Sintering Behavior of Ultra Fine MoCu Composite Powders 1-s2.0-S0263436813002011-Main (1)

8/13/2019 Sintering Behavior of Ultra Fine MoCu Composite Powders 1-s2.0-S0263436813002011-Main (1)

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The sintering behavior of ultra-ne MoCu composite powders and the sintering

properties of the composite compacts

Dezhi Wang a,b, Xiaojia Dong a, Pan Zhou a, Aokui Sun a, Bohua Duan a,b,a School of Materials Science and Engineering, Central South University, Changsha 410083, Chinab Key Laboratory of Nonferrous Metal Materials Science and Engineering, Ministry of Education, Central South University, Changsha 410083, China

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

Article history:

Received 8 May 2013Accepted 16 September 2013

Keywords:

MoCu

Ball-milling

Sintering

Microstructure

Properties

Nanocrystalline Mo25 wt.%Cu compositepowders were synthesizedby ball-milling, calcinating and subsequent

hydrogen reduction process. MoO3and CuO powders were used as precursors. The sintering behavior of ultra-

ne MoCu composite powders and the sintering properties of the composite compacts were investigated. The

densication, microstructure, hardness, electrical conductivity, thermal conductivity and coefcient of thermal

expansion were tested after solid phase sintering and liquid phase sintering. Relative density near 96% was

achieved for the specimen which was compacted under a very low pressure of 32 MPa and sintered at

1050 C. It reveals that high-energy ball milling increases the contribution of solid phase sintering of Mo and

Cu particles on the densication. The microstructure of the sintered compacts observed by scanning electron

microscopy showed homogenous dispersion of Mo and Cu phase. Thenal product showed good physical and

mechanical properties.

Crown Copyright 2013 Published by Elsevier Ltd. All rights reserved.

1. Introduction

Mo

Cu composites with 20

40% copper are widely used for theheavy dutyservice contacts dueto their excellent properties like lowco-

efcient of thermal expansion, wear resistance, high temperature

strength and prominent electrical and thermal conductivity [1]. The

conventional processes for fabrication of the MoCu composites are in-

ltration of copper to molybdenum skeleton and liquid phase sintering.

However, it is difcult to obtain high-density MoCu composites as a

resultof the mutual insolubility between Mo and Cu, or the high contact

angle of liquid copper on molybdenum [2,3]. The sinterabilityof MoCu

powders can be increased through an activated sintering process by

addition of a small amount of metal such as Co, Ni, or Fe. However,

such activators exhibit a negative inuence on the electrical and ther-

mal properties of the MoCu alloys[4]. Recently, many investigations

have been performed to synthesize the nanoscale MoCu powders,

since the sinterability can be improved by decreasing the particle size

and enhancing the homogeneity of the starting powders[58]. Modi-

cation of particle size and distribution can be achieved by mechanical

alloying (MA) [9]. Unlike conventional MoCu powders, the

sintering of nanocrystalline MoCu mixtures synthesized by MA is

signicantly enhanced at solid phase sintering temperature [4]. In

the case of MoCu composites prepared by MA, solid phase sintering

could have an adverse effecton therearrangement process duringliquid

phase sintering.Although most reports showthat the maximum relative

density is achieved during liquid phase sintering, it is undeniable that

solid phase sintering plays an important role in the densication of the

nanocrystalline materials[10,11].In our previous study [12], we have reported a simple route to

synthesize ultra-ne and well-dispersed MoCu nanocomposites. The

sintering behavior of ultra-ne composite powders and the sintering

properties of the composite compacts are investigated at present re-

search. It is conrmed that the contribution of solid phase sintering on

the densication is signicant.

2. Experimental procedures

The characteristics of the starting powders are listed in Table 1.

MoO3 and CuO powders with a mass radio of 3.6:1 were uniformly

blended and then calcined in air atmosphere at 530 C to obtain

CuMoO4MoO3mixtures. Ball milling was carried out in a Planet-Ball-

Grinding machine at a rotational speed of 400 rpm for 20 h in air atmo-

sphere. The ball milled CuMoO4MoO3 mixtures werenally reduced in

hydrogen atmosphere (dew point 30 C to 40 C) at 650 C for

1.5 h in a tube-type electrical furnace. The ow rate was 0.8 L min1,

and the height of powder bed was 12 mm.

The resultant MoCu nanocomposite powders were compacted in a

steel die under the pressure of 32 MPa to produce green parts. Sintering

was performed in a tube-type electrical furnace with a heating rate of

10 C min1 at different temperatures, ranging from 950 C to

1100 C, for 90 min under hydrogen atmosphere. The densities of the

sintered compacts were measured according to Archimedes' principle.

The hardness of the specimens was determined by a Vickers hardness

tester. The scanning electron microscope was used for microstructure

Int. Journal of Refractory Metals and Hard Materials 42 (2014) 240245

Corresponding author at: School of Materials Science and Engineering, Central South

University, Changsha 410083, China. Tel.: +86 731 88877221; fax: +86 731 88830202.

E-mail address:[email protected](B. Duan).

0263-4368/$ see front matter. Crown Copyright 2013 Published by Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.ijrmhm.2013.09.012

Contents lists available at ScienceDirect

Int. Journal of Refractory Metals and Hard Materials

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http://dx.doi.org/10.1016/j.ijrmhm.2013.09.012http://dx.doi.org/10.1016/j.ijrmhm.2013.09.012http://dx.doi.org/10.1016/j.ijrmhm.2013.09.012mailto:[email protected]://dx.doi.org/10.1016/j.ijrmhm.2013.09.012http://www.sciencedirect.com/science/journal/02634368http://www.sciencedirect.com/science/journal/02634368http://dx.doi.org/10.1016/j.ijrmhm.2013.09.012mailto:[email protected]://dx.doi.org/10.1016/j.ijrmhm.2013.09.012


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evaluation of the sintered samples. Electrical conductivity was exam-

ined using a four wire method by a micro-ohmmeter. Thermal conduc-

tivity and coefcient of thermal expansion were measured by thermal

diffusivity analyzer.

3. Results and discussion

The characteristics of the ultra-ne Mo25 wt.%Cu nanocomposite

powders synthesized by ball-milling have been reported in detail else-

where[12].Fig. 1 and Fig. 2show SEM and TEM images of the MoCu

composite powders. It can be seen that the MoCu powders compose

of superne spherical nanoparticles, with particle size ranging from 60

to 180 nm. These spherical particles exhibit high specic surfaces area,

regular shape and uniform size distribution, which make the MoCu

powders have excellent capability in sintering. Sintering behavior and

sintering properties are discussed in this work.

3.1. Sintering behavior

Fig. 3 shows the inuence of sintering temperature on shrinkage. As

sintering temperature rises, both radial and axial shrinkage of the

sintered specimens increase. Moreover, radial shrinkage is always higher

than axial shrinkage. This is because radial pressure is invariably lower

than axial pressure during the uniaxial pressing process. Hence the

green density in compact-pressing direction is higher than that in per-

pendicular to compact-pressing direction. When sintered at the same

temperature, radial shrinkage is always higher than axial shrinkage.

Theeffect of sintering temperature on density andrelative density ofsintered compacts are shown inFig. 4. It appears that the density and

the relative density of the specimens are increased by raising the

sintering temperature when sintered under 1100 C. At the tempera-

ture of 950 C, 1000 C and 1050 C, which are below the melting

point of copper, the sintering process is known as solid phase sintering.

During solid phase sintering, atomic diffusion ability was enhanced and

pores tended to decrease while the formation and growing of sintering

necks were accelerated. Meanwhile Cu element diffused from the bulk

to the surface of the composite particlesand linked to the other diffused

Cu elements from other particles to form a copper network all over the

structure. Formation of this network could improve the densication

[13,14]. So that an obvious increment in the densication was noticed

as the temperature increased. In contrast to solid phase sintering, densi-

ty and relative density are decreased when sintered at 1100 C (liquid

phase sintering). Nevertheless, both radial and axial shrinkage of the

MoCu samples are increased at 1100 C (Fig. 3). This phenomenon is

quite different from other researches [5,7,13,15]. Reasons for the re-

verse densication are concluded as follows. Firstly, some impurities

existing in powders vaporized into gases at the high temperature.

Then the gases were packaged in the specimens under the compacting

pressure. During sintering process, expands of these gasses resulted in

reverse densication. Secondly, pores which are resulted from the

large seepage of liquid copper increased during liquid phase sintering;

hence the density decreased, though the shrinkage increased.

Under the very low pressure of 32 MPa (most researches choose the

pressure of about 100 MPa), relative density of near 96% was achieved

for the compacts sintered at 1050 C by using MoCu composite pow-ders which were synthesized by coreduction of mechanical-activated

CuMoO4MoO3 mixtures. Owing to thenerMoCu nanoscale particles,

composite powders possess large specic surface area, high surface en-

ergy and high sinterability. Pores between particles are smaller; this

promotes diffusion rate and further facilitates the densication[16]. In

addition, the unique coated structure of MoCu composite powders

Table 1

Characteristics of raw powders.

Material Supplier Average particle

size (m)

Morphology Purity

(%)

MoO3 Tianjin Chemical

Development, China

1 Polygon 99.95

CuO Sinopharm Chemical

Reagent, China

5 Polygon 99.0

Fig. 1.SEM image of Mo

Cu composite powders.

Fig. 2.TEM image of MoCu composite powders.

Fig. 3.Effect of sintering temperature on shrinkage of sintered compacts.

241D. Wang et al. / Int. Journal of Refractory Metals and Hard Materials 42 (2014) 240245
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causes a homogeneous distribution of Mo phase and Cu phase, which is

benecial to sintering densication. Therefore, a maximum density of

near 96% was obtained at solid phase sintering temperature.

3.2. Microstructure of MoCu compacts

Fig. 5presents the cross-section microstructure of MoCu compacts

sintered for 1.5 h at the temperature of 950 C, 1000 C, 1050 C, and

1100 C. Large interparticle pores can be seen on the cross-section of

the specimen sintered at 950 C (Fig. 5a). The diffusion of Cu phase is

enhanced by increasing the sintering temperature to 1000 C, and the

distribution of Cu tends to be more uniform (Fig. 5b). The amount and

size of the pores are reduced, and a relatively dense and homogenousmicrostructure of the MoCu compacts is observed when sintered at

1050 C (Fig. 5c). Increasing the sintering temperature to 1100 C

results in signicant copper evaporation, generation of massive pores

and a decrease in density of the composite compacts (Fig. 5d).

Fig. 6shows the fractograph of MoCu samples sintered for 1.5 h at

1050 C and 1100 C. A dense and homogenous microstructure can be

seen apparently fromFig. 6(a), (c). On the whole, every Mo particle is

capsulated in continuous network structure of Cu. This network struc-

ture, an ideal sintering state, is favorable to strength, electrical conduc-

tivity and thermal conductivity of MoCu alloys.Fig. 6(b) andFig. 6(d)

reveal that the size of particles sintered at 1100 C is larger than that

sintered at 1050 C. In addition, a mass of pores come out as a result

of the loss of copper. When increasing the copper content, higher tem-

perature (over the melting point of copper) may cause a larger loss of

liquid copper.

Sintering properties of MoCu compacts.

Fig. 4. Effect of sintering temperature on density andrelativedensity of sintered compacts:

(a) density; (b) relative density.

Fig. 5.Cross-section microstructure of Mo

Cu compacts at different sintering temperature: (a) 950 C; (b) 1000 C; (c) 1050 C; (d) 1100 C.

242 D. Wang et al. / Int. Journal of Refractory Metals and Hard Materials 42 (2014) 240245
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3.2.1. Hardness

The effect of sintering temperature on the hardness is shown in

Fig. 7. It has been found that the tendency of Vickers hardness withdifferent temperature is similar to that of density (Fig. 4). Evidently,

the high density of the sintered samples is the main reason for high

hardness. During solid phase sintering, porosity of the sintered samples

decreases, and grain size grows as sintering temperature increases.

Hence the maximum value of hardness (214 HV) is achieved at the

temperature of 1050 C.

3.2.2. Electrical and thermal conductivity

Fig. 8 presents theelectrical and thermal conductivity of the sintered

compacts at different temperature. The electrical conductivity analysisresults present that the electrical conductivity increases gradually

from 950 C to 1050 C,and themaximum value of electrical conductiv-

ity (22.4 MS/m) is gained at 1050 C. When sintered at 1100 C (liquid

phase sintering), electrical conductivity of the compacts is lower than

that of those samples processed by solid phase sintering. The reason

Fig. 6.Fractograph of MoCu compacts at different sintering temperature: (a), (c) 1050 C; (b), (d) 1100 C.

Fig. 7.Effect of sintering temperature on the hardness of the sintered specimens.

Fig. 8. Effect of sintering temperature on electrical and thermal conductivity of the

sintered compacts.

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for this phenomenon is that the Cu phase inherently has a much higher

electricalconductivity than molybdenum.During liquid phasesintering,

a large number of pores reemerge inside the sintered compacts on

account of massive losses of copper. Thermal conductivity exhibits a

similar trend. From optical micrograph (Fig. 6c), we can clearly nd

out thatCu particles are connected together and almost every Mo parti-

cle is capsulated in continuous network structure of Cu. Consequently,

the heat can be fast transferred between Mo and Cu. Maximum value

of thermal conductivity is 147 Wm1 K1 for the samples sintered at

1050 C.

3.2.3. Coefcient of thermal expansion

Fig. 9 shows the coefcient of thermal expansion (CTE) of the

sintered samples at different temperature. The CTE analysis results indi-

cate that the CTE is unstable before 200 C because the temperature of

the samples is lower than the actual temperature during the primary

heating stage. Obviously, the CTE increases gradually from 200 C to

300 C, and the curves become at when the temperature is above300 C. It is known that the CTE of Cu is higher than that of Mo. Never-

theless, the higher CTE of Cu is constrained by the lower CTE of Mo in

MoCu composite[6]. Particles transportations and interdiffusion are

more easily occurring in nanoscaled MoCu composite obtained by

MA than those in conventional composite during solid phase sintering.

The distribution of Cu phase tends to be homogenous with the increas-

ing of solid phase sintering temperature; hence the CTE increases

gradually owing to the effect of Cu. When sintered at 1100 C (liquid

phase sintering), the sintered specimens reveal the lower CTE due to

the loss of Cu.

In summary, the overall performance of the Mo25 wt.%Cu compos-

ite sintered at 1050 C is improved slightly compared with other re-

searches [1719]. The comparison of the processing conditions and

properties of the composite from the present study to the prior researchstudies is listed in Table 2. However, it is worth reminding that the

compacting pressure in this study is 32 MPa, far lower than that

(100380 MPa) of other reports [8,16,20]. Supporting that we raise

the compacting pressure to a conventional degree, the density and the

relative density of MoCu composite may increase substantially. Then

other properties dependent on the relative density could have a corre-

sponding improvement.

4. Conclusion

In this work, we synthesized ultra-ne MoCu composite powders,

with nely dispersion and homogenous distribution of Mo and Cu

components. The following results were obtained:1. The superne MoCu powders showed remarkable sinterability at

1050 C which is below the melting point of copper (solid phase

sintering).

2. Maximum relative density (near 96%) of Mo25 wt.%Cu composites

compacted at the very low pressure of 32 MPa can be achieved at

low sintering temperature (1050 C). Homogeneous microstructure

and other excellent properties of sintered products were obtained

as well. Hardness, electrical conductivity, thermal conductivity and

coefcient of thermal expansion of the MoCu composites sintered

at 1050 C for 1.5 h are 214 HV, 22.4 MS/m, 147 Wm1 K1 and

8.5 106 K1, respectively.

3. Homogeneous microstructure could contribute to enhancing diffu-

sion rate of solid phase densication while the whole densication

process is controlled by solid phase diffusion. On the contrary, liquidcopper contributes little to densication of the Mo25 wt.%Cu com-

pacts, even produces some adverse effects, such as evaporation and

enhanced grain growth.

Acknowledgement

This work is supported by National Natural Science Foundation of

China (No. 51274246).

References

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Fig. 9. Coefcientof thermal expansion of the sintered compacts at different temperature.

Table 2

Comparison of the processing conditions and properties of the composite from the present study to the prior research studies.

Composition Processing conditions Properties of nal products

Sintering temperature

(C)

Sintering time

(min)

Atmosphere Relative density

(%)

Hardness Thermal conductivity

(Wm1 K1)

Electrical conductivity

(Ms/m)

CTE

(106 K1)

Mo25Cu 1050 90 H2 96 214HV 147 22.4 8.5

Mo30Cu[17] 1350 60 H2 90.12 68HRB 143.3 24.7 9.3

Mo30Cu[18] 1250 90 H2 92 52.8HRB 164.36 22.27

Mo18Cu1.5Ni[19] 1250 120 H2 99.2 72.5HRA 139 7.4

244 D. Wang et al. / Int. Journal of Refractory Metals and Hard Materials 42 (2014) 240245
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