MXPA99012029A - Coating a separated selective surface of an artic - Google Patents

Abstract

The present invention relates to a method for recovering an environmentally resistant coating 22 from a total thickness of the coating within a range of design thickness of the coating on a metal substrate 20 of an article 10, including the application of a metal restorative improved 30 to at least one separate local surface area 28, 29. Then at least the separate local surface area 28, 29 is coated with an environmentally resistant coating 32. To use the method for restoring a coating 22 on an article 10 that has experienced operation in service, and the separate surface area 28, 29 includes an undesirable amount of oxidation / corrosion products, first the oxidation / corrosion products of an outer portion 24 of the coating 22 are removed, while a coating 22 present in the areas is retained separate surfaces 28, 29, and the entire coating 22 is retained on the surface areas adjacent to the separate surface areas 28, 29. Then the restoration metal 30 is applied. To use the method to improve an existing coating 22, at least one separate surface area 28, 29 is selected based on an oxidation / corrosion etch pattern 16, 18 identified from similar articles 10 that underwent operation in service in an apparatus of a design for which article 10 is intended. Then the metal enhancer 30 is applied to selected area 28,

Description

COATING A SELECTIVE SURFACE SEPARATED FROM AN ARTICLE BACKGROUND OF THE INVENTION This invention relates to the restoration or improvement of a protective coating on an article, and more particularly, to the treatment of a separate local portion of the coating. Certain items, such as the components that operate in the hot gas path environment of gas turbine engines, are subject to significant temperature extremes, to an oxidizing atmosphere, and to pollutants, such as sulfur, sodium, calcium, and chlorine, which are present in the combustion gases. As a result of operation and service and exposure to this environment, the surfaces of the components, such as the blades and fins of the turbines, are subject to degradation by oxidation / corrosion. To protect the component substrate from excessive environmental attack, the surfaces of these components are usually treated with environmentally resistant coatings widely reported in the gas turbine engine technique. These environmental coatings are generally classified as diffusion coatings or overlays, distinguished by the processing methods or the degree of substrate consumption during deposition. The reported diffused aluminide coatings, which are based on the interdiffusion of Al applied with Ni from the substrate of a Ni-based superalloy to create an intermetallic surface layer, have been applied by a variety of methods, including package foundations, over of the package, vapor phase, chemical vapor deposit, and paste type coating. The thickness and aluminum content of the final product coating can be controlled by varying the coating parameters and the materials, such as the coating source materials, coating time, coating temperature, and aluminum activity. For example, this control is reported in a variety of Patents of the United States of America, including Number 3,544,348, - Boone et al. (Patented December 1, 1970), and Number 5,658,614 - Basta et al. (Patented 19). August 1997). It has been demonstrated that the performance of oxidation and corrosion resistance of the diffused aluminide coatings is improved by incorporating Pt, Rh, and / or Pd. To incorporate these elements, thin layers of these elements are deposited in general by electroplating or by means of physical vapor deposition, before the coating cycle with aluminide. One type of overlay, applied for the protection of oxidation and corrosion of Ni-based superalloy articles, includes Ni and Al, together with one or more other elements, such as Cr, Ta, Hf, Y and others. These coatings have been applied by deposition techniques, including plasma spray, crackling, electron beam, physical vapor deposition, among others. These processes are sometimes followed by aluminiuro application processes by diffusion, which improve the protection of the system's environment. During heat treatments and / or during the operation of the article, such as in a gas turbine engine, these overlapping coatings can diffuse into the substrate, consuming a portion of a load bearing wall, such as the wall of a turbine blade. Although the loss by oxidation / corrosion degradation or interdiffusion of the original coating composition that occurs during the engine service operation varies in intensity across the surfaces of a turbine blade, some surfaces experience very little attack or loss of power. composition, a current repair practice includes the complete removal of all coated surfaces from the diffused protective coatings or from the overlapping coatings prior to repair. This complete removal results in a loss of wall thickness, by the removal of the interdiffused region, which reduces the load support capacity of the component. Additionally, a complete removal of the coating creates problems with the maintenance of the designed cooling air flow patterns, and the flow rates for the air-cooled components, at the points where the cooling orifices, which communicate with The internal cooling passages intersect the external surface of the component, from which the coating has been removed, and must be replaced for the reuse of the component.

BRIEF COMPENDIUM OF THE INVENTION The present invention, in one form, provides a method for restoring an environmentally resistant coating, having a total coating thickness within a range of coating design thickness, on a metal substrate of a article having undergone service operation, the coating having an outer portion and an internal portion diffused with the metal substrate, for example, during the manufacture of the original coating, and / or after the subsequent operation in service. The external portion has at least a separate local surface area that includes an undesirable amount of oxidation / corrosion products resulting from exposure to the operation in service. The method comprises selectively removing the oxidation / corrosion products, at least from the surface area separated from the outer portion, while retaining any internal portion of the coating (when present) in the separated surface area, and retaining the inner and outer portions. external coating on the surface areas adjacent to the separated surface area. Then, at least the separated surface area is coated with a protective coating selected from aluminides and alloys including aluminum, using the selected coating parameters to maintain the total thickness of the coating substantially within the range of coating design thickness. In another form, the present invention provides a method for improving an existing environmentally resistant coating on an article, the coating of a total thickness of the coating within a range of coating design thickness. The method comprises selecting at least one local surface area separated from the coating most subject to oxidation / corrosion during the operation in service. Then, at least the separated surface area is coated with a protective coating selected from aluminides and alloys including aluminum, using selected coating parameters to maintain the total thickness of the coating substantially within the range of coating design thickness.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a diagrammatic perspective view of a gas turbine engine blade, from the concave side, showing the separate surface areas of heavier oxidation / corrosion local coating, resulting from the operation in service . Figure 2 is a photomicrographic sectional view including a portion of the environmentally resistant coating from which the oxidation / corrosion product has been removed. Figure 3 is a sectional photomicrograph view as in Figure 2, which shows, in addition, a bag within an additive coating layer, from which the corrosion has been removed. Figure 4 is a graph showing the effect of the thickness of the coating on the average tensile breaking life of a single Ni-superalloy glass material. Figure 5 is a diagrammatic sectional view as in Figure 2, showing the deposit of a restoration metal at the position of product removal. Figure 6 is a diagrammatic sectional view as in Figure 4, which includes the application of aluminide in the outer portions.

DETAILED DESCRIPTION OF THE INVENTION Each specific design of the turbine blade of the gas turbine engine has its own unique "attack pattern" of the environment, of more severe oxidation / corrosion of a protective coating, which occurs during the operation in service on an engine for which it has been designed. An etching pattern of separate selective surface areas of undesirable oxidation / corrosion is shown in the diagrammatic perspective view of Figure 1. In Figure 1, a turbine blade of a gas turbine engine for use in a high pressure turbine, shown from its concave side, generally at 10, comprises a base 12 and a blade 14 which includes thereon an environmentally resistant coating, a form of which is shown in the photomicrographs of Figures 2 and 3. The shapes of the Environmentally resistant coating include an aluminide coating, as well as an overlay of an alloy including Al. For example, widely reported overlays used in gas turbine engine technology, are the MCrAl or MCrAlY type of overlay, where M is at least one element selected from Fe, Ni, and Co, and Y represents any active element of oxygen. Shown on the concave side of the blade, on which a larger portion of the attack for a turbine blade generally occurs, are the separate local coating surface areas 16 and 18, which have been degraded during the engine service operation, to define an environmental attack pattern for that particular blade design. When the attack in a separate area exceeds the specified limits, removal of the oxidation / corrosion is required, and re-application of the coating must be conducted before the article can be returned to the service operation. The practice of the current state of the art includes removing all the surface coating, not only undesirably degraded portions, and reapplying the subsequent coating on all surfaces, not only of the attacked areas, without controlling the wall thickness with respect to the design limits. Typical design total coating thickness limits for aircraft engine turbine blades are in the range of approximately 25.4 microns to 127 microns for diffusion aluminides, and from approximately 25.4 microns to 254 microns for overlapping coatings. As discussed above, the complete removal of the coating can result in a detrimental thinning of the wall, and / or problems related to the openings of the cooling holes in the surface of the article. Overcoating an entire surface without controlling the total coating thickness can result in a significant increase in this thickness beyond the design limits. This increase in the total thickness of the coating can not only alter the patterns of air flow through a blade, but can also adversely affect the mechanical properties of the article. Numerous databases of existing mechanical properties show a strong correlation between the thickness of the coating and the key mechanical properties, such as resistance to stress rupture, high resistance to cycle fatigue, etc. Substantial decreases in mechanical properties can occur as coating thickness increases, especially in advanced nickel-based superalloys, where there is rapid interdiffusion between the substrate and the coating due to the high refractory element content of these alloys. Additionally thicker coatings are more susceptible to cracking than thinner coatings during thermal transients experienced during engine operation. Accordingly, the total thickness of the coating is selected for a particular article design, such as a turbine blade, to be within a design thickness scale, not only for considerations of air flow, but also to maintain the mechanical properties desired for the article during the operation. Typical data showing a relation of the thickness of the coating with the mechanical properties, and that the properties can be reduced by increasing the thickness, are the data included in the graph of Figure 4, which shows the effect of the coating thickness on the average tensile breaking life, of a single Ni-based superalloy glass material commercially used. The coating was a type of Pt-Al coating commercially used. In Figure 4, "inward" and "outward" refer to the predominant direction of diffusion during coating formation. Inward diffusion indicates that the coating is formed primarily by diffusion of the aluminum on the surfaces of the substrate, with a limited outward diffusion of nickel (ie, low temperature combined with high aluminum activity). Outward diffusion indicates that the coating is formed by outward diffusion of the nickel, along with inward diffusion of the aluminum (i.e., high temperature combined with lower aluminum activity). The present invention provides a method for the restoration or improvement of the environmental resistance of a coating on an article, while maintaining the design limits of the article, and substantially reducing the mechanical properties associated with the increase. in the thickness of the coating. For coated articles operated in service, this is done by selective removal of separate local surface areas, excessive oxidation / corrosion products, from the outer portion of a coating, without removing the inner portion of the coating, when present. The coating on the surface areas adjacent to the separate selective areas is retained during this removal of the undesirable products, and an inner portion of the coating on the substrate of the article can also be retained in the separate local areas. The removal of the oxidation / corrosion products can be achieved by mechanical or chemical elements, generally used in the art for these purposes. If the removal process is mechanical, it is required to mask the adjacent surfaces in general. If the removal process is chemical, it is not necessary to mask the adjacent surfaces in general, because the regions without oxidation / corrosion products are not affected by the ordinary chemicals used. The photomicrographs of Figures 2 and 3 show a practice of that portion of the present invention. In Figures 2 and 3, the substrate of article 20 was a Ni-based superalloy commercially used, sometimes referred to as Rene1 125 material, and where a commercially available aluminide CODEP coating, generally shown at 22. had been applied. coating 22 included an outer portion 24 and an internal portion 26 diffused with the substrate 20 in a manner well known in the art. During the operation of the gas turbine engine service, the local areas separated at 28 and 29 from the outer portion of the coating 24, experienced excessive oxidation / corrosion, with the area at 28 to a greater depth than the bag area at 29. To practice a form of the method of the present invention, this oxidation / corrosion was removed selectively from the locations separated at 28 and 29 by chemical elements, retaining the remainder of the outer portion of the coating 24 at location 29, coating the portion internal 26 below location 28, and retaining full coverage 22 over the areas adjacent to separate locations 28 and 29. If the selective separate surface areas from which oxidation / corrosion should be removed, include surface features such as openings or indentations For the discharge of cooling air, these characteristics can be masked if they use mechanical removal elements to avoid changing the airflow patterns of these characteristics. After the selective removal of oxidation / corrosion from locations 28 and 29, the surface coating areas adjacent to locations 28 and 29 were masked, and a restoration metal, such as Pt, Rh, and / or Pd, was selectively deposited in the recess at locations 28 and 29. The restoration metal , which in this example was Pt, was deposited to a thickness which, when the metal was diffused with the inner coating portion 26, is within a range of design thickness of the original coating. As mentioned above, generally for the high pressure turbine blades of a gas turbine engine, the coating design thickness for the overlapping coatings, such as the MCrAl type coating, is in the range of approximately 25.4 microns to 254 microns. Myrrhs, and for the diffusion aluminiuros is on the scale of approximately 25.4 microns to 127 microns. According to the above, the deposited Pt thickness is normally in the range of about 2.5 to 10 microns for these applications of gas turbine engine blades. As is well known and reported in the art, conveniently an element such as Pt, can be applied by electrodeposition. However, in an alternative way, the restoration metals can be applied by other methods, including spraying, sizzling, etc. Resulting from the deposit of Pt in this example, is the structure formed in the diagrammatic sectional view of Figure 5, where Pt 30 was electrodeposited selectively in the separate recess at location 28. After depositing the restoration metal 30 selectively in at separate locations 28 and 29, the masking of the outer surface was removed, and the restoration metal was heated to diffuse the metal inwardly from the inner portion 26. Typically, for a Pt deposit, the heat treatment was on the scale from about 900 to 1150 ° C, for a time, for example, from 0.5 to 4 hours, sufficient to diffuse the restoration metal into the underlying portion. Heat treatment at this point of the method, prior to the subsequent aluminiuum application, eliminates the need for prolonged exposure to high temperature during the aluminiide cycle, which in certain known methods is practiced to perform both the diffusion of Pt and the Aluminum application at the same time. Also, it provides significant flexibility in the selection of the aluminide application process, and in the parameters for the application of the surface aluminiuum substantially without increasing the total thickness of the coating beyond the coating design thickness, in accordance with the present invention. Following the diffusion heat treatment of the restoration metal, in this example aluminide was applied to the entire external surface, including the separate selective areas treated with Pt, as well as the other adjacent surfaces of the article. The aluminide application used the coating parameters that were selected to produce a coating portion of Pt-Al over the selective separated surface areas, and a surface enriched with aluminum in the adjacent areas, without a substantial increase in thickness in the areas adjacent, and while maintaining the total thickness of the coating within the design thickness range of the coating. An example of the resulting coating according to the present invention is shown in the diagrammatic sectional view of Figure 6, which shows the coating of Pt-Al 34 in the separate selective area previously identified as location 28, and the new coating portion. external enhanced with aluminum 32, without a substantial increase in coating thickness, within the range of coating design thickness. During the evaluation of the present invention, an air-cooled high-pressure turbine blade of a gas turbine engine was inspected, which had experienced service in the operation of the engine, to determine the oxidation / corrosion degradation on the surface of the blade. The blade was fabricated from a commercially-used Ni-based superalloy, sometimes referred to as the Rene '125 alloy, and included on the blade, an aluminide coating commercially available as the CODEP aluminide coating. The coating design thickness range for this article was from 25.4 microns to 101.6 microns, and the total coating thickness for the turbine blade as manufactured was in the range of approximately 50.8 microns to 76.2 microns. The inspection revealed separate local surface areas of oxidation / corrosion in an attack pattern on the blade similar to that shown in Figure 1, and to a degree requiring repair before the blade could be returned to the operational service. The surface of the blade was cleaned using a chemical cleaning process to remove surface contaminants, and to remove the oxidation / corrosion products identified in the attack pattern on the outer portion of the blade, in the separate local areas. The remainder of the coating was retained, generally as shown in Figures 2 and 3. The entire coating was also retained on the surfaces of the blade adjacent the separate selective surfaces. The surface areas of the coating outside the attack pattern were masked with standard electroplated lacquer. Then, the separated local areas from where oxidation and corrosion was removed, were electroplated with Pt to a thickness of approximately 2.5 microns. The masking was removed, and the Pt thus deposited was heated in a non-oxidizing atmosphere, at a temperature of about 1050 ° C, for about 2 hours, to diffuse the Pt into the underlying internal portion of the original coating. This resulted in a structure similar to that shown in Figure 5. After this diffusion heat treatment of the Pt deposit, the entire surface of the blade was subjected to aluminide, using a type of aluminide coating process in phase of commercial steam (on the package). The coating resulting from the practice of the present invention, represented by this example, had a total thickness of the coating within the design thickness range of the coating, while providing the blade with better resistance to oxidation and corrosion. As mentioned above, another form of the present invention comprises the practice of the method described above for the application of the Pt-Al coating in selective separate local surface areas, of a new, substantially unused, coated article, an example of which is a blade, to improve its resistance to oxidation and corrosion. The application of the Pt-Al coating on the separate areas is based on an identified attack pattern of similar articles that had experienced operation in service in the design apparatus for which the new article is intended. This invention provides a method for restoring or improving the environmental resistance of the 19 by a variety of published processes, while retaining the coating and coating portions as described above. Also, the metal application of restoration or improvement, as well as the application of aluminiuro, can be carried out by a variety of known processes, in the understanding that the considerations and thickness limitations of the present method are satisfied. coatings exposed to the service operation, such as in the gas trajectory environment of gas turbine engines, with coating application in separate local areas to match the actual needs for this improvement to the environment. This is done without a complete removal of the internal portions of a degraded coating, or without removal of the coating from the adjacent coated surfaces. This selective separate coating reduces the required amount of expensive metals, such as Pt. At the same time, it provides the ability to prevent damage to, or changes in, the surface characteristics of the article, for example, the characteristics of air flow in characteristics of blades, such as air cooling holes or surface indentations, as well as shore portions that can affect aerodynamic performance. The present invention has been described in relation to specific examples and embodiments which are intended to be typical of, rather than in some way limiting, the scope of the present invention. Those skilled in the art associated with this invention will understand that variations and modifications may be made to it without departing from the scope of the appended claims. For example, the oxidation / corrosion attack pattern will vary with each article design, and the removal of degraded surfaces can be performed

Claims (10)

1. A method for restoring an environmentally resistant coating 22 of a total thickness of the coating within a range of design thickness of the coating on a metal substrate 20 of an article 10 which has undergone operation in service, the coating 22 having a portion external 24 and an internal portion 26 diffused with the metal substrate 20, the outer portion 24 having at least a separate local surface area 28, 29 on which is an undesirable amount of oxidation / corrosion products resulting from exposure to the operation in service, which comprises the steps of: removing the oxidation / corrosion products at least from the separated surface area 28, 29 of the outer portion 24, while retaining any coating 24, 26 present in the separate surface area 28, 29 , and the internal and external portions of the cover 26, 24 are retained on the surface areas adjacent to the area s separate area 28, 29; and then applying an environmentally resistant coating 32 selected from the group consisting of aluminides and alloys including aluminum, at least the separated surface area 28, 29, using selected coating parameters to maintain the total thickness of the coating substantially within the range of coating design thickness. The method of claim 1, wherein at least the separated surface area 28, 29 is coated with at least one restoration metal 30 selected from the group consisting of Pt, Rh and Pd, to a thickness which, when diffuses with the inner portion of the cover 26, being within the range of design thickness of the coating. The method of claim 2, wherein the restoration metal 30 is heated at a temperature and for a time sufficient to diffuse the restoration metal 30 into the interior portion of the cover 26. 4. The method of the claim 1, wherein the protective coating 32 includes the entire outer portion 24. The method of claim 3, wherein: the article 10 is a gas turbine engine blade including an aerodynamic surface 14; the substrate 20 is a Ni-based superalloy; the separate local surface area 28, 29 is on the aerodynamic surface 14; the thickness range of the coating design is approximately 25.4 microns to 2.54 microns; the total thickness range of the coating is approximately 25.4 microns to 2.54 microns; the restoration metal 30 is Pt applied to a thickness in the range of about 1 to 10 microns; and, the restoration metal 30 is heated within the range of about 900 to 1150 ° C for about 0.5 to 4 hours. 6. A method for improving an existing environmentally resistant coating 22 on a metal substrate 20 of an article 10, which comprises the steps of: selecting at least a separate local surface area 28, 29 of the coating 22 more subject to undesirable oxidation / corrosion during the service operation, based on an oxidation / corrosion etch pattern 16, 18 identified from similar articles that would have experienced operation in service in the apparatus of a design for which article 10 is intended; masking the surface areas of the coating 22 adjacent the separate surface area 28, 29, leaving the exposed surface area 28, 29 exposed; and then applying an environmentally resistant coating 32 selected from the group consisting of aluminides and alloys including aluminum, at least the separate surface area 28, 29, using the selected coating parameters to substantially avoid a significant increase in the thickness of the coating. The method of claim 6, wherein at least the separate surface area 28, 29 is coated with at least one metal improver 30 selected from the group consisting of Pt, Rh, and Pd. The method of claim 7, wherein the improving metal 30 is heated at a temperature and for a time sufficient to diffuse the metal enhancer 30 into the existing coating 2

2. The method of claim 6, wherein the protective coating 32 includes the entire coating. The method of claim 8, wherein: article 10 is a gas turbine engine blade including an aerodynamic surface 14; the substrate 20 is a superalloy based on Neither; the separate local surface area 28, 29 is on the aerodynamic surface 14; the range of coating design thickness is approximately 25.4 to 254 microns; the metal enhancer 30 is Pt applied to a thickness in the range of about 1 to 10 microns; and the metal improver 30 is heated within the range of about 900 to 1150 ° C for about 0.5 to 4 hours. SUMMARY A method for restoring or improving an environmentally resistant coating 22 of a total thickness of the coating within a design thickness range of the coating on a metal substrate 20 of an article 10, includes the application of a restorative metal or enhancer. at least one separate local surface area 28, 29. At least the separate local surface area 28, 29 is coated with an environmentally resistant coating 32. To use the method for restoring a coating 22 on an article 10 that has experienced operation in service, and the separate surface area 28, 29 includes an undesirable amount of oxidation / corrosion products, first the oxidation / corrosion products of an outer portion 24 of the coating 22 are removed, while a coating 22 present in the separated surface areas 28, 29, and the entire coating 22 is retained on the surface areas ady acetes to the separated surface areas 28, 29. Then the restoration metal 30 is applied. To use the method to improve an existing coating 22, at least one separate surface area 28, 29 is selected based on an oxidation attack pattern. / Corrosion 16, 18 identified from similar articles 10 that underwent operation in service in an apparatus of a design for which article 10 is intended. The metal enhancer 30 is then applied to the selected area 28, 29. * * * * *