Kuang Fuming1,2
Key Laboratory of High Performance
Ship Technology,
Ministry of Education,
Wuhan University of Technology,
Wuhan 430063, China;
Reliability Engineering Institute,
School of Energy and Power Engineering,
Wuhan University of Technology,
Wuhan 430063, China
e-mail: fmkuang@whut.edu.cn
Zhou Xincong1
Key Laboratory of High Performance
Ship Technology,
Ministry of Education,
Wuhan University of Technology,
Wuhan 430063, China;
Reliability Engineering Institute,
School of Energy and Power Engineering,
Wuhan University of Technology,
Wuhan 430063, China
Huang Jian1
Key Laboratory of High Performance
Ship Technology,
Ministry of Education,
Wuhan University of Technology,
Wuhan 430063, China;
Reliability Engineering Institute,
School of Energy and Power Engineering,
Wuhan University of Technology,
Wuhan 430063, China
Zhou Xiaoran1
Key Laboratory of High Performance
Ship Technology,
Ministry of Education,
Wuhan University of Technology,
Wuhan 430063, China;
Reliability Engineering Institute,
School of Energy and Power Engineering,
Wuhan University of Technology,
Wuhan 430063, China
Tribological Properties of Nitrile
Rubber/UHMWPE/Nano-MoS2
Water-Lubricated Bearing
Material Under Low Speed and
Heavy Duty
This study sought to investigate the tribological properties of nitrile rubber (NBR)/
ultrahigh molecular weight polyethylene (UHMWPE)/nano-molybdenum disulfide (nanoMoS2) nanocomposites containing various quantities of nano-MoS2. The apparatus used
for these tests was a marine stern tube bearing testing apparatus SSB-100V that was
water-lubricated and was run at low speed under heavy duty conditions. The coefficient
of friction coefficient (COF), wear rate, and surface abrasion of the composite were
obtained to determine the effect of the addition nano-MoS2 and to obtain the optimum
nano-MoS2 content. The mechanical and physical properties of the rubber-plastic material met the requirements of the Chinese Ship standard CB/T769-2008 and U.S. military
standard MIL-DTL-17901C(SH). The experimental results showed that the nanocomposites that contained 9 phr nano-MoS2 (parts by weight per hundred parts of rubber materials) exhibited good comprehensive friction and wear properties. It is believed that the
experience achieved from this study can form a theoretical foundation for the improving
the properties of the subject rubber-plastic material. [DOI: 10.1115/1.4039930]
Keywords: water-lubricated stern tube bearing, nano-MoS2, tribological performance,
low speed and heavy duty
Wang Jun1
Key Laboratory of High Performance
Ship Technology,
Ministry of Education,
Wuhan University of Technology,
Wuhan 430063, China;
Reliability Engineering Institute,
School of Energy and Power Engineering,
Wuhan University of Technology,
Wuhan 430063, China
1
1
Present address: School of Energy and Power Engineering, 130 P.O. Box, 1178
Heping Road, Yujiatou Campus, Wuhan University of Technology, Wuhan 430063,
China.
2
Corresponding author.
Contributed by the Tribology Division of ASME for publication in the JOURNAL
OF TRIBOLOGY. Manuscript received July 7, 2017; final manuscript received April 2,
2018; published online May 7, 2018. Assoc. Editor: Zhong Min Jin.
Journal of Tribology
Introduction
Water-lubricated, rubber stern bearings have been widely used
in the marine and pump industries to increase the safety and reliability of marine propulsion systems and to reduce the pollution
from metal bearings that are lubricated with grease and oil [1]. In
particular, the remarkable performance of these materials onboard
American submarines during the battle of the Midway Islands
demonstrated to the world their superior qualities [2,3]. However,
C 2018 by ASME
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several shortcomings still exist in these materials. For instance,
the thickness of the water film formed between the moving pairs
is only 1/8 of the oil film, so that the designed pressure can
achieve only 1/3 that of an oil-lubricated bearing under similar
conditions [4]. When the drive shaft is rotating at low speed under
heavy duty, the bearing tends to suffer severe friction and wear,
because it is difficult to achieve a good fluid dynamic lubrication
condition [5,6]. Hence, it is crucial that systematic research be
conducted on the material development/modification of water
lubricated rubber stern bearings to improve their performance.
The U.S. Navy military has long equipped its warships with
rubber/ultrahigh molecular weight polyethylene (UHMWPE)
water lubricated bearings [7]. Orndorff has published his research
results on these bearings entitled “a new type of water-lubricated
bearing material nitrile rubber (NBR)/UHMWPE blend material,”
and in this report, the author describes a new material that exhibited good comprehensive performance that was termed SPA [8,9].
Molybdenum disulfide, MoS2 is an excellent solid lubricant that
has been called the “the king of lubrication” for many years,
because of its super self-lubrication properties [10]. In recent
years, MoS2 has frequently been used to enhance the tribological
properties of rubber. The coefficient of friction (COF) of an
MoS2/polytetrafluoroethylene composite containing quartz has
been reported to be is less than 0.005 when water-lubricated,
which demonstrates the ultrahigh lubrication behavior of this
combination exhibiting a stable friction factor and low wear rate.
MoS2 nanoparticles are known to enhance the mechanical and tribological properties of the rubber and, in turn, reduce the friction
noise and critical velocity [11]. However, a systematic study of
the effect of addition of various quantities of nano-MoS2 to a
rubber-plastic bearing material on the resultant lubrication behavior of the material, particularly at low speed and high duty conditions has not yet been conducted.
Using a standard formulation of NBR and UHMWPE, the
goal of this study was to develop a new material with good
friction-reducing and antiwear properties by adding an appropriate quantity of nano-MoS2 to the rubber. In these experiments,
the effect of the particle size of the MoS2 that was added to the
NBR/UHMWPE was determined. The same mass of MoS2 in
various particle sizes was individually added to the rubber to
elucidate the effect. The experimental conditions and wear test
results are detailed in Sec. 2. The other results are presented in
Sec. 3, followed by the discussions and conclusions in Secs. 4
and 5.
Table 1 Mechanical and physical properties of MoS2 and
nano-MoS2
Material
Content Mean thickness
MoS2
98.8%
Nano-MoS2 99.9%
1.41 lm
50 nm
Particle shape
Appearance
Lamellar structure Grayish black
Lamellar structure Grayish black
2
Experiment
2.1 Materials. Nano-MoS2 and MoS2 were purchased from
Shanghai Zaibang Chemical Industry Co., Ltd. (Shanghai, China).
The mechanical and physical properties of the nano-MoS2 and
MoS2 are listed in Table 1. Figure 1 shows the scanning electron
microscope (SEM) photographs of UHMWPE, nano-MoS2 and
MoS2. The UHMWPE (GUR4050, D50 ¼ 60 lm) was purchased
from the Celanese Corporation, which had a minimal molecular
weight of 7 106. Nitrile rubber and the auxiliary materials used
in this study were purchased from the Wuhan Park Rubber and
Plastic Product Co., Ltd., Wuhan, China.
The materials and formulations of the NBR/UHMWPE/nanoMoS2 are summarized in Table 2. The six prepared test samples
that individually received 0, 3, 6, 9, 12, and 15 phr nano-MoS2
were labeled as 1, 2, 3, 4, 5, and 6 in sequence. In addition, MoS2
was added to the NBR/UHMWPE matrix in the same quantities as
the nano-MoS2 to form a second set of test samples that were
labeled as 6, 7, 8, 9, 10, and 11 in sequence. During the preparation process of the NBR/UHMWPE/nano-MoS2 samples, the
combination of materials was mixed using a rubber and plastic
mixing device (LH-60, Kechuang Rubber and Plastic Machinery
Factory, Shanghai, China). The materials were mixed at 90 C for
13 min in an open two-roll laboratory mixing mill (Ø160 320,
Shanghai Rubber Machinery Factory, Shanghai, China). This process was repeated 20 times to achieve complete mixing at room
temperature. During the mixing, a silane coupling agent was
added to the mixture to promote the dispersion of the molybdenum disulfide particles. The optimum cure time (T, 90 min) was
determined using a rubber curometer (Youshen Electronic Instrument Company, Beijing, China). The vulcanization of the specimens was conducted using a compression molding press (Hangfa
Hydraulic Engineering Factory, Chengdu, China) at 15 MPa and
170 C for 90 min.
2.2 Experimental Apparatus and Wear Testing. All of the
wear experiments were conducted using a marine stern tube bearing testing machine (SSB-100V test-bed, Wuhan University of
Technology, China, as shown in Fig. 2) to determine the
Table 2 Formulation of sulfur system for NBR/UHMWPE/nanoMoS2 nanocomposite compounds
Materials
Amounts (phr)
Description
NBR
Carbon black
UHMWPE
Zinc oxide
Sulfur
Stearic acid
Accelerator MBTS
Nano-MoS2
100
40
12
5
1.5
1
1
0, 3, 6, 9, 12, 15
Mitsui N230S
220, 330 (carbon black)
60 lm, 7 106 molecular weight
—
—
—
Dibenzothiazyl disulfide
50 nm, microspherical
Fig. 1 SEM photographs of: (a) UHMWPE, (b) MoS2, and (c) nano-MoS2
061301-2 / Vol. 140, NOVEMBER 2018
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039, 0.65, 0.92, and 2.24 m/s as determined by the sliding
ratio, which was 50 mm. Each individual velocity was
maintained for 15 min at each speed level.
Wear test:
In this portion of the test, the specific pressure was set at
0.84 MPa, and the rotational speed of the test device was set to
0.16 m/s. Each specimen was tested for 10 h.
Fig. 2 The ship stern bearing test system and the physical
map of SSB-100V: 1–loading device; 2–test block; 3–alloy shaft;
4–pressure sensor; 5–level bar; 6–axial bearing; 7–flexible coupling; 8–measuring point of strain gauge; 9–coupling; and
10–coverter motor
Fig. 3 Test samples
Fig. 4 Schematic of sliding wear testing
tribological properties of the NBR/MoS2/UHMWPE in a waterlubricated condition.
The test specimens used in this study were comprised of an
NBR/UHMWPE/nano-MoS2 test block and Sn–Cu alloy shaft
(Fig. 3). The thickness, length and width of the rubber layer in the
test blocks was 12 mm, 25 mm, 25 mm. The roughness (Sa) of the
test block was 1.0860.05 lm following polishing with grit polishing paper. The counterpart was a Sn–Cu alloy shaft that had an
external diameter of 50 mm, a length of 57 mm, and a surface
roughness (Sa) of 0.65 lm. The wear test schematic diagram is
shown in Fig. 4.
To determine the tribology properties of the nano-MoS2/waterlubricated rubber-plastic material at low speed under heavy loads
(0.16 m/s and 0.84 MPa), friction tests and wear tests were conducted using the SSB-100V test-bed. One of the specimens that
was labeled #1 in Table 2 was used as a control that contained no
nano-MoS2 and was tested under the same conditions as the samples containing MoS2.
Friction test conditions:
(a) Running-in, this conditioning step was initially run on each
sample under the following conditions, the specific applied
pressure was 0.42 MPa, the sliding velocity of 0.21 m/s and
the water temperature was 20 C for 2 h.
(b) After conditioning, the specific pressure was set to
0.84 MPa, and the rotational speed of the test device was
varied to 20, 30, 40, 60, 100, 150, 250, 350, and 800 rpm.
The sliding velocities were 0.05, 0.08, 0.11, 0.16, 0.21,
Journal of Tribology
2.3 Measurement Techniques. The COF were measured and
calculated using the following equation (Eq. (1)):
f ¼
F
2T
¼
N dN
(1)
where T is the friction torque (Nm), d is the diameter of the shaft
(m), and N is the nominal load (N).
The wear weight loss of the test blocks in the experiments was
determined for the blocks labeled A, B, and the weights were
marked as mA and mB. The A blocks were the experimental group
and the B blocks were the control group. Just place B into water,
instead of rubbing with shaft. After each experiment, both blocks
were cleaned ultrasonically and dried in an oven at 40 C until
the weight of each sample was constant. The final weights of
experimental group and the control group were marked ma
and mb. Theoretically, rubber is impossible to dry in a short
time, so the weight change of the rubber was determined by
mA [ma (mb mB)], which took water absorption into account.
Regardless of whether or not the rubber was completely dry, the
mass variation during the soaking and drying processes were used
to directly determine the mass loss of the rubber. All of the weight
measurements were obtained in triplicate for each specimen to
ensure reproducibly.
The surface morphologies of the samples were examined using a
digital microscope system (VHX-2000, Keyence Corporation,
Shanghai, China). A scanning electron microscope (JSM-IT300,
JEOL Ltd., Tokyo, Japan) was used to examine the microstructure
of the wear surfaces, and X-ray photoelectron spectrometer (ESCALAB 250Xi, Thermo Fisher Scientific Co., Ltd., Beijing, China)
was used to determine the nature and content of the elements on the
worn surface. The worn weight losses of the materials were determined by weighing the samples using an electronic balance
(XS204, Mettler-Toledo International, Inc., Shanghai, China).
3
Experimental Results
The physical properties of the nanocomposite materials
conformed to the CB/T 769-2008 [12] and the MIL-DTL17901C(SH) standards [13], and the nano-MoS2 and MoS2 particles were dispersed in NBR matrix. The tribology properties of
NBR/UHMWPE/nano-MoS2 water lubricated bearing material
that was subjected to low speed and a heavy load were determined
by a comparative analysis of the COF, wear rate, and wear surface
morphologies of each test sample. The experimental results
showed that there were significant differences in the tribological
properties of the materials that contained the various nano-MoS2
and MoS2 additives.
3.1 Analysis of Mechanical Properties. Table 3 lists the
measured physical properties of the 11 test materials, which were
tested according to the specifications for CB/T 769-2008 and
MIL-DTL-17901C(SH) that are listed in Table 4. Also, the standards of shore hardness, Akron abrasion loss, tensile strength and
elongation at break and compression set are shown in Table 4.
Figure 5 shows the X-ray diffraction patterns of two sample
materials, NBR/UHMWPE/nano-MoS2 and NBR/UHMWPE/
MoS2. The diffraction spectra exhibited peaks at 2h ¼ 14.4 deg,
which is the characteristic diffraction peaks for MoS2. These
results showed that the MoS2 nanoparticles maintained their original layered structure in the NBR rubber.
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Table 3
Number
1
2
3
4
5
6
7
8
9
10
11
Physical property of materials which were added different phr nano-MoS2
Shore hardness (A)
Elongation at break (%)
Tensile strength (MPa)
Compression set (%)
Akron abrasion loss (cm3) (1.6 km)1
76.27
77.17
77.63
78.17
79.17
81.17
77.26
78.13
79.53
81.13
81.32
464
444
440
448
436
444
448
464
444
448
440
21.2
21.7
20.7
20.5
20.2
20.2
20.5
20.1
20.2
20.1
19.7
12
12
12
12
12
16
14
14
12
14
14
0.066
0.047
0.039
0.029
0.027
0.026
0.049
0.041
0.030
0.028
0.027
Table 4
Physical property standard of water-lubricated rubber stern tube bearing
Property
Tensile strength (MPa)
Compression set (%)
Elongation at break (%)
Shore hardness (A)
Akron abrasion loss (cm3) (1.6 km)1
CB/T 769-2008
MIL-DTL-17901C
16 (GB/T 528-92)
<40 (GB/T 7759.1)
300 (GB/T 528-92)
75–82 (GB/T 531)
0.40 (GB/T 1689)
>10.3 (ASTM D412)
—
>150 (ASTM D412)
65–90 (ASTM D2240)
—
Fig. 5 X-ray diffraction patterns of three materials
the range of 0.39–0.92 m/s and was constant when the sliding
velocity was greater than 0.92 m/s. All of these variations in the
COF were identical to the Stribeck curve, which are indicative of
boundary lubrication, mixed lubrication, and hydrodynamic lubrication. Similar trends have been reported previously [11,16,17].
In addition, as shown in both the inserts in the following figures,
when the quantity of nano-MoS2 or MoS2 was 9 phr, the COF was
its minimum.
To determine the effect of the particle size of the MoS2 and its
content on the tribological properties of the composite at low
speed and heavy duty, the COF of the test block/Sn–Cu alloy shaft
moving pairs was measured and the results are shown in Fig. 8.
The COF of the materials with no nano-MoS2 or MoS2 under the
same test conditions was also plotted as a reference. As shown, in
general, the COF initially decreased with the increase in the content of nano-MoS2 or MoS2 and then significantly increased as the
content of these materials was increased. In addition, the COF of
all the test samples that contained the nano-MoS2 was less than
that of the samples that contained microsized MoS2. It was found
that the COF was its lowest value when the test sample contained
9 phr nano-MoS2 or MoS2. In the former case, the COF was 0.076
and with MoS2 it was 0.084.
The degree of dispersion of the nano-MoS2 in the NBR matrix
was examined by SEM and energy dispersive spectroscopy (EDS)
analysis of the test materials. The typical nano-MoS2 particles,
which were inclined insert in the matrix shown in the illustration
in Fig. 6 expressed that the nanoparticles in the matrix were not
aggregate in the nanocomposites. And, the EDS of the cross section was used to analyze dispersity of nanoparticles. In order to
express the distribution more precisely, the edge of the sample
was chosen to analyze the surface energy. As can be seen in
Fig. 6, one could find that there was no obvious agglomeration on
the selected cross section, although the nano-MoS2 particles was a
little concentrated on the cross section with 15 phr nano-MoS2
in the illustration of Fig. 6(b). Similar results were found in
Refs. [14] and [15].
3.3 Analysis of Wear Rate. The wear rate is the weight of
the material that has been worn off the sample as a result of the
contact between the rubbing pair during the sliding wear process.
Figure 9 shows the wear rate of the test block. In general, this was
identical to the COF data, where the wear rate initially decreased
and then increased with an increase in the content of the
nano-MoS2 and MoS2. The wear rate of all the materials which
contained nano-MoS2 was less than that of the materials that contained the MoS2. In addition, the wear rate of materials that contained nano-MoS2 or MoS2 was less than the materials with no
additive. Therefore, in the case of the rubber-plastic sample with
9 phr nano-MoS2 or MoS2, the wear rate of the block was the
smallest, at 0.0011 mg/h and 0.0016 mg/h.
3.2 Determination of the Coefficient of Friction. The measured COF of the various test blocks and Sn–Cu alloy shafts are
shown in Fig. 7. As can be seen, the COF of the samples exhibited
an inverse relationship to the sliding velocities, and a rapid
decrease in the COF was produced by an increase in the sliding
speed from 0.052 to 0.39 m/s. The COF then slowly increased in
3.4 Analysis of Wear Surface Topographies. The wear
surfaces of the tested blocks under low speed of 0.16 m/s and
heavy-loads of 0.84 MPa were examined using laser interference
profilometer to investigate the wear mechanisms. Figure 10 shows
surface topographies of test blocks that included different additives. Three points in different parts were selected in a test block.
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Fig. 6 Analysis of surface energy spectra of cross section with (a) 9 phr and (b) 15 phr nano-MoS2
Fig. 7 Variation in the COF of the test specimens with various contents of (a) nano-MoS2 and (b) MoS2 at different sliding
speeds
By analyzing the wear surface topographies of test blocks and
then comparing with each sample, the scratches on the wear surface in the samples without nano-MoS2 were wider and nonordered, particularly at the first and second point in Fig. 10(a). In
order to ensure the accuracy of the measurements, surface roughness (the arithmetical mean deviation of the profile Sa) was used
to evaluate worn surfaces of all blocks. In addition, the mean of
Sa of three points on same block was calculated. As can be seen
in Fig. 11, with the increase of nano-MoS2 and MoS2 addition, the
Sa of the test block went down first and then went up, although
the variation of Sa of test blocks with MoS2 shows slight fluctuations. In addition, both MoS2 and nano-MoS2 reach their lowest
value.
To offer more intuitionistic information of surface morphologies, the wear surfaces of test blocks after the sliding wear test
were examined using a digital microscope VHX-2000 system.
Figure 12 shows the topography images and cross section profiles
of test block with 9 phr and 12 phr nano-MoS2 and MoS2. In general, the topographies of the test block with nano-MoS2 was more
flat than the block that contained MoS2. As shown in Figs. 12(b)
Journal of Tribology
and 12(c), the width and depth of scratches of the surface topographies of the test blocks with 9 phr nano-MoS2 and MoS2 was relatively narrow and shallow, and it also can be seen from the
contour that the surface was relatively flatter in comparison to the
other contours of the surface. These phenomena were consistent
with the analysis of wear rate shown in Fig. 9. When the content
of the nano-MoS2 and MoS2 reached 12 phr in the composites in
Figs. 12(d) and 12(e), the width and depth of the scratches of the
counterpart were wider and deeper, and the surfaces were rougher.
As can be seen from Fig. 12(a), in comparison to other pictures in
Fig. 12, the test block without nano-MoS2 and MoS2 exhibited
wider and deeper scratches.
4
Discussion
Based on the experimental test results, it was clear that the different additive contents had a significant effect on the COF, wear
rate, and surface topographies of the moving pairs. The layered
characteristics and low shear strength properties of the nanoMoS2 produced excellent self-lubrication properties [18]. What
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Fig. 8 Variation of COF between moving pairs under lowspeed of 0.16 m/s and heavy-loads of 0.84 MPa
Fig. 9 Variation of wear rate of test block under low speed of
0.16 m/s and heavy-loads of 0.84 MPa
follows is a discussion of (a) serious friction and wear at high
speed and heavy duty, (b) the influence of the additive content on
the tribological properties, and (c) a lubrication mechanism model
for the MoS2 nanoparticles.
4.1 Serious Friction and Wear Under Low Speed and
Heavy Duty. Figure 7 shows that the test block and Sn–Cu alloy
shaft moving pairs were subjected to boundary lubrication, mixed
lubrication, and hydrodynamic lubrication. When the sliding
velocity was less than 0.39 m/s, the COF was relatively high, and
the lubrication was worse, because the friction pairs were performing as a boundary lubrication. In addition, the lubricating
water film was destroyed under the heavy load of 0.84 MPa, so the
wear rate increased, because the real contact area increased, as
has been previously reported [19,20]. To obtain further insight
into the wear process, a SEM was used to examine the microstructure of the wear surfaces subjected to various velocities and heavy
duty loads at 0.84 MPa.
Figure 13 shows that the nanocomposite with different types
and contents of molybdenum disulfide had different wear surfaces
when subjected to different sliding velocities. Hydrodynamic
lubrication must be considered as the explanation for the effect of
the various sliding velocities on the wear rate [21]. At lower
velocity (at 0.052 m/s), it was difficult to form a water film
between the surfaces of the moving pairs, so there are many
061301-6 / Vol. 140, NOVEMBER 2018
highly deformed asperities on the surface that resulted from the
yielded plastic deformation. In Figs. 13(a) and 13(b), the wear
surfaces appear to have developed cracks when the sliding velocity was low. In addition, several deep pits appeared on the surface
as a result of the presence of solid particles that were extruded
from the pit due to the deformed asperities and UHMWPE particle
structure that occurred during the boundary lubrication. In
Fig. 13(c), it is evident that the condition of the wear surface is
better than that in Figs. 13(a) and 13(b). The reason for this was
that no cracks resulted because of the inorganic nanoparticles that
were present at the crosslinks of the polymer chain, which greatly
contributed to an increase in the tensile strength of the composite.
In addition, nano-MoS2 offers an energy transfer effect, which can
diminish the formation of microcracks and prevent the surface
from experiencing destructive cracking [22]. As the sliding velocity was increased, the deformed asperities became smaller as
more water accumulated in the moving pairs. Meanwhile, the
stretching and tearing phenomenon of the rubber-plastic material
was minimized when the sliding velocity increased. As shown in
Fig. 13, there was no obvious deformation on the wear surfaces at
a wear rate of 0.92 m/s. In addition, there were fewer pits on the
surface of the block and those that were present were shallow as a
result of the hydrodynamic lubrication resulting from the water
film.
4.2 Effect of Various Quantities of Nano-MoS2 Particle on
Tribological Properties. The results described in Sec. 3 indicated
that for the rubber-plastic materials containing 9 phr nano-MoS2
or MoS2 the COF and wear rate was relatively low. In addition,
the resulting surface topographies were found to be better than in
the other materials. As Figs. 8 and 9 show, at low speed and under
high load, both the COF and wear rate of modified rubber-plastic
material was found to initially decrease dramatically, and then
they increased in tandem with the increase in the content of the
nano-MoS2 or MoS2. In this study, the factors that were beneficial
for reducing friction and wear were called positive factors, while
the factors that exacerbated friction and wear were termed negative factors.
When the nano-MoS2 and MoS2 were incorporated into the
composite, there was a uniform distribution of MoS2 on the surface of the test block after the running-in process [23]. The unique
lamellar structure of the nano-MoS2 caused the interfacial
shear strength of the test block and the shaft to decrease, which
decreased the interfacial friction. As the quantity of the nanoMoS2 in the composite was increased, the amount of the nanoMoS2 on the contact surface increased, as did the nano-MoS2 in
free-state between the moving pairs, which increased the total
quantity of the available nano-MoS2. On the other hand, the
molecular chains of the MoS2 cross-links in the rubber polymer
transferred the impact and energy from the friction, which prevented crack propagation and stopped the development of destructive cracks [22,24,25]. In addition, the inorganic nanoparticle as
the crosslink points in the polymer contributed to an increase in
tensile strength of the composite. Therefore, the MoS2 nanoparticles reduced the friction and also prevented intensive wear.
These factors were the positive factors.
The COF and the wear rate of the moving pairs and the test
block with different additives were reduced to their minimum values when the content of additives was 9 phr. However, at concentrations beyond this quantity of additive, the COF and wear rate of
the test block increased. The reasons for these results are as follows: (a) as can be seen in Table 3, the mechanical and physical
properties tests of the blocks containing the nanoparticles showed
that the hardness of the material increased with the increase of the
nano-MoS2 and MoS2 in the composite [22]. The increase in hardness decreased the subsidence deformation of the block, which
precluded the formation of water sacs that produced poor lubrication. In addition, the real contact area decreased due to the
increase in the hardness of the test block. The unit area pressure
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Fig. 10 The worn surface morphologies of test block with (a) 0 phr nano-MoS2 and MoS2, (b)–(f) 3 phr, 6 phr, 9 phr,12 phr, 15
phr nano-MoS2, and (g)–(k) 6 phr, 9 phr,12 phr, 15 phr MoS2, respectively
Fig. 11 Variation of Sa (mean) of test block
increased, which resulted in an increase in the COF and wear rate.
(b) When the nano-MoS2 and MoS2 content in rubber-plastic
material was too high, the particles did not effectively combine
with the matrix. When excess additive was present in the matrix,
the MoS2 particles would form irreversible agglomerates to varying degrees [14,15,26–28]. This resulted in decreased wear performance in the material. These were termed the negative factors.
When the nano-MoS2 and MoS2 content exceeded 9 phr, the
negative impact of this increase on the tribological properties of
Journal of Tribology
the moving pairs was greater than positive effects. In other words,
when the amount of the additives exceeded the optimum concentration of 9 phr, the positive effects of the additives in reducing
friction were lost.
As shown in Figs. 8 and 9, it can be seen that the COF and wear
rate of the nanometer sized molybdenum disulfide composite
material was significantly lower than the micrometer scale molybdenum disulfide composite material. In addition, Fig. 12 shows
that the wear topographies of test block with added nano-MoS2
were flatter than the other materials that had additives with larger
sizes. Table 3 also shows that the tensile strength of the rubberplastic materials containing nano-MoS2 was higher than the materials containing MoS2 [22,29]. This result can be attributed to the
minute size of nanoparticles. The surface effect of the nanosized
particles was greater than the comparable microsized MoS2 particles, so the nano-MoS2 particles in test block were more likely
to physiochemically interact with the rubber, which created a
stronger bond between the two materials [30]. This produced a
lower COF and higher wear rate in the nanometer composite
material over that of the microsized composite material at the
same filler content.
4.3 Lubrication Mechanism Model of Nano-MoS2
Particles. Figure 14 is a plot of data that were extracted from
Table 3. As shown, the Akron abrasion loss of the test material
decreased with increasing molybdenum disulfide content and
became constant at 9 phr molybdenum disulfide content. However, in the water-lubricated condition, as shown in Fig. 9, the
wear rate increased as soon as the MoS2 content was 9 phr.
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Fig. 12 Wear surfaces of test block with (a) 0 phr nano-MoS2 and MoS2, (b) 9 phr
MoS2, (c) 9 phr nano-MoS2, (d) 12 phr MoS2, and (e) 12 phr nano-MoS2
Under the dry test conditions, the friction pair in the system
consisted of a rubber-plastic test block and a grinding wheel. The
surface roughness of the grinding wheel was quite high and was
much greater than the copper sleeve that was employed in the
water-lubricated test. When the pure rubber-plastic material and
061301-8 / Vol. 140, NOVEMBER 2018
the grinding wheel came into contact, the rubber and UHMWPE
particles on the sample surface were easily scraped and peeled
away from the rubber by the rough surface of the grinding wheel.
After a small amount of molybdenum disulfide was added to the
rubber, the molybdenum disulfide particles near the surface of the
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Fig. 13 SEM images of the wear surface of tested blocks with (a) 0 phr nano-MoS2 and MoS2, 9 phr, (b) MoS2, and (c) nanoMoS2 at various sliding velocity
Fig. 14 Volume wear rate in the case of dry friction (Akron
abrasion loss)
Journal of Tribology
block were easily torn off by tensile stress of the grinding wheel,
and the particles became firmly embedded in the surface of the
grinding wheel forming a self-lubricating layer that had excellent
lubricity. The greater the content of molybdenum disulfide in the
composite, the easier it was to form a continuous, intact antifriction layer on the wheel. The frictional behavior occurred primarily
at the surface between the sample and the antifriction layer, so the
amount of wear and tear on the bulk of the material decreased significantly. When the molybdenum disulfide content of the composite was 9 phr, a complete wear-reducing layer was formed
between the friction pairs. Therefore, as the content of molybdenum disulfide in the composite was increased, the amount of wear
of the material did not dramatically change.
The water-lubricated condition was influenced by the positive
and negative factors described in Sec. 4.2. When the content of
the additives was less than 9 phr, the positive impact on the tribological properties of moving pairs was greater than negative
effects, because of the antifriction layer and energy transfer provided by the MoS2 additives [25,30]. As the additive content was
continually increased, above the optimum concentration of 9 phr,
this activated the negative factors in the lubricity, because the
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Fig. 15 Analysis of surface energy spectra of copper
excess of MoS2 particles formed irreversible agglomerates that
increased the hardness of matrix, reducing the contact between
the moving pairs. In addition, in the water-lubricated condition,
the scouring effect of water and the extremely smooth surface of
the copper, caused the MoS2 to be partially scrubbed off the rubber composite. Therefore, this effect made it difficult to form a
uniform, friction-reducing layer on the surface of the copper
sleeve compared with the dry condition.
Undoubtedly, addition of MoS2 nanoparticles to the rubber
composite played a significant role in reducing the COF and wear
rate of the rubber-plastic material in wear tests. This was attributed to the layered and low shear strength properties of nanoMoS2 that produced excellent self-lubricating properties [18]. The
most likely reasons for this result were: (a) as the wear process
progressed, MoS2 nanolayers were gradually exposed at the wear
surface of test block forming a self-lubricating layer on the
surface of rubber-plastic material; (b) a portion of the poorly
adhering MoS2 nanolayers were deposited onto the surface of
the Sn–Cu alloy shaft, because of the continuous scraping of the
rough surface peaks. To validate this conclusion, and explain the
lubrication mechanisms of nano-MoS2 in the rubber-plastic material, EDS analysis was used to determine the type and content of
the elements on the wear surfaces of the Sn–Cu alloy. This block
of metal that was pressed against a rubber-plastic material that
contained 9 phr nano-MoS2 at 0.16 m/s and 0.84 MPa. As shown
in Fig. 15, the EDS surface scan the metal showed that molybdenum and sulfur were present in the same region of the contact surface of the copper sleeve. Therefore, the MoS2 apparently formed
a layer of molybdenum disulfide particles on the surface of the
Sn–Cu alloy shaft during the water-lubricated test.
The model of the lubrication mechanism of the nano-MoS2 in
the rubber-plastic material during the wear process is shown in
Fig. 16. When the sliding velocity was 0.16 m/s, the test block
directly contacted with the shaft due to boundary lubrication as
shown in Fig. 7. The nano-MoS2 particles mixed in the rubberplastic material bulged at the wear surface and easily formed a
surface contact with the surface of the shaft [23]. Then, as shown
in Fig. 16(b), along with the continuous scraping of the rough
peaks of the metal on the rubber-plastic material, the rubber and
nano-MoS2 mixture sheared away from the matrix. The nanoMoS2 particles were then transferred to the surface of the shaft to
form a MoS2 film layer that reduced the COF, while the rubber
particles were washed away by lubricating water.
5
Conclusions
The tribological properties of a NBR/UHMWPE/nano-MoS2
water-lubricated, bearing material were investigated using a
marine stern tube bearing testing machine under low speed and
heavy duty conditions. A comparative analysis of the COF, wear
rate and surface topographies was conducted to study the friction
and wear mechanism of the moving pairs. The NBR/UHMWPE
materials that contained nano-MoS2 or MoS2 exhibited excellent
tribological properties in comparison to compared the control
NBR/UHMWPE material with no additives. The optimum content
of nano-MoS2 or MoS2 in the NBR/UHMWPE was found to be 9
phr for use at low speed and heavy duty. The wear topographies
of rubber-plastic test blocks that included nano-MoS2 additives
were far better than the rubber-plastic materials containing MoS2.
The nano-MoS2 particles appeared to be easily transferred from
the matrix to the counterpart surface to form a continuous and
compact energy transfer film.
Funding Data
National Natural Science Foundation of China (51079119).
The Fundamental Research Funds for the Central Universities (2017-YB-020).
References
Fig. 16 Lubrication mechanism model of nano-MoS2 nanoparticles: (a) initial contact and (b) scrape and transfer
061301-10 / Vol. 140, NOVEMBER 2018
[1] Yan, X. P., Yuan, C. Q., Bai, X. Q., Li, X. U., Sun, Y. W., and Xing, S., 2012,
“Research Status and Advances of Tribology of Green Ship,” Tribology, 32(4),
pp. 410–420.
[2] Orndorff, R. L., Jr., 1985, “Water-Lubricated Rubber Bearings, History and
New Developments,” Nav. Eng. J., 97(7), pp. 39–52.
[3] Qin, H. L., Zhou, X. C., Zhao, X. Z., Xing, J. T., and Yan, Z. M., 2015, “A
New Rubber/UHMWPE Alloy for Water-Lubricated Stern Bearings,” Wear,
328–329, pp. 257–261.
[4] Zhou, X. R., Zhou, X. C., and Cheng, J. F., 2016, “Study on Tribological Properties of Modified UHMWPE for Water Lubricated Bearings,” Lubr. Eng., 12,
pp. 80–85.
[5] Tuononen, A. J., 2014, “Digital Image Correlation to Analyse Stick–Slip
Behaviour of Tyre Tread Block,” Tribol. Int., 69(69), pp. 70–76.
[6] Yan, Z., Zhou, X., Qin, H., Niu, W., Wang, H., Liu, K., and Tang, Y., 2015,
“Study on Tribological and Vibration Performance of a New UHMWPE/Graphite/NBR Water Lubricated Bearing Material,” Wear, 332–333, pp. 872–878.
Transactions of the ASME
Downloaded From: http://tribology.asmedigitalcollection.asme.org/ on 07/28/2018 Terms of Use: http://www.asme.org/about-asme/terms-of-use
[7] Rorrer, R. A. L., and Brown, J. C., 2000, “Friction-Induced Vibration of Oscillating Multi-Degree of Freedom Polymeric Sliding Systems,” Tribol. Int.,
33(1), pp. 21–28.
[8] Orndorff, R. L., 2000, “New UHMWPE/Rubber Bearing Alloy,” ASME J. Tribol., 122(1), pp. 367–373.
[9] Orndorff, R. L., and Spangler, R. C., 2003, “SPA Super Demountable Bearing,”
Duramax Marine LLC, Hiram, OH, U.S. Patent No. US6648510B2.
[10] Hu, K. H., Huang, F., Hu, X. G., Xu, Y. F., and Zhou, Y. Q., 2011, “Synergistic
Effect of Nano-MoS2 and Anatase Nano-TiO2 on the Lubrication Properties of
MoS2/TiO2 Nano-Clusters,” Tribol. Lett., 43(1), pp. 77–87.
[11] Dong, C., Yuan, C., Wang, L., Liu, W., Bai, X., and Yan, X., 2016,
“Tribological Properties of Water-Lubricated Rubber Materials After Modification by MoS2 Nanoparticles,” Sci. Rep., 6(1), p. 35023.
[12] Shipping industry of China, 2008, “Marine Whole Rubber Bearing CB/T 7692008,” Shipping industry of China, China.
[13] DoD, 2005, “Bearing Components, Bonded Synthetic Rubber,” United States
Department of Defense, Washington, DC.
[14] Xu, Y., Ji, K., Huang, Z., Zhao, H., and Dai, Z., 2014, “Tribological Behaviors
of Foamed Copper/Epoxy Resin Composites Augmented by Molybdenum
Disulfide and Multi-Walled Carbon Nanotubes,” Proc. Inst. Mech. Eng., Part J,
228(5), pp. 558–566.
[15] Hu, K. H., Liu, M., Wang, Q. J., Xu, Y. F., Schraube, S., and Hu, X. G., 2009,
“Tribological Properties of Molybdenum Disulfide Nanosheets by Monolayer
Restacking Process as Additive in Liquid Paraffin,” Tribol. Int., 42(1), pp.
33–39.
[16] Kraker, A. D., Ostayen, R. A. J. V., and Rixen, D. J., 2007, “Calculation of
Stribeck Curves for (Water) Lubricated Journal Bearings,” Tribol. Int., 40(3),
pp. 459–469.
[17] Zhou, K., Liu, J., Zeng, W., Hu, Y., and Gui, Z., 2015, “In Situ Synthesis, Morphology, and Fundamental Properties of Polymer/MoS2 Nanocomposites,”
Compos. Sci. Technol., 107, pp. 120–128.
[18] Chen, Z., Liu, X., Liu, Y., Gunsel, S., and Luo, J., 2015, “Ultrathin MoS2 Nanosheets With Superior Extreme Pressure Property as Boundary Lubricants,” Sci.
Rep., 5(1), p. 12869.
Journal of Tribology
[19] Dong, C. L., Yuan, C. Q., Bai, X. Q., Yang, Y., and Yan, X. P., 2015, “Study
on Wear Behaviours for NBR/Stainless Steel Under Sand Water-Lubricated
Conditions,” Wear, 332–333, pp. 1012–1020.
[20] Wang, F., Wu, Y., Cheng, Y., Wang, B., and Danyluk, S., 1996, “Effects of Solid
Lubricant MoS on the Tribological Behavior of Hot-Pressed Ni/MoS Self-Lubricating
Composites at Elevated Temperatures,” Tribol. Trans., 39(2), pp. 392–397.
[21] Yagi, K., and Sugimura, J., 2013, “Elastic Deformation in Thin Film Hydrodynamic Lubrication,” Tribol. Int., 59, pp. 170–180.
[22] Xia, W., Song, Z., and Yuxiang, C., 2011, Modification and Application of
Nitrile Rubber Composite Materials, China Petrochemical Press, Beijing,
China.
[23] Zhang, G., Rasheva, Z., and Schlarb, A. K., 2010, “Friction and Wear Variations of Short Carbon Fiber (SCF)/PTFE/Graphite (10 Vol. %) Filled PEEK:
Effects of Fiber Orientation and Nominal Contact Pressure,” Wear, 268(7–8),
pp. 893–899.
[24] Leblanc, J. L., 2000, “Elastomer–Filler Interactions and the Rheology of Filled
Rubber Compounds,” J. Appl. Polym. Sci., 78(8), pp. 1541–1550.
[25] Sarkawi, S. S., Dierkes, W. K., and Noordermeer, J. W. M., 2014, “Elucidation
of Filler-to-Filler and Filler-to-Rubber Interactions in Silica-Reinforced Natural
Rubber by TEM Network Visualization,” Eur. Polym. J., 54(1), pp. 118–127.
[26] Xie, H., Jiang, B., He, J., Xia, X., and Pan, F., 2016, “Lubrication Performance
of MoS2 and SiO2 Nanoparticles as Lubricant Additives in Magnesium AlloySteel Contacts,” Tribol. Int., 93(Pt. A), pp. 63–70.
[27] Sia, S. Y., Bassyony, E. Z., and Sarhan, A. A. D., 2014, “Development of SiO2
Nanolubrication System to Be Used in Sliding Bearings,” Int. J. Adv. Manuf.
Technol., 71(5–8), pp. 1277–1284.
[28] Tang, G., Zhang, J., Liu, C., Zhang, D., Wang, Y., Tang, H., and Li, C., 2014,
“Synthesis and Tribological Properties of Flower-Like MoS2 Microspheres,”
Ceram. Int., 40(8), pp. 11575–11580.
[29] Tang, Y., Yang, J., Yin, L., Chen, B., Tang, H., Liu, C., and Li, C., 2014,
“Fabrication of Superhydrophobic Polyurethane/MoS2 Nanocomposite Coatings With Wear-Resistance,” Colloids Surf., A, 459(14), pp. 261–266.
[30] Leblanc, J. L., 2002, “Rubber–Filler Interactions and Rheological Properties in
Filled Compounds,” Prog. Polym. Sci., 27(4), pp. 627–687.
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