Influence of graphite foils on I-CVI densification rate and microstructure of obtained pyrolytic carbon of carbon-carbon composites

Graphite foils were introduced when producing needle-punched felts. Two steps were adopted to densify the porous preforms, first by iso-thermal chemical vapour infiltration (I-CVI) to about 1.40 g/cm3 and then by impregnation with phenolic resin and carbonization till about 1.75 g/cm3. I-CVI rates were determined. The microstructure of the obtained pyrolytic carbon matrix of the ultimate carbon-carbon composites was investigated by polarized light microscopy (PLM) and transmission electron microscopy (TEM). Results show that the I-CVI densification rate is greatly enhanced by introducing graphite foil(s) in needle-punched felts and the thickness and number of the introduced graphite foils plays much role in the densification rate during carbon-carbon composite producing. Pyrolytic carbon matrix of equal texture resulting from PLM exhibit pronounced gradual changes when studied by TEM. Three differently textured layers, i.e. high-textured reaction layer, medium-textured transition layer and high-textured bulk layer, are observed in the fiber-matrix interfacial region. The thickness of the reaction layer in the 'between' sample (close to the graphite foils) is larger than that in the 'outside' sample (outside from the graphite foils). The variation in thickness of the reaction layer and the sub-micron structure of the different layers suggest the presence of thermal gradients within the preforms with graphite foils introduced, which makes infiltration much faster.