Preparation and mechanical properties of machinable alumina/mica composites

Using a mica-crystallizing glass powder in which a large amount of mica crystal was precipitated and a larger amount of MgF2 component was contained as the raw materials of mica, machinable alumina/mica composites were obtained at 1400 °C. In the firing process, magnesia component in the mica crystals reacted with alumina to form spinel at 1150–1200 °C. The reaction made the mica crystals melt. However, the mica crystals were precipitated again during the cooling. Because a larger amount of MgF2 component was contained in the mica-crystallizing glass powder, the nucleation of the mica crystals was caused during the cooling by the residual magnesium and fluorine in the liquid phase and succeedingly the mica crystals were precipitated. The precipitated mica crystals grew to anisotropicaly larger size than alumina grains, which lowered the bending strength and Vickers hardness and little heightened the fracture toughness.