Microstructures and mechanical behavior of carbon/carbon composites prepared by chemical vapor deposition

Abstract

The microstructure and mechanical behaviors of carbon/carbon (C/C) composites prepared by thermal-gradient chemical vapor deposition (CVD) technique are studied. The fibrous substrates used in this study are carbon mats and carbon cloths. The growth morphologies of pyrolytic carbon matrices within carbon mat substrates when preparing C/C composites by thermal-gradient CVD technique are investigated, and then a deposition mechanism of pyrolytic carbons on carbon fibers is proposed to explain the phenomena observed. The growth morphologies are a clear indication that pyrolytic carbon matrices mostly arise from the deposition of planar carbon molecules, and that the deposition mechanism of so-called laminar carbon matrix is similar to that of a massive carbon deposit with columnar structure. The variation in the preferred orientation of carbon matrix, revealed by optical reflectivity under polarized light, is believed to occur due to changes in the arrangement of planar carbon molecules. The variations in microstructure and structural parameters of pyrolytic carbon matrices are systematically investigated as a function of deposition condition. It is shown that the matrices, which were deposited at temperatures of 1100-1400℃ and from 10-70% propane concentrations, have wide ranges of density ($1.55-2.10 g/cm^3$) and crystallite size (23-35 Å ) as well as various microstructures with a strong dependence of the deposition condition. The matrices have either a columnar, smooth laminar, isotropic, or transition structure. The isotropic structure formed in the high temperature and low propane concentration region has lower density and smaller crystallite size, compared with the smooth laminar structure or the columnar structure. The transition structures are formed between the isotropic region and the smooth laminar region, and have an intermediate character in structure. The density and the crystallite size have a proportional relationship with each other. The structur...