Atmospheric Plasma Spray

Atmospheric Plasma Spray Process Overview

Atmospheric Plasma Spray Process Overview

The conventional plasma spray process is commonly referred to as air or atmospheric plasma spray (APS). Plasma temperatures in the powder heating region range from about 6000 to 15,000 °C (11,000 to 27,000 °F), significantly above the melting point of any known material. To generate the plasma, an inert gas—typically argon or an argon-hydrogen mixture—is superheated by a DC arc. Powder feedstock is introduced via an inert carrier gas and is accelerated toward the workpiece by the plasma jet. Provisions for cooling or regulating the spray rate may be required to maintain substrate temperatures in the 95 to 205 °C (200 to 400 °F) range. Commercial plasma spray guns operate in the range of 20 to 200 kW. Accordingly, spray rates greatly depend on gun design, plasma gases, powder injection schemes, and materials properties, particularly particle characteristics such as size, distribution, melting point, morphology, and apparent density.

Atmospheric Plasma Spray Process Overview

Components of Atmospheric Plasma System

  • Gas supply
  • Gas hoses
  • Gas regulators for plasma and powder carrier gases
  • Gas flow controls, usually rotameters or mass flowmeters/controllers
  • Plasma arc spray gun comprising a torch body, anode/nozzle, cathode, gas ring (gas injector), and powder injector
  • Electrical controls for arc current and voltage
  • Direct-current power supply, typically a constant-current type
  • High-frequency RF spark arc ignition source
  • Water-cooling circuit
  • Feedstock delivery: powder feeder
  • Water-cooled power cables
  • Hoses/cables
  • Safety interlocks and console purging

Coating Characteristics

In plasma sprayed coatings, the hot gas jet created by a plasma arc expands, entrains powder particles, heats the particles, and accelerates them toward the substrate, where they impact, deform, and resolidify to form a coating. The high degree of particle melting and relatively high particle velocity of plasma lead to higher deposit densities and bond strengths compared to most flame and electric arc spray coatings. The low porosity of plasma sprayed coatings can equal that of HVOF and detonation-gun-type coatings, exceeding 99%, depending, of course, on material properties, gun type, and configuration. The high quality of a plasma spray coating microstructure results from the high particle/ droplet kinetic energy, which effectively deforms the impacting particles, and the high degree of heating/melting that facilitates particle deformation and flow.

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