Characteristics of hydroxyapatite coating on Ti-6Al-4V substrate fabricated via sequent H2O2-oxidizing and RF-sputtering processes

Cite this paper: Vietnam J. Chem., 2020, 58(5), 654-660 Article 
DOI: 10.1002/vjch.202000064 
654 Wiley Online Library © 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH 
Characteristics of hydroxyapatite coating on Ti-6Al-4V substrate 
fabricated via sequent H2O2-oxidizing and RF-sputtering processes 
Nguyen Thi Truc Linh1*, Phan Dinh Tuan2 
1Department of Chemistry, Ho Chi Minh City University of Education, 
280 An Duong Vuong, ward 4, district 5, Ho Chi Minh City 70000, Viet Nam 
2Ho Chi Minh City University of Natural Resources and Environment, 
236B Le Van Sy, ward 1, Tan Binh district, Ho Chi Minh City 70000, Viet Nam 
Received April 28, 2020; Accepted August 18, 2020 
Abstract 
In this study, we created a hydroxyapatite (HAp) coating layer on top of the Ti-6Al-4V substrate via sequent H2O2-
oxidizing and RF-sputtering processes and determining the effect of H2O2-oxidizing state to the adhesion between HAp 
coating layer and Ti-6Al-4V substrate. The results showed that the H2O2-oxidized Ti-6Al-4V surface is rough and 
porous, which increases the adhesion strength of the HAp coating layer on the alloy substrate. The shear strength value 
of the HAp/H2O2-oxidized Ti-6Al-4V substrate was 69.3 MPa, significantly higher than that of the HAp/original Ti-
6Al-4V substrate (12.9 MPa). The X-ray photoelectron spectroscopy (XPS) proved the denser HAp coating layer 
covered the TiO2/Ti-6Al-4V substrate, consequently effectively prevented releasing of the unwanted toxic elements 
from the metallic implant. 
Keywords. Hydroxyapatite, Ti-6Al-4-V, implant, biomaterials. 
1. INTRODUCTION 
Titanium (Ti) alloys, including binary and tertiary 
compounds, have been applied in a lot of industrial 
fields, especially in biomedical (prostheses, 
implants) applications for many years owing to 
corrosion resistance[1], sliding wear resistance, 
superior tensile and fracture toughness.[2-4] The 
challenging issues include enhancing compatibility 
between biomedical materials and human bodies and 
ensuring sustainable clinical performance. Implanted 
Ti alloys in an organization may release unwanted 
toxic ions, which makes the surrounding tissue 
damaged. Consequently, human bodies may reject 
the implanted materials out.[5,6] Researchers 
modified the surface of Ti-alloys by coating with a 
non-toxic and bio-active ceramic such as 
hydroxyapatite (HAp, Ca10(PO4)6(OH)2).[7] The 
adhesion between HAp and Ti alloy is worthly 
considering because it directly affects the stability of 
the implanted material in bodies for the long-term 
applications.[8] Both coating and substrate 
characteristics determine the adhesion strength that 
relates to interacting forces such as chemical 
interaction or Van der Waals physical interaction, as 
well as mechanical anchorage. The bonding strength 
of the coating to a substrate could be significantly 
improved due to factors as follows: a denser 
microstructure and highly crystalline HAp coating 
layer, a rough and porous Ti–6Al–4V substrate. The 
methods measure the bonding strength of HAp 
coating layer to the Ti–6Al–4V substrate such as the 
indentation[9], the standard tensile adhesion test,[10] 
the standard adhesion test ISO 13779-4,[11] the 
standard adhesion test ASTM F1044-99,[12] the 
interfacial indentation test[13]. In other publications, 
pre-treating processes of Ti–6Al–4V substrate to 
form interfacial layers (TiO2 or TiN) improve the 
adhesion strength because the inner layers increase 
mechanical integrity[14] or reduce the decomposition 
of HAp.[12] The bonding strength between HAp 
coating layer and Ti–6Al–4V implant enhances 
thanks to the formation of a porous surface of Ti 
alloys. Our primary research investigated that the 
surface of Ti-6Al-4V oxidized in the H2O2 solution 
became a sponge-like nanostructured surface. Its 
feathers are considered as an advantage factor to 
form a denser microstructure of the HAp layer at the 
second state. Thus, the present study aims: (1) to 
create a HAp coating layer on top of the Ti-6Al-4V 
substrate via sequent H2O2-oxidizing and RF-
sputtering processes; (2) to determine the effect of 
H2O2-oxidizing state to the adhesion between HAp 
coating layer and Ti-6Al-4V substrate. 
Vietnam Journal of Chemistry Nguyen Thi Truc Linh, Phan Dinh Tuan 
© 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 655 
2. MATERIALS AND METHODS 
2.1. Preparation of Ti alloy samples 
Commercially procured Ti-6Al-4V alloy (Gr 5, 
ASTM, BAOJI TI-LEADER METAL 
PROCESSING CO.), with a diameter of 10 mm and 
a thickness of 1 mm, was used as matrix alloy 
material and substrate. The Ti-6Al-4V alloy 
substrates were sandblasted with SiC abrasive 
papers (120-2500 μm) and polished with the Struers-
DAP-U system, Denmark, then oxidized in an H2O2 
solution. The operation frequency of RF magnetron 
sputtering device (Angstrom Sciences, USA) was 
13.56 MHz and a power of 150 W, with the base 
pressure of 1.33×10−5 Pa and the working pressure 
of 7.32×10−5 Pa.[23] HAp target (Ca10(PO4)6(OH)2, 
High Purity Chemicals Lab. Corp., Grade: 99.99 %) 
was employed as the source material. Before 
sputtering, the chamber was held at the pre-
sputtering regime for 10 min (shutter closed) to 
remove any contaminations on the target surface and 
also to stabilize the deposition parameters. The 
oxidized Ti-6Al-4V substrates were fixed onto the 
substrate holder, which was inverted to face down 
and centrally positioned in parallel just above the 
source material with a target-to-substrate distance of 
40 mm. The deposition time was varied from 0.5 to 
2 hours to adjust the thickness of resulting films. 
The synthesized alloy coatings (i.e., HAp/H2O2-
oxidized alloy) were then annealed in a vacuum 
furnace (ACE Vac) at a pressure of 3340 Pa and a 
temperature of 400 C, reached with a ramp rate o ...  ionic 
compositions of blood plasma, interstitial fluid, and 
intracellular fluid.[21] 
Table 1: Elemental composition (at %) of TiO2/Ti-
6Al-4V and HAp/TiO2/Ti-6Al-4V samples 
Element 
Atom (%) 
TiO2/Ti6Al4V 
HAp/TiO2/Ti6Al4V 
HAp sputtering time: 0.5 h 
O 47.13 39.36 
Al 01.77 00.83 
Ti 50.58 27.41 
V 00.52 - 
P - 06.18 
Ca - 26.22 
Vietnam Journal of Chemistry Nguyen Thi Truc Linh, Phan Dinh Tuan 
© 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 657 
Logically, the HAp coating can play the role of 
an effective barrier to prevent releasing V, Al ions 
from the metallic implant due to the thickness of 
HAp coating layer, which is directly affected by the 
RF sputtering time. The XPS results of the denser 
HAp coating layer on top of the Ti-6Al-4V substrate 
via sequent H2O2-oxidizing and RF-sputtering 
processes at a longer sputtering time (2 hours) were 
shown in figure 3. 
Figure 3 (a-c) presents XPS spectra of Ti 2p, Al 
2p and V 2p of the dense HAp/H2O2-oxidized Ti-
6Al-4V surface, compared to H2O2-oxidized Ti-6Al-
4V substrate. The XPS data of the H2O2-oxidized Ti-
6Al-4V sample was discussed in our fundamental 
research with main features: a spin-split doublet of 
Ti 2p1/2 and Ti 2p3/2 at 463.4 eV and 457.8 eV are 
accounted for the oxidation state of TiO2 (+4 
oxidation state); a single peak at 73.2 eV 
corresponds to Al2O3 (+3 oxidation state); a minor 
peak at 515 eV is associated with V2O3 (+3 
oxidation state). However, all metallic elements of 
Ti-6Al-4V alloy cannot be detected by XPS analysis 
after depositing HAp on the H2O2-oxidized Ti-6Al-
4V substrate for 2 hours. 
Figure 3: Core level high resolution (HR) XPS spectra: 
(a) Ti 2p, (b) O 1s, (c) Al 2p, (d) V 2p, (e) Ca 2p and (f) P 2p 
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Vietnam Journal of Chemistry Characteristics of hydroxyapatite coating on 
© 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 658 
Figure 3d shows a higher O 1s peak located at 
around 529 eV of the H2O2-oxidized Ti-6Al-4V 
sample, which is mainly ascribed to the Ti-O bond 
as well as other minor Al- or V-related oxides.[22] A 
lower O 1s peak located at 530.06 eV of the HAp on 
the H2O2-oxidized Ti-6Al-4V substrate, which is 
attributed to the Ca-O bond. A peak at around 533 
eV corresponding to the P-O bond was not detected 
in the O1s spectrum. Still, a peak having low 
intensity at 132.27eV in the P 2p spectrum (figure 
3e) can be attributed to the pyrophosphate groups 
(NIST XPS database). Figure 3f shows the Ca 2p 
spectrum with a doublet band at 345.82 eV and 
349.45 eV, which is typical for calcium oxide in 
inorganic calcium-oxygen compounds. This 
attribution was supported by the result of the O1s 
spectrum with a peak at 530.06 eV, which is referred 
above. As a result, the atom ratio of Ca to P in the 
HAp coating layer synthesized by RF sputtering was 
higher than that in Ca10(PO4)6(OH)2) pure phase, and 
the crystallite creation of HAp phase was not 
perfect. 
In a study on the adhesion between HAp coating 
layer and Ti-6Al-4V substrate, the RF sputtering time 
is 2 hours, and the samples were annealed at 400 C 
to achieve a denser microstructure and highly 
crystalline HAp coating layer.[17] The surface 
morphology and bonding strength of HAp coating 
layer deposited on top of the H2O2-oxidized Ti-6Al-
4V substrate are compared with that of the original 
Ti-6Al-4V one. SEM images of HAp/TiO2/Ti-6Al-4V 
and HAp/Ti-6Al-4V samples are shown in Fig. 4.
(a) HAp/Ti-6Al-4V (b) HAp/TiO2/Ti-6Al-4V 
Figure 4: SEM images of HAp/TiO2/Ti-6Al-4V, compared to HAp/Ti-6Al-4V samples 
(HAp sputtering time: 2 h, after annealing) 
Figure 4a shows a plate surface without a special 
sign, while figure 4b exhibits a denser surface with 
few visible nanoholes (20-30 nm diameter). The 
H2O2-oxidizing state creates mechanical anchorages 
which support to get a stable coating on Ti-6Al-4V 
substrate. The surface features of the underlying 
H2O2-oxidized Ti-6Al-4V substrate (the porous, 
nanostructured surface morphology) are hidden by 
the progress of the development of HAp grains due 
to the thermal energy provided after annealing at 
600 °C. Therefore, the RF sputtering process is 
sequentially done after H2O2-oxidizing to form the 
dense HAp coating layer on the rough Ti-6Al-4V 
surface, which is waited for increasing mechanical 
interlocking between the coating and substrate; 
consequently, the bonding strength of the coating to 
the substrate is improved.[17] Moreover, the result 
also shows that the surface roughness of HAp can be 
controlled by changing the depositing time; 
therefore, choosing the appropriate sputtering time is 
dependent on the purpose of use. 
The shear strength values of the samples with 
and without oxidizing in H2O2 were measured (table 
2 and figure 5). 
Figure 5 shows a remarkable difference between 
the two groups: the peak force values of HAp/H2O2-
oxidized Ti-6Al-4V samples are in the range of 440-
547 N, while those of HAp/original Ti-6Al-4V ones 
are in the lower range of 62-80 N with an 
insignificant difference in fracture area values. As a 
result, the shear strength of HAp coating layer out of 
the Ti-6Al-4V substrate is around 12.9 MPa, which 
is significantly smaller than that of H2O2-oxidized 
Ti-6Al-4V substrate (approximately 69.3 MPa), 
table 2. The shear strength of the HAp coating layer 
without the TiO2 sub-layer is appropriate to the 
value, which was reported in the publication,[12] 
while that of the HAp coating layer with the TiO2 
inner layer is four times higher. The results prove 
the favorite role of H2O2-oxidizing state to the 
adhesion between HAp coating layer and Ti-6Al-4V 
substrate: a porous oxide interlayer may provide 
Vietnam Journal of Chemistry Nguyen Thi Truc Linh, Phan Dinh Tuan 
© 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 659 
better anchorage, improve the mechanical 
interlocking between the layer and substrate as well 
as prevent the formation or propagation of cracks 
during the annealing process. 
Table 2: Adhesion strengths of HAp coated Ti-6Al-4V substrates with and without oxidizing in H2O2 
Samples Peak force (N) Fracture area (mm2) Shear strength (MPa) 
No1-HAp/TiO2/Ti-6Al-4V 547 8 68.4 
No2-HAp/TiO2/Ti-6Al-4V 530 8 66.3 
No3-HAp/TiO2/Ti-6Al-4V 440 6 73.3 
No1-HAp/Ti-6Al-4V 78 6 13 
No2-HAp/Ti-6Al-4V 80 6 13.3 
No3-HAp/Ti-6Al-4V 62 5 12.4 
Figure 5: Adhesion strengths of HAp/TiO2/Ti-6Al-4V, compared to HAp/Ti-6Al-4V samples 
4. CONCLUSION 
In this study, the biocompatibility coating was 
formed on Ti-6Al-4V via sequent H2O2-oxidizing 
and RF-sputtering processes. 
The H2O2-oxidizing state plays a role in the 
creation of mechanical anchorage, which improves 
the adhesion between the coating and substrate. In 
contrast, the RF sputtering state forms a stable and 
biocompatibility HAp coating layer, which can play 
the role of an effective barrier to prevent releasing 
V, Al ions from the metallic implant. The 
combination of physics and chemistry methods in 
the modification of Ti-6Al-4V alloy for biomedical 
applications is efficiently proved. 
Acknowledgment. The research was supported by 
Ho Chi Minh City University of Education, Vietnam 
though the Project coded CS.2019.19.20. 
Conflicts of Interest. The authors declare no 
conflict of interest. 
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Corresponding author: Nguyen Thi Truc Linh 
Ho Chi Minh City University of Education 
280, An Duong Vuong, ward 14, district 5, Ho Chi Minh 70000, Viet Nam 
E-mail: linhntt@hcmue.edu.vn 
Tel.: +84- 919338187.