Diamond/Al metal matrix composites formed by the pressureless metal infiltration process

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Diamond particles are unique fillers for metal matrix composites because of their extremely high modulus, high thermal conductivity, and low coefficient of thermal expansion. Diamond reinforced aluminum metal matrix composites were prepared using a pressureless metal infiltration process. The diamond particulates are coated with SiC prior to infiltration to prevent the formation of A14C3, which is a product of the reaction between aluminum and diamond. The measured thermal conductivity of these initial diamond/Al metal matrix composites is as high as 259 W/m-K. The effects of coating thickness on the physical properties of the diamond/Al metal matrix composite, including Young's modulus, 4-point bend strength, coefficient of thermal expansion, and thermal conductivity, are presented.

I. INTRODUCTION During the past decade, the extent of chip integration within the semiconductor industry has increased dramatically. The feature size on electronic chips has decreased with the linewidths going from tens of microns in the early 70s to submicron dimensions today. Concomitantly, the number of logic circuits per chip has increased from the hundreds into the millions. These changes have led to very high power densities, approaching 10 W/cm 2 in some cases. To prevent large temperature rises attendant upon these high power densities, improved methods for removing heat from the chips are becoming increasingly necessary. One technique to accomplish this goal is through the use of a substrate or chip-carrier that has a high thermal conductivity to draw the heat away from the chip so it can be dissipated into the surroundings. Many such high conductivity substrate materials have been considered, including monolithic A1N, SiC, BN, and diamond.1 In addition, a number of composite materials have also been investigated, including C-fiber/Al metal matrix composites (MMC's), and particulate SiC-reinforced Al MMC's. 2 Recently, a new class of composite materials, diamond/Al MMC's, has been reported.2-3 Because the diamond has a very high thermal conductivity (700-2000 W/m-K), stiffness (1050 GPa), and a low coefficient of thermal expansion (CTE) (1.45 X 10" 6 /K between 25 and 100 °C), these composites offer promise for the most demanding applications where stiff, low CTE, high thermal conductivity substrates are required. This paper expands on these initial reports, discussing the processing and properties J. Mater. Res., Vol. 8, No. 5, May 1993

of diamond/Al MMC materials that are prepared by a pressureless metal infiltration process. II. EXPERIMENTAL PROCEDURE The approach used to prepare the diamond/Al MMC's is a pressureless metal infiltration process.4 Briefly, with this method the alloy composition and atmosphere are controlled to allow an Al alloy to wet and infiltrate beds or preforms of reinforcement material without an external pressure. For example, to form a SiC or A12O3 reinforced aluminum MMC, a Mg containing Al alloy is infiltrated into the reinforcement in a nitrogen atmosphere.5 Infiltration can occur over a wide ra

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