A. KOCIJAN: ELECTRODEPOSITION OF A HYDROXYAPATITE COATING ON A BIOCOMPATIBLE NiTi ALLOY
67–70
ELECTRODEPOSITION OF A HYDROXYAPATITE COATING ON A
BIOCOMPATIBLE NiTi ALLOY
ELEKTRODEPOZICIJA HIDROKSIAPATITA NA POVR[INO
BIOKOMPATIBILNE ZLITINE NiTi
Aleksandra Kocijan
Institute of Metals and Technology, Lepi pot 11, 1000 Ljubljana, Slovenia
aleksandra.kocijan@imt.si
Prejem rokopisa – received: 2017-09-04; sprejem za objavo – accepted for publication: 2017-10-20
doi:10.17222/mit.2017.144
The electrodeposition method was used to apply a hydroxyapatite coating (HAP) on the surface of a biocompatible NiTi alloy
with the aim to enhance the corrosion resistance of the substrate and therefore decrease the release of nickel ions from the
surface of the alloy. A uniform inner layer of HAP was formed and overlaid by clusters of spherical and flake-like morpho-
logical structures. The HAP coating exhibited super-hydrophilic wetting properties compared to the moderate hydrophilicity of
the NiTi alloy. The barrier properties of the HAP coating were investigated by using potentiodynamic measurements. The HAP
coating significantly enhanced the corrosion resistance of the NiTi alloy and confirmed the effective barrier properties of the
coating, which can be used to minimise the nickel release in biomedical applications.
Keywords: electrodeposition, hydroxyapatite, NiTi alloy, SEM
Za{~itna plast hidroksiapatita je bila z elektrodepozicijo nane{ena na povr{ino biokompatibilne zlitine NiTi z namenom izbolj-
{anja korozijskih lastnosti substrata in zmanj{anja izlo~anja nikljevih ionov. Tvorila se je zvezna notranja plast hidroksiapatita,
ki je bila prekrita s skupki okroglih in podolgovatih morfolo{kih struktur. Z meritvami stati~nih kontaktnih kotov smo potrdili
superhidrofilne lastnosti hidroksiapatitne prevleke v primerjavi z zmerno hidrofilnostjo osnovnega materiala. S potenciodinam-
skimi meritvami smo potrdili, da se z nanosom za{~itne plasti izrazito izbolj{a korozijska obstojnost zlitine NiTi in s tem
predstavlja ustrezno bariero za prepre~evanje izlo~anja nikljevih ionov v biomedicinskih aplikacijah.
Klju~ne besede: elektrodepozicija, hidroksiapatit, NiTi zlitina, SEM
1 INTRODUCTION
NiTi alloys are extensively used in biomedical appli-
cations due to their unique combination of properties
such as superelasticity, shape-memory effect, good
corrosion resistance and biocompatibility.1 In spite of its
outstanding properties, nickel release is still an important
issue of concern in using NiTi alloys for biomedical
applications.2 Research efforts have been directed to the
elimination of Ni release from the surface and therefore
improving the corrosion resistance and as well as the
biocompatibility of the implants.1–5 Nickel is known for
its diverse effects on human body, including allergic
reactions and toxicity.6 Studies on commercially avail-
able orthodontic wires showed increased Ni release rate
with time of exposure and type of solution, indicating the
need for better understanding the processes at the inter-
face between the implant and biological environment.2,7
Various surface modification techniques have been used
including mechanical, chemical and thermal treatments,
as well as bioactive coatings such as calcium phosphate
coatings.4,5,8,9 Hydroxyapatite coatings (HAP) are appro-
priate candidates for biocompatibility improvement since
they offer an excellent surface for cell growth.8,10 How-
ever, they cannot be used in load-bearing applications
due to their poor mechanical properties. Different me-
thods for the deposition of hydroxyapatite coatings are
known, such as plasma spraying, electrophoretic depo-
sition, sol-gel deposition, ion beam deposition and
electrochemical deposition.4,10–13 Electrochemical depo-
sition has the advantage of uniform coating formation,
low cost, room temperature and simple setup.14
In this paper, the electrodeposition of hydroxyapatite
coating on the surface of NiTi alloy was conducted. The
prepared coating was additionally alkaline treated. The
surface morphology, wetting and structural properties
were evaluated by using scanning electron microscopy
(SEM), contact-angle measurements and X-ray diffrac-
tion (XRD). The barrier properties of the prepared
coating were studied by potentiodynamic measurements
in simulated physiological solution.
2 EXPERIMENTAL PART
Materials
Discs of 20 mm diameter and with a thickness of
1 mm from 55 % Ni-45 % Ti alloy (NiTi) were em-
ployed as a substrate.
NiTi alloy substrate preparation
Prior to the application of the coating, the NiTi alloy
discs were ground mechanically with SiC emery paper
Materiali in tehnologije / Materials and technology 52 (2018) 1, 67–70 67
UDK 621.357.7:620.197:669.245:669.295 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(1)67(2018)
(up to 4000 grit), diamond polished (up to 1 μm), ultra-
sonically cleaned with ethanol and dried in warm air.
Coating preparation
The NiTi alloy discs were immersed in 1 M NaOH
solution at 80 °C for 1 h, rinsed in distilled H2O and
dried in warm air. The electrodeposition was performed
in a solution containing 0.1 M Ca(NO3)2 and 0.06 M
NH4H2PO4 at –2 V and pH = 6 for 30 min in a cell using
two electrode-system, where NiTi alloy was employed as
a cathode and a graphite rod as an anode. After the
electrodeposition the samples were treated with 1 M
NaOH solution at 80 °C for 1 h, rinsed in distilled H2O
and dried in warm air.
Scanning electron microscopy (SEM)
SEM analysis using FE-SEM JEOL JSM-6500F was
employed to investigate the surface morphology of the
coating which were sputtered with gold prior to imaging.
Contact-angle measurements
The static contact-angle measurements of water (W)
on the nanoparticle coatings were performed using a
Surface energy evaluation system (Advex Instruments
s.r.o.) and repeated at least five times for each sample.
The contact angles were determined by using Young-
Laplace fitting. All the contact-angle measurements were
carried out at 20 °C and ambient humidity.
X-ray diffraction
The composition and structural properties of the
hydroxyapatite coating were investigated using a PAN-
alyitical 3040 X-ray diffractometer fitted with a Cu-K

( = 0.154 nm) monochromated radiation source.
Electrochemical measurements
Electrochemical measurements were performed on
HAP-coated NiTi alloy and bare NiTi alloy samples in a
simulated physiological Hank’s solution, containing
8 g/L NaCl, 0.40 g/L KCl, 0.35 g/L NaHCO3, 0.25 g/L
NaH2PO4×2H2O, 0.06 g/L Na2HPO4×2H2O, 0.19 g/L
CaCl2×2H2O, 0.41 g/L MgCl2×6H2O, 0.06 g/L
MgSO4×7H2O and 1 g/L glucose, at pH = 7.8 and 37 °C.
All chemicals were from Merck, Darmstadt, Germany.
The measurements were performed by using BioLogic
Modular Research Grade Potentiostat/Galvanostat/FRA
Model SP-300 with an EC-Lab Software and three-
electrode flat corrosion cell, where working electrode
(WE) was the investigated specimen, reference electrode
(RE) was a saturated calomel electrode (SCE, 0.242 V
vs. SHE) and the counter electrode (CE) was a platinum
net. The potentiodynamic curves were recorded starting
the measurement at 250 mV vs. SCE more negative than
the OCP using a scan rate of 2 mV s–1.
3 RESULTS AND DISCUSSION
The SEM microscopy was used to investigate the
surface morphology and structure of electrodeposited
HAP coatings on the surface of NiTi alloy. A typical
micrograph of the HAP coating is presented in Figure 1.
The coating surface exhibited different morphological
characteristics from flake-like structure (Figure 1b) to
sphere-like particles (Figure 1c) on the top of the com-
pact HAP layer (Figure 1d). The mean size of spherical
particles was under 1 μm compared to larger flake like
particles. Both types of particles were randomly distri-
A. KOCIJAN: ELECTRODEPOSITION OF A HYDROXYAPATITE COATING ON A BIOCOMPATIBLE NiTi ALLOY
68 Materiali in tehnologije / Materials and technology 52 (2018) 1, 67–70
Figure 1: SEM images of surface morphology of HAP coating applied on the surface of NiTi alloy
buted and formed agglomerates over the bottom compact
HAP layer.
To analyze the surface wettability, five static con-
tact-angle measurements were performed with water (W)
on different spots all over the samples and used to deter-
mine the average contact-angle values with an estimated
error in the reading of ° ± 1.0°. The corresponding
static water contact angles are reported in Table 1. It was
shown that both types of investigated surfaces exhibited
hydrophilic characteristics. However, the electrodepo-
sition with further NaOH treatment significantly reduced
the water contact angle towards the complete wettability
limit. In the electrodeposition process CaHPO4×2H4O
(calcium hydrogenphosphate dihydrate) and Ca3(PO4)2
(calcium phosphate) are formed and further transformed
to more stable HAP after the NaOH treatment.15
Table 1: Static water contact angles (W) of HAP coating on the
surface of NiTi alloy and polished NiTi alloy
Substrate Contact angle w (°)
NiTi alloy 71
HAP coating on NiTi alloy < 5
A typical XRD pattern of a HAP coating is presented
in Figure 2. XRD results revealed the presence of
crystalline HAP phases, which were consistent with the
phases listed in the COD database (00-009-0432 and
01-072-1243). The main (h k l) indices for HAP: (002),
(102), (210), (211), (112), (202), (220), (221) and (222)
are indicated in Figure 2.
Figure 3 shows the potentiodynamic behaviour of
HAP coated NiTi alloy and bare NiTi alloy in a simu-
lated physiological Hank’s solution. The polarization and
the passivation behaviour of tested material after surface
modification was studied. The corrosion potential (Ecorr)
for the NiTi alloy in Hank’s solution was approximately
–173 mV vs. SCE and for the HAP coated NiTi alloy
was –270 mV vs. SCE. Following the Tafel region, the
both studied materials exhibited a broad passive region.
The passivation region for the HAP coated NiTi speci-
men was significantly moved to the lower corrosion-
current densities compared to bare NiTi alloy, indicating
the efficient barrier properties of the coating. The
application of HAP coating significantly enhanced
corrosion resistance of NiTi alloy which was confirmed
by the corrosion parameters calculated from the po-
tentiodynamic measurements. The results showed signi-
ficantly decreased corrosion-current densities and corro-
sion rate as well as increased polarisation resistances of
the specimens coated with HAP coating compared to
bare NiTi alloy (Table 2).
Table 2: Corrosion parameters calculated from potentiodynamic
measurements
Material E(I=0)(mV)
Icorr
(μA)
Rp
(k)
vcorr
(μm/year)
NiTi alloy –270 5.34 8.4 48
HAP-coated NiTi
alloy –173 0.62 35.8 6
4 CONCLUSIONS
The hydroxyapatite coating was successfully applied
on the surface of NiTi alloy by the electrodeposition
method. SEM evaluation revealed uniform inner layer of
HAP which was overlaid by smaller spherical and larger
flake-like morphological structures. XRD evaluation
revealed the presence of crystalline HAP phases. Wetting
properties evaluation showed that the electrodeposition
and further treatment with NaOH significantly reduced
the water contact angles of HAP coating towards the
complete wettability limit compared to moderate hydro-
philicity of NiTi alloy. Corrosion study showed that HAP
coating significantly enhanced corrosion resistance of
NiTi alloy and confirmed the effective barrier properties
of the applied coating, which is critical for biomedical
applications.
A. KOCIJAN: ELECTRODEPOSITION OF A HYDROXYAPATITE COATING ON A BIOCOMPATIBLE NiTi ALLOY
Materiali in tehnologije / Materials and technology 52 (2018) 1, 67–70 69
Figure 3: Potentiodynamic curves for HAP-coated NiTi alloy and
bare NiTi substrate in a simulated physiological Hank’s solution
Figure 2: XRD diffractogram showing the HAP phases present in the
coating
Acknowledgements
This work was carried out within the framework of
the Slovenian research project J2-7196: Antibakterijske
nanostrukturirane za{~itne plasti za biolo{ke aplikacije,
financed by Slovenian Research Agency (ARRS).
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