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SiCp/zirconia/mullite composites with various zirconia contents, fabricated by hot pressing, were exposed in air isothermally at 1000, 1200, and 1350℃ respectively for up to 500 hours. This study aims at the effect of zirconia content on the oxidation behavior of SiCp/zirconia/mullite composites. At 1000℃, the oxidation followed a consecutively parabolic kinetics during exposures for all composites except the zirconia/SiC (MZYlOO/SiC) composite. However, at temperatures > 1200℃, two different parabolic laws separately governed during exposures. Large difference in rate constant resulted from the effect of zircon, due to the interaction between zirconia and silica, which could significantly reduce the oxidation rate of the composites. The dramatic change of rate constant took place at an earlier stage when the zirconia content increased. This was attributed to the early formation of zircon when more zirconia particles were contained in the composites. It was shown that an evident critical volume fraction of zirconia existed for exposure both at 1000 and 1200℃. The oxidation rate was dramatically increased beyond 20 vol% ZrO2, while it was much slower below this content. The dramatic change in oxidation rate due to the variation of zirconia content could be explained by the percolation theory. On the other hand, the zirconia content also had a strong influence on the oxidation morphology. The composites with ZrO2 content less than the threshold value showed a small oxidation zone, while the composites with ZrO2 more than the threshold value revealed a large oxidation zone. The oxidation modes of SiC/zirconia/mullite composites with various zirconia contents could be further classified based on the features of the relationship of the silica layer thickness and the depth of the corresponding SiC particle below the outermost surface. Two basic oxidation modes together with mixed mode were observed in the exposed composites, depending on their zirconia content: (1) Below the threshold limit of zirconia content and without zircon formation, the composites exhibited oxidation behavior of Mode I, which was characterized by a small oxidation depth as well as a comparatively large gradient of the silica layer thickness versus depth curve. (2) When the zirconia content exceeded the threshold limit and did not form any zircon, the oxidation behavior exhibited the behavior of Mode II, which was characterized by a large oxidation depth as well as a relatively small gradient. (3) Below the threshold limit of zirconia content and with zircon formation, the oxidation mode exhibited the behavior of Mode I. (4) It was more complicated when the zirconia content exceeded the threshold limit and zircon formed after a relatively long exposure at 1200℃. Once the zirconia content fell below the threshold limit due to the replacement of zirconia by zircon, the oxidation behavior changed from Mode II to Mode I. If the zirconia content was still higher than the threshold limit after the replacement, the oxidation mode was Mode II.
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