Surface & Coatings Technology 205 (2011) 2871–2875

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Tribological behavior of electrodeposited Ni–SnO2 nanocomposite coatings on steel

Ikram ul Haq a,⁎, Tahir I. Khan b

a National Centre of Excellence in Physical Chemistry, University of Peshawar, Peshawar-25120, Khyber Pukhtoonkhwa, Pakistanb Department of Mechanical and Manufacturing Engineering, University of Calgary, Alberta, Canada

⁎ Corresponding author. Tel.: +92 91 9216676; fax: +E-mail address: ikramhq@yahoo.com (I. Haq).

0257-8972/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.surfcoat.2010.10.059

a b s t r a c t

a r t i c l e i n f o

Article history:Received 5 April 2010Accepted in revised form 26 October 2010Available online 31 October 2010

Keywords:ElectrodepositionNanocomposite coatingWear resistanceMicrohardnessCoefficient of frictionTin oxide

Ni–SnO2 composite coatings were successfully deposited onto steel substrates by the electrodepositionmethod from electrolyte solutions, containing submicron SnO2 particles. Results showed that an increase inthe amount of the SnO2 in the electrolyte solution led to an increase in the SnO2 content in the compositecoatings. Wear rate, friction coefficient and hardness of the composite coatings were determined as a functionof their particle contents. It was noted that wear rate and friction coefficient decreased, whereasmicrohardness increased with the increase in the SnO2 contents of the composite coatings. We believe thatuniformity in particle shape and size of SnO2 and their uniform distribution in the nickel matrix may havecontributed to the improved performance of the composite coating. In addition, the decrease in the frictioncoefficient of the composite coating with the increase in its SnO2 content pointed to the possible lubricationrole, played by the SnO2 particles.

92 91 9216671.

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© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Nickel has been used by many researchers as matrix metal fordepositing composite coatings on metallic substrate. The deposition ofcomposite coating is mostly carried out by electrolytic process from theelectrolyte dispersions, containing particles of different types of insolublesolidmaterials. For example, a number of studies have been published onresearch work which dealt with the deposition of nickel matrixcomposite coatings, reinforced with Al2O3 [1–3], CeO2 [4,5], SiC [6],TiO2 [7], ZrO2 [8], CeO2 [9] and SiC [10] particles on several substratemetals. Most of these results showed that the composite coatingsimproved various mechanical properties of the coatings, which includemicrohardness, wear resistance, etc. as compared to pure nickel coatings.However, the extent of the observed improvement varied from system tosystem and was strongly dependent upon the deposition history of thecomposite coatings and properties of the dispersed particles. As such, it isbelieved that there exists room for further improvement in mechanicalproperties of the electrodeposited nickel composite coatings by tailoringthe deposition conditions and bath composition, especially with respectto the properties of the dispersed phase.

Tin oxide (SnO2) is a semiconducting material with high thermalstability and has widely been used for many technological purposes,such as precursormaterial for tribofilm formation on the rubbing steelsurfaces [11], fillers in organic coating on glass bottles [12] andfabrication of gas sensors [13,14]. However, to our knowledge, noreport is available in the literature about using SnO2 as a particle

dispersion in nickel coatings. This work describes the synthesis ofmonodisperse system of submicron sized particles of SnO2 and theirincorporation in the nickel matrix by the electrodeposition process.The role of isoelectric point (IEP) of SnO2 in the co-deposition processis highlighted. The electrodeposition of Ni–SnO2 composite coatingson steel substrates and evaluation of its mechanical properties arereported for the first time. The effect of SnO2 particle content onelectrodeposited nickel coatings and changes in wear resistance,coefficient of friction and microhardness are investigated.

2. Experimental

2.1. Materials

Circular discs (diameter, 12 mm; thickness, 2 mm)weremade fromastainless steel rodwitha lowspeeddiamondsaw(Labcut1010). Stainlesssteel balls (diameter, 8 mm) of 1082 VHNwere used for the ball-on-discwear tests. Rectangular nickel bars (50 mm×12 mm×12 mm, Metal-men Sales Inc., purityN99%) were provided by Dr. T. I. Khan at theUniversity of Calgary, Alberta, Canada. Tin (IV) sulfate, hydrochloric acid,urea, and sodiumhydroxidewerepurchased fromMerck (Germany).De-ionized water was used for making stock and working solutions. Pyrexglass vessels were used for solution storage and carrying out reactions.For purification purposes, all solutions used for electrodeposition workwere filtered through a membrane filter before use.

2.2. Synthesis of SnO2 particles

The synthesis of SnO2 particles was performed by heating at 85 °C,aqueous solutions containing 3.7×10−3 mol/L tin(IV) sulfate,

10 20 30 40 50 60 70 80

aa

a

aa

Inte

nsit

y (a

.u.)

2 theta (degrees)

a, SnO2

Fig. 1. X-ray diffraction pattern (XRD) of the SnO2 particles shown in Fig. 2.

4 5 6 7 8-8

-4

0

4

8

12

pHIEP = 7.7

Zet

apot

enti

al (

mV

)

pH

Fig. 3. Zetapotential value of SnO2 particles as a function of pH in aqueous solution,containing 5×10−3 mol/L NiSO4, 1.5×10−3 mol/L H3BO3, and 1.0×10−3 mol/L NaCl.

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1.48×10−3 mol/L urea, and 0.1 mol/L hydrochloric acid for 1 h.Particles formed in the resulting mixture were separated by filtrationthrough a membrane filter and washed extensively with de-ionizedwater. The SnO2 in the form of white powder was dried at roomtemperature and then heated at 750 °C for 1 h at the rate of 5 °C/min.Particle morphology of the SnO2 powder was analyzed with scanningelectron microscope (SEM, JEOL, JSM-5910), whereas their crystallin-ity was assessed with x-ray diffractometer (XRD, JEOL JDX-3532)using Cu-Kα radiations. Moreover, Zeta potential of the dispersions(0.15 g/L) of heat treated SnO2 powder in aqueous electrolyte solution(5×10−3 mol/L in NiSO4; 1.5×10−3 mol/L in H3BO3; 1.0×10−3 mol/Lin NaCl) was measured with zetaphoremeter (CAD instrument,Z4000) as a function of pH in the pH range 4–8.3. pH value of eachof the dispersion was adjustedwith either 0.5 mol/L H2SO4 or 0.5 mol/L NaOH.

2.3. Electrodeposition process

In this case, a pure nickel bar and a polished stainless steel discwere employed as anode and cathode, respectively. The cathode wasfirst sonicated in acetone for about 10 min, and then dipped in0.1 mol/L hydrochloric acid for about 30 s. After this treatment, thecathode was extensively washed with de-ionized water. Both theelectrodes were dipped in 400 mL of the test electrolyte dispersions,positioned them in vertical plane and kept them 3 cm apart during allthe electrodeposition experiments. The test electrolyte dispersionswere 0–14 g/L in SnO2 particles, 0.5 mol/L in NiSO4, 0.15 mol/L inH3BO3, and 0.1 mol/L in NaCl. 500 mL double walled Pyrex glass vesselwas used as a reaction vessel for conducting all the electrodepositionexperiments. The temperature of the reaction vessel was adjusted to30±1 °C by circulating water from a thermostatic water bath. The pHof the dispersion was adjusted to ~5.2 with 0.1 mol/L sodium

Fig. 2. Scanning electron micrograph (SEM) of the SnO2 particles.

hydroxide solution. In each case, content of the reaction vessel wassonicated for 15 min and then kept stirred for 1 h. Both the electrodeswere connected to a DC power supply and a current density of 3 A/dm2 was supplied to the system for 14 min. In some cases, especiallyat high particles loading in the composite coatings, the currentincreased from 3.0 to 3.2 A/dm2. The test electrolyte dispersion wasstirred using a magnetic stirrer at a rate of 600 rpm. At the end of theeach experiment, the substrate (cathode) was removed from theelectrolyte dispersion and sonicated for 2 min in de-ionized water forremoving the loosely bound particles from the coated surface. Thesubstrate was then rinsed wither, dried in air, and kept in a dessicatorbefore characterization. The average thickness of the coating was~50 μm.

2.4. Microstructural characterization and wear assessment

Morphology of the SnO2 particles and surface morphology of thecoated substrates were examined by scanning electron microscope(SEM, JEOL, JSM-5910) coupled with energy dispersive x-ray analyzer(EDX, Inca-200). Vickers hardness indentation measurements fromthe coated substrates was determined with a microhardness tester(Shimadzu, HMV-2) using a load 4.6 N. Six indentation tests wereconducted for each sample and the averaged values were recorded.Wear and friction experiments were performed on the coatedsubstrates, using a ball-on-disc tribometer. In all cases, the rotating

0 2 4 6 8 10 12

0.5

1.0

1.5

2.0

2.5

3.0

SnO

2 co

nten

t in

coa

ting

(W

t%)

SnO2 content in bath mixture (g/L)

Fig. 4. Variation in SnO2 content in the composite coating as a function of the amount ofSnO2 particles dispersed in a nickel electrolyte bath.

Fig. 5. SEM images of the pure nickel (A) and Ni–SnO2 composite coatings, containing2.49 wt.% of the SnO2 particles (B). Deposition conditions: current, 3 A/dm2; stirring,600 rpm; temperature, 30 °C. Bath composition for (A): NiSO4, 0.5 mol/L; H3BO3,0.15 mol/L; NaCl, 0.1 mol/L; and (B) NiSO4, 0.5 mol/L; H3BO3, 0.15 mol/L; NaCl, 0.1 mol/L; SnO2, 8.1 g/L. (C) X-ray mapping of the samples shown at (A) with respect to Sn.

Fig. 6. Scanning electron micrograph (SEM) of the cross section of the Ni–SnO2

composite coating, displayed in Fig. 5B. Inset shows the magnified view of the coatingcross section.

0.0 0.5 1.0 1.5 2.0 2.5 3.04.0

4.5

5.0

5.5

6.0

6.5

Wea

r ra

te (

mm

3 /N

m)x

104

SnO2 content in coating (Wt%)

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

A

B

Fri

ctio

n co

effi

cien

t

Fig. 7. Variation in the wear volume (A) and friction coefficient (B) as a function of SnO2

content in the Ni–SnO2 composite coating.

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coated substrate was rubbed against the static stainless steel ball for5 min under an applied load of 15 N in the absence of any lubricant.The extent of wear was assessed from the width of the wear tracks,formed on the coated discs after the wear experiments. The frictionforce at the sliding contacts was monitored through a load cell, whichwas hooked with a computer for converting response of the load cellinto friction coefficient data.

3. Results and discussion

3.1. SnO2 particles

SnO2 particles were essentially prepared by the method describedelsewhere [15]. However, in this study we used slightly less acidicreactant mixture than reported earlier for obtaining relatively smallerparticles. These particles were amorphous in nature [15] which onheating at 750 °C converted into crystalline SnO2 particles (XRD,Fig. 1). SEM analysis showed that the heat treated particles weresubmicron in size and spheroidal with a uniform size distribution(SEM, Fig. 2).

Zetapotential of these particles was evaluated (Fig. 3) byelectrophretic mobility measurement in aqueous electrolyte solu-tions, identical in composition with those employed in elecrodeposi-tion experiments. Inspection of this figure indicated that the value ofzetapotential decreased with the increase in pH of the dispersions inthe pH range of 4 to 8.2 and changed its sign at the pH value ~7.7. Thelatter pH was designated as the isoelectric point (IEP) of the SnO2

particles. It is added that the value of IEP, determined in this study,was a bit at the higher pH side than that reported elsewhere [16] forthe SnO2 particles, which could be attributed to the difference incomposition of the electrolyte solutions used in these two studies.

Fig. 8. SEM images of the surfaces in Fig. 5A(A), and Fig. 5B(B) after the wear tests.Micrograph in (C) represents a magnified image of the worn surface in (B).

0.0 0.5 1.0 1.5 2.0 2.5 3.0225

300

375

450

525

Mic

roha

rdne

ss (

HV

)

SnO2 content in coating (Wt%)

Fig. 9. Variation in microhardness as a function of SnO2 content in the Ni–SnO2

composite coating.

250 300 350 400 450 500

4.8

5.2

5.6

6.0

6.4

Microhardness (HV)

Wea

r vo

lum

e (m

m3 /

Nm

)x10

4

Fig. 10. Variation in wear volume as a function of microhardness of the Ni–SnO2

composite coating.

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3.2. Characterization of the coatings

The SnO2 content deposited in the coatings was estimated fromEDX analysis and the results are depicted in Fig. 4. Each point in thisfigure represents the average value of six measurements at differentlocations on each coated sample. Inspection of this figure shows thatSnO2 content in the deposited coating increased with the increase inthe amount of SnO2 particles in the bath mixture and attained amaximum limit of ~2.65 wt.%, corresponding to 9 g/L of the SnO2

particles in the bath mixture. We believe that the dispersed SnO2

particles codepositedwith the nickel by electrostatic adsorption, sinceat the deposition pH (~5.2b IEP), the SnO2 particles carried a netpositive charge which facilitated their migration towards thenegatively charged steel substrate. The attainment of saturation ofthe SnO2 particles in the composite coating may be attributed to the

screening effect of the formed composite coating which might haveresulted in weakening of the electrostatic attraction between thesubstrate and the dispersed SnO2 particles in the nickel bath.

SEM examination revealed that surface morphology of the purenickel coating (Fig. 5A) was different from that of the Ni–SnO2

composite coatings (Fig. 5B) deposited under identical conditions.Furthermore, it can be noted from the micrograph in Fig. 5B that thesurface of the deposited coating consisted of uniformly distributedgrains, having a size close to the size of the SnO2 particles, shown inFig. 2. This suggests that the SnO2 particles were uniformly dispersedin the nickel matrix.

Furthermore, themagnified view of the cross section of the coating(inset in Fig. 6) also indicated the grainy appearance, which may beascribed to the embedded SnO2 particles in the composite coating.These results were also supported by the x-ray mapping (Fig. 5C) ofthe composite coating in Fig. 5B with respect to Sn, which indicatedthat the distribution of Sn in the composite coatingwas fairly uniform.

3.3. Wear and friction assessment

Wear and friction measurements were performed on nickelcoatings containing different concentrations of SnO2 particles inorder to study changes in tribological behaviour. These resultsindicated (see Fig. 7) that the wear rate of the coating decreased asthe content of SnO2 increased from 0.5 to 2.5 wt.%. A correspondingdecrease in the coefficient of friction was also recorded with

2875I. Haq, T.I. Khan / Surface & Coatings Technology 205 (2011) 2871–2875

increasing SnO2 content in the coatings. Most of these observationswere consistent with earlier research findings in which various metalcoatings have been reinforced with different types of materials, suchas CeO2 [4,5], SiC [6] and Al2O3 [17].

SEMmicrographs taken from the wear tracks are shown in Fig. 8. Acomparison of the wear track widths shows that for the same appliedload a greater width was observed for the pure nickel coatings(Fig. 8A) as compared to the one obtained with composite coating(Fig. 8B). The surface morphology within the wear tracks showedgreater plastic deformation for the pure nickel coatings compared tothe nickel composite coating. However, metal deformation andsmoothening of the wear track was observed in coatings containingthe SnO2 dispersion (Fig. 8C). This effect was also recorded in thecoefficient of friction measurements shown in Fig. 7, where a gradualdecrease in friction was observed with increasing SnO2 content. Theseresults pointed to the lubrication role of the SnO2 particles. Inaddition, it may also be concluded that the plastic deformation of thecoating at the steel ball/surface contact interface becomes moredifficult as content of SnO2 dispersion increased.

3.4. Microhardness measurements

The coated surfaces were subjected to microhardness indentationtests and the hardness profile as a function of SnO2 content in thecoating is shown in Fig. 9. These results showed that the microhard-ness values increased from 260 VHN to 490 VHN with a maximumdispersion content of 2.5 wt.%. It is worth mentioning that theobserved increase in the microhardness per unit amount of theSnO2 in the composite coating was relatively higher than thosereported for the matching content of the Al2O3 [1], CeO2 [4], SiC [6],and TiO2 [7] in the electrodeposited nickel based composite coatings.We believe that besides dispersion hardening and grain refining[18,19] the uniformity in particle shape and size of SnO2 and theiruniform distribution in the nickel matrixmay also have contributed tothe improved performance of the composite coating. However, thelatter aspect is further being investigated in the author's laboratory.

In addition, it may be noted (Fig. 10) that the increase in hardnessof the deposited coating led to the decrease in thewear volume,which

demonstrated the fact that increase in hardness decreased the plasticdeformation behaviour of the composite coating.

4. Conclusions

The successful incorporation of nano-sized SnO2 particles in a purenickel coating was achieved using a nickel bath. The Ni–SnO2

composite coating showed a significant increase in sliding wearresistance and a decrease in coefficient of friction with increasingSnO2 content. This improvement in tribological behavior has beenattributed to an increase in coating hardness and a correspondingchange in plastic deformation behavior of the coatings.

Acknowledgement

We are thankful to the National Centre of Excellence in PhysicalChemistry, University of Peshawar, Pakistan and the Department ofMechanical and Manufacturing Engineering, University of Calgary,Alberta, Canada, for facilitating this work.

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