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Atomization and Sprays
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ISSN Druckformat: 1044-5110
ISSN Online: 1936-2684

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Atomization and Sprays

DOI: 10.1615/AtomizSpr.v16.i3.30
pages 279-298

SHOCKWAVE ATOMIZATION: PHYSICAL MECHANISMS OF A MODULATED DC PLASMA TORCH DURING SPRAY COATING

Baruch Boris Gutman
SUNY

ABSTRAKT

This work is an attempt to understand the physical mechanisms of using DC torch modulation technology for spraying. The influence of the plasma disturbances created by means of arc current modulation and the influence of these disturbances on thermodynamic parameters and dynamic viscosity of the modulated plasma jet are investigated. As a result of this investigation, the phenomena causing the modulation effects—both in the plasmatron and in the mechanisms influencing the plasma technology parameters—have become understandable. It is shown that inserting hydrocarbons into the air plasma jet is a contributing factor for the transition of the torch to the laminar spraying mode (with vortex stabilization of the arc), with a high-coefficient use of powder. The physical estimations obtained have made the mechanisms of the formation of the sprayed coatings understandable. The results of interaction between the pulse-modulated plasma jet and the wire and powdery material being sprayed have been scrutinized. When plasma spraying is performed with DC current pulses superimposed in a reverse and direct polarity to the arc, the through-gas permeability of the coating is reduced by the order of magnitude. The most important explanation behind the phenomena is the disintegration of particles sprayed specific to the modulation process. By modulating the plasma arc current, sequential plasma shock waves disintegrate the spray particles, up to the size of the nanoparticles, and accelerate them toward the target substrate. The plasma arc current is precisely controlled to assure a short-time constant in the plasma, so that rapid changes in the plasma arc current form plasma shock waves that strongly impact the spray particles. There is a brief description of the advantages and shortcomings of plasma spraying technology, HVOF, cold spraying, plasma spraying with modulation, and detonation technologies. Each technology has important features, such as high velocity of spraying particles, melted spraying material, and nonmelted spraying material; however, these technologies have rigid requirements with regard to the size of spraying powder. In one case it is possible to neglect these requirements (to a certain degree); plasma technology with modulation combines all advantages of the above-mentioned technologies, and it has been applied in civil, military, and space techniques.


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