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Carbon dots as an additive for improving performance in water-based lubricants for amorphous carbon (a-C) coatings
Journal Pre-proof Carbon dots as an additive for improving performance in water-based lubricants for amorphous carbon (a-C) coatings Jinzhu Tang, Shuqing Chen, Yulong Jia, Ying Ma, Hongmei Xie, Xin Quan, Qi Ding PII:
S0008-6223(19)30963-7
DOI:
https://doi.org/10.1016/j.carbon.2019.09.055
Reference:
CARBON 14625
To appear in:
Carbon
Received Date: 6 July 2019 Revised Date:
12 September 2019
Accepted Date: 20 September 2019
Please cite this article as: J. Tang, S. Chen, Y. Jia, Y. Ma, H. Xie, X. Quan, Q. Ding, Carbon dots as an additive for improving performance in water-based lubricants for amorphous carbon (a-C) coatings, Carbon (2019), doi: https://doi.org/10.1016/j.carbon.2019.09.055. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Carbon Dots as an additive for improving performance in water-based lubricants for Amorphous Carbon (a-C) Coatings Jinzhu Tanga, *, Shuqing Chena, Yulong Jiaa, Ying Maa, Hongmei Xiea, Xin Quana, *, Qi Dingb, c, ∗ a
College of Materials Science and Engineering, Yangtze Normal University,
Chongqing, 408100, China b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics,
Chinese Academy of Sciences, Lanzhou, 730000, China c
Qingdao Center of Resource Chemistry and New Materials, Qingdao, China
ABSTRACT As a new type of carbon material, carbon dots (CDs) with exceptional properties are regarded as a type of potential water-based lubricant additive. In this study, it is first demonstrated that CDs perform excellently when used as lubricant additives for water-lubricated amorphous carbon (a-C) contacts. The results show that the introduction of CDs into water at a concentration of 0.1 wt.% could reduce friction and wear in a-C contacts by 33% and 80%, respectively. Additionally, the lubricating performance of CDs is highly sensitive to its adding concentration. The excellent tribological performance of CDs results from the minimized CDs sediment particles (deposited in situ on worn a-C surfaces) and the reduced friction and wear via nano-bearing and nano-filling mechanisms. With high CDs concentrations, however,
∗
Corresponding author. Tel.: +8602372791828; fax: +8602372791818. E-mail address: [email protected] (J.Z. Tang); [email protected] (X. Quan);[email protected] (Q. Ding).
micron and submicron-sized sediment particles form when friction occurs and cause undesirable abrasive wear. For water-lubricated a-C contacts, therefore, the CDs concentration is the crucial determinant of lubrication performance that has to be appropriately controlled. 1. Introduction Compared with conventional oil and grease lubricants, water-based lubricants afford the inimitable advantages of being non-polluting and energy resource-saving. This is because water is environmentally friendly, abundant, and a renewable resource. Water-based lubricants have been applied as cutting, machining, or drilling fluids in metal-forming operations and oil extraction. In view of the low viscosity, and inadequate lubricity of water as well as the corrosion problem that it creates, however, the further application of water-based lubricants in the industry is considerably limited [1]. Lubricant additives are widely used to improve the tribological performances of base fluids [2–8]. The most commonly used additives for water are polymers, water soluble polyols, and nanomaterials. Compared with conventional organic lubricant additives, nanomaterials exhibit superior chemical stability with substantially lower toxicity and less harmful emissions, which conform with the idea of using environmentally friendly and energy-saving materials [9–11]. Nanomaterials have been extensively explored as novel lubricant additives for aqueous environments. Among the variety of nanoparticles, two-dimensional graphene and graphene oxide could provide excellent lubricating effects because of their superior properties, such
as high elastic modulus, self-lubricating behavior, and good thermal conductivity. For example, Kinoshita et al. [9] reported that graphene oxide (GO) performed a positive function in remarkably reducing friction between stainless steel and tungsten carbide contact. Elomaa et al. [12] evaluated the tribological properties of GO as a water additive to lubricate the contact between stainless steel and diamond-like carbon. It was reported that the GO nanosheets significantly reduced the wear volume loss. The aforementioned carbon nanomaterials, however, may experience the embedded stability problem between micro-bulges of rubbing interfaces [13, 14]. The morphology and size of particles also affect the lubricating performance. In general, the spherical morphology is favorable for rolling mechanisms and small particles could more easily enter into the frictional interfaces and consequently improve tribological performance. As a new type of carbon material, carbon dots (CDs) have attracted considerable interest in various fields of research because of their unique properties, such as superior biocompatibility, high specific surface area, and robust chemical inertness [15–19]. Recently, Wang et al. [17] synthesized ionic liquid-capped carbon quantum dots with anion responsiveness. They also found that N-doped CDs-based fluorescent probes exhibited good selectivity and high sensitivity toward Hg2+, Cu2+, S2O32−, and Cr (VI) ions [18, 19]. For tribological application, positive results have been reported regarding CDs in oil-based lubricants [20–23]. Wang et al. [22] synthesized branched polyelectrolyte-grafted CDs, which exhibited excellent friction-reduction and anti-wear performances when added to polyethylene glycol.
Carbon dots are potential water-based lubricating additives because of their good solubility in water. Research investigations that are relevant to the water lubrication behavior of CDs, however, remain limited. Liu et al. [24] successfully synthesized ionic liquid-modified CDs by the one-pot pyrolysis method and evaluated its lubrication properties in water via a four-ball tester. They found that CDs–IL (ionic liquid) exhibited exceptional effectiveness in reducing friction and wear in steel contacts; they attributed the improved performance to the boundary tribofilm formed by the absorption and deposition of CDs–IL. Tang et al. [25] studied the tribological properties of CDs and CDs–IL as water-based lubricant additives for steel contacts and found that both additives reduced friction and wear. They observed that the latter exhibited a better performance than the former. They further suggested that the IL group is conducive to the formation of a good adhesion film of CDs–IL onto the rubbing surfaces. Xiao et al. [26] employed sulfur-doped CDs to enhance the lubricity of water for Si3N4-vs-steel and Si3N4-vs-Si3N4 contacts. The results showed that CDs effectively reduced friction and wear in the two types of tribopairs. These enhancements could be attributed to the rolling effect, interlayer shear sliding effect, and tribofilm formation. More recently, Wang et al. [27] found that CDs, as eco-friendly nanoadditives in water-based lubricants, exhibited good lubricating property and inhibition effect for 316 stainless steel. They suggested that the CDs-absorbed protective film formation, nano-filling effects, and nano-bearing effects are the main mechanisms of improved performances. In summary, CDs are considered as promising additives for water-based lubricants with lubrication effects and
mechanisms that vary in different tribopairs. Further studies are therefore necessary particularly for tribopairs that possess excellent tribological properties in a water environment. Amorphous carbon (a-C) coatings have low coefficients of friction and wear rates in water, and their deposition has been recognized as a highly effective approach to improve the tribological performance of tribopairs under water-lubricated conditions [28–31]. The a-C, however, exhibits low reactivity with chemically based additives because of their chemical inertness [32–34]. It has been found that graphene and its derivatives could significantly improve the tribological performances of a-C contacts [35, 36]. Theoretically, because of their small size and high water solubility, CDs should provide better lubricating effects for a-C contacts. To the best of our knowledge, however, there is no literature that reports on this subject. In this study, the tribological performance of CDs as additives for water-lubricated a-C/a-C contacts is investigated. The possible changes in the boundary lubricating mechanisms of CDs at different concentrations are examined. In particular, the remarkable synergetic effects between a-C and CDs at an exceedingly low concentration of 0.1wt.% are elucidated based on the observation of the morphology, distribution, and chemical composition of CDs on the worn a-C surface after tribological tests. 2. Experiment details 2.1. Materials Natural graphite powder (~325 mesh and 99.9% pure (metal basis)) is purchased from Qingdao Huatai Lubricant Sealing S&T Co. Ltd. Analytical grade anhydrous
sodium carbonate and sodium nitrate are purchased from Aladdin Reagent Co., Ltd. Sulfuric acid (98%) and potassium permanganate (99.5%) are purchased from Sinopharm Chemical Reagents Co., Ltd. The GO nanosheets are purchased from Beijing Bailingwei Technology Co., Ltd. 2.2. CDs preparation The CDs are synthesized from natural graphite powder via the improved Hummers’ method as presented by Gaoquan Shi [37]. Graphite, 1.0 g in weight, is dispersed in 100 mL of H2SO4 (98%) and stirred at room temperature. Next, 43.0 g of NaNO3 is added to the mixture, which is cooled to 0 °C. It is thereafter mechanically stirred at 150 rpm in an ice bath; 3.0 g of KMnO4 is slowly added to maintain the temperature of the suspension below 20 °C. The suspension is then made into an oil bath and heated at 40 °C for 40 min. The temperature is increased to 120 °C, and the suspension is stirred for 12 h. After the reaction is completed, the reaction system is naturally cooled to room temperature. Deionized water (500 mL) is added to the reaction system, and Na2CO3 is cautiously added to adjust the pH value to 3. The mixture is filtered through a 100-nm microporous membrane to remove large-sized particles. The solution is then centrifuged in a Cence GL-21M high-speed centrifuge at 10 000 rpm for 1 h to further remove any large particles. The solution is placed inside a 25-mL dialysis bag (molecular weight cut-off: 1000 Da) and dialyzed in 500-mL distilled water for 3 d; the water is changed every 2 h until the end of the experiment. Finally, after freezing and drying to achieve complete desiccation, the CDs black powder is obtained. 2.3. Preparation of a-C coating
The non-hydrogenated a-C film without doping elements is coated on GCr15 steel discs and balls with a diameter of 6 mm by UDP650 magnetron sputtering deposition system (Teer Coatings Ltd.), and the high-purity graphite target is used as the carbon source. The substrates are ultrasonically cleaned with acetone and alcohol in succession for 20 min, dried with N2 gas, and thereafter placed in a vacuum chamber. The ambient pressure in the chamber is approximately 1.3 × 10−4 Pa. Prior to deposition, the substrates are sputter-cleaned for 30 min using Ar plasma, and then a thin Cr intermediate layer is deposited to improve adhesion. Finally, the a-C layer is deposited using the graphite target as carbon source. The basic properties of the a-C film are listed in Table 1. Table 1. Basic properties of as-deposited DLC coating. Item
Properties
Coating method
Physical Vapor Deposition (PVD)
Carbon source
High-purity graphite
Transition layer
Cr interlayer
Thickness (µm)
3.0
Surface roughness, Ra (nm)
5.9±0.6
Hardness (GPa)
10.6±0.4
Young’s Modulus (GPa)
160.6±5
2.4. Tribological Evaluation
The CDs are dissolved in deionized water at concentrations of 0.05, 0.1, 0.5, 1.0, and 1.5 wt.%. The prepared CDs can be easily dissolved in water because there are several carbon–oxygen functional groups present (Fig. S2). The tribological properties of water with CDs are examined using a UMT-3 tribometer with a ball-on-disk geometry in ambient air (30% relative humidity) at room temperature (25 °C). The tribological test is performed at a frequency of 5 Hz with an amplitude of 5 mm and under a constant load of 15 N. This exerts an average Hertzian contact pressure of 1.0 GPa. The test duration is 30 min, and the resulting total sliding distance is 90 m. The influence of test conditions on the lubrication performances of CDs is also investigated by varying the applied load (5–20 N) and reciprocating frequency (1–20 Hz). The tribological performances of the GO nanosheet in water-lubricated a-C contacts are also compared. All experiments are repeated 3–4 times to ensure that the results are statistically accurate. 2.5. Characterizations The diameter and height of CDs are measured by transmission electron microscope (TEM, Tecnai G2) and atomic force microscope (AFM, Bruker Dimension Icon). The statistical size distribution of CDs is estimated using “nano measure 1.2” which is an image analysis software. The chemical compositions of CDs are
characterized
using
Raman
spectroscopy
Fourier-transform infrared spectroscopy (FT-IR,
(Raman,
Lab
JY-HR800),
Nicolet iS50), and X-ray
photoelectron spectroscopy (XPS, ESCALAB 250Xi). The rheological properties of deionized water, CDs solution, and GO dispersion are investigated via the RS6000 rheometer. After the tribological tests, the worn a-C surfaces are cleaned with alcohol in an
ultrasonic bath. The morphology of worn surfaces and wear volume are measured by JEM-5600LV scanning electron microscope (SEM; JEOL) and MicroXAM 3D profiler (ADE Phase-Shift), respectively. The specific wear rate coefficient, K, is calculated using the following equation by Archard and Hirst: K=V/(FS), where V is the wear volume, F is the applied load, and S is the total sliding distance. Raman spectroscopy (Lab JY-HR800, Horiba, λ: 532 nm) and XPS (ESCALAB 250Xi, ThermoFisher Scientific) spectroscopy are employed to evaluate the structural changes in the a-C surface after the tribological tests. 3. Results and discussion 3.1. CDs and CDs solution Characterizations As shown in Fig. 1, the CDs are similar spherical particles with diameters in the narrow 2–6 nm range and are well-dispersed without aggregation. Their heights are in the 1–4 nm range, which suggests that they consist of 2–6 graphene layers. The hollow onion-like structure of CDs is clearly observed in high-resolution TEM (HRTEM) images (Fig. 1a).
Figure 1. (a) TEM image with HRTEM image (inset); (b) size distribution; (c) AFM image; (d) height profile of prepared CDs.
In the Raman spectrum (Fig. 2(a)), the D band at 1364 cm−1 and G band at 1604 cm−1 with an intensity ratio (ID/IG) of 0.92 are observed with a high degree of graphitization. Figure 2(b) shows that there are four peaks in the 3680–3100, 1711, 1580, and 1051 cm−1 regions that correspond to the vibrations of OH, C=O, −C=C−, and C–O–C bonds, respectively [38]. The FT-IR spectrum reflects that the prepared CDs have numerous oxygenated functional groups. It can be observed in Fig. 2(c) that the prepared CDs present four main C1s peaks, which could be assigned to C–C/C=C (285.0 eV), C–OH (286.2 eV), C–O–C (287.5 eV), and C=O/COOH (288.4 eV).
Figure 2. (a) Raman spectrum, (b) FT-IR spectrum, and (c) C1s XPS spectra of CDs. Figure 3 shows the variations of apparent viscosity (η) and shear stress (τ) of deionized water, CDs solution (0.1 wt%), and GO dispersion (0.1 wt%) with an increased shear rate ( γ& ). As shown in Fig. 3(a), when γ& varies from 0 to 100 s−1, the η values of deionized water and CDs solution (0.1 wt.%) are considerably close and exhibit extremely weak dependence on γ& ; both fluids exhibit the typical Newtonian behavior. In comparison, the GO dispersion (0.1 wt%) is a type of non-Newtonian fluid with considerably unstable η values at various shear rates. Figure 3(b) presents the relationship between τ and γ& , showing that the addition of GO nanosheets could significantly increase the viscous force of water when the shear rate exceeds 10 s−1. This must be related to the large lateral size of GO nanosheets, which block the liquid medium flow in shear. In summary, because of the size effect, the prepared CDs
exhibit a considerably slight influence on the rheological behavior of water than the GO nanosheets at the same adding concentration.
Figure 3. (a) Apparent viscosity curves and (b) shear stress curves of deionized water, CDs solution (0.1 wt%), and GO dispersion (0.1 wt%) with increased shear rate. 3.2. Tribological results The coefficient of friction (COF) variation curves with respect to time in contacts lubricated with different CDs concentrations are shown in Fig. 4(a). It can be observed that in deionized water, the COF gradually increases and thereafter attains a steady level after a running-in period. The addition of a small amount of CDs (less than 1 wt.%) can result in a COF value that is lower than that obtained when deionized water is used as lubricant. In particular, at the optimal concentration of 0.1 wt.%, the lowest and relatively stable COF, approximately 0.03, could be achieved. Friction and wear are simultaneously reduced by 33% and 80%, respectively, compared with those that result from the use of deionized water as lubricant (Fig. 4(c) and 4(d), respectively). With the increase in the additive concentration from 0.5 to 1.0 wt.%, however, the friction curves become unstable and exhibit peaks and fluctuations. As the CDs concentration increases to 1.5 wt.%, the addition of CDs
results in negative effects on friction reduction, and the wear rate significantly increases by 38%.
Figure 4. (a) COF variation in a-C contacts with different additive concentrations as a function of sliding time; (b) variation of COF with time when lubricants are deionized water, water with 0.1 wt.% CDs, and water with 0.1wt.% GO; (c) average COFs and (d) wear rates of a-C contacts lubricated with CDs dispersion and GO dispersion with different additive concentrations. For comparison, the effect of GO nanosheets as water-based lubricant additives on the tribological performance of a-C/a-C contacts is also investigated. The friction curves when GO dispersions are employed as lubricant are presented in Fig. S3. At lower concentrations (0.05 and 0.1 wt.%), it is evident from the friction curves that the GO nanosheets deteriorate the lubrication performance of water as indicated by the intensive fluctuation and rapid increase in sliding time. As shown in Fig. 4(b), with the same adding concentration of 0.1 wt.%, the COF when GO is used as additive eventually increases to 0.075, which is more than twice
that when CDs are employed. When the additive concentration further increases from 0.5 to 1.5 wt.%, the GO gradually exhibits better friction reduction and anti-wear performances than the CDs at the same concentration, as shown in Fig. 4(c) and 4(d). However, the friction curves of GO are not stable and gradually increase to the values of pure water (Fig. S3). The different tribological performances between CDs and GO according to the adding concentration may originate from different lubricating mechanisms because of the size effect as discussed in later sections. In the aspect of lubrication efficiency for a-C contacts, however, the CDs are better than GO because the former requires a substantially lower adding content (only 0.1%) to achieve the best performances. Figure 5 presents the tribological performance of the 0.1 wt% CDs solution under different applied loads and at various sliding speeds (reciprocating frequency). It can be observed that the CDs solution could provide effective friction reduction performance under most test conditions, except for those under extreme conditions (e.g., low velocity (1 Hz) and high load (20 N)).
Figure 5. Variation of COF with (a) reciprocating frequency and (b) normal load; variation of wear rate with (c) reciprocating frequency and (d) normal load.
The anti-wear performance, however, is reliable under all the test conditions, and the variation trend of the COF (resulting from the use of the CDs solution) with sliding speed or normal load is considerably close to that of deionized water. This may be related to the fact that the addition of CDs has practically no effect on the rheological behavior of water. 3.3. Characterizations of worn a-C surfaces Figure 6 shows the morphology and cross-sectional profile of the wear track on the a-C surface after the tribological tests. It shows that in deionized water, the wear track exhibits deep narrow grooves and some small white areas. The worn surface lubricated with water containing 0.1 wt.% CDs is considerably smooth and without grooves. The maximum wear depth is 47 nm, which is considerably lower than that when pure water is used as lubricant (88 nm). This indicates that CDs could remarkably alleviate the abrasive wear when the concentration is 0.1 wt.%. The abrasive damage, however, becomes severe with increased CDs contents; this suggests that excessive CDs concentration could result in further abrasive wear.
Figure 6. SEM images of worn a-C surfaces lubricated with (a) deionized water, (b) water with 0.1 wt.% CDs, (c) water with 0.5 wt.% CDs, and (d) water with 1.5 wt.% CDs; insets are corresponding surface profiles obtained by MicroXAM 3D profiler.
The high-magnification SEM image in Fig. 7(a) shows that in pristine deionized water, the worn a-C surface with tightly packed and rounded micro-regions maintains its original appearance. In comparison, with the introduction of CDs into a-C contacts, it is observed that the nano-scale valleys on a-C surfaces are filled with nanoparticles, demonstrating the nano-filling effect. When the concentration of CDs is raised to 0.5 wt.%, the nano-filling effect becomes more evident; however, some micron-sized particles could also be observed on the worn a-C surface shown in Fig. 6 (c). The increased abrasive wear caused by lubricants with high CDs concentrations must therefore be related to these micron-sized particles, which may behave as abrasive particles that cause three-body abrasion. Different from the CDs that extensively exist in the nano-valleys in the wear track on the a-C surface, the GO materials are found to aggregate near the wear track edges after tribological tests, as shown in Fig. S4.
Figure 7. High-magnification SEM images of worn a-C surfaces lubricated with (a) deionized water, (b) water with 0.1 wt.% CDs, and (c) water with 0.5 wt.% CDs; insets are micro-sized particles shown in Fig. 6(c). To investigate the particles formed in CDs dispersions when friction occurs, the lubricants with CDs are collected with a pipette and diluted with ethyl alcohol after the tribological tests. After ultrasonic treatments for 5 min, the mixture is dropped onto silicon wafers for SEM observation. As shown in Fig. 8(b), the 0.1 wt.% CDs dispersion maintains good dispersion stability without observable particles; this state is similar to that before the tribological tests are conducted. Numerous visible
particles are present in the dispersion, however, after tribological tests when the CDs concentration is increased to 0.5 wt.%. The SEM image shows that the sizes of formed particles increase with the increase in CDs concentration. When the concentration is 0.1 wt.%, the particles are in submicron scale and nanoscale; when the concentration exceeds 0.5 wt.%, most of the generated particles grow into the micron scale. The amount and size of generated particles when friction occurs thus exhibit strong dependence on the concentration of CDs in water.
Figure 8. SEM images of wear debris from (a) water with 0.1 wt.% CDs, (c) water with 0.5 wt.% CDs, and (e) water with 1.5 wt.% CDs after tribological test; photographs (in white light) of CDs dispersions before tribological test, and collected dispersions after tribological test of (b) water with 0.1 wt.% CDs, (d) water with 0.5 wt.% CDs, and (f) water with 1.5 wt.% CDs. The chemical composition changes on a-C surfaces after tribological tests are investigated via Raman spectroscopy and XPS. As shown in Fig. 9(a), the Raman
spectrum obtained from the worn a-C surface lubricated with deionized water is considerably similar to that of as-deposited a-C, only with slightly increased D peaks. This indicates the slight graphitization of the a-C surface when friction occurs; this is a well-recognized lubricating mechanism of a-C films in aqueous environments. By contrast, the Raman spectrum obtained from the worn a-C surface lubricated with water containing 0.1 wt.% CDs is considerably similar to the spectrum of as-prepared CDs, indicating the deposition of dissolved CDs onto the a-C surface when friction occurs. It can thus be concluded that the observed nanoparticles on the a-C surface (Fig. 7b) are composed of close-packed CDs. It is noted that after the tribological tests, the relative intensity of the “disorder” in the D band to the crystalline G band (ID/IG) increases from 0.92 (for pristine CDs) to 1.03. This indicates that the graphite-like structures of CDs must be smaller and have a higher “disorder” intensity in the sliding friction process [39, 40].
Figure 9. Raman spectra of wear tracks on a-C when lubricated with (a) deionized water and (b) water with 0.1 wt.% CDs; insets are Raman spectra of as-deposited a-C and CDs. The presence of CDs sediments deposited on the aforementioned worn surfaces is also confirmed by XPS spectra. Figure 10(a) shows that the C1s peak of the a-C film lubricated with pure water has no evident change after the tribological tests. On
the other hand, the overall shape of the C1s peak of the worn a-C surface lubricated with 0.1-wt.% CDs solution exhibits some similarities to that of the CDs spectrum. Figure 10(b) presents the detailed comparison of C1s peaks between CDs and worn a-C surfaces lubricated with the CDs solution. The peaks are automatically calculated by function (80% Gaussian + 20% Lorentzian) and multi-peak assignment to derive the analytical peaks. The full width at half maximum of fitted peaks is set at 1.23 eV. As shown in Fig. 8(b), there are four types of C atoms: centered at 285.0 eV (C– C/C=C), 286.2 eV (C–OH), 287.5 eV (C–O–C), and 288.4 eV (C=O/COOH), which are the same as those of CDs (Fig. 2(c)). The CDs structures are therefore not completely damaged by friction; however, the relative intensities of peaks are different. The relative intensity of peaks of oxygenated functional groups, especially C–O–C, is significantly reduced after the tribological tests. This may be the reason for the increased structural disorder and low water solubility of CDs [35].
Figure 10. (a) C1s XPS spectra of as-deposited a-C films and worn a-C surfaces lubricated with deionized water; (b) fitting results of C1s peak in XPS spectra of worn a-C surface lubricated with water containing 0.1 wt.% CDs. 3.4. Size effect on lubricating mechanisms of CDs in a-C contacts
As presented above, the lubricating performance of CDs exhibits a strong dependence on the CDs concentration, which is correlated with the size of generated sediment particles. Generally, the particles function as the third body between two rubbing surfaces, and their effects on lubrication performance vary with particle size and rubbing surface characteristics. The beneficial effects of nanoparticles on anti-wear and friction reduction performance could be ascribed to mechanisms, such as the nano-bearing effect, self-healing effect, and tribolayer formation in frictional interfaces. The a-C surface characteristics, such as chemical inertness, low adhesiveness, and high smoothness with nanoscale roughness, are unique. The size effect is therefore the key factor for determining the primary tribological mechanism of nanoparticles. As confirmed by Figs. 6 and 7, most of the sediment particles are in nanometer scale when the CDs concentration is 0.1 wt.%, whereas micron and submicron sediment particles are generated when the CDs concentration is increased to 0.5 wt.%. The results of Raman and XPS spectra suggest that the formed particles originate from the aggregation of dissolved CDs when friction occurs. As shown in Fig. 11, in the sliding interface, the oxygen-containing groups on the CDs surface are mechanically broken by shear stress, which activates the CDs surface. The activated CDs particles tend to cohere via strong interactions among dangling bonds to form larger particles; this deteriorates the solubility and subsequent in-situ deposition of CDs onto a-C surfaces. On the one the hand, the deposited carbon particles, could smoothen the a-C surface by filling the nano-valleys on the a-C surface and reduce contact stresses. On the other hand, they could act as nano-bearings and alleviate the
shear stress between two rubbing surfaces by rolling. The friction-induced in-situ deposition mechanism of CDs is also confirmed by experimental results, which indicate that higher applied loads could result in a shorter running-in period, as shown in Fig. S5.
Figure 11. Schematic of formation and lubrication mechanisms of CDs sediment particles during tribological tests. The in-situ deposition of CDs also explains the variation of the CDs solution lubricating behavior under different test conditions. Figure S6 presents the COF when the 0.1 wt.% CDs solution is used as lubricant under different loads and sliding speeds (Fig. 5) in the form of Stribeck curves. When deionized water is used as lubricant, a special inverse Stribeck curve is obtained; this is a typical phenomenon that occurs in a-C contacts, as reported by Kalin et al. [42]. The uniqueness of the Stribeck curve of a-C contacts is that the friction coefficients under extreme boundary lubrication conditions (low ηU/P value) are lower than those under milder conditions (high ηU/P
value). The higher friction under milder conditions is assumed to be related to the extra viscous stress caused by liquid lubricants under shearing stress. When the lubricant is a 0.1 wt% CDs solution, the curve exhibits a similar variation trend as that when the lubricant is pure water; this can be ascribed to the similar rheological behavior between water and the 0.1 wt% CDs solution (Fig. 3). However, under extreme boundary conditions where the effect of liquid lubricants is negligible, the friction when the CDs solution is used is slightly higher than that when deionized water is employed as lubricant. This may be because of the increased contact area, As (F=τsAs, where τs is the shear stress between two solids in contact), as a result of the filling effect of CDs. In contrast, under milder conditions, the friction coefficient of the CDs solution is lower than that obtained when the lubricant used is water. The viscosity of water and CDs solution are considerably similar (similar viscous stress, τl=η γ& ); the lower friction generated by the CDs solution must be attributed to the nano-bearing effects of CDs particles. Higher CDs concentrations, however, lead to higher contents of activated CDs and further result in the deposition of more large-sized particles. The size of deposited particles could further grow to micron scale when friction occurs (Fig. 8) because of the constant cohesion of activated CDs and the aggregation of sediment particles. The formed micron-sized particles in the high-concentration CDs solution could not fill the nano-valleys on the a-C surface when friction occurs; these particles could cause extra plowing wear (Fig. 6) because of the high contact stress exerted on the a-C surface. On the contrary, under low-concentration conditions, small sediment particles
are eventually collected in the valleys on the a-C surface; this effectively prevents the further growth of particles when friction occurs. For the GO nanosheet, which has a considerably larger lateral size (approximately 0.2–3.0 µm) than CDs, the lubrication mechanism involves surface adhesion and shearing along the basal plane of its crystalline lamellar structure under sliding friction stress. The a-C surface, however, is inert and has low adhesiveness. The GO nanosheet could therefore not stably remain on the a-C surface by filling the nanoscale valleys; this is different from the behavior of CDs on the surface when friction occurs. The GO nanosheets are thus prone to be pushed out from the contact region and aggregated at the wear track edge (Fig. S4), resulting in the gradual COF increase with test time. In particular, under low-concentration conditions where full coverage could not be ensured, the lubrication could even become worse than when the lubricant is water. This is because the flow of water is blocked by the GO as indicated by the remarkable increase in shear stress with the GO dispersion in the rheological test (Fig. 3). 4. Conclusion In order to improve the lubrication performance of water, the CDs are selected as eco-environmental lubricant additives. Their lubrication effects on a-C/a-C contacts at different additive concentrations are firstly investigated systematically. The tribological tests demonstrate that the CDs are highly efficient lubricating water additives for a-C/a-C contacts and exhibit remarkable friction and wear reduction effects at a considerably low concentration (0.1 wt.%). It is also found that the tribological properties of CDs solutions in a-C contacts are highly sensitive to addition concentrations. The amount and dimensions of CDs sediment particles that
are deposited in situ on worn a-C surfaces at different concentrations when friction occurs perform a crucial function in determining the lubrication effects of CDs. At the optimal concentration, most of the formed CDs sediment particles are nanoscale in size. These particles could reduce friction and wear via nano-bearing and nano-filling mechanisms. At high CDs concentrations, the formed micron-sized sediment particles cause undesirable abrasive wear. To obtain further understanding regarding the lubrication mechanism of CDs, the structure–function relationship will be investigated in our future work. Acknowledgements This research was financially supported by the Scientific and Technological Research Program
of
Chongqing
Municipal
Education
Commission
(Grant
No.
KJQN201801432), the Chongqing Research Program of Basic Research and Frontier Technology (Grant No. cstc2018jcyjAX0755), the National Natural Science Foundation of China (Grant No. 51705508), and the Open Project of State Key Laboratory of Solid Lubrication (LSL-1706). Moreover, we would like to extend our thanks to Dr. Ruochong Zhang for her kind suggestions.
References
[1] F. Chinas-Castillo, J. Lara-Romero, G. Alonso-Nunez, J.D.O. Barceinas-Sanchez, S. Jimenez-Sandoval, Friction reduction by water-soluble ammonium thiometallates, Tribol. Lett. 26 (2) (2007) 137-144. [2] W. Junming, W. Jianhua, L. Chunsheng, Z. Gaiqing, W. Xiaobo, A study of 2,5-dimercapto-1,3,4-thiadiazole
derivatives
as
multifunctional
additives
in
water-based hydraulic fluid, Ind. Lubr. Tribol. 66 (3) (2014) 402-410. [3] A. Petrov, P. Petrov, M. Petrov, Research into water-based colloidal-graphite lubricants for forging of carbon steels and Ni-based alloys, Int. J. Mater. Form. 3 (1) (2010) 311-314. [4] Q. Chen, X. Wang, Z.T. Wang, Y. Liu, T.Z. You, Preparation of water-soluble nanographite and its application in water-based cutting fluid, Nanoscale Res. Lett. 8 (2013) 1-8. [5] Z.Y. He, Y.Q. Wu, J.H. Lei, L.P. Xiong, J.W. Qiu, X.S. Fu, Tribological property study of novel water-soluble triazine derivative, Proc. IMechE. J:J. Eng. Tribol. 228 (4) (2014) 445-453. [6] Y.G. Wu, Z.G. He, X.Q. Zeng, T.H. Ren, E. De Vries, E.L. van der Heide, Tribological properties and tribochemistry mechanism of sulfur-containing triazine derivatives in water-glycol, Tribol. Int. 109 (2017) 140-151. [7] Y.L. Wu, Z.Y. He, X.Q. Zeng, T.H. Ren, E. de Vries, E. van der Heide, Tribological and anticorrosion behaviour of novel xanthate-containing triazine
derivatives in water-glycol, Tribol. Int. 110 (2017) 113-124. [8] L.P. Xiong, Z.Y. He, S. Han, J. Tang, Y.L. Wu, X.Q. Zeng, Tribological properties study of N-containing heterocyclic imidazoline derivatives as lubricant additives in water-glycol, Tribol. Int. 104 (2016) 98-108. [9] H. Kinoshita, Y. Nishina, A.A. Alias, M. Fujii, Tribological properties of monolayer graphene oxide sheets as water-based lubricant additives, Carbon 66 (2014) 720-723. [10] X. Pei, L. Hu, W. Liu, J. Hao, Synthesis of water-soluble carbon nanotubes via surface initiated redox polymerization and their tribological properties as water-based lubricant additive, Euro. Polym. J. 20 44 (8) (2008) 2458-2464. [11] H. Xie, B. Jiang, J. Dai, C. Peng, C. Li, Q. Li, F. Pan, Tribological Behaviors of Graphene and Graphene Oxide as Water-Based Lubricant Additives for Magnesium Alloy/Steel Contacts, Materials 11 (2) (2018) 206. [12] O. Elomaa, V.K. Singh, A. Iyer, T.J. Hakala, J. Koskinen, Graphene oxide in water lubrication on diamond-like carbon vs. stainless steel high-load contacts, Diam. Relat. Mater. 52 (2015) 43-48. [13] H. Lu, W. Tang, X. Liu, B. Wang, Z. Huang, Oleylamine-modified carbon nanoparticles as a kind of efficient lubricating additive of polyalphaolefin, J. Mater. Sci. 52 (8) (2017) 4483-4492. [14] B. Wang, W. Tang, H. Lu, Z. Huang, Ionic liquid capped carbon dots as a high-performance friction-reducing and antiwear additive for poly(ethylene glycol), J. Mater. Chem. A 4 (19) (2016) 7257-7265.
[15] Z.L. Wu, Z.X. Liu, Y.H. Yuan, Carbon dots: materials, synthesis, properties and approaches to long-wavelength and multicolor emission, J. Mater. Chem. B 5 (21) (2017) 3794-3809. [16] X. Sun, K. Yin, B. Liu, S. Zhou, J. Cao, G. Zhang, H. Li, Carbon quantum dots in ionic liquids: a new generation of environmentally benign photoluminescent inks, J. Mater. Chem. C 5 (20) (2017) 4951-4958. [17] B. Wang, W. Tang, H. Lu, Z. Huang, Hydrothermal synthesis of ionic liquid-capped carbon quantum dots with high thermal stability and anion responsiveness, J. Mater. Sci. 50 (16) (2015) 5411-5418. [18] B. Wang, Y. Lin, H. Tan, M. Luo, S. Dai, H. Lu, Z. Huang, One-pot synthesis of N-doped carbon dots by pyrolyzing the gel composed of ethanolamine and 1-carboxyethyl-3-methylimidazolium chloride and their selective fluorescence sensing for Cr(vi) ions, Analyst 143 (8) (2018) 1906-1915. [19] B. Wang, H. Tan, T. Zhang, W. Duan, Y. Zhu, Hydrothermal synthesis of N-doped carbon dots from an ethanolamine–ionic liquid gel to construct label-free multifunctional fluorescent probes for Hg2+, Cu2+ and S2O32−, Analyst 144 (9) (2019) 3013-3022. [20] H. Huang, H. Hu, S. Qiao, L. Bai, M. Han, Y. Liu, Z. Kang, Carbon quantum dot/CuSx nanocomposites towards highly efficient lubrication and metal wear repair, Nanoscale 7 (26) (2015) 11321-11327. [21] W. Shang, T. Cai, Y. Zhang, D. Liu, S. Liu, Facile one pot pyrolysis synthesis of carbon quantum dots and graphene oxide nanomaterials: All carbon hybrids as
eco-environmental lubricants for low friction and remarkable wear-resistance, Tribol. Int. 118 (2018) 373-380. [22] Z. Mou, B. Wang, H. Lu, H. Quan, Z. Huang, Branched polyelectrolyte grafted carbon dots as the high-performance friction-reducing and antiwear additives of polyethylene glycol, Carbon 149 (2019) 594-603. [23] W. Ma, Z. Gong, K. Gao, L. Qiang, J. Zhang, S. Yu, Superlubricity achieved by carbon quantum dots in ionic liquid, Mater. Lett. 195 (2017) 220-223. [24] X. Liu, Z. Huang, W. Tang, B. Wang, Remarkable Lubricating Effect of Ionic Liquid Modified Carbon Dots as a Kind of Water-Based Lubricant Additives, Nano 12 (09) (2017) 1750108. [25] W. Tang, B. Wang, J. Li, Y. Li, Y. Zhang, H. Quan, Z. Huang, Facile pyrolysis synthesis of ionic liquid capped carbon dots and subsequent application as the water-based lubricant additives, J. Mater. Sci. 54 (2) (2019) 1171-1183. [26] H. Xiao, S. Liu, Q. Xu, H. Zhang, Carbon quantum dots: An innovative additive for water lubrication, Sci. China Technol. Sci. 62 (4) (2019) 587-596. [27] Y. Hu, Y. Wang, C. Wang, Y. Ye, H. Zhao, J. Li, X. Lu, C. Mao, S. Chen, J. Mao, L. Wang, Q. Xue, One-pot pyrolysis preparation of carbon dots as eco-friendly nanoadditives of water-based lubricants, Carbon 152 (2019) 511-520. [28] F. Zhou, K. Adachi, K. Kato, Sliding friction and wear property of a-C and a-CNx coatings against SiC balls in water, Thin Solid Films 514 (1) (2006) 231-239. [29] H. Li, T. Xu, C. Wang, J. Chen, H. Zhou, H. Liu, Tribochemical effects on the friction and wear behaviors of a-C:H and a-C films in different environment, Tribol.
Int. 40 (1) (2007) 132-138. [30] Q. Wang, F. Zhou, X. Ding, Z. Zhou, C. Wang, W. Zhang, L.K.-Y. Li, S.-T. Lee, Structure and water-lubricated tribological properties of Cr/a-C coatings with different Cr contents, Tribol. Int. 67 (2013) 104-115. [31] A. Tanaka, M. Suzuki, T. Ohana, Friction and Wear of Various DLC Films in Water and Air Environments, Tribol. Lett. 17 (4) (2004) 917-924. [32] M. Kalin, I. Velkavrh, J. Vižintin, L. Ožbolt, Review of boundary lubrication mechanisms of DLC coatings used in mechanical applications, Meccanica 43 (6) (2008) 623-637. [33] J. Kogovsek, M. Kalin, Various MoS2-, WS2- and C-Based Micro- and Nanoparticles in Boundary Lubrication, Tribol. Lett. 53 (3) (2014) 585-597. [34] M. Kalin, J. Kogovsek, M. Remskar, Nanoparticles as novel lubricating additives in a green, physically based lubrication technology for DLC coatings, Wear 303 (1-2) (2013) 480-485. [35] L. Zhang, J. Pu, L. Wang, Q. Xue, Synergistic Effect of Hybrid Carbon Nanotube-Graphene Oxide as Nanoadditive Enhancing the Frictional Properties of Ionic Liquids in High Vacuum, ACS Appl. Mater. Interfaces 7 (16) (2015) 8592-8600. [36] L. Zhang, J. Pu, L. Wang, Q. Xue, Frictional dependence of graphene and carbon nanotube in diamond-like carbon/ionic liquids hybrid films in vacuum, Carbon 80 (2014) 734-745. [37] Y. Sun, S. Wang, C. Li, P. Luo, L. Tao, Y. Wei, G. Shi, Large scale preparation
of graphene quantum dots from graphite with tunable fluorescence properties, Phys. Chem. Chem. Phys. 15 (24) (2013) 9907-9913. [38] D. Pan, J. Zhang, Z. Li, M. Wu, Hydrothermal Route for Cutting Graphene Sheets into Blue-Luminescent Graphene Quantum Dots, Adv Mater. 22 (6) (2010) 734-738. [39] R. Atchudan, S. Perumal, D. Karthikeyan, A. Pandurangan, Y.R. Lee, Synthesis and characterization of graphitic mesoporous carbon using metal–metal oxide by chemical vapor deposition method, Micropor. Mesopor. Mat. 215 (2015) 123-132. [40] S. Kundu, R.M. Yadav, T.N. Narayanan, M.V. Shelke, R. Vajtai, P.M. Ajayan, V.K. Pillai, Synthesis of N, F and S co-doped graphene quantum dots, Nanoscale 7 (27) (2015) 11515-11519. [41] Z.-C. Yang, M. Wang, A.M. Yong, S.Y. Wong, X.-H. Zhang, H. Tan, A.Y. Chang, X. Li, J. Wang, Intrinsically fluorescent carbon dots with tunable emission derived from hydrothermal treatment of glucose in the presence of monopotassium phosphate, Chem. Commun. 47 (42) (2011) 11615-11617. [42] M. Kalin, I. Velkavrh, Non-conventional inverse-Stribeck-curve behaviour and other characteristics of DLC coatings in all lubrication regimes, Wear 297 (1-2) (2013) 911-918.
S0008-6223(19)30963-7
DOI:
https://doi.org/10.1016/j.carbon.2019.09.055
Reference:
CARBON 14625
To appear in:
Carbon
Received Date: 6 July 2019 Revised Date:
12 September 2019
Accepted Date: 20 September 2019
Please cite this article as: J. Tang, S. Chen, Y. Jia, Y. Ma, H. Xie, X. Quan, Q. Ding, Carbon dots as an additive for improving performance in water-based lubricants for amorphous carbon (a-C) coatings, Carbon (2019), doi: https://doi.org/10.1016/j.carbon.2019.09.055. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Carbon Dots as an additive for improving performance in water-based lubricants for Amorphous Carbon (a-C) Coatings Jinzhu Tanga, *, Shuqing Chena, Yulong Jiaa, Ying Maa, Hongmei Xiea, Xin Quana, *, Qi Dingb, c, ∗ a
College of Materials Science and Engineering, Yangtze Normal University,
Chongqing, 408100, China b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics,
Chinese Academy of Sciences, Lanzhou, 730000, China c
Qingdao Center of Resource Chemistry and New Materials, Qingdao, China
ABSTRACT As a new type of carbon material, carbon dots (CDs) with exceptional properties are regarded as a type of potential water-based lubricant additive. In this study, it is first demonstrated that CDs perform excellently when used as lubricant additives for water-lubricated amorphous carbon (a-C) contacts. The results show that the introduction of CDs into water at a concentration of 0.1 wt.% could reduce friction and wear in a-C contacts by 33% and 80%, respectively. Additionally, the lubricating performance of CDs is highly sensitive to its adding concentration. The excellent tribological performance of CDs results from the minimized CDs sediment particles (deposited in situ on worn a-C surfaces) and the reduced friction and wear via nano-bearing and nano-filling mechanisms. With high CDs concentrations, however,
∗
Corresponding author. Tel.: +8602372791828; fax: +8602372791818. E-mail address: [email protected] (J.Z. Tang); [email protected] (X. Quan);[email protected] (Q. Ding).
micron and submicron-sized sediment particles form when friction occurs and cause undesirable abrasive wear. For water-lubricated a-C contacts, therefore, the CDs concentration is the crucial determinant of lubrication performance that has to be appropriately controlled. 1. Introduction Compared with conventional oil and grease lubricants, water-based lubricants afford the inimitable advantages of being non-polluting and energy resource-saving. This is because water is environmentally friendly, abundant, and a renewable resource. Water-based lubricants have been applied as cutting, machining, or drilling fluids in metal-forming operations and oil extraction. In view of the low viscosity, and inadequate lubricity of water as well as the corrosion problem that it creates, however, the further application of water-based lubricants in the industry is considerably limited [1]. Lubricant additives are widely used to improve the tribological performances of base fluids [2–8]. The most commonly used additives for water are polymers, water soluble polyols, and nanomaterials. Compared with conventional organic lubricant additives, nanomaterials exhibit superior chemical stability with substantially lower toxicity and less harmful emissions, which conform with the idea of using environmentally friendly and energy-saving materials [9–11]. Nanomaterials have been extensively explored as novel lubricant additives for aqueous environments. Among the variety of nanoparticles, two-dimensional graphene and graphene oxide could provide excellent lubricating effects because of their superior properties, such
as high elastic modulus, self-lubricating behavior, and good thermal conductivity. For example, Kinoshita et al. [9] reported that graphene oxide (GO) performed a positive function in remarkably reducing friction between stainless steel and tungsten carbide contact. Elomaa et al. [12] evaluated the tribological properties of GO as a water additive to lubricate the contact between stainless steel and diamond-like carbon. It was reported that the GO nanosheets significantly reduced the wear volume loss. The aforementioned carbon nanomaterials, however, may experience the embedded stability problem between micro-bulges of rubbing interfaces [13, 14]. The morphology and size of particles also affect the lubricating performance. In general, the spherical morphology is favorable for rolling mechanisms and small particles could more easily enter into the frictional interfaces and consequently improve tribological performance. As a new type of carbon material, carbon dots (CDs) have attracted considerable interest in various fields of research because of their unique properties, such as superior biocompatibility, high specific surface area, and robust chemical inertness [15–19]. Recently, Wang et al. [17] synthesized ionic liquid-capped carbon quantum dots with anion responsiveness. They also found that N-doped CDs-based fluorescent probes exhibited good selectivity and high sensitivity toward Hg2+, Cu2+, S2O32−, and Cr (VI) ions [18, 19]. For tribological application, positive results have been reported regarding CDs in oil-based lubricants [20–23]. Wang et al. [22] synthesized branched polyelectrolyte-grafted CDs, which exhibited excellent friction-reduction and anti-wear performances when added to polyethylene glycol.
Carbon dots are potential water-based lubricating additives because of their good solubility in water. Research investigations that are relevant to the water lubrication behavior of CDs, however, remain limited. Liu et al. [24] successfully synthesized ionic liquid-modified CDs by the one-pot pyrolysis method and evaluated its lubrication properties in water via a four-ball tester. They found that CDs–IL (ionic liquid) exhibited exceptional effectiveness in reducing friction and wear in steel contacts; they attributed the improved performance to the boundary tribofilm formed by the absorption and deposition of CDs–IL. Tang et al. [25] studied the tribological properties of CDs and CDs–IL as water-based lubricant additives for steel contacts and found that both additives reduced friction and wear. They observed that the latter exhibited a better performance than the former. They further suggested that the IL group is conducive to the formation of a good adhesion film of CDs–IL onto the rubbing surfaces. Xiao et al. [26] employed sulfur-doped CDs to enhance the lubricity of water for Si3N4-vs-steel and Si3N4-vs-Si3N4 contacts. The results showed that CDs effectively reduced friction and wear in the two types of tribopairs. These enhancements could be attributed to the rolling effect, interlayer shear sliding effect, and tribofilm formation. More recently, Wang et al. [27] found that CDs, as eco-friendly nanoadditives in water-based lubricants, exhibited good lubricating property and inhibition effect for 316 stainless steel. They suggested that the CDs-absorbed protective film formation, nano-filling effects, and nano-bearing effects are the main mechanisms of improved performances. In summary, CDs are considered as promising additives for water-based lubricants with lubrication effects and
mechanisms that vary in different tribopairs. Further studies are therefore necessary particularly for tribopairs that possess excellent tribological properties in a water environment. Amorphous carbon (a-C) coatings have low coefficients of friction and wear rates in water, and their deposition has been recognized as a highly effective approach to improve the tribological performance of tribopairs under water-lubricated conditions [28–31]. The a-C, however, exhibits low reactivity with chemically based additives because of their chemical inertness [32–34]. It has been found that graphene and its derivatives could significantly improve the tribological performances of a-C contacts [35, 36]. Theoretically, because of their small size and high water solubility, CDs should provide better lubricating effects for a-C contacts. To the best of our knowledge, however, there is no literature that reports on this subject. In this study, the tribological performance of CDs as additives for water-lubricated a-C/a-C contacts is investigated. The possible changes in the boundary lubricating mechanisms of CDs at different concentrations are examined. In particular, the remarkable synergetic effects between a-C and CDs at an exceedingly low concentration of 0.1wt.% are elucidated based on the observation of the morphology, distribution, and chemical composition of CDs on the worn a-C surface after tribological tests. 2. Experiment details 2.1. Materials Natural graphite powder (~325 mesh and 99.9% pure (metal basis)) is purchased from Qingdao Huatai Lubricant Sealing S&T Co. Ltd. Analytical grade anhydrous
sodium carbonate and sodium nitrate are purchased from Aladdin Reagent Co., Ltd. Sulfuric acid (98%) and potassium permanganate (99.5%) are purchased from Sinopharm Chemical Reagents Co., Ltd. The GO nanosheets are purchased from Beijing Bailingwei Technology Co., Ltd. 2.2. CDs preparation The CDs are synthesized from natural graphite powder via the improved Hummers’ method as presented by Gaoquan Shi [37]. Graphite, 1.0 g in weight, is dispersed in 100 mL of H2SO4 (98%) and stirred at room temperature. Next, 43.0 g of NaNO3 is added to the mixture, which is cooled to 0 °C. It is thereafter mechanically stirred at 150 rpm in an ice bath; 3.0 g of KMnO4 is slowly added to maintain the temperature of the suspension below 20 °C. The suspension is then made into an oil bath and heated at 40 °C for 40 min. The temperature is increased to 120 °C, and the suspension is stirred for 12 h. After the reaction is completed, the reaction system is naturally cooled to room temperature. Deionized water (500 mL) is added to the reaction system, and Na2CO3 is cautiously added to adjust the pH value to 3. The mixture is filtered through a 100-nm microporous membrane to remove large-sized particles. The solution is then centrifuged in a Cence GL-21M high-speed centrifuge at 10 000 rpm for 1 h to further remove any large particles. The solution is placed inside a 25-mL dialysis bag (molecular weight cut-off: 1000 Da) and dialyzed in 500-mL distilled water for 3 d; the water is changed every 2 h until the end of the experiment. Finally, after freezing and drying to achieve complete desiccation, the CDs black powder is obtained. 2.3. Preparation of a-C coating
The non-hydrogenated a-C film without doping elements is coated on GCr15 steel discs and balls with a diameter of 6 mm by UDP650 magnetron sputtering deposition system (Teer Coatings Ltd.), and the high-purity graphite target is used as the carbon source. The substrates are ultrasonically cleaned with acetone and alcohol in succession for 20 min, dried with N2 gas, and thereafter placed in a vacuum chamber. The ambient pressure in the chamber is approximately 1.3 × 10−4 Pa. Prior to deposition, the substrates are sputter-cleaned for 30 min using Ar plasma, and then a thin Cr intermediate layer is deposited to improve adhesion. Finally, the a-C layer is deposited using the graphite target as carbon source. The basic properties of the a-C film are listed in Table 1. Table 1. Basic properties of as-deposited DLC coating. Item
Properties
Coating method
Physical Vapor Deposition (PVD)
Carbon source
High-purity graphite
Transition layer
Cr interlayer
Thickness (µm)
3.0
Surface roughness, Ra (nm)
5.9±0.6
Hardness (GPa)
10.6±0.4
Young’s Modulus (GPa)
160.6±5
2.4. Tribological Evaluation
The CDs are dissolved in deionized water at concentrations of 0.05, 0.1, 0.5, 1.0, and 1.5 wt.%. The prepared CDs can be easily dissolved in water because there are several carbon–oxygen functional groups present (Fig. S2). The tribological properties of water with CDs are examined using a UMT-3 tribometer with a ball-on-disk geometry in ambient air (30% relative humidity) at room temperature (25 °C). The tribological test is performed at a frequency of 5 Hz with an amplitude of 5 mm and under a constant load of 15 N. This exerts an average Hertzian contact pressure of 1.0 GPa. The test duration is 30 min, and the resulting total sliding distance is 90 m. The influence of test conditions on the lubrication performances of CDs is also investigated by varying the applied load (5–20 N) and reciprocating frequency (1–20 Hz). The tribological performances of the GO nanosheet in water-lubricated a-C contacts are also compared. All experiments are repeated 3–4 times to ensure that the results are statistically accurate. 2.5. Characterizations The diameter and height of CDs are measured by transmission electron microscope (TEM, Tecnai G2) and atomic force microscope (AFM, Bruker Dimension Icon). The statistical size distribution of CDs is estimated using “nano measure 1.2” which is an image analysis software. The chemical compositions of CDs are
characterized
using
Raman
spectroscopy
Fourier-transform infrared spectroscopy (FT-IR,
(Raman,
Lab
JY-HR800),
Nicolet iS50), and X-ray
photoelectron spectroscopy (XPS, ESCALAB 250Xi). The rheological properties of deionized water, CDs solution, and GO dispersion are investigated via the RS6000 rheometer. After the tribological tests, the worn a-C surfaces are cleaned with alcohol in an
ultrasonic bath. The morphology of worn surfaces and wear volume are measured by JEM-5600LV scanning electron microscope (SEM; JEOL) and MicroXAM 3D profiler (ADE Phase-Shift), respectively. The specific wear rate coefficient, K, is calculated using the following equation by Archard and Hirst: K=V/(FS), where V is the wear volume, F is the applied load, and S is the total sliding distance. Raman spectroscopy (Lab JY-HR800, Horiba, λ: 532 nm) and XPS (ESCALAB 250Xi, ThermoFisher Scientific) spectroscopy are employed to evaluate the structural changes in the a-C surface after the tribological tests. 3. Results and discussion 3.1. CDs and CDs solution Characterizations As shown in Fig. 1, the CDs are similar spherical particles with diameters in the narrow 2–6 nm range and are well-dispersed without aggregation. Their heights are in the 1–4 nm range, which suggests that they consist of 2–6 graphene layers. The hollow onion-like structure of CDs is clearly observed in high-resolution TEM (HRTEM) images (Fig. 1a).
Figure 1. (a) TEM image with HRTEM image (inset); (b) size distribution; (c) AFM image; (d) height profile of prepared CDs.
In the Raman spectrum (Fig. 2(a)), the D band at 1364 cm−1 and G band at 1604 cm−1 with an intensity ratio (ID/IG) of 0.92 are observed with a high degree of graphitization. Figure 2(b) shows that there are four peaks in the 3680–3100, 1711, 1580, and 1051 cm−1 regions that correspond to the vibrations of OH, C=O, −C=C−, and C–O–C bonds, respectively [38]. The FT-IR spectrum reflects that the prepared CDs have numerous oxygenated functional groups. It can be observed in Fig. 2(c) that the prepared CDs present four main C1s peaks, which could be assigned to C–C/C=C (285.0 eV), C–OH (286.2 eV), C–O–C (287.5 eV), and C=O/COOH (288.4 eV).
Figure 2. (a) Raman spectrum, (b) FT-IR spectrum, and (c) C1s XPS spectra of CDs. Figure 3 shows the variations of apparent viscosity (η) and shear stress (τ) of deionized water, CDs solution (0.1 wt%), and GO dispersion (0.1 wt%) with an increased shear rate ( γ& ). As shown in Fig. 3(a), when γ& varies from 0 to 100 s−1, the η values of deionized water and CDs solution (0.1 wt.%) are considerably close and exhibit extremely weak dependence on γ& ; both fluids exhibit the typical Newtonian behavior. In comparison, the GO dispersion (0.1 wt%) is a type of non-Newtonian fluid with considerably unstable η values at various shear rates. Figure 3(b) presents the relationship between τ and γ& , showing that the addition of GO nanosheets could significantly increase the viscous force of water when the shear rate exceeds 10 s−1. This must be related to the large lateral size of GO nanosheets, which block the liquid medium flow in shear. In summary, because of the size effect, the prepared CDs
exhibit a considerably slight influence on the rheological behavior of water than the GO nanosheets at the same adding concentration.
Figure 3. (a) Apparent viscosity curves and (b) shear stress curves of deionized water, CDs solution (0.1 wt%), and GO dispersion (0.1 wt%) with increased shear rate. 3.2. Tribological results The coefficient of friction (COF) variation curves with respect to time in contacts lubricated with different CDs concentrations are shown in Fig. 4(a). It can be observed that in deionized water, the COF gradually increases and thereafter attains a steady level after a running-in period. The addition of a small amount of CDs (less than 1 wt.%) can result in a COF value that is lower than that obtained when deionized water is used as lubricant. In particular, at the optimal concentration of 0.1 wt.%, the lowest and relatively stable COF, approximately 0.03, could be achieved. Friction and wear are simultaneously reduced by 33% and 80%, respectively, compared with those that result from the use of deionized water as lubricant (Fig. 4(c) and 4(d), respectively). With the increase in the additive concentration from 0.5 to 1.0 wt.%, however, the friction curves become unstable and exhibit peaks and fluctuations. As the CDs concentration increases to 1.5 wt.%, the addition of CDs
results in negative effects on friction reduction, and the wear rate significantly increases by 38%.
Figure 4. (a) COF variation in a-C contacts with different additive concentrations as a function of sliding time; (b) variation of COF with time when lubricants are deionized water, water with 0.1 wt.% CDs, and water with 0.1wt.% GO; (c) average COFs and (d) wear rates of a-C contacts lubricated with CDs dispersion and GO dispersion with different additive concentrations. For comparison, the effect of GO nanosheets as water-based lubricant additives on the tribological performance of a-C/a-C contacts is also investigated. The friction curves when GO dispersions are employed as lubricant are presented in Fig. S3. At lower concentrations (0.05 and 0.1 wt.%), it is evident from the friction curves that the GO nanosheets deteriorate the lubrication performance of water as indicated by the intensive fluctuation and rapid increase in sliding time. As shown in Fig. 4(b), with the same adding concentration of 0.1 wt.%, the COF when GO is used as additive eventually increases to 0.075, which is more than twice
that when CDs are employed. When the additive concentration further increases from 0.5 to 1.5 wt.%, the GO gradually exhibits better friction reduction and anti-wear performances than the CDs at the same concentration, as shown in Fig. 4(c) and 4(d). However, the friction curves of GO are not stable and gradually increase to the values of pure water (Fig. S3). The different tribological performances between CDs and GO according to the adding concentration may originate from different lubricating mechanisms because of the size effect as discussed in later sections. In the aspect of lubrication efficiency for a-C contacts, however, the CDs are better than GO because the former requires a substantially lower adding content (only 0.1%) to achieve the best performances. Figure 5 presents the tribological performance of the 0.1 wt% CDs solution under different applied loads and at various sliding speeds (reciprocating frequency). It can be observed that the CDs solution could provide effective friction reduction performance under most test conditions, except for those under extreme conditions (e.g., low velocity (1 Hz) and high load (20 N)).
Figure 5. Variation of COF with (a) reciprocating frequency and (b) normal load; variation of wear rate with (c) reciprocating frequency and (d) normal load.
The anti-wear performance, however, is reliable under all the test conditions, and the variation trend of the COF (resulting from the use of the CDs solution) with sliding speed or normal load is considerably close to that of deionized water. This may be related to the fact that the addition of CDs has practically no effect on the rheological behavior of water. 3.3. Characterizations of worn a-C surfaces Figure 6 shows the morphology and cross-sectional profile of the wear track on the a-C surface after the tribological tests. It shows that in deionized water, the wear track exhibits deep narrow grooves and some small white areas. The worn surface lubricated with water containing 0.1 wt.% CDs is considerably smooth and without grooves. The maximum wear depth is 47 nm, which is considerably lower than that when pure water is used as lubricant (88 nm). This indicates that CDs could remarkably alleviate the abrasive wear when the concentration is 0.1 wt.%. The abrasive damage, however, becomes severe with increased CDs contents; this suggests that excessive CDs concentration could result in further abrasive wear.
Figure 6. SEM images of worn a-C surfaces lubricated with (a) deionized water, (b) water with 0.1 wt.% CDs, (c) water with 0.5 wt.% CDs, and (d) water with 1.5 wt.% CDs; insets are corresponding surface profiles obtained by MicroXAM 3D profiler.
The high-magnification SEM image in Fig. 7(a) shows that in pristine deionized water, the worn a-C surface with tightly packed and rounded micro-regions maintains its original appearance. In comparison, with the introduction of CDs into a-C contacts, it is observed that the nano-scale valleys on a-C surfaces are filled with nanoparticles, demonstrating the nano-filling effect. When the concentration of CDs is raised to 0.5 wt.%, the nano-filling effect becomes more evident; however, some micron-sized particles could also be observed on the worn a-C surface shown in Fig. 6 (c). The increased abrasive wear caused by lubricants with high CDs concentrations must therefore be related to these micron-sized particles, which may behave as abrasive particles that cause three-body abrasion. Different from the CDs that extensively exist in the nano-valleys in the wear track on the a-C surface, the GO materials are found to aggregate near the wear track edges after tribological tests, as shown in Fig. S4.
Figure 7. High-magnification SEM images of worn a-C surfaces lubricated with (a) deionized water, (b) water with 0.1 wt.% CDs, and (c) water with 0.5 wt.% CDs; insets are micro-sized particles shown in Fig. 6(c). To investigate the particles formed in CDs dispersions when friction occurs, the lubricants with CDs are collected with a pipette and diluted with ethyl alcohol after the tribological tests. After ultrasonic treatments for 5 min, the mixture is dropped onto silicon wafers for SEM observation. As shown in Fig. 8(b), the 0.1 wt.% CDs dispersion maintains good dispersion stability without observable particles; this state is similar to that before the tribological tests are conducted. Numerous visible
particles are present in the dispersion, however, after tribological tests when the CDs concentration is increased to 0.5 wt.%. The SEM image shows that the sizes of formed particles increase with the increase in CDs concentration. When the concentration is 0.1 wt.%, the particles are in submicron scale and nanoscale; when the concentration exceeds 0.5 wt.%, most of the generated particles grow into the micron scale. The amount and size of generated particles when friction occurs thus exhibit strong dependence on the concentration of CDs in water.
Figure 8. SEM images of wear debris from (a) water with 0.1 wt.% CDs, (c) water with 0.5 wt.% CDs, and (e) water with 1.5 wt.% CDs after tribological test; photographs (in white light) of CDs dispersions before tribological test, and collected dispersions after tribological test of (b) water with 0.1 wt.% CDs, (d) water with 0.5 wt.% CDs, and (f) water with 1.5 wt.% CDs. The chemical composition changes on a-C surfaces after tribological tests are investigated via Raman spectroscopy and XPS. As shown in Fig. 9(a), the Raman
spectrum obtained from the worn a-C surface lubricated with deionized water is considerably similar to that of as-deposited a-C, only with slightly increased D peaks. This indicates the slight graphitization of the a-C surface when friction occurs; this is a well-recognized lubricating mechanism of a-C films in aqueous environments. By contrast, the Raman spectrum obtained from the worn a-C surface lubricated with water containing 0.1 wt.% CDs is considerably similar to the spectrum of as-prepared CDs, indicating the deposition of dissolved CDs onto the a-C surface when friction occurs. It can thus be concluded that the observed nanoparticles on the a-C surface (Fig. 7b) are composed of close-packed CDs. It is noted that after the tribological tests, the relative intensity of the “disorder” in the D band to the crystalline G band (ID/IG) increases from 0.92 (for pristine CDs) to 1.03. This indicates that the graphite-like structures of CDs must be smaller and have a higher “disorder” intensity in the sliding friction process [39, 40].
Figure 9. Raman spectra of wear tracks on a-C when lubricated with (a) deionized water and (b) water with 0.1 wt.% CDs; insets are Raman spectra of as-deposited a-C and CDs. The presence of CDs sediments deposited on the aforementioned worn surfaces is also confirmed by XPS spectra. Figure 10(a) shows that the C1s peak of the a-C film lubricated with pure water has no evident change after the tribological tests. On
the other hand, the overall shape of the C1s peak of the worn a-C surface lubricated with 0.1-wt.% CDs solution exhibits some similarities to that of the CDs spectrum. Figure 10(b) presents the detailed comparison of C1s peaks between CDs and worn a-C surfaces lubricated with the CDs solution. The peaks are automatically calculated by function (80% Gaussian + 20% Lorentzian) and multi-peak assignment to derive the analytical peaks. The full width at half maximum of fitted peaks is set at 1.23 eV. As shown in Fig. 8(b), there are four types of C atoms: centered at 285.0 eV (C– C/C=C), 286.2 eV (C–OH), 287.5 eV (C–O–C), and 288.4 eV (C=O/COOH), which are the same as those of CDs (Fig. 2(c)). The CDs structures are therefore not completely damaged by friction; however, the relative intensities of peaks are different. The relative intensity of peaks of oxygenated functional groups, especially C–O–C, is significantly reduced after the tribological tests. This may be the reason for the increased structural disorder and low water solubility of CDs [35].
Figure 10. (a) C1s XPS spectra of as-deposited a-C films and worn a-C surfaces lubricated with deionized water; (b) fitting results of C1s peak in XPS spectra of worn a-C surface lubricated with water containing 0.1 wt.% CDs. 3.4. Size effect on lubricating mechanisms of CDs in a-C contacts
As presented above, the lubricating performance of CDs exhibits a strong dependence on the CDs concentration, which is correlated with the size of generated sediment particles. Generally, the particles function as the third body between two rubbing surfaces, and their effects on lubrication performance vary with particle size and rubbing surface characteristics. The beneficial effects of nanoparticles on anti-wear and friction reduction performance could be ascribed to mechanisms, such as the nano-bearing effect, self-healing effect, and tribolayer formation in frictional interfaces. The a-C surface characteristics, such as chemical inertness, low adhesiveness, and high smoothness with nanoscale roughness, are unique. The size effect is therefore the key factor for determining the primary tribological mechanism of nanoparticles. As confirmed by Figs. 6 and 7, most of the sediment particles are in nanometer scale when the CDs concentration is 0.1 wt.%, whereas micron and submicron sediment particles are generated when the CDs concentration is increased to 0.5 wt.%. The results of Raman and XPS spectra suggest that the formed particles originate from the aggregation of dissolved CDs when friction occurs. As shown in Fig. 11, in the sliding interface, the oxygen-containing groups on the CDs surface are mechanically broken by shear stress, which activates the CDs surface. The activated CDs particles tend to cohere via strong interactions among dangling bonds to form larger particles; this deteriorates the solubility and subsequent in-situ deposition of CDs onto a-C surfaces. On the one the hand, the deposited carbon particles, could smoothen the a-C surface by filling the nano-valleys on the a-C surface and reduce contact stresses. On the other hand, they could act as nano-bearings and alleviate the
shear stress between two rubbing surfaces by rolling. The friction-induced in-situ deposition mechanism of CDs is also confirmed by experimental results, which indicate that higher applied loads could result in a shorter running-in period, as shown in Fig. S5.
Figure 11. Schematic of formation and lubrication mechanisms of CDs sediment particles during tribological tests. The in-situ deposition of CDs also explains the variation of the CDs solution lubricating behavior under different test conditions. Figure S6 presents the COF when the 0.1 wt.% CDs solution is used as lubricant under different loads and sliding speeds (Fig. 5) in the form of Stribeck curves. When deionized water is used as lubricant, a special inverse Stribeck curve is obtained; this is a typical phenomenon that occurs in a-C contacts, as reported by Kalin et al. [42]. The uniqueness of the Stribeck curve of a-C contacts is that the friction coefficients under extreme boundary lubrication conditions (low ηU/P value) are lower than those under milder conditions (high ηU/P
value). The higher friction under milder conditions is assumed to be related to the extra viscous stress caused by liquid lubricants under shearing stress. When the lubricant is a 0.1 wt% CDs solution, the curve exhibits a similar variation trend as that when the lubricant is pure water; this can be ascribed to the similar rheological behavior between water and the 0.1 wt% CDs solution (Fig. 3). However, under extreme boundary conditions where the effect of liquid lubricants is negligible, the friction when the CDs solution is used is slightly higher than that when deionized water is employed as lubricant. This may be because of the increased contact area, As (F=τsAs, where τs is the shear stress between two solids in contact), as a result of the filling effect of CDs. In contrast, under milder conditions, the friction coefficient of the CDs solution is lower than that obtained when the lubricant used is water. The viscosity of water and CDs solution are considerably similar (similar viscous stress, τl=η γ& ); the lower friction generated by the CDs solution must be attributed to the nano-bearing effects of CDs particles. Higher CDs concentrations, however, lead to higher contents of activated CDs and further result in the deposition of more large-sized particles. The size of deposited particles could further grow to micron scale when friction occurs (Fig. 8) because of the constant cohesion of activated CDs and the aggregation of sediment particles. The formed micron-sized particles in the high-concentration CDs solution could not fill the nano-valleys on the a-C surface when friction occurs; these particles could cause extra plowing wear (Fig. 6) because of the high contact stress exerted on the a-C surface. On the contrary, under low-concentration conditions, small sediment particles
are eventually collected in the valleys on the a-C surface; this effectively prevents the further growth of particles when friction occurs. For the GO nanosheet, which has a considerably larger lateral size (approximately 0.2–3.0 µm) than CDs, the lubrication mechanism involves surface adhesion and shearing along the basal plane of its crystalline lamellar structure under sliding friction stress. The a-C surface, however, is inert and has low adhesiveness. The GO nanosheet could therefore not stably remain on the a-C surface by filling the nanoscale valleys; this is different from the behavior of CDs on the surface when friction occurs. The GO nanosheets are thus prone to be pushed out from the contact region and aggregated at the wear track edge (Fig. S4), resulting in the gradual COF increase with test time. In particular, under low-concentration conditions where full coverage could not be ensured, the lubrication could even become worse than when the lubricant is water. This is because the flow of water is blocked by the GO as indicated by the remarkable increase in shear stress with the GO dispersion in the rheological test (Fig. 3). 4. Conclusion In order to improve the lubrication performance of water, the CDs are selected as eco-environmental lubricant additives. Their lubrication effects on a-C/a-C contacts at different additive concentrations are firstly investigated systematically. The tribological tests demonstrate that the CDs are highly efficient lubricating water additives for a-C/a-C contacts and exhibit remarkable friction and wear reduction effects at a considerably low concentration (0.1 wt.%). It is also found that the tribological properties of CDs solutions in a-C contacts are highly sensitive to addition concentrations. The amount and dimensions of CDs sediment particles that
are deposited in situ on worn a-C surfaces at different concentrations when friction occurs perform a crucial function in determining the lubrication effects of CDs. At the optimal concentration, most of the formed CDs sediment particles are nanoscale in size. These particles could reduce friction and wear via nano-bearing and nano-filling mechanisms. At high CDs concentrations, the formed micron-sized sediment particles cause undesirable abrasive wear. To obtain further understanding regarding the lubrication mechanism of CDs, the structure–function relationship will be investigated in our future work. Acknowledgements This research was financially supported by the Scientific and Technological Research Program
of
Chongqing
Municipal
Education
Commission
(Grant
No.
KJQN201801432), the Chongqing Research Program of Basic Research and Frontier Technology (Grant No. cstc2018jcyjAX0755), the National Natural Science Foundation of China (Grant No. 51705508), and the Open Project of State Key Laboratory of Solid Lubrication (LSL-1706). Moreover, we would like to extend our thanks to Dr. Ruochong Zhang for her kind suggestions.
References
[1] F. Chinas-Castillo, J. Lara-Romero, G. Alonso-Nunez, J.D.O. Barceinas-Sanchez, S. Jimenez-Sandoval, Friction reduction by water-soluble ammonium thiometallates, Tribol. Lett. 26 (2) (2007) 137-144. [2] W. Junming, W. Jianhua, L. Chunsheng, Z. Gaiqing, W. Xiaobo, A study of 2,5-dimercapto-1,3,4-thiadiazole
derivatives
as
multifunctional
additives
in
water-based hydraulic fluid, Ind. Lubr. Tribol. 66 (3) (2014) 402-410. [3] A. Petrov, P. Petrov, M. Petrov, Research into water-based colloidal-graphite lubricants for forging of carbon steels and Ni-based alloys, Int. J. Mater. Form. 3 (1) (2010) 311-314. [4] Q. Chen, X. Wang, Z.T. Wang, Y. Liu, T.Z. You, Preparation of water-soluble nanographite and its application in water-based cutting fluid, Nanoscale Res. Lett. 8 (2013) 1-8. [5] Z.Y. He, Y.Q. Wu, J.H. Lei, L.P. Xiong, J.W. Qiu, X.S. Fu, Tribological property study of novel water-soluble triazine derivative, Proc. IMechE. J:J. Eng. Tribol. 228 (4) (2014) 445-453. [6] Y.G. Wu, Z.G. He, X.Q. Zeng, T.H. Ren, E. De Vries, E.L. van der Heide, Tribological properties and tribochemistry mechanism of sulfur-containing triazine derivatives in water-glycol, Tribol. Int. 109 (2017) 140-151. [7] Y.L. Wu, Z.Y. He, X.Q. Zeng, T.H. Ren, E. de Vries, E. van der Heide, Tribological and anticorrosion behaviour of novel xanthate-containing triazine
derivatives in water-glycol, Tribol. Int. 110 (2017) 113-124. [8] L.P. Xiong, Z.Y. He, S. Han, J. Tang, Y.L. Wu, X.Q. Zeng, Tribological properties study of N-containing heterocyclic imidazoline derivatives as lubricant additives in water-glycol, Tribol. Int. 104 (2016) 98-108. [9] H. Kinoshita, Y. Nishina, A.A. Alias, M. Fujii, Tribological properties of monolayer graphene oxide sheets as water-based lubricant additives, Carbon 66 (2014) 720-723. [10] X. Pei, L. Hu, W. Liu, J. Hao, Synthesis of water-soluble carbon nanotubes via surface initiated redox polymerization and their tribological properties as water-based lubricant additive, Euro. Polym. J. 20 44 (8) (2008) 2458-2464. [11] H. Xie, B. Jiang, J. Dai, C. Peng, C. Li, Q. Li, F. Pan, Tribological Behaviors of Graphene and Graphene Oxide as Water-Based Lubricant Additives for Magnesium Alloy/Steel Contacts, Materials 11 (2) (2018) 206. [12] O. Elomaa, V.K. Singh, A. Iyer, T.J. Hakala, J. Koskinen, Graphene oxide in water lubrication on diamond-like carbon vs. stainless steel high-load contacts, Diam. Relat. Mater. 52 (2015) 43-48. [13] H. Lu, W. Tang, X. Liu, B. Wang, Z. Huang, Oleylamine-modified carbon nanoparticles as a kind of efficient lubricating additive of polyalphaolefin, J. Mater. Sci. 52 (8) (2017) 4483-4492. [14] B. Wang, W. Tang, H. Lu, Z. Huang, Ionic liquid capped carbon dots as a high-performance friction-reducing and antiwear additive for poly(ethylene glycol), J. Mater. Chem. A 4 (19) (2016) 7257-7265.
[15] Z.L. Wu, Z.X. Liu, Y.H. Yuan, Carbon dots: materials, synthesis, properties and approaches to long-wavelength and multicolor emission, J. Mater. Chem. B 5 (21) (2017) 3794-3809. [16] X. Sun, K. Yin, B. Liu, S. Zhou, J. Cao, G. Zhang, H. Li, Carbon quantum dots in ionic liquids: a new generation of environmentally benign photoluminescent inks, J. Mater. Chem. C 5 (20) (2017) 4951-4958. [17] B. Wang, W. Tang, H. Lu, Z. Huang, Hydrothermal synthesis of ionic liquid-capped carbon quantum dots with high thermal stability and anion responsiveness, J. Mater. Sci. 50 (16) (2015) 5411-5418. [18] B. Wang, Y. Lin, H. Tan, M. Luo, S. Dai, H. Lu, Z. Huang, One-pot synthesis of N-doped carbon dots by pyrolyzing the gel composed of ethanolamine and 1-carboxyethyl-3-methylimidazolium chloride and their selective fluorescence sensing for Cr(vi) ions, Analyst 143 (8) (2018) 1906-1915. [19] B. Wang, H. Tan, T. Zhang, W. Duan, Y. Zhu, Hydrothermal synthesis of N-doped carbon dots from an ethanolamine–ionic liquid gel to construct label-free multifunctional fluorescent probes for Hg2+, Cu2+ and S2O32−, Analyst 144 (9) (2019) 3013-3022. [20] H. Huang, H. Hu, S. Qiao, L. Bai, M. Han, Y. Liu, Z. Kang, Carbon quantum dot/CuSx nanocomposites towards highly efficient lubrication and metal wear repair, Nanoscale 7 (26) (2015) 11321-11327. [21] W. Shang, T. Cai, Y. Zhang, D. Liu, S. Liu, Facile one pot pyrolysis synthesis of carbon quantum dots and graphene oxide nanomaterials: All carbon hybrids as
eco-environmental lubricants for low friction and remarkable wear-resistance, Tribol. Int. 118 (2018) 373-380. [22] Z. Mou, B. Wang, H. Lu, H. Quan, Z. Huang, Branched polyelectrolyte grafted carbon dots as the high-performance friction-reducing and antiwear additives of polyethylene glycol, Carbon 149 (2019) 594-603. [23] W. Ma, Z. Gong, K. Gao, L. Qiang, J. Zhang, S. Yu, Superlubricity achieved by carbon quantum dots in ionic liquid, Mater. Lett. 195 (2017) 220-223. [24] X. Liu, Z. Huang, W. Tang, B. Wang, Remarkable Lubricating Effect of Ionic Liquid Modified Carbon Dots as a Kind of Water-Based Lubricant Additives, Nano 12 (09) (2017) 1750108. [25] W. Tang, B. Wang, J. Li, Y. Li, Y. Zhang, H. Quan, Z. Huang, Facile pyrolysis synthesis of ionic liquid capped carbon dots and subsequent application as the water-based lubricant additives, J. Mater. Sci. 54 (2) (2019) 1171-1183. [26] H. Xiao, S. Liu, Q. Xu, H. Zhang, Carbon quantum dots: An innovative additive for water lubrication, Sci. China Technol. Sci. 62 (4) (2019) 587-596. [27] Y. Hu, Y. Wang, C. Wang, Y. Ye, H. Zhao, J. Li, X. Lu, C. Mao, S. Chen, J. Mao, L. Wang, Q. Xue, One-pot pyrolysis preparation of carbon dots as eco-friendly nanoadditives of water-based lubricants, Carbon 152 (2019) 511-520. [28] F. Zhou, K. Adachi, K. Kato, Sliding friction and wear property of a-C and a-CNx coatings against SiC balls in water, Thin Solid Films 514 (1) (2006) 231-239. [29] H. Li, T. Xu, C. Wang, J. Chen, H. Zhou, H. Liu, Tribochemical effects on the friction and wear behaviors of a-C:H and a-C films in different environment, Tribol.
Int. 40 (1) (2007) 132-138. [30] Q. Wang, F. Zhou, X. Ding, Z. Zhou, C. Wang, W. Zhang, L.K.-Y. Li, S.-T. Lee, Structure and water-lubricated tribological properties of Cr/a-C coatings with different Cr contents, Tribol. Int. 67 (2013) 104-115. [31] A. Tanaka, M. Suzuki, T. Ohana, Friction and Wear of Various DLC Films in Water and Air Environments, Tribol. Lett. 17 (4) (2004) 917-924. [32] M. Kalin, I. Velkavrh, J. Vižintin, L. Ožbolt, Review of boundary lubrication mechanisms of DLC coatings used in mechanical applications, Meccanica 43 (6) (2008) 623-637. [33] J. Kogovsek, M. Kalin, Various MoS2-, WS2- and C-Based Micro- and Nanoparticles in Boundary Lubrication, Tribol. Lett. 53 (3) (2014) 585-597. [34] M. Kalin, J. Kogovsek, M. Remskar, Nanoparticles as novel lubricating additives in a green, physically based lubrication technology for DLC coatings, Wear 303 (1-2) (2013) 480-485. [35] L. Zhang, J. Pu, L. Wang, Q. Xue, Synergistic Effect of Hybrid Carbon Nanotube-Graphene Oxide as Nanoadditive Enhancing the Frictional Properties of Ionic Liquids in High Vacuum, ACS Appl. Mater. Interfaces 7 (16) (2015) 8592-8600. [36] L. Zhang, J. Pu, L. Wang, Q. Xue, Frictional dependence of graphene and carbon nanotube in diamond-like carbon/ionic liquids hybrid films in vacuum, Carbon 80 (2014) 734-745. [37] Y. Sun, S. Wang, C. Li, P. Luo, L. Tao, Y. Wei, G. Shi, Large scale preparation
of graphene quantum dots from graphite with tunable fluorescence properties, Phys. Chem. Chem. Phys. 15 (24) (2013) 9907-9913. [38] D. Pan, J. Zhang, Z. Li, M. Wu, Hydrothermal Route for Cutting Graphene Sheets into Blue-Luminescent Graphene Quantum Dots, Adv Mater. 22 (6) (2010) 734-738. [39] R. Atchudan, S. Perumal, D. Karthikeyan, A. Pandurangan, Y.R. Lee, Synthesis and characterization of graphitic mesoporous carbon using metal–metal oxide by chemical vapor deposition method, Micropor. Mesopor. Mat. 215 (2015) 123-132. [40] S. Kundu, R.M. Yadav, T.N. Narayanan, M.V. Shelke, R. Vajtai, P.M. Ajayan, V.K. Pillai, Synthesis of N, F and S co-doped graphene quantum dots, Nanoscale 7 (27) (2015) 11515-11519. [41] Z.-C. Yang, M. Wang, A.M. Yong, S.Y. Wong, X.-H. Zhang, H. Tan, A.Y. Chang, X. Li, J. Wang, Intrinsically fluorescent carbon dots with tunable emission derived from hydrothermal treatment of glucose in the presence of monopotassium phosphate, Chem. Commun. 47 (42) (2011) 11615-11617. [42] M. Kalin, I. Velkavrh, Non-conventional inverse-Stribeck-curve behaviour and other characteristics of DLC coatings in all lubrication regimes, Wear 297 (1-2) (2013) 911-918.