Effect of Applied Voltage on Properties of Micro-arc Oxidation
Coating on TC4 Alloy
Wei Zhou, Yunlong Zhang
*
, Chaojun Cui, Zhigang Wang, Haifeng Zhang and Qingxiang Yang
Anyang institute of technology, Huang he street, Anyang city, P. R. China
Keywords: Micro-arc oxidation, Yttrium nitrate, Micro-hardness
Abstract: In order to resolve biological toxicity of TC4 titanium alloy, the micro-arc oxidation (MAO) technology was
introduced to fabricate ceramic coating on the surface of TC4 titanium alloy. Yttrium nitrate was introduced
into the silicate electrolyte system as additive. The micro-structure, phase composition, surface morphology
and micro-hardness of MAO coatings were characterized. When applied voltage increased, the content of
rutile-TiO
2
in the coating increased. The maximum value of surface roughness was about 11μm as applied
voltage was 300V. The maximum micro-hardness was about 5210MPa.
1 INTRODUCTION
Ti-based alloys are widely used in aerospace, weapon
and other fields owing to their low density, high
specific strength and excellent corrosion resistance
(Boyer, 1996; Wang, 2015; Wang, 2017). TC4 alloy
has biological toxicity, so it is seriously limited in the
clinical medicine applications. Therefore, various
modifying techniques such as salt cyaniding (Lai and
Wu, 1993), plasma immersion ion implantation
treatment (Yilbas and Shuja, 2000), laser treatment
(Yerokhin, 2000), micro-arc oxidation (Mandl, 2007)
and PVD process, were introduced to
improve performances for practical applications. The
main purpose of surface treatment is to keep
vanadium elements in titanium alloy matrix and
avoid its releasing.
Micro-arc oxidation technology (MAO) has
distinguished advantage to prepare ceramic coating
on the Mg, Ti and Al alloys. The ceramic coating can
improve both wear and corrosion resistance of alloys.
Rare earth materials have important scientific
research value, so they are widely used in the field of
metal matrix modification. In this work, yttrium
nitrate was introduced into the silicate electrolyte
system as additive. The TC4 alloy was oxidized
under different applied voltages by MAO method.
The micro-structure, phase composition, surface
morphology and micro-hardness of MAO coatings
were characterized.
2 EXPERIMENTAL
Before micro-arc oxidation treatment, TC4 alloy
were cut into the size of 20mm×20mm×2mm. The
specimens were polished by SiC sandpapers with grit
sizes of 600#, 1000# and 2000#, respectively. And
they were rinsed by distilled water, acetone several
times and dried. The electrolyte is composed of 30g/L
Na
2
SiO
3
·5H
2
O, 10g/L EDTA-2Na, 2g/L KF and
1.5g/L Y(NO
3
)
3
in aqueous solution. In MAO
process, applied voltage was designed as 240V, 270V,
300V, 330V and 360V, respectively. And the
samples were correspondingly nominated as S1, S2,
S3, S4 and S5. The MAO equipment with power of
100kW was composed of an AC power supply and
ultrasonic systems. The stainless steel container and
TC4 alloy were used as cathode and anode separately.
For comparison, MAO was carried out at 330V
without Y(NO
3
)
3
in the electrolyte, and the sample
was nominated as S6. After MAO treatment, the
samples were rinsed with distilled water and dried.
X-ray diffraction device (Bruker D8 Advance) was
used to analyse the phase structures with a scan rate
of 4°/min. Surface morphology and roughness was
analysed by Olympus self-focusing microscope and
field emission scanning electron microscope. At least
five areas within 480μm×640μm were measured to
calculate surface roughness. The hardness was tested
by digital micro-hardness tester (HVS-1000A,
Huayin L.L.C., China). The average hardness was
surveyed at least five spots. The Olympus
164
Zhou, W., Zhang, Y., Cui, C., Wang, Z., Zhang, H. and Yang, Q.
Effect of Applied Voltage on Properties of Micro-arc Oxidation Coating on TC4 Alloy.
DOI: 10.5220/0008187101640167
In The Second International Conference on Materials Chemistry and Environmental Protection (MEEP 2018), pages 164-167
ISBN: 978-989-758-360-5
Copyright
c
2019 by SCITEPRESS – Science and Technology Publications, Lda. All rights reserved
metallographic microscope was utilized to take
images of indentations.
3 RESULTS AND DISCUSSION
The XRD patterns of MAO coatings on the TC4 alloy
with different applied voltages is illustrated in
Figure 1 The coatings are composed of rutile-TiO
2
and anatase-TiO
2
. As the applied voltage increased,
the intensity of diffraction peaks of rutile-TiO
2
increased gradually, while those from TC4 alloy
became weaker. It was possible that TiO
2
coating
became thicken as applied voltage increased,
therefore the intensity of diffraction peaks from TC4
matrix became weak. Meanwhile, the enhanced
diffraction peaks of rutile-TiO
2
meant the increased
content of rutile-TiO
2
. As applied voltage increased,
the heat released per unit time also increased, which
facilitated the transformation from anatase to rutile.
20 30 40 50 60 70 80 90
S6
S5
S4
S3
S2
S1
×
×
×
×
Ti
Anatase-TiO
2
Rutile-TiO
2
×
Intensity (a.u.)
Diffraction angle (2-theta)
Figure 1: XRD patterns of TiO
2
coating on the TC4 alloy
with different applied voltages.
The surface morphology of TiO
2
coatings was
showed in Figure 2. It was obvious that many micro
pores evenly distributed in the TiO
2
coating. The
similar distribution characteristics appeared in all
samples. For comparison, larger pore structure was
found in S6. The introduction of Y(NO
3
)
3
in the
electrolyte would helpful in reducing the pore size.
Three dimensional topographic contour map of
TiO
2
coatings were rendered in Figure 3. The
surfaces of the five samples S1-S5 were relatively flat
on macro level.
Figure 4 showed the surface roughness of the
samples. The surface roughness started to increase as
the applied voltage increased, then reduced when the
applied voltage reached 300V. The maximum value
of surface roughness was about 11μm. It was noted
that the additive Y(NO
3
)
3
in the electrolyte would
decrease striking voltage remarkably and make the
coating smooth. The striking voltage averagely
reduced in the range of 15~25V. In addition, cooling
treatment with frozen ice was used to control the
temperature of electrolyte. The above conditions
would be benefit for obtaining smooth and compact
micro-arc oxidation coatings.
Figure 2: Surface morphology of TiO
2
coatings with
different applied voltages, a) S1, b) S2, c) S3, d) S4, e) S5
and f) S6.
Indentation morphologies of the samples were
showed in Figure 5 The original TC4 alloy had much
deeper indentation than others. With the increase of
applied voltage, the depth of indentation decreased.
And the minimum indentation occurred as applied
voltage reached 330V.
The micro-hardness of TiO
2
coatings was
illustrated in Figure 6 The micro-hardness of the
coatings showed a trend of gradual increase as the
applied voltage increased to 330V. And the
maximum value was about 5210MPa. The TiO
2
coating was directly grown on the TC4 matrix, so it
was beneficial for improving the micro-hardness.
b)
a)
E
e)
d)
c)
f)
a)
b)
Effect of Applied Voltage on Properties of Micro-arc Oxidation Coating on TC4 Alloy
165
Figure 3: Three dimensional topographic contour maps of
the samples: a) S1, b) S2, c) S3, d) S4 and e) S5.
240 260 280 300 320 340 360
0
2
4
6
8
10
12
14
16
18
20
Surface roughness (Micron)
Applied voltage (V)
Figure 4: Surface roughness of the samples.
Figure 5: Indentation morphologies of the samples, a)
original TC4 alloy, b) S1, c) S2, d) S3, e) S4 and f) S5.
Figure 6: Micro-hardness of TiO
2
coatings.
a)
c)
e)
d)
b)
a)
b)
c)
d)
e)
f)
MEEP 2018 - The Second International Conference on Materials Chemistry and Environmental Protection
166
4 CONCLUSIONS
Micro-arc oxidation technology was utilized to
grow TiO
2
coating on the TC4 alloy with different
applied voltages in the silicate electrolyte system
containing yttrium nitrate as additive. The
micro-structure and phase composition of the
MAO coatings were characterized. As the applied
voltage increased, the intensity of diffraction peaks
of rutile-TiO
2
increased gradually. The maximum
values of surface roughness and micro-hardness
were about 11μm and 5210MPa.
ACKNOWLEDGEMENTS
The authors were grateful for fund support by the
science and technology research projects from
Anyang city (project “thermal conductivity
behaviour research of copper matrix hybrid materials
with wear-resisting/low expansion for aviation
electric contact field”), the scientific research
projects in education department of Henan province
(No.18A430006), the higher education teaching
reform research and practice project of Henan
Province (No. 2017SJGLX117), the scientific
research project from Anyang Institute of
Technology (No. BSJ2017007, No. BSJ2018018, No.
BSJ2017004). Meanwhile, part of the data in this
paper was provided by Key Laboratory of Aerocraft
Simulation Design and Airborne Equipment of
Anyang City.
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