Tribological properties of sodium dodecyl sulfate aqueous dispersion of graphite-derived carbon materials

Abstract

The graphene oxide nanosheets (GOS) and reduced graphene oxide nanosheets (RGOS) were dispersed in 0.1 wt% anionic surfactant sodium dodecyl sulfate (SDS) aqueous solution to be functionalized with surfactant layers. The tribological properties of GOS and RGOS in 0.1 wt% SDS aqueous solution were studied using a four-ball tribometer. The RGOS and GOS at optimal concentration in SDS aqueous solution improved the anti-wear and friction reduction properties of SDS aqueous solution. GOS show better friction reduction properties than RGOS. The outstanding lubrication performance of GOS could attribute to their stable dispersion, thin laminated graphitic structure with high-lubricity and nanosize characteristics which can enter the contact area easily. Our findings provide the opportunity to use graphene-related materials for water-based lubricant applications.

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1. Introduction

Carbon forms (such as graphite,¹ graphite fluoride² and diamond like carbon³) are valuable as low-friction engineering materials, and are often used as lubricants in industry. Various forms of graphite-derived nanomaterials (i.e., nanotubes, nano-onions, nano-plates, etc.) to reduce the wear and friction in general are gaining high momentum.^4,5 Graphite is well known to have low-friction behavior ascribe to the low resistance to shear between neighboring atomic layers.⁶ Graphene derived from graphite is planar sheets of sp²-bonded carbon atoms, which are densely packed in a honeycomb crystal lattice.⁷ Graphene is also expected to exhibit low friction and wear rate in practically lubricating applications. Filleter et al.⁸ explored friction and dissipation in epitaxial graphene films, revealing that bilayer graphene as a lubricant outperforms even graphite due to reduced adhesion. Junfei Ou et al.⁹ found the reduced graphene oxide nanosheets (RGOS) assembled on silicon substrate have better friction–reduction properties than graphene oxide nanosheets (GOS) at nanoscale due to low adhesion force. Zhang¹⁰ et al. found different chemical functionalization adding ends affect the nanoscale frictional characteristics of graphene nanoribbons. However, these tribological studies of graphene have mostly focused on nano- to microscale friction and wear behavior^8–10 and have greatly increased our understanding of the fundamental lubrication mechanisms involved.

Macroscale tribological studies of graphene have remained relatively unexplored but are urgently needed to realize graphene's diverse tribological applications. Diana Berman¹¹ et al. reported that graphene layers on sliding steel surfaces can reduce wear and friction largely in dry nitrogen. Huang et al.¹² investigated the tribological properties of RGOS as additive in paraffin oil. They found that the frictional behaviour and antiwear ability of the lubricating oil were improved when graphite nanosheets were added to the paraffin oil at the optimal concentration. Lin¹³ et al. improved tribological properties of based oil using stearic acid-modified graphene nanosheets as a lubricant additive. Water-based lubricants possess significant advantages, such as reduced environmental pollution, high thermal conductivity, fire resistance, low cost and so on. To date, research on graphene as additive for water-based lubrication has been rarely referred to. The availability of stable graphene aqueous dispersion in large quantities is prerequisite for water-based lubrication because effective usage of graphene as lubricant depends on its nanoscale size and graphitic structure. Here the chemically exfoliated method has been used to make graphite oxide which disperses in water forming a stable colloidal suspension of GOS at large scale by ultrasonic treatment. RGOS were obtained by the chemical reduction of exfoliated GOS. The prevention of aggregation of RGOS is of particular importance because RGOS' hydrophobic nature. Then RGOS and GOS were dispersed in aqueous solution with the assistance of the anionic surfactant sodium dodecyl sulfate (SDS) to make chemically water-dispersible.¹⁴ Then the chemically functionalized GOS and RGOS can readily form stable aqueous colloids as lubricant. The tribological properties including the friction coefficient and wear scar diameter (WSD) were comparatively investigated using four-ball tribometer. The possible mechanism GOS and RGOS as additive in SDS aqueous solution was discussed. Our studies make it possible to process graphene-related materials at low-cost for water-based lubricant applications.

2. Experimental

2.1 Preparation of GOS and RGOS dispersion

The production of stable GOS and RGOS dispersion is prequisitive for water-based tribological applications. Graphite oxide was synthesized from spectral natural graphite (about 50 μm, Baotou Carbon Co., Ltd.)¹⁵ by the modified Hummer's method originally presented by Kovtyukhova and colleagues. Natural graphite (0.5 g) were mixed with 50 ml H₂SO₄ (98%) in a 200 ml beaker. The mixture was stirred for 30 minutes in an ice bath. Potassium permanganate (4 g) was added to the suspension under vigorous stirring. The addition was carefully controlled to keep the reaction temperature below 20 °C. The ice bath was then removed and the mixture was stirred at room temperature for 2 hours. The mixture gradually became pasty and the color turned to light brownish. Then mixture was diluted by adding DI water slowly with stirring. The diluted suspension was stirred at 90 °C for another 1 hour. The H₂O₂ (50 ml, 30%) was added to the mixture. For purification, the mixture was washed by repeated rinsing and centrifugation with 5% HCl then DI water several times until the pH of the filtrate reach neutral. The graphite oxides were dried in a vacuum oven at 60 °C.

For the reduction procedure, 15 mg of the as-synthesized graphite oxide were dispersed in DI water by ultrasonic treatment to give a brown dispersion of GOS. Graphite oxide readily exfoliates to GOS to be dispersable in water due to the presence of oxygen-containing groups including carboxylic acid and hydroxyl moieties. The pH of this solution was adjusted to 10. The anhydrous hydrazine hydrate solution (98%, 10 ml) was directly added into 100 ml GOS dispersion in a nitrogen-filled dry box by vigorous stirring in a silicon oil bath (85 °C) for 4 h. The final RGOS were obtained by filtration and thoroughly rinsed with large amounts of water several times to remove impurities. RGOS aqueous solution is hard to keep stable for long time. Then GOS and RGOS can readily disperse in 0.1 wt% SDS aqueous solution to generate stable colloids. Of great significance is that the successful formation of relatively stable graphene dispersions enables the use of water-based tribological application.

2.2 Characterization of RGOS and GOS

Raman scattering was performed on a JY-HR800 Raman spectrometer using a 533 nm laser source. Fourier transform infrared spectroscopy (FTIR) spectra were recorded on a Nicolet vacuum FTIR spectrometer over a range from 500 to 4000 cm⁻¹ with detector. Transmission electron microscopy (TEM, JEM2000EX) was used to characterize the morphology and structure of GOS and RGOS. Atomic force microscope (AFM) images were acquired by ASYLUM 5500 AFM system using tapping mode under ambient conditions.

Raman spectroscopy is a powerful nondestructive tool to distinguish ordered and disordered crystal structures of carbon. G band (sp²) is usually assigned to the E₂g phonon of C sp² atoms, while D (sp³)band is a breathing mode of point phonons of A1g symmetry. Fig. 1 shows the Raman spectra of GOS (a) RGOS (b). Raman spectra of chemically exfoliated GOS exhibit the D-band peak at 1346 cm⁻¹ and the G-band peak at 1588 cm⁻¹. This can be due to defects introduced into the GOS during preparation by Hummer's method. When the GOS is chemically reduced, the G band is shifted upward to 1595 cm⁻¹, intensity of the D band at 1360 cm⁻¹ of increases substantially. Then, an increased D/G intensity ratio of RGOS compared to that of the exfoliated GOS is observed. Therefore, upon ultrasonic reduction, de-oxygenation takes main responsibility for the increased intensity ratio of D/G of RGOS. Under reduction process with ultrasonic treatment, GOS would split into numerous smaller domains, and more defects are supposed to be generated. Then the average size of the in-plane sp² domains of the RGOS becomes smaller in size than exfoliated GOS.

Fig. 1 Raman spectrum of GOS (a) and RGOS (b).

IR spectrum (400 to 4000 cm⁻¹) was measured using Nicolet IR100 FT-IR Spectrometer with pure KBr as the background. Fig. 2(a) and (b) gives the FTIR spectrum of GOS and RGOS, respectively. The GOS exhibits the following characteristic IR features: peak at 3410 cm⁻¹ attributed to the hydroxyl (O–H) stretching vibrations of the C–OH groups, a weak peak at 1634 cm⁻¹ assigned to the C=O stretching vibrations of the-COOH groups, a strong band at 1380 cm⁻¹ assigned to the O–H deformations of the C–OH groups, and a strong band at 1045 cm⁻¹ attributed to C=O stretching vibrations, peak at about 1720 cm⁻¹ comes from stretching of the carbonyl groups (–COOH). The GOS with abundant oxidized groups at both the edges and in the plane reduces the van der Waals forces between the layers, increasing the dispersion of GOS in DI water to a greater extent upon ultrasonication. So GOS can form stable suspensions in aqueous solution without the assistance of surfactant. Fig. 2(b) indicate that a variety of the oxygen-containing functional groups have been thoroughly removed from the GOS after chemical reduction. However, if the oxygen functionality is removed to yield RGOS, the RGOS lose their water dispersability, aggregate, and eventually precipitate in DI water.

Fig. 2 FTIR Infra-red spectrum of GOS (a) and RGOS (b).

However, the RGOS dispersion in DI water can only keep stable for several hours before reaggregation and precipitation. The dispersing of RGOS needs the assistant of surfactant. The photography of GOS and RGOS dispersion in 0.1 wt% SDS aqueous solution was shown in Fig. 3. As shown in Fig. 3, dispersing state of RGOS and GOS is stable for several days. Thus, the functionalization of RGOS with SDS surfactants enhanced the dispersion of RGOS in water.

Fig. 3 Dispersing state of GOS (a) and RGOS (b) in SDS aqueous solution.

To further confirm the structures of GOS and RGOS, a few milliliters of RGOS SDS dispersion were dropped onto holey carbon mesh grids for TEM (JEM-200CX) analysis. The representative TEM image of the dispersed GOS and RGOS showed in Fig. 4(a) and (b). Fig. 4(a) showed the GOS with large area. However, distributed and isolated RGOS on the top of the carbon film were observed in Fig. 4(b). A typical TEM image of RGOS showed that the size of RGOS is about one square micrometer and much smaller than GOS. Sonication is responsible for chemo-mechanical breaking of the suspended RGOS into smaller pieces. The fragmentation of RGOS is controlled to a certain degree by the ultrasonic treatment.

Fig. 4 TEM image of GOS and RGOS.

As shown in Fig. 5(a), TEM images display layer structure of RGOS. The re-stacked parts with thickness of more than a few layers can also be seen in TEM image with higher magnification. The main framework of ROGS does not show great damages. And the layer structure of RGOS will be beneficial to preserve all the excellent mechanical and tribological properties of graphite. AFM is currently used to define the thickness of RGOS precisely. The RGOS 0.1 wt% SDS aqueous dispersion was dropped on silicon substrate for AFM characterization. Fig. 5(b) shows the AFM image of the dispersed RGOS. The exfoliated RGOS are flat sheets with an average thickness of about 1.7 nm and its lateral dimension is about 1 × 1 μm. The thickness of the RGOS is larger than that of the pure mechanical exfoliated graphene owing to the adsorption of SDS molecules on the surface.¹⁶ After the final reduction step, the lateral dimensions of RGOS range from several hundred nanometers to several micrometers.

Fig. 5 TEM (a) and AFM (b) image of the RGOS.

2.3 Tribological test of RGOS and GOS SDS aqueous dispersion

The GOS and RGOS were ultrasonicated in 0.1 wt% SDS aqueous solution for 30 minutes to be functionalized with self-assembled surfactant layers. GOS with covalently attached oxygen-containing groups generated in synthesis enables better dispersion than RGOS at high concentration in SDS aqueous solution. Typical concentration of 0.1 wt% GOS and RGOS in SDS solution has been diluted to five different concentrations for tribological test.

The friction and wear tests were conducted on a commercial four-ball tribometer MRS-10A (made in JINAN Xinshijin of China). Commercially available steel balls with grade two standard, diameter of 12.7 mm and a HRc of 59–61 were used in our experiments. All tests were conducted at room temperature and rotating rate of 1450 rpm. The test duration was 30 minutes. The friction coefficients were measured and recorded simultaneously during the whole wear test. The mean value and standard deviation of friction coefficient was calculated during the whole wear test. Load applied for the wear test produces a circular wear scar on each ball. The WSD means the diameter of the circular wear scar. The WSD of all low three steel balls were measured by coordinated optical microscope of four-ball tribometer to evaluate the relationship between scar diameter and the applied load. The average value of WSD of all low three steel balls was cited to define the anti-wear properties of lubricant for comparison.

3. Results and discussion

The relationship between friction coefficient (average and standard deviation during the whole test time) and the concentration of GOS and RGOS was given in Fig. 6 at load of 200 N. The friction coefficient of SDS aqueous solution decreases after the addition of GOS and RGOS dispersion in SDS aqueous solution. It is evident that both GOS and RGOS have friction reduction properties in SDS aqueous solution. The friction coefficient decrease with concentration of GOS increasing. The friction coefficient decrease with the concentration of RGOS increasing when the concentration is lower than 0.075 wt%, then increases with the concentration increasing to 0.1 wt%. The best concentration is at 0.1 wt% for GOS and 0.075 wt% for RGOS. Also the GOS have better friction reduction capability in SDS aqueous solution than RGOS. It can be deduced that the stable RGOS dispersion only can be obtained at a relatively low concentration about 0.075 wt%. The reason that friction coefficient increase at the concentration of 0.1 wt% is RGOS dispersion is hard to keep stable in water even if with the assistance of SDS moleculars.

Fig. 6 Friction coefficient as a function of RGOS and GOS additive at different concentration (200 N).

Fig. 7 gives the relationship between the concentration and the WSD (the average value of the lower three stationary balls) at the load of 200 N. The size of WSD decreases with the concentration of GOS and RGOS increasing. The GOS have better anti-wear properties than RGOS. The best anti-wear properties were obtained at the concentration with 0.1 wt% for both the GOS and RGOS. Higher concentration of GOS and RGOS allows the higher coverage density of them on the surface of wear scar. The exhibited the better friction-reduction, anti-wear properties can be assigned to the stable dispersion compared with the RGOS as well as the better graphitic structure.

Fig. 7 The WSD as a function of the GOS and RGOS concentration (200 N).

The relationship between friction coefficient and the loads is given in Fig. 8 at the concentration of 0.1 wt% GOS and RGOS. The friction coefficients decrease with increasing load lubricated with SDS solution containing 0.1 wt% RGOS and GOS. The GOS and RGOS in SDS aqueous solution have good friction-reduction and anti-wear properties at four different loads.

Fig. 8 Friction coefficient as a function of load with an additive of GOS and RGOS.

Fig. 9 gives the relationship between the load and the WSD lubricated with 0.1 wt% GOS and RGOS in 0.1 wt% SDS aqueous solution, respectively. The WSD increases with the load. Also both GOS and RGOS have better lubrication action under high loads than low loads.

Fig. 9 The WSD as function of load.

Lubricants need to be situated on the surfaces of workpiece to minimize the amount of friction and wear generated in tribotest.^17,18 During the wear process, the GOS and RGOS fill with the two mating surface of steel–steel sliding system. The GOS and RGOS serve as effective spacers to prevent rough contact and wear between the two mating wear surfaces under loads where fluid lubricants are normally squeezed out. The graphilic structures would provide very easy shear and slide, so the friction coefficient decreased greatly.¹⁹ The friction-reduction capability of GOS and RGOS as an additive in water can be explained by the self-lubricant graphitic structure. It is well known that the SDS moleculars is one of the good dispersant. Moreover, the SDS molecules could adsorb on the surface and form the effective membrane by hydrophobic interaction. The hydrophilic-hydrophobic balance between interplanar van der Waals force and electrostatic repulsion of GOS regulates the solution to achieve stable dispersion. The nanoscale size of GOS allows them to easily enter the contact area, and thin laminated graphitic structure with high-lubricity. RGOS easily aggregate because of the van der Waals interactions between each sheets and render great difficulties for tribological applications.²⁰ SDS functionalization could overcome such a limitation. However, RGOS could have precipitation over critical concentration of 0.1 wt% in SDS solution. Then, the dispersion and high graphitic structure of GOS and RGOS dominate tribological properties including friction-reduction and anti-wear capability of as lubricant additive in water. The tribological performance of another popular nanocarbon material, oxidized carbon nanotube (CNT), was also investigated as additive of water-based lubricant in previous studies. CNT bound tightly to surface of workpiece might consequently experience radial and axial deformations, which would change their geometry. Interestingly, the result reported here shows an opposite trend as the result obtained from our previous experiments with CNT.⁵ However, our results are compatible with previous results on graphene research reported in the literature. Thus, increasing the concentration of the grapheme flakes on sliding contact interface during sliding tests could potentially lower friction and wear further.

4. Conclusions

The GOS and RGOS were dispersed in SDS aqueous solution to be functionalized with surfactant layers by ultrasonic treatment. The GOS and RGOS at an optimal content as water-based lubricant additive greatly improved the wear resistance and friction reduction capacity. The overall improvement of tribological properties could attribute to the self-lubricated graphitic structure, stable dispersion and nanosize characteristics which facilitate it enter the rough contact surfaces. The GOS have better anti-wear and friction reduction than the RGOS because of better dispersion and higher graphilic crystalline. GOS and RGOS SDS aqueous dispersion exhibit better anti-wear and friction-reduction under higher applied load. Further understanding is needed in tribological performance including friction reduction, anti-wear of graphite derived carbon nanomaterial as additive for the water-based tribological applications in the near future.

Acknowledgements

This work was supported by the Natural Science Foundation of China (Grant no. 51002030) and open funding of State Key Laboratory of Mechanical Transmission (Grant no. SKLMT-KFKT-200904), International cooperation Project in Suzhou (Grant no. SH201117), in part by the New Century Training Program Foundation for the Talents by the Ministry of Education (Grant no. NCET-11-0095).

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