Volume 6 Paper 107
Breakdown of Anodic Films on Titanium and its Suppression by Alloying
H. Habazaki, M. Uozumi, H. Konno, K. Shimizu, P. Skeldon, G.E. Thompson and G.C. Wood
Keywords: Anodic titania, crystallization, anatase, ionic transport, TEM
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JCSE Volume 6 Paper 107
Submitted 6th July 2003, published 22nd September 2004
Breakdown of Anodic Films on Titanium and its Suppression by Alloying
H. Habazaki, M. Uozumi, H. Konno
School of Engineering, Hokkaido University, Sapporo 060-8628, Japan, mailto2('Habazaki','eng.hokudai.ac.jp')K. Shimizu
Chemical Laboratory, Keio University, Hiyoshi 4-1-1,
Yokohama 223-8521, Japan
P. Skeldon, G.E. Thompson, G.C. Wood
Corrosion and Protection Centre, UMIST, P.O. Box 88, Manchester M60 1QD, UK
§1 Breakdown of anodic
films on titanium, associated with their crystallization, and influences of
alloying additions on the crystallization have been examined by transmission
electron microscopy of ultramicrotomed sections, combined with Rutherford backscattering spectroscopy and glow discharge
optical emission spectroscopy.�� Anodic
oxide films have been formed on
sputter-deposited titanium and its alloys containing a range of valve metals at
a constant current density of 50 A m-2 in 0.1 M ammonium pentaborate electrolyte.�
In the case of titanium, anatase develops at relatively
low voltage in the inner film region, formed by anion ingress.� In contrast,
the outer film region, formed at the film/electrolyte interface, is composed only of amorphous oxide. Oxide crystals
are found near the plane separating the two regions, which is located at a
depth of 35-38% of the film thickness. Oxide zones, of size ~ 1 nm, with a
relatively ordered structure, developed at the metal/film interface, are
considered to lead to transformation of the inner region structure.
Incorporation into the film of alloying element species, such as aluminium,
molybdenum, silicon and zirconium species, suppresses the crystallization such
that uniform amorphous anodic oxides can grow to high voltages.�
However, the effectiveness of the suppression of crystallization is
dependent upon the particular alloying element species; small amounts of
alloying elements that form oxides with strong metal-oxygen bonds tend to
suppress the crystallization more effectively.
Keywords: Anodic titania, crystallization, anatase,
ionic transport, TEM
§2 Titanium is one of the
more important engineering metals, partly due to its bio-compatibility and high
corrosion resistance. Of relevance to these properties, anodic films formed on
titanium have been examined extensively. Various electrochemical
investigations, including ac impedance spectroscopy, photoelectrochemistry
and scanning electrochemical microscopy, in addition to ellipsometric
studies, have demonstrated the changes in the growth behaviour and film properties
with anodizing conditions [1-10].� The dependence of film properties on the
grain orientation of the titanium substrate has been also reported [11-15].� From
these investigations, it is known that an amorphous-to-crystalline transition
occurs on titanium at relatively low voltages of ~10 V, in contrast to growth of amorphous oxides
on aluminium and tantalum to high voltages.�
Crystallization of the anodic films on titanium induces
electron-conducting paths, allowing oxygen evolution on crystalline regions to
occur, which has been associated with the breakdown of the film .
§3 Direct evidence of the formation of crystalline oxide on
titanium has been obtained by in-situ and ex-situ Raman spectroscopy [17-20]
and transmission electron microscopy (TEM) [20, 21].� Most studies have demonstrated the
transformation of amorphous oxide to anatase, with rutile forming at increased voltages. Although an electrostriction model has been
proposed , the precise mechanism of crystallisation is
uncertain, partly due to lack of information about the in depth distribution of
crystalline oxides in the films.
§4 In this paper, recent investigations
by the authors of the crystallization of anodic titania,
studied using TEM, and its suppression by alloying additions, such as
aluminium, molybdenum, silicon and zirconium, have been considered further in
order to gain insight into the mechanism of crystallisation.� �
§5 Layers of titanium, and titanium alloys containing various concentrations of aluminium, molybdenum,
silicon and zirconium, 200-300 nm thick, were prepared by d.c.
magnetron sputtering using a 99.9% titanium target, of thickness 6 mm and of
diameter 100 mm, with small pieces of alloying
elements placed on the sputter-erosion region for preparation of
alloys.� The sputtering chamber was
firstly evacuated to <1 x 10-5 Pa, with deposition carried out in
99.999% argon at about 2 x 10-1
Pa.� The compositions of the alloys were
determined by Rutherford backscattering spectroscopy
(RBS) using 2.0 MeV He2+
ions.� The scattered ions were detected
at 170� to the incident beam direction, which was normal to the specimen
surface.� The data were analysed using
the RUMP program.�
working area about 2 x 10-4 m2, were anodized in a
two-electrode cell at a constant current
density of 50 A m-2 to selected voltages in various stirred
electrolytes at 293 K. The counter electrode consisted of a titanium sheet.
Electron transparent sections, of nominal thickness 10 nm, of anodized
specimens were then prepared using a LeicaUltracut-S ultramicrotome for
subsequent observation by TEM in a JEOL JEM-2000FX instrument operating at 200
§7 Elemental depth profiling of the anodic films was carried out by radio
frequency (rf) glow discharge optical emission
spectroscopy (GDOES) using a JobinYvon 5000 rfinstrument in
an argon atmosphere of 538 Pa by applying rf of 13.56
MHz and power of 40 W.� The signals were
detected from an area of 4 mm diameter.
Results and Discussion
Voltage-time responses of titanium in
§8 Voltage-time curves of
the sputter-deposited titanium during anodizing in various electrolytes are
shown in Fig. 1. In all electrolytes, except for 14.6 M phosphoric acid, the voltage
increases linearly with time to only about 10 V, after an initial voltage surge
of about 2 V at the beginning of anodizing. Then, a progressive increase in the
slope is observed, and finally, the voltage increases relatively slowly.� In the case of anodizing in sodium sulphate
electrolyte, a sudden drop in voltage marks the beginning of the region of slow
voltage rise.� In the final stage, gas
evolution was evident on the surface of the specimens. During anodizing in 14.6
M phosphoric acid electrolyte, the voltage increases linearly with time to a
slightly higher voltage of about 45 V, and then increases more slowly with gas
evolution from the surface. Thus, unlike aluminium and tantalum, uniform film
growth on titanium to high voltages is not achieved.
§9 Figure 1. Voltage-time curves of the sputter-deposited titanium during anodizing at
50 A m-2 in various electrolytes at 293 K.
SEM and TEM observations of anodic
films on titanium
§10 Scanning electron microscopy of the surface of titanium,
anodized to 80 V, revealed the presence of many cavities and swellings, associated
with gas evolution . Thus, gas should be
generated not from the surface of the oxide film, but within the oxide film.
Figure 2 shows a transmission electron micrograph of a stripped anodic film
formed to 20 V in 0.1M ammonium pentaborate electrolyte. The film was stripped
by immersing the titanium specimen anodized to 20 V in a 10 wt% bromine-methanol
solution.� The micrograph reveals
bubble-like features, possibly associated with oxygen generation even at this
low voltage. The generation of gas, within the anodic film, may be one of the
reasons for the progressive increase in the slope in the voltage-time response
(Fig. 1), since gas bubbles obstruct ionic transport through the oxide layer
and hence, increase the resistance of the film. The selected area electron
diffraction pattern, shown in Fig. 2, corresponds to anatase,
such that an amorphous-to-crystalline transition occurs below 20 V. Similar images can be seen by using high-resolution,
low voltage scanning electron microscopy (Fig. 3). The left- and right-side
images were obtained using out-lens and in-lens detectors respectively. The
regions with light appearance on the right appear dark on the left, possibly
due to the presence of oxygen gas in the region. The formation of
crystalline oxide in the film formed to 20 V is confirmed further by TEM of an ultramicrotomed section (Fig. 4). The film is of thickness ~ 35-40 nm, with irregular
film/electrolyte and metal/film interfaces associated mainly with the rough
surface of the as-deposited titanium.�
Lattice fringes, with spacing of ~ 0.35 nm, are revealed in the middle
of the anodic film, indicating nanocrystals of anatase,
in agreement with the electron
diffraction pattern in Fig. 2. Further, nanoscale
bubbles have developed around the nanocrystals, due to local generation of
oxygen.� The outer 30% of the film is
composed of an amorphous oxide, with no lattice fringes evident.
§11 Figure 2.� Transmission electron micrograph of
a stripped anodic film formed on sputter-deposited titanium to 20 V in 0.1 M
ammonium pentaborate electrolyte at 293 K.
§12 Figure 3.� Low voltage, high-resolution
scanning electron micrographs of the sputter-deposited titanium anodized to 20
V in 0.1 M ammonium pentaborate electrolyte at 293 K.
§13 Figure 4.�
Transmission electron micrograph of an ultramicrotomed
section of the sputter-deposited titanium anodized to 20 V in 0.1 M ammonium pentaborate electrolyte at 293 K.
§14 The distribution of the oxide crystals can be
correlated with ionic transport during film growth.� Amorphous
titania is formed both at the film/electrolyte and metal/film interfaces by
cation egress and anion ingress respectively, with the transport number
of cations being 0.35-0.38 [23-25].
Electrolyte-derived species are incorporated into the outer part of anodic
films and stabilize the amorphous structure [26, 27].
In the present film, boron species
are distributed in the outer ~19% of the film due to their outward migration .� Amorphous titania,
free of boron species, is present below the boron-containing film region.� Hence, the formation of amorphous material in
the film region generated at the film/electrolyte interface is not only due to
the presence of boron species.
§15 In contrast to the amorphous structure of the outer layer,
the inner film layer, formed at the metal/film interface by the inward
migration of O2-/OH-
ions, contains nanocrystals.� Their
formation may be associated with oxide regions with a relatively ordered
structure, of size ~1 nm, formed at the metal/film interface, that arise due to
the structure of the metal, impurity elements in the substrate and growth stresses
at the interface .� Contributions may also arise from
electrostriction .� Ionic transport, under an electric field of
~5 x 108 V m-1, should favour crystal growth at sites of
early nucleation now located at 35-38% of the film thickness presuming that the
crystals are immobile, similar to γ-Al2O3 nuclei
in anodic alumina .� As will be evident from later consideration
of alloys, immobile precursor nuclei may also be present at this depth. In
general, crystalline oxides have higher ionic resistivities,
and hence higher electric fields than the corresponding amorphous oxides .� Thus, the probability of excitation of
electrons in the valence band, formed by overlapping of O 2p orbitals in crystalline titania, to the conduction band is
enhanced, leading to the oxidation of O2- ions to form O2
molecules and later development of bubbles. The growth of the anodic film on
titanium is illustrated schematically in Fig. 5.
§16 Figure 5.� Schematic
diagram illustrating the formation of crystalline oxide in an anodic film on
Influence of alloy additions on the
structure of anodic titania
§17 As described above,
incorporated foreign species in anodic films generally stabilize the amorphous
structure.� Since oxide crystals are
developed in the inner part of anodic titania film,
where the film materials are formed at the metal/film interface, it is expected that incorporation of foreign species into the inner region of anodic titania stabilises the amorphous structure, resulting in uniform film growth to
high voltages. The incorporation into this region can be achieved readily by
alloying of titanium; the alloying element species should distribute at least
throughout the film region formed at the alloy/film interface, unless the
species are mobile inwards.
§18 Here, an example of
the addition of aluminium to titanium is considered. Figure 6 shows
voltage-time responses of the sputter-deposited titanium and Ti-Al alloys, with
several aluminium contents, during anodizing in 0.1 M ammonium pentaborate electrolyte. It is evident that the linear
voltage increase, suggesting uniform film growth, extends to higher voltages
with an increase of the aluminium content in the alloy. After the linear
voltage increase, the slope of the response progressively increases and
subsequently dielectric breakdown occurs, when sparking and gas evolution were evident.
§19 Figure 6.� Voltage-time curves of
sputter-deposited titanium and Ti-Al alloys with several compositions during
anodizing at 50 A m-2 in 0.1 M ammonium pentaborate
electrolyte at 293 K.
§20 Growth of uniform
anodic films during the linear voltage increase has been confirmed by TEM
observations. An example of a transmission electron micrograph of an
ultramicrotomed section of the Ti-26.0 at% Al alloy anodized to 50 V is given
in Fig. 7.� Unlike the film formed on
titanium (Fig. 4), the anodic film formed on this alloy, with flat and parallel
alloy/film and film/electrolyte interfaces, does not contain bubbles and is
apparently featureless, indicating the amorphous structure. The thickness of
the film is 100 � 3 nm, corresponding to a formation ratio of 2.0 nm V-1.� A similar uniform film, of 178 � 3 nm
thickness, was formed on the Ti-38.0 at% Al alloy to 100 V, with a
corresponding formation ratio of about 1.8 nm V-1. �The formation ratio for the film on the Ti-32
at% Al was 1.9 nm V-1, between those of the previous alloys.� Evidently, the increased aluminium content
reduces the formation ratio, due to the smaller formation ratio of anodic
alumina, i.e. 1.2 nm V-1 , compared with that of anodic titania, about 2.0 nm V-1
. The formation ratio for the
film on the Ti-17.5 at% Al alloy was not determined, since uniform growth was
limited to low voltages.
§21 Figure 7.� Transmission
electron micrograph of an ultramicrotomed section of
the sputter-deposited Ti-26 at% Al alloy anodized to 50 V in 0.1 M ammonium pentaborate electrolyte at 293 K.
8.� (a) Transmission electron micrograph of an ultramicrotomed section of the sputter-deposited Ti-26 at%
Al alloy anodized to 160 V in 0.1 M ammonium pentaborate
electrolyte at 293 K. (b) High resolution image near a
§23 In the region where progressive increase of the slope
in the voltage-time curves is observed, formation of nanocrystals and voids
within the anodic films has been found, as shown in the example of the Ti-26.0
at% Al alloy that had been anodized to 160 V (Fig. 8). Relatively large voids,
associated with oxygen generation, are formed at ~40%
of the film thickness from the film/electrolyte interface. From the high
resolution image (Fig. 8(b)), formation of nanocrystals, with lattice spacing
of 0.35 nm, is evident in the vicinity of voids. The film region containing
nanocrystals and voids, possibly filled with oxygen, is located close to the
plane separating film regions formed at the alloy/film interface by anion
ingress and at the film/electrolyte interface by cation egress, since the
transport numbers of cations in amorphous anodic alumina (~0.4)
and in amorphous anodic titania (0.35-0.38) are
similar. GDOES depth profiles of the anodic film formed on the Ti-32.0 at% Al
to 100 V (Fig. 9) reveal that both titanium and aluminium ions are distributed
throughout the film thickness.� The wavy profiles
of titanium and aluminium in the films are due to optical interference of light
emitted from the respective elements. The steeper increase in the
titanium profile at the film surface, compared with that of aluminium, suggests
formation of a thin outer layer of TiO2 due to titanium ions
migrating slightly faster than aluminium ions.�
Incorporation of boron species in the outer 32% of the film is evident, indicating its outward mobility in
growing film. The composition of the anodic film was also confirmed
from RBS analysis, indicating that the anodic films formed on the Ti-26 at% Al
and Ti-32.0 at% Al alloys to 100 V mainly consist of uniformly distributed
units of Al2O3 and TiO2. The resolution of RBS
was insufficient to identify any thin outer layer of TiO2.� From these results, the formation of
nanocrystals at about 40% of film thickness from the film/electrolyte interface
cannot be attributed to a different composition in
9.� GDOES depth
profiling analysis of the anodic film formed on the sputter-deposited Ti-32 at%
Al alloy to 100 V in 0.1 M ammonium pentaborate
electrolyte at 293 K.
§25 Development of bubbles
and crystalline oxide at the film region close to the plane separating
film regions formed by anion ingress and cation egress is also evident for Ti-6
at% Si  and Ti-10.5 at% Zr alloys . From these results, it
is likely that pre-cursor nuclei exist in the air-formed oxide.� These pre-cursors are immobile in the anodic
film and may arise due to reaction of the alloys and titanium with residual
oxygen in the sputtering chamber. During sputtering, substrates are heated to ~
373 K by the sputtering process, with pre-cursor nuclei possibly developing
during cooling of substrates. Ageing
of the as-deposited Ti-6 at% Si alloy in the laboratory atmosphere for one month
promotes the crystallization of the anodic film. During ageing of the
alloy, crystal nuclei, that are immobile during anodic film growth, probably
develop in the air-formed film .�
§26 Formation of
crystalline oxides in anodic titania can be suppressed
up to high voltages by incorporation of sufficient amounts of alloying element
species from the substrates. As described above, the linear voltage increase
with anodizing time for the Ti-Al alloys extends to higher voltages with
increased aluminium content in the alloy (Fig. 6). However, the amounts of
alloying elements required to suppress the formation of crystalline oxides to
high voltages, e.g. 100 V, are strongly dependent upon the particular alloying
elements. Figure 10 shows the voltages at which the region of linear voltage
increase with time terminates during anodizing at constant current density of
several titanium alloys, with alloying levels up to 30 at%. Obviously, silicon
is most effective in extending uniform film growth, i.e. suppression of the
formation of crystalline oxides, to high voltages, while relatively large
amounts of aluminium are required for growth of uniform films to high voltages.
From this Figure, the amount of alloying element required to grow uniform films
to 100 V increases in the order of silicon, molybdenum, zirconium and
aluminium.� This order is in approximate agreement
with that of the strengths of metal-oxygen bonds of their oxides, i.e. single
metal-oxygen bond energies of Si4+-O, Mo6+-O, Zr4+-O
and Al3+-O are 465, 359, 276 and 281 kJ mol-1 respectively
. Aluminium ions incorporated into anodic titania have a similar mobility to titanium ions, such that
both cations are distributed throughout most of the film thickness, as
described previously. However, other incorporated alloying element species have
mobilities different from that of titanium ions.� The authors have found that there is a good
correlation between the mobilities of outwardly
mobile incorporated species in anodic amorphous titania , as well as in anodic alumina
, and their single metal-oxygen bond energies:
species with stronger metal-oxygen bonds migrate more slowly. Thus, it is
likely that the alloying element species, which migrate more slowly, and hence
are more concentrated in the inner film region relative to the alloy
composition, suppress more effectively the formation of crystalline oxides in
anodic titania. The amorphous-to-crystalline
transition of anodic titania involves local re-ordering
of ionic arrangements, which is possibly assisted by ionic transport during
film growth under the high electric field. The slower migrating species may impede
such ordering of ionic arrangements, and hence, the amorphous structure of
anodic oxides is sustained to high voltages.
§27 Figure 10.� Compositional dependence of
the maximum voltage for the linear voltage increase in the voltage-time
response of the titanium alloys during anodizing at 50 A m-2 in 0.1 M
ammonium pentaborate electrolyte at 293 K.
§28 An amorphous-to-crystalline
transition of anodic oxide formed on titanium occurs during anodizing in 0.1 M
ammonium pentaborate electrolyte to ~20 V. Oxide crystals, which consist of anatase, develop in the film region where film materials
are formed at the metal/film interface by anion ingress, while the remaining
outer region formed at the film/electrolyte interface by cation egress consists
only of amorphous oxides. Oxide zones, of size ~ 1 nm, with a relatively
ordered structure, developed at the metal/film interface, are considered to
lead to transformation of the inner region structure. The incorporation of foreign species from the substrate suppresses the
amorphous-to-crystalline transition, resulting in the uniform film growth to
high voltages. Species incorporated from the substrate, such as aluminium,
molybdenum, silicon, and zirconium species, are distributed at least throughout
the film region formed by anion ingress, which cannot be achieved by
electrolyte-derived species. The slower migrating species, with stronger
metal-oxygen bonds, that impede local re-ordering of ionic arrangements, suppress
more effectively the formation of oxide crystals.
Thanks are due to Mr.Y. Uchida of
Horiba Ltd., Tokyo, Japan
for the provision of time on JobinYvon 5000 rf
GDOES instrument. The present work was supported in part bythe
Light Metal Educational Foundation, Inc.
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