US 2904451 A
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p 15, 1959 c. J. SCOTT m-AL 2,904,451
VAPORIZATION COATING PROCESS AND ALLOY THEREFOR I Filed Dec. 5, 1957 lnven tofs: Charles dScot=b Kenneth S. G.Per=twee y flzrf TheiT' A L- T'TWGH.
2,904,451 Patented Sept. 15, 1959 dice VAPORIZATION COATING PROCESS AND ALLOY THEREFOR Charles J. Scott, Cleveland Heights, and Kenneth S. G. Pertwee, Cleveland, Ohio, assignors to General Elee: tric Company, a corporation of New York Application December 5, 1957, Serial No. 700,799
6 Claims. (Cl. 117-97) Our invention relates to the formation of coatings by the vaporization of silver, and more particularly to a novel method and alloy for forming such coatings.
In the formation of metal coatings by thermal evaporation, it is desirable to effect the vaporization of the metal by applying it directly to an electrically energized vaporizer or filament of refractory metal which is preferably located in an evacuated space. The filament must be of a metal of high melting point and low vapor pressure so that, for practical purposes, it should be a metal such as molybdenum, tungsten, tantalum or columbium.
Some metals can be successfully vaporized from such filaments whereby they melt and spread over the filament bp capillary attraction so that satisfactory evaporation then occurs. However, silver does not wet the filament and therefore tends to ball up and fall from the filament in droplets instead of vaporizing. In order to overcome this disadvantage, it has been the practice to apply the silver to the filament in the form of an electroplated coating and the results have been generally satisfactory. However, that method entails certain disadvantages. For instance, as applied to the formation of reflecting coatings on glass electric lamp bulbs, each plated filament is used to coat a single bulb after which it must be replated with silver before being used to coat another bulb. It will be apparent that a considerable amount of labor and space is required to perform the plating operation; moreover, the spoilage of filaments is high due to continual handling.
In accordance with the present invention, we have found that the above-mentioned disadvantages are overcome by placing the silver on the filament or vaporizer in the form of an alloy of the silver with small amounts of both silicon and lithium.
Small amounts of silicon alone added to the silver caused it to wet the vaporizer or reduced the surface tension of the silver and allowed it to spread out on the vaporizer, but it apparently also raised the melting point. At any rate, after a few operations of the vaporizer, it required a higher temperature to evaporate the alloy and as a result of overheating the vaporizer the reflecting coatings were sometimes discolored. Moreover, When silicon alone is alloyed with the silver it leaves a clinker type residue of a black material on the vaporizer after a few uses thereof and subsequent loads of silver ball up in the vaporizer, so that it must then be replaced. Such replacement is a distinct disadvantage for automatic high-speed operation. The addition of lithium alone to silver apparently lowered its melting point and did not leave a residue, but it did not always spread smoothly on the vaporizer and showed a tendency to ball up in the vaporizer.
However, it was found that by adding both lithium, which lowered the vaporizing temperature, and silicon, which wet the vaporizer, we obtained the advantages of both and the disadvantages of neither. The lithium apparently acts as a scavenger and prevents the formation of the residue left when silicon alone is added, so that the original vaporizer filament can be used indefinitely and the process continued by merely supplying another quantity of alloy for each bulb which is to be coated.
In general, it is desirable to maintain the amounts of both silicon and lithium at a minimum consistent with adequate wetting and avoidance of the aforesaid residue. We have found that best results are obtained with a composition by weight of 0.06% lithium, 0.04% silicon, balance essentially silver. For high reflectivity, we prefer to employ silver of high purity, for example 99.95% or better, depending upon the economical aspect. The presence of copper is especially to be avoided since it results in discoloration of the reflector coating.
In the coating of certain types of lamp bulbs, a portion of the silver coating is removed by dissolving it with an acid such as a mixture of chromic and sulphuric acids and water, and excessive amounts of lithium would tend to affect the Wetting of the surface by the acid, and it may also have adverse effect on the glass itself. Excessive amounts of silicon would tend to leave a residue on the filament and, in any event, the reflectivity of the silver would be adversely affected by excessive amounts of any foreign material. 011 the other hand, the use of silicon alone, alloyed With the silver, results in the formation of the dark residue on the filament, whereas the use of lithium alone is likewise not satisfactory because of inadequate wetting of the vaporizer.
In those cases where a portion of the reflecting coating is to be removed by the conventional practice of dissolving it with acid and then Washing the bulb with Water, We have found that excessive amounts of lithium cause the coating to peel off when it is Washed, the lithium ap-' parently being dissolved by the water. In that case We prefer to limit the lithium content to not more than about 0.06%, and satisfactory results may be obtained with a composition in the range of 0.04-0.08% silicon, 0.02- 0.06% lithium, balance silver. When the finished coating is not to be Washed with Water, as by employing a mask over the portion of the bulb not to be coated, then satis factory results may be obtained by compositions in the range of about 0.04-l.9% silicon, 0.02l.8% lithium, balance silver. However, in either case we prefer the composition 0.04% silicon, 0.06% lithium, balance silver.
For good results, it is necessary that the alloy be formed by melting the silver, silicon and lithium in the indicated amounts in a vacuum. The vacuum melting process removes gas from the silver and promotes smooth evaporation of the silver from the vaporizer filaments. The gas normally present in silver may cause discoloration and bombardment or sputtering during vaporization from. the filament.
In preparing the alloy, the silver may be placed in a graphite crucible in a vacuum furnace and melted by induction heating to a temperature of about 1200 C. The heating is continued until bubbling ceases, which may be for about three or four minutes, and then the indi cated proportions of silicon and lithium are dropped into the molten silver, the induction heating is immediately terminated and the melt is poured into a mold. For ease of handling when adding the silicon and lithium, they may be Wrapped in silver foil the weight of which is, of course, taken into account. Alternatively, the lithium and silicon may be added to the molten silver in the form of separate silver-lithium and silver-silicon master alloys. The cast alloy is then preferably rolled and drawn to ribbon form about 0.010 inch thick and about 0.110 inch Wide.
The accompanying drawing is an elevation, partly in section, illustrating one form of apparatus for carrying out the method in connection with the coating of one type of electric lamp bulb.
Referring to the drawing, the glass bulb 1 is carried by a rubber gasket 2 which may constitute part of one of the heads of a rotary vaporizing machine. The gasket 2 is arranged to make a vacuum-tight seal with the neck portion 3 of the bulb and is provided with a conduit 4 for connection to a source of vacuum. The gasket 2 also supports a pair of conductors 5 which extend into the interior of the bulb 11 and which support a helically coiled vaporizer filament 6 of one of the aforesaid refractory metals preferably a stranded molybdenum wire. Other vaporizer forms may be employed, such as a basket of stranded molybdenum wire in the form of a vertically disposed conical coil with its apex lowermost. A quantity of the silver-silicon-lithium alloy suflicient to coat the interior of the bulb 1 tothe desired thickness is placed in the vaporizer coil 6, preferably in the form of a length of crimped ribbon 7.
After the ribbon 7 has been placed in the vaporizer 6.and the bulb 1 has been mounted in place on the gasket 2, the bulb is evacuated through the conduit 4 to a suitable pressure of 5 microns or less. The vaporizer 6 isthen heated to a temperature above the melting point of the alloy, preferably about 1800" C., by application of'an electrical current through the conductors 5. After the strip 7 has evaporated and deposited on the interior surface of the bulb 1 in the form of a reflecting metallic film, the bulb is removed and the coating is subsequently removed from the end face 8 thereof by dissolving it in a suitable acid in known manner. The vaporizer 6 is then supplied with another length of ribbon 7 for coating another bulb.
What we claim as new and desire to secure by Letters Patent of the United States is:
1. The method of coating articles by vaporizing silver from, a filament of metal selected from the group consisting of molybdenum, tungsten, tantalum and columbium, wherein the silver is placed on the filament as an alloy consisting of 0.02-1.8% lithium, 0.041.9% silicon and. the balance essentially silver, and is heated on said filament, and wherein the silicon causes the silver to wet and adhere to the filament and the lithium serves to prevent formation on the filament of a dark residue otherwise left thereon due to the presence of the silicon, and wherein by continued application of heat the silver is vaporized and caused to deposit on the said article.
2. The method of coating articles by vaporizing silver from a filament of metal selected from the group consisting of molybdenum, tungsten, tantalum and columbium, wherein the silver is placed on the filament as an alloy consisting of about 0.06% lithium, 0.04% silicon and the balance essentially silver, and is heated on said filament, and wherein the silicon causes the silver to wet and adhere to the filament and, the, lithium serves to prevent formation on the filament of a dark residue otherwise left thereon due to the presence of the. silicon-,and wherein by continued application. of. heat; the. silver is vaporized and caused to deposit on the saidiarticle.
3. The method of coating articles by vaporizing silver from a filament of metal selected from the group consisting of molybdenum, tungsten, tantalum and columbium, wherein the silver is placed on the filament as an alloy consisting of 0.020.06% lithium, 0.04-0.08% silicon and the balance essentially silver, and isheated on said filament, and wherein-the-silicon causes the. silver to wet and adhere to the filament and the lithium serves,
to prevent formation on the filament of a dark residue otherwise left thereon due to the presence of the silicon, and wherein by continued application of heat the silver is vaporized and caused to deposit on the said, article, andsubsequently dissolving the silver off a, portion. of, the:
References Cited in the file of this patent UNITED STATES PATENTS 1,863,645 Assmann June 21, 1932' 1,982,774 Winkler et al. Dec. 4, 1934 2,196,307 Hensel et al Apr. 9; 1940 2,221,285 Hensel et al Nov. 12, 1940 2,793,115 Bredzs et al May 21, 1957' 2,819,982 Westerveld et a1 J an. 14, 1958