US2450852A - Method of coating by evaporating metals - Google Patents

Description

Oct. 5, 1948. w. COLBERT ETAL I 2,450,852

METHOD OF COATINGv BY EYAPORATING METALS Filed Dec. 3, 1946 8 INVENTORS wllfiamlifi'alkgl: Y Adhurkbkmrzdz ATTORNEYS Patented Oct. 5, 1948 METHOD OF COATING BY EVAPORATING METALS William H. Colbert and Arthur R. Weinrich,

Brackenridge, Pa., assignors to Libbey-Owens- Ford Glass Company, Toledo, Ohio, s

tion of Ohio Application December a, 1946, Serial No. 713,698

Our present invention relates to a novel method of coating by evaporating metals. It has to do, more particularly, with the coating or' wettin by capillary attraction, of a filament which, for example, may be formed from a coil of ordinary tungsten wire, tantalum, molybdenum or columbium wire, by various metals in the form of an alloy with small amounts of magnesium which metals to be evaporated in pure form normally do not wet these metallic filaments, and the application of said metals or metal alloys by deposition resulting from thermal evaporation, to the face or surface of an article, such as a piece of glass, porcelain, plaster, metal, plastic, Cellophane, paper, or the like, to provide a reflective or metallized surface coating for said article. The inven tion also hasto do with securing wetting and with thermal evaporation of such metals from tungsten, tantalum, molybdenum, or columbium filaments alloyed or coated with magnesium by the application to the filaments of the pure metal desired to be evaporated.

Thi application is a continuation-in-part of our co-pending application, Serial No. 552,290, filed September 1, 1944, now Patent No. 2,413,606. Methods and apparatus have previously been employed to apply coatings of metals by thermal evaporation to the faces or surfaces of such articles to produce mirrors, reflectors or metallized materials for other purposes. In these methods it is desirable to efieot the thermal evaporation 14 Claims. (01. 117-107) of the metal, such as silver, copper, gold or aluminv'm, by applying the metal directly to an electrically energized and thus heated tungsten or other metallic filament which is preferably located within a vacuumized chamber; The metals which may be used as filaments for such evaporations must obviously be of high melting point and also of low vapor pressure at the elevated temperatures at which the metalsapplied to the fila ments evaporate. Thus tungsten, tantalum,

molybdenum and columbium have represented.

the only practical materials for such use. Platinum also has been used to a small degree but its high cost is generally prohibitive. While iron and nickel are of relatively low vapor pressure, they are of such relatively low melting pointthat filaments made from them rapidly burn out. I

With these filaments many of the metals can be readily evaporated. Thus, for example, aluminum, magnesium, vanadium, barium, strontium, iron, nickel, cobalt, manganese, thorium, chromium and titanium when applied to filaments of tungsten, tantalum, molybdenum or columbium,

will on heating in a vacuum, melt and spread over the filament by capillary attraction and satisfactory evaporatiorr of these metals then occurs from the large amount of surface which the molten metal covers.

However, with a large number of metals which it is desirable to be able to thermally evaporate and which from their vapor pressure at elevated temperatures should readily evaporate, it has been found dlfiicult, ifnot impossible, to carry out satisfactory deposition of such coatings by thermal evaporation. Thus, for example, silver and copper while readily lending themselves to thermal evaporation from a crucible, cannot be evaporated readily from a coil of tungsten, tantalum, molybdenum or columbium when applied to a filament of these metals and heated by electrical resistance. The silveror copper on melting shows no affinity for the metallic filaments and almost immediately after melting collects into a drop and falls off of the filament. This lack of ability to wet tungsten, tantalum, molybdenum and columbium occurs also with the metals gold. zinc, tin, antimony, cadmium, bismuth, lead, thallium and indium. With each of these metals the use of the four available coil filaments as a means of evaporating these metals has not been possi-. ble, and less desirable means of heating have been necessary where it became necessary to evaporate these under practical conditions repeatedly in the commercial production of mirrors and metallic coated articles. As each of these metals, after melting, pulls together into droplets and falls oil? the filaments, there has resulted a wastage of the metal whenever it has been attempted to evaporate them from these filaments and there have been continuous failures of the apparatus to function due to the loss of the metals from the heated wires; and where any metal has been evaporated the amounts s0 evaporated have always been un certain and Without control. We have found that we may use tungsten, tantalum molybdenum or columbium as filaments for the evaporation of metals which do not wet these filaments by causing them to wet such filamentsby the application to such filaments, or the forming thereon, of alloys of those metals which we desire to evaporate with small amounts of magnesium such as amounts ranging'from 0.1% to 5%, which is characterized in that it will also form an alloy with tungsten, tantalum, molybdenum and columbium in the presence of the metal we desire to evaporate. Thus, for example, we may add small amounts of magnesium to silver and when such alloys are melted on a tungsten filament the silver will be found to wet the tungsten filament and to spread itself by capillary attraction over the surface of the tungsten wires. In the absence of the magnesium the silver melts, draws itself into a. droplet and falls off the filament wire because it does not wet the same. Not all metals have been found to act in this manner. Alloying the normally non-wetting metals among themselves, such as adding lead to silver, does not seem to bring about any desirable improvement in the wetting characteristic. In each case, however, it is found that the magnesium readily forms alloys with tungsten, tantalum, molybdenum and columbium and also forms alloys with copper, silver, gold, zinc, tin, antimony, cadmium, bismuth, lead, indium and thallium. It has also been found that magnesium when heated reduces tungsten oxide, molybdenum oxide, tantalum oxide, and columbium oxide, to the metals, and that the removal of coatings of these oxides. normally present upon filaments composed of tungsten, molybdenum, tantalum, or clumbium, aids in securing the desired wetting since silver, copper, gold, zinc, tin, antimony, cadmium, bismuth, lead, thallium and indium do not wet the oxide-coated filament. Thus, the magnesium readily brings about the desired .wetting and it appears clearly that this is accomplished through the reduction of the oxide coating upon the filaments and upon the mutual alloying tendency which these metals possess. The magnesium may also be added to the filament either as an alloy with the tungsten, tantalum, molybdenum or columbium, or preferably it may be employed as a surface coating or surface alloy with such filaments. Filaments containing small amounts of magnesium are found to wet readily when the pure otherwise non-wetting metals are fused thereon. Thus the wetting is brought about in the presence of the magnesium nd it is immaterial as to whether the magnesium is supplied in an alloy applied to the filament or exists in the filament surface.

One of the objects of our invention is to provide an improved and satisfactory method of process of evaporating metals which normally do not wet heater filament coils of tungsten, tantalum, mo-

lybdenum, or columbium, by alloying the metal so as to cause the metals to wet the coils of such filaments and to coat the coils by capillary attraction so that thermal evaporation can then be carried out.

Another object of our invention is to apply to a tungsten, tantalum, molybdenum or columbium filament a metal to be evaporated which does not wet such filament coils, alloyed with a suitable proportion of magnesium which alloys with the filament and brings about a proper wetting or coating of the filament wires by capillary attraction under the influence of heat applied tothe filament.

As another object of our invention there is provided an improved method or process whereby a metal which is to be evaporated and which does not normally wet heater filaments of tungsten, tantalum. molybdenum and columbium is alloyed with magnesium and applied to such a filament, and by securing a wetting and coating of the filament by capillary attraction of the metal desired to be evaporated may be deposited upon the face or surface of an article, by thermal evapporation, to provide such article with a reflective or metallized surface coating.

As a further object there are provided filaments which have been pre-alloyed, with small amounts or magnesium which may be used directly o e lminus 5 millimeters or better.

4 crate the normally nonwetting metals since such alloyed filaments are found to be wetted readily by the molten pure metals desired to be evaporated.

A further object of our invention is to apply to a tungsten, tantalum, molybdenum or columbium filament, a metal such as copper, silver, gold, zinc, tin. antimony, cadmium, bismuth, lead, indium or thallium, alloyed with a suitable proportion of another metal which brings about a desirable wetting or coating of the filament metal by capillary attraction under the influence of heat applied to the filament and thus permits thermal evaporation of the metals.

Generally speaking, and in accordance with our present invention, the metal to be evaporated which normally does not wet the heater filament is applied alloyed with small amounts of magnesium to the extent of 0.1% to 5% or more, providing wetting characteristics to the coils of a filament which may be formed from tungsten, tantalum, molybdenum or columbium. Thus in order to thermally evaporate copper, zinc, gallium or arsenic, which are metals of the chemical periodic table arrangement found in series 5 or the metals silver, cadmium, indium, tin and antimony, which include metals of series 7, or the metals gold, thallium, lead and bismuthwhich in the periodic arrangement include series 11, all of which metals do not wet filaments made of tungsten, tantalum, molybdenum or columbium, we first bring about a satisfactory wetting and adhesion of these metals to the filaments by applying the metals to the filaments as an alloy with small amounts of magnesium and then by energizing the filament and thus heating it we cause a melting of the metals. When the applied metal alloys are thus melted they react with any oxide coatings upon the filaments and reduce such to the metals tlmgsten, tantalum, molybdenum or columbium and then they apparently alloy to some degree with the metal comprising the heater filament wire and by reason of such the molten metals wet the filament wires and by capillary attraction are drawn out over the surface of the coils. The molten metal which has thus covered considerable surface of the heated coil and is held thereto by capillary adhesion is thereafter evaporated uniformly from the heater coils to apply a surface coating of a metallic or reflective nature to an article such as a piece of glass, porcelain, silica, mica, plastic, metal, Cellophane, resin, or other support material, by deposition resulting from the thermal evaporation of the metal from the filament. The operations of thermal evaporation may with some of the metals, be carried out at normal pressure but generally are preferably carried out in vacuum chambers known tothe art and within a high vacuum, which may be of the order of one millimeter down to 10 to the It is very necessary that the metal to be evaporated wet and coat the coil surfaces in order that the metal wili evaporate uniformly in all directions. By securing such wetting action the thermal deposition of these metals, in addition to being made possible, has Keen found by our process to give uniform coat- We may also proceed to secure the objects within the scope of this invention by supplying magnesium in the filament either as an alloy with the tungsten, tantalum, molybdenum or columbium. or as a surface coating; or as a surface alloy on such filaments. When the pure metals such as silver, copper or gold which will not normally wet about the desired wetting andadherence of the molten metal to the heated filaments.

As will be shown more fully later, pieces of the metal to be evaporated and which have been previously alloyed with the metal which brings about the alloying with and wetting of the filament, may be hung onto the loops or coils of the filament. v

The foregoing and other objects and advantages of the present invention will appear from the following description and appended claims when considered in connection with the accompanying drawings forming a part of this specification wherein similar reference characters designate corresponding parts in the several views.

In said drawings:

Figure 1 is a perspective view, partly broken away, illustrating one suitable apparatus for carrying out our improved method or process.

the pure metal to beevaporated to such filaments and the wetting action upon the filament resulting from capillary attraction after the pure metal is melted.

Suitable apparatus employed by us, and illustrated in Figure 1, comprises, as shown, a supporting base Ili upon which is mounted a housing, shown as a whole at ii. The housing ll may be in the form of a bell-jar or the like having a dome-like or semi-spherical top portion or enclosed end and a bottom open end having a surrounding flange or projection I! which is adapted to rest upon the top face or surface of the supporting base l0.

' Within the chamber provided by the housing ll, we have shown a suitable work-piece support ii for supporting a work piece, such as a plate or piece of glass, plastic, plaster, paper, porce- Figure 2 is a perspective view of a fragment of an electric filament showing the application of a suitable metal alloy to several of the coils or convolutions thereof, and illustrating one phase of the method or process of wetting or coating the filament by said alloy; and

Figure 3 is a view similar to Figure 2 showing the filament after the completion of the wetting process by the alloy of Figure 2.

Figure 4 is an enlarged, cross-sectional view of a tungsten, tantalum, molybdenum or columbium electric resistance filament precoated with a thin layer of magnesium.

Figure 5 is another enlarged, cross-sectional view showing a filament of tungsten, tantalum, molybdenum or columbium which is alloyed only in-the surface of the filament with magnesium.

Figure 6 is also an enlarged, crosssectional view of a filament formed from tungsten, tantalum,

molybdenum or columbium, which is alloyed throughout with a small amount of magnesium and upon' which the pure metals, on melting, will directly show a good wetting action.

Figure '7 is a perspective view of a filament containing magnesium such as shown in Figures 4, 5 and 6 and upon which pieces of the pure metal to be evaporated have been hung.

Figure 8 is a'perspective view after the filament and pieces of metal of Figure '7 have been heated to effect a wetting of the filament.

Before explaining in detail the present invention it is to be understood that the inventionis not limited in its application to the details of construction and arrangement of parts illustrated in the accompanying drawings, since the invention is capable of other embodiments and of being practised or carried, out in various ways. It is to be understood also that the phraseology or terminology employed herein is for the purpose of description and not of limitation.

Referring now to the drawings, we have show a suitable apparatus for carrying out our im-.

proved method or process, as well as one suitable metal alloy and the steps of applying the alloy or a filament by a wetting action resulting from capillary attraction. We have also shown suitable alloyed filaments and'the steps of applying lain, metal, .or the like H, in upright position.

Located within the chamber and mounted upon the supporting base I0, is a pair of upright supporting posts l5 between which is carried or supported, in substantially horizontal position, an electric filament. IS. The filament, as shown, is in the form of a coiled wire made of tungsten, tantalum, molybdenum or columbium, or an alloy of these with magnesium or precoated with a layer of magnesium, whose opposite ends are attached to brackets ll mounted upon the supporting posts l5 and adjustable thereon so as to vary the position or location of the filament IS with relation to the supporting base Hi.

The chamber provided by the housing ll may, if desired. be completely evacuatedof air through outlet pipe or. conduit Illa and have a high vacuum created therein by means of suitable air evacuating and vacuum creating means, such as a pump (not shown).

In accordance with one method embodying our invention which is to be performed or carried out Within the chamberedhousing II, we preferably provide a metal alloy which may consist of silver, copper or gold or other normally non-wet ting metal and approximately 1% to 5% or more of magnesium. Pieces of this preformed silver and magnesium alloy, for example-or copper and magnesium alloy, for example. or gold and magnesium alloy, for example, several of which are shown at l8, in Figures 1 and 2, are bent and hung on the loops or convolutions lfia of the filament l-B composed of pure tungsten, tantalum, molybdenum or columbium in the manner shown.

It is known that silver, copper and gold lend themselves admirably to thermal evaporation but they have no wetting afimity for tungsten, tantalum, molybdenum or columbium surfaces and therefore silver, copper or gold aone is un-w satisiactory for coating the filament I-G formed from either of these metals by a wetting action effected by capillary attraction. Wetting of the tain percentage of magnesium, preferably 0.1%

to 5% or more, with the silver, copper or old to form the alloy II, the magnesium will serve to filament is energized and thus heated and will act to cause the silver, copper or gold to also cling to or wet the filament. An early stage or phase of the wetting action of the filament Itby the alloy i8 is shown generally at ll, Figure 2. As the wetting action by capillary attraction continues, the two metals of the alloy will proceed to wet the coils of the filament if and in fact, will substantially wet or. coat and cover the surfaces of the filament. In Figure 3, we have illustrated several of the coils or loops "a of the filament as being coated at 20 by the alloy from which the pieces I 8 are formed.

Thus, by including magnesium with the silver, copper or gold, as an alloy, it is possible to quickly and effectively coat or wet the filament ll by capillary attraction. Since, therefore. the alloy builds up onto the surfaces of the filament in substantially the manner illustrated in Figure 3, there will be a relatively uniform coating or wetting of the filament and a uniform dependable evaporation of the silver, copper or gold. Heretofore. when attempts were made to wet the filament by-the use of the silver, the copper or the gold alone, only small portions of the molten metal would cling to the filament as droplets hanging from the lower ends of the coils of the filament, with the major portion of the molten metal dropping or falling off the coils. This was particularly undesirable since it was practically impossible to produce, by thermal evaporation, an even surface coating by deposition, or to control the deposition to desired coating deposit thicknesses on the surface of an article, such as the article it, to which it was desired to apply a refiectlve surface coating. By virtue of the fact that the silver, copper or gold did not properly wet the tungsten or other metal filament but had a tendency to drop oil said filament, the process of coating with these metals by deposition was unsatisfactory, slow and painstaking because only a small portion or percentage of the filament received the metal coating. Considerable shutting down and starting over again was required when most of the gold, silver or copper on first melting dropped oil. the coils and no evaporation was secured. Thus great waste occurred. the process was considerably slowed down, and the coating produced by deposition, if any, was uneven or spotty and unsatisfactory because of such uneven character thereof on the surface of the article coated. Commercial production under such uncertain conditions was impossible.

It is to be understood that in carrying out our method or process as described above, in the chamber of the housing ii, the chamber depending upon the metal being evaporated, may be at atmospheric pressure, or it may be evacuated of air and a vacuum created therein. Thus, after the pieces I! of the silver and magnesium alloy or copper and magnesium alloy or gold and magnesium alloy, as the case may be, have been applied to the coils of the filament It and the work piece ll mounted upon its support l3 within the chamber, a vacuum of 10 to the minus 3 millimeters or better is created and the filament I6 is then energized and therefore heated so as to melt the alloys and to cause the hot magnesium to reduce any oxide coatings on the filaments and to start in motion the wetting action of the filament by capillary attraction, as explained above. After the wetting action has been completed as 8 illustrated generally at I. in Figure 3, the filament I0 is heated further, whereby silver, copper or gold in the coating ll of the alloy will be thermally evaporated and transferred by deposition to the face or surface of the work piece It which. as shown, is disposed in a position opposite the filament l8.

We have found that the desired wetting may also' be brought about by applying pure metals as pieces, as shown at 28 in Figure '1, to a filament of tungsten, tantalum. molybdenum or columbium, indicated at it which has been precoated at 24, Figure 4, pre-alloyed at 28, Figure 6, or surface-alloyed at 2!, Figure 5, with magnesium. In carrying out the operation, the filament it is mounted as in Figure 1 and the pure metal pieces 23 of Figure 7 are applied thereto. After the object It to be coated has been placed in position the bell Jar ii is lowered, the vacuum is created, and electric current is applied to the filament through the electrodes II to cause melting of the applied pieces and a wetting of the coil as illustrated in Figure 8. By continued application of heat to the coil the applied metal will be thermally evaporated and deposited upon the object ll.

During the evaporation of the higher boiling metals such as gold, silver and copper, some of the maa'nesium is also distilled while in the case of the lower boiling metals such as lead and zinc, where the filaments are operated at relatively low temperatures, little of the magnesium whether present originally as an alloy with the metal to be distilled or present in or on the filament is distilled. Thus. in the latter case practically pure lead or zinc coatings are deposited whereas in the case of gold, silver or copper an alloy of magnesium and these metals is thermally deposited upon the object being coated.

The precoating of filaments with magnesium as shown in Figure 4 may be accomplished by thermally evaporating in a vacuum a coating of the magnesium upon the cold filaments. If the filaments also carry the metal to be evaporated, such as silver, etc., the latter also would become coated with the magnesium and upon heating the coated filaments, good wetting by silver, etc., is

secured with subsequent evaporation. Such mag nesium-coated filaments may also be heated to cause the magnesium to surface-alloy with the filament to produce structures as illustrated by Figure 5. Where it is desired to introduce magnesium into the filament as a uniform alloy this may be accomplished by introducing magnesium into a molten alloy and thereafter in the known manner producing wire by pulling such alloy through dies.

From the foregoing it will be seen that we have provided an improved method or process for ap' plying certain metals to a filament of tungsten, tantalum, molybdenum or coiumbium by causing a wetting resulting from capillary attraction through the presence of small quantities of magnesium and applying heat from the filament, and i lvays. 'll'hus, we may applyseparate pieces of a preformed alloy of'such metals with magnesium and these will wet the pure metallic filaments, or we may apply the pure metals to be evaporated to a filament containing some magnesium either in its surface or throughout,

While we have referred to the use of tungsten, tantalum, molybdenum, or columbium as suitable metals from which the coiled filament or element l6 may be formed, other suitable metals may be. used for this purpose. W have mentioned these metals particularly since their high melting points and low vapor pressures at the boiling temperatures of other metals make these the practically desirable metals for use as such filaments.

We have described our improved method or process as preferably being carried out in a vacuumized chamber in which the step of wetting the filament takes place, as does also the step of thermal evaporation of the metal to effect its deposition upon the workpiece to provide a reflective coating thereupon.

Obviously also in the case of the most readily volatile metals, such as cadmium and zinc, the melting of the metals and the wetting of the filaments, as well as the evaporation of the readily volatile metals, such as cadmium or zinc, may be carried out under atmospheric conditions of pres sure if desired, while employing a suitable inerting of tungsten, tantalum, molybdenum and co-- lumbium wherein the metal is heated on such filament as an alloy with magnesium which causes the metal desired to be evaporated to wet, to adhere to, and to spread out over the filament surfaces and by the continued application of heat to evaporate, and depositing a coating thereof on said polished support material.

2. The method of coating surfaces which comprises evaporating metals from a filament made of a metal selected from the group consisting of tungsten, tantalum, molybdenum "and columbium wherein-the metal is heated on such filament as an alloy with magnesium which causes the metal desired to be evaporated to wet, to adhere to, and to spread out over the filament surfaces and by the continued application of heat, to evaporate and to deposit upon said surfaces.

3. The method of coating articles by evaporating silver from a filament selected from the group consisting of tungsten, tantalum, molybdethereon.

4. The method of coating 9. support material by evaporating silver within a vacuum from a filament selected from the group consisting of tungsten, tantalum. molybdenum and columbium wherein the silver is heated on such filament in the presence of magnesium and wherein said magnesium causes the silver desired to be evaporated to wet, to adhere to, and to spread out over the filament surfaces and by the continued application of heat, to evaporate within the vacuum and to coat by deposition said support material.

5. The method of coating a support by evaporating copper from a filament selected from the group consisting of tungsten, tantalum, molybdenum and columbium wherein the copper is heated on such filament in the presence of magnesium and wherein said magnesium causes the copper desired to be evaporated to wet, to adhere to, and to spread out over the filament surfaces and by the continued application of heat, to evaporate and to coat by deposition thereof on a surface of the support.

6. The method of coating a support by evaporating copper within a vacuum from a filament selected from the group consisting of tungsten, tantalum, molybdenum and columbium wherein the copper is alloyed with a relatively small amount of magnesium and is heated on such filament and wherein said magnesium causes the copper desired to be evaporated to wet, to adhere to, and to spread out over the filament surfaces and by the continued application of heat, to evaporate within the vacuum, and to coat the support by depositionv 7. A method according to claim 4 wherein gold is substituted for silver.

8. A method according to claim 1, wherein the magnesium is present in a relatively small amount.

9. A method according to claim 2, wherein the magnesium is present in an amount less than 5%.

10. A method according to claim 3, wherein the magnesium is present in an amount under 5%.

11. A method according to claim 4, wherein the magnesium is present in a relatively small wherein the metal is heated on said filament. in

the presence of magnesium and wherein said magnesium causes the metal desired to be evaporated to wet, to adhere to, and to spread out over the filament surfaces and by the continued application of heat, to evaporate within the vacuum and to coat by deposition said support material.

14. A method according to claim 13, wherein the support material is a polished support material.

WILLIAM H. COLBERT. ARTHUR R. WEINRICH.

REFERENCES CITED The following references are of record in the file of this patent:

Number Caldwell, article in Journal of Applied Physics,

V01. 12, Nov. 1941, 13 888 779-781.

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