Plasma spray may be the most common method by which thermal sprayed coatings are applied.
The process is cost-conservative, and is an excellent fit for applications where extremely low porosity is not a priority and where substantial amounts of material must be applied. It is also the only method by which ceramics can be efficiently thermal sprayed.
- Traction Surfaces
- Roll Surfacing
- Most Typical Thermal Spray Applications
Like other thermal spray processes, the purpose of a plasma system is to heat and project particles of the material to be applied toward the work surface with as little waste and overspray as possible. Predating the HVOF system, the development of the plasma gun focused only partially on imparting velocity to the powder.
The plasma arc that gives the gun its name was developed as an electrical replacement for the flame used in older flame spray systems. In these guns, the primary objective is to heat the particulate enough before impact that the softened material can easily deform enough to mechanically bond to the surface profile. By connecting opposing poles of a high-potential power supply to a central electrode and concentric nozzle, a high temperature (50,000°F) arc can be created between the two. With the addition of a high-flow-rate inert gas such as argon or nitrogen along the axis of the arc electrodes, the plasma stream can be pushed forward out the front of the gun.
Powder is injected into this hot plasma flame near the front of the nozzle. The arc gas and powder carrier gas expand rapidly in the heat of the plasma flame, and the subsequent velocity propels the hot powder particulate forward.
Plasma systems can require some patience during initial setup of parameters, but, once established, the gun can typically run for hours without stopping. This makes it an ideal tool for extremely large parts as material can be applied nearly continuously, stopping only for nozzle inspection or other routine checks.
Without the velocity found in HVOF applications or detonation systems, plasma coatings will typically exhibit less density, and, occasionally, more oxidization. Conversely, they will show lower oxide content and porosity than arc wire or flame combustion deposits.