Preparation And Characterization Of A Ceramic Matrix Composite Reinforced With Sic Fibers Of High Electrical Resistivity

Silicon carbide fiber reinforced ceramic matrix composites are ideal candidates for aerospace applications because of its superior properties,such as oxidation resistance at elevated temperatures,high specific strength and high toughness.The ceramic matrix composites prepared by using silicon carbide fibers with different electrical resistivity can meet a variety of electromagnetic requirements for aircraft service environments.In this dissertation,La₂O₃ coating was prepared on 2D SiC fiber cloth with high electrical resistivity by dip-coating process.Then the ceramic preform was obtained by impregnating ceramic suspension.Finally,the SiC_f/BANS composite was prepared by precursor infiltration and sintering process.This dissertation explored the preparation process of La₂O₃ coating,studied the effect of coating on the oxidation resistance of fiber,and analyzed the effect of dispersant and milling time on the stability of the suspension.Subsequently,the formation mechanism of the matrix was proposed through the phase composition and microscopic morphology of the matrix.Then,the mechanical properties and dielectric properties of the composites were tested and the effect of the fibers on the properties of the composites were analyzed.Finally,the reinforcement mechanism and fiber damage mechanism are proposed by testing the antioxidation resistance of the composites.The main conclusions are as follows:?1?The coating research suggests that the La₂O₃ coating of fiber prepared by 3impregnation of 4wt%La?CH₂COOH?₃·5H₂O solution has high coverage integrity and small strength damage.It was also found that the La₂O₃ coating can improve the oxidation resistance of the fiber below 700°C.However,the coating loses its protective properties due to thermal mismatch and gas bubbling when the temperature reaches 800?.In addition,the preparation process of ceramic preforms was further studied.The optimum ratios of stable suspensions with ethanol and water as the dispersion medium were obtained,and the composites were prepared by precursor impregnation sintering process subsequently.?2?The results of the phase and micro-morphology of the BANS matrix show that the water glass and kaolin in the BANS matrix are glassy,Al₂O₃ powder was encapsulated by them at low temperature.When the temperature rises to 800°C,Al₂O₃ dissolves into the molten glass phase to form NaAlSi₂O₆ and NaAlSiO₄;The borate formed by BN hydrolysis will heal the matrix at high temperature.With the addition of silicon carbide fiber,the flexural strength of composites increased from 52.42 MPa to 108.46 MPa,the fracture characteristics changed from brittle fracture to ductile fracture,the dielectric constant decreased,and the loss tangent increased.The La₂O₃coating can reduce the porosity of composites by reducing the wetting angle between the fiber and the matrix.Fiber pullout and coating debonding at the composites indicate that the La₂O₃coating was a weak interface.?3?The mechanical properties of composites after heat treatment shows that the flexural strength increased from 108.5MPa to 124.4MPa when the temperature increased from 500°C to 700°C,and duration of heat treatment impact slightly on the flexural strength of composites.However,the flexural strength of the composites rapidly decreases to 60.8 MPa when the temperature rises to 800?.Between 500-700°C,the flexural strength of composites is dominated by the matrix.For one thing,the bending strength of composites increases with the decrease of micro-cracks in the matrix.For another,the load-bearing mode of the fiber changes with the reduction of defects in matrix,and the stress can be transferred more effectively.When the temperature rises to800°C,the flexural strength of composites is dominated by fiber damage,which includes the combined effects of oxidation of fibers,matrix corrosion and coating failure.