Microstructure of the pyrocarbon matrix in carbon/carbon composites

When chemically vapour deposited from propane within fibrous preforms, pyrocarbon does not always exhibit the same microstructure in the whole preform. Texture profiles (or extinction angle profiles measured by polarized light optical microscopy) depend markedly on the residence time of the reactive gas in the hot zone and to a lesser extent on the infiltration temperature. For low residence times, a transition is observed from a smooth laminar (weakly anisotropic) texture at the external surface to an in-depth rough laminar (highly anistropic) texture. This transition can be explained on the basis of maturation of the gas phase within the preform. For high residence times, the reverse transition occurs, with a decrease of pyrocarbon anisotropy from the surface to the internal part of the preform. This opposite behavior can be associated to an external gas phase maturation followed by an internal depletion.