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Copyright 2007 American Scientic PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol. 7, 16, 2007

Synthesis of CuW Nanocomposite byHigh-Energy Ball Milling

T. Venugopal, K. Prasad Rao, and B. S. Murty

Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600036, India

The CuW bulk nanocomposites of different compositions were successfully synthesized by high-energy ball milling of elemental powders. The nanocrystalline nature of the CuW composite pow-der is conrmed by X-ray diffraction analysis, transmission electron microscopy, and atomic forcemicroscopy. The CuW nanocomposite powder could be sintered at 300400 C below the sinteringtemperature of the un-milled CuW powders. The CuW nanocomposites showed superior densi-cation and hardness than that of un-milled CuW composites. The nanocomposites also have threetimes higher hardness to resistivity ratio in comparison to Oxygen free high conductivity copper.

Keywords: CuW Nanocomposite, Immiscible System, High-Energy Ball Milling, ElectricalResistivity.

1. INTRODUCTION

Applications such as electronic packaging, manufacturingof electrodes or contact terminals require materials withhigh strength as well as high thermal and electrical con-ductivity. Cu is the most suitable material for such appli-cations, but suffers from lower strength. Traditionally Cuis strengthened by oxide and carbide dispersions but atthe expense of its electrical conductivity.13 In the recenttimes metalmetal composites are gaining importance. Inthis type of composites, both the matrix and dispersionsare metals and hence an insignicant reduction in electricalconductivity is expected due to dispersion. The immisci-ble systems are ideal cases for the metalmetal compos-ites. Cu and W are completely immiscible even in moltenstate due to a high positive heat of mixing: +35 kJ/mol.4As these elements are immiscible even in liquid state,solid state synthesis process is more appropriate to syn-thesize CuW nanocomposites with Cu as matrix andW as dispersoid. High-energy ball milling is one of thewidely used solid state processing technique to producenanocrystalline materials.511 A number of reports on high-energy ball milling of Cu and W deal with W rich sideof phase diagram.1215 Costa et al. gave a detailed accounton sintering behavior of WCu ball milled powders.15

They postulated that formation of composite particle dur-ing milling is a prerequisite to obtain good densica-tion in WCu powders. Raghu et al. studied mechanicalmilling of Cu-10.2 wt% W mixture and noted metastablemutual solubility aided by oxygen impurity.16 Xiong et al.

Author to whom correspondence should be addressed.

studied the mechanical alloying (MA) of W70Cu30 alloyand reported the substitutional solid solution formationin this composition.17 Gaffet et al. carried out a detailedstudy on entire range of CuW system by high energy ballmilling.18 They reported amorphous phase formation in thecomposition range from pure W to Cu70W30. It is evidentfrom the literature survey that the studies on the Cu richside of the CuW system are limited. The present studythrows light on the feasibility of Cu rich CuW nanocom-posites as promising materials for electronic industry.

2. EXPERIMENTAL DETAILS

The materials used in this study were 99.7% pure elec-trolytic Cu powder and 99.7% pure reduced W powderwith a particle size of

RESEARCHARTICLE

Synthesis of CuW Nanocomposite by High-Energy Ball Milling Venugopal et al.

Philips CM12 transmission electron microscope (TEM).The ball milled powders were compacted into pellets of12 mm in a hydraulic press at a pressure of 700 MPa.The pellets were sintered in a reducing atmosphere at 400,500, 600, and 700 C. The densities of the pellets wereestimated precisely by using Archimedes principle. Thesintered pellets were also analyzed by XRD. A Dimen-sion 3100 with Nanoscope IV, Digital Instruments, USA,Atomic force microscope (AFM) was used in tapping modewith a micro activated probe to assess the homogeneity andgrain size of nanocomposites. Hardness of nanocompositeswas measured by using Vickers micro-hardness tester at aload of 200 g. The electrical resistivity of the nanocompos-ites was also measured by using Vander-pauw technique.CuW composites were also made by compaction and sin-tering of un-milled Cu and W elemental powders in orderto compare its behavior with that of nanocomposites syn-thesized by high-energy ball milling.

3. RESULTS AND DISCUSSION

Figure 1(a) shows the XRD patterns of Cu-25 wt% Wpowder mixture at different stages of ball milling. It isclearly evident from the XRD patterns that with increasingthe time of milling, the X-ray peaks become broader. TheX-ray peak broadening can be attributed to the combinedeffect of ne grain size and lattice strain. No peak shift isobserved in both Cu and W indicating that no solid sol-ubility of either Cu in W or W in Cu could be achievedduring milling. There have been reports suggesting thathigh-energy ball milling process results in extended solidsolubility in many systems.7920 However, as Cu and Ware completely immiscible with very high positive heat ofmixing no solubility is observed in the present case. Thisresult is in conformity with the earlier reports by Aboudet al.13 and Gaffet et al.17 who also found no solubility ofCu and W in each other on mechanical alloying. The XRDpatterns of all other compositions showed a similar trend.Figure 1(b) shows the XRD patterns of all the compositionsafter ball milling for 20 h. It is evident from these XRD pat-terns that the XRD peaks of both Cu and W broaden withincrease in W content of the elemental powder mixtures.The crystallite size of the Cu and W for Cu-25 wt% W

composite powder calculated from X-ray peak broadeningis presented in Figure 2(a) as a function of milling time.With increase in milling time the crystallite size of bothCu and W were decreased. After 20 h of high-energy ballmilling the crystallite size of Cu is 36 nm and that of Wis 38 nm. Figure 2(b) shows the crystallite size of Cu andW for all the compositions after ball milling for 20 h. Itis interesting to note a signicant reduction in crystallitesize of W as a function of composition of the blend, whilethat in case of Cu is less signicant. The crystallite sizeof Cu and W reach more or less the same value of below40 nm after 25 wt% W in the blend. The crystallite size

40 50 60 70 80 90 100

(a) Cu-25 wt% W

2 (degrees)

5 h

20 h

15 h

10 h

0 h

- Cu- W

Inte

nsity

(A.U

.)

40 50 60 70 80 90 100

20 h of milling(b)

W - 30 wt%

W - 25 wt%

W - 20 wt%

W - 15 wt%

W - 10 wt%

W - 05 wt%

- Cu- W

Inte

nsity

(A.U

.)

2 (degrees)Fig. 1. XRD patterns of (a) Cu-25 wt% W nanocomposite powdersafter different milling times. (b) CuW nanocomposite powders of var-ious compositions after 20 h of ball milling.

of Cu is 29 nm and that of W is 33 nm for a composi-tion of Cu-30 wt% W after 20 h of milling. The presentresults conrm that the ner grain size can be obtainedin compositions with higher W content. Cu has an FCCstructure and is more ductile than W which has BCC struc-ture. The inherent nature of the high-energy ball millingis such that the ductile powder particles get attened bythe ball-powder-ball collisions, while the less ductile pow-der particles tend to fragment. The fragmented brittle pow-der particles get embedded in the ductile particles duringmilling. This resulted in the reduced ductility of the com-posite particles. As the W content increases the ductility ofthe composite particles also reduces which results in moreof fragmentation. Hence a ner grain size is obtained incomposite powders with higher W content.Figure 3 shows the TEM dark eld image of Cu-

20 wt% W nanocomposite powder after 20 h of ballmilling. The inset of the Figure 3 represents the

2 J. Nanosci. Nanotechnol. 7, 16, 2007

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Venugopal et al. Synthesis of CuW Nanocomposite by High-Energy Ball Milling

5 10 15 20

40

60

80

100

120Cu-25 wt.%W(a)

Crys

tallit

e siz

e (nm

)

Milling Time (h)

Crystallite Size of CuCrystallite size of W

5 10 15 20 25 3020

40

60

80

100

12020 h of milling

(b)

Crystallite Size of CuCrystallite Size of W

Crys

tallit

e siz

e (nm

)

wt% Tungsten

Fig. 2. (a) Crystallite size of Cu and W for Cu-25 wt% W as a functionof milling time. (b) Effect of composition on crystallite size of Cu andW after 20 h of ball milling.

diffraction pattern with continuous rings, which conrmsthe nanocrystalline nature of the composite powder. Thebright areas in the micrograph correspond to W crystallites.The crystallite size calculated from X-ray peak broadeningis in close agreement to that of TEM observations.Figure 4(a) represents the densication behavior of

CuW nanocomposite powders at various sinteringtemperatures. The green densities of the CuW nanocom-posite pellets vary from 80% to 85% of their respectivetheoretical densities. With increase in sintering tempera-ture, the density of the pellets increases. For compositionsCu-5 to 20 wt% W, the peak density of about 95% wasreached at 500 C and for the compositions Cu-25 wt% Wand Cu-30 wt% W, the peak density was obtained at600 C. The sintered density of the nanocomposites wasin the range of 93% to 95% of their respective theoreticaldensity, irrespective of their composition at 600 C withoutany further change at higher temperatures. Figure 4(b)shows the densication behavior of un-milled CuW pow-der mixture at various sintering temperatures. The densityof the green pellets was in the range of 77% to 81% oftheir respective theoretical densities. During sintering these

Fig. 3. Microstructure of Cu-20 wt% W ball milled powders after 20 hof ball milling. The inset shows the selected area diffraction pattern fromthe sample.

CuW composites reached their peak density over a tem-perature range of 900 and 1000 C. It has been observedthat the green density of the pellets decreased with increasein W content. This may be due to the poor deformationbehavior of W. From these results one can also observe thatthe amount of W in Cu plays a dominant role in obtaininggood density. With increase in W content the nal densitywas lower and also the sintering temperature to achieve agiven density increased. This may be attributed to the highmelting point and low diffusion rates of W. One can alsoobserve that the ball milled powders attains a higher den-sity than that of un-milled powders. During high-energyball milling, the harder W particles are embedded in theCu particles as Cu is more ductile than W. As millingcontinues the Cu particles and the embedded W particlesbecome ner leading to nanocomposites. During sinteringof milled nanocomposite particles, the Cu phase in eachparticle sinters together due to its lower melting point. Inaddition, the nanocrystalline nature of the crystallites andthe milling induced defects also help in increasing the dif-fusivities leading to better sinterability of the milled com-posites in comparison to unmilled composites.Figure 5 shows the crystallite size of as milled CuW

nanocomposites and after sintering at 500 C. In general,the grain growth of the CuW composites during solid-statesintering is negligible because Cu and W are completelyimmiscible. In an immiscible system, the diffusion of onespecies into other is restricted. As diffusion is one of theessential requisites for grain growth, in such immisciblesystems not much of grain growth is expected on heating.This is clearly evident from Figure 5, wherein the crystallitesize of both Cu and W do not signicantly increase by sin-tering at 500 C. It is also interesting to see that an increasein the W content of the nanocomposite reduces the graingrowth of both Cu and W. Increase in W content reduces

J. Nanosci. Nanotechnol. 7, 16, 2007 3

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400 500 600 70060

70

80

90

100milled for 20 h

(a)

Cu-5 wt% WCu-10 wt% WCu-15 wt% WCu-20 wt% WCu-25 wt% WCu-30 wt% W

Gre

en

% T

heor

etica

l Den

sity

Temperature C

600 700 800 900 100060

70

80

90

100unmilled

(b)

Cu-5 wt% WCu-10 wt% WCu-15 wt% WCu-20 wt% WCu-25 wt% WCu-30 wt% W

Gre

en

% T

heor

etica

l Den

sity

Temperature C

Fig. 4. Densication behavior of CuW (a) 20 h ball milled and(b) un-milled powders at various temperatures.

5 10 15 20 25 3020

40

60

80

100

120

140

160Crystallite size of Cu after sinteringCrystallite size of W after sinteringCrystallite size of Cu before sinteringCrystallite size of W before sintering

Crys

tallit

e siz

e (nm

)

wt% Tungsten

Fig. 5. Crystallite size of Cu and W as measured from X-ray peakbroadening after sintering at 500 C for all the compositions.

the probability of CuCu and WW contacts and increasesthe probability of CuW interfaces and hence their ten-dency for grain growth due to the immiscible nature of thetwo components.Figures 6(a) and (b) are the AFM images of

Cu-10 wt% W and Cu-20 wt% W bulk nanocompositesobtained in tapping mode technique. The nano-crystallinenature of the composites is clearly evident from theseimages. It is also important to note that the crystallite arener in case of 30 wt% W sample in comparison to 10 wt%sample. Figures 7(a) and (b) represents the AFM phasecontrast images of Cu-10 wt% W and Cu-20 wt% W bulknanocomposites. AFM phase contrast image is obtained bymapping the phase lag of the cantilever oscillation duringtapping mode scan. This phase lag is very sensitive to elas-tic properties of the materials and hence can distinguish themicro constituents based on their elastic properties. As Cuand W have quite different elastic modulii, one can easilysee the difference between the two in such an image andthus this image is similar to back scattered electron imagein SEM, where the contrast arises from the difference in theatomic number (number of electrons) of the constituents.

Fig. 6. AFM images of pallets sintered at 500 C clearly reveals thenano grains of Cu and W in (a) Cu-10 wt% W, (b) Cu-20 wt% W.


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Venugopal et al. Synthesis of CuW Nanocomposite by High-Energy Ball Milling

Fig. 7. AFM phase contrast images of pallets sintered at 500 Cshows the distribution of Cu and W in (a) Cu-10 wt% W, (b) Cu-20 wt% W.

Table I. Hardness and resistivity of CuW composites.

Electrical Hardness/resistivity HR ratioMilling resistivity Ratio (HR ratio) normalized

Composition condition Hardness (MPa) (-cm) MPa/-cm with OFHC Cu

OFHC Cu 369a 1.72a 214.5 1Cu-5% W unmilled 491 1.97 249.0 1.16

milled 1286 2.04 629.6 2.93Cu-10% W unmilled 526 2.26 232.5 1.08





milled 2529 3.94 641.4 2.99

aFrom Ref. [22].

5 10 15 20 25 30400

800

1200

1600

2000

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2800

2.0

2.5

3.0

3.5

4.0

4.5Hardness of composite (unmilled)Hardness of Nanocomposite

Har

dnes

s (M

Pa)

Wt% Tungsten

Resistivity of composite (unmilled)Resistivity of Nanocomposite

Res

istiv

ity (

-cm

)

Fig. 8. Effect of composition on the hardness and electrical resistiv-ity of CuW bulk nanocomposites and CuW composites made fromun-milled powders.

In the above images, the light color corresponds to Cuand dark color corresponds to W. A homogeneous distri-bution of Cu and W is evident from these images. Theseimages also indicate that the microstructure is ner in caseof 30 wt% W sample in comparison to 10 wt% W sample.The Vickers micro-hardness was measured on sintered

bulk CuW nanocomposites and also on sintered CuWcomposites synthesized from un-milled powders. Theseresults are presented as a function of composition inFigure 8. The nanocomposites showed a superior hardnessvalues than that of un-milled composites. This is due to thener grain size of nanocomposites, in agreement with Hall-Petch relation. From this gure one can also observe thatthe hardness of composites synthesized from ball milledpowders and un-milled powders increases signicantlywith an increase in W content. This increase in hardness isdue to the dispersion strengthening of W in Cu matrix.Figure 8 also shows a plot of electrical resistivity

measured on the sintered nanocomposites and also onsintered CuW composites synthesized from un-milled

J. Nanosci. Nanotechnol. 7, 16, 2007 5

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5 10 15 20 25 300

1

2

3

4H

R ra

tio n

orm

aliz

ed w

ith O

FHC

Cu

wt.% Tungsten

Cu-W composite (unmilled)Cu-W Nanocomposite

Fig. 9. Effect of composition on the normalized hardness to electricalresistivity (HR) ratio of CuW composites made from milled and un-milled powders.

powders as a function of composition. The resistivity ofthe nanocomposite increases with increase in W content.Two reasons may be cited for this increase in resistiv-ity. Firstly, with increase in W content the grain size isreduced. A ner grain size material will have higher vol-ume of grain boundary area which acts as obstacles forcurrent conduction path. Secondly, with increase in W con-tent the less conductive material in the composite increasesand hence the resistivity of the composite increases. Thisfact is also evident from the resistivity plot of compositefrom un-milled powders. The resistivity of CuW compos-ite from un-milled powders also increases with increasein W content. The resistivity of Cu-30 wt% W nanocom-posite is about 3.94 -cm. Cheng et al. reported theresistivity of 5 h ball milled copper as 3.36 -cm.21

The resistivity of oxygen free high conductivity (OFHC)Cu is 1.71 -cm,22 which indicates that the resistivityreduces to about 51% of that of OFHC Cu in case ofnanocrystalline Cu and with the addition of 30% W in thenanocomposite it further decreases only by 7% reachingabout 44% of that of OFHC Cu. The hardness and elec-trical resistivity values of all the composites are given inTable I. The hardness to resistivity ratio (HR ratio) wascalculated and normalized with that for OFHC Cu and theresults are shown in Table I. Figure 9 shows the variationof normalized HR ratio of CuW composites as a func-tion of composition. Interestingly, CuW nanocompositesshowed a higher normalized HR ratio than unmilled CuWcomposites. This indicates that the increase in hardness ismuch higher than the increase in the resistivity in case ofnanocomposites. The results indicate that an increase inHR ratio of about three times is possible with the CuWnanocomposites in comparison to the OFHC Cu, whichis a signicant increase. These results suggest that the

CuW nanocomposites have good promise as high strengthconductors.

4. CONCLUSIONS

(1) CuW bulk nanocomposites were successfully synthe-sized by high energy ball milling process followed by coldcompaction and sintering.(2) The sintering temperature of the nanocomposite pow-ders is 300400 C lower than that for un-milled CuWcomposite powders.(3) Nanocrystallinity of the milled CuW nanocompositesis retained even after sintering to near theoretical density(95%), which is evident by AFM and XRD crystallitesize analysis.(4) W delayed the grain coarsening of Cu by pinning thegrain boundaries.(5) The bulk nanocomposites showed superior hardnessthan that of un-milled composites.(6) The hardness to electrical resistivity ratio ofCu-30% W nanocomposite is three times higher than thatof OFHC Cu.

References and Notes

1. C. Biselli, D. G. Morris, and N. Randall, Scripta Mater. 30, 1327(1994).

2. T. Venugopal, K. Prasad Rao, and B. S. Murty, Mater. Sci. Eng.A393, 382 (2005).

3. S. J. Hwang and J. H. Lee, Mater. Sci. Eng. A405, 140 (2005).4. A. R. Miedema, Philips Tech. Rev. 36, 217 (1976).5. A. Gleiter, Prog. Mater. Sci. 33, 223 (1989).6. C. C. Koch, Annu. Rev. Mater. Sci. 19, 121 (1989).7. B. S. Murty and S. Ranganathan, Int. Mater. Rev. 43, 101 (1998).8. C. Suryanarayana, Prog. Mater. Sci. 46, 1 (2001).9. D. L. Zhang, Prog. Mater. Sci. 49, 537 (2004).

10. C. Suryanarayana, Adv. Eng. Mater. 7, 983 (2005).11. D. B. Witkin and E. J. Lavernia, Prog. Mater. Sci. 51, 1 (2006).12. T. Aboud, B. Z. Weiss, and R. Chaim, Nanostruc. Mater. 6, 405

(1995).13. J. C. Kim, S. S. Ryu, Y. D. Kim, and I. H. Moon, Scripta Mater.

39, 669 (1998).14. Y. D. Kim, N. L. O, S. T. Oh, and I. H. Moon, Mater. Lett. 51,

420 (2001).15. F. A. da Costa, A. G. P. da Silva, and U. U. Gomes, Powder Tech.

134, 123 (2003).16. T. Raghu, R. Sundaresan, P. Ramakrishnan, and T. R. Rama Mohan,

Mater. Sci. Eng. A304, 438 (2001).17. C. S. Xiong, Y. H. Xiong, H. Zhu, T. F. Sun, E. Dong, and G. X.

Liu, Nanostruc. Mater. 5, 425 (1995).18. E. Gaffet, C. Louison, M. Harmelin, and F. Faudot, Mater. Sci. Eng.

A134, 1380 (1991).19. T. H. Keijser, J. I. Langford, E. J. Mittemeijer, and A. B. P. Vogels,

J. Appl. Cryst. 15, 308 (1982).20. O. Drbohalav and A. R. Yavari, Acta Metall. Mater. 43, 1799 (1995).21. S. Cheng, E. Maa, Y. M. Wang, L. J. Kecskes, K. M. Youssef, C. C.

Koch, U. P. Trociewitz, and K. Han, Acta Mater. 53, 1521 (2005).22. P. Robinson, Properties and Selection: Non Ferrous Alloys and

Special-Purpose Materials, ASM Handbook, edited by R. L. Steven,ASM International, Materials Park, Ohio (1990), Vol. 2, p. 265.

Received: 26 February 2006. Revised/Accepted: 13 September 2006.


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