Plasma Spray Gun for Internal Coatings

The invention concerns a plasma spray gun with a cooled electrode and burner nozzle for insertion in pipes and bores of work pieces and for coating the internal surfaces of said work pieces.

A preferred field of application for such plasma spray guns is the coating of contact surfaces of the blade roof and turbine disc within the holder grooves of the turbine disc in the case of aircraft gas turbine engines.

In a known plasma spray gun of this type, the reduction of the geometrical dimensions of the burner nozzle-electrode pairing allowed the coating of the internal surfaces to be carried out in the required spray layer quality in bores of minimal inner diameter of 70 mm. In the known inner burner, plasma spray energy, plasma gas discharge and spray powder injection on the one hand and geometrical reduction of the burner nozzle electrode pairing on the other are coordinated so that practically any spray powder, for whose melting standard burners needed a flight path of up to 150 mm within the plasma flame, is molten after a flight path of about 35 mm. The spray spacing between the plasma spray gun and the substrate surface as well as the geometrical dimensions of the total inner burner define the minimal tube or bore diameter, with which coating can be performed with the same spray layer quality. Thus the latter is fixed in advance by the normal design of the plasma spray gun. It would be possible by reducing the plasma energy, the plasma gas amount, and the amount of injected powder to decrease the plasma flame length and thus the spray spacing in order to coat bores of smaller diameter as well; but this would only be possible at the expense of the spray layer quality.

An object of the invention is to provide a plasma spray gun of the type named above which makes possible a coating of higher quality on the internal surfaces of tubes and bores having minimal inner diameters of about 25 mm with increased spraying efficiency.

This is provided by this invention in that:

(a) the electrode is designed in the area of its head to be rotation-asymmetrical,

(b) the diameter of the electrode is smaller than the minimal inner diameter of the burner nozzle,

(c) the burner nozzle on the end facing away from the electrode has at least one partial area with an inner diameter which is larger than its minimal inner diameter, and

(d) the powder injector has a flat exit cross-section.

Using such a design for the plasma spray gun, the described burner nozzle electrode pairing ensures that the injected powder particles are melted with a very short flame length and thus flight path. Not only is the flame length shortened but the plasma flame is elliptically shaped as well, which leads both to an increase in the geometrical spray efficiency based on the spray jet diameter as well as to an equallized thickness of the sprayed layer during each spraying passage.

The electrode has advantageously two diametrically opposed bevellings on its semispherical head.

Advantageously the burner nozzle is expanded conically from its minimal inner diameter away from the electrode into an exit area having an inner annular surface of larger inner diameter.

The longitudinal axis of the flat exit cross-section of said powder injector is expediently arranged perpendicular to the connecting line between the bevellings of said electrode.

In order to optimize the heat discharge from the plasma spray gun and thus both to maintain the required spray layer quality by means of constant burner output as well as to increase the service life of the burner components, the electrode and the burner nozzles are expediently cooled by two separate water circuits.

To support this effect in addition a nozzle ring can provide for surface cooling and for blow-out of spray dust via an annular gas protective sleeve. As an alternative a separate lead can be provided via which a gas cooling and blow-out of the spray dust is effected directly at the burner nozzle. With such a design of the plasma spray gun there is an additional discharge of the reflected spray dust from the bore surface to be coated, which leads to a higher quality of the coating.

Further the burner advantageously consists of a stable cast portion with all the elements which are not subject to wear and tear and a portion capable of being opened which carries the elements subjected to wear including the electrode, the burner nozzle and the powder injector for easy replacement. All the components which are naturally subjected to attrition during the operation of the gun can thus be easily and simply exchanged.

The portion capable of being opened has advantageously two foldable semi-shells which are separated by an insulating plate.

For a further increase in the service life of the replaceable burner nozzle, the latter is sealed by O-rings against its cooling channel and the seat of said O-rings is designed so that they abut at the most on only one of four sealing surfaces directly on the burner nozzle and abut at least two of the four sealing surfaces on cooled components which are good heat conductors. Further channels for direct coolant access from the cooling channel to said O-rings are advantageously provided.

Using the plasma spray gun according to the invention, the distribution and melting on of the injected powder particles are performed in a broad coating spot whereby the substrate material, despite the small spray spacing, can be coated without excessive thermal stresses which is especially important in the case of thin-walled tubes. The additional gas cooling supports this effect.

The invention is explained in more detail below by the embodiments and with reference to the drawings.

FIG. 1 is a longitudinal section through an embodiment of an invented plasma spray gun for inner coatings;

FIG. 2 is an enlarged partial cut-out of the burner head in FIG. 1 shown schematically;

FIG. 3 is a schematic side sectional view of the electrode and burner nozzle of said plasma spray gun;

FIG. 4 is a schematic frontal view of the arrangement in FIG. 3;

FIG. 5 is a schematic illustration of the coating efficiency and layer thickness distribution in the static spray diagram in the case of a rotation symmetrical burner nozzle electrode configuration;

FIG. 6 is a schematic illustration of the coating efficiency and layer thickness distribution in the static spray diagram with a burner nozzle electrode configuration according to the invention,

FIG. 7 is a schematic illustration of the burner nozzle holder and sealing thereof;

FIG. 8 is an example of the supply by two separate coolant water circuits;

FIG. 9 is a schematic illustration of a turbine disc with turbine blade and internally coated holder groove.

The plasma spray gun 1 for internal coatings shown in FIGS. 1 and 2 has a stable cast portion 2 with all the elements which are not subject to wear, and an openable portion 3. The latter portion 3 consists of a cathode semi-shell 4 and an anode semi-shell 5 which are separated by an insulating plate 6, designed to be folded up, and held together by a clamp 7. On the stably cast portion 2 there is a nozzle ring 8 with nozzle apertures 9, via which a gas protective sleeve can be produced around the plasma spray gun for surface cooling and for the blow-out of the spray dust. Instead of this nozzle ring 8 or additionally thereto, a separate lead 31 can be guided directly into the area of the burner nozzle.

In the cathode semi-shell 4 an electrode 10 is secured so as to be easily exchangeable. An insulating and replaceable gas distribution ring 11 is inserted in the insulating plate 6. In the anode semi-shell 5 a burner nozzle 12 which is fixed with an extension lash is inserted to be easily replaceable. A powder injector 13 with a flat exit cross-section is also inserted so as to be easily replaceable in said anode semi-shell 5.

In the cathode semi-shell 4 there is a cooling channel 14 for the cooling of the electrode 10 while anode semi-shell 5 has a cooling channel 14 to cool the burner nozzle 12. Both cooling channels are charged in parallel with coolant, for example water, gas or liquid carbon dioxide.

Portion 2 represents the burner shaft, portion 3 the burner head. After release of the clamp 7 the cathode semi-shell 4 and the anode semi-shell 5 are folded away from each other in order to provide access to the gas distribution ring 11 optionally for its replacement together with the insulating ring 6. Electrode 10 has a semi-spherical head 15 with diametrically opposed bevellings 16. The diameter of electrode 10 is smaller than the minimal diameter of the burner nozzle 12. This nozzle 12 is conically expanded proceeding from its minimal inner diameter away from electrode 10 into an exit area with an inner ring surface 17 of larger inner diameter.

On the bevellings 16 the electric arc 18 formed between electrode 10 and burner nozzle 12 is suppressed and is concentrated on the undisturbed spherical surface of the head 15. This causes a plasma flame 19 which is pressed flat. Due to the conical expansion of the burner nozzle 12 towards the inner ring surface 17, the length of the plasma flame 19 is substantially shortened. The flat outlet cross-section of the powder injector 13 ensures that the powder injection corresponds to the flattened plasma flame 19.

FIG. 5 shows schematically the coating efficiency distributed over the plasma jet cross-section, taken by means of a static spray diagram on a substrate layer and the corresponding layer thickness in the case of a conventional rotation-symmetrical electrode-burner nozzle configuration. In a zone I of the spray jet the result is high coating efficiency with a practically constant growth rate per coating unit of time, in a zone II there is strongly decreasing coating efficiency as spacing from the centre increases and in a zone III there is almost no connecting spray layer any longer. The zones I and II are defined by concentric circles.

FIG. 6 shows the coating efficiency and layer thickness distribution for an inventive rotation-asymmetrical electrode burner nozzle configuration. The zones I and II are strongly bevelled elliptically, while the width of zone II is very small. The layer thickness within zone I is practically constant and drops off in zone II over its small width to zero. This produces a strong increase in the geometrical efficiency based on the spray jet diameter.

FIG. 7 shows that the burner nozzle 12 is sealed by two O-rings 21, 22 against its associated cooling channel 20. Both the O-rings 21, 22 abut respectively only one of the four sealing surfaces on the burner nozzle 12. A second sealing surface of the O-rings 21, 22 is formed for their thermal protection on the insulating plate 6 or on the insulating body 23, whereas the O-rings 21, 22 abut on their two other sealing surfaces the good thermally conducting components which are cooled by cooling channel 20. From cooling channel 20 additional channels 24, 25 are provided for direct access by the coolant to the O-rings 21, 22. This provides especially good heat protection for the endangered O-rings 21,22.

FIG. 8 shows the leads to the plasma spray gun 1. via a water inlet 26 coolant is supplied parallel to the cooling channels 14 and 20 and is again removed via a water outlet 27. On water inlet 26 the plus pole is connected and the minus pole is connected to water outlet 27. Insulating pipes 28 are provided in the ducts for the corresponding insulation of the coolant circuits from the electrical leads. Plasma gas is supplied via a connection 29 and spray powder via a connection 30. Air or gas can be supplied in the area of the gun via an additional lead 31.

FIG. 9 shows a preferred field of application for the inventive plasma spray gun. In holder grooves 32 of a turbine disc 33 the blade bases 34 of turbine blades 35 are inserted. Coatings 36 are provided using the invented plasma spray gun on the contact surfaces of the blade base 34 and the holder groove 32. It is the object of the coatings 36 to prevent frictional wear, frictional welding and/or dimensional variation of the walls of the grooves in the area of the turbine. These stresses on the holder groove 32 are caused by the necessary installation not free being from play of the turbine blades 35 in the holder grooves 32. These stresses occur above all when starting up and stopping the turbine. They are also relatively large because of the weight of the titanium of titanium alloys that are employed.

For the coating for example a CuNiIn spray layer can be used. The coatings 36 are applied flat and broad-tracked in 3 segments, advantageously each applied in one burner passage.

Below the individual performance and spray data are given as examples of the use of a machine burner according to the prior art, an inner burner according the prior art and an inventively designed inner burner:

Spray powder:

NiAl 95/5%

particle size range: -325 mesh

grain configuration: Ni-spheres with externally superimposed Al-particles.

plasma flame: Ar/H.sub.2 mixture.

Coating parameters for densely sprayed strongly adhesive plasma spray layer:

A. Machine burner according to the prior art:

B. Inner burner according to prior art:

C. Inner burner designed according to the invention: