24G09
Effect of Hydrogen on Nitriding using Atmospheric-Pressure Plasma Jet
大気圧プラズマジェットを用いた窒化処理における水素の効果
Hirokazu Nagamatsu, Ryuta Ichiki, Yuta Yasumatsu, Shuichi Akamine, Seiji Kanazawa
永松寛和, 市來龍大，安松勇太，赤峰修一，金澤誠司
Oita University
700 Dannoharu, Oita 870-1192, Japan
大分大学工学部 〒870-1192 大分市旦野原700
The nitriding treatment is important surface hardening technology of metals in industry. We performed
nitriding using atmospheric-pressure plasma jet. By adding hydrogen gas to prevent oxidization, wesucceeded in hardening steel surface. Moreover, we found that the hardness profile is changed by the wayof hydrogen gas introduction.

N2 Gas

R4 mm
18 mm
4 mm
air: 20 mm
N2: 200 mm
Fig.1. Schematic of the pulsed-arc plasma jet nozzle.
6Voltage

421 kHz21

200-2-1

Current-4-6

Current [A]

2. Experimental Procedure
We treat disk-shaped samples (20 mm indiameter, 4 mm in thickness) of SKD61 steel (AISIH13), the surface of which has been polished tomirror finish. Experiments are performed in aclosed container. The steel sample is set on theceramic heater mounted in the container. Thenozzle of the PA plasma jet is inserted into thecontainer from the upper end. The jet nozzle iscomposed of the inner and outer electrodes, asshown in Fig. 1. Nitrogen gas (20 L/min) is
introduced through the jet nozzle. The container ispurged by the nitrogen gas prior to nitriding
treatment. The pulsed-arc discharge is generated in
the nozzle, where the applied voltage is 4 kV, thefrequency is 21 kHz, and the discharge current is 1
A, as shown in Fig. 2. Afterglow of the pulsed-arcdischarge is forced out by nitrogen gas through thenozzle orifice of 4 mm in diameter. This develops
to the plasma jet plume, the length of which

Pulsed Power Supply

Voltage [kV]

1. Introduction
We paid attention to the plasma nitriding which
is one of the surface hardening technology ofmetals. As a new surface-hardening treatment underatmospheric pressure, we are researching nitriding
by the atmospheric-pressure pulsed-arc (PA)plasma jet. We have adopted the PA plasma jet for
the following reasons. First, the generation of N
atoms has been confirmed in the PA plasma jetplume [1,2]. Second, the gas temperature in theplume is relatively low so that sample surface will
be neither melted nor roughened. We have recently
succeeded in nitriding for steels using PA plasmajet [3-5]. Here, we present the details of thenitriding such as the hardened area of the samples
surface, dependence of nitriding on several
experimental parameters, especially, the effects ofhydrogen gas introduction on nitriding.

050100150-2200Time [s]Fig.2. Typical voltage-current wavefoms.

becomes around 20 cm in N2 atmosphere [1]. The
temperature of the steel sample is maintained at
approximately 500 ºC by the ceramic heater. Thenitrogen plasma jet is sprayed to the samples
surface. The treatment duration is 2 h.

Radial Position, r [mm]-6 -4 -2 0 2 4 6 8Mode B

Fig.3. Illustration of the way of H2 introduction.
Mode A: addition of H2 gas from container side.
Mode B: introduction of H2 gas through the jet nozzlewith N2 gas.

In our previous investigation, we found that only
spraying nitrogen plasma jet leads to oxidization of
the sample surface. Therefore, it is necessary to addhydrogen gas to the treatment atmosphere fromcontainer side to prevent oxidization, as shown inMode A of Fig. 3 [3-5]. Moreover, we haveinvestigated another way of hydrogen gas
introduction through the jet nozzle with nitrogengas, as shown in Mode B of Fig. 3. Mode B has apotential to decrease the hydrogen gas flow rate
needed for nitriding.
3. Results
The hardness profiles of SKD61 cross-section in
the case of Mode A and Mode B are shown in Fig.
4 (a) and (b), respectively. Fig. 4 (a) shows that thehardened layer is the thickest under the center ofplasma jet plume irradiation (r = 0 mm). However,
the thickness becomes smaller with increasing r. On
the other hand, Fig. 4 (b) shows that the nitrided
area for Mode B is considerably different from thenitrided area for Mode A.
We performed optical emission spectrometry of
the plasma jet plume for investigating elementary
process. As a result, we observed that the spectral
intensity of NH radical (336.1 nm in wavelength) isprominent. The dependence of NH spectrum
intensity on the H2 gas flow rate is shown in Fig. 5,
where H2 gas is introduced by Mode B. Theintensity is the highest when the H2 gas flow rate is
19 mL/min, which becomes smaller with decreasing
H2 gas flow rate. The finite intensity at 0 mL/min of
H2 is due to residual H2O. The dependence ofnitriding on NH radical is under investigation.
Acknowledgments
This work was(22760214).

supportedby KAKENHI

10(a)Hv0.0140120060110080

1000

10090012020800(b)

700

40600

60500

80

Fig.4. Hardness profile of SKD61 sample. (a): Mode A(H2: 3.7 L/min). (b): Mode B (H2: 0.9 L/min)

Wavelength: 336.1 [nm]Intensity [a.u.]Mode A

Depth from Surface, d [m]20050100150200

H2 Gas Flow Rate[mL/min]Fig.5. Dependence of NH radical spectral intensity
on H2 gas flow rate in the case of Mode B.
References[1] Y. Takemura, Y. Kubota, N. Yamaguchi, and T. Hara,
IEEE Trans. Plasma Sci. 37 (2009) 1604.
[2] Y. Kubota, R. Ichiki, T. Hara, N. Yamaguchi, and Y.
Takemura, J. Plasma Fusion Res. Series 8 (2009)
740.
[3] R. Ichiki, H. Nagamatsu, T. Iwao, S. Akamine, and S.
Kanazawa, Abstract of the 8th Asian-European
International Conference on Plasma Surface
Ingineering, p. 58 (2011).
[4] H. Nagamatsu, R. Ichiki, T. Iwao, S. Akamine, And
S. Kanazawa, Abstract of the 8th Asian-European
International Conference on Plasma Surface
Ingineering, p. 134 (2011).
[5] Japanese Patent Application No. 2011-170515.