Plasma Spray Deposition Processes
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temperature corrosive media and to enhance mechanical wear and erosion resistance. Several thousand parts within an aircraft gas turbine engine have protective coatings, many of them plasma sprayed. In fact, plasma spraying has emerged as a major means to apply a wide range of materials on diverse substrates. The process can be readily carried out in air or in environmental chambers and requires very little substrate surface preparation. The rate of deposit buildup is rapid and the costs are sufficiently low to enable widening applications for an ever increasing variety of industries. The Plasma When an electric arc forms within a gas, between positive and negative electrodes, the discharge gives rise to a breakdown of the dielectric nature of the gas, and electrical conductivity is achieved. Electrons and ions are formed, which are accelerated toward the positive and negative electrodes, respectively. These rapidly moving particles collide with neutral atoms and molecules, creating further ionization and, thus, an avalanche effect. The net result is a gaseous collection of energetic electrons and ionized molecules — a plasma.1 The dictionary definition of the word plasma is traced from the Greek and is "to form or mold." The aptness of this description will be seen in the discussion of the plasma spray gun. A given volume of a gas, said to be a plasma, contains both ions and electrons and, overall, is neutral. The density of a plasma, in electrons per cubic meter, ranges from 104 e/m 3 for plasmas in interstellar space to 1024 e/m3 for those in magnetohydrodynamic energy conversion systems.
Thermal plasmas, having those energetic particle temperatures of interest here, have electron densities of the order of 1016 e/m3. In fact, for thermal plasmas, the collective density of particles is some 1,000 times that of a low pressure plasma. Such plasmas have pressures sufficiently high enough to enable energy exchanges among the constituents, i.e., the light, energetic electrons and the heavy, slowly moving ions. This energy exchange between the electrons and the much heavier ions leads to an efficient transfer of energy, thus increasing the plasma temperatures, permitting equilibrium to be reached. This also enables the thermal plasma to be treated using the equations of thermodynamics, permitting predictions of the plasma's behavior. In this way the energy extracted by the electrons from the electric field is transferred to enthalpy and to an increase in temperature, as depicted in Figure 1. The relationship between heat content and temperature is linear, but at certain temperatures for given atoms and molecules, the deviations from linearity are large due to dissociation and ionization. Argon needs only to be ionized to become associated with the plasma but hydrogen and nitrogen, being bimolecular, undergo dissociation as well as ionization and, therefore, require a greater energy to enter the plasma state. This enhanced energy will yield an increased enthalpy within the plasma. Gases, however, differ in thermal cond
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