Microstructure and tribological properties of

electrodeposited Ni–Co alloy deposits

Liping Wanga,b, Yan Gaoa,b, Qunji Xuea, Huiwen Liua, Tao Xua,*

aState Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics,

Chinese Academy of Sciences, Lanzhou 730000, PR ChinabGraduate School of the Chinese Academy of Sciences, Beijng 100039, PR China

Received in revised form 24 August 2004; accepted 30 August 2004

Available online 20 October 2004

www.elsevier.com/locate/apsusc

Applied Surface Science 242 (2005) 326–332

Abstract

Ni–Co alloys with different compositions and microstructures were produced by electrodeposition. The effects of Co content

on the composition, surface morphology, phase structure, hardness and tribological properties of Ni–Co alloys were investigated

systemically. Results showed that the morphology and grain size of alloys are mainly influenced by the Co content and the phase

structure of Ni–Co alloys gradually changed from fcc into hcp structure with the increase of Co content. The hardness of Ni–Co

alloys with a maximum around 49 wt.% Co followed the Hall–Petch effect. It was found that the improvement of wear resistance

of Ni-rich alloys with hardness increase fits Archard’s law. In addition, the Co-rich alloys exhibited much lower friction

coefficient and higher wear resistance when compared with Ni-rich alloys. It has been concluded that hcp crystal structure in Co-

rich alloys contributed to the remarkable friction–reduction effect and better anti-wear performance under the dry sliding wear

conditions.

# 2004 Elsevier B.V. All rights reserved.

Keywords: Ni–Co alloy; Electrodeposition; Structure; Tribological properties

1. Introduction

Ni–Co alloys have been investigated as important

engineering materials for several decades because of

their unique properties, such as high-strength, good

wear resistance, heat-conductive, electrocatalytic

* Corresponding author. Tel.: +86 931 496 8169;

fax: +86 931 496 8169.

E-mail address: lpwang@lsl.ac.cn (T. Xu).

0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved

doi:10.1016/j.apsusc.2004.08.033

activity [1–5]. Additionally, the use of Ni–Co alloys

has been extended to the production of three-

dimensional, complex-shaped finished components

by the electroforming technique [6,7]. The investiga-

tions on the electrodeposited Ni–Co alloys have

shown that their microstructure and properties were

found to depend strongly on the Co content, which can

be controlled by the experimental parameters, such as

bath composition, temperature, pH value, and current

density, etc. [1,3,8]. The effects of plating parameters

.

L. Wang et al. / Applied Surface Science 242 (2005) 326–332 327

on the composition and morphology of Ni–Co

deposits were compared in many literatures [4,5,9].

Golodnitsky et al. recently studies the effects of Co

content on the tensile strength, internal stress and

high-temperature oxidation of Ni–Co alloys [3]. Their

activities for the oxygen evolution reaction and

hydrogen evolution reaction were also studied on

electrodeposited Ni–Co ultramicroelectrodes [10,11].

Moreover, much interest is focused on the magnetic

properties of Ni–Co alloys due to the application of

these alloys in various magnetic devices, especially in

microsystem technology for manufacture of sensors,

actuators and inductors [12,13]. It is reported that the

magnetic properties of Ni–Co alloy are greatly

influenced by the composition and phase structure

of Ni–Co alloy [14]. Unfortunately, there are very

limited studies focused on the friction and wear

properties of Ni–Co alloys as a function of their

microstructure and composition.

In the present paper, Ni–Co alloys with different Co

content were electrodeposited on AISI-1045 steel

substrates. The composition, microstructure, mechan-

ical, and tribological properties of Ni–Co alloys were

compared systemically in order to specifically

correlate the structure and tribological properties of

Ni–Co alloys.

2. Experimental

Ni–Co alloys were electrodeposited from a typical

Watts-type electrolyte, containing Nickel sulfate

(200 g/l), sodium chloride (20 g/l), boric-acid (30 g/

l), sodium lauryl sulfate (0.1 g/l) and cobalt sulfate (0–

80 g/l). In addition, pure Ni was also produced for

comparison purpose. The Ni–Co alloys were depos-

ited on AISI-1045 steel substrates by choosing a

current density of 3 A/dm2 at a bath temperature of

45 8C. The anode was a pure Ni plate. The pH of the

bath was kept at 4.0 adjusted by ammonia water or

dilute sulfuric-acid. Before deposition, the substrates

were mechanically polished to a 0.10–0.12 mm

surface finish, the substrate was then degreased in

acetone with ultrasonic cleaning for 5 min, rinsed in

the running water to remove contamination on the

substrate surface. After than, the steel substrates were

activated for 20 s in the 20 vol.% HCl solution, and

finally rinsed with distilled water.

The surface morphology and microstructure of the

alloy deposits were investigated using a JSM-5600Lv

scanning electron microscopy (SEM). The composi-

tions of Ni–Co alloys were determined with energy

dispersive X-ray spectroscopy (EDS) analysis tool

attached to SEM. The crystal structure and phase

composition of alloy deposits were studied by X-ray

diffraction (XRD). Microhardness of the deposits was

determined using a Vicker’s microhardness indenter

with a load of 50 g for 10 s, indentations were made on

the 50 mm thick deposits. 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

(Center for tribology, Inc., California, USA) at room

temperature with a relative humidity of 45–55% under

dry sliding conditions. 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 3 N with a sliding speed of 55 mm s�1. The

friction coefficient and sliding time were recorded

automatically during the test. The wear volume loss

was measured using a surface profilometer, wear rates

of all the alloy deposits were calculated using the

equation of K ¼ V=SF, where V is the wear volume

loss in mm3, S the total sliding distance in m and F the

normal load in N.

3. Results and discussion

3.1. Composition of Ni–Co alloys

The dependence of the composition of Ni–Co alloys

on the concentration of Co2+ ions in the electrolyte at a

fixed concentration of Ni2+ ions is presented in Fig. 1. It

is clearly observed that the Co content in alloy deposits

increased gradually with the increase of Co2+ con-

centration in the electrolyte. Note that the percentage of

Co in the alloys was always higher than in the

electrolyte in agreement with [3,5], which is confirmed

by the anomalous codeposition of Ni–Co alloy. Namely,

the less noble metal (Co) is preferentially deposited. A

generally accepted explanation for these anomalous

phenomena was the change of the near-electrode pH,

the formation of metal hydroxyl and their competitive

adsorption [15,16].

L. Wang et al. / Applied Surface Science 242 (2005) 326–332328

Fig. 1. The alloy compositions as a function of Co2+ concentrations

in the baths.

Fig. 2. SEM morphologies of Ni–Co alloy deposits with their Co

contents of (a) 0 wt.%, (b) 7 wt.%, (c) 27 wt.%, (d) 49 wt.%, (e)

66 wt.%, (f) 81 wt.%, (g) high-magnification of Ni–49 wt.% Co

alloy.

3.2. Morphology and phase structure of Ni–Co alloys

Typical surface morphologies of Ni–Co alloys with

different Co content are shown in Fig. 2b–f,

respectively. Fig. 2a shows a typical morphology of

a Watt Ni deposit, which has relatively large grain size

(3–10 mm) and showed polyhedral crystallites.

Sequentially increasing Co content from 7 to

49 wt.% (Fig. 2b–d) results in a gradual decrease in

the grain size of the Ni–Co alloy down to a sub-micron

grain size. When the Co content reached the 49 wt.%,

close observation of SEM morphology at high-

magnification (Fig. 2g) revealed that the Ni–Co alloys

have spherical cluster surface piled with a large

number of equally sized grains with spherical-shape.

At above 49 wt.% Co, the grain size of Ni–Co

deposits, however, increased with the increase of Co

content in alloys. When increasing Co content up to

81 wt.%, the morphology of the Ni–Co alloys changes

dramatically, and with less compact structure, the

Ni–Co alloy showed a rather regularly branched

structure with extended acicular 3–6 mm length

crystallites (Fig. 2f).

The phase composition and structure of pure Ni and

Ni–Co alloys with different Co contents were

investigated using XRD shown in Fig. 3. As can be

seen from Fig. 3a, the pure Ni deposit exhibits face-

centered cubic (fcc) lattice with remarkable (2 0 0)

growth orientation, which can be attributed to the

largest grain size of pure Ni. With the codeposition of

Co, the Ni–Co solid solution was formed. As can be

seen in Fig. 3b–f, both the crystal structure and phase

composition are mainly dependent on the Co contents

in alloys. For the Ni-rich alloys with Co content lower

than 49 wt.%, the Ni–Co alloys show complete fcc

phase structure, which is in agreement with previously

reported results [2,3]. Furthermore, the (1 1 1) growth

orientation gradually increased with the increase of Co

content, and the FWHM of the Bragg line for the

(1 1 1) peak also increased correspondingly, which is

in accordance with the gradual reduction of grain size

when increasing the Co content from 0 to 49 wt.% in

Ni–Co alloys as shown in Fig. 2b–d. Moreover, when

L. Wang et al. / Applied Surface Science 242 (2005) 326–332 329

Fig. 3. XRD patterns of Ni–Co alloy deposits with their Co contents

of (a) 0 wt.%, (b) 7 wt.%, (c) 27 wt.%, (d) 49 wt.%, (e) 66 wt.%,

(f) 81 wt.%.

the Co content was increased to 66 wt.%, the presence

of (1 0 0) peak demonstrated the initial formation of a

hexagonal close packed (hcp) lattice, indicating that

the crystal structure of the Ni–Co alloy changed from

complete fcc lattice into a mixed (majority of fcc) +

(minority of hcp) phase as shown in Fig. 3e. At above

Fig. 4. Microhardness as function of Co conten

81 wt.%, as shown in Fig. 3f that a very strong hcp

(0 0 2) texture with pronounced (1 0 0) and (1 1 0)

peaks were observed, which is commonly observed in

both conventionally electrodeposited Co and nano-

crystalline Co [17,18]. Therefore, it can be concluded

that the phase structure of Ni–Co alloys gradually

changed from fcc into hcp with the increase of Co

content as shown in Fig. 3.

3.3. Microhardness of Ni–Co alloy deposits

Fig. 4a presented the microhardness of Ni–Co

alloys as a function of Co content in alloys. It is clearly

that microhardness of Ni–Co alloys increased initially

with Co content varying from 0 to approximately

49 wt.%, and then gradually decreased as Co content

increased further above 49 wt.%. The explanation to

this gradual reduction of microhardness is the gradual

increase of grain size with the increase of Co content

in Co-rich alloys as shown in Fig. 2e–f. Note that the

microcrystalline Ni–Co alloys show the maximum

hardness at approximately 49 wt.% Co, which can be

associated with the smallest grain size as mentioned in

microstructure analysis sector.

Normally, strengthening of polycrystalline materi-

als by grain size refinement is technologically

attractive because it generally does not adversely

affect ductility and toughness [19], which can be

represented by the classical Hall–Petch effect:

H ¼ H0 þ kd�0:5 (1)

where H0 is hardness constant, k constant, and d

diameter of grain. The hardness change with average

grain size (d) of the Ni–Co deposits is shown in Fig. 4b

t (a) and d�0.5 (b) of the Ni–Co deposits.

L. Wang et al. / Applied Surface Science 242 (2005) 326–332330

Fig. 6. The comparison of friction coefficients vs. sliding time

between Ni-rich and Co-rich alloy deposits.

in the form of a Hall–Petch plot. It is obvious that the

Ni–Co alloy deposits exhibit a nearly constant Hall–

Petch gradient; such a relationship has also been

observed on pure Ni, pure Co and pure Zn from other

studies [20,21].

3.4. Friction and wear properties

The effect of Co content on friction coefficient of Ni–

Co alloys were shown in Fig. 5. It is observed that the

friction coefficient of pure Ni and Ni–Co alloys with Co

content lower than 49 wt.% (Ni-rich alloys) were quite

close. With the further increase of Co content, the Co-

rich alloys showed excellent friction–reduction beha-

vior. The Co-rich alloy deposit with Co content higher

than 81 wt.% exhibited the smallest friction coefficient

(more than two times lower than Ni and Ni-rich alloys),

followed by Ni–66 wt.% Co alloy (a litter lower than Ni

and Ni-rich alloys) under identical wear test conditions.

In addition, the friction coefficients of Co-rich alloys

were much more stable than that of Ni-rich alloy

deposits (see Fig. 6). Combined with the XRD analysis,

the close friction coefficient for Ni and Ni-rich alloy can

be attributed to the same fcc crystal structure they have.

In case of Ni–66 wt.% Co alloy, a mixed fcc/hcp phase

with smaller ratio of hcp phase structure led to the

gradual reduction of friction coefficient. Furthermore, as

for the Ni–81 wt.% Co alloy, dramatic reduction of

friction coefficient was observed due to the higher ratio

of hcp phase structure. Hence, we can conclude that the

reduction in friction coefficient of Co-rich alloys with

Fig. 5. Friction coefficient as function of Co content in the Ni–Co

alloy deposits.

the increase of Co content can be associated with the

change of crystal structure from fcc to hcp crystal phase.

The variation of the wear rates of Ni–Co alloys as a

function of Co content and microhardness of alloys are

shown in Fig. 7. It is observed that all Ni–Co alloy

deposits in this study have lower wear rates when

compared with pure Ni deposit. Moreover, the wear

rate of Ni–Co alloys slowly decreased with the

increase of Co content from 6 to 49 wt.%. It is clear

that when the Co content is lower than 49 wt.%, the

gradual decrease of wear rates with the increase of Co

content was attributed to the microhardness increase

from 315 to 462 HV. Above improvement of wear

resistance with hardness increase, in this study due to

the grain size reduction, could be expressed using

Archard’s law mostly used in adhesive wear condi-

tions [22,23], since the wear mechanism of Ni and

Fig. 7. Wear rates as function of Co content in the Ni–Co alloy

deposits.

L. Wang et al. / Applied Surface Science 242 (2005) 326–332 331

Ni-rich deposits is mostly the adhesive wear as

evidenced by SEM morphology of worn surface in

Fig. 8a and b. Thus, the Archard’s law can be

expressed as:

Q ¼ KLN

H(2)

where Q is the volumetric wear loss, N the applied

load, L the total sliding distance, K the wear coefficient

and H the hardness of the wear surface. Under the

same wear conditions, the wear rate is proportional to

the inverse microhardness of materials. The data of

wear rate for microcrystalline Ni–Co alloys with Co

content lower than 49 wt.% fit Archard’s law very

well. However, with further increase in Co content

above 49 wt.%, the wear rates of Co-rich alloys

decreased rapidly in spite of the fact that the hardness

also decreased. The wear rate of Co-rich alloy with

approximately 81 wt.% Co content is more than one

order of magnitude lower than that of pure Ni and Ni-

rich alloys. This reverse-Archard law may be caused

by special hcp crystal structure of Co-rich alloys. This

agree well with the reduction in friction coefficient for

Co-rich alloys, namely, the lower and stable friction

coefficient of Co-rich alloys caused by hcp phase

structure resulted in the less wear loss, while the

Fig. 8. Worn surface of Ni–Co alloy deposits

higher friction coefficient of Ni-rich alloys due to

fcc phase structure led to the more wear loss. More

important is the fact that the Co-rich alloys exhibited

excellent wear resistance and anti-friction behavior.

The difference in the wear behavior of Ni–Co

alloys can be further verified by the worn surface

morphologies of Ni-rich and Co-rich deposit as shown

in Fig. 8a–c. For the pure Ni deposit and Ni-rich alloy

with completely fcc crystal structure, the wear track

(Fig. 8a and b) shows the larger extent of adhesion

wear and severe deformation in the sliding direction

under the combined stresses of compression and shear,

which results in larger wear rate of pure Ni and Ni-rich

alloys. Furthermore, larger tendency for plastic

deformation, this in turn increased the probability

of formation of asperity junctions resulting in higher

and unstable friction coefficient for Ni and Ni-rich

alloys. Compared with pure Ni and Ni-rich alloys, a

densification of the worn surface of Co-rich alloy

seems to take place, the worn surface of Co-rich alloy

with hcp crystal structure revealed slight adhesion

wear and rather smooth surface with smaller damaged

regions, only some light grooves and scars are noted

on the worn surface (Fig. 8c). This resulted in the

better wear resistance of Co-rich alloy than Ni-rich

alloys. That is also the reason why the friction

: (a) 0% Co; (b) 27% Co; (c) 81% Co.

L. Wang et al. / Applied Surface Science 242 (2005) 326–332332

coefficients of Co-rich alloys were much more stable

and more than two times lower than that of Ni and

Ni-rich alloys. It is evident that the high the amount of

hcp phase structure, the better the friction and wear

behavior will be. Above evidence suggests that the

crystal structure is indeed a dominant factor, which

influences the friction and wear behavior of Ni–Co

alloys. Hence, it clearly demonstrates that hcp crystal

structure in Ni–Co alloys contributed to the remark-

able friction–reduction effect and better anti-wear

performance of Co-rich alloys.

4. Conclusions

(1) The Co content in Ni–Co alloys increased

gradually with the increase of Co2+ concentration

in the electrolyte, which is confirmed by the

anomalous codeposition of iron group metals.

(2) S

urface morphology of Ni–Co alloys changed

from regularly polyhedral crystallites into sphe-

rical cluster surface when increasing Co content

from 7 to 49 wt.% and the morphology of the

Ni–Co alloys with 81 wt.% Co showed a rather

regularly branched structure. Both the crystal

structure and phase composition are mainly

dependent on the Co content in alloys. The phase

structure of Ni–Co alloys gradually changed from

fcc into hcp with the increase of Co content.

(3) M

icrohardness of Ni–Co alloys increased initially

with Co content increasing from 0 to approxi-

mately 49 wt.%, and then gradually decreased as

Co content increased further above 49 wt.%. The

hardness change of Ni–Co with grain size follows

Hall–Petch effect.

(4) F

or the Ni-rich alloys, the improvement of wear

resistance with hardness increase fits Archard’s

law. In addition, the Co-rich alloys exhibited

much lower friction coefficient and higher wear

resistance than Ni-rich alloys. It has been

suggested that hcp crystal structure in Co-rich

alloys contributed to the remarkable friction–

reduction effect and better anti-wear performance.

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 Chinese Academy of Sciences for

financial support of this research work.

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