Processing, microstructure and mechanical properties of extruded mica glass-ceramics

Glass-ceramics with oriented mica crystals have been produced by extrusion and subsequent thermal treatment of glasses of the system SiO2–Al2O3–MgO–Na2O–K2O–F. The formation of a ring-fiber texture of plate-like mica crystals was observed by electron microscopy and X-ray diffraction. The study of the crystallization behavior of glasses extruded at different temperatures revealed that subsequent heat treatments solely result in oriented crystal growth if mica nuclei or crystals have been formed during the extrusion process. The strongly textured material showed notable anisotropic properties. By comparison to mica glass-ceramic with randomly oriented crystals, fracture toughness and bending strength increased by about 100% upon texture introduction. The force of crack deflection increased with the angle between the crack front and the basal planes of mica crystals. The strengthening effect through crystal alignment is attributed to the occurrence of a centric ring-fiber texture, which reduces the size and number of surface flaws generated by the cleavage planes of mica crystals. Machinability, however, decreased by about 50% in the textured material.