Composites: Part A 81 (2016) 282–288
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Tribological properties of fluorinated graphene reinforced polyimide composite coatings under different lubricated conditions Xiangyuan Ye a,b, Xiaohong Liu a,⇑, Zhigang Yang a, Zhaofeng Wang a, Honggang Wang a, Jinqing Wang a,⇑, Shengrong Yang a a b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China University of Chinese Academy of Sciences, Beijing 100080, PR China
a r t i c l e
i n f o
Article history: Received 9 July 2015 Received in revised form 15 November 2015 Accepted 17 November 2015 Available online 24 November 2015 Keywords: A. Polymer-matrix composites (PMCs) B. Wear B. Wettability
a b s t r a c t The tribological properties of polyimide (PI) and PI/fluorinated graphene (FG) nanocomposites, as a new class of graphene reinforced polymer, are investigated using a ball-on-disk configuration under different lubricated conditions of dry sliding, water lubrication and oil lubrication. Experimental results reveal that single incorporation of FG can effectively improve the tribological performance of PI under all the three conditions. In addition, compared to the results under dry sliding, the phenomenon that the friction coefficient decreases while the wear rate increases under water lubrication condition is observed and researched in detail. The worst anti-wear performance under water-lubricated condition can be ascribed to the fact that the water can be adsorbed by the polar imide radicals of the PI and PI/FG nanocomposite, therefore leading to the property deterioration of the PI and PI/FG nanocomposite coatings. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction The fluorinated graphene (FG), as the youngest graphene derivative, has become one of the biggest hotspots in materials science [1–4]. Numerous experimental and theoretical studies [5,6] indicate that the FG is endowed with excellent mechanical properties of the graphene, and also possesses the distinctive performances which are determined by the structure of graphene combined with fluorine. Based on the excellent performances, the FG brings a wide prospective application in different fields, such as superhydrophobic materials , organic electronics , hydrogen storage  and gas separation . Beyond its application for these fields, FG also has huge potential for lubrication. According to the report , FG can reduce the friction because of the low interlayer interaction induced by the repulsive electrostatic forces between F atoms at the interfaces. At the same time, our previous work indicates that FG can be an effective lubricant additive to enhance the friction reduction and anti-wear performances of lubricated oil . However, up to now, there is no systematic study paying attention to the tribological properties of the FG enhanced polymer, although some polymer/FG composites have been prepared successfully [13–16]. ⇑ Corresponding authors. Tel.: +86 931 4968076; fax: +86 931 8277088. E-mail addresses: [email protected]
(X. Liu), [email protected]
(J. Wang). http://dx.doi.org/10.1016/j.compositesa.2015.11.029 1359-835X/Ó 2015 Elsevier Ltd. All rights reserved.
The polyimide (PI) possesses excellent thermostability and mechanical properties, which has been widely applied in the microelectronics and aerospace industries [17–20]. However, the high friction coefficient and wear rate of pure PI go against its further application . Thus, utilizing nanofillers to enhance the tribological properties of PI has been the center of attention. For example, carbon nanofibers [22,23], carbon nanotube  and graphene oxide  have been successfully applied to improve the tribological properties of PI. Based on our previous work , FG can be regarded as one kind of novel nanofiller to enhance the thermal and mechanical properties of PI matrix. In addition, the enhancement in wear resistance of a material is always related to its excellent mechanical performances . Therefore, it can be expected that the FG will be an outstanding nanofiller to improve the tribological properties of PI. At the same time, taking into consideration that PI and its composites are usually applied in different lubricated conditions of water lubrication or oil lubrication ; however, there is no systematic study on their tribological behaviors in these situations. Therefore, in this work, the PI and PI/FG nanocomposite coatings are firstly prepared on the steel blocks. Then, their corresponding tribological properties are tested and analyzed under drying sliding, water-lubricated and oil-lubricated conditions in order to find the addition effect of the FG on the tribological properties of the PI matrix under these conditions.
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2. Experimental section 2.1. Materials N,N0 -dimethylacetamide (DMAc) and liquid paraffin are obtained from Tianjin Chemical Reagent Company. Pyromellitic dianhydride (PMDA) and N-methyl-2-pyrrolidone (NMP) are purchased from Sinopharm Chemical Reagent Co., Ltd. 4,40 oxydianiline (ODA) is purchased from Shanghai Kefeng Chemical Reagent Co., Ltd. Fluorographite (FGi) is bought from Shanghai CarFluor Ltd. All chemicals are analytical grade and used as provided. Ultrapure water (>18 MX cm) is used in this work. 2.2. Preparation of the FG The preparation process of the FG is described clearly in our previous paper . Briefly, 100 mg FGi is dispersed in 20 mL NMP and the mixed solution is heated at 60 °C for 2 h. Then, the ultrasonication is carried out to further exfoliate FGi. The resultant product is filtered and freeze-dried to get the FG. 2.3. Preparation of the PI and PI/FG nanocomposite coatings A two-step pathway is applied to prepare the PI and PI/FG nanocomposite coatings as follows: (1) the syntheses of the poly
(amide acid) (PAA) and PAA/FG nanocomposite coatings, (2) the subsequent thermal imidization of these coatings to achieve PI and PI/FG coatings. Firstly, 2.0 g ODA is dissolved completely in 30 ml DMAc with stirring in a three-necked flask. Secondly, the calculated amount of the FG is ultrasonic dispersed in solution as described above for 2 h to get stable FG dispersion, and then 2.18 g PMDA is added to react with ODA by stirring in the ice-water bath for 4 h. Finally, the solution is cast on the steel block substrate and dried at 80 °C for 6 h in a vacuum oven to remove the residual solvent, followed being heated at 80 °C for 2 h, 135 °C for 2 h, and 300 °C for 2 h to perform thermal imidization. The PI and PI/FG nanocomposite coatings (PI, PI/FG-0.25, PI/FG-0.5, PI/FG-0.75 and PI/FG-1) containing 0 wt.%, 0.25 wt.%, 0.5 wt.%, 0.75 wt.% and 1 wt.% of FG are prepared by the above-mentioned experimental processes. 2.4. Characterizations Transmission electron microscopy (TEM, Tecnai G2 TF20, operated at 200 kV) and atomic force microscopy (AFM, Agilent Technologies 5500) are applied to observe the morphology and microstructure of the FG. Microhardness values of samples are tested by Nanomechanical Test Instruments (TI-950, Hysitron Inc.). Macrotribological tests are run on an UMT-2MT tribometer (USA, CETR) in a ball-on-plate contact configuration. Commercially available steel ball (u = 6 mm) is used as the stationary upper
Fig. 1. TEM image (a), energy-dispersive spectrum (EDS) (b) and AFM images (c) of the prepared FG. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 2. Optical photos of the PI and PI/FG nanocomposite coatings. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
counterpart, while the PI nanocomposite coatings are mounted onto the flat base and driven to reciprocally slide at a distance of 0.5 cm. The friction coefficient-versus-time curve is recorded automatically and at least three repeated measurements are performed for each frictional pair. The wear volume is measured on a MicroXAM 3D non-contact profilometer (ADE Phase Shift Inc., USA) with phase mode and each data point is carried out through five measurements. The electronic balance weigh is made by Sartorius (BP 221 S) with the metering accuracy of 0.1 mg. Water contact angles (WCA) of the coating samples are determined using a DSA100 contact angle meter, and data are average values of at least five repeat measurements for each sample. The scanning electron microscope (SEM) (JSM 5600LV) is utilized to observe the morphology of the coatings. The viscosity of water and liquid paraffin are tested by Kinematic viscosity meter (SYP1003-III).
3. Results and discussion 3.1. Characterization of the prepared FG As disclosed in SEM image of Fig. 1(a), the prepared FG is transparent with a few micrometers in lateral size. The AFM images (Fig. 1(c)) illustrate that the thickness of the FG nanosheet is about 4 nm, corresponding to the layer numbers of 3–5 based on the fact that the thickness of the FG monolayer is 0.67–0.87 nm. The EDS detects the elementary composition of the prepared FG, except the Cu resulted from the Cu grids, the existed elements are C and F, which suggests that the prepared FG is extraordinarily pure, as shown in Fig. 1(b). Therefore, the pure FG with a high aspect ratio (the ratio of lateral size to thickness) is obtained, which can play significant role in enhancing tribological properties.
Fig. 3. SEM image of worn surface for the samples of PI (a), PI/FG-0.5 (b) and PI/FG-1 (c); (d) the histogram of friction coefficient and wear rate of all coatings under drying sliding condition. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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3.2. Morphology of the PI and PI/FG nanocomposite coatings As shown in Fig. 2, the prepared PI and PI/FG nanocomposite coatings are smooth and flat with the thickness of about 0.1 mm. Due to optical opacity of the FG, it can be clearly found that the color of the nanocomposite coatings becomes darker and darker with the content increase of the FG. The excellent film-forming ability of the PI and PI/FG nanocomposites on the steel surface is the precondition to test the tribological properties successfully. Meanwhile, the dispersion of the FG has been tested by the TEM. As shown in Fig. S1 in the Supporting Information (SI), on one hand, the FG is dispersed well in the PI matrix; on the other hand, the dispersion of the FG is denser and denser with the increase of the FG content. In order to investigate the tribological properties of the PI and PI/ FG nanocomposite coatings more comprehensively, the tribological tests are carried out on an UMT-2MT tribometer at a load of 20 N and a frequency of 3 Hz for 30 min under different lubricated conditions of drying sliding, water lubrication and oil lubrication.
3.3. Tribological properties under drying sliding condition The tribological properties of the PI and PI/FG nanocomposite coatings under drying sliding condition are tested and the results are displayed in Fig. 3. Clearly, the worn surfaces of the PI and PI/ FG nanocomposite coatings exhibit severe plastic deformation with some microcracks, as shown in Fig. 3(a)–(c). Specifically, larger and denser microcracks (marked with red cycle) are created on the worn surface of the PI (Fig. 3(a)) and the PI/FG-1 (Fig. 3(c)) while
smaller and sparser microcracks are observed on the worn surface of the PI/FG-0.5 (Fig. 3(b)). Such finding suggests that the addition of 0.5 wt.% FG can effectively enhance the tribological properties of the PI matrix. The variations of friction coefficient and wear rate of the PI/FG nanocomposite coatings related to the contents of FG is summarized in Fig. 3(d). Obviously, with the content increase of the FG, although the friction coefficient of PI/FG nanocomposite coatings changes slightly while the wear rate displays a trend of first decrease and then increase. When the addition content of the FG is 0.5%, the wear rate of the PI/FG nanocomposite coating reaches a minimum value of 0.87 105 mm3/N m, which is a 37.4% decrease compared to the pure PI (1.39 105 mm3/N m). The similar results also can be reflected by the size changes of the wear surfaces and the wear debris which are shown in Figs. S2 and S3 in the SI.
3.4. Tribological properties under water-lubricated condition Under the water-lubricated condition, the tribological properties of the PI and PI/FG nanocomposite coatings are shown in Fig. 4(a)–(c) and large wear debris are generated on the worn surfaces of the PI, the PI/FG-0.5 and PI/FG-1, indicating that the molecular chains of both the PI and PI/FG nanocomposite coatings are liable to break to form large wear debris under water-lubricated condition. The friction coefficient and wear rate of the PI and PI/FG nanocomposite coatings are summarized in Fig. 4(d). Obviously, with the content increase of the FG, the friction coefficient of the PI/FG nanocomposite coatings does not decrease obviously, while the wear rate reduces from 2.41 104 mm3/N m for the pure PI to 1.51 104 mm3/N m
Fig. 4. SEM image of worn surface for the samples of PI (a), PI/FG-0.5 (b) and PI/FG-1 (c); (d) the histogram of friction coefficient and wear rate of all the coatings under waterlubricated condition. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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for the PI/FG-0.5. The variation trend of the wear rate is consistent with the size changes of the worn surface and the wear debris, as shown in Figs. S4 and S5 in the SI. 3.5. Tribological properties under oil-lubricated condition Under the oil-lubricated condition, the tribological properties of the PI and PI/FG nanocomposite coatings are shown in Fig. 5. For the sample of pure PI, there is obvious scuffing appeared on its worn surface (Fig. 5(a)), while no scuffing almost visible on the worn surface of the PI/FG-0.5 (Fig. 5(b)). However, for the sample of PI/FG-1, the scuffing appears again on its worn surface (Fig. 5 (c)). The morphology variation of the worn surface bespeaks the variation trend of the wear rate. As clearly summarized in Fig. 5 (d), the addition of FG significantly enhances the anti-wear performance of the PI and trivially reduces the friction coefficient. Especially, the PI/FG-0.5 shows the minimum wear rate value of 1.42 106 mm3/N m, which is a 61.1% decrease compared with the value of 3.65 106 mm3/N m for the pure PI. The size reduction of the worn surface also demonstrates that the PI/FG-0.5 possesses the best tribological properties, as shown in Fig. S6 in the SI. As discussed above, under the three conditions, the tribological properties of the PI and PI/FG nanocomposite coatings exhibit similar trend: the friction coefficient changes slightly, while the wear rate reduces significantly when the content of the FG is 0.5%. Taking into account the report that excellent mechanical properties accompany outstanding tribological properties , and the fact that the addition of FG can greatly improve the mechanical properties of the tensile stress and elongation at break of the PI (when the addition of the FG is 0.5%, the tensile stress of the PI/FG nanocom-
posite film reaches a maximum value of 110.6 MPa, which is a 30.4% increase compared to the pure PI (84.8 MPa)) . On the other hand, even though the addition of FG can enhance the microhardness of the PI (234 MPa for PI, 243 MPa for PI/FG-0.25, 246 MPa for PI/FG-0.5, 239 MPa for PI/FG-0.75 and 249 MPa for PI/FG-1), there is no significant difference among different PI/FG composite samples. Therefore, for all the prepared composites, the sample of PI/FG-0.5 with the superior mechanical properties of the tensile stress and elongation at break resulting from the appropriate adding dosage of FG possessed the low deformation and fragmentation, leading to the smallest wear rate.
3.6. Comparison of tribological properties under the three conditions In order to research tribological properties under the three conditions, the friction coefficient and wear rate of the same specimen under different lubricated conditions are compared. As shown in Fig. 6, no matter the PI or the PI/FG nanocomposite coatings, the friction coefficient and wear rate exhibit similar variation trend. More specifically, the friction coefficients of the PI and PI/FG nanocomposite coatings are about 0.4 under drying sliding condition, about 0.2 under water-lubricated condition while about 0.03 under oil-lubricated condition, which is summarized in Fig. 6(a). The typical friction process for different lubricating conditions has been given in Fig. S8. However, it should be noticed that although the friction coefficient reduces obviously under water lubrication, the wear rate does not reduce correspondingly, while it increases greatly compared with the one under drying sliding condition for the same specimen, as displayed in Fig. 6(b).
Fig. 5. SEM image of worn surface for the samples of PI (a), PI/FG-0.5 (b) and PI/FG-1 (c); (d) the histogram of friction coefficient and wear rate of all the coatings under oillubricated condition. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 6. The histogram of friction coefficient (a) and wear rate (b) of the PI and PI/FG nanocomposite coatings under the three conditions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Under drying sliding condition, the immediate contact between the surface of the specimen and the steel ball generates high friction coefficient. Under water-lubricated condition, the friction coefficient is reduced due to the formation of water-boundary lubricated layer between the specimen and the steel ball, because the distilled water can be used as a kind of polar lubricant and reduce the direct contact zone of the specimen with the counter face, further reduce the friction coefficient . Due to the similar mechanism and the excellent lubricated property of the oil itself, the oil-boundary lubricated layer between the specimen and the steel ball leads to the lowest friction coefficient under all the three conditions. Herein, the lubricated property of the oil is better than that of the water, which may be attributed to the fact that the viscosity of the oil (liquid paraffin) is 51 times higher than the water at the temperature of 30 °C (41.63 mm2/s for liquid paraffin and 0.81 mm2/s for water). The higher viscosity of the oil results in the fact that the thickness of the oil-boundary lubricated layer is bigger than the water-boundary lubricated layer , which urges better boundary lubricated effects to further reduce the friction coefficient. Generally, liquid-boundary lubricated layer can reduce friction coefficient and often accompany with reduction of wear rate; however, this is not always true [29–32]. Under water-lubricated condition, the phenomenon that the friction coefficient decreases while the wear rate increases can be ascribed to the fact that the water can change the properties of the PI and PI/FG nanocomposite coatings . According to the report , the polar imide radicals of the PI molecules are liable to adsorb water and the adsorbed water can lead to swelling and softening of polymer matrix, causing the reductions in hardness and strength of polymer, and then making it easier to detach debris from the worn surface of polymer . Therefore, the wettability of the PI and PI/FG nanocomposite coatings was subsequently investigated. As shown in Fig. 7, as for water contact angle, there is no obvious difference between the PI and the PI/FG nanocomposite coatings, although the FG is hydrophobic. At the same time, the water contact angles of all the specimens are less then 90°, indicating the surfaces of the specimens are hydrophilic. Meanwhile, in order to investigate the water absorbing capacity of the PI and PI/FG nanocomposite coatings, each specimen with the mass of 100 mg is immersed in water for 30 min. The water absorbing capacity of the PI and PI/FG is about 13 mg(water)/g(PI), demonstrating the addition of FG almost cannot apparently influence the water absorbing capacity of the PI, as clearly shown in Fig. S9 in the SI. Based on the above investigations, it can be concluded that the polar imide radicals, the hydrophilic property and the water absorbing capacity of the PI and PI/FG nanocomposite coatings cause the
Fig. 7. The water contact angle of the PI and PI/FG nanocomposite coatings under the three conditions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
specimens to absorb water under water-lubricated condition. The absorbed water will diffuse through the microcracks (as shown in Fig. 3(a)–(c)) to swell and soften the polymer, further leading to the strength decrease of the surface layer. As a result, the debris (as shown in Fig. 4(a)–(c)) are detached from the worn surface of the PI and PI/FG nanocomposite coatings; correspondingly, the wear rate of the specimens under water-lubricated condition is much higher than that under the drying sliding condition. For comparison, the oil absorbing capacities of the PI and PI/FG nanocomposite coatings are also investigated. After each specimen (100 mg) has been immersed in liquid paraffin for 30 min, the weight of each specimen is hardly changed. The results illustrate that the PI and PI/FG nanocomposite coatings cannot absorb liquid paraffin; thusly, the optimal performance of friction reduction and anti-wear under oil-lubricated condition is attributed to the excellent tribological properties of liquid paraffin. 4. Conclusions The tribological properties of the PI and PI/FG nanocomposite coatings have been investigated under drying sliding, waterlubricated and oil-lubricated conditions. The results illustrate the addition of FG can effectively enhance the anti-wear performance of the PI; thereinto, the sample of PI/FG-0.5 displays the best tribological properties under the three conditions. Its superior wear resis-
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tance is attributed to the increase in mechanical properties of the tensile stress and the elongation at break resulting from the FG. At the same time, the phenomenon that all specimens have the maximum wear rate under the water-lubricated condition is found and investigated detailedly. The fact of the water absorbing property of the PI and PI/FG nanocomposite coatings takes the dominant place in increase the wear rate under the condition of water lubrication compared with those under the conditions of drying sliding and oil lubrication. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 51375474, 51575507 and 51205385), 863 Plan (Grant No. 2015AA034602) and the ‘‘Funds for Young Scientists of Gansu Province (145RJYA280)’’ scheme.
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