22P063
Trial Application of Pulse-Modulated Induction Thermal Plasmas toHigh-Speed Nitridation with N2 /H2 Injection
トーチ下流部から N2 /H2 を混入した
パルス変調誘導熱プラズマによる Ti 試料高速表面改質
Yosuke Haruta, Kenta Fujimoto, Yasunori Tanaka and Yoshihiko Uesugi
春多洋佑, 藤本健太, 田中康規, 上杉喜彦
Kanazawa University, Kakuma-machi, Kanazawa, 920-1192, Japan
金沢大学 〒 920-1192 金沢市角間町This report describes a trial application of pulse-modulated induction thermal plasma (PMITP) at 15 kW
to high-speed surface nitridation of Ti. Influence of the pulse modulation (PM) of the coil current wasinvestigated on the surface temperature and surface composition of the specimen. Gas composition wasalso measured by quadrupole mass spectroscopy (QMS). It was found that the PM could provide the highspeed nitridation of the specimen with low thermal damage.

1. Introduction
The inductively coupled thermal plasma (ICTP) is
widely used for many materials processings [1, 2] because of its high enthalpy and high reaction activity.
To obtain a further eﬀective radical source for highspeed surface modification process, we have developed
a pulse-modulated induction thermal plasma (PMITP)
system [1, 3]. This PMITP system can modulate thecoil current amplitude of the order of several hundreds
amperes almost into a rectangular waveform. Until
now, we found from the optical emission spectroscopy(OES) that Ar-N2 -H2 PMITP can produce more excited nitrogen atoms into the reaction chamber withless heat flux compared to a non-modulated thermal
plasma [3].
In the present report, we measured the gas composition around the surface position of the specimen irradiated by Ar-N2 -H2 PMITP. For this measurement,a quadrupole mass spectrometer (QMS) with a watercooled probe was used. In addition, the Ar-N2 -H2PMITP was actually irradiated to the Ti specimen withmeasuring surface temperature of the specimen. The
surface composition of irradiated specimen was also
analyzed by X-ray diﬀractometer (XRD).
2. Experimental setup and conditions
Fig. 1 illustrates a configuration of the plasma torch
for PMITP. The plasma torch is composed of two coaxial quartz tubes with a 330 mm length. Argon gas
was supplied as a sheath gas along the inside quartztube. Nitrogen and hydrogen gas mixture was radially injected from eight holes in the frange. The distance between the holes and coil-end is 110 mm. Total gas flow rate including N2 /H2 mixture was fixedat 100 slpm. Nitrogen gas flow rate QN2 was set to2 and 4 slpm. Hydrogen gas flow rate was fixed at 1slpm. Pressure in the chamber was fixed at 230 torr.
The quantity “On-time”, which is the time duration
with higher current level (HCL), was set to 10 ms, andGas
Gas

GasArGas

Gas

Gas (sheath gas)

155 mm(8 turns)

Gas

200 mmGas

N2 +H2(quenching gas)
Water-cooled movablespecimen Holder

330 mm

110 mmr180 mm

Ti
Cooling waterRadiation
thermometer
QMSFig. 1: Schematic of plasma torch and chamber.

“Oﬀ-time”, the time duration with lower current level(LCL), was set to 5 ms for PMITP. We also defined theshimmer current level “SCL”, as a ratio of LCL/HCL.
A time-averaged input power was fixed at 15 kW for
any modulation condition. A Ti specimen was locatedon the water-cooled specimen holder, which was installed downstream of the chamber. This holder wasfixed at 200 mm below the coil-end. Surface temperature was measured using a radiation thermometer. Irradiation time of Ar-N2 -H2 PMITP was set to 180 s.
For gas composition measurement, a capillary inside a water-cooled tube was installed at 180 mm below the coil-end. The water-cooled tube was used toavoid too much heat flux from the thermal plasma tothe capillary. Another tip of this capillary was connected to the QMS, which measures mass spectra. The
tip of capillary with the water-cooled tube can be movedin radial position to measure radial distribution of thegas composition.
3. Results and discussions
3.1. Gas composition around downstream portion
Irradiation of nitrogen atom to the specimen may
have large eﬀect for nitridation using thermal plasmas
because the nitrogen atom has much higher density ina thermal plasma than nitrogen ion. Thus, we here focused on the nitrogen atom, which appears at m/z=14

Ion current peak [pA]

References[1] T. Ishigaki, et al, Sci. Technol. Advanced Master. 6,111(2005).
[2] W. R. Chen, et al, Surface Coating Technol., 197,109(2005).
[3] Y. Tanaka, et al, Appl. Phys. Lett., 89, 031501(2006).
30

N2=4 slpm
m/z=1443%SCL53%SCL63%SCL

20
1078%SCL
100%SCL

0-60-50-40-30-20-10 0 10 20 30 40 50 60
Radial position r [mm]Fig. 2: Radial distribution of ion current for m/z=14 in mass
spectra.
1300

m/z=1425Temperature [ C ]201250N2= 4 slpm

oIon current peak [pA]30

N2=4 slpm

1510

N2=2 slpm

1200
N2= 2 slpm

1150

51100

040 50 60 70 80 90 100
Shimmer current level, SCL [%]40 50 60 70 80 90 100
Shimmer current level, SCL [%]Fig. 3: Ion current for Fig. 4: Dependence of inm/z=14 in mass spectra at frared radiation temperature of
Ti specimen surface on SCL.
radial position of 0 mm.
α-TiN0.3 (1 0 1)
100%SCL

163%SCL
43%SCL

α-TiN0.3 (1 0 1)

43%SCL
63%SCLTiN (2 0 0)

2

TiN (1 1 1)

3N2 4 slpm

TiN (2 0 0)

N2 2 slpm

TiN (1 1 1)

4Intensity [kcps]

100%SCL036 38 40 42 44

2θ(a) N2 =2 slpm

36 38 40 42 442θ(b) N2 =4 slpm

Fig. 5: XRD diﬀraction diagram of Ti specimen.
TiN ratio [TiN/α-TiN0.3]

in mass spectra. Fig. 2 shows the radial distribution ofion current for m/z=14 in mass spectra for Ar-N2 -H2
PMITP with QN2 =4 slpm. The ion current of m/z=14in mass spectra is considered to increase monotonouslywith nitrogen atomic density. In case of 100%SCL,
the ion current becomes lower around the center axisof the reaction chamber (r=0 mm) than at other radial position. This means that the nitrogen atomic density is lower at r=0 mm because of ionization due tohigh temperature of the thermal plasma. On the otherhand, reducing SCL increases the ion current at r=0mm compared to those at other radial positions. Fig. 3
shows the ion current for m/z=14 in mass spectra at
r=0 mm for QN2 =4 and 2 slpm as a function of SCL.
This figure implies that the nitrogen atomic density at
r=0 mm increases with reducing SCL, not only the excited nitrogen atom found from the OES measurement.
This result indicates that the PMITP has a capability toprovide more nitrogen atom onto the specimen.
3.2. Surface temperature of Ti specimen
Fig. 4 shows the surface temperature of a specimen
after 180 s irradiation by Ar-N2 -H2 PMITP as a function of SCL for diﬀerent QN2 s. The surface temperature at 100%SCL reaches to almost 1300 ◦ C. On theother hand, reducing SCL to 50% can decrease thesurface temperature to about 1140 ◦ C. This result indicates that PMITP can decrease the heat flux to thespecimen, which can lower thermal damage.
3.3. XRD analysis of Ti surface
Irradiated surface of Ti specimen was analyzed byXRD. Fig. 5 shows the XRD results for diﬀerent SCLs.
The spectral peaks of TiN can confirmed to appear,
and they were increased by using PMITP. On the otherhand, the peak of α-TiN0.3 was decreased. Here “TiNratio” was defined as the ratio of TiN peak (2θ=42.6◦ )to α-TiN0.3 peak (2θ=39.4◦ ) in XRD spectra to evaluate the nitridation degree. Fig. 6 shows the result of“TiN ratio” versus diﬀraction angle for diﬀerent QN2 s.
We can see that reducing SCL from 100% to 63% increases “TiN ratio”, and then further reducing SCL
from 63% to 43% decreases it for QN2 =4 slpm. In
this way, use of PMITP can promote nitridation of Ti
surface compared to the conventional non-modulated
thermal plasma. The PMITP can be a promising toolfor eﬀective nitridation process.
4. Conclusion
The experimental results showed that the PMITP
provides more nitrogen atom density and less heat flux
onto the specimen surface compared to conventional
non-modulated thermal plasma. The XRD analysis
indicated that nitridation of Ti was promoted around
63%SCL, i.e. the modulation condition. The above results imply that the PMITP can be a promising tool foreﬀective nitridation process.

2.5
2.0
1.5
1.0
N2=2 slpm

0.5

N2=4 slpm

0.040

506070 80
SCL [%]

90100Fig. 6: The ratio of TiN peak (2θ=42.6◦ ) to α-TiN0.3 peak(2θ=39.4◦ ) defined as “TiN ratio”.