Determination of the thickness and crystalline structure of TiO2 coatings made by anodic oxidation
of Ti-6Al-4V
Vera, M. L.,1 Alterach, M.A.,1 Rosenberger, M. R.,1 Lamas, D. G.,2 Schvezov, C. E.,1 and Ares, A.E.1
1 CONICET
2 Consejo
- Universidad Nacional de Misiones - Posadas-Misiones Argen Argentina
Nacional de Investigaciones Científicas y Técnicas - Buenos Aires Argentina
INTRODUCTION
At present Ti-6Al-4V alloy is used in prosthetic devices for
human implants due to the excellent corrosion resistance, bioand hemo-compatibility, which are known to be associated to
the formation of TiO2 on the surface. Oxide films in metals
can also be produced by anodic oxidation, resulting in films
thicker than the natural oxides [1]. By anodizing at low voltages smooth and amorphous films can be obtained [1]. For
haemocompatible applications smooth and crystalline TiO2
films are required [2]. One way to obtain such kind of films is
doing a thermal treatment after anodic oxidation.
TiO2 films on Ti-6Al-4V substrates were produced by anodic oxidation at different voltages with and without thermal
treatment. Thicknesses and phases were determined. The results of the influence of process parameters on color, thickness, and crystalline structure of the films are presented.
FIG. 1: Optical micrographs of the samples.
based on the Snell Law and the other on the modified Bragg
Law [3, 4]. Both methods employed intensity of the reflected
beam measured as a function of angle of incidence of the Xray beam (Fig. 2a).
Using the Snell Law, the thickness of the film t1 is calculated through the Equation t1 = λ/2∆α. Where λ in [nm] is the
wavelength of the X-ray and ∆α in [rad] is the difference in
the angle of incidence between two consecutive peaks of the
oscillations observed in Fig. 2a. In this figure it is seen that
∆α decreases with applied voltage that is, with film thickness.
The thicknesses t1 are plotted in Fig. 2b with black squares.
It is observed that t1 increases linearly with applied voltage
and they can be fitted as t1 = 2.49 [nm/V] x V, with R2 = 0.99,
where V is the applied voltage in [V]. The XRR curve for
the specimen anodized at 50V has a deviation in reading at
low angles due to misalignment of sample and beam; however, it does not affect thickness determination. Thickness
of the film obtained at 60V was not determined because ∆α
becomes smaller with increasing thickness and less than the
angular resolution chose in the experiments.
The film thickness using the modified Bragg Law can be
calculated using α2 = αc 2 + (m + ∆m)2 x (λ/2t2 )2 . Where α
is the angle of incidence at each maximum and minimum of
the oscillations in the intensity of the reflected beam shown in
Fig. 2a; m is the order of the reflection; ∆m takes the value
of ∆m = 0 for the peaks and ∆m = 12 for the valleys due to the
film is less dense than the substrate [4]; αc is the critical angle
of incidence at which there is total reflection. The relation
between α2 and (m + ∆m)2 is linear and the thicknesses t2
were calculated from the slope of the linear relation resulting
for each film. The correlation between thickness and applied
voltage gives a relation t2 = 2.3 [nm/V] x V, (R2 = 0.99) which
is represented in Fig. 2b by the black triangle marks.
The values of the constants of linearity obtained with both
methods are very similar; 2.49 and 2.3nm/V. The average
value of 2.4nm/V is adopted. There is a direct relation among
voltage, color and thickness of TiO2 films which permits to
estimate the thickness.
EXPERIMENT
As substrates rectangular plates of Ti-6Al-4V (10 x
20)mm2 in surface area and 2mm in thickness were employed.
The plates were polished to mirror finishing. The process of
anodic oxidation was carried out in 1M H2 SO4 solution at
ambient temperature using the Titanium alloy as anode and a
Platinum wire as cathode. A fixed voltage was applied during
1min and the range of applied voltages was between 10 and
60V. Thermal treatment (TT) of 1h at 500o C was performed to
samples anodically oxidized at 20V y 40V and to a substrate
which resulted thermal oxidized. The anodized surfaces were
observed by optical microscope. The thickness of the coatings
was determined by means of X-ray reflectometry (XRR) and
the crystalline phases were determined using X-ray diffraction
(XRD) employing a glancing angle of incidence of 1o . These
studies were performed at the D12A-XRD1 beamline of the
LNLS, with a wavelength of 1.55015 Å.
RESULTS AND DISCUSSION
The films obtained by anodic oxidation and thermal treatment present different colors of interference as observed by
the naked eye depending on the applied voltage as shown in
Fig. 1. In order to obtain the relation between applied voltage
and thickness, the thicknesses of the oxides were determined
using XRR; two different calculation methods were used; one
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Brazilian Synchrotron Light Laboratory
or it is too thin to detect the crystal structure of the oxides.
Similarly, the thermal oxidized sample (TT) does not present
peaks of the TiO2 crystalline phases. However, the diffractograms of the anodized with heat treatment samples show
peaks of anatase phase of TiO2 .
FIG. 3: Diffractograms of the samples. A = anatase; α = α-Titanium
corresponding to the substrate.
CONCLUSION
1) Anodic oxidation of Ti-6Al-4V in H2 SO4 1M produces
TiO2 films with different interference colors depending on the
applied voltages. 2) There is good agreement between Snell
and modified Bragg Laws for thickness calculation. 3) There
is a linear relation between thickness and applied voltage: t
= 2.4nm/V x V, which permit a quickly thicknesses estimation. 4) In anodic oxides obtained up to 60V there were not
detected crystalline phases of TiO2 . 5) Heat treatment after
anodizing produces crystalline TiO2 in anatase phase and a
slightly thickness increasing.
FIG. 2: a) XRR graphs of the samples. b) Film thickness vs. applied
voltage obtained from Snell and Modified Bragg Laws.
ACKNOWLEDGEMENTS
This work has been supported by the Brazilian Synchrotron
Light Laboratory (LNLS) / Brazilian Biosciences National
Laboratory (LNBio) under proposal D10A - XRD2 8100, the
CONICET (Argentina) and the ANPCyT (Argentina, PICT
36981 and PAE 22411).
The thickness for the thermal oxidized sample (TT) was determined using Snell Law and found to be 20nm. The non
regular form of the XRR curves of the anodized with heat
treatment samples suggests that heat treatment leads to another layer of oxide, then the maximum thicknesses of the
samples, are the sum of the thickness obtained by each process, i.e., 68 and 112nm respectively for 20V TT and 40V TT.
In the curve of substrate oscillations are not noticeable so it is
not possible to estimate the thickness of natural oxide formed
by air exposure.
In the diffractograms of Fig. 3 obtained for the anodic oxides up to 60V only the peaks corresponding to the substrate
are visible, indicating that either the oxide film is amorphous
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[1]
[2]
[3]
[4]
2
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Brazilian Synchrotron Light Laboratory