Bright nitriding using atmospheric-pressure pulsed-arc plasma jet based on NH emission characteristics

Bright nitriding of a steel surface is demonstrated by an atmospheric-pressure pulsed-arc plasma jet, keeping the original metallic luster and low roughness. As the hydrogen fraction in the nitrogen–hydrogen gas mixture increases, the optical emission intensity of NH radicals decreases. Following this fact, we examine a method to suppress the formation of a compound layer, in which the hydrogen fraction increases from the conventional value of 1% to larger values for suppressing excess nitrogen supply. The hydrogen fraction of 4% provides a nitrided surface without a compound layer, where surface reflectance is largest and surface roughness is lowest. However, the nitriding treatment produces a dark spot several millimeters in diameter at the sample center due to local oxidization.