Plasma Spray Coating Method

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Plasma spray deposition or plasma spraying is a process that combines particle melting, quenching and consolidation in a single operation. The process involves injection of powder particles (metallic, ceramic or cermet powders) into the plasma jet created by heating an inert gas in an electric arc confined within a water-cooled nozzle. The temperature at the core of the plasma jet is 10,000-15,000 K. The particles injected into the plasma jet undergo rapid melting and at the same time are accelerated. These molten droplets moving at high velocities (exceeding 100 meters/second) impact on the surface of the substrate forming adherent coating [1,2]. The coating is incrementally built up by impact of successive particles by the process of flattening, cooling and solidification. By virtue of the high cooling rates, typically 105 to 106 K/sec., the resulting microstructures are fine-grained and homogeneous. An overview of plasma spray process including some of the important applications would be given in this article.

Plasma spraying has certain unique advantages over othercompeting surface engineering techniques. By virtue of the high temperature(10,000-15,000K) and high enthalpy available in the thermal plasma jet, anypowder, which melts without decomposition or sublimation, can be coated keepingthe substrate temperature as low as 500C. The coating process is fastand the thickness can go from a few tens of microns to a few mm. Plasmaspraying is extensively used in hi-tech industries like aerospace, nuclearenergy as well as conventional industries like textiles, chemicals, plasticsand paper mainly as wear resistant coatings in crucial components.

Thermalplasma for plasma spray deposition is generated using plasma spray gun orplasma spray torch. The spray system also includes DC power supplies, coolingwater system, gas feeding system, powder feeder and control console. The plasmatorch consists of a cathode, made of thoriated tungsten and a nozzle shapedcopper anode. Both the electrodes are water-cooled. The electrodes areseparated by an insulating block made of nylon that has provision for gasinjection. Powder to be spray deposited is injected through an injection portlocated at the nozzle exit.

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