Modular cascaded thermal spray plasma gun comprising a configurable nozzle for shaping and / or deflecting a plasma jet

Abstract

The present invention relates to a modular cascaded thermal spray plasma gun comprising one or more cathodes, a modular neutrode stack, an anode, a gun body, an interface to a configurable nozzle and a configurable nozzle. The interface is coaxially aligned with the anode and comprises feedthroughs for cooling water. A configurable nozzle for a modular cascaded thermal spray plasma gun comprises cooling water inlet and outlets matching in form and location to corresponding feedthroughs of the interface. The configurable nozzle comprises an annular recess for coaxial alignment with an axial protrusion of an anode and a central plasma channel further comprising a transition zone for shaping the cross section of a plasma jet, a deflection zone for deflection the plasma jet and a plasma jet outlet.

Description

[0001] Modular Cascaded Thermal Spray Plasma Gun Comprising a Configurable Nozzle for Shaping and / or Deflecting a Plasma Jet

[0002] Cross-Reference to Related Application

[0003] This International Application claims the benefit of priority of U.S. Provisional Application No. 63 / 710,312 filed October 22, 2024, the disclosure of which is expressly incorporated by reference herein in its entirety.

[0004] Field of the invention

[0005] The present invention relates to cascaded thermal spray plasma guns in general and more particularly to modular cascaded thermal spray guns that may be used for coating internal surfaces of parts or surfaces with a confined space in between them or complex geometries. The invention also relates to a configurable nozzle mounted at an upstream end of an anode of a modular cascaded thermal spray gun, with the nozzle being configurable for deflecting a plasma jet in longitudinal direction and for shaping the cross section of a plasma jet.

[0006] Background

[0007] Thermal spray methods, such as plasma spraying, are coating methods using a heat source to melt a coating material and deposit the molten material on a substrate using a process gas. The coating material droplets molten and accelerated towards the substrate solidify rapidly on the surface of the substrate or on a previously applied coating layer in the form of splats that overlap and build up the coating layer. Thermal spray plasma processes use an arc between an anode and a cathode to ionize a process gas to generate a plasma jet which serves as a heat source for melting coating material in the form of powder particles. The electrical power supplied to non-cascaded thermal spray plasma guns, mainly for coating internal surfaces, is currently limited to 30 kW, the net power available to melt the coating material for this type of thermal spray plasma guns ranges from 7 to 18 kW. Cascade-type thermal spray plasma guns allow higher operating voltages and the creation of stabilized plasma arcs, showing less fluctuations in the gun voltage than for non-cascaded plasma guns. The electrical power supplied to cascaded thermal spray plasma guns is up to 160 kW, resulting in up to 100 kW of power available for melting coating material. In cascaded thermal spray plasma guns, the anode is separated in axial direction from the cathode by one or more electrically insulated neutrodes forming a neutrode stack in axial direction. In non-cascaded thermal spray plasma guns the length of the arc may vary due the attachment point of the arc on the anode moves in axial direction within the anode over time. In cascaded thermal spray plasma guns the arc length is defined and constrained by the length of the neutrode stack. For cascaded thermal spray plasma guns, the neutrode stack inserted in between the cathode and the anode extends the length of the plasma arc. The longer arc path in cascaded thermal spray plasma guns allows to increase operating power, even without using secondary gases such as He, H2 or N2 and to significantly reduce voltage instabilities. The increased power level of cascaded thermal spray plasma guns compared to non-cascaded thermal spray guns allows for higher material deposition rates and thus for reduced coating process times. An additional benefit of using cascaded thermal spray plasma guns is that the same power level can be achieved using a higher voltage and consequently a lower current than used for non-cascaded thermal spray plasma guns. This allows to use smaller power supplies and to lower operating cost for cascaded thermal spray guns compared to non-cascaded thermal spray guns.

[0008] One of the drawbacks of using cascaded thermal spray guns however is the axial length of the gun. Due to the insertion of a neutrode stack between the cathode and the anode, the axial length of cascaded thermal spray plasma guns is increased compared to non-cascaded thermal spray plasma guns. In thermal spray plasma guns, cascaded as well as non-cascaded ones, the plasma jet is created in axial direction of the gun. To apply a uniform coating with the desired coating characteristics, a certain spray distance as measured between the nozzle of the thermal spray gun and the substrate to be coated needs to be maintained during the coating process. In addition to this, the impact angle of the plasma jet transporting the molten coating material particles on the substrate to be coated should be close to 90°. An impact angle of close to 90° of the plasma jet on the substrate to be coated allows to achieve a higher deposition efficiency of the coating material on the substrate compared to lower angles and complex geometries. Coating of internal surfaces of parts or of surfaces with a space in between them requires the insertion of a thermal spray plasma gun into an internal volume of a part defined by the internal surfaces or into confined spaces defined by surfaces having a confined space in between them. Therefore, the use of cascaded thermal spray plasma guns for internal coating of parts or for coating of parts having a confined space is limited to parts having comparatively large internal diameters or a wide distance in between the surfaces to be coated.

[0009] Internal coating of parts is required for a large variety of parts like for, but not limited to, cylinder liners for the use in combustion engines, internal diameters of shafts, in-or outlets of turbochargers and combustors or transition ducts of gas turbines. Also, for coating of vane and blade assemblies of gas turbines, the space between the surfaces to be coated is limited. Internal coating of parts and coating of surfaces in confined spaces, especially for parts with increased axial length and / or complex geometries such as bends and curves along the travel path of the thermal spray gun impose dimensional restrictions in terms of length and diameter of the spray gun to be used.

[0010] Especially the application of Dense Vertically Cracked (DVC) coatings exhibiting 20-200 cracks / inch with cracks extending to more than 50% of the coating thickness using a thermal barrier coating (TBC) on the inside of parts for gas turbines using Yttria Stabilized Zirconia (YSZ) coating powders leads to long coating process times for applying the coating using non-cascaded thermal spray plasma guns.

[0011] According to a prior art approach, as disclosed in US 9,272,360 B2, a thermal spray plasma gun is mounted on an extension arm coupled to a robot to coat an interior diameter of a component having a curve in axial direction. Using an extension arm with variable length with a thermal spray plasma gun mounted on one end of the extension arm allows to coat interior diameters of parts with variable lengths and also of parts having bends and curves to a limited extent by introducing the gun into the part. However, as the impact angle of the plasma jet created is targeted to be close to 90° and as a certain spray distance need to be maintained during coating, this prior art approach of mounting a thermal spray plasma gun transversely to an extension arm can only be applied for parts with a large internal diameter or a large distance between surfaces to be coated.

[0012] According to another prior art approach, as disclosed in US 4,970,364, a noncascaded thermal spray plasma gun is mounted on an extensionarm, with the plasma gun creating a plasma jet in longitudinal direction of the arm. An anode also serving as a nozzle then deflects the plasma jet created at an angle of 45° from the longitudinal axis of the plasma gun. The gun disclosed in US 4,970,364 allows to coat tubes with internal diameters as little as 30 mm with a corrosion resistant coating using noncascaded thermal spray plasma gun technology. The spray nozzle is configured to be the anode of the thermal spray plasma gun and thus is referred to as “anodic spray nozzle”. The electric arc attachment may move axially along the entire plasma bore of the anodic spray nozzle as well as circumferentially around the plasma bore. This prior art approach allows to coat internal surfaces of parts with diameters as little as 30 mm using non-cascaded thermal spray plasma gun technology. However, due to the limited deflection angle of the plasma and as a consequence of a high incidence angle of the plasma jet to the surface to be coated as well as the use of non-cascaded thermal spray plasma gun technology, the use of this approach for coating internal surfaces of longer parts is limited due to the limited feed rates of the powder because limited feed rates extend the coating process time. A longer coating time for a single part also increases chances of embedding unwanted unmolten particles in the coating. With increasing process time, especially during the coating of internal surfaces, material feedstock is building up around the material injector and around the plasma nozzle. If those particle accumulations baked to the surface of the plasma nozzle become too big, they fall off and are transported via the plasma jet to the surface to be coated and create irregularities within the coating, with the irregularities decreasing the coating quality.

[0013] According to still another prior art approach as disclosed in US 10,612,122 B2, a cascaded thermal spray plasma gun is equipped with a plasma extension connected to a downstream end of an anode. In this prior art approach, the plasma jet is guided through a plasma extension to a nozzle mounted at a downstream end of the plasma extension to deliver plasma and coating material at extended locations from the anode The plasma extensions disclosed therein exceed a length of 150 mm, have a constant inner diameter of the extension and may be curved to allow the plasma jet to reach confined spaces along curved part surfaces. The plasma nozzle referred to in US 10,612,122 B2 may also deflect the plasma jet by up to 90° compared to the longitudinal axis of the extension to ensure an impact angle of close to or 90° on the surface to be coated. In this prior art approach, coating material is injected into the plasma jet at the plasma jet outlet of the nozzle which is mounted at a downstream end of a plasma extension. Along the travel path of the plasma jet within the plasma extension a loss of enthalpy is incurred. To compensate for this enthalpy loss within the plasma extension, this prior art approach requires a high enthalpy of the plasma jet at the downstream end of the anode to ensure proper melting of the coating material. To compensate heating energy loss occurring within the plasma extension, this prior art approach uses >75 vol% of molecular gases for the plasma gas to ensure that the plasma exits the extension module with an enthalpy of >15 kJ / g. To calculate the enthalpy of the plasma at the exist of the anode (HAM), this prior art approach uses following formula:

[0014] U*I

[0015] HAM = * J]

[0016]

[0017] G

[0018] U is the voltage supplied to the plasma gun in Volt, I the current at the plasma gun in Ampere, q the thermal efficiency of the plasma gun, and G is the mass in Grams. This prior art approach calculates the thermal efficiency of a cascaded thermal spray plasma gun being in the range of 0.43-0.66. In order to calculate the enthalpy of the plasma at the exit of the extension module (HEXIT), this prior art approach uses following formula:

[0019] HEXIT = HAM * £1]

[0020] With £q being the total thermal efficiency including the thermal losses in the extension and within the nozzle. For an extension length of 150 mm using HAM of 30 kJ / g, a maximum length of the extension of 250 mm is calculated to reach HEXIT of at least 15 kJ / g whereas the maximum length of the extension is calculated as being 1’083 mm for HAM of 80 kJ / g. One of the drawbacks of this prior art approach is that the high enthalpy of the plasma jet at the downstream exit of the anode leads to a high heat transfer to the inner surface of the plasma extension. Without countermeasures, the anode would quickly melt and / or wear down. To overcome this issue, this prior art approach teaches to apply a vortex distributor inducing a swirl to a plasma gas injected upstream of the cathode end and to inject a second plasma gas axially located between a cathode module and an anode module.

[0021] Taking the above said into account, it becomes clear that prior art approaches either focus on deflecting a plasma jet within an anode or on mounting a nozzle deflecting a plasma jet at a downstream end of a plasma extension. The prior art approaches therefore fail to offer solutions for a modular cascaded thermal spray plasma gun comprising a configurable nozzle that can be adapted to various coating geometries and to coating-material and substrate-specific process parameters to deflect and / or shape the plasma jet.

[0022] The prior art approaches mentioned also require the acquisition and maintenance of thermal spray plasma guns specifically for the coating of external surfaces of parts and of thermal spray plasma guns specifically adapted for the coating of internal surfaces. The use of specific thermal spray plasma guns for different coating processes and / or parts to be coated increases setup times of the thermal spray machinery as well as it increases changeover times between coating of different part geometries. In addition to this, the maintenance of a variety of specific thermal spray plasma guns incurs considerable cost for stock-keeping of spare parts.

[0023] For internal coating of parts having an internal diameter or distance within a confined space to be coated of 50 mm or more, especially for elongated parts with a curved interior surface the prior art approaches also fail to offer solutions without the need to use high-enthalpy plasma jets to reach high feed rate & high net power

[0024] Objective of the invention The objective of the present invention therefore is to provide a modular cascaded thermal spray plasma using configurable nozzles which allow for the deflection and / or shaping of the cross section of the plasma jet created.

[0025] It is a further objective of the invention to provide a thermal spray plasma gun platform being adaptable to a variety of coating process parameters and a variety of parts requiring coating of either external or internal surfaces without incurring the need of acquiring specific thermal spray plasma guns for each purpose.

[0026] Summary of the invention

[0027] According to the invention, this objective is met by a modular cascaded thermal spray plasma gun comprising one or more cathodes, a modular neutrode stack comprising a rear neutrode, more than two annular neutrodes inserted into a modular neutrode stack housing and a neutrode stack end piece, an anode, a gun body, an interface and a configurable nozzle.

[0028] According to one embodiment of the present invention, a modular thermal spray plasma gun is equipped with an interface having a standardized interface at the downstream end allowing for a quick exchange of configurable nozzles. The standardized interface allows mating inlets and outlets for cooling water and one or more outlets for powder entrained in carrier gas and feedthrough of current, water and coating powder in carrier gas located at a downstream end of the interface with corresponding openings in a configurable nozzle. The standardized interface allows angular positioning of the configurable nozzle as well. An annular extension of the anode in axial direction in conjunction with an annular recess on the configurable nozzle allows the configurable nozzle to be concentrically aligned with the axis of the central plasma bore of the anode.

[0029] In one embodiment, the modular cascaded thermal spray plasma gun comprises one or more cathodes at an upstream end of the modular cascaded thermal spray plasma gun, a modular neutrode stack, an anode, a gun body, an interface and a configurable nozzle at a downstream end of the modular cascaded thermal spray plasma gun, the neutrode stack comprising a rear neutrode at an upstream end of the neutrode stack, more than one annular neutrodes at a downstream end of the neutrode stack, wherein the neutrodes are electrically isolated from each other, a neutrode stack housing and a neutrode stack end piece, the anode located upstream of the neutrode stack comprising an axial protrusion and an electrically isolating annular ring at an upstream end of the anode, the interface located upstream of the anode comprising axial cooling water feedthroughs, the configurable nozzle comprising a mating surface to the interface, a recess and cooling water inlets and cooling water outlets, wherein the form and location of the cooling water inlets and the cooling water outlets on the mating surface of the configurable nozzle correspond to the form and location of the axial feedthroughs of the interface.

[0030] In some embodiments, the configurable nozzle is detachably fixed to the gun body and is angularly aligned with the interface.

[0031] According to some embodiments, the configurable nozzle is coaxially aligned with the anode by an axial protrusion of the anode in conjunction with a corresponding recess in the configurable nozzle or vice versa

[0032] In some embodiments, the configurable nozzle is electrically isolated from the anode by an electrically isolating annular ring located radially towards the outside of the axial protrusion of the anode and protruding axially beyond the axial protrusion of the anode.

[0033] In some embodiments, the interface comprises recesses around the axial feedthroughs for accommodating gaskets at the upstream and downstream end of the interface.

[0034] In embodiments, an annular sealing on the inner diameters prevents cooling water from flowing to a central bore.

[0035] In some embodiments, the interface further comprises axial powder transportation channels. In some embodiments, the anode of a modular cascaded thermal spray plasma gun comprises an expansion step at a downstream end of the anode with the expansion step inner diameter being at more than or equal to 1.5 and less than or equal to 2 times greater than the diameter of the anode’s central plasma bore.

[0036] In further embodiments, the modular neutrode stack of the modular cascaded thermal spray plasma gun is configured to accommodate at least 2 annular neutrodes, preferably at least 4 annular neutrodes and even more preferably 6 annular neutrodes.

[0037] According to another aspect of the invention, a configurable nozzle allowing to configure a plasma jet exiting a plasma jet opening in terms of angle in respect to a longitudinal axis of a thermal spray plasma gun and in terms of shape of the cross section of the plasma jet is proposed. The configurable nozzle can also be configured to varying distances between the anode and the plasma jet in axial and radial direction of the thermal spray plasma gun, to different outside diameters of the nozzle and to different deflection curves of the plasma jet within the nozzle, The configurable nozzle is preferably produced using an additive manufacturing method and is connected to the standardized interface and optionally electrically insulated to the gun body and anode respectively.

[0038] According to an embodiment of a configurable nozzle for a modular thermal spray plasma, the configurable nozzle comprises at an upstream end a anode mating surface, a recess and two or more axial bores, a central plasma channel, at least one tubular cooling water inflow channel and at least one cooling water outflow channel, wherein the central plasma channel comprises a transition zone at an upstream end of the central plasma channel, a deflection zone and a plasma jet outlet at a downstream end of the central plasma channel and a central axis.

[0039] In some embodiments of a configurable nozzle the recess is arranged to coaxially align the recess axis with an anode axis. In some embodiments of a configurable nozzle the transition zone comprises a circular upstream transition zone end wherein the diameter of the central plasma bore at an upstream transition zone end corresponds to an inner diameter of an anode or to the inner diameter of an anode expansion step.

[0040] According to embodiments of a configurable nozzle, the transition zone comprises a downstream transition zone end having a circular, oval or elliptical bore for the central plasma channel with the area of the cross section of the downstream transition zone end perpendicular to the central plasma channel corresponding to the area of the plasma jet outlet. The area of the plasma jet outlet is equal to or larger than 19,6 mm2and less or equal to 314,1 mm2, preferably equal to or larger than 33,1 mm2and less than or equal to 95 mm2.

[0041] In some embodiments, the central plasma channel comprises a deflection zone arranged to deflect a plasma jet from an axis of rotational symmetry of the upstream transition zone end radially towards the outside of the configurable nozzle by an angle of 0° to less than or equal to 90°.

[0042] According to some embodiments, the central axis in the deflection zone comprises at least an upstream curvature and preferably a downstream curvature with the upstream and the downstream curvatures having opposite signs, wherein the radius of a downstream curvature is greater than or equal to 20 mm and wherein the radius of an upstream curvature is at least 2 times the radius of the downstream curvature and preferably less than or equal to 100 mm.

[0043] In some embodiments, the central axis comprises in the deflection zone comprises a spline curve or elliptical curve of the central plasma channel to deflect a plasma jet from an axis of rotational symmetry of the upstream transition zone end radially towards the outside of the configurable nozzle.

[0044] In some embodiments, the central axis in the deflection zone comprises an upstream part of the central axis and a downstream part of the central axis starting at the upstream end of the transition zone end wherein both central axes each form a straight line with the straight lines intersecting at an angle of more than 0° and less than or equal to 90°.

[0045] In case of a cylindrical central plasma bore, the central axis is the axis of rotational symmetry of the central plasma bore, in case of an elliptical central plasma bore, the central axis is coaxial to the intersecting point of the minor and major axis of the ellipse and in case of an oval central plasma bore, the central axis is defined being coaxial to the intersection of its principal axes of symmetry, typically corresponding to the geometric center of the oval shape.

[0046] In some embodiments, the at least one tubular cooling water inflow channel is closer to the central plasma channel than the at least one tubular cooling water outflow channel. Within an annular groove arranged at an upstream end cap of the configurable nozzle cooling water flowing upstream is redirected to flowing downstream of the configurable nozzle.

[0047] In accordance with embodiments, it has been found that the inventive modular cascaded thermal spray plasma gun equipped with an inventive configurable nozzle allows to coat internal surfaces of parts within confined spaces of 50 mm or more even if using plasma gas comprising less than 75 vol% of molecular gases. In addition to this, it has been surprisingly found that nozzles can be configured to create oval or slot shaped plasma jets without deteriorating the characteristics of the coatings applied using so-formed plasma jets. By forming an oval or slot-shaped plasma jet, the highest length of which is perpendicular to the longitudinal axis of the nozzle, a wider spray plume can be created, which even further decreases coating time by widening the material deposition during a coating pass. The modular cascaded thermal spray gun disclosed thus allows to use a single thermal spray gun platform requiring a minimum of parts to be exchanged for adapting the thermal spray gun to the coating of internal or external surfaces. This inventive solution therefore also reduces the need of keeping thermal spray guns on stock for coating a large variety of parts and therefore reduces the cost of stockkeeping spare parts as well as it reduces setup and changeover times. It is preferred that the inventive solutions are further improved by configuring noncascaded thermal spray plasma guns to be used with inventive configurable nozzles.

[0048] The invention shall now be further exemplified with the help of figures.

[0049] Brief Description of Drawings

[0050] Figure 1 depicts a cross section of a conventional thermal spray plasma gun.

[0051] Figure 2 illustrates a cross section of a prior art cascaded thermal spray plasma gun.

[0052] Figure 3 shows a cross section of an inventive modular cascaded thermal spray gun.

[0053] Figure 4 is a representation of the interface between a cascaded gun body and a configurable nozzle and of an anode to be used in conjunction with the interface.

[0054] Figure 5 shows the interface between the interface and a configurable nozzle.

[0055] Figure 6 illustrates a configurable nozzle creating a deflected and formed plasma jet.

[0056] Figure 7 shows a cross section of the configurable nozzle of Fig. 6.

[0057] Figure 8a to 8c are representations of configurable nozzles having different deflection angles and generating different shapes of plasma jet cross sections at the plasma jet outlet of the nozzle.

[0058] Detailed description

[0059] The particulars shown herein are by way of example and for purposes of illustrative discussion of the embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the present invention. In this regard, no attempt is made to show structural details of the present invention in more detail than is necessary for the fundamental understanding of the present invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the present invention may be embodied in practice.

[0060] Fig. 1 illustrates a cross-section of a conventional, non-cascaded thermal spray plasma gun. Cooling water (101) is supplied through a water hose from a junction and monitoring box not shown in Fig. 1 to a water inlet at the thermal spray plasma gun, being the positive gun connection point. The cooling water flows through a predetermined path within the thermal spray plasma gun through the nozzle (105) acting as the anode to the electrode (115) acting as the cathode of the thermal spray plasma gun and subsequently to a gun a water outlet (119), being the negative gun connection point to a water hose back to the junction and monitoring box. An arc (113) is ignited between the nozzle (105) and the electrode (115), being electrically isolated by an insulator (103) by applying a voltage. The plasma gas (121) is ionized and forms a plasma jet (111) which melts the coating powder supplied (109) through injectors (107) using a carrier gas and propels the molten particles towards a substrate not shown in Fig. 1 on which the molten particles solidify rapidly and form a coating layer.

[0061] In Fig. 2 a prior art cascaded thermal spray plasma gun is shown. In contrast to a noncascaded thermal spray plasma gun as shown in Fig. 1, a rear neutrode (219) and one or more neutrodes (205) are inserted in the form of a stack in between the cathode (201) and the anode (203). Each of the neutrodes is electrically insulated to the neighboring neutrodes by an annular spacer. The arc generated between the cathode (201) and the anode (203) has a length constrained by the axial distance between the cathode (201) and the anode (203), with this distance being defined by the axial length and the number of neutrodes (205) stacked in between the cathode and the anode (203). Cooling water is supplied to the positive pole (209) of the gun and flows through channels radially towards the outside of the neutrode stack housing (217) through the anode (203) and subsequently cools the cathode (201) inserted in a holder before it exits the negative pole (213) of the plasma gun. Plasma gas that may comprise one or more gases is supplied through the plasma gas supply (211). The arc (215) created between the cathode (201) and the anode (203) is then used to generate a plasma jet exiting the anode (203). The anode (203) has an upstream end towards the cathode (201) and a downstream end towards the powder injectors (207). Downstream of the plasma jet and outside of the main plasma gun body, powder entrained in carrier gas is injected through one or more powder injectors (207) into the plasma jet.

[0062] A preferred example of a modular cascaded thermal spray plasma gun is shown in Fig. 3. The modular cascaded thermal spray plasma gun shown in Fig. 3 comprises a cathode (301) inserted in a water-cooled cathode holder (311), a modular neutrode stack comprising six annular neutrodes (305) and a rear neutrode (317) electrically isolated from each other. The rear neutrode (317) and the annular neutrodes (305) are inserted in a neutrode stack housing (307). At the upstream end of the neutrode stack housing (307), a neutrode stack end piece (329) is tightened in axial direction of the neutrode stack to press the neutrodes together in axial direction. The modular cascaded thermal spray plasma gun further comprises an anode (303), a gun body (309) and an interface (313) having a standard interface for mechanical connection, water supply and powder feedthrough and a configurable nozzle (315) attached to said standardized interface. For starting the gun operation, a voltage is applied to an ignition cable connection point (319) and an arc is ignited between the rear neutrode (317) adjacent to the cathode (301). Once this starting arc has been established, the arc attachment to the rear neutrode (317) is then transferred to the anode (303), thus in operation an arc between cathode (301) and anode (303) is present. The arc established is the used to ionize plasma gas supplied to a plasma gas connection point (321) to create a plasma jet not shown in Fig. 3 within the anode’s central bore (323). In contrast to prior art approaches, where the anode serves as nozzle, the inventive solution separates the anode part, which provides and attachment point of the arc on the internal central plasma bore, from the nozzle. On one hand, this allows a more compact gun body in radial dimension to the central plasma bore and on the other hand, this allows to form and deflect the plasma jet within the nozzle.

[0063] Cooling water connected to the positive connection point (325) of the plasma gun flows along the annular neutrodes (305) forming a neutrode stack in recesses not shown in Fig. 3 radially towards the outside of the annular neutrodes (305) of annular sealing rings (337) and subsequently through the anode (303) and through the interface (313) and the configurable nozzle (315). The recesses open to an outer periphery of the annular neutrodes (305) are axially aligned to form a plurality of cooling water flow channels. At the upstream end of the configurable nozzle the cooling water is redirected radially towards the outside and subsequently flowing back radially towards the outside of the configurable nozzle (315) through the interface (313) and through recesses applied on the outside of the neutrode stack housing (307) before leaving the plasma gun at the negative connection point (327) of the plasma gun.

[0064] Annular neutrodes (305) and a rear neutrode (317) are pressed together in axial direction by a neutrode stack end piece (329) screwed into the neutrode stack housing (307). The water-cooled anode (303) is held in place in axial direction by a gun body end piece (313) which is detachably mounted to the gun body (309). The anode is radially aligned with the anodes’ central plasma bore (323) by an axial extension (331) of the neutrode stack end piece and an annular groove (333) of the anode (303). In axial direction between downstream end of the neutrode stack and the anode an electrically isolating part (335) extending from the anode’s central plasma bore (323) of the neutrode stack radially outwards to the concentric surface of the recess of the anode and further extending axially along the concentric surface of the axial extension of the neutrode is used to electrically isolate the neutrode stack from the anode.

[0065] In embodiments, the number of individual annular neutrodes assembled in the neutrode housing ranges from 3 to 6, not including the upstream neutrode segment. Using a different number of annular neutrodes allows to adapt the voltage level specifically to individual coating process requirements. To adapt the modular cascaded thermal spray plasma gun for accommodating neutrode stack housings containing a varying number of annular neutrodes, the modular cascaded thermal spray plasma gun may comprise a gun body (309) being split into three parts (the individual parts not shown in Fig. 3), the split gun body assembly comprising a rear gun body axially extending from the upstream end of the plasma gun downstream to the ignition cable connection point (319), an exchangeable intermediate gun body, adaptable to the length of the neutrode stack housing, axially extending from the downstream end of the rear gun body to the downstream end of the neutrode stack housing and a front gun body. In some embodiments, rear, intermediate and front gun body parts are axially secured by screws.

[0066] The modular cascaded thermal spray plasma gun disclosed can be adapted to a variety of parts to be coated, either on external surface or on internal surfaces by using neutrode stack housings having a varying number of neutrodes and by using configurable nozzles. This inventive solution reduces setup times for thermal spray coating machines and reduce standstill times for exchanging anodes and nozzles as well as it reduces the complexity of the spare parts management by reducing the number of different spare parts to be maintained and kept on stock. In addition to this, the separation of the anode and nozzle allows to shape and deflect a plasma jet without incurring excessive wear on the anode and reduces the risk of the arc attachment point moving downstream and outside of the anode.

[0067] Fig. 4 shows a cross section of an inventive interface between a cascaded thermal spray plasma gun body (409) and a configurable nozzle (413) as well as it shows an anode (423) to be used in conjunction with the interface. Compared to Fig. 3, the cross section is tilted by 22.5° in clockwise direction looking from the downstream end of the configurable nozzle to show the internal cooling water flow channels. The anode (423) comprises an annular recess (425) to concentrically align the axis of the anode (423) with the central bore of the neutrode stack (429). The diameter of the anode’s central plasma bore (421) may correspond to the inner diameter of the central bore of the neutrode stack (429). In between an axial annular extension (427) of an annular neutrode (401), an optional electrically isolating part (403) extending from the central bore of the neutrode stack (429) radially towards the outside to a surface of the annular recess being concentric to the central plasma bore and further extending axially towards the neutrode is mounted. Annular sealing rings (405) and (407) prevent cooling water flowing through the neutrode stack to the anode from entering the central plasma bore by flowing radially towards the central plasma bore or radially towards the outside. The anode may further comprise an axial protrusion of the anode (441) at a downstream end comprising a radial expansion step (415). The radial expansion step allows on to prevent the arc attachment point within the anode to move outside of the anode towards the downstream end. The radial expansion step has an inner diameter in the range of 1.5 times the inner diameter of the anode’s central plasma bore to 2 times the diameter of the anode’s central plasma bore (421). Preferably the inner diameter of the expansion step is 1.75 times the diameter of the anode’s central plasma bore. The axial length of the expansion step is in the range of 5 to 10 mm, preferably in the range of 6 to 7 mm. In some embodiments, the anode may comprise a liner insert (435) to prevent wear of the inner diameter of the anode by the electric arc. A thread (431) is used to mount the anode (423) to the gun body (409).

[0068] An interface (411) is detachably mounted to the gun body (409) using screws not shown in Fig. 4 which also align the interface in angular direction with the gun body. The interface comprises axial cooling water feedthroughs (437) and (439) for the feedthrough of cooling water, the radial location and form of which correspond to the location and form shown in Fig. 5. The interface may also comprise one or more axial powder transportation channels not shown in Fig. 4 for the feedthrough of powder entrained in carrier gas. An electrically isolating annular ring (417) electrically isolates the anode (423) from a configurable nozzle (413). The electrically isolating annular ring (417) is radially located in between the inner diameter of an upstream annular recess (445) in the mating surface of the configurable nozzle and the axial protrusion (441) of the anode. The electrically isolating annular ring (417) may also extend axially beyond the axial protrusion (441) of the anode. The electrically isolating annular ring (417) may also extend in radial direction towards the anode’s central plasma bore (421) as shown in Fig. 4. In some embodiments, the complete interface (411) may be made from electrically isolating material. One or more annular sealing rings (433) prevent cooling water from entering the central plasma bore at the downstream end of the interface piece (411).

[0069] Recesses (419) formed around the axial cooling water feedthroughs (437) and (439) and / or axial powder transportation channels accommodate gaskets (443) on the upstream and on the downstream end of the interface to prevent cooling water and / or powder and / or carrier gas from flowing radially towards the in- or outside of the respective channels. The gaskets (443) may also be made from an electrically isolating material to electrically isolate the interface (411) from the configurable nozzle (413) and / or the gun body (409). In Fig. 5, the downstream connection of a configurable nozzle to the interface as shown in Fig. 4 is depicted. A recess (501) in the mating surface (513) to the interface ensures that the central plasma bore of the nozzle (515) is coaxially aligned with the central plasma bore of the anode. A conically shaped transition bore (511) extends axially from the recess (501). The nozzle is secured to the interface (411) by two or more screws inserted into the fixation bores (507). In some embodiments, one of the fixation bores (507) may be adapted to accommodate a pin for precise angular alignment of the nozzle. In other embodiments, a bayonet catch is used to mount the nozzle to the interface. Cooling water inlets (505) in the form of slots with rounded edges, or of oval shape or round shape placed radially towards the central plasma bore allow water to be fed from the interface to the nozzle. Cooling water outlets (503) located radially towards the outside of the nozzle allow the cooling water to flow back to the interface having cooling water inlets and outlets corresponding to the number and geometry of cooling water in- and outlets of the nozzle. An equal number of cooling water in- and outlets is located around the circumference of the nozzle. The number and geometry of the cooling water in- and outlets is determined based on the cooling water flow required. In the example shown, a total of 8 cooling water flow inlets and 8 cooling water flow outlets is shown. In the example shown in Fig. 5, an individual cooling water inlets (505) has a length of 6.5 mm and a height as measured in radial direction of the nozzle of 1.5 mm. The cooling water outlets shown in the example have a length of 6.5 mm and a height of 2 mm. However, the height of the cooling water in- and outlets measured in radial direction may range from 1 to 3 mm, preferably from 1 to 2 mm, whereas the length of the cooling water in- and outlets may range from 4 to 8 mm, preferably being in the range of 5 to 7 mm. Grooves (509) allow to remove or apply screws to mount the interface to the gun body. In some embodiments, one or more axial powder transportation channels for powder entrained in carrier gas are integrated into the nozzle by forming channels within the nozzle.

[0070] In Fig. 6 a configurable nozzle for the internal coating of parts is shown. The mating face (601) at the upstream end of the configurable nozzle is mating with the end face of the interface (411). An oval shaped plasma jet outlet (603) is located at the downstream end of the nozzle. In some embodiments, the plasma jet outlet (603) may be round, oval or elliptical. The plasma jet is also deflected within the nozzle from a direction along the axis of the nozzle’s central plasma bore such that the plasma jet is leaving the plasma jet outlet (603) inclined with respect to the nozzle’s central bore axis. Forming the plasma jet to an oval or elliptical shape allows to generate a wider plasma jet, which increases the coating width along the travel path of the thermal spray plasma gun along the bore. This reduces the time needed for coating parts without deteriorating the characteristics of the coating applied.

[0071] The exemplary configurable nozzle shown in Fig. 6 has an outer diameter of 40 mm measured at the downstream end of the nozzle and a length of 90 mm. In embodiments, the outer diameter of the configurable nozzle as measured at the downstream end ranges from 25 to 90 mm, preferably from 30 mm to 50 mm. The length of the configurable nozzle ranges from 15 mm to 275 mm, preferably from 15 mm to 150 mm.

[0072] The cross section of a configurable nozzle as shown in Fig. 7 illustrates that a configurable nozzle for use with a modular thermal spray plasma gun, preferably a modular cascaded thermal spray plasma gun, comprises a recess (715) located at the upstream end of the configurable nozzle to coaxially align the nozzle axis with the anode axis, two or more axial bores (713) for mounting the nozzle to the interface and / or for connecting powder hoses to the interface and for aligning the configurable nozzle in angular direction with the interface, one or more tubular cooling water inflow channels (709), one or more tubular cooling water outflow channels (711) and an annular groove (707) for redirecting cooling water flowing in the cooling water inflow channel (709) flowing towards the downstream end of the nozzle to a flow towards the upstream end of the nozzle in the cooling water outflow channel (711). The configurable nozzle further comprises cooling water in- and outlets (not shown in Fig.

[0073] 7) located in the nozzle’s mating surface (723) to the interface at the upstream end of the nozzle to which the tubular cooling water channels (709) and (711 ) are connected. The form and radial and angular location of the cooling water in- and outlets on the nozzle’s mating surface correspond to the form and radial and angular location of the water feedthroughs of the interface. The nozzle further comprises a central plasma channel (703) further comprising a central axis (733), the central plasma channel (703) may further comprise a transition zone (701), a deflection zone (719) and a plasma jet outlet (705). The transition zone (701) comprises a circular upstream transition zone end (725) having an axis of rotational symmetry and a downstream transition zone end (727). In embodiments where the plasma jet does not need to be deflected from the central axis of the anode and where the shape of the cross section of the plasma jet outlet is round, the central plasma channel (703) may only comprise a transition zone (701) and a plasma jet outlet (705). At the upstream transition zone end (725), the inner diameter of the bore of the transition zone may correspond to the diameter of the radial expansion step (415) of the anode. The area of the plasma jet opening (705) corresponds to the area of the central plasma bore at the downstream transition zone end (727). The inner diameter of the bore of the downstream transition zone end (727) ranges from 5 to 20 mm and is preferably within the range of 6.5 to 11 mm. Thus, the area of the plasma jet outlet (705) is within the range of 19,6 mm2to 314,1 mm2, preferably between 33,1 mm2and 95 mm2. The conical angle of the transition zone lies within the range of 20-40°, preferably in the range of 25-35°. The cone may be opening towards the upstream transition zone end (725) or towards the downstream transition zone end (727). Within the transition zone, the shape of the plasma jet may be transformed from a round cross section to an oval or elliptical cross section. To achieve this, the bore of the downstream transition zone end (727) may have the form of a circle, an oval or an ellipsis. Within the deflection zone (719), a plasma jet being coaxial to the anode’s central plasma bore and to the axis of rotational symmetry of the circular upstream transition zone end (725) may be deflected by an angle radially towards the outside of the nozzle. The deflection angle may range from 0 to 90°. To minimize the complexity of nozzle variants, nozzles with 15°, 30°, 45°, 60°, 75° and 90° deflection angle may be used. In the deflection zone, the central axis (733) of the plasma channel may comprise a spline curve, radii or elliptical curves and may have at least one upstream curvature (729) and preferably also a downstream curvature (731) with the upstream curvature (729) and the downstream curvature (731) having opposite signs. The radius of the downstream curvature (731) is 20 mm or more whereas the radius of the upstream curvature (729) is at least 2 times higher than the radius of the downstream curvature (731), preferably less than or equal to 100 mm. The central axis (733) of the plasma channel may also comprise two or more straight lines intersecting. The deflection of the plasma jet using a spline curve or radii allows to reduce energy loss of the plasma within the nozzle and thus to achieve the same coating results using lower power levels.

[0074] The end cap (717) of the nozzle shown in Fig. 7 comprises an annular groove (707) to redirect the cooling water flow and sealing rings (721) for preventing cooling water flowing out of the downstream end of the nozzle. In some embodiments, the end cap (717) of the nozzle is integrated into the nozzle, such that only one part is formed. The use of 3D printing techniques for manufacturing allows to implement tubular cooling water channels and powder flow channels within the nozzle as well as they allow to implement spline curves or radii for the plasma channel. Applying such complex geometries was not possible using drilling or milling.

[0075] Fig. 8a shows a nozzle configured for deflecting a plasma jet by 45° using deflection zone with a single curvature and generating a round cross section of the plasma jet. In Fig. 8b, a nozzle configured for deflecting a plasma jet by 45° whilst the plasma jet cross section is changed from a round shape at the upstream end of the transition zone to an oval or oval shape at the downstream end of the transition zone.

[0076] Fig. 8c shows a nozzle configured for deflecting a plasma jet by 90° using a deflection zone with two curvatures with opposite signs and a transition zone to shape a plasma jet having an oval cross section.

[0077] Experiments have shown that the modular cascaded thermal spray plasma gun having a configurable nozzle can be operated using a maximum electrical power of 120 kW and that the inventive modular cascaded thermal spray plasma gun equipped with a configurable nozzle allowing to deflect and shape a plasma jet’s cross section can be used to coat internal surfaces of parts having a diameter of 50 mm or more.

[0078] It is preferred that the inventive solutions are further improved by integrating the interface between the configurable nozzle and the gun body into the gun body. In an improvement of the inventive solution for a configurable nozzle, powder transportation channels (not shown in Fig. 7) to transport powder entrained in carrier gas to powder injectors (not shown in Fig. 7) screwed into the upstream end of the nozzle are integrated into the nozzle radially in between the tubular cooling water channels or radially towards the outside of the tubular cooling water channels. Integrating powder transportation channels into the nozzle body allows to reduce powder build up on the outside of the nozzle and thus increases the time in between maintenance cycles. To allow rapid exchange of powder injectors showing wear on the internal diameter or powder build up on the outside it is proposed the use powder injectors screwed into the nozzle body around the plasma jet outlet. This requires applying a thread to the outlet of the powder transportation channel and a thread on the outside of the injectors.

[0079] It is noted that the foregoing examples have been provided merely for the purpose of explanation and are in no way to be construed as limiting of the present invention. While the present invention has been described with reference to an exemplary embodiment, it is understood that the words which have been used herein are words of description and illustration, rather than words of limitation. Changes may be made, within the purview of the appended claims, as presently stated and as amended, without departing from the scope and spirit of the present invention in its aspects. Although the present invention has been described herein with reference to particular means, materials and embodiments, the present invention is not intended to be limited to the particulars disclosed herein; rather, the present invention extends to all functionally equivalent structures, methods and uses, such as are within the scope of the appended claims. List of reference signs

[0080] APS Atmospheric Plasma Spray DVC Dense Vertically Cracked TBC Thermal Barrier Coating YSZ Yttria Stabilized Zirconia

Claims

Claims1. A modular cascaded thermal spray plasma gun comprising one or more cathodes (301) at an upstream end of the modular cascaded thermal spray plasma gun, a modular neutrode stack, an anode (303), a gun body (309), an interface (313) and a configurable nozzle (315) at a downstream end of the modular cascaded thermal spray plasma gun,the modular neutrode stack comprising a rear neutrode (317) at an upstream end of the neutrode stack, more than two annular neutrodes (305) at a downstream end of the neutrode stack, wherein the neutrodes are electrically isolated from each other, a neutrode stack housing (307) and a neutrode stack end piece (329),the anode (303) located downstream of the neutrode stack comprising an axial protrusion (441) and an electrically isolating annular ring (417) at an upstream end of the anode,the interface located upstream of the anode comprising axial cooling water feedthroughs (437) and (439),the configurable nozzle (315) comprising a mating surface (513) to the interface, a recess (501) and cooling water inlets (505) and cooling water outlets (503),characterized in that the form and location of the cooling water inlets (505) and the cooling water outlets (503) on the mating surface of the configurable nozzle correspond to the form and location of the axial cooling water feedthroughs (437) and (439) of the interface.

2. A modular cascaded thermal spray plasma gun according to claim 1 , characterized in that the configurable nozzle (315) is detachably fixed to the gun body (309).

3. A modular cascaded thermal spray plasma gun according to claim 1 , characterized in that the configurable nozzle (315) has an angularly alignedoutlet geometry with respect to the gun axis (313).

4. A modular cascaded thermal spray plasma gun according to claims 1 , characterized in that the configurable nozzle (315) is coaxially aligned with the anode (303) by an axial protrusion of the anode (441) in conjunction with a corresponding recess (715) in the configurable nozzle.

5. A modular cascaded thermal spray plasma gun according to claim 1 , characterized in that the configurable nozzle (315) is electrically isolated from the anode (303) by an electrically isolating annular ring (417) located radially towards the outside of the axial protrusion of the anode (441) and protruding axially beyond the axial protrusion of the anode.

6. A modular cascaded thermal spray plasma gun according to claim 1 , characterized in that the interface (313) comprises recesses around the axial feedthroughs for accommodating gaskets at the upstream and downstream end of the interface.

7. A modular cascaded thermal spray plasma gun according to claim 1 , characterized in that the anode (303) comprises an expansion step (415) at a downstream end of the anode with the expansion step inner diameter being at more than or equal to 1.5 and less than or equal to 2 times larger than the diameter of the anode’s central plasma bore (421).

8. A modular cascaded thermal spray plasma gun according to claim 1 , characterized in that the modular neutrode stack is configured to accommodate at least 2 annular neutrodes, preferably at least 4 annular neutrodes (305) and even more preferably 6 annular neutrodes.

9. A configurable nozzle for a modular thermal spray plasma gun according to claim 1 , the configurable nozzle comprising at an upstream end a nozzle mating surface (723), a recess (715) and two or more axial bores (713), a central plasma channel (703) further comprising a central axis (733), at leastone tubular cooling water inflow channel (709) and at least one cooling water outflow channel (711), characterized in that the central plasma channel comprises a transition zone (701) at an upstream end of the central plasma channel, a deflection zone (719) and a plasma jet outlet (705) at a downstream end of the central plasma channel.

10. A configurable nozzle according to claim 9, characterized in that the recess is arranged to coaxially align the recess axis with an anode axis.11.A configurable nozzle according to claim 9, characterized in that the transition zone comprises a circular upstream transition zone end (725).

12. A configurable nozzle according to claim 11 , characterized in that the diameter of the central plasma bore at an upstream transition zone end (725) corresponds to an inner diameter of an anode.

13. A configurable nozzle according to claim 11 , characterized in that the diameter of the central plasma bore at an upstream transition zone corresponds to an inner diameter of an anode expansion step.

14. A configurable nozzle according to claim 9, characterized in that the transition zone comprises an oval or elliptical downstream transition zone end (727).

15. A configurable nozzle according to claim 9, characterized in that the area of the cross section of the downstream transition zone end perpendicular to the central plasma channel corresponds to the area of the plasma jet outlet (705).

16. A configurable nozzle according to claim 15, characterized in that the area of the plasma jet outlet (705) is equal to or larger than 19,6 mm2and less or equal to 314,1 mm2, preferably equal to or larger than 33,1 mm2and less than or equal to 95 mm2.

17. A configurable nozzle according to claim 9, characterized in that the central plasma channel comprises a deflection zone arranged to deflect a plasma jetfrom an axis of rotational symmetry of the upstream transition zone end (725) radially towards the outside of the configurable nozzle by an angle of 0° to less than or equal to 90°.

18. A configurable nozzle according to claim 17, characterized in that in the deflection zone (719) the central axis (733) comprises at least an upstream curvature (729) and preferably a downstream curvature (731) with the upstream and the downstream curvatures having opposite signs.

19. A configurable nozzle according to claim 9, characterized in that the at least one tubular cooling water inflow channel (709) is closer to the central plasma channel than the at least one tubular cooling water outflow channel (711).

20. Method for manufacturing a configurable nozzle according to claim 9, characterized in that the configurable nozzle is manufactured by means of an additive manufacturing process.

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