Metal working – Method of mechanical manufacture – Combined manufacture including applying or shaping of fluent...
Reexamination Certificate
1999-04-16
2001-07-03
Hughes, S. Thomas (Department: 3726)
Metal working
Method of mechanical manufacture
Combined manufacture including applying or shaping of fluent...
C029S424000, C427S421100, C427S427000, C118S504000
Reexamination Certificate
active
06253441
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the coating of a surface of a metallic article, and, more particularly, to such coating conducted with a patterned mask that permits coating in some areas and prevents coating in other areas of the surface.
BACKGROUND OF THE INVENTION
Nickel-base superalloy components of gas turbines are sometimes coated with aluminum and then heated to diffuse the aluminum into the surface of the article. The aluminum-rich surface is oxidized to produce an adherent aluminum oxide scale on the surface of the article. The aluminum oxide scale is an effective barrier against further oxidation and corrosion of the component in service.
The aluminum coating is typically applied by a vapor phase deposition process. In one such approach, aluminum fluoride gas is contacted to the component surface under conditions such that the compound decomposes to leave a layer of aluminum deposited on the surface. The aluminum diffuses into the surface during the deposition and any post-deposition heat treatment, producing the aluminum-enriched surface region.
It is sometimes the case in such deposition processes that a first portion of the surface of the article is to be left uncoated, and a second portion of the surface of the article is to be coated with aluminum. The uncoated portion may be required for any of several reasons, such as allowing attachment of other structure to the article by brazing. Any brazed joint between the aluminum coating and another structure is ordinarily brittle, so that the brazed joint must be made to the uncoated first portion.
In order to prevent deposition of aluminum from the aluminum-containing gas, the first (uncoated) portion of the surface of the article is physically covered with a mask. The mask prevents contact of the aluminum-containing gas to the first portion. Available maskants usually include sources of Ni
+2
and Cr
+3
ions in a binder complex with Al
2
O
3
particles. These maskants are intended to prevent the coating vapors from reaching the surface of the article, and to prevent depletion of the alloy components from the surface of the first portion of the surface.
The present inventors have observed that, after removal of the maskant from the first portion of the substrate surface, in some cases there are surface oxides on the first portion of the surface that impede subsequent brazing operations. It is believed that these surface oxides result from the chemical interaction of the maskant with the masked surface. The surface oxides cannot be readily removed in many cases, because their removal may cause damage to the aluminide coating. In addition, there may be surface depletion of elements from the substrate due to diffusional processes, altering the chemistry of the surface region of the substrate in an adverse manner.
These same types of problems arise in relation to other coatings as well, where masks are used. There is a need for an improved approach to the coating of a portion of an article surface with a coating such as an aluminide, where some of the surface must remain uncoated.
BRIEF SUMMARY OF THE INVENTION
The present invention provides techniques for masking of the surfaces of articles which are to be coated. In one approach, a method for processing a substrate article comprises the steps of furnishing a substrate, and masking a first portion of the substrate article with a maskant. The maskant includes a foil layer overlying and contacting the first portion of the substrate, leaving a second portion of the substrate exposed, and a mask layer overlying and contacting the foil layer. The foil layer is made of a material selected appropriately for the composition of the substrate, such as nickel for a nickel-base substrate, cobalt for a cobalt-base substrate, or titanium for a titanium-base substrate. The mask layer may be for example a conventional masking layer or a carbon masking layer. The method further includes processing the substrate article to deposit a coating on the second portion of the substrate, without removing the maskant. The coating may be any operable coating material. The coating is preferably a metal such as aluminum, but may be other metals compounds, ceramics, or organic compounds.
In a variation of this approach, a method for processing a substrate article comprises the steps of furnishing a substrate, and masking a first portion of the substrate article with a maskant. The maskant includes a graphite mask layer, preferably a freestanding graphite template piece that may be reused, overlying the first portion of the substrate, leaving a second portion of the substrate exposed. The masked surface is processed to deposit a coating on the second portion of the substrate, without removing the maskant.
The present approach may be used with a foil layer between the substrate and the mask, so that after coating the first portion of the substrate is suitable for brazing without further processing. If the foil layer is omitted and the graphite template piece is used as the mask, the masking costs are reduced due to the reusability of the mask. Additionally, a passage may be provided through the mask template piece to allow access of the aluminiding gas to the interior of a hollow substrate article. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiments, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to these preferred embodiments.
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Brown Terri K.
Wheat Gary E.
General Electric Company
Hess Andrew C.
Hughes S. Thomas
Jimenez Marc
Narciso David L.
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