| Carbon fiber/silicon carbide(Cf/SiC)composites have considerable potential to be widely used in aerospace field as thermostructural materials in place of superalloys due to higher strength to density ratio,higher temperature-resistance and better thermal shock resistance.Precursor impregnation and pyrolysis(PIP)process used to prepare Cf/SiC composites has attracted wide attention in the last 20 years due to its advantages of near-net forming and low cost.Interface layers have significant effects on mechanical performance of Cf/SiC composites.Pyrolytic carbon(PyC)layers and carbon nanotubes(CNTs)have been extensively used to improve the strength and toughness of Cf/SiC composites through obtaining proper interface bonding strength.Although PyC layers prepared by chemical vapor impregnation has advantages of thickness controllable and orderly structure,this method needs large energy consumption,high cost and precise control.Therefore,it is necessary to exploit an easy and cost-effective way to prepare PyC.Uniform dispersion of carbon nanotubes is crucial factor to ensure its effect on strengthening and toughening of composites,but the commercial surfactants are difficult to achieve good dispersion of CNTs.Therefore,it is necessary to synthesize a kind of surfactant.In addition,the effects of CNTs on mechanical performance of Cf/SiC composites have not been investigated systematically.Plenty of works listed as follows have been done in this dissertation to solve the above problems.Impregnation and pyrolysis of phenolic resin solution was exploited to prepare PyC layers on the surface of carbon fibers as an easy and cost-effective way.However,the PyC layers directly prepared on carbon fibers were non-uniform and incomplete,and its effects on the improvement of mechanical performance of Cf/SiC composites were limited.By adjusting oxidation condition of carbon fibers and concentrations of phenolic solution,the control about uniformity and thickness of PyC layers were achieved.Experimental results showed that Cf/SiC composites with uniform PyC layers had better mechanical properties than those with non-uniform PyC layers.In addition,Cf/SiC composites containing PyC layers with thickness of approximately 120 nm achieved the best tensile strength and work of fracture,with 26.2% and 71.8% improvement from those of Cf/SiC composites without PyC layers.It was mainly ascribed to the moderate interface bonding strength.A new dispersant named as AEP was synthesized by chemical modification of phenolic resin.AEP could be used to disperse CNTs in aqueous suspensions.By adjusting concentration of CNTs,sonication power and concentration of CNTs,uniform CNT aqueous suspensions were achieved with lots of single CNTs in suspensions.This work created the condition for uniform distribution of CNTs on carbon fibers.The results of ultraviolet-visible spectroscopy and scanning electron microscope showed AEP had better dispersability than sodium dodecyl benzenesulfonate in dispersing CNTs in aqueous suspensions.The mechanism was revealed to be the strong π-π interaction between AEP and CNTs according to the Raman spectrometry results.The influences of carbon nanotubes(CNTs)introduced at three different phases of carbon fiber/pyrolytic carbon/silicon carbide composites on the microstructure and properties of the composites were investigated systematically.The results showed CNTs positions had significant effects on the properties of composites.Compared with those of the CNTs-free composites,the tensile strength and work of fracture of the composites with CNTs introduced at PyC/SiC interface and the composite with CNTs introduced at Cf/PyC interface were not enhanced obviously.However,the composites with CNTs introduced at PyC/PyC interface improved 54.9% and 130.3% in tensile strength and work of fracture from the CNTs-free composite,and the corresponding microstructure and mechanism were unclosed.The work strongly confirmed that interface design was very important for mechanical properties of Cf/SiC composites. |
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