Materials Science and Engineering C 46 (2015) 290–300
Contents lists available at ScienceDirect
Materials Science and Engineering C
journal homepage: www.elsevier.com/locate/msec
Strontium-substituted hydroxyapatite coatings deposited via
a co-deposition sputter technique
A.R. Boyd a,⁎, L. Rutledge a, L.D. Randolph b, B.J. Meenan a
a
b
Nanotechnology and Integrated Bioengineering Centre (NIBEC), School of Engineering, University of Ulster, Shore Road, Newtownabbey, Co. Antrim, BT37 0QB Northern Ireland, UK
Louvain Drug Research Institute, Université Catholique de Louvain, Brussels, Belgium
a r t i c l e
i n f o
Article history:
Received 28 July 2014
Received in revised form 22 September 2014
Accepted 21 October 2014
Available online 23 October 2014
Keywords:
Strontium substituted hydroxyapatite
RF magnetron sputtering
Co-deposition
X-Ray Photoelectron Spectroscopy (XPS)
X-Ray Diffraction (XRD)
a b s t r a c t
The bioactivity of hydroxyapatite (HA) coatings can be modified by the addition of different ions, such as silicon
(Si), lithium (Li), magnesium (Mg), zinc (Zn) or strontium (Sr) into the HA lattice. Of the ions listed here, strontium substituted hydroxyapatite (SrHA) coatings have received a lot of interest recently as Sr has been shown to
promote osteoblast proliferation and differentiation, and reduce osteoclast activity. In this study, SrHA coatings
were deposited onto titanium substrates using radio frequency (RF) magnetron co-sputtering (and compared
to those surfaces deposited from HA alone). FTIR, XPS, XRD, and SEM techniques were used to analyse the different coatings produced, whereby different combinations of pure HA and 13% Sr-substituted HA targets were
investigated. The results highlight that Sr could be successfully incorporated into the HA lattice to form SrHA
coatings. It was observed that as the number of SrHA sputtering targets in the study were increased (increasing
Sr content), the deposition rate decreased. It was also shown that as the Sr content of the coatings increased, so
did the degree of preferred 002 orientation of the coating (along with obvious changes in the surface morphology). This study has shown that RF magnetron sputtering (specifically co-sputtering), offers an appropriate
methodology to control the surface properties of Sr-substituted HA, such as the crystallinity, stoichiometry,
phase purity and surface morphology.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
The nature of the initial interaction between calcium phosphate
(CaP) thin films and osteoblasts can be mediated by the outermost
surface properties of that material. As such, the phase, crystallinity,
stoichiometry, composition and morphology of the CaP surfaces
are seen as key parameters that must be accurately controlled in
order to influence their potential biofunctionality with respect to
osteoblasts. Hydroxyapatite [HA–Ca10(PO4)6(OH)2] has been extensively studied due to the structural and chemical similarities demonstrated with the main inorganic constituent of bone tissue and
teeth. However, it is well documented that biological hydroxyapatite, which forms the mineral phases of calcified tissues (enamel,
dentin and bone), differ from pure and synthetically produced HA.
Biological apatite is comprised of a mixture of calcium phosphate
phases, such as tricalcium phosphate (TCP), carbonated hydroxyapatite (CHA) and calcium-deficient hydroxyapatite (CDHA). In this
regard, synthetic HA exhibits a Ca/P ratio of 1.67, whereas biological
apatite deviates significantly from this value and are known to be
⁎ Corresponding author at: Room 25B14, Nanotechnology and Integrated Bioengineering
Centre (NIBEC), School of Engineering, University of Ulster, Shore Road, Newtownabbey,
Co. Antrim, BT37 0QB Northern Ireland, UK.
E-mail address: [email protected] (A.R. Boyd).
http://dx.doi.org/10.1016/j.msec.2014.10.046
0928-4931/© 2014 Elsevier B.V. All rights reserved.
calcium-deficient (Ca(10 − x)(HPO4)x–(OH)2 − x), with a Ca/P ratio
which can be as low as 1.5 [1]. Numerous impurities such as fluoride,
magnesium, sodium, potassium, carbonate and chloride can also be
found in naturally occurring bone. In addition to this trace amounts of elements such as strontium, barium, copper, zinc, and iron are commonly
associated with biological apatite and may be seen as substituents in the
apatite structure [2]. Therefore, one approach to control the osteoblastic
response of HA coatings, both in vitro and in vivo, could involve the use
of substituted HA, incorporating different ions, such as silicon (Si) [3], lithium (Li) [4] magnesium (Mg) [5], carbonate (CO3) [6], zinc (Zn) [7], silver
(Ag) [8], fluoride, (F) [9], chloride (Cl) [10], potassium (K) [11], copper
(Cu) [12], sulphate (SO4) [13], tantalum (Ta) [14], cerium [15] or strontium (Sr) [16] into the HA lattice, in order to mirror the complex chemistry of human bone. Many reports of the use of these substituted materials
can be found in the literature, both as bone substitute materials and as
coatings [17–19]. Of the different substitutions listed above, strontium
substituted hydroxyapatite (SrHA) coatings have received a lot of interest
recently as strontium (Sr) has been shown to have the dual benefit of promoting bone formation and reducing bone resorption, in vivo. Furthermore, it has been shown that strontium has the ability to enhance preosteoblastic cell replication, to activate the Wnt/β-caterine signalling
pathway that will therefore stimulate the formation of new bone through
osteogenesis and differentiation into osteoblasts [20–23]. It has also been
shown in a number of other studies [24,25] that strontium has the ability
A.R. Boyd et al. / Materials Science and Engineering C 46 (2015) 290–300
to inhibit the activity of osteoclasts. It has been reported that the optimum
strontium concentration for enhanced osteoblast activity, including
proliferation and differentiation could be observed when the substitution
of strontium into HA was at a high level of 3–7% [26].
A number of different deposition methods have been investigated
to deposit Sr-substituted HA surfaces, including Radio Frequency (RF)
magnetron sputtering [16], plasma spraying [27], sol–gel methods
[28], pulsed laser deposition [29], micro-arc oxidation [30] and coblasting [17]. Of the methods highlighted here, RF magnetron sputtering
has been shown to be particularly useful for the deposition of Ca–P coatings due to the ability of the technique to provide greater control of the
coating's properties, along with improved adhesion between the substrate and the coating [30–32]. Specifically, RF magnetron sputtering allows co-deposition of multiple target materials simultaneously and
provides an alternative and simple method to produce Sr-substituted
HA coatings [32]. A previous study by Ozeki et al. focused on the deposition of Sr-substituted HA coatings, albeit it using a single sputtering
target of mixed composition (Sr-substituted HA powder) [16]. To date,
however, to the best of the authors' knowledge, there is no available
information on the utilisation of co-sputter deposition as a method to deposit Sr-substituted HA surfaces using RF magnetron sputtering, with the
primary objective of creating a surface with specific surface chemistry and
morphology commensurate with enhanced biofunctionality.
The present work was undertaken in order to study the co-sputter
deposition of Sr-substituted coatings from a custom designed RF magnetron sputtering facility utilising three sputtering targets (referred to
as sources). In particular, the influence of different target compositions
and configurations on the properties of the Sr-substituted sputter
deposited coatings produced at a low discharge power level (150 W)
were investigated. A low discharge power level was chosen for this
study as the quality and consistency of the targets used could be guaranteed throughout the sputter deposition runs. All of the coatings produced were characterised after post-deposition annealing to 500 °C
using Fourier Transform Infrared Spectroscopy (FTIR), X-Ray Diffraction
(XRD), X-Ray Photoelectron Spectroscopy (XPS), Scanning Electron
Microscopy (SEM) and stylus profilometry.
2. Materials and methods
2.1. Substrate preparation
For this study coupons of chemically pure titanium (cpTi), Titanium
International Ltd (15 mm × 15 mm × 0.5 mm) were abraded using a
succession of 800, and 1200 grade SiC papers. The coupons were twice
sonicated for ten minutes consecutively in acetone, isopropyl alcohol
and distilled de-ionised water. The abraded coupons were then dried
thoroughly in a convection oven at 70 °C for twelve hours.
2.2. Sputtering procedure
Sputtering targets, 76 mm in diameter and 5 mm thick, were produced by dry pressing hydroxyapatite [HA — (Plasma Biotal Captal-R)]
and 13% Biphasic Calcium Phosphate [SrHA — (Himed Inc. NY, USA)]
powders (at a load of 40 kN for 10 min with a loading rate of 10 kN
per minute). RF magnetron sputtering was performed using a cluster
of three high vacuum Torus 3 M sputtering sources in a custom designed
system (Kurt J. Lesker Ltd, USA) each operating with a 13.56 MHz RF
generator and an impedance matching network (Huettinger, GmbH,
Germany). The sources were all mounted at 65° to the substrate surface.
For deposition from the sputtering targets onto the cpTi substrates, the
RF power in the sputtering system was ramped up slowly to provide an
initial break-in phase, thereby minimising any thermal shock effects.
The break-in prior to deposition from the HA target was conducted at
a ramp rate of 5 Watts (W) per minute (all with the source shutter
closed). The base pressure was below 5 × 10−6 Pa, with an argon gas
flow rate (BOC, 99.995%) of between 15–20 Sc cm and a throw distance
291
of 100 mm. To produce the co-deposited coatings with different levels
of Sr-substitution, targets combinations in the three source multitarget configuration were utilised as outlined in Table 1. The fragility
of the HA and 13SrHA targets limited their power absorption capacity
and consequently deposition was performed at 150 W for 5 h under
the same atmospheric conditions as were used for the target break-in
procedure. The power density for these HA targets was approximately
3.3 W cm−2. After sputter deposition, the Ca–P coatings were thermally
annealed in order to enhance their crystallinity. The samples were subjected to a ramp rate of 5 °C per minute to 500 °C (from room temperature) with a soak time of 2 h and a ramp rate of 5 °C per minute back
down to room temperature.
2.3. Characterisation of the Ca–P powders and coatings
Fourier Transform Infrared (FTIR) spectroscopy of the samples was
carried out using a BIORAD FTS 3000MX Excalibur series instrument
with a PIKE Diffuse Reflectance Infrared Fourier Transform Spectroscopy
(DRIFTS) accessory. Samples were analysed from 4000–400 cm−1 in
absorbance mode at a resolution of 4 cm−1 with 20 scans per sample.
X-Ray Diffraction (XRD) of the samples was performed using a
Bruker D8 Discover Diffractometer fitted with a Gobel Mirror. A Cu Kα
X-Ray radiation (λ = 1.540 Å) source was employed with diffraction
scans obtained at a tube voltage of 40 kV and a tube current of 40 mA.
Each diffraction scan was recorded at 2θ values from 20–50° with a
step size of 0.04° and a scan dwell time for each increment of 30 s. For
the grazing incidence angle XRD studies of Ca–P coatings on the cpTi
substrates the tube angle was set to 0.75°.
X-Ray Photoelectron Spectroscopy (XPS) of the samples was undertaken out using a Kratos Axis Ultra DLD spectrometer. Spectra were recorded by employing monochromated Al Kα X-Rays (hν = 1486.6
electron volts (eV)) operating at 5 kV and 15 mA (75 W). The base pressure was 1.33 × 10−7 Pa and the operating pressure was 6.66 ×10−7 Pa.
A hybrid lens mode was employed during analysis (electrostatic and
magnetic), with an analysis area of approximately 300 μm ×700 μm
and a take-off angle (TOA) of 90° with respect to the sample surface.
Wide energy survey scans (WESS) were obtained at a pass energy of
160 eV. High resolution spectra were recorded for O1s, Ca2p and P2p
and C1s (incorporating the Sr3p peak) at a pass energy of 20 eV. The
Kratos charge neutraliser system was used on all samples with a filament current of 1.95 A and a charge balance of between 3.3 and 3.6 V.
Sample charging effects on the measured BE positions were corrected
by setting the lowest BE component of the C1s spectral envelope to
285.0 eV, i.e. the value generally accepted for adventitious carbon surface contamination [34]. Photoelectron spectra were further processed
by subtracting a linear background and using the peak area for the
most intense spectral line of each of the detected elemental species to
determine the % atomic concentration.
The deposition rate (and thickness) of the different Ca–P coatings
were determined using a Dektak 8 stylus profilometer (Veeco Instruments Inc., USA). Measurements were taken across 10 step height
positions on each sample created by masking titanium coated silicon
substrates with aluminium foil prior to deposition in the sputtering
system. A 12.5 μm diameter diamond tipped stylus was employed
with scans lengths of 1000 μm at a load of 15 mg.
Table 1
Target configuration versus coating thickness.
Target configuration
Coating thickness (nm)
3 HA
2HA/1SrHA
1HA/2SrHA
3SrHA
445
341
248
204
±
±
±
±
16
17
14
11
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A.R. Boyd et al. / Materials Science and Engineering C 46 (2015) 290–300
3. Results
3.1. Characterisation of the Ca–P precursor powders
The properties of the HA and the Sr-substituted HA precursor
powders were characterised using FTIR, XRD and XPS to determine
the nature of the material prior to its use as a target for the sputter deposition of Ca–P coatings.
Fig. 1(a) shows a typical FTIR spectrum of the HA precursor powder
in the range 4000–500 cm−1. Absorption bands characteristic of P–O
stretching vibrations can be observed between 1080 and 950 cm− 1
while O–P–O bending vibrations can be seen between 605 and
550 cm−1 [32]. The bands at 3570 and 632 cm−1 are indicative of O–
H stretching and O–H librational bands, respectively [30,31]. In addition
to these spectral modes, weak bands associated with carbonate (CO2−
3 )
were observed at 1409, 1434, 1543 cm−1, along with an asymmetrical
envelope centred at 869 cm−1 [6]. These results may indicate that this
substitution at both the OH− (A-site) and PO3−
HA material has CO2−
3
4
(B-site) positions [6,33]. Fig. 1(b) highlights the FTIR results for the
SrHA precursor powder. Absorption bands characteristic of P–O
stretching vibrations can be observed between 1100 and 950 cm− 1
while O–P–O bending vibrations can be seen at 610 and 550 cm− 1
[31–33]. The weak band at 3567 cm−1 is indicative of an O–H stretching
vibration, however the peak expected for the O–H libration at around
630 cm−1 is clearly absent. Peaks associated with carbonate (CO23 −)
were also observed at 873, and between 1550–1400 cm− 1 [6]. The
groups
peak observed at 873 cm−1 may also be associated with HPO2−
4
[34,35]. As with the HA powder, the FTIR results for the SrHA powder insubstitution in both
dicate that this HA material has significant CO2−
3
the A and B sites [6]. It should be noted that the envelope relating to
the P–O stretching bands between 960 and 1100 cm−1 for the SrHA
powder has significantly lower peak resolution when compared to the
HA powder. These observations would be consistent with Sr substituting for calcium (Ca) in the HA lattice [36,37].
The XRD pattern for the HA powder shown in Fig. 2(a) is equivalent
to HA, with diffraction peaks observed at 2-theta (2θ) values that correspond closely to those indicated for HA from the International Centre for
Diffraction Data (ICDD) file #00-09-0432. The peak positions signify
that the material contains only HA and no additional impurity phases.
The XRD pattern for the SrHA powder is shown in Fig. 2(b). As expected
for a Sr-substituted material, the peak positions are shifted to slightly
lower 2-theta (2θ) values and show significant broadening [36–38].
Fig. 3(a) shows a typical XPS wide energy survey-scan recorded as
B.E. [0–600 eV] versus intensity in counts for the HA precursor powder.
Peaks corresponding to Ca2s (438.8 eV), Ca2p3/2 (347.5 eV), Ca2p1/2
(351.1 eV), P2p (133.6 eV), P2s (191.1 eV), O1s (531.6 eV), O AugerKLL
(764.0 eV) and Ca AugerLMM (963.8 eV) are clearly observed (and further reported in Table 2), and correspond closely to those reported in
the literature for HA [31,32]. No other elemental species were detected
at least at the detection limits of the instrument (~0.1 at.% concentration). The Ca/P ratio of the HA precursor powder, as determined by
XPS was 1.67 ± 0.25 and matched the value for stoichiometric HA
(1.67), as shown in Table 3. Fig. 3(b) shows a typical XPS wide energy
survey-scan recorded as B.E. [0–600 eV] versus intensity in counts for
the SrHA precursor powder. Peaks corresponding to Ca2s (438.8 eV),
Ca2p3/2 (347.6 eV), Ca2p1/2 (351.0 eV), P2s (191.0 eV), O1s
(531.6 eV), Sr3p3/2 (268.9 eV), Sr3p1/2 (280.1 eV), Ca3s (44.0 eV), O2s
(23.0 eV) and Ca3p (25.0 eV) are all clearly evident (as reported in
Table 2). The envelope for P2p and Sr3d peaks overlap significantly
and can be seen around 133.6 eV. The spectral envelope observed at
133.6 eV consists of an overlap of the Sr3d and P2p peaks because the
Sr3d5/2 (133 ± 0.5 eV), Sr3d3/2 (135 ± 0.5 eV) and the P2p (132–
133 eV) lines are located so close together. All of these peaks observed
correspond closely to those reported in the literature for Sr-substituted
HA [39,40]. No other elemental species were detected here for the SrHA
powder. The Ca/P ratio, Ca + Sr/P and Sr + Ca/Sr ratios of the SrHA powder, as determined by XPS was 1.33 ± 0.24, 2.02 ± 0.33 and 6.69 ± 0.28
respectively. The Sr/Sr + Ca ratio value for the SrHA powder (as shown in
Table 3) was slightly below the value expected for this material [16].
However, it should be noted that XPS is a surface analysis technique
and only obtains information from the top 5–10 nm of any sample.
From these findings it is clear that purity, crystallinity and stoichiometry of the HA powder was as expected and confirmed the presence of HA,
contamination. In comparison,
which also contained low levels of CO2−
3
Fig. 1. FTIR spectra for (a) HA powder, (b) 13SrHA powder.
A.R. Boyd et al. / Materials Science and Engineering C 46 (2015) 290–300
293
Fig. 2. XRD patterns for (a) HA powder, (b) 13SrHA powder.
the results for the SrHA powder confirmed the presence of significant
levels of Sr-substitution within the lattice, albeit at values below that
expected. Furthermore, there is evidence of A–B CO2−
3 substitution within
the SrHA powder, which is not unexpected [41].
3.2. Characterisation of the Ca–P coatings
The coatings produced from the HA, and the different SrHA/HA
target combinations were analysed using FTIR, XRD, XPS, stylus profilometry and SEM to assess the surface properties after thermal annealing at 500 °C. The as-deposited CaP and Sr substituted CaP coatings
were not analysed in this study as previous work by the authors has
shown that these surfaces are amorphous and require thermal annealing
in order to produce crystalline coatings [31,32]. Therefore, all coatings
that are described here are those that have been annealed to 500 °C.
The thicknesses of the various different coatings are reported in Table 1.
It is observed that as the number of Sr targets utilised in the deposition
runs increases the deposition rate decreases significantly.
The FTIR spectrum of the coatings produced from the HA targets (3
HA) were indicative of crystalline HA, as shown in Fig. 4(a). Well resolved P–O stretching vibrations were observed as expected between
1100–950 cm−1. O–P–O bending vibrations are also present between
620–560 cm−1 [31,32]. Hydrogen phosphate bands (HPO2−
4 ) can also
be observed at 1120 and 582 cm−1 [32]. A very weak O–H librational
band is observed around 630 cm−1 as a weak shoulder, with a further
peak associated with O–H groups observed at 3568 cm− 1. This peak
may be associated with O–H stretching groups within the film [31,32].
The absence of strong OH functional groups, commonly observed at approximately 632 cm−1 and 3570 cm−1, indicate a degree of dehydroxylation within the Ca–P crystal structure deposited from the HA targets
under the conditions employed here. A further peak associated with
O–H groups can be observed at 3541 cm− 1 and may be associated
with loosely bound O–H groups, or disordered water molecules trapped
species were
within the coating [40,42]. No peaks indicative of CO2−
3
observed in the FTIR spectrum for the HA derived coatings. The peak positions in the FTIR spectra for the coatings derived from the 2HA/1SrHA
target configuration were largely similar to those observed for the HA
derived coating as shown in Fig. 4(b). Furthermore, the OH− peak at
around 3570 cm−1 is still clearly present, along with the OH− libration
at 632 cm−1. However, the OH− libration is only present as a very weak
shoulder in the coatings derived from the 2HA/1SrHA target configuration, which would be expected for a coating where Sr substitutes for Ca
bands are observed
in the HA lattice [38,43]. Furthermore, weak CO2−
3
between 1550–1400 cm−1 and 890–820 cm−1 [6,33]. However, this
peak between 890–820 cm−1 may also be due to the presence of
and/or SrCO3 [43]. For those coatings produced from the 1HA/
HPO2−
4
2SrHA and 3SrHA target configurations, the phosphate bands (PO2−
4 )
and hydrogen phosphate bands (HPO2−
4 ) bands are observed in largely
similar peak positions as would be expected for SrHA [38,43]. However,
the OH stretching vibrations around 3570 cm−1 and the OH− librations
around 632 cm−1 are clearly absent for these coatings. Similar weak
bands are also observed for both the coatings produced from the
CO2−
3
1HA/2SrHA and 3SrHA target configurations. These results would be typical for coatings whereby the Sr content is increasing [36–38,44].
The peaks observed in the XRD patterns for the Ca–P coatings derived from the HA targets (3 HA), as shown in Fig. 5(a), correspond
closely to those in the ICDD file #09-0432 for HA. Furthermore, the intensity of the 002 reflection at 25.9° 2θ is higher than would be expected
for a HA coating, which suggests that this coating may have a 002 preferred orientation and would be line with expectations for coatings deposited under the conditions employed here [36–38]. The XRD pattern
for the 2HA/1SrHA derived coating is shown in Fig. 5(b), however, as expected for a Sr-substituted material, the peak positions are shifted to
slightly lower 2-theta (2θ) values [44,45]. No significant peak broadening was observed for this particular coating. A similar effect (in relation
to lower 2θ peak values) is again observed in Fig. 5(c) for the 2HA/
1SrHA and 3SrHA derived coatings, as shown in Fig. 5(c) and (d),
respectively. Furthermore, the intensity of the 002 peak appears to increase with increasing Sr content in the coatings. There is also a change
in the relative peak intensities of the 211 and 112 peaks with increasing
Sr content, as shown in Fig. 5. In the case of all the HA and SrHA derived
coatings, no significant amorphous background hump was observed. In
addition to this no other Ca–P phases were detected in the XRD pattern
for any of the coatings.
The XPS results for the CaP coatings deposited from the HA
targets, as highlighted in Figs. 6(a), 7(a)–(d) and Table 2, shows peaks
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A.R. Boyd et al. / Materials Science and Engineering C 46 (2015) 290–300
Fig. 3. XPS Wide Energy Survey Scans (WESS) for (a) HA powder, (b) 13SrHA powder.
corresponding closely to those reported for HA in the literature and those
obtained for the precursor powder used to produce the sputtering targets
[31,32]. The Ca/P ratio for the HA derived coatings were 1.79 ± 0.13, as
reported in Table 3. The Ca/P ratio of the HA derived coating is slightly
higher than that expected for stoichiometric HA, however, it is line with
expectations for coatings produced under the conditions employed here
[34–36]. The peak positions reported for all the SrHA derived coatings
were all in line with those expected for Sr-doped CaP materials, with
clear evidence for the presence of Sr highlighted by the presence of
the Sr3p3/2 and Sr3p1/2 peaks at approximately 269.0 and 280.0 eV, respectively [39,40]. These results are clearly shown in Figs. 6(b)–(d),
7(e)–(p) and Table 2. No other elemental species were detected for
these coatings. The Ca/P and Ca + Sr/P ratios for the 2HA/1SrHA derived
coatings were 1.38 ± 0.24 and 1.46 ± 0.17, respectively (as shown
in Table 3). For the 1HA/2SrHA derived coatings, the Ca/P ratio is comparable to the 2HA/1SrHA (1.34 ± 0.24), however, the Ca + Sr/P increases
significantly to 1.62 ± 0.45. For the 3SrHA derived coatings both the Ca/P
and Ca + Sr/P ratios increase to 1.51 ± 0.11 and 1.95 ± 0.14, respectively
(as shown in Table 3). If the Sr/Ca + Sr ratios are also compared for the
different Sr-substituted coatings (as shown in Table 3), the ratios increase
as the number of Sr targets in the deposition process increases. It is clear
from the results highlighted here that with increasing Sr-content in the
Table 2
XPS peak positions for the different target materials and coatings.
Sample
HA powder
13% SrHA powder
HA coating
2HA/1SrHA coating
1HA/2SrHA coating
3SrHA coating
Peak positions—B.E. (eV)
C1s
O1s
Ca2p3/2/Ca2p1/2
P2p/Sr3d5/2/Sr3d3/2
Sr3p3/2/Sr3p1/2
285.0
285.0
285.0
285.0
285.0
285.0
531.3
531.6
531.2
531.5
531.3
531.1
347.4/350.9
347.6/351.0
347.4/351.0
347.6/351.1
347.6/351.0
347.4/350.8
133.4
133.6
133.5
133.6
133.5
133.4
–
268.9/280.1
–
269.6/279.9
269.6/279.9
269.7/279.8
A.R. Boyd et al. / Materials Science and Engineering C 46 (2015) 290–300
Table 3
Comparative XPS data for the different target materials and coatings.
Sample
Ratios
Ca/P
HA powder
13% SrHA powder
HA coating
2HA/1SrHA coating
1HA/2SrHA coating
3SrHA coating
1.67
1.33
1.79
1.38
1.34
1.51
±
±
±
±
±
±
0.25
0.24
0.13
0.24
0.14
0.11
Sr + Ca/P
Sr + Ca/Ca
–
2.02
–
1.46
1.62
1.95
–
6.69
–
1.06
1.20
1.30
± 0.33
± 0.17
± 0.45
± 0.14
± 0.28
± 0.02
± 0.08
± 0.02
sputtering targets there is an increase in Sr content of the sputter deposited coatings (albeit at lower levels than might have been expected),
especially for the 1HA/2SrHA and 3SrHA derived coatings.
SEM was utilised here to determine the surface morphology of each
of the different coatings produced. SEM analysis of the abraded cpTi
substrate shows a surface that has random abrasion scratches running
across its surface (not shown here). These abrasion scratches dominate
the substrate surface and can range in width from 0.5 to 2.0 μm. Small
hillocks, pits and fissures, which vary in size up to 2.0 μm, are also
seen regularly across the surface, particularly between the polishing
scratches. The corresponding SEM results for the CaP coating derived
from HA targets on the cpTi substrate, as illustrated in Fig. 8(a) shows
a relative tendency for the coatings to mimic the substrate topography
and to conform to striations and defects produced by the abrasion of
the substrate. Again, there is evidence from the SEM to suggest that
there has been significant in-filling of the larger pits on the substrate
surface, which is typical of previous studies utilising similar RF
magnetron sputter deposited calcium phosphate coatings [38]. Furthermore, the SEM analysis of the HA derived coatings as shown in
Fig. 8(a) indicates the presence of spherical features of around 150–
300 nm in diameter. These spherical features are observed right across
the entire surface of the coating, however, they do tend to be more evident around abrasion scratches and surface defects and have dimensions similar to those observed previously deposited on similar
surfaces [31,32]. In comparison to this surface, the 2HA/1SrHA derived
surface has a significantly reduced amount of spherical features, as can
be seen in Fig. 8(b). These features are of similar dimensions and also
295
tend to be found around abrasion scratches and surface defects. For
the 1HA/2SrHA and 3SrHA derived coatings very few spherical features
are observed on the surface, as illustrated in Fig. 8(c) and (d), respectively. It is evident from these SEM images that as the Sr content of
the coatings increases the surface features observed become much less
prominent across the SrHA surfaces.
4. Discussion
RF magnetron sputter co-deposition from HA and Sr-substituted HA
targets in a tri-source sputtering system was utilsed in this study in
order to create a range of different SrHA coatings on titanium substrates
with varying amounts of Sr. Before sputtering the HA and 13% Srsubstituted HA target materials (SrHA) were analysed using FTIR, XRD
and XPS techniques. The purity, crystallinity and stoichiometry
(1.67 ± 0.25) of the HA powder was as expected and confirmed the
presence of carbonated HA. From the FTIR, XRD and XPS results for
the Sr-substituted HA powder (13% substituted HA) it is clear that the
purity, crystallinity and stoichiometry of these Sr-substituted HA powders were largely as expected and confirmed the Sr-substitution of Ca
within the HA lattice. The XPS results highlighted that the Ca/P ratio
and Ca + Sr/P ratio of the SrHA powder, was 1.33 ± 0.24 and 2.02 ±
0.33, respectively. The corresponding Sr/Sr + Ca ratio for the SrHA powder is 6.69 ± 0.28, well below that expected for the different powder
materials. As previously stated, XPS is a surface specific analytical
technique (5–10 nm) and the values reported here may not reflect the
chemistry of the bulk of the material. The corresponding FTIR spectra
highlighted that the presence of Sr in the HA lattice resulted in signifipeaks between 1200–
cant peak broadening with respect to the PO3−
4
900 cm−1. These peaks are also seen to shift to slightly lower
wavenumbers, which is indicative of Sr incorporation in the HA lattice
[43]. The Sr-substituted HA powder also became more dehydroxylated
as a result of the presence of Sr, with the OH− stretching and librational
bands in the FTIR spectra around 3570 cm−1 and 632 cm−1 becoming
much less prominent (or totally absent in the case of the OH− librational
band). Peak broadening, shifting and dehydroxylation are all indicative of
Sr incorporation of the HA lattice [36]. There is also a significant increase
peaks between 1550–1400 cm−1 and at
in the intensity of the CO2−
3
groups by
870 cm−1, indicating substitution of both the OH− and PO3−
4
Fig. 4. FTIR spectra for (a) HA coating, (b) 1Sr 2HA coating, (c) 2Sr 1HA coating and (d) 3Sr coating.
296
A.R. Boyd et al. / Materials Science and Engineering C 46 (2015) 290–300
Fig. 5. XRD patterns (a) HA coating, (b) 1Sr 2HA coating, (c) 2Sr 1HA coating and (d) 3Sr coating.
2−
CO2−
3 (A–B type substitution) [6,36,45] Despite this, no significant HPO4
peaks were observed in the FTIR spectra for any of the precursor powders
as might have been expected for a Sr-substituted CaP material [37].
Typically, Sr substitution for Ca in the HA lattice results in an increase in
and HPO2−
groups (especially CO2−
groups) due to the
both CO2−
3
4
3
increased lattice strain caused by the larger Sr ions [36,40]. The XRD results for the Sr-substituted powder also highlighted that due to the incorporation of Sr into the HA lattice, the peak positions shifted to slightly
lower 2-theta (2θ) values. The peaks also exhibited significant broadening
with increasing Sr content as can be observed in Fig. 2. This would be in
Fig. 6. XPS Wide Energy Survey Scans (WESS) for (a) HA coating, (b) 1Sr 2HA coating, (c) 2Sr 1HA coating and (d) 3Sr coating.
A.R. Boyd et al. / Materials Science and Engineering C 46 (2015) 290–300
Fig. 7. High resolution XPS spectra for (a–d) HA coating, (e–h) 1Sr 2HA coating, (i–l) 2Sr 1HA coating and (m–p) 3Sr coating.
Fig. 8. SEM images for (a) HA coating (b) 1Sr/2HA coating, (c) 2Sr/1HA coating and (d) 3Sr derived coatings.
297
298
A.R. Boyd et al. / Materials Science and Engineering C 46 (2015) 290–300
line with expectations and is due to substitutional strain in the lattice [35,
36,46]. The XRD results also highlighted that no other CaP phases were
detected in any of the Sr-substituted powders. The XRD results for the
Sr-substituted HA powders corroborate those results highlighted from
the XPS and FTIR analyses confirming that the SrHA powder contains appreciable levels of Sr, with Sr substituting for Ca in the HA lattice. This usually results in structural disorder along the c-axis. It is well documented
that a lack of hydroxylation within the HA lattice affects the degree of
atomic ordering in HA [45,47]. Interestingly, for the Sr-substituted HA
powder used here to produce the sputtering targets, no pronounced
002 preferred orientation was observed in the diffraction pattern. Previous studies on Sr-substituted apatites have shown that as the Sr content
in HA increases, so does c-axis orientation [36]. A typical random crystallite orientation was observed for all of the Sr-substituted powders used to
produce the sputtering targets in this body of work.
After sputtering these different materials onto the cpTi substrates
distinct changes were observed in their surfaces properties as determined using FTIR, XPS, XRD, stylus profilometry and SEM analyses.
The surface Ca/P ratio of the thin films deposited from three HA targets
was determined to be 1.79 ± 0.13, which is slightly higher than that expected for stoichiometric HA [36,37]. This value is in line with the expectation of coatings produced under the conditions employed here
[31,32]. Furthermore, no Sr is detected on the surface of these coatings
(as confirmed by the XPS results). The coatings produced from the
three HA targets also show the expected 002 preferred c-axis orientation, as determined by XRD analyses. Typically, 002 preferred orientation in HA coatings results in an elevated Ca/P ratio, which is in line
with the Ca/P ratio determined here [48]. To add to this, the FTIR results
highlight that these coatings produced from the HA precursor powders
contain no trace of CO2−
3 within the lattice (as might have been expected) but do show evidence of significant HPO24 − functional groups due to
the strong peaks around 1120 and 580 cm−1 (as shown in Fig. 4(a)).
The HA derived coatings are also slightly dehydroxylated. In this case,
the OH− libration is only observed as a shoulder around 630 cm−1. Typically, it is expected that if HA is dehydroxylated then the charge balance
within the crystal structure must therefore be provided by other means.
HA is understood to undergo carbonate (CO2−
3 ) substitution within the
crystal lattice at multiples sites [6,45]. The FTIR results obtained here
groups in the lattice
cannot confirm the presence (or absence) of CO2−
3
of any of the samples due to the observed specular reflectance in the
spectra above 1200 cm−1, therefore a direct determination of CO23 −
substitution cannot be made. However, the XPS analyses do suggest
the presence of CO2−
3 on the surface of the coatings due to the peak observed around 288–290 eV in the C1s envelope (as shown in Fig. 7(c)).
substitution in
This would suggest incorporation of low levels of CO2−
3
these HA derived coatings. It is assumed that the HA derived coating deposited here is not a calcium deficient hydroxyapatite (CDHA) material
[Ca10 − X[(PO4)6 − X(HPO4)X](OH)2 − X], given that this would result in
a material that has a Ca/P ratio less than 1.67 [45]. The presence of a
Ca10 − X − Y / 2[HPO4)(PO4)]6 − X − Y(CO3)Y(OH)2 − X species in the
HA derived coating is more likely. The slightly elevated Ca/P ratio observed here in the XPS results may also be a consequence of calcium enrichment of the uppermost surface of the HA derived coatings, as has
been observed in coatings produced previously under similar conditions [31,32]. This has been described previously, whereby when such
surfaces are thermally processed at high temperatures (above 500 °C),
weakly bound PO34 − species are lost as a consequence of thermal
processing.
In comparison, the XPS results for the 2HA/1SrHA derived coatings
were in line with expectations for a Sr-substituted HA material. Peaks
for Sr3p3/2 and Sr3p1/2 can be clearly observed around 269.6 and
279.9 eV in addition to the expected peaks for a CaP material [31,32,
39]. Figs. 6(b) and 7(e)–(h) show representative wide energy survey
scans (WESS) and high resolution spectra for the 2HA/1SrHA derived
coatings. The Ca/P ratios (and corresponding Ca + Sr/P ratio) for the
2HA/1SrHA coatings were 1.38 ± 0.24 and 1.46 ± 0.17, respectively,
as reported in Table 3. Although the Ca/P ratio does not change significantly for the 1HA/2SrHA coatings, the Sr + Ca/P does increase to
1.62 ± 0.45. For the 3SrHA both the Ca/P ratio and the Sr + Ca/P ratio
both increase, as highlighted in Table 3. Furthermore, if the Sr/Ca + Sr
ratios of the Sr-substituted coatings are also compared to one another
(as shown in Table 3), it is evident that the Sr levels within the coatings
increase slightly as the number of targets utilised in the deposition process increases. These results would be in line with expectations when
compared to a previous study by Ozeki et al., whereby SrHA coatings
were deposited from a single Sr doped HA target [16]. Furthermore,
the Sr content of the coatings deposited in that study were lower than
that of the original precursor target material and the same trend is observed in this study. Interestingly, as the number of Sr targets in the deposition process increased, the deposition rate appears to decrease
significantly, as highlighted in Table 1. This reduced deposition rate
with increasing Sr content of the sputtering targets observed in our
study would be consistent with expectations, however it does contradict the previous results of Ozeki et al. [16]. These results also highlights
the fact that under the conditions employed here, the deposition rate of
SrHA materials is lower than that observed for the HA derived coating
set. Therefore, the sputter yield for Sr atoms (Atomic Weight (AW) —
88) does not appear to be as high as the Ca (AW — 40) or P atoms
(AW — 31). Typically, it would be expected that the Sr atoms undergo
less scatter in the plasma due to their atomic weight, however, given
the relatively low sputtering power employed in this study it is less likely that all of the Sr atoms have the required energy or momentum to
reach the substrate surface (in line with the number of SrHA targets
used in the deposition process) [55]. Despite this it should be pointed
out that although both studies have considered sputter depositing coatings from HA targets with different amounts of Sr content, the conditions employed in both studies were distinctly different. Specifically,
Ozeki et al. [16] employed a single sputtering source (mounted axially
to the substrate) whereas in this study, three sputtering sources were
employed, each mounted at a 65° angle to the substrates' surface. Furthermore, the study reported here has employed a co-deposition process with the number of SrHA and HA targets varying between the
different deposition runs, as outlined in Table 1. It is therefore possible,
that the experimental conditions employed during deposition in this
study may have had a significant impact on the dynamics of the
sputtering process, and as a consequence, the Sr content and deposition
rate for the different coatings produced [32].
Considering all of the results obtained here, this could suggest a formula such as Ca10 − X − Y(Sr)Y(HPO4)X(PO4)6 − X(OH)2 − X for the 2HA/
1SrHA and 1HA/2SrHA coatings. The presence of CO23 −, which is not
within the formula described here, could be explained by the presence
of low levels of SrCO3 [43]. Weak bands, possibly associated with
SrCO3 were observed between 890–820 cm−1 in the FTIR spectra for
all of the SrHA coatings (as shown in Fig. 4(b)–(d)). Alternatively, the
presence of small amounts of carbonated CaP materials (such as
Ca10 − X − Y/2[HPO4)(PO4)]6 − X − Y(CO3)Y(OH)2 − X) in combination
with Ca10 − X − Y(Sr)Y(HPO4)X(PO4)6 − X(OH)2 − X (or labile HPO4
groups) cannot be entirely ruled out for both the 2HA/1SrHA and
1HA/2SrHA derived surfaces. Although the presence of SrCO3 could
be inferred from the FTIR results no SrCO 3 peaks were detected in
the corresponding XRD patterns or XPS spectra (at least within the
detection limits of the technique) [37]. Therefore, the 2HA/1SrHA
and 1HA/2SrHA derived surfaces may be better explained by the formula Ca10 − X − Y − Z(Sr)Y(HPO4)X(PO4)6 − X − Z(CO3)Z(OH)2 − X − Z, as this
allows for both a Ca/P and Sr + Ca/P lower than 1.67. The extremely
high Ca + Sr/P ratio observed for the 3SrHA derived coatings in this
study were perhaps unexpected, when compared to those for the
2HA/1SrHA (1.46 ± 0.17) and the 1HA/2SrHA (1.62 ± 0.45) derived
coatings. The formula Ca10 − X − Y − Z(Sr)Y(HPO4)X(PO4)6 − X − Z(CO3)Z
(OH)2 − X − Z (or a combination of Ca10 − X − Y / 2[HPO4)(PO4)]6 − X − Y
(CO3)Y(OH)2 − X with Ca10 − X − Y(Sr)Y(HPO4)X(PO4)6 − X(OH)2 − X)
can't explain the high Ca + Sr/P ratio (1.95 ± 0.14) of the 3SrHA derived
A.R. Boyd et al. / Materials Science and Engineering C 46 (2015) 290–300
coatings in isolation. It is likely that this higher than expected Ca + Sr/P
ratio could be due to Ca enrichment of the uppermost surface of the
3SrHA coating, with the higher amount of Sr present in the sample likely
to be a contributory factor also.
The corresponding XRD results for all of the Sr-substituted coatings
highlight that they have peak positions that are shifted to slightly
lower 2-theta (2θ) values with increasing Sr content, as can be observed
in Fig. 5. In addition the XRD results also show that the SrHA derived
surfaces all exhibit a preferred 002 c-axis orientation. The intensity of
the 002 peak (and therefore the 002 c-axis orientation) also appears to increase relative to the other peaks (namely the 211 peak) with increasing
Sr content in the coatings (commensurate with the number of Sr containing targets in the process increasing). There is also a change in the relative
peak intensities of the 211 and 112 peaks with increasing Sr content, as
shown in Fig. 5(b) to (d) for the SrHA surfaces. These observations
would be in line with previous studies on Sr-substituted HA, whereby it
has been shown that as the Sr content in HA increases, so does c-axis orientation [36]. Interestingly, no other Ca-P phases were detected in the
XRD pattern for any of the Sr-doped annealed coatings in this study,
whereas a study undertaken by Ozeki et al. highlighted the presence of
additional unwanted CaP phases after sputtering from Sr-doped HA targets [55]. As highlighted previously, the coatings produced here were deposited using distinctly different experimental conditions, and as such it
might be expected that different results were observed between the
two studies. In comparison, the SEM results here indicate that as the Sr
content of the HA coatings produced here increases, the morphology of
the resultant coatings changes, which would be in line with expectations
[26,48]. In the results observed here, as shown in Fig. 8, the presence of increased levels of Sr, particularly for the 1HA/2SrHA and 3SrHA coatings
appears to inhibit the formation of the regularly distributed spherical features more readily observed for the HA and 2HA/1SrHA derived surfaces.
When these results are taken in combination with the FTIR, XPS and
XRD results for the Sr-doped coatings, it is obvious that with increasing
Sr content (and an increasing degree of preferred 002 orientation),
there are observable differences in the surface morphology (specifically
with respect to the size and distribution of surface features). This would
be in line with the observations of Capuccini et al. [26], Kim et al. [48],
Oliveria et al. [38], and Wei et al. [39]. Other studies, which have specifically studied Sr-substituted apatite coatings, have contradicted these
findings (as a consequence of Sr doping), in that they did not observe
any significant difference in the surface morphology between such Srsubstituted surfaces and those produced from pure HA [49]. Again, significantly different experimental methods may explain these observable
differences.
Previous work by others has highlighted that a preferred 002 orientation can be beneficial and enhance the cellular response in vitro [48].
Further to this, it has also been reported that with standard CaP materials (namely HA), the bioactivity attributed to Ca-P materials originates
due to the actions of free Ca ions [50]. This suggests that the 3SrHA coatings produced here, with the high Sr + Ca/P and Sr + Ca/Ca ratios may
provide appropriate surfaces to enhance the cellular response. However, SrHA coatings with increased levels of Sr content, such as the
3SrHA derived coatings may be more prone to dissolution in physiological conditions when compared to the other coatings produced here.
This is largely due to the increased ionic radius of Sr (which substitutes
for Ca in the lattice) [29,42,43]. This typically results in an increase in the
lattice parameters (a and c), with the Ca(II) site thought to be the preferred site for Sr substitution [18]. This degradation behaviour of the
SrHA material is a consequence of a number of different factors, however, the degree of Sr substitution is obviously seen as highly significant
given the effect it has on the organization of the HA lattice [16,26]. It is
therefore obvious that by controlling the deposition parameters, most
notably, the choice and arrangement of the sputtering targets (as
utilised in this study), the Sr content, stoichiometry and surface morphology of the coatings can be controlled. Consequently, this can help
regulate the micro-environment in close proximity to the biomaterial's
299
surface and play a vital role in the success, or otherwise of the SrHA biomaterial, both in vitro and more importantly in vivo. In general, the effect of Sr-substitution in HA has also been well documented as having a
positive effect, both in vitro and in vivo through enhancing osteoblast
activity (proliferation), differentiation and reducing osteoclast activity [37]. Furthermore, Sr has also been implicated in antimicrobial activity as well [17]. With respect to the surfaces created here, all of the SrHA
coatings appear to be of high quality with Sr clearly substituting for Ca
within the HA lattice (to varying levels). All of the SrHA coatings appear
and HPO2–
to contain low levels of both CO2−
3
4 , with all of them, apart
from the 3SrHA having a higher than expected Sr + Ca/P ratio. This
would suggest that the 3SrHA derived coatings produced here exhibit
a range of properties that would be commensurate with those expected
to deliver significant benefits, both in vitro and in vivo, when compared
to pure HA [16].
5. Conclusion
The aim of this particular work was to study the feasibility of using
co-sputter deposition for creating Sr-substituted HA (SrHA) coatings
from a three source RF magnetron sputtering system. In particular, the
influence of different target configurations on the properties of the Srsubstituted sputter deposited coatings produced at a low discharge
power level (150 W) were investigated. The results presented herein
demonstrate that co-deposition sputtering can be used to produce Srsubstituted HA coatings. The results also indicate that all of the different
and HPO2−
. When
SrHA coatings produced contain low levels of CO2−
3
4
1 or 2SrHA targets where utilised (referred to as 2HA/1SrHA or 1HA/
2SrHA coatings, respectively), the Ca/P and Sr + Ca/P ratios were
lower than that expected (1.67). This could be explained by the formula
Ca10 − X\Y − Z(Sr)Y(HPO4)X(PO4)6 − X − Z(CO3)Z(OH)2 − X − Z, which allows for the lower stoichiometries observed. However, the coatings deposited when 3SrHA targets were utilised (referred to as 3SrHA)
produced coatings with a much higher Sr + Ca/P ratio which cannot
be explained by the same formula. The higher stoichiometry observed
here is most likely due to Ca enrichment of the surface along with the
presence of additional Sr ions. The presence of SrCO3 cannot be ruled
out for any of these coatings, however, they would only be present to
very low concentrations on the surface of the coatings as indicted by
the results obtained here. It was also observed that as the number of
SrHA targets in the deposition runs increased (from 0 to 3), the deposition rate decreased. However, all of the coatings produced did not appear to contain any other impurity CaP phases, with XRD data
showing peaks corresponding to that of ICDD file #09-0432 for HA, albeit shifted to lower 2θ values due to the incorporation of Sr into the
HA lattice for Ca. In addition to this, as the Sr content in the coatings increased the degree of preferred 002 orientation seemed to increase, producing a range of different morphologies as a consequence. RF
magnetron sputtering, employing a co-deposition approach, therefore
offers a means to control the attendant surface properties of Srsubstituted HA, such as the crystallinity, stoichiometry, phase purity
and surface morphology. However, these surfaces would need to be investigated by rigorous in vitro and in vivo testing to provide further evidence of their potential clinical utility.
Acknowledgments
The authors would like to acknowledge the PhD funding provided by
of the Department for Employment and Learning (Northern Ireland) to
support this work.
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