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

Effect of carrier gases(H₂ 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-H₂ is obviously higher than that from CH₄-CO₂, while the densification rate from CH₄-H₂ is lower than that from CH₄-CO₂with a further increase of infiltration time. When the carrier gas is switched from H₂ to CO₂, the average bulk density of the compositeincreases from 1.626 to 1.723 g/cm³, the maximum radial density gradient decreases from 0.074 to 0.056 g/cm³, 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.