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Surface & Coatings Technology 191 (2005) 1–6
Effects of bivalent Co ion on the co-deposition of nickel and
nano-diamond particles
Liping Wang, Yan Gao, Huiwen Liu, Qunji Xue, Tao Xu*
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China
Received 17 September 2003; accepted in revised form 26 March 2004
Available online 19 June 2004
Abstract
Nano-diamond particles were co-deposited on AISI-1045 steel substrates with nickel from a Watts-type bath by conventional
electrodeposition methods. The effects of Co2 + additives on the co-deposition of nano-diamond particles, the surface morphology,
microhardness, surface roughness and tribological properties of nanocomposite coatings were investigated. Results in this paper showed that
the addition of Co2 + in the Ni/diamond plating bath significantly improved the amount and uniformity of dispersed nano-diamond particles
in the metal matrix. Moreover, it has been established that the nanocomposites obtained with the addition of Co2 + in the Ni/diamond plating
bath produced much higher hardness and excellent anti-wear performance with lower friction coefficient when sliding against a steel ball. It
has been assumed that Co2 + may act as a cationic stimulator, which promoted the co-deposition of nano-diamond particles with nickel and
thus intensified the positive contribution of the embedded nano-diamond particles.
D 2004 Elsevier B.V. All rights reserved.
Keywords: Electrodeposition; Nano-diamond; Nanocomposite coatings; Nickel
1. Introduction
Electrodeposited composite coatings have been widely
developed over the past decades for various engineering
applications due to the interesting possibilities it offers.
Many researches have focused on the co-deposition of
micro-sized particles such as metallic powder, silicon car-
bides, oxides, polymer and diamond, etc. [1–7]. By com-
bining the properties of heterogeneous matrix metal and
various kinds of particles, many new function materials
were created with more comprehensive applications. How-
ever, the co-deposition of metal matrix with these micro-
sized particles led to poor dispersion of particles in sus-
pension, bad surface quality and weak bonding strength
between matrix and particles [3]. Several studies have
found that the co-deposition of nano-sized or submicron
particles with metal matrixes is superior to micro-sized
particles on qualities of the composite coatings. Among
these ultra fine particles, ultradispersed diamond particles
0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.surfcoat.2004.03.047
* Corresponding author. Fax: +86-931-496-8169.
E-mail address: [email protected] (T. Xu).
are increasingly attracting considerable scientific and tech-
nological interest by virtue of their unique mechanical and
tribological properties including higher hardness, lower
friction coefficient and inertness to chemical attack. But
recent researches have proved that these nano-sized particles
are more difficult to be co-deposited with metal than coarse
particles [2–5]. Hence, how to improve the co-deposition
content of these nano-sized particles has attracted much
attention recently.
Since the particle surface state is of great importance to
the co-deposition process, it has been widely supposed that
the above problem can be overcome significantly by the
surface modification of these particles [8,9]. More probably,
surface modification can be achieved through organic sur-
face-active additives; Attempts to increase the incorporation
of the co-deposited particles using various organic surfac-
tants in the electrolyte have been reported by many research-
ers [5,8,9]. On the other hand, inorganic additives for
modification of the particle surface state are much more
important and practical, since the addition of organic addi-
tives can cause such disadvantages as instability in the
electrolyte, high stress or brittleness of the composite
deposits, etc. Unfortunately, few studies have concentrated
Table 1
Basic bath compositions and plating conditions
Compositions and conditions
Nickel sulfate, NiSO4�6H2O (g/l) 300
Nickel chloride, NiCl2�6H2O (g/l) 50
Boric acid, H3BO3 (g/l) 40
Additive brightener A (g/l) 0.5
Nano-diamond content (g/l) 10
Temperature (jC) 45
L. Wang et al. / Surface & Coatings Technology 191 (2005) 1–62
on effects of inorganic additives on the co-deposition
process at present.
In the present work, the Ni-based nanocomposites were
deposited on AISI-1045 steel substrates by electroplating.
The effects of Co2 + on the co-deposition of nano-diamond
particles with nickel were investigated by evaluating the
microstructure, microhardness, surface roughness and tribo-
logical properties of the nanocomposite coatings.
pH 3–5Current density (A/dm2) 1–5
Stirring speed (rpm) 100–500
2. ExperimentalThe nickel and nano-diamond particles were electro-
deposited on the steel substrates from modified Watts-type
bath. The nano-diamond particles used in the experiments
were in spherical or spherical-like shapes and had an
average size of 3–10 nm obtained by explosive detonation,
as shown in Fig. 1 [10]. Prior to the co-deposition, the
diamond particles were ultrasonically dispersed in the bath
for 10 min. The basic compositions of the electrolyte and
plating conditions are shown in Table 1. Each experiment
was carried out with a fresh solution.
AISI-1045 steel plates were used as cathodes, the anode
was a pure Ni plate. Before the co-deposition, the substrates
were mechanically polished to a 0.08–0.12 Am surface
finish, then a sequence of cleanings were performed to
remove contamination on the substrate surface, the steel
substrates were activated for 20 s in a mixed acidic bath.
During the co-deposition process, the bath was slowly
stirred by a magnetic stirrer in order to keep the particle
well dispersed and prevents them from sedimentation in the
bulk of electrolyte-suspension. The temperature of the
electrolyte was maintained at a set value by an automatic
controller. The pH value of the electrolyte was adjusted by
H2SO4 or NH3�H2O. The thickness of the coatings were
fixed to 25 Am.
The surface morphology and microstructure of the
coatings were investigated using a JSM-5600 Lv scanning
electron microscopy (SEM) equipped with Kevex sigmakenergy dispersive X-ray spectroscopy (EDS) analysis tool.
Fig. 1. TEM image of nano-diamond particles obtained by explosive
detonation.
A surface profilometer was employed to measure the
surface roughness. Microhardness of the coatings was
determined using a Vicker’s microhardness indenter with
a load of 25 g for 5 s, for the selected load, the substrate
effects on hardness can be avoided, the final value quoted
for the hardness of a deposit was the average of 10
measurements.
The tribological behavior was tested on a reciprocating
ball-on-disk UMT-2MT tribometer at room temperature
with a relative humidity of 45–55% under dry sliding
conditions. A AISI-52100 stainless steel ball (diameter 4
mm with hardness of RC 62) was used as the counter body;
all tests were performed under a load of 1 N with a sliding
speed of 65 mm s� 1. The friction coefficient and sliding
time were recorded automatically during the test. The wear
volume was measured using a surface profilometer, the wear
rates of all coatings were calculated using the equation of
K =V/SF, where V is the wear volume in mm3, S is the total
sliding distance in m and F is the normal load in N.
3. Results and discussion
3.1. Effect of Co2+ on co-deposition and mechanical
properties of coatings
Ni/diamond composites were deposited under a current
density of 1.5 A dm� 2 and a pH of 4.2, while Ni–Co/
diamond composite coatings were obtained with 2.5 g dm� 3
Co2 + in the Ni/diamond plating bath under the above
mentioned plating conditions. The comparative hardness
results of composites and pure nickel (including Ni–Co)
coatings are shown in Fig. 2. Ni/diamond and Ni–Co/
diamond composites exhibited much higher hardness as
compared to the Ni and Ni–Co coatings, which is linked
with the dispersion hardening effect caused by the incorpo-
ration of nano-diamond particles in the metal matrix. More-
over, the Ni–Co/diamond composite coating showed the
highest microhardness. It is well known that the hardness
and other mechanical properties of metal matrix composites
depend on the amount of the dispersed phase and the
mechanical characteristics of the matrix [11]. For the above
two matrixes, the hardness value of the Ni–Co matrix is
slightly higher than that of a Ni matrix. However, with the
Fig. 3. SEM surface morphology of Ni (a) and Ni–Co (b) coatings.
Fig. 2. Microhardness of Ni, Ni–Co (matrix) and corresponding nano-
composites deposited under the same plating conditions.
L. Wang et al. / Surface & Coatings Technology 191 (2005) 1–6 3
addition of Co2 + in the Ni/diamond plating bath, the micro-
hardness of composite coating increased from HV687 to
HV826. It is obvious that this hardness improvement is
probably due to the higher content of nano-diamond par-
ticles embedded in the Ni–Co matrix.
In an attempt to clarify the effects of Co2 + on the co-
deposition of nano-diamond particles, typical SEM surface
morphology of Ni, Ni–Co coatings and corresponding
composites are shown in Figs. 3 and 4. It can be seen
from Fig. 3 that the Ni and Ni–Co coatings both have a
rather smooth surface, only some stripes were observed on
the surface of Ni coating, which may be caused by the
addition of brightener. It is evident in Fig. 4 that nano-
diamond particles and agglomerates with size in submicron
range are distributed in the Ni and Ni–Co matrixes
through co-deposition process. It is believed that many
nano-diamond particles agglomerated in the electrolyte and
then co-deposited into the metal matrix. However, the
SEM surface morphology and corresponding EDS analyses
of both type of composites indicated that a significant
increase of nano-diamond particles incorporated in the
metal matrix with the addition of Co2 + in the Ni/diamond
plating bath. In addition, the distribution of diamond
particles in the composite coatings deposited from the
electrolyte with Co2 + is much more uniform than that of
composite coatings without Co2 + in the electrolyte. The
above results showed that the addition of Co2 + in the Ni/
diamond plating bath significantly improved the amount
and uniformity of dispersed nano-diamond particles in the
metal matrix, which agrees well with the hardness im-
provement with the Co2 + addition in the Ni/diamond
plating bath. This is also the reason why the surface
roughness of the Ni–Co/diamond composite layer was
higher than that of Ni/diamond composite layer. Guglielmi
[12] proposed a successive two-step adsorption mechanism
for the incorporation of inert particles during the co-
deposition process, namely loose adsorption and strong
adsorption steps. The second step is thought to be the rate-
determined step in the co-deposition of particles, which
was assumed to be electric field assisted. Based on the
above results, in view of previous reports [8,13], it is
assumed that Co2 + are adsorbed on nano-diamond par-
ticles much easier compared with the Ni2 +, which might
give particle surfaces a more positive charge, and hence,
increases the forces of electrostatic attraction between the
diamond particles and the negatively charged cathode. As
a result, a higher content of nano-diamond particles were
co-deposited in the matrix.
3.2. Effect of Co2+ on tribological property of coatings
The results of wear tests for the Ni and Ni-based
coatings under the dry sliding conditions are summarized
in Table 2. It is observed that both type of Ni-based
composite coatings exhibited lower wear rates when com-
pared with pure Ni coatings, which can be attributed to the
dispersion-strengthening effect with the incorporation of
diamond particles, which is also in agreement with the
hardness results. Similar observations have been made in
case of other particles reinforced composites [14,15]. How-
ever, though the Ni–Co coating showed the lower hardness
as compared with the Ni/diamond composite coating,
against our expectations, the Ni–Co coating exhibited
lower wear rate than Ni/diamond coating. This may be
attributed to the formation of a Ni–Co solid solution.
Furthermore, the lowest wear loss was observed on the
Fig. 4. SEM surface morphology of Ni-based composites and corresponding EDS analyses (a) without the Co2 + addition (b) with the Co2 + addition deposited
for 90 min.
L. Wang et al. / Surface & Coatings Technology 191 (2005) 1–64
Ni–Co/diamond composite coating, which was seven times
and five times lower than that of Ni and Ni/diamond
coatings, respectively.
The corresponding evolution of friction coefficients
versus time is illustrated in Fig. 5. The Ni, Ni–Co and
Ni/diamond coatings all exhibited higher friction coeffi-
cients around 0.5–0.7. It is very interesting to note that
the Ni–Co/diamond composite coating, with the highest
surface roughness, showed the lowest and much more
stable friction coefficient among these four types of
coatings. Moreover, the friction coefficient of Ni/diamond
composite coating was higher than that of pure Ni
coating.
The improvement in wear resistance of Ni–Co and Ni-
based composites is confirmed by SEM images of the
worn surface as shown in Fig. 6. For the pure Ni coating,
the wear track (Fig. 6a) shows the larger extent of
adhesion wear and severe delamination in the sliding
Table 2
Wear test results of Ni-based coatings
Coatings Surface roughness
(Ra/Am)
Wear loss rate
(10� 5 mm3/Nm)
Ni 0.15 14.57
Ni–Co 0.14 6.02
Ni/diamond 0.20 10.10
Ni–Co/diamond 0.28 2.08
direction under the combined stresses of compression
and shear, which results in largest wear rate of pure Ni
coatings. Compared with the Ni coating, the worn surface
of Ni–Co coating revealed slight adhesion wear and
smaller damaged regions (Fig. 6b). Moreover, some wide
and continuous grooves are observed on wear track of Ni/
diamond coating, also many cracks were observed (Fig.
6c). However, as shown in Fig. 6d only some light and
non-continuous grooves are noted on the worn surface of
composite coating with the addition of Co2 + in the Ni/
diamond plating bath (Fig. 6d).
Fig. 5. Typical curve of the friction coefficient of Ni-based coatings under
dry sliding wear conditions.
Fig. 6. Worn surface morphology of the coatings under dry sliding conditions: (a) Ni coating (b) Ni–Co coating (c) Ni/diamond coating (d) Ni–Co/diamond
coating (the arrows show the sliding directions).
L. Wang et al. / Surface & Coatings Technology 191 (2005) 1–6 5
It is evident that the composite coatings obtained with the
addition of Co2 + in the Ni/diamond bath exhibited much
better anti-wear performance with the lower and stable
friction coefficient than Ni/diamond coatings when sliding
with the steel ball. This may be explained as follows: the
reinforced particles will reduce the direct contact between
the matrix and the steel ball during the sliding wear test
[16,17], which reduced the adhesion wear between the two
pure metal counter bodies and impede the wear process. As
mentioned above, the addition of Co2 + in the electrolyte
results in a more uniform distribution and higher content of
diamond particles in the matrix, which enhances the load
bearing capability [18] and avoids direct metal to metal
contact and therefore, as a result, improved the anti-wear
performance. However, in case of the Ni/diamond compo-
sites with smaller amount of diamond particles, after a short
period time of wear test, the diamond particles in the Ni
matrix is easily pulled out of the matrix, which produces the
abrasion wear when the diamond particles move along with
the steel ball, and as a result, the steel ball can easily damage
the surface of the coatings, thus results in some cracks on the
wear track and an increase in the friction coefficient in
comparison with pure Ni coating and leads to more materials
removal. On the other hand, compared to Ni, Co has better
wetting ability with diamond [19]. The higher the bonding
strength, the better the wear resistance will be [20], so this is
reasonable that Ni–Co/diamond composites have better anti-
wear performance. Hence, the favorable effects of diamond
particles on the anti-wear performance of composite coatings
were attributed to the higher content of uniform-dispersed
diamond particles in the matrix and better wetting and
bonding ability between the particles and matrix when add-
ing Co2 + in the Ni/diamond plating bath.
4. Conclusions
1. The addition of Co2 + in the Ni/diamond plating bath can
promote the co-deposition of nano-diamond particles
with nickel and achieve a more uniform distribution of
diamond particles in the matrix. It is assumed that the
Co2 + may act as a cationic stimulator for co-deposition
of nano-diamond particles.
2. The addition of Co2 + in the Ni/diamond plating bath
exhibited positive contribution to the mechanical and
tribological properties of Ni-based nanocomposites. The
nanocomposites obtained with the addition of Co2 + in
the Ni/diamond plating bath produced much higher
hardness and excellent anti-wear performance with
lower friction coefficient, which was attributed to the
higher content of well-distributed diamond particles in
the Ni–Co matrix and better wetting and bonding
ability between the nano-diamond particles and Ni–Co
matrix.
Acknowledgements
The authors gratefully acknowledge the National Natural
Science Foundation of China (Grant No. 50172052,
50271080 and 50323007), the 863 Program of China (No.
2003AA305670) and ‘‘Top Hundred Talents Program’’ of
L. Wang et al. / Surface & Coatings Technology 191 (2005) 1–66
Chinese Academy of Sciences for financial support of this
research work.
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