| Carbon/carbon(C/C)composites are carbon composites with carbon matrix reinforced by carbon fiber(CF),which have the characteristics of low density(<2.0g/cm~3),high specific strength,high specific modulus,low coefficient of thermal expansion and excellent high temperature mechanical properties,and play an irreplaceable role in aerospace and civil fields.However,during the fabrication process of C/C composites,ring cracks are formed in the matrix due to thermal mismatch,and the mechanical properties of C/C composites are sharply weakened due to the shortening of fibers and the destruction of structural continuity when they are machined into thin-walled and sharp components,which is attributed to the brittleness and low strength of carbon matrix in C/C composites.There are a large number of carbon matrix which has not been strengthened and toughened between micron-sized carbon fiber bundles.In this paper,a hybrid preform of reduced graphene oxide and carbon fiber(rGO@CF)was prepared by a loading-hydrothermal method,and the rGO was bridged between the fiber bundles to form a network structure,and then the pyrolytic carbon matrix was deposited by isothermal chemical vapor infiltration(CVI)process to prepare C/C composites reinforced by rGO and carbon fiber.The effects of hydrothermal conditions such as external load and concentration of graphene oxide(GO)solution on the morphology and structure of the hybrid preforms were investigated.The morphology and structure of rGO@CF/C composites were analyzed,the effects of rGO content on the mechanical properties of C/C composites were discussed,and the enhancement mechanism of rGO@CF/C composites was clarified.The main conclusions are as follows:(1)The effects of external load and GO solution concentration on the morphology and structure of rGO@CF hybrid preforms prepared by hydrothermal method were investigated.The results show that GO is reduced to rGO in the rGO@CF hybrid preforms,and the rGO and carbon fibers are bridged to form a network structure;the surface of the preforms is easy to form enriched and stacked graphene sheets,and with the increase of external load and go solution concentration,the graphene sheets are interconnected to form a larger area of graphene stack.(2)The effect of rGO on the deposition rate and microstructure of CVI pyrocarbon was investigated.The deposition rate of pyrolytic carbon can be increased by rGO in the first 40 h of deposition,and the deposition rate is reduced after 40 h because the network structure formed by rGO and carbon fiber easily leads to the formation of closed pores.With the increase of rGO content,the graphitization degree of the matrix carbon increases,and the structure of the pyrolytic carbon gradually changes to a higher texture.When the concentration of GO dispersion is 8 mg/m L,the texture of the matrix carbon decreases.rGO increases the specific surface area of the preform,which can provide more active sites for the nucleation of pyrolytic carbon,resulting in a"fine grain effect".(3)The effect of rGO on the flexural and compressive properties of C/C composites was investigated,and the reinforcement mechanism of rGO@CF composites was revealed.Compared with pure CF/C composites,the flexural strength of rGO@CF/C composites increased by 12.9%,the compressive strength perpendicular to Z axis increased by 112%,and the compressive strength parallel to Z axis increased by 28%when the GO dispersion concentration was 2 mg/m L.The rGO can effectively improve the pyrolytic carbon matrix around it,make the pyrolytic carbon texture more orderly,and reduce the internal defects of pyrolytic carbon.Meanwhile,with the help of the network structure formed by rGO and carbon fiber,rGO reduces the grain size of pyrolytic carbon,thus realizing the reinforcement of C/C composites. |
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