US2681876A - Refractory coated article - Google Patents

Description

V. J. DE SANTIS ETAL REFRACT'ORY COATED ARTICLE Filed Jan. 24, 1949' STEP-#l STEP #2 STEP #4 REF/FACTORY O/Y/f HFPL /E INVENTORS f//A/cEA/T I oss/)NTIS F A HUNTER P JKRZAK ATTORNY Patented `une 22, 1954 UNITED STATES P'EENT OFFICE Hunter,

Lake Elu, Eil.,

and Charles P.

Majlrrzak, South Orange, N. J., assignors to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application January 24, 1949, Serial No. 72,403

(Cl. 14S-31.5)

14 Claims. l

rl'his invention relates to improvements in the protection of metallic surfaces against adverse iniiuences at high temperatures. In a more particular sense, the invention is concerned with the provision of a coating upon a metallic surface which will protect the underlying metal against the adverse eects, particularly oxidation, which result from exposure of the coated article to oxidizing or otherwise deleterious atmospheres at temperatures of 750 C. or above.

Recent technological developments in the iieldV of gas turbines, jet-propelled and rocket-propelled missiles have resulted in the need for new materials capable of resisting the high temperatures encountered in operation of such devices, particularly materials capable of Withstanding such very high temperatures over an extended period of time. The temperatures encountered under these conditions are so much higher than those encountered in equipment of the types heretofore known that a Whole new field of technology has developed in this connection.

It has long been recognized that many of the high-inelting-point metals in their elemental or in alloyed condition retain their tensile strength to the extent of at least 5000 pounds per square inch at operating temperatures of as high as 800 C. and in some instances 1000 C. Among these refractory metals are tantalum (M. P. ca. 2770 C.), columbium (M. P. ca. 1950 0.), molybdenum (M. P. ca. 26'70" C.) and tungsten (M. P. ca. 3370" C.). However, in order that these metals may retain their strength at these high temperatures, it has been necessary that the other operating conditions, especially the atmosphere, be subject to control in order that oxidizing gases and other substances capable of reactinor With the metals at these temperatures may be excluded. For instance, even at 500o C. in an oxidizing atmosphere tungsten and molybdenum form volatile oxides so rapidly that little is left of a substantial mass of metal after less than 100 hours under these conditions, and tantalum and columbium, when thus -l oxidized, become brittle in 48 hours or less. At higher operating temperatures, say 1000 C., these efects are produced after merely a few minutes, and, reducing or neutral atmospheres no longer afford protection to the metals because then the metals Will reduce water Vapor or carbon monoxide present with resultant oxidation of the metal, and, if a hydrocarbon gas or other carbonaceous material be present, the metals will decompose them with progressive carbonization and resultant embrittlement of the metal. Inasmuch as hydrocarbon vapors and Water vapor are commonly, indeed necessarily, present in the atmosphere around hightemperature operating parts of gas engines, it is obvious that these metals are unsuited to such use, more especially at temperatures materially exceeding about '750 C`. for, under these conditions, the deleterious effects produced by the surrounding atmosphere reacting with the metal cause far-reaching changes Within the body of the metal resulting in structural distortion and lack of strength, of ductility and of resilience.

It has long been recognized that carbon is one of the most thermo-resistant materials generally available. It is cheap, easily Worked, generally available in large quantities, and can withstand temperatures far exceeding those which result in the structural enfeeblement of refractory metal articles. The physical strength of carbon is not apparently aiected to any material degree by elevated temperatures and thus this material has found high favor among workers fabricating apparatus to be used under these conditions. Carbon possesses several disadvantages, however, Which have limited the extent to which it can be utilized in such equipment: the extreme brittleness of carbon makes it very diicult to fabricate into delicate structures; furthermore if such structures are fabricated, they are very fragile and easily damaged in transit or during normal handling under conditions of use. Additionally, carbon burns if heated above a dull red heat, about 759 C., in an oxidizing atmosphere or in an atmosphere containing a read-ily-reducible, oxygencontaining compound such as Water vapor. In this connection it will be recalled.v that nearly all combustion gases, as in gas turbines for instance, contain water vapor and thus at elevated temperatures carbon would burn in these gases. A further disadvantage of carbon is that it is a porous material quite permeable to gases and this permeability increases with increase of temperature. For these reasons and other reasons connected with particular requirements of individual equipment, carbon has not been found satisfactory for use in many types of high-.emperature equipment, particularly equipment which of necessity must be structurally delicate While retaining substantial ruggedness so that it is not damaged under normal condition of transport and use.

It has also been suggested to coat refractory metals mentioned above with various mixtures of silica or other refractory metal oxides but this did not result in articles capable of withstanding temperatures above 750 C. because of reaction of the coatings with the underlying metal resulting in oxidation thereof, although this reaction proceeds at a relatively low rate at lower temperatures.

One of the objects of the present invention is to provide a method for protecting the surface of a refractory metal exposed to very-high temperatures so that deleterious atmospheric iniuences, especially the presence of oxidizing and hydrocarbon gases, will not affect the underlying metal.

A further object of the invention is to provide a composite structure that can be utilized in equipment functioning at high temperatures which retains its physical ruggedness and strength, even when exposed to temperatures of 1000" C. or materially thereabove, and which possesses the desirable properties of the refractory metals without the disadvantages, above mentions, that characterized them.

Other objects of the invention will be apparent to those versed in the art to Which the invention relates upon consideration of the fcllowing disclosure.

The accompanying single figure of drawing is a diagrammatic sectional illustration of the process and products, intermediate and final, constituting the subject matter of this invention.

Regarded in certain of its broader aspects, the present invention comprises providing, upon a base of refractory metal, a layer of refractory carbides, formed in situ upon the metal surface, over which is applied an outer coating of a material capable of successfully resisting high temperatures and which normally aects the base metal, but is prevented from doing so by the barrier layer of carbides.

In a more specific sense, the present' invention comprises a base of tantalum, molybdenum, tungsten or the like, including alloys thereof or alloys in which one or more of the mentioned metals is present in a substantially predominant proportion, with a layer of carbides comprising a carbide of one or more of said metals and silicon carbide, titanium carbide or zirconium carbide or a mixture or alloy of the same, formed, in situ, on the base metal and bearing an outermost layer of refractory material, such as a noble or refractory metal, particularly a metal of the platinum group, carbon or mixed carbides including metallic carbides.

in accordance with this invention the desired protective coating is produced upon the refractory metal substantially as follows: a layer of refractory oxide is applied to the metal surface by dipping, spraying or electrophoresis, or in any other manner which can be used to produce va thin, though uniform, adherent lm of coating on the metallic surface; this coating is then sintered and partially dissolved or otherwise integrated with the surface by heating to an appropriately high temperature, e. g., l7002000 C. in vacuo or inert atmosphere; thereafter the sintered oxide coating is covered with a coating of carbon, either, per se, in divided state or in the form of a carbonaceous material, so that the base metal carries a coating of oxide with an outside coating of carbon or carbonaceous substance. The article so coated is then fired at a temperature of at least l700 C., preferably under vacuum, to cause reaction between the oxide coating and 4 the overlying carbon coating, it 'being understood that the carbon content of the outside coating is so related to the oxide content of the underlying coating that the reaction between the carbon and the oxide does not fully spend the carbon but instead an excess of carbon remains after the reaction has taken place. This reaction between the refractory metal oxide and the carbon results in the formation of a layer of mixed cr alioyed refractory carbides, together with the excess carbon above mentioned, upon the refractory metal surface, the carbides being those of the base metal and of the metal moiety of the oxide layer. This fired coating, which is tenaciously adherent to the base metal, is then provided with a further coating of a material resistant to adverse atmospheric conditions at temperatures of 750 C. or above, including temperatures of the order of l000 C, or substantially thereabove, particularly platinum or a metal of the platinum group, if the final product is to be used in an oxidizing or carbonaceous atmosphere, or carbon if it is to be used in an atmosphere that is inert to carbon at these temperatures. The supericial appearance of the finished product resembles that of an ordinary carbon or metal article, depending on the material selected for the iinal layer, but upon more detailed examination it is found that underneath the exterior portion of the coating lies a hard layer of refractory metal carbides having a metallic crystalline appearance, which is extremely resistant to high temperatures, e. g., a mixture or alloy of tantalum carbide with zirconium carbide is capable of resisting temperatures up to about 3900 C. without substantial impairment of its physical strength or other properties. it is especially noteworthy in this connection that the carbide layer itself, as distinguished from a carbon layer on the refractory metal surface, does not migrate at elevated temperatures into the body of the metal, nor does it allow overlying normally migratory layers to migrate.

Referring now to the single figure of drawing, it will be noted that it illustrates in fragmentary, largely-diagrammatic, sectional view, an article provided with a protective coating pursuant to the present invention. The refractory metal base plate i, preferably formed of tantalum, columbium or the like, is provided with a refractory metal oxide coating 2, preferably zirconium dioxide, which can be applied by spraying, dipping or electrophoretically from an organic liquid bath. After this coating has been applied, it is consolidated upon the base metal by sintering in vacuo at a temperature of at least about 1700 C. so that it forms a thin film approximately .0005 inch thick continuously covering the base.

The sintered coated base is then provided with a second coating consisting of carbon which can be applied by any conventional method suited for application of coating compositions, e. g. spraying, dipping or electrophoretic deposition, so that there is provided upon the refractory oxide layer a carbon coating 3 approximately .003 inch thick. The coated article is then red at a temperature of at least 1700o C. in a vacuum whereby the carbon, which is present in excess of the stoichiometric proportion, reacts with the refractory Oxide to produce a layer of carbides, and a further layer 5 by reaction of carbon with the immediately subjacent refractory metal base to produce a layer of mixed carbides or alloy of carbides of the refractory metal base and the metal moiety of the refractory oxide. The completion cf these reactions may be readily detected by absence of oxides of carbon from the vacuum outlet.

Thereafter a further coating 6 of refractory metal or carbon is applied and consolidated to yield a finished article which consists of the refractory metal base i bearing an immediately adjacent layer 5 composed of mixed or alloyed car bides with anv overlying layer 4 of refractory carbides with an outermost layer 6` of refractory metalor carbon. Migration of the material from the layer B into the refractory metal base l is prevented by the intervening layers 4 and 5 containing refractory metal carbides.

To facilitate an understanding of the subject matter of the present invention certain specic embodiments thereof will be hereinafter described in detail but it is clearly to be understood that these examples are provided by way of illustration, not by way of limitation, of the present invention.

Example I An article of tantalum is coated with silica by making the article an electrode in electrophoresis of a bath containing finely divided particles of silica. When a coating of the desired thickness has been obtained, i. e. a thickness of about .003 inch, the article is removed from the bath, then dried and fired at a temperature of about 1000o C. This causes the silica coating to become sintered'. After cooling, a coatingof carbon is applied over the silica coating and then the coated article is rered in Vacuum at about 1.500c C. until the reaction between the carbon and the silica is completed to form a barrier layer or silicon carbide on the metal base. It is4 to be understood that the amount of carbon coating applied to the silica is such as to exceed that minimum amount necessary to cause full conversion of the silica to silicon carbide and under these conditions there is produced upon the metal base acoating comprising a mixture of silicon carbide and tantalum carbide. After a layer of a metal of the platinum group or car- :bon has been applied to this mixed carbide layer by a conventional method, for instance spraying, sputtering, electrophoresis, dipping, painting, etc., there is obtained a coated article which is resistant to temperatures up to about '750 C. At temperatures above this point there is some,

though almost insubstantial migration of the f carbide layer into the interior of the underlying metal with resultant impairment of the resistance of the coated metal to migration of the outer coating at the high temperatures involved.

Example II An article formed of molybdenum is treated as described in Example I except that all rings are at slightly lower temperatures than are there indicated, so that there is produced upon the article a barrier coating of the mixed carbides of molybdenum and silicon. This carbide barrier coating possesses substantially the same characteristics, physically and chemically as are possessed by the tantalum carbide silicon carbide barrier coating obtained by practice of the process described in Example I.

Example III The process as described in Example. I is repeated except that the silica is replaced with zirconia and the iirst and second rings are at a temperature of 2000 C. This results in an article bearing a barrier coating of the mixed or alloyed carbides of tantalum and zirconium which is resistant to atmospheric conditions at temperatures of well above '750 C. or thereabouts. Otherwise theproperties of the finished coated article are substantially those as set forth in Example I.

Example IV The procedure of Example III is. repeated except. that molybdenum is substituted for the tantaluln as the base metal and except that all iirings are at slightly lower temperatures than are there indicated. Under these circumstances, the barrier coating is a mixture of the carbides of zirconium and molybdenum which is resistant to elevated temperatures up to. well above about 750 C.

Example V The process as described in Example I is repeated except that the silica is replaced with titania, and the first and second firings are at a temperature of 2000o C. This results in an article bearing a barrier coating of the mixed or alloyed carbides of tantalum and titanium which is resistant to atmospheric conditions at temperatures of wellv above '750 C. or thereabouts. Otherwise the properties of the finished coated article are substantially those as set forth in Example I.

Erample VI The procedure of Example V is repeated except that molybdenum is substituted for the tantalum as the base metal and except that all firing-s are at slightly lower temperatures than are there indicated. Under these circumstances, the barrier coating is a mixture of the carbides of titanium and molybdenum which is resistant to elevated temperatures up to well above about 75@0 C.

It will be recognized by those skilled in the art to which this invention relates that the invention is not limited to any specic manner of applying the successive coatings on to the refractory base. Thus the coatings of 'the refractory oxide, of the carbon or carbonaceous material and the final` coating of refractory metal of the platinum group or carbon may be effected by any procedure known in the art for producing Such coatings including the operations of dipping the article in a coating composition, forming the coating by applying a coating composition by spraying or producing a coating by electrophoresis of a bath containing charged particles of the coating substances in a size suited for electrophoretic deposition. The initial sintering of the oxide coating can be effected within a substantially wide range of temperatures provided the temperature is sufficient to effect a satisfactory integration of the particles of refractory oxide with the underlying core so that during subsequent, processing whereby the oxide is converted to the carbide, a coating of carbides is obtained upon the underlying metal. For this purpose, temperatures of about 1500" C. for silica, about 2000* C. for zirconia and titania on a tantalum base,v or lower temperatures on a molybdenum base, have been found to be quite adequate but higher or lower temperatures may be used. if peculiar conditions of operation make these changes desirable. The final firing operation whereby the stratied refractory metal oxide and carbon coatings are converted into a refractory metal carbide layer on the refractory metal surface may be effected at a temperature approximating, that used in the first firing' operation. It is desirable, though not essential, that the second firing take place either under vacuum or in a noble gas which facilitates conversion of the refractory metal oxide to the corresponding carbide.

While from the foregoing it will be understood that the refractory metal carbide layer produced upon the refractory metal base pursuant to this invention inherently possesses entirely satisfactory resistance to migration by an overlying layer of a platinum group metal at very high temperatures, nevertheless a layer of carbon may in some instances be preferable to the layer of platinum group metal and in this instance too migration is prevented. The article having an outermost layer of a platinum group metal applied on top of the carbide layer possesses to the fullest measure the qualities of resistance to unfavorable atmospheric conditions which are characteristic of the metal and permanently retains these properties inasmuch as migration is prevented. One explanation of these phenomena is as follows: the mixed or alloyed carbides in the barrier layer are chemically inert toward the core metal even at the high operating temperatures involved at which the core metal normally is very reactive toward carbon, thus the barrier remains saturated with carbon and thus does not migrate. The result is that a surface coating which normally would migrate into the underlying metal is excluded effectively from such migration and so remains on the surface to resist oxidation, etc., as if the entire body were formed from the same substance as this outside layer.

In our joint application, Serial No. 72,404, filed January 2li, 1949, now Patent 2,552,535, we disciose an electrode for electron discharge devices, wherein a base metal core is nrst coated with a barrier layer, similarly as disclosed in the present application, over which a layer of non-emissive material is applied, the barrier layer preventing migration of the non-emissive material to the base metal core oi the electrode.

It will be understood that in the foregoing specification and in the following claims the term refractory metal includes the elements molybdenuni, tantaluin, tungsten and columbium tgether With mixtures thereof and alloys in which one or more of these metals is the predominant constituent imparting its characteristics to the alloy. The term refractory oxide includes zir conium oxidey titanium dioxide and silicon dioxide or mixtures thereof with similar or related refractory oxides wherein the zirconium oxide or the silicon dioxide is the chief or predominant component imparting its qualities as the principal properties ci the mixture. Vhen reference is made herein to carbon each of the elemental ailotropic forms of the element is meant, although for obvious practical considerations graphite or soot is the form in which the element is most likely to be used when producing the carbon coating on the refractory oxide or mixed refractory metal carbide coating.

While We have described above the principles of our invention in connection with specic apparatus, it is to be clearly understood that this description is made only by Way of example and not as a limitation to the scope of our invention.

We claim:

l. A composite structure that comprises a refractory metal base formed of a metal chosen from the class consisting of tantalum, columbium, molybdenum, tungsten and alloys in which at least one of said metals is the predominant component and a hard crystalline barrier layer 8 upon the base that comprises refractory carbides consisting essentially of zirconium carbide and a carbide chosen from the class consisting of the carbides of tantalum, columbium, molybdenum and tungsten.

2. A composite structure that comprises a refractory metal base formed of a metal chosen from the class consisting of tantalum, columbium, molybdenum, tungsten and alloys in which at least one of said metals is the predominant component and a hard crystalline barrier layer upon the base that comprises refractory carbides consisting essentially of zirconium carbide and a carbide of at least one of said base metals.

3. A composite structure as dened in claim 1 in which the refractory metal is tantalum and one or the carbides is a carbide of tantalum.

4. A composite structure as dened in claim 1 in which the refractory metal is columbium and one of the carb-isles is a carbide of columbium.

5. composite structure as defined in claim 1 in which the refractory metal is tungsten and one of the carbides is a carbide of tungsten.

6. A composite structure as defined in claim 1 in which the refractory metal is molybdenum and one or" the carbides is a carbide of molybdeum.

7. 'lin a process for producing a protective coating' upon a refractory metal base chosen from the class consisting of tantalum, columbium, molybdenum, tungsten and alloys in which at least one of said materials is the predominant component, the steps of applying to said base a coating oi iinely divided zirconium oxide, firing to cause sintering of the oxide coating, applying a layer of carbon on to the sintered oxide coating, and then firing at a temperature of at least C-D C. to produce a coating of carbidcs of the base metal and of zirconium.

8. Process as defined in claim 7 in which the refractory metal of the base is tantalum.

9. Process as defined in claim 7 in which the refractory metal of the base is columbium.

10. Process as defined in claim 7 in which the refractory metal cf the base is tungsten.

1l. Process as defined in claim 7 in which the refractory metal of the base is molybdenum.

12. rThe method or providing a molybdenum body with a resistant coating which comprises depositing on the surface of said body the element zirconium under conditions producing a molybdenum-zirconium layer on said body and reacting the said layer with carbon to produce a molybdenum-zirconium carbide layer integrally bonded to the molybdenum body.

13. A molybdenum body having a resistant coating therein consisting essentially of the carbides of molybdenum and zirconium integrally bonded to the molybdenum body.

14. A molybdenum body having a resistant coating thereon consisting essentially of the carbides of molybdenum and silicon integrally bonded to the molybdenum body.

References Cited in the rile of this patent UNITED STATES PATENTS Number Name Date 1,362,138 Elsey June 7, 1932 2,051, 2S Dester Aug. 25, 1936 2,497,110 Williams Feb. 14, 1950 FOREIGN PATENTS Number Country Date 842,981 France June 22, 1939

Claims (1)

1. A COMPOSITE STRUCTURE THAT COMPRISES A REFRACTORY METAL BASE FORMED OF A METAL CHOSEN FROM THE CLASS CONSISTING OF TANTALUM, COLUMBIUM, MOLYBDENUM, TUNGSTEN AND ALLOYS IN WHICH AT LEAST ONE OF SAID METALS IS THE PREDOMINANT COMPONENT AND A HARD CRYSTALLINE BARRIER LAYER UPON THE BASE THAT COMPRISES REFRACTORY CARBIDES CONSISTING ESSENTIALLY OF ZIRCONIUM CARBIDE AND A CARBIDE CHOSEN FROM THE CLASS CONSISTING OF THE CARBIDES OF TANTALUM, COLUMBIUM, MOLYBDENUM AND TUNGSTEN.

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