Method For Evaporating Alloy

Abstract

A method is described for evaporating an alloy from a single crucible in a vacuum enclosure. A solid bar of alloy is fed upwardly into the crucible from the bottom thereof. The alloy is heated to form a covering pool and to produce evaporation. The feed rate of the solid bar is regulated to maintain a substantially constant level of molten alloy relative to the crucible.

Description

This invention relates to vacuum evaporation of metals and, more particularly, to a method for evaporating a molten alloy from a single crucible in a vacuum enclosure.

Vapor deposition, that is, the evaporation of material and subsequent condensation thereof on a substrate to be coated, has been successfully used in a variety of manufacturing operations. One particular vapor deposition technique which is of advantage where high purity is desirable involves the utilization of a cooled crucible and means for heating the surface of molten material contained in the crucible. Surface heating may, for example be accomplished by means of a suitably directed electron beam, plasma beam, or laser beam. By cooling the crucible, a skull of solidified molten material forms between the molten material and the crucible wall, isolating the molten material from the crucible and preventing any reaction between the molten material and the crucible material.

Although satisfactory for many types of materials, heretofore known methods of vapor deposition have frequently encountered difficulty in evaporating alloys. This is particularly true for alloys having constituent materials of widely different volatilities, such as alloys of iron plus rare earth metals. For a given heating energy input and beam density the the evaporation rate depends upon the efficiency of the vapor deposition system. The efficiency of the system, however, is affected by a wide variety of different factors which cause changes in the heat balance of the system. Such changes can result in variation in the amount of solid alloy that lies between the cooled crucible walls and the molten alloy. The melting and freezing of alloy at the skull creates segregation of the alloy's components and thus may cause changes in the composition of the molten alloy in the crucible. This may result in an undesired variation in the composition of the deposit on the substrate. Evaporation of alloy constituents from separate crucibles simultaneously is one way of avoiding segregation problems, but greatly increases the cost and complexity of the system. Thus, evaporation of an alloy from a single crucible may be preferable.

In order to replenish the molten material in the crucible for alloy evaporated therefrom during typical vapor deposition operations, feed stock is introduced into the crucible. One form of feeding is by utilizing a wire fed into the path of the heating beam or directly into the molten alloy in the crucible through the open top of the crucible. Many important alloys are not available in wire form, however, for lack of commercial value of the alloy in wire form, or because of difficulty in fabricating the alloy because of brittleness. Alloys that are commercially available in wire form are frequently full of dissolved gas and generate a considerable amount of undesirable spitting when melted into the molten pool.

Cast alloy bars are sometimes preferable to wire for feeding purposes because cast bars are generally obtainable commercially in more pure form than wire and in a wider variety of available alloys. Such cast bars are generally greater than one-half inch in diameter requiring, in most instances, a relatively slow movement of the bar into the molten pool to replenish the molten alloy at the rate at which it is being evaporated. This may be difficult due to variations in the heat pattern of the system which make the bar melt back at varying rates and therefore cause significant changes in the actual feed rate of new alloy into the molten pool. Under such circumstances, a heat imbalance occurs in the system with the consequent undesirable changes in alloy composition previously pointed out.

A further difficulty encountered in vapor deposition systems, particularly when the crucible in which the molten alloy is contained is cooled to form a skull, is that condensate has a tendency to form between the skull and the water cooled crucible along the top edges thereof. During operation, such condensate builds up and may wedge between the skull and the crucible, leading to increased mechanical pressure and an attendant increase in the rate at which heat is transferred from the molten material to the cooled crucible. If the molten material is permitted to cool off, such as between coating operations, the solidified alloy will frequently shrink away from the supporting crucible. Upon reheating, the same mechanical pressures mentioned above will generally not develop, and the rate of heat transfer is therefore not the same as that which existed during the previously described situation. Such changes between successive operations in the ratio of the volume of liquid to the volume of solid alloy results in changes in the alloy composition of the molten pool for reasons previously described, and such changes cannot easily be predicted.

It is, therefore, an object of the invention to provide an improved method for evaporating a molten alloy from a single crucible in a vacuum enclosure.

Another object of the invention is to provide a method for evaporating alloys having constituent materials of widely different volatilities wherein improved uniformity of vapor composition is obtainable.

It is another object of the invention to provide a method for evaporating an alloy wherein the replenishment rate of evaporant is closely controllable.

Other objects of the invention will become apparent to those skilled in the art from the following description, taken in connection with the accompanying drawing in which a vapor deposition system for performing the method of the invention is illustrated schematically.

Very generally, the method of the invention comprises heating the surface 11 of a molten alloy 12 in a crucible 13 to produce evaporation. A solid bar 14 of alloy, of substantially the same cross section as the crucible interior, is fed upwardly into the crucible from the bottom thereof to replenish alloy evaporated therefrom. The feed rate of the solid bar is regulated to maintain a substantially constant level of molten alloy relative to the crucible.

Referring now more particularly to the drawing, the vapor deposition system illustrated therein includes a vacuum enclosure 16, the interior of which is evacuated by a suitable vacuum pumping system 17. A substrate 18 to be coated with a layer 19 of condensed vapor is positioned within the enclosure 16 in the path of the vapor by suitable supporting means, not illustrated.

The surface 11 of the molten alloy 12 contained within the crucible 13 is heated by means of an electron beam 21. The electron beam 21 is produced by an electron gun including an emissive filament or emitter 22 disposed within a recess in a backing electrode 23. An anode or accelerating electrode 24 is disposed adjacent the open side of the recess in the backing electrode to accelerate electrons out of the recess, as indicated by the dotted lines in the drawing. The electron beam thus developed is deflected through a generally curving path onto the surface 11 of the molten alloy 12 by means of a magnetic field having curving lines of flux which are concave with respect to the surface 11. The magnetic field is established between a pair of pole pieces 26, one of which is shown. An electromagnetic coil 27 is disposed around a low-reluctance core 28 which extends between the pole pieces 26 to establish the pole pieces at opposite polarities. Suitable means, not illustrated, are provided for supplying the required electrical potentials and currents to the various times described. An electron beam gun generally of the type described is shown and described in greater detail in U. S. Pat. No. 3,177,535.

The crucible 13 is provided with an internal passage 29 through which a suitable coolant, such as water, may be circulated. The crucible is supported within the vacuum enclosure 16 by suitable supports, not illustrated.

A large number of factors contribute to the evaporation rate. Two factors which are of substantial importance are the overall efficiency of the system and the size of the area of the surface to which heat is being added by impingement of the electron beam or other heating beam. The efficiency of a vacuum deposition system may be defined by the expression (H.sub.t /H.sub.a).times.100 where H.sub.t is the theoretical heat required to produce the evaporation rate desired and H.sub.a is the actual heat input. Thus, the fewer the heat losses in the system, the higher the efficiency will be. Where higher temperatures are required to produce evaporation, greater heat losses result, lowering the efficiency. For example, cadmium, which has a relatively low evaporation temperature, may be evaporated in many instances at efficiencies of greater than 80 percent. On the other hand, tantalum, which has a relatively high evaporating temperature, is usually evaporable at efficiencies which are typically less than 10 per cent.

As previously mentioned, the evaporation rate also depends upon the sharpness of the area of impingement of the beam on the surface 11 of the alloy 12 in the crucible 13. For given material and beam power, the larger the spot or impingement area, the lower the evaporation rate. The relationship, however, is not linear and the effects of variation in the impact area are much less for lower evaporation temperature materials. Of course, the evaporation rate becomes less if the beam is swept over the entire surface of the molten material, rather than impinging upon a particular area thereof. Such beam sweeping may be desirable to produce a more uniform evaporation from the entire surface of the molten material. Basically, however, the evaporation rate is most easily controlled by controlling the total power input or heat input of the electron beam or other heating beam, and by controlling the power density of such beam.

In order to replenish alloy evaporated from the crucible, the solid bar 14 of replenishment alloy is fed upwardly into the crucible from the bottom thereof. The bar is of the same cross section as the crucible interior and therefore constitutes the bottom or support for the molten alloy 12. The region of the bar adjacent the crucible 13 remains solid, due to cooling of the crucible, until just below the surface of the molten pool. The beam power and density are controlled so that the pool diameter at the surface is equal to the inner diameter of the crucible so the pool completely covers the solid part of the bar 14. If a fluctuation in heating occurs, causing melting of the bar 14 earlier, the pool surface area does not change, helping to keep the evaporation rate constant. Moreover, the avoidance of exposure of the solid end of the bar, and the avoidance of the formation of an exposed skull aids in maintaining a constant pool composition. The feed rate is controlled by the feed rollers 31 and 32 which engage the sides of the bar and feed it upwardly into the crucible. The rate of feed may be controlled by controlling the rate of rotation of the feed rollers 31 and 32 by suitable means, not illustrated. For operations which are to be continuous over an extended period of time, relatively long length of the bar 14 may be accommodated by feeding the bar through a suitable vacuum valve 33 in the wall of the enclosure 16. A thermal insulation 34 may be provided at the lower end of the bar 14 to minimize heat losses through the bar from the crucible.

The feed bar 14 is moved upwardly at a rate which may be appropriately adjusted by the operator through visual observation of the liquid level in the molten pool to maintain such level as nearly constant as possible. Examples of typical feed rates are set out below, and the average rate of feed depends on the evaporation rate. If an error of feed should occur, or conversely if a fluctuation in the heat balance of the system occurs, causing the liquid level of the pool to drop, the liquid-solid interface between the molten pool and the solid bar also drops, tending to keep the liquid inventory constant. This helps to maintain the evaporation rate and the pool composition relatively constant. This is of great advantage when evaporating alloys having constituents of widely different volatilities, since steady state conditions aid in maintaining constant composition in the condensate. In addition, the constant moving of the feed stock bar upwardly produces an abrading action which prevents condensate from accumulating between the bar and the walls of the cooled crucible.

By feeding the replenishment stock into the crucible in the manner described above, it is easy to maintain a constant level of the molten pool by appropriate adjustments in the feed rate. Moreover, the rate of heat transfer out of the crucible is generally constant, reducing the likelihood of changes in the skull thickness and attendant melting and freezing of metal which produces segregation and changes in the pool composition. Since the heat loss, when evaporating in accordance with the invention, may be maintained substantially constant, the pool depth may also be maintained substantially constant, creating an equilibrium condition wherein the feed rate equals the evaporation rate.

In order to more fully illustrate the advantages of the invention, the following examples are set forth. The invention, however, is not intended to be limited to such examples. Typical feed rates, in the following examples, may be about 1 or 2 inches per hour, and typical evaporation rates may be about 1 or 2 pounds per hour. The following examples illustrate that the invention is of particular advantage where elements of widely differing volatilities are present, such as with ratios of volatility of two or more to one. The actual volatility ratio may not necessarily be consistent with the volatilities of the elements by themselves, since compounds may form (due to intermetallic reactions) which have different volatilities from the elements alone.

EXAMPLE I

Evaporating an Fe 25 Cr alloy in accordance with the invention is capable or producing a condensate on a substrate which ranges from 23 to 27 percent chromium utilizing a 2-inch-diameter feed bar and crucible interior and utilizing 25 kilowatts of electron beam power. In such an alloy, the ratio of volatilities is about three to one, chromium to iron. Evaporation wherein replenishment stock is fed into a similar size crucible from the top, using similar beam power and an identical composition alloy, is typically only capable of holding a .+-.5 percent variation in chromium composition.

EXAMPLE II

Using a 2-inch-diameter crucible and a 2-inch-diameter feed stock bar of an 80 percent nickel 20 percent iron alloy, a 10 -kilowatt electron beam may be utilized to produce evaporation of the alloy with a range of chromium composition in the condensate from 79.8 to 80.2 percent. In such an alloy, the ratio of volatilities is about two to one, iron to nickel. Operations utilizing more conventional methods may be expected to produce a range of nickel composition in the condensate from 79.5 to 80.5 percent.

EXAMPLE III

Using a 2-inch inner diameter crucible and a 2-inch-diameter alloy bar comprised of 25 percent chromium, 6 percent aluminum and 0.4 percent yttrium, the balance iron, a 15 -kilowatt electron beam may be used to produce evaporation and a condensate with a composition as follows:

Chromium: 23% to 27%

Aluminum: 5% to 7%

Yttrium: 0.2% to 0.5%

Using previously known methods, typical composition percentages have about twice the composition spread. The yttrium is about one twenty-fifth as volatile as the aluminum, the chromium about three times the aluminum, and the iron about the same as the yttrium. The ratios of volatilities are not necessarily consistent with the volatilities of elements above because of the formation of compounds due to intermetallic reactions.

EXAMPLE IV

An alloy containing 18 percent chromium, 8 percent nickel and the balance iron may be evaporated in accordance with the method of the invention, using a 2-inch-inner-diameter crucible and a 2-inch-diameter feed stock bar. The resultant condensate contains from 16.5 to 19.5 percent chromium and from 7to 9 percent nickel. The iron in such a situation is about twice the volatility of the nickel, and the chromium about six times that of the nickel. Using previously known methods, evaporation of the same alloy may result in a condensate ranging from 14 to 23 percent chromium and from 5 to 12 percent nickel.

EXAMPLE V

An alloy containing 6 percent aluminum, 4 percent vanadium and the balance titanium may be evaporated in accordance with the invention using a 2-inch inner diameter crucible and a feed stock bar of about the same diameter. The resultant condensate contains from 41/2 to 71/2 percent aluminum and 31/2 to 41/2 percent vanadium. The aluminum is about 50 times as volatile as the titanium, and the vanadium is about one-half the volatility of the titanium. Previously known methods may be expected to produce twice the range of variation in composition.

EXAMPLE VI

An alloy containing 70percent copper and 30 percent nickel may be evaporated from a 2-inch inner diameter crucible and a feed stock bar of about the same diameter. The resultant condensate contains from 65 percent to 75 percent copper, the balance nickel, with the volatility of copper being about 50 times the volatility of nickel. Previously known methods may b expected to hold a composition range of 50 to 90 percent copper, the balance nickel.

It may therefore be seen that the invention provides an improved method for evaporating a molten alloy from a single crucible in a vacuum enclosure. Improved uniformity of condensate composition is obtainable and the replenishment rate of evaporant is closely controllable.

Various modifications of the invention in addition to those shown and described herein will become apparent from the foregoing description and accompanying drawing. Such modifications are intended to fall within the scope of the appended claims.

Claims

What is claimed is:

1. A method for evaporating an alloy from a single crucible in a vacuum enclosure, said alloy, in the molten condition, including at least two metals one of which is at least twice as volatile as the other, comprising: feeding a solid bar of alloy of substantially the same cross section as the crucible interior upwardly into the crucible from the bottom thereof, heating the bar of alloy in the crucible to form a molten pool completely covering the solid end of the bar, heating the surface of the molten pool to produce evaporation, and regulating the feed rate of the solid bar to maintain a substantially constant level of molten alloy relative to the crucible and thereby replenish the alloy at substantially the same rate as its evaporation.

2. A method according to claim 1 wherein the crucible is cooled.

3. A method according to claim 1 including controlling the heat losses out of the bottom of the solid bar to maintain them substantially constant.

4. A method according to claim 1 wherein the heating is accomplished by means of an electron beam.

5. A method for evaporating an alloy, from a single cooled crucible in a vacuum enclosure, said alloy, in the molten condition, including at least two metals one of which is at least twice as volatile as the other, comprising: cooling the crucible, feeding a solid bar of alloy of substantially the same cross section as the crucible interior upwardly into the crucible from the bottom thereof, heating the surface of the bar of alloy in the crucible to form a molten pool completely covering the solid end of the bar and to produce evaporation, controlling the cooling rate and the heat losses out of the bottom of the solid bar to maintain a substantially balanced condition of heat removal to heat input, and regulating the feed rate of the solid bar to maintain a substantially constant level of molten alloy relative to the crucible and thereby replenish the alloy at substantially the same rate as its evaporation.