Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
1998-12-22
2001-05-08
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C428S621000, C428S652000, C428S670000, C428S678000, C428S680000, C416S24100B
Reexamination Certificate
active
06228510
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to gas turbine engines, and more particularly, to a diffusion barrier layer applied to airfoils in the turbine portion of a gas turbine engine.
DISCUSSION OF THE PRIOR ART
The current coatings used on airfoils exposed to the hot gases of combustion in gas turbine engines for both environmental protection and as bond coats in thermal barrier coating (TBC) systems include aluminides of nickel and platinum. These coatings are applied over substrate materials, typically nickel-base superalloys, to provide protection against oxidation and corrosion attack. These coatings are formed on the substrate in a number of different ways. For example, a nickel aluminide, NiAl, typically is grown as an outer coat on a nickel base superalloy by simply exposing the substrate to an aluminum rich environment at elevated temperatures. The aluminum diffuses into the substrate and combines with the nickel to form the outer coating of NiAl. A platinum aluminide (PtAl) coating is formed by electroplating platinum over the nickel-base substrate to a predetermined thickness. Then, exposure of the platinum to an aluminum-rich environment at elevated temperatures causes the growth of an outer layer of PtAl as the aluminum diffuses into and reacts with the platinum. At the same time, Ni diffuses outward from the substrate, while aluminum diffuses inward through the platinum. Thus, complex structures of (Pt,Ni) Al are formed by exposing a substrate electroplated with Pt to an atmosphere rich in aluminum at elevated temperatures. As the aluminum diffuses inward toward the substrate and Ni diffuses outward through the Pt, PtAl
x
phases precipitate out of solution so that the resulting Pt-NiAl intermetallic also contains precipitates of PtAl, intermetallic, where x is 2 or 3.
Aluminides are also used as bond coats in thermal barrier systems, being intermediate between the substrate and an additional thermally resistant ceramic coating such as yttria-stabilized zirconia (YSZ) which is applied over the aluminide. However, the process for forming these diffusion aluminides is essentially the same, that is to say, the substrate is exposed to aluminum, usually by a pack process or a CVD process at elevated temperatures, and the resulting aluminide is grown into the surface.
Over time in the hot gaseous environment of a gas turbine engine, the coatings, whether applied as an environmental coating or as a bond coat in a thermal barrier system eventually degrade as a result of one or a combination of ongoing processes: erosion due to the impingement of hot gases on the airfoils; corrosion due to reaction of contaminants in the products of combustion with the airfoil surfaces; and oxidation. In order to repair an airfoil after service, it is necessary to remove not only the corrosion products and oxidation products, but also the previously applied coatings, if they haven't already been removed. Because the coatings are grown into the substrate by a diffusion process, this involves removing a portion of what was once substrate material. Because the parts are thin, this repair process limits the number of times that the airfoils can be reused since minimum allowable wall thicknesses cannot be violated.
What is desired is a method of forming protective aluminide coatings on airfoils for use in gas turbine service in which the growth of the coating into the substrate is either eliminated or minimized.
BRIEF SUMMARY OF THE INVENTION
The present invention is directed toward a protective coating that at once provides a protective coating for a nickel-based superalloy substrate and simultaneously acts as a diffusion barrier for superalloy components, such as airfoils, used in gas turbine applications.
An advantage of the present invention is that it extends the life of a turbine airfoil by reducing the growth of the protective aluminide coating into the substrate material. This permits the subsequent removal of the coating to occur with minimal impact on the initial wall thickness of the airfoil, so that the airfoil life can be extended through additional repair cycles in excess of the number that currently can be performed.
Another advantage of the system of the present invention is that the diffusion barrier layer slows the outward migration of alloying elements such as Co, Cr, W, Re, Ta, Mo and Ti from the substrate during high temperature operation so that the mechanical and metallurgical properties of the substrate is maintained.
Still another advantage of the system of the present invention is that the diffusion barrier layer retards the inward migration of aluminum from the coating so that the aluminum content of the coating is not depleted as rapidly as might otherwise occur.
The present invention provides for an article for use in a high temperature oxidative environment such as is found in a gas turbine engine comprising a nickel base superalloy substrate. Overlying the nickel base superalloy substrate is a tightly adherent metallic coating comprised of cobalt or nickel, or combinations of cobalt and nickel. The metallic layer is applied over the substrate and an outer coating of aluminide is formed by exposing the metallic layer to a high concentration of aluminum at an elevated temperature. The aluminum thus has to diffuse through the aforesaid metallic coating comprised of Co, Ni, or combinations of Co and Ni, retarding or delaying its reaction with the superalloy substrate.
The metallic diffusion barrier layer of the present invention may be applied to new airfoils prior to aluminiding or it may be applied to airfoils removed from service as part of the repair cycle. The metallic diffusion barrier of the present invention would be applied after removal of preexisting coatings for airfoils removed from service.
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.
REFERENCES:
patent: 3544348 (1970-12-01), Boone
patent: 4176433 (1979-12-01), Lee et al.
patent: 4897315 (1990-01-01), Gupta
patent: 4965095 (1990-10-01), Baldi
patent: 5116691 (1992-05-01), Darolla et al.
patent: 5516380 (1996-05-01), Darolla et al.
patent: 5716720 (1998-02-01), Murphy
patent: 5820337 (1998-10-01), Jackson et al.
patent: 5824423 (1998-10-01), Maxwell et al.
patent: 5975852 (1999-11-01), Nagaraj et al.
patent: 04032546 (1992-02-01), None
patent: 9605331 (1996-02-01), None
patent: 9613622 (1996-05-01), None
Chen Keng N.
Hu Genfa
Ngiam Shih T.
General Electric Company
Gressel Gerry S.
Hess Andrew C.
Jones Deborah
Young Bryant
LandOfFree
Coating and method for minimizing consumption of base... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Coating and method for minimizing consumption of base..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Coating and method for minimizing consumption of base... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2547028