Patent Application
20060051212
1. Field of the Invention
The present invention relates generally to a coated turbine blade and to a process of coating the turbine blade. More particularly, the present invention is directed to a coated turbine blade that forms part of a turbine wheel assembly that is used, for example, in an auxiliary power unit of an aircraft, and a process for coating such a turbine blade. It will be appreciated, however, that while turbine blades will be disclosed for use in auxiliary power units, they are not limited to such an application.
2. Description of Related Background Art
An auxiliary power unit (APU) provides the primary source of power in an aircraft when it is on the ground. An APU may also provide an auxiliary source of power to an aircraft when it is in the air.
A critical component of an APU is a turbine wheel assembly. As shown in
Until recently, the life span of a typical APU was relatively short, for example, providing 2000 hours of running time. After this time, which may give the APU a life span of a year or so depending on how frequently the airplane is in use, the APU was typically overhauled (replacing parts, such as the turbine wheel assembly, as opposed to servicing and maintaining them).
In a conventional turbine blade for use in an APU as discussed above, a portion of the turbine blade is coated with a corrosion resistant material. For example, and with reference to
Due to recent advancements in design and technology, APU's with a life span of 8,000 to 10,000 hours operating time or even more are now available. With these longer lasting APU's, however, it has been found that conventional turbine blades are unable to provide a satisfactory service life. Specifically, areas of the turbine blade that are not treated with a corrosion resistant material are prone to fatigue failure propagated by stress concentrations (e.g., pitting, scaling, cracking, etc.) caused by hot corrosion, sulfidation, and the like, after prolonged exposure to elevated temperatures, pollutants, particulates and corrosive dust.
One attempt to address this problem has been to treat the bottom surface of the platform with an aluminide coating applied by a diffusion process, such as a pack cementation process or a chemical vapor deposition (CVD) process. In the pack cementation process, the parts or areas to be coated are packed in an aluminum-rich powder. Heat is then applied to diffuse the aluminide into the coated part. This aluminide coating process has found some use in coating the underside of the platform of turbine blades. There are limitations, however, in both the aluminide coating itself and the coating process, that leave room for improvement.
Accordingly, there is a need for providing a turbine blade with an improved corrosion-resistant protective coating and a process for applying such a coating.
A general object of the present invention is to provide an improved turbine blade.
Another object of the invention is to provide a turbine blade with a coating to reduce or even prevent corrosion, and an improved process for coating the turbine blade.
In accordance with one aspect of the invention, a turbine blade comprises a platform having a top surface, a bottom surface, and a peripheral sidewall between the top and bottom surfaces. An airfoil protrudes from the top surface of the platform. A blade mount extends from the bottom surface of the platform for attaching the turbine blade to a turbine wheel. A corrosion resistant protective coating of MCrAlY is applied on the airfoil, the top surface, the peripheral sidewall and the bottom surface of the platform.
In accordance with another aspect of the invention, a turbine blade comprises a platform having a top surface, a bottom surface, and a peripheral sidewall between the top and bottom surfaces. An airfoil protrudes from the top surface of the platform. A blade mount extends from the bottom surface of the platform for attaching the turbine blade to a turbine wheel. A corrosion resistant protective coating of MCrAlY is applied on the bottom surface of the platform.
In accordance with still another aspect of the invention, a turbine blade comprises a platform having a top surface, a bottom surface, and a peripheral sidewall between the top and bottom surfaces. An airfoil protrudes from the top surface of the platform. A blade mount extends from the bottom surface of the platform for attaching the turbine blade to a turbine wheel. A corrosion resistant protective coating of MCrAlY is applied on the airfoil, the top surface, and the peripheral sidewall of the platform.
In accordance with another aspect of the invention, a distance from a top edge of the airfoil to a bottom edge of the blade mount is about two inches or less, and a distance between the bottom surface of the platform to the blade mount is about ½ inch or less.
Also, the protective covering preferably coats the airfoil, the top surface, the peripheral sidewall and recesses in the bottom surface of the platform in their entirety.
Preferably, the protective coating covering the bottom surface of the platform is applied using a high-velocity oxygenated fuel (HVOF) process, and the protective coating covering the airfoil, the top surface, and the peripheral sidewall of the platform is applied using an electron beam physical vapor deposition (EBPVD) process.
In accordance with another aspect of the invention, there is a method of coating a turbine blade comprising a platform, including a top surface, a bottom surface, and a peripheral sidewall, an airfoil protruding from the top surface of the platform, and a blade mount extending from the bottom surface of the platform, for attaching the turbine blade to a turbine wheel. The method comprises the steps of applying a first corrosion-resistant protective coating of MCrAlY to the bottom surface of the platform, and applying a second corrosion-resistant protective coating of MCrAlY to the airfoil, the top surface, and the peripheral sidewall of the platform.
Preferably, the method includes one or more of the additional steps of using the same compound as the first protective coating and the second protective coating, using CoCrAlY as the first and second protective coatings, and applying the second protective coating after the first protective coating is applied.
The method preferably also includes the step of masking off portions of the turbine blade except for the bottom surface of the platform, and applying the first protective coating using a high-velocity oxygenated fuel process, and/or the step of masking off portions of the turbine blade except for the airfoil, the top surface of the platform and the peripheral sidewall, and applying the second protective coating using an electron beam physical vapor deposition process.
In accordance with yet another aspect of the invention, a turbine wheel assembly comprises a rotatable disc having an outer periphery, and a plurality of turbine blades affixed to the disc at the outer periphery. Each of the turbine blades comprises a platform having a top surface, a bottom surface and at least one peripheral sidewall. An airfoil protrudes from the top surface of the platform. A blade mount extends from the bottom surface of the platform for attaching the turbine blade to the disc. A corrosion resistant protective coating of MCrAlY is applied on one or more of the airfoil, the top surface, the peripheral sidewall and the bottom surface of the platform.
A better understanding of these and other objects, features, and advantages of the invention may be had by reference to the drawings and to the accompanying description, in which preferred embodiments of the invention are illustrated and described.
A turbine blade 20 according to a preferred embodiment of the invention is illustrated in
By way of example only, and with reference to
The turbine blade is preferably a unitary body formed, for example, by investment casting, and is preferably made from a Nickle alloy; more preferably, the turbine blade is fabricated by an Equiaxed casting of MAR-M 247 Nickle alloy. Of course, other methods of forming the turbine blade (as either a unitary or composite body), and alternative materials, can be used without departing from the scope of the invention.
In the embodiment shown in
As best shown in
A corrosion resistant protective coating is applied to portions of the turbine blade 20 to help prevent, among other things, stress concentrations caused by corrosion (that could potentially lead to crack propagation and ultimately fatigue failures of the turbine blades). Preferably, the corrosion resistant protective coating comprises a metallic compound, such as an MCrAlY alloy (of e.g., CoCrAlY or NiCoCrAlY). Preferably, the corrosion resistant coating comprises a CoCrAlY alloy coating. CoCrAlY alloy provides excellent corrosion resistance (per mil of coating thickness), can be effectively applied at optimum thicknesses, such as about 2 to about 6 mils, and exhibits good ductile characteristics. Finally, the CoCrAlY coating can be applied (at least in part) by an HVOF process, which has been found to be very effective in coating relatively small scale turbine blades like those shown in
Other metallic CrAlY (MCrAlY) alloys, such as NiCrAlY could also be used in situations where resistance to oxidation is important.
While it is preferable to use a single type of corrosion resistant protective coating on the turbine blade, two or more different corrosion resistant protective coatings may be used to coat the same or different parts of a turbine blade, depending on the particular application and the environment in which it is intended to be used.
The corrosion resistant protective coating may be applied to one or more of the airfoil 22, the top surface 32 of the platform 24, the peripheral sidewall 34 of the platform 24, and the bottom surface 36 of the platform 24. The corrosion resistant protective coating may be applied to these elements in whole or in part, depending on the particular needs of the application. Preferably, however, the corrosion resistant protective coating is applied to the airfoil 22, the top surface 32 of the platform 24, and the peripheral sidewall 34 of the platform 24 in their entirety. The corrosion resistant protective coating is preferably applied to a substantial portion of the bottom surface 36 of the platform 24, but not to the bottom surface 36 in an area adjacent to the blade mount 26. Preferably, a distance g between the bottom of the coated area and the top of the blade mount is preferably at most ½ inch. In one particularly preferred embodiment shown in
The corrosion resistant protective coating may be applied by any suitable application process. In one preferred embodiment, the corrosion resistant protective coating is applied using an HVOF process in combination with an EBPVD process. Both of these general processes are, per se, known in the art. Preferably, a corrosion resistant protective coating is applied to the bottom surface 36 of the platform 24 by an HVOF process, while a corrosion resistant protective coating is applied to the airfoil 22, the upper surface 32 of the platform 24, and the peripheral sidewall 34 of the platform 24 by an EBPVD process. In a particularly preferred embodiment, the corrosion resistant protective coatings applied by both the HVOF and EBPVD processes are CoCrAlY alloy coatings. When HVOF and EBPVD processes are used to coat different portions of a turbine blade with the corrosion resistant protective coating, it is preferable that the HVOF process be performed prior to the EBPVD process.
In the embodiment depicted in
In one preferred method of coating a turbine blade in accordance with the subject invention, the surfaces of the turbine blade 20 are masked off, except for the bottom surface 36 of the platform 24. Then, a first corrosion resistant protective coating is applied using an HVOF process. After the masks are removed, any over-spray from the HVOF process on the airfoil 22, top surface 32 of the platform 34, or the peripheral sidewall 34 of the platform is preferably removed by hand polishing, or the like. Next, the surfaces of the turbine blade 20 are masked off, except for the airfoil 22, the top surface 32 of the platform 24, and the peripheral sidewall 34 of the platform 24. Then a second corrosion resistant protective coating is applied using a EBPVD process. Preferably, both the first and second protective coatings are the same; more preferably, both are CoCrAlY alloy coatings.
Various other processes, including cleaning of the turbine blade, shot peening of the coated surfaces, and the like may be performed on the turbine blade, as necessary or desirable for a given application and/or to achieve a desired finish.
While the turbine blade 20 depicted in
The embodiments discussed herein are representative of preferred embodiments of the invention and are provided for illustrative purposes only. They are not intended to limit the scope of the invention. Although specific structures, dimensions, components, methods, etc., have been shown and described, such are not limiting. Modifications and variations are contemplated within the scope of my invention, which is intended to be limited only by the scope of the accompanying claims.
