In-situ Grown SiC Nanowires And Its Application In SiC_f/SiC Composites

Due to their low density,high specific strength,high specific modulus,excellent thermostability,corrosion resistance and low-induced radioactivity under fusion environment,SiC_f/SiC composites have great potentials in the applications of aeronautic&aerospace,and fusion systems.However,as a promising high-temperature structural material,their mechanical properties still need to be improved.Compared to their bulk counterpart,SiC nanowires are expected to have super strength and modulus,and have been confirmed to be a kind of promising reinforcement in enhancing the mechanical properties of composites.In this thesis,SiC nanowires were in-situ grown in the SiC fiber fabrics via an in situ chemical vapour deposition(CVD)route with preceramic polymers as the raw materials,and SiC_f/SiC composites were fabricated by polymer infiltration and pyrolysis(PIP)process.Firstly,the CVD processes for preparing SiC nanowires were optimized.Then,SiC nanowires were prepared with Fe(NO3)3 as catalysts.The modification for loading catalysts into the fabric was studied.The technological parameters in the CVD process and mechanism of SiC nanowires were investigated.Finally,the effects of SiC nanowires on the mechanical properties of SiC_f/SiC composites were studied,and the reinforcement mechanisms of SiC nanowires were also discussed.Without any processing,no products were synthesized in the SiC fiber fabrics via the pyrolysis of PCS,the fabric was an unsuited substrate for the growth of SiC nanowires.Due to the iron impurities in the SiC powders and porous structures,SiC naowires were successfully grown in fabrics by PCS pyrolysis route with SiC powders as an assist.But the as-grown SiC naowires were non-uniform.The preparation of SiC nanowires with Fe(NO3)3 as catalysts were studied.Fabrics have been firstly modified by an EDTA/DMF suspension before immersed in the Fe(NO3)3 solution.Carboxyl groups could be introduced on the fibers surfaces during the modification,and the interfacial adhesion strength between fibers and iron ions were improved,resulting in a homogeneous deposition of catalytic particles and dense SiC nanowires grown on the entire surface of fiber surfaces.SiC nanowires in large areas with lengths of micrometers were grown on the fabrics by pyrolysis of PCS and PMS,but a uniform SiC/graphite core-shell layer with a thickness of 250 nm were synthesized by pyrolysis of LPVCS.The formation of SiC nanowires is discussed by the vapor-liquid-solid(VLS)mechanism.Some SiC nanowires with special morphology were found: bamboo-like nanowires,beaded nanowires,curved nanorods,bending nanorods.At the CVD process,the changing of technological parameters lead to different SiC nanowires products.The optimized parameters for SiC nanowires growth were: the Ar flux at 5ml/min,reaction temperature at 1300 ℃,reaction time of 60 min.Due to the net structures of SiC nanowiers,the efficiency of LPVCS’s impregnation was decreased,and SiC_f/SiC composites with SiC nanowires as interface exhibited low density and high porosity.With in-situ grown SiC nanowires in the fabrics,the SiC_f/SiC composite showed 5% increased flexural strength and 53% increased fracture toughness,the PyC-SiC_f/SiC composite showed 29% increased flexural strength and 34% increased fracture toughness.The improvement of mechanical properties could be attributed to mechanisms such as SiC nanowires bridging,SiC nanowires pullout,and the elastic deformation of the SiC nanowires.However,the improvement was limited which may be a result of two factors:(1)the introduction of SiC nanowires lead to a high volume of micropores in the matrix during the PIP process,(2)the bonding strength between the nanowires and the fibers was weak,the bonding strength between the nanowires and the matrix was strong,the pullout of SiC nanowires were limited.