In-Situ Investigation Of Oxidation Of ZrB₂and SiC At High Temperatures Under Low Oxygen Pressures

Ultra-high temperature ceramics are the promising materials for applications at ultra high temperatures, due to their unique properties, such as high melting temperatures (>3000℃), high thermal conductivities, and resistance to erosion/corrosion, thermal shock resistance and medium coefficient of thermal expansion. However, most of such materials are non-oxide ceramics, such as carbide and boride, which suffer oxidation at high temperature under oxidizing atmosphere. Oxidation may damage their structural stability. In this paper, we observed the oxidation process of ZrB2and SiC at atomic scale in real time by an environmental transmission electron microscope and investigated the oxidation mechanisms at high temperatures under low oxygen pressures.The dissertion investigated the oxidation of ZrB2nanoparticles at1500℃under oxygen pressure of5x10-2Pa. The results show the oxidative decomposition of ZrB2into ZrO2and B2O3. ZrO2particles nucleate on the surface of ZrB2with a diameter less than1nm, and its number increases with time. The ZrO2particles grow up and coalese into one particle finally. The B2O3evaporates completely simultaneously.The oxidation behaviors of micro-ZrB2were studied by using in-stu and ex-situ methods. The results demonstrate that, as oxidation temperature rises up, the diameter of oxidatively formed ZrO2increases and the density of ZrO2nucleus decreases. It is achievable to obtain a protective ZrO2film on ZrB2at relative low temperature under appropriate pressure, for example,1070℃and oxygen partial pressure of6Pa.The oxidation of the3C-SiC(111) plane was investigated. It is revealed that active oxidation occurs at temperature range1000—1300℃under oxygen pressure of～5x10-2Pa, and the reaction follows linear laws with an activation energy of0.353±0.002eV calculated by Arrhenius equation. The thickness of SiOx film depends on oxidation rate of SiC, decomposition and evaporation of the film and radiation damage of electron beam. Furthermore, we observed irregular lattice fringes of the Si-C biayers in amorphous SiOx film.