Thermal Conductivity and Tensile Properties of Carbon Nanofiber-Reinforced Aluminum-Matrix Composites Fabricated via Powder Metallurgy: Effects of Ball Milling and Extrusion Conditions on Microstructures and Resultant Composite Properties

Abstract

Carbon nanofiber (CNF)-reinforced aluminum-matrix composites were fabricated via ball milling and spark plasma sintering (SPS), SPS followed by hot extrusion and powder extrusion. Two mixing conditions of CNF and aluminum powder were adopted: milling at 90 rpm and milling at 200 rpm. After milling at 90 rpm, the mixed powder was sintered using SPS at 560 °C. The composite was then extruded at 500 °C at an extrusion ratio of 9. Composites were also fabricated via powder extrusion of powder milled at 200 rpm and 550 °C with an extrusion ratio of 9 (R9) or 16 (R16). The thermal conductivity and tensile properties of the resultant composites were evaluated. Anisotropic thermal conductivity was observed even in the sintered products. The anisotropy could be controlled via hot extrusion. The thermal conductivity of composites fabricated via powder extrusion was higher than those fabricated using other methods. However, in the case of specimens with a CNF volume fraction of 4.0%, the thermal conductivity of the composite fabricated via SPS and hot extrusion was the highest. The highest thermal conductivity of 4.0% CNF-reinforced composite is attributable to networking and percolation of CNFs. The effect of the fabrication route on the tensile strength and ductility was also investigated. Tensile strengths of the R9 composites were the highest. By contrast, the R16 composites prepared under long heating duration exhibited high ductility at CNF volume fractions of 2.0% and 5.0%. The microstructures of composites and fracture surfaces were observed in detail, and fracture process was elucidated. The results revealed that controlling the heating and plastic deformation during extrusion will yield strong and ductile composites.