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Article

Comparative Study of the Friction Behavior of Functionalized Graphene Oxide Additives Under Electric Stimulations

by
Linghao Zhang
1,
Qiuyu Shi
2 and
Xiangyu Ge
1,*
1
School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China
2
China Electric Power Research Institute Co., Ltd., Beijing 102209, China
*
Author to whom correspondence should be addressed.
Lubricants 2024, 12(12), 455; https://doi.org/10.3390/lubricants12120455
Submission received: 14 November 2024 / Revised: 16 December 2024 / Accepted: 17 December 2024 / Published: 19 December 2024
(This article belongs to the Special Issue Tribology of 2D Nanomaterials and Active Control of Friction Behavior)
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Abstract

:
Electro-regulated friction is a widely adopted approach for reducing friction, with graphene oxide (GO) emerging as a promising lubricating additive due to its electro-responsive frictional behaviour. However, with the wide variety of functionalized GO additives available, each exhibiting distinct properties, it remains unclear which type demonstrates the most effective electro-regulated friction-reducing performance, limiting their broader industrial application. In this study, the frictional behaviour of three functionalized GO additives under electric stimulation was investigated along with an analysis of the corresponding worn surfaces. The findings reveal the role of functional groups in determining the tribological performance of functionalized GO additives and the mechanism of electric stimulation. Notably, the formation of ester groups during the friction process of GO-OH enhances the adsorption of GO additives onto steel surfaces, resulting in superior friction-reducing properties. Under lubrication with GO-OH additives, negative electric stimulation promotes the generation of ester groups and transitions the lubrication regime to mixed lubrication, thereby contributing to friction reduction. This work provides new insights into the tribological performance of functionalized GO additives and the mechanisms underlying their electro-regulated behaviours, laying a foundation for the design of GO additives with superior lubrication performance for practical engineering applications.

1. Introduction

Economic losses and energy wastage resulting from frictional and wear-related issues in mechanical components are substantial. These losses amount to nearly 6% of developed countries’ gross national product and 23% of global energy consumption annually [1,2,3,4]. Modern advanced equipment, including aerospace, medical, and machine tools, requires friction reduction to ensure the longevity and stability of the components. Electro-regulated friction has become one of the prevalent approaches [5,6,7].
GO, as one of the graphene-family members, has weak interlayer interactions and excellent electrical conductivity [8,9,10]. Graphene-family materials have demonstrated macroscopic superlubricity as additives in non-electrical environments [11,12,13,14]. In electric-stimulation environments, Kim et al. and Bao et al. reported that the morphology of GO can be modified through electric stimulation, whereby a weak electric field can be used to control the macro-scale arrangements of GO [15,16]. Lang et al. and Wang et al. reported that graphene friction behaviour can be influenced by electric stimulation and that there exists a field strength threshold beyond which the friction behaviour is changed [17,18]. Cho et al. reported that the modification of the interfacial frictional behaviour of graphene-based thin films was determined by various potentials and humidity at the microscopic scale [19]. Gus et al. reported the use of field effects to examine the dynamic tuning of friction at the graphene interfaces [20]. Most recently, we showed the capacity of electric stimulation with GO additives to reduce friction in steel surfaces [21]. The surface morphology of GO-adsorbent membranes can be modified using electric stimulation, resulting in a parallel or perpendicular alignment to the sliding direction, which enables the regulation of the coefficient of friction (COF). These studies offer theoretical foundations for the graphene-family additives to reduce friction through electric stimulations. The oxygen-containing groups of GO consist of epoxy, hydroxyl, carboxyl, etc., which play a significant role in influencing the mechanical properties and friction performance of GO [22]. Nevertheless, which functionalized GO additives display better electro-regulated friction behaviours remains unclear, which constrains the GO materials used for industrial applications.
This work examines the role of functionalized GO additives in terms of friction behaviour. The additives in the discussion are hydroxylated GO (GO-OH), carboxylated GO (GO-COOH), and aminated GO (GO-NH2). The hydroxyl and carboxyl groups were selected as the most representative groups of GO. Since -OH and -COOH are negatively charged, the positively charged -NH2 was selected for comparison. Polyols are employed as lubricants due to their widespread use in the lubrication of metal interfaces [23,24,25,26,27].

2. Materials and Methods

2.1. Materials

The functionalized GO nanosheets include GO-OH, GO-COOH, and GO-NH2. Polyethylene glycol (PEG) includes PEG200, PEG400, and PEG600, and the purity of PEG is more than 99%. Diethylene glycol dimethyl ether (G2) has a purity exceeding 99.5%. All reagents were supplied by Aladdin Biochemical Technology Co., Ltd., Shanghai., China. and did not undergo purification prior to use. The functionalized GO nanosheets (1 mg) were first combined with G2 (100 mg) to obtain a mixed solution. The resulting mixed solution was subjected to ultrasound treatment at 40 °C for 15 min. Then, the mixed solutions were further mixed with PEG to obtain composite solutions of PEG + G2 + GO additives (GO-NH2, GO-COOH, and GO-OH). The composite solutions exhibited a functionalized GO nanosheet concentration of 0.1 wt‰. The dimensions and structure of the functionalized GO nanosheets were analyzed using an atomic force microscope (AFM; Icon, Bruker, Karlsruhe, Germany) and a high-resolution transmission electron microscope (HRTEM; 2100F, JEM, Tokyo, Japan). The defects, vibrational features, and chemical groups of functionalized GO nanosheets were characterized using Raman spectroscopy (HR-800, Horiba, Paris France) and X-ray photoelectron spectroscopy (XPS; PHI Quantera II, Ulvac-Phi, Chigasaki, Japan).

2.2. Friction Experiments

The influence of electric stimulation on the friction behaviour of functionalized GO are studied using a multifunctional friction and wear tester (UMT-3, Bruker, Karlsruhe, Germany). The tribological pair consists of a GCr15 ball and a GCr15 disc (the surface morphology of the ball and disc is shown in Figure 1a,b). Both surfaces are relatively smooth, with surface roughness values (Rq) of 10 nm for the ball and 15 nm for the disc. The ball’s surface shows no noticeable furrows, while the disc surface contains some pits. Prior to testing, both balls and discs are subjected to an ultrasonic cleaning process using ethanol and pure water (each step lasting for 15 min), then dried using nitrogen. Prior to commencing each test, 20 µL of the composite solution are applied to the contact area. The relative sliding velocity is 200 mm/s and the load range falls between 2 and 20 N. Two forms of electric contact are used to examine the impact of electric stimulation on the friction behaviour of functionalized GO. When the ball is connected to the positive pole and the disc is connected to the negative pole, it is recorded as a positive electric stimulation. Conversely, it is recorded as a negative electric stimulation. All tests are conducted at room temperature with a relative humidity of between 20% and 35%. To ensure the reliability of the experimental results, three replicates of each experiment are conducted and the average COF values with standard deviations are reported. The COF values are varied by modifying the height of the platform and the verticality of the loading instrument to ensure that the COF values remain consistent for both clockwise and counterclockwise rotations.

2.3. Surface Characterization

Following the completion of the friction test, the wear scar diameter (WSD) and Rq were observed by a three-dimensional white-light interferometer (Nexview, ZYGO Lamda, Middlefield, OH, USA). The surface morphology of the worn surface was examined by the scanning electron microscope (SEM, S4800, HITACHI, Tokyo, Japan). The defects and chemical groups in the functionalized GO nanosheets in the worn regions were characterized by the XPS and Raman spectroscope.

3. Results

3.1. Material Characterization

AFM results revealed that the single-layer thickness of the functionalized GO nanosheets was about 1.5–3.0 nm (Figure 1c–e). HRTEM results revealed the layer structure of the functionalized GO nanosheets (Figure 1f–h). The thickness of the functionalized GO nanosheets was about 3–6 nm. As shown in Figure 1i, the functionalized GO exhibited evidence of defects and lattice damage. The D bands (1340 cm−1) indicated defects, vacancies, and distortions within functionalized GO nanosheets, while the G band (1590 cm−1) indicated the stretching of C-C bonds [28,29,30]. Moreover, the 2D band at 2685 cm−1 and the D+G band at 2930 cm−1 were observed, and the more prominent G peaks in comparison to the 2D peaks indicated the multilayer structure of the functionalized GO nanosheets. Before the friction test, the ID/IG ratio for all three functionalized GOs was approximately 0.93–0.96. XPS results showed that the concentrations of carbon and oxygen in all functionalized GO nanosheets were approximately 74–75 at% and 24–25 at%, respectively, and the GO-NH2 nanosheets exhibited a nitrogen concentration of approximately 1% (Figure 1j). Furthermore, the 284.8 eV peak indicated the C-C and C-H (Figure 1k,m,n), which implies the graphene ring structure [31,32]. For the GO-NH2 nanosheets, the C-NH2 bonds (286.8 eV) and NH-C(=O)-NH bonds (288.2 eV) were identified in the C1s spectrum (Figure 1k). In the N1s spectrum (Figure 1l), the -NO2 bond (405.8 eV), and N-H bond (401.2 eV) were also identified, representing the amino groups [33,34,35]. For the GO-OH nanosheets, the C-OH band at 286.7 eV (Figure 1m) represented the hydroxyl groups [36,37]. For the GO-COOH nanosheets, the O-C=O band at 288.3 eV (Figure 1n) represented the carboxyl groups [38].

3.2. Friction Experiments

The comparison of three composite solutions (Figure 2a) revealed that the GO-COOH nanosheets exhibited poor dispersion. During friction experiments, three composite solutions demonstrated disparate changes in terms of COF and current output under the same electric stimulation of −1.0 V (Figure 2b). The COFs of the three composite solutions were 0.0386 (for GO-OH), 0.0609 (for GO-COOH), and 0.0619 (for GO-NH2), and the current outputs were 22.45 mA (for GO-OH), 17.22 mA (for GO-COOH), and 14.17 mA (for GO-NH2). Further experiments were conducted to investigate the influence of different functionalized GO additives on friction performances. The results (Figure 2c,d) demonstrated that the COF values under negative electric stimulations were smaller than those under positive electric stimulations, and the COF variation in the GO-OH additives was notably more stable than that of GO-COOH and GO-NH2 additives within negative electric stimulation. Subsequently, experiments were conducted to investigate the effects of load and viscosity upon the friction performance of functionalized GO additives within a negative electric stimulation. The COF values of GO-NH2 and GO-COOH exhibited a decrease in response to an increase in load from 2 N to 10 N and increased when the load further increased to 20 N. The COF values of GO-OH decreased as the load increased from 2 N to 20 N, showing a better lubricating performance than GO-NH2 and GO-COOH additives. The COF of all functionalized GO additives increased with the lubricant viscosity increasing (Figure 2f). In summary, GO-OH additives show better lubricating performance at negative electric stimulation and high loads and PEG200 shows the smallest COF among the base oils.

4. Discussion

4.1. Influence of the Functionalized GO Additives

To investigate the reasons behind the stable friction exhibited by GO-OH additives under negative electric stimulations, the worn surfaces of the GO-OH, GO-COOH, and GO-NH2 were analyzed. The WSDs and average contact pressures (Figure 3a–c) are similar (18–20 MPa) for three functionalized GO additives; thus, the average contact pressure does not represent the main factor influencing friction stability. The worn surfaces Rq of both the balls and discs increases greatly to above 100 nm and 35 nm, respectively, compared to the initial surfaces. The worn surfaces Rq of both the balls (104 nm) and discs (35 nm) of GO-OH were smaller than those of GO-COOH (116 nm for ball and 47 nm for disc) and GO-NH2 (137 nm for ball and 36 nm for disc). The substantial disparity in COF between the various lubrication regimes depicted on the Stockton curve necessitates the estimation of the film thickness (hc) and film-thickness ratio (λ) for steady-state lubrication. This is achieved using the Hamrock–Dowson formula [39]. The hc values were 261.26 nm for GO-OH, 275.75 nm for GO-COOH, and 278.88 nm for GO-NH2. The λ is estimated as 2.38 for GO-OH, 2.20 for GO-COOH, and 1.97 for GO-NH2, which indicates a mixed lubrication. Mixed lubrication is a lubrication regime where both the lubricating film (fluid lubrication) and surface asperities (boundary lubrication) share the load. It occurs when the film is not thick enough to fully separate the surfaces, leading to partial contact and a mix of fluid lubrication and boundary lubrication. Based on that, the lubrication regime affects the friction stability but is not the primary factor. Concerning worn surface morphology (Figure 3j–o), it is similar for both the balls and discs lubricated with the three functionalized GO additives, exhibiting dense furrows compared to the initial surfaces. Consequently, the surface morphology of the balls and discs can be considered a non-primary factor. Thus, there must be other factors responsible for the stable friction generated by GO-OH additives.
Figure 4 and Figure 5 illustrate the existence of chemical bonds on the worn surface. The C1s scan indicates the detection of C-C (284.8 eV), C-N (285.8 eV), C-O (286.0/286.2 eV), N-C=O (288.2 eV), and -COOH (288.2/288.4 eV) [40,41,42]. The detection of C=O bonds indicated the potential for the existence of a functionalized GO-adsorbent membrane, as PEG does not have C=O bonds. The N1s scan indicates the detection of -NH2 (399.2/399.4 eV), -CONH (400.4 eV), and -CH3-NH2 (401.2 eV) [35,37,42]. The O1s scan indicates the detection of Fe-O in Fe3O4 or FeOOH (529.8/530.5 eV), O-C=O/N-C=O (531.0/531.4 eV), C-OH (533.3/533.6 eV), and C=O (532.2/532.4 eV) [30,43,44,45]. The Fe2p scan indicates the detection of FeOOH (711.6 eV) and FexOy (724.8 eV) [46,47]. It is summarized that the worn surface of GO-OH and GO-COOH contains ester groups while GO-NH2 contains amide groups. The ester functional groups have been proven to facilitate GO adsorption onto the metal surface, which may account for the more stable friction performance of GO-OH in comparison to GO-NH2 [48].
To investigate the influence of GO-OH and GO-COOH additives on friction performance and demonstrate the adsorption of functionalized GO on worn surfaces, Raman spectroscopy (Figure 6) was conducted on the worn surfaces. The appearance of the D-band (~1340 cm−1) and the G-band (~1560 cm−1) provides further confirmation of the hypothesis that functionalized GO adsorption membrane may be present on the worn surface [49,50]. The ID/IG ratio is employed to elucidate the correlation between the intensity of the D and G peaks, whereby a higher ID/IG ratio is indicative of a relatively greater prevalence of defects. The number of defects is directly proportional to the COF, assuming the same working conditions [51]. The ID/IG ratio is observed to be higher for GO-COOH (1.16) than for GO-OH on the ball (1.07), whereas GO-NH2 exhibits a higher ID/IG ratio on the disc (1.23). This may explain why GO-OH provides a more stable friction state under various negative electric stimulations. It is posited that a combination of enhanced surface quality, the ester group generation, and fewer defects result in a stable friction state of GO-OH when compared to the GO-COOH and GO-NH2 additives.

4.2. Influence of the Direction of Electric Stimulations

This part discusses the effect of the direction of electric stimulations on the friction behaviour of GO-OH additives. The WSDs and average contact pressures are essentially identical (19–20 MPa) for both positive and negative electric stimulation (Figure 7a–c). However, the WSD markedly declines without electric stimulation, while the average contact pressure (62 MPa) is considerably higher. Regarding surface roughness (Figure 7d–i), the integrated surface-produced absent electric stimulation is smoother than that produced with positive or negative electric stimulation. hc was calculated, and the resulting values were 261.26 nm (negative electric stimulation), 120.47 nm (no electric stimulation), and 272.63 nm (positive electric stimulation). λ is estimated as 2.38, 1.39, and 0.96 for negative electric stimulation, no electric stimulation, and positive electric stimulation, respectively. Both the negative electric stimulation and non-electric stimulation lubrication regimes were situated within the mixed lubrication, and the positive electric stimulation lubrication regime was in the boundary lubrication. The results demonstrated that the lubrication regime may be the primary factor influencing friction performance. Figure 7j,m illustrate that both the ball-worn and disc-worn surfaces under negative electric stimulation have dense furrows. Figure 7k,n illustrate the worn surface morphology in the absence of electric stimulation. It can be observed that the ball-worn surfaces exhibit dense furrows while the disc-worn surfaces display both dense furrows and some pits. Figure 7l,o illustrate the worn surface morphology under positive electrical stimulation. It can be observed that the ball-worn surfaces exhibit deep furrows and adhesion while the disc-worn surfaces display shallow furrows and a few pits. Consequently, the worn surface morphology is similar for all conditions and is not a primary factor contributing to better friction performance. Notably, the lubrication regime of negative electric stimulation falls within the mixed lubrication, which should be one of the primary factors contributing to the smallest COF value.
To further investigate how the negative electric stimulation affects the friction performance of GO-OH and thus achieves a superior friction-reducing effect, we analyse chemical bonds present on worn surfaces (Figure 8 and Figure 9). The C1s scan indicates the detection of C-C (284.8 eV), C-OH (286.1/286.3 eV), and C=O (288.1/288.4 eV). The detection of C=O bonds indicated the potential for the existence of a functionalized GO-OH-adsorbent membrane. The O1s scan indicates the detection of Fe-O in Fe3O4 or FeOOH (529.8/530.5 eV), O-C=O (531.1/531.3 eV), C-OH (533.3/533.6 eV), and C=O (532.0/532.6 eV). The Fe2p scan indicates the detection of FeOOH (711.7 eV) and FexOy (725.2 eV). The findings indicate that comparable chemical compounds are produced on the worn surface in response to different electric stimulations. However, it is noteworthy that the proportion of ester groups on the worn surface is higher under negative electric stimulation compared to those without electric stimulation and that the production of Fe-related oxides is diminished. The rise in the proportion of the ester group facilitates the formation of a GO-OH-adsorbent membrane. Furthermore, the reduction in Fe-related oxides was identified as a contributing factor to the friction reduction in our previous study [52]. In summary, the proportion of tribochemical products generated with different electric stimulations is the primary factor influencing the friction performance of GO-OH.
To substantiate the hypothesis that the GO-OH-adsorbent membrane was formed on the worn surface, the worn surface was subjected to a Raman spectroscopy analysis (Figure 10). The appearance of the D-band (~1340 cm−1) and the G-band (~1560 cm−1) provides further confirmation of the hypothesis. Without electric stimulations, the ID/IG ratios of both balls and discs were at a maximum (1.23 for the ball and 1.35 for the disc), resulting in high friction. It is noteworthy that there were no significant variations in the ID/IG ratios of the GO-OH nanosheets before and after the friction test under positive electric stimulation, indicating that GO-OH nanosheets may not participate in the lubrication process. This may explain the larger COF value obtained under positive electric stimulations.

5. Conclusions

This study provides a comparative analysis of the friction performance of different functionalized GO additives. The results demonstrate that GO-OH nanosheets exhibit superior friction-reducing behaviours compared to GO-COOH and GO-NH2 nanosheets when used as lubricating additives for steel surfaces. Specifically, the ester groups formed during the friction process of GO-OH additives enhance the adsorption of GO nanosheets onto the steel surface, leading to a more stable friction state compared to GO-NH2 additives. Additionally, the defects of all three functionalized GOs increase during the friction process. However, the GO-OH exhibits the lowest increase in defects, which contributes to achieving greater stability relative to GO-COOH additives. Under lubrication with GO-OH additives, negative electric stimulation promotes the formation of ester groups, transitioning the lubrication regime to mixed lubrication and further reducing friction. This work elucidates the distinct frictional performance of functionalized GO additives and their underlying mechanisms, providing a valuable foundation for the design of functionalized GO materials with enhanced friction-reducing properties.

Author Contributions

Conceptualization, X.G. and Q.S.; data curation, L.Z.; funding acquisition, X.G.; investigation, Q.S. and L.Z.; supervision, X.G.; writing—original draft, L.Z.; writing—review and editing, X.G. and Q.S. All authors have read and agreed to the published version of the manuscript.

Funding

This work is funded by the National Key R&D Program of China (2023YFB3405700), the National Natural Science Foundation of China (52205180), the Foundation of the Beijing Key Laboratory of Long-life Technology of Precise Rotation and Transmission Mechanisms (BZ0388202402), and the Science and Technology Innovation Program of the Beijing Institute of Technology-Original Basic Frontier Interdisciplinary Innovation Special Program.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

Author Qiuyu Shi was employed by China Electric Power Research Institute Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. (a,b) The morphology of the initial ball and disc; (c–e) AFM images: (c) GO-NH2, (d) GO-OH, (e) GO-COOH; (f–h) HRTEM images: (f) GO-NH2, (g) GO-OH, (h) GO-COOH; (i) Raman spectra; (j–n) XPS spectra: (j) full spectra, (k) C1s for GO-NH2, (l) N1s for GO-NH2, (m) C1s for GO-OH, and (n) C1s for GO-COOH.
Figure 1. (a,b) The morphology of the initial ball and disc; (c–e) AFM images: (c) GO-NH2, (d) GO-OH, (e) GO-COOH; (f–h) HRTEM images: (f) GO-NH2, (g) GO-OH, (h) GO-COOH; (i) Raman spectra; (j–n) XPS spectra: (j) full spectra, (k) C1s for GO-NH2, (l) N1s for GO-NH2, (m) C1s for GO-OH, and (n) C1s for GO-COOH.
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Figure 2. (a) A dispersion comparison of the composite solutions; (b) current and COF changes in the composite solutions under −1.0 V; (c,d) friction test results of functionalized GO additives with different voltages stimulation at (c) 2 N and (d) 10 N; (e) the load effect (200 mm/s, −1.0 V), and (f) the viscosity effect (10 N, 200 mm/s).
Figure 2. (a) A dispersion comparison of the composite solutions; (b) current and COF changes in the composite solutions under −1.0 V; (c,d) friction test results of functionalized GO additives with different voltages stimulation at (c) 2 N and (d) 10 N; (e) the load effect (200 mm/s, −1.0 V), and (f) the viscosity effect (10 N, 200 mm/s).
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Figure 3. Worn surfaces of the three composite solutions after friction tests. (a–c) WSD and average contact pressure; (d–f) Rq of balls, (g–i) Rq of discs; (j–l) morphology of balls, (m–o) morphology of discs. Three sets of tests were conducted using the following operational conditions: 2 N, 200 mm/s, and −1.0 V.
Figure 3. Worn surfaces of the three composite solutions after friction tests. (a–c) WSD and average contact pressure; (d–f) Rq of balls, (g–i) Rq of discs; (j–l) morphology of balls, (m–o) morphology of discs. Three sets of tests were conducted using the following operational conditions: 2 N, 200 mm/s, and −1.0 V.
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Figure 4. XPS images for the ball-worn surfaces of three composite solutions after friction tests.
Figure 4. XPS images for the ball-worn surfaces of three composite solutions after friction tests.
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Figure 5. XPS images for the disc-worn surfaces of three composite solutions after friction tests.
Figure 5. XPS images for the disc-worn surfaces of three composite solutions after friction tests.
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Figure 6. Raman images for the (a) ball-worn and (b) disc-worn surfaces of three composite solutions after friction tests.
Figure 6. Raman images for the (a) ball-worn and (b) disc-worn surfaces of three composite solutions after friction tests.
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Figure 7. Worn surfaces of GO-OH after friction tests with various electric stimulation, (a–c) WSD and average contact pressure; (d–f) Rq of balls, (g–i) Rq of discs; (j–l) morphology of balls, (m–o) morphology of discs. Three sets of tests were conducted using the following operational conditions: 2 N and 200 mm/s.
Figure 7. Worn surfaces of GO-OH after friction tests with various electric stimulation, (a–c) WSD and average contact pressure; (d–f) Rq of balls, (g–i) Rq of discs; (j–l) morphology of balls, (m–o) morphology of discs. Three sets of tests were conducted using the following operational conditions: 2 N and 200 mm/s.
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Figure 8. XPS images for the ball-worn surfaces with different electric stimulations.
Figure 8. XPS images for the ball-worn surfaces with different electric stimulations.
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Figure 9. XPS images for the disc-worn surfaces with different electric stimulations.
Figure 9. XPS images for the disc-worn surfaces with different electric stimulations.
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Figure 10. Raman images for the (a) ball-worn and (b) disc-worn surfaces.
Figure 10. Raman images for the (a) ball-worn and (b) disc-worn surfaces.
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MDPI and ACS Style

Zhang, L.; Shi, Q.; Ge, X. Comparative Study of the Friction Behavior of Functionalized Graphene Oxide Additives Under Electric Stimulations. Lubricants 2024, 12, 455. https://doi.org/10.3390/lubricants12120455

AMA Style

Zhang L, Shi Q, Ge X. Comparative Study of the Friction Behavior of Functionalized Graphene Oxide Additives Under Electric Stimulations. Lubricants. 2024; 12(12):455. https://doi.org/10.3390/lubricants12120455

Chicago/Turabian Style

Zhang, Linghao, Qiuyu Shi, and Xiangyu Ge. 2024. "Comparative Study of the Friction Behavior of Functionalized Graphene Oxide Additives Under Electric Stimulations" Lubricants 12, no. 12: 455. https://doi.org/10.3390/lubricants12120455

APA Style

Zhang, L., Shi, Q., & Ge, X. (2024). Comparative Study of the Friction Behavior of Functionalized Graphene Oxide Additives Under Electric Stimulations. Lubricants, 12(12), 455. https://doi.org/10.3390/lubricants12120455

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