Coating processes – Coating by vapor – gas – or smoke – Metal coating
Reexamination Certificate
1998-11-23
2001-03-27
Beck, Shrive (Department: 1762)
Coating processes
Coating by vapor, gas, or smoke
Metal coating
C427S253000, C427S255390
Reexamination Certificate
active
06207233
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a process for coating an airfoil to protect it against oxidation and corrosion during operation and to a particular apparatus for use in chemical vapor deposition processes to form an oxidation and corrosion resistant coating on external and internal surfaces of said airfoil.
Aluminide coatings are applied on nickel-based superalloy turbine airfoils to protect the airfoils against oxidation and corrosion during operation in the turbine section of engines. These coatings are formed by the deposition of aluminum onto the surfaces of the airfoils. A reaction occurs between the nickel in the airfoil material and the deposited aluminum to form nickel-based aluminides. At high temperatures, in the presence of air, the aluminum in the nickel aluminide coating forms a thin, adherent aluminum oxide at the surface of the coating. This aluminum oxide provides a barrier against further oxidation and corrosion of the turbine airfoil. The external surfaces of turbine airfoils in most engines are coated with aluminides. The performance requirements of the engines determine whether aluminide coatings also are required on the internal surfaces of the airfoils.
Currently, there are two processes used in the prior art to internally coat airfoils. One process employs a slurry technique and is used for airfoils that operate in the low pressure turbine section of an engine. The second process uses chemical vapor deposition to coat the internal surfaces of the airfoils for the high pressure turbine section of an engine. Different coating processes are employed for the two different types of airfoils because the high pressure turbine section of an engine operates at a higher temperature and pressure than the low pressure turbine section of an engine. As a result, the coating applied to high pressure turbine airfoils must have higher temperature capacity and must be more robust than those applied to low pressure turbine airfoils.
U.S. Pat. No. 5,366,765 to Milaniak et al. describes a slurry technique for coating internal passages in low pressure turbine airfoils. The slurry described in this patent cannot be used to coat the internal passages of high pressure turbine airfoils for the following reasons:
(1) the slurry produces a coating that is too brittle;
(2) the coating is too thick to apply to the internal cooling passages of high pressure turbine airfoils; and
(3) it is not compatible with the processes used to coat the external surfaces of airfoils.
Chemical vapor deposition processes used to coat airfoils employ an apparatus known as a coat boat as generally shown in FIG.
1
. During the coating process, turbine airfoils
10
are placed in an upright position within a compartmentalized, large metal box or coating fixture
12
, called a coat boat. The figure illustrates a typical coat box arrangement. To coat the internal passages of the airfoils
10
, chemicals
14
are placed in a compartment
16
below the airfoils. The airfoils are mounted on specialized plumbing tools
18
that allow vapors to flow through the internal cooling passages of the airfoils. Argon gas is introduced into the lower compartment
16
via inlet
20
to force the coating vapors through the internal areas of the airfoils. These vapors react with the internal surfaces of the airfoil to produce an aluminide coating. At the same, chemicals
14
in an upper compartment
22
create vapors which react with the external surfaces of the airfoil to form an aluminide coating thereon. There are problems however associated with this coat boat process. The problems include (1) the large coat boat which is heavy and ergonomically unsafe to handle, (2) excessive time to coat airfoils due to the large volume of the coat boat, and (3) a fixed capacity due to the fact that the process is a batch process.
Thus, there remains a need for an apparatus and a coating process which eliminates the problems associated with employment of a coat boat in the chemical vapor deposition processes currently employed. There is also a need for an apparatus and a coating process which allows the external and internal surfaces of an airfoil to be coated during a single cycle.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an apparatus for use in chemical vapor deposition processes for forming a corrosion and oxidation resistant coatings on the surfaces of airfoils.
It is a further object of the present invention to provide an apparatus and process as above which produces coatings of desired thickness and allows the internal surfaces of an airfoil to be coated simultaneously with the external surfaces of the airfoil.
It is yet a further object of the present invention to provide an apparatus and a process which is ergonomically acceptable, reduces coating time and cost, increases coating capacity and the flexibility of single piece coating.
The foregoing objects are attained by the coating composition and the coating process of the present invention.
In accordance with the present invention, a coating apparatus is provided which comprises a can defining an internal chamber. A hollow sleeve which itself defines an internal space is located within the chamber. The hollow sleeve defines with the can an annular space. A plurality of perforations are located in the wall of the hollow sleeve for communicating the annular space with the internal space defined by the sleeve. A lid for enclosing the can and the hollow sleeve is secured to the can for forming a closed compartment.
In accordance with a preferred embodiment of the present invention the lid includes a slot for supporting a workpiece in the form of an airfoil within the internal space defined by the hollow sleeve. The perforations in the hollow sleeve are sized so as to provide sufficient communication of chemical vapors from the annular space to the internal space so as to coat the airfoil by chemical vapor deposition during the coating process. The perforations are preferably sized between about 0.05 to 0.20 inches in diameter.
In accordance with the process of the present invention a chemical coating composition is located in the annular space between the can and the hollow sleeve. An activator is located within the internal space defined by the hollow sleeve below the airfoil. The can which is sealed by the lid is then heated for sufficient time and temperature so as to affect chemical vapor deposition of the coating chemical composition on the airfoil.
REFERENCES:
patent: 3978251 (1976-08-01), Stetson et al.
patent: 4687684 (1987-08-01), Restall et al.
patent: 4828933 (1989-05-01), McGill et al.
patent: 5071486 (1991-12-01), Chasteen
patent: 5407704 (1995-04-01), Basta et al.
Bose Krishnangshu
LaFlamme David W
Magyar Lester J
Perry Terry T.
Bachman & LaPointe P.C.
Beck Shrive
Chen Bret
United Technologies Corporation
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