Preparation of machinable cordierite/mica composite by low-temperature sintering

In order to fabricate machinable cordierite/mica composite at low temperatures, the mica-composition glass powder was mixed with the conventional magnesia, alumina and silica powders which are raw materials of cordierite, compacted and fired in a sealed platinum container. By the addition of the 40 mass% mica-composition glass powder, machinable cordierite/mica composite was obtained. The machinability was caused by the interlocking microstructure of mica developed in the composite. In the firing process, mica crystallized at about 730 °C, cordierite was suddenly formed at 1050–1100 °C and the densification progressed markedly at 1000–1100 °C. The formation and sintering of cordierite were strongly promoted by a small amount of gaseous fluorine and/or fluorides. It was considered that fluorine and fluorides such as AlF3 evaporated from the mica-composition glass at >800 °C and gaseous HF was formed in the sealed platinum container by the reaction of fluorine with water evaporated from the glass.