Coating processes – Coating by vapor – gas – or smoke – Metal coating
Reexamination Certificate
1998-12-22
2001-05-01
Meeks, Timothy (Department: 1762)
Coating processes
Coating by vapor, gas, or smoke
Metal coating
C427S253000, C427S295000, C427S328000, C427S398100
Reexamination Certificate
active
06224941
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed toward a process for applying aluminide coatings to superalloy components used in gas turbine applications, and more particularly to an improved vapor phase aluminiding process to coat nickel-based and cobalt-based superalloy components used in the turbine section of gas turbine engines.
2. Discussion of Prior art
Aluminide coatings are applied to provide protection of superalloy turbine components from gas turbine engines to protect the substrate material by many different processes. One such process is set forth in U.S. Pat. No. 3,837,901, ('901) now expired, to Seybolt, incorporated herein by reference and assigned to the assignee of the present invention. In the 901' patent, an aluminide coating is applied by embedding the turbine components in a bed of powders having aluminum as a source. Generally, the active powders set forth in this patent were iron-aluminum compounds mixed with inert alumina, and the powders were activated by passing a halide gas through the carrier while heating the bed to a temperature in the range of about 1650° F. to about 2000° F. Subsequent improvements in the process have included modifications in the aluminum powder sources, modifications in the powder sizes and improvements in the gas delivery systems. These processes have generally come to be known as “pack processes” or vapor phase aluminide processes. These processes have several infirmities. First, since they involve producing coatings requiring specific compositions, they must be carefully mixed so that the coating compositions can be attained. However, once depleted, the mixed metal powders are not readily recyclable, cannot be replenished and present a disposal problem. A second problem associated with these pack processes is that the measuring and moving as well as disposing of the powders is labor intensive. Third, the process, while producing a good protective coating, yields a coating of varying thickness that is not easily controlled. Finally, as temperature demands of gas turbines have increased, cooling passages have been added to the turbine components. The powders of the pack processes frequently clog these channels, and the removal of these powders from these fine cooling passages is a further problem.
What is desired is a coating method that produces a good quality aluminide coating while avoiding the problems associated with the prior art pack processes.
BRIEF SUMMARY OF THE INVENTION
The present invention provides an improved process for applying aluminide coatings to superalloy components used in gas turbine applications. As compared with the prior available vapor phase aluminiding techniques for applying these aluminide coatings, the processes of the present invention provides an aluminide coating that has a more even coating thickness, while maintaining the advantage of the relatively thin coatings typically associated with vapor phase aluminiding processes.
A further advantage of the present invention is that the process is less labor intensive and more environmentally friendly, since heavy powders are not involved, eliminating the need to move these powders or to dispose of these powders. The pellets used in the present invention are easier to segregate and reprocess, if needed.
In accordance with the present invention, an improved process for applying aluminide coatings to superalloy components used in gas turbine applications comprises a series of steps, the first of which is to place the superalloy components into a retort with an aluminum-containing source. Air is then evacuated from the retort by introducing an inert gas into the retort. The retort is then heated, typically by placing the retort into a furnace, to a preselected temperature. While maintaining the preselected temperature, the inert gas is purged from the retort by introducing hydrogen gas. The hydrogen gas in the retort is then reduced to a preselected pressure below atmospheric pressure, by imposing a partial vacuum, while the temperature is held constant. Next, a halide-containing gas is introduced into the retort. This gas reacts with the aluminum source in the retort at the preselected temperature, creating a vapor of aluminum rich gas. The vapor of aluminum-rich gas passes over the surface of the superalloy substrate interacting with it to deposit a thin, substantially uniform coating until a thin coating is obtained. Hydrogen gas is then reintroduced into the retort to purge the gases from the retort. The process of introducing and purging the halide-containing gas into the retort can be repeated to uniformly increase the thickness of the coating as desired. After the desired thickness is achieved, the pressure of the gases in the retort are again reduced below atmospheric pressure, insert gases are introduced into the retort and the retort is cooled.
Thus, it can be seen that an advantage of the present invention is that a uniform, yet thicker aluminide coating can be achieved with the process of the present invention, if so desired.
Another advantage of the present invention is that since powders are not required to be used as an aluminum source or as filler material, the tendency for cooling holes to become plugged with powders in typical turbine components such as airfoils is eliminated. Finally since powders are not required, the labor intensive powder preparation process involving the accurate weighing and mixing of powders can be eliminated.
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: 4965095 (1990-10-01), Baldi
patent: 5264245 (1993-11-01), Punola et al.
patent: 5492726 (1996-02-01), Rose et al.
patent: 5928725 (1999-07-01), Howard et al.
Chen Keng N.
Yow Kwok H.
Chen Bret
General Electric Company
Gressel Gerry S.
Hess Andrew C.
Meeks Timothy
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