Note: Descriptions are shown in the official language in which they were submitted.
~24733~
METHOD OF MAKING ~E~SIFIED Si3N4/OXYNITRIDE
COMPOSITE WITH PREMIXED SILICON
AND OXYGEN CARRYING AGENTS
TECHNICAL FIELD
The invention relates to the technology of making
silicon nitride by reacting silicon powder under a
nitrogenous atmosphere and then densifying the nitrided
body by heat fusion.
BACKGROUND OF T~E INVENTION
AND PRIOR ART STATEMENT
In the art of making silicon nitride, it is
conventional to add certain oxides to the raw material
from which the fully dense silicon nitride body is
constituted, such oxides act as pressing aids or sintering
15 aids (see U.S. patent 4,143,107). The presence of these
oxides has required higher temperatures, pressures, and
pressing times to reach full densification, during hot
pressing or sintering, than what is optimally desired for
a more economical process.
Generally, compounds other than oxides have been
introduced to silicon nitride as a result of chemical
reaction during hot pressing or sintering (see UOS~
patents 4,102,598; 4,341,874; and 4,350,771). Rarely have
compounds other than oxides been introduced to Si3N4 prior
25 to hot pressing or sintering. In this latter case,
oxides were added to a silicon metal mixture and ~he
mixture was nitrided to react the oxides and form a
composite of silicon nitride, oxyni~rides, and a small
amount of silicate. Under this disclosure, the nitriding
30 step was not able to optimally fulfill the dual roles
~L~,4'7335
imposed upon it, that is: (a) to efficiently convert all
of the silicon to silicon nitride, and (b) at the same
time form only a selected silicon oxynitride in a
controlled optimum amount.
What is needed is the ability ~o introduce
chemical modifications to the mixture prior to nitriding
or heat treatment, which modifications allow for: (a) a
more eficient conversion of silicon to silicon nitride,
(b) a reduction in the time and temperature required in
subsequent steps to hot press or sinter ~he mixture to a
fully densified object, ~c) freedom to decrease the amount
of silicate forming oxide to a much lower controlled
amount to optimize physical characteristics in the final
object, and (d) closer control and proportioning of the
desired secondary phase chemistry in the final product
without total reliance on the vagaries of chemical
reaction during any reaction heating involved in the
process such as during nitriding, hot pressing, or
sintering.
2Q Continuing research, as exemplified herein,
underscores the need for selectivity of silicon
oxynitrides and the need for greater silicon conversion
efficiency. Certain of these oxynitrides are more
desirable than others and more desirable than silicates or
25 oxides in promoting lower temperatures and pressures
needed for processing. This selectivity is due in par~ to
the fact that certain of the~e compounds have a higher
degree of solubility for silicon nitride, thus tending to
promote a reduction in at least one of the processing
30 parameters (temperature, pressure, or hot pressing time)
needed for full dissolution of the silicon nitride during
the heat fusion step~
SUMMARY OF HE INVEN~ION
The invention is an improved method of making a
35 more densified silicon nitride comprising object under
;33~
less stringent heating conditions without sacrificing the
physical properties now attainable by the most preferred
prior art methods of making silicon nitride.
In a method where silicon powder is subjected to
5 sequen~ial heating, including first a nitriding heating to
form silicon nitride and secondly a heating to fuse the
resulting silicon nitride, the invention comprises having
yttrium silicon oxynitride present with said silicon or
silicon nitride prior to heating to fuse. A major
10 proportion of the yttrium silicon oxynitride is of the
YloSi6O24N2 phase. The yttrium silicon oxynitride serves
to dissolve the silicon nitride during the second heating
to fuse, the dissolved yttrium silicon oxynitride
converting to a more stable phase as a result of the
15 second heating to fuse.
Preferably, the Y10Si6O24N2 phase is
independently prepared and added to the low oxygen
containing silicon powder prior to the first nitriding
heating, the YloSi6O24N2 phase remaining substantially
20 unreacted during the nitriding heating. Alternatively, a
yttrium silicon oxynitride of the YSiO2N phase and excess
SiO2 (generally present as a surEace film on the silicon
metal) may be proportioned to be present with the silicon
powder prior to nitriding heating, the YSiO2N and excess
25 SiO2 reacting during nitriding heating to form the
YloSi6O24N2 phase prior to heating to fuse.
Preferably, the yttrium silicon oxynitride is
added in an amount of 6-18% by weight of the resulting
mixture and the Ylo5i6o24~2 phase is generally
3Q proportioned to constitute at least 75% of the added
o~ynitride. The remaining oxynitride phase is preferably
of the YlSiO2N phase, which converts substantially to
YloSi6O24N2 during nitridlng (using available SiO2 present
as a film on the silicon metal). Preferably, the heating
35 to fuse is carried out under atmospheric conditions and,
~733~
advantageously, the YloSi6O24N2 converts to the ~SiO2~l
phase under such conditions after full consolidation has
been reached.
The invention is also the making of a densified
5 silicon nitride/oxynitride composite, by the steps of:
(a) shaping a substantially homogeneous particulate
mixture of (i) silicon powder carrying .4-3.5% by weight
SiO2 as a surface oxide, (ii) independently prepared
yttrium silicon oxynitride in an amount of 6-18~ by weight
10 of the mixture and selected to effectively dissolve
silicon nitride at a temperature of 1760C, without the
use of pressure, or less, (iii) a glass forming oxide
effective to form a controlled small amount of glass
silicate with other elements of the mixture on heating,
15 and (iv) up to 2% by weight of Y2O3 effective to react
with any excess silica during heating to form a silicon
oxynitride, said shaping being carried out to form an
object of less than required dimension and density; (b)
heating the body in a nitriding atmosphere without the use
20 Of pressure to produce a silicon nitride/oxynitride
comprising object; and (c) densifying the shaped body by
heat fusion, with or without the use of pressure, to
constitute an object of required density and dimension.
Preferably, the heat fusion is carried out at a
25 temperature effective to form a solution of the silicon
nitride, glass (such as ~n aluminu~ containing silicate),
and yttrium silicon oxynitride~ i.e., in the range of
1650-1760C. The yttrium silicon oxynitride, along with
available aluminum containing silicates, serve to dissolve
30 the silicon nitride during heating to fuse.
Advantageously, the glass forming oxide is added in an
amount of .4-3.0% by weight of the mixture and is selected
from the group consisting of A12O3, MgO, CeO2, ZrO2, HfO2,
B2O3, and BeO2. Advantageously, the oxynitride forming
35 oxide is selected from the group consisting of Y2O3 and
rare earth oxides.
~733~ -
The product resulting from the practice of the
method herein is inventively characterized by a silicon
nitride/oxynitride composite, wherein the oxynitride
consists substantially of the YSiO2N phase in the
crystalline form, and a glass silicate thinly coats the
crystallites of silicon nitride and YSiO2N. The silicate
coating is preferably in a thickness of 2-10 angstroms and
has li~tle or no microporosity.
It is preferable that nitriding heating be
carried out in a gaseous atmosphere containing essentially
nitrogen and helium and a small proportion of hydrogen,
the pressure of the atmosphere being maintained at about
2.7 psig. The nitriding is preferably carried out at an
ultimate temperature of about 2000-2600F (1093-1427C)
for a period of about 72-160 hours.
The densifying step may be preferably carried out
by either the use o hot pressing to an ultimate
temperature level of about 3000-3200F under a pressure of
about 2500-4000 psi for a period of about .25-3 hours, or
sintering whereby the body is heated to a temperature of
3000-3200F (1650-1760C) for a period of .5-12 hours
without the use of pressure.
BEST MODE FOR CARRYING OUT THE INVENT _
A preferred mode for carrying out the present
invention is as follows.
1. Mixture Forming
A homogeneous powder mixture of 5i) silicon
powder (carrying .4-3.5% SiO2 by weight of the silicon as
a surace oxide, (ii) independently prepared yttrium
silicon oxynitride in an amount of 6-18~ by weight of the
mixture, (iii) .4-3% by weight of the mixture of a glass
forming oxide effective to form a controlled small amount
of glass silicate with other elements of the mixture upon
L7~3~i
heating, and (iv) up to 2~ by weight of the mixture of
Y2O3 effective to react with silica during nitriding
heating to form a silicon oxynitride.
The silicon powder is selected to have a purity
5 of 98% or greater and a starting average particle size of
2.5-3.0 microns with no particles or hard agglomerates
exceeding 10 microns in size. The major trace metal
contaminants experienced with such impurity include, as a
maximum: Fe - 1.0%, Ca - .02%, Al - .5%, and Mn - .09%.
1~ Nonmetallic contaminants include, as a maximum: carbon -
.05%, and 2 ~ less than .5%. Silicon is selected so that
after blending and comminution the oxygen content is less
than 1.75%. Comminution increases oxygen conten~
therefore~ the starting silicon contains oxygen levels
15 substantially lower than 1.75%.
The yttrium silicon oxynitride is optimally the
YloSi6O24N2 phase and secondarily the YSiO2N phase which
can be converted to the YloSi~O24N2 phase prior to
heating to fuse, preferably independently prepare~. The range ot- addi~ion
(6-18% of the mixture) is ~ased on the following
considerations: if more than about 18~ of the oxynitride
is employed, the secondary phases in the final product
will undesirably begin to dominate the physical properties
thereof; if less than about 6% of the oxynitride is
25 employed, considerable difficulty will be encountered in
achieving full density in the resulting product.
Instead of optimally adding the yttrium silicon
oxynitride in the form of an independently prepared
phase where at least 75% is of the type YloSi6O24N2, the
30 YSiO2N phase may be used as a substitute to facilitate use
of higher levels of oxygen in the silicon metal. Thus,
~7~33~
--7--
during nitriding heating, the YSiO2N and excess SiO2 will
react to form YloSi6024N2. YloSi6024N2 must constitute a
major portion of the second phase content, at least prior
to heating to fuse, because this phase per~its (a)
5 dissolution of Si3N4 at a lower temperature and/or
accompanying pressure and thus reduces the time and energy
requirements for achieving full densification, and (b)
closer control and proportioning of the desired secondary
phase chemistry in the final product. Use of
1~ independently prepared oxynitrides permits (a) more
efficient conversion of silicon to silicon nitride during
nitriding heating, and (b) freedom to decrease the amount
of glass forming oxide to a much lower controlled amount
to optimize physical characteristics in the final product.
The glass forming oxide is preferably selected to
be A1203 having a purity of at least 99.5% with an average
particle size of 2-3 microns or less and no particles
greater than 10 microns and with a crystal size of less
than .5 microns. Other glass forming oxides that can also
2n be used include MgO, CeO2, ZrO2, HfO2, B203, and BeO2.
The glass forming oxide reacts with excess SiO2 during
heating to form a small, but controlled, amount of
protective amorphous silicate coating the oxyni~ride
crystallites. This glass coating is usually in the
25 thickness range of 2-10 angstroms and has little or no
microporosity. The glass is useful to prevent high
temperature oxidation of the crystalline phase in the
final product, particularly when the product is used as a
cutting tool. More than 2~ gl~ss is to b~ generally
3Q avoided since such glass would have a dele~erious effect
on strength at elevated temperatures and decrease its
utility as a cutting tool.
The oxide, added to form an oxynitride with any
excess silica, i5 preferably Y203 having a purity of at
35 least 99.99~ and a surface area greater than 6.3 m2/gm and
a crystal size of less than .5 microns.
~'733~;
--8--
The mixture is blended by being charged into an inert
milling jar along with grinding media in the form of Si3N4
based cylinders, the Si3N4 cylinders being of the same
composition as the mixture. The mixture is blended and/or
milled for 48 hours at 64 rpm and then separated from the
milling media by the us~ of a ~10 mesh screen. The
blending and/or milling is preferably carried out dry, but
can be wet, with some accompanying disadvantages.
2. Shaping
The milled or comminuted mixture i5 then shaped
to form a body of a general dimension and configuration
desired. It is preferable for simple shapes tha~ shaping
be carried out by loading the milled mixture into a cold
pressing die arrangement and pressed at 1400-1500 psi to
form the desired shaped compact with an accompanying
density of about 1.4 gm/cm3 or greater. However, the
shaping step can be carried out successfully by other
modes such as extrusion, heating agglomeration in a die,
hydrostatic pressing, or by the preferred cold compaction
2Q above. Slip casting is somewhat difficult because of the
number of elements involved in the mixture system
therefore requiring unusually sensitive control.
3. ~eating to Nitride
The compact is heated in a nitriding atmosphere,
25 without the use of pressure, to produce essentially a
silicon nitride comprising body containing 5-17% yttrium
silicon oxynitride. When the silicon metal has a low
silica content (i.e., less than 1.7% after milling), the
YloSi6O24N2 phase is added directly; such phase will
3Q remain unreacted during the nitriding treatment. When the
silicon metal contains a hiyh oxygen content, it is
preferable to substitute YSiO2N for the YloSi6O24N2 phase;
the latter phase will then be produced as a result of the
presence of sufficient SiO2 and the nitriding heating.
35 The nitrided body may contain up to .5~ free silicon and
733~
g
unreacted oxygen carrying agents. The body will have a
size greater than and a density less than the object to be
formed.
To carry out the heating, the compact is
preferably placed in an enclosed furnace, evacuated to a
pressure of less than one micron, and heated at a fast
rate, i.e., 500C/hr (932F/hr) to 649C (1200F). The
furnace is then filled with a gaseous mixture consisting
of 72~ by weight nitrogen, 25% by weight helium, and 3% by
weight hydrogen, a~ a pressure of about 2.7 psig. The
total O~ and H2O content in such a gaseous mixture is less
than 4 ppm. The temperature of the furnace is then
increased in steps to 1200-1700F (649-927C) at 500F/hr
(278C) to 1700-2000F (927-1093C) at 200F/hr (111C),
and through ni~riding temperatures of 2000-2600F
(1093-1427C) at a much slower rate. Fresh (99.999% pure)
nitrogen is intermittently supplied to the furnace to
replace the nitrogen consumed in forming silicon nitride
and possibly additional oxynitrides. Nitrogen is added
when the pressure drops below 2.4 psig and brought back up
to a maximum pressure of 2.7 psig. Once complete
nitridation is achiev~d, the ma~erial is then cooled to
room temperature at a rate of 250F/hr (139C/hr).
The nitrided body will preferably consist of
silicon nitride (at least 60% of which is in the alpha
form), 5-17% yttrium silicon oxynitride in the YloSi6024N2
phase with possible minor amounts of YSiO2N crystalline
phase, and the remainder aluminosilicate glass possibly
containing Y2O3, and up to .5% of silicon and unreacted
3Q Y2O3 or A12O3. The N-melilite phase (Y2O3.Si3N4~ will be
present in an amount no greater than .5% by weight of the
body. The body will have a density of at least 1.9
gm/cm3. The minimum alpha/beta Si3N4 ratio is
approximately 2:1.
~733~;
--10--
4. ~eating to Fuse
Full densification is preferably carried out by
sinteriny for a period sufficient to densify the shaped
body to a product of re~uired dimension and density.
5 Sintering heating is carried out at a temperature level
effective to sinter, but without sublimation of the
silicon nitride (temperatures and times are selected
according to body composition and size); such temperatur~
is preferably in the range of 3000-3200F (1650-1760C)
1~ and advantageously about 1725C for eight hours (for a
median composition of 4-6 cubic inches~. The heating can
be carried out in an inert atmosphere with or without the
use of mechanical or atmospheric over-pressure, or with or
without the use of a blanket of ceramic packing medium
15 such as loose silicon nitride powder. For the preferred
mode, the heat-up to the sintering temperature is staged
first at a rate of about 600C/hr (1080F/hr) to a
temperature level of about 60QC (1112F) under a vacuum
(less than 20 microns), held for one hour, and then heated
2n at a rate of about 600C/hr (1080F/hr) to the sintering
temperature of 1725C (3137F), and held at this
temperature for a time sufEicient ~such as 2-12 hours) to
permit achieving full theoretical density. Nitrogen gas,
with up to 5% helium, is introduced to the furnace during
~5 heating to sintering (the last heat-up rate of 600C/hr).
The gas may also have controlled amcunts of 2
Alternatively, a simple shaped nitrided body can
be hot pressed ~o produce the silicon nitride comprising
object of required dimension and densi~y. A pressing
3a fixture having graphite wall~ is used to carry out hot
pressing. The walls and shaped body are both coated with
a slurry of boron nitride and dried. The pressing
fixture, with the shaped body therein, is placed in the
hot pressing furnace. The heating and pressing is c~rried
35 out preferably in increments: (1) a mechanical loading of
33~
--11--
100 p5i is applied at room temperature to the body; ~2)
the temperature is increased to 1800F (~82C) and
pressure increased to 500 psi; (3) the temperature is then
increased to 2500F (1371C) and the pressure is
5 simultaneously increased to 2500 psi; (4) the temperature
is finally increased to the ultimate hot pressing
temperature of 3200F (1760C) and pressure increased to
3700-4000 psi, the latter conditions being maintained
until at least 99% or desirably 99.5% of full theoretical
10 density is achieved. This usually requires .25-3.0 hours
at the ultimate hot pressing temperature, optimally about
60 minutes. The object is then cooled at any rate, even
quenched, to room temperature.
5. Resulting Product
The resulting object will consist essentially of
beta silicon nitride, 5-17% by weight yttrium silicon
oxynitride crystallites, predominantly of the YSiO2N
phase, and enveloped by up to 2% of a protective glass
silicate in the thickness range of 2-10 angstroms and
20 having little or no microporosity. The object preferably
possesses a hardness of 88.0-92.0 on the 45-N scale for
isotropic materials (but as low as 87.0 for materials not
isotropic), a density of 3.2-3.45 gm/cm3, an average
fracture strength above 85,000 psi at 1200C in a 4-poin~
25 bend test9 and an oxidation resistance that provides
little or no increase in weight by the object after 450
hours in air at 1000C. Some oxynitrides of the
Y105i6024N2 and Y4Si20N2 phases can be present up to 10
of the second phase content.
30 Examples
A series of six samples were prepared by the
nitriding and sintering sequence and were tested as to
density to illustrate how variations in the chemistry
facilitate or deny obtaining the advantages of this
35 invention. The variations and results are su~marized in
Table 1.
33~ii
-12-
In sample 1, a mixture was prepared with silison
powder containing therein 1~ or less by weight SiO2, 10.3~
YloSi6024N2 (H phase), and 1.5% glass forming oxide
(A1203), The sample 1 mixture was compacted, nitrided, and
S sintered in accordance with the conditions of the
preferred mode. The measured density of the compact prior
to nitriding was 1.48 gm/cm3; it increased to 2.29 gm/cm3
after nitriding, and rose to 3.31 gm/cm3 after sintering.
The presence of the Ylosi624N2 phase facilitated
sintering at 1725C for eight hours, and allowed for
control of the amount of YSiO2N phase in the secondary
phase of the final product without degrading the high
density in the final product. Increasing the A1203
content, as shown in sample 3, decreased the sintering
time for achieving full density to four hoursO Final
oxynitride phase content did not change significantly from
that in Sample 1.
When the YSiO2N phase is added to the mixture (in
place of the YloSi6024N2 phase), without sufficient SiO2
to react with such phase, the final density in the product
is lowered, even though all other conditions remain the
same (see sample 2). Sample 6 demonstrates how adding the
YlSiO2N phase with sufficient but extra amounts of SiO2 to
react with the YSio2N phasP produces an enhanced density.
Sample 4 illustrates how up to 25~ of the added
oxynitride can be the YSiO2N phase, with ~he remainder
YloSi6024N2, while achi~ving relatively satisfactory
density with equivalent processing conditions.
Lastly, sample 5 illustrates how noncompliance
30 with the requirement for adding, or having present before
sintering, a major quantity of the YloSi6024N2 phase
results in poor final density and the presence of unwanted
secondary phases in the final product (such as the
N~melilite phase).
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