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TECHNICAL FIELD
This invention is concerned with an improved fiber reinforced ceramic
material. The invention is particularly directed to a reaction-bonded
silicon nitride (RBSN) matrix material that is reinforced with fiber.
Because of their lightweight, excellent oxidation resistance,
high-temperature strength, environmental stability, and nonstrategic
nature, silicon-based ceramics are candidate materials for high
performance advanced gas turbine and diesel engines. However, the use of
these materials is severely limited because of their inherent flaw
sensitivity and brittle behavior. The reinforcement of ceramics by high
strength, high modulus, continuous length ceramic fibers should yield
stronger and tougher materials. Glass matrix composites reinforced by
polymer derived silicon carbide fibers have clearly demonstrated the
feasibility of obtaining strong and tough materials. These newly developed
composites, however, are presently limited in temperature capability by
matrix properties, interfacial reactions, and by thermal instability of
the fibers above about 1000.degree. C.
A reaction-bonded silicon nitride (RBSN) material has been suggested for
use in advanced heat engine applications. The RBSN material is
lightweight, has ease of fabrication, and near net shape capability.
However, the use of RBSN material for structural applications is limited
because of the low strength of the material as well as its brittle
behavior.
In the past preforms for SiC fiber reinforced RBSN composites were prepared
by either slip casting or plasma spraying. In the slip casting method, a
slurry comprising silicon powder, fugitive polymer binder, and a
dispersing medium is poured into a mold provided with a cavity filled with
ceramic fibers oriented in the desired directions. The mold is agitated
ultrasonically to remove entrapped molecules between fibers and slurry and
to distribute the fibers in the slurry. The slurry and the fibers are
heated in an oven at a controlled rate to drive off fugitive polymer
binders and the dispersing medium. The resulting preform is further
densified by conventional ceramic processing methods, such as reaction
bonding or sintered reaction bonding.
In the plasma spray method, composite preforms are prepared by spraying
molten silicon onto an array of fibers. These preforms are further treated
in a nitrogen atmosphere at an elevated temperature to convert the silicon
to a silicon nitride matrix.
One of the problems encountered in using these prior art procedures is the
lack of control over the distribution and alignment of the fibers in the
composite. Also the lack of a suitable surface coating on the fibers
caused a fiber/matrix reaction which resulted in poor composite strength
and brittle behavior. The use of high temperature densification methods
additionally produced in intrinsic weakening of the fibers.
It is, therefore, an object of the present invention to provide a strong
and tough SiC/RBSN composite material for use at temperatures up to
1400.degree. C.
Another object of the invention is to provide a method of making such a
composite material by utilizing optimum processing variables and using a
SiC fiber having a suitable surface coating.
BACKGROUND ART
Yajima et al U.S. Pat. No. 4,158,687 discloses composite materials that are
reinforced with SiC fibers which are produced by imbedding the fibers in,
or layering with, a powdery matrix (Si.sub.3 N.sub.4), pressing, and
sintering. Organosilicon binders may be used, but the matrix is not
reaction bonded.
Reaction bonding is disclosed in U.S. Pat. Nos. 4,285,895 to Mangels et al
and 3,819,786 to May. In the Mangels et al patent reaction bonded Si.sub.3
N.sub.4 is densified by heating under nitrogen gas pressure in the
presence of a densification aid which is an oxide of Mg, Y, Ce, or Zr. In
the May patent silicon nitride articles are made from a dough-like mixture
of Si and binder. The articles are heated in nitrogen after hot-milling
and the removal of trichloroethylene from the binder at 130.degree. C.
U.S. Pat. No. 3,926,656 to Mangels is concerned with a multiple mixture
containing silicon powder. An injection molding composition of Si powder,
paraffin wax, zinc stearate, and Fe.sub.2 O.sub.3 is used to form molded
parts which are nitrided to form Si.sub.3 N.sub.4 articles.
U.S. Pat. No. 4,004,937 to Masaaki discloses the use of nickel oxide as a
sintering aide for silicon nitride. The silicon nitride ceramic material
contains at least one of MgO, ZnO, and NiO in addition to at least one of
Al.sub.2 O.sub.3, Cr.sub.2 O.sub.3, Y.sub.2 O.sub.3, TiO.sub.2, and
SnO.sub.2. The mixed powders are formed into a green compact and sintered
in an inert gas.
DISCLOSURE OF INVENTION
A SiC fiber reinforced reaction bonded silicon nitride matrix composite
material (SiC/RBSN) is produced by hot pressing alternate layers of mats
of specially coated SiC fibers and silicon monotapes. The resulting
silicon carbide/silicon preforms are nitrided at elevated temperatures in
N.sub.2 or N.sub.2 /H.sub.2 to form a composite material comprising SiC
fibers and a reaction bonded silicon nitride matrix. This composite
material may be used in advanced engines operating at temperatures above
about 1200.degree. C.
DESCRIPTION OF THE DRAWING
The details of the invention will be described in connection with the
accompanying drawings in which
FIG. 1 is an enlarged cross section view of a chemical vapor deposited
silicon carbide fiber utilized in the composite material of the present
invention,
FIG. 2 is a graph showing the composition profile of the carbon rich
coating on the surface of the silicon carbide fiber shown in FIG. 1,
FIG. 3 is a photomicrograph showing a typical cross section of a SiC/RBSN
composite material produced in accordance with the present invention
showing the fiber distribution, and
FIG. 4 is a graph showing the tensile stress-strain behavior for a 20 vol %
SiC fiber/RBSN composite material at room temperature showing linear and
non-linear ranges.
BEST MODE FOR CARRYING OUT THE INVENTION
The fiber reinforced reaction bonded silicon nitride matrix material of the
present invention utilizes a commercially available silicon powder having
a high purity. To reduce particle size and promote reactivity during
nitridation, the powder is milled for 7 to 24 hours in an attrition mill
utilizing Si.sub.3 N.sub.4 balls, a Si.sub.3 N.sub.4 container, and a
Si.sub.3 N.sub.4 arm. A typical milling charge is 297 grams of silicon
powder, 3 grams of nickel oxide (NiO), and 1000 grams of heptane solvent.
The amount of the nitriding aid, NiO, ranges from one to seven percent by
weight. While NiO is the preferred nitriding aid, it is contemplated that
other materials, such as Fe.sub.3 O.sub.4, MgO, and Al.sub.2 O.sub.3, may
be used for this purpose.
After this milling operation, the silicon powder is dried in an oven to
evaporate the milling medium, which is heptane. The dried silicon powder
is used for preparing silicon monotape.
The chemical analysis, average surface area, and particle size of the
powder before and after attrition milling are shown in table 1. It is
apparent from the table that after the attrition milling there is a
significant increase in the oxygen and carbon contents and essentially no
increase in the iron content. The surface area of the powder increased
from about 1.6 m.sup.2 /g to about 10 m.sup.2 /g while the average
particle size decreased from 6 .mu.m to 0.4 .mu.m.
TABLE I
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Surface
Average
Oxygen
Carbon
Nitrogen
Iron
Area,
Particle
Material wt % wt %
wt % wt %
m.sup.2 /g
Size, .mu.m
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As-Received
0.43 0.025
0.0004
0.60
1.644
6.0
Silicon Powder
Attrition Milled
1.20 0.31
0.07 0.60
10.216
0.4
Silicon Powder
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Silicon carbide fibers are produced by chemical vapor deposition (CVD) from
methyltrichlorosilane onto a heated carbon monofilament which is drawn
continuously through a conventional depositon reactor. Different surface
coatings are deposited onto the silicon carbon fibers by introducing
hydrocarbon gas or a mixture of hydrocarbon gas or silane vapor near the
exit port of the reactor. An enlarged view of the fiber cross section is
shown in FIG. 1.
The fiber consists essentially of a silicon carbide sheath 10 surrounding a
pyrolytic graphite coated carbon core 12. The silicon carbide sheath 10
has an outer diameter of about 142 .mu.m while the graphite coated carbon
core 12 has a diameter of about 37 .mu.m. The silicon carbide sheath 10 is
entirely comprised of columnar B-SiC grains growing in a radial direction
with a preferred {111} orientation.
The fiber has a surface coating 14 comprising an overlayer with a high
silicon/carbon ratio on top of an amorphous carbon layer. The total
thickness of the coating 14 is about 2 .mu.m. The composition profile of
the carbon rich coating 14 is shown in FIG. 2. The average room
temperature tensile strength of the starting fiber is greater than 3.8
GPa.
A silicon carbide fiber mat is prepared by winding the silicon carbide
fiber shown in FIG. 1 on a metal drum at a predetermined spacing. The
fiber is then coated with a polymer slurry by using a paint brush or a
pressure spray gun. This polymer slurry comprises a low glass transition
temperature polymer and a solvent. The fiber mat is dried and cut to the
required dimensions.
The silicon carbide fiber reinforced reaction bonded silicon nitride matrix
composite material of the present invention utilizes both the silicon
carbide fiber mats and silicon monotapes. These silicon monotapes are
produced by mixing the previously milled silicon powder, NiO, a fugitive
polymer binder, such as a polybutylmethacralate or polytetrafluoroethylene
known commercially as Teflon, and a standard solvent. This material is
mixed in a blender for about 15 minutes, and the contents are filtered to
remove excess solvent.
The resulting slurry is poured on a hot skillet to drive off the remaining
solvent. The resulting polymer dough is rolled to a desired thickness to
form the silicon monotape. This tape is then cut to a predetermined size.
Alternate layers of silicon carbide fiber mat and silicon monotape are
stacked in an open channel molybdenum die. These alternate layers are then
hot pressed in a vacuum furnace to form a green compact or preform. The
silicon cloth and molybdenum die parts are separated by graphfoil to
prevent any reaction between the preform and the die. As shown in Table
II, two types of preforms were produced to illustrate the beneficial
technical effects of the invention. One preform contained about 23 volume
fraction of SiC fibers while the other contained about 40 volume percent.
The volume fraction of fibers in each green compact was varied by
controlling the fiber spacing or by adjusting the thickness of the silicon
monotape.
TABLE II
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23 Vol % 40 Vol %
SiC/RBSN SiC/RBSN Unreinforced
Property Composite Composite RBSN
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Density, gm/cc
2.19 2.36 2.53
Matrix Porosity
0.4 0.4 0.2
Elastic Modulus, GPa
165 230 185
4-Point Bend Ultimate
Strength, MPa
As-fabricated 727 868 262
1200.degree. C.
736 -- 400
1400.degree. C.
592 -- 345
Tensile Ultimate
Strength, MPa
As-fabricated 352 536 --
Tensile Strain, %
1st Matrix Cracking
0.13 0.13 --
Ultimate Fracture
0.25 0.3 0.14
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These preforms are hot pressed in two stages. In the first stage the
molybdenum die is heated to about 600.degree. C. in a vacuum furnace at
2.degree. C./min. This material is then maintained at this temperature for
about 1/2 hour to remove the fugitive binder present in the fiber mats and
silicon monotapes.
In the second stage the die temperature is raised to about 1000.degree. C.
and pressed at 35 MPa to 138 MPa for about 15 minutes to about one hour.
The die is cooled to room temperature under vacuum conditions. The
composite preform is then removed from the die. It is contemplated this
hot pressing could be accomplished in another inert environment, such as
nitrogen, instead of using a vacuum.
The composite preform is then transferred to a horizontal nitridation
furnace comprising a recrystallized Al.sub.2 O.sub.3 reaction tube having
stainless steel end caps. A nitriding gas, N.sub.2 or N.sub.2 +4% H.sub.2,
having a commercial purity is flowed through the furnace before, during,
and after nitridation. The nitridation is performed at about 1100.degree.
C. to 1400.degree. C. for between 30 to 70 hours. A cross section of the
resulting component material is shown in FIG. 3.
Physical and mechanical property data for unidirectionally reinforced
SiC/RBSN composite materials fabricated in accordance with present
invention and commercially available unreinforced RBSN matrix material at
room and elevated temperatures are shown in Table II. Comparison of this
data shows that the ultimate fracture strength and strains of the
composites were significantly higher than those of unreinforced RBSN
matrix material. Also, the composite strengths increased with the increase
in fiber volume fraction.
Measurement of room temperature axial tensile strength for 30 volume
percent silicon carbide/RBSN composite material after 100 hour exposure at
1200.degree. C. and 1400.degree. C. was 316 MPa and 323 MPa. These values
are similar to the value 350 MPa measured for as-fabricated composites.
This demonstrated the thermal 30 stability of the composite material.
When the SiC/RBSN composite material is stressed in tension in a direction
parallel to the fiber, the composite extends elastically until the RBSN
matrix fractures. At this stress level, in contrast with unreinforced
RBSN, the composite retains its shape because of fiber bridging of the
matrix cracks. The stress-strain behavior for a typical 20 vol % SiC/RBSN
composite specimen tested in tension at room temperature is shown in FIG.
4. This test exhibited a first linear range 20 separated from a second
linear range 22 by a non-linear range 24.
In addition, because of their high modulus, the SiC fibers bear more load
than the matrix they replace. Thus, the composite stress at which the
matrix fractures is greater than that for an unreinforced matrix. On
further stressing the composite above the first matrix fracture, the
material continues to deform with multiple matrix cracking until the
ultimate fracture strength of the fiber is reached. Therefore, the
composite is stronger than the unreinforced matrix, and it is tougher as
manifestd by a high strain to failure and an ultimate non-catastrophic
fracture which is fiber controlled. Because of the excellent thermal
stability and creep resistance of the CVD SiC fiber, the composite
deformation and fracture behavior is temperature independent to
1200.degree. C.
While the preferred embodiment of the invention is disclosed and described
it will be apparent that various modifications may be made to the
composite material without departing from the spirit of the invention or
the scope of the subjoined claims.
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