Preparation And Interface Modification Of Silicon Nitride Fiber Reinforced Wave-Transparent Composite Materials

Silicon nitride fibers are widely used as the reinforcement in composite materials because of their high specific strength and modulus,low dielectric constant,and high electrical resistivity.They are intensively utilized in wave-transparent antenna radomes,hypersonic vehicles,and other aerospace applications.This study evaluates the machenical properties,compositions,microstructures,and high-temperature resistance in nitrogen and air atmosphere of silicon nitride fibers.The SiON coatings were prepared on silicon nitride fibers and the tensile strength of coated fibers was investigated.This experiment includes preparing Si₃N_4f/SiO₂ composites by sol-gel craft and comparing the mechanical properties between Si₃N_4f/SiO₂ and SiO_2f/SiO₂composites.And the microstructures and mechanical properties of Si₃N_4f/SiO₂-BN composites prepared via sol-gel method and precursor infiltration and pyrolysis?PIP?route were studied.In this study,the Si₃N₄ fibers were employed to characterize the high-temperature behavior under a nitrogen and air atmosphere,respectively.The results show that the elevated temperature provoked the oxidation reaction from silicon nitride to silicon dioxide with cracks.This change was accompanied by a fall in the tensile strength of the filament.And the relatively slow strength degradation indicated that the fibers would possess a relatively long life under the surviving temperature of 1200?.In condition of nitrogen atmosphere,the fibers retained relatively high strength and maintained amorphous state after 1400?heat-treatment.When temperature reached to 1500?,silicon nitride grains increased dramatically and led a huge degradation of strength.According to the high crystallization temperature,the Si₃N₄ fibers can keep a desirable performance under the temperature of 1400?.The preparation of SiON coating for Si₃N₄ fibers from PHPS,the effects of coatings on the mechanical properties of fibers,the compositions and constructures of SiON coatings were studied.The preparing conditions:the Si₃N₄ fibers were dipped in acetone and dispersed by ultrasonic,then dipped in the 5wt%PHPS solution and heated in air atmosphere at 100?.The results show that the coating has a composition of SiON.The tenxile strength of fibers was increased by 24.1%with the coating prepared from5wt%PHPS solution.While the tensile strength of fibers surpass 1200?was lower than uncoated fibers.Si₃N_4f/Si O₂ composites were prepared via sol-gel method and the compositions,constuctures and mechanical properties of which were investigated.The results show that there were no interface interactions.The tensile strength of coated Si₃N_4f/SiO₂composites was increased 15.8%compared with uncoated composites at room temperature.And the tensile strength of coated Si₃N_4f/SiO₂ composites after 1000?and 1200?heat treatment was 95.01MPa and 57.59MPa,respectively,and the tensile strength of SiO_2f/SiO₂ composites was 83.52MPa and 28.54MPa,which means the thermal stability of Si₃N_4f/SiO₂ composites is higher than SiO_2f/SiO₂ composites.Si₃N_4f/Si O₂-BN composites were prepared via sol-gel and PIP methods,of which the compositions,constructures and mechanical properties were studied.The results show that the interface of composites was consisted of Si₃N₄ and B₂O₃.And the structure of the interface was mainly about the Si-N and Si-O-B-N.The tensile strength of Si₃N_4f/SiO₂-BN composites was fine under 1000?with fiber pull-outs,which was mainly because the curing effects of the fused SiO₂ and SiON.While the tensile strength of Si₃N_4f/SiO₂-BN composites after 1200?heat treatment decreased dramatically,the reasons of which included the increasment of microcracks,the interface interaction and the decreament of the mechanical properties of the fibers.