| Carbon fiber reinforced Carbon matrix (C-C) composites were initiated in the 1960's and developed extensively in the 1970's. They have excellent mechanical, thermal, and chemical properties. They also have uniquely high temperature properties. C-C composites are widely used in the aircraft, space, military, nuclear, and sports industries.; C-C composites are usually fabricated by impregnating the carbon fiber preform with phenolics resin or pitch. Then the matrices are carbonized and graphitized. After that, the Chemical Vapor Infiltration (CVI) technique is used to further densify the porous preform. Several types of CVI processes are developed, including isothermal, thermal gradient, pressure gradient, and plus CVI technique. The isothermal CVI technique is the only one used widely in commercial industries.; The CVI process is influenced by reaction temperature, pressure, gas composition, gas concentration, and nature of the preform. The processing parameters have been studied to understand the CVI process, to achieve the ultimate goal of uniformly densifying the preform in as short a time as possible. The effects of temperature, gas composition, and gas concentration on the CVI process is studied in this research. Temperature affects the CVI process by affecting the chemical reaction rate. A low temperature results in a low CVI rate and a low final density. Uniform densification, however, can be achieved. At high temperatures, the CVI rate is high, resulting in the premature closure of open porosity. So that the final density is low and uniformity of the density is poor. An intermediate temperature is most suitable for the CVI process. When a methane/propylene mixture is used as the precursor gas, the densification results are better than without propylene in the reactant gas in terms of the final density, uniformity of the density, and processing time. Adding hydrogen prevents the formation of soot and tar.; The microstructure of the CVI carbon is anisotropic. At a low temperature and/or a low propylene flow rate, the structure is a smooth laminar. A rough laminar structure is observed at high temperatures and/or high propylene flow rates. The CVI carbon is partially ordered and graphitizable. Before heat treatment, the crystallite size of the CVI carbon is about 40 A. After heat treatment at 2000 and 2200{dollar}spcirc{dollar}C, the crystallite size becomes 216 and 250 A. A mathematical model is developed employing the "overlap" concept. The model fits the experimental data. |
