High-temperature Oxidation-resistant Coating Alloy

Abstract

A coating alloy for the gas turbine engine super-alloys is described which consists primarily of nickel, aluminum and a reactive metal such as yttrium, particularly at the composition, by weight, 14-30 percent aluminum, 0.01-0.5 percent reactive metal balance nickel. A preferred embodiment also includes 15-45 weight percent chromium.

Parent Case Text

This is a division of Application Ser. No. 734,740, filed June 5, 1968.

Description

BACKGROUND OF THE INVENTION

the present invention is directed to oxidation-resistant alloys, particularly alloy compositions having application as coatings on the superalloys utilized in the gas turbine engine industry.

A nickel-base superalloy is typically a nickel-chromium solid solution, hardened by the additions of aluminum and titanium to precipitate the intermetallic compound or gamma prime phase Ni.sub.3 (Al,Ti). The contemporary superalloys also usually contain cobalt to raise the solvus temperature of the gamma prime phase, refractory metals such as tungsten or tantalum for solution strengthening, and carbon, boron and zirconium to promote ductility and fabricability.

A limiting factor in the application of the current superalloys to jet engine hardware is their susceptibility to oxidation at very high temperatures with a consequent progressive loss of substrate material. For this reason, the nickel-base superalloys are generally coated with a composition different from and more oxidation-resistant than the structural alloy. In most instances the requisite layer of icnreased oxidation resistance is provided by reacting aluminum with the surface of the alloy to form an aluminide which in turn oxidizes to provide a surface oxide layer through which the transport rates of the reacting species are low. Typical of the processes of this type is that described in the U.S. Pat. to Joseph No. 3,102,044.

Although the aluminide coatings as currently provided significantly enhance the lifetimes of superalloy hardware, the theoretically expected behavior of the nickel aluminide intermetallic compound is not in fact realized in dynamic oxidizing environments. This is the result of thermal shock spalling. In the dynamic environment of a gas turbine engine, for example, temperature fluctuations caused by the mixing of the hot combustion gases with cooler secondary air or those associated with varying power levels give rise to thermally-induced strains at the metal-oxide interface which are sufficiently large to eventually spall the oxide layer. This layer then reforms by the consumption of more aluminum from the intermetallic coating phase. In general, this is a rapidly recurring process and the aluminum is more rapidly depleted from the coating phase than would be the case in a truly isothermal environment wherein no thermal shock spalling would occur.

In a copending application of the same assignee, Ser. No. 734,706, filed June 5, 1968, entitled NICKEL BASE SUPERALLOY RESISTANT TO OXIDATION-EROSION, by D. H. Boone et al., there is described a nickel-base superalloy system having an oxidation-erosion resistance significantly superior to the conventional super-alloys. While the utilization of this alloy obviates the need for coatings for satisfactory oxidation resistance, such coatings may be advantageous in some circumstances, perhaps for economic reasons. In such instances the coating herein described will be seen to have particularly advantageous properties.

SUMMARY OF THE INVENTION

This invention relates to coating alloys of the type generaly identified as the nickel-aluminum intermetallics. It contemplates a basic nickel-aluminum alloy of relatively specific chemistry containing as an essential ingredient one or more of the reactive metals.

It has been found that two factors contribute to the improved oxidation-erosion resistance of the alloys of the present invention. First, the chemistry of the alloy is formulated such that, upon oxidation, essentially a single oxide, specifically alumina, is formed rather than other oxides or mixtures of oxides. This is done through maintenance of a particular aluminum level in the alloy. Secondly, the alloy is provided with at least a minor amount of retained reactive metal such as yttrium, scandium, thorium, or lanthanum and the other rare earth elements.

In terms of their composition, the alloys of the present invention consist of, by weight, 14-30 percent aluminum, 0.01-1 percent reactive metal, balance nickel together with, on an optional basis, one or more of alloying ingredients compatible with the basic alloy chemistry. Specifically, the compatability of the optional ingredients must be such that they do not interfere with the basic oxidation mechanism of the alloy.

A preferred embodiment of the invention comprises an alloy consisting essentially of, by weight, about 14-25 percent aluminum, 0.01-0.5 percent reactive metal, 15-45 percent chromium, balance nickel. This alloy possesses both oxidation-erosion and sulfidation resistance.

The most preferred coating alloy consists essentially of, by weight, 15-20 percent aluminum, 20-35 percent chromium, 0.05-0.3 percent reactive metal, balance nickel.

BRIEF DESCRIPTION OF THE DRAWING

The drawing is a graph depicting the oxidation-erosion behavior of an alloy of the present invention as compared to certain representative contemporary materials.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Although not so confined, the alloys of the present invnetion find particular utility in imparting long-term, oxidation-erosion resistance to the gas turbine superalloys, when utilized as coatings thereon, in the dynamic oxidizing environments of gas turbine engines. Representative of the centemporary superalloys requiring such oxidation protection is the alloy identified in the industry as B-1900, the nominal composition of which, by weight, is as follows: 8 percent Cr, 10 percent Co, 1 percent Ti, 6 percent Al, 6 percent Mo, 4.3 percent Ta, 0.11 percent C, 0.015 percent B, 0.07 percent Zr, balance Ni.

As previously mentioned, the prior art coatings are, in general, most commonly provided by reacting aluminum with the deoxidized surface of the article to be protected and an aluminide layer is formed with consumption of the substrate components. This aluminide layer in turn oxidizes to form the desired inert barrier oxide. However, because of the complex nature of most of the contemporary alloys, and because the coating composition thereon is derived in part from the components of the substrate alloys, it is difficult to control the coating composition so as to cause the formation of a suitable barrier oxide resistant to thermal shock spalling. This is particularly true in the case of the contemporary coatings after exposure to an oxidizing environment for an extended period of time, because in the reformation of the oxide barrier at this point the oxides reform as mixtures of many oxides due to the preferential depletion of certain species with time. Such mixtures are more prone to thermal shock spalling than the single oxide.

The alloys of the present invention are in themselves oxidation resistant and do not depend for their protective effect upon a reaction with the substrate material. Their particular formulation is such that the most desirable barrier oxide is preferentially formed in a high temperature oxidizing environment and this oxice is significantly more resistant to thermal shock spalling than that formed on competitive coatings.

The desired results in this case are achieved with a basic alloy containing, by weight, 14-30 percent aluminum, 0.01-1 percent reactive metal, balance nickel. Of course, whatever oxidation-erosion does occur with this coating, or with other coatings for that matter, results in the loss of aluminum from the system. A relatively high aluminum content is, accordingly, preferred from a durability standpoint. In addition, below about 14 percent, or possibly in some instances as low as about 12 weight percent aluminum, complete surface coverage by the desired protective oxide is not formed. The upper limit of the aluminum content, on the other hand, is established primarily by mechanical considerations. Aluminum contents in excess of about 31.5 weight percent result in the development of a brittle hyperstoichiometric beta phase of the aluminide which, while satisfactory in terms of its oxidation resistance, is in terms of its suitability to the dynamic conditions associated with jet engine operation generally unsatisfactory because of its poor mechanical properties.

Those materials which promote adherence of the oxide to the underlying substrate will include those having an affinity for oxygen approximating or exceeding that of aluminum. As used herein, however, the term "reactive metal" has reference to the elements yttrium, scandium, thorium, and lanthanum and the other rare earths, including mixtures of the same.

In those environments where not only oxidation but sulfidation may also be a problem, as is the case with many if not most gas turbine engine systems, 15-45 weight percent chromium is advantageously included in the coating composition. With the chromium addition, the aluminum content of the alloy is preferably reduced and limited to a maximum of about 25 weight percent to forestall the formation of a brittle phase or phases as previously mentioned. Experimentation has also revealed that, as a general rule, the higher chromium contents are to be preferred, about 30 percent chromium representing about the optimum amount from a sulfidation standpoint.

As the best balance between chemical and physical properties, the most preferred alloy composition corresponds to, by weight, 15-20 percent aluminum, 20-35 percent chromium, 0.05-0.3 percent reactive metal, balance nickel.

Those skilled in the art will recognize that certain other elements are known to be compatible with the basic chemistry of the present alloys. Accordingly, other elements such as cobalt, iron or tantalum may be advantageously added to the alloy as required in certain applications for modification of the mechanical, diffusional or hot corrosion characteristics of the coatings.

The alloys are relatively easily prepared by the conventional arc melt-drop cast technique. Among the compositions so prepared and tested were the following, by weight:

Ni -- 16% Al -- .5%Y

Ni -- 25% Al -- .5% Y

Ni -- 30% Al -- .5% Y

Ni -- 16% Al -- .1% Sc

Ni -- 30% Al -- .25% Sc

Ni -- 12% Al --.85% Y

Ni -- 12% Al -- .6% Nd

Ni -- 20% Cr -- 14.5% Al -- .5% Y

Ni -- 30% Cr -- 15% Al -- .1% Sc

Ni -- 15% Cr -- 15% Al -- .1% Sc

Ni -- 15% Cr -- 12% Al -- 4% Ta -- .25% Sc

With respect to the processes whereby the alloy is applied as a coating to the surface to be protected, the necessary presence in the alloy of the reactive metals precludes synthesis of these alloys in coating form by the widely used slurry or simple pack cementation techniques. It appears, however, that various of the other methods discussed in the literature including vapor deposition, plasma spraying, mechanical bonding, electrolysis, electrophoresis, gaseous ion plating and sputtering may be adapted to applying the specific compositions herein discussed. Several of these techniques have been utilized in connection with this invention as discussed in the following examples:

Example 1

A sputtering target of, by weight, Ni-26 percent Al-0.12 percent Y was prepared by a standard arc melting process. A 2.5 mil coating of this composition was deposited on a specimen of B-1900 alloy by a sputtering process. Basically this method consists of bombarding the target of correct coating composition with high energy argon ions which causes sublimation of the target material. The sublimed atoms are then condensed on the substrate alloy to form a coating of essentially the same composition as the original target material. The whole process is carried out in a vacuum of a few microns of argon.

The coated specimen of this example was tested in a hot, high velocity gas stream generated by the combustion of propane in air. The coating protected the specimen from oxidation damage for 115 hours at 2,000.degree. F and for a subsequent period of 37 hours at 2,100.degree. F at which time the test was terminated to permit metallographic examination of the specimen.

Example 2

A sputtering target of, by weight; Ni-30 percent Cr -- 12 percent Al -- 0.5 percent Sc was prepared as above. A one mil coating of this composition was deposited on a B-1900 specimen by the sputtering technique described above.

Testing of this specimen was conducted in a hot high velocity propane exhaust stream contaminated with 0.4 percent sulfur (sulfur/fuel ratio) and 3.5 ppm sea salt (salt/air ratio) to simulated gas turbine hot corrosion (sulfidation) conditions. At a specimen temperature of 1,650.degree. F, the test article survived for a total of 330 hours. Uncoated B-1900 is catastrophically attacked under these test conditions.

Example 3

An ingot of the composition, by weight, Ni -- 28 percent Cr -- 14 percent Al -- 0.4 percent Y was prepared by a standard melting method. A B-1900 erosion bar was coated to a thickness of 4.5 mils of this composition by electron beam evaporation. Subjected to dynamic oxidation-erosion in JP5R fuel exhaust at 2,000.degree. F, the erosion bar was protected from oxidation for 208 hours.

It has thus been clearly established that the alloys of the present invention are effective in imparting long-term oxidation-erosion protection to the superalloys. While the invention has been described in connection with certain preferred embodiments and examples, these will be understood to be illsutrative only. Numerous improvements to and variations of the present invention, some of which are discussed herein, will be evident to those skilled in the art from the teachings herein and will, in the true spirit of the invention, be embraced within the scope of the appended claims.

Claims

We claim:

1. An oxidation-erosion resistant coating alloy which consists essentially of, by weight, 14-25 percent aluminum, 15-45 percent chromium, 0.01-0.5 percent yttrium, up to 10 percent of an alloying ingredient selected from the group consisting of cobalt, iron and the refractory metals, balance essentially nickel.

2. An oxidation-erosion resistant coating alloy which consists essentially of, by weight, 15-20 percent aluminum, 20-35 percent chromium, 0.05-0.3 percent yttrium, balance essentially nickel.