JP3002688B2 - Thermal injection plasma device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a plasma apparatus capable of thermally spraying a powder material to coat a workpiece.

[0002]

2. Description of the Related Art There is known a plasma apparatus in which powder of a metal or other material is transported to a plasma gun and introduced into a plasma stream generated by the plasma gun. The plasma flow generated by the power supply or inert gas flow, typically under a low pressure source, is
From the gun, the powder is poured onto a target, such as a workpiece on which a powder is deposited and coated. The powder may be preheated with a plasma gun before being introduced into the plasma stream, but will melt when entrained by the plasma stream, resulting in a relatively dense coating on the workpiece.

One example of such a plasma apparatus is 1982
U.S. Pat. No. 4,328,257 to Mühlberger et al., Issued May 4, 1998 and co-assigned with the present application. In the plasma apparatus described in Mühlberger et al., A vacuum pump as a low pressure source is connected to a closed container containing a plasma gun and a work, and a plasma stream is injected from the plasma gun to the work at a supersonic speed. In. The powder is heated by a powder supply mechanism, transported to the side of the plasma gun, and introduced into the plasma flow.

Prior examples of plasma-related devices for supplying plasma and heated powder include U.S. Pat. No. 3,598,944 issued Aug. 10, 1971 to U.S. Pat.
It is disclosed in U.S. Pat. No. 3,839,618 to Mühlberger issued October 1, 741. Wemer et al. Describe heating particulate matter prior to introduction into the plasma heating space of an apparatus for forming spherical particulates of nuclear fuel. The Mühlberger patent describes a plasma device that uses electrical resistance tubes to preheat powder. Using a heated carrier gas, the powder is supplied through a pair of such tubes from a separate source. After the powder is heated by these tubes, it is introduced into the lumen of the plasma gun. The tube is resistively heated using a DC power supply connected to the tube.

[0005] US Patent 38396 to Mühlberger
No. 18 claims that the diameter of the powder particles is less than 44 microns, preferably much smaller. The relatively small particle size of 50 microns or less, typically required by prior art heated powder transport equipment, can be said to be a characteristic limit of such equipment. Small particles are required to facilitate preheating and dissolution so that they are rapidly accelerated in a plasma stream to provide a reasonably high concentration of coating on the workpiece.

[0006] However, small particles of less than 50 microns in diameter have considerable limitations. First of all, such particles are relatively expensive to manufacture, especially in the case of heat-resistant substances or gas-adsorbing substances. Heat-resistant substances such as tungsten and molybdenum have relatively high melting points, and gas-adsorbing substances such as barium, titanium and tantalum oxidize rapidly. Further, such particles have poor viscosity or flowability and are relatively difficult to convey through devices such as heating tubes with relatively small internal diameters. The particles are also prone to a high degree of surface oxidation, making it difficult to supply the particles to the workpiece in a relatively pure and non-oxidized state. Since the surface area to weight ratio of these particles is relatively small, thermal energy transfer is reduced, making heating control of the particles more difficult.

[0007] On the other hand, relatively large powder particles having a diameter of about 50 microns or more are advantageous over small particles in several important respects. In addition to the relatively low manufacturing costs,
Such particles have excellent fluidity. Since the particles are easily produced in a form having high purity and low surface oxidation, it is easy to coat the workpiece with a relatively high concentration and non-oxidation. An important disadvantage of this large particle is that it is difficult to completely dissolve in the plasma stream using conventional equipment and techniques. Due to the difficulty in dissolving the particles, it is very difficult to apply a sufficient coating to the workpiece. Even if such a powder is preheated using a resistance heating transport tube, coating of the workpiece is difficult to accomplish.

[0008] heating of the powder powder particles, as well as various characteristics of the size of ordinary aforementioned different particles, without different materials of different softening, considering solubility characteristics, it has been performed by standardized methods . Also, as with other properties of powder transport,
Adjustment of the powder temperature had to be done with a plasma gun or just outside.

Therefore, it is desirable to provide a plasma device that can control thermal injection by modifying all parts of the plasma device that affect thermal injection in an optimal manner. Such an improved device approach involves the ability to successfully heat jet a variety of powdered materials with different particle sizes and different softening and dissolving properties.

A more specific purpose of the improved plasma device is to:
To form a uniform film with a relatively high concentration on the work,
Includes the ability to inject relatively large particles of powdered material. Also, a heated powder transport device that can preheat powder particles of any size so that the particles are almost completely dissolved in the plasma stream, while the particles can flow relatively smoothly and continuously through the powder supply device. Is also a useful point.

[0011]

According to the plasma apparatus of the present invention ,
The control by the various components of the device is optimized to enhance the heat injection process. Powder different materials and sizes are all fed into the plasma stream in a relatively uniform preheat state, as can be optimized particles melt in the plasma stream, improved preheating device is used. In addition, plasma
The gun is adjusted to optimize particle density and acceleration in the plasma stream. Furthermore, it is possible to provide a device for controlling the heating of particles in the plasma stream, which can improve the coating applied to the workpiece.

The improved hot powder injection apparatus and technique according to the present invention allows the use of relatively large particle powders. The temperature of the spray device is carefully controlled so that the walls and other parts of the device in contact with the particles are kept below the melting point of the material of the particles. This prevents the surface of the particles from melting or sticking to the jetting device. At the same time, the temperature of the injection device is set in consideration of the softening point and viscosity of the powder material. The particles are thus heated to the highest possible temperature without the particles clogging the injection device or losing the viscosity required for good flow in the device.

The choice of injector length and other characteristics is such that upon introduction into the plasma gun, the particles are relatively uniformly heated to a temperature close to the injector temperature. As a result, the large particles in the plasma stream are relatively completely melted, and a dense workpiece can be coated. The length and density of the plasma stream can also be adjusted to enhance the melting of the particles, so that suitable modified plasma devices can be used. Such an improved plasma apparatus may increase the number of power supplies connected to the electrodes and control the heating of particles in different regions of the plasma flow in order to further optimize the thermal injection processing.

In a preferred embodiment of the improved hot powder injector according to the invention, the powder is transported from the powder feeder to the plasma gun using a resistive heating transport tube, but at the opposite end of the transport tube. The unit is connected to a power supply. The powder source is adjusted to provide a transport tube having a surface temperature below, but not too low of, the melting point of the powdered material being transported. The mixture of the carrier gas and the powder particles is directed into the carrier tube at a pressure sufficient to enable it to be carried at a standard flow rate within the carrier tube. The length of the transfer tube is carefully selected depending on other operating factors so that the powder particles are relatively uniformly heated to a temperature close to the temperature of the transfer tube wall before being sent to the plasma gun. . Other things being equal, the longer the length of the conveying tube, the longer the powder particles stay in the tube. The longer the residence time, the smaller the temperature difference between the tube and the particles, and the powder particles are discharged from the conveying tube at a uniform temperature very close to the temperature of the conveying tube.

The hot-powder conveying tube is arranged concentrically inside the outer reflecting tube so as to be heated more efficiently. Cooling water is pumped at a certain pressure into the outer double wall of the reflector tube while an inert gas is circulating in the gap between the powder transport tube and the reflector tube to prevent oxidation.

In the case of powdered materials whose softening point is slightly lower than the melting point, the improved powder injection device uses a plasma gun adapted for a thermal injection to discharge the powder therefrom. It is also possible to If the transfer device has the ability to heat the powder to a temperature just below the melting point, the powder particles can be directly sprayed onto the workpiece from the transfer device. In such a configuration, the transfer device is provided with a small-diameter outlet nozzle in order to maintain the pressure of the transfer device.

[0017] The plasma device may include one or more additional power supplies connected between the electrodes in the plasma stream to control the temperature of the powder particles in the plasma stream. In such an embodiment, one may be provided just outside the gun hole and the other may comprise a pair of annular electrodes disposed intermediate the plasma gun and the workpiece. A DC power supply connected between the pair of annular electrodes increases the density of the plasma flow and enhances the heating of the powder. In another embodiment, a plurality of electrodes spaced along the plasma flow are connected to a plurality of different DC power supplies to allow independent control of powder particle temperature in different regions along the plasma flow. ing. In still another embodiment, the lowermost electrode is provided at a position relatively close to the work, and after cleaning the work to avoid overheating of the work, the electrode between the conversion arc power supply and the work is removed. Try to disconnect. In yet another embodiment, a power supply connected to one or more electrodes has a negative terminal connected in series with the cathode of the plasma gun to achieve a desired plasma temperature distribution. In yet another embodiment, a permanent magnet or electromagnet member surrounding a portion of the plasma flow is provided to shape the plasma flow, disperse the plasma temperature at the electrodes, and avoid overheating.

In the configuration using the mixed particles, if necessary or desirable, at least a part of the surface of the powder particles is treated with a suitable coating member such as chromium, so that the adhesion of the powder particles having different compositions is improved. Is to be increased. The coating is done by mixing, plating or ball mixing.

[0019]

BRIEF DESCRIPTION OF THE DRAWINGS A better understanding of the present invention may be obtained by reading the following detailed description with reference to the accompanying drawings, in which: FIG.

FIG. 1 is a view showing a plasma apparatus having an improved hot powder injection apparatus according to the present invention and another feature of the present invention. The apparatus shown in FIG. 1 is a plasma-resistant container that has pressure resistance, insulation, and can maintain a vacuum.
It has a chamber 10. The chamber 10 has a cylindrical main body.
It is defined by 12 and an upper lid 13 connected thereto. The body of the plasma chamber 10 has a bottom collector cone 14. The cone 14 is connected to and associated with a related device for treating discharged gas and particles while maintaining a desired environmental pressure.

The downwardly flowing plasma stream is created by a plasma gun 16 mounted inside the top lid 13. The position of the gun 16 is controlled by a plasma gun actuator 18. The plasma chamber 10 is preferably configured as a double-walled water-cooled container, and the upper lid 13 is detachable so as to access the operation unit. The gun actuator 18 supports and controls the plasma gun 16 via a sealed bearing and coupler on the wall of the top lid 13.

The powder supply device 20 has the improved hot powder injection device shown in FIG. 2 and is connected to the upper lid 13. The powder feeder 20 supplies the heated powder in a controlled manner into the plasma stream via a device connected to the plasma gun 16. The hot powder supplied from the powder supply 20 is carried into the plasma stream generated by the plasma gun 16.

The downwardly flowing plasma flow from the plasma gun 16 strikes the workpiece 24. The work 24 is supported by a conductive work holder 25, which is internally cooled, and is positioned during operation by a shaft extending through the chamber body 12 to an external work moving device 26. Or be moved. There is a position 28 near one end of the work 24 and not in contact with the work 24. The interior of the dummy sting 28 is similarly cooled and is connected to the dummy sting actuator 30 through the side wall of the chamber body 12. The workpiece holder 25 and the dummy sting 28 can be adjusted as an insertion position with respect to the center axis of the chamber 10 and can be energized. This is done so that the holder 25 and the sting 28 can be maintained at selected potentials for conversion arcing during various aspects of operation.

Under the workpiece 24 and the dummy sting 28, the collector cone 14 directs excessively blown gaseous or particulate matter to the baffle filter module 32. The baffle filter module 32
A water-cooled baffle to initially combine the overspray and an in-line baffle to extract most of the entrained particulate matter
A line filter unit. Baffle filter
The effluent through the module is then directed to a vacuum manifold 38 through another water cooling device, a heat conversion module 36. Vacuum manifold 38 has an over-spray filter collection device that extracts almost all particles remaining in the effluent. The vacuum manifold 38 communicates with a vacuum pump 42 that has sufficient capacity to maintain the desired environmental pressure within the chamber 10. In particular, at ambient pressures between 0.6 atm and 0.001 atm, a static pressure is generated which is sufficient to generate supersonic plasma.

Overspray filter module 32
Also, the heat conversion module 36 is desirably a water-cooled device with a double wall, but any well-known type widely used in plasma devices may be used.

To facilitate operation and repair of various parts of the apparatus, the entire apparatus is mounted on rollers and rails, conventional viewing windows, water cooling access doors, and insulated feeds for electrical connection. The through plate was not shown or described in detail. The work support and the operation control device are desirably attached to a hinged front approach door 43 in the chamber main body 12.

The electric energy is supplied to the operation unit of the apparatus through a fixed bus bar 44 mounted on the upper lid 13 of the chamber. A flexible, water-cooled cable comprises a plasma power supply 46, a high frequency power supply 48, and a cathodic conversion arc power supply 40.
Is connected via a bus bar 44 to a plasma gun 16 for generating a plasma flow. The plasma power supply 46
An essential potential difference is provided between the electrodes of the plasma gun 16. The high frequency power supply 48 is applied to a DC power supply having a plasma power supply 46 by applying a high frequency discharge to the plasma gun 16.
Used to initiate an arc discharge. Thereafter, a cathodic conversion arc power supply 50 connected between the plasma gun 16 and the workpiece 24, in the present invention, supplies a concatenated cathodic conversion arc discharge therebetween.

[0028] The use of a water booster pump 52 is required to allow sufficient cooling water to flow through the interior of the plasma gun 16. The plasma gas source 54 provides a suitable ionized gas for generating a plasma stream. The gas used here is argon alone or argon seeded with helium or hydrogen. However, it is well known to those skilled in the art that other gases can be used.

Control over how the sequence of operation of the system of FIG. 1 determines the operating speed and width of the various operating systems is provided by the system control console 56. plasma·
The gun 16 is operated independently under the control of the plasma control console 58. The functions performed by these consoles and the circuitry inside the consoles are well known and have not been shown or described in detail. The conversion arc control circuit 60 includes a cathode arc power supply 50.
In some cases. Much of the matter described with respect to FIG. 1 is similar to the earlier mentioned U.S. Pat. No. 4,328,257 to Mühlberger et al., And one or more components of a plasma device may require additional explanation. Will be mentioned as needed.

The powder supply device 20 shown in FIG.
This is shown in detail in FIG. The powder supply 20 has a powder feeder 70 for flowing an inert gas containing a selected amount of powder to be supplied to the plasma gun. Any suitable conventional powder feeder may be used. One example is the powder feeder described in U.S. Pat. No. 4,808,042 to Mühlberger et al., Issued Feb. 28, 1989. In this case, the powder feeder 70 uses a mixed gas of argon and hydrogen as a carrier gas,
Feed the powder at a standard flow rate of 60 grams.

The powder carrying inert gas is supplied via a conduit 71 to a powder fitting 72 at a first end 74 of a hot powder injector 76. The hot powder sprayer 76 has a second end 78 mounted inside the side of the plasma gun 16 on the opposite side and consists primarily of an elongated, high resistance heating tube 80. The heating tube 80 is substantially continuous with the powder fitting 72 and extends from a power connection fastener 82 at a first end 74 through a manifold 84 near the first end 74 to a manifold at a second end 78. It extends to tube 86. The heating tube 80 passes through the manifold 86 and passes through the plasma tube.
It extends halfway through the side wall 88 of the anode of the gun 16 and terminates in a hole 92 inside the side wall 88. The hole 92 extends to an internal hole 94 inside the anode 90. At the anode 90, a plasma stream is formed by the plasma gun 16 in a conventional manner.

The heating tube 80 has a DC power supply to both ends of the heating tube 80.
Made of material that can be heated in response to 96 applications. In this example, the heating tube 80 is made of tungsten and is approximately 10 feet long, 0.375 inch OD, 0.171 ID.
Inches. The power supply 96 is a power connection fastener at the first end 74.
It has a positive terminal connected to 82 and a negative terminal connected to the anode of plasma gun 16. And, the plasma gun 16 is electrically connected to the opposite end of the heating tube 80 at the second end 78. The power supply 96 is connected to the heating tube 80 through
A 120 kW DC power supply that can supply up to 00 amps.

As described below, power supply 96 is regulated to provide the desired voltage drop and current through heating tube 80. As a result, the heating tube 80 is heated to a desired temperature along the longitudinal direction. The heating tube 80 is heated in order to heat the powder to a desired temperature while being transported to the hole 94 of the plasma gun 16 through the length of the powder feeder 70 by the inert gas for powder transport. is there.

Since the reflecting tube 98 having a larger diameter than the outer periphery is arranged substantially concentrically around the outer periphery of the heating tube 80, the heating efficiency of the heating tube 80 is improved. The concentric arrangement of the reflector tube 98 around the heating tube 80 is maintained by a ceramic spacer 99. The reflecting tube 98 has a hollow space between the inner wall and the outer wall of the concentric tube, and is provided between the manifolds 84 and 86. Reflector tube 98
May be made of stainless steel. The reflecting tube 98 is
It has a polished surface inside the inner wall of the tube, and the surface reflects heat generated from the heating tube 80 back to the heating tube 80. By providing the reflecting tube 98 in this manner, it is possible to heat the heating tube 80 using a smaller amount of current supplied from the power supply 76.

The reflecting tube 98 is cooled by the water flowing from the high-pressure water pump 100. First, the water flowing from the pump 100 passes through the inside of the power connection fastener 82 and cools the fastener 82. Water flowing out of the power supply fitting 82 passes through the conduit 102 and enters the manifold 84. The water that has flowed out of the manifold 84 enters the hollow interior of the reflector tube 98 as shown by the arrow 104, flows along the longitudinal direction of the reflector tube 98, and reaches the manifold 86. At manifold 86, the cooling water exits the reflector tube and reaches drain 106. In this example, the high-pressure pump 100
Run about 7 gallons of water per minute.

The manifold 84 also receives the inert gas from the inert gas source 108 and directs the inert gas into the space between the outside of the heating tube 80 and the inside of the reflecting tube 98. The inert gas passes through a hole in the ceramic spacer 99 and passes through tube 80 and tube
Flow along 98 to manifold 86. Then, the inert gas is discharged from the manifold 86 through the hole 110. The inert gas has a function of preventing the heating tube 80 and the reflecting tube 98 from being oxidized.

According to the present invention, the surface temperature of the heating tube 80 is
Of the powder supplied from the powder feeder 70 through the heating tube 80
The heating tube 80 will never be heated to a point above the melting point . The surface temperature of the heating tube 80 causes the powder to soften.
However, it is better that the temperature is slightly lower than the temperature at which the powder cannot flow freely . As a typical measure for a powdery substance whose melting point and softening point are already known, the temperature between the melting point and the softening point is the temperature at which the flow of the powder becomes poor and the powder adheres or clogs. Is to measure After measuring such temperature, the heating tube should be approximately 300 degrees Fahrenheit (149 degrees Celsius).
Degrees) . The temperature of the heating tube 80 in the apparatus shown in FIG. 2 can be carefully controlled in the following manner.

Adjusting the other parameters, provided that the surface temperature of the tube does not reach the melting point of the powder, the powder particles arrive at the second end and the plasma gun
Until guided to the 16 holes 94, the powder particles are heated relatively uniformly until the temperature is relatively close to the surface temperature of the heating tube 80. The powder discharge temperature is influenced by the volume and flow rate of the inert gas and the powder carried by said gas. At a given tube temperature, gas volume, gas flow rate, powder volume and powder flow rate, the heating tube 80
By changing the length of the powder, the powder discharge temperature at the plasma gun 16 can be changed. In particular, the length of the heating tube 80 in the apparatus shown in FIG. 2 is considerably longer than the resistive heating powder transport tube in the prior art, and therefore the heating tube 80
A powder discharge temperature that is quite close to the surface temperature of the powder. Thus, it is avoided that the powder discharge temperature is much lower than the melting point of the powder. This means that a diameter of 50
This is particularly advantageous for relatively large powder particles, such as greater than microns. Such large particles, when heated to a relatively high discharge temperature, melt much more easily in the plasma stream to achieve the desired relatively uniform, dense coating.

FIG. 3 is a table showing the relationship between the current flowing through the heating tube 80 and the functions of the surface temperature and the powder discharge temperature of the heating tube 80. Here, for the device of FIG.
1.450 amp arc current, 52 volt arc voltage,
The case where a plasma gun 16 operating at a total power of 75.4 kW is used is shown. It will be seen that because the power supply 96 is tuned to increase the heating tube current, the surface temperature of the tube is increasing in a substantially linear manner. At a tube current of 400 amps, the surface temperature of the tube is slightly below 1,250 degrees Fahrenheit (677 degrees Celsius) . As the tube current increases to 800 amps, the surface temperature of the tube is accordingly increased to about 2000 degrees Fahrenheit (1093 degrees Celsius).
Degree) . The tube temperature point indicates the condition after the equilibrium is established in the heating tube 80. The heating tube 80
When current is first applied to the tube, it takes at least several minutes for the tube to heat up to equilibrium. Similarly, when the voltage applied to the heating tube 80 suddenly increases or decreases by a significant amount, a small amount of time is required before equilibrium is established.

The adjustment of the power supply 96 is performed as described above.
The heating tube current causes the tube surface temperature to be slightly lower than the melting point of the powder being supplied to the plasma gun 16. As a result, the surface of the powder particles
It will not cause serious powder flow problems, such as sticking to the inner walls of tubes or clogging tubes. In addition to tube surface temperature, FIG. 3 also shows the change in powder discharge temperature as a function of tube current. It can be seen that the powder discharge temperature increases in a substantially linear manner with increasing tube current. In the case of the apparatus of FIG. 2, for a given volume and flow of gas and a given volume and flow of powder, the powder discharge temperature is about
It increases from 750 degrees (399 degrees Celsius) to a temperature slightly lower than 1,750 degrees F (954 degrees Celsius ) at 800 amps tube current.

The tube current is restricted by the melting point and softening point of the powder. Nevertheless, the adjustment of the powder discharge temperature for a given current depends on the optimum range of the flow rate and the amount of the inert carrier gas carrying the powder and the powder flowing through the heating tube 80. It depends on the hot powder injection device. In such a case, as described below, a heating tube
By choosing a longer length of 80, the powder discharge temperature can be raised to a temperature below, but relatively close to, the tube surface temperature.

FIG. 4 is a table showing the relationship between the length of the heating tube and the powder discharge temperature as a function thereof. FIG.
Presupposes that the volume and flow rate of the carrier gas and the powder carried by it are kept at constant, optimal values. The tube temperature, also shown in FIG. 4, is shown as being approximately constant. As mentioned above, it is assumed that as the length of the tube changes, the adjustment of the tube current is adjusted so that the tube surface temperature is slightly below the melting point of the powder. Temperature difference Δ
t is the difference between the powder discharge temperature and the tube temperature. It is desirable that Δt be small, ideally reduced to zero, and that the powder discharge temperature be close to or equal to the tube surface temperature. This guides the powder to the plasma gun 16 at as high a temperature as possible and prevents the powder particles from melting while passing through the heating tube 80.

As shown in FIG. 4, as the length of the tube increases, the powder discharge temperature gradually increases,
Δt decreases. Eventually, a point is reached where the powder discharge temperature is approximately equal to the tube surface temperature. Although this is considered the optimal operating range, the economics and practicality of the required tube length and the power required to heat such a length of tube to the desired tube surface temperature are reduced. It is selected in consideration of it. In many cases, the longer the length of the tube, the longer the residence time of the powder in the tube, so that a powder discharge temperature equal to or close to the tube surface temperature can be obtained. As the residence time in the tube increases, the uniformity of the heating of the powder tends to increase. This tendency is particularly remarkable in the case of relatively large powder particles in which the temperature inside the particles remains at room temperature for a long time even after the outer skin or surface temperature is increased. Heating the large powder particles to or near the tube surface temperature promotes the melting of the powder as it is directed into the plasma flow in the plasma gun 16. In order to achieve a relatively high deposition efficiency and a relatively uniform and high-density coating on a workpiece, it is necessary that the particles in the plasma stream are almost completely melted.

FIG. 5 is a table showing the relationship between the current flowing through the heating tube 80 and the deposition efficiency as a function thereof. The deposition efficiency is expressed as a percentage of the powder that is sprayed by the plasma gun 16 and changes into a film of the work 24. In FIG. 5, the tube current is shown in the range between 700 and 800 amps.

Using the hot powder injector 76 shown in FIG. 2, the curve labeled "no extension tube" in FIG. 5 was obtained. Deposition efficiency ranges from about 2% at 700 amps to 800
You can see that it has risen to about 10% in amps.

The addition of the heating tube extension of FIG. 2A to the apparatus of FIG. 2 improves the deposition efficiency. The extension device of FIG. 2A has a 16-length stainless steel tube 112 connecting the powder accessory 72 and the conduit 71 leading to the powder feeder 70. Power supply 1
Numerals 14 are connected to both ends of the tube 112 to supply the tube 112 with resistive heat.

The curve labeled "with extension tube" in FIG. 5 shows the adhesion efficiency when using the extension device shown in FIG. 2A. As the current flowing from the power supply 96 increases from 700 amps to 800 amps, the independent power supply 114 connected to the tube 112 is tuned to generate a current that increases from 45 amps to 80 amps. This allows
The voltage drop on tube 112 decreased from 55 volts to 49 volts and then rose to 56 volts. From FIG. 5, FIG.
When the extension device is added, the deposition efficiency is considerably improved even in the same current range as 700 to 800 amps in the case of the heating tube 80. In particular, the tendency is remarkable at a high current value even in the above range. The deposition efficiency gradually increases from about 6% at 700 amps to about 8% at 750 amps. Above 750 amps, deposition efficiency increases at a much higher rate, reaching a value of 22% at 800 amps.

As described above, it is desirable that the heating tube 80 be heated to a temperature between the softening point and the melting point of the powder. Experiments have shown that for a given powder material, the flow of the powder becomes poor and the particles of the powder become soft to the point where they begin to stick together or to the wall of the heating tube, potentially clogging Has been shown to cause problems. Such a critical point is at a temperature below the melting point but above the softening point. Once the critical point has been measured, the heating tube 80 is heated to a lower temperature, leaving a difference of 300 degrees F (149 degrees C ) from the critical point.

Experiments are needed because the softening properties vary with the powdered material. For example, tungsten carbide can be heated to a few degrees difference from its melting point without loss of flow smoothness or danger of clogging . On the other hand, aluminum has a melting point of about 12
It is 00 degrees (649 degrees Celsius), a softening point of Fahrenheit to about 800 degrees (and
Mr. 427 degrees) . When aluminum powder is sprayed, the heating tube 80 should be heated to a temperature much higher than 700 degrees Fahrenheit (371 degrees Celsius) without causing any clogging or loss of smooth powder flow. Is impossible. Under such circumstances, additional heating may be required in the plasma stream, as described below. However, the invention makes it possible to heat as much of the powder as possible before it is transported to the plasma gun and in proportion to the quantity introduced into the tube.

In the case of a powdery substance that can heat the heating tube 80 to a temperature close to the melting point of the powder, such as tungsten carbide, the plasma gun 16 is not included,
A configuration is possible in which the hot powder injection device 76 is used as the only device that performs thermal injection. Such a configuration is shown in FIG.

The hot powder sprayer 76 is essentially the same as that of FIG. 6 except for the second end 78 which is integral with the workpiece 24 and is shown therein. The heating tube 80 terminates in a nozzle member 116 having a reduced diameter portion 118. The member 118 maintains the pressure of the heating tube 80 while maintaining that the hot powder reaches the work 24 and is discharged from the member 118 at a speed sufficient to form a film. The hot powder sprayer 76 can heat the powder almost uniformly to a temperature just below the melting point, so that the powder that is relatively soft and almost melting can be coated on the workpiece 24 in a fairly uniform manner.

While the hot powder injector 76 of FIG. 2 is advantageous when using relatively large powder particles, such as 50 microns in diameter, such a device is useful with powder particles of almost any size. This is because the heating tube 80 is kept at a temperature much lower than the melting point, so that even small particles are not excessively heated. Instead, they heat up more quickly and over the short entire length of the tube 80 to the wall temperature of the heating tube 80.
The mixed powder used for hot spraying is almost always
Particle diameter varies. In such a case, device 76
Before the powder is discharged from the device 76, it is ensured that the large powder as well as the small powder is heated to a temperature close to or close to the temperature of the heating tube 80.

[0053] heated powder delivery device 76 of FIG. 2 but can be thermally most of the powder to the extent that would almost completely melted the particles in the plasma stream, but the device 76 the preheating plasma device Form only one of several stages. The second stage is formed by the plasma gun 16. The third stage is formed by the plasma flow. As will be described later, the plasma flow may have a fourth stage if equipped with an additional power supply, electrodes, etc. for separately controlling the heating of the powder particles.

The second stage with the plasma gun 16 is used to change the velocity and density of the particles. The particle velocity is required to be at least such that an acceptable film can be formed on the work 24. Also, the larger the particles, the more difficult it is to accelerate as soon as they enter the plasma flow. plasma·
By adjusting the plasma flow in the gun 16, the particle velocity can be optimized. If it becomes necessary to change the plasma density, such changes can be made by changing the flow rate of the inert gas or the arc power supply in the plasma gun 16.

The third stage, including the plasma flow, is also used to change the powder conditions for optimal thermal injection. It has been found that, despite the supersonic velocity of the plasma flow, the temperature of the powder particles can still be varied. By increasing the reversing arc power supply, more energy is added to the plasma stream, causing more heating to the particles in the plasma stream. If the powder particles are very large or made of a material that cannot be heated to near the melting point by the heating tube 80, more particles will be introduced into the plasma stream if near complete melting does not occur. It is also possible to increase the conversion arc power supply for heating.

FIG. 7 shows an example of a configuration provided with a fourth region forming apparatus that controls the plasma stream by further heating the powder. The plasma device of FIG. 7 comprises a plasma gun 16 and a hot powder transport device 76 for transporting hot powder particles to the plasma gun 16 in the manner described above. The plasma source 46, shown in FIG. 1 and described in connection with this, creates the required potential difference between the electrodes of the plasma gun 16. On the other hand, the conversion arc current source 50 shown and described in FIG.
This creates a voltage drop with the workpiece 24 that results in a negative conversion arc. The plasma gun 16 forms a plasma stream 120 in turn. The plasma flow 120 shown in dashed lines in FIG. 7 flows from the plasma gun 16 to the workpiece 24. The hot powder guided to the plasma gun 16 from the hot powder injection device 76 is applied to the plasma gun 16.
On the plasma flow 120 caused by the work
The plasma stream 120 continues to accelerate and heat the powder particles while the powder particles are being carried by the plasma stream 120 to form a coating on 24.

The plasma apparatus shown in FIG.
Has an annular electrode 122 connected to it. in this way,
Ring electrode 122 is not only DC power supply 124 but also plasma power supply
A convenient electrode is also formed for 46. The DC power supply 124 connects between the annular electrode 122 and a second annular electrode 126 disposed at a position between the plasma gun 16 and the work 24.

As previously seen, the energy of the plasma stream 120 causes additional heating of the powder particles. However, the potential difference between the second annular electrode 126 and the annular electrode 122 provided by the second annular electrode 126 and the plasma acceleration and heating power supply 124 can be used for further acceleration and heating of the powder particles. This is effective for relatively large powder particles. This is because it is more difficult to accelerate or completely melt than with relatively small powder particles. As a result, film processing of the work 24 having higher uniformity and higher density becomes possible. The DC power supply 124 is tuned to change the amount of energy added to the plasma flow by the power supply.

In the configuration of FIG. 7, a single electrode 126 and a DC power supply 124 connected to the single electrode 126 are added. However, as shown in FIG. Is also good. FIG. 8 shows an additional electrode 1
FIG. 7 is the same as FIG. 7 except that it has DC power supplies 132 and 134. A DC power supply 132 is connected between the electrodes 126 and 128, and a DC power supply 134 is connected between the electrodes 128 and 130. The electrodes 126, 128, 130 effectively divide the plasma flow 120 into four different regions. The four areas are DC power supplies 124, 132, 1
34 is controlled to some extent independently. DC power supply 12
4, 132, 134 are adjusted to provide the same or different voltages desired.

The configuration of FIG. 8 is particularly effective when the plasma flow 120 is very long and the conversion arc power supply 50 cannot properly apply a voltage. However, such an arrangement may be necessary to control the temperature and, to some extent, the speed of the powder particles in different regions along the plasma flow 120, when having a plasma flow of any length that is necessary or desirable. ,It is valid.

Although the electrodes 126, 128, 130 are illustrated as being equally spaced between the electrode 122 and the workpiece 24, such spacing can be varied if desired. Further, by changing the combination between the electrodes and connecting a DC power supply, or by inverting one or a plurality of power supply electrodes, it is possible to change the effect of heating or accelerating the region.

In the configuration of FIG. 8, the conversion arc power supply 50 is directly connected to the work 24. In addition to obtaining the desired plasma operation on the workpiece 24, such a direct connection is necessary if the workpiece 24 must be cleaned before the workpiece 24 is sprayed with powder from the hot powder injector 76 onto the workpiece 24. Become. However, direct connection as in the case of connection between each power supply 50 and the work 24 is necessary when the work 24 must be cleaned before powder is sprayed on the work 24. However, if the power supply 50 and the work 24 are directly connected as in the case of the connection, considerable heat is generated in the work 24. For workpieces 24 made of a relatively fragile material that cannot withstand the heat generated by the direct extension of the power supply, it may be desirable to disconnect the conversion arc power supply 50 from the workpiece 24 during injection. The configuration shown in FIG. 9 has a switch 140 between the conversion arc power supply 50 and the work 24. In addition, the electrode 130
May be lowered slightly from the position shown in FIG. In all other respects, the plasma device of FIG. 9 is similar to that of FIG. During the cleaning of the work 24, the switch 140 is closed and connects the conversion arc power supply 50 and the work 24 directly. As seen above, such a direct connection is required to clean the work 24. As soon as the cleaning process is completed, switch 140 is opened to disconnect conversion arc power supply 50 from work 24. switch
With the 140 opened, the plasma spraying operation is performed. When the electrode 130 is brought very close to the work 24, the coating operation is performed on the work 24 without connecting the conversion arc power supply 50 to the work 24. In this way, the workpiece 24 does not need to receive the strong heat generated when the power supply is directly connected.

The arrangement of FIG. 10 having a plasma gun 16, a plasma power supply 46, and electrodes 122, 126, 128 is a variation of the arrangement of FIG. 8 utilizing a direct connection between the power supply and the cathode 142 of the plasma gun 16. is there. Cathode of plasma gun 16 142
Is the common emitter of the electrons in the plasma gun 16. It has been found that by connecting one or more DC power sources to the cathode 142, the desired plasma temperature distribution can be achieved for certain applications. However, only the negative pole of the power supply can be directly connected to the cathode 142.

The reason why the negative electrode of the plasma power supply 46 is always connected to the cathode 142 is that this connection method is a common method of connecting the power supply of the plasma gun. In the configuration of FIG. 10, the negative terminals 144 and 146 of the DC power supply connected to the electrodes 126 and 128 respectively are connected to the cathode 124 of the plasma gun 16.
Also connected directly. The DC electrodes 144 and 146 have almost the same functions as the DC power supplies 124 and 132 in FIG. However, because the power sources 144, 146 are directly connected to the cathode 142 of the plasma gun 16, the poles of the electrodes must be connected in a relationship as shown in FIG. In the configuration of FIG. 8, as long as the negative terminal of another DC power supply in a plasma configuration, such as a DC power supply 134 connected to the cathode 130, is connected to the cathode 142, such a power supply may be used, if necessary, for the cathode of the plasma gun 16. Can be connected directly to 142. Thus, one or more DC power sources are connected to the cathode of the plasma gun 16.
Connecting directly to 142 has been found to be effective in achieving the desired plasma temperature distribution in some applications.

Connecting one or more DC power supplies to the cathode 142 requires connecting the negative terminal of such a power supply to the cathode 142. In contrast, other power sources in the plasma device can be connected with different electrode configurations. This is illustrated by the plasma device of FIG. 11, which is the same as the configuration of FIG. 10 except that there is a DC power supply 148. Since DC power supply 148 is not directly connected to cathode 142,
The electrodes may be arranged such that the positive electrode is connected to the electrode 126 and the negative electrode is connected to the electrode 128 as shown in FIG. However, if necessary, reverse the polarity of DC power supply 128,
Special temperature characteristics along the longitudinal direction of the plasma flow 120 may be obtained.

FIG. 12 is similar to the plasma apparatus of FIG. 8, and includes a plasma gun 16, a plasma power supply 46, an electrode 122,
Shown is a portion of a plasma device having 126, 128 and DC power supplies 124, 132. Further, the configuration of FIG. 12 has different permanent magnet members 150 and 152. The first permanent magnet member 150, which is hollow and generally cylindrical, has an electrode
It is located between 122 and the electrode 126 and surrounds the plasma flow 120 therebetween. A permanent magnet member 152, also hollow and generally cylindrical, is positioned between the electrodes 126 and 128 and the plasma flow 120 between them.
Is surrounded by

The normal outline of the plasma flow 120 is shown by the solid line in FIG. However, the presence of the permanent magnet members 150, 152 allows the plasma flow 120 to be shaped advantageously. Thus, since the permanent magnet member 150 is provided, the plasma flow between the electrode 122 and the electrode 126 is
The outline of 120 can be changed from the outline shown by solid line 154 to the configuration shown by dashed line 156. Similarly, the provision of the permanent magnet member 152 allows the portion of the plasma flow 120 between the electrode 126 and the electrode 128 to be reshaped to the general shape indicated by the dashed line 156. When the general shape of the plasma flow 120 is thus deformed by the permanent magnet members 150 and 152, the electrode 1
At 26 and 128, the electrons in the plasma stream 120 swirl in a circular fashion. This occurs as a result of the heat distribution caused by the plasma at the electrodes 126, 128 to prevent overheating of the electrodes 126, 128.

The polarities of the permanent magnet members 150, 152 are shown in FIG. 12 for illustrative purposes only. Such polarity is
It can be inverted if desired, but still achieve the beneficial effects of plasma flow shaping. Although two permanent magnet members 150, 152 are shown in FIG. 12, additional permanent magnet members may be added to relate to the overall plasma current between the electrodes 128, 130 shown in FIG. Further, the permanent magnet members 150 and 152 may have shapes and configurations other than the hollow cylindrical shape.

FIG. 13 shows another configuration in which an electromagnet 158 is provided between the electrodes 122 and 126 so as to surround a part of the plasma flow 120. The electromagnet 158 has a generally cylindrical shape and can be used to deform the general shape of the plasma flow 120 to obtain a more uniform temperature distribution at the electrode 126 as shown in FIG.
It produces the same effect as 150 and 152. Unlike the permanent magnet members 150, 152 of FIG. 12, the electromagnet 158 must be connected to a DC power source to obtain the required magnetic field.

The DC power supply 160 is connected between the first end of the electromagnet 158 and the electrode 126. At the same time, the second end of the opposite electromagnet 158 is connected to the cathode 142 of the plasma gun 16. By connecting the electromagnets 158 in this manner, a single DC power supply 160 supplies voltage to the electromagnets 158 and provides the required voltage between the plasma gun 16 and the electrodes 126.
However, when the common DC power supply 160 is used, the voltage applied to the electromagnet 158 and the voltage applied to the electrode 126 cannot be adjusted separately. DC power supply 160 to electromagnet 158
The DC power supply 160 is effectively connected directly to the cathode 142 by connecting to the cathode 142 of the plasma gun 16 via Must connect.

An electromagnet having a shape and configuration different from the electromagnet 158 can be used. Further, the electromagnet 158 may be configured as a permanent magnet instead of the above-described electromagnet member that requires a DC power supply.

[0072] Occasionally, the powder used in the thermal spray process is a mixture of different materials. In this case, some materials may not be easily mixed with other materials particularly when heated, which may cause a problem. To overcome this problem, it is possible to treat such immiscible particles so that they adhere to particles of other substances. It is known that a substance such as chromium, when applied to a part or the entire surface of a particle that is difficult to adhere, has an increased adherence. Chromium can be applied using a variety of techniques, such as mixing at room temperature, mixing with balls, and electroplating.
Chromium coating is particularly effective for oxidized particles that are difficult to mix with other particles.

Although the present invention has been shown and described with preferred embodiments of the invention, it will be understood by those skilled in the art that various changes may be made in form and detail without departing from the spirit and scope of the invention. You will understand.

[Brief description of the drawings]

FIG. 1 is a combination of a block diagram and a partially broken perspective view of an improved hot powder injection device according to the present invention and a plasma device having other features of the present invention.

2 is a cross-sectional view of an improved hot powder injection device of the device of FIG. 1 and is a front view of a heating tube extension device used in connection with the hot powder injection device of FIG. 2A to increase deposition efficiency. .

FIG. 3 is a table showing the relationship between the current applied to heat the powder injection tube of the apparatus of FIG. 2 and the powder temperature and the tube temperature, which are functions of the current.

FIG. 4 is a chart showing the relationship between the length of a powder transport tube of a device similar to the device of FIG. 2 and the powder discharge temperature as a function thereof for a predetermined powder speed and the state of a carrier gas in the powder transport tube. It is.

FIG. 5 shows the current applied to resistively heat the powder transport tube and the function of the current in the plasma device for the apparatus of FIG. 2 and for the case where an extension pipe is attached to the apparatus. 5 is a table showing the relationship between the powder particles sprayed on and the coating efficiency.

FIG. 6 is a cross-sectional view of a portion of the improved powder injection device of FIG. 2 modified to perform thermal spraying using a plasma gun for certain powdered materials.

FIG. 7 is a conceptual diagram illustrating a portion of an improved plasma device in which the addition of a power supply and an electrode ring enhances the heating of powder particles in a plasma stream within the device.

FIG. 8 is a conceptual diagram illustrating a portion of an improved plasma device that controls the heating of powder particles in different regions along the plasma flow by adding a power source and electrodes.

FIG. 9 is a conceptual diagram showing a part of a plasma device similar to the plasma device of FIG. 8, in which a conversion arc power supply can be cut off from the work in order to prevent overheating of the work. is there.

FIG. 10 is a conceptual diagram showing a part of a plasma device similar to the plasma device of FIG. 8 and having a DC power supply for an electrode connected in series to a cathode of the plasma gun.

11 is a conceptual diagram showing a part of a plasma device similar to the plasma device of FIG. 10 and having a DC power supply connected to the plasma gun in a manner other than in series.

FIG. 12 is a plasma apparatus similar to the plasma apparatus of FIG. 8, in which a plurality of permanent magnet members are disposed near a place where a plasma flow flows, so that the plasma flow is formed and the plasma heat at the electrodes is dispersed. It is a conceptual diagram which shows a part of apparatus.

FIG. 13 is a conceptual diagram showing a part of a plasma device similar to the plasma device of FIG. 12, in which an electromagnet is used instead of a permanent magnet.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Plasma chamber 16 Plasma gun 18 Plasma gun operation device 20 Powder supply device

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C23C 4/00-4/16

Claims (27)

(57) [Claims] 1. A method for transferring heated powder to a plasma gun, comprising the steps of: preparing powder particles having a particle size of 50 μm or more; A step of heating the powder particles to a temperature of, and a step of allowing the powder particles to stay in the powder transport tube for a time sufficient to reach a temperature substantially equal to that of the powder transport tube, and passing the powder particles through the powder transport tube. A method comprising: 2. The method according to claim 1, wherein the flow rate of the powder particles in the powder transport tube is given, and the temperature of the powder is substantially equal to that of the powder transport tube before the powder reaches the outlet of the transport tube. A method comprising selecting the length of the transport tube such that the powder is heated such that 3. The method of claim 1, wherein the temperature immediately below the melting point temperature is above the softening point until the powder is heated to a critical point temperature at which the powder begins to stick and clog. Determined by lowering the safety margin by: 4. A method for transferring heated powder from a powder feeder supplying a powder having a given melting point in a given flow rate and having a particle size of 50 μm or more to a plasma gun, wherein the particle size is 50 μm or more. Preparing powder particles having the following formulas: connecting the powder feeder and the plasma gun with a tube to supply powder at a given flow rate from the powder feeder to the plasma gun; Heating the powder to a temperature close to, but not reaching, the melting point, and heating the powder to a temperature close to the tube temperature while feeding the powder to the plasma gun for a given flow rate, powder particle size, and tube temperature Setting the length of the tube to a length that is similar to the length of the tube. 5. An apparatus for transferring a heated powder from a powder source to a plasma gun, comprising: a hollow heating tube connecting the powder source and the plasma gun; a power source connected to both ends of the heating tube; And a hollow reflecting tube which is disposed substantially concentrically with respect to and which reflects heat radiated by the heating tube. 6. The apparatus of claim 5, wherein said reflector tube comprises cylindrical inner and outer walls concentric and having a space therebetween, further comprising a space between the cylindrical inner and outer walls of the reflector tube. A means for flowing a cooling fluid therethrough. 7. The apparatus according to claim 5, wherein the reflecting tube has a reflecting inner wall, and furthermore, the heating tube is concentrically mounted in the reflecting tube. An apparatus comprising a plurality of ceramic spacer elements. 8. The apparatus according to claim 5, further comprising a space between the reflection tube and the heating tube, and further causing an inert gas to flow through the space between the reflection tube and the heating tube. A device comprising means. 9. The apparatus according to claim 5, wherein the heating tube comprises a powder passing through the heating tube such that the powder is conveyed from a powder source to a plasma gun at a temperature greater than or substantially equal to the surface temperature of the heating tube. An apparatus having a length determined by the flow rate of the fluid and the surface temperature of the heating tube. 10. A thermal injection device, comprising: a powder source for conveying powder in a carrier gas under pressure; an end connected to the powder source, and a powder source in the carrier gas under pressure. A hollow heating tube receiving and terminating in a nozzle having a neck with a reduced diameter at the other end, and a power supply coupled to the heating tube to heat the heating tube to a temperature close to the melting point of the powder. And means for positioning the work so as to be spaced from the end of the heating tube and directly face the end of the heating tube in order to supply the powder onto the work and apply the coating of the powder. Heat injection device. 11. A plasma gun supplying plasma stream to the workpiece from a position spaced apart with respect to the workpiece, means for supplying powder to said plasma stream, placed in a plasma stream between the workpiece and the plasma gun And a DC power supply connected to the plasma gun and the electrodes to provide a potential difference therebetween, and a DC power supply arranged to surround the plasma flow between the plasma gun and the electrodes, forming a magnetic field and And a hollow magnetic member for applying the heat of the plasma flow to the electrodes. 12. A plasma gun supplying plasma stream to the workpiece from a position spaced apart with respect to the workpiece, means for supplying powder to said plasma stream, placed in a plasma stream between the workpiece and the plasma gun And a DC power supply connected to the plasma gun and the electrodes for providing a potential difference therebetween, the electrodes including a plurality of electrodes spaced along the plasma flow. And the DC power supply includes a plurality of power supplies for supplying a potential difference between a plurality of pairs of electrodes. 13. A plasma gun supplying plasma stream to the workpiece from a position spaced apart with respect to the workpiece, means for supplying powder to said plasma stream, placed in a plasma stream between the workpiece and the plasma gun A DC power supply connected to the plasma gun and the electrodes to provide a potential difference therebetween, a conversion arc power supply connected to the plasma gun, and a conversion power supply provided between the conversion arc power supply and the work. A plasma device comprising: a switch; 14. The apparatus according to claim 13, wherein the electrode is disposed adjacent to a workpiece. 15. The apparatus according to claim 14, further comprising: a second DC power supply; and a second DC power supply connected to the work so that the second DC power supply is disconnected from the work except during cleaning of the work. Means for selectively connecting, a plasma device comprising: 16. A work, a plasma gun having a cathode spaced from the work and applying a plasma flow to the work, an electrode disposed in a plasma flow between the plasma gun and the work, And a DC power supply having a positive terminal connected to the electrode and a negative terminal connected to the cathode of the plasma gun. 17. The apparatus of claim 16, wherein said electrodes comprise a plurality of electrodes spaced apart from each other in a plasma flow between a plasma gun and a workpiece, and wherein each of said DC power supplies comprises A plasma device comprising a plurality of DC power supplies having a positive terminal connected to different electrodes and a negative terminal connected to a cathode of a plasma gun. 18. The apparatus according to claim 16, wherein said electrodes comprise a plurality of electrodes spaced apart from each other in a plasma flow between a plasma gun and a workpiece, and At least one of which has a positive terminal connected to one of the plurality of electrodes and a negative terminal connected to the cathode of the plasma gun, at least one of which is connected to one of the plurality of electrodes. And a negative terminal connected to another one of the plurality of electrodes. 19. A work, a plasma gun separated from the work to supply a plasma flow to the work, an electrode disposed in a plasma flow between the plasma gun and the work, and connected to the electrode. A magnetic member disposed around the plasma flow between the plasma gun and the electrode, the magnetic member forming a magnetic field to form the plasma flow, thereby dispersing heat generated at the electrode. A plasma device comprising a combination of: 20. The apparatus according to claim 19, wherein said electrodes comprise a plurality of electrodes spaced apart from the electrodes and disposed in a plasma flow between the plasma gun and the workpiece; A plurality of DC power supplies coupled to different ones of the plurality of electrodes, wherein the magnetic member is disposed around a plasma flow and each is disposed above a different one of the plurality of electrodes. A plasma device, comprising: a magnetic member. 21. The plasma apparatus according to claim 19, wherein said magnetic member comprises a hollow, substantially cylindrical permanent magnet member. 22. The plasma apparatus according to claim 19, wherein the magnetic member comprises a substantially cylindrical coil and a DC power supply connected to the coil. 23. The apparatus of claim 19, wherein said plasma gun has a cathode, said magnetic member comprises a substantially cylindrical coil, and said DC power supply is a negative terminal connected to an electrode. And a negative terminal connected to the first end of the coil, and further comprising means for connecting a second terminal on the opposite side of the first terminal of the coil to a cathode of the plasma gun. A plasma device characterized by the above-mentioned. 24. The method according to claim 1, further comprising at least partially coating the surface of the powder particles of the first material, wherein the powder particles of the first material and the first material are different second and powder particles mixed material, and a step of forming a mixed powder, changing the nature of the surface of the powder particles of the first material with a coating on the surface of the powder particles of the first material and, how to, characterized in that to promote the mixing of the powder particles of the second material. 25. The method of claim 24, wherein at least partially coating the surface of the first powder particles comprises coating the surface of the first powder particles with chromium. And how. 26. The method according to claim 1, further comprising the step of providing powder particles of the first and second materials; and wherein the first and second materials are attracted to each other. A step of coating at least a part of the surface of the powder particles of the material with a third material; and a step of mixing the powder particles of the first and second materials. A method comprising modifying the surface properties of powder particles of a first material by coating a third material to facilitate mixing with the powder particles of a second material. 27. The method of claim 26, wherein said first material is a metal oxide and said third material is chromium.