Information
-
Patent Grant
-
6589668
-
Patent Number
6,589,668
-
Date Filed
Wednesday, June 21, 200024 years ago
-
Date Issued
Tuesday, July 8, 200321 years ago
-
Inventors
-
Original Assignees
-
Examiners
- Jones; Deborah
- McNeil; Jennifer
-
CPC
-
US Classifications
Field of Search
US
- 427 252
- 427 253
- 428 670
- 428 650
- 428 621
- 428 628
- 428 610
- 428 680
- 416 241 R
-
International Classifications
- B32B1500
- F03B312
- C23C1606
-
Abstract
Method for forming on a superalloy or other metallic substrate a platinum graded, outward single phase diffusion aluminide coating on a surface of the substrate by depositing a layer comprising Pt on the substrate and then gas phase aluminizing the substrate in a coating chamber having a solid source of aluminum (e.g. aluminum alloy particulates) disposed therein close enough to the surface of the substrate to form at an elevated substrate coating temperature a diffusion aluminide coating having an inner diffusion zone and outer additive single (Ni,Pt)Al phase layer having a concentration of platinum that is relatively higher at an outermost coating region than at an innermost coating region adjacent the diffusion zone.
Description
FIELD OF THE INVENTION
The present invention relates to forming a platinum modified diffusion aluminide coating on a superalloy component, such as a gas turbine engine blade and vane, exposed to high service temperatures.
BACKGROUND OF THE INVENTION
Advancements in propulsion technologies have required gas turbine engines to operate at higher temperatures. This increase in operating temperature has required concomitant advancements in the operating temperatures of metallic (e.g. nickel and cobalt base superalloy) turbine engine components to withstand oxidation and hot corrosion in service. Inwardly grown and outwardly grown platinum modified diffusion aluminide coatings have been formed on superalloy turbine engine components to meet these higher temperature requirements. One such inwardly grown platinum modified diffusion coating is formed by chemical vapor deposition using aluminide halide coating gas and comprises an inward diffusion zone and an outer two phase [PtAl
2
+(Ni,Pt)Al] layer. The two phase Pt modified diffusion aluminide coatings are relatively hard and brittle and have been observed to be sensitive to thermal mechanical fatigue (TMF) cracking in gas turbine engine service.
One such outwardly grown platinum modified diffusion coating is formed by chemical vapor deposition using a low activity aluminide halide coating gas as described in U.S. Pat. Nos. 5,658,614; 5,716,720; 5,989,733; and 5,788,823 and comprises an inward diffusion zone and an outer (additive) single phase (Ni,Pt)Al layer.
An object of the present invention is to provide a gas phase aluminizing method using one or more solid sources of aluminum for forming on a substrate surface an outwardly grown, single phase diffusion aluminide coating that includes an outer additive layer having a graded Pt content from an outer toward an inner region thereof.
SUMMARY OF THE INVENTION
The present invention involves forming on a substrate, such as a nickel or cobalt base superalloy substrate, a platinum modified diffusion aluminide coating by depositing a layer comprising platinum on the substrate and then gas phase aluminizing the substrate in a coating chamber having a solid source of aluminum (e.g. aluminum alloy particulates) disposed therein close enough to the substrate surface as to form at an elevated coating temperature an outwardly grown diffusion aluminide coating having an inner diffusion zone and outer, single phase (Ni,Pt)Al additive layer having a concentration of platinum that is relatively higher at an outermost coating region than at an innermost coating region adjacent the diffusion zone. Gas phase aluminizing can be conducted with or without a prediffusion of the platinum layer into the substrate.
The present invention also envisions forming on a substrate a platinum graded, single phase diffusion aluminide coating at a first surface area of the substrate and concurrently a different diffusion aluminide coating at a second surface area of the substrate in the same coating chamber.
The present invention is advantageous to form on a nickel or cobalt base superalloy substrate an outwardly grown platinum modified diffusion aluminide coating having an outer, single phase (Ni,Pt)Al additive layer with a Pt content that is relatively higher at an outermost coating region than at an innermost coating region adjacent to a diffusion zone to impart oxidation and hot corrosion resistance thereto and improved ductility as compared to conventional two phase platinum modified diffusion coatings.
The above objects and advantages of the present invention will become more readily apparent from the following description taken with the following drawings.
DESCRIPTION OF THE DRAWINGS
FIG. 1
is an elevational view of a gas turbine engine blade having an airfoil region, a root region and a platform region with a damper pocket or recess beneath the platform region and located on the concave side and convex side of the airfoil.
FIG. 2
is an elevational view of a pin fixture to be positioned in the root end of a turbine blade for conducting coating gas through internal cooling passages of the turbine blade.
FIG. 3
is a partial schematic view of a coating chamber in which the turbine blades are coated. The coating chamber comprises a cylindrical annular chamber with a lid and having a central passage to receive a lifting post as illustrated in FIG.
4
.
FIG. 3
a
is partial enlarged elevational view of the turbine blade with the damper pocket proximate a source of aluminum.
FIG. 4
is a schematic sectional view of the retort showing a plurality of coating chambers positioned therein on a lifting post.
FIG. 5
is a photomicrograph at 475 X of an outwardly grown diffusion aluminide coating having an inner diffusion zone and outer single phase additive layer having a concentration of platinum that is relatively higher at an outermost coating region than at an innermost coating region adjacent the diffusion zone. The topmost layer of
FIG. 5
is not part of the coating and is present only to make the metallographic sample.
DESCRIPTION OF THE INVENTION
An exemplary embodiment of the invention involves forming on a nickel base superalloy, cobalt base superallloy, or other substrate an outwardly grown diffusion aluminide coating characterized by having an inner diffusion zone and outer, additive single phase (Ni,Pt)Al layer having a concentration of platinum that is relatively higher at an outermost coating region than at an innermost coating region adjacent the diffusion zone. The single phase (Ni,Pt)Al layer comprises a platinum modified nickel aluminide where platinum is in solid solution in the aluminide.
The substrate typically comprises a nickel or cobalt base superalloy which may comprise equiaxed, directionally solidified and single crystal castings as well as other forms of these materials, such as forgings, pressed powder components, machined components, and other forms. For example only, the substrate may comprise the PWA 1484 nickel base superalloy having a nominal composition of 10.0% Co, 8.7% Ta, 5.9% W, 5.65% Al, 5.0% Cr, 3.0% Re, 1.9% Mo, 0.10% Hf, and balance Ni (where % is in weight %) used for making single crystal turbine blades and vanes. Other nickel base superalloys which can be used include, but are not limited to, PWA 655, PWA 1422, PWA 1447, PWA 1455, PWA 1480, Rene N-5, Rene N-6, Rene 77, Rene 80, Rene 125, CSMX-4, and CMSX-10 nickel base superalloys. Cobalt based superalloys which can be used include, but are not limited to, Mar-M-509, Stellite 31, and WI 52 and other cobalt base superalloys.
For purposes of illustration and not limitation, the invention will be described herebelow with respect to forming the outwardly grown, graded platinum modified diffusion aluminide coating on a selected region of a gas turbine blade
10
as illustrated in FIG.
1
. The turbine blade comprises the aforementioned PWA 1484 nickel base superalloy. The turbine blade is made as a single crystal investment casting having an airfoil region
10
a
with a leading edge
10
b
and trailing edge
10
c
. The airfoil includes a concave side
10
d
and convex side
10
e
. The turbine blade
10
includes a root region
10
f
and a platform region
10
g
between the root region and airfoil region. The root region can include a plurality of fir-tree ribs
10
r
. The platform region includes a pair of damper pockets or recesses
12
(one shown in
FIG. 1
) with one damper pocket being located on the platform region at the concave side
10
d
and the other on the platform region at the convex side
10
e
of the airfoil region. Each damper pocket
12
is defined by an overhanging surface
12
a
of the platform region log and a side surface
12
b
thereof that has a surface extent defined by the dashed line L in FIG.
1
. Damper pocket surface
12
a
extends generally perpendicular to damper pocket surface
12
b.
The platform region
10
g
also includes external first and second peripheral end surfaces
13
a
at the respective leading and trailing edges, first and second peripheral side surfaces
13
b
disposed at the concave and convex sides, upwardly facing surfaces
14
that face toward the airfoil region
10
a
, and outwardly facing surfaces
15
that face toward and away from the root region
10
f.
The turbine blade
10
includes an internal cooling passage
11
illustrated schematically having cooling air inlet openings
11
a
,
11
b
at the end E of the root region
10
f
. The internal cooling passage
11
extends from the inlet openings
11
a
,
11
b
through root region
10
f
and through the airfoil region
10
a
, the configuration of the passage
11
being simplfied for covennience. In the airfoil region, the cooling passage
11
communicates to a plurality of exit openings lie at the trailing edge
10
c
where cooling air is discharged.
The exemplary turbine blade
10
described above is coated externally and internally with a protective outward diffusion aluminide coating in order to withstand oxidation and hot corrosion in service in the turbine section of the gas turbine engine.
In a particular embodiment offered for purposes of illustration and not limitation, the damper pocket surfaces
12
a
,
12
b
are gas phase aluminized pursuant to the invention to form an outwardly grown, platinum graded single phase diffusion aluminide coating of the invention locally on surfaces
12
a
,
12
b
, while an outwardly grown, Pt-free nickel aluminide diffusion coating is formed on the external surfaces of airfoil region
10
a
and the surfaces
13
a
,
13
b
,
14
of platform region log. The root region
10
f
and surfaces
15
of the platform region
10
g
are uncoated. The surfaces of the internal cooling passage
11
are coated to form a Pt-free outward diffusion aluminide coating.
For purposes of illustration and not limitation, the following steps are involved in coating the turbine blade
10
with the coatings described above. In particular, the investment cast turbine blades
10
are each subjected to multiple abrasive blasting operations where the damper pocket surfaces
12
a
,
12
b
are blasted with 240 mesh aluminum oxide grit at 10 to 40 psi with a 3 to 7 inch grit blast nozzle standoff distance.
In preparation for electroplating of platinum on the damper pocket surfaces
12
a
,
12
b
, the external surfaces of each turbine blade
10
, other than damper pocket surfaces
12
a
,
12
b
, are masked by a conventional peel type of maskant, while the internal cooling passage
11
is filled with wax.
Each masked turbine blade then is subjected to an electroplating operation to deposit a platinum layer on the damper pocket surfaces
12
a
,
12
b
only. For purposes of illustration only, a useful electroplating solution comprised of a conventional aqueous phosphate buffer solution including hexachloroplatinic acid (Pt concentation of 1 to 12 grams per liter, pH of 6.5 to 7.5, specific gravity of 16.5 to 21.0 Baume′, electrolyte temperature of 160 to 170 degrees F.) and a current density comprised 0.243-0.485 amperes/inch
2
to deposit a platinum layer. A suitable platinum plating solution including hexachloroplatinic acid is described in U.S. Pat. Nos. 3,677,789 and 3,819,338. A hydroxide based aqueous plating solution is described in U.S. Pat. No. 5,788,823. The platinum layer can be deposited in an amount of 0.109 to 0.153 grams/inch
2
, typically 0.131 grams/inch
2
, on damper pocket surfaces
12
a
,
12
b
. These electroplating parameters are offered merely for purposes of illustration as other platinum electroplating solutions and parameters can be employed. The platinum layer also can be deposited on surfaces
12
a
,
12
b
by techniques other than electroplating, such as including, but not limited to sputtering and other deposition techniques.
After plating, the maskant and the wax in internal passage
11
are removed from each turbine blade. The maskant and wax can be removed by heating the blades to 1250 degrees F. in air. The blades then are high pressure spray washed internally in deionized water followed by washing in a washer available from Man-Gill Chemical Company, Magnus Division, which is operated at medium stroke for 15 to 30 minutes at 160 to 210 degrees F. water temperature. The turbine blades then are dried for 30 minutes at 225 to 275 degrees F.
After cleaning as described above, the turbine blades
10
can be subjected to an optional prediffusion heat treatment to diffuse the platinum layer into the superalloy substrate at the electroplated damper pocket surfaces
12
a
,
12
b
. In particular, the turbine blades can be heated in a flowing argon atmosphere in a retort to 1925 degrees F. for 5 to 10 minutes. At the end of the prediffusion heat treat cycle, the turbine blades are fan cooled from 1925 degrees F. to 1600 degrees F. at 10 degrees F./minute or faster to below 900 degrees F. under argon atmosphere. The turbine blades then are removed from the retort. The airfoil region
10
a
and platform region
10
g
are then subjected to abrasive blasting using 240 mesh aluminum oxide grit at 40 to 60 psi with a 3 to 5 inch grit blast nozzle standoff distance. The root region
10
f
and damper pocket surfaces
12
a
,
12
b
are shielded and not grit blasted. The prediffusion heat treatment can be optional in practicing the invention such that the turbine blades with as-electroplated damper pocket surfaces
12
a
,
12
b
can be gas phase aluminized directly without the prediffusion heat treatment.
The turbine blades
10
with or without the prediffusion heat treatment then are subjected to a gas phase aluminizing operation pursuant to the invention in a coating chamber,
FIG. 3
, disposed in a coating retort, FIG.
4
.
Prior to gas phase aluminizing, a pin fixture
20
comprising an hollow pins
20
a
and
20
b
on a base plate
20
c
is adhered to the end E of the root region
10
f
. The pins
20
a
,
20
b
extend into and communicate to the respective openings
11
a
,
11
b
of the internal passage
11
at the root end, FIG.
2
.
Maskant then is applied to root region
10
f
and surfaces
15
in FIG.
1
. The maskant can comprise multiple layers of conventional M-1 maskant (stop-off comprising alumina in a binder) and M-7 maskant (sheath coat comprising mostly nickel powder in a binder), both maskants being available from Alloy Surfaces Co., Inc., Wilmington, Del. For example, 2 coats of M-1 maskant and 4 coats of M-7 maskant can be applied to the above surfaces. These maskants are described only for purposes of illustration and not limitation as any other suitable maskant, such as a dry maskant, can be used.
For purposes of illustration and not limitation, gas phase aluminizing of the turbine blades to form the coatings described above is conducted in a plurality of coating chambers
30
,
FIGS. 3 and 4
, carried on supports
40
a
on lifting post
40
positioned in coating retort
50
. Each coating chamber
30
comprises a cylindrical, annular chamber
30
a
and a lid
30
l
, the chamber and lid having a central passage
30
p
to receive lifting post
40
as illustrated in FIG.
4
.
Each coating chamber includes therein a lower chamber region
31
a
and upper coating chamber region
31
b
. A plurality of turbine blades
10
are held root-down in cofferdams
34
in upper chamber region
31
b
with the hollow pins
20
a
,
20
b
adhered on the root ends extending through respective pairs of holes in the bottom walls of the cofferdams
34
and wall W
1
so as to communicate the hollow pins
20
a
,
20
b
to lower chamber
31
a
. In
FIG. 3
, each pin
20
b
and the corresponding holes in each cofferdam
34
and wall W
1
are hidden behind pin
20
a
. The root regions
10
f
of a plurality of blades
10
are held in beds
37
of alumina (or other refractory) particulates in annular cofferdams
34
, FIG.
3
. Although only one blade
10
is shown so held in each cofferdam
34
for sake of convenience, the root regions
10
f
of a plurality of blades
10
typically are so held circumferentially spaced apart in each cofferdam
34
. The root regions
10
f
are placed in each cofferdam
34
with the respective pins
20
a
,
20
b
communicated to the lower chamber region
31
a
and the alumina particulates of bed
37
then are introduced into the cofferdams
34
to embed the root regions
10
f
in the alumina particulates to an extent shown in
FIG. 3
a
. Inner and outer gas seals
30
i
,
30
o
are formed between the lower chamber region
31
a
and upper chamber region
31
b
by alumina grit filled and packed in the spaces between the annular chamber walls as illustrated in FIG.
3
.
The lower chamber region
31
a
includes a solid source S
1
of aluminum (e.g. aluminum alloy particles) received in annular open wire basket B
1
to generate at the elevated coating temperature to be employed (e.g. 1975 degrees F. plus or minus 25 degrees F.) aluminum-bearing coating gas to form the diffusion aluminide coating on the interior surfaces of the cooling passage
11
of each turbine blade. An amount of a conventional halide activator (not shown), such as for example only AlF
3
, is used to initiate generation of the aluminum-bearing coating gas (e.g. AlF gas) from solid source S
1
at the elevated coating temperature to be employed. An argon (or other carrier gas) ring-shaped inlet conduit
32
is positioned in the lower chamber region
31
a
to discharge argon carrier gas that carries the generated aluminum-bearing coating gas through the pins
20
a
,
20
b
and the cooling passage
11
for discharge from the exit openings
11
e
at the trailing edge of the turbine blades. Each conduit
32
is connected to a conventional common source SA of argon (Ar) as shown in
FIG. 4
for the two topmost chambers
30
by individual piping
33
extending through the retort lid to a fitting (not shown) on each conduit
32
. Each piping
33
is connected to a common pressure regulator R and a respective individual flowmeter FM outside the retort to control argon pressure and flow rate. For sake of convenience, the argon source SA, pressure regulator R, flowmeter FM, and piping
33
are shown only for the two topmost coating chambers
30
in the retort
50
. Each conduit
32
of each of the other coating chambers
30
is connected in similar fashion to the common argon source SA and the common regulator R by its own piping (not shown).
The aluminum activity in the solid source S
1
(i.e. the activity of aluminum in the binary aluminum alloy particles S
1
) is controlled to form the desired type of diffusion aluminide coating on interior cooling passage surfaces at the elevated coating temperature. The aluminum activity in source S
1
is controlled by selection of a particular aluminum alloy particle composition effective to form the desired type of coating at the particular coating temperature involved. For purposes of illustration and not limitation, to form the above described outward type of diffusion aluminide coating on the interior cooling passage surfaces, the source S
1
can comprise Co-Al binary alloy particulates with the particulates comprising, for example, 50 weight % Co and balance Al. The particulates can have a particle size of 4 mm by 16 mm (mm is millimeters). The activator can comprise AlF
3
powder sprinkled beneath each basket B
1
. During transport through the cooling passage
11
by the argon carrier gas, the aluminum-bearing coating gas will form the outward diffusion aluminide coating on the interior cooling passage surfaces.
For purposes of illustration and not limitation, to internally coat up to 36 turbine blades in each coating chamber
30
to form the above outward aluminide diffusion coating in internal passage
11
, about 600 grams of AlF
3
powder activator can be sprinkled in each lower chamber region
31
a
beneath each basket B
1
and 60-75 pounds of Co-Al alloy particulates placed in each basket B
1
in each lower chamber region
31
a
. The outward diffusion aluminide coating so formed on internal passage walls has a microstructure comprising an inner diffusion zone and a single NiAl phase outer additive layer and has a total thickness in the range of 0.0005 to 0.003 inch for purposes of illustration.
The upper chamber region
31
b
includes a plurality (three shown) of solid sources S
2
of aluminum received in three respective annular open wire baskets B
2
on horizontal chamber wall W
1
with aluminum activity of sources S
2
controlled by the binary alloy composition to form the desired diffusion aluminide coating on the exterior surfaces of the airfoil region
10
a
and on platform surfaces
13
a
,
13
b
and
14
. A conventional halide activator (not shown), such as for example only, aluminum fluoride (AlF
3
) powder, is sprinkled beneath the baskets B
2
on wall W
1
in an amount to initiate generation of aluminum-bearing coating gas (e.g. AlF gas) from solid sources S
2
in upper chamber region
31
b
at the elevated coating temperature (e.g. 1975 degrees F. plus or minus 25 degrees F.) to be employed. For purposes of illustration and not limitation, to form the above outwardly grown, Pt-free nickel aluminide diffusion coating on the exterior surfaces of the airfoil region
10
a
and platform surfaces
13
a
,
13
b
and
14
, the sources S
2
can comprise a Cr-Al binary alloy particulates with the particles comprising for example, 70 weight % Cr and balance Al. The particulates can have a particle size of 4 mm by 16 mm. The activator can comprise AlF
3
powder. To coat 36 turbine blades in each coating chamber to form the above outwardly grown, Pt-free nickel aluminide diffusion coating, about 35 grams of AlF
3
is sprinkled beneath baskets B
2
on the wall W
1
of each coating chamber and 140 to 160 pounds of Cr-Al alloy particulates are placed in each basket B
2
in each upper chamber region
31
b
. The outwardly grown, Pt-free nickel aluminide diffusion coating includes an inner diffusion zone proximate the substrate and an outer, Pt-free additive single phase NiAl layer and typically has a total thickness in the range of 0.001 to 0.003 inch.
Pursuant to an embodiment of the invention, the upper chamber region
31
b
also includes solid sources S
3
of aluminum (e.g. binary aluminum alloy particles) disposed in the annular cofferdams
34
. The solid sources S
3
have a predetermined aluminum activity in the solid sources S
3
and are in close enough proximity to the damper pocket surfaces
12
a
,
12
b
to form thereon a diffusion aluminide coating
100
,
FIG. 5
, different from that formed on the surfaces of airfoil region
10
a
and platform surfaces
13
a
,
13
b
and
14
at the elevated coating temperature. The activity of aluminum in the sources S
3
is controlled by selection of a particular binary aluminum alloy particle composition effective to form the desired type of coating at the particular coating temperature involved.
In particular, the diffusion aluminide coating
100
formed only on damper pocket surfaces
12
a
,
12
b
includes an inner diffusion zone
100
a
and outer, additive Pt-bearing single phase (Ni,Pt)Al layer
100
b
,
FIG. 5
, having a concentration of platinum that is relatively higher at an outermost coating region (e.g. outer 20% of the additive layer thickness) than at an innermost coating region adjacent the diffusion zone
100
a
. This is in contrast to the above outwardly grown, Pt-free diffusion aluminide coating formed on the surfaces of airfoil region
10
a
and platform surfaces
13
a
,
13
b
and
14
to have an outer, additive single phase NiAl layer that is devoid of platinum. The coating
100
typically has a total thickness (layer
100
a
plus
100
b
) in the range of 0.001 to 0.003 inch, typically 0.002 inch.
For purposes of illustration and not limitation, the solid sources S
3
can comprise the same aluminum alloy particulates as used in beds S
2
(i.e. 70 weight % Cr and balance Al particles of 4 mm by 16 mm particle size) but positioned within a close enough distance D to the lowermost extent of damper pocket surface
12
a
delineated by the dashed line in
FIG. 1
to provide, at the elevated coating temperature, a higher aluminum species activity in the aluminum-bearing coating gas proximate the damper pocket surfaces
12
a
,
12
b
than is provided at the surfaces of the airfoil region
10
a
and upwardly facing surfaces of the platform region
10
g
by the solid sources S
2
as a result of their being more remotely spaced from the airfoil surfaces and platform surfaces.
For purposes of illustration only, to coat
36
turbine blades in each coating chamber
30
, 5 to 10 pounds of the Cr-Al alloy particulates (70 weight % Cr and balance Al) are placed in each cofferdam
34
with the upper surface of the source S
3
positioned within a close enough distance D,
FIG. 3
a
, of from ⅜ to ½ inch to the lowermost extent of damper pocket surface
12
a
defined by the dashed line L to form the above graded platinum concentration (Pt gradient) through the thickness of the outer additive layer
100
b
. On the other hand, the sources S
2
typically are spaced a distance of about 1.00 inch at their closest distance to the surfaces of the airfoil region
10
a
and platform surfaces
13
a
,
13
b
and
14
.
The solid sources S
3
alternately can comprise aluminum alloy particulate having a different composition from that of solid sources S
2
. The composition (i.e. activity) of the solid sources S
3
and their distance from the damper pocket surfaces
12
a
,
12
b
can be adjusted empirically so as to form the above graded platinum concentration through the thickness of the outer additive layer
100
b.
Gas phase aluminizing is effected by loading the coating chambers
30
having the turbine blades
10
and sources S
1
, S
2
, S
3
therein on the supports
40
a
on lifting post
40
and placing the loaded post in the retort
50
,
FIG. 4
, for heating to an elevated coating temperature (e.g. 1975 degrees F. plus or minus 25 degrees F.) in a heating furnace (not shown). The elevated coating temperature can be selected as desired in dependence upon the compositions of solid aluminum sources S
1
, S
2
, S
3
, the composition of the substrates being coated and coating gas composition. The coating temperature of 1975 degrees F. plus or minus 25 degrees F. is offered only for purposes of illustration with respect to coating the PWA 1484 nickel base superalloy turbine blades described above using the sources S
1
, S
2
, S
3
and activators described above.
During gas phase aluminizing in the coating chambers
30
in the retort
50
, the solid source S
1
in the lower chamber region
31
a
generates aluminum-bearing coating gas (e.g. AlF gas) which is carried by the carrier gas (e.g. argon) supplied by piping
33
and conduits
32
for flow through the internal cooling passage
11
of each turbine blade to form the outward diffusion aluminide coating on the interior cooling passage surfaces. The spent coating gas is discharged from the exit openings
11
e
at the trailing edge of each turbine blade and flows out of a space SP between the coating chamber
30
a
and loose lid
30
l
thereon into the retort
50
from which it is exhausted through exhaust pipe
52
.
The aluminum-bearing coating gas generated from sources S
2
, S
3
in the upper chamber region
31
b
forms the different diffusion aluminide coatings described above on the damper pocket surfaces
12
a
,
12
b
and the exterior surfaces of the airfoil region
10
a
and platform surfaces
13
a
,
13
b
and
14
. The coating gases from sources S
2
, S
3
are carried by the argon flow from gas discharge openings lie out of chamber
31
b
through space SP into the retort
50
from which it is exhausted via pipe
52
.
For forming the different internal and external aluminide diffusion coatings described in detail above on the PWA 1484 alloy turbine blades
10
, the coating chambers
30
and retort
50
initially are purged of air using argon flow. During gas phase aluminizing, a coating chamber argon flow rate typically can be 94 cfh (cubic feet per hour) plus or minus 6 cfh at 30 psi Ar plus or minus 2.5 psi. The retort argon flow is provided by the common argon source SA and the common pressure regulator R connected to piping
35
that extends through the retort lid behind the post
40
in
FIG. 4
to the bottom of the retort where the argon is discharged from the piping
35
. Piping
35
is connected to a flowmeter FM
1
downstream of the common regulator R to control argon pressure and flow rate. A retort argon flow rate typically can be 100 cfh Ar plus or minus 6 cfh at 12.5 psi plus or minus 2.5.
The elevated coating temperature can be 1975 degrees plus or minus 25 degrees F. and coating time can be 5 hours plus or minus 15 minutes. The elevated coating temperature is controlled by adjustment of the heating furnace temperature in which the retort
50
is received. The heating furnace can comprise a conventional gas fired type of furnace or an electrical resistance heated furnace. After coating time has elapsed, the retort is removed from the heating furnace and fan cooled to below 400 degrees F. while maintaining the argon atmosphere.
The coated turbine blades then can be removed from the coating chambers
30
, demasked to remove the M-1 and M-7 maskant layers, grit blasted with 240 mesh alumina at 15-20 psi with a 5 to 7 inch nozzle standoff distance, and washed as described above to clean the turbine blades. The coated turbine blades then can be subjected to a diffusion heat treatment (1975 degrees F. plus or minus 25 degrees F. for 4 hours), precipitation hardening heat treatment (1600 degrees F. plus or minus 25 degrees F. for 8 hours followed by fan cool from 1600 degrees F. to 1200 degrees F. at 10 degrees F./minute or faster to below 900 degrees F.) , abrasive blasting using 240 mesh alumina grit at 15 to 20 psi with a 5 to 7 grit blast nozzle standoff distance, then conventionally heat tint inspected to evaluate surface coverage by the diffusion aluminide coating, which heat tint inspection forms no part of the present invention.
FIG. 5
illustrates a typical diffusion aluminide coating
100
formed on damper pocket surfaces
12
a
,
12
b
as including inner diffusion zone
100
a
and outer, additive single phase (Ni,Pt)Al layer
100
b
having a concentration of platinum that is relatively higher at an outermost coating region (e.g. outer 20% of the additive layer thickness) than at an innermost coating region adjacent the diffusion zone
100
a
. For example, the outer additive (Ni,Pt)Al layer typically will have a Pt concentration of 25 to 45 weight % and possibly up to 60 weight % in the outer 20% of the outer additive layer
100
b
and an Al concentration of 20 to 30 weight % and possibly up to 35 weight % in the outer 20% of the outer additive layer
100
b
. In contrast, the outer, additive (Ni,Pt)Al layer typically will have a Pt concentration of 10 to 25 weight % in the inner 20% of the outer additive layer
100
b
adjacent the diffusion zone
100
a
and an Al concentration of 20 to 25 weight % in the inner 20% of the outer additive layer
100
b
adjacent the diffusion zone
100
a
. The black regions in the additive layer
100
b
in
FIG. 5
are oxide and/or grit particles present at the original substrate surface.
The Table below illustrates contents of elements at selected individual areas of the outer, additive single phase (Ni,Pt)Al layer
100
b
formed on damper pocket surfaces of PWA 1484 turbine blades. The compositions were measured at different depths (in microns) from the outermost surface of the outer additive layer
100
b
toward the diffusion zone by energy dispersive X-ray spectroscopy. The samples were measured before the diffusion and precipitation hardening heat treatments. The area designations I
2
, I
3
indicate samples coated in the inner basket of FIG.
3
. Microns is the depth from the outermost surface of the additive layer
100
b
.
TABLE I
|
|
ELEMENTAL COMPOSITION
|
(WEIGHT %)
|
SAMPLE/AREA/DISTANCE
|
FROM SURFACE, MICRONS
Al
Cr
Co
Ni
Pt
|
|
1-I2-2
28.7
4.3
1.9
31.8
33.4
|
5
30.5
3.2
2.7
29.3
34.3
|
8
27.5
5.8
2.1
23.8
40.7
|
11
31.8
1.7
4.9
45.5
16.1
|
14
31.1
1.3
6.9
47.3
13.4
|
17
24.5
12.3
7.9
48.2
7.1
|
20
19.1
14.4
8.9
50.0
7.6
|
23
8.7
30.5
6.6
50.7
3.8
|
1-I3-2
26.9
2.1
1.0
28.4
41.6
|
5
26.7
2.2
1.8
26.3
43.1
|
8
28.5
1.7
2.5
34.1
33.2
|
11
27.1
1.6
3.3
35.4
32.6
|
14
24.1
2.7
5.3
41.3
26.6
|
17
16.6
16.9
4.8
36.5
25.1
|
20
11.3
27.5
8.7
34.9
17.7
|
23
6.1
41.9
11.6
29.8
10.6
|
|
The Table reveals a distinct Pt gradient in the outer, additive layer
100
b
from the outermost surface thereof toward the diffusion zone
100
a
in the as-aluminized condition. Gradients of Al, Cr, Co and Ni are also evident.
The present invention is advantageous to provide an outwardly grown platinum modified diffusion aluminide coating having a single phase additive outer layer with a Pt content that is relatively higher at an outermost coating region than at an innermost coating region adjacent a diffusion zone to impart oxidation and hot corrosion resistance thereto and improved ductility as compared to conventional two phase platinum modified diffusion coatings.
Although the invention has been described in detail above with respect to forming the outwardly grown platinum modified diffusion aluminide coating having the outer, graded Pt single phase additive outer layer,
FIG. 5
, only on the damper pocket surfaces
12
a
,
12
b
, the invention is not so limited.
Such outwardly grown, graded platinum modified diffusion aluminide coating can be formed at other regions of turbine blades and vanes (referred to as airfoils). For example, some or all of the exterior surfaces of the airfoil region
10
a
and/or platform region
10
g
can be coated pursuant to the invention to form the outwardly grown, graded platinum modified diffusion aluminide coating,
FIG. 5
, thereon. To coat the entire airfoil region
10
a
, the airfoil region would be platinum electroplated as described above and the distance of the airfoil region to the aluminum sources S
2
would be reduced to form the outwardly grown, graded platinum modified diffusion aluminide coating of
FIG. 5
thereon.
Although the invention has been described in detail above with respect to certain embodiments, those skilled in the art will appreciate that modifications, changes and the like can be made therein without departing from the spirit and scope of the invention as set forth in the appended claims.
Claims
- 1. A method of forming a platinum modified diffusion aluminide coating on a substrate, comprisingdepositing a layer comprising platinum on the substrate, disposing the substrate in a coating chamber having a solid source comprising aluminum therein, wherein said substrate and said solid source are disposed so proximate one another as to form on said substrate at an elevated coating temperature an outwardly grown diffusion aluminide coating including an inner diffusion zone and additive layer on said inner diffusion zone, said additive layer having a single phase with a concentration of platinum that is relatively higher at an outermost region than at an innermost region thereof adjacent said diffusion zone, and heating said substrate and said solid source to said coating temperature to form said diffusion aluminide coating on said substrate.
- 2. The method of claim 1 wherein said coating is formed without a prediffusion of said layer before said heating.
- 3. The method of claim 1 wherein said coating is formed with a prediffusion of said layer at least partially into said substrate before said heating.
- 4. The method of claim 1 wherein said solid source of aluminum comprises an alloy of aluminum wit another metal and is positioned close enough to said substrate t form said coating at said coating temperature.
- 5. The method of claim 4 wherein said solid source comprises a binary aluminum alloy particulate bed disposed in said coating chamber.
- 6. The method of claim 4 including providing a halide activator in said coating chamber.
- 7. The method of claim 1 wherein said additive layer comprises (Ni,Pt)Al single phase.
- 8. A substrate comprising a nickel base superalloy having an outwardly grown diffusion aluminide coating formed on at least a surface area thereof by the method of claim 1 to include said inner diffusion zone and said additive layer having in said single phase said concentration of platinum that is relatively higher at said outermost region than at said innermost region thereof adjacent said diffusion zone.
- 9. A method of forming different diffusion aluminide coatings on a substrate, comprisingdepositing a layer comprising Pt on a first surface area of the substrate and not on a second surface area of the substrate, positioning the substrate in a coating chamber with said first surface area thereof relatively proximate to a first solid source comprising aluminum and with said second surface area relatively remote from said first solid source and relatively proximate to a second solid source comprising aluminum, and gas phase aluminizing the substrate by heating the substrate, first solid source, and second solid source to an elevated coating temperature to form on said first surface area a platinum-bearing diffusion aluminide coating having an inner diffusion zone and additive layer on said inner diffusion zone, said additive layer comprising a single phase having a concentration of platinum that is relatively higher at an outermost region than at an innermost region thereof adjacent said diffusion zone, and to form a platinum-free diffusion aluminide coating on said second surface area of said substrate.
- 10. The method of claim 9 wherein said gas phase aluminizing is conducted without a prediffusion of said layer.
- 11. The method of claim 9 wherein said gas phase aluminizing is conducted with a prediffusion of said layer at least partially into said substrate.
- 12. The method of claim 9 wherein said first solid source comprises an alloy of aluminum with another metal and is positioned close enough to said first surface area to form said platinum-bearing diffusion aluminide coating at said coating temperature.
- 13. The method of claim 12 wherein said first solid source comprises a binary aluminum alloy particulate bed disposed in said coating chamber proximate said first surface area.
- 14. The method of claim 13 wherein said second solid source comprises a binary aluminum alloy particulate bed disposed in said coating chamber relatively remote from said first surface area and relatively proximate said second surface area.
- 15. The method of claim 9 including providing a halide activator in said coating chamber.
- 16. The method of claim 9 wherein said different diffusion aluminide comprises an inner diffusion zone and outer additive NiAl layer free of platinum.
- 17. The method of claim 9 wherein said first surface area comprises surfaces forming a damper pocket of a gas turbine engine blade.
- 18. The method of claim 17 wherein said second surface area comprises an airfoil of a gas turbine engine blade.
- 19. A substrate comprising a nickel base superalloy coated by the method of claim 9 to have said platinum-bearing diffusion aluminide coating formed on said first surface area and said platinum-free diffusion aluminide coating formed on said second surface area.
- 20. The method of claim 1 wherein an outer 20% of the thickness of said additive layer has a platinum concentration of 25 weight % to 60 weight & Pt.
- 21. The method of claim 20 wherein said other 20% of the thickness of said additive layer has a platinum concentration of 25 weight % to 45 weight % Pt.
US Referenced Citations (20)