Effects of infiltration conditions on the densification behavior of carbon/carbon composites prepared by a directional-flow thermal gradient CVI process

Multiple quasi three-dimensioned carbon fibre preforms with disk-like shape were simultaneously densified by a directional flow thermal gradient CVI process. The effects of infiltration conditions, including temperature (ranging from 850 to 1050 °C), temperature gradient (5 and 10 °C/mm), pressure (2.5, 5.0, 7.5 and 9.5 kPa) and the type of carrier gas (N₂ or H₂), on the densification behavior of the resultant carbon/carbon composites were investigated. The results showed that lower temperatures (below 900 °C), a larger temperature gradient and higher pressure are favorable for higher average bulk density and homogeneous infiltration. Carbon/carbon composites disks with an average bulk density of 1.78 g/cm³ were achieved in one CVI cycle at a total pressure of 9.5 kPa. It was also found that adding N₂ carrier gas has no pronounced influence on the densification of the preforms. As compared to N₂, H₂ had positive effects on the densification of the preforms for temperatures above 900 °C, but it had negative effects on the densification when the control temperature was as low as 850 °C.