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-CO2 with 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 to0. 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.