Fabrication Of Gradient SiNO/SiC Fibers By Nitridation Pyrolysis

Ceramic fibers have wide range of application prospects in the military and civilian as superb reinforcement of resin matrix,ceramic matrix,and metal matrix composites duo to its excellent properties such as light weight,high strength and high modulus,high temperature resistance,oxidation resistance and corrosion resistance.Ceramic fibers as important strategic resource,the study of ceramic fibers also have always been a hot research topic in the world.Against this background,a series of novel SiNO/SiC fibers with gradient structure were prepared by combined the technical route of preparation of SiC fibers by polymer derived ceramics（PDC）and fabrication of Si₃N₄ fibers by nitridation pyrolysis with Polycarbosilane（PCS）,which using PCS electron beam cross-linked fibers as raw materials,adjusted nitriding depth by controlled nitriding temperature in the NH₃ atmosphere and then pyrolyzed at high temperatures in N₂ atmosphere.The composition and structural of the fibers were studied in detail,and the influence law on the properties was discussed in combination with the structure of the fibers.It was found that the ceramic yield of SiNO/SiC fibers were retained about 76%after nitridation pyrolysis,and it was slightly decreased with the increase of nitriding or pyrolysis temperature.The N elements introduced by the fibers are mainly present in the SiO_xN_y and Si₃N₄ structures,and the N element decreases gradually from the surface to the internal content,the Si elements is relatively homogeneous.The C content is decreased while the C is opposite with the increase of nitriding temperature due to the nitriding decarburization reaction.SiNO/SiC fibers structure model cauld be considered as the composed of combine SiO_xN_y,Si₃N₄ at the surface and SiC,free C at the core.Fibers surfaces are dense and smooth,cross-sections dense and no obvious holes or defects.When the nitriding temperature is 550℃,the mechanical properties of the fibers have somewhat decrease,but the resistivity decreases from 6.35*10³ Ω·cm to 6.00*10² Ω·cm,the absorption capacity of electromagnetic waves is also improved accordingly with the increase of the pyrolysis temperature.While the nitriding temperature rises to 600℃,the mechanical properties of the fibers also decrease,the change of fiber crystallinity is small due to the increase of N content in the fibers and did not reach the crystallization temperature of Si₃N₄,and the change of resistivity and absorbing property is not significant with the increase of the pyrolysis temperature.