Composite by infiltration

A composite comprised of a continuous matrix phase having a ceramic filler phase distributed therein is produced by shaping a mixture of ceramic filler and a solid constituent of the matrix into a compact and infiltrating the compact with a molten constituent of the matrix which combines with the solid constituent by dissolution and/or diffusion to produce the matrix in situ.

This invention relates to the production of a composite comprised of an 
inorganic matrix and ceramic filler by forming a mixture of the ceramic 
filler and a solid constituent of the matrix into a compact, and 
infiltrating the compact with a molten constituent of the matrix to form 
the matrix in situ. 
The following patent applications are assigned to the assignee hereof and 
are incorporated herein by reference: 
Ser. No. 803,172 filed Dec. 2, 1985 for "Composite by Infiltration" by W. 
B. Hillig discloses the production of a composite by shaping a ceramic 
filler into a porous compact and infiltrating the compact with liquid 
alkaline earth silicate or alkaline earth aluminosilicate. 
Ser. No. 740,444, filed June 3, 1985 for "Composite of Si.sub.3 N.sub.4 by 
Infiltration" by M. K. Brun and W. B. Hillig discloses a composite 
produced by infiltrating the open pores of a sintered polycrystalline 
silicon nitride body with a member selected from the group consisting of 
barium fluoride, calcium fluoride, magnesium fluoride, strontium fluoride, 
cerium fluoride, dysprosium fluoride, gadolinium fluoride, lanthanum 
fluoride, samarium fluoride, yttrium fluoride and a mixture of said 
fluoride and a metal oxide. 
Ser. No. 759,815, filed July 29, 1985 for "Composite by Infiltration" by W. 
B. Hillig discloses the production of a composite by forming a porous 
compact of a ceramic member selected from the group consisting of boron 
carbide, hafnium carbide, hafnium nitride, niobium carbide, niobium 
nitride, silicon carbide, silicon nitride, tantalum carbide, tantalum 
nitride, titanium carbide, titanium nitride, vanadium carbide, vanadium 
nitride, zirconium carbide and zirconium nitride, and infiltrating the 
compact with a member selected from the group consisting of barium 
fluoride, calcium fluoride, magnesium fluoride, strontium fluoride, cerium 
fluoride, dysprosium fluoride, gadolinium fluoride, lanthanum fluoride, 
samarium fluoride, yttrium fluoride, and a mixture of said fluoride with a 
metal oxide. 
There are certain limitations in infiltrating a molten matrix material into 
a compact to form a ceramic composite. The molten matrix material may not 
have the requisite fluidity characteristic to function well as an 
infiltrant. For example, its viscosity may be so high as to require an 
excessive amount of infiltration time, or it may be so high as to prevent 
infiltration. Also, the matrix material may have an excessively high 
melting point, or the molten material may require an impractically high 
temperature to impart a useful lower viscosity. High infiltration 
temperatures not only make a process economically unfeasible, but also are 
likely to affect the compact material deleteriously. 
The present process enables infiltration to be carried out at significantly 
lower temperatures within commercially useful time periods. 
Briefly stated, the present process for producing a composite comprised of 
a matrix phase and a ceramic filler phase and having a porosity of less 
than about 10% by volume comprises forming a mixture of ceramic filler and 
a solid constituent of said matrix, shaping said mixture into a compact 
having an open porosity ranging from about 30% by volume to about 95% by 
volume of the compact, contacting said compact with an infiltrant which is 
the remaining constituent of said matrix phase, said infiltrant 
constituent having a liquidus temperature lower than 1900.degree. C. and a 
viscosity of less than 30 poises at its liquidus temperature, heating the 
resulting structure to an infiltration temperature ranging from the 
liquidus temperature of said infiltrant constituent to below the 
temperature at which there is significant vaporization of said infiltrant 
constituent, said infiltrant constituent having a liquidus temperature at 
least 100.degree. C. lower than that of said matrix phase and/or a maximum 
viscosity at least 50% less than that of said matrix phase at infiltration 
temperature, and infiltrating the resulting liquid infiltrant constituent 
into said compact to produce said composite. 
In another embodiment of the present process, wherein upon completion of 
the infiltration step the solid constituent of the matrix in the compact 
has not dissolved and/or diffused into the infiltrated constituent of the 
matrix to produce the matrix phase, the infiltrated compact is subjected 
to a homogenization step. Specifically, it is heated at a homogenization 
temperature at which all or substantially all of the solid constituent 
dissolves and/or diffuses into the infiltrated constituent forming the 
matrix phase in a period of time of less than 100 hours to produce the 
present composite. 
In the present process, all or substantially all of the constituents of the 
matrix composition combine and ultimately transform to the phase 
equilibrium state determined by the overall combined composition. 
Therefore, the overall composition of the matrix is determined by the 
composition of the infiltrating liquid, the composition and amount of 
solid constituent incorporated into the compact, and the porosity of the 
compact. 
Briefly stated, the present composite is comprised of a ceramic filler 
phase ranging in amount from about 5% by volume to about 70% by volume of 
the composite, and a continuous interconnecting matrix phase ranging in 
amount from about 30% by volume to about 95% by volume of the composite. 
The matrix phase is an inorganic composition having a liquidus temperature 
of at least about 1200.degree. C. and which generally ranges from about 
1200.degree. C. to about 2200.degree. C.

FIG. 1 is a cross section of a structure 1 which illustrates one embodiment 
of the present process prior to infiltration. Graphite cylinder 7 and 
graphite base 2 have a coating of boron nitride 4 and 3 to prevent any 
sticking and facilitate removal of the resulting composite. Porous compact 
5 is comprised of a cold-pressed powder mixture of the ceramic filler and 
a solid constituent of the matrix phase. A layer of granules of infiltrant 
6 comprised of the remaining constituent of the matrix phase is shown in 
contact with compact 5 and covers its entire top surface. 
FIG. 2 shows a cross section of a free standing assembly 12 of a layer of 
granules of infiltrant constituent 11 in contact with the upper surface of 
porous compact 10 comprised of a mixture of ceramic filler and solid 
constituent of the matrix. Assembly 12 is set on graphite base 8 having a 
boron nitride coating 9 to prevent sticking. 
Graphite cylinder 7 and bases 2 and 8 are a convenience and are not 
required for carrying out the present process. However, structures 
chemically inert to the ceramic filler and matrix constituents such as 
graphite cylinder 7 and base 2 provide greater precision in the making of 
a finished product and also provide better control of the amount of 
infiltrant which is needed to penetrate the compact. 
The present ceramic filler is a polycrystalline inorganic material which is 
a solid at processing temperature. Specifically, the ceramic filler of the 
composite has the characteristic of being stable at the temperatures 
necessary for processing or it is not significantly affected by the 
processing temperatures. In the present process, the ceramic filler is 
sufficiently inert so that no significant reaction, and preferably no 
reaction detectable by scanning electron microscope, occurs between it and 
the constituents of the matrix phase or the matrix phase. Also, the 
ceramic filler is at least sufficiently wettable by the infiltrant 
constituent to allow the present infiltration to occur by capillarity. 
Preferably, the present infiltrant constituent of the matrix has a contact 
or wetting angle against the filler of less than 90.degree. C. The present 
process has no significant deleterious effect on the ceramic filler. 
Generally, the filler functions as a reinforcing, toughening, matrix grain 
size controlling material and or abrasion resisting material. 
The particular ceramic filler or mixture of fillers used depends largely on 
the particular properties desired in the composite. Preferably, the 
ceramic filler is a carbide, nitride, boride, silicide or other similar 
ceramic refractory material. Ceramic oxides are not useful as fillers in 
the present invention. 
Representative of ceramic carbides useful in the present process is the 
carbide of boron, chromium, hafnium, niobium, silicon, tantalum, titanium, 
vanadium, zirconium, and mixtures and solid solutions thereof. For 
example, the useful carbides include B.sub.4 C, Cr.sub.3 C.sub.2, HfC, 
NbC, SiC, TaC, TiC, VC and ZrC. 
Representative of the ceramic nitrides useful in the present process is the 
nitride of hafnium, niobium, silicon, tantalum, titanium, vanadium, 
zirconium, and mixtures and solid solutions thereof. For example, the 
useful nitrides include HfN, NbN, Si.sub.3 N.sub.4, TaN, TiN, VN and ZrN. 
Examples of ceramic borides are the borides of hafnium, niobium, tantalum, 
titanium, vanadium, zirconium, and mixtures and solid solutions thereof. 
More specifically, representative of the useful borides are HfB.sub.2, 
NbB, NbB.sub.2, TaB, TaB.sub.2, TiB.sub.2, VB, VB.sub.2 and ZrB.sub.2. 
Examples of useful silicides are TaSi.sub.2, MoSi.sub.2 and WSi.sub.2. 
The ceramic filler can be in any desired form such as, for example, a 
powder or filament or mixtures thereof. Generally, when the filler is in 
the form of a powder, it is characterized by a mean particle size which 
generally ranges from about 0.1 micron to about 1000 microns, preferably 
from about 0.2 micron to about 100 microns, and more preferably from about 
0.5 micron to about 25 microns. 
In one embodiment of the present invention, to produce a compact of 
particular porosity, or of high density, or a composite of particular 
microstructure, a particle size distribution of filler powder can be used 
with fractions of coarse or coarser particles being admixed with fractions 
of fine or finer particles so that the fine particles fit into the voids 
between the large particles and improve packing. Optimum distribution is 
determinable empirically. 
As used herein, filament includes a whisker, discontinuous fiber or 
continuous fiber of filler. Generally, the discontinuous filaments have an 
aspect ratio of at least 10, and in one embodiment of the present 
invention it is higher than 50, and yet in another embodiment it is higher 
than 1000. Generally, the lower their aspect ratio, the higher is the 
packing which can be achieved in the compact since the small fibers 
intertwine or interlock. Also, generally, the higher the aspect ratio of 
the discontinuous fiber for a given volume fraction of filament, the 
better are the mechanical properties of the compact. In cases where the 
filaments are continuous in length, a large packing fraction is possible, 
for example, by arranging them in parallel or weaving them into cloth. 
Generally, the filament ranges from about 0.1 micron to about 20 microns 
in diameter and from about 10 microns to about 10 centimeters in length. 
The filaments are used to impart desirable characteristics to the 
composite, such as improved stiffness strength, and toughness. In general, 
the greater the packing density of filaments, the greater is the 
improvement of such properties. Also, fibers with large aspect ratios 
usually are more effective in producing such improvement than are fibers 
having small aspect ratios. 
In one embodiment of the present process, a mixture of filler powder and 
filaments is used to produce a compact of desired microstructure. The 
particular desired mixture of powder and filaments is determinable 
empirically. 
Mixtures of ceramic filler powders and/or filaments can be produced by a 
number of conventional techniques. 
In carrying out the present process, a mixture of the ceramic filler and 
solid constituent of the matrix composition is formed. The solid 
constituent can be comprised of a single component or a mixture of 
components of the matrix composition. Mixtures herein can be produced in a 
conventional manner. The solid constituent should be used in the amount 
required to produce the desired amount of matrix phase. The solid 
constituent is solid at infiltration temperature and is soluble and/or 
diffusible in the infiltrant constituent at an elevated temperature to 
produce the matrix phase in situ. The average particle size of the solid 
constituent is determinable empirically. It should be of a size which 
allows the formation of the present matrix phase by the present process, 
i.e., a matrix phase which is of homogeneous or substantially homogeneous 
composition. The smaller the particle size of the solid constituent, the 
faster it will dissolve and/or diffuse into the infiltrant constituent. If 
the solid constituent dissolves into the infiltrant at too fast a rate, 
portions of the dissolved solid constituent may be washed away so that the 
composition of the resulting matrix phase will be highly inhomogeneous. If 
the solid constituent is too large in size, it will dissolve or diffuse 
too slowly into the infiltrant to be practical. Generally, the solid 
constituent has a particle size ranging from about 2.mu. to about 200.mu.. 
The mixture of filler and solid constituent can be produced in a 
conventional manner. For example, the filler and solid constituent of the 
matrix can be admixed in a liquid medium in which they are inert under 
ambient conditions using, for example, a propeller blender, and the 
resulting dispersion can be dried in air at ambient temperature. 
Preferably, a uniform or substantially uniform mixture is formed. 
The mixture comprised of ceramic filler and solid constituent of the matrix 
can be formed into a compact, i.e. preform or green body, of desired shape 
and size by a number of conventional techniques. For example, the mixture 
can be extruded, injection molded, die pressed, isostatically pressed or 
slip cast to produce the desired compact. Any lubricants, binders or 
similar materials used in shaping the compact should have no significant 
deleterious effect on the resulting composite. Such materials are 
preferably of the type which evaporate or burn off on heating at 
relatively low temperatures, preferably below 500.degree. C., leaving no 
significant residue. 
Preferably, the compact is formed into the shape and size required of the 
composite to allow the production of the required composite directly. The 
compact can be in any form desired, such as, for example, it can be hollow 
and/or of simple shape and/or of complex shape. The terms compact or 
preform refer to a non-sintered body prepared for infiltration later by 
the molten matrix material. 
The ceramic filler in the compact has a particle or filament size, or a 
ratio of filaments and powder which is predetermined by the particular 
microstructure desired in the resulting composite. 
The open porosity of the compact ranges from about 30% by volume to about 
95% by volume, and preferably from about 35% by volume to about 75% by 
volume, of the compact. The open porosity of the compact depends mostly on 
the composition of the matrix desired in the resulting composite. 
Specifically, the open porosity of the compact determines the amount of 
liquid infiltrant which can be introduced into the compact to combine with 
the solid constituent to form the matrix phase. The open porosity of the 
compact also corresponds to the maximum volume fraction of matrix phase 
attainable in the composite. To produce a composite containing the matrix 
phase in an amount ranging from about 30% by volume to about 95% by volume 
of the composite, the compact should have an open porosity ranging from 
about 30% by volume to about 95% by volume of the compact, respectively. 
By open porosity of the compact or body herein, it is meant pores or voids 
which are open to the surface of the compact or body thereby making the 
interior surfaces accessible to the ambient atmosphere. 
Generally, the present compact has no closed porosity. By closed porosity 
it is meant herein closed pores or voids, i.e. pores not open to the 
surface of the compact or body and therefore not in contact with the 
ambient atmosphere. 
Void or pore content, i.e. both open and closed porosity, can be determined 
by standard physical and metallographic techniques. 
Preferably, the pores in the compact are small, preferably between about 
0.1 micron and about 10 microns, and at least significantly or 
substantially uniformly distributed through the compact thereby enabling 
the production of a composite wherein the matrix phase is at least 
significantly or substantially uniformly distributed. 
In the present process, the infiltrant constituent of the matrix has a 
liquidus temperature ranging from greater than about 1000.degree. C. to 
less than about 1900.degree. C., frequently from greater than about 
1100.degree. C. to less than 1700.degree. C., and more frequently from 
about 1200.degree. C. to about 1600.degree. C. Also, the present 
infiltrant constituent has a viscosity at its liquidus temperature of less 
than about 30 poises, frequently less than about 10 poises, more 
frequently less than about 5 poises, and preferably less than about 1 
poise. In addition, the infiltrant constituent of the matrix has a 
liquidus temperature which is at least 100.degree. C. lower than that of 
the matrix, and/or it has a maximum viscosity which is at least 50% less 
than that of the matrix phase at a given infiltration temperature. 
Preferably, the infiltrant constituent has a liquidus temperature which is 
at least 300.degree. C. lower, and more preferably at least 500.degree. C. 
lower than that of the matrix. Also, preferably, the infiltrant 
constituent has a maximum viscosity which is at least 70%, more preferably 
at least 80% less than that of the matrix phase at a given infiltration 
temperature. By liquidus temperature herein, it is meant the temperature 
at which melting of the material is complete on heating. 
The compositions of the solid and infiltrating constituents of the matrix 
can be determined by a number of techniques. For example, to determine the 
infiltrant constituent from the phase diagram showing the matrix phase, a 
composition or compound can be selected having a liquidus temperature at 
least 100.degree. C. lower than that of the matrix, or a composition or 
compound can be selected because of its known low viscosity, or because 
its composition indicates that its viscosity should be low. Melt viscosity 
of a selected infiltrant composition or compound can be determined in a 
standard manner, or it can be calculated by various techniques disclosed 
in the art. For example, G. Urbain, "Viscosite et Structure de 
Silicoalumineux Liquides", Rev. int. Htes Temp. et Refract.J, 1974, t. II, 
pp. 133-145, discloses models for estimating the viscosity of melts of 
alkaline earth aluminosilicates, and J. D. Mackenzie, "The Discrete Ion 
Theory and Viscous Flow in Liquid Silicates", Transactions of the Faraday 
Society, No. 419, Vol. 53, Part 11, November 1957, discloses models for 
estimating the viscosity of melts of alkali and alkaline earth silicates. 
Preferably, the composition selected as the infiltrant constituent of the 
matrix composition has the lowest liquidus temperature and viscosity. Upon 
final selection of an infiltrant constituent, the solid constituent would 
be comprised of the remaining matrix composition and could also be 
selected from the phase diagram showing the matrix phase. 
If desired, the specific compositions of the solid and infiltrant 
constituents of the matrix can be determined empirically. The sum of the 
constituents should produce the matrix phase by dissolution and/or 
diffusion. 
The specific amounts of solid and liquid constituents needed to produce the 
required amount of matrix phase can be determined by a number of standard 
techniques. For example, amounts can be determined from the chemical 
equation showing the formation of the matrix composition from the solid 
and infiltrant constituents. In the present process, there is no reaction 
between the solid and infiltrant constituents. 
The present process can be illustrated with reference to FIG. 3 for the 
production of a composite wherein the matrix phase is comprised of 
CaO.2Al.sub.2 O.sub.3. From FIG. 3 it can be seen that CaO.2Al.sub.2 
O.sub.3 has a relatively high liquidus temperature of the order of about 
1760.degree. C. On the other hand, 12CaO.7Al.sub.2 O.sub.3 has a liquidus 
temperature of about 1360.degree. C. and would be a useful infiltrant if 
its viscosity at its liquidus temperature is less than 30 poises. Also, 
from FIG. 3, it can be seen that the solid constituent would have to be 
Al.sub.2 O.sub.3. Since FIG. 3 shows a number of compounds containing 
Al.sub.2 O.sub.3 in amounts higher than that contained in 12CaO.7Al.sub.2 
O.sub.3, Al.sub.2 O.sub.3 should be soluble and/or diffusible in 12 
CaO.7Al.sub.2 O.sub.3 at an elevated temperature to produce CaO.2Al.sub.2 
O.sub.3. The following chemical equation can be used to determine the 
required amounts of the constituents: 
EQU 12CaO.7Al.sub.2 O.sub.3 +17Al.sub.2 O.sub.3 .fwdarw.12(CaO.2Al.sub.2 
O.sub.3) (1) 
In a preferred embodiment, the present process is directed to forming a 
matrix phase in situ comprised of an alkaline earth aluminosilicate. These 
materials generally have a high liquidus temperature and/or high 
viscosity. Specifically, in the embodiment the matrix composition can be 
represented as BaO.Al.sub.2 O.sub.3.2SiO.sub.2, 2CaO.Al.sub.2 
O.sub.3.SiO.sub.2, CaO.Al.sub.2 O.sub.3 2.SiO.sub.2, 2MgO.2Al.sub.2 
O.sub.3.5SiO.sub.2, 4MgO.5Al.sub.2 O.sub.3.2 SiO.sub.2, SrO.Al.sub.2 
O.sub.3. SiO.sub.2, 2SrO.Al.sub.2 O.sub.3.SiO.sub.2 and 6SrO.9Al.sub.2 
O.sub.3.2SiO.sub.2 wherein each oxidic constituent can vary from the 
stoichiometric formula. These alkaline earth aluminosilicates can also be 
represented in terms of their oxidic constituents, i.e. MO, Al.sub.2 
O.sub.3 and SiO.sub.2, by the general formula xMO.Al.sub.2 
O.sub.3.zSiO.sub.2 where M=Ba, Ca, Mg, Sr and mixtures thereof, where x is 
1, 2, 4 or 6, y is 1, 2, 5 or 9 and z is 1, 2 or 5. Each oxidic 
constituent in such stoichiometric formula can range up to .+-.50%, 
preferably less than .+-.10% from its stoichiometric composition. 
As an illustration, cordierite (2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2) has a 
viscosity of about 36 poise at its liquidus temperature of about 
1540.degree. C. The matrix composition 2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2 
can be synthesized from a combination of components, e.g., 
EQU (a) 2(MgO..4Al.sub.2 O.sub.3.1.3SiO.sub.2)+[1.2Al.sub.2 O.sub.3 
+2.4SiO.sub.2 ] (2) 
or 
EQU (b) 2(MgO.O0.4Al.sub.2 O.sub.3.1.3SiO.sub.2)+[0.4 Mullite+1.6SiO.sub.2 ](3) 
or 
EQU (c) 2(MgO.0.4Al.sub.2 O.sub.3.2.1SiO.sub.2)+[0.4 Mullite] (4) 
In the above, the () means liquid and the [] means solids. The infiltrant 
constituent in (a), (b) and (c) has a liquidus temperature at least 
100.degree. C. lower than that of cordierite, a viscosity lower than 20 
poises at liquidus temperature and a viscosity which is at least 50% lower 
than that of cordierite at infiltration temperature. 
In carrying out the present process, the infiltrant constituent is placed 
in contact with the compact and such contact can be in a number of forms. 
Preferably, to inhibit its vaporization during infiltration, the 
infiltrant powder is compacted into a pressed powder form or it is used in 
the form of large granules. Preferably, a layer of infiltrant is deposited 
on as large as possible a surface area of the compact to promote 
infiltration. In one embodiment of the present invention, an aqueous 
slurry of infiltrant powder is coated on the surface of the compact and 
dried leaving a coating, preferably a continuous coating, of infiltrant. 
Preferably, the amount of infiltrant in contact with or deposited on the 
compact is sufficient to infiltrate the compact so that infiltration can 
be completed in a single step. However, if desired, the compact can be 
partially infiltrated and the infiltration repeated until the desired 
composite is produced. 
Should the filler contain desorbable material on its surface, the structure 
comprised of the infiltrant in contact with the compact preferably is 
heated initially to an elevated temperature below the melting point of the 
infiltrant, typically from about 800.degree. C. to below the melting point 
of the infiltrant, for a period of time sufficient to degas the compact, 
typically for about 10 minutes. Degassing temperature and time are 
determinable empirically. Generally, such degassing is necessary when the 
filler has desorbable material on its surface, such as hydrogen chloride, 
which would lead to gas evolution during the infiltration causing gas 
pockets or gross porosity. The completion of degassing is indicated by the 
stabilization of the pressure in the furnace. 
After degassing, if any, the temperature is increased to a temperature at 
which the infiltrant constituent is liquid and the compact is solid to 
infiltrate the liquid infiltrant into the open pores of the compact. The 
infiltration temperature ranges from the liquidus temperature of the 
infiltrant constituent to a temperature at which no significant 
vaporization of infiltrant occurs. Generally, with increasing infiltration 
temperature, the viscosity of the infiltrant decreases. The particular 
infiltration temperature is determinable empirically, and typically it 
ranges from greater than about 1000.degree. C. to about 1900.degree. C., 
frequently from about 1150.degree. C. to about 1750.degree. C., and more 
frequently from about 1210.degree. C. to about 1650.degree. C. Preferably, 
to prevent significant vaporization of the infiltrant, infiltration is 
carried out at as low a temperature as possible, and preferably no higher 
than about 50.degree. C. above the liquidus temperature of the infiltrant. 
To ensure infiltration of the compact, the entire compact should be at or 
above the liquidus temperature of the infiltrant during infiltration. 
Infiltration time can vary, but generally infiltration is completed within 
about an hour. 
Generally, the heating rate to below or just below the melting point of the 
infiltrant ranges up to about 100.degree. C. per minute. Commencing just 
below the melting point of the infiltrant, i.e. preferably within about 15 
degrees of the onset of the melting, and continuing to the maximum 
infiltration temperature, the heating rate preferably ranges from about 
1.degree. C. to about 10.degree. C. per minute, more preferably from about 
1.degree. C. to about 5.degree. C. per minute, to facilitate controlled 
infiltration of the liquid infiltrant into the porous compact. Overheating 
may cause significant vaporization of the infiltrant and may interfere 
with the present infiltration and also may cause undesirable deposition in 
the heating apparatus. 
If, upon completion of the infiltration step all or substantially all of 
the constituents have not combined to produce the matrix phase, which is 
determinable in a conventional manner, the infiltrated compact is 
homogenized, i.e., it is heated at a homogenization temperature at which 
all or substantially all of the solid constituent dissolves and/or 
diffuses into the infiltrant constituent to produce the matrix phase in a 
period of time of less than 100 hours. The homogenization temperature can 
range widely and is determinable empirically. Generally, it ranges from 
about 50% of the absolute liquidus temperature to about the absolute 
liquidus temperature of the matrix phase, and frequently, from higher than 
about 1000.degree. C. to a temperature at which the infiltrant constituent 
is liquid. Preferably, homogenization is completed in less than 50 hours, 
more preferably less than 30 hours and most preferably less than 10 hours. 
Homogenization should have no significant deleterious effect on the 
composite. 
At the completion of infiltration or homogenization, the matrix phase may 
be amorphous and/or polycrystalline. If it is amorphous, or partly 
amorphous, the composite can be cooled at a rate determinable empirically 
which may make it polycrystalline, and frequently this requires quenching. 
However, for some amorphous matrix compositions, an annealing of the 
composite at an elevated temperature below the homogenization temperature, 
determinable empirically or known in the art, may be required to induce 
crystallization. Also, for some matrix compositions, the inclusion of a 
nucleating agent in the starting materials may be required to produce a 
polycrystalline matrix phase. 
Generally, after infiltration or homogenization, the rate of cooling can 
vary and is not critical, but it should have no significant deleterious 
effect on the composite. Specifically, cooling should be at a rate which 
avoids cracking of the composite, and this is determinable empirically 
depending largely on the geometry and size of the piece. Generally, a 
cooling rate of less than about 50.degree. C. per minute is useful for 
small bodies of simple shape and a cooling rate as great as about 
20.degree. C. per minute or higher is useful for large bodies of complex 
shape. Preferably, the composite is cooled to ambient temperature prior to 
removal from the heating apparatus. 
The present process is carried out in an atmosphere in which the ceramic 
filler and matrix constituents are inert or substantially inert, i.e., an 
atmosphere which has no significant deleterious effect thereon. 
Specifically, the process atmosphere should be one in which no significant 
reaction between the filler and matrix or matrix constituents takes place. 
Reaction involving the filler will degrade the mechanical properties of 
the resulting composite. Preferably, the process atmosphere maintains the 
inertness of the filler so that no reaction between the filler and matrix 
or matrix constituents takes place which is detectable by scanning 
electron microscopy. Also, the process atmosphere should be non-oxidizing 
with respect to the ceramic filler. The particular process atmosphere is 
determinable empirically and depends largely on the ceramic filler used. 
Generally, the process atmosphere can be comprised of or contain nitrogen, 
a noble gas, preferably argon or helium, and mixtures thereof. However, 
when the filler is a ceramic carbide, the process atmosphere preferably 
should contain at least a partial pressure of carbon monoxide determinable 
empirically or by thermodynamic calculation which is at least sufficient 
to prevent reaction or significant reaction between the carbide and matrix 
or matrix constituents. Also, when the filler is a ceramic nitride, the 
process atmosphere preferably should contain at least a partial pressure 
of nitrogen determinable empirically or by thermodynamic calculation which 
is at least sufficient to prevent reaction or significant reaction between 
the nitride and the matrix or matrix constituents, and preferably the 
atmosphere is nitrogen. 
The pressure of the process atmosphere can vary widely and is determinable 
empirically or by thermodynamic calculations and depends largely on the 
dissociation and/or reaction pressures of the particular ceramic filler 
and infiltrant and the temperature required for infiltration. More 
specifically, the process atmosphere can range from below to above ambient 
pressure, and preferably it is at ambient, i.e. atmospheric or about 
atmospheric. When the process atmosphere is at reduced pressure, typically 
it can range from about 0.1 torr up to ambient, and frequently, it ranges 
from about 100 torr to about 400 torr. When the process atmosphere is 
above ambient, it is convenient to restrict it to below 10 atmospheres. 
Any excess infiltrant on the surface of the composite can be removed by a 
number of techniques, such as, for example, by gentle scraping or 
abrading. 
The present composite does not contain any significant amount of any 
constituent or component of the matrix or of any reaction product of 
ceramic filler and matrix or matrix constituent. Preferably, the present 
composite does not contain any constituent or component of the matrix or 
any reaction product of ceramic filler and matrix or matrix constituent 
which is detectable by X-ray diffraction analysis, and more preferably, 
which is detectable by scanning electron microscopy. 
The composite produced by the present process has a porosity of less than 
about 10% by volume, preferably less than about 5% by volume, more 
preferably less than about 1% by volume, and most preferably, it is 
pore-free, i.e., it is fully dense. 
Generally, in the present process, there is no significant loss of the 
components used to form the composite. 
The matrix phase is continuous and interconnecting, and generally, it is 
distributed evenly or substantially evenly through the composite. Also, 
generally, the matrix phase envelops more than 25% by volume, preferably 
more than 50% by volume, of the individual filler members, i.e., particles 
and/or filaments. 
The present invention makes it possible to fabricate a composite of the 
desired shape and size directly. For example, the composite produced by 
the present process can be in the form of a flat body, a crucible, a 
hollow shaped article, a long rod, a gasket, or a wear resistant part such 
as a bushing. Since the present composite can be produced in a 
predetermined configuration of predetermined dimensions, it would require 
little or no machining. 
The composite produced by the present process has a number of uses 
depending largely on its particular composition and microstructure. For 
example, it may be useful as a high temperature structural material, as a 
vane, or as a wear resistant part such as a bushing. 
The invention is further illustrated by the following examples where the 
procedure was as follows unless otherwise noted: 
Each preform, i.e. compact, had an open porosity of roughly about 50% by 
volume of the preform. The pores in the preform ranged from about 0.1 
micron to about 10 microns. 
The composite produced in each example was diamond polished for microscopic 
examination. 
EXAMPLE 1 
This example was directed to the formation of a cordierite phase in situ. 
One gram of 2-5 micron silicon carbide powder was hand mixed with 1 gram of 
a 3:2 (molar basis) blend of SiO.sub.2 and Al.sub.2 O.sub.3 powders 
respectively followed by sieving the mixture. The Al.sub.2 O.sub.3 and 
SiO.sub.2 powders had an average particle size of about 2-5.mu.. The 
mixture was pressed into a disk-shaped compact of substantially uniform 
thickness of about 3 mm. 
The compact was placed on a carbon felt mat and its top surface was covered 
with a layer of 3.5 grams of MgSiO.sub.3 granules. 
The resulting structure was placed in a carbon mold such as shown in FIG. 1 
and heated by induction under a reduced pressure of about 1 torr to about 
1650.degree. C. where it was held for about 10 minutes and then subjected 
to a rapid cool-down at a rate of about 100.degree. C. per minute to 
ambient temperature. At the infiltration temperature of about 1650.degree. 
C., MgSiO.sub.3 has a viscosity at least 50% less than that of cordierite. 
Examination of the resulting composite showed that infiltration of the 
MgSiO.sub.3 into the compact occurred resulting in a dense composite 
having no measurable open porosity. The composite had a continuous 
interconnecting matrix phase which appeared glassy. On the basis of 
calculations based on the available free volume in the compact in which 
the molten MgSiO.sub.3 could flow, it was known that the composition of 
the matrix phase corresponded to that of cordierite. No attempt was made 
to crystallize the matrix to the cordierite structure. 
EXAMPLE 2 
This example was directed to the formation of a strontium feldspar 
(SrO.Al.sub.2 O.sub.3.2SiO.sub.2) phase in situ. 
1.5 grams of 2-5 micron silicon carbide powder was hand mixed with 0.5 gram 
of a 1:1 (molar basis) blend of Al.sub.2 O.sub.3 and SiO.sub.2 powders 
followed by sieving the mixture. The Al.sub.2 O.sub.3 and SiO.sub.2 
powders had an average particle size of about 2-5.mu.. The mixture was 
pressed into a disk-shaped compact of substantially uniform thickness of 
about 3 mm. 
The compact was placed on a carbon felt mat and its top surface was covered 
with a layer of 3.0 grams of SrSiO.sub.3 granules. 
The resulting structure was heated in an induction furnace contained in an 
enclosure under a reduced pressure of about 1 torr to about 1600.degree. 
C. where it was held for about 10 minutes and then subjected to a rapid 
cool-down at a rate of about 100.degree. C. per minute to ambient 
temperature. SrSiO.sub.3 has a liquidus temperature at least 100.degree. 
C. lower than that of strontium feldspar, and at the infiltration 
temperature of about 1600.degree. C., its viscosity is at least 50% less 
than that of strontium feldspar. 
Examination of the resulting composite showed that infiltration of the 
SrSiO.sub.3 into the compact occurred resulting in a dense composite 
having no measurable open porosity. The composite had a continuous 
interconnecting matrix phase which appeared glassy. On the basis of 
calculations based on the available free volume in the compact into which 
the molten SrSiO.sub.3 could flow, it was known that the composition of 
the glassy matrix phase corresponded closely to that of strontium 
feldspar. No attempt was made to crystallize the matrix to the strontium 
feldspar structure.