Structure/oxidation behavior relationship in the carbonaceous constituents of 2D-C/PyC/SiC composites

Abstract

Oxidation tests were performed (at 500 < T < 900° C), on 1. (a) as-received T300 fiber, 2. (b) PyC-infiltrated tows 3. (c) C/PyC/SiC composite. The goal was to assess which carbonaceous constituent exhibits the highest reactivity with oxygen in the composites. TGA and TEM were used to establish the relationship between the behaviour of the materials in oxygen and their nano/microtexture. In the as-processed state, the overall oxidation rate of the fiber is higher than that of the PyC interphase, whereas both constituents behave in a similar way after an annealing treatment at 1600°C. Beyond 700°C, the rate-determining step in the oxidation of uncoated fibers is diffusion transport in the external stagnant boundary layer. Below 700°C, it is a mixed in-pore diffusion/surface reaction mode for the as-received fibers and mainly surface reaction for the annealed fibers. The oxidation of the fiber is selective. In cross-section, it occurs more rapidly in the outer zone, owing to the high porosity of this specific zone. The occurrence of a high porosity zone very near the PyC interphase plays a key role in understanding the effect of environment on the mechanical behaviour of the composites.