Available online at www.sciencedirect.com
Thin Solid Films 516 (2008) 5162 – 5167
www.elsevier.com/locate/tsf
Evaluation of adhesion strength and toughness
of fluoridated hydroxyapatite coatings
S. Zhang a,⁎, Y.S. Wang a , X.T. Zeng b , K.A. Khor a , Wenjian Weng c , D.E. Sun a
a
School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798
b
Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075
c
School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, PR China
Available online 13 July 2007
Abstract
Adhesion strength and fracture toughness are two crucial mechanical properties for bioceramic coatings on metal implants directly affecting
successful implantation and long-term stability. In this study, the adhesion strength of sol–gel derived FHA coatings on Ti6Al4V substrates was
measured by pull-out tensile test, and the toughness was assessed by energy release method. With increase of the degree of fluoridation, the
adhesion strength increases up to about 40% and the fracture toughness increases about 200 to 300%. Contrary to the wide-spread belief, it is
interesting to note that after soaking in the Tris-buffered physiological saline solution (for 21 days), the adhesion strength increases about 60% as
compared with the as-deposited coating, instead of decreasing. The mechanism of the increase is discussed.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Adhesion strength; Toughness; Fluoridated hydroxyapatite coating; Nanoindentation
1. Introduction
Hydroxyapatite (HA)-coated metallic implant has long been
recognized as the preferred hard tissue replacement/repair, especially
as load-bearing implants in orthopaedics and dentistry [1,2]. The
combination possesses the biological properties of HA and the
excellent mechanical properties of metallic substrate thus delivers a
reliable implant for patients. The HA coating also serves as
protection against corrosion of the otherwise bare metal in biological
environment [3]. It has been reported that the survival rate was
initially high for HA-coated implants; however, the high degradation
rate of HA coating in biological environment is a serious stability
concern, which could cause detrimental effect on adhesion
properties, resulting in undesirable debris and even delamination,
which eventually leads to the failure of the implant [4–6].
Recently, fluoridated hydroxyapatite (FHA, Ca10(PO4)6
(OH)2−xFx) has attracted much attention in replacement of
pure HA coating on metallic implants because it demonstrates
significant resistance to biodegradation while maintains comparable biocompatibility [7,8]. Fluorine is an essential element
⁎ Corresponding author. Tel.: +65 6790 4400; fax: +65 6791 1859.
E-mail address: [email protected] (S. Zhang).
0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.tsf.2007.07.063
for the development of human bones and teeth as well as the
prevention of dental caries [9,10]. Documented results indicate
that partial substitution of fluoride ions for OH− groups reduces
the solubility product of HA by ∼ 3.5 orders of magnitude [7].
Presence of fluoride ions also enhances the proliferation and
differentiation of osteoblastic cells and promotes bone regeneration [11,12]. Coating of HA and FHA can be realized via
thermal spraying [13], magnetron sputtering [14], pulsed laser
deposition [15] and sol–gel dip coating, [16] etc. In comparison,
sol–gel method has the advantages of composition homogeneity, low cost, ease in operation and doping of ions thus is used
widely, which is also the choice of method in this study.
Long-term stability of the bioactive ceramic-coated implants
comes from at least two critical aspects: low solubility of the
coating and high adhesion strength between coating and
substrate [17]. Solubility of HA can be reduced by incorporation of fluoride ions into HA lattice structure. But very few
reports are available on adhesion improvements, not to mention
adhesion studies after in vitro dissolution test. The present study
concentrates on the adhesion strength of sol–gel derived
fluoridated hydroxyapatite coatings on Ti6Al4V before and
after dissolution tests. Along with the adhesion measurement,
coating toughness is also evaluated.
S. Zhang et al. / Thin Solid Films 516 (2008) 5162–5167
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2. Experimental
2.1. Coating preparation and characterization
The preparation of dipping-sols and deposition of FHA
coatings were described in details in our previous work [16,18].
In brief, the selected precursors, i.e. calcium nitrate tetrahydrate
(Ca(NO3)2·4H2O, Sigma-Aldrich, AR), phosphorous pentoxide
(P2O5, Merk, GR) and hexafluorophosphoric acid (HPF6,
Sigma-Aldrich, GR) were dissolved in absolute ethanol
respectively for preparation of the dipping-sols. The designed
degree of substitution of OH− by F− was indicated by the x
value in the general formula of FHA, (Ca10(PO4)6(OH)2−xFx),
where x was selected as 0, 1 and 2, the subsequent coatings
obtained were labeled as F0, F1 and F2 respectively. Titanium
alloy (Ti6Al4V) slab of 20 × 30 × 1.2 mm polished to grade
#1200 of SiC sandpaper were used as substrate. The dipping run
was repeated 4 times for a final coating thickness of ∼ 1.5 μm.
In order to investigate the influence of dissolution behavior on
the adhesion strength, in vitro dissolution tests were carried out
by soaking FHA coatings in a Tris-buffered physiological saline
solution (TPS) (0.9%NaCl, pH7.4) at a constant temperature of
37 °C for 3 weeks. After that, the samples were taken out
followed by washing with DI water for 3 times, and then were
prepared for tensile tests as described in Section 2.2.
Surface roughness (Rq) of FHA coatings before and after
soaking in TPS solution was determined with a non-contact optical
profiler (Wyko NT2000, Veecco Instruments Inc. USA). Results of
X-ray diffraction analysis, coating surface morphology as well as
composition analysis were reported in our previous work [18,19].
2.2. Pull-out tensile test
The adhesion strength of the FHA coating on the metallic
substrate was measured using a Universal Instron mechanical
testing system (Instron 5569). A clamping fixture was designed
to avoid misalignment during the uniaxial tensile test: An eight-
Fig. 1. Schematic illustration of the pull-out tensile test for the evaluation of
adhesion strength.
Fig. 2. Pull-out adhesion strength of FHA coating before and after soaking in TPS
solutions. ⁎ indicates a significant increase of adhesion strength with respect to F0
(as prepared coatings); ⁎⁎ indicate a significant increase of adhesion strength with
respect to F0 (after soaking in TPS for 21 days).
millimeter diameter Al rod was glued onto the coating surface
with epoxy resin (Epoxy Adhesives DP460, 3M, ScotchWeld™, USA) and cured at room temperature for 24 hours; the
rod-sample combination then slide into a steel holder such that
the rod stuck out of the top opening for clamping onto the
Instron (Fig. 1). In this way, the lower top surface of the hollow
holder provided intimate contact with the sample surface which
transmitted the downward pulling force via a fixture attached to
the lower housing of the holder. In testing, the rod was pulled at
a cross-head speed of 1mm/min until the coating failure. The
SEM was used to evaluate the failure mode at the fracture
surface. A one-way ANOVA test was conducted to assess the
statistical significance of the adhesion and toughness results.
2.3. Toughness measurement
The toughness measurement of ceramic films and coatings is
still an unresolved issue since there is no international standard
or test procedure so far [20]. Since the nanoindentation-based
energy release method looks more convincing, we adopt the
energy method in this study. In the energy method, the energy
difference is examined before and after the crack formation and
propagation. This energy difference is considered responsible
for the through-thickness cracking in the coating. The energy
release is obtained from a “step” that is observed in the loaddisplacement curve during the indentation, thus the toughness
of the coating is determined via [21,22]:
DU
E
KIC ¼
d
ð1Þ
t 2pCR dð1 m2c Þ
Where νc is the poisson's ration of the coating, 2πCR is the
crack length in the coating plane, t and E are the coating
thickness and elastic modulus respectively, ΔU is the strain
energy difference before and after cracking.
Nanoindentation was carried out using a NANO Indenter XP
system (MTS Nano Instruments, USA) with a Berkovich
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S. Zhang et al. / Thin Solid Films 516 (2008) 5162–5167
Fig. 3. Typical adhesion failure surface of the FHA coatings without TPS soaking.
indenter. This instrument monitors and records the dynamic
load and displacement of the indenter during indentation
process. Indentations and associated cracks were observed
using SEM, and the length of cracks of the indentation was
analyzed using Image-Pro Plus image measurement software.
3. Results and discussion
3.1. Adhesion strength of the as-prepared FHA coatings
The measured nominal adhesion strength between the
coating and the Ti6Al4V substrate is shown in Fig. 2. By
“nominal” we do not distinguish “adhesion failure” and
“cohesion failure”. Without fluoridation (sample F0), the
adhesion strength is about 19 MPa. With incorporation of
fluoride ions (F1 and F2), the adhesion strength increases
significantly (p b 0.05) to about 26–27 MPa. There is no
significant difference between F1 and F2 (p N 0.05).
The adhesion strength between the coating and substrate
comes from two aspects, the mechanical interlocking and the
chemical bonding. In current work, since the substrates had the
same finish, the mechanical interlocking can be considered
identical. Therefore, the increase in the adhesion strength is
attributed to the stronger chemical bonds, which were
developed at the coating-substrate interface during coating
deposition process, especially at the firing stage. Our previous
studies obtained by scratch testing show that increasing fluoride
content accompanies increased scratch adhesion [18]. Meanwhile, formation of chemical bonds at the interfacial area
promises higher shear strength [23]. We proposed a tow-step
process to explain the formation of chemical bonds at the
interface [18]. According to this model, the presence of fluorine
ions not only modify the chemisorption and physisorption
properties of interface during the dipping and drying process,
but also attract more oxygen near to the interface to form
complex compound, e.g. Ti–O–Ca–P–F etc., during the firing
process. Incorporation of fluorine ions reduces the residual
stress (tensile) as much as 50%, thus will also contribute to
higher adhesion strength [23]. Weng et al. [24] also reported
higher adhesion strength (20.6 MPa) of FHA coating as
compared to pure HA coating (15 MPa). Lee et al.'s [25]
results confirmed that the adhesion strength improved from
∼20 MPa to ∼ 40 MPa when HA was fluoridated.
As fluoridation is up to the extent of x = 1 (sample F1),
further increase in fluoridation has no significant effect on
adhesion (Fig. 2). A typical adhesion failure topography of the
coatings is shown in Fig. 3. There are 3 different failure modes: 1)
the adhesion failure between the coating and substrate; 2) the
S. Zhang et al. / Thin Solid Films 516 (2008) 5162–5167
5165
Fig. 4. Typical adhesion failure topography of the FHA coatings after soaking in TPS for 21 days.
cohesion failure which happens inside the coating and 3) the
“adhesive failure” that happens between the epoxy and the
coating. Adhesion strength measurements the how strong the
coating bonds with the substrate; cohesion strength measures how
strong the coating itself holds together; the “adhesive failure”
signals poor bonding between the epoxy and the top (i.e., the outer
most) coating surface. A mixed failure is commonly observed,
consisting of all the three failures. From Fig. 3, the total area
fraction of adhesion and cohesion failure is only about 20%–30%,
the rest being “adhesive failure” (due to insufficient bonding or
adhesiveness between the coating surface and the epoxy). As
such, the adhesion strength should be a lot higher than 19 MPa for
HA and 27 MPa for FHA, thus the adhesion strength of the FHA
coatings are a lot more than the minimum 15 MPa using the pullout tensile test as stipulated by ISO standards (ISO 13779) [26] for
biomedical applications. As reference, the adhesion strength of
plasma sprayed HA coatings are usually in the range of ∼20–
30 MPa [27], and that of sol–gel method is generally lower than
30 MPa [8].
show significant increase in adhesion strength.of 55%, 65% and
66% for F0, F1 and F2 respectively. Though the small
improvement (∼ 43 MPa to ∼ 46 MPa) from different
concentrations of F is not believed to be real (p N 0.05), the
improvement from HA to FHA is statistically significant
(p b 0.05). This is contrary to the published results. Usually it
is believed that soaking in simulated body fluid results in
degradation of adhesion. The reasons of our improvement will
be discussed later.
Fig. 4 depicts typical adhesion failure topography of FHA
coatings after soaking in TPS for 21 days before the pull-out test
3.2. Adhesion strength of FHA coatings after soaking in TPS
Fig. 2 also plots the adhesion strength of the coating after
immersion in TPS for 21 days. Surprisingly, all the coatings
Fig. 5. Schematic illustration of the contact between the coating and the epoxy
resin.
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S. Zhang et al. / Thin Solid Films 516 (2008) 5162–5167
is conducted. Similar to that without soaking, a mixed failure is
observed. However, the area fraction of the adhesive failure
drastically drops to 30–40% (from 70–80% before soaking). In
other words, the total area fraction of adhesion and cohesion
failure increased from around 25% to around 65%. This
increase contributes to the increase in the overall adhesion
strength (c.f., Fig. 2, marked as TPS) to a value of around
45 MPa after soaking. The increase in area fraction of total
adhesion and cohesion failures after TPS soaking attributes to
the increase of surface roughness as a result of soaking: The
surface roughness (Rq) was around 267 ± 7, 293 ± 9 and 315 ±
15 nm for as-prepared F0, F1 and F2 respectively; after
immersion in TPS for 21 days, these values increased to 358 ±
10, 357 ± 12 and 370 ± 10 nm (or an increase of 34%, 22% and
17% respectively). Rougher surface provides more contact area
for epoxy-coating interface, as illustrated in Fig. 5. Since the
“adhesive failure” is a lot less after soaking, the “after soaking”
adhesion results should be more representative of the real
adhesion strength (still, the real adhesion strength should be
higher than 43 MPa for our FHA coatings because the test still
has some “adhesive failure”).
In the case of thermal sprayed HA coatings where “after
soaking” adhesion always tend to decrease (up to 75%). For
instance, the strength dropped from 27 MPa as tested before
soaking to 19 MPa after soaking in SBF for 2 weeks [28]. The
reason behind the drop was the existence of coating cracks
Fig. 7. Calculated fracture toughness of FHA coatings in relation to the degree of
fluoridation: ⁎ indicates a significant increase of fracture toughness with respect
to F0.
intrinsic to thermal spray deposited HA coatings. The impurity
phases (e.g. CaO, TCP etc.) may serve as the crack initiation
source, and the transversal cracks across the coating thickness
[13] will serve as the channels that lead the solution into the
bulk of the coating and the coating/substrate interface. Chemical
dissolution of the coatings inside the coating and at the interface
weakens the cohesion in the coating and the adhesion at the
interface, giving rise to decrease in overall adhesion strength
after SBF soaking. In contrast, our coatings are completely
dense, no surface cracks or through coating cracks exist (see
Ref. [18] which has the surface and cross-section of the coatings
used here), sipping of the solution into the coating or into the
interface areas is not possible. However, chemical dissolution
that takes place on the surface results in rougher surface, which
effectively aids the epoxy/coating bonding which, in turn,
helped reducing the area fraction of the “adhesive failure”.
3.3. Toughness of FHA coatings
Fig. 6. Typical SEM micrograph of indentations (a) and load-displacement curve
of indentation (b) on coating surface.
A typical SEM micrograph and the corresponding indentation curve of a coating are shown in Fig. 6. The coating around
the indenter bulges upwards (Fig. 6a), indicating delamination
and buckling of the coating. In the load-displacement curve, a
step is (Fig. 6b) caused by energy release corresponding to the
coating delamination and buckling during nanoindentation
process. Based on equation (1), the toughness of the coatings
are calculated and summarized in Fig. 7. Obviously, the
incorporation of fluorine has significant effect on the coating
fracture toughness: ∼ 0.12 MPam 1/2 of HA increases to
0.26 MPam1/2 for F1 and to 0.31 MPam1/2 for F2. Though
the difference between F1 and F2 is insignificant (p N 0.1), the
increase from “without” to “with” fluorine is more than doubled
and statistically significant (p b 0.01).
The reasons of increase in toughness are mainly attributed to
the following aspects caused by the incorporation of fluoride
ions. Firstly, the incorporation of fluoride ions causes a higher
elastic modulus (E) (c.f. ""Eq. (1)). The elastic moduli were
measured as about 47, 54 and 74 GPa for F0, F1 and F2
S. Zhang et al. / Thin Solid Films 516 (2008) 5162–5167
respectively [23]. Similar trend is also reported regarding the
enhancing effect of incorporated fluoride ions on Young's
modulus of FHA in Ref. [29]. Secondly, high adhesion strength
also benefits the fracture toughness. As discussed above,
fluoride ions incorporated into HA lattice structure give rise to
higher adhesion strength. From the scratch test results, the
coating-substrate interface becomes more ductile with fluoridation [18]. As such, the crack propagation is more restricted,
resulting in a smaller crack length (2πCR). Finally, residual
stress also plays a role in toughness. The presence of tensile
stress favors crack opening in indentation [30]. Therefore,
reduction of tensile stress reduces crack-sensitivity, thus
improves toughness. The decrease in tensile residual stress
was indeed observed with increase of fluoridation degree [23].
Meanwhile, the reduction of residual stress also benefits the
adhesion strength, which also indirectly contributes to the
coating toughness.
4. Conclusions
The nominal adhesion strength of sol–gel derived fluoridated hydroxyapatite (Ca10(PO4)6(OH)2−xFx) coatings on Ti6Al4V
substrates is measured by pull-out tensile test. The strength
ranges from ∼ 19 MPa for pure hydroxyapatite (x = 0) to
∼ 26 MPa for x = 1. After soaking in Tris-buffered physiological
saline solution for 21 days, the adhesion strength increases to
∼ 30 MPa in the case of pure HA and to over 40 MPa in the case
of FHA. With incorporation of fluorine (x = 1), the toughness of
the coating doubles as compared with HA. Both adhesion and
toughness do not have statistically significant improvement as
the fluoridation increases from x = 1 to x = 2.
Acknowledgements
This work is supported by the Agency for Science
Technology and Research, Singapore (A⁎Star) through project
032101 0005 and SIMTech-NTU collaboration project U03-S389B.
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