氧化鋯含量對碳化矽╱氧化鋯╱莫來石複合材料之高溫氧化行為的影響

Abstract

本研究是以熱壓法製備碳化矽╱氧化鈷╱莫來石之複合材料，並且將之暴露在1000, 1200, 以及1350℃的空氣中進行各種不同時間的氧化實驗。研究的主要目的是要探討各種不同氧化鈷含量對此複合材料氧化行為的影響。其主要探討的方向針對氧化鈷含量對此複合材料的氧化速率，氧化機構以及氧化模式均有一詳細探討。並且獲得下列結果： 在1000℃時，複合材料之氧化速率遵循一連續地拋物線速率，唯一例外的是碳化矽╱氧化鈷複合材料 (MZY100/SiC) 呈現出兩階段不同的氧化速率此乃因為裂縫形成之故。在1200℃時，氧化速率不再遵循一連續性的拋物線速率，但可將之區分為兩段不同階段的拋物線速率分別控制整個氧化過程，造成此一巨大的氧化速率差別主要是因為基地中的氧化鋯與碳化矽被氧化後生成的產物silica (SiO2) 產生反應後生成zircon，而改變氧在基地的擴散行為，導致氧化速率的降低，且氧化速率的改變會隨著氧化鋯含量的增加而有提前發生的現象。SiC/zirconia/mullite複合材料的氧化速率會隨著氧化鋯含量的增加而增加，特別是當此複合材料在1000以及1200℃氧化時存在一氧化鋯的臨界體積百分比，當氧化鋯體積百分比超過20 vol%時，其氧化速率急速增快，反之若低於臨界體積百分比，其氧化速率明顯下降，此一現象可用percolation theory解釋之。再者，氧化鋯含量亦對此複合材料的氧化型態有明顯的影響，當氧化鋯含量低於臨界百分比時，複合材料具有一很小的氧化影響區，反之，當高於此臨界值時，則顯現有一很大的氧化影響區。 本研究亦以TEM或SEM觀察氧化試片的縱切面，藉由SiC的深度（即該SiC與複合材料表面的距離）與該SiiC的氧化層厚度的關係，探討不同成分複合材料的氧化模式。當氧化鋯含量低於臨界體積百分比且無zircon相產生時，具有一較陡的氧化層厚度變化梯度以及有一較淺的氧化深度曲線，其氧化模式可歸類為Mode I。反之，若超過臨界值且亦無zircon相生成時，具有一較平緩的氧化層厚度變化梯度但具有一較深的氧化深度曲線，可歸類為Mode II的氧化行為。當低於臨界值且有zircon相產生時，其氧化模式仍可歸類為Mode I的氧化行為。然而當氧化鋯含量超過臨界值且有zircon相生成時，其複合材料的氧化行為可化分為兩種不同情形：（一）當基地中氧化鋯含量被zircon取代並且低於臨界值時，可被歸類為混和模式 (mixed mode) 的氧化行為，其中在氧化初期階段屬於Mode II，而後屬於Mode I的氧化行為模式。（二）當部分氧化鋯含量雖被zircon取代，但其含量仍高於臨界值時，其氧化模式則仍可歸類為Mode II的氧化行為。

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.