High Integrity CoCrAl(Y) Coated Nickel-Base Superalloys

1. Field of the Invention

This invention relates to a high temperature oxidation and corrosion resistant coated nickel-base superalloy article comprising (a) a nickel-base superalloy article, and (b) a first CoCrAl(Y) coating composition consisting essentially of, on a weight basis, approximately 26-32% chromium, 3-9% aluminum, 0-1% yttrium, the rare earth elements, platinum or rhodium, and the balance nickel. An aluminide overcoating can be applied to the CoCrAl(Y) coated superalloys and constitute another embodiment of my invention.

2. Description of the Prior Art

Evans et al. in U.S. Pat. No. 3,676,085 describe coated nickel-base superalloys wherein the coating composition consists essentially of, on a weight basis, 15-40% chromium, 10-25% aluminum, 0.01-5% yttrium or the rare earth elements and the balance cobalt. Evans et al. teache that when the aluminum content of the CoCrAl(Y) coating is below about 10% there is insufficient aluminum present in the coating system to provide the desired long term durability in the coating.

Unexpectedly, I have found that nickel-base superalloys when coated with CoCrAl(Y) coatings having an aluminum content of less than 10% have outstanding physical and chemical properties, i.e. significant oxidation and corrosion resistance and high coating-substrate interface integrity. These outstanding properties are not associated with nickel-base superalloys when coated with the coating systems described by Evans et al. referenced above.

This invention embodies a high temperature oxidation and corrosion resistant coated nickel-base superalloy article comprising (a) a nickel-base superalloy article of manufacture, and (b) a first CoCrAl(Y) coating composition consisting essentially of, on a weight basis, approximately 26-32% chromium, 3-9% aluminum, and 0-1% yttrium, the rare earth elements, platinum or rhodium, and the balance nickel.

My invention is more clearly understood from the following description taken in conjunction with the accompanying drawings, where:

FIG. 1 is a photomicrograph (500.times.) of a CoCrAl(Y) coated nickel-base IN-738 superalloy having a coating composition of Evans et al. U.S. Pat. No. 3,676,085, i.e. Co-22Cr-13Al-1Y. This figure illustrates the low integrity of a CoCrAl(Y) coated nickel-base superalloy of the prior art, i.e. a coating which has a significant and a substantial tendency to separate from a superalloy substrate thereby failing to give the oxidation and corrosion resistant coating integrity desired for nickel-base superalloys.

FIG. 2 is a photomicrograph (500.times.) of a CoCrAl(Y) coated nickel-base IN-738 superalloy having a coating composition of my invention, i.e. Co-29Cr-6Al-1Y.

This figure illustrates the high integrity of a CoCrAl(Y) coated nickel-base superalloy of my invention, i.e. a coating which does not have a significant or substantial tendency to separate from a superalloy substrate thereby giving the oxidation and corrosion resistance coating integrity desired for nickel-base superalloys. The coated nickel-base superalloys of my invention have a thermal expansion coefficient value .alpha. as measured in inches per inch per .degree. F. (in./in./.degree. F.) over a temperature range of (i) 100.degree.-1200.degree. F. of 8.45 to 9.05 in./in./.degree. F. and (ii) 100.degree. to 1740.degree. F. of 9.45 to 10.5 in./in./.degree. F.

A presently preferred nickel-base superalloy employed in my invention "IN-738" is of the following general composition:

This superalloy has a thermal expansion coefficient value .alpha. measured in in./in./.degree. F. over the temperature ranges set out above of 8.7 .+-. 0.1 and 9.7 .+-. 0.1, respectively.

A presently preferred CoCrAl(Y) coating employed in my invention "GT-29" is of the following general composition: Co-29Cr-6Al-1Y. This coating has a thermal expansion coefficient .alpha. measured in in./in./.degree. F. over the temperature ranges set out above of 8.8 .+-. 0.1 and 9.9 .+-. 0.1, respectively.

The nickel-base superalloys and CoCrAl(Y) alloys employed in my invention can be prepared by any method well-known to those skilled.

The CoCrAl(Y) coatings can be applied to the nickel-base superalloys by means, such as physical or chemical vapor deposition, or any other means well-known to those skilled in the art for the application of CoCrAl(Y) coatings to superalloys. Among the coating techniques that can be used are those described in

Flame Spray Handbook, Volume III, by H. S. Ingham and A. P. Shepard, published by Metco, Inc., Westbury, Long Island, New York (1965),

Vapor Deposition, edited by C. F. Powell, J. H. Oxley and J. M. Blocher, Jr., published by John Wiley & Sons, Inc., New York (1966), etc.

In general, the CoCrAl(Y) coated nickel-base superalloys can have any coating thickness sufficient to give a desired oxidation and corrosion resistance. Generally economic and effective coating thicknesses are 1-20 mils for most commercial applications. In preferred embodiments, where electron-beam techniques are employed the coating thicknesses range from 1-5 mils and where plasma flame spray techniques are employed the coating thicknesses range from 3-10 mils. In another preferred embodiment where an aluminide overcoating is employed, the aluminide overcoating -- including any duplex heat treatment where the aluminide overcoating is heated for periods of time of from 30 or 60 to 120 minutes at elevated temperatures of 850.degree. to 1200.degree. F. in air, argon, etc., for the purpose of diffusing aluminum into the CoCrAl(Y) coating -- the aluminuide process is carried out in a manner which limits the aluminum penetration into the CoCrAl(Y) coating to a distance no nearer than a 1/2 mil measured from the interface of the nickel-base superalloy and the CoCrAl(Y) coating. This aluminide diffusion penetration limitation is essential to the integrity of the CoCrAl(Y) nickel-base superalloy interface since as indicated hereinbefore (as illustrated by FIG. 1) an increase in the aluminum content of the CoCrAl(Y) coating to levels of 10% or more deleterious affects the integrity of the coating composition.

My invention is further illustrated by the following examples:

An experimental series was designed to study the expansion match characteristics of nickel-base superalloys and CoCrAl(Y) compositions as well as their oxidative and corrosion resistance.

Test specimen pins of IN-738 were prepared which had been lightly abraded with a No. 3 alumina powder. The resulting pins were 4.4 centimeters long and 0.25 cm. in diameter. A series of CoCrAl(Y) ingots having the compositions set out hereafter in Table I were electron-beam deposited on the abraded IN-738 pin substrates at a deposition rate of approximately 0.1 mils per minute while the pins were rotated at approximately 10 revolutions per minute. The coatings were deposited at various pins substrate temperatues, e.g. 1022.degree. F., 1292.degree. F., 1562.degree. F. and 1832.degree. F. The CoCrAl(Y) coated pins were thermal cycled during deposition over a temperature range of from approximately 1832.degree. F. to 70.degree. F. (room temperature).

Mellographic examination via photomicrographs -- illustrated by FIGS. 1 and 2 -- shows that the CoCrAl(Y) compositions of Evans et al. are not suited to nickel-base superalloys defined herein since high aluminum CoCrAl(Y) coatings as deposited on the nickel-base superalloy IN-738 separate from the substrate during thermal cycling over a temperature range of from 1832.degree. - 70.degree. F.

Another series of CoCrAl(Y) coated IN-738 pin samples were prepared as described in Example I above -- having the coating compositions set out hereafter in Table II -- were subjected to a burner rig test which simulated conditions used in a marine gas turbine engine under highly corrosive conditions. The test was run to coating failure using a diesel fuel containing 1% by weight of sulfur and 467 parts per million sea salt at a temperature 1600.degree. F. coupled with thermocycling to room temperature 3 to 5 times per week. The CoCrAl(Y) coated IN-738 samples were evaluated and characterized according to hours to failure, failure being defined as a condition wherein the results of the burner rig corrosion test conditions set out in Table II hereafter: