Surface & Coatings Technology 205 (2010) S51–S56

Contents lists available at ScienceDirect

Surface & Coatings Technology

j ourna l homepage: www.e lsev ie r.com/ locate /sur fcoat

Fabrication of TiC/WC ultra hard nanocomposite layers by plasma electrolysis andstudy of its characteristics

M. Aliofkhazraei, A. Sabour Rouhaghdam ⁎

Department of Materials Engineering, Faculty of Engineering, Tarbiat Modares University, P.O. Box: 14115-143, Tehran, Iran

⁎ Corresponding author. Tel.: +98 912 6905626; fax:E-mail addresses: sabour01@gmail.com, khazrayie@

(A. Sabour Rouhaghdam).

0257-8972/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.surfcoat.2010.04.010

a b s t r a c t

a r t i c l e i n f o

Available online 10 April 2010

Keywords:Tungsten carbideCeramic matrix nanocompositePlasma electrolysisNanostructure

Ultra hard ceramic based matrix nanocomposite layers of tungsten carbide (WC) on matrix of titaniumcarbide were fabricated in an organic electrolyte. The dependence of WC amount in nanocomposite coatingswas investigated in relation to the temperature of electrolyte, WC concentration in bath, current density andstirring rate. It was shown that volume percentage of WC in the layer can be affected by these parameters.The fabricated phases were studied with X-ray diffraction and their microhardness values were reached nearto 2580 Hv0.5. Increasing of the WC nanoparticles concentration in the electrolyte in a constant stirring ratewill lead to an increase in content of nanoparticles in the nanocomposite layers. Concentration of WCnanoparticles in the bath illustrated specific level for increasing of tungsten percentage in the nanocompositelayers. Concentration of WC nanoparticles in the bath had an optimum level around 6 g l−1 for achieving theminimum nano-roughness on the surface of nanocomposite layer. It may vary for different current densities.

+98 21 66960664.modares.ac.ir

l rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Carburizing is a diffusion-based surface modification process, inwhich the metallic surface of the component will saturate by smallcarbon atoms and may also form a hard carbide layer as the topcoating [1–3]. As a result of the tensile stress, formed by diffusion ofcarbon atoms among metallic lattice, a hardness profile will appearfrom the outer surface of the component toward its core [4–6]. Thishardness profile usually makes the substrate appropriate for applica-tions with high tribological properties [7–9].

This process can be applied to different metallic substrates. It isalso effective in the case of titanium-based alloys. These alloys areusually used in the applications that need high strength in addition tolight weight, but their relatively poor mechanical properties must beimproved by such procedures as carburizing [10–12].

Various methods have been reported in the literature forcarburizing titanium alloys. They include plasma treatment [13], gascarburizing [14], ion implantation [15], and salt bath carburizing [16].Each of these methods has its own advantages and limitations, whichmust be considered for their applications to industrial components[17–19].

One of the recent methods developed for carburizing titaniumalloys is based on the plasma electrolysis [20,21]. In this method, thetarget sample will immerse in an organic-based electrolyte and arelatively high negative potential will be applied to it [22]. Carbon

atoms (or nitrogen, boron, etc. atoms in the case of carbonitriding,boronizing etc.) will diffuse from the organic electrolyte to the surfaceof the sample. By precise controlling of the effective parameters, auniform carbide-based layer can be synthesized on the surface of thetarget [23–26].

This method has also the advantage and ability of easy and fastfabrication of novel (micro/nano) composite coatings by addition ofthe appropriate powder of the second phase in the electrolyte. In thiswork, plasma electrolytic carburizing (PEC) was performed in itsregular electrolyte by adding tungsten carbide (WC) nanopowder asthe second phase for possible improvement of the different propertiesof the coatings. The fabricated layers were analyzed by regulartechniques and the effects of the effective parameters, related to theadded nanopowder, were also studied.

2. Experimental procedure

Annealed, commercially pure titanium (CP-Ti) discs (20 mmØ×5 mm) were abraded by different grades of abrasive SiC papers(Buhler) followed by polishing on a soft cloth with the addition of 0.1micron diamond slurry to obtain a near-mirror surface finish(Ra∼400 nm). The samples were then ultra-sonically cleaned inpure ethyl alcohol. Pulsed bipolar nanocrystalline plasma electrolyticcarburizing (PBNPEC) treatment was performed for 2 h in a coolingvessel to maintain the constant temperature of the electrolyte at itsrelative studied level (Fig. 1). The treatment electrolyte wascomposed of a mixed organic solution of chemically pure glyceroland dissolved sodium carbonate (to provide a desirable electricalconductivity of electrolyte) with addition of WC fine nanopowders.

Fig. 1. Schematic illustration of coating setup, (1) cooling water outlet (2) window forobservation sparking surface (3) cylindrical (coin shape) sample (4) magnetic bar forstirring of nanoparticle suspension in electrolyte (5) nanoparticle suspension in organicbased electrolyte (6) cooling water inlet (7) stainless rod for holding sample andelectrical connection (8) wires for electrical connection to power supply (9) powersupply for applying pulsed current.

S52 M. Aliofkhazraei, A. Sabour Rouhaghdam / Surface & Coatings Technology 205 (2010) S51–S56

The average size of WC nanoparticles, calculated by figure analysis ofdifferent SEM nanostructures was around 49 nm (Fig. 2).

The work pieces were biased to rectangular-shape pulsed currentproduced from a 20 kW power supply. Frequency and duty cycle ofthe pulsed current were maintained constant at 10 kHz and 40%,respectively during the coating process. The peak voltages on thecathodic and anodic directions were maintained constant at 700 and200V, respectively and the average current density was changed bychanging the electrical conductivity of the electrolyte (changing theconcentration of sodium carbonate). Current density and othermentioned parameters of the applied current were in their averagemagnitude. An AISI 316 stainless steel cylinder with the dimensions of15 cm dia×22 cm formed the anode of the electrochemical system ofcoating process, while the target samples were connected as thecathode of the system. Morphology and quantitative analysis of thecoatings were observed by means of scanning electron microscopy(SEM) using a Philips XL-30 scanning electron microscope. Tungstencontent of the obtained layer was determined by EDS analysis of theSEM. Hardness of the nanocomposite layers was measured by aBuhler-Micromet1 micro-hardness tester with 500 g load in a relativelong loading time of 20 seconds (for a rough ensuring of load bearingcapability of the fabricated layers). Linear surface profiles wereplotted for the surfaces of coated samples by a Surtronic 25 (Taylor-

Fig. 2. Typical SEM nanostructure of used WC nanoparticles for performing nanocompo-site layers.

Habson) roughness checker and analysed by Taly profile (goldpackage) software (from Taylor-Habson Company). The XRD patternsof the nanocomposite layers were plotted by Phillips X'pert with CuKα radiation (λ=1.54178°A).

3. Results and discussion

Fig. 3a illustrates the typical surface of fabricated nanocompositelayer. As shown, the nanoparticles are homogenously distributed overthe surface. Distribution of the nanoparticles will be affected by thesuspension conditions and the current density. In the present study,the effects of these parameters were studied. Fig. 3b illustrates thecross section of coating. As it can be seen in this figure, coating isapproximately uniformwith some porosity. EDS analysis was done ondifferent spots along the coating and it showed approximatelyconstant concentration of tungsten in the layers (there was a lightincrease near the top surface of the coating (around 15% more)). TheXRD and GAXRD pattern (Fig. 4) confirms the formation of a TiC/WCnanocomposite layer. The concentration of WC nanoparticles wasincreased slightly by decreasing GAXRD angle which means that theamount of WC nanoparticles was increased toward the top surface ofnanocomposite layer. Average grain sizes was determined by Schererequation around 51, 58, 72 and 89 nm for the layers by 1, 5 and 10° ofglancing angle and also simple XRD, relatively. Tungsten carbidenanoparticles probably act as new sites for grain growth and hencedecrease the final size of grains.

3.1. Effect of electrolyte temperature

Dependency of the tungsten concentration in nanocomposite layerwas chosen as an indicator of the presence of WC nanoparticles in thelayer. Fig. 5 shows the effect of different temperatures of theelectrolyte on tungsten surface concentration of the layers, deter-mined by average EDS tests in different average current densities.

Fig. 3. SEM nanostructure of (a) surface (b) cross section of nanocomposite layer.

Fig. 4. X-ray diffraction pattern of nanocomposite layer.

S53M. Aliofkhazraei, A. Sabour Rouhaghdam / Surface & Coatings Technology 205 (2010) S51–S56

Increasing of the current density from 0.2 to 1.2 A/cm2 will lead to anon-stop increasing of tungsten percentage in the nanocompositelayer. According to this figure, this increase of tungsten will beaccelerated by decreasing of the electrolyte temperature. Consideringa constant current density, it can be understood that decreasing of theelectrolyte temperature will always affect tungsten percentage in thenanocomposite layer, especially in the temperature range of 45 to65 °C. In contrast, by considering a constant temperature line,increasing of the current density will be effective on the temperaturesequal to and below 65 °C. This behavior can be related to themechanism of cathodic plasma electrolysis, probably due to theincrease of WC absorption toward the surface of the sample at highcurrent densities. Higher current densities will increase the flow ofnanoparticles through the surface while lower electrolyte tempera-tures will cause more uniform gas in the gaseous envelope around thesample [27–29]. This will lead to more diffusion of the radicals fromthe electrolyte and fabrication of coatings with more embeddednanoparticles. The resulted sparks have more ability to carry and

Fig. 5. Relation among W content in coating and applied cu

embed the nanoparticles from electrolyte into the nanocompositecoating due to the less agitation around the sample (the agitation nearthe sample surface by the breakdown of gas bubbles rather than theregular electrolyte agitation for dispersion of nanoparticles). Hence,the surface concentration of tungsten is higher for the coatingssynthesized at lower electrolyte temperatures. Fig. 6 illustrates theeffect of the same parameters on the hardness of coatings. Here, thepresence of more nanoparticles, which is the main reason of theincrease of hardness of the achieved layers, causes more hardness ofthe coatings fabricated at higher current densities and lower tem-peratures of the electrolyte. The increase of the differences betweenthe two lines in this figure is also related to the lower agitation inlower current densities, due to smaller sparks.

3.2. Effect of electrolyte agitation and WC concentration in the electrolyte

Fig. 7a–b show the effect of electrolyte agitation and nanoparticlesconcentration in the electrolyte on tungsten content of the coating.

rrent density in different temperatures of electrolyte.

Fig. 6. Relation among surface hardness of coating and applied current density in different temperatures of electrolyte.

Fig. 7. Relation among W content in coating and (a) WC nanoparticle concentration in electrolyte with different stirring rates and (b) temperatures of electrolyte in differentconcentrations of WC nanoparticles in electrolyte.

S54 M. Aliofkhazraei, A. Sabour Rouhaghdam / Surface & Coatings Technology 205 (2010) S51–S56

Fig. 8. Typical surface linear profile of nanocomposite layer.

S55M. Aliofkhazraei, A. Sabour Rouhaghdam / Surface & Coatings Technology 205 (2010) S51–S56

Agitation will lead to the decrease of nanoparticles concentration inthe electrolyte in front of the sample surface. Hence, a higher agitatedelectrolyte will lead to lower concentration of tungsten in the coating.Contrary to the previous trend, the increase of nanoparticlesconcentration in the electrolyte will lead to thier more presence infront of the sample surface and thus sparks on the surface will embedmore nanoparticles into the coating. The results shown in Fig. 7 can beexplained by the Guglielmi two-step adsorption model [30,31] (likeelectrodeposition) in which at higher nanoparticle concentrationsin the bath, the adsorption and consequently the percentage ofnanoparticles in the nanocomposite layers will increase. Also theamount ofWC nanoparticles in the nanocomposite layer will decreasealong with the increase of the stirring rate. Due to the agglomerationof nanoparticles in the suspension at relatively high concentrations(more than 8 g/l), the effective nanoparticle concentration willremain constant. This will causeWC percentage in the nanocompositelayer to remain constant.

3.3. Study of surface condition and roughness

A typical surface profile of the fabricated layers can be seen inFig. 8. This profile is relatively smooth with no sharp falling (related tothe possible mini-cracks). It is also an example of different measure-ments, which, in addition to SEM figure, confirms a uniform surface of

Fig. 9. Relation among surface roughness of coating and WC nanopa

nanocomposite layer with no sharp cracks. The presence of nano-particles in the fabricated layer will avoid propagation of sharp cracks.It is even useful for increasing fatigue resistance of the coatings asshown for other nanocomposite layers [32–34]. More uniformdistributed nanoparticles will act better in this phenomenon.Roughness values of the nanocomposite layers were calculated to beapproximately between 1.6 and 4.9 microns. Fig. 9 illustrates thechanges of roughness with the change in the concentration of WCnanoparticles in the electrolyte. The increase of roughness is due tothe agglomeration of WC nanoparticles on the surface of the treatedsample (Fig. 10). Concentration of WC nanoparticles in the electrolytehas an optimum level for achieving the minimum roughness onthe surface of the nanocomposite layer at higher current densities. Infact, the increase of nanoparticles concentration in the electrolyte andthe increase of the current densities have similar effects on surfaceroughness. Higher current densities will lead to big sparks with moredamaging effects and their effects will show themselves on lowconcentrations of nanoparticles in the electrolyte. For the layersfabricated by 2 to 6 g/l of WC nanoparticles in electrolyte, the increaseof roughness is lesser for higher current densities as compared withhigh increase of roughness at lower current densities. This effectcan be seen in the left side of the Fig. 9 and in different slopes of thetrend lines. From this point of view, 6 g/l is an optimum concentrationof the nanoparticles for higher current densities. Concentrations of

rticle concentration in electrolyte in different current densities.

Fig. 10. Agglomeration of nanoparticles in the surface of nanocomposite layer.

S56 M. Aliofkhazraei, A. Sabour Rouhaghdam / Surface & Coatings Technology 205 (2010) S51–S56

nanoparticles in electrolyte higher than 6 g/l will lead to sharp in-crease for roughness of nanocomposite coatings.

4. Conclusions

Nanocomposite layers of tungsten carbide (WC) on ceramic-matrix of titanium carbide were fabricated successfully by cathodicplasma electrolysis. In this process, the nanoparticles were approx-imately homogenously distributed over the surface and along thecoating. There was a light increase of their concentration near the topsurface.

The results show that increasing of current density from 0.2 to1.2 A/cm2 will lead to a non-stop increase of tungsten percentage inthe nanocomposite layer. It will accelerate at low temperatures of theelectrolyte. Considering a constant temperature, increasing of currentdensity will be effective on the temperatures equal to and below65 °C. Presence of more nanoparticles is the main reason for theincreasing in the hardness of the achieved nanocomposite layers. Itwas causedmore hardness of the coatings fabricated at higher currentdensities and lower temperatures of the electrolyte. A maximumhardness near 2580 Hv0.5 was achieved under the best conditions.

Agitation of electrolyte will lead to the decrease of nanoparticlesconcentration in the electrolyte in the front part of the sample surface.Hence higher agitated electrolyte will lead to lower concentrations oftungsten in the coating. The increase of nanoparticles concentration inthe electrolyte will lead to their more presence in front of the samplesurface and as a result, the sparks on the surface will embed morenanoparticles into the coating.

Surface linear profile is relatively smooth with no sharp falling andin addition of SEM figure confirms a uniform surface of nanocompo-

site layer with no sharp cracks. Roughness values of the nanocompo-site layers were calculated as approximately between 1.6 and 4.9 μm.Concentration of the WC nanoparticles in the electrolyte showed anoptimum level for achieving a minimum roughness on the surface ofthe nanocomposite layer at higher current densities.

Acknowledgement

The authors would like to express their thanks to Dr. E. Matykina(University of Manchester, UK) for her helpful discussions throughinvestigation about the different aspects of anodic plasma electrolysis.Financial support of Iranian Nanotechnology Initiative Council is alsoacknowledged.

References

[1] K. Zheng, S. Zhang, Mater. Chem. Phys. 26 (1990) 57.[2] B.S. Suh, W.J. Lee, Thin Solid Film. 295 (1997) 185.[3] B. Selçuk, R. Ipek, M.B. Karami, V. Kuzucu, J. Mater. Process. Technol. 103 (2000)

310.[4] G. Straffelini, L. Versari, Eng. Fail. Anal. 16 (2009) 1448.[5] P. Cavaliere, G. Zavarise, M. Perillo, Comput. Mater. Sci. 46 (2009) 26.[6] M. Tsujikawa, D. Yoshida, N. Yamauchi, N. Ueda, T. Sone, S. Tanaka, Surf. Coat.

Technol. 200 (2005) 507.[7] N. Tsuji, S. Tanaka, T. Takasugi, Surf. Coat. Technol. 203 (2009) 1400.[8] J. Georgiev, T. Pieczonka, M. Stoytchev, D. Teodosiev, Surf Coat Technol. 180–181

(2004) 90.[9] W. Liang, X. Xiaolei, X. Bin, Y. Zhiwei, H. Zukun, Surf. Coat. Technol. 131 (2000)

563.[10] M. Shindo, H. Nakajima, J. Nucl. Mater. 166 (1989) 278.[11] T.S. Kim, Y.G. Park, M.Y. Wey, Mater. Sci. Eng. A 361 (2003) 275.[12] N. Tsuji, S. Tanaka, T. Takasugi, Mater. Sci. Eng. A 488 (2008) 139.[13] N. Tsuji, S. Tanaka, T. Takasugi, Mater. Sci. Eng. A 499 (2009) 482.[14] A. Zhecheva, W. Sha, S. Malinov, A. Long, Surf. Coat. Technol. 200 (2005) 2192.[15] X.M. Zhu, M.K. Lei, Curr. Appl. Phys. 5 (2005) 522.[16] T.I. Wu, J.K. Wu, Surf. Coat. Technol. 90 (1997) 258.[17] J.P. Souchard, P. Jacquot, M. Buvron, Mater. Sci. Eng. A 140 (1991) 454.[18] M.H. Jacobs, Mater. Des. 14 (1993) 33.[19] D.C. Lou, J.K. Solberg, T. Børvik, Mater. Des. 30 (2009) 3525.[20] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Surf. Coat. Technol. 122

(1999) 73.[21] D.J. Shen, Y.L. Wang, P. Nash, G.Z. Xing, Mater. Sci. Eng. A 458 (2007) 240.[22] M. Tarakci, K. Korkmaz, Y. Gencer, M. Usta, Surf. Coat. Technol. 199 (2005) 205.[23] X.M. Li, Y. Han, Electrochem. Commun. 8 (2006) 267.[24] X.M. Li, Y. Han, Appl. Surf. Sci. 254 (2008) 6350.[25] X.M. Li, Y. Han, Y.S. Li, Surf. Coat. Technol. 201 (2007) 5326.[26] M.A. Béjar, R. Henríquez, Mater. Des. 30 (2009) 1726.[27] X. Nie, C. Tsotsos, A. Wilson, A.L. Yerokhin, A. Leyland, A. Matthews, Surf. Coat.

Technol. 139 (2001) 135.[28] E.I. Meletis, X. Nie, F.L. Wang, J.C. Jiang, Surf. Coat. Technol. 150 (2002) 246.[29] M. Aliofkhazraee, A. Sabour Rouhaghdam, J. Alloys Compound. 462 (2008) 421.[30] N. Guglielmi, J. Electrochem. Soc. 119 (1972) 1009.[31] P. Berçot, E. Peña-Muñoz, J. Pagetti, Surf. Coat. Technol. 157 (2002) 282.[32] E. Broszeit, Thin Solid Film. 95 (1982) 133.[33] M.H. Zhu, Z.R. Zhou, Ph. Kapsa, L. Vincent, Wear 250 (2001) 650.[34] B. Jiang, B. Xu, S. Dong, Y. Yi, P. Ding, Surf. Coat. Technol. 202 (2007) 447.