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coatings
Article
Tribological Properties of New Cu-Al/MoS2
Solid Lubricant Coatings Using Magnetron
Sputter Deposition
Ming Cao 1ID , Lan Zhao 2, Libin Wu 3and Wenquan Wang 1,*
1
College of Materials Science and Engineering, Jilin University, No. 5988 Renmin Street, Changchun 130025,
China; caoming16@mails.jlu.edu.cn
2Advanced Materials Research and Development Center, Zhejiang Industry and Trade Vocational College,
No. 717 Fudong Road, Wenzhou 325003, China; cm0624@mail.zjitc.net
3College of Chemistry and Materials Engineering, Wenzhou University, No. 276 Xueyuan Middle Road,
Wenzhou 325035, China; 16451282236@wzu.edu.cn
*Correspondence: wwq@jlu.edu.cn; Tel.: +86-136-0440-4468; +86-0431-85-094-375
Received: 10 February 2018; Accepted: 26 March 2018; Published: 6 April 2018
Abstract:
The increasing demands of environmental protection have led to solid lubricant coatings
becoming more and more important. A new type of MoS
2
-based coating co-doped with Cu and Al
prepared by magnetron sputtering, including Cu/MoS
2
and Cu-Al/MoS
2
coatings, for lubrication
applications is reported. To this end, the coatings were annealed in an argon atmosphere
furnace. The microstructure and the tribological properties of the coatings prior to and following
annealing were analyzed using scanning electron microscopy, energy dispersive spectrometry, X-ray
diffractometry (XRD) and with a multi-functional tester for material surface properties. The results
demonstrated that the friction coefficient of the Cu/MoS
2
coating was able to reach as low as 0.07,
due to the synergistic lubrication effect of the soft metal Cu with MoS
2
. However, the wear resistance
of the coating was not satisfied. Although the lowest friction coefficient of the Cu-Al/MoS
2
coatings
was 0.083, the wear resistance was enhanced, which was attributed to the improved the toughness
of the coatings due to the introduction of aluminum. The XRD results revealed that the
γ2
-Cu
9
Al
4
phase was formed in the specimen of Cu-Al/MoS
2
coatings. The comprehensive performance of the
Cu-Al/MoS
2
coatings after annealing was improved in comparison to substrate heating, since the
heat-treatment was beneficial for the strengthening of the solid solution of the coatings.
Keywords:
Cu/MoS
2
coatings; Cu-Al/MoS
2
coatings; tribological properties; annealing treatment;
γ2-Cu9Al4phase
1. Introduction
The typical materials that can be used as solid lubrication coatings are mainly soft metals
(Au, Ag, Cu, Pb), metal compounds (metal oxides, metal halides, metal sulfides, metal selenides),
organic materials (Polytetrafluoroetylene, Polyethylene, Polyamide) and et al [
1
–
4
]. Among all the
metal sulfide systems (FeS, WS
2
, MoS
2
, ZnS), MoS
2
is widely used, due to its excellent lubricating
properties. The excellent lubrication performance of MoS
2
is due to the hexagonal crystal structure of
the corresponding layer. MoS
2
exhibits strong chemical bonds within the layers (Mo-S bonds) and
weak Van der Waals bonds between individual MoS
2
layers, which cause it to have an anisotropic
characteristic. Furthermore, the anisotropy of its crystal structure can improve its load capacity,
because the edge planes of MoS
2
have high reactivity and hardness [
5
]. Conversely, as is commonly
known, the usage environment has a significant effect on the performance of MoS
2
-based lubricating
coatings; for example, the service temperature [
6
], the humidity [
7
] and the contact load [
4
] will all
Coatings 2018,8, 134; doi:10.3390/coatings8040134 www.mdpi.com/journal/coatings
Coatings 2018,8, 134 2 of 16
affect the service life of MoS
2-
based lubricant coatings. To improve the comprehensive performance
of MoS
2
-based coatings, a large amount of research has already been conducted. The Physical Vapor
Deposition (PVD) method is one of the most efficient methods for preparing MoS
2
-based lubricant
coatings. Experimental results reveal that metal doping presents a simple and economical method of
improving the comprehensive properties of MoS2-based coatings. It is generally believed that doped
soft metals, such as Au [
8
–
10
] and Pb [
11
,
12
], can reduce the friction coefficients of coatings due to
the synergistic lubrication effects, while such coatings have the disadvantage of poor wear resistance.
In contrast, the co-doping of metals such as Ti [
13
,
14
], Ni [
15
], Cr [
16
,
17
], Al [
10
,
18
], Zr [
19
], Nb [
20
]
and Mo [
21
] would be appropriate for reducing the friction coefficient and improving the mechanical
properties of the coatings. Complementary multi-metal properties can be achieved by using magnetron
co-sputtering, which makes up for the deficiency of single-metal. Therefore, there is great potential
value in research into the coatings of binary metal co-doped MoS
2
. Until recently, few studies had been
carried out on the preparation of MoS
2
composite coatings through binary metal co-doping. ILIE [
21
]
studied the MoS
2
/(Ti, Mo) composite coatings through Ti-Mo co-doping, and found that the apparent
scratches occurred during wear testing, and the overall performance was worse when compared to
MoS
2
/Ti, indicating that the addition of Mo did not improve the friction properties of the coatings.
Ren and co-workers [
22
] fabricated MoS
2
-based coatings by means of the co-sputtering of Pb-Ti,
one soft and one hard material, and determined that the surface roughness of the coatings decreased
as the Pb content increased. Additionally, the microstructure of cross-section turned from porous
columnar into the dense amorphous. The hardness and elastic modulus of the coatings decreased as
the Pb content increased, and the tribological properties were improved. Adversely, Pb is an element
that causes environmental problems, while the processing of Ag and Au is not easy to control for a
high sputtering yield. Since copper and copper alloys are known to be good lubricant additives [
23
,
24
],
Cu was selected to be the soft metal incorporating into MoS
2
, with a view to the practical application
of the lubricant coatings. In terms of the strengthening and potential oxidation resistance at higher
ambient temperature of the solid solutions, Al was selected as a binary metal for doping into MoS
2
.
For the purposes of thermal stress reduction and the improvement of the surface uniformity of the
coatings, subsequent annealing of the coatings was performed.
Firstly, a series of Cu-MoS
2
coatings was prepared by co-sputtering under various power values of
the Cu target. Through the optimization of the process parameters, the Cu-MoS
2
coatings with superior
tribological properties were obtained. Subsequently, the co-sputtered Cu-Al/MoS
2
coatings were
prepared to further optimize the MoS
2
-based coating process. Finally, the prepared composite coatings
were annealed under argon atmosphere to obtain superior tribological properties. Through this process,
Cu-Al/MoS
2
composite coatings with a new composition system were discovered. The optimal process
parameters of the composite coatings were obtained based on the analysis of the relationship between
the microstructure and the tribological properties.
2. Materials and Methods
2.1. Preparation of Coatings
The composite lubrication coatings were deposited by a DEER DE500 magnetron sputtering
system. A
Φ
50 mm target of Al (99.999%) and a
Φ
50 mm target of Cu (99.995%) with direct current
(DC) sputtering power, as well as a
Φ
50 mm target of MoS
2
(99.99%) with radio frequency (RF) power
had been used. The coatings were deposited on polished SUS 201 steels and single-crystal silicon
wafers synchronously. The test steels of 20
×
20
×
1 mm
3
in dimensions were designed for friction
tests. In addition, the steel had an uncoated average roughness (R
a
) below 24 nm (mirror finish) and a
hardness of 30 HRC. The silicon wafers were used for structural analysis. The substrates were initially
cleaned ultrasonically in acetone and an alcohol bath for 20 min, consequently rinsed in deionization
water, as well as dried under nitrogen and placed in the vacuum chamber. Prior to coating deposition,
an aluminum intermediate layer of approximately 35 nm in thickness was deposited on the substrate.
Coatings 2018,8, 134 3 of 16
Then deposition of the main bulk of the MoS
2
-based coatings was performed for 3 h. The base pressure
in the sputter chamber was about 5
×
10
−4
Pa and the working pressure was 1 Pa. Argon was utilized
as the working gas for sputtering. The target-substrate distance was 16 cm and the substrate rotation
speed was 5 rpm. As presented in Table 1, a series of MoS
2
-based coatings was prepared by adjusting
the power values of the Cu and Al target, while the power values of the MoS
2
target was retained
at 100 W. To improve the bonding force between the substrate and the coating, various substrate
temperatures were also utilized in preparing the coatings.
Table 1. Parameters for fabrication of MoS2-based composite coatings.
Sample No Target Power/W Substrate Heat Temp./◦C
MoS2Cu Al
Cu5 100 5 0 Unheated
Cu10 100 10 0 Unheated
Cu20 100 20 0 Unheated
Cu40 100 40 0 Unheated
Cu50 100 50 0 Unheated
Cu60 100 60 0 Unheated
Cu10(100 ◦C) 100 10 0 100
Cu10(200 ◦C) 100 10 0 200
Cu5Al5 100 5 5 Unheated
Cu5Al10 100 5 10 Unheated
Cu10Al5 100 10 5 Unheated
Cu10Al10 100 10 10 Unheated
Cu5Al5(100 ◦C) 100 5 5 100
Cu5Al5(200 ◦C) 100 5 5 200
2.2. Annealing of Coatings
It is generally recognized that post-annealing release of residual stress of the coatings [
25
], without
significant phase and lattice constant changes in the as-deposited coatings, even subsequently to
annealing under vacuum at higher temperature [
26
]. Adversely, the results in [
27
] demonstrated that
the MoS2layers became crystalline following annealing at approximately 350 ◦C. In order to observe
the effect of annealing temperature on the Cu-Al/MoS
2
composite coatings properties, the coatings
were annealed at 300
◦
C, 400
◦
C and 500
◦
C under argon atmosphere based on the phase diagram of
Cu-Al [
28
]. The chamber of controlled atmosphere furnace was evacuated down to a residual pressure
of 3 Pa and consequently was purged with argon to remove oxygen. These processes were repeated
three times. The argon pressure during annealing was 0.06 MPa. The heating rate was 5 ◦C/min and
the holding time was 2 h. Finally, the coatings were cooled down to the room temperature while the
furnace was turned off. The microstructures and tribological properties of the coatings along with
subsequent annealing at various temperatures were analyzed and compared.
2.3. Structure Characterization
The 3D-reconstruction of the surface roughness of the composite coatings was obtained with a
scanning electron microscope (SEM; Phenom XL, Phenom-World B.V., Eindhoven, The Netherlands).
Both the microstructure and cross-sectional morphology of the coatings were observed using a field
emission scanning electron microscopy (FESEM; Hitachis-4800, Hitachi Co. Ltd., Tokyo, Japan).
The elemental compositions of the coatings were characterized with an X-ray energy-dispersive
spectrometer (EDS; Horiba EX250, Horiba Co. Ltd., Kyoto, Japan). The phase structures of the coatings
were examined by an X-ray diffractometer (XRD; Bruker D8 Advance, Bruker Inc., Karlsruhe, Germany)
with Cu Kαradiation. The scanning range was from 10◦to 65◦and the scanning rate was 5◦/min.
Coatings 2018,8, 134 4 of 16
2.4. Tribological Experiments
A reciprocating friction and wear tester (MFT-4000, HuaHui Instruments technology Co.
Ltd., Lanzhou, China) was utilized in the tribological testing of the coatings with a ball-on-plate
configuration. The friction and wear characteristics of the coatings were evaluated at 22
±
2
◦
C,
while the relative humidity in testing was approximately 60
±
10%. The tribological testing were
carried out with 2 Hz reciprocating frequency, 5 mm stroke length, 10 N normal forces and a GCr15
steel ball (
Φ
4 mm; hardness HRC60) as the friction pair. The corresponding theoretical initial Hertz
contact pressure was 1.24 GPa. The average friction coefficients were automatically recorded with a
computer. Subsequently to tribological testing, the SEM and optical microscope (OM; Zeiss Scope A1,
Carl Zeiss, Gottingen, Germany) were utilized to observe the morphology of the wear tracks. The
adhesion quality of the MoS
2
-based coatings to the steel substrate was obtained by use of the MFT-4000
scratch tester with a 0.2 mm tip radius diamond indenter. Testing parameter was a scratch length of 5
mm at a load rate of 100 N/min. The different depth profiles of the wear tracks were examined with a
surface profilometer (JB-4C; Shanghai Taiming Optical Instrument Co. Ltd., Shanghai, China), that was
the grinding crack depth along with the grinding crack width was assumed consistent. The wear
scar volumes were calculated according to the Archard wear equation [
29
] by measuring the average
cross-sectional area of track profiles and then multiplying for the track length. The result was used to
compare wear rates qualitatively for the errors would exist in the measurements of cross-sectional area
with the JB-4C type surface profilometer.
3. Results and Discussion
3.1. Coating Composition and Structural Characterization
Figure 1depicts the surface and cross-sectional micrographs of the composite coatings prepared
with various process parameters. Figure 1a presents a pure MoS
2
coating with worm-shaped surface
morphology, which was consistent with the results reported in [
30
]. The cross-section morphology of
the MoS
2
coating was lamellar perpendicularly to the surface. Figure 1b–h presents the micrographs of
the Cu/MoS
2
coatings prepared using magnetron co-sputtering and the DC power of Cu target ranged
from 5 W to 60 W. The surfaces of the Cu40, Cu50 and Cu60 coatings turned into coarse grained shapes,
whereas the cross-section displayed irregular hill shapes with an incompact microstructure. The surface
morphologies of the Cu5, Cu10 and Cu20 coatings were smooth, mainly displaying low-sized particles
of approximately 20–50 nm, whereas the cross-section presented a dense morphology. Adversely,
a low amount of grains of approximately 100–300 nm existed on the coating surfaces of the Cu15 and
Cu20 coatings. The cross-sectional microstructure of the Cu10 coating was lamellar perpendicularly
to the surface, which was more compact compared to the pure MoS
2
coating. The cross-section of
the Cu20 coating was densely columnar perpendicularly to the surface. Figure 1i,j depict the surface
micrographs of the Cu10 coatings deposited at 100
◦
C and 200
◦
C. The surface morphology of the
coatings changed from granular to worm-like as the temperature increased from 100
◦
C to 200
◦
C,
and the cross-sectional structure became looser. The results demonstrated that the appropriate amount
of Cu-doping changed the growth mode of MoS
2
, which led to the transformation of the coatings from
columnar to densely granular. As the DC sputtering power increased, the granular microstructure
on the surface grew rapidly and the surface roughness increased. Consequently, the power range
of 10 W–20 W was appropriate for preparing a uniform and dense coating. The substrate-heating
accelerated the formation of granular microstructure and increased the sputtering rate of Cu under the
same sputtering power. The worm-like morphology in the Cu10 (200
◦
C) coating similarly to the pure
MoS
2
coating was observed. Similar results were found in the literature [
31
]. The results indicated that
the increasing of substrate-heating temperature was beneficial to the formation of MoS2.
Coatings 2018,8, 134 5 of 16
Figure 1. SEM surfaces and cross-sectional morphologies of composite coatings on silicon wafers.
Figure 1k–n presents the surface and cross-sectional micrographs of the coatings, prepared
simultaneously using magnetron co-sputtering of the Cu and Al targets. A granular surface
morphology with tiny gaps was observed in the Cu5Al5 coating. Figure 1n illustrates certain particles
with diameter in the total range of 200–500 nm were generated on the Cu10Al10 coating surface.
The cross-sectional morphology of the Cu5Al5 coating was dense granular, whereas the other three
coatings were columnar. Figure 1o,p presents the morphologies of the Cu5Al5 composite coatings,
as prepared with the substrates heated to 100
◦
C and 200
◦
C. Figure 1o presents the granular surface
morphology of the Cu5Al5 (100
◦
C) coating with apparent agglomeration, and the tiny gaps between
the agglomerated particles decreased. Moreover, the surface morphology of the Cu5Al5 (200
◦
C)
coating turned into a worm-like shape, whereas the cross-section demonstrated loose microstructure,
as presented in Figure 1p, which was consistent with the Cu10 (200
◦
C) coating. The microstructure
analysis of the bimetallic co-sputtered coatings revealed that Al-doping promoted the dense growth of
the coatings. Also, to avoid the high-sized grains on the coating surface, the sputtering power of Al
and Cu targets should also be reduced properly.
Figure 2shows the morphologies of the MoS
2
-based coatings prior to and following annealing
at 300
◦
C, 400
◦
C and 500
◦
C under argon atmosphere. Figure 2a presents the surface microstructure
of the MoS
2
coatings prior to and following annealing. There was no apparent change occurred in
the surface morphology subsequently to various annealing temperatures. Figure 2b presents the
Coatings 2018,8, 134 6 of 16
tendency of single grains to agglomerate on the Cu10 coating, whereas the gaps among agglomerations
reduced gradually as the temperature increased to 500
◦
C. Figure 2c presents that the large grain on
the Cu20 coating surface disappeared following annealing at 300
◦
C. The gaps among the surface
grains were gradually reduced and the grain size increased following annealing at 400
◦
C. The grain
size of the coating surface increased significantly after annealing at 500
◦
C. Figure 2d–f presents the
surface morphology of the Cu5Al5 coatings prior to and following annealing at different substrate
temperatures. The grain-size and compactness of the Cu5Al5 coating surface increased gradually
as the annealing temperature increased. The Cu5Al5 (100
◦
C) and Cu5Al5 (200
◦
C) coating surfaces
also exhibited good uniformity and compactness as the annealing temperature increased. The surface
morphology of the Cu5Al5 (200
◦
C) was similar to the Cu10 (200
◦
C) coating, which indicated that
similar effects in the Cu-Al/MoS2coatings might exist while the substrate was heated.
Figure 2. SEM morphologies of composite coatings at various heat treatment temperatures.
Table 2presents the atomic percentages of the elements in the Cu/MoS
2
coatings. The typical EDS
spectrum of Cu10 and Cu20 were shown in Figure 3a,b. The percentage of Cu atoms in the coating
increased gradually as the sputtering power increased. The content of Cu in the coating increased from
9.57 at% to 25.13 at%, which indicated the higher sputtering efficiency of Cu within the power range of
10 W–20 W. The ratios of N
S
/N
Mo
in the Cu5–Cu50 coatings were lower than the pure MoS
2
coating,
which occurred due to the reverse sputtering of S during the deposition. The reverse sputtering would
decrease the deposition rate, which was good for the MoS
2
formation [
32
]. The value of N
S
/N
Mo
in
the Cu5 coating was below 1, whereas fewer S atoms were not conducive to the formation of the MoS
2
(002) basal plane [
22
] which affected the tribological properties of the coatings. The substrate-heating
was beneficial to the deposition of Cu atoms, thereby further affecting the tribological properties of
the coatings.
Coatings 2018,8, 134 7 of 16
Table 2. Quantitative results of Cu/MoS2coatings elemental analysis, determined through EDS.
Composite Coating Cu/at.% NS/NMo
MoS21.50
Cu5 7.80 0.78
Cu10 9.57 1.18
Cu20 25.13 1.34
Cu40 40.69 1.42
Cu50 46.21 1.50
Cu60 47.99 1.78
Cu10 (100 ◦C) 28.55 1.30
Cu10 (200 ◦C) 35.50 1.76
Figure 3. EDS spectrum corresponding to the (a) Cu10; (b) Cu20; and (c) Cu5Al5.
The EDS analysis results of the Cu-Al/MoS
2
coatings are presented in Table 3. The typical EDS
spectrum acquired from the area of 10
×
10
µ
m
2
on the Cu5Al5 coating was shown in Figure 3c.
The results indicated that Al-doping promoted the deposition of Cu atoms on the substrate, whereas
the substrate heating significantly inhibited the deposition of Cu atoms on the substrate. This might be
due to the fact that a competition occurred as a result of partial substitution of Al with Cu. Being the
coatings might form a solid solution or an intermediate phase during sputtering and annealing [
33
].
According to [
34
], the
α
-Cu phase would preferentially be produced in the MoS
2
-based coatings
during the co-sputtering of Cu and Al, followed by a
γ2
-Cu
9
Al
4
intermediate phase with the minimum
activation energy. In addition, the N
Cu
/N
Al
value of the Cu5Al5 coating shown in Table 3was the
most consistent with the
γ2
-Cu
9
Al
4
intermediate phase. Another consideration is the strength and
hardness of the Cu-Al/MoS
2
coatings might increase, whereas the brittleness of the coatings might
also be caused by the new phase.
Table 3.
Quantitative results of Cu-Al/MoS
2
composite coatings elemental analysis, determined
through EDS.
Composite Coating Cu/at.% Al/at.% NCu/NAl NS/NMo
MoS21.5
Cu5Al5 24.53 10.97 2.24 1.54
Cu5Al10 16.26 8.78 1.85 1.92
Cu10Al5 28.05 4.07 6.88 1.91
Cu10Al10 27.07 5.98 4.52 1.72
Cu5Al5 (100 ◦C) 10.95 5.63 1.94 1.39
Cu5Al5 (200 ◦C) 11.42 3.44 3.32 1.58
Figure 4a presents the X-ray diffraction patterns of the Cu/MoS
2
coatings. It demonstrated that
the copper phase increased gradually as the Cu target power increased. The crystallization peak of
Coatings 2018,8, 134 8 of 16
the coatings prepared at low sputtering power of the Cu target was weak, and an amorphous-based
structure of the coatings could be deduced, which was consistent with the result found in [
22
]. Figure 4b
presents the XRD patterns of the Cu10 coating subsequently to annealing and the Cu10 coating with
the substrate heating. It could be observed that the annealing temperature had an apparent effect
on the crystalline of the coatings. When the substrate was heated to 200
◦
C, the crystallization peak
between 10
◦
and 15
◦
was enhanced. It meant that substrate heating was in favor of the crystallization
of MoS
2
, and MoS
2
tent to be formed with the (002) basal plane parallel to the surface. The XRD
patterns of the annealed coatings also proved that annealing could be more beneficial to enhance the
crystallization of the coatings than substrate heating.
Figure 4. XRD spectrum of Cu/MoS2and Cu-Al/MoS2composite coatings.
Figure 4c,d presents the XRD patterns of the Cu5Al5 coating with substrate heating and the
Cu5Al5 coating following annealing. To a certain extent, the Cu and Al co-doping into MoS
2
inhibited
the formation of the MoS
2
crystal structure due to the formation of solid solution or intermediate
phase. Simultaneously, the XRD peaks of aluminum phase were not apparent, caused by the Al
solid solubility not being exceeded and the overlapping with the MoS2 (002) peak at 2
θ
= 13
◦
in
Figure 4c [
18
]. In addition, the XRD patterns of the Cu5Al5 (200
◦
C) coating also revealed the apparent
diffraction peaks of Cu
3
MoO
2
O
9
and Al
2
O
3
. Figure 4d presents the XRD patterns of the Cu5Al5
coating annealed under argon atmosphere, which demonstrated that the
γ2
-Cu
9
Al
4
phase was formed
in the coatings, as reported in [
34
]. The results indicated that the annealing had beneficial effects on
the formation of mesophase in the Cu-Al/MoS2coatings.
3.2. Friction and Wear Evaluation
Figure 5presents the friction coefficient curves of the Cu/MoS
2
coatings. The friction coefficient
decreased firstly then increased as the Cu target power increased, as presented in Figure 5a. The friction
coefficients of the Cu10 and Cu20 were even lower compared to the pure MoS
2
coating. The average
friction coefficient of the Cu10 coating was 0.07, which was the lowest among the Cu/MoS2coatings.
In Figure 5b, the friction coefficient of the Cu10 (200
◦
C) was 0.08, which was lower compared to
Coatings 2018,8, 134 9 of 16
the Cu10 (100
◦
C), whereas both were higher than the Cu10 coating. The soft metal Cu doping into
MoS
2
could reduce the friction coefficient of the MoS
2
-based composite coatings. The EDS results
demonstrated that the friction coefficient of the Cu10 coating with the N
S
/N
Mo
ratio of 1.18 was
the lowest, which was consistent with the results found in [
35
]. It meant that the Cu10 coating was
relatively easy to obtain a MoS
2
(002) basal plane, parallel to the substrate. In addition, the morphology
of the Cu10 coating with smooth and dense surface, dense cross-sectional lamellar mentioned above
were beneficial to maintain the low friction coefficient of the coatings in the humid environment based
on the [
36
]. In Figure 5c,d, the Cu10 and Cu20 coatings annealed under argon atmosphere displayed
an overall friction coefficient increase. When the heat treatment temperature increased to 500
◦
C,
the Cu10 coating had the lowest average friction coefficient (0.088). Moreover, the minimum average
friction coefficient of the Cu20 coating was 0.074, when the heat treatment temperature increased
to 400
◦
C. The results indicated that the annealing had more influence on the friction coefficient
than substrate heating. The average friction coefficients were reduced as the annealing temperature
increased in general.
Figure 5.
Friction coefficients of Cu/MoS
2
composite coatings at various heat treatment temperatures.
Considering that the friction coefficient of the Cu5Al5 coating (0.083) was the lowest among the
Cu-Al/MoS
2
coatings, only the friction coefficient curves of the Cu5Al5 coatings treated at various
temperatures were displayed in Figure 6. Figure 6a presents the friction coefficients of the Cu5Al5
coatings at various substrate heat-treatment temperatures and the friction coefficient of Cu5Al5 (100
◦
C)
coating had a maximum value of 0.164. The friction coefficients, as presented in Figure 6b–d, decreased
as the heat treatment temperature increased. The friction coefficient of the Cu5Al5 coating annealed at
400
◦
C was minimum (0.076), where the Cu5Al5 (100
◦
C) had the lowest friction coefficient (0.068) at
the same temperature. The friction coefficient of the Cu5Al5 (200
◦
C) coating annealed at 500
◦
C had
the lowest value of 0.075. Similar to the Cu/MoS
2
coatings, the annealing was more effective than the
substrate heating on the reduction of friction coefficient. Above all, the recommended heat treatment
temperature was generally in-between 400 ◦C and 500 ◦C.
Coatings 2018,8, 134 10 of 16
Figure 6.
Friction coefficients of the Cu5Al5 composite coatings at various heat treatment temperatures.
Figure 7presents the typical OM micrographs of the wear tracks o the MoS
2
-based coatings.
As presented in Figure 7a,e, the wear debris was pushed to the edges of the wear tracks, and there were
obvious furrows along with wear tracks on the Cu/MoS
2
coatings. Following annealing, the higher
wear resistance of the Cu/MoS
2
coatings would be inferred by the OM micrographs. The results might
be caused by a higher density of the coatings according to the SEM morphologies in Figure 2. The wear
track of the Cu5Al5 coating, as presented in Figure 7i, was relatively narrower compared to the
Cu/MoS
2
coatings, whereas a low amount of wear debris accompanied with shallow furrows existed
along the sliding direction. This indicated that the wear degree was significantly lower compared
to the Cu/MoS
2
coatings. It also illustrated that the co-doping of Al and Cu could be conducive to
the abrasion resistance of the coatings for the formation of the new mesophase. The worn surfaces
presented in Figure 7were covered with grooves parallel to the sliding direction, which were typical
features of the abrasive wear.
Figure 7. OM micrographs of MoS2-based composite coating wear tracks following heat treatment.
Coatings 2018,8, 134 11 of 16
3.3. Wear Mechanisms
Scratch testing can be used to evaluate the mechanical failure modes of the coatings. Figure 8
demonstrated the features of gross spallation in the Cu10 coatings case (Figure 8a), brittle tensile
cracking followed by gross spallation in the Cu20 coatings case (Figure 8b), conformal cracking
followed by gross spallation in the Cu5Al5 (100
◦
C) coatings case (Figure 8d), conformal cracking
followed by bulking spallation in the Cu5Al5 coatings case (Figure 8c) and the Cu5Al5 (200
◦
C)
coatings case (Figure 8e). The hardness of coatings with the tensile cracking followed by spallation
was generally considered higher than the coating with conformal cracking followed by spallation and
buckling failure, which meant that the Cu20 coatings had a higher hardness. Bulking spallation in the
Cu5Al5 and Cu5Al5 (200
◦
C) coatings meant that the MoS
2
co-doping with Cu and Al improved the
toughness of the coatings. The typical wear mechanism of these coatings was the adhesive wear from
the results presented in Figure 8.
Figure 8.
Scratches of (
a
) Cu10; (
b
) Cu20; (
c
) Cu5Al5; (
d
) Cu5Al5 (100
◦
C); (
e
) Cu5Al5 (200
◦
C) coatings
annealed at 500 ◦C.
The SEM morphologies and 3D reconstructions of the worn surfaces are presented in Figure 9.
Adhesive traces were observed on the worn surfaces of the Cu10 and Cu5Al5 (200
◦
C) coatings
(Figure 9a,d). It was clear that these severe damages resulted from the shearing and rupture of the
adhesive junctions between two contacting surfaces. The Cu phase on the friction surface of the Cu10
coating might be softened by braking, which could generate adhesion and lead to material transfer
(Figure 9a). The SEM micrographs of typical worn surfaces demonstrated negligible wear scar in
the Cu20 coating (Figure 9b), which might be benefit for the enhancement of surface hardness as
mentioned above.
The deeper grooves caused by the hard asperities in Figure 9c indicated a primary mechanism of
abrasive wear. Hence, a significantly low amount of oxide patches might exist in wear traces according
to the EDS analysis (Figure 10a). The EDS results of the Cu5Al5 coating in Figure 10b indicated that a
small amount of iron filings produced, while the content of oxygen was less than the Cu10 coating
(Figure 10a). Therefore, an oxidation-abrasion mechanism could occur during tribo-testing of the
coatings. The worn surface of the Cu5Al5 coating was characterized by typical features of adhesive
and abrasive wear, considering the results in Figure 8c. The iron filings or oxide patches on the friction
surface could lead to severe wear and deep grooves on the surface.
Coatings 2018,8, 134 12 of 16
Figure 9. 3D reconstructions and SEM surface morphologies of wear tracks in coatings annealed at 500 ◦C.
Figure 10. EDS analysis of worn surfaces. (a) Cu10; (b) Cu5Al5 annealed at 500 ◦C.
3.4. Oxidation Resistance and Wear Resistance
The oxidation resistance was enhanced according to the EDS testing results on the wear tracks
as presented in Figure 11. The Cu5Al5/MoS
2
coating had the best antioxidant properties among all
coatings, especially following the annealing at 500
◦
C. This was because the mesophase promoted
the densification of the coatings, and then improved the abrasion resistance and increased the
anti-oxidation ability of the coatings during wear tests.
Figure 12 presents a qualitative comparison of the wear rate of MoS
2
-based coatings prior to
and following annealing. The results showed that both the pure MoS
2
and Cu/MoS
2
coatings had
weak wear resistance. By doping soft metal Cu with MoS
2
, the friction coefficient of the coatings
decreased due to the synergistic lubricating effects. The wear resistance of the Cu/MoS
2
coating
was not satisfactory without heat treatment. By analyzing the oxidation potential of Cu, namely the
Coatings 2018,8, 134 13 of 16
φθ
(Cu
2+
/Cu
+
) = 0.153 V and
φθ
(Cu
2+
/Cu) = 0.337 V, the oxygen absorption performance of Cu during
wear test was weak. Following annealing, the wear rates of the Cu/MoS
2
coatings began to decrease,
especially subsequently to annealing at 500
◦
C. The SEM micrographs indicated that the dense surface
of the coating might be the key factors that affected antioxidant properties.
Figure 11.
Atomic percentage of oxygen in wear tracks of the coatings annealed at 400
◦
C and 500
◦
C.
Figure 12. Wear rate of MoS2-based coatings at various heat treatment temperatures.
The abrasion resistance of the coatings co-doped with Cu and Al was significantly enhanced.
On the one hand, the addition of aluminum improved the toughness of the coatings; and on the other
hand, the formation of Cu-Al mesophase had contributed to the antioxidant property and moisture
resistance of the Cu-Al/MoS
2
coatings. The wear life of the Cu-Al/MoS
2
coatings was thereby
improved. Given the oxidation potential of
φθ
(Al
3+
/Al) =
−
1.63 V, therefore, Al in the coatings oxidize
to Al
2
O
3
was easier than Cu. The protective Al
2
O
3
layer would help to inhibit the oxygen further
infiltrate into the coating matrix. Following annealing at 400
◦
C, the wear rate of the Cu5Al5 coating
decreased to be the lowest. The loose worm-like surface morphology of the Cu5Al5 (200
◦
C) coating
led to an increasing wear rate, but subsequently to an annealing at 500 ◦C, the wear rate decreased.
4. Conclusions
•
The Cu/MoS
2
composite coatings with various Cu contents were prepared by magnetron
sputtering and the tribological properties were tested at room temperature. The Cu10 coating had
the lowest friction coefficient among all Cu/MoS
2
coatings, whereas the friction coefficient of the
Cu5Al5 coating was the lowest in all Cu-Al/MoS
2
coatings. The average friction coefficients of
the Cu10 and the Cu5Al5 coatings were both lower than the pure MoS
2
coating. The abrasion
resistance of the coatings co-doped with Cu and Al was significantly enhanced through wear
Coatings 2018,8, 134 14 of 16
rate qualitative comparison. It was very probable that the
γ2
-Cu
9
Al
4
phase in the co-sputtered
Cu-Al/MoS2coatings improved the wear resistance.
•
Through comparison and analysis of the microstructures observed by SEM, the Cu5Al5
coating with enhanced tribological properties had a smooth surface and a dense cross-sectional
non-columnar microstructure. The co-sputtering deposition with Cu and Al led to a smooth
surface on the coating. This would lead to the flaking avoidance of the coatings in the friction
experiments as well. Following annealing, the grain growth further promoted the compactness of
the Cu5Al5 coatings and the wear resistance improvement all at the same time. That was both the
low friction coefficient and low wear rate were maintained under the test conditions.
•
The surface morphologies of the Cu/MoS
2
and Cu-Al/MoS
2
coatings turned into a worm-like
shape, and the cross-section morphology exhibited a loose microstructure. While the substrate
was heated to 200
◦
C, the friction coefficient and wear rate of the coatings increased. Following
annealing at 300
◦
C, 400
◦
C and 500
◦
C, the friction coefficient decreased slightly. It meant
that the annealing was more effective than the substrate heating on the friction coefficient
reduction. Considering the wear rate, the appropriate range of heat treatment temperature
was 400 ◦C–500 ◦C.
•
The scratch test demonstrated the failure mode of conformal cracking followed by bulking
spallation in the Cu-Al/MoS
2
coatings, being different from the gross spallation of the Cu10
coating and tensile cracking followed by gross spallation in the Cu20 coating, which had a higher
abrasion resistance for the probable cause that the addition of aluminum improved the toughness
of the coatings. The worn surface of the Cu5Al5 coating was characterized by typical features of
adhesive and abrasive wear.
Acknowledgments:
The authors gratefully acknowledge the financial research support of the Project of New
Century 551 Talent Nurturing in Wenzhou (2016 No.30). The authors would also like to thank the Zhejiang
Provincial Key Laboratory for Cutting Tools for offering access to their instruments and expertise, the Zhejiang
Provincial Key Laboratory of Carbon Materials for providing the sample preparation equipment along with
the SEM and XRD facilities, as well as the Wenzhou City Key Laboratory of Materials Forming and Tooling
Technology for offering the MFT-4000 device and surface roughness testing equipment.
Author Contributions:
M.C. and W.W. conceived and designed the experiments; M.C., L.Z. and L.W. performed
the experiments; M.C. analyzed the data; M.C., L.Z. and W.W. cooperatively wrote this article.
Conflicts of Interest: The authors declare no conflict of interest.
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