Metal treatment – Process of modifying or maintaining internal physical... – Producing or treating layered – bonded – welded – or...
Reexamination Certificate
1999-05-26
2001-02-20
Jenkins, Daniel J. (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Producing or treating layered, bonded, welded, or...
C148S527000, C428S680000
Reexamination Certificate
active
06190471
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to protective layers on nickel-base superalloy articles, and, more particularly, to the fabrication of such articles where the protective layer has a high content of hafnium and/or zirconium.
In an aircraft gas turbine (et) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is combusted, and the resulting hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of gas turns the turbine, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forwardly.
The hotter the exhaust gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the exhaust gas temperature. However, the maximum temperature of the exhaust gases is normally limited by the materials used to fabricate the turbine vanes and turbine blades of the turbine. In current engines, the turbine vanes and blades are made of nickel-based superalloys and can operate at temperatures of up to 1900-2100° F.
Many approaches have been used to increase the operating temperature limit and operating lives of the turbine blades and vanes. The compositions and processing of the materials themselves have been improved. The articles may be formed as oriented single crystals to take advantage of superior properties observed in certain crystallographic directions. Physical cooling techniques are used. In one widely used approach, internal cooling channels are provided within the components, and cool air is forced through the channels during engine operation.
In another approach, a protective layer or a ceramic/metal thermal barrier coating (TBC) system is applied to the turbine blade or turbine vane component, which acts as a substrate. The protective layer, with no overlying ceramic layer, is useful in intermediate-temperature applications. The currently known protective layers include diffusion aluminides and NiCoCrA1Y(X) overlays, where X is typically hafnium, silicon, and/or tantalum.
A ceramic thermal barrier coating layer may be applied overlying the protective layer, to form a thermal barrier coating system. The thermal barrier coating system is useful in higher-temperature applications. The ceramic thermal barrier coating insulates the component from the exhaust gas, permitting the exhaust gas to be hotter than would otherwise be possible with the particular material and fabrication process of the substrate.
Although superalloys coated with such protective layers and ceramic/metal thermal barrier coating systems do provide substantially improved performance over uncoated materials, there remains an opportunity for improvement in elevated temperature performance and environmental resistance. It has recently been discovered that incorporating hafnium, silicon, yttrium, and/or zirconium in the protective environmental coating improves its environmental resistance and adherence to the substrate. However, available techniques for applying protective layers with additions of hafnium, silicon, yttrium, and/or zirconium have not proved to be sufficiently reproducible for adoption in commercial fabrication operations. There is a need for an improved approach to preparing substrates having protective layers containing hafnium and/or zirconium. The present invention fulfills this need, and further provides related advantages.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a method for preparing a superalloy article with a protective layer thereon, wherein the protective layer has an elevated hafnium and/or zirconium content, and the articles made thereby. The approach retains excellent properties of the underlying substrate, while achieving the benefits of these protective-layer modifying elements. The approach is reliable and reproducible, and therefore suited for production operations.
In accordance with the invention, a method for preparing a superalloy article having a protective layer thereon comprises the steps of selecting a nominal nickel-base superalloy composition, and preparing a modified nominal nickel-base superalloy composition. The modified nominal nickel-base superalloy composition has an excess of a protective-layer modifying element over that of the nominal nickel-base superalloy composition, where the protective-layer modifying element is hafnium or zirconium. The protective-layer modifying element is preferably present in the modified nominal nickel-base superalloy composition in an amount of from about 0.2 to about 2.0 percent by weight, preferably about 1.0 percent by weight, for the case of hafnium, and/or in an amount of from about 0.1 to about 0.5 percent by weight, preferably about 0.25 percent by weight, for the case of zirconium, and combinations thereof. The method further includes processing the modified nominal nickel-base superalloy composition into a single crystal substrate having the shape of the article, and applying a protective layer to a surface of the substrate. The as-applied protective layer has a lower concentration of the protective-layer modifying element than the substrate. The protective layer modifying element is diffused from the substrate into the applied protective layer, providing an enhanced level of the protective layer modifying element in the protective layer.
The protective layer may be a diffusion aluminide or a NiCoCrA1Y(X) overlay, or other type of layer that benefits from the presence of increased amounts of hafnium and/or zirconium. Optionally, a ceramic layer may be deposited overlying the enhanced protective layer to form a thermal barrier coating system.
In prior fabrication approaches, the hafnium, silicon, yttrium, or zirconium have been added to the material deposited on the surface of the substrate that forms the protective layer. These fabrication techniques have proved to be insufficiently reproducible and reliable. In the present approach, by contrast, hafnium and/or zirconium in an elevated amount is added to the substrate alloy, and then diffused outwardly into the protective layer. This permits the protective layer which need not have hafnium, silicon, yttrium, or zirconium as it is deposited, to be deposited by more conventional techniques that are reliable and reproducible. There is concern that the presence of the protective-layer modifying element in the substrate in excessive amounts may adversely affect its mechanical properties. For this reason, only hafnium and/or zirconium, and not silicon and/or yttrium, are added in extra amounts to the substrate alloy.
Further, the hafnium and/or zirconium are added to the substrate in a specific narrow compositional range such that the benefits of their increased levels in the protective layer are realized without adverse effects on the mechanical properties of the substrate. The protective-layer modifying element is preferably present in the modified nominal nickel-base superalloy composition in an amount of from about 0.2 to about 2.0 percent by weight, preferably about 1.0 percent by weight, for the case of hafnium; and/or in an amount of from about 0.1 to about 0.5 percent by weight, preferably about 0.25 percent by weight, for the case of zirconium. Combinations of hafnium and zirconium within these compositional ranges are operable.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, 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 this preferred embodiment.
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patent: 5891267 (1999-
Darolia Ramgopal
Walston William S.
Coy Nicole
General Electric Company
Hess Andrew C.
Jenkins Daniel J.
Narciso David L.
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