U.S. patent application number 09/774550 was filed with the patent office on 2002-08-01 for thermal barrier coating applied with cold spray technique.
This patent application is currently assigned to Siemens Westinghouse Power Corporation. Invention is credited to Seth, Brij B., Subramanian, Ramesh, Wagner, Gregg P..
Application Number | 20020102360 09/774550 |
Document ID | / |
Family ID | 25101584 |
Filed Date | 2002-08-01 |
United States Patent
Application |
20020102360 |
Kind Code |
A1 |
Subramanian, Ramesh ; et
al. |
August 1, 2002 |
Thermal barrier coating applied with cold spray technique
Abstract
A process (20) for applying a thermal barrier coating (51) to a
turbine component (50) including the step (34) of depositing a bond
coating layer (56) by a cold spray process. The layer of bond
coating material may have different depths (80,82) in different
areas of the component (50), and it may have different compositions
(60,62) across its depth. The precise control afforded by the cold
spray material deposition step allows the surface of the bond
coating material layer to be formed with a predetermined surface
roughness or with a plurality of micro-ridges (86) in order to
optimize its bond to the overlying ceramic insulating layer
(52).
Inventors: |
Subramanian, Ramesh;
(Oviedo, FL) ; Wagner, Gregg P.; (Apopka, FL)
; Seth, Brij B.; (Maitland, FL) |
Correspondence
Address: |
Siemens Corporation
Intellectual Property Department
186 Wood Avenue South
Iselin
NJ
08830
US
|
Assignee: |
Siemens Westinghouse Power
Corporation
|
Family ID: |
25101584 |
Appl. No.: |
09/774550 |
Filed: |
January 30, 2001 |
Current U.S.
Class: |
427/419.1 ;
427/180; 427/201; 427/383.1 |
Current CPC
Class: |
C23C 28/3455 20130101;
C23C 28/3215 20130101; C23C 24/04 20130101; Y02E 20/16
20130101 |
Class at
Publication: |
427/419.1 ;
427/180; 427/201; 427/383.1 |
International
Class: |
B05D 003/02; B05D
001/36 |
Claims
We claim as our invention:
1. A method of applying a thermal barrier coating comprising the
steps of: providing a substrate material; directing particles of
bond coating material toward a surface of the substrate material at
a velocity sufficiently high to cause the particles to deform and
to adhere to the surface to form a layer of bond coating material;
applying a layer of ceramic insulating material over the layer of
bond coating material.
2. The method of claim 1, further comprising the step of
controlling at least one of the velocity and a size of the
particles to produce a predetermined roughness on the layer of bond
coating material.
3. The method of claim 1, further comprising the step of directing
particles of a barrier layer material toward the surface of the
substrate at a velocity sufficiently high to cause the particles to
deform and to adhere to the substrate surface to form a barrier
layer prior to the step of directing particles of a bond coating
material.
4. The method of claim 1, further comprising the step of directing
particles of the bond coating material toward a first portion of
the substrate surface to form a first thickness of bond coating
material on the first portion and directing particles of the bond
coating material toward a second portion of the substrate surface
to form a second thickness of bond coating material on the second
portion.
5. The method of claim 1, further comprising the step of varying
the composition of the particles of bond coating material during
the step of directing particles of bond coating material to produce
a layer of bond coating material having varying properties across
its depth.
6. The method of claim 1, wherein the step of directing particles
of a bond coating material further comprises controlling the
particles to form a plurality of micro-ridges on the surface of the
layer of bond coating material.
7. A method of fabricating a component for a gas turbine engine,
the method comprising the steps of: forming a substrate comprising
a superalloy material into a predetermined shape appropriate for
use in a gas turbine engine; and directing particles of an MCrAlY
material toward a surface of the substrate at a velocity
sufficiently high to cause the particles to deform and to adhere to
the surface to form an overlay layer of MCrAlY material.
8. The method of claim 7, further comprising the step of directing
particles of the MCrAlY material toward the surface of the
substrate at a velocity not sufficiently high to cause the
particles to deform and to adhere to the surface but sufficiently
high to clean the surface of the substrate.
9. The method of claim 7, further comprising the step of directing
particles of the MCrAlY material toward a first portion of the
substrate surface remote from a cooling hole to form a first
thickness of MCrAlY material remote from the cooling hole and
directing particles of the MCrAlY material toward a second portion
of the substrate surface proximate the cooling hole to form a
second thickness of MCrAlY material less than the first thickness
proximate the cooling hole.
10. The method of claim 7, further comprising the step of directing
particles of the MCrAlY material toward a first portion of the
substrate surface to form a first thickness of MCrAlY material on
the first portion and directing particles of the MCrAlY material
toward a second portion of the substrate surface to form a second
thickness of MCrAlY material on the second portion.
11. The method of claim 7, further comprising the step of
controlling at least one of the velocity and a size of the
particles to produce a predetermined roughness on the layer of
MCrAlY material.
12. The method of claim 7, further comprising the step of applying
a layer of ceramic insulating material over the layer of MCrAlY
material.
13, The method of claim 7, further comprising continuing the step
of directing particles until the overlay layer of MCrAlY material
is formed to a thickness of between 12 and 25 mils.
14. A method of fabricating a component for a gas turbine engine,
the method comprising the steps of: forming a substrate into a
predetermined shape useful in a gas turbine; defining a first
surface area of the substrate and a second surface area of the
substrate as a function of the environment to which the
predetermined shape will be exposed during the operation of the gas
turbine; directing particles of a bond coating material toward a
surface of the substrate at a velocity sufficiently high to cause
the particles to deform and to adhere to the surface to form a
layer of bond coating material having a first thickness over the
first surface area and a second thickness over the second surface
area.
15. A method of fabricating a component for a gas turbine engine,
the method comprising the steps of: forming a substrate into a
predetermined shape appropriate for use in a gas turbine engine;
depositing a plurality of layers of an overlay coating on a surface
of the substrate by directing particles of a coating material
toward the surface at a velocity sufficiently high to cause the
particles to deform and to adhere to the surface; and changing the
composition of the particles between a first composition for a
first of the plurality of layers to a second composition for a
second of the plurality of layers.
16. The method of claim 15, further comprising the steps of:
forming a first of the plurality of the layers to be a diffusion
barrier by selecting the first composition to comprise at least one
of rhenium, tantalum, platinum and alloys thereof.
17. The method of claim 15, further comprising the steps of:
controlling at least one of the velocity and a size of the
particles to produce a predetermined roughness on a top of the
plurality of layers; and depositing a ceramic insulating layer onto
the top of the plurality of layer.
18. A method of forming a part for a turbine, the method comprising
the steps of: providing a substrate; applying a layer of a bond
coat material to the surface of the substrate by directing
particles of MCrAlY material toward the surface at a velocity
sufficiently high to cause the particles to deform and to adhere to
the surface; applying a layer of platinum to only a selected
portion of the layer of bond coat material by directing particles
of platinum toward the surface of the layer of bond coat material
at a velocity sufficiently high to cause the particles to deform
and to adhere to the surface; subjecting the layer of platinum to a
diffusion heat treatment process; and applying a layer of a ceramic
insulating material over at least the selected portion of the layer
of bond coat material.
Description
[0001] This invention relates generally to the field of materials
technology, and more specifically to the field of thermal barrier
coatings for high temperature applications, and specifically to a
process for manufacturing a turbine component by applying layers of
a thermal barrier coating using a cold spray technique, and to a
component manufactured with such a process.
BACKGROUND OF THE INVENTION
[0002] It is well known that the power and efficiency of operation
of a gas turbine engine or a combined cycle power plant
incorporating such a gas turbine engine may be increased by
increasing the firing temperature in the combustion portions of the
turbine. The demand for improved performance has resulted in
advanced turbine designs wherein the peak combustion temperature
may reach 1,400 degrees C. or more. Special materials are needed
for components exposed to such temperatures. Nickel and cobalt
based super alloy materials are now used for components in the hot
gas flow path, such as combustor transition pieces and turbine
rotating and stationary blades. However, even super alloy materials
are not capable of surviving long term operation in a modern gas
turbine without some form of insulation from the operating
environment.
[0003] It is known to coat a superalloy metal component with an
insulating ceramic material to improve its ability to survive high
operating temperatures in a combustion turbine environment. One
thermal barrier coating system 10 in common use today is
illustrated in FIG. 1. A ceramic top coat 12 applied to a super
alloy substrate structure 18, with an intermediate metallic bond
coat 16. An example of a commercially available super alloy
material 18 is IN738 made by Inco Alloys International, Inc. A
common ceramic insulating material 12 is yttria stabilized zirconia
(YSZ). Hafnia or scandia stabilized zirconia may also be used as
layer 12, or alternatively, yttrium aluminum garnet (YAG). The bond
coat layer 16 provides oxidation resistance and improved adhesion
for the thermal barrier coating layer 12. Common bond coat
materials 16 include McrAlY and MCrAlRe, where M may be nickel,
cobalt, iron or a mixture thereof. The metallic bond coat material
16 has the additional function of supplying aluminum to form a
thermally grown oxide (TGO) layer 14, which may be formed
substantially of aluminum oxide. The oxide layer 14 develops during
manufacturing heat treatment operations and during the operating
service of the turbine, and it may grow from 0 to 15 micrometers
thick through the life of the coating 10. This oxide layer provides
oxidation resistance for the underlying super alloy 18 and provides
an improved bond between the ceramic layer 12 and the metallic bond
coat 16. The thermally grown oxide layer may alternatively be grown
on a platinum enriched bond coat or platinum aluminide. To achieve
such an embodiment, a layer of platinum is first applied to the
surface of the bond coat layer 16 and then is diffused into the
bond coat layer 16 by a diffusion heat treatment.
[0004] The metallic bond coat layer 16 is known to be applied by
any one of several thermal spray processes, including low pressure
plasma spray (LPPS), air plasma spray (APS) and high velocity
oxy-fuel (HVOF). Such processes propel the MCrAlY material in a
molten plasma state against the surface of the super alloy
substrate 18 where it cools and solidifies to form a coating. Such
thermal spray process are known to result in a significant amount
of porosity and the formation of oxygen stringers in the bond coat
layer 16 due to the inherent nature of the high temperature
process. The release of heat from the molten particles of MCrAlY
and the transfer of heat from the high temperature gas used in the
thermal spray process also result in a significant increase in the
surface temperature of the super alloy substrate material 18 during
the metallic bond coat 16 application process. Such elevated
temperatures result in localized stresses in the super alloy
material 18 upon the cooling of the coating layer. Furthermore, a
post-deposition diffusion heat treatment is necessary to provide
the required metallurgical bond strength, and such treatment may
have adverse affects on the material properties of the underlying
substrate.
[0005] The known processes for manufacturing thermal barrier
coating systems have numerous limitations, such as the creation of
residual stresses, the formation of coating layers containing voids
and porosity, the need for specialized thermal spraying equipment
that is not adaptable for field repair operations, and a high cost
of manufacturing. Thus, an improved process is needed for
manufacturing components having a thermal barrier coating.
BRIEF DESCRIPTION OF THE INVENTION
[0006] The present inventors have recognized that a cold spray
process is beneficial for the application of a metallic bond coat
layer of a thermal barrier coating for a combustion turbine engine
part. The cold spraying of bond coat powders allows for the
deposition of a dense oxidation and corrosion resistant coating on
both new and service-run gas turbine components. Because a cold
spray process produces a coating having essentially no porosity and
no oxygen stringers, the performance of the bond coating during the
operation of the component will be improved when compared to prior
art flame or thermally sprayed coatings.
[0007] Because the area to which a coating is applied may be
limited and controlled during a cold spraying process, new
components may be fabricated using a cold spray process for filling
gaps or discontinuities in a bond coat layer during the original
manufacturing process. Furthermore, components that have been
damaged by mishandling or by out-of-specification machining may be
repaired using a cold spray technique.
[0008] In one embodiment of the invention, the thickness of a bond
coat layer is varied along a surface of a turbine component, with a
thicker coating being applied in those areas of the component
exposed to the highest temperatures during turbine operation. In a
further embodiment of the invention, the composition of a bond coat
layer is varied along a surface of a turbine component. This may be
particularly advantageous to reduce the consumption of an expensive
material, such as platinum, by limiting the application of such
material to only those portions of the component where the
resulting benefit is necessary. Such variations in coating
applications may be accomplished without masking, thereby
eliminating process steps and eliminating the geometric
discontinuity normally associated with the edge of a masked
area.
[0009] In a further embodiment of the present invention, the
composition of a bond coat layer applied by a cold spray process is
varied along a depth dimension. The material of the first layers to
be applied is selected to minimize interdiffusion with the
underlying substrate material, and the material of the top layers
is selected to optimize resistance to oxidation and corrosion.
[0010] A process for manufacturing a turbine component in
accordance with the present invention is more economical than prior
art processes because there is no need for a high temperature heat
treatment following the deposition of the bond coat. As a result,
the initial interdiffusion zone between the substrate and bond coat
is minimized. Thus, the amount of aluminum available in the bond
coat layer for the subsequent formation of a thermally grown oxide
layer is increased when compared to components formed with prior
art processes. This results in improved performance of the thermal
barrier coating system.
[0011] In a further embodiment of this invention, the roughness of
the surface of a bond coat layer is controlled to a desired value
by controlling the parameters of a cold spray process. A desirable
degree of roughness may be obtained without the need for
post-deposition processing.
[0012] These and other objects and advantages of the invention are
provided by way of example, not limitation, and are described more
fully below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] 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:
[0014] FIG. 1 is a cross-sectional view of a prior art thermal
barrier coating system.
[0015] FIG. 2 illustrates the steps of a method for providing a
thermal barrier coating system wherein a bond coat layer is
deposited by a cold spray process.
[0016] FIG. 3 illustrates a thermal barrier coating applied by a
cold spray process and having varying material properties across
the depth of the bond coat layer.
[0017] FIG. 4 is a partial cross-sectional view of a turbine blade
having a thermal barrier coating including a bond coat layer with
varying thickness.
[0018] FIG. 5 is a partial cross-sectional view of the fillet area
of a gas turbine blade showing the orientation of a cold spray
nozzle during a bond coating material deposition step.
DETAILED DESCRIPTION OF THE INVENTION
[0019] U.S. Pat. No. 5,302,414 dated Apr. 12, 1994, incorporated by
reference herein, and re-examination certificate B1 5,302,414 dated
Feb. 25, 1997, describe a cold gas-dynamic spraying method for
applying a coating, also referred to as cold spraying. That patent
describes a process and apparatus for accelerating solid particles
having a size from about 1-50 microns to supersonic speeds in the
range of 300-1,200 meters per second and directing the particles
against a target surface. When the particles strike the target
surface, the kinetic energy of the particles is transformed into
plastic deformation of the particles, and a bond is formed between
the particles and the target surface. This process forms a dense
coating with little or no thermal effect on the underlying target
surface.
[0020] The present inventors have recognized that a cold spray
process may be beneficial for the application of a metallic bond
coat layer of a thermal barrier coating system. A process 20 for
providing a thermal barrier coating incorporating a bond coat layer
applied by a cold spray process is illustrated in FIG. 2 and is
described more fully below.
[0021] A gas turbine engine is designed to have various components
exposed to the hot gas flow path during the operation of the
turbine. Such components may include, for example, rotating blades,
stationary vanes, combustor baskets, ring segments, transitions,
etc. Such components are manufactured by first providing a
substrate material at step 22 formed to an appropriate shape in
accordance with the engine design. The substrate material may be
any material known in the art for such applications, and may
include wrought, conventionally cast, directionally solidified (DS)
and single crystal (SC) materials. The substrate material may be a
nickel or cobalt based superalloy material.
[0022] On the basis of the design temperatures within the turbine,
certain areas of the component may require a thermal barrier
coating. One such thermal barrier coating includes a layer of
ceramic insulating material deposited over a metallic bond coat
layer applied directly to the surface of the substrate. The area of
the component to be coated with the bond coat layer is identified
at step 24, and the required thickness of the coating is selected
at step 28. A bond coat layer, such as a layer of MCrAlY having a
thickness of between 75-200 micrometers, not only provides a
surface suitable for the application of overlying ceramic layers,
but it also functions as a thermal insulator for the substrate
material, and as a barrier against the migration of oxygen and
other corrosive materials into the substrate material. The bond
coat layer may be used without an overlying ceramic insulating
material in some applications.
[0023] The material to be cold sprayed to form the desired coating
is selected at step 26. As is described in U.S. Pat. No. 5,302,414,
the material may be prepared to contain particles ranging in size
from about 1-50 microns by any known melt/atomization process. The
desired surface roughness of the applied coating is selected at
step 30. The surface roughness obtained may be affected by varying
the cold spray application parameters selected at step 32. An
initial layer of bond coat material is applied to a substrate at
step 34 utilizing the material selected at step 26 and the process
parameters selected at step 32 to obtain a coating with a desired
thickness and surface roughness. A decision is then made at step 36
if an additional thickness of coating is desired on the same area
of the component. If not, a decision is made at step 38 if there is
an additional area of the component to be coated. If not, the
component is then processed through any remaining manufacturing
steps, such as applying a ceramic coating layer over the bond coat
layer at step 40. The cold spray material deposition step 34
involves directing particles of the coating material toward the
target surface at a velocity sufficiently high to cause the
particles to deform and to adhere to the target surface. Such
process will provide a dense coating of the material having a very
low incidence of porosity and oxygen stringers. As a result, the
bond coat layer applied at step 34 will provide improved resistance
to oxidation of the substrate and reduced susceptibility to
spalling of the overlying ceramic layer when compared to prior art
thermally deposited bond coat layers. Particles of from 0.1 to 50
microns may be accelerated to speeds of from 500-1,200 meters per
second. A feed rate of from 0.1 to 2 grams per second may be
deposited while traversing across a target surface at an advance
rate of between 0.01-0.4 meters per second.
[0024] The inventors have found that the use of a cold spraying
process 34 facilitates the deposition of a bond coat layer 56
having varying material properties across the depth of the coating,
as illustrated in FIG. 3. A turbine component 50 includes a
substrate 58 insulated by a thermal barrier coating system 51
including ceramic layer 52 deposited on a bond coat layer 56. A
thermally grown oxide layer 54 is formed by the oxidation of
aluminum diffusing from within the bond coat layer 56. The bond
coat layer 56 is deposited on the substrate 58 with a cold spraying
process in step 34 of FIG. 2. It is known in the art to provide a
separate barrier layer between the substrate 58 and the bond coat
layer 56 in order to minimize the interdiffusion of materials
between the two layers. Known barrier layer materials include
rhenium, tantalum, platinum, and alloys thereof. The present
inventors have recognized that the function of a separate barrier
layer may be incorporated into the bond coat layer 56 by forming
the bond coat layer 56 to have varying materials properties across
its depth. The first portion 60 of the bond coat layer 56 is formed
by cold spraying particles of material selected for its resistance
to the diffusion of aluminum and other materials into or out of the
substrate material 58. For example, the particles selected at step
26 of FIG. 2 may be a mixture of MCrAlY with a high percentage of
rhenium, tantalum, platinum or alloys thereof. The second portion
62 of the bond coat layer 56 is formed by cold spraying particles
of material selected to maximize the coating's oxidation and
corrosion resistance, such as an MCrAlY having about 16-25 weight
percent chromium, 6-15 weight percent aluminum, 0.1-0.5 weight
percent yttrium, and the balance nickel and/or cobalt. After one or
more layers 60 are applied to the substrate 58, a decision is made
as indicated at step 42 of FIG. 2 to apply a different composition
of coating material for subsequent layers 62. FIG. 4 is illustrated
as having two portions 60,62 within the bond coat layer 56,
however, more than two such portions may be used. In one
embodiment, a continuously variable coating layer may be applied,
with the composition of the particles cold sprayed onto the
substrate being gradually changed from a first initial chemistry to
a second final chemistry. Alternatively, a single step change in
the material composition may be provided.
[0025] The thickness of a bond coat layer may be selected at step
28 to be different for different portions of the substrate,
depending upon the localized environment to which those various
portions will be exposed during the operation of the component.
FIG. 4 illustrates a portion of a gas turbine blade 70 wherein a
bond coat layer 76 has a first thickness in a first area 80 and a
second, larger thickness in a second area 82. In this embodiment,
the thickness is purposefully reduced proximate a cooling air hole
84 formed in the substrate material 78. A cold spraying nozzle may
be controlled to apply a coating to only a small, well-defined
area. The thickness of the coating is relatively constant in the
target area, for certain nozzle designs an area having a radius of
from only 3 mm to 12 mm, and it tapers smoothly to zero a short
distance outside that area. Areas of different thickness may be
obtained by varying the application parameters from one area to
another along the surface of the component, or by applying more
layers to one area than another. The resulting change in thickness
from an area 80 to another 82 may thereby be formed to be a gradual
change without the usual step change in thickness associated with
the use of a masking process.
[0026] In order to improve the adhesion of a subsequent top coat,
it may be desirable to provide the top surface of a bond coat layer
with a predetermined degree of roughness. An MCrAlY layer applied
by prior art low pressure plasma spray methods will exhibit a
roughness of about 56 microns Ra in the as-deposited condition.
This surface is acceptable for receiving an insulating layer
applied by an APS process, however, a smoother surface is needed to
receive an insulating layer applied by an electron beam plasma
vapor deposition (EBPVD) process. Because an EBPVD process provided
improved performance for the thermal barrier coating, it is
preferred for those parts in a combustion turbine exposed to the
hotest temperatures. Accordingly, prior art parts having a bond
coat layer applied by LPPS need to be polished to about a 1-2
micron Ra finish, such as by being tumbled in a polishing media for
up to eight hours. Not only does such polishing add time and
expense to the fabrication process, it is also known to create
near-surface defects which increase the risk of oxidation of the
bond coating and spalling of the overlying ceramic layer. To move
from a rougher surface finish to a smoother finish with the same
material of a cold sprayed bond coat layer, the application
parameters selected at step 32 of FIG. 2 may be adjusted to include
one or more of the following: a higher gas temperature; a higher
gas velocity; and a smaller particle size. Such changes may be
incorporated on only the topmost layers or throughout the entire
depth of the coating. It may also be desirable to deposit a
plurality of micro-ridges 86 on the surface of a bond coat layer by
controlling the cold spray process parameters to provide an
engineered surface topography. Such micro-ridges may be on the
order of a few to 10 or more micrometers in height and may be
placed at equal or variable intervals across the surface of the
bond coat material. Such micro-ridges may be deposited by using a
specially designed nozzle capable of creating such a fine line on
the target surface, or they may be created by the incremental
movement of the nozzle between succeeding passes across the
surface.
[0027] The impact of solid particles upon a target surface during a
cold spraying process results in the plastic deformation of the
particles and the localized heating of the deformed particles. The
kinetic energy released by the deceleration of the particles upon
their collision with the target surface becomes available for the
fracturing of the crystal bonds within the particles. For this
reason, different coating materials react differently to the cold
spraying process, as illustrated in Table 2 of U.S. patent
application Ser. No. 5,302,414. Preferred bond coating materials
for thermal barrier coating systems include MCrAlY, which is
relatively non-ductile material when compared to other commonly
sprayed metals, such as copper, aluminum, nickel and vanadium. In
order to promote the plastic deformation of MCrAlY particles upon
high speed impact with a substrate surface, it is preferred to use
the nanophase form of the MCrAlY alloy. Such material may be formed
by processes known in the art by controlling the cooling rate
during formation of the particles, or by the agglomeration of finer
particles. The efficiency of a manufacturing process for producing
turbine components having a thermal barrier coating may be improved
by applying the base coat for the thermal barrier coating using a
cold spraying process. The prior art device of FIG. 1 required a
difusion heat treatment process following the thermal spraying of
the bond coat layer 16 onto the substrate material 18. Accordingly,
following the thermal spray application of an MCrAlY coating layer
16 to a super alloy substrate material 18, the component is
subjected to a heat treatment process, such as heating to a
temperature of approximately 1050-1150 degrees Centigrade for 1-4
hours. No such heat treatment step is necessary following cold
spray material deposition step 34, thereby eliminating the time and
cost required for such an operation during the manufacturing of a
turbine part such as turbine blade 70 of FIG. 4.
[0028] The elimination of the diffusion heat treatment step has the
additional advantage of reducing the amount of aluminum that is
diffused into the substrate 70 from the bond coating layer 76. The
growth of TGO layer 74 is dependant upon the availability of
aluminum for diffusion from the bond coating layer 76. Accordingly,
the TGO layer 74 will have the potential for a greater oxidation
and corrosion thickness on a cold sprayed bond coat layer 76 than
on a thermally applied layer having the same thickness because
there is more aluminum available within the bond coat layer 76.
[0029] The cost of producing a turbine component may also be
reduced as a result of the selective application of expensive
alloying elements to only those specific areas of the component
where the benefits of that expensive element are required. For
example, it is known that the addition of platinum to an MCrAlY
layer will improve the adhesion of an overlying ceramic coating.
With prior art electroplating processes, a layer of platinum would
be applied to the entire surface of a component exposed in an
electroplating bath. However, the process 20 of FIG. 2 takes
benefit of the selective coating capabilities of a cold spraying
process to allow the designer to specify the addition of platinum
over only a selected area of the component. Once a layer of bond
coat material is deposited in step 34 of FIG. 2, an additional area
to be coated may be identified in step 38. That area may be defined
in step 24 to be a sub-set of the previously coated area, for
example, only those portions of a gas turbine blade that are
exposed to the highest of operating temperatures. An expensive
material such as platinum may be specified in step 26 and applied
to the sub-set area at step 34. In this manner, the quantity of the
expensive material used, and the attendant cost, is minimized.
Parameters may be selected at step 32 for the application of the
expensive material to maximize the coefficient of particle
utilization in order to reduce the cost of the coating further.
[0030] FIG. 5 illustrates the advantageous application of a cold
spray material deposition step in the fillet area 88 of a turbine
blade or vane 90. Prior art thermal spray techniques are known to
present difficulties in applying a coating at the intersection of
two surfaces 89,91, such as fillet area 88, because it is difficult
to cause the coating material to impact the target surface at near
a right angle. Because a cold spray nozzle 92 may be constructed to
be physically small and to deposit a thin line of material, for
example as small as 1 micron line width, it is possible to
manipulate the nozzle 92 to remain perpendicular to the target
surface 94 as it passes across the fillet area 88. FIG. 6
illustrates nozzle 92 in three positions as it progress across the
surface of blade 90 to deposit an overlay coating 96 of MCrAlY over
a superalloy blade substrate 98. Accordingly, the thickness of the
overlay coating 96 may be maintained nearly constant throughout the
fillet area 88, or it may purposefully be made to have a different
thickness in different areas.
[0031] To optimize the adhesion of the layer 96 to the substrate
material 98, it is desired to have a metal to metal contact between
the layers. Any contamination, oxidation or corrosion existing on
the surface of the substrate 98 may adversely impact the adhesion
of the coating layer 96. A separate cleaning step, such as grit
blasting with alumina particles, may be used to clean the target
surface. However, such process may leave trace amounts of the
cleaning material on the surface. After even a short period of
exposure to moisture in air, the target surface may begin to
oxidize. Handling or storing of the component after the cleaning
step may introduce additional contaminants to the previously clean
surface. The environment of the prior art thermal spraying
processes also contributes to the oxidation of the substrate during
the coating process due to the presence of high temperature, oxygen
and other chemicals. The parameters selected at step 32 for the
cold spray process of step 34 may be chosen to produce a desired
halo effect of particles at the fringe of the spray area where the
speed of approach to the target surface is insufficient to cause
the particles to bond to the surface of the substrate 98, but is
sufficiently high to produce a desired grit blast/cleaning effect.
The halo effect is caused by the spread of particles away from a
nozzle centerline due to particle interaction or by specific nozzle
design. When the nozzle 92 is directed perpendicular to the target
surface 89,91,94, the halo may be generally circular around a
generally circular coating area. The halo effect and cleaning
action may also have an elliptical shape caused by a
non-perpendicular angle between the nozzle centerline and the plane
of the substrate target surface of so desired. The halo effect
provides a cleaning of the target surface coincident to the
application of the coating layer 96, thereby improving the adhesion
of the coating when compared to a prior art device wherein some
impurities or oxidation may exist on the target surface at the time
of material deposition.
[0032] It is known in the art to apply a layer of MCrAlY material
by a thermal spraying process to form an overlay layer for
protecting a superalloy substrate material from corrosion during
operation as a part of a gas turbine engine. After operation within
a turbine for a period of time, such parts are known to require a
new coating of overlay material, as the original layer will be
degraded by corrosion and/or erosion. Prior art turbine parts have
been coated with up to 12 mils of such overlay material. Thicker
layers are not practical because of the limitations of the known
thermal spraying techniques. A process in accordance with this
invention may be used to apply more than 12 mils of such overlay
materials, for example coatings up to 20 mils thick or up to 25
mils thick. Because such layers are applied using a cold spray
technique, there is no adverse build-up of stresses within the
overlay layer or within the underlying substrate material.
Accordingly, a turbine part having such a thicker layer of overlay
material may remain in service longer before it must be re-coated
with new overlay material.
[0033] 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.
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