Note: Descriptions are shown in the official language in which they were submitted.
107~g~;~
The present invention relates to a method for
producing a fiber composite consisting of continuous silicon
carbide fibers and metallic silicon and more particularly, a
method for producing a three dimensional fiber composite
having a high strength and maintaining excellent properties
of novel continuous silicon carbide fibers having extremely
excellent hardness, mechanical strength, heat resistance,
oxidation resistance and corrosion resistance by using
metallic silicon for said continuous silicon carbide fibers
as a binder.
Heretofore, as refractory materials having a high
strength, use has been made of ceramics sintered articles
having a high melting point, cermets or ceramics composite
materials reinforced with ceramics fibers or whiskers having
a high melting point. However, the ceramics sintered
articles are usually weak in the mechanical shock, particu- `
larly are poor in the mechanical strength at a high tempera-
ture. Furthermore, since the cermets contain a metal as the
matrix, they have a relatively high resistance against the
mechanical shock but when the cermets are used under an
oxidizing atmosphere at a high temperature, the metal is
readily oxidized and softened, so that the life for maintain-
ing the properties at a high temperature is short and the
scope of application is considerably limited.
The ceramics composite materials reinforced by
fibers or whiskers are considerably influenced in the property
or practicability by the reinforcing fibers or whiskers.
Namely, the reinforcing effect due to the fibers or whiskers
is increased by arranging said fibers or whiskers in a certain
direction but when short fibers or whiskers are used, the
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107',~9~9
length is short, so that it is difficult to arrange these
materials in the uniform direction throughout the whole
matrix and the matrix is readily broken at end portion of
the fibers or whiskers owing to the concentration of shearing
stress of the matrix and further the arranging step is
complicated, so that there is a defect that the production
cost becomes high.
Moreover, the length of the whiskers is up to
about 20 mm and the fineness is not constant and the whiskers
having constant properties are difficult to be obtained. In
addition, the production step is fairly complicated, so that
the production cost becomes high.
There is a process for improving the above described
arranging ability and preventing the shear breakage by using
ceramics continuous fibers and as these continuous fibers,
use has been made of the fibers composed of fused quartz,
alumina single crystal or carbon.
However, the fused quartz fibers are poor in
Young's modulus, alumina fibers are high in the production
cost and poor in the thermal shock resistance and carbon
fibers cannot endure use in an oxidizing atmosphere at a
high temperature and therefore the respective fiber is
limited in the use application.
Furthermore, concerning the carbon fibers, a
process wherein carbon fibers are immersed in fused Si at a
temperature lower than 1,500C to concurrently conduct the
conversion of carbon fibers into silicon carbide and the
formation of SiC-Si composite, has been described in "THE
ENGINEER", for Nov. 1974. However, the carbon fibers are
complicated in the production step, so that the production
~(~77969
cost is high, and said fibers include a fairly large amount of pores and
amorphous carbon and consequently the homogeneity in properties and the
uniformity are poor, so that the silicon carbide fibers obtained by immersing
the fibers in the fused metallic silicon are uneven in the fineness and the
strength is locally different. Furthermore, when metallic silicon penetrated
into pores in the above described carbon fibers solidifies and contracts,
the adhesion of silicon in the pores and the newly formed fibrous silicon
carbide becomes weak resulting into lowering of the strength. Accordingly,
the composite material composed of silicon carbide fibers and metallic
silicon, which is formed of carbon fibers and the fused metallic silicon,
is not only high in the cost but also the strength is low and is not uni-
form.
; An object of the present invention is to provide a method for
producing SiC-Si composite material having homogeneity in properties and
high strength in a low production cost which obviates various defects of
the above described SiC-Si composite material produced from the carbon
fibers and metallic silicon.
The method of the present invention will be explained in more
detail.
The present invention can provide SiC-Si composite materials having
a high strength, which are composite materials of continuous fibers having
a high strength, by forming piles of a bundle form, a network form, a rope
form, a layer form and the like from high strength continuous silicon
carbide fibers and filling spaces in the pile with fused silicon to tightly
adhere the fibers and silicon.
; The term "pile" used herein means the above described "bundle
form", "network form", "rope form" and "layer form" and the like of the
continuous silicon carbide fibers.
"High Strength continuous silicon carbide fibers" which can be
advantageously used in the present invention may be produced by the
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production method disclosed in Canadian Patent Application No. 250,637
already filed by the inventors of the present invention. More particularly,
in o:rder to produce such fibers organosilicon low molecular weight compounds
of the following groups (1)-(10) can be used as the starting material.
(1) Compounds having only Si-C bond.
(2) Compounds having Si-H bond in addition to Si-C bond.
(3) Compounds having Si-Ha~ bond.
(4) Compounds having Si-N bond.
(5) Compounds having Si-OR ~R is alkyl or aryl group) bond.
(6) Compounds having Si-OH bond.
~7) Compounds having Si-Si bond.
(8) Compounds having Si-O-Si bond.
~9) Esters of organosilicon compounds.
(10) Peroxides of organosilicon compounds.
From at least one of the organosilicon low molecular weight
compounds belongingtothe above described groups (1)-(10), organosilicon high
molecular weight compounds having silicon and carbon as the main skeleton
components, for example, the compounds having the following molecular
\
~77~6~
structures, are produced by polycondensation reaction using at least one
process of irradiation, heating and addition of a catalyst for the poly-
condensation
(al ~Si.(C)n-Si-O-
~b2 Si-O-CC)n-O-
CC1 -Sl~ (Cln~
(d~ The compounds having the above described skeleton
c~mponents (a)~(c~ as at least one of partial structures in
linear, ring and three dimensional structures or mixtures of
the compounds having the above described skeleton components
` (a~-(c~
From at least one of the organosilicon high molecular weight
compounds containing at least one of the above described molecuiar structures,
if necessary added with or reacted with a small amount of at least one of
oTganic metal compounds, metal complexes and organic polymers other than
the above described two compounds, is prepared a spinning liquid and then
; the spinning liquid can be spun into fibers having various lengths and
uniform fineness. The spun fibers are heated at a low temperature within
a temperature range of 50-400C under an oxidi~ing atmosphere and then
preliminarily heated at a temperature of 600-1,000C under at least one
atmosphere of vacuum, inert gases, C0 gas, hydrocarbon compound gas,
organosilicon compound gas and hydrogen gas to form the preliminarily
~ .
~ ~ -6_
:
~0779G9
heated continuous silicon carbide fibers. Ilowever, the
above described preliminary heating is able to get along
even under the above described atmosphere containing at
least one of an oxidizing gas, a hydrocarbon compound gas
and hydrogen gas in a partial pressure of less than 10 mmHg.
The above described preliminarily heated fibers are baked at
a temperature of 1,000-2,000C under at least one of atmos-
pheres of vacuum, inert gases, CO gas, CO2 gas, hydrocarbon
compound gas, organosilicon compound gas and hydrogen gas to
form continuous silicon carbide fibers.
Various properties of SiC fibers having a fineness
of about 10 ~ obtained by baking at 1,300C under vacuum
according to the above described method of the prior invention
are shown in ~he following Table 1. A bundle of the continuous
SiC fibers is shown in a photograph.
Table 1
Size of crystal grain Average diameter 33
Density 2.5 - 3.1 g/cm3
Hardness 9 (Mohs)
Tensile strength 300 - 500 Kg/mm2
_
Young's modulus (2.0-4.0)X10 4 Kg/mm2
Even if the fibers are kept at
Oxidation resistance 1,300C for 100 hours in air, the
weight variation is not observed.
Even if rapid heating and quenching
Thermal shock cycle of 25C t 1,000C is repeated
resistance more than 1,000 times, the texture
does not vary.
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In the above described silicon carbide fibers obtained by baking
the spun fibers consisting mainly of the organosilicon high molecular weight
compound, free carbon of more than 0.01% usually remains and the remaining
amount varies depending upon various conditions, such as the baking tempera-
ture, baking time and baking atmosphere. This free carbon reacts with the
fused silicon to form SiC at the boundary of SiC fibers and Si. Therefore,
the bonding of SiC fibers to the Si is promoted by the tight adhesion
owing to the local chemical reaction due to the free carbon at the boundary
of SiC fibers and the metallic Si in addition to the adhesion due to the
wettability and the more strong bonding can be attained, so that the above
described free carbon acts a very advantageous function in the formation
of SiC-Si composite material. Furthermore, in SiC fibers produced as above,
the size of the crystal grains is several ten A as shown in the above
described Table 1, so that a number of microscopic unevenness (projection
and recess) on the surface of the fiber becomes very large per unit area
and the fused silicon gets into the uneven portions and the contact area
becomes considerably larger and the adhesion of the fibers and the metallic
silicon becomes very strong due to the wettability and this is a great
characteristic of the present invention.
The merit of the present invention that the spaces of SiC fiber
pile are filled with Si consists in the following points. Si is different
from the other metals and even if silicon in the fused state contacts
with SiC mainly constituting the fibers, the fused silicon does not cause
any reaction by which the properties of SiC fibers are deteriorated and
further has a good wettability and the very tight bonding between Sic
fibers and Si can be obtained owing to the mutual diffusion of Sic and Si.
As a further advantage if fibers are used as produced as described above,
free carbon usually remaining in such fibers reacts with Si to form Sic and
this Sic has the function that the bonding of SiC fibers and Si is greatly
i~creased. Furthermore, silicon itself is relatively small in decrease of
~077969
the toughness and strength even at a high temperature and silicon is
preferable as the metal capable of maintaining the above described pro-
perties of the composite material to a high temperature. As mentioned
above, it has been found that the fibers and the silicon to be used in the
present invention are the most preferable raw materials for constituting
the composite material for production of the fiber composite having a high
strength due to strong adhesion.
Although there are various processes for production of metal-fiber
composite material, it is advantageous to form a composite material from
the fibers to be used in the present invention and silicon through either
of the following four processes.
(1) The fiber pile is merely immersed in the fused Si under
vacuum or an inert gas.
(2) This process is similar to the above process (1) but the
fibers are passed through a vessel containing the fused silicon
and are pulled up or pulled down to form a fiber bundle.
(3) Each fiber or the fiber pile is coated with the fused Si and
then the fibers are subjected to a hot press.
11~7~9~9
(4) The fibers are put in a mold and then the fused silicon is poured
into the mold or the fibeTs and the solid Si are put in a mold and then
the mold is heated to a tempe~ature higher than the melting point of
t:he Si to foTm the composite molding.
In these processes, it is more effective that the fibers and the
fused silicon are integrated under a reduced pressure and then the atmosphere
is changed into a pressed state, whereby the bonding degree is more increased.
By using the above described processes, it is possible to obtain the homo-
geneous and strong fiber composites without forming pores and hollow gaps
between the fibers and the metal.
It is preferable that a content of the silicon in the fiber
composites according to the present invention is 5-35% by weight. When
the content is less than 5%, the spaces in the fiber pile cannot be fully
filled, so that it is impossible to obtain a satisfactory fiber composite
composed of the fibers and the metal and the homogeneity and strength of
the composite material are not satisfied On the other hand, when the
content is more than 35%, the space in the fiber pile becomes too broad
and the influence due to the property of the silicon is larger than that
of the fibers, so that the high strength and heat resistance of the fibers
cannot be maintained.
The following examples are given for the purpose of illustration
of this invention and are not intended as limitations thereof~
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Example 1
An example for producing the continuous silicon
carbide fibers to be used in the present invention will be
explained hereinafter.
Dimethyldichlorosilane and sodium were reacted to
produce dimethylpolysilane. 250 g of dimethylpolysilane was
charged in an autoclave having a capacity of 1 Q and air in
the autoclave was substituted with argon gas and then the
reaction was effected at 470C for 14 hours. After comple-
tion of the reaction, the formed polycarbosilane was discharged
as N-hexane solution. This N-hexane solution was filtrated
to remove impurities and then N-hexane was evaporated under
a reduced pressure, after which the residue was heated in an
oil bath at 280C under vacuum for 2 hours to effect concen-
tration. Polycarbosilane was obtained in an yield of 40
based on dimethyldichlorosilane. A number average molecular
weight of the formed polycarbosilane was 1,700. By using a
usual spinning apparatus, the polycarbosilane was heated and
melted at 330C under argon atmosphere to form a spinning
melt and the spinning melt was spun at a spinning rate of
200 m/min to obtain polycarbosilane fibers. The fibers were
heated by raising the temperature from 20C to 190C in air
in 6 hours and this temperature was kept for l hour to
effect an unfusing treatment. The thus treated fibers were
heated to 1,300C at a temperature raising rate of 100C/hr
under vacuum of lxlo-3 mmHg and this temperature was kept
for 1 hour to form SiC fibers. The formed SiC fibers had an
average diameter of 15 ~, an average tensile strength of
350 Kg/mm2, an average Young's modulus of 23x103 Kg/mm2
and a specific gravity of 2.70 g/cm3.
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From silicon and the silicon carbide fibers produced as described
above, fiber composites were produced by varying the content of the metallic
silicon as shown in the following Table 2~
The fibers produced as described above and having a length of
50 m~ were formed into a bundle and said bundle was s~t in an alumina
crucible C12~xSOL~m3~ This c~ucible was suspended at an uppeT portion of
a heating vessel under vacuum of lx10-3 mnHg. At the lower portion in this
vessel was placed a tank made of alumina for charging the fused metallic
silicon and the tank was heated and the metallic silicon in the tank was
kept at the fused state at 1,500C~ Then the cTucible w~s put down and
immersed in the fused metal in the tank and the immersing was kept for 5
minutes~ Thereafter, 10 atmospheric pressures of argon gas was applied
to the vessel and said pressure was kept for 10 minutes and then the
c~ucible was taken out from the tank. The thus formed fiber composite was
worked into a rod having lO~x40Lmm and said rod was tested with respect
to various properties~ ~:
The content of the silicon in each fiber composite and the
properties of this composite are shown in the following Table 2~
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1077g~9
Table 2
'~ Content of _
\ metallic
\ ilicon 5 10 20 30
. Property \ __ _
Density (g/cm3) 2.7 2.6 2.5 2.4
Hardness (Mohs) 9 8_~ 8_9 8
__ _ _
Tensile strength
~Kg/mm23 320-460 30a-410 270 330 250-300
Young's modulus . _
(Kg/mm2) (10-15)x10 (8-13)x10 (7-ll)x10 (6-10)x10
Oxidation resist- _
ance. Weight
increase in air nearly 0 0-1 1-2 2-3
at 1,000C for
50 hours (%)
As seen from the above Table 2, continuous SiC fiber composites
produced by using silicon are composites having various excellent propeTties.
Example 2
Silicon carbide fibers obtained in Example 1 were formed into a
network having a mesh area of 0.1 0.3 mm2 and an outer size of 30mmX30mmXlmm~
This network was placed on a bottom of dies (female die) for a hot press
made of alumina and then metallic silicon powders were deposited in a
thickness of about 3 mm on the ~etwork and the assembly was heated at
1,450C under vacuum of lx10 4 mmHg~ As soon as the temperature was
raised to 1,450C, the temperature was decreased at a rate of 5C/min,
while applying a pressure of 100 Kg~cm . However, the dies have been
previously designed so that when the fused metallic silicon was pressed
by the male die, the
1~779~9
superfluous fused silicon other than the fused si]icon which fills the
mesh, was overflowed into the space between the male die and the female
die~ A composite sheet in which silicon carbide fibers of the network are
included in the metallic silicon was produced. The content of the fibers
in this sheet was about 80% by weight as the result of analysis~ The
stsength and the other properties of this composite sheet were similar to
those of the corresponding composition shown in Table 2 but it has been
found that this sheet is excellent particularly in elastic modulus for
bending~ Therefore, this sheet can be used as a heat resistant sheet under
a temperature up to 1,350C
Example 3
In such a silicon carbide mold that a pipe having an outer
diameter of 30 mm, an inner diameter of 25 mm and a length of 15 mm can
be obtained, were arranged continuous silicon carbide fibers having a
length of 100-150 mm produced in Example 1 and the mold was placed under
yacuum of lxlO 3 mnHg. Into this mold was poured silicon previously
fused by heating at l,500C. In this manner, a cylindrical silicon
carbide fiber composite molding was obtained. The content of the fibers
in this composite was 70% by weight and the strength and the other pro-
perties of this composite were similar to those of the coTresponding
composition shown in the above described Table 2. Such a molding is
excellent in the elasticity, so that even if the molding has an oval form,
such a lding is not easily broken, so that this molding is advantageously
used as a ~lexible circular pipe for a high temperature.
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Example 4
Silicon previously fused in argon gas was charged in a continuous
casting vessel ~50cmx50c~x50cm~ and about 200 continuous SiC fibers produced
in Example l were passed downwards through said vessel and passed through a
hole provided at the center of the bottom of the vessel to collect the
fibers and wound up at a rate of 1 m/min in such a manner that Si fiber
bundle was twisted~ Before the winder, a cooling coil was provided and
the twisted fiber bundle coated with the fused silicon was passed through
said coil, whereby the superfluous fused silicon was taken away and said
ibeT bundle was cooled and the fused metallic silicon was solidified.
The weight ratio of the fibers in the formed SiC fiber composite
having a rope form was about 90% and the properties of this composite are
equal to those of the corresponding composition in the above Table 2. Such
a fiber composite having a rope form was relatively excellent in the elastic
modulus for bending, so that this composite seems to endure the use under a
stress at a high te~perature.
The above described examples showed a few embodiments having the
typical shapes, which can be obtained as the continuous SiC fiber composite,
~ut in addition to these embodiments, the composites having various shapes
2~ can be obtained depending upon the shape of the fiber piles or the shape
of the mold.
As mentioned above, the continuous silicon carbide fiber composites
having excellent mechanical strength, heat resistance, oxidation resistance
and corro~ion resistance
~15
1077969
can be obtained in a relatively low cost according to the
present invention and it can be expected that these compo-
sites can be very advantageously used in many fields where
use must be made under severe conditions, such as super high
temperature, super high pressure and corrosive atmosphere. -
For example, these composites can be used for various
crucibles, various nozzles, turbine blades, engine con-
stituting materials, wear resistant parts, heat resistant
materials, aircraft materials and the like.
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