Professor, Institute for Materials Research, Tohoku University, Japan

Speech Title: Optimization of the operating conditions of a nitrogen microwave-induced plasma for surface treatment of steel substrate

Abstract: An Okamoto-cavity could be employed to generate a nitrogen microwave-induced plasma (MIP) as a reaction source for the surface nitridation of steel samples, because it had an excellent performance to form a nitride layer on the surface having a several-micrometer-thickness within only 1 mm [1]. The MIP was also employed as an excitation source for emission spectrometric analysis [2]. In this paper, the emission spectra of excited species from the plasma are investigated to optimize the discharge conditions for the surface treatment. The spatial distribution of their emission intensities is measured by using a two-dimensionally imaging spectrograph, comprising a collimator optics, an imaging spectrograph, and a charge-coupled device detector. The emission images of band heads of nitrogen molecule and nitrogen molecule ion extend towards the axial as well as the radial direction of the plasma with increasing microwave power. The number density of nitrogen excited species is elevated at larger powers, thus activating any chemical reactions on the steel substrate. On the other hand, the growth rate of the nitride layer is drastically elevated when the microwave power increased from 600 to 700 W, which does not necessarily correspond to the variation in the emission intensities of the excited species of nitrogen. This result is probably because the growth process is dominantly determined through thermal diffusion of nitrogen atom after it enters into the substrate, which would be controlled by heating of the plasma.

[1]S. Sato, Y. Arai, N. Yamashita, A. Kojyo, K. Kodama, N. Ohtsu, Y. Okamoto, K. Wagatsuma, Appl. Surf. Sci., 258 (2012) 7574. [2]Y. Arai, S. Sato, K. Wagatsuma, ISIJ Int., 53(11) (2013), 1993.

Keywords: Atomic emission spectrometry, spectral imaging, nitrogen microwave-induced plasma, Okamoto cavity, surface nitriding