Microstructure and mechanical properties of Al2O 3/ZrO2 eutectic ceramic composites prepared by explosion synthesis

Article abstract of DOI:10.1016/j.jallcom.2012.11.064

Al₂O₃/ZrO₂ eutectic ceramics were prepared by explosion synthesis using Al and Zr(NO₃)₄ as raw materials. The Al₂O₃/ZrO₂ eutectic microstructure showed that the rod-like ZrO₂ phases with a diameter 200 nm were embedded orderly in Al₂O₃ matrix. With the increasing of the reaction temperature, the volume content of the Al ₂O₃/ZrO₂ eutectic increased accordingly, and the diameter and phase spacing of the rod-like ZrO₂ decreased, which was mainly attributed to the sufficient diffusion at higher temperature and the large degree of supercooling. Due to the fine microstructure of the Al ₂O₃/ZrO₂ eutectic, the Vickers hardness and fracture toughness can reach 17.7 GPa and 10.3 MPa m^1/2, respectively.

Full text of DOI:10.1016/j.jallcom.2012.11.064

Journal of Alloys and Compounds 551 (2013) 475–480
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Microstructure and mechanical properties of Al2O3/ZrO2 eutectic ceramic
composites prepared by explosion synthesis
⇑
Yongting Zheng , Hongbo Li, Tao Zhou, Jing Zhao, Pan Yang
Center for Composite Materials and Structures, Harbin Institute of Technology, 150001 Harbin, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 March 2012
Received in revised form 31 October 2012
Accepted 10 November 2012
Available online 19 November 2012
Al2O3/ZrO2 eutectic ceramics were prepared by explosion synthesis using Al and Zr(NO3)4 as raw materi-
als. The Al2O3/ZrO2 eutectic microstructure showed that the rod-like ZrO2 phases with a diameter 200 nm
were embedded orderly in Al2O3 matrix. With the increasing of the reaction temperature, the volume con-
tent of the Al2O3/ZrO2 eutectic increased accordingly, and the diameter and phase spacing of the rod-like
ZrO2 decreased, which was mainly attributed to the sufficient diffusion at higher temperature and the
large degree of supercooling. Due to the fine microstructure of the Al2O3/ZrO2 eutectic, the Vickers
hardness and fracture toughness can reach 17.7 GPa and 10.3 MPa m1/2, respectively.
Keywords:
ZrO2–Al2O3
Eutectic ceramic
Explosion synthesis
Microstructure
Mechanical properties
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
laser heated floating zone method [10,11], micro pulling down
method [12,13], combustion synthesis method [14], and so on.
Ceramic matrix composites offer improved toughness and
strength compared with monolithic ceramics due to the energy
dissipation of crack initiation, growth and deflection at or along
the interface of the two phases [1]. Eutectic ceramics are a para-
digm of ceramic matrix composites with a fine microstructure on
micrometer or nanometer scale [2], which have outstanding ther-
mal stability and mechanical properties as compared with mono-
lithic and conventional ceramic matrix composites [3,4]. The
mechanical properties of two phase eutectic ceramics are better
than that of either constituent alone, because of the strong con-
straining effects provided by the interlocking microstructure [5].
In the past, much attention was focused on metallic eutectics,
and many progresses in field of the eutectic growth and micro-
structure have been achieved in metallic eutectics. However, less
effort was devoted to eutectic ceramics. The reason is that the
eutectic temperature of eutectic ceramics is much higher than that
of metallic eutectics and the preparation of eutectic ceramics was
difficult. Recently, most efforts of eutectic ceramics had been
made on Al2O3-based eutectics, especially the Al2O3–ZrO2 eutectics
[6–8], due to the outstanding creep resistance of sapphire along the
c-axis. In the last two decades, as the development of processing of
eutectic ceramics, several preparation techniques for eutectic
ceramics have been studied fully, such as Bridgman method [9],
In this paper, explosion synthesis was introduced to prepare
Al2O3/ZrO2 eutectic ceramics and the effects of reaction parameters
(temperature and pressure) on the microstructure and mechanical
properties were discussed. Explosion synthesis, as a novel prepara-
tion method, is a simple procedure and an energy saving method
for preparing refractory ceramics, cermets and metal-based com-
posites. Explosion synthesis method has some particular advanta-
ges compared with other methods, which was conducive to the
studying of reaction mechanism and the formation of eutectic
structure. Because of its super high reaction velocity, explosion
synthesis can be regarded approximately as an adiabatic process,
which is convenient for the analysis of physicochemical character-
istics of the reaction. In addition, explosion synthesis can reach
super high temperature easily, which can be used to prepare vari-
ous refractory eutectic ceramics. What is more, this method can
obtain high cooling rate and supercooling degree because the cool-
ing system can be directly mounted on the reactor without consid-
eration of the heating apparatus. The high cooling rate is very
important to achieve fine microstructure and excellent properties
of eutectic ceramic.
2. Experimental procedure
The reactant powders used in this experiment were Al (6
lm, purity > 99.5%,
Northeast Light Alloy Co., Ltd., China) and Zr(NO3)4 (2 m, purity > 99.9%, Zibo
l
Rongruida Micro Materials Plant, China). The explosion reaction for preparing
eutectic ceramic can be described as follows:
⇑
Corresponding author. Tel./fax: +86 451 86402392.
E-mail address: zhengyt@hit.edu.cn (Y. Zheng).
Al þ ZrðNO3Þ4 ! Al2O3 þ ZrO2 þ N2
ð1Þ
0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2012.11.064

476
Y. Zheng et al. / Journal of Alloys and Compounds 551 (2013) 475–480
Fig. 1. Binary phase diagram of Al2O3–ZrO2 eutectic.
Fig. 2. XRD patterns of as-synthesized products (AZ1) under different condition: (a)
in closed reactor and (b) in open reactor.
According to the phase diagram of Al2O3–ZrO2 shown in Fig. 1, eutectic phase
can be formed at the 42.6 wt% ZrO2 at the temperature of about 2135 K. Through
adding appropriate amount of Al2O3 and ZrO2 into reactant, the reaction tempera-
ture and Al2O3/ZrO2 ratio in products can be adjusted to study the reaction mech-
anism and eutectic microstructure. The reactant components with adiabatic
temperature of 2300, 2500, 2700 and 2900 K, which were marked as AZ1, AZ2,
AZ3 and AZ4, respectively, were selected to study the effect of reaction temperature
on the morphology and microstructure of the Al2O3/ZrO2 eutectic. The raw materi-
als were mixed by ball milling for 12 h using alumina milling-media with a ball-to-
powder ratio of 3/1. Then the mixed powders were cold-isostatically pressed into a
cylindrical compact with a diameter of 20 mm and height of 40 mm. The density of
the compact was about 56% of the theoretical value.
The combustion reaction was carried out in a high-pressure SHS reactor, which
consisted of ignition system, reaction chamber, water cooling system and gas evac-
uation system. An alumina insulator crucible, in which laid the powder compacts,
was put in the reaction chamber, and a Ni–Cr coil was mounted on the reactant
as electrical ignition source. The reaction was ignited by a large electric current
in short time.
reactor exhibit explosion mode, and the reaction completed
instantaneously.
Because the explosion synthesis of the Al2O3/ZrO2 eutectic cera-
mic was completed instantly, the reaction can be regarded as an
adiabatic procedure. The adiabatic temperature was calculated
through thermodynamics analysis, as shown in Fig. 3. The mea-
sured reaction temperatures were marked by black square in
Fig. 3. It indicated that the adiabatic temperature increased linearly
with the increasing Al content, and a temperature platform oc-
curred at the Al2O3/ZrO2 eutectic transformation temperature
(2135 K). To obtain Al2O3/ZrO2 eutectic ceramics, the reaction tem-
perature of the explosion synthesis should be higher than the eu-
tectic transformation temperature.
The reaction temperature curves of sample AZ1 measured by
W/Re thermocouples were shown in Fig. 4. As the figure indicated,
explosive reaction has very quickly reaction velocity, and a rela-
tively sharp rise in reaction temperature occurred once the reac-
tant mixture is ignited. Because the reactions in closed reactor
proceeded as explosion mode, the reactant compacts melted and
then deposited in the bottom of the reactor. At the top of the sam-
ple, the thermocouples was contacted with the molten products
with a short time span, subsequently, the temperature was de-
creased sharply, as shown in Fig. 4(a). However, the temperature
curve in the bottom was descended slowly after the sharp rise,
In order to the rapid solidification of melted Al2O3 and ZrO2, the explosion syn-
thesis reactions take place in a high pressure closed reactor with water cooling sys-
tem, with an inside diameter of 25 mm and height of 75 mm. The gas released by
the reaction can generated pressure of about 10–20 MPa. The reaction temperature
was measured by W/Re thermocouples. Although the standard measured tempera-
ture range of W/Re thermocouples is lower than 2573 K, higher temperature up to
3000 K can be calculated from thermoelectric potential by extrapolation method in
a short time span. Meantime, the combustion synthesis reaction was also carried
out in unclosed reactor to study the effect of the air atmosphere on the reaction
and composition of the eutectic ceramics.
The composition of the eutectic ceramics was tested by Dmax-rb X-ray Diffrac-
tomer. The morphology of the cross-sections of the products was examined by
scanning electron microscopy (SEM) at 20 kV using a FEI Quanta 2000F. Energy dis-
persive X-ray spectroscopy (EDS) was used to elemental and microstructure analy-
sis. Fracture toughness and Vickers hardness was measured on Vickers Hardness
Tester (HVS-5, Huayin Experimental Instrument Co., Ltd., China) with the loading
force of 5 kg and loading time of 30 s.
3. Results and discussion
Fig. 2 shows the XRD patterns of the Al2O3/ZrO2 ceramics pre-
pared in closed and open reactor. As the figure indicated, the prod-
ucts were both composed of ZrO2 and Al2O3, but small trace of
Zr7O8N4 was observed in the product prepared in the open reactor.
The formation of Zr7O8N4 phase could be attributed to the different
atmosphere in the open reactor compared with that in closed reac-
tor. In closed reactor, Zr(NO3)4 can decompose and release suffi-
cient amount of oxygen, and the high oxygen partial pressure
ensure the stability of the ZrO2, while in the open reactor, the large
content of oxygen escaped and relatively larger nitrogen content
promoted the formation of Zr7O8N4 phase. What is more, the reac-
tion velocity in closed condition is much faster than that in open
condition. Experimental results show that the combustion velocity
in open reactor is about 1 cm/s, however, the reaction in closed
Fig. 3. Relationship between the Tad and Al content.

Y. Zheng et al. / Journal of Alloys and Compounds 551 (2013) 475–480
477
Fig. 5. The relationship between reaction temperature and pressure.
was relatively large. Many ZrO2 particles, which have not trans-
formed to eutectic, were found. The reason is that the mutual dif-
fusion of the Al2O3 and ZrO2 was insufficient under the
temperature of slightly higher than eutectic point in short time.
With the reaction temperature increasing, the Al2O3/ZrO2 eutectic
area was larger and the eutectic structure was finer, and also the
content and size of the ZrO2 particles decreased.
When the reaction temperature was 2300 K, which is lower
than the melting point of Al2O3 and ZrO2, the solid Al2O3 and
ZrO2 particles cannot adequately diffuse to form eutectic melt in
short time because of low diffusion coefficient in solid state and
small contact area between particles. However, temperature fluc-
tuation can exist in the samples during the explosion synthesis,
and local temperature in the regions where Zr(NO3)4 aggregated
may rise up to the melting point of Al2O3. So a small quantity of
Al2O3/ZrO2 eutectic formed, as shown in Fig. 6(a). With the increas-
ing of the reaction temperature, the molten fraction of Al2O3 in-
creased, and contact area between Al2O3 and ZrO2 increased
accordingly because of wetting behavior, and the diffusion degree
of Al2O3 and ZrO2 was enhanced because of higher temperature
and longer retention time, thus the eutectic volume was increased,
as shown in Fig. 6(b) and (c). When the reaction temperature
reached 2900 K, the Al2O3 and ZrO2 phases were melted and dif-
fused quickly and fully in liquid state, and the good eutectic struc-
ture formed (Fig. 6(d)).
With the increasing of the reaction temperature, the micro-
structure of the Al2O3/ZrO2 eutectic became much finer. When
the reaction temperature was lower than the melting point of each
component, large number of Al2O3 and ZrO2 particles existed in the
samples and crystallization of the eutectic can be regarded as het-
erogeneous nucleation process, which usually has a small degree of
supercooling. So the eutectic structure was relatively coarse be-
cause of the small degree of supercooling, as shown in the
Fig. 6(a) and (b). Under high reaction temperature, the molten
Al2O3 and ZrO2 are in good mutual soluble condition, and no solid
phase particles were existed in the melt. The formation of eutectic
structure can be considered as the homogeneous nucleation pro-
cess, which usually has a large degree of supercooling. Due to the
metal reactor was made by high-strength steel and equipped with
water cooling system, the molten solution have high degree of
supercooling. So the Al2O3/ZrO2 eutectic exhibited fine microstruc-
ture with fine phase spacing.
Fig. 4. Temperature profile of the explosion synthesis for Al2O3/ZrO2 eutectic (AZ1):
(a) top of the sample and (b) bottom of the sample.
as shown in Fig. 4(b), which was attributed to the heat conduction
of the melt.
In the closed reactor, large content of nitrogen was released and
produced high pressure during the explosion synthesis under high
temperature in a short time, and the high pressure will raise the
boiling point of Al2O3 and ZrO2. Choosing proper reactant ratio of
the Al to ZrO2 can assure the appropriate reaction temperature to
melt Al2O3/ZrO2 eutectic. According the reaction (1), the pressure
in the reactor produced by the released nitrogen gas and evapora-
tion of Al2O3 and ZrO2 was calculated theoretically based on Van
der Waals equation [15]:
½P þ 1ðn2a=V2ÞꢀðV ꢁ nbÞ ¼ nRT
ð2Þ
where P is the pressure, V is the volume, n is the number of moles, T
is the reaction temperature, R is the ideal gas constant, a and b is the
gas constant of nitrogen. Fig. 5 shows the relationship between the
reaction temperature and the pressure in the reactor. It can be
found that the pressure in reactor increased with the increasing of
reaction temperature. A suddenly rise in pressure occurred at the
eutectic temperature of 2135 K because of the phase change in reac-
tant system. In addition, another dramatic increase occurred at the
boiling point of Al2O3 and ZrO2 (about 4500 K).
Fig. 6 shows the morphology and distribution of Al2O3/ZrO2 eu-
tectic prepared under different reaction temperatures. In general,
the content of Al2O3/ZrO2 eutectic in the products increased with
the increasing of reaction temperature; meantime, the diameter
and phase spacing of the rod-like ZrO2 decreased. Under reaction
temperature of 2300 K, the content of Al2O3/ZrO2 eutectic was
small, and the diameter and phase spacing of the rod-like ZrO2
The phase spacing (k) of the Al2O3/ZrO2 eutectic was decreased
with the increasing of the reaction temperature. It is known that
the k is controlled largely by the degree of supercooling. J. LLorca

478
Y. Zheng et al. / Journal of Alloys and Compounds 551 (2013) 475–480
Fig. 6. Morphology and distribution of the Al2O3/ZrO2 eutectic prepared under reaction temperature of (a) 2300 K, (b) 2500 K, (c) 2700 K and (d) 2900 K.
[2] has reported the formulas of relationships between phase spac-
ing and degree of supercooling:
kD
T ¼ K
ð3Þ
in which k is the phase spacing and
D
T is the degree of supercooling,
K is constant. According the above formulas, higher degree of super-
cooling is favorable for the refining of the phase spacing.
In summary, higher reaction temperature improve the mutual
solution of Al2O3 and ZrO2, which contributed to the formation of
larger area of Al2O3/ZrO2 eutectic; meantime, higher reaction tem-
perature led to higher cooling rate and larger temperature gradient,
and this also favor the formation of fine eutectic microstructure.
Fig. 7 shows the typical fractography of the Al2O3/ZrO2 eutectic
(AZ4). The ZrO2 phases embedded in Al2O3 matrix presented rod-
like shape, and the diameter was about 200 nm. This fine structure
was mainly attributed to the high cooling rate in the explosive reac-
tion. Meantime, the fractography shows the pull-out of the ZrO2
rods and the crack deflection, which can efficiently hinder the crack
propagation and greatly improve the bending strength and fracture
toughness. This is the reason that eutectics have relative high
mechanical properties at room and elevated temperatures.
Fig. 8(a) shows the back scattered electron image of the Al2O3/
ZrO2 eutectic structure (AZ4). As the figure showed, the randomly
orientated Al2O3/ZrO2 eutectic colonies can be observed more
clearly, and three directions of the ZrO2 rods can be found. In the
enlargement of the eutectic crystal, rod-like ZrO2 distributed uni-
formly with same diameter and interphase spacing, as shown in
Fig. 8(b).
Fig. 7. SEM fractography of the fracture of Al2O3/ZrO2 eutectic (AZ4).
temperature, and then uniform liquid phase was formed by the dif-
fusion between the liquid Al2O3 and ZrO2. The uniform degree of
the eutectic melt was determined by the reaction temperature
and reaction time. At last, the eutectic ceramic was obtained with
large degree of supercooling during the cooling process.
Typical eutectic microstructure mainly presented three models:
lamellas, rod and globule. The volume fraction of the two phases
decided whether lamellas or rod eutectics structure formed [16].
According to thermodynamics analysis, the morphology and
The formation characteristic of the Al2O3/ZrO2 eutectic struc-
tures is the simultaneous growth of the two phases from the melt.
Firstly, Al2O3 and ZrO2 phases were melt under high reaction

Y. Zheng et al. / Journal of Alloys and Compounds 551 (2013) 475–480
479
Fig. 8. Back scattered electron image of the Al2O3/ZrO2 eutectic (AZ4): (a) large eutectic area, (b) enlargement of eutectic structure.
ꢀ
ꢁ
1=2
distribution of the both eutectic phases should satisfy the mini-
mum energy principle, namely, minimum interfacial area and en-
ergy. Based on the [2], ZrO2 rods in ZrO2/Al2O3 eutectic are
anticipated to form when the volume fraction is less than 28%,
while the lamellas form when volume fraction is larger than 28%.
To study the effect of reaction temperature on the morphology
and distribution of the ZrO2/Al2O3 eutectic, excess Al was added
to the reactant, and this decreased the volume fraction of ZrO2
phase, so ZrO2 exhibited the trend to grow in the rod structure than
lamellas.
E
c3Hv
KIC ¼ 0:018P
ð5Þ
where E is Young’s modulus (Pa), P is the load (N), and c the crack
length (mm).
Fig. 10 shows the Vickers hardness of the Al2O3/ZrO2 eutectic
prepared under different reaction temperature. As the discussion
above, with the increasing of the reaction temperature, the
Al2O3/ZrO2 eutectic volume increased and the microstructure re-
fined as well, so the Vickers hardness increased accordingly.
Clearly crack path was observed only in sample of AZ4, so the frac-
ture toughness of AZ4 was calculated, and the value was
Fig. 9 shows the Vickers indent and the enlargement of the
crack in sample of AZ4. The values of Vickers hardness and fracture
toughness was calculated by the following equation [17]:
10.3 MPa m1/2
.
2 sinð68ꢂ ÞP
The key factor to improve the mechanical properties of the
eutectics was to obtain fine eutectic microstructure. In this exper-
iment, the high undercooling rate generated fine structure of
Al2O3/ZrO2 eutectic, which improves the crack propagation resis-
tance and the mechanical properties effectively. The Vickers hard-
Hv ¼
ð4Þ
d2
where HV is Vickers Hardness (Pa), P is the indenter load (N), and d
is the mean indent diameter (m).
The fracture toughness was calculated by the following equa-
tion [18]:
ness and fracture toughness reached 17.7 GPa and 10.3 MPa m1/2
,
respectively. However, it is noted that the hardness values were
Fig. 9. Vickers indent (a) and the enlargement of the crack (b) in sample of AZ4.

480
Y. Zheng et al. / Journal of Alloys and Compounds 551 (2013) 475–480
accordingly, and the diameter and phase spacing of rod-like ZrO2
decrease, which is mainly attributed to the large degree of super-
cooling and temperature gradient. With the Al2O3/ZrO2 eutectic
microstructure changing finer, the mechanical properties im-
proved. Vickers hardness and fracture toughness can reach
17.7 GPa and 10.3 MPa m1/2, respectively.
Acknowledgement
This work was supported by the National Natural Science Foun-
dation of China (No. 91016014) and Key Laboratory Fund Project of
Science and Technology on Advanced Composites in Special Envi-
ronments Laboratory.
References
[1] W.B. Hilling, Mater. Sci. Res. 20 (1986) 697–712.
[2] J. LLorca, V.M. Orera, Prog. Mater. Sci. 51 (2006) 711–809.
[3] Y. Waku, N. Nakagawa, J. Mater. Sci. 36 (2001) 1585–1594.
[4] J. Lee, A. Yoshikawa, T. Fukuda, Y. Waku, J. Cryst. Growth 231 (2001) 115–120.
[5] A. Sayir, S.C. Farmer, Acta Mater. 48 (2000) 4691–4697.
[6] J.Y. Pastor, J. LLorca, P. Poza, I. de Francisco, R.I. Merino, J.I. Pe~na, J. Eur. Ceram.
Soc. 25 (2005) 1215–1223.
Fig. 10. Vickers hardness of the products prepared under different temperature:
2300 K (AZ1), 2500 K (AZ2), 2700 K (AZ3) and 2900 K (AZ4).
[7] C. Oelgardt, J. Anderson, J.G. Heinrich, G.L. Messing, J. Eur. Ceram. Soc. 30
(2010) 649–656.
[8] Y. Mizutani, H. Yasuda, J. Cryst. Growth 244 (2002) 384–392.
[9] Y. Waku, N. Nakagawa, Nature 389 (1997) 49–52.
[10] F.A. Huamán-Mamani, M. Jiménez-Melendo, M.C. Mesa, P.B. Oliete, J. Alloys
Comp. (2012), http://dx.doi.org/10.1016/j.jallcom.2012.01.105.
[11] H.J. Su, J. Zhang, C.J. Cui, L. Liu, H.Z. Fu, J. Cryst. Growth 30 (2007) 448–456.
[12] J.H. Lee, A. Yoshikawa, J. Cryst. Growth 231 (2001) 179–185.
[13] J. Llorca, J.Y. Pastor, P. Poza, J.I. Peña, I. de Francisco, A. Larrea, J. Am. Ceram.
Soc. 87 (2004) 633–639.
lower than those reported in the reference [14,19]. The reason is
that the Al2O3/ZrO2 eutectic in the references was prepared by
combustion synthesis melt-casting under ultra-high gravity, and
at the same time the sintering aids of Y2O3 was added to improve
the solidification.
4. Conclusions
[14] Z.M. Zhao, L. Zhang, J. Zheng, Scripta Mater. 53 (2005) 995–1000.
[15] M.J. Klein, Physica 73 (1) (1974) 28–47.
[16] K.A. Jackson, J.D. Hunt, Trans. AIEE 236 (1966) 1129–1142.
[17] J.H. Gong, J.J. Wu, Z.D. Guan, J. Eur. Ceram. Soc. 19 (1999) 2625–2631.
[18] H. Lee, R.F. Speyer, J. Am. Ceram. Soc. 85 (2002) 1291–1293.
[19] L. Mei, P.L. Mai, J.T. Li, K.X. Chen, Mater. Lett. 64 (2010) 68–70.
Al2O3/ZrO2. eutectic ceramics were fabricated by explosion syn-
thesis. The microstructure of Al2O3/ZrO2 eutectic shows rod-like
ZrO2 phase with a diameter smaller than 200 nm embedded
orderly in Al2O3 matrix. With the increasing of the reaction tem-
perature, the volume fraction of the Al2O3/ZrO2 eutectic increases