2. influence of electrodeposition parameters on the deposition rate and microhardness of...

8/10/2019 2. Influence of Electrodeposition Parameters on the Deposition Rate and Microhardness of Nanocrystalline Ni

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In uence of electrodeposition parameters on the deposition rate and microhardnessof nanocrystalline Ni coatings

Jin-Xing Kang a ,b , , Wen-Zhen Zhao a , Gao-Feng Zhang ba School of Materials Science and Engineering, Xi'an Jiaotong University, Xi'an 710049, Chinab The Engineering Institute, Air Force Engineering University, Xi'an 710038, China

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

Article history:Received 2 April 2008Accepted in revised form 3 January 2009Available online 22 January 2009

PACS:81.15.-z

Keywords:Nanocrystalline nickelCoatingsDeposition rateHardnessDirect current deposition

In this paper, nanocrystalline nickel (nc-Ni) coatings were prepared by a direct current electrodepositiontechnique. Their microstructure and microhardness were investigated by a high-resolution transmissionelectron microscopy and a microhardness tester. It is found that the electrodeposition parameters,including content of C 7 H4 NO3 SNa2H2 O, temperature and current density, have signi cant in uences onthe electrodeposition rate and microhardness of nc-Ni coatings. The electrodeposition rate increases withthe current density stepwise. The largest electrodeposition rate is achieved at 60 C. It decrease when thetemperature is larger than 60 C. The electrodeposition rate decreases with the increased content of C7 H4 NO3 SNa2H2 O. The microhardnesses of the nc-Ni coatings are higher on the condition of the largercurrent density, lower temperatureor higher content of C 7 H4 NO3 SNa2H2 O. But, it remains stable when thecurrent density is in the range of 700 1000 A m 2 . The relationship between the mean grain sizes andmicrohardness ts for the Hall Petch function, approximately.

2009 Elsevier B.V. All rights reserved.

1. Introduction

The synthesis of nanostructured materials through electrodeposi-tion has seen advances from a laboratory scale phenomenon to oneof practical industrial relevance. Potentially there are a very largenumber of pure metals, alloys, composites, and ceramics which can beelectrodeposited with grain sizes less than 100 nm, for example, Ni,Co, Pd, Cu, Ni P, Ni Fe, Ni W, Zn Ni, Ni Fe Cr and Ni SiC [1 3].Among the electrodeposited nanocrystalline materials, great attentionhad been paid to nanocrystalline nickel (nc-Ni) coatings due to thehigh brightness, high hardness and high corrosion resistance ability.Erb et al. prepared nc-Ni coatings by a electropulsing depositionmethod [4,5]. Imre et al. studied the nc-Ni coatings prepared by adirect current deposition [6]. Because of the complex preparationprocess, there are great differences in above investigations [7 14].

In order to obtain a nc-Ni coating with high quality, it is necessaryto control the electrodeposition parameters (such as, current density,current ef ciency and time) and to understand the relationship be-tween the electrodeposition parameters and the properties of nc-Ni

coatings. Therefore, the purpose of this paper is to study the effects of the electrodeposition parameters on the electrodeposition rate andmicrohardness of nc-Ni coatings.

2. Experimental procedure

The nc-Ni coatings were prepared by a direct current deposition.The used reagents of the solutions were analytically pure. The maincomponents and contents are listed in Table 1 . The remaining wasdistilledwater. ThepH values of thesolutions were kept at 4 by addingthe dilute solution of HCl. Ni plates (99.9%) were used as anode. Thesample was cathode, which was made of Ti 6Al 4V. The dimension of the samples was 50 mm 30 mm 2.5 mm. Before the electrodeposi-tion, the samples were sanded and polished. Then they were washedby alkaline, acid and distilled water in turn. After the procedure, thetreated samples were put into the solutions. Two groups of exper-iments were made in present work. The temperature of the solutionwas hold at 60 C for the rst group, the values of the current densitywere 500, 700, 900, 1100,1300 and 1500 A m 2 . For the second group,the current density was xed at 1300 A m 2 , the temperatures were50, 60, 70 and 80 C, respectively. During the experiments, all theelectrodeposition time was 90 min.

Microstructure of the nc-Ni coatings was observed by a JEM-2100F high-resolution transmission electron microscope (HRTEM).The thickness was measured after the coatings were peeled off from

Surface & Coatings Technology 203 (2009) 1815 1818

Corresponding author. School of Materials Science and Engineering, Xi'an JiaotongUniversity, Xi'an 710049, China. Tel.: +86 29 82665183; fax: +86 29 82663453.

E-mail address: [email protected] (J.-X. Kang).

0257-8972/$ see front matter 2009 Elsevier B.V. All rights reserved.

doi: 10.1016/j.surfcoat.2009.01.003

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the matrix. The deposition rates were estimated by the ratio of thethickness to the time. The hardness of the coatings was determinedby a MH-5 type microhardness tester. The applied load was 50 g andthe loading time was 5 s.

3. Results and discussion

3.1. Microstructure of nc-Ni coatings

Fig. 1(a) shows the mean grain sizes of nc-Ni coatings depositedat different current densities. The deposition temperature is 60 C.The grain size of nc-Ni deposited in No. 1 solution increases with theincrease of current density till 900 A m 2 . Then the grain size de-creases when the current density is greater than 900 A m 2 . For No. 2solution, the grain size decreases with the current density, similar toRashidi's results [15] . The decrease of grain size can be explained byTafel formula [1]. Overpotential of cathode increases with increase of the current density. Therefore, the driving force of nucleation is en-

hanced and nucleation rate increases. As a result, the grain size de-creases. Also, hydrogen evolution is inevitable in the cathode [2].Hydrogen can provide more crystal nucleus during the reductionreaction. It may lead to ne Ni grains. Fig. 1(b) illustrates the meangrain sizes of nc-Ni at different temperatures. The current density is1300 A m 2 . It can be found that the grain size is larger at highertemperatures. The temperature of the solutions is of particular im-portance in controlling the grain sizes. At higher temperatures, the

Ni ions can diffuse in longer distance. The growth rate of grain maybe larger than the nucleation rate. Therefore, the mean grain sizeincreases with the temperature [14] . According to the above results,it can be concluded that the grain size of nc-Ni is in the range of 14

85 nm, no matter what electrodeposition condition was applied.Furthermore, the effect of the temperature on the grain size is largerthan that of current density [15 18].

Fig. 2(a) shows the a bright TEM eld image of a nc-Ni coatingthrough No. 1 solution on the condition that the applied currentdensity and temperature are 1300 A m 2 and 60 C, respectively. Theinserted gureis its diffraction image. It can be seen that the grain sizeof the nc-Ni coating is about 30 nm. Fig. 2(b) is a HRTEM image. Thegrain boundary of the grains is clear, characterized by a high-angleboundary.

Table 1Composition of the electrodeposition solutions, the remaining is distilled water (unit:g L 1 )

Number NiSO 4 7H2 O NiCl2 6H2 O H3 BO3 C12 H25 OSO2 Na C7 H4 NO3 SNa2H2O

No. 1 300 45 40 0.05 3No. 2 300 45 40 0.05 5

Fig. 1. (a) Mean grain sizes of nc-Ni at various current densities. The depositiontemperature is 60 C. (b)Relationships between the mean grain sizes of nc-Ni and

temperature of solutions. The current density is 1300 A m 2

.

Fig. 2. A bright eld image and diffraction pattern (a) and a high-resolution electronmicroscopy image (b) of a nc-Ni coating prepared through No. 1 solution, the current

density and temperature are 1300 A m 2

and 60 C, respectively.

1816 J.-X. Kang et al. / Surface & Coatings Technology 203 (2009) 1815 1818


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3.2. Deposition rate of nc-Ni coatings

According to Faraday's law, the quantity of an electrochemicalreaction occurred on an electrode is proportional to the quantityof electric charge. Taken the current ef ciency into account, thethickness of the coating ( d) can be derived from the ratio of theproduct mass to the electro equivalent, that is,

d = ik k tE

1

Where ik is current density, k is current ef ciency, t is time, E iselectro equivalent of Ni and is the density of Ni.

E and of Ni are equal to 1.095 g/(Ah) and 8.907 g/cm 3 . Accordingto Eq. (1), the deposition rate V can be estimated as Eq. (2):

V = 2:04910 6 k ik 2

Where the units of V and ik are mm min 1 and A m 2 , respectively, k is a dimensionless unit. According to above derivation, thedeposition rate is correlated with the current density and currentef ciency [1].

Fig. 3 showsthat the deposition rates of thenc-Ni increasewith thecurrent density at 60 C. It can be seen that the deposition rateincreases stepwise. There are two terraces whenthe current density isin the range of 500 700Am 2 and 1300 1500 A m 2 for No.1 and No.2 solutions.

At a constant current ef ciency, the relationship between thedeposition rate and current density should be linear according toEq. (2). In order to obtain nc-Ni grains, the additive of C 7 H4 NO3-SNa2H2 O was added in the electrodeposition solutions, which canaccelerate the formation of crystal nucleus and inhibit the growthof grains. But, at the same time, the deposition process was hin-dered with the increase of the additive content. In some range of current density, Ni 2+ in the solutions moves to the cathode, andpasses through the double electrode layer. Subsequently, Ni atom

Fig. 3. Relationship between the electrodeposition rates and current densities at 60 C.

Fig. 4. Electrodeposition rates of nc-Ni at various temperatures at 1300 A m 2

.

Fig. 5. (a) Microhardness of the nc-Ni coatings deposited at various current densities.(b) Relationship between microhardness of the nc-Ni coatings and temperatures.

Fig. 6. Relationship between the hardness and the mean grain sizes deposited at

different electrodeposition parameters.

1817 J.-X. Kang et al. / Surface & Coatings Technology 203 (2009) 1815 1818


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