Microstructures and Properties of Graphite Nanoflake/6061Al Matrix Composites Fabricated via Spark Plasma Sintering

Two types of graphite nanoflakes (GNFs), GNFA for 30-100 μm in diameter and less than 100 nm in thickness, and GNFB for 0.5-10 μm in diameter and less than 20 nm in thickness, were used to fabricate GNF/6061Al matrix composites with GNF fractions ranging from 5 to 15 wt.% via spark plasma sintering (SPS) at 610 °C under a load of 35 MPa. The effects of GNF size and content on microstructures and properties of the composites were investigated. The results show that uniform mixing of GNFs in the 6061Al powder was achieved through mechanical and ultrasonic stirring. When the GNFs were well dispersed, the composites were dense. An interfacial zone of 15-18 nm in thickness was formed and composed of two layers, a poorly crystalline layer and an amorphous layer. No Al4C3 was detected in the interfacial zone. The relative densities, bending strengths, thermal conductivities (TCs), and coefficients of thermal expansion (CTEs) (room temperature to 100 °C) of the 10 wt.% GNFA/6061Al matrix composites were 98.5%, 120 MPa, 155 W m−1 K−1 in the X–Y direction and 61 W m−1 K−1 in the Z direction, and 14.2 ppm K−1 in the X–Y direction and 12.1 ppm K−1 in the Z direction, respectively. Those of the 10 wt.% GNFB/6061Al matrix composites were 97.8%, 70 MPa, 110 W m−1 K−1 in the X–Y direction and 90 W m−1 K−1 in the Z direction, and 15.4 ppm K−1 in the X–Y direction and 14.7 ppm K−1 in the Z direction, respectively. The GNFB/6061Al matrix composites showed lower differences of TC and CTE between the X–Y and Z directions. Therefore, the anisotropy of the microstructures and properties of the composites in three dimensions were significantly reduced.