Coating of continuous carbon fibers with double layers by chemical vapor deposition

Abstract
A theoretical approach for the description of the chemical vapor infiltration (CVI) of continuous carbon fiber bundle is used to predict the homogeneity of the infiltration into a fiber bundle as a function of the temperature. Required input for the model is the rate constant of the chemical reaction and the effective diffusion coefficient, including the geometric parameter of the fiber bundle. The mechanical properties of SiC coated carbon fibers are measured in a single filament test and evaluated using the Weibull statistics. SiC nanolayer of about 45 nm thickness decreases the fiber tensile strength to 88 % of the value of an uncoated fiber. On the other hand, pyrolytical carbon (pyC) films improve their mechanical properties. Combining these two effects in applying a double layer coating consisting of pyC/SiC or pyC/TiN in a single step process, it is possible to slightly increase the tensile strength of the carbon fibers. SEM and TEM analysis also show a uniform distribution of the layer thickness within the fiber bundle and confirm the layer thickness measured experimentally