Preparation And Performance Optimization Of Low Cost Carbon Fiber Preform Reinforced SiOC Composites

Carbon fiber reinforced silicone resin derived SiOC ceramic（C/SiOC）composite is a high-temperature structural material with good application prospect due to its performance/cost ratio.It is.In order to further reduce the cost,based on the previous work of our research group,carbon fiber needled felt with low price was selected as the reinforcement,and C/SiOC composites were prepared by precursor impregnation pyrolysis（PIP）process.In view of the low fiber volume fraction of carbon fiber needled felt,the interfacial bonding state and fiber volume fraction of composites were improved by introducing interfacial C coating and hot molding process,so as to improve the mechanical properties of C/SiOC composites.After obtaining desirable mechanical properties at room temperature,the high temperature stability in inert atmosphere and oxidation resistance in static air of C/SiOC composites was studied,which laid a foundation for their application as high temperature structural materials.Interfacial C coating was prepared on the surface of carbon fiber by liquid-phase dipping and pyrolysis met hod using phenolic solution as raw material.The effect of interfacial C coating on the mechanical properties of C/SiOC composites at room temperature was studied.It was found that interfacial C coating not only can hinder the adverse interfacial reaction between fiber and matrix,but also weaken the interfacial bonding,thus improving the mechanical properties of composites at room temperature.By optimizing the concentration of phenolic solution and the cycles of dipping and pyrolysis,a dense C coating with moderate thickness was obtained.As a result,the bending strength of C/SiOC composites at room temperature was increased from 214MPa to 267 MPa,and the fracture toughness was increased from 11.5 MPa·m^1/2 to 13.7MPa·m^1/2.The hot molding process was introduced in the first cycle of PIP route.By optimizing the concentration of precursor solution,molding temperature and pressure,the fiber volume fraction and matrix content in the first cycle were effectively improved without damaging the structure of needled felt.Accordingly,the bending strength at room temperature and fracture toughness of C/SiOC composites were increased to 331MPa and 16.0 MPa·m^1/2,respectively.If the interfacial C coating and molding are coupled together,that is,the molding process is introduced during the preparation of the interfacial C coating,the room temperature mechanical properties of C/SiOC composites can also be significantly improved.The bending strength and fracture toughness can reach 292 MPa and 16.0MPa·m^1/2,respectively,which is equivalent to the effect of hot molding process.It was found that the content of SiOC matrix in the fiber bundle was relatively low and the distribution was not uniform.Thus the higher mechanical properties were not obtained.In comparison with three-dimensional braided（3d-B）C/SiOC composites in our previous work,the needled felt C/SiOC composites in this study show the same mechanical properties even though its fiber volume fraction（18.2%）was about half of that in 3d-B composites.The cost-effective advantage of the needled felt C/SiOC composites is evident.The high temperature stability of C/SiOC composites with interfacial C coating and hot molding in oxygen-free environment was investigated.The results show that Si atoms would diffuse into the carbon fiber at high temperature,result in chemical damage of C fiber and reducing the strength.Interfacial C coating can effectively hinder the diffusion of Si atoms,and hot molding can reduce the diffusion of Si atoms to carbon fibers.Therefore,the strength retention of the two composites after heat treatment at 1400℃for 1 h in inert atmosphere was as high as 90%.The matrix becomes loose due to the obvious activation of carbothermal reduction reaction,resulting in the further decrease of the strength,the strength retention rates was 83.7%and 87.1%at 1500℃,respectively.In addition,it is found that properly increasing the thickness of interfacial C coating prolong the service life of C/SiOC composites at high temperature without significantly sacrificing its mechanical properties at room temperature.Due to the influence of ambient pressure on the carbothermal reduction reaction of SiOC matrix,the strength would decrease significantly after heat treatment at 1400℃in vacuum for 1 h,the strength retention rate was 31.4%.The oxidation behavior of C/SiOC composites in static air was investigated.It was found that the micro cracks and open pores provide channel for the diffusion of oxygen to the interior of the composite,and then oxidize the carbon fiber,carbon coating and free carbon in the matrix,which responsible for the oxidation failure of the composite.XCT detection indicated that the z-direction fiber in the needled felt was the weak point of oxidation due to most of the holes oncentrated around the z-direction fiber.After oxidation at 1200℃for 0.5 h in air environment,the strength retention rate of the material is 77.7%,which can be slightly increased to 80.1%by silicone oil sealing treatment.The insufficient improvement was ascribed to exposion of the Z-direction fiber.In contrast,the fiber was effectively protect by mullite coating with Al₂O₃-Si O₂sol as raw material.As a result,the strength retention rate of the material increased to95.3%,significantly improved oxidation resistance was observed.