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ORIGINAL PAPER
Enhanced tribology properties of ZnO/Al2O3 compositenanoparticles as liquid lubricating additives
Qiang Chen • Shaohua Zheng • Shikuan Yang •
Wei Li • Xiaoyun Song • Bingqiang Cao
Received: 2 November 2011 / Accepted: 5 December 2011 / Published online: 17 December 2011
� Springer Science+Business Media, LLC 2011
Abstract Monodispersed and hydrophobic ZnO/Al2O3
composite nanoparticles are prepared by a nonhydrolytic
sol–gel method. ZnCl2 and AlCl3 are dissolved in acetone
and used as precursors. Oleic acid is adopted as an oxygen
donor. The tribology properties of the prepared ZnO/Al2O3
composite nanoparticles are studied by the four-ball fric-
tion and thrust ring friction test. It is demonstrated that the
average friction coefficient and the wear scar diameter are
reduced by 37.5 and 26.2%, respectively, in comparison
with pure lubricating oil. Moreover, the ZnO/Al2O3 com-
posite nanoparticles bear the merits of ZnO and Al2O3
when used as lubricant additives, exhibiting both excellent
antifriction and antiwear behaviors simultaneously. The
ZnO/Al2O3 composite nanoparticles improve the lubrica-
tion effect not only by turning the sliding friction into
rolling friction, but also forming a hard Al2O3 protective
film onto the thrust-ring surface containing ZnO/Al2O3
nanoparticles, which have much potentiality in industrial
applications.
Keywords Lubricant additives � Antifriction � Antiwear �ZnO/Al2O3 � Nonhydrolytic sol–gel
1 Introduction
Nano-structured materials have shown tremendous appli-
cation potentialities in a wide range of areas, including in
catalytic regions, magnetic areas, sensor and molecular
electronics devices [1–4]. In recent years, the exploration
of the use of nanoparticles in tribology have received more
and more attention owing to their unique properties in
lubrication and tribology, such as in anti-wear, in reducing
friction, and with high load capacity [5–7]. Currently,
many researchers have devoted to investigating the mech-
anisms about how to enhance the lubricant properties when
nanoparticles are used as lubricant additives. Mechanisms
of friction reduction when nanoparticles are added have
been ascribed to the transition from sliding to rolling effect,
forming protective film, third-body effect, self-mending
effect and the others [8–11]. Ye et al. [12] found that Ni-
MoO2S2 nanoparticles as lubricating additive exhibit very
good EP behavior and possess good anti-frictional perfor-
mance with a smooth transition from fluid film lubrication
at low temperature to solid film lubrication at elevated
temperature. Battez et al. [13] found that CuO nanoparti-
cles as lubricant additives exhibited reductions in friction
compared to the base oil and the antifriction behavior of
the nanoparticles on the wear surfaces can be attributed to
third body and tribosinterization mechanisms. Importantly,
it is found that there are interesting interactions when two
or more nanoparticle lubricant additives are introduced
together, such as adduct effect, synergy and antagonism
effects [14–16]. Although these effects have obvious
influence on the performance of lubricating oil when
nanoparticles were used as additives, the present study is
few and highly needed.
On the other hand, nanoparticle additives in the lubri-
cant have a strong tendency to agglomerate due to its high
Q. Chen � S. Zheng (&) � W. Li � X. Song � B. Cao (&)
Key Laboratory of Inorganic Functional Materials in
Universities of Shandong, School of Material Science and
Engineering, University of Jinan, Jinan 250022, China
e-mail: [email protected]
B. Cao
e-mail: [email protected]
S. Yang
Department of Engineering Science and Mechanics,
Pennsylvania State University, University Park, PA 16802-6812,
USA
123
J Sol-Gel Sci Technol (2012) 61:501–508
DOI 10.1007/s10971-011-2651-0
surface energy and poor consistency between the material
surface and the lubricant [17, 18], which usually restricts
their application as lubricant additives. The methods which
can fabricate nanoparticles with good dispersion stability in
lubricating oil are of prime importance. Sol–gel [19, 20]
process is a favorable method for its low-temperature
synthesis of nanoparticles. Aqueous sol–gel chemistry is
rather complex, mainly due to the high reactivity of the
metal oxide precursors and the double role of water as
ligand and solvent. In many cases, the three reaction types
(hydrolysis, condensation, and aggregation) occur almost
simultaneously and are difficult to control individually. So,
slight changes in experimental conditions result in altered
particle morphologies, which is a serious issue regarding
the reproducibility of a synthesis protocol. Furthermore,
the quality of final product extremely depends on the
amount of the water and pH. All of these largely limit the
widespread applications of sol–gel method. Thus, in order
to synthesize nanoparticles simply and unrestrictedly, it is
necessary to improve the aqueous sol–gel method. The
present paper describes an innovative and simple sol–gel
process for the preparation of nanoparticles at relatively
low temperature without any water and out of the limit of
pH, that is, the nonhydrolytic sol–gel [21–24] method.
Nonhydrolytic sol–gel method is a condensation reaction
by various ways of heating reactants, leading to a forma-
tion of sol or gel, without the hydrolytic of metal alkoxides.
Here, in this contribution, ZnO and Al2O3 nanoparticles
are used as lubricant additives together, demonstrating
fantastic properties in both antifriction and antiwear aspect.
It is also found that the monodispersed ZnO/Al2O3 com-
posite nanoparticles can be synthesized by the nonhydro-
lytic sol–gel method. The prepared ZnO/Al2O3 composite
nanoparticles show good dispersion stability in the lubri-
cant oil. The friction and wear behaviors of the lubricant
added with different concentrations of ZnO/Al2O3 are
studied through the four-ball friction test and thrust ring
friction test, exhibiting better properties compared with the
case when pure ZnO or Al2O3 nanoparticles are used as
lubricant additives. Then, the possible lubrication mecha-
nism is discussed in detail, especially the interactions
between ZnO and Al2O3 nanoparticles.
2 Experimental section
2.1 Nanoparticle preparation
First, the precursors (ZnCl2 and AlCl3) were dissolved in a
reasonable amount of acetone under continuous magnetic
stirring for 30 min. Then, the acetone was added dropwise
to oleic acid at room temperature under continuous stirring
for 10 h. Next, the mixture was placed into a stainless steel
autoclave, sealed and kept at 140 �C for 10 h under
autogenous pressure. At last, the organosol containing
different nanoparticles (ZnO, Al2O3 or ZnO/Al2O3) were
obtained.
The amount of the starting materials was added as
nZnCl2: noleic acid = 1:3; nAlCl3: noleic acid = 1:4; the con-
centration of ZnCl2 and AlCl3 in acetone is 0.1 g/ml. Molar
ratio of ZnCl2 and AlCl3 (1:1, 1:2, 1:3) was adjusted to
obtained ZnO/Al2O3 composite nanoparticles with differ-
ent elements content. As comparison, the pure ZnO and
Al2O3 were also prepared by adjusted amount of ZnCl2 and
AlCl3.
2.2 Characterization of dispersion stability
The dispersion stability of nanoparticles in lubricant was
investigated by absorption evolutions and sedimentation
tests. The ZnO/Al2O3 composite nanoparticles were added
into 20 # mechanical oil at a concentration of 1 wt%. The
absorbance of pure lubricating oil and oil with nanoparti-
cles was measured every 12 h in the absorption evolutions
test with a UV spectrophotometer with a light source of
190 nm. At the same time, the pure lubricating oil and oil
with nanoparticles were kept at room temperature to
compare the sedimentation of nanoparticles in oil.
2.3 Characterization of the tribological properties
The as-prepared ZnO, Al2O3 and ZnO/Al2O3 composite
nanoparticles were ultrasonically dispersed for 30 min into
20 # mechanical oil with different mass concentration of
0.05, 0.1, 0.5 and 1.0%. The nanoparticles are well-dis-
persed in the lubricating oil (without precipitation for at
least 1 month). The four-ball friction test and thrust ring
friction test of the lubricating oil with different nanopar-
ticles were performed using a friction-abrasion testing
machine (MMU-10G, Jinan). The structural sketch maps of
the four ball test and thrust ring friction test were shown in
Fig. 1.
Fig. 1 Structural sketch map of a the four ball test and b thrust ring
friction test
502 J Sol-Gel Sci Technol (2012) 61:501–508
123
2.3.1 Four-ball friction test
The antiwear properties were tested by the four-ball fric-
tion test, during which three balls located below were
tightly fixed and the top ball was driven and rotated on the
three supporting balls. Four-ball wear test parameters were
set as follows: temperature at 75 �C, speed of 1,450 round/
min, load at 147 N, and time of 30 min. The ball was in
accordance with the steel ball GB/308-89 manufacturing,
GCr15, two steel balls, diameter 12.7 mm; hardness
64–66 HRc. The size of wear scar diameter (WSD) was
observed with a metallographic microscope and then the
WSD was measured and averaged.
2.3.2 Thrust ring friction test
The antifriction properties were tested by thrust ring fric-
tion test. In thrust ring friction measurement, the test loop
was suspended in order to push the ring in the form of
surface contact friction. Thrust ring wear test parameters
were set as follows: temperature at 75 �C, speed of 1, 200
round/min, load at 200 N, and time of 30 min. The thrust
ring is 45 # steel, hardness of 45–50 HRC, with outside
diameter of 50 mm, inner diameter of 42 mm, and thick-
ness of 5 mm; test ring is 45 # steel, hardened to 44–46
HRC, with outside diameter of 54 mm, inner diameter of
38 mm, and thickness of 10 mm. After thrust ring friction
test, the friction coefficient was recorded automatically by
a data terminal processing system with a computer. Scan-
ning Electron Microscopy (SEM) was employed to observe
the morphologies of the transferred films formed on the
thrust ring surface during the sliding process. In addition,
Energy-dispersive X-ray spectrometer (EDS) was used for
determination of the element content in transferred films
formed on the thrust ring surface.
3 Results and discussion
3.1 Dispersion stability analysis
Figure 2 shows the XRD patterns of as-prepared particles
and particles after a heat treatment under different tem-
perature for 2 h. There is no diffraction peaks can be
observed for the as-prepared nanoparticles at 140 �C,
which means the obtained nanoparticles are amorphous
ZnO and Al2O3 or poorly crystallized. After heat treat-
ments at 200 and 800 �C, several peaks were observed as
shown in Fig. 2c. By comparison the XRD peaks of pure
ZnO and Al2O3 prepared by similar synthesis routes, the
phase of ZnO, Al2O3 and a new zinc aluminate (Zn2AlO4)
were observed, which means the annealing induced crys-
tallization and crystal growth occurs.
Figure 3 is the TEM images and size distribution his-
tograms of different particles. It is shown that the as-
obtained particles were monodispersed with the average
size of about 53, 66 and 124 nm, respectively. During the
preparation process, the nanoparticles were modified by
oleic acid to prevent their aggregations. Macromolecular
chains grafted on the surface of nanoparticles induced
repulsive force and steric hindrance effect, preventing the
agglomeration effect of the nanoparticles [25–27]. There-
fore, the suspension of the lubricating oil after the intro-
duction of modified nanoparticles is very stable, which is
verified by the following absorption evolution and sedi-
mentation tests.
Figure 4 shows the curves of the absorbance changes
of pure oil and lubricating oil with 1 wt% nanoparticles
as a function of time. As is shown in Fig. 4, the
absorbance of pure oil waves up and down around 2.06,
while absorbance of oil with 1 wt% ZnO/Al2O3 com-
posite nanoparticles waves up and down around 2.3. The
two cures in Fig. 4 have almost the same trend. This
indicates that the lubricating oil dispersed with nano-
particles were very stable without any precipitation in
300 h.
The oil was kept at room temperature and the results of
sedimentation tests of ZnO/Al2O3 composite nanoparticles
suspended in lubricating oil were shown in Fig. 5. It is
shown that such solution exhibits good turbidity. This
behavior is typical of well-dispersed suspensions and
smaller particles have much slower deposition rates, which
might be counter balanced by Brownian motion [28]. Even
after 28 days, the solution containing ZnO/Al2O3 com-
posite nanoparticles remained turbid. It indicates that the
suspension of the mechanic oil after the introduction of
modified nanoparticles is very stable.
Fig. 2 XRD patterns of nanoparticles after a heat treatment under
different temperature for 2 h, (a) ZnO/Al2O3, 140 �C; (b) ZnO/Al2O3,
200 �C; (c) ZnO/Al2O3, 800 �C; (d) Al2O3, 800 �C; (e) ZnO, 800 �C
J Sol-Gel Sci Technol (2012) 61:501–508 503
123
Fig. 3 TEM images and size
distribution histogram of
different particles a and d ZnO;
b and e ZnO/Al2O3; c and
f Al2O3
Fig. 4 Absorbance-time line chart of (a) oil and (b) oil with 1 wt%
ZnO/Al2O3 nanoparticlesFig. 5 Images of pure oil and oil with 1 wt% ZnO/Al2O3 composite
nanoparticles for different time
504 J Sol-Gel Sci Technol (2012) 61:501–508
123
3.2 Influence of nanoparticle concentrations
on the friction and wear behaviors
The variation of friction coefficient with time at different
additive concentrations of ZnO/Al2O3 composite nano-
particles is shown in Fig. 6a. The friction coefficient
obviously decreased with increasing concentration of ZnO/
Al2O3 composite nanoparticles up to 1 wt%. The data show
that the friction coefficient of the samples studied has a
smallest value when 0.1 wt% of ZnO/Al2O3 composite
nanoparticles existed in the lubricating oil. If physical
mixture of similar ZnO and Al2O3 nanoparticles were
added, the biggest friction coefficient reduction under an
optimized concentration was 9.8%, which is only quarter of
ZnO/Al2O3 composite nanoparticles (37.5%).
The effect of additive concentration of ZnO/Al2O3 on
the WSD is shown in Fig. 7. The size of the WSD gradu-
ally decreases as the addition of the ZnO/Al2O3 nanopar-
ticle increases in the oil. When the amount of the ZnO/
Al2O3 nanoparticles addition [0.1 wt%, the size of the
WSD turns to become large. However, the size of WSD
created after the addition of ZnO/Al2O3 nanoparticles (\1
wt%) in the lubricant oil is always smaller than that created
in pure oil.
According to previous studies, as the concentration of
nanoparticle increases, the friction and WSD decreases at
the beginning, but if further increase the nanoparticle
concentration, the friction and WSD increases due to the
agglomeration and precipitation of nanoparticles [29, 30].
As a result, the addition of a small amount of ZnO/Al2O3
composite nanoparticles can reduce the friction and
improve the antiwear abilities of the lubricating oil. This is
in good agreement with our experimental results. Consid-
ering the friction coefficient and the WSD value evolutions
as a function of the ZnO/Al2O3 addition amount, the
optimal concentration of ZnO/Al2O3 composite nanopar-
ticles appears to be 0.1 wt%. This proportion was chosen to
investigate the effect of mass fraction of ZnO in ZnO/
Al2O3 composite nanoparticles added into the lubricating
oil on the friction and wear behaviors.
3.3 Effect of the mass fraction of ZnO in ZnO/Al2O3
on friction and wear behaviors
Figure 8 shows the variation of the friction coefficient and
the WSD with the mass fraction of ZnO in ZnO/Al2O3,
which was measured for the lubricating oil with 0.1 wt%
ZnO/Al2O3 composite nanoparticles. It can be seen that
pure ZnO particles addition in oil has the best anti-friction
effect, but the lowest anti-wear effect. Oppositely, pure
Al2O3 introduction in the oil has the best anti-wear effect,
but the lowest anti-friction effect. Interestingly, ZnO/Al2O3
composite nanoparticles have a comparable antifriction
behavior with pure ZnO nanoparticles, but tremendously
enhanced antiwear behaviors caused by the Al2O3 species.
This indicates that the good anti-friction effect from ZnO
and the good anti-wear effect from Al2O3 can be perfectly
inherited by the ZnO/Al2O3 composite nanoparticles
without sacrificing one aspect (antifriction or antiwear) too
much. It is also found that different antifriction and anti-
wear performance can be obtained by adjusting the mass
fraction of ZnO in ZnO/Al2O3 composite nanoparticles
when testing under lubricating oil doped with 0.1 wt%
ZnO/Al2O3 composite nanoparticles. The possible empiri-
cal equations of the friction coefficient and WSD changed
with the mass fraction of ZnO in ZnO/Al2O3 were calcu-
lated by a linear fitting analysis to the data, as shown in
Eqs. 1 and 2,
yf ¼ �0:0023xþ 2:0577; R2f ¼ 0:94; ð1Þ
yw ¼ 33:68xþ 312:36; R2w ¼ 0:96; ð2Þ
Here, yf in Eq. 1 and yw Eq. 2 represent the friction
coefficient and the WSD, respectively; x in Eqs. 1 and 2
represents the mass fraction of ZnO in ZnO/Al2O3
Fig. 6 Variation of friction coefficient with time measured by thrust-
ring tester, a lubricating oil with different concentration of ZnO/
Al2O3 (1:1) composite nanoparticles, b lubricating oil with different
concentration of physical mixture of ZnO and Al2O3 (1:1)
nanoparticles
J Sol-Gel Sci Technol (2012) 61:501–508 505
123
composite nanoparticles. According to Eqs. 1 and 2, the
friction coefficient decreases and the size of the WSD
increases in a nearly linear way as the increase of the mass
fraction of ZnO in ZnO/Al2O3, testing under lubricating oil
doped with 0.1 wt% ZnO/Al2O3 composite nanoparticles.
In consequence, the difference of lubricating effect among
ZnO, Al2O3 and ZnO/Al2O3 was explained by an analysis
of the worn surface.
3.4 Analysis of worn surface
Figure 9 shows the SEM images of wear surface after
thrust-ring test and Fig. 10 shows EDS of transferred films
formed on the surface of thrust-ring after a friction test.
Figure 9a is an original surface of the thrust-ring as ref-
erence. From Fig. 9b, we can see that the wear scars of
thrust-ring rubbed under pure oil are very deep and wide
after a friction test. There are obvious scratches and fur-
rows on the surface layer. It indicates that the surface of the
thrust-ring has very serious wear and loss. From Fig. 9c,
we can see that the surface of thrust-ring rubbed under oil
doped with 0.1 wt% ZnO/Al2O3 nanoparticles before
ultrasonic washing was covered with a layer of transferred
film dispersed with some particles. The scratches and the
furrows were filled with particles. According to Fig. 10a,
some proportion of Zn, Al and O element exists in the
transferred film. Fe, part of C and O come from the thrust-
ring and contaminations. This indicates that ZnO/Al2O3
composite particles transferred onto the thrust-ring surface
together with the lubrication and a protective film was
formed during the rubbing process. At the presence of the
ZnO/Al2O3 composite particles, friction between pairs was
changed from sliding friction to rolling friction with the
nanoparticles as bearings. Therefore, the friction coeffi-
cient was reduced as shown in Fig. 6.
From Fig. 9d, we can see that the scratches of friction
surface after friction test were smoother and narrower than
that of Fig. 9b. According to Fig. 10b, some Al element
still exists on the friction surface, while no Zn element can
be found on the friction surface after ultrasonic washing for
30 min. This indicates that some proportion of Al2O3
particles are difficult to be removed by forming hard pro-
tective film through embedding into the thrust-ring surface
under instantaneous high-temperature and continuous load,
Fig. 7 Images of wear scar rubbed under oil doped with ZnO/Al2O3 nanoparticles at different concentration of a 0 wt%; b 0.05 wt%; c 0.1 wt%;
d 0.5 wt%; e 1.0 wt%
Fig. 8 Variation of friction coefficient and WSD with mass fraction
of ZnO in ZnO/Al2O3 tested under lubricating oil added with 0.1 wt%
ZnO/Al2O3 composite nanoparticles (the lines in the image is for
guiding eyes)
506 J Sol-Gel Sci Technol (2012) 61:501–508
123
while ZnO particles just transferred on the friction surface
by physical adsorption and can be easily ultrasonically
removed. This can explain why the Al2O3 exhibits better
anti-wear effect but lower anti-friction effect than ZnO, as
shown in Fig. 8.
4 Conclusions
In summary, the monodispersed ZnO/Al2O3 composite
nanoparticles are prepared by a nonhydrolytic sol–gel
method. The performance of the lubricating oil using ZnO/
Al2O3 composite nanoparticles as additives is better in
comparison with pure ZnO or Al2O3 nanoparticle additives,
which was investigated by the thrust-ring test and the four-
ball test. In exploring the lubricant performance, it is found
that there is an optimal concentration of nanoparticle
additives, which is 0.1 wt% for the tested ZnO/Al2O3
composite nanoparticles. Compared with the pure ZnO and
Al2O3, the ZnO/Al2O3 composite nanoparticles as lubricant
additives exhibit good antifriction and antiwear behaviors
simultaneously owing to the adduct effect between ZnO
and Al2O3 nanoparticles. The ZnO/Al2O3 nanoparticles
improve the lubrication properties by turning the sliding
friction into rolling friction and forming hard Al2O3 pro-
tective film after transferring to the friction surface. This
study underlies that composite nanoparticle additives (e.g.,
ZnO/Al2O3) have industrial application potentialities in
improving the lubricant oil performances.
Acknowledgments The authors acknowledge the financial support
from National Science Foundation of China (11174112) and the
Program for New Century Excellent Talents in University of MOE,
China. BC thanks the Oversea Taishan Scholar Professorship
(TSHW20091007) tenured at University of Jinan.
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