The compaction, sintering, and mechanical properties of Al 2O3-CeO2 composite nanopowders

Article abstract of DOI:10.1111/j.1551-2916.2011.04638.x

In the present work, compaction, sintering behavior, and mechanical properties of Al₂O₃-CeO₂ composite nanopowders were investigated. Al₂O₃ and Al₂O ₃-CeO₂ composite n

Full text of DOI:10.1111/j.1551-2916.2011.04638.x

J. Am. Ceram. Soc., 94 [10] 3488–3493 (2011)
DOI: 10.1111/j.1551-2916.2011.04638.x
© 2011 The American Ceramic Society
ournal
J
The Compaction, Sintering, and Mechanical Properties of
Al2O3–CeO2 Composite Nanopowders
Sayed Ali Hassanzadeh-Tabrizi and Ehsan Taheri-Nassaj†
Department of Materials Science and Engineering, Tarbiat Modares University, PO Box 14115-143, Tehran, Iran
In the present work, compaction, sintering behavior, and
mechanical properties of Al2O3–CeO2 composite nanopowders
were investigated. Al2O3 and Al2O3–CeO2 composite nano-
powders were prepared by a sol–gel method. The effect of
CeO2 on compressibility, sintering rate, final density, and
mechanical properties was studied. The aggregation strength of
nanopowders reduced from 158 to 101 MPa by adding 5 wt%
CeO2. The amount of CeO2 had a strong effect on the sinter-
ing behavior and final microstructure of nanopowders and it
inhibited alumina grain growth and suppressed densification
process. The activation energy for grain growth increased from
428 to 554 kJ/mol by adding 5 wt% CeO2 to the nano-
powders. The addition of 5 wt% CeO2 improved fracture
toughness (by 28%) and flexural strength (by 17%) with
respect to monolithic Al2O3.
oped yttria- and ceria-toughened alumina ternary ceramic
composite for cutting tool application with good wear prop-
erties.19 However, the subject of effect of ceria on the sinter-
ing behavior and mechanical properties of alumina have not
been broadly studied yet.
The aim of this work was the investigation of compaction
and sintering process of alumina–ceria composite nanopow-
der prepared by a sol–gel method containing different
amounts of ceria. In addition, mechanical properties of the
composites were studied.
II. Experimental Procedure
The Al2O3–CeO2 composite nanopowder was prepared
by a sol–gel method using AlCl3·6H2O (Merck, Darmstadt,
Germany), Al powder with d50 = 37.5 lm (M. A. University)
and CeCl3·7H2O (Merck). For preparation of the sol, first
AlCl3·6H2O and CeCl3·7H2O were dissolved in deionized
water and HCl. Then the aluminum powder was added to the
solution. The amounts of ceria in composite samples were
5, 10, and 15 wt%. One specimen was prepared by the same
processing, without cerium oxide powder (pure alumina).
The molar ratios of Al/AlCl3 and HCl/H2O were 2 and 0.18,
respectively. The precursor solution was stirred at 100°C for
3 h and was aged at 60°C until the batch set to a rigid gel. It
was then dried at 80°C for 48 h. More details about the
processing of nanopowder are mentioned elsewhere.20 The
resultant synthesis product (gel) has been calcined at 900°C for
30 min to remove volatile components. Figure 1 shows the
TEM image of the nanopowders. To reduce the amount of
hard agglomerates, the calcined nanopowder was wet-plane-
tary milled using a high dense alumina jar and high pure alu-
mina balls for 2 h. The powders with different CeO2 contents
were cold isostatically pressed into compacts at the pressure of
400 MPa. The compacts were then sintered in a tube furnace
under air atmosphere at various temperatures for 3 h with a
heating rate of 6°C/min. Some bars were sintered inside a
dilatometer in static air.
I. Introduction
APID developments in ceramics have introduced a vari-
ety of ceramic composites for many engineering appli-
R
cations. Alumina (Al2O3) is one of the more widely used
advanced ceramic materials because of its unique mechanical,
electrical, and optical properties. However, alumina is a brit-
tle ceramic material and it is strengthened by the addition of
secondary phase ceramic particles, fibers, and whiskers in the
alumina matrix.1–4 For instance Shi et al.5 prepared Al2O3–
SiC composites by hot pressing with Al2O3 and SiC powders
as raw materials. They reported that, adding SiC particles to
the Al2O3 improves mechanical properties. Li and Gao6
manufactured Al2O3–25 vol% YAG composite with good
strength characteristics. Tuan et al.7 prepared Al2O3–ZrO2
composite by sintering at 1600°C and showed that the
strength and toughness of alumina are enhanced by adding
zirconia. Lee et al.8 also reported that, fracture toughness of
alumina is enhanced with dispersion of nanometer sized par-
ticles of TiO2.
Ceria (CeO2) is not only widely used in polishing of opti-
cal glass but also used as a cleaning agent in glass manufac-
turing. Cerium oxide has been widely investigated because of
its multiple applications, such as its use as a catalyst, an elec-
trolyte material of solid oxide fuel cells.9–14
Nowadays, the alumina–ceria supported system constitutes
one of the most widely used mixed-oxides for elimination of
pollutants in automobile exhaust gases.15–17 In addition this
composite can be used as a cutting tool. Senthil Kumar
et al.18 developed the alumina–ceria ceramic tool inserts and
reported that those exhibited better mechanical properties
than the pure alumina inserts. In another work they devel-
The crystalline structure of the samples was determined by
X-ray diffraction using Philips X-pert model with CuKa radia-
tion. The densities of the sintered specimens were measured by
the Archimedes method, which gave reproducible results to
within 0.2%. The microstructures of powders and sintered
specimens were observed by transmission electron microscopy
(TEM; CM200-Phillips, Eindhoven, Netherlands) and scan-
ning electron microscopy (SEM; XL30-Phillips, Eindhoven,
Netherlands). An image analyzer was used to measure the
mean grain size of the samples.
The strength and fracture toughness were determined by
three-point bending. The sintered bars (2.5 mm 9 5 mm 9
25 mm) were polished to 1 lm. The polished edges of the
bend bars were beveled to limit failure due to edge flaws.
The fracture toughness value was calculated according to
ASTM E399. The fracture toughness measurements were per-
formed on single edge-notched bar (SENB) specimens with a
span of 20 mm, and a nearly half-thickness notch (~2.5 mm)
R. Bordia—contributing editor
Manuscript No. 28823. Received October 26, 2010; approved April 23, 2011.
†Author to whom correspondence should be addressed. e-mail: taheri@modares.
ac.ir.
3488

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Properties of Al2O3–CeO2 Composite Nanopowders
3489
Fig. 1. The TEM micrograph of the nanopowders.
Fig. 2. The XRD patterns of Al2O3–15 wt% CeO2 composite heat-
treated at various temperatures for 3 h.
was made using a low speed diamond cutting saw (Buehler,
Isomet, BUEHLER LTD, Lake Bluff, IL). The notch had a
nominal thickness of 0.3 mm. The flexural strength and the
fracture toughness were conducted in air at room temperature
(Instron Universal Testing Machine-1196, High Wycombe,
Buckinghamshire, U.K.) with a crosshead speed of 1 mm/min.
At least seven specimens were tested for each series of compo-
sition. For estimating the fracture toughness, the following
relation was used21:
PfS
BW3=2
3a1=2
ꢀ
KIC
¼
fðaÞ
ð1Þ
2ð1 ꢀ aÞ3 2
À
Á
að1 ꢀ aÞ 3:49 ꢀ 0:68a þ 1:35a2
fðaÞ ¼ 1:9887 ꢀ 1:326a
2
ð1 þ aÞ
ð2Þ
where Pf, S, B, and W are experimentally measured fracture
load, support span, specimen width, and specimen depth,
respectively. The notch depth, a, enters the equation through
a, where a = a/W.
The hardness was measured by Vickers indentation (Hv)
on polished surfaces with an indentation load of 5 kg and
loading time 15 s.
III. Results and Discussion
(1) Characterization and Compressibility of the
Synthesized Nanopowder
The X-ray diffraction patterns of the Al2O3–15 wt% CeO2
heat-treated for 3 h at the temperature ranges from 200° to
1300°C are shown in Fig. 2. As can be seen, the powder
heat-treated at 200° and 400°C seems to be amorphous. By
increasing the heat-treatment temperature, to 600°C, CeO2
with cubic structure is detected. g-Al2O3 and h-Al2O3 are
formed at 800°C. The peak intensity of these phases increases
with increasing the temperature. In addition, the broadening
of the peaks decreases which shows growing of the nanocrys-
tallites. However, a-Al2O3 is formed at 1100°C. The XRD
patterns of the composites with lower CeO2 contents were
also similar to those of Al2O3–15 wt% CeO2.
The calcined powders at 900°C were compacted by cold
isostatically pressing (CIP), applying pressures between 25
and 400 MPa. For the Al2O3 and Al2O3–5 wt% CeO2 pow-
ders, the values of the relative green densities were plotted as
a function of log P (Fig. 3). The compressibility curves are
characterized by two linear regions with intersection points
at PY = 158 MPa and PY = 101 MPa for Al2O3 and Al2O3–
Fig. 3. The relative green densities of nanopowders as a function
of log compaction pressures (P) for (a) pure Al2O3 and (b) Al2O3–
5 wt% CeO2.
5 wt% CeO2 powders, respectively. As can be seen the PY of
Al2O3–5 wt% CeO2 is lower than that of Al2O3. It was
shown that the powders synthesized by the wet chemical
method are built up from three microstructural elements: (1)
primary crystallites, (2) crystallites densely packed into small
and strong aggregates, and (3) weak and highly porous
agglomerates consisting of the aggregates.22 This type of the
powder structuring is clearly seen in Fig. 1 of the present

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Journal of the American Ceramic Society—Hassanzadeh-Tabrizi and Taheri-Nassaj
Vol. 94, No. 10
work. For the wet synthesis of powders the compressibility
curves are characterized in the wide range of compaction
pressure by three linear regions at low, middle, and high
that, YAG has a strong effect on the sintering behavior of
alumina and it suppresses densification process. Figure 5(b)
shows the densification rate as a function of temperature. It
is observed from this figure that the addition of CeO2 shifts
somewhat the onset of sintering toward higher temperatures;
moreover, CeO2 increases the temperature of maximum
shrinkage rate. Schehl et al.32 reported that, adding of dif-
ferent second phases, i.e. ZrO2, YAG, and mullite to the
alumina, increases the onset of shrinkage. They explained
that, when pure alumina is doped over the solubility limit
with large size cations, a pinning effect takes place at grain
boundaries due to the formation of second phase which
results in a delay in the densification. The maximum rate of
sintering for Al2O3–10 wt% CeO2 and Al2O3–15 wt% CeO2
was also reduced which resulted in lower sintered densities.
The grain growth analyses and microstructural studies
have been done for Al2O3 and Al2O3–5 wt% CeO2 which
achieved nearly full density. The effect of CeO2 on grain size
of samples is shown in Fig. 6. The addition of CeO2 was
found to significantly reduce grain size compared to that of
Al2O3 despite the higher sintering temperature. The effect has
been attributed to the Zener pinning of grain boundaries by
CeO2 particles.
In addition, the change of grain size as a function of sin-
tering temperature presents two distinct regions. At the first
region, the rate of grain growth was slow. In this stage of
sintering (usually between 0.65 and 0.9 theoretical density),
according to the solid-state sintering mechanism, dispersed
open pores could pin grain boundaries and hinder grain
boundary migration, for which the grain growth was sup-
pressed. In the second region, the final grain size of the sin-
tered samples was sharply increased. It shows a significant
grain growth at the final stage of sintering. It was confirmed
23
pressures with two intersection points at PY1 and PY2
.
The
region of low and middle pressures was investigated in Refs.
[22–25]. For all investigated powders the value area of PY1
and relative green density are 30–60 MPa and 18%–25%,
respectively. At pressures below PY1, the agglomerates are
only rearranged and partially fragmentized without a change
in their internal microstructure. At the pressure around and
higher than PY1, compression of the agglomerates occurs,
which causes changes in their internal microstructure.22
Therefore, the value of PY1 was denoted as the average com-
pression strength of the agglomerates. The regions of middle
and high pressures were investigated in Refs. [23,26,27]. The
value area of PY2 and relative green density are 100–
850 MPa and 35%–50%, respectively. The value of PY2 was
denoted as the average compressive strength of the aggre-
gates or “joining pressure.”23,28 At this stage, the aggregates
lose their separate identity and the compaction behavior is
similar to the non-aggregated powders.23,28 The strength of
the agglomerates and aggregates depends heavily on the
method of synthesis.22,23 In the present work, the values of
compaction pressure (and relative green density) at the inter-
section point are 158 MPa (~33%) and 101 MPa (~32%) for
Al2O3 and Al2O3–5 wt% CeO2 powders, respectively.
According to above discussion it can be concluded that, the
PY in the present work is the average compressive strength
of the aggregates.
The strength of the aggregates (LogPY) as a function of
calcination temperatures for Al2O3–5 wt% CeO2 is as shown
in Fig. 4. It is obvious that the strength of the aggregates
increases with increasing calcination temperature. By increas-
ing the calcination temperature, surface diffusion can be acti-
vated and it can cause sintering and formation of necks
between particles,29 which result in an increase in strength of
the aggregates. In addition, the dependency of calcination
temperature on strength of the aggregates is linear. This
result is in accordance with Bukaemskiy et al.23 findings.
(2) Sintering Behavior of the Synthesized Nanopowder
The densities of the samples are shown in Fig. 5(a) for vari-
ous sintering temperatures. The pure Al2O3 and the compos-
ite with 5 wt% CeO2 both achieved nearly full density when
sintered at 1585° and 1630°C, respectively, although some
porosity remained after sintering. When 10% and 15 wt%
CeO2 were added, the density decreased. The reduction of
density by adding second phase has also been reported in
other alumina base composites. Shapiro et al.30 showed that,
the density of alumina decreases by adding 5 vol% SiC parti-
cles. Hassanzadeh-Tabrizi and Taheri-Nassaj31 also reported
Fig. 5. The relative densities (a) and densification rates (b) of the
samples with different CeO2 contents as a function of sintering
temperature.
Fig. 4. The strength of the aggregates (LogPY) as a function of
calcination temperatures for Al2O3–5 wt% CeO2.

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Properties of Al2O3–CeO2 Composite Nanopowders
3491
Fig. 6. The grain size of the Al2O3 and Al2O3–5 wt% CeO2
samples versus sintering temperatures.
that the open pores (referring to the intermediate stage of
sintering) collapsed to form closed pores, after which the
final stage of sintering started. Such a collapse resulted in a
substantial decrease in pore pinning, which triggers the accel-
erated grain growth.33
The grain growth kinetic is deduced by analyzing the grain
size as a function of annealing time in accordance with the
classical theory for grain growth34:
Dn ꢀ Dn0 ¼ Kðt ꢀ t0Þ
ð3Þ
ð4Þ
ꢂ
ꢁ
ꢃ
K ¼ K0 exp ꢀQ RT
where D is the average grain size of Al2O3 at time, t, D0 is
the average grain size at time, t = 0, n is the growth expo-
nent, K is a rate constant, K0 is a pre-exponential constant,
Q is the activation energy of grain growth, R is the ideal gas
constant and T is the absolute temperature of sintering. The
pure Al2O3 samples fit an n = 2 dependence and the compos-
ite samples with 5 wt% CeO2 fit an n = 3 dependence. As
described by Brook,35 a grain growth exponent of 2 indicates
that solid state mass transport in a pure system is the domi-
nant mechanism for coarsening while n = 3 indicates that
mass transport in a system containing second phase occurs
(for the case of boundary controlled migration). Figure 7
Fig. 7. The plots of ln(DnꢀD0n) versus 1/T for the calculation of
activation energy (Q) for the (a) Al2O3 and (b) Al2O3–5 wt% CeO2
samples.
n
shows (DnꢀD0 ) against 1/T for Al2O3 and Al2O3–5 wt%
CeO2 samples, with activation energies indicated in the fig-
ures. It clearly shows that the composite sample exhibits
higher activation energy for grain growth, which may be
used to explain why CeO2 adding leads to grain growth sup-
pression.
Figure 8 presents the SEM micrographs and EDS spec-
trums of polished and thermally etched surfaces of pure
Al2O3 and Al2O3–5 wt% CeO2. The grains are fairly equi-
axed in shape, without any observable abnormal or aniso-
tropic grain growth. The contrasted dark and bright areas
within the images on the composites correspond to the Al2O3
and CeO2 phases, respectively. EDS spectrums confirm the
presence of more Ce among the second phase particles with
respect to the matrix. The source of Au in the EDS spec-
trums corresponds to the Au element used for coating the
specimens. The CeO2 particles were uniformly distributed in
matrix of composites. This homogeneous distribution is due
to the homogeneous mixing of the starting precursors
obtained in the sol–gel method.
Fig. 8. The SEM micrographs and EDS spectrums of polished and
thermally etched surfaces of the (a) pure Al2O3 and (b) Al2O3–
5 wt% CeO2 samples.
temperatures (ST) and relative densities (RD) of the samples
are shown inside the figure. A significant improvement in
fracture toughness was obtained for the Al2O3–5 wt% CeO2
composite. With increase in CeO2 content to 10 and 15 wt%,
the fracture toughness was observed to decrease for the
(3) Mechanical Properties of the Sintered Samples
The room temperature flexural strength and fracture tough-
ness of the samples are shown in Fig. 9. The sintering

3492
Journal of the American Ceramic Society—Hassanzadeh-Tabrizi and Taheri-Nassaj
Vol. 94, No. 10
composites specimens. A maximum improvement of fracture
toughness by ~28% (KIc = 4.1 MPa m1/2), with respect to
pure Al2O3, was obtained for the Al2O3–5 wt% CeO2
composite. This result is in a good agreement with Senthil
Kumar et al.18 studies. They reported that, the fracture
toughness of the pure alumina insert increases from 3.4 to
4.7 MPa m1/2 by adding 5 wt% CeO2 powder. One mecha-
nism for toughness increase in particulate-reinforced ceramic
composites arises from the development of coefficient of ther-
mal expansion (CTE) mismatch-induced residual stresses.36–38
In the composites under investigation, the second phase
particles (CeO2) have higher thermal expansion coefficient
) than the matrix (Al2O3; am
19
(ap ~13 9 10ꢀ6 Kꢀ1
~8 9 10ꢀ6
K
ꢀ1 39). This is likely to result in the development
of residual compressive stresses in the matrix during cooling
from the heat-treatment temperatures. According to a theory
proposed by Taya et al.37 development of such compressive
residual stresses in the matrix can result in the toughening of
particulate reinforced ceramic composites. In addition, crack
deflection could occur by dispersed ceria particles which
results in an improvement in mechanical properties.40 The
composites with 10 and 15 wt% CeO2 had lower fracture
toughness than Al2O3–5 wt% CeO2 which is due to lower
densities. A trend similar to the fracture toughness was
observed for bending strengths. A maximum improvement in
flexural strength of ~17% (rf ~381 MPa) over that of mono-
lithic Al2O3 was obtained for the Al2O3–5 wt% CeO2 com-
posite which is due to the development of residual
compressive stresses in the matrix. The reduction of strength
for composites with CeO2 contents higher than 5 wt%,
implies that the strength is limited by other factors such as
the presence of porosities and flaws which often limit the
Fig. 10. The hardness of samples as a function of CeO2 contents
(ST, sintering temperature; RD, relative density).
strength improvement with mixed powder routes.41,42 In the
present work the size of porosities and flaws are higher than
the grain size. Therefore, the highest size of flaw controls the
strength of the samples and the existence of the flaws has an
important effect on the strength.
Figure 10 shows the hardness of the samples as a function
of CeO2 contents. The hardness of Al2O3–5 wt% CeO2 com-
posite is about 18.5 GPa, which is comparable with that of
pure Al2O3 (~19.1 GPa). The hardness of Al2O3–5 wt%
CeO2 composite obtained with our samples are higher than
those reported by Senthil Kumar et al.18 (1680 HV). As it is
known, the hardness of ceramic materials is influenced by
the grain size.43,44 Although Senthil Kumar et al. did not
report the grain size of their samples after sintering but the
particle sizes of their raw materials (Al2O3 = 1.2 lm and
CeO2 = 5 lm) were relatively higher than the grain sizes of
our samples after sintering. In addition, the densities of the
samples in this work (~98%) are higher than those reported
by Senthil Kumar (~96%). Therefore, the higher hardness of
the composites in this work may be due to lower grain sizes
and higher densities of the samples. However, the hardness
of the composites with CeO2 contents higher than 5 wt%
appears to be less than that of pure Al2O3. The reduction
trend of the hardness with increasing of CeO2 contents is due
to the lower hardness of the CeO2 in comparison with the
Al2O3. In the case of Al2O3–10 wt% CeO2 and Al2O3–15
wt% CeO2 composites, the residual porosities also have an
important effect on reduction of hardness.
IV. Conclusions
In the present study, compaction, sintering, and mechanical
properties of Al2O3–CeO2 composite nanopowder were inves-
tigated. Al2O3 and Al2O3–CeO2 composite nanopowders
were prepared by a sol–gel method. The compressibility
curves of the nanopowders were characterized by two linear
parts. The aggregation strength of nanopowders reduced
from 158 to 101 MPa by adding of 5 wt% CeO2. The addi-
tion of CeO2 inhibited both sintering and grain growth pro-
cesses. The activation energy for grain growth increased from
428 to 554 kJ/mol by the addition of 5 wt% CeO2 to the
nanopowders. A fracture toughness improvement by 28%
and flexural strength improvement by 17% with respect to
monolithic Al2O3 were recorded by adding 5 wt% CeO2.
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