| With the development of hypersonic vehicles,the traditional thermal protection materials(TPM)can not meet the requirements of the thermal protection system(TPS)on hypersonic vehicles.The new generation of the toughness uni-piece fibrous reinforced oxidation-resistant composite(TUFROC)can satisfy the demand of the service environment of the future hypersonic vehicles for high temperature,long time,huge heat flux,high enthalpy,and high dynamic pressure.In this paper,aiming at the urgent needs of the high-temperature zone and the large area of the hypersonic vehicles for the novel TUFROC,the preparation technology of the composite was optimized,the mechanical and thermal properties of the composite were studied,the anti-oxidation coatings on the composite were constructed,and the oxidation and ablation resistance of the coatings was investigated.The SiBOC ceramic was prepared through the sol-gel method,which used the methyltrimethoxysilane(CH3Si(OCH3)3),dimethyldimethoxysilane((CH3)2Si(OCH3)2),and tetrahydroxydiboron(B2(OH)4)as the main raw materials.The effects of the temperature and B on the high-temperature microstructure evolution,crystal ization behavior,and the thermal stability were studied.The results showed the whole inorganic process of SiBOC precursor was completed at 1000°C.During the temperature rose from1000°C to 1400°C,the phase separation proceeded steadily,leading to the transfomation of the Si CxO4-x(0?x?2)and BCyO3-y(0?y?2)units to the Si C4and B(OSi)3units.Moreover,the B accelerated this transformation and the generation of Si C in phase separation.Besides,the introduction of B increased the graphitization degree of the free carbon,which could inhibit the occurrence of carbothermal reduction reaction,and thus improved the thermal stability of SiBOC ceramic.The SiBOC ceramic modified carbon fiber woven(SiBOC/Cf)composites were prepared by the precursor infiltration and pyrolysis(PIP)method,and the morphologies,mechanical properties,thermal properties,and oxidation resistance of the SiBOC/Cfcomposites were studied.The results revealed that more SiBOC ceramic was introduced into the carbon fiber needled felt preform with the increase of PIP times,promoting the coverage effect of SiBOC ceramic on the carbon fibers.The densities of the SiBOC/Cfcomposites were in the range of 0.76-0.79 g/cm3,and the porosities were in the range of63.4-67.2%.The compressive strengths of SiBOC/Cf composites in z direction and x/y plane were 3.1 and 9.0 MPa,respectively,at RT,decreasing to 1.8 and 3.2 MPa,respectively,at 1500°C.The tensile strength in x/y plane was 10.0 MPa at RT but decreased to 4.3 MPa at 1500°C.Furthermore,the thermal conductivities of SiBOC/Cfcomposites in z direction and x/y plane were 0.374 and 1.638 W/(m·K),respectively,at RT,ascending to 1.348 and 3.869 W/(m·K),respectively,at 1200°C.The coefficient of thermal expansion(CTE)increased from 2.374×10-6K-1at 400°C to 5.282×10-6K-1at1700°C.Besides,the weight loss of SiBOC/Cf composites was 23.2 wt%after 60-min oxidation at 800°C,increasing to 26.5 wt%after oxidation at 1000°C for 60 min.A novel joint process of the precursor pyrolysis assisted sintering and the rapid sintering was proposed to construct a Ta Si2-based coating and a Mo Si2-Si O2-Si C(MSS)/Zr B2-Mo Si2-Si C(ZMS)double-layer coating on the SiBOC/Cf composites.The oxidation and ablation resistance of the coated composites was evaluated by the oxygen-acetylene ablation test and the arc-jet wind tunnel test,and the ablation behavior,the microstructure evolution in the ablation process,and the ablation mechanism were investigated.Additionally,the oxidation and ablation resistance of these two kinds of coating systems were compared with each other.The results indicated that under the condition of 1600-1800°C/1000 s in the oxygen-acetylene ablation test,the mass recession rates of the MSS/ZMS coating coated samples increased from 0.47 g/(m2·s)to0.98 g/(m2·s)while those of the Ta Si2-based coating coated samples increased from 0.50g/(m2·s)to 0.99 g/(m2·s),and the linear recession rates of the Ta Si2-based coating coated samples increased from 0.5×10-4mm/s to 1.8×10-4mm/s.Under the condition of1700°C/1000-1500 s,the mass recession rates of the MSS/ZMS coating coated samples ascended from 0.72 g/(m2·s)to 0.86 g/(m2·s)while the counterparts of the Ta Si2-based coating coated samples ascended from 0.74 g/(m2·s)to 0.84 g/(m2·s),and the linear recession rates of the Ta Si2-based coating coated samples ascended from 1.2×10-4mm/s to 1.4×10-4mm/s.After the arc-jet wind tunnel test,a small area of the outer layer coatings closer to the heat flux direction was scoured away while the rest of outer layer coatings was just slightly ablated.Fortunately,the inner layer coatings could maintain the surface morphology.For the MSS/ZMS coating coated samples,the mass and the linear recession rates were 0.67 g/(m2·s)and 5.3×10-4mm/s,respectively,and the highest back-side temperature was 40°C.For the Ta Si2-based coating coated samples,the mass and the linear recession rates were 0.68 g/(m2·s)and 6.1×10-4mm/s,respectively,and the highest back-side temperature was 60°C.Overall,the thermal insulation performance and the anti-erosion ability of the MSS/ZMS coating were superior to those of the Ta Si2-based coating.Moreover,the ablation mechanism could be considered as the successive ablation of outer layer coatings and inner layer coatings,which continuously protected the interior SiBOC/Cf composites. |
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