Oxidation Behavior of Ceramic Fibers from the Si-C-N-O System and Sub-Systems

摘要

The oxidation behavior of various fibers from the Si-C-N-O system has been investigated. The oxidation kinetics and the activation energies of the Si-C-O systems are similar to those of crystalline SiC. The oxygen and free carbon in the ceramic have only little influence on the oxidation behavior. This feature is due to the fact that the oxidation kinetics in these materials is controlled by the diffusion of oxygen through the silica layer. The parabolic constants of the N-containing Si-C-N-O-derived systems are lower than those of SiC and their activation energies are significantly higher. Their values range between those of SiC and Si_3N_4. The oxidation mechanism might be related to a complex diffusion/reaction regime via the formation of an intermediate silicon oxynitride layer. The oxidation behavior these complex systems is not clearly related to the presence of oxygen nor free carbon in the initial ceramic. It might rather be governed by the N/Si ratio of the ceramic, limiting the nitrogen concentration of the silicon oxynitride layer at the silica/substrate interface and therefore affecting the diffusion/reaction kinetics.