Strengthening in CNT-Al composites produced by High-Energy Ball

Milling

Xiaofei Wanga, Xiaolan Caib*

Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093

alangzixiaofei1984@163.com,

bcxl9761@163.com

Keywords: carbon nanotubes, aluminum matrix composites, High-Energy Ball Milling, mechanical properties

Abstract. In this paper, carbon nanotubes (CNT)-reinforced aluminum (Al) matrix composites were

fabricated by High-Energy Ball Milling, the objective was to investigate the evolvement of particle

size, density and hardness of CNT-Al composites with increasing wt% CNT, and analyzed the

micrographs of mixture powders at different milling time. The results showed that the addition of

CNT can play a role of grinding aid to refine grain, improve the hardness and decrease the density, and

CNT can be homogeneous dispersed in the matrix with increasing ball-milling time, it also showed

that too much CNT was no help on hardness, this attributed to clustering of CNT, the proper addition

of CNT was 2wt%, and the mixture powders could reached a state of equilibrium between fracturing

and cold-welding at 75min.

Introduction

Carbon nanotubes (CNT) have attracted much attentions due to their unique properties. CNT have

Young,s modulus ranging from 1 to 5 TPa, CNT possess relatively low density ranging from 1.2 to 1.8

g/cm3, while the Young,s modulus of graphite is 1 TPa, and its density is 2.26 g/cm

3[1].

In the field of metal matrix composites[2-6], aluminum-based composites is an important area due

to their low density and good workability, its properties are attractive for diverse industrial

applications, mainly to meet the demanded materials by the aeronautic industry and automobile

manufacturing.

CNT-reinforced aluminum (Al) matrix can be successfully produced by ball-milling, and also

CNT can achieve uniform dispersion in aluminum matrix. This article studied the particle size

distributions of the aluminum-CNT mixture powders, the effect of milling time and wt% CNT on the

hardness and density of CNT-reinforced aluminum composites were also studied, and investigated the

morphological evolution of the mixture powders with increasing ball-milling time.

Experimental

Multi-wall carbon nanotubes (MWNTs) (CVD-grown MWNTs of 40~60 nm average diameter and

5~15 µm length, provided by ShenZhen NanoPort Company, whose purity is about 95% as claimed

by the producer) and commercial pure aluminum (mean particle size is about 10 µm) were used as the

starting materials. Took some CNT in concentrated nitric acid for 2 h in order to remove the impurity,

then CNT were washed with distilled water until the washings showed no acidity, in order to disperse

CNT, the as-received CNT were treated in ethanol by using an ultrasonic shaker for 15 min in order to

retain uniform distribution, the CNT were then dried in a dry oven until ethanol and moisture were

completely removed.

Both pure aluminum and CNT were putted in 2 L High-Energy Ball Milling jar, giving the ball to

powder ratio of 25:1, the jar, filled with argon and added 1 gram stearic acid as a process control agent

in order to minimize cold welding of the aluminum particles and also prevent powders sticking to the

balls and jar walls. The jar was then agitated using a ball-milling at 400 rpm for 90 min, the mixed

Advanced Materials Research Vols. 236-238 (2011) pp 2336-2339Online available since 2011/May/12 at www.scientific.net© (2011) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.236-238.2336

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powders were then put into a rectangle opening mould compaction at 500MPa, the compacts were

subsequently sintered in a vacuum furnace at 550±5˚C for 60min.

The particle size distribution of the mixed powders were analyzed using laser particle size

analyzer, the morphological evolution of the mixed powders was analyzed using scanning electron

microscope (SEM), the density of the composites were measured using the Archimedes technique and

the mechanical properties were evaluated using Brinell hardness.

Results and discussion

Fig.1 (a) shows the SEM image of raw CNT, it is evident that CNT were tangled together, most of

them were not straight and localized kinks and bends. Fig.1 (b) shows the SEM image of purified

CNT, they were uniformly dispersed, and their surfaces were very clean, no other impurities particles

can be found in SEM image.

(a) Raw CNT

(b) Purified CNT

Fig.1 SEM images of CNT

Table 1 shows the effect of increasing wt% CNT on the median diameter, D50, where take D50 as

the 50% size of the cumulative particle size distribution. It is evident that at the same ball-milling

time D50 become smaller with increasing wt% CNT, this indicate that CNT play a role of the

grinding aids to refine grain. It also can be seen that the addition of 4wt% CNT has decreased D50 by

about 26% at 90min, compared to ball-milled pure aluminum powders at the same conditions. But

the addition of 5wt% CNT would lead to D50 became big on the contrary, it is proposed that proper

amount of CNT have a function of refined grain, but when the addition of CNT is too much, there

will be much CNT among aluminum powders, this will absorb the energy transmitting into the

milling jar to prevent the fracture of aluminum powders, but at the same time the input energy

maintain unchanged, so the structure of CNT will be damaged, this will reduce the mechanical

properties of the composites, so the proper amount of CNT is very important.

Table1 The effect of wt% CNT on D50 at the same time

Ball-milling time (min) 15 30 45 60 75 90

D50/(µm)-pure Al 13.24 19.07 21.87 24.66 29.65 33.97

D50/(µm)-Al+1 wt %CNT 12.7 18.71 21.46 24.51 29.58 33.17

D50/(µm)-Al+2 wt %CNT 12.54 18.16 21.22 24.41 29.18 32.88

D50/(µm)-Al+3 wt %CNT 12.16 17.83 20.24 23.75 28.33 31.71

D50/(µm)-Al+4 wt %CNT 12.02 14.82 18.16 20.9 23.11 25.07

D50/(µm)-Al+5 wt %CNT 12.85 16.76 18.66 21.88 22.44 25.6

In order to further understand the effect of ball-milling time on the microstructure of mixture

powders, this study took 3wt% CNT samples for example to observe the SEM micrographs, as shown

in Fig.2, it is clear that the distribution of CNT in aluminum powders was different with various

ball-milling time, the amount of CNT on the surface of aluminum powders decreased with increasing

Advanced Materials Research Vols. 236-238 2337

ball-milling time, there are two reasons for this phenomenon: one is CNT were homogeneous

dispersed in the aluminum powders at the process of milling; the other is CNT were gradually buried

under the aluminum surfaces. It is also evident that CNT became shorter and some of them embedded

in the aluminum powders with increasing ball-milling time, there are only little CNT can be seen on

the surface of aluminum powders at 90 min.

a. Milling 30min b. Milling 60min c. Milling 90min

Fig.2 The effect of milling time on SEM micrographs of 3wt% CNT samples

Fig. 3 shows the results of the hardness measurements. It is clear that both increasing ball milling

time and wt% CNT (from 0 to 5) leaded to the increase in hardness of the composites. For the pure

aluminum that ball-milled, it also can be seen that the hardness have increased by about 95.2% at 90

min, compared to the pure aluminum that un-milled. For the samples that added various wt% CNT,

the hardness measurement have been increased by about 6.8%, 52.9%, 51.4%, 48.4%, 52.6% at

90min, for the 1, 2, 3, 4 and 5wt% CNT samples, respectively, compared to the pure aluminum that

ball-milled. It is deduced that the process of milling leaded to strain hardening of the mixture powders

that increased hardness of composites. The hardness was increased highly with the addition of CNT, it

was because CNT were homogeneous dispersed in the aluminum matrix with increasing ball-milling

time, they could play the role of transmit load. The highest hardness was observed for the 2wt%

samples, when continued to increase the amount of CNT, the hardness became decreased, this could

be attributed to the presence of CNT clusters in the composites.

10 20 30 40 50 60 70 80 90 10020

25

30

35

40

45

50

55

60

65

70

75

Hardness (HB)

Milling time (min)

0wt% CNT

1wt% CNT

2wt% CNT

3wt% CNT

4wt% CNT

5wt% CNT

unmilled

Fig.3 The effect of ball-milling time and wt% CNT on hardness of composites

The effect of ball-milling time and wt% CNT on density of composites are presented in Fig.4. It is

evident that the density of every sample increased with increasing ball-milling time, because the

aluminum powders we used was spheroid, they became flake-like at the process of milling as seen in

Fig.2, this would decreased the porosity among the composites, but the density have slightly

decreased for all of the samples at 90 min. We all known that sufficient ball-milling time is better for

dispersion of CNT in aluminum powders, but the density would decreased when milling too long (90

min) due to higher porosities inherent to the sintering of large-diameter powders[7]. The density

decreased as increased the amount of CNT, it can be attributed to the presence of CNT cluster, but

2338 Application of Chemical Engineering

clustering was significantly improved because CNT was uniformly dispersed in the aluminum

powders with increasing ball-milled time, and most of the CNT were embedded in the aluminum

powders after sufficient ball-milling (about 75min), so the density reached to the highest at 75 min.

10 20 30 40 50 60 70 80 90 1002.45

2.50

2.55

2.60

2.65

2.70

Density (g/cm

3)

Milling time (min)

0wt% CNT

1wt% CNT

2wt% CNT

3wt% CNT

4wt% CNT

5wt% CNT

Fig.4 The effect of ball-milling time and wt% CNT on density of composites

Conclusions

The results reported here suggest that High-Energy Ball Milling is an active route to produce CNT-Al

composites, and clustering was significantly improved in the mixture powders, both hardness and

density of the composites were increased with increasing ball-milling time. But we have to point out

that the length of CNT could be shorten or damaged when ball-milling too long time (above 75min),

and also the amount of CNT in the aluminum matrix is very important to affect the mechanical

properties, so careful choice of the condition of ball-milling would eventually affect the mechanical

properties of CNT-Al composites.

References

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[2] Kuzumaki T, Miyazawa K, Ichinose H, et al. Materials Research Society, Vol.13(1998), p.2445

[3] Zhong R, Cong HT, Hou PX. Carbon, Vol.41(2003), p.848

[4] George R, Kashyap KT, Rahul R, et al.Scripta Materialia, Vol.53(2005), p.1159

[5] Esawi AMK, Mostafa AEB. Compsites Science and Technology, Vol.68(2008), p.486

[6] Wang L, Choi H, Myoung JM, et al. Carbon, Vol.47(2009), p.3427

[7] Xu CH, Sun DM. Materials Science and Engineering: A, Vol.491(2008), p.338

Advanced Materials Research Vols. 236-238 2339

Application of Chemical Engineering 10.4028/www.scientific.net/AMR.236-238 Strengthening in CNT-Al Composites Produced by High-Energy Ball Milling 10.4028/www.scientific.net/AMR.236-238.2336