July/August 2004

Tiny tubes boost for metal matrix composites

Novel light metal matrix composites reinforced by carbon nanotubes have been produced by a Swiss research group using powder metallurgical methods. The magnesium composites exhibited a substantially higher Young's modulus than unreinforced sintered magnesium…

Since their discovery by Ijima in the early 1990s [1], carbon nanotubes (CNT) have been the subject of considerable research efforts to characterise and understand their remarkable mechanical, electrical and thermal properties. In particular, their high Young's moduli of up to 1.2 TPa make CNT the ultimate high-strength fibres for use as reinforcements in composite materials [2]. In recent years, the research has been focused on the development of nanotube reinforced polymer-based composites [3], and only a few studies have been concerned with the manufacture of nanotube-reinforced metal-matrix composites. This remains almost a virgin field [3-4].

To begin their investigation, the Swiss team produced multi-wall carbon nanotubes by catalytic vapour deposition at the Institute of Physics of Complex Matter - EPFL Lausanne. The route chosen was the catalytic decomposition of acetylene in a fixed-bed flow reactor [5]. Commercially available magnesium powder (99.8 per cent purity, average particle size 38 ?m, Alfa Aesar) was used as base metal powder. Mg-2wt%CNT powder mixtures were prepared by dry blending for four hours in a Turbula T2C mixer.

The blends were placed in double-action graphite tooling consisting of a die and two cylindrical pistons. Disk-shaped compacts (Ø 53 mm x 5mm) were obtained by hot pressing at 600°C in a vacuum atmosphere, under a pressure of 50 MPa for 30 min. Finally, the compacts where hot isostatic pressed at 600°C for 60 min under an argon pressure of 1800 bar. The density of the sintered parts was measured by the Archimedes method.

Mechanical properties where evaluated by tensile tests and Young's modulus measurements in a resonant apparatus (free-free bar), which allows one to tune the resonant frequency f (in the kHz range) in flexural mode for specimens 4 mm x 1 mm x 50 mm. The Young's modulus E is given by:

where l and a are the specimen length and thickness respectively, and r is the density [6]. Specimens were obtained by spark-machining. Scanning electron microscopy observations of specimens subjected to flexural fracture were performed in a Philips XL 30 FEG microscope.

The measured density of the sintered compacts was up to 98 per cent of the theoretical density calculated from a mixture law. The stress-strain curves measured in tensile tests (Figure 1) exhibit a ductile metallic behaviour, which suggests good bonding between carbon nanotubes and the magnesium matrix. Tensile properties such as yield strength, rupture strength and ultimate strain, are similar to those measured in unreinforced sintered magnesium.

yield strength s_0.2 89 MPa,

tensile strength s_TS 140 MPa

strain after fracture e_f ~ 3 %

Resonant measurements show that the addition of 2 wt% CNT results in an improvement of more than 9 per cent in the Young's modulus, compared with unreinforced sintered magnesium (Figure 2). Further improvement of E can be obtained by improving interface adhesion and load transfer from the matrix to the reinforcement. It could be accomplished by applying appropriate surface treatments to the nanotubes [5] (see "Coating the tubes" below).

Another way to improve mechanical properties could be the use of electric-arc discharge processed multi-wall or single-wall nanotubes instead of CVD processed multi-wall nanotubes. However, the arc-discharged technique cannot be used to produce large quantities of nanotubes [3] at the moment.

Figure 1: Stress-strain behaviour of Mg-2wt%CNT composites measured in tensile tests.

Scanning electron microscopy of the fracture surface of an Mg-2wt% CNT specimen (Figure 3) reveals that carbon nanotubes are uniformly dispersed in the magnesium matrix.

More work in progress

This uniform dispersion of reinforcement, together with the overall performance of the processed composites, show that potential sources of weakness such as nanotube agglomerates can be avoided by using appropriate mixing and sintering processes.

Carbon nanotube reinforced magnesium was produced by dry blending base powders followed by hot pressing and hot isostatic pressing. The microstructure and mechanical properties of Mg-2wt%CNT were characterised by scanning electron microscopy, tensile tests, and resonant bar measurements. A uniform dispersion of nanotubes in the magnesium matrix was observed. Although yield strength, rupture strength and ultimate strain are similar to those observed in unreinforced sintered magnesium, the addition of 2wt%CNT results in an improvement in Young's modulus of more than 9 per cent. A method of coating nanotubes with magnesium was developed. This is intended to improve interface bonding strength in sintered components.

The feasibility of manufacturing metal-matrix composites reinforced with carbon nanotubes has been assessed and the same processing steps used for Mg-2wt%CNT used to produce Al 2wt%CNT. Aluminium-CNT compacts of density up to 96 per cent theoretical have been obtained and this research is in progress.

Figure 2: Young's modulus of unreinforced and carbon nanotube reinforced magnesium.

Figure 3: Microstructure of a rupture surface of Mg-2wt%CNTs.

Coating the tubes

Carbon nanotubes have recently been coated with various inorganic materials such as alumina, silica and titanium dioxide. The work featured here focused on metallic magnesium coatings. Purified multi-wall CNTs were used as plain or modified by the surfactant sodium dodecyl sulphate - SDS. The impregnation was carried out with and without solvent for both types of CNTs. With magnesium chloride as a source of magnesium, a thick layer of magnesia was obtained over SDS-treated CNTs, but plain tubes showed only slight coverage. However, homogenous coverage of magnesium was obtained on plain CNTs when the impregnation was carried out using organometallic and inorganic sources. Figure 4a shows a representative tracking electron microscope (TEM) image of a plain multi-wall CNT after coating with magnesium. The coating is about 2nm thick and the nanotube is homogenously covered. EDX analysis (Figure 4b) clearly shows the presence of magnesium as coating material.

Figure 4a. A tracking electron microscope micrograph of a multi-wall CNT with MG coating. White arrows indicate the homogenous coating of Mg with a thickness of 2nm.

Figure 4b. The EDX analysis spectrum reveals a significant peak of Mg. The Cu peaks are due to the TEM grid used to support samples and the Ti peak is an artefact of the EDX set-up. No significant peak of O Ka at 0.523 keV is visible.

The team
The work on which this article is based was published as Carbon nanotube-reinforced metal matrix composites given at EuroPM 2003 in Valencia. The team that carried it out was drawn from the Material Design Group at the University of Applied Sciences of Western Switzerland, Haute Ecole Valaisanne and the Institute of Physics of Complex Matter at the Swiss Federal Institute of Technology Lausanne. Led by Professor Efrain Carreño-Morelli, they were J Yang, R Schaller and C Bonjour.

References
1. Ijima, S, Nature, 1991, 354, 56.
2. Salvetat, J-P; Briggs, G A D; Bonard, J-M; Bacsa, R R; Kulik, A J; Stöckli, T; Burnham, N A; Forró, L; Phys. Rev. Lett., 1999, 82, 944.
3. Thostenson, E T; Ren, Z; and Chow, T-W; Comp. Sc. Tech, 2001, 61, 1899.
4. Xu, C L; Wie, B Q; Ma, R Z; Liang, J; Ma, X K; and Wu, D H; Carbon, 1999, 37, 855.
5. Hernadi, K; Ljubovic, E; Seo, J W; and Forró, L; Acta Mater., 2003, 51, 1447.
6. Vittoz, B ; Secrétan, B; and Martinet, B ; J. Appl. Math. Phys., 1963, 14, 46.