Effect of carrier gases on densification rate, bulk density and microstructure of carbon/carbon composites

Effect of carrier gases(H-2 and CO2) on the densification rate, bulk density and microstructure of carbon/carbon composites fabricated by isothermalchemical vapor infiltration from methane (CH4) was investigated. In the initial 50 h, the densification rate obtained from CH4-H-2, is obviously higher than that from CH4-CO2, while the densification rate from CH4-H-2 is lower than that from CH4-CO2 with a further increase of infiltration time. When the carrier gas is switched from H-2 to CO2, the average bulk density of the compositeincreases from 1. 626 to 1. 723 g/cm(3), the maximum radial density gradient decreases from 0. 074 to 0.056 g/cm(3), the matrix changes from the pure rough laminar to hybrid rough laminar pyrocarbon with overgrowth cones, and the average degree of graphitization reduces from 62.7% to 50. 8%. These significant changes are caused by the fact that CO2 can effectively reduce the surface deposition rate but does not inhibit the in pore infiltration, and thatdefects are formed in the deposits by a CO2 introduction in gas phase and the resulting overgrowth cones deteriorate the texture degree of pyrocarbon.