Tribology International 53 (2012) 150–158
Contents lists available at SciVerse ScienceDirect
Tribology International
journal homepage: www.elsevier.com/locate/triboint
Comparing tribological behaviors of sulfur- and phosphorus-free
organomolybdenum additive with ZDDP and MoDTC
Lili Yan a,n, Wen Yue a, Chengbiao Wang a, Danping Wei b, Bo Xu c
a
School of Engineering and Technology, China University of Geosciences, Beijing 100083, China
Research Institute of Petroleum Processing, SINOPEC, Beijing 100083, China
c
School of Material Science & Engineering, Beijing Institute of Technology, Beijing 100081, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 November 2011
Received in revised form
7 March 2012
Accepted 4 April 2012
Available online 13 April 2012
A new kind of sulfur- and phosphorus-free organomolybdenum oil-based additive N, N-bis
(2-hydroxyethyl)-dodecanamide molybdate (NNDM) was prepared. Its tribological performances as
additive in base oil 150SN were examined on a four-ball tester, and compared with those of ZDDP and
MoDTC under boundary lubrication condition. The tribofilm NNDM generated on the worn surface was
analyzed by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Results
showed that NNDM blend oil exhibited excellent load-carrying capacity, significantly reduced friction
coefficient and wear rate of worn surface, which could be attributed to high amount of long-chain
alkylamide and MoOx in NNDM tribofilm.
& 2012 Elsevier Ltd. All rights reserved.
Keywords:
Sulfur- and phosphorus-free
Organomolybdenum additive
Lubrication
Tribological behavior
Introduction
It is well known that the addition of an appropriate additive to
an oil-based lubricant can effectively increase the mechanical
efficiency, decrease the energy consumption, and reduce friction
and wear of machinery equipment. And more importantly, with
the consideration of environment and people’s health, to reduce
or eliminate some harmful elements like phosphorus, sulfur and
ash in the lubrication additives is an urgent direction of developing environment-friendly and efficient additives [1–4]. Moreover,
it is believed that sulfur and phosphorus (SP) oxides and metallic
ash formed in the engine oil can reduce catalyst effectiveness and
block filters, thereby degrading the exhaust after-treatment
system during extended engine operation. Because of this, the
International Lubricant Standardization and Approval Committee
(ILSAC) proposed the GF-4 Performance Standards to limit the
concentrations of phosphorus (0.08% maximum) and sulfur (0.50%
maximum) in the finished passenger car engine oil [5,6].
The additive zinc dialkyl dithiophosphate (ZDDP), a typical
additive containing sulfur, phosphorus and zinc, is used almost
universally in engine oils as an anti-oxidant and anti-wear agent.
It can form a glassy phosphate film on the surface of Fe-based
materials to reduce wear and friction [7–10]. Another kind of
widely used additive is molybdenum dialkyldithiocarbamate
n
Corresponding author. Tel.: þ086 10 82320255; fax: þ 86 10 82322624.
E-mail addresses: [email protected] (L. Yan), [email protected] (W. Yue).
0301-679X/$ - see front matter & 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.triboint.2012.04.002
(MoDTC), which can effectively reduce wear and friction in the
boundary lubrication regime and promote the fuel economy in
engine oils through the formation of tribofilms containing primarily MoS2 and other molybdenum oxides [11–14]. It should be
recognized that a lubricant additive containing active elements
such as phosphorus, sulfur and some metal elements can always
provide good lubricating properties. More generally, it is considered that sulfur provides some extreme pressure property to
smooth rubbing surface and is valuable in preventing scuffing or
severe wear in high-sliding parts of an engine such as slider
followers, and phosphate provides protection against mild wear
in the high-temperature piston and bearing regions as well as
parts of the cam system [15,16]. However, once a lubricant
additive contains low or zero phosphorus and sulfur, its antiwear and friction-reducing properties must be intensively influenced. Thus, a great deal of research efforts has been recently
devoted to the development of new environment-friendly additives with low- or zero-SP but without reducing the wear and
friction performances [16]. Research on new additives was mainly
on the compounds containing boron [17–19], nitrogen [20–22]
and organometals [7,23,24]. However, organoboron additives
without SP showed shortages in anti-wear and hydrolysis-resistance. Nitrogen heterocyclics without SP exhibited good wear resistance and corrosion inhibition but poor friction reduction. Copper
nanoparticles were also studied as potential anti-wear agents especially in the last decade, but possessed bad dispersive ability [25,26].
They all still cannot be used to take the place of the traditional
phosphors- and sulfur-containing additives. The zero-sulfur and
L. Yan et al. / Tribology International 53 (2012) 150–158
phosphorus organic molybdenum compound has shown its better
friction-reducing properties and less corrosion [27]. Hu et al. have
synthesized organic molybdate ester without SP, and reported its
good anti-wear synergism with ZDDP, i.e. friction coefficient 0.055–
0.08 and WSD 0.4–0.5 mm; but molybdate ester alone was not
effective in reducing friction and wear [28]. Liu et al. have prepared
molybdenum coordination compound without SP, and reported its
friction-reducing and anti-wear abilities at different loads using a
SRV test rig with ball-on-disk configuration [29]. It is worth to note
that the ability of the lubricants carrying oxygen to the interface
strongly affects their friction and wear behavior, since oxygencontaining compounds absorb through the polar oxygen-carrying
group and react with rubbing surfaces and thus oxygen is carried to
the interface to change the surface films [30,31]. Our author Wei has
reported that nitrogen-containing heterocyclics with oxygen atoms
attached to the ring structure were particularly effective in reducing
wear [21,22]. This paper aims to design and explore a new kind of
sulfur- and phosphorus-free organic molybdenum compound with a
ring structure containing oxygen and nitrogen, and expects this
additive to reach the anti-wear and friction-reducing level of ZDDP
and MoDTC in the present market. This will be helpful for reducing
the use of additives containing sulfur and/or phosphorus, such as
ZDDP and MoDTC.
In this paper, a new kind of sulfur- and phosphorus-free
(SP-free) organic molybdenum compound was prepared, in which
a nitrogen atom, two oxygen atoms and a molybdenum atom
were introduced to the ring structure of the molecule. Low-price
molybdenum trioxide and dodecanoic acid were selected to
prepare the SP-free organic molybdenum compound N, N-bis
(2-hydroxyethyl)-dodecanamide molybdate (coded as NNDM). Its
tribological performances as additive in base oil 150SN were
compared with those of ZDDP and MoDTC under boundary
lubrication, and then the composition and compound states of
tribofilms on the worn surfaces were analyzed by modern surface
analysis instruments. Based on the results the tribological mechanism of NNDM was discussed as well.
Experimental details
Preparation of the N, N-bis (2-hydroxyethyl)-dodecanamide
molybdate (NNDM)
AR grade chemicals (dodecanoic acid, chlorine sulfide, dichloromethane, diethanolamine, molybdenum trioxide) were used
for the synthesis of N, N-bis (2-hydroxyethyl)-dodecanamide
molybdate (NNDM). The synthetic routine is shown in Scheme 1,
containing three steps. Progress of all the reactions was monitored
by thin-layer chromatography (TLC), using CCl4–Et2O (4:1) as eluent
and GC. In step (1), 0.1 mol dodecanoic acid was put into a 250 ml
151
bottom rounded flask and then was dissolved in chlorine sulfide to
form a solution, which was stirred at room temperature and
monitored by TLC. The unreacted chlorine sulfide was removed by
a
rotatory
evaporator,
and
then
the
intermediate
I dodecanoyl chloride was obtained. Thereafter, in step (2) the
prepared dodecanoyl chloride and 0.13 mol diethanolamine were
dissolved in alkaline dichloromethane to form a solution, whose pH
values were controlled between 9 and 10. The solution was stirred
below 0 1C, monitoring by TLC. The thin-layer chromatography
indicated that there were no dodecanoyl chloride and diethanolamine existing in the mixture, and the reacted solution was washed
by distilled water, and then was extracted by ethyl acetate to
separate the organic phase from the water phase. The obtained
organic phase was separately washed by dilute hydrochloric acid
and saturated sodium bicarbonate solution, dried with anhydrous
magnesium sulfate, and removed the volatile solvents by distillation
at vacuum. The intermediate II, N, N-bis (2-hydroxyethyl)-dodecanamide was obtained. The intermediate II was further purified
through silica gel column chromatography (v (CH2Cl2): v
(CH3OH)¼ 4:1). The corresponding pure product was detected by
liquid chromatography tandem mass spectrometry, the mobile
phase were acetonitrile and water (30:70, v:v), the detection
wavelength was 254 nm, the column temperature was 30 1C, and
the flow rate was 1.0 ml/min. The results showed the purity of the
intermediate II is above 99%. In step (3), a certain amount of toluene
as a solvent was added in a bottom-rounded flask equipped with
Dean Stark tube with condenser nitrogen atmosphere was provided
for the reactor. The intermediate II and 0.001 mol/L MoO3 aqueous
solution were mixed in the flask. The mixture was refluxed for
several hours until no more moisture of the action was produced.
Subsequently, the volatile solvents were removed by distillation at
vacuum, and the mixture was further purified to obtain the final
product NNDM, which possessed excellent oil solubility. The compound NNDM was then characterized by elementary analysis,
infrared spectroscopy (IR), nuclear magnetic resonance hydrogen
spectrum (1H-NMR) and thermogravimetric analyses (TGA).
The C, H and N contents of NNDM were evaluated by Vario EL III
element analyzer. Infrared spectrum was recorded using a PerkinElmer Spectrum GX FTIR spectrometer. 1H-NMR was obtained on a
JNM-ECA400 nuclear magnetic resonance spectrometer at 400 MHz,
using CDCl3 as solvent. The thermal stability of NNDM was measured
on a Mettler Toledo thermogravimetric and simultaneous differential
thermal combined analyzer of type TGA/SDTA851, over a temperature range from 30 to 600 1C under N2 atmosphere, at a rate of
increase of 10 1C/min.
Tribological tests
The friction and wear tests were carried out on a MS-10JR
four-ball friction and wear tester with a ball-to-ball contact
Scheme 1. Synthesis outline of the additive NNDM.
152
L. Yan et al. / Tribology International 53 (2012) 150–158
configuration. The maximum non-seizure load (PB) was obtained
according to national standards GB/T3142-82, which are similar
to the ASTM D2783 standards. The friction and wear tests were
conducted at a load of 392 N (corresponding to Hertz mean
contact stress of 2.293 GPa), linear speed of 0.461 m/s, test
duration of 60 min, at ambient temperature. A temperature
sensor is located at the bottom of the oil bath to monitor the test
temperature in real time. Lubricant temperature was increased by
the friction heat, which kept increasing during rubbing but
maintained within room temperature to 70 1C. The ratio of
minimum lubricating film thickness to surface roughness is
known as the lambda ratio, while the minimum lubricating film
thickness can be calculated using Dowson and Hamrock film
thickness equation for an elastohydrodynamic point contact
[32]. In current research work, the calculated lambda ratios
indicated all the test conditions were in the boundary lubrication
regime. During each test friction coefficients were measured and
recorded, and at the end wear scars were measured by a microscope. All the tests have been repeated for three times and a good
repeatability was recorded. The ball specimens in these tests were
made of GCr15 bearing steel (AISI52100) with diameter of
12.7 mm and hardness of 64 HRC. They were ultrasonically rinsed
with petroleum ether and ethanol solution for 10 min before
each test.
The base oil used was 150SN (Group II, viscosity 29.5 mm2 s 1
at 40 1C, viscosity 8.2 mm2 s 1 at 100 1C, viscosity index 109).
NNDM, ZDDP and MoDTC were applied as additives, whose active
elemental compositions were listed in Table 1. The viscosity of
NNDM was determined according to the GB/T 265. The viscosities
of the lubricants as a function of concentration of NNDM were
shown in Fig. 1. It can be seen that when the concentration of
NNDM increased by 1% the viscosity of the lubricant increased by
almost 0.056 mm2 s 1, which can be recognized as a small
deviation [33]. Three series of tests were carried out under
the lubrication of 150SN containing NNDM, ZDDP and MoDTC,
Table 1
Active elemental compositions of additives tested.
Additive
ZDDP
MoDTC
NNDM
Content weight (%)
Zn
Mo
P
S
N
10
–
–
–
10
14
8
–
–
15
11
–
–
–
3
respectively. The additives concentrations (mass fraction) in
the base oil were 0.0%, 0.5%, 1.0%, 1.5%, 2.0%, 2.5% and 3.0%,
respectively.
Surface analysis
Morphologies of the rubbing surfaces were analyzed using
JSM-6460LV model low vacuum scanning electron microscope
(SEM) with energy dispersive spectroscope (EDS). The EDS technique, giving elemental information, will not identify specific
chemical species. X-ray photoelectron spectroscope (XPS) is
therefore used to complement EDS to provide very surface
sensitive ( 5 nm) information by probing only the wear film
and analyzing the composition as a function of depth. XPS
analyses were performed on a PHI Quantera SXM XPS facility.
This instrument employs a high-power rotating anode and
monochromatised Al Ka X-ray source. A binding energy of
284.6 eV for contaminated C was used as a reference for charge
correction. Surfaces of these specimens were sputtered at a rate of
5 nm/min by Ar ion (sputtering depth on the standard SiO2
sample). Before analyzed by XPS, the specimens were cleaned in
the mixed solution of petroleum ether and absolute ethanol to
remove residual oil and contaminants.
Casa XPS software was used to analyze the XPS curves
obtained from long scans and the quantitative analyses of
the peaks were performed using peak area sensitively factors.
XPS handbook [34] was used to find the chemical species corresponding to the binding energies of the peaks.
Results and discussion
Characterization of NNDM
IR was employed to characterize the main functional groups of
NNDM. The IR spectra of NNDM are shown in Fig.2. It can be found
that the peak located at 3436 cm 1 was the stretch vibration
absorption peak of –OH, and the peaks located at 2854 cm 1 and
2925 cm 1 were the stretch vibration absorption peaks of –CH3
and –CH2–, respectively. The band at 1463 cm 1 could be assigned
to –CH2– as bend vibration, and l169 cm 1 to –CH3 as bend
vibration. The bands between 1700 and 1460 cm 1 were corresponding to in-plane deformation vibration of N–C. The signal at
around 1648 cm 1 was the absorption peak of unsymmetrical
stretching vibration of CQO group. The peak at 1072 cm 1 was
9.0
80
Transmittance (%)
Viscosity (100°C, mm2 s-1)
100
8.5
60
40
20
8.0
0
1
2
3
Concentration (wt. %)
4
5
Fig. 1. Viscosities of the lubricants at 100 1C under different concentrations of NNDM.
0
4000
3500
3000
2500
2000
1500
Wavenumber (cm-1)
Fig. 2. FTIR spectra of NNDM.
1000
500
L. Yan et al. / Tribology International 53 (2012) 150–158
assigned to C–H from secondary amine. The bands between 900
and 600 cm 1 was the stretch vibration absorption band of C–C.
The peak at 722 cm 1 was the plane swing absorption of –(CH2) n(n44). The stretch vibration absorption peak of MoQO end
groups was shifted to 945 cm 1, comparing with 995 cm 1 of
original MoQO, which indicated that the Mo atom bonded with
hydroxyl.
To further determine the structure of NNDM, the compound
was investigated by 1H-NMR spectroscopy. Typical 1H-NMR
spectra of NNDM are illustrated in Fig. 3. As shown in Fig. 3,
two distinct high resonances were clearly observed, namely the
peaks at 0.88 and 1.27 ppm which were assigned to the –CH3 and
–(CH2)8–, respectively. The peak areas of signals c (d ¼1.55 ppm)
and d (d ¼2.18 ppm) were nearly the same, which were attributed
to methylene protons of (CH2)8–CH2– and –CH2–CQO, respectively. In addition, the other methylene protons (–NCH2– and –
OCH2–) of the Mo-containing ring responded to the peaks at 3.09
and 3.84 ppm (e and f). All the above characterization showed
that the desired compound was synthesized successfully.
The element analysis results were: experimental—C: 45.08%,
N: 3.38%, H: 7.44%; theoretical—C: 46.49%, N: 3.39%, H: 7.51%.
The thermo-stability of NNDM was studied with TGA.
According to the results of Fig. 4, the thermal decomposition
temperature was about 295 1C, which revealed that the additive
153
NNDM possessed excellent thermal stability, and was appropriate
to be used as lubricant additive.
Friction and wear behaviors
Fig. 5 shows the comparisons of the maximum non-seizure load
(PB values) under different concentrations of NNDM with those of
ZDDP and MoDTC. As shown in Fig. 5, the PB values of 150SN
containing NNDM increased with the increase of NNDM concentration, and reached a stationary maximum with NNDM concentration higher than 1.5%, and then did not increase obviously with
an further increase of NNDM concentration, but still higher than
that of 150SN. This meant that NNDM possessed good loadcarrying capacity. In Fig. 5, it was obvious that the PB values of
150SN containing NNDM were smaller than those of 150SN
containing ZDDP or MoDTC, and the rank of maximum non-seizure
load was ZDDP4MoDTC4NNDM, which was identical to the rank
of the concentrations of S element and the P element.
Fig. 6 displays that average friction coefficients varied with
concentrations of the additives in 150SN. Results showed that the
1000
NNDM
ZDDP
MoDTC
900
PB (N)
800
700
600
500
400
0.0
0.5
1.0
1.5
2.0
Concentration (%)
2.5
3.0
Fig. 5. Variations of maximun non-seizure load (PB values) under lubrications
with different concentrations of NNDM, ZDDP and MoDTC.
Fig. 3. 1H-NMR spectra of NNDM in CDCl3 at 25 1C.
0.13
100
Average friction coefficient
90
TG (%)
80
70
60
50
40
NNDM
ZDDP
MoDTC
0.12
295°C
0.11
0.10
0.09
0.08
0.07
30
0.06
0.0
20
50
100 150 200 250 300 350 400 450 500 550 600
Temperature (°C)
Fig. 4. TGA curve of NNDM.
0.5
1.0
1.5
2.0
Concentration (%)
2.5
3.0
Fig. 6. Variations of average friction coefficients with different concentrations
of NNDM, ZDDP and MoDTC (392 N, 60 min).
154
L. Yan et al. / Tribology International 53 (2012) 150–158
friction coefficients of the three lubricants decreased with the
additives concentration. It was obvious that the friction coefficients of 150SN containing NNDM were always smaller than
those of 150SN containing ZDDP under all tested concentrations.
The friction coefficients of 150SN containing NNDM firstly
decreased slowly when the concentration was lower than 1.5%,
and then decreased sharply when the concentration was higher
than 2.0%, while those of ZDDP and MoDTC showed little changes
when the concentration was higher than 2.0%, which demonstrated that NNDM had better friction-reducing behavior than
ZDDP and MoDTC at higher concentrations. When the concentration was up to 3.0%, the friction coefficient of 150SN containing
NNDM reached the minimum (0.066), which was about 46% lower
than that of base oil 150SN (0.124). It indicated that NNDM had
excellent friction-reducing property.
Fig. 7 exhibits the variations of the worn scar diameters with
different concentrations of additives. It can be found that the
WSDs of all additives decreased with the increasing dosage, and
the WSDs lubricated by 150SN containing NNDM were always
smaller than those lubricated by 150SN containing ZDDP or
MoDTC. When the concentration was 3.0%, the WSD lubricated
by 150SN containing NNDM reached the minimum (about
0.43 mm), which was nearly half of that lubricated by base oil
150SN. It seemed that NNDM had better anti-wear behavior than
ZDDP and MoDTC under all the tested concentrations. It also
meant that the tribofilm NNDM formed on the worn surface
exhibited a notable wear-resistance behavior.
0.9
NNDM
ZDDP
MoDTC
WSD (mm)
0.8
0.7
0.6
0.5
0.4
0.0
0.5
1.0
1.5
2.0
Concentration (%)
2.5
3.0
Fig. 7. Variations of wear scar diameters with different concentrations of NNDM,
ZDDP and MoDTC (392 N, 60 min).
SEM with EDS analysis
In order to find some clues for understanding the mechanism
of reactions on metal surface lubricated with base oil containing
the additives, SEM and EDS analysis of the worn surfaces were
carried out. Fig. 8 shows the SEM images of the rubbing surfaces
lubricated by 150SN and 150SN containing 3.0% ZDDP, 3.0%
MoDTC and 3.0% NNDM. It can be found that the SEM micrographs show similar snatches for all the lubricants, but the
furrows on the worn surfaces lubricated by 150SN and 150SN
containing ZDDP were deeper than those lubricated by 150SN
containing MoDTC or NNDM. These morphologies are in agreement with the corresponding friction and wear test. It might be
related to the different compositions of the tribofilms formed by
oils containing different additives.
Table 2 illustrates the EDS quantification of the tribofilms formed
on worn scars lubricated by 150SN, 150SNþ3% ZDDP, 150SNþ3%
MoDTC and 150SNþ3% NNDM. For the purpose of supporting the
data from quantification, the corresponding EDS spectra were given
in Fig. 9. Fig. 9(b) demonstrates the presence of phosphorus and
Fig. 8. SEM morphologies of the worn surfaces lubricated by different lubricants (392 N, 60 min): (a) 150SN; (b) 3.0% ZDDP þ150SN; (c) 3.0% MoDTC þ 150SN;
(d) 3.0% NNDMþ 150SN.
L. Yan et al. / Tribology International 53 (2012) 150–158
oxygen peaks which might be an indication of a phosphate glass
[7–10], with the zinc cation (from ZDDP), iron and chromium (from
the metal surface). Fig. 9(c) shows the presence of molybdenum and
sulfur (from MoDTC), which might be a sign of a layered-structure
MoS2 [11–13]. Additionally, Fig. 9(d) displays the existence of
oxygen and molybdenum peaks which implied the formation of
molybdenum oxides, with nitrogen (from NNDM). Based on a
significant amount of research [7–14], the three additives exhibited
different ways of interactions with the steel surface, in spite of their
similar wear mechanisms seen in the SEM micrographs.
XPS analysis
For further understanding the tribological mechanism of
NNDM, XPS analysis of the worn scar lubricated by 3.0% NNDM
under the load of 392 N were carried out. Fig. 10 depicts the XPS
spectra of several typical elements on (a) the original worn
surface and (b) the worn surface after 1 min sputtering and their
peak fitting analysis. Table 3 lists the corresponding element
concentration on the worn surface detected by XPS.
According to the element composition data in Table 3 and the
peak intensity in Fig. 10, it can be seen that the elements C, N, O,
Mo and Fe were obviously detected on the rubbing surface of steel
ball. With the increasing of detection depth, the concentrations of
C and N decreased while those of O, Mo and Fe increased,
suggesting that carbon-containing compounds and nitrogencontaining compounds existed mainly on the top layer of the
Table 2
EDS quantification (at. %) of the tribofilms formed on worn scar lubricated by
different lubricants.
Lubricant
EDS (at. %)
C
150SN
25.32
150SN þ 3% ZDDP
60.43
150SN þ 3% MoDTC 29.03
150SN þ 3% NNDM 31.46
O
N
Cr
Fe
Mo
P
S
Zn
3.60
9.19
8.15
3.51
–
–
–
0.77
1.25
0.53
0.60
0.88
69.83
26.25
61.57
61.90
–
–
0.55
1.48
–
1.33
–
–
–
0.47
0.10
–
–
1.8
–
–
155
tribofilm. The element C possessed two different chemical states
in (a), the peaks of C1s located at 284.6 eV and 288.6 eV binding
energies were adsorptive carbon and organic carbon, which was
from the carbon chain of NNDM. But the peak at 288.6 eV
disappeared in (b), suggesting that the carbon chain of NNDM
was mainly adsorbed on the worn surface. In case of N 1s, the
binding energies located in the region of 397 400 eV, which
could be assigned to organic N-containing compounds [35], and
corresponding to –N–C– structure in the molecular of NNDM.
Moreover, Mo 3p peak (394.3 eV) was found in the XPS spectrum
of N 1s, which might be an interference to identify the N 1s peak
and complicated N-containing organic film [36]. The peak intensity of N1s in (a) was obvious, while that in (b) was not distinct,
which indicated nitrogen-containing layer existed mainly in the
outer layer of the tribofilm.
As far as the spectra of O 1s were concerned, a wide peak
appeared at the range of 528–532 eV, indicating the existence of
iron oxides and molybdenum oxides [12,34], which were probably due to a series of tribo-chemical reactions, including decomposition of NNDM additive to Mo element during the friction
process and some oxidation reactions.
In case of Fe element, the binding energies of Fe 2p in (a) were
709.2 eV, 710.7 eV and 712.6 eV, corresponding to the chemical
states of FeO, Fe2O3 and Fe–C respectively [34]. Instead of the
binding energies of Fe2O3 and Fe–C, a binding energy at 711.2 eV
was obvious in (b) of Fe 2p, corresponding to the chemical states
Fe3O4 [34]. In addition, the peak intensity of Fe 2p in (b) increased
as comparison to that in (a), which meant that the contents of
iron oxides in the tribofilm increased with the increasing of
film depth.
The Mo 3d peak presented a split doublet that correlates with
Mo 3d5/2 and Mo 3d3/2. By comparing the measured spectra with
those of Mo compounds as references, two peaks with Mo 3d5/2
at 232.6 eV and 229.5 eV were attributed to MoO3 and MoO2,
respectively. Moreover, the difference in binding energy between
Mo 3d5/2 and Mo 3d3/2 was 3.3 eV and the peak area ratio of Mo
3d5/2 to Mo 3d3/2 was 3:2 [28]. And it can be seen from Table 3
that the atomic concentrations of Mo and O on worn scar after
1 min sputtering increased in comparison with those on original
Fig. 9. EDS spectra of worn surfaces lubricated by (a) 150SN; (b)3.0% ZDDP þ150 SN; (c)3.0% MoDTC þ 150 SN; (d)3.0% NNDM þ150 SN ( 392 N, 60 min ).
156
L. Yan et al. / Tribology International 53 (2012) 150–158
C 1s
284.6
Mo 3p
organic N1s
N 1s
(b)
Intensity
Intensity
(b)
(a)
Organic C
(a)
280 282 284 286 288 290 292 294
Binding Energy (eV)
FeOX
392
394
396
398
400
Binding Energy (eV)
FeO 2p3/2
Fe3O4 2p3/2
O 1s
402
404
Fe 2p
MoOx
Intensity
Intensity
(b)
(b)
Fe2O3 2p3/2
Fe-C
(a)
(a)
526
528
530
532
534
Binding Energy (eV)
536
700 705 710 715 720 725 730 735
Binding Energy (eV)
MoO3 3d5/2
Mo 3d
Intensity
MoO3 3d3/2
(b)
MoO2 3d5/2
(a)
226 228 230 232 234 236 238 240 242 244
Binding Energy (eV)
Fig. 10. XPS spectra of C1s, N1s, O1s, Fe 2p and Mo 3d on the original worn surface (a) and the worn surface after 1 min sputtering and (b) lubricated by 3.0% NNDM.
Table 3
XPS quantification (at. %) of tribofilm formed on the worn scar.
Elements
C
O
(a) Worn scar
(b) Worn scar after 1 min sputtering
63.72
51.23
17.74
22.62
N (Mo)
5.15
1.13
Fe
Mo
2.98
13.95
10.41
11.07
worn scar, which suggested that more MoOx were formed in
deeper layer of NNDM tribofilm.
Discussion
From the variations of tribological curves versus NNDM concentration (Figs. 6 and 7), it could be understood that the additive
NNDM without SP showed better friction-reducing behavior than
the additive ZDDP with SP, and especially showed better antiwear property than the additives ZDDP with SP and MoDTC with
S. When the concentration of NNDM reached 3.0%, friction
coefficient and wear scar diameter were 0.066 and 0.43 mm
respectively, which were smaller than those obtained from the
tests running on the base oil 150SN, 150SN containing ZDDP and
150SN containing MoDTC.
In the boundary lubrication regime, the formation of an adsorption layer or the production of a surface chemical reaction film is the
determining factor in minimizing friction and wear [37]. The XPS
analysis results of the tribofilm formed by 150SN containing NNDM
(Fig. 10 and Table 3) demonstrated that under the boundary
lubrication, the additive NNDM first adsorbed on the metal surface,
and then decomposed and reacted with metal surface during the
rubbing process to form a stable tribofilm on the rubbing surface.
This tribofilm was composed of a reaction layer and an adsorption
layer. Therein, the reaction layer originated from the tribochemical
reactions of Mo element and O element in NNDM, while the
adsorption layer was formed by the coordination of long-chain
alkylamide with the metal surface. And two O atoms in the ring of
NNDM molecule structure, possessing two pairs of lone electrons
respectively, had strong electroaffinity [31], which was endowed
NNDM with the strong coordinate capacity with metal ions or metal
surface, which was helpful to enhance the tribochemical reaction
between NNDM and the rubbing surface. Moreover, with the
increasing of film depth, the contents of nitrogen and carbon
decreased, and the contents of molybdenum oxides and iron oxides
L. Yan et al. / Tribology International 53 (2012) 150–158
increased, it could be concluded that the long-chain alkylamidecontaining adsorption layer mainly existed on the top surface, while
the molybdenum oxides- and iron oxides-containing reaction layer
mainly existed in the inner layer. During the rubbing process, the
outer layer could protect the inner layer from being quickly rubbed
away, and then the inner reaction layer could maintain on the
rubbing surface for a longer time, thus effectively prevented the
direct contact of the surface asperities and inhibited wear and weld.
When the concentration of NNDM reached 3.0%, the contents of O,
N, Mo and carbon chains in the oil were higher than those under
other concentrations of NNDM, which was favorable to form higher
contents of long-chain alkylamide in the adsorption layers and
MoOx in the tribochemical reaction film, as a result, the smaller
WSD and coefficient were obtained (Figs. 6 and 7), which meant
NNDM effectively decreased the friction and wear.
From Fig. 5, it seemed that the PB values of oils containing
NNDM were smaller than those of oils containing ZDDP or
MoDTC, but larger than those of base oil 150SN. It has been
known that in the EP region, the surface layer was easily erased
away and fresh metal surface exposed continuously, so under this
rigorous condition the best additive should be the one which
reacted most rapidly with the freshly exposed metal surface to
generate the most effective protective film [38]. Especially, the
sulfur element and phosphorus element in the compounds could
react quickly with the metal surface to form S-rich or P-rich
tribofilm, which was helpful to improve the extreme pressure
properties and load carrying capacity [16,31]. ZDDP which contains SP decomposes into a glassy film containing polyphosphate,
sulfides (ZnS and FeS) and oxides (Fe2O3, ZnO) of tens or hundreds
of nanometers thick to prevent wear [7–10]. S-containing MoDTC
could mainly form low friction MoS2 and MoOx films to reduce
friction and wear [11–14]. But it is impossible for NNDM which
does not contain SP to form tribofilms with sulfide or phosphate.
The tribofilm NNDM-containing oil formed was mainly consisted
of long-chain alkylamide, MoOx and FeOx. Thus, the load-carrying
capacity of NNDM-containing oil was worse than those of ZDDPor MoDTC-containing oils, but better than that of base oil 150SN.
Conclusions
The main conclusions can be drawn from this research as follows:
(1) A new kind of sulfur- and phosphorus-free organomolybdenum
compound, as an effective wear-resistance and friction-reducing
additive, was successfully synthesized and characterized.
(2) When lubricated by NNDM-containing oil, friction coefficients
of the rubbing surfaces were smaller than those when
lubricated by ZDDP-containing oil under all test conditions,
and the WSDs of the surface lubricate by NNDM-containing
oil were smaller than those lubricated by ZDDP- and MoDTCcontaining oils. Especially, when the steel surface was lubricated by 150SN containing 3% NNDM, the friction coefficient
and the wear scar diameter can be decreased by 46% and 50%,
respectively, compared with that lubricated by 150SN.
(3) High amount of long-chain alkylamide and molybdenum oxides
in NNDM tribofilm formed on the rubbing surface played an
important role of improving the friction-reducing and wearresistance performances.
Acknowledgments
The authors are grateful to the financial supports of National
Natural Science Foundation of China (51005218), Fundamental
Research Funds for the Central Universities (2010ZY51, 2011YXL020).
157
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