碳化矽/氧化鋯/氧化鋁複合材料之高溫氧化行為

Abstract

本研究是以熱壓法製備碳化矽/氧化鋯/氧化鋁之複合材料，並且將之暴露在1000℃、1200℃以及1350℃的空氧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氧化鋯含量對此複合材料氧化行為的影響。研究結果顯示，碳化矽/氧化鋯/氧化鋁複合材料的氧化速率會隨著氧化鋯含量的增加而增加，特別是此複合材料存在一氧化鋯的臨界體積百分比，在1000℃時，氧化鋯的臨界體積百分比為20 vol%，在1200和1350℃，氧化鋯的臨界體積百分比皆為15 vol%，當氧化鋯體積百分比超過其臨界值時，則氧化的速率會迅速增加，反之，若低於臨界體積百分比，氧化的程度並不明顯。SiCp/ ZrO2/ Al2O3複合材料經氧化後，試片截面可分成三個以顏色區分的區域，白色和黑色部份為已經氧化的部份，灰色為未氧化的部份，在1000℃時，SiCp/ ZrO2/ Al2O3複合材料會產生cristobalite，1200℃氧化除了有cristobalite的產生外，亦有zircon的生成，1350℃氧化則除了有cristobalite、zircon的產生外，還有mullite的生成，mullite主要是大量生成在複合材料的表面，這些產物的生成對複合材料的氧化行為有很大的影響。在1000及1200℃時，複合材料之氧化速率遵循一連續地拋物線速率，1350℃時，氧化速率不再遵循一連續性的拋物線速率，造成此一巨大的氧化速率差別主要是因為在長時間的氧化下，碳化矽被氧化後生成的產物silica(SiO2) 和zirconia產生反應後生成zircon，當zirconia的含量低於臨界值時，氧化速率會開始變慢，且silica(SiO2)與基地中的氧化鋁產生反應後生成mullite，而改變氧在基地的擴散行為，導致氧化速率的降低，氧化速率的改變會隨著mullite含量的增加而有提前發生的現象。

SiC(p)/zirconia/alumina composites with various zirconia contents, fabricated by hot pressing, were exposed in air isothermally for up to 500 hours at 1000、1200 and 1350℃, respectively. The effect of zirconia content on the oxidation behavior of SiC(p)/zirconia/alumina composites was demostrated. The result showed as the higher zirconia content the higher oxidation rate. Meanwhile, the critical volume fraction of zirconia content obtained at 1000、1200 and 1350℃ was 20 vol%、15 vol% and 15 vol% . The oxidation rate was dramatically increased beyond the critical volume fraction. On the other hand, the composites with zirconia content less than the critical volume fraction showed slower oxidation rate. After SiC(p)/zirconia/alumina composites was oxidized, cross section reveal three region which had different color. White region and black region had oxidized, while grey region was unoxidized. The oxidation product cristobalite was present in 1000、1200 and 1350℃. Zircon was present in 1200 and 1350℃. Mullite was only present in 1350℃ and great majority produce in the composites surface. These oxidation product had important effect for the oxidation behavior of SiC(p)/zirconia/alumina composites. At 1000 and 1200℃, the oxidation followed a consecutively parabolic kinetics during exposures for all composites. However, at temperatures ≧1350℃, two difference 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. And the product of mullite, due to the interaction between alumina and silica, which also could reduce the oxidation rate of the composites. The dramatic change of rate constant took place at an earlier stage when the alumina content increased. The was attributed to the early formation of mullite when more alumina paticles were contained in the composites.