Microstructural evolution of Hi-Nicalon SiC fibers annealed and crept in various oxygen partial pressure atmospheres

The effects of environments and load conditions on the decomposition behaviour and microstructural evolution of Hi-Nicalon fibres were investigated. Three kinds of atmospheres (oxygen content from 21% to 0.1 ppb) were employed for heating the Hi-Nicalon fibres under no load and also under an applied load. The surface morphology and structural evolution of the fibres were studied by SEM, XRD and HRTEM. The degradation behaviour of fibres was also examined. Results showed that, after the fibres were annealed and crept in air, a silica layer with cracks was formed on the fibre surface. Under the creep load, the silica layer became thicker and porous due to the oxidation mechanism change from diffusion of ionic oxygen to transportation of oxygen molecules. An oxygen-enriched amorphous layer was formed at the fibre surface in the case of annealing in an argon flow, whereas SiC crystals were produced by the gas-phase reaction on the fibre surface when the fibre was crept in an argon flow. In an ultra high-purity argon flow, SiC crystals grew on the surface of both annealed and crept fibres. Growth of beta-SiC grain was enhanced under low oxygen partial pressure atmospheres and creep load. 19 refs.