US3423562A - Glow discharge apparatus - Google Patents

Description

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CLAUDE K. JONES, sTuART w. MARTIN, fw. Lm

THEIR ATTORNEY:

Jan. 21, 1969 Filed June 24, 1965 y C. K. JONES ETAL GLOW DISCHARGE APPARATUS POWER SUPPLY INVENToRs'.

CLAUDE K. JONES, l

STUART W. MARTIN,

BY w. @WMM THEIR ATTORNEY.

c. K. JONES ETAL 3,423,562

GLOW DISCHARGE APPARATUS Jan. 21, 1969 Filed June 24, v1965 sheet 3 of 5 POWER SUPPLY FIGB VOLTAGE Ec I l I i CATHODE DISTANCE ANODE INVENTORSI CLAUDE K. `10m-2s, STUART w. MARTIN,

THEIR ATTORNEY.

'United States Patent Claims ABSTRACT OF THE DSCLOSURE A glow discharge apparatus is provided for treatment of nonuniform' workpieces at uniform temperatures. The apparatus includes an outer metallic gas-tight chamber with an insulated cathode extending therein through a gas-tight insulated conection. A hollow electrically conductive container having a removable cover for insertion of the workpieces is ldisposed in the chamber and electrically connected to the cathode. A -glow is induced on the interior walls of the container and surfaces of the nonuniform workpieces therein lby providing an opening in the container wall large enough to preclude glow reinforcement within said opening as well as to admit the glow inducing means so as to act on the interior walls of the container, however yet arranged to prevent substantial radiation loss from the workpieces to the outside of the container through said opening.

This invention relates to an improved glow discharge apparatus for nitriding or otherwise treating [a plurality of workpieces in a glow discharge at uniform temperature Without regard to size, shape, material or arrangement of the workpieces within the apparatus.

Various arrangements have been disclosed for treatment of workpieces in the presence of a glow discharge, using various types of gases at low pressure inside a vacuum chamber. The gas ions subject the workpiece to high energy bombardment under the influence of the difference in electrical potential between anode and cathode to perform such operations as heating, nitriding, denitriding, etc. The energy lost by the Abombarding ions serves to heat the workpiece (which is usually connected as the cathode) to an elevated temperature. The ultimate temperature attained by a particular workpiece is -due to the energy lost by the ions plus the additional energy received by the workpiece by radiation from surrounding objects, minus the radation loss of the workpiece itself.

Since single workpieces are usually symmetrical, there has been little `ditiiculty in obtaining uniform temperature in a glow discharge, since a sin-gle object can be arranged within a symmetrical glow discharge chamber so as to radiate, and to receive radiation in substantially the same manner in all directions. Difliculties arise in attempting to treat a plurality of objects uniformly with the glow discharge process, Where it is desired that the objects attain uniform temperature. It has been proposed, in this regard, to arrange identical objects in a uniform and symmetrical manner so that each workpiece is disposed the same with respect to all other workpieces and the Walls of the container in a radial pattern. This Imethod is cumbersome and results in odd shaped glow discharge apparatus which is suitable only for workpieces of a given preselected shape.

It has also been known to employ suitably disposed radiation shields within the vacuum chamber to attempt to control or reduce heat radiated from the object under treatment. Such radiation shields are partially effective, but great care must be exercised in placement of the shields, since their ultimate temperatures are primarily a function of radiation gains and losses, which are not easily predetermined. f

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Where a plurality of objects of nonuniform size, shape, and arrangement lare to be treated, it has previously been difficult, if not impossible, to cause the various portions of the workpieces to achieve a uniform temperature. When nitriding in a glow discharge, for example, it is necessary to achieve this uniform temperature, in order to obtain a nitrided surface of uniform depth and quality, as well as to prevent distortion during the nitriding process. Also, it may occasionally be desirable to nitride objects of different material at the same time, and unless the temperature of the parts is accurately known, the result of the treatment cannot be accurately predetermined.

Accordingly, one object of the present invention is to provide an improved glow discharge apparatus suitable for treating a plurality of workpieces at uniform temperature in a glow discharge.

Another object of the invention is to provide apparatus for nitriding a plurality of objects in a glow discharge, the objects being of nonuniform size, shape, or arrangement within the apparatus.

Still another object of the invention is to provide an improved process for nitriding multiple workpieces in a glow discharge at uniform temperature.

The subject matter of the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of practice, together with further objects and advantages thereof,l may best be understood by reference to the following `description taken in connection with the accompanying drawings in which:

FIG. l is an elevation view, partly in section, of glow discharge apparatus illustrating a preferred embodiment of the invention,

FIG. 2 is a transverse cross section of the apparatus, taken along lines II-II of FIG. 1,

FIGS. 3 through 7 are simplified schematic views of modied forms of the invention, and

FIG. 8 is a graph for aid in understanding the operation of the invention.

Briey stated, the invention is practiced by completely surrounding the workpieces with a container which is connected as the cathode together with the workpieces so that it is also subjected to the glow discharge, and providing additional means enabling the glow discharge process to take place inside the container without substantially reducing the containers effectiveness as a radiation control device.

Referring now to FIG. 1, of the drawing, a metal bellshaped dome 1, cooled by suitable cooling tubes 2, is attached to a metal supporting base 3 by bolts (not shown) and provided with a gas-tight seal, such as O rings 4. Dome 1 and base 3 are of electrically conductive material, serve as au anode since they are connected to the positive terminal of a regulated power supply 5 providin a DC voltage. A vacuum pump 6 and a regulated gas supply 7 provide for exacuation of the chamber and control of the treatment gas. For example, if the parts are to be nitrided, gas supply 7 will supply a suitable mixture of nitrogen and hydrogen, while pump 6 maintains the chamber at a few millimeters of mercury pressure.

The negative terminal of power supply 5 is connected to an internal cathode 8, via a metal plate 9 attached to base plate 3 and insulated therefrom =by nylon screws 10 and an insulating plate 11. Cathode 8 is insulated from base 3 by an insulating tube 12, and the tube is protected from the glow discharge by a metal sleeve 13 at a oating electric potential.

Although the bell-shaped dome 1 and base 3 are connected as the anode, an auxiliary anode is provided by means of a thin metal cylinder 14 supported from the base 3 by metal legs 15 so as to be electrically connected thereto.

In accordance with the invention, a plurality of nonuniform workpieces such as cylinder 15, rectangular prism 16, pyramidal-shaped member 17 are disposed in a thin metal container 18. It will be understood that nonuniform as applied herein means nonuniform as to size, or shape, or arrangement, or material, or any combination of these. The container 18 is electrically connected to cathode 8. The workpieces 15, 16, and 17 are resting on the bottom of metallic container 18, so that container and workpieces are all at the electrical potential of the cathode 8.

Container 18 includes a removable cover 19, so that after the workpieces have been inserted, the container will completely surround the workpieces in so far as the possibility of radiant loss is concerned. However, it should be specifically noted that one or more holes 20 are provided in container 18 in order to allow the ions to migrate or move to the inside of the container. As will be more particularly noted in the description of the operation of the device, hole 20 should be large enough in relation to the size of the container so as not to interfere with ion movement in accordance with the various inlluences, such as space charge distribution and gas pressure. We have found that the diameter of the hole should be at least 1/100 of the distance from the hole to the most remote location inside the container. On the other hand, the hole should be as small as possible and so disposed in the container wall as not to allow substantial radiant heat loss therethrough as will be explained. If more than one hole is used, the number should be kept to a minimum.

Dotted lines have been used to indicate the presence of the glow layer which surroundsA the objects 15, 16 and 17. The glow layer also surrounds container 18, both inside and out, since it isalso vconnected as the cathode.

Reference to FIG. 2 of the drawing, illustrates a sectional plan view of the apparatus. In this ligure, To represents the outside temperature of the vacuum dome 1, Ta represents temperature prevailing at the auxiliary anode 14, and T15, T16, T11 and T18 are temperatures prevailing at the Irespective cathode-connected parts. EaL represents the electrical potential at theanode, and Ec represents the electrical potential at the cathode.

The operation ofthe invention will nowy be described with reference to FIGS. 2 and 8. In FIG. 8, which is a conventional presentation of space charge distribution in a glow discharge, it will be seen that substantially all of the voltage drop occurs closetothe cathode, while there is very little change in potential for most of the distance between anode and cathode. The foregoing graph of FIG. 8, of course, applies whenl a stable glow is in progress and is seen in practice by the fact that the brightest glow lies close to the surfaces of the cathode-connected members, as illustrated by dotted lines 15a, 16a, 17a, and 18a in FIG. 2. This illustrates the fact that, at a considerable distance from the anode and at a point almost up to the cathode, the space potential is still at a value approaching E11. However, since there is a slight drop in space potential from the anode to the inside of the container, there is a distinct influence causing the ions to move through hole 20 to the inside ofthe container. e

Providing that hole 20 is made sufficiently large, the glow will take place inside of the container'and there will be practically no reduction of the energy available for ion bombardment of lthe workpieces 15 16 and 17, despite the fact they are not in direct line with the surrounding anode 14. In other words, by suitably selecting the size of one or more holes such as 20, the vions inside container 18 will still have substantially the same ener-gy available for transforming lthe workpieces as if the container were not present.

Uniformity of temperature of all portions of workpieces 15, 16 and 17, despite their irregularity in size, shape, and disposition., is achieved by the fact that container 18 completely surrounds the parts and is itself connected as a cathode and also subjected to substantially the same glow discharge energy as the workpieces. Heat gains and losses by convection in this type of apparatus are negligible. Each member in the apparatus receives and gives up energy by mutual radiation from and to surrounding objects in accordance with the Stefan-Boltzmann law, where the net intensity of energy ow is proportional to the difference in absolute temperatures raised to the fourth |power. Therefore, differences between the temperatures of workpieces 15, 16 and 17 and the temperatures of their surroundings would ordinarily cause great nonuniformity of temperature in such a nonsymmetrical arrangement.

In accordance with the present invention, the container 18 is subjected to the same glow discharge intensity as the members within the container. Initial variations in intensity of the radiation pattern from pieces 15, 16 and 17 as it falls on container 18 are believed to be eliminated by conductor and temperature averaging in the walls of the container. Hence its temperature T18 will be ultimately the same as each of the'objects inside the container, i.e., T15, T16 and T11. Each object will see other objects at the same temperature and hence there will be no net radiant energy ow between container and the parts inside it. By this means, the irregularity of the workpieces is no longer of any consequence.

By utilizing the above specified thin metal container 18 and by providing the relatively large hole 20, glow reinforcement, sometimes called the hollow cathode effect, within the hole 20= is avoided. If glow reinforce-V ment should occur within the hole 20, it would thereby become a hot spot which would lprevent the workpieces from seeing the vsame temperature in all directions and would defeat any attempt to `surround the workpieces with a uniform temperature. This condition would be aggrevated by the use of a long hole in a thick wall which hol-e would provide a greater area for glow reinforce- `lent.

The auxiliary anode illustrated in FIG. 2 is at a temperature Ta depending primarily upon radiation losses from container 18 (since the anode receives little or no temperature increase due to the action of the glow). The outer vacuum dome 1 will be at a temperature slightly elevated above room temperature, primarily due to radiation losses from the auxiliary anode which is heated by container 18 as explained above.

By way of example, the members 15, 16, 17 might be at a temperature of 1000 F., container 18 at a temperature of 1000 F. plus or minus a few tenths of a degree, and the wall of chamber 1 would be on the order of room temperature plus F. The addi-tion of container 18 and the fact that the glow covers its insidel and outside surfaces might at rst be thought to `substantially increase the energy required in lthe process. This is not true, however, since most of the energy required is to compensate for radiation losses. Since 'the container is merely substituted for the group of workpieces in so far as its relation to the outside surroundings is concerned, the radiation losses are no greater than before and, for some configurations, they mightbe less than if there were no container 18.

Although the arrangement `shown in FIG. 1 has lbeen highly satisfactory, FIGS. 3 through 7 illustrate suitable modications of the invention operating on the same principle.

In FIG. 3, the vacuum chamber is shown as 21, power supply 22, gas supply 213, with electrode 24 leading 'into the vacuum chamber'through gas-tight insulator 25. Electrically connected to the cathode 2 4, as before, is a container 26 containing workpieces 27. Instead of allowing the glow current to enter container 26 through a hole,

however, an auxiliary anode 28, preferably a thin circular disk, is disposed inside container 26. This is preferably accomplished by a lead 2'9 connected electrically to the top of the anodic Vacuum chamber 21 and passing through a hole 30a in the removable lid 30 of container 26.

In the arrangement of FIG. 3, the glow will again cover the inside surface of the container, as well as the surfaces of workpieces 27. The inner surface of container 26 will be at the same temperature as parts 27, as before. The anode 28, which would otherwise be cold is heated by radiation from above and below to assume substantially the same temperature as the sidewalls of container 26, and hence is acting in this case as a hot anode.

Reference to FIG. 4 illustrates another modification of the invention. Here, the arrangement is similar to that in FIG. l, except that instead of having an auxiliary anode cylinder surrounding a container 26, the walls of the vacuum chamber 21 'serve as the only anode. Here the container 26 is provided with suitable holes 31 selected in accordance with the same criteria as discussed in connection with FIG. l. The holes must not be made so large as to allow substantial radiation losses therethrough to the relatively cold walls of vacuum chamber 21. Hence the arrangement is somewhat less satisfactory for attaining uniform temperature than the arrangement shown in FIG. 1 where the cylindrical auxiliary anode serves to block some of this radiation loss, being at an intermediate ltemperature. However, the arrangement of FIG. 4 is less expensive and may be suitable in some cases where Very precise temperature control is unnecessary.

FIGS. and 6 illustrate yet another modication 0f the invention, FIG. 6 'being a cross section taken along lines VI-VI of FIG. 5. The workpiece container 33 is made by ybending a sheet of metal with an increasing radius `so that the walls overlap and are spaced from one another to provide a vertical gap 34 for `access of ions to the interior as before. Since radiation from the workpieces is along straight lines, the workpieces will all see the same temperature as long as they are placed out of direct line of the gap 34, i.e., above 4and to the right of line 35. Of course, the container 33 is connected to the cathode as before and is provided with a cover 36 for inserting the parts lto 'be treated.

FIG. 7 is a modification illustrating the use of a variable size opening. A container 40 has a cover 41 with an opening 42. The opening is shielded against radiation losses `by a disk 43 which is arranged to be movable to vary the effective area of the opening. In order to accomplish this variation, the disk 43 is attached to a rod 44 operated by an outside handle 45. A suitable gland seal 46 prevents leakage. Additional fixed holes 47 may be employed in conjunction with the variable opening 42. The foregoing arrangement may be used to reduce the glow energy on the workpieces during the initial processing and then the disk 43 may lbe used to increase the opening after the parts are at an increased temperature.

Thus it will be seen that the invention provides means to treat a plurality of workpieces in a glow discharge without particular regard to size, shape and disposition of the parts. They are simply placed in the cathodic container at random, which completely surrounds them from a radiation standpoint except for suitable means to induce the glow discharge inside the -container without substantial radiation loss from the parts to the outside surroundings. This is conveniently accomplished Iby small holes in the container, overlapping walls or by an interior auxiliary anode.

It has been found that one added advantage of the invention is a reduction of the possibility of damage to the workpieces by a-rCing. The cathodic container is subjected to the glow and is in a more direct line with the anode than are the workpieces. Hence, in the event of an arc, it has been found that the arc is more likely e to occur on the surface of the container than on the surfaces of the workpieces, thereby protecting them from damage.

Modifications other than those disclosed will become apparent to those skilled in the art, .and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of this invention.

What is claimed as new and desired to secure by Letters Patent of the United States is:

1. In a glow discharge apparatus having outer metallic walls forming a gas-tight chamber and an insulated cathode extending into the chamber through a gas-tight insulated connection, said apparatus having a plurality |of workpieces therein connected to said cathode for the purpose of treating the same in a glow discharge at uniform temperature, the combination of:

a hollow electricallyconductive container disposed in said chamber and electrically connected. to said cathode, said container having walls surrounding said workpieces and provided with a removable cover for the insertion of said workpieces therein, and

means for inducing a glow on the interior walls of the -container as well as on the surfaces of the workpieces therein, said means including at least one opening through the container wall large enough to preclude glow reinforcement within said opening as well as to admit the glow inducing means so as to act on the interior walls and the workpieces but yet arranged to prevent substantial radiation loss from the workpieces to the outside of the container through said opening.

2. The combination according to claim 1 wherein an auxiliary anode is disposed in said chamber and surrounds said container so as to reduce radiation loss through said opening.

3. The combination according to claim 1 wherein an auxiliary anode is disposed in said chamber, and which al-so extends into the interior of said container Iby a lead passing through said opening.

4. The combination according t=o claim 1 including means for varying the size of said opening while the glow discharge is taking place.

5. The combination according to claim 1 wherein said container comprises a substantially cylindrical sidewall member electrically connected to said cathode along with the workpieces, said member having said opening arranged along the cylindrical sidewall.

6. The combination according to claim 5 wherein said opening is a hole, the diameter of which is at least 1/100 of the distance from the hole to the most remote part of said container.

7. The combination according to claim 5 wherein said opening is a gap defined by overlapping and spaced wall portions of said cylindrical sidewall member.

8. Glow discharge apparatus for nitriding a plurality of workpieces of nonuni-form size, shape or disposition at uniform temperature comprising:

an outer Vessel having metallic walls providing a gastight chamber with an insulated cathode extending into the cham-ber through a gas-tight connection,

a source of electrical power for connecting the outer vessel as the anode and connected to said cathode for providing a glow discharge in said chamber,

means for providing a low pressure nitrogenous atmosphere in said chamber,

a hollow electrically conductive container disposed in said chamber and electrically connected to said cathode, said container having a removable'cover for insertion of the workpieces and having walls surrounding the workpieces, and

means for inducing a glow on the interior walls of the container as Well as on the surfaces lof the workpieces therein, said means including at least one opening in the container wall large enough to admit the glow inducing means to the interior of the container and to prevent glow reinforcement within said opening, but arranged with respect to the workpieces to substantially impede radiation loss therefrom to the outside of the container through said opening. 9. A process for treating a plurality of workpieces of non-uniform size, shape or disposition at uniform temperature in a glow discharge comprising:

providing an electrically conductive hollow container having `at least one opening in the walls thereof,

dimensioning said opening to provide a passage large enough to preclude glow reinforcement and to admit a glow inducing means, connecting both workpieces and container as the cathode,

arranging said pieces inside the container so that radiation losses therefrom through said opening will be negligible, and

inducing a glow to take place on the interior of the container and on the workpieces through said opening.

10. The process according to claim 9 including the'additional step fof varying the-size of said opening while the glow is taking place.

References 'Cited UNITED STATES PATENTS RICHARD M. WOOD, Primary Examiner.

5 WD. BROOKS, Assistant Examiner.

U.S. Cl. X.R. 313-782

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