Plasma generating apparatus and method

Abstract: A plasma spray apparatus (20) which comprises means (22) for generating a first plasma of ionized plasma gas; means (24) for generating a second plasma of ionized plasma gas intersecting the first plasma; means (86) for extending and electromagnetically coalescing the first and second plasmas into a free-standing plasma of ionized plasma gas; and means (80) for supplying feedstock into the free-­ standing plasma, whereby the feedstock is heated and accelerated in particulate form. A method of plasma spraying which comprises generating a first plasma of ionized plasma gas; generating a second plasma of ionized plasma gas intersecting the first plasma; extending and electromagnetically coalescing the first and second plasma into a free-standing plasma of ionized gas; and feeding a feedstock through the intersection of the first and second plasmas into the free-standing plasma, the free-standing plasma heating and accelerating the feedstock in particulate form as a spray suspended in the plasma gas.

Description: Plasma torches were developed primarily as a high temperature heat source and are now widely used commercially for cutting, welding, coating and high temperature treatment of materials. Conventional direct current commercial plasma torches or guns include a pointed rod-like cathode generally formed of thoriated tungsten axially located within a bore in the body portion of the gun and an annular anode located downstream of the cathode having a nozzle orifice coaxially aligned with the cathode. A plasma-forming gas, typically argon or mixtures of argon and helium or argon and hydrogen, is introduced into the body portion of the gun such that the gas flows in an axial direction around the cathode and exits through the anode nozzle orifice. Plasma generation occurs in the gun in the arc region between the anode and cathode. The plasma is typically formed by initiating an arc between the anode and cathode using a high-frequency starting pulse, wherein the arc heats and ionizes the plasma gas to temperatures of about 12,000 K. The heated and expanded plasma gas is then exhausted at high speed through the nozzle orifice. The gas flow through the gun can be axial or introduced in a manner so as to cause a vortex-type flow. The electrical characteristics of the plasma arc are determined by the gas flow rate, gas composition, anode nozzle orifice diameter and the electrode spacing. Where the plasma gun is used for spraying a coating, the feedstock is usually in powder form suspended in a carrier gas and injected radially into the plasma effluent, either internally or externally of the nozzle exit depending on the gun manufacturer. Because the temperature drops off sharply in the plasma after it exits the anode nozzle, the powder is preferably introduced as close as possible to the point of plasma generation. US-A-2 806 124 is an early disclosure of the basic principles of plasma technology and US-A- 3 246 114 includes an early disclosure of a commercial plasma gun. Because of the geometry of a plasma gun and potential cathode deterioration, as discussed below, it is not possible to introduce the feedstock material axially through a conventional plasma spray gun, although the potential advantages have long been recognized. In a typical plasma jet coating apparatus, the feedstock powders are introduced radially into the plasma stream downstream from the plasma origin, either perpendicular to the axis or inclined in a direction with or counter-current to the flow of the plasma jet. As will be understood, the plasma interferes with particle penetration with a resistance that requires particle momentum sufficient to penetrate to the axis of the plasma jet. The particle momentum is provided by the carrier gas. Further, thermal spray powders never have an absolutely uniform particle size and generally include a broad distribution of particle sizes. Carrier gas flow rate must further be adjusted dependent upon the particle size, wherein the smaller or lighter particles require a greater carrier-gas flow rate. Nevertheless, the particle injection velocity distribution will be broad even for a narrow particle size distribution and blends or mixtures of feed powders have very limited commercial applications. Therefore, heat and momentum transferred to the injected particles will vary over a wide range, resulting in a broad range of velocity and surface temperature distribution upon impact of the particles with the target or substrate. Because of the greater momentum of the larger or heavier particles, the larger particles will penetrate through the plasma jet and become entrained in the outer, colder gas region or ejected out of the plasma jet, resulting in unmelted fringe regions of the deposit coating. Very small or light particles of low momentum will fail to penetrate the plasma jet and will also be included in the fringe area. Very small particles which enter the plasma jet core may also overheat and vaporize. Therefore, only a fraction of the particles enter the core of the plasma jet and are deposited as a highly dense layer on the target substrate. The unmelted or partially melted particles may affect the density of the deposit. In a typical application, the deposition efficiency (i.e., the ratio of material fed into the plasma jet gun compared to the portion which actually forms the coating) is typically low, usually well below 70% for high melting materials, such as for example, oxide ceramics and intermetallic compounds. Unreactive gases, such as, for example, argon or helium, are employed as the plasma gas to avoid erosion or deterioration of the cathode electrode. As described above, the cathode is normally formed of thoriated tungsten and the electrode is operated at temperatures above 1000 degrees Centigrade. Diatomic gases, such as, for example, hydrogen or nitrogen, may be added to the inert plasma gas to enhance the power output of the plasma jet torch. However, reactive gases, such as, for example, oxygen, cannot be employed because reactive plasma gases would result in oxidation corrosion of the cathode. The use of reactive gases or reactive gas mixtures will cause the cathode to undergo local deterioration, thereby causing the cathode point of arc origination to wander, resulting in plasma arc instability or "arc wandering". However, it would be desirable in a number of applications to utilize certain reactive gases, such as, for example, oxygen or oxygen bearing gas mixtures as the plasma forming gas. For example, certain plasma jet applications result in oxygen depletion of the feedstock. The utilization of oxygen, for example, as the plasma gas would result in restoration of oxygen in the resulting coating and eliminate the requirement of a post-spray oxygen replacement anneal. It would also be very desirable to raise the operating power level of conventional plasma jet guns without decreasing energy efficiency or deterioration of the electrical components. In a typical plasma jet gun, the energy efficiency decreases as the operating energy level increases because of the inherently high electrical current operation and energy losses in the gun and power cables. Presently, energy is increased in a plasma jet gun by raising the current. Since the power input to a plasma jet gun is a product of the voltage and the current (Power = VxI), it would be desirable to raise the operating power level by increasing the plasma voltage rather than the current. Since the operating voltage is directly related to the plasma-forming gas used, as well as the cathode-anode spacing, it would be desirable to adjust these parameters for optimum operation. However, as described above, plasma forming gas selection is restricted to the group of unreactive or inert gases to avoid cathode deterioration. Cathode-anode spacing is limited due to the problems of initiating and maintaining stable plasma arc conditions with large interelectrode spacing. Thus, the present plasma jet technology is limited in at least three important respects. First, radial injection of powdered feedstock results in poor deposition efficiency, reduced density of the deposit and requires a narrow range of feedstock particle size where uniform coatings are required. Second, reactive gases or reactive gas mixtures cannot be used as the plasma-forming gas to avoid deterioration of the cathode and arc wandering. Finally, the operating power level of conventional plasma jet guns cannot be significantly increased without decreasing the energy efficiency. Various attempts have been made to avoid the problems of radial feed of plasma jet guns without commercial success. The principal solutions proposed by the prior art include (a) hollow cathode plasma guns, (b) RF (radio frequency) guns and (c) a plurality of plasma guns with a single feed. The hollow cathode gun, as the name implies, utilizes a hollow cathode tube, rather than a conventional rod-shaped cathode. The RF plasma gun employs a rapidly alternating electric field generated by a radio-frequency coil which replaces the arc as the plasma source. Although the hollow cathode and RF plasma guns have commercial promise, neither system has achieved commercial success. As evidenced by US-A- 3 140 380 others have tried to merge two or more plasma effluents into a "joint plasma effluent into which a coating material is fed and reduced to substantially molten particles" for deposition on a substrate. In the prior art apparatus disclosed in US-A- 3 140 380, a plurality of plasma guns or "plasma generating means" are "displaced symmetrically" with relation to a common axis such that the "plasma effluents are directed to intercept at a point and merged to form a join plasma effluent". The plasma effluents from the individual plasma torches are then fed through a nozzle opening in the common axis and wire or powdered feedstock is fed through the nozzle opening in the common axis. As will be understood, this method of forming a "joint plasma effluent" does not result in a single or coalesced free-standing plasma and the impinging plasma effluent results in turbulence at the point of impingement through which the feedstock is fed. Further, the temperature of the plasma effluent at the point of impingement through which the feedstock is fed is substantially lower than the temperature of the plasma cores, resulting in lower efficiency than would be obtained for a true axial feed, wherein the feedstock particles are fed into the plasma core. This attempt to provide an axis feed for plasma spraying has not found commercial applications and the thermal spray industry therefore continues to utilize radial feed for plasma torches. The prior art also includes other attempts to combine two or more plasmas as disclosed in US-A-3 770 935. In the plasma jet generator disclosed in US-A- 3 770 935, a positive plasma jet torch is aligned at a right angle to a negative plasma jet torch, such that the plasmas meet and function as a plasma jet torch of straight polarity to achieve a high arc voltage and improved efficiency. However, the plasma jet generator must utilize an inert plasma gas and radial feed of the feedstock. This system has not been introduced commercially and does not overcome the problems with radial feed as described above. The prior art also includes numerous examples of transferred arc plasma guns or torches. Transferred arc plasma torches, wherein the substrate is connected electrically to the gun, has achieved commercial acceptance in many applications. It is also possible to utilize a second annular anode electrode, downstream of the primary anode, to transfer the plasma axially as disclosed in US-A- 2 858 411. Transferred arc technology has not, however, resulted in a commercial axial feed plasma gun utilizing powdered feedstock, which is a primary object of the present invention. Thus, although the problems of radial feed in commercial plasma spray apparatus have long been recognized, the prior art has failed to solve the problems described above in a commercially successful plasma spray system. There is, therefore, a long-felt need for an axial feed plasma spray system which has not been met by the prior art. International Publication Number WO 86/02024 discloses an electric arc plasma spray apparatus with several coalescing plasmas and means for supplying particulate matter, the latter being heated and transported towards a surface. The publication also describes a method to coat surfaces by mixing particulate material with a column of plasma. This publication, however, does not disclose a means for electromagnetically coalescing several plasmas into a suspended plasma of ionized plasma gas. JP-A-61-230300 discloses a plasma generator in which a plurality of electrical arcs are used to generate plasma streams. The streams are directed towards a common point within a bore of an anode and the plasma streams thus merge. The present invention provides plasma spray apparatus and method which generate a suspended electromagnetically coalesced stable plasma permitting true axial feed in a plasma spray system. According to the present invention there is provided a plasma spray apparatus which comprises a plurality of pilot plasma spray guns for generating respective intersecting plasmas of ionized plasma gas, said pilot plasma spray guns being symmetrically angularly displaced about a common axis and each comprising an annular anode and a centrally disposed rod shaped cathode; first plasma gas supplying means for supplying an inert plasma gas to the pilot plasma spray guns; electrode means comprising a main transfer electrode located downstream of said pilot plasma spray guns and having a bore which is coaxial with said common axis and into which the plasmas of ionized gas are directed for extending and electromagnetically coalescing the plasmas into a suspended plasma of ionized plasma gas; a second plasma gas supply means for injecting plasma gas tangentially into the bore of the main transfer electrode; means for supplying electrical power to the plasma spray guns generating the first and second plasmas of ionized gas and to the main transfer electrode; and means for supplying feedstock along said common axis into the suspended plasma, wherein the anodes of the first and second pilot plasma guns are electrically insulated from the main transfer electrode, and whereby the feed stock is heated and accelerated in particulate form. By injecting the reactive plasma gas tangentially into the nozzle bore a vortex may be formed of the plasma gases, constricting the suspended plasma in the nozzle bore. Preferably, the second plasma gas supply means supply a reactive plasma gas to the main electrode bore. Preferably, said bore in said main electrode is formed as a through going nozzle bore. In accordance with a further aspect of the present invention, there is a method of plasma spraying which comprises generating a plurality of intersecting plasmas of ionized plasma gas in respective plasma spray guns which are symmetrically angularly displaced about a common axis, are supplied with ionizable plasma gas and each comprise an annular anode and a centrally disposed rod shaped cathode; directing said first and second plasmas of ionized plasma gas through a bore which is coaxial with said common axis and is in a main transfer electrode disposed downstream of said plasma spray guns; and passing an electrical current through said first and second plasmas of ionized gas in said main transfer electrode which is electrically insulated from said first and second pilot plasma spray guns, thereby extending and electromagnetically coalescing said first and second plasmas into a suspended plasma of ionized gas; feeding a separate ionizable plasma gas tangentially into the suspended plasma; and feeding a feedstock along said common axis through the intersection of the first and second plasmas into the suspended plasma, the suspended plasma heating and accelerating the feedstock in particulate form as a spray suspended in the plasma gas. By feeding the separate ionizable plasma gas into the suspended plasma, the suspended plasma can be extended. The tangential feeding of the separate ionizable plasma gas may generate a vortex constricting the suspended plasma. It can also further heat the suspended plasma. Preferably, the ionizable plasma gas supplied to the plasma spray guns generating the first and second plasmas of ionized is substantially inert and the separate ionizable plasma gas supplied into the suspended plasma is reactive. The plasma spray apparatus and method of this invention generates a free-standing electromagnetically coalesced stable plasma through which feedstock may be fed, eliminating problems with conventional radial plasma guns. The plasmas generated by the pilot plasma guns are directed into the throat of the main transfer electrode bore and a second plasma gas is supplied to the throat of the main transfer electrode bore which is ionized and coalesced with the plasmas generated by the pilot plasma guns, generating a free-standing electromagnetically coalesced plasma. The second plasma gas may be a conventional inert or unreactive plasma gas or more preferably a reactive plasma gas increasing the energy of the free-standing plasma and providing additional advantages. The feedstock may then be fed through the bore of the transfer electrode and the free-standing electromagnetically coalesced plasma, uniformly heating the feedstock and permitting the use of a wide range of feedstock material forms and types, including particulate feedstock having dissimilar particle sizes and densities, slurries, sol-gel fluids and solutions. Feedstock, in particulate or rod form, may be fed through the axis of the suspended plasma, resulting in improved efficiency, including improved heat transfer and uniform heating of the feedstock, thereby eliminating the problems of radial feed. Further, the plasma generating apparatus and method of this invention may utilize reactive gases or reactive gas mixtures as the plasma forming gas, without resulting in deterioration of the cathode or arc wandering. Finally, the operating power level of the plasma jet torch of this invention may be significantly increased, without decreasing the energy efficiency of the system or damaging the electrical components. The plasma spray apparatus of the present invention includes at least two, more preferably three or four pilot plasma guns. The pilot plasma guns may be conventional plasma generating torches, each including a pair of electrodes and means supplying a substantially inert ionizable plasma gas between the electrodes, wherein the ionizable plasma gas flows through an arc generated between the electrodes, establishing a plasma of ionized gas. In particular the pilot plasma guns may each include a rod-shaped cathode, an annular body portion surrounding the cathode in spaced relation, an annular anode downstream of the cathode having a nozzle opening axially aligned with the cathode, and means for supplying an inert plasma gas to the annular body portion which flows around the cathode and exits the anode nozzle opening. The pilot plasma guns are angularly displaced such that the plasmas generated by the pilot plasma guns intersect the common axis. The individual plasmas generated by the pilot plasma guns are extended and electromagnetically coalesced into a suspended plasma by means of a transferred current established to the main electrode, such that the plasmas generated by the pilot plasma guns are directed into the nozzle bore of the main anode. The pilot plasmas may be generated by a conventional direct current power means connected to the rod-shaped cathodes and the annular anodes, forming an electric arc through which the inert plasma gas flows, ionizing the gas and forming a plurality of plasmas which intersect in the throat of the main anode. In such an embodiment, the throat of the main anode is preferably cone-shaped to receive and direct the individual plasmas generated by the pilot plasma guns into the nozzle bore of the main anode. The power means in such preferred embodiment further includes a source of direct current connected to the cathodes of the pilot plasma guns and the main anode establishes a transferred current which electromagnetically coalesces the pilot plasmas, forming a suspended coalesced plasm in the main transfer electrode bore, through Which the feedstock is fed. The second ionizable plasma gas may be fed into the throat of the main electrode and ionized, extending the suspended plasma and adding to the heat generated and transferred to the feedstock. Although the second plasma gas may be an inert plasma gas or the same plasma gas used in the pilot plasma guns, the second plasma gas is more preferably a reactive plasma gas or a reactive gas mixture in certain applications, adding to the energy generated by the suspended plasma when ionized and providing the advantages described above. Thus, the plasma spray apparatus of the present invention is capable of including any suitable ionizable gas as the plasma gas, depending upon the requirements of the particular application. As described, the feedstock may then be fed axially through the common axis of the pilot plasma guns, resulting in a true axial feed plasma spray apparatus. Powdered or particulate feedstock may be fed through a feedstock supply tube extending through the common axis of the pilot plasma guns to the point of intersection of the pilot plasmas in the throat of the main transfer electrode. Alternatively, the feedstock may be supplied to the nozzle bore of the main electrode in the form of a wire or rod. The feedstock is then fed through the intersection of the pilot plasmas into the suspended plasma in the main transfer electrode bore, uniformly heating and accelerating the feedstock and improving the deposition efficiency of the system. Still, alternatively, the feedstock may be in liquid form, such as, for example, a solution, a slurry or a sol-gel fluid, such that the liquid carrier will be vaporized or reacted off, leaving a solid material to be deposited. The plasma generating apparatus and method of the present invention thus eliminates the long-standing problems with radial feed plasma spray apparatus. Because the feedstock is fed aixally through the plasma spray apparatus of the present invention, deposition efficiency is improved and a greater range of particle sizes may be used, reducing the cost of the feedstock. Further, various blends of particulate feedstock may be utilized, including blends of particles dissimilar in size and density. Furthermore, much larger particles than are normally employed in commercial plasma spraying may be used due to the extended residence time in the hot zone. Further, reactive gases, including oxygen and blends of reactive gases including oxygen, may be used as the main plasma gas in the plasma spray apparatus of the present invention, increasing the range of applications for the plasma spray apparatus of this invention. Finally, the operating power level of the plasma spray apparatus of this invention may be increased by increasing the plasma voltage, rather than the current, and selecting the plasma-forming gas utilized. The present invention will now be further described with reference to and as illustrated in the accompanying drawings, but is in no manner limited thereto. In the drawings:- Figure 1 is a front elevational view of the plasma spray apparatus of the present invention in partial cross-section.; Figure 2 is an exploded perspective view of the housing of the present invention; Figure 3 is a plan view of a section taken along lines 3-3 of Figure 1; Figure 4 is a top view of the housing of the present invention; Figure 5 is a top view of a support block adapted to receive four pilot plasma guns in the present invention with magnetic field lines shown schematically; Figure 6 is a front elevational view of a portion of the main anode and disc of the present invention with plasma streams shown diagrammatically; Figure 7 is a diagrammatic perspective representation of the magnetic field lines coalescing the plasma streams.; Figure 8 is an alternative support block adapted to receive three pilot plasma guns; and Figure 9 is a front elevational view of a portion of the main anode and disc of the present invention in another embodiment in which a wire feedstock is fed to intersecting plasmas. Referring now to Figure 1 of the drawings, plasma spray apparatus 20 is shown generally in one embodiment having first pilot plasma gun 22 and second plasma gun 24, the latter being shown partially in cross-section. Pilot plasma guns 22 and 24 are of the conventional type in which a centrally disposed, rod-shaped cathode 26 is provided having a cone-shaped free end 28. Rod-shaped cathode 26 is secured in position by frictional engagement with retainer 30, one end of which is closed by closely fitting cap 32. As will be appreciated by those skilled in the art, cap 32 may be threaded onto retainer 30 such that rod-shaped cathode 26 can be replaced when worn. However, as will be more fully described hereinafter, in the present invention, the unique construction of the present invention may often reduce cathode wear so that replacement is less frequent. A ring of dielectric material such as, for example, a ceramic insulator 34 is provided to electrically isolate rod-shaped cathode 26 and its retaining structures from annular anode 36. Annular anode is secured in place by electrically insulating sheath 38 through which electrical lead 40 extends to make electrical contact with annular anode 36. Similarly, electrical lead 42 extends through retainer 30 making electrical contact with rod-shaped cathode 26. Annular anode 36 is provided with nozzle opening 46 through which a pilot plasma is directed during start-up of plasma spray apparatus 20. In some applications, rod-shaped cathode 26 will include internal passages through which a cooling medium such as water may be circulated to dissipate heat from rod-shaped cathode 26 developed during plasma operation. A similar heat exchange channel (not shown) is also preferably provided in annular anode 36 for the purpose of dissipating the extreme heat generated by the pilot plasma stream. Annular space 48 defined between the inner surface or wall of annular anode 36 and rod-shaped cathode 26 comprises a portion of a plasma gas passage which extends from plasma gas source 50 through a channel in insulating sheath 38 and retainer 30. As illustrated, retainer 30 inlcudes a portion which is spaced slightly from rod-shaped cathode 26 to permit the flow of plasma gas through a similar annular space provided by ceramic insulator 34 into annular space 48. Hence, when the appropriate electrical potentials are applied to rod-shaped cathode 26 and annular anode 36, and an electric arc is established via high frequency oscillator 52 (another high frequency oscillator 54 is provided in the elctrical circuit for pilot plasma gun 22) which extends from cone-shaped end 28 of rod-shaped cathode 26 to annular anode 36. As plasma gas is then flowed from plasma gas source 50 through annular space 48, the plasma gas encounters the electric arc which ionizes the plasma gas in the known manner, forming pilot plasma stream 56. Pilot plasma stream 56 emerges from nozzle opening 46. It is to be understood that the term "plasma gas" as used herein shall be defined as any gas or mixture of gases which ionizes when passing through an electric arc of suitable electrical characteristics. As will be understood more fully hereinafter, a significant feature of the present invention is that it permits a final, coalesced suspended plasma stream to be formed which includes an active or reactive gas such as, for example, oxygen without causing accelerated deterioration of rod-shaped cathode 26. However, for operating pilot plasma guns 22 and 24, an inert gas, preferably argon, is used as the plasma gas. Other suitable plasma gases will be known to those skilled in the art. Pilot plasma guns 22 and 24 are mounted in housing 58 at support block 59 such that they are displaced symmetrically about a common axis 60. As will be explained more fully hereinafter, although in this particular embodiment only two pilot plasma guns (22 and 24) are provided, it is preferred that plasma spray apparatus 20 be equipped with three pilot plasma guns in block 59' as shown in Figure 8 or four plasma pilot guns in block 59" as shown in Figure 5 of the drawings. In each case, the pilot plasma guns are symmetrically arranged about common axis 60 with each pilot plasma gun axes (62 and 62' in Figure 1) intersecting at an included angle of preferably less than about 60 degrees. In other words, the included angle between axis 62 and axis 60 is preferably less than about 30 degrees as is the included angle between axis 62′ and axis 60. Bores 64 and 66 in block 59 closely receive, respectively, pilot plasma gun 24 and 22 in rigid engagement. In this embodiment, and referring again to Figure 1 of the drawings, block 59 in countersunk at bores 64 and 66 to provide a shoulder or rim on which insulating sheath 38 abuts. Further, a dielectric ferrule 68 is provided as a sheath surrounding a portion of annular anode 36 to electrically insulate annular anode 36 from block 59. A polyester material is suitable for this purpose. Block 59 may be formed of any readily machinable metal such as, for example, brass. As shown in Figure 4, block 59 may be machined with four bores, two of which are plugged with plugs 65 and 67. Thus, block 59 can be easily adapted for 2 or 4 pilot plasma guns. It will also be understood that block 59″ shown in Figure 5 includes two additional bores for two additional pilot plasma guns (not shown). In this four-part configuration, each bore is spaced 90 degrees from each adjacent bore. In Figure 8, block 59′ is adapted to receive three pilot plasma guns spaced 120 degrees apart. In both arrangements, the bores are configured to support the pilot plasma guns angularly, preferably about 30 degrees or less off centre axis 60. This symmetry is important to provide a stable intersection of the pilot plasma streams. Block 59 is provided with annular heat exchange chamber 70 which is in flow communication with heat exchange passage 72 of jacket 74. In this manner, coolant 76 is flowed during operation through port 78 into heat exchange passage 72 whereby it is circulated through annular heat exchange chamber 70 to cool block 59. Where, as in the preferred embodiment, more than two pilot plasma guns are employed, additional bores may be provided symmetrically in block 59 as previously described. Referring now to Figures 1 and 2 of the drawings, in order to provide feedstock axially along axis 60, feedstock supply tube 80 is provided disposed in block 59 at bore 82. Feedstock supply tube 80 is closely received within bore 82 in frictional engagement with block 59. Feedstock supply tube 80 is open at its terminal end which extends into chamber 84 of block 59 and provides the means by which a feedstock material, such as, for example, a particular composition is delivered to the plasma along axis 60. As will be more fully explained, a solid feedstock in the form of a rod or the like may be suitable in some applications. Also, it will be noted that pilot plasma guns 22 and 24 extend into chamber 84 at their nozzle opening ends. Housing 58 further includes main anode 86 having a central bore or passage 88 extending the length thereof. Main anode 86 is formed of an electrically conductive material such as, for example, copper and includes an annular channel 90 through which a coolant is circulated via heat exchange passage 72. In other words, annular channel 90 and heat exchange passage 72 are in flow communication. In this particular embodiment, disc 92 is provided interposed between block 59 and main anode 86. As will become apparent, this configuration permits easy fabrication and assembly. Disc 92 has a centrally disposed bore 94 which is conical in shape and which mates with main anode 86 at a corresponding conical portion of bore 88. In this manner, conical throat 96 is defined in which axes 62 and 62' intersect. The included angle of conical throat 96 will typically be approximately 60 degrees or correspond to the angle of impingement of the pilot guns. Conical throat 96 and bore 88 are in axial alignment with axis 60. It will also be noted that in this embodiment main anode 86, disc 92, and block 59 are secured in position in jacket 74 with bolt 98. As will become more apparent during the description of the operation of plasma spray apparatus 20, it is preferable to coat conical throat 96 and a portion of disc 92 with a layer of dielectric material 100 such as, for example, aluminium oxide. In addition to reducing erosion of the surfaces defining conical throat 96, dielectric layer 100 serves to extend the length of main transferred plasma-arc or suspended plasma 102 by preventing the contacting of the coalesced plasma stream until after it enters the bore of the main transfer anode. The significant advantages of extending suspended plasma 102 in this manner will be described in detail in connection with the description of the method of the present invention. Main anode is formed of a highly conductive material such as, for example, a copper alloy or the like. Disc 92 may be formed of a durable metal or a refractory oxide. As shown best in Fig.3 of the drawings, in this embodiment of the invention disc 92 serves as a gas manifold having a network of channels or gas passages. In this regard, annular gas channel 104 is shown adapted to receive a plasma-forming gas from plasma gas source 106 as illustrated in Fig.1. Referring to Figs.1, 2 and 3, plasma gas moves from gas source 106 through passage 108 which is a bore extending through jacket 74 of housing 58. In flow communication with passage 108, a second annular gas passage 110 is provided in jacket 74. Main anode 86 also has a plurality of microbores 112 which are in flow communication with annular gas passage 110 and with annular gas channel 104. In flow communication with annular gas channel 104, a plurality of tangential gas passages 114 are provided which facilitate the introduction of plasma gas from a secondary plasma gas source 106 into conical throat 96 in a spinning or whirling manner. Although a path of introduction more direct than that provided by the tangential geometry of gas passages 114 may be suitable, by flowing plasma gas into conical throat 96 in the preferred manner, the whirling motion of the plasma gas which is imparted creates a plasma vortex within passage 88. This vortex helps constrict suspended plasma 102 along with other factors, such that it is a highly-collimated stream. It should be noted that the gas manifold can be provided in a similar manner directly in main anode 86. A plurality of O-rings 116 are also provided which conform to annular channels in the various structures of housing 58 such that substantially hermetic seals are attained. Numerous variations and modifications of plasma spray apparatus 20 will be apparent which are consistent with the principles of the present invention. For example, in most applications housing 58 will be encased in an electrically insulating material. Also, plasma spray apparatus 20 may be adapted to permit robotically-controlled spraying or hand-held spraying. Further, although plasma spray apparatus 20 is illustrated having two, three or four symmetrically disposed pilot plasma guns, five or more pilot plasma guns may be suitable or desirable in a particular application. In operation, and in accordance with the method of the present invention, plasma spray apparatus 20 is preferably utilized to apply a sprayed coating of a material such as, for example, a metal or ceramic to a target substrate. Other applications such as, for example, the processing of materials and the production of free-standing articles including near-net shapes are also preferred herein. Plasma spray apparatus 20 may also be suitable for use in high-temperature cutting or heating operations. Referring again to Figures 1 and 2, rod-shaped cathode 26 of pilot plasma gun 24 is electrically connected to the negative terminal of an electrical power source 118 via lead 42. In the same fashion, the rod-shaped cathode (not shown) of pilot plasma gun 22 is connected to the negative terminal of power source 118 with electrical lead 122. Annular anode 36 of pilot plasma gun 24 is electrically connected to the positive terminal of power source 123 via lead 40. Annular anode 124 of pilot plasma gun 22 is electrically connected to the positive terminal of power source 125 by lead 126. All power sources in the present invention preferably provide direct current. As previously stated, a first high frequency oscillator 52 and a second high frequency osciallator 54 are provided in the circuit for initiating an electric arc or "pilot arc" between each pilot plasma gun cathode and its respective annular anode. That is, high frequency oscillators 52 and 54 serve to initiate an electric arc between rod-shaped cathode 26 and annular anode 36 of pilot plasma gun 24 and, in pilot plasma gun 22, between annular anode 124 and its corresponding rod-shaped cathode (not shown). During start-up a first plasma gas, such as, for example, argon, is flowed from plasma gas source 50 into annular space 48 and outwardly through nozzle opening 46 of pilot plasma gun 24. Plasma gas flow is initiated in pilot plasma gun 22 in the same manner. Switches 128 and 129 are then closed momentarily, activating high-frequency oscillators 52 and 54 and simultaneously connecting power sources 123 and 125 to pilot plasma guns 24 and 22, respectively, thereby initiating and establishing pilot arcs in the pilot plasma guns. A steady direct current maintains the electric arcs. As plasma gas flows toward nozzle openings 46 and 130 of pilot plasma guns 24 and 22, respectively, preferably under pressure, it passes through the pilot arcs causing the plasma gas to ionize in the known manner. The plasma gas may be introduced axially or alternatively, "whirling" for form a vortex if desired. Non-transferred pilot plasma streams 56 and 132 are thus formed which intersect in conical throat 96 as shown also in Figs. 6 and 9. Switch 134 is then closed electrically energizing main anode 86. As will be appreciated by those skilled in the art, and as will be more fully explained hereinafter, the electromagnetic fields which are associated with charges in motion provide forces that affect the interaction of pilot plasma streams 56 and 132 at their point of intersection and the characterisitics of suspended plasma 102. Moreover, as main anode 86 is energized, the electromagnetically coalescing pilot plasma streams 56 and 132 in conical throat 96 are drawn through conical throat 96 into the straight bore portion of passage 88. This occurs because the intersecting pilot plasma streams have the properties of a "flexible conductor" and thus generate electromagnetic fields which cause the plasmas to be attracted to one another, causing the plasmas to coalesce in conical throat 96. The intersecting streams are drawn toward the positive charge of main transfer anode 86 which is in electrical connection with power source 118 at its positive terminal via lead 136. (It will be noted that in this embodiment, jacket 74 is in electrical connection with main anode 86. Other arrangements may be suitable.) By providing dielectric layer 100 in conical throat 96, in the preferred embodiment, the coalescing pilot plasma streams 56 and 132 move toward the exposed surfaces of main anode 86 in the straight bore position of passage 88. Dielectric layer 100 prevents pilot plasma streams 56 and 132 from "short-circuiting" with main anode 86 or disc 92 prior to electromagnetically coalescing. Also, in this manner, the electromagnetically coalesced plasma stream is extended into the straight bore portion of main transfer anode 86. By lengthening the plasma in this fashion, the plasma voltage is increased, producing an increase in the plasma energy density. High plasma energy densities are desirable because they facilitate thermal energy transfer to the feedstock and increase particle velocities. A second or main plasma gas from plasma gas source 106 is flowed under pressure into conical throat 96 via passage 108, annular gas passage 110, microbores 112 and tangential gas passages 114, the latter of which, as stated, open into conical throat 96. While it is preferred that an inert ionizable, plasma-forming gas be employed in forming pilot plasma streams 56 and 132 to prevent accelerated deterioration of the rod-shaped cathodes, a significant advantage of the present invention is the ability to form a plasma stream which includes an active or "reactive" gas such as, for example, oxygen which is detrimental to the cathode material. This is made possible by the present invention since an inert gas can be used in pilot plasma guns 22 and 24, thus protecting the rod-shaped cathodes, and an active gas then introduced downstream of the pilot plasma guns at conical throat 96. The use of a reactive gas may be desirable to alter the chemical composition of feedstock as it is sprayed and also permits higher operating voltages, since the latter is a function of the composition of the plasma gas. As plasma gas is flowed from tangential gas passages 114, it creates a vortex which further serves to collimate suspended plasma 102. The spin of the secondary plasma-forming gas is illustrated best in Figure 6 of the drawings as arrow G. As secondary plasma gas enters conical throat 96, it is ionized by the electrically energetic converging pilot plasma streams 56 and 132. The resulting hot, whirling rapidly-expanding plasma gases combine with pilot plasma streams 56 and 132 and, through the forces due to the expansion of hot gases and electromagnetic influences, the plasma is drawn into the straight bore portion of passage 88, forming suspended plasma 102 which emerges at a high velocity from plasma discharge opening 138. The tightly constricted suspended plasma 102 makes electrical contact with main anode 86 to complete the circuit. This occurs near plasma discharge opening 138 in passage 88 or at outer face 142 of main anode 86. After start-up is completed, switches 128 and 129 of Figure 1 may be opened such that the annular anodes of the pilot plasma guns are disconnected from the circuit. Pilot plasma streams 56 and 132 continue to flow into conical throat 96 because they are electrically linked to main anode 86 via suspended plasma 102 which is maintained by a steady direct current. It will be appreciated by those skilled in the art that one of the significant advantages of plasma spray guns in general is their ability to generate high temperatures, often exceeding 12,000 K. These high temperatures make plasma spraying ideal for processing and spraying refractory oxides and other heat-resistant materials. To prevent thermal deterioration of the various parts of plasma spray apparatus 20, and referring now to Figures I and 2 of the drawings, coolant is circulated through housing 58 in the coolant passages previously described. Coolant is removed at coolant exit 140. By cooling main anode 86 at the straight bore portion of passage 88, the regions of passage 88 immediately adjacent the interior walls of main anode 86 are cooled, producing a phenomenon known as "thermal pinch". Accordingly, a sheath of cooler, non-ionized gas is maintained near the walls of main anode 86. This non-conductive sheath constricts the electric field lines of suspended plasma 102 serving to further concentrate or constrict the plasma stream. A magnetic pinch is also provided which will now be explained. Pilot plasma streams 56 and 132 converge symmetrically at the intersection of axes 60, 62 and 62', as shown in Figure 1. Pilot plasma streams 56 and 132 (and any additional pilot plasma streams where more than two symmetrically disposed pilot plasma guns are utilized) deflect uniformly at the point of intersection. The uniform deflection is brought about in part by the kinetic interacting forces of the intersecting plasmas and the symmetrical geometry. Further, each pilot plasma stream has an associated circumferential magnetic field, induced by the transferred DC electric current between each of the cathodes of the pilot plasma guns and the main anode, illustrated by arrows A, B, C, and D in Figures 5 and 7. In addition, a magnetic field E is present which encircles the converging pilot plasma streams. Due to the superposition of the various magnetic vector components, magnetic field E serves to draw the individual plasma streams together as shown most clearly in Fig.7. The magnitude of this constricting magnetic pinch increases adjacent the point of intersection of the pilot plasma streams. This increasing magnetic pinch causes the individual pilot plasma streams to electromagnetically coalesce to form a stable coalesced plasma stream. The magnetic pinch increases the pressure, temperature and velocity of suspended plasma 102. The magnitude of this magnetic pinch is proportional to the combined current conducted by the pilot plasma streams and suspended plasma 102. After suspended plasma 102 is fully established, a feedstock material is supplied to the point of intersection of the pilot plasmas. Referring again to Figure I of the drawings, in one embodiment a particulate feedstock is injected through feedstock supply tube 80 which, as stated, is in axial alignment with axis 60. It is a significant advantage of the present invention that axial injection of feedstock can be achieved without disturbing the plasma arc. This is made possible by the angular arrangement of pilot plasma guns 22 and 24. The disadvantages of radial feed in prior art plasma spray apparatus are thus obviated by the present invention. Thus, the present invention provides uniform heating of the axially injected feedstock particles. Particle velocity is also extremely uniform. Supersonic particle velocities may be achieved. In most instances, the feedstock will be injected under pressure through the use of an inert carrier gas. By controlling the various operating parameters of plasma spray apparatus 20, including particle injection velocity, precise control over particle velocity and temperature can be achieved. Hence as feedstock enters the electromagnetically coalescing pilot plasma streams, it is entrained and accelerated in suspended plasma 102 at its region of highest enthalpy. The heated, high-velocity particles are directed toward a target substrate which they impact to form a dense, uniform deposit. High deposition efficiencies are thereby achieved. Ceramics, such as, for example, refractory oxides, metals and even polymers may be sprayed in this manner. One particularly preferred application is the fabrication of metal and ceramic matrix composites. Other methods of axially injecting feedstock in the present invention are also suitable, including fluid feed of materials such as, for example, slurries, solutions and sol-gel fluids, or the use of feedstock in the form of wires or rods. In particular, and referring now to Fig.9 of the drawings, in one embodiment of the present invention, the feedstock comprises rod 148 which is advanced by rollers 150 into the intersecting pilot plasma streams 56 and 132. Because pilot plasma streams 56 and 132 are electrically energized at their point of intersection, by applying an opposite electrical bias to rod 148, rod 148 becomes an electrode which may form an arc with the intersecting pilot plasmas. This electric feedstock arc and the heat generated by the intersecting pilot plasmas rapidly melts the tip of advancing rod 148. The molten feedstock is atomized by the intersecting pilot plasmas and moves into suspended plasma 102 in the manner previously described. It is an important advantage of the present invention that exceptionally high power levels can be obtained with plasma spray apparatus 20. Operating powers of 100 kw or greater for the cathode to main anode circuit may be continuously sustained. After start-up, a steady direct current of from about 75 to about 125 A and a voltage of about 100 to 200 V between each rod-shaped cathode and main transfer anode 86 is established. The preferred voltage of the pilot plasma guns is from about 15 to about 30 V. The preferred current is from about 10 to 30 A. Hence, suspended plasma 102 may be energized at voltages from about 10 to about 50 times higher than the combined power of the individual pilot plasma guns. It will be appreciated by those skilled in the art that an increase in plasma arc voltage increases the energy of the plasma stream. The flow rates of the plasma-forming gases into plasma spray apparatus as well as the injection velocity of feedstock may vary widely depending upon the desired temperatures, velocities and particle residence times. As an example of preferred operating parameters, preferred and most preferred ranges are set forth in Table I below (PPG = pilot plasma gun; MP = main plasma; F = feedstock): Preferred Most Preferred PPG plasma gas Ar Ar PPG gas flow 5-20 SCFH 7 SCFH PPG nozzle opening ( 1.52-4.83 mm (0.06-0.19 in ( 2.29 mm (0.09 in PPG voltage 15-30 V 24 V PPG current 10-30 A 20 A MP discharge opening (4.83-9.65 mm (0.19-0.38 in (6.35 mm (0.25 in MP gas Ar, O 2 , N 2 CH 4 , He, H 2 Ar/H 2 MP gas flow 50-200 SCFH 75 SCFH MP voltage 50-250 V 150 V MP current 200-500 A 350 A F feed rate (powder) (0.45-9.08 kg/h (1-20 lb/h (2.72 kg/h (6 lb/h F feed rate (wire) (2.27-45.36 kg/h (5-100 lb/h (18.14 kg/h 40 lb/h MP discharge opening to substrate distance (50.8-304.8 mm (2-12 in (152.4 mm (6 in.

Claims: A plasma spray apparatus which comprises a plurality of pilot plasma spray guns (22,24) for generating respective intersecting plasmas of ionized plasma gas, said pilot plasma spray guns (22,24) being symmetrically angularly displaced about a common axis and each comprising an annular anode (36) and a centrally disposed rod shaped cathode (28); first plasma gas supplying means (50) for supplying an inert plasma gas to the pilot plasma spray guns (22,24); electrode means comprising a main transfer electrode located downstream of said pilot plasma spray guns (22,24) and having a bore (88) which is coaxial with said common axis and into which the plasmas of ionized gas are directed for extending and electromagnetically coalescing the plasmas into a suspended plasma of ionized plasma gas; a second plasma gas supply means (104,114) for injecting plasma gas tangentially into the bore (88) of the main transfer electrode (86); means for supplying electrical power to the plasma spray guns (22,24) generating the first and second plasmas of ionized gas and to the main transfer electrode (86); and means (80) for supplying feedstock along said common axis into the suspended plasma, wherein the anodes (36) of the first and second pilot plasma guns (22,24) are electrically insulated from the main transfer electrode (86), and whereby the feed stock is heated and accelerated in particulate form. An apparatus according to claim 1, wherein the second plasma gas supply means supply a reactive plasma gas to the main electrode bore. An apparatus according to claim 1, wherein said bore (88) in said main electrode (86) is formed as a through going nozzle bore. A method of plasma spraying which comprises generating a plurality of intersecting plasmas of ionized plasma gas in respective plasma spray guns (22,24) which are symmetrically angularly displaced about a common axis, are supplied with ionizable plasma gas and each comprise an annular anode (36) and a centrally disposed rod shaped cathode (28); directing said first and second plasmas of ionized plasma gas through a bore (88) which is coaxial with said common axis and is in a main transfer electrode (86) disposed downstream of said plasma spray guns (22,24); and passing an electrical current through said first and second plasmas of ionized gas in said main transfer electrode (86), which is electrically insulated from said first and second pilot plasma spray guns (22,24); thereby extending and electromagnetically coalescing said first and second plasmas into a suspended plasma of ionized gas; feeding a separate ionizable plasma gas tangentially into the suspended plasma; and feeding a feedstock along said common axis through the intersection of the first and second plasmas into the suspended plasma, the suspended plasma heating and accelerating the feedstock in particulate form as a spray suspended in the plasma gas. A method according to claim 4, in which the ionizable plasma gas supplied to the plasma spray guns is substantially inert and the separate ionizable plasma gas supplied into the suspended plasma is reactive.

Inventors: HERMAN HERBERT, MARANTZ DANIEL RICHARD, HERMAN, HERBERT, MARANTZ, DANIEL RICHARD
Applicants: HERMAN HERBERT, MARANTZ DANIEL RICHARD, HERMAN, HERBERT, MARANTZ, DANIEL RICHARD
Application year: 1989
Main classification: H05H1
IPC codes: H05H1, B05B7