Method Of Depositing Heat Fusible Material And Apparatus Therefor

Abstract

A plasma spray device is described which can be used to deposit heat fusible powdered material onto a substrate to form a continuous film. An electric arc is formed between two electrodes to provide a hot gas plasma which is projected through a nozzle. Powdered fluent material is added to the laminar flowing plasma and is carried on the surface of the plasma during the passage of the plasma through an enlarged nozzle, the powder converging at the tip of the plasma jet. The heat from the arc forms the hot plasma and the powdered material is melted so that the material coalesces to form a continuous film when it strikes the substrate.

Parent Case Text

This is a continuation-in-part of application, Ser. No. 581,594, filed Sept. 23, 1966, which was a continuation-in-part of application, Ser. No. 536,229, both abandoned.

Description

This invention relates in general to the deposition of heat fusible material on a substrate and the apparatus for so depositing and, more particularly, to a method and apparatus of so depositing using a plasma flame.

In modern day technology there is a continuing demand for surface treatments of various materials to provide a property to the surface that is not inherent in the base material. Typical of such desired surface characteristics are resistance to corrosion, coloring, smoothness to promote easier uniform flow of a fluid thereover, a parting agent for molding, and the like.

In the typical attempts to obtain coatings on substrates, pulverulent materials having the properties desired in the coating are somehow attached to the surface to be coated. This has been done chemically in which the material may be provided in a solvent, but more often than not, the pulverulent material has been attempted to be attached to the substrate by heat.

The desired effect is to have the coating firmly adhered to the substrate so as to resist cracking, chipping or spalling.

The present invention provides a method of coating a substrate by depositing a heat fusible material thereon in which there is first created a plasma in a laminar flowing gas in a generally cylindrical confined region, with the gas flowing into such region in laminar flow in a converging annular stream about an electrode so that there is created a laminar flowing plasma. This laminar flowing plasma is constricted during approximately the first half of the laminar flow in the region and then is abruptly expanded to produce a subregion of turbulence around the outer surface of the jet only, leaving the main stream of the plasma to continue to flow in a laminar manner.

At the turbulent subregion, fluidized heat fusible material is added to the jet and carried on its surface, converging at the tip of the jet, and being melted by the heat of the plasma within that distance and then applied to a workpiece to be coated. It is necessary to create the subregion of turbulence at the surface of the jet in order to pick up the powdered material. Otherwise, the laminar flowing plasma produces a pressure within the nozzle and prevents the entry of the powdered material. The abrupt expansion causes a pressure drop, drawing the fusible material into the nozzle and carrying it along on the surface of the laminar flowing plasma.

The invention further provides that the converging annular gas stream that flows around the electrode have a boundary in the form of two constricting cones, with the cone angle of the inner one being greater than the outer one and preferably in the range of 10.degree. to 30.degree. difference.

The invention also contemplates a plasma spray device in which there is an electrode assembly having a central electrode surrounded by a cooled housing through which gas passes to provide an annular blanket of gas over the electrode. Gas is admitted to the spray device upstream from the arc electrodes in an annular space which contains a collar having a plurality of spaced holes for creating a laminar flow through the remainder of the spray device. The passageway of the nozzle is generally cylindrical and has at least one fluidized particle inlet through the wall of the nozzle while having at the inlet end an inwardly finely tapered constricting portion that terminates abruptly in a large expansion portion.

The invention additionally calls for the introduction of means to introduce the heat fluidized fusible material into the passageway of the nozzle at a position adjacent the termination of the constricting portion thereof.

The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this specification, but for a better understanding of the invention, its operating advantages and specific objects obtained by its use, reference should be had to the detailed explanations of the preferred embodiment of the invention along with the illustrations in the accompanying drawings.

In the drawings:

FIG. 1 is a vertical section through a plasma spray device embodying the present invention; and

FIG. 2 is a fragmented partially schematic illustration of the angle relationship between the electrode and the nozzle of the present invention .

It has been discovered that if a plasma is created in laminar flowing gas i.e., flowing gas having a Reynold's number of less than 2,200, that the noise level and the power consumption by plasma flame spraying can be considerably reduced. This invention adds to that discovery by a further finding that the mixing of pulverulent material in the plasma flame, and the general stability of the flame is improved; and the power consumption is further reduced in a more stable flame when the plasma flame is slightly compressed for a portion of its length while maintaining laminar flow, if at the end of such constriction or compression there is an abrupt increase in flow area.

An example of a plasma spray device utilizing this principle is shown in Figure 1 wherein there is an electrode assembly 10 having a cooled central electrode 12 and an elongated generally cylindrical cooled electrode nozzle 14 having a longitudinal passageway 16 therethrough. A power supply means 18 is arranged to deliver power through the leads 20, 22 to the nozzle electrode and central electrode respectively.

A cylindrical wall 24 surrounds the central electrode 12 to form an annular gas chamber 26 to which a gas may be delivered by a line 28. At one end of the chamber 26 there is an annular gas distributor having a multiplicity of distributing holes 30 placed therein at an angle toward the central axis of the central electrode so that the centerlines of the holes 30 converge at a point along the central axis.

The passageway 16 has at its inlet end a generally conical recess 32 while the interior wall of the nozzle 14 that forms the passageway 16 has a converging or inwardly finely tapered constricting portion 16A running from the maximum diameter at the electrodes to the junction 16B in the center portion thereof (see Figure 1). The passageway 16 of the nozzle 14 abruptly increases in diameter at the point 16B to provide a larger generally uniform diameter passageway 16C for the outlet or expansion portion of the nozzle 14.

The central electrode 12 has a conically shaped end 12A which is positioned within the conical inlet 32 to the passageway 16 so that it is coaxial with the centerline of such passageway, and forms, in conjunction with the conical recessed portion 32, the boundary of an annular converging nozzle which encloses the gas stream flowing from the distributing holes 30. It should be noted that the cone angle of the conical electrode end 12A is greater than the cone angle of the plane of the nozzle inlet 32, i.e., as seen in Figure 2, angle A is greater than angle B.

At the center portion of the nozzle electrode 14 there is a plurality of powder inlets 34 to generally L-shaped passages 36, the outlet end of which opens into the longitudinal passageway 16 at the juncture point 16B of the constricting section 16A and the expansion portion 16C. Further, the centerlines of the outlets 36 are such that they are tangent to an imaginary concentric circle of a lesser diameter than the bounding walls of the passage 16C so that the fluidized powder is applied to the plasma stream at an angle to the nozzle axis and is thereby carried on the surface of the gas stream rather than being injected into the body of the moving gas. The powder, when applied in this manner rides on the surface of the jet and converges at the tip.

The operation of the spray device is as follows: a plasma producing gas, i.e., a gas which may be ionized, is introduced into the inlet chamber 26 by the conduit 28 under pressure. The gas is then directed inwardly in a generally converging annular stream by the converging distributing passages 30 whereupon the converging annular stream flows between the central electrode 12A and the nozzle conical recess 32 at which point the stream becomes a constricted converging annular stream of gas. Thereafter, this stream of gas flows along the passageway 16 at a velocity wherein its Reynold's number is in the laminar flow region. Due to the slight or finely tapered shape of the constricting portion 16A of the passageway 16, the laminar flowing gas has its velocity increased while being maintained in the laminar flow condition. The plasma is caused to be set up in the passageway 16 upon the application of power across electrodes 12 and 14 and forms, as indicated in Figure 1, a generally uniform cylindrical plasma having a slight annular enveloping layer of flowing gas.

Upon reaching the juncture point 16B wherein the nozzle flow area suddenly increases, the laminar flowing plasma continues, but in appearance has a gradual or narrow cone angle of conical shape. The abrupt change in cross section marks the point where the plasma flame begins to diminish in diameter and appears to create a stability position. Fluidized pulverized material which is heat fusible is introduced into passages 36 and due to the entrance angle of these passages, the pulverized material is forced into a revolving path as it is carried along the nozzle 16C. The powdered material is carried through the supply conduits (not shown) and the passages 36 by a compressed gas which should be an ionizable gas and preferably of the same type as introduced into chamber 26. The mixture of powdered material and gas entering at point 16B flows through the nozzle and forms a cone around the plasma flame as the material is carried to the workpiece (not shown).

In Figure 2 there is illustrated in simple form just those portions of the central electrode 12A and the nozzle to illustrate the relationship between the cone angle A of the central electrode 12 and the cone angle B of the recess 32 of the nozzle 14. Performance of the plasma spray device has indicated that the relationship between angle A and angle B greatly affects the length, the diameter, and the stability of the plasma flame within the passageway 16 of the nozzle. Moreover, satisfactory performance has always been obtained when the cone angle A was slightly greater than the cone angle B and stable flames which can be maintained within passageway 16 have only been obtained when the difference between cone angles A and B is within the range of 10.degree. to 30.degree. positive, that is, the difference of cone angle A is always greater than cone B by a difference in the range of 10.degree. to 30.degree.. Further, it appears important that the plasma producing gas as it annularly flows to and around the central electrode, should be directed by the distributing holes 30 at an angle which produces an annular converging conical stream in order to maintain a stable plasma flame within the passage 16. A stream so produced flows through the conical arc space and into passageway 16 in a straight converging manner. The preferred cone angle of the stream and therefore, of the distributing holes 30, is within the range of 19.degree. to 35.degree..

It is also important to note that the included angle between the recess surface 32 and the nozzle surface 14 should be in the range of 60.degree. to 120.degree. while the included conical angle A of the central electrode 12A should be in the range of 80.degree. to 140.degree..

For the sake of simplicity, there has not been illustrated the shape and configuration of cooling passages which are necessary to maintain the metal in the nozzle electrode 14 and the central electrode 12A in usable condition. It should be remembered that a plasma jet within the passage 16 may have a temperature in the range of 15,000.degree. to 20,000.degree. C. and that all metal parts in close proximity to such plasma must be protected by some form of cooling and the preferred way is water cooling which can be done in a known manner.

Because the heat flow from the plasma to the parts may be very high, it is necessary that such parts be made of metal having a high heat conductivity so that the heat that they receive may be rapidly dissipated into the coolant. On the other hand, it has been found that some may react with the plasma forming gas. For instance, aluminum reacts with nitrogen if that is the plasma forming gas, and copper may react catalytically with tetrafluoroethylenes, hereinafter designated as PTFE.

Although the plasma spray device described above may be used for applying powdered metals, ceramic powder, and various plastic compositions, to workpieces, the method and apparatus has been found particularly useful in applying PTFE powder to substrates of various compositions. In so doing, the workpiece need only be given ordinary cleaning treatment and not anything extremely special such as acid cleaning, sand blasting, and the like.

Accordingly, utilizing the method described above and establishing plasma within the passageway 16 wherein the central electrode has a conical angle of 120.degree. and the recess inlet 32 has a cone angle of 90.degree., while the diameter of the passageway 16 is initially 0.312 inches constricting to 0.296 inches at junction point 16B, ionizing gas having the composition of N.sub.2 /He ratio of 6 to 1 is caused to flow through the above device at a rate of 1.5 to 2.0 c.f.m. Through the particle distributing passages 34 there is caused to flow PTFE powder having a generally spherical shape wherein all of the particles have a major dimension in the range of 30 to 50 microns and flowing at a rate of 0.01 pounds of PTFE per c.f.m. of ionizing gas. The workpiece is placed 2 to 3 inches from the exit of the expansion passage 16C. The plasma flame spraying by such a device will build a coating of PTFE on a substrate at the rate of 1 mil per minute per 36 square inches of surface coated. Under these conditions the interior of the nozzle 14 bounding the passage 16 should be coated with a thickness of at least 0.00015 inches of an alloy very high in nickel, i.e., above 90 percent, whereas the main body of the nozzle 14 would be of copper. The coating of the interior nozzle 14 can be applied by any of the well-known processes including electro plating and vapor deposition.

In carrying out the process described above, the electrical consumption reduces from 400 amps with a straight longitudinal passageway 16, to 150 to 200 amps with the constricted passageway illustrated and claimed herein. Under these conditions, the voltage will then be in the range of 50 to 100 volts.

PTFE particles having a generally spherical size and having a major dimension in the range of 10 to 50 microns can be made by oven baking virgin powder in a known manner.

In selecting the proper combination of materials to be used in the spray device itself as well as the cone angles to the two electrodes, one must start with the characteristics of the heat fusible material to be sprayed and the ionizable gas to be used in the plasma, bearing in mind that at elevated temperatures chemical reactions can occur which may not under ordinary high-temperature conditions react.

The method and apparatus described herein produces coatings of heat fusible material on substrates which are characterized by their extreme tenaciousness and performance, while at the same time due to the utter simplicity of the device and method, being most economical.

While in accordance with the provisions of the statutes there has been illustrated and described herein a specific form of the invention now known, those skilled in the art will understand that changes may be made in the form of the method or the apparatus thereof without departing from the spirit of the invention covered by the claims and that certain features of the invention may some times be used to advantage without a corresponding use of other features.

Claims

What I claim is:

1. A plasma spray device comprising an electrode assembly including a conical central electrode and a conical recess electrode in axial alignment, means for forcing an ionizable gas over the surface of the conical electrode and through the conical recess electrode to form a laminar flowing plasma between the two electrodes, a hollow passageway associated with the recess electrode and haVing an open end spaced from the electrodes to form a nozzle for the expulsion of the laminar plasma, said hollow passageway also formed with an annular abrupt step between the electrodes and the outlet nozzle which increases the diameter of the passageway to thereby produce a subregion of turbulence in the laminar flowing plasma adjoining the inside surface of the cylindrical passageway, conduits for carrying fluent particles to the plasma, said conduits having their exit ports in the hollow passageway at its largest diameter adjoining said abrupt step and disposed so as to direct the particles onto the surface of the laminar flowing plasma, gas injection means around the conical central electrode which includes a plurality of conduits spaced around the said conical central electrode and angularly disposed so as to direct the gas to travel through the space between said conical central electrode and said conical recess electrode, the cone angle of the recessed electrode being smaller than the cone angle of the conical central electrode and the difference between the two angles being within the range of 10.degree. to 30.degree., and electrical means connected to the two electrodes for forming an electrical arc therebetween.

2. A device according to claim 1 in which the inside surface of said hollow passageway is an alloy of iron and nickel.

3. A device according to claim 1 in which said conduits spaced around the conical central electrode are formed in an insulated disc positioned around the support for the conical electrode.

4. A device according to claim 1 wherein the said central electrode has a conical angle of approximately 120.degree., and said recess electrode has a conical angle of approximately 90.degree., and said hollow passageway gradually diminishes from the electrode end to said annular step.

5. A method of depositing a heat fusible material on a substrate comprising creating a plasma in a laminar flowing gas in a generally cylindrical confined region by an electric discharge between two electrodes, injecting the gas into said region in laminar flow through an annular constriction of gradually changing dimensions adjoining said electrodes, passing the plasma through a constricted conduit while maintaining laminar flow, expanding the plasma at the end of said constricted conduit to produce a subregion of turbulence as a result of the sudden change in diameter of the conduit, said turbulence restricted to the surface of the plasma stream, projecting at least one stream of fluidized heat fusible material into said subregion of turbulence for conveying the material by the plasma stream to a substrate to be coated.

6. A method according to claim 5 in which the confined region is defined by two conical concentric electrode surfaces with the cone angle of one being different that that of the other.

7. A method according to claim 6 in which the constricted conduit is of gradually changing diameters.

8. A method according to claim 7 in which the cone angle of the inner electrode is greater than that of the other electrode.

9. A method according to claim 6 in which the cone angle of the inner cone surface is greater than that of the outer cone surface and the difference is between 10.degree. and 30.degree..

10. A method according to claim 9 in which said confined region is enclosed by an envelope of an alloy that is nonreactive with the ionized gas and the heat fusible material.

11. A method according to claim 9 in which the heat fusible material is PTFE, said alloy envelope being high in nickel, and the ionized gas containing significant amounts of nitrogen.

12. A method according to claim 9 in which said fluidized particles have a major dimension within the range of 30 to 50 microns and having generally spherical shape.