Carbon/carbon processing by forced flow-thermal gradient chemical vapor infiltration (FCVI) using propane

A 2 3 factorial statistically designed experiment was used to study fabrication of carbon/carbon composites using the forced flow-thermal gradient chemical vapor infiltration process. Propane, diluted with hydrogen, was used as the feed. The independent variables were the temperature of the bottom of the preform holder, the concentration of the reagent, and the total flow rate. The response variables were infiltration time, final porosity, rate of weight gain, and uniformity of infiltration. It was found that the infiltration time was affected only by the temperature of the preform and the concentration of the reagent. Infiltration times as low as 7 hours were achieved. The bulk density of the final composite was found to be independent of the operating conditions, provided the process was continued until a fixed pressure drop was achieved. Density variations with position within a given composite were no more than ± 0.08 g cm −3. Deposition efficiency ranged from 3 to 14% and increased with temperature while decreasing with flow rate. The values of the coating thickness near the hot and cold sides of the composite were used to calculate the rate constant for carbon deposition from propane. The activation energy was found to be from ~23.6 Kcal mol −1 and the rate constant was given by ln k s(cm s −1) = 2.2–23610/RT.