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| United States Patent |
5,334,426
|
|
Smith
|
August 2, 1994
|
High temperature ceramic composite
Abstract
This invention provides a shaped ceramic composite article comprising
ceramic oxide fiber(s), a first coating comprising a carbonaceous matrix
which includes boron nitride particles in contact therewith, and a second
coating comprising silicon carbide. The inventive composite article is
useful in applications requiring good heat resistance and mechanical
properties, such as gas fired radiant burner tubes, gas burner nozzle
liners, heat exchangers, thermowells, core busters or flame dispersers,
and other gas fired furnace components.
| Inventors:
|
Smith; Robert G. (St. Paul, MN)
|
| Assignee:
|
Minnesota Mining and Manufacturing Company (St. Paul, MN)
|
| Appl. No.:
|
747647 |
| Filed:
|
August 20, 1991 |
| Intern'l Class: |
B32B 018/00 |
| Field of Search: |
501/95,96,99
428/902,283,288,289,688,668,704,34.4,34.5,34.6,36.1,36.3,668,408,323,698
|
References Cited
U.S. Patent Documents
| 3565683 | Feb., 1971 | Morelock | 117/215.
|
| 3672936 | Jun., 1972 | Ehrenreich | 117/46.
|
| 4605588 | Aug., 1986 | Simpson et al. | 428/288.
|
| 4642271 | Feb., 1987 | Rice | 428/698.
|
| 4650775 | Mar., 1987 | Hill | 501/95.
|
| 4735850 | Apr., 1988 | Eitman | 428/283.
|
| 4751205 | Jun., 1988 | Hill | 501/95.
|
| 4752503 | Jun., 1988 | Thebault | 427/248.
|
| 4766013 | Aug., 1988 | Warren | 427/228.
|
| 4894286 | Jan., 1990 | Gray | 428/446.
|
| 4961990 | Oct., 1990 | Yamada et al. | 428/240.
|
| 4970095 | Nov., 1990 | Bolt et al. | 427/226.
|
| 4981822 | Jan., 1991 | Singh et al. | 501/95.
|
| 5017522 | May., 1991 | Hegedus | 501/95.
|
| 5071600 | Dec., 1991 | Deleeuw | 501/95.
|
| Foreign Patent Documents |
| 0495570 | Jul., 1992 | EP.
| |
Other References
"Metal Oxide Fibers Create Forms for Silicon Carbide Deposition", Design
News, Sep. 7, 1987, by Cahners Publishing Company.
"SICONEX.TM. Fiber-Reinforced Ceramic", Data Sheet, New Products
Department, 3M Industrial and Consumer Sector, 3M, Aug. 1, 1991.
|
Primary Examiner: Robinson; Ellis P.
Assistant Examiner: Speer; Timothy M.
Attorney, Agent or Firm: Griswold; Gary L., Kirn; Walter N., Allen; Gregory D.
Claims
What is claimed is:
1. A shaped composite article comprising ceramic oxide fiber, said ceramic
oxide fiber having in said composite a surface which is available for
coating, a first coating at least partially covering said surface of said
ceramic oxide fiber available for coating to provide a surface which is
available for overcoating, and a second coating at least partially
covering that portion of said surface which is available for overcoating,
wherein said first coating comprises a carbonaceous matrix which includes
boron nitride particles in contact therewith, and said second coating
comprises silicon carbide, with the proviso that said second coating
covers at least a portion of said first coating.
2. The composite article according to claim 1 wherein said first coating
covers at least about 1 percent of said surface available for coating and
said second coating covers at least about 50 percent of said surface area
available for overcoating.
3. The composite article according to claim 1 wherein said ceramic oxide
fiber is present in the range from about 20 to about 50 percent by weight,
said carbonaceous matrix is present in the range from about 0.2 to about
20 percent by weight, said boron nitride is present in the range from
about 0.2 to about 15 percent by weight, and said silicon carbide is
present in the range from about 50 to about 75 percent by weight, based on
the total weight of said composite article.
4. The composite article according to claim 1 wherein said boron nitride
particles have a particle size in the range from about 0.5 to about 30
micrometers.
5. The composite article according to claim 1 wherein a fracture surface of
said composite article is brushy.
6. The composite article according to claim 1 wherein said ceramic oxide
fiber is at least one of woven, braided, knitted, or wound.
7. The composite article according to claim 1 wherein said ceramic oxide
fiber has a diameter in the range from about 1 to about 50 micrometers.
8. The composite article according to claim 1 which is a furnace element.
9. The composite article according to claim 1 which is a radiant heat
burner tube.
10. The composite article according to claim 1 wherein said ceramic oxide
fiber is alumina fiber.
11. The composite article according to claim 1 wherein said ceramic oxide
fiber is aluminosilicate fiber.
12. The composite article according to claim 1 wherein said ceramic oxide
fiber is aluminoborosilicate fiber.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a shaped ceramic composite article comprising
ceramic oxide fiber(s), a first coating comprising a carbonaceous matrix
which includes boron nitride particles in contact therewith, and a second
coating comprising silicon carbide. In another aspect, the present
invention provides a method of making the same.
2. Description of the Related Art
Radiant burner tubes are used in high temperature, corrosive environments
such as that found in industrial heat treating furnaces and aluminum
melting furnaces. The three most common types of commercially available
radiant burner tubes are metal alloy (e.g., nickel-based superalloy)
tubes, ceramic (e.g., silicon carbide) monolith tubes, and ceramic
composite (e.g., ceramic fibers and ceramic cloth in a ceramic matrix)
tubes. The upper use temperature of such tubes is typically in the range
from about 900.degree. (1650.degree. F.) to about 1260.degree. C.
(2300.degree. F.).
Although monolithic silicon carbide radiant burner tubes with an upper use
temperature up to about 1260.degree. C. are available, such tubes are
typically very brittle and prone to fail, a common problem of
conventional, shaped ceramic monoliths.
While it is possible to select a ceramic composite from which to prepare a
radiant burner tube which meets most, but not necessarily all, of the
requirements for use in high temperature, chemically corrosive
environments, it is only possible by taking great care in the selection
and by making some compromises in properties.
A commercially available ceramic composite radiant burner tube is marketed,
for example, under the trade designation "SICONEX" by the Minnesota Mining
and Manufacturing Company (3M) of St. Paul, Minn. "SICONEX" radiant burner
tubes are a ceramic-ceramic composite comprised of aluminoborosilicate
ceramic fibers, a carbonaceous layer, and a silicon carbide layer.
"SICONEX" radiant burner tubes are prepared by braiding, weaving, or
filament winding aluminoborosilicate ceramic fibers in the shape of a
tube, or alternatively, fashioning aluminoborosilicate ceramic cloth into
a tube shape. The ceramic fiber tube shape is treated with a phenolic
resin which is cured, producing a rigidified article. The rigidified
article is heated in an evacuated chamber such that the cured phenolic
resin is carbonized. The article is then coated with silicon carbide via
chemical vapor deposition at temperatures ranging from about 900.degree.
to about 1200.degree. C. to provide a semi-permeable, chemically resistant
coating of silicon carbide. The resultant rigid ceramic composite tube is
useful at high temperatures in corrosive environments.
The upper use temperature of "SICONEX" radiant burner tubes under typical
operating conditions is about 1260.degree. C. Above about 1260.degree. C.
such tubes typically exhibit properties characteristic of ceramic
monoliths (i.e., brittleness). There is a long-standing need to improve
the upper temperature limit and the mechanical characteristics of such a
composite.
While there have been many approaches to improving mechanical
characteristics of ceramic composites, such efforts have rarely been
coupled with a significant improvement in the high temperature performance
of the composite.
For example, U.S. Pat. No. 3,672,936 (Ehrenreich) discloses a reinforced
carbon article which comprises a carbon fiber shape bonded by a carbon
binder and having incorporated within the article the in situ reaction
product of carbon and a boron-containing additive which comprises a
material selected from the group consisting of boron, boron nitride, boron
silicide, and refractory metal borides. The reinforced carbon article is
made by forming a carbon fiber shape, dispersing the boron-containing
additive within at least a portion of the carbon fiber shape, impregnating
the carbon fiber shape with a carbonizable binder, and heating the shaped
article to carbonize the binder and to form in situ the reaction product
of carbon and the boron-containing additive.
U.S. Pat. No. 3,565,683 (Morelock) teaches a method of depositing a
boro-carbon coating onto filaments, wherein an electrically heated surface
of a pyrolytic carbon coated fused silica or quartz filament is passed
through a liquid, thermally decomposable boron compound such as boron
trichloride dissolved in a non-polar organic solvent such as benzene. The
heated portion of the filament produces an envelope of solvent vapor and
boron trichloride gas which are pyrolytically decomposed and carbon and
boron are simultaneously deposited on the fiber.
U.S. Pat. No. 4,605,588 (Simpson et al.) discloses a process for creating a
substantially uniform boron nitride barrier coating on the surface of
oxide-based ceramic fibers, wherein an oxide-based ceramic fiber
containing boron is heated for about 5-90 minutes in a nitriding
atmosphere of ammonia, hydrogen and nitrogen at a temperature of between
about 2200.degree.-2600.degree. F. to diffuse boron from the fiber to the
surface or slightly within the fiber where it reacts to form the boron
nitride coating.
U.S. Pat. No. 4,642,271 (Rice) discloses a ceramic fiber composite material
comprised of boron nitride coated ceramic fibers (e.g., SiC fibers,
Al.sub.2 O.sub.3 fibers, and graphite fibers) embedded in a ceramic matrix
(e.g., SiC, ZrO.sub.2, 96% SiO.sub.2 with 4% B.sub.2 O.sub.3, mullite,
cordierite, and carbon).
U.S. Pat. No. 4,650,775 (Hill) describes a thermally-bonded fibrous product
composed of a sintered blend of aluminosilicate fibers, silica powder, and
boron nitride powder.
U.S. Pat. No. 4,751,205 (Hill et al.) teaches a thermally-bonded fibrous
product composed of a sintered blend of ceramic fibers, low-grade silica
material, and boron nitride.
U.S. Pat. No. 4,752,503 (Thebault) discloses a thin, refractory,
intermediate adhesive layer of laminar structure (e.g., pyrocarbon or
boron nitride) deposited in an oriented fashion by chemical vapor
deposition onto reinforcing fibers, wherein the intermediate layer has a
greater elongation at break than the matrix and has a thickness of between
0.2 and 3 micrometers.
U.S. Pat. No. 4,766,013 (Warren) describes a fibrous ceramic matrix
composite article said to be useful in corrosive environments. The
composite article comprises a porous carbon fibrous substrate or other
suitable high temperature fibrous substrate which may include a pyrolytic
carbon or appropriate chemical vapor deposited sheath formed about each
fiber of the substrate; a chemically vapor deposited metallic carbide,
oxide, boride or nitride coating over the coated fibers of the substrate;
and an impermeable metallic carbide, oxide, boride, or nitride outer
protective layer formed about the entire periphery of the coated
substrate.
U.S. Pat. No. 4,970,095 (Bolt et al.) teaches an improved method for
depositing boron nitride coatings on ceramic fibers.
U.S. Pat. No. 4,981,822 (Singh et al.) discloses a composite article
produced by depositing a slurry of infiltration-promoting material and
organic binding material on a layer of boron nitride-coated fibrous
material forming a tape therewith on drying, firing the tape to burn out
organic binding material, and infiltrating the resulting porous body with
a molten solution of boron and silicon. Patentees state that in carrying
out the inventive process, the boron nitride is to be coated on the
fibrous material to produce a coating thereon which leaves no significant
portion, and preferably none, of the fibrous material exposed.
SUMMARY OF THE INVENTION
The present invention provides a shaped composite article comprising
ceramic oxide fiber(s), the ceramic oxide fiber(s) having in the composite
a surface which is available for coating, a first coating at least
partially covering the available surface of the ceramic oxide fiber(s) to
provide a surface which is available for overcoating, and a second coating
at least partially covering that portion of the surface which is available
for overcoating, wherein the first coating comprises a carbonaceous matrix
which includes boron nitride particles (i.e., boron nitride regions or
islands) in contact therewith (preferably at least partially embedded
therein), and the second coating comprises silicon carbide, with the
proviso that the second coating covers at least a portion of the first
coating. The boron nitride particles are preferably encapsulated by the
carbonaceous matrix or by the carbonaceous matrix and silicon carbide.
The term "carbonaceous" as used herein means a carbon matrix or coating
wherein substantially all of the carbon is amorphous. The carbonaceous
matrix in regard to an individual fiber typically has a thickness in the
range from greater than zero to about 1 micrometer. Preferably, the
carbonaceous matrix has a thickness in the range from greater than zero to
about 50 nanometers. The minimum thickness of the carbonaceous matrix is
that which is needed to provide a rigidified article suitable from the
process described herein to make a composite article of the present
invention. While matrix thicknesses greater than about 1 micrometer are
useful, there is no significant improvement when the thickness exceeds
about 1 micrometer, thus, such alternatives are not economical.
The term "carbonaceous matrix which includes boron nitride particles in
contact therewith" as used herein refers to a carbonaceous matrix having
boron nitride particles dispersed with the carbonaceous matrix, boron
nitride particles at least partially embedded in the carbonaceous matrix,
or boron nitride particles otherwise attached or adhered to a surface of
the carbonaceous matrix.
A certain portion of the exposed surface area of the ceramic fiber(s)
within the shaped composite article is available for coating. The term
"available surface for coating" refers to that portion of the exposed
surface area of the ceramic fiber(s) available for coating. For example,
the surface area of a fiber(s) which would be unavailable for coating
includes that which, due to braiding, weaving, knitting, or winding of the
fiber(s), is in contact with itself or with another fiber(s).
A certain portion of the exposed surface area of the ceramic oxide fiber(s)
having the first coating thereon within the shaped composite article is
available for overcoating. The term "available surface for overcoating"
refers to that portion of the exposed surface of the ceramic oxide
fiber(s) having the first coating thereon available for overcoating (i.e.,
the sum of the exposed surface area of the first coating available for
overcoating and the remaining exposed surface area of the ceramic oxide
fiber(s) which was available for coating, but was not covered by the first
coating).
Generally, the first coating covers at least about 1 percent of the surface
available for coating and the second coating covers at least about 50
percent of the surface available for overcoating.
Preferably, the first coating covers at least 90 percent of the surface
available for coating and the second coating covers at least about 90
percent of the surface available for overcoating. Most preferably, the
first coating covers about 100 percent of the surface available for
coating and the second coating covers about 100 percent of the surface
available for overcoating.
Preferably, the ceramic oxide fiber(s) are present in the range from about
20 to about 50 percent by weight, the carbonaceous matrix is present in
the range from about 0.2 to about 20 percent by weight, the boron nitride
is present in the range from about 0.2 to about 15 percent by weight, and
the silicon carbide is present in the range from about 50 to about 75
percent by weight, based on the total weight of the composite article.
More preferably, the ceramic oxide fiber(s) are present in the range from
about 25 to about 35 percent by weight, the carbonaceous matrix is present
in the range from about 0.5 to about 6 percent by weight, the boron
nitride is present in the range from about 0.75 to about 6 percent by
weight, and the silicon carbide is present in the range from about 60 to
about 75 percent by weight, based on the total weight of the composite
article.
The composite article of the invention preferably comprises a plurality of
ceramic oxide fiber(s) such as, for example, a yarn comprising a plurality
of individual ceramic oxide fibers.
Preferably, the ceramic oxide fibers are selected from the group consisting
of alumina fibers, aluminosilicate fibers, and aluminoborosilicate fibers.
The most preferred fibers are aluminoborosilicate fibers.
A preferred method of making a composite article according to the present
invention comprises the steps of:
(a) providing a shaped, rigidified article comprising ceramic oxide
fiber(s), the ceramic oxide fiber(s) having in the shaped, rigidified
article a surface which is available for coating, a coating of cured
organic resin which includes boron nitride particles in contact therewith,
wherein the coating covers at least a portion of the surface of the
ceramic fiber(s) available for coating, and wherein the organic resin is
capable of being carbonized;
(b) carbonizing the cured organic resin to provide a first coating at least
partially covering the surface of the ceramic oxide fiber(s) available for
coating to provide a surface which is available for overcoating, the first
coating comprising a carbonaceous matrix which includes boron nitride
particles in contact therewith; and
(c) depositing a second coating comprising silicon carbide onto at least a
portion of the surface available for overcoating, with the proviso that
the second coating covers at least a portion of the first coating,
to provide the composite article of the invention.
The composite article of the invention can be any of a variety of shapes
including, for example, a hollow tube, sheets, cones, and complex shapes.
The term "complex shape" as used herein refers to a variety of shapes in
which the ceramic oxide fiber can be formed, and processed according to
the method described herein to make the composite article of the
invention.
Particularly useful embodiments of the present invention include gas-fired
radiant heat burner tubes, gas burner nozzle liners, heat exchangers,
thermowells, core busters or flame dispersers, and other furnace elements
(including other gas fired furnace components or elements).
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a perspective of a core buster or flame disperser in accordance
with the present invention.
FIG. 2 is a perspective of a stepped burner liner in accordance with the
present invention.
FIG. 3 is a perspective of a type of burner liner in accordance with the
present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The invention provides a shaped, rigid, ceramic article which exhibits good
toughness and high temperature resistance, wherein high temperature
resistance means minimal degradation of the mechanical properties at a
temperature of about 1000.degree. C. Typically, a preferred composite
article according to the present invention is capable of use up to a
temperature of about 1500.degree. C., and is generally semi-permeable to
gas (e.g., air). Preferably, the inventive composite article exhibits good
composite properties.
Preferably, the ceramic oxide fiber(s) comprising the inventive composite
article include at least one of alumina fiber(s), aluminosilicate
fiber(s), and aluminoborosilicate fiber(s).
Methods for making alumina fibers are known in the art and include, for
example, the method disclosed in U.S. Pat. No. 4,954,462 (Wood et al.),
the disclosure of which is incorporated herein by reference.
Suitable aluminosilicate fibers are described in U.S. Pat. No. 4,047,965
(Karst et al.), the disclosure of which is incorporated herein by
reference. Preferably, the aluminosilicate fibers have an alumina content
in the range from about 67 to 77 percent by weight and a silica content in
the range from about 33 to about 23 percent by weight, based on the total
oxide content of the fiber.
U.S. Pat. No. 3,795,524 (Sowman), the disclosure of which is incorporated
herein by reference, teaches a method for making aluminoborosilicate
fibers. Suitable aluminoborosilicate fibers preferably have an alumina to
boria mole ratio in the range from about 9:2 to about 3:1.5, and a silica
content in the range from greater than zero to about 65 percent by weight,
based on the total oxide content of the fiber.
Preferably, the ceramic oxide fibers are polycrystalline or amorphous and
polycrystalline.
The ceramic oxide fibers preferably have a diameter in the range from about
1 to about 50 micrometers. More preferably, the diameter of the fibers is
in the range from about 10 to about 25 micrometers. The cross-section of
the fibers may be circular or elliptical.
Typically, individual ceramic oxide fibers are grouped together to form a
yarn. Generally, the ceramic oxide yarn has a diameter in the range from
about 0.2 mm to about 1.5 mm. Yarn diameters in the specified ranges
typically have superior textile qualities as compared to yarns with
diameters outside these ranges. Such yarns typically comprise in the range
from about 780 to about 7800 individual ceramic oxide fibers. Preferably,
the yarn comprises in the range from about 1560 to about 4680 individual
fibers.
Preferably, the ceramic oxide yarn is ply-twisted because such a
construction has better strength than a yarn which is not ply-twisted.
Suitable alumina yarns are commercially available and include those
marketed by the Minnesota Mining and Manufacturing Company (3M) of St.
Paul, Minn., under the trade designation "NEXTEL 610 CERAMIC FIBER."
Commercially available aluminoborosilicate yarns include those marketed
under the trade designations "NEXTEL 312 CERAMIC FIBER" and "NEXTEL 440
CERAMIC FIBER" from 3M.
The ceramic oxide yarn can be formed into a desired shape using
conventional techniques known in the art including, for example, braiding,
knitting, or weaving the yarn into the desired shape (e.g., a hollow
tube); braiding or weaving the yarn into a cloth or fabric and forming the
cloth into a desired shape (e.g., a hollow tube or a flat sheet); or
winding the yarn around a mandrel (e.g., helically winding or
cross-winding the yarn around a mandrel).
More complex shapes can be made by forming the fiber(s) into the desired
shape using conventional shaping techniques. Complex shapes can be formed,
for example, by stitching ceramic oxide cloth together with ceramic oxide
fiber or yarn. Although the ceramic oxide cloth may be stitched together
before or after the (cured or uncured) organic resin is applied to the
ceramic oxide cloth, it is preferable to stitch the cloth together before
the organic resin is applied. Examples of complex shaped articles in
accordance with the present invention are illustrated in FIGS. 1-3.
The organic resin can be any suitable organic-based resin which is
compatible with the method described herein for making the article of the
invention and which is capable of being carbonized. Preferably, the
organic resin which is coated onto the ceramic oxide fiber(s) is a
phenolic resin, wherein "phenolic resin" is a term that describes a wide
variety of resin products which result from the reaction product of
phenols with aldehydes. Phenolic resins include, for example, acid
catalyzed phenolic resins and base catalyzed phenolic resins. Phenolic
resins are commercially available, for example, under the trade
designations "DURITE-SC-1008" from Borden Chemical of Columbus, Ohio, and
"BKUA-2370-UCAR" (a water-based phenolic resin solution) from Union
Carbide of Danbury, Conn.
The organic resin can be coated onto the fibers using conventional coating
techniques including brush coating, pour coating (i.e., pour the resin
onto the fibers and allow the excess resin to drain off), dip coating,
roll coating, spray coating, etc.
In order to more easily coat the fibers with the organic resin, the
viscosity of the resin is usually lowered by adding a compatible organic
solvent such as acetone or methanol to the resin, or by adding water to a
water-based phenolic resin solution.
Boron nitride particles can be incorporated into the inventive composite
article during one or more steps in the process, for example, (1) boron
nitride particles can be dispersed in the organic resin before the resin
is coated onto the ceramic oxide fiber(s); (2) boron nitride particles can
be applied to the organic resin prior to curing (e.g., boron nitride
particles can be applied to coated organic resin before the organic resin
has dried); (3) a dispersion of boron nitride particles can be coated onto
the dried resin; (4) a dispersion of boron nitride particles can be
applied to the cured resin; (5) a dispersion of boron nitride particles
can be applied to a ceramic oxide fiber, yarn, or cloth before the organic
resin is applied; or (6) boron nitride particles can be incorporated into
ceramic oxide fabric, for example, by applying dry boron nitride particles
to the ceramic oxide fabric or by rubbing boron nitride particles into the
ceramic oxide fabric, before the organic resin is applied.
It is also within the scope of the present invention to apply organic resin
or organic resin having boron nitride particles dispersed therein to a
ceramic oxide fiber or yarn before the fiber or yarn is braided, knitted,
woven, or wound.
The boron nitride particles typically have a particle size in the range
from about 0.5 to about 30 micrometers. Preferably, the boron nitride
particles have an average particle size of less than about 1 micrometer.
Boron nitride particles are commercially available, for example, under the
trade designation "CERAC B-1084, BORON NITRIDE POWDER" from Cerac of
Milwaukee, Wis.
For a phenolic resin or phenolic resin/organic solvent blend having boron
nitride particles dispersed therein, the preferred amount of boron nitride
is in the range from greater than zero to about 20 percent by weight,
based on weight of the phenolic resin or phenolic resin/organic solvent
blend. More preferably, the boron nitride content of a phenolic resin or
organic resin/organic solvent having boron nitride particles dispersed
therein is in the range from about 4 to about 16 percent by weight, based
on the weight of the phenolic resin or phenolic resin/organic solvent
blend, and, most preferably, it is in the range from about 4 to 12 percent
by weight.
The organic resin is preferably dried (i.e., solvents, liquid vehicles, and
other volatile constituents are removed) prior to curing. The organic
resin can be dried using drying techniques known in the art including air
drying, heating, etc.
The organic resin can be cured using conventional curing techniques
including heating.
Boron nitride particles can be added to the dry or cured organic resin by
providing boron nitride particles and a means for attaching the particles
to the dried or cured organic resin. A preferred method of adding boron
nitride particles to the dried or cured organic resins is to disperse the
boron nitride particles in a liquid vehicle (e.g., acetone or petroleum
naphtha, also known as "odorless mineral spirits," commercially available
from Union Chemical/Division of Union Oil Co. of California, Rolling
Meadows, Ill.; or from Phillips Petroleum Company of Borger, Tex., under
the trade designation "SOLTROL 130"), and then coat (e.g., dip coating,
brush coating, spray coating, etc.) the dried or cured resin coated
article with the dispersion. Because the boron nitride particles have a
tendency to settle, the dispersion is preferably continually agitated
during coating. To further aid in dispersing the boron nitride in the
liquid vehicle, conventional ultrasonic dispersion techniques can be used.
The cured organic resin is carbonized using conventional techniques
including heating the rigidified article in a furnace chamber at a
temperature, for a time, and in an atmosphere sufficient to carbonize the
cured organic resin. Heating can be, for example, by resistive heating or
induction heating. An appropriate carbonizing atmosphere is a
non-oxidizing atmosphere. Such an atmosphere can be provided, for example,
by evacuating the furnace chamber, by flowing a non-oxidizing gas (e.g., a
reducing gas, such as H.sub.2 ; a neutral gas, such as N.sub.2 ; or a
combination thereof) through a partially evacuated furnace chamber, or by
blowing a non-oxidizing gas through an unevacuated (i.e., a furnace
atmosphere at atmospheric pressure or at a pressure in excess of
atmospheric pressure) furnace chamber.
Typically, the cured resin is carbonized by heating it under a pressure in
the range from about 5 to about 200 torr (preferably, in the range from
about 5 to about 100 torr) at a temperature in the range from about
200.degree. to about 1000.degree. C. (preferably, in the range from about
250.degree. to 500.degree. C.) for about 10 minutes to about 2 hours.
The preferred rate at which rigidified article is heated is that which
minimizes the processing time yet allows reaction and removal of volatile
constitutes from the cured organic resin at a rate sufficient to minimize
or to prevent spalling, cracking, etc., of the resulting carbonaceous
matrix.
Preferably, the rigidified article is heated according to the following
schedule:
room temperature to about 250.degree. C. at about 5.degree. to about
35.degree. C./minute (more preferably at about 5.degree. to about
15.degree. C./minute);
250.degree. to about 450.degree. C. at about 5.degree. to about 15.degree.
C./minute (more preferably at about 5 to about 10.degree. C./minute ); and
450.degree. to about 1000.degree. C. at about 5.degree. to about 35.degree.
C./minute (more preferably at about 20.degree. to about 35.degree.
C./minute).
The preferred gas flow rate of a non-oxidizing gas is dependent on the size
of the furnace chamber. For example, the preferred gas flow rate for a
91.4 cm (3 feet) long, 7.6 cm (3 inch) diameter quartz tube is in the
range from about 1.5 to about 10 liters per minute.
The carbonized resin comprising the boron nitride particles can be
overcoated with silicon carbide, for example, by chemical vapor
deposition. Such coating methods are known in the art and include, for
example, the method disclosed in U.S. Pat. No. 4,980,202 (Brennen et al.),
the disclosure of which is incorporated herein by reference.
Suitable commercially available silicon carbide precursors include, for
example, dimethyldichlorosilane also known as "DDS," and
methyltrichlorosilane also known as "MTS."
Typically, the rigidified, shaped article comprising ceramic oxide
fiber(s), cured organic resin, and boron nitride particles is placed in a
chemical vapor deposition chamber (e.g., a quartz chamber), which is then
evacuated. While flowing a non-oxiding gas through the evacuated chamber,
the furnace is heated (e.g., resistively or inductively) to the desired
carbonization temperature. Silicon carbide is then coated over the at
least partially carbonized organic resin by introducing a silicon carbide
precursor (e.g., DDS or MTS) into the chamber. Typically, the silicon
carbide precursor is introduced in the chamber by bubbling a non-oxidizing
gas through a suitable liquid silicon carbide precursor (such as DDS or
MTS, which are highly volatile), or by independently introducing a gaseous
silicon carbide precursor (such as DDS or MTS) into the chamber through a
separate gas line. Typically, the chamber is evacuated to a pressure in
the range from about 5 to about 50 torr. The preferred flow rates of the
silicon carbide precursor and non-oxidizing gas are dependent on the size
of the furnace chamber. For example, the preferred flow rates for a 91.4
cm (3 feet long), 7.6 cm (3 inch) diameter quartz tube are in the range
from about 0.15 to about 20 liters per minute for the non-oxidizing gas
and from about 0.15 to about 20 liters per minute for the silicon carbide
precursor.
The time and temperature typically required to provide a composite article
comprising in the range from about 50 to about 75 percent by weight
silicon carbide is in the range from about 4 to about 30 hours, depending
on the size of the article and from about 900.degree. to about
1000.degree. C., respectively. A composite article according to the
present invention comprising about 50 percent by weight silicon carbide
typically has better strength and toughness than does a composite article
according to the present invention comprising less than about 50 percent
by weight silicon carbide. Although the strength of the composite article
according to the present invention improves with increasing amounts of
silicon carbide, such increase in strength relative to the increased
processing cost is generally not economically justified.
Typically, the composite article of the invention exhibits "composite"
fracture properties rather than "monolithic" fracture properties. Ceramic
composites comprising fibers generally have fibers sticking out from the
fracture surface (i.e., exhibiting what is termed "fiber pullout"). The
fracture surface of such a composite article having such fiber pullout is
described as being "brushy." A brushy fracture surface is characteristic
of a composite article having ceramic oxide fibers which have not fused
together or fused to the matrix material. By contrast, a ceramic composite
having fibers which fuse together or to the matrix is characteristic of a
monolith. Fracture properties of a composite article having composite
properties are characterized by producing on fracturing a fracture surface
which is populated with the fracture ends of numerous ceramic fibers in a
brush-like array. A monolith will, however, fracture catastrophically like
a glass plate.
A preferred composite article according to the present invention typically
has good composite properties and high temperature resistance which make
them useful in high temperature applications (e.g., 1500.degree. C.).
Useful embodiments of the articles of the invention include radiant burner
tubes and furnace elements, including gas fired radiant burner tubes, gas
burner nozzle liners, heat exchangers, thermowells, core busters, or flame
dispersers, and other furnace components or elements (e.g., other gas
fired furnace components or elements).
Objects and advantages of this invention are further illustrated by the
following examples, but the particular materials and amounts thereof
recited in these examples, as well as other conditions and details, should
not be construed to unduly limit this invention. All parts and percentages
are by weight unless otherwise indicated.
EXAMPLE 1
Example 1 illustrates the preparation of coupons (i.e., small sheets) of a
composite article according to the present invention.
A woven ceramic cloth of aluminoborosilicate fibers having a boria content
of about 2 percent (BF-22 weave; commercially available under the trade
designation "NEXTEL 440 CERAMIC FIBER" from 3M) was heat-cleaned in a
furnace at a temperature of about 550.degree. C. for about 30 minutes in
air. The furnace temperature was uniformly ramped from room temperature
(i.e., about 21.degree. C.) to about 550.degree. C. over a period of about
1 hour. The heat-cleaned cloth was cut into 30.5 cm by 30.5 cm squares.
Each cloth square to be coated with a carbonaceous precursor was laid onto
a 1.6 mm (1/16 inch) thick aluminum sheet which had been covered with a
smooth layer of aluminum foil.
A carbonaceous precursor was prepared by adding about 4 parts by volume of
phenolic resin (percent solids of the phenolic resin was about 60 to 64
percent; density of the phenolic resin was about 1 g/cm.sup.3 ;
commercially available under the trade designation "DURITE SC-1008" from
Borden Chemical of Columbus, Ohio) to about 95 parts by volume of reagent
grade acetone (density of the acetone was about 0.8 g/cm.sup.3) and
stirring the resin/acetone for about 1 to 2 minutes. Several of the
ceramic cloth squares were saturated with the carbonaceous precursor. The
carbonaceous precursor was poured onto individual cloth squares in an
amount sufficient to saturate the cloth. After the cloth was saturated
with the carbonaceous precursor, any excess carbonaceous precursor was
drained off by tilting the aluminum foil covered aluminum sheet on which
the cloth was laid. About 190 grams of the carbonaceous precursor (i.e.,
phenolic resin/acetone) remained in a cloth after it was drained. Each
aluminum foil covered aluminum sheet and cloth were placed in an exhaust
hood to allow the solvent in the carbonaceous precursor to evaporate under
ambient conditions. The saturated cloth was considered dry when the
solvent odor was no longer observed (i.e., no longer sensed by smell).
About 184 grams of solvent evaporated.
A second aluminum sheet with an aluminum foil covering was superimposed on
the dried cloth with the edge of the aluminum sheet in alignment with the
juxtaposed sheets clamped at several points around the edge. The
carbonaceous precursor was cured by heating the sandwiched aluminum
sheets, aluminum foil, and saturated cloth in a 200.degree. C. preheated
oven for about 20 minutes. After heating, the assembly was removed from
the oven and allowed to cool to room temperature. The cloth was then
removed from the sandwich assembly and the above-described coating process
was repeated until the cured resin provided an add-on weight of about 4
weight percent of the cloth. A fresh sheet of aluminum foil was used for
each coating process.
The cloth rigidified with 4 weight percent resin was cut into 7.6
cm.times.1.3 cm pieces or coupons.
About 5 grams of boron nitride powder (hexagonal crystal form) having an
average particle size of less than about 1 micrometer (density of the
boron nitride was about 2.25 g/cm.sup.3 ; commercially available under the
trade designation "CERAC B-1084" from Cerac of Milwaukee, Wis.) was
dispersed in about 29 grams petroleum naphtha (commercially available from
Union Chemical) division of Union Oil Co. of California, Rolling Meadows,
Ill.) and about 29 grams of acetone. While stirring the boron nitride
dispersion, each rigid, 7.6 cm by 1.3 cm coupon was dipped into the
dispersion for several seconds. Each dip coated coupon was allowed to
drain and then dry under ambient conditions. When dry, the boron nitride
powder formed a thin whitish coating over the surface of each coupon.
Each dried coupon was mounted in a wire fixture and loaded into a
conventional quartz chemical vapor deposition furnace tube. The 91.4 cm (3
foot) long quartz tube was about 7.6 cm (3 inches) in diameter. Hydrogen
gas was passed through the quartz tube while the furnace was resistively
heated to about 1000.degree. C., during which time at least a portion of
the cured resin carbonized. The heating schedule was as follows:
______________________________________
room temperature (about 25.degree. C.)
about 18 minutes
to about 250.degree. C.
250.degree. C to 450.degree. C.
about 20 minutes
450.degree. C to 1000.degree. C.
about 45 minutes
______________________________________
At about 1000.degree. temperature, the flow of hydrogen gas was replaced
with a flow of hydrogen gas which had been bubbled through
dimethyldichlorosilane (i.e., a silicon carbide precursor). Byproducts and
unreacted gases exited at the end of the tube opposite that into which the
precursor was introduced. The exit gas flowed through the vacuum pumping
system and then through a scrubbing system. The pressure within the quartz
tube during the reaction of the silicon carbide precursor was in the range
from about 2 to about 15 torr. The flow rate of the precursor gas provided
about 0.15 liter per minute of dimethyldichlorosilane gas and about 1
liter per minute of hydrogen gas.
The reaction time was about 4 hours. The average silicon carbide content of
the resulting composite articles was about 65.3 percent. The results are
summarized in Table 1, below.
Control A was prepared as described above for Example 1 except boron
nitride particles were not incorporated in the construction of the
composite article (i.e., the dip coating step was skipped). Control B was
prepared as just described for Control A except the woven ceramic cloth
used was that marketed by 3M under the trade designation "NEXTEL 312
CERAMIC FIBER."
The mechanical strength of each sample, including Control A and B, was
measured using a conventional 4-point mechanical flexure test. The
specific test procedures used are described in ASTM D-790-86, which is a
standard test method for flexure properties (of insulating materials). An
average of 5 tests of each of Example 1, Control A, and Control B are
given above in Table 1, below.
An examination of the fracture surfaces of samples tested using the 4-point
mechanical flexure test using a conventional optical microscope at about
50.times. revealed that Example 1 and Control Example B had composite
fracture properties characterized by a "brushy" fracture surface. In
contrast, Controls A and B did not exhibit a brushy fracture surface.
EXAMPLE 2
This example illustrates that boron nitride particles can be incorporated
into the composite article by adding boron nitride powder to the organic
resin before it is coated.
Example 2 was prepared and tested as described in Example 1, except the
boron nitride particles were added to the phenolic resin before the resin
was coated onto the ceramic cloth.
The phenolic resin having boron nitride powder dispersed therein was
prepared as follows. About 4 ml of phenolic resin (DURITE SC-1008) was
blended with about 95 ml of reagent grade acetone by stirring the
ingredients for about 1 to 2 minutes. About 6 grams of boron nitride
powder (CERAC B-1084) were added to the phenolic resin/acetone blend. The
ingredients were stirred for about 2 minutes.
Fracture surfaces of the Example 2 samples tested revealed composite
fracture properties characterized by a "brushy" fracture surface.
The results are provided in Table 1, below.
TABLE 1
__________________________________________________________________________
Composite article
Weight % SiC
Flexure
contains boron
SiC deposition
of composite
strength,
Example
Fiber substrate
nitride particles
time, hours
article MPa (psi)
__________________________________________________________________________
1 "NEXTEL 440
Yes 4 65.3 36.1 (5240)
CERAMIC FIBER"
2 "NEXTEL 440
Yes 4 65.3 32.3 (4720)
CERAMIC FIBER"
Control A
"NEXTEL 440
No 4 57.3 22.3
(3240)*
CERAMIC FIBER" 18.6 (2700)*
Control B
"NEXTEL 312
No 4 65.7 38.6 (5600)
CERAMIC FIBER"
__________________________________________________________________________
*Two "Control A" Examples were prepared
EXAMPLE 3
This example illustrates the burst strength of a composite tube according
to the present invention.
A 7.6 meter (25 foot) roll of a 5.1 cm (2 inch) diameter braid sleeving
(style AS-40) made of aluminoborosilicate fibers having a boria content of
about 2 percent (commercially available under the trade designation
"NEXTEL 440 CERAMIC FIBER" from 3M) was heat-cleaned by heating it in an
air atmosphere furnace at a temperature of about 600.degree. C. for about
4 hours (including ramping the temperature of the furnace from room
temperature to about 600.degree. C. at a rate of about 5.degree. C. per
minute).
Portions of the heat-cleaned sleeving were fitted onto 5.1 cm (2 inch)
diameter mandrels. Each fitted sleeving portion was trimmed to about 25.4
cm (10 inches) in length. Each mandrel was supported over a catch tray in
an exhaust hood such that it could be constantly and uniformly rotated
about its long axis.
About 5 parts by volume of phenolic resin (DURITE SC-1008) was added to
about 95 parts by volume of reagent grade acetone. The ingredients were
stirred for about 2 minutes to provide a blend. About 6.8 parts by weight
of boron nitride powder (CERAC B-1084) were added to the phenolic
resin/acetone blend.
Each mounted sleeve was coated with the phenolic resin/acetone/boron
nitride dispersion while rotating the mandrel. The amount of dispersion
coated was sufficient to completely cover the mounted sleeving. Because
the boron nitride powder had a tendency to settle, the dispersion was
continuously agitated while it was poured onto the mounted sleeving.
Each coated sleeving was rotated for about 40 minutes to allow the solvents
present in the dispersion to evaporate, as well as to prevent the
dispersion or components thereof from settling in one location.
The phenolic resin was cured by heating the mounted, coated sleeving for
about 35 minutes in an air atmosphere oven preheated to about 210.degree.
C. After curing, the mounted, coated sleeving was removed from the oven
and allowed to cool to room temperature.
The addition of the cured phenolic resin/acetone/boron nitride dispersion
increased the weight of the sleeving about 13 to 14 percent.
The length of each cured sleeve was trimmed to about 20.3 (8 inches) and
removed from the mandrel. Each cured sleeve was mounted in a conventional
induction heated chemical vapor deposition furnace and processed as
described in Example 1, except the quartz tube was about 61 cm (2 feet) in
length with a diameter of about 20.3 cm (8 inches). The silicon carbide
precursor was methyltrichlorosilane rather than dimethyldichlorosilane,
the pressure within the furnace was about 60 torr, the reaction
temperature was about 1000.degree. C. the flow rate of the precursor
provided about 1.5 liter per minute each of methyltrichlorosilane gas and
hydrogen gas, and the silicon carbide deposition time was about 10 hours.
The average silicon carbide content of the two sleeves prepared was about
64.3 percent.
Control C was prepared as described above for Example 3 except no boron
nitride was added to the phenolic resin/acetone blend.
The burst strength of Example 3 and Control C were measured using an
internal pressurization to failure test. Specifically, a bladder filled
with hydraulic oil was fitted inside a 5.1 cm (2 inch) long section of the
Example 3 tube. A pressure transducer was mounted such that it was capable
of monitoring the internal pressure of the tubular shaped sample. The
pressure of the hydraulic oil filled bladder was increased until the
tubular sample burst. The pressure at which the sample burst is related to
the burst strength of the sample by the following equation,
##EQU1##
wherein P is the pressure at which the tube burst, d is the inner diameter
of the tube, and t is the wall thickness of the tube. The average burst
strength of ten 5.1 cm (2 inch) sections of Example 3 and ten 5.1 cm (2
inch) sections of Control C are given in Table 2, below.
TABLE 2
______________________________________
Weight % SiC
SiC deposition
of composite
Burst strength,
Example time, hours article MPa (psi)
______________________________________
3 10 64.3 58.5 (8490)
Control C
9 66.7 44.6 (6475)
______________________________________
An examination of the fracture surfaces of the burst tubes revealed that
Example 3 exhibited a brushy fracture surface typical of a composite,
whereas Control C exhibited a smooth fracture surface typical of a
monolithic article.
EXAMPLES 4 TO 8
These examples illustrate the effect of the boron nitride content on the
composite properties of a composite article according to the present
invention.
Examples 4, 5, 6, 7, and 8 were each prepared and tested as described for
Example 3 except the amount of boron nitride powder added to the phenolic
resin/acetone blend was about 2, 4, 6.5, 8.5, and 10 percent,
respectively, and two 5.1 cm (2 inch) sections of each sample were tested.
The results are provided in Table 3, below.
TABLE 3
__________________________________________________________________________
Amount of boron
nitride particles Weight % SiC
Burst
added to phenolic resin/
SiC deposition
of composite
Fiber
strength,
Example
acetone blend, wt. %
time, hours
article pullout
MPa (psi)
__________________________________________________________________________
4 2 7 64.5 No 55.4 (7920)
5 4 11.5 64.9 Yes 68.2 (9740)
6 6.5 24 64.5 Yes 55.5 (7935)
7 8.5 24 65.6 Yes 49.0 (7000)
8 10 24 65.8 Yes 52.1 (7445)
__________________________________________________________________________
An examination of the fracture surfaces of each of Examples 4, 5, 6, 7, and
8 revealed that all but Example 4 exhibited a brushy fracture surface.
EXAMPLE 9
A 198 cm (78 inch) section of a 8.3 cm (3.25 inch) diameter braided
sleeving made of aluminoborosilicate fibers having a boria content of
about 2 percent (commercially available under the trade designation
"NEXTEL 440 CERAMIC FIBER" from 3M) was heat-cleaned as in Example 3. The
fibers were in triaxial weave. The heat-cleaned sleeving was mounted onto
a 8.3 cm (3.25 inch diameter) metal mandrel. The mandrel was supported
over a catch tray in an exhaust hood so that it could be constantly and
uniformly rotated about its long axis.
About 65 parts by volume of a phenolic resin (DURITE SC-1008) were added to
about 582 parts by volume of reagent grade acetone. The ingredients were
stirred for about 2 minutes. About 10.4 parts by weight of boron nitride
powder (CERAC B-1084) were added to the phenolic resin/acetone blend.
The mounted sleeve was coated with the phenolic resin/acetone/boron nitride
dispersion while rotating the mandrel. The amount of dispersion coated was
sufficient to completely cover the mounted sleeving. Because the boron
nitride powder had a tendency to settle, the dispersion was continuously
agitated while it was poured onto the mounted sleeving.
The coated sleeving was rotated for about 40 minutes to allow the solvents
present in the dispersion to evaporate, as well as to prevent the
dispersion or components thereof from settling in one location.
The phenolic resin was cured by heating the mounted, coated sleeving for
about 1 hour in an air atmosphere oven preheated to about 177.degree. C.
After curing, the mounted, coated sleeving was removed from the oven and
allowed to cool to room temperature.
The addition of the cured phenolic resin/acetone/boron nitride dispersion
increased the weight of the sleeving about 13.2 percent.
The length of the cured sleeve was trimmed to about 183 cm (72 inches),
removed from the mandrel, and coated with silicon carbide as described in
Example 3 except the quartz tube was about 243.8 cm (96 inches) in length
with a diameter of about 33 cm (13 inches), the pressure within the
furnace was about 20 torr, the flow rate of the precursor provide about 8
liters per minute each of methyltrichlorosilane gas and hydrogen gas, and
the silicon carbide deposition time was about 28 hours. The amount of
silicon carbide deposited increased the weight of the cured sleeve about
200 percent.
An examination of a fracture surface of the Example 9 tube revealed a
brushy fracture surface typical of a composite.
Various modifications and alterations of this invention will become
apparent to those skilled in the art without departing from the scope and
spirit of this invention, and it should be understood that this invention
is not to be unduly limited to the illustrative embodiments set forth
herein.
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