Oxidation of SiC powders for the preparation of SiC/mullite/ alumina nanocomposites

The oxidation behaviour of two types of SiC powder of differing particle size and morphology distribution has been studied in the present work; one submicron-sized and the other micron-sized. It has been observed that the onset-temperature for significant oxidation of the SiC powder of smaller particle size is much lower than that for the SiC powder of larger particle size; namely, about 760 C as compared with about 950 C. Furthermore, the rate and extent of oxidation of the former SiC powder is much higher than that of the latter SiC powder. Interestingly, however, the SiC powder of smaller particle size exhibits more controllable oxidation behaviour in the context of the preparation of SiC/mullite/alumina nanocomposites, i.e., in terms of the extent of oxidation, and hence the amount of silica formed as an encapsulating outer layer and the resulting core SiC particle size, than the SiC powder of larger particle size. The SiO2 layer formed was amorphous when the SiC powders were oxidized below 1,200 C, but crystalline in the form of cristobalite when they were oxidized above 1,200 C. Since the presence of amorphous silica can accelerate the sintering of the nanocomposite, oxidation of the chosen SiC powder should thus take place below 1,200 C.