Influence of pulse frequency on the microstructure and wear resistance of electrodeposited Ni–Al2O3 composite coatings

Abstract

Nickel matrix composite coatings reinforced with sub-microsized Al2O3 particles were produced by pulse electrodeposition and the effect of pulse frequency on the microstructure, hardness and wear resistance of Ni–Al2O3 composite coatings were investigated. The results showed that the pulse frequency significantly influenced the preferred orientation of Ni–Al2O3 composite coatings; the texture of the coatings progressively changed from a strong (111) preferred orientation to a random orientation when pulse frequency increased. The hardness of composite coatings decreased slightly with the increase of volumetric content of alumina particles. The wear behaviors of composite coatings under dry sliding wear and oil-lubricated wear conditions were different significantly. The wear resistance of Ni–Al2O3 coatings decreased with the increase of incorporated alumina particles under dry sliding wear condition, which was largely influenced by the microstructure of Ni matrix due to the presence of adhesive wear. However, the wear resistance of composite coatings increased with the increase of volumetric content of the reinforcements under oil-lubricated wear condition, which mainly depends on the volumetric content of incorporated alumina particles because the adhesive wear can be avoided under oil-lubricated wear condition.

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Influence of pulse frequency on the microstructure and wear resistance of
electrodeposited Ni–Al2O3composite coatings
Li Chena,b, Liping Wanga,b, Zhixiang Zenga,b, Tao Xua,⁎
aState Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China
bGraduate School of Chinese Academy of Science, Beijing 100039, PR China
Received 9 October 2005; accepted in revised form 6 December 2005
Available online 18 January 2006
Abstract
Nickel matrix composite coatings reinforced with sub-microsized Al2O3particles were produced by pulse electrodeposition and the effect of
pulse frequency on the microstructure, hardness and wear resistance of Ni–Al2O3composite coatings were investigated. The results showed that
the pulse frequency significantly influenced the preferred orientation of Ni–Al2O3composite coatings; the texture of the coatings progressively
changed from a strong (111) preferred orientation to a random orientation when pulse frequency increased. The hardness of composite coatings
decreased slightly with the increase of volumetric content of alumina particles. The wear behaviors of composite coatings under dry sliding wear
and oil-lubricated wear conditions were different significantly. The wear resistance of Ni–Al2O3coatings decreased with the increase of
incorporated alumina particles under dry sliding wear condition, which was largely influenced by the microstructure of Ni matrix due to the
presence of adhesive wear. However, the wear resistance of composite coatings increased with the increase of volumetric content of the
reinforcements under oil-lubricated wear condition, which mainly depends on the volumetric content of incorporated alumina particles because the
adhesive wear can be avoided under oil-lubricated wear condition.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Pulse electrodeposition; Ni–Al2O3composite coatings; Preferred orientation; Hardness; Wear resistance
1. Introduction
Particle-reinforced metal matrix composites generally exhib-
ited wide engineering applications due to their enhanced
hardness, better wear and corrosion resistance when compared
to pure metal or alloy [1]. Composite electroplating has been
identified to be a technologically feasible and economically
superior technique for the preparation of such kind of
composites [2]. Over the past decades, successful co-deposition
of ultra-fine particles such as metallic powder, silicon carbides,
oxides, diamond and polymers with metal or alloy matrix have
been reported and their corresponding structures and properties
were investigated by many researchers [3–6]. The structure and
properties of composite coatings depend not only on the
concentration, size, distribution, and nature of the reinforced
particles, but also on the type of solution used and electroplating
parameters (current density, temperature, pH value, etc.).
Among these factors, the type of applied current is one of the
most important parameters [7,8].
It is well known that pulse electrodeposition is one of the
most effective methods in fabrication of metals and alloys due
to its independently controllable parameters and higher
instantaneous current densities when compared to traditional
DC electrodeposition [9]. The properties of metals and alloys
can be controlled and improved by modifying their microstruc-
ture when using pulse current. As a result, the effects of pulse
electrodeposition parameters on the microstructure and proper-
ties of metals and alloys have been reported in many literatures.
Jeong et al. have produced nanocrystalline nickel by pulse
electrodeposition and investigated the effect of grain size
reduction on the wear resistance of pure nickel [10]. Yang et al.
investigated the effect of pulse parameters on the morphology
and corrosion resistance of nickel deposits and the results
showed that electrodeposition of nickel using pulse plating
could lower porosity and improve corrosion resistance when
Surface & Coatings Technology 201 (2006) 599–605
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⁎Corresponding author. Tel.: +86 931 496 8169; fax: +86 931 496 8169.
E-mail address: lzchenli1981@yahoo.com.cn (T. Xu).
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doi:10.1016/j.surfcoat.2005.12.008

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compared to direct current plating [11]. In addition, pulse
electrodeposition of alloys has received considerable attention
recently. Brooks and Erb have produced nanocrystalline γ-
phase Zn–Ni alloys [12] and found that pulsed electrodeposits
presented a smaller grain size and better corrosion properties
than the deposits prepared with an equivalent continuous
current density [13]. The use of pulse current shifted the
polarization curve and greatly affected the composition of alloy
deposits [14]. Smooth, bright and nanocrystalline Ni–Cu alloys
were obtained by precise control of the pulse parameters. The
results showed that the Knoop microhardness for pulse current
plated samples was higher than the direct current plated sample
and the internal stress was lower for the PC sample [15].
Numerous literatures on pulse electrodeposition of metals
and alloys have been reported. However, there are very limited
studies focused on the pulse electrodeposition of nickel
composite coatings [16–18]. In the present paper, the effect of
pulse frequency on the microstructure, hardness and wear
resistance of pulse electrodeposition Ni–Al2O3 composite
coatings have been investigated.
2. Experimental procedures
The plating electrolyte was a Watts-type bath. The
composition and the experimental parameters are shown in
Table 1. The Al2O3particle was used as obtained and the
average particle size of the Al2O3 particles used in the
experiment was about 0.6 μm. Before the co-deposition, the
suspended solution was stirred for 6 h, and then was subject to
ultrasonic vibration for 30 min. Then the pH of this bath was
adjusted to 4 by appropriate additions of HCl or NH3solutions.
AISI-1045 steel plate with an area of 0.05 dm2on one side
was used as the cathode while the other surface of the substrate
was blocked with a PVC adhesive tape; the anode was a pure Ni
plate. Prior to electroplating, the substrates were mechanically
polished to a 0.08–0.12 μm surface finish, and then a sequence
of cleanings was performed to remove contamination on the
substrate surface. The steel substrates were activated in a mixed
acidic bath at room temperature before electroplating. This
substrate was placed parallel to a vertically oriented nickel plate
at a distance of 0.05 m in the above bath. During the co-
deposition process, the bath was slowly stirred by a magnetic
stirrer in order to keep the particles dispersed and prevent
sedimentation in the electrolyte suspension. After depositing for
3 h, the composite coating was washed in running water. Then,
it was cleaned ultrasonically in distilled water for 10 min. The
thickness of the produced composite coatings was in the range
of 100–120 μm.
The crystal structure of the composite coatings were studied
by X-ray diffractometry (Philips X' Pert-MRD). The surface
morphology of the coatings were observed using a JSM-5600Lv
scanning electron microscopy (SEM) and the percentage of co-
deposited Al2O3 particles was evaluated by using energy
dispersive X-ray spectroscopy (EDS) analysis tool. Hardness of
the coatings was determined using a Vicker's microhardness
indenter with a load of 25 g for 10 s. The final value quoted for
the hardness of a coating was the average of 10 measurements.
The wear tests were performed on a reciprocating ball-on-
disk UMT-2MT tribometer (Center for Tribology, Inc.,
California, USA) at room temperature with a relative humidity
of 45–55% both under dry sliding wear and oil-lubricated wear
conditions, respectively. An AISI-52100 stainless steel ball
(diameter 3 mm) was used as the counter body. Dry sliding wear
tests were performed under a load of 1 N with a sliding speed of
55 mm s−1; oil-lubricated wear tests were performed under a
load of 20 N with a sliding speed of 110 mm s−1. Wear rates of
all coatings were calculated on the basis of the volumetric loss,
which was measured using a surface profilometer. Wear rates of
all the deposits were calculated using the equation of K=V/SF,
where V is the wear volume loss 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 pulse frequency on the microstructure of
composite coatings
The XRD patterns of Ni–Al2O3composite coatings prepared
by different pulse frequency are illustrated in Fig. 1. As can be
seenfrom Fig.1,the compositecoatings exhibited face-centered
cubic (fcc) lattice with different orientation which was
influenced by the pulse frequency. It can be clearly observed
that the composite coating exhibited obvious (111) preferred
orientation at low pulse frequency, and the diffraction intensity
of the (200) fiber orientation increased with the increase of pulse
frequency. When the pulse frequency increased to 1000 Hz, the
composite coating exhibited a random orientation. It indicated
that low pulse frequency could produce preferred orientation
easier than high pulse frequency. The influence of pulse
frequency on the texture coefficient of the composite coatings
is tabulated in Table 2. The texture coefficient is expressed as
I ¼ 100 ? Ihkl=ðI111þ I200þ I220þ I311Þð1Þ
where I is the texture coefficient.
As shown in Table 2, the texture coefficients of the
composite coatings were affected by pulse frequency. The
texture coefficient of the composite coatings prepared under
different frequencies was in the order of (111)N(200)N(311)N
(220). The texture coefficients of the (200) plane increased
while those of the (111) (220) (311) decreased with the increase
Table 1
Basic bath compositions and electrodeposition conditions
Compositions and conditions
NiSO4·6H2O (g l−1)
NiCl2·6H2O (g l−1)
H3BO3(g l−1)
Sodium dodecyl sulfate (g l−1)
Al2O3particle (g l−1)
Temperature (°C)
pH
Current density (A dm−2)
Pulse duty cycle
Pulse frequency (Hz)
300
50
40
0.1
20
40–45
4
3.0
50%
10–1000
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of pulse frequency. The change of texture coefficient has direct
relation with the directional array of metal ions along (200)
plane under impulse of cathodic overpotential in the electro-
depositing process. The atomic density of (200) plane is lower
than that of (111) plane in face-centered cubic structure, so the
surface energy of (200) plane is higher than that of (111) plane.
In ordinary crystallization process, atoms always incline to
crystallization in (111) close-grained plane which has lower
surface energy. A greater crystal growth rate was obtained in
pulse electrodepositing process, the increased fresh atoms were
incapable of migrating to (111) plane, and some of them would
rest on (200) plane which has higher surface energy, which
consequently accelerated the growth of (200) plane and
influenced the preferred orientation [19]. A lower pulse
frequency means a longer cycle and longer ‘off-time’ when
the duty cycle is constant, which gives more time to the nickel
atoms to migrate to the most stable position during the intervals
than under higher frequency. Therefore, more nickel atoms rest
on the (111) plane and made the coatings exhibit obvious (111)
preferred orientation. The nickel atoms have less time to migrate
to (111) plane when the pulse frequency increased, so the
probability of fresh atoms resting on (200) plane increased with
the increase of pulse frequency, which made the coatings show
random orientation with pronounced (111) and (200) peaks.
3.2. Effect of pulse frequency on alumina co-deposition and the
hardness of composite coatings
Fig. 2 shows the effect of pulse frequency on the amount of
incorporated alumina particles into a nickel matrix. The volume
fraction of alumina particles in composite coatings increased
gradually with the increase of pulse frequency. This can be
further verified by SEM observation as shown in Fig. 3.
Bahrololoom and Sani [20] have investigated the effect of duty
cycle and pulse frequency on the incorporation of alumina
particles in nickel matrix; they showed that increasing pulse
frequency decreased the amount of incorporated alumina
particles slightly, which is in disagreement with the results of
this investigation. Electrodeposition of composite coatings from
suspension of alumina particles includes transport and electro-
chemisorption of alumina particles and metal matrix capture
particles to form a cathodic coating. The movement and
deposition of alumina particles depend directly on the time that
the current was applied. In fact, the total time that the current
was applied was the same at per second in different pulse
frequency, but there are more cycles of current pulses at higher
frequencies. It means a shorter ‘off-time’ compared to lower
frequency when the duty cycle is constant, which makes nickel
atoms capture alumina particles in time. It is possible that some
alumina particles which has adsorbed on the cathodic surface
desorbed by the stirring suspension during the longer ‘off-time’
when applied lower pulse frequency. Consequently, the
incorporated alumina particles increased slightly with the
increase of pulse frequency.
As shown in Fig. 4, the hardness of Ni–Al2O3composite
coatings decreased gradually with the increase of pulse
frequency. This indicates that higher hardness is obtained at
lower pulse frequency. In fact, the hardness of composite
coatings is influenced by two aspects: one is the hardness of
metal matrix which is determined by the microstructure of
coatings, the other is the amount of incorporated hard particles,
which increase the hardness of these electrocomposite coatings.
In order to investigate the influence of pulse frequency on the
hardness of metal matrix, pure nickel coatings without any
alumina particles were prepared under the same conditions. Fig.
4 indicates that the hardness of pure nickel coatings decreased
with the increase of pulse frequency, which is in good
0
2
4
6
8
10
12
14
0100 200 300 400 500 600 700 800 900 1000
Pulse frequency(Hz)
Al2O3 Vol%
Fig. 2. Effect of pulse frequency on the alumina content of Ni–Al2O3composite
coatings.
0
1020 30 40506070 8090
100 110
(311)
(220)
(200)
(111)
f=10Hz
f=500Hz
f=1000Hz
Intensity
2θ /degree
Fig. 1. XRD diagram of Ni–Al2O3composite coatings prepared under PC
conditions.
Table 2
The texture coefficients of the Ni–Al2O3composite coatings
Pulse frequency (Hz)10 5001000
I111
I200
I220
I311
67.5
16.4
1.4
14.7
66.1
21.1
0.9
11.9
47.6
46.4
0.5
5.5
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agreement with the investigation results of Devaraj and
Seshadri [21]. Although the volume content of incorporated
alumina particles increased slightly with the increase of pulse
frequency, the final effect was that the hardness of composite
coatings decreased slightly. The decrease of the hardness for the
pure nickel coatings with the increase of pulse frequency can be
explained in terms of microstructure of pure nickel coatings
prepared in different frequency. It has been reported that the
microstructure corresponding to (100) texture is associated to
coatings that present the minimum of hardness [22–24]. In fact,
we observe a (100) orientation detected by the intensive line
(200). Wyllie [25] also suggested that the hardness of a coating
was greatly influenced by the crystal orientation, the greatest
hardness being found in coatings with a high degree of preferred
orientation and the least hardness in coatings with random
orientation. As described above, the microstructure of compos-
ite coatings changed from (111) preferred orientation to a
random orientation when the pulse frequency increased, and
hence, it is reasonable that the composite coatings with random
orientation obtained at high frequency exhibited lower hardness
despite the high percentage alumina particles they contained. It
can be concluded that the microstructure of composite coatings
is an important factor for determining the hardness.
3.3. Effect of pulse frequency on the wear resistance of
composite coatings
3.3.1. Wear resistance of composite coatings under dry sliding
wear condition
The volumetric wear rates of prepared composite coatings
under dry sliding wear condition are shown in Fig. 5. As evident
from this figure, the wear rate of Ni–Al2O3composite coatings
increased with the increase of pulse frequency. However, the
amount of incorporated alumina particles increased slightly
with the increase of pulse frequency as seen in Fig. 2. By
combining Figs. 2 and 5, it is interesting to note that the
composite coatings which contained high percentage of alumina
particles have poor wear resistance. As shown in Fig. 6, the
morphology of worn surface of composite coating prepared
under low frequency was smooth, and there was less adhesive
wear on the worn surface. The adhesive wear enhanced
progressively with the increase of pulse frequency. The severe
(a)(b)
(c)
Fig. 3. Backscattered SEM micrograph of Ni–Al2O3composite coatings prepared with different pulse frequencies (a) f=10 Hz (Al2O3, 7.7 vol.%), (b) f=500 Hz
(Al2O3, 10.2 vol.%), (c) f=1000 Hz (Al2O3, 11.9 vol.%).
300
350
400
450
500
550
600
10 100294500 714 1000
Pulse frequency(Hz)
Microhardness HV
Ni-Al2O3 coatings
Pure Ni
Fig. 4. Effect of pulse frequency on the hardness of Ni coatings and Ni–Al2O3
composite coatings.
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adhesive wear occurred in the composite coating prepared under
high frequency. Not surprisingly, the presence of the severe
adhesive wear causes larger wear losses.
In fact, besides the volumetric content of the reinforced
particles, the anti-wear performance of a composite coating also
depends on the property of metal matrix. In order to clarify the
wear behavior of Ni–Al2O3 composite coatings under dry
sliding wear condition, we should take into account the
microstructure of the composite coatings. As described above,
the composite coatings prepared at low frequency exhibited a
strong (111) preferred orientation, which could avoid severe
adhesive wear that happened during the wear process owing to
low surface energy of (111) plane. Consequently, composite
coatings obtained at low frequency showed better wear
resistance even though they contained less alumina particles.
But for composite coatings prepared at high frequency, the Ni
crystallites exhibited random orientation, the severe adhesive
wear would take place between coatings and frictional mate
under dry sliding wear condition, which could be attributed to
the higher surface energy of (200) plane. This indicates that the
0
1
2
3
4
5
6
7
8
9
10
11
12
0 100 200 300 400 500 600 700 800 900 1000
Pulse frequency(Hz)
Wear rate(x10-4mm3/Nm)
Fig. 5. Effect of pulse frequency on the wear rate of Ni–Al2O3composite
coatings under dry sliding wear condition.
(a)(b)
(c)
Fig. 6. SEM photograph of the worn surface under dry sliding wear condition of Ni–Al2O3composite coatings prepared with different pulse frequencies: (a) f=10 Hz,
(b) f=500 Hz, (c) f=1000 Hz.
0
2
4
6
8
10
12
0 100 200 300 400 500 600 700 800 900 1000
Pulse frequency(Hz)
Wear rate(x10-7mm3/Nm)
Fig. 7. Effect of pulse frequency on the wear rate of Ni–Al2O3composite
coatings under oil-lubricated wear condition.
603L. Chen et al. / Surface & Coatings Technology 201 (2006) 599–605