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 microstru cture of the carbon fiber were carried out in the present paper. The experi mental results revealed that tensile strength and fracture strain of the ca rbon fiber were decreased after annealing at temperatures of 850-1100 degre esC in HCl atmosphere, but the elastic modulus was not varied. The degradat ion of strength and strain were responsible for the oxidation weight loss. Both oxidation weight loss and strength loss of the carbon fiber was obviou sly 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 stre ngth was decreased from 2850 to 1423 MPa. After oxidation, microstructure o f the carbon fiber exhibited three zones: a skin outer zone, a porous thin zone, and the bulk. laminated structure. During the chemical vapor infiltra tion process, the effects of the carbon interfacial layer lie in protecting the carbon fiber by filling the micro-depressions on the surface of the ca rbon fiber and by separating the carbon fiber with the oxidation atmosphere . (C) 2001 Elsevier Science Ltd. All rights reserved.