Enhancing tribological, mechanical, and thermal properties of polyimide composites by the synergistic effect between graphene and ionic liquid

Materials and Design 189 (2020) 108527

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Enhancing tribological, mechanical, and thermal properties of polyimide composites by the synergistic effect between graphene and ionic liquid Hong Ruan a,1, Qiu Zhang a,1, Weiqiang Liao a, Yuqi Li a,b,⁎, Xiaohua Huang a, Xu Xu a, Shaorong Lu a a Key Laboratory of New Processing Technology for Nonferrous Metals and Materials, Ministry of Education, College of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, China b Faculty of Engineering Science, University of Bayreuth, Universitätsstr. 30, 95440 Bayreuth, Germany

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• The mechanical properties of polyimide could be improved by the addition of ionic liquid functionalized graphene. • The thermal properties of polyimide could be enhanced by the addition of ionic liquid functionalized graphene. • A synergistic improvement in tribological properties was obtained by adding ionic liquid and graphene.

a r t i c l e

i n f o

Article history: Received 8 October 2019 Received in revised form 24 January 2020 Accepted 25 January 2020 Available online 27 January 2020 Keywords: Ionic liquid Graphene Synergistic effect Polyimide Tribological properties

a b s t r a c t Developing self-lubricating composites with excellent tribological, thermal, and mechanical properties is a key aspect to solving the wear of materials. Herein, ionic liquid functionalized graphene (ILFG) were prepared by a π-π stacking interface design strategy. Then, a range of ILFG was filled into the polyimide (PI) matrix to prepare ILFG/PI composites (IGPI), and their tribological properties were investigated with a plate-on-ring apparatus under dry sliding conditions. When the content of ILFG was 0.4 wt%, the friction coefficient and specific wear rate of IGPI decreased by 38.2% and 25% compared to pure PI, respectively. A synergistic lubrication mechanism of ionic liquid (IL) and graphene was proposed for the formation of tribofilm under dry sliding. In addition, the mechanical and thermal properties of IGPI were also significantly improved by the synergy of IL and graphene. When the content of ILFG was 0.4 wt%, the tensile strength and modulus of IGPI composites increased by 51.9% and 56.5% relative to pure PI, respectively. The decomposition temperature of 5% weight loss (T5%) of IGPI with 0.4 wt% ILFG could reach 581.3 °C, an increase of 52.6 °C compared to that of PI, and the glass transition temperature improved by 14 °C. © 2020 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction ⁎ Corresponding author at: Key Laboratory of New Processing Technology for Nonferrous Metals and Materials, Ministry of Education, College of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, China. E-mail addresses: [email protected] (Y. Li), [email protected] (X. Xu), [email protected] (S. Lu). 1 These authors contributed equally to this work.

With rapid technological advancements, the frequent use of mechanical equipment has made the problem of friction and wear particularly prominent. It is reported that approximately 30% of the energy consumption in automobiles alone is overcoming friction, and the

https://doi.org/10.1016/j.matdes.2020.108527 0264-1275/© 2020 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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tribology is also responsible for material losses and CO2 emissions [1,2]. Due to the anti-fouling, anti-friction, shock absorption, and noise reduction properties, polymer-based self-lubricating composites are ideal for use in harsh applications in the mechanical and automobile industries [3–5]. Polyimide (PI), a polymer-based self-lubricating material, is capable of resisting wear and extending the life of the components. PI has been extensively applied in the aerospace, maritime, mechanical, and auto industries, owing to its prominent thermal, mechanical, and tribological properties [6–9]. Nevertheless, pure PI cannot be used under some harsh conditions, showing disadvantages such as high friction coefficient, high wear rate, and high brittleness. In recent years, in order to broaden the practical application of PI, commercial and research groups across the world are continually striving to synthesize PI composites tailored by fillers to serve specific operating conditions [10–12]. Since the discovery of graphene (GN) in 2004 by Geim, the tribological properties of GN and GN-based lubricant composites have appealed to a range of interests from researchers in recent years [13,14]. It is proven that the incorporation of GN has a positive impact on promoting the tribological performance of the polymer matrix, and the friction mechanism can be attributed to the fact that GN gradually form a transfer film with a lubricating effect on the surface of the friction pair [15,16]. GN, a two-dimensional nanomaterial composing of carbon atoms in a sp2 hybrid manner, are endowed with extraordinary inherent properties such as mechanical, thermal, electrical, magnetic, and tribological properties. Yet, due to the presence of van der Waals forces in the GN structure, it easily agglomerates in the polymer matrix when employing it as a filler, which results in poor dispersion and affects the stability of the transfer film on the friction pair [17–19]. To obtain high-performance GN-based PI composites, the following problems need to be solved: the first problem is improving the dispersion of GN in the PI matrix and its interface compatibility with the matrix [20]. The other problem is that modified GN has the ability to form a stable transfer film. To solve the mentioned problems, surface treatment of GN is required [21]. Ionic liquids (IL), composed of relatively large organic cations and weakly coordinating inorganic anions, generally possess a unique structure and unique characteristics [22]. Therefore, IL can be an excellent modifier for GN functionalization [23]. Gusain et al. [24] synthesized graphene-IL hybrid nanomaterials using covalent grafting of imidazolium rings to facilitate the dispersion of graphene-IL in polyethylene glycol synthetic lube base oil. However, the covalent modification of GN will affect its own structure and its performance cannot be optimized, so the non-covalent modification of GN such as π-π stacking is worth considering. Wang et al. [25] prepared fluoroether rubber composites by IL-modified graphene (TrGO-IL) and demonstrated that the modification of IL on graphene makes TrGO-IL form a strength interface with the rubber matrix. Fan et al. [26] reported a 27% friction reduction and a 74% wear reduction for multi-alkylated cyclopentanes by lubrication with IL-functionalized GN, which is supposed to be the result of the stable transfer film formation between sliding surfaces. Zhao et al. [27] proved that IL-modified graphene oxide (GO) possesses more uniform dispersion in epoxy resins and better lubrication enhancement than as-exfoliated GO. Saurín et al. [28] acknowledged an improved wear resistance of the epoxy composites containing IL and GN and mechanically mixed IL and GN have a plasticizing effect on epoxy resin. The reason why IL has excellent tribological properties is that the anion and cations constituting IL usually contain various frictional active elements such as N, F, B, or P, and these active elements decompose and distribute under certain conditions on the surface of the metal frictional pair to form a boundary lubrication film in the friction test [29,30]. Currently, the existence of a synergistic lubrication mechanism between IL and GN is not fully understood. Meanwhile, the problem of the dispersion of GN in the matrix affecting the performance of the composites has not been fully solved. In view of van der Waals forces and cation-π and π-π stacking interactions between IL and GN, we prepared IL-functionalized GN (ILFG) in an effective and simple way in this work.

After verifying the modification of GN by IL, ILFG was filled into the PI matrix via the solution blending method, and the tribological properties and friction mechanism, mechanical properties, and thermal properties of the PI and its composites were systematically explored. 2. Material and methods 2.1. Materials 1-Butyl-3-methylimidazolium tetrafluoroborate (IL) was obtained from Lanzhou Institute of Chemical Physics of the CAS. Graphene nanosheets (lateral sizes: 0.5–3 μm, thicknesses: 0.55–3.74 nm) were acquired from Ningbo Institute of Industrial Technology of the Chinese Academy of Science. 4,4′-Oxydianiline (ODA) and 4,4′-oxydiphthalic anhydride (ODPA) were purchased from Aladdin Reagent Co., Ltd. and Shanghai Adamas Reagent Co., Ltd., respectively. N-methylpyrrolidone (NMP) was purchased from Beijing Innochem Sci. &Tech. Co. Ltd. 2.2. Preparation of ILFG First, 0.9 g IL and 0.09 g GN was ground in a mortar for 10 min and the mixture was dissolved in 20 mL absolute ethanol for ultrasonic dispersion for 1 h, followed by mechanical stirring at 80 °C for 24 h. After the reaction completed, the suspension was filtered and washed with absolute ethanol several times to remove excess IL, and then the obtained filtrate was dried under vacuum to obtain a black product; that is, GN functionalized with IL was obtained. The loading of IL on GN is approximately 22 wt%. 2.3. Preparation of PI and IGPI composites As shown in Fig. 1, PI and its composites were prepared by synthesizing the precursor of PI (polyamic acid, PAA) and thermal amidation. 0.0025 mol ODA and 3 mL NMP were placed in a clean and water-free flask, and the magnetic stirrer was turned on to adjust the magnet's rotational speed to 600 rpm/min. Afterwards, 0.0025 mol ODPA and 4 mL NMP were introduced to the system. The above process was controlled within 15 min. Finally, the reaction system was kept stirring at room temperature (RT) under N2 for 22 h to obtain a viscous PAA solution. Next, ILFG sonicated for 2 h in 3 mL NMP were added to the PAA solution and stirring was continued for 2 h. To facilitate the characterization of the properties of the composites, the resulting ILFG/PAA solution was molded onto a dry, smooth, and flat glass or steel substrate. It was then thermally imidizated according to the procedure in a high temperature oven. The program for thermal imidization was set to 70 °C for 4 h; 100 °C, 200 °C, and 300 °C for 1 h at each temperature. Finally, after successfully removing the solvent and bubbles and completing the curing, the IGPI composites were obtained. According to the above experimental procedures, the PI, GN/PI and IGPI composites with ILFG content (0.08 wt% (IGPI-1), 0.24 wt% (IGPI-3), 0.4 wt% (IGPI-5), 0.56 wt% (IGPI-7)) were prepared. 2.4. Characterization of ILFG and IGPI composites A Fourier-transform infrared spectra (FTIR) of GN and ILFG in a wavenumber range from 500 to 4000 cm−1 were characterized by an infrared spectrometer (Thermo Nicolet Nexus 470, USA) using a KBr wafer. A Thermo Scientific DXR Raman Microscope with 532 nm laser excitation (DXR, USA) was applied to record Raman spectra of GN and ILFG. The ESCALAB 250Xi instrument (ESCALAB 250Xi, USA) was used to record the X-ray Photoelectron Spectroscopy (XPS). A Field Emission Transmission electron microscope (TEM, JEM-2100F, Japan) was applied to observe the structures of GN and ILFG. In line with the National Standard of China (GB1040-92), the mechanical properties of PI and its composites were tested on a universal tensile tester (UTM4503BLXY, China) with the rate of 2 mm/min, where the shape of the sample is

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Fig. 1. The preparation process of IGPI composites.

dumbbell-shaped, 35 mm long and 2 mm wide, and the test for each film was repeated at least three times. A Field Emission Scanning electron microscope (FESEM, S-4800, Japan) was employed to observe the fractured surface morphologies. The thermal decomposition temperatures of PI and its composites were analyzed via a thermo-gravimetric analyzer (TGA Q-500, Belgium) at a heating rate of 10 °C/min up to 800 °C under flowing N2. The glass-transition temperature (Tg) of samples were acquired by a differential scanning calorimetry spectrometer (DSC-204, Germany) at a heating rate of 10 °C/min under flowing N2.

2.5. Tribological testing The tribological performance of PI and its composites was conducted on a high-speed block-on-ring friction and wear test machine (MRH-3A, China) at room temperature. The digital photograph and schematic diagram of the friction pair are displayed in Fig. 2. During the friction process, the tribological properties were compared by testing the friction coefficient of PI and its composites under different loads (15 N, 20 N, 25 N, and 30 N) and rotation speeds (0.26 m/s, 0.52 m/s, 0.78 m/s, and

Fig. 2. The digital photograph and schematic diagram of the friction pair.

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1.04 m/s) at RT. The counterpart was a GCr15 (C 0.95–1.05 wt%, Mn 0.25–0.45 wt%, Si 0.15–0.35 wt%, Pr 0.025 wt%, Sr 0.025 wt%, and Cr 1.40–1.65 wt%) steel ring in contact with the composites to be tested; the real-time data generated after the sensor responded to the rotation of the ring and signaled to the online coefficient of friction acquisition system. When the test was completed, the resulting coefficient of friction was the average of all data points throughout the test. Using a vernier caliper to measure the width of the specimens, the specific wear rate was calculated via the following formula [32]: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi13 0 2 2 πR h h h R2 − A5  arcsin@ − B4 2R 2 180 4 2

W¼

Ld

;

where B is the width of composite specimens (mm), R is the radius of the rotating ring (mm), h is the wear track of composite sample (mm), W is the specific wear rate (mm3/Nm), d is the friction travel (mm), and L is the applied load (N). 2.6. Characterization of worn surfaces and tribofilms The worn surfaces and wear tracks of specimens were observed by FESEM. FESEM, Energy Dispersive X-ray Spectroscopy (EDS, S-4800, Japan), Thermo Scientific DXR Raman Microscope and XPS were applied to verify the tribofilms. 3. Results and discussion 3.1. Analysis of IL functionalized GN The functionalization process of GN is confirmed by FTIR spectra as is seen in Fig. 3(a). In the spectra of GN, there are two significant absorption peaks appearing at approximately 3444 cm−1 and 1634 cm−1, which may be caused by the bending vibration peak of C-OH or the vibration peak of water molecules. In addition, the absorption peaks at

2921 cm−1 and 1578 cm−1 correspond to the C\\H stretching vibration and the weak bending vibration absorption peak of C\\C, respectively, indicating that small amounts of C\\OH and\\CH3 exist on the surface of GN in this work. In the spectra of ILFG, the peaks at 3413 cm−1 and 3081 cm−1 are attributed to the H\\C_C\\H anti-symmetric stretching and NC(H)N stretching on the imidazole ring of the IL [33]. The stretching vibrations of\\CH2 and\\CH3 in the IL should be responsible for the absorption peaks at 2870 cm−1 and 2959 cm−1, respectively. The feature peaks at 1632 cm−1 and 1569 cm−1 ascribe to the H\\C_C\\H shear vibration peak, and the appearance of characteristic peaks at 1461 cm−1 and 1385 cm−1 is due to the C\\N and C_N in the IL's structure, respectively. Meantime, the strong peak at a wavenumber of 1079 cm−1 in the spectra of ILFG is assigned to the B\\F bond, which is from the IL [31]. Consequently, FTIR spectra provide evidence that GN has been modified by IL. Raman spectroscopy is a very effective means of characterizing carbon materials because it can provide information on characteristics such as electronic structure, phononic structure, and defect structure. The Raman spectrum of GN and its derivatives mainly includes the defect peak (D peak) generated by the sp3 hybrid carbon atom vibration and the tangential vibration peak (G peak) reflecting the sp2 carbon atom in the hexagonal plane. Raman spectroscopy data of GN and ILFG are presented in Fig. 3(b). The degree of defects in GN materials can be reflected by the intensity of the D-band to that of the G-band (ID/IG). ID/IG of GN is approximately 0.21, while that of ILFG rises to 0.45, which could explain the fewer defects in unmodified GN and that the introduction of IL leads to slight defects [34]. Simultaneously, we can see that the peak shape of ILFG is similar to that of GN, but the position of the G peak of ILFG is slightly shifted, meaning that the modification of GN by IL has a slight effect on the lattice of GN. Fig. 3(c) and (d) is the TEM micrographs of GN and ILFG. Based on the morphology, it is seen that GN and ILFG are both roughly in the form of a sheet, revealing that the non-covalent bond modification of IL could not exert an obvious effect on the structure of GN. Furthermore, a slight agglomeration phenomenon was generated due to the strong van der Waals force between the layers and the stability of the GN

Fig. 3. (a) The FTIR spectra of GN and ILFG; (b) the Raman spectra of GN and ILFG; the TEM micrographs of GN (c) and ILFG (f).

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structure. However, as shown in Fig. 3(d), after introducing the imidazole ring on the surface of GN, the layers and transparency of ILFG became thinner and lighter, respectively, representing an improvement in dispersion. X-ray photoelectron spectroscopy (XPS) provides further evidence for the detection of chemical states of typical elements in samples. Comparing Fig. 4(a) and (b), it can be clearly seen that GN only include the C element from the carbon skeleton and the O element in the oxygencontaining group in the partial base or edge, but ILFG contain three additional elements: N, F, and B. In addition, the C1s peak is fitted, and the result is shown in Fig. 4(c). The peak appearing at the binding energy 286.8 eV is from the C\\N bond, while the N1s band appears again at 402.5 eV. With a lower binding-energy shoulder at 400.4 eV, the peak is also caused by the C\\N bond, which confirms the interaction of N atoms in IL with C atoms in GN and the adsorption of NC(H)N on the IL imidazole ring to the surface of GN [35]. Additionally, combining Fig. 4(d) with (f), BF4 in IL should be responsible for the peaks at 194.6 eV (B1s) and 686.5 eV (F1s) [31]. The results indicate that the imidazole ring in the IL structure interacts with sp2-hybridized GN through π-π, cation-π non-covalent bonds and van der Waals interactions. This analysis is consistent with the previous experimental results such as FTIR and Raman; thus, it is evident that GN was successfully modified by IL. The dispersion of the filler has a critical impact on the properties of the composites. To investigate the effect of IL functionalization on GN, the dispersion comparison experiments of GN and ILFG in deionized water, NMP, absolute ethanol, and DMF are carried out, as shown in Fig. S1(a)–(d). All suspensions (1.5 mg/mL) are sonicated at a power of 100 W for 1 h before the experiment and then allowed to stand for two days. The dispersion experiment of ILFG in PAA is also tested and the results obtained are shown in Fig. S1(e). Apparently, GN precipitates and agglomerates in each solvent to varying degrees, while ILFG shows better dispersion and stability. Causes of the phenomenon are likely because IL inserts into the GN layers and attaches on the surface of GN, thereby increasing the interlamellar spacing, which can effectively prevent the agglomeration of GN. 3.2. The tribological properties and mechanism of PI and IGPI materials In this work, the friction and wear behaviors of PI and its composites with ILFG at different loads (15 N, 20 N, 25 N, and 30 N) and rotation

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speeds (0.26 m/s, 0.52 m/s, 0.78 m/s, and 1.04 m/s) under dry friction at RT are systematically investigated to determine the superiority of ILFG, as is shown in Fig. S2. The trend of almost all of the friction test results are similar under the selected conditions, hence the tribological performance of PI and its composites under 15 N and 1.04 m/s are taken as an example. Based on this, the coefficient of friction and specific wear rate of PI and its composites are summarized in Fig. 5. A significant improvement in the coefficient of friction and specific wear rate of PI and its composites indicates that the synergistic effect between GN and IL is beneficial to the tribological performance of the PI matrix. When incorporating 0.4 wt% ILFG, the coefficient of friction of IGPI-5 is reduced by 38.2% in comparison with that of pure PI and is much lower than other PI-based composites containing GN and ILFG. In addition, the wear rate of IGPI-5 reaches the minimum value which decreased by 25% compared with pure PI. The SEM micrographs of the wear surfaces of PI and its composites are displayed in Fig. 6(a)–(f), the worn tracks of which are shown in Fig. S3(a)–(f). As shown in Fig. 6(a)–(f), the worn surfaces of PI and PI-based composites exhibit typical fatigue wear and abrasive wear characteristics. The PI exhibits a rough and discontinuous wear surface with many microscopic cracks, deep and wide furrows, and a large amount of wear debris, whereas smaller and sparser cracks, flatter surfaces, and less wear debris are observed on the worn surfaces of the PIbased composites containing the increased content of ILFG, except for IGPI-7 (Fig. 6(f)). In Fig. S3(a)–(f), the width of the wear tracks of the PI and its composites has a tendency to narrow and then widen as the ILFG content increases. In particular, the PI with 0.56 wt% ILFG possesses a minimum width of 0.2 μm. Combining the information from Figs. 6 and S3, it is evident that the synergistic effect between GN and IL can improve the anti-friction and wear resistance of the PI matrix. The results could be attributed to the good dispersion of ILFG in the matrix and the continuous formation of tribofilm on the surface of the friction pair by ILFG during the friction process [36]. Based on the above surmises, SEM-EDS, Raman and XPS spectra are conducted to further explore friction behavior and mechanism and the results are displayed in Fig. 7. Through observing the rubbed surfaces lubricated with the GN/PI and IGPI-5 in Fig. 7(a)–(b), it is visible that there is a large amount of wear debris scattered on the rubbed surface of the GN/PI-lubricated friction pair, and a narrow and small-scale tribofilm with a load carrying capacity and a matrix protection ability are formed (dark area in Fig. 7(a)). Instead, there is less wear debris on the surface

Fig. 4. XPS wide region spectra of GN (a) and ILFG (b), C1s spectrum of ILFG (c), XPS fine region spectra of B (d), N (e), and F (f) elements of the ILFG.

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Fig. 5. The coefficient of friction and specific wear rate of PI, GN/PI, IGPI-1, IGPI-3, IGPI-5, and IGPI-7 composites at RT under dry friction (15 N; 1.03 m/s; 30 min).

and a wider, more continuous tribofilm after the friction test of IGPI-5, which proves that the tribofilm is more stable and firmer on the metal friction pair. EDS spectra can be used as evidence to confirm the existence of elements in the transfer film. EDS spectra obtained from regions

of the surfaces of Fig. 7(b) is shown in Fig. 7(d); elements (C, N, B, and F) found inside the wear track could be attributed to a tribofilm derived by chemisorption of the ILFG. Meanwhile, the migration of the D-peak in the Raman spectrum (compared to that in Fig. 2(b)) of the tribofilm

Fig. 6. The SEM micrographs of the wear surfaces of (a) PI, (b) GN/PI, (c) IGPI-1, (d) IGPI-3, (e) IGPI-5 and (f) IGPI-7 composites at RT under dry friction (15 N; 1.03 m/s; 30 min).

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Fig. 7. SEM micrographs of the tribofilm of GN/PI (a) and IGPI-5 (b); (c) Raman spectroscopy of tribofilm formed by IGPI-5; (d) EDS elemental surface distribution images of tribofilm formed by IGPI-5; XPS spectra of Fe2p (e) and F1s (f) of the worn surfaces lubricated with the IGPI-5 composites.

formed by IGPI-5 in Fig. 7(c) proves that the ILFG have become graphitized due to mechanical effects and frictional sliding. In the XPS spectra of the tribofilm in Fig. 7(e)–(f), Fe2p and F1s appear at about 710.9 eV and 685.1 eV, respectively, which can be attributed to the decomposition of the F element in the IL during the friction process; then, it generates a tribochemical reaction with the Fe element of the rotating ring, thus generating FeF2 with a lubricating effect. Combined with the above analysis, the friction mechanism can explain that ILFG fully utilize the excellent tribological properties of IL and GN in the friction process, such as synergistic lubrication, and the schematic of friction mechanism is displayed in Fig. S4. Specifically, the IL with frictional active elements on the surface of the contact area are partially decomposed at certain conditions [37], which can help ILFG to stably adsorb on the surface of the friction pair and generate a tribochemical reaction, thereby forming a tribofilm with a high load-carrying capacity and anti-wear performance and effectively improving the tribological properties of the IGPI composites. 3.3. The mechanical properties of PI and IGPI materials To further study the synergistic enhancement between IL and GN, the mechanical properties of IGPI have been investigated. The mechanical properties of PI and its composites, including tensile strength, modulus, and elongation at break, are depicted in Fig. 8(a)–(b). It can be

found that the tensile strength and modulus of the PI matrix show a trend of increasing first and then decreasing with the increase of ILFG content. In particular, the tensile strength and modulus of IGPI reach a maximum of 92.3 MPa and 1392 MPa when the content of ILFG is 0.4 wt%, which is a significant increase of 51.9% and 56.5% compared to these of PI, respectively. The improvement can be attributed to two reasons: First, the excellent dispersion and high degree of orientation of GN modified by IL in the PI matrix, and second, the efficient load transfer from PI to ILFG [38]. Meanwhile, the elongation at break exhibits a downward trend, indicating that ILFG could act as a physical cross-linking point, hindering the fluidity of the polymer chains, thereby increasing the brittleness of the composite and preventing plastic deformation. The enhancement of mechanical properties proves that the addition of ILFG can effectively improve the load-carrying capacity of the PI matrix, which is beneficial to improve its tribological performance, and the finding is consistent with the results obtained from the friction tests. Additionally, the mechanical properties of IGPI composites begin to deteriorate as the content of ILFG increases to 0.56 wt%, possibly due to a small amount of aggregation of excess GN, which easily leads to defects (stress concentration) and thus has a negative impact on mechanical properties [39]. To better explain the mechanical properties of ILFG/PI materials, the fractured surface morphologies of PI and its composite films are observed by SEM with the results shown in Fig. 9(a)–(f). Fig. 9(a) shows

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Fig. 8. The mechanical properties of PI and IGPI materials: (a) Tensile strength and tensile modulus; (b) elongation at break.

the typical plastic deformation behaviors of PI and its composites, and the fractured surface of PI film is flat and smooth with few obvious appearances of the fracture, while those of PI-based composite films become rough and coarse with the incorporation of ILFG. Particularly, the fractured surface morphology of IGPI-5 film in Fig. 9(e) displays more uniform, dense, and distinct cracks, pits, interface voids, and craters, which is the result of strong interfacial adhesion and fine

compatibility between the ILFG and the PI matrix. The excellent dispersion of the fillers is beneficial to improving the stress concentration of the IGPI composites and transfers stress from the PI matrix to the ILFG, thereby enhancing the mechanical properties of the composite films. Compared to the fracture surface morphology of IGPI-5 film, the fewer concave pits in the fracture surface of GN/PI, IGPI-1, and IGPI-3 films means that the GN and ILFG have weak interfacial bonding with the

Fig. 9. SEM images of tensile fracture surface of (a) PI, (b) GN/PI, (c) IGPI-1, (d) IGPI-3, (e) IGPI-5 and (f) IGPI-7 film.

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Fig. 10. The thermal properties of PI and IGPI composite films: (a) TGA curves; (b) DSC curves.

matrix and poor ability to withstand external forces. With the increase of ILFG content, excessive ILFG in the matrix will agglomerate, leading to the poor mechanical performances of IGPI-7 composites.

3.4. The thermal properties of PI and IGPI materials The excellent thermal stability of engineering plastics is an important factor to greatly broaden its potential applications. Thus, it is valuable to consider the synergy between IL and GN on the thermal properties of the PI matrix. The thermal stability of PI and IGPI films was investigated by TGA and DSC in nitrogen and the resulting thermal decomposition temperature (e.g., decomposition temperature of 5% weight loss, T5%) curves, DSC (e.g., glass transition temperature, Tg) curves, and corresponding detailed data for the PI and PI matrix composites are presented in Fig. 10(a)–(b) and Table 1. Compared with T5% of PI (528.7 °C), all films possess higher T5%, especially IGPI with 0.4 wt% ILFG, whose T5% value increase by 52.6 °C. The reason for the change in thermal decomposition temperature of IGPI may be that IL interacts with GN through π-π, cation-π and van der Waals interactions, thereby improving the dispersion of modified GN in the PI matrix and thus improving the interfacial compatibility and stability of ILFG with the matrix. Furthermore, the thermal mobility of the PI molecular chain near the sheets' surface is limited by the physical interlocking and interfacial adhesion, therefore the relaxation of the PI polymer chain requires more fracture energy during the heating process. In Fig. 10(b), the trend of Tg improvement is similar to T5% with increase in ILFG content. When the loading of ILFG reaches 0.4 wt%, Tg shows an increasing trend up to 265.2 °C compared with PI (251.2 °C). Causes of the above improvements of the IGPI films are likely that uniform dispersion of ILFG in the polymer phase and strong covalent adhesion to the PI matrix hinder the movement of the polymer chain during the glass phase transition, which requires more heat [40].

4. Conclusions In this study, GN modified by IL was prepared through the π-π, cation-π and van der Waals interactions, and then filled a series of ILFG into the PI matrix by solution blending to prepare PI-based composites. IL can intercalate into the GN layer and adsorb on the surface of GN, increasing the distance between the GN layer and the interlayer, preventing the agglomeration of GN, which improves the dispersion and interfacial compatibility of GN in the matrix; hence, such PI-based composites are endowed with excellent tribological, mechanical, and thermal properties. Under 15 N and 1.03 m/s, the coefficient of friction and specific wear rate of IGPI-5 are 0.21 and 2.61 × 10−5 mm3/N·m, which is a 38.2% and 25% decrease compared with that of PI, respectively. The friction mechanism is that the frictional active element from IL makes the ILFG easily adsorb to the surface of the friction pair, and a tribochemical reaction occurs, thereby forming a tribofilm on the surface of the friction pair. Meanwhile, the synergy between IL and GN manifests by an increase in the mechanical properties of the PI matrix. The tensile strength and tensile modulus of IGPI-5 films increased by 51.9% and 56.5%, respectively, compared to those of neat PI. The minimum elongation at break is 15.8% for IGPI-5. T5% (581.3 °C) and Tg (265.2 °C) of IGPI-5 all present the maximum value, which is increased by 52.6 °C and 14 °C, respectively, relative to pure PI. It is anticipated that our current work would inform ongoing efforts to exploit a more efficient method to prepare a high temperature self-lubricating composites and for better understanding the synergistic effect of IL and GN in tribological properties. CRediT authorship contribution statement Hong Ruan: Formal analysis, Writing - original draft, Resources. Qiu Zhang: Investigation, Writing - review & editing. Weiqiang Liao: Data curation. Yuqi Li: Conceptualization, Supervision. Xiaohua Huang: Data curation. Xu Xu: Writing - review & editing, Funding acquisition. Shaorong Lu: Writing - review & editing, Supervision. Declaration of competing interest

Table 1 The data of T5%, T10% and Tg of PI and PI-based composite films. Sample Category

Decomposition temperature of 5% weight loss, T5% (°C)

Decomposition temperature of 10% weight loss, T10% (°C)

Glass transition temperature, Tg (°C)

PI GN/PI IGPI-1 IGPI-3 IGPI-5 IGPI-7

528.7 532.3 552.2 579.9 581.3 567.7

551.8 572.5 584.0 602.4 611.7 592.9

251.2 254.8 256.6 260.3 265.2 257.6

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors gratefully acknowledge the financial support of the National Science Foundation of China (51605109, 51763009, and 51563005), the Guangxi Natural Science Foundation

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