MINIATURISED PLASMA TORCH

Abstract

 A plasma torch (100) creating a plasma jet, being able to treat the walls of a cavity and comprising a cathode (110), an anode (120), an electrical potential floating component (140) and insulated discs (150) located between the other three components, said three components defining a plasma channel (130) having a first zone (133) with a reduced diameter (D1) near the cathode and a second zone (134) with an increased diameter near the anode, a section of the plasma channel between the cathode and the first zone having a conical shape decreasing to the first zone, the plasma torch comprising a specific combination of the reduced diameter, an angle (α1) of the conical face of said section, a distance between the cathode and the floating component and a distance (La) between the cathode and the anode such as the floating component is an ephemeral anode during an electrical arc ignition.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] For spraying material in a molten state onto a substrate surface, plasma spray processes are well established. Such processes are using a plasma torch, i.e. a system for creating a plasma with a plasma jet escaping from a nozzle-like component. Usually, a DC plasma is created between a cathode (e.g. with a negative electrical potential polarity) and an anode (e.g. with a positive potential in respect to the cathode elements) in a plasma torch and is guided through a plasma channel to an outlet nozzle where the powder may be injected, melted and accelerated.

2. Prior Art

[0002] In the majority of the plasma spray systems, which are commercially used in these days, the plasma jet is created by means of a high current arc discharge between a pin-shaped cathode component and a hollow cylinder anode component. Thereby, the coating material which has to be molten and axially accelerated is introduced into the plasma torch from the side in the region of the anode component which at the same time forms the outlet nozzle.

[0003] Significant differences may exist in the design of the plasma torch system, which may affect the efficiency of the coating deposition process and the capability to coat certain component geometry. The output power (e.g. enthalpy) density, the plasma voltage and current fluctuation are the key parameters of the plasma torch.

[0004] Today we can differentiate plasma spray torches with a non-controlled plasma arc (plasma channel) length and torches with a controlled plasma arc (plasma channel) length.

[0005] The design of plasma torch with a non-controlled plasma arc length has been miniaturised to be able to coat internal surface of bores or cavities up to a minimum internal diameter of 50 mm. Unfortunately, this miniaturisation step has forced the engineers to reduce the output power (enthalpy) available to melt the powder. This gives an important limitation in coating deposition efficiency and coating quality.

[0006] Moreover, in the plasma spray systems without electrical arc length stabilization, which are existing today, only one part of the gas is effectively ionized. Due mainly to the strong restrike effect of the electrical arc, this has the consequence that the gas is not continuously and homogeneously ionized meaning that the electrical heat transfer does not happen continuously.

[0007] In such condition, some powder particles are melted but other are less melted. This has an impact on the deposition efficiency and on the coating properties. Additionally, an important part of the energy of the plasma torch is lost by heat transfer from the plasma torch to the walls of the relatively narrow plasma channel.

[0008] A difference with the non-controlled plasma arc length is that the controlled plasma arc length comprises electrical potential floating rings between the cathode and the anode.

[0009] The documents EP0249238A2 and US005225652A describe plasma guns comprising an elongate plasma channel extending from a cathode to an anode. The plasma channel is defined by the interior of a plurality of annular electrical floating potential rings, which are electrically insulated from each other.

[0010] An advantage of such elongated plasma torches is to increase the heat transfer of the electrical power in the plasma gas, to stabilize the plasma arc in its length and in the time developing a higher thermal energy than a short plasma torch. This means that there is much more plasma enthalpy available to melt the powder at the powder injection position.

[0011] Another advantage of such elongated plasma torches is to stabilize the electrical arc length promoting a uniform melting of the powder e.g. more homogeneous coating properties.

[0012] Due to the design and complexity needed to construct such elongated plasma channels, one of their disadvantage is that they are quite massive and up today nobody has achieved to reduce their dimensions while creating a stabilized high-level plasma energy density. This is an important limitation for the high feed rate of coating material on plasma systems to be used to coat the internal surface of a bore or cavities of small diameter as for example motor blocs in the automotive industry.

OBJECTS OF THE INVENTION

[0013] It is the aim of the present invention to provide a plasma torch which can be operated at higher DC voltage and lower voltage and current fluctuations such that the operating life of the parts of the system which are subject to wear is increased.

[0014] It is still the aim of the present invention to provide a plasma spray system in which the material to be sprayed is better and more uniformly processed (melted) to improve the quality of the coating of a substrate.

[0015] It is still the aim of this invention to provide a plasma torch with no need of an additional ignition system between the cathode and a floating electrode ring to create an electrical conductive plasma gas channel. This will allow the very easy integration of the invented plasma system (torch) on already existing systems.

[0016] It is the aim of the invention to provide a plasma torch with optimised output power (e.g. enthalpy) density, plasma voltage and current fluctuations.

[0017] It is the aim of the present invention to provide a compact arc elongated and stabilize plasma spray system (e.g. reduction of plasma arc fluctuations) for spraying powder or gas or liquid or suspension materials, which has an improved efficiency for the coating of internal size of bores of cavities below 200 mm diameter.

[0018] It is still the aim of the present invention to provide a plasma spay system able to deposit material at a high feeding rate for internal surface of bores of cavities below 200 mm diameter.

SUMMARY OF THE INVENTION

[0019] For this purpose, it is proposed, firstly, a plasma torch creating a plasma jet, the plasma torch being able to be inserted into a cavity of an industrial or mechanical assembly to treat the walls of said cavity, the plasma torch comprising a cathode, an anode, at least one electrical potential floating component and insulated discs, the insulated discs being located between the cathode, the anode and the at least one floating component, the cathode, the anode and the at least one floating component defining a plasma channel having a first zone with a reduced diameter near the cathode and a second zone with an increased diameter located between the first zone and the anode, a section of the plasma channel between the cathode and the first zone having an essentially conical shape with decreasing diameter in direction to the first zone, the plasma torch comprising a specific combination of the reduced diameter, an angle defined by the conical face of said section, a distance between the cathode and the at least one floating component the closest of the cathode and a distance between the cathode and the anode such as the at least one floating component the closest of the cathode is an ephemeral anode during the first instants of an electrical arc ignition.

[0020] Various additional features may be provided, alone or in combination:

• the reduced diameter of the first zone is comprised between 3 millimeters and 12 millimeters, the angle defined by the conical face of said section is comprised between 45 degrees and 135 degrees, the distance between the cathode and the at least one floating component the closest of the cathode is comprised between 0.5 millimeters and 9 millimeters, the distance between the cathode and the anode is comprised between 5 millimeters and 30 millimeters.
• the anode comprises a reduced portion compared to a maximum diameter of the second zone of the plasma channel;
• the anode comprises a conical inner surface which has a diameter decreasing from the floating component to a free end of the anode;
• the reduced portion has a diameter equal to the reduced diameter;
• the maximum diameter of the second zone is comprised between 4 millimeters and 14 millimeters;
• the second zone of the plasma channel has an essentially cylindrical shape;
• the second zone of the plasma channel has an essentially conical shape with increasing diameter from the first zone to the anode, an angle defined by the conical face of the second zone being smaller than 90 degrees;
• an outer diameter of the plasma torch is less than 50 millimeters;
• the cavity to be treated has an internal diameter below 200 millimeters.

[0021] It is proposed, secondly, a plasma spray system for spraying material melted by a plasma jet, the plasma spray system comprising a plasma torch as previously described and devices for feeding the material into the plasma jet.

[0022] Various additional features may be provided, alone or in combination:

• the devices for feeding the material comprise a tube having a free end which is perpendicularly or axially aligned with regard to the plasma jet;
• the material feeding into the plasma jet comprises a powder material and/or a gaseous material and/or a liquid and/or a liquid suspension;
• it comprises a tube in which are confined all the cabling and/or conduct, the tube being setting up perpendicularly to the direction of the plasma channel.

[0023] It is proposed, thirdly, a method of coating a cavity of an industrial or mechanical assembly to treat the walls of said cavity comprising insertion of a plasma spray system as previously described inside the cavity and rotating the plasma spray system to coat the internal surfaces of the cavity.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In the following, preferred embodiments of the apparatus according to the invention will be further described, with reference to the accompanying drawings, in which:

• Figure 1 shows a plasma spray system for spraying powder or gaseous or liquid or suspension materials according to one embodiment;
• Figure 2 shows details of the realization and stabilization of the plasma jet according to one embodiment;
• Figure 3 shows an overview of the inside plasma torch with stabilized plasma according to one embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

[0025] The invention provides a plasma spray system for spraying powder or gaseous or liquid or suspension materials in internal surfaces of bores or cavities of a diameter lower than 200 mm.

[0026] It is obvious that with the invented plasma system, bores or cavities of diameter higher than 200mm may be also coated.

[0027] The system of the invention comprises a plasma torch 100 adapted to create an elongated arc Ea with a fixed arc plasma length La and one or more devices 200 for axially or perpendicularly feeding the powder or gaseous or suspension material into the plasma torch 100.

[0028] The electric arc Ea transforms a gas injected in the torch 100 into a plasma jet Pj.

[0029] In one embodiments, the fixed arc plasma length La is comprised between 5 millimeters and 30 millimeters. The fixed arc plasma length La is, for example, equal to 15 millimeters.

[0030] The plasma torch 100 comprises at least one cathode 110 component, and at least an annular or not annular anode 120 component located to a certain distance La from the cathode 110 component and a plasma channel 130 extending from the cathode 110 component(s) to the anode 120 component(s). The plasma channel 130 has a first end 131 close to the cathode 110 component as well as a second end 132 at the anode 120 component. The plasma channel 130 is delimited and defined, respectively, by the anode 120 component as well as by a single or a plurality of annular electrically potential floating ring 140 components, which are electrically insulated from each other by annular insulated discs 150.

[0031] In one embodiment, the floating ring 140 components are made of copper.

[0032] The electrically floating rings 140, which form the plasma channel 130, are separated from each other by the annular insulating discs 150 which are offset with regard to the wall of the plasma channel 130 by a certain amount in order to avoid an excess thermal load of these insulating discs 150.

[0033] Therefore, the wall of the plasma channel 130 is not continuous, but interrupted by gaps between the electrically floating rings 140.

[0034] The plasma channel 130 may have a first zone 133 with a reduced diameter D1 located in that region of the plasma torch 100, which is near to the cathode 110 component, and a second zone 134 with increased diameter located between the first zone 133 with a reduced diameter D1 and the anode 120 component.

[0035] In one embodiment, the reduced diameter D1 of the first zone 133 is comprised between 3 millimeters and 12 millimeters. The reduced diameter D1 is, for example, equal to 6 millimeters.

[0036] The first zone 133 with a reduced diameter D1 has the effect to compress and ionize efficiently the neutral gas therefore to transfer efficiently the ionized gas situated at the beginning of the plasma channel 130.

[0037] The result is that, as far as the gas flow parameters are concerned, the pressure and the temperature of the ionized gas is efficiently increased and that, as far as the electric parameters are concerned, an improved heating transfer of the electrical power to the plasma gas may be achieved at the end 132 of the plasma channel 130 e.g. at the anode 120 output of the plasma torch 100.

[0038] According to one embodiment, the one of the annular electrical potential floating ring 140 which is closest to the cathode 110 member extends at least up to the narrowest part of the first zone 133 having a reduced diameter D1.

[0039] According to one embodiment, the distance between the cathode 110 and the floating ring 140 which is the closest to the cathode 110 is comprised between 0,5 millimeters and 9 millimeters.

[0040] The combination of the specific diameter reduction, the conical angle of the floating annular ring 140 close to the cathode 110, the distance between the cathode 110 and said floating annular ring 140 and the distance between the cathode 110 and the anode 120 enables the creation of an ephemeral anode by said floating ring 140 during the first instants of the electrical arc Ea ignition creating an electrical capacitive effect which creates a preliminary arc between the cathode 110 and said floating annular ring 140 close to the cathode 110. This preliminary arc ionizes the gas in the plasma gun channel 130 which allows to establish the stable main electrical arc Ea between the cathode 110 and the anode 120 of the plasma torch 100.

[0041] This magneto-hydrodynamics configuration of the plasma torch 100 avoids to use an external ignition system coupled between the floating annular ring 140 close to the cathode 110 and the cathode 110 itself. Such external ignition system would require an additional integration in the plasma torch 100 of an ignition electrode with cabling increasing so the complexity and the dimension of the plasma torch 100. This physical process is the key for a simple plasma ignition process and allows to integrate the invented plasma torch 100 in any standard existing system.

[0042] As shown in figure 1, the section between the first end 131 of the plasma channel 130 and the first zone 133 is, in one embodiment, conical

[0043] In one embodiment, a first angle α1 of the conical first zone 133 is comprised between 45 degrees and 135 degrees. The first angle α1 is, for example, equal to 90 degrees.

[0044] In one embodiment, the maximum diameter D2 of the second zone 134 is comprised between 4 millimeters and 14 millimeters. The maximum diameter D2 is, for example, equal to 8 millimeters.

[0045] According to different embodiments, the second zone 134 of the plasma channel 130 extending from the first zone 133 to the anode 120 member has an essentially cylindrical shape or an essentially conical shape with increasing diameter from the first zone 133 to the anode 120 component.

[0046] As shown in figure 1, a second angle α2 of the conical second zone 134 is, in one embodiment, smaller than 90 degrees. The second angle α2 is, for example, equal to 53 degrees.

[0047] Since the invention provides that the plasma channel 130 has an increased or increasing diameter from the zone 133 of the cathode 110 with reduced diameter D1 towards the anode 120, the heat losses is reduced which means that there is more heat energy transferred in the plasma gas than transferred in the cooling system of the plasma torch 100 through the copper walls of the plasma channel 130 defined by the floating rings 140. This has as consequences that with less electrical power, we can better transfer the heat to the particles to be melted and the thermal load on the substrate due to the reduced quantity of electrical power used is better controlled.

[0048] According to one embodiment, the annular anode 120 comprises a reduced portion 122 compared to the second zone 134 of the plasma channel 130.

[0049] According to one embodiment, the reduced portion 122 has a diameter equal to the reduced diameter D1 of the first zone 133 of the plasma channel 130.

[0050] According to one embodiment, the reduced portion 122 has a different diameter than the reduced diameter D1 of the first zone 133 of the plasma channel 130.

[0051] According to one embodiment, the annular anode 120 member has a conical inner surface 123 which has a diameter decreasing from the one of the annular electrical floating potential rings 140 components which is closest to the anode 120 member to the reduced portion 122 closed to a free end 121 of the anode 120 member.

[0052] Said reduced portion 122 of the annular anode 120 increases speed of the plasma jet and its pressure. The increase of the pressure of the plasma jet improve heat transfer between the particle and the plasma gas. The increase of the speed of the plasma jet increase the kinetic energy transferred to the melted particles which improves the compaction of the particle on the substrate which increases the coating adhesion and cohesion. This feature ensures better energy transfer between the plasma spray and the powder and a better compaction of the melted particles at the substrate.

[0053] In an embodiment, if the powder or gaseous or liquid or suspension material is axially fed into the plasma torch 100, the feeding is located close to the first end 131 of the plasma channel 130 or may be integrated to the cathode 110 component. In another embodiment, if the powder or gaseous or liquid or suspension material is perpendicularly fed into the plasma torch 100, the feeding is located at the exit of the second end 132 of the plasma channel 130.

[0054] The axially feeding may occurs through a hole of the cathode 110 and the perpendicular feeding at the end 132 of plasma channel 130 created by the anode 120.

[0055] The devices 200 for perpendicular feeding the powder or gaseous material or liquid or liquid suspension into the plasma torch 100 comprise a tube 210 member having a free end 211 and which is perpendicularly aligned with regard to said plasma channel 130.

[0056] In one embodiment, said free end 211 of the tube 210 defines an angle with the plasma channel 130.

[0057] Figure 1 shows a longitudinal sectional view of the plasma spray torch 100 (apparatus) having a single cathode 110 and a single anode 120 component. The main body of the plasma torch 100 is composed of a high electrical insulating material which may be a high temperature polymer material like polyether ether ketone (PEEK), polytetrafluoroethylene (PTFE) etc. or a tough ceramic material such a mix of aluminum oxide (AI2O3) and zirconium dioxide (ZrO2), Boron nitride (BN), silicon nitride (Si3N4), etc...

[0058] In this main body is integrated a water-cooling network 300 which allow to cool down efficiently all the components of the plasma torch 100. This cooling network is essential to guarantee a long-life time of the plasma torch 100.

[0059] In Figure 2 we observe the plasma channel 130 with a defined length La between the cathode 110 component and the anode 120 component and which is build up by two floating rings 140. The plasma channel 130 is responsible for the optimal heat transfer between the electrical arc Ea and the plasma gas.

[0060] In one embodiment, the floating ring 140 the closest to the cathode 110 has a length Lr1 between 2 millimeters and 10 millimeters. The length Lr1 of this floating ring 140 is, for example, equal to 5 millimeters.

[0061] In one embodiment, the floating ring 140 the closest to the anode 120 has a length Lr2 between 1.5 millimeters and 8 millimeters. The length Lr2 of this floating ring 140 is, for example, equal to 4 millimeters.

[0062] The electrical arc Ea is created by a DC electrical potential difference between the cathode 110 and the anode 120.

[0063] The cathode 110, each floating ring 140 and the anode 120 are electrically insulated each other by the insulated discs 150 made of high temperature material such as ceramic (bore nitride, silicon nitride, etc...) and high temperature polymers.

[0064] The electrical power supply, the gas supply and the material feed supply and the water-cooling supply 300 to the head of the plasma torch 100 is set up perpendicularly to the direction of the plasma jet Pj which allow to confine all the cabling and/or conduct in a tube 400 of a diameter D0 below 50 mm (see Figure 3).

[0065] In this tube 400 we may have also additional tubing 500 for compressed air or inert or reactive gas to allow:

1) to cool down the substrate and the plasma torch 100 during the coating process and/or

2) to produce an inert gas protection to minimize the material reaction with the surrounding atmosphere and/or

3) to produce a coating material transformation with the interaction between the coating material and the reactive gas surrounding the plasma jet Pj.

[0066] According to one embodiment, an outer diameter D0 of the plasma torch 100 is less than 50 millimeters.

[0067] Due to the specific design of the electrical floating rings 140, the plasma torch 100 do not require an additional specific ignition system to switch on the plasma in the plasma torch 100.

[0068] Compared to plasma torches of the state of the art with approximately the same dimensions, the plasma torch 100 according to the invention is able to produce lower current fluctuations (around 5V against around 10V for the state of the art, the other parameters being identical) and a higher output power (around 15kW to 18kW against around 10kW to 12.5kW for the state of the art, the other parameters being identical). With the state of the art of plasma torches with approximately the same dimension as the plasma torch 100, a maximum output power of 16kW can be reached, with the new invented plasma torch 100 an output power up to 30kW can be reached.

[0069] The plasma torch 100 according to the invention is able to produce an output power until 35kW with a current until 600A.

Claims

1. A plasma torch (100) creating a plasma jet, the plasma torch (100) being able to be inserted into a cavity of an industrial or mechanical assembly to treat the walls of said cavity, the plasma torch (100) comprising a cathode (110), an anode (120), at least one electrical potential floating component (140) and insulated discs (150), the insulated discs (150) being located between the cathode (110), the anode (120) and the at least one floating component (140), the cathode (110), the anode (120) and the at least one floating component (140) defining a plasma channel (130) having a first zone (133) with a reduced diameter (D1) near the cathode (110) and a second zone (134) with an increased diameter located between the first zone (133) and the anode (120), a section of the plasma channel (130) between the cathode (110) and the first zone (133) having an essentially conical shape with decreasing diameter in direction to the first zone (133), the plasma torch (100) being characterized in that it comprises a specific combination of the reduced diameter (D1), an angle (α1) defined by the conical face of said section, a distance between the cathode (110) and the at least one floating component (140) the closest of the cathode (110) and a distance (La) between the cathode (110) and the anode (120) such as the at least one floating component (140) the closest of the cathode (110) is an ephemeral anode during the first instants of an electrical arc (Ea) ignition.

2. A plasma torch (100) according to the preceding claim in which the reduced diameter (D1) of the first zone (133) is comprised between 3 millimeters and 12 millimeters, the angle (α1) defined by the conical face of said section is comprised between 45 degrees and 135 degrees, the distance between the cathode (110) and the at least one floating component (140) the closest of the cathode (110) is comprised between 0.5 millimeters and 9 millimeters, the distance (La) between the cathode (110) and the anode (120) is comprised between 5 millimeters and 30 millimeters.

3. A plasma torch (100) according to one of the preceding claims in which the anode (120) comprises a reduced portion (122) compared to a maximum diameter (D2) of the second zone (134) of the plasma channel (130).

4. A plasma torch (100) according to one of the preceding claims in which the anode (120) comprises a conical inner surface (123) which has a diameter decreasing from the floating component (140) to a free end (121) of the anode (120).

5. A plasma torch (100) according to one of the preceding claims in which the reduced portion (122) has a diameter equal to the reduced diameter (D1).

6. A plasma torch (100) according to one of the preceding claims in which the maximum diameter (D2) of the second zone (134) is comprised between 4 millimeters and 14 millimeters.

7. A plasma torch (100) according to one of the preceding claims in which the second zone (134) of the plasma channel (130) has an essentially cylindrical shape.

8. A plasma torch (100) according to any of claims 1 to 6 in which the second zone (134) of the plasma channel (130) has an essentially conical shape with increasing diameter from the first zone (133) to the anode (120), an angle (α2) defined by the conical face of the second zone (134) being smaller than 90 degrees.

9. A plasma torch (100) according to one of the preceding claims in which an outer diameter (D0) of the plasma torch (100) is less than 50 millimeters.

10. A plasma torch (100) according to one of the preceding claims in which the cavity to be treated has an internal diameter below 200 millimeters.

11. A plasma spray system for spraying material melted by a plasma jet, the plasma spray system comprising a plasma torch (100) according to one of the preceding claims and devices (200) for feeding the material into the plasma jet.

12. A plasma spray system according to the preceding claim in which the devices (200) for feeding the material comprise a tube (210) having a free end (211) which is perpendicularly or axially aligned with regard to the plasma jet.

13. A plasma spray system according to any of claims 11 or 12 in which the material feeding into the plasma jet comprises a powder material and/or a gaseous material and/or a liquid and/or a liquid suspension.

14. A plasma spray system according to any of claims 11 to 13 where in it comprises a tube (400) in which are confined all the cabling and/or conduct, the tube (400) being setting up perpendicularly to the direction of the plasma channel (130).

15. A method of coating a cavity of an industrial or mechanical assembly to treat the walls of said cavity comprising insertion of a plasma spray system according to any of claims 11 to 14 inside the cavity and rotating the plasma spray system to coat the internal surfaces of the cavity.

Drawing