Method of making chemical vapor infiltrated composites

Method/apparatus for making a composite wherein reinforcement or filler material, such as, for example, ceramic fibers or whiskers, are confined in a microwave-transparent envelope, the reinforcement or filler material is microwave heated, and a gaseous reactant stream is flowed through the envelope so as to infiltrate interstices of the heated reinforcement or filler material and chemically vapor deposit a matrix, such as a ceramic matrix, from the interior toward the exterior of the heated material. The envelope imparts a desired shape and size to the composite. After infiltration, the envelope typically is removed from the composite.

FIELD OF THE INVENTION 
The present invention relates to composites and to a chemical vapor 
infiltration (CVI) method/apparatus for making same wherein a matrix is 
formed by microwave-assisted, forced reactant flow chemical vapor 
infiltration from the interior toward the exterior of reinforcement or 
filler material, such as ceramic reinforcing fibers, confined in a 
microwave-transparent envelope that includes an interior shape for 
imparting a desired configuration to the composite formed therein. 
BACKGROUND OF THE INVENTION 
Ceramic matrix/ceramic fiber composites having a thin-walled shape, such as 
a tube, shell, etc., have been fabricated by chemical vapor infiltration 
(CVI) wherein a fibrous preform is heated isothermally and exposed to a 
reactive vapor which includes gaseous chemical precursors for the ceramic 
matrix for long periods of time (e.g. a week or more). During this time, 
the gaseous chemical precursors infiltrate the fibrous preform and react 
to deposit the ceramic matrix material on the surfaces of the individual 
preform fibers and thereby form the matrix of the composite. The rate of 
the matrix-forming reaction is governed by the temperature of the preform 
and gaseous reactants, the concentrations of reactants, and the 
concentration of byproduct gases released by the reaction. 
An illustrative reaction for depositing a SiC matrix would involve hydrogen 
and methyltrichlorosilane (MTS), CH.sub.3 SiCl.sub.3, as gaseous 
reactants. At high temperature (e.g. greater than 700.degree. C.), these 
reactants can deposit SiC on the fiber surfaces with HCl being a byproduct 
gas that is released by the reaction and that inhibits the deposition of 
SiC on the fiber surfaces. 
In this particular isothermal CVI process, the concentration of MTS is 
maximum at the preform outer surface and diminishes toward the interior of 
the preform, while the concentration of the HCl byproduct gas increases 
towards the interior of the preform. Both of these effects result in 
preferential deposition of SiC near the outer surface of the fiber 
preform. Eventually, the surface porosity of the preform is sealed by the 
deposited SiC, leaving a core region of the preform inadequately 
infiltrated with the ceramic matrix. One attempt to minimize these adverse 
effects has involved conducting the CVI process at relatively low 
temperatures using diluted reactant gases. However, even after very long 
times (e.g. on the order of 2 months), it has been virtually impossible to 
make a ceramic/ceramic composite with a cross-section thicker than about 
12 mm (millimeters). Even then, the resulting composite microstructure 
exhibits undesirable porosity. 
Another attempt to minimize these adverse affects has involved establishing 
a temperature gradient across the fiber preform from one side of the 
preform to the other so that matrix deposition occurs preferentially at 
the hotter preform side and the cooler preform surface porosity remains 
open for a longer duration throughout the CVI process. For example, work 
at Oak Ridge National Laboratory has demonstrated that ceramic 
matrix/ceramix fiber composites can be made relatively rapidly (e.g. on 
the order of one day) by maintaining a sharp temperature gradient through 
the preform to provide a relatively cold side and a relatively hot side 
while concurrently imposing a flow of gaseous reactants through the 
preform in the direction of the increasing temperature; i.e. flowing the 
reactant gases from the cooler preform side to the hotter preform side. By 
this temperature gradient/forced parallel reactant flow CVI technique, the 
ceramic matrix is deposited preferentially near the hot side of the 
preform such that the zone of deposition moves from the hot side toward 
the colder side as the CVI process is continued. However, this technique 
is limited to making ceramic/ceramic composites having very simple 
geometries, such as flat plates and tubes as a result of the need for the 
temperature gradient across the preform. 
It is an object of the invention to provide an improved method/apparatus 
for making a composite wherein a matrix is microwave-assisted chemical 
vapor deposited from the interior toward the exterior of reinforcement or 
filler material confined in a microwave-transparent, shaping envelope in a 
manner that permits a wide variety of composite sizes (large and small) 
and configurations (simple and complex) to be made in a relatively rapid 
time. 
It is another object of the invention to provide an improved 
method/apparatus for making a surface composite on a monolithic body using 
microwave-assisted chemical vapor deposition from the interior toward the 
exterior of reinforcement or filler material disposed on the monolithic 
body. 
It is still a further object of the invention to provide a composite 
wherein the composite includes a matrix chemical vapor deposited from the 
interior toward the exterior of the reinforcement or filler material of 
the composite as a result of microwave heating of the reinforcement or 
filler material during infiltration. 
It is still a further object of the invention to provide a composite 
including relatively thick and thin sections that are microwave-assisted, 
chemical vapor infiltrated with a matrix material. 
It is still another object of the invention to provide a composite wherein 
the composite is confined in a microwave-transparent envelope that imparts 
a desired shape to the composite. 
SUMMARY OF THE INVENTION 
The present invention involves method/apparatus for making a composite 
wherein reinforcement or filler material, such as, for example, ceramic 
fibers or whiskers, are confined in a microwave-transparent envelope 
preferably having an interior configuration corresponding to the composite 
shape desired, the reinforcement or filler material is microwave heated in 
the envelope, and a gaseous reactant stream is flowed through the envelope 
from am inlet to an outlet thereof so as to infiltrate interstices of the 
heated reinforcement or filler material and chemically vapor deposit a 
matrix material, such as a ceramic matrix material, from the interior 
toward the exterior of the heated reinforcement or filler material. After 
infiltration, the envelope typically is removed from the composite. 
The reinforcement or filler material can comprise ceramic, glass, carbon, 
polymeric or other fibers, filaments, chopped fibers or filaments, 
whiskers, flakes, and particulates disposed in the envelope. The 
reinforcement or filler material can be oriented in a particular direction 
in the envelope or in random manner. The reinforcement or filler material 
can be preformed by weaving, braiding, lay-up, and other techniques to 
provide a preshaped body (preform) that is disposed in the envelope for 
infiltration. 
The matrix material can be the same as or different from the reinforcement 
or filler material. 
In one embodiment of the invention, after removal from the envelope, the 
composite is microwave heated and a gaseous reactant stream is flowed past 
the heated composite in free flow manner so as to infiltrate an outer, 
uninfiltrated portion of the composite and chemically vapor deposit a 
matrix material to complete infiltration of the composite. 
In another embodiment of the invention, the gaseous reactant stream is 
flowed through the envelope in a manner to force the reactant stream 
through interstices of the heated reinforcement or filler material in a 
direction generally perpendicular to a temperature gradient established 
therein by the microwave heating so as to deposit the matrix material. 
In still another embodiment of the invention, a microwave susceptor 
material, such as, for example, carbon, is provided at an interior portion 
of the reinforcement or filler material so that the susceptor material is 
initially microwave heated and raises the temperature of the interior 
portion of the reinforcement or filler material to a high enough 
temperature for direct coupling thereof with the microwave energy. 
In still another embodiment of the invention, the gaseous reactant stream 
includes precursor materials to generate in-situ at the reinforcement or 
filler material at least one of the chemical compounds needed to form the 
matrix material. The chemical compound is catalytically generated by 
contact of the precursors with the heated material. For example, the 
reactant stream can include CO.sub.2 and H.sub.2 that catalytically react 
in contact with the heated reinforcement or filler material to form water 
that, in turn, will react with Al.sub.2 O.sub.3 in the reactant stream to 
form and deposit an Al.sub.2 O.sub.3 matrix. Generation of the water 
reactant at the reinforcement or filler material avoids unwanted, 
premature reaction of the AlCl.sub.3 and H.sub.2 O and deposition of the 
resultant Al.sub.2 O.sub.3 at unwanted regions of the CVI apparatus. 
In still a further embodiment of the invention, a surface composite is 
formed on a monolithic body by disposing reinforcement or filler material 
on the monolithic body in an optional microwave-transparent envelope, 
microwave heating the reinforcement or filler material, and flowing a 
gaseous reactant stream in contact with the material so as to chemically 
vapor deposit a matrix material from the interior toward the exterior 
thereof and thereby form a surface composite on the underlying monolithic 
body. 
The present invention also involves a composite comprising a matrix and 
reinforcement or filler material in the matrix wherein the matrix is 
chemical vapor infiltrated from the interior toward the exterior of 
reinforcement or filler material. The composite can have a simple or 
complex configuration with relatively thick and thin sections and be small 
or large in size as determined by the interior configuration of the 
shaping envelope. 
The present invention also provides an article of manufacture, comprising a 
monolithic body having a composite formed in-situ on a surface thereof 
wherein the composite comprises reinforcement or filler material chemical 
vapor infiltrated from the interior toward the exterior thereof with a 
matrix material. 
The aforementioned objects and advantages of the invention will become more 
readily apparent from the following detailed description taken with the 
following drawings.

DETAILED DESCRIPTION 
Referring to FIG. 1, apparatus in accordance with one embodiment of the 
invention is schematically illustrated. Although the apparatus is 
illustrated and discribed herebelow with respect to making a 
ceramic/ceramic composite, the invention is not so limited. For example, 
the present invention also can be practiced to make composites where the 
reinforcement or filler material and the matrix material comprise glass 
(e.g. glass/glass composites), carbon (e.g. carbon/carbon composites) and 
other non-metallic materials such as possibly polymeric materials, as well 
as various combinations thereof to make composites such as glass/ceramic 
composites, carbon/glass composites, carbon/ceramic composites, etc. 
wherein the materials set forth can comprise the reinforcement or filler 
material 12 or the matrix material. 
The apparatus is illustrated as including microwave transparent tubular 
envelope 10 for confining ceramic or other reinforcement or filler 
material 12 therein. The envelope 10 can be oriented vertically as shown, 
horizontally or at other orientations. The reinforcement or filler 
material 12 can comprise fibers, filaments, chopped fibers and filaments, 
whiskers, flakes, particles, and other reinforcement or filler shapes 
disposed in the envelope 10. The reinforcement material can be used to 
enhance the mechanical properties of the composite in one or more 
directions thereof. Reinforcement materials such as fibers can be used 
uncoated or coated with a debond layer to provide improved composite 
properties. Alternately, the filler material can be used merely to reduce 
the amount of CVI matrix needed to form the composite body. 
For example, as illustrated in FIG. 1, the reinforcement material 12 can 
comprise ceramic or other fibers 13 oriented in a particular direction to 
impart anisotropic mechanical properties to the ceramic/ceramic or other 
composite formed. Alternately, the reinforcement material 12 also can 
comprise randomly oriented, loose ceramic or other whiskers (not shown) 
that can impart improved generally isotropic mechanical properties to the 
composite formed. 
The reinforcement or filler material 12 can be preformed by conventional 
two or three dimensional weaving, two or three dimensional braiding, ply 
lay-up or stacking, felt-making techniques as well as other techniques to 
provide a preshaped body (preform) that can be disposed in the envelope 10 
for chemical vapor infiltration of the pores or interstices of the 
material 12 to form the matrix therein. 
Ceramic reinforcement or filler material can comprise a wide variety of 
ceramic materials including for purposes of illustration, not limitation, 
alumina, mullite, aluminoborosilcate, silicon carbide, NICALON SiC-like 
material in the form of fibers, filaments, chopped fibers or filaments, 
whiskers, flakes, particles, and the like which can be randomly oriented, 
aligned in one or more directions, or preformed by weaving, braiding and 
the like as described hereabove. Similar forms of glass, carbon, 
polymeric, and other reinforcement or filler materials can be used for 
making composites other than ceramic/ceramic composites, for example, as 
discussed hereabove (e.g. glass/glass composites, carbon/carbon 
composites, glass/ceramic composites, etc.). 
In effect, the tubular envelope 10 acts as a container or mold in which the 
reinforcement or filler material 12 is confined in a desired configuration 
and size while the matrix is formed in-situ by CVI. The envelope 10 
includes an inlet 10a and outlet 10b for flow of the gaseous reactant 
stream 25 employed in the CVI process. The envelope 10 is made from a 
material that is substantially impervious to the gaseous reactant stream 
25 and that is substantially non-reactive with the reinforcement or filler 
material 12. For example, the envelope 10 can be made of 
microwave-transparent quartz glass, high silica glass, borsilicate glass, 
or possibly a ceramic material such as mullite or other low dielectric 
loss ceramic material. By "microwave-transparent" is meant a low 
electrical conductivity, low dielectric loss material through which 
microwaves can readily pass to heat the contents of the envelope 10. 
The envelope 10 includes an interior wall 10c that is configured to define 
a shaping cavity or chamber for confining the porous reinforcement or 
filler material 12 in the desired shaped and sized mass for infiltration 
of interstices of the material 12 with the matrix. The envelope 10 can be 
formed to desired interior shape and size by conventional glass or ceramic 
forming techniques. Ceramic/ceramic and other composites having a simple 
shape or a complex shape including relatively thick and thin sections can 
be made using the appropriately configured envelope 10. For example, 
referring to FIG. 4, fiber material 12' having relatively thick and thin 
sections T1 and T2 is shown for purposes of illustration, not limitation, 
in an envelope 10' having a complementary interior configuration to form 
thick and thin composite sections. 
The ceramic and other reinforcement or filler material 12 typically is 
positioned in the envelope cavity prior to placement of the envelope 10 in 
the microwave heating device 20. Loose, randomly oriented reinforcement or 
filler material 12 can be simply poured or otherwise introduced into the 
envelope cavity 10d until the cavity is filled and retained therein by 
suitable means. Alternately, reinforcement or filler material 12 in the 
form of fibers can be wedged or engaged in the envelope 10 as set forth in 
the examples hereafter. Ceramic or other reinforcement or filler material 
12 in the form of a preformed porous body or component can be inserted in 
the cavity through the inlet or outlet 10a, 10b or through a suitable 
envelope opening (not shown) that can be subsequently suitably sealed. The 
preform can be supported in appropriate position in the envelope cavity by 
engagement of the preform with the interior envelope wall or by one or 10 
more microwave-transparent positioning filament(s) (not shown) for 
suspending the preform therein, one or more microwave-transparent supports 
(not shown), or other positioning means. 
To effect CVI of the matrix, the envelope 10 is located in a conventional 
single mode or multi-mode microwave heating device or oven 20 in order to 
heat the ceramic reinforcement or filler material 12 to an appropriate 
temperature to deposit the ceramic or other matrix material on the 
reinforcement or filler material 12 in the envelope. Microwave heating of 
the reinforcement or filler material 12 results in temperature gradients 
within the material 12 with the interior of the material 12 being at a 
higher temperature than the exterior surface region of the material 12. 
The decreasing temperature gradient from the interior toward the exterior 
of the material 12 occurs as a result of the microwave energy being 
converted to heat throughout the volume of the material 12 while heat is 
dissipated from the exterior surface of the material 12. Microwave heating 
thereby can provide temperature gradients such that the rate of deposition 
of the matrix material (which is temperature dependent) is higher at the 
interior of the material 12 than at the exterior. In FIG. 1, the 
temperature gradient extends in an outward direction through the material 
12 as a result of the cylindrical shape of the material 12. Microwave 
heating of the material 12 in accordance with the invention thus takes 
advantage of the temperature dependence of the deposition rate of matrix 
material to provide interior-to-exterior infiltration of interstices of 
the the material 12 with ceramic matrix material while the material 12 is 
confined to a particular desired shape by the envelope 10. 
These matrix deposition kinetics overcome the aforementioned adverse 
concentration effects observed heretofore in isothermal CVI processes. 
Thus, it is possible in accordance with the invention to form the 
composite by depositing the matrix material preferentially from the 
interior and building the deposit through interstices of the reinforcement 
or filler material 12 toward the exterior thereof. This 
interior-to-exterior matrix deposition not only enables much thicker 
composites to be formed but also enables achievement of much higher 
deposition rates since the invention is not limited to the lower 
temperatures and dilute reactants used in practicing the aforementioned 
isothermal CVI process. This interior-to-exterior matrix deposition can 
produce lower final composite porosity than that produced by the 
isothermal CVI process. 
A single mode microwave heating oven (model CMPR 250) available from 
Wavemat Corp., 4419 Plymouth Oaks Blvd., Plymouth, Mich. 48170, was used 
in the examples set forth herebelow, although the invention is not limited 
to any particular microwave heating oven. Multi-mode microwave heating 
ovens can be used for commercial production of ceramic/ceramic composites 
having large sizes and complex shapes. Smaller, more simply configured 
composites may be better produced in single mode microwave ovens. 
A susceptor material, such as carbon, can be provided at the interior of 
the reinforcement or filler material 12 in order to facilitate microwave 
heating to the desired CVI temperature, especially when the material 12 
(such as Al.sub.2 O.sub.3 fibers) exhibits a low electrical conductivity 
and/or low dielectric loss. The susceptor material at the interior of the 
material 12 is initially heated by the microwave energy and quickly raises 
the temperature of the proximate material 12 to a level where the material 
12 will couple with the microwave energy for continued heating to the CVI 
10 temperature. The susceptor material can be introduced at the interior 
of the reinforcement or filler material 12 before or after the material is 
placed in the envelope 10, or the susceptor material can be incorporated 
on fibers, fabric, and the like constituting the material 12. One 
technique for providing susceptor material at the interior of the material 
12 in accordance with the invention is described in example 1 set forth 
herebelow. The envelope 10 is positioned in the microwave heating device 
20 with the inlet 10a communicated to a source or supply 22 (shown 
schematically) of gaseous reactants to provide a gaseous reactant stream 
25 through the envelope 10 and with the outlet 10b communicated to a 
suitable pump 24 (schematically shown) for establishing a desired forced 
reactant stream flow rate through the envelope 10 sufficient to infiltrate 
interstices of the microwave heated material 12 and chemically vapor 
deposit a ceramic or other matrix material from the interior toward the 
exterior of the heated material 12 as a result of the microwave heating 
effect. 
The ceramic or other matrix material can have the same composition as the 
reinforcement or filler material 12 or a different composition depending 
on the composite properties required for a given service application. 
FIG. 1 illustrates a preferred apparatus of the invention wherein the 
gaseous reactant stream 25 is flowed through the envelope 10 in a manner 
to force the gaseous reactant stream through interstices of the heated 
reinforcement or filler material 12 in a direction generally perpendicular 
to the outward temperature gradient established therein by the microwave 
heating. 
The supply 22 for the gaseous reactant stream 25 can comprise a 
conventional generator used heretofore in CVI manufacture of 
ceramic/ceramic composites to provide a reactant stream that will deposit 
a ceramic matrix on the material 12. For example, in depositing alumina 
(Al.sub.2 O.sub.3) matrix material on alpha alumina fibers as described in 
Example 1, the supply 22 can comprise a conventional AlCl.sub.3 generator 
wherein chlorine gas is passed over a bed of Al pellets at an elevated 
temperature suitable to form AlCl.sub.3 gaseous reactant. The AlCl.sub.3 
gaseous reactant reacts with water proximate the heated alumina fiber 
material 12 in the envelope 10 to form Al.sub.2 O.sub.3 which is deposited 
onto the fiber material 12. 
In practicing the present invention to deposit Al.sub.2 O.sub.3 matrix 
material, the water reactant preferably is catalytically generated by 
providing CO.sub.2 and H.sub.2 along with AlCl.sub.3 in the reactant 
stream 25 and catalytically reacting the CO.sub.2 and H.sub.2 by contact 
of the reactant stream with the heated material 12 in the envelope 10. 
Water vapor thereby is generated at the interior of the reinforcement or 
filler material 12 for immediate reaction with the AlCl.sub.3 to form and 
directly deposit Al.sub.2 O.sub.3 matrix material on the material 12. 
Formation of the water reactant in this manner avoids homogenous 
nucleation of Al.sub.2 O.sub.3 particles that might otherwise occur if 
water vapor is present in the bulk reactant stream flowing into the 
envelope 10. 
The composition of the matrix precursors of the gaseous reactant stream 
flowed through the envelope 10 will be selected in dependence on the 
ceramic or other matrix material to be deposited on the reinforcement or 
filler material 12. For purposes of further illustration but not 
limitation, example 1 sets forth a gas reactant stream composition for 
CVI'ing an Al.sub.2 O.sub.3 matrix about alpha alumina fibers and example 
2 sets forth a gaseous reactant stream composition for CVI'ing a silicon 
carbide matrix about NICALON silicon carbide-like fibers. Other reactant 
stream precursors can be selected as appropriate for depositing ceramic 
matrices of other compositions as those skilled in the art will 
appreciate. 
In practicing the invention, the reinforcement or filler material 12 
residing in the envelope 10 is first microwave heated by the microwave 
heating device 20 to an appropriate temperature for CVI of the matrix 
material. After the desired temperature is reached, the gaseous reactant 
stream is flowed through the envelope 10 in a manner to be forced to 
infiltrate interstices of the heated material 12 and deposit the matrix 
material therein. As mentioned hereabove, the gaseous reactant stream is 
flowed through the envelope 10 in a manner to force the gaseous reactant 
stream through interstices of the heated material 12 in a direction 
generally perpendicular to the radial temperature gradient established 
therein by the microwave heating. 
The respective flow rates of the gaseous reactants comprising stream 25 are 
controlled by respective conventional mass flow controllers (not shown). 
The reaction byproduct gas pressure downstream of the material 12 is 
controlled by the vacuum pump 24 and a pressure control system in response 
to the pressure controller. The flow rate of the gaseous reactant stream 
through interstices of the material 12 is controlled by the differential 
pressure established across the material 12 in the envelope 10. 
One pressure control system for use in the invention comprises a pressure 
sensor 30 and pressure controller 32 (e.g. MKS Model 250 pressure 
controller available from MKS Instrument, Inc.) that controls introduction 
of make-up air to the vacuum pump conduit 24a via make-up air valve 35 
(e.g. MKS Model 248 feed back valve) and air ejector 37 located upstream 
of an oversized vacuum pump 24, FIG. 1. 
Alternately, the downstream byproduct gas pressure can be controlled using 
pressure sensor 30 and pressure controller 32 to directly control a vacuum 
pump 24 that is sized appropriately for the pressures involved; i.e. a 
pump that is not oversized. 
The ceramic or other matrix material is deposited in dependence on the 
temperature gradient established by the microwave heating such that matrix 
deposition occurs from the interior toward the exterior of the shaped 
material 12. The shape of the deposited matrix material will correspond to 
the shape of the material 12 as confined and determined by the interior 
configuration of the envelope 10. As deposition takes place from the 
interior toward the exterior of the material 12, the entire shape of the 
material 12 can be infiltrated with matrix material with the exception of 
a relatively thin outermost surface region or zone z disposed about the 
matrix inflitrated core K, FIG. 3. For example, a thin outermost zone 
remains uninfiltrated or open to reactant flow in order for the reactant 
stream to infiltrate the material 12 and for the byproduct gases (e.g. HCl 
when AlCl.sub.3 and water react to form Al.sub.2 O.sub.3) to be exhausted 
to the outlet 10b. When the material 12 comprises ceramic or other fibers, 
the outermost, uninfiltrated zone may have a width of a few fiber 
diameters or more. 
The microwave-assisted deposition of the matrix material allows virtually 
any shape and size of composite to be made in accordance with the 
invention. Thus, reinforcement or filler material 12 in complex shape 
having relatively thick and thin sections (see FIG. 4) can be infiltrated 
with matrix material. Microwave heating insures that the thicker sections 
of the material 12 will be heated to the highest temperature where 
infiltration will be initiated and progress toward the exterior of the 
section(s). 
After the reinforcement or filler material 12 is infiltrated with the 
ceramic matrix with the exception of the outermost, uninfiltrated zone, 
the composite is removed from the microwave heating device 20, and the 
envelope is removed from the composite. The envelope 10 can be removed by 
grinding, sandblasting, or other suitable means. 
After the envelope 10 is removed, the outermost, uninfiltrated zone of the 
composite is infiltrated using the apparatus schematically illustrated in 
FIGS. 2-3 wherein like features of FIG. 1 are designated using like 
reference numerals primed. The apparatus of FIG. 2-3 differs from the 
apparatus of FIG. 1 in flowing the gaseous reactant stream 25' past the 
outer surface of the microwave heated composite C in free flow manner in a 
reaction vessel 11' so that interstices of the outermost, uninfiltrated 
zone Z are infiltrated with the reactant stream 25' and the ceramic or 
other matrix is chemically vapor deposited throughout the outermost zone Z 
to provide complete infiltration of the composite C. The envelope 10 of 
FIG. 1 directly confining the material 12 to a particular size/shape is 
not needed or used in the apparatus of FIGS. 2-3. Instead, the composite C 
is placed in a simple reaction vessel 11' through which the gaseous 
reactant stream 25' is flowed past the outer composite surface as shown in 
FIGS. 2-3. The composite C can be suspended in position in the vessel 11' 
by one or more microwave-transparent filaments, by one or more 
microwave-transparent supports or other suitable means. Thus, the 
finishing of the composite C is achieved by removing the envelope 10 of 
FIG. 1 and infiltrating interstices of the outermost zone Z thereof in the 
apparatus of FIGS. 2-3. 
The present invention can be used to produce bulk ceramic/ceramic or other 
composites of various shapes and sizes as determined by the interior 
configuration of the envelope 10. Moreover, the present invention can be 
used to make a ceramic/ceramic or other composite shell by disposing the 
reinforcement or filler material 112 on a suitable mandrel 160 heatable by 
microwave energy, FIG. 5, in a CVI reaction vessel 111. The material 112 
optionally can be confined on the mandrel by a microwave-transparent 
envelope (not shown), such as silica glass, through which the gaseous 
reactant stream 125 can be flowed. The material 112 can be infiltrated 
with ceramic or other matrix material from the interior toward the 
exterior by microwave heating in the manner described hereabove with 
respect to FIG. 1 (if an envelope 10 is used) or FIGS. 5 (if no envelope 
10 is used) with concurrent flow of the gaseous reactant stream 125 
through interstices of the material 112. The mandrel and optional envelope 
then are removed from the composite shell to free it for final finishing 
of the outermost, uninfiltrated surface zone in the manner described 
hereabove. 
The present invention also can be used to form a surface composite on a 
monolithic body, such as a monolithic ceramic body. For example, there are 
many potential service applications of ceramic materials where adequate 
toughness could be provided by a surface composite layer applied over a 
monolithic body of the same or different ceramic material, since the 
strength-limiting cracks in many ceramic articles originate at the 
surface. If the article surface could be rendered sufficiently tough, it 
would not be necessary for the entire article to have enhanced toughness 
to resist surface crack propagation. Thus, an article of manufacture 
fabricated with a core or body 250 comprising a monolithic ceramic and 
relatively thin layer 252 of ceramic/ceramic or other composite bonded to 
the surface, FIG. 6, would be advantageous. Such an article can be made in 
the manner described hereabove for making a ceramic/ceramic composite or 
other shell wherein the mandrel 160 is replaced by the monolithic ceramic 
or other body 250. The preferential heating attributable to the microwave 
energy causes the matrix material to be deposited from the outer surface 
of the monolithic body outward through the surface reinforcement or filler 
material. The envelope, if used, then is removed from the composite 
covered monolithic body shell to free it for final finishing of the 
outermost, uninfiltrated surface zone in the manner described hereabove 
with respect to FIGS. 2-3. A monolithic ceramic body having a crack 
resistant ceramic/ceramic or other composite surface layer bonded thereon 
is made. 
The embodiments described hereabove involve using microwave heating of the 
reinforcement or filler material as confined in a desired shape and size 
by the microwave-transparent envelope 10 to provide matrix deposition from 
the interior toward the exterior of the material. The resulting composite 
structures, whether bulk composites, shells, or surface layers, will be 
significantly more dense, stronger, and tougher than like composite 
structures produced by the aforementioned isothermal CVI process. 
The following examples are offered for purposes of illustrating the 
invention in greater detail, but not to limit the invention. 
Example 1 
Alumina fiber/alumina matrix composites were individually formed by 
positioning a respective fiber preform comprising alumina fiber tows 
having a collective diameter of 16 mm in a respective silca glass tube 
(constituting the envelope 10 of FIG. 1) having an inner diameter of 16 
mm. Each preform comprised numerous tows of alumina fibers oriented 
parallel to one another along the same axis. Tows comprising 200 alumina 
filaments (filament diameter of 20 microns) or 1000 alumina filaments 
(filament diameter of 10 microns) were used in this example. 
Before placement of each preform in the silica glass tube, an interior 
portion of the preform was impregnated with a sugar solution, dried in an 
oven, and pyrolyzed by heating in a separate furnace in the absence of 
oxygen to provide a carbon-containing limited interior zone in the fiber 
preform. The carbon functioned as a susceptor material at the interior of 
the preform during heating in a microwave oven. 
Each preform was microwave heated in a single mode Wavemat oven operated at 
2.45 gegahertz (HGz) using apparatus similar to that of FIG. 1. Initial 
heating of the carbon susceptor material caused a rapid increase in the 
temperature of the alpha alumina preform fibers to the point that they 
coupled directly with the microwave energy for continued heating. The 
carbon was thermodynamically unstable under the matrix deposition 
conditions and thus was removed early in the matrix deposition process. 
Each preform was heated to a surface temperature between 500-1000 degrees 
C. by the aforementioned Wavemat microwave prior to introduction of the 
gaseous reactant stream 25 to the silica glass envelope. The reactant 
stream comprised 6 vol. % AlCl.sub.3 vapor, 28 vol. % CO.sub.2, 28 vol. % 
H.sub.2, and 38 vol. % N.sub.2 as a carrier gas for depositing Al.sub.22 
O.sub.3 matrix material. A partial vacuum of from 20-50 torr was 
maintained on the downstream side of the preform and a reactant gas stream 
pressure of from 80 to 610 torr was maintained on the upstream side of the 
preform for 8-14 hours. Deposition of alumina matrix material was observed 
to occur from the interior of the preform toward the exterior as 
determined by the microwave induced temperature gradient to produce a 
cylindrical composite shape corresponding to the inner diameter of the 
silica glass tube used as the envelope 10. The deposited alumina in the 
preform represented from 50% to 100% of the theoretical maximum amount 
based on the amount of AlCl.sub.3 supplied to the preform. FIG. 7 is an 
SEM (scanning electron microscope) photomicrograph of a ceramic/ceramic 
composite formed pursuant to this example showing an alumina matrix and 
alumina fibers. 
Example 2 
Silicon carbide fiber/silicon carbide matrix composites were individually 
formed by helically winding 1000 fiber tows of Nicalon silicon 
carbide-type fibers (fiber diameter of 10-15 micrometers) on a 6.4 mm 
diameter SiC rod to form a preform. The total outer diameter of the wound 
rod was 8-12 mm. Each preform was suspended in a respective silica glass 
tube constituting the reaction vessel 11 of FIG. 2). Silica tubes having 
an inside diameter of 16 mm or 40 mm were used. 
Each preform was microwave heated in a single mode Wavemat oven operated at 
2.45 gegahertz to provide a preform surface temperature of 750 to 1000 
degrees C prior to introduction of the gaseous reactant stream to the 
silica glass vessel. The reactant stream comprised methyltrichlorosilane 
(MTS) in hydrogen diluted with nitrogen. The H.sub.2 :MTS ratio used was 
10:1 or 5:10, the N.sub.2 was from 15 to 25 vol.%, and the reactant gas 
stream pressure in the vessel 11 was maintained between 40-120 torr. 
Deposition of the silicon carbide matrix material was conducted for 8-24 
hours. Relatively high density SiC/SiC surface composites with residual 
porosity of less than 10% were produced by this example on the SiC rod. 
FIG. 8 is SEM photomicrograph of a ceramic/ceramic surface composite 
formed pursuant to this example. 
While the invention has been described in terms of specific embodiments 
thereof, it is not intended to be limited thereto but rather only to the 
extent set forth in the claims which follow.