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  Oxidation-resistant coatings bonded to metal substrates, and related articles and processesUSPTO Application #: 20060141160Title: Oxidation-resistant coatings bonded to metal substrates, and related articles and processes Abstract: An article is described, which includes a metal-based substrate and an oxidation-resistant coating bonded to the substrate by a bonding agent, such as a braze material. The oxidation-resistant coating material is often an aluminide- or MCrAlX-type coating, and can be one which contains relatively high amounts of aluminum. The coating is often very smooth, for maximum aerodynamic efficiency. The oxidation-resistant coating can be applied and bonded to the substrate by a variety of methods, using slurries, braze tapes, or metal foils. Coating repair methods are also described. (end of abstract)
Agent: General Electric Company Global Research - Niskayuna, NY, US Inventor: Wayne Charles Hasz USPTO Applicaton #: 20060141160 - Class: 427372200 (USPTO) Related Patent Categories: Coating Processes, With Post-treatment Of Coating Or Coating Material, Heating Or Drying (e.g., Polymerizing, Vulcanizing, Curing, Etc.) The Patent Description & Claims data below is from USPTO Patent Application 20060141160. Brief Patent Description - Full Patent Description - Patent Application Claims
TECHNICAL FIELD [0001] The invention disclosed herein generally relates to protective coatings for metal substrates. More specifically, the invention is directed to oxidation-resistant coatings for high-temperature substrates, and related processes. BACKGROUND OF THE INVENTION [0002] Components formed of specialty materials like superalloys are used in various industrial applications, under a diverse set of operating conditions. In many cases, the components are provided with coatings which impart several characteristics, such as corrosion resistance, heat resistance, oxidation resistance, and wear resistance. [0003] Oxidation-resistant coatings are often critical if the underlying component is exposed to an oxidizing atmosphere for an extended period of time. This is especially true in the case of components formed of aluminum-containing superalloys, which are often used in gas turbine engines which operate at elevated temperatures, e.g., 1000.degree. C.-1150.degree. C. In the absence of a protective coating, the oxidizing atmosphere can deplete the superalloy of aluminum. Since aluminum can greatly enhance the oxidation-resistance of the protective coatings, the loss of aluminum can be detrimental to the integrity of the superalloy. [0004] While increasing the aluminum content in the protective coatings often improves oxidation-resistance, the increase may be detrimental to other properties. For example, higher aluminum levels can decrease coating ductility, causing cracking in the coating during service. This in turn results in the loss of the hermetic, protective nature of the coating. [0005] Many of the oxidation-resistant coatings for superalloys are formed from conventional alloys of the formula MCrAlX, where M is iron, nickel, or cobalt. "X" is one of the elements mentioned below, in a more detailed description of the coating. In many instances, the oxidation-resistant coating is used as the most external layer of a component, e.g., a turbine engine blade. In that case, the coating often has to be very smooth, for maximum aerodynamic efficiency. [0006] Some of the thermal spray techniques are often used to deposit oxidation-resistant coatings with a desired surface texture. Examples of the thermal spray processes are high velocity oxy-fuel (HVOF) and vacuum plasma spray (VPS). Each of these techniques has attributes which make it suitable for a given situation. For example, VPS applications are often desirable when it is critical that the final coating be substantially free of internal oxides. [0007] While these types of thermal spray techniques are quite suitable for applying oxidation-resistant coatings under many circumstances, they exhibit drawbacks in other situations. For example, VPS and HVOF techniques are sometimes not effective for applying the coatings to regions of a substrate which are somewhat inaccessible. The spray equipment may be too large and cumbersome for such regions. As an illustration, it can be very difficult to thermally spray a coating on a flange of a turbine engine part. Moreover, applying the coating to any internal cavity in the part can be problematic. [0008] Furthermore, thermal spray processes may include one or more masking steps. These steps can be very time-consuming. Thus, it is often very difficult to carry out local repairs using the processes. [0009] It should thus be apparent that new methods for efficiently applying oxidation-resistant coatings to a substrate would be welcome in the art. The methods should be capable of providing a coating with substantially the same quality as coatings applied by thermal spray processes. Moreover, it would be desirable if the new methods were capable of applying the coating to inaccessible regions of a substrate. Furthermore, the methods should be compatible with any other fabrication processes to which the substrate is being subjected. It would also be very beneficial if the new methods allowed one to readily change the composition of the oxidation-resistant coating, to satisfy the needs of a particular substrate. It would also be desirable if the coating method were applicable to local (small area) application, for "new make" and in-situ repair of oxidation-resistant coatings. SUMMARY OF THE INVENTION [0010] One embodiment of the present invention is directed to a an article, comprising a metal-based substrate and an oxidation-resistant coating bonded to the substrate by a bonding agent. The oxidation-resistant coating materials are often aluminide- or MCrAlX-type coatings, discussed in detail below. In some preferred embodiments, the oxidation-resistant coating is an "aluminum-rich" coating. Such coatings often contain about 10 atomic % to about 50 atomic % aluminum. [0011] The bonding agent is often a braze material. Such materials usually include at least one metal selected from the group consisting of nickel, cobalt, iron, and a precious metal. Very often, the braze material comprises at least about 40% by weight nickel. [0012] The oxidation-resistant coating can be applied and bonded to the substrate by a variety of methods, discussed in more detail below. For example, the coating material and the bonding agent material can be pre-mixed to form a slurry. The slurry can be applied to the substrate by various techniques, and then heated to fuse the coating to the substrate with the bonding agent. [0013] As an alternative, the oxidation-resistant coating and the bonding agent can first be formed into a "sheet", i.e., a green tape or a metal foil. Green tapes can be fabricated by a variety of techniques, as described below. The sheet can then be cut to a desired size, and temporarily attached to the substrate. Sufficient heat can then be applied to fuse the sheet to the substrate. In some embodiments, two separate green sheets could be used--one formed of an oxidation-resistant coating material, and the other formed of the bonding agent material. [0014] Techniques for fabricating the metal foils are also discussed below. The foil can be sized and attached to the substrate by various means. Sufficient heat is then applied to fuse the foil to the substrate. [0015] Another embodiment of the invention is directed to a method for replacing an oxidation-resistant coating applied over a metal-based substrate. The existing oxidation-resistant coating (or portions thereof) is first removed by conventional techniques. The new oxidation-resistant coating is then applied as described herein, e.g., in the form of a slurry, tape, or foil. The new coating can then be fused to the substrate. [0016] Further details regarding the various features of this invention are found in the remainder of the specification, and in the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS [0017] FIG. 1 is a cross-sectional photomicrograph of a coating system which includes an oxidation-resistant coating fused to a substrate. DETAILED DESCRIPTION OF THE INVENTION [0018] As mentioned above, the article of this invention includes a metal-based substrate. A wide variety of metals and metal alloys can be used as the substrate. The term "metal-based" in reference to substrates disclosed herein refers to those which are primarily formed of metal or metal alloys, but which may also include some non-metallic components, e.g., ceramic sections. Usually, the substrate is a heat-resistant alloy, e.g., superalloys which typically have an operating temperature of up to about 1000-1150.degree. C. (The term "superalloy" is usually intended to embrace complex cobalt- or nickel-based alloys which include one or more other elements, such as aluminum, tungsten, molybdenum, titanium, and iron.) Superalloys are described in various references, such as U.S. Pat. Nos. 5,759,932; 5,399,313 and 4,116,723, all incorporated herein by reference. High temperature alloys are also generally described in Kirk-Othmer's Encyclopedia of Chemical Technology, 3rd Edition, Vol. 12, pp. 417-479 (1980), and Vol. 15, pp. 787-800 (1981). The type of substrate can vary widely, but it is often in the form of a jet engine part, such as a turbine nozzle or blade. [0019] Various types of oxidation-resistant coatings can be bonded to the substrate. Many of them comprise an alloy of the formula MCrAlX, where M is selected from the group consisting of Fe, Ni, Co, and mixtures of any of the foregoing. In that formula, X is selected from the group consisting of Y, Ta, Si, Hf, Ti, Zr, B, C, and combinations thereof. These types of coatings are generally well-known in the art, and described, for example, in U.S. Pat. No. 6,372,299 (Thompson et al), which is incorporated herein by reference. Many of these types of alloys have a general composition, by weight, of about 5% to about 30% chromium; about 4.5% to about 12.5% aluminum; and about 0.1% to about 1.2% yttrium; with M constituting the balance. Continue reading... Full patent description for Oxidation-resistant coatings bonded to metal substrates, and related articles and processes Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Oxidation-resistant coatings bonded to metal substrates, and related articles and processes patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. 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