TiB₂-Al-wollastonite Synergistic Modification Of High Silica Fiber/Boron Phenolic Ceramicizable Composites

Based on the urgent demands of hypersonic technology for high-performance thermal protection materials used in extreme environments such as high enthalpy,high pressure,and strong shear,ceramizable resin matrix composites are currently one of the protective materials that meet the needs of thermal insulation and thermal protection.Silicon-based composite materials are ablation materials with SiO₂ as the main component,also known as melting-gasification type ablation materials.They are simple in process,low in thermal conductivity,perfect in density and good in ablation resistance in the thermal protection system of the aircraft.However,since the latent heat of evaporation of SiO₂ is only one-third of the average sublimation heat of carbon,and the high-temperature ablation performance is far less than that of ceramic-based thermal protection materials,these shortcomings greatly limit the application of silicon-based materials.In order to alleviate or improve these shortcomings,this paper will focus on the high silica fiber/boron phenolic resin composite material,explore the theoretical structural changes and the corresponding performance relationship of resin and fiber from low temperature to high temperature on the temperature axis,and analyze the material failure principle and high temperature ceramic transformation process.By introducing the ultra-high temperature ceramic TiB₂,and utilizing the thermal material properties of the ceramic filler,a ceramic polymer matrix composite material with excellent ablation resistance was prepared.The gradient design of the material is carried out in combination with the flux to promote the ceramic transformation of the molten silica liquid layer and the resin cracking product,preventing or slowing down the thermal peeling of the molten layer on the surface of the material,thereby improving the ablation resistance of the material.In this paper,the high silica fiber/boron phenolic resin composite material is first prepared by the prepreg-molding process,and wollastonite is added to the material as a high temperature stabilizer.Changes and ceramic transformation processes were studied.The most intuitive result of static ablation is the color change of the material.The maximum decomposition temperature of the boron phenolic resin matrix is500℃.The connecting structure of the three-dimensional network is broken between the carbon chains（methylene groups）,and the active groups are oxidized.The boron element is separated from the resin system and oxidized to B₂O₃,and various volatile small molecules are produced at the same time.The weight loss of organic resins is the main reason for the weight loss of composites at low temperature.The fragments produced by the cracking contain a large number of benzene ring structures,and the benzene ring is dehydrogenated into amorphous carbon at 700℃-800℃,at which time the material almost loses its mechanical strength.At 1200 ℃,"fishbone" like holes appeared on the surface of the material,and they healed with the increase of temperature,forming a dense eutectic layer.XRD test shows that the resulting eutectic melts are silicate and borate,and the high silicon-oxygen melt has heterogeneous nucleation to form cristobalite phase.The research on high silica fiber plain weave shows that the sizing agent and other defects on the fiber surface will cause the fiber to absorb water spontaneously,which will affect the interfacial strength of the composite material,and the average weight loss rate after drying treatment is 2%.The fiber cloth melts at about 1200℃,and 1200℃-1400℃ is an important process of silicon melt formation and internal structure rearrangement.A large number of glassy SiO₂ nucleates and transformes into cristobalite phase,and the macroscopic fiber color changes from white to yellowish transparent,it has completely lost its mechanical strength.The silicon series at 1600℃ is polycrystalline,which means that the optimum working temperature of high silica fiber does not exceed 1200℃.On the basis of high silica fiber/boron phenolic resin composite material（original composite material）,the ultra-high temperature ceramic TiB₂ was further added to prepare TiB₂ modified high silica/phenolic resin composite material（TiB₂-composite material）,and the research on ultra-high temperature ceramics was carried out.Study on the effect of adding ultra-high temperature ceramics on the ceramic performance and high temperature thermal protection performance of ablation materials.Infrared analysis shows that the addition of TiB₂ will not affect the curing of boron phenolic,but the oxidation of ceramics will delay the cracking process of the resin.The weight gain of TiB₂ in the air atmosphere can reach 184.8%,and the overall weight loss rate of the TiB₂-composite is 4.62% higher than that of the original material,indicating that the addition of TiB₂ can significantly reduce the mass loss.In addition to the TiO₂ and cristobalite phases,the phase composition of the new ceramic layer also contains aluminosilicates and borides.XPS analysis captures Si C,an important product of the carbothermic reduction reaction,which can dispersion-strengthen the surface melt and effectively improve the stability of the high-temperature melt.The appearance of Ti N provides an effective proof for nitrogen fixation in the material.Using flux metal aluminum and low-melting glass frit for gradient design,the innermost layer is the core layer with high metal content,and the transition layer with gradient concentration of glass frit from the inside to the outside is used to prepare a gradient composite material,ablative properties.The results show that the ablation rate of the gradient composites is better than that of the single-filler composites（Al-composite and glass frit-composite）.Stable support core layer after the original structural features disappear,the new metal skeleton has higher structural strength than the glass phase skeleton.The cladding of glass melt at high temperature can close pores and cracks,and improve the resistance to heat flow erosion and oxidation.The gradient composite material forms a ceramic layer with certain strength after ablation treatment,and the bending strength after cooling（treatment at 1400 ℃）is as high as 24.11 MPa.Low-temperature resin cracking fragments,inorganic mineral fillers and glass frit undergo solid-phase reaction in silicon melt to form various silicates and other solid solutions.Together with the quartz phase and cristobalite phase formed in silicon-based melt,they form a multiphase ceramic layer significantly improves the ablation resistance of silicon-based ablation composites.