Preparation And High Temperature Properties Of C_f/SiCN Composite Prepared By PIP Process

Carbon fiber reinforced silicon carbonitride composite material(C_f/SiCN)has high specific strength,high specific modulus,high temperature resistance,oxidation resistance,wear resistance,and good thermal stability.It is an aerospace thermal structural material with great application potential.The precursor impregnation and cracking(PIP)method is the main method for preparing C_f/SiCN composites.The PIP method has the advantages of simple equipment,low preparation cost and no pollution to the environment.Polysilazane HTT 1800 was used as the precursor of ceramics to study the solidification and cracking mechanism of polysilazane ceramic precursors.The 3D needled carbon fiber preform and 2D laminated carbon fiber cloth were used as reinforcements respectively,the effects of different pyrolysis temperatures and cycles of impregnation,solidification and pyrolysis on the microstructure,mechanical properties and interfacial bonding conditions of the composites were investigated,and the optimum process parameters for the preparation of C_f/SiCN composites by PIP method were obtained;and the mechanical properties,microstructure,and microscopic interface of composites under different temperature shocks and service temperatures were studied.The polysilazane precursor undergoes solvent and oligomer evolution(below250~?),vinyl polyaddition reaction and deamination polycondensation reaction(250~?~500~?),and inorganicization process(above 500~?),at the temperature of900~? it is basically transformed into amorphous SiCN ceramics,when the pyrolysis temperature rises above 1100~?,the ceramic gradually devitrifies the?-SiC phase.The optimal process parameters for preparing C_f/SiCN composites by PIP method are curing temperature of 250~? and pyrolysis temperature of 900~?.The C_f/SiCN composites prepared by using 3D needled carbon fiber preforms have better density and mechanical properties.The composite material has been immersed in six cycles of impregnation,curing and pyrolysis processes,and the resulting SiCN composite has a density of 1.60g/cm~3.The open porosity was 19.0%,the room temperature flexural strength was 75 MPa,and the elastic modulus was 66 GPa.When the thermal shock temperature rises from 780~? to 1180~?,the residual flexural strength retention of C_f/SiCN composites decreases from 93.3%to 14.6%,and the fracture mode of the composites changes to brittle fracture;when the thermal shock temperature difference reaches 980~? or above,the carbon fiber surface is completely Oxidation,residual large number of pores,serious internal carbon fiber oxidation,detackification between some fibers and the matrix,material degradation;thermal shock temperature reached 1180~? when the fiber is completely oxidized,fibrin oxidation residue hole cracks produce significant cracks,so that the material properties Dramatically degraded.The flexural strength of composites decreases with the increase of temperature.When the temperature is lower than 800~?,the composites still have high strength.When the temperature reaches 1000~?,the bending strength of the composites decreases to 35 MPa and brittle fracture occurs.With the increase of temperature,the oxidation of carbon fiber on the surface of composite material is intensified.When the temperature reaches 800~?,the carbon fiber on the surface of the composite material is completely oxidized,and there is a large number of voids on the surface,and there is no obvious crack.In the composites with elevated temperature,the appearance of fiber pullout appears to be reduced,and the fiber shrinkage occurs with the matrix.When the service temperature reaches 1000~?,the internal carbon fibers of the composite are basically oxidized,and the remaining matrix has no obvious cracking phenomenon,so that the material retains certain strength,but is brittle.fracture.When the heat treatment temperature of the composite material is below 1000~?,the composite surface still exhibits an amorphous SiCN structure and the ceramic matrix structure is stable.When the heat treatment temperature reaches 1200~?,the ceramic matrix is oxidized and the diffraction peaks of alpha-cristobalite appear in the XRD pattern.The mechanical properties of the ceramic matrix have dropped dramatically.