Pressureless infiltration processing and thermal properties of SiC_p/Al composites with high SiC particle content

ZHOU Xian-liang(周贤良), ZOU Ai-hua(邹爱华), HUA Xiao-zhen(华小珍),

ZHANG Jian-yun(张建云), RAO You-hai(饶有海)

Department of Material Engineering, Nanchang Institute of Aeronautical Technology,

Nanchang 330034, China

Received 28 July 2006; accepted 15 September 2006

Abstract: SiC_p/1060Al, SiC_p/ZL101, SiC_p/ZL102 composites with SiC_p volume fraction of 55% were fabricated by pressureless infiltration. The microstructure was examined and thermal properties were characterized for SiC_p/Al composites. The results show that the composites are dense and macroscopically homogeneous. With the increase of temperature, the mean linear coefficient of thermal expansion(CTE) at 25-200 ℃ of the composites increases and ranges from 7.23×10^-6 to 10.4×10^-6 K^-1, but thermal conductivity declines gradually at the same time. With the increase of Si content in the Al matrix, CTE of the composites declines and thermal conductivity also declines but not linearly, when Si content is up to 7%, the average thermal conductivity is 140.4 W/(m?K), which is close to that of the SiC_p/1060Al composite (144.6 W/(m?K)). While Si content is 11.7%, the average thermal conductivity declines markedly to 87.74 W/(m?K). The annealing treatment is better than the solution aging treatment in reducing CTE and improving thermal conductivity of the composites. Compared with conventional thermal management materials, SiC_p/Al composites are potential candidate materials for advanced electronic packaging due to their tailorable thermo-physical properties.

Key words: SiC_p/Al composite; pressureless infiltration; thermal expansion coefficient; thermal conductivity

1 Introduction

SiC particle with high volume fraction reinforced aluminum alloy matrix (SiC_p/Al) composites are currently considered potential candidate materials for advanced electronic packaging and thermal management applications[1-3], due to the improved mechanical properties such as high specific strength and specific modulus, and the excellent thermo-physical properties such as low thermal expansion coefficient, high thermal conductivity and low specific gravity[4-5]. If volume fraction of SiC particle as high as 55% can be achieved, it is expected that SiC_p/Al composites will have thermal conductivity in excess of 120 W/(m?K), and a coefficient of thermal expansion similar to those of alumina substrate or semiconductor[6], necessary for electronic packaging applications.

Some researches on the fabrication and properties of SiC_p/Al composites have been reported, ZHANG et al[7] fabricated the SiC_p/2009A1 composites with SiC volume fraction of 15% through high energy milling with powder metallurgy(PM) technique and studied the effect of fabricating processes on the microstructure and properties of the composites. LI et al[8] researched on porosity of SiC_p/ZL101 composites by stirring cast, and VELHINHO et al[9] made use of centrifugal casting to prepare SiC_p/Al composites with SiC volume fraction of 12%-20%. However, most of these were obtained with high-pressured complex equipment or vacuum, and meanwhile it is difficult to produce high volume fraction composites by these methods and there exists non- uniformity for structures. So recently, high volume fraction SiC_p/Al composites have been fabricated by using the infiltration processing. Some experiments have been carried out to investigate the SiC_p/Al composites prepared by pressure infiltration[10-13], but only few of them[14] focused on the study of pressureless infiltration. Despite ZHANG and CUI[15], and SHI et al[16-18] fabricated SiC_p/Al composites by pressureless infiltration successfully and studied the reaction mechanism primarily. However, according to their research, two factors are necessary during the process of infiltration, one is that N₂ is used as protection atmosphere, the other is that Mg is contained in the Al matrix..

So, the present work is an attempt to improve the pressureless infiltration processing on the foundation of Lanxide-method[19], by which high volume of SiC_p/Al composites based on the matrix of Al-Si alloys were fabricated , with no magnesium and nitrogen and making Al melt infiltrating spontaneously into SiC particulate. Meanwhile, microstructures and thermo-physical properties of SiC_p/Al composites were analyzed, in order to provide some profitable information to the practice of applying SiC_p/Al composite in electronics packaging.

2 Experimental

2.1 Raw materials and fabrication process

In this experiment, SiC particles whose average size was 90 μm were used as reinforced-phase, and the matrix alloys whose chemical compositions are shown in Table 1 are respectively pure Al(1060), ZL101 and ZL102.

Table 1 Chemical composition of matrix alloy (volume fraction, %)

The SiC_p/Al composites, with a 55% volume fraction of SiC particles, were fabricated by pressureless infiltration[20], and Fig.1 shows the flowchart describing the process of fabrication.

Fig.1 Flowchart on fabrication process of SiC_p/Al composite

2.2 Test method

In order to exclude the stress in the composite, each specimen was all put under the condition of annealing treatment (400 ℃, 2 h,  air-cooled) before the test, besides the former heat treatment, SiC_p/ZL101 was also put under the condition of solution aging treatment (535 ℃, 3 h, cold-water-quenched+200℃, 3 h, air-cooled). Slices with 3 mm in thickness were cut down from the specimens and machined to analyze their structure and properties. The microscope examination of the composite was performed by scanning electron microscope(SEM) QUANTA-200, the ingredient of interphase was analyzed by X-ray diffraction(XRD) ADVANCE-D8, The linear coefficient of thermal expansion(CTE) was measured by Netzsch DIL 402EP and the average CTE was acquired according to the change curve of temperature-displacement in this experiment, and finally the thermal diffusivity was tested by Netzsch LFA-447 Nanoflash, the test temperature in the later two was both from 25 to 200 ℃.

3 Results and discussion

3.1 Microstructure and phases

The microstructure of SiC_p/Al composite is shown in Fig.2. From Fig.2, it can be seen that the distribution of SiC particles in Al matrix is homogeneous and no conglomeration and the structure of the composites is dense and no stomas.

The XRD pattern of SiC_p/Al composite is shown in Fig.3. It is noticed that the SiC_p/Al composites are mainly made up of three phases, Al, SiC and Si. Al and Si are the phase of matrix alloy, while SiC is the reinforced-phase, and it indicates that no new phase appears in the composite.

Fig.2 Microstructure of SiC_p/Al composite

Fig.3 XRD pattern of SiC_p/Al composite

3.2 Thermal properties

As can be seen from Fig.4, the CTE of SiC_p/Al composites increases with increasing temperature. When the temperature is beyond 130 ℃, the increase of rate begins to become smaller. In addition, the CTE of SiC_p/Al composites are remarkably lower in comparison with pure Al matrix alloy (23×10^-6 K^-1[21]). And the experiment results show that the average linear CTE of three composites between 25 ℃ and 200 ℃ are respectively 9.62×10^-6, 8.71×10^-6, 8.28×10^-6 K^-1, and the average CTE is below 10×10^-6 K^-1, which is basically near to that (4.1×10^-6 K^-1) of the CMOS chip. It is also found that the increase of Si content in matrix alloy can effectively reduce the CTE of the composite. Just as shown in Fig.4, when the Al matrix is ZL102 (Si content for 11.7%), the CTE of SiC_p/ZL102 composite can be adjusted to 7.23 × 10^-6 K^-1.

Fig.4 CTE of SiC_p/Al composites with different temperatures

The main reasons for these phenomena are that, as for the composites, there are many factors influencing their thermal expansion, and two major of which are as follows[21,22]. The first is the thermal expansion nature of matrix alloy; the second is the binding at the interface between matrix and reinforcements. On the one hand, the CTE value of matrix alloy will increase with temperature; on the other hand, the constraint role of the interface between matrix and reinforcements will be weakened, all which result in the increase of CTE of composite with rising temperature. In Fig.4, it is also seen that CTE of the composites decreases as Si content in the matrix alloy increases. It is suggested that the control of CTE value of composite may be adjusted by the content of matrix alloy element. Though the CTE of the composite is slightly higher than that of the alumina substrate, they can still meet the requirements of electronic packaging materials, as in such cases, when the composites and alumina substrate are in welding cooling time, the alumina substrate will be in the state of pressure stress, which is conducive to the practical applications of electronic components.

As can be seen from Fig.5, as the temperature rises, the thermal conductivities of composites drops gradually, ranging in the range of 80-166 W/(m?K), which surpasses that of the traditional material named Kovar alloy (the thermal conductivity is 17 W/(m?K)), and when the temperature is up to 200 ℃, the thermal conductivities of SiC_p/1060, SiC_p/ZL101 are still higher than 120 W/(m?K). Moreover, as the Si content increases, the thermal conductivity of the composites declines but it is not linear. When Si contents is 7%, the average thermal conductivity is 140.4 W/(m?K) which is close to that of the SiC_p/1060 composite (144.6 W/(m?K)). While the Si content is 11.7%, the average thermal conductivity declines markedly to 87.74 W/(m?K).

Fig.5 Thermal conductivity of SiC_p/Al composites at different temperatures

On one hand, the thermal conductivity is in reverse proportional to temperature, this is because that, in the particulate reinforced metal matrix composites (PRMMCs), the matrix transfers heat depending on free electrons, and the SiC particles depending on the vibration of crystal lattice named phonon[23]. When they make up of composites, the phonons and free electrons have an influence on heat conductivity together. As is also known by heat conduction mechanism of solid: the thermal conductivity of SiC_p/Al is proportional to transport mean free path of phonons and free electrons. And in the temperature interval of 25-200 ℃, the transport mean free path of free electrons is basically invariable for Al matrix, but the transport mean free path of the phonon (λ_s) is in reverse proportion with temperature(t) for SiC crystal materials, therefore the changing tendency of thermal conductivity of SiC_p/Al composite is consistent with that of SiC at 25-200 ℃, which is also in reverse proportion with temperature as shown in Fig.5, and it demonstrates the characteristic of heat conduction for crystal.

On the other hand, as Si content in matrix alloy increases, the thermal conductivity of the composites decreases, especially for SiC_p/ZL102 composite, the thermal conductivities of them decrease evidently and the difference of thermal conductivity between SiC_p/ZL101 and SiC_p/ZL102 is relatively large. The reasons for these phenomena are due to two main factors. Firstly, a few Si element can be separated out of Al matrix alloy under the annealing treatment, which will cause electron scatter seriously during heat conduction, therefore, lead to the decrease of the thermal conductivity of composite. Secondly, this is mainly because the matrix of ZL101 consists of relatively much primary (α(Al)), partly α+Si eutectic structure, few Si under the annealing treatment at room temperature, but ZL102 nearly consists of  α+Si eutectic structure just as shown in Fig.6, the existence of these eutectic interphase holds up respective heat conduction continuity at both sides, which weakens the capacity of heat transmission. So the thermal conductivity of SiC_p/ZL102 drops obviously.

Fig.6 Microstructures of matrix alloys: (a) ZL101; (b) ZL102

3.3 Influence of heat treatments on thermal proper- ties of SiC_p/Al composites

As well known, because of the difference in the CTE of Al and SiC particles, and the contraction of Al will be restrained, therefore comparatively larger remaining tensile stress will appear in the interphase of SiC and Al. In order to compare the influence of different heat treatments on the CTE and thermal conductivity, thus some certain thermal treatment is required before application of the composite. In this study, the CTE and thermal conductivity SiC_p/ZL101 composite were analyzed under the annealing and solution aging conditions, and the results can be seen from Figs.7 and 8.

Fig.7 CTE of SiC_p/ZL101 under different heat treatment conditions

Fig.8 Thermal conductivity of SiC_p/ZL101 under different heat treatment conditions

As can be seen from Fig.7 that the annealing and solution aging treatment have few effects both on the curve shape and value of CTE, when compared with the solution aging treatment, the CTE of composite under annealing treatment is relatively lower, this is mainly because the resisting tensile of matrix alloy as either the elastic strain or micro submits is smaller during the process of temperature rising. Because the elastic strain is relatively large and the micro submits is easy to happen, the macroscopic CTE is comparatively smaller. But the ZL101[21] is not typical Al alloy of the solution aging treatment, therefore the effect of solution aging treatment is not so obvious, thus the difference between the two CTE is not obvious.

It is can be seen that the heat conductivity for the annealing treatment is higher than that of the solution aging treatment from Fig.8. The main reason for this is that, the composite for the annealing treatment is easy to separate out the second phase, when the temperature is quite high. As is known that the free electron and the phonon have an influence on the heat conduction of particulate reinforced metal matrix composites (PRMMCs) together, it is because that these two phases are intensified to hinder the movement of the phonon, resulting in increasing thermal resistance of the materials. Therefore the thermal conductivity for the solution aging treatment is lower than that for the annealing treatment.

4 Conclusions

1) High volume of SiC_p/Al composites which reveals fairly uniform distribution of the constituent phases are fabricated with lower cost by using the pressureless infiltration.

2) With the rise of temperature, CTE of the composite materials increases and the heat conductivity declines gradually. CTE decreases apparently in comparison with their matrix alloy, and the linear CTE can be adjusted within (7.23-10.4)×10^-6 K^-1.

3) With increasing Si contents in matrix alloys, CTE of the composites declines and the thermal conductivity also declines but it is not linear, when Si content is 7%, the average thermal conductivity is 140.4 W/(m?K) on the verge of the composites based on the matrix of pure Al (144.6 W/(m?K)), while the Si content is 11.7%, the average thermal conductivity declines markedly to 87.74 W/(m?K).

4) The annealing treatment is better than the aging treatment to reduce the coefficient of thermal expansion and improves the thermal conductivity of composites.

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(Edited by LI Xiang-qun)

Foundation item: Project(0450100) supported by the Natural Science Foundation of Jiangxi Province; Project(2006[167]) supported by the Ministry of Education in Jiangxi Province, China

Corresponding author: ZHOU Xian-liang; Tel: +86-791-3863012; E-mail: zhouxianliang@hotmail.com