Materials and Design 32 (2011) 3548–3553

Contents lists available at ScienceDirect

Materials and Design

journal homepage: www.elsevier .com/locate /matdes

Short Communication

Fabrication of low friction bronze–graphite nano-composite coatings

T. Nickchi a,b,⇑, M. Ghorbani b,c, A. Alfantazi a, Z. Farhat d

a Department of Materials Engineering, The University of British Columbia, Vancouver, BC, Canadab Department of Materials Science and Engineering, Sharif University of Technology, P.O. Box 11356-8639, Tehran, Iranc Institute of Nano Science and Nano Technology (INST), Sharif University of Technology, Tehran, Irand Materials Engineering Program, Department of Process Engineering and Applied Science, Dalhousie University, Haliflex, NS, Canada

a r t i c l e i n f o a b s t r a c t

Article history:Received 24 November 2010Accepted 25 January 2011Available online 28 January 2011

0261-3069/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.matdes.2011.01.044

⇑ Corresponding author. Address: Department ofUniversity of British Columbia 309-6350 Stores Rd VanTel.: +1 604 822 6964; fax: +1 604 822 3619.

E-mail address: tirdadn@interchange.ubc.ca (T. Ni

This paper addresses the fabrication of bronze–nano-graphite coatings containing 30 nm sized graphiteparticles. Addition of Poly Vinyl Pyrrolidone (PVP) and dispersing the suspension by ultrasound was uti-lized to stabilize the electroplating solution. Pulsed current electrodeposition was used and the depositswere characterized in terms of graphite content, particle dispersion in coating and morphology. The wearbehavior of the coating was compared to bronze–micro-graphite coatings prepared with the same exper-imental conditions. The present coating showed improved wear resistance in comparison to bronze–micro-graphite coatings at the same graphite content.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

The superior wear, corrosion and high temperature oxidationresistance of composite coatings has motivated research in thefield of metal finishing. The process of composite electroplatinghas been studied progressively for, at least, the last 50 years. Ini-tially, researchers focused on experimental parameters regardingenhanced desired properties. Several excellent reviews summariz-ing the effect of process parameters are available [1–3]. In addition,there are some more fundamental studies with the aim ofunderstanding the process of particle co-deposition. Among these,Guglielmi [4] proposed the most plausible model.

During recent years, various nano-sized particles becameincreasingly available, as such, studies in the field of compositeelectroplating became directed toward producing composite coat-ings containing nano-sized particles. Not only new applications areintroduced for these ‘‘nano-composite’’ coatings, e.g. photoelectro-catalysis properties of PbO–TiO2 nano-composites [5], but also tra-ditional characteristics of these coatings like hardness, wear andcorrosion resistance are reportedly enhanced. For example, Lowet al. [3] have compared various nickel-based micro- and nano-composite. Their comparison shows that the hardness of nano-composite is appreciably higher than that of micro-composite ofthe same type. Other relevant recent research in the field of

ll rights reserved.

Materials Engineering Thecouver, B.C. Canada V6T 1Z4.

ckchi).

nano-composite coatings studied electrodeposition characteristicsand wear behavior of the coating [6–9].

One of the main problems associated with fabrication of nano-composite coatings is that the amount of nano-sized particles, code-posited in the growing metallic matrix, is lower in comparison withmicro-sized particles. However, by comparing the ‘‘particle density’’variable rather than ‘‘volume content’’, this difference would dimin-ish. In fact, many recent studies on mechanism of nano-sized parti-cle co-deposition are based on particle density [10–12].

Since nano-sized particles are more prone to agglomeration inelectroplating bath, the role of solution treatment prior to electro-plating is much more crucial in nano-composite electroplatingthan micro-composite electroplating. This necessitated the study-ing of dispersion of particles in the bath. Nevertheless, since sus-pensions are usually characterized in dilute solutions, very littleinformation is available on the behavior of nano-sized particlesand stability of electroplating suspensions. One of the recent stud-ies in this field examined the size distribution and zeta potential ofZrO2 particles in nickel electroplating bath [13].

Other shortcoming in the field of nano-composite electrodepos-ition is the characterization of wear resistant nano-composite coat-ings with attention to the effect of particle size; the coatings areusually studied individually and there are a few studies aboutthe effect of particle size on wear behavior of composites [10,14].

Due to enhanced hardness and stable lubrication properties, al-loyed copper–graphite composite coatings have received attention[15,16]. In this paper, we stepped forward producing bronze–graphite nano-composite coating with the aim of understandingthe effect of particle size on tribological behavior of bronze/graph-ite coatings as a self-lubricant coating. Our recent results regardingself-lubricating bronze–graphite micro-composite coatings

T. Nickchi et al. / Materials and Design 32 (2011) 3548–3553 3549

[15,16], revealed that pulsed current would be beneficial inpromoting the particle content and reducing the agglomerationof particles in the coating. Therefore, in this study, pulsed currentelectrodeposition was employed for fabrication of the coatingsand the tribological behavior of bronze–graphite nano-compositecoatings were compared with micro-composite coatings preparedin the same experimental condition.

2. Experimental

The cyanide–stannate bronze electroplating bath containing30 nm sized graphite particles (MTI corporation) with a purity of99.9% and specific surface area of >60 m2/g was used. Despite theenvironmental issues associated with cyanide containing electro-plating baths, this bath type was chosen, in order to make refer-ences and comparisons with our previous work [16]. Thecomposition of the bath and operating condition is CuCN 29 g/l,K2SnO3 35 g/l, KCN 35 g/l and KOH 10 g/l (pH = 10). In addition, anon-ionic surfactant (Poly Vinyl Pyrrolidone, Merck, extra pure)was added to the electroplating bath. Before each experiment,composite electroplating bath was sonicated with ultrasonichomogenizer (Bandelin, snoplus HD2070, 70 W, 20 kHz) for15 min.

Cathodes with dimensions of 30 � 30 mm were made of coldrolled low carbon steel. Prior to electroplating, specimens wereground down to 600 grit SiC emery paper, degreased ultrasonicallyin acetone and pickled in 15 vol.% HCl solution. Anodes were madeof 10%Sn bronze with the same active area as the cathode. A stan-dard oscilloscope measured the pulsed waveform, which was ap-plied by the pulse generator. The pulse wave specifications wereaverage current density of 10 mA cm�2, frequency of 10 Hz, andduty cycles of 1, 0.5, 0.1, and 0.05. The surface morphology of thecoatings and the dispersion of graphite nano-particles in thebronze matrix were examined by Scanning Electron Microscope(SEM). Graphite content of the coating was determined by imageanalysis. Cross-section of the samples were ground, polished andetched using diluted HNO3 solution. Images from each samplewere taken and processed using image analysis tools. Furthermore,some samples were cross-analyzed by chemical method. The LECOcombustion carbon and sulfur analyzer was used for chemicalanalysis. In this method, the sample is fully combusted in the ana-lyzer’s crucible using highly purified oxygen. Afterwards, the mag-nitude of the resulting carbon dioxide is measured. This gives anindirect analysis of carbon content of the coating down to0.005 wt.% in the sample. It is noteworthy that, the coating was de-tached from the steel substrate to remove any source of error forthis experiment.

Wear resistance and coefficient of friction of the as-depositedcoatings were measured using a pin-on-disk apparatus connectedto a personal computer to measure the coefficient of friction. Wearweight loss during wear was determined with an accuracy of0.1 mg. The counter-face pin was made of SAE 52100 steel withhardness of 62 HRC and diameter of 7 mm. Wear experiments wereperformed using a load of 2 kgf, speed of 100 rpm and track radiusof 15 mm similar to our previous works [16,17].

Fig. 1. Stability of 10 g/l and 20 g/l graphite nanoparticle suspensions as a functionof PVP concentration, Zhu et al. curve is reproduced for comparison form Ref. [20]and is in a aqueous solution of ammonia with pH = 10 and graphite concentration of1 g/l.

3. Results and discussion

Initial results using graphite nano-particles without any addi-tives in the solution showed that graphite particles did not embedin the coating effectively. To overcome this hurdle, as a first step,different additives were added to the solution in an attempt todisperse the graphite particles in the bath and enhance the processof particle electrocodeposition. The stability of the suspension wastaken as a criterion for dispersion of nano-sized particles in the

bath. The stability test consists of holding the graphite particlecontaining solution in glass test tubes and measuring the heightof the suspended fraction of the prepared solutions over time.Then, the suspended fraction defined as the ratio of the suspendedheight to the overall height of the solution in the tube is reported.

As the first choice, Sodium Dodecyl Sulphate (SDS) was chosendue to its high performance in dispersion of carbon nano-tubes inaqueous solutions [18]. Our results also showed that graphitenanoparticles are effectively dispersed in the electroplating bath,resulting in stable suspensions. However, composite electroplatingfrom this bath revealed that graphite particles were not embeddedthe coating. This might be related to the fact that SDS is an anionicsurfactant and would negatively charge the particles and thus inhi-bit the particle co-deposition. This phenomenon was previouslyobserved: Hovestad and Janssen [1] addressed that negativelycharged silica particles are not likely to reach the cathode surfacewhile alumina particles do because of positive surface charge.Because of the problems associated with negative surface charge,Cetyl Trimethyl Ammonium Bromide (CTAB) was chosen as acationic surfactant, with the aim of positively charging the graphiteparticles and facilitating the approach of particles to the cathodesurface. CTAB was added to the suspension at various concentra-tions below the micelle concentration and the stability of the sus-pension was evaluated. However, this surfactant failed to dispersethe particles effectively and to stabilize the suspension. One of thepossible explanations for this behavior is that since graphite is ini-tially negatively charged in the solution, addition of CTAB in-creased the zeta potential only to values around zero, and hence;the affinity of agglomeration was rather increased [1]. Further-more, literature survey shows that CTAB was previously utilizedonly in acidic and neutral electrodeposition solutions, for exampleNi–SiC [19], and no report of successful deposition in alkaline solu-tions using CTAB was found.

Addition of non-ionic surfactants to the solution is an alterna-tive solution to facilitate metal ion adsorption on the particle andthere are reports indicating an increase of particle incorporationrate in the metal deposit, by addition of non-ionic surfactants [1].Therefore, the literature was surveyed to find appropriate non-io-nic surfactant, which has previously proved to be effective at highvalues of pH. Zhu et al. [20] reported that Poly Vinyl Pyrrolidone(PVP) effectively stabilizes aqueous suspensions of graphite in di-lute alkaline solutions. Due to similar solution pH range and non-ionic structure, PVP was also examined. As Fig. 1 demonstrates,addition of PVP seemed effective in producing a stable suspension.

Fig. 2. Effect of pulse duty cycle on graphite content of the coating, at pulsefrequency of 10 Hz and average current density of 10 mA cm�2.

3550 T. Nickchi et al. / Materials and Design 32 (2011) 3548–3553

The results showed that PVP is able to stabilize the suspension of10 g/l and 20 g/l graphite nanoparticles when sufficient amountof PVP is added to the electroplating bath. Nonetheless, since PVPis a long chain polymer molecule, and it probably enters thecoating to some extent, electroplating from 20 g/l graphite stable

Fig. 3. Back scattered image of bronze–graphite nano-composite cross-section obtained a(b) 0.5, (c) 0.1 and (d) 0.05.

suspension containing PVP resulted in very brittle coatings. There-fore, the nano-sized graphite concentration of 10 g/l and PVP con-centration of 0.5 wt.% was selected as the optimal suspension. Inorder to obtain better dispersion, the suspension was sonicatedwith ultrasonic homogenizer for 15 min before each experiment.During the sonication time, the solution was treated for 2 s follow-ing a relaxation time for 2 s which was necessary for preventingthe solution from over heating.

After optimizing the dispersion of particles in the solution,pulsed electrodeposition of bronze–graphite nano-composite coat-ing was carried out. Previous results [16] showed that pulse dutycycle possesses the greatest effect on particle content in bronze/graphite system. This led us to investigate the effect of pulse dutycycle. Fig. 2 shows that, although not as apparent as bronze/micro-graphite coating [16], an increase in graphite content of the coatingis observed as a result of reducing the duty cycle from 1 to 0.05.This was because a shorter duty cycle means a longer OFF time.This permits particles to adsorb on the surface and embed intothe growing metal matrix in the subsequent ON time.

Fig. 3 shows the cross-section of the coating prepared at variousduty cycles. The dark regions are the graphite particles dispersed inthe bright bronze matrix. It shows that less agglomeration and bet-ter dispersion of particles is observed by decreasing the duty cyclefrom 1 to 0.05. A decrease in duty cycle results in more relaxationtime at the interface. This allows sufficient time for detachment of

t frequency of 10 Hz, average current density of 10 mA cm�2 and duty cycle of (a) 1,

Fig. 4. Secondary electron image of surface morphology of bronze–graphite nano-composite coatings obtained at frequency of 10 Hz, average current density of 10 mA cm�2

and duty cycle of (a) 1, (b) 0.5, (c) 0.1 and (d) 0.05.

Fig. 5. Friction–distance curve of bronze, bronze–micro-graphite and bronze–nano-graphite coatings obtained at frequency of 10 Hz, average current density of 10 mA cm�2

and duty cycle of 0.1. Bronze and bronze–micro-graphite curve is reproduced from Ref. [16].

T. Nickchi et al. / Materials and Design 32 (2011) 3548–3553 3551

Fig. 6. Coefficient of friction of bronze–micro-graphite and bronze–nano-graphiteversus graphite content of the coating. Bronze–micro-graphite curve is reproducedfrom Ref. [16].

3552 T. Nickchi et al. / Materials and Design 32 (2011) 3548–3553

agglomerated particles from the interface towards the bulk solu-tion. Finally, this leads to selective embedding of smaller particles,which is seen as better dispersion of the particles in the coating.

The effect of pulse duty cycle on surface morphology of nano-composite coatings was also investigated. Fig. 4 shows the overallmorphology of the surface as affected by altering the duty cycle. Itshows that a decrease in duty cycle results in more tendency ofcauliflower structure formation. This structure usually appearswhen electrodeposition is under mass transfer control [21]. Thesituation take place when duty cycle decreases and consequentlypeak current density is increased at constant average current den-sity. Thus, the morphology is affected by mass transfer effects in-duced by instantaneous current density variation, while thepresence of graphite nano-sized particles prevent the formationof pyramidal shaped morphology as observed in our previous work[16].

Fig. 5 represents the coefficient of friction of bronze–nano-graphite coatings along with that of bronze and bronze–micro-graphite coatings prepared by the same pulse waveform repro-duced from our previous work [16]. It shows that the coefficient

Fig. 7. Wear weight loss of bronze–nano-graphite and bronze–micro-graphite coatings vRef. [16].

of friction of nano-composite coating is between that of bronzeand bronze–micro-graphite coating. It also shows that there isslight oscillation in nano-composite curve that is a characteristicof adhesive wear mechanism. It should be mentioned that the dis-tance of 100 m was appropriate for evaluating the wear character-istics of this coating thickness based on the rotation rate and loadparameters. Coefficient of friction of other nano-composite coat-ings was also measured. The results showed that, althoughdecreasing the duty cycle promotes the graphite content of thenano-composite coatings from 3 vol.% to 6 vol.%, there is no mean-ingful difference between magnitudes of friction coefficient be-tween coatings obtained at duty cycles of 1, 0.5, 0.1 and 0.05.

It would be interesting to compare the composite coatings atthe same graphite content. Fig. 6 shows that bronze–nano-graphitecoatings exhibit lower coefficient of friction than that of bronze–micro-graphite coatings at the same graphite content. In otherwords, low coefficient of friction would be achieved at lowergraphite content of coating using nano-sized graphite particles.This would probably be related to the fact that smaller particleswould more easily and more effectively cover the adjacent tribo-surfaces and thus reduce the coefficient of friction.

Weight loss measurements of bronze-based composite coatingsare presented in Fig. 7. It shows that, as duty cycle decreases andgraphite content of coatings increases, weight loss of bronze–micro-graphite coatings increases, while that of bronze–nano-graphite decreases. In our previous paper [16], it was foundthat by decreasing the duty cycle, the hardness of electrodepositedbronze is increased and so is the wear resistance. It is observed inthis work that the wear resistance of the nano-composite coatingis increased as pulse duty cycle decreases. Additionally, the lowvolume fraction of graphite nano-sized particles is not expected tosignificantly decrease the hardness of the coating. It could be thusassumed that enhanced wear resistance of the bronze–graphitenano-composite is mainly due to increase of hardness of the bronzematrix.

4. Conclusion

This study showed that a combination of PVP as surfactant andultrasonic treatment, prior to electroplating, would result inappropriate dispersion of nano-sized graphite particles in

ersus pulse duty cycle. Bronze–micro-graphite weight loss data are reproduced from

T. Nickchi et al. / Materials and Design 32 (2011) 3548–3553 3553

cyanide–stannate electroplating bath and effective embedment ofparticles in the bronze coating. A decrease in pulse duty cycle re-sults in less agglomeration of particles in the coating. Coefficientof friction of bronze–graphite nano-composite coatings was inthe range of 0.24, which was obtained at as low volume fractionof graphite as 3 vol.%. This behavior shows that nano-sized graph-ite particles facilitate the lubricating properties of the coating moreeffectively than micron-sized graphite particles at the same graph-ite content. Wear weight loss of the nano-composite decreased aspulse duty cycle decreased. This probably implies that the wearweight loss is controlled by hardness; however, further investiga-tion on the hardness of the nano-composite coating is necessary.

References

[1] Hovestad A, Janssen LJJ. Electrochemical codeposition of inert particles in ametallic matrix. J Appl Electrochem 1995;25:519–27.

[2] Musiani M. Electrodeposition of composites: an expanding subject inelectrochemical materials science. Electrochim Acta 2000;45:3397–402.

[3] Low CTJ, Wills RGA, Walsh FC. Electrodeposition of composite coatingscontaining nanoparticles in a metal deposit. Surf Coat Technol2006;201:371–83.

[4] Guglielmi N. Kinetics of the deposition of inert particles from electrolyticbaths. J Electrochem Soc 1972;119:1009–12.

[5] Amadelli R, Samiolo L, Velichenko AB, Knysh VA, Luk’yanenko TV, Danilov FI.Composite PbO2–TiO2 materials deposited from colloidal electrolyte:electrosynthesis, and physicochemical properties. Electrochim Acta2009;54:5239–45.

[6] Xu R, Wang J, He L, Guo Z. Study on the characteristics of Ni–W–P compositecoatings containing nano-SiO2 and nano-CeO2 particles. Surf Coat Technol2008;202:1574–9.

[7] Frade T, Bouzon V, Gomes A, da Silva Pereira MI. Pulsed-reverse currentelectrodeposition of Zn and Zn–TiO2 nanocomposite films. Surf Coat Technol2010;204:3592–8.

[8] Cardinal MF, Castro PA, Baxi J, Liang H, Williams FJ. Characterization andfrictional behavior of nanostructured Ni–W–MoS2 composite coatings. SurfCoat Technol 2009;204:85–90.

[9] Gül H, Kılıç F, Aslan S, Alp A, Akbulut H. Characteristics of electro-co-depositedNi–Al2O3 nano-particle reinforced metal matrix composite (MMC) coatings.Wear 2009;267:976–90.

[10] Garcia I, Fransaer J, Celis J. Electrodeposition and sliding wear resistance ofnickel composite coatings containing micron and submicron SiC particles. SurfCoat Technol 2001;148:171–8.

[11] Vereecken PM, Shao I, Searson PC. Particle codeposition in nanocompositefilms. J Electrochem Soc 2000;147:2572–5.

[12] Shao I, Vereecken PM, Cammarata RC, Searson PC. Kinetics of particlecodeposition of nanocomposites. J Electrochem Soc 2002;149:C610–4.

[13] Simunkova H, Pessenda-Garcia P, Wosik J, Angerer P, Kronberger H, Nauer GE.The fundamentals of nano- and submicro-scaled ceramic particlesincorporation into electrodeposited nickel layers: zeta potentialmeasurements. Surf Coat Technol 2009;203:1806–14.

[14] Srivastava Sr M, William Grips VK, Jain A, Rajam KS. Influence of SiC particlesize on the structure and tribological properties of Ni–Co composites. Surf CoatTechnol 2007;202:310–8.

[15] Afshar A, Ghorbani M, Mazaheri M. Electrodeposition of graphite–bronzecomposite coatings and study of electroplating characteristics. Surf CoatTechnol 2004;187:293–9.

[16] Nickchi T, Ghorbani M. Pulsed electrodeposition and characterization ofbronze–graphite composite coatings. Surf Coat Technol 2009:3037–43.

[17] Ghorbani M, Mazaheri M, Afshar A. Wear and friction characteristics ofelectrodeposited graphite–bronze composite coatings. Surf Coat Technol2005;190:32–8.

[18] Zhang J, Gao L. Dispersion of multiwall carbon nanotubes by sodium dodecylsulfate for preparation of modified electrodes toward detecting hydrogenperoxide. Mater Lett 2007;61:3571–4.

[19] Ger M. Electrochemical deposition of nickel/SiC composites in the presence ofsurfactants. Mater Chem Phys 2004;87:67–74.

[20] Zhu H, Zhang C, Tang Y, Wang J, Ren B, Yin Y. Preparation and thermalconductivity of suspensions of graphite nanoparticles. Carbon 2007;45:226–8.

[21] Popov KI, Djokic SS, Grgur BN. Fundamental aspects of electrometallurgy.Kluwer Academic Publishers; 2002.