| Carbon/carbon composites are unique materials in that they combine the desirable properties of fiber-reinforced composites, such as high specific strength, stiffness, and toughness, with the refractory properties of carbon, namely, retention of strength at high temperatures in an inert atmosphere and resistance to creep and thermal shock. The applications of these materials include rocket nose cones, aircraft disk brakes, and heat shields. The commercial processing methods used to fabricate carbon composites are very slow and highly inefficient. These limitations add considerably to the cost of the components and limit the applications of this material. Consequently, there is a need to develop a process to fabricate carbon composites rapidly and economically. It has been shown that the forced flow-thermal gradient chemical vapor infiltration (FCVI) process can be used to fabricate SiC/SiC composites rapidly. However, the application of this process to carbon composites has been sparse. The present work showed that composites with porosities of only 7-9% can be fabricated within 3-14 h by FCVI process using propylene, propane, or methane as the reagent. The process was also optimized, using a statistically designed experiment for both propylene and propane. The infiltration time was reduced to 2.75 h for propylene and 7 h for propane. This is two orders of magnitude faster than the conventional CVI process, which takes 400-600 h to densify the composites. The composites fabricated in the statistical study were characterized for microstructure and tested for mechanical strength. In addition, a 1-D model was developed for the FCVI process. Finally, it was shown that the FCVI process was flexible enough to deposit a laminated matrix within a carbon preform and to protect the carbon composites internally against oxidation. |
