US3211572A - Coating metal surfaces with refractory metals - Google Patents

Description

A. R. GLOBUS 3,211,572

COATING METAL SURFACES WITH REFRACTORY METALS Oct. 12, 1965 Filed March 27, 1963 INVENTOR ALFRED GLOBUS BY W l Qywma ATTORNEYS United States Patent O 3,211,572 COATING METAL SURFACES WITH REFRACTORY METALS Alfred R. Globus, Forest Hills, NX., assignor to Consolidated Astronautics Inc., Long Island City, N.Y., a

corporation of Delaware Filed Mar. 27, 1963, Ser. No. 268,440 7 Claims. (Cl. 117-50) This invention relates to new and useful improvements in the coating of metal surfaces with refractory metals.

The invention more particularly relates to a process for depositing refractory metals, such as tungsten, tantalum, molybdenum, and the like on porous metal surfaces, such as sintered iron surfaces and the like. The invention also relates to certain novel products produced by this process.

The invention and its objects will be fully understood from the following description read in conjunction with the drawing which is a flow-sheet depicting the process in accordance with the invention.

In accordance with the invention the metal surface to be coated is first impregnated with a molten light metal, such as sodium, potassium, cesium, rubidium, calcium, magnesium or lithium. The impregnated surface is then contacted with a refractory metal halide vapor under reaction conditions of elevated temperature and preferably reduced pressure causing exchange of the halogen atom between the refractory metal and light metal, depositing the refractory metal and converting the light metal to the halide salt.

The starting metal surface to be coated may be any metal surface which is sutliciently porous to allow impregnation with the molten light metal, at least to a superficial depth. Preferably, the porous metal surface is the surface of a sintered metal article made by the conventional powdered metallurgy techniques, as for example, a sintered iron article made by pressing iron powder to a density short of the theoretical, and then sintering in the conventional manner in order to develop adequate shape followed by final finishing, if necessary or desired.

In addition to sintered iron surfaces, the process in accordance with the invention is also applicable for the coating of the surfaces of other sintered articles, as for example, porous stainless steel crucibles, chromiumcobalt alloy jet turbine blade and pump parts of chromium iron.

The metal part to be treated in accordance with the invent-ion is immer-sed in a molten bath of the light metal, such as the sodium, potassium, lithium or calcium so that the light metal impregnates the surface to at least a superficial depth and preferably to a depth of about 1 to 2 mm.

The actual depth of penetration depends upon the porosity of the surface, the pressure of the molten liquid as is determined by the depth of the bath and whether external, such as inert gas pressure is applied, and the capillary and wetting effect of the light metal on the metal surface.

The impregnated surface is then contacted with a refractory metal halide vapor under conditions of tem- 3,211,572 Patented Oct. l2, 1965 perature at which a reaction will take place with an exchange of the halide atoms between the refractory metal and the light metal, reducing the refractory metal to metallic form and converting the light metal to the halide salt. Thus, for example, when employing parts made -of iron whose surface is impregnated with sodium and contacted with the vapor of molybdenum pentachloride, the following reaction takes place:

Suitable refractory metals include any high melting heat, wear or chemical resistant metals, which are available in the form of halide vapors under practical operational conditions. These include: Tungsten, tantalum, molybdenum, zirconium, chromium, columbium, vanadium, and also boron and silicon.

Examples of the halides include: fluorides, chlorides, bromides and iodides.

The reaction at ordinary pressures generally requires temperatures in excess of 800 degrees C., as for example temperatures between 900 and 1000 degrees C., but under vacuum, as for example a vacuum between about l and 25 micron/Hg, this temperature may be reduced to as low as 500 or 600 degrees C. due to the fact that the volatilization of the alkali or alkaline earth metal becomes appreciable at these temperatures under vacuum as does the halide formed. The light metal halide formed must diffuse from the surface, as a complete coating of this halide on the surface will block the reaction from proceeding further. Under the reaction conditions the halide of the reactive metal is removed as rapidly as it forms since it is volatilized and `carried away by the stream of excess refractory metal halides. For this reason while the reaction proceeds rapidly and satisfactorily with sodium or potassium at ordinary pressures, it is preferable to use a fairly high vacuum, as for example 1 to 25 micron/Hg when using calcium or magnesium.

It is preferable to use a halide which will produce a soluble salt in order to facilitate the cleaning of this salt after the reaction, as for example, by washing with water. For this reason, when using calcium, the chloride or bromide is preferable to the fluoride as the fluoride salt is not water-soluble. Also the halides having the metal in the lowest valence state possible to effect higher proport-ional deposition are preferred.

It is also possible in accordance with the invention to effect the coating with more than one refractory metal by utilizing a mixture of refractory metal halide vapors or by contacting the impregnated :surface with several different vapors `in sequence. Thus, for example, by mixing the vapor of tungsten and molybdenum chloride, the subsequent metallic surface will be coated with mixed tungsten and molybdenum which, however, may not be a true tungsten-molybdenum alloy. The surface, however, appears uniform and the acid resistance of the surface is extremely marked and will show no signs of etching even after immersion in concentrated hydrochloric acid at room temperature for 24 hours.

It is preferable in accordance with the invention to pass the refractory metal halide vapor in Contact with the impregnated surface in an inert gas stream, as for example a stream of argon.

i to any appreciable degree,

Most preferable is the use of a halide content of by weight of the gas, but for greater alloying it is possible to use as little as 3% by Weight, or for faster coating to use pure halide. The preferred procedure is to pass argon or helium over the halide of the refractory metal held at an elevated temperature so that the argon will entrain several percent by weight of the halide. This is allowed to diffuse into a chamber in which the parts are placed which is maintained at the desired temperature. As the argon-halide mixture is introduced, gradual deposition of the refractory metal of the halide takes place with the formation of an increased quantity of alkali halide in the gaseous phase. The more concentrated the refractory metal halide in the gaseous phase, the more quickly the deposition will take place and the less diffusion through the metal of the base part.

Complete reaction of all the alkali metal is necessary as otherwise the resistance of the base metal is detrimentally affected. Under high vacuum, using sodium the reaction is completed in 2 hours, but at ordinary pressures it may take 16-20 hours. The time is also a function of the gas diffusion of the halide of the refractory metal to the parts surface and the alkali halide away from the same.

Several methods can be used alternatively to determine .when the reaction is complete, depending on the metal being deposited. One is immersion in diluted hydrochloric acid (room temperature). If bubbles appear on sample, reaction is not complete. Brinnel hardness tests are applicable to the harder metals. A ferrie chloride staining technique is satisfactory for coatings to be used as protective surfaces.

The reaction is preferably effected in an enclosed reactor lined with a ceramic, such as dense alumina, zirconia,

etc. Greater efficiency may be obtained by using reactors of greater lengths or those set in series. Excellent results are also obtained when using rotary reactors which are rotated slowly.

It is possible to use the reactor for both steps, i.e. the alkali metal impregnation .and the subsequent refractory metal halide vapor contacting. In this case the reactor is rst lled With the alkali metal, then emptied of the excess alkali metal and used for the reaction with the refractory halide.

The waste gas from the reactor can be washed with Water to recover the refractory metal by precipitation as an insoluble acid or various salts of low solubility which may then be reconverted for re-use. Total recoveries of the refractory metals may be of the order of as high as 90-96%.

The coating obtained in accordance with the invention because of the gradual deposition of the metal and the comparatively high temperatures employed is diffused into the surface at some depth and is often alloyed to a great degree. The coated surface is essentially the pure deposited metal with a gradual transition from refractory metal through various alloyed compositions to pure base metal in the direction away from the surface toward the core. This is particularly true where the initial surface treated is relatively porous, as for example, in connection with sintered iron. The coatings obtained are thus entirely distinct from those obtained by ordinary plating, as for example, electrolysis or vacuum sputtering which produces a metal coating which is essentially in the form of a separate coating layer. When the initial metal treated is of a material, as for example, stainless steel, products of high chemical and heat-resistance are produced. Even with superficial penetration in the starting temperature excellent coatings are obtained. Since, however, in this latter case, diffusion does not take place such coatings are often coarsely crystalline and rough and are often well suited for the production of solid catalysts where this rough surface is of advantage.

The process in aCCOrdance with the invention may be With it micron/Hg.

used wherever it is desirable to form coatings of refractory metals, as for example, heat and chemical resistance purposes on articles. Thus, for example, coatings may be produced on titanium, beryllium, etc. which are of excellent resistance to chemicals and heat.

The following examples are given by way of illustration and not limitation.

EXAMPLE l A sintered iron plate is immersed in a molten bath of sodium so that the same is impregnated with sodium to a depth of about 11/2 The impregnated plate is then placed in a closed chamber lined with dense alumina which is maintained at a temperature of about 950 C. A stream of argon containing 15% by weight of MoCl5 is passed into the chamber in contact with the iron plate. After 20 hours the plate is removed. The plate has a coating of molybdenum the concentration of which progressively increases toward the surface with the surface constituting a pure molybdenum coating.

EXAMPLE 2 Example l is repeated, except that the sodoium impregnated iron plate is maintained in a alumina-lined high vacuum chamber maintained at a vacuum of about 10 The temperature is maintained at about 600 C. and the plate removed after about 3 hours.

EXAMPLE 3 Examples l and 2 may be repeated using in place of the iron; (a) porous stainless steel; (b) chromium cobalt alloy; (c) chromium iron; in place of the sodium; (a) potassium; (b) cesium; (c) rubidium; (d) calcium; (e) magnesium; and (f) lithium; and in place of the molybdenum chloride; (a) tungsten chloride; (b) tantalum chloride; (c) zirconium chloride; (d) columbium chloride; (e) vanadium chloride; (f) boron chloride; and (g) silicon chloride, or the corresponding tiuorides, bromides and iodides. In each case the base will be coated with the refractory metal corresponding to the halide.

While the invention has been described with reference to certain embodiments, various changes and modifications which fall within the spirit of the invention will become apparent to the skilled artisan. The invention therefore, is only intended to be limited by the appended claims or their equivalents wherein I have endeavored to claim all inherent novelty.

I claim:

1. A process for depositing refractory metal on a sintered porous metal surface which comprises impregnating the porous metal surface with a molten light metal selected from the group consisting of sodium, potassium, and lithium, and thereafter contacting the impregnated surface with an inert gas stream containing a halide vapor of a refractory metal selected from the group consisting of tungsten, tantalum, and molybdenum under reaction conditions of elevated temperature sufficient to cause exchange of the halide atoms between the refractory metal and light metal, depositing the reduced refractory metal as a surface coating diffused into the sintered porous metal surface forming an alloy therewith and converting the light metal to a halide salt.

2. Process according to claim 1 in which said contacting of the impregnated surface with a refractory metal halide vapor is effected under vacuum.

3. Process according to claim 1 in which said light metal and the halide of said metal halide vapor form a soluble salt.

4. Process according to claim 1 in which said inert gas is argon.

5. Process according to claim 4 in which said argon has a concentration of refractory metal halide vapor of about 10-25% by weight.

6. Process according to claim 1 iu which said porous 2,671,954 metal surface is a sintered iron surface. 2,689,807 7. Process according to claim 1 in which said porous 2,801,462 metal surface is a surface selected from the group con- 2,915,384 sisting of titanium ferrous metal chromium alloy and 5 2,930,712 cobalt alloy surfaces. 3,061,462 3,071,491

References Cited bythe Examiner UNITED STATES PATENTS Lewin 29-182.1 Kempe 117-107.2 Wagner et a1. 29-182.1 Walsh 117-107.2 X Homer et al 117-50 Samuel 117-107.2 Horn et a1 117-50 JOSEPH B. SPENCER, Primary Examiner.

1,306,568 6/19 Weintraub 117 107 2 10 RICHARD D- NEVIUS, WILLIAM D- MARTIN,

Examiners.

Claims (1)

1. A PROCESS FOR DEPOSITING REFRACTORY METAL ON A SINTERED POROUS METAL SURFACE WHICH COMPRISES IMPREGNATING THE POROUS METAL SURFACE WITH A MOLTEN LIGHT METAL SELECTED FROM THE GROUP CONSISTING OF SDIUM, POTASSIUM, AND LITHIUM, AND THEREAFTER CONTACTING THE IMPREGNATED SURFACE WITH AN INERT GAS STREAM CONTAINING A HALIDE VAPOR OF A REFRACTORY METAL SELECTED FROM THE GROUP CONSISTING OF TUNGSTEN, TATALUM, AND MOLYBDENUM UNDER REACTION CONDITIONS OF ELEVATED TEMPERATURE SUFFICIENT TO CAUSE EXCHANGE OF THE HALIDE ATOMS BETWEEN THE REFRACTORY METAL AND LIGHT METAL, DEPOSITING THE REDUCED REFRACTORY METAL AS A SURFACE COATING DIFFUSED INTO THE SINTERED POROUS METAL SURFACE FORMING AN ALLOY THEREWITH AND CONVERTING THE LIGHT METAL TO A HALIDE SALT.

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