hydrogen, improves the wear behavior of the titanium
alloy Ti-15-3 by reducing the COF and decreasing the wear
loss.
The above results on nitrided titanium alloy Ti-15-3 show that
the alloy has poor nitridability. Even though the beta transus
temperature of the alloy is low, i.e., 750-770 C, the alloy has to
be heated to temperatures >800 C for effective nitriding to take
place. This implies that irrespective of beta transus temperatures,
titanium alloys have to be heated to temperatures greater than
800 C for effective nitriding. Hydrogen dilution helps in
increasing the nitrogen intake as can be seen from the hardness
data and EDS results. It is known that hydrogen removes the
surface oxide by etching and also increases ionization of nitrogen
in the plasma. The removal of the oxide barrier and the
availability of more nitrogen ions and atoms due to increased
ionization increase the intake of nitrogen by the sample. Even
then, the nitriding kinetics is low at temperatures lower than
800 C. With argon dilution, the sample surface is sputtered
during the nitriding process, and also the partial pressure of
nitrogen is lowered. While sputtering removes the oxide as well
as nitrided layers, the reduced nitrogen partial pressure decreases
the intake of nitrogen and helps in controlling the composition of
nitrided layer. Analyses of the nitrided layers by EDS, micro
Raman, and XPS show that intake of nitrogen is lower at
temperatures lowers than 800 C. The lower intake of nitrogen
leads to the formation of a phase and predominantly Ti
2
N
formation, as seen in XRD. Depth profiling of the nitrided layers
by XPS shows that diffusion of nitrogen into the interior of the
sample is low. All these factors determine the hardness, the
hardness profile, and the wear behavior of the nitrided samples as
discussed above.
4. Conclusions
Beta titanium alloy, Ti-15V-3Cr-3Al-3Sn (Ti-15-3) was
plasma nitrided in low-pressure RF plasma using nitrogen
and nitrogen diluted with hydrogen and argon at tempera-
tures less than and greater than the beta transus temperature
of the material. The results show that effective nitriding of
the alloy can occur only at temperatures higher than the beta
transus temperature, and even then the intake of nitrogen is
low. XRD shows the formation of alpha phase, and
predominantly, Ti
2
N. Micro Raman spectra, EDS, and XPS
show that nitrogen concentration in the nitrided layer is low.
Depth profiling by XPS shows that nitrogen profile in the
material is shallow. This is reflected in the hardness and its
profile in the nitrided layers. Wear studies show a low COF
and lower wear loss for plasma-nitrided samples, especially
with hydrogen dilution.
Acknowledgments
The study was carried under the 11th five-year plan projects on
enhancement of knowledge base in aerospace materials project
SIP-SED-05 funded by CSIR-NAL. The authors would like to
thank Director, NAL for permission to publish this research; and
Head, SED for constant encouragement. The authors would like
to thank Mr. Siju, Mr. NT. Manikandanath, Mr. Praveen, and
Mr. Muniprakash for various characterizations.
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2632—Volume 22(9) September 2013 Journal of Materials Engineering and Performance