Mechanochemically synthesized Al2O3-TiC nanocomposite

Article abstract of DOI:10.1016/j.jallcom.2009.10.206

Al₂O₃-TiC nanocomposite was synthesized by ball milling of aluminum, titanium oxide and graphite powder mixtures. Effect of the milling time and heat treatment temperatures were investigated. The structural evolution of powder particles after different milling times was studied by X-ray diffractometry and scanning electron microscopy. The results showed that after 40 h of ball milling the Al/TiO₂/C reacted with a self-propagating combustion mode producing Al₂O₃-TiC nanocomposite. In final stage of milling, alumina and titanium carbide crystallite sizes were less than 10 nm. After annealing at 900 °C for 1 h, Al₂O₃ and TiC crystallite sizes remained constant, however increasing annealing temperature to 1200 °C increased Al₂O₃ and TiC crystallite size to 65 and 30 nm, respectively. No phase change was observed after annealing of the synthesized Al₂O₃-TiC powder.

Full text of DOI:10.1016/j.jallcom.2009.10.206

Journal of Alloys and Compounds 491 (2010) 411–415
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Mechanochemically synthesized Al O –TiC nanocomposite
2
3
∗
E. Mohammad Sharifi , F. Karimzadeh, M.H. Enayati
Department of Materials Engineering, Nanotechnology and Advanced Materials Institute, Isfahan University of Technology, Isfahan 84156-83111, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Al O –TiC nanocomposite was synthesized by ball milling of aluminum, titanium oxide and graphite
2
3
Received 11 August 2009
Accepted 23 October 2009
Available online 29 October 2009
powder mixtures. Effect of the milling time and heat treatment temperatures were investigated. The
structural evolution of powder particles after different milling times was studied by X-ray diffractometry
and scanning electron microscopy. The results showed that after 40 h of ball milling the Al/TiO2/C reacted
with a self-propagating combustion mode producing Al2O3–TiC nanocomposite. In final stage of milling,
Keywords:
Nanostructured materials
Ceramics
◦
alumina and titanium carbide crystallite sizes were less than 10 nm. After annealing at 900 C for 1 h,
◦
Al2O3 and TiC crystallite sizes remained constant, however increasing annealing temperature to 1200 C
increased Al2O3 and TiC crystallite size to 65 and 30 nm, respectively. No phase change was observed
after annealing of the synthesized Al2O3–TiC powder.
Mechanochemical processing
©
2009 Published by Elsevier B.V.
1
. Introduction
The structural evolution during mechanochemical process was
studied.
Al O -based ceramic matrix composites are widely used in
2
3
2
.
Experimental
many applications, especially as excellent cutting tools [1]. Studies
have shown that the addition of TiC phase to alumina matrix can
improve toughness, hardness and also thermal shock resistance at
TiO2 (99% purity, rutile), Al (99.5% purity, gas atomized) and graphite (99.9%
purity) were mixed to produce, according to reaction (1), Al O -based nanocom-
posite containing 45 vol.% TiC.
2
3
◦
temperatures up to 800 C [2].
Al O –TiC composite is often manufactured by hot pressing
2
3
4Al + 3TiO2 + 3C = 2Al O + 3TiC
(1)
2
3
of Al O and TiC powder mixture [3,4]. However, inhomoge-
2
3
◦
= −1035 kJ/mol, ꢁH◦
neous blend of starting powders, grain coarsening during sintering
and weak interfaces between the ceramic particles deteriorate
ꢀG
= −1072.3 kJ/mol
2
98
298
◦
The negative value of ꢀG 298 suggests that reaction (1) is thermodynamically
mechanical properties of Al O –TiC composite fabricated by this
◦
2
3
favorable at room temperature. ꢀH 298 value is also negative, indicating that this
reaction is exothermic.
method. One possible route for introducing hard particles in
the matrix of composites is the mechanochemical processing
5]. Mechanochemical synthesis involves mechanical activation of
Ball milling of powder mixture was carried out in a planetary ball mill at room
temperature and under argon atmosphere. The ball milling media were hardened
chromium steel vial (150 ml) with five hardened carbon steel balls (20 mm). The
ball-to-powder weight ratio and the rotational speed of vial were 10:1 and 500 rpm,
respectively. The milling was interrupted at selected times and a small amount of
powder was removed for further characterizations. Phase transformation and aver-
age crystallite size were evaluated during milling by X-ray diffractometry (XRD) in a
Philips X’ PERT MPD diffractometer using filtered Cu K␣ radiation (ꢂ = 0.15406 nm).
The morphology of milled powder particles was examined by scanning electron
microscopy (SEM) in a Philips XL30 at an accelerating voltage of 30 kV. Crystallite
size and internal strain of specimens were calculated from broadening of XRD peaks
using the Williamson–Hall method [9].
[
solid-state displacement reactions in a ball mill. Thus, mechan-
ical energy is used to induce chemical reactions. The chemical
precursors are typically consisted of mixtures of oxides, chlorides
and/or metals that react either during milling or during subsequent
heat treatment to form a composite powder [6,7]. This technique
can lead to in situ formation of interpenetrating phase compos-
ites with nanosized microstructures. An important characteristic of
such phases is that they exhibit properties and performance much
improved over their conventional microcrystalline counterparts
ꢀ
ˇ cos ꢃ = 0
.9ꢂ
D
+ 2
(ε) sin ꢃ
2
[
8].
In this work, in situ formation of Al O –TiC nanocomposite
2
3
where ꢃ is the Bragg’s diffraction angle, D is the average crystallite size, ε is the aver-
age internal strain, ꢂ is the wavelength of the radiation used and ˇ is the diffraction
peak width at half maximum intensity. The average internal strain can be estimated
from the linear slope of ˇ cos ꢃ versus sin ꢃ, while the average crystallite size can be
estimated from the intersect of this line at sin ꢃ = 0.
was investigated by utilizing mechanochemical process starting
from titanium oxide, aluminum and graphite powder mixtures.
Prepared Al2O3–TiC nanocomposite powder was isothermally annealed to study
the thermal behavior of as-milled powders. Powder samples were placed inside cap-
sules and then annealed at 900 and 1200 C for 1 h under flowing argon atmosphere
∗ Corresponding author. Tel.: +98 312 5201676; fax: +98 311 3912752.
E-mail address: e.mohamadsharifi@ma.iut.ac.ir (E. Mohammad Sharifi).
◦
0
925-8388/$ – see front matter © 2009 Published by Elsevier B.V.
doi:10.1016/j.jallcom.2009.10.206

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E. Mohammad Sharifi et al. / Journal of Alloys and Compounds 491 (2010) 411–415
Fig. 3. The XRD patterns of Al2O3–TiC powder (a) as-milled for 60 h; (b) and (c) after
◦
subsequent annealing at 900 and 1200 C for 1 h.
During ball milling in room temperature reactions (2) and (3)
◦
are thermodynamically favorable due to their negative ꢀG 298.
The occurrence of the reaction at ambient temperatures can be
however hindered by kinetic barriers. It is now well known that
mechanochemical processing can provide the means to overcome
the kinetics limitations of the displacement reactions [8].
Fig. 1 shows the XRD patterns of powder mixture as-received
and after different milling times. The diffraction pattern of initial
powder mixture shows all expected peaks of Al, TiO2 (rutile) and
graphite. With increasing milling time to 10 h, the (0 0 2) reflec-
tion of graphite disappeared indicating that graphite particles were
heavily deformed and fractured into fine slices [10] and randomly
mixed with Al and TiO2 powders. In addition with increasing
Fig. 1. XRD patterns taken from the Al/TiO2/C powder mixture after different milling
times.
in a conventional tube furnace. After annealing the capsules were cooled in air. The
structural evolutions occurred during annealing were determined by XRD.
3
. Results and discussion
milling time, the intensity of Al and TiO peaks decreased and their
width increased as a result of refinement of crystallite size and
enhancement of lattice strain.
The variation of crystallite size and lattice strain of the Al and
TiO2 powders as a function of milling time, is presented in Fig. 2.
Both Al and TiO2 powders achieved a saturated crystallite size of
about 35 nm after 20 h of milling time. As expected the lattice strain
2
3
.1. Phase evolution and reaction mechanism
Al O –TiC nanocomposite was synthesized according to reac-
2
3
tion (1) which involves two reactions: reduction of TiO2 by Al to
form elemental Ti followed by the reaction of Ti with C to form
TiC. These two reactions can be represented as a thermite reaction,
reaction (2), and a subsequent synthesis reaction, reaction (3):
of Al and TiO was increased during milling time as result of increas-
2
4
Al + 3TiO = 2Al O + 3Ti,
(2)
ing number of lattice defects. Refinement of the crystallite size to
nanometer range and increasing the defect densities can promote
the reaction kinetics by providing short-circuit diffusion paths [8].
2
2
3
◦
= −500.2 kJ/mol, ꢁH◦
ꢀ
G
= −521.2 kJ/mol
298
298
XRD patterns after 40 h showed no crystalline TiO and Al peaks.
2
Meanwhile several additional peaks corresponding to ␣-Al O and
2
3
Ti + C = TiC,
(3)
TiC compounds developed on XRD patterns. The crystallite size of
-Al O and TiC phases after 60 h of milling time were calculated
␣
2
3
◦
= −180 kJ/mol, ꢁH◦298 = −183.7 kJ/mol
ꢀG
298
to be about 10 nm.
Fig. 2. Variation of (a) crystallite size and (b) lattice strain of Al and TiO2 phases with milling time.

E. Mohammad Sharifi et al. / Journal of Alloys and Compounds 491 (2010) 411–415
413
Fig. 4. SEM micrographs of as-received elemental powder particles: (a) Al, (b) TiO2 and (c) graphite.
It was noticed that after 40 h of milling time the outside tem-
combustion mode. Reaction (1) can be further promoted by the
dynamically maintained high reaction interface areas, as well as
the short-circuit diffusion path provided by the large number of
defects such as dislocations and grain boundaries induced during
ball milling.
Combustion type reactions require a critical mechanical energy
to be initiated. This means that a reaction which is highly exother-
mic can only take place after a certain time of milling [5,14]. As
mentioned earlier as milling time was increased to about 40 h,
reaction (2) occurred. The heat released from exothermic reaction
(2) provides the activation energy for TiC formation according to
reaction (3).
perature of vial, measured by pyrometer, increased rapidly to
◦
7
2 C suggesting that a combustion reaction occurred between Al
and TiO . The mechanochemical reactions generally fall into two
2
categories, namely (a) those which occur during the mechani-
cal activation process and there the reaction enthalpy is highly
negative and (b) those which occur during subsequent thermal
treatment and here the reaction enthalpy is only moderate. The
first type of reaction takes place in two distinct modes, i.e., either
self-propagating combustion reaction or a progressive reaction
8,11,12].
It has been suggested that the value of adiabatic temperature
Tad), which is the maximum temperature achieved under adia-
[
(
batic conditions as a consequence of the evolution of heat from the
reaction, should be above 1800 K to yield self-propagating com-
bustion reaction in a thermally ignited system [13]. However, the
minimum Tad required for the occurrence of self-propagating com-
bustion reaction during ball milling can be significantly reduced
3.2. Thermal stability
Nanocrystalline structures are thermodynamically metastable
because of the large excess free energy stored in their grain
boundaries. Significant grain growth was observed in several
nanocrystalline materials [17,18]. The study of thermal behavior
of nanostructured materials is therefore important with respect to
their possible applications.
[
14,15]. This reduction in Tad by mechanical activation implies that
those reactions with Tad < 1800 K can also take place with self-
propagating combustion mode by mechanical activation process.
The value of Tad can be calculated using following relation:
Thermal stability of Al O –TiC nanocomposite structure was
2
3
investigated by annealing of ball-milled powders at 900 and
1200 C for 1 h. The XRD patterns of samples as-milled for 60 h
ꢁ
Tad
ꢂ
◦
Q = (−ꢀH◦298) =
Cp(Products)dT
(4)
and after subsequent annealing are shown in Fig. 3. Annealing
2
98
◦
at 900 C caused only a slight narrowing of XRD peaks indicating
◦
◦
where Q is the heat of reaction, ꢀH 2 is the enthalpy change at
98 K (room temperature) and Cp the heat capacities of products.
The Tad for reaction (1) calculated using relation (4) and ther-
modynamic data [16] was found to be 2547 K which is much higher
than the critical value of 1800 K. Consequently, during milling
that no significant crystallite growth occurred at 900 C. In con-
trast after annealing at 1200 C the XRD peaks of Al O and TiC
98
◦
2
2
3
phases were completely separated due to the crystallite growth
and consequent width reduction of crystalline peaks. Nevertheless
Al O and TiC crystallite size remained in nanometer range, 65 and
2
3
of Al/TiO /C mixture, reaction (1) takes place in self-propagating
30 nm, respectively even after annealing at high temperature of
2

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E. Mohammad Sharifi et al. / Journal of Alloys and Compounds 491 (2010) 411–415
Fig. 5. SEM micrographs of Al2O3–TiC powder particles after (a) 5 h, (b) 20 h, (c and d) 40 h and (e) 60 h of milling times.
◦
1
200 C. No additional change in phase composition occurred in
After 5 h of milling time the aluminum powder particles are exten-
sively flattened (Fig. 5a). In mean while, TiO2 and graphite powder
particles are dispersed between the Al layers. By increasing milling
time to 20 h, because of work hardening of Al powder, the rate of
fracturing increased and as a result the size of powder particles
decreased. At this stage, the morphology of powder particles was
almost equiaxed with a wide size distribution of 1–20 ␮m (Fig. 5b).
The morphology of powder particles after combustion reaction,
powder particles after annealing.
3
.3. Morphological changes
Fig. 4 shows the morphology of as-received Al, TiO and graphite
2
powder particles. The aluminum powder particles were irregular
in shape with a size distribution of 20–100 ␮m. TiO2 powders with
spherical morphology and mean particle size of 0.2 ␮m show a ten-
dency to agglomeration. The graphite particles were angular and
flaky in shape with a mean particle size of 30 ␮m.
Fig. 5 shows morphology of powder particles after different
milling times. The plastic deformation, flattening and cold weld-
ing of aluminum powders occur in the early stage of ball milling.
4
0 h of milling time, are shown in Fig. 5c and d. Powder particles
adhere together and are agglomerated due to the heat released by
the combustion reaction. Further milling up to 60 h led to a signif-
icant decrease in powder particle size because of fragmentation of
the brittle constituent phases, Al O3 and TiC (Fig. 5e). The average
2
powder particle size after 60 h of milling time was about 1 ␮m.

E. Mohammad Sharifi et al. / Journal of Alloys and Compounds 491 (2010) 411–415
415
4
. Conclusions
Mechanochemical behavior of Al/TiO /C powder mixture was
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to the broadening of Bragg’s peaks as a result of reduction of
the crystallite size as well as microstrain induced in the pow-
der particles. On future milling however the Al/TiO /C reaction
occurred with a combustion mode producing Al O –TiC structure.
The Al/TiO /C reaction mode accords well with that predicted by
corresponding Tad value. It was found that Al O and TiC phase had
a nanocrystalline structure. No significant grain growth or phase
change observed after annealing of ball-milled Al O –TiC powder
at 1200 C for 1 h.
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