Research on Low-Temperature Material Processings Using Supersonic Plasma Jets

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In low-pressure plasma spraying, a plasma jet generator with a supersonic expansion nozzle is useful for spraycoating hard and large-area films adhering strongly to substrates. In the expansion nozzle, the pressure and the electron density drastically decrease downstream, and therefore the plasma is in thermodynamical nonequilibrium state. In this study, titanium nitride reactive spraying was carried out under a low-pressure environment using a DC arc plasma jet generator. Titanium powders were injected using a hollow cathode with argon gas, and the plasma gas was nitrogen or nitrogen and hydrogen mixture. Microstructure and properties of the coatings were examined using SEM and XRD. A dense and high-quality TiN coating with a Vickers hardness of 2000 was formed at a low substrate temperature of 700 oC with a low input power of 5.3 kW. All results showed that the supersonic plasma jet in thermodynamical and chemical nonequilibrium state had high potentials for reactive spraying.