Effect of Organic Additives on the Hardness of Ni Electrodeposited

from Sulfamate and Watt’s Solutions*1

Hiroaki Nakano1, Hideaki Tsuji2, Satoshi Oue1, Hisaaki Fukushima1,Feng Yang3;*2 and Wenhuai Tian3

1Department of Materials Science & Engineering, Kyushu University, Fukuoka 819-0395, Japan2Department of Materials Process Engineering, Kyushu University, Fukuoka 819-0395, Japan3Department of Materials Physics and Chemistry, University of Science and Technology Beijing, Beijing, China

Ni electrodeposition was galvanostatically conducted over a current density range 1–5000 A/m2 from unagitated sulfamate (323 K) andWatt’s solutions (313 K) containing polyethylene glycol (PEG) or saccharin to examine the effect of the C and S contents in the deposit and thetexture on the hardness of the deposited Ni. The C content in Ni deposited from Watt’s solutions containing PEG was significantly higher thanthat from the sulfamate solution. The S content in deposits from sulfamate and Watt’s solutions containing saccharin differed slightly, except at2000 A/m2. The hardness of the deposited Ni increased with increasing C and S contents, and the rate of increase in hardness decreased at Scontent above 0.02 mass%. The texture of the deposited Ni changed from a field-oriented texture to an unoriented dispersed type when C and Swere co-deposited with Ni by the addition of PEG and saccharin. The hardness of the unoriented dispersed type of Ni deposit was greater thanthat of the field-oriented type of Ni deposit. [doi:10.2320/matertrans.M2011217]

(Received July 19, 2011; Accepted September 2, 2011; Published October 13, 2011)

Keywords: nickel electrodeposition, texture, crystal orientation, hardness, saccharin, polyethylene glycol

1. Introduction

Electrodeposited Ni has been extensively used in dyes,molds, and screens for printing electroforming films becauseof its excellent toughness, corrosion resistance, and hard-wearing properties.1–4) Electroforming films are requiredto reduce the residual internal stress of thick films, andsulfamate solution has been used to reduce the internal stressof a deposited Ni film. However, sulfamate ions are reportedto decompose by anodic oxidation when an insoluble anodeis used or when a soluble anode is passivated.1,2,5–7) Watt’ssolution has been used widely for plating, and can be appliedto electroforming.8)

Previously, authors have reported the effect of currentdensity and organic additives on the texture, crystal orienta-tion, and hardness of the Ni deposited from sulfamate andWatt’s solutions.9,10) The hardness of the deposited Niincreases with the addition of organic additives. Althoughthe increase in hardness of the deposited Ni with organicadditives seems to be due to both the co-deposition of theadditives and a decrease in the particle size of Ni, there havebeen very few studies on the effect of the additive content onthe texture and hardness of the deposited Ni.

In this study, the effect of organic additives on the hardnessand texture of the Ni deposited from sulfamate and Watt’ssolutions was examined from the viewpoint of co-depositionof C and S derived from the additives. Polyethylene glycol(PEG), which is known to have a straight-chain structure andto be stable in an electrolytic solution, and saccharin toreduce the internal stress of the deposited Ni were used as theorganic additives.

2. Experimental

Table 1 shows the compositions of sulfamate and Watt’ssolutions and the electrolysis conditions for Ni deposition.The electrolytic solutions were prepared by dissolvingreagent grade Ni(NH2SO3)2�4H2O (1.24 mol/L), NiCl2�6H2O (0.04 mol/L), and H3BO3 (0.49 mol/L) for the sulfa-mate solution, and NiSO4�6H2O (0.91 mol/L), NiCl2�6H2O(0.19 mol/L), and H3BO3 (0.49 mol/L) for the Watt’ssolution in distilled and deionized water. The pH of bothsolutions was adjusted to 4.2 using sodium hydroxide. PEG[HO(C2H4O)nH] (mean molecular weight 3� 103) (0.1 g/L)or saccharin (3 g/L) was added to the electrolyte as anorganic additive. Ni electrodeposition was performed inunagitated solutions under galvanostatic conditions of 1 to1500 A/m2 at 323 K (sulfamate solution) and 313 K (Watt’ssolution). Copper sheets measuring 1� 2 cm2 and Ni sheetsmeasuring 4� 4 cm2 were used as the cathode and anode,respectively. In some experiments, electrodeposition wasconducted at a high charge to investigate the hardness of Niand the C and S contents in the deposit resulting from theorganic additives. Next, copper sheets measuring 5� 5 cm2

and non-activated Ni buttons (CROWNS made by theFALCONBRIDGE Company and Ni �99:98%) in a Tibasket were used as the cathode and anode, respectively.Continuous electrolysis was conducted using circulating 7 Lsolutions at a charge of 19440 C/cm2. The cathode potentialswere measured using a saturated KCl, Ag/AgCl referenceelectrode [0.199 V vs. the normal hydrogen electrode (NHE),25�C]. The potentials were plotted with reference to theNHE.

The C and S contents in the deposited Ni were measuredusing an infrared absorption method. The crystal orientationof the deposited Ni was determined using its X-raydiffraction intensity. The particle size of the deposited Niwas calculated by the Scherrer’s equation11) using the half-

*1This Paper was Originally Published in Japanese in J. Japan Inst. Metals

75 (2011) 406–411.*2Postgraduate Student, Kyushu University, Present address: College of

Science, China University of Petroleum, Beijing, China

Materials Transactions, Vol. 52, No. 11 (2011) pp. 2077 to 2082#2011 The Japan Institute of Metals

width of the X-ray diffraction pattern of maximum reflectionintensity. The cross-sectional texture of the deposited Ni wasinvestigated by an electron back-scattering pattern (EBSP).Prior to EBSP, Cu was electrodeposited on the deposited Ni,and the cross section was polished using an emery paper andbuffed. This was then electropolished in a solution containingsulfuric acid (10 vol%), methanol (10 vol%), and ethanol(80 vol%) at 10 V for 1 min. In EBSP, the crystallographicorientation of the normal direction (ND) with respect to thecross section of the deposited Ni and the orientation of thereference direction (RD) (i.e., the normal direction to surfaceof the deposited Ni) were examined. Vickers hardness of thedeposited Ni was measured for a cross section at a load of0.2 kg for 15 s.

3. Results and Discussion

3.1 Effect of additives on the texture of the deposited NiFigure 1 shows EBSP images of cross sections of Ni

deposited at 1000 A/m2 from Watt’s solution, which waseither additive-free or contained PEG or saccharin. Nideposited from the additive-free solution consisted of largecolumned crystals of a field-oriented texture12) at a middlestage of deposition (Fig. 1(a)). In the field-oriented texture,the preferred orientation of specific planes occurs towardthe electric field in deposition. The orientation of RD ofthe columned crystals was h100i, showing a field-orientedtexture of h100i. In the PEG-containing solution, thedeposited Ni showed a field-oriented texture identical to theadditive-free solution, but the width of the columned crystalswas smaller (Fig. 1(b)). The orientation of RD of columnedcrystals was predominantly h110i, unlike that of additive-freesolution. Ni deposited from the saccharin-containing solutionshowed an unoriented dispersed-type texture without thepreferred orientation of a specific plane (Fig. 1(c)).

Figure 2 shows EBSP images of cross sections of Nideposited at 1000 A/m2 from sulfamate solution, whichwas either additive-free or contained PEG or saccharin. Nideposited from additive-free solution formed large columnedcrystals of a field-oriented texture (Fig. 2(a)). The orientationof RD of the columned crystals was predominantly h100i, asfor the Watt’s solution. In the PEG-containing solution, thewidth of the columned crystals became smaller, and the ori-entation of RD of the columned crystals was predominantly

h110i (Fig. 2(b)). For the Ni deposited from the saccharin-containing solution, the field-oriented texture disappeared.There is a rare difference between Watt’s and sulfamatesolutions in the effect of PEG and saccharin on the texture ofthe deposited Ni. In both solutions containing PEG, a field-oriented texture was maintained, but the preferred orientationchanged from h100i for the PEG-free solution to h110i forthe PEG-containing solution. In both solutions containingsaccharin, the texture of the deposited Ni changed to theunoriented dispersed type12) without a preferred orientation.

3.2 Effect of additives on the C and S contents in thedeposited Ni

Figure 3 shows the C content in deposits obtained fromWatt’s and sulfamate solutions containing PEG. The errorbars in figure express the standard deviation of three samples.The C content in the Ni deposited from Watt’s solutionwas significantly higher than that from sulfamate solution.The C content in the deposits obtained from sulfamatesolution remained almost constant regardless of the currentdensity, while that from Watt’s solution clearly decreasedwith increasing current density. It was confirmed that theC contents in deposits obtained from Watt’s and sulfamatesolutions without PEG were minimal at 0.0008 mass% and0.0007 mass%, respectively. This suggests that the co-deposition of C results from PEG.

Figure 4 shows the S content in deposits obtained fromWatt’s and sulfamate solutions containing saccharin. Theerror bars in figure express the standard deviation of threesamples. The S content in the Ni deposited from Watt’s andsulfamate solutions differed slightly, except at 2000 A/m2.For Watt’s solution, the S content in deposits increased withcurrent density, showing an inverse current density depend-ence of C content in deposits. It was confirmed that Scontents in deposits obtained from Watt’s and sulfamatesolutions without saccharin were minimal at 0.0001 mass%and 0.0009 mass%, respectively. This suggests that the co-deposition of S results from saccharin.

Total polarization curves were measured for Ni solutionswith and without additives to investigate why the C content inthe Ni deposited from Watt’s solution was significantlydifferent from that from sulfamate solution. Figure 5 showsthe total polarization curves obtained from Watt’s solution.PEG polarized the cathode potential at potentials less noble

Table 1 Solution compositions and electrolysis conditions.

Ni(NH2SO3)2�4H2O (mol/L) 1.24

NiCl2�6H2O (mol/L) 0.04 Current density (A/m2) 1�5000

Sulfamate H3BO3 (mol/L) 0.49 Temperature (K) 323 (Sulfamate)

soln. Polyethylene glycol (g/L) 0.1 313 (Watt’s solution)

Saccharin (g/L) 3.0 Amount of charge (C/cm2) 150, 19440

pH 4.2

NiSO4�6H2O (mol/L) 0.91 Cathode Cu (1� 2 cm2, 5� 5 cm2)

NiCl2�6H2O (mol/L) 0.19 Anode Ni (4� 4 cm2), Ni chips

Watt’s H3BO3 (mol/L) 0.49 Quiescent bath

soln. Polyethylene glycol (g/L) 0.1

Saccharin (g/L) 3.0

pH 4.2

2078 H. Nakano et al.

than�0:4 V. The polarization effect was large, particularly atpotentials less noble than �0:7 V. In a saccharin-containingsolution, the polarization of the cathode was large at around�0:4 V, but the polarization effect was smaller than observedin the PEG-containing solution at potentials less noble than�0:5 V.

Figure 6 shows the total polarization curves obtained fromsulfamate solution. In a PEG-containing solution, the cathodepotential shifted to less noble direction at potentials lessnoble than �0:3 V, but the degree of shift was smaller thanthat in Watt’s solution. The polarization effect of PEGwas smaller than in Watt’s solution, particularly at around

3µm5µm 4µm

Dep

osit

ed N

i

Dep

osit

ed N

i

(a) (b) (c)

ND ND NDRD RD111

001 101ND

RD

TD

Fig. 1 EBSP images of cross section of Ni deposited at 1000 A/m2 from Watt’s solution additive-free or containing additives.

[(a) additive-free, (b) PEG-containing, (c) saccharin-containing]

5µm 5µm 3µm

Dep

osit

ed N

i

Dep

osit

ed N

i

111

001 101ND

RD

TD

(a) (b) (c)

ND ND NDRD RD

Fig. 2 EBSP images of cross section of Ni deposited at 1000 A/m2 from sulfamate solution additive-free or containing additives.

[(a) additive-free, (b) PEG-containing, (c) saccharin-containing]

Effect of Organic Additives on the Hardness of Ni Electrodeposited from Sulfamate and Watt’s Solutions 2079

�0:7 V. In a saccharin-containing solution, the cathodepotential slightly polarized at potentials less noble than�0:7 V, but the polarization effect was barely observed atpotentials more noble than �0:7 V. The suppression effect ofNi deposition and hydrogen evolution by additives is thoughtto be because of its adsorption on the cathode.13) Figures 5and 6 show that the suppression effect of Ni deposition andhydrogen evolution by PEG is larger in Watt’s than that insulfamate solution. This indicates that PEG adsorbs on thecathode more strongly in Watt’s than sulfamate solution. Thehigher C content in the Ni deposited from Watt’s solution, asshown in Fig. 3, is attributed to the adsorption effect of PEG.

The preferred orientation of Ni deposited from Watt’sand sulfamate solutions changed from h100i to h110i on theaddition of PEG, as shown in Figs. 1 and 2. In a saccharin-containing solution, the preferred orientation of Ni disap-peared and the texture became the unoriented dispersed type.Pangarov calculated the relative values of two-dimensionalnucleation work on various crystal planes.14,15) Assumingthat the two-dimensional nuclei with the smallest nucleationwork were formed at a given crystallization overpotential,he showed the overpotential dependence of the preferredorientation for various metals deposited from aqueoussolutions. According to Pangarov, the preferred orientationsof Ni with a face-centered cubic structure are h111i, h100i,and h110i at low, middle, and high overpotentials for Nideposition, respectively. In other words, the preferredorientations of Ni are shifted from h111i to h100i and h110i(in this order) as the overpotentials for Ni depositionincrease. In this study, Ni deposited from the additive-freesolution had a preferred orientation of h100i, correspondingto that at a middle overpotential for Ni deposition, assuggested by Pangarov. In the PEG-containing solution, thepreferred orientation of h110i corresponds to that at a highoverpotential for Ni deposition. Since PEG increased theoverpotential for the cathode reaction as shown in Figs. 5 and6, the crystal orientation in the PEG-containing solution canbe explained in terms of Pangarov’s calculations. However,in a saccharin-containing solution, the deposited Ni showedunoriented type of texture, despite the fact that the saccharinrarely changed the overpotential for the cathode reaction, asshown in Figs. 5 and 6. This observation cannot be explainedby Pangarov’s calculations. Pangarov also suggested that thework of the two-dimensional nucleation on various crystalplanes could change, regardless of the overpotential, whencertain foreign additives were adsorbed on the cathode.14,15)

In this study, saccharin might affect the nucleation andgrowth of a specific crystal plane, and suppress the preferredorientation of the deposited Ni.

3.3 Effect of C and S contents in the deposit on thehardness of deposited Ni

Figure 7 shows the relationship between the C content inthe deposit and the hardness of the Ni deposited from Watt’sand sulfamate solutions. For both Watt’s and sulfamatesolutions, the hardness of the deposited Ni increased with C

0.16

0.12

0.08

0.04

0

C C

onte

nt in

Dep

osit

(m

ass%

)

104103 102

Current Density, I/A m-2

Watt’ssulfamate

n=3

Fig. 3 C content in deposits obtained from Watt’s and sulfamate solutions

containing PEG.

Current Density, I/A m-2

Watt’ssulfamate

104103 102

0.05

0

0.04

0.03

0.02

0.01

S C

onte

nt in

Dep

osit

(m

ass%

) n=3

Fig. 4 S content in deposits obtained from Watt’s and sulfamate solutions

containing saccharin.

Cathode Potential, E/V vs. NHE

104

-0.4 -0.6 -1.2 0 -0.2 -0.8 -1.0

103

102

10

10-1

1Cur

rent

Den

sity

, I/A

m-2

additive-free PEG saccharin

Fig. 5 Total polarization curves in Watt’s solutions with and without

additives.

Cathode Potential, E/V vs. NHE

104

-0.4 -0.6 -1.20 -0.8 -1.0

103

102

10

10-1

1

additive-free PEG saccharin

-0.2

Cur

rent

Den

sity

, I/A

m-2

Fig. 6 Total polarization curves in Ni sulfamate solutions with and without

additives.

2080 H. Nakano et al.

content in the deposit. The hardness of the Ni deposited fromsulfamate solution was enhanced by a slight C content in thedeposit.

Figure 8 shows the relationship between the S content inthe deposit and the hardness of the Ni deposited from Watt’sand sulfamate solutions. Although the hardness of Nideposited from both solutions increased with the S contentin the deposit, the rate of increase in hardness reduced at Scontents in the deposit above 0.02 mass%. The hardness ofthe Ni deposited from both solutions was plotted on a curve,which shows the relationship between the S content in thedeposit and the hardness of Ni.

Figure 9 shows the texture of the Ni deposited from Watt’ssolution and the relationship between the current density andthe hardness of Ni. The value in h i represents the preferredorientation of Ni deposited from an additive-free solution.The hardness of the deposited Ni was enhanced by PEG andsaccharin, irrespective of the current density. The Ni with anunoriented dispersed-type texture deposited from a solutioncontaining PEG or saccharin showed a particularly highhardness. In the PEG-containing solution, only the Nideposited at 500 A/m2 showed an unoriented dispersed-typetexture. The C content in the deposit increased withdecreasing current density, as shown in Fig. 3, and the co-deposition of C seemed to cause the unoriented dispersed-type texture. In the additive-free solution, the Ni depositedat 500 and 1000 A/m2 showed a field-oriented texture witha preferred orientation of h100i, while the Ni depositedat 2000 A/m2 showed a preferred orientation of h110i. The

hardness of the Ni deposited at 1000 A/m2 was almostidentical to that deposited at 2000 A/m2, revealing littleeffect of crystal orientation on the hardness of the depositedNi with a field-oriented texture.

Figure 10 shows the texture of the Ni deposited fromsulfamate solution and the relationship between the currentdensity and the hardness of the Ni deposited. The hardnessof the deposited Ni was higher for a solution containing PEGor saccharin than that for additive-free solution, irrespectiveof current density. The hardness of the Ni deposited from aPEG-containing solution increased with decreasing currentdensity, while the hardness of the Ni deposited from asaccharin-containing solution increased with current density.When the Ni deposited from solutions containing PEG orsaccharin showed the unoriented dispersed-type texture, thehardness became particularly large. This trend was identicalwith that observed in Watt’s solution.

Figure 11 shows the relationship between the hardness andparticle size of the Ni deposited from Watt’s solution. Thehardness of the deposited Ni increased with decreasing Niparticle size. According to the Hall–Petch equation, sincethe grain boundary of the deposit resists slip deformation, anincrease in grain boundary or decrease in grain size enhancesthe yield stress of the deposited Ni.16) The presence of PEGand saccharin decreased the particle size of the deposited Ni,and as a result, the hardness of the deposited Ni increased.The decrease in the particle size of Ni in the presence ofPEG and saccharin occurs because the nucleation rate of Niprevails over the growth rate. It is well known that the

0

100

200

300

400

500

600

0 0.04 0.08 0.12 0.160

Watt’ssulfamate

C Content in Deposit (mass%)

Vic

kers

Har

dnes

s of

Ni,

Hv

Fig. 7 Effect of C content in deposit on the hardness of Ni deposited from

Watt’s and sulfamate solutions containing PEG.

0.01 0.02 0.03 0.04 0.050

S Content in Deposit (mass%)

100

200

300

400

500

600

0

Vic

kers

Har

dnes

s of

Ni,

Hv

Watt’ssulfamate

Fig. 8 Effect of S content in deposit on the hardness of Ni deposited from

Watt’s and sulfamate solutions containing saccharin.

100

200

300

400

500

600

0 104103 102

<100> <110><100> additive-free

PEG saccharin

Field Oriented Texture Type

Unoriented Texture Type

Current Density, I/A m-2

Vic

kers

Har

dnes

s of

Ni,

Hv

Fig. 9 Vickers hardness of Ni deposited from Watt’s solutions with and

without additives.

0

100

200

300

400

500

600

104103 102

Current Density, I/A m-2

Vic

kers

Har

dnes

s of

Ni,

Hv

additive-free PEG saccharin

Unoriented Texture Type

Field Oriented Texture Type

Fig. 10 Vickers hardness of Ni deposited from sulfamate solutions with

and without additives.

Effect of Organic Additives on the Hardness of Ni Electrodeposited from Sulfamate and Watt’s Solutions 2081

nucleation rate of deposits increases with the overpotentialfor deposition.17) PEG seems to increase the nucleation rateof Ni because of increases in the overpotential for deposition,as shown in Fig. 5. However, for saccharin-containingsolutions, because the overpotential for deposition wasbarely changed, the faster nucleation rate of Ni comparedwith the growth rate seems to be due to the fact that eitherthe co-deposited S originates the nucleation of Ni or the co-deposited S suppresses the growth of Ni.

Figure 12 shows the relationship between the hardness andparticle size of Ni deposited from sulfamate solution. Thehardness of the deposited Ni increased with decreasingparticle size of Ni, as in the case of Watt’s solution. Presenceof PEG and saccharin in the sulfamate solution decreased theparticle size of the deposited Ni and increased the hardnessof Ni.

As mentioned above, the hardness of the Ni deposited fromWatt’s and sulfamate solutions depended to a great extent onthe C and S contents in the deposit derived from PEG andsaccharin. The texture of the deposited Ni changed fromthe field-oriented type to the unoriented dispersed type bydeposition of C and S with Ni. It has been reported that theunoriented dispersed type of deposited metal generally showsa smooth surface composed of dispersed small crystals,irrespective of the base metal and the electric field.12) In thisstudy, increasing the C and S contents in the deposit led todeposited Ni of an unoriented dispersed type with smallparticle size. As seen from Figs. 11 and 12, the hardness of

the deposited Ni deviated from the Hall–Petch equation,indicating that the hardness of Ni cannot be explained by theNi particle size only. The hardness of the deposit is knownto increase with the strain of the crystal lattice at the grainboundary and within grains.18) The strain of the crystal latticeincreases by co-deposition of C and S, both of which have anatomic radius smaller than that of Ni. The hardness of theNi deposited from Watt’s and sulfamate solutions changedaccording to the C and S contents in the deposit. This seemsto be caused by changes in the crystal texture or in theparticle size and strain of the crystal lattice of the depositedNi because of the co-deposition of C and S.

4. Conclusion

The effect of PEG and saccharin on the hardness of Nideposited from Watt’s and sulfamate solutions was inves-tigated from the viewpoint of the co-deposition of C and Sand the crystal texture of Ni. The C content in the Nideposited from PEG-containing solutions was significantlyhigher for Watt’s solutions than that for sulfamate solutions.There was little difference in S content in deposits betweenWatt’s and sulfamate saccharin-containing solutions, exceptat 2000 A/m2. The hardness of the deposited Ni increasedwith increasing C and S contents in the deposits, and the rateof increase in hardness became small at S contents above0.02 mass%. The texture of the deposited Ni changed fromthe field-oriented type to the unoriented dispersed typewhen C and S were deposited with Ni by the addition of PEGand saccharin. The hardness of the deposited Ni increasedwith unoriented dispersed-type texture.

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100

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0

Vic

kers

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s of

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Particle Size of Deposited Ni, L-1/2/ nm-1/2

0 0.02 0.04 0.06 0.08 0.10

additive-free PEG saccharin

Fig. 11 Effect of particle size of Ni on the hardness of deposits obtained

from Watt’s solutions with and without additives.

100

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0

Vic

kers

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s of

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Particle Size of Deposited Ni, L-1/2/ nm-1/2

0 0.02 0.04 0.06 0.08 0.10

additive-free PEG saccharin

Fig. 12 Effect of particle size of Ni on the hardness of deposits obtained

from sulfamate solutions with and without additives.

2082 H. Nakano et al.