Spark plasma sintering of nanosized amorphous silicon nitride powder with a small amount of sintering additive

Article abstract of DOI:10.1111/j.1551-2916.2009.03575.x

Dense and fine-grained β-Si₃N₄ ceramics were successfully obtained with a small amount of sintering additives, 1.5 mass% Y₂O₃ and 0.5 mass% Al₂O₃, using nanosized amorphous Si₃N₄ powder by spark plasma sintering at temperatures of 1500°-1800°C and a pressure of 30 MPa under N ₂. The β-Si₃N₄ ceramics were composed of equiaxed grains with an average size of 300 nm. A higher sintering temperature was required for the densification of submicrometer-sized α-Si ₃N₄ powder with the small amount of the additives. The use of nanosized amorphous Si₃N₄ powder accelerated the densification and the transformation to the β-phase.

Full text of DOI:10.1111/j.1551-2916.2009.03575.x

J. Am. Ceram. Soc., 93 [6] 1544–1546 (2010)
DOI: 10.1111/j.1551-2916.2009.03575.x
r 2010 The American Ceramic Society
ournal
J
Spark Plasma Sintering of Nanosized Amorphous Silicon Nitride
Powder with a Small Amount of Sintering Additive
w,z
Mikinori Hotta, Takanori Shinoura, Naoya Enomoto, and Junichi Hojo
Department of Applied Chemistry, Faculty of Engineering, Kyushu University, Fukuoka 819-0395, Japan
1
0,12,15
Dense and fine-grained b-Si N ceramics were successfully ob-
ports on the fabrication by SPS.
The total amount of the
3
4
tained with a small amount of sintering additives, 1.5 mass%
Y O and 0.5 mass% Al O , using nanosized amorphous Si N
sintering additives of 45 mass% was used for the densification
of Si N powders by the SPS technique, which was the same
2
3
2
3
3
4
3
4
powder by spark plasma sintering at temperatures of 15001–
8001C and a pressure of 30 MPa under N . The b-Si N ce-
amount of additive as that by conventional sintering techniques.
We reported that fine-grained b-Si ceramics were pre-
pared with a general amount of sintering additive of 6 mass%
1
3 4
N
2
3
4
ramics were composed of equiaxed grains with an average size of
00 nm. A higher sintering temperature was required for the
densification of submicrometer-sized a-Si N powder with the
3
Y O and 2 mass% Al O using submicrometer-sized a-Si N
2
3
2
3
3
4
5
and nanosized amorphous Si
more, at the amount of the sintering additives, the densification
of the nanosized amorphous Si powder was easier to proceed
than that of the submicrometer-sized a-Si N . The use of nano-
3 4
N powders by SPS. Further-
3
4
small amount of the additives. The use of nanosized amorphous
Si N powder accelerated the densification and the transforma-
4
3 4
N
3
5
tion to the b-phase.
3
4
3 4
sized amorphous Si N is expected to fabricate nanostructured
Si ceramics that reduce the amount of sintering additives for
3 4
N
I. Introduction
3 4
ILICON NITRIDE (Si N ) ceramics are one of the most prom-
the densification.
In the present study, nanosized amorphous Si N powder was
3
4
sintered with a relatively small amount of Y
O
2 3
2 3
and Al O ad-
S
ising materials for structural applications at high tempera-
ture. However, Si N is very difficult to densify by solid-phase
ditives by SPS, and densification, phase transformation, and
microstructure of the Si N sintered bodies were studied. Also,
3
4
3
4
sintering because of its strong covalent nature and low self-
diffusion coefficients of Si and N. In general, the conventional
these results were compared with those of the products prepared
using submicrometer-sized a-Si powder.
3 4
N
3 4
densification of Si N ceramics is achieved by liquid-phase sinte-
ring with metal oxides additives such as MgO, Al O , and Y O .
2
3
2
3
In contrast, the grain-boundary phase derived from the sintering
additives degraded the high-temperature mechanical properties,
II. Experimental Procedure
Nanosized amorphous Si powder was prepared by a vapor-
phase reaction from SiCl and NH gases. The amorphous Si N
powder consisted of spherical particles with an average size of 80
nm (number mean diameter via SEM analysis). The total oxy-
1
and the corrosion and oxidation resistance of Si
N
3 4
ceramics.
3 4
N
Therefore, in order to improve these properties, it is necessary to
minimize the amount of sintering additives used for dense Si
ceramics. The densification of Si using conventional sinte-
4
3
3
4
N
3 4
3 4
N
ring techniques such as gas-pressure sintering and hot pressing
typically required the total amount of sintering additives of 5–10
mass%.
gen content of the nanosized amorphous Si
mass%. As a reference, submicrometer-sized a-Si N powder (b-
phase: o5%, total oxygen content: 1.2 mass%, SN-E10 grade,
Ube Industries Ltd., Ube, Japan) with an average particle size of
170 nm (number mean diameter) was also used as a starting
N powder was 4.8
3 4
3
4
2,3
Spark plasma sintering (SPS) technique can heat specimens
rapidly because the pulsed direct current used in this technique is
possible to pass through the graphite die and punch rods. Thus,
4
material. 1.5 mass% Y
HP grade, Shin-Etsu Chemical Co. Ltd., Tokyo, Japan) and 0.5
mass% Al (a-phase, average particle size: 500 nm, AKP-20
grade, Sumitomo Chemical Co. Ltd., Tokyo, Japan) powders
were added to Si powders as sintering additives. These pow-
2 3
O (average particle size: 300 nm, UU-
the entire sintering process can be completed in a short time,
leading to suppression of grain growth during sintering at high
temperature. Accordingly, densification of low-sinterable mate-
rials was accelerated and a fine-grained microstructure was
2 3
O
3 4
N
5
–10
formed by SPS.
pulse electric current sintering (PECS),
The SPS technique is sometimes called as
field-assisted sinte-
ders were mixed with a small amount of ethanol, dried, and then
sieved with a pore-opening size of 300 mm. The powder mixture
was filled in a graphite die of 15 mm in inner diameter and sin-
tered at a temperature of 15001–17001C for a holding time of 30
min and at 18001C for 1 min under a uniaxial pressure of 30
1
1–13
1
4
15
ring technique, and plasma-assisted sintering, because the
generation of spark discharge and/or plasma during SPS process
has not been verified. By using the SPS process, low-sinterable
oxides, nitrides, and carbides, which are difficult to densify by
conventional sintering techniques, have been fabricated as fully
MPa in N atmosphere by SPS (SPS-515S, SPS Syntex Inc.,
2
Kanagawa, Japan). The mixed powder was heated at rates of
150 and 3001C/min. The overshoot of the temperature of about
dense materials. With regard to Si
N
3 4
ceramics, there were re-
1
51C occurred at the rapid heating rate of 3001C/min, although
little overshoot of the temperature was monitored at 1501C/min.
The heating temperature on the surface of the die was measured
with a radiation thermometer. The relative densities of the sin-
tered samples were determined by the Archimedes method. The
phase composition and phase transformation of the samples
were evaluated using X-ray diffractometry (XRD; MiniFlex,
Rigaku Corp., Tokyo, Japan) with CuKa radiation for sintered
samples. The content ratio of the a- and b-Si N phases in
M. Nygren—contributing editor
Manuscript No. 26936. Received October 8, 2009; approved November 24, 2009.
This work was performed as a Research Fellow of the Japan Society for the Promotion
of Science (No.07570).
wAuthor to whom correspondence should be addressed. e-mail: hotta@jwri.
osaka-u.ac.jp
Present address: Joining and Welding Research Institute, Osaka University, Osaka 567-
z
3
4
0
047, Japan
the samples was estimated from the peak intensities using the
1
544

June 2010
Rapid Communications of the American Ceramic Society
1545
1
6
equation proposed by Gazzara and Messier. The sintered
specimens were polished with a 3-mm diamond slurry, and
then etched by plasma in CF and O gases. The etched sur-
(
a)
4
2
faces were observed by scanning electron microscopy (SEM;
S-5200, JEOL Ltd., Tokyo, Japan). The average grain size (num-
ber mean diameter) of the Si N bodies was determined from the
3
4
linear intercept length of 100 grains in the SEM pictures.
III. Results and Discussion
3 4
In the products prepared from nanosized amorphous Si N
powder at 15001–17001C for a holding time of 30 min at a heat-
ing rate of 1501C/min, Si N O was slightly detected as a sec-
2
2
ondary phase by XRD analysis. The formation of Si
be due to a reaction of Si with the surface oxide in amor-
phous Si particles during the long sintering process at high
temperatures, i.e., 17001C for 30 min. In a previous study of the
sintering of nanosized amorphous Si powder with a typical
O was not observed
2 2
N O may
1 µm
3
N
4
3
N
4
2
3
5
N
4
(b)
amount of Y
2 3 2 3 2 2
O and Al O additives, Si N
in the products.
Figure 1 shows the effect of sintering temperature on the rel-
ative density and a-phase ratio of specimens sintered at 15001–
1
7001C for 30 min and at 18001C for 1 min at 1501C/min using
nanosized amorphous Si N and submicrometer-sized a-Si N
4
3
4
3
3 4
powders. In the submicrometer-sized a-Si N powder, with
sintering at 15001C, the relative density of the specimen was
about 60%. When the temperature increased to 17001C, the
density increased to 93%. At 18001C for 1 min, the specimen
with a high density of 498% was obtained. The transformation
from a- to b-phase occurred with the increasing density of the
specimens. With sintering at 18001C, only the b-phase formed.
In nanosized amorphous Si N powder, on the other hand, only
b-phase was observed even at 15001C and the amorphous phase
directly transformed into the crystalline b-phase. The density of
the specimens reached 95% even at 15001C, although the dens-
1 µm
Fig. 2. Scanning electron micrographs of the etched surfaces of Si
3
4
3 4
N
ceramics prepared at 17001C for 30 min at 1501C/min using nanosized
amorphous Si N (a) and submicrometer-sized a-Si N powders (b).
ification of the submicrometer-sized a-Si N powder required a
3
4
3
4
3
4
high temperature of 18001C. This suggested that the densificat-
ion of nanosized amorphous Si powder was achieved at a
lower temperature than that of the submicrometer-sized a-Si N
3 4
N
3
4
Figure 2 shows SEM micrographs of the etched surfaces of
the Si ceramics prepared at 17001C for 30 min at 1501C/min
using nanosized amorphous Si and submicrometer-sized
a-Si N powders. The Si N ceramics obtained from nanosized
powder with a relatively small amount of sintering additive. The
3 4
N
high sinterability of nanosized amorphous Si powder might
3 4
N
3 4
N
be caused by a smaller particle size and a larger amount of total
oxygen content of the nanosized amorphous Si N powder. The
3
4
3
4
3
4
amorphous Si N powder were composed of finer equiaxed and
3 4
3 4
transformation from a- to b-phase for Si N ceramics is mainly
elongated grains, compared with those from submicrometer-
sized a-Si powder. The growth of elongated Si grains was
quite noticeable in Si N ceramics prepared using submicrom-
3
dependent on the densification. Consequently, because nano-
N
3 4
3 4
N
sized amorphous Si N powder was fully densified at a lower
4
3
3
4
temperature than the submicrometer-sized a-Si N powder,
3
4
eter-sized a-Si N powder. In addition, a residual pore was
3
4
the transformation to b-phase might be completed at a lower
temperature.
clearly observed in the Si
micrometer-sized a-Si powder. This supported that the dens-
ification of submicrometer-sized a-Si N powder was not
3 4
N ceramics obtained from the sub-
3 4
N
3
4
achieved at 17001C as shown in Fig. 1. The use of nanosized
amorphous Si powder enables to achieve densification at a
lower temperature, because nanosized amorphous Si had a
higher sinterability compared with the submicrometer-sized
a-Si . Therefore, the grain growth was inhibited during sinte-
ring, resulting in the production of fine-grained Si ceramics.
3 4
N
3 4
N
3 4
N
3 4
N
Using rapid sintering at a heating rate of 3001C/min and at
8001C for 1 min, dense b-Si N ceramics with a relative density
of 96% were successfully fabricated without the formation of
1
3
4
Si O in the product. A SEM micrograph of the b-Si ce-
N
2 2
3 4
N
ramics is shown in Fig. 3. The nearly equiaxed grains with an
average size of 300 nm (number mean diameter via SEM im-
ages) were observed in the fully dense b-Si
3
N
4
ceramics. Suga-
numa et al. and Schneider et al. reported dense Si
ceramics consisting of fine and equiaxed Si N grains from the
1
2
15
3 4
N
3
4
3 4
submicrometer-sized a-Si N powder using SPS. Nishimura
1
0
et al. revealed how to form a homogeneous microstructure
without elongated grains using nanosized b-Si N powder by
3
4
2
SPS. Sze
sized amorphous Si
phase. For the densification of these Si
´
pvo
¨
lgyi and Mohai reported that hot pressing of nano-
powder produced Si O as a secondary
powders, the total
Fig. 1. The effect of sintering temperature on the relative density and
ceramics prepared at 15001–17001C for a holding
time of 30 min and at 18001C for 1 min at a heating rate of 1501C/min.
a-phase ratio of Si
N
3 4
N
3 4
2 2
N
N
4
3

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546
Rapid Communications of the American Ceramic Society
Vol. 93, No. 6
using nanosized amorphous Si
N
3 4
powder. When rapid sinte-
ring at a heating rate of 3001C/min and at 18001C for 1 min was
used, fully-dense b-Si N ceramics consisting of equiaxed grains
with an average size of 300 nm were prepared without forming
3
4
Si O.
2 2
N
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in the long sintering at 15001–17001C for 30 min at 1501C/min