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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: taoxu@lsl.ac.cn (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 3–5

Current density (A/dm2) 1–5

Stirring speed (rpm) 100–500

2. Experimental

The 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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