Coating processes – Spray coating utilizing flame or plasma heat – Metal or metal alloy coating
Reexamination Certificate
1999-09-28
2002-04-16
Bareford, Katherine A. (Department: 1762)
Coating processes
Spray coating utilizing flame or plasma heat
Metal or metal alloy coating
C427S454000, C427S250000, C427S376800, C427S383700, C427S405000, C427S419200
Reexamination Certificate
active
06372299
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to protective coatings for metal substrates. More particularly, it is directed to improved thermal barrier coatings applied to metals designed for high temperature applications.
Superalloys are often the materials of choice for components intended for high-temperature environments. As an example, turbine blades and other parts of turbine engines are often formed of nickel-based superalloys because they need to maintain their integrity at temperatures of at least about 1000° C.-1150° C. Protective coatings, often referred to as thermal barrier coatings or “TBC”s, effectively increase the operating temperature of turbine components by maintaining or reducing the surface temperature of the alloys used to form the various engine components.
Most TBC's are ceramic-based, such as a material like yttria-stabilized zirconia. For a jet engine, the coatings are applied to various surfaces, such as turbine blades and vanes, combustor liners, and combustor nozzles. Usually, the TBC ceramics are applied to an intervening bond layer which has been applied directly to the surface of the metal part. The bond layer is often critical for improving the adhesion between the metal substrate and the TBC. Bond layers are usually formed from a material like “MCrAlY”, where “M” represents a metal like iron, nickel, or cobalt.
The term “superalloy” is usually intended to embrace complex cobalt- or nickel-based alloys which include one or more other elements such as aluminum, chromium, tungsten, molybdenum, titanium, and iron. The quantity of each element in the alloy is carefully controlled to impart specific characteristics, e.g., mechanical properties such as high-temperature strength. Aluminum is a particularly important component for many superalloys, because of its function in the precipitation-strengthening of the alloy.
If the superalloy is exposed to an oxidizing atmosphere for an extended period of time, it can become depleted in aluminum. This is especially true when the particular superalloy component is used at the elevated temperatures described above. The aluminum loss can occur by way of various mechanisms. For example, aluminum can diffuse into the bond coat, be consumed during oxidation of the bond coat, or be consumed during oxidation at the bond coat/substrate interface. The last-mentioned mechanism is particularly severe in porous bond coats, such as air plasma-sprayed (APS) bond coats. Aluminum-loss from the substrate is accelerated if the TBC or bond coat is removed during the service life of the component.
Since loss of aluminum can be detrimental to the integrity of the superalloy, techniques for countering such a loss have been investigated. At elevated temperatures, the substrate can be partially “replenished” with aluminum which diffuses from an adjacent MCrAlY-type bond coat. However, the amount of aluminum diffusion into the substrate from the bond coat is usually insufficient.
One method for increasing the aluminum content of the superalloy in its surface region is sometimes referred to in the art as “aluminiding”. In such a process, aluminum is introduced into the substrate by a variety of techniques. In the “pack aluminiding” process, the substrate is immersed within a mixture or pack containing the coating element source, filler material, and halide energizer. At temperatures about 850-1100° C., chemical reactions within the mixture yield an aluminum-rich vapor which condenses onto the substrate surface, and subsequently diffuses into the substrate.
While aluminiding successfully provides aluminum to the substrate and substrate-bond coat interface, there are some disadvantages associated with such a technique. For example, the resulting high-aluminum surface layer can be brittle. Deposition of an overlay bond coat on a brittle surface can sometimes be difficult.
It should thus be apparent that new methods for increasing the aluminum content of the superalloy surface and thereby increasing its oxidation life would be welcome in the art. These methods should prevent the formation of a brittle layer between the substrate and any subsequently-applied layer. Moreover, the new methods should result in a surface which is very amenable to deposition of subsequently-applied layers. It would also be very advantageous for the new methods to be capable of providing aluminum to a bond coat layer, to compensate for aluminum consumed in the bond coat by way of oxidation.
SUMMARY OF THE INVENTION
In one embodiment, the invention is directed to a method for providing a protective coating on a metal-based substrate, comprising the following step:
(a) applying an aluminum-rich mixture to the substrate to form a discontinuous layer of aluminum-rich particles in a matrix of metallic bond coat alloy, wherein the amount of aluminum in the particles exceeds the amount of aluminum in the metallic bond coat alloy by about 0.1 atomic % to about 40 atomic %, and wherein the total amount of aluminum in the mixture is in the range of about 10 atomic % to about 50 atomic per cent.
In a second embodiment, the invention is directed to a method for providing a protective coating on a metal-based substrate, comprising the following steps:
(a) applying an aluminum-rich mixture to the substrate to form a discontinuous layer of aluminum-rich particles in a matrix of metallic bond coat alloy, wherein the amount of aluminum in the particles exceeds the amount of aluminum in the metallic bond coat alloy by about 0.1 atomic % to about 40 atomic %, and wherein the total amount of aluminum in the mixture is in the range of about 10 atomic % to about 50 atomic %; and then
(b) applying at least one coating layer over the discontinuous layer of aluminum-rich particles.
Aluminum diffuses from the aluminum-rich layer into the superalloy substrate, as discussed below. The discontinuous nature of the aluminum-rich layer prevents embrittlement.
In preferred embodiments, substantially all of the aluminum-rich material comprises non-oxide particles. Moreover, in many preferred embodiments, the aluminum rich layer is formed of two components. Component (I) usually comprises particles of aluminum and a second metal, such as nickel, while component (II) usually comprises particles of an alloy of the formula MCrAlY, where M is a metal like Fe, Ni, Co, or mixtures which comprise any of the foregoing. The aluminum-rich layer can be applied by plasma spray techniques, such as air plasma spray or vacuum plasma spray, or by high velocity oxygen fuel (HVOF).
In some embodiments, the layer formed with the aluminum-rich mixture is heat-treated after being applied, to allow diffusion of aluminum into the superalloy. Moreover, in certain embodiments, a conventional metallic bond layer is applied over the aluminum-rich layer, prior to deposition of a thermal barrier coating. The heat treatment mentioned above can alternatively be carried out after deposition of the thermal barrier coating.
Another aspect of this invention is directed to an article, comprising:
(i) a metal-based substrate; and
(ii) an aluminum-containing layer over the substrate, comprising a discontinuous layer of aluminum-rich particles. In many preferred embodiments, the article may also include a thermal barrier coating disposed over the aluminum-containing layer.
As mentioned previously, the aluminum-containing layer can be formed from a mixture of a component based on particles of aluminum and nickel, along with a component based on a conventional MCrAlY alloy. Moreover, a metallic bond layer can be disposed between the aluminum-containing layer and the thermal barrier coating.
Further details regarding the various aspects of this invention are provided in the remainder of the disclosure.
DETAILED DESCRIPTION OF THE INVENTION
A variety of metals or metal alloys can be used as the substrate for the present invention. The term “metal-based” in reference to substrates disclosed herein refers to those which are primarily formed of metal or metal alloys, but which may also include some non-metallic compon
Gray Dennis Michael
Jackson Melvin Robert
Thompson Anthony Mark
Bareford Katherine A.
DiConza Paul J.
Ingraham Donald S.
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