Thermal Plasma Spraying
Plasma Spraying as a Thermal Spray Technology
Plasma spraying is a thermal spray technology in which finely ground metallic and non-metallic materials are deposited on a substrate in a molten or semimolten state (Kudinov et al., 1990; Zhukov & Solonenko, 1990; Pawlowski, 1995; Vurzel & Nazarov, 2000; Fauchais, Vardelle, & Dussoubs, 2001; Fauchais, 2004; Knight, 1991, 1998, 2005; Yoshida, 2005). Two other technologies are combustion spray and wire-arc spray. The thermal plasma heat source is usually based on direct-current (DC)-arc or radiofrequency inductively coupled plasma (RF-ICP) discharge. It provides very high temperatures, over 8000 K at atmospheric pressure, which allows melting of any material. This is actually a major distinctive feature of plasma spraying. Although thermal plasma enables extremely high temperatures, the operational melting temperature in plasma spraying is usually kept at least 300 K lower than the vaporization or decomposition temperature to avoid losses in energy efficiency. Ground materials are either injected into plasma (in the case of RF discharges) or in the plasma jet (in the case of DC arcs), where they are heated, melted or softened, accelerated, and directed toward the surface or substrate being coated. Particles or droplets making impact with the substrate rapidly cool, solidify, cool further, contract, and build up incrementally to form a deposit. The basic coating building blocks, known as “splats,” typically undergo cooling rates in excess of in the case of metals (Jackson et al., 1981).