US3139658A - Production of tungsten objects - Google Patents

Description

solving out the substrate material.

United States Patent 3,139,658 PRODUCTION OF TUNGSTEN OBJECTS Abner Brenner, Chevy Chase, Md, and Walter E.

Reid, Jr., and Jean H. Connor, Washington, l).C., as-

signors to the United States of America as represented by the Secretary of the Navy N0 Drawing. Filed Dec. 8, 1961, Ser. No. 158,144

1 Claim. (Cl. 222titl) (Granted under Titie 35, U.S. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The invention relates to the formation of tungsten objects and more specifically, to an improved method of producing tungsten objects by chemical deposition from the vapor phase. The present application is an improvement over the invention disclosed in copending application Serial No. 33,063, filed May 31, 1960, now Patent No. 3,072,983, granted January 15, 1963. That application discloses a novel method of coating objects with pure tungsten, superior to any pre-existing known method. Specifically, there is disclosed a method of tungsten coating by reducing vaporous tungsten hexafiuoride in the presence of hydrogen.

There is also disclosed a method of forming objects of pure tungsten by coating tungsten by the above-described process on a pattern or a master and the dis- The present invention relates to an improvement in this method of forming articles of tungsten.

Tungsten has many useful properties, exemplary of which are its high melting point, great hardness, corrosion resistance, resistance to chemical attack and retention of mechanical characteristics at temperatures as high as 2500 C. Unfortunately, the very properties that make tungsten so useful make it difficult to fabricate. Its high melting point makes casting of the metal almost prohibitive, and its hardness makes machining difiicult. Furthermore, at room temperature, the metal in the massive state is too brittle to shape by bending and spinning. For the above reasons the development of a process by means of which tungsten could be fabricated into coniplicated shapes was long sought after, but, heretofore unattainable by the methods of the prior art.

The method described above in pending application Serial No. 33,063 was a long step forward in attaining this goal. However, the process as then conceived and practiced was not consistently successful, since cracking in the tungsten object occurred in some instances.

Accordingly it is an object of the present invention to provide a process for producing shaped objects of pure tungsten.

Another object of the invention is to produce tungsten objects in an efficient manner, free of cracking.

Still another object of the invention described herein, is to provide a method for increasing the wall thickness of intricately shaped objects of tungsten.

Yet another object is to provide articles of tungsten, formed from smooth thick coatings, and produced at a faster rate than previously attainable.

Other objects and many of the attendant advantages of this invention will become readily apparent as the same becomes better understood by reference to the following detailed description and appended claims.

The present invention relates to the vapor deposition of tungsten via the hexafluoride on a thin-walled, heated object in the presence of hydrogen. This process may be performed by any number of structural arrangements several of which are commercially available.

An example of such a structural arrangement includes a cylindrical ceramic vessel in which the object to be coated is supported. Surrounding the vessel is an electrical heating element or induction coil which is utilized to elevate and maintain the temperature of the object to be coated. The tungsten hexafluoride, a gas at room temperature, and the hydrogen gas each flow through a separate flow meter into the reaction vessel. As the gas mixture flows past the heated object the herein described reaction takes place; that is, substantially pure tungsten is deposited on the surface of the object. It will be understood that many other combinations of structural arrangements will perform this process equally as well as that ar rangement just mentioned and that the structural arrangement used to perform the herein disclosed process does not form any part of the invention and is disclosed herein only for the purpose of better describing the process.

Performance of the instant process produces a tungsten coating of a much higher quality than could be anticipated from results of tests of similar processes with other halide compounds. That is, the instant process deposits a coating of substantially pure tungsten on any object which can be raised to the reaction temperature. The deposited coating is of a quality much higher than that heretofore obtained by commercial processes. A smooth evenly distributed layer of tungsten is deposited on an object.

A very important feature of this invention is the increased rate at which the tungsten can be deposited. That is, if this process is compared to similar processes, all normally variable factors being equal, the rate at which a coating is deposited will be substantially higher. It appears that two factors permit this improved plating rate. Firstly, the absence of side reactions or, more specifically, the production of lower valent halide compounds is eliminated, secondly, the rate of reaction of the tungsten hexafluoride is greater than that of similar compounds. r

The reaction of the instant process is entirely a surface reaction. That is, the only reaction is on the surface of the object being coated. This means that there are no lower valent halide compounds formed in the gas itself which would impede the reactive process.

The process has another desirable feature in that it is insensitive to the normal variables which generally affect chemical reactions of this type. The temperature at which the reactive process takes place appears to be the only critical variable. The effect of pressure in comparison to similar processes, has been proven to be sub stantially negative insofar as the quality of the coating is concerned. The rates at which the tungsten hexafluoride and the hydrogen gas are applied to the object have been shown to not affect the quality of the coating. That is, the amount of tungsten hexafluoride gas and the amount of hydrogen gas that is made available for the reaction does not substantially affect the quality of the coating. Obviously, it would affect the rateat which the reaction takes place and, consequently, the efficiency of the reaction. That is, if the flow of hydrogen were to be entirely stopped the reaction would, naturally cease. If the hydrogen flow is much less than the flow of tungsten hexafluoride the amount of hydrogen determines the amount of tungsten deposited. As a practical matter the most efiicient operation of the process dictates that there be an excess of hydrogen. However, as mentioned above, no matter what the rate of flow of hydrogen or tungsten hexafluoride or the relative flow thereof, the coating remains of very high quality and the process does not produce lower valent fluorides.

As stated above the only critical variable is the temperature at which the reaction takes place, and the optimum temperature varies with the type of material being a coated. However, extensive experimentation has shown that a temperature of approximately 300 C. is the lowest temperature at which the reaction will take place. The term temperature of reaction is defined as the temperature of the objects or articles being coated. It has been determined that for materials, the temperature which most efficiently causes deposition of the tungsten, as well as that which gives the highest degree of adhesion is approximately 650 C. However, this temperature may be Varied depending on the material. There appears to be no definite upper temperature limit to the reactive process. However, the rate of reaction becomes quite rapid at higher temperatures and an even coating thickness is difficult to obtain above about 900 C. Again this temperature varies slightly with the type of material being coated.

The above-mentioned features of the present invention combine to give it an efliciency which heretofore, was not i found to be possible in this type of process. In other words, the percentage of tungsten that is converted to pure tungsten exceeds that of similar processes. This factor, as well as, the operational simplicity of this process as compared to similar processes make the instant process very inexpensive and commercially desirable. The tungsten coating which results from the present process is of substantially improved quality as compared to tungsten which has heretofore been produced commercially.

The deposit is in the form of a smooth coating which can be made to any thickness by continuing the deposition for an adequate length of time. In an hour or two, coatings inch thick can be produced. If necessary, the coating can be smoothed mechanically and the deposition repeated as often as required. Smooth coatings. A; inch thick have been deposited without the need of smoothing by machining.

Since thick coatings can be produced, it is now possible to form complete articles of tungsten by depositing the metal on a master or mold which is subsequently removed. The mold can consist of any one of a number of metals of which copper and nickel are preferred because they withstand the high temperature involved and can be easily removed by dissolution in nitric acid, wherein tungsten is not attacked.

As mentioned previously the formation of shaped objects on a mold has been achieved by the instant process but not consistently because of cracking. The cracking is believed to be caused by the difference in the coefficient of thermal expansion of tungsten and the master. Tungsten has a much lower coefficient of thermal expansion than most other metals. It is about 4 1O W C.

as compared to about 12X l- C. for iron and 16 10- C. for copper. This difference in thermal expansion causes the tungsten coating to crack when it is heated or cooled over the temperature interval of room temperature to 650 C. The likelihood of the tungsten deposit cracking becomes less as its thickness, and hence its strength becomes greater.

It was observed that cracking of the tungsten was less likely to occur on cooling down an object from 650 C. than on heating up from room temperature to 650 C. This is because on cooling, the tungsten shell contracted less in size than the master upon which it was deposited. Thus, it was put under a compressive stress. However, on reheating the object to build up a thicker coating, the tungsten expanded less than the master and thus was put under a tensile stress. Like many brittle materials, apparently, the tungsten could withstand the compressive stress much better than a tensile stress. Therefore, it did not crack ifslowly cooled (while on the master) down to room temperature, but cracked on being reheated. y

Since the stresses developed in heating and cooling de V pend on the strength of the master, the latter was made weaker by being used in the form of a thin shell, for

example inch thick. It is possible that if the initial tungsten coating were suiiiciently thick, the master would deform, rather than the tungsten cracking.

The cracking of tungsten objects, deposited upon masters, was obviated by three improvements of procedure: (1) The heating and cooling were done slowly. For example, the cooling was done over a period of perhaps an hour, and the heating up was done even morely slowly. (2) Masters having thin Walls were used. (3) In case the tungsten shell had to be built up thicker by a second deposition, the master was first dissolved out, leaving the tungsten shell. The latter was then unattached to any metal and could be heated and cooled with impunity.

As an embodiment of the invention, the improved process is illustrated by the production of a nozzle insert for a rocket nozzle. The item will be hereafter referred to as an elzzon.

First a mandrel is prepared. It consists of stainless steel and is made in two sections which are bolted together.

The master which is to serve as the base for tungsten deposition is electroformed upon the mandrel by depositing either copper or nickel upon the mandrel to a thickness of about inch. The outer surface of the electroform is machined to the dimensions required for the inside dimensions of the elzzon. The stainless steel mandrel is then removed by exerting a tensile stress. Metals do not adhere well to stainless steel, and the master sep-.-

arates from it without much difliculty.

' The use of the master made by this method involves a machining operation. This can be avoided by electroforming the master on the inside of a mold instead of on the outside. The tungsten is then deposited upon the surface of the master which was originally adjacent to that of the mold. This procedure is illustrated by the process of making a master for an elzzon having a throat 5 inches in diameter.

Making the master, starts with the preparation of a stainless steel mold, the inside surface of which has the contours that are desired on the inside surface of the elzzon. The mold is made into two longitudinal halves which are held tightly together by a metal band. Copper or nickel is deposited on the inside surface of the mold to the desired thickness of to Ms inch. The deposit bridges the dividing line between the two halves. The two halves of the mold are then separated, liberating and freeing the master. The outside surface of the master has the exact contour of the inside surface of the mandrel mold and, hence, requires no machining. Tungsten is deposited upon the outside surface of the master to the required thickness in the manner previously described.

In preparation for the tungsten deposition, the inside of the master is plugged with graphite or any other solid inert material, so that no appreciable deposition will take place on the interior. Deposition is to be done only on the outside surface. The master is then placed in a ceramic vessel which has the coil of an induction heat unit wound on the outside. The master is heated by induction to 650 C. while the mixture of gases consisting of tungsten hexafluoride and hydrogen in about 1 to 4 ratio by volume (the exact ratio is unimportant) is passed over the body. After about 0.01 or 0.02 inch of tungsten has deposited upon the master, the plated object is cooled down very slowly and the copper or nickel base components used in an electron beam furnace have been made using a procedure similar to that described above.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claim the invention may be practiced otherwise than as specifically described.

What is claimed is:

The method of forming an object composed of substantially pure tungsten including the steps of depositing tungsten onto a heated, solid, thin-walled master by subjecting the master to a vaporous mixture comprising tungsten hexafluoride and hydrogen, slowly cooling the coated object for the purpose of preventing cracking of said object, to a suitable temperature for inspection and measurement of said object, thereafter increasing the thickness of the object wall Without cracking same by slowly reheating said object and master in an atmosphere of tungsten hexafluoride and hydrogen, and depositing tungsten to the desired wall thickness by repetitions of the foregoing steps as necessary and desirable, slowly cooling said coated object, dissolving said master with an acid and recovering an object of substantially pure tungsten.

References Cited in the file of this: atent UNITED STATES PATENTS 193,669 Niedringhaus July 31, 1877 2,572,702 Davis Oct. 23, 1951 2,834,690 Marvin Q. May 13, 1958 2,885,310 Olson et al. May 5, 1959 3,072,983 Brenner et a1. Jan. 15, 1963 FOREIGN PATENTS 742,304 Great Britain Dec. 21, 1955 OTHER REFERENCES Powell, Campbell, Gonser: Vapor Plating, Wiley & Sons, 1955;page 56.