Effect of boron on the microstructure and properties of graphite flakes/copper composites fabricated by vacuum hot pressing

Boron (B) powder was added directly into the copper (Cu) matrix by powder metallurgy method, and then the contents were mixed with graphite flakes (GFs) to fabricate GFs/Cu composites by vacuum hot pressing. The effects of B content (0–2.5 wt%) on the thermal conductivity (TC) and coefficient of thermal expansion (CTE) of GFs/Cu composites in X–Y (basal plane of GF) and Z directions were investigated in this study. The results showed that in X–Y direction, the TC of 50 vol% GFs/Cu composites increased significantly from 563.00 to 634.87 W m−1 K−1 when B content was 0.4 wt%; however, it dropped gradually with the further increase in B content. The CTE of 50 vol% GFs/Cu composites in Z direction decreased gradually from 9.50 to 3.63 ppm K−1 with the increase in B content from 0 to 2.5 wt%. This improvement in TC and CTE is attributed to changes in interface structure. The fluidity of the Cu matrix and the wettability of interface between GFs and Cu were improved due to the addition of B, which was found to be conducive for the Cu matrix to spread on the rough GFs surface and thus filling the gaps and microvoids caused by the insufficient filling of the Cu matrix. As a result, the interfacial thermal resistance between GFs and Cu reduced and the TC of the composites increased significantly.