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Claims  |
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What is claimed is:
1. A composite consisting essentially of a continuous interconnecting
polycrystalline mullite phase ranging from about 40% by volume to about
80% by volume of said composite and a polycrystalline inorganic non-oxide
filler phase ranging from about 20% by volume to about 60% by volume of
the composite, said mullite phase encapsulating at least about 50% by
volume of said filler phase and encapsulating and being intermixed with
the balance of said filler phase, said filler phase being distributed at
least significantly uniformly in said composite, said composite having a
porosity of less than about 5% by volume of said composite.
2. The composite according to claim 1 wherein said filler is particulate in
form.
3. The composite according to claim 1 wherein said filler is in the form of
filaments.
4. The composite according to claim 1 wherein said filler is comprised of a
mixture of particles and filaments.
5. The composite according to claim 1 wherein said mullite has an average
grain size less than about 15 microns.
6. The composite according to claim 1 wherein said composite contains a
glassy phase ranging from a detectable amount up to about 10% by volume of
said composite.
7. The composite according to claim 1 wherein said porosity is less than
about 1% by volume.
8. A process for producing a composite comprised of polycrystalline mullite
phase ranging from about 20% by volume to about 99.9% by volume of said
composite and a polycrystalline inorganic filler ranging from about 0.1%
by volume to about 80% by volume of the composite, said composite having a
porosity of less than about 10% by volume, which consists essentially of
forming a mixture of amorphous alumino-silicate glass powder, alumina
powder and filler in amounts required to produce said composite, said
glass powder consisting essentially of from about 15% by weight to about
40% by weight Al.sub.2 O.sub.3 balance SiO.sub.2 and having a liquidus
temperature below about 1800.degree. C., said glass powder, alumina powder
and filler being present in amounts required to produce said composite,
said filler being a solid in said process and not significantly affected
by said process, and hot pressing the mixture at a temperature ranging
from about 1500.degree. C. to about 1750.degree. C. at which said glass is
fluid but below its liquidus temperature under a pressure at least
sufficient to force the fluid glass to encapsulate at least about 50% by
volume of said filler and encapsulate and intermix with the balance of
said filler forming a continuous interconnecting phase, said hot pressing
of said mixture being carried out in a non-oxidizing atmosphere, said
alumina dissolving in and reacting with said glass producing said
composite.
9. The process according to claim 8 wherein said filler is in the form of a
powder.
10. The process according to claim 8 wherein said filler is in the form of
filaments.
11. The process according to claim 8 wherein said filler is comprised of a
mixture of powder and filaments.
12. The process according to claim 8 wherein said composite contains a
glassy phase from a detectable amount up to about 20% by volume of said
composite.
13. The process according to claim 8 wherein said composite contains a
glassy phase ranging from a detectable amount up to about 20% by volume of
said composite and wherein said composite is heated at a temperature
ranging from about 1200.degree. C. to about 1500.degree. C. to convert
said glassy phase to polycrystalline mullite.
14. A process for producing a composite comprised of polycrystalline
mullite phase having an average grain size of less than about 15 microns
ranging from about 20% by volume to about 99.9% by volume of said
composite and a polycrystalline inorganic filler phase ranging from about
0.1% by volume to about 80% by volume of said composite, said composite
having a porosity of less than about 10% by volume, which consists
essentially of forming a mixture of amorphous alumino-silicate glass
powder, alumina powder, polycrystalline mullite nucleating agent powder
and filler, said glass powder consisting essentially of from about 15% by
weight to about 40% by weight Al.sub.2 O.sub.3 balance SiO.sub.2 and
having a liquidus temperature below about 1800.degree. C., said glass
powder, alumina powder, nucleating agent and filler being present in
amounts required to produce said composite, said nucleating agent having
an average particle size of less than about 5 microns and ranging from
about 0.1% by weight to about 10% by weight of the total weight of said
glass and alumina, said filler being a solid in said process and not
significantly affected by said process, and hot pressing the mixture at a
temperature ranging from about 1500.degree. C. to about 1750.degree. C. at
which said glass is fluid but below its liquidus temperature under a
pressure at least sufficient to force the fluid glass to encapsulate at
least about 50% by volume of said filler and encapsulate and intermix with
the balance of said filler forming a continuous interconnecting phase,
said hot pressing of said mixture being carried out in a non-oxidizing
atmosphere, said alumina dissolving in and reacting with said glass
producing said composite.
15. The process according to claim 14 wherein said filler is in the form of
a powder.
16. The process according to claim 14 wherein said filler is in the form of
filaments.
17. The process according to claim 14 wherein said filler is comprised of a
mixture of powder and filaments.
18. The process according to claim 14 wherein said nucleating agent is used
in an amount of less than 1% by weight and has a particle size ranging
between about 0.1 micron to about 0.5 micron and said polycrystalline
mullite phase has an average grain size of about 1 micron.
19. The process according to claim 14 wherein said composite contains a
glassy phase ranging from a detectable amount up to about 20% by volume of
said composite and wherein said composite is heated at a temperature
ranging from about 1200.degree. C. to about 1500.degree. C. to convert
said glassy phase to mullite.
20. The composite according to claim 1 wherein said filler is a carbide of
a member selected from the group consisting of boron, hafnium, niobium,
silicon, tantalum, titanium, vanadium, zirconium and a mixture and solid
solution thereof.
21. The composite according to claim 1 wherein said filler is a boride
selected from the group consisting of HfB.sub.2, NB, NbB.sub.2, TaB,
TaB.sub.2, TiB.sub.2, VB, VB.sub.2, ZrB.sub.2 and a mixture and solid
solution thereof.
22. The composite according to claim 1 wherein said filler is a ceramic
nitride.
23. The composite according to claim 1 wherein the filler is a ceramic
silicide.
24. The composite according to claim 1 wherein said filler is silicon
nitride.
25. The composite according to claim 1 wherein said mullite phase
encapsulates more than 90% by volume of said filler phase.
26. The process according to claim 14 wherein said filler is a carbide of a
member selected from the group consisting of boron, hafnium, niobium,
silicon, tantalum, titanium, vanadium, zirconium and a mixture and solid
solution thereof.
27. The process according to claim 14 wherein said filler is a boride
selected from the group consisting of HfB.sub.2, NbB, NbB.sub.2, TaB,
TaB.sub.2, TiB.sub.2, VB, VB.sub.2, ZrB.sub.2, and a mixture and solid
solution thereof.
28. The process according to claim 14 wherein said filler is a ceramic
nitride.
29. The process according to claim 14 wherein said filler is a ceramic
silicide.
30. A composite consisting essentially of a continuous interconnecting
mullite phase ranging from about 20% by volume to about 99.9% by volume of
said composite and a polycrystalline inorganic filler phase ranging from
about 0.1% by volume to about 80% by volume of the composite, said filler
phase being a carbide of a member selected from the group consisting of
boron, hafnium, niobium, silicon, tantalum, titanium, vanadium, zirconium,
and a mixture and solid solution thereof, said mullite phase encapsulating
at least about 50% by volume of said filler phase and encapsulating and
being intermixed with the balance of said filler phase, said filler phase
being distributed at least significantly uniformly in said composite, said
composite having a porosity of less than about 5% by volume of said
composite.
31. The composite according to claim 30 wherein said filler is particulate
in form.
32. The composite according to claim 30 wherein said filler is in the form
of a filaments.
33. The composite according to claim 30 wherein said filler is comprised of
a mixture of particles and filaments.
34. The composite according to claim 30 wherein said mullite has an average
grain size less than about 15 microns.
35. The composite according to claim 30 wherein said composite contains a
glassy phase ranging from a detectable amount up to about 10% by volume of
said composite.
36. A composite consisting essentially of a continuous interconnecting
polycrystalline mullite phase ranging from about 20% by volume to about
99.9% by volume of said composite and a polycrystalline inorganic filler
phase ranging from about 0.1% by volume to about 80% by volume of the
composite, said filler being a member selected from the group consisting
of HfB.sub.2, NbB, NbB.sub.2, TaB, TaB.sub.2, TiB.sub.2, VB, VB.sub.2,
ZrB.sub.2, and a mixture and solid solution thereof, said mullite phase
encapsulating at least about 50% by volume of said filler phase and
encapsulating and being intermixed with the balance of said filler phase,
said filler phase being distributed at least significantly uniformly in
said composite, said composite having a porosity of less than about 5% by
volume of said composite.
37. The composite according to claim 36 wherein said filler is particulate
in form.
38. The composite according to claim 36 wherein said filler is in the form
of filaments.
39. The composite according to claim 36 wherein said filler is comprised of
a mixture of particles and filaments.
40. The composite according to claim 36 wherein said mullite has an average
grain size less than about 15 microns.
41. The composite according to claim 36 wherein said composite contains a
glassy phase ranging from a detectable amount up to about 10% by volume of
said composite.
42. A composite consisting essentially of a continuous interconnecting
polycrystalline mullite phase ranging from about 20% by volume to about
99.9% by volume of said composite and polycrystalline silicon nitride
filler phase ranging from about 0.1% by volume to about 80% by volume of
the composite, said mullite phase encapsulating at least about 50% by
volume of said filler phase and encapsulating and being intermixed with
the balance of said silicon nitride filler phase, said filler phase being
distributed at least significantly uniformly in said composite, said
composite having a porosity of less than about 5% by volume of said
composite.
43. The composite according to claim 42 wherein said filler is particulate
in form.
44. The composite according to claim 42 wherein said filler is in the form
of filaments.
45. The composite according to claim 42 wherein said filler is comprised of
a mixture of particles and filaments.
46. The composite according to claim 42 wherein said mullite has an average
grain size less than about 15 microns.
47. The composite according to claim 42 wherein said composite contains a
glassy phase ranging from a detectable amount up to about 10% by volume of
said composite.
48. A composite consisting essentially of a continuous interconnecting
polycrystalline mullite phase ranging from about 20% by volume to about
99.9% by volume of said composite and polycrystalline molybdenum
disilicide filler phase ranging from about 0.1% by volume to about 80% by
volume of the composite, said mullite phase encapsulating at least about
50% by volume of said filler phase and encapsulating and being intermixed
with the balance of said filler phase, said filler phase being distributed
at least significantly uniformly in said composite, said composite having
a porosity of less than about 5% by volume of said composite.
49. The composite according to claim 48 wherein said filler is particulate
in form.
50. The composite according to claim 48 wherein said filler is in the form
of filaments.
51. The composite according to claim 48 wherein said filler is comprised of
a mixture of particles and filaments.
52. The composite according to claim 48 wherein said mullite has an average
grain size less than about 15 microns.
53. The composite according to claim 48 wherein said composite contains a
glassy phase ranging from a detectable amount up to about 10% by volume of
said composite.
54. A composite consisting essentially of a continuous interconnecting
polycrystalline mullite phase ranging from about 20% by volume to about
99.9% by volume of said composite and a polycrystalline inorganic filler
phase ranging from about 0.1% by volume to about 80% by volume of the
composite, said mullite phase encapsulating at least about 50% by volume
of said filler phase and encapsulating and being intermixed with the
balance of said filler phase, said filler phase being in the form of
filaments and being distributed at least significantly uniformly in said
composite, said composite having a porosity of less than about 5% by
volume of said composite.
55. A composite consisting essentially of a continuous interconnecting
polycrystalline mullite phase ranging from about 40% by volume to about
80% by volume of said composite and a polycrystalline inorganic filler
phase ranging from about 20% by volume to about 60% by volume of the
composite, said mullite phase encapsulating at least about 50% by volume
of said filler phase and encapsulating and being intermixed with the
balance of said filler phase, said filler phase being in the form of
filaments and being distributed at least significantly uniformly in said
composite, said composite having a porosity of less than about 5% by
volume of said composite.
56. The composite according to claim 55 wherein said filler is a carbide of
a member selected from the group consisting of boron, hafnium, niobium,
silicon, tantalum, titanium, vanadium, zirconium and a mixture and solid
solution thereof.
57. The composite according to claim 55 wherein said filler is a boride
selected from the group consisting of HfB.sub.2, NB, NbB.sub.2, TaB,
TaB.sub.2, TiB.sub.2, VB, VB.sub.2, ZrB.sub.2 and a mixture and solid
solution thereof.
58. The composite according to claim 55 wherein said filler is a ceramic
nitride.
59. The composite according to claim 55 wherein said filler is ceramic
silicide.
60. The composite according to claim 55 wherein said composite contains a
glassy phase ranging from a detectable amount up to about 10% by volume of
said composite. |
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Claims |
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Description |
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The present invention relates to reactive hot pressing and to a composite
comprised of a continuous interconnecting phase of polycrystalline mullite
and a polycrystalline inorganic filler phase.
In copending U.S. Ser. No. 747,536 entitled MULLITE BY REACTIVE HOT
PRESSING filed on even date herewith in the names of W. B. Hillig and S.
Musikant and assigned to the assignee hereof and incorporated herein by
reference, there is disclosed the hot pressing of a mixture of
alumino-silicate glass and alumina having a composition corresponding to
mullite and containing a nucleating mullite powder to produce a dense
polycrystalline mullite body having an average grain size of less than 15
microns.
Polycrystalline mullite, a polycrystalline aluminum silicate phase of
composition 3Al.sub.2 O.sub.3.2SiO.sub.2 or close to 3Al.sub.2
O.sub.3.2SiO.sub.2 can contain from about 71.5 weight % to about 76 weight
% Al.sub.2 O.sub.3 and it has a melting point generally above 1820.degree.
C. depending on its specific composition.
Mullite is an attractive high temperature structural ceramic material
because of its high melting temperature, its relatively low thermal
expansivity and thermal conductivity compared with alumina. However, pure
mullite ceramics are relatively difficult materials to produce.
Conventionally made material does not have outstanding strength.
Fracture toughness is also a major consideration. One effective way to
enhance such toughness is by incorporating fibrous high strength
reinforcement materials into the structure. It should be possible to
toughen mullite in this way by making it the continuous matrix phase in
which the fibers are embedded. Candidate fiber materials include SiC and
Si.sub.3 N.sub.4 whiskers or filaments. However, the presence of stiff
filaments in the green (unconsolidated) structure interferes with normal
sintering. Such fibers act like rods which oppose shrinkage during
sintering which is needed to produce a fully dense final structure.
The present invention provides a means of avoiding the anti-shrinkage
characteristics of fibers.
The present invention utilizes reactive hot pressing, a technique which
involves hot pressing a mixture of materials which will undergo some type
of chemical reaction or transformation during the heat treatment. The
resulting material will then have the same overall chemical composition as
the starting material, but its phase content will be different.
More specifically, in one aspect, this invention is directed to a process
for forming a composite of controlled geometry useable up to the melting
point of polycrystalline mullite. The starting mixture is comprised of
amorphous alumino-silicate glass powder, alumina powder and filler. The
filler should be stable at the processing temperature, and not reactive,
or not significantly reactive, with the mullite phase that is formed in
situ.
The mixture is then hot pressed. As the temperature is raised, the glass
starts to soften and at some temperature it becomes sufficiently fluid so
that its yield stress will be overcome by the pressure applied to the die,
forcing the fluid glass to fill interstices and surround the particles
and/or filaments of filler forming a continuous interconnecting phase. The
alumina dissolves in the glass reacting with it to form mullite in situ.
A typical example of the present invention is shown in the following
reaction:
SiC+alumino-silicate glass+Al.sub.2 O.sub.3 =SiC+mullite
SiC in the above example is the present filler and does not participate to
any significant extent in the reaction. It remains as an inert phase while
the reaction between the glass and alumina takes place.
One of the advantages of the present method is that it will yield a
composite with a continuous mullite phase which encapsulates at least
about 20% by volume of the filler and/or filaments and which either
encapsulates or is intermixed with the balance of the filler particles
and/or filaments.
Those skilled in the art will gain a further and better understanding of
the present invention from the detailed description set forth below,
considered in conjunction with the accompanying figure forming a part of
the specification which is a phase diagram for the SiO.sub.2 --Al.sub.2
O.sub.3 system showing the present mullite phase.
Briefly stated, the present process for producing a composite comprised of
polycrystalline mullite phase ranging from about 20% by volume to about
99.9% by volume of the composite and a polycrystalline inorganic filler
ranging from about 0.1% by volume to about 80% by volume of the composite
comprises forming a mixture of amorphous alumino-silicate glass, alumina
and filler in amounts required to produce said composite, said glass being
comprised of from about 15% by weight to about 40% by weight Al.sub.2
O.sub.3 balance SiO.sub.2 and having a liquidus temperature below about
1800.degree. C., and hot pressing the mixture at a temperature at which
said glass is fluid but below its liquidus temperature under a pressure at
least sufficient to force the fluid glass to encapsulate at least about
20% by volume of said filler and encapsulate and intermix with the balance
of said filler forming a continuous interconnecting phase, said alumina
dissolving in and reacting with said glass producing said composite.
In a preferred embodiment, the present process produces a composite
comprised of polycrystalline mullite having an average grain size of less
than about 15 microns in an amount ranging from about 20% by volume to
about 99.9% by volume of the composite and a polycrystalline inorganic
filler ranging from about 0.1% by volume to about 80% by volume of the
composite comprises forming a mixture of amorphous alumino-silicate glass,
alumina, polycrystalline mullite nucleating agent and filler, said glass
being comprised of from about 15% by weight to about 40% by weight
Al.sub.2 O.sub.3 balance SiO.sub.2 and having a liquidus temperature below
about 1800.degree. C., said glass, alumina and filler being present in
amounts required to produce said composite, said nucleating agent having
an average particle size of less than about 5 microns and ranging from
about 0.1% by weight to about 10% by weight of the total weight of said
glass and alumina, and hot pressing the mixture at a temperature at which
said glass is fluid but below its liquidus temperature under a pressure at
least sufficient to force the fluid glass to encapsulate at least about
20% by volume of said filler and encapsulate and intermix with the balance
of said filler forming a continuous interconnecting phase, said alumina
dissolving in and reacting with said glass producing said composite.
The present alumino-silicate glass can be formed by a number of techniques
and can be produced by conventional glass making techniques from a mixture
of Al.sub.2 O.sub.3 and SiO.sub.2. The present alumino-silicate glass is
an amorphous material comprised of from about 15% by weight to about 40%
by weight Al.sub.2 O.sub.3 balance SiO.sub.2. Preferably, it contains as
much Al.sub.2 O.sub.3 as practical since with increasing Al.sub.2 O.sub.3
content, the fluidity of the glass increases. Frequently, however, the
glass contains from about 20% by weight to about 30% by weight Al.sub.2
O.sub.3 since glass with a higher content of Al.sub.2 O.sub.3 requires
significantly higher temperatures for its preparation and the required
high temperature equipment may not be conventionally available.
The present alumino-silicate glass has a liquidus temperature below
1800.degree. C. and which generally ranges from above about 1600.degree.
C. to less than about 1800.degree. C. depending on its particular Al.sub.2
O.sub.3 content, i.e. the higher the Al.sub.2 O.sub.3 content the higher
is its liquidus temperature. By the liquidus temperature of the glass
herein it is meant that temperature at which no crystalline material can
exist stably. Also, the higher the Al.sub.2 O.sub.3 content of the glass,
the more fluid it is at a given temperature. For example, at 1300.degree.
C. the glass having a 40% by weight Al.sub.2 O.sub.3 and 60% by weight
SiO.sub.2 is fluid and has a viscosity of about 20,000 poise, whereas the
glass containing 15% by weight Al.sub.2 O.sub.3 is practically not fluid
being about 50,000 times more viscous. By a fluid glass herein it is meant
a plastic deformable glass.
The present glass and alumina powders are used in amounts required to
produce the mullite phase. The average particle size of the glass and
alumina powders can vary, and generally, it is less than about 10 microns,
and preferably, it is submicron. Preferably, to insure production of a
uniform or substantially uniform distribution of filler in the composite,
the average particle size of the glass and alumina powders should not be
larger than the smallest dimension of the filler.
In the preferred embodiment of the present process, a nucleating agent
comprised of polycrystalline mullite powder is used to produce a composite
having a polycrystalline mullite phase of uniform or substantially uniform
grain size with an average grain size of less than about 15 microns. The
present nucleating agent constitutes the microcrystalline growth centers
from which further growth into the final mullite grains occurs. The
nucleating mullite powder preferably is of uniform or substantially
uniform particle size. Its average particle size depends largely on the
average grain size desired in the mullite phase and is determinable
empirically, and generally, it is less than about 5 microns. Ordinarily,
the finer the size of the nucleating agent, the finer will be the grain
size of the resulting mullite. Preferably, the average particle size of
the nucleating mullite powder is about 20% of the desired average grain
size of the resulting mullite phase. In the present invention, the
nucleating powder preferably has an average particle size which is less
than about 2.5 microns, more preferably less than about 1 micron, and most
preferably it ranges between about 0.1 micron to about 0.5 micron. In a
preferred embodiment, a nucleating powder having an average particle size
ranging between about 0.1 micron to about 0.5 micron is used to produce
the present composite wherein the mullite phase has an average grain size
of about 1 micron.
The particular amount of nucleating mullite powder used is determinable
empirically and depends largely on the amount of alumino-silicate glass
and alumina present. The nucleating powder must be present in at least an
amount which provides sufficient growth centers to produce the mullite
grain size desired. Generally, in the present process, the nucleating
powder is used in an amount ranging from about 0.5% by weight to about 10%
by weight, preferably from about 1% by weight to about 5% by weight of the
total weight of the alumino-silicate glass and alumina. An amount of
nucleating powder less than about 0.5% by weight may not be enough to be
operable whereas an amount in excess of about 10% by weight may have a
detrimental effect on the viscosity of the glass. Generally, the finer the
size of the nucleating powder, the less of it is needed to produce the
present preferred composite.
The present filler is any inorganic material which is a solid at the
processing temperature and which does not react or does not react to any
significant extent with mullite, alumina, or the alumino-silicate glass.
More specifically, the filler used in the present process as the inert
phase in the composite has the characteristic of being stable at the
temperatures necessary for processing or it is not significantly affected
by the processing temperatures. Also, in the present process, the filler
is relatively inert so that the favored reaction will be between the
reactants to form mullite. The present process has no significant effect
on the filler.
The particular filler or mixture of fillers used in the present process
depends largely on the particular composite desired, i.e., the particular
properties desired in the composite.
Generally, the filler is a ceramic material. Representative of ceramic
carbides useful in the present process are the carbides of boron, hafnium,
niobium, silicon, tantalum, titanium, vanadium, zirconium, and mixtures
and solid solutions thereof.
Still other useful fillers are the ceramic borides such as the borides of
hafnium, niobium, tantalum, titanium, vanadium, zirconium, and mixtures
and solid solutions thereof. More specifically, representative of the
borides are HfB.sub.2, NbB, NbB.sub.2, TaB, TaB.sub.2, TiB.sub.2, VB,
VB.sub.2 and ZrB.sub.2. Still other useful fillers are the ceramic
nitrides, such as silicon nitride, silicides such as molybdenum disilicide
and other similar ceramic refractory materials.
The 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 and this mean
particle size generally can range from about 0.1 micron to about 1000
microns, and preferably, it ranges from about 0.2 micron to about 100
microns, and more preferably it ranges from about 0.5 micron to about 25
microns.
In one embodiment of the present invention, to produce a composite of high
density, or of a particular microstructure, a particle size distribution
of the 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 void between the large silicon carbide
particles and improve packing. The optimum distribution is determinable
empirically.
As used herein, filament includes a whisker or fiber of filler. Generally,
the present filler filament has an aspect ratio of at least 5, and in one
embodiment of the present invention it is higher than 50, and yet in
another embodiment of the present invention it is higher than 1000.
Generally, the lower the aspect ratio of the filament, the higher is the
packing which can be achieved in the resulting composite since the small
filaments intertwine or interlock. Also, generally, the higher the aspect
ratio of the filaments, the better are the mechanical properties of the
resulting composite. Generally, the present filament can range in diameter
from about 0.1 micron to about 20 microns, and can range in length from
about 3 microns to about 10 centimeters.
In one embodiment of the present process, a mixture of filler powder and
filaments is used to produce a composite of desired density, mechanical
strength or microstructure. The particular desired mixture of powder and
filaments is determinable empirically.
A mixture of filler powders of distributed size or a mixture of filler
powder and filaments can be produced by a number of techniques. For
example, fractions of filler powders of distributed size or filler powder
and filaments can be admixed in water at ambient pressure and temperature
using, for example, a propeller blender, and the resulting dispersion can
be dried in air at ambient temperature.
In carrying out the present process, a uniform or at least a substantially
uniform mixture is formed of the components, i.e. filler, alumina powder,
alumino-silicate glass powder and, when it is used, nucleating agent. The
glass and alumina are used in the amounts required to react to form the
continuous phase of mullite ranging from about 20% by volume to about
99.9% by volume of the total volume of the composite.
The amount of the filler used also depends on the particular composite
desired. In the present process, it is used in an amount which produces a
composite wherein the phase of filler ranges from about 0.1% by volume to
about 80% by volume of the total volume of the composite.
The components can be admixed by a number of conventional techniques such
as, for example, ball milling, vibratory milling or jet milling, to
produce a uniform or substantially uniform mixture. The more uniform the
mixture, the more uniform is the microstructure, and therefore, the
properties of the resulting composite.
Representative of these mixing techniques is ball milling. Milling may be
carried out dry or with the charge suspended in a liquid medium inert to
the ingredients. Typical liquids include ethyl alcohol and acetone. Wet
milled material can be dried by a number of conventional techniques to
remove the liquid medium.
Hot pressing of the mixture is preferably carried out in a non-oxidizing
atmosphere. More particularly, hot pressing of the mixture is carried out
in a protective atmosphere in which the mixture is inert or substantially
inert, i.e. an atmosphere which has no significant deleterious effect on
it. Representative of the hot pressing atmospheres is nitrogen, argon,
helium or a vacuum.
In one embodiment of the present process, the mixture can be pressed in a
conventional manner, generally by die pressing at room temperature, to
produce a desired compressed form or preform before it is placed in the
hot press.
In carrying out the present process, the mixture is hot pressed under a
pressure and temperature and for a sufficient period of time to produce
the present composite. Maximum hot pressing temperature generally ranges
from about 1500.degree. C. to about 1750.degree. C., and preferably from
about 1550.degree. C. to 1700.degree. C., depending upon the particular
aluminosilicate glass used and is determinable empirically. Temperatures
lower than about 1500.degree. C. generally are not high enough to
sufficiently soften the glass to produce the present composite. On the
other hand, temperatures above 1750.degree. C. are too close to the
maximum liquidus temperature of the glass. At or close to the liquidus
temperature, the alumina is likely to dissolve too quickly in the glass
and react with it too rapidly forming a substantial amount of
polycrystalline mullite prematurely thereby preventing production of the
present continuous interconnecting phase of polycrystalline mullite.
Specifically, the hot pressing temperature must be one which allows the
glass to form a continuous interconnecting phase around the filler before
it is substantially or totally reacted to produce the polycrystalline
mullite phase.
The heating rate to the present maximum hot pressing temperature is
determinable empirically. It should be sufficiently rapid to prevent
dissolution of the alumina in the glass to any significant degree, and
thereby prevent any significant formation of crystalline mullite below the
maximum hot pressing temperature. Such heating rate may be as low as about
30.degree. C. per minute, but preferably, it is at least about 50.degree.
C. per minute, and most preferably it is about 200.degree. C. per minute.
The maximum heating rate in the present process is limited only by the
equipment. The formation of polycrystalline mullite in a significant
amount below hot pressing temperature prevents production of the present
continuous phase of which encapsulates at least about 20% by volume of the
filler.
The hot pressing pressure can vary and should be at least sufficient to
confine the material in the hot press and make the mullite reaction take
place during the present reactive hot pressing. Hot pressing pressure can
range to a maximum pressure which is limited by the available pressing
equipment. Typically, hot pressing pressure ranges from about 2000 psi to
about 8000 psi.
In the present process, there is no significant loss of the reactants or
components forming the present composite.
The present composite can contain a glassy phase in an amount of less than
about 20% by volume, preferably less than about 10% by volume, more
preferably less than about 5% by volume, and still more preferably less
than about 1% by volume, of the total volume of said composite. Even more
preferably, the present composite contains only a detectable amount of
glassy phase. There | | |
