Abstract: Needled carbon fibre integrated preforms were densified by directional flow thermal gradient CVI process from carbon source gas C₃H₆, and the effects of introducing active carrier gas (H₂) and inert carrier gas (N₂) on the bulk density and microstructure distribution of C/C composites obtained were investigated, respectively. The results show that the type of carrier gases has noticeable influences on the radial distribution of density and microstructure of C/C composites. When N₂ is used as carrier gas, the average bulk density of the resultant disk is 1.54 g/cm³ after 400 h infiltration, and the density distribution discrepancy (Δρ) of the disk along the radial direction is 0.24 g/cm³. Furthermore, the smooth laminar pyrolytic carbon is subjected to form at the exterior part of the C/C disks. Unlike N₂, when H₂ is used as carrier gas, a complete rough laminar pyrolytic carbon matrix and C/C disks with average bulk density of 1.67 g/cm³ can be obtained within the same time, and the density distribution discrepancy of the disk along the radial direction is only 0.11 g/cm³ , which means the density homogeneity of the radial distribution of C/C disks is more than doubled. It is also shown that no matter what type of carrier gas is employed, the circumferential density profile is very homogeneous (Δρ≤0.02 g/cm³) and the lower density zone is found to be the middle position which is adjacent to the inner part of the C/C disks.

Key words: C/C composites; CVI; carrier gas; pyrolytic carbon