Plasma Spray Gun And Method For Applying Coatings On A Substrate

Abstract

A method and plasma spraying device for more efficiently depositing heat fusible materials on a substrate. The improved efficiency referred to above is achieved by altering the flow characteristics of a gaseous material as it enters a plasma forming environment such as that produced by a pair of spaced-apart arcing electrodes. The flow of gas is controllably altered by a gas distribution ring which is capable of producing a linear or axial gas flow in combination with a helical gas flow. As the mixed flow of gas is converted into a plasma, its speed is accelerated and the axial flow component is gradually converted into a spiraled or helical flow. Whereupon, the heat fusible material introduced into the plasma is thermally liquified and ejected at or near the speed of sound.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is directed generally to plasma guns and particularly to an improved means and method for introducing a gas into a plasma-forming environment.

2. State of the Art

The use of plasma guns for converting a gaseous medium through electrical energy into heat and thereby achieving a high temperature, high velocity, gaseous stream suitable for applying a coating on a substrate is well known (e.g., U.S. Pat. Nos. 3,676,638 and 3,312,566 and British Pat. No. 1,087,173).

In such plasma guns, a coating material is introduced into a high temperature plasma stream (8,000.degree.F and higher), thermally liquified and ejected from the gun on the substrate at or near sonic and supersonic speeds.

The plasma's high temperature is obtained by applying a voltage sufficient to cause arcing between a pair of spaced-apart electrodes. In so doing, electrons are released from one of the electrodes (cathode) and as the electrons gain kinetic energy from the field, they move at accelerating velocities toward the other electrode (anode).

When a free electron field has been developed, the atoms and/or molecules of the plasma forming material (normally a gas), which has been introduced therein, collide with the free electrons. During these collisions some of the kinetic energy of the electrons is transformed and absorbed by the molecules as heat energy. As the temperature of the gas increases, some of the molecules or atoms are ionized yielding additional electrons. As ionization continues, the collisions become more frequent increasing the conversion of kinetic energy to energies of heat and ionization. Eventually the gaseous material will take on a characteristic which is normally referred to as a "high temperature plasma" state.

In order for this high temperature plasma state to be sustained, it is necessary that a continuous source of electrons be provided and that a continuous supply of plasma-forming material be made available. U.S. Pat. No. 2,960,594 identifies these prerequisities as (1) minimum power requirements (for providing the necessary electrons), and (2) minimum gas flow (for providing the prerequisite number of ions).

This patent further states, in effect, that if the power requirements and/or if the gas flow falls below these minimums the plasma-forming environment will not be sustained and a "flash back" condition occurs. Flash backing will normally result in a severe drop in temperature and loss of the plasma state.

With plasma guns currently available, relatively high power and gas flow requirements are required to sustain a plasma-producing environment. This, of course, results in a relatively high cost of operation. As a result, the use of plasma guns has been limited in commercial operations. In order for plasma guns to gain broader acceptance and in order that they may bcome more economically competitive with other spraying means, it would be highly desirable if the above minimum requirements could be lowered. In so doing, the cost of operation would not only be reduced but the life of the electrodes could also be extended thus reducing the down time and expense incurred for replacement.

SUMMARY OF THE INVENTION

A reduction in power and flow requirements for operating a plasma gun has been achieved by the apparatus and methods of this invention which comprise generally a substantially closed chamber wherein a first electrode (anode) defines a nozzle outlet from the chamber and a second electrode (cathode) extends into the chamber in spaced relationship to the first electrode. A means is provided for introducing an arc forming electric current across the electrodes. A plasma forming gas is introduced into the arc area in a particular manner such that the kinetic energy of the electrons emanating from the electrodes is effectively transformed into energies of heat for absorption by the plasma forming gas. In one embodiment, this is achieved by means of a specially designed gas ring which is capable of providing a substantially linear gas flow in combination with a helical or vortical gas flow. As the gas travels through the nozzle towards the outlet, the axial gas flow is gradually converted into a helical or vortical gas flow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional side view of the plasma spraying device;

FIG. 2 is an exploded cross-sectional side view of the main elements of the spraying device;

FIG. 3 is an isometric of the brass housing shown in FIG. 1;

FIG. 4 is an isometric of the insulated housing shown in FIG. 1; and

FIG. 5 is an enlarged partially cut away isometric of the gas distribution ring wherein the gas flow components emanating therefrom are illustrated.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown in FIGS. 1-4, the plasma spraying device has a handle housing 10 made from an insulating material such as rubber, plastic, synthetic resin and the like. A hollow electrical conductive cathode connector 12 runs longitudinally through the handle. One end of the cathode connector is adapted with a connecting means 14 for receiving a water-cooled electrical cable 16. The other end of the cathode connector is adapted with an electrical conductive bushing 18 for receiving and holding the male end of a bored water exit assembly 20 which in turn is adapted to receive and hold the male end of a bored cathode holder 22. An O-ring 24 is provided to insure a water-tight connection between the bushing 18 and the water exit assembly 20. Another O-ring 26 is provided to serve a similar purpose between the water exit assembly and the cathode holder 22.

A tungsten or thoriated tungsten hollow cathode 30 has a threaded rear section for screwing (not shown) into the cathode holder. The front end of the cathode has a conically shaped head 32 which is circumscribed by a gas distribution ring 34 capable of distributing a plasma forming gas into the area around the cathode in a particular manner. The design of the gas distribution ring and its use shall be subsequently described in greater detail. The water exit assembly 20, the cathode holder 22, the cathode 30 and the gas distribution ring 34 are encompassed and held in a fixed operative position by a bored insulated housing 36 constructed from an electrically non-conductive material such as Nylon, Teflon, and the like. A channel 38, traversing the insulated housing 36, is adapted at one end with a fitting 40 for receiving a plasma inlet line 42. The other end of the channel opens into an annular gas chamber 44 which is in communication with a plurality of slanted channel openings 45 and with at least one other opening 46 via an intermediate groove cut into the outer wall of the gas distribution ring. A bored disc plate 48 constructed from a refractory material such as aluminum oxide is mounted to the front face of the insulated housing 36. The inner circular edge of the bored disc plate 48 is in close proximity to the channel openings 45 and 46. The outer edge of the bored disc is adjacent to a plurality of water carrying passageways 50 bored axially through the insulated housing 36.

A centrally bored copper anode 54 having a water chamber 56 formed by an annular groove which is in communication with a plurality of axially bored water carrying passageways 57 is held to the insulated housing 36 by an overriding bored anode holder 58. The copper anode 54 is held in a position such that it is in proximal spaced relationship to the cathode 30.

The anode holder 58 is provided with a traversing conduit 60. One end of the conduit is adapted with a fitting 62 for receiving a water inlet line 64. The other end of the traversing conduit 60 is in communication with the water chamber 56 of the copper anode 54. The anode holder also contains a plurality of bored water carrying passageways 65 for directing cooling water toward outlet 16.

Mounted to the front face of the anode holder 58 is a centrally bored brass housing 70 having a conduit 72 radially bored therein. One end of the conduit is adapted with a fitting 74 for receiving an inlet line 76 for feeding a heat fusible material into a flared chamber 77 which is in communication with the outlet end 78 of the gun's nozzle. The outlet end 78 is formed by an open ended tubular member 79 having a frustrum shaped flared end piece 83 extending toward the anode holder 58 from a bored end piece 80. The flared end piece 83 is in spaced relationship with a matching flared end section 84 formed around the bore of the brass housing 70. The space between the two flared pieces 83 and 84 forms the flared chamber 77.

A plurality of axially bored water carrying passageways 86 circumscribe the bore in the brass housing 70. These passageways are in communication with an annular cavity 88 of the bored end piece 80. A second set of axially bored water carrying passageways 90, of larger diameter, circumscribe passageways 86 and are also in communication with the annular cavity 88. Cooling water enters the annular cavity 88 through passageways 86 and leaves the cavity through passageways 90. An O-ring 91 insures a seal between the flared chamber 77 and the water carrying passageways 86 and 88. The bored end piece 80 is held in sealing engagement against the brass housing 70 by means of an O-ring 92 and an annular lip 94 extending outwardly from the bored end piece 80.

A number of threaded bolts 96 extend through a corresponding number of matched openings positioned around the peripheral edge of the lip 94, the brass housing 70, the anode holder 58, the insulated housing 36 and engage a threaded opening in the handle housing 10 for holding these elements in an aligned and fixed position. Additional O-rings 98 are provided for insuring a sealing engagement between the above elements near the openings through which the bolts 96 pass.

As shown in FIG. 5, the gas ring 34 earlier referred to comprises a ceramic sleeve 100 having an annular groove 102 along one of its end. A plurality of inwardly slanted conduits 104 extend from the groove to the interior wall 106 of the ceramic sleeve. The conduits are characterized in that they all converge (if extended) at a central focal point at or near the vicinity of the conical head of the cathode. A tangential opening 108 which is in communication with the annular groove 102 via an intermediate surface groove 109 is positioned anterior to the exit openings of the slanted conduits for directing a portion of the gas introduced as a spiraled component about the gas linearly introduced through the slanted conduits 104. With this type of gas ring a major portion of the gas introduced into the area of the cathode possesses a linear component while a minor portion is introduced tangentially to provide a spiral or vortical component which in effect circumscribes the linear component.

As the gas passes through the bore (nozzle) 77 of the anode 54 and the bore in the brass housing 70, the linear components of the gas gradually take on a spiraling characteristic until reaching the end of the central bore where the flow is essentially completely helical or vortical in nature with little if any of the linear component remaining.

With the above gas distribution ring for introducing a plasma forming gas into the plasma gun, a substantially lower minimum gas flow can be realized along with lower power requirements. This phenomenon is demonstrated by the following example in which the operating parameters and the test results are reported in Table 1 which immediately follows the example.

A plasma gun of the type shown in the drawings and hereinabove described possessed a nozzle having a diameter of about 0.228 inches and a length of about 1.375 inches. Approximately 20 cubic feet per hour of plasma forming gas was introduced into the gun through the means provided. The gas introduced comprised a mixture of 8 cfh nitrogen and 12 cfh argon. An electrical input of 95 amperes and 47 volts was applied to the cathode having a diameter of about 0.40 inches. A heat fusible material comprising primarily tungsten carbide and having a particle size of about 25-50 microns was introduced into the heated plasma at a rate of about 0.05 lbs./min. As the gas and softened heat fusible material exited from the gun at a velocity approaching the speed of sound, they were directed against a sheet of aluminum. Upon contact the heat fusible material solidified forming a thin, evenly distributed coating across the surface of the aluminum sheet.

TABLE 1 __________________________________________________________________________ Min. Nozzle Nozzle Gun Est. Cath. Dia. Length Flow Temp. Gas Mix. Dia. Inches Inches c.f.h. F.degree. Amps Volts % N.sub.2 % Ar Inches __________________________________________________________________________ .25+ 1.25 19 8500.degree. 60-100+ 47 40 60 0.40 .375 1.25 19 8500.degree. 50 45 40 60 0.40 __________________________________________________________________________

To better understand the specific features of the plasma gun above described, a more detailed presentation of certain specific operational and design features is now presented.

OPERATION OF PLASMA GUN

In coating a substrate with a thermally fusible material, a plasma forming gas is introduced through line 42 into the gas chamber 44 via intermediate conduit 38. The gas passes through the channels 104 and 108 of the gas distribution ring 34 and exits about the conically shaped cathode head 32. The gas is emitted as a substantially linear component converging at a point slightly in front of the cathode head. Concomitantly a spiraled flow component is also emitted about the substantially linear component.

Generally, the linear component of the gas introduced into the plasma forming environment will constitute at least 10% of the total volume of gas introduced.

Preferably the linear component will constitute between about 80 and 90 percent of the total gas volume and the helical or vortical component will constitute between about 10 and 20 percent of the total gas volume. As the gas is converted into a plasma and moves toward the exit end of the nozzle, the linear component will gradually take on a vortical component. About midway down the nozzle, the vortical component constitutes between about 30 and 40 percent of the total gas flow and the linear component constitutes between about 60 and 70 percent of the total gas flow.

A direct current is now applied to the spaced-apart electrodes 30 and 54 causing an arc to develop between them. The gas passing through the electrical arc is thermally energized by the electrons released from the cathode transforming the gas into a "high temperature plasma." After the linear component of the hot plasma is gradually converted into a substantially helical flow, a finely divided thermally fusible material is introduced into the flow of hot plasma through line 76 and into the flared chamber 77 via intermediate conduit 72. Because of the chamber's flared characteristics, the heat fusible material is introduced countercurrent to the flow of hot plasma. The heat fusible material is thermally liquified as it contacts the hot plasma and is ejected with the hot plasma gas through the nozzle portion 78 upon a substrate.

PLASMA GUN COOLING SYSTEM

During operation of the plasma gun, a flow of circulating cooling water is introduced into the gun via line 64 and into a water chamber 60 of anode 54. The water flows from the water chamber 60 through a number of water carrying passageways 57 in the anode and through a series of interconnecting water carrying passageways 65 and 86 in the anode holder 58 and the brass housing 70 respectively. The water finally enters a turnaround water chamber 88 in the guns end piece 80. At this point the direction of water flow is reversed and enters water carrying passageways 90 in the brass housing and eventually into the hollow cathode 30 via a number of interconnecting water carrying passageways 65 and 50 carried by the brass housing 58 and the insulated housing 36 respectively. The water exits through line 16 via water cooled electrical cables 12 located in the handle housing 10.

ELECTRODES

The cathode preferably has a conical head as shown in the drawings; however, a rounded or blunt head can also be advantageously used.

Normally, the cathode's conically shaped head will have an included angle of between 45.degree. and 60.degree. and a diameter of between 0.10 inches and 0.125 inches. The length of the cathode can be varied or adjusted to provide a distance betweeen the two electrodes which will produce an arc best suited for a particular use. In instances where a temperature of about 8,000.degree.F is desired and where the gun is to be utilized for spraying a material on a substrate, the distance between the two electrodes will normally be between about 0.015 inches and 0.100 inches.

The second electrode or the anode nozzle preferably has a length of between 1.125 inches and 1.375 inches and an internal diameter of between 0.200 inches and 0.250 inches. Normally the anode will be constructed from an electrical conductive material such as copper.

POWER PARAMETERS

With electrodes of the type above described and with a gas distribution ring of this invention, a direct current of between 30 and 200 amperes and a voltage of between 30 and 90 volts are normally used. The power requirements may vary to a degree depending on the type and amount of plasma producing gas that is introduced into the electrode area. For example, a diatomic gas will normally require lower power requirements than a monotomic gas. Further, the degree of ionization and the gas temperatures desired are also factors which must be taken under consideration in determining the optimum power requirements. In most cases, the most suitable conditions can be imperically determined for the particular use intended. In all cases, though, substantially lower power requirements are required when the gas distribution ring or if the gas flow characteristics wherein described are employed.

PLASMA FORMING GASES

To achieve operating parameters wherein a minimum gas flow of around 20 and 30 c.f.h. and an average amperage input of between 50 and 100 amperes are used, a plasma producing gas comprising a volume ratio of between 2 to 1 and 1 to 1 of monotomic gas to diatomic gas is preferred. Excellent results have been obtained wherein a mixture containing 60 percent of argon and 40 percent of nitrogen have been used. As a general rule, argon is more easily ionized than nitrogen at relatively low energy levels. Mixtures of the above two gases will normally require an energy level of between those required for the individual gases. Certain gas combinations also appear to be more suitable for achieving a particular temperature, especially if the temperature is below 10,000.degree.F. For example, if temperatures of under 1,000.degree.F are desired, a mixture of gases comprising 60 percent argon and 40 percent nitrogen is preferred. However, if a temperature in excess of 10,000.degree.F. is desired, a mixture comprising 50 percent argon and 50 percent nitrogen can be used to advantage.

COOLANTS

Generally a coolant such as water will be circulated through the gun as earlier described. However, other coolants such as glycol, refrigerants, etc., can also be used. In some cases, circulated air or other heat absorbing gases may also be used.

Normally the amount of liquids circulated will vary depending on the degree of cooling desired. In order to maintain optimum electrode life, it is preferred that the electrodes be maintained at a low temperature.

Generally, as the power requirements are increased, the volume of coolant introduced or recirculated is likewise increased assuming that the other operating parameters remain relatively constant.

SPRAYING HEAT FUSIBLE MATERIALS

Finely divided heat fusible materials are introduced into the plasma stream and emitted on a substrate in the manner earlier described.

Generally, though, the distance between the nozzle and the substrate is about 6 to 8 inches when the gun is being operated at an amperage of between 50 and 100 amperes. The distance is normally longer if the heat fusible material has a relatively low melting point and at a shorter distance if it has a relatively high melting point. The heat fusible material will melt or be softened upon contact with the heated plasma and will then be accelerated to speeds approaching sonic or supersonic speeds.

Most all of the synthetic thermoplastic materials such as polyethylene, polypropylene, polyamides, polyvinylchloride, polystyrene or polytetrafloroethylene are particularly suitable for coating either alone or in combination. Other materials such as glass, ceramics, resins, cellulose butyrate, and the like may also be used. The material to be deposited is usually introduced as fine particulates having a particle size of between -125 mesh and -200 mesh.

Any conventional material may be used as a substrate such as the metals, woods, plastics, ceramics, glass and the like.

While the invention has been described with reference to specific embodiments, it should be understood that certain changes may be made by one skilled in the art and would not thereby depart from the spirit and scope of this invention which is limited only by the claims appended hereto.

Claims

I claim:

1. A fluid distribution ring solely for use on an electric plasma spraying device comprising a substantially circular ring member having slanted tubular primary openings and at least one secondary opening extending from the outside to the inside surfaces of said ring, said primary openings being characterized by their ability to direct a major portion of a fluid passing through said openings to a focal point positioned along the axis of said ring to provide a substantially linear flow component and said secondary opening being characterized by its ability to direct a minor portion of said fluid tangentially along the inner surface of said ring to provide a substantially helical flow component.

2. The fluid distribution ring of claim 1 wherein said ring contains an annular groove for directing said fluid into said primary openings.

3. The fluid distribution ring of claim 2 wherein said ring contains a second groove along its outer surface which intersects said annular groove for directing fluid into said secondary opening.

4. The fluid distribution ring of claim 3 wherein the secondary opening exits from the inner surface of said ring at a point anterior to said primary openings.

5. A plasma spraying device comprising a substantially closed chamber, a first electrode defining a substantially elongated nozzle outlet from said chamber, a second electrode extending into said chamber and in spaced relation to said first electrode, a means for introducing an arc forming electric current across said electrodes and a means for introducing a plasma forming gas into the area of said arc wherein said means includes a gas distribution ring for directing a major portion of said gas along a substantially linear flow path and a minor portion of said gas along a substantially helical flow path which circumscribes the linear flow path.

6. The plasma spraying device of claim 5 wherein said linear flow component is gradually converted to an increasingly helical flow component as said gas passes beyond said first electrode.

7. The plasma spraying device of claim 5 wherein said gas distribution ring comprises a substantially circular ring member having slanted tubular primary openings and at least one secondary opening extending from the outside to the inside surfaces of said ring, said primary openings being characterized by their ability to direct a major portion of a fluid passing through said openings to a focal point positioned along the axis of said ring to provide a substantially linear flow component and said secondary opening being characterized by its ability to direct a minor portion of said fluid tangentially along the inner surface of said ring to provide a substantially helical flow component.

8. The plasma spraying device of claim 7 wherein said gas distribution ring includes an annular groove for directing said fluid into said primary openings.

9. The plasma spraying device of claim 8 wherein said gas distribution ring includes a second groove along its outer surface which intersects said annular groove for directing fluid into said secondary opening.

10. The plasma spraying device of claim 9 wherein said secondary opening exits from the inner surface of said ring at a point anterior to said primary openings.

11. A method for producing a high temperature plasma in an electric plasma spraying device, comprising introducing a major portion of a plasma forming gas into an area of a plasma producing electrical arc in a path which provides a substantially linear flow component and concomitantly introducing a minor portion of said plasma forming gas into a plasma producing arc along a path which provides a substantially helical flow component.

12. The method of claim 11 wherein the linear flow component is gradually converted into a helical flow component as the plasma forming fluid is being converted into a high temperature plasma.