Coating processes – Solid particles or fibers applied – Uniting particles to form continuous coating with...
Reexamination Certificate
2002-07-26
2004-03-16
Parker, Fred J. (Department: 1762)
Coating processes
Solid particles or fibers applied
Uniting particles to form continuous coating with...
C427S191000, C427S201000
Reexamination Certificate
active
06706319
ABSTRACT:
FIELD OF THE INVENTION
This invention relates in general to the field of materials technology and more specifically to the field of abrasive coatings for high temperature applications. In particular, the present invention pertains to an abrasive coating and a process for depositing that coating on component parts of a turbine combustion engine where the hard particles are co-deposited with a matrix material by means of a cold spraying process. Together, the hard particles and matrix material form an abrasive coating that provides a protective layer for the component parts so they are wear, erosion and abrasion resistant when used in high temperature environments such as a gas turbine.
BACKGROUND OF THE INVENTION
It is well known that increasing the firing temperature in the combustion portions of a turbine may increase the power and operational efficiency of a gas turbine engine or a combined cycle power plant incorporating such a gas turbine engine. The demand for improved performance has resulted in advanced turbine designs wherein the peak combustion temperature may reach 1,400 degrees C. or more. Special materials are needed for components exposed to such temperatures. Nickel and cobalt based superalloy materials are now used for components in the hot gas flow path, such as combustor transition pieces and turbine rotating and stationary blades. An example of a commercially available superalloy material is IN738 made by Inco Alloys International, Inc.
A metallic bond coat layer may be initially applied to the surface of a component to provide oxidation resistance and improved adhesion of an overlaying ceramic coating. Common metallic bond coat materials include MCrAlY and MCrAlRe, where M may be nickel, cobalt or iron or a mixture thereof. It is known in the art to apply the metallic bond coat layer by any one of several thermal spray processes, including low-pressure plasma spray (LPPS), air plasma spray (APS) and high velocity oxy-fuel (HVOF). Such processes propel the MCrAlY or MCrAlRe material, or other suitable materials, in a molten plasma state against the surface of the superalloy substrate where it cools and solidifies to form a coating. Such thermal spray processes are known to result in a significant amount of porosity and the formation of oxygen stringers in the metallic bond coat layer due to the inherent nature of a high temperature process. The release of heat from the molten particles of the metallic bonding materials and the transfer of heat from the high temperature gas used in a thermal spray process also result in a significant increase in the surface temperature of the superalloy substrate material during the metallic bond coat application process. Such elevated temperatures result in localized stresses in the superalloy material upon the cooling of the coating layer, which may have an adverse affect on the performance specifications of the superalloy component. Furthermore, a post-deposition diffusion heat treatment is necessary to provide the required metallurgical bond strength, and such treatment may also have adverse affects on the material properties of the underlying substrate.
To optimize the adhesion of the metallic bond coat to the superalloy substrate, it is desired to have a metal-to-metal contact between the layers. Any contamination, oxidation or corrosion existing on the surface of the substrate may adversely impact the adhesion of the coating layer. A separate cleaning step, such as grit blasting with alumina particles, is known in the art and may be used to clean the target surface. However, such process may leave trace amounts of the cleaning material on the surface. After even a short period of exposure to moisture in air, the target surface may begin to oxidize. Handling or storing of the component after the cleaning step may introduce additional contaminants to the previously clean surface. The environment of the prior art thermal spraying processes also contributes to the oxidation of the substrate during the coating process due to the presence of high temperature, oxygen and other chemicals. An improved process in the art is desirable to minimize the risk of oxidation during the application process.
It is also known in the art that the operational specifications of certain components within gas turbine engines require that hard particles abrade the coatings of other surfaces such as a turbine blade tip abrading the interior coating of a ring segment during operation. For example, U.S. Pat. No. 5,702,574 discloses a jig and the process by which the tip portion of a gas turbine blade is provided with hard particles embedded within a matrix material. The tip of the blade is designed to run against the inside surface of a blade encapsulating ring segment during operation of the gas turbine. As little clearance as possible is desired between the blade tips and the inside surface of the ring segment in order to minimize bypass flow of air and other gases past the tips of the blades. The material covering the inside surface of the ring segment is designed to be softer than the material on the blade tips so that as the abrasive material on the blade tips interacts with the interior surface of the ring segment, a very small gap is formed between the blade tips and the ring segment, which minimizes gas losses during operation of the turbine. In accordance with the '574 patent, a plurality of blades may be mounted in a hollow jig having at least one ring of circumferentially disposed apertures through which the tips of the blades are inserted. The tips of the blades are then provided, by electrodeposition, with a coating of hard particles embedded within a matrix.
Electrodeposition is well known in the art and employed in the disclosure of U.S. Pat. No. 5,702,574 first identified above. For instance, the disclosed process includes situating the turbine blade tips within a jig such that they are encountered by a plating solution having hard particles entrained therein. As the particles encounter the tips they tend to settle on the tips where they become embedded in a metal that is being simultaneously plated out. This electrodeposition process, as well as other similar processes employing solutions such as electroplating or electroless plating, does not provide a means for precisely controlling the placement of abrasive particles on the blade tips, if desired.
Additionally, the invention disclosed in U.S. Pat. No. 5,702,574 includes deposition of an infill material by means of vibrating the jig assembly in order to coat regions of the blade tips that might otherwise be depleted of abrasive particles. Also, U.S. Pat. No. 5,076,897 discloses a similar vibration means used to plate infill of MCrAlY around abrasive particles deposited on portions of the blade tips. While electrodeposition and similar processes achieve good bonds they typically take several hours to perform and, in the case of depositing abrasive particles on the tips of turbine blades known in the art, must be performed in conjunction with rather elaborate apparatus that contribute to the cost of manufacture.
The known processes used to deposit abrasive particles within a matrix material on the tips of turbine blades, for example, have limitations such as they expose the underlying substrate to high temperatures, are time consuming, expensive and don't necessarily achieve an optimum deposition of particles. The known apparatuses used in conjunction with these processes may be relatively elaborate and not easily adaptable for field repair, which increases the costs of manufacture or repair. Thus, an improved process is needed for depositing abrasive particles dispersed within a matrix material that will entrap the abrasive particles, sufficiently bond to a substrate, resist oxidation and possess sufficient mechanical properties to maintain its shape on the substrate.
BRIEF SUMMARY OF THE INVENTION
The present invention uses a process, referred to herein as a cold spray process, to deposit hard particles that act as an abrasive onto a substrate to form an abrasive coatin
Seth Brij B.
Wagner Gregg P.
Parker Fred J.
Siemens Westinghouse Power Corporation
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