α-SiAlON ceramics obtained by slip casting and pressureless sintering

Article abstract of DOI:10.1111/j.1151-2916.2003.tb00027.x

Dense α-SiAlON ceramics were obtained by pressureless sintering of green compacts prepared using slip casting. The rheological properties of the reaction SiAlON suspension were optimized to achieve a high degree of dispersion with a high solids volume fraction, which resulted in homogeneous and relatively dense green bodies with high sintering ability, which could be densified by pressureless sintering at 1750°C for 2 h. The sintered samples revealed a high degree of uniformity and almost fully dense microstructures that consisted of many small, elongated grains homogeneously dispersed in the fracture surfaces, which had properties comparable with those of other SiAlONs obtained using hot pressing.

Full text of DOI:10.1111/j.1151-2916.2003.tb00027.x

J. Am. Ceram. Soc., 86 [2] 366–68 (2003)
journal
␣-SiAlON Ceramics Obtained by Slip Casting and Pressureless Sintering
Xin Xu, Marta I. L. L. Oliveira, and Jos e´ M. F. Ferreira
Department of Ceramics and Glass Engineering, CICECO, University of Aveiro, 3810–193 Aveiro, Portugal
Dense ␣-SiAlON ceramics were obtained by pressureless sin-
tering of green compacts prepared using slip casting. The
rheological properties of the reaction SiAlON suspension were
optimized to achieve a high degree of dispersion with a high
solids volume fraction, which resulted in homogeneous and
relatively dense green bodies with high sintering ability, which
could be densified by pressureless sintering at 1750°C for 2 h.
The sintered samples revealed a high degree of uniformity and
almost fully dense microstructures that consisted of many
small, elongated grains homogeneously dispersed in the frac-
ture surfaces, which had properties comparable with those of
other SiAlONs obtained using hot pressing.
II. Experimental Procedure
The starting powders used to prepare the SiAlON ceramics were
␣-Si
(d50 ϭ 0.38 ␮m, Alcoa Chemicals, Pittsburgh, PA), AlN (d50 ϭ 2
␮m, H. C. Stark), and Y (d50 ϭ 0.75 ␮m, H. C. Stark). An
N (d50 ϭ 0.38 ␮m, H. C. Stark, Berlin, Germany), Al O
3
4
2
3
O
2 3
azeotropic mixture of 60 vol% methylethylketone (MEK;
Riedel-de Ha e¨ n, Germany) and 40 vol% ethanol (E; Merck,
Darmstadt, Germany) was selected as solvent. A dispersant (Hy-
permer KD1, Imperial Chemical Industries PLC, England) also
was used.
For the preparation of starting reaction SiAlON powders,
weighed batches of 76.92 wt% Si N , 13.46 wt% AlN, 5.77 wt%
3
4
Y O , and 3.85 wt% Al O powder mixtures were planetary
2
3
2 3
milled for 4 h in 2-butanol, using Al O jars and Si N balls. After
2
3
3 4
I. Introduction
the raw reaction SiAlON powders were dried and sieved through
an 80 ␮m sieve, they were mixed with a solution of the dispersant
in the solvent, and planetary milled for 4 h to prepare a stable
suspension. Slip-casting experiments were conducted by pouring
the suspensions into Teflon rings set on absorbent plaster blocks.
The green bodies obtained by slip casting then were placed inside
a powder bed composed of ϳ30 wt% BN and 70 wt% starting
reaction SiAlON powder and sintered in a graphite furnace under
nitrogen-gas atmosphere. The powders were initially heated to
1200°C at a rate of 20°C/min, and then to various temperatures at
a rate of 4°C/min, followed by a 0.5 h hold. The holding time was
prolonged to 2 h only at 1750°C. After the samples were sintered,
they were cooled to room temperature at a rate of 20°C/min.
The green compact density was measured using the Archimedes
method in mercury. The microstructures of the fracture surfaces of
the green bodies and sintered bodies were observed using scanning
electron microscopy (SEM/EDS; Model S-4100, Hitachi, Ltd.,
Tokyo, Japan). X-ray diffractometry (XRD; Model D/MAX-C,
Rigaku Co., Tokyo, Japan) was used for phase identification.
Vickers indentation (30 kg load) was used to measure hardness and
fracture toughness.
IALON ceramics have attracted great attention because of their
unique combination of excellent properties. ␤-SiAlON ce-
1
S
ramics have been widely studied because of their good sintering
ability and relatively high fracture toughness associated with
rodlike grains. Recently, it has been found that ␣-SiAlON also can
2
evolve elongated grains, which means dense ␣-SiAlON can take
advantage of high hardness, high fracture toughness, and high
potential of cleaning-up grain boundaries. Thus, the extensive
3
–5
interest in ␣-SiAlON has increased.
However, most studies on ␣-SiAlON ceramics are based on
high-cost hot-press sintering and gas-pressure sintering, because
-SiAlON ceramics are difficult to pressureless sinter because of
␣
the small amount of transient liquid phase during the last stage of
sintering. Green bodies with high density and homogeneity have
improved performance in terms of sintering ability and the
homogeneity of the sintered microstructure. Moreover, inhomoge-
neities in green bodies are likely to inhibit sintering and to degrade
the mechanical properties and reliability of SiAlON ceramics.
Therefore, it is desirable to find powder-processing routes that
produce green compacts of high density and homogeneity to
produce materials with optimum properties.
Slip casting is regarded as one of the most suitable forming
techniques for large-scale fabrication of advanced ceramic com-
ponents of either very simple or very complicated shapes.
III. Results and Discussion
6
–8
Furthermore, this manufacturing method does not require expen-
sive equipment, and the overall processing costs are potentially
moderate. The purpose of the present work is to propose slip
casting and pressureless sintering as a route for developing
high-density, reliable ␣-SiAlON ceramics with high mechanical
properties.
Slip casting requires homogeneous stable suspensions with
optimized rheological properties. When 3 wt% KD1 is used, the
reaction SiAlON powder can be easily dispersed in the selected
9
,10
solvent up to solids loading as high as 60 vol%.
In this work,
a 50 vol% suspension has been selected to prepare green and
sintered compacts. The flow behavior of the suspension has been
9
reported previously. The suspension exhibits a near Newtonian
flow behavior with a low apparent viscosity value of 100 mPa⅐s at
Ϫ1
1
00 s , so that it can be easily handled and successfully used for
slip casting. Because of optimum flow behavior, the particles can
flow and pack individually and easily move to suitable positions
during deposition, which results in a green body with 68.3% of
theoretical density.
High levels of green density and homogeneity enhance sintering
reactivity. Figure 1 shows the evolution of sample density during
the sintering process The densification curve agrees quite well
R. Rice—contributing editor
Manuscript No. 186990. Received May 6, 2002; approved November 22, 2002.
Supported by Funda c¸ a˜ o para Ci eˆ ncia e a Tecnologia of Portugal under Project No.
POCTI/CTM/39419/2001 and Grant Nos. SFRH XXI/BPD/1626/2000 and PRAXIS
XXI/BD/18065/98.
3
66

February 2003
Communications of the American Ceramic Society
367
melt and preferential dissolution of AlN enriches the melt compo-
sition in AlN, which triggers transient precipitation of supersatu-
1
2
rated ␤-SiAlON. This evidence is also supported by the decrease
in the amounts of ␣-Si N and AlN and the increase of the
3
4
amounts of ␣-SiAlON and ␤-SiAlON. Densification accompanies
phase transformation, and it proceeds through pore elimination by
particle rearrangement and cooperative flow of the particle/liquid
mixture within the temperature range of 1400°–1700°C. The
supersaturated ␤-SiAlON is transient and is not compatible with
the phase diagram; therefore, it transforms to ␣-SiAlON again
after the complete dissolution of the starting powders during the
last stage of the sintering process.
The above results show that the obtained green bodies can be
easily pressureless sintered at a relatively low temperature of
1
750°C. This can be attributed to the relatively high green density
and to the homogeneously dispersed small pores, as reported in
1
4
Ref. 9. Using a theoretical model, Shaw has concluded that it is
desirable to have a fraction of small pores as large as possible to
maintain a high driving force for sintering, and high powder-
packing densities must be achieved for optimum sintering. Kamiya
et al. have drawn a similar conclusion that, if the pore size is too
large, the improvement of the structure and achievement of full
density is impossible.
Fig. 1. Evolution of density during the sintering process
1
5
with those observed in earlier publications on liquid-phase sinter-
3
,4,11–13
ing of SiAlON.
However, a sintered density of Ͼ98% of
theoretical has been obtained after sintering at 1750°C for 2 h.
Figure 2 shows the reaction sequence of the formation of
One difficulty in pressureless sintering of monophase
␣-SiAlON ceramics results from the fact that liquid formed during
1
3
␣
-SiAlON. Y O and Al O start to react with each other at
sintering is gradually exhausted by the formation of ␣-SiAlON.
However, in our obtained green body, as shown in Fig. 2, the Y
and Al
cates that the homogeneous dispersion of additives favors the
2
3
2 3
relatively low temperatures and seem to completely disappear after
heat treating the mixture at 1300°C, while the amounts of raw
O
2
3
O disappear completely at low temperature, which indi-
2 3
␣
-Si N and AlN start to decrease above 1400°C and seem to
3 4
1
6
completely disappear at
-SiAlON phase forms at temperatures Ͼ1400°C, and a rapid
increase in its amount occurs in the temperature range of 1400°–
700°C. The crystalline phase yttrium aluminum garnet (YAG),
a
temperature of ϳ1700°C. The
formation of liquid at lower temperature, which allows fast
densification to occur. Furthermore, the small-sized and homoge-
neously dispersed pores increase the driving force for densifica-
␣
1
4
1
tion. Thus, the green body reaches near theoretical density before
which is the most likely compound in the intermediate stage of
sintering Si N using Y O and Al O as additives, is detected in
transformation to ␣-SiAlON is completed.
3
4
2
3
2 3
the temperature range of 1300°–1500°C. An AlN polytypoid is
detected at 1750°C but completely disappears after holding for 2 h
at this temperature. Finally, no other crystalline phases except
␣
-SiAlON can be found in the sample.
Figures 1 and 2 show that the sintering process starts at
ϳ1300°C; YAG is observed at low temperature, which indicates
that liquid phase starts to form at ϳ1300°C. However, as shown in
Fig. 1, the amount of shrinkage achieved through the primary
rearrangement process is very small, because the viscosity of the
liquid phase in this temperature range is considered to be too high
to allow migration of the particles. Figure 2 shows that the initial
dissolution of AlN and Si N is very limited. When the sintering
3
4
temperature is increased further, nitrides (Si N and AlN) start to
3
4
dissolve in the oxide liquid phase, and ␣-SiAlON grains nucleate
and grow through a solution–reprecipitation process. At the same
time, because of better wettability, AlN initially traps the oxide
Fig. 2. Evolution of the relative amounts of crystalline phases formed
during sintering
Fig. 3. Fracture surface of (a) green sample and (b) sample sintered at
1750°C for 2 h.

3
68
Communications of the American Ceramic Society
Vol. 86, No. 2
Figure 3 shows SEM micrographs of fracture surfaces of the
References
green sample and of the sample sintered at 1750°C for 2 h. A good
homogeneity is observed in the green and the sintered samples.
The grain size in the sintered sample is small, and many small
rodlike ␣-SiAlON grains appear dispersed in the matrix. The
rodlike grains can improve the fracture toughness and the reliabil-
ity of the as-produced ceramics, obviously through crack deflec-
tion, crack bridging, and grain pullout. The uniformly dispersed
liquid phase stimulates the dissolution of nitride precursors and
leads to high supersaturation in the liquid phase, which promotes
1
T. Ekstr o¨ m and M. Nygren, “SiAlON Ceramics,” J. Am. Ceram. Soc., 75 [2]
2
59–76 (1992).
2
3 4
I-W. Chen and A. Rosenflanz, “A Tough SiAlON Ceramics Based on ␣-Si N
with a Whisker-Like Microstructure,” Nature (London), 389, 701–704 (1997).
3
S. Kurama, M. Herrmann, and H. Mandal, “The Effect of Processing Conditions,
Amount of Additives, and Composition on the Microstructures and Mechanical
Properties of ␣-SiAlON Ceramics,” J. Eur. Ceram. Soc., 22 [1] 109–19 (2002).
4
P. L. Wang, J. H. Zhang, J. B. He, and D. S. Yan, “Formation Behavior and
Microstructure of Multi-Cation Containing Neodymium and Ytterbium or Yttrium,”
J. Eur. Ceram. Soc., 20, 1987–95 (2000).
5
1
7
Z. Shen, H. Peng, and M. Nygren, “Formation of in Situ-Reinforced Microstruc-
the in situ formation of elongated ␣-SiAlON grains.
After the obtained ␣-SiAlON ceramics are pressureless sintered
ture in ␣-SiAlON Ceramics: I,” J. Mater. Res., 17 [2] 336–42 (2002).
6
F. F. Lange, “Powder Processing Science and Technology for Increased Relabil-
–
3
at 1750°C for 2 h, they have a density of 3.29 g⅐cm , a Vickers
ity,” J. Am. Ceram. Soc., 72 [1] 3–15 (1989).
1
/2
7
hardness of 19.5 GPa, and a fracture toughness of 5.3 MPa⅐m
.
R. K. Govila, “Strength of Slip-Cast, Sintered Silicon Nitride,” J. Am. Ceram.
Soc., 73 [6] 1744–51 (1990).
The results are comparable with that of hot-press sintering, which
can be attributed to the great sintering reactivity and high density
of slip casting green bodies.
8
T. M. Shan and B. A. Pethica, “Preparation and Sintering of Homogeneous Silicon
Nitride Green Compacts,” J. Am. Ceram. Soc., 69 [2] 88–93 (1986).
9
X. Xu, M. I. L. L. Oliveira, and J. M. F. Ferreira, “Slip Casting of Reaction
SiAlON Bodies in Organic Media,” J. Mater. Sci. Lett., 20, 2043–44 (2001).
1
0
X. Xu, L. P. Huang, M. Oliveira, and J. M. F. Ferreira, “Study on Slip Casting
Reaction SiAlON Suspensions,” Key Eng. Mater., 224–226, 673–78 (2002).
IV. Conclusions
1
1
J. W. T. van Rutten, H. T. Hintzen, and R. Metselaar, “Densification Behavior of
Ca-␣-SiAlONs,” Ceram. Int., 27, 461–66 (2001).
A homogeneous green body with a density as high as 68.3%
theoretical has been obtained from a well-dispersed 50 vol%
reaction SiAlON suspension. Densification of ␣-SiAlON proceeds
via liquid-phase sintering. The high starting density, the fine
pore-size distribution, and the high degree of homogeneity ob-
tained in the green bodies without particle agglomerates are the
important points to assure significant reactivity during the sinter-
ing process, resulting in Ͼ98% of theoretical density after sinter-
ing at 1750°C for 2 h, and in sintered microstructures with many
in situ-formed elongated ␣-SiAlON grains. The sintered
1
2
S. L. Hwang and I-W. Chen, “Reaction Hot Pressing of ␣-SiAlON and ␤-SiAlON
Ceramics,” J. Am. Ceram. Soc., 77 [1] 165–71 (1994).
1
3
K. Watari, T. Nagaoka, and S. Kanzaki, “Densification Process of ␣-SiAlON
Ceramics,” J. Mater. Sci., 29, 5801–807 (1994).
1
4
T. M. Shaw, “Liquid Redistribution during Liquid-Phase Sintering,” J. Am.
Ceram. Soc., 69 [1] 27–34 (1986).
15
H. Kamiya, K. Isomura, G. Jimbo, T. Hotta, and J. Tsubaki, “Influence of Powder
Properties and Green Microstructure on the Sintering Behavior of Si N ,” J. Ceram.
3 4
Soc. Jpn., 101, 277–82 (1993).
1
6
J. Yang, F. J. Oliveira, R. F. Silva, and J. M. F. Ferreira, “Pressureless
Sinterability of Slip-Cast Silicon Nitride Bodies Prepared from Coprecipitation-
Coated Powders,” J. Eur. Ceram. Soc., 19, 433–39 (1999).
␣
-SiAlON ceramics have properties comparable to those of other
17
H. Emoto and M. Mitomo, “Control and Characterization of Abnormally Grown
SiAlONs obtained through hot pressing.
Grains in Silicon Nitride Ceramics,” J. Eur. Ceram. Soc., 17, 797–804 (1997).
Ⅺ