U.S. patent application number 09/200803 was filed with the patent office on 2001-07-12 for curable masking material for protecting a passage hole in a substrate.
Invention is credited to BOROM, MARCUS PRESTON, BREWER, JAMES ANTHONY, HASZ, WAYNE CHARLES, SZALA, LAWRENCE EDWARD, VENKATARAMANI, VENKAT SUBRAMANIAM.
Application Number | 20010007708 09/200803 |
Document ID | / |
Family ID | 25051344 |
Filed Date | 2001-07-12 |
United States Patent
Application |
20010007708 |
Kind Code |
A1 |
VENKATARAMANI, VENKAT SUBRAMANIAM ;
et al. |
July 12, 2001 |
CURABLE MASKING MATERIAL FOR PROTECTING A PASSAGE HOLE IN A
SUBSTRATE
Abstract
A method for temporarily protecting at least one passage hole in
a metal-based substrate from being obstructed by at least one
coating applied over the substrate is disclosed. The method
includes the following steps: (a) filling and covering the hole
with a curable masking material which forms a protrusion over the
hole; (b) curing the masking material; (c) applying at least one
coating over the substrate and the masking material, wherein the
coating does not substantially adhere to the protrusion; and then
(d) removing the masking material to uncover the passage hole.
Usually, there are an array of holes, and they serve as conduits
for cooling gasses for an engine component. The curable masking
material exhibits substantially non-Newtonian flow characteristics
which make it especially suitable for forming protrusions of the
proper size and shape on the coating-side of an engine part.
Another embodiment of this invention relates to a the curable
masking material itself, and to articles which include such a
material.
Inventors: |
VENKATARAMANI, VENKAT
SUBRAMANIAM; (CLIFTON PARK, NY) ; BREWER, JAMES
ANTHONY; (SCOTIA, NY) ; BOROM, MARCUS PRESTON;
(NISKAYUNA, NY) ; HASZ, WAYNE CHARLES; (POWNAL,
VT) ; SZALA, LAWRENCE EDWARD; (SCOTIA, NY) |
Correspondence
Address: |
GENERAL ELECTRIC COMPANY
CRD PATENT DOCKET RM 4A59
P O BOX 8
BLDG K1 SALAMONE
SCHENECTADY
NY
12301
|
Family ID: |
25051344 |
Appl. No.: |
09/200803 |
Filed: |
November 27, 1998 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09200803 |
Nov 27, 1998 |
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08758328 |
Dec 3, 1996 |
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5902647 |
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Current U.S.
Class: |
428/139 ;
428/596 |
Current CPC
Class: |
Y10T 428/12361 20150115;
B23P 2700/06 20130101; Y02T 50/60 20130101; Y10T 428/24322
20150115; C23C 4/01 20160101; Y10T 428/24281 20150115; C23C 14/042
20130101; Y10T 428/24273 20150115; B05D 1/32 20130101; Y10T
428/24289 20150115; Y10T 428/24339 20150115 |
Class at
Publication: |
428/139 ;
428/596 |
International
Class: |
B32B 003/10 |
Claims
What is claimed:
1. A method for temporarily protecting at least one passage hole in
a metal-based substrate from being obstructed by at least one
coating applied over the substrate, comprising the following steps:
(a) filling and covering the hole with a curable masking material
which forms a protrusion over the hole; (b) curing the masking
material; (c) applying at least one coating over the substrate and
the masking material, wherein the coating does not substantially
adhere to the protrusion; and then (d) removing the masking
material to uncover the passage hole.
2. The method of claim 1, wherein the hole is filled and covered by
an extrusion technique.
3. The method of claim 1, wherein the metal-based substrate has a
first surface and a second surface, and the masking material is
extruded from the first surface, through the length of the passage
hole, to extend beyond the second surface.
4. The method of claim 3, wherein the extrusion is carried out by
applying a layer of the masking material over the first surface,
and then forming a pressure differential between the first surface
and the second surface, causing the material to move to the second
surface.
5. The method of claim 3, wherein the masking material is extruded
under a pressure of about 1 psi to about 30 psi.
6. The method of claim 3, wherein a sheet of material is placed
over the first surface after the masking material has been applied
to the first surface for extrusion, and extrusion is then carried
out by applying pressure on the sheet of material.
7. The method of claim 6, wherein the sheet of material comprises a
rubber or a saran-type material.
8. The method of claim 1, wherein the average height of the
protrusion above the substrate is at least about 50% greater than
the expected thickness of the coating subsequently applied in step
(c).
9. The method of claim 1, wherein the curable masking material
exhibits a substantially non-Newtonian flow characteristic.
10. The method of claim 9, wherein the curable masking material
exhibits a Bingham solid characteristic.
11. The method of claim 9, wherein the curable masking material
comprises a thermosetting resin and at least one additive selected
from the group consisting of fillers, plasticizers, and liquid
crystal materials.
12. The method of claim 11, wherein the thermosetting resin is
selected from the group consisting of epoxy resins and alkyd
resins.
13. The method of claim 12, wherein the epoxy is derived from
bisphenol A.
14. The method of claim 11, wherein the thermosetting resin is a
phenolic.
15. The method of claim 14, wherein the phenolic is a
phenol-formaldehyde polymer.
16. The method of claim 11, wherein the filler is an organic
material or a mixture of an organic material and an inorganic
material.
17. The method of claim 16, wherein the organic material comprises
a substance selected from the group consisting of rubber, wax,
gums, cellulosic materials, wood, sawdust, walnut shell powder, and
any combination of the foregoing.
18. The method of claim 11, wherein the filler comprises an
inorganic material.
19. The method of claim 18, wherein the inorganic material is
selected from the group consisting of graphite, silica, alumina,
magnesium oxide, talc, alkaline earth carbonates, zirconium basic
carbonate, sulfates, sulfides, halides, phosphates, borates,
borosilicates, slate flour, clay, and any combination of the
foregoing.
20. The method of claim 10, wherein the curable masking material
comprises a thermoplastic resin and at least one additive selected
from the group consisting of fillers, plasticizers, and liquid
crystal materials.
21. The method of claim 20, wherein the thermoplastic resin is
selected from the group consisting of acrylic resins, thermoplastic
polyesters, polyamides, thermoplastic polyimides, polycarbonates,
polyphenylene ethers, polyolefins, styrene-based resins, copolymers
of any of the foregoing; and mixtures of any of the foregoing.
22. The method of claim 21, wherein the filler is an organic
material or a mixture of an organic material and an inorganic
material.
23. The method of claim 1, wherein the masking material is removed
in step (d) by a technique which comprises pyrolysis.
24. The method of claim 1, wherein the masking material is
substantially removed in step (d) by treatment with a solvent which
dissolves the material.
25. The method of claim 1, wherein the metal-based substrate is a
superalloy.
26. The method of claim 1, wherein the metal-based substrate is a
component of a gas turbine engine.
27. The method of 26, wherein the component is a combustion
chamber.
28. The method of claim 1, wherein the coating applied in step (c)
comprises a thermal barrier coating.
29. The method of claim 1, wherein the substrate contains a row of
passage holes.
30. The method of claim 1, wherein the substrate contains an array
of passage holes.
31. The method of claim 3, wherein the second surface is a hot
surface exposed to elevated temperature, and the first surface is a
cold surface exposed to a temperature less than that to which the
hot surface is exposed.
32. The method of claim 31, wherein the hot surface is exposed to a
temperature of at least about 1200.degree. C.
33. The method of claim 31, wherein the passage holes are cooling
holes.
34. The method of claim 1, wherein step (c) comprises applying a
bond coat and then a thermal barrier coating over the bond
coat.
35. A method for applying a thermal barrier coating to a
high-temperature alloy substrate which includes cooling holes,
wherein the cooling holes remain clear after the thermal barrier
coating has been applied, comprising the following steps: (i)
filling and covering the holes with a curable masking material
which forms a protrusion over each hole; (ii) curing the masking
material; (iii) applying a thermal barrier coating over the exposed
masking material, wherein the thermal barrier coating does not
substantially adhere to the protrusions; and then (iv) removing the
masking material to uncover the cooling holes.
36. The method of claim 35, wherein a bond coating is applied
between steps (ii) and (iii), and the bond coating also does not
substantially adhere to the protrusions.
37. The method of claim 35, wherein the holes are filled and
covered by an extrusion technique.
38. The method of claim 35, wherein the curable masking material
exhibits a substantially non-Newtonian flow characteristic.
39. The method of claim 36, wherein the average height of the
protrusion above the substrate is at least about 50% greater than
the expected, total thickness of the bond coating and the thermal
barrier coating.
40. The method of claim 35, wherein the substrate is subjected to
an adhesion-enhancing pre-treatment between step (ii) and step
(iii).
41. The method of claim 40, wherein the pre-treatment comprises
abrasion of the substrate surface.
42. The method of claim 36, wherein the bond coating and the
thermal barrier coating are independently applied by a plasma
technique or by electron beam-chemical vapor deposition.
43. The method of claim 35, wherein the masking material is removed
in step (iv) by a technique which comprises pyrolysis.
44. A method for repairing a damaged thermal barrier coating
applied over a substrate which includes at least one passage hole
extending from a first surface to a second surface on which the
thermal barrier coating had been applied, comprising the following
steps: (A) removing the damaged thermal barrier coating from the
second surface, uncovering any passage hole in the substrate; (B)
filling and covering the hole with a curable masking material which
forms a protrusion over the hole; (C) curing the masking material;
(D) applying additional thermal barrier coating material over the
substrate and the masking material, wherein the coating does not
substantially adhere to the protrusion; and then (E) removing the
masking material to uncover the passage hole.
45. A curable masking material for protecting at least one passage
hole in a metalbased substrate from being obstructed by the
subsequent application of at least one coating to the substrate,
wherein said masking material comprises an extrudable resin
composition which is thermally stable at elevated temperatures, and
wherein the masking material, when cured, does not serve as an
adhesion site for any coating which is subsequently applied.
46. The curable masking material of claim 45, comprising a
thermosetting resin and at least one rheology-modifying additive,
wherein the weight-ratio of thermosetting resin to the total amount
of additive is in the range of about 90:10 to about 50:50.
47. The curable masking material of claim 46, wherein the
thermosetting resin is an epoxy resin or a phenolic resin.
48. The curable masking material of claim 45, comprising a
thermoplastic resin and at least one rheology-modifying
additive.
49. An article, comprising: (I) a substrate which includes at least
one passage hole extending from a first surface to a second
surface; (II) a curable masking material which fills the passage
hole and covers the hole to form a protrusion on the second surface
over the hole; and (III) at least one coating applied over the
second surface, wherein the coating does not substantially adhere
to the protrusion.
Description
TECHNICAL FIELD
[0001] This invention relates generally to coatings technology.
More particularly, it concerns the use of protective coatings which
contain open holes axially aligned with open holes in a
substrate.
BACKGROUND OF THE INVENTION
[0002] Substrates which are fabricated in an industrial setting are
usually subjected to a variety of processing steps. For example,
metal substrates, after being cast, may undergo many procedures to
achieve a final product, such as grinding, cold-working, cleaning,
annealing, grit-blasting, further cleaning, and the like. There may
be a variety of designed features on or in the substrate which are
incorporated early on in processing, and which must be preserved
through all subsequent fabrication steps.
[0003] Turbine engines provide a good illustration. The "substrate"
may be a turbine blade, or may be a combustion chamber (combustor),
for example. The parts are often made from high temperature
metallic alloys, often referred to in the art as "superalloys".
When turbines are used on aircraft, they are typically run at a
temperature as high as possible, for maximum operating efficiency.
Since high temperatures can damage the alloys used in the engine, a
variety of approaches have been used to raise the operating
temperature of the metal components. One approach calls for the
incorporation of internal cooling channels in the component,
through which cool air is forced during engine operation. Thus, the
"designed feature" in this instance is a pattern of cooling holes
which extend from one surface of the part to another. For example,
the holes may extend from a cooler surface of a combustor to a
"hot" surface which is exposed to combustion temperatures of at
least about 1200.degree. C. The cooling holes are usually formed in
the substrate by specialized laser-drilling techniques. Cooling air
(usually provided by the engine's compressor) is fed through the
holes from the cooler side to the hot side of the combustor wall.
As long as the holes remain clear, the rushing air will assist in
lowering the temperature of the hot metal surface and preventing
melting or other degradation of the component.
[0004] Another technique for protecting the metal parts and
effectively raising the practical operating temperature of an
aircraft engine involves the use of a thermal barrier coating
(TBC). The TBC is usually ceramic-based. TBC systems frequently
also include a bond coat which is placed between the ceramic
coating and the substrate to improve adhesion. The use of TBC's in
conjunction with the battery of cooling holes is sometimes the most
effective means for protecting an engine part. However,
incorporation of both systems can be very difficult. For example,
the cooling holes sometimes cannot be formed in the engine part
after a TBC has been applied, since lasers usually cannot
effectively penetrate both the ceramic material and the metal to
form the pattern of holes.
[0005] If the cooling holes are formed prior to the application of
the TBC system, they may become covered and at least partially
obstructed when the TBC is applied. Complete removal of the
ceramic-metal material from the holes can be very time-consuming
and ineffective, if not impossible. Any obstruction of the holes
during engine operation can interfere with the passage of cooling
air, can waste compressor power, and can possibly lead to engine
component damage due to overheating.
[0006] Even if a type of laser could satisfactorily penetrate the
TBC, registration and alignment difficulties would remain. For
example, there would be no suitable technique for ensuring that the
hole being drilled through the TBC is properly aligned with the
hole previously drilled in the substrate itself.
[0007] From this discussion, one can readily understand that new
methods for protecting certain features on metal substrates during
subsequent processing steps would be welcome in industry. Of
particular interest in the area of turbine engines would be new
methods for providing open holes which communicate through various
coating layers on engine parts.
[0008] These new methods should protect the designed features, but
should also allow the particular features to become fully exposed
after the other processing steps are complete. Furthermore, the
techniques should also be useful for repairing TBC systems while
retaining open holes axially aligned with previously-formed open
holes in the substrate.
[0009] Moreover, the techniques involved should be completely
compatible with the other processing steps, and should not
adversely affect the substrate. For example, the strength and
integrity of a turbine engine part should be completely retained
after the treatment to protect the cooling holes has been
completed.
SUMMARY OF THE INVENTION
[0010] The needs discussed above have been met by the discoveries
outlined herein. One embodiment of this invention is directed to a
method for temporarily protecting at least one passage hole in a
metal-based substrate from being obstructed by at least one coating
applied over the substrate, comprising the following steps:
[0011] (a) covering the hole with a curable masking material which
forms a protrusion over the hole;
[0012] (b) curing the masking material;
[0013] (c) applying at least one coating over the substrate and the
masking material, wherein the coating does not substantially adhere
to the protrusion; and then
[0014] (d) removing the masking material to uncover the passage
hole.
[0015] As discussed previously, the substrate often includes a row
or an array of passage holes. They are frequently cooling holes
within turbine engine components. The curable masking material
exhibits substantially non-Newtonian flow characteristics which
make it especially suitable for forming protrusions of the proper
size and shape on the coating-side of an engine part. The material
can be thermoplastic or thermosetting, and is usually used in an
admixture with at least one filler or other rheology-modifying
additive.
[0016] Another embodiment of this invention relates to the curable
masking material itself, which comprises an extrudable resin
composition which is thermally stable at elevated
temperatures--usually up to a temperature of at least about
350.degree. C. The material exhibits substantially non-Newtonian
flow characteristics, e.g., those of a Bingham solid. It may be an
epoxy or phenolic resin, for example, used in conjunction with at
least one organic or inorganic filler like graphite or silica. When
cured, the masking material ideally does not serve as an adhesion
site for protective coatings which are subsequently applied. The
masking material is easily removed from the substrate after any
related processing operations have been completed.
[0017] Numerous other details regarding these and other embodiments
of the present invention are provided below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a schematic, isometric view of a typical substrate
which contains holes or passageways.
[0019] FIG. 2 is a cross-sectional view of a portion of a substrate
similar to that of FIG. 1.
[0020] FIG. 3 is another cross-sectional view of a portion of a
substrate similar to that of FIG. 1, wherein the featured hole has
been filled and covered with a maskant material.
DETAILED DESCRIPTION OF THE INVENTION
[0021] The metal-based substrate can be any metallic material or
alloy which is to be covered (at least in part) by some sort of
coating. "Metal-based" refers to substrates which are primarily
formed of metal or metal alloys, but which may also include some
non-metallic components, e.g., ceramics or plastics. The holes
which are in the substrate usually extend from one surface to
another surface, and may constitute a variety of shapes. The
surface to which at least one coating is subsequently applied is
the "hot" surface (described above) when the substrate is a
component of a turbine engine. The distance between the surfaces,
which is usually equivalent to the thickness of the substrate, is
usually in the range of about 10 mils to about 250 mils, and most
often, in the range of about 20 mils to about 125 mils. Very
often--especially when used as cooling passageways in some sort of
engine part most of the holes are substantially circular, thus
having a cylindrical shape through the body of the substrate. The
diameter of the holes often is in the range of about 5 mils to
about 500 mils. In some embodiments, the diameter is in the range
of about 5 mils to about 100 mils. The holes may be substantially
perpendicular to the substrate surface, or they may be situated at
an angle, e.g., at least about 10 degrees relative to a horizontal
outer surface. This will of course depend on the function of the
holes. When they are used as cooling passageways in an engine part,
they are often situated at an angle in the range of about 20
degrees to about 80 degrees relative to the horizontal outer
surface. Moreover, the depth of the hole for that type of end use
(i.e., the "length" of a hole if it is situated at an angle) is
usually in the range of about 50 mils to about 700 mils. There are
usually about 50 to about 120 holes per square-inch of the
substrate surface.
[0022] As mentioned above, the holes are initially filled and
covered with a curable masking material (sometimes referred to
herein as a "maskant"). The characteristics of the masking
material--both in its uncured and cured states are important to the
operability of this invention. When the maskant is to be extruded
through the hole, it should exhibit substantially non-Newtonian
flow characteristics. In other words, the maskant should be able to
flow easily through the holes only under the application of force,
either by pressure or vacuum. The portion of the maskant material
which leaves the hole and moves onto the surrounding surface no
longer experiences a substantial shearing force, i.e., it is
relatively free-standing, and resists "slumping". This "flow and
freeze" characteristic results in the formation of the "bumps" or
protrusions which cover each hole, as discussed below. The
protrusions may increase slightly in size as more material exits
the hole, but the general shape of the protrusions is retained.
[0023] There are several types of materials having non-Newtonian
characteristics which are suitable for the present invention. The
characteristics of these materials are well-known in the art of
fluid mechanics and chemical engineering, and are described, for
example, in Ullmann's Encyclopedia of Industrial Chemistry, Fifth
Edition; Volume B2, VCH Publisher (1988), pp. 8-17 to 8-18; and
Volume B1, pp. 5-25 to 5-28; in the Kirk-Othmer Encyclopedia of
Chemical Technology, 3rd Edition, Vol. 10, p. 600-603 (1980.); and
in U.S. Pat. No. 5,304,331 (W. Leonard et al), incorporated herein
by reference. One class of maskant material is referred to as
pseudoplastic or "shear-thinning". Here, the ratio of shear stress
to shear rate (i.e., the viscosity) is a decreasing function of the
shear rate, as described in Volume B1 of the Ullmann reference.
These materials usually have a viscosity, at a given
use-temperature, of about 1 centipoise to about 5000 centipoises,
and preferably, in the range of about 10 centipoises to about 2000
centipoises.
[0024] Another class of suitable material which is particularly
preferred for the present invention is referred to as a
Bingham-solid material, which exhibits a "yield stress". In other
words, the shear stress must exceed a certain value before the
material begins to flow. The material may at that point begin to
display a linear "rate-of-shear versus shear stress" behavior
similar to that of a Newtonian system, as described in the
Kirk-Othmer reference mentioned above. These materials usually have
a viscosity, at a given use-temperature, of about 1 centipoise to
about 15,000 centipoises, and preferably, in the range of about 10
centipoises to about 5000 centipoises. Bingham materials generally
exhibit the "flow and freeze" characteristic which results in the
formation of the discrete protrusions which cover the surface of
each hole.
[0025] It should be emphasized that the maskant material useful for
the present invention, while exhibiting substantially non-Newtonian
behavior, may exhibit a small degree of Newtonian behavior. As an
illustration, a material which exhibited absolute Bingham solid
(non-Newtonian) characteristics might be expected to exit the hole
and take the form of an inclined rod extending up from the surface
(i.e., a geometry similar to the hole itself), rather than taking
the shape of an elliptical dome. Thus, it should be understood that
the preferred maskant material described herein should have a flow
characteristic which is generally sufficient to form the
protrusions described herein, with the purpose of the protrusions
for this invention also being kept in mind.
[0026] There are other important requirements for the maskant. It
should be thermally stable (i.e., not melting, degrading, or
substantially softening) under the time and temperature conditions
encountered during processing of the particular substrate. Thus,
when the substrate is used in high-temperature applications, the
maskant should be stable up to a transient temperature of at least
about 500.degree. C. during the time period when subsequent
coatings are being applied--especially coatings which require
elevated temperatures for deposition, such as TBC systems.
"Transient" as used herein indicates that deposition temperatures
for techniques such as plasma spray may vary somewhat, depending on
the location of the heat source (e.g., the torch) relative to the
surface being coated. Preferably, the maskant should be stable to a
transient temperature of at least about 650.degree. C. This is
especially true when the substrate is a turbine engine part, such
as a combustor liner, which may be coated with a bond layer at
deposition temperatures of about 350.degree. C., and may then be
coated with a TBC at a transient temperature of up to about
600.degree. C.
[0027] In preferred embodiments, the maskant, once cured to form
the protrusions which cover the holes, has a particular surface
characteristic which prevents substantial adherence of a
subsequently-applied coating to its surface. (By the prevention of
"substantial adherence", it is to be understood that very minor
amounts of exposed maskant-protrusions might be covered by the
applied coatings, e.g., less than about 5% of the surface area of
the protrusions, but the protrusions are still generally free of
any of the applied coating.) This characteristic is particularly
significant when the substrate is a turbine engine part and the
subsequently-applied coatings are bond coatings and/or TBC's, as
described below. The exposed protrusions of maskant are relatively
easy to remove (i.e., to "unplug" the holes in the substrate), in
contrast to the situation in which the protrusions are covered by
coatings--especially refractory coatings.
[0028] Moreover, the cured maskant should be relatively hard and
abrasion-resistant, so that subsequent pre-treatment and coating
steps do not degrade the protrusions and prematurely uncover any of
the substrate holes. Examples of pre-treatment of the masked
substrate prior to the application of coatings include etching with
solvents or acidic solutions, and/or grit-blasting with abrasive
media.
[0029] The cured maskant should also be easily removable from both
the coatings and the substrate holes after it has performed its
function, i.e., after further coatings have been applied to the
substrate and fired or cured. As described below, the maskant can
be removed by a variety of techniques, such as pyrolysis or
dissolution with solvents. The techniques used should preferably
result in a minimum of maskant-residue within the holes. For
example, on average, less than about 5%, and preferably, less than
about 1% of the total inner surface area of the holes should be
covered by any residue.
[0030] Various polymer systems fulfill the requirements stated
above, as those skilled in the polymer arts understand. Most of the
systems include a resin and at least one filler. Thermosetting
resins are preferred in some embodiments because of the fact that
they are often heat resistant up to at least about 400.degree. C.
and they are usually quite hard and abrasion-resistant. Because
they are not easily processible in their cured state, these
materials are used for this invention in an uncured
(non-cross-linked) or partially-cured state, based in part on the
Theological requirements discussed above.
[0031] One example of a suitable thermosetting resin is an epoxy.
Such materials are well-known in the art and described in many
sources, such as Organic Polymer Chemistry, by K. J. Saunders,
Chapman and Hall, 1973; and in The Condensed Chemical Dictionary,
10th Edition, 1981, Van Nostrand Reinhold Company. Many of these
materials are liquid-epoxies, and are based on starting compounds
such as bisphenol A ((2,2-bis (4'-hydroxyphenyl)propane)) and a
co-reagent such as epichlorohydrin. As is known in the art, epoxies
are often supplied as 2-component systems, e.g., a resin like
bisphenol A-epichlorohydrin, and a curing agent such as a tertiary
amine, a polyfunctional amine, or an acid anhydride. When the
2-component system is mixed (which can be accomplished in an
extrusion system), the product quickly cross-links into a polymer
which satisfies the needs of this invention. Those of skill in the
art realize that various other epoxy resins could also be used
herein, as long as they conform to substantially all of the
parameters outlined above.
[0032] Phenolic resins are another type of preferred thermosetting
polymers for the present invention. Many phenolics are heat
resistant up to at least about 400.degree. C. The chief class of
phenolic resins are the phenol-formaldehyde materials, which are
known in the art and described, for example, in the Saunders text
mentioned above. Within this class of materials, "resoles" are
prepared by reacting phenol with a molar excess of formaldehyde,
under alkaline conditions, while "novolacs" are usually prepared by
reacting a molar excess of phenol with formaldehyde, under acidic
conditions. In general, polymerization is often carried out in two
stages: (1) formation of a low molecular weight, soluble resin, and
then (2) curing operations to covert the soluble resin to a
cross-linked, high-molecular weight product. In the present
invention, a phenolic material in the first stage can be extruded
through the substrate holes as described below. When cured by
conventional techniques in the second stage, i.e., after the
protrusions have been formed, this type of maskant material is very
hard and solvent-resistant, making it very tolerable of the
potential pre-coating treatments mentioned above.
[0033] Various other types of thermosetting resins may be used as a
maskant for the present invention, although they may vary in the
degree to which they can be readily extruded through (or otherwise
be made to cover) the substrate holes in their uncured or
non-cross-linked state. One other example is a thermosetting
polyester, which is usually a polycondensation product of a
dicarboxylic acid with a dihydroxy alcohol. These resins can be
cross-linked through their double bonds with compatible monomers to
become thermosetting. Diols like propylene glycol are often used to
prepare these types of polyesters, in conjunction with unsaturated
acids or anhydrides like maleic anhydride. Alkyd resins, which are
well-known in the art, are included within the broad definition of
polyesters, and are also thought to be useful for the present
invention.
[0034] Silicone resins which cross-link are also suitable maskant
materials for some embodiments of the present invention. These
materials are usually siloxane polymers based on a structure of
alternating silicon and oxygen atoms with various organic radicals
attached to the silicon. In preferred embodiments, the silicones
should not be the type which convert to a high degree of silica
when cured, since the silica may be a difficult residue to remove
from the substrate holes.
[0035] As mentioned above, thermoplastic resins can also be very
suitable as maskant materials for this invention. Some of the
advantages of these materials are their ease-of-handling (once
softened), and amenability to extrusion. Examples are the polyamide
or "nylon" resins, which are described in many references, such as
the Saunders text and U.S. Pat. No. 4,824,915 (Aycock et al),
incorporated herein by reference. They are prepared by well-known
methods, such as the reaction of a diamine and a dicarboxylic acid.
Non-limiting, commercial examples of these materials are polyamide
6; {fraction (6/6)}; 11; 12; {fraction (6/10)}; and {fraction
(6/12)}; and compatible mixtures thereof.
[0036] Thermoplastic polyesters such as polyethylene terephthalate
(PET) and polybutylene terephthalate (PBT) (or mixtures or
copolymers thereof) could also be used as the maskant material.
These materials are well-known in the art and described in a
variety of references. Their preparation is also well-known. For
example, PET is usually prepared by an ester interchange
reaction.
[0037] Polycarbonates, which are somewhat related to the
thermoplastic polyesters, may also be used as the maskant material.
These materials are also known in the art, and are available, for
example, under the Lexan product designation of General Electric
Company. They can be made by a variety of methods, e.g., the
reaction of bisphenol A and phosgene; or a transesterification
reaction involving diphenyl carbonate.
[0038] Polyimides could also be used, i.e., polymers having the
imide group (--CONHCO--) in the polymer chain. A particular type of
polyimide is referred to as a "polyetherimide", and is available
from General Electric Company under the Ultem product
designation.
[0039] A wide variety of thermoplastic acrylics could also be used
for this invention, all of which are known in the art. Many are
based on acrylic acid or methacrylic acid, or on esters of these
compounds. Some are also based in part on acrylonitrile and/or
acrylamide. Acrylic copolymers are included herein within the
definition of "acrylics".
[0040] Those of skill in the polymer arts could certainly conceive
of other polymers or polymer mixtures which may be suitable for the
present invention, based on the parameters outlined herein.
Non-limiting examples would include various styrene-based resins
(e.g., polystyrene; rubber-modified polystyrene;
acrylonitrile-butadiene-styrene (ABS)); polyphenylene ether;
emulsion-type systems such as latex emulsions; polyurethanes;
polyolefins; sulfur-containing polymers such as polyphenylene
sulfide; and mixtures of any of these polymers with each other;
with elastomers; or with any of the other materials mentioned
herein.
[0041] As mentioned above, the polymer or polymer mixture will
usually be used in conjunction with at least one additive which
influences its rheological characteristics. (Thus, the maskant
material is sometimes referred to herein as a polymer "system").
The additive is usually a filler, a reinforcing agent, or an
extending agent. (For the purpose of this discussion, "filler" is
meant to include reinforcing--and extending agents, as well as
"thickening agents", although these other terms sometimes refer to
different types of materials utilized for other purposes). Fillers
are well-known in the art. In general, the weight-ratio of the base
polymer or polymers to total rheological modifier is usually in the
range of about 90:10 to about 50:50. Selection of a particular
additive will depend on various factors, such as its ability to
affect the viscosity of the polymer system; its compatibility with
the base polymer or polymer mixtures; and the type and amount of
residue attributable to the additive when the polymer system is
removed from the substrate hole.
[0042] Organic fillers are often preferred for this invention. This
is due in part to the fact that any residue resulting from their
use is minimal, and can be easily removed with the rest of the
maskant. Non-limiting examples of organic fillers are paraffins or
other waxes; gums, cellulosic materials such as methylcellulose;
wood, sawdust, walnut shell powder; or a rubber, such as isoprene.
The rubber could be utilized in a form which would facilitate
mixing with the base polymer, e.g., in the form of an emulsion. In
some preferred embodiments, cellulosic materials would be the
organic filler of choice.
[0043] A variety of inorganic fillers are also very suitable for
use in some embodiments of this invention. Non-limiting examples
include graphite, silica, alumina, magnesium oxide, talc, alkaline
earth carbonates, zirconium basic carbonate, sulfates, sulfides,
halides, phosphates, borates, borosilicates, slate flour, clay, and
any combination of the foregoing. Graphite is often the most
preferred inorganic filler. Fumed silica may also be preferred for
some embodiments, if its residue can be adequately removed when the
maskant is removed from the substrate holes.
[0044] Mixtures of organic and inorganic fillers may also be
suitable for the present invention. As a non-limiting example, a
mixture of rubber with silica, or a mixture of a cellulosic
material with silica, in a weight ratio (in each instance) ranging
from about 90:10 to about 10:90, may be appropriate in some
circumstances. Separate mixtures of either organics or inorganics
are also possible, e.g., a mixture of graphite and silica in the
case of inorganics.
[0045] As alluded to earlier, the total amount of filler used will
be determined in large part by the desired Theological nature for
the polymer system being used as a maskant. The level of filler
should thus be that which provides the polymer system with a
substantially non-Newtonian flow characteristic. In preferred
embodiments, the level is such that the polymer system has a
shear-thinning flow characteristic. In especially preferred
embodiments, the polymer system has a Bingham solid characteristic,
as described above. These flow characteristics are stated here in
terms of the use-temperature of the polymer system, i.e., the
temperature at which it is delivered to the substrate holes, via
extrusion or some other process. For thermoset-based polymer
systems, this may typically be room temperature, whereas for
thermoplastic-based systems, this may be the softening point or
melting point of the polymer.
[0046] However, the level of filler should not be so high as to
cause adherence of subsequently-applied coatings to the protrusions
formed when the polymer system is cured. (Excessive amounts of some
fillers are thought to cause this undesirable adhesion). In
general, the level of total filler will be in the range of about 1%
by weight to about 50% by weight, based on the weight of the
polymer system. In preferred embodiments, the level of total filler
will be in the range of about 5% by weight to about 20% by weight.
Those of ordinary skill in the art will be able to select the most
appropriate type and level of filler, based on this teaching and
related experimentation.
[0047] A variety of other additives may be used in the polymer
system. Most of them are well-known in areas of chemical
processing. As but one illustration, many are described in the
Kirk-Othmer Encyclopedia of Chemical Technology, 4th Edition, Vol.
5, pp. 610-613. Non-limiting examples include binders,
plasticizers, emollients, lubricants, and surface
tension-modifiers.
[0048] Still another additive which may be suitable is a liquid
crystal polymer or oligomer. Liquid crystal materials are usually
organic and fall into one of three classes: smectic, nematic, or
cholesteric. Special mention is made of these materials because
many of them exist in a transition state between solid and liquid
forms. They may therefore prove to be advantageous in adjusting the
Theological nature of the maskant.
[0049] The choice of a particular additive will obviously depend on
the needs of a given polymeric system, as well as other factors,
e.g., the type and level of residue the additive might leave when
the maskant is removed from the substrate holes. Usually, these
additives are used at a level of less than about 5% by weight of
the total polymer system.
[0050] As described previously, substrates related to the present
invention often contain a multitude of passage holes. An exemplary
substrate 10 is depicted in FIG. 1. It has a first surface 12, over
which at least one coating is subsequently applied, and a second
surface 14. Passage holes 16 extend throughout hole length section
18 to exit through hole bottom 20. The substrate thickness
(designated by dimension "x") is usually in the range of about 10
mils to about 250 mils, and most often, in the range of about 20
mils to about 125 mils. As further depicted in FIG. 2, the holes
are often inclined relative to the first surface, e.g., in the case
of a turbine engine component which contains cooling holes. Other
details regarding the passage holes have already been
discussed.
[0051] The holes can be filled and covered with the maskant by a
variety of techniques. However, in preferred embodiments, the holes
are covered by way of an extrusion technique. In one specific
embodiment, the maskant is extruded into each hole 16 from first
surface 14, through hole length 18, exiting each hole at second
surface 12.
[0052] Extrusion is a well-known, fundamental technique for the
processing of polymers. It is described, for example, in the
Kirk-Othmer treatise mentioned above. In general, it involves the
forging of the material of choice through a metal-forming die,
followed by cooling or chemical hardening (i.e., curing). Higher
viscosity materials are fed into a rotating screw of variable
pitch, which forces the material through the die with considerable
pressure. Many different types of extrusion equipment are
commercially available. An exemplary extrusion apparatus and method
for Bingham solid-type materials is provided in the Leonard et al
patent referenced above.
[0053] In the present instance, the "die" can be the substrate
itself, which contains the holes through which the maskant is
extruded. Existing extrusion equipment can be readily modified
without undue effort, so that the exit-end of the screw terminates
at or near the inner surface of the substrate. Conventional
extruders have one or more entry points, e.g., "hoppers", which can
accommodate maskant materials or maskant precursor materials in
liquid or solid form. The extruders also have standard heating
mechanisms (e.g., heating bands), so that the maskant material can
be brought to the appropriate extrusion temperature.
[0054] As those of ordinary skill in the molding arts understand,
the Theological characteristics required for this invention and for
a processing operation like extrusion, in general, depend on many
factors, such as viscosity, temperature, flow rate, and die design.
All of these parameters can be adjusted for the present invention,
by way of equipment control, atmospheric control, and masking
material-composition. As an illustration, the pressure used for
extrusion of the maskant through a hole having an average length of
about 50 mils to about 700 mils and an average hole diameter of
about 5 mils to about 500 mils is usually in the range of about 1
psi to about 30 psi.
[0055] The pressure can be applied from a source other than the
extruder itself. For example, after a layer of maskant is spread on
the first surface of the substrate, covering the hole entrances, a
pressure differential could be formed between the first surface and
the second surface by conventional techniques, extruding the
maskant through each hole.
[0056] In some embodiments, it may be desirable to spread a thin
(e.g., about 0.5 mil to about 100 mils) sheet of material over the
curable masking material after the maskant has been applied over
the first surface of the substrate. The sheet can be made of a
material which would not adhere to the maskant, e.g., a saran- or
rubber-type sheet in the case of many of the maskant materials
listed above. This sheet would help to prevent the "blow-through"
of the maskant completely through one or more of the holes.
[0057] The maskant is then cured by conventional means. Those of
skill in the polymer arts understand that thermosetting materials
are usually cured by the action of a catalyst and/or the use of
high temperatures. Thermoplastic materials such as polyamide or
PET, which are initially heated for extrusion or other processing
steps, will cool down quickly after processing to harden into final
form. Thus, "curing" as used herein refers to any type of steps
which put the maskant material into final form. When needed, heat
can be applied to a polymer or polymer precursor by conventional
techniques, e.g., a convection oven, heat lamp, etc.
[0058] FIG. 3 is a cross-sectional view illustrating a cured
maskant 40 filling up and covering a hole 30 which extends through
substrate 20, which includes coating layers 22 and 24 (discussed
below). The maskant material in this particular embodiment was
extruded through the hole from the hole entrance 38 on first
surface 34, exiting the hole at second surface 32, and forming
protrusion 36, which is hemispherical in shape. As described above,
the special Theological characteristics of the maskant allowed the
formation of the protrusion as soon as the extrusion pressure was
terminated.
[0059] In preferred embodiments, the height of the protrusion,
illustrated in FIG. 3 by dimension "y", is at least about 50%
greater than the expected, total thickness of coatings to be
applied (and cured or fired) on the substrate. In especially
preferred embodiments, the height of the protrusion is at least
about 100% greater than the expected coating thickness. For the
situation in which the substrate is a superalloy covered by a bond
coat and a TBC of typical thickness, the height of the protrusion
is usually in the range of about 20 mils to about 100 mils. The
height of the protrusion is pre-selected in part by the amount of
maskant which would be most appropriate for retaining the hole
geometry and keeping the holes covered during any of the subsequent
processing steps. As an example, if subsequent coating steps
involved a considerable amount of grit blasting in preparation,
larger protrusions (within the general ranges stated above) might
be appropriate, to ensure that hole protection and hole geometry is
maintained, despite any erosion.
[0060] As mentioned previously, the coatings applied to turbine
engine substrates are usually TBC's and intervening bond layers.
The compositions and techniques for forming these layers are
well-known in the art. The bond layer, which is often very
important for improving the adhesion between the metal substrate
and the TBC, is usually formed from a material like "MCrAlY", where
"M" represents a metal like iron, nickel, or cobalt. Very often,
the bond coating may be applied by a variety of conventional
techniques, such as PVD, plasma spray (e.g., air plasma), CVD, or
combinations of plasma spray and CVD techniques. The TBC itself can
be applied by a variety of techniques, such as plasma spray or
electron beam physical vapor deposition (EB-PVD). Usually, the bond
coat has a thickness in the range of about 1 mil to about 10 mils,
and preferably, in the range of about 3 mils to about 7 mils.
Typically, the TBC has a thickness in the range of about 5 mils to
about 100 mils, and preferably, in the range of about 10 mils to
about 40 mils.
[0061] Details regarding various techniques for applying the bond
coat and the TBC can be found, for example, in Kirk-Othmer's
Encyclopedia of Chemical Technology, 3rd Edition, Vol. 15, (1981)
and Vol. 20 (1982); in Ullmann's Encyclopedia of Industrial
Chemistry, Fifth Edition; Volume A6, VCH Publisher (1986); in
Scientific American, H. Herman, September 1988; and in U.S. Pat.
No. 5,384,200, incorporated herein by reference. Thus, one of
ordinary skill in the art can easily become familiar with various
process details which may be relevant, e.g., cleaning of the
surface prior to deposition; grit blasting (or some other form of
abrasion) to remove oxides and roughen the surface; substrate
temperature; and plasma spray parameters (when employed), such as
spray distances (gun-to-substrate); selection of the number of
spray-passes; powder feed rate, torch power, plasma gas selection;
angle of deposition; post-treatment of the applied coating (e.g.,
deburring); and the like.
[0062] As mentioned previously, the subsequently-applied coatings,
e.g., both the bond coating and the TBC, do not significantly
adhere to the protrusions formed of cured maskant. This
characteristic is particularly advantageous, since the exposed
protrusions and the underlying remainder of maskant are relatively
easy to remove. Many of the maskant materials can be efficiently
removed by a pyrolytic technique. As an example, thermosetting
materials like epoxies and phenolics can be "burned out" of the
hole and surrounding substrate surface area at temperatures in the
range of about 300.degree. C. to about 900.degree. C. The most
appropriate temperature can be predetermined by reference to the
thermal characteristics of the particular resin or resins which
constitute the polymer system for the maskant. The heating
temperature for the burn-out can be supplied via any conventional
technique, such as an oven or any type of torch. The time required
may vary, but is usually in the range of about 20 minutes to about
300 minutes, and more often, in the range of about 30 minutes to
about 180 minutes. The heating technique should be one which will
not damage the substrate or any of the coatings applied thereon.
After pyrolysis is complete, any residue remaining in or around the
holes can be removed by various techniques, such as agitation,
alone or in combination with brushing techniques or gas-blasts
(e.g., air).
[0063] It may not be necessary to pyrolyze a thermoplastic material
used as the maskant, since such a material may simply flow out of
the hole when the material is heated to a temperature at or around
its melting point. Burning or one of the techniques mentioned below
could then be used to more thoroughly clean the hole of the
maskant.
[0064] As mentioned above, other techniques, in combination with
each other or with pyrolysis, can sometimes be used to remove the
protrusions and the underlying maskant. As an example, a solvent or
solvent mixture (sometimes heated) which dissolves or solubilizes
the maskant material could be applied to the substrate, or the
substrate could be dipped in the solvent. To illustrate,
polyamide-based maskants can usually be dissolved in hot phenols,
or in cresols or formic acids, while phenolics could be dissolved
in a hot caustic solution. The solvent or solvent mixtures should
be those which do not adversely affect the substrate or the
overlying coatings.
[0065] The holes in the substrate at this stage are free of any
obstruction, and are therefore capable of performing their
function, e.g., serving as the passageway for cooling air.
Meanwhile, the substrate has also been provided with one or more
coatings which also perform a specific function, e.g., acting as a
thermal barrier in a high-heat environment. Thus, another aspect of
this invention is directed to the curable masking material itself,
which comprises an extrudable resin composition which is thermally
stable up to a temperature of at least about 350.degree. C. The
material exhibits non-Newtonian characteristics, as discussed
above. When cured, the masking material ideally does not serve as
an adhesion site for protective coatings which are subsequently
applied. The masking material is also easily removed from the
substrate after any related processing operations have been
completed, e.g., after the deposition of a bond coat and a TBC.
[0066] Yet another aspect of the present invention is directed to
an article which comprises
[0067] (I) a substrate which includes at least one passage hole
extending from a first surface to a second surface;
[0068] (II) a curable masking material which fills the passage hole
and covers the hole to form a protrusion over the hole; and
[0069] (III) at least one coating applied over the second surface
of the substrate, wherein the coating does not substantially adhere
to the protrusion.
[0070] Details regarding the article can be found in the remainder
of the teachings herein. In use, the article is further treated to
remove the curable masking material and re-open the passage holes,
as described previously.
[0071] Still another embodiment of the present invention relates to
a method for repairing a damaged thermal barrier coating applied
over a substrate which includes at least one passage hole extending
from a first surface to a second surface on which the thermal
barrier coating had been applied, comprising the following
steps:
[0072] (A) removing the damaged thermal barrier coating from the
second surface, uncovering any passage hole in the substrate;
[0073] (B) filling and covering the hole with a curable masking
material which forms a protrusion over the hole;
[0074] (C) curing the masking material;
[0075] (D) applying additional thermal barrier coating material
over the substrate and the masking material, wherein the coating
does not substantially adhere to the protrusion; and then
[0076] (E) removing the masking material to uncover the passage
hole.
[0077] The repaired TBC has the advantageous properties described
above.
[0078] The following example is not meant to limit the scope of the
claimed invention. It merely illustrates an embodiment of the
invention.
EXAMPLE
[0079] A two-component, commercial epoxy resin system (Epotek 730)
was used in this example. The material was combined with 2% (by
weight) of a silica material from Cabot Corporation; Cab-O-Sil M-5.
The resulting mixture was then extruded through 0.020 inch-diameter
cooling air holes from the underside of several substrate-sections
(each approximately 4 inch.times.4 inch, with a thickness of 62.5
mils) of a turbine engine combustor liner. The cooling air holes
were inclined at an angle of about 20 degrees relative to the upper
surface of the substrate sections. Uniaxial pressure was applied at
a pressure level of about 2 to 5 psi, to cause the resin to fill
the holes and protrude on the upper-side, i.e., the surface to be
coated. The protrusions had an average height of approximately 40
mils. They were in the general shape of elliptical domes, and
covered the hole openings.
[0080] The epoxy material was then cured at about 100.degree. C.
for 3 hours in a convection oven. The substrate-sections were then
grit-blasted to prepare the surface for coating. A standard
"NiCrAlY" bond coat was then applied to the substrate by air plasma
spray, to a thickness of 0.005 inch, at a deposition surface
temperature of about 200.degree. C. A standard, yttria-stabilized
zirconia thermal barrier coating was then applied by air plasma
spray, at a deposition surface temperature of about 300.degree. C.
The TBC had a thickness of about 0.010 inch. Neither the bond
coating nor the TBC adhered to the protrusions.
[0081] The epoxy material was then pyrolyzed at 600.degree. C. for
about 60 minutes in a forced-air convection oven, and the residue
from the pyrolysis was cleaned from the holes by brushing and
shaking.
[0082] Measurement of air flow through the substrate-section holes
had been taken prior to any treatment. Those measurements were
compared with those taken after the masking, coating, and
pyrolysis/cleaning steps were carried out. The results demonstrate
that (1) there was no reduction in the quantity of air; and (2)
there was no adverse change in the flow pattern of the cooling air
through the holes.
[0083] While preferred embodiments have been set forth for the
purpose of illustration, the foregoing description should not be
deemed to be a limitation on the scope of the invention.
Accordingly, various modifications, adaptations, and alternatives
may occur to one skilled in the art without departing from the
spirit and scope of the present invention.
[0084] All of the patents, articles, and texts mentioned above are
incorporated herein by reference.
* * * * *