Effect of bimodal cBN particle size distribution on thermal conductivity of Al/cBN composite fabricated by SPS

Abstract

Cubic boron nitride (cBN)-particle-dispersed-aluminum (Al) matrix composites were fabricated in solid-liquid co-existent state by Spark plasma sintering (SPS) process from the mixture of cBN powders, Al powders and Al-5 mass%Si powders. As the cBN powders, two kind of powders, monomodal cBN powders of 390 μm in diameter and a bimodal cBN powder mixture of 390 μm and 39 μm in diameter, were used. The microstructures and thermal conductivities of the composites fabricated were examined. These composites were all well consolidated by heating at a temperature range between 798 K and 876 K for 1.56 ks during SPS process. No reaction at the interface between the cBN particle and the Al matrix was observed by scanning electron microscopy for the composites fabricated under the sintering conditions employed in the present study. Although the relative packing density of the monomodal composite decreased from 99.5% to 89.5% with increasing the cBN volume fraction in a diamond volume fraction range between 50% and 60%, that of the bimodal composite was higher than 98.6% in a cBN volume fraction range up to 65%. The thermal conductivity of the bimodal composite was 306-325 W/mK, which is higher than that of the monomodal composite in a diamond volume fraction range higher than 45%. The coefficients of thermal expansion of the composites were a little higher than the theoretical values estimated by the upper line of Kerner's model, indicating the bonding between the cBN particle and the Al matrix in the composite is weak a little.