Elsevier

Materials Chemistry and Physics

Volume 226, 15 March 2019, Pages 344-349
Materials Chemistry and Physics

Microstructure and mechanical properties of ABOw and nickel-coated MWCNTs reinforced 2024Al hybrid composite fabricated by squeeze casting

https://doi.org/10.1016/j.matchemphys.2019.01.053Get rights and content

Highlights

  • Homogeneous nickel coating was prepared on MWCNTs by electroless plating method.

  • Reduced porosity and homogeneous MWCNTs were achieved in the composite with Ni-coating.

  • Enhanced modulus, strength and hardness were found in hybrid composites with MWCNTs(Ni).

Abstract

Homogeneous distribution of multi-walled carbon nanotubes (MWCNTs) in aluminum matrix composites and optimization of the MWCNTs-aluminum interfacial strength are challenging for MWCNTs/Al metal-matrix composites (MMCs). Here, in order to improve the wettability and the interfacial bonding between MWCNTs and aluminum matrix alloy, a uniform nickel layer was created on MWCNTs by electroless plating. Hybrid preforms comprising of aluminum borate whisker (ABOw) and MWCNTs with/without nickel coating were prepared. It was found that the presence of ABOw and nickel coating ensured the homogeneous distribution of MWCNTs in composites. The presence of the nickel coating on MWCNTs also helps in decreasing the porosity of composite prepared by squeeze casting technique. Compared with ABOw/2024Al and ABOw + MWCNTs/2024Al composites, improvements in elongation and hardness were obtained in the ABOw + MWCNTs(Ni)/2024Al composite, which may be attributed to the reduced porosity, increased interfacial bonding strength and the crack deflection induced by dispersed MWCNTs. However, the addition of MWCNTs did not change the main fracture mechanism: ABOw break and ABOw-aluminum interfacial de-bonding dominated the fracture process in ABOw and MWCNTs(Ni) reinforced composites.

Introduction

Multi-walled carbon nanotubes (MWCNTs) have attracted much attention as a kind of high performance reinforcement for composite materials due to their low density, high specific surface area, remarkable electrical and mechanical properties [[1], [2], [3], [4]]. In the last two decades, many researchers have dedicated to study MWCNTs reinforced aluminum composites and great progress has been made [[5], [6], [7], [8]]. However, gaps still exist in the strengthening efficiency of carbon nanotubes that can be achieved in practice compared with that in theory. In fact, the poor wettability between carbon nanotubes and aluminum matrix and the extremely high specific surface area of carbon nanotubes, which make it a great challenging to attain uniform dispersion state in aluminum matrix, are responsible for the descent strengthening efficiency. In addition, the reaction between carbon and aluminum is thermodynamically a spontaneous reaction. The interfacial reaction product, aluminum carbide, which is brittle and is prone to hydrolysis in wet air atmosphere, significantly affects the load transfer between aluminum matrix and carbon nanotubes. Besides, the interfacial reaction also breaks the integrity of MWCNTs, and thus reduces the strengthening effect of MWCNTs as well. Consequently, the homogenous distribution of carbon nanotubes and interfacial reaction control are key factors that dominate the mechanical properties of MWCNTs/Al composites. Therefore, the synthesis strategy optimization and interfacial modification are critical issues for the improvement of mechanical properties of the composites, which are crucial for the application of the MWCNTs/Al composite [3].

Efforts have been made to improve the interfacial bonding between MWCNTs and aluminum using electroless plating. For example, copper [9,10], cobalt [11] and nickel [12,13] have been successfully coated on MWCNTs, which not only can increase the bonding between MWCNTs and metal matrix, but also maintain the excellent intrinsic properties of the MWCNTs [9]. By using electroless deposition, Susumu et al. [14] obtained improved wettability between MWCNTs and aluminum matrix. Jagannatham et al. [15] fabricated copper coated CNTs reinforced aluminum composites, and found improved compressive strength when compared with the uncoated one. In Adnan Maqbool's research, 1.0 wt% copper coated CNTs/Al composite was fabricated by using electroless plating and powder metallurgy method [16]. Compared with the uncoated MWCNTs/Al composite, improved MWCNTs dispersion and tensile strength were obtained in the coated one. Above all, the creation of a metal coating on MWCNTs through electroless plating can be efficient in improving the wettability between MWCNTs and aluminum thus enhancing the mechanical properties of the composite, which shows broad application potential in fabricating high-performance aluminum matrix composites.

On the other hand, hybrid composites, which contain two or more than two kinds of reinforcements, have been investigated extensively due to the combination and compromise of the superior properties of all reinforcements [17]. By using squeeze casting technique, Zhang et al. [18] fabricated alumina fiber and particle reinforced aluminum matrix hybrid composites, which in possession of higher modulus and strength but lower elongation compared with composites contain monolithic reinforcement. In fact, owning to the deformation incompatibility, reinforcements in micron scale are usually not favorable for the improvement in ductility [[19], [20], [21]]. In our previous work [22], 2024Al hybrid composites reinforced with SiC whiskers and SiC nano-particles were fabricated by squeeze casting technique. Compared with monolithic SiC whisker reinforced 2024Al composites, composites with the addition of SiC nano-particles showed obvious improvement in ultimate tensile strength and modulus. In Deng's research [23,24], micron and submicron SiC particles reinforced magnesium composites were fabricated by stir casting technique and hot extrusion processes. The addition of submicron SiC particles significantly enhanced the grain refinement effect and improved the mechanical properties of the composites by the enhanced Orowan strengthening mechanism. It should be noted that the superiority of hybrid composites over monolithic ones was reflected in the improvement of mechanical properties, and in the optimization of the reinforcement distribution. For example, Deng et al. reported that in bimodal SiC particles (micron and submicron) reinforced Mg composites fabricated by stir casting technique, the introduction of micron SiC particles were favorable for improving the dispersion of submicron SiC particles [23]. Noting that uniform dispersed MWCNTs in Al matrix is difficult to achieve in monolithic MWCNT/Al composites [25,26], the development of hybrid composites might inspire new strategies of fabricating high-performance MWCNTs reinforced Al composites with improved MWCNTs dispersity. Therefore, the present work intends to combine the abovementioned two factors together by introducing the surface modification on MWCNTs and another micron-sized reinforcement into MWCNTs/Al composite, aiming to fabricate higher performance aluminum matrix composites.

Here, micron-scale ABOw and nano-scale MWCNTs hybrid aluminum composites were designed and fabricated. The introduction of micron-scale ABOw provides a giant surface for MWCNTs to adhere, which is favorable for the homogeneous dispersion of MWCNTs. To improve the wettability between MWCNTs and aluminum alloy, MWCNTs were coated with nickel by electroless plating. The effect of plating parameters, including pH and plating time etc., on the morphology of nickel layer was investigated, by which the optimized electroless plating parameters were obtained. Composite reinforced with uncoated MWCNTs was also fabricated by the same methods as a counterpart. The effects of nickel coating on the microstructure and mechanical properties of composite were revealed. The fracture mechanisms affected by nickel-coated MWCNTs were analyzed in detail. Even though hybrid composites reinforced by micron-scale whiskers and submicron- or nano-scale reinforcements have been reported before, while the combination of hybrid reinforce and electroless plating in fabricating MWCNTs/Al composites have not been reported yet. Our research might bring new sight into a new fabrication strategy for high-performance MWCNTs/Al composites.

Section snippets

Electroless plating of nickel on MWCNTs

The MWCNTs were supplied by Shenzhen Nanotech Port Company, with diameter ∼20–50 nm and length from hundreds of nanometers to micrometers. Before electroless plating, the MWCNTs were firstly purified by refluxing in a mixed solution of 98% H2SO4 and 67% HNO3 with volume ratio 3:1 for 48 h. The transmission electron microscope (TEM) and the corresponding high-resolution TEM (HRTEM) images of the purified MWCNTs are shown in Fig. 1(a) and (b), respectively. It can be seen from Fig. 1(a) that the

Morphology of nickel coated MWCNTs

Fig. 3 displays the TEM bright field image of nickel coated MWCNTs. As we can see from Fig. 1(a) that the TEM bright field image of the raw MWCNTs exhibits grey and transparent contrast, therefore, one may distinguish the coated nickel layers between continuous and discontinuous by the transparency of the TEM bright field images. As shown in Fig. 3(a), nano-scale discontinuous nickel particles were formed on the surface of the MWCNTs when plated at pH = 8.25 for 30 min, where the transparent

Conclusions

A nickel layer was coated on MWCNTs to improve the wettability and interfacial bonding strength between MWCNTs and aluminum matrix. ABOw and MWCNTs(Ni) reinforced 2024Al composite was fabricated by squeeze casting technique. The microstructure and mechanical properties were investigated. The main conclusions can be drawn as follows:

  • 1)

    Continuous nickel layer with uniform thickness was coated on MWCNTs by electroless plating at room temperature with pH = 9.5 and coating time 60 min.

  • 2)

    Reduced porosity

Acknowledgement

MFQ, XXZ and LG greatly acknowledge the financial supports from National Key R&D program of China (grant number 2017YFB0703103), National Natural Science Foundation of China (NSFC) (grant number U1537201), China Postdoctoral Science Foundation (grant number 2017M620114) and the Fundamental Research Funds for the Central Universities (grant number HIT. NSRIF.201801).

References (35)

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