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A LaserStrobe (TM); Control Vision system was employed to examine water-stabilized plasma (WSP), gas stabilized plasma (GSP), and single wire arc plasma (SWAP) technologies. Visualization of the plasma spray process in each of these technologies has, in some instances, been made possible for the first time. Parameter optimization for the three processes was accomplished. This technology has significantly added to the theoretical and scientific knowledge of plasma diagnostics and plasma processing.
The goal of this program is to fundamentally understand the details of the thermal spray process. Key aspects to analyze include plasma-particle interactions, particle-substrate interactions, and molten particle impact dynamics.[1] Employing a Laserstrobe (TM); system enables a real-time visualization of the thermal spray process.[2] The system bridges the gap of theory to application by allowing particle velocity measurements to be made and particle flow trajectories and patterns to be visualized. The Control Vision; system is also used to verify that the various thermal spray control settings are optimum and that the plasma torch is operating within optimum limits.
The Laserstrobe (TM); Control Vision System uses two lasers (2 pulsed nitrogen laser strobes with a 5 ns pulse width) with fiber optic feed to transport and focus intense 337 nm light to the viewing area of interest. The reflected laser light from the site is brighter then either direct or reflected light from the thermal spray process. The high speed electronic shutter of 50 ns to 5 ms duration (at 1 to 30 frames/sec) is synchronized with the laser flashes. The laser is also synchronized with the framing of the video sensor and is fired once for each captured video frame. Temporal and spectral filtering results in an image which is free of process light. The moving particles are effectively frozen in place and subsequently recorded onto video tape.
The system control unit permits visual capturing of a particle displaced in flight to be measured as a function of time. These double exposures are used to calculate the velocities of the particles. The time it takes the particle to move from its original position to its double exposed position is known (i.e., the laser synchronization time). The captured video frame is imported into a Macintosh Quadra supporting NIH software (Image 1.49) where the distance between the two particles is measured. The velocity of the particle is then calculated from the laser synchronization time and particle displacement.
The Laserstrobe (TM); system provides visualization of only the central part of the jet which extends a few orifice diameters downstream of the nozzle corresponding to the jet core region. Figure 1 shows the core of the plasma and arc discharge attachment to the anode. The length of the plasma core was observed to vary as the system operates. The luminous core extends up to distances of 8 diameters downstream of the jet. Prior to this visualization technique, the stability of the plasma was only observed through variations in the output power. The plasma arc discharge has now been directly observed to be attached to the anode and traverse back and forth along it's width as the anode rotates in front of the nozzle exit. A "pinching" effect of the hot plasma core region is also observed resulting from a sheath of cooler gas, that is neither ionized nor conductive, encircling and constricting the plasma jet. [3] This pinching effect increases the temperature of the plasma as well as centerline velocities. [4] The outer shear layer as well as the downstream flow field was not observed due to entrainment and mixing of the plasma jet with the colder ambient air.
The particle velocity is dependent on the size of the particle as well as it's location within the plume. Figure 2 shows the powder injection into the plasma plume. Some vaporization is observed as "comet trails" behind the particles. Many particles are seen to become entrained in a flow pattern (parallel to central flow field) near the central regions of the core.[5] Particles sufficiently melted will impact the substrate and bond with previously deposited material. By correlating LaserStrobe (TM); images with deposit microstructures and operating conditions, powder injection was optimized with a feed angle of approximately 40_ from horizontal at a feed rate of approximately 38 kg/hr (for alumina feedstock).
Table 1 provides the mean, maximum, and minimum velocities at an average centerline distance of 5 cm downstream of the nozzle. Particles that were measured were mostly on the outer fringes of the plasma plume due to obscuring effects of the plume. Secondly, an average centerline distance is given because particle measurements (30 particles for WSP and 50 particles for SWAP) were made in a range of distances along the centerline to provide an adequate sample size.
The structure of a Metco 3MB (Metco Inc., Westbury, NY) gas stabilized plasma jet was investigated using the Laserstrobe (TM) system. It was observed that pinching of the luminous core occurred within 2 to 3 diameters downstream of the jet exit, Fig. 3. This occurrence is most likely due to entrainment of cold ambient air into the core of the jet.[4] Examination of the particle streaks indicate that the particle paths extend across the luminous core region of the jet. This suggests there is a large variation in the thermal environment for the externally introduced particles into the jet.
The Laserstrobe (TM); system examined the SWAP gun and allowed direct visualization of the degree of atomization, Fig. 4. If the wire feed rate is too high, the wire will be pushed through the core of the plasma before it is fully melted. In this case, the unmelted wire can be visualized along with a larger particle size distribution for the atomized metal. The larger particles directly affect the deposit quality by producing a more rough coating. The wire feed rate was monitored using the Control Vision; system to ensure that complete atomization of the wire occurred; therefore, resulting in consistently smaller particle sizes.
The control settings were chosen to remove the plasma plume from the SWAP gun in the recorded images. This allowed discrimination of the double exposed particles and, hence, particle velocity, since the laser synchronization time and particle displacement can be measured. Table 1 summarizes these measurements for the atomized aluminum wire with respect to distance from the wire. An advantage of using Control Vision; is that specific particles of interest are selectively chosen for measurement; unlike laser doppler velocimetry (LDVM) which measures the whole family of particles whereupon no particles are specifically discriminated.[6] Velocity data as a function of distance away from the centerline of the nozzle is thus made possible, along with a visual record of process phenomena.
4. ACKNOWLEDGMENTS
This work is supported by the National Science Foundation under grant number MSS-9311053 as well as by Flame Spray Industries, Inc. (Stony Brook, NY) and Control Vision, Inc. (Idaho Falls, ID).
5. REFERENCES