A high-velocity, pulsed wire arc spraying apparatus has been proposed and partly developed in an effort to improve the quality of coatings deposited by thermal spray techniques. In this apparatus, material from a wire arc is atomized and propelled toward a deposition substrate by a repetitively pulsed plasma jet. As explained below, this development is prompted by (1) the observation that the particle velocities attainable in traditional wire arc spraying are too low to enable the deposition of dense, high-quality coating materials that are often desired and (2) the expectation that higher spray velocities should result in superior coatings.
In traditional wire arc spraying, an arc is drawn between the proximate tips of two insulated metallic wires and the wires are continuously fed toward the arc as the arc consumes them. (Sometimes one of the wires is replaced by a nonconsumable electrode.) A steady stream of gas (usually compressed air) is directed through the arc to atomize the molten wire material in the arc and propel the resulting droplets toward a deposition substrate. One principal disadvantage of the conventional wire arc spray technique has been the low particle velocity obtained, generally limited to ≈ 250 m/s as a consequence of the fundamental physical nature of the steady gas stream. It is desirable to increase the velocity in order to minimize droplet cooling in the cold gas stream, to eliminate in-flight oxidation of the droplets, and to produce zero porosity coatings.
The UTRON Pulsed Wire Arc Spray device integrates the excellent accelerating characteristics of pulsed plasmas with the simplicity of wire arc systems. The basic approach is to replace the compressed gas stream used in conventional wire arc spray with a repetitive, high-pressure, high-momentum flux pulsed plasma jet. The plasma jet is generated by a so-called capillary arc discharge in a gas or liquid located between electrodes at each end of a long narrow ceramic tube. It is called a capillary discharge due to the high length/diameter ratio, typically about 10 or so. The high-speed plasma jet exiting from a nozzle at the one open end of the capillary is oriented to aim the jet through the arc between the wire tips.
The plasma jet can be formed from a wide range of working fluids, but the work to date has utilized either cryogenic liquid argon or gaseous argon. Liquid argon can be injected as a thin stream into the capillary region either continuously or through fast-acting valves or check valves. The injection orifice, located at the rear end of the capillary, is sized to admit a quantity of liquid equivalent to room-temperature gas at up to 30 atm (~3 MPa). A highvoltage spark discharge triggers electrical breakdown through the gas or vaporizing liquid, which then triggers the main arc discharge along the length of the capillary. Highest performance is achieved with liquid injection since that introduces sufficient mass of gas to attain high pressure without exceeding temperature limits of the ceramic wall. The pulsed discharge quickly raises the pressure and temperature of the working fluid in the capillary to about 1 kbar (100 MPa) and 1 eV (11,600 K), resulting in a high-velocity jet exiting through the nozzle at the end of the tube.
The coating-material wires are positioned through holes in the expansion nozzle attached to the front end of the capillary tube. The arc discharge between the wires can occur either a few microseconds before or after initiation of the plasma jet discharge. In the latter case, the plasma jet itself provides a convenient breakdown path for the wire arc.
The pulsed plasma jet strips the molten metal from the wire tips, atomizes the droplets, and rapidly accelerates the resulting atomized metal droplets to speeds comparable to the plasma. These droplets are then deposited on a substrate or other piece of equipment. The entire process lasts about 100 to 250 µs and can be repeated hundreds of times per second.
The effectiveness of the pulsed wire arc spray device in atomizing and accelerating droplets derives from the extremely high-momentum flux (ρν2) of the pulsed plasma jet. The high-momentum flux is achieved mainly by an order of magnitude increase in velocity ν, but also by a similar increase in density ρ when liquid injection is utilized.
To date, the Pulsed Wire Arc Spray device has been operated at single pulse energies up to 4.2 kJ with liquid argon, and at repetition rates as high as 20 Hz at lower energy. Deposition rates as high as 2 kg/h were demonstrated, limited only by the available power supply. Experimental measurements established droplet velocities of up to nearly 1,500 m/s in acceleration distances as short as 3.2 cm, thus making this an ideal process for spraying the inner walls of confined regions (such as cylinder bores and pipes) where space is limited.
This work was performed by F. Douglas Witherspoon, Russell W. Kincaid, and Dennis Massey of UTRON Inc. for Marshall Space Flight Center. UTRON has been awarded U.S. Patent 6,001,425 for this pulsed high-velocity wire arc spraying device. For further information about the current status of this invention, please contact Dr. Russell Kincaid or Dr. Douglas Witherspoon at UTRON at (703) 369-5552 or by e-mail at
Inquiries concerning rights for the commercial use of this invention should be addressed to the Patent Counsel, Marshall Space Flight Center; (256) 544-0021. Refer to MFS-31491.
