| With the progress of modern aerospace technology,a high mach of new hypersonic spacecraft has become the focus of research around the world.X series of spacecraft as one of the representatives,thermal protection system has become a key technology to control the service capability of high-speed aircraft.For the service environment,the material used in the system has a light,high temperature,anti-oxidation ablation and high reliability of the basic characteristics to meet the needs of thermal protection system.Continuous carbon fiber reinforced ceramic matrix composites as a porous material,due to its high porosity,low density,good high temperature stability makes it an important candidate material.In addition,as a new type of rigid porous ceramic matrix insulation material,the introduction of SiBCN ceramics compared to the traditional SiOC ceramics can further improve its heat resistance.Continuous carbon fiber reinforced ceramic matrix composites(CFRCMC)with high strength,high hardness,low specific gravity,chemical resistance and other advantages,has been focal point of material science during the past few decades.Due to its excellent mechanical properties,CFRCMC has a wide application in the aerospace,automotive industry and national defense industry.In addition,compared with the traditional metal or alloy materials,CFRCMC has better in-plane tensile properties,as well as the advantages of lower density.However,the properties of CFRCMC still have some limitations,mainly due to the weak interface between carbon fiber and matrix.Graphene oxide grafted carbon fibers(CF-g-GO)and nanosilica grafted carbon fibers(CF-gnanosilica),two kinds of novel composite reinforcements were studied in this thesis.After impregnation and pyrolysis process,CF-g-GO/Si BCN-Ⅰ ceramic composites and CF-gnanosilica/SiBCN-Ⅱ ceramic composites were fabricated via precursor infiltration and pyrolysis(PIP)method combined with in situ polymerization method.Two composites chemical group composition,element distribution and content changes,and the mechanical properties and thermal physical properties were discussed in detail.Using(3-aminopropyl)triethoxysilane and(3-glycidyloxypropyl)trimethoxysilane,two silane coupling agents modified graphene oxide and carbon fiber,respectively.Graphene oxide grafted carbon fiber composite were prepared by ring opening reaction.The preparation technology of graphene oxide grafted carbon fiber and the changes of the chemical composition on the interface of carbon fiber before and after grafting process were also investigated.FTIR,XPS and SEM characterizations showed that graphene oxide had grafted onto the carbon fibers surface efficiently.Tensile test showed that grafted graphene oxide can bridge the defects on the carbon fiber surface in a certain extent,which emerged in oxidation process.The method was beneficial to improve the tensile strength and fracture toughness of the carbon fibers,and could enhance the fibers surface roughness and interfacial mechanical occlusion effect at the same time.The synthesis mechanism of carbon fiber/nanosilica hybrid structure and the chemical compositions and structure changes before and after grafting process was studied in the paper.1,4-phenylene diisocyanate as a monomer was used to synthesize the macromolecular coupling agent PPDI,nanosilica particles as reinforcing phase,nanosilica grafted carbon fiber was prepared by the nucleophilic addition reaction between isocyanate and hydroxyl groups.FTIR,XPS,SEM and other tests showed that nanosilica particles were uniformly and orderly coated on the carbon fibers surface,and the grafting ratio and grafting density of silica nanoparticles were also controlled by changing the reaction conditions.The tensile test was used to characterize the mechanical properties of nanosilica grafted carbon fiber.The results showed the contents of silica nanoparticles grafted onto the carbon fiber surface would directly influenced the mechanical properties of carbon fibers.When the carbon fibers and nanosilica mass ratio was 5:1,modified carbon fibers had the best mechanical properties,tensile strength and fracture toughness were increased to 3.74 GPa and 34.25 J·m-3 compared with carbon fiber precursor.The tensile strength and fracture toughness were increased by 45% and 132%,respectively.AFM measurement results showed that the silica nanoparticles on the carbon fiber surface not only greatly increased the carbon fiber surface roughness and specific surface area,but also effectively enhanced the mechanical properties of modified carbon fiber.The graphene oxide grafted continuous 3D carbon fibers as novel reinforcement,boron trichloride,methyltrichlorosilane and hexamethyldisilazane as monomers,CF-gGO/SiBCN-Ⅰ ceramic composites were prepared by in-situ polymerization and PIP process.XPS and FTIR indicated that the composites were mainly composed of Si,B,C and N and the bonding mode was composed of B-N bond,Si-N bond and Si-C bond.The oxidation resistance of CF-g-GO/SiBCN-Ⅰ ceramic matrix composites in air atmosphere was studied by thermogravimetric analysis.The results showed that when the composite temperature raised to 1400 °C,the loss weight was 26%,which was mainly due to BN oxidation and partial oxidation of the fiber during 700~836 °C.Silica glass phase and boron oxide glass phase generated would prevent further oxidation of the carbon fibers.The compression tests showed that the compressive strength increased with the densities of the composites.After four cycles of impregnation and pyrolysis process,the composites obtained the best compression performance,the x/y direction of the compression strength and compression modulus were 5.56±0.52 MPa and 87.92±8.13 MPa,z direction of the compressive strength and compressive modulus respectively to 0.91±0.32 MPa and 35.10±5.07 MPa,respectively.Nanosilica modified continuous 3D carbon fiber as novel reinforcement,boron trichloride,trichlorosilane and hexamethyldisilazane as monomers,CF-g-nanosilica/ SiBCN-Ⅱ ceramic composites were also prepared by in-situ polymerization and PIP process.The compression results showed that the nanocomposites have better mechanical properties SiBCN-Ⅱ precursor at the same PIP cycles,compared to the SiBCN-?.x/y direction of the resulting composite The compression strength and compression modulus on the x/y direction were 7.36±0.67 MPa and 223.53±16.15 MPa and the compressive strength and compressive modulus on the z direction were 0.92±0.25 MPa and 46.4±9.31 MPa,respectively,which were significantly higher than that of the composite material SiBCN-I synthesis.It was attribute that SiBCN-Ⅱ precursor after pyrolysis at 1400 °C had higher ceramic yield and structural stability.The dynamic oxidation behavior of CF-g-nanosilica/SiBCN-Ⅱ ceramic matrix composites in air atmosphere was investigated by thermogravimetric analysis.The results showed that the oxidation process consists of three stages: the first stage,the composites had no obvious oxidation behavior during 28~826 °C;the second stage,when the temperature raised from 826 °C to 900 °C,the sample weight loss rate soared,which was owing to BN oxidation of SiBCN ceramic on the carbon fibers and cause the pores appearance in the composites,then caused carbon fiber oxidation and leaded to weight loss;the third stage was 900~1200 °C,Silica glass phase and boron oxide glass phase generated to bridge the oxidation process pores,and effectively protected the carbon fiber from damaging at high temperature oxidation environment.Therefore,SiBCN ceramic coating was beneficial to enhance the oxidation resistance of carbon fiber in high temperature environment. |
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