Plasma Spray Method and Apparatus

FIG. 1 is a simplified cross section view of apparatus implementing the concepts of the present invention;

FIG. 2 is a diagrammatic represntation of the temperature profile of the plasma at various stations along the passageway through the nozzle assembly; and

FIG. 3 is a graph illustrating the velocity of the plasma and the powder particles along the passageway through the nozzle assembly.

Plasma spray apparatus of the present invention is described in detail in FIG. 1. The apparatus principally includes a conventional plasma generator 10 of the type referred to in the prior art section of this specification and a nozzle extension assembly 12. The generator is capable of producing a high velocity stream of high energy plasma and the nozzle extension assembly is operative on that stream in preparing the plasma for the injection of powder particles of coating material to be sprayed. The principal elements of the generator 10 include a pintle shaped cathode 14 and an anode 16. A cylindrical wall 18 of the anode defines a passageway 20 through the anode. The cylindrical wall is adapted to receive an electrical arc eminating from the cathode. The generator further includes means 22 for flowing a gaseous medium, such as helium or argon, through the electric arc between the cathode and the anode to generate the high velocity, high energy plasma. In the embodiment of the invention illustrated the generator must be capable of producing a plasma stream which is characterized by an average stream velocity on the order of two thousand feet per second (2000 fps) and an average plasma temperature within the stream on the order of fifteen thousand degrees Fahrenheit (15,000.degree. F.). The Metco 3MB Plasma Gun with G Nozzle is known within the industry to be capable of producing such an effluent. Other plasma guns are likely to have utility in performing the concepts of the present invention. To the extent that such guns product effluents differing in characteristics from that of the Metco Gun, corresponding deviations in the detail design of the nozzle extension assembly are anticipated. Nevertheless, such a modified nozzle extension assembly will include the principal features hereafter described.

The nozzle extension assembly 12 abuts directly against the generator 10 and has an elongated passageway 24 which is in alignment with the passageway 20 through the anode of the generator. As illustrated, the passageway 24 extends through a tubular finned member 25. The passageway of the presently illustrated device is both shorter and wider than the corresponding passageway of the device illustrated in the copending parent to this continuation-in-part application. Effluent from the generator is dischargeable directly into the passageway 24 of the extension assembly. Conduit means 26 is adapted to flow cooling medium such as water through the extension assembly. A plasma cooling zone 28 is located at the upstream end of the passageway 24 and is provided for reducing the temperature of the plasma prior to the injection of the particles of coating material. The passageway 24 at the cooling zone extends for an axial length of approximately one inch (1 in.) and has a diameter of two hundred eighty seven thousandths of an inch (0.287 in.). The diameter of the passageway at the cooling zone and the diameter of the anode passageway to which the extension assembly aligns are made to correspond. In the embodiment illustrated the cross sectional area of the passageway 24 at the cooling zone is greater than the cross sectional area defined by the cylindrical wall 18 of the anode to which the electric arc is struck. The remaining geometric dimensions and parameters are sized from this basic dimension.

A plasma acceleration zone 30 along the passageway 24 immediately downstream of the cooling zone is provided for accelerating the cooled plasma stream. In this embodiment the acceleration zone is not only adapted to recover velocity lost in the cooling zone, but is adapted to accelerate the cooled plasma to velocities well in excess of the plasma velocity entering the nozzle extension. Within the acceleration zone of the illustrated nozzle the diameter of the passageway is reduced to approximately one hundred fifty two thousandths of an inch (0.152 in.) from an initial diameter of two hundred eighty seven thousandths of an inch (0.287 in.). This represents a cross sectional area reduction of approximately one-fourth (1/4), although somewhat greater or lesser area reductions are likely workable.

A powder particle introduction zone 32 along the passageway 24 immediately downstream of the acceleration zone 30 is provided for admitting or injecting powder particles of coating material into the cooled and accelerated plasma stream. Particles are flowable into the passageway through one or more powder ports 34. Two diametrically opposed powder ports are illustrated. With two ports as illustrated powder delivery rates on the order of eight pounds per hour (8 lbs./hr.) are attainable. The passageway in the introduction zone is approximately one hundred fifty two thousandths of an inch (0.152 in.) in diameter. Plasma velocities entering the introduction zone are on the order of eleven to fourteen thousand feet per second (11,000-14,000 fps).

A plasma/particle confining zone 36 is provided along the passageway 24 downstream of the particle introduction zone for enabling the particles to be accelerated by the plasma stream before the particles are discharged from the apparatus. The confining zone extends to a distance of approximately one inch (1 in.) downstream from the point of powder introduction. The passageway 24 in the confining zone opens to a diameter of approximately three hundred seventy thousandths of an inch (0.370 in.) at the end of the nozzle assembly. This represents a cross sectional area increase from the injection zone at approximately six (6) times the injection zone area. Particle velocities on the order of two thousand feet per second (2000 fps) are attainable in the described apparatus.

As has been previously discussed, the effluent upon which the nozzle extension assembly is operative is in a high energy state. The electric arc between the cathode and the anode breaks down the structure of the gas molecules to produce a plasma stream containing an assemblage of ions, electrons, neutral atoms and molecules. The stream is characterized by an average temperature and a thermal spike at the core of the stream which greatly exceeds that average temperature, perhaps one-third (1/3) greater. The temperature profile across the stream is illustrated in FIG. 2 and the thermal spike is readily viewable in the representation at the upstream end of the plasma cooling zone 28. As the plasma passes through the cooling zone, the average temperature is reduced on the order of two thousand degrees Fahrenheit (2000.degree. F.) or ten to fifteen percent (10-15%) from fifteen thousand degrees Fahrenheit (15,000.degree. F.) to thirteen thousand degrees Fahrenheit (13,000.degree. F.). Of equal importance the temperature of the plasma in the core ie even more greatly reduced from twenty thousand degrees Fahrenheit (20,000.degree. F.) or greater to around fifteen thousand degrees Fahrenheit (15,000.degree. F.), or within approximately two thousand degrees Fahrenheit (2000.degree. F.) or approximately fifteen percent (15%) of the average plasma temperature in that region. As the plasma passes through the acceleration zone the plasma has reached a nearly uniform temperature on the order of twelve thousand degrees Fahrenheit (12,000.degree. F.). Substantial elimination of the thermal spike to provide a nearly uniform plasma temperature profile at the point of powder injection is important. The above described normalization of the plasma temperature is illustrated by FIG. 2.

Powders are injected into the stream through the ports 34 and are heated by the plasma. The particles are accelerated by the plasma. Approximate corresponding plasma or gas velocities (curve A) and particle velocities (curve B) are illustrated in FIG. 3. As the particles progress downstream through the nozzle assembly the powder particles are heated to a plasticized state. The nearly uniform plasma temperature profile causes all particles to be heated to the same degree of softness, and a homogeneous stream of particles emanating from the nozzle results. Rates of cooling flow to the nozzle extension assembly are controlled to yield plasticized powders in the stream at the point of incidence on the substrate to be coated. The average temperature of the plasma discharging from the nozzle extension assembly is on the order of ten thousand degrees Fahrenheit (10,000.degree. F.) or two-thirds (2/3) of the originally provided average temperature.

The particular apparatus described has been specifically developed for the deposition of nickel alloy or cobalt alloy powders such as those typified by the NiCrAlY composition described as follows:

14-20 wt. % chromium;

11-13 wt. % aluminum;

0.10-0.70 wt. % yttrium;

2 wt. % maximum cobalt; and

balance nickel.

Particles sized to the order of five to forty-five (5-45) microns have been successfully applied. Additionally, the nozzle extension apparatus is well suited to the deposition of Haynes Stellite Alloy No. 6, a hard facing alloy which is procurable from Stellite Division of Cabot Corporation, Kokomo, Indiana. The Stellite Alloy No. 6 is utilized in the automotive industry as, for example, a coating material improving the wear resistance of valves of internal combustion engines.

The concepts of the present invention enable high energy levels to be initially imparted to the plasma stream in acceleration of the stream within the carrying passageways. Although reductions in plasma temperature along the passageway can be achieved by reducing the power input to the generator, the resultant energy in the plasma stream is correspondingly reduced and the acceleration effects of the plasma on the powder are not as great. The ability of the plasma to accelerate rapidly within the generator is not inhibited substantially by the reduction in plasma temperature in the nozzle assembly.

Those skilled in the art will recognize that empirical temperature and velocity measurements within a plasma stream are without the state of the art to accurately measure. Applicants have, therefore, analytically quantified conditions and states within the plasma stream to aid in understanding the inventive concepts taught. Actual temperature and velocity conditions may vary from those recited in the specification without departure from the fundamental concepts taught and claimed.