Production of NO in nitrogen-based active screen plasma nitriding devices, a disadvantage or an opportunity?

Abstract

Nitric oxide (NO) can have both negative and positive effects on life. Therefore, it is crucial to control its production and destruction, and plasmas associated with reactive surfaces can effectively serve these functions. In this study, we observed the emission of NO radical using optical emission spectroscopy during nitriding experiments in active screen plasma nitriding (ASPN) with pure nitrogen. This phenomenon can be attributed to the contamination of the nitriding reactor surfaces by water vapor when the reactor is opened to introduce the samples for treatment.

In N₂–20% H₂ plasmas, the emissions of both NO and iron are very low. However, when the hydrogen supply is cut off, we observed that, after a few minutes, the intensities of the NO and iron emission lines become comparable to those obtained when the plasma is generated in nitrogen alone.

We conducted GDOES and XPS characterizations of the surfaces of samples fixed on the grid and treated in both N₂ and N₂–10% H₂ plasmas under similar contamination conditions. The results showed that the grid is nitrided in the N₂-H₂ plasma mixture, while no nitriding occurs in pure nitrogen plasma. A substantial quantity of NO-containing compounds (nitrosonium and nitrite or nitrito species) is detected when the treatment is performed solely in nitrogen plasma.

We then discussed the production of NO and proposed a surface reaction between the active nitrogen species generated in the plasma and the iron oxide formed through surface segregation on the grid. This reduction reaction consumes active nitrogen, resulting in no nitriding of the grid. In N₂-H₂ plasmas, the active hydrogen reduces the oxides formed on the grid, allowing the active nitrogen to facilitate nitriding.

In N₂-H₂ mixtures, an oxynitride composed of the metallic elements of stainless steel forms on the surface, complicating the sputtering of iron in the compound regime. We also explored the potential for controlling the cleanliness of the plasma nitriding process and the possibility of fixing nitrogen through the production of NO.