Processing of Al/SiC composites in continuous solid–liquid co-existent state by SPS and their thermal properties

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Abstract

Silicon carbide (SiC)-particle-dispersed-aluminum (Al) matrix composites were fabricated in a unique fabrication method, where the powder mixture of SiC, pure Al and Al–5mass% Si alloy was uniquely designed to form continuous solid–liquid co-existent state during spark plasma sintering (SPS) process. Composites fabricated in such a way can be well consolidated by heating during SPS processing in a temperature range between 798 K and 876 K for a heating duration of 1.56 ks. Microstructures of the composites thus fabricated were examined by scanning electron microscopy and no reaction was detected at the interface between the SiC particle and the Al matrix. The relative packing density of the Al–matrix composite containing SiC was higher than 99% in a volume fraction range of SiC between 40% and 55%. Thermal conductivity of the composite increased with increasing the SiC content in the composite at a SiC fraction range between 40 vol.% and 50 vol.%. The highest thermal conductivity was obtained for Al–50 vol.% SiC composite and reached 252 W/mK. The coefficient of thermal expansion of the composites falls in the upper line of Kerner’s model, indicating strong bonding between the SiC particle and the Al matrix in the composite.

Introduction

High-performance thermal management materials should have high thermal conductivities [1], [2] and low coefficients of thermal expansion (CTE) for maximizing heat dissipation and minimizing thermal stress and warping, which are critical issues in packaging of microprocessors, power semiconductors, high-power laser diodes, light-emitting diodes (LEDs), and micro-electro-mechanical systems (MEMs) [3], [4], [5], [6]. Packaging base materials well known are Cu–W [7], AlN [8], BeO [9] and Al/SiC [10]. Among these materials, particularly, Al–matrix composites containing dispersed silicon carbide (SiC) particles have received the most attention as potential candidates for a variety of uses in advanced electronic packaging. They are currently competing with established materials such as Cu/W or Cu/Mo in the electronic packaging industry [11].

High thermal conductive composites consisting of Al matrix and SiC particles have been reported by Chu et al., who fabricated a SiC-particle-dispersed Al–matrix composite by a metal infiltration technique and obtained a thermal conductivity of 163–186 W/mK for Al composites containing 50–65 vol.% SiC [12]. Al–matrix composites containing SiC particles are to date mainly fabricated by liquid metal infiltration into SiC preforms [12], [13], [14]. This process often uses Al alloys with Si and/or Mg additions to avoid the formation of interfacial products such as Al4C3, leading to a detrimental effect on the thermo-mechanical properties of the composites. When Si and Mg dissolve into the Al matrix, on the other hand, the thermal conductivity of the matrix decreases and lowers the thermal conductivity of the composites. In addition, it is difficult to make SiC particles dispersed homogeneously in the Al composite when it is fabricated by a metal infiltration technique. This is especially true when the composite contains a lower volume fraction of SiC, say less than 50 vol.%, because of the difference in packing density between SiC and Al.

Spark plasma sintering (SPS) applied as a new method for preparing high performance of Al–matrix composites containing SiC has been presented recently [15]. The main advantage of the SPS process is to fabricate various materials for relatively short sintering times at low temperatures [16], [17], [18], [19], [20], [21], [22], leading to the prevention of Al4C3 at the interface between Al and SiC. Chu et al. fabricated SiC-particle-dispersed Al–matrix composites by SPS and obtained a thermal conductivity of 224 W/mK for an Al–55 vol.% SiC composite [23]. However, pores always appear in their study as an unavoidable structure in the Al–matrix composites containing SiC fabricated in solid state by SPS, which leads to insufficient wetting between Al and SiC. This results in weak Al/SiC interfaces, especially, at high volume fractions of SiC particles. Pores considerably decrease the thermal conductivity of the Al–matrix composites due to scattering of the heat flow [24].

To fabricate high-performance thermal management materials with very high thermal conductivities and low CTEs, we have recently initiated a series of investigations to fabricate diamond-particle-dispersed metal–matrix composites [25], [26], [27], [28], [29], [30], [31], [32]. In our recent study, to improve the above-mentioned drawbacks in structural uniformity, interfacial reaction and residual pores in the composites, a new processing technique during SPS was proposed [28], [30], [31]. In our previous work, diamond-particle-dispersed Al–matrix composites were fabricated in continuous solid–liquid co-existent state during SPS, and the high relative packing density was obtained along with strong bonding between the Al matrix and the diamond particles. A thermal conductivity of 552 W/mK was obtained at a diamond volume fraction of 50 vol.% [28], [30], [31].

In the present work, Al–matrix composite containing dispersed-SiC particles was fabricated by a SPS process of continuous solid–liquid co-existent state, similar to that used for Al–matrix composites containing dispersed diamond particles. That is, in the present study, the powder mixture of SiC, pure-Al and Al–Si alloy was used for SPS processing and processing temperatures employed were lower than the melting point of Al. SPS processing parameters were optimized for the fabrication of high thermal conductive Al–matrix composite containing SiC particles. Experimental results of thermal properties and packing density were obtained from the Al–matrix composite and they are reported herein along with their microstructures observed by scanning electron microscopy.

Section snippets

Experimental procedure

According to three-dimensional Euclidean space, the maximum volume fraction of filler particles in Al–matrix composite can ideally be 74 vol.% without cavities or voids if filler particles are spherical and of mono size. Due to some experimental constraints, empirically, Al–matrix composites are commonly produced with around 50 vol.% of fillers when roughly mono-sized filler particles are used. In the present study, SiC particles (GMF-60FH2, mean particle diameter being about 110 μm, provided by

Densification of composites during fabrication by SPS

The relative packing density of the composite and the change in die temperature were investigated as a function of sintering time for various volume fractions of SiC. Some results for the packing density obtained for Al–50 vol.% SiC composite were depicted in Fig. 3. As seen in the figure, the packing density is naturally low (about 60%) prior to SPS and remains uncharged until sintering time becomes 40 s, where temperature reaches about 350 K. The packing density then increases abruptly with

Summary

Al–matrix composite containing dispersed silicon carbide particles were fabricated in a unique fabrication method where continuous solid–liquid co-existent state of the powder mixture of SiC, pure Al and Al–5mass% Si alloy was designed to use in spark plasma sintering process. The composite was well consolidated by heating in a temperature range between 798 K and 876 K for 1.56 ks. Scanning electron microscopy detected no reaction at the interface between the SiC particle and the Al matrix. The

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