Study of Crystallization of Potassium Fluormica Glass-Ceramics for CAD/CAM applications

Objectives: Fluormica glass-ceramics were one of the first glass-ceramic developed for engineering and dental application. Its excellent machinability due to its unique ‘’house of cards’’ microstructure makes it a perfect candidate to be used with dental CAD/CAM. Literature has shown the crystallization of fluorphlogopite crystal phase crystallises following two precursor phases; chondrodite and norbergite. This work studied the crystallization process of fluormica glass-ceramic using a glass composition very close to fluormica crystal phase stoichiometry.
Methods: Parent glass was produced with composition 47.2SiO2, 16.7Al2O3, 9.5K2O, 14.5MgO, 8.5B2O3, 6.3F2 (Wt%). The produced glass was characterized using Differential Scanning Calorimetry [DSC], X-ray Diffraction [XRD] and Simultaneous thermal analysis (STA). It was then converted to fluormica glass-ceramics using a two-step heat treatment by holding glass at 660°C for 1 hour then another hold at (700, 750, 800, 850, 900 and 1000°C) for 5 hours. The glass-ceramics produced from each heat treatment was characterized using XRD and 19F ,27Al MAS-NMR.
Results: DSC results indicated bulk crystallisation as the dominant mechanism. XRD showed the crystallization of potassium fluorphlogopite crystal phase in all heat treatments except for the one at 700°C. The latter temperature produced completely amorphous. No precursor phases were detected by XRD. 19F MAS-NMR spectra of different heat treatments showed a sharp peak at -177 ppm in glass-ceramics, while that of the parent glass and glass-ceramic heat treated at 700°C showed a broad peak at -177ppm. 27Al MAS-NMR spectra showed that Al was present largely as Al(IV) in both glass and glass-ceramics with a development of extra sharp peak in all heat treated glass-ceramics except for the one heat treated at 700°C.
Conclusions: Glasses with chemical compositions close to fluormica stoichiometry directly crystalise to fluorphlogopite crystal phase without any precursor phases at lower nucleation and crystalisation temperatures. This could significantly improve material proeperties for CAD/CAM applications.