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Article

The Preparation of MoS2/Metal Nanocomposites Functionalized with N-Oleoylethanolamine: Application as Lubricant Additives

by
Yaping Xing
,
Zhiguo Liu
,
Weiye Zhang
,
Zhengfeng Jia
*,
Weifang Han
*,
Jinming Zhen
and
Ran Zhang
School of Materials Science and Engineering, Liaocheng University, Liaocheng 252059, China
*
Authors to whom correspondence should be addressed.
Lubricants 2024, 12(9), 319; https://doi.org/10.3390/lubricants12090319
Submission received: 14 August 2024 / Revised: 8 September 2024 / Accepted: 11 September 2024 / Published: 14 September 2024
(This article belongs to the Special Issue Preparation, Tribological Behavior, and Applications of Lubricant Additives)
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Versions Notes

Abstract

:
In this study, MoS2 nanosheets have been prepared and treated ultrasonically with silver ammonia solutions. The MoS2/Ag precursor was reduced using dopamine (DA) as reducing and linking agent at room temperature, and it was subjected to a hydrothermal treatment to produce MoS2/Ag nanocomposites (denoted as MoAg). The MoAg samples were functionalized with N-oleoylethanolamine to improve dispersion in the base oil component of additives. Use of the functionalized MoAg (denoted as Fc-MoAg) as a lubricant additive for steel balls resulted in effective friction reduction and anti-wear. This work avoids ion exchange during exfoliation, and the Ag+ has been reduced to nano-silver particles by dopamine to enlarge the layer spaces of MoS2. Taking the case of lubrication with base oil containing Fc-Mo0.6Ag15, the wear scar diameters and coefficients of friction of the steel balls were 0.428 and 0.098 mm, respectively, which were about three-fifths base oil. In addition, MoS2/Cu and MoS2/Ni nanocomposites were synthesized and the tribological properties associated with steel/steel balls assessed. The results demonstrate that all MoS2/metal composites exhibit enhanced tribological behavior in the steel/steel pair tests. Both nanocomposite synergy and the tribofilm containing sulfide, oxide, carbide, and other compounds play important roles in achieving reduced friction and improved anti-wear. The friction and wear properties of base oil containing Fc-MoAg and commercial additives were evaluated using a four-ball wear tester with steel/steel, steel/zirconia and zirconia/zirconia pairs. The base oil containing Fc-MoAg delivered smaller coefficients of friction (COFs) and/or scarring groove depths than those observed with the use of pure base oil and base oil containing commercial additives.

1. Introduction

In recent decades, research has increasingly focused on nanomaterials due to enhanced associated properties including friction reduction and wear resistance, energy saving, and eco-friendliness [1,2]. Two-dimensional nanomaterials (2D), such as graphene, MoS2, and BN are effective in terms of friction reduction and anti-wear applications because of the lower van der Waals forces between the layers of these materials [3,4,5,6]. Layered MoS2 are widely used metal sulfides due to their favorable crystallographic structure, featuring layers of transition metal atoms between chalcogen atoms, forming easily sheared lamellae with excellent frictional properties [2,5,7]. In addition, composites of MoS2 with nano-metal, graphene, and/or oxides have been widely studied because of the associated synergistic effects [8]. Sun et al. [7] prepared nitrogen-doped carbon quantum dot (N-CQD) nanoparticles using a solvothermal method and mixed the particles with MoS2 in a nanofluid to investigate the tribological properties applied to steel/steel pairs. The inclusion of N-CQD resulted in a 31.0% decrease in the coefficients of friction (COFs) and wear rates, which was attributed to the effectiveness of the tribofilms containing sulphur and nitrogen.
MoS2 nanosheets can be prepared in several different ways, such as mechanical exfoliation, chemical vapor deposition (CVD) on substrates, chemical exfoliation through intercalation, etc. Mechanical exfoliation is an effective method to prepare few-layer MoS2 nanosheets. However, the yield is low and the method is time consuming. CVD has been used to synthesize high-quality MoS2 nanosheets in a controllable manner with different compositions. But the method is not easy to scale up. Chemical exfoliation is a simple method to prepare few-layer MoS2 nanosheets [4,9]. Exfoliation represents an alternative approach to produce nano-sheets. Moreover, ion and surfactant solutions and ionic liquids have served as promotors in MoS2 nano-sheet synthesis [4,9,10,11]. The application of ultrasonic, shear and/or thermal technologies can facilitate ion intercalation in the interlayers and weaken interlayer interaction [4]. Following exfoliation, the ions and/or chemical agents must be exchanged (by ion exchange) or removed by washing. The hydrothermal synthesis of MoS2 involves treatment of sodium molybdate and thiourea at high temperatures for extended periods [1].
Carbon nanomaterials, metal oxides, and other nano-particles have been employed to improve the tribological properties of MoS2 [6,8]. Doping metal elements in MoS2 is regarded as an effective way to further improve the lubrication performance of MoS2 [12]. The tribological properties of MoS2/nano-metal were found to be far superior to those of the constituent phases [13]. The soft metal and lubricious debris diffuse to the surface to provide lubrication to the contact surfaces. The MoS2/nano-metal was transferred to the sliding contacts to form an anti-friction tribofilm [14]. Soft metal nanoparticles, such as Cu and Ag, can contribute to improved tribological properties [3,15,16]. These soft particles may be embedded in the worn surface and form tribofilms and/or trapped at the asperity level and stay on the surface. The authors have prepared Ag/polydopamine (PDA) nanoparticles at room temperature, added the particles to base oil, and investigated the tribological properties [16]. The results revealed that tribofilms containing C, O, N, Ag and Fe of steel balls were formed on the worn surfaces of steel–steel pairs, which improved friction reduction and anti-wear capability.
The motivation of this work is that interlamellar van der Waals interaction between adjacent atomic/molecular layers of 2D materials furnishes low shearing strength and reduces the friction under the tribo-stress to overcome wear. MoS2 nanosheets have been prepared by applying ultrasound treatment of Tollens’ reagent. Synthesis of MoS2/metal particles has utilized dopamine (DA) as a reducing and linking agent at room temperature and/or hydrothermal treatment. The MoS2/metal particles were functionalized with N-oleoylethanolamine and the resultant tribological properties and mechanism investigated. This work avoids ion exchange during exfoliation, and the Ag+ has been reduced to nano-silver particles by dopamine to enlarge the layer spaces of MoS2. Both nanocomposite synergy and the tribofilm containing sulfide, oxide, carbide, and other compounds play important roles in achieving reduced friction and improved anti-wear. The additives can be used in the automobile industry.

2. Experimental Procedures

2.1. Materials and Synthesis

Silver nitrate (AgNO3), dopamine (DA) hydrochloride, and MoS2 were purchased from Aladdin Biochemical Technology Company (Shanghai, China) and used as supplied. The commercial gear oil, and base oil (denoted as BO) classified as 120BS, were obtained from Liaocheng Manxiandi Lubricating Oil Company (Liaocheng, China) and used as supplied (shown as Table 1). AISI-52100 steel balls and zirconia balls (φ12.7 mm) were purchased from Jinan Shunmao Experimental Apparatus Company (Jinan, China) and Zhejiang Jienaier New Materials Company (Jiaxing, China), respectively, and used as supplied.
A silver ammonia solution was prepared as reported previously [16]. A known mass (5.0 g) of MoS2 powders was added to 100 mL silver ammonia solution with stirring for 30 min and then processed by ultrasonic exfoliation for 60 h at 250 W to produce the nanosheets. Then, 0.6 g exfoliated MoS2 was transferred into 15 mL silver ammonia solution and stirred, with the addition of 0.25 g DA and continual stirring at room temperature for 24 h. The resultant mixture was transferred to a hydrothermal reactor and heated at 160 °C for 12 h to produce the nanocomposite (denoted as Mo0.6Ag15).
The N-oleoylethanolamine and MoAg were mixed in absolute ethyl alcohol with vigorous stirring at 60 °C for 1 h and then subjected to reduced pressure distillation to generate Fc-MoAg, as illustrated in Figure 1a. The Fc-MoAg was added to the BO with stirring at concentrations of 0.5, 1.0, 1.5 and 2.0 wt.%. The sizes of Fc-MoAg were stable at about 500–750 nm, and there was no evidence of precipitation after 15 days (see Figure 1b). Wear experiments were performed using a four-ball wear tester at a 392 N load and 1450 rpm for 30 min for steel balls, which was in line with the Chinese national standard GB/T 3142-82 [17]. On the other hand, wear tests utilizing steel/steel, steel/zirconia and zirconia/zirconia pairs under BO lubrication with/without functionalized MoAg and commercial additives were executed with a 98 N load and 500 rpm for 30 min, which are non-standard parameters. Separate tests were conducted for each concentration, three repeated measurements were carried out, and the averaged values of the coefficients of friction and wear scar diameters of all low balls (calculated through the 3D surface profiler of the low balls) are reported in this paper. The error values were the differences between the measured values and averaged values.

2.2. Characterization

A high-resolution transmission electron microscope and a field-emission scanning electron microscope equipped with energy-dispersive X-ray spectroscopy (EDXA, Kevex Sigma, St. Louis, MO, USA) were used to analyze sample micro-structure. The X-ray diffraction (XRD) patterns were collected on a Bruker D8 Advanced X-ray Diffractometer (Bruker, Billerica, MA, USA) using Cu Kα radiation in the 2θ range between 10° and 80° with a scanning rate of 8° min−1. The ESCA LAB Xi+ X-ray photoelectron spectrometer (XPS) was employed to evaluate the chemical structure of the worn surface of composites. Fourier transfer infrared spectroscopy (FT-IR) analysis of the composites before and after modification was conducted using a Bruck IFs66v spectrometer (Bruker, Billerica, MA, USA). Thermogravimetric analysis (TGA) of the composites before and after modification employed a NETZSCH STA499 (Shenzhen, China) simultaneous thermal analyzer operated from room temperature to 800 °C at a heating rate of 10 °C/min in a N2 atmosphere. The confocal Raman spectroscopy analysis was conducted on a Horiba LabRam HR evolution spectrometer equipped with a 532 nm laser.

3. Results and Discussion

3.1. Structure

The morphology and corresponding elemental mapping of bulk MoS2, exfoliated MoS2 and Mo0.5Ag20 were determined by FE-SEM and EDXA, respectively. It can be seen (Figure 2a,b) that the bulk MoS2 exhibits a lamellar stacking structure. After ultrasonic treatment, the bulk material is broken into smaller sheets with a thickness of less than 100 nm (Figure 2c,d). In the case of MoAg, the MoS2 nanosheets were uniformly covered with nanoparticles, which can be attributed to PDA/Ag particles (Figure 2e,f). The TEM image presented in Figure 2g reveals the presence of particles with a diameter of ca. 20 nm that cover the surface of the MoS2 sheets (with the thickness of less than 20 nm), confirmed by selected area electron diffraction analysis [18]. The high-resolution TEM image in Figure 2h shows inter-layer spaces of ca. 0.7 nm associated with the MoS2 sheets, which exceeds that of bulk MoS2 (0.62 nm) [19]. The analysis has determined a lattice spacing of ca. 0.24 nm for Ag (111), where a significant Ag (111) component was found in the inter-layer spaces of the MoS2 sheets, suggesting the successful insertion of Ag. The amorphous species present on the MoS2 sheets and Ag particles can be attributed to polydopamine.
The SEM image, elemental mapping, and EDS analysis presented in Figure 3 have further confirmed the presence of MoS2 (with an atomic ratio of Mo:S = 1:2) and the uniform distribution of C, O, Ag, and N on the MoS2 nano-sheets. The FT-IR and TG spectrum of the Fc-MoAg means that the condensation of N-oleoylethanolamine and polydopamine and the functionalization of the composites were executed (shown as Figure S1) [16,20,21,22].

3.2. Tribological Behavior

The wear scar diameter (WSD) concentration curves and COF–concentration curves are shown in Figure 4 for BO containing Fc-MoxAg20 with varying Mo content (x = 0.4, 0.6, and 0.7). A smaller WSD was observed for the steel balls in the case of the three lubricants relative to pure base oil. At a concentration of 1.0 wt.%, the WSD of the steel balls lubricated with BO-Fc-Mo0.6Ag20 was ca.0.440 mm, smaller than that recorded with BO (0.680 mm), BO-Fc-Mo0.4Ag20 (0.520 mm) and BO-Fc-Mo0.7Ag20 (0.465 mm). The corresponding optical images show that the worn surfaces with BO possess deeper furrows than that with BO-Fc-Mo0.7Ag20. The average WSDs for the steel balls lubricated with BO-Fc-Mo0.6Ag20 were lower at the higher concentrations and converged with BO-Fc-Mo0.7Ag20 at 2.0 wt.%. Smaller COFs were recorded for the steel balls lubricated with BO including additives when compared with pure BO, with significantly lower values achieved when using BO-Fc-Mo0.6Ag20. At a concentration of 2.0 wt.%, the average COF for steel balls lubricated with BO-Fc-Mo0.6Ag20 is ca. 0.092, significantly lower than the value (ca. 0.120) observed for the two other lubricants. The COF–time curves of BO with three additives (1.0 wt.%) were lower than that of base oil (shown as Figure 4c).
The WSD–concentration, COF–concentration curves and COF–time curves for BO-Fc-Mo0.6Agy treated with different volumes of silver ammonia are shown in Figure 5, where y = 10, 15, and 20. The steel balls lubricated with BO-Fc-MoAg exhibited smaller WSDs and COFs than lubrication with BO. Comparing the three additives, the steel balls lubricated with BO-Fc-Mo0.6Ag15 showed the smallest WSD at concentrations in excess of 1.0 wt.% (Figure 5a). Taking the case of lubrication with BO-Fc-Mo0.6Ag15 at 1.5 wt.%, the WSDs of the steel balls decreased from 0.681 mm (using BO) to 0.428 mm, which is also smaller than that obtained with BO-Fc-Mo0.6Ag10 (0.512 mm) and BO-Fc-Mo0.6Ag20 (0.493 mm). The COF–concentration curves shown in Figure 5b reveal lower COF values for the steel balls lubricated with base oil containing the three additives at each concentration. Use of BO-Fc-Mo0.6Ag15 generated lower COFs than the other lubricants at concentrations higher than 1.0 wt.%. At a concentration of 1.5 wt.%, the average COF of balls lubricated with Fc-Mo0.6Ag15 is ca. 0.098, lower than that recorded for pure BO (0.153), BO-Fc-Mo0.6Ag10 (0.112), and BO-Fc-Mo0.6Ag20s (0.107). The COF–time curves of BO with three additives (1.5 wt.%) were smoother and lower than that of base oil (shown as Figure 5c). The volume of silver ammonia solution was accordingly fixed at 15 mL.
The WSD–concentration and COF–concentration curves of steel balls lubricated with BO containing functionalized Mo0.6Ag0, Mo0Ag15 and Mo0.6Ag15 are presented in Figure 6. It is immediately evident that the WSDs of steel balls tested with the three lubricants are smaller than that recorded for pure BO. A comparison of the curves reveals smaller WSDs for lubrication with BO-Fc-Mo0.6Ag15 relative to the BO containing Fc-Mo0.6Ag0 and Fc-Mo0Ag15. At a concentration of 1.5 wt.%, the WSD of the BO-Fc-Mo0.6Ag15 is ca. 0.44 mm, roughly 75% of the values for BO-Fc-Mo0Ag15 and the BO-Fc-Mo0.6Ag0. Moreover, the COFs of balls lubricated with BO-Fc-Mo0.6Ag15 are smaller than the other lubricants (Figure 6b). At a concentration of 1.5 wt.%, the average COF recorded for balls tested with the BO-Fc-Mo0.6Ag15 lubricant was ca. 0.098, significantly lower than BO-Fc-Mo0Ag15 (0.149) and BO-Fc-Mo0.6Ag0 (0.151). The COF–time curves of BO and BO with three additives (1.5 wt.%, respectively) with respect to time is displayed in Figure 6c, respectively. It is interesting that the COF–time curve of BO with BO-Fc-Mo0.6Ag15 is lower and smoother than that of the other three lubricants. The results suggest a synergy of MoS2 and Ag that influences WSD and COF [23,24].
The effect of the hydrothermal treatment on the WSD–concentration and COF–concentration response is presented in Figure 7 for functionalized Mo0.6Ag15. The steel balls lubricated with BO containing additives that were hydrothermally treated (at 160 °C) exhibited smaller WSDs and COFs than the use of additives without hydrothermal treatment. At a concentration of 1.5 wt.%, the WSD of steel balls lubricated with BO containing functionalized Mo0.6Ag15 without hydrothermal treatment decreased from 0.680 mm using pure BO to 0.480 mm. At the same concentration, the average COF of steel balls lubricated with BO containing Fc-Mo0.6Ag15 without hydrothermal treatment was 0.131, representing a significant decrease relative to pure BO (0.153). The COF–time curves and standard deviation of BO and BO with the two additives (1.5 wt.%, respectively) are displayed in Figure 7c, respectively. It should be noted that the use of BO containing Fc-Mo0.6Ag15 with hydrothermal treatment served to lower COFs to a greater extent than without hydrothermal treatment. The results demonstrate that the hydrothermal treatment of the additives improved friction reduction and anti-wear performance.

3.3. Discussion

The results presented have indicated that sample treatment temperature and the MoS2 and silver content affected the tribological properties of steel balls under lubrication by functionalized MoAg. The synergism involving the MoS2 sheets and Ag particles served to improve the anti-wear capability of steel balls in BO containing Fc-MoAg. This synergism was further evaluated by measuring the WSDs and COFs of the steel balls under the lubrication of BO with a mixture of MoS2 sheets and Ag particles to give an equivalent Mo0.6Ag15 content, where the mixture is denoted as Mo0.6/Ag15Ns. The results are presented in Figure 8. The WSD–concentration and COF–concentration curves establish that both lubricant systems reduce friction and improve anti-wear. At a concentration of 1.0 wt.%, the WSDs of steel balls lubricated with BO-Fc-Mo0.6/Ag15 and BO-Fc-Mo0.6Ag15 were decreased from 0.680 mm for pure BO to 0.580 and 0.490 mm, respectively. At a higher concentration (1.5 wt.%), the COFs of steel balls lubricated with BO-Fc-Mo0.6/Ag15 and BO-Fc-Mo0.6Ag15 decreased from 0.150 for pure BO to 0.148 and 0.090, respectively. The COF–time curves and standard deviation of BO and BO with the two additives (1.5 wt.%) are displayed in Figure 8c, respectively. The results demonstrate that the Mo0.6Ag15 composite prepared using the hydrothermal procedure delivers better friction reduction and anti-wear than that achieved using mixtures of the composite components.
The test results have revealed that MoAg synthesized using the in situ hydrothermal method improved the tribology properties of BO. In order to probe the underlying wear mechanism, the XRD patterns of MoS2 before and after exfoliation are shown in Figure 9. The peaks at 14.2°, 32°, 39.5°, 44°, 50° and 58° are attributed to the (002), (100), (103), (006), (105) and (110) planes, respectively (JCPDS card reference 37-1492) [25]. The Bragg equation (2dsinθ = nλ, where n = 1, and λ = 0.154 nm) was used to estimate the interplanar spacing. The diffraction peak due to the MoS2 (002) plane was shifted from 14.47° to 14.36° after exfoliation, corresponding to an increase in the interplanar spacing of (002) from 0.612 nm to 0.616 nm [26]. All the MoS2 XRD peaks were moved to lower 2θ following exfoliation, including the (103) plane, representing an increased interplanar spacing of the crystal face [27,28,29,30]. The Scherrer equation was used to calculate the size (D) of the MoS2 crystal in the direction perpendicular to the (002) plane:
D = Kλ/(L·cosθ)
where λ is the X-ray wavelength (0.154 nm), K represents the Scherrer constant (0.94), θ (°) is the Bragg angle, and L represents the peak full width at half maximum (FWHM, radians) [3]. Application of the Scherrer equation has revealed a decrease in size from 54.39 nm to 50.82 nm after exfoliation. The results indicate that exfoliation resulted in an increased interplanar spacing and decreased crystal size.
The XRD analysis of Mo0.6Ag15 synthesized with and without hydrothermal treatment is presented in Figure 10. The diffraction peaks at 14.2°, 39.5°, 44°, 58°, and 50° correspond to the (002), (103), (006), (110) and (105) planes of MoS2, respectively. The peak at 38.0° is due to the Ag (111) plane (Figure 10a). The diffraction peaks of the composites were shifted to lower values following hydrothermal treatment, suggesting that this step resulted in an increased composite interplanar spacing. The diffraction peak due to the MoS2 (002) plane was shifted from 14.37° to 14.26° following the hydrothermal step, corresponding to an interplanar spacing increase from 0.616 nm to 0.621 nm. Furthermore, the Ag (111) diffraction peaks were moved to the left and exhibited an increased intensity, indicating that the high temperature and pressure treatment improved the degree of crystallinity and increased the interplanar spacing. The Ag size in the direction perpendicular to (111) (based on the Scherrer equation) increased from 36.09 nm to 38.94 nm as a result of the hydrothermal treatment.
Raman spectroscopic analysis of the composites before and after hydrothermal treatment is shown in Figure 11a. The ID/IG values were increased from 0.84 to 0.86 as a result of the high temperature and pressure treatment, suggesting the formation of a new carbon phase due to PDA. The characteristic peaks of hexagonal MoS2 at 378 cm−1 and 401.2 cm−1 correspond to the E12g and A1g modes of in-plane and out-of-plane vibrations in the S-Mo-S linkages of the MoS2 layer (Figure 11b) [1,31,32,33,34,35]. These wavenumbers are lower than the values associated with the E12g (391 cm−1) and A1g (411 cm−1) modes of bulk MoS2 [18]. The red shifts of the A1g and E12g bands for MoAg can be attributed to the decreased interlayer Van der Waals force and the influence of intercalation agents, respectively. The gap between the E2g and A1g peaks for Mo0.6Ag20 decreased from 25.2 cm−1 to 24.9 cm−1 as the composite treatment temperature was increased from room temperature to 160 °C, suggesting a decrease in the thickness of the nanosheets [36,37]. Exposure to high temperature and pressure can promote a stripping of MoS2 and decrease the van der Waals forces.
An XPS analysis (C1s, N1s, O1s, Mo3d, S2p and Ag3d) of Mo0.6Ag15 was conducted before and after hydrothermal treatment. The results are presented in Figure S2 [38,39,40,41]. The characterization results have established that the hydrothermal synthesis of MoAg promoted growth of Ag nanoparticles with increased space between the MoS2 layers and decreased thickness of the MoS2 sheets. Moreover, the PDA was transferred into an organic matrix, and azide and the MoS2 and Ag components in the composite were connected through Mo-O, S-O, and Ag-O bonds after hydrothermal treatment.
The worn surfaces under lubrication with BO containing Fc-Mo0.6Ag15 were characterized to determine the wear mechanism. The optical or SEM images, 3D skeleton maps and corresponding cross-sectional curves of the steel balls using pure BO and BO with Fc-Mo0.6Ag15 are shown in Figure 12a–g, respectively. The worn surfaces associated with pure BO are coarse, with a diameter and depth of wear scars of ca. 0.68 mm and 0.75 µm, respectively. In contrast, the worn surface for BO containing Fc-Mo0.6Ag15 was much smoother, with a diameter and depth of scarring of 0.43 mm and 0.21 µm, respectively, significantly reduced when compared with pure BO. Comparing optical and SEM images of the samples, BO with Fc-Mo0.6Ag15 surfaces displayed a small, completely round form and a shallow groove. This event demonstrated that the tribolayers could have penetrated between the two bodies in operation, thus improving the tribological performance by reducing COF and WSD. Possibly, soft metal compounds may get trapped at the asperity level and stay on the surface, rendering easy sliding and improved tribology. The elemental maps and energy dispersive X-ray analysis (EDXA) profile have established the distribution of C, N, O, Mo, S, Ag and Fe on the worn surfaces of steel balls lubricated with BO containing Fc-MoAg (Figure S3). The results of XPS analyses of the worn surfaces lubricated with BO containing Fc-MoAg are shown in Figure 12h–n. The C1s signal has been deconvoluted into three peaks at 284.82 eV, 286.14 eV, and 288.32 eV, corresponding to C-C, C–N/C–S, and O–C=O bonds, respectively [42,43]. The high-resolution N1s peaks at 397.09 eV, 404.61 eV, 399.27 eV and 400.71 eV are assigned to nitride, nitrites, organic matrix/cyanides and ammonium salt, respectively. Moreover, the O1s peaks associated with the worn surfaces correspond to metal oxides (530.05 eV), carbonates (531.13 eV), sulfates (531.76 eV), and nitrates (533.07 eV). Weak Mo3d peaks at 232.41 eV and 229.59 eV are attributed to MoO3 and MoS2, respectively [44]. In addition, weak S2p signals at 160.47 eV, 167.62 eV and 169.20 eV are due to FeS, sulfite and sulfate, respectively [44,45]. The weak S2p signals at 162.07–163.27 eV are due to MoS2 [44,45]. The weak Ag3d spectrum could not be identified, which might be attributed to the small percentage of silver on the worn surfaces. The Fe2p peaks at 710.84 eV, 712.18 eV and 706.70 eV are ascribed to Fe2O3, FeS and iron, respectively [46]. The XPS results demonstrate that a lubrication film containing MoS2, FeS, MoO3, Fe2O3, carbon-nitrogen compounds and other compounds was formed, which served to lower the COFs and WSDs during testing [47]. The results have established that the expanded interlayer spacing of the MoS2 sheets due to exfoliation and/or hydrothermal treatment and the tribo-films containing sulfide, oxide, and carbon-nitrogen compound served to improve the tribological properties of steel balls lubricated with BO containing Fc-MoAg.
Functionalized Mo0.6Cu15 and Mo0.6Ni15 were synthesized under the same condition and used as BO additives to assess the tribological behavior of the composites with other metals. The FE-SEM and corresponding elemental mapping have shown that Cu and Ni were uniformly incorporated within the MoS2 sheets (Figure S4). At a concentration of 1.5 wt.%, the WSDs recorded for Fc-Mo0.6Cu15 and Mo0.6Ni15 decreased from 0.68 mm in pure BO to 0.56 mm and 0.58 mm, respectively (shown as Figure S5). The FE-SEM and EDS mapping analysis of the worn samples under lubrication with BO containing Fc-Mo0.6Cu15 and Fc-Mo0.6Ni15 has revealed smooth surfaces with tribo-films containing Fe, C, O, Mo, S, and Ni or Cu that protect the steel balls from damage (Figure S6).
The four-ball wear tests utilizing steel/steel, steel/zirconia and zirconia/zirconia pairs under BO lubrication with/without functionalized Mo0.6Ag15 (wt. 2.0%) and commercial additives (11.5 wt.%) generated the results presented in Figure 13. The average COFs for BO, additives and BO containing Fc-Mo0.6Ag15 were 0.152, 0.13 and 0.095, respectively (Figure 13a). The COFs of the steel balls lubricated with BO containing Fc-Mo0.6Ag15 were unchanged over the first twenty minutes, and then exhibited a sharp increase to 0.12. The COFs of steel balls lubricated with BO oil and BO containing commercial additives were larger and showed a greater degree of fluctuation when compared with the BO containing additives. The 3D skeleton image and corresponding cross-sectional curves of the worn surface in BO containing commercial additives exhibited a WSD and depth of groove of ca. 0.410 mm and 2.20 µm, respectively. The deeper grooves associated with the wear scars under BO containing commercial additives lubrication indicate that BO oil containing Fc-Mo0.6Ag15 (1.5 µm groove depth) offers superior anti-wear (Figure 13a). The COF–time curves for the steel/zirconia pairs are shown in Figure 13b under lubrication with a load of 98N and a speed of 500 rpm for 30 min. It can be seen that the three profiles are essentially time invariant, and the average COFs for BO, BO with functionalized Mo0.6Ag15 and commercial additives were ca.0.180, 0.130, and 0.140, respectively. This response has established enhanced and near equivalent friction reduction for steel/zirconia using BO with Fc-Mo0.6Ag15 and commercial additives. Moreover, the depth of the worn grooves for the combined BO and-Fc-Mo0.6Ag15 was 0.08 µm, appreciably smaller than the value using BO (0.141 µm) or lubricants with commercial additives (0.713 µm). The WSD of steel/zirconia in BO with Fc-Mo0.6Ag15 was ca.0.200 mm, lower than the values recorded for BO (0.260 mm) and commercial oil (0.220 mm). The COF–time curves and corresponding 3D profiles and cross-sectional curves for the zirconia/zirconia pair are shown in Figure 13c, yielding values for BO containing Fc-Mo0.6Ag15 and commercial additives of 0.148 and 0.151, respectively, lower than that recorded for BO alone (0.16). The groove depth for BO was 0.906 µm, greater than that observed for BO with Fc-Mo0.6Ag15 (0.840 µm) and commercial additives (0.679 µm). Both BO containing Fc-Mo0.6Ag15 and commercial additives exhibit improved friction reduction relative to pure BO for the three test pairs. The BO containing Fc-Mo0.6Ag15 delivered lower COFs, WSDs and groove depths than commercial additives in the case of the steel/steel and steel/zirconia pairs. Moreover, the COFs and depth of the wear scar grooves associated with the zirconia/zirconia pair were smaller when using BO containing Fc-Mo0.6Ag15 and commercial additives than pure BO.
The results show that the MoS2/metal composites exhibit enhanced tribological behavior as lubricant additives. Both nanocomposite synergy and the tribofilm play important roles in achieving reduced friction and improved anti-wear. The exfoliation, permeation of Ag+, and growth of nano-silver decrease the size and thickness of MoS2 nanosheets and enlarge the interplanar spacing of the (002) plane, which decreases Van der Waals forces in the perpendicular direction and provides good lubrication. On the other hand, the tribofilm containing sulfide, oxide, carbide, and other compounds protects the steel balls from damage to improve the tribological properties (shown as Figure 14).

4. Conclusions

The interlamellar van der Waals interaction between adjacent atomic/molecular layers of 2D materials furnishes low shearing strength and reduces the friction under the tribo-stress to overcome wear. We have prepared MoAg nanocomposites using a hydrothermal procedure with in situ reduction using dopamine. The MoAg was functionalized with N-oleoylethanolamine and exhibited enhanced friction reduction and anti-wear properties as a lubricant additive in 120BS oil. The composite formed by combining MoS2 with Cu/Ni nanoparticles also showed preferable tribological properties as base oil additives. A tribofilm, containing MoS2, FeS, Fe2O3, MoO3, carbon-nitrogen compounds, and carbon, formed during testing served to lessen friction and wear. The interplanar spacing of the MoS2 (002) plane was enlarged by exfoliation and permeation of Ag+. The hydrothermal treatment resulted in increased Ag particle size and a stripping of MoS2, which provided good lubrication. The wear scar diameters and coefficients of friction of the steel balls lubricated with base oil containing Fc-Mo0.6Ag15 were 0.428 and 0.098 mm, respectively, which were about three-fifths base oil. The tribological properties of base oil with functionalized Mo0.6Ag15 (wt. 2.0%) and commercial additives have been evaluated for steel/steel, steel/zirconia and zirconia/zirconia pairs. The base oil containing Fc-Mo0.6Ag15 showed lower COFs, WSDs and groove depths for the steel/steel pairs and steel/zirconia pairs when compared with commercial additives and base oil. The COFs and wear scar grooves in the case of zirconia/zirconia were lower for lubrication with base oil containing Fc-Mo0.6Ag15 and commercial additives relative to pure base oil.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/lubricants12090319/s1, Figure S1. (a) FT-IR spectrum of Fc-MoAg; (b) TGA curve of MoAg before and after functionalization. Figure S2. The XPS curves of the Mo0.6Ag15Ns with (a–f) and without (g–l) hydrothermal treatment, respectively. Figure S3. The elemental maps of the worn surfaces lubricated with base oil containing Fc-Mo0.6Ag15 (a–g) with the corresponding EDXA profile (h). Figure S4. The FE-SEM and corresponding images of MoCuNs (a–d) and MoNiNs (e–h), respectively. Figure S5. (a) WSD-concentration curves, and (b) COF-concentration curves for BO containing functionalized Mo0.6Ag15, Mo0.6Cu15 and Mo0.6Ni15. Figure S6. The FE-SEM and corresponding mapping images of the worn surfaces under the lubrication of BO containing Fc-MoCuNs (a–g) and Fc-MoNiNs (h–n), respectively.

Author Contributions

Y.X.: Data curation, Formal analysis, Investigation, Methodology, Writing-original draft. Z.L.: Data curation, Investigation, Methodology. W.Z.: Visualization, Investigation. Z.J.: Conceptualization, Investigation, Funding acquisition, Project administration, Supervision, Writing—review and editing. W.H.: Conceptualization, Investigation, Funding acquisition, Writing—review and editing. J.Z. and R.Z.: Writing—review and editing, Supervision. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Project of Natural Science Foundation of China (Grant No. 52105190), and Shan Dong Province Nature Science Foundation (Grant ZR2020ME133 and ZR2020QE044).

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) Schematic illustration of the synthesis process of Fc-MoAg; (b) sizes of Fc-MoAg in base oil after 0 day, 15 days, and 60 days.
Figure 1. (a) Schematic illustration of the synthesis process of Fc-MoAg; (b) sizes of Fc-MoAg in base oil after 0 day, 15 days, and 60 days.
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Figure 2. FE-SEM images of MoS2 (a,b), exfoliated MoS2 (c,d) and MoAg (e,f); TEM images of MoAg (g,h).
Figure 2. FE-SEM images of MoS2 (a,b), exfoliated MoS2 (c,d) and MoAg (e,f); TEM images of MoAg (g,h).
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Figure 3. SEM image (a), EDS elemental maps and EDS analysis of MoAg (160 °C) (b–h).
Figure 3. SEM image (a), EDS elemental maps and EDS analysis of MoAg (160 °C) (b–h).
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Figure 4. (a) WSD–concentration curves, (b) COF–concentration curves for BO containing Fc–MoxAg20 with different MoS2 contents and (c) COF–time curves for BO and BO containing Fc–MoxAg20 with different MoS2 contents (concentration: 1.0 wt.%), respectively.
Figure 4. (a) WSD–concentration curves, (b) COF–concentration curves for BO containing Fc–MoxAg20 with different MoS2 contents and (c) COF–time curves for BO and BO containing Fc–MoxAg20 with different MoS2 contents (concentration: 1.0 wt.%), respectively.
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Figure 5. (a) WSD–concentration curves, (b) COF–concentration curves for BO containing Fc-Mo0.6Agy with different volumes of silver ammonia and (c) COF–time curves for BO and BO containing Fc-Mo0.6Agy with different volumes of silver ammonia (concentration: 1.5 wt.%), respectively.
Figure 5. (a) WSD–concentration curves, (b) COF–concentration curves for BO containing Fc-Mo0.6Agy with different volumes of silver ammonia and (c) COF–time curves for BO and BO containing Fc-Mo0.6Agy with different volumes of silver ammonia (concentration: 1.5 wt.%), respectively.
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Figure 6. (a) WSD–concentration curves, (b) COF–concentration curves for BO containing functionalized Mo0.6Ag0, Mo0Ag15, and Mo0.6Ag15 and (c) COF–time curves for BO and BO containing functionalized Mo0.6Ag0, Mo0Ag15, and Mo0.6Ag15 (concentration: 1.5 wt.%), respectively.
Figure 6. (a) WSD–concentration curves, (b) COF–concentration curves for BO containing functionalized Mo0.6Ag0, Mo0Ag15, and Mo0.6Ag15 and (c) COF–time curves for BO and BO containing functionalized Mo0.6Ag0, Mo0Ag15, and Mo0.6Ag15 (concentration: 1.5 wt.%), respectively.
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Figure 7. (a) WSD–concentration curves, (b) COF–concentration curves for BO containing functionalized Mo0.6Ag15 with or without hydrothermal treatment and (c) COF–time curves for BO and BO containing functionalized Mo0.6Ag15 with or without hydrothermal treatment (concentration: 1.5 wt.%), respectively.
Figure 7. (a) WSD–concentration curves, (b) COF–concentration curves for BO containing functionalized Mo0.6Ag15 with or without hydrothermal treatment and (c) COF–time curves for BO and BO containing functionalized Mo0.6Ag15 with or without hydrothermal treatment (concentration: 1.5 wt.%), respectively.
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Figure 8. (a) WSD–concentration curves, (b) COF–concentration curves for steel balls lubricated with BO containing functionalized Mo0.6Ag15 and Mo0.6/Ag15, and (c) COF–time curves for steel balls lubricated with BO and BO containing functionalized Mo0.6Ag15 and Mo0.6/Ag15 (concentration: 1.5 wt.%), respectively.
Figure 8. (a) WSD–concentration curves, (b) COF–concentration curves for steel balls lubricated with BO containing functionalized Mo0.6Ag15 and Mo0.6/Ag15, and (c) COF–time curves for steel balls lubricated with BO and BO containing functionalized Mo0.6Ag15 and Mo0.6/Ag15 (concentration: 1.5 wt.%), respectively.
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Figure 9. XRD patterns (a–c) for bulk MoS2 before and after exfoliation.
Figure 9. XRD patterns (a–c) for bulk MoS2 before and after exfoliation.
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Figure 10. XRD patterns (a–c) for MoAg synthesized with and without hydrothermal treatment.
Figure 10. XRD patterns (a–c) for MoAg synthesized with and without hydrothermal treatment.
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Figure 11. Raman spectra (a,b) of Mo0.6Ag15 before and after hydrothermal treatment.
Figure 11. Raman spectra (a,b) of Mo0.6Ag15 before and after hydrothermal treatment.
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Figure 12. The optical images, 3D profile images, SEM images and cross-sectional curves of the worn surfaces lubricated with base oil without (a–c) and with (d–g) additives, respectively; the XPS spectra of worn surfaces lubricated with BO-c-Mo0.6Ag15 (h–n).
Figure 12. The optical images, 3D profile images, SEM images and cross-sectional curves of the worn surfaces lubricated with base oil without (a–c) and with (d–g) additives, respectively; the XPS spectra of worn surfaces lubricated with BO-c-Mo0.6Ag15 (h–n).
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Figure 13. COF–time curves, corresponding 3D profile images and cross-sectional curves of the worn surfaces for lubrication using base oil with commercial additives (11.5 wt.%), and with/without Fc-Mo0.6Ag15 (wt. 2.0%) in the case of (a) steel/steel, (b) steel/zirconia, and (c) zirconia/zirconia pairs, respectively.
Figure 13. COF–time curves, corresponding 3D profile images and cross-sectional curves of the worn surfaces for lubrication using base oil with commercial additives (11.5 wt.%), and with/without Fc-Mo0.6Ag15 (wt. 2.0%) in the case of (a) steel/steel, (b) steel/zirconia, and (c) zirconia/zirconia pairs, respectively.
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Figure 14. The sketch map of friction and wear mechanism.
Figure 14. The sketch map of friction and wear mechanism.
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Table 1. The physical property of lubricants.
Table 1. The physical property of lubricants.
Viscosity (mm2/s, 40 °C)Viscosity (mm2/s, 100 °C)Flash Point °CPour Point °C
Base oil (denoted as BO)263.5623.8289−15
Commercial additives—115–135≧180—
BO containing commercial additives (11.5 wt.%)315.2328.53253−13
BO containing nano composites.285.1225.1283−14.5
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Xing, Y.; Liu, Z.; Zhang, W.; Jia, Z.; Han, W.; Zhen, J.; Zhang, R. The Preparation of MoS2/Metal Nanocomposites Functionalized with N-Oleoylethanolamine: Application as Lubricant Additives. Lubricants 2024, 12, 319. https://doi.org/10.3390/lubricants12090319

AMA Style

Xing Y, Liu Z, Zhang W, Jia Z, Han W, Zhen J, Zhang R. The Preparation of MoS2/Metal Nanocomposites Functionalized with N-Oleoylethanolamine: Application as Lubricant Additives. Lubricants. 2024; 12(9):319. https://doi.org/10.3390/lubricants12090319

Chicago/Turabian Style

Xing, Yaping, Zhiguo Liu, Weiye Zhang, Zhengfeng Jia, Weifang Han, Jinming Zhen, and Ran Zhang. 2024. "The Preparation of MoS2/Metal Nanocomposites Functionalized with N-Oleoylethanolamine: Application as Lubricant Additives" Lubricants 12, no. 9: 319. https://doi.org/10.3390/lubricants12090319

APA Style

Xing, Y., Liu, Z., Zhang, W., Jia, Z., Han, W., Zhen, J., & Zhang, R. (2024). The Preparation of MoS2/Metal Nanocomposites Functionalized with N-Oleoylethanolamine: Application as Lubricant Additives. Lubricants, 12(9), 319. https://doi.org/10.3390/lubricants12090319

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