th

obla

s he bas cuof trohn mtheain the preferential orientation and grain size of the electrodeposits and scanning

en inteeased hom teson wie been

Surface & Coatings Technology 212 (2012) 94100

Contents lists available at SciVerse ScienceDirect

Surface & Coatin

l seElectrodeposition is onemethod for producing this type ofmaterials,specically via pulse plating. Severalmetals have been electrodepositedusing this method including Ni, Cr, Zn and Cu [812]. Nickel appearsparticularly suitable because it combines two factors: structures witha ner grain size can be obtained due to higher instantaneous currentdensities compared with the conventional DC method, and the pulseacts on the formation mechanism of the nickel deposit. The electrode-position of Ni is known to be a highly inhibited process. During thedeposition of the Ni ions, species such as Hads, H2 and Ni (OH)2 are

texture have also been assessed [8,15,17]. However, although theyare of great importance in industrial processes, less attention hasbeen paid to the mechanical properties of nickel electrodeposits[1821].

The microindentation method with the evaluation of the universalhardness (HU) and Young's modulus (E) is commonly used for themechanical characterization of thin lms [2225]. These parametersare obtained from the loadingunloading curves measured with acomputer-controlled microindentation instrument [26].adsorbed on the cathode surface affecting thephase. During the process of pulse plating, thof these species is perturbed through the acathodic current (or potential) over a short tim

Corresponding author.E-mail address: enrique.fatas@uam.es (E. Fats).

0257-8972/$ see front matter 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.surfcoat.2012.09.027related to large volume, grain boundary sliding,

deposits have been widely studied. For example, the surface rough-ness or surface nishing has been examined using a variety ofwaveforms [14,15]. Microstructure [16], electric transport [17] andfractions of grain boundaries, triple junctionsetc. [47].1. Introduction

Nanocrystalline materials have beof their unique properties such as incrtance, superplastic extensibility at rosaturation magnetization in comparicounterparts [13]. These effects havAll deposits exhibited a preferential orientation, and the highest performing lms have been analyzed withrespect to their mechanical properties. The effect of grain size, in the nanometer range, has been determinedfor variations in the microhardness with current density conrming the HallPetch relationship. Thesestudies allow lms to be tailored with good mechanical performance for various technological applicationsby simply selecting the appropriate electrodeposition conditions.

2012 Elsevier B.V. All rights reserved.

nsively studied becauseardness and wear resis-mperature and reducedth their coarser-grained

The characteristic parameters of this method [13] are the cathodicpulse-on time (ton), the rest or pulse-of time (toff) and the peak cur-rent density (ip), which are shown schematically in Fig. 1. In addition,other variables such as the duty cycle (), pulse frequency (f) or aver-age current density (iav) have been used.

The effects of pulsed currents on the properties of nickel electro-electron microscopy used to evaluate their morphology.

Microhardness have been performed to obtEffect of the pulse plating parameters onnickel electrodeposits

A. Ibez, R. Escudero-Cid, P. Ocn, E. Fats Departamento de Qumica Fsica Aplicada, Universidad Autnoma de Madrid, Campus Cant

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

Article history:Received 24 April 2012Accepted in revised form 17 September 2012Available online 25 September 2012

Keywords:Pulse electroplatingNickel

Nanocrystalline nickel lmadditive-free sulfamate-typto the parameters of variouThe mechanical propertiesknown as the universal micof elastic recovery have beethe mechanical response of

j ourna l homepage: www.egrowth of the Ni metale adsorptiondesorptionpplication of a periodice.

rights reserved.e mechanical properties of

nco, 28049 Madrid, Spain

ave been prepared via pulse electrodeposition on steel substrates from anth. The mechanical properties of the lms have been analyzed with respectrrent programs.hese lms were determined using dynamic microindentation measurementsardness test. The hardness, plastic component, Young's modulus and percenteasured. Increasing the peak current density was more effective at improvinglms than varying the pulse-on time. In addition, X-ray diffraction studies

gs Technology

v ie r .com/ locate /sur fcoatRecent research has indicated that materials with the same hard-ness value do not exhibit the same value for Young's modulus. Thetwo parameters cannot be directly related: a high hardness valuedoes not always correspond to a high value of Young's modulus. Forthis reason, general relationships between these two parametershave been proposed. Musil et al. [27,28] studied these relationshipsand found that the ratios H/E* and H3/E*2, in which H refers to theplastic component of the hardness and E* is the effective Young's

For the universal microhardness test, the load must be carefullychosen to avoid the inuence of the substrate, which can be problem-atic in soft coatings on hard substrates such as the measurements ofpaint on steel, or in hard coatings on soft substrates. Therefore, forpracticality, the maximum penetration depth must not exceed 10%of the total lm thickness. However, one must always carefully con-sider intermediate situations, e.g. coatings and substrates of similarhardness in which the substrate can exert inuence at larger penetra-

im

Table 2Pulse-plating parameters used to investigate the effect of ton and ip.

ton (ms) toff (ms) ip (mA cm2) iav (mA cm2) Plating time (min)

1 12 400 30 14.52.1 12 400 60 83.5 12 400 90 57.2 12 400 150 31 12 780 60 7.51 12 1170 90 51 12 1950 150 3

95A. Ibez et al. / Surface & Coatings Technology 212 (2012) 94100modulus (E*=E/(1v)2, v being the Poisson's ratio), provide an im-proved value for a material's mechanical resistance to plastic defor-mation. High values indicate a good mechanical resistance to plasticdeformation as they combine a high value of hardness and an appro-priate degree of elasticity to accommodate loads or impacts over awider area.

The aim of this work is to obtain nickel lms via pulse plating andto relate the variables of this method (pulse-on time and peak currentdensity) with the mechanical properties of the deposits to tailor thenickel lms for specic applications [29].

2. Experimental procedures

Nickel electrodeposits were obtained from a sulfamate-type bath de-scribed in Table 1. Boric acid was added because of its buffering capacity,while NiCl2 improved both the anodic and cathodic reactions. The tem-perature was adjusted to 55 C and the measured pH was 3.8. The bathwas additive-free because, although they produce deposits with a nergrain size, the additives that are incorporated into the deposit causenot only sulfur and carbon contamination but also embrittlement.

Electrodeposition was performed in a typical electrochemical cellwith a two-electrode conguration containing an 18/8 stainlesssteel plate cathode and a pure nickel foil anode. The electrodepositionwas carried out galvanostatically under constant stirring with a PAR363 potentiostat/galvanostat coupled to a PAR 175 universal pro-grammer to adjust the parameters of the pulsed currents. The pulseplating parameters (ton and ip) and the average current density (iav)are provided in Table 2 and were chosen within a range previouslyshown to produce ultra-ne-grained nickel deposits [8]. Thepulse-of time (toff) was held constant at a value higher than thepulse-on time because, as has been discussed in the literature [17],if toff ton, the plating conditions approach the DC behavior. Thus,4 m thick electrodeposits were obtained in all cases by adjustingthe plating time to get a constant net charge of 27 Ccm2.

Mechanical characterization was carried out using a Fischerscope

Fig. 1. Schematic representation of the characteristic parameters of the pulse method.HV 100 tester with a Vickers indenter. Fig. 2 shows one of the indenta-tions left. Hardness was calculated according to the following formulataking into account the indenter's imprint as a function of the penetra-tion depth (h):

HU F26:43h2

: 1

Table 1Bath composition and plating conditions.

Ni(NH2SO3)4H2O 70 g/lNiCl26H2O 15 g/lH3BO3 40 g/lpH 3.8Temperature 55 Ction depths. In this situation, one should analyze the hardness ten-dency through an optimal load test. Fig. 3 shows the hardnessprole for this test; the hardness corresponding to the nickel coatingextends to approximately 20 mN. By increasing the load, a decreasein hardness can be clearly observed due to the effect of substrate.Thus, a nal test load of 20 mN, applied stepwise, with a load timeof 20 s was chosen. Fifteen tests were performed along the surfaceto obtain a mean value. Fig. 4 provides the hardness proles for thecoatings obtained by varying the ton. No inuence of the substratecan be observed for the samples because the microhardness remainsconstant with the applied load, except at low loads (up to 5 mN) atwhich irregularities can be attributed to surface effects. Furthermore,the microhardness values are markedly higher than that of the steelused as a substrate (1.69 GPa). Fig. 5 shows the loadingunloadingcurves for the same samples. The maximum indentation depth doesnot exceed 0.4 m, thereby satisfying the condition that the indenta-tion depth should not exceed 10% of the coating thickness.

To estimate the effective Young's modulus, a value of 0.31 waschosen for the Poisson's ratio [30].

Morphological characterization was carried out using a PhilipsXL30 scanning electron microscope, and the structure determinedvia X-ray diffraction with a SIEMENS D5000 diffractometer (CuK ra-diation, =1.540589 ). The grain size was evaluated on the basis ofthe Scherrer expression, which takes into account the peak broaden-ing at the X-ray diffraction patterns [31]. The lm thickness was mea-sured using X-ray uorescence with a Fischerscope X-ray XUVMSystem.Fig. 2. Indent left.

0 20 40 60 80 100

2.0

2.4

2.8

3.2

3.6

4.0

4.4

Substract influence

Coating hardness

HU

(GPa

)

F (mN)Fig. 3. Microhardness prole for the optimum load test.

0.1 0.2 0.3 0.4 0.50

5

10

15

20

1 ms 2.1 ms 3.5 ms 7.2 ms

F (m

N)

h (m)

Fig. 5. Test curves of the electrodeposits of nickel obtained by varying the time-onpulse.

96 A. Ibez et al. / Surface & Coatings Technology 212 (2012) 941003. Results and discussion

3.1. Effect of the pulse-on time

To investigate the effect of the pulse-on time, values from 1 to7.2 ms were applied maintaining constant a pulse-of time and ca-thodic peak current density (see Table 2). Fig. 6 provides themicrohardness values for this set of electrodeposits and indicates asteep rise in the microhardness when the pulse-on time increasesfrom 1 to 3.5 ms with a slight decrease for the deposit obtained at apulse-on time of 7.2 ms. Grain renements up to the nanometerrange are known to produce a strain-hardening effect thus increasingthe microhardness values. In this research, the grain sizes obtainedwere approximately 2124 nm, which can explain the high valuesof the microhardness, but cannot account for the observed variationsbecause the grain size variations are small and practically within thelimits of experimental error. Therefore, no conrmation of the HallPetch relationship, a linear relationship between the hardness andthe reciprocal square root of the grain size, could be established.

Thus, the increase in microhardness with pulse-on time must beprimarily due to the microstructure, especially if one considers thatthe electrodeposition of nickel is a highly inhibited process in whichthere is a competition between the adsorbed chemical species (suchas Hads and Ni (OH)2) or hydrogen evolution to affect the crystal0 5 10 15 20

2.0

2.5

3.0

3.5

4.0

4.5

5.0

3.5 ms

7.2 ms

2.1 ms

1 ms

HU

(GPa

)

F (mN)Fig. 4. Universal microhardness prole vs. applied load of the nickel lm obtained withdifferent pulse-on time.orientation of the deposit. The X-ray diffraction patterns (Fig. 7) indi-cate a change from the strong predominance of the (200) plane atshort pulse-on times to a decrease in the intensity in the X-ray dif-fraction line (200) with increasing pulse-on time.

A more detailed study of the preferential orientations of our lmswas carried out through the evaluation of the orientation index (M)[12]. For each plane, (110) for example, the orientation index was cal-culated as follows:

IFR 110 IF 110 = IF 110 IF 200 f g 2

IR 110 I 110 = I 110 I 200 f g 3

M 110 IR 110 =IFR 110 4

in which IF (110) is the X-ray diffraction intensity in the JCPDS cardsand I (110) is the intensity in the experimental data. A positive devi-ation of M from 1 indicates a preferential orientation of that plane. Incontrast, an M of less than 1 points to a depression in the orientation.

This research indicates no substantial change in the orientationindex of plane (111), which has an M value less than 1 indicating adepressed orientation. The plane with the preferential (or highest)orientation is the (200) plane whose value of M decreases with1 2 3 4 5 6 7 8

2.4

2.8

3.2

3.6

4.0

4.4

4.8

5.2

HU

/ GPa

ton / ms

Fig. 6. Variation of the microhardness with pulse-on time (toff=12 ms; ip=400 mA cm2).

of 1 and 3.5 ms. The surface features vary from a ake-like morphol-ogy when ton is 1 ms, to a ne grained morphology with aggregationsof grains in larger domains when ton is 3.5 ms. The surface roughnessis also reduced for the deposits obtained at greater pulse-on times; inthis case the deposit with a more compact and smooth morphologyalso has the highest hardness values.

90 80 70 60 50 40

(311) (220)(200) (111)7.2 ms

3.5 ms

2.1 ms

1 ms

Inte

nsity

(a. u

.)

2

Fig. 7. XRD patterns of the nickel electrodeposits obtained at different pulse-on times.

Fig. 9. SEM micrographs of the nickel electrodeposits showing the inuence of ton(a) 1 ms and (b) 3.5 ms, with toff and ip maintained constant at 12 ms and 400 mA cm2

respectively.

97A. Ibez et al. / Surface & Coatings Technology 212 (2012) 94100increasing pulse-on time (Fig. 8). The corresponding increase in hard-ness with pulse-on time could indicate that the (200) plane, with alower degree of packing than the (111) plane, could act as a slidingsystem. Thus, as this dominance of orientation decreases (a decreas-ing M value), the hardness increases due to a reduction in the numberof sliding planes.

These results indicate that the predominance of the (200) planedoes not favor an increase in the microhardness, but favors its de-crease. The highest microhardness values correspond to pulse timesof 3.5 and 7.2 ms, providing lms with similar microstructures anda less prevalent preferential orientation of the (200) plane.

Under DC conditions, nickel electrodeposits (obtained with aWatt's-type bath) develop the (200), (220), (420) or (422) preferen-tial orientations [32,33]; while, if pulse electrolysis is employed (alsowith a Watt's type bath), the preferential orientation changes from arandom texture to a (200) preferential orientation with increasingpulse-on time [8]. In our case (with a sulfamate-type bath) a some-what different situation exists in which the predominance of the(200) plane does not favor the microhardness.

Changes in the crystal orientation with increasing pulse-on timecan be ascribed to factors such as an increase in the overpotentialwith increasing pulse-on time that affects the structural developmentand the morphology of the surface as observed in Fig. 9, which pro-

vides the SEM micrographs of the deposits obtained with ton values

1 2 3 4 5 6 7 8

0,5

1,0

1,5

2,0

2,5M111M200M220M311

Orie

ntat

ion

inde

x (M

)

ton (ms)

Fig. 8. Variation of the orientation index with pulse-on time.As shown in Fig. 10a, the values obtained for Young's modulus arelow compared with those for the deposit obtained at 3.5 ms, which isless elastic. Although the deposits obtained at 3.5 and 7.2 ms havesimilar hardness values, their Young's moduli are quite different;they exhibit differing degrees of elasticity. Therefore, the mechanicalproperties of the Ni electrodeposits could be tailored for differenttechnological applications through the proper selection of electro-chemical parameters. For these cases, analysis of the H3/E*2 ratiocan better indicate the mechanical resistance of a material to plasticdeformation. A plot of this relationship (Fig. 10b) demonstrates thatthe maximum value occurs at a pulse-on time of 7.2 ms, indicatinga material that is nearly as hard as the one obtained at 3.5 ms, butthat is much more elastic.

3.2. Effect of the peak current density

To study the effect of the peak current density, values wereapplied from 400 to 1950 mA cm2 with a ton of 1 ms and theremaining parameters (ton and toff) held constant (Table 2). As previ-ously noted, one of the primary advantages to the pulse platingmethod is the ability to apply high current densities. Such an increasein the current density can cause an increase in the nucleation rate,

400 800 1200 1600 2000

2.5

3.0

3.5

4.0

4.5

5.0

HU

(GPa

)

ip(mA cm-2)

Fig. 11. Variation of the microhardness with peak current density (ton=1 ms; toff=12 ms).

98 A. Ibez et al. / Surface & Coatings Technology 212 (2012) 941001 2 3 4 5 6 7 8180

200

220

240

260

280E

(GPa

)

0.0016

0.0018

0.0020

ton (ms)

a

bproducing microstructures with a lower grain size, usually in thenanometer range. This, in turn, results in increased hardness values.

Fig. 11 provides the hardness values obtained. A signicant in-crease can be observed in the hardness compared with that of pureNi foil (3.52 GPa). Microhardness values between 3 and 4 GPa havebeen obtained under similar experimental conditions using aWatts-type bath [20]. Microhardness values of approximately 7 GPa,measured via nanoindentation tests, have been reported with shorterpulses and higher currents for much thicker Ni coatings obtainedfrom a nickel sulfate solution [21]. As expected, the microhardness in-creases as the peak current density increases, reaching a value of ap-proximately 5 GPa corresponding to a grain size of 17 nm, which canbe explained if one considers that increasing the current densityyields large overpotentials that increase the nucleation rate. Theplot of the microhardness versus the reciprocal square root of thegrain size conrms the HallPetch strengthening in the range ofultra-ne grained nickel deposits (Fig. 12).

An examination of the X-ray diffraction patterns (Fig. 13) indi-cates a different evolution for the preferential orientation comparedwith that found in the pulse-on time. The strong (200) preferentialorientation at 400 mA cm2 changes to another strong (220) prefer-ential orientation at 780 mA cm2 and changes again at the highestpeak current densities to a random (111)/(220) predominance. Thisresult indicates a predominance of the (111) plane over the othertwo, which yields an improved hardness in the electrodeposited

1 2 3 4 5 6 7 80.0006

0.0008

0.0010

0.0012

0.0014

H3 /E

*2 (G

Pa)

ton (ms)

Fig. 10. (a) Young's modulus and (b) H3/E*2 relation for the deposits obtained withvariation of the pulse-on time.nickel lms. However, the mechanism that yields the predominanceof the (111) plane cannot be directly assessed because of the com-plexity of the nickel electrodeposition mechanism.

Twomicrographs from this set of deposits (780 and 1950 mA cm2)are shown in Fig. 14. The lm in Fig. 15a indicates a similar morphology,although less compact and ake-like, to that obtained at 400 mA cm2

(Fig. 9a). Both deposits exhibit similar hardness values. In contrast,Fig. 15b, which corresponds to the deposit obtained at the highest peakcurrent density, shows a highly compact morphology and smoothnesswith grains grouped in larger domains. Thus, as with the effect of thepulse-on time, a relationship can be observed between the compactnessof the surface and the hardness values, although this does not establish acausal relationship between the two properties.

The values of Young's modulus (Fig. 15a) are similar to thoseobtained at pulse-on times of 1, 2.1 and 7.2 ms, with the exceptionof the more elastic deposit at 780 mA cm2. As previously discussed,the combination of these two parameters through the H3/E*2 ratio(Fig. 15b) provides a better understanding of the overall mechanicalresponse of the material to plastic deformation. The higher valuescorrespond to higher peak current densities because they combinean increased hardness value with an allowable Young's modulusvalue. The H3/E*2 ratios for the deposits obtained with a pulse-ontime of 1 ms and peak current densities of 1170 and 1950 mA cm20.20 0.21 0.22 0.23 0.24 0.25

2.5

3.0

3.5

4.0

4.5

5.0

5.5

HU

(GPa

)

d-1/2(nm-1/2)Fig. 12. Relation between the microhardness and the grain size plotted as the HallPetch relationship.

are double than that obtained for the deposit with a pulse-on time of7.2 ms and a peak current density of 400 mA cm2.

Taking into account the microhardness and Young's modulusvalues obtained by varying the pulse-on time and the peak currentdensity, one can infer that a pulse-on time of 3.5 or 7.2 ms and apeak current density of 1950 mA cm2 are the optimum conditions

for obtaining a deposit with excellent mechanical properties. How-

90 80 70 60 50 40

(220)(311) (200)(111)1950 mA cm-2

1170 mA cm-2

780 mA cm-2

400 mA cm-2

Inte

nsity

(a. u

.)

2

Fig. 13. XRD patterns of the nickel electrodeposits obtained at different peak currentdensities.

400 800 1200 1600 2000

0.0005

0.0010

0.0015

0.0020

0.0025

0.0030H

3 /E*2

(G

Pa)

ip (mA cm-2)

Fig. 15. (a) Young's modulus and (b) H3/E*2 relation for the deposits obtained with dif-ferent peak current densities.

99A. Ibez et al. / Surface & Coatings Technology 212 (2012) 94100Fig. 14. SEMmicrographs of the nickel deposits obtained with a peak current density of(a) 780 and (b) 1950 mA cm2, and with ton 1 ms and toff 12 ms.400 800 1200 1600 2000

170

180

190

200

210

220

230

240a

E (G

Pa)

0.0035

0.0040b

ip (mA cm-2)ever, the experiments carried out under these conditions produceddull and partially burned deposits indicating that, due to the com-plexity of the nickel electrodeposition process, the variables ton andip are not independent and their effects cannot simply be added.

4. Conclusions

Nanocrystalline nickel (grain size20 nm) has been producedfrom an additive-free sulfamate-type bath under pulse plating condi-tions. Increasing the pulse-on time and the peak current densityresulted, in both cases, in an increase in the microhardness and themechanical resistance of the lms.

An increase in the peak current density, which is related to in-creases in the overpotential and nucleation rate, was more effectiveat improving the mechanical response of the lms.

Films with the highest microhardness values were obtained at apulse-on time of 1 ms and a peak current density of 1950 mA cm2,while the most elastic lms were obtained at ton=1 ms and ip=780 mA cm2. In addition, lms with high hardness values but lowelasticity corresponded to ton=3.5 ms and ip=400 mA cm2.

The crystal orientation of the deposits changed with the varyingdeposition conditions. The best mechanical properties were obtainedwith random (111)/(200) and (111)/(220) preferential orientations

(with variations in the applied pulse-on time and peak current density,respectively).

The morphology of the deposits also improved with improve-ments in the mechanical parameters (HU and H3/E*2).

Acknowledgments

Financial support is acknowledged from MICINN (Spain) underproject no. CTQ2010-17338. The authors also gratefully acknowledgeCentro de Tratamientos de Supercie (C. T. S.), Spain, for providingfacilities in which to carry out our research.

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100 A. Ibez et al. / Surface & Coatings Technology 212 (2012) 94100

Effect of the pulse plating parameters on the mechanical properties of nickel electrodeposits1. Introduction2. Experimental procedures3. Results and discussion3.1. Effect of the pulse-on time3.2. Effect of the peak current density

4. ConclusionsAcknowledgmentsReferences