Microstrusture And Properties Of Pyrocarbon Matrix Composites Strengthened By Carbon Nanotubes

Carbon fiber reinforced pyrocarbon matrix(C/C)composites have a status with non-substitute in the aerospace fields.But due to the different coefficient of thermal expansion between carbon fiber and pyrocarbon(PyC)matrix,the pyrocarbon around the carbon fiber in C/C composites that ware fabricated from the carbon fiber preform will crack during the high temperature treatment.And the higher temperature of the heat treatment or the more times of heat treatment can cause more and larger cracks,and the mechanical preperties of the composites decrease significantly.In addition,the compressive strength and shear strength which are determined by the strength of the PyC matrix will also decrease greatly.As a result,they all reduce the stability and reliability of the C/C composites using as thin-wall,sharp-angled and complex monolithic component,which limit the development and application of C/C composites.This work designed to introduce carbon nanotubes(CNTs)into the PyC matrix by different growth way,in order to improve the mechanical properties of C/C composites.And the effect of catalyst residue on the oxidation resistance of C/C composites was also discussed.Copper(Cu)is usually not only used to catalyze the growth of CNTs,but also can be used as the transpiration material in high temperature environment to improve the ablation resistance of C/C composites.So,the Cu was used to catalyze the growth of CNTs during fabricating C/C-Cu composites to improve the mechanical properties.The results showed that when the content of Cu was approximately equal,the in-situ CNTs strengthened C/C-Cu(ICNTs-C/C-Cu)not only achieved improved mechanical properties compared with C/C-Cu composites,but also had better ablation resistance compared with C/C-Cu composites.However,the catalytic oxidation effect of the catalyst residue of Cu accelerated the oxidation erosion of C/C composites greatly,and it go against the composites using in high temperature environment.The growth of CNTs induced by Cu is in the way of tip-growth mode,and the catalytic can move with the growth of CNTs.So,based on the temperature gradient in the thermal gradient chemical vapor deposition equipment,the growth of CNTs and the formation of PyC can be carried out at the same time in different temperature zone,and the temperature gradient can prompt the CNTs growing continously in the way of epitaxial growth.As a result,the epitaxial growing CNTs reinforced C/C(ECNTs-C/C)composites can be fabricated in the same atmosphere environment in one step.The ECNTs-C/C composites achieved not only higher densification rate and lower porosity,and also no catalyst residue of Cu in the ECNTs-C/C composites.Moreover,the ECNTs-C/C composites had higher mechanical properties and ablation resistance than that of traditional C/C composites.In the ablation thermal shock test,the epitaxial growing CNTs reinforced PyC matrix could relieve the thermal stress induced by the mismatch of CTEs between carbon fiber and PyC matrix,and making the possibility of cracking and the number of cracks decreased.Combining the formation of PyC and the growth of CNTs together,a one-step method was developed to fabricate nano carbon/carbon(Carbon nanotubes reinforced pyrocarbon matrix,ECNTs/C)composites without carbon fiber.During the fabrication process,the method not only decreased the diffusion distance of carbon source gas in the“preform”and made the porosity of ECNTs/C nanocomposites(<0.5%,density:1.76g/cm~3)much lower than that of traditional C/C composites(>3.00%,densidy:>1.80g/cm~3),but also the catalytic Cu could move with the specific temperature zone that was fitted to the growth of CNTs and not left in the obtained nanocomposites.Beniftting from the large amount and disorderly distributed CNTs,the ECNTs/C nanocomposites achieved higher mechanical strength and lower desity at the same time comparing with traditional C/C composites and isotropic PyC material,and showed excellent isotropic properties in every direction.Before breaking,the ECNTs/C only happened elastic deformation.Even after several cyclic loading-unloading process,the ECNTs/C nanocomposites also kept the original mechanical strength.In addition,due to the compact and uniform microstructure,the ECNTs/C nanocomposites also achieved better ablation resisitance comparing with C/C composites and some matrix modified C/C composites,and its mass ablation rate and linear ablation rate was 0.62mg/s and 1.24μm/s,respectivesly.Moreover,the effect of heat treatment on the microstructure and mechanical properties fo the CNTs/Ccomposites was also discussed.During the heat treatment process,the microstructure was changing from disordered into ordered,and the higher temperature,the more ordered microstructure would be.When at 1500℃,the heat treatment had little influence on the microstrucuture of the ECNTs/C nanocomposites,and mianly the residual stress was released.As a result,the flexural strength of the ECNTs/C nanocomposites was decreased by8.09%at 1500℃,while the compressive strength was increased by 9.85%.When the residual stress had been released and the temperature was higher than 1500℃,microstructure of the ECNTs/C nanocomposites changed significantly,and the mechanical properties decreased continuously with increasing the heat treatment temperature.When the heat treatment temperature increased to 2450℃,the flexural strength and compressive strength was decreased by 16.28%and 28.72%,respectively.