ORIGINAL PAPER

Friction and Wear Properties of Electrodeposited Nickel–TitaniaNanocomposite Coatings

X. J. Sun Æ J. G. Li

Received: 26 October 2005 / Accepted: 24 July 2007 / Published online: 21 September 2007

� Springer Science+Business Media, LLC 2007

Abstract A novel nanostructure metallic composite

coatings consisting of nanocrystalline nickel matrix (aver-

age grain size: 10 nm) and dispersed titania nanoparticles

(average grain size: 12 nm) have been electrodeposited on

copper substrates from a modified Watts bath. Micro-

structure, friction, and wear properties of the coatings were

investigated by means of X-ray diffraction, scanning

electron microscopy, Vickers microhardness test, and ball-

on-disk friction test. The results showed that the content of

titania nanoparticles in the nanocomposite coatings could

be controlled by changing the concentration of the sus-

pending titania nanoparticles in the electrolytes. The

friction coefficient and wear rate of the nanocomposite

coatings decreased with increasing the content of titania

nanoparticles while sliding against 440C stainless steel at

room temperature.

Keywords Electrodeposition � Nanocomposite �Nickel � Titania � Friction � Wear

Introduction

Investigations on preparation, microstructure, and proper-

ties of particle reinforced metal matrix nanocomposite

materials with grain size of both matrix and dispersed

particles less than 100 nm have been increased to meet the

particular demands of low friction coefficient and high

wear resistance for current advanced technological appli-

cations, such as aerospace, defence, automobile, and

nuclear power industries. Recently, nanometer scale

microstructure design by grain size refinement and hard

inert particle dispersion became powerful tools to obtain

superior mechanical properties of conventional metallic

materials [1–3]. Erb et al. reported that electrodeposited

nanocrystalline nickel was about five times harder than

conventional microcrystalline nickel [4, 5]. Lu and his co-

workers represented the Superplastic Extensibility of

nanocrystalline copper at room temperature, which was

never been found in microcrystalline copper [6]. The above

works indicated that nanocrystalline metals generally have

much improved mechanical properties as compared with

their microcrystalline counterparts. Based on the principle

of dispersion-strengthening, further improvement in

mechanical performance of the nanocrystalline metals can

be achieved by incorporating ceramic nanoparticles into

metal matrix [7]. Generally, electrodeposition is a single

step deposition process under relatively low temperature

followed without heat treatment. The advantages of this

established technique used to fabricate nanocrystalline

metallic coatings include low cost and industrial applica-

bility, easy operation, flexibility, large size availability and

high deposition rate, and almost full density [8]. Numerous

nickel matrix nanocomposite coatings containing multi-

farious types of hard inert nanoparticles, such as TiO2, SiC,

Al2O3, Si3N4, ZrO2 and diamond, etc., have been electro-

deposited from different electrolytes in which the

nanoparticles were suspended [9–17]. Friction and wear

properties of electrodeposited nickel–diamond and nickel–

CeO2 nanocomposite coatings were examined [12, 18], and

it was found that titania nanoparticles can be used as

X. J. Sun (&) � J. G. Li

Institute of Materials Science and Engineering, Lanzhou

University, Lanzhou 730000, P.R. China

e-mail: xjsun@lzb.ac.cn

X. J. Sun

State Key Laboratory of Solid Lubrication, Lanzhou Institute of

Chemical Physics, Chinese Academy of Sciences, Lanzhou

730000, P.R. China

123

Tribol Lett (2007) 28:223–228

DOI 10.1007/s11249-007-9254-5

lubricant under liquid lubrication condition [19, 20].

However, little work has been concerned with the friction

and wear properties of nanostructure metallic coatings

containing titania nanoparticle up to now, In our previous

work, anatase titania nanoparticles with average grain size

in 12 nm have been obtained by sol–gel method and

nanocrystalline nickel with average grain size in 10 nm

have been successfully electrodeposited, and the titania

nanoparticles as the dispersive phase were found to play a

key role on the improvement of the mechanical and cor-

rosion resistance performances of the nanocrystalline

nickel coatings [21, 22]. In this article, the primary target

of our work is the expanded exploration on the friction and

wear properties of the electrodeposited nickel–titania

nanocomposite coatings.

Experimental Procedure

A polished pure copper sheet (15 · 10 · 1 mm) and a pure

nickel sheet (40 · 20 · 5 mm) were used as cathode and

anode, respectively. Before electrodeposition, the copper

substrates were ultrasonically cleaned in acetone and eth-

anol, degreased in 2.0 M sulfuric acid, rinsed with distilled

water, activated in 1:1 hydrochloric acid for about 3 min,

and again rinsed with distilled water. The nickel–titania

nanocomposite coatings were electrodeposited on copper

substrates from a modified Watts bath in which the titania

nanoparticles were suspended. The composition of the

electrolyte for nanocrystalline nickel electrodeposition was

250 g/L six-hydrated nickel sulfate, 25 g/L boric acid, and

5 g/L saccharin. 100 or 200 g/L anatase titania nanoparti-

cles with grain size of 12 nm were added in the electrolyte

when preparing nickel–titania nanocomposite coatings.

Before electrodeposition, the electrolyte was treated by

intense ultrasonic irradiation combined with high-speed

mechanical agitation for about 8 h to disperse the particles

uniformly. During the electrodeposition, the electrolyte

was slowly stirred to keep the particles well suspended.

The pH value of the electrolyte was kept at a constant of

3.0 ± 0.2, and the bath temperature was kept at 30 ± 2 �C.

The electrodeposition was carried out by using a SMD-30

plating power supply (made in Dashun Co. Handan, China)

at a direct current density of 10 A/dm2. The XRD analysis

was performed on a Rigaku D/max-2400 X-ray diffrac-

tometer at room temperature. The as-deposited

nanocomposite coatings were first examined by X-ray

diffraction (XRD) to determine average grain size and

phase content. The average grain sizes were calculated

from the diffraction data by Scherrer equation, and the

phase content was estimated by the K-value method. The

surface morphology was observed with a JSM-5600LV

scanning electron microscope (SEM). The sliding friction

test was conducted on a reciprocating ball-on-disk tribo-

meter. All of the counterparts are 440C stainless steel balls

with 3.0 mm diameter and hardness of 58 ± 2 HRC. The

friction test conditions were listed as follows: the sliding

speed was 0.1 m/min, the moving amplitude was 10 mm

and the normal load was 3.0 N. The environmental

humidity and temperature were 20–30% RH and 15–18 �C,

respectively. The friction coefficient was calculated by

dividing the friction force with normal load. A Shimadzu—

M type microhardness tester calibrated with standard

sample was used to determine the Vickers microhardness,

and the reported value was an average of 10 tests carried

out on different locations in the center section of the

coating samples with a load of 100 g for 25 s. The wear

rates were calculated from the worn volume which was

measured by a profilometer. The uncertainties of the mi-

crohardness test and wear rate test were 0.2 GPa and

1.5 · 10–15 m3, respectively.

Results and Discussion

The X-ray diffraction patterns of the electrodeposited

nanocrystalline nickel coatings containing various con-

centrations of titania nanoparticles are shown in Fig. 1. It is

found that the relative intensity of the titania diffraction

peaks compared to the nickel peaks increased with

increasing content of the titania particles in the electrolyte,

indicating that content of the incorporated particles

increase with increasing concentration of the titania parti-

cles suspended in the electrolyte. The broader peaks of

both nickel and titania in the X-ray diffraction pattern are

indicative of small grains of both the matrix phase and the

dispersive phase in the electrodeposited composite coat-

ings. The average grain size of titania and nickel in the

coatings were calculated from the diffraction data accord-

ing to the Scherrer equation to be 12 and 10 nm,

respectively. The deviation of the average grain size

determination is less than 10%. The contents of titania

particles in the coatings were calculated from the diffrac-

tion results through a K-value method to be 6 and 11 wt.%

while the particle concentrations of the electrolytes were

100 and 200 g/L, respectively.

To describe the friction characteristic of the coatings

sliding against 440C steel, two types of friction coefficients

were introduced. One is directly proportional to the applied

load and the other is independent of the contact area. In the

case of solid–solid sliding friction condition, the two fric-

tion coefficients are conventionally defined, respectively,

as ls = Fs/P and lk = Fk/P, where Fs is the force just

sufficient to prevent the relative motion between two sur-

faces, and Fk is the force needed to maintain relative

motion between two sliding surfaces. The definition of the

224 Tribol Lett (2007) 28:223–228

123

two friction coefficients can be traced back to Amonton’s

law of friction, and easily be distinguished: ls represents

the friction opposing the beginning of relative motion, and

lk represents the friction opposing the continuance of rel-

ative motion once the motion has started [23]. As shown in

Fig. 2a, b, the ls and lk are dependent on the content of the

nanoparticle in the coatings. It is clearly seen that both ls

and lk decreased with increasing the content of titania

nanoparticle. This result reveals that the friction coefficient

between nanocrystalline nickel and 440C steel can be much

lowered by incorporating titania nanoparticles. The similar

results were found by several authors in kinds of electro-

deposited nickel-based metallic composite coating systems,

the reported particles including SiC, CeO2, diamond,

Al2O3, WC, Si3N4, etc. [12, 18, 24–28]. These results

indicate that the microstructure design through the fine

particles embedded into electrodeposited metal matrix is a

simple and effective way to obtain metallic lubricating

coatings.

Figure 3 shows the dependence of wear rate and Vickers

microhardness on the content of titania nanoparticles in the

composite coatings. One can see that the microhardness of

the nanocomposite coatings increase with increasing the

content of titania nanoparticles. This result means that

mechanical properties of the nanocrystalline metal coatings

can be tailored by incorporating nano-sized hard inert

particles. This can be attributed to particle-strengthening

effect by dispersing titania nanoparticles which inhabit the

grain boundaries of the nanocrystalline nickel coatings, and

acts as obstacles to the grain movement and grain bound-

aries migration under an cyclic normal load during sliding

friction test [7]. It is also observed in Fig. 3, the titania–

nickel nanocomposite coatings exhibit much lower wear

rates than the nanocrystalline nickel, indicate that the wear

resistance of nanocrystalline nickel coatings is obviously

improved by incorporating titania nanoparticles.

Figure 4 shows the SEM photographs of as-deposited

and worn surfaces of the pure nanocrystalline nickel and

the nanocrystalline nickel-based composite coatings

containing 11 wt.% titania nanoparticles. Comparing the

micrographs, an obvious difference can be found between

the coatings with and without titania nanoparticles.

20 40 60 80 100 120

0

500

1000

1500

2000

2500

3000

3500

4000

Nickel

Titania (anatase)

1 0 g/L2 100 g/L3 200 g/L

2

3

1

Inte

nsity

2Theata (degree)

Fig. 1 XRD patterns of nickel–titania nanocomposite coatings elec-

trodeposited from the electrolytes with various concentration of

titania nanoparticles

µ s

0.60

0 6% 11%

6%0 11%

0.50

0.40

0.30

0.20

0.10

0.00

µ k

0.30

0.25

0.20

0.15

0.10

0.05

0.00

0 20 40 60 80 100

0 20 40 60 80 100

(a)

(b)Nc

Nc

Fig. 2 Friction coefficients of nickel–titania nanocomposite coatings

containing varies contents of titania nanoparticles with the cycle

number Nc. (a) static friction coefficient, (b) kinetic friction

coefficient

Particle content (wt.%)

Wea

r ra

te (

10-1

5 m3 )

0 2 4 6 8 10 125.0

6.0

7.0

8.050

Vickers microhardnessWear rate

30

40

20

10

0

HV

(G

Pa)

Fig. 3 Variation of microhardness and wear rate of nickel–titania

nanocomposite coatings with the content of titania nanoparticles in

the coatings

Tribol Lett (2007) 28:223–228 225

123

Figure 4a, b are the surface morphology of the as-

deposited coatings, showing the evolvement from particle

free to 11 wt.% titania nanoparticles. The nickel nano-

crystalline coatings exhibit a more regular surface

(Fig. 4a), while the coatings incorporated with titania

nanoparticles show up nodule-like surface (Fig. 4b), and

the grain size of nickel in the nanocomposite coatings is

smaller than that in the nanocrystalline nickel coatings,

confirming that titania nanoparticles are uniformly dis-

tributed in the nanocrystalline nickel matrix. According

to Orowan dispersion strengthening theory, besides

intrinsic properties of the nanoparticles, the size and the

distribution of the particle are the two key factors to

result in the superior mechanical behaviors compared to

pure metallic matrix. Figure 4c, d show the worn surface

micrographs of the pure nanocrystalline nickel coatings

and the nickel–titania nanocomposite coatings containing

11 wt.% titania nanoparticles under dry sliding against

440C stainless steel at room temperature. The micro-

cracks and the tearing debris are clearly visible on the

worn track of the pure nanocrystalline nickel coatings,

but almost no visible cracks on the worn surface of the

nanocomposite coatings containing 11 wt.% titania

nanoparticles. The worn surface of nanocomposite coat-

ings appears to be smoother than that of the pure

nanocrystalline nickel coatings. Our experimental results

reveal that the incorporation of titania nanoparticles in

the nanocrystalline nickel can markedly reduce the wear

rate of this kind of coatings, and the wear resistance can

be further enhanced by increasing the content of titania

nanoparticles in the nanocomposite coatings. The

improvement of the friction and wear properties by the

titania nanoparticles can be attributed to both dispersion-

strengthening and particle-strengthening effects, which

agrees with other similar works [18, 29]. The titania

nanoparticles inhabit the grain boundaries and the triple

junctions to obstruct the grains of the nanocrystalline

nickel from moving and recrystallization, which conse-

quently enhances the mechanical behaviors of the

nanocomposite coatings and further prevents the micro

crack growing along the grain boundaries. During the

friction process, the well-distributed titania nanoparticles,

which are located near the coatings surface, reduced the

real area of metal–metal contact, which consequently

reduces the adhesion between the metal–metal contacts

instead metal–ceramic–metal contact type appears. Fur-

thermore, nano-sized titania particles may act as a tool to

reduce the friction and enhance the wear resistance of

the electrodeposited nanocrystalline nickel coatings.

According to the principle of particle-strengthening, we

have designed a novel nanostructure by the type of par-

ticulate metal matrix composite coatings. The matrix phase

of the coating is electrodeposited nanocrystalline nickel

and dispersed phase is nano-sized nanocrystalline titania

particles prepared by sol–gel method. A series of this kind

of coatings varies with particle contents have been elec-

trodeposited from a modified Watts bath by controlling the

particle concentration. Comparing with nanocrystalline

Fig. 4 SEM photographs of

nanocrystalline nickel and

nickel–titania nanocomposite

coatings at 5,000 magnification.

(a) as-electrodeposited

nanocrystalline nickel, (b) as-

electrodeposited nickel–titania

nanocomposite coatings

containing 11 wt.% titania

nanoparticles, (c) worn surface

of nanocrystalline nickel, (d)

Worn surface of nanocomposite

coatings containing 11 wt.%

titania nanoparticles

226 Tribol Lett (2007) 28:223–228

123

nickel electrodeposits, much higher hardness, lower fric-

tion coefficient and higher wear resistance have been

reached. Namely, electrodeposition of nanostructure

metal–ceramic composite coatings breaks a new path for

properties control through microstructure design of nano-

structure metallic materials. Thus, the particular demands

for advanced industry applications, such as enhanced

mechanical properties, lubrication and wear resistance,

etc., could be reached by adjusting electrodeposition

parameters of the established industrial technology.

Conclusions

Friction and wear properties of the electrodeposited

nickel–titania nanocomposite coatings were investigated.

The coatings consist of nanocrystalline nickel matrix and

titania nanoparticles. The average grain size of nickel and

titania were determined by X-ray diffraction method to be

10 and 12 nm, respectively. The phase content of the

titania nanoparticles in the composite coatings increased

from 6 wt.% to 11 wt.% with increasing the concentration

of the nanoparticles suspended in the electrolytes from

100 g/L to 200 g/L. The surface morphology, Vickers

microhardness, friction coefficient, and wear rate closely

depended on the content of the titania nanoparticle.

Compared with the nanocrystalline nickel coatings elec-

trodeposited under the same conditions, titania–nickel

nanocomposite coatings exhibited smoother surfaces,

higher hardness, and lower friction and higher wear

resistances.

Acknowledgments The authors sincerely thank Prof. J. Z. Zhao

and Mr. D. K. Song for their kind help in scanning electromicro-

scope observation and X-ray diffraction analysis. Thanks to Prof. X.

G. Hu, Prof. W. M. Liu, Dr. L. P. Wang, Dr. H. Yang and Dr. Z. B.

Lu for their helpful discussions. Thank Prof. W. T. Tysoe, Mrs.

Linda Singer and two reviewers for their kind attention and hard

works on the revision and publication of this article. The financial

support by the National Science Foundation of China (Grant Nos.

50421502, 50371035, and 50272068) and the National Basic

Research Program of China (Grant No. 2007CB607600), the Spe-

cialized Research Fund for the Doctoral Program of Higher

Education of China and the Opening Fund of State Key Laboratory

of Solid Lubrication, Chinese Academy of Sciences (Grant No.

0401) are also acknowledged.

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