J. Biomedical Science and Engineering, 2008, 1, 190-194
Published Online November 2008 in SciRes. http://www.srpublishing.org/journal/jbise JBiSE
Fabrication and characterization of HAp /Al2O3
composite cating on titanium substrate
Zhou-Cheng Wang, Yong-Jin Ni, Jin-Cong Huang
Department of Chemical and Biological Engineering, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, China. Correspondence
should be addressed to Zhou-Cheng Wang (zcwang@xmu.edu.cn).
Received July 5, 2008; revised September 28, 2008; accepted September 28, 2008
HAp/Al2O3 composite coating was fabricated
onto micro-arc oxidized titanium substrate
using a combination of electrophoretic depo-
sition and reaction bonding process. SEM,
EDS and XRD were employed to characterize
the titanium substrate and as-prepared coat-
ings. The interfacial bonding strength of the
sintered composite coating was tested by
shear strength testing experiment. Results
show that the green form composite coating
can be easily sintered with no cracks and de-
composition at 850, the bonding strength to
the substrate is significantly improved com-
pared with the single HAp coating.
Keywords: Hydroxyapatite; Composite coating;
Electrophoretic deposition; Reaction bonding
Titanium and its alloys are widely used for dental and
orthopedic implants, because of their high mechanical
properties, chemical stability, and biocompatibility. Due
to its poor osteoconductive properties, coating of bioac-
tive hydroxyapatite (Ca10(PO4)6(OH)2, HAp), which has
similar chemical and crystallographic structure to the
main inorganic phase of human bone tissues, onto bio-
medical titanium implants has attracted widespread in-
terest in the orthopaedics biomedical field [1].
Many techniques have been investigated for depositing
HAp onto metallic implants, including plasma spray [2],
thermal spray [3], sol-gel processing [4], electrolytic
deposition [5] and electrophoretic deposition [6]. Among
these techniques, plasma spraying is the most developed
process and has been used in clinical practice; however,
this process suffers from facts that it requires complex
and costly equipment and being a line-of-sight process
which is difficult to apply uniform coatings on implants
with complex geometries.
Electrophoretic deposition (EPD) is a colloidal form-
ing technique where charged, colloidal particles in a sta-
ble suspension are deposited onto a positively charged
substrate by the application of electric field [7]. EPD has
recently gained increasing interest in the processing of
advanced ceramic materials and coatings not only be-
cause of its coast-effectiveness requiring simple appara-
tus, but also it offers important advantages in the deposi-
tion on substrates of complex geometry [8]. As for many
ambient-temperature powder coating processes, the de-
posit is in the form of a loosely packed particles which
must be subsequently densified by heating the coated
implant to elevated temperatures [9]. Electrophoretic
deposition of HAp coating onto metallic substrate has
gained wide interest and previous researches have dem-
onstrated that EPD is an attractive method for formation
of biomedical implants and a number of advantages of
this method have been suggested [10]. However, most of
the reports demonstrated that bonding strength between
HAp coating and titanium substrate is commonly low and
far from the requirement for clinical application.
The main problem associated with the EPD process is
the difficulty in the sintering of the coatings. First, high
sintering temperature is required for full densification of
the green coatings [9]. Lower sintering temperature leads
to weakly bonded and lowly-densified coatings, whereas
higher temperature can result in degradation of the metal
substrate and decomposition of HAp coating. Decompo-
sition of the HAp coating is undesirable as it leads to an
enhanced in vivo dissolution rate. Sintering temperatures
ideally should be below 1000 [9]. Second, the thermal
expansion coefficient of titanium substrate is much lower
than that of HAp (αTi= 8.7×10-6 /K, αHAp=13.6×10-6 /K),
so large thermal contraction mismatch would arise and
tend to induce the formation of cracks when cooled from
the elevated temperatures; besides, a significant firing
shrinkage during sintering will lead to the formation of
cracks in coatings as well.
In the present work, reaction bonding Al2O3 with rela-
tively lower thermal expansion coefficient
(αAl2O3=8.3×10-6 /K) was introduced into the HAp coating
to shorten the thermal expansion coefficient difference
with the titanium substrate. Meanwhile, the reaction
bonding process would overcome problems caused by the
firing shrinkage during sintering [7]. Both the two ad-
vantages have been proved to be beneficial in avoiding
the formation of cracks and improvement of bonding
strength of ceramic coatings [11].
SciRes Copyright © 2008
Z. C. Wang / J. Biomedical Science and Engineering 1 (2008) 190-194 191
SciRes Copyright © 2008 JBiSE
Surface modification of the titanium substrate was also
concerned. Chemically stable TiO2 can act as a bonding
layer and chemical barrier, which improves the interfacial
bonding, and prevent in vivo release of metal ions [12].
Among the methods used to produce the oxide layer on
titanium substrate, the micro-arc oxidation (MAO)
method has gained much interest, where anticorrosive
and rough oxide coatings can be easily fabricated [13-14].
Rough morphology has been proved to be beneficial in
mechanically anchoring the as-deposited coating [15]. So
MAO surface modification was done prior to the deposi-
tion of composite coating.
Commercially available titanium plates which were
shaped in a size of 30mm×10mm×0.8mm, were used as
the substrate materials. All the specimens were mechani-
cally polished with SiC water-proof abrasive papers.
Then they were degreased in a certain base solution and
pickled in an acid solution containing 100 mL/L HF and
300 mL/L HNO3. After that, the specimens were treated
with MAO in a sulfate solution. A platinum plate was
used as the counter electrode.
For preparation of the EPD suspension, HAp and Al
particles with an average particle size of 0.5 μm were
dried previously and dispersed in absolute ethanol. Drops
of nitrate were used to adjust the pH value of the suspen-
sion to 4-5 approximately. Then the suspension was
stirred in an ultrasonic agitator for 1 hour and aged for 1
day to allow full charging of the particles dispersed. Prior
to the deposition, the suspension was again stirred in the
ultrasonic agitator for 30 min. EPD was carried out using
applied voltages in the range 30-40 V for 30 s. Two par-
allel stainless steel plates were used as the anodes. The
deposited specimens were dried in air and then stored in
a drying container.
The heat treatment was done in a tubular electric re-
sistance furnace. The furnace was heated at a rate of
5/min to 660 and held at this temperature for 2 hours
to allow the occurrences of melting and oxidation of Al
particles. The temperature was then increased at
Figure 1. Schematic diagram of bonding strength testing.
5/min to 850 or 900 and held for 2 hours for sin-
tering. At last, the furnace was cooled in a rate of
1/min to room temperature. During the treatment, Ar
atmosphere of high purity was controlled in a proper ve-
locity to flow through the tube to protect the substrates
from excessive oxidation in elevated temperatures.
The surface and cross-sectional morphologies of the
MAO titanium and as-prepared coatings was observed by
LEO1530-FESEM (Germany). The element composition
of the composite coating was analyzed through EDS
(Oxford, England) attached to the FESEM. The phase
composition and thermal stability of the composite coat-
ing were analyzed by X-ray diffraction (Panalytical
X’pert, Philips).
The interfacial bonding strength was tested by shear
strength testing experiment according to ASTM-F1044
standard. This test relies on a bonding agent to remove
the coating with applied shear force as shown in Figure 1.
The value of the bonding strength can be calculated from
the fracture force over the stressed area.
Many researches have demonstrated that surface modify-
Figure 2.SEM images of the surface morphology of MAO treated
Figure 3.EDS spectrum of the composite coating.
192 Z. C. Wang / J. Biomedical Science and Engineering 1 (2008) 190-194
SciRes Copyright © 2008 JBiSE
(a) (b)
Figure 4. SEM images of the surface morphology of HAp/Al2O3 composite coating sintered at 850. (a: ×100; b: ×20,000).
(a) (b)
Figure 5. SEM images of the surface morphology of single HAp coating sintered at 850. (a: ×100; b: ×20,000).
Figure 6. Cross-sectional morphology of HAp/Al2O3 composite
coating sintered at 850.
Figure 7. XRD patterns of HAp/Al2O3 composite coating. (:
TiO2 (Rutile); : Cubic-Al2O3; : Al; : Decomposition products;
The others: HAp)
Z. C. Wang / J. Biomedical Science and Engineering 1 (2008) 190-194 193
SciRes Copyright © 2008 JBiSE
cation is essential to guarantee coating adhesion to me-
tallic substrate [14]. MAO is an advance coating process
for forming oxide layer on some anodic metal substrate
which is accompanied by visible plasma-like sparking at
the anode surface [13]. Prior to EPD of the composite
coating, the titanium substrate was treated by MAO in a
sulfate solution under constant current density of 50
mA/cm2. Surface morphology in Figure 2 shows that the
oxide film fabricated is rough and porous, which has
been proved to be beneficial for mechanically anchoring
the as-deposited coating [15].
The green form of HAp/Al composite coating was
co-deposited onto the titanium substrate from a suspen-
sion containing 10 g/L HAp and 10 g·L Al particles using
an applied voltage of 30 V for 30 s. The chemical com-
positions of the composite coating were determined
through EDS element analysis. Relating spectrum is
shown in Figure 3. The results show that elements Al,
Ca, P and O are all present in the composite coating,
which confirms that the co-deposition of HAp and Al
under the present condition is feasible. Quantitative
analysis based on the spectrum shows that the weight
percentage of Al in the composite coating was about
17.5%, and the Ca/P mole ratio was about 1.70, which is
approximately equal to the stoichiometric ratio of HAp,
and this confirms the chemical stability of HAp in the
Figure 4 shows the surface morphology of the
as-prepared HAp/Al2O3 composite coating after sintered
at 850. The surface morphology of single HAp coating
fabricated under the same conditions is also shown in
Figure 5 as comparison. As shown in Figure 4 (a), no
cracks were observed from the surface of the composite
coating; while for the single HAp coating as shown in
Figure 5 (a), numerous cracks were found because of the
firing shrinkage during sintering. Figure 4 (b) shows that
the composite coating was well sintered. It is obvious
that particles in the composite coating bond with each
other and grain size grows; however, for the single HAp
coating shown in Figure 5 (b), most particles appear to
remain stand-alone, bonding among the particles is not as
full as in the composite coating and there is nearly no
growth of grain size. It can be excluded that the sintering
property is greatly improved by the addition of Al to the
green form coating. The irregular shape of grains in the
composite coating implies the presence of liquid phase
during sintering which is known to be beneficial in pro-
moting the mass transport and bonding among grains.
The volume expansion associated with the oxidation re-
action of AlAl2O3 partially compensates for the sinter-
ing shrinkage and prevents the formation of cracks.
Figure 6 shows the cross-sectional morphology of the
composite coating after sintered at 850, which presents
an impression of a layered interfacial structure. A dense
oxide layer, which came from a composite oxidation of
MAO and heating oxidation, is present between the
composite coating and substrate. The oxide layer acts as
a bonding layer to bond the composite coating and sub-
strate together, which is able to improve the interfacial
bonding strength. Besides, the dense oxide layer is bene-
ficial in preventing the ion release from the metallic sub-
strate [13].
Figure 7 shows XRD patterns of the sintered coatings.
All the diffraction patterns confirm the presence of
rutile-TiO2 as the inner bonding layer shown in Figure 6,
and HAp as the main phase in the composite coating.
The sharp and clear reflections corresponding to HAp
confirm the phase purity and high crystallinity which is
critical for in vivo stability of the implants. The confir-
mation of the existence of Al2O3 in the HAp coating puts
the lowering the thermal expansion coefficient of the
composite coating into effect and contributes to the in-
crease of interfacial bonding strength.
The thermal stability of the as-prepared composite
coating was also studied to determine a proper heat
treatment condition. At the sintering temperature of
850, no signs of HAp decomposition can be found in
the relating XRD pattern. While sintered up to 900,
several peaks (marked by “”) of new phase arise in the
corresponding XRD pattern and imply the thermal de-
composition of HAp phase; but reflections characteristic
for HAp still well match its reference pattern, which
suggests that decomposition degree of HAp is not serious.
Hence, in order to guarantee the chemical and structural
integrity of HAp and its in vivo properties, the sintering
temperature should necessarily be controlled to be below
Interfacial bonding strength of the as-prepared
HAp/Al2O3 composite coating was tested. Results are
shown in Tab.1 where each value represents a statistic
average of three test data. Obviously the bonding
strength of HAp/Al2O3 composite coating is commonly
higher than that of single HAp coating under the two
sintering temperature. The improvement of interfacial
bonding can mainly be attributed to the application of
reaction bonding process. The process overcame the
cracking problem and improved the sintering property of
the coating as analyzed above. On the other hand, the
formation of reaction bonding Al2O3 with relatively
lower thermal expansion coefficient shortens the thermal
expansion coefficient difference between the coating and
the titanium substrate, and improves the interfacial
bonding as well.
Rough and porous oxide film, which has been proved to
be beneficial in mechanically anchoring the as-deposited
coating, was fabricated on titanium substrate through the
MAO technique. The electrophoretic co-deposition of
HAp and Al powder was achieved successfully to form
HAp/Al composite coating. The sintering temperature of
the composite coating should be controlled to be below
900 due to the thermal decomposition of HAp phase.
A crack-free and adhesive HAp/Al2O3 composite coating
was then successfully fabricated using the combination
of electrophoretic deposition and reaction bonding proc-
194 Z. C. Wang / J. Biomedical Science and Engineering 1 (2008) 190-194
SciRes Copyright © 2008 JBiSE
ess. The reaction bonding process promotes the coating’s
sintering densification and improves the substrate’s oxi-
dation resistance during the heat treatment. In compari-
son with the single HAp coating, the HAp/Al2O3 com-
posite coating exhibits much higher bonding strength.
Dr Zhou-Cheng Wang thanks for the financial support from Natural
Science Foundation of China (20573086), National Key Technology
R&D Program of China (2007BAE05B04) and National Basic Re-
search Program of China (973 Program) (2007CB935603).
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