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C-Cu composites strengthened with in-situ grown carbon nanotubes
Journal of Alloys and Compounds 694 (2017) 1054e1060
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Microstructure and flexural properties of C/C-Cu composites strengthened with in-situ grown carbon nanotubes Gang Kou a, Ling-jun Guo a, *, Zhao-qian Li b, Jian Peng a, Jie Tian c, Cai-xia Huo a a
State Key Laboratory of Solidification Processing, Carbon/Carbon Composites Research Center, Northwestern Polytechnical University, Xi'an 710072, China Shanghai Aerospace Equipment Manufacturer, Shanghai 200245, China c Shanghai Composites Science & Technology Co., Ltd, Shanghai 201112, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 August 2016 Received in revised form 7 October 2016 Accepted 16 October 2016 Available online 17 October 2016
Carbon nanotubes (CNTs) reinforced carbon/carbon-copper (C/C-Cu-CNTs) composites were fabricated by thermal gradient chemical vapor infiltration (TGCVI). The effect of copper and CNTs on the microstructure and flexural properties of carbon/carbon (C/C) composites were investigated. Results showed that the flexural strength of C/C-Cu-CNTs composites was larger than that of pure C/C composites and C/ C-Cu composites. Copper particles coated on carbon fibers reduced the contact area between pyrolytic carbon matrix and carbon fibers, leading to the decrease of flexural strength by 17.34% in direction Z and 22.44% in direction X, respectively. However, CNTs catalyzed by copper particles linked adjacent pyrolytic carbon layers together as a bridge and made propagation resistance of cracks increased, which strengthened the matrix. The flexural strength of C/C composites was improved by 33.61% in direction Z and 23.94% in direction X, respectively. © 2016 Elsevier B.V. All rights reserved.
Keywords: Microstructure Flexural properties Carbon-carbon composites Copper Carbon nanotube
1. Introduction Carbon/carbon (C/C) composites have been applied in aviation and aerospace fields for their superior thermal and hightemperature mechanical properties such as high thermal conductivity, low density, high specific strength, excellent chemical and ablation resistance. But with the development of new spacecraft, materials with lower ablation rate at high-temperature and highpressure environment are expected, researches on improving the properties of C/C composites draw much attention [1e3]. In order to improve its mechanical and ablation properties, many high melting point additives have been introduced into C/C composites. But low melting point additives are rarely considered to modify C/C composites. Due to the low cost, outstanding machining properties and prominent short-term ablation resistance as a result of “selftranspiration” technique of copper, a few researchers have introduced it into C/C composites to improve its properties [4e6]. Results illustrated that as the ablation rate being reduced, the flexural properties decreased, and some increased. Because of chemical stability, excellent mechanical strength, high surface area and extraordinary high electrical conductivity,
* Corresponding author. E-mail address: [email protected] (L.-j. Guo). http://dx.doi.org/10.1016/j.jallcom.2016.10.151 0925-8388/© 2016 Elsevier B.V. All rights reserved.
carbon nanotubes (CNTs) have attracted many researchers' attention [7e10]. The special characteristics of CNTs render them as promising materials in a wide range of potential applications such as: field emission devices, high strength yarns, hydrogen storage, nano-mechanical devices, viscoelastic materials and biomedical materials. So CNTs are natural candidates for reinforcement of advanced structural composites. For example, CNTs have been used as additive in metal [11e13], polymer [14,15], cement [16] and ceramic [17] matrix composites. Q. Song et al. [18] conducted CNTs depositing on unidirectional carbon fiber preforms by electrophoretic deposition and employed the CNT-attached fibers to make composites with multi-scaled reinforcement to reinforce C/C composites. The introduction of CNTs decreased the probability of annular cracks occurring in matrix caused by thermal stress during fabrication. Thus enhanced the anti-destroy capacity, and the refined matrix also leaded destructive cracks to spread along multiple paths. H. Zhang et al. [19] prepared in-situ grown CNT reinforced C/C composites by thermal gradient chemical vapor infiltration using ferric chloride as catalyst. The test revealed that the doping of CNTs into matrix increased the interface area between reinforcement and matrix and linked adjacent pyrolytic carbon layers together as a bridge, which can avoid the formation of concentric crack around carbon fiber to a certain extent. K.H. Hung et al. [20] conducted CNTs grown on carbon fibers and employed
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the CNT-attached fibers to make composites with multi-scaled reinforcement for modulate composites' intrinsical properties. Their results showed that increasing interfacial area and pulling out of nanotubes can improve the tensile strength of as-prepared composites. H.P. Li et al. [21] fabricated carbon nanotube reinforced hydroxyapatite (HA) composites with a double in situ synthesis process, CNTs were synthesized in situ in HA powders by chemical vapor deposition with the further encapsulation of CNTs using HA by a sol-gel method. The result illustrated that the flexural strength of the composite is 1.6 times higher than that of pure hydroxyapatite. In order to achieve the increase in mechanical properties of C/C composite, copper was introduced into C/C composites to catalyze the growth of CNTs. However, the catalyst was often used to catalyze the growth of CNTs [10,22] or as additions to improve the properties of C/C composites only [5,6,23]. There are few reports that not only study the effect of catalyst on the growing of CNTs, but also use it to modify the properties of C/C composite. In this work, the CNTs were introduced into two-dimensional carbon fiber felts by in-situ catalyzed with copper particles coated on carbon fibers, and then the C/C-Cu-CNTs composite was fabricated by densifying the 2-D carbon fiber felts within CNTs via thermal gradient chemical vapor infiltration (TGCVI). The microstructure and flexural mechanical properties of the fabricated C/C-Cu-CNTs and C/CCu composites were investigated further. 2. Experiment 2.1. Preparation of the materials 2D needle-punched carbon felts with a density of 0.4 g/cm3 were used as starting materials in this work. The carbon fibers were 12 k PAN-based (T300, Toray, Japan) with a mean filament diameter of 7 mm. The carbon felts were fabricated by over-lapping layers of 0 non-woven carbon fiber cloth, short-cut fiber web and 90 nonwoven carbon fiber cloth repeatedly with needle-punching in the vertical direction. Three kinds of C/C composites samples were prepared from the preform carbon felts. The pure C/C composites were fabricated directly from the preform without additive copper by thermal gradient chemical vapor infiltration (TGCVI) with the density of 1.80 g/cm3. The C/C-Cu and C/C-Cu-CNTs composites were fabricated including the following steps. First, the preform was soaked into saturated CuSO4 aqueous solution for 2 h, and drying followed. Then, the soaked preform was treated under 800 C in H2 atmosphere, and a preform with about 24 wt% copper was prepared finally. The treated preform was densified to obtain the C/C-Cu composites. Moreover, the treated preform was densified by isobaric chemical vapor infiltration (CVI) for 20 h under a lower natural gas flow at 1050 C, and finally densified by TGCVI to obtain C/C-Cu-CNTs composites. Fig. 1 shows the structure schematic of the prepared three kinds of C/C composites.
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Fig. 1. Schematic of the structure of C/C (a), C/C-Cu (b) and C/C-Cu-CNTs (c).
Three-point bending tests were carried out to determine the mechanical properties of the obtained composites. Rectangular bars of 30 mm 5 mm 3 mm along the axial direction and X direction of the carbon fibers in non-woven cloth were cut from samples as shown in Fig. 2. Tests were performed on an SANS universal mechanical machine (CMT5304-1 kN). The span was 20 mm. The cross-head speed was 0.2 mm/min and the loading direction was perpendicular to and parallel to the non-woven cloth respectively. Five specimens were tested for each composite at least. The loads and deflections values were recorded as a function of time. The nominal bending stress (s in MPa) and the nominal outer fiber strain (ε in %) were calculated from the following equations.
s¼
3FL 2bd2
(2)
ε¼
6sd 100 L2
(3)
where F is the load (N), L is the span (mm), b is the specimen width (mm), d is the specimen depth (mm) and s is the deflection or displacement (mm). A ductility factor (FD) was used to determine the ductility of the composites [1]. FD can be calculated from the following equation.
FD ¼ 1
Esecant Eorigin
!
¼1
εlin εt
where Esecant is secant modulus (the slope of the line from the origin to the stress at failure in the stressestrain curve), Eorigin is elastic modulus (the slope of the linear part of the stressestrain curve), εlin
2.2. Properties test and morphology analysis The density of the samples was determined by the Archimedes principle. First, weighting the dried samples as m0 (g). Second, putting the samples in boiling water for 2 h, taking out and letting cool, then weighting as m1 (g). Third, weighting the samples in deionized water as m2 (g). Finally, the density (r in g/cm3) of the samples was calculated by the following equation:
r¼
m0 m1 m2
(4)
(1) Fig. 2. Schematic of the 2D needle-punched carbon felts.
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is the strain in the linear part of the stressestrain curve at failure and the εt means the strain at failure. A Leica DMLP polarized light microscope (PLM) was used to observe the textures of obtained composites. The microstructure and fracture morphology of each composite was observed with scanning electron microscopy (SEM, Tescan Mira3).
around carbon fibers and shows light activity in pure C/C composites [28]. In Fig. 4, it shows light activity clearly, and its center is perpendicular to the direction of fiber. In its light activity center, one carbon nanotube lies there, liking carbon fiber in polarized light microscopic morphology of pure C/C composites. So the result is consistent with the growing morphology of CNTs on carbon fibers above.
3. Results and discussion 3.1. Microstructure Fig. 3 shows the carbon nanotube growing around carbon fibers in C/C composites before final densification. In the early formation stage of CNTs, it starts from the surface of carbon fiber, and a few carbon nanotubes with the length of a few micrometers and the diameter less than 1 mm distributes on carbon fiber as shown in Fig. 3(a). And the EDS pattern shows catalysts are packed in the CNT, especially in the end. With increasing densification time, the CNTs grow much longer as shown in Fig. 3 (b). The CNTs distribute around the carbon fibers, and carbon fibers are coated by pyrolytic carbon. The length of CNTs is up to more than 200 mm, and its diameter shows a wide distribution from about 100 nm to 1 mm, which are much larger than others [8,9,24]. Moreover, some CNTs present a bamboo shaped microstructure which are reported in Refs. [25e27]. In Fig. 3 (c), a cluster of multi-walls CNTs generate in the pores in C/C composites, and their diameter are uniform (500 nm). They most grow straightly, but some fork to grow branches. In addition, the wall of CNTs is thin, many carbon pieces distribute on the CNTs and make its surface much rough. Moreover, the polarized light microscopic morphology of C/CCu-CNTs composites was introduced to demonstrate that CNTs generated on carbon fibers. In general, the pyrolytic carbon forms
3.2. Flexural properties Usually, matrix-modifying influences the mechanical properties
Fig. 4. Polarized light microscopic morphology of C/C-Cu-CNTs composites.
Fig. 3. SEM images of carbon nanotubes in C/C-Cu-CNTs composites before final densification: (a) CNTs initiated on carbon fibers; (b) CNTs around carbon fibers; (c) multi-walls CNTs and (d) EDS pattern corresponding to the area marked in (a).
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of C/C composites significantly. In Refs. [4,6], results showed that the C/C-Cu composites possessed better flexural strength than pure C/C composites. In this work, the matrix-modified C/C-Cu composites possessed a lower flexural strength comparing with pure C/ C composites as shown in Fig. 5. Its flexural strength is only 165.60 MPa loading in Z and 150.76 MPa loading in X, respectively. However, through the way that CNTs grew in-situ in C/C composite catalyzed by Cu particles, the flexural strength of C/C composites is not decreased any more, but promoted instead. Its flexural strength is up to 267.67 MPa loading in Z and 240.93 MPa loading in X, while it is 200.34 MPa and 194.40 MPa for pure C/C composites. The difference of flexural strength between different loading directions are greater for C/C-Cu-CNTs composites. The reason is that the pores in carbon felts distribute differently in direction Z and X, leading to an uneven distribution of Cu particles in different directions. And pores in C/C composites influence its
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strength greatly. During the densification process, pyrolytic carbon and CNTs generated at the same time. With prolonging densification time, pyrolytic carbon and CNTs made some big pores closed or closed partly, causing fresh carbon source unable to flow into the pores and generate pyrolytic carbon to fill them anymore. Meanwhile, they distributed in carbon felts randomly and made data scatter of the flexural strength of C/C/-Cu-CNTs composite increased finally. But for pure C/C composites and C/C-Cu composites, though pores existed in carbon felts, there was no effect of CNTs on the densification process. So the closed pores in fabricated composites reduced not only in size, but also in quantity, and they distribute uniformly. The density of fabricated composites (pure C/ C: 1.80 g/cm3; C/C-Cu: 1.83 g/cm3; C/C-Cu-CNTs:1.82 g/cm3) also shows that pores in C/C-Cu-CNTs composites is more than that in C/ C-Cu composites. In spite of the same copper content in C/C-Cu and C/C-Cu-CNTs composites, the density of C/C-Cu-CNTs composites is less than that of C/C-Cu composites, so it illustrates that there are more pores in C/C-Cu-CNTs composites. In general, the pores in C/C composites is unavoidable. Due to the concentration of stress in pores, the presence of porosity in C/C composites generally leads to a negative influence on the mechanical properties, resulting in lower strength [29]. However, though pores in C/C-Cu-CNTs composites is more than that in C/CCu composites, duo to the great mechanical properties of CNTs, the strengthening effect of CNTs is greater than the reduction effect of pores on mechanical properties. So, the mechanical strength C/CCu-CNTs composites is larger than that of C/C-Cu composites in theory. Moreover, CNTs also grew in these pores. Because these pores had been closed, CNTs were not covered by pyrocarbon anymore, but they linked the walls of pore. So, CNTs can prevent the cracks propagating to some extent.
Fig. 5. Flexural strength of different samples.
Fig. 6. Flexural stress-strain curves: (a) loading in Z and (b) loading in X.
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In addition, carbon felts compose of non-woven carbon cloth and short-cut carbon fiber web layer by layer. In non-woven carbon cloth layers, copper particles attached to its surface and not enter its fiber bundles. So in this layer, the strength is similar to that of pure C/C composite. But in short-cut carbon fiber web layers, copper particles distribute uniformly. CNTs can grow evenly in short-cut carbon fiber web layers, resulting increased strength for C/C-Cu-CNTs composites. As a result, the flexural strength of C/CCu-CNTs composites are different in different directions. Fig. 6 shows the typical flexural stress-strain curves of the samples loading in direction Z and X. In Fig. 6, it shows the similar characteristics of stress-strain curves in two directions for different obtained composites. For pure C/C composites, the displacement increases with increasing load, and it shows the particular cracking characteristic when the load is larger than the maximum load. It does not arise brittle fracture. But for C/C-Cu and C/C-Cu-CNTs composites, the stressstrain curves show the same variation tendency comparing with C/C composites. Moreover, they fracture in a brittle way when the load is up to the maximum. In linear stage, the slope of stress-strain curve increases in the order: C/C, C/C-Cu, C/C-Cu-CNTs, showing that the flexural modulus of C/C-Cu-CNTs composites is the largest. Table 1 shows the flexural modulus and ductility factor FD in detail. The FD increases with the ductility property improving,
Table 1 Mechanical properties of the C/C composites. Sample
C/C C/C-Cu C/C-Cu-CNTs
Density (g/cm3)
1.80 1.83 1.82
Flexural strength (MPa)
Flexural modulus (GPa)
FD
Z
X
Z
X
Z
X
200.34 165.60 267.67
194.40 150.76 240.93
8.85 14.64 15.32
11.10 13.43 14.51
0.696 0.479 0.491
0.456 0.404 0.446
while the modulus changes in a contrary tendency. In Table 1 and Fig. 5, it shows the C/C-Cu-CNTs composites possesses the largest flexural strength and the lowest ductility, and C/C-Cu composites has the worst flexural strength and medium ductility, while the flexural strength and ductility of pure C/C composites is between C/C-Cu composites and C/C-Cu-CNTs composites. So CNTs catalyzed by Cu particles in C/C-Cu composites shows positive effect on the flexural properties. Fig. 7 shows the fracture morphology of C/C-Cu and C/C-CuCNTs composites after three-point bending tests. In the fracture morphology, it shows the characteristic morphology of C/C composites [1,19] that involve fiber debonding and fiber pull-out for C/C-Cu and C/C-Cu-CNTs composites. In Fig. 7 (a) and (b), the fracture is flat, and it shows some copper particles in the fracture and distribute around carbon fibers or on the wall of holes which carbon fibers are pulled out. But for the fracture of C/CCu-CNTs composites, some CNTs distribute in the fracture as shown in Fig. 7 (c) (labeled by arrows). They scatter between carbon fibers and keep a morphology as shown in Fig. 3 (a). While in local fracture as shown in Fig. 7 (d), it shows a few small holes in the fracture surrounded by pyrolytic carbon. Comparing the diameter of the holes with CNTs in Fig. 3, it illustrates that CNTs are pulled out from the holes (labeled by arrows). Moreover, the directions of the holes are different, it is similar to the CNTs generating on carbon fibers as shown in Fig. 3 (a). And there is some differences of the diameter and shape between holes, so it also demonstrates that the holes are the legacy of CNTs. 3.3. Discussion The structural investigations and mechanical testing yield complementary information about correlation between matrix microstructure and mechanical properties of investigated composites. The form of the stress-strain curves correlates with the morphology of fracture morphology. For pure C/C composites, the
Fig. 7. Fracture morphology of C/C composites: (a) and (b) C/C-Cu, solid arrows show the copper particles around carbon fibers (inset of figure refers to the EDS pattern corresponding to area marked by red box); (c) and (d) C/C-Cu-CNTs, solid arrows show the fracture of CNTs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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the adhesion between carbon matrix and carbon fibers, leading to a lower flexural properties compared with pure C/C composite. By controlling fabricating process, CNTs catalyzed by copper particles grew in C/C composites successfully, and the flexural strength of C/ C composites was improved by 33.61% in direction Z and 23.94% in direction X, respectively. And it is a meaning way to introduce copper particles into C/C composites to improve the ablation, thermal and electric properties. Acknowledgments
Fig. 8. Morphology of carbon fibers covered with catalyst particle.
pyrolytic carbon matrix coated on the carbon fibers completely. Even there is interface between the matrix and fibers, it represented good adhesion between them and possesses modest flexural strength. However, the adhesion between pyrolytic carbon matrix and carbon fibers decreased when carbon fibers were coated with copper particles. The reason is that there is a bad wettability between copper and carbon fiber, copper covers carbon fibers in the way of particles instead of whole layer as shown in Fig. 8. Seen from Fig. 8, the bright particles are copper particles, it shows the copper particles attach to carbon fibers and distribute around carbon fibers uniformly. During densification, pyrolytic carbon forms on carbon fibers, but it cannot coat carbon fibers completely due to the copper particles on fibers as shown in Fig. 1 (b). There are many copper particles distributing between pyrolytic carbon and carbon fibers as shown in Fig. 7 (a) and (b). It results in a poor adhesion between the pyrolytic carbon matrix and carbon fibers. So the C/C-Cu composites showed a faded flexural strength compared with pure C/C composites. For C/C-Cu-CNTs composites, though the copper particles coated on carbon fibers weaken the strength of C/C composites, CNTs generating on carbon fibers catalyzed by copper strengthen it instead. The root of CNTs combine with carbon fiber greatly, and copper catalysts go follow with the front of CNT as shown in Figs. 3 (a) and Fig. 1 (c). So it not only makes the interface between carbon fiber and pyrolytic carbon matrix increased, but also strengthens the matrix in C/C composite with CNTs by linking adjacent pyrolytic carbon layers together as a bridge and makes propagation resistance of cracks increased. Although there are CNTs generated in C/C-Cu-CNTs composites, some raised structures still survive on carbon fibers, which some copper particles are packed in them. It demonstrates that some catalyst particles have no effect on the growing of CNTs, they are packed on carbon fiber by pyrolytic carbon as shown in Fig. 7 (a) and (b). The packed copper particles can melt and evaporate under ultra-high temperature and make the surface temperature of C/C composites decreased to protect C/C composites in ablation. 4. Conclusions Copper was introduced into C/C composites to generate CNTs and improve the flexural strength of C/C composites. The flexural strength of pure C/C, C/C-Cu and C/C-Cu-CNTs composites was 200.34 MPa, 165.60 MPa, 267.67 MPa loading in direction Z and 194.40 MPa, 150.76 MPa, 240.93 MPa loading in direction X, respectively. The additive copper particles on carbon fibers faded
This work has been supported by the National Natural Science Foundation of China (Grant Nos. 51432008 and U1435202), the Innovation Foundation of Shanghai Aerospace Science and Technology (SAST201470) and the Research Fund of State Key Laboratory of Solidification Processing (NWPU), China (Grant No. 142-TZ2016). References [1] B. Reznik, M. Guellali, D. Gerthsen, R. Oberacker, M.J. Hoffmann, Microstructure and mechanical properties of carbonecarbon composites with multilayered pyrocarbon matrix, Mater. Lett. 52 (2002) 14e19. [2] Q.L. Yuan, Y.L. Li, H.J. Li, S.P. Li, L.J. Guo, Quasi-static and dynamic compressive fracture behavior of carbon/carbon composites, Carbon 46 (2008) 699e703. [3] Y. Jia, K.Z. Li, S.Y. Zhang, L. Li, J.J. Ren, Microstructure and mechanical properties of multilayer-textured 2D carbon/carbon composites, J. Mater. Sci. Technol. 30 (2014) 1202e1207. [4] L.P. Ran, K. Peng, M.Z. Yi, L. Yang, Ablation property of a C/CeCu composite prepared by pressureless infiltration, Mater. Lett. 65 (2011) 2076e2078. [5] Y.J. Fu, L.Y. Zhang, G. Chen, Preparation of a carbon nanotube-copper nanoparticle hybrid by chemical reduction for use in the electrochemical sensing of carbohydrates, Carbon 50 (2012) 2563e2570. [6] L. Liu, H.J. Li, X.H. Shi, W. Feng, B. Feng, C. Sun, Effect of Cu particles on the ablation properties of C/C composites, Solid State Sci. 25 (2013) 78e84. [7] L. Yang, D. Chu, L. Wang, X. Wu, J. Luo, Synthesis and photocatalytic activity of chrysanthemum-like Cu2O/Carbon Nanotubes nanocomposites, Ceram. Int. 42 (2016) 2502e2509. [8] Y.J. Su, Y.Z. Zhang, H. Wei, Z. Yang, E.S. Kong, Y.F. Zhang, Diameter-control of single-walled carbon nanotubes produced by magnetic field-assisted arc discharge, Carbon 50 (2012) 2556e2562. [9] N. Nafeie, M. Montazer, N.H. Nejad, T. Harifi, Electrical conductivity of different carbon nanotubes on wool fabric: an investigation on the effects of different dispersing agents and pretreatments, Colloids Surf. A Physicochem. Eng. Aspects 497 (2016) 81e89. [10] Y.Y. Li, L.J. Guo, Y.W. Wang, H.J. Li, Q. Song, A novel multiscale reinforcement by in-situ growing carbon nanotubes on graphene oxide grafted carbon fibers and its reinforced carbon/carbon composites with improved tensile properties, J. Mater. Sci. Technol. 32 (2016) 419e424. [11] M. Jagannatham, S. Sankaran, P. Haridoss, Microstructure and mechanical behavior of copper coated multiwall carbon nanotubes reinforced aluminum composites, Mater. Sci. Eng. A 638 (2015) 197e207. [12] K. Kang, H. Park, J. Kim, C. Lee, Role of spray processes on microstructural evolution, and physical and mechanical properties of multi-walled carbon nanotube reinforced Cu composite coatings, Appl. Surf. Sci. 356 (2015) 1039e1051. pez, R. Poyato, A. Morales-Rodríguez, A. Ferna ndez-Serrano, [13] A. Gallardo-Lo ~ oz, A. Domínguez-Rodríguez, Hardness and flexural strength of singleA. Mun walled carbon nanotube/alumina composites, J. Mater. Sci. 49 (2014) 7116e7123. [14] L. Liu, C.Y. Jia, J.M. He, F. Zhao, D.P. Fan, L.X. Xing, Interfacial characterization, control and modification of carbon fiber reinforced polymer composites, Compos. Sci. Technol. 121 (2015) 56e72. [15] Z. Eslami, F. Yazdani, M.A. Mirzapou, Thermal and mechanical properties of phenolic-based composites reinforced by carbon fibres and multiwall carbon nanotubes, Compos. Part A Appl. Sci. 72 (2015) 22e31. [16] S.J. Park, D.G. Lee, Development of CNT-metal-filters by direct growth of carbon nanotubes, Curr. Appl. Phys. 6 (2006) e182ee186. [17] H. Mei, Y.W. Xu, Y.Y. Sun, Q.L. Bai, L.F. Cheng, Carbon nanotube buckypaperreinforced SiCN ceramic matrix composites of superior electrical conductivity, J. Eur. Ceram. Soc. 36 (2016) 1893e1898. [18] Q. Song, K.Z. Li, H.J. Li, Q.G. Fu, Increasing the tensile property of unidirectional carbon/carbon composites by grafting carbon nanotubes onto carbon fibers by electrophoretic deposition, J. Mater. Sci. Technol. 29 (2013) 711e714. [19] H. Zhang, L.J. Guo, Q. Song, Q.G. Fu, H.J. Li, K.Z. Li, Microstructure and flexural properties of carbon/carbon composite with in-situ grown carbon nanotube as secondary reinforcement, Prog. Nat. Sci. Mater. 23 (2013) 157e163.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Microstructure and flexural properties of C/C-Cu composites strengthened with in-situ grown carbon nanotubes Gang Kou a, Ling-jun Guo a, *, Zhao-qian Li b, Jian Peng a, Jie Tian c, Cai-xia Huo a a
State Key Laboratory of Solidification Processing, Carbon/Carbon Composites Research Center, Northwestern Polytechnical University, Xi'an 710072, China Shanghai Aerospace Equipment Manufacturer, Shanghai 200245, China c Shanghai Composites Science & Technology Co., Ltd, Shanghai 201112, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 August 2016 Received in revised form 7 October 2016 Accepted 16 October 2016 Available online 17 October 2016
Carbon nanotubes (CNTs) reinforced carbon/carbon-copper (C/C-Cu-CNTs) composites were fabricated by thermal gradient chemical vapor infiltration (TGCVI). The effect of copper and CNTs on the microstructure and flexural properties of carbon/carbon (C/C) composites were investigated. Results showed that the flexural strength of C/C-Cu-CNTs composites was larger than that of pure C/C composites and C/ C-Cu composites. Copper particles coated on carbon fibers reduced the contact area between pyrolytic carbon matrix and carbon fibers, leading to the decrease of flexural strength by 17.34% in direction Z and 22.44% in direction X, respectively. However, CNTs catalyzed by copper particles linked adjacent pyrolytic carbon layers together as a bridge and made propagation resistance of cracks increased, which strengthened the matrix. The flexural strength of C/C composites was improved by 33.61% in direction Z and 23.94% in direction X, respectively. © 2016 Elsevier B.V. All rights reserved.
Keywords: Microstructure Flexural properties Carbon-carbon composites Copper Carbon nanotube
1. Introduction Carbon/carbon (C/C) composites have been applied in aviation and aerospace fields for their superior thermal and hightemperature mechanical properties such as high thermal conductivity, low density, high specific strength, excellent chemical and ablation resistance. But with the development of new spacecraft, materials with lower ablation rate at high-temperature and highpressure environment are expected, researches on improving the properties of C/C composites draw much attention [1e3]. In order to improve its mechanical and ablation properties, many high melting point additives have been introduced into C/C composites. But low melting point additives are rarely considered to modify C/C composites. Due to the low cost, outstanding machining properties and prominent short-term ablation resistance as a result of “selftranspiration” technique of copper, a few researchers have introduced it into C/C composites to improve its properties [4e6]. Results illustrated that as the ablation rate being reduced, the flexural properties decreased, and some increased. Because of chemical stability, excellent mechanical strength, high surface area and extraordinary high electrical conductivity,
* Corresponding author. E-mail address: [email protected] (L.-j. Guo). http://dx.doi.org/10.1016/j.jallcom.2016.10.151 0925-8388/© 2016 Elsevier B.V. All rights reserved.
carbon nanotubes (CNTs) have attracted many researchers' attention [7e10]. The special characteristics of CNTs render them as promising materials in a wide range of potential applications such as: field emission devices, high strength yarns, hydrogen storage, nano-mechanical devices, viscoelastic materials and biomedical materials. So CNTs are natural candidates for reinforcement of advanced structural composites. For example, CNTs have been used as additive in metal [11e13], polymer [14,15], cement [16] and ceramic [17] matrix composites. Q. Song et al. [18] conducted CNTs depositing on unidirectional carbon fiber preforms by electrophoretic deposition and employed the CNT-attached fibers to make composites with multi-scaled reinforcement to reinforce C/C composites. The introduction of CNTs decreased the probability of annular cracks occurring in matrix caused by thermal stress during fabrication. Thus enhanced the anti-destroy capacity, and the refined matrix also leaded destructive cracks to spread along multiple paths. H. Zhang et al. [19] prepared in-situ grown CNT reinforced C/C composites by thermal gradient chemical vapor infiltration using ferric chloride as catalyst. The test revealed that the doping of CNTs into matrix increased the interface area between reinforcement and matrix and linked adjacent pyrolytic carbon layers together as a bridge, which can avoid the formation of concentric crack around carbon fiber to a certain extent. K.H. Hung et al. [20] conducted CNTs grown on carbon fibers and employed
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the CNT-attached fibers to make composites with multi-scaled reinforcement for modulate composites' intrinsical properties. Their results showed that increasing interfacial area and pulling out of nanotubes can improve the tensile strength of as-prepared composites. H.P. Li et al. [21] fabricated carbon nanotube reinforced hydroxyapatite (HA) composites with a double in situ synthesis process, CNTs were synthesized in situ in HA powders by chemical vapor deposition with the further encapsulation of CNTs using HA by a sol-gel method. The result illustrated that the flexural strength of the composite is 1.6 times higher than that of pure hydroxyapatite. In order to achieve the increase in mechanical properties of C/C composite, copper was introduced into C/C composites to catalyze the growth of CNTs. However, the catalyst was often used to catalyze the growth of CNTs [10,22] or as additions to improve the properties of C/C composites only [5,6,23]. There are few reports that not only study the effect of catalyst on the growing of CNTs, but also use it to modify the properties of C/C composite. In this work, the CNTs were introduced into two-dimensional carbon fiber felts by in-situ catalyzed with copper particles coated on carbon fibers, and then the C/C-Cu-CNTs composite was fabricated by densifying the 2-D carbon fiber felts within CNTs via thermal gradient chemical vapor infiltration (TGCVI). The microstructure and flexural mechanical properties of the fabricated C/C-Cu-CNTs and C/CCu composites were investigated further. 2. Experiment 2.1. Preparation of the materials 2D needle-punched carbon felts with a density of 0.4 g/cm3 were used as starting materials in this work. The carbon fibers were 12 k PAN-based (T300, Toray, Japan) with a mean filament diameter of 7 mm. The carbon felts were fabricated by over-lapping layers of 0 non-woven carbon fiber cloth, short-cut fiber web and 90 nonwoven carbon fiber cloth repeatedly with needle-punching in the vertical direction. Three kinds of C/C composites samples were prepared from the preform carbon felts. The pure C/C composites were fabricated directly from the preform without additive copper by thermal gradient chemical vapor infiltration (TGCVI) with the density of 1.80 g/cm3. The C/C-Cu and C/C-Cu-CNTs composites were fabricated including the following steps. First, the preform was soaked into saturated CuSO4 aqueous solution for 2 h, and drying followed. Then, the soaked preform was treated under 800 C in H2 atmosphere, and a preform with about 24 wt% copper was prepared finally. The treated preform was densified to obtain the C/C-Cu composites. Moreover, the treated preform was densified by isobaric chemical vapor infiltration (CVI) for 20 h under a lower natural gas flow at 1050 C, and finally densified by TGCVI to obtain C/C-Cu-CNTs composites. Fig. 1 shows the structure schematic of the prepared three kinds of C/C composites.
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Fig. 1. Schematic of the structure of C/C (a), C/C-Cu (b) and C/C-Cu-CNTs (c).
Three-point bending tests were carried out to determine the mechanical properties of the obtained composites. Rectangular bars of 30 mm 5 mm 3 mm along the axial direction and X direction of the carbon fibers in non-woven cloth were cut from samples as shown in Fig. 2. Tests were performed on an SANS universal mechanical machine (CMT5304-1 kN). The span was 20 mm. The cross-head speed was 0.2 mm/min and the loading direction was perpendicular to and parallel to the non-woven cloth respectively. Five specimens were tested for each composite at least. The loads and deflections values were recorded as a function of time. The nominal bending stress (s in MPa) and the nominal outer fiber strain (ε in %) were calculated from the following equations.
s¼
3FL 2bd2
(2)
ε¼
6sd 100 L2
(3)
where F is the load (N), L is the span (mm), b is the specimen width (mm), d is the specimen depth (mm) and s is the deflection or displacement (mm). A ductility factor (FD) was used to determine the ductility of the composites [1]. FD can be calculated from the following equation.
FD ¼ 1
Esecant Eorigin
!
¼1
εlin εt
where Esecant is secant modulus (the slope of the line from the origin to the stress at failure in the stressestrain curve), Eorigin is elastic modulus (the slope of the linear part of the stressestrain curve), εlin
2.2. Properties test and morphology analysis The density of the samples was determined by the Archimedes principle. First, weighting the dried samples as m0 (g). Second, putting the samples in boiling water for 2 h, taking out and letting cool, then weighting as m1 (g). Third, weighting the samples in deionized water as m2 (g). Finally, the density (r in g/cm3) of the samples was calculated by the following equation:
r¼
m0 m1 m2
(4)
(1) Fig. 2. Schematic of the 2D needle-punched carbon felts.
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is the strain in the linear part of the stressestrain curve at failure and the εt means the strain at failure. A Leica DMLP polarized light microscope (PLM) was used to observe the textures of obtained composites. The microstructure and fracture morphology of each composite was observed with scanning electron microscopy (SEM, Tescan Mira3).
around carbon fibers and shows light activity in pure C/C composites [28]. In Fig. 4, it shows light activity clearly, and its center is perpendicular to the direction of fiber. In its light activity center, one carbon nanotube lies there, liking carbon fiber in polarized light microscopic morphology of pure C/C composites. So the result is consistent with the growing morphology of CNTs on carbon fibers above.
3. Results and discussion 3.1. Microstructure Fig. 3 shows the carbon nanotube growing around carbon fibers in C/C composites before final densification. In the early formation stage of CNTs, it starts from the surface of carbon fiber, and a few carbon nanotubes with the length of a few micrometers and the diameter less than 1 mm distributes on carbon fiber as shown in Fig. 3(a). And the EDS pattern shows catalysts are packed in the CNT, especially in the end. With increasing densification time, the CNTs grow much longer as shown in Fig. 3 (b). The CNTs distribute around the carbon fibers, and carbon fibers are coated by pyrolytic carbon. The length of CNTs is up to more than 200 mm, and its diameter shows a wide distribution from about 100 nm to 1 mm, which are much larger than others [8,9,24]. Moreover, some CNTs present a bamboo shaped microstructure which are reported in Refs. [25e27]. In Fig. 3 (c), a cluster of multi-walls CNTs generate in the pores in C/C composites, and their diameter are uniform (500 nm). They most grow straightly, but some fork to grow branches. In addition, the wall of CNTs is thin, many carbon pieces distribute on the CNTs and make its surface much rough. Moreover, the polarized light microscopic morphology of C/CCu-CNTs composites was introduced to demonstrate that CNTs generated on carbon fibers. In general, the pyrolytic carbon forms
3.2. Flexural properties Usually, matrix-modifying influences the mechanical properties
Fig. 4. Polarized light microscopic morphology of C/C-Cu-CNTs composites.
Fig. 3. SEM images of carbon nanotubes in C/C-Cu-CNTs composites before final densification: (a) CNTs initiated on carbon fibers; (b) CNTs around carbon fibers; (c) multi-walls CNTs and (d) EDS pattern corresponding to the area marked in (a).
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of C/C composites significantly. In Refs. [4,6], results showed that the C/C-Cu composites possessed better flexural strength than pure C/C composites. In this work, the matrix-modified C/C-Cu composites possessed a lower flexural strength comparing with pure C/ C composites as shown in Fig. 5. Its flexural strength is only 165.60 MPa loading in Z and 150.76 MPa loading in X, respectively. However, through the way that CNTs grew in-situ in C/C composite catalyzed by Cu particles, the flexural strength of C/C composites is not decreased any more, but promoted instead. Its flexural strength is up to 267.67 MPa loading in Z and 240.93 MPa loading in X, while it is 200.34 MPa and 194.40 MPa for pure C/C composites. The difference of flexural strength between different loading directions are greater for C/C-Cu-CNTs composites. The reason is that the pores in carbon felts distribute differently in direction Z and X, leading to an uneven distribution of Cu particles in different directions. And pores in C/C composites influence its
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strength greatly. During the densification process, pyrolytic carbon and CNTs generated at the same time. With prolonging densification time, pyrolytic carbon and CNTs made some big pores closed or closed partly, causing fresh carbon source unable to flow into the pores and generate pyrolytic carbon to fill them anymore. Meanwhile, they distributed in carbon felts randomly and made data scatter of the flexural strength of C/C/-Cu-CNTs composite increased finally. But for pure C/C composites and C/C-Cu composites, though pores existed in carbon felts, there was no effect of CNTs on the densification process. So the closed pores in fabricated composites reduced not only in size, but also in quantity, and they distribute uniformly. The density of fabricated composites (pure C/ C: 1.80 g/cm3; C/C-Cu: 1.83 g/cm3; C/C-Cu-CNTs:1.82 g/cm3) also shows that pores in C/C-Cu-CNTs composites is more than that in C/ C-Cu composites. In spite of the same copper content in C/C-Cu and C/C-Cu-CNTs composites, the density of C/C-Cu-CNTs composites is less than that of C/C-Cu composites, so it illustrates that there are more pores in C/C-Cu-CNTs composites. In general, the pores in C/C composites is unavoidable. Due to the concentration of stress in pores, the presence of porosity in C/C composites generally leads to a negative influence on the mechanical properties, resulting in lower strength [29]. However, though pores in C/C-Cu-CNTs composites is more than that in C/CCu composites, duo to the great mechanical properties of CNTs, the strengthening effect of CNTs is greater than the reduction effect of pores on mechanical properties. So, the mechanical strength C/CCu-CNTs composites is larger than that of C/C-Cu composites in theory. Moreover, CNTs also grew in these pores. Because these pores had been closed, CNTs were not covered by pyrocarbon anymore, but they linked the walls of pore. So, CNTs can prevent the cracks propagating to some extent.
Fig. 5. Flexural strength of different samples.
Fig. 6. Flexural stress-strain curves: (a) loading in Z and (b) loading in X.
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In addition, carbon felts compose of non-woven carbon cloth and short-cut carbon fiber web layer by layer. In non-woven carbon cloth layers, copper particles attached to its surface and not enter its fiber bundles. So in this layer, the strength is similar to that of pure C/C composite. But in short-cut carbon fiber web layers, copper particles distribute uniformly. CNTs can grow evenly in short-cut carbon fiber web layers, resulting increased strength for C/C-Cu-CNTs composites. As a result, the flexural strength of C/CCu-CNTs composites are different in different directions. Fig. 6 shows the typical flexural stress-strain curves of the samples loading in direction Z and X. In Fig. 6, it shows the similar characteristics of stress-strain curves in two directions for different obtained composites. For pure C/C composites, the displacement increases with increasing load, and it shows the particular cracking characteristic when the load is larger than the maximum load. It does not arise brittle fracture. But for C/C-Cu and C/C-Cu-CNTs composites, the stressstrain curves show the same variation tendency comparing with C/C composites. Moreover, they fracture in a brittle way when the load is up to the maximum. In linear stage, the slope of stress-strain curve increases in the order: C/C, C/C-Cu, C/C-Cu-CNTs, showing that the flexural modulus of C/C-Cu-CNTs composites is the largest. Table 1 shows the flexural modulus and ductility factor FD in detail. The FD increases with the ductility property improving,
Table 1 Mechanical properties of the C/C composites. Sample
C/C C/C-Cu C/C-Cu-CNTs
Density (g/cm3)
1.80 1.83 1.82
Flexural strength (MPa)
Flexural modulus (GPa)
FD
Z
X
Z
X
Z
X
200.34 165.60 267.67
194.40 150.76 240.93
8.85 14.64 15.32
11.10 13.43 14.51
0.696 0.479 0.491
0.456 0.404 0.446
while the modulus changes in a contrary tendency. In Table 1 and Fig. 5, it shows the C/C-Cu-CNTs composites possesses the largest flexural strength and the lowest ductility, and C/C-Cu composites has the worst flexural strength and medium ductility, while the flexural strength and ductility of pure C/C composites is between C/C-Cu composites and C/C-Cu-CNTs composites. So CNTs catalyzed by Cu particles in C/C-Cu composites shows positive effect on the flexural properties. Fig. 7 shows the fracture morphology of C/C-Cu and C/C-CuCNTs composites after three-point bending tests. In the fracture morphology, it shows the characteristic morphology of C/C composites [1,19] that involve fiber debonding and fiber pull-out for C/C-Cu and C/C-Cu-CNTs composites. In Fig. 7 (a) and (b), the fracture is flat, and it shows some copper particles in the fracture and distribute around carbon fibers or on the wall of holes which carbon fibers are pulled out. But for the fracture of C/CCu-CNTs composites, some CNTs distribute in the fracture as shown in Fig. 7 (c) (labeled by arrows). They scatter between carbon fibers and keep a morphology as shown in Fig. 3 (a). While in local fracture as shown in Fig. 7 (d), it shows a few small holes in the fracture surrounded by pyrolytic carbon. Comparing the diameter of the holes with CNTs in Fig. 3, it illustrates that CNTs are pulled out from the holes (labeled by arrows). Moreover, the directions of the holes are different, it is similar to the CNTs generating on carbon fibers as shown in Fig. 3 (a). And there is some differences of the diameter and shape between holes, so it also demonstrates that the holes are the legacy of CNTs. 3.3. Discussion The structural investigations and mechanical testing yield complementary information about correlation between matrix microstructure and mechanical properties of investigated composites. The form of the stress-strain curves correlates with the morphology of fracture morphology. For pure C/C composites, the
Fig. 7. Fracture morphology of C/C composites: (a) and (b) C/C-Cu, solid arrows show the copper particles around carbon fibers (inset of figure refers to the EDS pattern corresponding to area marked by red box); (c) and (d) C/C-Cu-CNTs, solid arrows show the fracture of CNTs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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the adhesion between carbon matrix and carbon fibers, leading to a lower flexural properties compared with pure C/C composite. By controlling fabricating process, CNTs catalyzed by copper particles grew in C/C composites successfully, and the flexural strength of C/ C composites was improved by 33.61% in direction Z and 23.94% in direction X, respectively. And it is a meaning way to introduce copper particles into C/C composites to improve the ablation, thermal and electric properties. Acknowledgments
Fig. 8. Morphology of carbon fibers covered with catalyst particle.
pyrolytic carbon matrix coated on the carbon fibers completely. Even there is interface between the matrix and fibers, it represented good adhesion between them and possesses modest flexural strength. However, the adhesion between pyrolytic carbon matrix and carbon fibers decreased when carbon fibers were coated with copper particles. The reason is that there is a bad wettability between copper and carbon fiber, copper covers carbon fibers in the way of particles instead of whole layer as shown in Fig. 8. Seen from Fig. 8, the bright particles are copper particles, it shows the copper particles attach to carbon fibers and distribute around carbon fibers uniformly. During densification, pyrolytic carbon forms on carbon fibers, but it cannot coat carbon fibers completely due to the copper particles on fibers as shown in Fig. 1 (b). There are many copper particles distributing between pyrolytic carbon and carbon fibers as shown in Fig. 7 (a) and (b). It results in a poor adhesion between the pyrolytic carbon matrix and carbon fibers. So the C/C-Cu composites showed a faded flexural strength compared with pure C/C composites. For C/C-Cu-CNTs composites, though the copper particles coated on carbon fibers weaken the strength of C/C composites, CNTs generating on carbon fibers catalyzed by copper strengthen it instead. The root of CNTs combine with carbon fiber greatly, and copper catalysts go follow with the front of CNT as shown in Figs. 3 (a) and Fig. 1 (c). So it not only makes the interface between carbon fiber and pyrolytic carbon matrix increased, but also strengthens the matrix in C/C composite with CNTs by linking adjacent pyrolytic carbon layers together as a bridge and makes propagation resistance of cracks increased. Although there are CNTs generated in C/C-Cu-CNTs composites, some raised structures still survive on carbon fibers, which some copper particles are packed in them. It demonstrates that some catalyst particles have no effect on the growing of CNTs, they are packed on carbon fiber by pyrolytic carbon as shown in Fig. 7 (a) and (b). The packed copper particles can melt and evaporate under ultra-high temperature and make the surface temperature of C/C composites decreased to protect C/C composites in ablation. 4. Conclusions Copper was introduced into C/C composites to generate CNTs and improve the flexural strength of C/C composites. The flexural strength of pure C/C, C/C-Cu and C/C-Cu-CNTs composites was 200.34 MPa, 165.60 MPa, 267.67 MPa loading in direction Z and 194.40 MPa, 150.76 MPa, 240.93 MPa loading in direction X, respectively. The additive copper particles on carbon fibers faded
This work has been supported by the National Natural Science Foundation of China (Grant Nos. 51432008 and U1435202), the Innovation Foundation of Shanghai Aerospace Science and Technology (SAST201470) and the Research Fund of State Key Laboratory of Solidification Processing (NWPU), China (Grant No. 142-TZ2016). References [1] B. Reznik, M. Guellali, D. Gerthsen, R. Oberacker, M.J. Hoffmann, Microstructure and mechanical properties of carbonecarbon composites with multilayered pyrocarbon matrix, Mater. Lett. 52 (2002) 14e19. [2] Q.L. Yuan, Y.L. Li, H.J. Li, S.P. Li, L.J. Guo, Quasi-static and dynamic compressive fracture behavior of carbon/carbon composites, Carbon 46 (2008) 699e703. [3] Y. Jia, K.Z. Li, S.Y. Zhang, L. Li, J.J. Ren, Microstructure and mechanical properties of multilayer-textured 2D carbon/carbon composites, J. Mater. Sci. Technol. 30 (2014) 1202e1207. [4] L.P. Ran, K. Peng, M.Z. Yi, L. Yang, Ablation property of a C/CeCu composite prepared by pressureless infiltration, Mater. Lett. 65 (2011) 2076e2078. [5] Y.J. Fu, L.Y. Zhang, G. Chen, Preparation of a carbon nanotube-copper nanoparticle hybrid by chemical reduction for use in the electrochemical sensing of carbohydrates, Carbon 50 (2012) 2563e2570. [6] L. Liu, H.J. Li, X.H. Shi, W. Feng, B. Feng, C. Sun, Effect of Cu particles on the ablation properties of C/C composites, Solid State Sci. 25 (2013) 78e84. [7] L. Yang, D. Chu, L. Wang, X. Wu, J. Luo, Synthesis and photocatalytic activity of chrysanthemum-like Cu2O/Carbon Nanotubes nanocomposites, Ceram. Int. 42 (2016) 2502e2509. [8] Y.J. Su, Y.Z. Zhang, H. Wei, Z. Yang, E.S. Kong, Y.F. Zhang, Diameter-control of single-walled carbon nanotubes produced by magnetic field-assisted arc discharge, Carbon 50 (2012) 2556e2562. [9] N. Nafeie, M. Montazer, N.H. Nejad, T. Harifi, Electrical conductivity of different carbon nanotubes on wool fabric: an investigation on the effects of different dispersing agents and pretreatments, Colloids Surf. A Physicochem. Eng. Aspects 497 (2016) 81e89. [10] Y.Y. Li, L.J. Guo, Y.W. Wang, H.J. Li, Q. Song, A novel multiscale reinforcement by in-situ growing carbon nanotubes on graphene oxide grafted carbon fibers and its reinforced carbon/carbon composites with improved tensile properties, J. Mater. Sci. Technol. 32 (2016) 419e424. [11] M. Jagannatham, S. Sankaran, P. Haridoss, Microstructure and mechanical behavior of copper coated multiwall carbon nanotubes reinforced aluminum composites, Mater. Sci. Eng. A 638 (2015) 197e207. [12] K. Kang, H. Park, J. Kim, C. Lee, Role of spray processes on microstructural evolution, and physical and mechanical properties of multi-walled carbon nanotube reinforced Cu composite coatings, Appl. Surf. Sci. 356 (2015) 1039e1051. pez, R. Poyato, A. Morales-Rodríguez, A. Ferna ndez-Serrano, [13] A. Gallardo-Lo ~ oz, A. Domínguez-Rodríguez, Hardness and flexural strength of singleA. Mun walled carbon nanotube/alumina composites, J. Mater. Sci. 49 (2014) 7116e7123. [14] L. Liu, C.Y. Jia, J.M. He, F. Zhao, D.P. Fan, L.X. Xing, Interfacial characterization, control and modification of carbon fiber reinforced polymer composites, Compos. Sci. Technol. 121 (2015) 56e72. [15] Z. Eslami, F. Yazdani, M.A. Mirzapou, Thermal and mechanical properties of phenolic-based composites reinforced by carbon fibres and multiwall carbon nanotubes, Compos. Part A Appl. Sci. 72 (2015) 22e31. [16] S.J. Park, D.G. Lee, Development of CNT-metal-filters by direct growth of carbon nanotubes, Curr. Appl. Phys. 6 (2006) e182ee186. [17] H. Mei, Y.W. Xu, Y.Y. Sun, Q.L. Bai, L.F. Cheng, Carbon nanotube buckypaperreinforced SiCN ceramic matrix composites of superior electrical conductivity, J. Eur. Ceram. Soc. 36 (2016) 1893e1898. [18] Q. Song, K.Z. Li, H.J. Li, Q.G. Fu, Increasing the tensile property of unidirectional carbon/carbon composites by grafting carbon nanotubes onto carbon fibers by electrophoretic deposition, J. Mater. Sci. Technol. 29 (2013) 711e714. [19] H. Zhang, L.J. Guo, Q. Song, Q.G. Fu, H.J. Li, K.Z. Li, Microstructure and flexural properties of carbon/carbon composite with in-situ grown carbon nanotube as secondary reinforcement, Prog. Nat. Sci. Mater. 23 (2013) 157e163.
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