Preparation And Study Of C_f/Carbon Aerogels Composite And ZrB₂-SiC Coating

The preparation of thermal protection system（TPS）with light weight,high strength and excellent thermal insulation performance is one of the difficulties in the development of hypersonic vehicle key components.The TPS consists of an ani-oxidation layer and an insulating layer,which are prepared separately and then used in combination.This design and preparation have wide designability,but it increases the complexity of the combination process,reduces the reliability of the system and increases the risk of the whole machine.In order to innovate preparation of TPS,reduce the overall density of the material and strengthen the mechanical properties,we studyed and prepared a kind of lightweight C_f/carbon aerogel composite material with dense ZrB₂-SiC ultra high temperature ceramic coating in this paper.It is expected to one of candidate for TPS,at the same time,it can provide novel idea on the preparation of the thermal protection system for researchers.In this study,low-density continuous carbon fiber braid was used as reinforcement to connect the ceramic coating and carbon aerogel.C_f/resorcinolformaledhyde organic aerogel was prepared by sol-gel method,and then C_f/carbon aerogel was obtained by carbonization at inert atmosphere.At last,the ZrB₂-SiC ceramic coating was prepared.In this paper,resorcinol and formaldehyde is chosed as the monomer,hexamethylenetetramine（HMTA）as catalyst and ethanol as solvent.By sol-gel reaction at 75℃,carbon fiber reinforced resorcinol formaldehyde-organic aerogel is prepared.Carbon fiber reinforced carbon aerogel composite material can be received under carbonization process.The mechanical properties are strengthened by repeating the above steps.It is found that the mechanical properties of composites are different in different stress directions,which is caused by the array of carbon fiber braid.When the impregnation times reaches at four,the compressive strength and bending strength at XY direction of the composite increase by 5 times and 27 times reaching 10.93 MPa and 30.27 MPa,respectively.However,the density of composite material increases from 0.22 g/cm3 to 0.40 g/cm3,which merely increases less than one time.When the compressive strain increased to 50% and bending strain increased to 20%,the material won’t damage,which is significantly different from the traditional carbon aerogel materials.The carbon fibers play a important role of strengthening and toughening.In the test of high and low temperature resistance of the composite material,when the cycling temperature ranges from-200℃ to 200℃ and the cycle time is 10 mins,the compressive strength of the composite material doesn’t decay when the cycle times increase to 20.Through high concentration ZrB₂ slurry brushing and several cycles of SiC precursor impregnation,solidification and pyrolysis process,the surface micropores are closed and the dense ceramic coating is prepared.The maximum bending strength of the surface coating reaches 114.32 MPa,and the toughness of the material is improved by comparing with pure phase ceramics.In this study,high strength and low density C_f/carbon aerogel is prepared by organicinorganic conversion method,and dense ZrB₂-SiC ceramic coating is integrated on the surface.This preparation scheme can be applied to design and prepare many kinds of gradient materials.The composite prepared in this paper has the properties with low density and high strength,which is expected to be a candidate material for thermal protection system.