| Cf/C composites are widely used in aerospace,nuclear,military and other fields due to its high specific strength,high specific modulus and excellent strength retention in high temperatures.The traditional method prepared high-performance Cf/C composites is mainly isothermal chemical vapor infiltration?ICVI?,but it has serious problems of long densification cycle and high cost to prepered Cf/C composites.In order to solve this problem,using cyclohexane as a carbon precursor,this research based on self-made experimental device which combine precursor vaporization and pyrolytic carbon deposition,prepare annular Cf/C composites by liquid phase vaporization TG-CVI method.Effects of process conditions on the densification behavior,microstructure and properties of Cf/C composites are studied by means of PLM,SEM,XRD and other three-point bending methods.The main works and contents include the following aspects:?1?For Cf/C composites using carbon felt as prefabricated,effects of deposition temperature and furnace pressure on the densification behavior,microstructure and properties of Cf/C composites are investigated.The results show that with the increase in deposition temperature,the average density and flexural strength of composites significantly increases and radial density difference is reduced as well as the pyrolysis carbon transitions from medium texture to high texture.With the furnace pressure increases,the average density of composites first increases and then decreases while the difference in radial density firstd ecreases and then increases and the pyrolytic carbon transitions from high texture to medium texture.The optimum process parameters of carbon felt is 1000°C-20kPa.The carbon felt with initial density of0.140g·cm-33 can be densified to 1.662g·cm-33 within 20h,and the densification rate is up to 0.07610g·cm-3·h-1.The composites with the pseudoplastic fracture characteristic is rough layer pyrolytic carbon and the bending strength is about 59.74MPa.?2?For Cf/C composites using carbon fiber mesh tire as prefabricated,effects of furnace pressure on the densification behavior,microstructure and properties of Cf/C composites were investigated.The results show that with the increase of furnace pressure,the average density of composites first increases and then decreases,the uniformity of radial density increases,and the thermal carbon decomposes from high texture to medium texture.The flexural strength of composites decreases first and then increases.The optimum process parameters of net tire is 1000°C-10kPa.Under this process condition,the net tire with an initial density of 0.340g·cm-3can be densified to 1.657g·cm-33 within 20h,and the densification rate is about0.06585g·cm-3·h-1.The composites with pseudoplastic fracture characteristic is rough layer pyrolytic carbon and the bending strength is about 49.71MPa.?3?The effects of preform structure on the microstructure of Cf/C composites were studied under the same process conditions for dense carbon felt and carbon fiber mesh prefabricated.The results show that under 1000?-20kPa,the pyrolytic carbon structure of composites with carbon felt as prefabricated is mainly RL structure grown in a tapered manner.However,the pyrolytic carbon of composites with mesh tire as prefabricated is mainly a SL structure which grows in a smooth manne.The initial As/Vr value is the main factor affecting the pyrolytic carbon structure.?4?Large temperature gradient and high-concentration of gaseous carbon source precursors are main reasons to rapidly prepare Cf/C composites by liquid phase vaporization of TG-CVI. |
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