass-ceramics with different fluormica content was different, and it increased with increasing fluormica content. While the addition of fluormica was less than 50 wt%, the sintering mechanism was a viscous flow one. In this range of fluormica content, there is a quantitative relationship between the fluormica addition and the relative density of glass-ceramic.The fluormica addition had a considerable effect on the drilling speed, and other processing parameters had small effect on it. The processing parameters had different effects on the mechanical properties of fluoramphibole glass-ceramics. The optimal processing parameters for fabricating fluoramphibole glass-ceramics obtained by the analysis of variance are listed below: 30 wt%³5 wt% fluormica additions, the size of 100 mesh¹30 mesh for the glass particles, the forming pressure of 150MPa²00MPa for the green compact, the sintering temperature of 860℃⁹00℃and the firing time of 2 hours. The glass-ceramics fabricated by these optimal processing parameters had good mechanical properties and machinability, they could be turned, drilled, tapped and milled using ordinary metal working tools, and components with a thin wall thickness 0.8 mm and intact screws could be machined .