US 3350219 A
Description (OCR text may contain errors)
Oct. 31, 1967 A. J. SHALER EVAPORATING METAL EMPLOYING POROU S MEMBER Filed July 7. 1966 u 7 TA T TJFLLL FUILJIIZFIL 0 F L Hull/I'll llll ///// II/rl/ /////fl m T N E V m AMOS J. 5f/4L 5? BY mwww%.
United States Patent Ofifice 3,359,219 Patented Oct. 31, 1967 3,350,219 EVAPORATING METAL EMPLOYING POROUS IVEMBER Amos J. Shaler, State College, Pa., assignor to Stackpole Carbon 'Company, St. Marys, Pa., a corporation of Pennsylvania Filed July 7, 1966, Ser. No. 565,347 5 Claims. (Cl. 117-107) ABSTRACT OF THE DISCLOSURE The invention relates to the coating of objects with metals. A porous member is disposed within a chamber at a pressure of non-oxidizing gas of from a few microns to less than one atmosphere. The member is made from material adapted to withstand high temperatures and which neither alloys nor reacts with the coating metal. The size of the connected pores of the member is such as to hold molten coating metal within them by capillary force, and the member has a surface exposed to the object to be coated. Coating metal is supplied to the member and heated within the pores to a temperature at which the vapor pressure of the metal is substantially in excess of that of the gas in the chamber. Under these conditions the coating metal is evaporated from said surface to said object without nucleate boiling at a much higher rate than it can from a free surface.
This application is a continuation-in-part of my copending application Ser. No. 278,278, now abandoned filed May 6, 1963.
This invention relates to the evaporation of metals to form vapor for coating the surfaces of objects.
Evaporation of metals for coating purposes has been accomplished in various ways. In some cases the metal to be evaporated is placed upon or within a coil of tungsten wire disposed near the surface to be coated inside an evacuated chamber. An electric current is passed through the coil to heat it. As the metal evaporates, the resulting vapor recondenses upon the cold surface of the article being coated. Such a system has the disadvantages of inefficient use of electric power, low rate of evaporation, waste of the metal evaporated, non-uniformity of the density of the vapor reaching the surface being coated, and limitation in the size and shape of both the evaporating device and the object to be coated.
An improvement on the foregoing has been the substitution for the tungsten coil of boats of various designs made of refractory materials. Auxiliary means are used for heating the boats to evaporate the metal contained in them. Some drawbacks remain, however, and additional ones are introduced. That is, boiling of the metal contained in a boat necessarily involves non-uniformity of evaporation and spitting of liquid metal droplets upon release of the bubbles from the surface of the boiling metal, with consequent non-uniform coating. On the other hand, if boiling is avoided the rate of evaporation becomes so low as to become uneconomical. Moreover, the direction of evaporation must necessarily be upward only, since gravity requires that the evaporating surface, which is the top of the molten metal in the boat, must be horizontal, flat and upward-facing.
It is among the objects of this invention to avoid the above-mentioned disadvantages encountered heretofore.
A particular object is to provide a method of and an apparatus for evaporating metals rapidly, economically and with production of uniform vaporization.
Other objects will appear from the following specification.
In accordance with this invention, a porous element having fine pores occupied by metal to be evaporated and having an exposed surface is disposed in a chamber at a pressure of inert or reducing, i.e., non-oxidizing gas from a few microns to less than one atmosphere, and the element is heated to a temperature to melt the metal and sufiiciently high to cause the vapor pressure of the metal to be substantially in excess of the pressure of gas in the chamber, whereby evaporation of the metal takes place without bubble formation (boiling) because the surface tension pressure of the metal in the pores is sufficiently greater than the vapor pressure of the metal at the prevailing temperature to prevent formation and escape of bubbles of the metal vapor. In this way I am able to attain rapid and smooth evaporation with production of uniform coatings of the metal where the process is applied to a coating operation.
In illustration of this reference may be made to evaporation of aluminum the normal boiling point of which is 2740 K. at which temperature the surface tension is 770 dynes per centimeter. This surface tension tends to squeeze shut any bubble present in the liquid with a pressure that is greater the smaller the bubble. For a bubble to grow it must, accordingly, have an internal opposing vapor pressure that is at least equal to the surface tension pressure, plus the pressure of gas, if any, outside the porous element. If the liquid is in a pore of 10 microns diameter, the largest bubble than can be formed spontaneously will have that diameter. At the normal boiling point of aluminum a bubble that size has a surface tension pressure of 45 p.s.i.a., i.e., three atmospheres. However, at 2740" K. the vapor pressure of aluminum is only one atmosphere so that under these conditions bubble formation, or boiling, cannot occur in the 10 micron pore. This means that I am able to operate my evaporative process at temperature above the normal boiling point. Thus, the following table shows the temperatures at which aluminum boiling can start in pores of various sizes and in the presence of various outside pressures.
As may be seen from the foregoing data, with a 10 micron pore and in a chamber at 0.1 atm. pressure, I can effect evaporation at a temperature as high as 3030 K., 290 K. above the normal boiling point Without boiling. At that temperature the vapor pressure is 43 p.s.i.a. which will give an extremely fast rate of evaporation and deposition. On the other hand, evaporation from a boat can be effected only up to 2300 K. without starting boiling while the aluminum vapor pressure will be only 1.5 p.s.i.
As will be observed from the foregoing data for aluminum, the finer the pores the higher the temperature of operation may be without boiling. Also, if the pores are too coarse there is little benefit to be had. Of course, different pore diameters will be needed according to the metal to be evaporated and the desired rate of evaporation. Because the molten metal wets the walls of the pores in the porous element, it is held in the pores and cannot easily flow out, as is well known in the art of infiltrating porous powder-metallurgy products; the molten metal does not fall out any more than water falls out of a saturated fine sponge of a similar configuration.
Preferably the porous element, or evaporator, is an electrical resistor that is heated by passing an electric current through it. The heat of the resistor may be used to melt and feed to the resistor metal to be evaporated.
The preferred embodiment of the invention is illustrated in the accompanying drawings, in which FIG. 1 is a plan view of suitable apparatus;
FIG. 2 is an enlarged combined end view and cross section taken on the line IIII of FIG. 1;
FIG. 3 is a vertical section taken on the line IIl1II of FIG. 2; and
FIG. 4 is a fragmentary horizontal section taken on the line IVIV of FIG. 2.
Referring to the drawings, a housing, or chamber, 1 of any suitable construction is formed to permit objects that are to' be coated to be passed through it. For example, if metal plates, strips or sheets are to be coated, the opposite ends of the housing may be provided near its bottom with aligned horizontal slots 2, through which the work 3 can enter and leave the housing as shown in FIG. 3. The inside of the housing may be provided with a suitable conveyor 4 for carrying the work through the housing. The entrance and exit are provided with means to seal them as well as possible, such as flexible strips 5 or soft rubber rollers or other sealing devices that extend across the upper and lower surfaces of the work in engagement therewith. This scaling is desirable because the housing should be evacuated or filled with a non-oxidizing gas while the coating operation is proceeding. To evacuate or fill the chamber, it may be provided with one or more ports 6 connected by suitable conduits 7 to a vacuum pump and, if desired, to a supply of inert or reducing gas.
Supported above the conveyor 4 inside housing 1 by means of brackets 9 or the like is a case 10, the bottom of which is provided with a central slot 11 (FIG. 3) that extends across the conveyor. This case supports a porous element 12 that extends along the slot and has an exposed lower surface facing downwardly. The porous element is made of a material that can withstand temperatures above the melting point of the metal that is to be evaporated, so that molten metal can infiltrate and saturate the element and evaporate from its exposed surface. Of course, the material of this element should be one that will not react or alloy with the metal being evaporated but will be spontaneously infiltrated by it. Although the heating may be done by auixiliary heaters, the simplest way to do it is to make the porous element from an electrical resistance material, such, for example, as titanium diboride sintered to a high strength. The opposite ends of the resistor, projecting from the ends of the case 10, then are connected in an electric circuit that heats the resistor to a high temperature. As shown in FIGS. 2 and 4, the busbars 13 of the circuit may be connected to the projecting ends of the resistor by water cooled clamps 14.
It is preferred to mount the porous element or resistor in an inverted channel 16 formed in a substantially nonporous refractory supporting bar 17 having greater resistance, a slightly greater coefficient of thermal expansion than the resistor, and not to be wettable by the metal being evaporated at the temperature of evaporation. For example, the supporting bar may be made of boron nitride or titanium nitride. The top and opposite sides of the resistor are engaged by the walls of the channel. The thickness of the bar 17 above the resistor is important. It should be such that in service, when the resistor is at a temperature high enough to evaporate the metal being used, the temperature of the upper surface of the bar will be just above the melting point of that metal. The supporting bar may be held firmly by the case by providing opposite sides of the bar with longitudinal grooves 18 (FIG. 3), into which the bottom wall of the case at opposite sides of slot 11 projects, leaving room in the slots to accommodate the expansion and distortion of the bottom wall.
The upper part of the supporting bar is provided with a row of vertical passages 20 connecting its upper surface with the top of the resistor inside the bar. Mounted on top of the bar around each of these passages is a vertical refractory ceramic tube 21 that extends up through a hole in the top of the case and is clamped between the bar and the top of the housing. Preferably, the inside of each tube tapers upwardly slightly. Inside each tube is placed a body 22 of metal to be evaporated, such as a rod of aluminum when a steel sheet or other material is to be coated with aluminum. The diameter of the rod should be approximately equal to the width of the lower surface of the porous resistor below it. Resting on top of each rod is a plug 23 that is loose in the tube. The plug is made of a material of low electrical and thermal conductivity and lower density than the metal to be evaporated, such as porous alumina, fire brick or porcelain. A rod-receiving opening in the top of the housing above the tube normally is closed by a threaded cap 24, between which and plug 23 a light coil spring 25, made for example of tungsten ribbon is compressed to continually hold the lower end of the rod firmly against the top of supporting bar 17. All of the elements inside of the case are surrounded by thermal insulating material 26, for example magnesia, which fills the case.
In service, electric current is supplied to the porous element 12 which, because of its electrical resistivity, becomes hot and heats the surrounding supporting bar 17. When the temperature of the upper surface of the latter reaches the melting point of the metal in tubes 21, such as the aluimnum rods 22, the lower ends of those rods melt and the molten metal flows down through passages 20 in the bar and infiltrates the porous resistor. The temperature of the resistor is maintained substantially above the melting point of the metal, and in the preferred embodi-ment even higher than its normal boiling point, so the molten metal at the lower surface of the resistor evaporates downward toward the metal sheet 3 on the conveyor. As fast as evaporation progresses, more metal from the rods continuously replenishes the supply inside the porous resistor. Since the molten metal is highly dispersed in the porous element in a complex network of interconnecting tortuous passages of very small diameter, it cannot readily boil (in the thermodynamic sense, i.e. by nucleation and growth of vapor bubbles) even if, as explained above, its vapor pressure substantially exceeds the pressure of other gases inside the chamber but must evaporate uniformly at a rapid rate, thereby maintaining a uniform vapor density away from the surface of the porous element. The porous element is not heated to a temperature so high above the boiling point of the metal (at the chamber pressure utilized) as to start the process of nucleate boiling, which can occur at a temperature so high that vapor bubbles of diameter less than the diameter of the largest pores are thermodynamically stable.
Since the porous element is continuously cooled by evaporation of the vaporized metal it can absorb a quantity of heat that otherwise would cause the material of the porous element to melt or dissociate.
This method and apparatus produce a uniformly dense quantity of vapor, which is a larger quantity than can be produced by an equivalent device of the coil or boat type.
My apparatus is self-correcting in all important respects. If a local decrease in the supply of infiltrated metal occurs, the local electrical resistance of the porous clement would increase and thereby supply extra heat to the nearest metal supply rods above the depleted spot to produce a more ample supply of molten metal. Capillary pressure normally prevents the metal from dripping out of the porous element, but should liquid metal as such, instead of vapor, come out of the porous element at some point along its length through local overheating and coat its bottom surface the electric current then would have an easier path along the coated surface, so the portion of the element above that surface would then be underheated and its supply of liquid metal would decrease, thus correcting the imbalance. Since these two self-correction features ensure that the electrical resistance of the porous element is constant along its length, the heat developed is everywhere the same, and so is the rate of evaporation. In case the supply of electric power is temporarily increased, the rates of evaporation and of metal supply would both rise, exerting a cooling effect by absorbing latent heat of vaporization and of melting, thus smoothing over the transient power increase and preventing damage to the element. Should the porous element break, it would immediately cease to conduct electricity, and evaporation and melting would cease so that no harm would befall the workpiece nor the rest of the apparatus. Finally, if the operator happened to forget to replace the metal supply rods when they are used up, the electrical resistance of the porous element would rise as it becomes depleted of molten metal, thus giving him warning by means of an ammeter or other warning device to add a fresh supply of metal but meanwhile reducing the heat input so that no harm could befall the apparatus.
As will be understood, the exposed face need not point down and it may be contoured to correspond to the contour of an object to be coated. If it points up, a continuous layer of molten metal would tend to form between it and the bar 17, shorting the electrical circuit. To avoid this, the porous element may be heated, instead of by direct resistance, by an auxiliary heater, such as an insulated tungsten rod threaded through a hole fashioned in the length of the element. In addition, the passages 20 would be replaced by tubes leading to one or two metal supply units similar to those shown by 21, 22, 23, 24, and 25, but disposed elsewhere, for example to the right and left of case (FIG. 2) and in front of or behind the bar 17 (FIG. 3) and separately heated.
According to the provisions of the patent statutes, I have explained the principle of my invention and have illustrated and described what I now consider to represent its best embodiment. However, I desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.
1. A method of coating an object with metal comprising supporting the object to be coated and a porous element in a chamber in a non-oxidizing gas at a pressure from a few microns to less than one atmosphere, supplying molten coating metal to the pores of said element, and heating the element and its contained metal to a temperature at which the vapor pressure of the metal is substantially in excess of the pressure of said gas, and thereby rapidly evaporating said metal without bubble formation from the pores of said element to form a coating on said object.
2. A method according to claim 1, said metal being supplied continuously to said element as evaporation occurs.
3. A method according to claim 1, said element being an electrical resistance element, and passing an electric current through the element to effect said heating.
4. Apparatus for coating objects with metals comprising a chamber, a porous element disposed within the chamber, means for maintaining in said chamber an atmosphere of non-oxidizing gas at a pressure from a few microns to less than one atmosphere, means for conducting coating metal to said element to fill its pores, means for heating said element and its contained metal to a temperature at which the vapor pressure of the metal is substantially in excess of the pressure of said gas, and means for supporting an object to be coated to receive vapor of coating metal evaporated from the pores of said element.
5. Apparatus according to claim 4 comprising a substantially horizontal porous element having an exposed lower surface disposed in said chamber, a refractory member provided with an inverted channel snugly receiving said element and provided with a row of vertical passages connecting the top of said member with the top of said element, a vertical tube mounted on said refractory member around each of said passages and adapted to receive bodies of metal to be evaporated, means for maintaining in said chamber non-oxidizing gas at a pressure less than one atmosphere, and means for heating said porous element and refractory member to a temperature high enough to melt the lower ends of said metal bodies and to bring the metal in said element to a temperature at which its vapor pressure is substantially in excess of the pressure of said gas.
References Cited UNITED STATES PATENTS 2,363,781 11/1944 Ferguson 1l7-107 2,490,902 12/ 1949 Goodwin 10l-327 3,085,913 4/1963 Caswell 117107 3,117,887 1/1964 Shepard et al 1l7l07.1
ALFRED L. LEAVITT, Primary Examiner. A. GOLIAN, Assistant Examiner.