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Article
Au/Graphene Oxide Nanocomposite Synthesized in
Supercritical CO2 Fluid as Energy Efficient Lubricant Additive
Yuan Meng, Fenghua Su, and Yangzhi Chen
ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 25 Oct 2017
Downloaded from http://pubs.acs.org on October 25, 2017
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ACS Applied Materials & Interfaces
Au/Graphene Oxide Nanocomposite Synthesized in
Supercritical CO2 Fluid as Energy Efficient Lubricant Additive
Yuan Meng, Fenghua Su* and Yangzhi Chen
School of Mechanical and Automotive Engineering, South China University of Technology,
Guangzhou, China
ABSTRACT
Au nanoparticles are successfully decorated onto graphene oxide (GO) sheets with the aid of
supercritical carbon dioxide (ScCO2) fluid. The synthesized nanocomposite (Sc-Au/GO) was
characterized by X-ray diffraction (XRD), Raman spectroscopy, thermal gravimetric analysis
(TGA) and transmission electron microscopy (TEM). The characterization results show that the
Au nanoparticles are featured with face-centered cubic crystal structure and disperse well on the
GO nanosheet surfaces with average diameters of 4-10 nm. The tribological behaviors of
Sc-Au/GO as lubricating additive in PAO6 oil were investigated using a ball-on-disc friction
tester, and a control experiment by respectively adding GO, nano-Au particles, and Au/GO
produced in the absence of ScCO2 was performed as well. It is found that Sc-Au/GO exhibits the
best lubricating performances among all the samples tested. When 0.10 wt. % Sc-Au/GO is
dispersed into PAO6 oil, the friction coefficient and wear rate are respectively reduced by 33.6%
and 72.8% as compared with that of the pure PAO6 oil, indicating that Sc-Au/GO is an energy
efficient lubricant additive. A possible lubricating mechanism of Sc-Au/GO additive in PAO6 oil
has been tentatively proposed on the basis of the analyzed results of the worn surface examined
by scanning electron microscopy (SEM), Raman spectroscopy and X-ray photoelectron
spectroscopy (XPS).
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KEYWORDS: Graphene oxide; Au nanoparticles; Supercritical CO2; Lubricating additive;
Friction and wear
1. INTRODUCTION
In modern industry, reducing friction and wear of mechanical elements becomes a critical factor
for enhancing durability of mechanical components and improving machine efficiency. Using
lubricant additives has been widely regarded as a feasible strategy to improve the lubricating
performances of lubricants. Besides traditional organic molecules, numerous metal and metallic
oxide nanoparticles have been proven to be effective lubricating additives for reducing friction
and wear.1-12 For instance, Chen et al.1 discovered that when nano-nickel particles were added in
synthetic PAO6 oil, decreased friction and wear and increased loading-carrying capability were
achieved. Gusain et al.2 reported that CuO nanorods as additives in PEG 200 and 10W-40 oil
exhibited excellent friction reduction and anti-wear behaviors. Recently, several reports have
shown that superior friction-reducing and anti-wear properties can be achieved by the
introduction of trace amount of precious metal nanoparticles, such as silver, gold, and palladium
as lubricating additives without significant increasing of the costs.13-17 Li et al.13 dispersed 0.5
wt. % dialkyldithiophosphate coated Ag nanoparticles into liquid paraffin and obtained 51 %
reduction in wear scar diameter and 60 % enhancement in PB value. Wang et al.14 found that the
addition of traces of Au nanoparticles (1.02 × 10-3 wt. %) in ionic liquid could reduce the friction
coefficient and wear volume by 13.8 % and 45.4 % respectively. It is extremely challenging to
completely explain the lubricating mechanism of nanoparticles as lubricating additive to date,
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because of the changeable friction environment and various types of nanoparticles employed.
Several hypotheses such as deposited film, cold welding, surface alloying and rolling effect have
been proposed to have close relation with the anti-wear mechanism of nanoparticle additives. It
has also been realized that the features of chemical composition, grain size and morphology of
nanoparticles had important effects on their lubricating performances.
Besides metal and metallic oxide nanoparticles, lots of layered materials including graphene,
graphite, molybdenum disulfide (MoS2) and hexagonal boron nitride (h-BN) have also been
widely considered as effective solid lubricants.18-33 The unique anisotropic crystal structure
imparts the layer materials with strong covalent intralayer and weak van der Waals interlayer
interactions, which lead to effective lubrication eventually. Among these materials, graphene has
been considered as the most promising and attractive material due to its extreme strength and
easy shearing capability on the densely packed and atomically smooth sheets.18-27
Tabandeh-Khorshid et al.18 found that 1 wt. % graphene nanoplatelets greatly improved
tribological properties of the filled aluminum matrix. Liang et al.19 discovered that in-situ
exfoliated graphene in deionized water could offer 81.3% and 61.8% reduction in friction
coefficient and wear scar diameter, respectively. Such intriguing anti-wear abilities from
graphene was further confirmed by Gupta et al.20, as they found that the friction coefficient and
wear rate were remarkably reduced to 70% and 50% through introducing optimized
concentration of rGO into neat oil. They thought that the graphene sheets were linked with PEG
molecules through hydrogen bonding and thus provided sufficiently lower shear strength for the
formed boundary tribofilm.
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Since nanoparticles and layered materials are both excellent lubrication materials, it is quite
appealing to decorate nanoparticles on layered material substrates as integrated composite as
advanced lubricative agent. In fact, several researches have performed such recombination and
found that the physicochemical properties including the tribological property can be greatly
improved.34-40 Song et al.34 synthesized α-Fe2O3 nanorod/graphene oxide composites in a
hydrolysis process and found that the friction and wear properties of this composite was better
than that of graphene oxide (GO) and the mechanical mixture of α-Fe2O3 and GO when
employed as lubricating additives in paraffin oil. Although the nanocomposites of layered
materials dotted with nanoparticles have great potential to obtain superior lubrication
performances, it is still a challenge to achieve well-defined nanostructure and optimal
combination of those base ingredients at nanoscale dimension, which has direct influences on the
play of ingredients’ capability.35,36
Supercritical fluid technique is regarded as an effective and efficient technique for preparing high
quality nanocomposites.41-47 Supercritical carbon dioxide (ScCO2) is the most widely employed
supercritical medium, thanks to its readily accessible supercritical conditions and various unique
properties including gas-like diffusivity, extremely low viscosity, near-zero surface tension and
excellent mass-transfer activity. ScCO2 can easily wet substrates, increase loading weight and
make metal nanoparticles decorated on substrate surfaces uniformly. Several reports documented
that the nanocomposites fabricated by the ScCO2 technique usually presented more regular
microstructure and better macro-properties than those prepared without the aid of ScCO2.41,42
In this work, we establish a facile in situ one-step reduction route for synthesizing Au/graphene
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oxide nanocomposite (Sc-Au/GO) with the aid of ScCO2 fluids. The tribological performances of
this nanocomposite used as lubricating additive in PAO6 oil are investigated by a ball-on-disc
friction tester. Other nanomaterials including GO, nano-Au particles, and Au/GO produced in air
are also tested as control samples. The lubrication model of the friction pairs lubricated by the
Sc-Au/GO dispersed PAO6 oil is discussed and proposed. The findings here provide a viable
strategy for preparing nanoparticle/graphene oxide composites with remarkable tribological
performances readily for potential industrial applications.
2. EXPERIMENTAL SECTION
2.1. Chemicals
All chemicals used in this work were of analytical grade. Gold precursor (HAuCl4·4H2O) was
purchased from Shanghai Zhanyun Chemical Co., Ltd. Glucose (C6H12O6) was purchased from
Shanghai Rich Joint Chemical Reagent Co., Ltd. Natural graphite with average size around 44
µm was supplied from Tianjin Fuchen Chemical Reagent Factory. Sodium dodecyl sulfate (SDS)
was supplied from Sinopharm Chemical Reagent Co., Ltd. Oleyamine (C18H37N) was purchased
from Shanghai Hansi Chemical Industry Co., Ltd. Ethanol was produced by Tianjin Fuyu Fine
Chemical Co., Ltd. Carbon dioxide of high purity was used in our experiments.
2.2. Synthesis of Sc-Au/GO
Graphene oxide (GO) was prepared by following a modified Hummers method48,49 with natural
graphite powders as raw materials, and applied as substrate to load Au nanoparticles. Glucose
aqueous dispersion (2 mL, 9.1 wt. %) was prepared by dissolving glucose in deionized water. In
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a typical experiment to synthesize the nanocomposite of Sc-Au/GO, suitable amounts of GO,
AuHCl4·4H2O and SDS were dissolved and ultrasonically dispersed in 48 mL of ethanol,
respectively. The suspension was then mixed with 2 mL glucose aqueous dispersion to obtain the
final reaction suspension. Subsequently, the reaction suspension was loaded into a stainless
autoclave of 100 mL, which was preheated in advance. After flushed with carbon dioxide gas for
2 min, the autoclave was sealed. The autoclave was pressurized by CO2 to 12 Mpa and heated up
to 120 oC. The reaction suspension was maintained at this pressure and temperature and
vigorously stirred by magnetic agitation at 450 rpm for 2 h, and then cooled to room temperature
naturally. After the autoclave was depressurized, the dark precipitate was separated from the
suspension by centrifugation, and then washed with copious ethanol and deionized water
repeatedly. After vacuum-dried at 65 oC for 8 h, the precipitate was collected and denoted as
Sc-Au/GO for further characterizations and friction tests. The gold nanoparticles (nano-Au) were
also synthesized using the same method without the addition of GO. In addition, the
nanocomposite of Au/GO was also prepared by the same fabrication process but without the
introduction of ScCO2.
2.3. Preparation of Sc-Au/GO Dispersed Oil
Lubricating performances of nanoparticle additive are seriously influenced by its dispersity and
stability in neat oil. To obtain well-dispersed and stable oil dispersions, all the synthesized
nanomaterials including GO, nano-Au, Au/GO and Sc-Au/GO are modified by oleylamine.
Typically, 100 mg nanomaterials were dispersed in 20 mL ethanol solution of oleylamine (15
mg·mL-1). After sealed in a 100 mL glass flask, the mixture was gently refluxed with slow
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magnetic stirring of 60 rpm at 90 oC for 24 h. The products were washed by hot ethanol to
remove excess modifiers and then dried naturally in air. The modified nanomaterials with desired
mass fraction were ultrasonically dispersed in PAO6 synthetic oil to obtain the dispersed oil
samples.
2.4. Characterizations
XRD analyses of the nanomaterials were carried out using a Philips X’pert X-ray diffractometer
at 40 kV and 40 mA with Cu-κα radiation. The diffraction data were recorded for 2θ angles
between 5o to 90o. The morphology and microstructure were observed with a field-emission
transmission electron microscope (TEM, JEOL JEM-2010F). Raman spectra were recorded by a
multichannel confocal micro-spectrometer (Dilor Labram-1B, excitation laser of 20 mW and 532
nm). Thermal gravimetric analysis (TGA, STA 449C, Germany) was performed in air
atmosphere at a heating rate of 10 oC·min-1 from room temperature to 800 oC.
Lubrication characteristics of the neat PAO6 oil and the nanomaterial dispersed oils were tested
by a ball-on-disc friction tester (MS-T3000, Lanzhou Huahui Instrument Technology Co., Ltd.,
China). The tests were performed at room temperature and ambient humidity under applied load
of 10 N for 30 min. The bottom disc is rotated against the stationary upper ball with a speed of
0.1 m·s-1. The upper ball with diameter of 6.5 mm is composed of GCr15 steel (AISI 52100) and
the bottom disc with sliding contact radius of 10 mm consists of stainless steel (AISI 40300).
Prior to installment, the steel ball and disc were cleaned ultrasonically in petroleum ether and
dried in air. The friction coefficients were recorded automatically by a strain sensor. The wear
track on the disc was examined by a Taylor profilometer (Talysurf CLI 1000) to compute the
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wear rate. The cross section area (Ss) of the wear track is obtained directly using the analysis
software (TalyMap Universal 3.2.0). The wear volume (Vw) is calculated for a half-ellipse track
according to Eq. (1) and the load-normalized wear rate (Wr) is calculated according to Eq. (2).:
= ∙ 2
(1)
/ = / ∙ (2)
Where Ss = cross section area of wear tracks (mm2), L1 = radius of circular tracks (10 mm). L2 =
sliding distance (180 m), F = applied normal load (10 N). Each tribo-test and wear track
measurement was carried out at least three times to ensure standard deviations less than 5%.
The morphology of the wear scar on the ball and the wear track on the disc was observed using a
scanning electron microscope (SEM, JEOL JSM 6700F). X-ray photoelectron spectroscopy
(XPS, Kratos Axis Ultra DLD; Al-κα radiation) and Raman spectroscopy were employed to
analyze the chemical states of typical elements on the wear track surface.
3. RESULTS AND DISCUSSION
3.1. Composition and Morphology
X-ray diffraction (XRD) patterns of GO, nano-Au, Au/GO and Sc-Au/GO are presented in
Figure 1. The typical diffraction peak at 10.2o is attributed to the graphene (001) lattice plane of
GO. However, it completely disappears in the patterns of Au/GO and Sc-Au/GO, which is
probably due to effective prevention of graphene oxide restacking and aggregation caused by the
deposited Au nanoparticles.46,50 We postulate that the dotted Au nanoparticles can exfoliate the
GO layers and prevent them from restacking orderly. In the XRD patterns of nano-Au, Au/GO
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and Sc-Au/GO, the peaks at 38.2o, 44.4o, 64.6o, 77.5o, 81.7o are respectively ascribed to gold
(111), (200), (220), (311) and (222) crystallographic planes (JCPDS No.04-0784), confirming
that the dotted Au nanoparticles on GO sheets adopted with face-centered cubic (FCC) crystal
structure. Moreover, these peaks in Sc-Au/GO are broader than those in Au/GO at the same 2θ
position, signifying that the smaller grain sized Au crystals in Sc-Au/GO were obtained.
Figure 1. XRD patterns of GO, nano-Au, Au/GO and Sc-Au/GO.
Next, Raman spectroscopy was conducted to further examine the structural features of these
samples. As shown in Figure 2, two neighboring sharp peaks can be found for GO in Raman
spectra, the G band (κ-point phonon of A1g symmetry) at around 1580 cm-1 and the D band (E2g
phonon of C sp2 atoms) at around 1350 cm-1. Interestingly, the intensities of the D and G bands
in Sc-Au/GO and Au/GO are distinctly increased due to the adsorption of Au nanoparticles. Such
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phenomenon has been well documented in previous reports 40,51for the nanocomposites of
carbonous materials decorated by metal nanoparticles. The so-called surface enhanced Raman
scattering (SERS) is mainly caused by chemical enhancement effect.40,51 Note that, the G/D ratio
is an important parameter for analyzing the structural properties of graphene oxide. The G/D
ratios for GO, Sc-Au/GO and Au/GO are 1.06, 0.99 and 0.84, respectively. It can be noted that,
the D band exhibited a higher increase than the G band for the composites especially for Au/GO,
indicating that more defects are presented upon the decoration of Au nanoparticles. No other
peaks below 1000 cm-1 are observed, which suggests that no impurity of any metallic oxides
exist in these nanocomposites. In addition, a relatively weak but broad 2D band of GO is located
at ~2701 cm−1, which is ascribed to the graphite-like structures.25 GO restacked during the drying
process in vacuum chamber and formed the graphite-like structures which can be broken by the
following ultrasound treatment. When Au nanoparticles are decorated on GO sheets, the layer
interspaces of GO sheets are expanded by the nanoparticles and the restacking is also impeded,
leading to decreased layers of GO sheets in the nanocomposites of Au/GO and Sc-Au/GO.
Consequently, the 2D bands of Au/GO (at ~2650 cm−1) and Sc-Au/GO (at ~2688 cm−1) are
downshifted and become narrower as compared to that of GO.
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Figure 2. Raman spectra of GO, Au/GO and Sc-Au/GO.
To unveil the components in the composites, thermogravimetric analysis (TGA) was then
performed. Figure 3 shows the TGA curves of GO, Au/GO and Sc-Au/GO. The weight loss
below 100 oC is ascribed to the evaporation of adsorbed water. GO shows the largest water
weight loss, which is probably due to a large number of water molecules absorbed by a multitude
of oxygen-containing groups on its surface. The loss in the range of 100~300 oC originates from
the thermal decomposition of the oxygen-containing groups, while the loss between 300~600 oC
is owing to the pyrolysis of carbon structures that are converted to carbon dioxides in air. All
three curves exhibit similar pattern with the increasing of temperature, indicating a very weak
reduction degree of GO in Au/GO and Sc-Au/GO nanocomposites. Meanwhile, it can be seen
that GO is nearly burned out with few remnants remained (4.81 wt. %), as the temperature
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reaches 800 oC. In contrast, the eventual residual weights of Au/GO and Sc-Au/GO are 16.77
wt. % and 38.65 wt. %, respectively. The final residues of Au/GO and Sc-Au/GO should be the
leftover Au nanoparticles because of the chemical inertness of gold at high temperature. The
higher mass fraction of Au nanoparticles in Sc-Au/GO than that in Au/GO is closely related to
the function of ScCO2 during the synthesis process.41,45-47 The unique properties such as gas-like
diffusivity, extremely low viscosity and excellent mass-transfer activity of ScCO2 are beneficial
for transferring more precursors onto GO surfaces, leading to more metal particles anchored on
GO nanosheets.
Figure 3. TGA curves of GO, Au/GO and Sc-Au/GO.
The shape and surface microstructure of the samples were then observed by electron microscopic
techniques. The representative TEM images of nano-Au, GO and the nanocomposites are
presented in Figure 4. As shown in Figure 4a, GO sheet resembles a transparent crumpled paper
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and no micropores or mesopores are observed on the sheet surface. Figure 4b shows that the
as-prepared nano-Au particles display spherical shape but heavy aggregation is observed with
large chunks easily identified. Apparently, it is easy to recognize the influence of ScCO2 on the
morphology and microstructure of the nanocomposites by comparing the TEM images of Au/GO
(Figure 4c) and Sc-Au/GO (Figures 4d, e). The Au nanoparticles in Au/GO are dispersed
inhomogeneously on GO sheets and show a wide size range between 10 and 90 nm. In sharp
contrast, the homogeneous Au grains with average diameter of 4-10 nm are uniformly dispersed
on the exfoliated GO sheets in Sc-Au/GO, as shown in Figures 4d, e. According to the previous
reports,41,42 ScCO2 is conductive to exfoliate of GO sheets, enhances the adhesion and dispersion
of metal precursors on the sheet surfaces, and prevents the growth of metal crystals from out of
control during the synthesis process. Accordingly, the Au nanoparticles with smaller grain size
and high-quality dispersion are achieved in the nanocomposite of Sc-Au/GO, which agrees well
with the XRD results. Additionally, HR-TEM image of typical Au nanoparticles on GO sheet
taken from Figure 4e is shown in Figure 4f. The lattice spacing of 0.2355 nm corresponds well to
the (111) lattice plane of typical FCC Au crystal.
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Figure 4. Representative TEM images of (a) GO, (b) Nano-Au particles, (c) Au/GO and (d, e)
Sc-Au/GO; (f) Typical HR-TEM image of Au nanoparticles on GO sheets from Sc-Au/GO.
3.2. Lubricating Performances
As we known, the dispersity and stability of nanoparticle additives in neat oil play a decisive role
in the lubricating performances of the dispersed oil. Figure 5 shows the stabilities of the neat
PAO6 oil and the oils dispersed with different nanomaterials after 10 days’ standing. It is
observed that few precipitates are observed at the bottom of the glass bottles, indicating the
qualified dispersity and stability of these oil samples.
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Figure 5. Photographs of the neat PAO6 oil and the nanomaterial dispersed oil samples after
standing for 10 days.
Figure 6a shows the variations of friction coefficients (FCs) lubricated with the neat PAO6 oil
and the different dispersed oil samples as a function of sliding distance. The FC of the neat PAO6
oil increases from 0.094 in the beginning to 0.120 at around 50 m of sliding and then maintains
at this level till the end of sliding, which indicates the lubrication state in neat oil most likely
belongs to a mixed lubrication (ML) regime that contains dry contact (DC) and boundary
lubrication (BL) during the sliding process. The dispersed oils with these nanomaterials present
lower FCs almost during the entire sliding process, when compared to the neat PAO6 oil. The
FCs of the nano-Au dispersed oil are lower than that of the GO dispersed oil at the initial 50 m of
sliding and become higher with the further increase of sliding distance. The nanocomposite
dispersed oils show much lower and steadier FCs than GO or nano-Au dispersed oil, indicating
that an improved lubrication state is achieved during the sliding process.
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The average FCs and the wear rates (Wrs) of the corresponding bottom discs lubricated with
these oils are shown in Figure 6b. It is clear that these nanomaterials are effective for reducing
friction coefficient and wear rate as lubricating additive. The nanocomposites of Sc-Au/GO and
Au/GO exhibit better lubricating abilities than the individual nanomaterial of Nano-Au or GO,
which confirms the synergistic effect of GO sheets and Au nanoparticles.34,36 In addition, Figure
6b also reveals the different friction-reducing and antiwear abilities of Au/GO and Sc-Au/GO,
which is due to their different microstructures and morphologies (Figures 4c, d). The smaller
grain size and more uniform dispersion of Au nanoparticles anchored on GO surfaces might be
favorable for the release of the friction-reducing and antiwear potentials of Au nanoparticles and
GO in the nanocomposite. As a result, the Sc-Au/GO dispersed oil presents the better lubricating
ability than the Au/GO dispersed one.
Figure 6. (a) Friction coefficient curves for the pure PAO6 oil and the dispersed oils respectively
with 0.10 wt. % GO, nano-Au, Au/GO, and Sc-Au/GO with increasing sliding distance; (b)
Average friction coefficients (FCs) and wear rates (Wrs) of the corresponding discs lubricated
with these oils.
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It is well known that, the concentration of additive in lubricating oil plays an important role to
determine lubricative characteristics. Effect of the Sc-Au/GO concentration on the FCs and Wrs
of the Sc-Au/GO dispersed oil is shown in Figure 7. The FCs and Wrs decrease firstly and then
increase slowly with increasing concentration of Sc-Au/GO. And there exist a narrow bottom in
the curve where the lowest FC and Wr are presented. At the bottom, the FC and Wr are reduced
by 33.6% and 72.8% respectively, in comparison with the neat PAO6 oil. With adding Sc-Au/GO
into oil and increasing its concentration, more and more nanocomposites deposit on wear surface
and thus greatly reduce the roughness of the surface, and the corresponding lubrication state
gradually goes up into a good lubrication regime.52,53 Nevertheless, excessive concentration of
Sc-Au/GO in oil will lead to GO piling up between friction pairs, thus blocking the oil film, and
oil film will become much more discontinuous, even causing a dry friction.53 As a result, the FC
and Wr will increase beyond the bottom point of the curves (about 0.10 wt. %).
Figure 7. Friction coefficients (FCs) and wear rates (Wrs) versus Sc-Au/GO concentration in the
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Sc-Au/GO dispersed oil.
3.3. Analyses of Wear Interfaces
Figure 8 shows the typical SEM images of the wear scars on the balls and the wear tracks on the
corresponding discs lubricated with different oil samples. As shown in Figures 8a, d, the wear
scar on the ball lubricated by the neat PAO6 oil is very big and presents many block wear debris
and deep furrows, indicating severe scuffing and adhesion wear occurred. It can be concluded
that the contact pairs lubricated with the PAO6 oil belongs to the mixed lubrication state. In
contrast, the wear scars on the balls lubricated by the Au/GO dispersed oil (Figures 8b, e) and the
Sc-Au/GO dispersed oil (Figures 8c, f) become much smaller and shallower and have a small
number of wear debris, which indicates the improvement in lubrication state and the reduction in
friction and wear. During the sliding process, GO sheets and Au nanoparticles in the
nanocomposites easily deposit on contact pair surfaces and form a protective film,13-16,19 which
can smooth the surfaces and thus reduce friction and wear. Figures 8g-i shows the wear tracks on
the discs lubricated with different oil samples. The width of wear tracks on the discs matches
well with the size of wear scars on the corresponding balls. As shown in Figure 8g, the wear
track on the disc lubricated by the pure PAO6 oil is fluctuant and very deep and contains lots of
wear debris. In contrast, the wear tracks on the discs lubricated by the nanocomposite dispersed
oils become clean and small, as shown in Figures 8h, i. In general, the wear surface lubricated by
the Sc-Au/GO dispersed oil are the evenest and smoothest and show the smallest wear on the
contact pairs, as shown in Figures 8c, f and i. The results are in good accordance with the
previous findings in Figure 6, further verifying the best antiwear ability of Sc-Au/GO among all
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these nanomaterials as lubricating additives in PAO6 oil.
Figure 8. Typical SEM images of (a-f) wear scars on upper balls and (g-i) wear tracks on bottom
discs lubricated with different oil samples. (a, d, g) PAO6 oil, (b, e, h) PAO6 + 0.10 wt. %
Au/GO and (c, f, i) PAO6 + 0.10 wt. % Sc-Au/GO.
The 3D simulated images of the wear tracks on the discs lubricated with the PAO6 oil and the
nanocomposite dispersed oils are examined by a Taylor profilometer and shown in Figure 9. It is
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clear that the wear track lubricated by the PAO6 oil (Figures 9a, b) is extremely rough and has
huge peak and many wide, deep furrows. As 0.10 wt. % Au/GO or Sc-Au/GO are dispersed in
the oil, the generated scratches and furrows become much smaller and the surface roughness
greatly decreases, as shown in Figures 9c-f. These simulated images give a visual representation
of the decreased wear volume of the discs lubricated by the Au/GO and Sc-Au/GO dispersed oils.
The Wr of the wear tracks on the discs lubricated with the PAO6 oil, the Au/GO dispersed oil and
the Sc-Au/GO dispersed oil are calculated as 3.268×10-6, 1.155×10-6 and 8.889×10-7 mm3(Nm)-1,
respectively, which proves the excellent lubricating effects of the nanocomposites in PAO6 oil.
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Figure 9. Taylor profilometer 3D simulated images of the wear tracks on discs lubricated with (a,
b) PAO6 oil, (c, d) 0.10 wt. % Au/GO dispersed oil and (e, f) 0.10 wt. % Sc-Au/GO dispersed
oil.
Figures 10a-c displays the optical microscopy images of the bare steel surface and the wear track
surfaces lubricated with the PAO6 oil and the Sc-Au/GO dispersed oil, and Figure 10d presents
the Raman spectra of the typical position (marked with point) on these surfaces to confirm the
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deposition action of the Sc-Au/GO additive during rubbing process. The surfaces of the bare
steel in Figure 10a and the wear track in Figure 10b are relatively bright and clean, but large dark
areas spread on the wear track surface in Figure 10c. As shown in Figure 10d, a dark area (point
5 in Figure 10c) in the wear track surface lubricated with the Sc-Au/GO dispersed oil exhibits
strong D band and G band of GO in the Raman spectrum. The two bands with weaker intensities
are also observed on the bright area (point 4) of Figure 10c. The Raman spectra of the bare steel
surface (point 1) and the bright/dark areas (points 2 and 3) on the wear track surface lubricated
by the PAO6 oil (Figure 10b) prove no existence of graphene oxide debris. Therefore, the
analysis result verifies the deposition behavior of Sc-Au/GO in dispersed oil during rubbing
process, and the whole wear track surface are almost covered by the nanocomposite after
tribological test (Figure 10c).
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Figure 10. Optical microscopy images of (a) bare stainless steel surface and wear track surfaces
lubricated by (b) PAO6 oil and (c) 0.10 wt. % Sc-Au/GO dispersed oil; (d) Raman spectra of the
marked points (1-5) on these surfaces.
To further reveal the friction-reducing and antiwear mechanism of the Sc-Au/GO dispersed oil,
the valence states of several typical elements on the wear tracks of the discs are examined by
XPS measurements. Figure 11 shows the curve-fitted XPS spectra of C1s, O 1s, Fe2p and Au4f on
the bare steel surface (Figures 11a-c) and on the wear track surfaces of discs lubricated with the
neat PAO6 oil (Figures 11d-f) and with the Sc-Au/GO dispersed oil (Figures 11g-j). The
corresponding mass fractions of these elements are determined by XPS analysis and summarized
in Table 1. The peaks from C1s electrons can be de-convoluted into four sub-peaks at 284.6,
285.2, 286.1, 288.5 eV, and they can be attributed to different chemical components. The
sub-peaks in Figure 11a mainly correspond to the contaminated carbons and the carbons from
bare steel substrate; ones in Figure 11d are mainly ascribed to the contaminated carbons and the
oxidation products of oil. The C1s spectrum in Figure 11g is different from the ones in Figures
11a, d. The strong sub-peak at 284.6 eV corresponding to the carbon from GO indicates the
existence of Sc-Au/GO on the wear track, which agrees well with the analysis in Figure 10.
Furthermore, the enhanced C-O signal at 286.1 eV than that in Figure 11d can also demonstrate
the successful deposition of Sc-Au/GO during the rubbing process, because this increased signal
arises from the functional groups of GO in Sc-Au/GO. The O1s spectrum in Figure 11b with five
peaks around 530, 530.3, 531.3, 532 and 533.7 eV are attributed to the O1s electrons in FeO,
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Cr2O3, C=O, FeOOH and absorbed oxygen, respectively. It can be reasonably concluded that the
bottom disc surface was oxidized to some extent during the polishing process. As to the O1s
spectra in Figure 11e, strong signal from Fe2O3 can be found without the presence of FeO signal,
indicating further oxidation occurred to some extent on wear track during the sliding test.
Moreover, the enhanced C=O intensity also proves the possible oxidization of neat oil during the
rubbing process. In the O1s spectra of the wear track lubricated with the Sc-Au/GO dispersed oil
(Figure 11h), the Fe2O3 signal (530 eV) is substituted by Fe3O4 (529.8 eV), which indicates a
higher oxidation degree of Fe element than that on bare steel surface and a lower oxidation
extent than that on the wear track lubricated with PAO6 oil. In contrast, Figure 11h shows the
strongest C=O signal that is from plentiful oxygen-containing groups of GO, which further
confirms the deposition of Sc-Au/GO nanocomposites during the rubbing process. The
oxidization extent can be clearly recognized by further analyzing the Fe2p spectra in Figures 11c,
f and i. In Figure 11c, a part of the iron elements at zero valence state are oxidized to FeO or
FeOOH on the bare steel surface. After sliding process lubricated with the PAO6 oil, the iron
elements on wear track (Figure 11f) were completely oxidized and transformed to higher valence
state of Fe3+. The Fe2p spectrum in Figure 11i shows more Fe2+ than that in Figure 11f, signifying
the relatively weak oxidization degree of Fe0 elements. The above analyses of the Fe2p spectra
correspond well with the analysis results of the O1s spectra once again. The XPS spectrum of Au4f
on the wear track lubricated by the Sc-Au/GO dispersed oil is shown in Figure 11j. The strong
signals of Au4f7/2 peak at 83.80 eV and Au4f5/2 peak at 87.45 eV are detected on this wear surface,
which suggests that Au particles are released from Sc-Au/GO nanocomposite and deposit on the
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sliding steel surfaces during the rubbing process. And the Au nanoparticles show chemical
inertness without any chemical reactions as expected.
Figure 11. Curve-fitted XPS spectra of typical elements on (a-c) the bare steel surface and on the
wear tracks of steel discs lubricated with (d-f) PAO6 oil and with (g-j) 0.10 wt. % Sc-Au/GO
dispersed oil.
Table 1 presents that the wear track lubricated by the Sc-Au/GO dispersed oil has much higher
relative mass fraction of C and lower mass fractions of Fe, Cr and Ni than the bare steel surface
and the wear track lubricated by the PAO6 oil. Au element only exists on the wear track
lubricated by the Sc-Au/GO dispersed oil. It can be reasonably inferred that Sc-Au/GO in
dispersed oil are able to form block deposition film with good coverage on rubbing surfaces,
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which finally leads to the significant increase of C element and sharp decreases of other
elements.
Table 1. Relative mass fraction of typical elements on different disc surfaces
Sample
Mass fraction (%)
C
O
Fe
Cr
Ni
Au
Bare steel disc
25.23
28.01
27.34
14.53
4.89
/
Disc by PAO6
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2.71
0.62
0.52
3.4. Lubricating Mechanism Analysis
The lubrication models of the pure PAO6 oil and the Sc-Au/GO dispersed oil are illustrated in
Figure 12. In case of lubrication with the neat PAO6 oil, two contact surfaces scratch with each
other and many abrasive particles are produced because of the friction force. The stiff abrasive
particles slide under high friction stress, leading to the contact surface becoming extremely rough.
Once the surface roughness exceeds the oil film thickness, the dry contact will happen, and wide
and deep grooves and furrows are formed on the wear surface (Figures 8a, d, g and Figures 9a, b).
But as the Sc-Au/GO nanocomposites are added into neat oil, the nanocomposites with oil can
penetrate into the interface of contact pairs and gradually deposit and accumulate in original and
eventually produced pits and grooves. Quickly, these defects are repaired and the contact
surfaces become flat and smooth (Figures 8c, f and Figure 9e), resulting in a decrease in
frictional force. The surface roughness of the rubbing surfaces is largely decreased compared
with that lubricated with neat PAO6 oil. Finally, the deposited nanocomposites form block
physical deposition film and uniformly cover the rubbing surface, as confirmed by Figures 10
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and 11, which results in fewer abrasive particles produced. The reduction of abrasive particles is
helpful for maintaining the smooth contact surfaces and reducing the wear volume of friction
pairs. Moreover, the modified Sc-Au/GO can absorb base oil, which thickens the oil film and
prevents the friction pairs from direct contact. As a result, the lubrication state in the Sc-Au/GO
dispersed oil has transferred to good boundary lubrication from the mixed lubricating in the pure
PAO6 oil, which leads to a significant improvement in friction reduction and antiwear ability. In
addition, the components of Sc-Au/GO, i.e., Au nanoparticles and GO sheets, both are good
lubricating nanomaterials and show a synthetic lubricating effect. The GO nanosheets are
exfoliated by the decorated Au nanoparticles,36,38 so the interlamination sliding becomes easy,
which can reduce the friction force effectively. A few GO sheets may be deformed and ruptured
by friction force and heat after a period of sliding time,25,27 and then the dotted Au nanoparticles
are exposed and released. The released Au nanoparticles can repair the ruptured film and thus
continue to improve the durability of deposition film.
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Figure 12. Schematic of the lubricating models of the pure PAO6 oil and the Sc-Au/GO
dispersed oil for the ball-on-disc friction tester.
2. CONCLUSIONS
In summary, Au nanoparticle decorated-graphene oxide nanocomposite (Sc-Au/GO) has been
successfully prepared by a facile chemical reduction in the supercritical carbon dioxide (ScCO2)
fluid. The anchored Au nanoparticles show narrow grain size range from 4 to 10 nm and uniform
distribution on GO sheet surfaces due to the unique properties of ScCO2. The as-prepared
Sc-Au/GO is highly dispersed in the neat PAO6 oil as lubricating additive after modifying with
oleylamine. The friction coefficient and wear rate are reduced up to 33.6% and 72.8%
respectively, as traces of Sc-Au/GO (0.10 wt. %) were added in the neat oil. The
friction-reducing and antiwear abilities of Sc-Au/GO are superior to the contrastive additives of
GO, Au nanoparticles, and Au/GO produced by traditional method. During the sliding process,
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the nanocomposite of Sc-Au/GO deposits on contact surfaces and forms block physical
protective film, which can significantly smooth the contact surface and reduce the roughness.
Meanwhile, the Sc-Au/GO can absorb oil molecules, which is beneficial to forming good
boundary lubricating state on the sliding surface. In addition, the Au nanoparticles and the GO
nanosheets in the Sc-Au/GO present synergistic lubricating effect, leading to the superior
lubricating performances as compared to that of the single components (GO and nano-Au) of the
nanocomposite. The deposition protective film as well as the synergistic lubricating effect
corresponds to the superior lubricating performances of the Sc-Au/GO dispersed oil. The
findings here not only provide a new type of excellent lubricating materials, but also can shed
light on preparing nanoparticle/graphene oxide nanocomposites with remarkable properties
readily for potential industrial applications.
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]
ORCID
Fenghua Su: 0000-0002-6953-4663
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
The authors are grateful to the financial support of the National Natural Science Foundation of
China (Nos. 21473061 and 51575191), the Guangdong Natural Science Funds for Distinguished
Young Scholar (grant: 2015A030306026), and the Science and Technology Planning Project of
Guangdong Province (grant: 2016A010102009).
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