Multi-walled carbon nanotube-aluminum matrix composites prepared by combination of hetero-agglomeration method, spark plasma sintering and hot extrusion

Multi-walled carbon nanotubes (MWCNTs) with outstanding mechanical properties are considered as an ultimate reinforcement for conventional metals such as aluminum (Al), though they require uniform dispersion and intimate contacts with the host metal matrix. However, bundled structure of chemically stable, pristine MWCNTs has been the main challenge to achieving the expected structural reinforcement. In addition, the sole contribution of MWCNTs in strengthening the metal matrix has not been recognized yet, due to the concurrent strengthening mechanisms such as metal's work hardening, grain refinement, etc. Here, we prepared MWCNT-Al matrix composite powders with uniform MWCNT dispersion by using hetero-agglomeration principle, and fabricated fully dense 1.0 vol% MWCNT-Al matrix composite bulk by using spark plasma sintering (SPS) and a subsequent hot extrusion process, with a 40% improved tensile strength and an elongation to failure of 27.3% similar to that of cast pure Al. During the SPS process, the nanoscale surface defects of MWCNTs were infiltrated by momentarily formed liquid Al and an intimate Al₄C ₃-free interface was formed between MWCNTs and bare Al matrix. After the hot extrusion process, straight MWCNTs aligned in the extrusion axis found intimate contacts with the dynamically recovered, crack-free Al matrix. Our study suggests that the tensile improvement realized in our extruded/SPSed composites is directly originated from an effective load transfer at the MWCNT/Al interface, because no evidence of Al work hardening or the formation of interfacial Al₄C₃ crystals was detected.