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Hindawi Publishing Corporation
Journal of Nanomaterials
Volume 2015, Article ID 710462, 11 pages
http://dx.doi.org/10.1155/2015/710462
Research Article
Synthesis and Characterization of Molybdenum Disulfide
Nanoflowers and Nanosheets: Nanotribology
S. V. Prabhakar Vattikuti and Chan Byon
School of Mechanical Engineering, Yeungnam University, Gyeongsan 712-749, Republic of Korea
Correspondence should be addressed to Chan Byon; cbyon@ynu.ac.kr
Received 20 March 2015; Accepted 4 May 2015
Academic Editor: Xiaogang Han
Copyright © 2015 S. V. P. Vattikuti and C. Byon. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
This paper reports the solvothermal synthesis of MoS2 nanoflowers and nanosheets. The nanoflowers have a mean diameter of
about 100 nm and were obtained using thioacetamide (C2 H5 NS) as a sulfur source. The few layered nanosheets were obtained
using thiourea (CH4 N2 S) as a sulfur source. The obtained powders were characterized using powder X-ray diffraction (XRD),
scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), and transmission electron microscopy (TEM).
The lubricating effect of MoS2 nanoflowers and nanosheets were analyzed using four-ball test, the topography of the wear scar was
analyzed using SEM, EDS, and 3D surface profilometry. The relationship between the tribological properties and morphology of
the materials was determined. It is observed that the engine oil containing the MoS2 nanomaterials penetrated more easily into the
interface space, and it formed a continuous film on the interface surface. The tribological performance showed that the synthesized
nanosheets had superior antiwear and friction-reducing properties as a lubrication additive compared with nanoflowers. Also,
the wear scar of balls lubricated with nanoflowers revealed a larger diameter compared to nanosheets. In conclusion, nanosheets
dispensed in oil have better tribological performance compared to nanoflowers oil in terms of capability to reduce friction.
1. Introduction stress due to their layered structure with strong intralayer
covalent bonds and weak interlayer van der Waals bonds,
Two-dimensional (2D) and three-dimensional (3D) func- which decreases friction between interfaces [15].
tional nanostructured materials have received great attention These materials exist in numerous forms, such as
due to their inherent physicochemical properties, such as fullerene-like nanoparticles, flake-like structures, and nan-
high specific surface-to-volume ratio, anisotropy, chemical otubes [16, 17]. These materials are suitable for adding to
inertness, photocorrosion resistance, and excellent tribologi- lubrication fluids. MoS2 nanoparticles perform very well in
cal performance [1, 2]. Such materials are applicable in var- boundary-lubricated contact, particularly with steels [18–20].
ious fields, including lubricants, energy storage, field-effect However, the key parameters under different contact condi-
transistors, and catalysis [1–4]. Novel lubricants containing tions with respect to morphology have yet to be determined.
nanoparticles could provide extended reliability and major Lubricants mixed with nanoparticles are known to be the
energy savings in severe friction and wear conditions, which most effective approach to reducing the friction and wear
would impact industries related to sustainable engineering of at contact interfaces; however, the presence of solid particles
heavy equipment and support energy self-reliance [5–10]. may also lead to oil starvation for lubrication regimes [21].
Tenne et al. discovered spherical fullerene-like nanopar- Explaining the tribological performance of inorganic
ticles of MoS2 and WS2 nanotubes in 1992 [11]. MX2 (M = Mo, nanomaterials depends upon several mechanisms and effects:
W and X = S, Se) are well known for their solid-lubrication (a) rolling friction [22], (b) the inorganic nanoparticles
properties. Molybdenum disulfide (MoS2 ) has a layered acting as spacers to prevent direct contact between the
hexagonal crystal structure, which is mostly important for asperities [23], and (c) third-body material transfer to form
solid lubrication or as an additive for lubricating oils [12–14]. a thin lubricious film [24, 25]. However, the efficiency of the
These materials offer low shear resistance to any applied shear lubrication mechanism depends on various conditions andPage 2
2 Journal of Nanomaterials
intrinsic characteristics, including the morphology, crystal 2.1. Preparation of Nanoflowers. To obtain the MoS2 flower-
structure, shape, and size of the particular nanoparticles. For like structure, ammonium heptamolybdate tetrahydrate
example, in the case of fullerene-like MoS2 nanoparticles, ((NH4 )6 Mo7 O24 ⋅4H2 O), citric acid (C6 H8 O7 ), and thioac-
different modes of deformation and destruction are exhibited etamide (C2 H5 NS) were used as the starting materials and
when in contact with surfaces [18]. Kalin et al. demonstrated the sulfur source. The synthesis methodology was performed
that the adhesion of thin MoS2 nanosheets on a surface with an optimized molar ratio of Mo to citric acid of 1 : 2.
has four possible mechanisms: (i) exfoliation of individual Starting, 1.4 g of ammonium heptamolybdate tetrahydrate
nanotubes into nanosheets due to shear stress, (ii) nanosheet and 0.61 g of citric acid were dissolved in distilled water under
aggregation, (iii) nanotube deformation at the surface, and magnetic stirring and kept at 130∘ C on a hot plate for 25 min.
(iv) some nanotubes being damaged [26]. However, these The suspension was continuously stirred and refluxed with
mechanisms have been difficult to prove. a final pH of 4. Then, 2.36 mL of thioacetamide in water
The main negative aspect of a nanofluid is sedimentation was added dropwise to the solution. Finally, the precipitate
of the dispersed particles in the fluid due to poor compat- solution was placed in a Teflon-lined stainless-steel autoclave.
ibility between the dissimilar phases. This poses a problem
of deteriorating the tribological properties of nanofluids [27, 2.2. Preparation of Nanosheets. The ammonium heptamolyb-
28] due to small gaps between the asperities: the supply of date tetrahydrate, citric acid, and thiourea (CH4 N2 S) were
nanoparticles to the contact is inadequate or interrupted at used as the starting materials and the sulfur source to obtain
high contract loads in the boundary-lubricant regime [26]. It the MoS2 sheet-like structure. Starting, 1.3 g of ammonium
is believed that better lubrication is maintained only through heptamolybdate and 0.49 g of citric acid were dissolved in
the entrapment of multilayered or flake-like solid particles. distilled water under magnetic stirring and kept at 90∘ C
Useful tribological properties have also been ascribed to their on a hot plate for 25 min. The ammonia water is added to
structure, such as chemical inertness and longevity. However, suspension solution with continuous stirring to adjust the pH
experimental confirmation of the tribological properties (and at 4. Then, 1.27 g of thiourea in water was added dropwise to
thus the lubrication mechanisms) of inorganic MoS2 flake- the solution with continuous stirring on a hot plate for 5 min.
like nanoparticles has remained limited. Finally, suspension solution was transferred into a 40 mL
Recently, various synthesis processes have been devel- Teflon autoclave.
oped to prepare MoS2 nanomaterials, including chemical For both samples, the autoclave was maintained at 160∘ C
vapor deposition (CVD), thermal reduction, high-temper- for 10 h. The reactor was cooled to room temperature; the
ature sulfurization, laser ablation, and sol-gel methods [27– obtained black precipitates were collected by centrifugation
35]. Using these approaches, different morphologies (fulle- and then filtered and washed three times with distilled water
rene-like, nanotube, nanosphere, and nanorods) have been and ethanol. The obtained powders were finally dried under
tested and used in practical applications. However, the syn- vacuum at 140∘ C for 8 h.
thesis methods generally require complex technologies and
harmful organic surfactants. The solvothermal method has
attracted much interest due to its versatility and potential to 2.3. Lubricants Preparation. Experiments were performed
fabricate nanoparticles for applications. using Durasyn-170 oil (polyalphaolefin (PAO)), which is a
To the best of our knowledge, there have been limited typical synthetic oil for automotive applications that has a
reports using solvothermal methods to control the mor- density of 27.7 kg/m3 at 15∘ C. The experiments were carried
phology and properties of MoS2 nanomaterials with proper out with the base oil and with base oil containing 0.05, 0.1,
selection of the sulfur source. We report an optimized 0.5, and 1 wt% of nanoflowers and nanosheets additives. A
procedure for solvothermal synthesis of MoS2 nanoflowers relatively various concentration of synthesized MoS2 mate-
and nanosheets assembled with a few lamellar layers. Thioac- rials was used to determine the effect of morphology on
etamide and thiourea were used as the respective sulfur the tribological performance. The suspensions of oil and
sources. The aim of this work is to investigate the lubricating nanoadditives were thoroughly mixed with a magnetic stirrer
and antiwear behavior of the synthesized MoS2 materials. for 3 hours. The MoS2 materials were added to the base
The materials were tested under the same conditions in oil and mixed with hexane, and stability measurement was
dispersion in engine oil and in the boundary lubrication done using a dynamic light scattering system (Nano ZS (ZEN
regime. The tribofilm and wear debris were characterized 3600)). The kinematic viscosity of the oils was observed using
to understand the lubrication mechanisms. In addition, the an Ultra Programmable Rheometer as per ASTM standards.
optical properties and band energies of these MoS2 materials
are reported. 2.4. Tribological Tests. The coefficient of friction and wear
scar of all samples were studied using a four-ball tribometer.
2. Experimental Procedure The four-ball test machine is used to estimate the wear pre-
ventive characteristics of the lubricant. The test force was kept
In order to obtain the optimal conditions for synthetic constant at 40 Kgf, and 1000 rpm was applied for 60 minutes
procedure, a series of trials were carried out. All the chemicals at 75∘ C. Steel balls with a 12.5 mm diameter and hardness
were used in this synthesis without further filtration. Figure 1 of HRC 65 were used. The scar diameter of ball serves
shows a schematic diagram of the synthesis procedure and as a characteristic of the lubricant. In dissipative systems,
formation of MoS2 nanoflowers and nanosheets. extreme pressure at contacts is a major issue, depending onPage 3
Journal of Nanomaterials 3
∘ Nanoflowers
(NH4 )6 Mo7 O24 4H2 O + C6 H8 O7 Stir at 130 C
for 25 min
+
C2 H5 NS
Stir at 90∘ C Add
for 25 min Transfer
(NH4 )6 Mo7 O24 4H2 O + C6 H8 O7
+
CH4 N2 S
Final solution Autoclave Lubricant
Nanosheets
Figure 1: Schematic diagram of the formation of MoS2 nanoflowers and nanosheets.
the potential applications, which is why we selected the four- JCPDS card number 371492
(002)
ball tribometer for these studies.
(100)
The crystalline structure of the samples was estimated (b)
(103)
(110)
using powder X-ray diffraction (XRD) using a Shimadzu
Labx XRD 6100 with Cu-K𝛼 radiation (𝜆 = 0.14056 nm).
Intensity (a.u.)
The morphologies of the samples were observed by scan-
ning electron microscopy (SEM) on a Shimadzu Corpora-
tion Superscan SSX-550 SEM-EDS. Transmission electron
microscope (TEM) analysis of the samples was carried out (a)
with a Hitachi H-7000 of 100 KV. The phase purity was
estimated using a Fourier transform infrared spectroscope
(FTIR) (Avatar 370) with a spectral range of 400–4000 cm−1 .
Thermogravimetry analysis was done using a DTG-60/60H 20 30 40 50 60 70
TG/differential thermal analyzer under an argon gas at 900∘ C Position [2𝜃]
at a heating rate of 9∘ C per minute. The topography of the
wear scar was studied by SEM and 3D surface profilometer. Figure 2: XRD patterns of MoS2 : (a) nanoflowers and (b) nanosh-
The chemical composition of the tribofilm formed on the eets.
worn surfaces of the upper ball was examined using EDS
mapping.
believed that this open-ended structure would offer more
effective triboactive mechanisms for easy distribution on the
3. Results and Discussion interfacing surfaces. Interestingly, the sample obtained using
thiourea as the sulfur source formed multilayer nanosheets of
The powder X-ray diffraction (XRD) studies were carried out a size of a few nanometers, as shown in Figures 3(c) and 3(d)
to analyze the crystal structure of the MoS2 nanoparticles with different magnifications.
as a function of the processing conditions as shown in TEM images of the nanoflowers are shown with different
Figure 2. Both samples exhibit the crystallite nature of MoS2 magnifications in Figures 4(a), 4(b), and 4(c). The images
materials with an XRD pattern indexed at 14∘ , 32∘ , 39.5∘ , show that the nanoflowers are rounded and loosely connected
and 58∘ corresponding to the (002), (100), (103), and (110) to each other with a narrow size distribution and regular
crystal planes of the MoS2 structure, consistent with the spherical structure, which is in good agreement with the SEM
corresponding standard card (JCPDS card number 371492). results. This means that the nanoflowers have well-defined
No impurity peaks or other phases were observed. shape and are uniform in both morphology and particle size
SEM images (Figures 3(a) and 3(b)) show that the distribution. Figures 4(d), 4(e), and 4(f) show the structures
sample obtained with C2 H5 NS as the sulfur source comprises of the nanosheets with different magnifications. The edge
uniform MoS2 nanoflowers with an average size of 100 nm. lengths of the nanosheets are controllable at the nanometer
The nanoflowers are well defined and rounded with a large scale, typically from 40 nm to a few tens of nanometers, and
amount of agglomeration of open-ended structure. It is their thickness is 10 to 50 nm.Page 4
4 Journal of Nanomaterials
1 𝜇m 200 nm
(a) (b)
1 𝜇m
200 nm
(c) (d)
Figure 3: SEM images of MoS2 : ((a) and (b)) nanoflowers and ((c) and (d)) nanosheets.
200 nm 100 nm 100 nm
(a) (b) (c)
200 nm 100 nm 100 nm
(d) (e) (f)
Figure 4: TEM images of MoS2 : ((a), (b), and (c)) nanoflowers and ((d), (e), and (f)) nanosheets.Page 5
Journal of Nanomaterials 5
0.00 0.9
16.00 ∘
310 C
0.8
−20.00
DTA (𝜇V)
14.00
TGA (mg)
0.7
Absorbance
12.00 −40.00
0.6
(b)
10.00
−60.00 0.5
0.00 200.00 400.00 600.00 800.00 0.4
Temperature (∘ C) (a)
0.3
Figure 5: TG-DTA curves of MoS2 nanosheets.
200 400 600 800 1000
Wavelength (nm)
Figure 7: UV-Vis spectra of MoS2 : (a) nanoflowers and (b) nanosh-
eets.
The UV-Vis optical absorption spectra were recorded at
room temperature in the wavelength region of 250–750 nm,
T (%)
1100 as shown in Figures 7(a) and 7(b). The nanoflowers and
(b)
nanosheets have strong absorption in the visible-light region.
However, the absorption of the nanosheets compared to
1650 nanoflowers is stronger due to the higher light harvesting
(a) behavior of nanosheets with active edge sites. The absorption
900
bands appeared at 340 and 610 nm, which are attributed to
480 the direct excitonic transition at the 𝐾 and 𝑀 point of the
500 1000 1500 2000 2500 3000 3500 4000 Brillouin zone [37]. The indirect band gap was calculated by
Wavenumber (cm−1 ) the Tauc equation using the optical absorption data near the
band edge [38, 39]:
Figure 6: FT-IR spectra of MoS2 : (a) nanoflowers and (b) nanosh-
eets.
(𝛼ℎ])1/2 = 𝐴 (ℎ] − 𝐸𝑔 ) , (1)
where 𝛼 is the absorbance, ℎ] is the incident photon energy,
TG-DTA curves of the MoS2 nanosheets are shown in and 𝐴 is a constant. The band gaps (𝐸𝑔 ) are determined by
Figure 5. The nanosheets display 5% weight loss occurring linear fit extrapolation onto the 𝑥-axis. A plot of (𝛼ℎ])1/2
above 730∘ C when heating to 900∘ C throughout the analysis. versus photon energy (ℎ]) gives the band energies of the
This can be attributed to dehydroxylation of the material, nanoflowers and nanosheets by the intercept of the tangent
which is favorable for recrystallization and growth of the to the 𝑥-axis, as shown in Figures 8(a) and 8(b). The band
nanosheets. The prominent exothermic peaks at 495∘ C and energies of the MoS2 nanoflowers and nanosheets were
700∘ C correspond to the decomposition of surfactants and estimated to be 2.72 and 2.83 eV, respectively, which are fairly
the sheet crystallization phase. As a result, the decomposition close to earlier reports [37, 40].
of physisorbed solvent occurred during the preparation The agglomeration and stability of nanomaterials are
using heat-assisted magnetic stirring. Furthermore, there is quantified based on the zeta potential absolute value, which
a possibility that recrystallization could occur above 800∘ C, designates the static repellency of nanomaterials dispensed
which reflects the stable state. in oil, as shown in Figure 9. The maximum zeta potential
FT-IR spectra of the nanoflowers and nanosheets are absolute value of the nanoflowers and nanosheets in oil was
shown in Figures 6(a) and 6(b). There are broad absorption obtained at 0.1 wt%, which is ascribed to the optimum con-
bands at 480 cm−1 , 900 cm−1 , 1100 cm−1 , and 1650 cm−1 for centration with maximum stability of the nanosheets in the
both samples. The band at 480 cm−1 is due to the Mo-S base oil. The bigger value designates improved dispersion of
bond, and that at 900 cm−1 is due to the S-S bond [36]. the nanomaterials in the base oil. If the absolute value of zeta
The absorption band between 1100 cm−1 and 1650 cm−1 is potential is higher than 30 mV, then the nanofluid becomes
ascribed to the stretching vibrations of the hydroxyl group stable [41]. For nanoflowers and nanosheets dispersed oils,
and Mo-O vibrations [36]. The absorption band at 3500 cm−1 the absolute values of zeta potential are 32 and 34 mV, respec-
formed by the stretching vibration of hydroxyls vanished in tively. Thus, the stability of both oils is indicated to be good.
the nanosheet sample, which was confirmed by the TG-DTA In the case of nanosheets dispersed oil, the zeta potential of
curve (Figure 5). 0.6 wt % concentration is shown to be smaller than 1 wt% duePage 6
6 Journal of Nanomaterials
0.9
0.48
0.8
0.45
0.7
0.42
(𝛼h?)1/2
(𝛼h?)1/2
0.6
0.39
0.5 0.36
0.4 0.33
0.3 0.30
1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 2.0 2.2 2.4 2.6 2.8 3.0 3.2
Photon energy, h? (eV) Photon energy, h? (eV)
(a) (b)
Figure 8: Tauc plots of MoS2 : (a) nanoflowers and (b) nanosheets.
35
36
30 34
32
30
Zeta potential (mV)
25 28
Zeta potential (mV)
26
24
20 22
20
18
15 16
14
12
10 10
8
6
5
0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0
Concentration (wt%) Concentration (wt%)
27∘ C 27∘ C
35∘ C 35∘ C
(a) (b)
Figure 9: Variation of zeta potential with MoS2 (a) nanoflowers and (b) nanosheets for different concentration at 27∘ C and 35∘ C.
to low pH value of the oil. Therefore, the zeta potential is of viscosity was observed with the different morphologies of
related with corresponding pH of the nanoparticles dispersed these nanomaterials.
oils. The dispersion nature of nanoadditives in oils depends Figure 11 shows the friction coefficient as a function
on process parameters such as the type, concentration (wt% of time with different concentrations of nanoflowers and
of nanoparticles), and nature of the base oil and temperature. nanosheets in oil. This data was obtained using a four-ball test
Figure 10 shows the viscosity of the base oil and oils with with a 40 Kgf load and a speed of 1000 rpm for 60 minutes.
0.1 wt% of nanoflowers and nanosheets added was estimated The friction coefficient of the pure base oil without any
in the temperature range of room temperature to 120∘ C. In additives increases with the applied load. Furthermore, the
all cases, the viscosity of lubricant decreases with increasing friction coefficient of the base oil containing nanoflowers or
temperature. In addition, the viscosity increases by 20% for nanosheets is always lower than that of the pure base oil. On
oil with nanoflowers and nanosheets added compared to the the other hand, the base oil containing nanosheets exhibits
base oil at the elevated temperature, and no significant change a lower friction coefficient than that with nanoflowers.Page 7
Journal of Nanomaterials 7
75 surfaces produced after the 1 h tests with lubrication using
Viscosity (cP)
180
the nanoflowers and nanosheets. Negligible wear scar for
160 the steel balls was observed during these experiments for oil
140 20
containing 0.1 wt% nanoflowers and nanosheets. There is a
120
40 50 60 70 80 90 clean film on the surface of the steel balls, and in the case
Temperature (∘ C)
Viscosity (cP)
B of nanosheets dispersed in oil, the tested balls have greater
100 C film formation than the nanoflower oil. The base oil with
D
80 nanosheets is easily adherent and causes plastic deformation
60 due to the contact pressure. The formation of tribofilm
containing nanosheets supports reducing the friction due to
40
slippage of the individual layers of nanosheets.
20 Interestingly, the average surface roughness (𝑅𝑎 ) value of
0 the nanoflowers and nanosheets dispersed in oils remained
20 40 60 80 100 120 constant as a function of the normal load. 𝑅𝑎 of the wear scars
Temperature (∘ C) of the nanoflower and nanosheet oils were low: approximately
Base oil 85.3 and 54.2 nm, respectively, as shown in Figure 12. The
Base oil + MoS2 nanoflowers results suggest some fracture occurs due to adhesive wear
Base oil + MoS2 nanosheets upon continuous sliding friction under the applied load. This
proves that the base oils with nanoflowers and nanosheets
Figure 10: Variation of viscosity with respect to temperature. have better antiwear capability than the pure base oil. MoS2
nanomaterials can easily react and form an abrasion-resistant
protective film at contact interfaces due to high surface
0.075 energy with many dangling bonds [16]. A firm boundary
0.070 lubrication effect between the friction pairs occurs when
protective tribofilms formed. This may yield a good ability
0.065
to resist the shear failure due to fine lubricity. The friction
Coefficient of friction
0.060 coefficient thus declines distinctly, and the surface quality of
0.055 the contact elements improves greatly, as shown in Figures
0.050 12(a) and 12(b).
SEM micrographs (Figures 13(a), 13(b), and 13(c)) show
0.045
the wear scars of the base oil with and without nanoflowers
0.040 or nanosheets. The rubbed surface was lubricated by the
0.035 base oil and had many wide and deep ruts compared to the
nanoflowers and nanosheets oils. We believe that the many
0.030
regular nanosheets penetrate more easily into the interface
0 1000 2000 3000 4000 5000 6000 7000 8000 with the lubricant than the nanoflowers. The nanosheets
Time (s) could form a continuous film on the rubbing surfaces due
0 wt% NF 0 wt% NF to strong adherence to contacts and enhance the tribological
0.05 wt% NF 0.05 wt% NF properties. The nanoflowers are so small that they can easily
0.1 wt% NF 0.1 wt% NF go into worn areas under compressive stress and perturb the
0.5 wt% NF 0.5 wt% NF hydrodynamic regime. Because of this, a higher coefficient of
1.0 wt% NF 1.0 wt% NF friction was obtained for nanoflowers rather than nanosheets,
which is in good agreement with Figure 11.
Figure 11: Friction coefficient curves of different concentrations of
MoS2 nanoflowers (black color line) and nanosheets (red color line) The presence and formation of a tribofilm on the worn
with respect to time. surface were examined with EDS for base oil containing
0.1 wt% nanoflowers and nanosheets, as shown in Figures
13(d), 13(e), and 13(f). Mo-S signals detected on the worn
surface indicate that the MoS2 nanomaterials settle and fill
This is due to the slippage of nanosheets at asperities and the furrows on the worn surface, although the Mo-S signal is
deformation into individual nanosheets to form a tribofilm much weaker than the Fe signal on the ball surface. Moreover,
at the interfaces, which reduces the coefficient of friction. The the surface scratches and the furrows became vanished. Both
friction coefficient (𝜇) is equal to T√6/3𝑊𝑟, where 𝑇 is the the friction coefficient and the wear scar diameter were
frictional torque in kg mm, 𝑊 is the normal load in kg, and minimal. In both cases (i.e., nanoflowers and nanosheets),
𝑟 is the distance between the center of the interface on the the surface edges were slightly ragged and obscured by metal
lower balls and the axis of rotation [42, 43]. particles at normal loads greater than 40 Kgf. As a result,
To evaluate the wear-resistance tribological properties worn surfaces revealed abrasive wear and no severe adhesive
of the MoS2 nanomaterials, a noncontact universal surface wear was noticed.
profilometer was used to measure the wear scar and debris The superior friction reduction and antiwear behavior of
of the balls. Figure 12 shows the micrographs of worn MoS2 nanoflowers and nanosheets dispersed in oil comparedPage 8
8 Journal of Nanomaterials
480.0 0.8157 1.0837
480.0
Y (𝜇m)
Y (𝜇m)
240.0 0.4079 240.0 0.5419
0 0.00 0 0.00
0 320.0 640.0 0 320.0 640.0
X (𝜇m)
X (𝜇m)
X direct profile X direct profile
640.0 640.0
352.08 93.57
292.55 69.95
0
H (𝜇m)
359.68
195.04
279.81
H (𝜇m)
97.52 −69.95
0 −139.90
−37.99 −186.24
0 160.0 320.0 480.0 640.0 0 160.0 320.0 480.0 640.0
X distance (𝜇m) X distance (𝜇m)
Roughness Information Roughness Information
Ra : 79.31 nm A.H: 166.382 nm Ra : 58.29 nm A.H: 39.517 nm
Rq : 91.90 nm A.W: 0.620 𝜇m Rq : 70.59 nm A.W: 0.638 𝜇m
Rt : 354.50 nm Rt : 282.28 nm
Rp : 217.01 nm Rp : 144.85 nm
R? : 137.49 nm R? : 137.43 nm
Y direct profile Y direct profile
480.0 480.0
0.377 271.01
0.264 219.66
H (𝜇m)
0.4848
H (𝜇m)
292.88
0.132 146.44
0 73.22
−0.132 0
−0.151 −21.87
0 120.0 240.0 360.0 480.0 0 120.0 240.0 360.0 480.0
Y distance (𝜇m) Y distance (𝜇m)
Roughness Information Roughness Information
Ra : 92.12 nm A.H: 337.717 nm Ra : 50.11 nm A.H: 86.130 nm
Rq : 118.69 nm A.W: 408.000 nm Rq : 55.78 nm A.W: 438.000 nm
Rt : 479.62 nm Rt : 194.50 nm
Rp : 284.44 nm Rp : 100.02 nm
R? : 195.18 nm R? : 94.48 nm
(a) (b)
Figure 12: Noncontact optical profile testing apparatus images of wear scar at 1000 rpm under 40 Kgf loads for 1 h: (a) base oil with MoS2
nanoflowers and (b) base oil with 1.0 wt% MoS2 nanosheets.
to pristine oil are attributed to development of tribofilms [13, 25, 26]. This mechanism helps to form a tribofilm and
between the contact interfaces [10, 12, 13, 20]. The wear adhere at counter parts, enhancing the tribological properties
mechanism of MoS2 nanosheets is ascribed to separation which are confirmed with 3D topography results (Figure 12).
of interlayers into individual layers due to weaker van der The tribological properties with dispersion of MoS2 additives
Waals or Coulombic repulsive interaction at contact pressure in oil are improved. The worn surface of the upper ballPage 9
Journal of Nanomaterials 9
541
Fe
406
270
Fe
135 Cr
C Si
50 𝜇m 0
1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00
(a) (d)
394
315 Fe
236
157
S Fe
78 C Si Mo Cr
50 𝜇m 0
1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00
(b) (e)
372
Fe
279
186
C
93 Mo Fe
Si S Cr
50 𝜇m
0
1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 1
(c) (f)
Figure 13: Wear scar of upper ball on contacted surface with different conditions: (a) base oil, (b) base oil with 0.1 wt% MoS2 nanoflowers,
and (c) base oil with 0.1 wt% MoS2 nanosheets. (d), (e), and (f) represent EDS spectra of (a), (b), and (c) conditions, respectively.
showed metal-to-metal contact at interfaces. Therefore, the From the friction tests, the performance of lubricating
nanosheets could reduce the coefficient of friction, since oil using these synthesized MoS2 materials as additives
the metal-to-metal contact is smaller than when nanoflow- is investigated using the four-ball test and results show
ers were used. Presumably, these results indicated that the that the optimal additive concentration of nanoflowers and
nanosheets are more suitable and helped the lubricant to nanosheets is 0.1 wt%. The friction test results revealed that
adsorb onto the metal surface very well, which reduced the the average friction coefficient was two times lower for the
friction. nanosheets than for the nanoflowers. It was concluded that
the significant improvement in tribological properties of oil
4. Conclusions with nanoflowers and nanosheets additives was due to the
beneficial tribofilm transferred on the contact surfaces when
We report the solvothermal synthesis of MoS2 nanoflowers particles entered the wear regime as evident from SEM and
and nanosheets using thioacetamide and thiourea as sulfur EDS studies.
source, respectively. The method presented enables large- The surface roughness analyses reveal that the roughness
scale production of unique and controllable morphologies of the friction surface is reduced and the surface gets
by suitable selection of the surfactant. The effects and smoother when MoS2 materials at optimum concentration
nature of morphology on the tribological properties have level are added to the lubricants. It was also concluded that,
been presented. Substantial friction and wear reduction was as the benefits of exfoliation mechanism, the nanoflowers and
achieved in boundary lubrication, and the beneficial effects nanosheets act as reservoir of low friction layers which cover
are attributed to the nanosheets rather than nanoflowers the interface surfaces with a thickness of a few monolayers.
when added to the lubricant oil. The results revealed tribo- Therefore, nanoflowers and nanosheets used as lubricating oil
logical effect of engine oil without additive and presence of additives exhibit wear reduction characteristics. The unique
nanoflowers and nanosheets. mechanical and tribological properties indicate that the MoS2Page 10
10 Journal of Nanomaterials
nanosheets are a promising material for autolocomotive rolling of steel strip,” China Petroleum Processing and Petro-
applications and help to contribute to successful commercial- chemical Technology, vol. 13, no. 1, pp. 64–69, 2011.
ization of hybrid lubricants. [14] M. Praveena, V. Jayaram, and S. K. Biswas, “Friction between a
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[15] J. Kogovšek, M. Remškar, and M. Kalin, “Lubrication of DLC-
The authors declare that there is no conflict of interests coated surfaces with MoS2 nanotubes in all lubrication regimes:
regarding the publication of this paper. surface roughness and running-in effects,” Wear, vol. 303, no. 1-
2, pp. 361–370, 2013.
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