Coating processes – Solid particles or fibers applied
Reexamination Certificate
1998-10-19
2001-02-27
Parker, Fred J. (Department: 1762)
Coating processes
Solid particles or fibers applied
C427S343000, C427S435000, C134S002000, C134S007000
Reexamination Certificate
active
06194026
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to manufacture of coated superalloy components, such as aluminide or MCrAlY overlay coated gas turbine engine blades and vanes, wherein the superalloy components are grit blasted during manufacture and treated to remove embedded grit particles prior to subsequent coating operations such that the coating is substantially free of entrapped grit particles.
BACKGROUND OF THE INVENTION
Abrasive media blasting using aluminum oxide, silicon dioxide and other grit particles is a commonly used processing step in the manufacture of nickel, cobalt and iron base superalloy gas turbine engine hardware, such as turbine blades and vanes, by casting and protectively coating the casting. Examples of processing or manufacturing operations using abrasive grit blasting include a) ceramic shell mold removal from the casting using coarse (e.g. 90 grit or larger) grit particles at high carrier air pressure (e.g. 60-90 psi) , b) surface cleaning of the casting using fine (e.g. 220 grit or smaller) particles at moderate carrier air pressure (e.g. 40-60 psi) prior to aluminizing to form a protective coating, c) roughening a casting surface using fine grit at moderate carrier air pressure prior to platinum or other electroplating operation, or d) roughening a casting surface using large grit at high carrier air pressure prior to overlay coating deposition by plasma or flame spraying or EB-PVD (electron beam-physical vapor deposition). Abrasive grit blasting unfortunately results in grit particles, such as alumina grit particles, becoming embedded as contaminants in the blasted surface of the superalloy component. In general, the severity of surface contamination with embedded grit particles increases with the carrier air pressure and the angle of incidence of the abrasive blast media relative to the casting surface.
Embedded abrasive oxide grit particles in protective diffusion aluminide or MCrAlY overlay coatings can have a detrimental effect on the performance of the coatings. Specifically, embedded grit particles can concentrate stresses (thermal and/or mechanical) applied to the coated engine component, and such stress concentration can result in cracking of the coating and/or the component. In addition, embedded grit particles can adversely affect the adherence of either outwardly grown diffusion aluminide coatings or MCrAlY overlay coatings of the known type (where M is Ni and/or Co and/or Fe), particularly when small grit particles cover a significant fraction of the surface area of the component prior to coating. Because of the undesirable effects of entrapped grit oxide particles, most gas turbine engine manufacturers limit the size and quantity of embedded grit particles permissible in protective coatings. Consequently, processes have been included in manufacture of such components as turbine blades and vanes to control surface contamination from embedded abrasive grit particles. Such processes have included chemical etching of the component surface and fluoride ion cleaning among others.
Grit blasting is inexpensive and widely used in manufacture of such superalloy components as turbine blades and vanes. Frequently, multiple grit blasting operations are incorporated into the production routing of such superalloy components as turbine blades and vanes from shell mold removal to aluminizing. However, in order to reduce manufacturing costs and processing time, surface finishing operations to remove embedded grit particles may be reduced in number to an extent that there can be a chronic problem meeting the aforementioned specifications of gas turbine engine manufacturers with respect to size and quantity of embedded grit particles in protective coatings. Currently, fluoride ion cleaning, ferric chloride etching and ultrasonic cleaning are used to reduce the presence of embedded grit particles at the pre-aluminide coating and/or pre-electroplating stage of manufacture. However, none of these processes is without serious limitations. For example, grit removal using HF/H
2
mixtures at high temperature produces unacceptable alloy depletion in the component and involves considerable expense. Chemical etching of the component using ferric chloride is disadvantageous in that such etching attacks and removes a portion of the alloy surface itself, and so, reduces the component wall thickness. Ultrasonic cleaning is disadvantageous because it only removes some of the embedded particles.
There thus is a need for a process which can be incorporated into the manufacture of superalloy components to remove embedded grit particles effectively without degradation of the component. The present invention has an object to satisfy this need.
SUMMARY OF THE INVENTION
The present invention provides for manufacture of superalloy component wherein the component is blasted with abrasive grit particles at one or more stages during the manufacture, the grit particles are removed from the surface of the component by treating the surface with a chemical medium effective to chemically attack (e.g. completely or partially dissolve) the abrasive grit particles in a manner that allows their removal from the surface without substantially chemically attacking or degrading the component (substrate), and then is coated with a protective coating such that entrapped grit particles are substantially reduced or eliminated from the coating. The chemically treated component surface can be contacted with a fluid, such as rinse water, in a manner to assist in removal of any undissolved grit particles loosened from the surface by partial particle dissolution.
The present invention provides an abrasive grit blasted superalloy component having a coating that is substantially free of entrapped grit particles on a surface that is free of chemical attack.
In an illustrative embodiment of the present invention, the component can be a cast nickel or cobalt base superalloy blade or vane freed from ceramic shell mold material by a knock-out operation followed by abrasive grit blasting using coarse abrasive oxide grit particles at high carrier air pressure and then subsequently surface treated prior to coating by abrasive grit blasting. After surface treatment, the component is immersed in an inorganic hydroxide bath having a high concentration of alkali or alkaline earth hydroxide with the bath at ambient (atmospheric) pressure and elevated temperature, such as at least 275 degrees F., for a time sufficient to at least partially dissolve oxide grit particles embedded in the surface of the component followed by rinsing in water. The hydroxide bath can comprise an inorganic hydroxide solution having at least 50 weight % of an alkali or alkaline earth hydroxide and the balance essentially water. An exemplary inorganic hydroxide bath can comprise a 90 weight % KOH and 10 weight % water solution at 650 degrees F. disposed in a vessel open to atmospheric pressure.
The above and other objects and advantages of the present invention will become more readily apparent from the following drawings taken in conjunction with the following detailed description.
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Cockerill Joel L.
Nordin Alexander J.
Schilbe John E.
Warnes Bruce M.
Howmet Research Corporation
Parker Fred J.
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