METHOD FOR IMPROVING OXIDATION AND SPALLING RESISTANCE OF DIFFUSION ALUMINIDE COATINGS

Abstract: Higher oxidation resistance and high-temperature life result when from 0.01 to 0.30 weight percent of an additive selected from the group consisting of zirconium, yttrium, and mixtures thereof is present in a nickel-base superalloy substrate bearing a diffusion aluminide coating.

Description: METHOD FOR IMPROVING OXIDATION AND SPALLING RESISTANCE OF DIFFUSION ALUMINIDE COATINGS BACKGROUND OF THE INVENTION This is a Continuation-In-Part of Serial Number 08/117,564, filed September 3, 1993, now abandoned, which in turn is a Continuation of Serial Number 07/764,251, filed September 23, 1991, now abandoned. Field of the Invention The invention relates to a coated superalloy article having a substrate of a nickel-base composition, with an oxidation resistant diffusion aluminide coating thereupon having improved resistance to spalling. Description of the Prior Art Nickel-base superalloy articles are used in applications requiring strength and oxidation resistance at elevated temperatures. These applications include components for high temperature gas turbine engines, such as gas turbine engine blades. In these high temperature applications, and particularly in gas turbine engine applications, the cyclic high temperatures to which components of these alloys are subjected many times surpasses the inherent oxidation resistance of the alloy. Consequently, it is known to use protective surface coatings to enhance oxidation resistance. Specifi¬ cally for this purpose, aluminide coatings are produced by introducing aluminum into the surface of a nickel-base superalloy article to provide an aluminum-rich diffused surface layer that serves to improve the oxidation resistance of the article by providing sufficient aluminum to develop a protective alumina scale on the article surface, with sufficient aluminum also being present to reform this scale as it spalls from the surface of the article as a result of heat cycling during use thereof. This nickel aluminide coating is also known to be highly resistant to diffusion of metal from the substrate to the surface, thus limiting loss of strengthening or otherwise beneficial elements in the substrate. For example, the diffusion zone, i.e. the zone of diffusion of substrate materials into the aluminide coating, has been observed to be limited to about one third of the thickness of the coating, below the surface zone. The effectiveness of diffusion aluminide coatings in improving surface oxidation resistance is materially affected by the resistance of the alumina scale to removal, such as by spalling. Hence, the adherence of the coating oxide scale to the article surface greatly influences the duration of the desired oxidation resistance upon cyclic high temperature exposure during typical applications. Strangman et al, in US 4,880,614, teach a ceramic thermal barrier coating system for superalloy components, which includes a high purity alumina interfacial layer between the metallic substrate and the ceramic overcoat to better resist spalling. The reference teaches the use of a diffusion aluminide coating on a zirconium containing superalloy, but requires additional layers over said diffusion aluminide to achieve protection of the substrate. Gostic et al, on the other hand, in US 4,878,965, teach the addition of small amounts of zirconium to a single crystal alloy composi¬ tion to improve oxidation resistance. However, Gostic et al specifically teach the use of the alloy compositions in a uncoated manner, to avoid the additional costs and complexities of aluminide coatings. SUMMARY OF THE INVENTION It is accordingly a primary object of the present invention to provide a diffusion aluminide coated oxidation resistant nickel-base superalloy article wherein the surface alumina scale formed upon oxidation is characterized by improved adherence and resistance to spalling. In accordance with the invention, an oxidation resistant article is provided having a nickel-base superalloy substrate comprising up to 8 percent by weight aluminum, 5 to 18 percent chromium, and a small but effective amount of an additive element selected from the group consisting of zirconium, yttrium, and mixtures thereof. A diffusion nickel aluminide layer is provided on the surface of the substrate, in the absence of any further coating. The diffusion aluminide layer is formed from an aluminum coating applied directly to the substrate, and nickel in the substrate. The article is characterized by a more adherent alumina scale formed at the surface of the diffusion aluminide coating resulting from the presence of a from 0.01 to 0.30 percent of zirconium or yttrium, or mixtures thereof, added to the substrate. The scale is formed at elevated temperatures, such as those encountered during use of the article in conventional high-temperature applications. Diffusion aluminide coatings formed on superalloy substrates including such an additive, preferably from about 0.02 to 0.15 percent zirconium, or from about 0.01 to 0.10 percent yttrium, or mixtures thereof, provide a significantly higher life than aluminide coatings on superalloy substrates having less of these elements. It is also noted that if hafnium is present in the alloy of the substrate, the effectiveness of the yttrium is enhanced. Such diffusion aluminide coatings may be used without the addition of further surface coatings, and are highly resistant to spalling. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1-3 are graphs showing the burner rig cyclic oxidation properties of test specimens. Fig. 4 is a bar graph comparing, for various test specimens, the shortest time to coating failure without regard to oxidation or spallation- erosion failure. DESCRIPTION OF THE PREFERRED EMBODIMENTS Articles prepared in accordance with the present invention comprise an oxidation resistant coated superalloy, wherein the nickel-base superalloy substrate has been modified by the addition of a small but critical amount of a specified additive element selected from the group consisting of zirconium, yttrium, and mixtures thereof. The substrate is coated with a diffusion aluminide coating, whereby nickel diffuses from the substrate alloy into the applied coating, thereby forming a nickel aluminide, and improving the oxidation characteristics of the coated substrate. We have found that the above cited elemental additives unexpectedly also diffuse from the substrate alloy through the aluminide coating to the surface thereof, and act to limit the flaking or spalling of the surface oxide during cyclic heating. This improvement in flaking or spalling is not observed in coated substrates in the absence of the additives of this invention, as demonstrated by Figure 1. The present invention constitutes an enhancement of diffusion coating oxidation resistance as opposed to an improvement in inherent oxidation resistance. This improvement to diffusion aluminide formation is particularly advantageous for superalloy substrates having lower levels of aluminum, e.g. those in which the aluminum content is insufficient to form an extensive alumina surface scale. The invention is applicable to both single crystal materials and to polycrystalline substrates. It is also to be noted that the addition of these elements to the substrate alloys has been found to improve the oxidation characteristics of the substrate alloys per se slightly, but insufficiently to use the alloy uncoated at high temperatures. The nickel-base superalloy substrates suitable for modification in accordance with this invention may comprise less than about 8 percent aluminum, from about 5 to about 18 percent chromium. Exemplary suitable substrate alloys are set forth in Table I. While specific composi¬ tions are set forth for two alloys particularly suited for this invention, the invention is clearly not limited to substrates of these compositions only. TABLE I COMPOSmONS OF SUBSTRATE ALLOYS (PERCENT BY WEIGHT) Alloy A Alloy B Ingredients min max min max Chromium 9.50 10.50 4.75 5.25 Cobalt 4.50 5.50 9.50 10.50 Molybdenum — — 1.70 2.10 Tungsten 3.75 4.25 5.60 6.20 Titanium 1.25 1.75 — — Rhenium — 2.80 3.20 Tantalum 11.75 12.25 8.40 9.00 Aluminum 4.75 5.25 5.50 5.80 Hafnium — 0.0300 (300 ppm) 0.05 0.15 Carbon — 0.0500 (500 ppm) — 0.050 (500 ppm) Manganese — 0.12 — 0.12 Silicon ~ 0.12 ~ 0.12 Phosphorus — 0.015 — 0.015 Sulfur — 0.015 — 0.015 Boron — 0.0030 (30 ppm) — 0.0030 (30 ppm) Iron — 0.20 -- 0.20 Copper — 0.10 — 0.10 Zirconium — 0.0075 (75 ppm) — 0.0075 (75 ppm) Lead — 0.0005 (5 ppm) — 0.0005 (5 ppm) Bismuth — 0.00003 (0.3 ppm) — 0.00003 (0.3 ppm) Selenium — 0.0001 (1 ppm) — 0.0001 (1 ppm) Tellurium — 0.00005 (0.5 ppm) — 0.00005 (0.5 ppm) Thallium — 0.00005 (0.5 ppm) _ 0.00005 (0.5 ppm) Nickel remainder remainder The diffusion nickel aluminide coating materials may be chosen from conventional high aluminum content diffusion coating materials. Aluminum may be transferred to the substrate by various coating techniques, such as gas phase deposition, low temperature pack coating, or high temperature pack coating. In gas phase deposition, gaseous aluminum trichloride may be passed over the heated substrate in a furnace at about 1500 - 2100°F. In the pack coating techniques, the substrate may be encased within a pack of particulate elemental aluminum or appropriate aluminum alloy, inert alumina, and an activator such as ammonium chloride, fluoride, or bifluoride, at about 1400°F (low temperature pack) or about 1900°F (high temperature pack). In pack coating, of course, it is also possible to achieve the desired result by placing the substrate over a bed of the particulate pack and subjecting it to gaseous deposition. It is also noted that an aluminum-silicon compound may be advantageously used in the pack, or, alternatively, may be applied to the surface of the substrate in the form of a slurry, utilizing a binder component such as nitrocellulose, and then heated to remove the binder materials and diffusion bond the aluminum-silicon compound, preferably comprising 90 percent aluminum and 10 percent silicon, to the substrate. Diffusion aluminide coatings may be categorized as either inward diffusion or outward diffusion. In the inward diffusion coating, aluminum diffuses inwardly from the coating into the substrate, whereas with the outward diffusion coating, nickel diffuses outwardly from the substrate into the coating. It is not uncommon to have both forms, i.e. both inward and outward diffusion, in the same coating. In either event, a surface coating of nickel aluminide, NiAl, is formed to a depth of about 1 to about 4 mils. The aluminum content of said coating layer is nominally from about 22 to about 32 weight percent, dependent upon coating method and/or temperature. A post coating diffusion heat treatment at about 1975 °F in an argon or hydrogen environment may also be employed. Exemplary coating compositions and techniques are as follows. Coating I designates a vapor deposition method for applying a diffusion aluminide coating in which the article to be coated is not in contact with a pack powder mixture. Coating II designates a pack process. In both coating methods, articles to be coated were thoroughly cleaned and free of dirt, oil, grease, stains and other foreign materials after having been conditioned by grit blasting with No. 220 or finer aluminum oxide grit. Articles subjected to Coating I were placed in a retort in such a manner that all surfaces thereof were out of contact with the source coating material. The retort was covered and placed in a furnace, and heated to about 1975±50°F, for sufficient time to produce the desired diffusion aluminide coating, to a depth of about 1 to about 2 mils, with a surface aluminum content of from about 25 to 28 weight percent. Articles subjected to coating II were packed in a retort so as to surround all areas to be coated with at least 0.50 inch of coating material. The retort was covered and placed in a furnace at about 1400±50°F for sufficient time to produce the desired coating thickness and aluminum content. After removal from the furnace and cleaning to remove any pack materials, the articles were heated at 1975±25°F in an argon or hydrogen environment as a diffusion heat treatment. The aluminide coating was from about 2 to about 4 mils thickness, with a surface aluminum content of from about 22 to 32 weight percent. In Coating II, the pack material comprised about 15 weight percent aluminum silicon powder, about 2.5 weight percent ammonium chloride, and about 82.5 weight percent alumina. In Coating I, the source of aluminum was cobalt aluminide, rather than aluminum silicon. In the present invention, the addition of a small but significant amount of zirconium or yttrium to the nickel-base superalloy substrate results in the presence of a solid solution thereof at the surface of the nickel aluminide layer. While the mechanism is not fully understood at this time, a synergistic effect has been found in that more adherent coatings, which are more resistant to both erosion, e.g. spalling, and to oxidation, are formed when from about 0.01 to 0.30 percent zirconium or yttrium is present in the substrate. This synergy is more pronounced for the yttrium additive in those substrate alloys comprising a small amount of hafnium, such as from 0.02 to 0.30 percent. The zirconium is preferably added in a concentration of from about 0.02 to 0.15 percent, most preferably from about 0.02 to 0.10 percent, and the yttrium is preferably added in a concentration of from about 0.01 to 0.10 percent, most preferably 0.01 to 0.04 percent by weight of the substrate alloy. To evaluate the effect of adding zirconium to the substrate, burner rig bars having a diameter of 0.468 inch with a length of 3.25 inches were fabricated as test specimens from both Alloy A and Alloy B. Additional test specimens were fabricated having zirconium additions in the range of from 0.10 to about 0.25 weight percent to each of Alloys A and B. Similarly, test specimens are fabricated having yttrium additions of from 0.01 to 0.05 percent to each of Alloys A and B. Diffusion aluminide coatings were applied to selected specimen bars in accordance with the methods set forth above for Coatings I and II. Then, coated specimens were subjected to burner rig oxidation testing at various temperatures to determine oxidation resistance, as measured by weight loss, and spalling resistance, as measured by diameter loss. In the burner rig, the temperature cycle during testing including heating to the indicated temperature for 57 minutes followed by forced air cooling for 3 minutes. Inspection for determining specific weight change, as a measure of oxidation, and specimen diameter, as a measure of spalling, were conducted at selected intervals after initiation of cyclic oxidation for the tests conducted at 2200°F and higher. The weight of the specimens was measured on a Sartorius Type 1602 MP I Scale. The oxidized surface area for all of the samples was estimated as 18 cm 2 . Specific weight change per square centimeter was calculated and plotted versus time. Minimum specimen bar diameters were measured with a flat blade dial vernier caliper at the hot spot center and plotted versus time. The tests were conducted to erode at least 30 mils from the base line Alloy B/Coating II bar specimen. Figs. 1-3 show the specific weight change of selected samples having zirconium addition as a function of oxidation and spalling, which indicates the adherence characteristics of the alumina scale formation. The diameter change of the specimens as a function of test time was also recorded and generally substantiated the specific weight change behavior of test specimens at time-temperature intervals. Similar results are obtained for samples having yttrium additions. Fig. 4 presents an indication of the shortest time to coating defeat or failure without regard to oxidation or spallation-erosion failure. By averaging the benefits at all temperatures from Fig. 4, the overall life of a coated article with zirconium addition to Alloy B, depending on temperature, can be approximated as 2 to 3.5 times the life of a coated article of Alloy B with no zirconium addition. Coating failure as reflected in Fig. 4 is a subjective observation, based upon the alumina (grey oxide) forming capability of the specimen surface. As the surfaces were depleted in aluminum during oxidation rig testing, other base metal atoms were incorporated into the surface scale, resulting in color changes to blue and green. Coating failure was designated as that point in time when 50 percent of the hot spot diameter no longer formed a grey alumina scale. It is to be understood that the above disclosure of the present invention is subject to considerable modification, change, and adaptation by those skilled in the art, and that such modifications, changes, and adaptations are to be considered to be within the scope of the present invention, which is set forth by the appended claims.

Claims: WHAT IS CLAIMED IS: 1. A method for improving the oxidation and spallation resistance of a diffusion aluminide coated nickel-base superalloy substrate comprising up to 8 percent by weight aluminum and from 5 to 18 percent by weight chromium, said method comprising adding to said substrate from about 0.01 to 0.30 percent by weight of an additive selected from zirconium, yttrium, and mixtures thereof, and applying directly to said substrate containing said additive a diffusion aluminide coating, with no intermedi¬ ate coatings and with no further coating thereupon. 2. The method of Claim 1, wherein said additive comprises from about 0.02 to 0.15 percent zirconium. 3. The method of Claim 2, wherein said substrate comprises, prior to adding said zirconium, 9.5 to 10.5 percent chromium, 4.5 to 5.5 percent cobalt, 3.75 to 4.25 percent tungsten, 1.25 to 1.75 percent titanium, 11.75 to 12.25 percent tantalum, and 4.75 to 5.25 percent aluminum, the balance nickel and incidental refractory materials. 4. The method of Claim 2, wherein said substrate comprises, prior to adding said zirconium, 4.75 to 5.25 percent chromium, 9.5 to 10.5 percent cobalt, 1.70 to 2.10 percent molybdenum, 5.60 to 6.20 percent tungsten, 2.80 to 3.20 percent rhenium, 8.40 to 9.00 percent tantalum, 5.50 to 5.80 percent aluminum, and 0.05 to 0.15 percent hafnium, the balance nickel and incidental refractory materials. 5. The method of Claim 1, wherein said additive comprises from about 0.02 to 0.10 percent zirconium. 6. The method of Claim 5, wherein said substrate comprises, prior to adding said zirconium, 9.5 to 10.5 percent chromium, 4.5 to 5.5 percent cobalt, 3.75 to 4.25 percent tungsten, 1.25 to 1.75 percent titanium, 11.75 to 12.25 percent tantalum, and 4.75 to 5.25 percent aluminum, the balance nickel and incidental refractory materials. 7. The method of Claim 5, wherein said substrate comprises, prior to adding said zirconium, 4.75 to 5.25 percent chromium, 9.5 to 10.5 percent cobalt, 1.70 to 2.10 percent molybdenum, 5.60 to 6.20 percent tungsten, 2.80 to 3.20 percent rhenium, 8.40 to 9.00 percent tantalum, 5.50 to 5.80 percent aluminum, and 0.05 to 0.15 percent hafnium, the balance nickel and incidental refractory materials. 8. The method of Claim 1 , wherein said additive comprises from about 0.01 to 0.10 percent yttrium. 9. The method of Claim 8, wherein said substrate comprises, prior to adding said yttrium, 9.5 to 10.5 percent chromium, 4.5 to 5.5 percent cobalt, 3.75 to 4.25 percent tungsten, 1.25 to 1.75 percent titanium, 11.75 to 12.25 percent tantalum, and 4.75 to 5.25 percent aluminum, the balance nickel and incidental refractory materials. 10. The method of Claim 8, wherein said substrate comprises, prior to adding said yttrium, 4.75 to 5.25 percent chromium, 9.5 to 10.5 percent cobalt, 1.70 to 2.10 percent molybdenum, 5.60 to 6.20 percent tungsten, 2.80 to 3.20 percent rhenium, 8.40 to 9.00 percent tantalum, 5.50 to 5.80 percent aluminum, and 0.05 to 0.15 percent hafnium, the balance nickel and incidental refractory materials. 11. The method of Claim 1 , wherein said additive comprises from about 0.01 to 0.04 percent yttrium. 12. The method of Claim 11, wherein said substrate comprises, prior to adding said yttrium, 9.5 to 10.5 percent chromium, 4.5 to 5.5 percent cobalt, 3.75 to 4.25 percent tungsten, 1.25 to 1.75 percent titanium, 11.75 to 12.25 percent tantalum, and 4.75 to 5.25 percent aluminum, the balance nickel and incidental refractory materials. 13. The method of Claim 11 , wherein said substrate comprises, prior to adding said yttrium, 4.75 to 5.25 percent chromium, 9.5 to 10.5 percent cobalt, 1.70 to 2.10 percent molybdenum, 5.60 to 6.20 percent tungsten, 2.80 to 3.20 percent rhenium, 8.40 to 9.00 percent tantalum, 5.50 to 5.80 percent aluminum, and 0.05 to 0.15 percent hafnium, the balance nickel and incidental refractory materials. 14. An oxidation resistant superalloy article comprising a substrate comprising nickel, less than 8 percent by weight aluminum, from 5 to 18 percent by weight chromium, and from about 0.01 to 0.30 percent by weight of an additive selected from zirconium, yttrium, and mixtures thereof, said substrate having only a diffusion aluminide coating thereup¬ on. 15. The article of Claim 14, wherein said substrate comprises 9.5 to 10.5 percent chromium, 4.5 to 5.5 percent cobalt, 3.75 to 4.25 percent tungsten, 1.25 to 1.75 percent titanium, 11.75 to 12.25 percent tantalum, 4.75 to 5.25 percent aluminum, and from 0.02 to 0.15 percent zirconium additive, the balance nickel and incidental refractory materials, and said additive is present in said diffusion aluminide coating as a solid solution. 16. The article of Claim 15, wherein said additive comprises from 0.02 to 0.10 percent zirconium. 17. The article of Claim 14, wherein said substrate comprises 9.5 to 10.5 percent chromium, 4.5 to 5.5 percent cobalt, 3.75 to 4.25 percent tungsten, 1.25 to 1.75 percent titanium, 11.75 to 12.25 percent tantalum, 4.75 to 5.25 percent aluminum, and from 0.01 to 0.10 percent yttrium additive, the balance nickel and incidental refractory materials, and said additive is present in said diffusion aluminide coating as a solid solution. 18. The article of Claim 15, wherein said additive comprises from 0.01 to 0.04 percent yttrium. 19. The article of Claim 14, wherein said substrate comprises 4.75 to 5.25 percent chromium, 9.5 to 10.5 percent cobalt, 1.70 to 2.10 percent molybdenum, 5.60 to 6.20 percent tungsten, 2.80 to 3.20 percent rhenium, 8.40 to 9.00 percent tantalum, 5.50 to 5.80 percent aluminum, 0.05 to 0.15 percent hafnium, and from 0.02 to 0.15 percent zirconium additive, the balance nickel and incidental refractory materials, and said additive is present in said diffusion aluminide coating as a solid solution. 20. The article of Claim 19, wherein said additive comprises from 0.02 to 0.10 percent zirconium. 21. The article of Claim 14, wherein said substrate comprises 4.75 to 5.25 percent chromium, 9.5 to 10.5 percent cobalt, 1.70 to 2.10 percent molybdenum, 5.60 to 6.20 percent tungsten, 2.80 to 3.20 percent rhenium, 8.40 to 9.00 percent tantalum, 5.50 to 5.80 percent aluminum, 0.05 to 0.15 percent hafnium, and from 0.01 to 0.10 percent yttrium additive, the balance nickel and incidental refractory materials, and said additive is present in said diffusion aluminide coating as a solid solution. 22. The article of Claim 21, wherein said additive comprises from 0.01 to 0.04 percent yttrium.

Inventors: FENTON RICHARD J, KHAN ABDUS S, MURPHY KENNETH S, FENTON, RICHARD, J., KHAN, ABDUS, S., MURPHY, KENNETH, S.
Applicants: UNITED TECHNOLOGIES CORP, UNITED TECHNOLOGIES CORPORATION
Application year: 1995
Main classification: C22C19
IPC codes: C22C19, C23C10, C23C14, F01D5, F02C7