WO1998038841A1

WO1998038841A1 - Plasma torch for plasma spraying equipment and pertaining anode

Info

Publication number: WO1998038841A1
Application number: PCT/DE1998/000599
Authority: WO
Inventors: Oliver Prause
Original assignee: Oliver Prause
Priority date: 1997-02-26
Filing date: 1998-02-26
Publication date: 1998-09-03
Also published as: EP1013155A1; DE19707699C1

Abstract

The invention relates to an electrode (20), specially an anode (20) made from a highly refractory metal electrode material for plasma spraying equipment, specially plasma torches (10). According to the invention, the electrode (20) has little or no material additives, specially little or no doping agents, thereby enabling enhanced serviceable life. The invention further relates to a plasma torch (10) with an inventive electrode (20) attached in the form of an anode, as well as to corresponding plasma spraying equipment.

Description

Plasma torch for plasma spraying systems and associated anode

The present invention relates to a plasma torch for a thermal plasma spray system, comprising a first electrode which can be connected as an anode to an associated current source and a second cathode which can be connected to the current source as a cathode.

Thermal plasma spraying systems generally have a so-called plasma torch. The plasma torch essentially consists of a cathode and an anode. The arrangement of the cathode and anode and the entire arrangement of the plasma torch is generally carried out in such a way that an electric current flows between the cathode and anode via an arc. A hot plasma is generated by supplying a gas flow to the arc. The plasma generated is used, for example, to coat any surface with or without filler materials (e.g. ceramic thermal barrier coatings on turbine blades) or to treat it thermally (e.g. surface hardening). To cool the thermally loaded plasma torch, a suitable supply and discharge of a cooling medium (e.g. water) to the plasma torch must generally be provided.

The electrodes for plasma torches are usually formed from a high-melting metal with one or more dopants that promote electron emissions, ie material additives with low electron work function. For powder-metallurgical manufacture of the electrodes, a metallic powder is usually mixed with the dopants, in bars or rods. be pressed and sintered. After sintering, the bars are formed several times until the required diameter is reached. The bars are then sanded and cut. As a rule, the electrodes are in a heat-conducting connection with a further metal (eg copper), which has good thermal conductivity, in order to dissipate the heat that occurs during operation to a cooling medium. For this purpose, the cut electrode blanks are placed in a graphite mold and cast in copper in an oven. Finally, this composite material is machined, whereby the added doping leads to improved machinability (eg drilling) of the high-melting metal (eg tungsten).

Conventional high current electrodes serving as cathode, which can be used in a plasma torch as a plasma torch cathode, are known for example from DE 35 44 657 A1. The published specification primarily describes high-current cathodes with a tungsten core and a high-melting coating and a low electron work function. The coating consists of tungsten and the tungsten added material additives which cause the low electron work function (so-called doping; according to claims 3 and 4 of the published patent application at least 4 percent by weight of ThO ₂ or CeO ₂ , preferably about 10 percent by weight of ThO ₂ or CeO ₂ ). The high-current cathodes are compared with a comparison cathode made of pure tungsten and with a conventional sintered cathode in order to demonstrate improved burn-off properties. The sintered electrode has an electrode body made of tungsten and added to the tungsten. doping which reduces the electron work function (2nd

Weight percent ThO ₂ ; according to the introduction to the description

Laid open a value customary in plasma technology in order to reduce the electron work function in tungsten from 4.4 eV to approximately 2.63 to 2.86 eV).

Further electrodes with electron emission-promoting doping are known from DE 27 55 213 C2 and DE 31 06 164 A1. EP 0 326 318 A2 and DE 36 18 600 C2 can be cited as a further prior art for electrodes in general. Further background information can be found in the following writings and articles: Oliver Prause: "The Influence of Electron Emission-Promoting Doping on the Production and Application of Tungsten Electrodes", Progress Reports VDI, Series 5: Basic and Materials, No. 470, VDI Verlag , Düsseldorf, 1997; A. Modinos: "The electronic work function of W-faces", in: Surf. Sei, No. 75, pp. 327-342, 1987; S. W. H. Yih, C. T. Wang: "Tungsten", Plenum Press, New York, 1979; M. Henzler, W. Göpel: "Surface Physics of the Solid", Teubner Verlag, Stuttgart, 1991.

The addition of electron emission-promoting doping leads, as is also made clear by the discussed prior art, to a significant improvement in the cathode properties. This can be explained by the fact that the doping reduces the ionization energy of the outer electrons compared to pure metal (eg tungsten). With a low electron work function and a given current density, the temperature in the is reduced in accordance with the Richard Dushmann equation Cathode. A low temperature consequently leads to a longer service life of the cathode. From this fact, which has been proven in practice, it was concluded that the anode service life is also improved by adding dopants. It was assumed that when the electrons re-entered the anode, the ionization energy was obtained as thermal energy. Since the service life of the anode is influenced by the heat load, it was concluded that the reduction in the ionization energy by adding dopants also leads to a reduction in the heat load in the anode, so that the service life is increased. Although electron-emitting dopants were added, the service life of the anode was still unsatisfactory.

Against this background, the invention has for its object to provide a plasma torch of the type specified in such a way that a long service life is achieved both for the first electrode serving as the anode and for the second electrode serving as the cathode.

This object is achieved by the plasma torch with the features of claim 1.

The second electrode serving as the cathode (more precisely: its active electrode section interacting with the arc, which is made of a high-melting metal electrode material and preferably comprises tungsten and / or molybdenum), according to the invention - like cathodes of the prior art - has one with regard to the electron work function we- substantial proportion of material additives in the form of doping with low electron work function (or - in other words - with energetically unstable electron configuration). As doping, elements with incompletely occupied d or f electrode shells, or / and with electron donor action, for example oxides and borides of the elements of the IIIb to IVb subgroup of the periodic table of the elements and the first three elements from the lanthanoid and actinide group, such as ThO ₂ , La ₂ O ₃ , CeO ₂ , Y ₂ O ₃ and LaB ₆ can be used.

According to the invention, the first electrode serving as the anode (more precisely: its active electrode section interacting with the arc, which is made of a high-melting metal electrode material and preferably comprises tungsten and / or molybdenum) has no material additions in the form of dopants with regard to the electron work function with a low electron work function (or - in other words - with an energetically unstable electron configuration).

Contrary to expectations, it was found that a substantial increase in the service life can be achieved for the anode by dispensing with electron-emission-promoting doping, quite in contradiction to the previously valid ideas about the (positive) influence of doping on the service life of anodes due to the reduction the ionization energy.

The result of increasing the service life by dispensing with doping can possibly be explained by the fact that in the first place A high thermal stability of the anode material is of crucial importance for the service life. Tungsten with the highest melting point of all metals is particularly suitable. Furthermore, it may be that the theoretically explainable low amount of energy practically does not occur when the electrons re-enter the anode due to the low ionization energy caused by the doping. This could be explained by the fact that, due to the high temperature and the sputtering effect of the arc, no layer near the surface with a low electron work function can form on the anode.

In the known anodes with doping, the following material-related causes may also have an adverse effect on the anode life: formation of microcracks due to evaporation of doping (eg ThO ₂ , La ₂ O ₃ ) that promotes electron emission;

- Evaporation of tungsten through oxidation with oxygen radicals, which arise from the thermal dissociation of the electron-emitting oxidic dopants;

- Local overheating of the anode by burning the moving arc at an energetically favorable point (eg micro cracks and / or surface morphological changes which are related to the doping). Local overheating of the anode material usually leads to failure of the plasma torch. On the one hand, the impressed electrical quantities can no longer be checked or controlled and, on the other hand, the material removal of the anode material leads to a deterioration in the quality of the the surface. As far as attributable to the doping, these disadvantages no longer occur according to the invention.

It has furthermore been shown that minor additions of material in the high-melting metal, in particular additions of sinter activating materials in the case of sintered electrodes (electrodes produced by a sintering process), have no significant negative influence on the service life of the anode material. This can possibly be explained by the fact that with complete or partial solubility of the sinter-activating material additives and the high-melting metal there is a homogeneous and finely dispersed distribution. In contrast to this, for example, the oxidic doping, due to the complete insolubility in the high-melting metal (eg tungsten), exists as comparatively coarse particles at the grain boundaries of the material structure. It can be assumed that the high thermal load on the anode and the sputtering effect of the arc always leads to a more or less pronounced evaporation of the anode material, in particular the electron emission-promoting material additives (doping). Accordingly, it would be conceivable that, for example, microcracks close to the surface occur at local points through selective and / or rapid evaporation of the dopants. These local points have an energetically higher-quality state compared to neighboring areas, so that the hot, electric arc preferably hits the anode at these points. This in turn leads to increased material removal, which ultimately leads to the so-called "sticking" of the arc and thus to the failure of the plasma torch. In contrast, a finely dispersed distribution of low-melting material additives leads to a more homogeneous material removal on the material surface. This would have the consequence that the local energetic differences are only slight and the arc does not find any preferred locations on the material surface of the anode. From this, a uniform material removal from the material surface can be derived and thus justify an extension of the service life.

In the case of sintered electrodes (the preferred embodiment of the electrodes according to the invention), sinter-activating material additives can thus be added to the metal powder both in the case of the first electrode serving as an anode and in the case of the second electrode serving as cathode, since sinter-activating material additives if they contain a small amount Make up the proportion of the electrode or the electrode material, have no significant influence on the service life. By adding sinter-activating material additives, the sintering temperature or sintering time can be reduced and / or the workability (e.g. drilling) of the high-melting sintered material (preferably sintered from high-melting tungsten powder) can be improved. Good results have been achieved with a weight fraction of the sinter-activating material additives, in particular nickel, in the metal powder of up to 1 weight percent. In particular, contents of approximately 0.12 to 0.5 percent by weight are recommended.

The electrodes according to the invention, in particular anodes, can be produced, for example, by the method known from German patent DE 44 42 161 C1 (cf. in particular Their claims 1 1 to 18), but in the case of the first electrode or anode, according to the invention, no or only an insignificant proportion of doping to promote electron emission is to be provided and under certain circumstances (possibly in deviation from claim 1 of DE 44 42 161 C1) a post-processing of the electrode surface interacting with the arc is indicated.

Preferred developments of the electrode according to the invention are specified in the subclaims. As already mentioned, the formation of both electrodes as sintered electrodes is particularly preferred, sinter-activating material additives being able to be used in their production. It has been shown that, in view of the longest possible anode service life for any material additives, a maximum value of the weight fraction should not be exceeded, the smaller the maximum value, the better the result. The maximum value proposed is a maximum of 1.5 percent by weight, preferably a maximum of 1.0 percent by weight, most preferably a maximum of 0.5 percent by weight. Since additions of materials that require electron emissions (doping) have a greater effect on the service life of the first electrode or anode than sinter-activating additions of material If the first electrode or anode for the doping is as small as possible, a maximum value of 1.0 percent by weight, preferably a maximum of 0.1 percent by weight, most preferably a maximum of 0.01 percent by weight is suggested, or it is best to dispense with it entirely. The invention further relates to an anode for a plasma torch according to the invention, namely an electrode serving as an anode, the electrode section of which, when interacting with an arc, directly interacts with an electrode (active electrode section; in particular in contrast to an optionally provided holding section of the electrode, for example made of copper, which is the active one Holds electrode section and can serve for cooling) consists predominantly or completely of at least one high-melting metallic electrode material. According to the invention, the active electrode section or the electrode material has no material additives or only a small proportion of material additives, in particular no or only an insignificant proportion - in particular insignificant with regard to the electron work function - of doping with low electron work function or energetically unstable electron configuration, such as Elements with incompletely occupied d- or f-electron shells, or / and with electron donor action, for example oxides and borides of the elements of the 111 ____ to IVb subgroup of the periodic table of the elements and the first three elements from the lanthanide and actinide groups, such as ThO ₂ , La ₂ O ₃ , CeO ₂ , Y ₂ O ₃ and LaB ₆ . The anode according to the invention can have the features specified above and in the claims with respect to the first electrode. As a rule, the anode will have a plasma passage channel. The anode is preferably a sintered electrode.

It goes without saying that an anode according to the invention can optionally also be composed of several sections. With regard to the plasma torch, it goes without saying that this may also have multiple anodes or / and multiple cathodes.

The invention also relates to a thermal plasma spray system with an anode according to the invention. In the plasma spraying system according to the invention, an electrode with electron-emitting doping according to the prior art is preferably to be provided as the cathode. It is therefore preferred that the plasma spraying system has a plasma torch according to the invention.

Further advantages and features emerge from the following description of an embodiment of the invention in connection with the drawing. Show it:

Figure 1 is a schematic sectional view of a plasma torch according to the invention with two electrodes: a cathode and an anode; and

Fig. 2 is a schematic sectional view of the anode according to

Fig. 1.

1 shows a longitudinal section of a cathode 22 and an anode 20 according to the invention as parts of a plasma torch 10.

The cathode 22 and the anode 20 as part of the plasma torch 10 are geometrically designed so that a cooling medium 40 (for example water) can be brought up to a material 32 with high thermal conductivity (for example copper), which with the actual electrode material 30 (which in operation with a

Arc directly interacting electrode section of the anode - here referred to as "active" electrode section) is in a thermally conductive connection. The cathode 22 and the anode 20 are arranged in such a way that an arc can form between the cathode 22 and the anode 20 (more precisely: between their "active" electrode sections which interact directly with the arc) in order to use a gas flow to introduce a plasma into the To produce an arc. The generated plasma is conducted via the plasma passage channel 50 in the direction of the surface to be treated.

In FIG. 2, the anode designated 20, also called a plasma nozzle, is shown as a composite of the electrode material 30 and the material 32 with high thermal conductivity.

The electrode material 30 of an anode 20 produced in the described embodiment using a sintering process consists of metal powder which essentially consists only of chemically pure tungsten or from 95.5 to 98.8% by weight chemically pure tungsten with a sinter-activating material additive, namely in the case of the here described example 0, 12 to 0.5 weight percent nickel, is composed. Depending on the composition of the metal powder, there are two different processing methods, in particular sintering methods.

In a known manner, when using chemically pure tungsten powder without the addition of materials, the metal powder in elastic, cylindrical hoses filled and sealed watertight. The metal powder is then compressed in a cold isostatic press at 2000 to 3000 bar. Alternatively, the metal powder can also be placed in a correspondingly shaped press die and then compressed mechanically and hydraulically. Then the pressed metal powder is sintered in a direct current passage - Coolidge process - at approx. 2600 ° to 3200 ° C and a holding time of 15 to 30 minutes in a hydrogen atmosphere. In a further working step, the sintered rod or the electrode material 30 is usually shaped at temperatures of 900 ° to 1600 ° C., the temperature depending on the degree of deformation, for example by hammering.

When using the metal powder with a sinter-activating material additive, the following processing method can be used: First, the metal powder and the sinter-activating material additive are prepared by dry mechanical mixing or by a wet chemical hydrometallurgical process. In the latter case, a liquid nickel nitrate solution is sprayed into tungsten trioxide and mixed, and subjected to a drying, reducing and sieving process. The prepared metal powder with the material additive is filled into elastic, cylindrical hoses, sealed watertight and cold isostatically compressed at 2000 to 3000 bar. The compacted metal powder is sintered in a furnace at temperatures of 1400 ° to 1600 ° C and holding times of 30 to 180 minutes in a hydrogen atmosphere. By sintering, the pressed metal powder becomes the metallic state of the electrode material 30 transferred, densities from 80 to 97% of theoretical

Density can be achieved.

The electrode material 30 made of chemically pure tungsten or of tungsten with sinter-activating material additive can be machined after the sintering process (for example, rotating and / or grinding and / or eroding), in particular by drilling and / or eroding the plasma passage 50 as Plasma nozzle serving anode 20 are introduced. The appropriately prepared electrode material 30 is finally placed in a suitably designed graphite mold. After adding copper as material 32 with high thermal conductivity to the graphite mold, it is placed in a lowering or gradient furnace. At temperatures of 1050 ° to 1200 ° C, the copper is completely melted in a hydrogen atmosphere and then continuously cooled in order to obtain a non-porous composite.

In principle, the cathode 22 can be produced in the same way using the two processing methods mentioned, although preferably additions of materials (doping) which require electron emissions are introduced, as is known in the prior art for anodes and cathodes for plasma burners. Such electron emission-requiring material additives are not provided according to the invention for the anode 20. After the material-removing processing described above, in particular after drilling and / or eroding the plasma passage 50, the anode 20 can be installed in a plasma torch 10 as part of a plasma spraying system. The cathode 22 and the anode 20 are electrically connected to a current source, the cathode 22 being connected to the negative pole and the anode 20 being connected to the positive pole of the current source.

The arc between cathode 22 and anode 20 is ignited by a high-frequency voltage superposition or capacitor discharge. Electrons emerge from the cathode 22, are accelerated by an electrical voltage in the direction of the anode 20 and reenter the electrode material 30 of the anode 20, as a result of which the electrical circuit is closed.

In a comparative study, different anode materials (tungsten with about 2 percent by weight ThO ₂ and tungsten with 0.5 percent by weight nickel) were tested. The following operating parameters were used for an atmospheric plasma spraying system (APS): Voltage: 30 - 80V

Current: 400 or 600A Type of current: DC

Gas type: 47 standard liter quantity (SLM) argon or

47 SLM Argon with 1 2 SLM hydrogen Cooling medium: water Compared to the conventional electrode material as anode with about 2 percent by weight ThO ₂ , the electrode material 30 according to the invention as the anode showed an improved service life behavior, in particular a smaller change in the surface morphology in the piasma passage channel 50.

The invention relates to an electrode, in particular anode, made of a high-melting metal electrode material for plasma spraying systems, in particular plasma torches. According to the invention, the electrode has no or only a small proportion of material additives, especially no or only a small proportion of doping, so that an improvement in the service life properties is achieved. Furthermore, the invention relates to a plasma torch with an electrode according to the invention, connected as an anode, and to a corresponding plasma spraying system.

Reference list

10 plasma torches

20 anode (plasma nozzle)

22 cathode

30 electrode material (sintered tungsten powder)

32 material with high thermal conductivity (copper) 40 cooling medium (water)

50 plasma passage channel

Claims

claims

A plasma torch (10) for a thermal plasma spray system, comprising: a first electrode (20) which can be connected as an anode to an associated current source, and a second electrode (22) which can be connected to the current source as a cathode, - an active electrode section of the first electrode (20) which, after ignition of an arc, interacts directly with the arc between the two electrodes (20, 22), consists of a high-melting metal electrode material (30) and, at the most, an insignificant proportion of material additives in the form of doping with low doping Having an electron work function, an active electrode section of the second electrode (22), which interacts directly with the arc after ignition of the arc between the two electrodes (20, 22), consists of a high-melting metal electrode material and one with regard to reducing the ionization energy on the elec- tron work function has a significant proportion of material additives in the form of doping with low electron work function.

2. Plasma torch (10) according to claim 1, characterized in that the proportion by weight of the doping of the active section of the first electrode (20) is at most 1.0 percent by weight, preferably at most 0.1 percent by weight, most preferably at most 0.01 percent by weight.

3. Plasma torch (10) according to claim 1, characterized in that the active electrode section of the first electrode (20) has no doping.

4. Plasma torch (10) according to any one of the preceding claims, characterized in that the electrode material (30) of the active section of the first electrode (20) and / or the electrode material of the active section of the second electrode (22) using a powder metallurgical method and is made using a metallic powder.

5. Plasma torch (10) according to claim 4, characterized in that the powder metallurgical process comprises a powder preparation process, a pressing process and a sintering process, optionally with subsequent material-removing or non-cutting processing, such as turning, drilling, eroding or flow pressing.

6. Plasma torch (10) according to claim 4 or 5, characterized in that at least one material additive in the form of a sinter-activating material is added to the metallic powder.

7. Plasma torch (10) according to claim 6, characterized in that at least one element of sub-group VIII of the periodic table of the elements is provided as the sinter activating material.

8. Plasma torch (10) according to claim 6 or 7, characterized in that the proportion by weight of the sinter-activating material is up to 1 percent by weight.

9. plasma torch (10) according to any one of claims 6 to 8, characterized in that the sinter-activating material comprises nickel, palladium and / or platinum.

10. Plasma torch (10) according to one of the preceding claims, characterized in that the electrode material

Tungsten and / or molybdenum comprises.

1 1. Plasma torch according to one of the preceding claims, characterized in that the first electrode (20) is designed as a plasma nozzle with a plasma passage (50).

1 2. Plasma torch according to claim 1 1, characterized in that the plasma passage channel (50) is provided in the active electrode section of the first electrode (20).

13. Electrode, in particular anode (20), for thermal plasma spraying systems or plasma torches (10), with an active electrode that directly interacts during operation with an arc. Trode section predominantly or completely of at least one high-melting metallic electrode material (30), characterized in that the active electrode section or the electrode material (30) has no material additives or only a small proportion of material additives, in particular none or only an insignificant one - in particular in Insignificant with regard to the electron work function - proportion of doping with low electron work function or energetically unstable electron configuration, such as elements with incompletely occupied d- or f-electron shells, or / and with electron donor action, for example oxides and borides of the elements of the IIIb to IVb subgroup of the periodic table of the elements and the first three elements from the lanthanoid and actinoid groups, such as ThO ₂ , La ₂ O ₃ ,

CeO ₂ , Y ₂ O ₃ and LaB ₆ .

14. Sintered electrode, in particular sintered anode (30), for thermal plasma spraying systems or plasma burners (10), which is produced by sintering from a high-melting metal powder, in particular tungsten powder, optionally with sinter-activating material additives, characterized in that an in operation with an arc Directly interacting active electrode section (30) or the metal powder, no material additives or only an insignificant - especially insignificant with regard to the electron work function - proportion of material additives in the form of doping with low electron work function or energetically unstable electron configuration, such as elements with incompletely occupied d or f electron shells, and / or with electron donor action, for example

Oxides and borides of the elements of the IIIb to IVb subgroup of the Periodic Table of the Elements as well as the first three elements from the lanthanide and actinide groups, such as

ThO ₂ , La ₂ O ₃ , CeO ₂ , Y ₂ O ₃ and LaB ₆ .

15. Anode (30) with the features of claim 13 or / and 14, for use in a plasma torch according to one of claims 1 to 12.

1 6. Anode according to claim 1 5, characterized in that the anode (30) has a plasma passage channel (50).

1 7. Plasma torch (10) for a thermal plasma spraying system with an anode (20) according to claim 1 5 or 1 6.

1 8. Plasma torch according to claim 1 7, characterized in that an electrode is provided as the cathode (22) according to the preamble of claim 13 or 14, wherein the

Operation with an arc directly interacting active electrode section of this electrode (cathode (22)) or its electrode material or the metal powder to reduce the ionization energy. Material additives in the form of doping with low electron work function or energetically unstable electron configuration, such as elements with incompletely occupied d or f-electrode shells, and / or with electron donor action, for example oxides and borides of the elements of the IIIb to IVb subgroup the periodic table of the elements and the first three

Elements from the lanthanoid and actinoid group, such as

ThO ₂ , La ₂ O ₃ , CeO ₂ , Y ₂ O ₃ and LaB ₆ .

19. Thermal plasma spraying system with an anode (20) or a plasma torch (10) according to one of the preceding claims.