ukeyrkey

maplre

outly pnie cugenhepacity. Detailed tribological tests and characterization showed that DC- and PC-

ly usedation, lhas berrent (ell asine ma

into the electroplatedss/strength, toughness,[30,31]. Under identi-idiamond composite

Surface & Coatings Technology 258 (2014) 12021211

Contents lists available at ScienceDirect

Surface & Coatin

l seposition of nanocomposite coatings has mainly been focused on deter-mining the optimum conditions for production: electrolysis conditions(composition of the electrolytic bath, presence of additives, pH value)

(611 Hv) is reported to be higher than that of a DC-plated composite(540 Hv) [32]. The PC-plated NiPSiC composite was also reported topossess better tribological behavior than the DC-plated deposit [33].rangeof engineeringapplicationsdue to their enhancedproperties [812].Oxides, carbides, diamond particles, nitrides, oxometallates and oil-containing microcapsules have all been incorporated into a nickel matrixto improve the tribological properties [1323]. Research on the electrode-

polymers in an electrolytic bath are incorporatedcoating to improve its properties, such as hardnewear/friction resistance, and corrosion resistancecal conditions, the microhardness of a PC-plated Nfor industrial applications dating back to 1970 [1].Composite coatings with incorporating different types of particles ex-

hibit distinctly improved properties, such as higher hardness, wear resis-tance and corrosion resistance compared to pure metal or alloy coatings[27]. Particle-reinforced metal matrix composites (MMCs) have a wide

period [15]. As a result, PC plating is a promising procedure. It can controlthe microstructure, composition and properties of electrodeposits byvarying the electrical parameters [28,29] and thus, can be used to depositMMC coatings through co-deposition. In the co-deposition process, ne(micro- and nanoscale) particles of metal, non-metallic compounds orand current conditions (type of imposed currendensity) [2227].

Corresponding author. Tel.: +90 380 73140 05; fax:E-mail address: harungul@duzce.edu.tr (H. Gl).

http://dx.doi.org/10.1016/j.surfcoat.2014.07.0020257-8972/ 2014 Elsevier B.V. All rights reserved.terials. Electrodepositionase particles dispersedjective of investigations

than large grains, as in bubble coalescence. Additionally, metals deposit-ed by the PC technique have less absorbed hydrogen than those pro-duced using a continuous current due to desorption during the offof composite coatings containing second phthroughout the metal matrix has been the ob1. Introduction

Electrolytic co-deposition is widecomposites due to its ease of preparOver the past decade and a half, thereconcentrated on conventional direct cuplating and electroless plating, as wdeposition processing for nanocrystallto obtain metal matrixow-cost and versatility.en an extensive researchDC) electroplating, pulseon production-scale co-

Pulse current (PC) plating is an established method of electrodepos-iting metals and alloys that signicantly affects the mechanism ofmetal crystallization. The pulse parameters (such as peak current densi-ty, duty cycle and frequency) can control the adsorption or desorption ofa species in the electrolyte and the surface diffusion in more ways thanDC plating. During the off period, small grains re-crystallize becausetheir high surface energy makes them less thermodynamically stablet and values of the current The purpose onanoscale Al2O3sults with our prwere deposited bPC method was

+90 380 731 31 24. 2014 Elsevier B.V. All rights reserved.Wear mechanisms coated nanocomposite layers yielded different wear mechanisms depending on the sliding velocity.Friction coefcientSurface damage its increased load bearing caEffect of PC electrodeposition on the structof NiAl2O3 nanocomposite coatings

H. Gl a,, M. Uysal b, H. Akbulut b, A. Alp b

a Duzce University, Gumusova Vocational School, Department of Metallurgy, 81850 Duzce, Turb Sakarya University, Department of Metallurgical & Materials Engineering, 54187 Sakarya, Tu

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

Article history:Received 6 November 2013Accepted in revised form 1 July 2014Available online 9 July 2014

Keywords:Nano-compositePC and DC electro co-depositionWear resistance

In this study, NiAl2O3metaltrolyte by pulse current (PC)tests were performed with aThe wear tests were carriedcomparedwith our previoustrodepositionmethod can sigposite coatings. For the samparticle content, more homoproved friction coefcients. T

j ourna l homepage: www.ere and tribological behavior

trix composite (MMC) coatingswere prepared from amodiedWatt's type elec-ating under current densities varying between 1 and 9 A/dm2. The tribologicalciprocating ball-on-disk apparatus sliding against a M50 steel ball ( 10 mm).at sliding velocities of 50, 100 and 150 mm/s under a constant load. The resultsublishedwork of DC electrodeposited coatings. The results showed that the elec-cantly affect themicrostructure and tribological behavior of NiAl2O3 nanocom-rrent density, PC electrodeposition creates coatings with higher co-depositedous particle distribution, higher wear resistance at high sliding distance and im-superior dispersion of Al2O3 nanoparticles in PC-coatedmaterials contributed to

gs Technology

v ie r .com/ locate /sur fcoatf this study is to co-deposit a layer of soft Ni and hardceramic particles by PC plating and to compare the re-eviously published work in which [27], the coatingsy DC method with same current densities. Since thesuggested to increase deposited particle content and

Administrator

Administrator

Administrator

Administrator

Administrator

Administrator

Administrator

Administrator

to provide better distribution, it was assumed to determine the opti-mum experimental conditions to obtain best wear resistance, frictioncoefcient, hardness, etc. Although there are several studies on thewear of NiAl2O3 nanocomposite coatings and numerous reports onDC and PC electrodeposition of metals and alloys, to the best of ourknowledge, there is no a comprehensive work to investigate the effectof current density between DC and PC plating techniques on the tribo-logical behaviors of NiAl2O3 nanocomposite coatings at differentsliding velocities. In the present work, Ni matrix composite coatingscontaining nanoscale Al2O3 particles were prepared using a pulseplating procedure to study their microstructure and tribological perfor-mance. The effect of current density, and thus the co-deposited percent-age and distribution of Al2O3 nanoparticles, on the microstructure andthe subsequent wear performance of both DC- and PC-coated materials

domly chosen areas and then average particle volume percent was cal-culated. A Rigaku D/MAX/2200/PC model device was used for X-ray

1203H. Gl et al. / Surface & Coatings Technology 258 (2014) 12021211analysis at a speed of 1/min over a range of 20100. The coating hard-ness was measured with a Vickers microhardness indenter (Leica

Table 1Bath and electrodeposition conditions for nano-Al2O3 reinforced MMC production.

Nickel sulfate (Ni2SO46H2O) (g/l) 300Nickel chloride (NiCl26H2O) (g/l) 50Boric acid (H3BO3) (g/l) 40Sodium dodecyl sulfate (g/l) 0.1Hexadecylpyridinium bromide (HPB) (mg/l) 200Alumina (Al2O3) (g/l) 20pH 4Temperature (C) 45Current density (A/dm2) 1, 3, 6 and 9Current typeDuty cyclePulse frequency

PC50%50 Hz

Plating time (h) 2were compared using reciprocating ball-on-disk tests under differentsliding speeds.

2. Experimental procedure

The plating electrolyte used for electrodeposition of the nanoparticle-reinforcedMMCswas aWatt's-type bath. The bath composition and elec-trodeposition conditions are shown in Table 1. These experimental pa-rameters were obtained by carrying out several studies to optimize(surfactant content, current density, Al2O3 content in the electrolyte andstirring speed, etc.) the values before attempting to compare the micro-structure and wear characteristics of DC- and PC-electroplated coatings.The average particle size of the -Al2O3 used for the reinforcing phasewas 80 nm. Several experiments were conducted to determine the suf-cient amount of surfactant to create a colloidal electrolyte. For this pur-pose, prior to deposition, 5 different measurements were carried out fordetermining the zeta potentials of the nanoparticle-suspended solutionswith a Malvern Zetasizer Nano Series Nano-ZS.

In the electrodeposition experiments, four different current densi-ties, 1, 3, 6 and 9 A/dm2, were studied to determine the optimum condi-tions for obtaining a homogeneousmicrostructure, thusmaximizing theimprovement of wear resistance by pulse electrodeposition method.The experimental results obtained from our previous DC coated coat-ings were used for comparison with PC method. Plating time was keptconstant at 2 h for each electroplating run. Before deposition, substrateswere polished with 600 mesh emery paper. Al2O3 nanoparticles weredispersed into the plating bath electrolyte by stirring magnetically for20 h and then treated in a high frequency homogenizator for 0.5 h. Mi-crostructural investigations were performed with a JEOL-JSM 6060LVinstrument. The particle volume percent were calculated directly fromthe 6060LV SEM image analysis program, which was based on phaseareamethod. Themeasurementswere carried out from10different ran-VMHT) and a load of 50 g for 15 s. At least 5 measurements were con-ducted on each sample and the results were averaged.

Wear and friction testswere performedwith a reciprocating ball-on-disk CSM tribometer in accordance with DIN 50324 and ASTMG99-95astandards at room temperature and at 5565% relative humidity underdry sliding conditions. The counterpart was a M50 steel ball ( 10 mm)with a hardness of 62 Rc. The system measures the friction coefcientand time-dependent depth proles using sensitive transducers. Thedepth transducer was located vertically on top of the sample. The testswere performed at a constant applied load of 1.0 N at sliding speeds of50, 100 and 150 mm/s. After each test, the amount of wear on the com-posite was calculated by measuring the wear width and depth using a3D surface proler (KLA Tencor P6) and low magnication optical mi-crographs. These measurements were also compared with the verticaltransducer depth proles, and thus, the wear rate of the compositeand the steel ball was determined. Lattice distortion and grain size ofthe Ni matrix were determined by calculating the lattice constantsusing basic reections from the crystal planes.

3. Results and discussions

3.1. Effect of current density on deposition

Figs. 1 and 2 show the effect of current density on the volume per-centage of Al2O3 in the deposited layers. The microstructures in Fig. 1show cross-sections of DC and PC co-deposited nanocompositeswhere-as, Fig. 2 presents the relationship between current type and currentdensity on the co-deposited Al2O3 content. The microstructures pro-duced by PC electrodeposition, shown in Fig. 1, exhibit more homoge-neous particle distribution than those produced by the DC method.Similar results were reported by different researchers and explainedby less agglomerated nano-ceramic particles in the case of PC currentapplication [34]. During the Ton time the applied pulse current resultedin a high driving force to tend the ceramic particles to adsorb on thecathode surface. However, during the Toff time the loosely adsorbednano-Al2O3 particles de-attached from the agglomerated state andmoved into the electrolyte. The volume percentage of Al2O3 in the DC-plated coatings increased signicantly with current densities up to3.0 A/dm2 (approximately 9 vol.%). Above 3.0 A/dm2, there was nomeasureable particle content increase in the deposited layer (shownin Figs. 1 and 2a). This maximum in the current density versus Al2O3volume percentage curve can be attributed to the transition fromactivation-controlled metal deposition to diffusion-controlled particletransfer [27]. As shown in Fig. 2a, for PC electrodeposition, however,the co-deposited particle content increased linearly with current densi-ty. It has been reported that PC coating is a more efcient depositionprocess for nanocrystalline NiAl2O3 composite coatings than that ofDC deposition and that it produces less agglomeration of the aluminananoparticles embedded in nanocrystalline Ni matrix [28].

The application of PC technique results in the production of compos-ite coatings with higher percentages of incorporation, and a more uni-form distribution of ceramic particles in the Ni matrix than thoseattained under DC regime [35]. The reason can be explained in termsof electro recrystallization. Electro crystallization occurs via two com-peting processes (i.e. the buildup of existing crystals and the formationof new ones) which are inuenced by different factors. The major rate-determining steps have been revealed to be charge transfer at the elec-trode surface and surface diffusion of adions on the crystal surface. Graingrowth is favored at low current density and high surface diffusionrates, while high current density (overpotential) and low surface diffu-sion rates promote the formation of new nuclei. If the average currentdensity is similar, PC plating can satisfy the latter two requirementssince it permits considerably higher overpotentials than the limitingDC current density [36]. However, in the current work the peak currentdensities in the PC technique were chosen as equal with the current

densities in the DC technique. Since the duty cycle in the PC technique

Administrator

Administrator

DC

PC

part

1204 H. Gl et al. / Surface & Coatings Technology 258 (2014) 12021211was 50%, this causes to apply lower current densities during theelectroplating compared with DC technique. Because of this reason,surface diffusion rate in the presence of PC is decreasing and causes toobtain coarser Ni grains. Therefore, the nucleation and growth mecha-nisms become surface diffusion dominant deposition [34]. On theother hand, in the PC technique, during the T the applied similar cur-

a) 1 A/dm2 DC b) 3 A/dm2

e) 3 A/dm2 d) 1 A/dm2 PC

Fig. 1.Cross sectional SEMmicrographsofMMCco-depositions showingdistribution of Al2O3PC, e) 3 A/dm2 PC and f) 9 A/dm2 PC.on

rent density PC provides more powerful effect to transfer the Al2O3 par-ticle through cathode. Therefore, at the Ton peak density applicationtime, PC permits higher overpotentials and increased entrapment ofthe Al2O3 particles [37]. The decrease in the concentration gradient intheDCmethodprevents to insert into cathode from the electrolyte cath-ode interface, which is a type of Nernst boundary layer. As it is known,Nernst boundary layer is formed because of the concentration differ-ence at very close section of the cathode electrode [38]. In the case ofPC technique, the negative effect of the Nernst boundary layer againstnano-particle entrapment on the cathode can easily be overcome.Therefore, similar results reported by many researchers about advan-tages of using PC technique. Karathanasis et al. [39] reported thatthere is a strong dependence of the percentage of the embedded parti-cles on the type of the applied current for composite coatings and theyfound PC is more dominant than DC technique. In our present study,specically the imposition of the PC regime leads to higher incorpora-tion percentage of particles compared to DC condition. Therefore, itseems that there is a proper combination of Ton and Toff at a givenduty cycle of 50%, which permits a sufcient replenishment of thecatholyte enriched in particles during Toff and adequate depositiontime Ton that allows the total engulfment of particles in the matrix. Ad-ditionally, Sheu et al. [37] also showed that pulse plating leads to higherco-deposition percentage of particles compared to DC, regardless of thecurrent density. This could be associatedwith prolonged relaxation timeToff that permits a satisfactory replenishment of Al2O3 particles in thecatholyte and therefore, leads to the increase in the particle incorpora-tion in the matrix.

Fig. 2b compares the XRD patterns of selected nanocomposites cre-ated by both current types at 9 A/dm2. These XRD results agree withthe SEM microstructures and quantitative analysis; the nano-Al2O3content is increased with PC electrodeposition. Co-deposition of Al2O3also affected the relative intensity of certain crystal planes in the XRDpatterns. An unreinforcedNi coating, deposited for comparison, exhibit-ed preferential growth along the (111) crystal plane. The growth orien-tations of co-depositedNiAl2O3 composite coatingswere not randomlyoriented for both DC and PC coated materials. For DC-deposited nano-

c) 9 A/dm2 DC

f) 9 A/dm2 PC

icles coatedwith currentdensities; a) 1A/dm2DC, b) 3A/dm2DC, c) 9A/dm2DC, d) 1A/dm2composite coatings, the (220) peak was faint, and the (311) peak be-came stronger with increasing Al2O3 content. The dominant planes are(111) and (200) for Ni in the DC coated materials and it seems thatthe DC deposit exhibits a mixed [211] + [100] orientation, with amore profound [211] orientation. zkan et al. [40] and Sohrabi et al.[41] reported the same results that, introducing the nano-ceramicparticles into the Ni coatings promoted to obtain high intensity (111)diffraction lines and thus, dispersion at the [211] direction. We havepreviously reported that the XRD patterns of nickel nanocompositecoatings reect textural changes dependent on the particle content ofthe deposited layer [27]. The crystallographic orientation of the PC-deposited coatings was somewhat different from that of the DC-coated samples. The PC-deposited nanocomposites exhibited obviouspreference for the (200) and (111) planes. This comprise the PC depositexhibits a mixed [100] + [211] orientation with a more profound[100] orientation. PC deposition is seen to produce a preferred orienta-tion more easily than DC deposition. This provides evidence that PCcoating provided preferential texture and the nano-Ni grains in the PCtechnique grown through the (200). Since the PC technique providedhigher preferential nucleation and growth along the (200) comparedwith DC technique, PC technique yielded coarser Ni grains.

High frequency effect can be an alternative reason to obtain coarsegrains in the PC coatedmaterials. According to the experimental resultsfrom the work of Lajevardi and Shahrabi [42], the [100] orientation be-comes dominantwhen the frequency is decreased. They can only obtainthat at the frequency level of 100 Hz, the (200) line produced very lowintensity. Since the applied frequency in this study is 50 Hz, we have at-tributed that the high orientation at [100] direction for PC coated mate-rials can be another reason. On the other side, Kollia and Spyrellis [43]have investigated the effect of pulse parameters on the textural and

particles' surface charge by absorbed molecules or ions, thereby pro-moting electrophoretic migration of the suspended particles.

3.2. Microhardness of composite coatings

Fig. 3 compares the microhardness of unreinforced Ni and NiAl2O3composite coatings produced by both DC and PC methods. The micro-hardness generally increased with nanoparticle content. This increaseis related to the dispersion hardening effect; the presence of Al2O3nanoparticles obstructs the movement of dislocations in the nickel ma-trix [44]. From Fig. 3, it can be seen that the microhardness of NiAl2O3nanocomposite coatings created with both DC and PC methods was

1205H. Gl et al. / Surface & Coatings Technology 258 (2014) 12021211% V

ol. A

l2O

3 in

coat

ings

0

3

6

9

12

15

18PCDCa)

20 g/l Al2O3200 mg/l HPBmicrostructural modications of the nickel electrodeposits. Based ontheir results, they deduced that at high duty cycles [211] is the preferredorientation and decreasing the duty cycle resulted in [100] orientation.

To ease the comparison of microstructural and tribological analytics,the same electrolyte was used for both the PC and DCmethods. As seenin Fig. 2c, the zeta potential of the electrolyte is very close to 0mVwhen100 mg/l HPB is added to the electrolyte. Any additional surfactant be-yond the baseline 100 mg/l would increase the zeta potential. It isknown that a high positive or negative zeta potential is critical forsuspending nanoparticles and preventing agglomeration during elec-trodeposition.Other authors studying electrodeposition indifferent sys-tems reported similar results. For example, Chen et al. [6] demonstratedthat enhanced deposition results are associatedwithmodication of the

higher than that of the pure Ni coating and increased with increasingnano-Al2O3 content. There are three reasons behind this increase [14,18,24,27]: particle strengthening, dispersion strengthening and grainrening. Particle strengthening is related to the incorporation of hardparticles at a volume percent above 20%. Dispersion strengthening is as-

Current Density (A/dm2)1 3 6 9

b)

Concentration of surfactant (HPB) (mg/l)0 100 200 300 400

Zeta

Pot

entia

l (m

V)

-30

-20

-10

0

10

20

30

c)

Fig. 2. a) The volume percentage of co-deposited Al2O3 particles in various current densi-ties for each current type, b) XRD patterns of composite coatings producedwith DC and PCcurrent types at a constant current density (9 A/dm2), and c) the relationship betweenamount of surfactant and zeta potential.sociated with the incorporation of ne particles (b1 m) at a volumefraction less than 15%; thematrix carries the load while the small parti-cles hinder dislocationmotion. The thirdmechanism involves thenucle-ation of small grains on the surface of incorporated particles, resulting ina general structural renement. The presence of these smaller grainsimpedes dislocation motion and increases microhardness. The resultsobserved in this study can be explained by the second and third mech-anisms. The ne particles incorporatedwithin the Nimatrix restrain thegrowth of Ni crystals and impede the motion of dislocations by way ofgrain rening and dispersive-strengthening effects.

In general, the hardness of coatings produced by DC deposition im-proved less than that produced by PC deposition. As discussed before,PC-deposited coatings containmore reinforcing nanoparticles; thus, im-proved hardness seems to be due to the increased concentration of thereinforcing hard particles in the coatings. The increase in hardness ob-served in PC-deposited coatings, however, is not as high as the increasednanoparticle content in such coatings would lead us to expect. This re-sult can be attributed to the smaller Ni matrix grain size and randomcrystallographic orientation in DC-deposited coatings.

3.3. Grain size and lattice distortion of composite coatings

The matrix grain sizes of nanocomposites deposited via both DC(studied previously) and PC methods were calculated from the XRDdata using Scherer's formula [27]. Fig. 4 shows the effect of current den-sity on thematrix grain size for the DC and PCmethods. Fig. 4 clearly in-dicates that the Ni grains were smaller in DC- than in PC-depositedcoatings. Although the electro co-deposited particle content was higherin the PC-coatedmaterials and matrix grains are expected to be renedby such an increase in particle content, the DC method yielded ner

Current Density (A/dm2)

Mic

roha

rdne

ss (H

v)

200

300

400

500

600

700DCPC

Ni 1 3 6 9

Fig. 3. Effect of current density on microhardness produced with direct and pulse current

composite coatings.

by lattice parameter mismatch at the coating and substrate interface,(2) thermal stresses arising from differing thermal expansion coef-cients at the substrate and coating interface, and (3) residual or intrinsicstress from particular plating conditions and bath composition.

3.4. Wear and friction properties

3.4.1. Effect of current density on wear and friction propertiesThe relationships between wear rate and current density in DC and

PC-plated nanocomposites are illustrated in Fig. 6. Fig. 6a clearly showsthat increasing current density, resulted in a signicant decrease in thewear rate for DC-plated nanocomposites. Increasing sliding speed caused

Fig. 5. Effect of current density on lattice distortion of the nickel matrix produced with di-rect (a) and pulse current (b) composite coatings.

1206 H. Gl et al. / Surface & Coatings Technology 258 (2014) 12021211matrix grains. This is because the surface diffusion is dominant in nucle-ation and growth in the PC coating compared with DC technique [34].Increasing the current density results in increasing the ner grains.Lajevardi and Shahrabi [42] found that in the (200) planewhen the cur-rent density increased from 2 A/dm2 to 8 A/dm2 the grain size of Ni wasreduced from 34 nm to 31 nm. The reason for this decrease in the parti-cle size is due to the changing of the preferred crystalline orientationand/or embedded particles content in the coating [42]. Same results re-ported bymany authors. Beltowska-Lehman et al. [45] reported that theaddition of larger Al2O3 particles results in a slight decrease of the aver-age matrix grain size with increasing current density. The presence ofnano-particles provides more nucleation sites by increasing the surfacearea of cathode in accordance with perturbing matrix growth andconsequently results in ner grain size [46]. In this study, increasingthe current density for both PC and DC caused to increase the co-deposited particle content and therefore, the grain size of matrix be-comes to be ner.

The colloidal particles in aqueous solution are in charged state.Consequently, a charged particle suspended in an electrolyte solutiontends to be surrounded by an ionic cloud. It was reported that thenano-ceramic particles could adsorb Ni2+ ions. There are two types ofspecies that include Ni2+ cations and Ni2+/ceramic particle clouds inelectrolyte. At high current densities, nickel ions and Ni2+/Al2O3 are ac-celerated to deposit on the cathode surface. Thismechanism is valid untilhigh amount of hydrogen evolution causes a reduction in the current ef-ciency as well as hindering the adsorption of nanoparticles to themetalsurface [35].

Lattice distortion of the Ni matrix for both the DC and PC techniques

Current Density (A/dm2)

Gra

in S

ize

(nm

)

45

50

55

60

65

70

75

80PCDC

1 3 6 9

Fig. 4. Effect of current density on grain size of the nickel matrix for each current type (di-rect and pulse current).was calculated using basic reections from the crystal planes as denedbyMisbah-Ul et al. [47]. The calculated lattice distortions demonstratedthat the composite matrix lattice constants depend on current densityand current type, as shown in Fig. 5. Increasing current density causednegative lattice distortions in the Ni matrix for both the DC and PCmethods (Fig. 5a and b). In general, these lattice distortionswere higherin DC- than in PC-deposited coatings. As previously discussed in theXRD analysis, PC electro co-deposition does not have a profound effecton the texture and, therefore, crystallographic orientation. Despite thedecreased (200) plane intensity, no signicant orientation change wasobserved in coatings deposited by PC nanoparticle co-deposition. Incontrast, DC electro co-deposition decreased the intensity of the maincrystal plane, (200), and increased the intensity of the (111) plane.Moreover, the intensity of other subsidiary crystal planes also increasedin the DC method. Therefore, it was inferred that the DC method exhib-ited smaller matrix grains. The origins of negative lattice distortion innanoparticle-reinforced MMCs can be summarized by three types ofstresses, as reported by El-Sherik and co-workers [48]; these threestresses are (1) lattice mist stresses resulting from distortion causedsharp increments in the wear rate for the coatings deposited with DCmethod. As evident from Fig. 6b, PC-deposited NiAl2O3 composite coat-ings withstood wear better than DC-deposited coatings, this can be at-tributed to the increased alumina particle content and homogenousparticle distribution. The higher Ni matrix grain size is another advanta-geous factor for increasing wear resistance in the PC coatedmaterials. Asdiscussed before, increasing the intensity of (002) plane results in higherductility in the Ni based coatings and this caused to increase plastic de-formation energy absorbability which prevents microcrack formationand subsequent delamination. As stated by zkan et al. [40] increasingthe intensity of the (002) plane deposition of Ni resulted in the growthin the [100] direction. The combined high ductility and higher amountof Al2O3 particle content resulted in better tribological properties provid-ing both resistance to deformation hardening and load carrying capacity.However, increasing particle content in the deposited layer with currentdensity no signicant change observed in the wear rate of PC-platednanocomposites (Fig. 6b). This was attributed to the increasing the par-ticle content in the deposited coatings and this may result in decreasingplastic deformation capability, which causes to reveal fatigue wear

Administrator

causes wear debris formation in the form of delamination failures,which reveals ne wear debris.

Fig. 7a and b shows the friction coefcient variation in DC- and PC-plated nanocomposites depending on sliding speed and current density.In general, it can be concluded that increasing sliding speed decreasedthe friction coefcient for both DC- and PC-plated nanocomposite coat-ings. DC and PC-plated nanocomposites have very similar friction coef-cients for all current densities at sliding speeds of 100 mm/s and150 mm/s, except in theDC-plated coatings, small increments in the fric-tion coefcient values have been observed with increasing current den-sity. The increment in the friction coefcient in the DC plated coatingscan be explained in terms of poor interfacial bonding between Ni andAl2O3 when compared with PC coated materials. The friction coefcientat 50 mm/s, however, exhibited signicantly different characteristicswith increasing current density. The friction coefcient for DC-platedcoatings were extremely high (approximately 0.7) at the 1 A/dm2 and3 A/dm2 current densities. A further increase in the current density re-sulted in a sharp decrease in the friction coefcient. In contrast, coef-cients for the PC-plated nanocomposite coatings remained around0.430.52, and no signicant variation have been observed by changingthe current density. As stated in the experimental section, the depositionprocess was carried out with a constant 200 mg/l surfactant and 20 g/lparticle concentration in the electrolyte. The interfacial bond betweenAl2O3 nanoparticles and the Ni matrix is thought to be one of the mostinuential factors in sliding wear resistance. It is known that PC deposi-tion provides not only a higher concentration of second phase nanopar-ticles in the electrodeposited layer but also better interface propertiesbetween the matrix and ceramic particles [51]. It is also evident from

1207H. Gl et al. / Surface & Coatings Technology 258 (2014) 12021211occurred because of microcrack formation. On the other hand, the effectof sliding speed on thewear rate of PC-produced composites is more in-teresting and impressive. Increasing sliding speed in the PC-plated nano-composites resulted in a remarkable decrease in thewear rate.When thewear rates of DC- and PC-coated nanocomposites are compared in thecase of 1 A/dm2 current density deposition condition, remarkably highwear resistance is observed for the PC-plated nanocomposites. For exam-ple, for the sliding speed of 150 mm/s, the wear rate was recorded as 17 104 mm3/Nm in the DC-plated material whereas the wear rate wasmeasured as 2 104 mm3/Nm in the PC-produced nanocomposite.Therefore, thewear rate of PC-deposited coatingwas found to be approx-imately 8 times lower than that of DC-plated material for 1 A/dm2 cur-rent density deposition condition. In the sliding wear, the decrease inthe wear rate by increasing sliding wear have been observed by severalauthors, studied in the dry wear conditions [49, 50]. Therefore, the de-crease in the wear rate by increasing the sliding distance is an expectedfeature in the electrodeposited Ni coatings. The unusual result here isthe increase in the wear rate with sliding distance in the DC coated ma-terials. This increase can be attributed to the insufcient interfacial inter-face bonding and inhomogeneous distribution of nano-Al2O3 nano-particles in the DC coated materials that could result in decreasing loadbearing capacity. As explained and discussed in the microstructure ofthe PC and DC deposited materials, the entrapment of the nano-Al2O3particles on the cathode is more effective in the PC technique sincepulse effect result in overcoming theNernst boundary layer and providesmore homogenous particle distribution. In the DC coated layers, the par-ticle de-attachment from the surface during sliding occurs because of theAl2O3 particle agglomeration. Increasing sliding speed result in increas-ing the stress concentration around the agglomerated particles and

our SEM micrographs that PC deposition promotes more homogenousdistribution and segregation free particle distribution. Thus, increasingsliding speed caused the wear rate of DC-coated materials to increase,

Fig. 6. Effect of sliding speed on the wear rate of NiAl2O3 composite coatings preparedwith different current types and densities, a) DC and b) PC.Fig. 7. Effect of sliding speed on friction coefcient of NiAl2O3 composite coatings pre-

pared with different current types and densities, a) DC and b) PC.

but PC-coated materials exhibited the opposite; increasing sliding speeddecreased thewear rate. This result implies that optimizing the load car-rying capacity depends on tribo-oxide formation, which governs thewear phenomena and, thus, decreases the wear rate.

Fig. 8 illustrates theworn surfaces of DC and PC-produced nanocom-posite samples tested at a sliding speed of 50 mm/s. Fig. 8ac shows themorphology of the worn surfaces of nanocomposite coatings depositedat different DC current densities. The worn surface of the 1 A/dm2 DCcoated Ni/Al2O3 nanocomposite sliding against M50 steel ball is rela-tively smooth but displays a few debris (Fig. 8a), indicating that thecoating experienced predominantly adhesive wear character associatedwith fatigue crack. Increasing current density from 1 A/dm2 to 9 A/dm2

resulted in decrease in the plastic deformation of the coatings due to theincreased quantity of co-deposited nanoparticles. Increasing currentdensity also increased the ne debris areas that are evidence for the de-lamination wear, which occurred because of microcrack formation. Thenano-particle agglomeration in the DC coated materials activated tostart micro crack formation and subsequent delamination with nedebris.

The worn surfaces of the PC-deposited nanocomposites tested at thesliding speed of 50 mm/s, shown in Fig. 8df. It is evident from thewornmicrographs, delamination cracks and smeared wear debris were ob-served, conrming that the wear process of NiAl2O3 composites isgoverned by a combination of abrasion and adhesion mechanisms.Since the PC coatedmaterials showed coarseNimatrix grains and prefer-ential growth at (200) plane, there are much more plastic deformationevidences compared with DC coated materials. At low current density,theworn surface (Fig. 8d) exhibited predominantly adhesive wear char-

corresponds to the higher amount of particle co-deposition in the PCmethod. At high current densities, the discrepancy in deformation ofthe nanoscale reinforcement phase and the matrix leads to stress con-centrations at the edges of the reinforcement phase, fueling the forma-tion of small debris. Thus, increasing the particle content leads to anincrease in debris (Fig. 8e and f).

In Fig. 9, the surfaces of DC- and PC-produced nanocomposites wornat the sliding speed of 150 mm/s are presented. The worn surfaces ofboth DC and PC coatedmaterials showmixed type of wearmechanismsof adhesive and abrasive. The wear of DC-coated materials starts withpredominantly abrasive mechanisms and continues with plastic defor-mation of wear debris leading surface hardening of smeared ductile Nimatrix; thereafter, followed by fatigue, which produces very smallwear debris associated with some particle agglomeration, most likelyby delamination (Fig. 9ac). Increasing the current density resulted indecreased formation of ne debris because increased co-deposited par-ticle content increases the load bearing ability. As seen in Fig. 9df, theworn surfaces of PC-produced coatings are different from those of DC-coated samples. Taking into account all the current density conditions,the surfaces of PC-coated materials were smoother than those of DC-coated samples. Moreover, signicantly larger quantities of very smalldebris were detected on PC-coated materials than on DC samples. ForPC-coated materials, increased current density resulted in decreasedne debris formation and increased surface smoothness. As shown inFig. 9df, the worn surfaces of the PC coated Ni/Al2O3 nanocompositessliding against steel ball is not only smooth and show the signs of slightfatigue and adhesion wear, which indicates that the coating is slightlydamaged by the counterpart steel compared with DC coated materials.

ris

t cu

1208 H. Gl et al. / Surface & Coatings Technology 258 (2014) 12021211acteristics caused by detachment of the smeared matrix after plastic de-formation and, later, deformation hardening (showed with the arrow).The extensive deformation and wear of samples tested at sliding speedsof 50mm/s is attributed to the low quantity of co-deposited particles. Ascan be seen from Fig. 8e and f, increasing the current density leads to de-crease the soft phase smearing on the worn surfaces and therefore, theamount of plastic deformation decrease. The width of the abrasivewear scar of the PC deposited Ni/Al2O3 nanocomposite coating is muchmore higher than that of the DC deposited Ni/Al2O3 coatings, which

a) 1A/dm2 DC

Abrasive groove

Debris

Deb

Deattachment

b) 3A/dm2 DC

e) 3 A/dm2 PCd) 1 A/dm2 PC

Fig. 8. SEMmorphology of the wear tracks of composite coatings preparedwith differen2 2 2 2DC, c) 9 A/dm DC d) 1 A/dm PC, e) 3 A/dm PC and f) 9 A/dm PC.As shown in Fig. 9d, some ne plows and scratches are observed onthe worn surface for the coating deposited at 1 A/dm2 current density,indicating the abrasive wear also occurred besides the adhesion and fa-tiguewear. Themixedmode of wearmechanism in the PC coatedmate-rials is attributed to the ductile structure of the Nimatrix because of thecoarser grain size and higher particle co-deposition compared with DCcoated materials. Homogeneous distribution of the particles resultedin decreasingmicrocrack formation contrary to the DC coatedmaterials.Therefore, the worn surfaces of the PC coated nanocomposites featured

Abrasive groove

MicrocrackMicrocrack c) 9A/dm2 DC

f) 9 A/dm2 PC

rrent types and current densities for 50mm/s sliding speed; a) 1 A/dm2 DC, b) 3 A/dm2

fere

1209H. Gl et al. / Surface & Coatings Technology 258 (2014) 12021211with less scufng, small plowing and some extend of abrasion. This re-sult indicates that some polishing took place, most likely by pull out ofalumina particles or material transfer during the sliding process. This

2

Abrasive groove

Debrisa) 1A/dm2 DC b) 3A/dm2 DC

e) 3 A/dm2 PCd) 1 A/dm2 PC

Fig. 9. SEM morphology of the wear tracks of composite coatings prepared with difb) 3 A/dm2 DC, c) 9 A/dm2 DC d) 1 A/dm2 PC, e) 3 A/dm2 PC and f) 9 A/dm2 PC.phenomenonwas also observed byHoua and Chen [28] in pulse electro-deposited NiW/Al2O3 composite coatings and was considered whenassessing the increased load bearing capacity. Because comparativelylower friction coefcients were obtained for PC-coated samples com-pared with DC-coated nanocomposite materials by increasing the cur-rent density, another reason for the surface smoothness could beoxidation, which produces tribo-oxide layers.

The surfaces of nanocomposites worn out at the sliding speeds of50 mm/s and 150 mm/s were also analyzed by EDS facility. Increasingsliding speed in DC-coated materials showed increased amount of Aland other components transferred from the steel ball. After the50 mm/s wear test, EDS analysis performed from several regions alongthe wear scar of DC- and PC-coated materials and conrmed the ab-sence of signicant oxygen content. This result shows that the slidingspeed was insufcient to generate heat at the interface between thesteel ball and nanocomposite surface. The PC-coated worn surfacesafter the 150 mm/s test exhibited a thick oxide transfer layer over themajority of the wear scar with the metallic coating exposed in large lo-calized regions within the scar. Similar results were also reported byLekka et al. [52]; at high sliding speeds, Ni-based composite coatingsunderwent tribo-oxidative wear, and localized EDS analysis revealedthe wear tracks to be partially covered by a nickel oxide layer (lightgray zone). The scars along the sliding direction were attributed tothird body abrasion causedmainly by detachment of nickel oxide akesthat interpose themselves between the deposit and the counter materi-al. Since the particle distribution is not homogenous in the DC coatedmaterials compared with PC produced coatings, the oxidized regionscan easily undergo delamination crack and the ne wear debris oc-curred. The delamination in the form of ne debris in the DC coatedma-terials prevented to form an effective tribo-oxidation layer.

Fig. 10ac depicts the original diagram of friction coefcient andsteel ball penetration (wear depth) changes versus sliding distance inDC- and PC-produced composites tested at different sliding speeds.For ease of comparison, only the diagrams for nanocomposites deposit-ed at 3 A/dm2 are presented. As seen from the diagrams for DC-

Abrasive groove

c) 9A/dm2 DC

f) 9 A/dm2 PC

nt current types and current densities for 150 mm/s sliding speed; a) 1 A/dm2 DC,produced nanocomposites, increasing sliding speed decreased the fric-tion coefcient and increased the amount of wear. Fig. 10b clearlyshows that the steel ball penetration sharply increased with increasingsliding distance, where the friction coefcient remained nearly stable byincreasing the sliding distance. In fact, the continuous increase in wear(ball penetration) with sliding distance is because of the wear charac-teristics of the nanocomposite. As stated previously, the predominantwear mechanism of the DC-coated materials for the 50 mm/s slidingspeed was delamination caused by rstly, adhesive plastic deformationand continuing surface hardening and then fatigue that produce verysmall wear debris. Since poor homogeneous distribution of nano-Al2O3 particles were produced in the DC method composite coatings ahigh wear rate obtained compared with PC method coated nanocom-posites tested under similar conditions. The PC deposited nanocompos-ite tested at 50 mm/s sliding speed, exhibit the high friction coefcientsince the smeared wear debris and a combining effect of abrasion andadhesion mechanisms. Increasing the sliding speed, the surfaces of thePC coated materials revealed smooth nature because of oxidation ofthe matrix phase leading signicant decrease in the friction coefcient.Therefore, this decrease at the high sling speeds was attributed to thetribo-oxides formation on the worn surfaces and surface smoothnesscombined with good load bearing capacity in the PC produced nano-coatings.

To reveal and make a better comparison between the wear mecha-nisms of the DC and PC coatedmaterials some selected worn nanocom-posite surfaces were scanned with 3D prolometry. The results arepresented in Fig. 11. For brevity, only the nanocomposites tested at50 mm/s are chosen. It can be seen from Fig. 10a which represents theworn surface of the DC plated composite produced at 1.0 A/dm2 thatthere is a very rough surface and that a severe surface damage occurred,revealing a large, deep valley. The rough surface is evidence that signif-icant amounts of wear products were smeared on the surface, including

a) 3A/dm2 DC/50mm/s b) 3A/dm2 DC/150 mm/s

in d

1210 H. Gl et al. / Surface & Coatings Technology 258 (2014) 12021211c) 3A/dm2 PC/50 mm/s

Fig. 10. Variation of friction coefcient and wear track depth for NiAl2O3 nanocomposites150 mm/s.agglomerated Al2O3 nano-particles (Fig. 11a). In the case of PC coatedworn surface a very smooth surface was obtained (Fig. 11b). Increasingcurrent density resulted in decreasing, smearing and scufng on thewear surface. It is probably because of increasing particle content inthe deposited layer made possible by the increasing current density inDC method (Fig. 11c). Applying PC deposited samples with a currentdensity of 9.0 A/dm2 yielded a smoother surface and fewer protrudedareas than the sample produced by DC plating. These results also

Fig. 11. 3D prolometry results of composite coatings preparedwith different current types andDC d) 9 A/dm2 PC.d) 3A/dm2 PC/150 mm/s

ifferent sliding speeds: (a) DC, 50 mm/s; (b) DC, 150 mm/s; (c) PC, 50 mm/s; and (d) PC,prove that PC plating produces better interfaces between nanocompos-ite constituents and results in better tribological behavior (Fig. 11d).

4. Conclusions

Several electrodepositedNiAl2O3 nanocomposite coatingswere pre-pared by DC and PC electrodeposition methods with the same currentdensity and similar experimental parameters. A detailed comparison

current densities andwear tested at 150 mm/s; a) 1 A/dm2 DC, b) 1 A/dm2 PC, c) 9 A/dm2

revealed the effect of electrodeposition method and current density onthe co-deposited nanoparticle content, particle distribution, matrix mi-crostructure, hardness, wear rate and friction coefcient. The followingconclusions can be drawn from this study:

1. Nanocomposites produced with PC electrodeposition contain higherquantities of Al2O3 nanoparticles and more homogeneous particledistribution than those produced with DC deposition. At a currentdensity of 9.0 A/dm2, the volume percent of Al2O3 was found to be8.81% and 12.7% for DC and PC deposition, respectively.

2. The microhardness of the coatings increased with dispersed nano-particle content. The nano-Al2O3 reinforced electrodeposited coat-ings yielded hardness values as high as 641 Hv with the DC methodand 656 Hv with the PC plating method.

3. Increasing current density decreased the wear rate in DC-platednanocomposites. However, increasing current density did not showsignicant change in the wear rate of PC-plated nanocomposites.

4. Increasing sliding speed in DC-plated nanocomposite materials in-creased their wear rate. Contrary to the DC-plated nanocomposites,increasing sliding speed resulted in a remarkable decrease in thewear rate of PC-plated materials and decreased the friction coef-cient for both DC- and PC-plated materials.

Acknowledgments

[12] M. Surender, B. Basu, R. Balasubramaniam, Trib. Int. 37 (2004) 743749.[13] B. Szczygie, M. Koodziej, Electrochim. Acta 50 (2005) 41884195.[14] T. Lampke, B.Wielage, D. Dietrich, A. Leopold, Appl. Surf. Sci. 253 (2006) 23992408.[15] M.D. Ger, Mater. Chem. Phys. 87 (2004) 6774.[16] F. Hu, K.C. Chan, Mater. Chem. Phys. 99 (2006) 424430.[17] A. Abdel Aal, Z.I. Zaki, Z. Abdel Hamid, Mater. Sci. Eng. A 447 (2007) 8794.[18] Q. Feng, T. Li, H. Yue, K. Qi, F. Bai, J. Jin, Appl. Surf. Sci. 254 (2008) 22622268.[19] M. Stroumbouli, P. Gyftou, E.A. Pavlatou, N. Spyrellis, Surf. & Coat. Tech. 195 (2005)

325332.[20] C.B. Wang, D.L. Wang, W.X. Chenc, Y.Y. Wang, Wear 253 (2002) 563571.[21] W. Wang, F.Y. Hou, H.i. Wang, H.T. Guo, Scr. Mater. 53 (2005) 613618.[22] T. Tsubotaa, S. Taniib, T. Ishidab, M. Nagata, Y. Matsumoto, Diam. Relat. Mater. 14

(2005) 608612.[23] S.A. Lajevardi, T. Shahrabi, J.A. Szpunar, Appl. Surf. Sci. 279 (2013) 180188.[24] H. Gl, F. Kl, M. Uysal, S. Aslan, A. Alp, H. Akbulut, Appl. Surf. Sci. 258 (2012)

42604267.[25] E.A. Pavlatou, M. Raptakis, N. Spyrellis, Surf. & Coat. Tech. 201 (2007) 45714577.[26] P. Gyftou, E.A. Pavlatou, N. Spyrellis, Appl. Surf. Sci. 254 (2008) 59105916.[27] H. Gl, F. Kl, S. Aslan, A. Alp, H. Akbulut, Wear 267 (2009) 976990.[28] H. Kung-Hsu, C. Yann-Cheng, Applied Surf. Sci. 257 (2011) 63406346.[29] D. Landolt, A. Marlot, Surf. Coat. Technol. 8 (2003) 169170.[30] C.T.J. Low, R.G.A. Wills, F.C. Walsh, Surf. Coat. Technol. 201 (2006) 371383.[31] B. Tushar, S.P. Harimkar, Surf. Coat. Technol. 205 (2011) 41244134.[32] H.W. Lee, S.-C. Tang, K.-C. Chung, Surf. Coat. Technol. 120121 (1999) 607.[33] K.H. Hou, W.H. Hwu, S.T. Ke, M.D. Ger, Mater. Chem. Phys. 100 (2006) 5459.[34] S.R. Allahkaram, S. Golroh,M.Mohammadalipour,Mater. Des. 32 (2011) 44784484.[35] V. Zarghami, M. Ghorbani, J. Alloys Compd. 598 (2014) 236242.[36] A.F. Zimmermann, D.G. Clark, K.T. Aust, U. Erb, Mater. Lett. 52 (2002) 8590.[37] H.H. Sheu, P.C. Huang, L.C. Tsai, K.H. Hou, Surf. Coat. Technol. 235 (2013).[38] J.H. Choi, J.S. Park, S.H. Moon, J. Coll. Inter. Sci. 251 (2002) 311317.[39] A.Z. Karathanasis, E.A. Pavlatou, N. Spyrellis, Electrochim. Acta 54 (2009) 25632570

(529535).[40] S. zkan, G. Hap, G. Orhan, K. Kazmanl, Surf. Coat. Technol. 232 (2013) 734741.[41] A. Sohrabi, A. Dolati, M. Ghorbani, A. Monfared, P. Stroeve, Mater. Chem. Phys. 121

1211H. Gl et al. / Surface & Coatings Technology 258 (2014) 12021211This work is supported by the Scientic and Technical ResearchCouncil of Turkey (TUBITAK) under contract number 106M253. The au-thors thank the TUBITAK MAG workers for their nancial support.

References

[1] Y.S. Dong, P.H. Lin, H.X. Wang, Surf. Coat. Technol. 200 (2006) 36333636.[2] M.R. Vaezi, S.K. Sadrnezhaad, L. Nikzad, Colloids Surf. A Physicochem. Eng. Asp. 315

(2008) 176182.[3] S.T. Aruna, V.K. William Grips, K.S. Rajam, J. Alloys Compd. 468 (2009) 546552.[4] K.H. Hou, M.D. Ger, L.M. Wang, S.T. Ke, Wear 253 (2002) 9941003.[5] S.C. Wang, W.C.J. Wei, Mater. Chem. Phys. 78 (2003) 574580.[6] L. Chen, L. Wang, Z. Zeng, J. Zhang, Mater. Sci. Eng. A 434 (2006) 319325.[7] S.A. Lajevardi, T. Shahrabi, J.A. Szpunar, A. Sabour Rouhaghdam, S. Sanjabi, Surf. Coat.

Technol. 232 (2013) 851859.[8] L. Wang, Y. Gao, H. Liu, Q. Xue, T. Xu, Surf. & Coat. Tech. 191 (2005) 16.[9] L. Chen, L. Wang, Z. Zeng, T. Xu, Surf. & Coat. Tech. 201 (2006) 599605.

[10] N.K. Shrestha, T. Takebe, T. Saji, Diam. Relat. Mater. 15 (2006) 15701575.[11] D.J. Riley, Curr. Opin. Colloid Interface Sci. 7 (2002) 186192.(2010) 497505.[42] S.A. Lajevardi, T. Shahrabi, Appl. Surf. Sci. 256 (2010) 67756781.[43] C. Kollia, N. Spyrellis, J. Appl. Electrochem. 20 (1990) 10251032.[44] G. Wu, N. Li, D. Zhou, K. Mitsuo, Surf. Coat. Technol. 176 (2004) 157164.[45] E. Beltowska-Lehman, P. Indyka, A. Bigos, M. Kot, L. Tarkowski, Surf. Coat. Technol.

211 (2012) 6266.[46] I. Gurrappa, L. Binder, Sci. Technol. Adv. Mater. 9 (2008) 4354.[47] I. Misbah-Ul, K.A. Hashmi, M.U. Rana, T. Abbas, Solid State Commun. 121 (2002)

5154.[48] A.M. El-Sherik, J. Shirokoff, U. Erb, J. Alloys Compd. 389 (2005) 140143.[49] R.A. Al-Samarai, K. Rafezi Ahmad, Y. Al-Douri, Int. J. Sci. Res. Publ. 2 (2012).[50] A.M. AI-Qutub, I.M. Allam, M.A. Abdul Samad, J. Mater. Sci. 43 (2008) 57975803.[51] A.Z. Karathanasis, E.A. Pavlatou, N. Spyrellis, J. Alloys and Comp 494 (2010)

396403.[52] M. Lekka, A. Lanzutti, A. Casagrande, C. Leitenburg, P.L. Bonora, L. Fedrizzi, Surf. Coat.

Technol. 206 (2012) 36583665.

Effect of PC electrodeposition on the structure and tribological behavior of NiAl2O3 nanocomposite coatings1. Introduction2. Experimental procedure3. Results and discussions3.1. Effect of current density on deposition3.2. Microhardness of composite coatings3.3. Grain size and lattice distortion of composite coatings3.4. Wear and friction properties3.4.1. Effect of current density on wear and friction properties

4. ConclusionsAcknowledgmentsReferences