U.S. patent application number 14/605224 was filed with the patent office on 2016-07-28 for porous ceramic materials for investment casting.
This patent application is currently assigned to GENERAL ELECTRIC COMPANY. The applicant listed for this patent is GENERAL ELECTRIC COMPANY. Invention is credited to Ayesha Maria GONSALVES, Cathleen Ann HOEL, Matthew Thomas JOHNSON, Frederic Joseph KLUG, John Thomas LEMAN, Thomas Francis MCNULTY, Michael Joseph O'BRIEN.
Application Number | 20160214165 14/605224 |
Document ID | / |
Family ID | 55701671 |
Filed Date | 2016-07-28 |
United States Patent
Application |
20160214165 |
Kind Code |
A1 |
HOEL; Cathleen Ann ; et
al. |
July 28, 2016 |
POROUS CERAMIC MATERIALS FOR INVESTMENT CASTING
Abstract
Methods and compositions for forming porous ceramic articles are
provided. The method is forming a composition including liquid
siloxanes, ceramic particles, a catalyst, and a pore-former;
shaping the composition; curing the composition to form a green
body; volatilizing the pore-former before or after curing the
composition; and firing the green body. The pore-former is a
silicon-bearing agent with an average molecular weight under 1300
grams per mole, or a molecule with a molecular weight under 1,000
grams per mole and a freezing point between -40.degree. C. and
25.degree. C. that is soluble in the liquid. The composition
comprises liquid siloxanes, ceramic particles, a catalyst, and a
pore-former, which is an agent with an average molecular weight
under 1300 grams per mole or a molecule with a molecular mass under
1,000 grams per mole and a freezing point between -40.degree. C.
and 25.degree. C. that is soluble in the liquid.
Inventors: |
HOEL; Cathleen Ann;
(Schenectady, NY) ; LEMAN; John Thomas;
(Schenectady, NY) ; MCNULTY; Thomas Francis;
(Ballston Lake, NY) ; O'BRIEN; Michael Joseph;
(Halfmoon, NY) ; GONSALVES; Ayesha Maria;
(Schenectady, NY) ; JOHNSON; Matthew Thomas; (Los
Angeles, CA) ; KLUG; Frederic Joseph; (Schenectady,
NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
GENERAL ELECTRIC COMPANY |
Schenectady |
NY |
US |
|
|
Assignee: |
GENERAL ELECTRIC COMPANY
Schenectady
NY
|
Family ID: |
55701671 |
Appl. No.: |
14/605224 |
Filed: |
January 26, 2015 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C04B 38/0605 20130101;
C04B 35/16 20130101; C04B 35/632 20130101; B22C 1/02 20130101; C04B
2111/00939 20130101; C04B 2235/3418 20130101; C08K 2003/2244
20130101; C08K 3/36 20130101; C08L 83/04 20130101; C04B 35/6325
20130101; C04B 2235/6022 20130101; B22C 9/04 20130101; C08L 83/00
20130101; C08G 77/20 20130101; C04B 2235/483 20130101; C08G 77/12
20130101; C04B 2235/606 20130101; B22C 9/10 20130101; C04B
2235/3244 20130101; C04B 38/0605 20130101; C04B 35/16 20130101;
C08L 83/04 20130101; C08K 3/36 20130101; C08K 2003/2244 20130101;
C08L 83/00 20130101; C08L 83/00 20130101 |
International
Class: |
B22C 1/02 20060101
B22C001/02; B22C 9/04 20060101 B22C009/04; B22C 9/10 20060101
B22C009/10 |
Claims
1. A method for creating a porous ceramic structure comprising:
creating a composition, wherein the composition comprises a liquid,
a plurality of particles disposed within the liquid, a catalyst
material disposed within the liquid, and a pore-forming agent
disposed within the liquid; wherein the liquid comprises one or
more siloxane species; the plurality of particles comprises a
ceramic material; the pore-forming agent comprises an agent that is
substantially inert with respect to the liquid, further comprising
(a) a silicon-bearing agent that has a number average molecular
weight less than about 1,300 grams per mole, or (b) a molecule
having a molecular mass less than approximately 1,000 grams per
mole and a freezing point between -40.degree. C. and 25.degree. C.
that is soluble in the liquid, or any combination of the two;
disposing the composition in any desired shape; volatilizing at
least a portion of the pore-forming agent; curing the composition
to form a green body; and firing the green body.
2. The method of claim 1, wherein the one or more siloxane species
of the liquid comprise an alkenyl functional group, a hydride
functional group, or any combination of the two.
3. The method of claim 1, wherein the pore-forming agent comprises
cyclohexane, benzene, p-xylene, o-xylene, cyclooctane, cyclononane,
bicyclohexyl, tridecane, dodecane, 3,4-dichlorotoluene,
cycloundecane, cycloheptane, 2,2,3,3-tetramethylpentane, and
2,2,5,5-tetramethylhexane, or combinations including one or more of
these.
4. The method of claim 3, wherein volatilizing at least a portion
of the pore-forming agent occurs before curing the composition to
form said green body and comprises lowering a temperature of the
composition to below the freezing point of the pore-forming agent,
freezing at least a portion of the pore-forming agent to
phase-separate said portion of the pore-forming agent from another
component of the composition, and reducing atmospheric pressure to
below a vapor pressure of the pore-forming agent but above a vapor
pressure of another component of the composition at said
temperature for a period of time sufficient to volatilize at least
some of said portion of the pore-forming agent.
5. The method of claim 4 wherein a pore-forming agent is
cyclohexane.
6. The method of claim 1, wherein the pore-forming agent is a
silicone-bearing agent comprising: decamethylcyclopentasiloxane;
tetramethyltetraphenyltrisiloxane; phenyltrimethylsilane;
tetra-n-butylsilane; p-tolyltrimethylsilane;
methyltri-n-trioctylsilane; dimethyldiphenylsilane;
methyltri-n-hexylsilane, hexamethyl disiloxane; octamethyl
trisiloxane; decamethyl tetrasiloxane; dodecamethyl pentasiloxane;
phenyl-tris(trimethylsiloxy)silane;
methyl-tris(trimethylsiloxy)silane;
3,3,3-trifluoropropyl-tris(trimethylsiloxy)silane;
tetrakis(trimethylsiloxy)silane; or a compound of the formula
(R.sub.3SiO).sub.3SiR or (R.sub.3SiO).sub.4Si wherein R is a
monovalent hydrocarbon, a halocarbon, or a halogenated hydrocarbon,
and with which the siloxane species of the liquid is not reactive;
or a combination including one or more of these.
7. The method of claim 6, wherein volatilizing at least a portion
of the pore-forming agent comprises raising the temperature of said
green body to above the cure temperature and up to the boiling
point for said silicon-bearing agent for a period of time
sufficient to volatilize said portion of the pore-forming agent
before firing.
8. The method of claim 6 wherein curing the composition to form a
green body comprises phase-separating at least a portion of the
pore-forming agent from another component of the composition.
9. The method of claim 7, wherein the silicon-bearing agent
comprises
1,3,5-tris(3,3,3-trifluoropropyl)trimethylcyclotrisiloxane.
10. The method of claim 3, wherein the pore-forming agent is
present at a concentration of at least about 5 percent by volume of
the entire composition.
11. The method of claim 3, wherein the pore-forming agent is
present in the composition at a concentration in a range from about
7 volume percent to about 35 volume percent of the entire
composition.
12. The method of claim 6, wherein the pore-forming agent is
present at a concentration of at least about 5 percent by volume of
the entire composition.
13. The method of claim 6 wherein the pore-forming agent is present
in the composition at a concentration in a range from about 7
volume percent to about 35 volume percent of the entire
composition.
14. The method of claim 1, wherein the pore-forming agent comprises
a mixture of components, wherein the components of the mixture have
freezing points, boiling points, or vapor pressures at a given
temperature that differ from each other.
15. The method of claim 14, wherein at least one component of said
mixture is said (a) silicon-bearing agent that has a number average
molecular weight less than about 1,300 grams per mole and at least
one component of said mixture is said (b) molecule having a
molecular mass less than approximately 1,000 grams per mole and a
freezing point between -40.degree. C. and 25.degree. C. that is
soluble in the liquid.
16. The method of claim 1, further comprising disposing the ceramic
body within an investment casting mold, and solidifying molten
metal within the investment casting mold.
17. The method of claim 1, wherein disposing comprises injecting
the material via a printer nozzle.
18. The method of claim 1, wherein disposing comprises disposing a
plurality of layers of the material in successive deposition
actions.
19. A composition comprising: a liquid that comprises one or more
siloxane species; a plurality of particles, comprising a ceramic
material, disposed within the liquid; a catalyst material disposed
within the liquid; and a pore-forming agent comprising an agent
that is substantially inert with respect to the liquid, further
comprising (a) a silicon-bearing agent that has a number average
molecular weight less than about 1,300 grams per mole, or (b) a
molecule having a molecular mass less than approximately 1,000
grams per mole and a freezing point between -40.degree. C. and
25.degree. C. that is soluble in the liquid, or any combination of
the two.
20. The composition of claim 19, wherein the pore-forming agent
comprises cyclohexane, benzene, p-xylene, o-xylene, cyclooctane,
cyclononane, bicyclohexyl, tridecane, dodecane,
3,4-dichlorotoluene, cycloundecane, cycloheptane,
2,2,3,3-tetramethylpentane, and 2,2,5,5-tetramethylhexane, or
combinations including one or more of these.
21. The composition of claim 20, wherein the pore-forming agent
comprises cyclohexane.
22. The method of claim 19, wherein the pore-forming agent is a
silicone-bearing agent comprising decamethylcyclopentasiloxane;
tetramethyltetraphenyltrisiloxane; phenyltrimethylsilane;
tetra-n-butylsilane; p-tolyltrimethylsilane;
methyltri-n-trioctylsilane; dimethyldiphenylsilane;
methyltri-n-hexylsilane, hexamethyl disiloxane; octamethyl
trisiloxane; decamethyl tetrasiloxane; dodecamethyl pentasiloxane;
phenyl-tris(trimethylsiloxy)silane;
methyl-tris(trimethylsiloxy)silane;
3,3,3-trifluoropropyl-tris(trimethylsiloxy)silane;
tetrakis(trimethylsiloxy)silane; and a compound of the formula
(R.sub.3SiO).sub.3SiR or (R.sub.3SiO).sub.4Si wherein R is a
monovalent hydrocarbon, a halocarbon, or a halogenated hydrocarbon,
and with which the siloxane species of the liquid is not reactive;
or a combination including one or more of these.
23. The method of claim 22, wherein the silicon-bearing agent
comprises
1,3,5-tris(3,3,3-trifluoropropyl)trimethylcyclotrisiloxane.
24. The composition of claim 19, wherein the pore-forming agent
comprises a mixture of said molecules, wherein the freezing point,
boiling point, or vapor pressure at a given temperature of one
molecule of said mixture differs from that of another molecule of
said mixture.
25. The composition of claim 19, wherein the pore-forming agent is
present at a concentration of at least about 5 percent by volume of
the entire composition.
26. The composition of claim 19, wherein the pore-forming agent is
present in the composition at a concentration in a range from about
7 volume percent to about 35 volume percent of the entire
composition.
27. The composition of claim 19, wherein the one or more siloxane
species of the liquid comprises an alkenyl functional group, a
hydride functional group, or any combination of the two.
Description
BACKGROUND
[0001] This disclosure generally relates to investment casting and,
more particularly, to materials for use in forming the ceramic
cores and shell molds employed in investment casting.
[0002] The manufacture of gas turbine components, such as turbine
blades and nozzles, requires that parts be manufactured with
accurate dimensions having tight tolerances. Investment casting is
a technique commonly employed for manufacturing these parts. The
dimensional control of the casting is closely related to the
dimensional control of a ceramic insert, known as the core, as well
as the mold, also known as the shell. In this respect, it is
important to be able to manufacture the core and shell to
dimensional precision corresponding to the dimensions of the
desired metal casting, e.g., turbine blade, nozzle, and the
like.
[0003] In addition to requiring dimensional precision in the
casting of the ceramic core, the production of various turbine
components requires that the core not only be dimensionally precise
but also be sufficiently strong to maintain its shape during the
firing, wax encapsulation, shelling, and metal casting processes.
In addition, the core must be sufficiently compliant to prevent
mechanical rupture of the casting during cooling and
solidification. Further, the core materials generally must be able
to withstand temperatures commonly employed for casting of
superalloys that are used to manufacture the turbine components,
e.g., temperatures generally up to and above 1,000.degree. C.
Finally, the core must be easily removed following the
metal-casting process. The investment casting industry typically
uses silica or silica-based ceramics due to their low thermal
expansion coefficient and superior leachability in the presence of
strong bases.
[0004] Investment casting cores and shells can be made using low
pressure injection molding techniques such as those described in
U.S. Pat. No. 7,287,573. The process described therein generally
includes dispersing a ceramic powder to form a slurry in a silicone
fluid, wherein the silicone fluid includes silicone species having
alkenyl and hydride functionalities. In some cases, the silicone
species are first dissolved in a volatile solvent, (e.g., aliphatic
and aromatic hydrocarbons that can be removed by heat treatment),
which are then added to the ceramic powder to form a ceramic slurry
and further processed. Once a stable suspension is formed, a
metallic catalyst is added and the desired part is formed.
Depending on the particular binder liquid and metallic catalyst
employed, a heating step may then be applied to effect a catalyzed
reaction among the siloxane species, thereby curing the formed
suspension into a green body. The silicone species cross-link in
the core or mold, yielding a dispersion of ceramic particles in a
rigid silicone-based polymeric matrix. The so-formed silicone
polymeric matrix may be substantially decomposed to produce a
silica char by further heating at a higher temperature.
[0005] The process described above provides improvements over
previously developed processes for forming ceramic cores for
investment casting. However, some opportunities for improvement
still exist. For instance, the solvent-free approach may result in
fairly dense material being formed in the green body; the lack of
porosity may lead to size-dependent shrinkage that can lead to
cracking during the firing step. This cracking can be particularly
acute in cases where the feature scale (i.e. the size difference
between small and large features on the core) is large. The use of
a solvent in the process may at least partially address this
problem, in that interconnected porosity may form in the core as a
result of the solvent removal prior to the curing step. In some
cases, inclusion of solvent in the process may result in the
emission of volatile organic compounds (VOCs), or create a need for
the use of special liquid-permeable molds designed for solvent
removal.
[0006] Accordingly, there remains a need in the art for improved
ceramic slurries and related processes that provide cores and other
ceramic bodies, such as those used in investment casting, having
desirable physical and mechanical characteristics.
BRIEF DESCRIPTION
[0007] Embodiments of the present invention are provided to meet
this and other needs. One embodiment is a method. The method
comprises creating a composition that comprises a liquid siloxane
species; a plurality of particles comprising a ceramic material
disposed within the liquid; a catalyst material disposed within the
liquid; and a pore-forming agent that is substantially inert with
respect to the siloxane species disposed within the liquid. The
pore-forming agent may comprise a silicon-bearing agent that has a
number average molecular weight less than about 1,300 grams per
mole, a molecule having a molecular mass less than approximately
1,000 grams per mole and a freezing point between -40.degree. C.
and 25.degree. C. that is soluble in the liquid, or any combination
of the two. The composition is disposed in any desired shape, the
pore-forming agent is volatilized, the composition is cured to form
a green body, and the green body is fired.
[0008] Another embodiment is a composition. The composition
comprises a liquid that comprises a siloxane species; a plurality
of particles, comprising a ceramic material, disposed within the
liquid; a catalyst material disposed within the liquid; and a
pore-forming agent disposed within the liquid that is substantially
inert with respect to the liquid. The pore-forming agent may
comprise a silicon-bearing agent that has a number average
molecular weight less than about 1,300 grams per mole, a molecule
having a molecular mass less than approximately 1,000 grams per
mole and a freezing point between -40.degree. C. and 25.degree. C.
that is soluble in the liquid, or any combination of the two.
DETAILED DESCRIPTION
[0009] Embodiments of the present invention include compositions
and methods for fabrication of porous ceramic bodies, such as
cores, for use in investment casting and in other applications
where a porous ceramic structure is advantageous. Some compositions
do not rely on VOC-emitting solvents to provide interconnected
porosity, and in some cases may be entirely free of such solvents,
thereby mitigating some of the shortcomings of previously described
techniques as noted above.
[0010] Approximating language, as used herein throughout the
specification and claims, may be applied to modify any quantitative
representation that could permissibly vary without resulting in a
change in the basic function to which it is related. Accordingly, a
value modified by a term or terms, such as "about", and
"substantially" is not to be limited to the precise value
specified. In some instances, the approximating language may
correspond to the precision of an instrument for measuring the
value. Here and throughout the specification and claims, range
limitations may be combined and/or interchanged; such ranges are
identified and include all the sub-ranges contained therein unless
context or language indicates otherwise.
[0011] In the following specification and the claims, the singular
forms "a", "an" and "the" include plural referents unless the
context clearly dictates otherwise. As used herein, the term "or"
is not meant to be exclusive and refers to at least one of the
referenced components being present and includes instances in which
a combination of the referenced components may be present, unless
the context clearly dictates otherwise.
[0012] As used herein, the terms "may" and "may be" indicate a
possibility of an occurrence within a set of circumstances; a
possession of a specified property, characteristic or function;
and/or qualify another verb by expressing one or more of an
ability, capability, or possibility associated with the qualified
verb. Accordingly, usage of "may" and "may be" indicates that a
modified term is apparently appropriate, capable, or suitable for
an indicated capacity, function, or usage, while taking into
account that in some circumstances, the modified term may sometimes
not be appropriate, capable, or suitable.
[0013] One embodiment of the present invention includes a
composition that may be usefully employed in, for example, a
process for making a porous ceramic body; such a process is
exemplified, but not necessarily limited to, the techniques
disclosed in the aforementioned U.S. Pat. No. 7,287,573, as well as
in U.S. Pat. No. 7,413,001, U.S. Pat. No. 7,487,819, U.S. Pat. No.
8,413,709, and U.S. patent application Ser. No. 14/280,993, filed
19 May, 2014, among others.
[0014] The composition is typically a slurry that includes ceramic
powders dispersed within a silicon-bearing liquid; the liquid may
also be referred to as a "binder" in the parlance of slurry
techniques. In particular, the liquid includes a siloxane species,
for instance, (a) one or more siloxane polymers--such as (but not
limited to) the so-called "Room Temperature Vulcanizable," (RTV)
systems well known in the silicones art, including as an example
RTV 615 (trade name of Momentive Performance Materials), as well as
other such silicone formulations that contain polymeric inputs; (b)
siloxane monomers; and/or (c) siloxane oligomers. The siloxane
species may include alkenyl and hydride functionalities. The
siloxane species used in the liquid is of a type referred to in the
art as "curable" or "reactive," meaning that under a given set of
processing conditions, the species will undergo a cross-linking
("curing") reaction; the process of curing is described in further
detail, below.
[0015] The siloxane species having alkenyl functionalities that may
be used as a binder liquid in the composition described herein are
alkenyl siloxanes of the general formula (I):
##STR00001##
wherein R.sup.1, R.sup.2, and R.sup.3 each independently comprise
hydrogen or a monovalent hydrocarbon, halocarbon, or halogenated
hydrocarbon radical; X a divalent hydrocarbon radical; and a is a
whole number having a value between 0 and 8, inclusive. The terms
"monovalent hydrocarbon radical" and "divalent hydrocarbon radical"
as used herein are intended to designate straight chain alkyl,
branched alkyl, aryl, aralkyl, cycloalkyl, and bicycloalkyl
radicals.
[0016] The siloxane species that include hydride functionalities
are hydrosiloxanes having hydrogen directly bonded to one or more
of the silicon atoms, and therefore contain a reactive Si-H
functional group.
[0017] Examples of alkenyl siloxanes useful in the present
disclosure include polyfunctional olefinic substituted siloxanes of
the following types:
##STR00002##
[0018] wherein R is a monovalent hydrocarbon, halocarbon, or
halogenated hydrocarbon; and R' is an alkenyl radical such as
vinyl, or other terminal olefinic group such as allyl, 1-butenyl,
and the like. R'' may include R or R', a=0 to 200, inclusive, and
b=1 to 80, inclusive, wherein a and b are selected to provide a
fluid with maximum viscosity of about 1,000 centistokes, and such
that the ratio of b/a allows for at least three reactive olefinic
moieties per mole of siloxane of formula (II) above.
[0019] Suitable alkyl/alkenyl cyclosiloxanes are of formula
(III):
[RR'SiO]x, (III)
wherein R and R' are as previously defined, and x is an integer 3
to 18 inclusive.
[0020] Other suitable functional unsaturated siloxanes may be of
the formula (IV):
##STR00003##
[0021] wherein R, R', and R'' are as previously defined. In some
embodiments, the ratio of the sum of (c+d+e+g)/f is .gtoreq.2.
[0022] Examples of unsaturated siloxanes include
1,3-divinyl-tetramethyldisiloxane, hexavinyldisiloxane,
1,3-divinyltetraphenyldisiloxane,
1,1,3-trivinyltrimethyldisiloxane,
1,3-dimethytetravinylldisiloxane, and the like. Examples of cyclic
alkyl-or arylvinylsiloxanes include
1,3,5-trivinyl-1,3,5-tri-methylcyclotrisiloxane,
1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane,
1,3-divinyloctaphenylcyclopentasiloxane, and the like.
[0023] Suitable polyfunctional hydride siloxanes include
compositions depicted below:
##STR00004##
[0024] wherein R is as defined previously, R''' may include R or H,
and a and b are defined as above, and selected such that the ratio
of b/a allows for at least three reactive Si-H moieties per mole of
siloxane of formula (V) above.
[0025] Suitable alkyl/hydride cyclosiloxanes are of formula
(VI):
[HRSiO]x, (VI)
[0026] wherein R is as previously defined, and x is an integer 3 to
18 inclusive.
[0027] Other suitable functional hydride siloxanes include:
##STR00005##
[0028] wherein R and R''' are as previously defined. In some
embodiments, the ratio of the sum of (c+d+e+g)/f is .gtoreq.2.
[0029] Examples of siloxane hydrides include
poly(methylhydrogen)siloxane,
poly[(methylhydrogen)-co-(dimethyl)]siloxane;
1,3,5,7-tetramethylcyclotetrasiloxane,
1,3,5,7,9-pentamethylcyclopentasiloxane, and other cyclic
methylhydrogen siloxanes; tetrakis(dimethylsiloxy)silane, and
organically modified resinous hydride functional silicates
corresponding to Formula (VII), with the composition
[HSi(CH.sub.3).sub.2O.sub.1/2].sub.2 (SiO.sub.2).
[0030] The siloxane species in the liquid may be selected so as to
include at least one alkenyl and hydride siloxane as described
above.
[0031] Additional terminally functional alkenyl or hydride
siloxanes described below in formulas (VIII) and (IX), alone or in
combination, may be added to augment the matrix composition in
order to adjust the viscosity of the uncross-linked matrix, effect
changes in the cured green body hardness, strength and strain, and
so on, as would be apparent to those skilled in the art in view of
the present disclosure.
##STR00006##
wherein R and R' are as previously defined; and n=0 to 500, in some
embodiments 0 to 30, and in particular embodiments 0 to 10.
[0032] It should also be apparent that in some embodiments a
satisfactory cross-linked network may be effected by combining one
component from each of A) a polyfunctional alkenyl or
polyfunctional hydride siloxane, as defined in Formulas (II)-(IV)
or Formulas (V)-(VII), respectively; and B) a terminally functional
alkenyl or hydride siloxane as defined in Formulas (VIII) or (IX)
respectively, restricted only such that the composition contains
both an alkenyl and a hydride functional species to allow
cross-linking between the complementary alkenyl and hydride
reactive functional groups.
[0033] The viscosity of the liquid binder, its theoretical
cross-link density, and resultant silica char yield may be adjusted
using the appropriate siloxane species and the stoichiometric ratio
of total hydride to alkenyl reactive functional groups. For
instance, the viscosity of the composition can vary from about 1 to
about 6000 centistokes, in some embodiments from about 1 to about
300 centistokes, and in particular embodiments from about 1 to
about 100 centistokes. The theoretical cross-link density, as
represented by the number average molecular mass of the shortest
formula repeat unit distance between reactive hydride or alkenyl
functional cross-link sites (abbreviated for the purposes of this
description as MW.sub.c,), can vary in some embodiments from about
30 to about 4,100 g/mole, in some embodiments from about 30 to
about 500 g/mole, and in particular embodiments up to about 150
g/mole. In other embodiments, such as embodiments in which the
binder includes a siloxane polymer, the MW.sub.c can be much
higher, such as, for example, up to about 35,000 g/mole. In some
embodiments, the MW.sub.c is in the range from about 10,000 g/mole
to about 35,000 g/mole. Such comparatively high MW.sub.c binders,
when processed in accordance with the techniques described herein,
may result in a softer, more compliant material with higher strain
to failure (in the green to dried state) and with lower cure
shrinkage than with lower MW.sub.c binders. To produce a suitably
hard and resilient cured material, the hydride to alkenyl molar
ratio is generally in the range from about 0.5 to 3, in some
embodiments in the range from about 0.5 to 2, and particular
embodiments in the range from about 1.0 to 1.75. In the particular
case of 1,3,5,7-tetramethylcyclotetrasiloxane and
1,3,5,7-tetravinyl-1,3,5,7-tetramethyl-cyclotetrasiloxane,
combinations in molar ratios from 0.5 to 2 gave silica yields upon
pyrolysis of the cured matrix at 1,000.degree. C. in air from 74 to
87% of the original mass.
[0034] Generally, the amount of reactive siloxane species included
in the ceramic slurry significantly affects the degree of hardness
of the resulting solid, shaped product. Using too low an amount may
result in unacceptably low strength, while too high an amount may
result in material that is difficult to process or lacks
sufficiently high ceramic solids content to provide the desired
properties. The selection of a desired reactive siloxane species
amount (that is, the amount of binder) thus depends on the target
application and the nature of the other inputs used in the
composition. For example, depending on the amount of other
components such as ceramic powder that is present, a useful green
product can be formed using greater than about 5 volume percent of
the reactive siloxane species, with some embodiments employing from
about 20 volume percent to about 40 volume percent of reactive
siloxane species as binder liquid relative to the total volume of
the composition. In certain embodiments, the one or more reactive
siloxane species comprises about 22 to about 37 volume percent of
the slurry composition, a range that has been found particularly
useful for producing articles for use in investment casting
processes, though such applications are not exclusively limited to
the use of compositions within this range.
[0035] The composition further comprises a catalyst material, such
as a metal-containing catalyst. Cross-linking of the siloxane
species may be accomplished by utilizing a catalyzed reaction of
the alkenyl groups and the silicon-bonded hydrogen groups.
Catalysts suitable for such reactions are well known and widely
used in the art, including, for example catalysts that include
metals such as platinum (Pt), rhodium, iron, palladium, or
combinations thereof, usually present in the form of compounds of
such metals. Specific examples include, but are not limited to, the
Pt divinylsiloxane complexes as described by Karstedt in U.S. Pat.
No. 3,715,334 and U.S. Pat. No. 3,775,452; Pt-octyl alcohol
reaction products as taught by Lamoreaux in U.S. Pat. No.
3,220,972; the Pt-vinylcyclosiloxane compounds taught by Modic in
U.S. Pat. No. 3,516,946; and Ashby's Pt-olefin complexes found in
U.S. Pat. Nos. 4,288,345 and 4,421,903. Often, catalyst is added as
a final ingredient to a pre-mixed slurry just prior to use of the
slurry; even at room temperature there is a certain rate of gelling
that may occur as the catalyst promotes cross-linking of the binder
liquid. Optionally, inhibitors may be added along with the catalyst
to prevent undue premature gelling before a part is cast. Such
inhibitors are well known in the art, an example of which is
provided in U.S. Pat. No. 4,256,870. Once the slurry mixture is
heated, the reaction rate is relatively high whereby polymerization
and cross linking of the species is achieved in a practical time
span. The amount of metallic catalyst included in the slurry
mixture is generally small as compared with the amount of species
in accordance with conventional curing and cross-linking
methods.
[0036] Ceramic powders suitable for use in the present disclosure
include, but are not intended to be limited to, oxides, carbides,
and/or nitrides; specific examples of such materials include,
without limitation, alumina (such as fused alumina), fused silica,
magnesia, zirconia, spinels, mullite, glass frits, tungsten
carbide, silicon carbide, boron nitride, silicon nitride, and
mixtures thereof. In particular embodiments, the ceramic powder
includes at least some silica, mixtures of silica and zircon, or
mixtures of silica and alumina. The ceramic powder in part provides
mechanical integrity for the finished product produced from the
slurry composition, and the amount of powder added to the
composition contributes to, among other things, the flow properties
of the composition and the strength of the green and finished
product. In some embodiments, the slurry composition includes at
least about 30 volume percent of the ceramic powder, and in
particular embodiments, at least about 50 volume percent. In many
applications, an excessively high concentration of ceramic powder
in the slurry composition may adversely affect the flow properties,
rendering the slurry too resistant to flow to allow practical
injection or other processing operations. Accordingly, in some
embodiments the composition includes up to about 70 volume percent
ceramic powder.
[0037] The particle size distribution of the ceramic powder may be
selected to attain desired rheological properties for the
composition, which in turn will depend in part upon the desired
application of the composition. Similarly, the powder morphology,
including sphericity/angularity, aspect ratio, and the like, may
also be optimized for a given application.
[0038] Other additives that may be present in the ceramic powder
include, but are not intended to be limited to, aluminum, yttrium,
hafnium, yttrium aluminate, rare earth aluminates, colloidal
silica, magnesium, zircon, and/or zirconium for increasing
refractory properties of the ceramic body. Alkali and alkaline
earth metallic salts are added in some instances to effect
devitrification of amorphous silica and promote the formation of
cristobalite, leading to a dimensionally stable ceramic body, as
desired for precision investment casting. Moreover, various
dispersants for ceramic powders are known in the art and are
appropriate for use in the present techniques. Care should be
exercised, however, to select a dispersant that does not interact
with the other components of the slurry composition. A particular
dispersant may be evaluated for suitability with a particular
combination of material components by mixing small amounts of the
respective components and judging the flow properties of the
resultant mixture, whether the resultant mixture exhibits a notable
yield point, and/or whether the mixture exhibits pseudoplastic
behavior. Typical dispersants include stearic acid, oleic acid, and
menhaden fish oil. Generally, the dispersant is used in a small
amount, by volume, as compared with the amount, by volume, of the
ceramic powder included in the mixture.
[0039] Further substances may be added to, for instance, modify the
ceramic powder surfaces for improved dispersion, for better flow of
the slurry, or for enhanced mechanical properties by providing for
covalent bonding between an agent absorbed on the surface and a
complimentary reactive functionality in the liquid siloxane matrix.
These surface-modifying agents may include reactive amino- or
alkoxy silanes such as hexamethyldisilazane or
methyltrimethoxysilane. Examples of a liquid siloxane
matrix-reactive agent may include substances such as 1,3-divinyl
tetramethyl disilazane or vinyltriethoxysilane. For purposes of
powder surface treatment, enhancement of the above properties may
result merely from addition of the agent to the liquid siloxane and
powder mixture during processing. Enhancement may also be effected
by treatment of the powder surface with the surface-modifying agent
in a separate step prior to slurry compounding, either in the
liquid phase, in more dilute solution in the presence of a solvent,
or in the gas-phase; treatment may be performed at room temperature
or at elevated temperature to speed reactivity and extent of
reaction. These and other aspects of surface functionalization are
well-known in the art.
[0040] The composition further comprises a pore-forming agent. As
mentioned above, conventional approaches to ceramic body processing
often results in a dense as-cured body that is relatively
impermeable to gaseous species entering or exiting the body.
Post-cure processes involve pyrolyzing the core--i.e. converting
silicone to silica--which involves the formation and/or release of
gases. A lack of permeability in the body may lead to structural
differences, particularly in features of differing cross-sectional
area. These structural differences may manifest themselves as
shrinkage differences that may cause cracks to form in the body.
Porosity in the as-cured body, created by the inclusion of a
pore-forming agent, may enable a more rapid transit of gaseous
species, which in turn may reduce the dimensional dependence of
shrinkage. Increasing pore volume by increasing pore size, number,
and/or interconnectedness may advantageously enhance such
properties.
[0041] A pore-forming agent is a component of the composition
(often, but not necessarily always, a liquid component) that is
removed before the cross-linking reaction that occurs during curing
of the binder liquid, or does not otherwise substantially decompose
during processing or substantively participate in such
cross-linking reaction. A pore former may be removed by being
volatized, before or after curing, such as by sublimation or
evaporation. For example, a composition containing a pore former
may be freeze-dried before curing, causing volatilization of the
pore-former and loss thereof from the composition, yielding a core
or mold with a plurality of pores within a cross-linked material
matrix once the composition is subsequently cured. Or, after being
cured to its green state, a core or mold can be heated to
volatilize pore-former remaining therein, leaving behind such a
plurality of pores within the cross-linked material matrix.
[0042] A pore-forming agent removed before curing in embodiments of
the invention may include a molecule with a molecular mass less
than approximately 1,000 g/mol and a freezing point between
-40.degree. C. and 25.degree. C. that is non-reactive but soluble
in the siloxane binder in the composition. Such agent has
appropriate volatility such that either it does not significantly
volatilize during handling or other activity before freeze-drying
or any volatilization that occurs before freeze-drying can be
compensated for by adding more pore-forming agent. It is also
substantially inert with respect to the liquid binder, meaning that
it does not chemically react with the liquid binder under typical
processing conditions to a degree that would significantly reduce
the amount of agent present before freeze-drying. Furthermore, it
is stable in that it undergoes no substantial decomposition or
other chemical change while present in the material. If
volatilization of only the pore-forming agent during freeze-drying
is preferred, a pore-forming agent should be selected such that its
vapor pressure is substantially higher than the vapor pressure of
all other binder components at the temperature at which
freeze-drying is performed. Examples of such a pore-forming agent
include, but are not limited to, cyclohexane, benzene, p-xylene,
o-xylene, cyclooctane, cyclononane, bicyclohexyl, tridecane,
dodecane, 3,4-dichlorotoluene, cycloundecane, cycloheptane,
2,2,3,3-tetramethylpentane, and 2,2,5,5-tetramethylhexane.
[0043] A pore-forming agent that is removed after curing in
embodiments of the invention may include, and in some cases may be
made entirely of, a silicon-bearing material such as a silane, a
siloxane, or mixtures of such. Such a pore-forming agent,
therefore, is advantageously free of current VOC emission-regulated
substances such as hydrocarbon-based solvents. It has a number
average molecular mass less than about 1,300 g/mol, and is
substantially inert with respect to the liquid binder, meaning that
it does not participate in the curing reactions of the liquid
binder under typical processing conditions to a degree that would
significantly reduce the amount of agent present after curing. Such
pore-forming agent may also be of low or limited miscibility in the
binder used, to enhance its phase separation from the rest of the
composition during curing and resulting in larger pore diameters
upon its removal. Furthermore, it is stable in that it undergoes no
substantial decomposition or other chemical change while present in
the material. It also has appropriate volatility such that it does
not significantly volatilize until post-cure processing, after
curing of the material. Use of pore-forming agents that are
miscible in a given binder system but show phase separation during
a cure step may be particularly advantageous.
[0044] The average molecular weight of a silicon-bearing
pore-forming agent is often correlated with its volatility; in
certain embodiments, the number average molecular mass of the
silicon-bearing agent is at least about 150 grams per mole, and in
particular embodiments at least about 200 grams per mole. In some
embodiments, the number average molecular weight of the agent may
range from either of these lower values up to about 500 grams per
mole
[0045] Examples of silicon-bearing materials suitable for use as,
or in, the pore-forming agent include cyclic siloxanes and linear
siloxanes. Cyclic siloxane compositions of use as pore-forming
agent have a general formula of
[RRSiO].sub.x (X),
where x is an integer from 3 to 18 and each R can be,
independently, any of the R as previously defined, except for
groups such as, for instance, alkenyl or hydride groups, that
substantially react with the binder liquid. One example of a cyclic
siloxane is decamethylcyclopentasiloxane, often referred to in the
art as D.sub.5 and commonly available commercially in substantially
pure form as, for instance, Momentive SF1202 or Dow-Corning 245
fluids. D.sub.5 has been shown in experiments to possess especially
favorable stability and volatility characteristics; this, combined
with its general availability, make it a particularly attractive
choice for use as the pore-forming agent. Another example is
1,3,5-tris(3,3,3-trifluoropropyl)trimethylcyclotrisiloxane, also
known in the art as D.sub.3.sup.F. In addition to substantially
pure, single components, commercially available mixtures of cyclic
siloxanes may also be advantageously employed, for example:
Dow-Corning 246, Dow-Corning 344, Dow-Corning 345, Dow-Corning 702,
Momentive SF1204, Momentive SF1256, Momentive SF1257, and Momentive
SF1258 fluids.
[0046] Examples of suitable linear siloxanes include without
limitation siloxanes following the formula
R.sub.3SiO(SiR.sub.2O).sub.xSiR.sub.3, where R is as defined
previously for Formula (X), and x=0-15. Specific examples of
suitable linear siloxanes include without limitation hexamethyl
disiloxane, octamethyl trisiloxane, decamethyl tetrasiloxane, and
dodecamethyl pentasiloxane; commercially available silicone fluids
such as Dow-Corning 704 (tetramethyltetraphenyltrisiloxane),
Dow-Corning OS-10, OS-20, and OS-30; linear PDMS mixtures such as
the Dow-Corning 200 fluids or equivalent Xiameter PMX-200 fluids in
the viscosity ranges from 0.65 to 10 cStokes
[0047] Examples of suitable branched siloxanes include without
limitation T- or Q-functional compounds following the formula
(R.sub.3SiO).sub.3SiR or (R.sub.3SiO).sub.4Si, where R is as
defined previously for Formula (X). Specific examples of suitable
branched siloxanes include without limitation
phenyl-tris(trimethylsiloxy)silane,
methyl-tris(trimethylsiloxy)silane,
3,3,3-trifluoropropyl-tris(trimethylsiloxy)silane, and
tetrakis(trimethylsiloxy)silane.
[0048] Other examples of silicon-bearing materials include silanes.
Examples of suitably non-reactive silanes include, without
limitation, phenyltrimethylsilane, tetra-n-butylsilane, p-to
lyltrimethylsilane, methyltri-n-trioctylsilane,
dimethyldiphenylsilane, and methyltri-n-hexylsilane.
[0049] In some embodiments, using a mixture of pore-forming agents
possessing vapor pressures at given temperatures that differ from
each other, as opposed to a pure substance with a single vapor
pressure at given temperatures, may be advantageous. When such a
mixture is used, the temperature and/or pressure ranges over which
the bulk of the agent's volatilization occurs during its removal,
pre- or post-cure, is spread out across a broader range, rather
than having all of the agent in the composition subliming or
evaporating at a single temperature or pressure, thereby decreasing
the probability of damaging the green body from internal pressures
caused by trapped vapor accumulation. In other embodiments, a
mixture of two or more pore-forming agents, a portion of which is
suitable for pre-cure removal and another portion of which is
suitable for post-cure removal, may be used, to advantageously
distribute volatilization across a broader range of conditions.
[0050] The amount of a pore-forming agent present in the
composition is a significant factor in determining the porosity of
the "green body", that is, the material that remains after curing
the binder and the pore-forming agent has been removed (whether
before or after curing). Typically the pore-forming agent is
present at a concentration of at least about 5 percent by volume of
the entire composition to form a sufficiently percolated network to
enable the transit of gases during processing of the green body. In
particular embodiments, a comparatively high degree of porosity in
the porous green body, such as at least about 7 volume percent, is
desirable to increase the likelihood of forming an interconnected
network of pores throughout the green body. On the other hand,
depending on the application, porosity in some cases may be
desirably controlled so as not to exceed a certain amount to
maintain acceptable strength in the green body and/or end product.
In some embodiments this upper limit of porosity is about 35
percent by volume, and thus in particular embodiments the
pore-forming agent is present in the composition at a concentration
in a range from about 7 volume percent to about 35 volume percent
of the entire composition. In other embodiments, the upper limit of
the pore-forming agent may be dictated by the volume fraction of
other components of the composition, such as the ceramic powder.
Where a composition has a relatively low loading of ceramic powder,
more volume is available for pore-forming agent, and thus, if
service conditions for the green product and/or the finished
product allow, even higher amounts of pore-forming agent may be
feasible.
[0051] One particular embodiment that applies advantages described
above is a composition that includes, at least in part, a liquid
("binder") comprising a siloxane species as noted previously, the
siloxane species comprising an alkenyl functional group and a
hydride functional group; a plurality of particles, comprising a
ceramic material, disposed within the liquid; a catalyst material
comprising a metal, and disposed within the liquid; and a
pore-forming agent disposed within the liquid, wherein the
pore-forming agent comprises cyclohexane and is present in the
composition at a concentration in a range from about 5 volume
percent to about 35 volume percent of the composition.
[0052] Another embodiment that applies advantages described above
is a composition that includes, at least in part, a liquid
("binder") comprising a siloxane species as noted previously, the
siloxane species comprising an alkenyl functional group and a
hydride functional group; a plurality of particles, comprising a
ceramic material, disposed within the liquid; a catalyst material
comprising a metal, and disposed within the liquid; and a
pore-forming agent disposed within the liquid, wherein the
pore-forming agent comprises 1,3,5-tris(3, 3,
3-trifluoropropyl)trimethylcyclotrisiloxane and is present in the
composition at a concentration in a range from about 5 volume
percent to about 35 volume percent of the composition.
[0053] As noted previously, the composition described herein may be
applied to fabricate a porous ceramic body, such as, for example, a
core or a shell for use in investment casting techniques. The
composition is disposed in a desired shape, such as by placing the
composition into a die that is the shape of a core, or such as by
coating the composition over a pattern to create a shell according
to practices known in the art, and the siloxane species are cured
(thereby forming a silicone-based polymeric matrix as noted
previously). The pore-forming agent is volatilized, either before
or after curing, to drive the agent out of the polymeric matrix,
leaving behind a plurality of pores within the matrix and thereby
resulting in the formation of a porous green body. The green body
may then be heated to produce a ceramic body.
[0054] It will be appreciated that in forming articles such as
shells, for instance, repeated cycles of coating (such as by
dipping a form into a quantity of slurry composition) and curing
may be employed to build up a desired shell thickness. Advanced
layer-by-layer techniques by so-called "additive manufacturing"
methods may also be employed to create complex objects, such as
cores and other articles, using the composition, whereby a thin
layer of the composition is disposed in a desired shape (such as by
injection via a printer nozzle or other deposition equipment), the
composition is cured, and then further successive deposition/cure
cycles are employed, building up mass layer-by-layer in accordance
with a three-dimensional part design, until a part of desired shape
is fabricated. In techniques that employ a layer-by-layer approach,
the pore-forming agent may be removed at any convenient point in
the process, such as (but not limited to) after all layers have
been deposited and cured. The compositions used to create different
layers may include pore-formers that differ from each other to
modify the relative porosity of different lamina of a green body so
formed and, ultimately, cross-sectional uniformity and/or strength
of the final ceramic product. For example, a pore-forming agent
used in creating one layer may be appropriate for pre-cure removal
while for that used in another layer may be appropriate for
post-cure removal. Or, ranges of pore-formers' vapor pressures in
the compositions used to create different layers may otherwise
differ.
[0055] If the catalyst is not already present in the slurry
composition, it may be added before the molding process. The
composition is then transferred, e.g., by extrusion, pouring,
syringe transfer, pressing, gravity transfer, and the like, into a
closed cavity of the mold. Where extrusion is used, the composition
is extruded, for example under low pressure (less than 50 psi),
into a die and then cured. The curing process is often accomplished
with heat for rapid manufacturing. However, room temperature
gelation may be desirable where a metal component (if present)
reactivity is excessive, e.g. aluminum, wherein the metal component
is disposed to react with available organic matter to produce
undesirable hydrogen gas bubbles. Other molding techniques,
including injection molding, may also be employed. Moreover, any
conventional additives known in the ceramic processing arts, for
example, mold release agents, may be included in the composition
for their known functions.
[0056] The temperature at which curing, that is, polymerization
and/or cross-linking, is desirably carried out in this method
("curing temperature") depends in large part on the particular
metallic catalyst compound and the particular species that are
included. The curing temperature is typically selected to be
greater than about room temperature, such as at about room
temperature to about 120.degree. C., and in particular embodiments
about 50.degree. C. to about 100.degree. C. Similarly, the time
necessary to form a firm polymer-solvent gel matrix is dependent on
the particular components of the composition. Generally, the mold
containing the slurry composition is heated at an elevated
temperature (i.e., greater than room temperature) for at least
about five minutes, and in some embodiments is heated for a period
of about 5 to about 120 minutes, to polymerize and cross-link the
siloxane species and form a firm silicone-based polymer matrix.
[0057] In some embodiments, before the cure step, the composition
may be brought to a freeze-drying temperature below the freezing
point of the pore-forming agent, while the pore-forming agent
remains present but unreacted, thus persisting to occupy space
within the composition. A desired porosity may also be obtained by
causing phase-separation between the pore-forming agent and the
remaining components of the shell or core composition by bringing
the composition to such a freeze-drying temperature. The
pore-forming agent is then removed and the space formerly occupied
by the pore-forming agent becomes desirable porosity within the
green body formed after subsequent curing. Removal of the
pore-forming agent is typically accomplished by lowering the
atmospheric pressure below the vapor pressure of the pore-forming
agent at the freeze-drying temperature, at which volatilization of
the agent is accomplished within a desirable time frame. To
preferentially volatilize the pore-forming agent during
freeze-drying, a pore-forming agent may be selected such that the
vapor pressure of the pore-forming agent is substantially higher
than the vapor pressures of all other binder components at the
freeze-drying temperature. Temperature and/or pressure may remain
constant throughout the entire method, or may be increased or
reduced, during the agent removal step to assist in attaining
preferred rates and duration of volatilization of the solidified
pore-former. In some embodiments, the atmospheric pressure may be
maintained near the vapor pressure of the pore-forming agent to
slow volatilization of the pore-former during processing, because
vapor production that occurs too quickly may cause vapor
accumulation internal to the green body, increasing risk of damage
to the body. A typical range for the freeze-drying temperature is
down to about -40.degree. C., but of course will in part depend on
specific material choices made in formulating any particular
instance of the composition.
[0058] In other embodiments, during the cure step, the pore-forming
agent remains present but unreacted, thus persisting to occupy
space within the cured matrix. A desired porosity may also be
obtained by causing phase-separation between the pore-forming agent
and the remaining components of the shell or core during the cure
step. The pore-forming agent is then removed after curing under
temperature and pressure conditions suitable to effect
volatilization of the agent, and the space formerly occupied by the
pore-forming agent becomes desirable porosity within the resultant
green body. Removal of the pore-forming agent is typically
accomplished by heating to a drying temperature at which
temperature, for a given pressure, volatilization of the agent is
accomplished within a desirable time frame. Ambient processing
pressure may remain constant throughout the entire method, or may
be adjusted, such as reduced, during the agent removal step to
assist in efficient liquid volatilization. In certain embodiments,
the drying temperature is greater than the temperature used to cure
the composition, to ensure that the agent persists throughout the
cure treatment. In some embodiments, the drying temperature is
maintained below the boiling point of the pore-forming agent to
better control the evolution of vapor during processing, because
vapor production that occurs too quickly may cause vapor
accumulation in the cured body, increasing risk of damage to the
body. A typical range for the drying temperature is up to about
300.degree. C., but of course will in part depend on specific
material choices made in formulating any particular instance of the
composition.
[0059] In some aspects, a composition described herein may provide
enhanced dimensional control compared with conventional systems, in
part through better dimensional stability during the drying step.
For example, filler particles trapped within a cured resin may tend
to remain in place, limiting the particle rearrangement commonly
observed during drying of conventional systems. As a result, drying
shrinkage in embodiments of the present invention may be
significantly decreased compared to conventional slurry-cast
systems.
[0060] The porous green body may be subsequently heated ("fired")
to decompose the silicone-based polymer of the green body, thus
forming a ceramic body that includes silica from the decomposed
polymer and ceramic material from the powder originally suspended
in the slurry composition. Firing may be accomplished by heating at
a firing temperature, for example, greater than about 475.degree.
C. Moreover, the ceramic body thus formed may be further processed
as desired; for instance, the ceramic body may be sintered to form
a body of adequate density for use in investment casting. Sintering
temperatures for various ceramic powders are well known in the art.
In a particular example, the porous green body may be heated in a
conventional kiln under an oxygen-containing atmosphere to a
temperature of about 900.degree. C. to about 1,650.degree. C. for
an aggregate period of about 2 to about 48 hours. The heating rate
is typically, but not necessarily, from about 5.degree. C. per hour
to about 200.degree. C. per hour.
[0061] The resultant ceramic body may be employed in an investment
casting process, such as by using the body as a mold core. In such
embodiments, the ceramic body has a shape that conforms to a
desired shape for an internal cavity in the part to be fabricated
by investment casting. For example, the ceramic body may have a
shape corresponding to the internal cooling passages of a
fluid-cooled machine component, such as an air-cooled turbine
blade. In keeping with well-known methods for investment casting,
the ceramic body is disposed within an investment casting mold
cavity, molten metal is poured or otherwise disposed within the
mold cavity (thereby immersing the ceramic body), and then the
molten metal is solidified within the mold. The ceramic body (core)
is removed by chemical leaching or other method, leaving behind a
space within the solidified metal part.
EXAMPLES
[0062] The following examples are presented to further describe the
techniques, but should not be read as limiting, because variations
still within the scope of embodiments of the present invention will
be apparent to those skilled in the art.
Comparative Example
[0063] A ceramic slurry composition with the reactive siloxane
binder system and powder mixture was produced by combining the
listed ingredients in the proportions specified in Table 1, below,
to make Formulation A. The reactive siloxanes were
1,3,5,7-tetravinyl 1,3,5,7-tetramethylcyclotetrasiloxane (also
known in the art as D.sub.4.sup.Vi) and a hydride-functional
organosilicate resin (CAS registry number 68988-57-8 corresponding
to Formula (VII) with the composition
[HSi(CH.sub.3).sub.2O.sub.1/2].sub.2(SiO.sub.2), also known in the
art as an M.sup.HQ resin). . The mass ratio of the reactive
siloxanes was maintained to give an approximately 1:1 Si-H to vinyl
molar ratio. Anhydrous sodium tetraborate was additionally included
as a mineralizer to enhance the conversion of amorphous silica to
cristobalite during sintering.
TABLE-US-00001 TABLE 1 Comparative Example Slurry Composition
Component Wt % D.sub.4.sup.Vi 7.27 M.sup.HQ resin 9.69
Na.sub.2B.sub.4O.sub.7 0.42 fumed silica 1.06 Zircon 39.76 fused
silica 21/25 41.80 Total 100.00
[0064] The above composition represents a ceramic slurry with a
ceramic (zircon +silicas) loading of about 63% by volume. The
components were mixed in a DAC1100-FVZ HS dual asymmetric
centrifuge (Flacktek, Landrum, S.C.), with intermittent
hand-mixing, for a total of 3 minutes, 10 seconds mix time at 1600
RPM. After cooling, the slurry was catalyzed by addition of 9.4
microliters of Karstedt's-type platinum catalyst (Heraeus CPPT3130,
10% Pt by weight) per 100 parts by weight of slurry. The resultant
catalyzed slurry was transferred into 6 oz. cartridges, degassed
under reduced pressure, then injected into fugitive organic
polymeric, plastic and metal molds using a manual caulker cartridge
gun (Techon Systems).
[0065] To create test bars for testing shrinkage and flexural
strength, molds were rectangular-shaped with internal dimensions 4
''.times.0.625''.times.0.2'' (L.times.W.times.H). The filled test
bar molds were then heated for 15 hours in an air-circulating oven
at 50.degree. C. to cure the reactive siloxane binder matrix.
[0066] After curing was completed, the cured test bar
specimen/molds were loaded into an electric furnace and fired in an
air atmosphere to a terminal temperature of 1000.degree. C. for
3hrs, with intermediate isothermal holds for 3 hrs. each at 150,
175, 200, 300, 500, and 650.degree. C. Following sintering at
1000.degree. C., the furnace was turned off and the contents
allowed to cool naturally back to room temperature. After firing,
the net linear shrinkage was measured with calipers and the room
temperature MOR (modulus of rupture) of the resultant sintered
ceramic test bars was measured on an Instron 4465 load frame in
4-point bend mode.
[0067] Permeability to air was measured as an indication of total
open pore volume and pore size. Test specimens for permeability
measurements were fabricated by either of two methods, referred to
herein as Method 1 and Method 2. For Method 1, slurry was injected
into a 1'' diameter cylindrical mold at least 1'' long. The slurry
was cured at 50.degree. C. to form a slug and 1''-diameter disks of
desired thicknesses were cut from the slug with a wet diamond saw
and excess water and particulate were cleaned off of the specimen.
For Method 2, slurry was injected into disk molds with diameters of
1.1-2.5'' and thickness of 0.074''. Slurry was cured in molds at
50.degree. C. then mechanically removed from molds. Disks were
measured as prepared or after heating to desired temperatures to
remove pore-former and/or firing them to form a ceramic body.
Permeability was measured with a capillary flow porometer by
measuring the volumetric flow rate of air across a circular area
with a fixed diameter of 1.27 cm and applied pressure differential
of 1 atmosphere.
Example 1
[0068] Comparable 63 volume % ceramic solids slurry formulations
and cylindrical specimens for permeability measurements and
rectangular test bars were produced and fabricated in the same
fashion as described for the Comparative Example, but with
decamethyl cyclopentasiloxane (D.sub.5) or 1,3,5-tris(3, 3,
3-trifluoropropyl) trimethylcyclotrisiloxane (also known in the art
as D.sub.3.sup.F) added to the slurry mixture as a pore-forming
agent. Formulations B and C were produced, with 15% of the total
slurry volume comprising D.sub.5, or D.sub.3.sup.F, respectively,
with the volume fraction of the reactive siloxane mixture reduced
accordingly to maintain a constant 63 volume % ceramic solids
loading in the uncured mixture.
[0069] The prepared slurries were catalyzed and injected into
cylindrical (for measuring permeability and loss of pore-former
during drying) test molds and rectangular (for measuring shrinkage
and flexural strength) test bar molds and cured as in the
Comparative Example. Test disks for measuring permeability were
fabricated following the procedure outlined in Method 1 where the
thickness of the disks made from Formulation A were 0.085'', the
thickness of disks made from Formulation B were 0.055'' and the
thickness of disks made from Formulation C were 0.013''. The
permeability values account for the different thicknesses and can
be compared to one another. After curing but before firing, test
disks were heated in an air-circulating oven for post-cure
pore-former removal. Temperature was raised at a rate of 5.degree.
C/hr until a temperature of 120.degree. C. was reached, which
temperature was then held for 10 hours. Temperature was then again
raised at a rate of 5.degree. C/hr until a temperature of
160.degree. C. was reached, which temperature was then held for 24
hours. Cured samples were weighed before drying at 120.degree.
C.-160.degree. C. and after such drying to determine percentage of
pore-former that was removed by drying at 120.degree. C.
-160.degree. C. Test bars were fired to 1000.degree. C. and tested
as in the Comparative Example. Mean test values for Formulations
A-C are given in Table 2.
TABLE-US-00002 TABLE 2 Test Data for Pre- and Post-Fired Ceramic
Parts Fabricated from Slurry Compositions Containing 15 Volume %
D.sub.5 or D.sub.3.sup.F as Pore-Former 160.degree. C. Dried
1000.degree. C. Pre-Firing Air 1000.degree. C. Fired Pore-
Permeability Fired Flexural Former (cm.sup.2/(sec- Linear Strength
Sample Pore- Loss at atm)) Shrinkage (psi) ID Former 160.degree. C.
(.+-.95% CI) (.+-.95% CI) (.+-.95% CI) A None NA 2.2 .times.
10.sup.-4 2.8% 1193 (4.9 .times. 10.sup.-4) (0.04) (73) B D.sub.5
95% 6.6 .times. 10.sup.-4 3.1% 2009 (1.9 .times. 10.sup.-4) (0.05)
(282) C D.sub.3.sup.F 57% 4.7 .times. 10.sup.-2 2.9% 2870 (8.0
.times. 10.sup.-4) (0.06) (368)
[0070] The foregoing data indicate that using a pore-former, such
as D.sub.5 or D.sub.3.sup.F, advantageously increases pre-firing
permeability and post-firing strength, without disadvantageously
affecting shrinkage. Of particular interest is the significant
increase of pre-fired permeability for slurry prepared with
D.sub.3.sup.F compared to slurry prepared with D.sub.5 even though
only half of the D.sub.3.sup.F pore-former was removed at
160.degree. C. This enhanced permeability is thought to be due to
phase separation of the D.sub.3.sup.F from the binder during the
curing step due to reducing miscibility of the D.sub.3.sup.F in the
binder as it cures and increases in molecular weight. In a separate
experiment, only the liquid binders, pore-forming agent and
catalyst were mixed together without any solids to form a clear,
colorless liquid. A mixture of the binders and D.sub.5 form a
clear, colorless liquid and the mixture turns to a clear, colorless
solid when cured indicating that any phase separation of the
D.sub.5 results in domains smaller than the wavelength of visible
light. Phase separation of D.sub.3.sup.F from the binders during
cure was evidenced by observation of the clear, colorless liquid
mixture turning to an opaque white color during cure indicating the
formation of a separate domain size larger than the wavelength of
visible light. This phase separation leads to pores with larger
diameters than those formed with a fully miscible pore-former, such
as D.sub.5. The pores in the D.sub.3.sup.F fired bar (Formulation
C) were observed to be larger than the pores in the D.sub.5 fired
bar (Formulation B) by scanning electron microscopy and as
estimated by Brunauer-Emmett-Teller surface area analysis. The
increased permeability is a desirable property to promote rapid
diffusion of oxygen into the center of the body and release of
gaseous by-products during firing, which would decrease flaws
caused by shrinkage gradients.
Example 2
[0071] Comparable 63 volume % ceramic solids slurry formulations
and cylindrical test disks were produced in the same fashion as
described for the Comparative Example, but with decamethyl
cyclopentasiloxane (D.sub.5) or cyclohexane added to the slurry
mixture as a pore-forming agent. Sample formulations D and E were
produced, with 30% of the total slurry volume comprising D.sub.5 or
cyclohexane, respectively, with the volume fraction of the reactive
siloxane mixture reduced accordingly to maintain a constant 63
volume % ceramic solids loading in the uncured mixture. For control
sample F, compacted pellets of dry pressed ceramic powders
(silicas, zircon and sodium tetraborate) were prepared by firing to
1050.degree. C. to sinter the particle network to 67.5% of the
theoretical density and tested for their permeability, as a
representation of the theoretically maximum attainable
permeability.
[0072] Samples fabricated with slurry formulation D were formed
into disks as described for Method 2 of the Comparative Example and
cured then dried as described for Examples A-C to volatilize
D.sub.5. After being injected into cylindrical molds as in the
Comparative Example, samples prepared with formulation E were
frozen at -40.degree. C. They were subsequently placed into a
freeze-drier with an initial temperature set-point of -40.degree.
C. The air pressure was reduced to 300 mTorr. The temperature was
ramped up to -20.degree. C. at 5.degree. C/hr and then to
20.degree. C. at a rate of 2.5.degree. C/hr to induce
volatilization of cyclohexane. The samples were held at 20.degree.
C. for at least 15 hrs before removal from the freeze-drier. The
samples were then cured at 50-75.degree. C. for at least 15 hr. The
slug of cured slurry was removed from the mold and sliced into
0.058'' thick disks to obtain the pre-firing samples. Measurement
of permeability of discs was as described for the Comparative
Example. Mean test values for Formulations D-F are given in Table
3.
TABLE-US-00003 TABLE 3 Test Data for Fired Ceramic Parts Fabricated
from Slurry Compositions Containing 30 Volume % D.sub.5 or
Cyclohexane as Pore-Former Air Permeability (cm.sup.2/(sec- Sample
atm)) ID State of Sample Pore-Former (.+-.95% CI) A Pre-fired
(Cured then dried None 2.2 .times. 10.sup.-4 at 160.degree. C.)
(4.9 .times. 10.sup.-4) D Pre-fired (Cured then dried D.sub.5 4.0
.times. 10.sup.-3 at 160.degree. C.) (7.4 .times. 10.sup.-4) E
Pre-fired (Freeze-dried then Cyclohexane 2.1 .times. 10.sup.-1
cured) (3.6 .times. 10.sup.-2) F Fired at 1050.degree. C. None 2.8
.times. 10.sup.-1 (Pressed (3.8 .times. 10.sup.-2) Pellets)
[0073] The foregoing data indicate that using a pore-former, such
as D.sub.5 or cyclohexane, advantageously increases pre-firing
permeability. In a separate experiment only the liquid binders,
pore-forming agent and catalyst were mixed together without any
solids to form a clear, colorless liquid. Phase separation of
cyclohexane from the binders was evidenced by observation of the
mixed liquid components appearing as a clear, colorless liquid at
20.degree. C., then appearing opaque when cooled to -40.degree. C.,
which is below the freezing point of cyclohexane. The formation of
porosity by a phase separating pore-forming agent, such as
solidified cyclohexane, and the subsequent removal of the phase
separated pore-forming agent, allows for permeability values in
excess of Formulation A, a slurry that does not contain a
pore-forming agent, and approaches that of the binder-free ceramic
system.
[0074] While only certain features of the invention have been
illustrated and described herein, many modifications and changes
will occur to those skilled in the art. It is, therefore, to be
understood that the appended claims are intended to cover all such
modifications and changes as fall within the true spirit of the
invention.
* * * * *