Sintering Behavior and Machinability of Phlogopite Glass Ceramics Containing Fe2O3

Mica-based glasses in the SiO2-Al2O3-MgO-K2O-F system were prepared by a sintering method to investigate the effects of different amounts of hematite (Fe2O3) on thermal and sintering behaviors besides machinability of the glasses by means of differential thermal analysis (DTA), X-ray diffraction, and scanning electron microscope techniques. DTA analysis on fine and coarse glass powders indicated that the main crystallization mechanism in this system occurred in the bulk rather than the surface. Increasing Fe2O3 content to 5 wt.% improved machinability of the glass ceramic. Fe2O3 led to the disruption of the glass matrix and facilitated the nucleation of the crystalline phase. Precipitation of sellite (MgF2) crystals as heterogeneous nucleating sites for potassium phlogopite crystals acted as a second contribution to the machinability of the 5 wt.% Fe2O3-containing sample. However, introducing more than 5 wt.% Fe2O3 to the base glass prohibited the nucleation of MgF2, and as a result, large micas formed within the glass. This together with precipitation of cordierite aggregates in highly doped glass with Fe2O3 led to lower machinability in these samples.