Applied Surface Science 253 (2007) 3154–3159
www.elsevier.com/locate/apsusc
Effects of water plasma immersion ion implantation on
surface electrochemical behavior of NiTi shape
memory alloys in simulated body fluids
X.M. Liu a, S.L. Wu a, Paul K. Chu a,*, C.Y. Chung a, C.L. Chu a,c,
K.W.K. Yeung b, W.W. Lu b, K.M.C. Cheung b, K.D.K. Luk b
b
a
Department of Physics & Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China
Division of Spine Surgery, Department of Orthopaedics and Traumatology, The University of Hong Kong, Pokfulam, Hong Kong, China
c
Department of Materials Science and Engineering, Southeast University, Nanjing 210018, China
Received 8 April 2006; accepted 2 July 2006
Available online 2 August 2006
Abstract
Water plasma immersion ion implantation (PIII) was conducted on orthopedic NiTi shape memory alloy to enhance the surface
electrochemical characteristics. The surface composition of the NiTi alloy before and after H2O-PIII was determined by X-ray photoelectron
spectroscopy (XPS) and atomic force microscopy (AFM) was utilized to determine the roughness and morphology of the NiTi samples.
Potentiodynamic polarization tests and electrochemical impedance spectroscopy (EIS) were carried out to investigate the surface electrochemical behavior of the control and H2O-PIII NiTi samples in simulated body fluids (SBF) at 37 8C as well as the mechanism. The H2O-PIII NiTi
sample showed a higher breakdown potential (Eb) than the control sample. Based on the AFM results, two different physical models with related
equivalent electrical circuits were obtained to fit the EIS data and explain the surface electrochemical behavior of NiTi in SBF. The simulation
results demonstrate that the higher resistance of the oxide layer produced by H2O-PIII is primarily responsible for the improvement in the surface
corrosion resistance.
# 2006 Elsevier B.V. All rights reserved.
PACS: 82.47.Wx
Keywords: NiTi; Plasma immersion ion implantation; Water plasma immersion ion implantation; Electrochemical impedance spectroscopy
1. Introduction
NiTi alloys have become important materials in biomedical
applications on account of their outstanding properties such as
the shape memory effect and super-elasticity [1,2]. Medical
applications of NiTi that have been reported include
orthopedics, dentistry, as well as components in medical
devices and instruments [3,4]. However, the high nickel content
has raised health concerns because nickel release into body
fluids can induce toxic and allergic responses [5], especially
when fretting is prevalent. In this respect, the proper surface
treatment can simultaneously achieve the goal of improved
* Corresponding author. Tel.: +852 27887724; fax: +852 27889549.
E-mail address: [email protected] (P.K. Chu).
0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.apsusc.2006.07.008
biocompatibility and surface corrosion resistance. It has been
shown that improvement of the corrosion resistance and
biocompatibility of NiTi can be accomplished by the formation
of a stable, uniform, and highly adherent oxide film on its
surface [6–9]. Many techniques such as chemical passivation,
anodic oxidation, electropolishing, thermal oxidation, and laser
surface melting have been proposed to modify the NiTi surface
[10–14]. In particular, plasma immersion ion implantation
(PIII) is an effective surface modification technique that excels
for samples and components with an irregular shape like
medical implants [15–17]. Water PIII is an efficient technique
to implant oxygen species such as H2O+, HO+ and O+ into the
materials with minimal energy spread compared to oxygen PIII
which introduces both O+ and O2+. Thus, H2O PIII may yield a
better surface oxide layer that can offer better enhancement in
the surface electrochemical properties. In our previous work,
X.M. Liu et al. / Applied Surface Science 253 (2007) 3154–3159
we have conducted H2O-PIII and successfully improved the
surface bioactivity of titanium metals and surface corrosion
resistance of magnesium alloys [18,19].
In this work, we used water PIII to improve the surface
properties of NiTi shape memory alloys. The composition and
morphology of the NiTi surface were investigated by X-ray
photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), respectively. In order to investigate the surface
electrochemical characteristics of NiTi before and after PIII,
electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests were conducted in simulated body
fluids (SBF). EIS is an effective tool to assess the surface
corrosion process and disclose the mechanism for thin films
fabricated on metals or alloys [20–22]. An electrochemical
system can be established using EIS and different equivalent
electrical circuit models can be adopted to explain the
phenomena that occur at the metal/film and film/solution
interfaces as well as inside the film [23–25].
2. Experimental
2.1. Water plasma immersion ion implantation
The materials used in this work were Ni–50.8 at.% Ti alloy
bars (SE508, Nitinol Device Company, Fremont, USA). The
samples were cut to 1 mm thick and 4.8 mm in diameter. Before
implantation, the specimens were mechanically polished with
200, 400, 800 and 1200 grit SiC papers sequentially, followed
by ultrasonic cleaning in acetone and deionized water and then
air dried. Before water plasma implantation, the samples were
sputtered for 5 min with argon plasma at 5 kV to remove
surface contaminants. Subsequently, water PIII was conducted
using the parameters listed in Table 1. After water PIII, the
samples showed a blue surface color.
2.2. Surface analysis
X-ray photoelectron spectroscopy (XPS, Physical electronics PHI 5802, Minnesota, USA) was used to obtain the indepth distributions of O, Ti and Ni. An aluminum X-ray source
with a power of 350 W was used, the vacuum was 2 108 Pa,
and the analysis area was approximately 0.8 mm2. Photoelectrons were detected at a take-off angle of 458. Depending on the
oxide layer thickness, a sputtering rate of either about 12 nm/
min or 4 nm/min was employed to obtain the depth profiles.
Atomic force microscopy (AFM, Auto Probe CP, Park
Scientific Instruments) was utilized to characterize the surface
Table 1
H2O-PIII instrumental parameters
RF power
Sample voltage
Pulse width
Frequency
Pulse duration
Base pressure
Working pressure
1000 W
40 kV
30 ms
100 Hz
240 min
1 105 Torr
3.4 104 Torr
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topography and roughness of the surface. It was operated in the
contact mode at room temperature.
2.3. Surface electrochemical tests
The samples for surface electrochemical tests were mounted
in epoxy to expose a surface area of 0.282 cm2. Electrochemical impedance spectroscopy was conducted in simulated
body fluids at 37 8C using a three-electrode cell consisting of a
pair of graphite rods as the counter electrode and a saturated
calomel electrode (SCE) as the reference electrode. The ion
concentrations in the SBF are similar to those in human body
fluids and they can be found in previous publications [26]. The
EIS measurements were performed at open-circuit conditions
using an EG&G PAR Model 283 potentiostat/galvanostat and a
PAR Model 5210 lock-in-amplifier. The EIS spectra were
obtained over the frequency range 100 kHz to 5 MHz at opencircuit potential with amplitude of 10 mV. The experimental
results were then fitted with the software, ZSimpWin 3.21.
Potentiodynamic measurements were performed on the NiTi
samples by a potentiostat (VersaStat II, EG&G, USA) at a pH
value of 7.42 in the SBF. A potential between 0.7 and +1.5 V
was applied to the samples with a scanning rate of 0.2 mV/s.
3. Results and discussion
3.1. Surface characterization
The XPS depth profiles of the H2O-PIII and control
(unimplanted) NiTi samples are depicted in Fig. 1. The near
surface of the H2O-PIII sample shows graded Ni, Ti and O
concentrations. The maximum oxygen content is observed at
the surface and it diminishes gradually with depth. Ti is
abundant at the surface and varies slightly with the depth. The
near surface structure is essentially TiO2 due to the fairly high
solubility of O in Ti and previous experiments have shown that
H2O-PIII into Ti produces surface TiO2 [18]. In addition, owing
to the high affinity on O to Ti compared to Ni, Ni is almost
absent in the near surface except in the vicinity of the interface
between the oxide and substrate. This is also due to the higher
preferential sputtering rate of Ni compared to Ti during H2OPIII [27]. The oxide layer thickness in the H2O-PIII sample
(Fig. 1a) is about 40 nm whereas an oxide layer only several nm
thick is observed on the control sample (Fig. 1b) arising from
natural surface oxidation. The intensity of Ni after H2O-PIII is
lower than that of the control sample as shown in the XPS
survey scan in Fig. 2. The dominant O 1 s peak on the H2O-PIII
sample confirms that there is large quantity TiO2 formed on the
H2O-PIII NiTi surface, and it is also confirmed by the shift of
the O 1 s peak to a higher energy. After H2O-PIII, Ti on the NiTi
surface is oxidized and the Ti 2p peak also shifts towards a
higher energy.
The morphology and roughness of the surfaces before and
after H2O-PIII are determined by atomic force microscopy. As
shown in Fig. 3(a), the surface of the control sample is
composed of flat regions, shallow scratches, and some
protrusions. After H2O-PIII, many fine and evenly distributed
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X.M. Liu et al. / Applied Surface Science 253 (2007) 3154–3159
Fig. 3. AFM images: (a) control sample; (b) H2O PIII sample.
protrusions (Fig. 3b) are observed on the surface, and they can
be attributed to energetic bombardment by the ions in the water
plasma, namely O+, OH+ and OH2+. Moreover, no obvious
pores can be observed on the surface. The root mean square
(RMS) surface roughness value of the control sample measured
over an area of 5 mm 5 mm is 7.04 nm which is slightly
higher than that of the H2O-PIII sample of 6.05 nm.
3.2. Potentiodynamic tests
Fig. 1. XPS depth profiles: (a) H2O-PIII sample; (b) control sample.
Fig. 2. XPS surface survey scans: (a) H2O-PIII sample; (b) control sample.
In the potentiodynamic tests, the breakdown potential (Eb) is
a measure of the stability and corrosion resistance of the surface
layer. A higher or more positive value of Eb indicates a larger
corrosion resistance [28]. As shown in Fig. 4, a significant
improvement in the corrosion resistance is evidenced by a large
shift of the breakdown potential Eb from 250 mV for the control
Fig. 4. Potentiodynamic polarization curves.
X.M. Liu et al. / Applied Surface Science 253 (2007) 3154–3159
NiTi to 1000 mV for the H2O-PIII sample. Moreover, the
passive current density measured from the H2O-PIII is smaller
by about a factor of 10 compared to the control. The results
show that the H2O-PIII sample has significantly better
corrosion resistance.
3157
The corrosion resistance of the H2O-PIII sample is closely
related to the properties of the surface oxide barrier. The
electrical resistance of the oxide layer is conventionally studied
by electrochemical impedance spectroscopy using AC voltage.
The Bode plot (magnitude and phase of impedance Z versus log
of frequency) is usually used to represent the result [29].
Fig. 5(a) shows the Bode plot obtained from the control NiTi
sample and the related simulated plot. From the Bode plot, the
phase angle is about zero at high frequencies, indicating that the
impedance is dominated by solution resistance in this frequency
range. The phase angle remains close to 808 over an
intermediate range of frequencies implying a near capacitive
response of the native oxide. Besides, at the low frequency
region, the phase angle shifts to a lower value due to
polarization resistance of the oxide layer [30]. The Bode
magnitude plot is characterized by two distinctive regions. In
the higher frequency region (1–100 kHz), the Bode magnitude
plot exhibits constant log jZj versus log( f) at high frequencies.
This is due to the response of the electrolyte resistance Rs. In the
low and mid frequency regions, the spectra display a linear
slope of about 1, which is indicative of the capacitive
behavior of the native oxide layer [31].
The electrochemical response of the control sample is best
simulated employing the equivalent electrical circuit, Rs(Q1R1),
as shown in Fig. 5(b). The Randle’s model is usually used to
simulate the oxide film [29]. In this model, Rs, corresponds to
the solution resistance in the SBF between the working
electrode and reference electrode. The resistance R1 represents
the electrical resistance of the native oxide layer. The element
Q1 is a constant phase element (CPE) that is defined by
ZQ = [C(jv)n]1, where C is the capacitance of an ideal
capacitor for n = 1 [32]. 0 < n < 1 represents the deviation
from an ideal capacitor, which is related to the surface
roughness [33]. The physical model (Fig. 5b) for this equivalent
circuit assumes that the oxide layer naturally formed on the
NiTi possesses resistance and capacitance [34]. The surface can
be envisaged as a parallel circuit of resistance R due to ionic
conduction through the oxide layer and capacitance C due to the
dielectric property [30] in order to assess the electrical
resistance of the native oxide layer on the NiTi surface. As
shown in Fig. 5, the single oxide layer model is adequate in
explaining the electrochemical behavior of the control NiTi
alloy exposed to SBF.
Fig. 6(a) exhibits the equivalent electric circuit,
Rs(Q1(R1(Q2R2))), used to simulate the impedance spectra
of the H2O-PIII sample. This model takes into account the
surface that is composed of an inner oxide layer (oxide layer
shown in the XPS profile in Fig. 1) and an outer oxide layer
with protrusions (observed in the AFM image) [33]. Q2 and
R2 are the inner oxide layer capacitance and resistance or the
charge transfer resistance, respectively while Q1 and R1
correspond to the outer oxide layer capacitance and resistance
(or the electrolyte resistance between the protrusions),
respectively. Fig. 6(b) compares the experimental and
simulated impedance results of the H2O-PIII sample in
SBF at 37 8C. The simulation is conducted using the model
Fig. 5. (a) Bode plot of control NiTi sample in SBF; (b) physical model and
equivalent circuit Rs(Q1R1).
Fig. 6. (a) Physical model and equivalent circuit Rs(Q1(R1(Q2R2))); (b) Bode
plot of water implanted NiTi sample in SBF.
3.3. Electrochemical impedance spectroscopy
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X.M. Liu et al. / Applied Surface Science 253 (2007) 3154–3159
Table 2
Result of electrochemical impedance spectroscopy measurements
Specimen
Control
H2O-PIII
Rs (V cm2)
42.5
43.02
R1 (V cm2)
7
1.7 10
1.1 106
Q1 (F cm2)
6
3.0 10
1.0 107
shown in Fig. 6(a). After water implantation, the NiTi alloy
has a two-time constant response [35]. In the high frequency
range, Q1 and R1 are the capacitance of the oxide layer and
resistance of the protrusions in the outer oxide layer. The low
frequency region of the impedance is related to the
capacitance Q2 and resistance R2 at the interface of the
inner oxide and electrolyte (SBF). The results presented in
Fig. 6 indicate that the physical model of a double oxide layer
can explain the electrochemical behavior of the NiTi alloy
H2O-PIII.
The information about the intrinsic characteristics of the
surface of the control and H2O-PIII samples can be obtained by
the aforementioned analysis. When the control sample is placed
in SBF, its impedance spectrum reveals a passive process with a
phase angle of about 808 over a wide range of frequencies. The
SBF penetrates the native oxide layer by diffusion and attacks
the substrate. However, the spectrum of the H2O-PIII is quite
different. The existence of the two-time constant response is
related to the difference in the surface structure which can be
seen from the physical model. The outer oxide layer with many
protrusions interacts with the surrounding SBF. Some SBF can
penetrate the outer oxide layer through defects produced during
the implantation process. Usually in this process, some pitting
can be produced at the interface between the oxide layer and
substrate. In our previous work, the surface of the control
sample was totally destroyed after potentiodynamic tests
whereas only some pitting occurred on the surface of the
oxygen plasma implanted sample [36].
Table 2 summarizes the simulation results obtained using the
relative equivalent electrical circuits. The simulation error is
smaller than 10%. It can be seen that the values of n1 are very
close to unity for the two types of samples. This indicates a near
capacitive behavior of the native oxide layer on the control as
well as the oxidized layer produced by H2O-PIII. A little higher
value of n1 for the H2O-PIII sample demonstrates a lower
roughness than the control as confirmed by the AFM results. The
oxide layer resistance of the H2O-PIII sample is composed of R1
and R2, especially R2 (charge transfer resistance). Moreover, the
value of n2 is comparatively lower. This indicates a dominant
resistive behavior of the oxide inner layer on the H2O-PIII
sample. The resistance of the control is only composed of R1
(passive oxide layer resistance) which is lower than the resistance
R2 of the H2O-PIII sample. The resistance of the oxide layer in the
H2O-PIII sample is found to be higher by a factor of 4 compared
to the control. A similar tendency can be observed in the
potentiodynamic curve (Fig. 4). The breakdown potential
increases from 250 mV for the control to 1000 mV for the
H2O-PIII sample while the passive current density decreases by a
factor of 10.
n1
R2 (V cm2)
Q2 (F cm2)
n2
0.88
0.91
8.0 107
1.1 107
0.80
4. Conclusion
The H2O-PIII technique can create an effective titanium
oxide layer on the surface of NiTi alloy. The surface
roughness of the control sample is higher than that of the
H2O-PIII sample. Two different physical models with related
equivalent electrical circuits are devised to simulate the EIS
results. The impedance of the surface of the H2O-PIII sample
is four times that of the control sample which has only a
native oxide. A similar tendency is observed in the
potentiodynamic corrosion tests. Due to the high resistance
of the oxide layer produced by water PIII, the breakdown
potential increases from 250 mV for the control to 1000 mV
for the H2O-PIII sample and the passive current density
decreases 10 times. Our results indicate that H2O-PIII is
effective in enhancing the surface electrochemical properties
of NiTi shape memory alloys.
Acknowledgements
The work was financially supported by Hong Kong Research
Grants Council (RGC) Central Allocation Grant #CityU 1/04C
and City University of Hong Kong Applied Research Grant
#9667002.
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