Al2O3 coating fabricated on titanium by cathodic microarc electrodeposition

Article abstract of DOI:10.1016/j.jallcom.2008.08.079

A Al₂O₃ coating was prepared on titanium substrate by cathodic microarc electrodeposition method in Al(NO₃)₃ ethanol solution. The coating thickness was about 80 μm when a 400 V cathodic potential was applied. T

Full text of DOI:10.1016/j.jallcom.2008.08.079

Journal of Alloys and Compounds 476 (2009) 356–359
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Al2O3 coating fabricated on titanium by cathodic microarc electrodeposition
Qian Jin, Wenbin Xue∗, Xijin Li, Qingzhen Zhu, Xiaoling Wu
Key Laboratory for Beam Technology and Materials Modification of Ministry of Education, Institute of Low Energy Nuclear Physics, Beijing Normal University, Beijing 100875, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A Al2O3 coating was prepared on titanium substrate by cathodic microarc electrodeposition method in
Al(NO3)3 ethanol solution. The coating thickness was about 80 ␮m when a 400 V cathodic potential was
applied. The morphology and phase constituent of the Al2O3 coating were investigated by scanning elec-
tron microscope (SEM) and X-ray diffraction (XRD). The isothermal oxidation at 700 ◦C and electrochemical
corrosion behavior of the coated titanium were analyzed. The coating was composed of ␥-Al2O3 and lit-
tle ␣-Al2O3 phases. The oxidation resistance of the titanium subjected to cathodic microarc treatment
was obviously improved. The polarization test indicated that the coated titanium has better corrosion
Received 19 July 2008
Accepted 23 August 2008
Available online 19 October 2008
Keywords:
Cathodic microarc electrodeposition
Titanium
Al2O3 coating
Properties
resistance.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
papers about this technique are less published. The two electrodes
are placed in the electrolyte and the workpiece acts as cathode,
Ti and its alloys are widely applied in aeronautics, chemistry,
shipbuilding, medical materials and other areas. However, the
industrial applications of Ti are limited by its poor corrosion resis-
tance in deoxidized acid, low wear resistance and poor oxidation
resistance at elevated temperature [1,2]. In order to improve the
properties of titanium, many surface treatment processes have
been applied, such as anodizing [3,4], physical vapor deposition
(PVD) [5,6], plasma and laser nitriding [7], thermal oxidation [8]
and ion implantation [9,10].
Microarc oxidation (MAO) or plasma electrolytic oxidation (PEO)
is a promoting method to prepare for ceramic coating on Ti and
its alloys [1,11,12]. During MAO treatment, the Ti workpiece acts
as anode. Many papers about microarc oxidation on Ti have been
published [1,12–14]. The MAO coating can significantly improve the
corrosion and wear resistance of Ti alloys [1,12], however, this coat-
ing is mainly composed of rutile and anatise TiO2, or Al2TiO5 phase
formed in NaAlO2 solution [13]. The diffusion coefficient of oxygen
in TiO2 and Al2TiO5 phases are rather high and the MAO coating
could not effectively impede the oxygen diffusion. Furthermore, a
pure alumina coating without TiO2 phase could not be fabricated
by microarc oxidation. Therefore, microarc oxidation process is not
an effective method to improve the oxidation resistance of Ti and
its alloys.
which is different from anodic microarc oxidation. When the critical
potential is reached, the vapor envelope over the cathode will be
broken out and microarc discharge happens. Then the ions in the
electrolyte will be deposited on the metal to form ceramic coatings.
In this study, we applied cathodic microarc electrodeposition to
fabricate a pure Al2O3 coating on Ti substrate. The morphology,
microstructure and phase constituent of the coating were ana-
lyzed. Thermal shock test was carried out to estimate the adhesion
between the coating and Ti substrate. The oxidation resistance of
the coating was evaluated. Furthermore, electrochemical corrosion
behaviors of coating and Ti substrate were analyzed.
2. Experimental procedure
The substrate materials were TA2 pure titanium. All samples were cut into the
size of 30 mm × 15 mm × 1 mm. The surfaces were ground to 800# emery paper, and
then cleaned with distilled water.
A 30 kW pulse power supply with the frequency of 50 Hz was used. Ti samples
and stainless steel bath acted as cathode and anode, respectively. The electrolyte was
0.4 mol/L Al(NO3)3·9H2O ethanol solution. The temperature of the solution was con-
trolled below 60 ◦C. After deposition, the coated sample was cleaned using distilled
water, and then dried by hot air. The coating thickness was measured by an eddy-
current thickness gauge. Table 1 displays the coating thickness can reach 80 ␮m
under 60 min electrodeposition.
A X’ PERT PRO MPD X-ray diffractometer (XRD) was used to examine the phase
components of the coating. The morphology and microstructure of the coating
were observed using a Hitachi S4800 scanning electron microscope (SEM) with
energy dispersive spectroscopy (EDS). Thermal shock test was performed for 100
cycles. In each cycle, the coated sample was heated to 700 ◦C and kept for 5 min,
then quenched into water. The longest crack on the coating surface was mea-
sured using an optical microscope. Isothermal oxidation of the coated sample and
Ti substrate was performed at 700 ◦C. The mass of the samples was measured
by an analytical balance with an accuracy of 10−4 g. Thus their oxidation kinetic
Cathodic microarc electrodeposition is a new method to produce
oxide coatings on metal substrates [15–17]. Up to now, the relevant
∗
Corresponding author. Tel.: +86 10 62207222; fax: +86 10 62231765.
E-mail address: xuewb@bnu.edu.cn (W. Xue).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.08.079

Q. Jin et al. / Journal of Alloys and Compounds 476 (2009) 356–359
357
Table 1
Deposition parameters of the cathodic microarc coating
Applied potential (V)
Deposition time (min)
400
60
Concentration of Al(NO3)3 (mol/L)
Coating thickness (␮m)
0.4
80
curves were obtained. Potentiodynamic polarizations in 3.5% NaCl solution were
carried out using a CS300UA electrochemical workstation to evaluate corrosion
behaviors before and after cathodic microarc electrodeposition on Ti substrate. A
three-electrode cell, with the Ti sample as working electrode, saturated calomel
electrode (SCE) as reference electrode and platinum coil as counter electrode, was
employed in this test. The area of samples was 1 cm2. After 5 min initial delay, scan
was conducted at 1 mV s−1 from −250 mV versus open circuit potential towards
more noble direction.
3. Experimental results
Fig. 2. Cross-sectional microstructure of cathodic microarc coating on Ti.
3.1. Morphology and microstructure of the coating
that the cathodic microarc electrodeposition and microarc oxida-
tion have different formation mechanism. The former depends on
the electrolyte deposition, but the latter mainly comes from the Ti
substrate oxidation.
The surface morphology of cathodic microarc coating on Ti is
indicated in Fig. 1. The coarse and rough surface of coating is com-
posed of granules. Melting particles and residual discharge pores
appear on the coating surface. These pores correspond to the spark
channels. This surface morphology in Fig. 1 is similar to that of
microarc oxidation coating on Ti [1,12,14], however, the pore quan-
tity of cathodic microarc coating seems to be less than that of
microarc oxidation coating. Furthermore, comparing to the MAO
coating [12,14], Fig. 1(b) displays that many deposition particles
are observed on cathodic microarc coating surface. That implies
The cross-sectional microstructure of coated sample is displayed
in Fig. 2. The thickness of coating is about 80 ␮m. The coating con-
sists of a porous outer layer and an inner layer. The inner layer
about 15 ␮m close to the coating/Ti interface adheres well with Ti
substrate. Table 2 provides EDS composition analysis results at dif-
ferent points of A, B, C as shown in Fig. 2. The A, B, C refer to the Ti
substrate, inner layer coating and outer layer coating, respectively.
A little amount of Al and O elements are detected in the Ti substrate
close to the coating/Ti interface (see point A in Table 2). That means
some elements from electrolyte under cathodic microarc discharge
also diffuse into the Ti substrate near the interface. In fact, when
the EDS testing point is only several microns away from the inter-
face, Al and O will not be detected in the Ti substrate. On the other
hand, the coating is mainly composed of Al and O elements, which
corresponds to Al2O3 (see point B and C in Table 2). However, a lit-
tle amount of Ti element was also detected in the coating of both
inner layer and outer layer. It confirms that a little amount of Ti near
the interface involves in the sintering process in cathodic microarc
discharge zone and some titanium atoms diffuse into the whole
coating. However, the Ti content in the cathodic microarc coating
is much lower than that in anodic microarc oxidation coating.
3.2. XRD analysis
Fig. 3 depicts XRD pattern of cathodic microarc coating on tita-
nium. The coating formed in Al(NO3)3 ethanol solution consists of
␣-Al2O3 and ␥-Al2O3 phases. The Ti peaks in Fig. 3 is contributed
from the Ti substrate. Comparing the relative intensity of feature
peaks of ␣-Al2O3 (2Â = 43.3◦) and ␥-Al2O3 (2Â = 45.9◦), it is found
that the ␥-Al2O3 content in the coating is much higher than ␣-
Al2O3 content. EDS analyses in Table 2 have identified the existence
of a little Ti element in the coating, but XRD pattern of coating in
Fig. 3 has not detected titanium oxide. That results from the low Ti
content in the coating.
Table 2
EDS composition analyses of the coating at different positions in Fig. 2
Element (wt.%)
A
B
C
Al
O
Ti
1.26
0.96
97.78
48.71
43.97
7.32
42.00
53.41
4.59
Fig. 1. Surface morphology of cathodic microarc coating on titanium, (b) is a mag-
nified image (a).

358
Q. Jin et al. / Journal of Alloys and Compounds 476 (2009) 356–359
Fig. 3. XRD pattern of cathodic microarc coating deposited on Ti in Al(NO3)3 ethanol
solution.
Fig. 5. Polarization curves of the coating and Ti substrate.
3.3. Thermal shock test
is observed that at 100 h isothermal oxidation, the weight gain of
Ti substrate is about five times higher than that of coated sample.
Hence, cathodic microarc electrodeposition is a potential method
to improve the high-temperature oxidation resistance of Ti and its
alloys.
In order to evaluate the thermal shock resistance of cathodic
microarc coating, the coated Ti sample of 80 ␮m thick was sub-
jected to 100 times thermal cycles by quenching into water from
700 ◦C. After 100 times thermal cycles, no cracks on the coating sur-
face were observed, meanwhile the coating was not delaminated
from the Ti substrates. That proves that the coated titanium by
cathodic microarc electrodeposition has better thermal shock resis-
tance. Fig. 3 shows that the cathodic microarc coating deposited in
Al(NO3)3·9H2O ethanol solution mainly consists of ␥-Al2O3, and
the content of ␣-Al2O3 phase is less. The ␥-Al2O3 phase has better
ductile property than ␣-Al2O3 phase, which is favor of improving
the adhesive strength of coating/Ti interface. On the other hand, the
porous layer in the coating benefits to decrease thermal stress and
depress the generation and propagation of the thermal cracks.
3.5. Corrosion resistant test
Potentiodynamic polarization curves of 80 ␮m coating and Ti
substrate are given in Fig. 5. The corrosion current density of coating
with 6.83 × 10−7 A cm−2 is nearly one order of magnitude lower
than that of Ti substrate with 4.7875 × 10−6 A cm−2. In addition, the
corrosion potential of coated sample is about 300 mV higher than
that of substrate. Hence, the cathodic microarc electrodeposition
also improves the corrosion resistance of titanium.
Figs. 1 and 2 show that the cathodic microarc coating is porous.
However, the coating close to coating/substrate interface has a thin
compact layer, which hinders the Cl− ions in the solution penetrate
into the Ti substrate. So this thin compact layer plays an important
role in improving the corrosion resistance of titanium.
3.4. Isothermal oxidation test
Fig. 4 depicts the mass variation of the Ti substrate and coating
sample with thermal oxidation time at 700 ◦C in air. It is indicated
that the oxidation resistance of the coated sample is much better
than that of the Ti substrate. Each curve in Fig. 4 consists of two
stages. In the initial 10 h, the mass of Ti substrate increases quickly.
After 10 h oxidation, the rate of its weight gain reduces, and the
weight gain almost increases linearly. However, for the coated sam-
ple, its weight gain is obviously lower than the Ti substrate. After
5 h oxidation, the weight gain of coated sample is rather low. It
4. Discussion
When the Ti sample as a cathode is immersed in electrolyte and
a negative potential is applied, many bubbles appear on Ti cath-
ode and gradually cover the whole sample. The main reaction at
cathode is the hydrogen gas evolution, following with electrolysis
of Al(NO3)3·9H2O. In this stage, no sparks appear, and the covering
hydrogen film provides a barrier layer for microarc discharge.
When the potential exceeds a threshold value, the hydrogen film
around Ti cathode is broken down. Small and weak sparks appear
on the local Ti surface. After several minutes, the whole surface is
covered with wandering sparks. The temperature in microarc or
spark discharge zone is high enough to cause some physical and
chemical reactions. Hence Al(OH)3 is deposited on the Ti substrate
and then sintered into Al2O3 phase:
Al3+ + 3OH− → Al(OH)3↓
2Al(OH)3 → Al2O3 + 3H2O
The OH− ions may come from reduction reactions of O2, NO3 and
H2O [11,18]:
(1)
(2)
−
NO3− + H2O + 2e− → NO2− + 2OH−
2NO2− + 3H2O + 4e− → N2O + 6OH−
O2 + 2H2O + 4e− → 4OH−
(3)
(4)
(5)
Fig. 4. Isothermal oxidation dynamics of Ti substrate and cathodic microarc coating
sample at 700 ◦C in air.

Q. Jin et al. / Journal of Alloys and Compounds 476 (2009) 356–359
359
2H2O + 2e− → H2 + 2OH−
(6)
(4) The oxidation rate of the coated Ti reduces about five times
under 700 ◦C isothermal oxidation test.
(5) By cathodic microarc treatment, the corrosion current density
of Ti substrate reduces about one order of magnitude.
When a thin Al2O3 coating deposits on Ti substrate, it becomes the
barrier layer. The cathode microarc discharge always happens at
the relatively weak areas of Al2O3 film, thus sparks could move on
the coating and form a uniform coating.
Acknowledgments
Different from anodic microarc oxidation, the coating fabri-
cated by cathodic microarc electrodeposition is mainly composed
of ␥-Al2O3 (see Fig. 3). In our previous papers about formation
mechanism of ceramic phases in anodic microarc oxidation coat-
ings on 2024 Al alloys [19,20], we have proposed that both ␣-Al2O3
and ␥-Al2O3 phases come from the rapid solidification of alumina
melt in microarc discharge zone, and high cooling rate is favor of the
formation of ␥-Al2O3 phase. In fact, this explanation may also apply
to the formation of cathodic microarc coating. As shown inFig. 2, the
cathodic microarc coating is porous, especially in the outer layer,
thus the cooling rate of melt in cathodic microarc discharge zone is
very high, even if in the interior of coating. Therefore, the cathodic
microarc coating in this work mainly consists of ␥-Al2O3 phase,
which is consistent with the XRD result in Fig. 3.
Although Ti substrate involves in the formation process of
cathodic microarc coatings, Ti content in the coating is rather low
(see Table 2). Thus a relative pure and continuous alumina coat-
ing may be formed on the Ti substrate. That results in a good high
temperature oxidation resistance for cathodic microarc coatings as
shown in Fig. 4. The cathodic microarc electrodeposition process is
a promising effective method to improve the oxidation resistance
of Ti and its alloys.
This research was sponsored by the New-star Program of Beijing
Science and Technology Committee (No. 9558102500), the SERF
for ROCS, SEM and Beijing Scientific Research Foundation for the
Returned Overseas Chinese Scholars.
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(1) Continuous Al2O3 coating was prepared by cathodic microarc
electrodeposition on Ti substrate in Al(NO3)3 ethanol solution.
(2) The coating is about 80 ␮m and mainly composed of ␥-Al2O3
and ␣-Al2O3. The ␣-Al2O3 content is less than 15%.
(3) Thermal shock test implied that the coating adheres well to the
substrate.