A novel approach to the uniformly distributed carbon nanotubes with intact structure in aluminum matrix composite
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Achieving a uniform dispersion of carbon nanotubes (CNTs) in metal matrix composites (MMCs) is a vital prerequisite for enhancing the mechanical properties of the samples. In this work, a novel strategy called little by little (LbL) adding was successfully adopted in order to achieve a uniform dispersion of CNTs. The Al composite powders reinforced with different amounts of CNTs (1, 2, 3, and 5 wt.%) were consolidated by spark plasma sintering (SPS). Based on scanning electron microscopy (SEM) as well as Raman spectroscopy, individual and uniform dispersion of CNTs without any serious structural damage was achieved in Al matrix. In this regard, the relative intensity ratio (ID/IG) of the raw CNTs was around 0.85 and that of LbL composite was ~ 0.9, indicating that the CNT structure experienced very little damage during the applied method. As a result of the uniform dispersion, there was no drastic reduction in mechanical property (microhardness) of the LbL-composites with increasing CNT content. Based on the tribological tests, it was found that the dominant wear mechanism of the LbL composite is abrasive wear accompanied by adhesion wear. Moreover, for the reference composites, produced by conventional wet milling process, the dominant wear mechanism was severe adhesive.
KeywordsAluminum Carbon nanotubes Metal-matrix composites (MMCs) Raman spectroscopy Uniform dispersion
Carbon nanotubes (CNTs) have been considered as ideal nano reinforcements for strengthening metal matrix composites due to their one-dimensional structures with large aspect ratios and unique mechanical, thermal, and electrical properties [1, 2, 3, 4, 5, 6]. In the past decade, increasing attention was particularly paid to CNT reinforced Al matrix composites (AMCs), because the light weight along with high strength made them interesting structural materials in aerospace and automobile industries, where the fuel economy and weight reduction are the first priority [7, 8, 9, 10]. It is worth noting that Al and its alloys are the first option for the composite matrix reinforced by CNTs. As mentioned before, AMCs are being considered as a group of newly advanced materials for their light weight, high strength, high specific modulus, low coefficient of thermal expansion, and good wear resistance properties . These composites are fast becoming favorite choice in numerous applications such as bearing sleeves, piston, gears, valves, and cylinder liners. In many of these applications excellent friction and wear performance are required [8, 9, 10]. On the other hand, because of the mentioned extraordinary mechanical, thermal and electrical properties of CNTs, they have been used to improve the properties of many different materials such as polymers, metals, and ceramics. Specifically, CNT-reinforced metal matrix composites possess a huge potential to be widely used in structural and functional applications, such as automobile, aerospace, sports, micro-electrochemical systems (MEMS), sensors, battery, and energy storage .
However, theoretical potential of CNTs has not yet been achieved in metal matrix composites [11, 12]. In order to acquire high strengthening potential of nanotubes in AMCs, a homogeneous dispersion of un-bundled CNTs is an essential issue. Nevertheless, nanotubes naturally tend to self-assemble into clusters even after attempts are made to disperse them as a result of the strong attractive force due to their high specific surface area [13, 14, 15]. Although many studies have been made to deal with nanotube dispersion, it is still a great challenge to uniformly distribute CNTs into AMCs with small structural damages [13, 16]. Recently, high energy ball-milling (HEBM) has been widely used to disperse CNTs into the metal matrix composites. It has been reported that mechanical milling can produce a composite with homogeneous distribution of CNTs . Despite the good dispersion that can be achieved by HEBM, the morphology and structure of CNTs is inevitably damaged during mechanical milling, which will be detrimental to the strengthening effect of CNTs . In other words, severe structural damages including CNT shortening and crystal-structure changes seem to be unavoidable at the sacrifice of appropriate CNT dispersion [17, 18, 19]. Apart from HEBM, several approaches, such as in situ synthesis , spray-drying, and nano-scale dispersion (NSD) [21, 22], have also been reported in order to produce homogeneous CNT/Al composites through powder metallurgy (PM). However, they still failed to achieve uniform and individual distribution of CNTs with intact structure. Based on the previous studies, there is a dilemma for the applied methods to keep balance between homogeneous dispersion and maintenance of the structural integrity of nanotubes. In this regard, a novel and unique development of a production process which promotes a homogeneous dispersion of CNTs in the metal matrix without damaging them, is necessary for obtaining nanocomposites with excellent mechanical and physical properties. Moreover, a significant progress in the future manufacturing of CNT-reinforced metal matrix composites is strongly essential when compared with the previous methods.
Herein, we reported for the first time a novel strategy called little by little (LbL) adding through which a uniform distribution of CNTs with intact structure can be achieved in Al composites. SEM as well as Raman spectroscopy was used to evaluate the quality of dispersion and structural integrity of CNT. Furthermore, microhardness and pin-on-disc wear tests were utilized to examine the mechanical and tribological behavior of the resultant composites.
2 Experimental procedure
Multiwall carbon nanotubes (MWCNTs) with outer diameter of around 20–40 nm and purity of more than 95% were manufactured by US Research Nanomaterials. Flaky Al powders (101,056 Merck, purity ˃ 90%) were also supplied in order to elevate the quality of CNT dispersion. Two techniques were used for preparing the powder mixture; typical solution ball milling for 1 h (named conventional method) versus novel method based on solution milling at 6 stages (named LbL). From now on, the novel applied method is called little by little and will be discussed later in details. In the solution (ethanol) milling, ball to powder weight ratio (BPR) and rotation speed were 20:1 and 300, respectively. The obtained composite powders (1, 2, 3, and 5 wt.% CNT/Al) were sintered at 500 °C, with a holding time of 10 min, a heating rate of 50 °C/min, and a pressure of 40 MPa in a Φ10 mm graphite mold, using a SPS apparatus installed at Ferdowsi University of Mashhad (Iran). Microstructures of the composites were studied by a scanning electron microscope (SEM, LEO 1450VP) equipped with energy dispersive spectrometry (EDS) analysis. A transmission electron microscope (TEM, LEO 912AB) with an operating voltage of 120 kV was also utilized. Raman spectroscopy (Teksan-Takram P50C0R10) was performed by using a 532-nm argon laser as the excitation source to evaluate the structural integrity of CNTs. In addition, a microhardness tester (Buehler model 1600.61025) with a load of 100 g and dwelling time of 10 s was carried out to examine the mechanical properties of the samples. The average values of 10 points were calculated and reported. Finally, in order to evaluate the tribological behavior of the composites, pin-on-disk wear test was performed at room temperature, using a normal load of 10 N and a sliding speed of 0.2 m/s. An abrasive SiC disc with a hardness of approximately 2500 HV was used as a counterpart. Prior to each test, surface of the samples was carefully polished and cleaned ultrasonically. The sliding distance was kept constant at 200 m.
3 Results and discussion
It is worth mentioning that the dispersion stage is very important for reducing the number of clustering. Although short sintering duration of SPS effectively reduces CNT agglomeration, clusters formed in the previous processing stages, namely mixing and dispersing stage, could be carried over in the next step. It was found that applying LbL method reduces CNT clusters and finally results into more homogeneous dispersion of nanotubes. However, future studies need to be directed at other important CNT-MMC systems to prepare industry-acceptable process maps under powder metallurgy approach to achieve individual dispersion of CNTs without clustering. The novel applied method, in this study, controlled clustering of CNTs and contributed to the significant decrease of CNT agglomeration and clusters, which is helpful to eliminate internal defects and stress concentration as well as effective load transfer ability.
Hc and Hm is hardness of composite and matrix, respectively. In addition, VCNT refers to volume fraction of CNT. The calculated data are also compared with the results of CNT/Al composites in other publications (see Fig. 6b). According to Fig. 6b, LbL-composites show the best strengthening efficiency of CNTs. It can be concluded that the applied method (LbL) could improve strengthening efficiency of CNT to the values higher than that of low energy blending (LEB), high energy ball milling (HEBM), nano-scale dispersion (NSD) as well as in situ and flake powder metallurgy methods with relatively better dispersion and less damage of CNT. The highest R of the Al composite containing ~ 2 wt.% CNT produced by flake powder metallurgy  was around 40, which is about 42% lower than the achieved value of R at the same amount of LbL-CNT in this study. The higher strengthening efficiency makes LbL-composite a promising material in the new class of high-performance MMCs. It can be concluded that CNT dispersion in the powder stage is very critical for obtaining high strengthening in the resultant composite. Using LbL technique leads to better distribution of nanotubes causing better CNT–matrix contact and load transfer, and finally, composite prepared by this method show better strengthening.
In conclusion, LbL method has been proved to be successful in the dispersion of CNTs without serious damage in Al matrix and can be used in other nanocomposites. It is convincing that LbL technique would provide capable thoughts for further developments of MMCs in wide range of applications in the future.
A novel and effective approach, namely little by little (LbL) adding, for the uniform dispersion of CNTs in Al matrix was successfully adopted in this study. In order to evaluate the effect of CNT content, Al composite powders reinforced with different amounts of CNTs (1, 2, 3, and 5 wt.%) were consolidated through spark plasma sintering (SPS). According to the SEM images and Raman spectroscopy data, it was found that the individual dispersion of CNTs with intact structures can be achieved in Al matrix through LbL method. The relative intensity ratio (ID/IG) of the raw CNTs was ~ 0.85 and that of LbL composite was ~ 0.9, indicating that the CNT structure experienced very little damage during LbL. It was revealed that with increasing CNT content from 1 to 2 wt.%, the microhardness values increase and thereafter decrease up to 5 wt.% at both conventional and LbL methods. However, unlike the reference samples, the variation of mechanical values of the LbL-composites was so smooth (~ 10%) while that of reference composites was around 55.5% which could be attributed to the uniform dispersion of CNT by LbL technique. In addition, the hardness values of LbL composites were higher than that of reference-composites, revealing the effectiveness of the applied method. As an instance, 2 wt.% CNT/Al LbL composite exhibited ~ 136% and ~ 8% increase in Vickers hardness, compared with pure Al and 2 wt.% CNT/Al Ref composite, respectively. Furthermore, comparing 5 wt.% CNT/Al composites produced through different methods, it was shown that around 51% increase in the hardness can be achieved by utilizing the LbL method. Based on the tribological tests, it was found that the dominant wear mechanism of the LbL composite is abrasive wear accompanied by mild adhesion wear, while for the reference composites, the dominant wear mechanism was severe adhesive. Furthermore, a reduction of around 20% in wear weight loss was achieved for 2 wt.% and 5 wt.% LbL-CNT/Al composites compared to the reference samples. Finally, a successful development of a production process that promotes a homogeneous dispersion of CNTs in the matrix without damaging them is essential for obtaining nanocomposites with excellent mechanical and physical properties, and of course a significant progress in the future manufacturing of CNT/metal matrix composite when compared with the previous methods is strongly needed. Beside the mentioned advantages of uniform dispersion of CNT in the matrix (in comparison with reference samples) without scarifying structural integrity of the tubes, which were achieved by LbL method, very small clusters of CNTs were still observed in the microstructure. Therefore, future work needs to focus on maintaining the integrity while achieving individual dispersion of CNTs without any clustering.
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Conflict of interest
The authors declare that there is no conflict of interest.