Production and Characterization of Copper-Niobium Composite Electrocoatings

Alain Robin1,a, Jorge Luiz Rosa1,b, Messias Borges Silva2,c

1Departamento de Engenharia de Materiais

2Departamento de Engenharia Química

Escola de Engenharia de Lorena – Universidade de São Paulo

12600-000 Lorena – SP – Brazil

a alain@demar.eel.usp.br, b jlrosa@demar.eel.usp.br, c messias@dequi.eel.usp.br

Keywords: Electrocomposite, Copper, Niobium, Design of experiments.

Abstract. Copper-niobium composite electrocoatings were obtained by co-electrodeposition in

acidic copper sulfate bath containing suspended niobium particles. The amount of incorporated

particles was evaluated using a Central Composite Design (CCD) with three factors of control

(cathodic current density, stirring rate and particle concentration in the bath) at three levels each. A

great influence of particle concentration was observed. The stirring rate also had influence but to a

lower extent and the cathodic current density was the least significant factor. The combination,

both cathodic current density and particle concentration at the highest levels and stirring rate at the

lowest level, led to the highest amount of incorporated particles. The behavior was not linear

between the high and low levels for all factors. The roughness of the composites was higher than

the pure copper coatings and increased with increasing current density. The microhardness of the

composite layers was higher than that of pure copper deposits obtained under the same conditions

due to copper matrix grain refinement and increased with the increase of both current density and

incorporated particle volume fraction.

Introduction

Electrodeposition is widely used for obtaining metallic layers such as copper, nickel,

chromium, zinc, tin and noble metals. In order to improve the properties of the coatings

(mechanical, wear and corrosion resistance), the incorporation of solid particles during metal

electrodeposition (co-electrodeposition) is an attractive method [1] This technique consists in

electrolyzing a solution containing the metallic salts and the particles in suspension. The electrolysis

parameters (composition of the bath, pH, temperature, cathodic current density, stirring rate) and

the particle characteristics (type, mean size and concentration in the bath) generally determine the

amount of incorporated particles, and consequently the composite coating properties.

The copper electroplates have high electrical and thermal conductivity, good ductility and

good corrosion resistance, but show low mechanical and wear resistance. In order to improve or

modify the properties of copper coatings, particles of ceramic [2], metallic or intermetallic [3] and

polymeric materials [4] can be incorporated to copper.

The aim of the present work was the production of copper-niobium (Cu-Nb)

electrocomposites from acidic copper sulfate bath and their characterization. The influence of

cathodic current density, particle concentration in the bath and stirring rate on the incorporated

particle volume fraction and on the characteristics of the coatings (morphology, roughness and

microhardness) was analysed. The properties of the composite coatings were compared with those

of pure copper deposits.

Experimental procedure

Cu-Nb composite coatings were obtained by co-electrodeposition in 150 gL-1

CuSO4 + 30

gL-1

H2SO4 electrolyte containing suspended Nb particles. The Nb particles of 15 µm mean size

Materials Science Forum Vol. 805 (2015) pp 184-189Online available since 2014/set/12 at www.scientific.net© (2015) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.805.184

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were maintained in suspension by magnetic agitation at 240, 400 and 550 rpm stirring rate and the

particle concentration in the bath was 10, 30 and 50 gL-1

.

The electrodeposition runs were performed on AISI 1020 carbon steel cathodes of 100mm x

6mm x 1mm dimensions, previously ground to a 600 grit finish. During the electrolysis, one

cathode was placed at the center of a cylindrical electrolytic copper anode of 40 mm diameter,

previously etched in dilute HNO3. Cathodic current densities of 10, 20 and 30 mAcm-2

were applied

for 7h40min, 3h50min and 2h33min, respectively, in order to obtain deposits of approximately 100

µm thickness.

The effect of current density, particle concentration and stirring rate of the electrolyte on the

incorporated particle volume fraction was studied. In order to reduce the number of experiments, a

Central Composite Design (CCD) with three-factors at three levels each (coted -1, 0 and +1) was

used. The factors of control were: (A) cathodic current density, (B) stirring rate and (C) particle

concentration in the bath. The corresponding values are presented in Table 1. A total of twelve

experiments, including four replicates at center point, was designed. The corresponding CCD is

shown in Table 2. The different experiments were performed randomly. Analysis of variance

(ANOVA) was used in order to analyse the effects of the factors of control and interactions. The

statistical analysis was performed using Statistica 6.0 software.

Table 1: Control factors and their respective levels.

Fator Low (-1) Intermediate (0) High (+1)

A: Cathodic current density / mAcm-2

10 20 30

B: Stirring rate / rpm 240 400 550

C: Concentration of Nb particles in the bath / gL-1

10 30 50

Table 2: Factor settings and incorporated particle volume fraction.

Experiment number

Level of control factor Incorporated

particle volume

fraction / % A B C

1

2

3

4

5

6

7

8

9

10

11

12

-1

+1

-1

+1

-1

+1

-1

+1

0

0

0

0

-1

-1

+1

+1

-1

-1

+1

+1

0

0

0

0

-1

-1

-1

-1

+1

+1

+1

+1

0

0

0

0

0.66

1.29

0.76

1.12

7.35

10.91

2.27

5.46

4.46

6.05

5.55

4.40

Scanning electron microscopy was used to evaluate the coating morphology, the distribution

and the amount of incorporated particles. The percentage of incorporated particles in the deposits

was determined by image analysis of polished cross-section photomicrographs using the Image

Tool 2.0 free software. The analysis was performed on about twenty photographs of each coating

taken at 1000 times magnification. The mean volume fraction of incorporated particles was

calculated from the twenty values obtained.

The roughness and microhardness of the composite coatings were also evaluated.

The morphological and mechanical characteristics of Cu-Nb electrocomposites were

compared to those of pure copper electrocoatings obtained under the same electrolysis conditions.

Materials Science Forum Vol. 805 185

Results and discussion

Coating morphology and incorporated Nb particle content

Figures 1a shows the surface morphology of a Cu-Nb composite layer. All Cu-Nb composite

coatings were well-crystallized. Nevertheless, the morphology of the deposits depend on

experimental conditions. The coatings became coarser and nodules appeared growing at the surface

as the cathodic current density increased. Figure 1b presents a SEM micrograph of composite

coating surface taken at higher magnification where the Nb particles (light regions) are cleary

identified.

Fig. 1: (a) Surface morphology of a Cu-Nb composite coating; (b) evidence of Nb particles in the

copper matrix.

Figure 2 shows the variation of surface roughness of pure copper and Cu–Nb composite

coatings as a function of cathodic current density. The pure copper deposits displayed substantial

smoothness (roughness lower than 2.0 µm) when compared to the composites and their roughness

did not depend significantly on current density. In the other hand, the roughness of the composite

coatings increased with the increase of current density. This increase of roughness is related to the

nodular growth of the composite coatings observed when current density increases. No noticeable

influence of stirring rate and Nb particle concentration in the bath on roughness was found. No

defined relation between roughness of the Cu–Nb composite coatings and incorporated Nb particle

content could be deduced from the results.

Figure 3 shows SEM micrographs of the cross-section of composite coatings obtained under

different experimental conditions. The Nb particles appear as light spots in the darker copper

matrix. The distribution of the Nb particles was homogeneous for all composite coatings but the

incorporated particle volume fraction was dependent on experimental conditions. It can be noted

that the size of the embedded particles range from 1 to 10 µm. The incorporated particle volume

fraction in the composite coatings obtained is reported in Table 2.

The Pareto chart of the effect of the factors on the incorporated particle volume fraction is

shown in Figure 4a. Pareto chart is a bar chart that displays the relative importance of various

factors and puts them in order from the most to the least significant. Figure 4a indicates with 95%

confidence that particle concentration in the bath (C) was the most significant factor, followed by

stirring rate (B). The interaction between these factors (B×C) was also significant.

186 A Special Issue in Memory of Dr. Lucio Salgado

10 15 20 25 300

1

2

3

4

5

6

7

pure Cu

Cu-Nb composites

Ro

ugh

ness R

a / µ

m

Current density / mA cm-2

Fig. 2: Ra mean surface roughness of pure copper and Cu-Nb composite coatings as a function of

cathodic current density.

Fig. 3: Cross-sections of Cu–Nb composites obtained under different experimental

conditions: (a) 30 mAcm−2

, 240 rpm, 10 gL−1

; (b) 20 mAcm−2

, 400 rpm, 30 gL−1

.

The values of F in the ANOVA of incorporated Nb particle volume fraction (Table 3)

confirm the significant effect of particle concentration in the bath (C), followed by stirring rate (B)

and (B×C) interaction. Current density (A) is less significant than (C), (B) and (B×C). (A×B) and

(A×C) interactions have no significance. In ANOVA, for a degree of freedom of 1 for the

numerator (effect) and 5 for the denominator (error), the factor is significant with 95% confidence

if F exceeds 6.61, and with 90% confidence for F higher than 4.06 [5] or if p-value is lower than

0.05 and 0.1, respectively.

The main effect plot for the amount of incorporated particles (Figure 4b) indicates that

particle concentration (C) has more influence, followed by stirring rate (B) and finally current

density (A). Moreover, the behavior between the high (+1) and low (−1) levels is not linear for the

three factors. From Figure 4b, the best condition of adjustment of control factors in order to have

the greatest incorporated particle volume fraction is current density (A) to the high level (+1),

stirring rate (B) to the low level (−1) and particle concentration to the high level (+1).

The relative unsignificance of the current density on the incorporated particle volume

fraction could be related to the diffusion control of the copper cation deposition. The increase of the

incorporated particle volume fraction with increasing particle concentration in the bath can be

explained by the higher number of particles reaching the cathode. Increasing stirring rate can lead to

the removal of freshly adsorbed particles due to collision factor.

Materials Science Forum Vol. 805 187

(a)

(b)

Fig. 4: (a) Pareto chart of the effect of factors and interactions on the incorporated Nb particle

volume fraction; (b) main effect plot for the amount of incorporated Nb particles; (A) current

density, (B) stirring rate and (C) particle concentration.

Table 3: Analysis of variance of average response of incorporated Nb particle volume fraction.

SS df MS F p

A 7,4885 1 7,48845 5,24497 0,070633

B 14,0450 1 14,04500 9,83723 0,025770

C 61,3832 1 61,38320 42,9932 0,001237

AxB 0,0512 1 0,05120 0,03586 0,857250

AxC 4,1472 1 4,14720 2,90473 0,149043

BxC 13,6764 1 13,67645 9,57909 0,027006

Error 7,1387 5 1,42774

Total 107,930 11

Microhardness

The variation of microhardness of pure copper and Cu–Nb composite coatings with cathodic

current density and incorporated Nb particle content is shown in Figures 5a and 5b, respectively. An

increase of microhardness with increasing cathodic current density occurs for both deposits (Figure

5a) but the microhardness of the composites was higher than that of pure copper. The increase in

microhardness of pure copper coatings with increasing current density was attributed to copper

grain refinement, due to a higher copper crystal nucleation rate. Comparing composite coatings with

nearly the same Nb particle content, the microhardness increased with increasing current density.

This proves that a reduction of the copper matrix grain size also occurred for the composites with

increasing current density. At constant current density, microhardness increases with increasing of

the incorporated Nb particle content (Figure 5b). Particle-strengthening which is related to the

incorporation of hard particles and volume fraction above 20%, as well as dispersion-strengthening

which is associated to the incorporation of fine particles (<1 µm) and volume fraction lower than

15% [6] cannot explain the higher microhardness of composites when compared to that of pure

copper coatings. The matrix grain refining due to the nucleation of small grains on the surface of the

incorporated Nb particles, resulting in a general structural refinement, is probably responsible for

the higher microhardness of composites.

188 A Special Issue in Memory of Dr. Lucio Salgado

(a)

10 15 20 25 3070

80

90

100

110

120

pure Cu

Cu-Nb composites

Mic

roha

rdne

ss /

VH

N

Current density / mA cm-2

(b)

0 2 4 6 8 10

70

80

90

100

110

120

10 mA cm-2

20 mA cm-2

30 mA cm-2

Mic

rohard

ness / V

HN

Incorporated Nb particle volume fraction / %

Fig. 5: Microhardness of pure copper and Cu-Nb composite coatings as a function of (a) cathodic

current density and (b) incorporated Nb particle volume fraction.

Conclusions

Cu-Nb composite electrocoatings with uniform Nb particle distribution were obtained on

carbon steel by electrolysis of acidic copper sulfate bath containing suspended Nb particles.

Using DOE (Design of Experiments) Nb particle concentration in the bath was shown to be

the most significant factor for the incorporation of Nb particles in copper, followed by stirring rate.

Current density presented low significance. Under our experimental conditions, the highest Nb

particle concentration, the lowest stirring rate and the highest current density led to the largest

amount of incorporated Nb particles.

The composite coatings were rougher than pure copper layers and their roughness increased

with the increase of current density.

The microhardness of the composite coatings was higher than that of pure copper due to

copper matrix grain refinement and increased with the increase of both cathodic current density and

incorporated Nb particle content.

Acknowledgments

A. Robin acknowledges CNPq (National Council for Development and Research - Brazil) for

financial support.

References

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[3] V. Medeliene, M. Kurtinaitiene, G. Bikulcius and V. Stankevic:Surf. Coat. Tech. Vol. 200

(2006), p. 6123.

[4] J.G. Tang, K. Hu, S.H. Fu, H.J. Qi, Z.S. Jia, K. Li, H.K. Pang and F.H. Wang:J. Appl. Polym.

Sci. Vol. 69 (1998), p. 1159.

[5] P.J. Ross: Taguchi Techniques for Quality Engineering: (McGraw-Hill, New York, 1996).

[6] D I. Garcia, J. Fransaer and J.P. Celis: Surf. Coat. Technol. Vol. 148 (2001), p. 171.

Materials Science Forum Vol. 805 189

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