Plasma Arc Flame Spraying

In the plasma process, a gas or a mixture of gases, such as argon, hydrogen, or nitrogen, is fed into the arc chamber of the plasma generator and heated by an electric arc struck between an electrode and the nozzle. The gas is heated to temperatures as high as 8300 °C (15,000 °F) to form a plasma, or ionized gas, that is accelerated through the nozzle. The ceramic powder, carried by a gas stream, is injected into the plasma, where it is heated, melted, and propelled toward the workpiece.

The higher-temperature heat source of the plasma arc imparts an energy content to the ceramic particles that is different from that in the combustion flame process. This necessitates some modification of the gun position. When the plasma process is used, higher-melting ceramic materials, such as the refractory metal carbides, can be deposited with a greater deposition rate.

The processing operations for plasma-arc spraying are similar to those discussed in the section on combustion flame spraying in this article. For a more detailed discussion of plasma-arc spraying, see the article "Thermal Spray Coatings" in this Volume.

Thermal barrier coating is one current application using plasma-arc spraying. Applied to certain high-temperature components, such as the inside of combustion chambers or the first-stage vane or blade of a gas turbine engine, thermal barrier coatings act to insulate the metal substrate thermally. Coatings are designed to provide as much as a 110 °C (200 °F) drop in temperature at 980 to 1095 °C (1800 to 2000 °F), but they should be used in a temperature gradient, such as is provided by air cooling the substrate or metal side. A thermal barrier could be a 150 to 200 pm (6 to 8 mils) undercoat of a high-temperature nickel-cobalt-chromium-aluminum-yttrium alloy, followed by 255 to 305 pm (10 to 12 mils) of yttria-stabilized zirconia or magnesium zirconate (MgOZrO2). If greater thickness for greater insulation is desired, thermal stresses resulting from application should be carefully considered. In laboratory applications, a thermal cycling test is used followed by a bench engine evaluation to qualify the coating and estimate service life.

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