The thermal expansion and mechanical properties of high reinforcement

content SiCp/Al composites fabricated by squeeze casting technology

Qiang Zhang*, Gaohui Wu, Guoqin Chen, Longtao Jiang, Bofeng Luan

School of Materials Science and Engineering, Harbin Institute of Technology, No. 92 West Dazhi Street, Harbin 150001, China

Received 16 November 2002; revised 14 May 2003; accepted 10 July 2003

Abstract

With mixing different sized SiC particles, high reinforcement content SiCp/Al composites (Vp 50; 60 and 70%) for electronic packagingapplications were fabricated by squeeze casting technology. The composites were free of porosity and SiC particles distributed uniformly in

the composite. The mean linear coefficients of thermal expansion (20100 8C) of SiCp/Al composites ranged from 8.3 to 10.8 1026/8C anddecreased with an increase in volume fraction of SiC content. The experimental coefficients of thermal expansion agreed well with predicted

values based on Kerners model. The Brinell hardness increased from 188.6 to 258.0, and the modulus increased from 148 to 204 GPa for the

corresponding composites. The bending strengths were larger than 370 MPa, but no obvious trend between bending strength and SiC content

was observed.

q 2003 Elsevier Ltd. All rights reserved.

Keywords: A. Metalmatrix composites; A. Particle-reinforcement; B. Coefficient of thermal expansion; B. Mechanical properties; Squeeze casting

1. Introduction

Particles reinforced aluminum matrix composites

(PRAMCs) have received much recognition in electronic

packaging and thermal management (such as power module

base plates, printed wiring board cores, microprocessor lids,

or electric enclosures), for their flexible fabrication

techniques, tailorable thermo-physical properties and excel-

lent specific mechanical properties [15]. Because tem-

perature cycling often exists in the operating environment of

electronic components, a reinforcement volume fraction of

.60% is necessary [1,5,6] for coefficient of thermalexpansion (CTE) matching with those of electronic

components or ceramic substrates to reduce thermally

induced stresses and increase the reliability of electronic

devices. The lifetime limitation of hybrid power modules

can be improved more than 10 times by replacing copper

with SiCp/Al baseplate [7].

Composites with 5575 vol% particle reinforcements

are produced by powder metallurgy [4,8] or pressureless

infiltration process [9,10]. However, limitations of these two

techniques include relatively higher cost, complicated

processing steps, longer production cycles or specific

protecting atmosphere. In contrast to them, squeeze casting

technology is believed to be an effective technique because

of the advantages of higher production rates, elimination of

expensive equipments, feasibility of mass production and

near-net products.

Accordingly, the purposes of present work are two-fold:

firstly, to produce high reinforcement content SiCp/Al

composites by squeeze casting process and examine their

microstructure; secondly, to investigate their thermal

expansion and mechanical properties, then to further

understand the feasibility of SiCp/Al composites for

electronic packaging.

2. Materials and experimental

2.1. Materials preparation

The fabrication of high reinforcement content composite

is related to the packing fraction of particles. It is believed

that the packing density of single size spheres is limited and

0.74 is the largest theoretical packing fraction. But in fact, it

is difficult to obtain such a higher packing. It was reported

1359-835X/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/S1359-835X(03)00253-7

Composites: Part A 34 (2003) 10231027

www.elsevier.com/locate/compositesa

* Corresponding author. Tel.: 86-451-864-2164; fax: 86-451-864-3922.

E-mail address: zhang_tsiang@sina.com (Q. Zhang).

that the packing fraction of random arrays of single size

spheres varied from 0.59 to 0.64, depending on the packing

conditions [11].

However, an increased packing fraction can be obtained

by mixing particles that have a proper particle size

distribution, because fine particles can pack more efficiently

around larger ones [1214]. Even a very high packing

fraction more than 90% could be achieved by mixing the

proper volume fraction of different particle sizes that

differed by several orders of magnitude [14].

On the basis of above analysis, three average particle

sizes of 20, 40 and 60 mm were used in the present study and

the particles were divided into three groups: (I) 20 mm SiC;

(II) 20 and 40 mm SiC with a weight ratio of 3:2; (III) 20 and

60 mm SiC with a weight ratio of 4:1. The composites of 50,

60 and 70% were produced by group (I), (II) and (III),

respectively. The aluminum matrix was an AlSi alloy

(4032Al) whose nominal compositions were listed in

Table 1. This alloy was chosen for the purpose of decreasing

CTE efficiently and of avoiding interfacial reaction between

aluminum and SiC. Table 2 summarized the properties of

SiC particle and 4032Al.

The SiCp/Al composites were fabricated by squeeze

casting technology, as illustrated in Fig. 1. Firstly, the SiC

particles with different sizes and weight ratios were mixed

in distilled water by mechanical stirring for 2 h, and dried

at 130 8C for 24 h. Then they were filled and pressed into

a mold to produce a SiC preform according to the given

volume fraction. After that, the preform was pre-heated in

a tool steel die. At the same time, the aluminum alloy was

melt, degassed, cleaned in a graphite crucible and heated

to 750800 8C. When the preform was heated to 500

600 8C, the molten aluminum was poured into the tool

steel die and a vertical pressure up to 100 MPa was

applied immediately to force molten aluminum to

infiltrate into SiC preform completely. The pressure was

maintained for about 5 min until the solidification was

complete.

2.2. Testing

An S-570 scanning electron microscope (SEM) was used

to examine the microstructure of as-fabricated SiCp/Al

composites. Prior to the following tests, the composites

were annealed at 410 8C for 3 h, and then furnace cooled.The CTE was measured on a DIL 402C Dilameter

(NETZSCH Corp.) with a heating rate of 5 8C/min. And ahelium atmosphere with a flowing rate of 50 ml/min was

used to keep the chamber temperature consistent. Based on

the measured curves of relative length changes versus

temperatures, the CTE was calculated between 20 and

100 8C. To diminish systematic errors, the dilatometer wascalibrated by measuring an alumina sample under identical

conditions.

Brinell hardness (HB) tests were performed on a HBV-30

double-purpose tester with a 1mm ball indenter. A load of

30 kgf was applied and maintained for 30 s. Brinell

hardness, instead of Vickers hardness measurement, was

used because Vickers hardness would be affected by the

reinforcements in an uncertain way depending on the

reinforcement position under and around the indentation.

Three-point bending tests with 30 mm span were carried out

at room temperature on an Instron5569 universal electron

tension testing system. The specimens were all 4 mm wide,

3 mm thick and 36 mm in length. Fractography observations

were carried out also on S-570 SEM.

Table 1

Chemical compositions of 4032Al (wt%)

Si Cu Mg Fe Ni Zn Al

4032Al 11.99 0.51.3 0.81.3 1.0 0.51.3 0.25 Bal.

Table 2

Properties of SiC particle [15] and 4032Al [16]

Density

(g/cm3)

CTE

( 1026/8C)

Youngs

modulus

(GPa)

Shear

modulus

(GPa)

Bulk

modulus

(GPa)

Poissions

ratio

SiC 3.18 4.7 450 192 225 0.17

4032Al 2.68 20.8a 69 29.7 77.5 0.33

a By experiments.

Fig. 1. Fabrication process of SiCp/Al composites.

Q. Zhang et al. / Composites: Part A 34 (2003) 102310271024

3. Results and discussion

3.1. SEM microstructure examination

Fig. 2 showed the SEM microstructure of as-fabricated

SiCp/Al composites. It was found that larger SiC particles

were necessary in larger volume fraction composites.

Though different SiC particles were used in the different

composites, SiC particles distributed uniformly in the

composite and fine SiC particles occupied the interstitial

positions around coarse particles efficiently. Three SiCp/Al

composites were all dense and macroscopically homo-

geneous, and seldom particles cluster was observed. A

dense microstructure was beneficial to electronic packaging

applications because of improvement in mechanical

strength and heat conductivity.

3.2. Thermal expansion analysis

The measured CTEs of SiCp/Al composites were 8.3, 9.7

and 10.8 1026/8C for the 70, 60 and 50% composites,respectively. The CTEs were reduced with an increase in

volume fraction of SiC. In a SiCp/Al composite, the thermal

expansion behavior is influenced by the thermal expansion

of aluminum matrix and the tightened restriction of SiC

particles. Since the restriction of SiC increased with the

increasing of particles content, the CTEs of SiCp/Al

composites were reduced. The CTEs were slightly higher

than those of ceramic substrates, but the difference in CTE

was favorable since it would make the substrates in

compression when cooling from elevated temperature to

ambient temperature during welding. The compressive

forces could reduce the probability of component cracking.

Several theoretical models were proposed to predict the

CTE of particulate composite. If the matrix modulus is

much smaller than that of reinforcement, the CTE of a

composite is expressed as rule-of-mixture [17] (ROM):

ac amVm apVp 1where a is the CTE, V is the volume fraction, and subscriptsc, m, p refer to the composite, matrix and particle,

respectively.

Turners model [18] considers the uniform hydrostatic

stresses and gives the CTE of a composite as:

ac amKmVm apKpVp

KmVm KpVp 2

where K is the bulk modulus.

Both the normal and shear stress are taken into account in

Kerners model [19], and the CTE of a composite is

expressed as:

ac amVm apVp VpVmap 2 am Kp 2 Km

VmKm VpKp 3KpKm=4Gm 3

where G is shear modulus.

Fig. 3 shows the comparison between above theoretical

predictions and experimental data. As could be seen,

experimental data were in good agreement with the

predicted values based on Kerners model, but deviated

from ROM and Turners model. This may be attributed to

the fact that normal and shear stress were included in

Kerners model, and ROM or Turners model could not

describe the complicated internal stresses inside a

composite.

Fig. 2. SEM micrographs of SiCp/Al composites. (a) 50% SiCp/Al composite, (b) 60% SiCp/Al composite, (c) 70% SiCp/Al composite.

Fig. 3. Comparison between theoretical predictions and experimental

CTEs.

Q. Zhang et al. / Composites: Part A 34 (2003) 10231027 1025

3.3. Mechanical properties

The mechanical properties of SiCp/Al composites were

shown in Table 3. As could be seen, the hardness of SiCp/

Al composites increased with the increasing of SiC

content. This could be ascribed to two factors. On one

hand, the effective loading volume increased as the

volume fraction of SiC increased. At the same time, SiC

particles were larger in the higher Vp composites. This

would enlarge the contact areas between indenter and

particle, then help to improve the hardness of SiCp/Al

composites.

The bending strengths of SiCp/Al composites were larger

than 370 MPa, with a maximum of 422 MPa. But no

obvious trend between bending strength and SiC content

was observed. It is believed that larger SiC particles in the

higher Vp composites tend to fracture brittly because larger

SiC particles had many larger defects and the bending stress

of SiC was dominated by the largest flaw. Therefore, besides

SiC content concerned, some other factors such as particle

size and particle defects would influence the bending

strength of the composite. Fig. 4 shows the bending

fractographs of SiCp/Al composites. The matrix failed in

a ductile manner, especially in the 50% SiCp/Al composite,

as shown in Fig. 4 (b). The SiC particles fractured brittly,

and some facets were found in the larger SiC particle,

indicating that fracture propagated through the particles

with shear mode.

Table 3

Mechanical properties of SiCp/Al composites

Brinell hardness Bending strength (MPa) Modulus (GPa)

Measured H-S model Measured/H-S model (%)

50% SiCp/Al 188.8(6.0) 422.0(12.6) 148(10.1) 164.3 90.1

60% SiCp/Al 208.7(5.5) 371.8(10.1) 165(7.3) 193.8 85.1

70% SiCp/Al 258.0(7.2) 390.7(10.4) 204(8.9) 231.4 88.2

Standard deviation was shown in parentheses.

Fig. 4. SEM bending fractographs of SiCp/Al composites (a) 50% SiCp/Al composite. (b) High magnification image of the framed area in (a). (c) 60% SiCp/Al

composite, (d) 70% SiCp/Al composite.

Q. Zhang et al. / Composites: Part A 34 (2003) 102310271026

However, the moduli of SiCp/Al composites increased as

the volume fraction of SiC increased. A Hashin-Shtrikman

model [17] (H-S model) was used to predict the modulus of

SiCp/Al composites. In this model, the modulus of a

composite is expressed as:

Ec EmEmVm EpVp 1EpVm EmVp 1 4

where E is the modulus.

The calculated data were also presented in Table 3. It

could be found that the measured moduli were lower than

the theoretical values, but reached their 85% of them,

indicating that H-S model was valid for the prediction of

composite modulus.

The density of SiCp/Al composite was about

3 103 kg/m3, resulting in a high specific modulus. Fig. 5illustrates the comparison of specific modulus among several

conventional packaging materials [1,3,5]. As could be seen,

the specific modulus of 70% SiCp/Al composite was more

than 3 times higher than those of preferred packaging

materialsKovar and Copper. A high specific modulus

would diminish electronic components while maintaining its

stiffness. Then the electronic devices could be made thinner

and lighter. Additionally, in cold plate design, the thermal

resistance was in inverse proportion to the thickness of plate

[20]. Therefore, the decrease in thickness could reduce

thermal resistance and enhance thermal diffusivity.

4. Conclusions

1. With mixing proper volume fractions of different particle

sizes, 50, 60 and 70% SiCp/Al composites were

fabricated by squeeze casting process, then the compo-

sites were macroscopically dense and homogeneous.

2. The linear CTEs of SiCp/Al composites ranged from 8.3

to 10.8 1026/8C, depending on the volume fraction of

SiC. The experimental CTEs were in good agreement

with predicted values based on Kerners model.

3. The Brinell hardness and modulus of SiCp/Al compo-

sites increased with an increase in the volume fraction of

SiC, and the bending strengths were larger than

370 MPa, but no obvious trend between bending strength

and SiC content was observed.

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Fig. 5. Specific moduli of conventional packaging materials and SiCp/Al

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Q. Zhang et al. / Composites: Part A 34 (2003) 10231027 1027

The thermal expansion and mechanical properties of high reinforcement content SiCp/Al composites fabricated by squeeze casting IntroductionMaterials and experimentalMaterials preparationTesting

Results and discussionSEM microstructure examinationThermal expansion analysisMechanical properties

ConclusionsReferences