US3574571A - Coatings for high-temperature alloys - Google Patents

Abstract

COLUMBIUM AND ITS ALLOYS ARE PROVIDED WITH TITANIUMMODIFIED COLUMBIUM SILICIDE COATING WHICH PROTECT THE METAL FROM OXIDATION OF HIGH TEMPERATURE. THE COATING IS FORMED BY CODEPOSITION OF TITANIUM AND SILICON IN A PACK-CEMENTATION PROCESS. DURING EXPOSURE TO OXIDATION AT HIGH TEMPERATURE AN OXIDE COAT IS FORMED ON THE OUTER ZONE.

Description

April 13, 1971 FRIEDRICH ETAL 3,574,571

is COATINGS FOR HIGH-TEMPERATURE ALLOYS 0? Filed April 14, 1964 2 Sheets-Sheet l Ti-Modified CbSi Cb-lZr Substrate l I Ti-Moditied Coiumbium Siiicide Coating on Cb-iZr Substrate 500x (As-Coated Condition) Oxide Zone (Oxidic Silicon) Ti-Moditied CbSi Diffusion Zone (Subsilicides of Substrate) CbiZr Substrate FIG. 2.

Ti-Modified Coiumbium Sii icide Coating on Cb-iZr Substrate 3mm L After Exposure in Static Air for I000 Hours at |800F e LEONARD A. FRIEDRICH EMANUEL C. HIRAKIS ems April 1 1971 L. A. FRIEDRICH ETA!- COATINGS FOR HIGHTEMPERATURE ALLOYS 2 Sheets-Sheet 2 Filed April 14, 1964 w w T m w m n N AC 181A I WU.

B nor- United States Patent 015cc 3,574,571 Patented Apr. 13, 1971 3,574,571 COATINGS FOR HIGH-TEMPERATURE ALLOYS Leonard A. Friedrich, West Hartford, and Emanuel C. Hirakis, Mansfield Center, Conn., assignors to United Aircraft Corporation, East Hartford, Conn.

Filed Apr. 14, 1964, Ser. No. 360,176 Int. Cl. B32p 3/00 US. Cl. 29-198 16 Claims ABSTRACT OF THE DISCLOSURE Columbium and its alloys are provided with titaniummodified columbium silicide coatings which protect the metal from oxidation of high temperatures. The coating is formed by codeposition of titanium and silicon in a pack-cementation process. During exposure to oxidation at high temperature an oxide coat is formed on the outer zone.

This invention relates to novel coatings for columbium and columbium base alloys that will protect the base metal or alloy from oxidation at high temperatures and to a method for creating such coatings.

More particularly, this invention relates to titaniummodified columbium silicide coatings for columbium and its alloys in which the coatings are created by methods, such as, vapor deposition, electrophoretic deposition, and the like. The invention also particularly relates to a method for obtaining vapor deposition of such titaniummodified silicide coatings on columbium base materials to produce a protective layer or zone over such materials providing an oxidation-resistant coating at high temperatures, such as, for example, temperatures up .to at least 2000 F. in air and even higher for short exposure times.

During exposure to oxidation at elevated temperatures an oxide coat is formed on the outer or exposed zone of the coating and a diffusion zone is formed between the titanium-modified columbium silicide coating and the substrate. Upon exposure the structure of the coating is thus to this extent altered. The oxide coat and diffusion zone once formed are stable in structure and function as part of the protective coating system.

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 are capable of forming alloys having high strength at high temperatures.

The need for structural materials for service at temperatures in excess of those obtainable with existing structural materials has stimulated interest in the metals having the highest melting points, or the refractory metals, particularly, chromium, columbium, tantalum, molybdenum and tungsten.

Molybdenum was once considered the chief prospect as a base metal in alloys for such usage. At the hightemperature service conditions needed, however, molybdenum not only oxidizes but the molybdenum oxide formed is volatile, and once the oxidation reaction begins it tends to progress rapidly until molybdenum is consumed at a catastrophic rate.

As an alloy base material for high-temperature service, columbium offers more promise, and considerable interest has been directed to its use as a structural alloy base for applications in high-temperature environments. Among the technically most important physical qualities of columbium as an alloy base are its high melting temperature (4474 F.) and its low neutron-capture crosssection. Columbium is, therefore, potentially useful as a structural material for containment vessels for high-temperature liquid metals.

Further, columbium is inherently a soft, ductile, readily fabricable material, and although it becomes too weak for practical structural uses at temperatures above 1200 F., it readily can be strengthened for use at much higher temperatures by alloying with various metals, and particularly by alloying with the refractory metals. Columbium is also a highly reactive metal in that it dissolves large quantities of oxygen and nitrogen, upon exposure to atmospheres containing even small amounts of these elements at modest temperatures.

Although columbium oxidizes rapidly at high temperatures, in contrast to molybdenum which oxidizes catastrophically, columbium oxide does not volatilize. It is thus potentially possible to prevent oxygen attack on columbium by coating the metal, and if premature localized coating failure should occur, to restrict such failure and oxygen attack to the localized site. Further advantages offered by columbium over molybdenum base alloys are that columbium base alloys are relatively more ductile and workable at low temperatures and columbium has a lower density than molybdenum.

The history of columbium alloy technology has, however, demonstrated the incompatibility of achieving oxidation resistance and high-temperature strength through alloying alone. Since the major uses for columbium base alloys are as structural components in high-temperature applications, it is apparent that useful classes of high-ternperature columbium alloys will require protective coatings in their normal high-temperature oxidizing environments.

A particularly important potential area of use for columbium base alloys as dictated by economic and technological factors is in structural materials designed for exposure to oxidizing environments at temperatures up to about 2000 F. (a temperature that clearly establishes utility for these alloys). Concomitantly, such alloys must be able to resist mechanical stresses for appreciable periods of time at these high temperatures.

About 500 F. is the maximum operating temperature to which columbium base alloys may be subjected for extended times in the uncoated condition Without serious oxidation, and at temperatures above 500 F. the oxidation problem becomes acute.

The art has previously recognized certain oxidation resistant intermetallic coatings that exhibit particular potential for protecting refractory metals (e.g., columbium, molybdenum, tantalum, and tungsten) from oxidation at high temperatures. In general, the more effective of these intermetallic coatings are silicides, aluminides and beryllides of the base metal.

In considering coatings for the refractory metals, both coating and substrate materials are important to the performance of the coated systems. For example, silicide coatings over columbium and molybdenum may perform quite differently with the difference in performance attributable to the substrate rather than the coating type. As an additional confirmation of the importance of the substrate, some species of coatings that are reliably protective over other of the refractory metals are ineffective over columbium and are susceptible to failure on columbium at high temperatures. Coating and substrate must be evaluated and treated as an integrated system. Success with a particular coating on a particular base metal does not mean the coating will be successful when used over a different base metal.

Several methods, such as. flame or plasma torch spraying, slurry application techniques, electrophoretic deposition, hot pressure bonding or vapor deposition, have been used for applying intermetallic coatings to columbium base alloys. A vapor deposition process that can be used advantageously to achieve some types of coatings is the so-called pack-cementation process, in which the object to be coated is surrounded by a particulate pack mixture containing, for example, (1) the metal to be reacted with or deposited upon the object to be coated (e.g., silicon, aluminum, beryllium), (2) an activator or energizer (usually a halide salt, such as NaCl, KF, NH I, NH Cl, and the like), and (3) an inert filler material (e.g., A1 SiO BeO, MgO, and the like).

This mixture, held in a suitable container (steel box, graphite boat or refractory oxide crucible, for example), is then heated to a desired coating temperature in a prescribed atmosphere and held for a length of time sufficient to achieve the desired coating. When conducted properly, the pack-cementation process may be used to produce controlled-thickness coatings on columbium, the major proportions of which may be compounds, such as CbA13, CbSi and the like.

The more favorable coatings for columbium (columbium aluminides, silicides, beryllides) possess certain intrinsic deficiencies such as rapid oxidation failure at low" temperatures (in the vicinity of 1300" F.) or at high temperatures (about 2000 F. and above). Perhaps the most serious deficiency of existing coatings for co lumbium, however, is their propensity for failing at localized sites.

Silicide coatings on columbium and its structural a loys are more stable than aluminides and have a better thermal expansion match with the substrate than beryllides which have such a severe thermal expansion mismatch that it prohibits their use. With columbium the silicides are thus of primary interest.

Silicide coatings on structural columbium alloy substrates, however, are prone to consumption by rapid oxidation at low (about 1300 F.) temperatures (this characteristic of silicide coatings is sometimes termed the silicide pest phenomenon). Modification of silicide coatings is thus highly desirable to impart sufficient longevity and reliability to give to them a utility they do not normally possess.

In view of the foregoing, it is a primary object of this invention to provide a titanium-modified columbium silicide coating composition that will protect columbium base alloys from deleterious effects of oxidation in static air at temperatures up to at least about 1800 F. for times in excess of 5000 hours.

Another object of this invention is to provide a titanium-modified columbium silicide coating for columbium and alloys thereof that overcomes the silicide pest phenomenon characterized by rapid consumption of columbium silicide coatings through oxidation at temperatures of about 1300 F. and that also overcomes rapid consumption of columbium silicide coatings through oxidation at high temperatures of about 2000 F. and above.

Further objects of this invention are to provide a titanium-modified silicide coating for columbium and its alloys that in addition to providing resistance to simple thermal oxidation will also be protective under other reasonably expected conditions of use, and to this end the protective coatings of this invention achieve good resis tance to thermal cycling, thermal shock, and formation of defects. They are also diifusionally stable, well-bonded to the substrate and resistant to spalling.

Other objects of this invention are to provide for columbium and its alloys:

1) A coating that in nominal thicknesses of 1% to 3 mils is capable of providing protection to exposures in static air for times in excess of 5000 hours at temperatures up to at least about 1800 F.;

(2) A coating that exhibits excellent resistance to thermal shock failure; and

(3) A coating that displays excellent resistance to the formation of defects at both higher and lower temperatures of exposure.

A still further object of this invention is to provide a method for coating columbium and its alloys with a titanium-modified silicide coating using a vapor deposition (pack-cementation) process that will achieve substantial 4 uniformity of the coating and yield an essentially uniform coating on even intricately shaped parts and at the edges and comers of parts.

Additional objects and advantages of the invention 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 practice of the invention, the objects and ad vantages being realized and attained by means of the compositions, methods and processes particularly pointed out in the appended claims.

The term diffusing temperature refers to those temperatures at which a diffusion zone forms by interdiffusion between the coating and substrate taking place at an appreciable rate. A preferred diffusing temperature for the coatings of this invention is 1800 F. At this temperature interdiifusion proceeds at an advantageous speed until a distinct diffusion zone is formed, and it is unnecessary and uneconomical to use a higher temperature. Once the difi'usion zone is formed it essentially stabilizes, and loss of the coating through diffusion into the substrate is prevented. Diffusing temperatures of from 1700 to 1900 F. are efficacious, and even higher temperatures up to 2300 F. would achieve the desired result, but at 2300" E, if the diffusion is accomplished in an oxidizing atmosphere, the life of the coated article would be relatively short.

Exposure to the diffusing temperature is normally effected during actual use of the coated articles and not as a separate step, and is thus simultaneous with exposure to an oxidizing environment at elevated temperatures. At about 1800 F. diffusion begins with exposure and continues until the structure of the coating system becomes essentially stabilized after about 25 hours of exposure.

To achieve the foregoing objects and in accordance with its purpose, this invention includes an article of manufacture having good resistance to oxidation in air at temperatures up to 2000 F. which article comprises a. core of metal selected from the group consisting of columbium and columbium base alloys, the article having a defect, spalling and thermal shock failure resistant coating or surface zone consisting essentially of columbium silicide (preferably CbSi modified by a titanium content of less than 4% of columbium silicide by weight.

The invention further comprehends an article of manufacture having good resistance to oxidation in air at elevated temperatures, which article comprises a core of metal selected from the group consisting of columbium and co umbium base alloys, the article having a defect, spalling, and thermal and mechanical shock failure resistant shell or coating, the shell or coating consisting essentially of a surface zone of columbium silicide (preferably CbSimodified by a titanium content of less than 4% by weight, of columbium silicide, and, after exposure to a diffusing temperature in a nonoxidizing atmosphere, a diffusion zone or subzone beneath the surface zone consisting essentially of subsilicides of the metal core.

The invention may also be described as including a new and improved article of manufacture having good resistance to oxidation in air at high temperatures up to 2000" R, which article comprises a core of metal selected from the group consisting of columbium and alloys thereof, the article having a thermal-shock failure resistant, defect resistant, spalling resistant and broad range oxidationresistant shell or coating, the shell or coating being characterized, after exposure to an oxidizing environment at a diffusing temperature effective to create interdiffusion between the coating and the metal core, by an exposed layer or surface zone, consisting essentially of oxidic sili con (normally SiO the shell or coating also having an intermediate layer or zone consisting essentially of columbium silicide (preferably CbSi modified by a titanium content of less than 4% titanium by weight of columbium silicide and an inner layer or zone consisting essentially of subsilicides of the metal core.

This invention also embraces as an article of manufacture, a refractory metal body, comprising a substrate selected from the group consisting of columbium and its alloys and having a defect, spalling, oxidation and thermal-shock resistant protective coating comprising, after exposure to a diffusing temperature in an oxidizing environment, an exterior exposed layer or zone consisting essentially of oxidic silicon (normally SiO a Sublayer or intermediate zone beneath the exterior or exposed layer or zone consisting essentially of columbium silicide (preferably CbSi modified by a titanium content of less than 4% titanium by weight of columbium silicide, and a second sublayer or inner zone beneath the first sublayer or intermediate zone consisting essentially of subsilicides of the substrate; the body being characterized by good resistance to oxidation at temperatures up to at least 2000 F.

In accordance with its purpose, this invention includes a method of producing a coated metal article having resistance to oxidation at high temperatures, which method comprises depositing a surface coating on a metal substrate, the substratebeing a metal selected from the group consisting of columbium and alloys thereof and the coating consisting essentially of columbium silicide (preferably CbSi modified by a titanium content of less than 4% by weight of columbium silicide.

One embodiment of such method inculdes a vapor deposition process of coating a fabricated base metal which process comprises surrounding a base metal selected from the group consisting of columbium and its alloys with a powdered pack of a finely ground source of silicon, a finely ground source of titanium, and a small amount of a volatilizable fluoride salt as active ingredients and an inert filler, heating the base metal and powdered pack for a time period sufficient to cause volatilization of the fluoride salt and to produce deposition of silicon and titanium on the surface of the base metal thereby effecting the creation of an exterior surface layer or zone on the base metal consisting essentially of columbium silicide (preferably CbSi modified by a titanium content of less than 4% titanium by weight of columbium silicide Such method may be extended to include the step of exposing a metal article (the exposure normally being effected during actual use), previously coated a described with Ti-modified columbium silicide, to an oxidizing environment at a diffusing temperature to effect the creation of an outer exposed layer or surface zone consisting essentially of oxide silicon, an intermediate layer or first subzone consisting essentially of columbium silicide (preferably CbSi modified by a titanium content of less than 4% titanium by weight of columbium silicide, and an inner layer or second subzone consisting essentially of subsilicides of the base metal.

As previously set forth, conventional silicide coatings on structural columbium alloy substrates are prone to rapid consumption through oxidation at low (about 1300 F.) temperatures (this tendency is sometimes referred to as the pest phenomenon) and at high (about 2000 F.) temperatures. At the latter temperature a rapid oxidation mechanism begins to occur which though different from the pest phenomenon, is similar in its undesirable end result.

Quite unexpectedly, and contrary to what one would expect from the usual behavior of silicide coatings, if the titanium-modified silicide coatings of this invention are used on columbium and columbium alloy substrates, the deleterious effects of both the low temperature pest phenomenon and high temperature rapid oxidation mechanism are essentially overcome.

The titanium-modified columbium silicide coatings of the present invention have thus proven to be particularly outstanding in their ability to protect columbium and its alloys from oxidation under a wide variety of conditions of use and at temperatures up to at least 2000 F. These coatings possess distinctly superior oxidation resistance to unmodified silicide coatings and overcome the tendency of titanium-free silicide coatings on columbium substrates 6 to fail at the critical temperatures of about 1300" F. and 2000 F., and above.

In accordance with this invention, typical substrates, in addition to unalloyed columbium, to which the titaniummodified silicide coatings of this invention have been applied, parts expressed as percentages by weight are as follows:

ALLOY 1 Columbium 99 Zirconium 2 (Cb-lZr).

ALLOY 2 Columbium 93 Vandadium 5 Aluminum 2 (Cb-5V-2Al).

ALLOY 3 Columbium 95 Zirconium 5 (Cb-SZr).

ALLOY 4 Columbium 94.5 Vanadium 5 Chromium 0.5 (Cb-5V-0.5Cr).

ALLOY 5 Columbium 87 Vanadium 7 Aluminum 3 Tungsten 3 In accordance with this invention, suitable fluoride salts are as follows:

AlF KF ZnF NiF FF3 CdF SnF The preferred fluoride salt for best results with the present invention is, however, aluminum fluoride (AlF For a clearer understanding of the invention, specific examples of the invention are set forth in this specification. These examples are merely illustrative and are not to be understood as limiting the scope and underlying principles of the invention.

In the embodiment forming the examples of this invention an alloy consisting essentially of Cb-lZr by weight, hereafter referred to as the Alloy, was selected as a representative substrate material. Other columbium-base alloys such as those set forth previously could have been used equally well as substrates to illustrate the new and desirable performance of the titanium-modified silicide coatings for columbium base alloys described in this specification.

EXAMPLE 1 Titanium-modified silicide coatings were applied to the Cb-lZr substrate or the Alloy by utilizing a packcementation process, which comprised embedding chemically cleaned specimens to be coated in a pack of the following mixture:

11 grams (90 cc.) of A1 0 powder (micron particle size) 15 grams of titanium powder (minus mesh) 15 grams of silicon powder (minus 200 mesh) 4 grams of AlF The packs, contained in Inconel containers, were then subjected to various thermal treatments ranging from 1800 to 2200 F., for times of from about 4 to 16 hours, preferably 16 hours at 1800 F.

The thickness of the coating deposited was controlled by the time temperature relationships used. A lower coating temperature is generally preferred for economy of operation, and this is particularly important when the process is scaled up.

In accordance with the invention, an argon flowthrough was maintained in the pack at atmospheric pressure or slightly above. This fiowthrough was equivalent to about 10 cc. of argon per minute through a 500 cc. canister.

When the reaction had proceeded to the extent desired, the canister was removed from the furnace and cooled under a flow of argon.

During this treatment, the silicon, titanium and substrate reacted to yield a titanium-modified CbSi structure in which titanium was present to the extent of less than 4% by weight of the CbSi The titanium content of the coating was determined by electron microprobe studies and X-ray diffraction analysis.

Thicknesses of the titanium-modified silicide coating varied from 1 /2 to 4 mils depending upon specific deposition conditions. Desired thicknesses of these coatings were from about 1 /2 to 3 mils, preferably about 2 mils.

The inert filler material is not limited to A1 since almost any refractory oxide filler, such as zirconia or beryllia, also works well.

The coating thus produced was found to protect the alloy for periods in excess of 5000 hours at temperatures of 1800 F. The coating is also protective over the alloy for shorter periods at even higher temperatures up to 2300 F.

Although many of the fiuoride salts will produce the desired results, ammonium fluoride (NH F) should not be used, since it leads to hydrogen and nitrogen embrittlement of the columbium. Aluminum fluoride (AlF is the preferred salt, and it works especially well because of its high vapor pressure.

EXAMPLE 2 A chemically cleaned specimen of the alloy was embedded in a pack of the following mixture:

11 grams (90 cc.) of A1 0 powder (micron particle size) 25 grams of titanium powder (minus 100 mesh) 15 grams of silicon powder (minus 200 mesh) 3 grams of potassium fluoride (KF) The pack, contained in an Inconel container, was purged for two hours with argon, and then subjected to thermal exposure in an argon atmosphere at a temperature of 1800 F. for 16 hours. After exposure, the pack was removed from the furnace and allowed to cool under a flow of argon. The resulting coating was about 3 mils thick and consisted essentially of a titanium-modified CbSi structure in which titanium was present in an amount of less than 4% by weight of CbSi This coating protected the substrate in excess of 5000 hours at 1800 F.

'EXAMPLE 3 A chemically cleaned specimen of the alloy was embedded in a pack of the following mixture:

11 grams (90 cc.) of ZrO (micron particle size) 15 grams of titanium powder (minus 100 mesh) 20 grams of silicon powder (minus 200 mesh) 4 grams of zinc difluoride (ZnF The pack, contained in an Inconel container, was purged for two hours with argon, and then subjected to thermal exposure in argon atmosphere at a temperature of 1800 F. for 16 hours. After exposure, the pack was removed from the furnace and cooled under a flow of argon. The resulting titanium-modified silicide coating was about 3 mils thick and consisted essentially of a titaniummodified CbSi structure in which titanium was present in an amount of less than 4% by weight of CbSi This 8 coating effectively protected the specimen for periods in excess of 5000 hours at temperatures up to 1800 F.

Analysis of the coatings of the foregoing examples as set forth above revealed that the titaniummodified silicide coatings of this invention on columbium base substrates in the as-coated condition are composed essentially of CbSi modified by a titanium content of less than 4% titanium by weight of CbSi FIG. 1 is a photomicrograph magnified 500 times showing a representative titanium-modified silicide coating of this invention over the alloy in the as-coated condition. This coating, as is characteristic of the coatings of this invention, is very uniform both in composition and thickmess; it is also well-bonded to the substrate and highly resistant to spalling.

FIG. 2 is a photomicrograph enlarged 500 times showing the titanium-modified silicide coating of this invention over the alloy after exposure for 1000 hours at 1800 F. As can be seen from the photomicrograph FIG. 2, the basic titanium-modified silicide coating remains essentially unchanged in composition and thickness; however, a narrow diffusion zone has been created between the substrate and the original coating by exposure at a diffusing temperature, and this diffusion zone is composed of subsilicides of the substrate. There is also a relatively thick layer of oxide formed during the exposure period.

As is characteristic of the coatings of this invention, and in accordance with the invention, the coating shown in FIG. 2 has fissured down to the diffusion band, but the fissures do not go through the diffusion and and the coating is stabilized and shows no signs of failure after 1000 hours at 1800 F. The fissures are essentially micron size in width and it is thought that by becoming filled with oxide on exposure to a high temperature oxidizing environment, further attack through the diffusion band is prevented.

FIG. 3 is a plot on a logarithmic scale showing continuous oxidation weight gain data for the alloy coated with the titanium-modified silicide coating of this invention. As shown in FIG. 3, although the continuous weight gain data is for hours only, it is apparent that the weight gain versus exposure time is a parabolic function. The curves have been extrapolated to 1000 hours on the basis of the parabolic behavior exhibited. It has been established by actual exposure of some specimens that the coatings remain stable with respect to weight gain for at least 5000 hours at 1800 F.

In accordance with the invention, the titanium-modified silicide coatings consisting essentially of columbium silicide (preferably CbSi modified by a titanium con tent of less than 4% titanium by weight of columbium silicide are created by codeposition of titanium and silicon in a pack-cementation process as previously described.

The desired coatings can be achieved with a pack mixture in which the weight ratio of titanium powder to silicon powder varies from 1.75:1 to 111.75, but preferably the ratio of titanium to silicon is l to 1.

The novel coatings of this invention for columbium and columbium base alloy substrates achieve an important, new and useful result. They possess distinct and unique advantages over the usual types of intermetallic protective coatings such as CbSi or CbAl Among the novel and unexpected beneficial results and advantages obtained from the coatings and methods for achieving the coatings of this invention are the following:

(1) The titanium-modified silicide coatings of this invention composed essentially of columbium silicide (preferably (CbSi modified by a titanium content of less than 4% by weight of columbium silicide exhibit excellent long term oxidation resistance at temperatures up to at least 2000 F., and are superior to the existing coating materials, typified, for example, by CbSi and CbAl (2) When compared with existing coating materials, such as CbSl2, the titanium-modified silicide coatings of this invention are highly resistant to the pest phenomenon or the low temperature (about 1300 F.) rapid oxidation failure that is characteristic of typical titaniumfree silicide coatings and the coatings of this invention retain this resistance even after repeated exposure to high-temperature environments, thereby exhibiting excellent thermal stability. In contrast, even some improved and modified silicide and aluminide coatings are prone to fail when subjected to relatively mild thermal cycling between low and high temperatures.

(3) The coatings of this invention impart long term reliability to columbium base structures operating in static air. For example, when used as pipes for liquid metal in power plants or heat exchangers, the protection afforded the columbium substrates by the coatings of this invention enables such power plants and exchangers to be operated at higher and more efiicient temperatures over longer periods of time.

(4) Upon exposure at a diffusing temperature the coatings of the present invention become diffusionally stable through the creation of a diffusion band composed essentially of subsilicides of the substrate between the deposited titanium-modified silicide coating and the substrate.

(5) Although a thermal expansion mismatch exists between the coatings of this invention and typical columbium base substrates, the coatings as deposited are thin enough so that they take on the characteristics of the substrate and expand and contract with the substrate. There is excellent adherence of the coatings to the substrate and they are thus well-bonded and highly resistant to spalling.

(6) The coatings of this invention are characteristically highly uniform in both composition and thickness, are mechanically stress resistant, and are compatible with columbium base substrates in that they form no low melting phases or volatile compounds.

As used in this specification the expression CbSi will be understood to include all those forms of columbium silicide in which the atomic ratio of columbium to silicon is of the order of 1:2. And the expression SiO will be understood to include all those forms of oxidic silicon in which the atomic ratio of silicon to oxygen is in the order of 1:2.

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

What is claimed is:

1. An article of manufacture which comprises a core of metal selected from the group consisting of columbium and columbium base alloys, the article having a defect, spalling, and thermal-shock failure resistant surface zone that is oxidation resistant at high temperatures, the surface zone consisting essentially of columbium silicide modified by a titanium content of less than 4% of the columbium silicide by weight.

2. An article having good resistance to oxidation in air at elevated temperatures, which article comprises a core of metal selected from the group consisting of columbium and columbium base alloys, the article having a defect, spalling and thermal and mechanical shock failure resistant coating consisting essentially of a surface zone of columbium silicide modified by a titanium content of less than 4% of the columbium silicide by weight and a diffusion zone which has been produced by exposure of the article at a diffusing temperature in a nonoxidizing atmosphere, the diffusing zone being beneath the surface zone and consisting essentially of subsilicides of the metal core.

3. An article of manufacture having good resistance to oxidation in air at high temperatures which comprises a core of metal selected from the group consisting of columbium and alloys thereof, the article having a thermalshock failure resistant, defect resistant, spalling resistant, and broad range oxidation-resistant shell which has been produced by exposure of the article to an oxidizing environment at a diffusing temperature effective to create interdifi'usion between the shell and the metal core, the shell being characterized by an exposed surface zone consisting essentially of oxidic silicon, an intermediate zone consisting essentially of columbium silicide modified by a titanium content of less than 4% titanium by weight of columbium silicide, and an inner zone beneath the intermediate zone consisting essentially of subsilicides 0f the metal core.

4. As an article of manufacture, a refractory metal body, comprising a substrate selected from the group consisting of columbium and its alloys and having an exterior exposed zone composed predominantly, after exposure at a diffusing temperature in an oxidizing environment, of oxidic silicon, an intermediate zone beneath the exposed zone composed predominantly of columbium silicide modified by a titanium content of less than 4% titanium by Weight of columbium silicide, and an inner zone beneath the intermediate zone composed predominantly of subsilicides of the substrate; the body being characterized by good resistance to oxidation at temperatures up to at least 2000 F.

5. A coated metal body comprising a substrate selected from the group consisting of columbium and its alloys and having a protective shell at least on that part of the substrate that is exposed to attack by oxygen at high temperatures, the coating consisting essentially of columbium silicide modified by a titanium content of less than 4% titanium by weight of columbium silicide, the shell being oxidation-resistant, thermal-shock failure resistant, spalling resistant, and defect resistant at high temperatures.

6. A coated metal body comprising a substrate selected from the group consisting of columbium and its alloys and having a protective shell, at least 011 that part of the substrate that is exposed to attack by oxygen at high tem peratures, the shell, after exposure of the article to an oxidizing environment at a diffusing temperature, having a surface zone consisting essentially of oxidic silicon, an intermediate zone beneath the surface zone consisting essentially of columbium silicide modified by a titanium content of less than 4% titanium by weight of columbium silicide, and an inner zone beneath the intermediate zone consisting essentially of subsilicides of the substrate; the shell being oxidation resistant, thermal-shock failure resistant, spalling resistant, and defect resistant at high temperatures.

7. The process of coating a fabricated base metal which process comprises surrounding a base metal selected from the group consisting of columbium and alloys thereof with a powdered pack of a finely ground source of silicon, a finely ground source of titanium, and a small amount of a volatilizable fluoride salt as active ingredients and an inert filler, heating the base metal and powdered pack for a time period sufiicient to cause volatilization of the fluoride salt and to produce codeposition of silicon and titanium on the surface of the base metal, thereby effecting the creation of an exterior surface zone on the base metal consisting essentially of columbium silicide modified by a titanium content of less than 4% titanium by weight of columbium silicide.

8. The invention as defined in claim 7, in which the process includes the further step of exposing the previously coated base metal to an oxidizing environment at a diffusing temperature to eifect the creation of an outer exposed surface zone consisting essentially of oxidic silicon, an intermediate zone beneath the surface zone consisting essentially of columbium silicide modified by a titanium content of less than 4% titanium by weight of columbium silicide, and an inner zone underneath the intermediate zone consisting essentially of subsilicides of the base metal.

9. The process of claim 7, in which the fluoride salt is AlF 10. The proces of claim 7, in which the ratio by weight of titanium to silicon is from 1.75:1 to 121.75.

11. The process of treating a metal from the group consisting of columbium and alloys thereof to render the surface of the metal resistant to oxidation at high temperatures, that includes, heating the metal to a temperature of from 1800 F. to 220 F. in a nonoxidizing atmosphere and in surface contact with a powdered mixture of silicon, titanium, a fluoride salt and an inert refractory material, to form thereby a protective surface shell on the metal consisting essentially of columbium silicide modified by a titanium content of less than 4% titanium by weight of columbium silicide.

12. The invention as defined in claim 11, in which the fluoride salt is A11 13. The invention as defined in claim 11, in which the metal is heated from 4 to 16 hours.

14. The invention as defined in claim 11, in which the heating step is carried out for 16 hours at 1800 F.

15. A method of producing a high temperature oxidation resistant, thermal-shock failure resistant, spalling resistant, and defect resistant surface shell on a metal article formed of a substrate selected from the group consisting of columbium and columbium base alloys, the coating surface shell consisting essentially of columbium silicide modified by a titanium content of less than 4% titanium by weight of columbium silicide; the method comprising the steps of: enclosing the article in a siliconizing and titanizing pack of powdered material containing a source of silicon, a source of titanium, and a small amount of a volatilizable fluoride salt as essential active ingredients and an inert filler, heating the article in the pack to a temperature higher than that causing volatilization of the fluoride salt, and maintaining this temperature for a discrete interval of time to effect the deposition of silicon and titanium onto the surface of the article, thereby creating an exterior surface shell consisting essentially of columbium silicide modified by a titanium content of less than 4% titanium by weight of columbium silicide.

16. The method of claim 15, that includes the further step of exposing the metal article with its protective shell to an oxidizing environment at a high temperature to effect the creation of a surface zone on the shell consisting essentially of oxidic silicon, an intermediate zone beneath the surface zone consisting essentially of columbium silicide modified by a titanium content of less than 4% titanium by weight of columbium silicide, and an inner zone beneath the intermediate zone consisting essentially of subsilicides of the substrate.

References Cited UNITED STATES PATENTS 3,081,530 3/1963 Wlodek 29198X 3,186,070 6/1965 Oxx Jr. 29198X 3,069,288 12/1962 Oxx, Jr., et a1 11771 3,086,886 4/1963 Kieffer et al. 117-l07 2,665,475 1/ 1954 Campbell et a1 117l06 REUBEN EPSTEIN, Primary Examiner US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 571 Dated April 13, 1971 Inventor(s) L. A. Friedrich et a1 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Claim 6, column 10, line 38, before "having" insert the shell H to Claim 11, column 11, line 7, change "220 F.

Signed and sealed this llpth day of September 1971 SEAL) Attest:

EDWARD M,FLETGHER,JR ROBERT GOTTSCHALK Atte ti offi Acting Commissioner of Pa FORM PO-1U5O (10-69) uscoMM-Dc 603

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