Carbon/carbon processing by forced flow-thermal gradient chemical vapor infiltration using propylene

The fabrication of carbon/carbon disks using the forced flow-thermal gradient chemical vapor infiltration process was studied using a 23 factorial statistical design. Propylene, diluted with hydrogen, was used as the reagent. The independent variables were the temperature of the bottom of the fibrous preform, concentration of the reagent, and total flow rate. The response variables were infiltration time, final porosity, rate of weight gain, and uniformity of densification. It was found that the infiltration time and rate of weight gain were affected by each of the three independent variables. It was also observed that the densification of the composites could be monitored by the in situ measurement of the back pressure. The composites were cut into 12 slices 1 cm long, 4 mm wide and 2 mm deep to measure the uniformity of densification. It was found that the density of the slices varied very little with position within an individual composite disk. Also, coating thickness as a function of position was measured for different composites. In most of the runs the coating thickness increased exponentially from the cold side to the hot side of the disk. The coating thicknesses near the cold and hot sides were used to calculate the rate constant for the pyrolysis of propylene in the preform. The activation energy was found to be ~21.0 kcal mol−1 and the rate constant is given by ln ks = 1.5–21034/RT.