Microstructural evolution of Hi-Nicalon (TM) SiC fibers annealed and creptin various oxygen partial pressure atmospheres

It is expected that in the future SiC fiber-reinforced ceramic-matrix compo sites (CMCs) will be used in high temperature and hostile environments. In this study, Hi-Nicalon(TM) SiC fibers were annealed and crept at 1500 degre es C for 1 hour in air, an argon flow and an ultra high-purity argon flow i n order to investigate the effects of atmospheres and load conditions on th e decomposition behavior and microstructural evolution of the fibers. After the fibers were annealed and crept in air, a silica layer with cracks was formed on the fiber 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 amor phous layer was formed at the fiber surface in the case of annealing in an argon flow, whereas SiC crystals were produced by the gas-phase reaction on the fiber surface when the fiber was crept in an argon flow. In an ultra h igh-purity argon flow, SiC crystals grew on the surface of both annealed an d crept fibers. Growth of beta-SiC grain was enhanced under low oxygen part ial pressure atmospheres and creep load. (C) 2000 Kluwer Academic Publisher s.