Experimental investigation for pulsed laser nitriding of metals

Abstract

Pulsed laser nitriding of active metals has been regarded as a promising method for performance enhancement of metals. Nevertheless, clarifying the complicated mechanisms of pulsed laser nitriding such as the nitrogen transport and the chemically active state of nitrogen poses challenges due to the rather fast and complex processes. In this work, the in situ momentum measurements, in situ laser-induced breakdown spectroscopy (LIBS) measurements, in situ transient off-axis Digital holography (TODH) measurements and energy dispersive spectroscopy measurements were implemented to investigate the pulsed laser nitriding. It was revealed experimentally that the laser-supported-detonation (LSD) may be the key, and the nitriding process could proceed effectively only when the illumination power density of laser can reach the threshold of generating LSD. The LSD-induced high-speed N+ ions and N atoms flow to the metal surface, which may be the major mechanism for the nitrogen transporting to the metal surface. Moreover, the experimental results of LIBS showed that the chemically active state of nitrogen is N+ ions and their lifetime is at the scale of 102 ns. The lifetime of N+ ions was found to be prolonged as the nitrogen pressure was increased. As indicated by TODH experimental results, the prolonged lifetime of N+ ions may be caused by the smaller shock wave velocity at the higher nitrogen pressure. These experimental findings in this work may clarify the nitrogen transport mechanism and the chemically active state of nitrogen, which may lead to a better understanding of the pulsed laser nitriding and provide the guidance for optimizing the related industrial design in the future.