Improved tensile strength of carbon nanotube-grafted carbon fiber reinforced composites

Accepted Manuscript Improved tensile strength of carbon nanotube-grafted carbon fiber reinforced composites Geunsung Lee, Minchang Sung, Ji Ho Youk, Jinyong Lee, Woong-Ryeol Yu PII: DOI: Reference:

S0263-8223(18)33876-5 https://doi.org/10.1016/j.compstruct.2019.04.037 COST 10865

To appear in:

Composite Structures

Received Date: Revised Date: Accepted Date:

26 October 2018 11 January 2019 5 April 2019

Please cite this article as: Lee, G., Sung, M., Youk, J.H., Lee, J., Yu, W-R., Improved tensile strength of carbon nanotube-grafted carbon fiber reinforced composites, Composite Structures (2019), doi: https://doi.org/10.1016/ j.compstruct.2019.04.037

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Improved tensile strength of carbon nanotube-grafted carbon fiber reinforced composites Geunsung Leea, Minchang Sunga, Ji Ho Youkb, Jinyong Leec, Woong-Ryeol Yua, a

Department of Materials Science and Engineering and Research Institute of Advanced Materials, Seoul National University, 599 Gwanangno, Gwanak-gu, Seoul 151-742, Republic of Korea b Department of Advanced Fiber Engineering, Inha University, 253 Yonghyun-dong, Nam-gu, Incheon 402-751, Republic of Koreas c High Temperature Composite Materials Group, The 4th R&D Institute-4, Agency for Defense Development, Daejeon 305-600, South Korea

Abstract Increased tensile strength of carbon nanotube (CNT)-grafted carbon fiber (CF) composites has been reported, but the mechanism of this increase is not yet clear. In this study, CNTgrafted CF unidirectional (UD) and woven composites were fabricated using a lowtemperature chemical vapor deposition (CVD) and resin transfer molding. Two types of CNTs (short and thin, long and thick) were successfully grown and grafted to CFs without degrading the CFs in the preforms. The CNT-grafted CFs exhibited increased interfacial shear strength (IFSS) similarly regardless of the CNT type. Interestingly, however, long and thick CNT-grafted CF UD and woven composites exhibited significant increases in tensile strength (about 20% and 30%), suggesting other mechanisms besides increased IFSS. The splitting crack initiation under the mixed mode condition was quantitatively characterized for the CNT-grafted CF UD composites, demonstrating that long and thick CNTs delayed the splitting crack initiation. Delayed fiber splitting and increased IFSS were concluded to be the main sources of increased tensile strength of CNT-grafted CF composites. Keywords: Carbon fibers; Carbon nanotubes; Composites; Tensile strength; Splitting crack

Corresponding author. Tel.: +82 2 880 9096; fax: +82 2 883 8197. E-mail address: [email protected] (W.-R. Yu)



1

1. Introduction Due to their excellent mechanical, electrical and thermal properties, carbon fibers (CFs) and their composites have been used in nearly all engineering fields, prompting vast amounts of research to improve their mechanical properties. Meanwhile, carbon nanotubes (CNTs) have emerged as a new reinforcement and have also stimulated a considerable amount of research. The hybridization of CNTs and CFs, e.g., CNT-grafted CFs, has emerged as an advanced and hierarchical material that can improve the reinforcing effect of CFs in composites and solve the dispersion problems of CNTs [1]. CNTs radially grafted on CFs can improve the radial stiffness [2] and axial tensile strength [3] of CF-reinforced composites, the interfacial shear strength (IFSS) of polymer composites [4] and the electrochemical performance of CF electrodes [5]. Most researchers have grafted CNTs onto individual CFs; this process has been extended to continuous processing and has enabled bulk quantities of CFs to be treated [6-8]. Others have grafted CNTs onto woven fiber preforms or felts [9]. To date, four different techniques have been reported to graft or attach CNTs onto fibrous materials, i.e., direct growth of CNTs on fibers by chemical vapor deposition (CVD) [7, 8, 10], electrophoretic deposition of CNTs on fiber surfaces [11], chemical reactions between functionalized CNTs and fibers [12], and spraying or coating of fibers with CNT-containing sizing agents [3, 13]. Direct growth via catalytic CVD has been widely used because it can radially and uniformly introduce CNTs on the CFs. As such, this study is more concerned with direct growth of CNTs on CF surface via CVD. As reinforcement, CNT-grafted CFs have two distinct advantages: improved fracture toughness in the radial direction of the fibers by radially-grown CNTs, and increased interfacial shear strength (IFSS). Kepple et al. [2] reported increased toughness of CNTgrafted CF reinforced composites in the radial direction, which resulted from the bridging effect of CNTs between CF lamellas. Grafting of CNTs onto CFs greatly increases the IFSS 2

[4, 14-20]. The effect of IFSS on the tensile strength of composites has been studied. The strength increases with increasing IFSS, but the optimal IFSS is identified with the maximum tensile strength due to multiple fiber fracture [21-24]. Although CNT-grafted CFs have great potential as reinforcement, there are few reports concerning any improvement in the mechanical properties of composites reinforced by them. This is attributed, in part, to the degradation of CFs incurred during the CVD process [18]. For example, the tensile and flexural strength of CNT-grafted CF reinforced plastics (CFRPs) decreased. Vargas et al. reported an increased in-plane shear strength for CNT-grafted CFRPs [25] but significant positive effects of the grafted CNTs on the fiber-direction properties, such as tensile strength, were not disclosed. Instead, decreased mechanical properties of the CNTgrafted CFRP were reported [9, 11]. In this study, we investigated the tensile strength of CNT-grafted CFRPs. Two types of carbon preforms (unidirectional (UD) and woven fabrics) were used and CNTs were grafted onto CFs in the preforms using a low-temperature CVD with Ni–Fe bimetallic catalysts [7]. This approach was reported as a means to grow CNTs without degrading the CFs. Then, the CNT-grafted CF preforms were infiltrated with epoxy resin and consolidated. The mechanical behavior of the CNT-grafted CFRPs was characterized using fractography, fourpoint bending testing with notched specimens, and in situ microscope observation of surface cracks. Finally, the thermal and electrical properties of the CNT-grafted CF composites were determined.

2. Experimental 2.1 Preparation of the CNT-grafted CF fabrics Catalyst for CNT growth was introduced on the surfaces of CFs in UD (T-300 grade 12K, stitched by glass yarn, ACP composites, 167.2g/m2, thickness = 0.2mm) and woven (T-300 3

grade 3K, plain weave; Entra Korea, 200g/m2, thickness = 0.2mm) preforms. An ethanol solution of 0.5 M FeCl3∙(H2O)6 and 1 M Ni(NO3)2∙(H2O)6 was used as the catalyst precursor solution. The CF preforms were soaked in the catalyst precursor solution for 30 min and dried at 70°C for 4 h to prevent wrinkles and twisting by rapid drying. This soaking method is a simple, cost-effective and mass-scalable process. Note that the Ni–Fe bimetallic catalysts were used to lower the CVD temperature, which is important for maintaining the mechanical properties of CFs [7]. The H2 ratio of the CVD atmosphere was adjusted to prepare CNTgrafted CFs of different CNT lengths and diameters. Total amount of the gas injected into furnace was 800 sccm and the volume fraction of each element was controlled by mass flow controller (MFC). Short and thin CNTs were grown on the CF surface by controlling the H2 content in the carrier gas (Ar) to 10% v/v (these materials are designated as S-CNT-gr-UD and S-CNT-gr-woven), while long and thick CNTs were grown on the CF surface by controlling the H2 content to 25% v/v (these materials are designated as L-CNT-gr-UD and L-CNT-gr-woven). 12.5% v/v of C2H2 gas was used as the carbon source to grow CNTs. The flow rates of the acetylene, hydrogen, and argon gas were 100, 70, and 630 sccm, respectively, for S-CNT-gr-UD and S-CNT-gr-woven samples, and were 100, 100, 175, and 525, sccm respectively, L-CNT-gr-UD and L-CNT-gr-woven samples. Additionally, CNTgrafted CF preforms were prepared using Fe-only catalyst and high-temperature CVD (760°C) to investigate the effect of monometallic and bimetallic catalysts on the mechanical properties of CNT-grafted CFs. The flow rates of the gases were set to be the same as those of the SCNT-gr-UD samples. The CVD process is detailed in Figure 1.

2.2 Fabrication of the CNT-grafted CF composites UD and woven composites were manufactured to examine the effect of CNT-grafted CFs on the mechanical properties. Four layers of CNT-grafted and As-received CF (without desizing 4

process) UD and woven preforms were placed in the mold, and epoxy resin and hardener (Epofix; Struers) mixed with 25:3 weight ratio. After mixing, resin was infiltrated into the preforms by a vacuum-assisted resin transfer molding process (for fast and uniform infiltration, a distribution mesh was placed on the preform). Then, the preforms were pressed to a thickness of 1 mm to control the fiber volume fraction. Finally, the composites were molded at 100 kgf/cm2 and 60°C. The target volume fractions of the composites were 42.5% and 47.4% for the UD and woven composites, respectively.

2.3 Morphological, mechanical and electrical characterization The morphologies of the CFs after the CVD process were observed by field-emission scanning electron microscopy (FE-SEM) (JSM 7600F; JEOL). The wall structure of the CNTs grafted on the CFs was observed by high-resolution transmission electron microscopy (HR-TEM) (3000F; JEOL). The fracture surface after tensile testing was examined using FESEM to identify the failure mechanism of the composites. Additionally, the surface cracks of the composites were observed in situ using an optical microscope during the tensile test, from which the mechanical behavior of the CNT-grafted CF UD and woven composites, in particular crack initiation and propagation, was investigated. Thermogravimetric analysis (TGA) (Mettler Toledo, 851e) was carried out to measure the fiber volume fractions of the CNT-grafted CF UD and woven composites, for which the TGA profiles of the as-received CFs, epoxy, CNT-grafted CF and as-received CF composites were obtained. TGA were performed with 10 oC/min of heating rate under nitrogen atmosphere. The mechanical properties of the CNT-grafted CFs were measured using a single-fiber tensile test using universal tensile machine (UTM, R&B RB307 Chamber) Each single fiber was placed on Tab made by photopaper (Printec, Semi Glossy Photo Paper) and attached by 5

an instant adhesive. The gage length of specimen was 20 mm and the crosshead speed was 2 mm/min (10% of gage length per min). The IFSS of the CNT-grafted CFs was measured using a micro-droplet debonding test and the same epoxy resin used in VARTM. To make a droplet on the CF, first the epoxy compound (containing hardener) was wetted at the tip of a wooden pin and transferred into a droplet on the CF surface. After the curing, the droplet size was measured using an optical microscope (Olympus BX53-P). Finally, the droplet was positioned below a razor zig and the CF was then pulled out using UTM, during which the force was measured. Details and schemes of the test are described in Figure 2. The tensile properties of the CNT-grafted CF composites were measured using a universal testing machine (Instron 5569). The specimen sizes were 150 × 15 × 1 mm3 (gage length = 100 mm) and 100 × 4 × 1 mm3 (gage length = 50 mm) for the woven and UD composites, respectively. Because it was difficult to prepare the specimen while keeping the alignment of the fibers, we could not follow the size of the specimen described in ASTM. Here, the crosshead speed was 1% of the gage length per minute for both composites. Four-point bending tests were carried out on notched UD composite specimens to measure the stress intensity necessary for splitting crack initiation [26]. Details of the specimen, including the notch shape and loading direction, are given in Figure 3. Single-edge cracks were introduced at three different fiber orientations, i.e., fiber, transverse and 45°inclined directions. The edge notch was 0.25 mm and the ratio of the notch depth to the specimen width was 0.4. The four-point bending test was carried out for five loading conditions for each crack orientation. All of the experiments were performed for the three composites (L-CNT-gr-UD, S-CNT-gr-UD, and as-received). The electrical conductivity of the woven CF composites was measured using an electrometer (6517A; Keithley) at several voltages (1, 10 and 100 V). Conductive epoxy (CW2400; Chemtronics) was applied to the polished specimen surfaces. The thermal 6

conductivity of the composites was measured using laser flash analysis (LFA 457 Microflash; Netzsch) using the same specimen used for tensile testing. Electrical and thermal measurements were made in the through-thickness and in-plane directions.

3. Results and discussion 3.1 Morphologies and mechanical properties of the CNT-grafted CFs The morphology of the CNT-grafted CF preforms, prepared by the thermal CVD process using the Ni–Fe bimetallic catalyst, was observed by FE-SEM. Figure 4 shows that the CNTs, in particular multiwalled CNTs, were uniformly grown on the CF surface. Morphology of CNTs were wavy because catalysts were deposited as particle, not film [27]. Diameter of CNTs was measured by ImageJ software from SEM images, for which 50 CNTs were selected for each specimen. Their diameters were varied by adjusting the H2 content of the CVD atmosphere; the average diameters of the CNTs were 22.1±1.7 and 49.2±8.1 nm for low (10%) and high (25%) H2 contents, respectively. The CNT lengths showed a similar trend to their diameters, i.e., as the H2 content increased, so did the length. It was not routine to measure the length of CNTs grown on the CF surface because of their curvature, clustering, and overlapping in TEM images. Therefore, the apparent thickness of single CF with grafted CNTs was measured and was then subtracted by the thickness of single CF without CNTs (i.e., the diameter of CF), defining the thickness of CNT-grated UD and woven fibers. The thickness of S-CNT-gr-UD and woven fibers increased 400 to 500nm while that of L-CNTgr-UD and woven fiber increases more than 1 ㎛. Contrary, CNT number density of the CNT-grafted CF for low H2 contents were higher than that of high H2 contents. After microdroplet debonding test, trails of the CNTs were investigated using SEM and were counted to measure the number of the CNTs grown on the CF surface in unit area (Figure 4 (g) and (h)). The number density of CNTs grown on S-CNT-gr-UD and L-CNT-gr-UD fibers 7

was 554(±24)/㎛ 2 and 336(±16)/㎛ 2, respectively. These results were attributed to the H2, which prevented the conversion of Fe particles to Fe3C, thereby accelerating CNT growth and developing more walls (and thus leading to thicker CNTs) [28], as confirmed by the wall structure of the CNTs (Figure 4 (c) and (f)). The amount of CNTs grafted on the CFs was about 0.3 wt% with respect to the CFs; this was determined by weighing the CF preform before and after the CVD process. Table 1 provides the tensile strengths and IFSSs of the CNT-grafted CFs, which were measured by single-fiber tensile and micro-droplet debonding tests, respectively. The tensile strengths of the CNT-grafted CFs in the UD and woven preforms were similar to those of the as-received CFs, demonstrating that the CNT-grafted CFs in the UD and woven preforms were prepared without degrading the CFs using the bimetallic catalyst. Note that the tensile strength of the as-received CFs in UD and woven preforms is slightly different, which can be explained by different CFs used for the preforms (section 2.1). The IFSSs of the CNT-grafted CFs for both UD and woven composites were much higher than those of the as-received CFs. Interestingly, the effect of CNT diameter (or length) on the IFSS was negligible, short and thin CNTgrafted CFs exhibited similar IFSS to long and thick CNT-grafted CFs. Since the increase in IFSS observed for the CNT-grafted CFs is most likely due to an increase in the interphase yield strength as well as an improvement in interfacial adhesion due to CNTs [29], the two cases showed a similar increase in IFSS regardless of the diameter and length of the CNTs grafted on the CFs.

3.2 Mechanical properties of the CNT-grafted CF reinforced composites The fiber volume fractions of the CNT-grafted CF UD and woven composites were measured. The TGA profiles of the as-received CFs, CNT-grafted CFs, epoxy, and as-received and CNT-grafted CF composites are shown in Figure 5, from which the fiber fractions in the 8

composites were calculated using the weight loss. First, the amount of sizing agent coated on the surface of CFs were measured by TGA. To remove the sizing agent, as-received CF fabric was ultrasonicated in acetone for 2 days. The fabric was further sonicated in acetone for 1 day to ensure that the CF were completely desized. These two desized CF fabrics showed almost the same TGA curve, indicating that the sizing agent was completely removed. Figure 5 (a) discloses that the amount of the sizing agents was 1.7 wt% with respect to total CFs. The epoxy was fully burned-out at 700°C, as shown in Figure 5 (c), while the asreceived and CNT-grafted CFs showed 2.1 and 0.4 wt% losses at 650°C, respectively (Figures 5 (a) and (b)). The CNT-grafted and as-received CF composites showed 47.9 and 48.2 wt% losses at 650°C, respectively (Figure 5(d)). The fiber volume fractions of the UD composites were calculated assuming densities of 1.7 and 1.13 for the CF and epoxy, respectively. Table 2 shows that the CF-reinforced composites in this study were well prepared to have target fiber volume fraction and their mechanical properties can be compared without the effect of the fiber volume fraction. The fiber volume fractions of the woven composites were also obtained using the same method, showing very similar fiber volume fraction of the three kinds of woven composites. The mechanical properties of the composites were measured by tensile testing. Figure 6 shows their representative stress–strain curves. The CNT-grafted CF UD and woven composites had higher tensile strengths than the as-received CF UD and woven composites, while their moduli were similar (see Table 2). The increase in tensile strength was slightly greater for the woven composites. For the woven composites, the long and thick CNT-grafted CF composites exhibited a greater increase in the tensile strength (32%) than the short and thin CNT-grafted CF composite (9%). Notably, the maximum strain dramatically increased from 2.07% for the as-received CF composite to 2.32% for the long and thick CNT-grafted CF composite. On the other hand, the UD composite showed increased tensile strength (19%), 9

but the improvement was not as great as observed in the woven composites. These improvements are due, in part, to increased IFSS, however, only IFSS cannot explain them because short and thin CNT-grafted CFs exhibited similar IFSS to long and thick CNTgrafted CFs as described in section 3.1. Therefore, the mechanism of increased tensile strength of the CNT-grafted CF woven and UD composites was further investigated as follows. The fracture surfaces of the CNT-grafted CF UD and woven composites were examined by FE-SEM to understand the failure mechanism (Figures 7 and 8). The as-received CF UD composite showed that some of CFs pulled out from the matrix (Figure 7 (a) and (b)), while the fracture surface of the CNT-grafted CF UD composite appeared stair-like (Figure 7 (c) and (d)), indicating a higher IFSS [23] (Table 1 and section 3.1). The amount of pulled-out CFs was lower in the CNT-grafted CF composite due to the increased IFSS, which improved load transfer [30]. Thus, the CFs fragmented into shorter fibers, and a higher tensile strength might be observed because of their stochastic properties. Furthermore, a surface of broken CFs was clearly visible on the as-received CF composite (Figure 7 (b)), while the CNTgrafted CF composite showed complex cracks attributed to the CNTs (Figure 7 (d)). It is likely that the CNTs grafted on the CFs inhibited crack propagation through the CF surface and lengthened the crack path. Consequently, the tensile strength and toughness of the CNTgrafted CF composites increased. A similar trend was observed for the woven composites. The as-received woven CF composite showed many pulled-out CFs with a few being multiply fractured (Figure 8 (a)), while the CNT-grafted CF woven composite showed large numbers of multiply fractured CFs (Figure 8 (b)), also indicating that IFSS of CNT-grafted CF woven composites increased. The fracture surface analysis confirms the effect of increased IFSS. Therefore, this analysis does not help explain the difference in tensile

10

strength between short (thin) and long (and thick) CNT grafted CF composites because the IFSS of the two composites were similar. In situ microscope observation for surface cracks of CF woven composites was conducted to investigate the mechanism underlying the increased tensile strength. Figure 9 shows that cracks developed in the interlamellar region for the as-received CF composites and short and thin CNT-grafted CF composite, resulting in clear separation of lamellas. However, in the case of the long and thick CNT-grafted CF composite, transverse cracks in the tow appeared first and then the composite suddenly fractured. This phenomenon was also supported by ex situ observations by FE-SEM (Figure 10). A load on the long and thick CNT-grafted CF woven composites could thus be well-distributed until fracture, while a load on the as-received and short (and thin) CNT-grafted CF composite concentrated the load on a specific area after tow pull-out. As a result, the long and thick CNTs grafted on the surface of the CFs can be thought to enhance the interlamellar shear properties. Cracks can initiate in UD composites along the fiber direction from pre-existing defects. Thus, the fiber splitting has an important influence on the mechanical properties of UD fiber reinforced composites. We investigated the effect of CNTs on the fiber splitting behavior of CF UD composites by determining the stress intensity for splitting crack initiation by following a published procedure [26]. From the results of the experiments with three fiber orientations for three kinds of CFs (five loading conditions for each orientation), the stress intensity factors for modes I and II can be obtained using Equation (1). (1) where a, W and B are the crack length and the width and the thickness of the specimen, respectively. M and F are the bending moment and shearing force at the crack position; these are calculated from the geometry and loading condition. The correction factors FI and FII are obtained from reference [26]. Figure 11 shows the stress intensity factors obtained using 11

Equation (1) for the three samples. From these values, the tensile and shear stress intensity factors (Kfσ and Kf) were obtained and plotted by following the method of [26]. Figure 12 shows the stress intensity required for the initiation of the splitting crack for each sample. The tensile stress intensity factor of the as-received CF UD composites increased from 1.56 to 1.94 for long and thick CNT-grafted CF UD composites, meaning enhanced resistance of the splitting crack initiation. In other words, splitting cracks were delayed or suppressed for the long and thick CNT-grafted CF UD composites due to a specific mechanism. However, the splitting crack resistance of the short and thin CNT-grafted CF UD composites were similar to those of the as-received CF composites, indicating that there was no such mechanism working in those composites. Previous studies manufactured the CNT-grafted CF reinforced composites and discussed about their mechanical properties [32-34]. Li et al. reported about similar increasing trend of the mechanical properties of CNT-grafted CF reinforced composites [31] and Rmanov et al. modeled CNT-grafted CF reinforced composites with various CNT distribution and morphologies [32, 33]. In their research, agglomerated CNTs were found to behave like stiff microscopic particles, resulting in additional stress concentration. Thus, inhomogeneous distribution of CNTs had negative effect on the mechanical properties of the composites. As a result, in their research, there were no significant increases in the tensile properties of composites. Inhomogeneous distribution of CNTs in the current study was observed in Figure 4 (c), (d), and (e), however significant increases in the tensile strength of CF composites were observed due to the bridge effect between CNTs grown on different CFs surface, which was not issued in the previous literature. In summary, as detailed in Table 1, the IFSSs of the two UD composites (S-CNT-gr-CF and L-CNT-gr-CF) are similar, but their tensile strengths in Table 2 are quite different. For the woven composites, the same trend can be observed. Therefore, it can be concluded that 12

IFSS alone cannot explain such large difference. We attributed the difference to a bridging effect of the CNTs considering the delay or suppression of fiber splitting crack in the long and thick CNT-grafted CF UD composites. Short and thin CNTs cannot form bridges between CFs, resulting in little contribution to the fiber splitting resistance. On the other hand, for the woven CF composites, the fiber splitting is mitigated by other orthogonal fibers, e.g., warp fibers for weft fiber splitting and vice versa. Therefore, a CNT bridge effect cannot be directly detected using the same test method as used for the UD composites in this study. Recall that the two (long and thick and short and thin) CNT-grafted CF woven composites with slightly different IFSS showed quite different tensile strengths. Therefore, we believe again that the bridge effect of CNTs in the woven composites resulted in the enhanced interlaminar shear properties, playing an important role in increasing the tensile strength of the long and thick CNT-grafted CF woven composites. 3.3 Electrical and thermal properties of the CNT-grafted CF reinforced composites We also measured the electrical and thermal conductivities of the CF woven composites (Table 3). The CNT-grafted and as-received CF composites had almost the same electrical conductivity in the in-fiber direction, but the electrical conductivity via the thickness of CNTgrafted CF woven composite was 62% higher than that of the as-received CF composite. Similarly, the thermal conductivity of the CNT-grafted CF woven composite was 51% higher than that of the as-received CF composite, but their thermal conductivities in the warp and weft directions were almost the same. These two results imply that there were some factors in the through-thickness direction that improved the electrical and thermal properties. We believe that the CNT bridges and network in the through-thickness direction enabled such increases because the CNTs were directly grown on the CF surfaces in the radial direction (Figure 4 (e)). It can be claimed that the measured electrical and thermal properties indirectly 13

support such a CNT bridge effect, which was discussed for the mechanism of the improved tensile strength of the CNT-grafted CF composites along with increased IFSS in Section 3.2.

14

4. Conclusions CNT-grafted CF UD and woven composites were fabricated using CNT-grafted CFs, which were prepared without degradation of their mechanical properties by low-temperature CVD using bimetallic catalysts. The IFSS increased for both composites due to the CNTs grafted on the CFs. Interestingly, the CNT-grafted CF UD and woven composites exhibited tensile strengths that were increased by 19% and 32%, respectively, over the as-received composites, which was partially attributed to the increased IFSS. Similar IFSSs but different CNT type (e.g., thick and long CNTs) significantly improved the tensile strength, which cannot be explained by the IFSS alone. The splitting crack initiation of the UD composites was investigated by measuring the tensile and shear stress intensity factors. These values revealed that long and thick CNT-grafted CF UD composites increased the resistance of the fiber splitting significantly compared with short and thin CNT-grafted CF UD composites, because long and thick CNTs could form bridges between the CFs. This bridge effect can be observed in long and thick CNT-grafted CF woven composites through enhanced interlamellar properties, increasing their tensile strength significantly. The bridge effect of CNTs were indirectly confirmed by higher measured electrical and thermal conductivities in the throughthickness direction of the CF composites.

Acknowledgement This work was supported by DAPA and ADD, and also by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science, ICT and Future Planning (MSIP) (NRF-2015R1A5A10 37627) and the Ministry of Education Science and Technology (MEST) as a project within the “Space Core Technology Development Program” (NRF2015M1A3A3A020 27377). This work was also supported by the Technology Innovation Program (10083615) funded by the Ministry of Trade, Industry & Energy (MOITE) in Korea. 15

The Institute of Engineering Research at Seoul National University provided research facilities for this work.

Data availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

Reference [1] Y.S. Song, J.R. Youn, Carbon, 43 (2005) 1378-1385. [2] K. Kepple, G. Sanborn, P. Lacasse, K. Gruenberg, W. Ready, Carbon, 46 (2008) 20262033. [3] G. Lee, K.D. Ko, Y.C. Yu, J. Lee, W.-R. Yu, J.H. Youk, Composites Part A: Applied Science and Manufacturing, 69 (2015) 132-138. [4] E.T. Thostenson, W.Z. Li, D.Z. Wang, Z.F. Ren, T.W. Chou, Journal of Applied Physics, 91 (2002) 6034-6037. [5] H. Qian, A. Bismarck, E.S. Greenhalgh, G. Kalinka, M.S. Shaffer, Chemistry of Materials, 20 (2008) 1862-1869. [6] R. Andrews, D. Jacques, A. Rao, F. Derbyshire, D. Qian, X. Fan, E. Dickey, J. Chen, Chemical physics letters, 303 (1999) 467-474. [7] G. Lee, J.H. Youk, J. Lee, I.H. Sul, W.-R. Yu, Diamond and Related Materials, 68 (2016) 118-126. [8] K.J. Kim, J. Kim, W.-R. Yu, J.H. Youk, J. Lee, Carbon, 54 (2013) 258-267. [9] A. Godara, L. Mezzo, F. Luizi, A. Warrier, S.V. Lomov, A. Van Vuure, L. Gorbatikh, P. Moldenaers, I. Verpoest, Carbon, 47 (2009) 2914-2923. [10] G. Lee, K.J. Kim, W.-R. Yu, J.H. Youk, International Journal of Nanotechnology, 10 (2013) 800-810. [11] E. Bekyarova, E. Thostenson, A. Yu, H. Kim, J. Gao, J. Tang, H. Hahn, T.-W. Chou, M. Itkis, R. Haddon, Langmuir, 23 (2007) 3970-3974. [12] X. He, F. Zhang, R. Wang, W. Liu, Carbon, 45 (2007) 2559-2563. [13] H. Zhang, Y. Liu, M. Kuwata, E. Bilotti, T. Peijs, Composites Part A: Applied Science and Manufacturing, 70 (2015) 102-110. [14] F.-H. Zhang, R.-G. Wang, X.-D. He, C. Wang, L.-N. Ren, Journal of materials science, 44 (2009) 3574-3577. [15] H. Qian, A. Bismarck, E.S. Greenhalgh, M.S. Shaffer, Composites Part A: Applied science and manufacturing, 41 (2010) 1107-1114. [16] H. Qian, A. Bismarck, E.S. Greenhalgh, G. Kalinka, M.S. Shaffer, Chem. Mater, 20 (2008) 1862-1869. [17] Q. Peng, X. He, Y. Li, C. Wang, R. Wang, P. Hu, Y. Yan, T. Sritharan, Journal of Materials Chemistry, 22 (2012) 5928-5931. [18] E.T. Thostenson, T.-W. Chou, Journal of physics D: Applied physics, 35 (2002) L77. 16

[19] P. Lv, Y.-y. Feng, P. Zhang, H.-m. Chen, N. Zhao, W. Feng, Carbon, 49 (2011) 46654673. [20] H. Qian, E.S. Greenhalgh, M.S. Shaffer, A. Bismarck, Journal of Materials Chemistry, 20 (2010) 4751-4762. [21] M.G. Bader, in: Science and Engineering of Composite Materials, 1988, pp. 1. [22] M.S. Madhukar, L.T. Drzal, Journal of Composite Materials, 25 (1991) 958-991. [23] H. Hatta, K. Goto, T. Aoki, Composites Science and Technology, 65 (2005) 2550-2562. [24] H. Hatta, M.S. Aly-Hassan, Y. Hatsukade, S. Wakayama, H. Suemasu, N. Kasai, Composites Science and Technology, 65 (2005) 1098-1106. [25] G. Vargas, J.Á. Ramos, J. De Gracia, J. Ibarretxe, F. Mujika, Journal of Reinforced Plastics and Composites, 34 (2015) 1926-1936. [26] K. Tohgo, A.S. Wang, T.-W. Chou, Journal of composite materials, 27 (1993) 10541076. [27] Zhang, Yingying, et al. ACS nano, 3 (2009) 2157-2162. [28] M.J. Behr, E.A. Gaulding, K.A. Mkhoyan, E.S. Aydil, Journal of Applied Physics, 108 (2010) 053303. [29] R.J. Sager, P.J. Klein, D.C. Lagoudas, Q. Zhang, J. Liu, L. Dai, J.W. Baur, Composites Science and Technology, 69 (2009) 898-904. [30] X.F. Zhou, H.D. Wagner, S.R. Nutt, Composites Part A: Applied Science and Manufacturing, 32 (2001) 1543-1551. [31] R. Li, N. Lachman, P. Florin, H. Daniel Wagner, B. L. Wardle, Composites Science and Technology, 117 (2015) 139-145. [32] V. Rmanov, S. V. Lomov, I. Verpoest, L. Gorbatikh, Carbon, 82 (2015) 184-194. [33] M. Mehdikhani, A. Matveeva, M. Ali Aravand, B. L. Wardle, S. V. Lomov, L. Gorbatikh, Composites Science and Technolohy 137 (2016) 24.

17

Table 1. Tensile strength and interfacial shear strength (IFSS) of CNT-grafted CFs in UD and woven preforms. The results are the average of at least 30 specimens and the values in parentheses are the standard deviations. Samples UD preform Woven preform

As-received CF S-CNT-gr-CF L-CNT-gr-CF As-received CF S-CNT-gr-CF L-CNT-gr-CF

Tensile strength (MPa) 4110 (370) 4120 (450) 4070 (320) 4030 (270) 4010 (410) 4080 (350)

Interfacial shear strength (MPa) 30 (2.7) 57 (8.2) 58 (10.8) 37 (2.8) 65 (6.9) 67 (10.2)

Table 2. Mechanical properties of the CF-reinforced composites fabricated in this study. The results are the average of at least eight specimens and the values in parentheses are the standard deviations.

Fiber volume fraction (%) Elastic modulus (GPa) Tensile strength (MPa) Maximum strain (%)

UD composites AsS-CNT- L-CNTreceived grafted grafted 42.8 42.4 42.5 (0.4) (0.5) (0.4)

Woven Composites AsS-CNT- L-CNTreceived grafted grafted 46.7 46.9 46.8 (0.4) (0.7) (0.7)

101.4 (1.98)

103.2 (2.42)

107.3 (1.79)

36.6 (0.71)

37.1 (0.82)

38.2 (0.88)

1232 (103.6)

1274 (81.63)

1470 (97.5)

473.2 (20.5)

531.1 (52.2)

627.3 (44.3)

1.44 (0.02)

1.5 (0.02)

1.66 (0.03)

2.07 (0.05)

2.12 (0.02)

2.32 (0.04)

18

Table 3. Electrical and thermal conductivities of CF woven composites. The results are the average of at least four specimens and the values in parentheses are the standard deviations.

Electrical conductivity (S/m) Thermal conductivity (W/mK)

In-plane Through-thethickness In-plane Through-thethickness

As-received CF composites 138.2 (7.2)

L-CNT grafted CF composites 148.8 (9.1)

23.7 (3.1)

38.4 (2.7)

2.72 (0.13)

2.78 (0.16)

0.607 (0.06)

0.945 (0.09)

19

Figure 1. Schematic diagram of the chemical vapor deposition (CVD) procedure used to fabricate the carbon nanotube (CNT)-grafted carbon fiber (CF) preform. (a) Bimetallic catalyst precursors used for the CVD. (b) Catalyst precursor solution and woven or UD carbon preform soaked in the solution. (c) Thermal CVD furnace and (d) temperature profile.

20

Figure 2. Schematic diagram of the microdroplet test for the carbon fibers.

21

Figure 3. Four-point bending test of single-edge-cracked specimens used to measure the stress intensity for splitting crack initiation. (a) Crack orientation with respect to fiber orientation and (b)– (f) various crack positions and loading conditions.

22

(a)

(b)

23

(c)

(d)

24

(e)

(f)

(g)

25

(h) 84 Figure 4. Morphologies of CNT-grafted CF preforms and CNT-grafted CFs. (a) Low and (b) high resolution scanning electron microscopy (SEM) images of CNT-grafted CF woven preform after the CVD process at low H2 content (10%) in the carrier gas. (c) Wall structure of CNTs grafted on a CF. (d) Low and (e) high resolution SEM images of the CNT-grafted CF woven preform after the CVD process at high H2 content (25%) in the carrier gas. (f) Wall structure of CNTs grafted on a CF. Trails of CNTs on (g) S-CNT-gr-UD fiber surface and (h) L-CNT-gr-UD fiber surface

26

27

(d) Figure 5. Thermogravimetric analyses of (a) CFs, (b) CNT-grafted CFs, (c) epoxy resin and (d) CF-reinforced composites.

28

Figure 6. Representative stress–strain curve for CNT-grafted CF (a) woven and (b) unidirectional (UD) composites.

29

(a)

(b)

30

(c)

(d) Figure 7. Fracture surface of CF UD composites after tensile testing. (a) and (b): As-received CF composites observed in the fiber direction and in the transverse fiber direction. (c) and (d): Long and thick-CNT-grafted CF composites in the fiber direction and in the transverse fiber direction.

31

(a)

(b) Figure 8. Fracture surface of CF woven composites after tensile testing. (a) As-received CF composite and (b) Long and thick CNT-grafted CF composite.

32

(a)

33

(b) Figure 9. Stress–strain curve and in situ microscope observation of the surface cracks of CF woven composites during tensile testing. (a) As-received CFs and long and thick CNTgrafted CF composites; cracks are highlighted in red. (b) Short and thin CNT-grafted CF composites.

34

(a)

(b) Figure 10. Morphologies of the CF woven composites after in situ microscope observation for surface cracks. (a) As-received and (b) long and thick CNT-grafted CF composites

35

Figure 11. Stress intensity factor (modes I and II) obtained from the four-point bending tests.

Figure 12. Tensile and shear stress intensity factors representing conditions for splitting crack initiation of CF UD composites.

36