Microstructure And Mechanical Properties Of C/C Composites Modified By Injection CVD-grown CNT

Carbon/carbon?C/C?composites have high temperature resistance,high anti-ablation and anti-erosion,and excellent mechanical properties at ultra-high temperature,and so on,which make them important candidates in aerospace application.For the commonly-used 2D C/C composites,the carbon matrix at the interlaminar and intralaminar regions,intra-bundles,and the interspaces of fabric body has not been reinforced.Destructive cracks can easily avert the intralaminar fibers and directly extends in the interlaminar matrix,which eventually leads to the low-stress failure of composites and significantly degrades the mechanical properties of C/C composites.Grafting carbon nanotubes?CNT?with nano diameter and micron length onto carbon fibers forming multiscale CNT-fiber preforms can provide efficient enhancements to the unreinforced carbon matrix without destroying the 2D stack design and then achieve multiscale strengthening and toughening in the composites on macro/micro/nano scale,which can substantially increase the comprehensive performance of C/C composites.However,at present,it is still unable to control the grafting morphology of in-situ grown CNT,which makes little contribution to the mechanical properties of C/C composites.In order to realize the control of CNT morphology and to enhance the role of CNT in reinforcing C/C composites,in this work,curved CNT,aligned CNT and radial CNT were grown on the surface of carbon fibers by injection chemical vapor deposition method.Explorations of three types of CNT in growth control,microstructural characters,growth mechanism and their damage to the carbon fibers were conducted.Based on these studies,effects of CNT morphology on the pyrocarbon?PyC?structure,fiber/matrix?F/M?interface state,distribution of micro-defects,mechanical properties and failure behavior of C/C composites were systematically investigated.The main conclusions are as follows:?1?Study on the growth control of the three types of CNT.Curved CNT was grown on the surface of carbon fibers by injection floating catalyst method using ethanol as carbon source and ferrocene as catalyst precursor.The optimal growth temperature was 800°C and catalyst concentration was 0.01 g/mL.A surface treatment by coating a SiO₂ interface layer could improve the alignment degree of CNT and achieved the vertical distribution of aligned CNT arrays on the carbon fibers.Radial CNT was grown on the surface of carbon fibers by injection fixed catalyst method using xylene as carbon source and ferrous sulfate as catalyst precursor.The optimal growth temperature was 850°C.During the growth process of the three types of CNT,the feeding of ethylenediamine contributed to promote and to keep the activity of iron catalysts,achieving the stable growth of CNT with large length-diameter ratio,high purity,uniform diameter distribution and excellent spatial distribution.For the injection floating catalyst method,growth temperature and catalyst concentration mainly affected the purity of CNT products.The higher growth temperature or the greater catalyst concentration would make it easier for the growth of small tubes and attachment of catalysts on CNT surfaces,leading to the decrease of CNT purity.For the injection fixed catalyst method,growth temperature mainly influenced the diameter of CNT.The higher the growth temperature was,the larger the CNT diameter becomed.The extending lengths of curved CNT,aligned CNT and radial CNT could be controlled by adjusting the growth time:the growth rates of the three types of CNT were 0.1-0.25?m/min,0.5-0.65?m/min and 0.03-0.06?m/min,respectively.Injection floating catalyst method was only able to grow CNT on the outer surface of fiber preforms,which could be used to prepare multiscale CNT-carbon cloth laminated preforms;injection fixed catalyst method could achieve the growth of CNT in the interior of fiber preforms,which was applicable to the fabrication of multiscale CNT-carbon felt preforms.?2?Study on the microstructural characteristics and growth mechanism of the three types of CNT as well as their damage to the carbon fibers.Both the curved CNT and aligned CNT grown by injection floating catalyst method had multiwalled bamboo-like structures with the diameter ranging from 10-70 nm and high crystallinity,which increased with the increase of growth temperature.Radial CNT synthesized by injection fixed catalyst method showed multiwalled hollow structures with large diameters ranging from 100-300 nm and low crystallinity,which decreased with the increase of growth temperature.The injection floating catalyst method proceeded by“base-growth model”,since the catalysts were always attached to the fiber surface during the growth process;for the injection fixed catalyst method,the catalysts were located at the tip of CNT during the growth process,which was classified as“tip-growth model”.According to the macro-mechanical investigations,long growth time and high growth temperature were more likely to degrade the tensile strength of carbon fibers;coating an interface layer on the fiber surface could decrease the thermal degradation and impeded the iron etching,which obviously increased the strength retention rate of fibers.?3?Effects of CNT morphology on the PyC microstructure and F/M interface state of C/C composites were studied.After introducing CNT,the carbon matrix of composites changed from single structure to two-layer structure,i.e,the CNT/PyC layer within the CNT length and the pure PyC layer out of the CNT reach.These two layers displayed significantly different microstructural morphologies under PLM and SEM.The CNT/PyC layer showed isotropic under PLM,while the optical activity of peripheral PyC layer was closed to that of pure C/C composites,but with smaller size and grain boundaries.CNT/PyC layer exhibited rough morphology under SEM,while the peripheral PyC layer was smooth similar to those in pure C/C composites,but having interlocked grain boundaries and distorted textures without annular cracking.Therefore,the strengthening mechanism of CNT on carbon matrix could be divided into direct reinforcement and indirect reinforcement.CNT extending into the surrounding matrix of fibers as continuous reinforcers directly stiffen the matrix?i.e.,direct reinforcement?.CNT also induced the dense nucleating of peripheral PyC forming interlocked grain boundaries via“fine-crystal effect”and then decreased the micro-defects such as annular cracks,thus improving the structure and mechanical properties of peripheral matrix.The CNT with better orientation and longer length would provide more significantly direct and indirect reinforcements to the carbon matrix.It is difficult to densify the curved CNT and aligned CNT in their three-dimentional networks due to the very small pores within them and thus the formed CNT/PyC showed a porous structure with many nano/submicro closed holes.It is easily to densify the radial CNT networks by virtue of their gradually increased pore size in the direction vertical to the fiber surface and to form a dense CNT/PyC layer.Therefore,in order to improve the densification of CNT-C/C composites,the CNT grafting mode with good orientation and relatively sparse distribution was imperative.XRD and Raman investigations revealed that these three types of CNT could efficiently improve the graphization degree within their reach,including decreasing their d₀₀₂ and increasing L_c and L_a,and the improvement in graphization degree depended on the crystallinity of CNT themselves.This improvement effect could extend to the region within⁵00 nm away from CNT,while for the PyC far from nanotubes,CNT could only change their size and texture orientation and had little influence on their internal crystalline structure.Additionally,the direct growth of CNT on carbon fibers results in an excessive strong F/M interface bonding,while the introduction of interface layers,to some extent,can optimize the F/M interface by weakening the bonding strength between carbon fiber and CNT/PyC layer.?4?Introduction of CNT efficiently enhanced the interlaminar shear strength and compressive properties of C/C composites,i.e.matrix-dominated performance.For the carbon-cloth laminated C/C composites,the enhancement in interlaminar shear strength was over 120%and over 270%for in-plane compressive strength and over 60%for out-of-plane compressive strength;for the carbon-felt C/C composites,the improvements in in-plane and out-of-plane compressive strengths were over 80%and 50%,respectively.SEM observations indicated that as the grafted CNT was curved and short,their refinement effect on the peripheral PyC was insufficient,and the destructive cracks were still rely on the long-straight grain boundaries and annular cracks of indirect reinforced matrix to extend,leading to the delamination failure of composites.When the grafted CNT had well orientation and long length,their increased direct and indirect reinforcements and lateral mechanical support endowed the carbon matrix with high ability of inhibiting and deflecting the destructive cracks,thus resulting in great higher matrix-dominated performance.?5?Simultaneous introduction of CNT and interface layer efficiently enhanced the flexural properties of C/C composites,i.e.fiber-dominated properties.Grafting long CNT could better strengthen the carbon matrix,while it also meant that the carbon fibers would be exposed to the CNT growth system for a long time,which aggravated the damage of surface structure.Furthermore,the strong F/M interface bonding strength created by the direct growth of CNT was adverse to the deflection of destructive cracks and leaded to the failure of fiber pullout.Both of them resulted in the brittle fracture surface and low flexural strength.Introducing PyC interface layer could not only protect the carbon fiber structure but also optimized the F/M interface,which was beneficial for the propagation of destructive cracks along the fiber axis and the subsequent fiber pullout,which significantly improved the flexural strength and fracture toughness.Therefore,if we want to fully exert the synergistic toughening mechanism of CNT and carbon fibers,it is necessary to modify both the carbon matrix and F/M interface.?6?Motivated by the ability of CNT to strengthen carbon matrix and to create mechanical interlocking at interface,radial CNT/PyC as bonding interlayer was introduced between C/C composites,which exhibited excellent joining effect.Shear strength tests revealed that,the averaged shear strength of CNT/PyC-bonded C/C was 19.2 MPa,104%higher than that of neat PyC-bonded C/C?9.4 MPa?.This great improvement resulted from the outstanding contributions of CNT involving enhancing interlayer-C/C interfacial interaction,inducing PyC texture to align perpendicular to the bonding surface and increasing the additional fracture process,which changed the failure of bonded C/C from bonding interlayer-fracture dominated mode to substrate-fracture dominated one.