Tribological properties of MoS2 coating for ultra-long wear-life and low coefficient of friction combined with additive g-C3N4 in air

The solid lubricant MoS2 demonstrates excellent lubricating properties, but it spontaneously oxidizes and absorbs moisture in air, and thus results in poor wear resistance and short wear-life. In this study, the additive g-C3N4 (CN) was successfully combined with MoS2 via hydrothermal synthesis as a solid lubricant for the first time. Meanwhile, a low friction coefficient (COF, μ = 0.031) and ultra-long wear-life of CN/MoS2 compared to pure MoS2 in air were demonstrated. The functional groups and good crystallinity of the lubricant material were characterized via Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD). The formed valence states in CN/MoS2 were analyzed via X-ray photoelectron spectroscopy (XPS). The characterized results of the scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) show the morphology and interior crystal phase structure of CN/MoS2. From the cross-section analysis, the presence of iron oxide nanoparticles lubricating film is synergistic with CN/MoS2 film during the friction process, resulting in its ultra-long wear-life. In particular, the friction mechanism of interlayer sliding friction combined with energy storage friction was analyzed and proposed.


Introduction
From a tribological perspective, the lubricity and anti-wear of contact materials determine their sliding stability and wear-life [1]. Solid lubricant materials commonly used include graphene, hexagonal boron nitride (HBN), diamond, molybdenum disulfide (MoS2), and polymers [2,3]. Among these, the well-known lamellar transition metal sulfide MoS2, equipped with the advantages of thermal stability, lower shear strength, and strong surface adhesion force, is a good lubricating material [4][5][6]. The MoS2 atomic-layered structure was formed via the strong covalent bond between Mo and S atoms (S-Mo-S), especially the single-layer MoS 2 prepared using chemical vapor deposition or mechanical exfoliation exhibited the feasibility as protective or lubricant layers for micro-and nano-devices, whereas weak van der Waals force between the layers [7][8][9]. By easy slippage and weak interactions between the layers, MoS 2 exhibits a low coefficient of friction (COF), and as an additive in lubricating oil and solid lubricants, its application in satellites and mechanical components has attracted significant attention [10,11]. However, pure MoS2 absorbs moisture easily in a humid environment and can be oxidized in a molecular (O 2 ) or atomic (O) oxygencontaining environment, which sharply increases the COF (μ > 0.2) and shortens the friction lifetime [10,12]. These factors reduce and limit its practical application value. Therefore, the superlubricity of MoS 2 , at the atomic or molecular scale under desirable structural orders or orientations, can be better achieved in a dry atmosphere or vacuum [13]. For overcoming these drawbacks, MoS 2 is primarily combined with materials such as graphene, polyurethane, and metal (Ti, Cr, Al, Cu, etc.) owing to its excellent friction, mechanical, and thermal properties [14][15][16][17][18]. Currently, a significant challenge is presented regarding the achievement of a long wear-life and low COF of MoS2 for its use as a solid lubricant in the macro-field [19]. However, metal additives in MoS 2 coatings limit its application because of the high cost. Non-metals, such as layered g-C3N 4 (CN) presenting the advantages of being cost-effective, readily available, sufficiently stable, and environmentally friendly, can theoretically replace metal materials [20,21]. At present, the polymer semiconductor CN that demonstrates tris-triazine units and weak van der Waals force among its layers is utilized in many fields [20,22]. In the field of lubrication, CN is often used as an additive in lubricating oil to enhance the friction performance. For instance, an effective boundary film was formed on the friction surface when CN was grafted with octadecylamine, whereby the wear resistance was improved [3]. Duan et al. [23] introduced CN as an additive in the base oil, as it can significantly improve the wear resistance of thermosetting polyimide. Simultaneously, the nano-composites CN/Cu and CN/MoS 2 were prepared to exhibit excellent wear resistance [22,24]. Some researchers have prepared CN fullerenes to achieve a low COF (μ = 0.01-0.02) [21]. Those studies have shown that CN, as an additive, exhibits good wear resistance at room temperature from 22 to 26 ℃ and in air.
Both MoS 2 and CN can be used as lubricating materials, however, to date, few studies have investigated CN for use as an additive in MoS 2 to form a nanocomposite solid lubricating material. Given the atomic layered structure of MoS 2 and CN, these can easily slide from one layer to another upon combination, and thereby improving the friction properties. In this study, the nano-composite CN/MoS 2 was synthesized as a lubricating material via a one-step hydrothermal method. The data show that the CN/MoS 2 composite has a low COF, ultralong wear-life, high crystallinity, and good stability, demonstrating the potential for practical application. Simultaneously, the friction mechanism of CN/MoS 2 in air was proposed and explained.

Preparation of CN powder
Urea (20 g) was weighed into a 50 mL crucible, covered, and placed into a muffle furnace. It was then calcinated at 550 ℃ for 4 h at a rate of 5 ℃/min. After it was cooled to 22-26 ℃, the yellow powder CN was obtained via grinding.

Preparation of composite CN/MoS 2
The following reagents were added to a beaker containing 80 mL deionized (DI) water: 0.1 g CN powder, 0.61 g sodium molybdate, 0.99 g thiourea, and 0.66 g citric acid. The mixture was transferred into a Teflon-lined reaction kettle following ultrasonic stirring for 30 min. The reactor was then placed in an oven at 190 ℃ for 24 h. After being cleaned with ethanol and DI water and then centrifuged, the mixture was dried in an oven at 80 ℃ for 8 h, following which the CN/MoS 2 composite powders were obtained. The steps to prepare pure MoS2 were the same as those mentioned above, except for the addition of CN.

Preparation of coating
Before the preparation of the coating, the surface of the steel substrate was roughened with 80-mesh grit and then dipped in acetone and stirred ultrasonically for 30 min. Stoichiometric CN/MoS 2 powder was dissolved and dispersed in a beaker

Tribological experiment
The friction experiments of the coatings were performed using a common friction machine (CSM, Switzerland). The counterpart ball was composed of commercial AISI 52100 steel ball (the diameter: Ø = 6 mm, the roughness: Ra ≤ 0.2 μm). All experiments were performed in an atmosphere with a humidity of approximately 30%±5% and room temperature of about 21 ℃ . The test frequency, reciprocating radius, sliding speed, and loading was 10 Hz, 5 mm, 10 cm/s, and 1 N (equivalent to a pressure of 0.5 GPa), respectively. The dynamic data of the COF were automatically saved and recorded using a computer. Each test was performed three times, and an average value was calculated to eliminate accidental errors. The experiment was terminated when the COF was higher than 0.12 and the intense noise was generated.

Characterization
The phase composition of the samples was determined using a Philips X'Pert MPD X-ray diffractometer (XRD; Cu Kα radiation, potential 40 kV, and current 150 mA) over a 2θ range of 5-75. The Fourier transform infrared (FTIR) spectroscopy spectra of the samples were characterized using a Thermo Nicolet Avatar 380 FTIR spectrometer. The microstructures of the powders were observed using a transmission electron microscope (TEM, FEI Tecnai G2 TF20, USA; accelerating voltage of 200 kV). The thin foils of the worn surface for highresolution transmission electron microscopy (HRTEM) observation were produced through a focused ion beam (FIB, Helios Nanolab 600, FEI Corporation, USA, liquid metal Ga as ion source). The worn surface of the coatings and the friction pairs were characterized using a scanning electron microscope (SEM, JSM-5600LV, JEOL Corporation, Japan, equipped with an energy dispersive spectrometer (EDS)). The chemical element valences of powders were determined using an X-ray photoelectron spectroscope (XPS, PHI-5702, Al Kα X-ray source), in which the binding energy of the carbon contaminant (C 1s = 284.6 eV) was used as the reference for calibration.

XRD and FTIR Analysis
The crystal structure and functional groups of three samples were examined via XRD and FTIR, respectively. As shown in Fig. 1(a), the diffraction peaks of CN were positioned at 2θ = 13.0° and 27.5°, corresponding to the (100) and (002) crystal planes [23,25], respectively. The two typical characteristic peaks represent interlayers stacked in conjugated aromatic systems [25]. Remarkable peaks occur at 2θ = 14°, 32.7°, and 58.3°, which are assigned to the (002), (100), and (110) planes of the hexagonal MoS2 (PDF#77-1716) [26,27], respectively. It is noteworthy that a blue-shift of MoS2 at 2θ = 12.6° (002) and red-shift of CN at 2θ = 13.0° (100) occurred in the XRD pattern, representing the lattice distortion that results from the changes in the lattice constant during the mutual doping of atoms during the synthesis [28].
In the FTIR pattern, the peaks at 808 and 1,237.4 cm 1 are attributed to the breathing mode of a triazine ring and the aromatic ring structure of CN, respectively [20]. Meanwhile, the peaks located at 1,407.1 and 1,638.9 cm 1 are caused by the respiration and extension of typical C-C and C=N heterocyclic structures in CN [23,29]. Besides, the peak at 3,432.2 cm 1 is attributed to the stretching vibration of residual -NH in the C-N ring and the vibration of -OH in the H2O adsorbed on the surface [20,23,29]. The characteristic peaks of CN and MoS2 exist in the FTIR pattern of CN/MoS 2 , suggesting that MoS 2 does not affect the functional group of CN in the composite during hydrothermal synthesis. All characterizations sufficiently indicate that the successfully synthesized nano-composite CN/MoS2 contains two structures of CN and MoS 2 .

Analysis of morphology
SEM and TEM were performed to investigate the morphology of the solid lubricant. Figure 2(a) shows that the surface of CN possesses a smooth and multilayer structure, while the flower-like MoS 2 assembled with several nanosheets is fully displayed in Fig. 2(b). Figure 2(c) illustrates that MoS 2 and CN in the composite are wrapped together, which indicates that the two materials were successfully combined. It can be discerned that the CN sheets are scattered at the bottom of the flower-like MoS 2 in Fig. 2(d). In the friction test, the reciprocating shear stress can enable nanospheres to be peeled off to obtain more MoS 2 lamellar. This provides a reliable basis for achieving a lower COF for pure MoS 2 and composite CN/MoS 2 .
Distributed elements in the composite can be detected via an energy dispersive X-ray detector (EDX). Figure 2(e) shows that the composite contains C, N, Mo, and S elements, which is consistent with the results of the XPS analysis. The crystal structure  (105) and (103) crystal plane, respectively [30]. The d-spacing corresponding to the (002) crystal plane is 0.62 nm, which is the inter-plane spacing of hexagonal MoS 2 [5,9]. This indicates that the crystal of MoS 2 formed well in the composite, which corresponds to the aforementioned XRD results. The illustration in the lower right corner of Fig. 2(f) shows the layered CN, and the inset in the upper right corner is the selected area showing the electron diffraction (SAED) pattern of the composite CN/MoS2.

XPS analysis
The elementary valence distribution in CN/MoS 2 was investigated via XPS. The full spectrum in Fig. 3(a) detected the composite containing C, N, O, Mo, and S elements, which is consistent with the EDX results mentioned earlier. The peaks of C 1s in Fig. 3(b) can be divided into 284.9, 286.1, and 288.15 eV, which correspond to C-C, C-O, and C-OOH bonds, respectively [21,23]. The high-resolution spectrum of N 1s was fitted to three peaks with binding energies at 397.07, 400.4, and 402.1 eV in Fig. 3(b). The highest intensity peak corresponds to the triazine ring (C-N=C) located at 397.07 eV. The binding energies at 400.4 and 402.1 eV are attributed to N-(C) 3 and hydrogen-bearing CN-H groups [23,29], respectively. The peak of N 1s occurs at 404.4 eV, mainly owing to the charge effect and positive charge localization in heterocycles [23,31]. From the XPS spectra of C 1s and N 1s, it can be concluded that C and N exist in the form of covalent bonds, which are more favorable for bond formation between CN and MoS 2 . The high-resolution spectrum of Mo 3d (Fig. 3(d)) shows that the peaks with a binding energy at 231.08 and 234.25 eV are attributed to Mo 3d 5/2 and Mo 3d 3/2 in MoS 2 , indicating that Mo is mainly in the form of Mo 4+ , meanwhile, a small peak of S 2s is located at 228.12 eV [32,33]. Figure 3(e) is a high-resolution spectra of S 2p and the binding energies at 163.1 and 162.2 eV are attributed to the negative bivalent S 2  ion orbit [9,34]. The results imply that the Mo and S atoms mainly combined with S-Mo-S covalent bonds in CN/MoS 2 . From the discussion above, it can be concluded that the material CN/MoS 2 was successfully synthesized.   Figure 4 shows the COFs of CN, MoS2, and CN/MoS 2 coatings with different ratios, under a load of 1 N, and a sliding speed of 10 cm/s. The optimum ratio of composite materials was confirmed as 65 wt% CN/MoS 2 , which revealed the lowest COF and the longest friction lifetime in Fig. 4(a). Meanwhile, the highest average COF was achieved by the composite with 75% CN content. The illustration insert in Fig. 4(b) shows a higher COF (μ = 0.7) for pure steel substrate, which is caused by the direct contact sliding between the counterpart ball and the steel substrate. Meanwhile, pure CN has a higher COF (μ = 0.08) and shorter wear-life, as a majority of the CN powder exhibits a larger particle size as well as a lamellar structure, whereby the valley bottom cannot be enriched. It is just attached to the surface of the micro-convex body, which is prone to being peeled off by the frictional force, following which poor adhesion is exhibited at the rough steel substrate surface. Therefore, the lubricating structure of the CN powder on the substrate surface is easily destroyed during the friction process, which shows an unsatisfactory lubricating property when it is used alone.

Friction and wear properties in air
Here, the running-in period shown in friction experiments, which can provide a stable and robust friction interface for the subsequent friction process, contributes to a longer wear-life. As shown in Fig. 4(b), the materials MoS 2 and CN/MoS 2 are compacted during the running-in period, and thus forming a dense film on the coating surface because of the reciprocating shear stress. The friction material will not peel off immediately; thus, the substrate is prevented from directly coming in contact with the counterpart ball. Simultaneously, the COF of pure MoS 2 tends to be stable and close to 0.025 in a dry environment; this lubrication performance of MoS 2 is unusual as per many studies. One reason for this is that MoS 2 microspheres with good adhesion and smaller particle size easily fill the rough surface of the substrate in a dry atmosphere [6]. Furthermore, MoS 2 microspheres can be exfoliated into lamella via the reciprocating shear stress during the friction process, which causes sliding friction to occur between the counterpart ball and the coating.
The noteworthy features are excellent COF and lifetime of the composite CN/MoS 2 coating. After a short running-in period, the average COF of the lubricant CN/MoS 2 is rapidly stabilized to 0.031, and its wear-life is significantly longer than that of pure MoS 2 and CN. Although the COF of CN/MoS 2 is slightly higher than that of pure MoS 2 , CN exerts excellent wear resistance and can significantly prolong the wear-life of MoS 2 . The change in COF caused by different loads in the friction tests was explored and shown in Fig. 4(c); the lowest COF is close to 0.02 when the load is 15 N, which indicates that the larger is the load, the lower is the COF. The reason is that the rate of increase in the contact area is lower than that of the increase in load. The abovementioned phenomena show that combining the solid lubricant MoS 2 with the additive CN can fully demonstrate their unique features. Under the conditions of a suitable addition amount, MoS 2 can significantly reduce the COF of CN while CN can increase the wear-life of MoS 2 , resulting in lower

COF and longer wear-life of CN/MoS 2 composites.
This provides the possibility for exploring the performance of CN in lubrication and preparing new solid lubricating materials.

Analysis of wear surfaces and optical microscope
To further explain the friction phenomenon from a microscopic perspective, a SEM and optical microscope were used to observe the wear scar. From the wear scar surface after the friction test in Fig. 5(a), a local shear fracture occurred on the substrate surface during relative sliding, which demonstrates the adhesive wear sliding mechanism [29]. Meanwhile, the lubricating material on the surface was consumed during the friction process, which continually increased the depth and width of the wear scar. Coinstantaneously, the wear debris appeared on the middle groove and the edge of the wear scar. Valleys filled with CN/MoS2 material on the wear track of the coating (both Figs. 5(a) and 5(d)) were also observed. Figure 5(d) shows that the material was exhausted and the substrate surface exposed after enduring friction; a few fragments of CN/MoS2 material in these valleys will continue to provide lubrication, and thus contributing to a longer wear-life. The results of wear track EDS mapping in Fig. 5(a) are the same as those of the line scan mapping of the materials for the contact region in Fig. 5(f); in addition to the elements of CN/MoS 2 , the presence of Fe suggests that the lubricating material on the surface was removed, and the counterpart ball was in sliding contact with the steel substrate. The presence of the O element was caused by surface oxidation that resulted in iron oxide during the friction test in air. The SEM images in Figs. 5(b) and 5(c) and the TEM image in Fig. 5(e) show the morphologies of materials on the wear scar surface after relative friction; the MoS2 microspheres and the large flaky fragments of CN were exfoliated from the multilayer nanosheet structure, which contributes to a longer friction life and a lower COF.

Surfaces of counterpart ball analysis
The SEM images of the counterpart ball contact surface after the friction test are shown in Figs. 6(a) and 6(b). Many scratches and grooves are formed on the surface of the counterpart ball, as well as padded debris. The lubricating materials peeled off when the coating was rubbed due to the shear stress, following which a rough and thin transfer film formed on the friction surface of the counterpart ball. In this case, a longer wear-life and lower COF  can be maintained, mainly due to the friction that occurs between the transfer film adhering to the counterpart ball and the composite material on the steel substrate. From the EDS mapping in Fig. 6(c), a certain amount of C, N, Mo, and S appeared on the worn surface, which confirmed the accuracy of the thin transfer film and debris observed in Fig. 6(a). Besides, the content of O and Fe can be observed due to the oxidation during the friction process in air.

Analysis of cross-section
To further elucidate the friction mechanism of the CN/MoS 2 coating, a thin cross-section of the wear scar surface was prepared by the FIB. A few flakes of MoS 2 and amorphous CN are presented after the friction process in Fig. 7(a) The above phenomenon indicates that under the shear stress, a tribochemical reaction occurred on the wear surface of the friction contact zone, resulting in the formation of metal nanoparticle boundary lubrication film with a small particle size of iron oxide. Subsequently, the friction pair was separated from the substrate during the friction process to prevent its direct contact [34]. Concurrently, the metal nanoparticle boundary lubrication film synergistic with the CN/MoS2 film is conducive to lessen the friction and wear of the coating and improve the friction lifetime [37,38].

Lubrication mechanism
The structural features of the CN nanosheets are conducive to reveal more potential reactive functional groups (confirmed by FTIR), which increase the chemical reaction with MoS 2 [29]. Figure 8(a) shows the two-dimensional (2D) layered atomic structure of the CN and MoS 2 . Among them, the CN heterocyclic compounds exist in a graphite-like structure and exhibit an interaction of stacking between layers, the H-bond inside the molecule is stronger than the van der Waals force between the CN and MoS 2 layers [23,33]. The MoS 2 can be perceived as hexagonal crystalline containing a S-Mo-S covalent bond, which is connected with the CN molecular layers by van der Waals force [33,39]. The CN/MoS 2 compound shows chemical activities and quickly adsorbed onto a friction surface to form a physical adsorption film [40]. That structural characteristic facilitates low COF and longer wear-life of CN/MoS2 coating.
Hence, a friction mechanism of CN/MoS2 coating is proposed in Fig. 8(b) based on adhesive wear and oxidation wear. To begin with, when the counterpart ball is in point-to-surface contact with the CN/MoS 2 coating, the reciprocating shear stress compacts the materials on the surface and in the valley to constitute a friction surface (the inset (2) in Fig. 8(b)). The MoS 2 nanospheres and CN composed of a layered structure are then prepared for sliding friction.
Furthermore, under the action of reciprocating shear stress, the MoS 2 microspheres are exfoliated to fragments of nanosheets during friction process (the inset (3) in Fig. 8(b)), and thus forming a thin and inter-laminar friction between MoS 2 or MoS2 and CN nanosheets, which effectively maintain the low COF. Subsequently, when the materials from the sliding surfaces are exhausted due to shear and removal, the CN/MoS 2 lubricant locked in the valley bottom continues to exert its lubricating | https://mc03.manuscriptcentral.com/friction function, and thereby maintaining a longer wear-life. Furthermore, the existence of CN reduces the oxidation of MoS 2 in the friction process to prevent its failure, and thus increasing the wear resistance of the materials.
After a period of friction, fragments of the lubricating materials were transferred to the counterpart ball to form a transfer film and wear debris, and then the friction mainly occurs between the lubricants retained on the substrate and transfer film as well as on the wear debris. Meanwhile, the tribochemical reaction that occurred on the wear surface of the friction contact zone, resulted in the metal nanoparticle boundary lubrication film of iron oxide, which synergized with the CN/MoS 2 film to prolong the friction lifetime. Therefore, a low COF is maintained and the wear resistance of the composites is further improved, which is also a reason for the ultra-long wear-life of composite lubricants.
Finally, with the friction experiment progressing, the transfer film on the counterpart ball gradually diminished, and the lubricating materials on the substrate surface and in the valley gradually depleted; this directly manifests itself as friction failure and the COF increases linearly. At this point, the lubricating material loses its lubrication.

Conclusions
The nano-composite CN/MoS 2 was successfully synthesized by hydrothermal synthesis and first applied to solid lubricating material in air. The material CN/MoS 2 has both the morphology and the functional group of a MoS 2 microsphere flower and CN layer. Simultaneously, the valence distribution in XPS shows the covalent bond formed between Mo and S. Because CN exhibits stability and wear resistance, which can significantly prolong the wear-life of MoS 2 as a water and oxygen cleaner while MoS 2 can lower the COF of CN. Thus, the composite CN/MoS 2 has a low COF (μ = 0.031) and long wear-life in air. This shows that the wearresistant additive CN can effectively inhibit oxidation and prevent the failure of MoS 2 . Besides, the transfer film on the counterpart ball and the storage materials retained in the valley contribute significantly to the lower COF and longer wear-life. The analysis of the cross-section results shows that the presence of iron oxide nanoparticles lubricating film is synergistic with CN/MoS 2 film during the friction process, which results in its excellent friction performance. Aiming at the outstanding lubrication performance under reciprocating shear stress, the friction mechanism of combining interlayer sliding friction and storage friction is proposed and analyzed.
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