Electrodeposited Co–Ni–Al O composite coatings

Surface and Coatings Technology 176(2004) 157–164

0257-8972/04/$ - see front matter� 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0257-8972Ž03.00739-4

Electrodeposited Co–Ni–Al O composite coatings2 3

Gang Wu *, Ning Li , Derui Zhou , Kurachi Mitsuoa, a a b

Department of Applied Chemistry, Harbin Institute of Technology, P.O. Box 411, Harbin 150001, PR Chinaa

Faculty of Engineering, Kyoto University, Kyoto 606-8283, Japanb

Received 27 September 2002; accepted in revised form 14 April 2003

Abstract

Composite coatings of Co–Ni–Al O were studied by electrolytic codeposition of Co–Ni alloys and Al O from a sulfamate2 3 2 3

electrolyte containing Al O particles. The influence of plating parameters on the content of the co-deposited Al O particles in2 3 2 3

Co–Ni alloys was investigated. The maximum value of co-deposited Al O can be achieved at a particle content of 80 g l iny12 3

bath, a current density of 3 A dm , a pH of 4.5, and a stirring rate of 100 rpm. In the process of codeposition, cathodicy2

polarization increases with the increase of Al O concentration in bath and cobalt ions in electrolyte caused the reduction of2 3

polarization while nickel ions do not change the polarization behavior. Surface morphology and microstructure of Co–Ni–Al O2 3

coatings were determined by means of scanning electron microscopy, atomic force microscopy and X-ray diffraction. It was foundthat the phase structure of solid solution cannot be varied by codeposition of Al O particles in Co–Ni alloys, and it only2 3

influences the growth and orientation of crystal planes. It was shown that the presence of Al O particles in deposit greatly2 3

improves the hardness and the wear resistance of composite coatings. However, the codeposition of Al O increases the tensile2 3

internal stress of Co–Ni–Al O deposit. The coefficient of thermal expansion and the thermal conductivity of Co–Ni–Al O2 3 2 3

composite coatings are varied with the increase of Co contents and the temperature.� 2003 Elsevier Science B.V. All rights reserved.

Keywords: Electrodeposition; Composite coating; Cobalt–Nickel alloy; Alumina

1. Introduction

Inert particles(SiC, WC, Al O , SiO , etc.) suspended2 3 2

in an electrolytic bath can be co-deposited during elec-trodeposition. Composite coatings produced by this tech-nique enhance physical and mechanical properties suchas wear and corrosion resistance as compared to thepure metal coatingsw1,2x. These improved propertiesmainly derive from the presence of particles dispersedin the metallic matrix and thus depend on the contentand nature of particles in the coatings. Among manymethods of preparing such dispersion-hardened alloys,electrodeposition is a simple and economic method ofproducing composite coatings. This technique involvesno high-temperature or high-pressure process. Further-more, the concentration and spacing of the particles indeposits can be controlled precisely by this methodw3x.

*Corresponding author. Tel.:q86-451-6413721; fax:q86-451-6221048.

E-mail address: [email protected](G. Wu).

Alloys based cobalt or nickel are widely applied indifferent industrial fields due to their significant resis-tance to wear and corrosion in high-temperatures. Theyare also used as protection coatings for element workingin aggressive environmentsw4,5x. However, these coat-ings still suffer both severe wear and oxide scaling atelevated temperaturesw6x.Recently, in order to improve the physical and

mechanical properties of coatings, many kinds of com-posite coatings based on cobalt or nickel were developedby electrodeposition method such as Ni–SiCw7,8x, Ni–ZrO w9x, Ni–Al O w10–12x, Ni–WC w13x, Ni–PSZ2 2 3

w14x, Co–Cr O w15x, Co–TiO w16x etc. Nevertheless,2 3 2

few articles have reported the preparation of Co–Nialloy matrixes containing dispersed particles. The use ofCo–Ni alloy can gives exceptional advantage in termof mechanical properties(hardness, chemical inertia andgood behavior in friction) w17,18x and physical proper-ties (electrocatalytic activityw19x, magneticw20x). Espe-cially, in the last few decades, the use of Co–Ni alloyshas been extended to the production of three-dimension-

158 G. Wu et al. / Surface and Coatings Technology 176 (2004) 157–164

Fig. 1. Relationship between cobalt contents in solution and cobaltcontents in deposit from the sulfamate electrolyte; pH 4.5,j s3k

A dm , Ts60 8C, 100 rpm.y2

al, complex-shaped finished components and uniquearticles by the method of electroformingw21x. Al O2 3

particle has many superior properties, such as low price,good chemical stability, high microhardness and wearresistance at high-temperaturew22x. Therefore, as asecond phase to strength composite materials, Al O is2 3

one of the economic and powerful materials. Takinginto account the various properties depending on boththe electroplating technique and hard particle addition,the aim of this work was to co-deposit Co–Ni–Al O2 3

composite coatings and to evaluate their properties fromthe view of technical and engineering application.

2. Experimental procedure

In order to obtain electrodeposited composite Co–Ni–Al O coatings, electrolytes were prepared having2 3

the following composition: 200–300 g ly1

Co(NH SO ) Ø4H O, 300–350 g ly12 3 2 2

Ni(NH SO ) Ø4H O and 20 ml l CH NO, to whichy12 3 2 2 3

20–140 g l a-Al O particles was added. Reagents ofy12 3

analytical purity and distilled water were used forpreparing the solution. Using an optical microscope, themean size of thea-Al O particles was estimated to be2 3

0.5 mm. The Al O particles were pretreated with2 3

acetone and warm 5% HNO to remove residual organic3

and impurities, then washed with distilled water anddried. In addition, an ultrasonic generator was used tominimize Al O particles agglomeration in the2 3

suspension.The electrodeposition experiments were conducted in

a 2-l capacity PVC container with heating facilities.Rectangular 80=40 mm copper plates were used as2

substrates for the cathodes. Separate rectangular 80=25mm cobalt(99.0%) and nickel(99.9%) anodes were2

used. To keep constant of metal ions concentration inelectrolyte, the dissolving rate of cobalt and nickelanodes was controlled by precise adjusting the currentdensity flowing the different anodes. The solution wasstirred with a magnetically driven Teflon coated stirringbar. The codeposition parameters were current density1–9 A dm , stirring rate 40–160 rpm, pH 3.0–5.0 andy2

temperature 50–608C. To investigate the factors affect-ing incorporation of Al O particles, different parameter2 3

variations and electrolytes containing different concen-tration of particles and metal ions were applied. Thevolume fraction of Al O particles in composite coatings2 3

(denoted byV ) was measured by gravimetric analysis.p

In order to assess the influence of alumina presence onthe properties of coatings, Co–Ni alloys were alsoobtained under the same condition from a bath withoutalumina. Comparative tests were conducted on Co–Niand Co–Ni–Al O coatings.2 3

The polarization curves for the electrolyte with dif-ferent concentration of particles and metal ions wereperformed using an EG&G PAR 273 Potentiostat–

Galvanostat controlled by an IBM compatible PC. Thescan rate was 2 mV s .y1

Surface morphology of the Co–Ni–Al O and Co–2 3

Ni coatings was determined by scanning electronmicroscopy (SEM) and atomic force microscopy(AFM). The surface composition of coatings was inves-tigated using electron dispersive spectroscopy. Phasestructure of the coatings was analyzed by the X-raydiffraction (XRD).Vickers hardness was measured by means of AKASHI

Hardness tester at 100 g loads. Wear resistance wasdetermined as weight loss per square centimeter afterbeing exposed 20 min at the rate of 200 rpm, using aface wear test machine under a constant load of 20 kgat the temperature of 3008C with GCrl5 abrasivewheels. The testing method of internal stress is the bentstrip (cantilever beam) technique.Coefficient of thermal expansion(CTE) was measured

using NETZSCH DIL 402C Thermal Dilatometer at thetemperature range 50–4008C; Thermal conductivitywas measured by the laser thermal conductivity detectorat the temperature range of 150–4008C.

3. Results and discussion

3.1. The deposition of Co–Ni–Al O coatings2 3

Fig. 1 shows the relationship between the Co2q

concentration in solution and Co contents in depositwith and without addition of Al O particles. Whether2 3

Al O particle was added or not, the percentage of Co2 3

in the deposit is always higher than that in the solution.This indicates the anomalous codeposition behavior ofcobalt–nickel alloys was not changed by addition ofAl O . Moreover, It can be seen that A1 O particles in2 3 2 3

electrolyte promotes the codeposition of cobalt andmakes the Co contents in deposit become higher thanthose of without Al O particles.2 3

The effect of Co concentration in electrolyte on2q

159G. Wu et al. / Surface and Coatings Technology 176 (2004) 157–164

Fig. 2. The effect of concentration of Co in solution onV ; 802qp

g l Al O , pH 4.5, j s3 A dm , Ts60 8C, 100 rpm.y1 y22 3 k

Fig. 3. Relationship between Al O concentration in bath andV from2 3 p

electrolyte containing: 300 g l cobalt sulfamate, 300 g l Nickely1 y1

sulfamate,j s3 A dm , pH 4.5, 100 rpm.y2k

Fig. 4. The effect of current density onV , from electrolyte containing:p

300 g l cobalt sulfamate, 300 g l nickel sulfamate and 80 g ly1 y1 y1

Al O , pH 4.5,Ts60 8C, 100 rpm.2 3

volume percentage of Al O particles in composite2 3

coatingsV is shown in Fig. 2. TheV increases withp p

an increase of Co concentration in bath and holds up2q

to 0.4 ratio of wCo xy(wCo xqwNi x), with further2q 2q 2q

marginal increment at higher Co concentration. The2q

codeposition of Al O is promoted by the adsorption of2 3

Co on its surface, which increases the surface positive2q

charge of Al O particles.2 3

Fig. 3 shows the relationship between concentrationof Al O particles in the electrolyte andV at different2 3 p

temperature. The ratio ofwCo xy(wCo xqwNi x) in2q 2q 2q

bath is 0.5. It can be noted that Al O in the deposit2 3

increases with an increase of Al O in the bath, tending2 3

to attain a steady value at Al O concentration of 802 3

g l , Furthermore, as can be seen from Fig. 3, electro-y1

lyte temperature of 508C is more beneficial to thecodeposition of Al O than that of 608C, but higher2 3

temperature is useful to reduce roughness and internalstress of coatings.Moreover, the curves are quite similar to the well-

known Langmuir adsorption isotherms, supporting amechanism based on an adsorption effect. The codepos-ition of Al O by the electrodeposition technique may2 3

be attributed to the adsorption of Al O particles on the2 3

cathode surface, as suggested by Guglielmi’s two-stepadsorption modelw23x. Once the particle is adsorbed,metal begins building around the cathode slowly, encap-sulating and incorporating the particles. The plateauobserved at higher particles concentration in bath maybe a result of saturation in adsorption on cathode surface.Fig. 4 shows the effect of current density on theVp

from a bath containing 80 g l of Al O particles andy12 3

0.5 ratio of wCo xy(wCo xqwNi x). It is observed2q 2q 2q

that theV increases initially with the current densityp

and reaches a maximum at 3.0 A dm . Beyond thisy2

current density, the co-deposited Al O content decreas-2 3

es. The maximum observed in the curve of currentdensity vs.V can be attributed to the transition fromp

an activation-controlled metal deposition reaction to a

diffusion-controlled of particles transfer. The energyrequired to deposit for metal ion that solvated andadsorbed on the particle surface is larger than that forthe freely solvated metal ionsw24x. Because of thedifference in activation energy needed for deposition,the initial low codeposition may be a result of thepreferred free metal ions deposition. As the currentdensity increases, this energy difference criterionbecomes less important, therefore, codeposition ofAl O increases. However, at higher current densities,2 3

the loose adsorption of particles at the cathode surfacemay become rate controlling. Because this is alwaysslower than the metal deposition rate, incorporationdecreases with increase in current density.The pH value of the electrolyte plays an important

role in the codeposition process. The effect of pH ontheV is shown in Fig. 5. As can be seen, theV attainsp p

the greatest value at pH 4.5.To promote incorporation of the Al O particles in2 3

deposit, the adsorption of metal ions on particle surface


Fig. 5. Effect of the pH value on theV ; from electrolyte containing:p


Al O , j s3 Adm , Ts60 8C, 100 rpm.y22 3 k

Fig. 7. Cathodic polarization curves for codeposition of Al OyCo–2 3

Ni from sulfamate electrolyte containing 300 g l cobalt sulfamate,y1

300 g l nickel sulfamate and different concentration of Al Oy12 3

particles.

Fig. 6. Effect of the stirring rate on theV from electrolyte containing:p


Al O , j s3 A dm , Ts60 8C, pH 4.5.y22 3 k


Ni in sulfamate electrolyte containing 300 g l nickel sulfamate, 80y1

g l Al O and different concentration of cobalt.y12 3

is more useful than that of hydrogen ionsw25x. Thedecrease of H ions concentration in electrolyte contrib-q

utes to the metal ions adsorption on the surface of theAl O particles, which led to an increase in codeposition2 3

of Al O with Co–Ni alloys.2 3

Since Al O particles should be transported to the2 3

cathode surface for the codeposition, the stirring ratestrongly affects the content of Al O in deposit. The2 3

influence of stirring rate onV is shown in Fig. 6. Thep

V increases with stirring rate and reaches a maximump

value at 100 rpm, then decreases with the stirring rate.The codeposition behavior of Al O particles with Co–2 3

Ni alloys is apparently controlled by particle transferwhen the stirring rate is smaller than 100 rpm. Collisionfactor may be the most important reason that causescodeposition of Al O particle decrease at very rapid2 3

stirring rate w26x. Another possible reason is perhapsthat the increasing streaming velocity of the suspensionmay also dislodge and sweep away the loosely adsorbedAl O particles on the cathode surface, causing a reduc-2 3

tion in codeposition.

3.2. The polarization behavior of Co–NiyAl O2 3

electrolyte

The cathodic polarization behavior of the Co–NiyAl O electrolyte containing different concentration of2 3

Al O particles is shown in Fig. 7. The curves represent2 3

two consecutive reduction reactions, in which H andq

Co yNi are reduced to H and Co–Ni alloy, respec-2q 2q2

tively. The reduction of H would occur more quicklyq

than the reduction of Co or Ni with the reduction2q 2q

potential increasing. Hydrogen evolution results in thedecrease in current efficiency and has a detrimentaleffect on codeposition of Al O . Fig. 7 also indicates2 3

that the Al O particles in the electrolyte cause an2 3

increase in the cathodic polarization, but the slope isunchanged. This means that the adsorption of Al O2 3

particles on cathode surface hinders the deposition ofCo and Ni , but does not significantly affect the2q 2q

electrochemical reaction mechanism.Fig. 8 shows the cathodic polarization behavior of the

electrolyte containing different Co and constant2q

Ni and Al O concentration. When the potential is2q2 3



Ni in sulfamate electrolyte containing 300 g l cobalt sulfamate, 80y1

g l Al O and different concentration of nickel.y12 3

Fig. 10. SEM micrographs of Co–Ni and Co–Ni–Al O coatings.(a) Ni-16Co; (b) Ni-40Co; (c) Ni-78Co; (d) Ni-20Co-8.5 vol.% Al O ;(e)2 3 2 3

Ni-42Co-8.7 vol.% Al O ;(f) Ni-80Co-8.7 vol.% Al O .2 3 2 3

polarized fromy0.75 toy1.25 V vs. SCE, the increaseof cobalt concentration led to an increase in the cathodiccurrent. However, from the Fig. 9, when different Niconcentration was added in the electrolyte containingconstant Co and Al O concentration, the cathodic2q

2 3

polarization behavior of Co–NiyAl O electrolyte is2 3

almost unchanged. This phenomenon shows that theadsorption of Ni on Al O surface is weaker than that2q

2 3

of Co and cannot influence the codeposition process.2q

3.3. Surface morphology and microstructure

Fig. 10 compares the SEM morphology of Co–Nicoatings and Co–Ni–Al O composite coatings. It is2 3

evident that Al O particles are uniformly distributed in2 3

the Co–Ni matrix by electrolytic codeposition. It can beseen that the Co–Ni deposit has a rather regular surface,whereas the Co–Ni–Al O composite coating develops2 3

a nodular surface structure. A smaller Co–Ni grain sizeis observed in the composite coatings compared to theCo–Ni deposits. Due to the adsorption of Al O on the2 3

cathode surface, it increases cathodic polarization andcontributes to developing fine grains. As the Co contentsin the Co–Ni and Co–Ni–Al O coatings increase from2 3

16 to 42%, there is a decrease in grain size and themorphology changes from a nodular aspect which isrelated to low Co content coating to a fibril one,associated with medium Co content. Simultaneously, thesurface structure became less compact. However, withthe further increase of Co content()78%), morecompact and fine granular morphology is observed inthe Co–Ni and Co–Ni–Al O coatings.2 3

In order to investigate the microstructure of Co–Ni–Al O composite coating more detailed, scanning anal-2 3

ysis and measurements were conducted using AFMmethod. The results are shown in Fig. 11, Sphericalgrowth is clearly visible on the surfaces of the compositecoatings. The microstructure of Co–Ni–Al O appears2 3

to be influenced by variations in Co contents in thecomposite coating. The microstructure of Ni-80Co-8.7vol.% Al O coating has a more uniform and fine2 3

structure than that of Ni-20Co-8.5 vol.% Al O .2 3

The phase composition and structure of Co–Ni alloysand Co–Ni–Al O coatings with different cobalt con-2 3

tents was investigated using XRD. As can be seen fromFig. 12, the crystal structures both Co–Ni coatings and


Fig. 11. AFM analysis results of Co–Ni–Al O composite coatings.(a) Ni-20Co-8.5 vol.% Al O ;(b) Ni-80Co-8.7 vol.% Al O .2 3 2 3 2 3

Fig. 12. XRD pattern of Co–Ni–Al O composite coatings.(a) Ni-2 3

16Co;(b) Ni-40Co; (c) Ni-78Co; (d) Ni-20Co-8.5 vol.% Al O ;(e)2 3

Ni-42Co-8.7 vol.% Al O ;(f) Ni-80Co-8.7 vol.% Al O .2 3 2 3

Fig. 13. The effect ofV on Vickers Hardness of Co–Ni–Al O com-p 2 3

posite coatings with same cobalt content.

Co–Ni–Al O composite coatings are mainly dependent2 3

on the cobalt contents in deposits. For the low-cobaltcoatings of Co–Ni and Co–Ni–Al O , it is confirmed2 3

the structure is single-phase solid solution of cobalt innickel with a face-centered cubic(fcc) lattice. Neverthe-less, for high-cobalt coatings of Co–Ni and Co–Ni–Al O , a solid solution of nickel in cobalt with a2 3

hexagonal close packed(hcp) lattice is obtained. TheCo–Ni and Co–Ni–Al O coatings with the low Co2 3

content all exhibit fcc nickel(1 1 1) growth orientationwith significant (2 0 0), (2 2 0) and (3 1 1) reflectionsas well. However, at high Co content, the Co–Nicoatings show very strong hcp cobalt(2 0 0) growthorientation, while Co–Ni–Al O composite coatings2 3

form strong hcp cobalt(1 0 0) texture. Therefore, It canbe concluded that the phase structure of Co–Ni alloyssolid solution is not changed by the codeposition ofAl O particles, but it obviously influences the growth2 3

and orientation of crystal planes in composite coatings,especially at high Co content.

3.4. Mechanical properties of Co–Ni–Al O coatings2 3

Microhardness of the composite coatings as a functionof V is shown in Fig. 13. As can be seen from Fig. 13,p

the hardness of the composite coating increases with theincrease ofV to a certain value at 5.2%, after whichp

the increase is marginal. The improvement in the hard-ness of composites coating is related to the dispersionhardening effect caused by Al O particles in the com-2 3

posite, which obstructs the shift of dislocation in cobalt–nickel alloys. Moreover, after annealing at 6008C for 1h, the microhardness of the Co–Ni–Al O composite2 3

coatings decreases. However, whenV in the deposit isp

increased from 5.2 to 8.7%, the effect of annealing onhardness is slight and the hardness remains above 520Hv. The likely explanation to the reduction of hardness


Fig. 14. The effect ofV on internal stress of Co–Ni–Al O compositep 2 3

coatings with same cobalt content.

Fig. 16. The CTE of the Co–Ni–Al O composite coatings.2 3

Fig. 15. The wear weight loss at 3008C and hardness of differentcoatings obtained by electrodeposition.(1) pure Nickel; (2) pureCobalt;(3) Ni-16Co alloy;(4) Ni-78Co alloy;(5) Ni-20Co-8.5 vol.%Al O ; (6) Ni-80Co-8.7 vol.% Al O .2 3 2 3

Fig. 17. Thermal conductivity of the Co–Ni–Al O composite2 3

coatings.

is the recrystallization of Co–Ni alloy and the decom-position of metal hydride during annealing. The latteroccurs at temperatures above 2008C by releasing hydro-gen w27x.Fig. 14 shows the effect of theV on the tensilep

internal stress of Co–Ni–Al O composite coatings. It2 3

is shown that an increasing stress from 20 to 57 MPa isa result of increasingV from 0 to 8.7%. The increasingp

trend of internal stress is caused by the adsorption ofAl O particles on cathode surface, which makes the2 3

current efficiency decrease and the large amount of Hoccluded in the deposit increase. Subsequently, thedesorption of H atoms causes tensile internal stressincreasew28x.Wear weight loss and hardness of different coatings,

including (1) Pure Nickel; (2) Pure Cobalt;(3) Ni-16Co alloy;(4) Ni-78Co alloy; (5) Ni-20Co-8.5 vol.%Al O ; (6) Ni-80Co-8.7 vol.% Al O are shown in Fig.2 3 2 3

15. The Ni-80Co-8.7 vol.% Al O coating has a mini-2 3

mum wear weight loss and maximum hardness. Thisresult reflects that the coating with maximum hardnesshas the best wear resistance at high-temperature. Accord-

ing to the data, the dispersion of Al O particles in Co–2 3

Ni alloys enhances the surface hardness and wearresistance. Furthermore, wear resistance and hardnessare dependent on the Co content. The high Co contentin composite coatings contributes to improving the wearresistance and hardness of composite coatings.

3.5. Thermal physical properties of Co–Ni–Al O2 3

coatings

When materials are to be employed at elevated tem-perature, knowledge of their thermal physical propertiesis important. Both CTE and thermal conductivity areimportant thermal physical properties. When a construc-tion during application is exposed to temperature chang-es, the mismatch of CTE between the thin films and thesubstrate may lead to residual stresses and produceadditional problemsw29x. The higher the thermal con-ductivity, the more efficient the cooling property. Highthermal conductivity also assures a uniform temperaturedistribution, which reduces thermally induced stresses,and thereby improves fatigue propertiesw30x.Relationship between the temperature and the CTE

and thermal conductivity of Co–Ni–Al O composite2 3

coatings are shown in Figs. 16 and 17. When tempera-


ture is lower than 2508C, CTE of two types ofcomposite coatings are almost equal. But as the temper-ature above the 2508C, CTE of Ni-80Co-8.7 vol.%Al O coating is higher than that of Ni-20Co-8.5 vol.%2 3

Al O coating. Fig. 17 shows the effects of temperature2 3

and cobalt content of Co–Ni–Al O coatings on the2 3

thermal conductivity. The experimental results indicatethat there is a tendency for the thermal conductivity todecrease with increasing temperature from 150 to 4008C. The thermal conductivity of Ni-20Co-8.5 vol.%Al O is lower than that of Ni-80Co-8.7 vol.% Al O at2 3 2 3

the same temperature. The reason maybe is that the Ni-20Co-8.5 vol.% Al O coating has high degree of2 3

porosity and intergranular cracking, as shown in Fig. 11by AFM analysis. These microstructure flaws impedethe flow of heat current in the composite coatings,reducing the conductivity. Furthermore, due to anincrease of cobalt content in coatings, the number ofscattering sites of conduction electrons increases and itimprove the thermal conductivity of Co–Ni–Al O com-2 3

posite coatings.

4. Conclusions

Co–Ni–Al O composite coatings were studied by2 3

electrolytic codeposition of Co–Ni alloys and Al O2 3

from a sulfamate electrolyte and the following conclu-sions can be obtained.(1) Al O (vol.%) in Co–Ni alloys was attained2 3

maximum value at a particle content of 80 g l , ay1

current density of 3 A dm , a pH of 4.5, and a stirringy2

rate of 100 rpm.(2) The anomalous codeposition of cobalt–nickel

alloys cannot be changed by addition of Al O in2 3

electrolyte. However, the addition of Al O particles in2 3

electrolyte promotes the deposition of Co in the deposit.The presence of Co in the electrolyte also increases2q

the codeposition amount of Al O particles. The cathod-2 3

ic polarization of Co–NiyAl O electrolyte increases2 3

with the increase of Al O concentration in bath. Cobalt2 3

ions in the electrolyte leads to the decrease of cathodicpolarization while nickel ions do not change the polari-zation behavior.(3) Surface morphology and microstructure of Co–

Ni–Al O coatings are mainly influenced by the cobalt2 3

contents. In the high-cobalt region, the coatings withhcp lattice structure have a more uniform and finesurface structure than those of the low-cobalt coatingswith fcc lattice structure. The phase structure of solidsolution cannot be varied by codeposition of Al O2 3

particles in Co–Ni alloys, and it only influences thegrowth and orientation of crystal planes.

(4) The codeposition of Al O particles in the deposit2 3

greatly improves the hardness and the wear resistanceof composite coatings. However, it also increases thetensile internal stress of Co–Ni–Al O coatings. The2 3

CTE and the thermal conductivity of Co–Ni–Al O2 3

composite coatings are varied with the increase of Cocontents and the temperature.

Acknowledgments

The authors would like to thank Shanghai BaoShanIron & Steel Company for the financial support.

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Electrodeposited Co–Ni–Al O composite coatings