Mechanism of Antiwear Property Under High Pressure of Synthetic Oil

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Mechanism of Anti-Wear Property under High Pressure of Synthetic Oil-Soluble Ultrathin MoS2 Sheets as Lubricant Additives Zhe Chen, Yuhong Liu, Selda Gunsel, and Jianbin Luo Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03851 • Publication Date (Web): 31 Dec 2017 Downloaded from http://pubs.acs.org on January 1, 2018

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Mechanism of Anti-Wear Property under High Pressure of Synthetic Oil-Soluble Ultrathin MoS2 Sheets as Lubricant Additives Zhe Chen1,2, Yuhong Liu1,*, Selda Gunsel3, Jianbin Luo1,* 1. State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, P. R. China 2. Sloan Automotive Laboratory, Massachusetts Institute of Technology, Cambridge MA 02139, U. S. 3. Shell Global Solutions (US) Inc., Shell Technology Center, Houston TX 77210, U. S.

*Corresponding authors E-mail addresses: [email protected] (Y. Liu), [email protected] (J. Luo).

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ABSTRACT Wear occurs between two rubbing surfaces. Severe wear due to seizure under high pressure leads to catastrophic failures of mechanical systems and raises wide concerns. In this paper, a kind of synthetic oil-soluble ultrathin MoS2 sheets is synthesized and investigated as lubricant additives between steel surfaces. It is found that, with the ultrathin MoS2 sheets, the wear can be controlled under the nominal pressure of about 1 GPa, while the bearable nominal pressure for traditional lubricants is only a few hundred MPa. It is found that when wear is under control, the real pressure between the asperities agrees with the breaking strength of ultrathin MoS2. Therefore, it is believed that, because of the good oil-solubility and ultra-small thickness, the ultrathin MoS2 sheets can easily enter the contact area between the contacting asperities. Then the localized seizure and further wear are prevented because there will be no metal-to-metal contact as long as the real pressure between the asperities is below the breaking strength of ultrathin MoS2. In this way, the upper limit pressure the lubricant can work is dependent on the mechanical properties of the containing ultrathin 2D sheets. Additionally, ultrathin MoS2 sheets with various lateral size are compared and it is found that sheets with larger size show better lubrication performance. This work discovers the lubrication mechanism of ultrathin MoS2 sheets as lubricant additives and provides an inspiration to develop a novel generation of lubricant additives with high-strength ultrathin 2D materials.

KEYWORDS: ultrathin MoS2 sheets, oil-soluble, lubricant additives, anti-wear, high pressure


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1. INTRODUCTION Friction is ubiquitous in mechanical systems and usually involves complex physical processes and chemical reactions. Among them, wear, which is usually defined as the removal of material from two surfaces under the mechanical action of the two surfaces rubbing together, is a phenomenon of wide concern. Wear occurs in dry as well as lubricated contacts. Although wear is widely used as a manufacturing technology (polishing and lapping) in various industries, large-scale wear of sliding interfaces usually leads to catastrophic failures of mechanical systems such as piston-ring pack in internal conbustion engines, gear boxes or bearings in aircraft engines, high-speed trains, and wind turbines. Tremendous effort has been put to defining wear mechanisms, determining wear rates, modelling/predicting wear and developing measurement methods for wear. However, wear, as a system function depending on materials, surface roughness, lubricants, environment, operating conditions, temperature, and other such factors, is still too complicated to be fully understood and further controlled.1 On the aspect of wear mechanism, past studies tend to focus on two phenomena: wear mechanism in the substrate beneath the surface and the chemical and physical mechanisms within the interfacial layer. According to existed researches, wear processes can be divided into categories depending on the nature of material displacement or removal. The principal categories depend very much on the text being consulted as nomenclature can vary. In Reference [2] they are classified as adhesive wear, abrasive wear, contact fatigue and corrosive wear. In hydrodynamic lubrication regime, sliding surfaces are separated by the lubricant film and fatigue-induced third-body abrasion is an important mechanism under long cyclic stresses. In boundary and mixed lubrication regimes, the occurrence of solid-to-solid contact, primarily at asperities, will lead to component failure due to local seizure. Avoidance of excessive wear


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and/or seizure in boundary lubricated contacts is achieved through applying formation of a protective film, derived from the complex additive packages, which have been developed over the last 60-70 years. Due to the increasingly strict stipulations limiting the amount of phosphorus in engine oil to minimize the negative impact on the emmision catalyst, to use nanoparticles as a substitution of the conventional phosphorus containing organic additives may become a promising trend.3,4 Among the potential nanoparticle additives, MoS2 could be the most anticipated one, not only because of its layered structure and easy inter-layer sliding, but also because it tends to absorb onto metal surfaces due to the highly polarized sulphur atoms.5 With the purpose to disperse MoS2 particles into lubricants, enormous efforts have been being made to reduce their size. From the first half of the 20th century, mechanical methods, such as ball milling, have been used and the size at the scale of micrometers or submicrometers can be achieved.6 But the size at this scale can not ensure the dispersibility of the MoS2 particles. So most of them are used as grease additives instead of lubricant oil additives.7 Recently, as nanotechnology develops, various kinds of nano-sized MoS2 particles with different shapes and structures, were successfully synthesized.8-10 Laboratory tests showed that the synthetic MoS2 nanoparticles are of better lubrication ability compared to the MoS2 particles produced through milling from bulk MoS2.1113

However, as the size of the particles dereases to the scale of nanometers, the specific surface

area increases sharply, resulting in strong agglomeration, which makes them still difficult to be dispersed. Additionally, the inter-layer sliding of MoS2 is very likely to be blocked by excessive dispersants or surfactants,14 which makes the dispersion problem even harder to be solved. Therefore, like any other particles, the long-time dispersion of MoS2 nanoparticles in lubricant oil remains an unsolved problem. Aiming at these challenges, we report a kind of synthetic oil-


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soluble ultrathin MoS2 sheets and test them as lubricant oil additives. An ultralow wear volume at high pressure, which has never been reported with other nanoparticle additives, is achieved with the synthetic ultrathin MoS2 sheets. 2. MATERIALS AND METHODS 2.1. Synthesis and Characterization of the ultrathin MoS2 sheets. The synthesis route is based on the work of Altavilla et al.15 Typically, 500 mg ammonium tetrathiomolybdate (Alfa Aesar, 99.95%) and 50 ml oleylamine (OAm, Acros, 80-90%) are loaded into a reactor and stirred under N2 flow. The temperature is kept at 355 °C for 1 hour. After the reactor cools down to room temperature, the product is collected and purified by repeated centrifugation and ethanol-washing to remove excess OAm. Then the product is dried in vacuum at 40 °C for 1 hour. X-ray photoelectron spectroscopy (XPS, Thermo Fisher ESCALAB 250Xi) with an Al K α Xray source and Fourier transform infrared spectroscopy (FTIR, PerkinElmer Spectrum 100) are used to determine the chemical condition of the as-synthesized product. Transmission electron microscope (TEM, JEOL JEM-2010) is used to observe the morphology of the synthetic MoS2. Confocal Raman microscopic system (HORIBA Jobin Yvon HR800) and X-ray diffractometer (XRD, BRUKER D8 Advance) are used to analyze the structure of the synthetic MoS2. The particle size distribution (PSD) of the synthetic MoS2 sheets is determined through the technique of dynamic light scattering (DLS) with a commercial device (Malvern Zetasizer Nano ZS) when they are dispersed in cyclohexane. 2.2. Lubricant sample preparation. The synthetic MoS2 sheets after centrifugation and drying aggregate with each other and it is very difficult to disperse them directly into base oil. Instead, an indirect dispersion method is used. The synthetic MoS2 sheets are fully dispersed in


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cyclohexane forming a solution fisrt. Then the solution is mixed with base oil. Finally, the cyclohexane in the mixture is removed within a rotary evaporator. A group V base oil, whose dynamic viscosity is 23.1 mPa·s at 50 °C and 6.16 mPa·s at 100 °C, is used in this research. The concentration of synthetic MoS2 sheets in the lubricant sample is 1 wt%, which is the same with other similar researches.16-18 2.3. Tribological Tests. The tribological tests are carried out with a commercial tribotester (Optimal SRV4) with the ball-on-disk mode. The ball is purchased from Shanghai Steel Ball Plant. Its diameter is 10 mm and its surface roughness (Rq) is about 20 nm. The disk is grinded with sandpapers and the surface roughness (Rq) is about 40 nm. Both the ball and the disk are made of bearing steel (AISI 52100) and their hardness is 60±2 HRC. Before the test, both the ball and the disk are cleaned in ultrasonic bath with organic solvents. Then 30 µL lubricant sample is placed on the disk before the ball touches the disk. During the test, the disk is fixed and the ball is pressed on the disk doing reciprocating motion with the stroke of 2 mm and the frequency of 50 Hz. The tests are performed under the laod of 50 N, 100 N and 150 N respectively. The temperature during the test is kept at 50 °C. One thing needs to note is that with the order to prevent excessive surface damage from extremely high initial pressure under high load, there is a prescribed 30-second running-in period at the beginning of each test. During the running-in period, the load is fixed at 50 N. Then the load rises to the assigned value. After the rubbing test, the ball and the disk are ultrasonically cleaned again to remove the lubricant and the wear debris. All the tests are repeated at least three times under each condition. 2.4. Analysis of the rubbing surfaces. The surface topography of the rubbed area is measured with a white light interferometer (ADE PHASE SHIFT MicroXAM). Scanning electron microscope (SEM, TESSCAN LYRA3) is used to examine the rubbed area. The chemical


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configurations of the rubbed area is determined with engery dispersive X-ray spectrometer (EDX, GENESIS XM). 3. RESULTS AND DISCUSSION Figure 1 shows the XPS analysis results of the as-synthesized product. All the binding energies of the characteristic peaks in the Mo3d and S2p XPS spectra (Figure 1a) correspond to the expected values of MoS2,19 proving that the as-synthesized product is MoS2. In the XPS wide spectrum of the as-synthesized product (Figure 1b), besides the elements of Mo and S, the element of N is also detected. Among all the reactants, only OAm contains the element of N. Thus it is assumed that some OAm molecules survive the cleaning process and still exist in the product. To further confirm the existence of OAm in the synthetic MoS2 sample, the technique of FTIR is used and the FTIR spectra of the base oil, OAm and the synthetic MoS2 are displayed in Figure 2. It can be found that the C=C-H bending, which is at 967 cm-1,20 exists in both spectra of OAm and the synthetic MoS2, which is an evidence of the existence of OAm in the synthetic MoS2 sample. Due to the unequally shared bonded pair, the S atom in MoS2 carries a partial negative charge and the H atom in amine carries a partial positive charge, so the OAm molecule is able to adhere to the surface of MoS2 through electrostatic force, as shown in the Table of Contents Graphic. In other words, MoS2 could be surface-modified by OAm molecules. This is believed to be the reason why not all the OAm molecules are removed by repeated washing with ethanol. According to the thermal analysis performed by Altavilla et al, the concentration of OAm in the washed product is about 38.3 wt%.15 Since the concentration of MoS2 in the lubricant sample is 1 wt%, the concentration of OAm in the lubricant sample is about 0.62 wt%. The electrostatic force between S atom and H atom shoud have an impact on the bonding between H atom and N atom in the amino group. It can explain the phenomenon that the peak of


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N-H stretching in amines around 3300 cm-1, which appears in the FTIR spectrum of OAm, does not exist in the spectrum of the synthetic MoS2 sample. Additionally, the good dispersibility of the synthetic MoS2 in cyclohexane further proves the surface-modification with OAm. As shown in Figure 3a, the suspension of the synthetic MoS2 in cyclohexane is very stable that there is no delaminating or precipitation after 1 year in ambient conditions. Without surface-modification, MoS2 particles cannot be dispersed in nonpolar liquid for such a long time.

Figure 1. (a) Mo3d (left) and S2p (right) XPS spectra with Mo3d3/2 at 231.9 eV, Mo3d5/2 at 228.7 eV, S2s at 226.1 eV, S2p1/2 162.8 eV and S2p3/2 at 161.7 eV. (b) XPS wide spectrum of the as-synthesized product.


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Figure 2. FTIR spectra of (a) the base oil, (b) OAm and (c) the synthetic MoS2.

Figure 3. (a) Stable suspension of the synthetic MoS2 in cyclohexane. (b) Hydrodynamic diameter of the synthetic MoS2 sheets dispersed in cyclohexane.

Figure 4 shows the TEM images of the synthetic MoS2. In the TEM image with relatively low magnification (Figure 4a), many eyelash-like structures can be found. Most of them are monolayered and some others are bi-layered. In the bi-layer structures, the distance between the two layers is about 0.65 nm, which matches the interlayer distance of natural MoS2. In the TEM image with relatively high magnification (Figure 4b), clear crystal lattice can be seen inside the eyelash-like structure. The lattice distance is about 0.27 nm, which equals to the distance between (100) lattice planes of natural MoS2. Moreover, the width of the mono-layer eyelash-


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like structure is about 0.42 nm, which is in accordance with the thickness of one S-Mo-S layer. Thus it is believed that each of these eyelash-like structures is a standing part of the synthetic MoS2. Therefore, the layer number of the synthetic MoS2 can be estimated through the eyelashlike structures. Except the mono-layer and bi-layer eyelash-like structures, tri-layer or multilayer ones are hardly to be found through TEM, so it is considered that most of the synthetic MoS2 is consist of one or two S-Mo-S layers. To further confirm the layer number of the synthetic MoS2, as shown in Figure 5, the Raman spectrum and the XRD spectrum of the synthetic MoS2 were measured respectively. In a typical Raman spectrum of MoS2, there are two 1 characteristic peaks, which are marked as of E2 g and A1g . The Ramanshift difference between

the two characteristic peaks are constant at around 27 cm-1 when the number of S-Mo-S layer is more than 6. However, if the layer number is less than 6, the Ramanshift difference shrinks exponentially as the layer number decreases.21 It can be found in Figure 5a that the Ramsnshift difference of the synthetic MoS2 is only 21.9 cm-1, which is much smaller than that of natural multilayer 2H-MoS2. Due to the impact of the OAm molecules adhered to the MoS2 surface, it is inaccurate to determine the layer number distribution based on the Raman spectrum. However, the Raman analysis should be sufficient to prove that the synthetic MoS2 is of few-layer structure and its thickness is only a few nanometes. As for the XRD pattern of the synthetic MoS2 (Figure 5b), it is possible to assign the reflection preaks of the family lattice planes of (100), (103) and (110), but the (002) plane, which reflects the stacking of S-Mo-S layers, is hardly recognizable. Therefore, the synthetic product can be described as ultrathin MoS2 sheets.


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Figure 4. TEM images of the synthetic MoS2.

When the synthetic MoS2 sheets are being observed with TEM, they stack with each other on the Cu grid randomly. Because the vision field and the depth of focus of TEM are both limited, it is very difficult to observe a whole piece of MoS2 sheet and measure its lateral size through TEM. However, thanks to the good dispersibility of the synthetic MoS2 sheets in cyclohexane, their size distribution can be measured through the technique of DLS when they are dispersed in cyclohexane. It should be noted that the size determined by DLS is called hydrodynamic size and it is the size of a sphere that moves in the same manner as the scatterer. As shown in Figure 3b, the hydrodynamic diameter of the synthetic MoS2 sheets is in the range from 90 nm to 800 nm and concentrates at 200 nm, which indicates that the lateral dimension of the ultrathin MoS2 sheets is at the scale of hundreds of nanometers.


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1 Figure 5. (a) Raman spectrum of the synthetic MoS2 with E2 g at 383.0 cm-1 and A1g at 404.9

1 cm-1 and Raman spectrum of natural 2H-MoS2 with E2 g at 381.0 cm-1 and A1g at 407.8 cm-1. (b)

XRD pattern of the synthetic MoS2 and standard XRD pattern of MoS2.

The tribological properties of the synthetic ultrathin MoS2 sheets containing lubricant sample were tested under the load of 50 N, 100 N and 150 N respectively and the test results are displayed in Figure 6. Figure 6a shows the COF during the test. It can be found that, for all the tests, the COF after the running-in period is relatively stable during the test. The value of the COF decreases as the load increases. On the contrary, as displayed in Figure 6b and 6c, both the wear volume on the disk and the size of the wear scar on the ball increases along with the load. Based on the size of the wear scar on the ball, the nominal pressure between the ball and the disk after the test can be calculated according to Equation 1, where Pn is the nominal pressure, F is the load and d is the diameter of the wear scar. The resulting nominal pressure between the ball


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and the disk at the end of the test is presented in Figure 6d. It is found that the final nominal pressure increases as the load increases. The phenomenon that lower COF and higher nominal pressure are achieved under higher load will be discussed later.

Pn =

4F πd2

(1)

Figure 6. (a) The COF during the tests, (b) the wear volume on disks, (c) the diameter of wear scar on balls and (d) the nominal pressure between the ball and the disk after the tests under 50 N, 100 N and 150 N respectively.

The surface topography of the area around the wear track on disks after the test are displayed in Figure 7a. In accordance with the wear volume shown in Figure 6b, the wear track on the disk gets wider and deeper as the load increases. It can be seen that, when the load is 50 N, although


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there is some material loss at both ends of the wear track, the wear in the middle part of the wear track is extremely light, while distinct scratches along the rubbing direction can be clearly seen in the wear tracks under the load of 100 N and 150 N. The magnified images of the central part of the rubbed areas, which are indicated with dot-line rectangles in Figure 7a, are displayed in Figure 7b. It can be found that there is still no visible scratches in the rubbed area under the load of 50 N. Moreover, the grinding traces, which are originally on the disk surface before the rubbing test, are almost unchanged and are easily distinguishable in the rubbed area. For the rubbed area under the load of 100 N, the grinding traces can also be recognized but some parts of the traces have been damaged by the scratches. As for the wear track under 150 N, the scratches are very deep and all the grinding traces in the rubbed area are demolished. Then the average cross-section profiles of the middle part of the rubbed area, which are 500 µm wide and indicated by two dash-lines in Figure 7b, are plotted in Figure 7c. The depth of the wear track under the load of 50 N is only about 90 nm and the surface profile is very smooth. The wear track depth is about 280 nm under 100 N and 540 nm under 150 N, and wear scratches along the rubbing direction can be easily told from the cross section profiles.


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Figure 7. (a) The surface topography of the rubbed area on the disk after the test under 50 N, 100 N and 150 N respectively. The areas in the dot-line rectangles in (a) are magnified and shown in (b) accordingly. The average cross-section profiles of the areas between the two dashlines in (b) are displayed in (c) accordingly.

Considering that the wear under the load of 50 N is very slight. The rubbed areas on both the ball and the disk are examined with SEM. Figure 8a shows the surface around the rubbed areas on the ball and the disk respectively. It can be found that both the wear scar on the ball and the wear track on the disk are not very distinguishable, indicating that the depth of the rubbed area is very small, which is in agreement with the result found through the white light interferometer (Figure 7). Then some parts inside the rubbed areas, which are indicated by dash-line rectangles in Figure 8a, are magnified and displayed in Figure 8b accordingly. The surfaces inside the


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rubbed areas are almost the same with those outside the rubbed areas. No visible scratches along the direction of rubbing can be found. Therefore, it is confirmed that the surfaces are well protected during the rubbing test with the synthetic ultrathin MoS2 sheets containing lubricant sample. The chemical conditions on the rubbed surface is analyzed with EDX. As shown in Figure 8c, it is found the EDX spectra inside and outside the rubbed area are almost the same with each other. The elements of Mo and S are not detected inside the rubbed area, indicating that little MoS2 adheres to the surface during the rubbing, which is unusual when MoS2 is used as solid lubricant or lubricant additive.

Figure 8. (a) SEM images of the rubbed area on the ball and on the disk. The areas in the dashline rectangles in (a) are magnified and displayed in (b) accordingly. (c) EDX spectra inside and outside of the rubbed area on the disk.


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Before any speculation of the lubrication mechanism, it is important to figure out the lubrication regime of the concerned situation. The lubrication regime can be determined based on the ratio of theoretical minimum film thickness to the combined surface roughness. The lubrication regime is hydrodynamic lubrication if the ratio λ is larger than 3, mixed lubrication if the ratio ranges from 1 to 3 and boundary lubrication when the ratio is smaller than 1.3 The ratio can be calculated through Equation 2, where hmin is the theoretical minimum film thickness, σ is the combined surface roughness, σ1 and σ2 are the surface roughness of the two contact surfaces.

λ=

hmin

σ

=

hmin

σ 12 +σ 2 2

(2)

For elastic ball on disk contact, the minimum lubricant film thickness can be calculated based on Hamrock-Dowson theory according to Equation 3,22 in which H min = hmin R , U = ηV E ' R ,

G = α E ' , W = F E ' R 2 , hmin is the minimum film thickness, R is the radius of the ball, η is the bulk viscosity of lubricating medium, V is the averaged linear velocity of the ball and the disk, E' is the effective elastic modulus, F is the normal load, α is the viscosity-pressure coefficient and k is the ellipticity parameter. H min = 3.63

U 0.68G 0.49 1 − e−0.68 k ) ( 0.073 W

(3)

However, after a period of rubbing, because of wear, the shape of the ball is no longer sphere, which makes the Hamrock-Dowson film thickness formula not applicable any more. To solve this problem, an approximate method is employed.23 According to Hertz theory, when a ball is pressed on a plane, the deformation of the ball can be calculated through Equation 4,24 where a is the radius of Hertz contact area, R is the radius of the ball, F is the normal load and E' is the effective elastic modulus. It can be found that, when the load is constant, the contact area grows


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larger as the radius of the ball increases. In the situation that there is wear on the ball, the pressure between the ball and the disk is distributed more evenly so the deformation is much smaller compared with the situation with no wear, so the part wore off on the ball can be regarded as the elastic deformation of an imaginary ball with larger size under the same load. The radius of the imaginary ball can be calculated through Equation 5, in which R' is the radius of the imaginary ball and a' is the radius of the wear scar on the ball. 1

 3RF  3 a=   2E ' 

(4)

a' R ' = ( )3 R a

(5)

The value of the parameters to calculate λ are listed in Table 1 and it turns out that λ = 0.52 at the end of the test under 50 N, indicating that it is in the boundary lubrication regime. Similar calculation shows that the lubrication regime for the tests under 100 N and 150 N is also boundary lubrication.

Table 1. Parameters for determining the lubrication regime for the test under 50 N Parameter

Symbol

Value

Elasticity modulus of the ball

E1

206 GPa

Elasticity modulus of the disk

E2

206 GPa

Poisson's ratio of the ball

ν1

0.3

Poisson's ratio of the disk

ν2

0.3

Surface roughness of the ball

σ1

20 nm

Surface roughness of the disk

σ2

40 nm

Sliding speed of the ball

V1

0.28 m/s

Sliding speed of the disk

V2

0 m/s

Radius of the ball

R

5 mm


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Viscosity of the lubricant

η

23.1 mPa·s

Viscosity-pressure coefficient of the lubricant

α

25 GPa-1

Normal Load

F

3N

Ellipticity parameter

k

1

Radius of the wear scar on the ball

a’

130 µm

Since the lubrication is in the boundary lubrication regime, the two sliding surfaces are not totally separated by the lubricant film and there exist contacts between opposite asperities. It has been proven through molecular dynamics simulation that when ultrathin 2D material is used as a coating film, it can largely reduce the friction and wear in the load scenarios where the coating itself is not damaged.25 That is to say, the pressure between the two contacting asperities is limited by the breaking strength of the ultrathin 2D material coating. It should be noted that the real pressure between contacting asperities is not equal to the nominal pressure between the contacting surfaces. Because of the roughness, the real contact area is only a small portion of the nominal contact area. The average real contact pressure between contacting asperities can be estimated through Equation 6, where Pa is the average real contact pressure, F is the normal load,

ϕ is the ratio between real contact area and nominal contact area, An is the nominal contact area and Pn is nominal contact pressure. Here in the rubbing test, the dispersed synthetic MoS2 sheets could be the film to protect the steel surfaces. It is reported that the breaking strength of monolayer MoS2 is in the range from 16 GPa to 30 GPa, which is much larger than that of steel.26 In the test under the load of 50 N, the final nominal pressure between the ball and the disk is about 0.94 GPa. Typically, the real contact area between two contacting rough surfaces is about 5-10% of the nominal contact area and the percentage of the real contact area increases after running-in or wear. Based on Equation 6, It can be found that the average real contact pressure between the ball and the disk at the end of the test under 50 N is in the range from 9.4


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GPa to 18.8 GPa, which is bearable for the ultrathin MoS2 sheets. For the tests under the loads of 100 N and 150 N, although the final nominal pressure is higher than that under 50 N, the average real contact pressure is still within the breaking strength of ultrathin MoS2.

Pa =

P F = n ϕ An ϕ

(6)

According to the analysis above, it is believed that the synthetic ultrathin MoS2 sheet is the reason for the extremely low wear. In this way, the ability for the ultrathin MoS2 sheets to enter the contact area is the key problem. Fortunately, the synthetic MoS2 sheets possess this ability due to their unique features. Firstly, the surface modification with OAm molecules makes them well dispersed in the lubricant without agglomeration. Secondly, the extremely small thickness of the sheets makes them easy to enter the contact area. As shown in Figure 9, when one asperity is moving to the other one, the ultrathin MoS2 sheets will enter the contact area instead of being pushed away. When the two opposite asperities contact each other, because of the ultrathin MoS2 sheet between them, there is no direct metal-to-metal contact. The contacting asperities may deform due to pressure but little adhesion and resulting wear is caused because of the protection from the ultrathin MoS2 sheet. After the two asperities leave each other, since no MoS2 is detected in the rubbed area (Figure 8c), the sheet floats back into the lubricant. As long as the concentration of the ultrathin MoS2 sheets is high enough, the process depicted in Figure 9 can take place between almost all the contacting asperities and the wear is controlled in this way.


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Figure 9. Schematic of the lubrication mechanism of the synthetic ultrathin MoS2 sheets as lubricant additives.

During the test, when the real contact pressure exceeds the breaking strength of ultrathin MoS2, even if the synthetic MoS2 sheet enters the contact area between two contacting asperities, it is not able to protect the asperities from localized seizure and wear. Usually, the height of asperities on rough surfaces follows normal distribution. Asperities with higher altitude bear larger pressure and are more likely to be worn off. After high asperities are removed, more asperities will contact with the opposite surface, enlarging the real contact area and decreasing the real contact pressure. Meanwhile, the increase of the nominal contact area, which is induced by wear, is also able to lower the real contact pressure. After the real contact pressure drops below the breaking strength of ultrathin MoS2, the synthetic ultrathin MoS2 sheets can start to protect the contacting asperities and wear rate is therefore largely reduced. At the beginning of the test under the load of 50 N, according to Hertz contact theory, the average contact pressure between the ball and the disk is about 1.15 GPa and the highest pressure, which appears at the center of the circular contact area, is about 1.73 GPa. According to Equation 6, the real contact pressure around the center of the contact area is in the range from


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17.3 GPa to 34.6 GPa, which slightly exceeds the breaking strength of ultrathin MoS2 and leads to mild wear. For the tests under the load of 100 N and 150 N, after the running-in period at 50 N, the load rises abruptly. The real pressure largely exceeds the breaking strength of ultrathin MoS2, and severe wear takes place. Larger load results in more plastic deformation and removal of the asperities with high altitude and makes the real contact area larger. Since the real contact pressure is constant, the nominal pressure is higher. Meanwhile, the occurrence of shoulder-toshoulder bumping between outstanding asperities is reduced as well, so the COF is also lowered. The lateral size of the ultrathin MoS2 sheets synthesized in this research is at submicroscale (Figure 3c). Another kind of ultrathin MoS2 sheets, whose lateral size is at nanoscale, was synthesized in our previous work.11 Both of these two kinds of ultrathin MoS2 sheets are surfacemodified with OAm molecules and the main difference is in the lateral size. Therefore, by comparing them under same test conditions, it is possible to preliminarily explore the effect of the lateral size on the tribological properties of the ultrathin sheets as lubricant additives. It can be found from Figure 10 that, when the load is 50 N, the average COF of the nanoscaled MoS2 sheets is higher than that of the submicroscaled ones, but the wear volume of the nanoscaled ones is much smaller. When the load is 100 N or 150 N, the nanoscaled sheets show less lubricating ability in both COF and wear volume. As discussed above, the ultrathin MoS2 sheets lubricate the sliding surfaces by covering the contacting asperities, so the coverage rate dominates the lubrication performance. Usually, there are gaps among adjacent ultrathin sheets. The gaps not only leave the area unprotected but also generate a rough sliding interface, which lead to high friction and severe wear. However, even if the concentration of the ultrathin sheets is high enough that the whole contact area is fully covered, the overlaps of adjacent sheets still make the sliding interface rough and lead to extra friction.27,28 It is believed that, after the


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ultrathin sheets enter the contact area, those with larger lateral size can cover the contact area with less gaps and overlaps, so the lubrication performance of the submicroscaled ultrathin MoS2 sheets is generally better than the nanoscaled ones. As for the exception occurred under the load of 50 N (Figure 10b), it is thought that the real contact area for each pair of contacting asperities is small under relatively low load, so it is easy to be covered. Nanoscaled ultrathin MoS2 sheets can enter the contact area more easily due to smaller size and therefore can cover the contact area better and lead to lower wear compared with the submicroscaled ones.

Figure 10. (a) The average COF and (b) the wear volume on the disk of the comparison tests of ultrathin MoS2 sheets with different lateral size.

With the purpose to further prove the lubrication mechanism proposed above, the synthetic MoS2 sheets containing lubricant sample is directly compared with OAm and the base oil, which are the two components of the lubricant sample besides the ultrathin MoS2 sheets. The test load


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and temperature are remained at 50 N and 50 °C respectively, but the test time duration is extended to 5 hours. The test results are summarized in Figure 11. For the synthetic ultrathin MoS2 sheets containing lubricant sample, the COF during the 5 hours is always stable around 0.125, which is almost the same with that of the 30-minute test. The wear volume after the 5hour rubbing is only about 37.8% larger than that after 30 minutes, while the size of the wear scar on the ball and the final nominal pressure keep the same with that of the 30-minute test. For the test of OAm, the COF is high at first and then drops to the similar level of the MoS2 containing sample, but the wear volume is about 55.5% larger than that of the MoS2 containing sample and the final nominal pressure is about 42.0% lower. As for the test of the base oil, both the COF and the wear is much higher than those of the MoS2 containing sample and thus the pressure the base oil can bear is much lower. The surface morphology around the rubbed areas of these tests are shown in Figure 12a and the average cross section profile of the middle part of the rubbed areas, which are 500 µm wide and are indicated with two dash-lines in Figure 12a, are displayed in Figure 12b. It can be found that the wear track of the MoS2 containing sample is shallower and smoother than those of OAm and the base oil. It is proven through these tests that the synthetic ultrathin MoS2 sheets is the key component to achieve the ultralow wear rate, while OAm and the base oil alone cannot achieve it. Moreover, the phenomena, that the wear rate decreases and the size of the wear scar does not change any more after the running-in period, agree with the lubrication mechanism, that the synthetic ultrathin MoS2 sheets enter the contact area and protect the surfaces after the real pressure decreases to the bearable range of the sheets, and therefore further confirms the proposed mechanism.


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Figure 11. (a) The COF during the tests, (b) the wear volume on disks, (c) the diameter of wear scar on balls and (d) the nominal pressure between the ball and the disk after the tests of the synthetic ultrathin MoS2 sheets containing lubricant, OAm, the base oil and a commercial engine oil.


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Figure 12. (a) The surface topography of the rubbed area on the disks after the tests of the synthetic MoS2 sheets containing lubricant, OAm, the base oil and a commercial engine oil. The average cross section profiles of the areas between the two dash-lines in (a) are displayed in (b) accordingly.

Then one kind of commercial engine oil is also tested under the same condition as a comparison to evaluate the anti-wear property of the synthetic ultrathin MoS2 sheets. The test result results and the surface morphology are also presented in Figure 11 and Figure 12. It can be seen that tribological properties of the engine oil is worse than the MoS2 containing sample at every aspect. Especially, the final nominal pressure between the ball and the disk of the test of the engine oil is about 43.4% lower than that of the MoS2 containing lubricant, indicating that the engine oil is not able to control wear under high pressure. The main anti-wear additive in the engine oil is zinc dialkyldithiophosphate (ZDDP). ZDDP protects the surface by forming a tribofilm. However, it is reported that both ZDDP and the generated tribofilm react with the steel surface and it results in corrosive wear.29,30 There is no chemical reaction between the synthetic


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ultrathin MoS2 sheets and the steel surface, so no material is lost in the form of corrosive wear when ultrathin MoS2 sheets are used as lubricant additives. As mentioned above, MoS2 is always regarded as a good lubricant because of its easy interlayer sliding and the tendency to adhere onto metal surfaces. However, as is found through EDX analysis (Figure 8c), the synthetic ultrathin MoS2 sheets show excellent anti-wear ability without adhesion onto the steel surfaces during the test under 50 N. As displayed in Figure 9, it is believed that the OAm molecules adhered to the MoS2 surface prevent the direct contact between the MoS2 sheets and the steel surfaces. Meanwhile, because of the few-layer structure of the ultrathin MoS2 sheets and the surrounding OAm molecules, the effect of interlayer sliding for the lubrication should be very weak. Except the good oil-solubility, the most contributory feature of the synthetic ultrathin MoS2 sheet for the anti-wear property under high pressure is its high mechanical strength. In this way, it is inspired that any kind of ultrathin 2D material sheets can work as lubricant additives as long as the dispersibility is satisfied. The bearable pressure of the lubricant depends on the mechanical properties of the containing ultrathin 2D materials sheets. Therefore, it is promising to develop soluble ultrathin 2D material sheets with high breakingstrength as a novel generation of lubricant additives. 4. CONCLUSIONS In this paper, a kind of oil-soluble ultrathin MoS2 sheets is synthesized through a solvothermal method. It is found that, when the ultrathin MoS2 sheets are used as lubricant additives between steel surfaces, the wear can be effectively controlled when the nominal load is as high as about 1 GPa. The lubrication mechanism is that, due to the oil-solubility and ultra-small thickness, the ultrathin MoS2 sheets can enter the contact area and prevent the contacting asperities from seizure and wear when the real pressure between the asperities is within the breaking strength of


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ultrathin MoS2. Moreover, it is found that the ultrathin MoS2 sheets with larger lateral size lead to lower COF and slighter wear. That is because, when the contact area is covered with larger ultrathin MoS2 sheets, there are less gaps and overlaps among the adjacent sheets, which generates a smoother sliding interface. Therefore, based on the lubrication mechanism proposed above, lubricant-dispersible ultrathin 2D material sheets with high mechanical property and large lateral size are favorable to result in good lubrication performance and have great potential to be a new generation of lubricant additives.

AUTHOR INFORMATION Corresponding Authors *Y.L.: e-mail, [email protected] *J.L.: e-mail, [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The work is financially supported by the National Natural Science Foundation of China (NSFC, 51522504), the National Postdoctoral Program for Innovative Talents and Shell Global Solutions (US) Inc.


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REFERENCES (1) Stachowiak, G. W. Wear: Materials, Mechanisms and Practice. John Wiley & Sons Ltd: West Sussex, England, 2006. (2) Williams, J. Engineering Tribology. Cambridge University Press: Cambridge, U.K., 2005. (3) Luo, J. B.; Lu, X. C.; Wen, S. Z. Developments and Unsolved Problems in NanoLubrication. Prog. Nat. Sci. 2001, 11 (3), 173-183. (4) Wong, V. W.; Tung, S. C. Overview of Automotive Engine Friction and Reduction Trends– Effects of Surface, Material and Lubricant-Additive Technologies. Friction 2016, 4 (1), 1-28. (5) Holinski, R.; Gänsheimer, J. A Study of the Lubricating Mechanism of Molybdenum Disulfide. Wear 1972, 19 (3), 329-342. (6) Winer, W. O. Molybdenum Disulfide as a Lubricant: A Review of the Fundamental Knowledge. Wear 1967, 10 (6), 422-452. (7) Devine, M. J.; Lamson, E. R.; Stallings, L. Molybdenum Disulfide Diester Lubricating Greases. NLGI Spokesman 1964, 27 (10), 320-326. (8) Feldman, Y.; Wasserman, E.; Srolovitz, D. J.; Tenne, R. High-Rate, Gas-Phase Growth of MoS2 Nested Inorganic Fullerenes and Nanotubes. Science 1995, 267 (5195), 222-225. (9) Hu, K. H.; Wang, Y. R.; Hu, X. G.; Wo, H. Z. Preparation and Characterization of Ball-Like MoS2 Nanoparticles. Mater. Sci. Tech. 2007, 23 (2), 242-246. (10) Nath, M.; Govindaraj, A.; Rao, C. Simple Synthesis of MoS2 and WS2 Nanotubes. Adv. Mater. 2001, 13 (4), 283-286.

(11) Chen, Z.; Liu, X. W.; Liu, Y. H.; Gunsel, S.; Luo, J. B. Ultrathin MoS2 Nanosheets with Superior Extreme Pressure Property as Boundary Lubricants. Sci. Rep. 2015, 5, 12869. (12) Rosentsveig, R.; Gorodnev, A.; Feuerstein, N.; Friedman, H.; Zak, A.; Fleischer, N.;


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Tannous, J.; Dassenoy, F.; Tenne, R. Fullerene-like MoS2 Nanoparticles and their Tribological Behavior. Tribol. Lett. 2009, 36 (2), 175-182. (13) Joly-Pottuz, L.; Dassenoy, F.; Martin, J. M.; Vrbanic, D.; Mrzel, A.; Mihailovic, D.; Vogel, W.; Montagnac, G. Tribological Properties of Mo-S-I Nanowires as Additive in Oil. Tribol. Lett. 2005, 18 (3), 385-393. (14) Aralihalli, S.; Biswas, S. K. Grafting of Dispersants on MoS2 Nanoparticles in Base Oil Lubrication of Steel. Tribol. Lett. 2013, 49 (1), 61-76. (15) Altavilla, C.; Sarno, M.; Ciambelli, P. A Novel Wet Chemistry Approach for the Synthesis of Hybrid 2D Free-Floating Single or Multilayer Nanosheets of MS2@Oleylamine (M=Mo, W). Chem. Mater. 2011, 23 (17), 3879-3885.

(16) Cizaire, L.; Vacher, B.; Le Mogne, T.; Martin, J. M.; Rapoport, L.; Margolin, A.; Tenne, R. Mechanisms of Ultra-Low Friction by Hollow Inorganic Fullerene-Like MoS2 Nanoparticles. Surf. Coat. Tech. 2002, 160 (2), 282-287.

(17) Lahouij, I.; Vacher, B.; Martin, J.; Dassenoy, F. IF-MoS2 Based Lubricants: Influence of Size, Shape and Crystal Structure. Wear 2012, 296 (1-2), 558-567. (18) Rabaso, P.; Ville, F.; Dassenoy, F.; Diaby, M.; Afanasiev, P.; Cavoret, J.; Vacher, B.; Le Mogne, T. Boundary Lubrication: Influence of the Size and Structure of Inorganic FullereneLike MoS2 Nanoparticles on Friction and Wear Reduction. Wear 2014, 320, 161-178. (19) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-Ray Photoelectron Spectroscopy: A Reference Book of Standard Spectra for Identification and Interpretation of XPS Data. Perkin-Elmer Corporation: Eden Prairie, Minnesota, U.S., 1992.

(20) Stuart, B. Infrared spectroscopy. John Wiley & Sons Ltd: West Sussex, England, 2004. (21) Lee, C.; Yan, H.; Brus, L. E.; Heinz, T. F.; Hone, J.; Ryu, S. Anomalous Lattice Vibrations


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of Single- and Few-Layer MoS2. ACS Nano 2010, 4 (5), 2695-2700. (22) Hamrock, B. J.; Dowson, D. Isothermal Elastohydrodynamic Lubrication of Point Contacts. 3. Fully Flooded Results. J. Lubr. Technol. 1977, 99 (2), 264-276. (23) Chen, Z.; Liu, Y. H.; Zhang, S. H.; Luo, J. B. Controllable Superlubricity of Glycerol Solution via Environment Humidity. Langmuir 2013, 29 (38), 11924-11930. (24) Wen, S.; Huang, P. Principles of Tribology. John Wiley & Sons (Asian) Pte Ltd: Singapore, 2012. (25) Klemenz, A.; Pastewka, L.; Balakrishna, S. G.; Caron, A.; Bennewitz, R.; Moseler, M. Atomic Scale Mechanisms of Friction Reduction and Wear Protection by Graphene. Nano Lett. 2014, 14 (12), 7145-7152. (26) Bertolazzi, S.; Brivio, J.; Kis, A. Stretching and Breaking of Ultrathin MoS2. ACS Nano 2011, 5 (12), 9703-9709. (27) Hölscher, H.; Ebeling, D.; Schwarz, U. D. Friction at Atomic-Scale Surface Steps: Experiment and Theory. Phys. Rev. Lett. 2008, 101 (24), 246105. (28) Meyer, E.; Lüthi, R.; Howald, L.; Bammerlin, M.; Guggisberg, M.; Güntherodt, H. J. SiteSpecific Friction Force Spectroscopy. J. Vac. Sci. Technol. B 1996, 14 (2), 1285-1288. (29) Gellman, A. J.; Spencer, N. D. Surface Chemistry in Tribology. J. Eng. Tribo. 2002, 216 (6), 443-461. (30) Ghanbarzadeh, A.; Parsaeian, P.; Morina, A.; Wilson, M.; Eijk, M. A Semi-Deterministic Wear Model Considering the Effect of Zinc Dialkyl Dithiophosphate Tribofilm. Tribol. Lett. 2016, 61 (1), 12.


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Table of Contents Graphic


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Mechanism of Antiwear Property Under High Pressure of Synthetic Oil

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