Preparation And Performance Of Ablation Resistant, Wave-Transparent Nitride Ceramic Matrix Composites And Radome

As one of key components of hypersonic missiles, the radome must possess multifunctions of thermal protection, electromagnetic transparence and load bearing to meet the requirements of the homing guidance systems. In this dissertation, based on the comprehensive review of aerospace wave-transparent materials and ablative materials at home and abroad, aiming at the requirements of different functional sections in the function separated hypersonic missile radome, the materials system in each functional section was designed, a new precursor for nitride ceramics, hybrid polyborosilazane (H-PBSZ) was synthesized, carbon fiber reinforced nitride matrix (CFRN) ablative composites and silica fiber reinforced nitride matrix (SFRN) wave-transparent composites were prepared by precursor infiltration and pyrolysis (PIP) process, and the preparation of the coating for ablative composites were explored primarily. Finally, integral formation of different functional sections was realized, and full sized monolithic radome was fabricated.H-PBSZ, a new precursor for nitride ceramics, was synthesized, and the curing and pyrolysis process of H-PBSZ was investigated. H-PBSZ was obtained from the copolymerization of borazine and perhydropolysilazane, which were synthesized separately. Chemical bonds of N-H, B-H, Si-H, B-N, Si-N and so on exist in the structure of H-PBSZ. H-PBSZ is stable in airtight atmosphere at low temperatures, and could be cured and solidified when heated. With the increase of atmospheric pressure in curing process, the volumn of bubbles originated from the precursor is decreased, and the floating velocity of bubbles is slowed down, the curing process can be calmly performed. Moreover, the reaction which emits gases can be restrained, and the amount of the gases is decreased. Therefore, the foaming phenomenon is effectively restrained. The cured H-PBSZ can be pyrolyzed in ammonia gas at high temperatures. With the increasing pyrolysis temperature, both the crystallinity and stability of pyrolytic product are increased. H-PBSZ is fully ceramized at 1600℃, and the ceramic yield is about 83wt%. The ultimate pyrolytic product of H-PBSZ is a mixture comprising BN and Si₃N₄. H-PBSZ is an appropriate preceramic precursor for PIP process.3D CFRN composites were prepared from H-PBSZ by PIP process for the first time. With the increase of PIP cycles, the density of the composites increases, the porosity decreases, and the composite becomes almost dense after four cycles; the mechanical properties are enhanced, including flexural strength and elastic modulus; and ablation resistance is improved. In the curing stage, with the increase of curing pressure, the foaming phenomenon is effectively restrained; the density and integrative properties of the composites are elevated. The density of 3D CFRN composite cured at 8 MPa reaches 1.65g/cm³. Both the density and mechanical properties of the composites increase as the pyrolysis temperature increases from 800 to 1300℃. The infiltration efficiency of the precursor and the relative density of the composites are hardly changed with the variation of pyrolysis temperature. When heat treated at 2100℃, Si₃N₄ in the matrix of 3D CFRN composites is decomposed, and the existence of B₄C and SiC is detected, which indicates the interfacial chemical reactions between nitride matrices and carbon fibers. The mechanical properties of heat treated composites decline severely.Ablation properties of 3D CFRN composites were characterized, and the variations of structure and morphologies of the composites during ablation were analyzed. Carbon fibers in the composites are protected effectively by nitride matrices when ablated, and the composites exhibit excellent ablation resistance. The ablation properties of 3D CFRN composites reinforced by T300 and T700 carbon fibers are almost the same. Nitride matrices display better ablation resistance than carbon fibers when the composite is ablated by the electric arc heater. Si₃N₄ on the ablated surface of 3D CFRN composite is decomposed, while BN and carbon fibers are crystallized. The ablation of 3D CFRN composites is a combinative process of thermochemical erosion and mechanical denudation.The effects of the type of carbon fibers and the fiber surface treatments on the mechanical properties of 3D CFRN composites were investigated. The mechanical properties of 3D CFRN composites reinforced by T300 and T700 carbon fibers are quite different. Lots of fiber pull-out can be observed on the fracture surface of the composite reinforce by T700 fibers, and it is stronger and tougher than that reinforced by T300 fibers. The different surface states of different carbon fibers result in different fiber/matrix combinations and different mechanical properties of the composites. The size on the surface of carbon fiber can be eliminated by both surface oxidation in air at 400℃and heat treatment in inert atmosphere at 1000℃, and the fiber can keep enough residual strength. Surface oxidation of carbon fiber can improve the mechanical properties of the composite reinforced by T700 fibers, while it is harmful to the composite reinforced by T300 fibers. The heat treatment of carbon fibers causes little improvement for the mechanical properties. The variation of the mechanical properties is attributed to the change of fiber/matrix combination derived from the surface treatments. The flexural strength and elastic modulus of 3D CFRN composite reinforced by surface oxidized T700 carbon fibers, cured by high atmospheric pressure curing process and pyrolyzed at 800℃are 268.4MPa and 67.6GPa, respectively.The preparation of erosion resistant coatings for 3D CFRN composite was investigated primarily. SiC was selected, and chemical vapor deposition (CVD) was performed to prepare the coatings. Liquid carborsilanes which could be used as the precursor for CVD SiC were synthesized. There is no corrosive element in the structure and there is no corrosive byproduct when liquid carbosilanes are used as the precursor for depositing SiC materials. With the increase of deposition temperature, liquid carbosilanes are pyrolyzed more completely, and pure SiC which is partly crystallized can be deposited at 900℃. SiC coatings are obtained when the flow rate of carrier gas is relatively low, while nanosized SiC powders are deposited when the flow rate of carrier gas is high enough. CVD SiC coating deposited on the surface of 3D CFRN composite is compact and hard, and the microhardness of the coating is about 2800～3200 kgf/mm² (HV). The ablation resistance of 3D CFRN composite can be improved considerably with the existence of SiC coating.3D SFRN composites were prepared using H-PBSZ conversion method. Compared with normal atmospheric pressure curing process, 3D SFRN composites cured by high atmospheric pressure curing process exhibit much more better integrative properties, the density is increases by 8%, the flexural strength and elastic modulus are increased by 35.5% and 102.4%, and the oxyacetylene linear ablation rate is decreased by 26.7%. With the increase of pyrolysis temperature, the embrittlement of silica fibers becomes more serious, and the mechanical properties of 3D SFRN composites decline gradually. The density of 3D SFRN composite cured by high atmospheric pressure curing process and pyrolyzed at 800℃reaches 1.83g/cm³, the flexural strength and elastic modulus are 148.2 MPa and 41.5GPa, and the oxyacetylene linear ablation rate and mass ablation rate are 0.11mm/s and 1.07×10^-2g/s. The dielectric properties of 3D SFRN composites prepared by different processes are relatively stable, the dielectric constant is 3.31～3.34, and the loss tangent is 3.8～4.9×10^-3.Aiming at the integral formation of the function separated monolithic radome, the materials system and preparing process of each functional section were optimized. As the candidate materials for each function section, 3D CFRN and 3D SFRN composites have similar coefficient of thermal expansion, and they display good compatibility during the preparing and ablation processes. Based on the application environment, the radome materials are confirmed to be reinforced by 2.5-dimensional fabric, which can ensure the mechanical properties both in longitudinal direction and in latitudinal direction. Hybrid fiber reinforced nitride (2.5D HFRN) composites are used in ablation-load bearing section, wherein carbon fiber is longitudinal and silica fiber is latitudinal; silica fiber reinforced nitride (2.5D SFRN) composites are used in wave-transparent section. The radome materials were prepared from H-PBSZ by PIP process. The longitudinal flexural strength of 2.5D HFRN composite is 259.1 MPa, the elastic modulus is 65.3GPa, the fracture toughness is 10.78 MPa·m^1/2, thermal conductivity and specific heat are 1.20W/m·K and 0.80 kJ/kg·K, and the oxyacetylene linear ablation rate and mass ablation rate are 0.058mm/s and 5.52×10^-3g/s. The longitudinal flexural strength of 2.5D SFRN composite is 142.7MPa, the elastic modulus is 40.3GPa, thermal conductivity and specific heat are 0.80W/m·K and 0.81 kJ/kg·K, the oxyacetylene linear ablation rate and mass ablation rate are 0.107mm/s and 1.06×10^-2g/s, the dielectric constant and the loss tangent are 3.34 and 4.0×10^-3, and the wave transmittance is higher than 70% in the frequency range from 7.5 to 18.5 GHz. The properties of both composites are excellent, and they can meet the requirements of hypersonic missile radomes.The technological process for preparing full sized radome was analyzed, the parameters of PIP process were optimized, and the machining opportunity was selected. The integral formation of different functional sections was realized, and full sized, thin-walled monolithic radome which had exact shape and accurate size was fabricated.