Patent Application
20110151140
This invention relates generally to coatings for metallic substrates, and more specifically to processes for providing nickel aluminide-based coatings on metallic substrates.
Processes for providing NiAl type coatings are discussed in U.S. Pat. Nos. 6,620,524 and; 6,933,058 and US Patent Publication, 20070207339. Exemplary processes to provide a NiAl based overlay coating include cathodic arc (ion plasma) deposition techniques using sources having a generally homogenous composition. This process results in uniform chemistry through the coating thickness, but can lead to defects in the coating associated with macroparticle transfer and subsequent shadowing. Discussion of macroparticles from cathodic arc processing may be found in A. Anders, “Cathodic Arcs: From Fractal Spots to Energetic Condensation”, Springer (2008).
The cathodic arc deposited NiAl coatings may suffer from macroparticles that are believed to reduce oxidation performance. Additionally, the consumable source (i.e., cathode) used in the deposition process is difficult to manufacture due to the high melting temperature, large freezing range, and low ductility (i.e., brittleness) of the NiAl material.
U.S. Pat. No. 6,964,791 discloses a two-step method for supplying a coating onto a metallic substrate. The method includes disposing a first coating layer onto said substrate comprising nickel, zirconium, and aluminum, and thereafter, disposing onto the first coating layer, a second coating layer comprising at least 90 atomic percent aluminum. Subsequent processing leads to the formation of a substantially single-phase reacted coating layer comprising nickel aluminide with a B2 structure. The reacted coating layer includes a higher aluminum concentration at the outer surface and a lower aluminum concentration near the coating/substrate interface.
The teachings of U.S. Pat. No. 6,964,791 demonstrate that substantially pure aluminum deposited over NiAl(CrZr) coating layer exhibits high interdiffusion when heated to 1079° C. (1975° F.), thus covering the macroparticles mentioned above. However, the coating exhibits a higher aluminum surface level and an aluminum compositional gradient throughout the thickness of the coating.
Further improvements in coatings and coating processes are still sought, in particular, to reduce costs, ease manufacturing difficulties, and shorten coating times. In addition, improvements in alloying capabilities are sought due to the brittle nature of NiAl materials.
The above-mentioned need or needs may be met by exemplary embodiments, which provide a method for forming a nickel aluminide based coating on a metallic substrate.
An exemplary method includes providing a first source able to supply aluminum content for a nickel aluminide based coating, providing a second source able to supply nickel and at least one alloy element content for the nickel aluminide based coating, providing a metallic substrate, disposing a nickel aluminide based coating precursor on at least a portion of the metallic substrate comprising aluminum content provided by the first source, and nickel and at least one alloy element content provided by the second source; and forming the nickel aluminide based coating from the coating precursor.
The coating precursor layers are disposed with sufficient relative thicknesses, so that following subsequent processing a nickel aluminide based coating may be provided on a metallic substrate.
Separation of a significant portion of the aluminum from the remainder of the coating composition in the consumable cathodes eliminates the difficulties associated with brittle NiAl cathodes.
The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the concluding part of the specification. The invention, however, may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which:
Exemplary embodiments are described herein with reference to the figures. In general terms, an exemplary coating is disposed on a metallic substrate by supplying the desired coating composition in at least two parts to provide a coating precursor. Subsequent processing of the coating precursor provides the desired coating.
In an exemplary embodiment, the coating composition, prior to deposition, is separated into at least two portions, and provided through separate “consumables”, e.g., cathodes for cathodic arc (ion plasma) deposition. In an exemplary embodiment, the portions of the coating composition provided by the consumable are very different in composition. One portion is substantially pure aluminum (i.e., aluminum source), and the other portion is nickel plus other alloying elements (i.e., NiX source). For example, the other alloying elements may include those elements desired for environmental resistance, strengthening, and the like, and theoretically may be substantially free of aluminum. For example, in addition to nickel, the second portion may include alloying elements such as chromium, zirconium, hafnium, silicon, yttrium, or combinations thereof. Additional alloying elements may include titanium, tantalum, rhenium, lanthanum, cerium, calcium, iron, gallium, and the like to provide desired characteristics in the final coating. The second portion is designated as “NiX” herein, where X represents any one or more alloying elements.
In other exemplary embodiments, the other portion (i.e., NiX) may include some aluminum content at reduced levels as compared to prior consumables and less than an aluminum content that would be required absent the first source of aluminum. The reduced aluminum in the NiX source will still provide the desired ease of manufacturing and provide aluminum diffusion to heat macroparticles, etc. Provision of some of the required aluminum content in the NiX portion reduces the amount of aluminum from the first portion aluminum source) needed to reach the target composition, thus providing process flexibility.
In an exemplary embodiment, the coating composition is disposed onto the substrate through a multi-layering cathodic arc (ion plasma) deposition process. Subsequent processing and/or heat treatment are utilized to provide a dense coating wherein at least the aluminum is distributed throughout the coating thickness, rather than presenting a higher aluminum content at the surface. Preferably, the coating comprises a substantially uniform distribution of the alloying elements. In an exemplary embodiment, a three layer coating process is utilized. A first layer may utilize the NiX consumable, followed by deposition of Al, and thereafter, another NiX layer. The thickness of each deposited layer is dependent upon the desired final coating composition. The layered coating precursor may be subjected to a homogenization heat treatment at a predetermined temperature for sufficient time for the layers to interdiffuse into a dense coating. In an exemplary embodiment the heat treatment is performed at about 1079° C. (1975° F.).
In other exemplary embodiments, additional Al and/or NiX layers may be utilized to form the layered coating precursor, and subsequently processed to provide the desired coating. In an exemplary embodiment the layered coating precursor is provided in a sufficient thickness so that a total coating thickness is nominally between about 12.7-254 microns (about 0.5-10.0 mil). In other exemplary embodiments, the total coating thickness is between about 12.7-76.2 microns (about 0.5-3.0 mil). The layers from each of the respective sources may be uniform in thickness, or may vary in thickness in order to achieve the desired coating.
Although described in terms of providing a layered coating precursor, those with skill in the art will appreciate that co-depositing NiX and Al would provide a coating precursor having near infinite number of very fine layers. Thus any number of layers may be possible, as long the target composition requirements are achieved. Minimum number of layers is two: 24a and 26. Maximum number of layers can be infinite where for example NiX and Al are co-deposited at the same time making a near infinite number of very fine layers. Any number of layers in between are possible as long as the final composition requirements are achieved.]
Exemplary coating compositions disclosed herein represent target or nominal compositions obtained by disposing layers of the NiX and Al onto the substrate. Table I lists exemplary compositions of as-deposited coatings, wherein a significant amount of the aluminum portion is supplied from a first consumable source (e.g., cathode) and the remaining portion is provided through one or more additional consumable sources (e.g., cathodes). The exemplary coating compositions provided in Table I generally correspond to the coating compositions set forth in U.S. Pat. No. 6,153,313, incorporated herein in its entirety.
In an exemplary embodiment, the as-deposited coating, applied via at least two different consumables (e.g., Al and NiX), includes zirconium content of at least 0.2 atomic percent in addition to nickel and aluminum. In other exemplary embodiments, the zirconium content is in the range of at 0.2 to 0.5 atomic percent, as set forth in U.S. Pat. No. 6,255,001, which is incorporated herein in its entirety.
In an exemplary embodiment, the as-deposited coating, applied via at least two different consumables (e.g., Al and NiX), includes a chromium content in a range of about 2 to 15 atomic percent, and a zirconium content of about 0.1 to 1.2 atomic percent, the balance nickel as set forth in U.S. Pat. No. 6,291,084, incorporated herein in its entirety. The balance includes an aluminum content of about 30 to 60 atomic percent of the as-deposited coating, preferably about 30 to 50 atomic percent, and more preferably an atomic ratio of 1:1 with the nickel content.
In an exemplary embodiment, the as-deposited coating, applied via at least two different consumables (e.g., Al and NiX), includes aluminum, nickel, and at least two modifying elements selected from zirconium, hafnium, yttrium, and silicon. In an exemplary embodiment, zirconium is at least one of the selected modifying elements. In an exemplary embodiment, the modifying elements, if present, may comprise from about 0.1 to about 5, more preferably from about 0.1 to about 3, and most preferably from about 0.1 to about 1, percent by weight of the as-deposited coating composition. In an exemplary embodiment, yttrium, when present, is included in an amount of from about 0.1 to about 1 percent by weight of the as-deposited coating. Exemplary coating compositions are set forth in U.S. Pat. No. 6,579,627, incorporated herein in its entirety.
The exemplary coatings disclosed herein may be particularly suitable for use as bond coats disposed between a substrate and an overlying thermal barrier coating, for example, 7 YSZ, as illustrated by optional Step 150. Alternately, the exemplary coatings disclosed herein may be suitable for use as environmental coatings (without an overlying thermal barrier coating) as will be appreciated by those having skill in the relevant art. Exemplary metallic substrates include nickel base superalloys, cobalt base superalloys, and iron base superalloys.
Thus, with appropriate thicknesses of deposited layers, and subsequent processing (e.g., diffusion heat treatment), a nickel aluminide based coating may be provided on a metallic substrate. Separating a significant portion of the aluminum from the remainder of the coating composition as disclosed herein eliminates the difficulties associated with brittle NiAl cathodes, eases manufacturing difficulties, and shortens coating times.
This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.
This Application claims priority and benefit of U.S. Provisional Patent Application Ser. No. 61/288,707 filed Dec. 21, 2009, the disclosure of which is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61288707 | Dec 2009 | US |
