Modeling the effect of microstructure on the coefficients of thermal expansion of pyrolytic carbon

摘要

The prediction of the coefficients of thermal expansion of carbon/carbon composites fabricated by chemical vapor infiltration (CVI) is important for their effective industrial applications. These composites consist of carbon fibers embedded in a matrix of pyrolytic carbon. The pyrolytic carbon matrix around the fibers has a cylindrically layered structure and each layer may have different coefficients of thermal expansion in the axial, radial and circumferential directions, i.e. it is anisotropic (Fig. 1a). The number of layers, their width, order and structure are influenced by the deposition parameters. The material of each layer is a pyrolytic carbon, which exhibits a broad variety of microstructures. On the basis of four different types of preferred orientation (or texture) there are four basic forms of pyrolytic carbon: isotropic (ISO), low- (LT), medium-(MT) and high-textured (HT) 1. Some experimental and theoretical investigations leading to the determination of coefficients of thermal expansion of pyrolytic carbon and pyrolytic graphite are published in Refs. 2-5, but all these investigations were performed for pyrolytic carbon without significant modifications of this material. The differences between the coefficients of thermal expansion for different modifications of pyrolytic carbon is very significant and knowledge of their values is very important for investigations of damage processes in carbon/carbon composites. The material model proposed in Ref. 6 for the identification of the unknown elastic properties of these pyrolytic carbon modifications utilize the real physical 2D spatial distribution of carbon structural units (so-called coherent domains) obtained directly from experiments (Fig. 1b).