Effect of aluminum powders on the oxidation resistance of Al₂O₃-MgO composites

MgO–Al₂O₃ and Al–Mg O–Al₂O₃ composites were prepared by using the sintered alumina, α-Al₂O₃ powders, high purity magnesia, and aluminum powders as raw materials, and the phenolic resin as binder. After molding, the samples were dried at 200 ℃ and then sintered at 1500 ℃ in oxidation atmosphere. The effect of aluminum powders on the oxidation resistance of Al₂O₃–MgO composites were investigated by X-ray diffractometer(XRD), scanning electron microscope(SEM), and energy dispersion spectrometer(EDS). The results show that, the major crystalline phases of the sintered samples without Al addition are α-Al₂O₃ and MgAl₂O₄, and the microstructures of the sintered samples are looser. The major crystalline phases of the sintered samples with Al addition are α-Al₂O₃ and MgAl₂O₄, and the newly formed phases are Al₄O₄C, Al₄C₃, and(Al₂OC)_x(AlN)_1-x; the microstructures of the sintered samples are more densification, and the sintering and properties of samples were improved. The internal and external components of the sintered samples with Al addition show the gradient changes, and the Al₄O₄C phase mainly appears in the internal region of the sample with residual aluminum. The oxygen partial pressure in the sintered samples decreases gradually from the surface to the interior due to the Al powder addition, the microstructures are densified by spinel, which is in-situ synthesized by oxidizing aluminum powders and MgO, preventing the further infiltration of oxygen. The whisker-like Al₄O₄C is obtained by the reaction of Al₂O formed in the sample and C.