Preparation of components having a partial platinum coating thereon

A curved component such as a turbine airfoil, shroud, or combustor centerbody is prepared with a platinum or a platinum-aluminide protective coating over only a portion of the surface thereof. The coating may serve as an environmental coating, or as a bond coat of a thermal barrier coating system. The partial coverage is achieved by depositing platinum only over a portion of the surface of the component, typically including the concave portion in the case of an airfoil, optionally depositing an aluminum layer, and optionally interdiffusing the platinum and aluminum layers.

FIELD OF THE INVENTION
 This invention relates to protective coatings on articles, and, more
 particularly, to platinum and platinum-aluminide coatings on aircraft
 components such as airfoils.
 BACKGROUND OF THE INVENTION
 In an aircraft gas turbine (jet) engine, air is drawn into the front of the
 engine, compressed by a shaft-mounted compressor, and mixed with fuel. The
 mixture is combusted, and the resulting hot combustion gases are passed
 through a turbine mounted on the same shaft. The flow of gas turns the
 turbine by contacting an airfoil portion of the turbine blade, which turns
 the shaft and provides power to the compressor. The hot exhaust gases flow
 from the back of the engine, driving it and the aircraft forwardly.
 The hotter the turbine gases, the more efficient is the operation of the
 jet engine. There is thus an incentive to raise the turbine operating
 temperature. However, the maximum temperature of the turbine gases is
 normally limited by the materials used to fabricate the turbine vanes and
 turbine blades of the turbine. In current engines, the turbine vanes and
 blades are made of nickel-based or cobalt-based superalloys that can
 operate at temperatures of up to 1900-2100.degree. F.
 Many approaches have been used to increase the operating temperature limits
 and operating lives of the airfoils of the turbine blades and vanes. The
 compositions and processing of the materials themselves have been
 improved. The articles may be formed as oriented single crystals to take
 advantage of superior properties observed in certain crystallographic
 directions. Physical cooling techniques are used. In one widely used
 approach, internal cooling channels are provided within the components,
 and cooler air is forced through the channels during engine operation.
 In another approach, a protective layer in the form of an environmental
 coating or a ceramic/metal thermal barrier coating (TBC) system is applied
 to the airfoil of the turbine blade or turbine vane component, which acts
 as a substrate. One of the currently known protective layers is a
 diffusion aluminide layer. A diffusion aluminide protective layer may be
 formed, for example, by electrodepositing a layer of platinum onto the
 surface to be protected, depositing a layer of aluminum over the platinum
 layer, and interdiffusing the two deposited layers.
 This protective layer, with no overlying ceramic layer, is useful in
 intermediate-temperature applications. For higher temperature
 applications, a ceramic thermal barrier coating layer may be applied
 overlying the protective layer, to form a thermal barrier coating system.
 The ceramic thermal barrier coating layer insulates the component from the
 exhaust gas, permitting the exhaust gas to be hotter than would otherwise
 be possible with the particular material and fabrication process of the
 substrate.
 Platinum is expensive, and therefore care is taken not to deposit more
 platinum than necessary. However, in existing practice excessive platinum
 is still used. There is a need for an improved approach to the preparation
 of airfoils with a platinum aluminide protective layer, which reduces the
 use of platinum. The present invention fulfills this need, and further
 provides related advantages.
 BRIEF SUMMARY OF THE INVENTION
 The present approach provides a technique for depositing platinum at
 selected locations of a component such as a turbine blade or turbine vane
 airfoil, so that the platinum aluminide protective layer is deposited only
 where needed. Expensive platinum metal is conserved. This technique allows
 full utilization of the volume within the electrodeposition tank, so that
 there is no reduction in part throughput. High efficiency and cost
 reduction in the production operation are thereby achieved. The approach
 is compatible with further processing procedures.
 A method for preparing an article comprises the steps of providing an
 article precursor having a curved surface with a first portion and a
 second portion, and positioning a deposition anode in facing relationship
 to the first portion of the curved surface, so that there is no anode in
 facing relationship to the second portion of the curved surface. The
 method further includes electrodepositing a platinum layer from solution
 onto the article precursor using the deposition anode, with deposition
 occurring primarily on the first portion of the curved surface.
 In one implementation, two airfoil precursors are provided, with each
 airfoil precursor having a convex suction side and a concave pressure
 side. The two airfoil precursors are positioned with their convex sides in
 a facing relationship, and two deposition anodes are respectively
 positioned in facing relationship to the concave sides of the airfoil
 precursors. A platinum layer is electrodeposited from solution onto the
 two airfoil precursors using the two deposition anodes, with deposition
 occurring primarily on the concave sides and some deposition on the
 leading edge of the airfoil. Other configurations for electrodeposition
 onto multiple components may also be utilized.
 The article precursor is a metallic article having the shape and
 substantially the dimensions of the final part, optionally with small
 dimensional reductions to account for the layers that are deposited in the
 processing. The article precursor is preferably a turbine blade airfoil or
 a turbine vane airfoil, but other articles such as a shroud or a combustor
 center body may be processed using the present approach. The preparation
 of the article may include depositing an aluminum layer overlying the
 platinum deposited on the precursor, and interdiffusing the platinum layer
 and the aluminum layer. The resulting coating serves as an environmental
 protection layer. To form a thermal barrier coating for even
 higher-temperature applications, a ceramic layer is deposited overlying
 the article precursor.
 Other features and advantages of the present invention will be apparent
 from the following more detailed description of the preferred embodiment,
 taken in conjunction with the accompanying drawings, which illustrate, by
 way of example, the principles of the invention. The scope of the
 invention is not, however, limited to this preferred embodiment.

DETAILED DESCRIPTION OF THE INVENTION
 FIG. 1 depicts a component article of a gas turbine engine such as a
 turbine blade or turbine vane, and in this illustration a turbine blade
 20. The turbine blade 20 includes an airfoil 22 against which the flow of
 hot exhaust gas is directed. (The turbine vane has a similar appearance in
 respect to the pertinent airfoil portion.) The turbine blade 20 is mounted
 to a turbine disk (not shown) by a dovetail 24 which extends downwardly
 from the airfoil 22 and engages a slot on the turbine disk. A platform 26
 extends longitudinally outwardly from the area where the airfoil 22 meets
 the dovetail 24. In some articles, a number of cooling channels extend
 through the interior of the airfoil 22, ending in openings 28 in the
 surface of the airfoil 22. A flow of cooling air is directed through the
 cooling channels, to reduce the temperature of the airfoil 22.
 As illustrated, the airfoil 22 portion of the turbine blade 20 is curved in
 an airfoil shape. There is a concavely curved side, termed the concave
 side 30 (also sometimes known as the "pressure" side of the airfoil), and
 a convexly curved side, termed the convex side 32 (also sometimes known as
 the "suction" side of the airfoil). A curved leading edge 31 separates the
 concave side 30 from the convex side 32 along one longitudinal margin of
 the airfoil. A more sharply defined trailing edge 33 separates the concave
 side 30 from the convex side 32 along the other longitudinal margin of the
 airfoil 22. The airfoil 22 terminates in a tip 42 remote from the dovetail
 24. In service, the pressurized hot combustion gas from the combustors is
 directed against the concave side 30. This concave side 30 therefore
 requires more protection against the incident hot combustion gas than does
 the convex side 32. To provide this protection, the concave side 30 is
 coated with a protective coating, either in the form of an environmental
 coating or a thermal barrier coating.
 FIG. 2 schematically illustrates the airfoil 22 portion, after the
 protective coating is applied. On the concave side 30, there is a platinum
 layer 34 adjacent to a substrate 36 made of a base metal. The base metal
 forming the substrate 36 is typically a nickel-base superalloy such as
 Rene' N5. The nickel-base superalloy has more nickel than another element,
 and is usually strengthened by gamma-prime precipitation. Rene' N5 has a
 nominal composition in weight percent of about 7.5 percent cobalt, about 7
 percent chromium, about 6.2 percent aluminum, about 6.5 percent tantalum,
 about 5 percent tungsten, about 1.5 percent molybdenum, about 3 percent
 rhenium, about 0.05 percent carbon, about 0.004 percent boron, about 0.15
 percent hafnium, up to about 0.01 percent yttrium, balance nickel and
 incidental impurities. An aluminum layer 38 overlies the platinum layer
 34. In practice, the platinum layer 34 and the aluminum layer 38 are
 deposited as separate layers, but are interdiffused with each other and
 the substrate 36 so that they merge partially or fully into a single
 platinum-aluminide layer 34/38. In the illustration, they are shown as the
 originally deposited layers 34 and 38 to facilitate the subsequent
 discussion of the deposition processes. On the convex side 32, there is
 only the aluminum coating 38, or there may be no coating at all. The
 aluminum coating 38 is typically interdiffused into the substrate 36, but
 is illustrated as a separate layer for purposes of facilitating the
 subsequent discussion.
 If no further layer is deposited overlying the aluminum layer 38, the
 platinum aluminide of the layers 34/38 is termed an "environmental
 coating". The environmental coating may be satisfactory for protecting
 turbine blades and vanes that are not exposed to the highest temperatures,
 such as the low-pressure turbine blades and vanes, or other components
 that are not exposed to high temperatures such as shrouds and center
 bodies. For those portions of the turbine blades and vanes that are
 exposed to the highest temperatures, such as the concave sides 30 of the
 high-pressure turbine blades and vanes, an additional ceramic layer 40 is
 deposited overlying the aluminum layer 38 (or interdiffused layers 34/38)
 to provide additional thermal protection. In this structure, the layers
 34/38 are termed a "bond coat". The bond coat 34/38 and the ceramic layer
 40 are together termed a "thermal barrier coating system".
 The ceramic 40, where present, is preferably from about 0.004 inches to
 about 0.030 inches thick, most preferably from about 0.005 to about 0.015
 inches thick. The ceramic layer 40 is operable in thicknesses outside this
 range, but is less desirable. Lesser thicknesses of the ceramic layer 40
 tend to give insufficient insulation to the substrate. Greater thicknesses
 of the ceramic layer 40 tend to add unnecessary weight to the article. The
 ceramic layer 40 is preferably yttria-(partially) stabilized zirconia,
 which is a zirconium oxide-base ceramic material containing from about 4
 to about 8 weight percent of yttrium oxide. Other operable stabilizing
 oxides and ceramic base materials may be used as well.
 FIG. 3 depicts a preferred approach for practicing a first embodiment of
 the invention, and FIG. 4 illustrates an apparatus used in the platinum
 electrodeposition. An article precursor 60 is provided, numeral 50. In the
 illustration of FIG. 4, the article precursor 60 is a turbine blade 22, as
 illustrated in FIG. 1. The article precursor constitutes the substrate 36,
 and may be formed of any operable material such as the Rene' N5 material
 discussed above. The article precursor 60, here the turbine blade 22, is
 positioned in an electrodeposition apparatus, numeral 52. The
 electrodeposition apparatus includes an electrodeposition tank 64, an
 anode 66, and a voltage source 68 that creates an electrical potential
 between the anode 66 and the substrate 36. The anode 66, illustrated as a
 flat plate generally coextensive with the concave side 30 in this case, is
 disposed in a facing relationship to the concave side 30 of the article
 precursor 60 serving as the substrate 36. The anode 66 is typically made
 of an electrically conductive material such as platinum-clad or
 platinum-plated titanium or niobium, and is positioned in close proximity
 to the concave side 30, typically about 1/8 inch to 1/2 inch from the
 substrate 36.
 FIG. 5 illustrates another embodiment. In this case, two airfoil precursors
 60 and 62 are mounted in the electrodeposition tank 64. The two airfoil
 precursors 60 and 62 are positioned with their respective convex sides 32
 in a facing relationship to each other, which may be viewed as a
 "back-to-back" relationship. One of the anodes 66 is positioned in facing
 relationship to the concave side 30 of each of the two airfoil precursors
 60 and 62. The structure of the embodiment of FIG. 5 is otherwise similar
 to that of FIG. 4, and the above description of the embodiment of FIG. 4
 is incorporated here.
 FIG. 6 illustrates yet another embodiment, where two airfoils 60 and 62 are
 positioned linearly in a configuration with their trailing edges 33
 adjacent and the concave sides 30 facing in opposite directions. The
 anodes 66 are placed in facing relationship to the concave sides 30. In
 this case, the anodes 66 are curved to approximate the curvature of the
 concave sides 30. The structure of the embodiment of FIG. 6 is otherwise
 similar to that of FIG. 4, and the above description of the embodiment of
 FIG. 4 is incorporated here.
 The embodiments of FIGS. 4-6 may be extended to include additional articles
 or sets of articles in the single electrodeposition tank 64.
 In all of the embodiments, the voltage/current source 68 is connected
 between each of the airfoil precursors 60 and 62, and its respective anode
 66. Equivalently, a single voltage/current source 68 may be used.
 The platinum layer 34 is deposited, numeral 54. The deposition is
 accomplished by placing a platinum-containing solution into the tank 64
 and depositing platinum from the solution onto the airfoil precursors 60
 and 62. An operable platinum-containing aqueous solution is
 Pt(NH.sub.3).sub.4 HPO.sub.4 having a concentration of about 4-20 grams
 per liter of platinum, and the voltage/current source 68 is operated at
 about 1/2-10 amperes per square foot of facing article surface. The
 platinum layer 34 about 5 micrometers thick is deposited in 1-4 hours at a
 temperature of 190-200.degree. F.
 Because of the positioning of the airfoil precursor 60 and its anode 66
 (and, wherein present, the airfoil precursor 62 and its respective anode),
 the great majority of the platinum is deposited on the concave side 30 of
 the airfoil precursors 60 (and 62). Some platinum is also deposited on the
 leading edge 31, which is acceptable. The convex side 32 is partially
 screened from deposition. An incidental amount of platinum which may be
 deposited on the convex sides 32 is not damaging and consumes very little
 platinum. In the embodiment of FIG. 5, the two facing convex sides 32 of
 the airfoils also aid in screening the deposition of platinum.
 Optionally, the platinum layer 34 may be interdiffused into and with the
 substrate 36 after the electrodeposition is complete but as part of step
 54, prior to the next step. If this pre-aluminiding interdiffusion step is
 performed, interdiffusion is accomplished by heating to a temperature of
 from about 1500.degree. F. to about 2210.degree. F., and for a time of
 from about 2 minutes to about 4 hours.
 The aluminum layer 38, if used, is deposited overlying the platinum layer
 34 (or interdiffused platinum/substrate region), numeral 56. The aluminum
 layer 38 is deposited by any operable approach, with vapor deposition
 preferred. In that approach, a hydrogen halide gas, such as hydrogen
 chloride, is contacted with aluminum metal or an aluminum alloy to form
 the corresponding aluminum halide gas. Other elements may be doped into
 the aluminum layer from a corresponding gas, if desired. The aluminum
 halide gas contacts the airfoil precursor 60, depositing the aluminum
 thereon. The deposition occurs at elevated temperature such as from about
 1825.degree. F. to about 1975.degree. F. so that the deposited aluminum
 atoms interdiffuse into the platinum layer 34 (or interdiffused
 platinum/substrate region) during a 4 to 20 hour cycle. This technique
 allows alloying elements to be deposited into the aluminum layer 38 if
 desired, from the halide gas. In this process, the aluminum layer 38 is
 also deposited on the convex side 32. Such deposition of aluminum on the
 convex side 32 is not harmful, and in fact forms a beneficial diffusion
 aluminide layer on the convex side 32 which resists oxidation in this less
 demanding region of the article. Aluminum is inexpensive.
 A significant amount of interdiffusion of the platinum layer 34, the
 aluminum layer 38, and the substrate 36 is achieved during the aluminum
 deposition step 56. Additional interdiffusion may be accomplished if
 desired by maintaining the structure at elevated temperature after the
 flow of halide gas is discontinued, numeral 58.
 The interdiffused platinum and aluminum layers form a protective layer that
 inhibits oxidation and corrosion damage to the airfoil 22, during exposure
 at intermediate temperatures. This protective layer is an environmental
 layer.
 If further protection is required because the airfoil is to be used at very
 high temperatures, the ceramic layer 40 is deposited, numeral 59. The
 ceramic layer 40 is typically applied only over the concave side 30 and
 thence over the interdiffused platinum-aluminide coating, but it could be
 applied over the convex side 32 if desired. The ceramic layer 40 may be
 applied by any operable technique, with electron beam physical vapor
 deposition (EB-PVD) being preferred for the preferred yttria-stabilized
 zirconia coating. The EB-PVD processing may be preceded and/or followed by
 high-temperature processes that may affect the distribution of elements in
 the bond coat. The EB-PVD process itself is typically conducted at
 elevated temperatures.
 The preceding discussion focused on deposition on a gas turbine airfoil.
 Other elements may be similarly processed, such as a gas turbine shroud 80
 illustrated in FIG. 7 or a gas turbine center body 82, illustrated in FIG.
 8.
 Although a particular embodiment of the invention has been described in
 detail for purposes of illustration, various modifications and
 enhancements may be made without departing from the spirit and scope of
 the invention. Accordingly, the invention is not to be limited except as
 by the appended claims.