Designs, Preparations And Properties Of The SiCf/SiC Radar Absorbing Materials With Sandwich Structures

This study is based on the high-temperature radar absorbing material (RAMs) requirements for the new generation military areocraft, such as supersonic cruise missile and battleplane. Owing to the advantages of excellent mechanical and dielectric properties, the homemade continuous SiC fiber-reinforced SiC matrix (SiCf/SiC) composites were chosen as the high-temperature RAMs. By the innovative design of one kind of RAMs with sandwich structures, and based on the sloven key technique of the dielectric property adjustment of the SiC fiber, the interphase preparation of the SiCf/SiC composites and the evolvement mechanism investigation of the dielectric properties of the SiCf/SiC composites with temperature, the SiCf/SiC RAMs with excellent absorbing properties were developed successfully for the first time in our country. This study has initiating meaning in the continuous fiber-reinforced ceramic matrix RAMs and establishs the foundation for the further investigations.The sandwich structural radar absorbing materials (SSRAMs) was put forward for the first time, and the parameter optimizations of the SSRAMs were done by the self-programing software. The results show that the excellent absorbing properties of the SSRAMs can be attained when the thickness of the SSRAMs is 5~6mm and the parameters of the dielectric layers and absorbing layers are appropriate. The absorbing bandwidth below -10dB can reach 12GHz. The parameter sensitive degree of the SSRAMs was analyzed, and it can be found that the parameter latitude of the SSRAMs is fine, which is good for the realization of the SSRAMs. The absorbing properties of the SSRAMs were validated by the resin matrix composites, and the experimental results are good agreement with the calculated results, which prove the validity of the optimization methods. At the same time, the property requirements for constitutes of SSRAMs are definite, and the theoretical guides for the investigations of the SiCf/SiC RAMs are founded.In order to meet the dielectric property requirements for the dielectric layers of the SSRAMs, the raw-SiCf/SiC composites, reinforced by the original KD-SiC fiber, were fabricated by the HP-PIP (hot press-precursor infiltration and pyrolysis) process, the flexural strength and fracture toughness of which are 268.8MPa and 12.9MPa·m1/2, respectively. The dielectric properties of the raw-SiCf/SiC composites have also been studied, and the results show that the dielectric constant of the raw-SiCf/SiC composites is remarbable, which is far away from the dielectric property requirements for the dielectric layers of the SSRAMs. XPS and HRTEM analysis show that a 20~30nm turbostratic pyrocarbon (PyC) layer exists on the surface of the original KD-SiC fiber, which is the prime factor that leads to the high dielectric constant of the raw-SiCf/SiC composites, so the PyC layer must be removed. By comparing with several decarbonization methods, the process of oxidation under air atmosphere was chosen. The effects of the oxidation parameters on the properties and microstrutures of the SiC fiber and 2D-SiCf/SiC composites were investigated systematically. It is found that the strength retention ratio of the SiC fiber is 89.6%, the electrical resistivity can reach 106?·cm and a SiO2 layer forms on the surface of the SiC fiber after 60min oxidation under 600℃. At the same time, the dielectric constant of the SiCf/SiC composites can meet the dielectric property requirements for the dielectric layers of SSRAMs. However, the flexural strength of the SiCf/SiC composites is only 48.4% that of the raw-SiCf/SiC composites. In order to make clear the reasons, the fracture surface morphology and interfacial microstructures of the SiCf/SiC composites were analyzed. It can be concluded that, after the PyC layer being removed, the strong interfacial bonding and large fiber damage occur in the SiCf/SiC composites, which result in the poor mechanical properties of the SiCf/SiC composites. So, the appropriate interphase must be prepared in the SiCf/SiC composites.To improve the interfacial characteristics of the SiCf/SiC composites, two kinds of interphase, SiC and BN, were investigated. The contiuous and compact SiC coating about 0.4μm thick was deposited on the SiC fiber using the optimal process parameters by chemical vapour deposition (CVD) process, and the mechanical properties of the SiCf/SiC composites can be improved obviously, the flexural strength of which is 1.5 times that of the SiCf/SiC composites without the CVD-SiC interphase. However, it is found that the dielectric constant of the SiCf/SiC composites with the CVD-SiC interphase is very large, which is attributed to the existence of the carbon-rich layer within the CVD-SiC interphase. With the low dielectric constant and layered-crystalline structural characteristics, the BN is the ideal interphase material for SiCf/SiC composites. The BN coating was fabricated by the dip-coating process, and it is found that the continuous and compact BN coating can be attained by the repetitious infiltration and heat treatment process with the low concentration precursor solution. The results show that the flexural strength of the SiCf/SiC composites can reach the maximum after 3 times of BN interphase treatment, which is 1.4 times that of the SiCf/SiC composites without BN interphase. In addition, the dielectric properties of the SiCf/SiC composites with the BN interphase is nearly the same as that of without BN interphase, which can meet the dielectric property requirements for the dielectric layers of the SSRAMs.Because the SiCf/SiC RAMs mostly serve under high-temperature environment, the high-temperature dielectric constant of the SiCf/SiC composites and high-temperature sheet resistance of the SiC fiber cloths were investigated for the first time in our country. Debye equation analysis and experimental results all show that both the real and imaginary part of the dielectric constant of the SiCf/SiC composites increase with the increasing of the temperature. Theε' increases from 6.3 at room temperature to 6.6 at 700℃, and theε" increases from 0.1 to 0.4. The measurement results of the electrical conductivity of the SiCf/SiC composites at high-temperature show that the loss due to conductivity is low. As a whole, the changes of the dielectric constant of the SiCf/SiC composites with temperature are insignificant, and the dielectric constant of the SiCf/SiC composites at different temperature all can meet the dielectric property requirements for the dielectric layers of the SSRAMs. The EDS and TG anaylsis show that somewhat oxidation exists for the SiCf/SiC composites, but mostly occurs on the surface of the SiCf/SiC composites. The oxidation leads to a slight decrease of dielectric constant after high-temperature testing. Theε' decreases from 6.3 to 6.1, and theε" from 0.1 to 0.04. The relationship between the temperature and sheet resisitance of the SiC fiber cloths under N2 atmosphere was studied, and the results show that the sheet resistance decreases nearly linearly with the increasing of temperature, which is due to the positive temperature-electrical conductivty coefficient (PTC) of the PyC. The sheet resistance of the SiC fiber cloths decreases from 120?/□and 160?/□at room temperature to 95?/□and 120?/□respectively, and the circulation characteristics of the temperature-sheet resistance are excellent. Under air atmosphere, the sheet resistance of the SiC fiber cloths increases remarkably above 400℃due to the oxidation of the PyC layer, so the oxidation of the SiC fiber cloths must be avoided in the SiCf/SiC RAMs.After solving the problems of the structural design, adjustment of dielectric properties, improvement of the mechanical properties, and evolvement mechanism investigation of dielectric properties with temperature of SiCf/SiC composites, the preparation conditions of SiCf/SiC RAMs have been possessed. The SiCf/SiC RAMs were fabricated according to the optimal parameters, and the reflectivity measurement results are in agreement with the calculated results, which all exhibit the excellent absorbing properties. The high-temperature reflectivity of the SiCf/SiC RAMs was also measured, and it is found that the range of the reflectivity change with temperature is small, and the absorbing properties are all excellent at different temperature. The evolvement mechanisms of the reflectivity of the SiCf/SiC RAMs with temperature were investigated, and it can be confirmed that the change of the sheet resisitance of the SiC fiber cloths with temperature due to the PTC characteristics of the PyC layer of the SiC fiber is the most important factor.