Study Of Hydrogen On The C / C Composite Materials Cvi Of Pyrocarbon Microstructure

The microstructure of CVI pyrocarbon of C/C composites has prominent influence on the properties of composites. How to obtain the needed microstructure of the CVI pyrocarbon by controlling parameters in the CVI process is always one of the research hotspot concerned. On the basis of experimental investigations and theoretical analyses, the relationship between control parameters and pyrocarbon microstructure of C/C composites fabricated by thermal gradient CVI process and by using hydrogen as the carrier gas has been researched for the first time. By the polarized observing and graphitization degree measuring of the materials it has been found that the hydrogen content in the gas has important influence on the pyrocarbon microstructure, and the CVI pyrocarbon microstructure can be effectively controlled by adjusting the hydrogen content in the gas during CVI process.The changing rule of the residence time of the reactant gas in the C/C composites with disk shape fabricated by thermal gradient CVI process has been carefully analysed. The distributions of the pyrocarbon microstructure of the materials can be well explained by using the above rule and the temperature distribution rule of the thermal gradient furnace.The influence of the different carrier gas on the pyrocarbon microstructure has been investigated and it has been found that the nitrogen in the CVI process has no obvious influence on the pyrocarbon microstructure which playing an inert role, and the addition of the hydrogen has the inhibition of the carbon deposition on the one hand which reduce the critical value of the residence time which is needed as the condition of the formation of RL pyrocarbon on the other hand.The influence of the hydrogen content in the reactant gas, controlling temperature and total pressure on the microstructure of pyrocarbon has been systematically researched for the first time. It has been found that the RL pyrocarbon can be obtained during the more wide range of temperature and pressure in the propylene/hydrogen system. The uniform RL pyrocarbon can be obtained in the whole material by adjusting hydrogen content in the reactant gas.On the basis of experimental results and the qualitatively analysis of the pyrocarbon deposition process, hydrogen adjusting model about pyrocarbon deposition mechanism has been brought forward: pyrocarbon deposition is actually the merging of two kinds of deposition based on respectively chemical absorption mechanism and physical absorption mechanism. The addition of the hydrogen content in the reactant gas adjust the relationship between two kinds of pyrocarbon deposition rate based on respectively chemical absorption mechanism and physical absorption mechanism, which change the pyrocarbon microstructure as the following sequence: SL-RL-SL-ISO. The forming conditions of the RL pyrocarbon has been summarized: X>Xc, vn