Lubrication-Enhanced Mechanisms of Bentonite Grease Using 2D MoS₂ with Narrow Lateral Size and Thickness Distributions

Abstract

2D MoS₂ with narrow lateral size and thickness distributions was introduced to promote the anti-friction and anti-wear properties of the bentonite grease (BG) in a state of boundary lubrication. Optical microscopy (OM), and 3D optical profilers (3D OP), Raman spectrometry (Raman), scanning electron microscope, energy dispersion spectrum (SEM-EDS), and X-ray photoelectron spectroscopy (XPS) were applied to characterize the wear surface of the GCr15 bearing steel/GCr15 bearing steel contact. It is found that the average friction coefficient (AFC), wear scar diameter (WSD), surface roughness and average wear scar depth of BG + 1.2 wt.% 2D MoS₂ were effectively reduced by approximately 22.15%, 23.14%, 55.15%, and 21.1%, respectilvely, compared with BG under the working condition of 392N, 75 °C, 1 h, and 1200 rpm. Raman, EDS and XPS results jointly demonstrated that a stable adsorbed film and a robust tribochemical film composed of Fe₂O₃, FeSO₄, Fe₂(SO₄)₃, FeSO₃, FeS, FeO and MoO₃, which further contributes to the enhancement of lubrication performance.

1. Introduction

Friction exists extensively in nature and industry, which is confirmed to be the main cause of energy consumption and equipment life shortening. Utilizing lubricants properly can not only effectively reduce the energy required to operate machinery and equipment, but also prolong the life of machinery and equipment, which can minimize the necessity for energy-intensive maintenance and repair [1]. Lubricants act as a barrier, preventing direct contact between surfaces in relative mutual motion, and reduce both the frictional force and wear. Modern lubricants are normally divided into liquid (the most important), gaseous and solid lubricants based on their physical state. In general terms, a lubricating grease is defined as a kind of lubricating fluid thickened to a solid or semi-fluid product by means of a thickener, which are particularly suitable for applications where a continuous oil supply is not feasible or frequent lubrication is impractical. A lubricating grease is formulated from a base oil, a thickener and various performance-improving additives [2]. Although the thickener accounts for a comparatively small proportion in a grease, which does regulate the foundamental properties of grease, such as the dropping point, consistency, water resistant and thermal stability. There are many different types of thickeners [3], which can be classified into two major types soap (lithium soap, sodium soap, aluminum soap, barium soap, calcium soap, lithium complex soap, complex sodium soap, complex aluminum soap, complex barium soap, complex calcium soap, etc.) and non-soap (polyurea, organophilic bentonite, modified attapulgite, fumed silica, fluoropolymers solid hydrocarbons, etc.), however, lithium soap is the most common thickener of greases due to their superior performance and ease of manufacturing over comparable technologies. Currently, the future of lithium greases is at a crossroads due to the unprecedented growth in electric vehicles (EVs), which results in a sharp increase in the price of lithium.

Bentonite grease (BG) is a type of clay thickened grease, which can be a potential replacement for lithium greases due to its significant advantages, such as temperature change resistance, great wear and tear protection, exceptional water tolerance, good mechanical or shear stability, and impressive adhesiveness [4]. Friction modifiers (FM) additives are components added to lubricans that help prevent metal-to-metal contact when loads are extremely high, which work chemically with the metal surfaces to form protective films that shield them from scuffing, welding, and wear [5]. Traditional FM additives containing S, P, and Cl elements have been used to enhance the lubricating performance of lubricants, for example, chlorinated paraffin [6], sulfurized isobutylene [7], tricresyl phosphate [8], amine phosphate [9], zinc dialkyl dithiophosphate [10] and thiophosphate amine salt [11]. However, traditional FM additives are environmentally unfriendly and are harmful to humans [12], for example, chlorine-containing additives have been prohibited by many countries due to their toxicity and corrosive problems and sulfur can result in sulfur pollution to the environment, whereas phosphorus can lead to eutrophication of water bodies. Due to the characteristics of outstanding high specific surface area, nanometer dimension effect, and unique physical and chemical properties [13,14], two-dimensional (2D) materials have been comprehensively investigated in the field of FM additives, for instance, graphene [15], black phosphorus (BP) [16], hexagonal boron nitride [17], transition metal dichalcogenides (TMDs) [18], two-dimensional molybdenum carbide (MXene) [19], layered double hydroxides (LDHs) [20] and covalent–organic frameworks (COFs). Top-down exfoliation is suitable for scale industrial production. The as-prepared 2D materials have wide distributions in lateral size and thickness, which is a dilemma for some applications requiring controlled nanosheet sizes: 2D material-assembled thin films [21,22,23,24], active site exposure of 2D material catalysts [25,26,27], functionalization of 2D material sheets [28,29,30,31] as well as the field-induced alignment order of 2D material liquid crystals. As indeed suitable candidates for traditional FM additives, achieving homodisperse in both lateral size and thickness of 2D materials, which is of great importance to maximize the potential of their extreme pressure and lubrication performance in lubricants.

In the present article, lubrication-enhanced mechanisms of bentonite grease using 2D MoS₂ with narrow lateral size and thickness distributions were systematically investigated by a four-ball tribometer. Optical microscopy (OM) and 3D optical profilers (3D OP) methods were adopted to characterize the worn surface. The composition and microstructure of the physical adsorption film and tribo-chemical film were analyzed by Raman spectrometry (Raman), scanning electron microscope (SEM), energy dispersion spectrum (EDS), and X-ray photoelectron spectroscopy (XPS). The lubrication-enhanced mechanisms of bentonite grease using 2D MoS₂ with narrow lat-eral size and thickness distributions was discussed.

2. Materials and Methods

2.1. Materials

As reported in our previous paper [26], 2D MoS₂ had been prepared by the ultrasound-assisted liquid-phase exfoliation method and the atomic force microscopy (AFM) study revealed a mean thickness of 22 nm, as shown in Figure 1. 150BS base oil (Typical characteristics showed in

Table 1. Typical physico-chemical properties of the 150BS base oil.

Test Description	Result	Method
Kinematic viscosity (mm2/s) (40 °C)	490.7	ASTM D445
Kinematic viscosity (mm2/s) (100 °C)	31.75	ASTM D445
Viscosity Index	95	ASTM D2270
Appearance	Clear to bright	Visual
Colour (ASTM) (Quantitative)	L2.0	ASTM D1500
Density (kg/m3) (15 °C)	0.9012	ASTM D4052
Density (kg/m3) (30 °C)	0.8917	ASTM D4052
Refractive index (20 °C)	1.46	ASTM D1218
Pour point (°C)	−6	ASTM D5950
Flash point (°C) (PMcc)	316	ASTM D92
Specific Gravity (60/60 °F)	0.9017	ASTM D4052
Total acid number (mgKOH/g)	0.01	ASTM D664
Cartxon residue (micro method) (wt.%)	0.41	ASTM D4530
Sulphur content (wt.%)	0.536	ASTM D4294
Water content (vol.%)	Nill	ASTM D95

) was commercially obtained from the PetroChina Karamay Petrochemical Company (Karamay, China). The commercial surface-modified organo-bentonite modified by HTMAB was purchased from ZheJiang AnJi Tianlong OrganicBentonite Co., Ltd. (Huzhou, China). GCr15 bearing steel balls made of GCr15 bearing steel with a diameter of 12.7 mm and an HRC59-61 were obtained from the SINOPEC Research Institute of Petroleum Processing Co., Ltd. (Beijing, China).

2.2. Preparation and Characterization of Bentonite Grease with 2D MoS₂

The bentonite greases were prepared in accordance with the literature [3]. Firstly, the 150BS base oil was poured into the vessel and next stirred. Secondly, organo-bentone was added to the 150BS base oil little by little, being stirred vigorously. When the 150BS base oil and the organo-bentonite had been well dispersed, acetone, at approximately 50 wt.% of the entire quantity of the thickener, was introduced to guarantee that the organo-bentone can be thoroughly dispersed throughout the 150BS base oil. After continuous stirring for 30 min, acetone was removed from the mixture via heating for 30 min at 80 °C. The 2D MoS₂ of specified mass (0, 0.3, 0.6, 0.9, 1.2, 1.5, 1.8 and 2.1 wt.%) were put into the mixture. Thirdly, when the mixture was cooled naturally to room temperature and then the mixture was ground three times using a three-roll mill to gain the samples, which were labeled as BG (BG-0), BG + 0.3 wt.% 2D MoS₂ (BG-0.3), BG + 0.6 wt.% 2D MoS₂ (BG-0.6) , BG + 0.9 wt.% 2D MoS₂ (BG-0.9), BG + 1.2 wt.% 2D MoS₂ (BG-1.2) , BG + 1.5 wt.% 2D MoS₂ (BG-1.5), 2D MoS₂ BG + 1.8 wt.% 2D MoS₂ (BG-1.8) and BG + 2.1 wt.% 2D MoS₂ (BG-2.1) for short, respectively. Schedule of all prepared grease samples are illustrated in Figure 2.

The consistency, thermal stability, colloid stability, evaporation loss and corrosive properties of the propared grease samples were evaluated on the basis of the GB/T 269, GB/T 3498, NB/SH/T 0324, GB/T 7325 and GB/T 7326 standards, respectively.

2.3. Tribology Tests and Analysis

The lubrication performance of eight grease samples is carried out via the four-ball tester using the SH/T 0204, which is similar to the ASTM D2266. The tribometer is displayed schematically in Figure 3, and a series of friction tests were conducted under atmospheric conditions with a relative humidity (RH) of about 75%–80%.

The friction coefficient (COF) was automatically measured and recorded in real time by the computer and the wear scar diameter (WSD) of the three fixed steel balls were scanned by optical microscope. To ensure the repeatability and accuracy of the data, friction tests were performed three times under the identical experimental conditions, and the average value was calculated. The parameters of the tested GCr15 steel balls are shown in

Table 2. Experimental conditions and basic properties of the steel balls utilized.

Parameter	BG-0	BG-0.3	BG-0.6	BG-0.9	BG-1.2	BG-1.5	BG-1.8	BG-2.1
Speed	1200 rpm
Load	392 N
Temerature	75 °C
Test Duration	60 min
Component	Elastic modulus (MPa)		Poisson ratio	Diameter	Rockwell harness (HR)		Surface roughness (Ra)
GCr15	2.085 × 105		0.3	12.7 mm	60 ± 1		0.256 µm

.

For spherical contact, the contact pressure can be estimated according to the Hertzian point contact theory [32]:

$$q_{max}={\displaystyle \frac{3p}{2\pi a^2}}$$

(1)

$$a={({\displaystyle \frac{3}{4}}\times {\displaystyle \frac{p{R}^{\prime }}{E^{\ast }}})}^{{\displaystyle \frac{1}{3}}}$$

(2)

$$w=3p\mathrm{c}\mathrm{o}\mathrm{s}\theta (\mathrm{c}\mathrm{o}\mathrm{s}\theta =\sqrt{6}/3)$$

(3)

$${\displaystyle \frac{1}{R^{\prime }}}={\displaystyle \frac{1}{R_{above}}}+{\displaystyle \frac{1}{R_{below}}}$$

(4)

$$E^{\ast }={({\displaystyle \frac{1-v_{above}^2}{E_{above}}}+{\displaystyle \frac{1-v_{below}^2}{E_{below}}})}^{-1}$$

(5)

Herein, q_max is the maximum contact pressure between the four GCr15 steel balls, p is the effective load, a is the Hertzian contact radius, E* is the equivalent Young’s modulus, R’ is the comprehensive radius, w is the total load (392N), R_above is the radius of the rotating upper ball, R_below are the radii of the lower three stationary balls, v_above and E_above are Poisson’s ratio and Young’s modulus of the rotating upper ball, v_below and E_below are Poisson’s ratio and Young’s modulus of the lower three stationary balls, respectively.

The $\lambda$ was estimated on the basis of the Dowson and Hamrock minimum film thickness formula [33]:

$$h_{min}=3.63{\displaystyle \frac{G^{0.49}{U}^{0.68}{R}^{\prime }}{W^{0.073}}}\left(1-0.61e^{-0.68k}\right)$$

(6)

$$\lambda ={\displaystyle \frac{h_{min}}{\sqrt{{\sigma }_1^2+{\sigma }_2^2}}}$$

(7)

where, h_min was the minimum oil film thickness, G = αE′, U = η₀u/E′R′, W = p/E′R′², k = 1.03(R_y/R_x)^0.64 = 1.03 were dimensionless material parameter, dimensionless speed parameter, dimensionless load parameter, ellipticity parameter, respectively. α and ${\eta }_0$ were the viscosity-pressure coefficient and the dynamic viscosity, respectively. u (0.461 m/s) was the relative sliding velocity of the two friction pairs, k was the ellipticity parameter, E′ = 2E* was the effective modulus of elasticity, σ₁ (2.1 μm) and σ₂ (2.1 μm) were the surface roughness of the worn area of the upper rotating ball and the lower stationary ball, respectively. Calculated from the formula, the maximum contact pressure was 2.34 GPa and the lambda ratio is 0.45 for these test conditions, which indicating that the contact area was firmly in a state of boundary lubrication under the four-ball tribometer.

After testing, the morphology of the worn surface was observed using a JEOL JSM-6610LV scanning electron microscope (JEOL, Tokyo, Japan) and a 3D optical profilers (Sensofar, Terrassa, Spain), and the elemental distribution and composition of the worn surface was identified and quantified using an Oxford X-Max 20 mm² energy dispersive X-ray spectrometer (Oxford Instruments, Oxford, UK). The Raman spectra of worn surfaces were measured using Raman spectrometry (LabRAM HR Evolution, HORIBA, Longjumeau, France) with a laser wavelength of 532 nm. For the sake of exploring the elemental composition and chemical state of the films on the worn surfaces, X-ray photoelectron spectroscopy (XPS) tests were performed using an ESCALAB 250Xi X-ray photoelectron spectrometer (Bruker, Karlsruhe, Germany) to probe the deposition of the ternary films.

3. Results and Discussion

3.1. Physico-Chemical Properties

The consistency, thermal stability, colloid stability, evaporation loss and corrosive properties of the BG containing 2D MoS₂ are presented in

Table 3. Effect of 2D MoS₂ on penetration, dropping point, oil separation, evaporation loss and copper corrosion of BG.

Parameter	BG-0	BG-0.3	BG-0.6	BG-0.9	BG-1.2	BG-1.5	BG-1.8	BG-2.1	Method
Penetration/0.1 mm	285	244	261	292	297	301	306	310	GB/T 269
Dropping point	278	292	290	285	286	287	287	291	GB/T 3498
Oil separation	0.93	0.54	1.46	1.22	0.92	0.91	1.42	1.64	NB/SH/T 0324
Evaporation loss	0.20	0.41	0.47	0.40	0.39	0.43	0.45	0.52	SH/T 0661
Copper corrosion	1a	1a	1a	1a	1a	1a	1a	1a	GB 7326

. As the addition of 2D MoS₂ in BG was increased, the thermal stability, colloid stability and evaporation loss values increased first, then decreased and increased in the end. The 2D MoS₂ introduced to BG could effectively enhance the holding performance of bentonite grease structural networks for base oil owing to its high adsorption property. The consistency of samples increased after adding 2D MoS₂, which means that 2D MoS₂ has an has a considerable thickening effect on the BG due to its big specific surface area and excellent dispersity. Additionally, 2D MoS₂ is proved to be uninfluential on the corrosion of BG.

3.2. Friction and Wear Performance

Figure 4a shows the curves of the COF variation with time of BG with different mass fraction of 2D MoS₂. Figure 4b displays the AFC and WSD variations with 2D MoS₂. As shown in Figure 4a, the tendency of COF among different samples can be distinctly discovered that the three periods turned up during the friction process. At the initial stage of test, COF value rised sharply indicating that there is severe friction, because of the inevitable surface wear occuring during the running-in process, which leads to pollution of the grease by the wear debris. The COF value has been changed to be more mild after continuing for around 300 s. When 1.2 wt.% 2D MoS₂ is introduced, COF is obvious lower than that of BG-0. Figure 4b reveals that AFC initially decreases and subsequently increases with the introduction of 2D MoS₂, implying that an appropriate addition of these nanosheets to BG can provide a certain anti-friction effect. Among them, the AFC and WSD of BG-1.2 are the smallest, reducing by 22.15%, 23.14%, respectively, which exhibits the best performance. When the addition amount is less than 1.2 wt.%, 2D MoS₂ easily penetrate and distribute evenly between the friction pairs, while the anti-friction effect is not as effective as BG-1.2 due to their insufficient quantity. On the other hand, when the addition is more than 1.2 wt.%, high specific surface area and strong surface energy of 2D MoS₂ leads to irreversible accumulation and agglomeration into abrasive particles, exacerbating wear on these particles and weakening their friction-reducing effect.

For purpose of further distinctly comparing the anti-wear properties of BG with different concentrations of 2D MoS₂, the 3D OP is used in Figure 5. In comparison with the surface roughness of wear surface lubricated by BG-0, the wear surface lubricated by BG-1.2 seems to have the smaller surface roughness of about 2.097 μm, reducing by 55.15%, as displayed in Figure 5a,b. After a testing time of 1 h, the worn surface lubricated by BG-0 are seriously worn and show considerable deep furrows and rough scratches along the sliding direction. Meanwhile, the surface roughness and the average wear depth was remarkably reduced (21.1%) after the introduction of layered structure 2D MoS₂ into the BG, which is consistent with the change in WSD, as illustrated in Figure 5c,d. Due to the their small size and sheet shape, the 2D MoS₂ in BG can lightly permeate into the friction surfaces and fill the gaps on surfaces to decrease surface roughness and generate a physical absorption film in the central worn surface ,which can avoid severe wear of the uneven peak of the steel balls as much as possible, resulting in promising anti-wear properties on account of the sliding effect between the two sliding surfaces by reason of the shear stress. This requires to be further testified by a Raman characterization.

3.3. Worn Surface Analysis

In order to detect the physical absorption film of 2D MoS₂ on the worn surface, Raman spectroscopy was utilized to anatomize the worn surfaces on the GCr15 bearing steel balls after the tribotest. The generation of 2D MoS₂ on the wear surface confirmed by the Raman results. The Raman spectra were recorded at the as prepared 2D MoS₂ additive, worn surface lubricated by BG-0 and BG-1.2, respectively, as shown in Figure 6. The notable fact is that the representative peaks at about 380 and 404 cm⁻¹ for the worn surface lubricated by BG-1.2, which are E¹_2g and A_1g modes were observed. The presence of the as prepared 2D MoS₂ additive on the worn surface verified by the appearance of similar patterns located at the same frequency as in the Raman spectra of the 2D MoS₂ additive itself. It demonstrates that 2D MoS₂ in BG can smoothly slide with oil into the point contact of the steel balls to avoid impetuous collision of the coarse peak of the steel balls. However, the physical absorption film may fracture due to the harsh working conditions. At the same time, a new film with superior mechanical properties could stand up to the scuffing and prevent the steel ball surfaces from severe crashes, which will be further investigated.

In order to elucidate the lubrication effect of 2D MoS₂ on the surface of the GCr15 bearing steel ball and preliminarily evaluate the appearance of the tribo-chemical film, the EDS and element mapping analyses on the wear surface lubricated by BG-0 and BG-1.2 were measured and the corresponding results are displayed in Figure 7 and Figure 8, respectively. The deep grooves and large pits can ba observed on the wear surface under the lubrication of BG-0, as illustrated in Figure 7. Five main elements were measured on the worn surface, i.e., C, O, Al, Si, and Fe. The detected Fe elements were primarily from the GCr15 bearing steel. However, the detected C, O, Al, Si elements originated from the 150BS base oil and organo-bentone, respectively.

After adding the 2D MoS₂ into BG-0, the pits and grooves on the worn surface lubricated by BG-1.2 were more smoother and smaller, as shown in Figure 8. For example, the mapping illustrated the generation of C, O, Al, Si, Fe, Mo, S, and the like. Even if the proportions of Mo and S elements account for 0.14% and 0.10% owing to the deficient concentration required for the detection limits for EDS, which given evidence of the 2D MoS₂ do work during the friction process and 2D MoS₂ made a large contribution to the significant improvement of lubricantion performance. Compared with worn surface lubricated by BG-0, two new elements Mo and S occured on the wear surface. It could be readily acquired that the Mo and S elements were enriched in the area of contact between the worn surface and GCr15 matrix, which was in accord with to the distribution of Hertz contact stress. Furthermore, the oxidation of the GCr15 bearing steel ball surfaces was visibly cut down in the middle area, where the thickness of the lubrication film was the least. It was shown that the fierce frictional process continually updated sliding GCr15 ball surfaces in the central area, which were hard to completely participate in the tribochemical reaction. In the mean time, the 2D MoS₂ as additives could play a role in avoiding direct crash of asperities during the friction period. Hence, a comparatively thorough tribofilm, abundant in Mo and S elements, which were mainly generated on the aregion of the GCr15 bearing steel ball surfaces with comparatively low contact stress.

For the sake of exploring the detailed chemical tribo-film formation mechanisms on the steel ball lubricated by BG-0 and BG-1.2, the chemical components on the worn surfaces were detected. The C 1s of 284.8 eV was utilized to calibrate the whole high-resolution XPS photoelectron spectra and subsequently the Gaussian–Lorentzian fitting was used to separate the peaks. Figure 9 presents the XPS survey of worn surface with BG-0 lubrication and showed the patterns of C 1s, O 1s, Si 2p, Al 2p, Fe 2p. As illustrated in Figure 9b, the greatest characteristic peak at 284.80 eV attributes to the C-C bond, the middle characteristic peak at 286.00 eV belongs to the C-O bond, and the smallest characteristic peak at 288.99 eV corresponds to the C=O bond, which originates from the organic matter in 150BS base oil or tribo-chemical film formed by the BG-0 during friction test [34]. As displayed in Figure 9c, the peak of 530.43 eV corresponds to metal oxides, and the characteristic peak near 533.38 eV belongs to the C–O bond [35]. Meanwhile, the peak of 102.29 eV (Figure 9d) and 74.14 eV (Figure 9e) both to refer to the Aluminosilicate which proves that the prepared bentonite grease is relatively stable, and the internal structural skeleton has not been significantly damaged. The peaks located at 711.37.6 and 725.63 eV (Figure 9d) ascribed to the Fe2p1/2 and Fe2p3/2 of –Fe(III)-O- of Fe₂O₃, and there were also two characteristic peaks at 714.14 eV and 728.11 eV on the worn surface of steel ball ascribed to the Fe2p1/2 and Fe2p3/2 of –Fe(II)-O- of FeO [36], respectively, indicating that a stable and robust tribo-film generated on the wear surface.

Figure 10 dispalys the XPS survey of wear surface with BG-0 lubrication and the spectra of C 1s, O 1s, Si 2p, Al 2p, Fe 2p, Mo 3d and S 2p are at around 284.8 eV, 531.84 eV, 102.08 eV, 56.08 eV, 712.08 eV, 233.08 eV and 168.51 eV, respectively, as shown in Figure 10a. Similar to BG-0, the three characteristic peak at 284.80 eV, 286.00 eV and 288.93 eV belong to the C-C bond, the C-O bond and the C=O bond, respectively, as illustrated in Figure 10b. As shown in Figure 10c, the peaks of O1s spectra at 530.33 eV, 531.84 eVand 533.38 eV could be attributed to the metal oxides, the C-O bond and sulfate. Fe elements participate in the metal oxidation reaction. In addition, the spectral peak of 533.38eV is FeSO₄, which can be reflected by the increase of the peak of Fe2p3/2 of Fe(II) near 712.47eV in Figure 10f and the appearance of 168.51 eV and 169.66 eV of SO4²⁻ spectral peaks in Figure 10h. Figure 10g is Mo 3d spectrum of worn surface. The peak at 288.97 eV and 232.02 eV ascribe to the Mo⁴⁺ in MoS₂ [37], which indicates the occurrence of 2D MoS₂ on the worn surface. The peaks located at nearby 232.80 and 235.98 eV correspond to Mo 3d 3/2 and Mo 3d 5/2, respectively, indicating that the the formation of the possible oxidative product molybdenum trioxide (MoO₃) in the tribo-chemical film [38]. Figure 10h is the S 2p spectra of wear surface. The characteristic peaks at 161.78 and 162.93 eV belong to S²⁻ in MoS₂ and FeS [39]. The characteristic peak vicinity 168.51 and 169.66 eV belong to S⁶⁺, which confirms that there is not only Fe oxide, but also ferrous sulfate (FeSO₄) or ferric sulfate (Fe₂(SO₄)₃) on the worn surface of the steel ball. In other words, S is oxidized during the friction test. However, the existence of weak peaks of 165.57 eV and 166.72 eV refering to SO₃²⁻ shows that the oxidation reaction on the worn surface of the steel ball is incomplete, and also indirectly confirms the existence of the intermediate FeSO₃. In Figure 10e, apart from the peak of aluminosilicate, a distinct peak at 99.22 eV is observed, indicating the presence of monatomic silicon. This can be ascribed to the layered structure of montmorillonite crystal within bentonite grease, which provides ample space for Mo elements from 2D MoS₂ to insert and form chemical bonds, leading to reduction of silicon element in the aluminosilicate. Conversely, no additional peaks are observed in the spectrum of Al 2p (Figure 10e) except for its aluminosilicate peak at 74.30 eV, suggesting that aluminum element does not participate in tribological reactions on the wear surface of GCr15 bearing steel ball. According to the above analysis, a stable adsorption film and a robust tribochemical film composed of Fe₂O₃, FeSO₄, Fe₂(SO₄)₃, FeSO₃, FeS, FeO and MoO₃ generated on the wear urface during frictional test, which account for the enhancement of lubricantion performance.

3.4. Lubrication Mechanism of 2D MoS₂

Based on the above, the schematic illustration of sliding process of GCr15 bearing steel/GCr15 bearing steel friction couples in bentonite grease with 2D MoS₂ has been shown in Figure 11. During the early stages of friction test, 2D MoS₂ can readily penetrate between the sliding friction couples and fill the gaps or adsorb to the worn surface owing to the advantages of the nanometer-sized effect. At the moment, the actual contact region between GCr15 bearing steel balls is much less than the cleavage strength of interatomic covalent bond (S-Mo-S) in 2D MoS₂ sheets is adequately more than actual contact pressure, exceeding 30 GPa [40], which indicates that 2D MoS₂ could bear extremely high Hertz contact pressure between asperities and alleviate the abrasion phenomenon, promoting the anti-wear and friction-reducing performance, which confirmed by Raman analysis.As the sliding proceeds, the lubrication state in the contact area changing to boundary lubrication is inevitable, leading to the physical adsorption film would rupture. However, the heat, plastic distortion and defects generated on the wear surface provided a suitable condition for the formation of subsequent tribo-chemical reactions, resulting in a new and stable tribofilm with superior mechanical properties could withstand plastic deformation and protect GCr15 bearing steel ball surface from serious abrasion. The above-mentioned tribo-film mainly composes of Fe₂O₃, FeSO₄, Fe₂(SO₄)₃,FeSO₃, FeS, FeO, and MoO₃ confirmed by XPS analysis. Therefore, 2D MoS₂ were more easily adsorbed on the GCr15 bearing steel/GCr15 bearing steel surfaces to generate a stable adsorption film and a robust tribochemical film composed of Fe₂O₃, FeSO₄, Fe₂(SO₄)₃,FeSO₃, FeS, FeO, and MoO₃, which is typically more ductile than the GCr15 bearing steel substrate and protect the GCr15 bearing steel/GCr15 bearing steel substrate from severe wear by avoiding direct metal-to-metal contact.

4. Conclusions

In summary, 2D MoS₂ with narrow lateral size and thickness distributions was introduced to enhance the friction-reducing and anti-wear performance of the bentonite grease. The relevant tribological mechanisms were illustrated.

a

The 2D MoS₂ as lubricating additives utilized in the bentonite grease have significant effects on its penetration, dropping point, oil separation, evaporation, copper corrosion and friction-reducing and antiwear properties.

b

The COF, WSD, surface roughness and wear scar depth of BG + 1.2 wt.% 2D MoS₂ were effectively reduced by approximately 22.15%, 23.14%, 55.15%, and 82.64%, respectilvely, in comparison with that of BG. In addition, the contact region was firmly in a state of boundary lubrication under the four-ball tribometer according to the calculation of the Dowson and Hamrock minimum film thickness formula.

c

Raman, EDS and XPS results collectively showed that a stable adsorption film and a robust tribochemical composed of Fe₂O₃, FeSO₄, Fe₂(SO₄)₃,FeSO₃, FeS, FeO, and MoO₃, which is typically more ductile than the GCr15 bearing steel substrate and protect the GCr15 bearing steel/GCr15 bearing steel substrate from severe wear by avoiding direct metal-to-metal contact.