US2924004A

US2924004A - Refractory metal bodies

Info

Publication number: US2924004A
Authority: US
Publication date: 1960-02-09
Anticipated expiration: 1977-02-09

Description

United States Patent 2,924,004 REFRACTORY METAL BODIES Ralph F. Wehrmann and Leonard F. Yntema, Waukegan, Ill., and Ivor E. Campbell, Gahanna, Ohio, assignors, by direct and mesne assignments, to Fansteel Metallurgical Corporation, North Chicago, 111., a corporation of New York No Drawing. Application February 5, 1953 Serial No. 335,396

8 Claims. (Cl. 29-198) This invention relates to refractory metal bodies of substantial worked strength and to other metal bodies of substantial strength which have been clad with a worked refractory metal such as molybdenum, which metal bodies carry thereon an exposed sintered skin comprising alloys or intermetallic compositions with the base metal of silicon and a component alloyed or reacted therewith to reduce its fusion temperature, such as one of the metals selected from the group consisting of nickel, aluminum, copper and magnesium, with or without minor contents of boron to modify the effect thereof. These sintered skins upon the base metal serve to impart a substantial resistance to oxidation in air at temperatures at which the metal is substantially attacked by oxidation with air, but not exceeding a temperature at which the base metal would tend to recrystallize and thereby lose its worked strength. The sintered skin is formed on the base metal, in accordance with the present invention, at temperatures which will not impair the worked characteristics of the base metal.

In its preferred aspects, the invention relates to molybdenum, or materials which have been clad with molybdenum, which have been worked to impart a substantially increased mechanical strength due to the working thereof, and which tend to recrystallize at temperatures above about 1,000 C., such as about l100 to 1300 C., at which temperatures the worked strength of molybdenum is lost by recrystallization. In carrying out the invention, the worked molybdenum or molybdenum clad metal base is coated with a skin coating at a temperature not exceeding about 1000 C., whereby the molybdenum or molybdenum clad base metal is made resistant to oxidation in air at such raised temperatures. The skin coating comprises mixtures .or eutectic alloys with silicon which may be sintered or will fuse at temperatures not exceeding 1000 C., such as mixtures, alloys, or eutectic compositions of silicon with one of the metals selected from the group consisting of nickel, aluminum, copper and magnesium.

Refractory metals, particularly molybdenum, have highly desirable mechanical properties at elevated temperatures. One desirable use therefor has been as electrical furnace heating elements. Other desirable uses are in oil burner nozzles, artillery piece nozzles, rocket nozzles, turbine blades and buckets, component parts of jet engines, ignition coils for burners, and valve seats for internal combustion engines. To obtain optimum utility for such refractory metals in these several high temperature uses, it usually has been necessary to exclude oxygen therefrom, and it is common for this purpose to supply a continuous flow of hydrogen to the heated metal parts to avoid oxidation at the raised temperatures.

In various companion copending applications to which the present invention is related, there are described methods of coating molybdenum and similar refractory metals with silicon (Campbell et al. Serial Nos. 150,398; 150,543 and 150,544, filed March 18, 1950); silicon combined with other metals such as boron (Yntema et al., Serial No. 299,216, filed July 16, 1952); chromium 2,924,004 Patented Feb. 9, 1960 (Yntema et al. Serial No. 315,184, filed October 16, 1952); and zirconium (Yntema et al., Serial No. 331,498, filed January 15, 1953, the effect of such coatings being to protect the molybdenum at high temperatures, such as above about 1500 C. and up through about 2000 C., against oxidation in air for long periods of time. Some of the methods disclosed therein for coating the metal are useful herein.

According to the present invention, silicon containing skin coatings are applied to the refractory base metals, such as molybdenum, at a temperature at which the base metal will not have recrystallized and thereby lost its Worked strength. It is accordingly a primary object of the present invention to coat the worked refractory metal base, such as molybdenum metal, with coating compositions which may be sintered to the worked molybdenum metal at a temperature, below about 1000 C., at which the molybdenum metal will not have lost its worked strength by recrystallization, and to sinter the coating composition to the molybdenum at such lower temperatures and thereby form a protective skin alloy or intermetallic composition with the molybdenum which protects the molybdenum against oxidation in air at temperatures below about 1000 C., at which temperatures the skin-clad molybdenum may be used without losing its worked strength.

We have found, according to the preferred practice of the present invention, that a binary mixture or alloy of the elements silicon and nickel, silicon and copper, silicon and aluminum, or silicon and magnesium, when ap plied to the surface of the base metal, such as molybdenum, as a coating thereover, may be sintered to an alloy or intermetallic composition with the molybdenum base at a temperature below about 1000 C. In some instances, simple binary mixtures of the element silicon and one of the elements nickel, copper, aluminum or magnesium may be directly sinterable at such low temperatures not exceeding 1000 C.; in other instances, such mixtures may be prealloyed or presintered to have such low sintering temperature for coating by sintering; and in still others, the addition of fluxing agents which are low-fusing and tend to dissolve any oxide impurities in these coating elements are useful to obtain the skin coating composition of such low-sintering temperature for low-temperature sintering to the "molybdenum base.

Minor additions of boron, which forms part of the intermetallic composition or alloy with the molybdenum and the other coating elements applied thereto as a skin coating, serve to modify the skin characteristics, particularly to lower the sintering temperature of the skin elements for alloying or combining with the molybdenum. The boron added to the binary coating composition has other beneficial effects upon the skin coating. It makes the coating more evenly fused, non-blistering, non-flaking, and substantially ductile.

Thus, the several elements nickel, aluminum, copper and magnesium, when mixed with silicon, are outstanding for their ability to form low sintering temperature skin coatings which substantially protect the molybdenum against oxidation in air at an intermediate temperature usually not exceeding about 1000 C., and have the further advantage that such skin coatings are highly ductile and have substantially the same ductility and/or creep characteristics as the molybdenum base metal per se, a property most desirable where the coated molybdenum is used at such intermediate temperature in air and under conditions requiring the high worked strength of the molybdenum base material. Such coated molybdenum metal products are particularly useful, for example, in turbine blade and bucket constructions, and as valve elements for internal combustion engines, where the high molybdenum worked strength characteristics in the presence of an oxidizing medium at an intermediate temperature, usually not exceeding about 1000 C., are highly desirable.

Several procedures are available for applying the coatings hereof to the molybdenum as a sintered skin thereon comprising an alloy or intermetallic composition with the worked molybdenum, and it will be generally preferred to use the method which provides the most readily sinterable coating at a temperature which does not exceed 1000 C., depending upon the melting point characteristics of the particular elements being coated. For example, silicon alone will fuse at temperatures above about 1400 C., and in combination with one of the metals, such as nickel, also having a fusion point above 1000 C., would comprise a binary mixture having a fusion point above 1000 C. Hence, these elements would not be applied alone as a simple mixture of elements to the base metal because the base metal would need to be heated above about 1000 C. to sinter such coating thereto, thereby causing loss of the worked characteristics of the base metal. However, silicon reacted with nickel forms various low melting eutectic mixtures; for example, trinickel disilicide (Ni Si and nickel monosilicide (NiSi) form a eutectic mixture at about 40 parts of silicon to about 60 parts of nickel, reacted by weight, which fuses at a temperature below 1000 C. Similarly, silicon and aluminum form a eutectic mixture of aluminum silicides at about 12 parts of silicon to 88 parts of aluminum. These ratios may, of course, be varied considerably, but such variation is desirably limited to obtain a mixture which will in any case fuse and be sintered with the molybdenum below about 1000 C.

In the case of more refractory compositions, either of simple mixtures of elements or prealloyed products thereof, a suitable fluxing material may be mixed therewith to reduce its fusion temperature. Such fluxing material is a low melting salt having the property of dissolving oxides, and may be typically an alkali fluoride such as sodium fluoride, ammonium fluoride or oxidized boron compounds such as borax, boric oxide, etc., which are all useful fluxing agents to reduce the temperature of the mixture of prealloyed compositions of the elements named above, when mixed therewith as a coating composition which may be sintered at a temperature below about 1000 C. to the molybdenum.

f the several methods available for coating of the molybdenum base, the simplest method is merely to mix the silicon with the element to be alloyed therewith in the skin, i.e., nickel, aluminum, copper or magnesium, mechanically adhere the mixed powders as a paint slurry containing a temporary binder substance upon the molybdenum and then sinter the coatings thereto. Where such sintering is readily effected without a flux, such as in the instance of silicon and aluminum, the simple mixture is readily applied in this manner, and where the mixture needs a flux, as in the case of the more refractory silicon and nickel, such flux as sodium or ammonium fluoride or others mentioned above will be mixed in very minor portions therewith and the slurry made up with a temporary binder, adhered by painting upon the molybdenum base, and then sintered at a temperature below about 1000 C., such as about 900 to 1000 C.

In an alternative method, the elements are obtained in a lower fusing temperature condition by prealloying them to a eutectic mixture, such as by preliminarily heating the nickel with the silicon in desired eutectic proportions; the prealloyed mixture is then ground to fine particles, suspended as a slurry or paint with a temporary binder which is then painted upon the molybdenum, the coating is dried and finally sintered at a temperature between about 900 and 1000 C. Even such prealloyed mixture may have the sintering temperature substantially reduced by adding fluxes thereto, such as sodium fluoride or other fluxes mentioned.

As another alternative procedure, instead of merely prealloying to desired eutectic of, for example, 5 9 a d nickel, the silicon may be fused together with sufficient nickel to form an alloy or compound corresponding to tri-nickel disilicide (Ni si a separate quantity of nickel and silicon reacted to form nickel silicide (NiSi), and the two compounds blended to the desired eutectic of minimum fusing point, which mixture is then formed into a slurry and painted upon the molybdenum base and ultimately fused thereto.

Again, as still another alternative procedure, where one of the elements may be applied at a relatively low temperature such as by hot-dipping molybdenum in a lowfusing element such as aluminum or magnesium, or electroplating the molybdenum in a cold state such as with elements like copper and nickel, these elements may be first applied by such relatively cold methods of electroplating or hot-dipping at a temperature below about 1000 C., and the second element, silicon, may be applied as a slurry in a carrier liquid upon the first coated element and ultimately sintered thereto at a temperature below about 1000 C. Thus, following such procedure, the molybdenum will be electroplated from a liquid bath with a thin coating of nickel, then the nickel-coated molybdenum will be painted with a liquid slurry of powdered silicon together with a temporary binder, and the dried coating will be sintered at a temperature of about 900 to 1000 C. to effect the final skin coating comprising the alloy or intermetallic composition of both silicon and nickel with the molybdenum base.

In the skin coatings hereof, the silicon may be present in quantity as low as about 5% and may vary upwards to about 50% by weight of the binary mixture with the secondary element nickel, aluminum, copper or magnesium. Each of these binary combinations gives desirable protective coatings for the molybdenum in air at temperatures not exceeding 1000 C. for long periods of time. However, within this range of proportions, it is most useful to select such proportions as give the lowest fusing temperature, in many cases obviating the need for a flux. Thus, in the case of silicon and nickel, the ratio of 35 to 45 percent, preferably about 40 percent, by weight of silicon to 65 to 55 percent, preferably 60 percent, by weight of nickel gives a minimum fusing eutectic mixture most readily applicable to the molybdenum base at low sintering temperatures not exceeding 1000 C. Of course, the wider range of proportions, as stated, may be used where a suitable flux is included in the composition. In contrast, in the case of aluminum, 5 to 20 percent, preferably about 12 percent, by weight of silicon is used to 95 to percent, preferably 88 percent, by weight of aluminum to obtain the optimum low-sintering temperature mixture. But again, the wider range of proportions may be used when a fluxing agent is included. Similar characteristic sinterings are obtained in mixtures of silicon with copper and silicon with magnesium; for example, 10 to 30 percent by weight of silicon would be used to 90 to 70 percent of copper, preferably 20 percent silicon to 80 percent copper for optimum eutectic ratio; magnesium, when substituted, would be in the same proportion as copper.

The fluxing agent is usually used in widely variable but minor proportions such as 5 to 25 percent by weight of the composition of silicon and other modifying metal. As indicated, the purpose of the fluxing agent is merely to reduce the temperature at which the binary mixture or prealloy will sinter to the molybdenum. While this invention is not to be limited by any theory advanced herein, it appears that impurities of the mixture are oxides of the coating elements, particularly silicon dioxide, which form a coating about the silicon and tend thereby to inhibit or excessively raise the temperature for its reaction or alloying with the other element and with the molybdenum, whereby the sintering temperature becomes unduly raised. The fluxing agent tends to dissolve silicon dioxide and analogous oxides at the temperature of the sintering. The flux may be of a character either to dissolve, reduce, or

perhaps volatilize the oxide by forming volatile reaction products therewith in the attack upon the silicon dioxide.

In an alternative procedure, the binary mixture may contain minor quantities of elemental boron. For example, to 15% of the total binary mixture may be additional boron which remains in the composition upon sintering to supply a fiuxing eifect, to impart also to the skin a flexibility and resistance to blistering in the sintering, and to give an even and smooth coat thereover. Such fiuxing eifect of elemental boron is particularly noticeable when the binary mixture is applied in proportions which are varied beyond that which is critical for a low-fusing eutectic composition, as set forth above.

The actual composition of the skin formed upon sintering with the molybdenum is one in which the quantity of molybdenum will vary from 50 to about 75%, increasing in molybdenum from the lowest to the highest in the region wherein the molybdenum is penetrated, the quantity of the molybdenum varying progressively to approach the pure molybdenum body. It appears that the predominant molybdenum compound in the skin is molybdenum disilicide and alloys of molybdenum with the other element such as nickel, copper, aluminum or magnesium. The compounds which are analogous to the binary mixture of the two elements applied as a binary complex or ternary complex with the molybdenum will also be present. Thus, the skin may also contain such components or compounds as, in the case of nickel, nickel alloyed with molybdenum, nickel silicides, nickel molybdenum silicides and, where boron has been added, there will be analogous complex borides of these several elements in admixture therewith as an alloy or intermetallic composition with the molybdenum. Similarly, where other elements such as aluminum, copper or magnesium have been substituted for the nickel, analogous compounds will be present.

It will be seen in the companion applications above referred to, that siliconizing at high temperatures substantially protects a refractory metal, particularly molybdenum, reacted therewith as a fused skin coating, alloy, or intermetallic composition with the molybdenum. It will accordingly be apparent that the present invention in its broader aspect provides a siliconized worked refractory metal, such as molybdenum, which has been siliconized by mixtures of other elements with the silicon to reduce its sintering temperature and thereby avoid loss of the worked characteristics of the molybdenum. The several elements nickel, aluminum, copper and magnesium, alone or in combination with a flux, and sometimes with boron, provide such means for reducing the sintering temperature of the silicon whereby it may be applied as a sintered coating upon the molybdenum at temperatures below about 1000 C. to impart, at least by the siliconizing, a substantial resistance to oxidation in air at high temperatures, but not exceeding about 1000 C., since these elements in combination with the silicon allow such reduced temperature sintering with or without a flux or with boron.

Other elements useful for reducing the sintering temperature of the silicon for purposes of siliconizing molybdenum at low temperatures may be substituted. However, further outstanding advantages are present in silicon combined with one of these several elements, nickel, aluminum, copper and magnesium, one of which is that the skin coating upon the molybdenum when one such element is present with the silicon has the further property of modifying the creep characteristics of the skin coating to approximately that of the molybdenum, whereby the coating is stable in long use and resists crazing and cracking. This is particularly desirable under the more rugged uses of the molybdenum which has been worked to allow such more rugged use of the molybdenum because of its greater strength. Another advantage in these silicon modifying elements for a coating is that they in themselves tend to enhance the resistance to oxidation in air when combined with the silicon. For this reason, their use in substantial proportions, and a proportion substantially exceeding that of the silicon as indicated above, are specifically desirable because of this property to enhance the resistance of the silicon to oxidation in air at the intermediate temperature not exceeding 1000 C.

Hence, while in its broader aspect the present invention provides a low temperature siliconized worked molybdenum or other refractory metal, in its more specific aspects the invention is a siliconizing of molybdenum with these particular elements applied for optimum sintering with or without fluxes and with or without boron, at temperatures not exceeding about 1000 C.

In the coatings referred to herein, the increased thickness of the coating upon the molybdenum base does not represent the total thickness of the coating per se inasmuch as in the application of the coating by sintering there is substantial penetration of the molybdenum base. However, the thickness of coating as referred to herein is a thickness increase measured from the original thickness of the metal prior to coating, and not the actual thickness of the alloy or intermetallic composition imparted as a skin to the molybdenum. Such thickness of coating usually exceeds about 0.5 mil and may range from about 1 to 5 mils, the thicker coatings usually afiording the greater protection.

Substantial penetration and interaction of the coating elements take place with the molybdenum, variable somewhat with the time and degree of heating as well as the method of application. For example, to effect penetration, the molybdenum, coated with a suspension of powdered or prealloyed elements dried thereto as a superficial skin coating with a temporary binder, must be heated at the sintering temperature for at least about 2 minutes and preferably at least about 5 minutes, say about 5 to 20 minutes; but heating for a period substantially exceeding 5 minutes does not considerably vary the protective results obtainable from such sintered coating. The temperature of sintering should not substantially exceed 1000 C., and in no case the actual recrystallizing point of the base metal. In the case of molybdenum, this is generally at temperatures between about 1100 and 1200" C., depending upon the purity thereof. Better penetration of the sintering coating is obtainable in most cases with the highest permissible temperature, i.e., about 1000" C. Lower temperatures than this, such as below about 900 C., give progressively inferior coatings, resulting from incomplete or non-penetrating sinterings.

The optimum protective condition of the skin coating is believed to be present when the ratio of the silicon to the molybdenum approximates the composition of molybdenum disilicide, i.e., approximately 37% silicon on the basis of the molybdenum present. Such composition varies progressively outwardly of the coating, and has progressively decreased proportions of molybdenum and greater proportions of silicon and one of the elements selected from the group consisting of nickel, aluminum, copper and magnesium, with or without boron, as pointed out above; and it varies progressively inwardly as the molybdenum is penetrated with the coating elements, progressively increasing in molybdenum content to the pure molybdenum base.

According to the preferred procedure herein, the silicon together with one of the elements nickel, aluminum, copper and magnesium, as powdered mixtures or as mixtures which have been prealloyed by heating and then powdering, with or without boron, and with or without a flux as may be necessary, are suspended as finely powdered particles not exceeding mesh and usually substantially less, such as less than about 325 mesh powders, in a liquid paint composition comprising a resin, preferably in alkyd resin such as Glyptal, dissolved in a volatile solvent. Coatings of the powders upon the molybdenum are mechanically applied by painting a slurry of the powdered coating materials in the liquid carrier upon the worked molybdenum base metal or upon a worked molybdenum clad base metal, such as steel, for example. The

elements are temporarily bonded by the carrier resin to the molybdenum by drying the wet coatings, and are subsequently sintered to an alloy or intermetallic composition of the applied elements as a skin upon the molybdenum to form an alloy or intermetallic composition therewith by heating at a sintering temperature of about 1000 C. or less. It will be understood that the molybdenum base metal, or other base metal which has been clad with molybdenum, has been worked prior to coating to develop maximum strength, and this coating and heat treatment are such as will not substantially affect the worked strength characteristics of the base metal.

The molybdenum may be first coated with a liquid slurry of one element, dried and sintered, then coated with a liquid slurry of another element, and dried and sintered. Preferably the molybdenum is either coated with a mixture of the finely powdered elements, with or without a flux and with or without boron, or with a prealloyed composition formed by presintering the binary mixture of elements, then adding a flux where necessary, or the element boron, then coating the molybdenum base with the same, suspended as a slurry in the liquid carrier and temporary binder, which is dried and sintered as described before, the procedure being repeated in several coatings until the desired thickness of coating of about 1 to mils is obtained. Thus, it may take 2 to 5 successive painted and dried coatings, with or without intermediate sinterings, to obtain the desired thickness of coating upon the molybdenum. Obviously, it is desirable in any case that the coating be applied uniformly in even thickness over the molybdenum to obtain a uniformly thick sintered coating. In the sintering, and primarily to prevent oxidation of the elements in air at raised temperature, which tends to raise the sintering temperature thereof, the heating will be effected in a reducing atmosphere such as hydrogen.

THE PAINT RESIN The resin of the paint is selected to be, preferably, a thermoplastic resin because of the superior properties of such resins, such as volatilizing and/or decomposing at the high temperatures used for sintering; leaving no substantial carbonaceous residue; and imparting fluidity, smoothness, and even thickness to the paint. Any typical thermoplastic paint resin which leaves no substantial carbonaceous residue when heated to temperatures approximating that of sintering, i.e., about 800 to 1000 C., may be used herein. Such resin will be understood to be a typical bonding resin to firmly adhere the powdered coating elements to the molybdenum and maintain the adhesion until the resin is completely volatilized and/or decomposed.

Alkyd resins are superior in this respect since they may be applied as a very smooth and even coating, leaving substantially no carbonaceous residue when heated. Thermosetting resins, such as Bakelite, are generally undesirable but may be used. Typical alkyds, such as are formed by reaction of polybasic organic acids or their anhydrides, such as phthalic acid, succinic acid, adipic acid, or their anhydrides, etc., with a polyhydroxy aliphatic alcohol such as glycerin, ethylene glycol, etc., are most desirable thermoplastic resins for use herein, of which the reaction product of phthalic anhydride with glycerin, i.e. Glyptal resin, is preferred. The resin is applied in proportions of from 5 to 15%, usually about by weight of the liquid carrier solvent.

THE SOLVENT Desirable solvents, particularly for alkyd resins, are ketones. Thus, there may be used acetone, methyl ethyl ketone, diethyl ketone, diacetone alcohol and, preferably for Glyptal, a mixture of diacetone alcohol and acetone in a ratio of about 7:3 by volume.

SILICON The silicon is preferably fine commercial elemental silicon, usually about 97% pure, used as a very finely powdered fraction. The silicon will be powdered to a particle size less than about 100 mesh, and usually will be less than 325 mesh size or even finer. A desirable form of silicon is obtained by further classifying 325 mesh silicon by stirring a slurry thereof in water or alcohol and pipetting otf successive portions near the upper surface to obtain an extremely fine elutriated silicon in this manner.

NICKEL, COPPER, ALUMINUM AND MAGNESIUM These elements are obtained in powdered form commercially in particle size usually less than 100 mesh and have a purity usually exceeding about such as 98% or purer. It is preferred that these elements be substantially free of oxides, which tend to raise the melting point thereof. However, other usual impurities which do not affect their normal melting points may be present. It is desirable that the elements be used even finer; for example, 100 mesh screened particles are available of which about 95% will pass a 200 mesh screen. Such finely powdered product may be obtained by ball-milling, preferably in the presence of inert or reducing gases; even finer particle sizes, i.e. powdered particles which will be finer than 325 mesh, may be obtained by elutriating the powdered suspension as described for silicon.

THE FLUX As indicated, typically useful fluxes are fluorides of alkali metals and ammonium and oxidized compounds of boron, such as borates of the alkali metals including the tetraborates, as well as oxides of boron such as boric oxide. Such fiuxing compounds tend to volatilize and/or decompose, but at least are fused at the sintering temperatures and are characterized by their ability to dissolve oxides of the elements hereinabove listed. Particularly in the case of oxides of silicon, it is believed that the fluoride type flux will dissolve the oxide and form volatile fluoride complexes therewith at the sintering temperature.

Other fiuxing materials known in the art, particularly typical welding fluxes which are fusible at the sintering temperature herein, may be used. In use, the flux, in ratio of about 5 to 25%, usually about 8 to 12%, by weight of the coating elements, may be ground therewith or added as a fine powder not exceeding about 100 mesh, and suspended in the carrier together with the elements used for coating, as described above, when needed to reduce the sintering temperature thereof.

BORON The boron used may be a commercial grade comprising about 91% of elemental boron, the remainder of which comprises the usual impurities. The boron is similarly used as a finely powdered product of a size comparable to that of the other elemental powders applied. It has some fiuxing effect to reduce the temperature of the sintered mixture of other elements used with the silicon, but primarily it is used to impart to the molybdenum an even ductile coating which tends to resist flaking and blistering when it is sintered and in ultimate hightemperature use.

PRESINTERING AND MIXING As indicated above, the silicon may be prealloyed as by presintering with one of the elements nickel, aluminum, copper or magnesium before coating as a skin upon the molybdenum. This presintering is effected by first mixing the powders in desired proportions, i.e. silicon with nickel, or silicon with one of the other elements, with or without boron, as finely divided powders, as described above, then heating to any temperature necessary to sinter the same to an alloy or intermetallic composition, such as at a temperature from 700 to 1500 C., in an inert atmosphere such as hydrogen, after which the cooled sintered product is finely ground to a powder of THE COATING COMPOSITION The resin is first dissolved in a high boiling solvent, as above identified, preferably Glyptal resin in a solvent comprising a 7:3 volumetric mixture of diacetone alcohol and acetone, in proportions of about to 15% by weight thereof, usually about by weight thereof. The powders are then stirred into the resin solution in the desired ratio to form a smooth paint slurry. The worked molybdenum or worked molybdenum clad metal in the form of wire, rod, flat sheet or irregular shapes is then coated with the paint slurry by hand brushing, dipping, or spraying with an airbrush type sprayer. The wet coated metal is then placed in an air drying oven or allowed to dry at any moderate temperature including air temperature, but usually not exceeding 110 C., to evaporate the solvent and produce an apparently dry and adherent coating of powder which may contain a flux temporarily bonded to the molybdenum with the resin.

SINTERING The coated worked molybdenum has its coating sintered by heating for a short period of time exceeding about 2 minutes, usually 5 to 10 minutes, in an inert or reducing atmosphere, such as hydrogen, in the temperature range of about 900 to 1000 C., whereby the resin binder,

such as Glyptal resin, is volatilized and/or decomposed, leaving substantially no residue of carbon; simultaneously, when heated in this range, with or without a flux as may be necessary, the silicon and one of the elements nickel, aluminum, copper or magnesium, with or without an addi tional quantity of boron, become sintered to an alloy or intermetallic composition skin including molybdenum upon the molybdenum. Alternatively, the molybdenum may have been first coated such as by electroplating with nickel, copper, or aluminum, or coated by hot-dipping in liquid metallic aluminum or magnesium, then coated with silicon powder alone, with or without a flux, and with or without boron, suspended as a powder in the liquid carrier, dried and then heated to a sintering temperature in hydrogen.

It is often desirable for successive applications of coatings to lightly abrade the coated surface to remove portions of the coating that may not have been tightly adhered by sintering, prior to applying the next coating for optimum adhesion of the coating and to obtain a final coating of even thickness and smoothness throughout. Usually, however, such abrasion may be applied only to the final sintering.

Several types of furnaces may be used to effect the sintering. Heating may be in a muffle furnace or in an induction heated furnace, or the molybdenum metal base may form the resistance of an electrical circuit whereby it becomes heated to the desired temperature range. This latter form of heating is herein termed electrical resistance heating. If desired, the dried coated molybdenum may be preheated in a reducing atmosphere to an intermediate temperature to more firmly set and partially or entirely decompose or volatilize the binder prior to heating at the sintering temperature.

Example 1 A mixture was made of 40 parts by weight of elemental silicon, 60 parts by weight of nickel, 10 parts by weight of powdered sodium fluoride, and 2 parts by weight of Glyptal resin. The mixture was wet with diacetone alcohol and acetone in the volumetric ratio of 7:3 to form a slurry of paint consistency. The nickel used was finely powdered nickel of 98% purity. The silicon used was 97% silicon. The powdered elements were obtained by milling in a ball-mill and the fractions used were separated by elutriation with water.

The following particle sizes for silicon were obtained:

. Percent Finer than 31 microns 31 Finer than 19 microns 28 Finer than 10 microns 20 Finer than 5 microns 12 Finer than 3 microns 7 Finer than 1 microns 2 The nickel had the following particle sizes:

Percent +31 mesh 3.1

--200 mesh+325 mesh 25.2

325 mesh 71.7

The slurry was applied to molybdenum having the usual enhanced mechanical strength obtained by a working. The single wet coating applied by brush or spray gun had a wet thickness of about 1.5 to 2 mils. The coated specimen was dried in air for about 15 minutes, then at 110 C. for about 30 minutes. A specimen was mounted as a filament between water cooled electrodes and heated by passing electric current therethrough while in an atmosphere of hydrogen gas. It was heated to 1000 C. and held at that temperature for 5 minutes. The cooled specimen was then recoated with the same slurry, dried and resintered in the same manner. A third coating was applied in the same manner and the product brushed and scraped to leave an even, sintered silicon-nickel coating of about 2 to 3 mils thickness. If desired, prior to coating, the molybdenum may be cleaned by rubbing lightly with emery paper and wiping clean with a cloth.

A cold-drawn -mil rod sample treated in the manner described above was tested against oxidation by heating in air to 900 C. It lasted more than 750 hours.

Example 2 A series of similar samples of the same composition as in Example 1 were made up and tested by varying the sintering time, the tests being performed by heating the samples obtained by sintering for various periods and finally testing by heating to 900 C. in air.

Heated in Air Hours sintering Time at 1,000 0., Minutes l Coating still on test after this time interval.

Example 3 Other variations of conditions are shown in the following table.

SIIIiIO'IERING CONDITIONS DURING COATING AND LIFE IN URS OF SO-MIL MOLYBDENUM ROD COATED WITH NICKEL AND SILICON IN THE RATIO OF 60 TO 40 Coatlng scraped after final sinter, before thickness of coating was measured.

! Specimen still on test.

11- Where an initial coating is sintered, scraping immediately before the application of the next coating may be unnecessary since unsintered material which would be scraped off may be adhered with the second application of a coat of powdered materials applied therewith.

A satisfactory coating of silicon and nickel was developed with three cycles of painting and sintering. The efiect of variations of the quantity of fluxing agent is indicated in this table, but it will be noted that no marked improvement of the sintered coating appears with such variation of quantity of the fiuxing agent. However, improved efiects are noted by heating at 1000 C. as compared to 900 C.; more resistant coatings are obtained with increased thickness of the coating. Sintering time should be at least two minutes, however, sintering time exceeding five minutes does not appear to efiect a marked improvement.

Example 4 A nickel coating was applied to a cold worked 80-mil molybdenum rod to a thickness of approximately 0.5 mil by electroplating. Thereafter the nickel coated molybdenum wire was painted with a slurry of -325 mesh silicon powder suspended in a 10% solution of alkyd resin in a 7:3 mixture of diacetone alcohol and acetone. The wet coating was dried at 110 C. and sintered at 1000 C. by electrical resistance heating in hydrogen. The coating operation was repeated until a total thickness of 3 mils was obtained. The coating, without using a flux, was found to resist oxidation in air at a temperature of 900 C. for a period exceeding 300 hours.

Example A mixture of 40 parts of silicon and 60 parts of finely divided nickel, both as powders averaging less than 325 mesh particle size, was prealloyed by sintering in hydrogen to form a eutectic mixture of nickel silicides comprising di-nickel trisilicide, Ni Si and nickel silicide, NiSi. The eutectic mixture of nickel-silicon compounds was milled to a particle size averaging less than 325 mesh, suspended as a slurry in a 10% Glyptal solution in diacetone alcohol and acetone in a 7:3 volumetric ratio, and painted upon cold worked 80-mil molybdenum rod. The coating was dried and sintered at 1000 C. and recoated with intermediate sinterings, to a total sintered thickness of approximately 3 mils. It was found to resist oxidation in air at 900 C. for a period exceeding 700 hours.

Example 6 In a similar manner, 12 parts by weight of silicon and 88 parts by weight of aluminum were prealloyed by presintering in hydrogen. The presintered mix was finely milled to a particle size less than 325 mesh and suspended in the same liquid carrier as in the previous examples, and sintered in a series of coatings and sinterings upon molybdenum for three coats to obtain a coating thickness of 2.5 mils. The sintering temperature used was 900 C. The cold worked 80-mil molybdenum rod which formed the base metal thereof lasted over 300 hours in air at 900 C.

Example 7 The following table, generally repreating the coatings of Example 6 but varying the proportions of aluminum to silicon, indicates in several mixtures where boron is used that the boron generally improves coatings, particularly wherein the ratio of silicon to aluminum is higher. In these mixes the prealloyed powder is as stated in the table, and where boron was used, one part of boron was added to nine parts of the powdered mixture of silicon and aluminum. It may be noted that in the examples of this table the silicon and aluminum are present as a prealloyed mixture of these elements with each other and with powdered boron. In contrast, the simple mixture of elements, in the absence of prealloying of the silicon and aluminum, does not give a good sintered coating at temperatures of 1000 C. or less without a flux.

LIFE IN HOURS OF GOLD WORKED -MIL MOLYBDENUM ROD COATED WITH SILICON-ALUMINUM ALLOYS OF DIFFERENT COMPOSITIONS No. of sintering Testing Lite, Weight Ratio of Si to Al Sintered Temp., Temp., Hours Coats 0. 0.

l Prealloyed powder mixed with boron. in ratio of 9 to 1.

Example 8 A mixture of silicon and copper, in the ratio of 20 parts of silicon to 80 parts of copper by weight, was suspended as a slurry in a 10% Glyptal resin solution in diacetone alcohol and acetone in a volumetric ratio of 7:3 and painted upon cold worked 80-mil molybdenum rod. Three coatings with intermediate sinterings, as in Example 1, were applied to obtain a total coated thickness of approximately 3 mils, the sintering temperature being approximately 1000 C. The mixed powders sintered satisfactorily without a flux and without prealloying. The coated rod was found to resist oxidation in air at 900 C. for a period exceeding hours.

Example 9 Thirty parts of silicon powder and 70 parts of magnesium metal free of oxides were prealloyed in hydrogen and mixed with sodium fluoride as a flux in the proportion of 10 parts of prealloyed silicon and magnesium to 1 part of sodium fluoride. The material was reduced to a particle size of less than 200 mesh and suspended as a slurry in 10% Glyptal solution in diacetone alcohol and acetone of 7:3 volumetric ratio. The slurry was painted upon worked 80-mil molybdenum rod, dried and sintered at 1000 C. in hydrogen, the coating and sintering operation being repeated to produce a coating of approximately 3 mils in thickness. The coated molybdenum rod was found to resist oxidation in air at 900 C. for a eriod exceeding hours.

As shown by the foregoing, very desirable coatings on worked molybdenum and similar refractory metals may be applied to protect the worked metal in air at temperatures below which the worked characteristics thereof will not be impaired, by mixing silicon with alloying components to reduce the sintering temperature thereof sufiiciently to allow application at a temperature below about 1000 C. Such alloying components as nickel, aluminum, copper and magnesium used with the silicon satisfactorily allow coating thereof by sintering at such temperatures; give siliconized coatings of highly improved oxidation-resistant characteristics while producing an alloy or intermetallic composition as a skin upon the molybdenum of creep resistance comparable to the base metal; and provide a stable and non-crazing coating. Substantial improvement in the application of such coating may be effected by inclusion of minor quantities at boron, particularly to allow a more ductile coating which is non-blistering or flaking, and to allow wider variation in proportions of eutectic mixtures with silicon of the above stated modifying components.

The invention is not to be limited to the in situ forms of molybdenum-silicon modified with an element selected from the group consisting of nickel, aluminum, copper and magnesium and, if desired, with further inclusions of boron, applied in some instances with a flux, as a skin upon a molybdenum base, since said skin may be preformed and applied either to a molybdenum base or to a base of another metal or alloy such as steel, tantalum, columbium, titanium, etc., which is to be protected from high temperature oxidation. This may be done by having a skin of molybdenum first applied to a steel base, for example, as by cladding or by electroplating same or by other known means for obtaining a molybdenum coating upon a metal base, the said molybdenum coated metal being worked in a manner known in the art to first impart thereto high worked strength characteristics, and then having a protective skin of the character described applied thereto at a temperature not substantially exceeding 1000 C., according to the procedures taught herein.

We claim:

1. As an article of manufacture, a refractory metal body comprising a molybdenum base protected against oxidation in air at elevated temperatures by an exterior layer composed predominantly of an alloy or intermetallic composition of molybdenum, silicon, and nickel in proportions by weight of approximately 50 to 75% of molybdenum, the balance being silicon and nickel in the approximate ratio of 40% silicon and 60% nickel.

2. As an article of manufacture, a refractory metal body comprising a molybdenum base protected against oxidation in air at elevated temperatures by an exterior layer composed predominantly of an alloy or intermetallic composition of molybdenum, silicon, and aluminum in proportions by weight of approximately 50 to 75% molybdenum, the balance being silicon and aluminum of approximately 11 to 12% silicon and 89 and 88% aluminum.

3. As an article of manufacture, a refractory metal body comprising a molybdenum base protected against oxidation in air at elevated temperatures by an exterior layer composed predominantly of an alloy or intermetallic composition of molybdenum, silicon and copper in proportions by weight of approximately 50 to 75 molybdenum, the balance being silicon and copper in the approximate ratio of about 20% silicon and about 80% copper.

4. As an article of manufacture, a refractory metal body comprising a molybdenum base protected against oxidation in air at elevated temperatures by an exterior layer composed predominantly of an alloy or intermetallic composition of molybdenum, silicon, aluminum and boron in proportions by weight of approximately 50 to 75% molybdenum, the balance being silicon, aluminum and boron in the approximate ratios of about to 50% silicon, 95 to 50% aluminum, and 5 to 15% of boron based on the combined proportions of the silicon and aluminum.

5. As an article of manufacture, a refractory metal body comprising a molybdenum base protected against oxidation at elevated temperatures by an exterior layer composed predominantly of an alloy or intermetallic com ssition with the molybdenum base of silicon and at least one element selected from the group c-nsisting of nickel, aluminum, copper and magnesium, the silicon and the selected element of said group being present in said exterior layer in the following proportions: of the total weight of the silicon and the selected element, from about 5% to about 50% by weight being silicon,

and from about to about 50% by weight being the selected element.

6. As an article of manufacture, a refractory metal body comprising a molybdenum base protected against oxidation in air at elevated temperatures by an exterior layer composed predominantly of an alloy or intermetallic composition with the molybdenum base of silicon, boron and at least one element selected from the group consisting of nickel, aluminum, copper and magnesium, the silicon, the boron and the selected element of said group being present in said exterior layer in the following proportions: of the total weight of the silicon and the selected element and the boron, from about 5% to about 50% by weight being silicon, from about 95% to about 50% by weight being the selected element and from about 5% to about 15% by weight being boron.

7. As an article of manufacture, a refractory metal body comprising a molybdenum base protected against oxidation at elevated temperatures by an exterior sintered layer composed predominantly of an alloy or intermetallic composition with the molybdenum base of silicon and at least one element selected from the group consisting of nickel, aluminum, copper and magnesium, the silicon and the selected element of said group being present in said exterior sintered layer in the following proportions: of the total weight of the silicon and the selected element, from about 5% to about 50% by weight being silicon, and from about 95% to about 50% by weight being the selected element.

8. As an article of manufacture, a refractory metal body comprising a molybdenum base protected against oxidation in air at elevated temperatures by an exterior sintered layer composed predominantly of an alloy or intermetallic composition with the molybdenum base of silicon, boron and at least one element selected from the group consisting of nickel, aluminum, copper and magnesium, the silicon, the boron and the selected element of said group being present in said exterior sintered layer in the following proportions: of the total weight of the silicon, the selected element and the boron, from about 5% to about 50% by weight being silicon, from about 95% to about 50% by weight being the selected element, and from about 5% to about 15% by weight being boron.

References Cited in the file of this patent UNITED STATES PATENTS 1,939,712 Mahoux Dec. 19, 1933 1,948,505 Bray Feb. 27, 1934 2,110,893 Sendzimir Mar. 15, 1938 2,305,555 Peters Dec. 15, 1942 2,313,410 Walther Mar. 9, 1943 2,612,442 Goetzel Sept. 30, 1952 2,665,475 Campbell Jan. 12, 1954 2,682,101 Whitfield June 29, 1954 2,690,409 Wainer Sept. 28, 1954 2,763,921 Turner Sept. 25, 1956 2,772,985 Wainer Dec. 4, 1956 FOREIGN PATENTS 169,916 Austria Dec. 23, 1951