Microstructure And Mechanical Performance Of In-situ Grown CNT Doped Carbon/Carbon Composites

Carbon/carbon (C/C) composites, as high-temperature materials, are widely used inaerospace industry. During the fabrication of C/C composites, microdefects, includingmatrix annular crack and fiber/matrix (F/M) interface relaxation, are unavoidable,which have a negative effect on their mechanical properties. To alleviate thislimitation, it is efficient to in-situ growing carbon nanotubes (CNTs) in carbon fiberpreform to prepare a multiscale hybrid structure, which can provide micro-and nano-reinforcements to carbon matrix and then improve mechanical properties of thecomposites. However, CNTs reported in the available work are always curved andhave a heterogeneous growth on carbon fibers, which contribute little to themechanical properties of C/C composites. To improve the reinforcing role of CNTson C/C composites, in this work, radially-straight CNTs were in-situ grown uniformlyin carbon fiber preform by catalytic chemical vapor deposition (CCVD). At the sametime, to explore the relationship between CNT growth morphology and mechanicalperformance of C/C composites, the effects of radially-straight CNTs and curvedCNTs on the densification behavior, matrix microstructures and defects, andmechanical prosperities of C/C composites were investigated. The main conclusionsare as follows:The CCVD growth of CNTs with two morphologies and the CNT length andyield controlling were investigated. The results show that: the optimized gowthtemperature for radially-straight CNTs by CCVD is1050°C using natural gas ascarbon source and Fe as catalyst; the carbon fiber surface treatment by concentratednitric acid can improve the growth uniformity of radially-straight CNT on fibers and amodified laminar flow field and a decreased residence time of reaction gas in CCVDreactor can improve the radially-straight CNT growth uniformity in carbon fiberpreform; the yield of radially-straight CNT can be controlled by adjusting catalystloading and growth time; the optimized gowth temperature for curved CNT by CCVDis770°C using C3H6as carbon source and Ni as catalyst; the yield of curved CNTand the thickness of curved CNT coating on carbon fibers can be controlledrespectively by adjusting catalyst loading and growth time. The microstructures andthe bonding between two types of CNTs and carbon fiber were investigated and theresults show that radially-straight CNT has a high crystallinity and its outer diameterand wall thickness increase and inner diameter decreases as a function of growth temperature; while, curved CNT has a relatively low crystallinity. The bonding forcebetween radially-straight CNT and carbon fiber is high showing an average bondingforce of60GPa, which is contributed to the formation of “graphite wall” that has asimilar structure with the nanotube wall; while, there is no direct structural linkbetween carbon fiber and most of the curved CNTs and thus the bonding forcebetween them is very low.The densification behaviors of C/C composites doped with the two types ofCNTs were investigated and the results show that: the densification rate ofradially-straight CNT-doped C/C composites is much higher than pure C/Ccomposites at different densification temperatures and partial pressures of precursorgas. Moderate densification temperature (1080°C) and moderate partial pressure ofprecursor gas (20kPa) are benefial to the densification rate and the densifydistribution uniformity of radially-straight CNT doped C/C composites. Thedensification rate enhances gradually with the content increase of radially-straightCNTs. Curved CNT only contributes the increase of the initial densification rate ofC/C composites at different densification temperatures and partial pressures ofprecursor gas; while, curved CNT-doped C/C composites has a decreased averagedensification rate compared with C/C composites and the decreasing degree enhanceswith the increasing of curved CNT content. To increase the densification rate ofcurved CNT-doped C/C, a lower densification temperature and a higher partialpressures of precursor gas are necessary; however, the density distribution of thecomposites will decrease. There are different densification modes in the C/Ccomposites doped with two types of CNTs: for radially-straight CNT-doped C/Ccomposites, the pyrocarbon shows a layer-by-layer deposition in the radial directionof carbon fiber and the pores between CNTs can be fully filled by pyrocarbon; while,for curved CNT-doped C/C composites, it is different for pyrocarbon to deposit in thenano-pores existed in the curved CNT coating and thus the pyrocarbon tend to depositin the spaces between curved CNT-coated carbon fibers, which results in theformation of abundant nanoscale pores in the composites with a low bulk density.The effects of CNTs on pyrocarbon matrix microstructures of C/C compositeswere investigated and the results show that: two layers of carbon matrices withdistinctive PLM morphologies have been observed after the introduction of CNTs.The first layer, which is closely around carbon fibers and is doped with CNTs, isisotropic under PLM with no optical activity though the prepation conditions would lead to medium-textured pyrocarbon. The first layer area enlarges with the increase ofCNT content. Other layer is near to the first layer, where no CNT exists and purepyrocarbon are deposited in this area, showing a similar optical activity with that inpure C/C composites but with smaller sizes and more abundant growing cores. Thefirst layer matrix has a compact structure in the radially-straight CNT-doped C/Ccomposites; while, a porous structure in curved CNT-doped C/C composites. Bothcurved CNT and radially-straight CNT have a ability of inducing the ordereddeposition of pyrocarbon; however, such a ability is limited with a action area rangingfrom hundreds of nanometers to few micrometers.XRD tests show that CNT-doped C/C composites have a smaller d002and a largerLc than pure C/C composites. Combined with the Raman analyses, it can be deducedthat the decreased d002and increased Lc result from the formation of high-texturedpyrocarbon around CNTs. Compared with that of curved CNT-doped C/C composites,the first layer matrix of radially-straight CNT-doped C/C composites show a better(002) in-plane order degree, which profits from the straight body of radially-grownCNTs. HRTEM analyses show that the first layer matrix shows a long-range disorderand short-range order structure. There is a close relationship between the formation ofisotropic carbon matrix around carbon fibers in CNT-doped C/C composites andrandom lay-out of CNTs and the related experiment demonstrates that formation ofisotropic carbon is unavoidable when densifying three dimensional CNT network fullof submicro-and nano-pores by chemical vapor infiltration.Mechanical properties of CNT-doped C/C composites were studies and theresults show that radially-straight CNTs endow the composites with greatly enhancedmatrix-dominated mechanical performances such as out-of-plane compression andinterlaminar shearing properties especially when the CNT content is high (resulting inover200%improvements than pure C/C composites) and decrease the mechanicalproperty anisotropy of C/C composites. However, radially-straight CNT-doped C/Ccomposites have a obviously decreased flexural properties compared with pure C/Ccomposites, due to the great damage of carbon fiber surface occurred during the CNTgrowth. SEM analyses show that the greatly-improved matrix-domianted mechanicalperformances benefit from the radial reinforcements from straight CNTs to F/Minterface and carbon matrix, which avoids the micro-defects that lead to the poormechanical performances of C/C composites. Curved CNTs can not provide the radialreinforcements to carbon matrix and facilitate themselves to pull out during the fracture of the composites, thus, they can not greatly increase the matrix-dominatedmechanical performances of C/C composites. However, curved CNTs can obviouslyimprove the pseudo-plastic fracture behaviors and increase the flexural performanceof C/C composites to some extents.Curved CNT-and radially-straight CNT-doped C/C composites have F/Minterfaces with different micro-mechanical properties: the former has a elasticitymodulus smaller than carbon fiber and carbon matrix, which is benefical to the carbonfiber pullout during the composite fracture that endows the composites with goodpseudo-plastic fracturecharacteristics; while, the latter has a higher elasticity modulussmaller than carbon matrix, which is harmful for itself to induce cracks to deflect thatinhibits the carbon fiber pullout during the composite fracture and leads to the brittlefracture of C/C composites. The investigations on the low-temperature thermalstability of C/C, curved CNT-doped C/C, and straight CNT-doped C/C compositesfurther demonstrate that both curved CNTs and straight CNTs can obviously stiffenF/M interface of C/C composites; however, compared with curved CNTs, straightCNTs can better reinforce carbon matrix and endow C/C composites with betterlow-temperature thermal stability.