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tribological properties
Materials Chemistry and Physics 243 (2020) 122677
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Polydopamine/FeOOH-modified interface in carbon cloth/polyimide composites for improved mechanical/tribological properties Zhe Lin, Jin Yang *, Xiaohua Jia, Yong Li, Haojie Song ** School of Materials Science & Engineering, Shaanxi Key Laboratory of Green Preparation and Functionalization for Inorganic Materials, Shaanxi University of Science & Technology, Xi’an, Shaanxi, 710021, PR China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� PDA transition layer and FeOOH nano particles were introduced on the sur faces of CFs. � FeOOH nanoparticles are uniformly coated on the surfaces of CFs. � PDA and FeOOH nanoparticles improved the interface between CFs and PI. � PDA-FeOOH-CFC/PI exhibited excellent mechanical and tribological properties.
A R T I C L E I N F O
A B S T R A C T
Keywords: Carbon fiber Polydopamine β-FeOOH Polyimide Mechanical properties Tribological properties
The performance of carbon fiber (CF) reinforced composites depend greatly on the interfacial structures of the fibers and matrix. Based on the problem, we proposed a novel and efficient method to improve their interfacial combination. Polydopamine (PDA) transition layer was coated on the CF by dip coating process, followed by growth of β-FeOOH on single CF, and then polyimide (PI) was introduced by immersion method. β-FeOOH with a special tetrahedral structure enabled more PI matrix to be embedded in the grooves, increasing the surface roughness of CF and contact area with PI. After modifying by PDA and β-FeOOH, the tensile strength and modulus of the composite increased by 46% and 35%, respectively. In addition, tribology test verified that the better wear rate was obtained after introducing PDA and β-FeOOH. All results proved the better reinforcing effect than the unmodified ones. This means that an alternative CF reinforced composites with high strength and wear resistance was developed.
1. Introduction Carbon fiber (CF) and carbon fiber reinforced composites (CFRCs) were widely used in aerospace, automotive, wind power, sports equip ment, electronic device and many other fields because of their excellent physical and chemical properties, such as high specific strength,
modulus, temperature resistance and chemical stability [1–7]. It is well known that the performance of CFRCs are typically depend on the interfacial strength of CF and polymer matrix [8–10]. However, virgin CF was difficult to form desired interfacial bonding with other matrix because of the chemical inertia and low surface energy, which limited the further application of CF in various fields [11,12]. Hence, modifying
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Yang), [email protected] (H. Song). https://doi.org/10.1016/j.matchemphys.2020.122677 Received 2 November 2019; Received in revised form 15 January 2020; Accepted 17 January 2020 Available online 18 January 2020 0254-0584/© 2020 Elsevier B.V. All rights reserved.
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Fig. 1. Preparation process of PDA-FeOOH-CFC/PI.
(PTFE), the typical hydrophobic polymer membranes [27,28]. Further more, the interaction between PDA film and matrix includes covalent and none-covalent chemical bonding, such as hydrogen bonding, π-π bonding and electrostatic bonding [29,30]. Most importantly, the PDA film on other matrix had excellent stability and durability in various environments, excepting in strong alkaline solutions [31]. Wan et al. [32] reported that the new adsorbent prepared by coating amine-functionalized CNTs with PDA was well dispersed in water and easy to be separated from the media. Song et al. [33] coated the outer layer of GO with PDA for surface modification, and used hydrothermal method to grow Cu on the surface of GO to prepare Cu/PDA/GO com posite material and found that the composite had remarkable lubricating effect in base oil. This means that PDA could supply a green, non-destructive and efficient method to modify the surface of certain materials. However, to the best of our knowledge, there has been few reports for the functionalization of CF with PDA as a transition layer to improve the interface between CF and resin matrix. In the present work, PDA was directly introduced as a transition layer between CF and PI matrix, and then β-FeOOH nanoparticles were grown on the surface of CF by hydrothermal method, PI was introduced onto the rough surface of CF by impregnation method in the final process. β-FeOOH contained lots of hydroxyl functional groups (electrostatic forces and hydrogen bond induced –OH formation), which led to strong surface chemical activity and firm interface [34]. The effects of β-FeOOH and PDA on thermal stability, mechanical and tribological properties of CF/PI composites were studied respectively. The satisfactory results were identified by the better mechanical and tribological properties of modified composites, which would provide a practical solution for the application of CFRCs in engineering.
the surface of CF to increase the interfacial bonding strength becomes a critical topic. Generally, CF modification methods were mainly divided into two types: changing the surface chemical composition or growing micro-nano structures on the surface of CF. Several traditional modifi cation methods have been proposed, such as wet chemical [13,14], electrochemical oxidation [15] and plasma treatment [16,17], coating treatment [18–21], and so on. The purpose of these surface treatments was to introduce chemically active functional groups or to change the microstructure of CF surface by etching [11]. However, these treatments have limited effect to improve the interfacial and mechanical properties of the composites. Recently, a variety of works have been reported to further improve the interface bonding strength between CF and resin matrix by grafting micro-nano structures directly on the surface of CF. Mei [22] introduced CNTs on the surface of CF by electrophoretic deposition, the result showed that after depositing CNTs on the surface of CF, the tensile strength and Young’s modulus increased by 9.86% and 12.40% respectively, but the oxidation of CF by nitric acid would destroy the properties of virgin CF unavoidably. Wang [23] grafted a size-controlled GO sheet onto the surface of CF to improve the interfacial properties of CF composites, the tensile strength of the modified composite was found to increase from 4.73 GPa to 5.02 GPa. Meanwhile, the pretreatment of CF and electrophoretic deposition environment later etched the surface of CF seriously. Other methods was developed to improve the interfacial bonding between CF and resin matrix, such as increasing the toughness of CF to form the interfacial interlock effect, but the damage of CF itself not be considered reasonably, so it is critical to develop a moderate method to modify the surface of CF. Chen [24] reported a novel method for enhancing the mechanical and tribological properties of CFRCs using polyamide-amine (PAMAM) as the transition layer, the satisfactory re sults indicated the feasibility of the transition layer. The introduction of transition layer not only protects the CF surface from damage but also provides more reactive groups, which could improve the interfacial bonding, further optimize the mechanical properties without destructing. Dopamine, one of the most important neurotransmitters, could be found from lots of animals and plants [25,26]. It can self-polymerize under moderate reaction conditions and form polydopamine (PDA) film on all types of surfaces, even on the poly(tetrafluoroethylene)
2. Experimental 2.1. Materials Ferric chloride hexahydrate (FeCl3⋅6H2O), sodium fluoride (NaF), dopamine hydrochloride, 4, 40 -oxidianiline (ODA), N, N-dimethylace tamide (DMAC) and pyromellitic dianhydride (PMDA) were supplied by Sinopharm Chemical Reagent Co., Ltd (China). Carbon fiber cloth (CFC) is a type of plain weave T700 fiber consisted of 3K filament produced by 2
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Fig. 2. (a) XRD patterns and (b) Raman spectra of CFC, FeOOH-CFC, and PDA-FeOOH-CFC composites.
Japan Toray. All chemicals were analytical grade and used as received. The commercial CFC was dipped in mixture of ethanol and acetone for 12 h, and then ultrasonic treatment in acetone for 2 h, and oven-dried before being used.
experiment, the rotational speed was 200–500 r/min (3.768–9.42 m/ min), the load was 200–1000 g, and the friction time was 60 min. Then cross-sectional area of the wear surface was measured by the depth-ofdepth 3D graph and the equation of specific wear rate (K, m3/Nm) was exhibited as follows:
2.2. Preparation of PDA-CFC and PDA-FeOOH-CFC
(3)
K ¼ ΔV /PL 3
The pretreated CFC was put into 200 mL Tris buffer and adjusted PH ¼ 8.5 by dilute HCl solution, adding 0.4 g of dopamine hydrochloride and reacting for 12 h under strong magnetic stirring. The product was washed with distilled water and ethanol to remove surface impurities, and finally dried at 60 � C, the final product was marked as PDA-CFC. For PDA-FeOOH-CFC, FeCl3⋅6H2O and NaF were added to distilled water and configured as 100 mL mixed solution and stirred for 2–3 h (the concentrations of FeCl3⋅6H2O and NaF were 0.1 mol/L and 0.125 mol/ L). Then, adding PDA-CFC and 100 mL ethanol to the above mixed so lution, magnetically stirring for 30 min and transferring to a singlenecked flask and reacting at 100 � C for 8 h. The product was marked as PDA-FeOOH-CFC.
Here, ΔV, P and L refer to abrasion loss (mm ), the applied load (N) and sliding length (m), respectively. The samples were repeated 3 times to obtain the average value. 2.6. Characterization The phase structure of CFC, FeOOH-CFC and PDA-FeOOH-CFC composites were investigated by X-ray diffraction (XRD, Bruker, Ger many) over the 2θ range 10–80� . The graphite structure (D and G band) of CFC, FeOOH-CFC and PDA-FeOOH-CFC were characterized by Raman spectroscopy (Thermo Fisher, America). The functional groups and chemical composition of CFC, FeOOH-CFC and PDA-FeOOH-CFC were characterized by X-ray Photoelectron Spectroscopy (XPS, ESCA LAB250Xi, Thermo Fisher, America). Thermal stability of CFC, FeOOHCFC and PDA-FeOOH-CFC and their composites were determined with a thermogravimetric analyzer (TG-DSC, NETZSCHSTA449C, Germany), the temperature went from 25 � C to 800 � C under N2 atmosphere. The morphology of the CFC, FeOOH-CFC and PDA-FeOOH-CFC as well as the tensile and wear surfaces were characterized by the Scanning Electron Microscopy (SEM, JEOL JSM-7200F, Japan).
2.3. Preparation of FeOOH-CFC/PI and PDA-FeOOH-CFC/PI composites The preparation process of PDA-FeOOH-CFC/PI is illustrated in Fig. 1. ODA (5 g) was evenly dispersed in DMAC (70 mL) ice bath system by continuous stirring. Then, 5.559 g of dianhydride (PMDA) was added to the three-necked flask in triplicate reaction for 5 h to obtain a pol yamic acid (PAA). The CFC was immersed into the PAA (15 wt%) for 6 times, then dried until reaching certain mass fractions to prepare pre cursor solution for the preparation of PI composite. Subsequently, the obtained prepregs were solidified at 70 � C for 3 h, then raise 30 � C and stay for 30 min each time from 70 � C to 300 � C.
3. Results and discussion The phase structures of CFC, FeOOH-CFC and PDA-FeOOH-CFC composites were investigated by XRD. The main diffraction peak at around 26� can be attributed to typical diffraction peak of CFs [35]. The diffraction peaks and positions of FeOOH-CFC were consistent with the tetragonal FeOOH (JCPDS no. 75-1594). For PDA-FeOOH-CFC, no different peaks were observed, which could be attributed to the diffraction peaks of PDA were overshadowed by dense β-FeOOH nano particles. The Raman spectra of CFC, FeOOH-CFC, and PDA-FeOOH-CFC composites were shown in Fig. 2b. There were two distinct peaks at about 1580 cm 1 and 1350 cm 1 appearing for CFC, which correspond to the graphite (G) and diamondoid (D) bands, respectively. The D band represents the amorphous structures of carbon, and the G band can be attributed to the graphitic structures of carbon. Generally, the peak in tensity ratio of D and G band (ID/IG) was a benchmark to characterize the degree of graphitization, and the lower value represented the closer to the ideal graphite structure [36]. The ID/IG values (ID/IG) of pre treated CFC was higher than other samples, indicating the pretreated
2.4. Tensile and bonding strength test The tensile properties of CFC/PI, FeOOH-CFC/PI and PDA-FeOOHCFC/PI composites were performed on a UTM 4104 microcomputer control electronic universal testing machine by controlling the tensile rate at 1 mm/min. The dimensions of the samples used for tensile strength tests were 30–40 mm in length, 10–20 mm in width, and 400 � 50 μm in thickness. 2.5. Friction and wear test The tribological properties of the CFC, FeOOH-CFC and PDA-FeOOHCFC samples were investigated by friction and wear tester (MS-T3000). Each sample was fixed on a rotatable disk, sliding friction was performed with a steel ball at a fixed position in this configuration. In this 3
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stability of CFC/PI composites, thermal scans of these samples were carried out from 25 � C to 800 � C by TGA under nitrogen atmosphere (Fig. 4). As shown in Fig. 4a, the three samples exhibited similar trends but varying degrees of weightlessness below 150 � C, which was due to the evaporation of water. A slight weight loss exhibited by CFC from 150 � C to 800 � C, which reflected the good thermal stability of CFC. There was a 2.78% and 8.64% reduction of FeOOH-CFC and PDAFeOOH-CFC, respectively. In addition, the TG analysis of PI matrix composites were shown in Fig. 4b. In general, CFC/PI, FeOOH-CFC/PI and PDA-FeOOH-CFC/PI had similar trend during the weight loss, the three curves exhibited the thermal weight loss of about 16%, which demonstrated that the introduction of PDA and FeOOH had little impact on the PI matrix composites. The surface morphologies of FeOOH-CFC and PDA-FeOOH-CFC were observed by scanning electron microscope (SEM). The rough surface of the CFC was well coated with β-FeOOH particles, which can be seen in Fig. 5a. The red frame region in Fig. 5b showed the surface of the CFs after magnification. In previous studies, the smooth surface of CFs affected the reaction with the matrix, and surface treatment could create some grooves and increased the area of reaction surface to improve the surface activity. In contrast with FeOOH-CFC, β-FeOOH was homoge neously distributed on CFC-PDA. Additionally, it could be noted that the most particles formed a definite shape and kept the same size (Fig. 5c and d). β-FeOOH and PDA immobilized successfully on CFC were confirmed powerfully from the SEM analysis. To investigate the mechanical properties of the composites, the tensile strength and modulus of CFC/PI, FeOOH-CFC/PI and PDAFeOOH-CFC/PI were analyzed (Fig. 6). Compared with CFC/PI, the tensile strength of FeOOH-CFC/PI composite increased to 42 MPa. After the addition of PDA, the tensile strength of composites showed dra matical increasement (from 149.61 MPa to 202.52 MPa), and the final tensile strength was almost twice of the original CFC/PI composite. Tensile modulus of PDA-FeOOH-CFC/PI composite reached 1.55 times of the original CFC/PI composite. The fiber fracture morphology of these composites after stretching was performed to evaluate the influence of PDA and β-FeOOH, as shown in Fig. 7. The interface (Fig. 7a) of CFC/PI was found that CF was pulled out from PI matrix. For FeOOH-CFC/PI (Fig. 7b), most fibers form the compact interfacial bonding with PI matrix, but there were still few fibers pulling out from the matrix. After the introduction of PDA transition layer, PDA-FeOOH-CFC/PI (Fig. 7c) was considered to exhibit better bonding strength between all fibers and matrix after tensile test. It indicates that β-FeOOH and PDA played synergistic roles in the process of CFC functionalization, the interface adhesion between CFC and PI matrix can be improved through rich chemical bond binding, and then effective stress can be transferred easily from PI matrix to CF. In addition, the thick amino functional
Fig. 3. XPS analysis of CFC, FeOOH-CFC and PDA-FeOOH-CFC. Table 1 Surface element compositions of CFC, FeOOH-CFC and PDA-FeOOH-CFC. CFC FeOOH-CFC PDA-FeOOH-CFC
C
N
O
F
Fe
Cl
78.42 66.43 63.79
2.34 2.69 3.74
19.24 20.36 21.18
3.63 3.58
\ 6.22 7.02
\ 0.67 0.69
CFC possessed more defects or disorders because sp2 carbon network was broken during the pretreated process [37]. For another, the lower value of ID/IG of FeOOH-CFC (ID/IG ¼ 0.89) and PDA-FeOOH-CFC (ID/IG ¼ 0.79) suggested that the introduction of PDA coating and FeOOH nanoparticles could change or heal the graphite layer of CFC [38]. XPS wide spectrum was exhibited to determine the chemical composition of the composites surface (Fig. 3) and the detail atom percentage was given in Table 1. It was shown in Fig. 3 that the surface of CFC was mainly composed of C and O elements and the atom per centage of C and O was 78.42% and 19.24%, respectively. For FeOOHCFC and PDA-FeOOH-CFC, the presence of Fe 2p peak (the atom per centage was 6.22% and 7.02%) proved the successful graft of FeOOH nanoparticles. Notably, the existence of F peak probably because the NaF was used to fabricate composites during experimental work. Further, the stronger N 1s peak (3.74%) on PDA-FeOOH-CFC revealed the presence of PDA on the surface of CF. In order to estimate the effect of surface modification on the thermal
Fig. 4. TG of (a) CFC, FeOOH-CFC and PDA-FeOOH-CFC, (b) PI, FeOOH-CFC/PI, and PDA-FeOOH-CFC/PI over the temperature range of 25–800 � C in a heating rate � of 5 C/min under a N2 flow. 4
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Fig. 5. SEM images of (a), (b) FeOOH-CFC and (c), (d) PDA-FeOOH-CFC.
and PDA-FeOOH-CFC/PI composites under the specific conditions were shown in Fig. 8a. The average friction coefficient of CFC/PI was 0.2929, and the friction coefficient of the FeOOH-CFC/PI composite containing FeOOH increased to 0.3438. The lower friction coefficient of CFC/PI could be attributed to the exposure of CFs (shown in Fig. 8b), and the graphite lubrication of CFs can effectively reduce the friction coefficient of the composite. Interestingly, the friction coefficient of PDA-FeOOH-CFC/PI showed the decreased trend compared to FeOOHCFC/PI. That is because the introduction of PDA increased the compatibility on the interface and the load can be partly applied to CF, avoiding large deformation and retaining the flatness of the composite. In order to study the influence of the external conditions on the tribological properties, friction testing under various applied loads and sliding speed were carried out. The results about average friction co efficients and wear rates of CFC/PI, FeOOH-CFC/PI, and PDA-FeOOHCFC/PI under variety sliding speed and load were presented in Fig. 9a–d. The change of wear surfaces of PDA-FeOOH-CFC/PI was shown in Fig. 9d. It can be noted that the same trend was appeared for three samples, friction coefficients under different sliding rate decreased rapidly and reached minimum at 300 r/min, after that, the friction co efficients increased gradually. In Fig. 9c, when load changed from 2 N to 5 N, the friction coefficients dropped dramatically and then rose steadily between 5 N and 10 N. High applied load increased the real contact area
Fig. 6. The tensile strength and tensile modulus of CFC/PI composite, FeOOHCFC/PI composite, and PDA-FeOOH-CFC/PI composite.
coating formed on the surface of CFC can increase the roughness of CFC significantly. The tribological properties of the three samples were investigated by sample-steel contacts. Friction coefficients of CFC/PI, FeOOH-CFC/PI,
Fig. 7. The SEM images of tensile fracture surfaces: (a) CFC/PI (b) FeOOH-CFC/PI (c) PDA-FeOOH-CFC/PI. 5
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Fig. 8. (a) Friction coefficients of CFC/PI composite, FeOOH-CFC/PI composite, and PDA-FeOOH-CFC/PI composite under the constant condition (8 N, 300 r/min, 60 min). Wear surfaces of (b) CFC/PI composite, (c) FeOOH-CFC/PI composite and (d) PDA-FeOOH-CFC/PI composite after friction tests (5 N, 300 r/min, 60 min).
Fig. 9. (a) Average friction coefficients and (b) wear rates of CFC/PI composite, FeOOH-CFC/PI composite, and PDA-FeOOH-CFC/PI composite under different sliding speed (5 N, 200-500r/min, 60min); (c) average friction coefficients of CFC/PI composite, FeOOH-CFC/PI composite, and PDA-FeOOH-CFC/PI composite under different load (2–10 N, 500r/min, 60min); (d) wear rates and wear surfaces of PDA-FeOOH-CFC/PI composite under different load (5–10 N, 500r/min, 60min).
6
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Fig. 10. Friction mechanism of PDA-FeOOH-CFC/PI.
PDA-FeOOH-CFC/PI composite was enhanced significantly.
between the steel ball and composite, at the same time, it can make the stainless steel ball compacted deeply into PI composites, producing high friction, and ultimately improving the friction coefficients and wear rates [39,40]. Moreover, the SEM images of the wear surfaces of PDA-FeOOH-CFC/PI at various load conditions were added in Fig. 9d. The wear surface under higher loads was rougher than those under lower loads. In addition, surficial deposit occurred and some fibers exposed, and PI matrix was crushed into pieces, which corresponds to the higher friction coefficient and wear rate under higher loads. The introduction of PDA transition film and β-FeOOH played a sig nificant role in wear resistance, Fig. 10 showed the friction mechanism of PDA-FeOOH-CFC/PI. Under certain loading and rotating speed, the stainless steel ball easily entered the composite to participate the friction process. For CFC/PI, the PI matrix was damaged easily during the fric tion process due to the weak interface between PI and CF, the CF and steel ball were contacted directly and PI matrix cannot protect the CF, which eventually leads to a high wear rate [41]. For FeOOH-CFC/PI, the introduction of FeOOH strengthened the interface between PI and CF, the effect was not obvious due to particle agglomeration, the wear rate had decreased but needs further improvement. But for PDA-FeOOH-CFC/PI, the introduction of PDA made the β-FeOOH par ticles distributed evenly on the surface of CF, and the load can be uni formly transferred to CF, avoiding the stress concentration caused by particle agglomeration. Furthermore, the mechanical interlock between β-FeOOH and PI matrix was improved and further reduced the pulled fibers when subjected to external load. After numerous cycles of friction, some fibers were broken in the wear area, the binding force between the PI matrix and the fibers was strong that the CF debris was pressed onto the composite surface again, and no cracks appeared on the wear sur face. Therefore, it was proved that the PDA-FeOOH-CFC/PI had better wear resistance performance.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work is supported by the National Natural Science Foundation of China (No. 51875330 and 51975342), and the Natural Science Foundation of Shaanxi Province (No. 2018JZ5003 and 2019JZ-24). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.matchemphys.2020.122677. References [1] S. Giraudet, Activated carbon fiber and textiles, in: A Volume in Woodhead Publishing Series in Textiles, Elsevier, Book, 2017. [2] P.E. Irving, C. Soutis, Polymer Composites in the Aerospace Industry, Elsevier, Book, 2014. [3] C.R. Lu, J. Wang, X. Lu, T. Zheng, Y.Y. Liu, Wettability and interfacial properties of carbon fiber and poly(ether ether ketone) fiber hybrid composite, ACS Appl. Mater. Interfaces 11 (2019) 31520–31531. [4] J. Liu, Y.L. Tian, Y.J. Chen, J.Y. Liang, L.F. Zhang, H. Fong, A surface treatment technique of electrochemical oxidation to simultaneously improve the interfacial bonding strength and the tensile strength of PAN-based carbon fibers, Mater. Chem. Phys. 122 (2010) 548–555. [5] A. Hendlmeier, L.I. Marinovic, S. Al-Assafi, F. Stojcevski, Sizing effects on the interfacial shear strength of a carbon fibre reinforced two-component thermoplastic polymer, Compos. Appl. Sci. Manuf. 127 (2019) 105622. [6] Y.N. Zhang, F.J. Xu, C.Y. Zhang, Tensile and interfacial properties of polyacrylonitrile-based carbon fiber after different cryogenic treated condition, Compos. B Eng. 99 (2016) 358–365. [7] T.T. Yao, Y.T. Liu, H. Zhu, X.F. Zhang, G.P. Wu, Controlling of resin impregnation and interfacial adhesion in carbon fiber/polycarbonate composites by a spraycoating of polymer on carbon fibers, Compos. Sci. Technol. 182 (2019) 107763. [8] X.Q. Zhang, X.Y. Fan, C. Yan, Interfacial microstructure and properties of carbon fiber composites modified with graphene oxide, ACS Appl. Mater. Interfaces 4 (2012) 1543–1552. [9] W.Q. Zhang, X. Deng, G. Sui, Improving interfacial and mechanical properties of carbon nanotube-sized carbon fiber/epoxy composites, Carbon 145 (2019) 629–639. [10] G.J. Ehlert, U. Galan, H.A. Sodano, Role of surface chemistry in adhesion between ZnO nanowires and carbon fibers in hybrid composites, ACS Appl. Mater. Interfaces 5 (3) (2013) 635–645. [11] Y. Liu, X. Zhang, C.C. Song, An effective surface modification of carbon fiber for improving the interfacial adhesion of polypropylene composites, Mater. Des. 88 (2015) 810–819.
4. Conclusion In summary, we introduced an alternative method to improve the interface bonding strength of CF and matrix. The adhesion of PDA increased the surface activity of CF, allowing β-FeOOH to grow uni formly. The coating of β-FeOOH could further functionalize CF and enhanced the compatibility of interface between CF and PI matrix. The modification method used β-FeOOH could introduce rich –OH into CF, which lead to the chemical cross-linking reaction. More importantly, the surface structure of the tetragonal system could form a mechanical interlock with the PI matrix, which improved the interfacial energy between their interfaces. The modified surface area increased the interfacial interaction between CF and matrix, and transferred stress effectively, therefore the mechanical properties and wear resistance of 7
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Polydopamine/FeOOH-modified interface in carbon cloth/polyimide composites for improved mechanical/tribological properties Zhe Lin, Jin Yang *, Xiaohua Jia, Yong Li, Haojie Song ** School of Materials Science & Engineering, Shaanxi Key Laboratory of Green Preparation and Functionalization for Inorganic Materials, Shaanxi University of Science & Technology, Xi’an, Shaanxi, 710021, PR China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� PDA transition layer and FeOOH nano particles were introduced on the sur faces of CFs. � FeOOH nanoparticles are uniformly coated on the surfaces of CFs. � PDA and FeOOH nanoparticles improved the interface between CFs and PI. � PDA-FeOOH-CFC/PI exhibited excellent mechanical and tribological properties.
A R T I C L E I N F O
A B S T R A C T
Keywords: Carbon fiber Polydopamine β-FeOOH Polyimide Mechanical properties Tribological properties
The performance of carbon fiber (CF) reinforced composites depend greatly on the interfacial structures of the fibers and matrix. Based on the problem, we proposed a novel and efficient method to improve their interfacial combination. Polydopamine (PDA) transition layer was coated on the CF by dip coating process, followed by growth of β-FeOOH on single CF, and then polyimide (PI) was introduced by immersion method. β-FeOOH with a special tetrahedral structure enabled more PI matrix to be embedded in the grooves, increasing the surface roughness of CF and contact area with PI. After modifying by PDA and β-FeOOH, the tensile strength and modulus of the composite increased by 46% and 35%, respectively. In addition, tribology test verified that the better wear rate was obtained after introducing PDA and β-FeOOH. All results proved the better reinforcing effect than the unmodified ones. This means that an alternative CF reinforced composites with high strength and wear resistance was developed.
1. Introduction Carbon fiber (CF) and carbon fiber reinforced composites (CFRCs) were widely used in aerospace, automotive, wind power, sports equip ment, electronic device and many other fields because of their excellent physical and chemical properties, such as high specific strength,
modulus, temperature resistance and chemical stability [1–7]. It is well known that the performance of CFRCs are typically depend on the interfacial strength of CF and polymer matrix [8–10]. However, virgin CF was difficult to form desired interfacial bonding with other matrix because of the chemical inertia and low surface energy, which limited the further application of CF in various fields [11,12]. Hence, modifying
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Yang), [email protected] (H. Song). https://doi.org/10.1016/j.matchemphys.2020.122677 Received 2 November 2019; Received in revised form 15 January 2020; Accepted 17 January 2020 Available online 18 January 2020 0254-0584/© 2020 Elsevier B.V. All rights reserved.
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Fig. 1. Preparation process of PDA-FeOOH-CFC/PI.
(PTFE), the typical hydrophobic polymer membranes [27,28]. Further more, the interaction between PDA film and matrix includes covalent and none-covalent chemical bonding, such as hydrogen bonding, π-π bonding and electrostatic bonding [29,30]. Most importantly, the PDA film on other matrix had excellent stability and durability in various environments, excepting in strong alkaline solutions [31]. Wan et al. [32] reported that the new adsorbent prepared by coating amine-functionalized CNTs with PDA was well dispersed in water and easy to be separated from the media. Song et al. [33] coated the outer layer of GO with PDA for surface modification, and used hydrothermal method to grow Cu on the surface of GO to prepare Cu/PDA/GO com posite material and found that the composite had remarkable lubricating effect in base oil. This means that PDA could supply a green, non-destructive and efficient method to modify the surface of certain materials. However, to the best of our knowledge, there has been few reports for the functionalization of CF with PDA as a transition layer to improve the interface between CF and resin matrix. In the present work, PDA was directly introduced as a transition layer between CF and PI matrix, and then β-FeOOH nanoparticles were grown on the surface of CF by hydrothermal method, PI was introduced onto the rough surface of CF by impregnation method in the final process. β-FeOOH contained lots of hydroxyl functional groups (electrostatic forces and hydrogen bond induced –OH formation), which led to strong surface chemical activity and firm interface [34]. The effects of β-FeOOH and PDA on thermal stability, mechanical and tribological properties of CF/PI composites were studied respectively. The satisfactory results were identified by the better mechanical and tribological properties of modified composites, which would provide a practical solution for the application of CFRCs in engineering.
the surface of CF to increase the interfacial bonding strength becomes a critical topic. Generally, CF modification methods were mainly divided into two types: changing the surface chemical composition or growing micro-nano structures on the surface of CF. Several traditional modifi cation methods have been proposed, such as wet chemical [13,14], electrochemical oxidation [15] and plasma treatment [16,17], coating treatment [18–21], and so on. The purpose of these surface treatments was to introduce chemically active functional groups or to change the microstructure of CF surface by etching [11]. However, these treatments have limited effect to improve the interfacial and mechanical properties of the composites. Recently, a variety of works have been reported to further improve the interface bonding strength between CF and resin matrix by grafting micro-nano structures directly on the surface of CF. Mei [22] introduced CNTs on the surface of CF by electrophoretic deposition, the result showed that after depositing CNTs on the surface of CF, the tensile strength and Young’s modulus increased by 9.86% and 12.40% respectively, but the oxidation of CF by nitric acid would destroy the properties of virgin CF unavoidably. Wang [23] grafted a size-controlled GO sheet onto the surface of CF to improve the interfacial properties of CF composites, the tensile strength of the modified composite was found to increase from 4.73 GPa to 5.02 GPa. Meanwhile, the pretreatment of CF and electrophoretic deposition environment later etched the surface of CF seriously. Other methods was developed to improve the interfacial bonding between CF and resin matrix, such as increasing the toughness of CF to form the interfacial interlock effect, but the damage of CF itself not be considered reasonably, so it is critical to develop a moderate method to modify the surface of CF. Chen [24] reported a novel method for enhancing the mechanical and tribological properties of CFRCs using polyamide-amine (PAMAM) as the transition layer, the satisfactory re sults indicated the feasibility of the transition layer. The introduction of transition layer not only protects the CF surface from damage but also provides more reactive groups, which could improve the interfacial bonding, further optimize the mechanical properties without destructing. Dopamine, one of the most important neurotransmitters, could be found from lots of animals and plants [25,26]. It can self-polymerize under moderate reaction conditions and form polydopamine (PDA) film on all types of surfaces, even on the poly(tetrafluoroethylene)
2. Experimental 2.1. Materials Ferric chloride hexahydrate (FeCl3⋅6H2O), sodium fluoride (NaF), dopamine hydrochloride, 4, 40 -oxidianiline (ODA), N, N-dimethylace tamide (DMAC) and pyromellitic dianhydride (PMDA) were supplied by Sinopharm Chemical Reagent Co., Ltd (China). Carbon fiber cloth (CFC) is a type of plain weave T700 fiber consisted of 3K filament produced by 2
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Fig. 2. (a) XRD patterns and (b) Raman spectra of CFC, FeOOH-CFC, and PDA-FeOOH-CFC composites.
Japan Toray. All chemicals were analytical grade and used as received. The commercial CFC was dipped in mixture of ethanol and acetone for 12 h, and then ultrasonic treatment in acetone for 2 h, and oven-dried before being used.
experiment, the rotational speed was 200–500 r/min (3.768–9.42 m/ min), the load was 200–1000 g, and the friction time was 60 min. Then cross-sectional area of the wear surface was measured by the depth-ofdepth 3D graph and the equation of specific wear rate (K, m3/Nm) was exhibited as follows:
2.2. Preparation of PDA-CFC and PDA-FeOOH-CFC
(3)
K ¼ ΔV /PL 3
The pretreated CFC was put into 200 mL Tris buffer and adjusted PH ¼ 8.5 by dilute HCl solution, adding 0.4 g of dopamine hydrochloride and reacting for 12 h under strong magnetic stirring. The product was washed with distilled water and ethanol to remove surface impurities, and finally dried at 60 � C, the final product was marked as PDA-CFC. For PDA-FeOOH-CFC, FeCl3⋅6H2O and NaF were added to distilled water and configured as 100 mL mixed solution and stirred for 2–3 h (the concentrations of FeCl3⋅6H2O and NaF were 0.1 mol/L and 0.125 mol/ L). Then, adding PDA-CFC and 100 mL ethanol to the above mixed so lution, magnetically stirring for 30 min and transferring to a singlenecked flask and reacting at 100 � C for 8 h. The product was marked as PDA-FeOOH-CFC.
Here, ΔV, P and L refer to abrasion loss (mm ), the applied load (N) and sliding length (m), respectively. The samples were repeated 3 times to obtain the average value. 2.6. Characterization The phase structure of CFC, FeOOH-CFC and PDA-FeOOH-CFC composites were investigated by X-ray diffraction (XRD, Bruker, Ger many) over the 2θ range 10–80� . The graphite structure (D and G band) of CFC, FeOOH-CFC and PDA-FeOOH-CFC were characterized by Raman spectroscopy (Thermo Fisher, America). The functional groups and chemical composition of CFC, FeOOH-CFC and PDA-FeOOH-CFC were characterized by X-ray Photoelectron Spectroscopy (XPS, ESCA LAB250Xi, Thermo Fisher, America). Thermal stability of CFC, FeOOHCFC and PDA-FeOOH-CFC and their composites were determined with a thermogravimetric analyzer (TG-DSC, NETZSCHSTA449C, Germany), the temperature went from 25 � C to 800 � C under N2 atmosphere. The morphology of the CFC, FeOOH-CFC and PDA-FeOOH-CFC as well as the tensile and wear surfaces were characterized by the Scanning Electron Microscopy (SEM, JEOL JSM-7200F, Japan).
2.3. Preparation of FeOOH-CFC/PI and PDA-FeOOH-CFC/PI composites The preparation process of PDA-FeOOH-CFC/PI is illustrated in Fig. 1. ODA (5 g) was evenly dispersed in DMAC (70 mL) ice bath system by continuous stirring. Then, 5.559 g of dianhydride (PMDA) was added to the three-necked flask in triplicate reaction for 5 h to obtain a pol yamic acid (PAA). The CFC was immersed into the PAA (15 wt%) for 6 times, then dried until reaching certain mass fractions to prepare pre cursor solution for the preparation of PI composite. Subsequently, the obtained prepregs were solidified at 70 � C for 3 h, then raise 30 � C and stay for 30 min each time from 70 � C to 300 � C.
3. Results and discussion The phase structures of CFC, FeOOH-CFC and PDA-FeOOH-CFC composites were investigated by XRD. The main diffraction peak at around 26� can be attributed to typical diffraction peak of CFs [35]. The diffraction peaks and positions of FeOOH-CFC were consistent with the tetragonal FeOOH (JCPDS no. 75-1594). For PDA-FeOOH-CFC, no different peaks were observed, which could be attributed to the diffraction peaks of PDA were overshadowed by dense β-FeOOH nano particles. The Raman spectra of CFC, FeOOH-CFC, and PDA-FeOOH-CFC composites were shown in Fig. 2b. There were two distinct peaks at about 1580 cm 1 and 1350 cm 1 appearing for CFC, which correspond to the graphite (G) and diamondoid (D) bands, respectively. The D band represents the amorphous structures of carbon, and the G band can be attributed to the graphitic structures of carbon. Generally, the peak in tensity ratio of D and G band (ID/IG) was a benchmark to characterize the degree of graphitization, and the lower value represented the closer to the ideal graphite structure [36]. The ID/IG values (ID/IG) of pre treated CFC was higher than other samples, indicating the pretreated
2.4. Tensile and bonding strength test The tensile properties of CFC/PI, FeOOH-CFC/PI and PDA-FeOOHCFC/PI composites were performed on a UTM 4104 microcomputer control electronic universal testing machine by controlling the tensile rate at 1 mm/min. The dimensions of the samples used for tensile strength tests were 30–40 mm in length, 10–20 mm in width, and 400 � 50 μm in thickness. 2.5. Friction and wear test The tribological properties of the CFC, FeOOH-CFC and PDA-FeOOHCFC samples were investigated by friction and wear tester (MS-T3000). Each sample was fixed on a rotatable disk, sliding friction was performed with a steel ball at a fixed position in this configuration. In this 3
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stability of CFC/PI composites, thermal scans of these samples were carried out from 25 � C to 800 � C by TGA under nitrogen atmosphere (Fig. 4). As shown in Fig. 4a, the three samples exhibited similar trends but varying degrees of weightlessness below 150 � C, which was due to the evaporation of water. A slight weight loss exhibited by CFC from 150 � C to 800 � C, which reflected the good thermal stability of CFC. There was a 2.78% and 8.64% reduction of FeOOH-CFC and PDAFeOOH-CFC, respectively. In addition, the TG analysis of PI matrix composites were shown in Fig. 4b. In general, CFC/PI, FeOOH-CFC/PI and PDA-FeOOH-CFC/PI had similar trend during the weight loss, the three curves exhibited the thermal weight loss of about 16%, which demonstrated that the introduction of PDA and FeOOH had little impact on the PI matrix composites. The surface morphologies of FeOOH-CFC and PDA-FeOOH-CFC were observed by scanning electron microscope (SEM). The rough surface of the CFC was well coated with β-FeOOH particles, which can be seen in Fig. 5a. The red frame region in Fig. 5b showed the surface of the CFs after magnification. In previous studies, the smooth surface of CFs affected the reaction with the matrix, and surface treatment could create some grooves and increased the area of reaction surface to improve the surface activity. In contrast with FeOOH-CFC, β-FeOOH was homoge neously distributed on CFC-PDA. Additionally, it could be noted that the most particles formed a definite shape and kept the same size (Fig. 5c and d). β-FeOOH and PDA immobilized successfully on CFC were confirmed powerfully from the SEM analysis. To investigate the mechanical properties of the composites, the tensile strength and modulus of CFC/PI, FeOOH-CFC/PI and PDAFeOOH-CFC/PI were analyzed (Fig. 6). Compared with CFC/PI, the tensile strength of FeOOH-CFC/PI composite increased to 42 MPa. After the addition of PDA, the tensile strength of composites showed dra matical increasement (from 149.61 MPa to 202.52 MPa), and the final tensile strength was almost twice of the original CFC/PI composite. Tensile modulus of PDA-FeOOH-CFC/PI composite reached 1.55 times of the original CFC/PI composite. The fiber fracture morphology of these composites after stretching was performed to evaluate the influence of PDA and β-FeOOH, as shown in Fig. 7. The interface (Fig. 7a) of CFC/PI was found that CF was pulled out from PI matrix. For FeOOH-CFC/PI (Fig. 7b), most fibers form the compact interfacial bonding with PI matrix, but there were still few fibers pulling out from the matrix. After the introduction of PDA transition layer, PDA-FeOOH-CFC/PI (Fig. 7c) was considered to exhibit better bonding strength between all fibers and matrix after tensile test. It indicates that β-FeOOH and PDA played synergistic roles in the process of CFC functionalization, the interface adhesion between CFC and PI matrix can be improved through rich chemical bond binding, and then effective stress can be transferred easily from PI matrix to CF. In addition, the thick amino functional
Fig. 3. XPS analysis of CFC, FeOOH-CFC and PDA-FeOOH-CFC. Table 1 Surface element compositions of CFC, FeOOH-CFC and PDA-FeOOH-CFC. CFC FeOOH-CFC PDA-FeOOH-CFC
C
N
O
F
Fe
Cl
78.42 66.43 63.79
2.34 2.69 3.74
19.24 20.36 21.18
3.63 3.58
\ 6.22 7.02
\ 0.67 0.69
CFC possessed more defects or disorders because sp2 carbon network was broken during the pretreated process [37]. For another, the lower value of ID/IG of FeOOH-CFC (ID/IG ¼ 0.89) and PDA-FeOOH-CFC (ID/IG ¼ 0.79) suggested that the introduction of PDA coating and FeOOH nanoparticles could change or heal the graphite layer of CFC [38]. XPS wide spectrum was exhibited to determine the chemical composition of the composites surface (Fig. 3) and the detail atom percentage was given in Table 1. It was shown in Fig. 3 that the surface of CFC was mainly composed of C and O elements and the atom per centage of C and O was 78.42% and 19.24%, respectively. For FeOOHCFC and PDA-FeOOH-CFC, the presence of Fe 2p peak (the atom per centage was 6.22% and 7.02%) proved the successful graft of FeOOH nanoparticles. Notably, the existence of F peak probably because the NaF was used to fabricate composites during experimental work. Further, the stronger N 1s peak (3.74%) on PDA-FeOOH-CFC revealed the presence of PDA on the surface of CF. In order to estimate the effect of surface modification on the thermal
Fig. 4. TG of (a) CFC, FeOOH-CFC and PDA-FeOOH-CFC, (b) PI, FeOOH-CFC/PI, and PDA-FeOOH-CFC/PI over the temperature range of 25–800 � C in a heating rate � of 5 C/min under a N2 flow. 4
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Fig. 5. SEM images of (a), (b) FeOOH-CFC and (c), (d) PDA-FeOOH-CFC.
and PDA-FeOOH-CFC/PI composites under the specific conditions were shown in Fig. 8a. The average friction coefficient of CFC/PI was 0.2929, and the friction coefficient of the FeOOH-CFC/PI composite containing FeOOH increased to 0.3438. The lower friction coefficient of CFC/PI could be attributed to the exposure of CFs (shown in Fig. 8b), and the graphite lubrication of CFs can effectively reduce the friction coefficient of the composite. Interestingly, the friction coefficient of PDA-FeOOH-CFC/PI showed the decreased trend compared to FeOOHCFC/PI. That is because the introduction of PDA increased the compatibility on the interface and the load can be partly applied to CF, avoiding large deformation and retaining the flatness of the composite. In order to study the influence of the external conditions on the tribological properties, friction testing under various applied loads and sliding speed were carried out. The results about average friction co efficients and wear rates of CFC/PI, FeOOH-CFC/PI, and PDA-FeOOHCFC/PI under variety sliding speed and load were presented in Fig. 9a–d. The change of wear surfaces of PDA-FeOOH-CFC/PI was shown in Fig. 9d. It can be noted that the same trend was appeared for three samples, friction coefficients under different sliding rate decreased rapidly and reached minimum at 300 r/min, after that, the friction co efficients increased gradually. In Fig. 9c, when load changed from 2 N to 5 N, the friction coefficients dropped dramatically and then rose steadily between 5 N and 10 N. High applied load increased the real contact area
Fig. 6. The tensile strength and tensile modulus of CFC/PI composite, FeOOHCFC/PI composite, and PDA-FeOOH-CFC/PI composite.
coating formed on the surface of CFC can increase the roughness of CFC significantly. The tribological properties of the three samples were investigated by sample-steel contacts. Friction coefficients of CFC/PI, FeOOH-CFC/PI,
Fig. 7. The SEM images of tensile fracture surfaces: (a) CFC/PI (b) FeOOH-CFC/PI (c) PDA-FeOOH-CFC/PI. 5
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Fig. 8. (a) Friction coefficients of CFC/PI composite, FeOOH-CFC/PI composite, and PDA-FeOOH-CFC/PI composite under the constant condition (8 N, 300 r/min, 60 min). Wear surfaces of (b) CFC/PI composite, (c) FeOOH-CFC/PI composite and (d) PDA-FeOOH-CFC/PI composite after friction tests (5 N, 300 r/min, 60 min).
Fig. 9. (a) Average friction coefficients and (b) wear rates of CFC/PI composite, FeOOH-CFC/PI composite, and PDA-FeOOH-CFC/PI composite under different sliding speed (5 N, 200-500r/min, 60min); (c) average friction coefficients of CFC/PI composite, FeOOH-CFC/PI composite, and PDA-FeOOH-CFC/PI composite under different load (2–10 N, 500r/min, 60min); (d) wear rates and wear surfaces of PDA-FeOOH-CFC/PI composite under different load (5–10 N, 500r/min, 60min).
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Fig. 10. Friction mechanism of PDA-FeOOH-CFC/PI.
PDA-FeOOH-CFC/PI composite was enhanced significantly.
between the steel ball and composite, at the same time, it can make the stainless steel ball compacted deeply into PI composites, producing high friction, and ultimately improving the friction coefficients and wear rates [39,40]. Moreover, the SEM images of the wear surfaces of PDA-FeOOH-CFC/PI at various load conditions were added in Fig. 9d. The wear surface under higher loads was rougher than those under lower loads. In addition, surficial deposit occurred and some fibers exposed, and PI matrix was crushed into pieces, which corresponds to the higher friction coefficient and wear rate under higher loads. The introduction of PDA transition film and β-FeOOH played a sig nificant role in wear resistance, Fig. 10 showed the friction mechanism of PDA-FeOOH-CFC/PI. Under certain loading and rotating speed, the stainless steel ball easily entered the composite to participate the friction process. For CFC/PI, the PI matrix was damaged easily during the fric tion process due to the weak interface between PI and CF, the CF and steel ball were contacted directly and PI matrix cannot protect the CF, which eventually leads to a high wear rate [41]. For FeOOH-CFC/PI, the introduction of FeOOH strengthened the interface between PI and CF, the effect was not obvious due to particle agglomeration, the wear rate had decreased but needs further improvement. But for PDA-FeOOH-CFC/PI, the introduction of PDA made the β-FeOOH par ticles distributed evenly on the surface of CF, and the load can be uni formly transferred to CF, avoiding the stress concentration caused by particle agglomeration. Furthermore, the mechanical interlock between β-FeOOH and PI matrix was improved and further reduced the pulled fibers when subjected to external load. After numerous cycles of friction, some fibers were broken in the wear area, the binding force between the PI matrix and the fibers was strong that the CF debris was pressed onto the composite surface again, and no cracks appeared on the wear sur face. Therefore, it was proved that the PDA-FeOOH-CFC/PI had better wear resistance performance.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work is supported by the National Natural Science Foundation of China (No. 51875330 and 51975342), and the Natural Science Foundation of Shaanxi Province (No. 2018JZ5003 and 2019JZ-24). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.matchemphys.2020.122677. References [1] S. Giraudet, Activated carbon fiber and textiles, in: A Volume in Woodhead Publishing Series in Textiles, Elsevier, Book, 2017. [2] P.E. Irving, C. Soutis, Polymer Composites in the Aerospace Industry, Elsevier, Book, 2014. [3] C.R. Lu, J. Wang, X. Lu, T. Zheng, Y.Y. Liu, Wettability and interfacial properties of carbon fiber and poly(ether ether ketone) fiber hybrid composite, ACS Appl. Mater. Interfaces 11 (2019) 31520–31531. [4] J. Liu, Y.L. Tian, Y.J. Chen, J.Y. Liang, L.F. Zhang, H. Fong, A surface treatment technique of electrochemical oxidation to simultaneously improve the interfacial bonding strength and the tensile strength of PAN-based carbon fibers, Mater. Chem. Phys. 122 (2010) 548–555. [5] A. Hendlmeier, L.I. Marinovic, S. Al-Assafi, F. Stojcevski, Sizing effects on the interfacial shear strength of a carbon fibre reinforced two-component thermoplastic polymer, Compos. Appl. Sci. Manuf. 127 (2019) 105622. [6] Y.N. Zhang, F.J. Xu, C.Y. Zhang, Tensile and interfacial properties of polyacrylonitrile-based carbon fiber after different cryogenic treated condition, Compos. B Eng. 99 (2016) 358–365. [7] T.T. Yao, Y.T. Liu, H. Zhu, X.F. Zhang, G.P. Wu, Controlling of resin impregnation and interfacial adhesion in carbon fiber/polycarbonate composites by a spraycoating of polymer on carbon fibers, Compos. Sci. Technol. 182 (2019) 107763. [8] X.Q. Zhang, X.Y. Fan, C. Yan, Interfacial microstructure and properties of carbon fiber composites modified with graphene oxide, ACS Appl. Mater. Interfaces 4 (2012) 1543–1552. [9] W.Q. Zhang, X. Deng, G. Sui, Improving interfacial and mechanical properties of carbon nanotube-sized carbon fiber/epoxy composites, Carbon 145 (2019) 629–639. [10] G.J. Ehlert, U. Galan, H.A. Sodano, Role of surface chemistry in adhesion between ZnO nanowires and carbon fibers in hybrid composites, ACS Appl. Mater. Interfaces 5 (3) (2013) 635–645. [11] Y. Liu, X. Zhang, C.C. Song, An effective surface modification of carbon fiber for improving the interfacial adhesion of polypropylene composites, Mater. Des. 88 (2015) 810–819.
4. Conclusion In summary, we introduced an alternative method to improve the interface bonding strength of CF and matrix. The adhesion of PDA increased the surface activity of CF, allowing β-FeOOH to grow uni formly. The coating of β-FeOOH could further functionalize CF and enhanced the compatibility of interface between CF and PI matrix. The modification method used β-FeOOH could introduce rich –OH into CF, which lead to the chemical cross-linking reaction. More importantly, the surface structure of the tetragonal system could form a mechanical interlock with the PI matrix, which improved the interfacial energy between their interfaces. The modified surface area increased the interfacial interaction between CF and matrix, and transferred stress effectively, therefore the mechanical properties and wear resistance of 7
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