| Accompany with the development of the integrated performance of aircraft,high requirement of assembly composites relating with the low density,low thermal conductivity,thermal resistance,long-term ablation resistance,and electromagnetic wave(EMW)absorption capacity because that takes service under harsh surroundings.For the C/C composites with low-density(?<1.0g/cm~3),owing to the excellent thermomechanical,heat-shielding,thermal-shock resistance,and good microwave absorption,they have widely applied to the field of aircraft thermal protection system(TPS).However,C/C composites are easily oxidated in a thermal oxygen environment and would restrict to engineering applications.It is the main route to enhance carbon yield ratio and thermostability through inducing the element Boron(B)into the chemical structure of phenolic resin.Indeed,by inducing ceramic SiOC into the C/C composites proposing with boron phenolic resin as the matrix phase,and its ablative resistance will be significantly improved.Therefore,it is necessary to study the preparation and performances of SiOC modified C/C composites with low-density.In this paper,the microstructural regulatory mechanism is revealed for the SiOC modified C/C composites with low-density through adjusting the process parameters of production.The relationship between the microstructure and performance are illuminated by investigating the key performances of the composites.The SiOC modified C/C composites with low-density proposed present excellent integrated performances.First,Boron phenolic resin was synthesized with the potassium borate as a catalyst,and by using the optimized productive variables determined by the experiments.Compared with traditional phenolic resin,a higher carbon yield ratio of boron phenolic resin was obtained after pyrolyzing treatment.The curing reaction mechanism of boron phenolic resin and the reason of influencing its surface tension was analyzed.It is revealed that the surface tension decreased with the increase of the action temperature and concentration.Second,taking the boron phenolic resin as the precursor,and on this basis,combining with SiOC ceramic precursor,SiOC modified C/C composites possessed lower density was successfully prepared by vacuum precursor infiltration pyrolysis(PIP)process.The influencing facts of PIP time and pyolysis cycles to the microstructure and density,and heat-insulating property of the C/C and SiOC composites were studied.The thermal oxidation resistance and stability were characterized under room temperature up to the 1000?in aerobic and argon atmospheres,respectively.Third,a study on the variation of ambient temperature affects the C/C and SiOC composites were implemented.The compressive and flexural performances,and damage mechanism for the both composites in the range from room temperature up to1200?were investigated.The effect of PIP cycles on the compressive performances of along the Z(vertical to carbon fiber pregreg layered)and X/Y(parallel to carbon fiber prepreg layered)directions were experimentally characterized.The transformation law of the compressive strength enhances with increasing the ambient temperature was revealed.The reason for two typical failure modes is illuminated through investigating the flexural property and fracture toughness property.By defining the mechanical damage variable,comparing with the experimental and FE simulation results,the damaged distribution zone and evolution rule of the composites under different ambient temperature were obtained.Finally,the ablative resistance and EMW absorption property of both C/C and SiOC modified composites with low-density were studied.The mass-and linear ablativities were separately inspected through the static-and dynamic thermal oxidation experiments.The reasons for the oxidation-and ablation resistance of the SiOC composites were clarified.Through inspected EMW attenuation under specific waveband(2-18GHz),the enhanced mechanism of EMW absorption for the SiOC modified composites with low-density was specified. |
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