Advances in Ma terials Physics and Che mist ry, 2012, 2, 45-48
doi:10.4236/ampc.2012.24B013 Published Online December 2012 (htt p://www.SciRP.org/journal/ampc)
Apatite Deposition on ZrO2 Thin Fi lms by DC Unbal an ced
Magnetron Sputtering
Arisara Thaveedeetrakul1, Virote Boonamnuayvitaya2, Nirun Witit-anun2,3
1Department of Chemical Engi neering, Faculty of Engineering, KMUTT, Bangkok, Thailand
2Department of physics, Faculty of Science, Burapha University, Chon Buri, Thailand
3Thailand Center of E xcellenc e in Physics /CHE, Ministry of Ed ucation, Bangkok, Thailand
Received 2012
ABSTRACT
Zirconia thin films deposited on 316L stainless-steel substrate were prepared by DC unbalanced magnetron sputtering from a metal-
lic zirconium target at low temperature with the target-to-substrate distance (dt-s) of 100 mm and sputtering power of 180 W. High
purity gas of Ar as the working gas and O2 as the reactive gas were used. The depositions were performed for 120 min at a total
pressure of 0.5 Pa. The effect of thermal treatment on the HA formation was investigated. The bioactivity was assessed by investi-
gating the formation of hydroxyapatite (HA) on the surface soaked in simulated body fluids (SBF). Films structure, surface mor-
phology and chemical composition of the ZrO2 films and HA formation were characterized by X-ray diffraction (XRD), scanning
electron microscopy (SEM), and FT-IR spectroscopy. The XRD results demonstrate the ZrO2 films are monoclinic phase. The an-
nealed films sho w the higher fil m crystalline due to the rearran gement of fil m stru cture. After b eing immersed th e samples in SBF,
the bone-like apatite was observed on all ZrO2 films, but a denser and more continuous HA layer were observed on annealed films
due to the crystallinity of ZrO2 films.
Keywords: Component; Zirconium Dioxide; Magnetron Sputtering; Hydroxyapatite; Simulated Body Fluid
1. Introduction
Thin films of zirconium dioxide (ZrO2) or zirconia are widely
used in protecti ve and thermal b arrier co atings [1], optical filter
[2] , oxygen sensor, microelectronic devices and biocompatibility
of bone implants [3-5]. It was considered an attractive ceramic
for biomedical application due to its inertness, high strength,
corrosion resistance, and fracture toughness. The ZrO2 have
three crystalline polymorphs, namely: monoclinic, tetragonal and
cubic [6]. Both the monoclinic and tetragonal phases exhibit
excellent biocompatibility properties on th eir surfaces [7].
Bioactivity is widely accepted as the essential requirement
for an artificial to exhibit chemical bonding to living tissues
upon the formation of a bone-like apatite layer on its surface in
any simulated body environment [8].
The formation of a bone-like apatite layer on biomaterials is
assumed to be the precondition for their osteoinductivity to
induce bone formation on the biomaterials in non-osseous site.
The research method of bone-like apatite formation in vitro
commonly is to immerse specimen in simulated body fluid (SBF)
and hydroxyapatite (HA) layer can be formed on all kinds of
bio active materials [9] . Th e in v itro tests of zirconium hydrogel
coating have found new bone-like apatite layer formation on
the surface [7]. Clinical studies of ZrO2 thin films are now
expected to be the useful as bone substitutes even under highly
loaded conditions such as is found in femoral and knee joint
since they exhibit high fracture toughness as well as high
bond-bonding ability [10].
There are many methods to prepare the ZrO2 thin films.,
however, the sputtering technique is a very attractive process
for producing metallic oxide with good uniformity at low
temperature [11]. This technique has not yet been well studied
for the deposition of ZrO2 films with the purpose of the
applications mentioned.
In this study, the ZrO2 thin films were deposited by DC unba-
lanced magnetron sputtering technique followed by the thermal
treatments. All samples were immersed in a SBF solution for
demonstration the bone-like apatite on the ZrO2 films. The
effect of the pretreatment on the formation of the HA was in-
vestigated.
2. Experimental
Reactive ma gn etro n s putt erin g, with zir coniu m target o f 54 m m
diameter, was used to produce the ZrO2 thin films with the
sputtering power of 180 W. The coatings were deposited on
316L-stainless steel type sub strate, 10 mm × 1 0 mm × 1 mm in
size. The chemical pretreatment of the substrates was carried
out by cleansing with propanol and acetone in ultrasonic bath
for 15 min. The sputtering gases argon with a purity of
99.999% and reactive oxygen with a purity 99.999% of which
flow rates of 1 and 4 sccm, respectively, were introduced to the
chamber separately and controlled by the mass flow controllers
(MKS type 247D). The target was sputt er cleaned for 15 min at
the argon pressure lower than 0.005 Pa to remove impurities
from target surface. The dt-s was adjusted to 100 mm and the
base pressure of the system was 0.005 Pa. The depositions were
performed for 120 min at a total pressure of 0.5 Pa. Some
as-deposited ZrO2 films were annealed in air at the temperature
of 800˚C for 1 h.

A. THAVEEDEETRAKUL ET AL.
46
The SBF solution that had ionic concentration close to human
blood plasma, as shown in Table 1, was prepared by dissolving
reagent-grade NaCl, NaHCO3, KCl, K2HP O 4•3H 2O, MgCl2•
6H2O, CaCl2, and Na2SO4 in ultra-pu re water. The so lution was
buffered at pH of 7.4 with 1M HCl and tris (hydroxymethyl)
aminomethane ((CH2OH)3CNH2) at 37˚C. The samples were
immersed into solution at 37˚C for 7 days. Subsequent to im-
mersion, the samples were removed from the solution, gently
rinsed with the ultra-pure water, and then dried at room tem-
perature.
The surfaces of the substrates before and after i mmersion of
SBF solution were analyzed via X-ray diffractometer with a
thin-film mode (TF-XRD) adjusted with CuKα radiation. The
measured 2θ angles were recorded from 20 to 40˚ at a step rate
1˚min-1. The morphology of the SBF-immersed samples was
observed by the scanning electron microscopy (SEM). FT-IR
spectroscopy was measured in transmission using the KBr pellet
techn ique.
3. Result and Discussion
3.1. Deposit ion of Monoclinic ZrO2 and Further
Annealing
It is well known that the sputtered films have lower packing
density and annealing improved the packing density and im-
proved crystallinity. X-ray diffraction patterns of the as-depos-
ited and annealed films are shown in Figure 1. The resu lts also
reveal that the as-deposited films are crystalline in nature. The
monoclinic phase was formed without any phase mixing (JCPDS
file no. 89-9066). The as-deposited and annealed films in Figure
1 were char acteri zed by th ree broad peaks with locating at 2θ ≈
28.2˚, 34.1˚, and 35.3˚, indicating the films contain monoclinic
phase with the M(
111
), (002) and (200) orientations, respect-
tively. The existence of pure monoclinic phase at high tem-
peratures observed in the present work is similar to that noticed
by Venkataraj et al. [12]. In addition, the ZrO2 thin film with
heat treatment in Figure 1(a) shows a narrower peak with
higher intensity, implying a higher crystallinity, compared with
the as-deposited film (Figure 1(b)). These results reveal that
the quality of ZrO2 film was improved after annealing. These
data indicate significant migration of atoms and thus changes in
film atomic structures at h igh temperatures.
The mean crystalline size in the ZrO2 films is determined
using the well-known Scherrer equation [13] on the basis of
ZrO2 peak with higher intensity as shown in Table 2. Th e c rystal
size calculated from the (
111
) peak of the anneal ed film is lar ger
than the as-deposited films due to the increase of the atom and
Table 1. Ionic concentrations of SBF in comparison with those of
human blood plasma.
Concentration
( mM) Blood Plasma SBF
Na+
K+
Ca2+
Mg2+
HCO3-
Cl-
HPO42-
SO42-
142.0
5.0
2.5
1.5
27.0
103.0
1.0
0.5
142.0
5.0
2.5
1.5
4.2
147.8
1.0
0.5
grain boundary mobility in the film [14]. These results reveal
that the quality of ZrO2 film was improved after anneali ng.
3.2. HA Formation on As-Deposited a nd A nnealing
ZrO 2 Films
The bioactivity of the ZrO2 films was assessed by SBF immer-
sion test s. The samples after incu bation in SBF for 7 d ays were
anal yzed by XRD to determi ne the cr ystal stru cture o f the newl y
formed layers. Figure 2 shows the formation of HA peaks as
well as the monoclinic ZrO2 peaks for both as-deposited and
annealed samples. Two new diffraction peaks at about 25.9˚
and 31.8˚ can be referred to crystalline apatite according to
JCPDS file no. 09-0432. The low intensity and broad peak in-
dicate that the amount of as-grown crystalline apatite on the
ZrO2 thin film is small and the crystallinity is low. When the
sample was annealed at 800˚C, the HA peak became more in-
tense and shaper (F igure 2(a)). This may ascribe to dense of
HA formation or high crystallinity of HA.
2 Theta
20 25 30 35 40
Intensity
M(111)
M(002)
M(200)
(a)
(b)
Figure 1. XRD patterns of the ZrO2 thin films deposited on 316L-
SS type substrate s : (a) annealing a t 800˚C and (b) as-deposited.
Tabl e 2. FWHM and cryst al size of zirc onia thin fi lms M(
111
).
Sample FWHM
(degrees) Crystal size (nm)
as-deposited
800˚C
0.90
0.59
18.45
28.16
2 Theta
20 25 30 35 40
Intensity
HA(002)
M(111)
HA(211)
M(002)
(a)
(b)
Figure 2. XRD patterns for the ZrO2 samples tested in SBF for 7
days: ( a) annealing at 800˚C and (b) as-deposited.

A. THAVEEDEETRAKUL ET AL.
47
The calcium and phosphate ions required for hydroxyapatite
generatio n on the film surface were derived from the S BF. The
result obtained from the present study indicates that Zr-OH
groups, abundant on the surface of the thin film, are able to
induce ap atite nucleatio n in a similar manner as Si -OH, Ti-OH,
and Ta-OH gro up do. Neverth eless, Uchida et al . [7] found the
apatite-forming ability of the gels with tetragonal or monoclinic
structure was apparently much higher than that of the gel with
amorphous structure. This implies that not all types of Zr-OH
groups , but only Zr-OH group with specific arrangements based
on tetragonal or monoclinic structure, are effective in inducing
apatit e nucleation.
The microstructures of the as-deposited and annealed ZrO2
films after soaked in SBF for 7 days were observed by SEM as
shown in Figure 3. Both the samples can induce hydroxyapa-
tite form on the films surface. The HA particl es was island -like
and spherically shaped with diameter size about 2 to 3 µm. For
the as-deposited sample (Figure 3(a)), only a small amount of
HA particles formed sparsely scattered on the surface of the
sample. The morphology is very similar to that of the deposited
apatite on the surface of zirconia gel through biomimetic
processing utilizing SBF [7]. However, a den ser and more con-
tinuous HA layer was observed on annealed films (Figure 3(b)).
The small HA particle size under the aggregation is about 600
nm in diameter. These results indicate that the annealed ZrO2
films with a high crystallinity are more bioactive compared
with low crystallized structure.
(a)
(b)
Figure 3. SEM micrographs of HA formation after soaking in S BF
solution for 7 days: (a) as-deposited and (b) annealing at 800˚C.
Wavenumber (cm
-1
)60012001800240030003600
Transmission (%)
H2OCO32-
PO43-
PO43-
OH-
CO32-
Figure 4. FT-IR spectra of HA formed on the sample s in SBF for 7
days.
In order to confirm the structure of the Ca-P layer, we per-
formed FTIR analysis, as shown in Figure 4. The phosphate
group itself has a tetrahedral symmetry; resulting in four vibra-
tional modes (symmetric stretch (ν1) at 958 cm-1; asymmetric
stretch (ν2) at 430 - 460 cm-1; (ν3) at 1041-1090 cm-1; (ν4) at
575 - 610 cm-1) [15]. An OH- absorption peak at 3440 cm-1 can
be seen in the spectra. The absorption peak at 1635 cm-1 can be
assigned to absorbed H2O groups, which is a common characte-
ristic of precipitates in aqueous solutions [16]. Furthermore,
peaks between 1400 - 1450 cm-1 are due to the C−O stretching
of C O32- groups, which indicate that the apatites formed on the
ZrO2 thin films are bone-like carbonate-containi ng apatite [ 16].
4. Conclusion
Zirconium dioxide thin films of monoclinic phase have been
deposited by DC unbalanced magnetron sputtering and the film
was subjected to thermal treatment. The annealed films show
the higher film crystalline due to the rearrangement of film
structure. After being immersed the samples in SBF, the
bone-like apatite was observed on all ZrO2 films, but a denser
and more continuous HA layer was ob served on annealed films
due to the crystallinity of ZrO2 films. These results show that
the sp ut tered Zr O 2 thin films exhibit good bioactivity.
5. Acknowledgements
This work was supported by the Royal Golden Jubilee of
Thailand Research Fund and the Department of Chemical En-
gineering at King Mongkut’s University of Technology Thon-
buri.
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