Fabrication of an Al2O3/YAG/ZrO2 ternary eutectic by combustion synthesis melt casting under ultra-high gravity

Article abstract of DOI:10.1111/j.1551-2916.2008.02911.x

This work presents a novel method for preparing an Al₂O ₃/YAG/ZrO₂ ternary eutectic whereby combustion synthesis melt casting has been combined with the ultra-high gravity (UHG) technique. The fabricated product had a rela

Full text of DOI:10.1111/j.1551-2916.2008.02911.x

J. Am. Ceram. Soc., 92 [2] 549–552 (2009)
DOI: 10.1111/j.1551-2916.2008.02911.x
r 2009 The American Ceramic Society
ournal
J
Fabrication of an Al O /YAG/ZrO Ternary Eutectic by Combustion
2
3
2
Synthesis Melt Casting Under Ultra-High Gravity
z
y,z
y
J
w,z
Rui Liang, Jun Pei, Jiangtao Li, Haibo Jin, and Kexin Chen
z
Department of Materials Science and Engineering, State Key Laboratory of New Ceramics and Fine Processing,
Tsinghua University, 100084 Beijing, China
yTechnical Institute of Physics and Chemistry, Chinese Academy of Sciences, 100080 Beijing, China
zGraduate School, Chinese Academy of Sciences, 100039 Beijing, China
J
School of Material Science and Engineering, Beijing Institute of Technology, 100081 Beijing, China
This work presents a novel method for preparing an Al O /
2
YAG/ZrO ternary eutectic whereby combustion synthesis melt
2
casting has been combined with the ultra-high gravity (UHG)
technique. The fabricated product had a relative density of
field can enhance the transmission of heat and mass, conse-
quently, the burning rate and the intensity of the thermite reac-
tion are increased. If such combustion reactions are conducted
under a high centrifugal force, then the essential conditions of
ultra-high temperature required for the eutectic ceramics to melt
along with a rapid cooling rate for refining the interphase spac-
ing during the subsequent solidification are fulfilled. In the pres-
ent work, we have investigated a novel method of combustion
3
3
99.3% of the theoretical one. Phase composition and micro-
structure analyses indicated that the application of UHG re-
sulted in a metal-free ceramic microstructure with no porosity or
microcracks. The microstructure comprises ZrO rods dispersed
2
in Al O . The product had 17.82 GPa Vickers hardness and
2
5
synthesis (CS) melt casting under UHG for producing an Al
2
YAG/ZrO ternary eutectic with good characteristic in terms of
2 3
O /
3
1
.51 MPa m fracture toughness.
/2
.
phase composition, microstructure, and mechanical perfor-
mance.
I. Introduction
1
N 1997, Waku et al. reported that a directionally solidified
II. Experimental Procedure
I
eutectic (Al O /YAG, where YAG is yttrium–aluminum
2
3
garnet,
3 5 12
Y Al O )
with
a
continuous three-dimensional
Powders of Al (purity 99.9%, 100 mm), Fe O (purity 99.99%,
2
3
interpenetrating microstructure exhibited outstanding high-
temperature mechanical properties (up to 2073 K), thermal
and microstructural stability, and oxidation resistance as
compared with conventional composites. The main factors
influencing the properties of the eutectic solidified from the
melt are the phase spacing (or eutectic lamella) and the eutec-
tic pattern. The phase spacing for each eutectic system depends
mainly on the solidification rate, while the eutectic pattern
depends on the volume fraction of each phase, the formation
of faceted or nonfaceted interphases, etc.
44 mm), Y O (purity 99.999%, 6.9 mm), and ZrO2 (purity
2
3
99.99%, 20 mm) were used as the starting materials. The reac-
tant powders, which had an Al O /Y O /ZrO molar ratio of
2
3
2
3
2
65.8/15.6/18.6, were well mixed by ball milling in ethanol media
for 2 h. The reaction of the reduction of Fe O was expected to
take place according to the following equation :
2
3
4
2
Al þ Fe2O3 ! Al2O3 þ 2Fe þ 836 kJ
(1)
The methods of fabricating eutectic oxides from a melt can be
classified into two groups: (a) unidirectional solidification in a
container and (b) pulling of a solid from the melt meniscus. The
The homogenized reactant powder was cold pressed into a
cylindrical graphite crucible with an inner diameter of 30 mm.
Each sample was 200 g. The density of the powder compact was
about 55% of the theoretical value. The crucible was mounted
on a Ni-based super alloy rotor in an apparatus that was spe-
cially designed and constructed in our laboratory for carrying
out the combustion synthesis meltcasting under ultra-high grav-
ity (CSMC-UHG). The CSMC-UHG apparatus is schemati-
cally represented in Fig. 1. The experimental procedure was as
follows: (a) the rotor started rotating to achieve an acceleration
2
Bridgman method is one of the former methods, whereby ther-
mal gradient is generally below 10 K/cm and interphase spacing
2
3
4
is usually larger than 10 mm. Higher thermal gradients (10 –10
K/cm) and consequently a smaller interphase spacing (o1 mm)
can be attained using the melt zone methods, which are suitable
for preparing high-performance fibers with submillimeter diam-
eters.
Highly exothermic combustion reactions are generally con-
sidered for generating sufficient heat to increase the temperature
of a system much above the melting point of products consisting
of both metals and ceramics, especially when the reactions are
carried out under ultra-high gravity (UHG). Because the UHG
2
of about 600 g (where g is the gravity acceleration, 9.8 m/s ) at
the end edge of the sample; (b) ignition of the mixture; (c) evac-
ꢀ5
uation of the chamber down to 10 MPa to remove reaction
gases; (d) cooling of the sample.
The crystallographic phase analysis was performed using
X-ray diffraction (XRD) (Model D/max-2500, Rigaku, Tokyo,
Japan). The microstructure of the samples was observed by
scanning electron microscopy (SEM) (Model S-4300, Hitachi,
Tokyo, Japan). The density of the samples was determined by
the Archimedes method by immersion in water. The hardness
and the fracture toughness were measured with a Vickers in-
dentation equipment using a load of 5 kg applied for 15 s, using
J. Ferreira—contributing editor
Manuscript No. 25424. Received October 30, 2008; approved November 20, 2008.
This work was supported by the National Natural Science Foundation of China (Grant
No. 50772116) and National High-Tech and New Materials Projects (Grant No.
2006AA03Z112).
5,6
2
1/2
3/2
wAuthor to whom correspondence should be addressed. e-mail: kxchen@mail.
tsinghua.edu.cn
the equations
where H is the Vickers hardness, K is the fracture toughness,
H 5 k P/d and KIC 5 k (E/H) (P/c ),
V 1 2
V
IC
5
49

5
50
Communications of the American Ceramic Society
Vol. 92, No. 2
Fig. 1. Schematic diagram of the apparatus designed and constructed for carrying out combustion synthesis melt casting under ultra-high gravity.
P is the indentation load, d is the indentation diagonal, E the
Young’s modulus of the material, attained from nanoindenta-
tion test, c is the crack length, and k and k are the dimension-
less constants determined experimentally. The mean values were
obtained from five different independent measurements.
From the density measurements, the relative density of the
product was found to be 99.3% of the calculated theoretical
value, indicating a highly dense material. This result agrees fairly
well with the SEM observations of the microstructure of the
products, shown in Fig. 3. The microstructure features an en-
tangled network with no pores or cracks. At a higher magnifi-
cation (Fig. 4), it is observed that the phase spacing is
submicrometer, i.e., r500 nm. Such a small scale is likely caused
by a large thermal gradient. The pattern consisting of a parallel
array shown in Fig. 4 distributes in the samples, which may have
been caused by local special cooling condition, and this requires
further study. According to the energy-dispersive spectroscopy
(EDS) elemental analysis, whose spectra are shown in Fig. 5, the
black zones are attributed to Al O , the gray zones to YAG, and
1
2
III. Results and Discussion
The X-ray diffractogram of the product presented in Fig. 2
shows that the product of CSMC-UHG under gravity condi-
2 3 2 2 3
tions of 600 g consists of Al O , YAG, and ZrO (YSZ, Y O -
stabilized ZrO ). This finding suggests that the molten Fe should
2
have been completely separated from the molten ceramic before
its solidification under the UHG field.
2
3
the fine white zones to ZrO . The Al peak in the spectra of ZrO
2
2
may be caused by the signals received from the Al element in
the neighboring areas, because the EDS beam spot is probably
larger than the relatively small ZrO area. The phases of YAG
2
and Al
other. The microstructure of the interface between the Al
and the YAG phases is ‘‘faceted to faceted.’’ The phase of ZrO2
2 3 2
O , with smaller areas of ZrO , penetrate into each
2 3
O
2 3
is largely dispersed in the form of rods in Al O , but it also
seldom appears in the phase of YAG.
The obtained dense microstructure, featuring the character-
istics of a composite material, resulted in good mechanical prop-
erties; specifically, the maximum Vickers hardness reached 17.82
GPa and the maximum fracture toughness reached 5.51
1
/2
.
MPa ꢁ m
IV. Discussion
At elevated temperatures of the combustion reaction, molten
iron (T 5 1809 K), molten ceramics phases in the form of ter-
nary eutectics (T 5 1990 K), and pores should coexist. Under
Fig. 2. X-ray diffractogram of an Al
prepared under UHG field. YAG, yttrium-aluminum garnet,
Al 12; YSZ; Y -stabilized ZrO ; UHG, ultra-high gravity.
2 3 2
O /YAG/ZrO ternary eutectic
a
m
Y
3
5
O
2
O
3
2
m

February 2009
Communications of the American Ceramic Society
551
Fig. 3. Microstructure of the as-fabricated Al
and (b) backscattered electron image.
2
O
3
/YAG/ZrO
2
ternary eutectic, observed by scanning electron microscopy: (a) secondary electron image;
where V is the moving rate of the dispersed phase, r the diameter
of the dispersed phase, a the UHG magnitude, r
the dispersed phase, r the density of the consecutive medium,
and Z the viscosity of the consecutive medium. According to Eq.
2), under a gravity of 600 g, a ceramic spherical droplet with a
1
the density of
2
(
diameter of 10 mm needs o1 s to travel a distance of 20 mm
through molten iron. For the case of a spherical pore under ex-
actly the same aforementioned conditions, the calculated time is
o5 s. Thus, complete separation of pores from molten Al
2 3
O is
possible under such high gravity values. Therefore, the UHG
field favors the fast elimination of pores from the molten ternary
eutectic ceramics, resulting in a highly dense microstructure,
which was observed in the experimental results.
The observed microstructure with the particular features of
the distribution of three phases in the ternary eutectics appar-
ently satisfies Jackson’s criterion. The Jackson interface rough-
2
Fig. 4. Submicrometer scale phase spacing observed by backscattered
electron.
ness parameter is defined as a ꢂ DSf =R, where DS
f
is the
entropy of fusion and R is the gas constant. If a42 for one of
the phases, then the growth of that phase is limited by the
nucleation rate and facets are easily produced. In the present
work, the calculated order for developing facets is
aYAG > aAl O3 > 2 > aZrO . Hence, the microstructure of
the UHG field, the separation of these three phases should obey
Stock’s law. Accordingly, the velocity of each phase can be cal-
culated by the following equation
2
2
the interface between Al O and YAG is anticipated to be ‘‘fac-
2
3
2
ðr ꢀ r Þ
agr ;
1
2
2
eted to faceted’’ (planar sharp interfaces), while the interface
between Al and YSZ should be ‘‘faceted to nonfaceted’’
V ¼ 3
Z
(2)
2 3
O
2 3 2
Fig. 5. Energy-dispersive spectroscopy elemental analysis of the component phases of the as-fabricated Al O /YAG/ZrO ternary eutectic.
YAG, yttrium–aluminum garnet, Y Al
3
5 12
O .

5
52
Communications of the American Ceramic Society
Vol. 92, No. 2
2
curved smooth interfaces). Therefore, the Al
(
phases showed a typically faceted morphology with triangular
2
O
3
and YAG
product obtained at UHG of 600 g had a fine and dense mi-
crostructure. No pores, cracks, or impurities of other phases
were registered. The final product had a relative density of
99.3% of the theoretical one, Vickers hardness 17.82 GPa,
or tetragonal shapes, while ZrO tends to grow in rods or lamel-
2
las instead of a faceted configuration. In the simplest case of
isotropic surface energy, ZrO rods are anticipated to form when
1
/2
2
and fracture toughness 5.51 MPaꢁ m
.
the volume fraction of the minor phase is o28%, while lamellas
The microstructure of the ternary eutectic consisted of an
entangled network of three phases, penetrating into each other
should form when it is above 28%. In the present study, the
theoretically calculated volume fraction of ZrO
2
was 13.6%.
Thus, ZrO intrinsically exhibited the tendency to grow in the
such that the interface between Al
faceted,’’ ZrO rods are distributed in Al
spacing is at a submicrometer scale.
2 3 2
The experimental results indicate the Al O /YAG/ZrO eu-
tectic material produced via the proposed method for further
consideration and experimentation in applications involving
mechanical and thermal fatigue stresses. Moreover, further
development and improvement of the UHG conditions, optimi-
zation of the mold shape design, and better control of cooling
will likely make CSMC-UHG a promising method for produc-
ing large-scale bulk eutectic ceramics with submicrometer phase
spacing.
2
O
3
and YAG is ‘‘faceted to
2
2
2 3
O , and the interphase
rod form than lamellas.
Although the different phases formed were separated from
each other with well-defined interfaces, there were no cracks
formed along or across these interfaces. This indicates both
strong interfacial adhesion and small residual stresses (likely due
to the mismatch of the thermal expansion coefficients or me-
chanical properties) along the interface. Accordingly, such
microstructural composite prepared via this particular method
is anticipated great vitality. Hence, it can be considered for fur-
ther experimentation with regard to potential applications that
involve mechanical and thermal fatigue conditions.
The above advantageous characteristics of the interfaces
among the phases, along with the dense microstructure with fea-
tures of composite material, resulted in a material with better
2 3 2
mechanical properties than those obtained for Al O /YAG/ZrO
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