Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2000-01-31
2001-06-05
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C428S642000, C428S680000, C428S681000, C428S469000
Reexamination Certificate
active
06242109
ABSTRACT:
BACKGROUND OF THE INVENTION:
Field of the Invention
The invention relates to a high-temperature-resistant component and a method of protecting a high-temperature-resistant component against oxidation.
U.S. Pat. No. 4,321,310 describes a composite coating system for protecting a metallic component, in particular at high temperatures. In particular, a coating system for a gas turbine blade is described. A gas turbine blade is subjected to great stresses by high temperatures and strong oxidation. In order to be able to withstand the high temperatures, gas turbine blades are frequently made of a nickel-based or cobalt-based superalloy which has a sufficiently high melting point. In general, it is also necessary to protect the surface of such a gas turbine blade against oxidation. This can be achieved by means of an MCrAlY coating. Here, M is at least one of the elements iron, cobalt or nickel, Cr is chromium, Al is aluminum and Y is yttrium. Oxidation protection is achieved by a very oxidation-resistant aluminum oxide being formed on the surface of such a coating. In the document cited, such a coating on a gas turbine blade is additionally provided with a ceramic coating. This ceramic coating acts as a thermal barrier layer. Between the thermal barrier layer and the oxidation layer, there is arranged an aluminum layer. This improves, in particular, the adhesion of the ceramic layer to the antioxidation layer.
WO 97/07252 describes a product for conducting a hot, oxidizing gas. In particular, this is a highly thermally stressed component of a gas turbine.
It is stated that the oxidation resistance of metallic protective layers comprising MCrAlY is impaired over the course of time by the protective layer being continuously depleted in aluminum. Such depletion occurs as a result of the oxide film being continually worn away during use of the component and aluminum continuously diffusing from the protective layer to the surface. Accordingly, the maximum life of a protective layer depends on its aluminum content. A high proportion by weight of aluminum in an alloy for a protective layer is therefore desirable in the interests of longevity. However, a high proportion by weight of aluminum leads to embrittlement of the alloy. This is because the aluminum is not present in elemental form in the alloy, but at least a significant proportion of it is in the form of intermetallic compounds, in particular intermetallic compounds of nickel and alumium or cobalt and aluminium. Different properties, in particular different coefficients of thermal expansion, of these intermetallic compounds result in the embrittlement mentioned. This greatly restricts the proportion by weight of aluminium in the protective layer. It is stated that alloying of gallium into the protective layer ensures the desired oxidation resistance, with the alloying-in of gallium simultaneously leading to an improvement in the ductility of the protective layer, i.e. the brittleness of the protective layer is reduced.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a high-temperature-resistant component that overcomes the above-mentioned disadvantages of the prior art devices and methods of this general type. It is a further object of the invention to provide a method of protecting a high-temperature-resistant component against oxidation.
With the foregoing and other objects in view, there is provided, according to the invention, a high-temperature-resistant and oxidation-resistant component comprising a nickel-based or cobalt-based superalloy base material and a gallium-based antioxidation coating.
The term “gallium-based” means that the antioxidation coating consists predominantly of gallium, i.e. pure gallium or gallium containing a minor amount of aluminum. The antioxidation coating does not have to cover the entire component; it is also possible for only one region of the component or a plurality of regions of the component to be coated. Such an antioxidation coating affords a resistance to oxidation at least comparable with that of an aluminum layer since gallium forms a very resistant oxide which effectively protects material located under this oxide layer. In particular, an antioxidation coating based on gallium has the advantage that it does not significantly embrittle an underlying base material containing iron, cobalt or nickel. From an antioxidation coating based on aluminum, aluminum would diffuse into such a base material also known as alitizing. This would result in the formation of intermetallic phases whose brittleness, as mentioned above, would lead to considerable embrittlement of the base material.
Gallium also diffuses into the base material. However, the intermetallic phases formed thereby are significantly more ductile than analogous intermetallic phases with aluminum instead of gallium. Accordingly, an antioxidation coating based on gallium leads to significantly lower embrittlement which is controllable in the layer properties.
Preferably, an antioxidation coating comprising gallium and a protective alloy selected from the class consisting of MCrAlY alloys, where
a) M is at least one metal selected from the group consisting of nickel, cobalt and iron,
b) Cr is chromium,
c) Al is aluminum and
d) Y is a metal selected from the group consisting of yttrium, scandium, lanthanum and cerium,
are applied to the base material. The protective alloy can be applied to the base material and the antioxidation coating can be applied to the protective alloy.
When such an antioxidation coating is based on aluminum, it is likely to cause embrittlement by formation of intermetallic phases containing aluminum. The improved oxidation protection provided by application of aluminium generally leads, therefore, to a barely controllable susceptibility to cracking in the protective layer. Accordingly, for example, gas turbine blades which have such a protective alloy, can only be provided with additional oxidation protection by application of aluminum with great difficulty, if at all. Such protective alloys have to be screened by means of complicated coverings from an application of an aluminum layer carried out on the other blade, or have to be freed of aluminum by subsequent, expensive machining. Applying gallium instead of aluminum allows these protective alloys to be provided with additional oxidation protection without being unduly embrittled. A zone of an intermetallic phase comprising gallium, nickel and/or cobalt is preferably present adjacent to the antioxidation coating.
Further preference is given to the component being a part of a gas turbine unit, in particular a gas turbine blade, or an element for a combustion chamber lining. It is just such components of a gas turbine unit which are subjected to very high temperatures of the order of 600-1400° C. and correspondingly severe oxidation. Reliable and long-lived oxidation protection is a key property for such components.
In a particularly preferred embodiment, the component is a gas turbine blade of a stationary gas turbine or a gas turbine blade of an aircraft engine.
The gas turbine blade preferably has a cooling channel with a cooling channel wall which is provided at least in part with the antioxidation coating. Gas turbine blades are frequently cooled. This cooling is generally achieved by the gas turbine blade being provided with an internal cooling structure comprising a multiplicity of cooling channels through which a cooling gas, e.g. air, is conveyed. These cooling channels are also subjected to oxidation which can be considerably reduced by means of an effective and long-lived antioxidation coating based on gallium according to this invention.
With the foregoing and other objects in view, there is also provided, according to the invention, a method of protecting a high-temperature-resistant component, in particular a gas turbine blade, made of a nickel-based or cobalt-based superalloy as base material against oxidation, wherein gallium is applied to the component to form an antioxidation coating. The thickness of the resultin
Greenberg Laurence A.
Jones Deborah
Lerner Herbert L.
Siemens Aktiengesellschaft
Stemer Werner H.
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