US1893286A - Method of carbonizing metals and alloys - Google Patents

Description

C. V. IREDELL Jan. 3, 1933.

Filed May 25, 1928 Patented Jan. 3, 1933 UNITED STATES PATENT OFFICE CHARLES V. IREDELL, OF BLOOMFIELD, NEW JERSEY, ASSIGN 'QR TO WESTINGHOUSE LAMP COMPANY, A CORPORATION OF PENNSYLVANIA METHOD OF CABBONIZING METALS AND ALLOYS Application filed May 25,

This invention relates to metallurgy and more particularly relates to methods of coating metals and comprises essentially in a method of forming an adherent gas-free carbon coating upon metal surfaces.

Inthe production of electron discharge devices particularly of the rectifier and os-. cillator types, the use of high plate and fgrid voltages result in pronounced heating 0 the refractory metals comprising these electrodes. In some types this heating effect is sufiicient to raise the temperature of these electrodes tobright redness. Under such temperature conditions these electrodes are then capable of themselves emitting electrons which disrupt the operating efliciency of these devices. One of the methods devised to overcome this heating effect is to cover the metal sur face of these electrodes with a material WhlCh Will more readily radiate and dissipate the heat energy of the device than the bright metallic surface heretofore employed. Black coatings have been found to serve this purpose admirably, and many methods have been developed wherein black carbonaceous coatings have been applied to these surfaces.

It has been found, however, that although such blackening by coating with carbonaceous material is effective in reducing and even eliminating back emission due to incandescin g the metal electrodes, the majority of such coatings have introduced an additional factor in that unless highly purified carbon is employed therein the contained impurities are broken down under the influence of the electron bombardment and the resultant liberated impurities either produce back emis- 40 sioneffects or are volatilized and destroy the emissive qualities of the filament.

Such carbonaceous coatings have also been difficult to cause to adhere to the metal surfaces, and are usually hard to free of ad- 45 sorbed and absorbed gaseous impurities.

One of the objects of this invention is to provide a method of coating metal surfaces with substantially pure carbon.

Another object of this invention is to provide a method whereby a closely adherent 1928. Serial No. 280,432.

I ment, and which is coated with a closely ad herent heat radiating carbon layer, substantially integral with but only superficially attached thereto.

Other objects and advantages will be apparent as the method is more fully disclosed.

My invention substantially is comprised of two olperations, each containing two steps.

1. eoxidizing and carbonizing.

2. Degasifying and surface cementing.

The carbonizing step in the first operation is dependent upon obtaining an oxide-free metal surface to deposit the carbon upon. I have also found that thesubsequent degasifying and surface cementing step is dependent upon this procedure as it is essential that these metal parts be free of oxygen carrying compounds.

This surface deoxidizing is obtained by means of heating the metal body in a reducinggas' (hydrogen) prior to carbonizing, thereafter the metal surface is carbonized by introducing into the reducing gas a proportion of a hydrocarbon of the paraflin series and heating the deoxidized metal to a temperature sufiicient to effect decomposition or dissociation of the hydrocarbon vapor. The two steps in this operation are made in a continuous or consecutive manner without intervening exposure to any other gases. The deposited carbon thereupon is formed in a dense coherent layer on the surface of the clean metal body. Upon cooling to room temperature this coating is treated by the second operation of my process to effect consolidation and firm cementationof the carbon upon the surface of the metal body.

In the second operation the coated surface 1 and the metal -body are mutually degasifie'd and cemented together by a second heating in y a high vacuo for a prolonged time interval. The temperature of heating is not suflicient to cause any more than superficial formation of metallic carbides but provides a means whereby these carbonized metals may be latterly exposed to atmospheric contamination without the formation of surface oxides which are decomposable under the influence of electronic bombardment, and normally inherent on most metal surfaces. The removal of this oxide film is essential in order to prevent the destruction of the activating constituent of the hot cathode in an electron-emitting device by interaction with the liberated oxygen of the electronically decomposed surface oxides.

In this manner I obtain, by the practice of m invention, a carbon coated metal body suita is for use in electron discharge devices which has a coating that is not only adherent sufiiciently to prevent the formation of oxide films thereon but of a sufficient thickness to substantially serve as a heat radiating surface, and moreover is of a degree of purity such that the hack emission of electrons due to the unavoidable heating effect produced during the operation of the electron discharge device is restrained and in most cases inhibited.

In the practice of my invention 1 preferably employ the formed and finished plate, grid and support material, although I have found that the coating I obtain by my process is smliciently adherent whenappiied to sheets so that the plate or anodes may be stamped or formed therefrom subsequert to the carbonizing step. Thestamped or formed electrodes may then be put through a degasifying or vacuum treating operation as described above.

In the practice of my invention 1 have employed various members of the hydrocarbons of the parafiin series, some of which are gaseous at room temperatures, such as methane, and others of which have higher boiling points but high vapor pressures and which may be carried over the heated metal surface by saturating an inert or reducing gas with the vapor of the hydrocarbon liquid at room temperatures.

The particular hydrocarbon of the parafdn series which i prefer to employ in the carbonizing step is commonly known as petroleum ether, which may be purchased on the market.

Petroleum other is a composite hydrocarbon having a high vapor pressure and a boiling point ranging from 30 C. to -C., and may be introduced into the carbonizing process as a Vapor by bubbling the hydrogen through the liquid petroleum ether in a manner shown and described in my specific description.

I have determined that the paratfin series a very pure carbon as a decomposition prod-' not. With the refractory metal bodies commonly employed in the forming of anodes, grids and grid and plate supports in electron discharge devices, such as iron, nickel, or refractory alloys of these metals with copper, chromium, manganese, etc., the carbon deposited at this temperature forms a close ly adherent film, which film is very diflicult to remove even by abrasion methods, particularly when the metal body has been previously freed of the surface adherent oxide layer and which film may subsequently be caused to uniformly adhere to the surface and prevent the formation of surface oxide compounds by suitable treatment in vacuo. As a specific embodiment of my process I will describe the procedure emploved in coating nickel with an adherent gas-free carbon layer.

In the accompanying drawing, Fig. 1 is a cross sectional schematic diagram of the apparatus employed in the first or deoxidizing and carbonizing step of myprocess; and,

Fig. 2 is a cross sectional schematic diagram of the apparatus employed in the second or degasification and cementation step of my process.

In Fig. 1, the carbonizing tube 1 which may be silica. iron, or any heat resistant al-.

loy is mounted in a furnace 2 which may be heated electrically or by gas as desired. Suitable connections such as 3 should be provided for passing the carbonizing gas through the carhonizing tube 1, and where there is considerable length to the tube 1, suitable piping 4 for reversing the gas flow, I prefer to provide a shunting means, such as indicated at 5, of passing the hydrogen through the petroleum ether without interrupting the flow of gas through the carbonizing chamber and carrying the resulting gas mixture vto the carbonizing tube 1. I also provide a method,such as indicated at 6. of effecting a refilling of the chamber 7 with petroleum ether as the supply therein becomes exhausted. Suitable valves 8 are provided where indicated to provide a way of controlling the process. In Fig. 2 the evacuating chamber give a small flame at the gas outlets 9 at the opposite ends of the carbonizing chamber 1. When the air has thoroughly been displaced. thechamber 1 may be heated to the carbonizing temperature. Furnace 2 should i The hydrogen is thenpassed gen is diverted through the petroleum ether by closing valve 8a and opening valves 8b and 8a, and the carbonizing continued within the indicated temperature range for a period of time suflicient to effect complete surface coating. By regulating the gas flow with respect to the diameter of the carbonizing tube, length of heating zone and surface area to be carbonized a uniform time interval may be established.

' In practice I have observed the following ichedules in the carbonizmg of nickel suraces Under these conditions the time interval is approximately one hour.

I have found in the practice of this carbonizing method that the direction of the gas flow through the carbonizing chamber should be reversed at frequent intervals. prefer to reverse the gas flow at 10 minute intervals, in order to equalize, if possible, the carbonizing efl'e'ct at both ends of the carbonizing chamber.

At the conclusion of the carbonizing process the furnace is turned off and the chamber and contents allowed to cool to temperatures below 300 (3., periodically reversing the hydrocarbon saturated gas ow until after the temperature has been lowered to 500 C. Thereafter until cooled to room temperature the hydrocarbon gas may be shut off and an atmosphere of pure hydrogen passed through the chamber.

The cooled carbonized metal parts are then cleaned of the excess loosely adherent carbon by brushing or by tumbling in sawdust, and are ready for the second step .in my process.

The second step comprises a high temperature vacuum treatment for the purpose of removing adsorbed or absorbed gases and in the accompanying drawing (Fig. 2) is similar to that used in the carbonizing procedure with the exception that instead of gas flowing through the chamber 1, the chamber is hermetically sealed and connected in any suitable manner to an exhaust system as indicated, wherein a vacuum approximating 0.25 mm. of mercury may be obtained within the chamber. Valve 8 is placed in between the evacuating means and the chamber to facilitate handling.

The carbonized metal parts, after cleaning off the excess carbon deposit, are placed in the chamber and heated by any suitable means to between 850 C. to 950 C. for from one to one and one-half hours or until a gas pressure of not more than 0.25 mm. has been maintained for a prolonged period of time. The heating of the metal parts during the degasification process should proceed from room temperatures to the maximum temperature with regular increases in the temperature gradient while evacuating the liberated gases continuously as liberated.

The degasified carbonized parts should then be cooled to room temperature before exposure to the air and until used in the construction of electron discharge devices sure to air prior to incorporation within theelectron discharge device.

Nickel parts so treated and carbonized substantially as described will be found to have thereon a closely adherent carbon coating which moreover is substantially free of electronically decomposable intermet-allic compounds which would deleteriously affect the operatingefiiciency of the hot cathode of an electron discharge device, and moreover will be found to be substantially free from deleterious and diflicultly evacuated gases. It is believed that the temperatures employed are not sufiicient to effect appreciable formation of metallic carbide formation except in so far as is necessary to cause adherence of the deposited carbon to the surface of the metal body.

When these metal parts are incorporated within an electron discharge device they will be found to more efiiciently radiate the heat energy than heretofore obtained by other methods of applying the carbon coating and.

thereby reducing and substantially eliminating that source of back-emission trouble which the uncoated electrode material norto cause the complete'cementation of the demally would develop. In addition thereto,

posited carbon to the surface of the metal ody. The apparatus employed as indicated the high degree 5f purity obtained in the carbon deposit substantially eliminates that source of back-emission which has been encountered in prior processes from the electronically decomposable compounds inadvertently included therein.

Having broadly outlined the scope of my invention and specifically described the same as regards to nickel, it is obvious that many departures from the specific process may be made without departing from the nature of the same. By the process as described with nickel and within the temperature and times specified, I have been able to successfully carbonize iron, various nickel alloys with manganese, iron, chromium, copper, etc. More refractory metals, such as molybdenum and tungsten would require substantially higher temperatures to produce the same effect as is at present obtained with nickel, iron or the alloys of these metals with other metals.

Such variations are anticipated as should fall within the scope of the following claims.

lVhat is claimed is:

1. The method of producing adherent carbon coatings on metal surfaces which comprises heating a metal first in an atmosphere of a gas reducing with respect to the oxygen compounds of the metal thereafter introducing into said gas atmosphere without intervening exposure of the metal to other gases a proportion of the vapor of a hydrocarbon of the paraflin series, and continuing the heating of the metal body, the temperature of heating approximately the decomposition temperature of the hydrocarbon vapor employed. I

2. The method of forming an adherent carbon coating on nickel surfaces comprising heating the nickel to approximately 800 C. in an atmosphere of hydrogen subsequently introducing into the hydro en atmosphere a proportion of the vapor oi a hydrocarbon of the paratlin series and continuing the heating until a suflicient depth of carbon coating has been obtained.

' 3. The process of forming an adherent carbon coating on the surface of refractory metals, the oxides of which are reducible by hydrogen, which comprises heating a metal body to approximately 800 C. in an atmosphere of pure'dry hydrogen to effect reduction and removal of the oxygen compounds of the same, and thereafter introducing into the hydrogen atmosphere the vapor of a hydrocarbon of the paraflin series and continuing the heating at the same temperature until surface carbonization has become effected.

4:. The process of forming an adherent carbon coating on the surface ofa nickel containing metal, which comprises heating a nickel containing metal in an atmosphere of pure dry'jhydrogen to temperatures approxi mating 800 C. to effect substantial removal of the oxygen containing compounds therein and thereon" and thereafter introducing into the hydrogen atmosphere the vapor of a hydrocarbon of the paraflin series, and continuing the heating at the same temperature until surface carbonization has become effected.

5. The process of forming an adherent carbon coating on a nickel surface which com prises heating the metal body in an atmosphere of pure dry hydrogen to temperatures approximating800 C. to effect substantial removal of the surface oxides and thereafter introducing into the hydrogen atmosphere the vapor of petroleum ether, andcontinuing the heating at the same temperature until surface carbonization has become effected.

6. The process of treating electrodes for use in electron discharge devices which comprises heating electrodes in a pure dry hydrogen atmosphere to effect reduction and removal of oxygen containing compounds therein and thereon and thereafter carbonizing the cleaned metal surface by heating the electrodes in an atmosphere containing a proportion of a hydrocarbon of the paraflin series to the decomposition temperature of the same, without intervening exposure to atmospheric gases.

7. The process of treating nickel electrodes for use in electron discharge devices which comprises heating nickel electrodes in a pure dry hydrogen atmosphere to temperatures approximating 800 (1, and thereafter introducing into the hydrogen atmosphere a proportion of the vapor of a hydrocarbon of the parafiin series and continuing the heating at approximately the same temperature for a suficient interval of time to effect surface carbonizing of the nickel electrode.

8. The process of treating electrodes for use in electron discharge devices which comprises deoxidizing the electrodes by heating in a pure dry hydrogen atmosphere,-carb onizing the cleaned, surface by heating in avapor of a hydrocarbonof the paraffin series without intervening exposure to atmospheric gases and thereafter effecting cementation of the carbon coating to the metal base and substantial degasification of the carbonized electrodes by heating the same in a high vacuo prior to utilizing the electrodes as incorporated parts of arr-electron discharge device.

9. The process of treating nickel electrodes for use in electron discharge devices which comprises deoxidizing the nickel electrodes by heating in pure dry hydrogen at temperatures 'a roximatin 800 C. carbonizin the PP z: a b

deoxidized surface by introducing into the hydrogen atmosphere a proportion of a vapor of a hydrocarbon of the parafiin series and continuing the heating without intervening exposure to atmospheric gases, and thereafter 'degasifying and cementing the carbon coating to the metal surface of the electrodes by heating them to approximately 900 C. in a vacuo of the order of .25 mm. of mercury prior to incorporating the electrodes into the electron discharge device.

.10. As an article of manufacture deoxidized nickel coated with a substantially pure adherent carbon 'film. I

11. As an article of manufacture, deoxidized nickel coated with an adherent substan-' tially pure degasified carbon film.

12. As an article of manufacture, an electrode for electron discharge devices composed of deoxidized nickel coated with an adherent substantially pure carbon film.

13. As an article of manufacture, an electrode for electron discharge devices composed of deoxidized nickel coated with an adherent gas-free substantially pure carbon film.

In testimony whereoi, I have hereunto subscribed my name this 23rd day of May 1928.

. CHARLES V. IREDELL.

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