Dependence of ground-state NH radical fluorescence in atmospheric-pressure pulsed-arc plasma jet on operating gas composition

We are developing our original nitriding technology with the atmospheric-pressure pulsed-arc plasma jet. Since atmospheric-pressure plasmas do not require vacuum equipment, the processing time becomes shorter, the capital cost becomes lower, and the operation becomes easier than those of the conventional low-pressure plasma nitriding. These advantages show the potential to realize high-mix low-volume production technology. However, the mechanism of nitriding has not yet been elucidated. In this experiment, the laser-induced fluorescence observation of ground-state NH radicals in the atmospheric-pressure plasma jet plume was performed in order to investigate the elementary process of NH radicals and the correlation between NH density and nitriding efficiency. We succeeded in detecting the fluorescence of NH radicals at 336 nm and 337 nm by irradiating the jet plume with a pulsed laser of ca. 305 nm, where a mixed gas of nitrogen and hydrogen is used as the operating gas. We found that increasing the hydrogen fraction in the operating gas from 0 to 0.5% increased the fluorescence intensity of NH, while increasing it from 0.5% to 5% decreased the fluorescence intensity. This tendency is qualitatively consistent with the nitriding efficiency of the plasma jet for steel surface.