Stoichiometry, crystallinity, and nano-scale surface morphology of the graded
calcium phosphate-based bio-ceramic interlayer on Ti-Al-V
J.D.Long,K.Ostrikov,andS.Xu
Advanced Materials and Nanostructures Laboratory, Natural
Sciences, Nanyang Technological University, 637616 Singapore
V. Ligatchev
School of Electrical and Electronic Engineering,
Nanyang Technological University, Nanyang Avenue,
Singapore 639798, Singapore
ABSTRACT
A plasma-assisted concurrent Rf sputtering technique for fabrication of biocompatible,
functionally graded CaP-based interlayer on Ti-6Al-4V orthopedic alloy is reported. Each
layer in the coating is designed to meet a specific functionality. The adherent to the
metal layer features elevated content of Ti and supports excellent ceramic-metal interfa-
cial stability. The middle layer features nanocrystalline structure and mimics natural bone
apatites. The technique allows one to reproduce Ca/P ratios intrinsic to major natural cal-
cium phosphates. Surface morphology of the outer, a few to few tens of nanometers
thick, layer, has been tailored to fit the requirements for the bio-molecule/protein attach-
ment factors. Various material and surface characterization techniques confirm that the
optimal surface morphology of the outer layer is achieved for the process conditions
yielding nanocrystalline structure of the middle layer. Preliminary cell culturing tests
confirm the link between the tailored nano-scale surface morphology, parameters of the
middle nanostructured layer, and overall biocompatibility of the coating.
INTRODUCTION
Calcium phosphate-based bio-ceramics have recently attracted a great deal of in-
terest as functional surface coatings in dental and orthopedic implants because of excel-
lent bio-activity, bio-compatibility, chemical and mechanical properties [1-5]. In particu-
lar, hydroxyapatite (HA, Ca
10
(PO
4
)
6
(OH)
2
, Ca/P=1.67) coatings reveal inspiring clinical
advantages in promoting efficient implant fixation and implant-to-bone adhesion shortly
after the implantation, as well as faster bone remodeling due to enhanced bi-directional
growth and formation of a bonding interlayer between bone and implant [1]. Further-
more, calcium phosphates with apatite-like structure are the major constituents of the
bone mineral phase, are compatible with various soft and muscular tissue types, and can
efficiently sustain protein attachment and growth [1]. Clinical applications of CaP-based
bio-ceramics for improved fixation between bone and implant pose a number of chal-
lenges for tailoring the coating quality specifications. The key quality factors include
stoichiometry, crystallinity, microstructure, metal-implant interfacial stability and several
others [6]. In addition, a successful technique for fabrication of a viable biocompatible
Mat. Res. Soc. Symp. Proc. Vol. 737 © 2003 Materials Research Society F3.42.1
coating would intrinsically imply a certain degree of replication of biological apatites,
featuring nano-crystalline structures in bone and dentin materials. Above all, surface
morphology with nano-scale features and excellent island uniformity, appears to be a
critical factor in promoting bio-molecule/protein - surface interactions.
Thus, fabrication of a graded bioceramic-implant structure capable of simultaneously sat-
isfying the basic requirements for the interfacial stability at the coating-implant interface,
controllable stoichiometry and crystallinity in the bulk, and the optimized, from the point
of view of sustained protein attachment and growth, nano-scaled surface morphology, is a
critical problem in the biomaterials research. In particular, a large number of the existing
techniques for CaP-based films (including HA) deposition suffer from poor coating-metal
implant interfacial bonding strength, excessive amorphosity or larger, than in natural apa-
tites, crystal size in the bulk, as well as irregular surface morphology features typically in
the micrometer range.
EXPERIMENTAL DETAILS
Here, we report on a new and efficient technique for synthesis of a graded CaP-based
biocompatible interlayer on orthopedic alloy Ti-6Al-4V, consistently satisfying all the
above requirements. The essential part of the method is a concurrent low-temperature
plasma-assisted Rf magnetron sputtering (PA-RFMS) of crystalline HA and metallic tita-
nium targets in low-pressure discharges of reactive gas mixtures of argon and water va-
por sustained in PSAC/PA-RFMS facility. The titanium target has purposely been intro-
duced to create a titanium-rich layer adjacent and stronger adhering to the implant-
simulating metallic sample. The discharges were sustained in the range of Rf powers of
P
in
= 300 - 700 W applied to a water-chilled Rf magnetron electrode with several rows of
concentrically positioned permanent magnets with specific polarities. The working pres-
sure p
0
was typically maintained in the range of 10 to 70 mTorr. In this pressure and
power range, large DC sheath potentials near the HA/Ti target surface, promoting high
sputtering yields, and eventually high film deposition rates, can be achieved. An electri-
cally floating substrate heater powered by an external temperature controller supports the
Ti-6Al-4V samples, with the surface being coated facing downwards, approximately 6
cm above the HA/Ti sputtering target. In the experiments, the substrates were negatively
biased with V
b
= 25 - 200 V. Chemical composition and elemental bonding states in the
interlayer were studied by VG ESCALAB 220i-XL spectrometer (XPS). The crystal
structure was characterized using SIEMENS D5005 X-Ray diffractometer (XRD) in a
lock coupled (θ − 2θ) mode with an incident x-ray wavelength of 1.540A (Cu Kα line).
Cross-sectional structure of the functionally graded structure was examined with the
Field Emission Scanning Electron Microscopy (FESEM). Nano-scaled features of the
surface morphology were studied by the Atomic Force Microscope in a contact mode
(AFM). Bio-ceramic-metal interfacial bonding strength was assessed via a Micro-scratch
teste. Further details of the PSAC/PA-RFMS sputtering facility, routine steps in pre-
deposition substrate treatment, as well as film characterization instruments and tech-
niques, can be found elsewhere [7,8].
F3.42.2
RESULTS AND DISCUSSION
XPS layer-by layer analysis of the composition and chemical bonding states reveals clear
presence of Ca, P, O, and Ti in the film, the concentration of the latter being gradually
increased towards the bottom layer of the coating. In addition, varying the DC substrate
bias V
b
and deposition pressure p
0
, one can efficiently control the elemental composition
and Ca/P ratio (Fig.1), which is a key factor defining the phases of apatite. In particular, a
near-stoichiometric HA (Ca/P ~ 1.67) can simply be achieved by applying DC bias of
V
b
= -100 V at p
0
=70 mTorr. The elemental presence in the bulk of the coating is meas-
ured to be approximately Ca (27 %), P (15 %), O (50 %), and Ti (5 %). Furthermore,
most of calcium and phosphorus in the film enter calcium phosphate phase, whereas tita-
nium is predominantly a part of CaTiO
3
and TiO
2
. Formation of the phosphate phases is
further supported by the results of FTIR analysis (Perkin Elmer Spectrum I spectrometer)
showing the characteristic absorption peaks of PO
4
3-
. Thus, the results of the XPS and
FTIR analysis confirm that stoichiometry, elemental composition, and bonding states in
the coating in question can indeed be tailored by varying the substrate bias, and under
certain conditions the near-stiochiometric phase of HA can be synthesized.
Figure 1. Ca/P ratio versus DC bias voltage
TheXRDspectrumofthefilmpreparedatP
in
=700 W and p
0
=70 mTorr is presented in
Fig.2, suggesting preferential crystal growth along (130) or (132) planes depending on
deposition parameters. Furthermore, the diffraction pattern suggests presence of mono-
clinic crystalline tetracalcium phosphate Ca
4
P
2
O
9
(TTCP) structure. It is remarkable that
the diffraction pattern is strongly affected by variation of the substrate bias and working
pressure. An amorphous phase is observed to be characteristic to weakly biased films. A
close examination of the broadening nature of the diffraction peaks reveals that the films
prepared at higher negative bias feature clear nanocrystal structures (see, for example,
(132) plane), and the (132) direction becomes a preferential crystal growth plane at sub-
strate biases exceeding 150 V. It is also worth noting that a high crystallinity of the coat-
ing can be achieved at lower working pressures and moderate substrate biases.
50 100 150 200
1
2
3
4
Ca/P Ratio
Bias Votage (V)
F3.42.3
Cross-sectional structure of the graded CaP-based film on Ti-6Al-4V has been analyzed
using FE SEM. The total thickness of the CaP-based interlayer appears to be approxi-
mately 200 nm, and it contains three major areas namely, adjacent to the implant alloy,
bulk area, and outer surface area (the latter to be in contact with bone tissues). Further-
more, cross-sectional SEM imaging (not shown here) suggests about formation of a func-
tionally-graded titanium-embedded CaP-based graded coating on Ti-6Al-4V alloy.
Figure 2. Typical XRD spectra from Ca-P-Ti thin films at different DC substrate bias.
Nano-scaled surface morphology of graded CaP-based crystalline interlayers was ana-
lyzed with the Atomic Force Microscopy. Fig.3 demonstrates remarkable changes in sur-
face morphology with variation of the substrate bias. Indeed, V
b
affects homogeneity of
distribution and architecture (e.g. sharpness) of the elements of morphology as well as
inter-element spacing and surface areas in the "humps" and "dips". We note that homo-
geneous distribution of very much similar by shape and size nano-sized morphology ele-
ments over the micro-scaled surface area is ideal for promoting biomolecule/protein at-
tachment and growth. Furthermore, sharp "humps" and smooth "dips" are especially de-
sirable for this purpose, which has recently been confirmed by cell culturing experiments
on ordered arrays of sharp pillars. The inspiring conclusion that can be drawn from the
AFM analysis is that all the above quality factors of the surface morphology of the outer
functional layer are met for the films prepared at V
b
= -100 V (Fig.3a). The latter sug-
gests direct link between the origin of nano-crystalline structure reflected in Fig.2, and
self-organization of the outer functional layer, which can be inferred as a possible reason
for preferred nano-crystalline self-assembly of natural apatites in various bone embodi-
ments. The AFM data thus confirm a challenging opportunity of controlling the nano-
scale surface morphology of the outer layer of the graded structure by varying the process
parameters.
Meanwhile, the entire coating strongly adheres to the Ti-6Al-4V alloy, which has con-
vincingly been confirmed by the interfacial bonding strength measurement with the Mi-
20 30 40 50 60
(260)
(303)
(143)
(141)
substrate
*
(132)
(130)
*
*
*
*
*
25 V
100 V
200 V
Intensity (a.u.)
2 θ (degrees)
F3.42.4
croscratch test. The critical load that the coating can withstand ranges from 2.0 N to 5.7
N. It is notable that the best interfacial strength is achieved for moderately-biased and
higher pressure-grown coatings with well-resolved nano-features. We remark that higher
gas pressures favor enhanced sputtering of the titanium target, thus eventually improved
doping of the bottom layer by titanium/titania in the atomic/molecular or nanocluster
states.
(a) (b)
Figure 3. AFM images of the outer nano-sized layer of Ca-P-Ti coating deposited at
–100 V (a) and –50 V (b) DC bias.
It is notable that preliminary in-vitro cell culturing tests on the graded CaP-based inter-
layer suggest promising biocompatibility of the coating. The relevant imaging reveals
well-preserved cyto-sceletal structure of COS7 monkey kidney cell series after up to 48
hours after cell culturing. The work is continuing into in-vivo assessments of these bioac-
tive HA films.
CONCLUSION
In summary, the plasma-assisted concurrent Rf sputtering in low-temperature, low-
pressure reactive environment offers a great deal of control of the major coating quality
parameters, including functional grading, stoichiometry, phase purity, chemical composi-
tion, crystallinity, crystal size, interfacial bonding strength, as well as of tailoring the
nano-scale features of the surface morphology of the outer thin (few nm) layer with spe-
cific bio-active functionality. Another unique features of this technique include, but are
not limited to, low-temperature, no need for post-treatment, easy deposition process con-
trol, etc. From practical considerations, the technique proposed is indeed promising to
mimic a number of the basic parameters of natural bone minerals via tailoring certain fea-
tures at nano-scales, such as nano-crystalline structure and surface morphology. Success-
ful cell culturing tests have demonstrated excellent in-vitro biocompatibility, which
sounds favorably in view of future in-vivo studies. Finally, the results obtained have fur-
ther supported the usefulness of nano-scale approach for assembly of functionally graded
F3.42.5
coatings and give us a hope that such an approach will soon be widely adopted for a
wider class of nano- and bio-materials.
ACKNOWLEDGMENTS
This work was supported in part by the Agency for Science, Technology, and Research of
Singapore (Project No. 012 101 00247). The authors are grateful to J. H. Lu for his assis-
tance in in-vitro analyses of the films. Fruitful discussions with N. Voelcker, S. Kumar, I.
Brown, and C. H. Diong are also gratefully acknowledged.
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