Enhanced interfacial bonding and mechanical properties in CNT/Al composites fabricated by flake powder metallurgy
Graphical abstract
Comparison of the mechanical properties and microstructure of 3 vol.% CNT/Al fabricated by slurry dispersion process (blue frame) and by slurry dispersion plus 2 h ball milling process (red frame).
Introduction
Carbon nanotubes reinforced aluminum matrix composites (CNT/Al) are regarded as the new generation lightweight structural materials for aerospace and automobile applications, expecting that the strength and Young's modulus of the composites could be significantly enhanced with the addition of CNTs [[1], [2], [3], [4], [5]]. During the past two decades, many methods have been developed to fabricate CNT/Al composites, such as high energy ball milling (HEBM) [6,7], accumulative roll bonding (ARB) [8,9], friction stir-processing (FSP) [10,11], flake powder metallurgy (Flake PM) [[12], [13], [14]], and nano-scale dispersion (NSD) [15,16], etc. However, the mechanical properties of CNT/Al composites are often found deteriorating seriously with increasing CNT content. Not only the ductility dropped dramatically but also the reinforcing efficiency decreased or even became negative with the increasing content of CNTs, especially when the CNT content was higher than 2 vol.% [7,[17], [18], [19]].
The deterioration of mechanical properties of CNT/Al composite with increasing CNT content is mainly attributed to two reasons. Firstly, it is extremely difficult to disperse larger amount of CNTs into Al matrix due to the strong Vander Waals interaction between CNTs [20]. The agglomeration of CNTs will lead to highly probable processing defects (voids or pores), which may greatly lower the strengthening effect and result in early initiation of cracks at defects [21]. Secondly, the specific surface area of CNT is so large that the interface/volume ratio is much higher than that of conventional metal matrix composites reinforced with micron-sized particles, and thus the mechanical properties are more sensitive to the interfacial bonding between CNT and Al matrix. Many studies have pointed out that the formation of chemical bonding/reaction bonding between CNT and Al would be benefit for load transfer and thus good strengthening efficiency and good ductility can be achieved [[22], [23], [24], [25]]. However, the interface reaction product, Al4C3, is brittle and supposed to be easily hydrolyzed in wet environment [26,27]. Therefore, the interface reaction degree should be restricted and the ideal interfacial bonding in CNT/Al system would be the physical bonding/diffusion assisted bonding, accompanied by limited chemical bonding/reaction bonding. The situation is that among the methods to fabricate high content CNT/Al composite, the solutions to achieve uniform dispersion are usually contradictory to that to good interfacial bonding. Such as the most frequently used HEBM technique, although the extension of the ball milling period can help to achieve relatively good dispersion of CNTs, the CNT structure would be seriously damaged and thus result in excess interface reaction [28]. Therefore, to achieve CNT/Al composites with comprehensive mechanical properties, it is urgently needed to develop effective fabrication methods that not only can disperse high content CNTs into Al matrix but also can achieve good interfacial bonding between CNT and Al matrix.
Among the methods, the Flake PM has shown great advantages in dispersing high content of nano reinforcements uniformly on Al nanoflakes [12,13,29,30]. Jiang et al. [12] showed that the dispersion capacity of CNTs was greatly enhanced by changing Al spherical particles into disc-shaped Al nanoflakes which had a thickness of few hundred nanometers and a diameter of dozens of microns, and the dispersion uniformity of CNTs was improved by surface modification of Al nanoflakes with Polyvinyl Alcohol (PVA), which leads to a strong bonding between carboxylated CNT and Al and prevents the re-aggregation during filtering and drying. The uniform dispersion and well maintained CNT structural integrity guaranteed the high reinforcing efficiency of CNTs. The 2 vol.% CNT/Al fabricated by the above process exhibited very high strengthening efficiency, an enhancement of more than 60% in tensile strength and an uniform elongation of 6% was obtained [13]. However, when applying the above method to fabricate CNT/Al with higher CNT content, the ductility of the composite also decreased dramatically [31]. This is mainly due to the poor interfacial bonding between Al nanoflakes and between CNT and Al matrix. Due to high CNT content, several layers of CNTs would be attached and overlapped on the surface of Al nanoflakes after slurry based dispersion, which making it very hard to establish effective bonding between neighboring Al nanoflakes. Although CNTs can be realigned and bonded with Al matrix during the extrusion, many CNTs remain aggregated along the interlayers/grain boundaries after extrusion. Additionally, the existence of the native Al2O3 skin (∼5 nm thick) that easily formed during the powder preparation process will also inhibit the direct bonding between CNT and Al [32,33]. Thus, for CNT/Al nanoflake powders with high CNT content (volume fraction), direct consolidation by sintering and extrusion leads to poor interfacial bonding and thus results in low ductility. Although a study by Chen et al. [34] showed that simultaneous enhancement in strength and ductility can be achieved by increasing the bonding strength between matrix and CNTs and between matrix grains through high temperature sintering. Our experiments found that for the high content CNT/Al composite made by Flake PM, no obvious improvement in mechanical properties was obtained by increasing the sintering temperature up to 630 °C. Therefore, to achieve strong and ductile high content CNT/Al composites by Flake PM, additional effort should be applied to break the Al2O3 skin and redistribute the CNTs that aggregated at interlayers and establish effective interfacial bonding between CNT and Al.
In this study, to improve the interfacial bonding between CNT and Al for CNT/Al composites reinforced with high content CNTs, a short time high energy ball milling is introduced as a subsequent process of the slurry based Flake PM, to break native Al2O3 skin and embed CNTs into Al matrix. The experimental results shown that with the aid of 2 h ball milling, both CNT dispersion homogeneity and interfacial bonding were significantly improved, and a simultaneous enhancement in strength and ductility can be achieved.
Section snippets
Experimental
The typical fabrication of 3 vol.% CNT/Al composites by the Flake PM method including two steps, the slurry based dispersion process and high energy ball milling process, as illustrated in Fig. 1.
- (1)
The slurry based dispersion process. The Al nanoflakes with a thickness of 500 nm were prepared by ball milling the pure water-atomized spherical Al powders (10 μm in diameter, 99.8% in purity, Bai Nian Yin (Zhejiang) co., Lt, China) in absolute ethyl alcohol for 4 h at 99 rpm in a 1 L Lab Stirring
Results and discussion
Fig. 2 show the morphology evolution of 3 vol.% CNT/Al powder. After the slurry based dispersion process, the Al nanoflakes were fully covered by CNTs while a few CNTs were entangled and overlapped due to the high CNT content (Fig. 2a). From the magnified SEM image (Fig. 2b), we can see that at least 2–3 layers of CNTs overlapped separately with each other. After 2 h ball milling, the CNT/Al nanoflake powders were transformed into large particles (>100 μm) as shown in Fig. 2c, and the magnified
Conclusions
In summary, the present study demonstrates that by applying a short time high energy ball milling to the pre-dispersed CNT/Al nanoflake powders prepared from slurry based dispersion process, CNTs can be redistributed more homogeneously and interfacial bonding between CNT and Al can be improved from non-bonding to physical bonding and partial reaction bonding. As results, 3 vol.% CNT/Al composites exhibited good mechanical properties with tensile strength enhanced by 65.7%, modulus increased by
Acknowledgments
The authors would like to acknowledge the financial support from the National Key Research and Development Program of China (Nos. 2017YFB1201105, 2016YFB1200506, 2016YFE0130200), the Natural Science Foundation of China (Nos. 51671130, 51371115), the Ministry of Education of China (No. B16032), Aeronautical Science Foundation of China (2016ZF57011), and Shanghai Science & Technology Committee (Nos. 17ZR1441500, 15JC1402100).
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