Composition, Sructure, And Properties Of Continuous Silicon Nitride Fibers

Silicon nitride fibers are widely used as reinforcing components 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 mechanical properties,compositional and structural stability,microstructure,and thermal stability in inert and oxidative atmosphere of silicon nitride fibers.The strength of silicon nitride fibers can be enhanced by repairing the surface flaws via coating.Silicon nitride fibers were prepared by nitridation of EB（electron beam）-cured polycarbosilane fibers at various NH₃ flow rates.The use of larger NH₃ amounts ensured a higher degree of decarbonization.However,overly high NH₃ flows resulted in a high free radical yield.Finally,we compared and discussed the compositional and structural changes of two representative fiber types and explained the underlying mechanism responsible for the oxygen content increase to elucidate the effect of residual free radicals,providing first-time evidence of their presence in silicon nitride fibers and showing that they were responsible for the high oxygen content of these fibers.The oxygen,carbon content and N/Si atomic ratio of silicon nitride fibers prepared by optimizing the flow of NH₃ were 0.81wt%,0.76wt%,and 1.13.Small-angle X-ray scattering experiments were carried out using the synchrotron radiation facility to characterize the defects in silicon nitride fibers.Silica coatings were prepared on silicon nitride fibers by a continuous dip-coating method.The results confirmed that the size of sol particles must match that of flaws on the fiber surface before these flaws can be effectively repaired.The SiO₂ coatings with uniform thickness were prepared from a sol solution with concentration of 0.75wt%,colloid size of 2–8nm,and then heat-treated at 400°C,and the strength of the fibers was improved by 26%by the treated coating.This study evaluates the thermal stability in inert atmosphere and the oxidative resistance of silicon nitride fibers.When the silicon nitride fibers were oxidized at1000°C for 1h,they possessed the highest tensile strength due to surface defect healing by the glassy phase,whereas those oxidized at higher temperatures showed lower strength than non-oxidized fibers.After treatment at 1200°C in air,they retained 63.4%of strength.The fibers with 0.81wt%oxygen maintained 61.0%of strength after 1h at1450°C in nitrogen,showing good thermal stability.When heat-treated in N₂,original fibers with the higher oxygen content exhibited the lower strength retention.The strength retention of the fibers containing 9.53wt%oxygen was 26.6%after 1h at1450°C in N₂.The fiber strength decreased in high temperature because exacerbation of existing flaws induced by thermal decomposition of SiN_xO_y.Moreover,surface oxidation or SiO₂ coatings of silicon nitride fibers adversely affected their thermal stability.