Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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HOT PRESSED SILICON CARBIDE
The invention herein described was made in the
course of or under a contract or subcontract thereunder
(or grant) with the Department of the Navy.
The chemical and physical properties of silicon
carbide make it an excellent material for high temperature
structural applications. These properties include good oxi-
dation resistance and corrosion behavior, good heat transfer
coefficients, low expansion coefficient, high thermal shock
resistance and high strength at elevated temperature. This
unique combination of properties suggests the use of silicon
carbide as components for gas turbines, check valves for
handling corrosive liquids, linings of ball mills, heat
exchangers for high temperature furnaces, pumps for die
casting machines and combustion tubes.
Heretofore, hot pressing of silicon carbide was
used to produce small specimens under closely controlled
condi~ions. Unfortunately, silicon carbide is not easily
sin~ered to densities approaching the theoretical density of
3.21 grams per cubic centimeter. A method of hot pressing
silicon carbide to uniform densities on the order of 9~/O of
the theoretical density with slight additions of aluminum
and iron aiding in densification i9 disclosed by Alliegro
et al., J. Ceram. Soc., Vol. 39, 11 (November 1956), pages
386-389. It was found that dense hot pressed silicon
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carbide containing 1% by weight of aluminum had a modulus
rupture of 54,000 psi at room temperature and 70,000 p9i at
1,371 CO However, it was also reported that the density of
hot pressed silicon carbide containing 3 mole percent boron
was s,ubstantially equivalent, i.e. 2.7 grams per cc., to
~ilicon carbide obtainable without impurities or additives.
,. ...
In the Canadian application of Prochazka entitled
a HOT PRESSED SILICON CARBIDE, Serial No.l9~, 95 ~ filed A ~
there is described an improved method of
making a dense silicon carbide ceramic by forming a
homogeneous dispersion of a submicron powder of silicon
carbide and a boron containing additive, e.g. elemental
boron or boron carbide, and hot pressing the dispersion at a
temperature of about 1900-2000 C. and at a pressure of about
5,000-10,000 psi for a sufficient time to produce a dense
nonporous silicon carbide ceramic. The advantage of boron
as a sintering aid, in comparison to other materials such as
alumina, aluminum nitride and other metallic compounds, is
that boron provides increased oxidation and corrosion
resistance at elevated temperature. This may be explained
by the fact that at high temperatures boron oxidizes to
B2O3, which iq volatile and evaporates, leaving the surface
o~ the silicon carbide body coated with pure silica which
provides protection to further oxidation. Aluminum additions,
on the other hand form alumina on oxidation which alloys th~
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protective silica coating on the silicon carbide surface,
decreases its viscosity and thus degrades the resistance to
oxidation. It is essential that the powder dispersion used
in the process described in the copending application be a
mixture of submicron sized particles in order to obtain the
high densities and strengths upon hot pressing. These fine
powders, however, tend to retain large amounts of oxygen
which will vary to some extent depending on the technique of
preparation. Thus, for instance, wet ball milled, commercial
abrasive grade silicon carbide having an average particle
size of about 1.5 microns contains about 3.5% oxygen, where-
as a silicon carbide powder obtained by the reduction of
silica gel by carbon typically contains up to 5.0% oxygen.
Thi9 oxygen is present as silica which is strongly bonded to
the silicon carbide and, in most cases, cannot be desorbed.
On hot pressing, some of the silica is incorporated into the
product and forms a second phase distributed along the edges
of the silicon carbide grains. The amount of oxygen
introduced into the material depends essentially on the
partial pressure of carbon monoxide and si~icon monoxide
within the powder during heating up to the hot pressing
temperature. When the pressure i9 high, the silica will be
stable and is ~ound as a glassy phase in the product. At
low gas pressures, on the other hand, the silica will react
with silicon carbide to form metallic silicon. Both the
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silica and silicon secondary phases affect high temperature
mechanical properties, especially creep resistance and
static fatigue. It is likely that small amounts of these
phases present in hot pressed boron doped silicon carbide
limit the high temperature properties of the material.
Very fine silicon carbide powders have been used to
allow densification at the lowest possible temperature and to
refine microstructures in order to increase mechanical
properties. However with submicron silicon carbide powders,
the development of the microstructure on hot pressing is
difficult to control and often exaggerated grain growth
degrades the mechanical properties of the hot pressed
material. This exaggerated grain growth is accompanied by a
crystallogrsphic transformation in silicon carbide at high
temperatures from the ~-SiC cubic starting material to the
hexagonal ~-6H-SiC The hexagonal silicon carbide tends to
grow into large tabular grains which become strength limit-
ing.
In accordance with the present invention, we have
discovered a method of making a dense silicon carbide ceramic
body by forming a substantially homogeneous dispersion of a
submicron powder of silicon carbide, a boron-containing
additive and a carbonaceous additive. The dispersion is then
hot pressed in an inert atmosphere at a temperature of about
1900-2000 C. at a pressure of about 5,000-10,000 psi ~or a
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sufficient time to produce a highly dense ceramic body. The
product obtained has a density of at least about 98% of the
theoretical density, contains no glassy phase, and has a
fine-grain uniform microstructure without the presence of
exaggerated grain growth. It is suitable as an engineering
material such as, for example, in high temperature gas
turbine applications
The invention is more clearly understood from the
following description taken in conjunction with the
accompanying drawing in which:
FIG. 1 is a photomicrograph of hot pressed silicon
carbide doped with boron and containing carbon prepared ac-
cording to the process of the present invention;
FIG. 2 i8 a photomicrograph of hot pressed silicon
carbide doped with boron and containing no carbonaceous
additive; and
FIG. 3 is a photomicrograph of the hot pressed
boron doped silicon carbide of Fig. 2 after being sub~ected
to sel~ctive etching with hydrofluoric acid.
It is essential that the powder dispersion is a
mixture of submicron particle sized powders in order to
obtain the high densities and strengths upon hot pressing.
These may be obtained by different techniques as, for
instance, by direct synthesis of a powder mixture of elements
or by carbon reduction of silica.
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A process for preparing silicon carbide powder
with excellent sintering properties is disclosed by Prener
in U.S. patent 3,085,863 entitled METHOD OF MAKING SILICON
CARBIDE. The patent teaches a process of making pure sili-
con carbide which includes the steps of forming a silica gelin sugar solution, dehydrating the gel to decompose the
sugar and to form a homogeneous, intimate, finely divided
mixture of silica and carbon, and heating the mixture in an
inert atmosphere to form silicon carbide. We have found that
it is preferable to modify this procedure by substituting
ethylsilicate for the silicon tetrachloride to eliminate the
inco~venience of vast amounts of hydrochloric acid relea~ed
on hydrolysi 8 .
The boron containing additive may be in the form
of a submlcron sized powder and further may be either as
elemental boron or boron carbide. Alternatively, the boron
may be added directly to the silica gel in the form of a
boron compound, such as boric acid during the preparation of
the silicon carbide powder. In order to obtain densifi-
cation, the amount of boron containing additive is critical,the amount of the additive being equivalent to about 0.3-3.0%
by weight of elemental boron. Experiments on pressure
sinterin8 of silicon carbide with the boron containing
addition indicate that there is a lower limit of efficiency
below which there is essentially no effect. This critical
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concentration appears to be equivalent to between 0.3-0.4%
by weight of boron. When this concentration is exceeded,
~ull densification is obtained in submicron powders at 1950
C. and a pressure of 5-10 K psi. A further increase in boron
concentration does not bring about enhancement of densifi-
cation, and, when the amount is equivalent to more than 3.0%
by weight of boron, exaggerated grain growth occurs together
with los~ of ~trength and oxidation resistance. The optimum
amount i8 about equivalent to one part by weight boron per
100 parts of silicon carbide.
This behavior i~ probably related to the ~olubility
limit of boron in silicon carbide which has to be exceeded in
order to get segregation of boron at grain boundaries and
the resulting effect. However, as there are limitations to
the degree of dispersion of boron in the silicon carbide
powder which can be achieved, it is advantageous to slightly
exceed the lower limit of effectiveness of boron, This
brings about safe densification throughout the compact and
eliminates islands of lower densification which may form
with low concentrations and incomplete mixings. Thus, for
the most part, an amount equivalent to 1% by weight of boron
is the minimal addition when elemental boron powder is
mechanically mixed with silicon carbide powders. On the
other hand, when boron is introduced as a solution of a boron
compound into the silica gel during preparation of silicon
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carbide powders, the most desirable dispersion is achieved
and an addition of only an amount equivalent to 0.4% by
weight of boron gives satisfactory results.
The critical part of this invention involves the
incorporation of a carbonaceous additive into the submicron
powder dispersion to suppress the exaggerated grain growth
in the microstructure. Previously, exaggerated grain growth
had been a critical problem because large grains are strength
limiting. Exaggerated grain growth of large tabular crystals
is linked to the ~-~ silicon carbide transformation and
proceed~ by a mechanlsm involving small amounts of oxygen
and a silicon metal phsse. Carbon additions suppress this
anomalous grain growth probably by the removal of residual
oxygen, In order to obtain the beneficial effect, there must
be a certain amount of carbon present in the homogeneous
powder dispersion after pyrolysis, Some of the carbon is,
of course, lost on pyrolysis and this may vary depending on
the heating rate and various other factors. However, the
carbon content of the powder dispersion after pyrolysis
should be about in the range of 0.5-3.0% by weight. More
than 3% by weight leads to poor microstructure, coarsening
of the grains snd inclusions of carbon which are expected to
cause a decrease in strength properties, whereas less than
0 5~/O by weight has no significsnt effect. The preferred
amount is such that the amount of carbon present after
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pyrolysis is about 1% by weight.
Since we are concerned with the carbon material
after pyrolysis, there are certain general funrtional
criteria which may be used to describe the characteris~ics
of the carbonaceous additive Firstly, compounds which
readily crystallize from solutions, such as sugar from an
aqueous solution, will tend to precipitate as crystals dur-
ing evaporation of the solvent. Such crystals turn into
relatively large carbon part~cles on pyrolysis and form un-
desirable inclusion~ in the microstructure of the finalproduct. Hence, compounds which do not crystallize from
solution are preferred. Secondly, compounds derived from
aliphatic hydrocarbons give low yields of carbon which more-
over varies with the rate of heating, so that no exact
control may be exercised over the carbon addition. The low
yield is therefore another serious limitation. For instance,
acrylic resins which yield about l~/o carbon on pyrolysis are
not effective.
High molecular weight aromatic compounds are the
preferred material for making the carbon addition since
these give high yield of carbon on pyrolysis and do not
crystallize Thu9, for instance, a phenol-formaldehyde
condensate-novolak which is soluble in acetone or higher
alcohols, such as butyl alcohol, may be used as well as many
of the related condensation products, such as resorcinol-
formaldehyde, aniline-formaldehyde, cresol-formaldehyde, etc.
_g_
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Similar compounds yield about 40-60% of carbon. Another
satisfactory group of compounds are derivatives of poly-
nuclear aromatic hydrocarbons contained in coal tar, such as
dibenzanthracene, chrysene, etc. A preferred group of
carbonaceous additives are polymers of aromatic hydrocarbons
such as polyphenylene or polymethylphenylene which are
~oluble in aromatic hydrocarbons and yield up to 90% of
carbon. However, the addition of elemental carbon directly
to the silicon carbide powder is impractical, since it is
very difficult to obtain the required degree of distribution
and, frequently, large amounts of carbon inclusions are
found after the hot pressing. Such inhomogeneities have, of
course, a detrimental effect on strength because they
initiate fractures and may entirely override the beneficial
effect of the carbon additions.
An excellent way to introduce carbon into the sub-
micron silicon carbide powders is by adding a solution of
the carbonaceous substance which is decomposçd to carbon on
successive heat treatment. In making the carbon addition,
the first step is to prepare a solution of the selected
carbonaceous compound in a convenient solvent preferably
having a moderately high melting point in case freeze drying
is to be used, The powder is then dispersed in the desired
amount of solution containing the necessary amount of the
organic compound. The volume of the solvent required is an
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amount sufficient to yield a thin slurry when the silicon
carbide powder is fully dispersed. The solvent is then
evaporated either directly from the liquid dispersion or by
freeze drying the dispersion and subliming off the solvent
in vacuum. This latter procedure has the advantage, that it
prevents inhomogeneities in the distribution of the additive
which is always introduced on drying in the liquid state due
to the migration of the solute. In this way, a uniform coat-
ing of the organic substance on the silicon carbide crystal-
lites is obtained which yields the desired degree of carbondistribution.
Another approach to improved carbon distribution
on a submicron particle size level is the application of ~et
milling. The silicon carbide powder is soaked with a
solution of, for instance, a novolak resin in acetone, dried
in air and heated up to 500 C. in nitrogen to pyrolyze the
resin. The actusl amount of carbon introduced by this
process i~ determined as weight grain after the pyrolysis or
by analysis of free carbon. The powder with the added carbon
is then ~et milled which greatly improves the distribution
of carbon and eliminates ma~or carbon grain9 in the hot
pressed product.
There are several critical parameters during hot
pressing which control the densification and microstructure
of the final product. The most important of the~e are
pressure, temperature, and time. while they will be
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discussed individually, it is readily apparent that these
conditions are interdependent~
The pressure range useful for full densification
is between about S,000 to 10,000 psi. For general uses and
for larger articles, the pressure is li~ited by available die
materials and design. Thus for solid graphite dies the
upper limit is about 5,000 psi and for graphite fiber-
wound dies the upper limit is about 10,000 psi. It is
advantageous to use a pressure close to the upper limit of
10,000 psi because the application of high pressure makes it
possible to keep the temperature low enough to control the
grain growth. Low pressures, below 5,000 psi, require the
use of h~gher sintering temperatures or longer pressing time
which will induce exaggerated grain growth.
r~,
lS The first indication of densification on heating
up is obtained at or slightly above 1,600 C. which may be
detected by the motion of the press ram. However, for all
practical purposes high density cannot be obtained below
1,900 C~ This limitation is imposed by the applicable pres-
sure. At 10,000 psi and 1,900C C~, a 96% relative density
i9 obtained in 10 minutes and a 98% relative density is ob-
tained in 30 minutes. By increasing the temperature to
1,950 C. and maintaining the pressure at 10,000 psi for 30
minutes, a density 3.20 g./cc. is reached corresponding to
99.6% of the theoretical. This pressing, when investigated
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metallographically, is pore-free. The microstructure ob-
tained on these latter pressing conditions is uniform and
fine-grained composed of about 3 microns sized grains. At
2,000 C. and 10,000 psi for 10 minutes the density obtained
is still high, near theoretical, but the grain morphology
changes. The grains become more elongated and isolated
large tabular cry~tals form in the fine grain matrix. This
exaggerated grain growth is more pronounced as the pressing
time i8 extended or as the temperature is further increased.
For instance, at 2,100 C. and 10,000 psi, tabular crystals
as long as 1 mm. grow in 10 minutes and a 180 large pores
develop and consequently the density drops to about 3.15
g./cc~ (98% of theoretical density). Thus, at the applied
pressure, there is a fairly narrow temperature region from
about 1,900 to 2,000 C., preferably near 1,950, at which
full densification along with a uniform microstructure can
be obtained. The dramatic exaggerated grain growth may be
attributed to the transformation of ~-SiC into the a-6H-SiC~
The time dependence in SiC densification, as in
other ~intering phenomena, is less significant in that press-
ing time cannot satisfactorily compensate for either decreased
temperature or pressure~ For instance, sintering at 1,900
and lO,000 psi yields 96% density in 10 minutes, and 98% in
30 minutes, while a total hold of 100 minutes brings only a
marginal improvement, The sintering is usually complete
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within a time range of 10-60 minutes
During hot pressing an atmosphere must be used
which is inert to the silicon carbide. Thus, oxidizing
atmospheres such as air cannot be used since they would tend
to oxidize the silicon carbide to silica, interfere with
sintering and degrade the high temperature properties~ Use-
ful inert atmospheres include argon, helium, and nitro~en.
As revealed by X-ray diffraction, electron diffrac-
tion and metallography, the fine grained SiC is composed of
~-SiC and a varying small amount of a-6H-SiC. Also present
are minute carbon grains resulting from limited distribution
of the carbonaceou~ additive as shown in Fig. l~ No
separate boron containing phase was detected which suggests
that boron formed a solid solution with SiC. One of the
main features of our invention is the preparation of an
essentially pore-free fine grained silicon carbide ceramic
with outstanding mechaniçal and thermal properties which are
essential for high temperature gas turbine application. The
density of the product is at least 98% of the theoretical
density.
My invention is further illustrated by the follow-
ing examples:
EXAMPLE I
A solution of 123 g. of sucrose and 1.09 g. of
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H3BO3 in 90 cc. of water was added to 100 g. of tetraethyl-
silicate and the liquids were heated for 13 hours at 80 C.
in a covered containér. After this time, the hydrolysis was
completed and the liquid turned into a soft gel. Alcohol
was first allowed to evaporate and then the temperature was
increased to 120 C. to drive off the excess water. Finally,
the ~ucrose was decomposed by heating the mass to 300 C.
for Z hours. The resulting powder was fired for 2 hours in
a carbon crucible at 1750 C. in argon. Excess carbon was
eliminated by refiring the powder at 700 C, in air and
SiO2 formed during this procedure was leached out using a 5%
solution of hydrofluoric acid~
An analysis of the yellow product showed in ppm:
300 Fe
50 Al
1200 2
8000 B
X-ray diffraction
identified ~-SiC
The specific surface area was 12 m2/g~ giving ulti-
mate crystalline size 0.2 u. This powder was hot-pressed to
densities greater than 99% theoretical at 1950 C. and
10,000 psi, without additional processing,
EXAMPLE II
In accordance with the procedure of Example I, a
submicron SiC powder doped with 0.4% boron was prepared ana
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contained 0.35% oxygen and less than 0.2% total metallic
impurities. The powder was dry ball-milled with 1% aluminum
stearate addition for five hours and after that, dispersed
in a 1% xolution of polymethylphenylene in toluene.
The solvent was then evaporated at room temperature
and the powder charged in a l-inch bore graphite die and hot-
pressed at 1950 C., 10,000 psi, for 30 minutes. The press-
ing had a density of 3.20 g./cc.; i.e., 99.5% of the
theoretical and a fine-grained uniform microstructure as
~hown in Fig. 1 (500X). It was noted that exaggerated
grain growth of a-6H silicon carbide was surpressed.
The same powder, when hot-pressed without carbon
addition, yielded densities varying between 3.00 to 3.18 and
its microstructure as represented by Fig. 2 (lOOX) showed
pronounced exaggerated grain growth; i.e., large tabular
grains, up to several hundred microns, in a fine-grained
matrix.
A sample prepared by hot pressing the boron doped
silicon carbide powder without carbon addition at a tempera-
ture of 1950 C., 10 K psi for 30 minutes, had a density of
3.15 g./cc. Selective etching of polished sections with a
5% HF solution indicated the presence of silica. A deta~led
examination of the photomicrograph shown in Fig. 3 (30,000X)
reveals that SiO2 is distributed along SiC grain edges but
does not form a continuous grain boundary phase.
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High-temperature mechanical properties were evalu-
ated with the following results:
l~/o Carbon as
No Carbon Poiymethyl
Addition Phenylene
Be~d Strength 44,000 psi 64,000 psi
(3 point bending at 1600 C.)
Creep rate at 1600 C. per sec.1.3xlO 8 1 5xlO 9
(bending)
Time to Fracture at 1600 C.:
40,000 psi 3 minutes>3000 minutes
50,000 p~i - >3000 minutes
60,000 p8i - 12 minutes
EXAMP~E III
Following the procedure of Example I, a submicron
SiC powder doped with 0.4% boron was prepared
Sixty g. of the powder were thçn dispersed in 80 cc.
of a 1% solution of phenol-formaldehyde novolac resin in
acetone, the solvent was evaporated in air and the powder was
calcined in a metal boat at 500 C. in flowing nitrogen
atmosphere. An analysis of free csrbon revealed the presence
of 0.71% Next, the powder was 3et milled to break up the
agglamerates of SiC and to improve the dispersion of carbon.
The resulting particle size was -2u, with about 90% of the
2$ product ln the submicron region.
The processed powder was then prepressed at 4,000
p9i into a disc, placed in a graphite die 2 inches in diameter
and hot-pressed at 1 torr of ambient pressure of argon at a
temperature of 1950 C. and a pressure of 10 K psi for 30
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minutes. The density was 3.19 g~/cc. or 99.3% of the
theoretical and an x-ray analysis gave lines of only ~-SiC.
The microstructure of the obtained material was
investigated on a thermally etched section. It revealed
slightly elongated grains with an aspect ratio of about 2,
average grain size was 3.5 microns. The largest detectable
grains wère about 15 microns, The modulus of rupture
measured in three point bending at room temperature was
71,000 psi and showed an increase with temperature.
In a second run, the powder was hot-pressed without
carbon addition under identical conditionsO The density
achieved was 97% of the theoretical and the microstructure
showed large tabular grains of a-6H-SiC having a length of
up to several hundred microns. The fraction of ~-SiC trans-
formed to ~-SiC was 20%~ The modulus of rupture at room
temperature was 39,000 psi and was clearly limited by
fracture initiation at the large grains intersecting the
tensile surface.
Flexural Strength of Hot Pressed SiC
( 3 point bending)
No carbon addition 0.7% carbon
Room temp. 39,000 71,900
1300 C. 38,700 80,000
1400 C. 37,500 81,900
1500 C~ 36,600 86,100
These results clearly indicate the improvement in
flexural strength of the hot pressed boron doped SiC in-
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corporating the carbonaceous additive as compared to a
similar sample to which there had been no carbon addition~
It will be appreciated that the invention is not
limited to the specific details shown in the examples and
illustrations and that various modifications may be made
within the ordinary ~kill in the art without departing from
the spirit and ~cope of the invention,
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