Full text of DOI:10.1111/j.1151-2916.2003.tb00007.x

J. Am. Ceram. Soc., 86 [2] 247–51 (2003)
journal
Bulk, Dense, Nanocrystalline Yttrium Aluminum Garnet by
Consolidation of Amorphous Powders at Low Temperatures and
High Pressures
,
†
Samrat Choudhury, Ashutosh S. Gandhi, and Vikram Jayaram*
Department of Metallurgy, Indian Institute of Science, Bangalore 560 012, India
Amorphous Al O –37.5% Y O powders, prepared using
decomposed phases can be densified under pressure, while remain-
ing below the critical temperature at which further gas evolution
occurs. The above considerations also extend to higher tempera-
2
3
2
3
spray pyrolysis followed by partial or complete thermal de-
composition, were hot-pressed at 315°–640°C and 500 or 750
MPa uniaxial pressure. Hot pressing of fully decomposed
amorphous powder at 450°–640°C at pressures up to 750 MPa
led to densification (up to 96%) as well as nanocrystallization
of yttrium aluminum garnet (YAG). When the pressure was
applied during heating, instead of after reaching the final
temperature, higher relative densities resulted. Fully crystal-
line starting powder did not densify. The low true density of
5
,6
tures and pressures (700°–900°C and 1 GPa),
under which
conditions the amorphous phase crystallizes during densification.
At 700°C, the relative density achieved is higher in the presence of
the metastable phases of ␥-Al O and tetragonal-ZrO2 than at
2
3
800°–900°C, where the equilibrium phases of ␣-Al O and
2
3
monoclinic-ZrO appear.
2
To investigate the generality of the above phenomena, an
amorphous material with composition corresponding to yttrium
aluminum garnet (YAG, Y Al O ) has been chosen for consoli-
؊3
the amorphous phase (3.1 g⅐cm ) was believed to be respon-
sible for the densification through enhanced ionic mobilities.
3
5 12
dation using a route similar to that described above for Al O –
2
3
ZrO . Precursor-derived amorphous phases in Al O –Y O crys-
tallize at temperatures as high as ϳ900°C, which allows a wide
2
2
3
2 3
7
I. Introduction
range of temperatures for hot pressing. Moreover, the radius of the
1
–3
T RECENTLY has been shown
that amorphous ZrO –Al O
2 3
3ϩ
4ϩ
8
2
Y
ion is slightly larger than that of the Zr ion and can be
I
powders, prepared using spray pyrolysis of aqueous solutions of
expected to lead to open, low-density structures similar to that
encountered in Al O –ZrO . The ability to produce the amorphous
nitrates, can be hot-pressed to high relative densities (95%–99%)
while retaining the amorphous state. Bulk nanocrystalline meta-
stable tetragonal-ZrO (Al O ) solid solutions with 8 nm grain size
have been produced by crystallization of the dense amorphous
samples. Under the conditions used for consolidation (uniaxial
pressures of 500–750 MPa and temperatures of 450°–650°C), the
amorphous powders exhibit densification behavior that is largely
independent of time. The relative density increases monotonically
and instantaneously with applied pressure. The true density of the
amorphous phase with composition ZrO –40 mol% Al O , at 3.4
2
3
2
counterpart of YAG allows the synthesis of a material with new
electronic and optical properties.
Chemical precursor synthesis of phase-pure and nanocrystalline
YAG has been widely studied. Metastable microstructures with
phases such as orthorhombic- or hexagonal-YAlO3 and ␣- or
2
2 3
␥
-Al O are encountered even in nominal compositions corre-
2 3
9,10
sponding to YAG (37.5 mol% Y O ).
YAG synthesis experi-
2
3
ments frequently have produced single-phase YAG as the first
11–15
2
2 3
(and only) product of crystallization,
although, occasionally,
Ϫ3
g⅐cm , is considerably less than that of the equilibrium phases
5.0 g⅐cm for a mixture of monoclinic-ZrO and ␣-Al O ). It is
an intermediate hexagonal-YAlO has served as the crystalline
Ϫ3
3
(
16–19
2
2 3
precursor to YAG.
Similar intermediate phases form during
20
suspected that the open structure of the metastable amorphous
phase can, in part, be responsible for the ease of plastic deforma-
tion at the relatively low temperatures of consolidation. The low
yield stress of the amorphous phase can be inferred from the low
room-temperature hardness of 4 GPa (compared with ϳ10 and 18
GPa for ZrO2 and Al O , respectively), and, subsequently, the
ability to plastically deform under uniaxial compression at stresses
of ϳ360 MPa at 600°–700°C has been established.
It also was found that as-sprayed, partially decomposed Al O –
ZrO powders ex hibit marked sinterability when heated to their
sintering temperature through the temperature range of complete
decomposition (up to 750°C). Sintering does not occur once the
thermal decomposition is complete. While this feature cannot be
exploited during hot pressing because of the need to allow volatile
species to escape, there remains the possibility that these partially
mechanochemical synthesis. These results indicate the need to
preserve molecular homogeneity during precursor synthesis, in-
cluding sol–gel and spray pyrolysis routes. In situations where
segregated phases appear (e.g., Y O , Y Al O , and YAlO ),
2
3
4
2
9
3
subsequent reaction is needed at higher temperatures to produce
16
2
3
garnet. However, it has been shown in recent reports that
phase-pure YAG powders can be synthesized through low-
temperature chemical precursor synthesis routes, such as spray
4
7
21,22
23
2
3
pyrolysis, coprecipitation,
glycothermal method,
and
24
2
mixed-metal citrate precursor method. Most of these routes yield
nanocrystalline YAG powders at temperatures Ͻ1000°C.
Processing of bulk, dense nanocrystalline YAG has proved to be as
difficult as processing other nanocrystalline oxides. The usual trade-
25–30
off between residual porosity and grain growth
sintering of nanometer-sized powder particles.
has been made in
Sintering tem-
21,31–33
peratures ϾϾ1000°C are usually required for obtaining high relative
density. In this context, the present research studies the processing of
bulk, nanocrystalline YAG, as well as the densification of spray-
pyrolyzed amorphous powders with the YAG composition of Al O –
J. Halloran—contributing editor
2
3
3
7.5% Y O at temperatures of Յ640°C.
2 3
Manuscript No. 187431. Received October 15, 2001; approved July 3, 2002.
Supported by a grant from the Department of Science and Technology, Govern-
ment of India.
II. Experimental Procedure
–1
An aqueous solution (200 g⅐L ) of Al(NO ) and Y(NO3)3
corresponding to the composition of YAG (37.5 mol% Y O ) was
2 3
*
Member, American Ceramic Society.
3 3
†
Author to whom correspondence should be addressed.
2
47

2
48
Journal of the American Ceramic Society—Choudhury et al.
Vol. 86, No. 2
sprayed with compressed air onto a Teflon-coated aluminum pan
maintained at 285°–305°C. The resulting partially decomposed
powder was studied using thermogravimetric analysis (TGA;
Model TG-171, Cahn Instruments, Cerritos, CA) at a heating rate
of 10°C/min to identify the temperature range of weight loss
accompanying complete thermal decomposition. Differential ther-
mal analysis (DTA; Polymer Laboratories, Epsom, U.K.) per-
formed at 10°C/min on the as-sprayed powder and X-ray diffrac-
tometry (XRD; Model JDX-8030, JEOL, Tokyo, Japan) performed
on heat-treated powders were used to identify the crystallization
temperature and the product of crystallization. Amorphous
powders with an increasing degree of thermal decomposition
were obtained by heat treatments at 365°, 450°, and 600°C,
whereas fully crystalline YAG powder was obtained by heat
treatment at 1000°C. The heating rate was 10°C/min and the
holding time was 1 min, after which the sample was air cooled.
Sedimentation was conducted in a 15 cm tall column of xylene
to obtain particles Ͻ20 ␮m.
The true densities of the heat-treated powders were measured
using pycnometry with specific gravity bottles of 10 mL volume
and xylene as the fluid medium. The powders were ground using
a mortar and a pestle to break agglomerates. A period of 10 d was
allowed to achieve full infiltration of the finest interparticle voids.
Uniaxial hot pressing was conducted at temperatures ranging
from 315° to 640°C, in a 5 mm diameter cylindrical die made of
nickel-based superalloy (Alloy 718, Midhani, Hyderabad, India).
In the cases of partially decomposed powders, hot pressing was
performed at a temperature lower than or equal to the heat
treatment to minimize the evolution of volatile matter and buildup
of gas pressure inside the compact. Most of the experiments were
performed under load control using a servo-hydraulic universal
testing frame (Model 9500 Console, DARTEC, Stourbridge,
Fig. 2. DTA of as-sprayed Al O –37.5% Y O powders show exother-
mic peak with maximum at ϳ921°C, which corresponds to the crystalli-
zation of YAG.
2
3
2 3
The relative density was quoted as 100%-% porosity. Transmis-
sion electron microscopy (TEM; Model 000FX-II, JEOL) was
performed on hot-pressed samples after they were crushed to fine
powders, mounted on a copper grid, and coated with carbon.
–
1
U.K.). The pressure was applied at 10.2 MPa⅐s , either at room
temperature or after the final temperature was reached. A few
experiments were performed under a constant displacement rate of
III. Results and Discussion
Figure 1 shows the TGA plot of the as-sprayed Al O –37.5%
2
3
–
1
Y O powder. The major weight loss, which follows the loss of
1
0 ␮m⅐s in a screw-driven universal testing frame (Instron,
2 3
adsorbed moisture up to ϳ100°C, occurs between 325° and 480°C,
which is attributed to the completion of thermal decomposition.
DTA reveals an exothermic peak at 921°C (Fig. 2). The endo-
therms at low temperatures correspond to the weight loss due to
thermal decomposition. As discussed below, this peak is due to the
crystallization of YAG. Based on the above information, the four
temperatures of heat treatment (365°, 450°, 600°, and 1000°C) of
the as-sprayed powder were selected for hot-pressing experiments.
XRD patterns of the as-sprayed powder in Fig. 3 show that it is
amorphous. After heat treatment at 600°C, broad and weak
Canton, MA) fitted with a temperature-compensated extensometer
that recorded the change in sample thickness during hot pressing.
3
Hot-pressed samples were checked for crystallinity using XRD
and for porosity using optical image analysis and, where possible,
the Archimedes method. Image analysis was done on optical
micrographs captured with a computer-controlled display (CCD)
camera at a magnification of 600. The porosity of a sample was
measured by specifying the gray-scale range of the pores (dark
regions) and measuring the area fraction of this range. The error
range of Ϯ1.5% in relative density was estimated by measure-
ments made by first including and then excluding fine pores (Ͻ5
crystalline peaks that correspond to Y O appear, which indicates
2
3
␮m), which had lighter contrast. Including these pores led to the
expansion of areas of larger pores and an overestimate of porosity.
Fig. 3. (a) XRD patterns of Al O –37.5% Y O powder heat-treated at
2
3
2 3
3
65°C shows that it is amorphous. (b) Heat treatment at 600°C yields
Fig. 1. TGA of as-sprayed Al O –37.5% Y O powders prepared by
predominantly amorphous phase, with some Y O because of minor
2
3
2
3
2 3
spray pyrolysis of aqueous nitrate solutions.
segregation during spray pyrolysis.

February 2003
Yttrium Aluminum Garnet by Consolidation of Amorphous Powders
249
Fig. 4. XRD pattern of Al O –37.5% Y O powder heat treated at 875°C
2
3
2 3
shows the beginning of YAG formation from the amorphous phase and
residual Y O (Y). Heat treatment at 1000°C produces single-phase YAG.
2
3
some segregation during spray pyrolysis. However, a sample
heated to 875°C (Fig. 4) shows the beginning of YAG formation
and some residual Y O , whereas, by 1000°C, only single-phase
2
3
YAG is formed, and no residual Y O is detected using XRD. The
2
3
fact that the heat treatment of the as-sprayed powder at these
temperatures produces YAG, and no other phase, indicates that the
7
segregation is minor. The possibility of forming YAG when the
major volume fraction has composition slightly leaner in Y O
2
3
1
5
than 37.5% cannot be ruled out. The influence of the small
amount of yttrium-rich regions on the densification behavior has
been neglected.
The true densities obtained using pycnometry measurements
of the powders heat treated to 365°, 450°, and 600°C are 2.7,
Ϫ3
3
.0, and 3.1 g⅐cm , respectively. Although the densities of the
Fig. 5. Photomicrographs of samples hot-pressed at 750 MPa at (a)
amorphous powders increase with degree of decomposition, the
values are considerably lower than that of the equilibrium
garnet, which has a density of 4.55 g⅐cm . Table I summarizes
6
40°C (sample 9 in Table I), and (b) 450°C (sample 4). Subsurface
porosity, corresponding to diffuse intermediate contrast, has been omitted
during porosity measurement.
Ϫ3
the results of the hot-pressing experiments. Representative
optical micrographs of polished samples on which porosity
measurements have been made after hot pressing are shown in
Fig. 5. Table I shows that the partially decomposed powders do
not achieve significant densities, irrespective of whether pres-
sure is applied during or after heating. On the other hand, the
powder that is fully decomposed at 600°C can be pressed to
relative densities as high as 96%, provided the load is main-
tained during heating. Other conditions remaining the same, the
relative density increases from 83% to 96% (samples 4 and 9,
respectively) as the hot-pressing temperature increases from
4
9
50° to 640°C. A significant amount of densification (up to
5%) occurs when predominantly amorphous powder is hot-
pressed (sample 6), as compared with fully crystalline powder
hot-pressed under the same conditions (sample 10). The final
relative density of sample 10 is very close to the initial density
expected after cold compaction. This implies that the crystalline
powder is not sinterable under the conditions used here. Indeed,
Table I. Results of Hot-Pressing Experiments on Amorphous and Crystalline Powders of
†
Al O –37.5% Y O Prepared Using Spray Pyrolysis
2
3
2
3
Applied
pressure (MPa)
Heat
treatment
(°C)
Hot-pressing
temperature
(°C)
Relative
density
(%)
Starting powder
condition
Cold
press
Hot
press
Time
(min)
Sample
1
2
3
Partially
365
450
315
400
450
50
50
500
500
30
30
0
70
72
78
decomposed
†
†
750
750
4
5
6
7
8
9
Fully
decomposed
600
450
550
600
600
600
640
0
30
0
0
15
0
83
78
50
500
†
‡
750
95, 91
84
50
100
750
750
84
96
†
†
750
750
§
1
0
Crystallized
1000
600
0
68
†
‡
§
Ϫ3
Heating under pressure. Repeated experiment. Density measured using Archimedes principle is 3.09 g⅐cm
.

2
50
Journal of the American Ceramic Society—Choudhury et al.
Vol. 86, No. 2
the extensive interconnected porosity precludes a ceramo-
graphic polish in sample 10.
Figure 6(a) shows a plot of the punch displacement against the
increasing pressure, and Fig. 6(b) shows displacement as a
function of time with the pressure held constant at 500 MPa during
hot pressing of powders with varying degrees of thermal decom-
position. The greater part of densification is achieved during
pressurizing with only a small increase with time when the
pressure is held constant at the hot-pressing temperature. XRD
traces of selected hot-pressed pellets are shown in Fig. 7. The
samples that reach near theoretical density have crystallized,
whereas the onset of crystallization is detected in samples hot-
pressed under less-severe conditions, e.g., at 450°C and 750 MPa
(sample 4) or 550°C and 500 MPa (sample 5). The grain size has
been estimated from X-ray peak broadening scanned in the range
of 20°–40° 2␪. The hot-pressed pellet, when heated to 1000°C
(sample 9), has a grain size (ϳ27 nm) comparable with that
obtained when the as-sprayed powder is heated to the same
temperature. This implies that crystallization during hot pressing
does not affect the subsequent grain growth.
In experiments in which pressure is applied during heating of
the amorphous powder compacts, densification up to ϳ80% occurs
before the initiation of crystallization. Densification and increase
in the volume fraction of YAG occur after the crystallization stage.
Figure 8 shows a bright-field TEM micrograph of the sample
pressed at 450°C and 750 MPa. Nanocrystals of YAG appear in the
matrix of the amorphous phase. Although the volume fraction of
the amorphous phase continues to decrease, it is expected to
continue to participate in the densification process. It is also likely
that the transformation to YAG assists densification. However, a
compact pressurized to 750 MPa after reaching 600°C (sample 7)
is crystalline after pressing, and its final relative density is 84%,
Fig. 7. XRD patterns of hot-pressed pellets of Al O –37.5% Y O
2 3
(sample details are presented in Table I). Crystalline peaks correspond to
YAG.
2
3
compared with 91%–95% obtained by heating compacts of sample
6
to 600°C at 750 MPa. Therefore, it is important to heat a compact
under pressure to obtain high relative densities. When the XRD
patterns of samples 6 and 7 are compared (Fig. 7), it is evident that
heating under load also leads to higher degree of crystallization.
Fig. 8. Bright-field TEM micrograph of Al O –37.5% Y O sample
2
3
2 3
hot-pressed at 450°C and 750 MPa shows nanocrystals of YAG (G) in the
matrix of amorphous phase (A). Inset shows electron diffraction pattern
with crystalline ring pattern corresponding to YAG and an amorphous halo.
Amorphous phase can continue to participate in densification, even as the
volume fraction transformed to YAG increases during hot pressing.
Fig. 6. (a) Change in the compact thickness as a function of applied
pressure during hot-pressing of Al O –37.5% Y O powders with varying
2
3
2 3
degrees of decomposition as indicated by the heat-treatment temperatures.
b) Change in the thickness as a function of time, at 500 MPa.
(

February 2003
Yttrium Aluminum Garnet by Consolidation of Amorphous Powders
251
4
A. S. Gandhi, “The Processing of Bulk Metastable Amorphous and Nanocrystal-
–Al Ceramics by Pressure Consolidation of Amorphous Powders”;
When the amorphous phase is exposed to pressure at lower
temperatures before significant crystallization, higher densification
occurs than if the pressure is applied only at temperatures of rapid
crystallization. The implication is that the contribution of the
amorphous phase to densification is dominant over any possible
densification induced by crystallization. The higher sinterability of
the amorphous phase is probably due to the open structure, as
indicated by its high molar volume (low density), as compared
with the crystalline phase (YAG), leading to higher ionic
mobilities.
line ZrO
2
2 3
O
Ph.D. Thesis. Indian Institute of Science, Bangalore, India, 2001.
5
V. Jayaram, R. S. Mishra, B. Majumdar, C. Lesher, and A. K. Mukherjee, “Dense
Nanometric ZrO –Al O from Spray-Pyrolysed Powder,” Colloids Surf. A, 133,
2
2 3
2
5–31 (1998).
6
R. S. Mishra, V. Jayaram, B. Majumdar, C. Lesher, and A. K. Mukherjee,
ZrO –Al O Nanocomposite by High-Pressure Sintering of Spray-Pyrolysed Pow-
“
2
2 3
ders,” J. Mater. Res., 14, 834–40 (1999).
7
C. K. Ullal, K. R. Balasubramaniam, A. S. Gandhi, and V. Jayaram, “Non-
Equilibrium Phase Synthesis in Al O –Y O by Spray Pyrolysis of Nitrate Precur-
2
3
2 3
sors,” Acta Mater., 49, 2691–99 (2001).
8
Y.-M. Chiang, D. P. Birnie, and W. D. Kingery, Physical Ceramics; pp. 15–16.
The applied pressure accelerates the crystallization of YAG, as
expected from the molar volume difference between the amor-
phous phase and YAG. Similar acceleration of crystallization to
tetragonal-ZrO (Al O ) solid solution, as well as its subsequent
Wiley, New York, 1997.
M. Nyman, J. Caruso, M. J. Hampden-Smith, and T. T. Kodas, “Comparison of
9
Solid-State and Spray-Pyrolysis Synthesis of Yttrium Aluminate Powders,” J. Am.
Ceram. Soc., 80 [5] 1231–38 (1997).
2
2
3
10
P. Apte, H. Burke, and H. Pickup, “Synthesis of Yttrium Aluminium Garnet by
evolution into the equilibrium phases, occurs in hot-pressing
experiments on ZrO –40% Al O , although at slightly higher
Reverse Strike Precipitation,” J. Mater. Res., 3, 706–11 (1992).
1
1
Y. Liu, Z.-F. Zhang, B. King, J. Halloran, and R. M. Laine, “Synthesis of Yttrium
2
2 3
5
Aluminum Garnet from Yttrium and Aluminum Isobutyrate Precursors,” J. Am.
Ceram. Soc., 79 [2] 385–94 (1996).
temperatures and hydrostatic pressures (700°C and 1 GPa). The
present results are similar to the densification of amorphous
12
Y. Liu, Z.-F. Zhang, J. Halloran, and R. M. Laine, “Yttrium Aluminum Garnet
ZrO –Al O , in that the high-pressure, low-temperature consoli-
2
2
3
Fibers from Metalloorganic Fibers,” J. Am. Ceram. Soc., 81 [3] 629–45 (1998).
1
3
dation of initially amorphous powders leads to dense compacts.
However, the overlap of crystallization and densification in the
present work appears to preclude the retention of the amorphous
phase.
G. Gowda, “Synthesis of Yttrium Aluminates by the Sol–Gel Process,” J. Mater.
Sci. Lett, 5, 1029–32 (1986).
1
4
R. V. Kamat, K. T. Pillai, V. N. Vaidya, and D. D. Sood, “Synthesis of Yttrium
Aluminium Garnet by the Gel Entrapment Technique using Hexamine,” Mater.
Chem. Phys., 46, 67–71 (1996).
1
5
T. Ogihara, K. Wada, T. Yoshida, T. Yanagawa, N. Ogata, K. Yoshida, and N.
Matsushita, “Preparation of Monodispersed Zirconia–Alumina and Yttria–Alumina
Particles,” Ceram. Int., 19, 159–68 (1993).
IV. Conclusions
16
R. S. Hay, “Phase Transformations and Microstructure Evolution in Sol–Gel-
Derived Yttrium Aluminum Garnet Films,” J. Mater. Res., 8, 578–604 (1993).
1
7
Amorphous powders of Al O –37.5% Y O prepared by spray
O. Yamaguchi, K. Takeoka, K. Hirota, H. Takano, and A. Hayashida, “Formation
of Alkoxy-Derived Yttrium Aluminum Oxides,” J. Mater. Sci., 27, 1261–64 (1992).
2
3
2 3
pyrolysis have been hot-pressed at low temperatures (315°–
40°C), under moderately high pressures (500–750 MPa). High
1
8
O. Yamaguchi, K. Takeoka, and A. Hayashida, “Formation of Alkoxy-Derived
6
Y Al O12,” J. Mater. Sci. Lett., 10, 101–103 (1990).
3
19
5
relative densities (96%) are obtained when fully thermally decom-
posed amorphous powders are used. The results establish the
generality of the phenomenon of densification of the amorphous
phase found in the ZrO –Al O system. Partially decomposed
S. D. Parukuttyamma, J. Margolis, L. Haiming, C. P. Grey, S. Sampath, H.
Herman, and J. B. Parise, “Yttrium Aluminum Garnet (YAG) Films through a
Precursor Plasma Spraying Technique,” J. Am. Ceram. Soc., 84, 1906–908 (2001).
2
0
X. Guo and K. Sakurai, “Formation of Yttrium Aluminum Perovskite and
Yttrium Aluminum Garnet by Mechanical Solid-State Reaction,” Jpn. J. Appl. Phys.,
9, 1230–34 (2000).
2
2 3
powders do not densify to high relative densities. Crystallization of
YAG with grain size of ϳ20 nm accompanies densification. This
feature allows the processing of almost fully dense nanocrystalline
YAG at temperatures as low as 600°C, under moderately high
pressures. Heating the compacts under pressure is important for
obtaining high relative densities. Crystallization precludes the
retention of amorphous phase in the dense compacts, in contrast
with the hot-pressing behavior of amorphous ZrO –Al O .
3
2
1
J. G. Li, T. Ikegami, J.-H. Lee, T. Mori, and Y. Yajima, “Reactive Yttrium
Aluminate Garnet Powder via Coprecipitation using Hydrogen Carbonate as the
Precipitant,” J. Mater. Res., 15, 1864–67 (2000).
22
H. Wang, L. Gao, and K. Niihara, “Synthesis of Nanoscaled Yttrium Aluminum
Garnet Powder by the Coprecipitation Method,” Mater. Sci. Eng., A288, 1–4 (2000).
2
3
M. Inoue, H. Otsu, H. Kominami, and T. Inui, “Synthesis of Yttrium Aluminum
Garnet by the Glycothermal Method,” J. Am. Ceram. Soc., 74 [6] 1452–54 (1991).
2
4
M. K. Cinibulk, “Synthesis of Yttrium Aluminum Garnet from a Mixed-Metal
Citrate Precursor,” J. Am. Ceram. Soc., 83 [5] 1276–78 (2000).
2
2 3
2
5
D.-J. Chen and M. J. Mayo, “Densification and Grain Growth of Ultrafine 3
mol% Y –ZrO Ceramics,” Nanostr. Mater., 2, 469–78 (1993).
M. J. Mayo, “Processing of Nanocrystalline Ceramics from Ultrafine Particles,”
Int. Mater. Rev., 41, 85–115 (1996).
2
O
3
2
2
6
Acknowledgments
2
7
R. Vassen and D. St o¨ ver, “Processing and Properties of Nanophase Non-Oxide
The authors are grateful to Mr. S. Sasidhara for assistance in the hot-pressing
experiments and to Mr. D. Patro for assistance in the preparation of the manuscript.
The superalloy material was supplied by Gas Turbine Research Establishment,
Bangalore.
Ceramics,” Mater. Sci. Eng. A, A301, 59–68 (2001).
28
2
D.-J. Chen and M. J. Mayo, “Rapid Rate Sintering of Nanocrystalline ZrO –3
mol% Y
2
O
3
,” J. Am. Ceram. Soc., 79, 906–12 (1996).
R. Chaim and M. Hefetz, “Fabrication of Dense Nanocrystalline ZrO
2 3
Y O by Hot Isostatic Pressing,” J. Mater. Res., 13, 1875–80 (1998).
30
29
2
–3 wt%
B. Bloch, B. G. Ravi, and R. Chaim, “Stabilization of Transition Alumina and
Grain Growth Inhibition in Ultrafine Al –5 wt% SrO Alloy,” Mater. Lett., 42,
1–65 (2000).
References
2 3
O
6
1
31
A. S. Gandhi, V. Jayaram, and A. H. Chokshi, “Phase Evolution and Densification
R. Manalert and M. N. Rahaman, “Sol–Gel Processing and Sintering of Yttrium
Aluminum Garnet (YAG) Powders,” J. Mater. Sci., 31, 3453–58 (1996).
of Spray-Pyrolysed ZrO –Al O Powders,” Mater. Sci. Forum, 243– 245, 227–32
2
2 3
3
2
(
1997).
R. C. Pullar, M. D. Taylor, and A. K. Bhattacharya, “The Sintering Behaviour,
2
A. S. Gandhi, V. Jayaram, and A. H. Chokshi, “Dense Amorphous Zirconia–
Mechanical Properties, and Creep Resistance of Aligned Polycrystalline Yttrium
Aluminium Garnet (YAG) Fibres, Produced from Aqueous Sol–Gel Precursor,”
J. Eur. Ceram Soc., 19, 1747–58 (1999).
Alumina by Low-Temperature Consolidation of Spray-Pyrolyzed Powders,” J. Am.
Ceram. Soc., 82 [10] 2613–18 (1999).
3
33
A. S. Gandhi, V. Jayaram, and A. H. Chokshi, “Low-Temperature Densification
M. Panneerselvam, G. N. Subanna, and K. J. Rao, “Translucent Yttrium
Behaviour of Metastable Phases in ZrO –Al O Powders Produced by Spray
Aluminum Garnet: Microwave-Assisted Route to Synthesis and Processing,”
2
2 3
Pyrolysis,” Mater. Sci. Eng., A304–306, 785–89 (2001).
J. Mater. Res., 16, 2773–76 (2001).
Ⅺ