Preparation And Properties Of SiC_f/SiC High-temperature Structural Microwave Absorbing Composites

The rapid development of high Mach number aircrafts,such as supersonic cruise missile and new generation stealthy warplane,makes it extremely important and imperative for improving the stealth capability of high-temperature sections of the aircrafts.Silicon carbide fiber reinforced silicon carbide matrix?SiC_f/SiC?composites have been considered as one of the most promising high-temperature structural radar absorbing materials?RAMs?,which possess many advantages such as high specific strength and modulus,high-temperature oxidation resistance,high fracture toughness,low density,adjustable electrical properties and so on.However,the researches on the microwave absorbing properties of SiC_f/SiC composites are still at the preliminary stage in domestic,especially that the reports on the high-temperature dielectric and microwave absorbing properties are rare.In the present study,SiC_f/SiC composites were firstly fabricated by chemical vapor infiltration?CVI?and precursor infiltration and pyrolysis?PIP?processes,and the evolvements of room and high temperature dielectric properties of the composites and their components were investigated.On this basis,the high-temperature oxidation process,inert fillers loading,lowtemperature air pre-oxidation and hybrid processes were applied to adjust the mechanical and high-temperature dielectric properties of the composites.According to the optimized processing parameters,the SiC_f/SiC microwave absorbing composites were fabricated and their hightemperature microwave absorbing properties were measured.Finally,the calculated reflection loss?RL?values were compared with the measured RL values to verify the feasibility of measured high-temperature complex permittivity,and the influencing factors on the high-temperature microwave absorbing properties of the composites were discussed.The main research contents and results are summarized as follows:The evolvements of mechanical and high-temperature dielectric properties of original CVI-SiC_f/SiC composites and the composites after oxidation treatment were investigated.The results indicate that,the KD-I SiC fibers with rich carbon layers show much higher electrical conductivity and complex permittivity than the SLF SiC fibers with Si-C-O phase,and the complex permittivity of the two fibers are 14.7-j18.6 and 2.7-j0.2 at 10 GHz.Owing to the poor impedance matching between the KD-I fibers and free space and the weak dielectric loss of SLF fibers,both of the two fibers show poor room-temperature microwave absorbing properties.Both the ?? and ?? of KD-I fibers drastically increase with the increasing temperatures,while the complex permittivity especially the ?? of SLF fibers show a small increasing range,and the complex permittivity of the two fibers increase to 20.9-j25.0 and 5.0-j0.37 at 700?,respectively.The CVI-Si C matrix is composed of highly crystalline ?-SiC and free carbon,which possesses high electrical conductivity and complex permittivity.Owing to the strong conduction loss caused by the free carbon and thermally excited carriers from SiC at high temperatures,the complex permittivity increases from 9.1-j8.5 to 20.0-j22.1 as the temperature increases to 700?.As a result,the complex permittivity of the fabricated KD-I and SLF SiC_f/SiC composites dramatically increases from 21.0-j23.0 and 13.0-j10.0 to 35.0-j43.0 and 25.0-j24.0 as the temperature increases to 700?,which results in weak impedance matching and poor microwave absorbing properties of the composites.After thermal oxidation at 700?-1000?,the complex permittivity of the composites merely decreases to a certain degree due to the large amount of residual SiC phase and free carbon.The high-temperature complex permittivity shows an obvious reduced enhancement with the rising temperatures,while the composites still possess poor high-temperature microwave absorbing properties.The mechanical properties of the composites deteriorate obviously after oxidation.In order to obtain excellent high-temperature microwave absorbing properties,the complex permittivity of the composites at room temperature should be appropriate and as low as possible,and it turns out that the CVI process is unsuitable for fabricating the high-temperature SiC_f/SiC microwave absorbing composites.The evolvements of mechanical and high-temperature dielectric properties of original PIP-SiC_f/SiC composites and the composites with micron-sized Al₂O₃ filler,nano-sized SiO₂ filler,or low-temperature air pre-oxidation treatment were investigated.The results indicate that,the original PIP-SiC_f/SiC composites exhibit a high complex permittivity?10.2-j7.2?owing to the abundant ?-SiC microcrystals and free carbon in the matrix.And also,the complex permittivity increases greatly with the temperature increases to 700??16.0-j14.5?.After introducing a 200?m BN interphase,the flexural strength of the composites is improved from 132 MPa to 238 MPa,and the BN interphase has little influence on the complex permittivity of SiC_f/SiC composites.The impregnation efficiency of SiC_f/BN/SiC composites can be effectively enhanced by introducing the Al₂O₃ or SiO₂ filler.It is noted that the flexural strengths of the composites increase firstly and then degrade as the filler contents increase.The micronsized Al₂O₃ filler can effectively suppress the matrix volume shrinkage and achieve the particle reinforced effect compared with the nano-sized SiO₂ filler,thus,the maximum flexural strength of the composites with Al₂O₃ filler is up to 295 MPa,and the composites with SiO₂ filler is improved to 275 MPa.Both the ?? and ?? of the composites decrease with the increasing filler contents.Owing to the higher specific surface areas of nano-sized filler,the complex permittivity of the composites with SiO₂ filler shows a larger reduction than that of the comp-osites with Al₂O₃ filler,and the high-temperature complex permittivity presents lower increments,which demonstrates that the RL values of the composites with specific thickness can be inferior to-8dB at room and high temperatures.The flexural strength deteriorates severely after pre-oxidation treatment owing to the increase of oxygen content in the matrix and the strengthening fiber/matrix interface.Due to the low complex permittivity of SiCxOy matrix and reduced free carbon content,both of the room and high temperature complex permittivity of the composites significantly decrease,and the RL values at 700? are below-8d B in the frequency range of 8.2-12.4GHz when the thickness varies in the range of 3.0³.4mm.However,the mechanical properties are poor when the high-temperature microwave absorbing properties are optimized.The evolvements of mechanical and high-temperature dielectric properties of SiC_f/BN/ SiC composites fabricated by CVI+PIP hybrid process or filler loading combined with preoxidation process were investigated.The results indicate that,both the flexural strength and complex permittivity of the composites increase with the increasing CVI time.And all the composites show relative larger room and high temperature complex permittivity than PIPSiC_f/BN/SiC composites,which also exhibit poor microwave absorbing properties.The introduction of SiO₂ filler during the following PIP processes shows little effect on the flexural strength and complex permittivity of the composites due to the high complex permittivity of CVI-SiC matrix.It is revealed that the complex permittivity of SiC_f/BN/Si C composites can be drastically reduced by the filler loading combined with pre-oxidation process.Owing to the low complex permittivity of SiCxOy matrix and Al₂O₃ filler,the high-temperature complex permittivity also exhibits lower values,and it increases from 3.4-j1.4 to 5.8-j2.7 at 700? for SiC_f/BN/SiC-Al₂O₃ composites after pre-oxidation under 200?-6h.It is displayed that the microwave absorbing property is enhanced with the rising temperatures.Although the flexural strength decreases with the increasing pre-oxidation cycles,the residual strength can reach 200 MPa owing to the enhancement by Al₂O₃ filler.According to the above optimized processing parameters,the SiC_f/BN/SiC microwave absorbing composites were fabricated and their high-temperature microwave absorbing properties were examined.And then,the influencing factors on the high-temperature microwave absorbing properties were discussed.The results indicate that,the RL values of SiC_f/BN/SiC composites pre-oxidized under 260?-6h are below-8dB in the whole X band at the temperature range of 600⁸00?,and the RL values of SiC_f/BN/SiC-Al₂O₃ composites pre-oxidized under 200?-6h are below-8dB at 800¹000?.The calculated RL values by the high-temperature complex permittivity are in agreement with the measured results,which manifests that the high-temperature microwave absorbing properties of the composites can be tailored and predicted by measuring the high-temperature complex permittivity.Owing to the low oxidation degree,the high-temperature RL values show small changes with the increasing holding time at the first examination,while the oxidation degree aggravates and the high-temperature microwave absorbing properties deteriorate seriously with the increasing examining cycles.It is also proven that the pore distributions and measurement directions of the composites have little influence on the high-temperature microwave absorbing properties.