Effects of chemical vapor infiltration atmosphere on the mechanical properties and microstructure of carbon fibers

Investigations of the effects of chemical vapor infiltration atmosphere on the mechanical properties of carbon fiber and examinations of the microstructure of the carbon fiber were carried out in the present paper. The experimental results revealed that tensile strength and fracture strain of the carbon fiber were decreased after annealing at temperatures of 850–1100°C in HCl atmosphere, but the elastic modulus was not varied. The degradation of strength and strain were responsible for the oxidation weight loss. Both oxidation weight loss and strength loss of the carbon fiber was obviously increased with the presence of HCl resulting from the by-product of MTS reaction. As oxidation weight loss was increased from 0 to 4.97%, the strength was decreased from 2850 to 1423 MPa. After oxidation, microstructure of the carbon fiber exhibited three zones: a skin outer zone, a porous thin zone, and the bulk laminated structure. During the chemical vapor infiltration process, the effects of the carbon interfacial layer lie in protecting the carbon fiber by filling the micro-depressions on the surface of the carbon fiber and by separating the carbon fiber with the oxidation atmosphere.