New pathways in plasma nitriding of metal alloys

In this paper, we report the effects of oxygen, hydrogen, and deuterium on nitrogen implanted stainless steel AISI 316. The samples were studied in situ by photoemission electron spectroscopy (XPS), nano-indentation (hardness), and scanning electron microscopy (SEM). At relative higher oxygen partial pressures, a surface potential barrier for nitrogen implantation is created by the oxygen absorption. The absorption process obeys a Langmuir isothermal law. The surface barrier is formed by the oxidation of metallic nitrides. The bulk properties such as nitrided layer thickness can be modeled by studying the surface properties. Hydrogen improves the nitrogen content on surface and, consequentially, the hardness in-depth. Surprising efficient oxygen elimination was detected using deuterium instead of hydrogen. This is due to the fact that deuterium improves the nitrogen chemical potential, augmenting the material hardness in depth up to 30% compared to the case when hydrogen is used. This phenomenon is interpreted by an increasing isotope residence time. These novel results suggest that new pathways can be opened in plasma nitriding processes using deuterium in industrial equipment for treatments of metal alloys with stable oxides on the surface.