Note: Descriptions are shown in the official language in which they were submitted.
RD--924 0
~6~3~3
Silicon nitride is a choice candidate material for
turbine applications because of its good high temperature
strength and creep resistance, low thermal expansion
coefficient and excellent oxidation resistance. So far,
the conventional method of producing large specimens of
dense silicon nitride is by hot~pressing with the helP
of an oxide flux, at temperatures greater than 1700 C.
Various oxide fluxes or densification aids, such as
MgO, Y2O3, ZrO2 and Ce2O3, permit the attainment of full
density in hot-pressed Si3N4. However, experience has
shown that these oxide additions produce a silicate glass
at grain boundaries which has a deleterious effect on the
high temperature creep and strength properties due to the
softening or melting of the glassy phase at temperatures
ranging from about 1000C to 1200C depending on the
oxide flux added. Consequently, most efforts to improve
the high temperature properties of Si3N4 containing an
oxide additive(s) have been directed towards improving
the refractoriness of the silicate "glassyl' phase by
composition control and crystallization methods.
In accordance with the present process no oxide
additive is used. Also, at the grain boundaries of the
-- 1 --
RD-9240
;8~3
present product there appears to be no detectable glassy
phase.
The present invention is directed to hot-pressing
a homogeneous particulate dispersion of silicon nitride and
magnesium silicide to produce a novel dense polycrystalline
body of silicon nitride which substantially retains its room
temperature mechanical properties at elevated temperatures
ranging up to about 1350C or higher in air, depending on
the purity of the starting powder.
lQ Those skilled in the art will gain a further and
better understanding of the present invention from the
detailed description set foxth below, considered in con-
junction with the figure, accompanying and forming a part of
the specification, which is a graph showing relative densities
of silicon nitride hot-pressed with magnesium silicide
additions at 1750C for 20 minutes under a pressure of 8000
pis~ The amount of Mg2Si is based on the amount of silicon
nitride. Specifically, the graph is a plot of the relative
; density of the hot-pressed silicon nitride body, i.e. the fractional
density of the theoretical density of silicon nitride (3.18
g/cc) vs. the amount of Mg2Si admixed with the starting silicon
nitride powder. The graph illustrates the present invention
and shows-the highLy dense bodies of silicon nitride which
can be produced particularly with'additions of about 2% by
weight of Mg2Si.
Briefly stated, the process of the present invention
comprises providing at least a significantly homogeneous
particulate'dispersion or mixture having an average particle
size which'is submicron of silicon nitride and magnesium
- 2 -
~ 3 RD-9240
silicide in an amount of about 0~5~/O by weight to about 3.0%
by weight based on the amount of sald 8ilicon nitride, and
hot-pressing said particulate dispersion in an atmosphere
of nitrogen at a temperature ranging from about 1600C to
about 1850C under ~ minimum pressure of about 2000 psi
to produce a pressed body having a density of at least 80%
of the theoretical density for silicon nitride.
The silicon nitride powder used in the present
process may be amorphous or of the ~-type or mixtures
thereof. These powders can also contain ~-~ilicon nitride
usually in an amount up to about 20 weight % of the total
amount of silicon nitride.
At present commercially available silicon nitride
powder in any significant amount is formed by nitridation
of silicon powder with the aid of catalysts which always
leave CaO, Fe2O3, and A12O3 as impurities, in a significant
amount, typically about 1 to 2%, Such a silicon nitride
powder is not useful in the present process because when it
is hot-pressed, even without an oxide flux, these impurities
. ,
combine with SiO2, which is inherently initially present in
silicon nitride or forms on firing, to produce a low meltin~
intergranular glassy phase.
In contrast, the present starting silicon nitride
powder is substantially pure but it can range somewhat in
purity, The necessary purity of the powder used depends
6 89 3 RD-9240
largely on the temperatures and loads at which the final
hot-pres~ed product will be used with the highest
temperatures of use generally requiring the mo~t pure
powders, Specifically, wlth increasingly pure powder the
resulting hot-pressed product increasingly retains its
room temperature propertie~ at high temperatures, i,e, the
more stable are the mechanical properties of the hot-pressed
product with increasing temperature.
The present silicon nitride powder may contain
certain metallic and non-metallic impurities in a limited
amount and these impurities are based on the total composition
of the starting silicon nitride powder, Specifically, the
powder should be free or substantially free of metallic
impurities which react with SiO2 or Si and 2 to form
low melting intergranular silicate glassy phase in a signi-
ficant amount. Those impurities which form such a glassy
phase include calcium, iron and aluminum and should not be
present in a total amount greater than about 0.1% by weight.
Al80, the present silicon nitride powder may have an
oxygen content ranging up to about 3% by weight, Normally,
the oxygen is present in the form of silica, The amount
of excess elemental silicon in the powder should not be
present in an amount higher than about 4% by weight because
appreciable amounts of residual elemental silicon may be
retained in the product, depending on the extent of nitridation
occurring during hot~pressing. Also, any elemental
lQ~S~3~3
-9240
silicon presen~ should be of submicron size and should be
substantially homogeneously dispersed throughout the powder.
Non-oxide impurities such as halogens which evaporate to a
significant ex~ent and which do not significantly deteriorate
the propertles of the hot-pressed silicon nitride body
may al~o possibly be present in amounts up to about 1% by
weight of the starting Rilicon nitride powder.
To produce a hot-pressed product which has
subs~antially stable mechanical propertie~ at high temperatures,
the preferred starting silicon nitride powder has a low
oxygen content, i,e, usually about 2% or less by weight of
- the powder, and essentially free of elemental silicon, Also,
it is free or substantially free of metallic impurities
~n total amount ranging up to about 0~05~/O by weight of the
powder. Such a powder can be synthesized, Alternatively,
to reduce its oxygen content and also remove its vaporizable
impurities, the silicon nitride powder can be calcined at a
temperature ranging from about 1300C to about 1500C in a
vacuum or in an atmosphere which has no significant
deteriorating affect on the powder such as helium, nitrogen,
hydrogan and mixtures thereof,
The present silicon nitride powder can be produced
by a number of processes, For example, in one process SiO2
is reduced with carbon in nitrogen below 1400C, Still other
processes react a si.licon halide with ammonia or a ni~rogen
RD-9240
~Q6~3
and hydrogen mixture to obtain either Si3N4 directly or via
precursors such as si (NH) 2 which are converted to Si3N4
b~ calcination yieldin~ silicon nitri~e which usually
contains oxygen and halogens at a 1% to 3% by weight level.
The powder can also be synthesized in a plasma from silicon
vapor and nitrogen.
Very pure silicon nitride powder can be formed by
a process set forth in U.S. Patent No~ ~J /~
dated ~ ]1q7g in the names of Svante
Prochazka and Charles D. Greskovich and assigned to the
assignee hereof. Specifically, this invention discloses
reacting silane and an excess amount of ammonia above
500C and calcining the resulting solid at between 1100C
to 1500C to obtain amorphous or crystalline silicon nitride.
In the present process magnesium silicide, Mg2Sl, is
used as a densifying agent. In contxast to magneslum
nitride which is highly hygroscopic at room temperature
which requires it to be used under nitrogen glove box
conditions, magnesium silicide is a friable solid which is
stable in air at room temperature and therefore presents no
problems with respect to formulation and mixing procedures
thereby permitting a significantly simpler and much more
practical preparation process. In the present process
magnesium silicide is used in an amount ranging from about
1~96893 RD-9240
0.5% by weight to about 3.0% by weight of the ~ilicon
nitride. The preferred amount of magnesium silicide i8
determinable empirically and it is the lowest amount
necessary to produce the hot-pressed body of de8ired den8ity
under the particular hot-pressing conditions used in the
present process. However, amounts of magnesium silicide
less than about 0,5% are not effective in producing the
present hot-pressed body with a density with at least about
80%. On the other hand, amounts of magnesium silicide higher
than about 3.0% by weight of the silicon nitride provide no
additional densification of the hot-pressed body.
In carrying out the present process at leas t a
significantly or substantially uniform or homogeneous
particulate disperqion or mixture having an average particle
size which is submicron of silicon nitride and the magnesium
silicide is formed. Such a dispersion is necessary to produce
a hot-pressed product with significantly uniform properties
and having a density of at least 80%, The silicon nitride
and magnesium silicide powders, themselves, may be of a
particle size which breaks down to the desired size in
forming the dispersion but preferably the starting silicon
nitride is submicron and the starting magnesium silicide is
less than 5 microns in particle size, and preferably, submicron.
Generally, the silicon nitride powder ranges in mean surface
area from about 2 square meters per gram to about 50 square
meters per gram which is equivalent to about 0,94 micron to
1~ q 6 89 3 RD-9240
0,04 micron, re8pectively, Preferably, the 8ilic~n nitride
powder ranges in mean surface area from aboue 5 square meters
per gram to about 25 square meters per gram which ie equivaleRt-
to about 0.38 micron to about 0,08 micron, respectively,
The silicon nitride and magnesium silicide powders
can be admixed by a number of technique~ such as, for example,
ball milling or vibratory milling, to produce a h~mogeneous g
dispersion. The more uniform the dispersion, the more uniform
are the microstructure and properties of the resulting dense
hot-pressed body,
Representative of these mixing techni,ques is ball
milling, preferably with balls of a material such as tungsten
carbide or silicon nitride which has low wear and which has no
significant detrimental effect on the properties desired in
the final product. If desired~ such milling can alsb be used
to reduce particle size, and to distribute any impurities
which may be present substantially uniformly throughout the
powder. Preferably, mLlling is carried out in a li~uid mixing
medium which is inert to the ingredients, Typical liquid
mixing mediums include hydrocarbons 6uch as benzene and
chlorinated hydrocarbons, Milling time varies widely and
depends largely on the amount and particle size of the
powder and type of milling equipment, In general, milling
time ranges from about 1 hour to about 100 hours, The
resulting wet milled material can be dried by a number of
conventional techniques to remove the liquid medium,
?i~
. 1096S~3 RD-9240 '.
Preferably, it is dried in a vacuum oven maintained ju~t
above the boiling point of the liquid mixing medlum~ ¦
The present powder di~persinn i8 hot-pressed in an
atmosphere o nitrogen which can range from atmo~pheric
pressure to superatmospheric pressure, generally, up to
about 5 atmospheres, The nitrogen inhibits or prevents
significant thermal decomposition of the silicon nitride
and thereby promotes its densLfication, In the present
invention no significant weight loss due to the thermal
decomposition of silicon nitride occurs, Gases such as
argon or helium are not useful at the lower pressing
temperatures, i,e, below about 1750C because they are too
expensive for commercial use, and at temperatures close to
or above 1750C they would not prevent thermal decomposition
of silicon nitride,
Thermal decomposition of silicon nitride may possibly
occur during the hot-pressing cycle to leave elemental
silicun in the product, ~y a significant thermal decomposition
o the silicon nitride herein it is meant a decomposition
which produces elemental silicon in the hot-pressed product
in an amount higher than about 2% by volume of the product,
This can be monitored by microstructural observation of polished
sections of the hot-pressed body,
The nitrogen gas used should be free of oxygen or
substantially free of oxygen so that there is no significant
oxygen pickup by the body being hot-pressed.
~96893 RD-9240
In carrying out the present proces~, the particulate
mixture or dispersion is hot-pressed, i.e. densified, at a
pressure and temperature and for a 8ufficient period of time
to produce the present dense product, Specifically, the
5 hot-pressing temperature ranges from about 1600C to about
1850C and applied pressure at such pressing temperature
ranges from about 2000 psi to a maximum pressure which i8
limited by available pressing equipment. Thus, for solid
graphite dies the upper limit is about 5000 psi and for ~
graphite fiber-wound dies the upper limit i8 about 15,000 psi. ~r
The specific temperature and pressure used is determinable
empirically and depends largely on the powder being pressed
and the specific dense product desired. The higher the
pressure, the lower is the pressing temperature required,
but as a practical matter, ~emperatures below 1600C will
not produce the present dense product. On the other hand,
temperatures higher than about 1850C are not practical since
the silicon nitride decomposes sub tantLally in the present
hot-pressing process at about 1900C resulting in material
loss. Preferably, for best results, the hot-pressing or
densification temperature ranges from about 1700C to about
1830C and the pressure ranges from about 5000 psi to about
10,000 psi, It is advantageous to use a pressure close to
the maximum available because the application of such high
pressure makes it possible to keep the pressing temperature
low enough to control grain growth, Generally, hot-pressing
!
-10-
., . , , , . ;, ~ " ,
lQ~68~3 RD-9240
t.
in the pre~ent process ii8 carried out at the desired temperature E
in a period of time ranging up to about 30 min~tes, and
longer period8 of time usually do not provide Rny 8igni- !
ficant advantage except at temperatures below 1700C where
there continues to be a conversion of ~- to the preferred
~-form of silicon nitride.
The composition of ~he silicon nitride in the
present product depends on the hot-pressing temperatures used
and ranges from a-silicon nitride alone to ~-silicon nitride .
~lone with all mixtures of the a- and ~-forms of ~ilicon
nitride alling within the range. Specifically, with hot-
pressing temperatures below about 1680C, the silicon nitride
in the resulting hot-pressed product may be all of the
a-form, or it may be comprised of a major amount of the
a-form and up tc about 25% by weight of the ~-form, based
on the total amount of silicon nitride depending on the
amount of a- which converts to the ~-form, and also on the
amount of the ~-form initially present in the powder, At
temperatures above about 1680C and ranging up to about
1750C, the silicon nitride in ~he resulting product is always
a mixture of a-and ~-forms of silicon nitride. At hot-
pressing temperatures above about L750C, the silicon
nitride in the product is usually only of the ~-type.
The morphology of the ~- and ~-silicon nitrides
in the hot pressed product is distinguishable, Speciiically,
1~96893 RD-9240
as determined by scanning electron microscopy and met~llo- ¦
graphically in combination with X-ray diffraction ~naly9i5,
the grains of ~-~ilicon nitride ~re su~stanti~lly equlaxed
in form whereas the ~-silicon nitride grain~ are elongated
in form, The ~-grains are always less than two microns in
size and normally les~ than one micron in size~ Preferabl~Jg
they have a grain size of one micron or less, The ~grains
are generally less than about 5 microns in length, and for
best results usually less than about 2 microns in length
and have a width usually less ~han about 0,5 micron. The .
strength of the present hot pressed product ;.ncreases wi.th
increasing content of ~-silicon ni.t~ide provided that such
grains arQ less than about 10 microns in length, The
~-grains are interp~netrati.ng usually forming a network which
resists fracture, At relatively high hot-pressing
temperatures and for rela~ively long periods of hot~pressing~
i,e, longer than about one hour, the ~ grains may grow to a
length of about 10 microns, Preferably, the present product
is comprised of only the ~-LOrm of silicon nitride since it
provides the most s~able properties,
The hot-pressed body o the present invention has a
densi.ty ranging from about 80~/o~ and preferably from about
96% to about 100% of the theoretical density of silicon
nitride, The product is comprised of silicon nitride and
some form of magnesium, The magnesium is present in an
amount rangin~ from about 0,3% by weight to about 1,9%
-12-
.... . , , . . , ~,. . ,............ , , , . ., " ,. , . , , , .:, .. .
.,, ., ~, .. . . ... .. . . .
~T
l~g6~3
by weight of the silicon nitride, The magnesium component
of the product is detectable or dbtenminable by techniques
BUCh as X-ray fluorescsnt analysis, emission ~pectro~copy
and chemical analy~is.
The presen~ hot-pressed product may Also contain
oxygen in some form in an amount up to about 3% by weight of
the product, Preferably, for high temperature applications,
the hot-pressed product contains oxygen in an amount le~s
than about 2% by weight of the product, Oxygen content may '.
10 be determined by techniques such as neutron activation analysis,
According to X-ray diffraction analysis or optical
microscopy, the hot-pressed product may be single phase or
polyphase. With reference to the hot-pressed product of the
present invention by the tenm single phase or primary phase
it is meant herein the silicon nitride phase, i.e, the a-form
or ~-form of silicon nitride and mixtures thereof, The single
phase hot-pressed body indicates dissolution of the magnesium
therein, Generally, when the magnesium silicide additive
is used in amounts of up to about 1% by weight of the silicon
nitride, it is difficult to detect a secondary phase in the
hot-pressed product and it is believed to be a single phase
material, However, when the magnesium silicide is used in
amounts of about 2% to 3% by weight o~ the silicon nitride
a secondary magnesium containing phase may be detected, and
it is believed to be some form of magnesium silicon nitride,
, ", " j " ", ~
.. . . . .. . . . . . . .. .. . .
~0 ~ 6 8~ 3 RD-9240
T~ls secondary phase may be present in an amount less than
about 5% by volume of the hot-pressed body,
In an alternative embodiment of the present invention,
free carbon, submicron in size, is admixed with the present
silicon nitride and magnesium silicide disper~ion to form
at least a significantly or substantially homogeneous
dispersion, Mixing can be carried out by the same techni~ues
used in forming the silicon nLtride and magnesium silicide
dispersion, The amount of free or elemental carbon ranges
from about 0,5% by weight to about 4% by weight, and preferably
from about 1% by weight to about 2% by weight, based on the
amount of silicon nitride, The particular amount of free
carbon used ;s determinable empirically but should be
sufficient so as to leave no ~ree carbon or no significant
amount of free carbon in the hot-pressed product. The free
carbon addition provides a number of advantages. It reacts
with surace oxide films which may be present on the silicon
nitride powder and also with oxygen present in the hot-
pressing environment thereby substantially reducing the amount
of oxygen in the final hot-pressed product, The free
carbon also reacts in the system to produce in situ very
fine-sized pure si.l.icon carbide particles less than about 2
microns in size, The presence o~ these fine silicon carblde
particles in the sili.con nitride matrix leads to improved
strength and fracture toughness because the thermal expansion
coefficient of silicon carbide is larger than that of silicon
.: ..
10~6~3 RD-9240
nitride and hence the silicon nitride matrix surrounding the
particles will be in a state of compression, thereby enhancing
the fra~ture energy as well as the high temperature 8trength
of the resulting hot-pressed product. In this embodiment
of the invention, the hot-pressed product contains discrete
and isolated particles of silicon carbide distributed
substantially uniformly throughout the hot-pressed body
in an amount ranging from about 1% to about 8% by volume
of the body. Amounts of free carbon which result in the
formation of silicon carbide in amounts higher than about
8% by volume of the hot-pressed body are not useful because
the larger volume of silicon carbide particles would result
in these particles being insufficiently isolated to provide
the desired state of compression.
In addition, if the starting powder contains free
silicon, the hot-pressed body may also contain free silicon
as a secondary phase, but such free silicon should be present
in an amount less than about 2% by volume of the hot-pressed
body.
The secondary phase or phases are discrete and dis-
tributed substantially uniformly throughout the present hot-
pressed body, Generally, the grains of the secondary phase
or phases are of about the same size or finer than the grains
of the primary phase,
The presence of a glassy phase ~ u~y de~mined by ælective
etching of the specimen and observing the pits formed by the
6 ~ 3 RD-9240
the etched out glassy phase and/or by deep etching o~ the
grain boundaries themsslves, Sectioning and polishing
of the present hot-pressed body and subjecting the polished
surface to acid solutions containing hydrofluoric acid
reveals no etching or no significant etching of the grain
boundaries which signifies essent~ally no detectable
evidence of an intergranular silicate phase at the grain
boundaries.
The present hot-pressed product usually exhibits
a preferred orientation of the grains in a direction per-
pendicular to the direction of the applied ho~-pressing
pressure, i,e. a preferred orientation in the plane per- ;
pendicular to the hot-pressing direction, As a reRult, a
test bar cut perpendicular to the hot-pressing direction
usually exhibits a tensile strength higher than that of a
test bar cut parallel to the plane of the hot-pressing
direction,
The present hot-pressed product is useful in
structural applicatisns such as components for gas turbines,
Specifically, the present process can produce hot-pressed
products in the form of simple shapes including cylinders,
plates and domes which retain their room temperature shape
and mechanical properties at high temperatures making these
bodies particularly useful for high temperature structural
applications,
-16-
RD-9240
~C~Q6~3
In the present invention, unless otherwise stated,
the density of the hot-pressed body is given as a fractional
density of the theoretical density of silicon nitride
(3.18 g/cc).
The invention is further illustrated by the following
examples wherein the procedure was as follows unless other-
wise stated:
In-house silicon nitride powder was prepared for
use in all the examples as disclosed in U.S. Patent No.
B 10 ~ ) 0~ ~ dated G7~ ~6 ~ J ~ql ~
Specifically, this powder was prepared in a furnace which
included an openended fused silica reaction tube 3.8 cm.
diameter placed in a tube furnace, i.e. except for its
open-end portions the reaction tube was located inside
the furnace, and connected on the downstream end to a
coaxial electrostatic separator operated between 5 and
15 XV and 0.2 to 0.5 mA. The outlet of the separator -
was terminated with a bubbler filled with an organic
solvent which ensured positive pressure in the system.
A liquid manometer indicated gas pressure in the reaction
tube. For each run the reaction tube was heated at a
length of 15 inches to a maximum temperature which was
600C for the silicon nitride powder used in Example 1 and
- which was 850C for the powder used in Examples 2 and 3, the
system purged with purified argon and the reactants were then
metered in. Electronic grade silane and anhydrous ammonia
RD-9240
~6~3
dried further by passing the gas through a column of calcium
nitride were metered in separately by coaxial inlets into
the reaction tube. The gas flow rates were adjusted to
0.2 standard cubic feet per hour (SCFPH) of SiH4 and 3.5
SCFPH of NH3. A voluminous, light-tan powder was collected
in the downstream end of the reaction tube and in the attached
electrostatic separator. After four hours the gas flow
of reactants was discontinued and the system was left to
cool off to room temperature under a flow of 0.5 SCFPH of
purified argon, and the powder was then recovered from the
reactor and separator. The product was a light-tan powder,
amorphous to X-rays, had wide absorption bands in its I.R.
spectra centered around 10.5 and 21.0 microns (characteristic
for silicon-nitrogen bonding), and contained no metals above
50 ppm determined by emission spectroscopy.
Surface area measurements were made by a low
temperature nitrogen absorption technique.
Oxygen content was determined by neutron activation
analysis.
Powder density was determined by a helium Null-
Pycnometer.
Before each hot-pressing run the system was evacuated
and back-filled with nitrogen gas and during the hot-pressing
ru~ and subsequent furnace cooling nitrogen was flowing
through the system at a flow rate of one cubic foot per hour.
- 18 -
' 9L~68~3 ~D-92~0
EXAMPLE 1
't
The in-house silieon nitride powder used in this
example had a specific surface area of 15m /g, ~ powder
density of 2,75g/cc and an oxygen content of 3,12% by
weight of the starting powder,
To 1 gram of the silicon nitride powder there was
added 0,03g Mg2Si, i.e, 3 weight % of magnesium ~ilicide powder
which corresponds to 1.9 weight % of elemental magnesium baqed
on the amount of silicon nitride, 10 cc o~ benzene with 0,02
gram of paraffin added as a binder, Mixing was carried out at
room temperature for 15 minutes in a silicon carbide mortar
and pestle ln air.
After drying in a vacuum oven at 50C and collection,
resulting dry powder dispersion1 which had an average particle
size which was submicron, was loaded into a graphite die
fitted with a 1 cm, diameter boron nitride insert, The
faces of the graphite plungers were coated with a boride
nitride slurry and dried before hot-pressing, The boron
n~tride material prevented reaction between the siLicon
nitride and graphite.
The thermal and pressure cycle for hot-pressin~
consisted of applying a pressure of 3,5 MPa(500 psi) at room
temperature and a pressure of 55 MPa ~ 000 psi) at 1100C,
There was a 1 minute hold at red heat ( ~ 800C~ to remove
~he paraf~in binder, The time to reach 1750C was about 15
minutes, After a soak time of 20 minutes at 1750C in ~e
-19-
, 1Cr~6893 RD_9240
nitrogen atmosphere under a pressure of 8000 psi, the ~,
power to the inductlon coils was turned-off, and ~he load
removed, The boron nitride was removed by grinding it ff !~
the resulting hot-pressed body before characterization, ~.
X-ray diffraction analysis of t~e hot-pres~ed body
showed that the silicon nitride phase was composed of about
50% by weight ~-silicon nitride phase and about 50% by weight
~-silicon nitride phase; no other phases could be detected,
However, observation of a polished section of the h~t-pre~sed
body by optlcal mLcroscopy at high (500-1000X) magnifications
showed the presence of unidentified discrete second phasei
particles usually less than 10 microns in size and in total
amount less than 2% by volume of the body, One of the
secondary phases had a high reflectivity and was probably
elemental silicon; the other secondary phase is believed
to be MgSiN~, The residual pores wers less than 5 microns
in size and usually smaller than 2 microns, The ~-sil.icon
nitride grains were elongated and not longer than 5 microns, .
with an average grain size of about 2 microns and an average
aspect ratio less than 4, The density of the hot-pressed body
measured to be 3,18g/cc, or 100% of the theoretical value of
3,1~g/cc, This run, i,e, the 3 wt,% Mg25i run, which
corresponds to 1,9 wt,% elemental magnesium, is shown in the
accompanying figure,
In the accompanying figure all of the plotted runs
were carried out in the same manner as the 3 wt,% Mg2Si
- ~ O -
iQq6~3~3 RD-9240
run except for the am~unt o~ Mg2Si used. SpecificaLly, the
control run at 0% Mg2Si produced a product with a density of
57%, at 0.5 wt.% Mg2Si (0.3 wt,% elemental Mg) the product
density was 74%, at 1 wt,% Mg2Si (0.6 wt,% elemental Mg~ the
product density was 87%, and at 2,0 wt,% Mg2Si.(1,3 Wt,% r
elemental Mg) the product density was 100%,
The hot-presseid product produced in those runs where '~
the magnesium silicide ranged from 0,5 wt,% to 2,0 wt,%,
showed substantially ~he same silicon nitride grain structure
as was seen for the 3 wt,% Mg2Si run, However, the product
produced i.n the 0,5 wt,% Mg2Si was found to be single phase
by X-ray diffracti.on analysis and optical microscopy,
The shape of the curve i.n the accompanying figure
: will be in part determined by thei mixing procedure used in
that better mixing procedures will in general give higher
densities for a given composi.tion especially for magnesium
silicide levels less than 1 weight % l)ecause it is difficult
to disperse small amounts of the magnesium silicide with a
mortar and pestle,
EXAMPLE 2
The in-house silicon nitride powder used in this
example was amorphous to X-rays and had a speci.fic s~lrface
area of 16m /g. This powder was calcined in N2 at 1450C
for 15 minutes to essentially crystalliæe all of the powder
into a-Si3N4, The oxygen rontent and specific surface area
- 2 1 -
RD-9240
~L096~3~3
of this calcined powder was 2.06 wt .% and 10.0~/g,
respectively,
40 g of this calcined, a-Si3N4 powder was mixed at ,~
room temperature with 0.80 g of Mg2Si, i.e. 2 wt.% Mg~Si
pow~, 25 cc of a solution of l~/o paraffin in benzene, and
250 cc of benzene in a polyethylene jar mill containing 1/4 inch
balls of Si3N4 grinding media, After mixing for 2 hours, the
resulting dispersion was dried in a vacuum oven at 50C for
1 day, The powder mixture, which was a significantly
homogeneous dispersion with an average particle size that
was submicron, was collected and placed in a 2 inch diameter
graphite die which was previously coated with a boron nitride
slurry and dried, The faces of the die plungers were also
coated with a boron nitride slurry and dried,
The powder mixture was hot-pressed at 1800C for
15 minutes in a nitrogen atmosphere at 10,000 ~si, The
density of the resulting hot-pressed body was 3,18g/cc, or
100% of the theoretical value, This hot-pressed body
was cut into a number of bars and evaluated,
X-ray diffraction analysis showed the sample was
composed of ~-Si3N~ plus a trace of a-Si3N4
Observation of a polished section by optical
microscopy showed ~he microstructure contained a small
amount of secondary phases of size smaller than 5 microns.
Partîc'es of a bright reflecting second phase were observed
1~ 9 6 ~ 3 RD-9240 '~'
~.
tnd believed to be elemental ~ilicon, micron to submicron in '~
size, and present in an amount less than about 1% by volume
of the hot-pressed product, Particles of another secondary
phase could be seen which i8 believed to be MgSiN2 and
present in an amount of less than about 2% by volume of the
body,
The hot-pressed product had a Knoop hardneRs
equal to 2250 kilograms per æquare millimeter for a load
of 200 grams,
Its resistance to oxidation was measured in air at
14L0~C for 21 hours and the resulting weight gain was 2,0
milligrams per cm2,
Observation of the gralns in the microstructure by
ehemical etching (8 minutes in boiling NH4F ~ HNO3 mixture)
reveals that the ~-Si3N4 grains are elongated and have an
average length of about 2 microns and an aspect ratio of
about 4, It was difficult to etch most of the grain boundaries
indicating no obvious detection of a glassy phase,
The fracture strength of the hot-pressed product
~n the 3 poin~ bending mode was determined, Its room
temperature strength had an average value of 64,000 psi in
air and at 1350C in air it had an average value of 52,000
psi indicating a retention of about 80% of its room temperature
strength value,
This indicates that at 1350~C the hot-pressed body o~
the present invention should maintain at least about 75% of
-23-
~ 93 RD-9240
its room temperature strength in air at 1350C in air.
In contrast, commercially available hot-pressed 9ilicon
nitride containing magnesium oxide as a densification aid
usually retains only about 40% of its room temperature
strength at 1350C in air,
EXAMPLE 3
The calci.ned silicon nitride powder disclosed
in Example 2 was also used in this example,
40 g of this calcined, a-Si3N~ powder was mixed with
1,2g of Mg2Si pow~ and 0,8 g of free carbon of submicron ,.
size, 25 cc of a solution of 1% paraffin in benzene, and
250 cc of benzene in a polyethylene jar mill containing 1/4
inch balls of Si3N4 grinding media, This was equival~nt to
3 wt,% Mg2Si and 2 wt,% free carbon based on the weight of
~ili.con nitride, After mixing for 2 hours, the resulting
dispersion was dri.ed in a vacuum oven at 50~ for 1 day, The
powder mixture, which was a significantly homogeneous dis-
persion with an average particle size that was submicron, was
collected and placed in a 2 inch di.ameter graphite di.e which
was previously coated with a ~oron nitri.de slurry and dried,
The faces of the die p].un~ers were also pre-coated with a
boron nitride slu,.ry and dried,
The powder mi.xture was hot-pressed at 1~00C or 20
minutes in a nitrogen atmosphere at 10,000 psi, The density
- of th~ hot-pressed sample was (3,19 g/cc) which is slightly
higher than the theoretical value (3.18g/cc)of pure Si3N4,
probably indicating the formation of SiC in the hot-pressed body.
~ 3 RD-92~0
X-ray diffraction analysis showed the sample was
composed of ~LSi3N4, plus a trace of C~-Si3N4. There was
no detection of free C, SiC or any other possible secondary
phases.
Observation of a polished section by optical microscopy
showed the microstructure contained a small amount of secondary
phases of a size smaller than 5 microns. Specifically,
observation of the polished section by optical microscopy under
reflected light shows the silicon nitride phase, i.e. the
primary phase, as dark grey, and in contrast there is a micron
to submicron dispersion of a discrete lighter grey phase which
; is believed to be silicon carbide and which appears to be
present in an amount less than about 4% by volume of the hot-
pressed product. In additio~, there appears to be a few bright
reflecting discrete particles of a phase which is believed to
be elemental silicon and it appears to be present in a trace
amount. In an occasional silicon carbide grain there is
evidence of a small unreacted particle of elemental carbon and
it is believed that such carbon can be eliminated with more
uniform mixing.
Observation of the grains in the microstructure by
chemical etching (8 minutes in boiling NH4F + HNO3 mixture)
revealed that the ~-Si3N4 grains are elongated and have an
average length less than 2 microns.
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