carbon fiber reinforced polymer composite for the lightning strike protection application

Composites Part B 180 (2020) 107563

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Composites Part B journal homepage: www.elsevier.com/locate/compositesb

Fabrication of the silver modified carbon nanotube film/carbon fiber reinforced polymer composite for the lightning strike protection application Qianshan Xia a, Hao Mei a, Zhichun Zhang a, Yaxin Liu a, Yanju Liu b, Jinsong Leng a, * a

National Key Laboratory of Science and Technology on Advanced Composites in Special Environments, No. 2 YiKuang Street, Science Park of Harbin Institute of Technology (HIT), Harbin, 150080, PR China b Department of Aerospace Science and Mechanics, No. 92 West DaZhi Street, Harbin Institute of Technology (HIT), Harbin, 150001, PR China

A R T I C L E I N F O

A B S T R A C T

Keywords: Carbon nanotube film Carbon fiber Polymer-matrix composites Lightning strike protection

Carbon fiber reinforced polymer (CFRP) composites with low density, corrosion resistance and excellent me­ chanical properties are widely applied in the aircraft industry, instead of the traditional metallic material. Their poor electrical conductivity leads to the vulnerability to the lightning strike (LS). In this paper, highly conductive silver modified carbon nanotube film (SMCNF) was developed via the electrophoretic deposition (EPD) method for protecting CFRP structures and components of the aircraft. Lightning strike protection (LSP) efficiency of the SMCNF/CFRP composite was characterized from the micro-scale to the macro-scale, and its possible protective mechanism was discussed. The compressive strength of the SMCNF/CFRP composite after the simulated LS test maintains 91.05% and is higher than that of the Cu mesh/CFRP composite (83.28%). Compared to commercial copper mesh LSP material, it can reduce the weight by 27.4% with the better residual compressive strength ratio. Therefore, highly conductive and lightweight SMCNF as a LSP layer can effectively reduce the damage of the CFRP matrix caused by the LS.

1. Introduction Due to low density, corrosion resistance, high specific stiffness and strength, carbon fiber reinforced polymer (CFRP) composites have been widely applied in the aircraft industry. Compared with traditional metallic materials, utilization of CFRP composite leads to the fuel con­ sumption decreasing and the fatigue resistance characteristic increasing. For instance, the CFRP utilization ratio of Boeing 787 structures and components has been up to 50% by weight [1]. However, CFRP com­ posite instead of metal brings a fatal flaw that is serious lightning strike (LS) damaging, owing to its poorer conductivity than the metallic ma­ terials [2]. Moreover, an airplane may undergo lightning strike more than once per year [3]. The LS with the electric current around 40–100 kA in microseconds causes the catastrophic failure to the aircraft systems [4], including vaporization and burning of the resin at lightning attached points [2], electromagnetic interference [5], and igniting fuel vapors [6]. With the increase of air routes, accidents caused by the LS occur frequently [7,8]. Therefore, lightning strike protection (LSP) of aircraft that contains CFRP composite structures and components is very important. The objective of the LSP is to form conductive paths on exterior

structures of the aircraft, to cause the lightning charges conducting quickly and reducing the damage to the plane. To improve the con­ ductivity of the CFRP, there are some LSP layers introduced, such as metal meshes [9], metal foils [10,11], metal nano-materials [12,13] and various composites [14]. At present, commercial LSP products are metallic mesh or foil prepared by copper or aluminum, which has enough conductivity to dissipate the LS energy. Owing to high density, utilization of metallic LSP material will increase the total structural weight of aircraft, which cannot satisfy the requirement for fuel saving. Moreover, metallic character and poor resin compatibility will cause corrosion and exfoliation of LSP systems in long-term application and vibration environment [11]. Meanwhile, metal mesh needs the isolation layer that is adhered on the composite surface, to prevent the electro­ chemical corrosion [11,15]. To overcome above drawbacks of metallic protection system, re­ searchers have paid attention to some lightweight and conductive fillers as potential alternatives to metallic LSP materials [16], such as nickel particles [5], carbon nanofibers [17], carbon nanotubes (CNT) [18], graphenes [19] and boron nitride nanotubes [18]. For instance, Zhang prepared aluminum/copper (Al/Cu) meshes modified by graphene, carbon fiber and indium tin oxide (ITO) as an alternative material for

* Corresponding author. E-mail address: [email protected] (J. Leng). https://doi.org/10.1016/j.compositesb.2019.107563 Received 25 June 2019; Received in revised form 18 October 2019; Accepted 29 October 2019 Available online 1 November 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. The photo of the EPD set-up and the schematic diagram of the EPD process.

two short rectangles rectangles (440 � 40 � 3 mm3), (100 � 55 � 3 mm3) and one square (443 � 443 � 3 mm3). Four long rectangles as walls and one square as the bottom were bonded into an EPD bath by moderate dichloromethane (Tianjin Fuyu Fine Chemical Co., Ltd.). Two short rectangle pieces were adhered on two walls of the EPD bath in the opposite direction for placing graphite electrode. The graphite plate (350 � 250 � 5 mm3) as an anode after cleaning was dried in a vacuum oven. The as-prepared CNF as a cathode was adhered on the polyethylene film with the same size and placed on the bottom of the EPD bath. Anode and cathode kept parallel, and their distance was about 15 mm. Finally, set-up and adjustable power supply were connected by wires and connectors, as shown in Fig. 1a).

LSP through galvanic corrosion method, to improve the conductivity of the composite [17]. When the ITO content reached 40 wt%, the con­ ductivity of the composite was 1366 S/cm. Wang added reduced gra­ phene oxides (RGO) on the CFRP composite surface for LSP through conductive protection [19]. The effect of the RGO content on its con­ ductivity has been analyzed. When the RGO content was 25.15%, the conductivity of the as-prepared composite reached 440 S/cm. However, some serious defects limit the application of the conductive nano-filler/CFRP composite. On the one hand, conductive nano-fillers are difficult to maintain good dispersion in the resin phase of the CFRP prepreg during the curing process. On the other hand, adding high content of conductive fillers into CFRP composites will cause their me­ chanical property reducing significantly. Carbon nanotube film (CNF) is a non-woven film of CNTs. CNFs possess superior properties of CNTs [20,21] and are mainly prepared by vacuum filtration [22] or chemical vapor deposition (CVD) [23] methods. Due to the excellent electrical conductivity, CNFs can be applied in widespread fields, including deicing [24], electrodes of bat­ tery and super capacitor [25–27], infrared stealth [28] and electro­ magnetic shielding [29]. Additionally, the density of the CNF is less than 0.83 g/cm3 [18] and is much lower than copper (8.9 g/cm3) and aluminum (2.7 g/cm3). Moreover, CNF possesses better interface compatibility with various resins compared to the metallic mesh, during the manufacturing process of CFRP composite. Therefore, the CNF as a promising alternative to metallic mesh can be applied in the LSP application. In this study, the silver modified carbon nanotube film (SMCNF) was fabricated via electrophoretic deposition (EPD) technology, to obtain excellent electrical conductivity. Then, it was directly integrated with CFRP prepregs through vacuum hot pressing to form SMCNF/CFRP composite for LSP, without adding any surface protection adhesive [30]. LSP efficiency of the SMCNF/CFRP composite was studied via simulated lightning strike tests. According to experimental results and analysis, the possible LSP mechanism of SMCNF was discussed in this work.

2.2.2. Preparation of the EPD solution 55 g of silver nitrate (AgNO3, Shanghai Shiyi Chemicals Reagent Co., Ltd.) and 2.5 g of magnesium nitrate (Mg(NO3)2, Damao Chemical Re­ agent Co., Ltd.) were dissolved in 500 and 4500 mL of DI water, respectively. Then AgNO3 solution was drop-wise added into Mg(NO3)2 solution with mechanical stirring 2 h. The mixing solution without any precipitate was transferred into the EPD bath [31]. 2.2.3. EPD of silver nanoparticles To improve the conductivity of the CNF, a small quantity of silver nanoparticles were only deposited on one side of CNF via the direct current (DC) electrophoretic deposition technique. In this research, we controlled the deposition time and applied DC voltages to acquire SMCNFs with different conductivity values. Therefore, the variation of the deposition time ranged from 60 to 360 s and the applied DC voltage ranged from 5 to 25 V. Finally, the as-prepared SMCNF was dried at 60 � C in an air-circulating oven for 10 h and peeled from the poly­ ethylene film. The size of the as-prepared SMCNF was 370 � 370 mm2. The schematic diagram of the EPD process is shown in Fig. 1b). 2.3. Preparation of CFRP, Cu/CFRP, CNF/CFRP and SMCNF/CFRP panels

2. Experimental

CFRP, copper mesh/carbon fiber reinforced polymer (Cu mesh/ CFRP), CNF/CFRP and SMCNF/CFRP panels were manufactured by the vacuum hot pressing technique. The CFRP prepreg (T300 125 g) used in this work was provided by the Weihai Guangwei Composites Co., Ltd., and its volume fraction of carbon fibers was 70 vol%. 32 layers of CFRP prepregs were paved together to form a CFRP laminate and the stacking sequence was [0� /90� ]16S. All the sizes of CFRP laminates were 370 � 370 � 4 mm3. The CFRP composite panel was integrally vac­ uumed in an autoclave under 0.4 MPa of the chamber pressure, during the curing process. Cu mesh (Dalian Yibang Science and Technology Co., Ltd, 73 g/m2), CNF and SMCNF were paved on the outmost layers of three CFRP laminates, respectively. Subsequently, Cu mesh/CFRP, CNF/ CFRP and SMCNF/CFRP composites were also integrally vacuumed in an autoclave under the same chamber pressure as the CFRP composite. Structural representation of SMCNF/CFRP composite, schematic profile

2.1. Preparation of the CNF The CNF was prepared by single-wall carbon nanotubes (SWCNTs, TNSR, Chengdu Organic Chemicals Co., Ltd.), and its areal density was 4 mg/cm2 according to the previous report [29]. The size of the as-prepared CNF was 370 � 370 � 0.05 mm3. To improve electrical conductivity and porosity, the as-prepared CNF was heated at 350 � C in an air-circulating oven for removing the residual surfactant (Triton X-100, Aladdin Chemical Reagent Co., Ltd.). 2.2. Preparation of the SMCNF 2.2.1. Fabrication of set-up A commercial polymethyl methacrylate sheet was cut into four long 2

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of preform fabrication, and temperature-time and pressure-time curves used in the curing process are shown in Fig. S1 a–c, respectively.

Table 1 Waveform data of D, B and C* waveforms.

2.4. Measurements Morphologies of specimens were observed by two scanning electron microscopes (SEM, TESCAN, VEGA3 and FE-SEM, Quanta 200F). Elec­ trical resistivity of the SMCNF was recorded by a four point probe re­ sistivity measurement system (Napson Resistivity Measurement System, RG-7C). The conductivity was obtained by calculating the electrical resistivity of nine measurement points of the sample for acquiring ac­ curate values. Measurement points locating on the sample are shown in Fig. S2. Silver particles of the SMCNF were characterized by an X-ray diffraction device (XRD, Panalytical-X’PERT) with Cu kα1 radiation in the 2θ range from 10 to 90� . Defects of CNTs of CNF and SMCNF were analyzed by the Raman spectrum system (Renishaw, InVia-Reflex) with a 532 nm of laser wavelength. Porous structures of specimens were characterized by a volumetric adsorption analyzer (Micromeritic, TriS­ tarII 3020) and calculated by the Barrett-Emmett-Teller (BET) method. The wettability of epoxy resin on the SMCNF surface was characterized through a contact angle measurement system (OCA20, GFR) at room temperature. The silver content of the SMCNF was measured by a Mettler Toledo TGA/DSC1 in air atmosphere. Photos of specimens were obtained by a digital camera (Sony, ILCE-6000L). Overhead view and profile view of specimens were observed by two types of microscopes (KEYENCE, VH-Z500R and OLYMPUS SZX2-FOF). For analyzing inter­ nal damages of samples after simulated LS tests, B-scan and C-scan im­ ages were recorded by non-destructive inspection devices. Damage areas of specimens were performed by a C-scan inspection system (Tecnatom, Composite Ⅸ). The damage depth of the specimen was characterized by a B-scan inspection device (Omni Scan MX). Atomic concentrations of CNF and SMCNF before and after the LS were analyzed by an X-ray photoelectron spectroscopy system (XPS, ThermoFishe-ESCALAB 250Xi). Temperature field variations of non-LS sides of different com­ posite panels were observed with an infrared camera (FLIR, FLIR A655sc) during LS tests. After LS tests, compressive strengths of speci­ mens were tested by an AG-1 Material Testing Machine with a crosshead speed of 1 mm/min to further evaluate their damage levels, according to American Society for Testing and Materials D7137 (ASTM D7137). The equipment of the compressive strength test is shown in Fig. S3. The compressive test was performed under displacement control, and sam­ ples were trimmed to 150 � 100 mm2 from corresponding composite panels.

Parameter

D waveform

B waveform

C* waveform

Ipeak/avg (kA) AI (A2s) Q (C)

100 0.25 � 106 ––

2 –– 10

0.4 –– 21

about 50 mm, and a fine copper wire was adhered on the top of the probe to connect the specimen. Parameters of component D, B and C* current waveforms are listed in Table 1 and current-time curves of D, B and C* waveforms are displayed in Fig. S5 a–c, respectively. 3. Results and discussion 3.1. The morphologies of the SMCNFs The effect of the deposition time on morphology of the SMCNF was analyzed by SEM observation. Surface morphologies of EPD sides of SMCNFs prepared through EPD applying a 5 V DC voltage for 0 s, 60 s, 120 s, 180 s, 240 s, 300 s and 360 s are shown in Fig. 2a–g, respectively. Compared with Fig. 2a–g, it can find that contents and sizes of silver (Ag) particles increase with the increasing of the deposition time, and their sizes grow from 200 nm to 2 μm. As shown in Fig. 2a–e, particle distri­ butions of different SMCNFs are uniform without obvious aggregation, and shapes of silver particles are spherical. At the initial stage of the EPD, silver particles grow at a low speed. When the deposition time exceeds 300 s, particle sizes increase at a high speed with obvious ag­ gregation. Silver ions gather along CNTs and grow asymmetrically, so that some silver flakes appear. As another important factor, applied voltage can affect morphology of the SMCNF surface. Silver particle morphologies of EPD sides of SMCNFs prepared through EPD applying 5 V, 10 V, 15 V, 20 V and 25 V DC voltages for 240s are displayed in Fig. 3a–e, respectively. Fig. 3a–e display that silver particle contents and sizes on SMCNF surfaces in­ crease with the increasing of the applied voltage. Silver particles on SMCNF surfaces aggregate seriously, and their shapes are different and complex. When the applied voltage is lower than 15 V, silver particle shapes convert from sphere to cube with the increasing of the voltage, owing to silver ions in the electrophoresis liquid migrating slowly. Acicular silver particles appear on the SMCNF surface which is prepared under an applied 15 V. After the applied voltage exceeding 15 V, some dendritic silver particles appear and SMCNF surfaces are uneven and porous. High applied voltage causes charged ions rapidly migrating in the electrophoresis liquid. Many silver ions quickly gather and grow along CNT templates, so that silver particles with asymmetrical shapes form and randomly stack on the SMCNF surface.

2.5. The simulated LS test The simulated LS generator was developed by Avic Hefei Hangtai Electrophysics Co., Ltd. According to the Society of Automotive Engi­ neers (SAE) Aerospace Recommended Practice 5412 (ARP 5412), CFRP, Cu mesh/CFRP, CNF/CFRP and SMCNF/CFRP composite panels were directly subjected to the D, B, and C* current combined waveform of Zone 2A, which contains a peak current of 100 kA. Action integral (AI) and electrical charge (Q) of the simulated LS process represent specific energy and total energy of the impulse current, respectively. The two parameters can be written as following equations: R Q ¼ idt (1) R 2 AI ¼ i dt (2)

3.2. The fundamental physical quantities and conductivities of the SMCNFs Light weight is a crucial factor for the aeronautical material. For acquiring the lightweight SMCNF, thicknesses and areal densities of SMCNFs prepared by different deposition time and applied voltages were studied. Fig. 4a displays thicknesses and areal densities of SMCNFs prepared through EPD applying a 5 V DC voltage for 0 s, 60 s, 120 s, 180 s, 240 s, 300 s and 360 s. Thicknesses and areal densities of SMCNFs increase slowly, when the deposition time is less than 240 s. However, thicknesses and areal densities of SMCNFs increase quickly, when the deposition time is more than 240 s. The reason is that silver particle contents and sizes increase with the deposition time increasing, which causes thicknesses and areal densities of SMCNFs increasing. At the initial stage of the EPD, sizes of silver particles attached on SMCNF surfaces are less than 1 μm, meanwhile, silver deposition contents in­ crease and the particles do not aggregate. Thicknesses and areal den­ sities of SMCNFs increase slightly. When the deposition time exceeds

where i is the time varying electrical current of simulated LS waveforms. Experimental setup and conditions before the simulated LS test and the composite panel during the LS test are shown in Figs. S4a and b, respectively. The specimen was fixed on a metal frame by some clamps, and the frame was connected to ground with two aluminum tapes for conducting the LS current from the specimen to the ground. The distance between spherical discharge probe and test side of the specimen was 3

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Fig. 2. SEM images of SMCNFs prepared under an applied voltage of 5 V for a) 0 s, b) 60 s, c) 120 s, d) 180 s, e) 240 s, f) 300 s and g) 360 s.

240 s, silver particle sizes grow rapidly, which are greater than 1 μm, and most of the particles aggregates seriously. Thicknesses and areal densities of the SMCNFs increase obviously. Fig. 4b is line charts of thicknesses and areal densities of SMCNFs with applied voltages. Thicknesses and areal densities of SMCNFs increase with the increasing of the applied voltage, resulting from silver particle contents and sizes

increasing. With the increasing of the applied voltage, silver particle shapes convert from sphere to branch, and the particles aggregate seri­ ously. It leads to the result that the SMCNF surface is porous. Thick­ nesses and areal densities of SMCNFs obviously increase with the increasing of the applied voltage. The conductivity of the SMCNF plays a role factor for its LSP 4

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Fig. 3. SEM images of SMCNFs prepared under applied voltages of a) 5 V, b) 10 V, c) 15 V, d) 20 V and e) 25 V for 240 s.

Fig. 4. Thicknesses and areal densities of SMCNFs prepared a) under an applied voltage of 5 V, for 60–360 s and b) under applied voltages of 5–25 V for 240 s.

property. The conductivity of the SMCNF is controlled by deposition time and applied voltage, to acquire the SMCNF with the applicable conductivity. Fig. 5a shows line charts of resistivity and conductivities of

SMCNFs with the EDP time. With deposition time increasing, conduc­ tivities of SMCNFs increase rapidly first. When the deposition time reaches 240 s, the conductivity of the SMCNF is 5091.65 S/cm, which is 5

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Fig. 5. Resistivity and conductivities of SMCNFs prepared a) under an applied voltage of 5 V for 60–360 s and b) under applied voltages of 5–25 V for 240 s.

Fig. 6. a) XRD patterns and b) Raman spectra of CNF and SMCNF.

about 9.7 times higher than CNF (525.11 S/cm). When the deposition time exceeds 240 s, conductivities of SMCNFs tends to level off at a constant value (about 5000 S/cm). It indicates that the long deposition

time cannot affect the conductivity of the SMCNF obviously. Resistivity and conductivities of SMCNFs prepared through EPD applying 5 V, 10 V, 15 V, 20 V and 25 V DC voltages for 240 s are shown in Fig. 5b. It can be

Fig. 7. a) Pore size distribution curves and b) adsorption and desorption isotherms of CNF and SMCNF, contact angles of epoxy droplets on c) non-EPD and d) EPD sides of the SMCNF. 6

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found that conductivities of SMCNFs level off at a constant value, when the applied voltage is lower than 15 V. Applying higher voltage, con­ ductivity values of SMCNFs decrease. It illustrates that the applied voltage has a negative influence on the conductivity of the SMCNF after silver particles over a certain amount. During the EPD process, silver particle sizes and contents on the SMCNF surface rapidly increase with the increasing of the voltage. The particles aggregate seriously and the SMNCF surface displays the porous structure. The contact area of silver particles reducing leads to their contact resistivity increasing, so that silver particles cannot provide the enhancement of the conductivity of the SMCNF. Based on above comparison and analysis, the optimal SMCNF is prepared under an applied voltage of 5 V for 240 s and it is studied by following characterizations for the LSP application. 3.3. XRD and Raman spectrum analysis of the SMCNF For identifying elemental silver of the SMCNF, XRD patterns of CNF and SMCNF are shown in Fig. 6a. Three characteristic peaks of the CNT of the CNF appear at 26.7� , 41.8� and 77.7� (red heart signs) and correspond to (002), (100) and (110) facets of the graphitic structure, respectively. Besides three diffraction peaks of CNT, the SMCNF con­ tains another five diffraction peaks, which appear at 38.3� , 44.6� , 64.6� , 77.5� and 82� and correspond to the (111), (200), (220), (311) and (222) facets (spade signs) of the standard spectrum of elemental silver (PDF#65–2871), respectively. It indicates that silver particles of the SMCNF are elemental silver. For analyzing the defect degree of CNTs generated by the EPD, Raman spectra of CNF and SMCNF are displayed in Fig. 6b. Wavenumbers of D-band and G-band peaks of CNF and SMCNF appear at 1345 cm 1, 1597 cm 1, 1345 cm 1 and 1598 cm 1, respectively. ID/IG values of CNF and SMCNF are 0.023 and 0.024, respectively. The tiny difference of ID/IG values means that the EPD process causes few defects.

Fig. 8. TGA curves of CNF and SMCNF.

weight percent of the CNF is 7.07% and its residues are oxides of residual catalysts in CNTs. While, the residual weight percent of the SMCNF is 31.28% and its remnants contain oxides of residual catalysts and elemental silver. The reason is that elemental silver is oxidized slowly and the product is silver oxide (Ag2O), as shown in Eq. (3). The decomposition temperature of the Ag2O is 250 � C, as shown in Eq. (4). Ag2O will rapidly decompose, when the temperature reaches 300 � C. According to two TGA curves, the Ag content of the SMCNF is only about 25.22%.

3.4. Pore structure and wettability of the SMCNF Some pores of CNF layer are filled by silver particles after the EPD. It will affect the ability of the epoxy resin to spread on the SMCNF surface. Thus, it is necessary to investigate pore features of the SMCNF after the EPD. Pore diameter distribution curves of CNF and SMCNF are shown in Fig. 7a. Maximum pore volumes of CNF and SMCNF appear at 14.5 nm and 31.3 nm, respectively. Ranges of pore diameter distributions of CNF and SMCNF are 3.8–45 nm and 1.8–52.2 nm, respectively. According to the International Union of Pure and Applied Chemistry’s (IUPAC) clas­ sification [32], both CNF and SMCNF are mesoporous materials. The mesoporous structure of CNF contains interspaces of CNT bundles, and that of SMCNF is formed by CNT bundles randomly overlapping and silver particles depositing. According to the IUPAC’s classification, both adsorption and desorption isotherms of CNF and SMCNF are type IV isotherms with H4 hysteresis loops as displayed in Fig. 7b. It also con­ firms that both the two films can be regarded as mesoporous materials and their pore shapes are slit-shape. The wettability of epoxy resin on the SMCNF surface is further investigated through contact angles of epoxy droplets on two sides of the SMCNF. Fig. 7c and d show contact angles of epoxy droplets on non-EPD and EPD sides of the SMCNF at room temperature are 15.9� and 25.3� , respectively. It indicates that the epoxy resin can spread on both two surfaces of the SMCNF. In addition, the viscosity of the epoxy resin will reduce as the temperature increases. According to the above analysis, epoxy resin of the CFRP prepreg can infiltrate into the SMCNF to form the stable composite.

4Ag þ O2 ¼ 2Ag2 O

(3)

2Ag2 O ≜ 4Ag þ O2 ↑

(4)

3.6. The morphology of the SMCNF/CFRP composite Morphologies of CFRP, Cu mesh/CFRP, CNF/CFRP and SMCNF/ CFRP panels fabricated via the vacuum hot pressing method are char­ acterized for predicting properties of LSP layers. Fig. 9a–d are photos of CFRP, Cu mesh/CFRP, CNF/CFRP and SMCNF/CFRP panels, respec­ tively. It can be found that sizes of all the composite panels are 360 � 360 mm2. Owing to better resin infiltration, surfaces of four types of panels are smooth, and it indicates that LSP layers are infiltrated by the epoxy resin, during the vacuum hot pressing process. In addition, the epoxy resin permeates through pores of the SMCNF, and some silver particles are taken away from the SMCNF surface to the resin layer of the composite surface. The surface color of the SMCNF/CFRP composite displays non-uniform. The surface roughness of the above four com­ posites is displayed with 3D images, as shown in Fig. S6. For further analyzing microstructures, surfaces of composite panels were observed by an optical microscope. Microscope images of four types of panels are displayed in Fig. 9e–h, respectively. In Fig. 9 e, well-organized mono­ filaments of carbon fibers are infiltrated fully with the epoxy resin, to form a compact CFRP composite. The diameter of a carbon fiber monofilament is about 10 μm. The golden mesh covered by the resin is the Cu mesh and its long pitch is about 2.5 mm, as shown in Fig. 9 f. The CNT network is not clear in Fig. 9 g, and it indicates that the CNF is infiltrated fully with the resin during the vacuum hot pressing process. Light points of the SMCNF/CFRP composite surface are silver particles in Fig. 9 h. The reason is some silver particles are taken away from the SMCNF by flowing resin. It proves that the SMCNF is infiltrated fully by the epoxy resin. Section morphologies of above four kinds of composite panels are displayed in Fig. 9i–l, respectively. It can be found that the layout of the carbon fiber is orthogonal in Fig. 9 i. Fig. 9 j shows that the diameter of a single copper wire is about 60 μm and the top surface of

3.5. The thermogravimetry analysis of the SMCNF Silver (Ag) content can affect the areal density of the SMCNF. Compared with thermogravimetry analysis (TGA) curves of CNF and SMCNF, the Ag content of the SMCNF can be confirmed. TGA curves of CNF and SMCNF in air atmosphere are shown in Fig. 8. The residual 7

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Fig. 9. Photos of a) neat CFRP panel, b) Cu mesh/CFRP panel, c) CNF/CFRP panel and d) SMCNF/CFRP panel; e)-h) surface morphologies of different panels corresponding to a)-d); i)-l) microstructures of cross sections corresponding to a)-d).

the copper mesh is covered by little resin. However, the Cu mesh/CFRP composite surface is not smooth. In Fig. 9 k, the CNF is embedded compactly into the CFRP matrix, which is covered by the resin. The thickness of the CNF is about 50 μm after vacuum hot pressing. The thickness of the SMCNF in the composite panel is about 70 μm and the SMCNF is closely bonded to the CFRP matrix in Fig. 9 l. There are some epoxy resin on the composite surface and the surface of the composite is smooth. It proves that the SMCNF can be fully infiltrated by the epoxy resin and form the SMCNF/CFRP composite with the compacted struc­ ture. The compacted structure is benefit to enhancement of LSP property of the protective layer, owing to impact resistance increasing.

damage zone around the LS attachment point, several small zones display resin exfoliating and carbon fibers cracking. The visual damage depth of the Cu mesh/CFRP panel is lower than that of the neat CFRP panel. Fig. 10 c and g are photo and microscopic image of the CNF/CFRP panel after the LS, respectively. Part of the CNF is lost and its damage edge is curly. Cracking carbon fibers are loose. The visual damage area of the CNF/CFRP composite panel that is about 125 � 145 mm2, while its visual damage depth reduces, compared with the CFRP panel. The reason is that part of the LS energy is conducted by the CNF. It indicates that the CNF as a protective layer can reduce the LS damage of the CFRP. Photo and microscopic image of the SMCNF/CFRP panel after the LS are displayed in Fig. 10 d and h, respectively. The damage area of the SMCNF is about 150 � 180 mm2 and its damage edges are fragmentized. Few carbon fibers crack in the central damage zone and the surface glossiness of the SMCNF disappears. Compared with Fig. 10a - c), the damage area of the SMCNF is the largest, while its damage depth obviously decreases. The reason is that re-solidified silver particles fix the CNF on the CFRP matrix surface and increase the time for consuming the LS energy. Fig. 10 i - l and m - p are C-scan and B-scan images of damage areas of the above four kinds of composite panels, respectively. In the C-scan image, the red zone stands for the undamaged zone and the color changing from red to black means that the damage depth increases. The zone of the B-scan inspection is displayed by the green square of the Cscan image. Fig. 10 i and m show the damage area of the neat CFRP composite panel is 113 � 104 mm2 and its deepest damage depth that locates in the center of the damage area is about 2.82 mm. Owing to poor electrical conductivity, the huge Joule heat generated by the LS current is conducted away from the LS zone slowly along the composite surface and causes lots of carbon fibers cracking. It leads the LS damage area of

3.7. The morphology of the SMCNF/CFRP composite after the LS test The strong electromagnetic field that is generated by the huge lightning current will ionize the air. Pressure around the LS zone will increase to 10 atm and causes carbon fiber cracking, resin exfoliating and other damage. Based on Joule’s law, the lightning current will generate the huge Joule heat at the lightning attachment point, and then the surface temperature of the composite increases to 3000–30000 � C and the CFRP composite decomposes seriously [33,34]. Figs. 10 a-d and e-h are photos and micrographs of above four composite panels after simulated LS tests, respectively. As shown in Fig. 10 a and e, LS damage of the CFRP panel without protective layer is serious, which contains a deeper damage depth and about 100 � 150 mm2 of damage area by the visual inspection. Fig. 10 b and f are morphologies of the Cu mesh/CFRP panel after the LS test. Some epoxy resin on the composite surface is carbonized and the color of the carbonization zone turns black. The visual damage area of the Cu mesh/CFRP panel is about 150 � 160 mm2 and it is much larger than that of the neat CFRP panel. Besides a circular 8

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Fig. 10. Photos of a) neat CFRP panel, b) Cu mesh/CFRP panel, c) CNF/CFRP panel and d) SMCNF/CFRP panel after LS tests; micrographs of e) neat CFRP panel, f) Cu mesh/CFRP panel, g) CNF/CFRP panel and h) SMCNF/CFRP panel after LS tests; C-scan images of i) neat CFRP panel, j) Cu mesh/CFRP panel, k) CNF/CFRP panel and l) SMCNF/CFRP panel after LS tests; B-scan images of m) neat CFRP panel, n) Cu mesh/CFRP panel, o) CNF/CFRP panel and p) SMCNF/CFRP panel after LS tests.

the neat CFRP panel is small and its damage depth is deep. Compared with Fig. 10i and j, the damage area of the Cu mesh/CFRP panel is about 128 � 168 mm2 and it is larger than that of the neat CFRP panel. Dis­ tribution of other damage zones displays scattered, besides the circle damage in the center of the composite panel. Its LS damages include Cu mesh losing, epoxy resin carbonizing and carbon fiber cracking. Many carbon fibers crack in the outer layer of the panel and a few carbon fibers rupture in the deeper position. It creates a damage depth of 2 mm. It indicates that the Cu mesh can effectively dissipate the LS energy. The damage area of the CNF/CFRP composite is 123 � 115 mm2 in Fig. 10 k and the damage zone mainly locates in the center of the panel. As shown in Fig. 10 o), its utmost damage depth is about 2.41 mm. Fig. 10 l shows the damage area of the SMCNF/CFRP composite panel is 163 � 155 mm2, and its shape is similar to a circle. Fig. 10 p) displays the damage depth of the SMCNF/CFRP panel is about 1.8 mm and the damage shape is similar to a circular arc. It illustrates that the damage area of the CFRP layer reduces gradually with the increase of the damage

depth. Compared with neat CFRP, Cu mesh/CFRP and CNF/CFRP composite panels after simulated LS tests, the damage area of the SMCNF/CFRP composite surface is largest and its utmost damage depth is lowest. It indicates that the SMCNF as an expendable layer can effectively dissipate the LS energy and reduces the damage caused by the LS. The micro-morphology of the LS damage zone of the SMCNF/CFRP composite panel is characterized, for analyzing its possible LSP mech­ anism. Surface and a single carbon fiber of the CFRP composite without the LS are displayed in Fig. 11a). The surface of the CFRP without the LS is smooth due to enough epoxy resin covering. Owing to resin wrapping, the single carbon fiber surface is smooth and its diameter is about 10 μm. Surface edges and carbon fibers of damage zones of the CFRP, Cu mesh/ CFRP, CNF/CFRP and SMCNF/CFRP panels are displayed in Fig. 11b, c, d and e, respectively. As shown in Fig. 11 b, most of the epoxy resin is oxidized. Many carbon fibers are exfoliated from the resin phase, and a little epoxy resin is still attached on carbon fiber surfaces. Moreover, the 9

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Fig. 11. SEM images of surface and a single carbon fiber of a) CFRP without the LS, and residual ablation edges of protective layers and cracking carbon fibers in LS areas of b) CFRP, c) Cu mesh/CFRP, d) CNF/CFRP and e) SMCNF/CFRP.

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LS current. The high surface temperature generated by the LS current leads the silver particles attached on the SMNCF surface melting. After the surface temperature reducing, the liquid silver solidifies. Decom­ position of the epoxy generates the gas and it impacts liquid silver during the solidification process. The re-solidified silver frame forms the porous structure. The silver frame can fix conductive networks of the SMCNF and increases the time for dissipating LS energy. However, liquid silver releases heat, during its solidification process. It seriously damages the CNT network Thus, CNT networks of the SMCNF are seri­ ously damaged, but conductive networks are connected by the silver frame. Compared with the CNF/CFRP panel, the fiber surface of the SMCNF/CFRP is smooth and its diameter is thick owing to more residual resin wrapping. Based on above morphology analysis, the highly conductive SMCNF can effectively dissipate the LS energy and reduce damage of the CFRP matrix. 3.8. Residual strength of the SMCNF/CFRP composite For evaluating LSP effects of different LSP layers, compressive strengths of different composite panels after LS tests are measured through compressive tests, as prescribed in the ASTM D7137. Compressive specimens of CFRP, Cu mesh/CFRP, CNF/CFRP and SMCNF/CFRP composites without simulated LS tests are considered as control specimens. As shown in Fig. 12, their ultimate compressive strengths are 292.1 MPa, 314.59 MPa, 288.51 MPa and 291.1 MPa, respectively. After simulated LS tests, ultimate compressive strength values of CFRP, Cu mesh/CFRP, CNF/CFRP and SMCNF/CFRP com­ posites are 176.58 MPa, 262 MPa, 231.82 MPa and 265.05 MPa, respectively. Compared with control specimens, ultimate compressive strengths of CFRP, Cu mesh/CFRP, CNF/CFRP and SMCNF/CFRP com­ posites after simulated LS tests reduce 39.55%, 16.72%, 19.65% and 8.95%, respectively. If the residual strength of the composite after the LS exceeds 80%, this composite can be reused by the appropriate main­ taining in the engineering application [30]. Thus, residual strength of the SMCNF/CFRP composite can satisfy the LSP requirement of the engineering application.

Fig. 12. Ultimate compressive strength values of different composite panels.

cracking carbon fiber displayed a needle shape with a thin diameter and its surface is smooth without resin wrapping. Owing to poor electrical conductivity, the CFRP composite cannot effectively dissipate the LS energy, so that the Joule heating causes serious heating damage of carbon fibers and epoxy. Most of the resin of the Cu mesh/CFRP com­ posite surface is oxidized and a few carbon fibers separate from the CFRP matrix as shown in Fig. 11 c. Compared with the neat CFRP, the surface damage of the Cu mesh/CFRP composite reduces obviously, and its diameter of the cracking carbon fiber is thicker than that of the neat CFRP panel. It illustrates that Cu mesh with excellent conductivity can effectively reduce the damage caused by the LS. Compared with Fig. 2 a, the morphology of the CNF after the LS changes obviously and the dense CNF transforms into many ‘CNT islands’ that are connected by some CNT bundles. The conductive network still exists in the CNF for dissi­ pating the LS energy. A few particles are attached on the CNF surface, which are generated by decomposition of the epoxy resin. Compared with Fig. 11 b, the cracking carbon fiber of the CNF/CFRP composite possesses a thick diameter and rough surface caused by a little residual epoxy resin attaching. It indicates that the CNF can dissipate part of the LS energy and reduces the heating damage caused by the LS. The white and porous structure is the re-solidified silver frame in Fig. 11 e. At the initial stage, the SMCNF with good conductivity effectively conducts the

3.9. The possible LSP mechanism of the SMCNF CNT defects of the SMCNF after LS tests are characterized by Raman spectra, to investigate damages caused by the LS. Fig. 13a and b are Raman spectra of CNF and SMCNF before and after the LS, respectively. As shown in Fig. 13 a, D-band and G-band intensities of the CNF before and after the LS change slightly, and their ID/IG values are 0.023 and 0.103, respectively. The CNT defect of the CNF after the LS increases by 4.5 times. It indicates that the heating effect generated by the LS current

Fig. 13. Raman spectra of a) CNFs and b) SMCNFs before and after LS tests. 11

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XRD patterns of the SMCNF before and after the LS are characterized, to analyze the influence of the LS on silver particles, as shown in Fig. 14. Compared with the pattern of the SMCNF before the LS, all the diffraction peak intensities of the CNT of the SMCNF after the LS reduce obviously and only the characteristic peak that exists at 26.7� is distinct. Moreover, all the diffraction peaks of the elemental silver still exist and their intensities reduce slightly. It illustrates that the LS energy causes serious damages of CNTs and silver particles still exist as the elemental silver. During the LS process, silver particles without oxidization can increase the dissipating time for the LS energy and effectively reduce the damage of the CFRP matrix. Variations of elemental contents of CNF and SMCNF before and after the LS are characterized by the X-ray photoelectron spectroscopy (XPS) for further analyzing LS damages. Fig. 15a and b are XPS survey spectra of the CNF before and after the LS, respectively. Both XPS survey spectra of the CNF before and after the LS contain C 1s and O 1s peaks. The oxygen element of the CNF before the LS is introduced on defects of CNTs during the synthesis process. Lightning current causes slight oxi­ dization of CNTs. The atomic percent of the carbon element of the CNF after the LS reduces 3.64% and that of the oxygen element increases 3.64%. Fig. 15c and d are XPS survey spectra of the SMCNF before and after the LS, and insert images of Fig. 15c and d are details of Ag 3d5/2 and Ag 3d3/2 peaks of the metallic Ag. Both XPS survey spectra of the SMCNF before and after the LS contains C 1s, O 1s and Ag 3d peaks. The atomic percent of the carbon element of the SMCNF reduces 9.49%, and the atomic percent of the oxygen and silver elements after the LS in­ creases 4.7% and 4.79%, respectively. Besides the Joule heat, the liquid silver also releases heat during its solidification process. CNTs are oxidized and decomposed by the heat, and the carbon element of the SMCNF reduces more than that of the CNF. Due to oxidization of CNTs,

Fig. 14. XRD patterns of SMCNFs before and after LS tests.

damages CNT structure. Compared with the CNF, intensities of D-band and G-band peaks of the SMCNF before and after the LS change obvi­ ously, as shown in Fig. 13 b. Their ID/IG values are 0.024 and 0.494, respectively. The CNT defect of the SMCNF after the LS increases by 20.6 times. It illustrates that CNT defects of the SMCNF caused by the LS energy are serious. The reason is that CNT layer of the SMCNF dissipates more LS energy in a long period.

Fig. 15. XPS survey scans of CNFs a) before the LS and b) after the LS; XPS survey scans of SMCNFs c) before and d) after LS tests; insert images in c) and d) are details of Ag 3d peaks. 12

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Fig. 16. Infrared thermal images of maximum temperature of non LS sides of CFRP panel a) without any protective layer, and protected by b) Cu mesh, c) CNF and d) SMCNF during LS tests.

the oxygen element of the SMCNF increases. It indicates that the SMCNF dissipates more LS energy than the CNF in a longer period. Based on Joule’s law, the LS current can generate a high temperature of 3000–30000 � C on the composite surface. The high temperature will cause serious damage of the CFRP matrix. Therefore, the research on ability of the protective layer to dissipate the heat is beneficial to analyze its LSP mechanism. Moreover, some parts of the aircraft need slight temperature variation of the non LS side of the composite during the LS process, such as the fuselage, engine, fuel tank and other parts. The CFRP composite has the poor conductivity, so that its ability to dissipate LS energy is also too poor. The LS energy conducts more along its through-thickness direction. The maximum temperature of the non LS side of the neat CFRP composite reaches 117.8 � C during the LS process as displayed in Fig. 16 a. The addition of the copper mesh in­ creases the ability of the composite surface to dissipate the LS energy, and the energy conducting along the through-thickness direction of the CFRP matrix is effectively reduced. As shown in Fig. 16 b, the maximum temperature of the non LS face of the Cu mesh/CFRP composite is 72.8 � C during the LS process. Because the electrical conductivity of the CNF is much lower than the copper, the LS current will generate a higher temperature on the CNF/CFRP composite surface. CNT networks are

easily broken by pyrolysis of CNTs, and it will reduce the dissipating time for the LS energy along the composite surface. The CNF/CFRP composite conducts more LS energy along its through-thickness direc­ tion than the Cu mesh/CFRP composite. The maximum temperature of the non LS side of the CNF/CFRP composite is 94.2 � C during the LS as shown in Fig. 16 c. Fig. 16 d presents that the maximum temperature of the non LS face of the SMCNF/CFRP composite is 54.8 � C during the LS, and it is lower than the Cu mesh/CFRP composite. The reason is that the LS energy conducts along the SMCNF/CFRP composite surface by the plane conduction, instead of the linear conduction of the Cu mesh. Moreover, CNTs are fixed on the conductive network by the re-solidified silver frame during the LS process. The SMCNF has a longer LS energy dissipating time along the composite surface. Thus, less LS energy con­ ducts along the through-thickness direction of the CFRP matrix. The SMCNF effectively reduces the non LS side temperature of the CFRP composite during the LS process. Based on the above analysis, schematic diagrams of LSP mechanisms of neat CFRP, Cu mesh/CFRP, CNF/CFRP and SMCNF/CFRP composites are shown in Fig. 17a and b, c and d, respectively. The arrow length stands for the ability to conduct the LS energy. As shown in Fig. 17 a, conductive paths of the CFRP composite for the LS current are carbon

Fig. 17. Schematic diagrams of protective mechanisms of a) neat CFRP panel, b) Cu mesh/CFRP panel, c) CNF/CFRP panel and d) SMCNF/CFRP panel. 13

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fibers and the energy dissipating mode is the linear conduction. In addition, the CFRP composite has the poor conductivity. The neat CFRP composite has the poor ability to dissipate LS energy, and it is damaged by the LS without effective conduction in a longer period. The LS energy causes the smaller surface damage and the deeper damage depth. When the epoxy resin decomposes and generates some gas under the high temperature, the through-thickness resistivity of the CFRP matrix in­ creases. The conductive layer can dissipate more LS energy along the composite surface [35]. Compared with the neat CFRP composite, the Cu mesh as a highly conductive layer can increase the ability of the composite to dissipate LS energy along its surface direction through the linear conduction. When the Cu mesh melts under the high temperature, the LS energy dissipating along the composite surface reduces rapidly and the residual energy still conducts along through-thickness direction of the CFRP matrix. The damage area of the Cu mesh/CFRP composite is much larger than the neat CFRP composite, and its depth damage re­ duces markedly. The CNF as a conductive layer can increase the ability to dissipate LS energy along the surface direction of the composite through the plane conduction as shown in Fig. 17 c. When the CNF layer is oxidized, its ability to dissipate LS energy reduces rapidly and part of the residual energy still conducts along through-thickness direction of the composite. The damage area of the CNF/CFRP composite is larger than neat CFRP composite, and its damage depth is lower. As shown in Fig. 17 d, the SMCNF with higher conductivity can effectively conduct more LS energy along the surface direction of the composite through the plane conduction at the initial stage of the LS process, compared with carbon fibers, Cu mesh and CNF. Silver particles melt under the high temperature and then solidify. The conductive network of the SMCNF is fixed by re-solidified silver frame and its ability to dissipate LS energy along the surface direction enhances, so that more LS energy conducts along the composite surface and less LS energy conducts along its through-thickness direction. The damage depth of the SMCNF/CFRP composite is lower than Cu mesh/CFRP and CNF/CFRP composites. It indicates that the SMCNF as a LSP layer can effectively reduce the LS damage of the CFRP matrix.

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4. Conclusion A series of SMCNFs were fabricated successfully via the electropho­ retic deposition method. The optimal SMCNF prepared under an applied voltage of 5 V for 240 s is appropriate for using as a LSP layer, owing to excellent conductivity and low density. Its conductivity is about 9.7 times higher than the CNF, and its areal density is only 53.3 g/m2. The SMCNF not only enhances the conductivity of the composite surface, but also provides re-solidified silver frame to fix the conductive network. It improves the ability of the SMCNF/CFRP composite to conduct LS en­ ergy along its surface direction and provides a longer dissipating time to reduce the LS damage. The SMCNF/CFRP composite maintains 91.05% compressive strength after the LS, which is little higher than the Cu mesh/CFRP composite (83.28%). Especially, the utilization of SMCNF as a LSP layer can reduce the weight by 27.4%, compared with commercial Cu mesh (73 g/m2). It indicates the SMCNF is a feasible strategy for CFRP lightning strike protection, instead of traditional metallic materials. Acknowledgements This work is supported by the National Natural Science Foundation of China [Grant Nos. 11225211, 11272106]. Project supported by the Foundation for Innovative Research Groups of the National Natural Science Foundation of China [Grant No. 11421091]. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.compositesb.2019.107563. 14

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