| With the development of hypersonic aricraft,traditional insulation materials can not meet the requirements of the vehicle thermal protection system.Thus,it is urgent to solve the brittleness of the traditional thermal insulation tiles with the lightweight thermal protection materials.The new generation of toughness uni-piece fibrous reinforced oxidation-resistant composite,has effectively solved the brittless of ceramic insulation tiles.Besides,the composites can meet the future aircraft environment need of long time high temperature reistance and anti-oxidation.In this paper,we are aiming at the windward side to prepare thermal protection materials and study on ablation properties of the composites.The lightweight,high porosity and low thermal conductivity of carbon-bonded carbon fiber?CBCF?composite was reinforcement and SiOC ceramic was matrix.The mechanical and thermal properties of composite in high temperature environment were discussed;then in-situ growth of SiC nanowire reinforced SiOC/CBCF composites was also studied.Finally,TaSi2-based multicomponents,multigradient with anti-oxidation coatings were designed and the anti-oxidation properties of the coating were investigated via static oxidation,oxygen acetylene flame and arc wind tunnel test.With methyltrimethoxysilane and dimethoxydimethylsilane as precursors,using Impregnation and pyrolysis?PIP?technology to obtain lightweight SiOC/CBCF composites with density of 0.54g/cm3.The thermophysical properties of SiOC/CBCF composites in high temperature were investigaged.The results demonstrated that the compressive strength decsended,while specific heat incaresed with the tempareature.The thermal conductivity and coefficient of expansion were 0.38±0.03W/m·K and2.883×10-6/°?,respectively at 100°? and increased to 8.10±0.04W/m·K and4.493×10-6/°?,respectively when the temperature rose to 1600°?.The thermal stability under air atmosphere of the SiOC/CBCF composites was investigated by static oxidatio test.The resulats showed that the time of reaching maximum of weight loss?40%50%?of the composites was decreased with the temperature.The residual strength of the composites was 2.15±0.31 MPa when the composites oxidated for 15 min at 500°? and decreased to 0.03±0.01 MPa after they were oxidated for 60 min.In order to increase the mechanical propertity of SiOC/CBCF composites,three-dimensional?3D?SiC nanowires were in-situ grown in SiOC/CBCF composites using poly?methylhydrosiloxane??PHMS?and 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxan?TMTV?as precursors.The evolution of structure and phrase of the SiOC ceramics were studied;based on the evolution of SiOC ceramics,we obtained the growth mechanism of in-situ growth of SiC nanowires in SiOC/CBCF composites.The growth mechanism of SiC nanowires belonged to vapor-solid mechanism,in which SiOC ceramic phase separation happened to generate SiO2 at high temperature,SiC nanocrystals and carbon free;then SiO2 produced Si steam due to carbothermal reduction reaction,and further free carbon reacts to form SiC nanowires.The mechanical properties of 3D SiC nanowires reinforced SiOC/CBCF composites were studied.The results shown that the bending strength in x/y direction increased at a rate of 80%and in z direction increased at a rate of 38%;the tensile strength in x/y direction increased at a rate of 50%and in z direction increased at a rate of 350%.The 3D SiC nanowires improved the mechanical properties of the composites by pull-out,briding and brittle fracture.In order to improve the poor oxidation resistance of SiOC/CBCF composites,a TaSi2-based multicomponents coating,including MoSi2 as radiation agent,borosilicate glass as self-healing phase and SiB6 as addivtives was designed.According to the special structure of the matrix material,a gradient coating which consisted of porous layer and dense layer was prepared.Through study of the densification of the coating process,the sintering method for preparing high density coating was rapid sintering in air atmosphere and the proper sintering temperature was 1350°?1450°? and the sintering time was 15 min at 1450°?.Besides,emissivity of the coating increased with the temperature.The emissivity of TaSi2-based coating was 0.8 at 1000°?,whereas the MoSi2-based coating was 0.87 at 1000°?.Increase the density of the coating can improve the emissivity of the coating;when the TaSi2-based coating was oxidated for60 min at 1500°?,the emissivity was significantly lower than the un-oxidized coating.The emissivity of oxidated coaing was 0.81 at 1000°?.The bonding strength between coating and matrix was higher than the strengh of the matrix iteself via tensile strength tests.The weight loss of TaSi2-based and MoSi2-based coating was 0.022%and 0.076%,respectively after the composites were oxidated for 160 min at 1500°? using static oxidation test.Oxyacetylene flame test showed that with the increase of surface tempeature,weight loss and ablation rate of the coating gradually increased.When the surface temperature was 1600°? after ablation for 1000 s,the weight loss rate and ablation rate were 1.03×10-3g/s and 1.2×10-3mm/s,respectivley.Plasma wind tunnel test showed that with the increase of heat flux,the ablation rate from 1.23×10-6g/s·cm increased to 9.39×10-6g/s·cm.The mechanism of oxidation ablation was also studied.With the increase of heat flux,the volatilization of borosilicate glass in the coating became serious.Lacking of the glass,Ta2O5 could not be bonded to the surface of the coating,and then it was peeled off gradually and a porous layer was exposed to the bottom.Finally,a hybrid thermal protection material?TPM?,consisting of TaSi2-based coating on carbon-bonded fiber?CBCF?composites and mullite fiber insulation tiles were prepared and the heat-insulating property of the hybrid structure was investigated by arc wind tunnel test. |
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