Field of the Disclosure
[0001] The present disclosure generally relates to a plasma spray coating apparatus, such
as a microplasma spray coating apparatus, for spray coating a workpiece and a method
for using the same.
Background of the Disclosure
[0002] Plasma coating methods and apparatus are known. For example, one such method and
apparatus for plasma flame spray coating material onto a substrate by means of passing
a plasma forming gas through a nozzle electrode, and passing an arc forming current
between the nozzle electrode and a rear electrode to form a plasma effluent. The method
includes introducing coating material into the plasma effluent, passing the plasma
effluent axially through a wall shroud extending from the exit of said nozzle electrode,
and forming a flame shroud for the plasma effluent. The coating is thereby applied
to the substrate.
[0003] One area where such technology is particularly advantageous is in connection with
coating various components, particularly aerospace components like gas turbine engines
and their components. For example, the blade roots of compressor blades can be coated
with material to meet dimensional tolerance requirements for sealing the compressor
blade with the compressor wheel and the like. Metallic coatings consisting of copper-nickel,
aluminum-copper, and other similar composition materials have been applied in this
regard using various conventional plasma spray coating processes. Typically, the coating
process requires the workpiece to be masked in areas where the material transfer is
not required and/or not desired. Furthermore, the workpiece is typically coated in
a dedicated facility such as a gas turbine engine manufacturing plant or repair shop.
Prior art methods and apparatus required masking the workpiece and applying the coating
in dedicated facilities because the coating equipment was large and not portable and
the spray pattern was too wide to accurately control the coating process. It would
be desirable to improve the accuracy of spray coating devices so that masking and
the like would not be required, as well as permitting hand spray coating repairs in
the field.
Summary of the Disclosure
[0004] In accordance with one aspect of the disclosure, a plasma spray apparatus for coating
at least a portion of a workpiece such as a gas turbine compressor blade is provided.
A plasma apparatus includes an anode, cathode, and an arc generator for generating
an electric arc between the anode and cathode. The apparatus includes an arc gas emitter
for injecting gas into the electric arc. The electric arc is operable for ionizing
the gas to create a plasma gas stream. A powder feeder provides powdered material
to the plasma apparatus. A powder injector nozzle is connected to the powder feeder
via a conduit. The powder injector nozzle extends through the anode and is operable
for injecting powdered material into the plasma gas stream.
[0005] In accordance with another aspect of the disclosure, a plasma spray apparatus for
coating a portion of a workpiece such as a gas turbine compressor blade is provided.
A plasma apparatus includes an anode, cathode, and an arc generator for generating
an electric arc between the anode and cathode. The apparatus includes an arc gas emitter
for injecting gas into the electric arc. The electric arc is operable for ionizing
the gas to create a plasma gas stream. A powder feeder provides powdered material
to the plasma apparatus. An electrode extending from a cathode housing and terminating
at a tip includes a substantially circular cross section along at least a portion
of a lengthwise axis. An angled surface extending from the tip toward the cathode
housing is formed on the electrode. A substantially flat edge having a predetermined
height defines a forward edge of the tip.
[0006] In accordance with another aspect of the present disclosure, a method for injecting
powdered material into a plasma gas stream is provided. A powder injector nozzle is
positioned through an anode in a plasma apparatus. Powdered material is transported
from a powder hopper to the powder injector nozzle. The powdered material is injected
into the plasma gas stream prior to being applied to a workpiece.
[0007] Other applications of the present invention will become apparent to those skilled
in the art when the following description of a preferred embodiment of the invention
is read in conjunction with the accompanying drawings.
Brief Description of the Drawings
[0008]
Fig. 1 is a schematic representing one embodiment of a microplasma spray apparatus
and a workpiece of the present disclosure;
Fig. 2 is an exploded, perspective view of one embodiment of a microplasma spray apparatus
constructed in accordance with the teachings of the disclosure;
Fig. 3 is an enlarged view of an electrode depicted in Fig. 2.
Fig. 4 is an assembled perspective view of the microplasma spray apparatus of Fig.
1, applying a coating to a workpiece; and
Fig. 5 is a flowchart describing one embodiment of a process for plasma spray coating
a workpiece.
While the following disclosure is susceptible to various modifications and alternative
constructions, certain illustrative embodiments thereof have been shown in the drawings
and will be described below in detail. It should be understood, however, that there
is no intention to limit the disclosure to the specific forms disclosed, but on the
contrary, the intention is to cover all modifications, alternative constructions,
and equivalents falling within the spirit and scope of the disclosure as defined by
the appended claims.
Detailed Description of the Disclosure
[0009] Referring now to Fig. 1, one embodiment of a plasma spray apparatus 10 schematically
represented by the dashed box outline is depicted. In generalized terms, the plasma
spray apparatus 10 includes a plasma gun 12 having an arc gas emitter 14, an anode
16, and a cathode 18. An electric arc 20 is generated between the anode 16 and cathode
18. A plasma stream 21 is formed when arc gas is injected from the arc gas emitter
14 and passes through the arc 20. A powdered material injector 22 dispenses powdered
material into the plasma stream which transports the powdered material to the workpiece
24 to form a coating thereon. The size of the plasma stream 21 created by the device
and/or the power used by the device determines whether the device is considered a
microplasma spray apparatus. When the plasma stream 21 is small and/or the power used
by the device is low, the device is considered a microplasma spray device. Fig. 1
displays a microplasma spray device.
[0010] For example, the powdered material can form a solid coating with a thickness of approximately
0.0015 to 0.006 inches (0.038 mm to 0.152 mm) in a desired location on the workpiece
24. The coating material may be virtually any metallic, non-metallic or intermetallic
powder, including the materials described above and ceramic-based materials.
[0011] In operation, an electric arc 20 is generated between the anode 16 and cathode 18
of the plasma gun 12. Arc gas such as, but not limited to argon, is emitted into the
electric arc 20 formed between the anode 16 and the cathode 18. It should be understood
that in practice the arc gas can be emitted prior to generating the electric arc.
The electric arc 20 ionizes the gas to create the plasma gas stream 21. The ionization
process removes electrons from the arc gas, causing the arc gas to become temporarily
unstable. The arc gas heats up to approximately 20,000°F to 30,000°F (11097°C to 16426°C)
as it re-stabilizes. The plasma stream cools rapidly after passing through the electric
arc.
[0012] While a number of different embodiments and structural variations can be constructed
to practice such an invention, the following describes one possible embodiment. Referring
now to Fig. 2, an exploded view of such a plasma spray apparatus is again referred
to by reference numeral 10. As will be described in detail below, the plasma spray
apparatus 10 is operable for coating a workpiece, including, but not limited to at
least a portion of a compressor blade 72 in a gas turbine engine (not shown). However,
it is to be understood that the teachings of disclosure can be used to coat myriad
other surfaces, including those on aircraft, land-based vehicles, weapons, sea-faring
vessels and the like.
[0013] In the depicted embodiment, the plasma spray apparatus 10 includes the aforementioned
plasma gun 12 having an anode 16 and a cathode 18. The cathode 18 is further depicted
to include an insulated body 26 with an electrode 28 extending therefrom. The cathode
18 can also include threads 30 for threadingly engaging the plasma gun 12. The cathode
18 can also include an O-ring seal 32 to seal the leak path that is created at the
interface between the cathode 18 and the plasma gun 12.
[0014] A powdered material injector 22 injects powdered material 34 into the plasma gas
stream 21. The powdered material 34 is heated and super plasticized in the plasma
stream 21 and is deposited on the compressor blade 72 (see Fig. 4) where it cools
and re-solidifies to form the coating. The powdered material injector 22 includes
a powder hopper 36 for holding and feeding the powdered material 34 into the plasma
stream 21. The hopper 36 can be connected to the plasma gun 12 through a conduit 38
such as a flexible hose or the like. The conduit 38 can be connected via a threaded
fitting 39 to a powder injector nozzle 40. The powder injector nozzle 40 can extend
through an aperture 42 formed in the anode 16. The powder injector nozzle 40 can threadingly
connect to the anode 16 via threads 43.
[0015] Conventional anodes are typically formed from a copper-tungsten alloy and provide
very limited service life of approximately 10 to 20 minutes in a plasma spray apparatus
10. Copper and other similar metals have melting temperatures that are lower than
the anode operating temperature. These metals can melt and cause the edge of the anode
16 to become molten and initiate cavitation erosion along an upper edge of the anode.
In order to produce high quality coatings, the edge of the anode must remain relatively
sharp. To achieve this, a commercially pure sintered tungsten material has been developed
to produce a more robust anode. Test results using anodes made from sintered tungsten
material has shown marked improvements in the erosion resistance over prior art anodes.
Utilizing commercially pure tungsten in the anode 16 has increased the service life
of the anode 16 to approximately between 10 and 20 hours.
[0016] A nozzle shroud 46 positioned on a forward wall 48 of the plasma gun 12 holds a nozzle
insert 50 and permits the electrode 28 to extend through a center aperture 52 formed
in the nozzle shroud 46. The nozzle insert 50 can be threadingly attached to an end
of the nozzle shroud 46. A shield gas cap 54 is positioned over the nozzle shroud
46. An insulator 56 is positioned between the shield gas cap 54 and the nozzle shroud
46 to electrically isolate the shield gas cap 54 from the nozzle shroud 46. The shield
gas cap 54 can be pressed fit onto the nozzle shroud 46 and over the insulator 56.
The shield gas cap 54 includes a plurality of through apertures 58 for permitting
shield gas to flow therethrough and shield the arc gas from ambient atmosphere. A
center aperture 60 formed in the shield gas cap 54 permits high velocity arc gas to
pass through and into the electric arc.
[0017] Cooling fluid, such as water or the like, can be utilized to cool the plasma gun
12. The cooling fluid is delivered to the plasma gun 12 via a cooling fluid hose 62.
The cooling fluid traverses through internal passages (not shown) in the plasma gun
12 and flows through an inlet passage 64, into an anode holder 66 and back through
an outlet passage 68. The cooling fluid reduces the temperature of the anode 16 during
operation of the plasma gun 12. The cooling flow rate may be approximately 0.1 to
1.0 gallons per minute. A second conduit 70 can be connected to the plasma gun 12.
The second conduit may be operable for providing electrical power, arc gas, and/or
shield gas to the plasma gun 12.
[0018] Referring now to Fig. 3, the electrode 28 of the cathode 18 is shown in an enlarged
view. The electrode 28 can have a circular cross-section, for example, of approximately
1/16
th inch (1.59 mm) in diameter, although other dimensions are certainly possible. The
electrode 28 can include a tip 65 that is tapered, for example, by machining at an
angle A to form a substantially flat upper surface 67. The angle A can range between
0 and 90 degrees, but in one embodiment the angle A ranges between approximately 8
and 10 degrees. A distal end of the tip 65 can then be machined flat to a desired
height B. In one embodiment the height B can range from .008 to .010 inches (0.2 mm
to 0.25 mm). For variably sized electrodes, the height B can be defined as approximately
between 10% and 20% of a diameter or a width of the electrode. The electrode can be
formed from any electrically conductive material such as a copper alloy, but has been
found to be advantageously formed from thoriated tungsten.
[0019] Referring now to Fig. 4, it is shown that a localized area of the compressor blade
72, such as a blade root 74, can be spray coated with powdered material 34. The plasma
gas stream 21 is directed toward the portion of the compressor blade 72 to be coated.
The plasma gun 12 is operated at a relatively low power range of between approximately
0.5 Kilowatts and 4 Kilowatts. The low power output of the plasma gun 12 significantly
reduces the heat flow into the compressor blade 72 over that of conventional coating
methods. The maximum surface temperature of the compressor blade 72 caused by the
coating process is approximately 200°F (93°C). Such low power output and resulting
low temperature on blade 72 allows the plasma gun 12 to apply powdered material 34
to a thin wall area of the compressor blade 72 without distorting the compressor blade
72 because the localized stresses caused by high thermal gradients do not exist.
[0020] The plasma gun 12 can apply coating material in narrow strips of, for example, about
0.5 to about 5 mm in width. This permits accurate surface coating even with a hand
held device. The narrow strips of coating substantially eliminate the need for masking
or otherwise covering the compressor blade.72 in areas where the coating is unwanted.
The narrow spray pattern is controlled by the nozzle opening size. The hand held version
of the plasma gun 12 can spray coatings on components even while they remain in an
installed condition, such as in an engine or the like.
[0021] The arc gas flow rate of the plasma apparatus 10 may be between approximately0.5
and 3 liters per minute, although other rates are certain possible.
As stated above, the arc gas and shield gas are typically argon, but any suitable
inert gas can be utilized as is known to those skilled in the art. The shield gas
flow rate could range between approximately 2 and 8 liters per minute for a typical
application.
[0022] The powder hopper 36 holds the powdered material 34 prior to being injected into
the plasma gas stream 21 by the powder injector 22. Powdered material 34 can be transferred
to the workpiece from between approximately 1 to 30 grams per minute. The plasma gun
12 can typically apply the coating from distances ranging from approximately 1.5 inches
to 8 inches (38 mmto 203 mm) to the workpiece, but can vary depending on the coating
application requirements. The plasma spray gun 12 provides unlimited angles of orientation
relative to the workpiece because the pressurized powder feed system uses carrier
gas to entrain and deliver the powdered material 34 to the plasma stream 21 and does
not rely on gravitation as prior art systems did.
[0023] Compressed carrier gas, such as an inert gas, flows through the powder injector 22.
Powdered material 34 can be entrained with the carrier gas as is known to those skilled
in the art. The carrier gas will flow through the powder injector 22 at any angle
of orientation and thus does not rely on gravitational forces to deliver powdered
material 34 to the plasma stream 21. The plasma stream 21 provides a venturi effect
with respect to the powder injector 22. The high velocity flow rate of the plasma
stream 21 across the powder injector 22 generates a low pressure region which augments
the flow rate of the carrier gas and the powdered material 34 through the powder injector
22.
[0024] The plasma spray gun 12 generates a relatively low noise level that ranges from between
40 and 70 decibels due to the low power output, thereby making the apparatus 10 suitable
for hand held application. Current U.S. government regulations require hearing protection
when environmental noise reaches 85 decibels. The plasma spray apparatus 10 can be
hand held or alternatively held in a fixture (not shown) such as one that is computer
controlled.
[0025] In one embodiment, a residual amount of electric current is transmitted from the
anode 16 to the powder injector 22. This residual current can cause preheating of
the powdered material 34 to occur which facilitates softening of the powdered material
34 prior to entering the plasma stream 21.
[0026] Referring now to Fig. 5, a block diagram generally describing the operation of the
plasma spray apparatus 10 and the plasma spray coating process is illustrated. Initially,
at block 80, arc gas is emitted from the nozzle insert 50. An electric potential is
generated between the anode 16 and the cathode 18 of the plasma spray gun 12 and is
directed through the arc gas, as described in block 82. Arc gas is directed through
the electric potential to create the plasma stream 21. At block 84, powdered material
34 is injected into the plasma stream 21. At block 86, the plasma stream heats the
powdered material 34 to a "super plasticized" condition such that the powdered material
34 is malleable when it is applied to a workpiece. At block 88, the powdered material
34 is applied to an unmasked substrate. The powdered material 34 then cools and solidifies
as a hard coating on the substrate.
[0027] While the preceding text sets forth a detailed description of certain embodiments
of the invention, it should be understood that the legal scope of the invention is
defined by the claims set forth at the end of this patent. The detailed description
is to be construed as exemplary only and does not describe every possible embodiment
of the invention since describing every possible embodiment would be impractical,
if not impossible. Numerous alternative embodiments could be implemented, using either
current technology or technology developed after the filing date of this patent, which
would still fall within the scope of the claims defining the invention.
1. A plasma spray apparatus (10) for coating a workpiece, comprising:
an anode (16), a cathode (18), and an arc generator generating an electric arc (20)
between the anode (16) and cathode (18),
a nozzle to emit arc gas into the electric arc (20), the electric arc operable for
ionizing the gas to create a plasma gas stream (21); and
a feeder extending through the anode (16) to provide powdered material (34) into the
plasma gas stream (21).
2. The plasma spray apparatus of claim 1, wherein the cathode (18) includes an electrode
(28) having a first end extending from a cathode housing and a second end terminating
at a tip (65), the electrode operable for conducting electric current.
3. The plasma spray apparatus of claim 2, wherein the electrode (28) includes a substantially
circular cross section along at least a portion of a lengthwise axis.
4. The plasma spray apparatus of claim 2 or 3, wherein the electrode (28) is formed with
a desired surface angle (A) extending from the tip (65) toward the cathode housing.
5. The plasma spray apparatus of claim 4, wherein the angle (A) is approximately 10 degrees.
6. The plasma spray apparatus of any of claims 2 to 5, wherein the tip (65) includes
a substantially flat forward edge.
7. The plasma spray apparatus of claim 6, wherein the flat forward edge of the tip (65)
is formed at a desired height (B).
8. A plasma spray apparatus for coating a workpiece, comprising:
an anode (16), a cathode (18), and an arc generator to generate an electric arc (20)
between the anode (16) and cathode (18),
a nozzle to emit arc gas into the electric arc (20), the electric arc ionizing the
gas to create a plasma gas stream (21); and
a feeder providing powdered material (34) to the plasma gas stream (21);
wherein the cathode (18) includes an electrode (28) having a taper terminating at
a tip (65) the taper including a substantially flat edge having a predetermined height
(B) at the tip (65).
9. The plasma spray apparatus of claim 8, wherein the surface angle of the taper is approximately
between 8 and 10 degrees.
10. The plasma spray apparatus of any of claims 7 to 9, wherein the height (B) of the
edge is approximately between 10% and 20% of a width of the electrode (28).
11. The plasma spray apparatus of any preceding claim, wherein the plasma spray apparatus
(10) is a microplasma spray apparatus.
12. The plasma spray apparatus of any preceding claim, wherein the feeder uses a carrier
gas to entrain the powdered material (34) through the anode (16).
13. The plasma spray apparatus of any preceding claim, further including a powder hopper
(36) for holding the powdered material (34) prior to the powdered material being injected
into the plasma gas stream (21).
14. The plasma spray apparatus of claim 13, wherein the powder hopper (36) and feeder
are combined in one apparatus.
15. The plasma spray apparatus of any preceding claim, further including a powder injector
nozzle connected to the feeder, the powder injector nozzle extending through the anode
(16) and injecting powdered material into the plasma gas stream (21).
16. The plasma spray apparatus of any preceding claim, wherein the plasma apparatus operates
at a power range of between approximately 0.5 Kilowatts and 4 Kilowatts.
17. The plasma spray apparatus of any preceding claim, wherein a maximum surface temperature
of the workpiece caused by the coating process is approximately 200°F (93°C).
18. The plasma spray apparatus of any preceding claim, wherein the plasma apparatus applies
the coating material in widths of about 0.5 mm to about 5 mm to the workpiece.
19. The plasma spray apparatus of any preceding claim, further including a shield gas
cap (54) having shielding gas injected therethrough.
20. The plasma spray apparatus of any preceding claim, wherein the powdered material (34)
is a metal alloy.
21. The plasma spray apparatus of any of claims 1 to 19, wherein the powdered material
(34) is a ceramic based coating.
22. The plasma spray apparatus of any preceding claim, further including a cooling system
for cooling the plasma apparatus.
23. The plasma spray apparatus of any preceding claim, wherein the plasma apparatus is
operable for spray coating a workpiece at any angle of orientation.
24. The plasma spray apparatus of any preceding claim, wherein the plasma apparatus generates
a noise level of between approximately 40 and 70 decibels.
25. The plasma spray apparatus of any preceding claim, further including a cathode shroud
(46) surrounding a portion of the cathode (18).
26. The plasma spray apparatus of claim 25, wherein the nozzle is positioned in a receiving
aperture formed in the cathode shroud (46).
27. The plasma spray apparatus of claim 25 or 26, further including a shield gas cap (54)
substantially encompassing the cathode shroud (46), the shield gas cap operable for
providing shielding gas as a barrier between the arc gas and an ambient atmosphere.
28. The plasma spray apparatus of claim 45, further including a shield cap insulator (56)
positioned between the shield gas cap (54) and the cathode shroud (46).
29. The plasma spray apparatus of any preceding claim, wherein the anode (16) is formed
from a commercially pure tungsten material.
30. The plasma spray apparatus of any preceding claim, wherein the anode (16) is formed
from sintered tungsten material.
31. A method for injecting powdered material (34) into a plasma gas stream (21), comprising:
positioning a powder injector nozzle through an anode (16) of a plasma spray apparatus
(50),
transporting powdered material (34) from a powder hopper (36) to the powder injector
nozzle; and
injecting the powdered material (34) into the plasma gas stream (21).
32. The method of claim 31, further comprising:
entraining powdered material (34) with carrier gas flowing from a powder feeder through
the injector nozzle.
33. The method claim 31 or 32, further comprising:
preheating the powdered material (34) in the powder injector nozzle with electric
current running through the anode (16).
34. A plasma spray apparatus (10), comprising:
an anode (16), cathode (18) and an arc generator for generating an electric arc (20)
between the anode (16) and the cathode (18);
a nozzle for emitting arc gas into the electric arc (20), the electric arc (20) ionizing
the gas to create a plasma gas stream (21); and
a feeder for providing powdered material (34) to the plasma gas stream;
wherein the feeder provides the powdered material (34) to the plasma gas stream (21)
from a direction other than above the plasma gas stream (21).
35. The plasma spray apparatus of claim 34, wherein the direction is from beneath the
plasma gas stream.
36. A plasma spray apparatus, comprising:
an anode (16) formed from a commercially pure tungsten material;
a cathode (18) operationally coupled to the anode;
an arc generator for generating an electric arc (20) between the anode (16) and cathode
(18);
a nozzle for emitting arc gas into the electric arc (20), the electric arc (20) ionizing
the gas to create a plasma gas stream (21); and
a feeder for providing powdered material (34) to the plasma gas stream.
37. The plasma spray apparatus of claim 34, 35 or 36, wherein the plasma spray apparatus
is a microplasma spray apparatus.