Oxidation behaviour of a SiC based fibre

摘要

The oxidation kinetics of Nicalon fibers is studied in the 900- 1200°C range in various atmospheres: dry and wet oxygen. These fibers are constituted of three phases (SiC nanocrystals, SiC{sub}xO{sub}y amorphous continuum, free carbon). They alsocontain residual hydrogen which forms water vapour during oxidation and influences the kinetics. Experiments show that the influence of water vapour is significant in the 0-0.75 kPa range . Residual hydrogen effect is minimized by a thermal pretreatmentof the fibers in helium at 1250°C. In these conditions, a diffusion controlled regime is established after a transient period which canno't be modelled by the classical linear-parabolic law characteristic of a mixed reaction-diffusion regime. The weakapparent activation energy obtained in dry oxygen (85 kJ/mole) for the diffusion mechanism, compared to the literature values (120 kJ/mole) for oxygen permeation through amorphous silica indicates that this process is not the only limiting step or thatsilica has a different texture depending on the nature of the substrate.