IL33767A - Forming aluminide coating on nickel-base and cobalt-base alloys - Google Patents

Description

33767/2 Forming aluminide coatings on nickel-base and cobalt base alloys UNITED AIRCRAPT CORPORATION C. 32057 The present invention relates in general to the formation of oxidation-resistant aluminide coatings on the superalloys, particularly;"those adapted to gas turbine engine use.

Diffused aluminum coatings for the protection of ferrous and non-ferrous metals from high temperature oxidation have been in use for over fifty years. Two major, processes have been used for the application of such coatings to gas turbine engine hardware, particularly that hardware derived from the nickel-base and cobalt-base superalloys. The first involves covering the surface of the metal to be protected with a slurry of aluminum in a liquid vehicle,, followed by drying and firing at an elevated temperature. The second process comprises the steps of embedding the article in;' a -dry powder of aluminum, an inert filler . such as. powdered alumina, and an. activator such as ammonium chloride, and heating the pack to some elevated temperature for a period, of time sufficient to form a coating of the desired thickness. This latter process is often referred to as pack aluminizing or pack cementation aluminizing. While various other elements may be added to the., pack mix either as rate-controllers .or to impart some - additional specific property to the coating, nevertheless all. such coatings achieved by these, processes consist primarily of intermetallic compounds, such as nickel aluminides or cobalt aluminides derived from the aluminum in the pack and elements from the substrate, and from which the basic oxidation resistance is derived.

. . .. In general, the thickness, composition and structure of a pack cementation coating are determined by the following controllable variables: (a) pack mix composition; (b) processing temperatures; (c) time at temperature ; and (d) any subsequent heat treatment of the coated component. Historically, the pack cementation processes have been performed in large retorts necessitating the use of lengthy times at temperature to obtain thermal equilibrium. Furthermore, to prevent large differences in coating thickness as a result of different thermal histories in such large containers, a pack mix of low time-sensitivity is generally employed, such a mix being characterized by a' low aluminum activity.

The use of low time-sensitivity pack mixes, however, can cause certain undesirable coated structures. These may be characterized as follows: (a) a large zone of carbides or other brittle phases of low oxidation resistance underlying the oxidation resistant coating. Such a layer is undesirable from the standpoint of mechanical properties of the coating-substrate system and also leads to excessive oxidation undercutting of the coating when and if the, outer layers are penetrated; (b) an enlarged region of unbalanced substrate chemistry underlying, the coating leading to the formation of sigma or other such phases which are detrimental to the mechanical properties of the coating substrate system; and (c) the inefficient use of certain substrate alloying elements, such as chromium, which are desirable to add to the hot corrosion resistance of the coating. . - The present invention contemplates a pack cementation process which develops coatings on the superalloys by an inward aluminum diffusion so as to incorporate desirable substrate alloying elements, in the coating and minimize deleterious phase formation. The unique features of the invention include the use of a pack mix of high aluminum activity and the formation of the coating at relatively low temperatures £0r processes, of this general nature.

In -a preferred embodiment, the article to be coated is immersed in a well blended mixture of powdered aluminum or aluminum alloy, inert filler, and an activator in a closed but not necessarily sealed container; the coating is formed at relatively low temperatures (64-9-871°C) in relatively short times; and, after removal, from the pack mix, the coated articles are subjected to a ductilizing heat treatment which is preferably matched to those strengthening heat treatments usually prescribed for that particular superalloy substrate.

In a particular preferred process, the pack mix is formulated as follows: aluminum powder (325 , mesh) 5-20 percent by weight; ammonium chloride (granular) 0.5-3 percent by weight; balance aluminum oxide (120 mesh).

In the most preferred process, the pack mix is formulated as follows: aluminum-silicon alloy powder (10-30 weight percent silicon) (325 mesh) 5-20, percent by weight; ammonium chloride (granular) , 0.5-3 percent by weight; balance aluminum oxide (120 mesh). . .

In an article entitled "Formation and Degradation Mechanisms of Aluminide Coatings. on Nickel^Base Superalloys" , Transactions of. the ASM, Volume.60, page 228, (1967) , the inventors., as co-authors, have described the basic mechanisms by which pack cementation and related aluminide coatings are formed. . ... , If the aluminum activity of the pack mix is such that the equilibrium coating phase is NiAl or phases lower in aluminum content then the coating will form only by the outward diffusion of nickel from-the substrate to form these phases on top of the substrate alloy. Virtually.no aluminum, motion is involved in the formation of the aluminide coating. Since the coating is formed by the removal of nickel from the substrate, the substrate becomes enriched, in the remaining alloying elements causing the formation of refractory under the NiAl layer. Nickel denudation which, extends even below this layer also causes the formation of other undesirable phases such as sigma below the carbide layer. Furthermore, analysis of the outer NiAl layer reveals very little chromium enrichment . which may be desirable for enhanced sulfidation resistance. This is frequently the mechanism involved in the prior art pack cementation processes.

On the other hand, if the, aluminum activity of the pack mix is such that the equilibrium coating phase is i2 l^ or other phases higher in aluminum content, as in the present process, then the' coating will form only by the inward diffusion , of aluminum and virtually no nickel motion is involved. Because of the mechanism invdlved, all of the substrate alloying elements are incorporated in the coating and there is no corresponding change in the chemistry of the substrate under the coating and, hence, no discernible carbide layer during application of the coating.

The Ni2 lj layer, however, is not a desirable coating for mechanical, property reasons, the l^ l^ being too brittle for practical use. However, after. removal from the pack mix, the coating-substrate system responds ; to heat treatment in the range of about .1038-120 °C. ,. depending on - the system, to cause further diffusion to occur thereby forming the more ductile NiAl phase. .Because the. drivin force incident to the presence of a high aluminum activity pack is no longer present, nickel diffusion from the substrate now occurs in combination with aluminum diffusion from the surface , layer to form a layer of NiAl beneath the original coating. Following the diffusion heat treatment . there is evidence of a carbide layer beneath the NiAl phase but it is thinner and, hence, less deleterious, to the coated component than that formed in those pack cementation processes characterized by outward nickel diffusion in the coating stage. Furthermore, if the diffusion is conducted in the temperature range where the sigma phase is stable some of this deleterious phase may form but in the present process in lesser quantities than in the conventional processes.

One, of the unique features of the present process is that the coatings are formed at relatively low temperatures of 64-9-971°C. and in relatively short times, 1-4 hours. This is in turn a function of a pack mix of high aluminum activity. Such temperatures cause no adverse mechanical property changes in the superalloy substrate such as occur in the usual processing cycles utilizing high temperatures 982-11 9°C. - long times (10-20 hours) , resultant from pack mixes characterized by a low aluminum activity . level. It is frequently a characteristic of these prior processes that rate inhibitors are included, to specifically reduce the level of activity.

Because the application of the coating is performed at low temperatures and for relatively short periods of time and because the pack mix is characterized by high aluminum activity, the pack cementation step can.be performed in air. This offers a major economic advantage over those processes requiring the use of inert gas cover and sealed containers. It is preferable, therefore, to utilize activator contents in the present process sufficiently high to insure a high activity at the coating temperature and, hence, I weight percent or more.

The ductilizing heat treatment, occurihg after removal of the coated component from the pack mix, can be devised to match the usual strengthening heat treatments employed with the superalloys.

If the article is coated on all surfaces prior to the ductilizing heat treatments, or if the uncoated portions are suitably protected, the ductilizing heat treatments may also passages are also to be coated, the low temperatures of the present process during immersion in the pack mix are sufficiently low to preclude sintering of the pack mix and, therefore, affords easy and complete removal of the mix from the internal passages.

In one example an aluminide coating was provided on a Udimet 700 alloy substrate. The pack mix employed utilized, by weight, 15 percent aluminum (-325 mesh), 3 percent ammonium chloride (granular), balance aluminum oxide (120, esh). The article was buried. in the mix in an.Inconel container, covered and inserted in a furnace held at 760°C and fitted for the continuous passage of argon. As previously mentioned, the coating has also been applied in unsealed containers. wherein the inert gas cover has been eliminated. At the end of one hour the container was .removed and cooled. The article was then removed from the pack mix; cleaned of any loosely adhering powder in flowing water; and air dried. Subsequently, the coated article was heat treated as follows: A hours at 1080°C. , 2 hours at 84-3°C. and 16 hours at 760°C The solution heat treatment of. A hours at 1171?C. can be performed before or after the coating treatment depending upon the final coating properties desired. Those skilled in the art will recognize the preceding as the standard heat treatment for Udimet 700.

Extensive oxidation-erosion and sulfidation testing has shown that in terms of coating efficiency, the present coatings are essentially equivalent, on an equivalent thickness basis, to the conventionally utilized processes, such as the slurry process of Joseph 3,102,04A, and the pack cementation process of Wachtell et al 3 ,257,230.

If it is desired to incorporate increased amounts of chromium, above that obtainable from the substrate alloy, a conventional pre-chromizing process can be employed. Using the the coating is formed by the inward diffusion of aluminum.

Furthermore, it has been found that coatings formed from pack mixes containing the aluminum-silicon alloys in the range of 10-30 weight percent silicon have improved oxidation-erosion and hot corrosion resistance.

From the foregoing description those skilled in the art will recognize not only the applicability of the process to the coating of various components, but fche alternative compositions available as coating material components, activators, inert dispersants, etc^ , in the pack mix, within the general requirement that the mix be such as to be characterized by high aluminum activity at the coating temperatures, particularly in the ragge of 69-87 Accordingly, .while the present invention has been described in connection with particular preferred embodiments and examples, various modifications, substitutions and alternative process details will be evident to those skilled in the art within the true spirit , and scope of the invention as set forth in the appended claims.

Claims (9)

What we claim is:

1. The method of imparting high temperature oxidation resistance to the nickel-base and cobalt-base alloys characterized in contacting the alloy to be protected to a pack mix of high aluminum activity; heating the pack to a temperature of 649-871°C. and this holding at /temperature for a sufficient period of time to provide a coating of the desired thickness ; and, after removal of the coated alloy from the pack mix, heat treating, the coated alloy to convert the aluminide coating to an aluminide having an aluminum content not sub- of stantially exceeding that the monoaluminide.

2. . The method according to claim 1 , characterized in that the pack mix consists of, by weight, 5-20 percent aluminum or aluminum alloy powder, .0.5-3 percent vaporizable halide activator, and balance inert refractory dispersant.

3. The method according to claim 1 and 2, characterized in that the pack mix consists essentially of, by weight, 5-20 percent aluminum-silicon alloy powder, the silicon content of the alloy being in the range of 10-30 percent, 0.5-3 percent ammonium chloride, balance aluminum oxide powder.

4. The method according to claim 1, characterized in that the process, is. performed in air.

5. The method according to claim 1 characterized in that the pack is held at temperature for a period of time sufficient to provide a. coating of 25.4-127^*· thickness.

6.. The method according to. claim 1 characterized in that the pack is. held at temperature for a period of 1-4 hours.

7. The method according to olaim 1 and 4, characterized in that the alloy is a nickel-base or cobalt-base superalloy; and the pack is held at a temperature of about 760°C.

8. The method according to claim 1 and characterized in that both the coating step and the subsequent heat treatments are performed in air.

9. A method of forming an oxidation resistant aluminide coating on a nickel-base and cobalt-base superalloy according to claims 1 to 8, substantially as hereinbefore described and claimed with reference to the description. - 10 - For the Appl ksn!u .