Effect of plasma immersion ion implantation of nitrogen on the
wear and corrosion behavior of 316LVM stainless steel
P. Saravanan a , V.S. Raja a,⁎, S. Mukherjee b
a
Corrosion Science and Engineering, Indian Institute of Technology Bombay, Powai, Mumbai-400 076, India
b
FCIPT, Institute for Plasma Research, Gandhinagar, 382044, Gujarat, India
Abstract
Low energy plasma immersion ion implantation (PIII) of nitrogen ions on vacuum arc melted 316L (316LVM) austenitic stainless steel has
been carried out at three different temperatures namely, 250 °C, 380 °C and 500 °C. X-ray diffraction analysis indicated that PIII results in the
formation mixed iron-nitrides along with expanded austenite phase at all temperatures. Microhardness measurements revealed a significant
increase in hardness after PIII treatment. Corrosion resistance in 3.5% NaCl increases when implanted for 3 h. The passive current density seems
to increase with treatment temperature. Wear measurements carried out using a pin-on-disc machine show an increase in wear resistance with rise
in treatment temperature.
Keywords: 316LVM; Plasma immersion ion implantation; Corrosion; Wear
1. Introduction
Most of the research and development in stainless steel
continues to generate new ideas for improving the mechanical and
corrosion properties of this important class of engineering
materials. Development of austenitic stainless steels with
improved properties had become wide spread in the 1980s.
Then the addition of nitrogen to these steels was made to improve
corrosion resistance and wear properties [1]. Since corrosion and
wear behavior of an alloy primarily concern its surface, surface
modification of austenitic stainless steels will be a viable route to
improve their corrosion and tribological properties [2].
Plasma immersion ion implantation (PIII) developed in 1988
[3] has been proved to be one of the most promising surface
modification techniques that can be exploited to modify the
surface chemistry of an alloy. PIII, which involves both the
implantation and diffusion of nitrogen, seems to be effective in
the temperature range of 250–500 °C for stainless steels [4]. By
this process it is possible to enhance simultaneously the wear
and corrosion resistance. The phase formed by PIII is called
‘expanded’ austenite [5,6] or γN phase [7,8]. However, despite
numerous investigations, the structure and formation of this
phase is not completely understood. These results are promising
in view of industrial applications, because this treatment on
austenitic stainless steel can be performed with techniques using
less energetic ions [9–12] such as low-energy high current
density ion implantation [13,14]. Using high current density, it
is possible to obtain nitrided layers several micrometers thick.
Hence the present study is concerned with examining how
low-energy high current density PIII as a surface modifying
technique affects the corrosion and wear behavior of vacuum
arc melted 316L (316LVM) stainless steel, and to have a
detailed study on the phase formed.
2. Experimental
316LVM stainless steel has the composition of 17.23% Cr,
14.85% Ni, 2.42% Mo, 0.029% C and 0.067% N. For plasma
immersion ion implantation the samples were polished systematically using silicon carbide emery papers starting from 120 to 4/0
grade and finally using alumina powder of 0.25 μm. The
experimental set-up for PIII comprises a vacuum chamber and
associated pumping systems, a substrate holder and heater, a
plasma source and a high-voltage pulsed power supply. At the
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Fig. 3. Hardness profile of untreated and PIII treated 316LVM SS for 3 h at
250 °C, 380 °C and 500 °C.
Fig. 1. X-ray diffractograms of 316LVM after 3 h PIII treatment at 250 °C,
380 °C and 500 °C.
center of the vacuum chamber a substrate holder usually in the
form of a disc was kept. Below the substrate holder, a heater was
kept connected to a 1:1 ratio transformer. The system was pumped
to a base pressure of 7.0 × 10− 3 Pa. From this the operating
pressure of 10− 2–10− 1 Pa was achieved by introducing nitrogen in
to the system. An electrically isolated thermocouple was used for
measuring substrate temperature. Before the start of implantation,
the surface oxide layer was removed by sputtering with argon gas
for 30 min. The substrate was negatively biased to a voltage of
1 keV, and the implantation was done at 100 mA current with the
dosage of 1.52 × 1019 ions/cm2 for 3 h and 3.04 × 1019 ions/cm2 for
6 h.
Implanted samples were characterized by X-ray diffraction
using an Expertpro diffractometer with CuKα radiation
(λ = 0.154 nm) as a source. Load dependent surface microhardness testing was carried out on samples using a Leica Vickers
indenter microhardness tester with the load varying from 15 to
300 g.
Fig. 2. X-ray diffractrograms of PIII treated 316LVM for three different
durations at 380 °C.
Corrosion behavior of implanted sample was studied by
using the cyclic polarization technique. The electrical connections were provided by soldering copper wire on the back side
of implanted samples. Thick araldite (epoxy) coating was
applied excluding the implanted surface used for corrosion
studies, care being taken to avoid crevice formation at the
interface. Polarization experiments were performed using the
EG&G Potentiostat/Galvanostat model 273 operated by m352
SoftCorrIII. Studies were carried out in 3.5% NaCl solution. For
the study, platinum sheet as a counter electrode and saturated
calomel electrode (SCE) as a reference electrode were used.
Potential was scanned at a rate of 0.5 mV/s.
The dry wear behavior of the 316LVM stainless steel was
studied by means of a Ducom friction monitoring machine TR20-M-2. It is a pin-on-disc machine, where the pin of 3 mm dia
was rubbed over the alumina coated 304 disc at about 200 rpm at a
constant load of 3.5 N. Data such as friction coefficient and wear
loss in terms of sample thickness were continuously recorded.
3. Results and discussion
Fig. 1 shows the X-ray diffractograms of 316LVM SS
subjected to plasma source nitrogen ion implantation for 3 h at
250 °C, 380 °C and 500 °C. In Fig. 2, the variations in X-ray
diffractrograms of this alloy for various time intervals of
implantation at 380 °C are displayed. In both figures, X-ray
diffractograms of untreated 316LVM SS are given for
comparison. From Fig. 1, it is clearly seen that the untreated
alloy exhibits the austenitic phase. At 250 °C, new broad peaks
appear just ahead of (111) and (200) planes of austenite. With
rise in temperature only one of the two sets of peaks is present in
the XRD pattern. This can be considered as either due to shift in
the austenitic peaks towards lower angles or the appearance of
the newly emerged peak towards the lower angles. However, a
close examination of the XRD pattern of the sample treated at
380 °C reveals that the peak corresponding to (111) of austenite
is very small. This indicates that the appearance of the peak is
something to do with the disappearance of peaks corresponding
to the austenite phase. Broadening of the new peak might be due
to a possible raise in the internal strain of the austenite lattice
and or a possible variation in the lattice parameter of the
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Fig. 5. Pitting potential (0.6 M (3.5%) NaCl at 25 °C) versus PREN for stainless
steel used in this study is located within the circled area [17].
Fig. 4. Cyclic polarization of 6h PIII treated and untreated 316LVM in 3.5% NaCl.
austenite phase along the thickness direction of the sample due
to the variation in the concentration of N. Because the diffusion
of N into the stainless steel (SS) at 250 °C might be very slow, it
can cause a steep concentration profile with respect to N. Since
N is expected to exhibit higher diffusion coefficients at 380 °C
and 500 °C than at 250 °C, the former samples do not exhibit a
steep concentration gradient such as that seen at 250 °C. The
above propositions are supported by the following observations.
While the alloy treated at 380 °C for 3 h does not show peak
broadening as compared to the alloy treated at 250 °C for 3 h,
the same alloy treated for 6 h at 380 °C shows a similar peak
broadening. This indicates that at high temperature the sample
needs to be implanted for longer time to build up a similar
concentration gradient, as the N flux in the sample is high. It is
possible that the N content of the austenite lattice may be too
high, so that the phase should now be called as nitrides.
The shape of the peak (decreasing intensity with increasing
angle) indicates that the concentration of N rich phase (to be
called either a nitride or a supersaturated austenite) decreases
with increasing depth in the sample. This is supported by the
fact that as the angle of diffraction (and incidence) increases, Xray signals come from deeper below the surface. The problem
that arises in distinguishing nitrides from an expanded austenite
lattice is that both exhibit a simple cubic structure. It is
interesting to note that, though X-ray diffraction data of various
austenitic stainless steels show similar patterns, as shown in the
present case, different authors have interpreted them differently.
Thus, Samandi et al. [15] and Mukerjee et al. [5] attributed them
to lattice expansion of the austenite phase, while Haen et al. [16]
attributed them to mixed nitrides such as FeN, γ′-Fe4N, Fe4N.
To make it simple the peaks are identified in a general way as
MXN. An MXN phase can, in turn, be attributed to any of the
following three phases: γ′-FeN, Fe4N, FeNiN.
4. Hardness
Load dependent microhardness measurements have been
carried out on the implanted samples. Fig. 3 shows the
microhardness of 3 h plasma ion implanted 316LVM SS at different temperatures. For comparison, the hardness of untreated
316LVM SS is also shown. Fig. 3 shows that the implanted
316LVM SS exhibits higher hardness than that of the untreated
alloy. The hardness profiles of 250 °C and 380 °C samples did not
differ much, whereas, at 500 °C, implanted samples showed the
highest hardness values. Further, from Fig. 3 it is clearly seen that
the hardness decreases with increase in load, and at higher loads it is
Table 1
Electrochemical parameters of 3 h PIII treated and untreated 316LVM at 3.5%
NaCl
Alloy
condition
Untreated
250 °C
380 °C
500 °C
Ecorr in
mV vs SCE
− 210
− 230
− 150
− 75
ipass in A/cm2 [at
100 mV vs SCE]
−7
6.3 × 10
3.16 × 10− 7
3.4 × 10− 7
3.02 × 10− 7
Breakdown potentials
mV vs SCE
1
2
3
108
120
120
25
–
490
500
550
–
1200
1200
1210
Fig. 6. Wear behavior of untreated and PIII treated 316LVM SS for 3 h at 250 °C
380 °C and 500 °C.
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almost constant, which suggests that nitrogen had diffused
uniformly.
the surface, and it gradually decreases near the substrate. This
will raise the PREN to 38, whereas earlier authors reported 5%
at the surface for plasma nitriding [18].
5. Electrochemical corrosion studies
6. Wear studies
Fig. 4 shows cyclic polarization curves of 316LVM SS
implanted for 3 h at different temperatures as well as that for
untreated material. Table 1 summarizes the data. Generally the
treated alloy shows a slight increase in passivity (as exhibited by
lower passive current density than that of the untreated alloy, as
seen in Table 1) and a marked increase in pitting resistance. The
following observations can be made on the data. The untreated
material shows passive current density (ipass) of about
6.3 × 10− 7 A/cm2, but the anodic current shows the fluctuation
even at low applied potential. The sample treated at 250 °C
shows a marginal decrease in corrosion potential as compared to
that of the untreated material. They have three break down
potentials, which suggest that the alloy might have three
different passive layers. Their final break down potential is
found to be 1210 mV. The passive current density (ipass)
decreased from 6.3 × 10− 7 A/cm2 (untreated) to 3.16 × 10− 7 A/
cm2 (treated). For the alloy treated at 380 °C, a shift in corrosion
potential (Ecorr) from − 210 mV untreated to − 150 mV was
observed. Further, all its break down potentials (380 °C) are
almost the same as those observed in case of 250 °C treated
material, eventhough a slight increase in passive current density
as compared to both 250 °C was noticed. 500 °C treated
material exhibited a slightly lower passive current density as
compared to 250 °C and 380 °C. But its corrosion potential
remained the same as that of 250 °C and 380 °C treated samples.
There were three breaks in the passive region. The first break
is lower than that of untreated material and other two break
down potentials are the same as those of the other two treatment temperatures. At all treatment temperatures, the first break
down might be due to nitride formed at the surface. From XRD
analysis it is clearly seen that a nitride phase dominates over
the austenitic phase, and this should lead to a decrease in the
break down potential.
Among various alloying elements added to stainless steel,
only Cr, Mo, and N were found to be effective to promote
localized corrosion resistance [17]. Pitting resistance of an
alloy is indexed using the pitting resistance equivalent number
(PREN). The relation between PREN and pitting potential is
shown in Fig. 5, to show how effective N is in enhancing the
pitting resistance of the stainless steel. Since untreated
316LVM SS contains 17.23% Cr, 2.42% Mo and 0.067% N,
its PREN turns out to be 26.28. From the figure the
corresponding pitting potential of 0.6 M (∼ 3.5%) NaCl
solution is around 100 mV (SCE) for the above composition.
This is in good agreement with the experimental results shown
in Fig. 4, where a sudden increase in current density occurred at
a potential of about 107 mV. As the plasma ion implanted
sample shows a pitting potential of around 1 V, it should
correspond to a PREN value of 38. Since only nitrogen has
been added at the surface, the increase in the PREN value is
attributable to enrichment of nitrogen at the surface. The
nitrogen content on the sample surface can be around 12% in
Fig. 6 shows the wear rate of untreated and 3h PIII treated
316LVM for different temperatures. It is evident that PIII
treatment has considerably improved the wear resistance of
316LVM SS. The samples treated at 500 °C and 380 °C for 3 h
show better wear resistance than that of the sample treated at
250 °C for 3 h. At the initial stage, the 250 °C treated sample
also exhibited good wear resistance, but soon it started behaving
like the untreated one. This suggests that only a thinner coating
existed on the sample treated at 250 °C, compared with that on
the 380 and 500 °C treated samples. Samples treated at 500 °C
and 380 °C show good wear resistance for longer duration,
possibly because of greater depth of nitride.
7. Conclusion
Low energy PIII treatment (1 keV) on 316LVM SS has
been carried out at different temperatures to implant nitrogen.
The XRD results reveal the existence of nitrides along with
the expanded austenite lattice in the implanted alloys. Broad
XRD peaks seen in the X-ray diffractograms are attributed to
the presence of mixed nitrides such as FeN, γ′-FeN, Fe4N.
Hardness of 316LVM SS has been found to increase with
increasing treatment temperature. This has been attributed to
increase in thickness of the nitrided layer. Corrosion
resistance 316LVM SS increases drastically when implanted
for 3 h. PIII treated 316LVM shows better wear resistance
than that of untreated material, and the extent of wear
resistance increases with increasing temperature of the sample
during implantation.
Acknowledgements
The authors acknowledge Department of Science and
Technology for providing financial support. Our thanks are
due to Dr. S Ramadurai, Mishra Dhatu Nigam Ltd. Hyderabad
for supplying the 316LVM sample.
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