Qi
Zhou
a,
Jingxia
Huang
a,
Jinqing
Wang
*b,
Zhigang
Yang
b,
Sheng
Liu
c,
Zhaofeng
Wang
*b and
Shengrong
Yang
b
aCollege of Materials Science and Engineering, Lanzhou University of Technology, Lanzhou, 730050, P. R. China
bState Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, P. R. China. E-mail: jqwang@licp.cas.cn; zhfwang@licp.cas.cn; Fax: +86 931 4968076; Tel: +86 931 4968076
cThe Technical Center of Zibo Entry-exit Inspection and Quarantine Bureau, Zibo 255031, P. R. China
First published on 12th October 2015
Nanocomposites consisting of zirconia (ZrO2) nanoparticles and reduced graphene oxide (rGO) nanosheets were successfully fabricated by a one-pot hydrothermal method. By regulating the proportion of the precursors of the GO colloidal suspension and zirconium oxychloride (ZrOCl2) solution, ZrO2 nanoparticles with a diameter of about 5 nm were uniformly anchored onto the rGO nanosheets. The combination mechanism of ZrO2 nanoparticles fully bonded onto rGO nanosheets is the formation of the monodentate or bidentate composites between the oxygen-containing groups of GO and Zr(IV) complex ions from hydrolysis of the ZrOCl2 solution. The dispersibility and tribological properties of the prepared composites were investigated as novel lubricant additives in paraffin oil. The results suggested that the oil with a small amount of nanocomposite (0.06 wt%) exhibits good dispersibility, excellent friction-reduction and anti-wear properties as well as a high load-bearing capacity caused by the synergistic effect of the rGO nanosheets and ZrO2 nanoparticles.
Graphene, a two-dimensional carbon structure arranged in a honeycomb lattice, has attracted great attention of scientists since its discovery in 2004,3 owing to its remarkable electronic, optical, mechanical, and thermal properties, high surface area and low density as well as its ease of functionalization.4,5 These features of graphene materials led to a broad range of potential applications in many areas, such as reinforced materials, hydrogen storage materials, nanoscale electronic devices, field-effect transistors and the electrochemistry field.6 In particular, graphene also has formidable mechanical properties, with a high Young’s modulus up to 1 TPa and a breaking strength of 130 GPa,7 and it also exhibited high-efficiency anti-wear and friction-reducing performances when used as a novel lubricating material.8,9 Therefore, the tribological properties of graphene are also of interest for application in a micro/nano-electromechanical system in addition to its excellent electronic and mechanical properties.10 For instance, Elomaa et al.11 investigated the friction and wear performances of water as a lubricant solvent containing 1 wt% of graphene oxide (GO) under a normal load of 10 N and the results showed that the friction coefficient decreased by 57% compared to pure water. Berman et al.12 demonstrated that graphene dispersed in ethanol could reduce friction greatly but was instantly removed from the sliding surface under an applied load of 5 N.
It is well known that graphene has outstanding tribological properties, but its high surface energy induces easy agglomeration, resulting in poor dispersibility. It’s fortunate that the situation can be improved by surface modification of graphene, but most methods are organic-grafted. Mungse et al.13 synthesized dual-layer alkylated graphene and used it as a lubricant additive; the result suggested that the lube oils containing the alkylated graphene (0.02 mg mL−1) exhibited long-term dispersion stability, as well as the low friction and wear characteristics under sliding contact between steel tribo-pairs. However, organics usually are not environmentally friendly. Moreover, the lower hardness of graphene in the horizontal direction, and inevitable presence of defects and grain boundaries in macroscopic samples14 has a significant effect on the widespread applications of graphene in the tribological field.
Zirconia (ZrO2) is one of the most important ceramic materials in industry and has potential applications to the tribological field, due to its high mechanical strength, good chemical durability and thermal stability, and prominent tribological properties as well as corrosion resistance and non-toxicity.15–17 Over the past few years, researchers have prepared graphene/ZrO2 composites through different means; however these composites were mainly used in other fields, for example, Shon et al.18 demonstrated that graphene is an effective reinforcing agent to improve the mechanical properties of ZrO2 composites. Rao et al.19 synthesized a surface modified graphene/ZrO2 nanocomposite, which displayed excellent removal efficiency towards 4-chlorophenol from aqueous solution when used as the adsorbent. Salimi et al.20 manifested that the reduced GO(rGO)/ZrO2 nanocomposite exhibited high performance as a novel architecture application to electrochemical sensing and biosensing platforms. Besides, Zheng et al.21 synthesized graphene/ZrO2 composite coatings using a plasma spraying technique and the results of a wear test indicated that the composite coatings exhibited excellent wear resistance and a low friction coefficient. Therefore, preparation of a composite consisting of ZrO2 nanoparticles and rGO nanosheets can not only satisfy these requirements of good lubricating properties and a high load-bearing capacity, but also improve the dispersibility by anchoring ZrO2 nanoparticles onto the surface of graphene to prevent the exfoliated rGO nanosheets from re-stacking.
Herein we introduce a simple and effective route to prepare the composite of rGO nanosheets homogeneously covered with ZrO2 nanoparticles through adjusting the volume ratios of the GO and zirconium oxychloride (ZrOCl2) solutions; meanwhile, a possible mechanism for coating the rGO nanosheets with ZrO2 nanoparticles is also presented. Furthermore, the application of the prepared composite as a lubricant additive has been investigated, and the results indicated that the tribological properties of the base oil of liquid paraffin could be improved by the addition of the composite under proper conditions. Moreover, the possible friction-reduction and anti-wear mechanisms have also been discussed. To the best of our knowledge, no systematic studies have been done about the rGO/ZrO2 nanocomposite as a lubricant additive even though graphene and its composites have been studied extensively in recent years.
The friction properties of the lubricant mixed with the rGO/ZrO2 composite were strongly influenced by its concentration. The experiments for determining the optimum concentration have been tested with a constant load of 50 N for 30 min to obtain the best lubricating performance. For comparison, the pure oil and oil separately mixed with natural graphite powders, ZrO2 nanoparticles, rGO nanosheets and commercial zinc dialkyl dithiophosphate (ZDDP) at the optimum concentration was also studied under the same conditions. After performing the friction testing using various materials as lubricant additives, the resulting wear tracks were observed by scanning electron microscopy (SEM) using a JSM-5600LV scanning electron microscope. Wear scars of the ball were measured by an Olympus STM6 microscope (Japan). The wear volume and the height profile of the tracks were obtained with a non-contact 3D surface profiler (model MicroMAXTM, ADE, USA). The wear rate (K) of the discs was calculated according to the formula below:
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Fig. 2 (a) XRD patterns of synthetic rGO, ZrO2 and the rGO/ZrO2 nanocomposite. (b) FTIR spectra of synthetic ZrO2, GO, rGO, and the rGO/ZrO2 nanocomposite. |
Fig. 2a presents the XRD patterns of the prepared rGO/ZrO2 nanocomposite and the control samples. Obviously, the pattern of rGO presents a broad diffraction peak at 22.4° (d ≈ 0.386 nm), suggesting that the hydrothermal reduction allowed the rGO nanosheets to tightly pack.26 The crystal structure of the ZrO2 nanoparticles exists as tetragonal and monoclinic, in good agreement with the standard cards (JCPDS PDF no. 49-1642 and JCPDS PDF no. 37-1484, respectively), but the peak of the former has the dominant position of the relative intensity and the quantity. Moreover, the average size of the ZrO2 nanoparticles calculated by the Debye–Scherrer formula is about 5 nm. The rGO/ZrO2 nanocomposite shows characteristic peaks of rGO (002) and ZrO2 of a tetragonal structure, indicating the presence of ZrO2 nanoparticles over the rGO nanosheets.
The FTIR spectra of ZrO2, GO, rGO and the rGO/ZrO2 nanocomposite are shown in Fig. 2b. Obviously, a broad peak at ∼3405 cm−1 attributed to the O–H stretching vibration of adsorbed water molecules appears in all samples. The FTIR spectrum of ZrO2 displays some peaks at 1635 cm−1 (O–H bending vibration of water molecules), 750 cm−1, 590 cm−1 and 505 cm−1 (Zr–O stretching vibrations).27,28 The spectrum of GO presents the typical peaks at 1729 cm−1 (CO stretching vibration of –COOH and C
O), 1629 cm−1 (C
C skeleton stretching vibration and O–H bending vibration of water molecules), 1420 cm−1 (O–H bending vibration of –COOH and C–OH), 1281 cm−1 (C–O stretching vibration of phenols, ethers and epoxy groups), 1121 cm−1 (C–O–C stretching vibration), and 1072 cm−1 (C–O stretching vibration of –OH).29,30 For rGO, the peaks of the oxygen-containing groups corresponding to GO are mostly weakened and have slightly disappeared, indicating that GO was reduced incompletely. However, for the rGO/ZrO2 nanocomposite, the characteristic peak of the C
O stretching vibration is shifted from 1729 to 1746 cm−1, which is attributed to the influence of coordination between O and Zr on the surface of the ZrO2 nanoparticles,38 and the relative intensity decreases significantly. Furthermore, the broad peak at 1420 cm−1 has separated into two small peaks at 1460 and 1398 cm−1, which can be assigned to the formation of either a monodentate complex or bidentate complex between the oxygen-containing groups of GO and Zr(IV) from the hydrolysis of the ZrOCl2 solution.31,32 So, it can be concluded that the ZrO2 nanoparticles are fully bonded onto the surface of the rGO nanosheets.
Fig. 3 provides the XPS analyses of GO and the rGO/ZrO2 nanocomposite to confirm the chemical changes. As shown in Fig. 3a, the C 1s XPS spectrum of GO unambiguously indicates a considerable degree of oxidation with four kinds of carbon atoms coming from the different functional groups: the non-oxygenated ring C (CC/C–C, 284.5 eV), the C in C–O bonds (285.3 eV), the C in O–C–O/C–OH (286.5 eV), and the C in HO–C
O (287.8 eV).33 In Fig. 3b, the C 1s XPS spectrum of rGO/ZrO2 also exhibits the same oxygen functionalities corresponding to the GO nanosheets, but the peak intensities of these components are much weaker than those in GO, proving considerable deoxygenation by the reduction process. In addition, the presence of an additional component at 285.6 eV corresponding to the C in the C
N bond of hydrazine34 is in good agreement with the XPS survey spectrum of the rGO/ZrO2 nanocomposite shown in Fig. 3d. These observations suggest that GO was deoxygenated by the hydrothermal process and nitrogen incorporation. In Fig. 3c, the binding energies of Zr 3d5/2 and Zr 3d3/2 are 183.5 and 185.7 eV, respectively, indicating that zirconium was oxidized into the Zr4+ state.35
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Fig. 3 (a) XPS spectrum of C 1s for the GO nanosheets, (b) the C 1s spectrum and (c) the Zr 3d spectrum for the rGO/ZrO2 nanocomposite. (d) The corresponding XPS survey spectrum. |
All characterizations clearly confirmed that ZrO2 nanoparticles have been successfully anchored onto the rGO sheets. The synthesis process of the rGO/ZrO2 nanocomposite is schematically illustrated in Fig. 4. When the ZrOCl2 solution was mixed with GO, the wealthy negative charges and the abundant oxygen-containing functional groups at the surface and edges of the GO nanosheets can attract and strongly bond with Zr(IV) complex ions coming from the hydrolysis of the ZrOCl2 precursor, such as [Zr(OH)2·4H2O]48+ and [Zr(OH)2+x·(4 − x)H2O]4(8−4x)+,36,37 which might be physically or chemically adsorbed onto the GO nanosheets. Subsequently, as the whole reaction is carried out, Zr(IV) complex ions transformed into stable ZrO2 nanoparticles by nucleation and further growth, meanwhile, the GO nanosheets were reduced into rGO under the experimental conditions of high temperature and pressure as well as the presence of hydrazine hydrate with excessive alkali. Finally, the ZrO2 nanoparticles were grown uniformly onto the rGO nanosheets to form the composites.
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Fig. 4 A sketch showing the preparation process of the rGO/ZrO2 nanocomposite. The inset shows the specific integrating process occurring in the hydrothermal reaction. |
For comparison, Fig. 7 presents the COF and the corresponding AFC as well as the wear volumes of wear tracks on the discs for pure oil, oil containing rGO/ZrO2 and the control samples at the same concentration of 0.06 wt%. Moreover, the corresponding mean area of wear scars for the steel ball, and the wear rates of wear tracks on the discs were calculated and are summarized in Table 1. As shown in Fig. 7a, the stability property of the COF for oil containing the rGO/ZrO2 nanocomposite and rGO nanosheets was similar, and it was smaller and more stable than the pure oil, oil with graphite, ZrO2 and ZDDP (the commercial and frequently used additive). In Fig. 7b, the AFC value of the pure oil is about 0.263, which is much higher than that of the others. In all testing samples, the AFC value of 0.118 for the oil mixed with the rGO/ZrO2 nanocomposite is lowest and is also closer to the AFC value of 0.119 for the oil mixed with rGO nanosheets. In Fig. 7c, the wear volume (1.820 × 105 μm3) and wear rate (33.70 μm3 N−1 m−1) as in Table 1 are the maximum for the test of the pure oil. Moreover, the addition of the ZrO2 nanoparticles has no obvious effect on the reduction of the COF for the pure oil, but it can increase the micro-hardness of the oil film formed between the friction pairs and thus improve the wear resistance of the oil film.40 Obviously, the wear volume of 0.116 × 105 μm3 and the wear rate of 2.15 μm3 N−1 m−1 for the oil containing the rGO/ZrO2 nanocomposite are lower than the values of the wear volume of 0.177 × 105 μm3 and the wear rate of 3.28 μm3 N−1 m−1 for the oil containing rGO nanosheets, and are also the lowest in all the testing samples.
Test conditions | The mean area of wear scars on balls (μm2) | Wear volumes on discs (μm3) | Wear rates on discs (μm3 N−1 m−1) |
---|---|---|---|
Pure oil | 1.922 × 105 | 1.820 × 105 | 33.70 |
Oil with graphite | 1.627 × 105 | 0.935 × 105 | 17.31 |
Oil with ZrO2 | 1.339 × 105 | 0.487 × 105 | 9.02 |
Oil with rGO/ZrO2 | 0.452 × 105 | 0.116 × 105 | 2.15 |
Oil with rGO | 0.461 × 105 | 0.177 × 105 | 3.28 |
Oil with ZDDP | 0.565 × 105 | 0.214 × 105 | 3.96 |
By comparison, it can be concluded that the lubricant containing various additives can reduce the wear to different degrees. Therein, the oil containing the rGO/ZrO2 nanocomposite presents the optimal tribological properties; namely, the AFC reduced from 0.263 to 0.118 and the wear rate reduced to 6.4% of the pure oil, which can be attributed to the fact that the unique layered structure and small size of the rGO/ZrO2 nanocomposite are convenient for entering the interfaces of the friction pairs, and also the easy shearing under the tribo-stressed zone due to the weak van der Waals interactions between their coordinated lamellas. Besides, the oil containing the rGO/ZrO2 nanocomposite has a high load-bearing capacity preventing the oil film from fracture, and the good dispersion stability of the composite in oil ensures their continuous supply on the tribological interfaces during the test.
After friction testing, the micrographs and wear depths of the different worn tracks characterized by the measurements of OM, SEM and the 3D surface profiler are displayed in Fig. 8. As shown in Fig. 8a1–a3, for the test performed with pure oil alone, the worn surfaces are the roughest with severe scuffing signs as well as wide and deep grooves, and the mean area of the worn scar on the ball is also the largest (1.922 × 105 μm2) in Table 1, which indicates that the boundary lubricating film at the contact surfaces has poor strength. By contrast, all additives used in our experiments can reduce wear to some extent, but the anti-wear ability is different. The additives of ZrO2 and graphite have a poor wear resistance so that the worn tracks are rough and the mean area of the worn scars on the ball are large, although they are lower than that of the pure oil. It is attributed to the fact that the agglomerate blocks of the ZrO2 nanoparticles act as abrasive particles and increase the roughness of the oil film, while the lubricating oil film containing graphite fractures and loses efficacy eventually. For the test performed with ZDDP, the tribological properties are better than the above two kinds of additives, which is attributed to the good lubricating property and load-bearing capacity. Apparently, the worn surfaces seem to be uniform and smooth, and the mean area of the worn scar on the ball is the minimum (0.452 × 105 μm2) in all testing samples for the oil containing the rGO/ZrO2 nanocomposite (in Fig. 8d1–d3). The results suggested that a good surface protective film formed on the frictional interfaces, which is in agreement with the low COF for the oil mixed with the composite. Furthermore, the rGO is also effective in reducing wear but is inferior to the as-prepared composite.
To further testify whether the tribological properties of the oil mixed with the prepared composite are better than those of the oil mixed with the rGO nanosheets, two experiments were performed under a high load as shown in Fig. 9. In Fig. 9a, the results display that the COF in sequence rose for the pure oil and oil containing rGO nanosheets after being tested for about 4 and 7 min under a constant load of 100 N, respectively. However, the COF for the oil containing the rGO/ZrO2 nanocomposite is stable and low in the whole test process. Furthermore, the maximum load-bearing capacity of the oil containing the composite could be elevated up to 450 N, although the COF gradually increased after being tested for 15 min at this load (Fig. 9b).
Based on the above research, the excellent tribological properties of the as-prepared rGO/ZrO2 nanocomposite acting as a novel additive in paraffin have been proven. The tribological model of the rGO/ZrO2 nanocomposite in paraffin oil is vividly depicted in Fig. 10. The possible lubricating mechanism can be described as follows: firstly, the ZrO2 nanoparticles anchored onto rGO nanosheets cover up the nanogaps of the rubbing surfaces, which prevents direct contact from friction pairs under contact stress. Moreover, the lamellar structure of the rGO nanosheets forms a conformal protective film between the contact interfaces under the friction stress, and it can provide low resistance for shear which thus results in the reductions of friction and wear.41 Similarly, such a finding was also supported by a formation of a MoS2 transfer film in the contact area, because the structure of MoS2 is similar to that of the graphene.42 In addition, the presence of ZrO2 nanoparticles on the surface of rGO nanosheets acts as a bearing, which contributes to the wear resistance and improves the load-bearing capacity of the lubricant.43 The excellent lubricating properties support the fact that the as-prepared composite is a kind of potential candidate as a lubricant additive in tribology.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra17440f |
This journal is © The Royal Society of Chemistry 2015 |