U.S. patent application number 10/649536 was filed with the patent office on 2004-04-29 for segmented thermal barrier coating and method of manufacturing the same.
This patent application is currently assigned to Siemens Westinghouse Power Corporation. Invention is credited to Burns, Andrew Jeremiah, Subramanian, Ramesh.
Application Number | 20040081760 10/649536 |
Document ID | / |
Family ID | 25445089 |
Filed Date | 2004-04-29 |
United States Patent
Application |
20040081760 |
Kind Code |
A1 |
Burns, Andrew Jeremiah ; et
al. |
April 29, 2004 |
Segmented thermal barrier coating and method of manufacturing the
same
Abstract
A ceramic thermal barrier coating (46) having a plurality of
segmentation gaps (44) formed in its top surface (56) to provide
thermal strain relief. The surface width of the gaps may be limited
to minimize the aerodynamic impact of the gaps. The gaps may be
formed as continuous grooves (68) extending along a flow path of a
fluid stream traveling over the thermal barrier coating. Such
grooves may be used in the fluid stream without removing the ridge
(60) created by splashing of molten material onto the surface of
the coating during a laser engraving process used to form the
grooves, since the fluid stream is flowing parallel to the ridge.
Preferred failure planes (A1, A2, A3) may be formed through the
thickness of the coating in order to stimulate the generation of a
fresh surface when a portion of the coating fails by spalling. The
bottom geometry of the gaps may be formed to have a generally
U-shape in order to minimize stress concentration. The gaps serve
to reduce the crack driving force along the interface between the
thermal barrier coating and an underlying bond coating.
Inventors: |
Burns, Andrew Jeremiah;
(Orlando, FL) ; Subramanian, Ramesh; (Oviedo,
FL) |
Correspondence
Address: |
Siemens Corporation
Intellectual Property Department
170 Wood Avenue South
Iselin
NJ
08830
US
|
Assignee: |
Siemens Westinghouse Power
Corporation
|
Family ID: |
25445089 |
Appl. No.: |
10/649536 |
Filed: |
August 26, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10649536 |
Aug 26, 2003 |
|
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09921206 |
Aug 2, 2001 |
|
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6703137 |
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Current U.S.
Class: |
427/269 ;
427/58 |
Current CPC
Class: |
Y10T 428/249953
20150401; F01D 5/288 20130101; F05D 2230/13 20130101; C23C 4/18
20130101; Y10T 428/249967 20150401; C23C 28/345 20130101; C23C
28/3215 20130101; Y10T 428/24314 20150115; Y10T 428/249981
20150401; C23C 28/3455 20130101 |
Class at
Publication: |
427/269 ;
427/058 |
International
Class: |
B05D 005/00; B05D
005/12 |
Claims
We claim as our invention:
1. A method of manufacturing an insulated component, the method
comprising: providing a substrate having a surface; depositing a
layer of ceramic insulating material on the substrate surface; and
forming a continuous gap in a top surface of the layer of ceramic
insulating material to define segments therein, the continuous gap
having a width at the top surface of less than 100 microns.
2. The method of claim 1, further comprising forming the continuous
gap to have a width of less than 75 microns.
3. The method of claim 1, further comprising forming the continuous
gap to have a width of less than 50 microns.
4. The method of claim 1, further comprising forming the continuous
gap to have a depth that does not extend through an entire
thickness of the layer of ceramic insulating material.
5. The method of claim 1, further comprising forming the continuous
gap using a laser engraving process.
6. The method of claim, 1, further comprising: forming a first
plurality of continuous gaps to a first depth into the layer of
ceramic insulating material; and forming a second plurality of
continuous gaps to a second depth into the layer of ceramic
insulating material.
7. The method of claim 1, further comprising forming the continuous
gap by: exposing the top surface to a first pass of laser energy
having a first parameter to form the continuous gap; and exposing
the continuous gap to a second pass of laser energy having a second
parameter different than the first parameter to change a geometry
of the continuous gap.
8. The method of claim 7, wherein the second pass of laser energy
has a wider beam footprint than that of the first pass of laser
energy.
9. The method of claim 7, wherein the second pass of laser energy
has a pulsation frequency that is greater than that of the first
pass of laser energy.
10. The method of claim 1, further comprising forming the
continuous gap using laser energy delivered through a fiber optic
cable.
11. The method of claim 1, further comprising forming the
continuous gap with a laser engraving process using a lens having a
focal length of at least 160 mm in order to reduce accumulation of
molten material splashed onto the lens during the laser engraving
process.
12. The method of claim 1, further comprising forming the
continuous gap to follow a direction of a fluid stream over the top
surface when the component is in use.
13. The method of claim 1, further comprising forming a plurality
of continuous gaps in the top surface at a spacing between adjacent
gaps of less than 750 microns.
14. The method of claim 13, further comprising forming the
plurality of continuous gaps in the top surface at a spacing
between adjacent gaps of less than 500 microns.
15. The method of claim 13, further comprising forming the
plurality of continuous gaps in the top surface at a spacing
between adjacent gaps in a range of 500-750 microns.
16. The method of claim 1, further comprising: depositing a first
layer of ceramic insulating material on the substrate surface;
forming a first plurality of continuous gaps in a top surface of
the first layer; depositing a second layer of ceramic insulating
material on the top surface of the first layer; and forming a
second plurality of continuous gaps in a top surface of the second
layer.
17 The method of claim 16, further comprising forming each of the
gaps in the top surface of the second layer to have a width at the
top surface of less than 100 microns.
18. A method of manufacturing an insulated component, the method
comprising: providing a substrate having a surface; depositing a
layer of insulating material on the substrate surface; forming a
gap in a top surface of the layer of ceramic insulating material by
applying a first material removal process to the top surface; and
reshaping the gap by applying a second material removal process to
the gap.
19. The method of claim 18, further comprising: forming the gap in
a top surface of the layer of ceramic insulating material by
exposing the top surface to a first exposure of energy having a
first parameter; and reshaping the gap by exposing walls defining
the gap to a second exposure of energy having a second parameter
different than the first parameter.
20. The method of claim 19, wherein the energy used for both the
first exposure and the second exposure is laser energy and the
second exposure of laser energy has a wider beam footprint than
that of the first exposure of laser energy.
21. The method of claim 19, wherein the energy used for both the
first exposure and the second exposure is laser energy and the
second exposure of laser energy has a pulsation frequency that is
greater than that of the first exposure of laser energy.
22. The method of claim 19, wherein the first exposure of energy
and the second exposure of energy utilize different forms of
energy.
23. The method of claim 18, wherein the gap is reshaped to have a
generally U-shaped bottom geometry.
24. A method of manufacturing an insulated component for use in an
air stream environment, the method comprising: applying a
heat-inducing process to a top surface of a layer of ceramic
insulation of a component to form a continuous groove bordered by a
ridge along the top surface; applying the heat-inducing process to
form the continuous groove and ridge to follow a direction of a
fluid stream over the top surface when the component is in use; and
using the component in the fluid stream without removing the
ridge.
25. The method of claim 24, further comprising using a
laser-engraving process to form the continuous groove to have a
width at the top surface of between 25-125 microns.
26. The method of claim 24, further comprising: forming a first
plurality of continuous grooves to a first depth into the layer of
ceramic insulation; and forming a second plurality of continuous
grooves to a second depth into the layer of ceramic insulation to
define a plurality of failure planes in the layer of ceramic
insulation.
27. A method of manufacturing an insulated component, the method
comprising: providing a substrate having a surface; depositing a
layer of ceramic insulating material on the substrate surface;
forming a first plurality of grooves to a first depth into the
layer of ceramic insulating material; and forming a second
plurality of grooves to a second depth into the layer of ceramic
insulating material.
28. The method of claim 27, further comprising forming the grooves
each to have a width at a top surface of the layer of ceramic
insulating material of less than 100 microns.
29. The method of claim 27, further comprising forming the grooves
each to have a width at a top surface of the layer of ceramic
insulating material of less than 75 microns.
30. The method of claim 27, further comprising forming the grooves
each to have a width at a top surface of the layer of ceramic
insulating material of less than 50 microns.
31. The method of claim 27, further comprising forming the grooves
each to follow a path of an air stream flowing over a top surface
of the layer of ceramic insulating material during use of the
component.
32. A method of manufacturing an insulated component, the method
comprising: providing a substrate having a surface; depositing a
first layer of ceramic insulating material on the substrate
surface; forming a first plurality of grooves into the first layer
of ceramic insulating material; depositing a second layer of
ceramic insulating material onto the first layer of ceramic
insulating material; and forming a second plurality of grooves into
the second layer of ceramic insulating material.
33. The method of claim 32, further comprising forming each of the
second plurality of grooves to have a width at a top surface of the
second layer of ceramic insulating material in the range of 25-125
microns.
34. The method of claim 32, further comprising forming each of the
second plurality of grooves to have a width at a top surface of the
second layer of ceramic insulating material of less than 100
microns.
35. A method of manufacturing an insulated component, the method
comprising: applying a bond coating to a surface of a component;
applying a thermal barrier coating to the bond coating to create a
bond coating/thermal barrier coating interface; and decreasing a
crack driving force at a location along the bond coating/thermal
barrier coating interface by engraving respective grooves to
respective partial depths into the thermal barrier coating on
opposed sides of the location.
Description
[0001] This application is a continuation-in-part and claims
benefit of the Aug. 2, 2001, filing date of co-pending U.S. patent
application Ser. No. 09/921,206.
FIELD OF THE INVENTION
[0002] This invention relates generally to thermal barrier coatings
and in particular to a strain tolerant thermal barrier coating for
a gas turbine component and a method of manufacturing the same.
BACKGROUND OF THE INVENTION
[0003] It is known that the efficiency of a combustion turbine
engine will improve as the firing temperature of the combustion gas
is increased. As the firing temperatures increase, the high
temperature durability of the components of the turbine must
increase correspondingly. Although nickel and cobalt based
superalloy materials are now used for components in the hot gas
flow path, such as combustor transition pieces and turbine rotating
and stationary blades, even these superalloy materials are not
capable of surviving long term operation at temperatures sometimes
exceeding 1,400 degrees C. In many applications a metal substrate
is coated with a ceramic insulating material in order to reduce the
service temperature of the underlying metal and to reduce the
magnitude of the temperature transients to which the metal is
exposed.
[0004] Thermal barrier coating (TBC) systems are designed to
maximize their adherence to the underlying substrate material and
to resist failure when subjected to thermal cycling. The
temperature transient that exists across the thickness of a ceramic
coating results in differential thermal expansion between the top
and bottom portions of the coating. Such differential thermal
expansion creates stresses within the coating that can result in
the spalling of the coating along one or more planes parallel to
the substrate surface. It is known that a more porous coating will
generally result in lower stresses than dense coatings. Porous
coatings also tend to have improved insulating properties when
compared to dense coatings. However, porous coatings will densify
during long term operation at high temperature due to diffusion
within the ceramic matrix, with such densification being more
pronounced in the top (hotter) layer of the coating than in the
bottom (cooler) layer proximate the substrate. This difference in
densification also creates stresses within the coating that may
result in spalling of the coating.
[0005] A current state-of-the-art thermal barrier coating is
yttria-stabilized zirconia (YSZ) deposited by electron beam
physical vapor deposition (EB-PVD). The EB-PVD process provides the
YSZ coating with a columnar microstructure having sub-micron sized
gaps between adjacent columns of YSZ material, as shown for example
in U.S. Pat. No. 5,562,998. The gaps between columns of such
coatings provide an improved strain tolerance and resistance to
thermal shock damage. Alternatively, the YSZ may be applied by an
air plasma spray (APS) process. The cost of applying a coating with
an APS process is generally less than one half the cost of using an
EB-PVD process. However, it is extremely difficult to form a
desirable columnar grain structure with the APS process.
[0006] It is known to produce a thermal barrier coating having a
surface segmentation to improve the thermal shock properties of the
coating. U.S. Pat. No. 4,377,371 discloses a ceramic seal device
having benign cracks deliberately introduced into a plasma-sprayed
ceramic layer. A continuous wave CO.sub.2 laser is used to melt a
top layer of the ceramic coating. When the melted layer cools and
re-solidifies, a plurality of benign micro-cracks are formed in the
surface of the coating as a result of shrinkage during the
solidification of the molten regions. The thickness of the
melted/re-solidified layer is only about 0.005 inch and the benign
cracks have a depth of only a few mils. Accordingly, for
applications where the operating temperature will extend damaging
temperature transients into the coating to a depth greater than a
few mils, this technique offers little benefit.
[0007] Special control of the deposition process can provide
vertical micro-cracks in a layer of TBC material, as taught by U.S.
Pat. Nos. 5,743,013 and 5,780,171. Such special deposition
parameters may place undesirable limitations upon the fabrication
process for a particular application.
[0008] U.S. Pat. No. 4,457,948 teaches that a TBC may be made more
strain tolerant by a post-deposition heat treatment/quenching
process which will form a fine network of cracks in the coating.
This type of process is generally used to treat a complete
component and would not be useful in applications where such cracks
are desired on only a portion of a component or where the extent of
the cracking needs to be varied in different portions of the
component.
[0009] U.S. Pat. No. 5,558,922 describes a thick thermal barrier
coating having grooves formed therein for enhance strain tolerance.
The grooves are formed by a liquid jet technique. Such grooves have
a width of about 100-500 microns. While such grooves provide
improved stress/strain relief under high temperature conditions,
they are not suitable for use on airfoil portions of a turbine
engine due to the aerodynamic disturbance caused by the flow of the
hot combustion gas over such wide grooves. In addition, the grooves
go all the way to the bond coat and this can result in its
oxidation and consequently lead to premature failure.
[0010] U.S. Pat. No. 5,352,540 describes the use of a laser to
machine an array of discontinuous grooves into the outer surface of
a solid lubricant surface layer, such as zinc oxide, to make the
lubricant coating strain tolerant. The grooves are formed by using
a carbon dioxide laser and have a surface opening size of 0.005
inch, tapering smaller as they extend inward to a depth of about
0.030 inches. Such grooves would not be useful in an airfoil
environment, and moreover, the high aspect ratio of
depth-to-surface width could result in an undesirable stress
concentration at the tip of the groove in high stress
applications.
[0011] It is known to use laser energy to cut depressions in a
ceramic or metallic coating to form a wear resistant abrasive
surface. Such a process is described in U.S. Pat. No. 4,884,820 for
forming an improved rotary gas seal surface. A laser is used to
melt pits in the surface of the coating, with the edges of the pits
forming a hard, sharp surface that is able to abrade an opposed
wear surface. Such a surface would be very undesirable for an
airfoil surface. Similarly, a seal surface is textured by laser
cutting in U.S. Pat. No. 5,9951,892. The surface produced with this
process is also unsuitable for an airfoil application. These
patents are concerned with material wear properties of an wear
surface, and as such, do not describe processes that would be
useful for producing a TBC having improved thermal endurance
properties.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The features and advantages of the present invention will
become apparent from the following detailed description of the
invention when read with the accompanying drawings in which:
[0013] FIG. 1 is a partial cross-sectional view of a combustion
turbine blade having a substrate material coated with a thermal
barrier coating having two distinct layers of porosity, with the
top layer being segmented by a plurality of laser-engraved
gaps.
[0014] FIG. 2 is a graphical illustration of the reduction in
stress on the surface of a thermal barrier coating as a function of
the width, depth and spacing of segmentation gaps formed in the
surface of the coating.
[0015] FIG. 3A is a partial cross-section view of a component
having a laser-segmented ceramic thermal barrier coating.
[0016] FIG. 3B is the component of FIG. 3A and having a layer of
bond inhibiting material deposited thereon.
[0017] FIG. 3C is the component of FIG. 3B after the bond
inhibiting material has been subjected to a thermal heat treatment
process.
[0018] FIG. 4A is a cross-section view of a gap being cut into a
ceramic material by a first pass of a laser having a first focal
distance, the gap having a generally V-shaped bottom geometry.
[0019] FIG. 4B is the gap of FIG. 4A being subjected to a second
pass of laser energy having a focal distance greater than that used
in the first pass of FIG. 4A to change the gap bottom geometry to a
generally U-shape.
[0020] FIG. 5 is a plane view of a gas turbine vane illustrating
segments formed in a thermal barrier coating by laser engraved
grooves extending along the path of a fluid stream traveling around
the vane.
[0021] FIG. 6 is a graph illustrating the impact of surface gaps on
the force needed to extend a crack between a thermal barrier
coating and an underlying bond coat.
[0022] FIG. 7 is a partial cross-sectional view of an insulated
component having a ceramic thermal barrier coating that is
segmented by a plurality of laser-engraved grooves formed to a
plurality of predetermined depths to define preferred failure
planes throughout the depth of the coating.
[0023] FIG. 8 is a partial cross-sectional view of an insulated
component having a ceramic thermal barrier coating that is formed
by a plurality of layers, with each layer segmented by a plurality
of laser-engraved grooves, thereby defining preferred failure
planes throughout the depth of the coating.
DETAILED DESCRIPTION OF THE INVENTION
[0024] FIG. 1 illustrates a partial cross-sectional view of a
component 10 formed to be used in a very high temperature
environment. Component 10 may be, for example, the airfoil section
of a combustion turbine blade or vane. Component 10 includes a
substrate 12 having a top surface 14 that will be exposed to the
high temperature environment. For the embodiment of a combustion
turbine blade, the substrate 12 may be a superalloy material such
as a nickel or cobalt base superalloy and is typically fabricated
by casting and machining. In other embodiments the substrate may be
a ceramic matrix composite material or any known structural
material. The substrate surface 14 is typically cleaned to remove
contamination, such as by aluminum oxide grit blasting, prior to
the application of any additional layers of material. A bond coat
16 may be applied to the substrate surface 14 in order to improve
the adhesion of a subsequently applied thermal barrier coating and
to reduce the oxidation of the underlying substrate 12.
Alternatively, the bond coat may be omitted and a thermal barrier
coating applied directly onto the substrate surface 14. One common
bond coat 16 is an MCrAlY material, where M denotes nickel, cobalt,
iron or mixtures thereof, Cr denotes chromium, Al denotes aluminum,
and Y denotes yttrium. Another common bond coat 16 is alumina. The
bond coat 16 may be applied by any known process, such as
sputtering, plasma spray processes, high velocity plasma spray
techniques, low or high velocity flame spray techniques, or
electron beam physical vapor deposition.
[0025] Next, a layer of insulating material such as a ceramic
thermal barrier coating 18 is applied over the bond coat 16 or
directly onto the substrate surface 14. The thermal barrier coating
(TBC) may be a yttria-stabilized zirconia, which includes zirconium
oxide ZrO.sub.2 with a predetermined concentration of yttrium oxide
Y.sub.2O.sub.3, pyrochlores, perovskites, mixed oxides of
pyrochlores, perovskites or other TBC material known in the art.
The TBC may be applied using the less expensive air plasma spray
technique, although other known deposition processes may be used.
The thermal barrier coating 18 may be formed of the same material
throughout its depth in one embodiment. In another embodiment, as
illustrated in FIG. 1, the thermal barrier coating includes a
first-applied bottom layer 20 and an overlying top layer 22, with
at least the density being different between the two layers. Bottom
layer 20 has a first density that is less than the density of top
layer 22. In one embodiment, bottom layer 20 may have a density
that is between 80-95% of the theoretical density, and top layer 22
may have a density that is at least 95% of the theoretical density.
The theoretical density is a value that is known in the art or that
may be determined by known techniques, such as mercury porosimetry
or by visual comparison of photomicrographs of materials of known
densities. The porosity and density of a layer of TBC material may
be controlled with known manufacturing techniques, such as by
including small amounts of void-forming materials such as polyester
during the deposition process. The bottom layer 20 provides better
thermal insulating properties per unit of thickness than does the
top layer 22 as a result of the insulating effect of the pores 24.
The bottom layer 20 is also relatively less susceptible to
interlaminar failure (spalling) resulting from the temperature
difference across the depth of the layer because of the strain
tolerance provided by the pores 24 and because of the insulating
effect of the top layer 22. The top layer 22 is less susceptible to
densification and possible interlaminar failure resulting there
from since it contains a relatively low quantity of pores 24, thus
limiting the magnitude of the densification effect. The combination
of a less dense bottom layer 20 and a more dense top layer 22
provides desirable properties for a high temperature environment.
In other embodiments, the density of the thermal barrier coating
may be graduated from a higher density proximate the top of the
coating to a lower density proximate the bottom of the coating
rather than changed at discrete layers.
[0026] The dense top layer 22 will have a relatively lower thermal
strain tolerance due to its lower pore content. For the very high
temperatures of some modern combustion turbine engines, there may
be an unacceptable level of interlaminar stress generated in the
top layer 22 in its as-deposited condition due to the temperature
gradient across the thickness (depth) of that layer. Accordingly,
the top layer 22 is segmented to provide additional strain relief
in that layer, as illustrated in FIG. 1. A plurality of segments 26
bounded by a plurality of gaps 28 are formed in the top layer 22 by
a laser engraving process. The gaps 28 allow the top layer 22 to
withstand a large temperature gradient across its thickness without
failure, since the expansion/contraction of the material can be at
least partially relieved by changes in the gap sizes, which reduces
the total stored energy per segment. The gaps 28 may be formed to
extend to the full depth of the top layer 22, or to a greater or
lesser depth as may be appropriate for a particular application. It
may be desired that the gaps do not extend all the way to the bond
coat 16 in order to avoid the exposure of the bond coat to the
environment of the component 10. The selection of a particular
segmentation strategy, including the size and shape of the segments
and the depth of the gaps 28, will vary from application to
application, but should be selected to result in a level of stress
within the thermal barrier coating 18 which is within desired
levels at all depths of the TBC for the predetermined temperature
environment. Importantly, the use of laser engraved segmentation
permits the TBC to be applied to a thickness greater than would
otherwise be possible without such segmentation. Current
technologies make use of ceramic TBC's with thicknesses of about 12
mils, whereas thicknesses of as much as 50 mils are anticipated
with the processes described herein.
[0027] Known finite element analysis modeling techniques may be
used to select an appropriate segmentation strategy. FIG. 2
illustrates the percentage of stress relief versus the ratio of the
gap spacing to the gap depth for a typical TBC system using the
following values for the properties of the coating and substrate:
E.sub.substrate=200 GPa, E.sub.TBC=40 GPa, gap depth (d)=200
microns, gap centerline spacing (S)=1,000 microns, and coating
thickness (D)=300 microns. FIG. 2 illustrates the percentage of
stress relief (as a percentage of the stress for a similar
component having no segmentation) at a point A on the surface of
the TBC coating midway between two gaps as a function of the ratio
of gap depth to TBC thickness (d/D) for each of several gap
centerline spacing values (S). For example, as can be appreciated
by examining the data plotted on FIG. 2, a gap spacing of S=1,000
microns is predicted to produce approximately a 50% reduction in
the stress at point A for a gap extending approximately two thirds
the depth of the coating. It may be appreciated from FIG. 2 that a
smaller spacing S between adjacent gaps will result in a greater
reduction in stress. A spacing S between adjacent gaps of less than
500 microns will provide a high degree of stress reduction in the
coating. However, there may be practical manufacturing issues that
make it difficult to create gaps with very small spacing S. In one
embodiment, the a spacing S between adjacent gaps in the range of
500-750 microns may be used, or in the range of 500-1000 microns,
or any spacing less than 750 microns or less than 1000 microns.
[0028] Laser energy is preferred for engraving the gaps 28 after
the thermal barrier coating 18 is deposited. The laser energy is
directed toward the TBC top surface 30 in order to heat the
material in a localized area to a temperature sufficient to cause
vaporization and removal of material to a desired depth. The edges
of the TBC material bounding the gaps 28 will exhibit a small
re-cast surface where material had been heated to just below the
temperature necessary for vaporization. The geometry of the walls
defining gaps 28 may be controlled by controlling the laser
engraving parameters. For turbine airfoil applications, the width
of the gap at the surface 30 of the thermal barrier coating 18 may
be maintained to be no more than 50 microns, or no more than 25
microns, or less than 125 microns, less than 100 microns, or less
than 75 microns. Various embodiments may have a gap width at the
surface 30 of between 25-125 microns (i.e. greater than 25 micron
and less than 125 micron), between 25-100 microns, between 25-75
micron between 25-50 micron, between 50-100 micron, between 50-75
micron, between 75-125 microns, or between 75-100 microns, for
example. Such gap sizes are selected to provide the desired
mechanical strain relief while having a minimal impact on
aerodynamic efficiency. Wider or more narrow gap widths may be
selected for particular portions of a component surface, depending
upon the sensitivity of the aerodynamic design and the predicted
thermal conditions. The laser engraving process provides
flexibility for the component designer in selecting the
segmentation strategy most appropriate for any particular area of a
component. In higher temperature areas the gap opening width may be
made larger than in lower temperature areas. A component may be
designed and manufactured to have a different gap width and/or
spacing (S) in different sections of the same component.
[0029] Furthermore, a bond inhibiting material, such as alumina or
yttrium aluminum oxide, may be disposed within the gaps on the gap
sidewalls in order to reduce the possibility of the permanent
closure of the gaps by sintering during long-term high temperature
operation. FIGS. 3A-3C illustrate a partial cross-sectional view of
a component part 32 of a combustion turbine engine during
sequential stages of fabrication. A substrate material 34 is coated
with a variable density ceramic thermal barrier coating 36 as
described above. A plurality of gaps 38, as shown in FIG. 3A, are
formed by laser engraving the surface 40 of the ceramic material.
Other methods and other forms of energy may be used to form the
gaps, for example, ion beam, electron beam, EDM, abrasive
machining, chemical etching, etc. A layer of a bond inhibiting
material 42 is deposited on the surface 40 of the ceramic,
including into the gaps 38, by any known deposition technique, such
as sol gel, CVD, PVD, etc. as shown in FIG. 3B. The amorphous state
as-deposited bond inhibiting material 42 is then subjected to a
heat treatment process as is known in the art to convert it to a
crystalline structure, thereby reducing its volume and resulting in
the structure of FIG. 3C. The presence of the bond inhibiting
material 42 within the gaps 38 provides improved protection against
the sintering of the material and a resulting closure of the gaps
38.
[0030] The inventors have found that a YAG laser may be used for
engraving the gaps of the subject invention. A YAG laser has a
wavelength of about 1.6 microns and will therefore serve as a finer
cutting instrument than would a carbon dioxide laser that has a
wavelength of about 10.1 microns. A power level of about 20-200
watts and a beam travel speed of between 5-600 mm/sec have been
found to be useful for cutting a typical ceramic thermal barrier
coating material. The laser energy is focused on the surface of the
coating material using a lens having a focal distance of about
25-240 mm. In one embodiment, a lens having a focal distance of 56
mm was used. In order to reduce the accumulation of molten material
splashed onto the lens during the laser engraving process, a lens
having a focal distance of at least 160 mm may be used. Typically
2-12 passes across the surface may be used to form the desired
depth of continuous gap.
[0031] It may be beneficial to change one or more parameters of the
energy used to create the gap between sequential applications of
energy to the insulation material. The geometry of the gap thus
formed may be affected by such a change in energy parameter(s). The
inventors have found that a generally U-shaped bottom geometry may
be formed in the gap by making a second pass with the laser over an
existing laser-cut gap, wherein the second pass is made with a
wider beam footprint than was used for the first pass in order to
reshape the walls defining the gap. The wider beam footprint may be
accomplished by simply moving the laser farther away from the
ceramic surface or by using a lens with a longer focal distance. In
this manner the energy from the second pass exposure will tend to
penetrate less deeply into the ceramic but will heat and evaporate
a wider swath of material near the bottom of the gap, thus forming
a generally U-shaped bottom geometry rather than a generally
V-shaped bottom geometry as may be formed with a first pass. This
process is illustrated in FIGS. 4A and 4B. A gap 44 is formed in a
layer of ceramic material 46. In FIG. 4A, a first pass of the laser
energy 48 having a first focal distance and a first footprint size
is used to cut the gap 44. Gap 44 after this pass of laser energy
has a generally V-shaped bottom geometry 50. In FIG. 4B, a second
pass of laser energy 52 having a second focal distance greater than
the first focal distance and a second footprint size greater than
the first footprint size is used to widen the bottom of gap 44 into
a generally U-shaped bottom geometry 54. The dashed line in FIG. 4B
denotes the gap shape from FIG. 4A, and it can be seen that the
wider laser beam tends to evaporate material from along the walls
of the gap 44 without significantly deepening the gap, thereby
giving it a less sharp bottom geometry. The width of the gap 44 at
the top surface 56 in FIG. 4A is wider than the width of the beam
of laser energy 48 due to the natural convection of heat from the
bottom to the top as the gap 44 is formed. Therefore, the width of
beam 52 can be made appreciably wider than that of beam 48 without
impinging onto the sides of the gap 44 near the top surface 56.
Since the energy density of beam 52 is less than that of beam 48,
the effect of beam 52 will be to remove more material from the
sides of the gap 44 than from the bottom of the gap, thus rounding
the bottom geometry somewhat. Such a U-shaped bottom geometry will
result in a lower stress concentration at the bottom of the gap 44
than would a generally V-shaped geometry of the same depth.
[0032] The bottom geometry of the gap 44 may also be affected by
the rate of pulsation of the laser beam 52. It is known that laser
energy may be delivered as a continuous beam or as a pulsed beam.
The rate of the pulsations may be any desired frequency, for
example from 1-20 kHz. Note that this frequency should not be
confused with the frequency of the laser light itself. For a given
power level, a slower frequency of pulsations will tend to cut
deeper into the ceramic material 46 than would the same amount of
energy delivered with a faster frequency of pulsations.
Accordingly, the rate of pulsations is a variable that may be
controlled to affect the shape of the bottom geometry of the gap
44. In one embodiment, the inventors envision a first pass of the
laser energy 48 having a first frequency of pulsations being used
to cut the gap 44. Gap 44 after this pass of laser energy may have
a generally V-shaped bottom geometry 50. A second pass of laser
energy 52 having a second frequency of pulsations greater than the
first frequency of pulsations is used to widen the bottom of gap 44
into a generally U-shaped bottom geometry 54. The dashed line in
FIG. 4B denotes the gap shape from FIG. 4A, and it is expected that
the more rapidly pulsed laser beam would tend to evaporate material
from along the walls of the gap 44 without a corresponding
deepening of the gap, thereby giving the gap a less sharp bottom
geometry. The bottom geometry 54 may further be controlled by
controlling a combination of laser beam footprint and pulsation
frequency, as well as other cutting parameters. If energy other
than laser energy is used to form the gap, similar or other changes
in energy parameter(s) may be used to provide a desired gap
geometry. Furthermore, the insulating material may be exposed to
more than one form of energy, e.g. laser energy then ion beam or
other combinations of forms of energy, to achieve a desired
geometry. Alternatively, combinations of methods may be used to
form a single gap, e.g., a chemical etch and the application of a
form of electromagnetic energy.
[0033] The laser energy may be delivered to the ceramic material 46
through a fiber optic cable. A fiber optic cable may be
particularly useful in applications where access to the ceramic
material surface 56 is limited. One or more lens could be used
downstream and/or upstream of the fiber optic cable to enhance the
power density and/or the focus of the energy.
[0034] When gap 44 is formed in the ceramic material 46 by a laser
engraving process or other heat-inducing process, a portion of the
molten material generated by the laser energy is splashed onto the
top surface 56 of the ceramic material proximate the gap 44 to form
a ridge 60 on opposed sides of the gap 44. The ridge 60 may have a
height above the original plane of the top surface 56 of about
10-50 microns for example. Ridge 60 would cause a perturbation and
downstream wake in an airflow passing over the ceramic thermal
barrier coating material 46. Accordingly, in prior art laser
drilling operations where laser energy has been used to drill
cooling fluid passages through a ceramic coating, such ridges 60
have been removed, such as by polishing, prior to use of the
component in an airfoil application such as a gas turbine blade.
The present inventors have realized that laser engraved gap 44 may
be used in an air stream application in its as-formed state
including ridge 60 provided that the axis of the gap 44 (i.e. its
longitudinal length along the gap perpendicular to the paper as
viewed in FIGS. 4A and 4B) is oriented along the direction of flow
of the air/fluid passing over the ceramic material 46. This concept
is illustrated in FIG. 5 where a gas turbine stationary vane
assembly 62 is seen in a top plan view showing an airfoil member 64
attached to a platform 66. The platform 66 is coated with a ceramic
thermal barrier coating into which a plurality of continuous laser
engraved grooves or gaps 68 are formed. The grooves 68 are formed
on the platform 66 in a pattern that coincides with the direction
of a fluid stream flowing over the platform 66. Because the fluid
is flowing parallel to a longitudinal axis of the groove 68, the
fluid dynamic impact of the ridge 60 (illustrated in FIGS. 4A and
4B but not in FIG. 5) adjacent the groove 68 is minimal.
Furthermore, the fluid stream will tend to sweep along the groove
68, thereby helping to keep the groove 68 free of debris that might
otherwise possibly accumulate in a cross-flow environment.
Continuous laser engraved grooves may also be formed on the airfoil
member 64 in a direction corresponding to the direction of the
fluid stream over the airfoil, i.e. from the leading edge toward
the trailing edge. In one embodiment, such grooves are formed
proximate the leading edge only, i.e. along the highest temperature
regions of the airfoil member 64. Similarly, the fillet area
between the airfoil member 64 and the platform 66 may be grooved in
a direction parallel to the air flow in a direction from the
leading edge to the trailing edge of the airfoil member 64. These
embodiments are provided by way of illustration and are not meant
to limit the present invention, which may include grooves with or
without ridges 60, and grooves parallel to, perpendicular to and/or
otherwise oblique to a direction of a fluid stream.
[0035] The laser engraved gaps 44 can be formed to have a shape
that is generally perpendicular to the top surface 56 of the
ceramic material 46; i.e. a depth dimension line drawn from the
center of the top of the gap to the center of the bottom of the gap
would be perpendicular to the plane of the surface 56. This may be
accomplished by keeping the laser beam 52 perpendicular to the
surface 56 as it is moved in any direction. Alternatively, if the
laser beam 52 is disposed at an oblique angle to the surface 56,
the beam 52 can be moved parallel to the direction of the oblique
angle along the laser line of sight so that the resulting gap 44
still remains perpendicular to the surface 56. The depth of the gap
44 may be less than 100% of the depth of the coating to avoid
penetrating an underlying bond coat, and it may be at least 50% of
the thickness of the ceramic coating, or between 50-67% of the
depth of the coating. Such partial depth gaps 44 not only relieve
stress in the coating, but they also serve as crack terminators for
a crack developing between the bond coat and the ceramic thermal
barrier coating. This aspect is illustrated in FIG. 6, which shows
the level of stress needed to drive a crack along the interface
between a bond coat and an overlying thermal barrier coating. As
illustrated in FIG. 6, the crack driving force decreases in the
region between the laser engraved gaps in the surface, thus
reducing the crack propagation velocity and consequently increasing
the coating spallation life when compared to a non-engraved
coating. FIG. 6 was developed using a finite element model assuming
no transient temperature dependence and depicting the stresses upon
cooling under stationary conditions.
[0036] One embodiment of the present invention utilizes a Model
RS100D YAG laser producing a pulsed laser light with a repetition
rate of 20 KHz with a power of 15 watts delivered with a 110
nanosecond pulse duration and 4.9 mJ/pulse. Two to six passes of
laser energy are made over the surface of a ceramic thermal barrier
coating through a 160 mm lens at a distance above the surface of
approximately 150-175 mm to produce a 75-100 micron wide groove
extending 50-67% of the coating depth. Additional parallel grooves
may be produced by repeating this process at a spacing of 500
microns away from the first groove.
[0037] FIG. 7 illustrates a partial cross-sectional view of an
insulated component 70 having a ceramic thermal barrier coating 72
covering a substrate material 74. A laser-engraving process is used
to form continuous grooves 76 extending to a partial depth into the
coating 72 from the top surface 78. Various ones of the grooves 76
are formed to extend to selected predetermined depths A1, A2 and A3
below the top surface 78 to form a multi-layered arrangement of
vertical segmentation within the coating 72. This arrangement is
selected to allow the coating to spall within discreet planes of
failure as a result of service-induced thermal stresses. By
tailoring the depths of the grooves 76 and the spacing S1, S2, S3
between grooves of the same depth, spallation can be forced to
occur at optimum levels, thereby resulting in a fresh, un-sintered
coating surface being exposed when an upper layer of the coating 72
spalls off. In this manner, the operating life of the coating 72
may be increased beyond that of a similar coating formed with no
grooves or with grooves all having a uniform depth. The groove
depths and spacing may be selected so that the stress induced in
the coating 72 during a known thermal gradient will reach a
critical level at a critical depth within the coating 72, such as
at depth A1. Once the critical stress level is achieved, the
coating will fail in a generally planar manner at the critical
depth, thereby exposing a new surface of the coating 72. It is also
possible to utilize a "seed" material at the critical depth to
change the interface properties between layers and to ensure that
failure propagation remains at the interface of the layers. For a
zirconia coating, the seed material may be organic, including
carbon, graphite, or polymer for example, or it may be inorganic,
including alumina, hafnia or other high temperature oxide material
having a thermal expansion characteristic or geometric or other
property different than the zirconia to enhance crack propagation
within the failure plane.
[0038] FIG. 8 illustrates another embodiment of an insulated
component 80 having a layer of ceramic thermal insulation 82 that
is formed to have three distinct segmented layers 84, 86, 88. Each
layer 84, 86, 88 is laser engraved after it is deposited and before
it is covered by a subsequent layer to include a plurality of
stress-relieving grooves 90. The vertically stacked grooves
optionally may be aligned with each other. The underlying grooves
may be partially but not fully filled in by the overlying coating
layer. In this embodiment, the thermal insulation 82 will
preferentially fail along the interface between the respective
layers 84, 86, 88. The depth of the layers and the segmentation
scheme are selected to allow the insulation 82 to spall along a
critical depth in response to an expected thermal transient,
thereby presenting a fresh layer of the insulation to the exterior
environment. The properties of the material, the thermal gradient
across the insulation, and the distance between vertical segments
are contributing factors that define the strain energy buildup and
subsequent spallation depth in such a coating. A coating may thus
be designed to have a plurality of layers of defined spallation
thicknesses, each providing a duration of exposure to the
surrounding high temperature environment. The total useful life of
the coating is the sum of the times leading to the spallation of
the various layers, and such time may well exceed the total
spallation life of an un-segmented coating.
[0039] While the preferred embodiments of the present invention
have been shown and described herein, it will be obvious that such
embodiments are provided by way of example only. Numerous
variations, changes and substitutions will occur to those of skill
in the art without departing from the invention herein.
Accordingly, it is intended that the invention be limited only by
the spirit and scope of the appended claims.
* * * * *