Plasma Treatment

the treatment of materials with low-temperature plasmas generated in arc or high-frequency plasmatrons. It may alter the shape, dimensions, or structure of the material or the state of the material’s surface. Examples of plasma treatment are cutting apart, surface cutting, application of coatings, hard facing, and welding. The destruction of rocks in plasma drilling is another example of plasma treatment.

Plasma treatment has found wide application owing to the plasma’s high temperature (about 10⁴ °K) by industrial standards, the broad power control range, and the possibility of concentrating the plasma flow on the object undergoing treatment. The effects of plasma treatment result from both the thermal and the mechanical action of the plasma (the bombardment of the object with plasma particles moving at a very high velocity—the dynamic pressure of the plasma jet). The specific power output transferred to the surface of the material by a plasma arc can reach 10⁵-10⁶ watts (W)/cm²; the corresponding figure for a plasma jet is 10³-10⁴ W/cm². If necessary, the heat flow can be dispersed to provide a “soft” even heating of the surface, an effect made use of in hard facing and in the application of coatings.

The cutting of metals is performed by a constricted arc formed between the anode (the metal being cut) and the cathode of the plasma torch. The arc’s flow channel is stabilized and compressed (an action that increases the arc’s temperature) by the nozzle of the torch and by blowing into the arc a stream of plasma-forming gas, which can be Ar, N₂, H₂, HN₃, or mixtures thereof. Chemically active plasmas are used to increase metal-cutting efficiency. For example, cutting with air plasma involves oxidation of the metal by O2, which contributes additional energy to the cutting process. Plasma arcs are used in the cutting of stainless and nickel-chromium steels, Cu, Al, and other metals and alloys that cannot be cut by oxygen cutting methods. The high productive capacity of plasma cutting permits the use of the technique in continuous assembly-line production processes. The power output of the devices can reach 150 kilowatts (kW). For nonconducting materials—such as concrete, granite, and thin sheets of organic materials—a plasma jet is used; the arc burns in the nozzle of the plasma torch between the torch’s electrodes.

The application of coatings in plasma spraying is done to protect parts that operate at high temperatures or in aggressive media or that undergo intensive mechanical action. The coating material can be of refractory metals, oxides, carbides, suicides, borides, or other materials. It is introduced in the form of a powder or wire into the plasma jet, in which it melts, is atomized, acquires a velocity of about 100–200 m/sec, and in the form of small particles with a size of 20–100 microns is applied to the surface of the object. Plasma coatings are distinguished by reduced heat conductivity and are very resistant to thermal shock. The power output of spray-coating devices is 5–30 kW; such devices can yield up to 5–10 kg of sprayed material per hour. Powders with spherically shaped particles are produced for powder metallurgy by feeding into a plasma jet materials whose particles on melting assume a spherical shape owing to surface tension forces. The particle size may be controlled within a range from several microns to 1 mm. Even finer (ultradispersed) powders with particle sizes of 10 or more nanometers are produced by the vaporization of the initial material in the plasma and the material’s subsequent condensation.

The capability of a plasma arc to penetrate metals to a great depth is made use of in metal welding. The advantageous form of the pool that is created permits the welding of fairly thick metal (10–15 mm) without preliminary preparation of the edges. Plasma arc welding has a high productive capacity and, because of the great stability of the arc, results in a weld of good quality. Low-power plasma arcs with currents of 0.1–40 amperes are convenient for the welding of thin sheets (0.05 mm) in the fabrication of membranes, bellows, and heat exchangers from Ta, Ti, Mo, W, and Al.