Processing of ceramic matrix composite using microwave energy

Studies of microwave processing of a ceramic composite were conducted on alumina reinforced by silicon carbide whiskers (10 vol%). The processing was accomplished with an industrial microwave heating system. Analyses were performed by mechanical testing methods, and by modern microanalysis methods including electron microscopy (scanning electron microscopy, transmission electron microscopy with energy-dispersive spectroscopy). X-ray photoelectron spectroscopy, and X-ray diffraction. The results indicated that microwave sintered silicon carbide whisker reinforced alumina matrix composite has an improvement of fracture toughness over conventionally sintered monolithic alumina. Crack deflection along the whiskers was the major toughening mechanism operating in the composite. Also, whisker bridging, whisker debonding, and whisker pullout all contributed to the improvement of fracture toughness of the composite. Grain-bridging toughening was also observed. The microstructure study showed that the interfacial bonding between the whiskers and the alumina matrix played an important role in the composite materials. The TEM study revealed that an amorphous interphase existed at the whisker-matrix interface. Strong interface interactions between the whiskers and the alumina matrix weakens the benefits of the whisker toughening effects. The samples sintered at 1650°C showed a 10% higher toughness than those sintered at 1550°C where a strong interface interaction was obvious. It was found that ultrafine grain size was a characteristic of the microwave sintered composite, which is directly related to the microwave internal volumetric sintering mechanism. The investigation also showed that without using sintering aids there was an amorphous layer on most of the alumina grain boundaries although the sintering temperature was 400 to 500°C lower than the melting temperature of the alumina. This observation provided direct evidence of the decrease of local melting temperature at the grain boundary area due to the effects of capillary and electromagnetic field.