Patent Application
20140220375
This invention relates to protective coatings and methods for forming the same.
Coatings are often applied to metallic surfaces to protect against wear, erosion, corrosion, oxidation or to lower surface temperatures. Coatings, such as oxidation-corrosion protection coatings for a metal, function by diffusing protective oxide forming elements like aluminum and chrome to the surface that is exposed to harmful externalities. Thermal barrier coatings (TBCs) are made up of a bond coating on the substrate and a top coating on the bond coating. Examples of bond coatings include diffusion aluminide bond coatings. The top coating is typically zirconia based and may comprise yttria, magnesia, ceria, scandia or rare earth oxide partially stabilized zirconia.
Application of these protective high temperature oxidation coatings can be by thermal spray and diffusion techniques. The top coating may be applied air plasma spray (APS) or electron beam physical vapor deposition (EB-PVD). EB-PVD has been used successfully in commercial applications of ceramic top coatings to aluminide diffusion bond coatings to create TBCs that are strain tolerant and have good spallation life for high thermal cycle applications. Likewise, application of top coatings using APS has been found to create microstructures with vertical cracks that improve TBC cyclic spallation life. However, attempts to apply this air plasma spray, dense vertically cracked (DVC) top coating to aluminide bond coatings have been unsuccessful due to lack of adhesion to the smooth surface of the bond coatings. In cases where DVC top coatings have adhered to a bond coating, the spallation life of the TBCs have been inferior to TBCs with bond coatings having two to three times the surface roughness.
Accordingly, there is a need for a simple and economically desirable method for preparing a bond coating surface to receive and adhere to a top coating for TBCs with improved strain tolerance and cyclic spallation life.
This disclosure addresses the above described need in the art by providing a method for forming a protective coating on a substrate comprising, applying a bond coating having a first surface roughness, ionizing an inert gas which flows into the surface of the bond coating so as to impart a second surface roughness to the bond coating greater than the first surface roughness, and applying a top coating to the bond coating. The inert gas is ionized and caused to flow into the surface of the bond coating by a reverse polarity current supplied to an electrode which removes at least one electron from the inert gas. The positively charged ions of the inert gas are repelled by the positively charged electrode and flow into surface of the bonding agent, causing particulate fragments of the surface of the bond coating to break off. Therefore, the ions create microscopic craters in the surface of the bonding agent. Consequently, this roughening of the surface of the bond coating improves the adherence of the top coating to the bond coating.
Other objects, features, and advantages of this invention will be apparent from the following detailed description, drawing, and claims.
As summarized above, this disclosure encompasses a method for forming a protective coating on a substrate, a method for preparing a surface to receive and adhere to a coating. In a particular embodiment, a method for improving the strain tolerance and the cyclic spallation life of a thermal barrier coating (TBC) is disclosed. Embodiments of this invention are described in detail below and illustrated in
A thermal barrier coating (TBC) 10 formed on a substrate 12 by a method in accordance with an embodiment of this invention is illustrated in
As show in
The surface of the bond coating 14 as applied to the substrate 12 has a first roughness that is inherently smooth. For example, a bond coating 14 made of aluminide has a surface roughness of less than about 60 microinches Ra, where Ra is the arithmetic mean of displacement values as calculated to quantify the degree of roughness achieved. The inherent smoothness of the bond coating 14 results in poor adherence of a top coating 16, particularly air plasma spray (APS) top coatings. Consequently, the bond coating 14 is roughened to improve adherence of the top coating 16 to the bond coating.
As shown in
The second surface roughness of the bond coating 14 may be between about 75 microinches Ra to about 750 microinches Ra. More particularly, the second surface roughness of the bond coating 14 may be between about 100 microinches Ra to about 600 microinches Ra. Still more particularly, the second surface roughness of the bond coating may be between about 150 microinches Ra to about 450 microinches Ra. This second surface roughness resulting from the creation of the micro-roughening network 18 on the bond coating 14 promotes adhesion and mechanical bonding of the top coating 16 to the bond coating.
The roughening of the bond coating 14 to create the micro-roughening network 18 may be manual or automated using a mechanical device such as a robot. In addition, the bond coating 14 may be roughened in multiple passes to impart the desired second surface roughness.
The ionizing of the inert gas may be accomplished by using a reverse transfer arc welding torch. The reverse transfer arc welding torch may be a gas tungsten welding torch, a plasma arc welding torch, or any arc welding torch with a plasma source. Although a reverse transfer arc welding torch may be used in the present invention to ionize the inert gas, it should be understood that an electric arc is not conducted from the electrode in the reverse transfer arc welding torch to the bond coating. The formation of an electric arc between the electrode 22 and the bond coating 14 may melt the bond coating or cause cracking in the bond coating. To prevent the formation of an electric arc, the electrode is positioned at least about three times further from the bond coating than the distance the electrode would be positioned for arc welding. For example, a gas tungsten welding torch is positioned about 0.5 inches to about 1 inch away from a surface to be welded. In contrast, a gas tungsten welding torch used in a method in accordance with the present invention is positioned about 1.5 inches to about 3 inches from the bond coating to prevent an electric arc from forming.
In addition, the ions 20 which roughen the surface of the bond coating 14 bombard the bond coating at a slow speed relative to the speed at which the electrons strike the electrode. Consequently, only small amounts of heat are carried to the bond coating 14. Conversely, the electrons strike the electrode 22 at a high velocity and carry a substantial amount of welding heat. This, the heat may be removed from the electrode by water-cooling, for example.
Once the micro-roughening network 18 is created on the bond coating 14, the top coating 16 may be applied to the bond coating as shown in
The methods of forming TBCs of this invention may be used in articles having a TBC. Examples of such articles include a gas turbine or a diesel engine. In addition, the embodiments of the TBC may be formed on nickel or cobalt based alloys.
The present invention is further illustrated below in an example which is not to be construed in any way as imposing limitations upon the scope of the invention. On the contrary, it is to be clearly understood that resort may be had to various other embodiments, modifications, and equivalents thereof which, after reading the description therein, may suggest themselves to those skilled in the art without departing from the scope of the invention and the appended claims.
An example of an embodiment of a method for forming a TBC is disclosed in this example. General techniques of forming a TBC are well known in the art and are disclosed, for example, in U.S. Pat. No. 5,830,586, the disclosure of which is expressly incorporated herein by reference in its entirety.
In this embodiment, the forming of the TBC comprises applying an aluminide diffusion bond coating to either a nickel or cobalt based superalloy substrate. This bond coating has a smooth surface which is not optimal for applying an air plasma sprayed top coat. Thus, inert gas argon is then ionized by a gas tungsten arc welding machine and used to roughen the surface of the bond coating. The electrode is positioned at a distance from the aluminide diffusion bond coating to insure that an electric arc does not form. The reverse polarity current then removes electrons from the argon and creates positively charged argon ions which are repelled by the positively charged electrode towards the aluminide diffusion bond coating. The gas tungsten arc welding machine is traversed at a rate of about 1 inch per minute to impart a surface roughness of 150 microinches Ra to about 450 microinches Ra onto the bond coating. A top coating is air plasma sprayed onto the micro-roughening network created on the bond coating. The air plasma spraying of the dense vertically cracked top coating improves strain tolerance and cyclic spallation life of the TBC.
It should be understood that the foregoing relates to particular embodiments of the present invention, and that numerous changes may be made therein without departing from the scope of the invention as defined from the following claims.
This application is a continuation of U.S. patent application Ser. No. 11/307,266 filed on Jan. 30, 2006, the disclosure of which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11307266 | Jan 2006 | US |
| Child | 14251097 | US |
