High toughness alumina ceramics with elongated grains developed

Vol. 46 No. 5
SCIENCE IN CHINA (Series E)
October 2003
High toughness alumina ceramics with elongated grains
developed from seeds
XIE Zhipeng ()1, GAO Lichun ()2,
XU Lihua (
)1 & WANG Xidong ()2
LI Wenchao ()2,
1. Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China;
2. University of Science and Technology Beijing, Beijing 100083, China
Correspondence should be addressed to Xie Zhipeng (email: [email protected])
Received February 22, 2003
Abstract
In the present paper, the influence of α-Al2O3 seeds and sintering methods on elongated grain growth and fracture toughness is investigated. The preparation of alumina ceramics
started with commercial aluminum hydroxide. Abrasives were introduced to the starting materials
by wet-grinding of high-purity alumina milling balls. Abrasives, playing the role of seeds, lowered
the transformation temperature of aluminum hydroxide to alumina. Microstructures with elongated
grains were developed by hot-pressing for the above calcined powders containing α-Al2O3 seeds,
and alumina grain shapes changed with the amount of seeds introduced. However, only equiaxed
grains were observed for the samples pressureless sintered. Fracture toughness of the alumina
ceramics was dramatically improved by elongated grains. For the sample hot-pressed at 1600k
for 2 h under 40 MPa pressure, fracture toughness reached 7.1 MPa⋅m1/2, which is much higher
than that of normal alumina ceramics without elongated grains. In addition, high flexural strength of
630 MPa for the hot-pressed samples was also obtained.
Keywords: alumina elongated grains, α-Al2O3 seeds, aluminum hydroxide, fracture toughness.
DOI: 10.1360/03ye0181
Alumina ceramics have been used in a wide variety of engineering fields because of their
excellent mechanical and electric properties as well as relatively low cost of manufacture. Now
such alumina ceramics with flexural strength as high as 7001000 MPa and Weibole mode of 40
have been prepared by using pure and ultra-fine powder[1,2]. However, the fracture toughness of
the materials is very low, typically around 3 MPam1/2. Therefore, much effort has been made on
the improvement of fracture toughness of alumina ceramics.
In order to increase fracture toughness of alumina ceramics, some researches developed
anisotropic grain growth through additives[3
ü5]
. Yasuoka et al.[6] have made alumina ceramics with
some elongated and platelet-like grains through introduction of several hundred ppm of SiO2. In
addition, Horn and Messing[7] also obtained microstructure with hexagonal platelets and elongated
grains by adding TiO2, increasing the fracture toughness up to 5.2 MPam1/2. Guo et al.[8,9] found
the formation of elongated grains when La2O3 was introduced as an additive to ZTA. Wu et al.[10]
obtained plate like alumina grains with fracture toughness of 4.5 MPam1/2 by adding CAS glass
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powder (CaO-Al2O3-SiO2) into fine alpha alumina powders and sintered the material in the range
from 1550 to 1600.
The method of inducing anisotropic grain growth by crystal seeds has been widely employed
in fabrication of silicon nitride[11,12]. For the alumina ceramic system, α-Al2O3 seeds were used to
promote the transformation from AlOOH or Al(OH)3 to alpha alumina. Messing[13] reported that
the transformation temperature from γ-AlOOH to alpha alumina was dramatically reduced when
α-Al2O3 seeds with 0.1 µm were added into γ-AlOOH powder. Further study on sintering of the
powder with seeds by Kwon[14] showed that a high density of sintered sample was reached at
1300. However microstructures obtained in the above work were mostly equiaxed. Only recently did Yoshizawa[15] fabricate alumina ceramics with high fracture toughness through developing elongated grains in microstructure where α-Al2O3 seeds were introduced into ultra-fine
aluminum hydroxide.
In the present work, commercial aluminum hydroxide was used as raw materials and fine
abrasives from α-alumina medium were introduced as seeds through wet-ball milling. The transformation from aluminum hydroxide to α- alumina and growth of elongated alumina grains were
investigated. High toughness alumina ceramics with elongated grains were fabricated.
1 Experiment procedure
Commercial aluminum hydroxide was used as starting powder with 1wt% impurities of SiO2,
Na2O, MgO, etc. The average particle size was 2.84 µm. α-Al2O3 seeds were introduced through
wet-ball milling where α-Al2O3 ball medium of high purity (99.97%), deionized water and the
starting powder were milled.
Abrasives from α-Al2O3 medium can act as the seeds, the amount
of which can be calculated according to the mass loss of Al2O3 ball medium after ball milling
process. The dried ground aluminum hydroxide containing α-Al2O3 seeds of abrasives was put in
an Al2O3 crucible and calcined in air in an MoSi2 resistance-heat furnace at 1100 for 2 h. The
resulting powder was cold-pressed to form the green cakes. The green cakes were hot-pressed at
1600 in Ar atmosphere in a high temperature furnace (FVPHP-R-5 FRET-20, Fujidempa Kogyd Co. LTD., Japan). Some carbon on the sample surface after hot-pressing was removed by
heating the sample in the MoSi2 furnace. For comparison, some green cakes were pressurelessly
sintered at 1550 and 1600.
Particle size and distribution were measured by a BI-XDC analyzer (Brookhaven Instrument
Corp., USA), and the morphology of the particles was examined with a transmission electron microscope (TEM, H-800, Hitachi Co.). Phases in the starting and calcined powders were investigated by XRD (D/MAX Rigaku). Sintered samples were cut and polished with a series of diamond pastes, and then thermally etched at the temperature of 100150 below sintering temperature. Microstructure of the samples was examined by S-450 scanning electron microscopy and
JEM-6300F scanning electron microscopy (SEM). For the measurement of mechanical properties,
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the sintered samples were cut, ground and polished into test bars of dimensions with 3 × 4 × 36
mm3 and 4 × 6 × 30 mm3 for flexural strength and fracture toughness measurement respectively.
Flexural strength measurements were carried out by the 3-point bending method, and fracture
toughness was determined by the single-edge notched-beam method with a notch width of 2.5 mm
and a span of 24 mm. The loading rate was 0.05 mm/min and there were 5 test bars for one point.
Fracture surfaces of sintered samples were examined by scanning electron microscopy (SEM).
2 Results and discussion
2.1 Influence of seed amount on aluminum hydroxide transformation
The amount of seed introduction can be controlled by changing the ball milling period. After
ball milling of 0, 24, 48 and 96 h, different
amounts of α-Al2O3 seeds, 0, 2.67wt%, 10.7wt%
and 23.0wt%, were obtained respectively. The
ball-milled aluminum hydroxide powders containing α-Al2O3 seeds were calcined at 1100
for 2 h and examined by XRD (fig. 1). XRD patterns show that the main phase of calcined starting
powders without any seed is one κ phase, while
there is only a small amount of α phase. The
strong background of the XRD pattern indicated
the presence of amorphous mass. With the introduction of abrasives, the amount of α-alumina in
the sample increased substantially. In sample 2
which contained 2.67 wt% seed of the abrasives,
the α-phase dominated as the main phase, still
with an amount of κ phase; while in samples 3
and 4, κ-phase was not found any more.
Fig. 1. XRD patterns of calcined Al (OH)3 with various
seed concentration: 1#, 0wt%; 2#, 2.67wt%; 3#, 10.70wt%;
4#, 23.00wt%..
Fig. 2 shows the particle size distributions of calcined powders with various seed
concentrations. The particle size distribution
became narrower when the seed concentration
increased. The average particle size (d50) also
decreased with the declining seed concentration. The higher the seed concentration, the
finer the calcined powder. The average particle size decreased from about 0.75 to 0.45 µm
Fig. 2. Particle size distribution with various seed concentration: 1#, 0wt%; 2#, 2.67wt%; 3#, 10.70wt%; 4#, 23.00wt%.
when seed concentration was raised from 0
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wt% to 23 wt%.
Why α-alumina seed influences calcination of Al(OH) 3 was investigated. The phase transformation took place through the following route in the present work:
Al(OH)3 x-Al2O3 κ-Al2O3 α-Al2O3
(1)
The added seeds can act as the sites for initial crystal nucleation during the nucleation process. Also crystal growth can take place on the surface of the seeds and crystal transformation can
be accelerated. Nyvlt[16] indicated that crystal seeds can be considered as a kind of preferred nucleation sites, which can effectively lower the temperature of crystal transformation. And these
seeds can provide active sites, and increase nucleation frequency. The high nucleation frequency
results in an increase in nucleation speed, and thus finer grains develop from the crystal nucleation
at certain temperature. The transformation temperature, from aluminum hydroxide to alumina after adding seeds, was remarkably lowered due to nucleation density increase and nucleation potential decrease as well as nucleation frequency and speed increase. From the point of reaction
dynamics, introduction of seeds can reduce the activation energy. Hungchan et al.[17] showed that the activation energy of the transformation from θ-Al2O3 to
α-Al2O3 was reduced from 700 to 650 kJ/mol when
ultrafine α-Al2O3 seeds were added from 3 to 17.5
mol%.
Morphology of the alumina particles from calcined aluminum hydroxide is shown in fig. 3. Fine
particles about 0.4 µm were obtained. Alumina particles show different morphologies, some being equiaxed and the other being platelets or short elongated
grains. It is thought that the equiaxed and elongated
grains are associated with the α-Al2O3 seeds because
Fig. 3. TEM micrograph of Al2O3 grains transformed from aluminum hydroxide under the presence
of seeds.
2.2
the seeds are introduced from abrasives of α-alumina
medium on milling, which are mainly platelets or
have irregular shapes.
Effect of seed content on grain morphology of sintered samples
The SEM micrographs (fig. 4) showed the evolution of Al2O3 grain shapes with various seed
concentrations. The samples were all hot-pressed at 1600 for 2 h under a pressure of 30 MPa.
The shape and size of Al2O3 grains were greatly affected by the seed concentration. The microstructure of sample 1 (fig. 4(a)) without the seed contained mainly equiaxed grains and occasionally some large plate like grains, while in sample 2 (fig. 4(c)), with a seed concentration around 3
wt%, the morphology consisted of hexagonal platelets with a diameter of about 10µm. In sample 3
(fig. 4(c)), the seed concentration increased to about 10 wt%, and the grain shape remained un-
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changed, but the platelet grains became finer. The diameter of platelets reduced to around 5 µm.
With a higher seed concentration of above 20 wt%, the grains shown in fig. 4(d) was no longer
hexagonal platelet-shaped. The anisotropic growth of grains exhibited a trend of elongation. Some
elongated grains with an aspect ratio 2 could be observed in the matrix. Meanwhile the grain
size became even finer; most of them were smaller than 5 µm.
Fig. 4. SEM of microstructures of Al2O3 ceramics with various seed concentration. (a) 0 wt%; (b) 2.67wt%; (c) 10.70wt%;
(d) 23.00wt%.
The above results show that in sintered samples, grain size is influenced by the concentration
of added seeds. A relationship between seed platelets of α-Al2O3 and microstructure has been
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given by Brandon[18]:
 π
Do = α D 
 4 Af

,

(2)
where α is a geometrical constant, D the platelet diameter, f the seed concentration, and A the aspect ratio. It can be concluded that grain size will be coarser under the condition of small seed
concentrations. On the contrary, a high seed concentration will produce the microstructure of finer
grains. In the present study, abrasives with irregular shapes which were obtained by grinding
high-purity alumina milling balls, were used as seeds. The variation in grain size follows approximately the above relationship. With an increase in seed concentration, the grain size decreases. In addition, grains grew into hexagonal platelets and then became elongated grains with
further increase in seed concentration.
2.3
Relationship between sintering conditions and elongated grain growth
The calcined seed-containing powders were pressurelessly sintered at 1550 and 1600 for 2
h. The SEM micrographs of the polished and thermally-etched samples are shown in fig. 5. The
microstructure contained uniformly equiaxed grains. However, the hot-pressed samples have
much different microstructure from the pressurelessly sintered samples shown in fig. 4(d). Many
elongated grains with an aspect ratio of >2 were formed, and the other grains also tend to have an
elongated shape. A typical elongated grain morphology can be clearly observed from a high magnification SEM shown in fig. 6. The elongated grains have a regular geometry shape and develop
in certain direction. In addition, the plane surface of the elongated grain growth is found to be vertical to the pressure force. This can be explained by crystal growth kinetics. There is a large resistance-force to the grain growth on the face of the (0001) plane parallel to the hot-press direction,
and a relatively small resistance-force to the grain growth on the face of the (1120) plane. In addition to the sintering pressure, the seed addition is another key factor for the elongated grain development. This point has been confirmed by hot-pressing the sub-micrometer alumina powder
without seeds, where elongated grains could not be observed any more. All this indicates that
seeds and sintering pressure are two key factors developing elongated grains. Seeds induce a nucleation of elongated grain and sintering pressures promote anisotropic grain growth.
2.4
Fracture toughness increase via alumina elongated grains
Mechanical properties of the samples, hot-press sintered at 1600 with different hold times
under 25 MPa, are summarized table1. Flexural strength are 550600 MPa for the samples with 2
and 4 h hold times, and 400450 MPa for the samples with 0.5 and 1 h hold time. All the above
samples show high fracture toughness up to 6 MPam1/2. The major reason for the high fracture
toughness can be attributed to elongated grains developed on sintering. Toughening mechanisms
of crack bridging, crack deflection and elongated grain de-bonding can be introduced to the ceramics when elongated grains are formed in ceramics.
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Fig. 5. SEM micrographs of equiaxed grain alumina produced
from pressure-less sintered.
533
Fig. 6. SEM micrographs of alumina elongated grain
developed under hot-pressing.
Table 1 Influence of hold time on mechanical properties of alumina ceramics with elongated grains
1600
0.5
6.02
433
Temperature/k
Hold time/h
Fracture toughness/MPa⋅m1/2
Flexural strength/MPa
1600
1
6.13
496
1600
2
6.08
612
1600
4
6.04
623
The above results can be explained using Griffith equation in ceramic fracture mechanics:
σf =
1
Y
2 Eγ i 1 K IC
=
,
C
Y C
(3)
where Y is a crack geometry factor, E Young’s modulus, γ the fracture energy, C the crack length,
and KIC the fracture toughness. Eq. (3) shows that the fracture toughness can be expressed as
K IC = 2 Eγ i .
(4)
According to eq. (2), one can raise the toughness of ceramics by increasing Young’s modulus (E)
and fracture energy (γ ). However, the enhancement of the Young’s modulus is limited, which is
not sensitive to microstructure. So the main way to reach high fracture toughness is to increase
fracture energy as high as possible. The fracture energy (γ) can be written as
γ i = (γ s + γ p + γ AE + γ D + γ etc ) µ ,
(5)
where γ s is the surface energy for obtaining new flat surface, γ p the consumption energy for plastic
deformation at crack tip, γ AE the sound consumption energy during rapture, γ D kinetic energy for
crack-extension or vibration, γ etc other consumption energy, and µ correction parameter of non-flat
surface. Thus we see that the essence of toughening is to introduce fracture energy absorption
mechanisms into materials and reduce dynamic force of crack-extension or increase
crack-extension resistance force. In the present study, elongated alumina grains produce the
toughening effect by grain pulling-out (fig. 7). In fig. 7 lots of regular depression pits on the frac-
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ture surface were observed, which were the trace left from grain pulling-out. It is the pulling-out
effect that enhances crack-extension energy and results in a high fracture toughness. This is similar to the toughening mechanism in silicon nitride ceramics with high fracture toughness via elongated grain development in microstructures[11,12].
Fig. 7. SEM micrographs of fracture surface with elongated grain debonding.
In order to investigate the effect of hot-pressing pressure on microstructures and mechanical
properties of the samples sintered from calcined powder with seeds, powder containing 30 wt%
seeds was sintered at 1600k under different pressures. The results are shown in table 2. For the
samples sintered under 0.1 MPa (i.e. pressureless sintering), fracture toughness and flexural
strength are 3.74 MPaCm1/2 and 302 MPa, respectively, and the samples contain mostly the equiaxed grains. For the samples sintered under 25 and 40 MPa, high fracture toughnesses of 6.1 and
7.1 MPaCm1/2 are reached respectively. The above results imply that the grain shape and fracture
toughness are closely related to sintering pressure for the samples containing alumina seeds. This
is because hot-pressing pressure can promote anisotropically grain growth in one dimension.
Table 2 Effect of sintering pressure on alumina properties (1600)
Sintering pressure/Mpa
Fracture toughness/MPam1/2
Flexural strength/MPa
0.1
3.74
302
25
6.08
612
40
7.10
630
3 Conclusions
(1) The phase transformation from aluminum hydroxide to α- alumina was completed at rela-
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tively low temperature of 1100k for 2 h, and fine aluminum powder with sub-micrometer was
obtained by introducing α-Al2O3 seeds produced from abrasives of high purity aluminum ball medium into the raw materials. As seed concentration increased from 0 to 23 wt%, the average particle size of the calcined powder changed from 0.75 to 0.45 µm and the particle size distribution
also became narrower.
(2) The morphology of hot-pressed samples was strongly affected by the seed concentration.
Grains in the samples without seeds were mainly exquiaxed in shape. The microstructure of ceramics consists of hexagonal platelets with a low seed concentration. When the seed concentration
was higher than 20 wt%, the hexagonal platelets changed to elongated grains, resulting in a microstructure of elongated grains of aluminum.
(3) The grain shape was also strongly affected by sintering pressure. The microstructure with
elongated grains was obtained through hot-pressing. Some elongated grains with an aspect ratio
>2 were formed, and the rest also shows the tendency to elongate grain growth. In contrast, only
equiaxed grains were observed for the pressure-less sintered samples. This indicates that α-Al2O3
seeds and sintering pressure are two key factors developing elongated grains. The former introduces nuclei for elongated grain growth and the latter promotes grain growth anisotropically.
(4) Fracture toughness of the alumina ceramics with an elongated grain microstructure was
greatly increased. For the sample hot-pressed at 1600k under 40 MPa, fracture toughness as high
as 7.1 MPaCm1/2 was achieved, which is two times the value for the normal alumina ceramics
without elongated grains.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant No.
50172022) and Key Laboratory for High Temperature Structural Ceramics and Engineering Ceramic Machining of Tianjin University.
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