| Due to the characteristics of low density,high temperature mechanical properties,excellent friction and wear properties and ablation resistance,C/C composites are one of the important light weight,high strength and high temperature resistant thermal structural materials in the aerospace and defense fields.Nevertheless,the uncontrollable pyrocarbon structure and prolonged preparation period remain as two fundamental obstacles that hinder its wider application in various fields.In this paper,we investigate the enhancement of pyrocarbon structure by adding graphene and carbon nanofibers(CNFs)into carbon fiber preforms.The densification rate,microstructure and properties of C/C composites were tested and characterized,and the controlled growth of pyrolytic carbon was realized.On the other hand,the rapid densification of C/C composites and the preparation of isotropic pyrocarbon were achieved through the introduction of an electromagnetic field to optimize the deposition process.The main contents are list as follows:Graphene-modified C/C composites were fabricated via impregnation of graphene and ICVI process.The influence of graphene on the densification rate,morphology,structure,mechanics,and tribological properties of the composites was investigated.Graphene enhances the active sites on the fiber surface,where gas molecules preferentially nucleate and grow,thereby increasing the initial deposition rate.After 45hours of deposition,the addition of graphene resulted in a composite with a density that was 0.11 g/cm~3 higher than that of the composite without graphene.Graphene leads to the formation of numerous delicate growth cones,while pyrocarbon transforms from a smooth laminar structure in pure C/C composites to a regenerated laminar structure.The bonding between carbon layers is strengthened and ring cracks are reduced.The introduction of graphene improved compression properties and friction and wear properties.Specifically,the compression strength of the C/C composite containing 0.66wt%graphene increased from 107.4 MPa to183.5 MPa,while exhibiting a minimal friction coefficient of 0.115 and volume wear.CNFs were synthesized via copper tartrate-catalyzed acetylene cracking,and then modified C/C composites with CNFs were prepared by ICVI process after being dipped into the preform under negative pressure.The impact of CNFs on the densification rate,morphology,structure,mechanics and frictional properties of the composites was investigated.The introduction of CNFs resulted in an increase in active sites and initial deposition rate.After 30 hours of deposition,the composite density with CNFs was found to be 0.10 g/cm~3 higher than that without CNFs.Simultaneously,CNFs facilitated the organized deposition of pyrocarbon,resulting in the formation of coarser nanofibers.When the thickness of CNFs reached 2μm,the induced effect of CNFS disappeared and the texture of carbon layer deteriorated.Only the active site effect remained,resulting in a large number of growth cones appearing and leading to a change in the structure of pyrocarbon to regenerated laminar.The pyrocarbon within the aggregates of CNFs exhibits overall disorder and isotropy.In preforms,CNFs are easy to distribute unevenly,resulting in a decrease in compressive strength,but deposition of pyrocarbon can reduce friction coefficient and volume wear.Compressive strength of the C/C composite with 0.53 wt%CNFs decreased from 114.4 MPa to 93.6 MPa,but friction coefficient was the lowest at 0.138.It was possible to prepare a macroscopically isotropic CNFs/C composite by adjusting the manufacturing process.The structure and properties of the composite were tested and characterized,as well as its properties compared to isotropic pyrolytic carbon.Pyrocarbon inside the composite grows layer by layer around the CNFs,and the growth cones are fine and broken,showing isotropy under polarized light,but its essence lies in the rough laminar structure.The microhardness and elastic modulus of composites are 208.85 Hv and 19.51 GPa,respectively,which is twice that of isotropic carbon of the same density.CNFs/C composite also has a lower friction coefficient and volume wear,making it an ideal replacement for isotropic carbon.A modified tube furnace was used to introduce electric fields and magnetic fields,and to observe the effects on pyrocarbon each had.The rapid preparation of C/C composites and the successful preparation of block isotropic carbon are realized by improving the electromagnetic field CVI process.The deflection of electric and magnetic fields on charged particles generated by carbon source can increase the deposition rate.The weight gain rate of samples without electric field deposition for 1 h is90.4%,which is lower than that of 150 V uniform electric field and 250Hz alternating electric field.The density of the sample deposited without electricity for 28 h was 1.09 g/cm~3,which was less than 1.20 g/cm~3 when2 A AC was applied.In the HCVI process,the addition of graphene has no significant effect on the densification rate and pyrocarbon structure,but reduces the mechanical properties of the material.The disk C/C composites with a density of 1.53g/cm~3 were prepared in 70 h by improved induction CVI process.A block isotropic carbon material with a density of 1.75 g/cm~3was prepared after 35 h by electromagnetic field CVD process.The internal ribbon-like pyrocarbon can improve the stability of friction coefficient and reduce volume wear.105 figures,10 tables,127 references... |
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