Carbon nanotubes coated hybrid-fabric composites with enhanced mechanical and thermal properties for tribological applications

Accepted Manuscript Carbon nanotubes coated hybrid-fabric composites with enhanced mechanical and thermal properties for tribological applications Junya Yuan, Zhaozhu Zhang, Mingming Yang, Fang Guo, Xuehu Men, Weimin Liu PII: DOI: Reference:

S1359-835X(17)30305-6 http://dx.doi.org/10.1016/j.compositesa.2017.08.006 JCOMA 4757

To appear in:

Composites: Part A

Received Date: Revised Date: Accepted Date:

15 February 2017 1 August 2017 2 August 2017

Please cite this article as: Yuan, J., Zhang, Z., Yang, M., Guo, F., Men, X., Liu, W., Carbon nanotubes coated hybridfabric composites with enhanced mechanical and thermal properties for tribological applications, Composites: Part A (2017), doi: http://dx.doi.org/10.1016/j.compositesa.2017.08.006

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Carbon nanotubes coated hybrid-fabric composites with enhanced mechanical and thermal properties for tribological applications Junya Yuana,b, Zhaozhu Zhanga,, Mingming Yanga,b, Fang Guoa, Xuehu Menc,*, Weimin Liua a

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese

Academy of Sciences, Tianshui Road 18th, Lanzhou 730000, P. R. China b

University of Chinese Academy of Sciences, Beijing 100049, P. R. China

c

School of Physical Science and Technology, Lanzhou University, Lanzhou, 730000, P. R. China

ABSTRACT Poor interfacial adhesion and inferior thermal property of resin matrix severely obstruct the continued development of fabric-reinforced polymer composites for potential advanced tribological applications. Herein, carbon nanotubes (CNTs) encapsulated by copolymer of polydopamine (PDA) and polyethylenimine (PEI) directly deposited onto hybrid Nomex/PTFE fabric leading to a hierarchical reinforcing structure was successfully carried out via a facile one-pot synthesis. The achieved organic-inorganic hybrid functional coating significantly increased the wettability, reactive functional groups and surface roughness of hybrid-fabric. Tensile and peeling tests show that the hierarchical composites exhibited 38.4% and 63% enhancement in tensile strength and interfacial bonding strength compared to that of the pristine fabric composites. Furthermore, CNTs modification forming percolating networks on hybrid-fabric within the resin matrix effectively promotes the thermal stability of the

Corresponding author. Fax: 86-931-4968098. E-mail: [email protected] (Z.Z. Zhang); [email protected] (X.H. Men) 1

fabric composites. Results of wear tests prove that the hierarchical composites exhibited outstanding tribological properties under varied applied loads. Keywords: A. Fabrics/textiles; B. Fiber/matrix bond; B. Thermal properties; B. Wear. 1. Introduction Fabric-reinforced polymer composites have been widely used in the high-performance manufacture fields, including aerospace, automotive, military and sporting equipment, owing to good processability, high strength to weight ratio and outstanding fracture toughness [1-2]. Especially, hybrid woven fabrics are becoming increasingly attractive due to their integrating the unique advantages of different fibers into single units, which enables the creation of hybrid materials with new or improved properties. [3]. Among various hybrid-fabric materials investigated, hybrid Nomex/PTFE fabric is chiefly preferred for self-lubrication materials usage, as it combines high specific-strength of Nomex fibers and exceptional self-lubrication of PTFE fibers [4]. However, the intrinsically chemical inertness, low surface energy and excessive smoothness characteristics of Nomex and PTFE fibers lead to poor interfacial adhesion properties with polymer matrixes, which largely hinder the practical applications of hybrid Nomex/PTFE fabric composites. Moreover, the inferior thermal property makes polymer matrixes highly susceptible to thermal effects generated from friction process and therefore present relatively weak anti-wear and load-carrying capacity [5]. These existing critical issues severely obstruct the advance in whole tribological properties of fabric-reinforced polymer composites. To overcome the limitations enforced by impediments of the inert fiber surface 2

and inferior thermal property of polymer matrixes, considerable efforts have been made by tailoring the surface chemical composition of fibers and/or introduction of reinforcing nanofillers [6，7]. Note that although significant progress has been achieved, their practical applications are largely hampered by fiber strength deterioration and nanofillers agglomeration. Recently, the utilization of nanoparticles for the modification of high-performance fibers to realize hierarchical reinforce structure has been appraised [8, 9]. Carbon nanotubes (CNTs) are ideal candidates in this regards, owing to their excellent mechanical, thermal and self-lubrication properties [10]. By anchoring the CNTs to fiber surface, there is a potential to alleviate the problem of CNTs agglomeration [11]. In addition, the suffusion of CNTs on fiber surface can provide simultaneous reinforcements to fiber/matrix interface and polymer matrix [12, 13]. Based on the above respects, numerous researches have been devoted to hybridizing carbon nanotubes (CNTs) and fibers to produce better fiber-reinforced composites，such as chemical vapor deposition (CVD) [14], electrophoretic deposition (EPD) [15], chemical reactions between functionalized CNTs and fibers [16] and coating of fibers with CNT-containing sizes [17]. Unfortunately, most of the aforementioned methods need high-temperature process, electricity-conductive surfaces or pretreatment of fibers, which are unavailable for hybrid Nomex/PTFE fabric

due

to

its

poor

heat-resistance,

electrically insulation and

super

chemical-inertness. Consequently, it is worthy to explore novel and facile methods for the hybrid Nomex/PTFE fabric without compromising its strength. Inspired by the notable and universal adherent ability of mussel adhesive proteins 3

on various substrates, oxidative self-polymerization of dopamine (DA) into polydopamine (PDA) coating has been regarded as a versatile approach for surface modification [18]. In addition, many surface modifications based on mussel-inspired PDA were performed to achieve the desirable performances [19]. This strategy is substrate-independent but relatively complicated and time consuming. Besides, PDA oligomers usually form aggregates easily by non-covalent interactions after a long deposition time, which may weaken the adhesive property of obtained PDA coating [20]. Recently, the co-deposition of PDA with PEI to construct the active layer for enhancing the compactness of PDA coating and meantime simplifying the time-consuming preparation procedures has been proposed. Thus, many researchers utilize hybrid PDA-PEI coating for substrate surface engineering or as an intermediate layer for subsequent immobilization of nanoparticles [21，22]. Though widely been investigated, to our knowledge, currently the potential use of hybrid PDA-PEI coating for one-step binding nanoparticles onto substrate surface has not been reported. Herein, take advantage of this facile mussel-inspired surface chemistry, we report a novel one-step dip-coating method that PEI is introduced to copolymerize with the dopamine to directly anchor CNTs onto hybrid Nomex/PTFE fabric surface. FESEM, FTIR, Raman spectra, XPS and TGA were used to verify the existence of the organic-inorganic hybrid functional coating. The interfacial properties and thermal stability of obtained hybrid-fabric composites were studied. Furthermore, the tribological behaviors and wear mechanisms of the fabric composites were systematically investigated. 4

2. Experimental Section 2.1. Materials MWCNTs (purity > 99.9%) with an average length of 10-30 μm and diameter of 20-30 nm were purchased from Nanjing XFNANO Materials Tech. Co. Ltd. Dopamine hydrochloride and Polyethylenimine were separately supplied by Sigma Aldrich and Meryer Chemical Tech. Co. Ltd. The hybrid Nomex/PTFE fabric was woven out of Nomex fibers and PTFE fibers provided by DuPont Plant (satin weave, volume fraction of Nomex to PTFE: 3:1). The phenolic resin adhesive (resol) was obtained from Xing-guang Chemical Reagent Co. Ltd, China. The rest chemicals were all of analytical grade and used as received. Prior to the modification, the hybrid Nomex/PTFE fabric was desized by washing with petroleum ether and ethanol sequentially in Soxhlet extractor and then dried at 50 ºC. The raw CNTs was oxidized by mixture of HNO3/H2SO4 (1/3, v/v) at 70 ºC for 3 h. 2.2. Preparation of CNT-modified hybrid Nomex/PTFE fabric The CNT-PDA-PEI@hybrid-fabric hierarchical reinforcing structure was prepared by a facile one-step method as illustrated in Fig. 1a. Specifically, Dopamine hydrochloride, PEI and Oxidized carbon nanotubes (OCNTs) were dissolved or dispersed in a Tris buffer solution (pH = 8.5, 20 mM) with a mass ratio of 1:1:1 by intense ultrasonic treatment for 30 min. The concentration was 2 mg/mL for each component in the solution. Hybrid Nomex/PTFE fabric sample was subsequently immersed in this freshly prepared solution at ambient temperature for 6 h under continuous

stirring.

The

resulting 5

fabric

sample,

denoted

as

CNT-PDA-PEI@hybrid-fabric, was rinsed thoroughly with deionized water and dried overnight at 60 ºC under vacuum. Another group of fabric sample was only immersed into a dopamine and PEI solution without OCNTs under the same conditions for comparison, which was denoted as PDA-PEI@hybrid-fabric. Moreover, CNTs and OCNTs modified by the dopamine/PEI solution were denoted as PDA-PEI@CNTs and PDA-PEI@OCNTs, respectively. 2.3. Preparation of hybrid-fabric reinforced phenolic composites The

pristine

hybrid-fabric,

PDA-PEI@hybrid-fabric

and

CNT-PDA-PEI@hybrid-fabric were separately immersed into the phenolic resin solution. Repetitive immersion and drying of these hybrid-fabrics were carried out until the weight fraction of hybrid-fabrics reached 70-75% in the composites. Subsequently, the obtained prepregs were cured at 180 ºC for 2 h under 0.2 MPa. The detailed data about samples and apparatus for wear and mechanical properties tests as well as the characterization methods of hybrid-fabric composites and their fibers were included in Supplementary Information. 3. Results and discussion 3.1. Mechanism of CNT-PDA-PEI@hybrid-fabric preparation The possible chemical reactions occur on the hybrid-fabric surface during chemically anchoring of CNTs as illustrated in Fig. 1b. Specifically, dopamine can be oxidized to a quinoid form and then quinoid can react with catechol via quinoid–phenol dismutation, or with amine via Michael addition or Schiff base reaction in an alkaline Tris buffer solution [23]. Hence, the self-polymerization of dopamine and their 6

copolymerization with PEI were proceeding simultaneously. The introduction of PEI catalyzes the polymerization of phenols and disintegrates large polyphenol conglomerations for enhancing the compactness of PDA coating and meantime simplifying the time-consuming preparation procedures [24]. More importantly, the PDA-PEI intermediate layer provides a coordination force to prompt subsequent immobilization of nanoparticles on the substrate surfaces [25]. The reactions gradually form PDA-PEI@CNTs aggregates in the solution and subsequently deposit onto the hybrid-fabric surfaces. Pre-oxidation of CNTs was carried out to achieve strong adhesion strength to the attached PDA-PEI copolymerization as well as stable dispersability and nanoparticle size-uniformity for good deposition effect. 3.2. Characterization of CNT-modified hybrid Nomex/PTFE fabric 3.2.1. Surface morphology of Nomex and PTFE fibers The surface morphology of Nomex and PTFE fibers before and after modification were characterized by FESEM and the results were shown in Fig. 2. Initially, the surface of desized Nomex and PTFE fibers with a few narrow grooves were relatively neat and smooth (Fig. 2a, d). Fig. 2b, e presented the surface morphology of Nomex and PTFE fibers after the co-deposition of dopamine and PEI. The Nomex and PTFE fiber surface were fairly covered by homogenous PDA-PEI layer but rarely discontinuous areas. In contrast, the different surface morphology and obvious increased surface roughness of CNT-PDA-PEI@Nomex (Fig. 2c) and CNT-PDA-PEI@PTFE (Fig. 2f) indicated CNTs with a dense yield and uniform dispersion were covered on the Nomex and PTFE fibers. 7

In an attempt to further identify the morphological features of CNTs coated on hybrid-fabric surface, additional SEM and TEM images for CNTs immersed into a dopamine and PEI solution without hybrid-fabric under the same conditions were shown in Fig. 3. As could be seen, both the pristine CNTs and OCNTs were enwrapped by the ultrathin PDA-PEI co-deposited film after polymerization reaction. Moreover, it was evident from the micrographs that comparatively higher degree of wrapping was achieved for OCNTs than that of pristine CNTs. This could be attributed to the fact that the OCNTs contained a larger quantity of hydroxyl and carboxylic functional groups which led to adequate level of vander Waals, hydrogen bond or chemical bond forces between OCNTs and PDA-PEI co-deposited film. Therefore, pre-oxidation of CNTs was carried out for better deposition effect. However, the pristine CNTs were also well enwrapped by PDA-PEI co-deposited film after polymerization reaction. These results indicated that the surface functional groups were not essential for PDA-PEI wrapping, and the co-deposition reaction of dopamine and PEI was substrate-independent, which was extremely important for anchoring CNTs to hybrid-fabric surface. In conclusion, the CNTs on the Nomex and PTFE fibers were enwrapped by the PDA-PEI co-deposited film. 3.2.2. Surface chemical structure and composition The surface chemical structure of the pristine and modified PTFE and Nomex fibers were analyzed by FTIR spectra and the results were shown in Fig. 4. Compared with the pristine PTFE fibers, PDA-PEI@PTFE fibers presented additional absorption peaks at around 3411 cm-1 and 3247 cm-1 assigned to the stretching vibrations of -OH 8

and N-H groups. The new absorption peaks at 2933 cm-1 and 2850 cm-1 corresponded to the methylene non-symmetric and symmetric stretching vibrations, respectively, while the new peaks at 1656 cm-1 and 1440 cm-1 were associated with the stretching vibrations of C=O and C-N [26]. The peak at around 1265 cm−1 was ascribed to the C-O stretching and primary amine vibration (Fig. 4a) [27]. These results indicated the successful coating of dopamine and PEI on PTFE fibers. With regard to Nomex fibers, the characteristic peak at 3297 cm-1 was due to the stretching vibration of N-H group. Absorption peaks were also observed at around 1600 cm−1, 1510 cm−1, 1467 cm−1 and 1403 cm−1, which corresponded to the benzene skeleton vibrations. After introduction of PDA-PEI, the absorption peak at 3405 cm-1 associated with -OH group showed a noticeable increment in the peak intensity, and the 3297 cm−1 peak of N-H group in pristine Nomex shifted to 3247 cm-1 ascribed to the stretching vibration of N-H from PEI. The overlapped peaks at 2933 cm-1 and 2850 cm-1 were enhanced obviously attributed to methylene stretching vibrations from PEI. Moreover, the benzene skeleton vibrations with the decreasing of peak intensities were observed (Fig. 4b). All of the changes confirmed the Nomex was successfully co-deposited by PDA-PEI. In addition, the FTIR spectra results of CNT-PDA-PEI@PTFE and CNT-PDA-PEI@Nomex were similar to those of PDA-PEI@PTFE and PDA-PEI@Nomex, which resulted from the special surface structure. The CNTs combined with the PDA-PEI formed an organic-inorganic hybrid functional coating and CNTs were wrapped by copolymer of dopamine and PEI. Raman spectroscopy was used for the characterization of CNTs on modified 9

hybrid-fabric surface. The spectra of pristine PTFE fibers showed two bands at 1378 and 1596 cm-1 ascribed to amorphous carbon [28]. After PDA-PEI coating, these bands were weakly present or completely absent attributed to the introduction of disordered PDA-PEI structure. As for the CNTs-PDA-PEI@PTFE, the typical D and G bands of the CNTs were observed at 1338 and 1575 cm−1, respectively (Fig. 4c) [29]. Pristine Nomex fiber showed bands at 1250, 1335, 1545 and 1603 cm−1 related to the stretching vibrations of C-C bonds in the aromatic rings of Nomex fiber [30]. The vibration at 1650 cm−1 was attributed to the stretching mode of C=O group presented in the main fiber structure. The relative intensities of these bands in PDA-PEI@Nomex also decreased, suggesting the successful coating of PDA-PEI. However, these bands belonging to the Nomex fiber overlapped with the D and G bands of the CNTs, and consequently, their intensity and width increased when CNTs were deposited on the fiber surface, as shown in Fig. 4d for the bands centered at 1335 and 1603 cm−1. The surface chemical composition changes of PTFE and Nomex fibers were further determined by XPS. Fig. 5a, e showed the survey scan spectra of pristine and modified PTFE and Nomex fibers and the corresponding elemental composition was listed in Table 1. The survey scan of pristine PTFE only displayed two peaks of C1s and O1s, while the new peak of N1s arose after decoration with PDA-PEI or CNTs-PDA-PEI. As for the Nomex, the N/C ratio increased from 8.2% of pristine Nomex to 11% of PDA-PEI@Nomex and 10% of CNTs-PDA-PEI@Nomex. In addition, the C1s core-level spectra of the pristine and modified PTFE and Nomex fibers were curve-fitted to track the surface functional group changes of fibers. The C1s 10

peak of pristine PTFE could be split into four peaks corresponding to C-C at 284.6 eV, C−F at 287 eV, CF2-CF2 at 292.4 eV and CF2/CF3 at 293.3 eV (Fig. 5b). As for the PDA-PEI and CNTs-PDA-PEI decorated PTFE, besides the four characteristic functional groups of PTFE, there were three other peaks appeared, including C–N at 285.7 eV, C-O at 286.5 eV and C=O at 288.3 eV, which was consistent with the chemical characteristics of functional coating (Fig. 5c, d). Compared with the pristine Nomex, the decline in C=O band and enhancement of C-N were observed in PDA-PEI@Nomex. Moreover, it was noted that the increase in C-O was observed for CNTs-PDA-PEI@Nomex. These changes were well consistent with FTIR and Raman spectroscopy results and further confirmed the successful introduction of PDA-PEI and CNTs-PDA-PEI coating. 3.2.3. TG analysis In order to further study the chemical components of original and modified hybrid Nomex/PTFE fabrics quantitatively, TG analysis was utilized and the results were shown in Fig. 6a and the corresponding mass loss rate or residue rate in different temperature ranges was listed in Table 2. In all of the fabric samples studied, initial weight loss stage appeared between room temperature and 100 °C ascribed to the moisture evaporation depending on the sample properties. Then the loss of weight was about 3% observed in PDA-PEI@hybrid-fabric and CNTs-PDA-PEI@hybrid-fabric during the second stage, while there was no obvious change of the weight for pristine hybrid Nomex/PTFE fabric. The weight loss was mainly attributed to the decomposition of the PDA-PEI coating on the hybrid-fabric. The main mass loss 11

observed in the third stage was ascribed to the carbonization of hybrid-fabric. After these three mass loss stages, the residue left rate for CNTs-PDA-PEI@hybrid-fabric was about 4% higher than pristine hybrid-fabric and PDA-PEI@hybrid-fabric, which was attributed to the presence of CNTs on hybrid-fabric surface. 3.2.4. Surface wettability The wettability and water permeability of the hybrid-fabrics was characterized by measuring the water contact angle (WCA). The original hybrid-fabric showed hydrophobicity, with a WCA beyond 90o in air (Fig. 6b). In contrast, the PDA-PEI/CNT-PDA-PEI

decorated

hybrid-fabric

specimens

all

presented

superhydrophilicity, leaving the undetectable apparent WCA. In addition, it was worth mentioning that water drops permeated through the CNT-PDA-PEI@hybrid-fabric (WCA turned into 0o) even faster than PDA-PEI@hybrid-fabric. It was known that both surface topological feature and chemical composition influenced the wettability of material. Herein, it was the intrinsic hydrophilicity of PDA-PEI coating and the well-defined micro-nano surface structures made the modified hybrid-fabric superhydrophilic. 3.3. Mechanical properties of CNT-modified hybrid-fabric composites The mechanical properties of the hybrid-fabric composites were examined by tensile and peeling tests. Fig. 7 showed the stress-strain curves and bonding strength of the original and modified hybrid-fabric composites and the corresponding tensile strength, Young's modulus and failure strain were listed in Table 3. It was observed that composites coated with PDA-PEI and CNT-PDA-PEI is mechanically stronger than the 12

original hybrid-fabric composites. In particular, the CNT-PDA-PEI@hybrid-fabric composites showed the best mechanical properties with tensile strength, Young's modulus, failure strain and bonding strength increased by 38.4%, 34.9%, 48.3% and 63% respectively. The suffusion of CNTs on the hybrid-fabric surface effectively improved the surface area, mechanical interlocking and local stiffening of hybrid-fabric/matrix interface, which imparted the strength to the interface by promoting stress transfer from resin matrix to the hybrid-fabric [31, 32]. In addition, PDA-PEI co-deposited film on the surface of CNTs was able to interact with the phenolic resin matrix, which facilitated the further improvement in interface bonding strength [33]. The fractured surface morphologies of the hybrid-fabric composites after tensile and peeling tests were examined by SEM in Fig. 8. As for the cross-sections, the obvious gaps between fibers and resin were observed in original hybrid-fabric composites (Fig. 8a), whereas the gaps lessened and the resin was adhered to the fabric tightly after deposition of PDA-PEI and CNT-PDA-PEI coatings (Fig. 8b, c), indicating the improvement of interfacial adhesion properties. Fig. 8d, e and f showed the peeling surfaces of the original and modified hybrid-fabric composites. As shown in Fig. 8d, the original hybrid-fabric composites presented smooth peeling surfaces, suggesting the pristine hybrid-fabric was easily detached from the resin matrix due to relatively poor interfacial bonding. In contrast, the outer layers of the hybrid-fabric was peeled off and adhered with the peeling surfaces observed in PDA-PEI@hybrid-fabric composites. With regard to the CNT-PDA-PEI@hybrid-fabric composites, many fibers were still adhered with matrix and some fibers fragmented into shorter fibers. These 13

results were attributed to that the CNTs coated on the hybrid-fabric surface, as an anchor to locally stiffen at the interface region, could stick into the phenolic matrix. In addition, the polar groups on CNTs anchored on the hybrid-fabric could react with phenolic matrix to form the chemical bonding, leading to a significantly enhancement in the interfacial properties of fiber-matrix. 3.4. Thermal stability of CNT-modified hybrid-fabric composites The thermal stability of original and modified hybrid-fabric composites were evaluated by TGA in Fig. 9, and the values of Tonset and Tmax obtained from the analysis were given in Table

4.

From the results,

PDA-PEI@hybrid-fabric and

CNT-PDA-PEI@hybrid-fabric composites showed increasing value of Tonset and Tmax, indicative of the improved thermal stability. The introduction of PDA-PEI co-deposited film on fabric surface enhanced the interfacial adhesion between reinforcing fabric and resin matrix, thereby limiting the chain mobility of phenolic resin leading to better thermal stability. For CNT-PDA-PEI@hybrid-fabric composite, CNT modification formed percolating networks on the fabric surface providing simultaneous reinforcements to the fabric/matrix interface and the resin matrix. In addition, this coating method also alleviated the agglomeration problem commonly observed when CNTs were freely dispersed in the resin matrix. The uniform dispersion of CNTs further restricted the free rotation of the resin matrix backbone and retarded the diffusion of oxygen into the resin matrix through tortuous paths and barrier effects, eventually retarding the thermal degradation of this hybrid-fabric composite and presenting the best thermal stability [34]. 14

3.5. Tribological properties of CNT-modified hybrid-fabric composites The variations in the specific wear rates and friction coefficients of pristine, PDA-PEI and CNT-PDA-PEI decorated hybrid-fabric composites as a function of load were comparatively shown in Fig. 10. It can be concluded that the wear rates of these three hybrid-fabric composites increased with applied load accompanied with a decrease in friction coefficients simultaneously. Increase in applied load made the sliding surfaces suffer even fierce compressive and shearing stresses and then induced added frictional heat generation [35]. In view of the poor thermal stability and heat transfer of phenolic resin matrix, plentiful frictional heat generation and accumulation at the sliding interface brought about softening or local melting of the resin matrix, which led to serious deterioration in matrix strength [36]. Phenolic resin then got easily separated from the hybrid-fabric along with microcutting, pulling out and fracturing of the fibers and ultimately resulted in higher wear. Meanwhile, the more debonded PTFE fibers exposed on the contact surface were pulverized into smaller debris and subsequently formed a smooth transfer film responsible for the decreasing friction coefficients. Moreover, the reduced shear strength of composites resulting from increasing temperature also led to the further decrease of friction coefficients. Compared to pristine hybrid-fabric composite, PDA-PEI@hybrid-fabric and CNT-PDA-PEI@hybrid-fabric composites exhibited lower wear rates under all range of the load. This significant improvement in wear resistance was expected to be a combined effect of increase in interfacial properties and thermal stability. In particular, CNT-PDA-PEI@hybrid-fabric composite presented even more outstanding wear 15

properties than that of PDA-PEI@hybrid-fabric. This was mainly attributed to the reinforcing and lubricating effects of CNTs. To investigate the friction and wear mechanisms of the hybrid-fabric composites, the worn surfaces of the samples were examined and the resulting morphology was shown in Fig. 11. For the pristine fabric composite, masses of phenolic resin were worn off and bundles of reinforced fibers were pulled out and cut off, which corresponded to the high wear rate (see Fig. 11a and d). With regard to the PDA-PEI@hybrid-fabric composite, increased amount of resin and decreased fiber damage were seen on the worn surface (Fig. 11b and e). It was deduced that the enhanced interfacial bonding between hybrid-fabric and resin matrix contributed to the effective stress transfer from matrix to reinforced fabric and inhibited the generation and expansion of microcracks, which consequently weakened the damage caused by high compressive and shearing stresses [37]. But, in case of CNT-PDA-PEI@hybrid-fabric composite, relatively smooth worn surface without broad exfoliation and pulling out (Fig. 11c and f) was observed. Strong interfacial shearing resistance of the CNT reinforced fabric composite restricted the ploughing and cutting mechanisms. The effective stress transfer from resin to CNTs suppressed the plastic deformation and cracking, which led to less removal of the covered resin from the worn track. In addition, continuous lubrication effect from CNTs was also responsible for the excellent wear resistance [38]. CNTs got exposed on the sliding surface, creating the interface and preventing a direct contact between the counter pin and composite, which greatly reduced the ploughing and scratching effects in the composite. 16

4. Conclusions In this study, a novel chemical method is developed to coat CNTs onto hybrid Nomex/PTFE fabric via co-deposition of PDA, PEI and functionalized CNTs as the organic−inorganic hybrid functional coating. CNTs wrapped in copolymer of PDA and PEI adhered homogenously on the surfaces of hybrid-fabric to form a CNT-PDA-PEI@hybrid-fabric

hierarchical

reinforcing

structure

without

compromising fibers strength. Mechanical testing and thermal analysis indicated that the inclusion of CNTs on fabric surfaces improved the interfacial adhesion and thermal stability of hybrid Nomex/PTFE fabric composites. Wear tests also showed that the hierarchical composites exhibited the optimal anti-wear and load-carrying capacity compared to that of pristine fabric composites and PDA-PEI@hybrid-fabric composites. More importantly, this method can be easily extended to other systems due to the versatile conjunction function of PDA-PEI hybrid coating. Acknowledgments The authors gratefully acknowledge the financial support from the National Science Foundation of China (Grant Nos. 51375472, and 51675252). References [1] Gao B, Zhang R, He M, Wang C, Liu L, Zhao L, et al. Interfacial microstructure and mechanical properties of carbon fiber composites by fiber surface modification with poly(amidoamine)/polyhedral

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Figure Captions Fig. 1 Schematic illustration of the preparation procedure (a) and mechanism (b) for the CNT-PDA-PEI@hybrid-fabric. Fig. 2 SEM images of the pristine (a and d), PDA-PEI co-deposited (b and e) and CNT-PDA-PEI modified (c and f) Nomex and PTFE fibers. The insets are the corresponding magnified images. Scale bars: a, b, c, d, e, f) 5μm, the insets of a, b, d, e) 23

3μm, the insets of c, f) 1μm. Fig. 3 SEM (a–d) and TEM images (e-h) of the nascent (a and e), oxidated (b and f), PDA-PEI wrapped nascent (c and g) and oxidated (d and h) CNTs. Scale bars: a, b, c, d) 800 nm, e, f, g, h) 400 nm. Fig. 4 FTIR (a and b) and Raman (c and d) spectra of pristine and modified PTFE (a and c) and Nomex (b and d) fibers. Fig. 5 XPS spectra (a and e) and corresponding high-resolution C 1s spectra of the pristine (b and f), PDA-PEI co-deposited (c and g) and CNT-PDA-PEI modified (d and h) PTFE and Nomex fibers. Fig. 6 Thermogravimetric curves of pristine and modified hybrid-fabric (a). Water contact angles (b) of Bonding faces (Ⅰ,Ⅱ and Ⅲ) and Wear faces (Ⅳ,Ⅴ and Ⅵ) of pristine (Ⅰ and Ⅳ), PDA-PEI co-deposited (Ⅱ and Ⅴ) and CNT-PDA-PEI modified (Ⅲ and Ⅵ) hybrid-fabric. Fig. 7 The stress-strain curves (a) and Bonding strength (b) of pristine, PDA-PEI co-deposited and CNT-PDA-PEI modified hybrid-fabric composites. Fig. 8 SEM images of the fracture surfaces (a, b and c) and peeling surfaces (d, e and f) of pristine (a and d), PDA-PEI co-deposited (b and e) and CNT-PDA-PEI modified (c and f) hybrid-fabric composites. Scale bars: 40 μm. Fig. 9 Thermogravimetric curves of pristine, PDA-PEI co-deposited and CNT-PDA-PEI modified hybrid-fabric composites. Fig. 10 Wear rate (a) and friction coefficient (b) of pristine, PDA-PEI co-deposited and CNT-PDA-PEI modified hybrid-fabric composites. 24

Fig. 11 SEM images of the worn surfaces for pristine (a), PDA-PEI co-deposited (b) and CNT-PDA-PEI modified (c) hybrid-fabric composites (82.5 MPa, 0.26 m/s, 120 min); (d–f) are magnified images of (a–c), respectively. Scale bars: a, b, c) 200 μm, d, e, f) 100 μm.

Surface composite (at.%) Fiber sample Nomex PDA-PEI@Nomex CNT-PDA-PEI@Nomex PTFE PDA-PEI@PTFE CNT-PDA-PEI@PTFE

C

O

N

73.25 76.46 77.82 35.79 53.18 52.33

20.75 15.22 14.44

6.01 8.32 7.74

8.46 6.27

6.7 4.21

F

64.21 31.67 37.19

Table 1 The relative atomic concentration in surface of fiber samples before and after modification. 25

Mass loss (%) Sample

Stage Ⅱ

StageⅠ 20-100℃

Pristine Nomex/PTFE fabric PDA-PEI@hybrid-fabric CNT-PDA-PEI@hybrid-fabric

2.31 2.9 2.9

100-400℃ 0.3 3.3 3.1

Residue left (%) Stage Ⅲ

Stage Ⅳ

400-600℃

600-800℃

11.28 11 15.5

Table 2 Results of TGA traces of hybrid-fabric in oxygen atmosphere.

26

0 0 0

Properties Composite sample Pristine hybrid-fabric composite PDA-PEI@hybrid-fabric composite CNT-PDA-PEI@hybrid-fabric composite

Tensile strength Young' modulus (MPa) (G (GPa)

Failure strain (%)

107.2±7.9

1.075±0.08

11.8±2.3

124.6±13.5

1.25±0.09

14.3±3.6

147.8±11.7

1.45±0.15

17.5±4.3

27

Table 3 Results of Tensile strength, Young' modulus and Failure strain of hybrid-fabric composites.

Temperature Composite sample

Tonset

Pristine hybrid-fabric composite PDA-PEI@hybrid-fabric composite CNT-PDA-PEI@hybrid-fabric composite

379.19 383.59 402.6

28

Tmax 551.2 558.6 562.32

Tonset temperature corresponding to a weight loss of 10 wt%. Tmax temperature corresponding to maximum rate of decomposition.

Table 4 Results of Tonset and Tmax of hybrid-fabric phenolic composites in oxygen atmosphere.

29

30

31

32

33