Reinforcing And Toughening Carbon/Carbon Composites By Nanofibers Doped Via Electrophoresis Deposition

Carbon fiber reinforced pyrocarbon matrix composites,or known as carbon/carbon(C/C)composites,are one of the most important ultrahigh-temperature thermal structural materials widely applied in aerospace industry.However during the fabrication process of C/C material,the microdefects,that matrix annular cracks and fiber/matrix interface relaxation ascribed to the low strength and brittle feature of pyrocarbon matrix,have directly impact on their mechanical properties and restrict their application in aerospace fields further,especially for the components with sharp corners and thin walls.Such limitations can be effectively alleviated by the in-situ growth of carbon nanotubes(CNTs)in carbon fiber preform by catalytic chemical vapor deposition(CCVD),which constructs a multiscale hybrid structure.This multiscale structure can provide micro-and nano-reinforcing for C/C composites.Unfortunately,it is unavoidable to damage the carbon fibers and subsequently decrease the fiber-dominated mechanical properties by growing CNTs via CCVD.In addition,some other disadvantages,mainly including the complex process,long fabrication period and high cost,also restrict the application of CCVD.In this work,CNTs and silicon carbide nanowires(Si CNWs)were introduced uniformly in carbon fiber preform by the electrophoretic deposition(EPD)method to realize micro and nano multiscale reinforcement for C/C composites.And the parameters of EPD and chemical vapor infiltration(CVI)were optimized to improve the homogeneous of C/C composites.Further,the effects of CNTs and Si CNWs on the densification behaviors,matrix microstructures and defects,and mechanical prosperities of C/C composites were investigated in order to explore the relationship between the nanofibers and mechanical performance of C/C composites.Additionally,the interfaces have been well discussed in respect of the designing of materials,which proposed to further improve the mechanical strengths and the fracture ductility of C/C composites at the same time.The main conclusions of the present work are shown as follows:The EPD introduction of nanofibers for fabricating multiscale preforms and the morphologies controlling were investigated.The results show that: the optimized parameters were EPD conditions of the voltage 40 V,distance 1cm,the CNTs concentration of 1.0g·L-1 and and the Si CNWs concentration of 0.5g·L-1 in isopropyl alcohol,respectively;the content of nanofibers can be controlled by adjusting EPD time,which are linearly dependent;the distribution of nanofibers on carbon fiber surface were uniformed without obvious aggregations.The depositing behavior and distributions of EPD nanofibers were analyzed and results show that the stacking density of randomly distributed CNTs with nanoscale pores is higher than that of Si CNWs with isotropy features and the pores with micrometer scale,owing to the difference of significant shape of CNTs and Si CNWs.The densification processes of C/C composites doped with CNT/Si CNW have been investigated.The average densification rates of CNT-C/C composites are slower than that of C/C composites under various CVI temperatures and precursor pressures.And the densification rates decrease with the increasing content of CNTs,and the density gradients will be arise while their initial densification rate are faster than that of C/C composites.Meanwhile,with the increase of Si CNW contents,the densification rates and density of Si CNW-C/C composites are faster than that of C/C composites during the whole CVI process.Also,the densification parameters of C/C composites doped with CNT/Si CNW were optimized and the results show that: the CVI temperature,precursor pressure and the residual time of the precursor gas were set to 1080 °C,20 k Pa and 0.1 s,respectively.Under such condition,multiscale preform can be densified moderately with satisfactory density distributions.The effects of 1D nanofiber on the densification behaviors are analyzed.Due to the presence of nanofibers,pyrocarbon precursors deposited on the surface of CNTs at first,blocking the pores,which ultimately impedes the diffusion of precursor gas inside the preforms and causes the increase of density gradients.While the pores in Si CNW/CF multiscale preforms were too relatively large to be blocked by the preferred deposition,so the density of the final samples was improved and the density gradient did not further increase.The influence of EPD nanofibers on pyrocarbon microstructures were characterized: the texture of pyrocarbon matrix transferred from medium texture into multi-level structures after introducing nanofibers,which may be marked as ISO-HT-MT matrix;as the contents of nanofibers increase,the proportions of ISO and HT increase.It has been confirmed that the growth of ISO texture is inevitable with dopeing of nanofibers.The growth of CNTs and Si CNWs induces the different microstructure of pyrocarbon: in HT zone,CNTs induces the “skin-core” structure in which HT pyrocarbon surrounds the CNTs,whereas no similar phenomena have been observed in the Si CNW doped zone.Compared with CNT-C/C composites,the pyrocarbon in Si CNW doped zone of Si CNW-C/C composites has smaller d002 and larger Lc value,which indicates its higher degree of graphitization.The influence of EPD nanofibers on mechanical properties of C/C composites are analyzed: for flexural performance,moderate content of EPD CNTs results in the improvements of approximately 35% compared with the original C/C composites and the further increase of CNT contents results in the decrease of sample density,the increase of porosity and the subsequent decrease of flexural strength.The introduction of CNTs on carbon fiber surface also causes the brittle fracture of the composite,because of the growth of ISO pyrocarbon induced by CNTs.For interlaminar shear performance,EPD CNTs can significantly increase the interlaminar shear strength by approximately 100% and the mode II fracture toughness of the composites increases by 43% accordingly.The reinforcing mechanisms of EPD CNTs can be summarized into ?direct reinforcing‘ and ?indirect reinforcing‘.?Direct reinforcing‘ refers to the improvements of pyrocarbon mechanical properties by introducing CNTs and ?indirect reinforcing‘ refers to the refinement of pyrocarbon grains.Compared with CNTs,doping Si CNWs can achieve better reinforcing results and the flexural and interlaminar shear strength can be improved by nearly 50% and 200% respectively.Also,Si CNWs causes the brittle fracture of C/C composites.In order to solve the brittle fracture caused by introducing nanofibers,various methods have been prepared to modify the fiber-matrix interface,such as HT interlayer and GO interlayer.The introducing of interlayers can significantly increase the fracture ductility of the composites with FD increased by 1100% over that of the nanofilbers direct doping,and further improve the strength utilization of nanofibers.The results indicate that introducing interlayers can construct ?hard matrix-soft interlayer-hard fiber? system in C/C composites,which is effective in improving the fracture strength and ductility of C/C composites.Additionally,we have introduced EPD Si CNWs in the anti-oxidation Si C coatings to further extent the applications of C/C composites.Our results successfully demonstrated that EPD Si CNWs is not only feasible but also effective in improving the performance of C/C composites with the coating under thermal shock conditions,which provides useful insights for the development of advanced anti-oxidation coating systems.