YTTRIUM ENRICHED ALUMINIDE COATINGS

Abstract: Methods are described for forming an yttrium enriched aluminide coating on the surface of a nickel or cobalt base superalloy article. In one preferred embodiment, a pack mixture for forming the coating consists essentially of, by weight percent, 5-35 of an Al-Y-Si alloy, 1-20 CoI₂, balance Y₂O₃.

Description: The present invention concerns a method for forming an yttrium enriched diffusion aluminide coating on a nickel or cobalt base superalloy article according to the precharacterizing portion of claim 1 and a powder mixture for forming an yttrium enriched aluminide coating according to the precharacterizing portion of claim 9. Aluminide coatings are widely used in the gas turbine engine industry to provide protection against oxidation and corrosion degradation to superalloy articles used in the engine. U.S. Patents which are indicative of the skill in the art relative to aluminide coatings include the following: 3,079,276, 3,276,903, 3,667,985, 3,801,353, 3,837,901, 3,958,047, 4,132,816, 4,142,023, 4,148,275 and 4,332,843. In general, aluminide coatings are formed by heating a powder mixture containing a source of aluminum, an activator, and an inert diluent in the presence of the article to be coated. The article may either be embedded in the powder mixture (and the process is termed a "pack cementation" process) or the article is in out-of-contact relation with the powder mixture (and the process is termed a "gas phase" process). The source of aluminum may be pure aluminum metal or it may be an alloy of aluminum such as Co₂Al₅, as disclosed in US-A-4,132,816 to Benden et al; US-A-3,958,047 to Baldi discloses the use of Ni₃Al as the source of aluminum; and US-A-4,332,843 to Ahuja discloses the use of Fe₂Al₅. Activators which have been used in the aluminiding process generally include halides of alkali or alkaline earth metals. See, e.g., the aforementioned patent to Benden. Aluminum oxide is typically added to the powder mixture as a buffer or diluent, in order to control the aluminum activity of the mixture. There are also references in the prior art that aluminum oxide prevents the powder mixture from sintering together during the coating process. See, e.g., US-A-No. 3,667,985 to Levine et al. US-A-3,794,511 to Baranow discloses that a nickel alloy having an aluminide coating which contains Misch metal has better resistance to sulfidation degradation than the same alloy with an aluminide coating containing no Misch metal. The coatings are produced by heating the article in an aluminum-Misch metal alloy powder, the alloy containing between 27 and 45 weight percent aluminum. In US-A-3,996,021 to Chang et al, a small amount of hafnium is added to a powder mixture containing an Al-Ti-C alloy, an activator such as NH₄F, and aluminum oxide. The powder mixture is said to produce an aluminide coating containing between 0.1 and 10 weight percent hafnium. US-A-3,993,454 to Giggins et al suggests that coatings which contain hafnium have better high temperature properties (e.g., resistance to oxidation and corrosion) than do coatings containing yttrium. Other patents which relate to diffusion coatings include US-A-2,801,187, 3,625,750, 4,123,595 and 4,156,042; US-A-Re. 26,001; and JP-A-55-82769. The PATENT ABSTRACTS OF JAPAN, vol. 5, number 156 (C-74) [828], 06.10.81 ; & JP-A-56 87661 discloses a method to improve the oxidation and corrosion resistance of an Al diffusion coated layer into a Ni superalloy by heating in a non oxidizing atmosphere a Ni superalloy metal article buried in a mixed powder containing Al powder, alloy powder of a rare earth metal and another metal such as Al, Fe or Ni, halide activator and inert carrier. Notwithstanding the developments in aluminide coatings, engineers still expend considerable effort in an attempt to develop coatings having improved properties. - The method and powder mixture for forming an yttrium enriched diffusion aluminide coating are defined according to the characterizing portions of claims 1 and 9, respectively. With the method and powder mixture of this invention, an yttrium enriched diffusion aluminide coating containing 20-35 weight percent aluminum and 0.2-2.0 weight percent yttrium is deposited on a nickel or cobalt base superalloy article. This coating has high temperature properties which are far superior to the diffusion coatings of the prior art. The invention coating is produced by heating the article in the presence of (i.e., embedded in or in out-of-contact relation with) a powder mixture which contains an alloy or mixture of aluminum, yttrium, and one or more of the elements from the group of silicon, chromium, cobalt, titanium, and nickel; a halide containing activator; and an inert material which is not reduced by yttrium containing vapors evolved during the deposition process. Preferably, the aluminum and yttrium are alloyed with each other and with a third constituent "X" which is one or more of the aforementioned elements silicon, chromium, cobalt, nickel, and titanium. X is more preferably silicon, chromium, or cobalt, and is most preferably silicon. The halide in the activator is preferably an iodide, and the most preferable activator to use with an aluminum-yttrium-silicon powder mixture is cobalt iodide. The filler material which is used is preferably yttrium oxide. A coating 0.025-0.089 mm (0.001 to 0.0035 inches) thick is formed on nickel base superalloys with a 982°C-1093°C (1,800-2,000°F), 4-20 hour coating cycle. In addition to containing 20-35% Al and 0.2-2.0% Y, the coating also contains elements from the base material, in amounts similar to prior art (yttrium free) aluminide coatings. The invention coatings have about 300% better oxidation life compared to prior art aluminide coatings which do not contain yttrium. The Figure is a photomicrograph of an yttrium enriched aluminide coating produced in accordance with this invention. The invention can be carried out using diffusion coating techniques known to those skilled in the art. For a representative example of such techniques, see US-A-3,544,348 to Boone et al and 4,132,816 to Benden et al. The aluminiding powder mixture comprises at least three parts. The first part is a metallic alloy or mixture containing aluminum, yttrium, and a third constituent designated "X", where X is one or a combination of the elements selected from the group consisting of silicon, chromium, cobalt, nickel, and titanium. The first part of the aluminiding powder mixture is preferably an alloy (rather than a mixture of elemental powders), and this alloy is referred to as an aluminum-yttrium-X alloy. Three aluminum-yttrium-X alloys are especially preferred in the practice of this invention. They are aluminum-yttrium-silicon (Al-Y-Si), aluminum-yttrium-chromium (Al-Y-Cr), and aluminum-yttrium-cobalt (Al-Y-Co). The most preferred alloy is Al-Y-Si. The composition of the aluminum-yttrium-X alloy should be about, by weight percent, 2-20 yttrium, 6-50 X, balance aluminum. A more preferred range is 2-12 yttrium, 8-48 X, balance aluminum. When X is chromium or cobalt, the preferred range is 30-44 chromium or cobalt, 2-12 yttrium, balance aluminum. When X is silicon, the preferred range is 6-20 silicon, 2-12 yttrium, balance aluminum. This particular range of alloys has a melting point slightly less than pure aluminum. The second part of the powder mixture is an activator which reacts with the aluminum and yttrium containing powder during the high temperature coating process to produce aluminum and yttrium containing vapors which are carried to the article surface which is to be coated. Preferably, the activator is a halide of any of the transition metals. The most preferred halide is iodide, and the most preferred transition metal halide is cobalt iodide, CoI₂. The use of the preferred activator CoI₂ ensures that, in general, yttrium diffuses into the coating simultaneously with aluminum, and that the yttrium is evenly distributed throughout the coating. While halide containing activators based on alkali or alkaline earth metals may also be used, the results obtained with CoI₂ are clearly superior. The third part of the powder mixture is an inert filler material which controls the activity of the aluminum and yttrium containing powder mixture, and also prevents the mixture from sintering together during the coating cycle. The filler metal used in this invention must have particular properties, due to the characteristics of the metallic Al-Y-X alloy. Due to the highly reactive nature of the yttrium containing vapors which are produced when the powder mixture is heated, the filler metal must not react with these vapors. In other words, the filler metal must not be reduced by yttrium, otherwise little or no yttrium will diffuse into the article being coated. Aluminum oxide, the filler metal used throughout the coating industry in prior art diffusion aluminide coating powder mixtures, will be reduced by yttrium if used in the invention method and form the more stable yttrium oxide; therefore aluminum oxide is not useful in the practice of this invention. Yttrium oxide will not be reduced in the invention method, and is therefore the preferred filler metal. Other possible filler materials are materials more stable than yttrium oxide (i.e., nonreactive with yttrium). The composition of the preferred powder mixture (i.e., aluminum and yttrium containing alloy, activator, and filler) is about, by weight percent, 5-35 aluminum-yttrium-X, where X is one or more of the elements selected from silicon, chromium, cobalt, nickel, and titanium; 1-20 of a halide activator; with the balance a filler material which is not reduced by yttrium at the elevated coating deposition temperature. Preferably, the mixture is 5-35 Al-Y-Si, 1-20 CoI₂, balance Y₂O₃. Most preferably, the mixture is 5-10 Al-Y-Si, 5-10 CoI₂, balance Y₂O₃. The invention may be better understood by reference to the following examples, which are intended to illustrate the features of the invention. In each example, the nickel base superalloy test specimens which were coated had the composition described in US-A-4,209,348 to Duhl et al. Example I A coating pack mixture which contained, by weight percent, 5 Al-Y-Si, 10 CoI₂, balance Y₂O₃ was prepared. The composition of the Al-Y-Si alloy was about 77Al - 11Y - 12 Si, and was in powder form, having an average particle size of about 10-40 microns. The CoI₂ activator was an anhydrous powder and the Y₂O₃ particle size was nominally 25 µm (microns). The powder mixture was thoroughly mixed and then the test specimens and pack mixture placed in a protective gas atmosphere (i.e., nonoxidizing) retort. After heating the retort to 1038°C (1,900°F) for 16 hours, an yttrium enriched aluminide coating having a thickness of 0.05-0.63 mm (0.002-0.0025 inches) was produced, and had a microstructure similar to that shown in the Figure. In cyclic burner rig oxidation tests at 1149°C (2,100°F), the coating had a life of about 255 hours per 25.4 µm (mil). Example II Test specimens were coated in the manner described in Example I with a pack mixture which contained 5 Al-Y-Si, 5 CoI₂, balance Y₂O₃. The Al-Y-Si alloy was the same as described in Example I. The 0.076 mm (0.003 inch) thick coating which was produced had a life of about 300 hours per 25.4 µm (mil) in a 1149°C (2,100°F) cyclic oxidation test. Example III COMPARATIVE EXAMPLE Techniques similar to those described in the aforementioned US-A-3,544,348 were used to apply a prior art (i.e., yttrium free) aluminide coating. The coating produced with these prior art techniques was 0.05-0.076 mm (0.002-0.003 inches) thick, and had a life of about 90 hours per 25.4 µm (mil) in 1149°C (2,100°F) cyclic oxidation tests. Thus, in comparison with the oxidation resistance of the coatings described in Examples I and II, the invention coatings have about 300% better resistance to oxidation degradation than do the coatings of the prior art. Example IV A coating according to this invention was produced by heating a powder mixture containing 10 Al-Y-Cr, 5 CoI₂, balance Y₂O₃ at 1038°C (1,900°F) for 6 hours. The Al-Y-Cr alloy composition was about 60Al - 38Cr - 2Y. A 0.05-0.063 mm (0.002-0.0025 inch) yttrium enriched coating was produced, which had a 1149°C (2,100°F) cyclic oxidation test life of about 180 hours per 25.4 µm (mil), which is about 200% better than the prior art aluminide coatings. Example V Test specimens were pack aluminided at 1038°C (1,900°F) for 6 hours in a powder mixture containing 20 Al-Y-Cr, 10 CoI₂, balance Y₂O₃. The composition of the Al-Y-Cr alloy was about 60Al - 34Cr - 6Y. The resultant 0.05-0.063 µm (0.002-0.0025 inch) yttrium enriched aluminide coating had a 1149°C (2,100°F) cyclic oxidation life of about 195 hours per 25.4 µm (mil). Example VI Test specimens were pack aluminided at 1038°C (1,900°F) for 6 hours in a powder mixture which contained 50 Al-Y-Co, 5CoI₂, balance Y₂O₃. The composition of the Al-Y-Co alloy was about 56Al - 6Y - 40Co. The resultant 0.063-0.076 mm (0.0025-0.003 inch) yttrium enriched aluminide coating had a 1149°C (2,100°F) cyclic oxidation life of about 140 hours per 25.4 µm (mil). This low life (compared with the lives of the invention coatings in the above examples) is due to the high metallic content (50%) in the pack mix. The high metallic content results in the diffusion of an excessive amount (i.e., greater than about 2%) of yttrium in the coating, which reduces the coating's melting point, and thereby its oxidation resistance. Preferably, the invention coatings contain a maximum of about 0.5% yttrium, most preferably about 0.3%. Example VII COMPARATIVE EXAMPLE Test specimens were pack aluminide coated in a powder mixture which contained 15% of a nickel-yttrium alloy, 1.5% NH₄F, balance Al₂O₃. After heating at 1093°C (2,000°F) for 4 hours, a 0.05 mm (0.002 inch) thick aluminide coating was formed. Chemical analysis of the coating indicated that it contained no yttrium. During the coating process, yttrium containing vapors apparently reacted first with the Al₂O₃ filler material, and reduced the Al₂O₃ to the more stable Y₂O₃. As a result, no yttrium diffused into the test specimen. Cyclic oxidation testing at 1149°C (2,100°F) indicated that the coating performed similarly to the yttrium free coatings of Example III. From the foregoing description, one skilled in the art can ascertain the essential features of this invention, and can make various changes and modifications of the invention to adapt it to various uses and conditions. For example, slurry or gas phase coating techniques may be used in place of pack techniques, as disclosed in, e.g., US-A-No. 4,374,183 to Deadmore et al and 4,132,816 to Benden et al, respectively. Furthermore, any of the other rare earth elements may be present in combination with, or substituted for, yttrium provided that an appropriate inert buffer is selected and the necessary process changes made (which can readily be determined by one skilled in the art). See, e.g., the following US-A-3,542,530; 3,918,139; 3,928,026; 3,993,454; 4,034,142; 4,535,033; and Re. 32,121.

Claims: A method for forming an yttrium enriched diffusion aluminide coating on a nickel or cobalt base superalloy article comprising the step of heating the article in the presence of a powder mixture comprising a halide activator and an inert carrier characterized in that the powder mixture consists of an aluminum-yttrium-X alloy, a halide of the transition metal activator, an yttrium oxide inert filler, wherein X is selected from the group consisting of silicon, chromium, cobalt, nickel, titanium and hafnium, or is an alloy or mixture thereof. The method according to claim 1, characterized in that X is silicon, chromium, or cobalt. The method according to claim 2, characterized in that X is silicon. The method according of claim 1, characterized in that the halide activator is cobalt iodide. The method according to claim 1, characterized in that the powder mixture consists of an Al-Y-Si alloy, CoI₂ activator, and Y₂O₃ filler material, to cause diffusion of aluminum and yttrium into the article surface. The method according to claim 1 characterized in comprising the step of disposing the article in a pack mixture consisting of, by weight percent, 5-10% Al-Y-Si, 5-10% CoI₂, balance Y₂O₃, therein the Al-Y-Si alloy contains 2-12% Y, 8-15% Si, balance Al; and heating the mixture to produce a 0.025-0.089 mm (0.001 to 0.0035 inch) thick coating. The method according to claim 6, characterized in that the Al-Y-Si alloy contains 11% Y, 12% Si, balance Al. The method according to claim 1 characterized in comprising the step of disposing the article in a retort in out-of-contact relation with a pack mixture in the retort, the pack mixture consisting of, by weight percent, 5-10% Al-Y-Si, 5-10% CoI₂, balance Y₂O₃, wherein the Al-Y-Si alloy contains 2-12% Y, 8-15% Si, balance Al; and heating the retort to produce a 0.025-0.089 mm (0.001 to 0.0035 inch) thick coating. A powder mixture for forming an yttrium enriched diffusion aluminide coating on the surface of a nickel or cobalt base superalloy article, comprising a halide activator and an inert filler characterized in consisting of by weight percent, 5-35% of an aluminum-yttrium-X alloy where X is selected from the group consisting of silicon, chromium, cobalt, nickel, titanium, and hafnium, or an alloy or mixture thereof; 1-20% of said halide activator being a halide of the transition metals; with the balance being an yttrium oxide inert filler. The mixture according to claim 9, characterized in consisting of 5-35% of an Al-Y-Si alloy, 1-20% CoI₂, balance Y₂O₃. The mixture according to claim 9, characterized in that X is silicon, and the powder mixture consists of 5-10% of an Al-Y-Si alloy, 5-10% CoI₂, balance Y₂O₃. The mixture according to claim 9, characterized in that X is silicon, and the powder mixture consists of 5% of an Al-Y-Si alloy, 5% CoI₂, balance Y₂O₃. The mixture according to claim 9, characterized in that the aluminum-yttrium-X alloy contains 2-20 yttrium, 6-50% X, with the balance aluminum. The mixture according to claim 9, characterized in that X is selected from the group consisting of silicon, chromium, and cobalt, and the aluminum-yttrium-X alloy contains 2-12% yttrium, 8-48% X, with the balance aluminum. The mixture according to claim 10, characterized in that the aluminum-yttrium-silicon alloy contains 2-12% yttrium, 8-15% silicon, with the balance aluminum. The mixture according to claim 9 characterized in consisting of by weight percent, 5-10% of an Al-Y-Si alloy, 5-10% CoI₂, balance Y₂O₃, wherein the Al-Y-Si alloy contains 2-12% Y, 8-15% Si, balance Al. The mixture according to claim 16 characterized in that the Al-Y-Si alloy contains about 11% Y, 12% Si, balance Al.

Inventors: GUPTA, DINESH KUMAR, MILANIAK, MICHAEL STEPHEN, OLSON, WALTER E.
Applicants: UNITED TECHNOLOGIES CORPORATION
Application year: 1987
Main classification: C23C10
IPC codes: C23C10