US3489535A - Coatings for refractory-metalbase alloys - Google Patents

Description

1970 N. s. BORNSTEIN ETAL 3,489,535

COATINGS FOR REFRACTORY-METAL-BASE ALLOYS Filed Oct. 5, 1966 3 Sheets-Sheet 1 FIG. 1

Sn RICH ENVELOPE ZONE Cb AI3 RICH ZONE Cb Sn RICH ZONE SUBALUMINIDE ZONE Cb-IZr ALLOY SUBSTRATE PHOTOMICROGRAPH OF COATED Cb-lZr ALLOY SUBSTRATE IN AS COATED CONDITION FIG. 2

Sn RICH ENVELOPE ZONE CbAI3 RICH ZONE Cb Sn RICH ZONE SUBALUMINIDE ZONE Cb-IZr ALLOY SUBSTRATE 500x PHOTOMICROGRAPH OF COATED CbiZrALLOY SUBSTRATE AFTER EXPOSURE IN ARGON AT 2000F FOR 100 HOURS INVENTOIB NORMAN S. BORNSTEIN LEONARD A. FRIEDRICH EMANUEL C. HIRAKIS ATTORNEYS Jan. 13, 1970 s. BORNSICIN T AL 3,489,535

COATINGS FOR REFRACTOHYIBTALBASE ALLOYS Filed Oct. 5, 1966 5 Sheets-Shea 2 FIG. 3

Sn RICH ENVELOPE ZONE Cb m RICH ZONE Cb Sn RICH ZONE SUBALUMIN'DE ZONE Cb-iZr ALLOY SUBSTRATE PHOTOMICROGRAPH OF COATED Cb-1Zr ALLOY SUBSTRATE AFTER EXPOSURE IN ARGON AT 2000F FOR 500 HOURS Cb Ala RICH ZONEJ Cb Sn RICH ZONE SUBALUMINIDE ZONE? Cb-lZr ALLOY SUBSTRATE 50C! PHOTOMICROGRAPH OF COATED Cb-iZr ALLOY SUBSTRATE AFTER EXPOSURE IN ARGON AT 2000F FOR 1000 HOURS INVENTORS NORMAN S. BORNSTEIN LEONARD A. FRlEDRICH EMANUEL C. HIRAKIS ATTORNEYS Jan. 13, 1970 5. o s'r m ET AL 3,489,535

COATINGS FOR REFRACTORY-METAL-BASE ALLOYS Filed Oct. 5, 1966 A 3 Sheets-Sheet 3 FIG. 5

Cb Al RICH ZONE Cb Sn RICH ZONE SUBALUIVIINIDE ZONE Cb-i Zr ALLOY SUBSTRATE 500x PHOTOMICROGRARH OF COATED Cb-lZr ALLOY SUBSTRATE AFTER EXPOSURE IN ARGON AT 2000F FOR 2500 HOURS Cb A13 RICH ZONE Cb3Sn RICH ZONE SUBALUMINIDE ZONE Cb--1 Zr ALLOY SUBSTRATE PHOTOIVIICROGRAPH OF COATED Cb-iZr ALLOY SUBSTRATE AFTER EXPOSURE IN ARGON AT 2000F FOR SOOO HOURS INVENTORS NORMAN S. BORNSTEIN LEONARD A. FRIEDRICH EMANUEL C. HIRAKIS ATTORNEYS United States Patent Ofice 3,489,535 Patented Jan. 13, 1970 US. Cl. 29194 17 Claims ABSTRACT OF THE DISCLOSURE Oxidation and contamination protective coatings are provided for columbium, tantalum, molybdenum, tungsten, and alloys of these refractory metals. The coatings have an exterior coating zone consisting essentially of from 40 to 50% Sn, from 27 to 33% Cr. from 14 to 18% Al, and from 7 to 11% Ti; and an interior coating zone, located between the exterior coating zone and the refractory metal substrate, consisting essentially of 65 to 75% Sn, 11 to 15% Al, 7 to 11% Ti, 2 to 6% Cr, to 4% Zn, and from 0 to 3% of an alkali or alkaline earth metal halide. Both coating zones are modified during their bonding to the substrate and in subsequent heat treatment and use by the diffusion of the refractory metal of the substrate into the coating zones. These coatings afford long time oxidation and contamination protection to refractory metal substrates at intermediate temperatures up to about 2000 F.

This invention relates to coatings for the refractory metals and their alloys that will protect such metals from atmospheric contamination at high temperatures.

More particularly this invention relates to two-zone thermally and mechanically stable coatings for the refractory metals that will protect such metals from atmospheric contamination for long periods of time in intermediate temperature environments.

The coatings of this invention are designed to protect refractory metal substrates at temperatures of from about room temperature up to at least about 2000 F. Although these coatings are primarily designed for use in protecting geometrically complex engineering structures and assemblies made from the refractory metals and their alloys, they are also particularly useful in coating laboratory test specimens of refractory metals.

As used in this specification and claims, the term refractory metals refers to those nonprecious refractory metals having melting points equal to or higher than the melting point of chromium (Cr), or 3407 F. (1875 C.). So defined, the refractory metals of this application in ascending order of their melting points are thus: chromium (Cr), vanadium (V), hafnium (Hf), columbium (Cb), molybdenum (Mo), tantalum (Ta), and tungsten (W). The term refractory metals as used herein also refers to alloys having refractory-metal bases, as well as to the refractory metals themselves. The invention in its most important aspects relates to protective coatings for Cb-base substrates.

For many years it has been generally known that the high-temperature strength properties of metals are closely related to their melting points. In general, metals having high melting points thus are capable of forming alloys having high strength at high temperatures. In recent years, the need for new structural materials for service at temperatures in excess of those that can be withstood by conventional structural materials has stimulated interest in those metals having the highest melting points, or the refractory metals and their alloys.

As alloy base materials for high-temperature service, a number of these metals have shown much promise in various high-temperature applications. Perhaps one of the most versatile and promising of these metals is Cb and considerable work has been done to develop it as a structural alloy base for uses in high-temperature environments.

Among the technically more important physical qualities of Cb as an alloy base are its high melting temperature (4474 F.) and its low neutron-capture cross-section. Further, Cb is inherently a soft, ductile, readily fabricable metal and, although it becomes too weak for practical structural uses at temperatures much above 1200 F., it is capable of being strengthened for use at much higher temperatures by alloying it with various other metals, particularly with other refractory metals. Disadvantageously, Cb is a highly reactive metal at elevated temperatures and will dissolve relatively large quantities of nitrogen and oxygen on exposure to atmospheres containing even small amounts of these elements at moderately elevated temperatures.

Because of the relative importance of Cb, much of the description that follows is based on the use of the coatings of this invention on Cb or Cb-alloy substrates. It will be understood, however, that the scope of the invention is not limited to coatings for Cb-base substrates, but includes coatings for the refractory metals generally. To some extent, the nature of the substrate, particularly as governed by the primary or preponderant element present, determines the formulation of the coating of this invention that is most effective for the substrate in question.

It is well known that no metal is completely resistant to surface contamination from exposure to air at elevated temperatures. Most metals that can be used at high temperatures without surface protection form a thin adherent protective oxide coating during initial exposure. This oxide coating insulates the base metal from further oxidation as long as it remains intact. The pure metals and alloys that exhibit this attribute of self-protection are, however, generally limited in their use to temperatures below about 1800 F.

The refractory metals and their alloys are essentially the only metals that retain suflicient strength at temperatures above about 1800 F. to make them useful at these temperatures. In recent years the refractory metals have been subjected to extensive study, investigation, and development. Various of the refractory metals that are available in sufficiently abundant supply for possible commercial development have been evaluated for numerous high-temperature uses. Unfortunately, none of the refractory metals has sufiicient resistance to oxidation or contamination in air at high temperatures to be used without protection.

The refractory metals do not form their own adherent and protective coatings within the temperature ranges of primary interest for their uses. Many of the most promising of these metals, such as Cb. Ta, and Mo are subject to extremely rapid or even catastrophic oxidation if unprotected in air at temperatures above 1000 F. Such oxidation vitiates and destroys the high-temperature strength of these metals. Accordingly, many efforts have been directed toward forming effective coatings for the refractory metals which inhibit or prevent their oxidation and contamination at high temperatures.

In the past, most of these efiorts have been directed to the production of coatings suitable for oxidation protection of the refractory metals at extreme temperatures and short exposure times, such as those which would be encountered by composite structures on atmospheric reentry. Very little effort has been devoted to the production of thermally and mechanically stable protective coatings that will provide long-time protection for refractory metal substrates at intermediate temperature environments, such as from room temperature up to about 2000" F. Coatings of this character are of particular utility in providing protection for test speciments being used for laboratory testing and development of refractory metal structures.

In the past, the absence of a protective coating suitable for long-time, intermediate temperature protection has required that such labotator specimens be tested in highly purified inert cover gas or extremely high vacuum (on the order of torr) test environments. The provision of such testing environments not only requires greatly increased testing expenditures, but also decreases the reliability of the specimens, and hence the value of the test results.

It is obvious that coatings which provide long-time, intermediate temperature protection also would be useful for many applications where refractory metal substrates are to be subjected to such conditions in use. An example of such a requirement would be in oxidation protective coatings for structural parts of advanced nuclear power plants.

The requirement that refractory metals be tested under an inert atmosphere, due to their oxidation-prone character, is discussed above. However, even when a controlled protective atmosphere is used, unacceptable contamination can result. In certain uses of the refractory metals, the presence of even very small amounts of oxygen in the base metal can have serious deleterious results, even though the strength of the metal members remains substantially unimpaired. This is particularly true when a structural member is used for containment of liquid metals. For example, Cb-alloys, because of their relative strength, availability, and fabricability are outstanding candidates as structural materials for liquid metal containment. Other refractory metals have also displayed favorable compatibility with alkali liquid metals, such as lithium (Li).

Pure Cb shows no susceptibility to solution attack by purified Li at temperatures up to 2200 F. However, when the oxygen in solution in Cb reaches a concentration of as little as a few hundred parts per million, Cb may be rendered sensitive to intergranular Li attack. Under these conditions Li will penetrate grain boundaries of Cb-base alloys and actually seep through the metal. The attack occurs at all temperatures above 1000 F. and is quite rapid, reaching completion in a few minutes.

While this susceptibility to Li attack in Cb may be reduced by alloying the Cb with zirconium (Zr) and heat treating, the Cb will remain susceptible to Li attack if oxygen atoms are present in amounts in excess of twice the Zr atoms present. Accordingly, when Cb or other refractory alloy substrates are desired to be used as structural materials for containment of liquid alkali metals, it is of the utmost importance that the refractory metals, and particularly Cb, be protected from oxidation at all stages of their manufacture. Thus, where these uses are contemplated, it is particularly important that the oxidation resistant coatings of this invention be used, even if fabrication is to be carried out largely or completely under inert environments.

Various types of coatings have been provided for Cband other refractory metal-base substrates in the prior art. For example, certain silicide and aluminide coatings have been used. Exemplary of the latter type are aluminum-silicon (such as Al-lOSi) and tin-aluminum (such as Sn-lOAl) coatings. However, none of these coatings have provided satisfactory long-time oxidation protection for the substrates at the intermediate temperatures needed for the requirements and uses described above.

In view of the foregoing, it is a primary object of this invention to provide new and improved protective coatings for refractory metal substrates at intermediate temperatures, whereby such substrates can be subjected to exposure to air at elevated temperatures up to ab ut 2000 F. for long periods of time without danger of oxidation or contamination, and to provide composite articles having refractory metal substrates and such new and improved protective coatings.

Another object of this invention is to provide new and improved thermally and mechanically stable coatings for refractory metal substrates which provide excellent p tection against oxidation and contamination of such substrates during their subjection to intermediate temperatures, of from about room temperature up to about 2000 F., for long periods of time.

Another object of this invention is to provide a suitable protective coating for refractory metal test specimens, to protect such specimens from oxidation during long-time testing in both impure inert and oxidative atmospheres.

Still another object of this invention is to provide new and improved two-zone coatings for refractory metal substrates that will protect the substrates from oxidation for long periods of time at intermediate temperatures up to about 2000 F.

Yet another object of this invention is to provide a two-zone coating for refractory metal substrates, each coating zone of which contains critical amounts of Al, Sn, Cr, and Ti.

A still further object of this invention is to provide improved coatings for refractory metal substrates that will protect the substrates from oxidation and contamination at intermediate temperatures for long periods of time, the coating having a self-healing Sn-containing surface zone which prevents contamination or oxidation of the u strate by cracking of the coatings or the formation of defects in the coatings.

Another object of this invention is to provide improved them from contamination during long exposure to air at intermediate temperatures up to about 2000 F. with a coating having a self-healing metallic subsurface zone backed up by a self-healing metallic surface zone in which the two zones cooperate together to prevent contamination or oxidation of substrates by cracking or formation of other defects in the coatings.

Yet another object of this invention is to provide a coating for refractory metal substrates that can be readily applied to both laboratory specimens and dimensionally large and geometrically complex engineering structures without sacrificing coating performance. These coatings are amenable to application and repairs in the field, and can be applied uniformly both in thickness and in composition over the entire surface of the substrate.

Additional objects and advantages will be set forth in part in the description that follows, and in part will be obvious from the description, or may be learned by the practice of the invention, the objects and advantages being realized and attained by means of the compositions, methods, and processes particularly pointed out in the appended claims.

To achieve the foregoing objects and in accordance with its purpose, this invention includes, as broadly described, a coated metal article having a substrate selected from the group consisting of the refractory metals and alloys thereof, and a coating having, in the as-a plied form:

(1) An exterior or surface zone consisting essentially of certain critical amounts of Sn, Al, Cr and Ti, and

(2) An interior coating zone interposed between the refractory metal substrate and the surface or exterior zone of the coating, described above, which also consists essentiall of certain critical amounts of Sn, Al, Cr, and Ti.

In accordance with the invention, upon heat treatment, the primary element or elements comprising the substrate alloy diffuse outwardly into both the interior and exterior coating zones and become elemental modifying constituenls of the coating itself. For example. if a Cb-base substrate is Coated according to the invention, Cb will diffuse into both coating zones during heat treatment and will become part of the resultant coating composition. Similarly, if a Cb-base alloy substrate containing given amounts of Ta, W, and M0 is coated, Cb, Ta, W, and Mo will all be present in the coating after heat treatment and in amounts roughly proportional to the relative amounts of each of these elements that is present in the substrate alloy composition.

These elemental additions to the as-applied coating compositions that occur through the phenomenon of diffusion contribute to and enhance the beneficial performance of the coating. The final or completed coatings of this invention, as contrasted with the as-applied coatings, may thus be described as substrate-modified.

In addition to Al, Sn, Cr, and Ti, the interior coating zone of the coatings of this invention can contain, as optional ingredients, zinc (Zn) and an alkali metal halide or alkaline earth metal halide activator.

In accordance with the invention, the interior or subsurface coating zone as-applied consists essentially of from 65 to 75% by weight of Sn, from 11 to by weight of Al, from 7 to 11% by weight of Ti, from 2 to 6% by weight of Cr, from 0 to 4% by weight of Zn, and from 0 to 3% by weight of an alkali metal halide or alkaline earth metal halide activator. Preferably, Zn when present is in an amount of from 1 to 4% by weight of the interior coating zone, and the halide activator is present in an amount of from 1 to 3% by weight of the interior coating zone.

The exterior or surface zone of the coatings of this invention as-applied consists essentially of from to by weight of Sn, from 27 to 33% by weight of Cr, from 14 to 18% by weight of Al, and from 7 to 11% by weight of Ti.

An optimum coating in accordance with this invention has an interior coating zone as-applied consisting essentially of about: 70% Sn, 13% Al, 9% Ti, 4% Cr, 2% Zn, and 2% HF; and an exterior coating zone as-applied consisting essentially of about: 45% Sn, 16% Al, 30% Cr, and 9% Ti.

The coatings of this invention are preferably applied to the surfaces of refractory metal articles by a spray or slurry deposition process. Generally, this process comprises:

(1) Forming an interior coating zone as-applied on the refractory metal substrate which consists essentially of Sn, Al, Cr, and Ti, and as optional ingredients, Zn and the halide activator, all in the amounts set forth above; and

(2) Applying over the interior coating zone a second coating composition, thereby forming an exterior or surface coating zone as-applied on the composite consisting essentially of Sn, Al, Cr, and Ti, all in the amounts set forth above.

First, following the application of the interior coating composition to the refractory metal-base article, and again, following the application of the exterior coating composition, the composite is fired to a predetermined temperature to produce a uniform and adherent coating on the substrate which is substantially impervious to gaseous contaminants, such as oxygen, at the intermediate elevated temperatures to which the products of this invention are intended to be subjected. During these firings some substrate-modification of the interior and exterior coating zones occurs through diffusion of substrate elements into the coating zones, as previously described.

Both of the coating zones of this invention are preferably applied to the substrate, or previously coated substrate, by a cold spray slurry process. In this process the coating composition is dispersed in a vaporizable diluent in an amount sufficient to give the composition a sprayable consistency and then sprayed onto the surface of the substrate. Generally, a binding or sticking agent is included in the suspension of the coating composition. The binding or sticking agent causes particles of the coating composition to adhere both to each other and to the substrate or the other coating composition previously applied to the substrate, as the case may be.

While, as pointed out above, a cold spray slurry process is the preferred method of applying the coating compositions of this invention, any other suitable methods, which will be readily apparent to those skilled in the art, can, of course, be used.

For ease of description in the bulk of the specification that follows the coatings of this invention will be described specifically as they would be used on a Cb-base substrate. Although this specific description may thus at times appear to apply only to use of the invention on Cb-base substrates, it will be understood that it is not so limited and that other refractory-metal-base substrates can be substituted for Cb with substantially equivalent results to those obtained with the Cb-base substrates on which the specific description is based.

When a Cb-base substrate is used with coatings of this invention, the Al in the interior coating composition forms an oxidation resistant intermetallic columbium aluminide composition (CbAl with the Cb of the substrate. This columbium trialuminide (CbAl provides the primary oxidation and contamination protection afforded by the interior coating zone for protection of the refractory metal substrate and provides important oxidation resistance at intermediate temperatures up to about 1800" F. To fulfill this important function of CbAl formation, A] is present in the interior coating zone as-applied in amounts of from 11 to 15% by weight of the total coating of that zone.

Sn is also a primary component of the interior coating composition and it performs a variety of functions in the coating. Al has a limited solubility in Sn, and because Sn liquefies at a relatively low temperature, it performs the useful role of carrying amounts of dissolved Al throughout the coating to bring Al into contact with unreacted or partially reacted Cb from the substrate thereby to form the oxidation resistant CbAl component of the coating.

This function of the Sn in the coating is important both in the initial formation of the CbAl and in elevated temperature service, when diffusion of Cb from the substrate into the coating, or recession of the coating into the substrate, can result in formation of subaluminides which do not have the oxidation resistance capacity of the desired Cb-trialuminide (CbAl The Sn is believed to transport Al throughout: the coating to provide Al for reaction during service with excess Cb which diffuses into the coating at heat-treatment temperatures and use temperatures, as previously described.

The Sn thus gives the coating self-healing properties, since its diffusion into a liquid phase with dissolved Al throughout the coating provides Al for the production of new Cb-trialuminides at any sites in the coating where oxidation resistance may have been reduced by formation of subaluminides less oxidation resistant than CbAl Sn also forms an intermetallic compound with Cb from the substrate, Cb Sn, which intermetallic compound is located between the substrate and the CbAl layer. This Cb Sn layer exhibits a coefficient of thermal expansion intermediate between that of the substrate and the main Cb-trialuminide coating zone and therefore acts as a thermal shock absorber for the coating.

To fulfill these varied and important functions, Sn is present in the interior coating composition as-applied in amounts, by weight, of from 65 to 75%. Less than 65% Sn in the coating is insuflicient to perform the foregoing functions, but the maximum permissible Sn content in the interior coating zone is 75%, because more than this would not allow for the requisite amounts of Al, Cr, and Ti in that coating zone.

The critical amounts of Ti and Cr that are present in both the interior and exterior coating zones of the coatings of this invention produce the greatly improved coating performance which is the essence of the invention. The presence of Ti and Cr in the coatings effectively counteracts the low temperature aluminide pest phenomenon characteristics which are usually exhibited by Cb-trialuminide coatings (powdering at temperatures of about 1200 to 1600 F.) For these reasons, it is necessary that the interior coating compositions of this invention contain Ti and Cr in the amounts specified. Additionally, the presence of these ingredients in the critical amounts specified results in the greatly improved coating life afforded by the coatings of this invention at all of the elevated temperatures contemplated by this invention, and particularly at temperatures of about 1200 F., 1600 F. and 2000 F., which are approximate temperatures representative of temperature ranges often encountered in actual use of refractory metal alloys of the type which are beneficially coated in accordance with this invention.

Although applicants do not wish to be bound by any particular theory as to the reasons for the improvement resulting from the addition of Ti and Cr to the coatings, it is believed that Ti replaces some Cb atoms and that Cr replaces some Al atoms in the Cb-trialumimide latice structure. These substitutions substantially improve coating performance over that normally obtained with unmodified Cb-trialuminide coatings. Additionally, it is believed that Ti and Cr are both soluble to some extent in Cb-trialuminides. The final coating also probably contains minor amounts of Ti-aluminides and Cr-aluminides, since both Ti and Cr form refractory aluminides. For one or more of these reasons, significant improvements in coating performance result from the inclusion of the specified amounts of Ti and Cr in the interior coating zone. To achieve these highly benefical coating attributes, the Ti content of the interior coating zone must be at least 7% and the Cr content of the interior coating zone must be at least 2% in the as-applied condition. The presence of more than 11% by weight of Ti and more than 6% Cr in the interior coating zone in the as-applied condition produces erratic coating performance and must be avoided, because the presence of Ti or Cr in excess of the stated amounts causes difficulties in bonding the exterior coating zone to the coated substrate.

In addition to the foregoing attributes of the coatings of this invention are the benefits resulting from the selfhealing properties of the coatings. In addition to Al, both Ti and Cr also have some solubility in liquid Sn, and thus the liquid Sn acts to supply fiaws in the coating with the reactive metals used-Al, Ti, and Cr. Until these reactive materials are entirely oxidized or consumed by reaction with the substrate or with orygen this self-healing mechanism of the coatings of this invention continues.

The self-healing properties of the coatings imparted by liquid Sn (containing dissolved Al, Ti, and Cr) are useful in correcting defects such as cracks that may be present in the coating after its initial formation. If such cracks or other defects are present, initial thermal cycling of the coating in use will generally effect healing of these defects, through the self-healing properties imparted by liquid Sn.

Sn in liquid phase is believed to transport A] through the coating by convection and gross carrying as well as through solution of Al in Sn. Because the liquid phase Sn carries Al and other particles to desired reaction sites, it promotes formation of a uniform coating at both minimum exposure times and minimum elevated temperatures.

One optional component of the interior coating composition is an activating agent comprising an alkali metal halide or alkaline earth metal halide. This activator is preferably present in the interior coating zone in the asapplied condition in amounts of from 1 to 3% by weight. The halide activating agent serves to flux the metal powders used in the production of the interior coating zone, particularly the Al, and promotes coalescence, wetting, fusion, and reaction of the metal powders to create the desired intermetallic composition.

The activating halide also serves to reduce oxide films on the metal powder particles-particularly on Al-as the coated substrate is heated, thereby promoting the desired intermetallic reaction.

The optional metal component of the interior coating composition, namely, Zn-like Ti and Cr, discussed aboveis believed to achieve various functions that improve coating performance, such as increasing long-term oxidation resistance, promoting self-healing properties, and deoxidizing. However, this ingredient is truly optional and can be omitted entirely from the interior coating composition. Zn should not be present in the interior coating zone in the as-applied condition, in any case, in an amount greater than 4% by weight, because at levels above 4% it can adversely affect the beneficial proporeties of the coating.

After the interior coating composition has been applied to the substrate, preferably by a cold spray slurry process, it is heat treated at from 1800 to ZOO-0 P. for from 1 to 16 hours, and preferably for from 1.5 to 4 hours. Optimum heat treatment is for 2 hours at 1950 F. This heat treatment produces an adherent interior coating zone on the substrate, formed from the interior coating composition. The interior coating composition is preferably applied to the article being coated in an amount of about 20 to 25 mg./cm. of surface area-optimum is about 23 mgfcmF-but these amounts are not critical.

Following this heat treatment, the exterior or surface coating composition consisting essentially of Sn, Cr, Al, and Ti is applied to the previously coated composite. The Sn and Al in this exterior coating composition provide a reservoir of excess amounts of these elements in the coating which modify the Cb-aluminides formed in the interior coating zone. For these purposes the exterior coating composition in the as-applied condition contains to by weight of Sn and 14 to 18% by weight of Al.

The Ti and Cr in the exterior coating composition also modify the Cb-aluminides of the second coating zone, and when present in the specified amounts, provide greatly improved coating performance for the reasons discussed above. The exterior coating composition, as-applied, contains from 7 to 11% Ti and from 27 to 33% Cr.

It will be noted that the amounts of Cr present in the exterior coating zone are substantially greater than the amounts of Cr present in the interior coating composition. The incorporation of these greater amounts of Cr in the exterior coating zone is possible because of the separation of this exterior or surface coating from the Cb (or other refractory metal) of the substrate by the intermediate or interior coating zone, thereby resulting in less quantitative diffusion of the substrate metal into the exterior coating zone.

If comparably large amounts of Cr were present in the interior coating zone, it could lead to a competing reaction between Cr and Cb in the interior coating for production of Cr-aluminides rather than the desired Cbaluminides. For this reason, only the much lesser amounts of Cr set forth above can be present in the interior coating zone. The high levels of Cr present in the exterior coating zone, however, contribute significantly to the improved performance of the coatings of this invention, particularly at temperatures approaching about 2000 F. The exterior coating zone in the as-applied condition cannot have a Cr content greater than 33%, however, because such higher levels tend to produce an undesirably low level of coating fluidity.

The exterior coating composition is also preferably applied by a cold spray slurry process in the form of a dispersion in a suitable vaporizable lacquer. After this coating composition is applied to the composite, which comprises the refractory metal substrate having the abovedescribed interior coating zone adjacent to it and the above described exterior coating zone superimposed on that interior coating zone, the composite is again heat treated at from 1800 to 2000" F. for from /2 to 16 hours, and preferably from 1.5 to 4 hours, to adherently and metallurgically bond the exterior coating zone onto the composite. Again, heat treatment at 1950 F. for about 2 hours is considered optimum.

The exterior coating composition is preferably adhered to the composite in an amount of about 20 to 25 mg./ cm. optimum is about 22 mg./cm. of surface area, although these amounts are not critical. The resulting product is a coated refractory metal article having excellent resistance to oxidation and contamination at intermediate temperature ranges up to about 2000 F. for long periods of time.

The overall fired thickness of the two-zone coatings of this invention is from about 3 mils to about 7 mils, and preferably is about 3 t mils.

"Ti-degradation of the mechanical and strength properties of the substrates coated in accordance with this invention is not a problem, because the amounts of Ti present in the coatings are relatively small, and the Ti is normally chemically tied in the coating, as an intermetallic compound or the like, and is not likely to migrate or diffuse into the substrate in an amount sufficient to cause any significant problem of substrate degradation.

Minor amounts of iron (Fe), manganese (Mn), and boron (B) can be present in the as-applied coatings of this invention without materially affecting coating performance. Total amounts of Fe, Mn, and B in excess of 3%, and amounts of any one of these elements in excess of 1%, however, can be detrimental and should not be used.

It will be understood that whenever parts of percentages are referred to in this specification and in the appended claims, this is intended to mean parts or percentages by weight, unless otherwise specifically indicated.

The alteration of the structure of the coatings of this invention in use is clearly shown by the photomicrographs FIGS. 1 through 6 which illustrate a Cb-lZr alloy coated in accordance with this invention and exposed in impure argon at about 2000 F. for times up to 5000 hours. The argon atmosphere was dynamic with about ten volume changes per hour, and the argon contained up to 2.5 p.p.m. of oxygen and 5 ppm. of water vapor.

FIG. 1 is a photomicrograph taken in polarized light and enlarged 500 times which shows the coated substrate in the as-applied condition, with a Cb Sn zone almost immediately adjacent to the Cb-lZr alloy matrix. The CbAl zone and the Sn-rich envelope zone are respectively superimposed on the Cb Sn first coating zone. The envelope zone comprises a Sn-rich matrix containing varying proportions of all of the coating elements. The envelope zone behaves as a reservoir, supplying Sn and Al for the growth of the CbAl and Cb Sn phases.

FIG. 2 is a photomicrograph taken in polarized light and enlarged 500 times which shows the same coated alloy after exposure in argon for about 100 hours at 2000" F. FIG. 2 reveals that exposure for this period produced some growth of the Cb Sn and CbAl zones at the expense of the envelope zone. No noticeable increase is evident in the very thin subaluminide zone (largely Cb Al) located between the Cb Sn zone and the substrate.

FIG. 3 is a photomicrograph taken in polarized light and enlarged 500 times which shows the coating on the CblZr alloy substrate after about 500 hours of exposure in the dynamic argon atmosphere at 2000 F. FIG. 3 reveals that after 500 hours there has been no significant growth of the subaluminide zone, but that there has been significant growth of the Cb Sn and CbAl zone, again at the expense of the envelope zone. The excess Sn and Al of the envelope zone have been largely depleted after 500 hours, although the envelope zone has not disappeared entirely at this time.

FIG. 4 is a photomicrograph taken in polarized light and enlarged 500 times which shows the coating of this invention on the Cb-lZr alloy substrate after exposure in the dynamic argon atmosphere for about 1000 hours at 2000 F. The envelope zone has now disappeared completely and a noticeable expansion of the subaluminide zone has occurred.

FIGS. 5 and 6 are photomicrographs taken in polarized light and enlarged 500 times which show the coating of this invention on the CblZr alloy substrate after exposure in an argon atmosphere at 2000" F. for 2500 hours and 5000 hours, respectively. These figures show the rapid growth of the subaluminide zone which commenced after disappearance of the envelope zone. They also show the formation of two phases within the subaluminide zone, with lower subaluminides (probably Cb Al) being formed at the substrate-coating interface.

FIG. 6 clearly shows that the rapid growth of the subaluminide zone, after disappearance of the envelope zone, occurs largely at the expense of the primary oxidation and contamination protective CbAl coating zone. However, it can be seen that this protective layer remained intact and capable of providing a substantial period of additional protection even after 5000 hours of exposure.

The coatings shown in the foregoing photomicrographs were those of Example 1, which are substantially the optimum coatings of this invention.

Each of the coating zones of this invention is preferably applied by a cold spray slurry process. This process i readily adaptable to scaling up from use on laboratory specimens to use on dimensionally large configurations Without any sacrifice in coating performance. It is also amenable to application of coatings or repairs of coatings in the field. This coating procedure is further desirable in that it does not require excessively high temperatures and hence does not produce or contribute to thermal damage or interstitial contamination of the refractory metal substrate.

One important advantage of the coatings of this invention is that they can be applied to complex engineering structures. The coatings have been tested to determine the feasibility of their use for such purposes, and were found to be entirely satisfactory.

Moreover, the cold slurry spray process is useful in that it can produce multi-component composites of various combinations of a wide variety of elements and compounds. It also achieves a uniformity in both thickness and composition of coating from place to plac on the workpiece surface. All that is required for the cold spray slurry process to be effective is a clean surface, spray coating or brushing of the slurry onto the area to be coated with blending into any already coated surfaces. and inertatmosphere heat treatment. The latter may be accomplished using portable apparatus which have been developed for annealing field welds.

Before coating, the surfaces of the substrate should be thoroughly cleaned of dust, dirt, or other foreign substances. This may be accomplished by water rinsing, liquid blasting, washing in suitable organic or inorganic solvents, or immersion in alkali cleaners or acid pickles. Care should b taken in cleaning the substrate to insure removal of all foreign matter.

After the surface has been cleaned, a metal powder mixture of the interior coating composition is dispersed in a suitable liquid diluent, and the resulting dispersion is applied to the substrate by spraying, brushing, dipcoating, or any other effective method. As pointed out above, spraying is generally preferred.

The diluent used in the preparation of the dispersion can be any compatible diluent, Any of the well-known diluents employed with resins and polymers in the paint industry may be used. Preferably, a readily volatilizable organic solvent or mixture of solvents is used. Examples of solvents that can be used are lower aliphatic alcohols, lower aliphatic ketones, lower alkyl esters or lower aliphatic acids, and lower hydrocarbons such as benzene and lower alkyl substituted benzene. Non-limiting examples of such diluents are methyl, ethyl, propyl, and butyl alcohols; acetone, methyl ethyl ketone, diethyl ketone, and octyl hexyl ketone; methyl acetate, butyl acetate, octyl 1 1 acetate, methyl propionate, octyl hexanoate; benzene, toluene, xylene, ethyl benzene; and the like.

The organic solvents mentioned are illustrative only and are not to be considered limiting. The main requirement of the volatile liquid substance or diluent is that it be reasonably safe to use, inexpensive, and sufiiciently liquid at ordinary temperatures to act as a dispersant for the metallic powders so that the dispersion can be sprayed or suitably coated on the specimen, and at the same time be sufficiently volatile to evaporate when exposed to atmospheric or other conditions as will be described below.

If desired, a binding or sticking agent can be added to the liquid diluent to hold the powder mixture to the surface of the substrate after evaporation of the solvent. Use of a binder enables the powders to adhere to the substrate for prolonged periods of time, thereby precluding the necessity of heat treating immediately after application of powder or of taking special precautions in handling the treated substrate to avoid loss of powders.

The binder should be one that will substantially completely decompose during heat treatment and that will preferably decompose at a temperature below the melting point of the lowest melting metal or combination Of metals used. Suitable binders or sticking agents that may be mentioned include nitrocellulose, naphthalene, and stearates. Other sticking or binding agents will be readily apparent to those skilled in the art.

Suitable wetting agents may also be added to the diluent if required. Moreover, low boiling organic compounds in small amounts can be added to the diluent to enhance its rapid evaporation.

In accordance with the invention, a dispersion of the metal powders of the interior coating composition, such as, for example, Sn, Al, Cr, Ti, Zn, and UP in a liquid diluent, or preferably in a lacquera diluent containing a hinder or sticking agentis deposited on the surface of a substrate to be coated in the manner already described. After application the solvent is allowed to evaporate and a mixture of metal powders is left on the substrate. If a sticking agent is added to the diluent, upon evaporation of the solvent the sticking agent will remain dispersed throughout the powder mixture in the coating, and will serve to hold the powder or dust on th substrate before heat treatment begins.

Evaporation of the volatile solvent, or a volatile portion of the lacquer, containing a sticking agent, may be conveniently brought about by allowing the coated substrate to be stored in an atmospheric environment at room temperature. If desired, suction or vacuum and elevated temperatures may be used to accelerate evaporation of th volatile solvent. Evaporation of the solvent leaves a fine layer of metallic powder mixture on the surface of the substrate to be heat treated.

The ratio of metallic powders to liquid diluent may vary from about 1:1 to l:l volume percent or higher. with the amount of diluent being adjusted to suit the particular method of application. A ratio of powder to diluent of 1:1 volume percent is satisfactory when it is desired to use a spatula to spread the coating on the surface to be protected. For spray application, however, the coating composition will be of proper consistency when the ratio of powders to diluent is about 1:10 volume percent. Still larger amounts of diluent may be used if desired; however, amounts of diluent in excess of a powder to diluent ratio of about 1:10 are of no particular advantage and increase the amount of diluent that must be evaporated from the coating.

The metallic powders may be mixed in the diluent or lacquer by any of the arts well known in the paint industry, or simply by using a Waring Blendor or a ball mill.

A preferred lacquer or diluent with binding or sticking agent for use with the coating compositions of this invention is nitrocellulose lacquer, i.e., nitrocellulose dissolved in an organic solvent such as amyl acetate.

After the solvent has been allowed to evaporate from the surface of the substrate, the resulting specimen is ready for heat treatment in a suitable furnace or oven to complete the formation of the interior coating zone on the substrate surface.

The foregoing description of the various suitable diluents and methods of application for the interior coating composition of this invention also apply to the preparation and application of the exterior coating composition. In other words, diluents suitable for use with the interior coating composition are also suitable for use with the exterior coating composition.

The metal powders used in the coating compositions of this invention preferably have a size range that will permit them to pass through a 200 mesh screen, although coarser particles up to a size that will pass through a mesh screen may also be used. Especially good results are obtained when the size range of metal powders is reduced to a size that will pass through a 325 mesh screen (43 microns), or between about 0 to 43 microns, and preferably between about 0 to 10 microns. As a general rule, it can be said that the finer the particles, the better will be the final coating produced. The mesh sizes referred to above are Tyler standard.

The use of a fine mesh metal powder helps to keep the powders in suspension and in a slurry and hence is desirable. The larger the particles are, the more differences in specific gravity of the powders produce tendencies to sep aration and make dispersion of the powders in liquid carriers more difficult. Further, as the particle size decreass, the surface area per unit weight increases and reaction is thus promoted by having powders of small particle size.

It should be noted, however, that the above advantages of fine particle size must be balanced against the increase in oxygen content of the coating that can result from the use of small particles having a larger total oxidized surface area.

The metal and halide activator dust or powders can be applied to the refractory metal substrates in any suitable manner. As pointed out above, the application of these powders in the form of a dispersion in a diluent is preferred. However, a fine film of the powders may be blasted or dusted onto the substrate, or any other suitable means can be used.

A preferred halide activator for use with the second coating composition is lithium fluoride (LiF). LiF is soluble in many of the organic solvents mentioned above, and when it is used as the activator, an organic solvent is selected in which it will readily dissolve. Similarly, when other halides of alkali metals and alkaline earth metals are used as activators, the particular halide used should be soluble in the particular organic solvent selected for use as the diluent.

The coatings of this invention are designed to be used on refractory metal substrates generally. They are most important, however, in providing desired oxidation and contamination protection to Cb or Cb-alloy substrates. It has been found that the beneficial properties of the coatings of this invention are most apparent where the substrates to which these coatings are applied are Cb-base alloys (i.e., alloys containing at least 40% Cb) which contain significant amounts (at least about 5%) of Ti.

For a clearer understanding of the invention specific examples of it are set forth below. These examples are merely illustrative and are not to be understood as limiting the scope and underlying principles of the invention in any way.

EXAMPLE 1 The specimen blanks used in this example were sheared from a Cb-lZr alloy sheet stock to a nominal size of 0.625 inch x 0.625 inch x 0.030 inch, and a 0.125 inch diameter hole was punched at one end of each sample to facilitate handling. The blanks were tumbled in a ball mill using porcelain balls and alumina grit, for 100 hours to relieve the edges of the specimens. The blanks were then etched for minutes in an acid solution consisting of HF, 28% HNO and 62% H O to remove surface contamination and were subsequently vacuum heat treated to provide stress relief. A typical heat treatment for these Cb1Zr alloy specimens was 16 hours at 1800 F.

Immediately prior to application of the coating, the specimen blanks were degreased in trichloroethylene vapor, immersed 5 minutes in a heavy-duty alkali cleaning solution at 150 F. rinsed in water, etched for 3 additional minutes in the above described acid etching solution at room temperature, rinsed again in tap water and then in deionized water, dried, and placed on spraying racks.

The interior coating composition was prepared by dry mixing the following high purity metal powders in the proportions indicated:

70% by weight of Sn powder (-325 mesh or finer;

9999+ percent purity),

13 by weight of Al powder (flaked Al powder or finer),

9% by weight of Ti powder (-325 mesh or finer;

99+ percent purity),

4% by weight of Cr powder (-325 mesh or finer;

999+ percent purity),

2% by weight of Zn powder (-325 mesh or finer;

999+ percent purity), and

2% by weight of LiF powder (-325 mesh or finer;

chemically pure grade).

This metal powder mixture was suspended in nitrocellulose lacquer (nitrocellulose dissolved in amyl acetate) by mixing in a Waring Blendor. To provide a quantity of interior coating composition suitable for spraying, approximately 50 grams of dry powder per 40 cc.s of lacquer were mixed together. After uniformly mixing, the coating was applied to the specimen blanks by spraying at a rate of about 23 mg./cm. of surface area. The interior coating composition thus applied had a sprayed-on thickness of about 3 mils. This coating composition was then permitted to dry in air with the aid of a heat lamp for at least 2 hours at temperatures up to about 250 F.

At the end of this time substantially all of the organic solvent had evaporated from the nitrocellulose lacquer leaving a coating composition on the specimen blanks of metal powders, UP and nitrocellulose as a binder or stick ing agent. The LiF powder dissolved in the nitrocellulose lacquer when the powder mixture was mixed with the lacquer. (Alternatively, the UP powder can be dissolved in the lacquer before it is mixed with the metal powders.) When the solvent was evaporated, the LiF precipitated out and remained substantially evenly distributed throughout the coating composition.

The specimen was then subjected to heating in an argon atmosphere furnace for 2 hours at a temperature of about 1950 F. The specimens were initially heated to about 225 F. in the furnace, at an argon gas flow rate of about 3 to 10 room temperature volume changes per hour, and then the temperature of the furnace was rapidly raised to the 1950 F. firing temperature. In accordance with preferred procedure, the time required to reach the firing temperature was less than 3 hours. After this heat treatment the interior coating zone had a thickness of about 2 mils.

Following this heat treatment of the interior coating zone, the specimens were cleaned by light brushing with a clean stainless steel brush to remove any powder deposit or other excess material remaining on their surfaces and to prepare the surfaces for application of the exterior coating composition.

The exterior coating composition was prepared by mixing high purity metallic powders in the following proport1ons:

45% by weight of Sn powder (-325 mesh or finer;

9999+ percent purity),

30% by weight of Cr powder (-325 mesh or finer;

99.9-lpercent purity),

16% by weight of Al powder (flaked Al powder or finer),

and

9% by weight of Ti powder (-325 mesh or finer; 99+

percent purity).

This metal powder mixture was also suspended in a nitrocellulose lacquer of the type described above by mixing in a Waring Blendor. To provide a quantity of this second coating composition suitable for spraying, approximately 50 grams of dry powder per 40 ccfs of lacquer were mixed together. After mixing, the coating was applied to the specimen blanks by spraying at the rate of about 22 mg./cm. of specimen surface. The exterior coating composition thus applied had a sprayed-on thickness of about 3 mils.

The exterior coating composition was then air dried with the aid of heat lamps for at least 2 hours at temperatures up to about 250 F. This resulted in evaporation of substantially all of the organic solvent from the coating. The composite was then heat treated in an argon atmosphere furnace, in the manner described above, for a period of /2 hour at 1950 F. The resultant article had a two-zone coating thickness of about 3 to 4 mils.

Coated specimens produced in the above manner (on Cb-lZr alloy substrates) were endurance tested to determine their air exposure lifetimes at 650 C. (1200 F.), 871 C. (1600 F.), and 1095 C. (2000 F.). The endurance test specimens were placed on slotted ceramic supports and inserted in preheated furnaces at the testing temperatures. The specimens were thermal-cycled to room temperature daily for examination during the first 1000 hours, and after that were examined once a week. Failure criterion was the first appearance of Cb-oxide on the specimens.

A total of Cb-lZr alloy specimens coated in the above manner were tested in two groups at 1200 F. in the manner described above. Testing of the first group of 42 samples was terminated after 4464 hours, and the second group of 48 samples was tested for 10,000 hours. A total of 32 of the 42 specimens in the first group had not failed when the test was ended after 4464 hours. Over half of the specimens in the second group (25 out of 48) had not failed after 10,000 hours of exposure, and 39 of the 48 second group specimens exhibited coating lifetimes in excess of 5000 hours.

A total of 91 Cr-lZr alloy specimens, prepared in the above manner, were endurance tested by the same procedure in air at 1600" F. These specimens were also thermal-cycled to room temperature once each day for the first 1000 hours of testing and thereafter once per week. The median coating life of these specimens was 1128 hours. With a number of specimens, protection for over 4000 hours was obtained, and instances of 10,000 hour protection at 1600 F. were also exhibited in this testing.

Four (4) specimens coated in accordance with this invention were exposed to air at 2000 F. to failure. At this temperature, the specimens exhibited coating lifetimes from 144 to 720 hours.

EXAMPLE 2 A number of Cb alloy specimen blanks consisting essentially of 1% by weight of Zr, 0.1% by weight of carbon, and balance essentially all Cb were prepared and coated in accordance with the procedure set forth in Example 1. Both interior and exterior coating compositions having the same ingredients as in Example 1 were applied to the Cb-base alloy specimen blanks in the same manner set forth in Example 1, with heat treatment after the application of each coating, in the manner described in that example. This procedure produced a metal article having a coating of the desired properties on the surface of the Cb-lZr-0.1C alloy substrate.

EXAMPLE 3 Eightly-five (85) Cb-base alloy specimen blanks consisting essentially of 8% by weight of Ti, 4% by weight of Mo, and balance essentially all Cb, were coated in accordance with the procedure set forth in Example 1. Interior and exterior coating compositions of the same ingredients as in Example 1 were applied to the Cb-base alloy specimen blanks in the same manner set forth in Example 1, with heat treatment of the character described in that example following the application of each coating composition. The resulting product had a coating of the desired properties on the surface of the Cb-8Ti-4Mo alloy substrate.

Some 42 of these coated specimen blanks were tested for coating endurance life in air at 1200 F. in the same manner set forth in Example 1. None of the 42 specimens had failed when the test was terminated after 4464 hours.

The remaining 43 specimen blanks were endurance tested in air at 1600 F., also in the manner set forth in Example 1. The median coating life of these specimens at 1600 F. was 3240 hours and 40 of the 43 specimens exhibited a coating life of at least 2400 hours.

EXAMPLE 4 A Cb-base alloy specimen blank consisting essentially of 15% by weight of Ti, 3% by weight of Al, and balance essentially all Cb was prepared in accordance with the procedures set forth in Example 1. Interior and exterior coating compositions of the same ingredients as in Example 1 were applied to the Cb-base alloy specimen in the same manner set forth in Example 1, with heat treatment following the application of each coating composition, in accordance with the procedure of that example. The resulting product had a coating of the desired properties on the surface of the Cb-lSTi-3Al alloy substrate.

A total of 94 of these coated specimens were subjected to oxidation exposure testing in accordance with the procedure of Example 1. Of the 47 specimens tested at 1200 F., 20 were unfailed when the testing was concluded after 10,000 hours. The median oxidation endurance lifetime of these specimens at 1200 F. was 9312 hours, and 40 of the specimens reached at least 6000 hours before failure.

The 47 specimens tested at 1600 F., also in accordance with the procedure of Example 1, exhibited a median coating life of 960 hours.

EXAMPLE A Cb-base alloy specimen blank consisting essentially by weight of Ti, 5% V, and balance essentially all Cb was prepared in accordance with the procedures set forth in Example 1. Interior and exterior coating compositions of the same ingredients as in Example 1 were applied to the Cb-base alloy specimen in the same manner set forth in Example 1 with heat treatment, in the manner set forth in that example, following the application of each of the coating compositions. The resultant product had a coating of the desired properties formed on the Cb15Ti-5V alloy substrate.

Ninety-one (91) specimen blanks prepared in accordance with this example were oxidation endurance tested in accordance with the procedure of Example 1. The fortytwo (42) specimens tested at 1200 F. exhibited a median coating life of 8688 hours, and 17 of these 42 specimens were unfailed when the testing was concluded after 10,000 hours.

The forty-nine (49) specimens tested at 1600 F. exhibited a median coating life of 456 hours, with 21 of the specimens affording protection for over 1125 hours.

EXAMPLE 6 A Cb-base alloy specimen blank consisting essentially of by weight of Ti and balance essentially all Cb was prepared in accordance with the procedures set forth in Example 1. Interior and exterior coating compositions of the same ingredients as in Example 1 were applied to the Cb-base alloy specimen, in the manner set forth in Example 1, with each coating being heat treated on the substrate following its application, in accordance with the procedures of Example 1. The resulting product had a coating of the desired properties formed on the surface of the Cb-2OTi alloy substrate.

Thirty-five (35) specimens prepared in accordance with this example were oxidation endurance tested by the procedure of Example 1.

The eighteen (18) specimens tested at 1200 F. were all unfailed when the testing was concluded after 4464 hours; and the seventeen (17) specimens tested at 1600 F. exhibited a median coating life of 2400 hours.

EXAMPLE 7 A Ta-base alloy specimen blank consisting essentially of 8% by weight of W, 2% by Weight of Hf, and balance essentially all Ta was prepared in accordance with the procedures set forth in Example 1. Interior and exterior coating compositions of the same ingredients as in Example 1 were applied to this Ta-base alloy specimen in the same manner as set forth in Example 1, with heat treatment of each coating composition following its application to the specimen, in accordance with the procedure of that example. The resulting product had a coating of the desired properties formed on the surface of the Ta- SW-ZHf alloy substrate.

EXAMPLE 8 A Mobase alloy specimen blank consisting essentially of 0.5% by weight of Ti, 0.08% by weight of Zr, 0.02% by weight of C, and balance essentially all Mo was prepared in accordance with the procedure set forth in Example l. Interior and exterior coating compositions of the same ingredients as in Example 1 were applied to this Mobase alloy specimen in the same manner as set forth in Example 1, with heat treatment of each coating composition following its application to the specimen, in accordance with the procedure of that example. The resulting product had a coating of the desired properties formed otn the surface of the Mo-0.5Ti0.08Zr-0.02C alloy subs rate.

EXAMPLE 9 A tungsten specimen blank consisting essentially of pure W was prepared in accordance with the procedures set forth in Example 1. Interior and exterior coating compositrons of the same ingredients as in Example 1 were applied to this W specimen in the same manner as set forth in Example 1, with heat treatment of each coating composition following its application to the specimen, in accordance with the procedure of that example. The resulting product had a coating of the desired properties formed on the surface of the W substrate.

The invention in its broader aspects is not limited to the specific details shown and described, but departures may be made from such details within the scope of the accompanyrng claims without departing from the principles pf the invention and without sacrificing its chief advanages.

What is claimed is:

1. A metal article comprising a refractory metal or a refractory metal-base alloy substrate, said refractory metal being selected from the group consisting of columbium, tantalum, molybdenum and tungsten, and an oxidation and contamination resistant coating on the substrate, the coating comprising an interior coating zone superimposed on and metallurgically bonded to the substrate, and an exterior coating zone superimposed on and metallurgically bonded to the interior coating zone, the interror coating zone consisting essentially of the refractory metal or refractory metal alloy of the substrate, which diffuses into the interior coating zone during the metallurgical bonding of the interior coating zone to the substrate,

and an interior coating composition which consists essentially, by weight, of:

from 65 to 75% of Sn,

from 11 to 15% of A1,

from 7 to 11% of Ti,

from 2 to 6% of Cr,

to 4% of Zn, and

0 to 3% of at least one alkali metal halide or alkaline earth metal halide; and the exterior coating zone consisting essentially of the refractory metal or refractory metal alloy of the substrate, which diffuses into the exterior coating zone during metallurgical bonding of the exterior coating zone to the interior coating zone, and exterior coating composition which consists essentially of:

from 40 to 50% of Sn,

from 27 to 33% of Cr,

from 14 to 18% of Al, and

from 7 to 11% of Ti.

2. The article of claim 1 in which the interior coating composition contains from 1 to 4% by weight of Zn, and from 1 to 3% by weight of at least one alkali metal halide or alkaline earth metal halide.

3. The article of claim 1 in which the substrate comprises Cb or a Cb-base alloy.

4. The article of claim 3 in which the substrate consists essentially of about 1% by weight of Zr and the balance essentially Ch.

5. The article of claim 3 in which the substrate consists essentially of about 1% by weight of Zr, about 0.1% by weight of C and balance essen'tially Cb.

6. The article of claim 3 in which the substrate comprises a Cb-base alloy which contains at least about 5% by weight of Ti.

7. The article of claim 6 in which the substrate consists essentially of about 8% by weight of Ti, about 4% by weight of Mo, and balance essentially Ch.

8. The article of claim 6 in which the substrate consists essentially of about 15% by weight of Ti, about 3% by weight of Al, and balance essentially Ch.

9. The article of claim 6 in which the substrate consists essentially of about by weight of Ti, about 5% by weight of V, and balance essentially Cb.

10. The article of claim 6 in which the substrate consists essentially of about by weight of Ti and balance essentially Ch.

11. The article of claim 1 in which the substrate comprises Ta or a Ta-base alloy.

12. The article of claim 11 in which the substrate consists essentially of about 8% by weight of W, about 2% by weight of Hf, and balance essentially Ta.

13. The article of claim 1 in which the substrate comprises M0 or a Mobase alloy.

14. The article of claim 1 in which the substrate consists essentially of about 0.5% by weight of Ti, about 0.08% by weight of Zr, about 0.02% by weight of C, and balance essentially Mo.

15. The article of claim 1 in which the substrate comprises W or a W-base alloy.

16. The article of claim 1 in which the interior coating composition consists essentially, by weight, of about 70% Sn, about 13% Al, about 9% Ti, about 4% Cr, about 2% Zn, and about 2% HF; and the exterior coating composition consists essentially, by weight, of about 45% of Sn, about 16% of Al, about 9% of Ti, and about of Cr.

17. The article of claim 16 in which the substrate comprises Cb or a Cb base alloy.

References Cited UNITED STATES PATENTS 3,078,554 2/1963 Carlson 29194 3,216,806 ll/1965 Santa 29198 X 3,360,350 12/1967 Sama 29-498 X HYLAND BlZO'l, Primary Examiner US. or. x11, 29 19s

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