Thermal Spraying Processes

Atmospheric Plasma Spray

In this process the coating material in powder form is melted in a hot plasma flame and propelled on to a prepared substrate surface to form a coating. When the spray contacts the prepared surface of a substrate material, the fine molten droplets rapidly solidify forming a coating.

Characteristics of Air Plasma Spray :

Material Form Powder
Heat Source Plasma Flame
Flame Temperature (°C) 12,000 to 20,000
Particle Velocity (m/sec) 200 to 500
Porosity (%) 2 to 10
Coating Adhesion (MPa) 40 to 70

Plasma spray system consists of a Plasma Spray Gun, Power Source, Powder Feeder, Control unit and Heat Exchanger. Plasma is formed inside the specially designed plasma spray gun, by introducing a high intensity electric arc between a cathode (tungsten electrode) and anode (nozzle) assembly. Plasma forming gas is introduced in this arc, where it gets ionised to form a plasma. When this plasma exits the special design convergent – divergent nozzle, it returns to its original state liberating extreme energy. The coating material in powder form is introduced in this hot flame, where it melts and is propelled by the flame on to the substrate surface.

The energy produced in a plasma is high resulting in very high temperatures at the plasma core. Thus ceramics with very high melting points are ideally sprayed with Air Plasma spray for achieving good quality coatings. The coatings produced by plasma spray are uniform, dense and have comparatively lesser porosity.

Many variants of ceramics are available that have different properties :

  • Aluminum Oxide : Wear Resitance & Abrasion Resitance
  • Aluminum Oxide + Titanium Oxide (Alumina Titania) : Wear Reisitance & Abrasion Resitance
  • Chrome Oxide : Corrosion, Wear and Abrasion Resistance
  • Zirconium Oxide (Zirconia) : High Temperature Barrier
  • Very good quality Metal, Alloys and Abradable coatings can also be achieved using Air Plasma spray. Air Plasma Spray also has the provision to spray coatings inside holes or pockets.

Advantages :

  • Largest choice of coating materials
  • High production spray rates
  • Process can be automated
  • Good bond strength and low porosity
  • High degree of flexibility
  • Very versatile

Disadvantages :

  • Porosity in the range of 2 - 10%
  • Lower bond strength in comparison to HVOF
  • Not feasible for carbide coatings

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