Fabrication And Mechanical Properties Of Carbon Nanotube(CNT) Reinforced Aluminum Matrix Composites By Powder Metallurgy Method

Carbon nanotube(CNT) has great application potential in fabrication of metal matrix composites(MMC) due to exceptional mechanical, electrical and chemical properties as well as low density. However, so far, there are many issues should be addressed. Challenges are mainly lie in:(i) large aspect ratio and strong Van der Waals’ force between the tubes make the CNTs trend to agglomerate into clusters, which result in difficulty in homogeneous dispersing them into metal matrix;(ii) the low bonding strength between CNT and metal matrix limited the full play of its role. To solve these two main issue, two methods were attempt to disperse the CNTs, one is the combination of solvent-assistant dispersion and mechanical ball milling, the other is direct mechanical ball milling of CNT and the 2014 Al powders, the CNT/2014 Al composites were successfully fabricated. The effect of surface nature of CNTs(untreated, carboxyl treated), nickel coated surface, aspect ratio, hybrid second nano particles, machanical ball milling time and the initial aluminum alloy powder size on the dispersion and the microstructure of the composites were investigated by FESEM and TEM. The influence of initial aluminum alloy powder size and ball milling time on the damage of the CNT were studied by Raman spectroscopy. The influence of the factors mentioned above on the mechanical properties were investigated by tensile and compressive test as well as micro-hardness test. The strengthening mechanism of CNT/2014 Al composites were also investigated by the study of interface between CNT and the matrix and the observation of the fractured surface, the results mayprovide certain reference meanings for the research on CNT reinforced metal matrix composites.The main results are as follows:1) An approach that combines solvent-assistant and mechanical ball milling process was proposed to improve the dispersion of CNT in the 2014Al; the investigation of the dispersion of CNT with different surface nature through this method reveals that the dispersion of the carboxyl treated CNT considerably improved compared the untreated one when the ethanol was used as solvent. The study of hybrid nano Si C on the dispersion of CNT reveal that the addition of Si C improve the dispersity of the CNT.2) The study of initial powder size of the 2014 Al and the ball mill time effect on the dispersion of CNT reveal that the CNT gradually embeded into the aluminum alloy powder with increasing ball milling time during ball milling. The larger the initial aluminum powder size, the harder for the dispersion of the CNT, CNT with higher aspect ratio makes it hard to disperse into the matrix.3) The study of initial 2014 Al alloy powder size and ball milling time on the damage of CNT during ball milling found that the damage to CNT increases with ball milling time before the embedding of CNT; almost no further damage occurred to the CNT with ball milling time after the embedding of CNT; the larger the initial powder size, the easier for the CNT to be embedded in the powder, such entrapment produces protect effects on the CNT.4) CNT/2014 Al composites with different kinds of CNT were designed, the influence of CNT concentration on the mechanical properties were revealed, the best content of the CNT with low aspect ratio was optimized to be 0.5 wt.%, at which content the composites exhibit best tensile properties; carboxyl treated CNT show much better enhance effect than the untreated one under the optimized CNT concentration; CNT with low aspect ratio exhibit more enhancement effect than the CNT with high aspect ratio. The best content of the CNT with high aspect ratio was optimized to be 2.0 wt.%, at which content the compositesexhibit best compressive properties; at this CNT loading, when the initial 2014 Al with powder size of 9 μm and ball milling time of 24 h were adopted, the composites show the best compressive properties; the strengthening effect of the uncoated CNT is more significant than the Ni uncoated one when the CNT loading below 2.0 wt.%.5) The investigation of the interface between CNT and Al found that a layer of discontinuous Al4C3 with a thickness of about 2~3 nm is benificial to the bonding between the CNT and the 2014Al; the rough surface of the carboxyl treated CNT produces an interlocking effect between the CNT and the matrix which is helpful to the stress transfer from the matrix to the CNT; a continuous layer of Al3 Ni with a thickness of 4-5 nm formed between the Ni coated CNT and the 2014 Al matrix, which is bad for the CNT to play the role in enhancing the composites.6) Through the analysis of the the investigation of the interface, fracture morphology and microstructure, it reveals that the main strengthening mechanism for the CNT/2014 Al composites is the stress transfer from the matrix to the CNT and the grain refinement strengthening due to the addition of CNT.