Fabrication, thermal expansion, thermal conductivity, and mechanical properties of high particulate volume fraction aluminum-matrix composites

High particulate volume fraction (up to 75 vol.%) aluminum-matrix composites are technologically important in electronic packaging as thermal conductors of low thermal expansion {dollar}(10times10sp{lcub}-6{rcub} spcirc{dollar}C{dollar}sp{lcub}-1{rcub}){dollar} and low density. The particles used included AlN, SiC and Al{dollar}sb2{dollar}O{dollar}sb3.{dollar} The use of AlN as the reinforcement resulted in superior high temperature (up to 600{dollar}spcirc{dollar}C) resistance and higher shear strength than the use of SiC, due to the reactivity between SiC and Al. The Al/Al{dollar}sb2{dollar}O{dollar}sb3{dollar} exhibited the lowest mechanical properties due to particle clustering in preform and incomplete infiltration in composite. The SiC-Al reaction may be diminished by using a silicon-containing Al matrix, but such a matrix is inherently brittle compared to pure Al, thus resulting in low tensile strength and low ductility in the composite.; The composites were fabricated by infiltration of liquid aluminum into a porous reinforcement preform. The SiC-Al reaction produced Si, which diffused toward the excess liquid metal around the preform, thus causing a relatively low Si concentration at the edge of the composite next to the excess aluminum. However, the non-uniform Si distribution did not result in a non-uniform mechanical property variation. In the case that a mold-Al reaction (as for a steel mold) occurred in addition to the SiC-Al reaction, a non-uniform mechanical property variation resulted.; The quality of the composites depended on the preform fabrication technology, which is more critical in high particulate volume (than in lower whisker/fiber) fraction preform fabrication due to much lower porosity and pore size for liquid metal infiltration. In this work, this technology which involved the use of binder and carrier, binder/carrier ratio, binder-reinforcement reactivity, and optimal preform baking, was developed for high volume fraction particulate preforms. The technology developed used an acid phosphate binder (with P/Al molar ratio = 23) in the amount of 0.1 wt.% of the preform, in contrast to the much larger binder amount used for whisker preforms. The preforms were made by filtration of a slurry consisting of the reinforcement particles, the binder and a carrier (preferably acetone), and subsequent baking (preferably at 200{dollar}spcirc{dollar}C) for the purpose of drying. Baking in air at 500{dollar}spcirc{dollar}C instead of 200{dollar}spcirc{dollar}C caused the AlN preforms to oxidise, thereby decreasing the thermal conductivity of the resulting Al/AlN composites. The reinforcement-binder reactivity was larger for AlN than SiC, but this reactivity did not affect the composite properties due to the small binder amount used.