MXPA99012032A - Renewing a thermal barrier coating system - Google Patents

Abstract

A thermal barrier coating (TBC) system, comprising a metallic bond coat on an article surface and an outer ceramic TBC is repaired by renewing without removing the bond coat. The outer ceramic TBC is removed to expose a bond coat substrate. At least one discrete local surface area of the article surface subject to bond coat degradation is selected from a thermal pattern unique to the article surface from service operation. At least one renewal metal selected from Pt, Rh, and Pd is applied at least to the selected discrete local surface area and heated to diffuse the renewal metal into the bond coat substrate. An environmental resistant coating selected from aluminides and alloys including aluminum is applied at least to the discrete local surface area over the renewal metal. Thereafter, an outer ceramic TBC is applied to the article surface.

Description

RENOVATION OF A THERMAL BARRIER COATING SYSTEM BACKGROUND OF THE INVENTION This invention relates to the renewal of a protective coating on an article, and more particularly, to the renewal of a thermal barrier coating (TBC) system, by renewing a bonding layer beneath the coating of thermal barrier. Certain components or articles that operate in the environment of the gas path of a gas turbine engine are subject to significant temperature extremes, and to degradation by oxidative and corrosive conditions. It is common practice in the gas turbine engine technique to apply a thermal barrier coating (TBC) system to the surfaces of these components to protect them from this environment, while also providing the opportunity to improve efficiency of the engine by making possible an increase in operating temperatures. Thermal barrier coating systems are generally comprised of a metal environmental inner lining, generally referred to herein as a tie layer, applied to a surface of the article, and an outer layer of insulating ceramic, generally applied directly to the surface of the article. link layer. Typical of these -i * »? ... i.,. yy. t. i and y • "" '"-» "i mn i". t i ii mmfcjama jM ^^ aaMuEMM-i external coatings of thermal barrier coating is one based on zirconia stabilized with yttria, for example, about 92 weight percent zirconia stabilized with about 8 weight percent yttria. A preferred method of applying or depositing a thermal barrier coating is by physical vapor deposition with electron beam, although plasma spray processes are widely used for burner applications of turbine engines. The device is sold commercially for these uses. This general type of thermal barrier coating system has been reported for some time, as evidenced by U.S. Patent Nos. 4,055,705 - Stecura et al. (Patented October 25, 1977); Number 4,095,003 - Weatherly et al. (Patented June 13, 1978 (; Number 4,328,285 - Siemers et al. (Patented May 4, 1982); Number 5,216,808 - Martus et al. (Patented June 8, 1993); 5,236,745 - Gupta et al. (Patented August 17, 1993) The bond layers most frequently used for turbine blades of gas turbine engines and burner components have been classified into two general types. a type of M Al superimposed, where M is at least one element selected from Fe, Ni and Co, * - • * '* "* - for example, MAl, MA1Y, MCrAl, and MCrAlY, the other is that of the extended aluminide coatings, both types have been widely used and have been reported in relation to the turbine technology The MCrAlY type coating has been applied by physical vapor deposition, including crackling, cathode arc, and electron beam, as well as by plasma spray processes.The composition, microstructure and thickness of the coating are controlled by the parameters The applied aluminide coatings have been applied by a variety of methods, including, as used in the art, package foundation, package top, vapor phase, chemical vapor deposition, and paste coating processes. The thickness and aluminum content of the final product coating has been controlled by varying the coating time, the coating temperature, and the aluminum activity of the coating and process waste. The performance of these coatings is often improved by incorporating elements such as Pt, Rh, Pd, Cr, Si, Hf, Zr, and / or Y. With any type of link layer, the elements of the link layer are interdiffused with a substrate of the article during processing or operation or both, producing a diffusion zone between the link layer and the underlying substrate of the article. The diffusion zone is considered as part of the link layer and, therefore, as the thermal barrier coating system. As used herein, the term "link layer substrate" is intended to mean at least a portion of the remaining link layer and diffusion zone between the link layer and the underlying substrate of the article. For gas turbine engine applications, the materials and processing methods selected for the thermal barrier coating system are selected to provide resistance to the fragmentation of the ceramic outer layer during the thermal cycle of the engine, as well as resistance to the oxidizing and corrosive environment in the event of a fragmentation event of the thermal barrier coating. During normal operation of the motor after the time, the thermal barrier coating system, including the bonding layer and the outer ceramic layer, will degrade in certain surface areas more subject to the strenuous operating conditions. It has been observed that the bonding layer is interdiffused with a substrate of the article in these surface areas during operation, to the extent that its protective capacity has been reduced below an acceptable level, necessitating the removal and reapplication of a protective coating. . A current practice in this repair is to remove the entire thermal barrier coating system, including the bonding layer, together with its diffusion zone, with the article substrate, and the outer ceramic layer.

After any required repair of the structure of the article, the entire thermal barrier coating system is reapplied, including a new bonding layer and a new external ceramic coating. However, this type 5 removal of the thermal barrier coating system, where the diffusion area of the bonding layer is removed, will lead to a thinning of the walls of the article. Numerous databases of mechanical properties have reported that they show a strong correlation of properties key mechanics (including drag resistance and high fatigue capacity of the cycle), and the thickness of the remaining wall. Consequently, this thinning of the wall can result in a reduction in the operational life, and in the subsequent possibility of repair of the article, as well as problems of airflow control, if the air cooling openings are involved.

BRIEF COMPENDIUM OF THE INVENTION The present invention, in one form, provides a method for renewing a thermal barrier coating system on the surface of an article, without removing the entire metal bond layer. The method comprises removing the external ceramic thermal barrier coating to expose a substrate of the bond layer, including any original bond layer and the diffusion zone of the gold layer. tm ^ mmm? m link. From a single thermal pattern for the article surface of the service operation, at least one local surface area separated from the surface of the article subject to degradation of the link layer during operation in service is selected. At least one renewal metal selected from Pt, Rh and Pd is applied to the substrate of the bond layer, at least in the separate local surface area; and the renewing metal is heated at a temperature and for a time sufficient to diffuse the renewal metal into the substrate of the bonding layer. Then an environmentally resistant coating selected from aluminiuides and alloys including aluminum, at least the local surface area separated, on the diffused renewal metal is applied. Subsequently, an outer ceramic thermal barrier coating is applied to the surface of the article.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a diagrammatic perspective view of a gas turbine engine blade, from the concave side, showing separate local surface areas subject to degradation of the coating, in a unique thermal pattern for the article, the operation in service. Figure 2 is a graphical comparison of the change in aluminum content of a link layer at different locations of the turbine blade, as a result of the operation in service. Figure 3 is a graphical comparison of the change in platinum content of a link layer at different locations of the turbine blade, as a result of the operation in service. Figure 4 is a graphical comparison of the differences in PtAl depletion in a binding layer, as a function of the fragmentation of an external ceramic thermal barrier coating. Figure 5 is a graphical comparison of the platinum content in the link layer, before and after the renewal of the link layer. Figure 6 is a graphical comparison of the aluminum content in the link layer, before and after the renewal of the link layer.

DETAILED DESCRIPTION OF THE INVENTION During the operation in service of a gas turbine engine, it is known that a hot operating article or component, such as a burner, an aerodynamic surface of a turbine blade or fin, etc., it experiments and exhibits a non-uniform thermal pattern across its surface and unique for the design of the particular component and motor. A typical single thermal pattern on the concave side of a surface aerodynamics of a gas turbine blade is shown in the perspective diagrammatic view of Figure 1. In Figure 1, a high pressure turbine blade of the gas turbine engine shown from the concave side of the airfoil generally in 10, comprises a base 12 and an aerodynamic surface 14, which includes on it a protective thermal barrier coating system. Shown on the surface of the article on the concave side of the aerodynamic surface 14 are the separate local surface areas represented by 16 and 18, which are subject to more strenuous thermal conditions during the operation in service of the blade in a motor. In some unique patterns, these areas 16 and 18 merge along the front edge of the aerodynamic surface. This type of thermal pattern results in a non-uniform degradation of the surface of the article, including a non-uniform diffusion of a surface coating, such as a bonding layer, into the article substrate, and / or oxidation of a film layer. exposed link. In the above-described type of the thermal barrier coating system, the surface areas of the article, for example, on an aerodynamic surface, subjected to the higher temperatures, experience a greater diffusion loss of the critical elements of the link layer towards within the substrate, and the potential for fragmentation of the thermal barrier coating, and the IlakMu »UA-_ ^ tA ^ áát ^ ^ subsequent exposure of the bonding layer to the oxidizing and corrosive atmosphere. The cooler locations on the surface of the components with this thermal barrier coating system can be virtually unaffected by the operation of the motor. The graphical comparisons of Figures 2 and 3 show, respectively, the depletion of aluminum and platinum from a link layer including these elements, in different locations on an aerodynamic surface of a turbine blade. This depletion in the form of diffusion of the elements towards the substrate of the article as a function of the thermal exposure, was presented under an external ceramic thermal barrier coating, in the absence of fragmentation of the thermal barrier coating. The graphical comparison of Figure 4 shows the effect of fragmentation, and the resulting direct exposure of the binding layer to an oxidizing atmosphere, on the amount of aluminum exhaustion of a link layer that included PtAl at the front edge of a aerodynamic surface of the turbine blade. Accordingly, it was recognized, in connection with the present invention, that the potentially harmful, as well as costly, removal described above of the entire link layer, including its diffusion zone into the article substrate, was unnecessary. In accordance with the present invention, only the selected surface areas in a single thermal pattern for the component, and more affected by this exposure, require a renovation in a thermal barrier coating system. An initial step in the practice of the present invention is the removal of the surface of the article, such as the surface of a blade, from the external ceramic thermal barrier coating, substantially without affecting the underlying substrate of the tie layer. A preferred removal method used in the evaluation of the present invention included directing a stream of water under pressure, for example, on the scale of about 0.35 to 1.4 kg / cm, to an externally treated external ceramic thermal barrier coating layer. . This removed the thermal barrier coating without damaging or removing the substrate from the binding layer, as defined above. In general, the methods for carrying out this removal, reported extensively in the current art, include the application of controlled pressure of abrasives, sand, vapors, liquids, etc. This removal exposes the substrate of the link layer, which may include any remaining original link layer, as well as the diffusion zone where the link layer has diffused into the substrate of the article. After removal of the external ceramic thermal barrier coating, at least one local surface area separated from the surface of the article subject to degradation of the bond layer was selected from a unique thermal pattern for the surface of the article, and observed from the operation in service. Examples of these separate local areas are the areas of the aerodynamic surfaces 16 and 18 of Figure 1; Another type of thermal pattern includes a united combination of the fused areas 16 and 18 along the front edge of the aerodynamic surface. A renewal metal selected from Pt, Rh, Pd, or combinations thereof was applied, at least to the exposed substrate of the bond layer, in the selected separate local area. When only the separate areas were to be coated, the rest of the surface was masked, for example, using standard electroplating tape. Other commonly used masking materials include non-conductive fastening tool, for example, plastic. The renewal metal was then applied, in one example by electrodeposition. The thickness of the applied renovation metal was varied based on the degree of interdiffusion of the bond layer with the article substrate in the selected spaced areas (eg, as shown in Figures 2, 3 and 4), the scheme of Global coating planned for the article, and consideration of the specific operational limitations unique to the article. Following its application, the renovation metal, and conveniently all the article to which the renovation metal was applied, was heated in a non-oxidizing atmosphere, at a temperature normally in the range of about 900 ° C to 1150 ° C, and it was maintained for a time, typically from about 0.5 to 4 hours, sufficient to diffuse the renewal metal into the underlying substrate of the bond layer. In one example, where the renewal metal was Pt applied to a thickness of about 6 microns, the temperature was about 1050 ° C, and the time was about 2 hours, to diffuse the renewal metal into the substrate of the link layer. This separate diffusion heat treatment eliminates the need, in another current practice, for a prolonged exposure to a high temperature during an aluminide application cycle, sometimes used to perform both the deposition of aluminum and the diffusion of an applied metal, such as Pt, into the underlying coating or substrate. Also, through the present invention, significant flexibility in the selection of process type and process parameters is allowed for a protective coating of the subsequently applied environment, for example, an aluminide. Subsequent to the application and diffusion of the renewal metal, an environmentally resistant coating was applied, for example, an aluminide or an alloy including aluminum, at least to the surface areas separated on the melt renewal metal, and preferably to the entire surface area of the article, such as the entire aerodynamic surface of a blade, including the surface areas to which a renovation metal was not applied in the previous steps. In one example, where the entire aerodynamic surface was coated with an environmental aluminide coating, the coating parameters were selected in such a way as to produce a modified aluminum aluminide coating for Pt renewal, for example, a coating of PtAl, in the selected separate local surface areas, while the adjacent surface areas not treated with the renovation metal, were enriched with aluminum without appreciable thickness change. As a result of this practice, the renewed bond coating of a restored protective capacity remained in the selected separate local areas. In addition, an improved environmental capacity was provided to the adjacent surface areas. The graphical comparison of Figure 5 shows an example of the platinum profile of the selected separate local areas treated from an aerodynamic surface of a turbine blade after the operation in service, and after the replacement of Pt and Al. The graphical comparison of Figure 6 shows an example of the aluminum profile of the aerodynamic surface of the turbine blade after the operation in service, and after the application of aluminide over the entire aerodynamic surface, as described above. In both cases, the capacity was renewed to levels close to the coating levels as it was produced. After the renewal of the bonding layer as described above, a new external thermal thermal barrier coating was applied on the bonding layer, using physical electron beam vapor deposition, in accordance with current commercial practice in the technique. The practice of the present invention restored the thermal barrier coating system without negative effects on the article, for example, a reduction in wall thickness as discussed above, or an unnecessary removal of the substrate from the bond layer. In addition, areas of the bond layer adjacent to the selected spaced areas treated with the renovation metal improved without a substantial increase in thickness. The present invention provides an efficient and effective method for repairing a thermal barrier coating system, by renewing the selected separated surface areas of an underlying bond layer, without removing a substrate from the bond layer. In one way, it improves the environmental capacity of the surface areas adjacent to the selected areas. Although the invention has been described in connection with specific examples and embodiments, it is intended that they are typical of, rather than in any way limiting, the scope of the present invention. It will be understood by those skilled in the art that the invention may have variations and modifications without departing from the scope of the appended claims.

Claims (6)

1. A method for renewing a thermal barrier coating system on the surface of an article 14, the thermal barrier coating system comprising a metal bond layer on the surface of the article, and an external ceramic thermal barrier coating, the which comprises the steps of: 10 removing the outer ceramic thermal barrier coating to expose a substrate of the tie layer; selecting, from a thermal pattern, unique to the surface of article 14 of the operation in service, at least a separate local surface area 16, 15 18 of the surface of article 14, subject to degradation of the link layer during the operation in service; applying to the exposed substrate of the binding layer, at least in the separate local surface area, at least one renewal metal selected from the group consisting of Pt, Rh and Pd; heating the renewing metal at a temperature and for a time sufficient to diffuse the renewal metal into the substrate of the bonding layer; 25 apply a medium resistant coating environment selected from the group consisting of aluminum and alloys including aluminum, at least the local surface area separated on the renewal metal; and then applying an external ceramic thermal barrier coating to the surface of article 14. The method of claim 1, wherein the environmentally resistant coating is applied to the entire surface of article 14. 3. The method of claim 1, wherein the renewing metal is heated to a temperature in the range of about 900 ° C to 1150 ° C, for a time on the scale of about 0.5 to 4 hours. 4. The method of claim 3, wherein: the renewal metal is Pt; the environmentally resistant coating is an aluminide; and the environmentally resistant coating is applied to the entire surface of article 14. 5. The method of claim 3, wherein: the renewing metal is Pt; the environmentally resistant coating is an alloy of the MAl type, wherein M is at least one metal selected from the group consisting of Fe, Co and Ni; and the environmentally resistant coating is applied to the entire surface of article 14. 6. The method of claim 5, wherein the MAl type alloy is an alloy of MCrAlY.