Process for preparing textured ceramic composites

A method for the preparation of a fibrous monolithic ceramic which exhibits non-brittle fracture characteristics from green monofilament ceramic fibers having a controlled texture. This method includes the steps of: (a) forming a first ceramic-laden composition includes a thermoplastic polymer and at least about 40 vol. % of a ceramic particulate into a substantially cylindrical core, (b) applying a layer of a second ceramic-laden composition includes a thermoplastic polymer and at least about 40 vol. % of a ceramic particulate which differs from the particulate contained in the first composition onto the core to form a substantially cylindrical feed rod having an average initial diameter, (c) extruding the feed rod to form a green ceramic monofilament fiber which has an average diameter that is less than the average diameter of the feed rod, and (e) arranging the green ceramic monofilament fibers into a desired configuration to provide a green fibrous monolith, wherein, during the extrusion step, each ceramic-laden composition has a viscosity which is approximately equivalent to that of each other ceramic-laden composition, and, if the ceramics present in the extruded ceramic monofilament fiber are sintered, a plane of weakness defined by the interface of the core and the layer is provided, the interface being relatively weaker than the core. The green fibrous monolith may be sintered to provide a fibrous monolith.

TECHNICAL FIELD OF THE INVENTION 
The present invention relates to methods for preparing textured ceramic 
composites, such as fibrous ceramic monoliths, using ceramic green fibers 
as well as to methods for the preparation of such ceramic green fibers 
having specific textures. 
BACKGROUND OF THE PRESENT INVENTION 
Attractive properties can be obtained from ceramic composites having a 
texture in which the distribution of two or more materials are well 
controlled. An example of such textured ceramic composites are fibrous 
monolithic ceramics. Unlike ordinary ceramics which abruptly and 
catastrophically suffer tensile fracture, fibrous monoliths have the 
unique property of non-brittle fracture--they gracefully split and 
delaminate, like, e.g., wood, thereby providing for non-catastrophic 
failure. This property is of great value in many applications, e.g., high 
temperature structural applications such as those encountered by engine 
components as well as a number of other automotive structural 
applications. 
A more detailed description of the structure and properties of such fibrous 
monolithic ceramics is provided in U.S. Pat. No. 4,772,524. This patent 
discloses a fibrous monolithic ceramic body as comprising a plurality of 
compacted, coated, and sintered fibers. These fibers comprise a core of a 
first ceramic composition, and a coating on that core of a different 
ceramic composition. This coating is referred to as a "debond phase," and 
serves as a "plane of weakness." The particular debond phase described in 
the '524 patent is said to be comprised of three ceramics--aluminum 
titanate, zirconia, and halfnia--all of which possess a tendency to 
spontaneously microcrack. By providing a layer of these microcracked 
ceramics, it was found that the desired "plane of weakness" was formed in 
the fiber. 
It is this plane of weakness which provides a fibrous monolith prepared 
using such a fiber, after sintering, with a non-brittle fracture 
characteristic. Specifically, the interface, which defines a plane of 
weakness, will function to deflect a crack in the coating, or "debond 
phase, from normal to the plane of weakness to a direction parallel to the 
plane of weakness. Thus, catastrophic failure of the fibrous monolith 
prepared using such fibers is avoided. 
The '524 patent further discloses a process for preparing such fibrous 
monolithic ceramics. This process comprises coating a fugitive cotton 
thread by passing that thread first through a suspension of the core 
composition, and subsequently through the coating composition, to provide 
a ceramic fiber. These fibers are then arranged together to form the 
desired fibrous monolith. 
Since the issuance of the '524 patent, new varieties of fibrous monolithic 
ceramics have been discovered. See, e.g., S. Baskaran et al., "SiC-Based 
Fibrous Monolithic Ceramics," Ceramic Sci. & Eng. Proc. 14 (9-10) pp. 
813-823; S. Baskaran et al., "Fibrous Monolithic Ceramics, I: Fabrication, 
Microstructure, and Indentation Behavior," J. Am. Cer. Soc'y 76 (9), pp. 
2209-16 (1993); S. Baskaran et al., "Fibrous Monolithic Ceramics, II: 
Flexural Strength and Fracture Behavior of the SiC/Graphite System," J. 
Am. Cer. Soc'y 76 (9) pp. 2217-24 (1993); S. Baskaran et al., "Fibrous 
Monolithic Ceramics, III: Mechanical Properties and Oxidation Behavior of 
the SiC/BN System," J. Am. Cer. Soc'y 77 (5) pp. 1249-55 (1994); S. 
Baskaran et al., "Fibrous Monolithic Ceramics, IV: Mechanical Properties 
and Oxidation Behavior of the Alumina/Ni System," J. Am. Ceramic Soc'y, 
77, (5) pp. 1256-62 (1994); and D. Popovic' et al., "Silicon Nitride and 
Silicon Carbide Fibrous Monolithic Ceramics" 42 Silicon Based Structural 
Ceramics (B. W. Sheldon et al. eds., Am. Cer. Soc'y, Westerville, Ohio, 
1994) pp. 173-86. In these newly discovered ceramic fibrous monoliths, the 
ceramic fibers from which they are prepared establish a plane of weakness 
therein by using a graphite layer or a boron nitride layer. The core 
composition, in contrast, was able to be prepared from a wide variety of 
ceramics including, e.g., silicon carbide, silicon nitride, and alumina. 
In conjunction with or shortly after the discovery of the aforementioned 
new materials, new methods for preparing the ceramic fibers used to 
fabricate fibrous monoliths were also discovered. Specifically, it was 
taught that the core of a green ceramic fiber could be prepared either by 
dry spinning or melt spinning a composition comprising a polymer and 
ceramic powder. To complete the ceramic fiber, it was further taught that 
the coating layer was to be subsequently applied by dipping the core into 
a slurry of the debond phase composition. 
Three U.S. patents have issued which involve the extrusion of a mixture of 
a ceramic powder and a polymer to form a fiber. The first patent, U.S. 
Pat. No. 4,908,340, discloses the extrusion of ceramic green fibers by 
melt spinning a mixture of thermoplastic polymers and ceramic powders. The 
second, U.S. Pat. No. 4,990,490, describes a process for the thermoplastic 
extrusion of green fibers from superconducting ceramics which are 
subsequently coated with metal powders. The third patent, U.S. Pat. No. 
5,041,248 describes the extrusion of green fibers by melt spinning 
polyisobutylene with ceramic powders. This patent further discloses that 
its thermoplastic extrusion process may be used to make sintered ceramic 
bars, rods, tubing, or fibers from ceramic-polymer mixtures. The mixtures 
are described as those in which the polymer acts as a fugitive vehicle, it 
being later removed during the heat treatment required to obtain a 
sintered ceramic product. 
In summary, fibrous monoliths have traditionally been fabricated using 
fibers that were prepared by the laborious process of dip-coating 
previously extruded solid ceramic cores in a coating composition 
comprising ceramics and polymers. While this provides a textured fiber, it 
is slow, inconvenient to set-up and use, difficult to control, and is 
unable to provide a uniformly-textured fiber. 
Thus, there exists a need for a more efficient method for preparing fibrous 
monolithic ceramics which exhibit non-brittle fracture characteristics 
using green ceramic fibers. The exists a further need for a method by 
which the texture of fibers used to prepare such monoliths can be more 
readily controlled. 
It is therefore an object of the present invention to provide a relatively 
efficient method for preparing fibrous monolithic ceramics which exhibit 
non-brittle fracture characteristics from green ceramic fibers. 
Another object of the present invention is to provide a relatively 
efficient method for preparing such green ceramic fibers despite the 
presence of high levels of ceramic particulate loading in any composition 
from which the fibers are prepared. 
A further object of the present invention is to provide a green ceramic 
fiber useful for preparing fibrous monolithic ceramics in which the 
texture of the fiber is precisely controllable within defined parameters. 
Yet another object of the present invention to provide a method for 
increasing the strength of fibrous monolithic ceramics. 
These and other objects and advantages of the present invention, as well as 
additional inventive features, will be apparent from the description of 
the invention provided herein. 
BRIEF SUMMARY OF THE INVENTION 
In one aspect, the present invention provides a method for the preparation 
of a fibrous monolithic ceramic which exhibits non-brittle fracture 
characteristics from green monofilament ceramic fibers having a controlled 
texture. This method comprises: (a) forming a first ceramic-laden 
composition comprising a thermoplastic polymer and at least about 40 vol. 
% of a ceramic particulate into a substantially cylindrical core, (b) 
applying a layer of a second ceramic-laden composition comprising a 
thermoplastic polymer and at least about 40 vol. % of a ceramic 
particulate which differs from the particulate contained in the first 
composition onto the core to form a substantially cylindrical feed rod 
having an average initial diameter, (c) extruding the feed rod to form a 
green ceramic monofilament fiber which has an average diameter that is 
less than the average diameter of the feed rod, and (e) arranging the 
green ceramic monofilament fibers into a desired configuration to provide 
a green fibrous monolith, wherein, during the extrusion step, each 
ceramic-laden composition has a rheology which is approximately equivalent 
to that of each other ceramic-laden composition, and, if the ceramics 
present in the extruded ceramic monofilament fiber are sintered, a plane 
of weakness defined by the interface of the core and the layer is 
provided, the interface being relatively weaker than the core. Of course, 
the green fibrous monolith may be sintered to provide a fibrous monolith. 
Another aspect of the present invention provides a method for the 
preparation of a fibrous monolithic ceramic which also exhibits 
non-brittle fracture characteristics. This method is the same as that 
described in the preceding paragraph, but further comprises extruding at 
least two of the green monofilament ceramic fibers substantially 
simultaneously to form a multifilament green ceramic fiber, and then using 
those multifilament fibers to prepare a green fibrous monolith. It is 
believed that the use of such multifilament fibers will cause the fibrous 
monolith to possess a greater strength as compared to a fibrous monolith 
prepared using monofilament fibers. 
Further aspects of the present invention provide methods for the 
preparation of the mono- and multi-filament fibers used in preparing the 
fibrous monolithic ceramics in which the texture of the fibers may be 
readily controlled. 
The invention may best be understood with reference to the accompanying 
drawings wherein illustrative embodiments are shown and in the following 
detailed description of the preferred embodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
One aspect of the present invention provides a method for preparing fibrous 
monolithic ceramics having non-brittle fracture characteristics using a 
textured ceramic fiber which has been prepared by the simultaneous 
extrusion of a particular ceramic feed rod. The feed rod comprises a core 
prepared from a first ceramic composition and a shell which surrounds that 
core that is prepared from a second, and different, ceramic composition. 
Each of the compositions used to prepare the aforementioned fiber comprise 
a ceramic particulate component and a thermoplastic polymer component, the 
latter acting as a fugitive carrier for the ceramic particulates, i.e., 
the polymer is removed when the fiber, or a monolith prepared therefrom, 
is subjected to high temperatures, such as those required to sinter the 
ceramic particles present in the compositions. 
Specifically, this aspect of the present invention comprises forming a 
first ceramic-laden composition comprising a thermoplastic polymer and at 
least about 40 vol. % of a ceramic particulate into a substantially 
cylindrical core. Subsequently, a layer of a second ceramic-laden 
composition comprising a thermoplastic polymer and at least about 40 vol. 
% of a ceramic particulate which differs from the ceramic particulate 
contained in the first composition is applied onto the core to form a 
substantially cylindrical feed rod. 
Prior to the discovery of the present invention, it was appreciated by 
those skilled in the art that the introduction of such relatively high 
levels of ceramic particulates, e.g., above about 40 vol. %, into 
thermoplastic polymers would make the extrusion of such loaded polymers 
very difficult. One of the reasons for this is the dramatic changes that 
occur in the rheology of the molten polymer mixture. Specifically, such 
mixes were known to possess, at least, significant yield stress and a much 
higher viscosity as compared to the unfilled polymer. Until the discovery 
of the present invention, then, such problems were thought to foreclose 
the ability of one to successfully extrude more than one such composition 
simultaneously, i.e., using two continuous extruders to feed two such 
molten polymer compositions through a complex coextrusion die, and still 
obtain a usable green ceramic fiber therefrom. One of the aspects of the 
present invention is the recognition that a desired geometry in the fiber 
product may be obtained by extruding a "controlled geometry" feed rod, 
i.e., the desired fiber geometry is created in the feed rod itself. This 
method allows a fiber of a particular desired texture to be prepared by 
simple piston extrusion through a simple extrusion die. 
In order to provide a fibrous monolith having non-brittle fracture 
characteristics, the components which comprise the compositions from which 
the feed rod is prepared should be selected so that, if one sinters the 
extruded ceramic fiber, a plane of weakness defined by the interface of 
the core and the layer is provided, wherein the interface is relatively 
weaker than the core. Although specific examples of materials which will 
provide a fiber having such attributes will be provided herein, the 
selection of such materials is well within the means of one skilled in the 
art due to the availability of published materials on the subject of 
fibrous monolithic ceramics. 
In preparing the ceramic-laden compounds used in the inventive methods, the 
fine ceramic powder will typically be blended with a fiber-forming polymer 
and, advantageously, one or more processing aids. Most fiber-forming 
thermoplastic polymers can be used in the compositions of the present 
invention, but preferred polymer systems are the highly flexible polymers 
and copolymers, advantageously ethylene polymers and copolymers, and 
preferably polyethylene, ethylene-ethyl acetate copolymers ("EEA") (e.g., 
DPDA-618NT, Union Carbide) and ethylene-vinyl acetate copolymers ("EVA") 
(e.g., ELVAX 470, E. I. DuPont Co.). 
A wide variety of powder ceramics may also be used in the ceramic-laden 
compositions, affording a wide flexibility in the composition of the 
ultimate textured ceramic composite. Advantageously, powders which may be 
used in the first ceramic-laden composition to provide the core of the 
feed rod include ceramic oxides, ceramic carbides, ceramic nitrides, 
ceramic borides, and silicides. Preferred powders for use in that 
composition include aluminum oxide, barium oxide, beryllium oxide, calcium 
oxide, cobalt oxide, chromium oxide, dysprosium oxide and other rare earth 
oxides, lanthanum oxide, magnesium oxide, manganese oxide, niobium oxide, 
nickel oxide, aluminum phosphate, lead oxide, lead titanate, lead 
zirconate, silicon oxide and silicates, thorium oxide, titanium oxide and 
titanates, uranium oxide, yttrium oxide, yttrium aluminate, zirconium 
oxide and its alloys, boron carbide, iron carbide, halfnium carbide, 
molybdenum carbide, silicon carbide, tantalum carbide, titanium carbide, 
uranium carbide, tungsten carbide, zirconium carbide, ceramic nitrides 
including aluminum nitride, cubic boron nitride, silicon nitride, titanium 
nitride, uranium nitride, yttrium nitride, zirconium nitride, aluminum 
boride, halfnium boride, molybdenum boride, titanium boride, zirconium 
boride, and molybdenum disilicide. 
In regard to the powders suitable for use in the second composition, the 
composition which provides the debond layer, there are advantageously 
included: agents known, from the available literature, to create weak 
interfaces such as fluoromica, tin oxide and lanthanum phosphate; agents 
known, from the available literature, to create porosity in a layer which 
function to create a weak interface; graphite powders and 
graphite-containing powder mixtures; and hexagonal boron nitride powder 
and boron nitride-containing powder mixtures. If a metallic debond phase 
is desired, reducible oxides of metals may be used, e.g., nickel and iron 
oxides, or powders of metals, e.g., nickel, iron, cobalt, or their alloys. 
In regard to ceramic powder size, ultrafine powders have been successfully 
used, e.g., HSY-3.0 zirconia (a specific surface area of 7.0 m.sup.2 /g 
and an average particle size of 1 micrometer, available from Daichi 
Kigenso) and Cabot.RTM. Black Pearl 2000 carbon black (a specific surface 
area of 1500 m.sup.2 /g and an average particle size of 12 nanometers, 
available from Cabot Corporation). However, relatively coarse powders, 
e.g., those having average diameters above about 5 .mu.m and up to about 
10 .mu.m, may also be used successfully in the ceramic-laden compositions, 
e.g., an 80% nickel/20% chromium alloy powder. 
The level of powder loading in each composition should range from at least 
about 40 vol. %, and may advantageously range from about 40 vol. % to 
about 70 vol. %. Preferably, the loading may comprise about 50 vol. %. 
These levels are selected in order to provide good sintering behavior of 
the powder. Additional amounts of processing aids, as described further in 
the following paragraph, may be added to improve the dispersion of the 
powder within the composition, particularly when the powder loading 
exceeds about 60 vol. %. 
As mentioned in the previous paragraph, a processing aid is advantageously 
included in the compositions in order to reduce the viscosity of the 
polymer compositions, aid in the dispersion of the powder in the 
compositions, and act as a lubricant for the compositions during 
extrusion. Many different oils, waxes, stearates, and fatty acids may be 
used, with preferred processing aids including methoxypolyethylene glycol 
having a MW of about 550 (e.g., MPEG 550 or Carbowax.RTM. 550, Union 
Carbide) and mineral oil, such as heavy mineral oil (Mineral Oil White, 
Heavy, Labguard.RTM.) or light mineral oil ((Mineral Oil White, Light, 
Labguard.RTM.) because they do not significantly degrade weaken or 
embrittle the green ceramic fiber. 
It was determined that the compounding or mixing of the powders with the 
polymer and processing aids may be accomplished according to procedures 
known in the art of plastics compounding, despite the fact that the filler 
loading is higher than that experienced in typical plastic compositions. 
As will be appreciated by those skilled in the art, however, different 
powder/polymer/processing aid combinations require slightly different 
compounding techniques for providing the proper dispersion of those 
components in the ceramic-laden composition. 
After the feed rod has been prepared from the appropriate ceramic/polymer 
compositions, it is extruded through an extrusion die to provide the 
desired green ceramic fiber. When the feed rod is the same shape as the 
orifice, as for example when both are round (the feed rod being 
substantially cylindrical), and certain other conditions are met, the flow 
field of the extrudate is such that there is little or no axial 
distortion. Thus, the present invention provides a method by which feed 
rods with a certain axially symmetric pattern or texture on a coarse scale 
can be extruded to form a nearly identical version of that pattern, but on 
a smaller scale. One is able to obtain such small scale axial texture in a 
ceramic fiber by the coextrusion of an axially symmetric feed rod. 
For example, consider the scenario wherein one desires to prepare a 300 
.mu.m diameter green fiber having a 250 .mu.m diameter core of material A, 
with 50 .mu.m thick cladding of material B. To produce this fiber, a 22 mm 
controlled geometry feed rod is prepared by molding material to provide a 
core rod (a solid cylinder) having a diameter of 18.3 mm, and combining 
this rod with a hollow cylindrical shell (e.g., provided by molding two 
half cylindrical shells that, when placed onto the core, will provide a 
complete cylindrical shell) that has been molded using material B. This 
hollow shell has a thickness of 3.7 mm, i.e., the shell has an inner 
diameter of 18.3 mm and an outer diameter of 22 mm. This feed rod is then 
suitably extruded through a 300 .mu.m orifice at the appropriate 
temperature and pressure to provide the desired 300 .mu.m green ceramic 
fiber. 
In contrast to the axial symmetry, however, is the effect extrusion has 
upon the composition in the radial direction. Specifically, in the radial 
direction, the flow field present during extrusion causes distortions 
which result in the preparation of a non-axially symmetric fiber. For 
example, when one provides a feed rod having a sequence of layers of 
different compositions in the axial direction, such layers can be 
dramatically extended by the flow field to produce a pattern which is 
rather like tree rings. It has been recognized, however, that this effect 
can be used in a positive manner to create another distinct type of 
fine-scale texture in radially coextruded ceramic fibers. 
Despite the foregoing, and as mentioned in a previous paragraph, preparing 
a ceramic fiber using a coextrusion method which possesses the same axial 
geometry as a two component feed rod is difficult. The extrusion will, if 
uncontrolled, cause distortion in the geometry of the feed rod as it 
passes through the extrusion die. In order to avoid such unwanted 
distortion, the rheology of the compositions being extruded should be 
substantially identical. Advantageously, and further, the temperature and 
rate at which the extrusion is conducted should also be selected so as to 
minimize the distortion in the geometry of the feed rod. One particular 
aspect of the rheology that should be controlled to retain this geometry 
is the viscosity of each composition. More specifically, each 
ceramic-laden composition should possess a viscosity which is 
approximately equivalent to that of each other ceramic-laden composition. 
Without such matching and careful control, flow instabilities between the 
two compounds will result, yielding a fiber which does not substantially 
replicate the original geometry of the feed rod. 
As with any extrusion process, the reduction ratio is another important 
parameter. Although certain reduction ratios are provided in the Examples 
which follow, they should be considered to be merely illustrative, and not 
limiting. In the particular case of multifilament coextrusion, which will 
be discussed in a subsequent section, it should be recognized that a wide 
variety of spatial scales of the extrudate can be obtained by varying the 
reduction ratios of the first and second extrusion steps. It should 
further be noted that three or more extrusion steps could be completed in 
series to provide a fiber of a very fine diameter. 
Moreover, while the Examples disclose the use of generally cylindrical feed 
rods, the claimed invention is not necessarily limited to that geometry. 
Other shapes could also be co-extruded in the manner of the present 
invention. 
If desired, and after the feed rod has been extruded and a green ceramic 
fiber has been provided in the manner of the present invention, one may 
apply at least one further layer of a ceramic-laden composition comprising 
a thermoplastic polymer and at least about 40 vol. % of a ceramic 
particulate onto the fiber. When this scenario is undertaken, one 
essentially forms another feed rod, a second feed rod. This second feed 
rod may then be extruded to provide a green monofilament ceramic fiber 
having multiple layers of ceramic-laden compositions. This step may, if 
desired, be repeated any number of times. A second, and related, scenario 
provides for a further layer of ceramic-laden composition being layered 
onto an existing layer before the feed rod is extruded, i.e., the core 
material may be surrounded by one, two, or several layers of ceramic 
compositions. In either scenario, however, the ceramic particulate in each 
further layer should differ from that contained in the composition onto 
which the one further layer is applied. 
After one ceramic fiber has been extruded, it may, if desired, be extruded 
with at least one other such fiber of the same or different composition 
and/or texture to provide a multifilament ceramic fiber. This may be 
achieved by molding a number of such fibers to form a new multifilament 
feed rod, and then extruding that newly formed feed rod. This process may 
be repeated any number of times to provide a multifilament fiber having 
very small, i.e., fine, filament diameters. Of course, the temperature and 
feed rate of the multifilament feed rod should be controlled to ensure 
that the geometry of the feed stock is not altered during extrusion. The 
multifilament green ceramic fibers provided by the foregoing process may 
be arranged to provide a green fibrous monolithic ceramic. It is believed 
that, because such fibers contain a great number of fine filaments, the 
monolith prepared from such fibers will possess greater strength that 
monoliths prepared using a monofilament fiber of an equivalent diameter. 
After the coextruded product, either monofilament or multifilament, has 
been prepared, it can further be shaped by known means to produce green 
ceramic articles, such as green fibrous monoliths. Typically, the 
coextruded fiber or fibers will be molded by pressing in an appropriate 
mold at a temperature and pressure which will cause the fibers to form a 
solid, dense body from the individual mono- or multi-filament fibers. Any 
shape which can be compression molded or otherwise formed by plastic 
deformation can be obtained with the coextruded product. The molded 
article thus obtained is a ceramic "green body." The ceramic green body so 
molded has the desired texture created by the arrangement of the 
coextruded fibers. For example, a uniaxially aligned fibrous monolith can 
be obtained by the uniaxial lay-up of the coextruded fibers prior to 
molding, a random felt fibrous monolith can be obtained by molding 
randomly arraigned coextruded fiber, or a woven architecture can be 
obtained by molding a shape from previously woven coextruded green fiber. 
The coextruded product permits a wide variety of composite architectures 
to be fabricated in a molded green body. 
Another important aspect of the coextruded product is that it serves as a 
ceramic green body, and hence can be treated to produce a ceramic article. 
This implies that the polymer and organic processing aids can be removed 
by one of the methods commonly employed in the field of ceramics, without 
damaging the molded article. One such example of polymer removal (or 
"binder burnout") is slow baking the green article to about 500.degree. 
C., with a heating schedule determined by the characteristics of the 
polymer, the powder, and the geometry of the molded article, using 
techniques known in the art of molded ceramics and powder metallurgy. The 
ceramic body formed thereby may then be densified, by sintering or 
pressure sintering, to produce a high quality ceramic article, e.g., a 
fibrous monolith. 
The conditions for sintering or pressure sintering are peculiar to the 
particular material. For a given material system, the densification 
conditions for textured ceramic composite made from coextruded product are 
similar to the conditions for the same material made with ordinary powder 
processing. Thus, such densification conditions can be readily determined 
by one skilled in the ceramic art. 
The following examples further illustrate the present invention but, of 
course, should not be construed as in any way limiting its scope. 
EXAMPLE 1 
This example illustrates the preparation of a silicon nitride/boron nitride 
monofilament fiber in accordance with one aspect of the present invention. 
1. Silicon nitride compound: 
A. Sinterable silicon nitride powder (E-10, UBE Chemical, Tokyo, Japan) 
mixed with sintering aids (9 wt % yttria powder and 3 wt % alumina 
powder): 37.23 g 
B. Ethylene Vinyl Acetate copolymer: 7.65 g 
C. Heavy Mineral oil: 3.21 g 
2. Boron nitride compound: 
A. Boron nitride powder (HCP, Advanced Ceramic Corporation, Cleveland, 
Ohio): 24.75 g 
B. Ethylene Vinyl Acetate copolymer: 10.34 g 
C. Methoxypolyethylene Glycol (MW 550): 0.75 g 
The two ceramic compounds were prepared separately. The mixing was carried 
out in a Brabender Plastograph blender whose mixing bowl was preheated to 
about 120.degree. C.-150.degree. C. The ethylene copolymers and a portion 
of the processing aid were first added to the mixing bowl until they were 
melted and the torque reached a steady value. The dry powders were then 
added incrementally so that they blended thoroughly with the molten 
polymer. The balance of the processing aid was added incrementally with 
the powder. This compounding was continued until the apparent viscosity of 
the compound, as indicated by the torque rheometer function of the 
Brabender Plastograph, reached the desired level. For this example, the 
viscosity of the boron nitride compound was adjusted by the addition of 
processing aid in an amount such that its viscosity approximately matched 
the viscosity of the silicon nitride compound, i.e., about 18,000 poise at 
170.degree. C. The mixed compound was then removed from the blender and 
cooled. 
A feed rod was molded from the silicon nitride compound using a 22 mm 
cylindrical mold. To accomplish this, granulated pieces of the compound 
were loaded into the mold and then molded at 150.degree. C. and 23.2 MPa. 
After cooling, the molded feed rod was ejected. An example of a feed rod 
prepared by this process is illustrated in FIG. 1. 
Cylindrical shells of the boron nitride compound were compression molded 
using a mold to create a shell of the desired dimensions. To accomplish 
this, granulated pieces of the compound were loaded into the mold and 
subsequently molded at 150.degree. C. and 8.9 MPa. After cooling, the 
shell segments were removed from the mold. An example of shells prepared 
by this process is illustrated in FIG. 2. 
A controlled geometry feed rod was then assembled by combining the silicon 
nitride core rod with the two boron nitride half-cylindrical shells. An 
example of a feed rod prepared in this manner is illustrated in FIG. 3. 
This feed rod was then loaded into the extrusion cylinder of a Bradford 
laboratory fiber extrusion machine (Bradford University Research, 
Bradford, UK). The cylinder was fitted with an extrusion die having a 285 
.mu.m diameter. Extrusion was commenced, and conducted at about 
165.degree. C. A monofilament coextruded silicon nitride/boron nitride 
fiber was obtained. 
The extrudate was collected continuously on a spooler. FIG. 4 illustrates a 
cross-section of a representative monofilament coextruded fiber prepared 
by the foregoing method, and FIG. 5 shows a longitudinal cross-section of 
that fiber. 
After arranging the fibers in a desired architecture and molding them to a 
desired shape, the green fibrous monolithic ceramic prepared thereby was 
baked to remove the binder. The resulting fibrous monolith was pressure 
sintered at 1750.degree. C. 
EXAMPLE 2 
This example illustrates the preparation of a silicon carbide/boron nitride 
monofilament fiber in accordance with one aspect of the present invention. 
1. Silicon carbide compound: 
A. Sinterable silicon carbide powder (B-10, H. C. Starck, Newton, Mass.) 
mixed with sintering aids (11.1 wt % aluminum nitride powder and 8.9 wt % 
alumina powder): 39.41 g 
B. Ethylene Vinyl Acetate copolymer: 8.35 g 
C. Heavy Mineral Oil: 2.54 g 
2. Boron nitride compound: 
A. Boron nitride powder (HCP): 27.0 g 
B. Ethylene Vinyl Acetate copolymer: 11.28 g 
C. Methoxypolyethylene glycol [MW 500]: 0.87 g 
The two ceramic compounds were prepared separately as in Example 1. For 
this example, both the silicon carbide compound and the boron nitride 
compound were adjusted to an apparent viscosity of about 13,000 poise at 
170.degree. C. The mixed compound was removed from the blender and cooled. 
As in Example 1, a feed rod was molded from the silicon carbide compound 
using a 22 mm cylindrical mold. Flat sheets of the boron nitride compound 
were compression molded by squeezing the granulated compound between flat 
steel sheets at a temperature of 80.degree.-150.degree. C. After cooling, 
the flat sheets were cut to the desired size and wrapped around the 
feedrod of the silicon carbide compound. The thickness of the sheets was 
adjusted to achieve the desired ratio of boron nitride to silicon carbide. 
A controlled geometry feedrod was then assembled from the boron nitride 
wrapped silicon carbide rod, and was extruded as in Example 1 to obtain 
monofilament coextruded silicon carbide/boron nitride fiber. The extrudate 
was collected continuously on a spooler. 
After arranging the fibers in the desired architecture and molding them to 
the desired shape, the resulting green fibrous monolithic ceramic was 
baked to remove the binder and the fibrous monolith is pressure sintered 
to provide the finished fibrous monolith article. 
EXAMPLE 3 
This example illustrates the preparation of an aluminum oxide/80% 
nickel-20% chromium alloy monofilament fiber in accordance with one aspect 
of the present invention. 
1. Aluminum oxide compound: 
A. Sinterable aluminum oxide powder (A16SG, ALCOA, Bauxite, Ark.): 44.55 g 
B. Ethylene Vinyl Acetate copolymer: 10.34 g 
C. Heavy Mineral oil: 2.32 g 
2. Nickel-Chromium compound: 
A. Nickel-Chromium powder: 92.40 g 
B. Ethylene Vinyl Acetate copolymer: 10.34 g 
C. Methoxypolyethylene Glycol [MW 550]: 0.35 g 
The two ceramic compounds were prepared separately as in the preceding 
examples with a Brabender Plastograph at about 150.degree. C. The aluminum 
oxide compound was adjusted to an apparent viscosity of 15,000 poise at 
150.degree. C., and the nickel-chromium compound to an apparent viscosity 
of about 13,000 poise at 150.degree. C. A core rod was molded from the 
aluminum oxide compound in the manner set forth in the prior examples, and 
cylindrical shells of the nickel oxide compound were compression molded to 
create a shell of the desired dimensions. A controlled geometry feedrod 
was then assembled by combining the core rod with the two half-cylindrical 
shells. This feed rod was loaded into the extrusion cylinder of a Bradford 
laboratory fiber extrusion machine. The cylinder was fitted with an 
extrusion die having a 285 micrometer diameter. A monofilament coextruded 
aluminum oxide/nickel oxide fiber was obtained by extruding the feed rod 
at about 150.degree. C. The extrudate was collected continuously on a 
spooler. 
After arranging the fibers into a desired configuration and molding them to 
obtain the desired green fibrous monolithic ceramic, the monolith was 
baked to remove the binder and then pressure sintered at temperature of 
about 1350.degree. C. to provide a fibrous monolith. 
EXAMPLE 4 
This example illustrates the preparation of a silicon carbide/boron nitride 
multifilament fiber in accordance with one aspect of the present 
invention. 
1. Silicon carbide compound: 
A. Sinterable silicon carbide powder mixed with sintering aids (11.1 wt % 
aluminum nitride powder and 8.9 wt % alumina powder): 36.13 g 
B. Ethylene Vinyl Acetate copolymer: 7.57 g 
C. Polyethylene Glycol [MW 550]: 0.75 g 
2. Boron nitride compound: 
A. Boron nitride powder: 24.75 g 
B. Ethylene Vinyl Acetate copolymer: 10.23 g 
C. Methoxypolyethylene Glycol [MW 550]: 0.75 g 
The components of each compound were mixed in the manner set forth in 
Example 2. The apparent viscosity of both the silicon carbide compound and 
the boron nitride compound were then adjusted to an apparent viscosity of 
about 14,500 poise at 150.degree. C. A controlled geometry feedrod was 
then prepared according to the method of Example 1, and 2.3 mm diameter 
monofilament fibers were provided by extrusion of the feed rods. An 
example of those extruded fibers is set forth in FIG. 6. 
The 2.3 mm monofilament extruded fibers were then cut to a length 
appropriate for a feed rod and bundled together. About 65-70 of the 2.3 mm 
fibers were tightly packed together into a molding cylinder, wherein they 
were molded at 150.degree. C. to form a multifilament feedrod. This 
multifilament feedrod was subsequently extruded at 110.degree. C. through 
a 2.3 mm orifice to produce multifilament rod, as illustrated in FIG. 7. 
After repeating the foregoing steps so that a number of multifilament 
fibers are prepared, a number of such fibers are arranged into a desired 
shape and molded to provide a green fibrous monolithic ceramic. An example 
of this ceramic is set forth in FIG. 8. The green ceramic monolith was 
then baked to remove the binder, and the resulting article pressure 
sintered to provide a fibrous monolith. 
EXAMPLE 5 
This example illustrates the preparation of a silicon nitride/boron nitride 
multifilament fiber in accordance with one aspect of the present 
invention. 
1. Silicon nitride compound: 
A. Sinterable silicon nitride powder mixed with sintering aids (9 wt % 
yttria powder and 3 wt % alumina powder): 37.23 g 
B. Ethylene Vinyl Acetate copolymer: 7.65 g 
C. Heavy Mineral oil: 2.16 g 
D. Methoxypolyethylene Glycol [MW 550]: 1.0 g 
2. Boron nitride compound: 
A. Boron nitride powder: 24.75 g 
B. Ethylene Vinyl Acetate copolymer: 10.34 
C. Methoxypolyethylene Glycol [MW 550]: 0.75 g 
The components of each composition were compounding in the manner set forth 
in Example 1. The apparent viscosity of both the silicon nitride compound 
and the boron nitride compound were then adjusted to an apparent viscosity 
of about 18,000 poise at 170.degree. C. A controlled geometry feedrod was 
then prepared by the method of Example 1, and 3 mm monofilament fibers 
were prepared by extrusion of the feed rod. About 30-50 of these 3 mm 
monofilament fibers were then cut to an appropriate length for a feed rod, 
and bundled together and molded, in the manner of Example 4, to form a 
multifilament feedrod. The multifilament feedrod was subsequently extruded 
at 110.degree. C. through a 3 mm orifice to produce multifilament fiber. 
After preparing a number of such multifilament fibers and arranging them 
into a desired configuration, the configured fibers were molded to provide 
a green fibrous monolithic ceramic. This monolith was then baked to remove 
the binder, and the resulting article pressure sintered to provide a 
fibrous monolith. 
EXAMPLE 6 
This example illustrates the preparation of an alumina/graphite 
multifilament fiber in accordance with one aspect of the present 
invention. 
1. Alumina compound: 
A. Sinterable alumina powder: 44.55 g 
B. Ethylene Vinyl Acetate copolymer: 10.23 g 
C. Polyethylene Glycol [MW 550]: 0.5 g 
2. Graphite compound: 
A. Graphite powder: 24.75 g 
B. Ethylene Vinyl Acetate copolymer: 11.26 g 
C. Polyethylene Glycol [MW 550]: 0.5 g 
Each of the compounds were compounded in the manner set forth in Example 1. 
The apparent viscosity of both the alumina compound and the graphite 
compound were adjusted to an apparent viscosity of about 19,000 poise at 
170.degree. C. A controlled geometry feedrod was then prepared in 
accordance with the method of Example 4, and 2.3 mm diameter monofilament 
fibers were then prepared by extruding the feedrod. These 3 mm 
monofilament coextruded fibers were then cut to a length appropriate for a 
feedrod, and bundled together and molded as in Example 4 to provide a feed 
rod. This multifilament feed rod was subsequently extruded through a 2.3 
mm orifice to produce a multifilament ceramic fiber. 
After preparing a number of such fibers, arranging the fibers into a 
desired shape, and molding the shaped article, a green fibrous monolithic 
ceramic was obtained. This monolith was then baked to remove the binder, 
and subsequently sintered to provide a fibrous monolith. 
EXAMPLE 7 
This example illustrates the preparation of a zirconia oxide/aluminum 
oxide/nickel oxide multifilament fiber in accordance with one aspect of 
the present invention. This example will further illustrate the 
preparation of a zirconia/nickel fibrous monolith, wherein an alumina 
interphase exists between the zirconia and the nickel. 
1. Zirconia oxide compound: 
A. Sinterable zirconia--3 mole percent yttria powder: 66.99 g 
B. Ethylene Vinyl Acetate copolymer: 10.34 g 
C. Methoxypolyethylene Glycol [MW 550]: 1.38 g 
2. Aluminum oxide compound: 
A. Sinterable aluminum oxide powder: 44.55 g 
B. Ethylene Vinyl Acetate copolymer: 10.34 g 
C. Methoxypolyethylene Glycol [MW 550]: 1.0 g 
3. Nickel oxide compound: 
A. Nickel oxide alloy powder: 73.37 g 
B. Ethylene Vinyl Acetate copolymer: 10.34 g 
C. Methoxypolyethylene Glycol [MW 550]: 1.38 g 
Each of the compounds were compounding in the manner set forth in prior 
Examples. The apparent viscosity of both the zirconia compound and the 
nickel oxide compound were adjusted so that they were similar. The 
aluminum oxide compound was prepared according to the method set forth in 
Example 6. A feedrod was then molded from the zirconia in the manner as 
described in previous examples. Flat sheets of the alumina compound and 
the nickel oxide compound were compression molded in accordance with the 
method of Example 2. The thickness of the sheets were controlled to 
achieve the desired ratio of nickel to zirconia and alumina to zirconia in 
the feed rod. The alumina sheet was then wrapped around the zirconia rod, 
and the nickel oxide sheet was wrapped around the alumina sheet to 
fabricate the controlled geometry feedrod. The three-component feedrod was 
then extruded to provide a 2.3 mm monofilament fiber. As in Example 4, a 
number of these fibers were then bundled to create a multifilament feed 
rod, which feed rod was then extruded as before to form multifilament 
fiber. 
After preparing a number of such multifilament fibers, arranging the fibers 
into a desired shape, and molding the shaped article, a green fibrous 
monolithic ceramic was obtained. This monolith was then baked to remove 
the binder, the nickel oxide was reduced to metallic nickel, and 
subsequently sintered to provide a fibrous monolith. The three-layer 
textured zirconium oxide/aluminum oxide/nickel green fiber is illustrated 
in FIG. 9. 
EXAMPLE 8 
This example illustrates the preparation of a "tree ring" texture in 
aluminum oxide/iron oxide fibers, using axial coextrusion, in accordance 
with one aspect of the present invention. 
1. Alumina compound: 
A. Sinterable alumina powder: 44.43 g 
B. Ethylene Vinyl Acetate copolymer: 7.57 g 
C. Methoxypolyethylene Glycol [MW 550]: 0.42 g 
2. Iron oxide compound: 
A. Iron oxide (hematite) powder: 57.64 g 
B. Ethylene Vinyl Acetate copolymer: 10.23 g 
C. Methoxypolyethylene Glycol [MW 550]: 0.5 g 
The aluminum oxide compound and the iron oxide compound were compounded as 
in previous examples. Using the method of Example 6, flat sheets, 0.5 mm 
thick, were pressed from the alumina and iron compounds. Using a punch, 22 
mm diameter discs were cut from these sheets. A feedrod was prepared by 
stacking these discs in a particular sequence, which was for this example, 
three white aluminum oxide discs, one red iron oxide disc, etc. After 
compression molding that stack at a temperature of 160.degree. C. and a 
load of 10,000N, solid feed rod with axial compositional variations was 
obtained. This feed rod was extruded at 180.degree. C. through a 2.3 mm 
orifice to obtain a green fiber with a texture similar to tree rings. An 
example of this fiber is shown in FIG. 10 (axial section) and FIG. 11 
(radial section). A uniaxial layup of these fibers was molded to produce a 
textured green ceramic article having a wood-like appearance, as 
illustrated in FIG. 12. 
EXAMPLE 9 
This example illustrates the preparation of silicon nitride/boron nitride 
fibers using "chip and washer" axial extrusion, in accordance with one 
aspect of the present invention. 
1. Silicon nitride compound: 
A. Sinterable silicon nitride powder mixed with sintering aids (9 wt % 
yttria powder and 3 wt % alumina powder): 36.50 g 
B. Ethylene Vinyl Acetate copolymer: 9.30 g 
C. Heavy Mineral oil: 2.16 g 
D. Methoxypolyethylene Glycol [MW 550]: 1.0 g 
2. Boron nitride compound: 
A. Boron nitride powder: 24.75 g 
B. Ethylene Vinyl Acetate copolymer: 10.34 g 
C. Methoxypolyethylene Glycol [MW 550]: 0.75 g 
The silicon nitride compound and the boron nitride compound were compounded 
as in previous examples. Using the method of Example 2, the silicon 
nitride compound and boron nitride compound were compression molded at 
150.degree. C. between flat steel sheets to provide flat sheets of each 
material of 0.3 mm in thickness. Using a punch, 22 mm diameter discs were 
cut from the silicon nitride sheets. "Washers" were cut from the boron 
nitride sheets, the washers having a 22 mm outer diameter and a 6 mm inner 
diameter. A 6 mm silicon nitride disc was then placed into the central 
hole of the boron nitride disc. A feedrod was prepared by stacking those 
multi-component discs in a particular sequence, which was for this example 
four grey silicon nitride discs, one white boron nitride disc, etc. 
After compression molding at 150.degree. C. and a load of 23.2N, a solid 
feed rod with axial compositional variations was obtained. This feed rod 
was extruded at 110.degree. C. through a 3 mm orifice to obtain a green 
fiber having a texture similar to tree rings. A uniaxial layup of these 
fibers was molded to produce a textured green ceramic. After molding a 
number of such fibers into a desired shape to provide a green fibrous 
monolithic ceramic, the monolith was baked to remove the binder. The 
resulting article was then pressure sintered to provide a fibrous monolith 
having a wood-like appearance. 
While the examples included herein utilize piston extrusion, it is believed 
that other extrusion methods known in the art could also be used. As one 
example, continuous bicomponent extrusion of highly loaded ceramic systems 
could be accomplished if properly designed bicomponent spinning dies were 
utilized. 
All of the references cited herein are hereby incorporated in their 
entireties by reference. 
While this invention has been described with an emphasis upon preferred 
embodiments, it will be obvious to those of ordinary skill in the art that 
variations of the preferred embodiments may be used and that it is 
intended that the invention may be practiced otherwise than as 
specifically described herein. Accordingly, this invention includes all 
modifications encompassed within the spirit and scope of the invention as 
defined by the following claims.