Patent Application
20070298277
Wherever possible, the same reference numbers are used throughout the drawings to refer to the same or like parts.
One embodiment of a turbine component that can be used with the method of the present invention includes turbine blade 10 shown in
The amorphous phosphate-containing coating 52 preferably has a thickness of from about 0.10 microns to about 10 microns and provides resistance to oxidation and hot corrosion at temperature greater than about 1000° F., including temperatures of about 1000° F. to about 2000° F. In addition, the amorphous phosphate-containing coating provides a barrier to oxygen diffusion. The barrier preferably provides sufficient resistance to oxygen diffusivity that the amorphous phosphate-containing coating 52 has an oxygen diffusivity of less than about 1×10−12 cm2/sec at temperatures of about 1400° C. (2552° F.). The reduced oxygen diffusivity of the amorphous phosphate-containing coating 52 allows for slow oxide scale growth and a fast transition from metastable aluminum oxide phases, such as theta or gamma, to the stable alpha aluminum oxide phase. The amorphous phosphate-containing coating 52 is also resistant and acts as a barrier to hot corrosion from deposited metal, sulfates, sulfites, chlorides, carbonates, various and sundry oxides and/or various salts in either particulate or gaseous form. The amorphous phosphate-containing coating 52 of the present invention, while adherent to the surface, is thin and compliant making it resistant to cracking under stress.
While the above has been described with respect to turbine blades, turbine vanes, seals, substrates such as under-platform components, turbine disks, shafts and other turbine components that come into contact with cooling air may also be coated with the amorphous phosphate-containing coating of the present invention.
The present invention also includes a method for coating the interior surfaces of a turbine blade and for coating hot section turbine component surfaces that come into contact with the cooling air of a gas turbine engine. The method includes providing a high pressure turbine component that comes into contact with cooling air within the gas turbine engine. The component may include underlying layers within the interior surfaces, such as aluminide layers, as well as exterior coating systems, not to be limited by, including diffusion aluminides, noble metal-modified diffusion aluminides, NiAl or MCrAlY overlays, and thermal barrier coatings. An optional surface preparation may be performed to clean the surface in preparation for coating. The surface preparation may be any surface preparation known in the art suitable for preparing a surface for subsequent coating. Thereafter, a liquid coating composition is provided including a liquid composition comprising a composition for forming amorphous phosphate-containing coating 52. An amorphous phosphate-containing coating 52 for use in the coating system of the present invention includes, but is not limited to an aluminophosphate coating, formed from a liquid mixture comprising a metal salt, alcohol and phosphorus pentoxide (P2O5). Aluminophosphate compounds for use in the amorphous phosphate-containing coating 52, compositions for forming an aluminophosphate amorphous coating and/or materials making up the coating composition for forming an aluminophosphate coating, are disclosed in U.S. Pat. Nos. 6,036,762 and 6,461,415 and U.S. Pat. Application Publications US 2006/0057407, US 2005/0106384, US 2004/0206267, US 2004/0011245 and 2003/0138673 which are incorporated by reference herein in their entirety. Examples of aluminophosphate coatings including compositions comprising amorphous, metastable aluminum phosphate having phosphate (e.g., PO4) and aluminum oxide compounds (e.g., AlO4) bonded within the amorphous coating. The application of the coating composition may take place using any conventional application method, including, but not limited to, injection, brushing, rolling, dipping, injecting, spin-coating, spraying and combinations thereof. In order to provide coating in selected areas, masking of the surface may be utilized. Masking may be useful in preventing coating of areas such as the exterior surfaces and dovetail pressure faces of the turbine blade.
The coated component is then dried and heated to cure the coating composition and form the amorphous phosphate-containing coating. The temperature curing is preferably greater than about 400° C. (752° F.), more preferably greater than about 600° C. (1112° F.) and still more preferably from about 500° C. (932° F.) to about 800° C. (1472° F.). The cured amorphous phosphate-containing coating 52 is adherent to the surface, has a thickness of from about 0.10 microns to about 10 microns, specifically, from about 0.1 microns to about 5 microns, more specifically from about 0.2 microns to about 2.0 microns, and is resistant to oxygen diffusion. In a preferred embodiment, the internal passages of a turbine blade 10 are coated with an amorphous phosphate-containing coating 52 comprising aluminophosphate.
In order to provide the amorphous-containing coating 52 in desired locations, while allowing other surfaces to remain uncoated, masking may be utilized. Masking includes covering a surface with a material or coating that may subsequently and substantially be removed and prevents coating of the underlying substrate. Suitable masking material include, but are not limited to, masking tapes which may be applied at room temperature and removed from the substrate surface after applying the amorphous phosphate-containing coating 52. For example, the coating method of the present invention has the advantage that the coating may be applied to only the internal surface of the turbine blade 10 with the use of masking. Prior to the coating material being applied to the turbine blade at room, simple masking methods including masking tape, releasable coatings and inert films may be applied to areas of turbine blade that do not require/desire coating including the external surface of the airfoil, the contact points with the disk of the dovetail, contact points with dampers, and over other coatings. The component may then be immersed (i.e., dipped) in an amorphous phosphate-containing coating precursor containing liquid composition. Following the precursor application and drying, the masking material can be removed before curing at elevated temperature to form the coating.
Optionally, the coating method of the present invention has the advantage that the coating may be applied to the interior surface of a turbine blade 10 without the use of masking. For example, the component may be immersed (i.e., dipped) in liquid phosphate coating precursor containing liquid composition and subsequently cured to form a coating on the entire surface of the turbine blade 10. The relatively thin low density coating has little or no detrimental effect on weight and provides properties, such as resistance to oxygen diffusion, that provide hot corrosion and oxidation resistance. Although not required on exterior surfaces having an aluminide coating and thermal barrier coating, the amorphous phosphate-containing coating 52 may be present on all surfaces of the turbine blade 10, including the exterior surfaces of the airfoil section 34 and coatings present thereon. In the embodiment wherein the entire blade, including the interior surfaces, are coated, adherence to coated surfaces, such as thermal barrier coatings on the airfoil section 34 may be weak and the amorphous phosphate-containing coating 52 may wear away during normal operation of the gas turbine engine. Additionally in the embodiment wherein the entire blade, including the interior surfaces, are coated, contact surfaces that are subject to high wear rates, such as the contact points with the disk of the dovetail or the contact points with blade dampers, will wear through the coating 52 early during operation of the gas turbine engine and not affect the performance of these contact surfaces. The remaining portions of the turbine blade 10 remain coated and the amorphous phosphate-containing coating 52 provides oxidation and hot corrosion resistance.
Two samples of RENE® N5 were provided. The first sample was coated with an amorphous phosphate-containing coating according to the present invention. A second, comparative sample was left uncoated. Both the first sample and comparative sample were subject to oxidation testing with 20 cycles per hour in a Mach 1 gas stream at 2150° F., where the samples were allowed to be cooled to about room temperature between cycles. The weight loss of the sample in grams was measured and are show in TABLE 1.
As shown in TABLE 1, the alloy coated with amorphous phosphate-containing coating had substantially greater resistance to oxidation than the uncoated material as evidence by the initiation of material weight loss at a longer test time (about 824 hours versus about 371 hours).
While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
