Thermal barrier coating ceramic structure

Details Thermal barrier coating ceramic structure Thermal barrier coating ceramic structure

2001-01-16

2002-09-24

Jones, Deborah (Department: 1775)

Stock material or miscellaneous articles

All metal or with adjacent metals

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

C428S629000, C428S633000, C428S472000, C428S701000, C427S255700, C416S24100B

Reexamination Certificate

active

06455173

ABSTRACT:

TECHNICAL FIELD
The present invention relates to protective coatings for metallic articles and more particularly to an improved ceramic topcoat of a thermal barrier coating system for superalloy substrates.
BACKGROUND INFORMATION
During gas turbine engine operation, hot section components such as combustors, turbine blade and vane airfoils, turbine frames, and exhaust nozzles are subject to oxidizing and corrosive, high temperature combustion effluent gas. Because these components often are subjected concurrently to high magnitude thermally and mechanically induced stress, the art has developed a variety of techniques in the design and manufacture of these components to ensure maintenance of structural and metallurgical integrity throughout the operating range of the engine. For example, components typically are manufactured from material compositions such as nickel- and cobalt-base superalloys having desirable properties at elevated, operating range temperatures. In the case of turbine airfoils, the selected alloy generally is formed by casting. For enhanced high temperature strength, grain structure advantageously may be controlled during solidification of the casting to produce a directionally solidified or single crystal structure, thereby providing greater strength for a given alloy composition.
In addition to component strength enhancement by selection of alloy composition and control of the casting process, both internal and external cooling schemes are employed extensively to maintain component temperatures below critical levels. Tailored film cooling of external surfaces and sophisticated turbulent flow cooling of serpentine shaped internal cavities in the cast airfoils are routinely utilized in advanced gas turbine engine designs respectively to decrease the thermal energy input to the component and reduce the temperature rise thereof. Despite efforts to optimize these varied approaches, both alone and in combination, advanced gas turbine engine efficiency is limited by the inability of the hot section components to achieve acceptable operating lives under increased mechanical and thermal loading.
An additional method employed by those skilled in the art of gas turbine engine design is the use of a relatively thin ceramic insulative outer layer on surfaces exposed to the effluent gas flow. The ceramic coating facilitates component operation at greater operating temperatures. This coating, generally referred to in the industry as a thermal barrier coating (“TBC”), effectively shields the metallic substrate of the component from temperature extremes. By reducing the thermal energy input to the component, higher combustion effluent gas temperatures and/or more efficient use of cooling flows are realized with a resultant increase in engine operating efficiency.
Conventional ceramic coatings are prone to delamination at or near the ceramic/substrate interface due to differences in coefficients of thermal expansion between the relatively brittle ceramic and the more ductile superalloy substrate. The ceramic may spall or separate from the component surface. This failure mechanism is aggravated and accelerated under conditions of thermal cycling inherent in gas turbine engine operation. In order to prevent premature failure of the ceramic, methods of providing strain tolerant ceramic coatings have been developed. Certain moderate service applications employ porous or pre-cracked ceramic layers. In more harsh operating environments, such as those found in advanced gas turbine engines, the art exploits strain tolerant open columnar ceramic crystal or grain microstructures, such as those described in U.S. Pat. No. 4,321,311 to Strangman, the disclosure of which is herein incorporated by reference. These columnar grain microstructures have a generally parallel grain orientation and are disposed in a normal direction, perpendicular to the surface of the substrate. They are considered to provide improved strain tolerance due to the segregated nature of the columnar grains which form intercolumnar gaps therebetween.
Substantial attention also has been directed to the use of an intermediate or bond coat layer disposed between the substrate and the ceramic layer. The bond coat employs a composition designed both to enhance the chemical bond strength between the ceramic topcoat and metal substrate as well as to serve as a protective coating in the event of premature ceramic topcoat loss.
There are presently two primary classes of bond coat compositions conventionally employed in multilayered TBC systems of this type. One type of metallic bond coat typically specified by gas turbine engine designers is referred to as MCrAlY alloy, where M is iron, cobalt, nickel, or mixtures thereof The other major constituents, namely chromium, aluminum and yttrium, are represented by their elemental symbols. As used herein, the chemical symbol “Y” signifies the use of yttrium as well as other related reactive elements such as zirconium, lanthanum, and mixtures thereof A conventional MCrAlY bond coat is described in U.S. Pat. No. 4,585,481 to Gupta et al., the disclosure of which is herein incorporated by reference. In coating a superalloy substrate, the MCrAlY bond coat first is applied to the substrate by a method such as physical vapor deposition (“PVD”) or low pressure plasma spraying.
The MCrAlY class of alloys are characteristically very resistant to oxidation at the elevated temperatures experienced by hot section components due to their ability to form a thin adherent protective external film of aluminum oxide or alumina. As used herein, the term “alumina” signifies predominantly aluminum oxide which may be altered by the presence of reactive elements to contain, for example, yttrium oxide or zirconium oxide. In addition to providing protection, the alumina film also provides a chemically compatible surface on which to grow the insulative ceramic topcoat. As known by those having skill in the art, the ceramic topcoat most commonly employed is zirconium oxide or zirconia, either partially or fully stabilized through the addition of oxides of yttrium, magnesium, or calcium. Conventional open columnar structured stabilized zirconia is grown on the alumina film by PVD in which the component to be coated is rotated at a constant rate in a ceramic vapor in a vacuum chamber. This coating system generally is considered to exhibit improved integrity under cyclic thermal conditions over ceramic coatings disposed directly on the metallic substrate, thereby providing the intended insulative protection to the underlying article over an extended period.
Another type of metallic bond coat routinely specified by those skilled in the art includes a class of materials known as aluminides. These are popular compositions for gas turbine engine components and include nickel, cobalt, and iron modified aluminides as well as platinum modified aluminides. Generally, aluminides are intermediate phases or intermetallic compounds with physical, chemical, and mechanical properties substantially different from the more conventional MCrAlY bond coats. Some aluminide compositions are known to be useful coatings in and of themselves for protecting iron-, cobalt-, and nickel-base alloys from oxidation and corrosion; however, some aluminides may be used as bond coats for ceramic topcoats in TBC systems.
The aluminide-based TBC system is similar to the MCrAlY-based TBC system insofar as the aluminide bond coat is first formed on the substrate surface by conventional diffusion processes such as pack cementation as described by Duderstadt et al. in U.S. Pat. No. 5,238,752 and Strangman in published U.K. Patent Application GB 2,285,632A, the disclosures of which are herein incorporated by reference. According to this method, aluminum from an aluminum halide gas in the pack mixture reacts and interdiffuses with the substrate surface over time at elevated temperature. Strangman discusses production of aluminide bond coats, for example, by reacting a nickel-, iron-, or cobalt-base superalloy article substrate with an aluminum rich va

Affiliated with

Also associated with

Rating

Thermal barrier coating ceramic structure does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Thermal barrier coating ceramic structure, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Thermal barrier coating ceramic structure will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2910809