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Tribological performances of epoxy resin composite coatings using hexagonal boron nitride and cubic boron nitride nanoparticles as additives
Chemical Physics Letters 732 (2019) 136646
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Tribological performances of epoxy resin composite coatings using hexagonal boron nitride and cubic boron nitride nanoparticles as additives
T
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Wenchao Zhaoa,b, Wenjie Zhaoa, , Zhiping Huanga,b, Gang Liua, Bin Wua a Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China b University of Chinese Academy of Sciences, Beijing 100049, China
H I GH L IG H T S
and CBN were added to the epoxy resin as binary nanofillers. • HBN BN nanoparticles modified by polydopamine displayed great dispersibility. • The • The tribological mechanisms of HBN and CBN were systematically studied.
A R T I C LE I N FO
A B S T R A C T
Keywords: Hexagonal boron nitride Cubic boron nitride Polydopamine Tribological performance
Hexagonal boron nitride and cubic boron nitride were modified by polydopamine to improve dispersion in epoxy resin (EP). The average coefficient of friction (COF) of EP-HBN@PDA coating decreased under dry and seawater condition compared to EP indicating BN possessed self-lubricating effect. COF decreased with CBN content raising in seawater, because seawater removed debris including BN particles and CBN with high hardness improved rigidity of coatings. Wear rates of EP-CBN@PDA coating decreased by 68.88% and 97.95% compared to EP-HBN@PDA, and binary-filled coating performed excellently under seawater, it revealed CBN was more effective than HBN in improving load carrying capacity of coatings.
1. Introduction As all we know, polymer composite materials have been applied widely as structural materials in the aerospace, automotive and chemical industries due to their lightweight and excellent tensile strength [1,2]. EP is a unique kind of polymer material because of its comparatively low shrinkage after curing, excellent mechanical properties, great chemical stability, and high strength of adhesive joints [3,4], which has been widely used in many fields like protective coating, composites and adhesives. But the EP polymer has a poor anti-wear performance that is due to the fact its network structure formed during curing [5,6]. Therefore, it is an effective method to add nanofillers to epoxy resin matrix to cope with harsh application environment. For instance, SiO2 [7,8], nanodiamond [9,10], carbon nanotubes [11,12], graphene [13,14], graphene oxide [15] and other nanofillers have been used widely in improving the tribological properties of EP. Boron nitride (BN) is a new type of micro-material, it has many excellent properties. Other than hexagonal structure, BN has many other crystal forms, such like cubic structure, amorphous structure and ⁎
wurtzite lattices. HBN which known as “white graphene” owns a crystalline structure same as graphene and got a lot of attention from research scholars [16–18]. HBN is an insulating isomorph of graphite with boron and nitrogen atoms occupying the inequivalent A and B sublattices in the Bernal structure. HBN is relatively inert and is expected to be free of dangling bonds or surface charge traps owing to the strong, in-plane, ionic bonding of the planer hexagonal lattice structure [19–21]. Different from graphene, the bonding “lip-lip” exiting in the layers of HBN is stronger than the Van der Waals force between the layers of graphene [22] And HBN possess electrical insulation, excellent lubricity, thermal and chemical stability [23,24]. Cubic boron nitride (CBN) is a very promising material as a hard coating because of its many desirable properties including a hardness second only to diamond, oxidation resistance and chemical inertness against iron. Miyake et al. [25] deposited CBN and HBN films onto a silicon substrate by a magnetically enhanced plasma ion plating method. The results by comparison demonstrated that the CBN film exhibited the highest wear resistance under all microscopic and macroscopic sliding loads, and the lubrication performance of CBN film was also obvious, the lubrication
Corresponding author. E-mail address: [email protected] (W. Zhao).
https://doi.org/10.1016/j.cplett.2019.136646 Received 8 July 2019; Received in revised form 29 July 2019; Accepted 30 July 2019 Available online 30 July 2019 0009-2614/ © 2019 Elsevier B.V. All rights reserved.
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ultrasonic treatment for 3 h in sonic bath afterwards. Then the reaction was magnetically stirred and oil bathed at 60 °C for 24 h. After that, we centrifuged the mixture and washed the mixture with deionized water several times until the color of the filtrate turned to be transparent. After drying treatment of the grey products at 80 °C for 24 h, we got the polydopamine-grafted BN.
life was long, and the friction was low. Yu et al. [26] prepared a batch of composite epoxy resin coatings enhanced with functionalized CBN and functionalized HBN successfully on 316L stainless steel. The results revealed that the presence of functionalized CBN or functionalized HBN fillers could significantly reduce friction and wear of epoxy, and CBN gave the composite EP coatings the best wear resistance, and HBN made the coatings’ friction reduction effect the best which was due to the selflubricating performance of lamella HBN sheet. Inspired by the above, choosing HBN and CBN to reinforce the tribological performances of epoxy resin is feasible. To the best of our knowledge, the combination of HBN and CBN as the filler to enhance the tribology performance was also rare. Therefore, the investigation of tribological performance of composite EP coatings under both of dry friction and seawater friction by means of adjustment of the content and the type of BN filler is significant and novel. The corresponding morphologies, structure properties, chemical composition and thermal stability were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectrum, X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TG). Composite coatings’ COFs were researched by UMT-3, and composite coatings’ wear rates were investigated by surface profile. The working mechanism of the composite coatings including multiple proportion of BN fillers was illustrated based on the experimental results obtained in this work.
We prepared six sets of samples (EP, F-HBN, HC21, HC11, HC12, FCBN), and the mass fraction of the polydopamine-grafted BN fillers in the composite material was 10 wt%. The preparation process was as follows: 0.15 g HBN@PDA powder was put into xylene with ultrasonic for 30 min to get a well dispersed suspension, and then 0.68 g EP was put into the suspension with ultrasonic for 30 min to obtain a uniform mixture. Then 0.17 g curing agent was put into the mixture, which was stirred for 5 min and degassed at 40 °C in a vacuum drying oven for 15 min to remove the air bubbles and the residual xylene. The final mixture was coated on the 304L stainless steel surface by a wire bar coater which was washed by acetone before use, then the composite coating was placed at room temperature to be cured for 5 days. The thickness of the obtained coating was 200 μm. In addition to different quality ratios, preparation processes of other coatings were the same with above process (see Fig. 1).
2. Experimental
2.4. Characterization
The series samples were prepared, for simplicity, we give each sample a code as listed in Table 1.
The crystalline structure of BN was characterized by XRD (D8 Advance Davinci, Bruker). Scanning the sample from 20° to 100° at a scan rate of 10° per minute. The observation of BN micromorphology before and after chemical modification was performed by SEM (S4800, Hitachi), and the thickness of coated polydopamine on the surface of BN was observed by TEM (Talos F200x, ThemoFisher). The element composition and element content of BN fillers before and after chemical modification were analyzed by XPS (Axis Ultra DLD, Kratos). Raman spectroscopy (Renishaw inVia Reflex, Renishaw) was used to analyze the molecular structure of the BN fillers. TG/DTA (Diamond TG/DTA, PerkinElmer) was carried to characterize the stability of BN fillers.
2.3. Preparation of EP-BN composite coatings
2.1. Materials The epoxy resin (TC-K44) and curing agent (T31) were provided by Jiangyin Chemicals Co., Ltd., China. Cubic boron nitride (CBN, average size is 5 μm) was supplied by FUNIK Ultrahard Material Co., Ltd., China. In addition, dopamine hydrochloride and hexagonal boron nitride (HBN, with particle size of 1–2 μm) were provided by Aladdin Reagent Co. Ltd., China. Solvent including xylene and alcohol were supplied by Sinopharm Chemical Reagent Co., Ltd., China. 2.2. Chemical modification of HBN and CBN
2.5. Tribological measurements
Because rare functional groups exist on the outside of BN nanopaticles, we performed surface hydroxylation of BN to get better graft effect. The process of dopamine-modified boron nitride was carried out according to the literature. First of all, BN was treated at 100 °C for 24 h in a 5 M NaOH solution [27,28], then we centrifuged the mixture and washed BN with deionized water to neutral status. After that, the centrifuged BN precipitation was dried at 60 °C for 24 h in the furnace, collected after cooling. We grafted polydopamine onto the hydroxylated BN to dispel the reunite of CBN and HBN for better dispersion in epoxy resin. We put 0.726 g Tris(hydroxymethyl)aminomethane into 600 ml deionized water, and adjusted pH value to 8.5 by 1 M hydrochloric acid. The dopamine (1.2 g) and hydroxylated BN were added and the color of the mixture turned into grey gradually. Did mixture
UMT-3 (CETR) was carried out to characterize tribological properties of the coatings with a reciprocating ball-on-plan friction experiment, the experimental scheme was as follows: the loading force was 5 N, the reciprocating sliding frequency was 5 Hz, the length of friction track was 5 mm and the sliding time was 20 min, and we equipped GCr15 ball with 3 mm in diameter as the stationary upper counterparts, acetone was used to remove oil stain on the GCr15 ball before testing. In addition to the above, all samples were tested in both of dry and seawater environment. In the friction process performed in seawater, the load was 10 N, other experimental parameters were the same with the above. At last, surface profiler (Alpha-Step IQ, KLA-Tencor) was carried out to investigate the wear tracks, and the wear rates of wear tracks were calculated by the formula below.
Table 1 Composition of filler and experimental code of the coatings. Composition
Code
Epoxy HBN@PDA HBN@PDA:CBN@PDA = 2:1 HBN@PDA:CBN@PDA = 1:1 HBN@PDA:CBN@PDA = 1:2 CBN@PDA
EP F-HBN HC21 HC11 HC12 F-CBN
W=
V S×d = F×L F×T×f×d×2
In which, W refers to wear rate (mm3 N−1m−1), V refers to wear volume (mm3), F refers to loading force (N), L refers to sliding distance (m), T refers to sliding time (min), d refers to length of wear tracks (mm), S refers to wear tracks’ cross-sectional area (mm2), f refers to sliding frequency (Hz). The sectional appearance of the specimens were characterized by 2
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Fig. 1. Preparation route of composite EP coating.
centering at about 1581 cm−1 also appeared after chemical modification which was resulted from the deformation of catechol moiety from polydopamine grafted. Other than this, TGA and XPS of BN powder before and after chemical modification can indicate that polydopamine has been chemically grafted on the surface of BN. Fig. 4(a) and (b) can depict the thermal degradation behaviors of the BN powder and the decorated BN powder in the temperature range of 30–800 °C. With the gradual increase of temperature, there was no loss in the quality of pristine BN as indicated in Fig. 4(a) and (b). The curve of HBN@PDA and CBN@PDA started to fall at the temperature about 330 °C. This revealed that a loss of mass happened to the polydopamine-decorated BN, and the loss was due to the fact the thermal decomposition of polydopamine. By calculating the weight loss rate of HBN@PDA and CBN@PDA, we can get the mass fraction of polydopamine on HBN@PDA was about 3.67% and the mass fraction of polydopamine on CBN@PDA was about 0.61%. As shown in Fig. 4(c) and (d), we analyzed the elemental content of BN powder sample before and after chemical modification by XPS. It can be found that the atomic percentage of B, N, C and O of pristine HBN were 28.49%, 30.9%, 30.45% and 10.16% respectively, but the atomic percentage of B, N, C and O of HBN@PDA were 13.53%, 15.32%, 54.95% and 16.2% respectively. The atomic percentage of C and O increased apparently and the atomic percentage of B and N decreased which
SEM (FEI Quanta FEG 250, FEI), and the surface morphologies of the wear tracks were investigated by SEM (FEI Quanta FEG 250, FEI). 3. Results and discussion 3.1. Composition characterization As shown in Fig. 2(a) and (c), pristine HBN showed a sheet with sharp edges and smooth surface, pristine CBN displayed irregular granular and uneven surface. After chemical modification, HBN@PDA sheets was covered by a substance and displayed smooth edges and rough surface, CBN@PDA displayed rough surface with many granular substance distributing on its surface, as can be seen in Fig. 2(b) and (d). Before and after chemical modification, the shape of HBN and CBN didn’t change, and what had changed resulted from the chemical graft of polydopamine. In Fig. 3(a), pristine HBN showed a sharp characteristic peak centered at 1366 cm−1 which was due to the E2g phonon mode and analogy to the G peak of graphene [29]. There was a new peak emerging at 1581 cm−1 which was related to the deformation of catechol moiety of polydopamine [30]. The pristine CBN showed two strong peaks at 1050 cm−1 and 1301 cm−1 respectively in Fig. 3(b), which were due to the scattering of the transverse optical (TO) and longitudinal optical (LO) phonon modes of CBN [31]. A new wide peak
Fig. 2. SEM images of HBN (a), HBN@PDA (b), and CBN (c), CBN@PDA (d). 3
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Fig. 3. Raman Spectra of HBN and HBN@PDA (a) and CBN and CBN@PDA (b).
resolution TEM images. As Fig. 5(b) and (c) showed, a continuous thin layer was observed on the surface of HBN@PDA and CBN@PDA, respectively, indicating that the polydopamine coated the nanoparticles successfully [33].
resulted from the introduction of polydopamine. It also can be found that the atomic percentage of B, N, C and O of pristine CBN were 32.91%, 32.85%, 21.69% and 12.55% respectively, but the atomic percentage of B, N, C and O of CBN@PDA were 13.95%, 16.81%, 54.53 and 14.71% respectively. The atomic percentage of C and O increased apparently and the atomic ratios of B and N decreased which resulted from the introduction of polydopamine. In Fig. 5 we could observe sharp peaks at 2θ = 26.82°, 41.72° and 43.97° on the curve of HBN and HBN@PDA which were corresponded to the (0 0 2), (1 0 0) and (1 0 1) faces [32]. Other than this, the strong sharp peaks appeared at 2θ = 43.31°, 50.41° and 74.14° of CBN and CBN@PDA which were corresponded to the (1 1 0), (2 2 0) and (2 2 0). It can be observed that the position of peaks of grafted BN were almost identical to the original which revealed that pristine BN and decorated BN were highly crystalline and the process of modification did not change BN’s structure [5]. Moreover, we can further confirm that functionalization of HBN and CBN was successful through the high-
3.2. Morphological characteristics From the cross-section morphology of the pure EP in Fig. 6, we could observe many long cracks and uniform sections without defects which indicated that the surface had brittle fracture. When the BN filler were added into EP matrix, the fracture surface became rough. As shown in Fig. 6(b), (c), (d), (e), (f), when the mass fraction of HBN@ PDA increased, the fracture surface became rougher and rougher, which indicated that the effect of granular CBN@PDA on the fracture surface was not as great as that of flaky HBN@PDA on the fracture surface. In addition, comparing Fig. 6(b) and (f), we could find that particles agglomeration on F-CBN fracture morphology was weaker than F-HBN,
Fig. 4. Thermal degradation of pristine HBN, HBN@PDA (a), and pristine CBN, CBN@PDA (b). XPS of pristine HBN and HBN@PDA (c), pristine CBN and CBN@PDA (d). 4
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Fig. 5. XRD patterns of HBN, HBN@PDA, CBN and CBN@PDA (a), TEM images of HBN@PDA (b) and CBN@PDA (c).
As shown in Fig. 7, the hardness value of composite EP coatings increased when the BN fillers were added into the matrix, the hardness value of F-HBN coating increased by 7.50% compared with the hardness of neat EP coating. Moreover, the hardness value of F-CBN coating increased by 17.58% compared with the hardness of neat EP coating. And when the binary BN fillers were added to the EP matrix, the hardness values were higher than F-HBN. This was due to the fact that
which was due to that the lamellar HBN had a larger specific surface area than the granular CBN and the interaction force between HBN was stronger than CBN. Other than this, the fracture form of EP-BN composite coatings was different from that of EP coating. It could be observed that the BN fillers inhabited the development of long cracks in the area which were marked by the yellow loop. The fracture form of EP-BN composite coatings was not brittle fracture.
Fig. 6. The cross-section morphologies of pure EP coating (a) and the composite EP coatings with different mass ratios as F-HBN (b), HC21 (c), HC11 (d), HC12 (e), FCBN (f). 5
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seawater sliding were much lower than COFs in dry sliding, because seawater played the role as lubricant in the friction process. It was observed that the average COF decreased when we add BN fillers into the epoxy matrix. The average COF of F-HBN coating decreased by 4.09% compared with EP coating, and the friction reduction effect was weaker than that of friction under dry sliding. It was due to the fact that the erosion of seawater weakened the friction reduction effect of HBN which was exposed in the interface where the friction pair contacted. Interestingly, when the binary BN fillers were added into the epoxy matrix, the average COF of coating decreased obviously compared with F-HBN coating which was different from dry sliding. The average COFs of HC21, HC11 and HC12 coating were decreased by 25.82%, 25.5% and 27.04% respectively compared with F-HBN coating which indicated that the introduction of CBN@PDA decreased the COFs of coatings. The average COF of F-CBN coating decreased by 33.37% and 30.53% respectively compared with EP coating and F-HBN coating. Firstly, F-CBN with high hardness increased the rigidity of the coatings compared to F-HBN. Secondly, the seawater could take away the BN particles which were exposed to the friction pair interface during wear process. Finally, seawater could take away the frictional heat produced in the sliding process to reduce plastic deformation and the spalling of composite coating [26]. At the same time, the BN fillers reduced the cracks expanding, and enhanced the toughness of the composite EP coating and reduced the peeling of coating surface [34,35]. Fig. 9 showed the cross-section profiles and wear rate of the composite coating in both of dry sliding and seawater sliding, respectively. The cross-section profiles clearly showed the wear of the composite EP coatings as shown in Fig. 9(a1) and (a2). EP showed a maximum depth of wear crack with 46 µm under dry sliding, and the depths of wear cracks produced on HC21 and F-CBN were 14 µm and 12 µm respectively under dry sliding. F-CBN and HC21 composite coating performed excellent anti-wear behaviors under dry condition with wear rates of 0.55 × 10−4 mm3/N·m and 0.63 × 10−4 mm3/N·m which were decreased by 87.12% and 85.24% respectively compared with pure epoxy (4.27 × 10−4 mm3/N·m). As can be seen from Fig. 9(b1) and (b2), in the case of seawater sliding, EP showed a depth of 68 µm, the depths of
Fig. 7. The hardness of pure EP coating and EP-BN composite coatings.
CBN possessed the second highest hardness and was granular, but HBN displayed a layered structure and was softer compared with CBN. 3.3. Tribological properties As shown in Fig. 8(a1), the curves of COFs increased sharply at first and achieved a steady state under dry sliding. It could be observed from Fig. 8(a2), the average COF of BN-EP composite coating decreased by 8.81% when we added HBN@PDA into the epoxy matrix compared with neat EP coating which was attributed to the layered structure of HBN showing excellent lubrication performance. However, the average COF started raising when the proportion of CBN@PDA increased, and the value was larger than epoxy coating when the filler was all CBN@ PDA. As all we know, CBN particles possessed high hardness. Other than this, when the composite coating was worn, CBN particles were exposed to the interface where the friction pair contacted, resulting in an increase in the COF. Fig. 8(b1) and (b2) showed us that COFs in
Fig. 8. COFs and average COF of HBN@PDA/CBN@PDA/EP coatings in dry sliding (a1, b1), and in seawater sliding (b1, b2). 6
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Fig. 9. Cross-section profiles of the HBN@PDA/CBN@PDA/EP coatings under dry sliding (a1), and seawater condition (b1). Wear rate of the HBN@PDA/CBN@ PDA/EP coatings in dry sliding (a2), and in seawater condition (b2).
Fig. 10(a) showed that many hollow pits existed on the bottom of the wear crack which was attributed to peeling of EP matrix. In addition, Fig. 10(b) and (c) represented that the degree of peeling was much weaker than EP coating. However, Fig. 10(d) and (e) showed that many long cracks which could expand gradually to broke the coating. And Fig. 10(f) showed that some short and tiny cracks generated because of the effect of CBN@PDA, CBN@PDA as a kind of hard filler could increase the load carrying capacity of the coating and decrease the cracks expanding. Fig. 11(a) showed the anti-wear properties of pure epoxy under seawater condition. It was found that EP coating was peeling off piece by piece. In addition, obvious wear can be observed on the crack of F-HBN (Fig. 11b), this damage was more like pitting. This was due to the fact that the neat EP exhibited brittle features and HBN@PDA filler increased the toughness of the composite coating. However, when the filler was composed of HBN@PDA and CBN@PDA, the bottom of cracks turned to be smoother and flatter, CBN@PDA raised the rigidity of the coating. Fig. 11(f) represented some light scratch generated on the coating whose filler was all CBN@PDA under seawater condition. Fig. 12 further revealed the wear mechanism of the composite EPBN coatings. No matter under dry sliding or seawater sliding, the coatings went through the occurrence of tiny cracks, cracks expanded wavily, fatigue wear and coating peeling off in friction process [36]. First of all, in the presence of shear stress, pure epoxy material with brittle features easily produced long and wide cracks. Afterwards, cracks expanded and peeling appeared in the matrix, leading to high wear rate of neat EP coating. In order to improve the tribological properties of epoxy coating, some fillers like BN were necessary needed, which not only can increase hardness but also improve toughness of the composite coatings. The average COF of EP coating decreased obviously indicting a friction reduction effect occurred by the incorporation of HBN@PDA under dry or seawater sliding. This was due to that the layered structure of HBN@PDA showed low shear force and excellent lubricity. However, anti-wear performance of F-HBN coating was not as good as F-CBN coating, because lower hardness could cause fatigue wear of the coatings, which in turn caused high wear rates. When the filler was all CBN@PDA, excellent anti-wear behavior could be
wear cracks of HC21, HC12 and F-CBN were 2.7 µm, 3.6 µm, and 1.3 µm respectively. The wear rate of HC21, HC12 and F-CBN coating were as low as 0.64 × 10−5 mm3/N·m, 0.55 × 10−5 mm3/N·m and as 0.28 × 10−5 mm3/N·m which were decreased by 98.13%, 98.39% and 99.18% compared with pure EP coating (3.415 × 10−4 mm3/N·m). It represented clearly the BN fillers would greatly reduce the wear rate of EP composite coating. Other than this, the EP-BN composite coating represented more excellent anti-wear properties under seawater condition compared to the dry sliding condition because of the effect of seawater which was mentioned above. The wear rates of F-HBN coating were 1.7673 × 10−4 mm3/N·m and 1.3689 × 10−4 mm3/N·m under dry sliding and seawater condition respectively which were larger than that of F-CBN with 0.55 × 10−4 mm3/N·m and 0.28 × 10−5 mm3/N·m. It was due to that HBN was not as hard as CBN, and the CBN with much higher hardness could increase the rigidity and the loading capacity of the composite coating greatly, but the enhancement of HBN was not as good as CBN. When the fillers were composed of CBN@PDA and HBN@PDA, the composite EP-BN coating also showed excellent anti-wear properties. Especially in seawater condition, when the proper amount of CBN@ PDA was added into the F-HBN coating, the wear rate was greatly reduced, which further demonstrated that CBN could greatly increase the load carrying capacity of the coating and reduce the wear rate of the EP-BN composite coating. At the same time, HBN was exposed to the interface where friction pair contacted during the process of sliding, which increased the lubrication effect under dry sliding condition. However, as the proportion of CBN content increased, the hard particles CBN in the wear debris exacerbated the damage of the coating when the coating started breaking under dry sliding. Figs. 10 and 11 represented the wear tracks microscopic morphologies of the composite EP coatings in both of seawater sliding and dry sliding. The wear cracks of composite coatings under dry sliding were wider than the corresponding wear cracks under seawater sliding, even though the load under seawater was 10 N compared with 5 N under dry sliding. Moreover, the morphologies of worn cracks bottom look smoother and flatter under seawater sliding than that under dry sliding. 7
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Fig. 10. Worn surface morphologies of the HBN@PDA and CBN@PDA/EP coating with different mass ratio of HBN@PDA and CBN@PDA, EP (a), F-HBN (b), HC21 (c), HC11 (d), HC12 (e), F-CBN (f) under dry sliding condition.
was caused by the self-lubricating effect from HBN with a lamellar structure. The COF of HC21, HC11, HC12 and F-CBN coating increased comparing with F-HBN because of the introduction of the filler CBN@ PDA under dry sliding, it was due to the fact that CBN particles were exposed to the interface where the friction pair contacts during wear process. However, the COF of F-CBN coatings decreased under seawater sliding, and the COF of F-HBN coating was larger than the COF of FCBN, it was due to that seawater removed the debris including HBN and CBN particles from friction pair interface and the enhancement of rigidity from CBN particles was stronger than HBN. Comparing the wear rates of F-HBN coating, F-CBN coating and EP coating, we could find out that BN fillers possessed excellent anti-wear effect not only under dry sliding but also seawater sliding. And when the binary fillers were added into the coating, the wear rates decreased greatly compared to FHBN, and the wear rate of F-CBN coating was much lower than F-HBN, it was due to the fact that CBN with higher hardness improved the loading capacity of coatings greatly compared to HBN.
observed for CBN@PDA/EP composite coating, including highest hardness and few cracks formation even under the applied load. The good anti-wear performance could be explained by the incorporation of CBN@PDA which increased the hardness and toughness of the coating. However, CBN@PDA/EP didn’t perform well in the case of friction reduction under dry sliding condition, because the hard particles CBN@ PDA which exposed from the coating would increase the roughness of the contacting surface. In addition, the composite coating containing both of CBN@PDA and HBN@PDA displayed great anti-friction and anti-wear performance, this was due to the synergy of CBN@PDA and HBN@PDA. 4. Conclusions The tribological performances of the EP composite coating with different mass ratios of HBN@PDA and CBN@PDA were explored by several characterization measurements. At first, HBN and CBN particles were modified by polydopamine for increasing the dispersibility and compatibility of BN fillers in EP. Secondly, by comparing tribological properties of different EP composite coatings, the COF decreased when the filler was all HBN@PDA under both dry and seawater sliding that
Declaration of Competing Interest The authors declared that there is no conflict of interest. 8
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Fig. 11. Worn surface morphologies of the HBN@PDA/CBN@PDA/EP coating with different mass ratio of HBN@PDA and CBN@PDA, EP (a), F-HBN (b), HC21 (c), HC11 (d), HC12 (e), F-CBN (f) under seawater condition.
Fig. 12. Friction and wear mechanism diagram of composite EP coatings enhanced via HBN@PDA and CBN@PDA fillers. 9
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Acknowledgements
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Tribological performances of epoxy resin composite coatings using hexagonal boron nitride and cubic boron nitride nanoparticles as additives
T
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Wenchao Zhaoa,b, Wenjie Zhaoa, , Zhiping Huanga,b, Gang Liua, Bin Wua a Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China b University of Chinese Academy of Sciences, Beijing 100049, China
H I GH L IG H T S
and CBN were added to the epoxy resin as binary nanofillers. • HBN BN nanoparticles modified by polydopamine displayed great dispersibility. • The • The tribological mechanisms of HBN and CBN were systematically studied.
A R T I C LE I N FO
A B S T R A C T
Keywords: Hexagonal boron nitride Cubic boron nitride Polydopamine Tribological performance
Hexagonal boron nitride and cubic boron nitride were modified by polydopamine to improve dispersion in epoxy resin (EP). The average coefficient of friction (COF) of EP-HBN@PDA coating decreased under dry and seawater condition compared to EP indicating BN possessed self-lubricating effect. COF decreased with CBN content raising in seawater, because seawater removed debris including BN particles and CBN with high hardness improved rigidity of coatings. Wear rates of EP-CBN@PDA coating decreased by 68.88% and 97.95% compared to EP-HBN@PDA, and binary-filled coating performed excellently under seawater, it revealed CBN was more effective than HBN in improving load carrying capacity of coatings.
1. Introduction As all we know, polymer composite materials have been applied widely as structural materials in the aerospace, automotive and chemical industries due to their lightweight and excellent tensile strength [1,2]. EP is a unique kind of polymer material because of its comparatively low shrinkage after curing, excellent mechanical properties, great chemical stability, and high strength of adhesive joints [3,4], which has been widely used in many fields like protective coating, composites and adhesives. But the EP polymer has a poor anti-wear performance that is due to the fact its network structure formed during curing [5,6]. Therefore, it is an effective method to add nanofillers to epoxy resin matrix to cope with harsh application environment. For instance, SiO2 [7,8], nanodiamond [9,10], carbon nanotubes [11,12], graphene [13,14], graphene oxide [15] and other nanofillers have been used widely in improving the tribological properties of EP. Boron nitride (BN) is a new type of micro-material, it has many excellent properties. Other than hexagonal structure, BN has many other crystal forms, such like cubic structure, amorphous structure and ⁎
wurtzite lattices. HBN which known as “white graphene” owns a crystalline structure same as graphene and got a lot of attention from research scholars [16–18]. HBN is an insulating isomorph of graphite with boron and nitrogen atoms occupying the inequivalent A and B sublattices in the Bernal structure. HBN is relatively inert and is expected to be free of dangling bonds or surface charge traps owing to the strong, in-plane, ionic bonding of the planer hexagonal lattice structure [19–21]. Different from graphene, the bonding “lip-lip” exiting in the layers of HBN is stronger than the Van der Waals force between the layers of graphene [22] And HBN possess electrical insulation, excellent lubricity, thermal and chemical stability [23,24]. Cubic boron nitride (CBN) is a very promising material as a hard coating because of its many desirable properties including a hardness second only to diamond, oxidation resistance and chemical inertness against iron. Miyake et al. [25] deposited CBN and HBN films onto a silicon substrate by a magnetically enhanced plasma ion plating method. The results by comparison demonstrated that the CBN film exhibited the highest wear resistance under all microscopic and macroscopic sliding loads, and the lubrication performance of CBN film was also obvious, the lubrication
Corresponding author. E-mail address: [email protected] (W. Zhao).
https://doi.org/10.1016/j.cplett.2019.136646 Received 8 July 2019; Received in revised form 29 July 2019; Accepted 30 July 2019 Available online 30 July 2019 0009-2614/ © 2019 Elsevier B.V. All rights reserved.
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ultrasonic treatment for 3 h in sonic bath afterwards. Then the reaction was magnetically stirred and oil bathed at 60 °C for 24 h. After that, we centrifuged the mixture and washed the mixture with deionized water several times until the color of the filtrate turned to be transparent. After drying treatment of the grey products at 80 °C for 24 h, we got the polydopamine-grafted BN.
life was long, and the friction was low. Yu et al. [26] prepared a batch of composite epoxy resin coatings enhanced with functionalized CBN and functionalized HBN successfully on 316L stainless steel. The results revealed that the presence of functionalized CBN or functionalized HBN fillers could significantly reduce friction and wear of epoxy, and CBN gave the composite EP coatings the best wear resistance, and HBN made the coatings’ friction reduction effect the best which was due to the selflubricating performance of lamella HBN sheet. Inspired by the above, choosing HBN and CBN to reinforce the tribological performances of epoxy resin is feasible. To the best of our knowledge, the combination of HBN and CBN as the filler to enhance the tribology performance was also rare. Therefore, the investigation of tribological performance of composite EP coatings under both of dry friction and seawater friction by means of adjustment of the content and the type of BN filler is significant and novel. The corresponding morphologies, structure properties, chemical composition and thermal stability were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectrum, X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TG). Composite coatings’ COFs were researched by UMT-3, and composite coatings’ wear rates were investigated by surface profile. The working mechanism of the composite coatings including multiple proportion of BN fillers was illustrated based on the experimental results obtained in this work.
We prepared six sets of samples (EP, F-HBN, HC21, HC11, HC12, FCBN), and the mass fraction of the polydopamine-grafted BN fillers in the composite material was 10 wt%. The preparation process was as follows: 0.15 g HBN@PDA powder was put into xylene with ultrasonic for 30 min to get a well dispersed suspension, and then 0.68 g EP was put into the suspension with ultrasonic for 30 min to obtain a uniform mixture. Then 0.17 g curing agent was put into the mixture, which was stirred for 5 min and degassed at 40 °C in a vacuum drying oven for 15 min to remove the air bubbles and the residual xylene. The final mixture was coated on the 304L stainless steel surface by a wire bar coater which was washed by acetone before use, then the composite coating was placed at room temperature to be cured for 5 days. The thickness of the obtained coating was 200 μm. In addition to different quality ratios, preparation processes of other coatings were the same with above process (see Fig. 1).
2. Experimental
2.4. Characterization
The series samples were prepared, for simplicity, we give each sample a code as listed in Table 1.
The crystalline structure of BN was characterized by XRD (D8 Advance Davinci, Bruker). Scanning the sample from 20° to 100° at a scan rate of 10° per minute. The observation of BN micromorphology before and after chemical modification was performed by SEM (S4800, Hitachi), and the thickness of coated polydopamine on the surface of BN was observed by TEM (Talos F200x, ThemoFisher). The element composition and element content of BN fillers before and after chemical modification were analyzed by XPS (Axis Ultra DLD, Kratos). Raman spectroscopy (Renishaw inVia Reflex, Renishaw) was used to analyze the molecular structure of the BN fillers. TG/DTA (Diamond TG/DTA, PerkinElmer) was carried to characterize the stability of BN fillers.
2.3. Preparation of EP-BN composite coatings
2.1. Materials The epoxy resin (TC-K44) and curing agent (T31) were provided by Jiangyin Chemicals Co., Ltd., China. Cubic boron nitride (CBN, average size is 5 μm) was supplied by FUNIK Ultrahard Material Co., Ltd., China. In addition, dopamine hydrochloride and hexagonal boron nitride (HBN, with particle size of 1–2 μm) were provided by Aladdin Reagent Co. Ltd., China. Solvent including xylene and alcohol were supplied by Sinopharm Chemical Reagent Co., Ltd., China. 2.2. Chemical modification of HBN and CBN
2.5. Tribological measurements
Because rare functional groups exist on the outside of BN nanopaticles, we performed surface hydroxylation of BN to get better graft effect. The process of dopamine-modified boron nitride was carried out according to the literature. First of all, BN was treated at 100 °C for 24 h in a 5 M NaOH solution [27,28], then we centrifuged the mixture and washed BN with deionized water to neutral status. After that, the centrifuged BN precipitation was dried at 60 °C for 24 h in the furnace, collected after cooling. We grafted polydopamine onto the hydroxylated BN to dispel the reunite of CBN and HBN for better dispersion in epoxy resin. We put 0.726 g Tris(hydroxymethyl)aminomethane into 600 ml deionized water, and adjusted pH value to 8.5 by 1 M hydrochloric acid. The dopamine (1.2 g) and hydroxylated BN were added and the color of the mixture turned into grey gradually. Did mixture
UMT-3 (CETR) was carried out to characterize tribological properties of the coatings with a reciprocating ball-on-plan friction experiment, the experimental scheme was as follows: the loading force was 5 N, the reciprocating sliding frequency was 5 Hz, the length of friction track was 5 mm and the sliding time was 20 min, and we equipped GCr15 ball with 3 mm in diameter as the stationary upper counterparts, acetone was used to remove oil stain on the GCr15 ball before testing. In addition to the above, all samples were tested in both of dry and seawater environment. In the friction process performed in seawater, the load was 10 N, other experimental parameters were the same with the above. At last, surface profiler (Alpha-Step IQ, KLA-Tencor) was carried out to investigate the wear tracks, and the wear rates of wear tracks were calculated by the formula below.
Table 1 Composition of filler and experimental code of the coatings. Composition
Code
Epoxy HBN@PDA HBN@PDA:CBN@PDA = 2:1 HBN@PDA:CBN@PDA = 1:1 HBN@PDA:CBN@PDA = 1:2 CBN@PDA
EP F-HBN HC21 HC11 HC12 F-CBN
W=
V S×d = F×L F×T×f×d×2
In which, W refers to wear rate (mm3 N−1m−1), V refers to wear volume (mm3), F refers to loading force (N), L refers to sliding distance (m), T refers to sliding time (min), d refers to length of wear tracks (mm), S refers to wear tracks’ cross-sectional area (mm2), f refers to sliding frequency (Hz). The sectional appearance of the specimens were characterized by 2
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Fig. 1. Preparation route of composite EP coating.
centering at about 1581 cm−1 also appeared after chemical modification which was resulted from the deformation of catechol moiety from polydopamine grafted. Other than this, TGA and XPS of BN powder before and after chemical modification can indicate that polydopamine has been chemically grafted on the surface of BN. Fig. 4(a) and (b) can depict the thermal degradation behaviors of the BN powder and the decorated BN powder in the temperature range of 30–800 °C. With the gradual increase of temperature, there was no loss in the quality of pristine BN as indicated in Fig. 4(a) and (b). The curve of HBN@PDA and CBN@PDA started to fall at the temperature about 330 °C. This revealed that a loss of mass happened to the polydopamine-decorated BN, and the loss was due to the fact the thermal decomposition of polydopamine. By calculating the weight loss rate of HBN@PDA and CBN@PDA, we can get the mass fraction of polydopamine on HBN@PDA was about 3.67% and the mass fraction of polydopamine on CBN@PDA was about 0.61%. As shown in Fig. 4(c) and (d), we analyzed the elemental content of BN powder sample before and after chemical modification by XPS. It can be found that the atomic percentage of B, N, C and O of pristine HBN were 28.49%, 30.9%, 30.45% and 10.16% respectively, but the atomic percentage of B, N, C and O of HBN@PDA were 13.53%, 15.32%, 54.95% and 16.2% respectively. The atomic percentage of C and O increased apparently and the atomic percentage of B and N decreased which
SEM (FEI Quanta FEG 250, FEI), and the surface morphologies of the wear tracks were investigated by SEM (FEI Quanta FEG 250, FEI). 3. Results and discussion 3.1. Composition characterization As shown in Fig. 2(a) and (c), pristine HBN showed a sheet with sharp edges and smooth surface, pristine CBN displayed irregular granular and uneven surface. After chemical modification, HBN@PDA sheets was covered by a substance and displayed smooth edges and rough surface, CBN@PDA displayed rough surface with many granular substance distributing on its surface, as can be seen in Fig. 2(b) and (d). Before and after chemical modification, the shape of HBN and CBN didn’t change, and what had changed resulted from the chemical graft of polydopamine. In Fig. 3(a), pristine HBN showed a sharp characteristic peak centered at 1366 cm−1 which was due to the E2g phonon mode and analogy to the G peak of graphene [29]. There was a new peak emerging at 1581 cm−1 which was related to the deformation of catechol moiety of polydopamine [30]. The pristine CBN showed two strong peaks at 1050 cm−1 and 1301 cm−1 respectively in Fig. 3(b), which were due to the scattering of the transverse optical (TO) and longitudinal optical (LO) phonon modes of CBN [31]. A new wide peak
Fig. 2. SEM images of HBN (a), HBN@PDA (b), and CBN (c), CBN@PDA (d). 3
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Fig. 3. Raman Spectra of HBN and HBN@PDA (a) and CBN and CBN@PDA (b).
resolution TEM images. As Fig. 5(b) and (c) showed, a continuous thin layer was observed on the surface of HBN@PDA and CBN@PDA, respectively, indicating that the polydopamine coated the nanoparticles successfully [33].
resulted from the introduction of polydopamine. It also can be found that the atomic percentage of B, N, C and O of pristine CBN were 32.91%, 32.85%, 21.69% and 12.55% respectively, but the atomic percentage of B, N, C and O of CBN@PDA were 13.95%, 16.81%, 54.53 and 14.71% respectively. The atomic percentage of C and O increased apparently and the atomic ratios of B and N decreased which resulted from the introduction of polydopamine. In Fig. 5 we could observe sharp peaks at 2θ = 26.82°, 41.72° and 43.97° on the curve of HBN and HBN@PDA which were corresponded to the (0 0 2), (1 0 0) and (1 0 1) faces [32]. Other than this, the strong sharp peaks appeared at 2θ = 43.31°, 50.41° and 74.14° of CBN and CBN@PDA which were corresponded to the (1 1 0), (2 2 0) and (2 2 0). It can be observed that the position of peaks of grafted BN were almost identical to the original which revealed that pristine BN and decorated BN were highly crystalline and the process of modification did not change BN’s structure [5]. Moreover, we can further confirm that functionalization of HBN and CBN was successful through the high-
3.2. Morphological characteristics From the cross-section morphology of the pure EP in Fig. 6, we could observe many long cracks and uniform sections without defects which indicated that the surface had brittle fracture. When the BN filler were added into EP matrix, the fracture surface became rough. As shown in Fig. 6(b), (c), (d), (e), (f), when the mass fraction of HBN@ PDA increased, the fracture surface became rougher and rougher, which indicated that the effect of granular CBN@PDA on the fracture surface was not as great as that of flaky HBN@PDA on the fracture surface. In addition, comparing Fig. 6(b) and (f), we could find that particles agglomeration on F-CBN fracture morphology was weaker than F-HBN,
Fig. 4. Thermal degradation of pristine HBN, HBN@PDA (a), and pristine CBN, CBN@PDA (b). XPS of pristine HBN and HBN@PDA (c), pristine CBN and CBN@PDA (d). 4
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Fig. 5. XRD patterns of HBN, HBN@PDA, CBN and CBN@PDA (a), TEM images of HBN@PDA (b) and CBN@PDA (c).
As shown in Fig. 7, the hardness value of composite EP coatings increased when the BN fillers were added into the matrix, the hardness value of F-HBN coating increased by 7.50% compared with the hardness of neat EP coating. Moreover, the hardness value of F-CBN coating increased by 17.58% compared with the hardness of neat EP coating. And when the binary BN fillers were added to the EP matrix, the hardness values were higher than F-HBN. This was due to the fact that
which was due to that the lamellar HBN had a larger specific surface area than the granular CBN and the interaction force between HBN was stronger than CBN. Other than this, the fracture form of EP-BN composite coatings was different from that of EP coating. It could be observed that the BN fillers inhabited the development of long cracks in the area which were marked by the yellow loop. The fracture form of EP-BN composite coatings was not brittle fracture.
Fig. 6. The cross-section morphologies of pure EP coating (a) and the composite EP coatings with different mass ratios as F-HBN (b), HC21 (c), HC11 (d), HC12 (e), FCBN (f). 5
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seawater sliding were much lower than COFs in dry sliding, because seawater played the role as lubricant in the friction process. It was observed that the average COF decreased when we add BN fillers into the epoxy matrix. The average COF of F-HBN coating decreased by 4.09% compared with EP coating, and the friction reduction effect was weaker than that of friction under dry sliding. It was due to the fact that the erosion of seawater weakened the friction reduction effect of HBN which was exposed in the interface where the friction pair contacted. Interestingly, when the binary BN fillers were added into the epoxy matrix, the average COF of coating decreased obviously compared with F-HBN coating which was different from dry sliding. The average COFs of HC21, HC11 and HC12 coating were decreased by 25.82%, 25.5% and 27.04% respectively compared with F-HBN coating which indicated that the introduction of CBN@PDA decreased the COFs of coatings. The average COF of F-CBN coating decreased by 33.37% and 30.53% respectively compared with EP coating and F-HBN coating. Firstly, F-CBN with high hardness increased the rigidity of the coatings compared to F-HBN. Secondly, the seawater could take away the BN particles which were exposed to the friction pair interface during wear process. Finally, seawater could take away the frictional heat produced in the sliding process to reduce plastic deformation and the spalling of composite coating [26]. At the same time, the BN fillers reduced the cracks expanding, and enhanced the toughness of the composite EP coating and reduced the peeling of coating surface [34,35]. Fig. 9 showed the cross-section profiles and wear rate of the composite coating in both of dry sliding and seawater sliding, respectively. The cross-section profiles clearly showed the wear of the composite EP coatings as shown in Fig. 9(a1) and (a2). EP showed a maximum depth of wear crack with 46 µm under dry sliding, and the depths of wear cracks produced on HC21 and F-CBN were 14 µm and 12 µm respectively under dry sliding. F-CBN and HC21 composite coating performed excellent anti-wear behaviors under dry condition with wear rates of 0.55 × 10−4 mm3/N·m and 0.63 × 10−4 mm3/N·m which were decreased by 87.12% and 85.24% respectively compared with pure epoxy (4.27 × 10−4 mm3/N·m). As can be seen from Fig. 9(b1) and (b2), in the case of seawater sliding, EP showed a depth of 68 µm, the depths of
Fig. 7. The hardness of pure EP coating and EP-BN composite coatings.
CBN possessed the second highest hardness and was granular, but HBN displayed a layered structure and was softer compared with CBN. 3.3. Tribological properties As shown in Fig. 8(a1), the curves of COFs increased sharply at first and achieved a steady state under dry sliding. It could be observed from Fig. 8(a2), the average COF of BN-EP composite coating decreased by 8.81% when we added HBN@PDA into the epoxy matrix compared with neat EP coating which was attributed to the layered structure of HBN showing excellent lubrication performance. However, the average COF started raising when the proportion of CBN@PDA increased, and the value was larger than epoxy coating when the filler was all CBN@ PDA. As all we know, CBN particles possessed high hardness. Other than this, when the composite coating was worn, CBN particles were exposed to the interface where the friction pair contacted, resulting in an increase in the COF. Fig. 8(b1) and (b2) showed us that COFs in
Fig. 8. COFs and average COF of HBN@PDA/CBN@PDA/EP coatings in dry sliding (a1, b1), and in seawater sliding (b1, b2). 6
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Fig. 9. Cross-section profiles of the HBN@PDA/CBN@PDA/EP coatings under dry sliding (a1), and seawater condition (b1). Wear rate of the HBN@PDA/CBN@ PDA/EP coatings in dry sliding (a2), and in seawater condition (b2).
Fig. 10(a) showed that many hollow pits existed on the bottom of the wear crack which was attributed to peeling of EP matrix. In addition, Fig. 10(b) and (c) represented that the degree of peeling was much weaker than EP coating. However, Fig. 10(d) and (e) showed that many long cracks which could expand gradually to broke the coating. And Fig. 10(f) showed that some short and tiny cracks generated because of the effect of CBN@PDA, CBN@PDA as a kind of hard filler could increase the load carrying capacity of the coating and decrease the cracks expanding. Fig. 11(a) showed the anti-wear properties of pure epoxy under seawater condition. It was found that EP coating was peeling off piece by piece. In addition, obvious wear can be observed on the crack of F-HBN (Fig. 11b), this damage was more like pitting. This was due to the fact that the neat EP exhibited brittle features and HBN@PDA filler increased the toughness of the composite coating. However, when the filler was composed of HBN@PDA and CBN@PDA, the bottom of cracks turned to be smoother and flatter, CBN@PDA raised the rigidity of the coating. Fig. 11(f) represented some light scratch generated on the coating whose filler was all CBN@PDA under seawater condition. Fig. 12 further revealed the wear mechanism of the composite EPBN coatings. No matter under dry sliding or seawater sliding, the coatings went through the occurrence of tiny cracks, cracks expanded wavily, fatigue wear and coating peeling off in friction process [36]. First of all, in the presence of shear stress, pure epoxy material with brittle features easily produced long and wide cracks. Afterwards, cracks expanded and peeling appeared in the matrix, leading to high wear rate of neat EP coating. In order to improve the tribological properties of epoxy coating, some fillers like BN were necessary needed, which not only can increase hardness but also improve toughness of the composite coatings. The average COF of EP coating decreased obviously indicting a friction reduction effect occurred by the incorporation of HBN@PDA under dry or seawater sliding. This was due to that the layered structure of HBN@PDA showed low shear force and excellent lubricity. However, anti-wear performance of F-HBN coating was not as good as F-CBN coating, because lower hardness could cause fatigue wear of the coatings, which in turn caused high wear rates. When the filler was all CBN@PDA, excellent anti-wear behavior could be
wear cracks of HC21, HC12 and F-CBN were 2.7 µm, 3.6 µm, and 1.3 µm respectively. The wear rate of HC21, HC12 and F-CBN coating were as low as 0.64 × 10−5 mm3/N·m, 0.55 × 10−5 mm3/N·m and as 0.28 × 10−5 mm3/N·m which were decreased by 98.13%, 98.39% and 99.18% compared with pure EP coating (3.415 × 10−4 mm3/N·m). It represented clearly the BN fillers would greatly reduce the wear rate of EP composite coating. Other than this, the EP-BN composite coating represented more excellent anti-wear properties under seawater condition compared to the dry sliding condition because of the effect of seawater which was mentioned above. The wear rates of F-HBN coating were 1.7673 × 10−4 mm3/N·m and 1.3689 × 10−4 mm3/N·m under dry sliding and seawater condition respectively which were larger than that of F-CBN with 0.55 × 10−4 mm3/N·m and 0.28 × 10−5 mm3/N·m. It was due to that HBN was not as hard as CBN, and the CBN with much higher hardness could increase the rigidity and the loading capacity of the composite coating greatly, but the enhancement of HBN was not as good as CBN. When the fillers were composed of CBN@PDA and HBN@PDA, the composite EP-BN coating also showed excellent anti-wear properties. Especially in seawater condition, when the proper amount of CBN@ PDA was added into the F-HBN coating, the wear rate was greatly reduced, which further demonstrated that CBN could greatly increase the load carrying capacity of the coating and reduce the wear rate of the EP-BN composite coating. At the same time, HBN was exposed to the interface where friction pair contacted during the process of sliding, which increased the lubrication effect under dry sliding condition. However, as the proportion of CBN content increased, the hard particles CBN in the wear debris exacerbated the damage of the coating when the coating started breaking under dry sliding. Figs. 10 and 11 represented the wear tracks microscopic morphologies of the composite EP coatings in both of seawater sliding and dry sliding. The wear cracks of composite coatings under dry sliding were wider than the corresponding wear cracks under seawater sliding, even though the load under seawater was 10 N compared with 5 N under dry sliding. Moreover, the morphologies of worn cracks bottom look smoother and flatter under seawater sliding than that under dry sliding. 7
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Fig. 10. Worn surface morphologies of the HBN@PDA and CBN@PDA/EP coating with different mass ratio of HBN@PDA and CBN@PDA, EP (a), F-HBN (b), HC21 (c), HC11 (d), HC12 (e), F-CBN (f) under dry sliding condition.
was caused by the self-lubricating effect from HBN with a lamellar structure. The COF of HC21, HC11, HC12 and F-CBN coating increased comparing with F-HBN because of the introduction of the filler CBN@ PDA under dry sliding, it was due to the fact that CBN particles were exposed to the interface where the friction pair contacts during wear process. However, the COF of F-CBN coatings decreased under seawater sliding, and the COF of F-HBN coating was larger than the COF of FCBN, it was due to that seawater removed the debris including HBN and CBN particles from friction pair interface and the enhancement of rigidity from CBN particles was stronger than HBN. Comparing the wear rates of F-HBN coating, F-CBN coating and EP coating, we could find out that BN fillers possessed excellent anti-wear effect not only under dry sliding but also seawater sliding. And when the binary fillers were added into the coating, the wear rates decreased greatly compared to FHBN, and the wear rate of F-CBN coating was much lower than F-HBN, it was due to the fact that CBN with higher hardness improved the loading capacity of coatings greatly compared to HBN.
observed for CBN@PDA/EP composite coating, including highest hardness and few cracks formation even under the applied load. The good anti-wear performance could be explained by the incorporation of CBN@PDA which increased the hardness and toughness of the coating. However, CBN@PDA/EP didn’t perform well in the case of friction reduction under dry sliding condition, because the hard particles CBN@ PDA which exposed from the coating would increase the roughness of the contacting surface. In addition, the composite coating containing both of CBN@PDA and HBN@PDA displayed great anti-friction and anti-wear performance, this was due to the synergy of CBN@PDA and HBN@PDA. 4. Conclusions The tribological performances of the EP composite coating with different mass ratios of HBN@PDA and CBN@PDA were explored by several characterization measurements. At first, HBN and CBN particles were modified by polydopamine for increasing the dispersibility and compatibility of BN fillers in EP. Secondly, by comparing tribological properties of different EP composite coatings, the COF decreased when the filler was all HBN@PDA under both dry and seawater sliding that
Declaration of Competing Interest The authors declared that there is no conflict of interest. 8
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Fig. 11. Worn surface morphologies of the HBN@PDA/CBN@PDA/EP coating with different mass ratio of HBN@PDA and CBN@PDA, EP (a), F-HBN (b), HC21 (c), HC11 (d), HC12 (e), F-CBN (f) under seawater condition.
Fig. 12. Friction and wear mechanism diagram of composite EP coatings enhanced via HBN@PDA and CBN@PDA fillers. 9
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Acknowledgements
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