Materials Science and Engineering A 504 (2009) 157–162

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Materials Science and Engineering A
journal homepage: www.elsevier.com/locate/msea

Friction and wear behavior of sintered magnesium composite reinforced with
CNT-Mg2 Si/MgO
Junko Umeda, Katsuyoshi Kondoh ∗ , Hisashi Imai
Joining and Welding Research Institute, Osaka University, 11-1, Mihogaoka, Ibaragi, Osaka 567-0047 Japana r t i c l e

i n f oArticle history:
Received 28 August 2008
Received in revised form 18 October 2008
Accepted 27 October 2008
Keywords:
Magnesium
SiO2
Mg2 SiCarbon nanotube
Friction coefﬁcient
Powder metallurgya b s t r a c t
In order to improve and evaluate the wear behavior of the sintered magnesium materials under dry sliding
conditions, the effect of carbon nanotubes (CNTs) and Mg2 Si/MgO compounds, which were reinforcementsof the sintered material, on the friction coefﬁcient and wear loss under dry conditions was discussed. Oneof the raw materials was amorphous and porous silica particles originated from rice husks, which were
coated with CNTs and contained nanotubes in the pores. The in situ synthesis of Mg2 Si and MgO via
deoxidization and oxidation reaction occurred from the elemental mixture of pure magnesium and CNTSiO2 composite particles via the spark plasma sintering process. The friction coefﬁcient of the composite
material was low and stable because of no adhesion and stick-slip phenomenon in contacting with the
SUS304 stainless steel ball as a counter material. The wear rate of sintered magnesium materials decreases
in increasing the content of CNTs and Mg2 Si. The friction coefﬁcient was proportional to the total wear
loss of magnesium materials. These results were due to both of the defensive by hard Mg2 Si dispersoids
and self-lubricant effect by network nanotubes on the sliding surface.
© 2008 Elsevier B.V. All rights reserved.

1. Introduction
Recently magnesium alloys are widely applied to structuralcomponents both to reduce the energy consumption and to
improve the mobility of personal digital assistants because they arelightest of the industrial metals in the practical use. They are, however, poor to the mechanical properties, such as Young’s modulus,
tensile strength, hardness and heat resistance. In particular, when
applying them to friction materials, the wear or seizure phenomena easily occur by contacting with the counter materials [1–3].
Therefore, the additives of hard particles and lubricants are effective to improve the mechanical and tribological properties of theconventional magnesium alloys [4–7]. On the other hand, carbon
nanotubes (CNTs), showing excellent characteristics such as hightensile strength, high elastic modulus, and high hardness, havebeen considered as useful and attractive additives to organic materials [8,9], bulky metals [10,11] and metallic coatings [12,13] fortribological and structural applications. In particular, according to
the theoretical consideration [14], a friction coefﬁcient between
the walls of multi-walled CNTs should be extremely low. That is,
CNT has a signiﬁcant self-lubricant property by nano-ball bearing effects [15]. The bundles of CNTs are easily formed during∗ Corresponding author. Fax: +81 6 6879 4369.
E-mail address: kondoh@jwri.osaka-u.ac.jp (K. Kondoh).
0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.msea.2008.10.054

the conventional mixing or blending under dry conditions due tovan der Waals forces at the most-surface carbon atoms between
CNTs. Unfortunately, they cause the decrease of mechanical andwear properties because their bundles have poor bonding to thematrix. In order to obtain a stable and low friction coefﬁcient, it
is important to control the nanotubes left out from the matrix ofthe composites during contacting with the counterparts in sliding. On the other hand, magnesium silicides (Mg2 Si) intermetallics
are one of the effective dispersoids to improve the mechanicaland tribological performance of magnesium alloys because of theirhigh hardness of 350–700 Hv [16]. They are easily synthesized by
the solid-state reaction of magnesium powder and silicon or silica particles with exothermic heat [17]. By the synthesis of Mg2 Si
compounds from the elemental mixture of Mg and SiO2 powders,the magnesium composites reinforced with Mg2 Si/MgO particles
show a good wear resistance and anti-offensive performance insliding wear test under oil lubricant [18]. In this study, CNT-SiO2
composite particles have been employed as the reinforcements ofthe sintered magnesium materials to obstruct the CNTs dropped
out from their matrix in sliding. Porous structured SiO2 particles,
originated from rice husks [19], were used as raw materials, anddipped into the solutions including un-bundled CNTs. The composite powders were elementally mixed with the magnesium powder,and sintered after cold compaction. The microstructural analysis
and evaluation of the mechanical and tribological behavior of thesintered magnesium reinforced with CNTs and Mg2 Si/MgO com-

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pounds. The effects of Mg2 Si hard particles and nanotubes on thefriction coefﬁcient, surface damages and wear loss of the sinteredmagnesium material were investigated by using the ball on disc
wear test equipment under dry conditions.

magnesium composite material including CNT-Mg2 Si/MgO reinforcements was used as a disk specimen of wear test.

2. Experimental

SEM observation on amorphous and porous silica particles
coated with CNTs was carried out. Differential thermal analysis(DTA, Shimadzu, DTG-60) was taken place on the silica particles
and the elemental mixture of silica and pure magnesium powders
from 295 K to 973 K under an argon gas supplement with 2.5 ml/s toevaluate the reaction behavior of the silica coated with CNTs and theelementally mixed powders. XRD and SEM-EDS analysis were used
to detect the in situ formed intermetallics of the magnesium composites during sintering. Rock-well type hardness (F-scale, 1.59 mm
diameter steel ball was used) of the sintered material was measured by using a hardness testing machine (Mitutoyo, ARK-600).
The density of each sintered magnesium material was measured
by the Archimedes method. The wear behavior of the P/M sinteredmagnesium composite reinforced with the CNT-Mg2 Si was investigated by using a ball on disk wear test equipment (RHESCA CO
LTD., FPR-2100) under an air condition. A SUS304 stainless steel ballwith 1.9 mm diameter was used as the counter material. The sliding speed was 55 mm/s, and applied load from the SUS304 ball wascontrolled at 0.5 N. The friction radius and sliding distance of thiswear test were 3 mm and 33 m, respectively. A change in the friction
coefﬁcient was automatically calculated from the measured friction
torque between the SUS304 ball and the sintered magnesium disk
specimen by PC.

2.1. Rice husk silica particles
In order to prepare amorphous silica particles with a purityof 99% or more from rice husks, the citric acid (C6 H8 O7 ) leachingtreatment was applied to them at 323 K for 1.8 ks for the removalof metallic impurities such as Ca, K, Na, via chelate reaction [20].
After water rinsing and air drying at 373 K of the rice husks, they
were combusted at 1073 K for 3.6 ks under an air supplement of
2.5 ml/s to control the crystallization of original amorphous silica during burning [21]. The fragmentation and reﬁnement of therice husk ashes were carried out by using a ball milling equipment(SEIWA GIKEN, RM-05) for 900 s under a dry condition. Their particle size was measured by Laser Diffraction Particle Size Analyzer(HORIBA, Partica LA-920), and their morphology was observed byﬁeld-emission scanning electron microscope (FE-SEM, JOEL, JSM6500F) with energy dispersive X-ray spectrometer (EDX). The mean
particle size and speciﬁc surface area of them were 12.6 ␮m and
187 m2 /g, respectively.
2.2. CNT-SiO2 composite particles
Multi-wall carbon nanotubes (MWCNTs, Bayer Material Science), having a diameter of about 10 nm and 1–3 ␮m length, wereused as raw materials. Another raw materials were reﬁned porous
silica particles with a large speciﬁc surface area of 180–200 m2 /g[22]. They were dipped into the surfactant solution dispersed with
un-bundled nanotubes to prepare the CNT-SiO2 composite particles. With regard to the solutions dispersed with completely
split nanotubes, polar zwitterions generally had a high solubility in water, but a poor solubility in most organic solvents [23].
3-(N,N-dimethylstearylammonio) propanesulfonate, a typical linear zwitterionic surfactant, was used in this study. It had bothhydrophobic and hydrophilic groups. Electrostatic interactions,having larger attractive forces than the van der Waals forces
between CNTs, occurred at the hydrophilic because of the positive and negative charges on their headgroups. As a result, theun-bundled CNTs were dispersed in the zwitterionic surfactant
solution [23]. By dipping the silica particles into the solution, theirsurface was covered with the un-bundled CNTs due to the negative
zeta potential of silica particles with −61.8 mV (pH 8.28) in the solution kept at 298 K. The CNTs also entered the continuous pores ofthe silica, and existed inside the particles. After air drying at 353 Kof the porous silica particles, SEM observation was carried out toclarify that the CNTs uniformly existed not only on the surface, butalso inside the pores of the silica particles.
2.3. Preparation of sintered magnesium reinforced with CNT-SiO2
composite particles
The elemental mixture of the above CNT-SiO2 composite particles and pure magnesium powders, having a mean particle size of182 ␮m and 98.8% purity, were prepared as the starting materials.
They were ﬁlled into the steel die with a diameter of 45 mm, andconsolidated at room temperature by using a 2000 kN hydraulicpress machine. Their green compacts with a relative density of
77–79% were sintered at 823 K by spark plasma sintering (SPS,
Syntex Inc., SPS-1030) under 30 MPa pressure in vacuum to synthesize Mg2 Si intermetallics via a solid-state reaction. The sintered2.4. Evaluation

3. Results and discussion
3.1. Reactivity and microstructures
Fig. 1 shows an amorphous silica powder, which contains nanotubes in the intra-pores. It consists of the primary ultra-ﬁne silica
particles less than 1–2 ␮m. The primary silica reveals a sphericalshape, not angular, because they were fractured during a long timeball milling process under a low energy density in milling. The
added CNTs are completely de-bundled, and dispersed on the silica
particles as shown by the arrows. Fig. 2 indicates DTA proﬁles of the
CNT-SiO2 composite particles and the elemental mixture of magnesium powders with the composite particles (CNT-SiO2 content;
10 mass%) in heating up to 973 K. No exothermic and endothermicheat is detected in the proﬁle of the CNT-SiO2 composite particles. That is, no reaction between nanotubes and amorphous silica
particles occurs, and silicon carbides (SiCs) are not formed in heating less than the melting point of pure magnesium (923 K). In thecase of the elemental mixture of magnesium powders with CNTSiO2 composite particles, the exothermic is observed at 720 K, andthe exothermic reaction with an extremely large heat occurs at
776 K. The previous study showed the sintering temperature at
765 K or more was enough to synthesize Mg2 Si intermetallics fromthe elemental mixture of magnesium and SiO2 powders by bothdeoxidization and oxidation reactions in a solid state [18]. From athermodynamic point of view as shown in the below, this reaction
reasonably occurs.
Si(s) + O2 (g) = SiO2 (s)G[at765 K] = −725(kJ/molO2 )2Mg(s) + O2 (g) = 2MgO(s)G[at765 K] = −1046(kJ/molO2 )Therefore, when heating the CNT-SiO2 composite particles, theignition temperature accompanying a large exothermic heat shownin Fig. 2 corresponds to the starting temperature to synthesize both
Mg2 Si and MgO via the deoxidization and oxidation reaction.

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159

Fig. 1. Granulated amorphous silica particles with porous structure, including un-bundled carbon nanotubes inside (CNT-SiO2 hybrid particles).

The structural evaluation by XRD analysis on the sintered magnesium composites is carried out when applying the sintering
temperature of 823 K in the SPS process. This sintering temperature is enough to accelerate the reaction between magnesium and
SiO2 powders as mentioned above. Fig. 3 indicates the XRD patternsof the CNT-SiO2 composite particles (a), the sintered pure magnesium (b), the sintered magnesium composites including CNT-SiO2
particles of 5 mass% (c) and 10 mass% (d). In the case of (a), the broad
proﬁle with no crystalline peak is detected. This means that silica
raw particles have completely amorphous structure. As shown in(b), Mg peaks are mainly detected in the proﬁle, and a small one of
MgO is also observed due to the original surface oxide ﬁlms of raw
magnesium powder. When using the elemental mixture of magnesium and SiO2 powders coated with CNTs, both peaks of Mg2 Si
and MgO are obviously detected in the proﬁles (c) and (d). Theirintensities of the sintered material containing 10 mass% CNT-SiO2
composite particles are extremely larger than those with the content of the composite particles with 5 mass%. This means that thecontent of Mg2 Si and MgO compounds via the solid-state synthesis
increases during sintering at 823 K. Furthermore, when applying
the annealing treatment at 1373 K in argon gas atmosphere to thematerial (d), no crystalline SiO2 peak is detected by XRD analysis. In general, the crystallization temperature of the amorphoussilica originated in rice husks is 1323 K or more [19], and the crystalline silica such as crystobalite or trydymite is detected in XRD
analysis. Accordingly, no crystalline peak of SiO2 detected in (d)Fig. 3. XRD patterns of CNT-SiO2 composite particles (a), sintered pure magnesium(b), sintered magnesium composites including CNT-SiO2 particles of 5 mass% (c) and
10 mass% (d).

indicates that the amorphous silica raw particles have been completely reacted with the magnesium powders to form both Mg2 Si
and MgO during SPS process at 823 K. Fig. 4 shows SEM-EDS analysis results of the sintered magnesium material including 10 mass%
CNT-SiO2 composite particles (a), compared to the P/M pure magnesium (b). As shown in Fig. 4(a), Si elements are detected at the
primary particle boundaries (PPBs) of the magnesium matrix. Thismeans that the synthesized Mg2 Si compounds exist at the PPBs,
and nanotubes are also dispersed at the same boundaries. On theother hand, the sintered pure magnesium material with no nanotube reveals network-structure of magnesium oxides at the PPBs
shown by arrows in Fig. 4b). However, it indicates a good metallurgical bonding between magnesium powders without any defects andpores at the boundaries. Table 1 shows Rock-well hardness (scaleF) measurements of the sintered magnesium composite reinforced
with CNT and Mg2 Si. With increase in the reinforcement content,
Table 1Rock-well hardness (scale F) of sintered magnesium composite reinforced with
CNT/SiO2 particles.

Fig. 2. DTA proﬁles of CNT-SiO2 hybrid particles and elemental mixture of magnesium with hybrid particles (content; 10 mass%) in heating up to 973 K under argon
gas supplement with 2.5 ml/s.

Ave.
Max.
Min.

10%

5%

0%

54.8
58.6
51.2

43.9
45.0
43.2

21.6
24.1
19.4

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Fig. 4. SEM-EDS analysis results of sintered magnesium including 10 mass% CNT-SiO2 hybrid particles (a) and pure magnesium (b).

the hardness gradually increases, and the composite with 10 mass%
CNT-SiO2 particles shows twice value as that of the pure magnesium with no reinforcement. When considering that each sinteredmaterial has a relative density of 98% or more, the increase of thehardness is due to the uniform distribution of Mg2 Si hard compounds [16] and un-bundled nanotubes in the matrix.
3.2. Tribological properties
Fig. 5 shows the changes of the friction coefﬁcient () of thesintered magnesium materials including CNT-SiO2 composite particles of 10 mass% (a) and 5 mass% (b), compared to the puremagnesium with 0 mass% composite particles (c), when employing a SUS304 stainless steel ball as the counter material in the dry
wear test. In the case of (a), the friction coefﬁcient is the moststable, and its mean value (0 ) is lowest of all specimens. The
0 value and its instability gradually increases with decrease in
the content of the additive CNT-SiO2 composite particles as shownin Fig. 5(b) and (c). The above results mean that the tribologicalbehavior becomes stable by the uniform distribution of the additive reinforcements of CNTs and Mg2 Si contained in the sinteredmagnesium material. Fig. 6 shows the damaged area of the sliding surface of each sintered magnesium disc specimen after wear
test observed by optical microscope. When containing the reinforcement of 10 mass% CNT-SiO2 composite particles (a), a slightly
abrasive wear, but no adhesion is observed on the sliding surface.
The black area corresponds to both Mg2 Si compounds and CNTs.
In the case of (b), the main damages are due to the abrasive wear
attacked by the counter material, but some plastically deformed
areas are also detected. The sintered pure magnesium with no additive (c) reveals both severe abrasive wear and adhesion at the sliding
surface. The macro-observation on the surface also shows the stripe
pattern damages vertical to the sliding direction. This is caused by a“stick-slip” phenomenon due to the repetition of the adhesion anddetachment of the magnesium on the counter material under thedry sliding condition [24]. As a result, the friction coefﬁcient changeshows a typical behavior with a sharp ﬂuctuation corresponding toa “stick-slip” phenomenon shown in Fig. 5(c). Fig. 7 indicates theSEM-EDS analysis result on the PPB, where the synthesized Mg2 Si

Fig. 5. Changes of the friction coefﬁcient () of sintered magnesium materials
including CNT-SiO2 composite particles of 10 mass% (a), 5 mass% (b) and 0 mass%(c), which are used as disc specimen, when employing a SUS304 stainless steel ballas counter material in the dry wear test.

J. Umeda et al. / Materials Science and Engineering A 504 (2009) 157–162

161

Fig. 6. Damaged area of sliding surface of each sintered magnesium disc specimen after wear test observed by optical microscope: CNT-SiO2 hybrid reinforcement of 10 mass%(a), 5 mass% (b) and 0 mass% (c).

Fig. 7. SEM-EDS analysis result on primary particle boundaries, where synthesized Mg2 Si and MgO compounds exist, of sliding surface of sintered magnesium composite
with reinforcement of 10 mass% after wear test.

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Fig. 8. Dependence of wear loss of magnesium composites on sliding distance (a), and relationship between total wear loss and mean friction coefﬁcient (b).

and MgO compounds exist, of the sliding surface of the sinteredmagnesium composite including 10 mass% CNT-SiO2 particles. In
the magniﬁed area of the reinforcement of Fig. 7(b), it consists of
the white particles, corresponding to Mg2 Si and MgO hybrid compounds, and un-bundled nanotubes. There is no detached Mg2 Si
and MgO compound from the sliding surface because of a strong
bonding between the magnesium matrix and their compounds via
the solid-state synthesis as mentioned above. In particular, thenetwork-structured CNTs are formed around the synthesized composite particles, and the individual CNT covers the sliding surface.
Accordingly, the reason why the sintered magnesium composite
material with CNTs and Mg2 Si/MgO particles showed a superior tribological behavior with a low and stable friction coefﬁcient is due
to both improved defensive by the synthesized Mg2 Si hard particles[17] and the self-lubricant and bearing effects of the networkstructured nanotubes dispersed in the matrix. Fig. 8 summarizes
the tribological properties of the sintered magnesium composite reinforced with un-bundled CNTs and Mg2 Si/MgO compounds
under dry conditions. Fig. 8(a) shows a change of the wear loss ofthe magnesium composite specimen as a function of the sliding distance. All specimens indicate that the wear loss gradually increases
with increase in the distance. However, the wear rate depends onthe content of CNT-SiO2 additives. In the case of 10 mass% CNT-SiO2
particles, the wear rate is constant through the wear test, and tribological behavior is stable. When the content of the additives is
5 mass%, it drastically increases at the initial stage of the wear test
until 200 s after starting the test. By considering the friction coefﬁcient changes shown in Fig. 5(b), the lubrication effect of CNTs on
the sliding conditions is not stable, and causes the large wear loss.
After that, the magnesium composite shows a lower wear rate. As
shown in Fig. 8(b), the total wear loss is proportional to the friction coefﬁcient. It means that both of the uniform distribution of
Mg2 Si hard particles and the lubricant effect of un-bundled CNTs
are effective to improve the tribological properties of the sinteredmagnesium composite by decreasing the friction coefﬁcient andwear loss during sliding.
4. Conclusion
The sintered magnesium composite material reinforced withthe carbon nanotubes and Mg2 Si/MgO compounds has been developed by powder metallurgy process when using the elemental
mixture of pure magnesium powders and amorphous porous sil-

ica particles containing CNTs as a starting material. During sintering
the mixture powder, the solid-state synthesis of Mg2 Si and MgO via
the deoxidization and oxidation reaction occurred. The tribologicalproperties of the sintered magnesium material were signiﬁcantly
improved by the additive reinforcement, and the friction coefﬁcient
was low and stable under the dry sliding condition. Hard Mg2 Si
intermetallic compounds distributed in the matrix of the composite were effective for improving the wear resistance in contacting
a SUS304 stainless steel ball. Nanotubes had an important role toform the lubricant condition at the sliding interface between thesintered magnesium and the counter material due to their network
structure on the sliding surface.
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