GB2056487A - Superalloy coating composition - Google Patents

Abstract

Coatings for iron-, nickel- and cobalt-base superalloys. The coatings are applied in order to provide good oxidation/sulfidation and thermal fatigue resistance for the substrates to which the coatings are applied. The coatings consist essentially of, by weight, 10 to 50% chromium, 3 to 15% aluminum, 1.0 to 15% metal mixture from the group consisting of tantalum, tungsten, manganese and combinations thereof, up to 5% reactive metal from the group consisting of lanthanum, yttrium and other rare earth elements, up to 5 percent of rare earth and/or refractory metal oxide particles, and the balance selected from the group consisting of nickel, cobalt and iron, and combinations thereof. Additions of titanium up to 5% and noble metals up to 15% are also contemplated, Tantalum makes up at least 20% of the metal mixture or 0.5% of the total coating weight, whichever is greater. Tungsten, manganese, or a combination thereof, make up at least 0.5% of the total coating.

Description

SPECIFICATION Superalloy coating composition RELATED APPLICATION This application is a continuation-in-part of applicants' copending application Serial No.43,146, filed May 29, 1 979, and entitled "High Temperature Oxidation and Sulfidation Resistant Coating".

BACKGROUND OF THE INVENTION This invention is concerned with coatings adapted to significantly improve the elevated temperature corrosion resistance of articles composed of iron-, cobalt- or nickel-based superalloys whereby more satisfactory performance and longer life for such articles can be obtained.

Elevated temperature exposure of metal articles is experienced in many situations. Metal components are subjected to such conditions, for example, in various aerospace applications and in land and marine operations such as in the case of components utilized in gas turbine engines.

In such applications, it is important to provide some means for preventing undue oxidation/sulfidation of the components involved since such corrosion can materially shorten the useful life of the components. Deterioration of components can also create significant performance and safety problems.

Various alloys, including most superalloys, are characterized by a degree of corrosion resistance, however, such resistance is significantly decreased when unprotected superalloy components are exposed at the operating temperatures involved in certain systems. For that reason, such components have been provided with coatings, such as aiuminide coatings, which increase the corrosion resistance at elevated operating temperatures.

Aluminide coatings are applied by methods such as the pack cementation process. In this process, the substrate chemistry and the processing temperature exert a major influence on coating chemistry, thickness and properties. Specifically, the coatings comprise a hard, brittle outer layer and a hard, brittle multi-phase sublayer that can crack when subjected to mechanically or thermally induced strain. This leads to poor fatigue properties, and the cracks can also materially reduce the corrosion resistance of the coated components.

Another class of coatings is the MCrAIY overlay coatings where M stands for a transition metal element such as iron, cobalt or nickel. MCrAIY coatings have been shown to have an advantage over aluminide coatings in providing extended life to turbine components. Specifically MCrAIY coatings generally demonstrate greater corrosion resistance than aluminide coatings and also greatly superior ductility.

Presently, these MCrAIY coatings are applied by vacuum physical vapor deposition. However, the fundamental nature of the vacuum physical vapor deposition process limits the composition of the coating that can be applied to an article. Specifically, with a single multi-element source, it is very difficult to deposit MCrAIY type coatings which contain other elements that have either very low or very high vapor pressures. Resorting to dual or multiple sources introduces a further degree of complexity to an already complex process which is undesirable from a production standpoint.

Another technique of applying MCrAIY coatings is plasma spraying. In plasma spraying, the heated alloy particles corresponding to the desired coatings composition are impinged on the preheated surface of the metal article at very high velocity and temperature. Such particles, upon contact with the metal article surface or with other applied particles, deform plastically and fuse and bond to the surface or to the other applied particles, thus producing a dense and adherent coating. Plasma spraying is particularly desirable since it is a generally less costly technique for producing the overlay coating and is not restricted by vapor pressure limitations as is the case with the vacuum physical vapor deposition processes.

Other attempts at improving elevated temperature corrosion resistance are described in U.S.

Patent No. 4,145,481, issued on March 1979. This process involves the location of a MCrAIY coating over a substrate to provide an overlay, and an aluminide coating was then added as an outer layer. This technique attempts to achieve the advantages of the ductility of the MCrAIY and the resistance to elevated temperature corrosion of the aluminide. Copending application Serial No.

847,253, filed on October 1977, takes the approach of utilizing first and second MCrAIY-type coatings on a substrate. A first coating is intended to provide a ductile layer with the second coating providing a layer having a greater resistance to elevated temperature corrosion.

Still other approaches, particularly from the standpoint of alloying ingredients and application techniques are described in the following U.S. Patents: Inventor Patent No. Date of Issue Gedwill, et al. 3,849,865 Nov. 26, 1974 Gedwill, et al. 3,869,779 Mar. 11, 1975 Hecht, et al. 3,928,026 Dec. 23, 1975 Bessen 3,957,454 May 18, 1976 Preston 4,005,989 Feb. 1,1977 In view of the fact that increasingly greater demands are placed on performance, particularly for compontnts subject to extreme temperature conditions, it is desirable to provide even greater improvements in the capabilities of coatings of the type described. The demand for requisite ductility while maintaining resistance to the corrosive effects of temperature and atmosphere is particularly critical.

Oxidation-sulfidation resistance and thermal fatigue resistance at temperatures above 1400OF is of great importance. Coatings suited for metal components which are subjected to a relatively low temperature (less than 1400OF) corrosive environment are, however, also of great value.

The low temperature corrosive environment, in particular, refers to the conditions that exist in liquid-fueled turbines burning fuel high in sulfur and vanadium content and operating in a marine environment. Substantial sulfidation (hot corrosion) has been observed in these types of engines, especially when they are operated at low power settings (low temperature). SO3 has been identified as an agent that can be responsible for this type of attack.

In such applications, it is important to provide some means of preventing the catastrophic corrosion since such corrosion can materially shorten the useful life of the components. Deterioration of components can also create significant performance and safety problems.

Some attempts have been made to develop coating compositions to combat the problem of "low temperature" corrosion occurring below 14000F. The following U.S. patents describe compositions and application techniques which might be used for this application: Patent No. Date of Issue Patentee 4,022,587 May 10, 1977 Wlodek 4,088,479 May 9,1978 Spengler 4,101,715 July 18,1978 Rairden These compositions are simply MCrAIY coatings. In view of the energy shortage, resulting in gas turbine engines burning "dirty" fuel containing large amounts of sulfur and vanadium, it is desirable to provide even greater improvements in the capabilities of coatings to provide corrosive resistance at temperatures below approximately 1400OF.

SUMMARY OF THE INVENTION The coating compositions of this invention are particularly resistant to oxidation sulfidation at elevated temperatures, are otherwise highly efficient in their performance at these temperatures, and are well suited for application to substrates by plasma spraying. In the broadest sense, the coatings consist esentially of from 10 to 50% chromium, 3 to 15% aluminum, a metal mixture from the group consisting of tantalum, tungsten and manganese and combinations thereof, and the balance selected from the group consisting of nickel, cobalt and iron and combinations thereof. The mixture of tantalum, tungsten and manganese is present in amounts from 1 to 15%, and tantalum comprises at least onefifth by weight of the mixture, or at least 0.5% of the total coating weight whichever is greater. The balance of the mixture consists of at least 0.5% manganese or tungsten, or combinations thereof.

Optionally, the coating may have up to 5% reactive metal from the group consisting of lanthanum, yttrium and the other rare earths. Also the addition of rare earth and/or refractory metal oxide particles to the aforementioned coating composition is contemplated; these ingredients preferably being individually utilized in amounts from about 0.05 up to about 2.0% by weight. Such additions can be beneficial to the over-all protective response of the coating because the metal oxide particles assist in pinning protective oxide scales. This pinning phenomenon results in superior adherence (less spalling) of the protective scale, thus increasing the over-all coating life. Additions of titanium up to about 5% and of noble metals up to about 15% are also contemplated.

It will be appreciated that in using the term "coating", reference to application of material to a substrate surface is generally intended. Use of the material as an abradable sealing material is, for example, contemplated.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 comprises a 500X magnification of the microstructure of a coating and substrate particularly suited for performance at high temperatures; Figure 2 comprises a 500X magnification of the microstructure of a coating and substrate particularly suited for performance at low temperatures; Figure 3 is a plan view of a test specimen having temperature profiles superimposed thereon; and, Figure 4 is an end view of the test specimen.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As set forth in the foregoing summary, the invention relates to a coating composition for superalloy substrates. Protection against oxidation at very high temperatures (in the order of 1400OF and higher) is particularly important, and a coating particularly suited to this consists essentially of from 5 to 35% cobalt, 10 to 35% chromium, 5 to 15% aluminum, a metal mixture from the group consisting of tantalum, tungsten and manganese and combinations thereof, and the balance nickel. The mixture is present in amounts from 1 to 1 5%, and tantalum represents at least one-fifth by weight of the mixture, or at least 0.5 of the total coating weight, whichever is greater. The balance of the mixture consists of at least 0.5% manganese or tungsten, or combinations thereof.

This coating may include alloying elements for various purposes, for example up to 5% reactive metal consisting of lanthanum, yttrium and the other rare earths. In the preferred form of the invention, these elements are utilized in an amount between 1.0 and 3.0% by weight of the coating composition.

Rare earth of refractory metal oxide particles in amounts beneficial to the over-all protective aspects of the coating are also contemplated because they assist relative to the pinning phenomenon.

As indicated, these additions are preferably in the range of 0.05 to 2.0% by weight.

A similar improvement in the coating life can be achieved by the addition of up to about 1 5 weight percent of a noble metal selected from the group consisting of platinum, rhodium or palladium. An addition of this type also improves the diffusional stability of the coating.

In formulating the metal mixture which makes up a critical portion of the coating composition, it is preferred that the mixture contain between 2 and 5% tantalum along with between 2 and 5% of material comprising tungsten, manganese or a combination thereof. It has been found, however, that the amount of tungsten preferably does not exceed 1.5% by weight. The other ingredients of the coating composition of this type are preferably employed within the limits of 10 to 20% cobalt, 1 5 to 25% chromium and 10 to 14% aluminum.

It is contemplated that the coating composition of this invention form the corrosion layer of the two layer MCrAiY coating described in the aforementioned patent application Serial No. 847,253.

The composition may also form the inner layer or the bond coat for a two layer or graded thermal barrier coating which is used to reduce metal operating temperatures and the effects of thermal transients in gas turbine engines. In such applications, the bond coat must be similar to the substrate, and the composition of the ceramic/metallic two layer or graded coating system must be such that stresses caused by thermal expansion mismatch between the bond coat and the compatible oxide outer layer are minimized. In addition, the inner layer must be fairly dense and the surface of this undercoat must be rough enough to provide an adherent surface for the oxide overcoat. Plasma sprayed compositions satisfy these requirements, and hence, are ideally suited for this application.

As noted, the above narrower composition ranges are best suited for higher temperatures although compositions within the broader ranges have utility at such temperatures. With reference to compositions suited for the coating of superalloy substrates where operating temperatures are less than abot 1400OF, the broader ranges for percentages of ingredients are recommended. Specifically, higher chromium content is desirable in certain instances with from 10 to 50% by weight chromium being an appropriate range. On the other hand, lower percentages of aluminum can be utilized and, therefore, an aluminum range from 3 to 15% is appropriate. Finally, titanium in amounts up to about 5% by weight is desirably included.

The intermediate temperature coating compositions may comprise either nickel, cobalt or iron base compositions with combinations of these ingredients being suitable but not required. Otherwise, the coating compositions include additions of a metal mixture comprising tantalum combined with tungsten or manganese or a combination thereof with preferred inclusions of a reactive metal and, alternatively, inclusions of rare earth and/or refractory metal oxide particles. In each instance, the amounts of these ingredients are utilized in the same ranges as expressed relative to the high temperature coatings.

Other uses for the compositions of the invention will be apparent to those skilled in the art, and it will also be appreciated that other alloying elements may be employed in the coatings in accepted amounts and in accordance with known practices.

The utilization of plasma spray techniques to deposit the coating compositions is preferred. The wide differences in the evaporation rate (or vapor pressure) between high vapor pressure elements like manganese or aluminum and low vapor pressure elements like tantalum or tungsten makes the deposition and composition control of this coating by vacuum physical vapor deposition difficult, if not impossible. In addition, compositions in accordance with this invention produce very dense coatings after plasma spraying. It will be appreciated, however, that process improvements or modifications in methods such as physical vapor deposition or ion plating could make coating by these methods possible, and the use of these methods is therefore contemplated. Additionally, techniques like sputtering or slurry sintering may also be considered.

After deposition of the coating, the coated articles preferably are subjected to an elevated temperature treatment in the range of 1 9000F to 22000F for a duration of one to 10 hours in an inert atmosphere (for example, in a vacuum or in an argon atmosphere) to promote interfacial bonding.

To illustrate the practice of the invention, a typical nickel-base superalloy of the type used in gas turbine engines, known as @N738, and having a nominal composition of 0.09% C, 16.0% Cr, 8.5% Co, 1,7% Mo, 2.5% W, 1.7% Ta, 3.5% Ti, 3.5% Al, 0.01% B, 0.03 Zr and the balance Ni, was provided as one substrate. A typical cobalt-base superalloy of the type used in gas turbine engines, known as Mar-M509 and having a nominal composition of 0.60% C, 23.4% Cr. 10.0% Ni, 7% W, 3.5% Ta, 0.23% Ti, 0.01% B, 0.45% Zr, 1.5% Fe, 0.10% Mn, 0.40% Si and the balance Co, provided a second substrate for testing.

A first series of coatings was produced by plasma spraying prealloyed powders. These powders were sprayed in a plasma arc ( > Mach 3 velocity) using argon and helium as primary and secondary gases, respectively.Spraying was performed in a chamber maintained at a dynamic pressure of 55 Torr.

The process parameters were: Gun to workpiece distance 16 in.

Primary gas (argon) 370 CFH at 225 psi Secondary gas (helium) 150 CFH at 250 psi Voltage 50 - 52 volts Current 1400 -- 1440 amps Powder flow 0.07 Ib./min Carrier gas (argon) 25 CFH at 100 psi Time for deposition 45 sec.

The articles were then subjected to heat treatment in vacuum for four hours at 1975 F.

The following table illustrates the compositions, tested and the test results: TABLE 1 PROPERTIES OF MCrAlY COATINGS

Impact Energy Average Life () Required (Hours) To Produce Composition (Wt %) Cracking After Cracks () Coating IN738 MAR-M509 Water Spray or Chips System Ni Co Cr Al Ta Mn W La Y Substrate Substrate Quench Test (In.-Lbs.) UTC ( ) NICoCrAlY Bal 23 18 13 0.3 100 190 No 3.0 MDC-35A Bal 15 20 12 2.5 0.5 107 No 3.0 MDC-34H Bal 10 20 17 0.6 186 Yes 1.0 MDC-1A Simple Aluminide 23(4) Yes 0.5 LDC-2E Platinum Aluminide 135 Yes 2.0 MDC-35B Bal 15 20 12 2.5 1.5 0.5 124 237 No 3.0 MDC-35C Bal 15 20 12 2.5 0.5 110 No 3.0 MDC-35D Bal 15 20 12 2.5 2.5 0.5 175 238 No 2.0-3.0 MDC-35E Bal 15 20 12 2.5 1.5 0.5 125 No 3.0 MDC-35M Bal 21 16 12 2.5 1.7 1.0 230 No 3.5-4.0 () Rlg Cycle : 2100 F/2 Min. + 1750 F/4 Min. +2100 F/2 Min. + Cool/2 Min. (5 ppm salt) () Results obtained from drop weight test.

( ) Composition conforming to United Technologies Patent No. 3,928,026.

(4) Result from one test.

A 500X photomicrograph of one of the coatings (MDC-35D) is shown in Figure 1; the thicknesses of the coatings typically were 0.004 inches, however, it is contemplated that the coating thickness vary between 0.0001 and 0.1 inches. The optical micrograph reveals the presence of a ductile matrix of gamma (Ni, Cr) containing a dispersion of beta (Ni, Co) Al intermetallic compound. The proportion of these two phases was about the same in all the MCrAIY coatings listed in Table I with the exception of MDC-34H; this high Al MCrAIY coating contained more of the beta phase. Electron microprobe analysis showed that the coating chemistry was not very much different from that of the chemistry of the powder.

The performance of the articles coated pursuant to this example was evaluated using a 0.7 Mach burner rig. The testing cycle was 21 000F/2 minutes; 1 7500F/4 minutes; 21 000F/2 minutes; air cool 1/2 minutes. Five (5) ppm salt solution was injected into the combustion products of JP5 fuel containing 0.2% sulfur. This cycle closely simulates the gas turbine engine environment for turbine vanes and blades, it highlights the oxidation phenomenon, and it imposes significant thermal stresses on the protection system.

It is seen from Table I that coatings produced in accordance with this invention exhibit substantially improved performance compared to a simple MCrAIY type system (UTC NiCoCrAIY, Patent No. 3,928,026). In addition, the corrosion resistance of the relatively low aluminum content material is similar to that of a very high aluminum content MCrAIY coating like MDC-34H which had more of the beta phase. A typical high Al content MCrAIY will have good oxidation resistance but poor ductility because of the high amount of beta phase; whereas, a low Al content MCrAIY will have good ductility but relatively poor oxidation resistance. Coatings produced in accordance with this invention demonstrate excellent oxidation resistance and, because of their relatively low aluminum content, exhibit excellent ductility as will be discussed.The coatings also reveal improved oxidation resistance compared to an advanced platinum aluminide coating like LDC-2E, and a simple aluminide coating such as MDC--1A.

The performance of the articles coated pursuant to this example was also evaluated by means of a water spray quench test and drop weight impact testing. The former test is a measure of coating ductility and consists of heating the coated airfoil sections to 21000F+1000F, holding the articles at this temperature for time periods of 1 5 minutes to two hours, and then quenching them in a water spray. It is to be noted that the thermal strains that are generated in this type of test are less severe than those encountered in cooled aircraft engine gas turbine blades and may be similar to those experienced in other types of gas turbine blades. The latter test is also a measure of coating ductility, highlighting the handling characteristics of the coating parts.It consists of dropping a one pound indentor from several heights onto the trailing edge of a coated airfoil section with the energy of impact being equal to the height in terms of inch-lbs. The tested specimens are evaluated, using a stereo microscope at 20X magnification, for appearance of defects (chips and cracks). The energy of impact necessary to produce cracks or chips on the trailing edges is taken as a measure of coating ductility. The higher this energy, the greater the coating ductility.

The results from these two tests are also shown in Table I. It is seen that articles coated pursuant to this example do not show any cracks after the water spray quench test unlike the high Al MCrAIY or the aluminides thereby confirming the ductility of coatings in accordance with this invention. Drop weight tests done on the trailing edges of coated airfoil sections indicate that articles coated pursuant to this example are able to withstand greater impact energy than high Al MCrAIY or aluminide coated articles. This also confirms that coatings in accordance with this patent are ductile.

The Table also illustrates the value of the use of the metal mixture of the invention. Thus, MDC-35A contains only tantalum and MDC-35C contains only manganese, while MDC-35B, D, E and M contain the mixture. The latter exhibit significantly improved coating life.

As indicated, the foregoing series of coatings illustrate the application of the invention for high temperature operations. A second series of coatings was developed for purposes of illustrating the application of the invention to intermediate temperature applications. The IN738 and MAR-M509 substrates previously referred to were also used in association with these coatings. The coatings were plasma sprayed onto the substrates in accordance with the parameters previously described and the same heat treatment in a vacuum for four hours at 1975OF was utilized.

A 500X photomicrograph of one of the coatings (MDC-36D) is shown in Figure 2. The thickness of the coatings typically was between 0.004 to 0.006 inches, and it is contemplated that the coating thickness vary between 0.0001 and 0.1 inches. Electron niicroprobe analysis revealed that the coating chemistry is substantially the same as that of the powder.

The performance of the articles coated pursuant to this example was evaluated using a 0.7 Mach burner rig. The testing cycle was 1 7500F/2 minutes; 1 4500F/4 minutes; 1 7500F/2 minutes; air cool/2 minutes. As shown in Figure 3, the peak temperature of 1750OF generated a spectrum of temperatures in the paddle test specimen, there being a variation of about 2000F (1 5500F to 17500 F) over the entire surface of the specimen. The same variation and isotherm pattern developed at the lower end of the cycle (14500 F), and the coatings that were tested were therefore exposed to temperatures from 1250OF to 1750OF. This is one of the conditions necessary for the aforementioned corrosion to take place, and the other conditions were created by adjusting the sulfur content in the fuel and by injecting salt solution into the combustion products. The fuel used was JP5 doped with ditertiary butyl disulfide to obtain 0.3% sulfur; and fifty (50) ppm salt was injected into the combustion products. These conditions closely simulate the gas turbine engine environment for turbine blades and vanes in marine applications, and they highlight the sulfidation (hot corrosion) phenomenon while imposing significant thermal stresses on the protection system.

The results of this test are shown in Table II: TABLE II BURNER RIG TEST RESULTS OF HIGH Cr LOW Al MODIFIED MCrAIY COATINGS

Composition (Wt %) Average Coating Life () System () Ni Co Cr Al Ti Ta Mn W La Y (Hrs.) UTC CoCrAlY ( ) Bal 23 13 0.6 912 GE CoCrAlY (4) Bal 29 6 1 1020 MDC-34U 29 Bal 26 6 0.6 936 MDC-34Y Bal 10 20 6 0.6 1445 MDC-36D Bal 30 9 2.5 1.7 1.0 1500 # (5) MDC-36E Bal 5 30 8 2.0 2.5 1.7 1.0 1700 # (5) (l) On both IN738 and MAR-M509 substrates (2) Rig cycle: 1750 F/2 min + 1450 F/4 min + 1750 F/2 min + cool/2 min (50 ppm salt) (3) Composition conforming to United Technologles' Patent No. 3,676, 085 (4) Composition conforming to General Electric Co. Patent No. 4,101,715 (5) # denotes "net yet failed" The term "average life" mentioned in the above refers to the approximate number of hours of test prior to the formation of an observable substrate metal oxide (which indicates that the coating has been penetrated under the burner rig test conditions) anywhere on the paddle specimen.It is seen that coatings produced in accordance with this invention unexpectedly exhibit substantially improved performance compared to a simple MCrAIY system such as the CoCrAIY of Patent No. 3,676,085, or the CoCrAIY specimen MDC-3 1 C. The test also revealed that additions of Ta, Mn and Ti improve the corrosion resistance in a fashion similar to the improvements observed in Table I.

Although the performance of these high Cr compositions was the same on both IN738 and MAR-M509 alloys, the microstructures were different. High Cr coating compositions on cobalt-base substrates tended to form a continuous layer of carbide (probably M23C6) at the interface. The tendency of this carbide formation was reduced by increasing the amount of Co in the coating. Coatings on Nibase alloys were free of carbide formation.

The performance of the articles coated pursuant to this example was also evaluated by means of water spray quench testing and drop weight impact testing of the types previously described. Coating ductility is important for gas turbine engine applications to ensure that the mechanical properties of the substrate alloy are not compromised.

The results from the first test showed that articles coated pursuant to this example do not show any cracks after the water spray quench test unlike the UTC CoCrAIY coating, thereby showing that these coatings are ductile. Drop weight tests done on the trailing edges of coated airfoil sections also indicate that articles coated pursuant to this example are able to withstand greater impact energy than the UTC CoCrAIY coating. This also confirms that coatings in accordance with this concept are ductile.

The coating compositions falling within the scope of this invention are suitable for a wide variety of superalloy substrates, and the particular examples of substrates referred to herein are not to be considered limiting. Thus, any substrate which can be satisfactorily coated with the composition of this invention by means of plasma spraying or any other suitable coating technique, and which will retain the coating in a satisfactory manner where elevated temperature performance is contemplated, will be suitable.

Coating compositions falling within the broader ranges expressed herein are generally useful for applications where sulfidation and/or oxidation resistance is desired. As explained, applicant's have also discovered that certain more limited ranges within the broad ranges provide particularly suitable performance for elevated temperature applications. The following claims are presented with this general understanding in mind.

It will be understood that various changes and modifications not specifically referred to herein may be made in the invention described without departing from the spirit of the invention particularly as defined in the following claims.

Claims (21)

1. A coating composition for nickel, cobalt and iron base alloys consisting essentially by weight of from 10 to 50% chromium, 3 to 15% aluminum, 1 to 15% of a metal mixture, and the balance selected from the group consisting of nickel, cobalt and iron, and combinations thereof, said metal mixture consisting essentially of at least 20% by weight tantalum and the balance tungsten or manganese or a combination thereof, said coating containing at least about 0.5% by weight tantalum and at least about 0.5% by weight of tungsten or manganese or a combination thereof.

2. A coating composition for application to nickel, cobalt and iron base alloys to be exposed to temperatures in excess of about l40O0Fconsisting essentially by weight of from 5 to 35% cobalt, to 35% chromium, 5 to 15% aluminum, 1 to 1 5% of a metal mixture, and the balance nickel, said metal mixture consisting essentially of at least 20% by weight tantalum and the balance tungsten or manganese or a combination thereof, said coating containing at least about 0.5% by weight tantalum and at least about 0.5% by weight of tungsten or manganese or a combination thereof.

3. A composition in accordance with Claims 1 or 2 comprising up to 5% by weight of a reactive metal selected from the group consisting of lanthanum, yttrium and the other rare earths.

4. A composition in accordance with Claim 3 wherein said reactive metal is present in an amount between 1 and 3% by weight of the composition.

5. A composition in accordance with Claims 1 or 2 including up to 5% by weight of a member selected from the group consisting of rare earth oxide particles and refractory metal oxide particles.

6. A composition in accordance with Claim 5 wherein said oxide particles are present in an amount between 0.05 and 2.0% by weight.

7. A composition in accordance with Claims 1 or 2 wherein the tantalum is present in an amount between 2.0 and 5.0% by weight, and including from 2.0 to 5.0 by weight of tungsten and manganese.

8. A composition in accordance with Claim 7 wherein the amount of tungsten does not exceed 1.5% by weight of the coating.

9. A composition in accordance with Claims 1 or 2 including from 10 to 20% by weight cobalt, from 15 to 25% by weight chromium, and from 10 to 14% by weight aluminum.

10. A composition in accordance with Claims 1 or 2 including from about 0.05 up to about 1 5 percent by weight of a noble metal selected from the group consisting of platinum, rhodium or palladium.

11. A composition in accordance with Claim 1 including up to about 5% by weight titanium.

12. In a superalloy component, the improvement comprising a coating composition on said component consisting essentially by weight of from 10 to 50% chromium, 3 to 1 5% aluminum, 1 to 15% of a metal mixture, and the balance selected from the group consisting of nickel, cobalt and iron, and combinations thereof, said metal mixture consisting essentially of at least 20% by weight tantalum and the balance tungsten or manganese or a combination thereof, said coating containing at least about 0.5% by weight tantalum and at least about 0.5% by weight of tungsten or manganese or a combination thereof.

13. In a superalloy component, the improvement comprising a coating composition on said component consisting essentially by weight of from 5 to 35% cobalt, 10 to 35% chromium, 5 to 15% aluminum, 1 to 15% of a metal mixture, and the balance nickel, said metal mixture ocnsisting essentially of at least 20% by weight tantalum and the balance tungsten or manganese or a combination thereof, said coating containing at least about 0.5% by weight tantalum and at least about 0.5% by weight of tungsten or manganese or a combination thereof.

14. A component according to Claims 12 or 1 3 wherein said coating is present in a thickness between 0.0001 and 0.1 inches.

15. A component in accordance with Claims 12 or 13 including a superalloy substrate, and at least two coating layers applied to said substrate, said composition comprising either or both of said layers.

1 6. A component according to Claims 12 or 13 wherein said coating comprises the bond coat for a thermal barrier coating.

1 7. A component according to Claims 12 or 13 wherein said coating is deposited by plasma spraying.

1 8. A coating composition according to any one of Claims 1 to 11 substantially as herein described with reference to any one of the coating compositions in U.S. application Serial No. 43146 of May 1979.

1 9. A superalloy component according to Claim 12 substantially as herein described with reference to any one of the coating compositions in U.S. application Serial No. 43146 of May 1979.

20. A superalloy component according to any one of Claims 1 3 to 1 7 substantially as herein described with reference to any one of coating compositions in U.S. application Serial No. 67097 of August 1 6, 1 979.

New claims or amendments to claims filed on 20/11/80.

New or amended Claims:

21. A coating composition for nickel, cobalt and iron base superalloys consisting by weight of (a) 10 to 50% chromium, (b) 3 to 1 5% aluminum, (c) tantalum or manganese or a mixture thereof with or without tungsten, the balance being one or more of nickel, cobalt and iron, the amount of (c) being at most 1 5% by weight but sufficient to impart to the composition a resistance to oxidation higher than that obtained in the absence of (c).