Synergistic effect of bimodal structure and Cu-coated CNTs on the properties of Cu/CNTs composites

Multi-walled carbon nanotube-reinforced Cu matrix composites (Cu/MCNTs) present a challenge in achieving high strength and good ductility. To address this issue, the bimodal structure composites were fabricated by vacuum mixing of coarse Cu powder (Cum) and fine Cu powder along with Cu-coated CNTs (CCNTs) followed by spark plasma sintering (SPS). Two sets of bimodal composites with 1 wt% Cu-coated CNTs were fabricated by varying the proportion of fine to coarse Cu powder (70:30 and 30:70). The unimodal composite was prepared by mixing Cum and 1 wt% functionalized CNTs in a ball mill for 1 h followed by SPS. Using electron backscatter diffraction (EBSD), the microstructure of the prepared composites was examined, and the influence of coarse grain and fine grain, along with CCNTs, on the structure-property correlation in the composites was investigated. The results show that the bimodal composites outperform the unimodal counterpart, and the bimodal composite with fine to coarse Cu grain ratio of 70:30 has the highest compressive strength (323 MPa), compressive strain (18.8%) electrical conductivity (85.4 ± 0.6% IACS), and thermal conductivity (295.4 ± 5 W/mK). The increase in strength of the bimodal composite is attributed to the Cu-coated CNTs and the tuning between coarse and fine grain. Load transfer, aided by the Cu coating on the CNT surface, is revealed to be the primary strengthening mechanism for bimodal composites. This work sheds light on the mechanisms of strengthening and the structure-property correlations in bimodal composites.