Effect of carrier gas on bulk density and microstructure distribution of carbon/carbon composites fabricated by thermal gradient chemical vapor infiltration

The effect of the type of carrier gas on the bulk density and microstructure distribution of carbon/carbon (C/C) composites fabricated by thermal gradient chemical vapor infiltration from propylene was investigated. The results show that the type of carrier gas has an appreciable influence on the radial distribution of density and microstructure of the C/C composites. When N2 is used as carrier gas, the average bulk density of the carbon disk is 1.54 g/cm3 after 400-h infiltration, and the radial density difference of the disk is 0.24 g/cm3. Furthermore, smooth laminar pyrolytic carbon forms at the outer wall of the disks. When H2 is used as carrier gas, an entirely rough laminar pyrolytic carbon matrix and C/C disks with average bulk density of 1.67 g/cm3 are obtained in the same time, and the radial density difference of the disk is only 0.11 g/cm3. No matter which of these two carrier gases is used, the lower density zone is found to be at or near the middle of the disks.