Tribological Performance Studies of Waterborne Polyurethane Coatings with Aligned Modified Graphene Oxide@Fe3O4.

In this study, the chemical graft method was used to connect modified graphene oxide (GO) and Fe3O4 through covalent bonds. To make full use of the tribological properties of graphene, aligned graphene oxide@Fe3O4/waterborne polyurethane (GO@Fe3O4/WPU) was prepared in a magnetic field and tribological experiments were carried out on it. The GO@Fe3O4 was characterized by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and a transmission electron microscopy (TEM). The characterization results show that Fe3O4 is successfully loaded on the surface of GO and GO@Fe3O4 has better dispersibility in WPU. Among the coatings without alignment inducement of GO@Fe3O4, 0.5 wt % GO@Fe3O4/WPU has the lowest friction coefficient and wear rate. In addition, the 0.5 wt % aligned GO@Fe3O4/WPU composite coating has the lowest friction coefficient and wear rate compared with nonaligned and pure WPU coatings. The excellent tribological properties of the aligned composite coating come from its ability to quickly form a uniform and continuous transfer film on the friction counterpair, which avoids direct friction between the friction counterpair and the coating.

Introduction

In the process of automobile manufacturing, the stamping and forming of the side periphern class="Chemical">al panels and the door inner panels have to undergo a series of complex deformations, such as bending and reverse bending.[1] Due to the high friction between the steel plate and the mold and the low hardness of the galvanized layer, serious friction problems will cause scrapping of the parts.[2,3] Coating with a lubricating coating on the sheet material can effectively improve its friction problems.[4−6] Among various polymers, WPU has a wide range of applications in the friction field because of its excellent mechanical properties, easy modification, environmental protection, and no pollution.[7−9] However, pure WPU cannot meet the requirements of antifriction and wear resistance under high-strength conditions. The tribological properties of the polymer can be improved by adding lubricants such as molybdenum disulfide,[10,11] graphite,[12] and polytetrafluoroethylene.[13]

With its extremely low coefficient of friction, atomic smooth surface, and low interlayer shear resistance, n class="Chemical">graphene has attracted considerable attention of many scholars in the field of lubrication,[14−16] especially as a nanoadditive to improve the tribological properties of polymers.[17,18] Since graphene is extremely easy to agglomerate and difficult to disperse in most solvents, surface modification of graphene by chemical modification is crucial to improve the dispersion and stability of graphene in the polymer matrix.[19−21] Mo et al. used KH550 to modify GO to obtain functionalized GO (FGO) with good dispersibility and compatibility in the polyurethane matrix. The results showed that the friction and corrosion resistance of the composite coating with 0.25–0.5% FGO are the best.[22]

Graphene is a two-dimenn class="Chemical">sional sheet material with anisotropy; its distribution in the polymer matrix will affect its excellent performance in composite materials.[23,24] Dai et al. used the coprecipitation method to modify Fe3O4 on the reduced graphene oxide (RGO) and realized the aligned FRGO/WPU composite material by the magnetic field induction method. The aligned FRGO/WPU composites showed excellent performance, and the dielectric loss is significantly higher than nonaligned FRGO/WPU composites.[25] Liu et al. synthesized reduced graphene oxide-Fe3O4@polyphosphazene/bismaleimide (RGO-Fe3O4@PZM/BMI) nanocomposites to improve the mechanical and tribological properties of BMI composites. In their study, the elastic strength and impact strength of composites containing 0.4 and 0.8 wt % RGO-Fe3O4@PZM increased by 31.3 and 54.5% compared with pure BMI. After adding 0.8 wt % RGO-Fe3O4@PZM, the wear rate of the coating dropped from 13.5 × 10–6 to 1.0 × 10–6 mm3/(N·m) and the frictional coefficient of the coatings reduced by three times at the same time.[26] To the best of our knowledge, studies have been reported on graphene and its composite coatings, which mainly focused on the research on the additive components of the coating, and the other properties of the composite coating, the ultrathin sheet structure, and excellent self-lubricating performance of graphene inspired us to study performance improvement brought by its aligned distribution.

To our knowledge, there are few studies that applied oriented n class="Chemical">GO@Fe3O4 to WPU coatings to improve the tribological properties. First, the Fe3O4 particles were loaded on GO through the covalent reaction of the amino groups in the modified Fe3O4 and the isocyanate groups in the GO (Figure ).[27−29] Then, the GO@Fe3O4 with different mass fractions were added to the WPU, and the GO@Fe3O4 were aligned in the coating by a magnetic-field-induced method (Figure ). The tribological performances of the aligned and the nonaligned GO@Fe3O4 composite coating were investigated. Finally, the friction and wear mechanism of the GO@Fe3O4/WPU composite coating was studied and discussed.

Figure 1

Schematic illustration for the preparation of GO@Fe3O4 composites.

Figure 2

Schematic diagram of the coating cured in a magnetic field.

Results and Discussion

Characterization of GO@Fe3O4

The FTIR spectra of Fe3O4, n class="Chemical">Fe3O4-KH550, and GO@Fe3O4 are shown in Figure . For Fe3O4, the Fe–O bond at 590 cm–1 is the characteristic absorption peak of Fe3O4.[30] In the Fe3O4-KH550 spectrum, the absorption peak at 569 cm–1 is attributed to the Fe–O bond stretching vibration, and the strong absorption peak near 1083 cm–1 is the stretching vibration of the Si–O–Si bond, indicating that Fe3O4 nanoparticles are modified with the silane molecules.[31] For the GO@Fe3O4 spectrum, the absorption peak at 569 cm–1 is assigned to the Fe–O bond, the peak near 1632 cm–1 is attributed to the stretching vibration of the skeleton carbon ring C=C and the C=O in the carbonyl functional group at 1796 cm–1, and the peaks at 1632 and 1796 cm–1 are characteristic peaks of GO, indicating the presence of GO and Fe3O4 nanoparticles in the composite.[32] The N–H bending vibration of the secondary amide appears at 1550 cm–1, indicating that GO and Fe3O4 are successfully grafted through chemical bonds.[33]

Figure 3

FTIR spectra of Fe3O4, Fe3O4-KH550, and GO@Fe3O4.

To study the changes of the GO inner structure before and after modification, n class="Chemical">GO, Fe3O4, and GO@Fe3O4 were characterized by XRD. As shown in Figure , there is a strong peak of GO around 2θ = 10.82°, which is the diffraction peak of GO on the (001) plane.[34] The intensity of the diffraction peak of the (001) crystal plane of GO@Fe3O4 is significantly weaker than GO, and the diffraction peak shifts to the left from 2θ = 10.82° (d = 8.1699 Å) to 2θ = 8.41° (d = 10.5050 Å) after Fe3O4 is loaded on GO, indicating that the interlayer distance expanded.[35] In addition to the diffraction peaks of GO, the diffraction peaks appearing at 2θ = 30.1, 35.4, 43.1, 53.4, 57.1, 62.6, and 73.9° are highly consistent with the cubic spinel crystal structure of Fe3O4 (JCPDS Card No. 19-0629).

Figure 4

XRD spectra of GO, Fe3O4, and GO@Fe3O4.

The formation of covalent bonds between n class="Chemical">GO and Fe3O4 was also confirmed by XPS, as shown in Figure . Figure b,c shows the C 1s and O 1s high-resolution XPS spectra of GO, respectively; Figure e,f shows the C 1s and O 1s high-resolution XPS spectra of GO-IPDI, respectively; and Figure h,i shows the C 1s and O 1s high-resolution XPS spectra of GO@Fe3O4, respectively. As can be seen from Figure b, the peaks at 284.6, 286.6, and 287.4 eV were assigned to C=C, C–O, and C=O bonds of GO, respectively.[36,37] An obvious N 1s peak appears in Figure d compared with Figure a, which represents that nitrogen-containing functional groups have been introduced on the GO surface.[38] It is observed from Figure e that 284.6, 286.8, and 288.4 eV correspond to C=C, C–O, and C=O, respectively. Moreover, a new peak appears at 286.2, which is a C–N covalent bond, indicating that IPDI has a surface modification effect on GO.[39,40] Fe 2p and Si 2p peaks appear in Figure g, indicating that Fe3O4 is introduced on the surface of GO. In Figure h, it is observed that 284.8, 286.3, 287.2, 288.2, and 289.1 eV correspond to C=C, C–N, C–O, O=CNH, and O–C=O, respectively. In Figure i, 530.0, 531.5, 532.5, and 533.3 eV correspond to Fe–O, C=O, C–O, and Si–O, respectively.[41] The results show that Fe3O4 is successfully grafted to GO.

Figure 5

XPS survey scan spectra of GO (a), GO-IPDI (d), and GO@Fe3O4 (g); C 1s high-resolution XPS spectra of GO (b), GO-IPDI (e), and GO@Fe3O4 (h); O 1s high-resolution XPS spectra of GO (c), GO-IPDI (f), and GO@Fe3O4 (i).

The micromorphologies of GO, Fe3O4, and GO@Fe3O4 were studied by TEM. As shown in Figure c, a large amount of Fe3O4 nanoparticles with a relatively uniform size is supported on the GO surface. Based on the above characterization, Fe3O4 is successfully grafted to GO.

Figure 6

TEM images of Fe3O4 (a), GO (b), and GO@Fe3O4 (c).

The dispersibilities of 0.5 wt % GO and GO@Fe3O4 in WPU are shown in Figure . Two solutions are in good dispersion after ultrasonic dispersion for 1 h. The 0.5 wt % GO/WPU starts to precipitate after 1 h of static and completely precipitates after 4 h, while the 0.5 wt % GO@Fe3O4/WPU has good dispersibility, indicating that the dispersion of modified GO in WPU becomes better.

Figure 7

Dispersion degree images of GO@Fe3O4 (left) and GO (right), (a) after dispersion, (b) after standing for 1 h, and (c) after standing for 4 h.

Characterization of the GO@Fe3O4/WPU Composite

To further study the interface interaction between GO@n class="Chemical">Fe3O4 and WPU, the cross sections of pure WPU and its composite were investigated by SEM. As shown in Figure a, when GO@Fe3O4 is not added to WPU, the cross section is relatively smooth and the cracks extend like a river, which is a characteristic of brittle fracture. When GO@Fe3O4 is added to WPU (Figure b–d), due to the introduction of a large number of interfaces and the compatibility of GO@Fe3O4 with WPU being better, the cross section of the composites becomes rougher. Compared with the cross section of pure WPU, the cross sections of GO@Fe3O4/WPU composites show a stronger interaction. Figure b is the fracture of the aligned composite. It can be seen that GO@Fe3O4 with an uneven and protruding cross section is covered by resin and embedded in the WPU, which is due to the uniform distribution of GO@Fe3O4 in the WPU and the good interface interaction between the Fe3O4 and WPU. The even dispersion of GO@Fe3O4 in WPU can promote GO to give full play to its excellent lubricating properties. The good interface interaction between GO@Fe3O4 and WPU can improve the mechanical properties of the composite coating, thus probably inhibiting the growth of microcracks in the friction process and contributing to enhancing the wear resistance of the composite coating. The agglomeration caused by excessive GO@Fe3O4 in the resin can be clearly seen in Figure c, which indicates that the interfacial interaction between the GO@Fe3O4 and WPU is weak. Hence, agglomeration may cause GO@Fe3O4 to peel off easily during the friction process, accordingly affecting the wear resistance of the coating. Nevertheless, when the added amount of GO@Fe3O4 is 0.5 wt %, its dispersion in the WPU is uniform, as shown in Figure d. However, the aligned arrangement of GO@Fe3O4 in WPU cannot be clearly seen in the sectional diagram of the aligned GO@Fe3O4/WPU composite. It may be due to the good interfacial interaction between the GO@Fe3O4 and the WPU. Further, the amount of GO@Fe3O4 added to the WPU is small, so the filler is covered with a large amount of resin.

Figure 8

SEM images of cross sections of WPU composites with and without GO@Fe3O4 additives, (a) pure WPU, (b) aligned GO@Fe3O4/WPU composite, (c) composite with 2.0% addition, and (d) composite with 0.5% addition.

Tribological Performance of the Nonaligned GO@Fe3O4/WPU Composite Coatings

Tribological experiments were carried out on nonn class="Chemical">aligned composite coatings with GO@Fe3O4 mass fractions of 0, 0.3, 0.5, 1, and 2 wt %. The effects of GO@Fe3O4 on the antifriction and wear resistance of WPU coatings are discussed. As shown in Figure , the friction coefficient of pure WPU is relatively high and fluctuates greatly, with an average friction coefficient of 0.8. With an increase of the GO@Fe3O4 content, the friction coefficient shows a gradual decrease trend, the friction coefficient curve also tends to be stable, and the fluctuation decreases. The average friction coefficient reaches the minimum at 0.5 wt %, which is 0.45. When the mass fraction gradually increases to 2 wt %, the friction coefficient of the GO@Fe3O4/WPU composite coating also increases, and the average friction coefficient reaches 0.72. Moreover, it is still lower than that of the pure WPU coating. Compared with the pure WPU coating and the coating with the added GO@Fe3O4 amount of 2 wt %, the tribological performance of the coating with the GO@Fe3O4 content not exceeding 1 wt % is more stable. This is due to the good lubricating effect of GO@Fe3O4 and the uniform dispersion in the resin. During the friction process, the relative sliding between graphenes enhances its excellent lubrication performance. At the same time, Fe3O4 nanoparticles with a spherical structure load on the surface of the graphene act as rolling bearings, which can convert sliding friction into rolling friction. In addition, a part of GO@Fe3O4 will adhere to the friction counterpair to form a transfer lubricating film, so GO@Fe3O4 has a certain antifriction performance. When the content of GO@Fe3O4 in WPU is high, GO@Fe3O4 will agglomerate seriously. The agglomeration seriously affects the interfacial bonding force between GO@Fe3O4 and WPU, which leads to the easy exfoliation of fillers from the WPU during the friction process so that its antifriction performance cannot be fully exerted.

Figure 9

Tribological performance of the GO@Fe3O4/WPU composite coatings, (a) real-time friction coefficient and (b) average friction coefficient and wear rate.

The wear rate of the GO@n class="Chemical">Fe3O4/WPU composite coatings is displayed in Figure . With the increase of the GO@Fe3O4 content, the wear rate of the composite coating shows a trend of decrease first and then increase. In Figure b, the wear rate of the pure WPU coating is the largest, reaching 26.67 × 10–4 mm3/(N·m). With the increase of the GO@Fe3O4 content, the wear rate of the composite coating gradually decreases; when the mass fraction is 0.5 wt %, the coating wear rate is the smallest, 4.48 × 10–4 mm3/(N·m); when the added amount exceeds 0.5 wt %, the wear rate of the composite coating gradually increases, reaching 17.49 × 10–4 mm3/(N·m) at 2 wt %. From the cross-sectional morphology of the coating in Figure , it can be seen that the composite with 0.5 wt % GO@Fe3O4 has better filler dispersion than the composite with 2 wt % GO@Fe3O4. The addition of GO@Fe3O4 to the WPU can deflect the crack growth path and hinder the rapid propagation of microcracks in the coating, thereby preventing the formation of wear debris during the wear process and reducing the wear rate of the coating.[42] However, excess GO@Fe3O4 will form agglomeration in the resin, which makes the crack expand more easily. The agglomeration will cause the coating to peel off during friction and then affect the wear resistance of the coating.

Figure 10

SEM images of the cross-sectional morphology of the GO@Fe3O4/WPU coatings, (a) 0.5 wt % GO@Fe3O4 and (b) 2 wt % GO@Fe3O4.

Tribological Performance of the Aligned GO@Fe3O4/WPU Composite Coatings

The friction coefficient and wear rate of the aligned n class="Chemical">GO@Fe3O4/WPU composite coating were tested, and the effect of GO@Fe3O4 alignment on the tribological performance of the composite coating was studied. Figure a indicates the influence of the GO@Fe3O4 content on the antifriction performance of the aligned composite coating. With the increase of the GO@Fe3O4 content, the average friction coefficients of aligned and nonaligned GO@Fe3O4/WPU coatings both decrease first and then increase. Aligned and nonaligned GO@Fe3O4/WPU coatings have the minimum average friction coefficient at 0.5 wt %, which are 0.45 and 0.25, respectively. When the mass fraction gradually increases to 2 wt %, the friction coefficient of the aligned GO@Fe3O4/WPU composite coating also increases, and the average friction coefficient reaches 0.4. Comparing Figures and 11, it can be clearly seen that the friction coefficient of the aligned composite coating is significantly lower than that of the pure WPU coating and the nonaligned GO@Fe3O4 coating. The average friction coefficient decreases from 0.8 to 0.25, indicating that the aligned distribution of GO@Fe3O4 can significantly improve the antifriction performance of WPU coating. Since the isocyanate functional group grafted on the modified GO surface reacts with the carbamate group in the WPU, it enhances the binding force between the matrix and filler. Therefore, a lot of energy can be absorbed during the friction process to prevent the growth of microcracks. Comparing Figure a with Figure a, it can be seen that the average friction coefficient of the aligned GO@Fe3O4/WPU coating (0.25) is lower than that of the nonaligned coating (0.45). The distribution of GO@Fe3O4 in the nonaligned composite coating is shown in Figure a. In the nonaligned composite coating, since GO@Fe3O4 presents a random morphology in the WPU, the shear force required to transfer GO@Fe3O4 from the coating to the friction counterpair is different during the friction process. The shearing force required for GO@Fe3O4 in the vertical direction is greater than for GO@Fe3O4 distributed in the horizontal direction, so an uneven and discontinuous transfer film may be formed, which limits the excellent antifriction performance of GO. By aligning the GO@Fe3O4 with a magnetic field, it can fully exert its antifriction effect, thereby improving the antifriction performance of the composite coating. During friction, the friction counterpair first acts on the resin part of the GO@Fe3O4/WPU coating. Then, the friction pair slides on the resin embedded with aligned GO@Fe3O4. Due to the aligned distribution of GO@Fe3O4 in the resin matrix, the shear force required for the exfoliation of GO during friction is smaller, which can reduce the friction resistance. In addition, the performance of each part of the coating is relatively average, so a uniform and continuous transfer film is formed easily on the surface of the friction pair during the friction process, which significantly reduces the friction coefficient of the GO@Fe3O4/WPU composite coating. When the GO@Fe3O4 content exceeds 0.5 wt %, the average friction coefficient of the composite coating begins to rise, but it is still lower than that of the pure WPU coating. This phenomenon is due to the agglomeration of GO@Fe3O4 in the matrix.[43] The agglomeration is easy to peel from the WPU, which will greatly affect the formation of an effective transfer film on the surface of the friction counterpair. In addition, it will cause more microcracks during the friction process, thus reducing the bearing capacity of the composite coating during the friction process and affecting the excellent antifriction performance of GO@Fe3O4 itself.

Figure 11

Friction coefficients of aligned GO@Fe3O4/WPU composite coatings: (a) real-time friction coefficient and (b) average friction coefficient and wear rate.

Figure 15

Mechanism scheme for the wear behavior of nonaligned composite coating (a, b) and GO@Fe3O4/WPU aligned composite coating (c, d).

Friction coefficients of aligned n class="Chemical">GO@Fe3O4/WPU composite coatings: (a) real-time friction coefficient and (b) average friction coefficient and wear rate.

The wear rates of pure WPU coating and n class="Chemical">aligned GO@Fe3O4/WPU composite coating are shown in Figure b. Comparing Figure b with Figure b, it can be seen that the wear rate of the aligned GO@Fe3O4/WPU composite coating is lower than that of the nonaligned coating and the wear rate of the pure WPU coating is the largest, up to 26.67 × 10–4 mm3/(N·m). The wear rate decreases with the increase of the GO@Fe3O4 content. The wear rate of the 0.5 wt % GO@Fe3O4/WPU coating is the smallest, which is 3.60 × 10–4 mm3/(N·m). As a result of the aligned and uniform distribution of GO@Fe3O4 in the resin, the stress during friction is evenly distributed and delays the formation of crack sources. It also can expand the crack propagation path and delay the role of wear debris, thereby improving the wear resistance of the coating. When the content of GO@Fe3O4 reaches 2 wt %, the wear rate increases to 14.81 × 10–4 mm3/(N·m). This indicates that the GO@Fe3O4 agglomerate seriously causes the microcracks in the coating to grow more easily. The wear mechanism of GO@Fe3O4/WPU will be discussed in the next section.

Wear Mechanisms of the Aligned GO@Fe3O4/WPU Composite Coatings

It can be seen from the above experimental results that adding n class="Chemical">GO@Fe3O4 to the coating can significantly improve the antifriction and wear resistance of the WPU coating. Figure shows the SEM images of the surface morphology of the composite coating with different GO@Fe3O4 contents. It can be seen that when the added amounts of GO@Fe3O4 are 0.3, 0.5, and 1 wt %, the dispersibility of the filler in the resin is better and there is no agglomeration phenomenon. The isocyanate groups on the surface of GO@Fe3O4 can covalently bond with the carbamate groups in WPU, which greatly improves the dispersibility of GO@Fe3O4 and its interfacial bonding force with the WPU.[44] When the amount of GO@Fe3O4 added reaches 2 wt %, the surface of the coating is rougher and a large number of agglomerates can be observed. When the amount of GO@Fe3O4 is higher, the nanomaterials tend to agglomerate in the resin to reduce the surface free energy. The coating is subjected to reciprocating friction pressure during the friction process. Due to the weak interfacial bonding, microcracks are prone to generate and propagate between GO@Fe3O4 and WPU. These microcracks are generated under the surface of the coating and are usually difficult to observe on the surface of the coating. When these internal microcracks continue to propagate and eventually connect with the coating surface, the wear debris may be peeled from the coating, thus affecting the wear rate of the coating.

Figure 12

SEM images of the surface morphology taken from the aligned GO@Fe3O4/WPU composite coating: (a) 0.3 wt % GO@Fe3O4 content, (b) 0.5 wt % GO@Fe3O4 content, (c) 1 wt % GO@Fe3O4 content, and (d) 2 wt % GO@Fe3O4 content.

SEM images of the surface morphology taken from the aligned n class="Chemical">GO@Fe3O4/WPU composite coating: (a) 0.3 wt % GO@Fe3O4 content, (b) 0.5 wt % GO@Fe3O4 content, (c) 1 wt % GO@Fe3O4 content, and (d) 2 wt % GO@Fe3O4 content.

The overall morphology of the coating worn surface and its corresponding elementn class="Chemical">al mapping image are observed in Figure . Figure a,f,k are SEM images of worn surfaces of the GO@Fe3O4/WPU coatings with 0.5, 1, and 2 wt %, respectively. Figure b–e,g–j,l–o shows the C, O, N, and Fe element mapping images on the worn surfaces of 0.5, 1, and 2 wt % coatings. From Figure , when the GO@Fe3O4 addition values are 0.5, 1, and 2 wt %, the surface of the coatings is worn to varying degrees; the 2 wt % coating surface wear is the most serious. Due to the high concentration of filler added to the WPU, agglomeration will occur in the coating, leading to easy exfoliation of the coating and the appearance of large pieces of debris during the friction process.[24] Besides, the elemental mapping image of the worn surface of the 2 wt % GO@Fe3O4/WPU coating shows that there were several places where the element distribution was blank, indicating that the coating on the surface of the steel plate is completely exfoliated. The result corresponds to the high wear rate shown in Figure b. However, the worn surface of the 0.5 wt % GO@Fe3O4/WPU coating was smooth, as seen from the SEM images. It can be observed from the elemental mapping images that the distribution of various elements in the coating is relatively uniform, with only a small amount of coating peeling, and its wear rate is the lowest. It may be transferred to the friction counterpair during the friction process, and a thin transfer film is formed on it.

Figure 13

SEM images of the worn surface and corresponding elemental mapping images taken from the aligned GO@Fe3O4/WPU composite coating: (a) 0.5 wt % GO@Fe3O4 content; (b–e) C, N, O, and Fe element distribution of 0.5 wt % GO@Fe3O4 content; (f) 1 wt % GO@Fe3O4 content; (g–j) C, N, O, and Fe element distribution of 1 wt % GO@Fe3O4 content; (k) 2 wt % GO@Fe3O4 content; and (l–o) C, N, O, and Fe element distribution of 2 wt % GO@Fe3O4 content.

SEM images of the worn surface and corresponding elemental mapping images taken from the n class="Chemical">aligned GO@Fe3O4/WPU composite coating: (a) 0.5 wt % GO@Fe3O4 content; (b–e) C, N, O, and Fe element distribution of 0.5 wt % GO@Fe3O4 content; (f) 1 wt % GO@Fe3O4 content; (g–j) C, N, O, and Fe element distribution of 1 wt % GO@Fe3O4 content; (k) 2 wt % GO@Fe3O4 content; and (l–o) C, N, O, and Fe element distribution of 2 wt % GO@Fe3O4 content.

The SEM images of the wear surface of aligned n class="Chemical">GO@Fe3O4/WPU composite coatings with different GO@Fe3O4 contents are further observed, as shown in Figure . Figure a shows the worn surface morphology of a pure WPU coating; adhesive wear occurs on the coating. The wear surface is rough and there is a lot of wear debris distribution, which indicates that microcracks are generated and spread inside the coating during the friction process. This phenomenon corresponds to the high wear rate of the coating shown in Figure b. Comparing Figure a–d, it can be seen that with the weight fraction of GO@Fe3O4 increasing, the amount of wear debris distributed on the wear surface is significantly reduced. It can be observed from Figure c that the wear surface of the 0.5 wt % GO@Fe3O4/WPU composite coating is the smoothest and has no obvious wear debris, which shows the best wear resistance. Due to the covalent attachment between the isocyanate group on the surface of GO@Fe3O4 and the aminomethyl ester in the WPU, good compatibility and interfacial interaction between the filler and the WPU matrix are achieved. When the microcracks inside the coating spread to the vicinity of the well-dispersed GO@Fe3O4, the aligned GO@Fe3O4 in the resin matrix will extend the crack path, absorb more energy, and prevent further expansion of microcracks; meanwhile, Fe3O4 nanoparticles loaded on the GO can fill the worn part of the coating surface, so the wear resistance of the composite coating is improved. Compared with the nonaligned composite coating, the friction shearing force of the separation between the GO layers in the aligned composite coating is smaller and the aligned GO@Fe3O4 in the resin matrix can be transferred to the friction counterpair faster. So it can quickly form a uniform transfer film on the surface of the friction pair during the friction process,[23,45] which slows down the shearing effect of the metal pair on the composite coating and ultimately improves the wear resistance of the composite coating. Moreover, as shown in Figure e, when the amount of GO@Fe3O4 is 2 wt %, it can be seen that the wear surface of the coating is very rough and accompanied by wear debris. This may be attributed to the agglomerate in the 2 wt % GO@Fe3O4/WPU, so its ability to prevent microcrack propagation is drastically reduced.[46] The agglomerated GO@Fe3O4 may hinder the formation of a continuous transfer film, resulting in increased wear debris on the wear surface.

Figure 14

SEM images of the worn surface taken from aligned GO@Fe3O4/WPU composite coatings with GO@Fe3O4 contents of (a) 0 wt %, (b) 0.3 wt %, (c) 0.5 wt %, (d) 1 wt %, and (e) 2 wt %.

SEM images of the worn surface taken from aligned n class="Chemical">GO@Fe3O4/WPU composite coatings with GO@Fe3O4 contents of (a) 0 wt %, (b) 0.3 wt %, (c) 0.5 wt %, (d) 1 wt %, and (e) 2 wt %.

Based on the above experimental data and conclun class="Chemical">sions, the mechanism model of wear behavior of the GO@Fe3O4/WPU composite coating is obtained as shown in Figure . Figure a shows the nonaligned composite coating with GO@Fe3O4 added. Figure b is the friction mechanism of the coating. When the microcracks expand to the vicinity of well-dispersed and aligned GO@Fe3O4 under the action of friction, GO@Fe3O4 will deflect and extend the crack path due to the good interfacial bonding force between GO@Fe3O4 and WPU. Moreover, the energy of crack growth is absorbed by GO@Fe3O4, and the growth of microcracks is hindered. Nonaligned coatings also have a certain wear resistance. Figure c represents the aligned composite coating with GO@Fe3O4. The figure shows that GO@Fe3O4 is uniformly dispersed in the composite coating. By magnetic field induction, GO@Fe3O4 is aligned. As shown in Figure d, the aligned distribution of GO@Fe3O4 in the coating can make the stress on the coating smaller and evenly dispersed. It slows down the formation of microcracks, thereby reducing the generation of wear debris and ultimately improving the wear resistance of the coating. Moreover, when the aligned distribution of GO@Fe3O4 is subjected to friction, the frictional shear force for separation between the GO is smaller and the performance of each part of the coating surface is more even. It is easier to quickly form a uniform transfer film on the friction counterpair, which can also reduce the friction coefficient of the composite coating.

Mechanism scheme for the wear behavior of nonaligned compon class="Chemical">site coating (a, b) and GO@Fe3O4/WPU aligned composite coating (c, d).

Conclusions

In this work, surface modification of GO and n class="Chemical">Fe3O4 was carried out by chemical grafting, and Fe3O4 was successfully loaded on the GO. Then, different concentrations of GO@Fe3O4 are added to the WPU and cured in a magnetic field to obtain aligned GO@Fe3O4/WPU composite coating. The tribological experiment results show that after adding GO@Fe3O4, the friction coefficient and wear rate of the composite coating are significantly decreased. The results lead to the following five conclusions.

The successful modification of GO-IPDI and Fe3O4-KH550 and the Fe3O4 successful loading on GO surfaces are confirmed by FTIR, XRD, XPS, and TEM characterization analyses.

GO@Fe3O4 has a certain antifriction performance, and the optimal mass fraction in the composite coating is 0.5 wt %. The wear rate and friction coefficient of the composite coating are reduced to 0.25 and 3.60 × 10–4 mm3/(N·m), respectively.

The addition of aligned GO@Fe3O4 can accelerate the formation of the transfer film and further improve the tribological properties of the composite coating.

The spherical Fe3O4 nanoparticles loaded on the surface of the GO can fill the worn parts of the friction surface and play a repairing role.

Alignment and uniform distribution can extend the n class="Chemical">crack propagation path, delay the formation of wear debris, and improve the wear resistance of the coating.

Therefore, the aligned GO@Fe3O4/WPU composite material has great application potential in the field of tribology.

Experimental Section

Materials

Iron oxide (Fe3O4, 20 nm, purity ≥ 99%), 3-aminopropyl triethoxy n class="Chemical">silane (KH550, purity ≥ 99%), and isophorone diisocyanate (IPDI, purity ≥ 99%) were purchased from Aladdin Industrial Corporation. Graphene oxide (GO, thickness: 4–8 nm; layers: 5–10; lateral dimension: 10–50 μm, purity ≥ 95%) was supplied by Suzhou Tanfeng Technology Co., Ltd. Dichloromethane (purity ≥ 99.5%), N,N-dimethylformamide (DMF, purity ≥ 99.5%), and other chemicals were provided by Sinopharm Chemical Reagent Co., Ltd. Waterborne polyurethane (WPU) was obtained from Anhui Huatai New Material Co., Ltd.

Preparation of GO@Fe3O4

The preparation process of GO@n class="Chemical">Fe3O4 is shown in Figure . First, 0.2 g of GO was dispersed in 20 mL of DMF, and 1.78 g of IPDI was added into a three-necked flask under a nitrogen atmosphere with stirring for 24 h. The solution was washed 3–4 times with dichloromethane and then dried out in an oven at 80 °C for 12 h before the next modification. Second, 0.1 g of Fe3O4 was dispersed in 21.6 mL of anhydrous ethanol by ultrasonication for 30 min, and then 12 mL of KH550 was added slowly into the suspension by mechanical stirring for 2 h. The resulting Fe3O4-KH550 was washed with anhydrous ethanol 3 times, followed by drying at 80 °C for 12 h.[47] Finally, 0.08 g of GO-IPDI was dispersed in 25 mL of DMF by ultrasonication for 30 min. Later, 0.08 g of Fe3O4-KH550 was added into the suspension by mechanical stirring for 24 h. Then, the solution was washed 3–4 times with dichloromethane and GO@Fe3O4 powder was obtained by drying in an oven at 80 °C for 12 h.

Fabrication of Aligned GO@Fe3O4/WPU Composite Coatings

To study the influence of GO@n class="Chemical">Fe3O4 on the tribological properties of WPU coatings, composite coatings with 0, 0.3, 0.5, 1, and 2 wt % GO@Fe3O4 were prepared. After that, the effect of alignment distribution of GO@Fe3O4 on tribological properties of WPU coatings will be analyzed. Figure shows a schematic diagram of the magnetic-field-aligned distribution of GO@Fe3O4. For preparing the aligned GO@Fe3O4/WPU composite coating, the coating was cured in a magnetic field environment.

The galvanized steel plate was selected as the matrix materin class="Chemical">al. The galvanized steel was sanded with 800-mesh sandpaper and then underwent absolute ethanol ultrasonic treatment to remove the dirt on the surface of the steel sheet before coating. To prepare the aligned GO@Fe3O4/WPU coating, GO@Fe3O4 with different mass fractions were added to 6 g of WPU and further GO@Fe3O4 and WPU were mixed thoroughly by the ball-milling method. Next, the processed steel sheet was brushed with a wire bar coater of 70 μm coating thickness to obtain the GO@Fe3O4/WPU coating, and the coating was placed in a 0.5 T magnetic field at 60 °C for 1 h to obtain the aligned GO@Fe3O4/WPU coating, as shown in Figure . Nonaligned GO@Fe3O4/WPU coating was obtained without a magnetic field.

Characterization

The successful modification of Fe3O4 and n class="Chemical">GO and the chemical bond connection between Fe3O4 and GO were proved by Fourier transform infrared spectroscopy (FTIR, Nicolet6700) and X-ray photoelectronic spectroscopy (XPS, Escalab 250Xi, China). The internal structure and defects of GO and GO@Fe3O4 were determined with X-ray diffraction (XRD, D/MAX-2550VB+/PC, Japan) and identified with Cu Kα radiation. The microstructures and morphologies of GO, Fe3O4, and GO@Fe3O4 were characterized by transmission electron microscopy (TEM, JEM-2100F, Japan). The morphologies of the cross section, the coating surface, and the worn surface were obtained by field emission scanning electron microscopy (FESEM, SU8010, China). The elemental component information was determined by an energy-dispersive spectrometer (EDS) in the FESEM. To study the dispersibilities of GO@Fe3O4 and GO in WPU, 0.5 wt % GO@Fe3O4/WPU and GO/WPU solutions were prepared, and the dispersion degrees of GO and GO@Fe3O4 in WPU were compared after being allowed to stand for a certain period of time.

Tribological Test of the Composite Coatings

The tribological properties of the n class="Chemical">aligned GO@Fe3O4/WPU and nonaligned GO@Fe3O4/WPU composite coatings were tested by high-speed reciprocating friction and a wear testing tribometer (HSR-2M, Zhong Ke Kai Hua Corporation, China) under dry conditions. Tribological experiments were carried out three times for each composite coating of different mass fractions, and the average value of the three experimental results was taken as the average friction coefficient of the coating. In the test, a GCr15 bearing steel ball (diameter: 6 mm) was used as the friction counterpair, the friction load was 3 N, the running speed was 220 t/min, the reciprocating distance was 5 mm, and the friction time was 30 min.

The wear rates of the aligned and nonn class="Chemical">aligned GO@Fe3O4/WPU composite coatings were calculated by the formula[48,49]where A is the wear rate of the coating (mm3/(N·m)), V is the volumetric wear measured by the HSR-2M surface wear measurement component, S represents the reciprocating distance (m), and F represents the friction load (N).