Coatings for turbine components

Details Coatings for turbine components Coatings for turbine components

1998-10-02

2001-01-02

Zimmerman, John J. (Department: 1773)

Stock material or miscellaneous articles

All metal or with adjacent metals

Composite; i.e., plural, adjacent, spatially distinct metal...

C428S621000, C428S662000, C428S670000, C416S24100B

Utility Patent

active

06168875

ABSTRACT:

The present invention relates to coatings for the blades and vanes of turbines and particularly relates to the bond coat that is used with a thermal barrier coating on turbine components.
BACKGROUND OF THE INVENTION
In order to improve the efficiency of gas turbines, it is necessary to apply ceramic thermal barrier coatings (TBC's) to the blade and vane components that are exposed to very high temperatures. These TBC's lower the material surface temperatures of the turbine blades/vanes and extend their life and reliability. In order to bond the TBC coatings to the ceramic surface of the blades/vanes, a bond coat is used which also provides oxidation and hot corrosion protection to the blades and vanes. Current bond coats are normally alumina forming systems such as platinum aluminide diffusion coatings or NiCoCrAlY overlays. Often other elements can be added to NiCoCrAlY overlays such as Si, Ta, etc. At high temperatures, oxygen diffuses through the ceramic TBC which results in oxide growth and cracks can initiate in the TBC. Eventually, due to stresses from the oxidation process and fatigue due to thermal cycling, the TBC can spall resulting in accelerated oxidation of the bond coat and possible failure of the entire coating system. Initially cracks are formed in the thermal barrier coatings due to the growth of oxide and thermal expansion differences between the TBC coatings, thermally grown alumina, and bond coats. Of course, cracking can also occur in TBC's for other reasons such as bond coat creep. The spallation of the TBC can result in accelerated oxidation of the bond coat. Normally, An, failure of the TBC occurs when the oxide thickness has grown to 5 to 25 microns below the ceramic TBC. To a large extent, for engines which are base loaded oxide growth of the bond coat can determine the life of the coating system.
SUMMARY OF THE INVENTION
The invention relates to improving the life of a thermal barrier coating (TBC) for turbine blades and vanes by the use of a high temperature bond coat with good oxidation resistance. Specifically, the invention relates to the use of an iridium-niobium (Ir—Nb) alloy bond coat under the TBC to firmly bond the TBC to the substrate or underlying layers. Between the bond coat and the substrate is an underlying protective coating of a low pressure plasma sprayed coating or a vapor deposited coating. The low pressure plasma sprayed coating is formed from a mixture of metal powders such as NiCoCrAIY which may also include other metals such as Si and Ta. Preferably, there is a diffusion barrier coating between the underlying protective coating and the blade/vane substrate to limit interdiffusion between the coatings and the substrate. The diffusion barrier can be a metallic system such as tantalum (Ta), nickel-tantalum (Ni—Ta), or rhenium (Re) or it can be a ceramic such as alumina which is especially effective when in an amorphous form. The bond coat is bonded to the underlying layers by a diffusion heat treatment. Further a preoxidation procedure can be performed on the bond coat in a high temperature oxidation furnace to form a desirable oxide structure on the surface of the bond coat prior to the application of the TBC.


REFERENCES:
patent: 4036601 (1977-07-01), Weimar et al.
patent: 5624721 (1997-04-01), Strangman
patent: 5914189 (1999-06-01), Hasz et al.
patent: 5993976 (1999-11-01), Sahoo et al.

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