The influence of carbon nanotube (CNT) morphology and diameter on the processing and properties of CNT-reinforced aluminium composites

Ball milling was used to disperse MWCNTs of two different morphologies (stiff and straight vs. bent and entangled) and diameters (very large diameter and 3.5 times smaller diameter) in aluminium powders, which were subsequently hot consolidated by hot extrusion. Characterization of the produced composites revealed that the CNT morphology plays an important role in affecting dispersion. It was found that the smaller diameter bent and entangled CNTs were more difficult to disperse with increase in CNT content compared to the larger diameter stiff and straight ones; which in turn affected the tensile properties and hardness of the composites. Furthermore, cold welding of the milled powders as well as carbide formation in the final composite was found to depend on the CNT diameter. The smaller diameter CNTs – having a larger effective interfacial contact area with the aluminium matrix compared to the larger ones for a given CNT wt.% – were found to reduce particle welding during milling and to be more affected by carbide formation. Nano-sized particles of aluminium oxide as well as nano-rods of aluminium carbide, in addition to CNT damage were observed upon TEM analysis of the smaller diameter CNTs.