Tribological properties of naphthyl phenyl diphosphates as antiwear additive in polyalkylene glycol and polyurea grease for steel/steel contacts at elevated temperature

Article abstract of DOI:10.1039/c3ra46591h

Naphthyl phenyl diphosphates: 1-naphthyl diphenyl phosphate (NDP) and 1,5-dihydroxynaphthalene bis(diphenyl phosphate) (DDP) were evaluated as the antiwear additives in polyalkylene glycol and polyurea grease at 200 °C. Results showed that they could effe

Full text of DOI:10.1039/c3ra46591h

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Tribological properties of naphthyl phenyl
diphosphates as antiwear additive in polyalkylene
glycol and polyurea grease for steel/steel contacts
at elevated temperature
Cite this: RSC Adv., 2014, 4, 6074
*
Xinhu Wu, Xiaobo Wang and Weimin Liu
Naphthyl phenyl diphosphates: 1-naphthyl diphenyl phosphate (NDP) and 1,5-dihydroxynaphthalene
bis(diphenyl phosphate) (DDP) were evaluated as the antiwear additives in polyalkylene glycol and
polyurea grease at 200 ꢀC. Results showed that they could effectively reduce the friction and wear of
sliding pairs compared with the cases without these additives. Furthermore, the tribological properties of
NDP and DDP were generally better than the normally used tricresyl phosphate (TCP) in PAG and
molybdenum disulfide (MoS2) in polyurea grease. Boundary lubrication films composed of Fe(OH)O,
Fe3O4, FePO4 were formed on the worn surface, which resulted in excellent friction reduction and
antiwear performance.
Received 12th November 2013
Accepted 18th December 2013
DOI: 10.1039/c3ra46591h
www.rsc.org/advances
has been used as an antiwear additive in jet oils for many years.1
Hydroquinone bis(diphenyl phosphate) (HDP) exhibited excel-
1. Introduction
lent friction-reducing and anti-wear performance in poly-
alkylene glycol (PAG) at 200 ꢀC.12 The excellent tribology
performances of phosphate esters can be explained that phos-
phorus containing additives can be lm-forming on the metal
surfaces at ꢁ200 ꢀC,13 and provide an extremely low friction
coefficient between 250 and 550 ꢀC.14 The lm can cause a
smoothing of the metal surface that is then able to proving a low
shear strength boundary layer. Consequently, phosphate esters,
especially that have advantages of high molecular weight and
high thermal and chemical stability as well as very low vapor
pressure, could be potential candidates as high temperature
lubricating additives.
1-Naphthyl diphenyl phosphate (NDP) and 1,5-
dihydroxynaphthalene bis(diphenyl phosphate) (DDP) are
halogen free, phosphorus based ame retardants and have
advantages of high molecular weight and low volatility,
indicating that they might be promising candidates as AW
additives under high temperature in lubricating additives.15
However, the existing studies of NDP and DDP are on the
effects of ame retardant,15 there have no studies on the
tribological properties of NDP and DDP, specically as AW
additives under harsh friction conditions. In the present
paper, the tribological properties of NDP and DDP in poly-
alkylene glycol and polyurea grease for steel/steel contacts
were evaluated on an Optimal SRV-IV oscillating friction and
wear tester at 200 ꢀC. The morphology and chemical
composition of worn surface were studied by SEM and XPS in
order to explore the lubrication mechanism of NDP and DDP
under high temperature.
With rapid advances in aircra, turbines, automobiles, trucks,
farm equipment, railroad equipment, industrial machinery,
etc., there is an increasing demand for lubricants that can
successfully operate under conditions of high loads, high
speeds and high temperatures.1–6 For instance, the working
temperature of aviation gas turbine engine oils oen exceeds
200 ꢀC. Under such high temperature, conventional lubricating
oils are subjected to such hot operating conditions, decompo-
sition of the petroleum ingredient eventually occurs, with the
formation of end products which are hard, abrasive, carbona-
ceous or coke-like materials which reduce clearance between
rubbing members and tend to promote wear. To address these
problems, various high-performance base uids or base greases
such as polyalpha olens (PAO), polyalkylene glycol (PAG),
lithium complex soap greases and polyurea greases, etc. were
developed to meet the demand for high temperature lubri-
cants.7–11 However, only a few high temperature lubricant
additives especially anti-wear (AW) additives were commercially
available, which limits the formulation of high temperature
lubricants. Thus it is essential to do some research on lubri-
cating additives under high temperature.
Many different types of phosphate esters have been exam-
ined as additives for lubricating oils or greases, with most
attention given to their potential as antiwear (AW) additives at
elevated temperature. For example, tricresyl phosphate (TCP)
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics,
Chinese Academy of Sciences, Lanzhou 730000, People's Republic of China. E-mail:
wangxb@lzb.ac.cn
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Table 1 Performance parameters for polyurea grease
2. Experimental section
2.1. Chemicals
Result
The chemical structures of NDP and DDP are given in Fig. 1,
which were synthesized using previously reported method.16
Diphenyl chlorophosphate ($99.0%), 1-naphthol ($99.0%) and
1,5-dihydroxynaphthalene ($98.0%) were obtained from J & K
Chemical. Tricresyl phosphate (TCP, $99.0%) were purchased
from Tianjin Chemical Reagent no. 1 Plant. Molybdenum
disulde (MoS2, particle size of 0.5 micron) was also commer-
cially obtained from Shanghai Shen Yu Industry & Trade Co.,
Ltd. All the chemicals were of the AR grade and utilized in this
work without further purication.
Item
Polyurea grease Test method
Work penetration (1/10 mm)
Dropping point (ꢀ)
320
290
1b
GB/T 269
GB/T 498
GB/T 7326
Copper corrosion (T2 copper,
100 ꢀC, 24 h)
Steel mesh sub-oil (100 ꢀC, 24 h) (%)
Evaporative capacity (99 ꢀC, 22 h) (%)
3.64
0.30
SH-T 0324
GB/T 7326
SH/T0048
SH/T0109
Similar viscosity (ꢂ10 ꢀC, 10 sꢂ1) (Pa S) 450
Shuilin loss of volume (38 ꢀC, 1 h) (%) 1.5
Synthesis of 1-naphthyl diphenyl phosphate (NDP) and 1,5-
dihydroxynaphthalene bis(diphenyl phosphate) (DDP): 1.44 g
(10.0 mmol) 1-naphthol, 2.68 g (10.0 mmol) diphenyl chlor-
ophosphate, 0.2 g AlCl3 and 50 mL toluene were introduced into
a dry 100 mL four-necked ask with a thermocouple, a dip tube
for N2 purge, a product condenser and a vacuum receiver. The
mixture was heated to 100 ꢀC while keeping the ask devoid of
moisture. Aer being stirred at this temperature for 20 hour,
toluene was removed and the resultant precipitate was washed
repeatedly with water and acetonitrile. A white powder 3.2 g
(yield of 85.0%) was obtained aer drying under vacuum at
80 ꢀC, until a constant weight. DDP was synthesized using same
method and a grey white powder product was obtained (yield of
89.0%). The structures of the prepared compounds were char-
acterized by infrared spectroscopy (FTIR), proton nuclear
magnetic resonance (1H-NMR, 400 MHz) and carbon nuclear
magnetic resonance (13C-NMR, 100 MHz) spectroscopy.
laboratory. Its typical properties are listed in Table 1. The base
grease and additives with different concentrations were mixed
thoroughly prior to the tests. Briey, the grease was combined
with 3 wt% of additives. For full dissolution, 97 wt% of the
polyurea grease was then added to the additivated grease by
agitation with a mechanical stirrer and nely ground three
times in a three-roller mill.
2.2. Thermal analysis
The thermal behaviours of the sample were studied on a STA
449C Jupiter®-simultaneous thermo-gravimetry and differential
scanning calorimetry (TG-DSC). A total of 5 mg of sample was
placed in the TGA sample holder. The temperature was pro-
grammed to increase from ambient temperatures to approxi-
mately 800 ꢀC, at a heating rate of 10 ꢀC minꢂ1 in air. The weight
loss was monitored in the TG-DSC analysis.
NDP: FTIR (KBr disc): 3070, 1590, 1490, 1231, 1205, 1130,
1022, 960, 773, 684, 565, 501 cmꢂ1 1H-NMR (DMSO-d6, 400
.
2.3. Tribology test
MHz), (d/ppm): 6.97 to 8.20. 13C-NMR (DMSO-d6, 100 MHz), (d/
ppm): 152.57, 152.51, 150.64, 150.34, 134.77, 129.84, 127.71,
126.43, 125.84, 125.60, 125.56, 125.33, 124.41, 121.44, 120.82,
120.45, 119.99, 115.41, 115.75.
The Optimol SRV-IV oscillating reciprocating friction and wear
test was performed with a ball-on-disk conguration. The upper
ball (diameter 10 mm, AISI 52100 steel, hardness of approxi-
mately 58–60 HRC) slides reciprocally against the lower
stationary disk (ø 24 mm ꢃ 7.9 mm, AISI 52100 steel). All the
DDP: FTIR (KBr disc): 3060, 1590, 1490, 1401, 1310, 1238,
1211, 1192, 1061, 990, 966, 938, 782, 756, 684, 567, 507, 481, 410
cmꢂ1. 1H-NMR (DMSO-d6, 400 MHz), (d/ppm): 6.75 to 7.83, 4.18,
1.30, 0.86. 13C-NMR (DMSO-d6, 100 MHz), (d/ppm): 152.50,
150.36, 150.29, 146.16, 146.09, 130.79, 129.80, 129.27, 128.67,
127.47, 126.69, 124.04, 122.32, 120.49, 120.44, 119.23, 117.0.
The selected lubricating base uid PAG was obtained from
HENUVAR Chemical Corporation and its typical properties was
listed in previous work.12 Polyurea grease was obtained from our
Fig. 1 Molecular structures of NDP of DDP.
Fig. 2 TGA curves of NDP and DDP in air atmosphere.
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SRV tests were conducted at 200 ꢀC, 25 Hz oscillation frequency, PHI-5702 multifunctional X-ray photoelectron spectrometer,
1 mm amplitude, for 30 min test duration, and the relative using Al Ka radiation as the exciting source. The binding
humidity is 20–40 wt%. The corresponding friction curves were energies of the target elements were determined at a pass
recorded automatically with a charter attached to the SRV test energy of 29.35 eV, with a resolution of about ꢄ0.3 eV, using
rig. The volume losses from lower discs were measured using a the binding energy of contaminated carbon (C 1s, 284.8 eV) as
MicroXAM-3D non-contact surface mapping microscope pro- ref. 17.
lometer. Three repetitive measurements were performed for
each wear of the discs, and the averaged values are reported in
this paper.
3. Result and discussion
The morphologies of the worn surfaces were analyzed by a 3.1. Thermal stability
JSM-5600LV scanning electron microscope (SEM). The X-ray
photoelectron spectrometry (XPS) analysis was carried out on a
seen that the two additives does not exhibit any weight loss
The TGA curves of NDP and DDP are presented in Fig. 2. It is
below 200 ꢀC and the decomposition temperatures for NDP and
DDP are 422.4 and 465.8 ꢀC, respectively. Furthermore, the
ꢀ
temperature for total decomposition is over 800 C, indicating
the high thermal stability. DDP exhibited better thermal
stability than NDP.
3.2. Tribological properties of NDP and DDP as additives in
PAG
ꢀ
The tribological properties of NDP and DDP in PAG at 200 C
were rst investigated. The coefficient of friction evolution and
wear volume of sliding discs are shown in Fig. 3. It is seen that
pure PAG has a relatively large friction coefficient, while the
Fig. 3 (a and b) Evolution of friction coefficient with time at 100 N for
PAG plus NDP and DDP additives at different concentrations; (c) wear Fig. 4 (a) Friction coefficient and (b) wear volume losses of the discs
volume losses of the steel discs lubricated by PAG with NDP and DDP at lubricated by PAG plus 0.4 wt% TCP, 0.4 wt% NDP and 0.4 wt% DDP
different concentrations after the constant load tests. (SRV load, 100 N; at 200 ꢀC; (SRV load, 100 N; stroke, 1 mm; frequency, 25 Hz; duration,
stroke, 1 mm; frequency, 25 Hz; duration, 30 min; temperature, 200 ꢀC). 30 min).
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Fig. 5 (a) Evolution of friction coefficient with time during a load ramp test from 100 to 500 N for PAG with different additives at 200 ꢀC; (b) wear
volume losses of the steel discs lubricated by PAG with different additives at 200 ꢀC. (load, 100–500 N; stroke, 1 mm; frequency, 25 Hz).
addition of 0.2 wt% NDP and DDP can dramatically reduce the
Fig. 5 shows a load ramp test stepped from 100 N up to 500 N
friction coefficient (Fig. 3a and b). When the concentration by 100 N intervals at a frequency of 25 Hz for PAG + 0.4 wt%
increases to 0.4 wt%, the friction coefficient decreases to a TCP, PAG + 0.4 wt% NDP and PAG + 0.4 wt% DDP at 200 ꢀC. The
comparatively low level. No signicant improvement can be test duration for each load is 5 min. We can see that the base oil
observed aer 0.5 wt% concentration. Meanwhile, the AW exhibits relatively high friction coefficient and the friction
properties of 0.4 wt% NDP and 0.4 wt% DDP increased by coefficient uctuated a few times. For 0.4 wt% NDP and 0.4 wt%
79 and 102 times as compared with the base oil (Fig. 3c), DDP, in the initial friction stage, the two additives may form
respectively. It shows the best friction reduction and AW boundary lubricating lms and the curve shapes were stable for
capability when the additives concentration of NDP and DDP the initial 400 s and 600 s, respectively (Fig. 5a). Then the
reach 0.4 wt% under the testing conditions. Moreover, as boundary lms might be worn away and could not complement
shown in Fig. 4, 0.4 wt% NDP and DDP in PAG can signicantly rapidly probably because of high loads, the friction coefficient
improve the friction reduction and antiwear properties of suddenly increased to 0.160, and aerward stabilized at the
PAG compared to base oil containing 0.4 wt% TCP, a very value equal to that of pure PAG. However, TCP as comparison
popular phosphorus containing additive, this result indicates exhibits relatively high friction coefficient all the time under the
that both of NDP and DDP are excellent additives of PAG, same conditions. Fig. 5b displays the wear volumes of the
exhibiting outstanding friction reduction and AW properties, lubricants increased in the following sequence: DDP < NDP <
especially during the constant load test at elevated TCP < PAG. The ability of NDP and DDP to stabilize friction is
temperature.
not that prominent, but the enhancement of the AW property is.
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Fig. 7 (a) Friction coefficient and (b) wear volume losses of the discs
lubricated by PG plus 4 wt% MoS2, 4 wt% NDP and 4 wt% DDP at 200 ꢀC;
(SRV load, 100 N; stroke, 1 mm; frequency, 25 Hz; duration, 30 min).
1–5 wt% NDP and DDP in polyurea grease. This is under the
same test conditions (at a constant load of 100 N, a temperature
of 200 ꢀC and the frequency was 25 Hz) as that in Fig. 3. Fig. 7
shows the variation in the friction coefficients and wear
volumes of the sliding discs under lubrication of polyurea
grease plus 4 wt% NDP and 4 wt% DDP at a constant load of 100
N and 200 ꢀC. and their tribological behavior were compared
with MoS2, which is one of the traditional AW additives for
lubricating greases.19,20 The results showed that the friction
coefficients of the additives and polyurea grease increased in
the following sequence: DDP < NDP < polyurea grease < MoS2,
and the wear volumes of the discs increased in the following
sequence: DDP < NDP < MoS2 < polyurea grease. The results
indicate that NDP and DDP as the additives in polyurea grease
have better friction reduction and AW properties than MoS2,
and the poor friction reduction performance of MoS2 might be
Fig. 6 (a and b) Evolution of friction coefficient with time at 100 N for
polyurea grease plus NDP and DDP at different concentrations; (c)
wear volume losses of the steel discs lubricated by polyurea grease
with NDP and DDP at different concentrations after the constant load
tests. (SRV load, 100 N; stroke, 1 mm; frequency, 25 Hz; duration, 30
min; temperature, 200 ꢀC).
This result further proves the friction-reducing and antiwear relevant to the oxidation of itself at high temperature and the
proprieties of NDP and DDP.
products of MoS2 oxidation can cause an increase of friction.21
Furthermore, it is clearly seen that the effect of additive
concentration on tribological properties of PAG and polyurea
grease is very different. In PAG, the friction is more sensitive to
3.3. PDP and TDP as additives for polyurea grease (PG)
In modern-day engineering, polyurea greases are increasingly the additives at low concentration and signicant friction
and widely used. This is because they have many excellent reduction and antiwear performance occur. This is in contrast
properties, such as high oxidation stability, high-temperature to that in polyurea grease, where the optimum concentration is
performance, and high mechanical stability.18 Fig. 6 displays 4 wt%. The possible reason might be relevant to the freedom of
the effect of the concentration on the tribological behavior of additives in PAG and polyurea grease. In PAG, the freedom of
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Fig. 8 (a) Evolution of friction coefficient with time during a frequency ramp test from 15 to 40 Hz for PG with different additives at 200 ꢀC. (b)
Wear volume losses of the steel discs lubricated by PG plus 4 wt% MoS2, 4 wt% NDP and 4 wt% DDP at 200 ꢀC. (load, 200 N; stroke, 1 mm;
frequency, 15–40 Hz).
Fig. 9 SEM morphologies of worn surfaces lubricated by PAG and different additives: (a and b) PAG, (c and d) 0.4 wt% NDP, and (e and f) 0.4 wt%
DDP (magnification: on the left is 80ꢃ, and on the right is 500ꢃ; load ¼ 100 N; stroke ¼ 1 mm; frequency ¼ 25 Hz; duration ¼ 30 min).
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Fig. 10 SEM morphologies of worn surfaces lubricated by polyurea grease and different additives: (a and b) PG, (c and d) 4 wt% NDP, and (e and f)
4 wt% DDP (magnification: on the left is 80ꢃ, and on the right is 400ꢃ; load ¼ 100 N; stroke ¼ 1 mm; frequency ¼ 25 Hz; duration ¼ 30 min).
additive molecules is fairly large and they can form boundary 200 ꢀC. The test duration for each frequency was 5 min. It is
lm at interface more rapidly once boundary lm is swept away, seen that 4 wt% MoS2, 4 wt% NDP and 4 wt% DDP also expe-
whereas in polyurea grease, the additive molecules at interface rienced a running-in time with larger friction coefficients,
cannot be supplied timely due to the non-owing character and aerward they could reduce friction and rendered a stable and
restricted motion of additive molecules.
smaller friction coefficient (Fig. 8a). Polyurea grease exhibits
Fig. 8 shows frequency ramp test amplied from 15 Hz up to much high friction coefficient (>0.5) under the same conditions,
40 Hz in 5 Hz, intervals. The test was conducted under a load of and so the test had to be stopped aer 9 min. Fig. 8b demon-
200 N for the three kinds of additives in polyurea grease at strates that wear volumes increased in the following sequence:
Fig. 11 XPS spectra of Fe 2p, O 1s, P 2p and N 1s of the worn surfaces lubricated by (a) PAG + 0.4 wt% NDP, (b) PAG + 0.4 wt% DDP, (c) PG + 4 wt%
NDP and (d) PG + 4 wt% DDP at 200 ꢀC. (SRV load, 100 N; stroke, 1 mm; frequency, 25 Hz; duration, 30 min).
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correspond to Fe(OH)O.26,27 Fig. 11c and d display the N1s
spectra of the worn surface lubricated by polyurea grease plus
4 wt% NDP and 4 wt% DDP at 401.0 eV, possibly corresponding
to nitrogen oxide or nitrogen double bond compounds.24,27 XPS
peak of P2p (Fig. 8a–d) appearing at 133.7 eV may correspond to
FePO4.12,24,28
From the above explanations, it is suggested that chemical
reactions occurred on the wear surface in the frictional process
with the generation of a surface-protective lm composed of
Fe(OH)O, Fe3O4, FePO4 and compounds containing nitrogen
oxide, etc., which signicantly contribute to the friction-
reducing and antiwear properties of NDP and DDP in PAG and
Table
measurements
2 Binding energies of typical elements from the XPS
Binding energy (eV)
Fe
Additive
O
P
N
a
b
c
PAG + 0.4 wt% NDP 712.1, 725.5
PAG + 0.4 wt% DDP 711.7, 725.5
PG + 4 wt% NDP
PG + 4 wt% DDP
531.5 133.5
531.7 133.6
711.7, 725.5
711.6, 725.5
531.7 133.6 401.1
531.5 133.5 401.0
d
4 wt% DDP < 4 wt% NDP < 4 wt% MoS2 < polyurea grease. This polyurea grease at elevated temperature.
is consistent with the result shown in Fig. 7. Both the friction
reduction and AW wear capability of DDP was prominent
during the test. These results verify DDP as additive in the
4. Conclusions
polyurea grease both in the constant load and variable Two naphthyl phenyl diphosphates: 1-naphthyl diphenyl phos-
frequency tests have better tribological performance for steel/ phate (NDP) and 1,5-dihydroxynaphthalene bis(diphenyl phos-
steel contacts at 200 ꢀC. This may be attributed to high thermal phate) (DDP) were synthesized and their tribological properties
stability, and the capability to form effective boundary lms.22,23 have been studied as high temperature friction-reducing and
ꢀ
However, the ability of NDP to reduce friction and wear in anti-wear additives in PAG and polyurea grease at 200 C. The
polyurea grease is not that prominent, the possible reason friction and wear test results show that the two additives have
might be relevant to the differences of thermal stability, phos- excellent friction reducing and antiwear properties for the
phorus content and molecular polarity between NDP and DDP. lubrication of steel/steel contacts. This may be attributed to high
thermal stability, and the capability to form effective boundary
lms. Furthermore, the tribological properties of NDP and DDP
were generally better than the normally used tricresyl phosphate
3.4. Surface analysis
Fig. 9 and 10 display the SEM micrographs of the worn surfaces (TCP) in PAG and MoS2 in polyurea grease. XPS analysis indi-
of steel discs lubricated by different lubricants at 100 N and 200 cated that the good tribological performance of NDP and DDP
ꢀC. It is clearly seen that the worn surfaces of steel discs lubri- are attributed to the formation of a boundary lubrication lm
cated by pure PAG and PG exhibit considerably wider and composed of Fe(OH)O, Fe3O4 and FePO4. The lm was generated
deeper wear scars, with a number of deep and narrow grooves on the worn surface, leading to good friction reduction and AW
(PAG: Fig. 9a and b; PG: Fig. 10a and b). Thus, severe scuffing performance at high temperature.
occurs in these cases. The addition of 4.0 wt% NDP in PG can
make the worn surface of the steel becomes relatively narrow
and shallow, showing that NDP has some certain AW property
Acknowledgements
(Fig. 10c and d). When 4.0 wt% DDP is added into PG, not only The authors are thankful for nancial support of this work by
does the width of wear scars become smaller in size, but the “973” Program (2013CB632301).
abrasions are also fewer and smoother (Fig. 10e and f). This is
consistent with previously measured wear volume. In marked
contrast, the wear scars of the steel discs lubricated by 0.4 wt%
References
NDP and DDP in PAG are much narrower and shallower, and
scuffing is greatly alleviated, indicating a signicantly improved
antiwear behavior by simple addition of NDP and DDP (PAG +
NDP: Fig. 9c and d; PAG + DDP: Fig. 9e and f). These results are
consistent with previously measured wear volume results and
indicate undoubtedly the excellent anti-wear properties of NDP
and DDP as additives for PAG.
To gain further insight into the lubricating mechanism of
NDP and DDP, Fig. 11 presents the XPS spectra and Table 2
gives the atomic percent on the worn steel surfaces lubricated
by 0.4 wt% NDP + PAG, 0.4 wt% DDP + PAG, 4 wt% NDP + PG
and 4 wt% DDP + PG, respectively. It can be seen that all peaks
of Fe2p (Fig. 11a–d) appear at approximately 711.6 and 725.5 eV,
which may correspond to Fe(OH)O and Fe3O4.24,25 Almost no
difference is observed in XPS peaks of O1s (Fig. 11a–d) on the
worn surfaces. The peak of O1s appears at 531.5 eV, which may
1 R. M. Mortier, M. F. Fox and S. T. Orszulik, Chemistry and
Technology of Lubricants, Springer, New York, 2010.
2 H. K. Trivedi, N. H. Forster and C. S. Saba, Tribol. Lett., 2004,
16, 231.
3 M. J. Neale, The Tribology Handbook, Butterworth-
Heinemann, Oxford, 1995.
4 B. K. Sharma, A. Adhvaryu, J. Perez and S. Z. Erhan, J. Agric.
Food Chem., 2006, 54, 7594.
5 C. J. Boner, Manufacture and Applications of Lubricating
Greases, Krieger, R. E. Publishing Co, Huntingdon, NY, 1971.
6 C. J. Boner, Modern lubricating greases, Scientic
Publications, England, 1976.
7 L. R. Rudnick and R. L. Shubkin, Synthetic Lubricants And
High-Performance Functional Fluids, Revised And Expanded,
Taylor & Francis, New York, 1999.
8 B. Cavdar, Wear, 1997, 206, 15.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 6074–6082 | 6081

View Article Online
RSC Advances
Paper
9 M. Masuko, S. Hirose and H. Okabe, Lubr. Eng., 1996, 52, 641. 20 J. P. Sheth, E. Yilgor, B. Erenturk, H. Ozhalici, I. Yilgor,
10 J. Kang and Y. Z. Zhao, Synth. Lubr., 2005, 2, 25. L. Garth and G. L. Wilkes, Polymer, 2005, 46, 8185.
11 T. Q. Guo, M. J. Jiang, X. C. Guo, C. S. Li, K. S. Cheng and 21 Y. Epshteyn and T. J. Risdon, Presented at the 12 Lubricating
S. H. Liu, Lubr. Eng., 2006, 1, 164.
12 G. Q. Zhao, X. H. Wu, W. M. Li and X. B. Wang, Ind. Eng.
Chem. Res., 2013, 52, 7419.
Grease Conference, NLGI-India Chapter, January 2010, pp.
28–30.
´
´
22 A. Hernandez Battez, R. Gonzalez, J. L. Viesca, D. Blanco,
13 R. Mandakovic, J. Synth. Lubr., 1999, 16, 13.
E. Asedegbega and A. Osorio, Wear, 2009, 266, 1224.
14 L. R. Rudnick, Lubricant Additives Chemistry and Applications, 23 H. Kamimura, T. Kubo, I. Minami and S. Mori, Tribol. Int.,
2nd edn, RSC, New York, 2009. 2007, 40, 620.
15 K. Tamura and C. Wang, U.S. Patent 2004254390A1, 16 24 M. R. Cai, Y. M. Liang, F. Zhou and W. M. Liu, ACS Appl.
December 2004. Mater. Interfaces, 2011, 3, 4580.
16 J. Zilberman, D. Can and A. Gregor, U.S. Patent 25 National Institute of Standards and Technology, NIST X-ray
20120022193 A1, 26 January 2012.
Photoelectron Spectroscopy Database, version 4.1, 2012,
17 J. F. Moulder, W. F. Stickle, P. E. Sobol and K. D. Bomben,
http://srdata.nist.gov/xps/.
Handbook of X-ray Photoelectron Spectroscopy, Perkin Elmer, 26 Y. B. Sun, L. T. Hu and Q. J. Xue, Wear, 2009, 266, 917.
Eden Prairie, MN, 1992.
18 F. L. Yu, R. L. Zhang, C. X. Xie and S. T. Yu, Tetrahedron,
2010, 66, 9145.
27 M. R. Cai, Y. M. Liang, M. H. Yao, Y. Q. Xia, F. Zhou and
W. M. Liu, ACS Appl. Mater. Interfaces, 2010, 2, 870.
28 M. H. Yao, Y. M. Liang, Y. Q. Xia and F. Zhou, ACS Appl.
Mater. Interfaces, 2009, 1, 467.
19 C. Roland, J. Twigg, Y. Vu and P. Mott, Polymer, 2007, 48, 574.
6082 | RSC Adv., 2014, 4, 6074–6082
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