Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2000-04-24
2002-05-28
Koehler, Robert R. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C075S255000, C148S527000, C148S535000, C148S537000, C419S008000, C419S009000, C419S053000, C419S054000, C427S189000, C427S190000, C427S191000, C427S192000, C427S197000, C427S198000, C427S205000, C428S546000, C428S548000, C428S678000
Reexamination Certificate
active
06395406
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to protective coatings for metal substrates. More particularly, it is directed to improved methods for preparing and applying ceramic coating compositions. The coatings are applied on metal substrates employed in high temperature applications, e.g., superalloy substrates used in turbine engines.
The power and efficiency of turbine engines typically increases as the operating temperature increases. However, the ability of the turbine to operate at elevated temperatures (e.g., 1000° C.-1150° C.) is limited by the ability of the turbine components to withstand the adverse effects associated with such operation. For example, turbine vanes and blades must be capable of withstanding the heat, oxidation and corrosion effects of the impinging hot gas stream, while still maintaining sufficient mechanical strength. The turbine components are often formed of a nickel-based superalloy, which usually must be covered with protective coatings which provide greater resistance to the effects of high temperatures, as compared to the superalloy itself.
Common examples of protective coating materials for turbine engine components are nickel aluminides and noble metal-aluminides, such as platinum aluminide. This type of material is usually deposited in several steps. First, the platinum is electroplated onto the blade, using P-salt or Q-salt electroplating solutions. The second step is usually carried out by an aluminiding technique. For example, the platinum layer can be diffusion-treated with aluminum vapor to form platinum aluminide.
When such a protective coating becomes worn or damaged, it must be carefully repaired, since direct exposure of the underlying substrate to excessive temperature may eventually cause the component to fail and adversely affect other parts of the engine. The coatings often have to be repaired several times during the lifetime of the component. The “overhaul” of the protective coating usually involves complete removal of the coating, followed by the application of a new coat of the material.
The aluminiding technique described above can sometimes be carried out to efficiently provide an aluminide-type repair coating. However, the process usually requires very high temperatures, often approaching the melting temperature of the base alloy itself. For this reason, it is often necessary to add a melting point suppressant to the repair alloy composition, to lower its melting temperature. In some situations, the addition of the melting point suppressant may compromise the integrity of the repair alloy.
Other aluminiding techniques can be used to apply protective coatings to the base alloy, either in a repair mode or as a “new make” coating. As an example, metal organic chemical vapor deposition (MOCVD) is sometimes used to aluminide a superalloy surface. However, MOCVD can be an expensive process, and is not always capable of producing the desired aluminide-type composition. Moreover, coating deposition rates for MOCVD are sometimes very slow. A lengthy coating process is often undesirable in a repair facility.
It should thus be apparent that new techniques for applying ceramic coatings-especially aluminide-type coatings-would be welcome in the art. The techniques should be efficient and not labor-intensive. They should also involve deposition of the coatings or their precursors at temperatures which are generally lower than those used in the prior art, e.g., at temperatures less than about 1000° C., and preferably, in the range of about 600° C. to about 1000° C. Moreover the techniques should be useful for coating selected portions of the substrate, e.g., only those sections which require repair. They should also preserve the integrity of the metal alloy surface.
BRIEF SUMMARY OF THE INVENTION
The present invention includes a method for preparing an aluminum alloy-containing coating composition, comprising the step of mixing a first slurry comprising a selected amount of aluminum with at least one additional slurry (i.e., a “second slurry”) comprising a selected amount of a second metal which forms an alloy with aluminum. (For brevity, the first slurry is sometimes referred to as the “aluminum slurry”). The amount of aluminum and the amount of the second metal in the respective slurries are substantially proportional to the desired proportion of aluminum to the second metal for the coating composition. The second metal is usually selected from the group consisting of noble metals, nickel, and mixtures thereof. In some embodiments, a third slurry containing a third metal can also be combined with the first and second slurries. As an example, the second slurry can include platinum, while the third slurry can include nickel, to form a nickel-platinum-aluminide coating.
Another embodiment of this invention is directed to a method for applying an aluminum alloy-containing coating to a metal-based substrate, comprising the following steps:
a) mixing a first slurry comprising a selected amount of aluminum with a second slurry comprising a selected amount of a second metal which forms an alloy with aluminum, thereby forming a slurry mixture, wherein the amount of aluminum and the amount of the second metal in the respective slurries are substantially proportional to a desired proportion of aluminum to the second metal for the coating;
b) applying the slurry mixture to the metal-based substrate;
c) heating the applied slurry mixture to remove at least a portion of the volatile material contained therein, forming a substantially devolatilized coating; and
d) subjecting the substantially devolatilized coating to a secondary heat treatment. Steps (c) and (d) can be combined into a single heating step with multiple stages. Moreover, a single slurry mixture containing all of the metals could be used, rather than employing separate slurries containing each metal, as described below.
A further embodiment is directed to a method for repairing a damaged or worn coating formed from an alloy of aluminum with at least one additional metal, applied over a metal-based substrate, comprising the following steps:
(i) removing the damaged or worn coating from a selected area on the substrate;
(ii) applying additional coating material over the selected area, by
(A) mixing a slurry comprising a selected amount of aluminum with a slurry comprising a selected amount of the additional metal, thereby forming a slurry mixture, wherein the amount of aluminum and the amount of the additional metal in the respective slurries are substantially proportional to a desired proportion of aluminum to the additional metal for the alloy;
(B) applying the slurry mixture to the metal-based substrate;
(C) heating the applied slurry mixture to remove at least a portion of the volatile material contained therein; and
(D) subjecting the coating material to a secondary heat treatment.
This process is effective for rapidly repairing worn coatings on various metal substrates, e.g., nickel-based superalloys used for turbine engine components.
Another embodiment of this invention is directed to a slurry composition. The slurry comprises aluminum and at least one additional metal which forms an alloy with aluminum. The additional metal is selected from the group consisting of a noble metal, nickel, and mixtures thereof.
An article also forms part of this invention. The article comprises:
(i) a metal-based substrate; and
(ii) a volatile-containing slurry of aluminum and at least one additional metal which forms an alloy with aluminum, applied over the substrate. When the volatile components in the slurry have been removed, a green coating remains. The green coating can receive a secondary heat treatment, e.g., a diffusion heat treatment.
Further details regarding the various features of this invention are provided in the remainder of the specification.
DETAILED DESCRIPTION OF THE INVENTION
The aluminum-containing slurry of this invention usually includes a dispersion of aluminum metal powder in an appropriate solution, which can be aqueous or organic. The organic phase
Cabou Christian G.
General Electric Company
Johnson Noreen C.
Koehler Robert R.
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