The plasma spray process is considered to be the most versatile of all the thermal spray processes. During operation, gases such as argon, nitrogen, helium, or hydrogen are passed through a torch. An electric arc disassociates and ionizes the gases. Beyond the nozzle, the atomic components recombine, giving off a tremendous amount of heat. In fact, the plasma core temperatures are typically greater than 10,000°C, well above the melting temperature of any material. Powder is injected into this flame, melted, and accelerated to the workpiece.
Plasma spraying was initially developed to spray ceramics and is still the premier process for applying them. Metals and plastics can also be sprayed with this technique. The particle velocities for plasma are higher than for those of flame and arc spraying and result in coatings that are typically denser and have a finer as-sprayed surface roughness. The trade-off of increased density, however, is that the maximum coating thickness for a given material is usually reduced. As both metals and ceramics can be effectively sprayed with the technique, plasma spraying lends itself to automation and to reducing process steps. For instance, ceramic coatings typically require a metallic bond coat to improve bond strength. With the plasma system, it’s possible to initially apply the bond coat and then immediately follow with the ceramic material.
Gartman Technical Services
