Patent Application
20180297156
The present invention generally relates to protective coatings on components, and, more particularly, to NiCoCrAlY and platinum-group metal aluminide coatings on gas turbine components having airfoils.
In gas turbine engines, 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 hotter the turbine gases, the more efficient the operation of the engine. Thus, there is 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.
A protective layer is applied to the airfoil of the turbine blade or turbine vane component, which acts as a substrate. Among the currently known diffusional protective layers are aluminide and platinum aluminide layers. The protective layer protects the substrate against environmental damage from the hot, highly corrosive combustion gases. This protective coating is approximately 38 μm to 76 μm (i.e., approximately 0.0015 to 0.0030 inch) thick, and provides a degree of protection against marine hot corrosion. Approximately half the thickness of the diffusion coating is part of the original blade thickness & the diffusion platinum aluminide coatings are effective in maintaining the cooling holes open after the coating process. Even with the use of these protective techniques, there remain problems to overcome in certain operating service conditions, particularly within marine turbine engines that are exposed to harsh conditions related to the salinity of the operating environments.
A more effective alternative coating, which is used widely in marine gas turbine applications, is approx. 254 μm (i.e., about 0.010 inch) with an “overlay” MCrAlX coating having a thickness range of about 177.8 μm to about 330 μm (i.e., about 0.07 inch to about 0.013 inch), where M is (Co and/or Ni), X is a reactive element such as Y, Hf, and the coating has a chromium concentration of 20% to 28%. The overlay coatings are typically deposited by a plasma spray process, and the composition of the coating can be tailored to mitigate marine hot corrosion.
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. Advanced turbine blades are cooled by cooling air from compressor discharge to reduce the blade temperature and enable a higher gas temperature for increased efficiency. Thus, it is important to keep the cooling holes open to prevent overheating of blades.
For gas turbines operating in marine environment, it is necessary for the coatings to resist corrosive attack from environmental corrodents. Deposits containing sodium sulfate have been recognized to be particularly corrosive to marine airfoils.
Cobalt based CoCrAlHf coatings with chromium content in the range of 20 to 25%, aluminum in the range of 9 to 11% have been utilized successfully to resist marine corrosion. The coatings are thick (relative to the size of cooling holes of advanced turbine blades), typically in the range of 177.8 μm to about 356 μm (i.e., about 0.07 inch to about 0.014 inch) and are deposited by a thermal spray process. Such coatings are deposited on new blades prior to drilling of holes, since the coatings can partially or completely close the holes during their application.
When the field returned blades are ready for repair, any and all the remaining CoCrAlHf coating is stripped off with an appropriate acid. Some manufactures require chemical cleaning with strong acid or alkali mixtures to remove field service debris and/or hot corrosion products prior to stripping. Others allow grit blasting to accomplish the same ends. Complex cooling passages in blades can accumulate dust or other debris in service, which may have to be removed with hot caustic at elevated pressure in an autoclave.
Full removal of coatings is universally accomplished by selective dissolution of the coating phase(s) by various simple or complex mixture of acids. Most procedures depend on selective attack of beta (NiAl or CoAl) phases. If coatings are depleted of beta phases, selective coating dissolution can be difficult or impossible, and residual coatings must then be removed by physical methods (e.g., belt grinding).
Re-coating of repaired blades with cooling holes is typically accomplished a diffusion aluminide or platinum aluminide process, (described above) which keeps the cooling holes open. Platinum aluminide is a diffusion coating, the composition and properties of the platinum aluminide coating depend, in part, on the chemistry of the underlying alloy or coating. It is necessary to remove all the original CoCrAlHf coatings, since platinum aluminide coating of any underlying CoCrAlHf coating will result in a brittle cobalt platinum aluminide, which is undesirable. Since the diffusion platinum aluminide coatings are relatively thin and have a composition that is rich in nickel, but deficient in chromium and cobalt, the marine hot corrosion resistance of platinum aluminide coating is inferior to that provided by the thicker CoCrAlHf coatings.
Thus, an improved method of repair such coatings is generally needed, particularly with gas turbine components used in marine environments.
Objects and advantages of the invention will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the invention.
A method is generally provided of forming a coating system on a surface of a superalloy component having film holes defined therein. In one embodiment, the method includes applying NiCoCrAlY on the surface of the superalloy component to form a NiCoCrAlY layer while keeping the film holes open (e.g., wherein the NiCoCrAlY layer has a chromium content that is higher than the superalloy component), then heating the NiCoCrAlY layer to a treatment temperature of about 900° C. to about 1200° C., then forming a platinum-group metal layer on the NiCoCrAlY layer, and then forming an aluminide coating over platinum-group metal layer.
In one particular embodiment, the NiCoCrAlY is applied onto an existing coating system on the surface of the superalloy component, wherein the existing coating system is a Co-based coating system that is substantially free from Ni.
Other features and aspects of the present invention are discussed in greater detail below.
The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which:
Repeat use of reference characters in the present specification and drawings is intended to represent the same or analogous features or elements of the present invention.
Reference now will be made to the embodiments of the invention, one or more examples of which are set forth below. Each example is provided by way of an explanation of the invention, not as a limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as one embodiment can be used on another embodiment to yield still a further embodiment. Thus, it is intended that the present invention cover such modifications and variations as come within the scope of the appended claims and their equivalents. It is to be understood by one of ordinary skill in the art that the present discussion is a description of exemplary embodiments only, and is not intended as limiting the broader aspects of the present invention, which broader aspects are embodied exemplary constructions.
Chemical elements are discussed in the present disclosure using their common chemical abbreviation, such as commonly found on a periodic table of elements. For example, hydrogen is represented by its common chemical abbreviation H; helium is represented by its common chemical abbreviation He; and so forth.
A coating system is generally provided for hot gas path components (e.g., airfoils) of turbine engines, along with methods of its formation. In particular, the coating system is useful on a superalloy component of a marine turbine engine, which is exposed to particularly corrosive operating environments. The methods and coating system is particularly useful during a repair of a component that has been used and damaged during use, either through an impact event or corrosion. Embodiments of the methods described herein leads to enhancement of the corrosion resistance of the existing coating by incorporating chromium and cobalt while keeping the cooling holes open. In one embodiment, the methods described herein allows for the retention of an existing marine Co-based coating system (e.g., a CoCrAlHf type coating that is substantially free from Ni) on surface of the used airfoil (or on the surface of a new airfoil), by adding the NiCoCrAlY materials. The resulting coating, with the platinum aluminide coatings thereon, may be thicker, less brittle, and may have a higher quantity of aluminum, leading to more resistance to oxidation than platinum aluminide on a cobalt-based material, due to the relatively lower diffusion of aluminum and platinum in a cobalt based material.
The coating system has a multiple layer construction chemistry, which includes at least a NiCoCrAlY layer, which may have a chromium (Cr) content that is higher than the underlying superalloy. The coating system is, in one particular embodiment, formed from a NiCoCrAlY layer and a platinum-group metal aluminide coating through a diffusion coating process, resulting in a coating system that includes NiCoCrAlY, Pt, and Al. The NiCoCrAlY layer has a chromium content that is higher than the superalloy component, both in its deposition composition and its composition following treatment.
The coating system can reduce the susceptibility of gas turbine components to property degradation such as low-cycle fatigue failures, while retaining the benefits associated with protective coatings that are applied to the components. The present approach may be accomplished as part of the normal production operation, without major modifications. Additionally, the use of any additional bond coating or other layer between the surface of the component and the coating system and/or within the construction of the coating system (e.g., between the NiCoCrAlY layer and the platinum-group metal aluminide coating) can be avoided in particular embodiments. That is, in this embodiment, the NiCoCrAlY layer is directly on the surface of the component, and/or the platinum-group metal aluminide coating is directly on the NiCoCrAlY layer to form the coating system. When a thermal barrier coating is present, the coating system is free from a bond coating between the NiCoCrAlY layer (e.g., the platinum-group metal aluminide coating of the coating system) and the thermal barrier coating (e.g., the thermal barrier coating is directly on the platinum-group metal aluminide coating of the coating system).
Referring to the drawings,
High pressure gas turbine blades, such as shown in
When the nozzle segment 10 is assembled with other nozzle segments to form a nozzle assembly, the respective inner and outer platforms of the segments form continuous inner and outer bands between which the vanes 12 are circumferentially spaced and radially extend. Construction of a nozzle assembly with individual nozzle segments is often expedient due to the complexities of the cooling schemes typically employed. The nozzle segment 10 depicted in
In one embodiment, the airfoil 6 of the turbine blade 5 of
The airfoil 6 of the turbine blade 5 of
Although described above and in
Referring to
Generally, the NiCoCrAlY layer has a composition of (by weight) that is based on nickel (Ni), which provides a good surface for subsequent PtAl deposition. Cobalt (Co) is present in the NiCoCrAlY layer to interact and bond with the Co remaining on the surface from the previous coating, which may still be present on the surface or may have diffused into the surface. In one particular embodiment, the NiCoCrAlY layer has a composition at deposition (i.e., prior to heat treatment and prior to forming additional layers thereon) that includes, by weight percent, about 16% to about 20% Cr (e.g., about 17% to about 19% Cr), about 9% to about 11% Al (e.g., about 9.5% to about 10.5% Al), about 19% to about 24% Co (e.g., about 21% to about 23% Co), about 0.05% to about 0.2% Y (e.g., about 0.07% to about 0.15% Y), up to about 0.5% Hf (e.g., about 0.05% to about 0.3% Hf, such as about 0.05% to about 0.2% Hf), up to about 1% Si (e.g., about 0.5% to about 0.9% Si, such as about 0.6% to about 0.8% Si), and the balance Ni.
Following deposition, the NiCoCrAlY layer 20 is heated to bond the NiCoCrAlY layer 20 onto the surface 13 of the component 5. In one embodiment, a portion of the NiCoCrAlY layer 20 diffuses into the component 5 to form a diffused portion 44. In one embodiment, the NiCoCrAlY layer 20 is heated to a treatment temperature of about 900° C. to about 1200° C. (e.g., about 1000° C. to about 1100° C.). The NiCoCrAlY layer 20 may be heated to the treatment temperature for a time sufficient to bond the NiCoCrAlY layer 20 onto the surface 13, such as for about 30 minutes to about 5 hours.
As stated, the NiCoCrAlY layer 20 may diffuse into the component 5 due to the heat treatment to form the diffused portion 44. In one embodiment, about 30% or less of the deposited thickness of the NiCoCrAlY layer 20 diffuses into the surface 13 of the component 5, such as about 5% to about 25% of the deposited thickness may diffuse into the component 5. Following heat treatment, the NiCoCrAlY layer 20 has a thickness extending from the surface 13 that is about 10 μm to about 100 μm (e.g., about 25 μm to about 50 μm). By keeping the NiCoCrAlY layer 20 relatively thin (i.e., less than 100 μm), any film holes defined within the surface can remain open even without the use of mask or other deposition blocking method.
Examples of deposition processes which can be used to deposit NiCoCrAlY layer without closing cooling holes, and resulting in a smooth coating (e.g., having a surface roughness of about 100 μm or less) include ion plasma deposition process, composite plating process, cold spray process, high velocity air plasma spray process.
Following heat treatment, a platinum-group metal layer 30 and an aluminide coating 34 may be formed onto the NiCoCrAlY layer 20, as shown in
In most embodiments, a suitable thickness for a platinum-group metal layer 30 is about 1 μm to about 10 μm (e.g., about 3 μm to about 7 μm). In the embodiment shown, the platinum-group metal layer 30 is formed directly on the NiCoCrAlY layer 20 due to this relatively thin nature of the platinum-group metal layer. As such, no other layer (e.g., a bond coating) is positioned between the NiCoCrAlY layer 20 and the platinum-group metal layer 30.
The platinum-group metal layer 30 can be formed via any suitable process. For example, the platinum-group metal layer 30 is, in one particular embodiment, deposited by an electrodeposition process as (e.g., electroplating), although sputtering, brush plating, etc. could alternatively be used. Plating can be performed at room temperature (e.g., about 20° C. to about 25° C.). In one embodiment, the electrodeposition process is accomplished by placing a platinum-group metal-containing solution (e.g., platinum-containing solution) into a deposition tank and depositing platinum-group metal from the solution onto the NiCoCrAlY layer 20. For example, when depositing platinum, the platinum-containing aqueous solution can include Pt(NH3)4 HPO4, and the voltage/current source can be operated at about ½-10 amperes per square foot of facing article surface. In the deposition, the platinum-group metal layer 30 is deposited onto the unmasked portion of the surface 13 (i.e., the trailing edge 24).
The platinum-group metal layer 30 may be heat treated, as desired. For example, the platinum-group metal layer 30 can be heat treated at a treatment temperature of about 900° C. to about 1200° C. In one embodiment, the platinum-group metal layer 30 is heat treated in a vacuum (e.g., at a treatment pressure of about 10 torr or less, such as at a treatment pressure of about 1 torr or less).
An oxidation-resistant coating is applied to the surface 13 of the airfoil 12 to further promote the oxidation resistance. In one particular embodiment, the oxidation-resistant coating is a diffusion aluminide coating 34, which may include aluminum intermetallics, gamma phase, gamma prime phase, or the like. The aluminide coating 34 is deposited overlying the platinum-group metal layer 30. The aluminide coating 34 can be formed to a thickness of about 2 μm to about 100 μm (e.g., about 25 μm to about 100 μm, such as about 35 μm to about 75 μm) by any suitable method. For example, the aluminide coating 34 can be deposited by any operable approach, such as aluminiding by pack cementation, or other processes including vapor phase aluminiding.
In one embodiment, the aluminide coating 34 is deposited via vapor phase aluminiding. For example, a hydrogen halide gas, such as hydrogen chloride or hydrogen fluoride, 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 surface 13, depositing the aluminum thereon. The deposition occurs at elevated temperature such as from about 900° C. to about 1125° C. during a cycle time (e.g., a 4 to 20 hour cycle). The aluminide coating 34 is preferably from about 12 to about 125 micrometers thick (such as about 25 μm to about 100 μm, for example about 35 μm to about 75 μm). The deposition technique allows alloying elements to be co-deposited into the aluminide coating 34 if desired, from the halide gas.
Because the deposition of aluminum is performed at elevated temperature, the deposited aluminum atoms interdiffuse with the platinum-group metal layer 30 (or interdiffused platinum/substrate region) and/or the material of the NiCoCrAlY layer 20 forming a coating system 22 on the surface 13 of the component 5.
In the embodiment shown in
Following heat treatment of the platinum-group metal layer 30 and the aluminide coating 34 shown in
In one particular embodiment, the outer portion has a nickel (Ni) content that is higher, in terms of weight percent, than the nickel content of the middle portion 31. Similarly, the inner portion has a nickel content that is higher, in terms of weight percent, than the nickel content of the middle portion. As such, the middle portion has a nickel content that this less than, in terms of weight percent, than the inner portion and/or the outer portion. In certain embodiments, for example, the outer portion has a nickel content of about 40% to about 50% by weight; the middle portion has a nickel content of about 30% to about 40% by weight; and the inner portion has a nickel content of greater than about 40% (e.g., greater than about 50%) by weight.
The coating system 22 is deposited and processed to have a smooth surface finish, e.g., about 3 μm or less of surface roughness (Ra), in order to promote the aerodynamics of the nozzle assembly. In one embodiment, the coating system 22 preferably has a surface roughness (Ra) of less than about 3 μm (e.g., about 0.75 μm to about 2.75 μm, such as about 1.25 μm to about 2.25 μm).
The thermal barrier coating 36 is deposited and processed to have a very smooth surface finish, e.g., about 1.5 μm Ra or less, in order to promote the aerodynamics of the nozzle assembly. In one embodiment, the thermal barrier coating 36 preferably has an as-deposited surface roughness (Ra) of less than about 3 μm. Thereafter, the surface of the environmental coating 36 preferably undergoes processing, preferably peening and then tumbling, to improve the surface finish of the environmental coating 36. Following peening and tumbling, the environmental coating 36 preferably has a surface roughness of not higher than about 2.0 μm Ra, with a typical range being about 1.3 μm to about 1.8 μm Ra on the concave surfaces and leading edges of the vanes, and about 0.5 μm to 1.0 μm Ra on the convex surfaces of the vanes.
In the embodiments shown in
As stated, the nozzle segment can have any number of airfoils (e.g., one (a singlet), two (a doublet), four, six, etc.). Different processing methods can be utilized, depending on the number of airfoils in the nozzle segments. In most embodiment, the film holes can be formed (e.g., drilled) prior to any coating is formed, and may be masked for any subsequent coatings to be applied if desired.
The present invention is generally applicable to components that operate within environments characterized by relatively high temperatures, and particularly to nozzle segments of the type represented in
Methods are also generally provided for forming a coating on a surface of component (e.g., an airfoil) and for repairing a coating on the surface of an airfoil. Referring to
Referring to
These and other modifications and variations to the present invention may be practiced by those of ordinary skill in the art, without departing from the spirit and scope of the present invention, which is more particularly set forth in the appended claims. In addition, it should be understood the aspects of the various embodiments may be interchanged both in whole or in part. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and is not intended to limit the invention so further described in the appended claims.
