Coating system for superalloys

Abstract

A protective coating system is provided for nickel-base and cobalt-base superalloys which is capable of imparting oxidation and corrosion resistance at elevated temperatures. The superalloy body is first coated by physical vapor deposition with a composition consisting essentially of chromium, aluminum and, optionally, a member selected from the group consisting of yttrium and the rare earth elements and at least one element selected from the group consisting of iron, cobalt and nickel. Thereafter the body is coated with an overlayer of aluminum by physical vapor deposition and heat treated to cause the interdiffusion of aluminum into the first coating layer and thereby greatly enhance the hot corrosion and oxidation resistance of the coating system.

Description

The superalloys are heat-resistant materials having superior strengths at high temperatures. Many of these alloys contain iron, nickel or cobalt alone or in combination as the principal alloying elements together with chromium to impart surface stability and usually contain one or more minor constituents, such as molybdenum, tungsten, columbium, titanium and aluminum for the purpose of effecting strengthening. The physical properties of the superalloys make them particularly useful in the manufacture of gas turbine engine components.

Heretofore, surface coatings have been used to protect the superalloy articles from high temperature oxidation and corrosion. Various coatings for superalloys have been described in the literature and of particular interest are coating compositions consisting essentially of chromium, aluminum and, optionally, a member selected from the group consisting of yttrium and the rare earth elements and a metal selected from the group consisting of iron, cobalt and nickel. Illustrative coatings wherein the compositions are given in weight percent are designated as follows:

Ingredients FeCrAlY CoCrAlY NiCrAlY ______________________________________ Chromium 25-29% 19-24% 20-35% Aluminum 12-14% 13-17% 15-20% Yttrium 0.6-0.9% 0.6-0.9% 0.05-0.30% Iron balance -- -- Cobalt -- balance -- Nickel -- -- balance ______________________________________

The application of the coating composition to a variety of substrates, such as nickel-base and cobalt-base superalloys may be achieved by physical vapor deposition in a vacuum chamber. During this procedure, the composition is thermally evaporated from a source heated, for example, by an electron beam, and a thin metal coating is condensed on the surface of the workpiece. Layers of the coating are formed as the workpiece is rotated until the thickness is, preferably, in the range of about 1-7 mils. Unfortunately, the deposited coating has radially oriented defects which are the sites of attack by oxidizing and/or corrosive atmospheres at high temperatures. Such defects can lead to premature failure of the coating.

Attempts to prolong the useful life of superalloys coated with a FeCrAlY alloy are disclosed by Elam et al., U.S. Pat. No. 3,528,861. The coating effectiveness was found to be limited by the formation of an intergranular precipitate during the coating deposition cycle. The effect of the detrimental precipitate was improved by shot peening or glass bead blasting to break up the precipitate into small particles which are more easily taken into solution by heat treatment.

In accordance with the present invention, I have invented a method of improving the high temperature corrosion resistance of a nickel-base or cobalt-base superalloy body by first coating the superalloy body by physical vapor deposition with a composition consisting essentially of chromium, aluminum, and, optionally, a member selected from the group consisting of yttrium and the rare earth elements, and at least one element selected from the group consisting of iron, cobalt and nickel and thereafter depositing a layer of aluminum, preferably 0.1 to 3.0 mil thick, onto the coated body by physical vapor deposition and heat treating this composite coating to increase the corrosion resistance of the body. The effectiveness of the coating system may be explained by the fact that the first coating layer exhibits flaws or boundaries that are oriented in a perpendicular direction to the deposition plane. Upon exposure to a corrosive environment, these flaws or boundaries are preferentially attacked resulting in premature failure of the coating. The application of an aluminum overcoat that is subsequently suitably heat treated prevents this type of failure and thereby substantially increases the life of the coated article. In addition, the concentration profile of our novel coating system indicates the presence of a high concentration of aluminum on the outer surface of the coating which may also contribute to the improved properties. The coated superalloy bodies prepared by our invention are particularly useful in making gas turbine engine components.

The invention is more clearly understood from the following description taken in conjunction with the accompanying drawing in which:

FIG. 1 is a photomicrograph (500X) of a Udimet 500 nickel-base superalloy body coated with a CoCrAlY coating.

FIG. 2 is a photomicrograph (500X) of a Udimet 500 nickel-base superalloy body coated with a first CoCrAlY coating and then an aluminum layer and heat treated for three hours at 1160.degree. C. in argon to form a composite coating according to the method of our invention.

FIG. 3 is a photomicrograph (500X) illustrating the effect of corrosion on a CoCrAlY coated superalloy body.

FIG. 4 is a photomicrograph (500X) illustrating the effect of corrosion on a coated superalloy body coated as shown in FIG. 2.

The superalloys are strong, high temperature materials which are particularly useful in gas turbine engines. A substantial listing of these materials is set forth by W. F. Simmons, Compilation of Chemical Compositions and Rupture Strengths of Superalloys, ASTM Data Series Publication No. DS9E, and may be represented by the nominal compositions in weight percent of the following superalloys:

Udimet Ingredient Rene 80 Rene 100 In-738 500 ______________________________________ C 0.17 0.18 0.17 0.08 Mn 0.2 0.50 0.20 0.75 Si 0.2 0.50 0.30 0.75 Cr 14.0 9.5 16.0 19.0 Ni *Bal. *Bal. Bal. Bal. Co 9.5 15.0 8.5 18.0 Mo 4.0 3.0 1.75 4.0 W 4.0 -- 2.6 -- Cb -- -- 0.9 -- Ti 5.0 4.20 3.4 2.9 Al 3.0 5.50 3.4 2.9 B 0.015 0.015 0.01 0.005 Zr 0.03 0.06 0.10 -- Fe 0.2 1.0max 0.50 4.0 Other -- 1.0V 1.75Ta -- ______________________________________

The first coating of our protective coating system is designated herein as "MCrAlY" or "MCrAl" coating wherein M is a member selected from the group consisting of iron, cobalt, and nickel. This coating is broadly defined as consisting essentially in weight percent of the following nominal compositions:

Ingredients Weight % ______________________________________ Chromium 14-35 Aluminum 4-20 Yttrium 0-3 Iron ) Cobalt ) Balance Nickel ) ______________________________________ Included in this formulation are the compositions designated hereinabove as FeCrAlY, CoCrAlY, and NiCrAlY and also includes these compositions in which yttrium is entirely absent. The MCrAlY or MCrAl coating is applied to the substrate by a physical vapor deposition technique which is described in considerable detail in Vapor Deposition, Edited by C. F. Powell et al., John Wiley & Sons, New York (1966). Accordingly, the coating is evaporated and deposited in a vacuum chamber. Typically, the metal alloy is heated by an electron beam focused on the metal alloy ingot to evaporate the metal to a vapor. During evaporation, the vapor condenses as a coating, preferably about 1-7 mils in thickness on the workpiece being coated. The material to be applied is heated in a high vacuum to a temperature at which its vapor pressure is about 10.sup..sup.-2 torr or greater whereupon it emits molecular rays in all directions. During coating the vacuum must be very high to permit the molecular rays to travel from their source without disturbance until they hit the surface of the object to be coated. A photomicrograph of a nickel-base superalloy coated with a CoCrAlY coating is shown in FIG. 1.

Then, the first coating is overcoated with a layer of aluminum, preferably 0.1 to 3.0 mils thick, by a physical vapor deposition technique already described and referred to in Powell et al. cited hereinabove. In a preferred embodiment, the aluminum layer is deposited by evaporation from an electron beam heated source. This technique is particularly useful because the aluminum layer can be deposited in the same equipment used to deposit the MCrAl or MCrAlY by simply substituting an aluminum source for the MCrAl or MCrAlY source.

Thereafter, the coated body is subjected to a heat treatment to cause interdiffusion of the aluminum overlayer into the first coating layer. Essentially while the aluminum, which has a low melting point of 660.2.degree. C., diffuses into the cracks and defects of the first coating, a simultaneous interdiffusion phenomena occurs during the heat treatment whereby the aluminum combines with the first coating to form a higher melting alloy. The minimum temperature for the heat treatment is about 950.degree. C., while the maximum temperature depends upon the superalloy substrate since it is undesirable to heat above the solutionizing heat treatment of the alloy involved. For nickel-base superalloys this tends to be in the range of about 1040.degree.-1230.degree. C. The heat treatment of cobalt-base alloys is much less complex than for nickel-base alloys and high temperature solution heat treatments are usually at about 1150.degree. C. The time of heat treatment is preferably in the range of about 0.25-5 hours. To avoid premature oxidation of the duplex coating surface prior to interdiffusion, the heat treatment should be performed in an inert atmosphere, e.g. helium, argon.

A photomicrograph of a nickel-base superalloy which has been coated with a heat treated duplex coating of CoCrAlY plus aluminum is shown in FIG. 2. The first coating of CoCrAlY exhibited flaws or boundaries that are oriented in a perpendicular direction to the deposition plane as shown in FIG. 1 which become sites for attack by high temperature oxidation and corrosion as shown in FIG. 3. Upon application of the heat treated duplex coating, any open defects of an MCrAlY coating become filled as shown in FIG. 2. FIG. 4 shows a photomicrograph of the heat treated duplex coating of CoCrAlY plus aluminum that has been subjected to a severe hot corrosion and oxidation environment consisting of a fused salt at 1650.degree. F. containing Na.sub.2 SO.sub.4 and V.sub.2 O.sub.5. It is to this unique coating system that we attribute the improved properties of high temperature oxidation and corrosion resistance.

Our invention is further illustrated by the following examples:

EXAMPLE I

Cast pins of Udimet 500 nickel-base superalloy 2 in. long were ground to a diameter of 0.170 in. They were placed into a vacuum electron beam furnace and the chamber was then evacuated to 10.sup..sup.-5 microns of mercury. The pins were rotated during deposition and a resistance heater was used to heat the pins. When the temperature reached 900.degree. C., the electron beam was focused on an ingot having the following nominal composition: Ingredient Weight % ______________________________________ Cobalt 64 Chromium 22 Aluminum 13 Yttrium 1 ______________________________________

Evaporation of the metal was at a constant power of 19.0 kilovolts and 275 milliamps for 30 minutes. A coating having a thickness of about 3 mils was deposited on the pins. The pins were cooled to room temperature in the vacuum. Then the chamber was filled to atmospheric pressure with air and a pure aluminum source was substituted for the CoCrAlY source. The chamber was again evacuated to a pressure of 10.sup.-.sup.5 mm. Hg and the pins were overcoated with a layer of aluminum about 1/2 mil thick by evaporation at a constant power of 19.0 kilovolts and 80 milliamps for three minutes. Finally, the pins coated with the duplex layer of CoCrAlY plus aluminum were heat treated for three hours at 1160.degree. C. in an argon atmosphere.

Upon examination of the sample it was noted that this procedure resulted in a penetration of about 1-2 mils of aluminum into the surface of the CoCrAlY coating and an essentially total filling of the open boundaries of the CoCrAlY coating.

A crucible test was then performed to test resistance to oxidation and corrosion of the samples coated only with the CoCrAlY coating and the second group subjected to the duplex coating procedure. Both groups of coated pins were immersed in a fused salt bath of 85.2 percent Na.sub.2 CO.sub.3, 0.5 percent NaCl, 13.0 percent V.sub.2 O.sub.5 and 1.3 percent Na.sub.2 SO.sub.4 (by weight) at a temperature of 900.degree. C. in an air atmosphere. After 16 hours the samples were removed.

It was observed that the samples coated only with the CoCrAlY coating were characterized by deep "spike" corrosion, but the pins subjected to the aluminizing overcoating procedure were substantially more resistant to attack by corrosion with little or no evidence of "spike" corrosion.

EXAMPLE II

Following the procedure of Example I, test pins having a 0.170 in. diameter were prepared from Udimet 500 nickel-base superalloy. The pins were coated by electron beam evaporation with the following nominal composition:

Ingredient Weight % ______________________________________ Nickel 70 Chromium 20 Aluminum 15 ______________________________________

After the pins were removed from the apparatus, the coating had a thickness of about 3 mils.

Thereafter some of the coated pins were overcoated with about 0.5 mil of aluminum by the physical evaporation technique. The samples coated with the duplex layer of NiCrAl plus aluminum were heat treated for three hours at 1160.degree. C. in argon.

Comparative high temperature oxidation and corrosion tests were performed on test samples coated only with the NiCrAl alloy and on test samples coated with a heat treated duplex layer of NiCrAl plus aluminum. In the crucible test, the pins were partially immersed in the fused salt mixture described in Example I for 16 hours at 900.degree. C. The samples coated only with the NiCrAl coating after being subjected to the hot corrosion and oxidation test showed the typical corrosion and "spike" penetration. The samples which had been protected by the heat treated duplex coating showed almost a complete absence of "spike" corrosion and that indicated that the heat treated aluminum overlay coating had filled in the defects of the initial coating.

EXAMPLE III

Following the procedure of Example I, cast pins of Udimet 500 nickel-base superalloy were coated by physical vapor deposition with the following nominal composition:

Ingredient Weight % ______________________________________ Iron 64.0 Chromium 25.0 Aluminum 10.0 Yttrium 1.0 ______________________________________

A coating having a thickness of about 3 mils was deposited on the pins. Thereafter some of the pins were duplex coated by the physical vapor deposition technique described in Examples I and II.

When subjected to the corrosion tests it was observed that the pins subjected to the duplex overcoat procedure were considerably more resistant to corrosion than those which had been coated only with the FeCrAlY coating.

It will be appreciated that the invention is not limited to the specific details shown in the examples and illustrations and that various modifications may be made within the ordinary skill in the art without departing from the spirit and scope of the invention.

Claims

I claim:

1. A method of improving the high temperature oxidation and corrosion resistance of a nickel-base or a cobalt-base superalloy body comprising the steps of:

a. coating the superalloy body by physical vapor deposition with a composition consisting essentially of chromium, aluminum, a member selected from the group consisting of yttrium and the rare earth elements, and at least one element selected from the group consisting of iron, cobalt and nickel, and

b. subjecting the coated body to an overcoat of aluminum deposited by physical vapor deposition, and

c. heat treating the duplex coating to cause the interdiffusion of aluminum with the first coating to increase the oxidation and corrosion resistance of the coating.

2. The method of claim 1, wherein said composition consists essentially in weight percent of 14-35 percent chromium, 4-20 percent aluminum, 0-3 percent yttrium and the balance being a member selected from the group consisting of iron, cobalt, nickel, and mixtures thereof.

3. The method of claim 1, wherein said composition consists essentially in weight percent of 25-29 percent chromium, 12-14 percent aluminum, 0-0.9 percent yttrium and the balance being iron.

4. The method of claim 1, wherein said composition consists essentially in weight percent of 19-24 percent chromium, 13-17 percent aluminum, 0-0.9 percent yttrium, and the balance being cobalt.

5. The method of claim 1, wherein said composition consists essentially in weight percent of 20-35 percent chromium, 15-20 percent aluminum, 0-0.30 percent yttrium and the balance being nickel.

6. The method of claim 1, wherein said heat treating is at a temperature in the range of about 950.degree. C. up to the solutionizing heat treatment of the superalloy body.

7. The method of claim 6, wherein said body is a nickel-base superalloy.

8. The method of claim 6, wherein said body is a cobalt-base superalloy.

9. The method of claim 6, wherein the first coating has a thickness of about 1-7 mils and the heat treated aluminum overcoating penetrates into the first coating to a depth of about 1-2 mils.