Method for forming a protective coating with enhanced adhesion between layers

A method for forming a protective coating on a substrate comprising, applying a bond coating to the substrate, the 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, wherein 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, and applying a top coating to the bond coating. Additionally, a method for preparing a surface to receive and adhere to a coating comprising roughening the surface to create a micro-roughening network on the surface. In addition, a method of improving strain tolerance and cyclic spallation life of a protective coating.

TECHNICAL FIELD

This invention relates to protective coatings and methods for forming the same.

BACKGROUND OF THE INVENTION

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.

SUMMARY OF THE INVENTION

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.

DETAILED DESCRIPTION OF EMBODIMENTS

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 inFIGS. 1A-C.

A thermal barrier coating (TBC)10formed on a substrate12by a method in accordance with an embodiment of this invention is illustrated inFIG. 1C. The TBC10comprises a bond coating14and a top coating16. Although this embodiment illustrates a TBC, it should be understood that this invention is applicable to other types of coatings.

As show inFIG. 1A, the bond coating14is applied to the substrate12. The substrate can comprise, but is not limited to, any nickel or cobalt based alloy. For example, the substrate may comprise a superalloy such as GTD-222 (51Ni19Co22Cr1.2Al2.3Ti.94Ta.8Nb2WCBZr). The bond coating14may be applied using various methods, including high velocity oxy-fuel spraying. Suitable materials for use as a bond coating14include, but are not limited to, aluminide diffusion bond coatings. These aluminide diffusion bond coatings may include modified or alloyed aluminides, chromium aluminide (CrAI), palladium aluminide (PdAI), platinum aluminide (PtAI), silicon modified aluminides, simple aluminide, and over aluminized MCrAIY, where M stands for Fe, Ni, Co, Si, Hf, Ta, Re, noble metals, or a mixture of Ni and Co or additional elements and combinations that well known to those skilled in the art. Additionally, aluminide diffusion bond coatings may be about 1 mil to about 4 mils thick.

The surface of the bond coating14as applied to the substrate12has a first roughness that is inherently smooth. For example, a bond coating14made 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 coating14results in poor adherence of a top coating16, particularly air plasma spray (APS) top coatings. Consequently, the bond coating14is roughened to improve adherence of the top coating16to the bond coating.

As shown inFIG. 1B, a micro-roughening network18is created on the surface of the bond coating14by using an electrode22to ionize an inert gas and cause the ions20to flow into the bond coating surface. To ionize the inert gas, the electrode22is supplied a reverse polarity current (not shown). This reverse polarity current is a direct current set at a high frequency to create the ions20in the inert gas. The reverse polarity current is also set at an amperage between about 0 and about 10 amperes. A higher amperage setting results in a roughness greater than a roughness that would result from a lower amperage setting. Once the electrode22is supplied a reverse polarity current, it removes at least one electron from the inert gas that is supplied adjacent to the bond coating14. The inert gas may be, but is not limited to argon. While argon may be used as the inert gas, it should be understood that any inert gas may be used, provided that it is may be ionized and used in roughening the bond coating14in accordance with the methods of the present invention. As a result of the removal of at least one electron, the inert gas is ionized to a positive charge and the positively charged electrode22repels the ions20toward the bond coating14. These ions bombard the bond coating14, causing particulate fragments to break off and microscopic craters to form. Thus, the ionized inert gas20imparts a second surface roughness to the bond coating14greater than the first surface roughness.

The second surface roughness of the bond coating14may be between about 75microinches Ra to about 750 microinches Ra. More particularly, the second surface roughness of the bond coating14may 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 network18on the bond coating14promotes adhesion and mechanical bonding of the top coating16to the bond coating.

The roughening of the bond coating14to create the micro-roughening network18may be manual or automated using a mechanical device such as a robot. In addition, the bond coating14may 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 electrode22and the bond coating14may 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 ions20which roughen the surface of the bond coating14bombard 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 coating14. Conversely, the electrons strike the electrode22at 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 network18is created on the bond coating14, the top coating16may be applied to the bond coating as shown inFIG. 1C. Adhesion and mechanical bonding of the top coating16to the bond coating14is improved by the micro-roughening network18. The top coating16may be applied by air plasma spray (APS), for example. APS is particularly suitable for application of a dense vertically cracked (DVC) top coating16. This DVC top coating16has vertical cracks within the top coating that consequently improve the TBC strain tolerance and cyclic spallation life. Suitable materials for use as the top coating16include, but are not limited to, ceramic materials. These ceramic materials may comprise yttria, magnesia, ceria, scandia or rare earth oxide partially stabilized zirconia. For example, the top coat may comprise yttria stabilized zirconia in an amount of 8% by weight of the top coat. In addition, the top coating16may be about 10 mils to about 100 mils thick.

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.