Surface & Coatings Technology 205 (2011) 4418–4424

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Surface & Coatings Technology

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Study on electrodeposition of Al on W–Cu substrate in AlCl3+LiAlH4 solutions

Qiang Chen a, Dun-qiang Tan a,⁎, Rui Liu a, Wen-xian Li b

a School of Material Science and Engineering, Nanchang University, Nanchang, 330031, Chinab School of Materials Science and Engineering, Central South University, Changsha, 410083, China

⁎ Corresponding author. Tel.: +86 13970080578.E-mail address: cntdunqiang@yahoo.com.cn (D. Tan

0257-8972/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.surfcoat.2011.03.058

a b s t r a c t

a r t i c l e i n f o

Article history:Received 2 December 2010Accepted in revised form 17 March 2011Available online 25 March 2011

Keywords:Non-aqueous solutionAluminumSurface morphologyElectrodeposition

Electrodepositing Al coatings on W90Cu10 foils in AlCl3+LiAlH4 Tetrahydrofuran–benzene electrolyte wasreported in this work. Effects of variations in molar ratios of AlCl3/LiAlH4, current density and plating time onsurface morphology and crystal orientations were studied, and the optimal values of three parameters werepreliminarily gained. Results showed that Al coatings displayed dense and cone-shaped micro-crystallites,and the particle size of coatings increased obviously with increasing current density or decreasing theproportion of AlCl3. It was indicated that all of the electrodeposits exhibited a strong preferred (111), (220)crystal orientation, and the (220) crystal orientation became strong along with electroplating time andcurrent density, but weak with the proportion of AlCl3 in baths. On the contrary, the changes of the (111)orientation exhibited an opposite trend.

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1. Introduction

W–Cu composites are commonly applied as a kind of thermalsubstrate for low expansion coefficient and high thermal conductivity[1–3], but the poor corrosion resistance prevents them for applica-tions in a wider scope. How to eliminate the deficiencies of W–Cu arebecoming urgent tasks. Aluminum coatings have excellent corrosionresistance and physicochemical properties, so if an aluminum coatingcould be obtained on W–Cu substrates as protective coating, it wouldendow W–Cu with better properties.

Manymethods can be employed for Al coating, such as hot dipping[4], thermal spraying [5], sputter deposition [6] and vapor deposition[7], but these techniques may be expensive, difficult to control layers’thickness or damage the specimens at high temperature [8]. On thecontrary, the method of electrodepositing has many advantages likemild conditions, uniform thickness distribution and unlimited sub-strates. Though various media of electroplating Al attracted a long-standing interest of scientists [9–14], no specific medium has beenreported yet, and the potentiality of this method seemed to bemodest. Therefore, whenwe try our best to discover newmedia, someclassic ones should be considered now.

Hydrides baths, consisting of AlCl3 and LiAlH4 in organic solvents,were firstly used to electroplate Al by D.E. Couch [15], numerousinvestigations were done on the preparation and mechanism of theplating solution and excellent Al coatings were obtained, and thechemistry of these solutions has been studied by various scientists[16–23]. Last decade, M.C. Lefebvre and B.E Conway have systemat-

ically reported the kinetic and mechanistic studies of AlCl3+LiAlH4

baths through the modern analytical techniques [24–26]. However,this method of electroplating Al, obtained after the efforts of severalgenerations, had been considered to be difficult to be used in own tothe health risks involved in volatile and carcinogenic solvents andaccurate industrial devices for the use of flammable media in galvanicprocesses. At present, the sealing and flame retardant techniques havea great development, and it would also be a great breakthrough torediscover the potentiality of the method with the help of theseassistive techniques.

In this work, electro-deposition of Al on tungsten–coppercomposites were investigated from AlCl3+LiAlH4 non-aqueoussolution, which has not been reported in the literatures before, andthe effects of current density and plating time on the crystal structureand orientation of Al coatings were studied, the optimum depositionconditions have been determined based on the study of processparameters, such as bath composition, current density and platingtime. And we also focused on the surface morphology of Al coatings,crystallographic orientation and nucleation processes.

2. Experimental

2.1. Materials

The commercial W–Cu foils (90%W and 10% Cu in mass ratio, usedas working electrode, purchased from Shanghai Leading MetalTechnology Co., Ltd) were cut to small disks (10 mm×15 mm×0.15 mm) and polished in H2SO4/H3PO4 electrolyte, which canremove the tungsten oxide and surface dirt, then rinsed and dried.Al plate (99.99%, used as counter electrode, manufactured in ourlaboratory) was dipped in NaOH, then rinsed with deionized water,

4419Q. Chen et al. / Surface & Coatings Technology 205 (2011) 4418–4424

acetone and dried. Anhydrous AlCl3 was supplied by Beijing ChemicalReagents Company, without further purification. LiAlH4 powder (98%)used as as-received was purchased from Aladdin. Tetrahydrofuran(THF) and benzene were obtained from Shanghai Reagent Co., Ltd andTHF was distilled from sodium and benzophenone.

2.2. Preparation of electrolyte and electrodepositing

The preparation of electrolyte and subsequent electrodepositionwere carried out under nitrogen atmosphere, which was similar to theone experimented in U. Bardi's paper [13]. In a 500 ml three-neckbottle as the electrochemical cell, working electrode and counterelectrode were insert though two necks of three, and the nitrogen wasinput though the least one. The LiAlH4 solution (dissolved in THF) wasdropped slowly into the cooled (0 to 5 °C) AlCl3 solution (dissolved inTHF and benzene). The obtained electrolyte was continuously stirredfor several hours to ensure uniformity. The W–Cu foils were theworking electrode and Al plate was used as counter electrode. Thedepositions were carried out immediately under predeterminedconditions after completing pretreatments. The process parametersin our study had bath composition, current density and plating time,the specific values were as followed: molar ratios of AlCl3/LiAlH4 (5:1,3:1, 1:1 and [AlCl3]+[LiAlH4]=1.5 mol/L), current density (1, 3, 5,and 7 A/dm2), plating time (5, 15, 30, 45, and 60 min). Following eachdeposition experiment, the deposit sample was rinsed with absolutealcohol, pure water and then dried in the air, and each result wasmeasured by repeated experiments.

2.3. Analysis and characterization of Al coatings

Surface morphology of the Al coatings was examined withmetallurgical microscope (XJP-3C) and emission scanning electronmicroscope (S3000N). The compositional analysis of the deposits wasconfirmed by X-ray, the crystalline structure was studied on a BrukerD8 Focus X-ray diffractometer with Cu Kα radiation.

3. Results and discussion

3.1. Micrographs of Al coatings

Fig. 1b shows an optical photo of a deposit, and Fig. 1a shows theoriginal sample surface. We found that the coating was silvery white,smooth, and its surface coverage was quite satisfactory.

Fig. 1. An optical photo of (a) original sample and (b) deposit manufactured in the

Fig. 2 shows the SEM morphologies of the deposits obtained fromdifferentmolar ratios of AlCl3/LiAlH4, andwe found that all of these Al-deposited samples were dense and homogeneous. The coatingsurfaces displayed similar faceted structures of coarse cone-shapemicro-crystallites in Fig. 2a, the other two deposits obtained in AlCl3/LiAlH4 3.0:1 and 5.0:1 showed similar cone structures with obviouschange of the cone size, and the cone size decreased significantly as themolar ratio increased, with an average size at the order of 5–10 μm.

The current density usually had a significant influence on the bright-ness, thickness distribution, deposition rate and microstructure of theelectrodeposits [8]. Electrodepositing Al coatings were performed withdifferent current densities in this paper, the results showed that thedeposits obtained from 1 to 7 A/dm2 were dense and homogeneous. Asshown in Fig. 3, when the current density was small enough, the depositshowed fine disc-shaped particles with size of 3–5 μm, and theseparticles becamecoarse and cone-sharp as the current density increased.In fact, the deposits at 1 to 5 A/dm2were quite smooth and bright, so theoptimum current densities could be determined to be in this range.

Fig. 4 shows SEM morphologies of Al coatings for different platingtime. The deposit surface showed isolated fine disc-shaped particleswhen plating just started (Fig. 4a). As time passed, fine particles wereconnected to be a coarse thin film (Fig. 4b). Finally, it appeared cone-shape micro-crystallites (Fig. 4c) and some secondary particles at theedge of them (Fig. 4d–f).

Therefore, controlling the current density and plating time canachieve the purpose of controlling the coating's thickness, andexcessive current density and plating time will lead to defects, suchas cracks and dendrites on the edge of deposit, as it shown in Fig. 5.Also, reasonable control of electroplating parameters will be helpful tothe quality of Al coatings. Our experiments showed that the optimalparameters were AlCl3/LiAlH4 3.0:1.0, current density 2–5 A/dm2,plating time 45–60 min.

3.2. Analysis on the crystal structure and orientation of Al coatingsformation

The chemical compositions of the samples electrodeposited underdifferent conditions were inspected with an EDAX and the resultsrevealed an identical spectrum. Fig. 6 shows the EDAX analyses of thedeposits obtained on the working electrodes after 45 min in 3:1 AlCl3/LiAlH4 at 302 K and 3 A/dm2. As expected, the deposits displayed astrong peak for aluminium, and slight peaks of oxygen. The detected Oin the Al deposits may results from the oxidation of Al. Table 1 showsthe components of the deposits clearly.

3.0:1.0 AlCl3/LiAlH4 bath at 298 K for 60 min with current density of 3 A/dm2.

Fig. 2. SEM morphologies of Al coatings on W–Cu obtained at 298 K for 45 min with current density of 5 A/dm2 at different molar ratios of AlCl3/LiAlH4 (a) 1.0:1.0, (b) 3.0:1.0 and(c) 5.0:1.0.

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Fig. 7 shows crystal structure analyses were carried out using XRD,and the acquired diffraction patterns of the obtained samples showedpatterns of Al andW–Cu Substrate. The diffraction peaks attributed topure Alwith a face-centered cubic structurewere clearly detected, andthe peak intensities of Al becamehigher as plating timewas increasing.

Preferred orientation of almost every metal that could beelectrodeposited have been widely studied, The orientation of ourobtained samples was studied by analyzing the results of XRD [27]and the Texture Coefficient for the (111), (200), (220), (311) and(222) reflections was calculated by using the following expression:

TChkl =IðhklÞ = I0ðhklÞ

∑n

i¼1IðhklÞ = I0ðhklÞ

× 100%

Where I(hkl) was the peak intensity of the (hkl) reflection forthe obtained samples, I0(hkl) was the peak intensity of the (hkl)

Fig. 3. SEM morphologies of Al coatings obtained at 298 K for 45 min with AlCl3/LiAlH4 3.0:1

reflection for the JCPDS card no. 01-089-2769, and n was the numberof peaks.

When the TC values of the reflections were the same, the crystalorientation was random, and if the TC of a crystal reflection wasgreater than the average value, the preferred orientation occurred onthat reflection, and the greater TC was, the stronger orientationwas. Itis shown in Fig. 7 that it had five reflections which were (111), (200),(220), (311) and (222) reflections, thus, n was 5, and the reflectionwhose TC was bigger than 20% was the preferred orientation.

The calculated results of the effects of plating time on preferredorientation are plotted in Fig. 8. It was shown that at the beginning ofcoatings formation, the crystal orientation was almost random, thereappeared to be no textures. As the plating time increased, the (200)and (311) TCs decreased quickly, the (111) and (220) crystallographicorientation were observed and the (222) reflection was essentiallyequal to that of a randomly referenced oriented sample, the strongestorientation of (111) were observed at 15 min. So the electrodeposits

.0 at different current densities (a) 1 A/dm2, (b) 3 A/dm2, (c) 5 A/dm2 and (d) 7 A/dm2.

Fig. 4. SEM morphologies of Al coatings at 298 K with AlCl3/LiAlH4 3.0:1.0, current density of 3 A/dm2 at different plating time (a) 5 min, (b) 15 min, (c) 30 min, (d) 45 min,(e) 60 min and (f) 75 min.

Fig. 5. Metallographic of (a) cracks and SEM morphology of (b) dendrite.

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Fig. 6. EDAX spectra of the deposit on cathode in 3.0:1.0 AlCl3/LiAlH4 for 45 min at302 K and 3 A/dm2.

Table 1Components of the deposit in 3:1 AlCl3/LiAlH4 for 45 min at 302 K and 3 A/dm2.

Element Weight % Atomic %

O K 1.33 2.22Al K 98.67 97.78Totals 100.00

5 15 25 35 45 55 650

10

20

30

40

50

Tex

ture

Co

effi

cien

t %

Plating time / min

(111)(200)(220)(311)(222)

Fig. 8. Texture coefficient from XRD reflections of the deposits obtained under differentplating time with current density of 3 A/dm2 in 3.0:1.0 AlCl3/LiAlH4 at 302 K.

1 3 5 70

10

20

30

40

50

Tex

ture

Co

effi

cien

t %

Current density / A/dm2

(111)(200)(220)(311)(222)

Fig. 9. Texture coefficient from XRD reflections of the deposits obtained with differentcurrent density in 3.0:1.0 AlCl3/LiAlH4 for 45 min at 302 K.

4422 Q. Chen et al. / Surface & Coatings Technology 205 (2011) 4418–4424

exhibited a preferred (111), (220) crystallographic orientation, andthe longer electroplating time was, the stronger the preferred (220)crystallographic orientation was.

According to the above method for analyzing crystallographicorientations, the results of the effects of current density on thepreferred orientation are plotted in Fig. 9, which was shown that all ofthe electrodeposits with different current density exhibited apreferred (111) and (220) crystallographic orientation too. The(111) intensity decreased along with current density and the (220)increased along with it; the (222) intensity decreased first, thenincreased; the (200) and (311) reflections were relatively weak andtheir intensities at 5 A/dm2 and 7 A/dm2 were the same.

Fig. 10 shows that results of effects of molar ratio of AlCl3/LiAlH4 onthe preferred orientation, and the electrodeposits exhibited a pre-ferred (111) and (220) crystallographic orientation, but the (111)intensity increased along with the content of AlCl3 and the (220)

10 20 30 40 50 60 70 80 90

(111

)

(200

)

(222

)(3

11)

(220

)

2 θθ (degree)

e

Rel

ativ

e in

ten

sity

/ a.u

(111

)

(200

)

(222

)(3

11)

(220

)d

(222

)(3

11)

(220

)

(200

)(111

)

c

b

(220

)

(311

)(2

22)

(222

)(3

11)

(220

)

(200

)

(111

)

aAlAl

WCuxWCux

(111

)

(200

)

Fig. 7. XRD patterns of the deposits onW-Cu obtained fromAlCl3/LiAlH4 3.0:1.0 at 302 Kwith current density of 3 A/dm2 under different plating time (a) 5 min, (b) 15 min,(c) 30 min, (d) 45 min and (e) 60 min.

decreased with it. The (222) intensity decreased first, then increased;the (200) and (311) reflections were relatively weak, the intensitiesincreased along with the content of AlCl3.

From above analyses, conclusions can be drawn that crystal orien-tation of Al deposits on Wu–Cu foils were effected markedly by the

1 3 50

10

20

30

40

50

Tex

ture

Co

effi

cien

t %

Molar ratio of AlCl3/LiAlH4

(111)(200)(220)(311)(222)

Fig. 10. Texture coefficient from XRD reflections of the deposits obtained at differentmolar ratios of AlCl3/LiAlH4with current density of 3 A/dm2 for 45 min at 302 K.

Fig. 11. (a) Cross-section by liquid nitrogen Quench and (b) Sketch map of coating.

4423Q. Chen et al. / Surface & Coatings Technology 205 (2011) 4418–4424

variations in plating time, current density and the content of AlCl3.It was indicated that all of the electrodeposits exhibited a preferred(111) and (220) crystal orientation. The (222) reflection was essen-tially equal to that of a randomly referenced oriented sample, and the(200) and (311) reflections were relatively weak. Moreover, the pre-ferred (220) crystal orientation became strong along with electro-plating time and current density, but weak with the proportion ofAlCl3 in baths, however the changes of the preferred (111) crystalorientation were almost in contrast with the (220) reflection.

3.3. Discussion of the formation of Al coatings

Layer growth and three-dimensional crystal growth were usuallytwo basic growth pattern for the coatings. The existence of isolatedfine disc-shaped particles andmerger growth proved that it was moresuitable for the latter to explain the formation of Al coatings.

At first, the fine disc-shaped particles appeared because the initialnucleation belonged to no-coherent epitaxial growth, under suitableconditions, they changed into three-dimensional crystal quickly, andsmall crystals were assembled into bigger particles, which maychanged the distribution of electric field and coursed surface particles’competitive growth. Then Merger growth happened between parti-cles spontaneously for reducing surface energy, with the increase ofparticles’ size, misorientation between adjacent particles expanded,the Merger growth got difficult. However, it was easier to form coarseparticles and rough boundary, which would provide good conditionsfor secondary nucleation, so we can see some small particles at theedge of some coarse ones.

In Fig. 11a, the interface between W–Cu substrate and Al wasshown, proving that there was a perfect adhesion between them. As itshown, the coating stratification was observed, it was fine and denseparticles closed to the substrate, and in the middle there were small

Fig. 12. Surface morphology of initial W90Cu10 foil.

holes coursed by competitive growth and different growth rate ofcrystals, then the particles on the surface were crowded dense andcoarse. Fig. 11b is the sketch map of coating formation.

However, compared to the ideal surface, the surface of W–Cu com-posite had many defects, such as grain boundaries, phase boundaries,voids, cracks and so on. At the beginning of Al electro-deposition, mostof fine disc-shape particles were attached to the high energy placeswhere the defects usually was, surface morphology of copper tungstenfoils is shown in Fig. 12, where dark gray particleswere tungsten, brightwhite particles was copper. Therefore, the electro-deposition on thesurface of W–Cu foil was special, and further studies were necessary onhow the complex surface of W–Cu foil effected on the nucleation andgrowth of Al coating.

4. Conclusion

Al coatings on W90Cu10 foils in AlCl3+LiAlH4 THF-benzene bathswere successfully obtained. The deposits displayed close cone-shapedmicro-crystallites with an average size at the order of 3–10 μm, andthe particle size of coatings increased obviously with increasingcurrent density or decreasing the proportion of AlCl3; all of thedeposits exhibited a strong preferred (111), (220) crystal orientation,and the (220) crystal orientation became strong along with electro-plating time and current density, but weak with the proportion ofAlCl3 in baths, the orientation changes of the (111) reflection werealmost in contrary to ones of the (220) reflection; and preliminaryexperimental results showed that the optimal Al coating can begained in 3.0:1.0 AlCl3/LiAlH4 with current density of 2–5 A/dm2 for45–60 min.

Acknowledgement

Thisworkwas supported by the Program for Chang Jiang Scholars andInnovative Research Team in University, China (Grant IRT0730). NationalKey Technology R&D program, China (Grant SQ2010BAJY1485).

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