Patent Application
20140010967
This application claims priority from German Patent Application No. 10 2012 106 078.2, filed on Jul. 6, 2012, which application is incorporated herein by reference in its entirety.
The invention relates generally to a plasma coating device, and more specifically to a plasma coating device for coating of a substrate.
Furthermore, the invention generally relates to a method for plasma coating of a substrate.
The state of the art comprises a diverse series of sample coating methods and devices. The methods may be selected according to the substrate, the layer to be coated thereon and in particular according to the layer thickness. Amongst suitable coating methods are, according to the initial state of the coating material, gas-phase deposition, dip coating, spraying methods, electroplating or powder coating. Some methods, amongst which in particular thermal spraying methods, plasma spraying, low-temperature plasma coating or laser-assisted powder deposition, are especially suitable for making a homogeneous coating on a substrate. All of these methods require a continuous and well-dosed amount of powder supply to a so-called coating torch.
To obtain coating layers with specific properties while inducing little thermal and mechanical stress into the substrate to be coated, especially fine-grained powders are used which have particle sizes of 20 μm and below. During the coating of the substrate, the powder is injected into the coating torch. The conveyance of such fine-grained powders is, however, challenging. Furthermore, fine-grained powders tend to form agglomerates that a processing gas stream may not be able to break up. Also, the operation of suitable types of pumps often becomes unstable when handling processing gases with high powder contents. Pumps and conveyance devices are prone to heavy erosion and blockage depending on the powder used. Also, the properties of the powder material itself have a significant impact. Particularly critical are abrasive, low-melting and hygroscopic powders.
U.S. Pat. No. 5,853,815 discloses a method to homogeneously coat a substrate, wherein a plasma stream covers the entire width of the substrate. For this purpose, a powder reservoir is connected via a feeding line directly with a plasma generation unit. A large pressure difference between the coating torch in the plasma generation unit and the environment of the coating torch generates a shock pattern. Thereby, the coating material is dispersed within the plasma stream and the resulting coating jet is widened.
German Patent No. DE 198 26 550 C2 proposes that powder may be extracted from a reservoir by mechanical means and converted, together with a carrier gas, into a powder aerosol. The powder aerosol may subsequently be stored in a container and be submitted to ultrasonic treatment in order to break up any particle agglomerates. This process is, however, very complex and not sufficiently reliable for powders prone to agglomeration.
Other documents of the state of the art, like e.g., DE 102 16 294 A1, DE 10 2005 032 711 A1, US 2007/059436 A1, U.S. Pat. No. 4,109,027 A, DE 43 28 021 A1, EP 0 120 810 A1 or WO 2010/060646 A1 disclose deposition devices or deposition methods that do not involve a plasma and therefore do not inject a deposition material (powder) into a plasma coating torch. Likewise, the problem remains unaddressed of how a constant extraction rate of deposition material is achieved.
The present invention broadly comprises a plasma coating device having a particle reservoir, a dosing device for dosing the particles stored in the particle reservoir, an all-round closed processing chamber, at least one transport line from the particle reservoir to the processing chamber, a suction pump being connected with the processing chamber such that a pressure gradient between the processing chamber and the particle reservoir is setable in such a way that there is a processing chamber pressure in the processing chamber and a particle reservoir pressure in the particle reservoir wherein the processing chamber pressure is lower than the particle reservoir pressure, and a plasma coating torch within the processing chamber that receives at a constant conveying rate the particles from the particle reservoir wherein the constant conveying rate is accurately adjustable by means of the transport line and mechanically or pneumatically generated particle aggregations within the transport line are avoided.
The present invention also broadly comprises a method for plasma coating of a substrate having the steps of: extracting in a regulated manner particles from a particle reservoir with a dosing device by means of a movable suction lance; supplying the particles via a transport line to a processing chamber, in which a plasma coating torch is provided; and setting a pressure gradient between the processing chamber and the particle reservoir by means of a suction pump connected to the processing chamber such that the pressure gradient determines a delivery rate of particles.
The object of the present invention is to provide a plasma coating device and a method for plasma coating of a substrate capable of conveying a powder or a mixture of powder and processing gas from a powder reservoir into a processing chamber ensuring a homogeneous, precisely dosed, pulsation-free and time stable powder conveyance between the powder reservoir and the processing chamber.
Accordingly, a plasma coating device for deposition of a coating onto a substrate is disclosed. The coating is produced from particles contained in a particle reservoir. A dosed quantity of particles is extracted from the particle reservoir by a dosing device and subsequently fed to an all-round closed processing chamber via a transport line. Pressure P1 within the processing chamber is lower than pressure P2 within the particle reservoir. The pressure gradient between processing chamber pressure P1 and particle reservoir pressure P2 is achievable and adjustable by means of a suction pump connected to the processing chamber. The powder particles in the powder reservoir are fed to a plasma coating torch inside the processing chamber via the transport line. The transport is precisely adjustable so as to avoid aggregation of mechanically or pneumatically-related particles in the transport line.
In an embodiment of the invention, the particle reservoir of the plasma coating device is arranged within a dosing chamber, and the dosing device is connected with the all-round closed processing chamber via the transport line. Due to their different purposes, the processing chamber and the dosing chamber are functionally separated.
Processing pressure P1 may be adjusted and adapted by a suction pump connected to the processing chamber via a suction line. Furthermore, a throttle valve can be provided in order to adapt the suction gas stream acting upon the processing chamber. For this purpose, this throttle valve is arranged between the suction line and the suction pump.
In another embodiment of the invention, a filter is provided for filtering the suction gas stream siphoned off from the processing chamber by the suction pump. This filter is arranged between the suction line and the suction pump and preferably downstream of the throttle valve. In the context of the present invention, the notion “suction gas stream” is to be understood as the stream composed of the mixture of particles and gas siphoned off from the processing chamber.
The pressure difference between particle reservoir pressure P2 and processing pressure P1 is adjusted between 50 mbar and 1000 mbar, preferably at 200 mbar. For this purpose, the absolute particle reservoir pressure P2 is adjusted between 900 mbar and 1500 mbar. The absolute processing pressure P1 in the processing chamber is adjusted to at most 1013 mbar, preferably below 500 mbar and more preferably still at 30 mbar.
The dosing device has a movable suction lance protruding into the particle reservoir. This setup is particularly advantageous if the deposition device is implemented as plasma coating device with a plasma coating torch inside a processing chamber.
With respect to the inventive method for coating a substrate, a dosed quantity of particles is extracted from a particle reservoir by means of a dosing device. Subsequently, these particles are conveyed via a transport line from which they are fed into the processing chamber. A pressure gradient is set between the particle reservoir and the processing chamber in order to determine a feed rate of particles into the processing chamber.
In an embodiment of the inventive method, the pressure gradient is generated by aspiration of the processing chamber. The value of the pressure gradient may be set by adjusting the aspiration power.
The setup of the proposed plasma coating device has the particular advantage that no pumping unit is required in the powder transport line between the powder reservoir and the processing chamber. Furthermore, one can get around of mechanical or pneumatic units in the transport path.
The nature and mode of operation of the present invention will now be more fully described in the following detailed description of the invention taken with the accompanying figures, in which:
At the outset, it should be appreciated that like drawing numbers on different drawing views identify identical, or functionally similar, structural elements of the invention.
While the present invention is described with respect to what is presently considered to be the preferred aspects, it is to be understood that the invention as claimed is not limited to the disclosed aspect. The present invention is intended to include various modifications and equivalent arrangements within the spirit and scope of the appended claims.
Furthermore, it is understood that this invention is not limited to the particular methodology, materials and modifications described and as such may, of course, vary. It is also understood that the terminology used herein is for the purpose of describing particular aspects only, and is not intended to limit the scope of the present invention, which is limited only by the appended claims.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this invention belongs. In the context of the present invention, the notion “suction gas stream” is to be understood as the stream composed of the mixture of particles and gas siphoned off from the processing chamber. Although any methods, devices or materials similar or equivalent to those described herein can be used in the practice or testing of the invention, the preferred methods, devices, and materials are now described.
According to the invention, the conveyance of particles 15 from dosing device 16 to processing chamber 20 is achieved by the generation of a suitable pressure gradient ΔP between dosing chamber 11 and processing chamber 20. Dosing chamber 11 and processing chamber 20 are functionally independent of each other. For this purpose, processing chamber pressure P1 is set within processing chamber 20. Also, dosing chamber pressure P2 within particle reservoir 14 is set to be higher than processing chamber pressure P1. The value of pressure gradient ΔP can be adjusted, thus enabling the powder 15 feed rate generated by pressure gradient ΔP to be accurately set. In this way, it is possible to avoid the formation of particle agglomerates caused by mechanical or pneumatic units or means in the conveyance path of particles 15 towards substrate 12. Avoiding the formation of such particle agglomerates allows a homogeneous, precisely dosed, pulsation-free and reliable powder conveyance between powder reservoir 14 and processing chamber 20.
In order to adjust pressure gradient ΔP between processing chamber 20 and particle reservoir 14, processing chamber 20 is equipped with suction line 24. Suction line 24 is connected to suction pump 30 generating processing pressure P1 within processing chamber 20 which is lower than particle reservoir pressure P2 within dosing chamber 11. The suction pump is controlled in order to maintain pressure gradient ΔP between processing chamber 20 and particle reservoir 14 within predetermined deviation limits at a given target value.
Processing pressure P1 generated by suction pump 30 takes absolute pressure values of 1013 mbar at most and preferably ranges below 50 mbar. Processing pressures P1 at 30 mbar are especially preferable, particularly because, in this value range, excess material accumulating in processing chamber 20, like e.g., loose powder 15 not adhering to coating 13, and also processing gas can be sucked out with suction stream 3 forming from processing chamber 20 towards suction pump 30. Particle reservoir pressure P2 preferably ranges from absolute values between 900 mbar and 1500 mbar.
By means of a suitable combination of an aspiration of processing chamber 20 and the dosing and fluidizing of the particles, pressure gradient ΔP between the two functionally separated chambers (processing chamber 20 and dosing chamber 11) can thus be achieved and the feed rate of particles 15 can be precisely set. The supply of the coating process with the coating material, e.g. the powder, is governed by pressure gradient ΔP and can thus be regulated by setting pressure gradient ΔP. In this way, the dosing and fluidizing process of powder particles 15 in a processing gas can be controlled. Pressure gradient ΔP is set such that the pressure difference between particle reservoir pressure P2 and processing pressure P1 ranges between 50 mbar and 1000 mbar, preferably around 200 mbar.
Throttle valve 26 is arranged downstream of suction line 24 and can provide an additional means of control and regulation of the pressure difference. By means of throttle valve 26, the suction power generated by suction pump 30 and acting upon processing chamber 20 can be adjusted without the suction power of suction pump 30 having to be changed itself. This fact allows a fast setting of the effective suction power acting upon processing chamber 20 and thus constituting an efficient means of regulation of plasma coating device 10. The control of additional components, such as suitable sensors (not shown), can optionally be automated by a control unit (not shown). Additionally, filter 28 may be mounted upstream of suction pump 30 to avoid undesired contamination of suction pump 30 with particles 15. The extracted gases can be exhausted from suction pump 30 via outlet 32. Suction pump 30 and throttle valve 26 are coordinately controlled in order to maintain pressure gradient ΔP between processing chamber 20 and particle reservoir 14 on the set target level within predetermined limits.
The invention has been described with reference to preferred embodiments. On the basis of the disclosure it is obvious to one skilled in the art that changes, modifications of the invention and in particular combinations of features disclosed in the context of different embodiments are contained in the protective scope of the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102012106078.9 | Jul 2012 | DE | national |
