US3023491A - Use of dioxane as a solvent for vapor plating molybdenum, tungsten and chromium from their hexacarbonyls - Google Patents

Description

United States Patent 3,023,491 USE OF DIOXANE AS A SULVENT FOR VAPOR PLATING MGLYBDENUM, TUNGSTEN AND CHROMIUM FROM m IEXACONYLS Elmer Robert Breining and John R. Whitacre, Dayton,

Ohio, assignors, by mesne assignments, to Union Carbide Corporation, New York, N.Y., a corporation of New York Filed Jan. 2, 1958, Ser. No. 706,843 3 Claims. (Cl. 29-194) This invention relates to coatings on the surfaces of bodies or substrates; more particularly the invention relates to the deposition of a coating by the thermal decomposition of a heat decomposable compound in the presence of vapors of the compound and vapors of a solvent for the compound. Still more specifically the invention relates to the deposition of a coating by the thermal decomposition of compounds selected from the group of molybdenum, tungsten and chromium carbonyls.

The thermal decomposition of molybdenum carbonyl, which is representative of the compounds under consideration, for example, has been known; however, to attain adequate deposition within reasonable plating times it has been necessary to employ vacuum or materially reduced pressures. This is due to the fact that the vapor pressure normally exerted by the molybdenum compounds is relatively low. While hereinafter molybdenum will be referred to in detail, it is to be understood that the same general characteristics and utilities apply to the carbonyls of tungsten and chromium.

This present invention provides for the effecting of the decomposition of the molybdenum compound While it is in the vapor state and associated with vapors of a solvent for the molybdenum compound. Further, normally associated with the vapors of solvent and vapors of the molybdenum component, in the practice of the invention, there is an entraining gas. Such combination provides for a relatively high concentration of the molybdenum compo nent in the vapor phase and relatively fast plating is achieved thereby.

It is therefore a principal object of this invention to provide a novel method for the deposition of coatings from thermally decomposable metal bearing compounds.

It is an important object of this invention to provide a novel method for the deposition of continuous, nonporous, adherent coatings on substrates of metals, including alloys such as stainless steel.

A particular object of the present invention is the provision of coated substrates in which the coating is deposited thermally by a vapor phase plated metal.

It has been found that molybdenum bearing compounds, such as molybdenum carbonyl, which is normally solid at room temperature, may be dissolved in cyclic ethers, for example, and that the molybdenum compound when the ether is heated exerts a considerable vapor pressure. This is in considerable contrast to the vapor pressure of the carbonyls (hexacarbonyls) of molybdenum, tungsten and chromium at room temperature which pressure is about 0.1 mm.

While plating with vapors from the solution in which the carbonyl component is dissolved is practical, it is preferable to increase the quantity of carbonyl in the vapor phase per unit of time by passing an entraining gas for the carbonyl component through the solution. Such entraining gas may, for example, be nitrogen, argon, carbon dioxide, or any other gas which is inert or substantially inert to chemical reaction, at temperatures up to the decomposition temperature of the metal carbonyls.

The ethers utilized in the practice of the invention should be maintained free of water and oxidizing constituents in order to avoid deterioration of the carbonyl com- 3,023,491 Patented Mar. 6, 1962 pound. In the practice of the invention the ether which is preferably employed is 1,4-dioxane. The cyclic ether exhibits a boiling point of 101 C. and when heated readily dissolves the carbonyls, which are white solid at normal temperatures and decompose thermally at about 150 C. Another cyclic compound found to be useful is tetrahydro furan; this S-member ring compound has a boiling point of about 65-66 C. and like the 6-membered ring compound dioxane is stable for the purpose of the invention.

The solid carbonyl dissolves in the dioxane only sparingly at room temperature, but is quite soluble at temperatures approaching the boiling point of the dioxane. Accordingly it is preferred to use the dioxane while heated in order to establish an adequate vapor pressure of the carbonyl. Preferably also the molybdenum component, such as the carbonyl, is provided in excess in contact with the solution in order that the solution will be maintained saturated throughout the procedure.

The coating procedure is accomplished by providing the object or substrate to be coated into the vapors containing the metal carbonyl component. The exposure to the vapors may be by a static method or by drawing the substrate through the vapors of the metal carbonyl component. Normally the temperature of the solution with which the vapors are in contact is -100 C. and the substrate itself is heated to at least the decomposition temperature of the metal carbonyl component. In the case of molybdenum carbonyl, for example, the temperature should exceed C. and preferably is well above this decomposition temperature-that is up to 315 C. (600 F.). Customarily operation in the range of ZOO-250 C. is satisfactory. The substrate temperature however will vary with the nature of the substrate, but must be sufficiently high such that it remains at plating temperature a sufficient length of time to effect a deposit. Such minimum temperature is generally about 150 C.. for a few seconds. Thicker deposits are attained by providing the substrate at a higher temperature and for a somewhat longer period of time in contact with the vapors. Thicknesses, however, may be built up by repeated presentation of the substrate to the vapors. The nature of the substrate will, of course, affect the maximum temperature to which the substrate may be heated; for example, substrates of most metals may be heated to 300 C., or more, While with many plastics a considerably lower temperature is necessary.

The invention will be more fully understood by reference to the following detailed description and accompanying drawings wherein:

FIGURE 1 schematically illustrates an apparatus arrangernent for the practice of the invention by a static method;

FIGURE 2 schematically illustrates an apparatus arrangement for the practice of the method of invention as applied to wire, tape, and similarly configurated materials; and

FIGURE 3 is a cross-sectional view of a wire metallized with molybdenum in accordance With the invention.

Referring to the drawings, the numeral 1 in FIGURE 1 designates generally apparatus useful in the practice of a static method for the substrate to be plated.

In FIGURE 1 a vessel 2 is provided with a closure 3 as of glass, for example. Within the vessel and substantially half filling the same is a solution 4 of molybdenum carbonyl in 1,4-dioxane; solid molybdenum carbonyl in contact with the solution being indicated at 5.

Through the closure 3 there is provided a stopper 6 of Teflon which receives a conduit 7 for the passage of the entraining gas to the solution as indicated at 8. The conduit 7 dips well into the solution. The numeral 9 designates an exhaust conduit passing through the stopper 10. Extending into the space 11 above the solution 4 is a substrate 12, such as a rod of stainless steel; this stainless steel rod is supported in a stopper 13 which also supports a thermocouple, the leads of which are indicated generally at 14. The rod is heated to about 277 C. (527 F.).

If desired this substrate 12 may be heated electrically, for example, during the course of the plating operation. The solution 4 is itself heated by an electric element, such as a hot plate, designated at 15 in FIGURE 1. Preferably the temperature of the solution is maintained at very nearly the boiling point of the solution in order that the quantity of carbonyl in the vapor phase will be as high as practicable. However, the solution may be as low as 90 C. and still provide adequate plating within a reasonable time.

Also the flow of the entraining gas is in the present instance such that a voluminous bubbling of the gas through the solvent is apparent. 0.25 cubic foot per minute is suflicient when about 50 grams of the solvent are employed in a beaker.

Referring now to FIGURE 2, the numeral 16 indicates a resistance heater of substantially cylindrical contour, through the center of which there passes a copper wire '17, unrolled in any conventional manner from a reel indicated at 18. The wire 17 in its traverse passes through a gas seal 19, chamber 20' and an outlet gas seal 21 to a. winding device 22. The chamber 20 is provided with an inlet port 23 and an exhaust port 24. The inlet port 23 is connected by a conduit 25 with a vaporizer 26, which itself consists of a tank 27 surrounded by a steam jacket 28. The jacket 28 maintains the temperature of the solution of dioxane and molybdenum carbonyl indicated at 29 to within the range of 90l00 C.

The entraining gas, which as previously indicated may be an inert gas, such as CO or nitrogen, is passed through a conduit 30 into the solution 29 and the vapors exit through the conduit 25 to the chamber 20. In the chamber 20 the vapors contact the wire 17, which has been heated to a temperature of 2SO275 C., and the product of decomposition passes through exhaust 24 to any suitable exhaust arrangement.

Referring now to FIGURE 3, the numeral 17 designates a section of copper wire and the numeral 18a indicates a hard coating of the metallic constitutent of the carbonyl completely surrounding the wire.

By way of further specific examples, molybdenum hexacarbonyl (or carbonyl) is placed in dioxane and the dioxane heated at a temperature of about 88 C., the hexacarbonyl readily enters soluiton and at 100 C. is completely in solution.

An iron substrate heated to 550 F. was exposed to vapors of the carbonyl and dioxane while the dioxane was heated to the boiling point. Carbon dioxide was passed through the solution and the iron substrate exposed to the vapors for a period of about 3 minutes. The deposit was hard, somewhat brittle, and provided a molybdenum coating on the substrate.

The above experiment was repeated utilizing tungsten carbonyl and maintaining the same temperature conditions, and the resulting coating was hard, also somewhat brittle, and tungsten contained.

III

Experiment II was repeated except for the substitution of chromium hexacarbonyl for the molybdenum hexacarbonyl. The resulting deposit, like the other resulting deposits, contained a metallic constituent and was hard and slightly brittle.

Chrominum hexacarbonyl was dissolved in tetrahydrofuran having a boiling point of about 63 -64 C. At 55 C. the chromium hexacarbonyl enters solution; in the present instance a saturated solution was formed as described hereinbefore. The steel substrate was heated to a temperature of 550 F. (287 C.) while the solution itself was heated to boiling with carbon dioxide passed therethrough; the temperature of the solution was indicated by thermometer in the solution to be about 170 F. C.). Exposition of the substrate to the vapors for a period of about three minutes provided a heavy coat.

Each of the carbonyls, that is the hexacarbonyl of chromium, tungsten and molybdenum, dissolves in a tetrahydro furan. While the tungsten carbonyl appeared to enter solution more easily, the products tend to be gray, but spectrographic analyses clearly indicate the presence of chromium, tungsten and molybdenum in the various experiments.

While the invention has been specifically described in connection with 1,4-dioxane and tetrahydro furan as the solvents for the carbonyls, it has been contemplated that it may be possible to employ other ethers, such as 1,3-dioxane, or mixtures of ethers, such as 1,3-dioxane and 1,4-dioxane. In'general 5 and 6 membered cyclic ether rings, which exhibit stability at the temperature of decomposition of the carbonyl, and dissolve the carbonyl component, are considered useful in the practice of the invention.

It is important to note that the pressure within the plating chamber may be substantially atmospheric and that the exhaust at 9 (FIGURE 1) and at 24 (FIGURE 2) may lead to the atmosphere.

Generally, since such gases contain in addition to the carbon dioxide and vaporized solvent some small amount of undecomposed carbonyl and some carbon monoxides, it is well to provide for the combustion of the exhaust or for the recovery thereof.

Also, it is to be noted that cooling the coated material to prevent oxidation in the atmosphere, while the coated material is highly heated, is frequently a desirable precaution against reaction of the atmosphere.

Also, the coating itself may be presented on the substate to the atmosphere to build up thicknesses where such is desired. Normally an exposure of three minutes to the vapors of a saturated solution is sufiicient to produce a coating thickness of 0.2 to 0.3 mm.

It will be understood that this invention is susceptible to modification in order to adapt it to diiferent usages and conditions and accordingly, it is desired to comprehend such modifications within this invention as may fall within the scope of the appended claims.

What is claimed is:

1. In the process of gas plating metal on metal substrates wherein deposition of the metal is effected by thermal decomposition of a compound of said metal in the vapor state and while associated with vapors of a solvent for said compound, said process comprising the steps of heating the substrate to the decomposition temperature of a metal carbonyl selected from the group of molybdenum, tungsten and chromium hexacarbonyls, dissolving the said metal carbonyl in dioxane and heating the resultant solution to form vapors of the metal carbonyl, heating the substrate to a temperature high enough to cause decomposition of the vapors of the metal carbonyl, passing an inert gas through the resultant heated metal carbonyl solution to entrain the carbonyl vapors and maintain a saturated solution of the carbonyl, contacting the heated substrate with said entrained carbonyl vapors while concurrently passing the inert gas through the saturated solution of the metal carbonyl to facilitate contact of the vapors of the metal carbonyl with the substrate whereby said vapors are thermally decomposed and the metal constituent deposited on the substrate at a relatively high rate of deposition.

2. In the process of gas plating metal on metal substrates wherein deposition of the metal is effected by thermal decomposition of a compound of said metal in the vapor state and while associated with vapors of a solvent for said compound, said process comprising the steps of heating the substrate to the decomposition temperature of a metal carbonyl from the group of molybdenum, tungstem and chromium carbonyls, dissolving such a carbonyl in 1,4 dioxane, heating the resultant solution in the presence of an excess of said metal carbonyl to provide a saturated solution of the carbonyl and vapors of the metal carbonyl, passing an inert entraining gas through the resultant heated solution to entrain the vapors of the metal carbonyl and maintain a saturated solution of the carbonyl, and exposing the substrate while heated to a temperature to cause thermal decomposition of the metal carbonyl vapor and between about 200 and 315 F. 15 2,880,067

whereby the same is thermally decomposed and the metal constituent deposited on the substrate.

3. As an article of manufacture, a gas plated article made in accordance with the process of claim 1.

References Cited in the file of this patent UNITED STATES PATENTS 2,375,482 Lyle May 8, 1945 2,477,554 McKeever July 26, 1949 2,638,423 Davis et a1. May 12, 1953 2,785,651 Pawlyk Mar. 19, 1957 2,824,828 Homer et al. Feb. 25, 1958 2,867,546 MacNevin Jan. 6, 1959 Closson Mar. 31, 1959

Claims (1)

1. IN THE PROCESS OF GAS PLATING METAL ON METAL SUBSTRATES WHEREIN DEPOSITION OF THE METAL IS EFFECTED BY THERMAL DECOMPOSITION OF A COMPOUND OF SAID METAL IN THE VAPOR STATE AND WHILE ASSOCIATED WITH VAPORS OF A SOLVENT FOR SAID COMPOUND, SAID PROCESS COMPRISING THE STEPS OF HEATING THE SUBSTRATE TO THE DECOMPOSITION TEMPERATURE OF A METAL CARBONYL SELECTED FROM THE GROUP OF MOLYBDENUM, TUNGSTEN AND CHROMIUM HEXACARBONYLS, DISSOLVING THE SAID METAL CARBONYL IN DIOXANE AND HEATING THE RESULTANT SOLUTION TO FORM VAPORS OF THE METAL CARBONYL, HEATING THE SUBSTRATE TO A TEMPERATURE HIGH ENOUGH TO CAUSE DECOMPOSITION OF THE VAPORS OF THE METAL CARBONYL, PASSING IN INERT GAS THROUGH THE RESULTANT HEATED METAL CARBONYL SOLUTION TO ENTRAIN THE CARBONYL VAPORS AND MAINTAIN A SATURATED SOLUTION OF THE CARBONYL CONTACTING THE HEATED SUBSTRATE WITH SAID ENTRAINED CARBONYL VAPORS WHILE CONCURRENTLY PASSING THE INERT GAS THROUGH THE SATURATED SOLUTION OF THE METAL CARBONYL TO FACILITATE CONTACT OF THE VAPORS OF THE METAL CARBONYL WITH THE SUBSTRATE WHEREBY SAID VAPORS ARE THERMALLY DECOMPOSED AND THE METAL CONSTITUENT DEPOSITED ON THE SUBSTRATE AT A RELATIVELY HIGH RATE OF DEPOSITION.

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