Method and device for coating powder- or granular-form particles

Abstract

The invention relates to a method and a device for coating powder- or granular-form particles by means of gas flow sputtering with a hollow cathode functioning as the target and arranged in an evacuable container together with an anode and a collection container. In order to work efficiently and with easier handling, according to the invention, after a metered introduction into an inlet opening, the powder- or granular-form particles to be coated pass through the evacuated interior of the hollow cathode in a free fall of 0.3 m to 1 m and are collected in a lower collection container.

Description

[0001] Method and device for coating powder or granular particles

[0002] The invention relates to a method and a device for coating powdered or granular particles by means of gas flow sputtering with a hollow cathode serving as a target, which is arranged in an evacuable container together with an anode and a collecting container.

[0003] In gas flow sputtering, the material to be sputtered is placed in a vacuum in the form of a "target." A noble gas plasma, usually argon, is ignited in the process vessel, with the target serving as the anode of the discharge. Through the intensive bombardment of the target surface with noble gas ions, the target material is sputtered and condenses on the substrates located in the chamber. Sputtering can be carried out at relatively low temperatures. One such process is described, for example, in DE 196 35 669 C1. For this purpose, a device is used that has a hollow cathode serving as the target. Inert gas is introduced into the hollow cathode interior via an inflow device, generating the desired hollow cathode glow discharge. If necessary, an additional quantity of charge carriers can be introduced using an additional hollow cathode.With the inert gas flow, the particles sputtered from the cathode pass through a housing opening onto the substrate arranged behind the opening, on which the particles released from the target are deposited and form a coating.

[0004] DE 198 04 838 A1 describes a process in which particulate materials such as metals, metal alloys, inorganic oxides, silicates, zeolites are provided with a metal coating, e.g. of cobalt, vanadium, copper or stainless steel. For this purpose, a hollow cathode plasma source is to be used consisting of a cathode material with a cavity arranged therein and an anode material arranged in the immediate vicinity and insulated therefrom, wherein the cavity is a single cavity or a plurality of cavities and the cavity, both individually and in the plurality, runs continuously from one side to another of the cathode material, is open on both sides and at least partially has a substantially cylindrical cross-section with a diameter of the individual cavity of 0.1 mm to 20 mm.In order to ensure sufficient coating of the powdered material in the plasma in the cathode cavity, the material to be coated should be passed through the plasma several times.

[0005] Based on this, it is the object of the present invention to provide a method and a device that works more efficiently and is easy to handle.

[0006] This object is achieved by a method according to claim 1 or by the device according to claim 6.

[0007] According to the invention, the powdered or granular particles to be coated are introduced into an inlet opening in a controlled manner into the evacuated interior of the hollow cathode, where they fall freely over a distance of 0.3 m to 1 m and are collected in a lower collecting container. For this purpose, the evacuable container has a hollow cathode serving as a target, an anode, and a collecting container. The evacuable container for coating the material has an inlet opening in the upper area for the powdered or granular particles and for a gas or gas mixture. This inlet opening opens into the hollow cathode with a vertical length of 0.5 m to 1 m and is arranged below the collecting container(s) for the coated particles. Due to the vacuum set at a pressure of a few Pa, the powder introduced into the cathode interior experiences a gravitational acceleration of 9.81 m / s. 20.3 s at a drop height of 0.5 m and 0.45 s at a drop height of 1 m. This means that if the drop height is increased fivefold, the coating times do not increase significantly. Overall, high deposition rates can therefore be achieved with a single pass through a hollow cathode. Further preferred measures arise from the subclaims. For example, a pulsed plasma is preferably used, which can increase process stability. According to a further embodiment of the invention, the particles to be coated are introduced together with a gas or a gas mixture, wherein preferably the particles are introduced via a centrally arranged tube and the gas or gas mixture is introduced via an annular space which extends around the tube for the particle supply.This measure can be used to create turbulent behavior of the powder or granular particles, so that a uniform coating is guaranteed due to the resulting swirling of these particles.

[0008] The diameter of the hollow cathode is tapered at the lower outlet, preventing uncontrolled discharge of the sputtering material and stabilizing the cathode. The coating material that does not deposit on the powder or granular particles is redeposited on the opposite side of the hollow cathode, ensuring a steady state in the event of a lack of or insufficient particle flow. The tapered interior of the hollow cathode at the lower outlet also prevents uncontrolled discharge of the sputtering material and prevents particle deposits inside the cathode.

[0009] According to a further embodiment of the invention, the backflow of the coated powder is prevented by a net that extends from the collecting container into an area above the outlet of the hollow cathode.

[0010] The device according to the invention preferably has a hollow cathode inner diameter of 10 mm to 30 mm and an outlet opening of the hollow cathode with a diameter of 4 mm to 6 mm, preferably 5 mm. The diameter taper of the cathode interior is realized in a preferably detachably attached cover with a conical interior. The detachably attached cover, which can be screwed on, for example, can thus be easily replaced or cleaned. As already mentioned, a mesh with a suitable mesh size is preferably used to prevent backflow of the coated powder.

[0011] The anode and the hollow cathode are water-cooled. The hollow cathode has a (target) insert, which can be made of precious metal, for example. The anode extends into the area below the hollow cathode and has holes to allow the passage of coated particles. These openings are preferably 30 mm to 40 mm in diameter.

[0012] According to a further embodiment of the invention, a metering device for the particle feed is provided above the inlet opening for the powdered or granular particles, in which metering device, in particular, a vibrator and an adjustable valve are arranged. This makes it possible to both avoid material build-up and ensure a uniform particle feed. In a special embodiment of the invention, the particle inlet, which projects into the hollow cathode interior, consists of a tube with a diameter of 0.8 mm to 1.2 mm (preferably 1 mm), which is arranged centrally to an annular space designed as a gas inlet and has a diameter of between 3 mm and 5 mm. This dimensioning has the advantage that the formation of a "burning plasma" in the inlet area is avoided and thus there is no possibility of melting of the introduced particles.

[0013] The efficiency of the device is optimized by arranging several hollow cathodes in parallel in a cathode array, surrounded by a common (ring) anode. Each of these hollow cathodes has an inlet for the particles and the gas or gas mixture. In this way, multiple hollow cathodes can carry out coatings. A collecting container is located beneath each hollow cathode. To ensure that the device can be sufficiently evacuated before the coating process, a rotary vane pump or a combination of a rotary vane pump and a Roots pump is preferably used. Only after the required negative pressure (vacuum) has been reached is the particle and gas supply released via the existing valve(s). The vacuum chamber and the collecting containers are potential-free. The selected power consumption per hollow cathode is approximately 2 kW to 8 kW, preferably 3 kW to 6 kW.

[0014] In specific embodiments, for example, an Al2O3 powder was coated with Cu. The Al2O3 powder had a grain size corresponding to a mesh size of 500. SiC, WC-graphite, or TisOs powders could be coated just as well.

[0015] The single figure shows a schematic structure of a device consisting of an evacuable container 10 with two water-cooled hollow cathodes 11 and a water-cooled anode 12, which extends into the area below the hollow cathodes 11 and has bores 121 through which the coated powder or granular material can reach one of the two collecting containers 13 shown.

[0016] The mesh 14, also shown, encloses the collecting containers 13 all around and extends upwards into an area above the outlet openings 121 of the hollow cathode interiors. The powder is supplied via tubular inlets 15, which are surrounded by an annular space 16 arranged concentrically thereto for the gas supply. The entire container 10 can be evacuated via a pump, preferably a combination of a rotary vane and Roots pump, which is connected to the outlet 101. The pipes 15 for supplying the powder particles have both an adjustable valve 151 and a vibrator 152, which causes the filling funnel to vibrate, thus preventing clumping during the powder supply.

[0017] The hollow cathode has a cover 111 in its lower area, which is attached to the rest of the hollow cathode in a removable manner. This cover has a tapered interior 112, which reduces the hollow cathode's inner diameter from a width of 1.5 cm to 3 cm to approximately 5 mm in diameter.

Claims

Claims 1. Method for coating powder- or granular particles by means of gas flow sputtering with a hollow cathode (11) serving as a target, characterized in that the powder- or granular particles to be coated, after metered introduction into an inlet opening (15), pass through the evacuated interior of the hollow cathode (11) in free fall for 0.3 m to 1 m and are collected in a lower collection container (13).

2. Method according to claim 1, characterized in that a pulsed plasma is set in the interior of the hollow cathode (11).

3. Method according to claim 1 or 2, characterized in that the particles to be coated are introduced together with a gas or a gas mixture, wherein preferably the particles are introduced into the hollow cathode interior via a centrally arranged tube (15) and the gas or gas mixture via an annular space (16) extending around the tube (15).

4. Method according to one of claims 1 to 3, characterized in that the diameter of the hollow cathode (11) tapers at the lower outlet.

5. Method according to one of claims 1 to 4, characterized in that the backflow of the coated powder is prevented by a mesh (14) which extends from the collection container (13) to an area above the outlet of the hollow cathode (11). Device for coating powder- or granular particles by means of gas flow sputtering with an evacuable container in which a hollow cathode (11) serving as a target and an anode (12) as well as collection containers (13) are arranged, characterized in that the container has an inlet opening (15) in the upper region for powder- or granular particles as well as for a gas or a gas mixture, which opens into the hollow cathode (11) with a vertical length of 0.3 m to 1 m, below which the collection containers (13) for the coated particles are arranged. Device according to claim 6, characterized in that the inner diameter of the cathode (15) is between 10 mm and 30 mm and the outlet opening has a diameter of 4 mm to 6 mm, preferably 5 mm, wherein the diameter reduction of the cathode interior (11 ) is realized in a preferably detachably fastened cover (111 ) with a conical interior (112).Device according to claim 6 or 7, characterized in that a mesh (14) is arranged in the container (10), extending from the collection container (13) to an area above the outlet of the hollow cathode (11) and serving to retain the coated powder. Device according to any one of claims 6 to 8, characterized in that the anode (12) is water-cooled and extends to the area below the hollow cathode (11), where the anode has bores (121) for particle passage. Device according to any one of claims 6 to 9, characterized in that the inner surface of the hollow cathode (11), serving as a target, is replaceable and preferably made of precious metal. Device according to any one of claims 6 to 10, characterized in that a metering device for particle supply is provided above the inlet opening (15), in which preferably a vibrator (152) and an adjustable valve (151) are arranged. Device according to any one of claims 6 to 11, characterized in that a tube (15) serving as a particle inlet, with an inner diameter of 0.8 mm to 1.2 mm, is provided and is arranged centrally to an interior space designed as a gas inlet with an inner diameter between 3 mm and 5 mm. Device according to any one of claims 6 to 12, characterized in that several hollow cathodes are arranged in parallel in a cathode array, which are surrounded by a common (ring) anode. Device according to any one of claims 6 to 13, characterized by a power input of 2 kW to 8 kW, preferably 3 kW to 6 kW.Device according to one of claims 6 to 14, characterized in that a rotary vane pump or a combination of a rotary vane pump and a rotary piston pump is used for evacuation.

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