MXPA99012033A - Improved coating and method for minimizing consumption of base material during high temperative service - Google Patents

Abstract

An improved coating and method for applying the coating to minimize the consumption of substrate material during high temperature service by formation of an intermediate metal aluminide layer. A layer of Ni or Co or combinations of Ni and Co are added to the substrate material as an overlay by a process such as electroplating or electroless plating. A diffusion aluminide is then formed at the surface by exposing the plated substrate to a source of aluminum at an elevated temperature.

Description

IMPROVED COATING AND METHOD TO MINIMIZE THE CONSUMPTION OF BASE MATERIAL DURING HIGH TEMPERATURE SERVICE FIELD OF THE INVENTION This invention relates to gas turbine engines, and more particularly, to a diffusion barrier layer applied to the blades in the turbine portion of a gas turbine engine.

DISCUSSION OF THE PREVIOUS TECHNIQUE Current coatings used on blades exposed to hot combustion gases in gas turbine engines, both for environmental protection and bond coatings in thermal barrier coating (TBC) systems, include Nickel and platinum aluminiides. These coatings are applied on substrate materials, usually nickel based super alloys, 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, is typically grown as an outer layer on a nickel-based superalloy, simply exposing the substrate to an environment rich in aluminum at elevated temperatures. The aluminum diffuses into the substrate, and combines with the nickel to form the external coating of NiAl. A coating of platinum iniuro alu (PtAl) is formed by electroplating platinum onto the nickel-based substrate to a predetermined thickness. Then, the exposure of the platinum to an environment rich in aluminum at elevated temperatures, causes the growth of an outer layer of PtAl, as the aluminum diffuses inward and reacts with the platinum. At the same time, the Ni diffuses out of the substrate, while the aluminum diffuses inward through the platinum. Accordingly, complex (Pt, Ni) Al structures are formed, exposing an electroplated substrate with Pt to an aluminum-rich atmosphere at elevated temperatures. As the aluminum diffuses inward towards the substrate, and the Ni diffuses outward through the Pt, the PtAlx phases precipitate out of solution, so that the resulting intermetallic Pt-NIAl also contains precipitates of Intermetallic PtAlx, where x is 2 or 3. The aluminiides are also used as link layers in thermal barrier systems, being intermediates between the substrate and an additional thermally resistant ceramic coating, such as zirconia stabilized with yttria (YSZ) , which is applied on aluminum. However, the process for forming these diffusion aluminides is essentially the same, that is, the substrate is exposed to aluminum, usually by a packet process or a CVD process, at elevated temperatures, and the resulting aluminide is grown inwardly. Of the surface. Over time, in the hot gaseous environment of a gas turbine engine, coatings, either applied as a coating to the environment, or as a bonding layer in a thermal barrier system, eventually degrade as a result of one or a combination of continuous processes: erosion due to the impact of hot gases on the blades; corrosion due to the reaction of the contaminants of the products of combustion with the surfaces of the blades; and oxidation. In order to repair a blade after service, it is necessary to remove not only corrosion products and oxidation products, but also coatings previously applied, if they have not been removed already. Because the coatings are grown into the substrate by a diffusion process, this involves removing a portion of what was once the substrate material. Because the parts are thin, this repair process limits the number of times the blades can be reused, because the minimum allowable wall thicknesses can not be violated.

What is desired is a method for forming protective aluminide coatings on the blades for use in the gas turbine service, where the growth of the coating in the substrate is eliminated or minimized.

BRIEF COMPENDIUM OF THE INVENTION The present invention relates to a protective coating that immediately provides a protective coating for a nickel-based superalloy substrate, and simultaneously acts as a diffusion barrier for superalloy components, such as blades, used in gas turbine applications. An advantage of the present invention is that it prolongs the life of a turbine blade by reducing the growth of the protective aluminide coating into the substrate material. This allows the subsequent removal of the coating, with minimal impact on the initial wall thickness of the blade, so that the life of the blade can be extended through additional repair cycles, in excess of the number that can be made in the present. Another advantage of the system of the present invention is that the diffusion barrier layer slows the outward migration of the alloying elements, such as Co, Cr, W, Re, Ta, Mo and Ti, from the substrate , during the operation at high temperature, in such a way that the mechanical and metallurgical properties of the substrate are maintained. Yet another advantage of the system of the present invention is that the diffusion barrier layer retards migration into the aluminum from the coating, so that the aluminum content of the coating is not depleted as rapidly as otherwise could occur. . The present invention provides an article for use in a high temperature oxidative environment, such as is found in a gas turbine engine comprising a nickel-based superalloy substrate. The overlay of the nickel-based superalloy substrate is a tightly adherent metallic coating, comprised of cobalt or nickel, or combinations of cobalt and nickel. The metal layer is applied on the substrate, and an external aluminide coating is formed by exposing the metal layer to a high concentration of aluminum, at an elevated temperature. The aluminum in this way has to diffuse through the aforementioned metallic coating, comprised of Co, Ni, or combinations of Co and Ni, retarding or delaying its reaction with the substrate of the superalloy. The metal diffusion barrier layer of the present invention can be applied to new blades prior to the application of aluminide, or it can be applied to blades removed from service as part of the repair cycle. The metallic barrier to the diffusion of the present invention would be applied after removing the previously existing coatings for the blades removed from the service. Other features and advantages of the present invention will become clearer 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.

DETAILED DESCRIPTION OF THE INVENTION The present invention comprises depositing a thin layer of a metal selected from the group consisting of Co, Ni, or combinations of Co and Ni, on the surface of a turbine engine component, such as a blade , before the formation of an aluminide protective coating. Typically, components such as blades are manufactured from superalloys, and often well-known Ni-based superalloys, such as Rene 80, Rene N5, Rene N6, Rene 142 and Rene 162. These superalloys include a number of elements of alloy. The nominal composition of Rene 80, for example, is, in percentage by weight, 0.17 percent C, 0.2 percent Mn, 0.2 percent Si, 14 percent Cr, 9.5 percent Co, 3 percent Al, 5.0 percent Ti, 4.0 percent Mo, 0.2 percent Faith, 0.015 percent B, 0.03 percent Zr, 4 percent W, and the rest of Ni and incidental impurities. Other superalloy substrate materials include similar elements in different percentages by weight. When Co, Ni or combinations thereof are applied on the surface of a blade, they initially serve as a new surface on which a protective coating of aluminide is grown. Of course, as is well known in the gas turbine art, the aluminide coating can serve as an environmental coating for this blade, or it can serve as an underlying bond layer to an upper thermal barrier layer. When the engine is taken out of service for repair or renovation, it is necessary to remove the existing coatings, as well as any oxidation or combustion products that may have formed on the external surfaces of the component parts of the turbine, in order to perform a renewal. Accordingly, in the present invention, the applied material is sacrificial, because it is used to form the protective aluminide coating, and minimizes the amount of substrate material that is removed during the renovation. Of course, for existing turbine components, Ni, Co, or combinations of Co and Ni can be added, after removing existing coatings, which have grown into the substrate, using existing procedures. This aggregate metal serves to replace the removed substrate material, and allows a new protective coating to be grown into the aggregate metal, rather than directly into the substrate, which has already experienced thinning as a result of the application and removal. conventional aluminum. In addition, the aggregate metal acts as a barrier to diffusion, to limit the migration into the Al, so that a minimum amount of substrate is consumed in the formation of the aluminide protective coating, and during service. The operating temperature of a gas turbine in the turbine portion of the engine is sufficiently high, approximately 1093 ° C and higher, so that the effects of diffusion processes can not be ignored. At these temperatures, there is a tendency for aluminum, which is added by a packet dust process or other equivalent process, to form an external aluminiide protective surface, diffuse through the aluminum layer, and migrate into the aluminum layer. substratum. There is also a tendency for substrate elements, such as Co, Re, Ta, Y, V, Mo, etc. , diffuse away from the substrate when they are present through the aluminide layer, to the outer surface, where they form oxides that are not tightly adherent. This diffusion process results in an adverse effect on the aluminide protective layer, where it becomes depleted of aluminum, altering its ability to act as a protective surface, and alters the composition of the underlying substrate, in which an area of diffusion where the chemical properties are altered, and therefore mechanical. Because the diffusion process depends on time and temperature, a relatively thick layer of Ni is preferred, in order to slow down the effects of the diffusion process. In another preferred embodiment, a thin layer of Co is applied to slow down the effects of the diffusion process, due to the diffusion of the aluminum inward through the cobalt layer, and the diffusion of the alloying elements outwardly. through the cobalt layer, it is significantly slower. Therefore, a thin layer of cobalt serves both as an effective surface for the formation of an aluminide protective coating, as a barrier to diffusion, to inhibit the migration of aluminum inwards, towards the substrate, and from the alloying elements towards the outside, moving away from the substrate. When the present invention is used to repair a superalloy component removed from the service of a gas turbine, it is first necessary to remove the oxidation and corrosion products that may have accumulated on the external surface as a result of exposure to the hot gases of the gas turbine. combustion or other pollutants. This is commonly done by degreasing and applying sandblasting. Then it is necessary to remove the coatings from the external surfaces of the component. This may include thermal barrier coatings, such as zirconia stabilized with yttria, which is removed by sandblasting, and aluminide bond layers or environmental aluminide coatings. The removal of the aluminide coatings is usually done by acid separation or fluoride ion cleaning (FIC). Of course, because many aluminiides of the prior art were applied, by growing the aluminiide into the substrate, this removal process reduces the thickness of the substrate. Subsequent to the removal of the coatings and cleaning with fluoride ion, the replacement of the superalloy components is virtually identical to the preparation for servicing the components of a new turbine. These components are usually aerodynamic surfaces, such as the blades or fins of the turbine. A thin layer of a metal, either Ni, Co, or combinations of Co and Ni, is deposited on the surface of the component to coat the surface. As noted above, because Al and other elements diffuse through H Co more slowly than through Ni, a thicker layer of Ni is required to effectively slow the diffusion sufficiently to be effective. The thickness of the deposited metal will vary from about 2 to about 60 microns, it being understood that the Co is applied at the lower end of the thickness range, and Ni is applied at the highest end of the thickness range, with an overlap in the thicknesses in the intermediate ranges. The metal can be deposited by any convenient technique that produces a uniform high quality coating thickness. These processes include electroplating, physical vapor deposition, plating without electrolysis, and chemical vapor deposit. It will be recognized that there may be certain trace impurities that do not adversely affect the characteristics and operation of the present invention present in the deposited metal. After the deposit, the component coated with The metal is exposed to an aluminum source at an elevated temperature, so that a diffusion aluminide is formed by reaction with the external surface of the metal. The aluminide diffusion can be made to grow by any well-known and proven methods in time, to grow these coatings, such as subjecting the component to a package process or to a process on the package. Other methods of success include chemical vapor deposition and application of aluminide in the vapor phase. Of course, it will be understood by experts in the field, who Through time, the Ni from the substrate will spread to ^ m ^ ^ through the Co, as will the Al from the coating, forming complex aluminiuros (Co, Ni) Al through the coating. The thickness of the aluminide coating will vary depending on the alloy system, but typically has a thickness after being exposed to the aluminum source, from about 2 to 90 microns. When Ni is applied in sufficient thickness, it can act to slow the diffusion of Al from the external surface to the substrate, and will also minimize the effect on the underlying substrate. However, the present invention is more effective when used to take advantage of the diffusion barrier characteristics of the Co. In a preferred embodiment, a thin layer of Co is applied to the substrate, typically up to a thickness of approximately 2 to 10 microns, and more preferably to a thickness of about 10 microns. Then a diffusion aluminide coating is formed as stipulated above. In this mode, the Co acts as a barrier to diffusion, slowing down the diffusion of aluminum into the superalloy substrate. The aluminide coating thus formed will have a reduced thickness, compared to that when the substrate is coated in the absence of a diffusion barrier. When a coated component is repaired by this method, a smaller amount of material will have to be removed, compared to the coated components. ______________________ .......... "___. conventional methods. In another preferred embodiment, a thin layer of Co is applied to the substrate, typically to a thickness of about 1 to 10 microns, and more preferably to a thickness of about 10 microns. Then a thin layer of a metal selected from the group consisting of Ni and Pt is deposited on the Co. The thickness of this second layer will vary from about 2 to 25 microns. More preferably, when the Pt is deposited, it is applied to a thickness of about 5 to 10 microns. These metal layers are deposited as stipulated above, using typical metal application techniques. Then an aluminide coating is formed as stipulated above. In this mode, the outer metal layer, either Ni or Pt, forms a protective aluminide. However, during the subsequent exposure to a high temperature, the Co acts as a barrier to diffusion. Because the Al diffuses inward from the aluminide, an additional protective coating of CoAl is formed on the surface, which becomes a complex coating of aluminide, (Ni, Co) Al or (Pt, Co) Al over time, depending on whether Ni or Pt is deposited on the Co. This complex aluminide will grow from the external surface of Co. Because Al diffuses faster than Ni, over time, the Al that is diffuses inwards from the external aluminide, and the Ni that diffuses outwards from the substrate, will be combined inside the Co, to form an additional NiAl complex in the vicinity of the Co / substrate interface. However, the effects of Co as a barrier to diffusion are such that this reaction is relatively insignificant, and the effect of this reaction on the Co / substrate interface is minimal, compared to current practices. The overall effects of the coatings of the present invention are to provide an effective protective aluminide coating to the superalloy turbine component for use in gas turbine applications, in such a way as to minimize the effects on the dimensional integrity of the component, while at the same time the adverse effects of the environment of the hot gas turbine on the superalloy substrate material are minimized, so that the chemical, and therefore mechanical, properties of the substrate are substantially maintained. Although the present invention has been described in connection with specific examples and embodiments, those skilled in the art will recognize that the present invention may have other variations and modifications within its scope. These examples and embodiments are intended as being typical of, rather than in some way limiting, the scope of the present invention, as presented in the appended claims. .... - _., _-_ ..

Claims (20)

1. A method for repairing a superalloy turbine component removed from the gas turbine service, which comprises: removing the oxidation and corrosion products from the exposed surfaces of the component under repair; remove the existing aluminide coatings from the exposed surfaces of the component under repair; then, deposit a thin layer of an element selected from the group consisting of Ni, Co and combinations of Ni and Co, on the surface of the component under repair, to coat the surface; Then, form a diffusion aluminide coating by exposing the coated surface to an aluminum source at an elevated temperature.

The method of claim 1, wherein the thin layer of an element selected from the group consisting of Ni, Co and combinations of Ni and Co is deposited on the surface of the component under repair, by a method selected from from the group consisting of electroplating, physical vapor deposition, plating without electrolysis, and chemical vapor deposition.

3. The method of claim 1, wherein the thin layer has a thickness of about 2 to 50 microns.

4. The method of claim 3, wherein the thin layer has a thickness of about 2 to 10 microns.

The method of claim 4, wherein the thin layer has a thickness of about 10 microns.

The method of claim 1, wherein the step of forming a diffusion aluminide coating is performed by a packet process.

The method of claim 1, wherein the step of forming a diffusion aluminide coating is performed by a process on the package.

The method of claim 1, wherein the step of forming a diffusion aluminide coating is performed by a chemical vapor deposition process.

The method of claim 1, wherein the step of forming a diffusion aluminide coating is carried out by a vapor phase aluminiide process.

10. A method for repairing a superalloy turbine component removed from the gas turbine service, which comprises: removing the oxidation and corrosion products from the exposed surfaces of the component under repair; remove the existing aluminide coatings from the exposed surfaces of the component under repair; then, deposit a thin layer of Co on the exposed surface of the component under repair; then, deposit a thin layer of an element selected from the group consisting of Ni and Pt, on the Co layer, superimposed on the surface of the component under repair, to coat the thin layer of Co; Then, form a diffusion aluminide coating by exposing the coated surface to an aluminum source at an elevated temperature.

The method of claim 10, wherein the thin layer of Co is deposited on the exposed surface of the component under repair, to a thickness of about 2 to 10 microns.

The method of claim 10, wherein the thin layer of an element selected from the group consisting of Ni and Pt, up to a thickness of about 2 to 25 microns, on the Co layer superimposed on the surface of the component in repair.

13. A superalloy turbine component for use in a gas turbine engine, which comprises: a superalloy base material substrate; a coating of diffusion aluminide superimposed on the superalloy base material, the diffusion aluminide coating being formed on the superalloy substrate, first depositing a layer of an element selected from the group consisting of Ni and Co, to coat the surface of the component, then forming the diffusion aluminide coating on the surface of the substrate, exposing the surface of the coated substrate to an aluminum source at an elevated temperature, such that the diffusion aluminide coating is substantially formed by diffusion of the aluminum into the coating of the substrate layer of the component, so 10 that the amount of material from the superalloy base material substrate which reacts with the aluminum source to form the diffusion aluminide coating, is substantially reduced, and the dimensional and chemical integrity of the base material substrate is substantially maintained. superalloy.

The superalloy turbine component of claim 13, wherein the diffusion aluminide coating superimposed on the superalloy base material is formed by first depositing a thin layer of Co on the substrate of superalloy material, and depositing then a thin layer of an element selected from the group consisting of Ni and Pt, before forming the diffusion aluminide coating.

15. A method for preparing a superalloy turbine component 25 for use in the turbine portion of a '**? * Jsimtetm' '- f? ü ', - -', f. * Ü ¡- > . . ._._. . i .__._ .., gas turbine engine, which comprises the steps of: cleaning the exposed surfaces of the component; then, depositing a thin layer of an element 5 selected from the group consisting of Ni, Co and combinations of Ni and Co, on the surface of the component under repair, to coat the surface; then, form a diffusion aluminide coating, exposing the coated surface to a source of aluminum at an elevated temperature.

16. The method of claim 1, wherein the thin layer has a thickness of about 2 to 50 microns.

17. The method of claim 3, wherein the thin layer has a thickness of about 2 to 10 microns.

18. The method of claim 1, wherein the step of forming a diffusion aluminide coating is performed by a packet process.

19. The method of claim 1, wherein the step of forming a diffusion aluminide coating is 20 performed by a process on the package. The method of claim 1, wherein the step of forming a diffusion aluminide coating is performed by a chemical vapor deposition process. d ^ M _? __ B__a > _____ * _