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Nitriding of Pure Titanium using Pulsed-Arc Atmospheric-Pressure Plasma Jet
Y. Yoshimitsu1, *R. Ichiki1, S. Kanda1, M. Yoshida2, S. Akamine1, S. Kanazawa1
1
Department of Electrical and Electronic Engineering, Oita University, Oita 870-1192, Japan.
2
Department of Mechanical Engineering, Shizuoka Institute of Science and Technology, Fukuroi, 437-8555, Japan
We have performed plasma nitriding of high-purity titanium using pulsed-arc plasma jet with N2/H2 mixture gas under
atmospheric pressure. As a result, the spectra of TiN and Ti2N were confirmed from X-ray diffraction. Moreover, the
diffusion layer with higher hardness than that of the base material was also formed beneath the nitride layers.
1. Introduction
Titanium and its alloys are widely used in medical
fields due to good biocompatibility and high corrosion
resistance, for example, for artificial joints, artificial heart
valves, and dental implants. It is reported that the
performance of titanium as a biomaterial was upgraded
more by nitriding or TiN coating [1-5]. Compared to coating
technologies such as PVD and CVD, nitriding has a plenty
of merits; e.g., extremely small size distortion and strong
adhesion of nitride layers and substrate. Moreover, it is well
known that nitriding can form a hard layer, called the
diffusion layer beneath the titanium nitride layer. The
consequent improvements of mechanical properties such as
surface hardness and wear resistance will upgrade titanium
further as biomedical components. Gas nitriding and
low-pressure plasma nitriding are used in general, but they
have inconvenient points such as long-treatment time, the
need of vacuum systems.
In order to eliminate these shortcomings of
conventional nitriding, we have developed a new pulsed-arc
(PA) plasma-jet nitriding technology operated under
atmospheric pressure.
2. Experimental Procedures
Fig. 1 shows the schematic diagram of PA plasma jet
system. N2/H2 mixture gas used as operating gases is
introduced from the upper part of the coaxial cylindrical
electrode nozzle at the flow rate of 20 slm, where the H 2
ratio is 1 %. This N2/H2 mixture ratio is the optimal value
for steel nitriding found in our previous study [6,7]. The
purities of N2 and H2 gas are 99.5 % and 99.97 %,
respectively. Pulsed-arc discharge is generated by a
low-frequency power source (plasmatreat, FG3001).
Low-frequency voltage pulses (4-5 kV in height and 21 kHz
in repetition) are applied to the inner electrode, and the outer
electrode is grounded. The maximum of the discharge
current is approximately 1 A. The afterglow generated by PA
discharge is spewed out from the orifice in 4 mm diameter,
forming the jet plume.
Plasma-jet nitriding was performed in an air-tight
container made of stainless steel to purge the residual
oxygen by operating gases. The container is not evacuated
with any pumping system so that the pressure inside the
container is kept at ca. 1 atm.
We used high-purity titanium JIS TP270 as a sample.
The sample size is 20×20×4 mm3. All sample surfaces were
mirror polished. The polished samples were ultrasonically
cleaned in acetone bath. The micro-hardness of as-received
substrate ranges from 100 to 150 Hv0.01. The treatment
temperature depends on the gap of sample and jet-nozzle,
where the gap and treatment temperature is 3 mm and 900
ºC, respectively. Irradiation time of PA plasma jet is 2 h.
Formation of nitride layers was confirmed by X-ray
diffraction (XRD) and from micrograph of sample
cross-section observed with a metallographic microscope.
The hardness profile was detected with a micro Vickers
hardness tester (Akashi, HM-124).
3. Experimental Results
Entire surface of treated sample turned to golden color
corresponding to that of TiN. Fig. 2 shows XRD patterns of
untreated and treated sample surface. We confirmed the
formation of TiN and Ti2N. Fig. 3 shows micrograph of
treated sample cross-section in the vicinity of the
jet-spraying center. Two layers were observed on the surface
in Fig. 3. From the fact that the upper layer displays golden
color, it follows that the upper and lower layers correspond
to TiN and Ti2N layers, respectively. The thickness of upper
layer is 2-4 m, and that of lower layer is 10-12 m.
However, the surface roughness appears to increase. We
consider that the roughening is attributed to erosion by
undesirable PA-discharge between the inner electrode and
HV Pulsed-Power Source
N2/H2 mixture gas
Outer electrode
Inner electrode
Pulsed-arc discharge
Fig. 1 Schematic diagram of pulsed-arc plasma jet system
the sample surface due to the short gap.
Fig. 4 shows the hardness profile of cross-section of
treated sample, where the hardness is displayed with
gray-scale. Vertical and horizontal axes are the radial
position (0 mm is the center of jet-spraying), and the depth
from the surface, respectively. We found that the surface of
sample was partially hardened as shown Fig. 4. The
maximum of the surface hardness was more than 900 Hv0.01
corresponding to that of nitride layer formed by plasma
nitriding. The hardness of the other hardened zones,
corresponding to the diffusion layer, ranges from 200 to 600
Hv0.01.
4. Conclusions
We have succeeded in nitriding of pure titanium using
pulsed-arc atmospheric-pressure plasma jet. The XRD peaks
evidenced the formation of the nitride layers. The existence
of the nitride layers were confirmed also by metallographic
observation of sample cross-sections. The hardness of
treated titanium surface was found to increase by
micro-Vickers test. In conclusion, the formation of titanium
nitride layers and diffusion layers was demonstrated for the
first time using PA atmospheric-pressure plasma.
In the present experiment, changing the gap of
sample-jet nozzle varies not only the treatment temperature
but also the condition of nitrogen supply. However, we have
investigated the gap dependence to evaluate how the quality
of titanium nitriding varies. We have also conducted the
experiments of adding argon into operating gases of PA
plasma jet, and we would like to report the results in the
conference. Moreover, we are planning to examine
biomedical properties of treated titanium such as
biocompatibility and bacterial adhesion.
(103)
(200)
(112)
(004)
(110)
(102)
(101)
(002)
(101)
(a) Untreaed
α-Ti
TiN
Ti2N
(b) Treaed
Fig. 2 X-ray diffraction patterns. (a) Untreated sample. (b)
Treated sample.
resin
Upper layer
Lower layer
Acknowledgements
We are truly grateful to Mr. Masaki Sonoda, Oita Industrial
Research Institute, for his valuable technical assistance.
Fig. 3 Micrograph of the sample surface cross-section taken
in the vicinity of the center of jet-spraying.
Radial Position [mm]
Depth from Surface [ m]
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Fig. 4 Hardness profile of the sample cross-section.