Full text of DOI:10.1557/JMR.2000.0141

Sintering of nanopowders of amorphous silicon nitride under
ultrahigh pressure
Ya-Li Li,a) Yong Liang, and Fen Zheng
Department of Micro-Crystalline and Laser Processing, Institute of Metal Research, Chinese
Academy of Sciences, 72 Wenhua Road, Shenyang 110015, People’s Republic of China
Xian-Feng Ma and Suo-Jing Cui
Laboratory of Ultrahigh Pressure, Changchun Institute of Applied Chemistry, Chinese Academy of
Sciences, 159 Renming Street, Changchun 130022, People’s Republic of China
(Received 17 May 1999; accepted 31 January 2000)
Nanopowders of amorphous silicon nitride were densified and sintered without
additives under ultrahigh pressure (1.0–5.0 GPa) between room temperature and
1600 °C. The powders had a mean diameter of 18 nm and contained 5.0 wt% oxygen
that came from air-exposure oxidation. Sintering results at different temperatures were
characterized in terms of sintering density, hardness, phase structure, and grain size. It
was observed that the nanopowders can be pressed to a high density (87%) even at
room temperature under the high pressure. Bulk Si3N4 amorphous and crystalline
ceramics (relative density: 95–98%) were obtained at temperatures slightly below the
onset of crystallization (1000–1100 °C) and above 1420 °C, respectively. Rapid grain
growth occurred during the crystallization leading to a grain size (>160 nm) almost 1
order of magnitude greater than the starting particulate diameters. With the rise of
sintering temperature, a final density was reached between 1350 and 1420 °C, which
seemed to be independent of the pressure applied (1.0–5.0 GPa). The densification
temperature observed under the high pressure is lower by 580 °C than that by hot
isostatic pressing sintering, suggesting a significantly enhanced low-temperature
sintering of the nanopowders under a high external pressure.
I. INTRODUCTION
shown that Si3N4 ceramics free of additives that were
densified under high pressure exhibited improved me-
chanical and high temperature compared to those sin-
tered with additives.3
Silicon nitride ceramics (Si3N4) have superior physi-
cal, chemical, and high-temperature mechanical proper-
ties and have become one of the most studied advanced
ceramics, as an important candidate for a variety of tech-
nical and engineering applications. Common dense Si3N4
polycrystalline materials are generally fabricated by
liquid-phase sintering of powders with the addition of
various sintering aids (such as Al2O3, Y2O3, Yb2O3,
ZrO2, etc.) at high temperature (1750–2000 °C). The
existence of metallic glass phases at grain boundaries
resulting from the additives greatly impairs the high-
temperature mechanical properties of the materials.1,2
Thus, the sintering of Si3N4 ceramics without additives is
an important approach to reducing impurity phases in the
sintered bodies and hence achieving Si3N4 ceramics with
the intrinsic properties of the materials. Studies have
Si3N4 ceramics cannot be densified in their “pure”
state using traditional sintering routes, owing to their
strong covalent bonding and much lower bulk diffusion
rate. However, they can be densified under ultrahigh high
pressure (UHP) (>1.0 GPa) and hot isostatic pressing
(HIP), which were conducted by several groups of re-
searchers.3–7 Prochazaka et al.4 and Yamada et al.5 sin-
tered Si3N4 powders at 5 GPa between 1500 and 1600 °C
and observed that the densification behavior of the ma-
terials strongly depended on powder composition and
phase structure. They observed that crystalline Si3N4
powders (␣ phase) were densified to a near full density
under the conditions, but the amorphous powders with a
stoichiometric composition exhibited poor sintering be-
havior.4,5 The reason the amorphous powders were
poorly sintered was not discussed.
a)Address all correspondence to this author.
Present address: National Institute for Research in Inorganic Ma-
terials, Research Center for Advanced Materials, 1-1 Namiki,
Tsukuba, Ibaraki 305-0044, Japan.
In the above UHP sintering studies, common coarse
particles (micrometers or submicron in diameters) were
used as the starting powders. Densification of nano-size
ceramic powders by the UHP method, as a main route to
e-mail: yalili@nirim.go.jp
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Y-L. Li et al.: Sintering of nanopowders of amorphous silicon nitride under ultrahigh pressure
produce bulk nanophase ceramics, has received increas-
ing interest in recent years and has been applied to a
variety of ionically bonded oxide nanoceramics (such as
TiO2, ZrO2, ZnO2, etc.).8,9 However, relatively less work
has concentrated on densification of the covalent Si3N4
nanoceramics under ultrahigh high pressures. Recently, a
HIP method was used to sinter amorphous Si3N4 pow-
ders by Symons and Danforth et al.10–12 The densifica-
tion of the powders was observed at very high sintering
temperature (2050 °C) in this process, and such high
temperature is unfavorable for the control of grain
growth. Thus, application of higher pressures is expected
to reduce the sintering temperature of the materials.
Pechenik et al.13,14 recently pressed nano Si3N4 powders
by diamond anvil at 5.0 GPa from liquid-nitrogen tem-
peratures to 500 °C and reported the formation of trans-
parent amorphous Si3N4 samples by the cryogenic
densification. However, densification of the nano-size
powders at higher temperature under ultrahigh high pres-
sure has not yet been reported.
Because the nano-size Si3N4 powders are generally
present in amorphous form, obtaining crystallized ceram-
ics requires sintering at least above the crystallization
temperature (>1000–1300 °C). On the other hand, sinter-
ing the amorphous powders below the crystallization
temperature may generate bulk amorphous ceramics. In
the present work, we report densification of nanosize
powders without additives under ultrahigh pressure
(1.0–5.0 GPa) between room temperature and 1600 °C.
The sintering results obtained at different temperature
regimes are presented, and the behavior of densification
and grain growth were investigated.
FIG. 1. (a) TEM micrograph and (b) particulate distribution [obtained
by small-angle x-ray scattering (SAXS)] of the starting amorphous
Si3N4 nanopowders.
II. EXPERIMENTAL
A. Starting powders
The starting Si3N4 powders were produced by laser-
induced vapor phase reactions from a mixture of silane
and ammonia gases. Synthesis of the powders was car-
ried out on a 5-kW CO2 laser-assisted powder synthesis
reactor with a yielding ability of 1 kg/day that was es-
tablished in the Institute of Metal Research, Chinese
Academy of Sciences. The synthesis principle is similar
to that reported previously in detail by Cannon et al.15,16
The powders are amorphous as checked by x-ray diffrac-
tion (XRD) and have a Brunauer–Emmett–Teller (BET)
specific area of 109 m2/g. The mean particle diameter
was calculated to be 18 nm using the equation d ס
6/(␴␳), where ␴ denotes the BET specific area and ␳
represents the solid density of the amorphous Si3N4. The
solid density of the amorphous Si3N4 powders was de-
termined by pycnometry which is equal to 2.90 g/cm3 as
reported previous.10 The BET-derived diameter is in
good agreement with the particle size estimated by trans-
mission electron microscopy (TEM) observation which
is in range of 15–20 nm [Fig. 1(a)]. Small-angle x-ray
scattering (SAXS) measurements showed that the par-
ticle diameter was distributed in a very narrow range
below 50 nm [Fig. 1(b)]. The powders were pure white in
color. Elemental analysis gave compositions (wt%) of
Si 57, N 37.2, and O 5.8. The oxygen and nitrogen con-
tents were determined by melt extraction method using a
He and infrared (IR) detection (LECO TC 436 Nitrogen/
Oxygen Determinator) with a standard deviation of <1%.
The elemental compositions of the powders correspond
to a Si/N ratio of 1.5, which is equal to that for stoichoi-
metric Si3N4. The appreciable amount of oxygen in the
powders resulted from a hydrolysis reaction between the
silylamino groups on the surface of as-synthesized pow-
ders and water vapors as the powders were exposed in the
air.10,17 The reaction is extremely pronounced for the
powders of nano-sizes due to their very large specific area,
which resulted in an oxygen content of 5.8 wt% in the
powders after 0.5 year of storage in air. However, other
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Y-L. Li et al.: Sintering of nanopowders of amorphous silicon nitride under ultrahigh pressure
metallic impurities in the powders are below ppm level.
Samples sintered at various pressures and tempera-
tures were cut and polished using 1-␮m diamond paste
and were then characterized in term of bulk density,
phase composition, grain size, and hardness. Density was
measured according to Archimedes’ principle using wa-
ter as the liquid medium. The density of porous or
cracked samples was measured after coating a thin layer
of organic resin to prevent water penetration into the
samples during the measurements. Phase structure was
identified by x-ray diffraction (Perkin D/max-rA diffrac-
tometry) using Cu K␣ radiation. Grain size was calcu-
lated from the line broadening of the diffraction peaks
(301), for ␣ and (411) for ␤ phases, according to the
Scherrer equation (D ס 0.89␭/␤ cos ␪).19 Hardness was
performed on polished surface of specimens on a digital
microhardeness tester (Micromet 11, Buehler Ltd. USA)
under loading of 0.2–1.0 kg with a duration of 10 s. Ul-
traviolet and visible (UV/VIS) transmission spectra of
the translucent/transparent specimens of Si3N4 glass ob-
tained by pressing were recorded on the UV/VIS spec-
troscopy (model 721, Shanghai). The samples for the
spectra measurements were a pellet of the sintered Si3N4
(2–5 mm thick) with two polished surfaces.
The low level of metallic impurities is one of many superior
properties of the laser-synthesized nanopowders.15,16
B. Compaction, sintering, and characterization
The amorphous Si3N4 nanopowders were shaped using
a pistol-cylindrical mold under uniaxial pressing prior to
the high-pressure experiments. The shaped powder com-
pacts were subsequently densified and sintered on a
cubic-anvil pressure apparatus which can generate a
pressure in range of 1–10 GPa. The cubic-anvil (WC), as
used in the present study, has a larger pressure cell than
that of diamond anvil cell (DAC), as used by Pechenik et
al.13 in their studies, hence allowing processing of rela-
tively larger sized samples. Also, the cubic-type anvil
can generate pressures more uniform in the pressure cell
than that of the uniaxial pressure by the diamond anvil
cell, which is expected to enhance densification of pow-
ders during pressing. For the shaping and compacting
experiments, all the procedures were conducted in air
without outgassing. About 3 g of each powder batch was
pressed (400 MPa) in a stainless steel mold to form a
cylindrical pellet 15 mm in diameter and 17 mm in
height which fit the size of the high-pressure cell. The
precompacted columns were then inserted in a graphite
sleeve and capped with graphite discs at each end. The
graphite sleeve was used as a heater during sintering. The
sample was then placed inside a bulk pyrophyllite which
served as solid pressure-transmitting medium during
pressing. Sintering was done by first applying pressure to
1.0–5.0 GPa, raising the temperature, holding at the tem-
perature for 30 min, and then turning off the heater and
unloading. The temperature inside the pressure cell was
detected by a 30 RhPt–6% RhPt thermocouple. The tem-
perature in the pressure cell was calibrated against an
electric current for heating that was actually monitored to
control sintering temperatures (±20 °C). Pressure was
calibrated by the standard method using the phase tran-
sition points of Bi, Ti, and Ba with a reading deviation of
±0.2 GPa.18
III. RESULTS AND DISCUSSION
A. Compacting at room temperature
The amorphous Si3N4 nanopowders were first pressed
at room temperature ( 25 °C) between 1.0 and 5.0 GPa
in order to study their compaction behavior without heat-
ing. It was observed that the powders can be densified to
a high density even at room temperature under the high
pressure. Two typical pressing results, together with
those sintered at high temperature, are listed in Table I. A
bulk density of 2.49–2.53 g/cm3 was obtained for the
compacts pressed between 1.0 and 5.0 GPa for 30 min.
The corresponding relative density is between 84 and
87%, taking 2.90 g/cm3 as the solid density of the amor-
phous Si3N4 as determined by He pycnometry.10–12 This
compact density is much higher than the theoretical
TABLE I. Typical results on the densification and sintering of the amorphous Si3N4 nanopowders under high pressure.
Sintering
temp.
(°C)
Relative
densitya
(%)
Pressure
(GPa)
Sintering density
(g/cm3)
Phase composition
(XRD)
Samples
Sample appearance
1
2
3
4
5
6
7
1.5
5.0
5.0
5.0
5.0
5.0
5.0
25
25
850
2.49 ± 0.05
2.53 ± 0.05
2.63 ± 0.03
2.75 ± 0.03
2.95 ± 0.02
3.01 ± 0.01
3.02 ± 0.01
84
87
90
95
93
95
93
Amorphous
Amorphous
Amorphous
Amorphous
␣ phase
Brown opaque
Brown translucent
Brown translucent
Brown translucent
Dark opaque
950
1000
1300
1420
␣ + ␤
␤ phase
Gray opaque
White opaque
aCalculated using 2.90 g/cm3 as theoretical density of amorphous sample and 3.18 g/cm3 as that of crystalline Si3N4.10
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Y-L. Li et al.: Sintering of nanopowders of amorphous silicon nitride under ultrahigh pressure
packing density of 74% for equal-size, closely packed
high pressure are not exhibited commonly by Si3N4
green bodies pressed from coarse powders, indicating the
enhanced densification behavior of the nanosize powders
under a high external pressure.
hard spheres, which indicates the occurrence of signifi-
cant plastic deformation of the nano-size particles under
the high pressure. It was observed that the green bodies
compacted at room temperatures exhibited strong bond-
ing characteristic and can be ground and polished to form
a mirrorlike surface. Figure 2(a) shows the appearance of
one specimen which was obtained by pressing at 5.0 GPa
for 30 min. It is noted that, after the pressing, the samples
changed their appearance from the off-white color of the
starting powders to brown or light brown ones. Vicker
hardness measurements performed on the polished sur-
face of a compact pressed at 5.0 GPa gave hardness val-
ues in the range of 710–752 kg/mm2. The high
compaction density and the strong bonding characteris-
tics of the nano-size powder compacts formed under the
Pechenik et al. pressed laser-synthesized amorphous
Si3N4 powders by a diamond anvil at 5.0 GPa using liq-
uid nitrogen as lubricants and obtained transparent Si3N4
specimens, from which they deduced that a high density
was achieved in the compacted amorphous Si3N4 ceram-
ics.13 But no compact density was reported, perhaps due
to the small size of their specimens (0.15 × ␾ 0.2 mm),
precluding a routine determination of the compact den-
sity of the specimens. The amorphous samples pressed
in the present study at room temperature were not
transparent in appearance. But the hardness value
(710–752 kg/mm2) is nearly equal to, and even slightly
higher than that, 700 kg/mm2, of their cryogenically
pressed amorphous Si3N4 materials. Also, the hardness
of our amorphous Si3N4 ceramics (710–752 kg/mm2) is
significantly higher than that of the compacts pressed by
Pechenik et al. at room temperatures at 5.0 GPa
( 450 kg/mm2). The enhanced densification at room
temperature in our case was probably ascribed to the
application of cubic-anvil pressure, which could generate
“quasi-isostatic” pressure and hence promoted interpar-
ticle sliding during densification, compared with the uni-
axial pressure as used by Pechenick et al.13
B. Sintering below crystallization
temperature (<1000-–1100 °C)
Sintering experiments were done between 500 and
1600 °C under 1–5 GPa. XRD identified that sintered
specimens obtained below 1000–1100 °C remained
amorphous. The sintered density of the amorphous speci-
mens obtained between 800 and 950 °C was between
2.63 and 2.75 g/cm3, depending on sintering tempera-
tures (Table I). A sintered density of 2.75 g/cm3, ob-
tained at 950 °C at 5.0 GPa, corresponds to a relative
density of 95%. This high relative density indicates that
the nano-size powders can be almost fully densified be-
low the crystallization temperature under sufficient high
pressure. Therefore, bulk Si3N4 amorphous ceramics can
be formed at sintering temperatures slightly below that
of the onset of crystallization. The Vicker hardness of
the amorphous Si3N4 bodies sintered between 800
and 950 °C was determined to be in the range of
830–1206 kg/mm2, depending on sintering temperatures.
Interest in the bulk amorphous Si3N4 arises in part
from its optical transmittance. As mentioned above,
Pechenik et al.13,14 pressed amorphous Si3N4 nanopow-
ders at 5.0 GPa at liquid-nitrogen temperature and ob-
served the formation of transparent amorphous Si3N4
under this condition. In our case, the dense amorphous
compacts obtained at room temperature are opaque. But
FIG. 2. (a) Appearance of the amorphous Si3N4 compacts pressed
from the amorphous Si3N4 nanopowders at 5.0 GPa at room tempera-
ture for 30 min and (b) the crystalline Si3N4 (␤ phase) sintered at
1420 °C at 5.0 GPa for 30 min. Note the pure white color of the
sintered and crystallized Si3N4 ceramic, which is a result of nonaddi-
tive sintering and differs from the normal gray or dark-gray color of
sintered Si3N4 with additives.
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Y-L. Li et al.: Sintering of nanopowders of amorphous silicon nitride under ultrahigh pressure
the amorphous materials pressed between 800 and
It was observed that the appearance of the sample after
sintering changed greatly with sintering temperature at
T > 1000 °C. The samples sintered between 1000 and
1350 °C were generally irregularly broken into small
pieces, while crack-free specimens were easily obtained
at T > 1350 to 1420 °C. Different from the translucent
and brown appearance of the amorphous Si3N4 that
formed below 1000–1100 °C, the samples obtained at
T > 1350 to 1420 °C were translucent and pure white in
color [Fig. 2(b)]. The white color of the Si3N4 ceramics
is seldom observed in conventional sintered Si3N4 ce-
ramics. It is a result of the high purity of the starting
Si3N4 powders and of nonadditive sintering.
Figure 4 shows the variation of Vicker hardness with
sintering temperatures at 5.0 GPa. The hardness in-
creases steeply between 850 and 1000 °C which may be
due to the increase of bulk density of the specimens with
the increasing temperatures in this temperature range. At
T > 1000 °C, the hardness increases slowly with sintering
temperature. The sintered bodies obtained at 1420 °C
consisted of ␤ phase exhibited a hardness around
1648 kg/mm2. This hardness value is comparable to that
of the hot-isostatically sintered Si3N4 ceramics with ad-
dition of sintering aids (Y2O3 + Al2O3).20
950 °C at 5.0 GPa are visibly translucent (2–5 mm thick).
The lack of transparency for our materials may be due
to the larger dimensions of our specimens (2–5 mm
thick) than those demonstrated by Pechenik et al.
(0.1 mm thick).14 It was observed that the degree of
transparency increased with sintering temperatures be-
tween 25 and 950 °C and changed with powder compo-
sitions: low oxygen powders (2.0 wt% oxygen) and Si-
rich powders gave less translucent or opaque specimens.
The amorphous Si3N4 ceramics obtained at 950 °C at
5.0 GPa were characterized by UV/VIS absorption spec-
trum in the wavelength ranging from 250 to 850 nm,
showing a gradually decreased absorption with the in-
creasing wavelength above 450 nm. The absorption be-
havior is similar to that shown by Pechenik et al. for their
cryogenically compacted specimens.14 Furthermore, an
optical transmission spectrum between wavelength of
250 and 850 nm was recorded for a translucent amor-
phous Si3N4 plate (2 mm thick), which shows an increas-
ing transmission above wavelength of 450 nm, with a
transmittance attaining 85% at 850 nm (Fig. 3).
C. Sintering above crystallization
temperature (>1000 °C)
2. Grain growth
1. General sintering results
The grain size of the crystallize Si3N4 sintered at
5.0 GPa above crystallization temperatures (>1000 °C)
was measured by XRD according to the line broadening
of diffraction peaks. The grain size of ␣–Si3N4 formed at
1000 °C was determined to be 160 nm, which is almost 1
order of magnitude greater than the starting particle
diameter (18 nm). This indicates that a rapid grain
growth occurred following the nucleation of ␣ phase dur-
ing crystallization. Figure 5 shows the variation of grain
size with sintering temperature which was obtained at
5.0 GPa. With increasing temperature, the grain size in-
Crystalline Si3N4 specimens were obtained when sin-
tering temperature was above 1000 °C. XRD analysis
revealed that the specimens sintered between 1000 and
1100 °C at 5.0 GPa contained predominately ␣–Si3N4
phases with ␤ as a minor phase. At T ס 1100 to 1420 °C,
a mixture of ␣ and ␤ phase was obtained, and at
T > 1350 to 1420 °C, the sintered bodies were almost
entirely ␤ phase.
FIG. 3. UV/VIS transmission spectrum of the amorphous Si3N4 ce-
ramics sintered at 950 °C at 5.0 GPa. The sample for the measurement
was a thin plate, 2 mm thick with polished surfaces.
FIG. 4. Variation of Vicker hardness with sintering temperatures
at 5.0 GPa.
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Y-L. Li et al.: Sintering of nanopowders of amorphous silicon nitride under ultrahigh pressure
FIG. 6. Variation of sintered density with sintering temperatures at
three different pressures (1.0, 1.5, and 5.0 GPa).
FIG. 5. Variation of grain size with sintering temperature at 5.0 GPa.
creases slowly between 1100 and 1350 °C, followed by
an accelerated growth above 1300 °C. The slower grain
growth between 1100 and 1350 °C may be related to the
dynamically controlled solid nucleation and growth of
new ␤ phases in the ␣-phase matrix that first crystallized
from the amorphous structure during crystallization. The
accelerated grain growth at T > 1350 °C is attributed to
the growth by combination of ␤ grains that occurred after
the completion of ␣/␤ phase transition. The rapid grain
growth at the sintering temperature right above the crys-
tallization temperature makes it difficult to control grain
size below 100 nm. However, because most of densifi-
cation is nearly completed below 1300 °C, it is possible
to control the grain size below 200 nm while simulta-
neously ensuring a higher bulk density ( 93%). To ac-
quire nanocrystalline bodies with a grain size below
100 nm, an alternative possible route is to process the
dense amorphous bodies by a subsequent high-
temperature anneal, as another approach which is cur-
rently being undertaken.
density, which seems to be independent of the pressure
applied (1.0–5.0 GPa). This may be caused by generation
of viscous flow of silicon oxide phases in the sintered
bodies at the elevated temperature which allow densifi-
cation of the materials at a relatively low pressure. The
reduction of viscosity of the silicon oxide phases may be
caused by the existence of OH groups in the starting
powders12,17 which was found to have a remarkable ef-
fect on lowering the viscosity of silica even in ppm level,
in particular, above 1420 °C.12,21
It is noted that the final density (3.02 ± 0.01 g/cm3)
attained at T > 1420 °C is lower than the theoretical
density of crystalline Si3N4 (3.18 g/cm3), which corre-
sponds to a relative density of 94% with respect to crys-
talline Si3N4. However, this does not mean that the
sintered bodies were not fully densified under the sinter-
ing conditions. The lower relative density was due to the
existence of low density silicon oxide or silicon oxyni-
tride phases (Si2ON2 density: 2.82 g/cm3). Assuming all
the 5.8 wt% oxygen is present as silicon oxynitride in the
sintered bodies (formation of Si2ON2 phases were de-
tected under certain conditions), calculation gives a com-
promise density value of 3.03 g/cm3 for the two phase
composite (Si3N4–Si2ON2). This value is nearly equal to
the bulk density of the specimens sintered at T >1420 °C,
implying that the materials were fully densified at
T > 1420 °C.
Finally, it is interesting to note that the relative density
of 94% (with respect to crystalline Si3N4; density 3.18 g/
cm3) obtained in this experiment is equal to that obtained
by HIP sintering of similar powders as reported by Sy-
mons and Danforth.10,11 However, the densification tem-
perature of 1420 °C in the present case is 580 °C lower
than that of HIP sintering, in which the final density was
attained at temperature as high as 2050 °C. This indicates
that application of high pressure can significantly en-
hance densification of the amorphous Si3N4 ceramics
and reduce sintering temperatures.
3. Effects of pressure and sintering temperature
on densification
The variation of density with sintering temperature at
three different pressures (1.5, 2.5, and 5.0 GPa) is shown
in Fig. 6. It is seen that the density increases with tem-
perature along a similar trend at the three different pres-
sures (1.5, 2.5, and 5.0 GPa), i.e., a remarkable increase
at lower temperature, typically below the crystallization
temperature (1000 °C), followed by a slow increase at
higher temperatures. In the low-temperature region, the
effects of pressure on density are more pronounced than
that in the high-temperature region, indicating a large
contribution of pressure to the densification of the amor-
phous powders at lower temperatures. As temperature
increases above 1350–1420 °C, the sintered density un-
der different pressures trends to approach the same final
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Y-L. Li et al.: Sintering of nanopowders of amorphous silicon nitride under ultrahigh pressure
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IV. SUMMARY AND CONCLUSIONS
Amorphous Si3N4 nanopowders were densified under
1.0–5.0 GPa at room temperature and 1600 °C. It was
observed that the nanopowders can be compacted to a
high density (87%) even at room temperature. Upon
heating, dense bulk amorphous Si3N4 ceramics were ob-
tained at temperature slightly below that of the onset of
crystallization (1000–1100 °C). Crystalline Si3N4 ceram-
ics that were pure white in color were obtained at sinter-
ing temperatures above 1420 °C. However, a rapid grain
growth occurred during crystallization which made it
difficult to control grain size below 100 nm by the
high-pressure sintering. Under an applied pressure of
1.0–5.0 GPa, a final density of 95–98% was attained at
1420 °C which seems to be independent of the pressure.
This densification temperature is 580 °C lower than that
previously achieved in HIP sintering for the same sin-
tered density, indicating the significantly enhanced den-
sification of the amorphous Si3N4 at relatively lower
temperatures under high pressure.
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ACKNOWLEDGMENT
This work was supported by the Chinese Foundation
of Nature and Science under Grant No. 159472013.
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