US 3140380 A
Description (OCR text may contain errors)
July 7, 1964 G. A. JENSEN DEVICE FOR COATING SUBSTRATES Filed Sept. 8. 1961 JENSEN INVENTOR.
ATTORNEYS GERALD A United States Patent DEVICE FOR COATING SUBSTRATES Gerald A. Jensen, Lowell, Mass., assignor to Avco Corporation, Cincinnati, Ohio, acorporation of Delaware Filed Sept. 8, 1961, Ser. No. 136,812 11 Claims. (Cl. 219-76) This invention relates to devices for coating substrates and, in particular, to are plasma spray-coating devices.
An electrical arc is a source of extremely high heat energy. A fluid, when interacted with an electric arc, ab-
sorbs heat from the arc and partially dissociates into free electrons and ions. The partially dissociated fluid is electrically neutral and is generally called a plasma. Eventually, when the plasma leaves the vicinity of the arc, the electrons and ions tend to recombine. During the recombination process, heat is give up to the surrounding environment. The plasma thus acts as a source of heat energy.
An arc plasma spray-coating device of the type being considered in this discussion is described in the Yenni Patent 2,982,845. It will be noted that a rod of coating material is fed laterally into a plasma eflluent generated by an arc plasma generating device A. This approach has at least two limitations. There is a tendency for the rod to melt unevenly and to produce large particles of coating material. A poor coating inevitably results. Large particles typically are not heated sufficiently or accelerated to a high enough velocity to produce a suitable bond with a substrate; additionally, particle deposition velocity will also be low since:
(1) The rod impedes the flow of the efiiuent; and
(2) Energy must be expended in changing the lateral movement of rod material to a generally axial direction.
As may also be expected, the rod deflects the plasma efiiuent so that rod particles have a lateral velocity component when leaving the plasma generator and therefore, tend to disperse quickly. g
A second suggested configuration for an arc plasma spray-coating device calls for the rod of coating material to be fed through a hollow center electrode-a hollow version of electrode 10 in FIGURE 1 of the Yenni patent,
for example-directly through the arc.
Experience has shown that the arc spot should remain stationary on the center electrode, generally the cathode electrode, for long life. On a ring electrode, such as would result if the rod material were fed through the center of the center electrode, there is a tendency for the arc to move around. This type of operation increases the erosion rate of the center electrode.- Additionally, the
alternate heating and cooling of portions of the center electrode ring causes severe heat stresses. In many cases,
.these stresses have been suflicient to cause the electrode to crack or disintegrate.
An additional important limitation of feeding coating material through the arc is the tendency for this material to cause instability in the arc. Nozzle openings of arc plasma generating devices are small, 4" diameter being relatively large. It would be very difficult to a spray a A rod with a A" nozzle opening. Some of the rod material would adhere to the nozzle and build up to a point where the flow of. the plasma efiluent would be completely blocked. At the same time as the throat area 3,140,380 Patented July 7, 1964 level required to operate the arc at a given power, it is desirable to have operating voltages as high as possible which means operating at higher are chamber pressures. When the throat'diameter is increased, the only way that the arc chamber pressure can be kept at the desired value is to increase the rate of mass flow of working fluid. Manifestly, this is costly and inefiicient.
There are many advantages to spraying large rods, and /2 diameter rods, for example. Manifestly, the problems of maintaining proper are parameters with reasonable fluid mass flow increase sharply in severity.
It is an object of the invention to provide a device for coating substrates which avoids the limitation and disadvantages of prior art devices.
It is another object of the invention to provide an arc plasma spray-coating device for coating substrates which comprises two or more plasma effluents merging into a joint plasma eflluent into which a coating material is fed and reduced to substantially molten particles and deposited on a substrate. 1
It is still another object of the invention to provide an arc plasma spray-coating device which is reliable, operates with low fluid mass flow, and does not affect the stability of the plasma generating means.
In accordance with the invention, a device for coating substrates comprises a plurality of separate arc devices for generating a plurality of plasma effluents. The plasma effluents are directed to intercept at a point and merge to form a joint plasma eflluent.
The are plasma spray-coating device also includes means for feeding a rod of coating material into the joint plasma efl'luent.
In addition, provision may be incorporated in arc plasma spray-coating device for supplying a secondary high velocity fluid stream for accelerating the coating particles from the plasma effluent to a desired deposition velocity.
The novel features that are considered characterstic of the invention are set forth in the appended claims; the invention itself, however, both as to its organization and method of operation, together with additional objects and advantages thereof, will best be understood from the following description of a specific embodiment when read in conjunction with the accompanying figure which is a plasma spray-coating device embodying the principles of the present invention.
There is represented in this figure a device for coating substrates and, more particular, an arc plasma spray-coating device 10. The are plasma spray-coating device 10 includes a first plasma generating means 11 and a second plasma generating means 12. Plasma. generators 11 and 12 are preferably identical in construction and performance and are angularly displaced symmetrically with relation to an axis 13. I
Plasma generator 11 includes a center electrode 14 and a nozzle electrode 16. The electrode 14 and the electrode 16 are disposed. in a coaxial relationship. Electrodes 14 and 16 are connected to an electric power supply 18 as indicated. When plasma generator 11 is energized, an
are 19 is generated across electrodes 14 and'16. A fluid,
nected to an external gas supply means (not shown).
Such gas may be any suitable arc gas such as argon, nitrogen, hydrogen, or helium.
The gas flowing through the are 19 absorbs heat from the are 19 and is converted into a plasma effluent 17 which leaves the plasma generator 11 by way of the nozzle electrode 16.
As is obvious from FIGURE 1, plasma generator 12 1 =19 is identical to plasma generator 11. It includes a center electrode 23, a nozzle electrode 24, an arc 26 and a gas supply passage 27. In a manner similar to that indicated above, plasma generator 12 produces a plasma effiuent 28. Plasma effluent 28 merges with plasma efiluent 17 to form a joint plasma efiiuent 29.
The are plasma spray-coating device also includes means for supplying coating material to the joint plasma effluent 29 where such material is heated into minute molten or plastic particles 22 and accelerated to a substrate surface (not shown). A passage 39 which is coaxial with axis 13 and midway between the plasma generators 11 and 12 is provided in the arc plasma spray-coating device 10. A rod 34 formed from a desired coating material is inserted in the passage with one end projecting into the joint plasma efiiuent 29 at or near the junction of plasma effluents 17 and 28. The rod 34 is continuously fed by a feed mechanism shown schematically as rollers 36. Manifestly, passage 30 or a modification thereof may be used to provide coating material in the form of particles.
The symmetrical relationship shown in FIGURE 1 between plasma generators 11 and 12 and rod 34, in addition to locating the end of rod 34 at or near the junction of the plasma efiluents l7 and 28 generate a pointed end from which a coating material is uniformly removed from the rod 34. In FIGURE 1 two plasma generators are shown and accordingly, the point on rod 34 takes the form of a chisel point. Where three or more plasma generators are used, it is clear that the point will approach a conical shape.
It will be noted that the coating material does not interfere with the operation of the plasma generators 11 and 12. Their operating parameters such as voltage, current mass flow rate, etc. are determined independently of the material being used to coat a substrate. It will also be noted the deposition of coating material in the arc plasma spray device 10 may be minimized since its exit aperture can be designed independently of the plasma generators 11 and 12 to avoid coating deposition within the arc plasma spray-coating device 10. Theparticles 22 leave rod 34 in substantially the same direction as the direction of flow of the plasma effiuent 29. Higher particle velocities are thus achieved.
In the event it is desired to provide an independent means for accelerating the particles 22, the arc plasma spray-coating device 10 includes means, in the form of passages 31 and 32, through which a blast of air or other suitable fluid may be directed into the joint plasma efiiuent 29 to accelerate the particles 22. The aforementioned accelerating means may also be used to prevent or minimize dispersion of the joint plasma effluent 29.
A coating material in the form of a rod is preferred over powdered coating material, where a large surface is to be coated in a short time; i.e., where high deposition rates are desired. To transform a rod into sprayable particles requires a large amount of heat, much more than is required to spray powdered coating material. The heat must also be applied uniformly for the reasons discussed heretofore. The use of multiple plasma generating devices and merging their plasma efiluents meets both the requirement of high heat and uniform heat distribution uniquely.
Obviously, since the rod is set centrally into the joint plasma efliuent, heating over the entire surface of the rod is achieved.
Additionally, the use of the single high-heat source, a large arc plasma generator for example, is avoided. A single source is less efficient, more costly and requires more frequent replacement of components than a plurality of arc plasma generating units having an aggregate heat output equaling the output of a single source.
The various features and advantages of the invention are thought to be clear from the foregoing description. Various other features and advantages not specifically enumerated will undoubtedly occur to those versed in the art, as likewise will many variations and modifications of the preferred embodiment illustrated, all of which may be achieved without departing from the spirit and scope of the invention as defined by the following claims.
1. A device for coating substrates comprising: means for generating a first plasma efiluent; means for generating a second plasma effluent which merges with said first plasma effluent to form a joint plasma effiuent; and means for supplying coating material to the joint plasma effiuent.
2. A device for coating substrates as defined in claim 1 which includes, in addition, means for supplying a high velocity fluid stream to the joint plasma efiluent for accelerating the emission of said coating material from said joint plasma effiuent.
3. A device for coating substrates as defined in claim. 1 in which said means for generating said first and second plasma eflluent comprise arc plasma generating means.
4. A device for coating substrates comprising: a plurality of angularly placed means for generating a plurality of merging plasma effluents for forming a joint plasma effluent; and means for supplying coating material into said joint plasma eflluent.
5. A device for coating substrates as defined in claim 4 which includes in addition means for supplying a highvelocity fluid stream to said joint plasma effiuent for accelerating the emission of said coating material from said joint plasma effluent.
6. A device for coating substrates comprising: a pair of angularly displaced co-planar means for generating a pair of co-planar merging plasma effluents for forming a joint plasma effiuent; and means for feeding, in the plane defined by said pair of plasma eflluents, a rod of coating material to said joint plasma efiluent.
7. A device for coating substrates as defined in claim 6 in which said rod bisects the angle separating said generating means.
8. A device for coating substrates as defined in claim 6 in which the means for generating plasma comprises are plasma generating means.
9. A device for coating a substrate which uses coating material in the form of a rod comprising: nozzle means including an exit orifice aligned with a central axis; a plurality of arc plasma generating means in an angularly spaced relationship with respect to the central axis for generating a plurality of plasma effiuents directed toward the exit orifice and merging adjacent to the exit orifice into a joint plasma effinent emerging from the exit orifice; and means for feeding the rod to the exit orifice and into the joint plasma effluent.
10. A device for coating a substrate which uses coating materials in the form of a rod comprising: nozzle means including an exit orifice aligned with a central axis; a plurality of arc plasma generating means in an angularly spaced relationship with respect to the central axis for generating a plurality of merging plasma effiuents to form a joint plasma efiluent emerging from the exit orifice; means including a second passage aligned with the central axis through which the rod is insertable; and means for continuously feeding the rod to the exit orifice and the joint plasma effluent at a predetermined rate to generate sprayable particles.
11. A device for coating a substrate as defined in claim 10 which includes means for supplying a high velocity stream of fluid to the joint plasma efiluent for accelerating the sprayable particles.
References Cited in the file of this patent UNITED STATES PATENTS 1,037,979 Perkins Sept. 10, 1912 1,243,795 Apple Oct. 23, 1917 2,754,225 Gfeller July 10, 1956 2,806,124 Gage Sept. 10, 1957 2,982,845 Yenni et al. May 2, 1961