Effect of dispersion method on the deterioration, interfacial interactions and re-agglomeration of carbon nanotubes in titanium metal matrix composites

Highlights

• Sonication assisted ball milling was proved effective in dispersion of carbon nanotubes into titanium.

• An optimal energy in achieving uniform dispersion of carbon nanotubes in titanium matrix was calculated.

• The developed dispersion method improved the mechanical properties of the carbon nanotubes reinforced titanium composites.

• Raman spectroscopy is a useful tool in analyzing the evolution of carbon nanotubes.

Abstract

The influence of various synthesis techniques on the dispersion and evolution of multi-walled carbon nanotubes (MWCNTs) in titanium (Ti) metal matrix composites (TMCs) prepared via powder metallurgy routes has been investigated. The synthesis techniques included sonication, high energy ball milling (HEBM), cold compaction, high temperature vacuum sintering and spark plasma sintering (SPS). Powder mixtures of Ti and MWCNTs (0.5 wt.%) were processed by HEBM in two batches: (i) ball milling of the mixtures (Batch 1) and (ii) ball milling of Ti powder alone, followed by a further ball milling with sonicated MWCNTs (Batch 2). Both batches of the powder mixtures were pressed at 40 MPa into green compacts and then sintered in vacuum. Batch 2 powder mixtures were also consolidated using SPS. The crystallinity and sp² carbon network of the MWCNTs were characterized through analyzing the characteristic Raman peak ratio (I_D/I_G) of each processed sample. X-ray diffraction (XRD) was used for phase identification. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to study the morphology of the MWCNTs in the powder mixtures. The evolution of MWCNTs during the fabrication process and mechanical properties of the sintered compacts were discussed in conjunction with the formation of nano-crystalline TiC.

Introduction

The extraordinary mechanical and thermal properties of multi-walled carbon nanotubes (MWCNTs) advocate their use as a promising reinforcement in metal matrix composites (MMCs). Their tubular morphology and superior mechanical properties (e.g., Young's modulus in the terapascal (TPa) range) make them an ideal reinforcement material for metal matrices [1]. Much research has been carried out to assess the attributes of MWCNTs reinforced aluminum (Al) [2], [3] and titanium (Ti) [4], [5], [6]. MWCNTs reinforced titanium metal matrix composites (TMCs) have the potential to offer significantly enhanced elastic modulus, thermal conductivity and other tribological properties of interest [5], [7], [8]. Improvement in these properties is particularly attractive for weight savings in the aerospace and automotive industries through replacing existing conventional materials. Recent developments in CNTs (0.35 wt.%) reinforced TMCs have shown significant enhancements in ultimate tensile strength and yield strength [4], [5]. The presence of sp² carbon-carbon (C–C) bonds in the MWCNTs imparts them excellent mechanical and thermal properties [9], [10]. In general, the superior properties and chemical stability of carbon nanotubes (CNTs) are attributed to the sp² C–C bonds t in their outer shells [10], [11], [12].

In the synthesis of MWCNTs reinforced TMCs, achieving a uniform dispersion of MWCNTs in the Ti metal matrix while retaining their sp² carbon network is a key challenge. MWCNTs tend to agglomerate because of their nano-scale dimensions and the strong Van der Waals forces among individual tubes [13]. In order to solve the dispersion problem, high energy ball milling (HEBM) via powder metallurgy routes has emerged as an effective and economic technique to disperse MWCNTs in the metal matrices [1], [14]. However, it has been reported that harsh milling conditions produce defects in carbon nanotubes (CNTs) [15], [16]. These structural defects generate non-sp² disorders in the C–C network of MWCNTs, leading to loss in their unique properties. Deterioration of the strong sp² carbon network in MWCNTs results in the formation of vacancies and open edges in MWCNTs [17], [18]. The formation of highly reactive sp³ C–C amorphous phase is also capable of triggering interfacial reactions at the metal-reinforcement interface during ball milling, thus favoring the formation of carbides [19], [20]. In metal matrix composites (MMCs), the matrix transfers the load to the reinforcement during loading. Consequently, the matrix-reinforcement interfaces play an important role in defining the strengthening mechanism of the MMCs [21], [22]. It is therefore important to retain the essential load-bearing capability of MWCNTs during the various processing stages. Ultra-sonication is a physical method which can break up large agglomerates of CNTs to achieve uniform dispersions [23]. The de-bundling is attained through the interactions of the shock waves, produced by the sonicator, with the CNTs clusters in a dispersant medium, e.g., ethanol [24], [25], [26], [27], [28], [29], [30]. As a result, the dispersion and de-bundling of the CNTs are governed by the key sonication parameters including sonication time, sonication energy and intervals [31], [32]. The formation of open edges, vacancies and defects in the side walls of CNTs constitutes the non-sp²structural disorders in CNTs during ball milling, leading to a loss in their unique properties due to the conversion of sp² to sp³ hybridized carbons [33]. Furthermore, collapse, buckling and flattening of inner and side walls of MWCNTs have been reported previously when harsh ball milling conditions are employed [34]. The formation of non-sp² defects in the sp² C–C network of MWCNTs not only changes the morphology of the CNTs but also downgrades their unique properties. It has been found that the load-bearing capability of CNTs depends upon their tubular and structural integrity [35]. Hence processing techniques should be optimized to achieve a better dispersion while retaining the tubular structure of MWCNTs with high aspect ratios [36]. CNTs experience severe bending stresses during HEBM and sonication but their ultra-high elastic modulus (~ 1 TPa) allows effective stress relief upon unloading. On the other hand if such severely stressed CNTs are embedded into the metal matrices during composite processing stages then the residual stresses may lead to their breakage and the formation of a multi-layered graphite structure with a partial amorphous state [37], [38].

To date, the influence of different powder processing techniques on the sp² C–C network of MWCNTs and the resultant MWCNTs reinforced TMCs have not been adequately studied as yet. In this study, MWCNTs-Ti composites were synthesized using each of the following approaches, HEBM, sonication assisted HEBM, cold compaction, high temperature vacuum sintering and spark plasma sintering (SPS). The evolution of the MWCNTs during different stages of processing and their dispersion mechanism in the Ti matrix were quantitatively studied. The morphological changes and dispersion mechanism of MWCNTs, and the formation of the crystalline phase of TiC in the powder mixtures and sintered composites were investigated.