セラミックス基盤工学研究センター年報 (2008). Vol. 8, 7-11

Li Pengfei

論文

Study on Morphologies and Structures of Ceramic Coating Preparedby Micro-Arc Oxidation on TC4 Alloy in Different Electrolytes
Li Pengfei*(College of Materials Science and Engineering, Inner Mongolia University of Technology, Huhhot, 010051, CHINA)
Abstract: In this work, the effects of concentration and mixture ratio of Ca(CH3COO)2•H2O and Na5P3O10 in electrolyte on surface
morphology, and phase structure of ceramic coating prepared by micro-arc oxidation (MAO) on TC4 titanium alloy were studied.
A suitable formula of electrolyte for bio-modification of titanium by MAO was obtained, namely, Ca(CH3COO)2•H2O 20g/L andNa5P3O10 9.3g/L. Using this electrolyte with appropriate process conditions, the ceramic coating was prepared with porous structure
and combined well with the substrate. Scanning electron microscope (SEM) equipped with the energy dispersive spectrometry (EDS)
was used for observation and evaluation of the morphology. The pores distributed equably on the surface of ceramic coating andthe size of pores was in micron order. The ceramic coating was composed of titanium dioxide in different crystal types whichhave been characterized by the X-ray diffraction (XRD). The anatase titanium dioxide was a major component.
Key words: Ceramic Coating; Micro-Arc Oxidation; TC4; Electrolyte

characteristic.
By means of the MAO technique, high quality ceramic
coatings with high micro-hardness, high adhesion, highstrength and high wear resistance can be obtained. The
coatings can be used in many application fields, such as
automotive, aerospace, textile engineering and medicine,etc.
MAO is a multifactor-controlled process. The qualities
of MAO ceramic coating are affected by technicalparameters like composition of electrolyte, temperatureof electrolyte, alloy composition, voltage, current density,
handling time, etc. High quality coatings can only beformed through the optimization of deposition
parameters.
The effect of the electrolyte composition onmorphologies and structures of ceramic coating on TC4
alloy prepared by MAO has been investigated in this work.

1. Introduction
Titanium and its alloy have become important medical
materials [1-6] for the human body hard organization to
repair and/or replace, because they provides smaller
elastic modulus close to the bone tissue of body, goodchemical stability, corrosion resistance, and excellent
biological compatibility, in addition to relatively low
costs for fabrication.
The hydroxyapatite (HA) is an inorganic material
which can be chemically connected with the bone tissue.
It has been thought as the best bioceramics in biologicalcompatibility at present[7].
The hydroxyapatite coating can be deposited directlyon the surface of titanium alloy through such processes
as laser melting or sol-gel method，but theses
hydroxyapatite decompose easily and the integraty withthe substrate is poor. Extensive application of theseprocesses has been restricted[8, 9].
Micro-arc oxidation (MAO) is a novel technique[10, 11]
to deposit ceramic coatings on the surface of metals andalloys. The ceramic coating formed by MAO can beadhered tightly to its substrate, which is a most important

2. Experimental details
The substrates of TC4 (Ti6-Al4-V) titanium alloy with
30 mm in length and 20 mm in width and 1 mm thickness
were cleaned with acetone in a ultrasonic cleaner and*The author comes from Inner Mongolia University of Technology who is training at the research group of Professor Nobuo Ishizawa
in the CRL under the auspices of Japan Bank for International Cooperation (JBIC) from 6 October 2008 to 28 March 2009.
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Study on Morphologies and Structures of Ceramic Coating Prepared by Micro-arc Oxidation on TC4 Alloy in Different Electrolytes

ceramic coating formed on the TC4 alloy. The phasecomposition of the ceramic coating was characterizedwith X-ray diffraction (XRD).

rinsed with water to remove the mechanicalcontamination and grease. The electrolyte was composedof the analytical reagent grade Ca(CH3COO)2•H2O,
Na5P3O10, Na2SiO3•9H2O, and deionized water.
The special MAO equipment with adjustable frequency
and voltage was used for the MAO process, consistingof a stainless steel tank and a high voltage asymmetricac power supply. One output of the power supply wasconnected to the stainless steel tank; the other wasconnected to the sample immersed in electrolyte. Figure
1 shows an illustration of the special MAO equipment
system.

3. Results & discussion
3.1. Effects of electrolytes on morphologies of ceramic
coating
During MAO process, numerous micro-arc discharges
occurred. When a micro-arc discharge is extinguished,it left a pore on the coating surface. Thus the MAO
coatings usually have a porous surface layer, which
originates in the high temperature treatments along
discharge channels during MAO process.
The gray ceramic coating was prepared on the surface
of samples by MAO. Some typical SEM micrographs of
MAO ceramic coating formed on the samples in different
electrolytes are shown in Figs. 2 and 3.
When the concentration of Ca(CH3COO)2•H2O is heldat 20g/L, we can see from Fig. 2 that the porosity of theceramic coating increases with decreasing the Na5P3O10
concentration. The pore sizes range from several
micrometers to less than one, and the average sizes ofthe samples are not larger than 5mm.
When the atomic ratio of Ca and P elements in theelectrolyte was 3:1, the surface of the ceramic coating has
partially appeared protuberant like a volcano shape asshown in Fig 2, 1-f.

Fig.1 Illustration of Micro-Arc Oxidation equipment system.
1. Power supply, 2. Sample, 3. Stainless steel tank, 4. Electrolyte,
5. Mixing device, 6. Valve.

The process parameters adopted in the experiment are;anodic voltage 460 V, cathodic voltage 160 V, frequency
800 Hz, handling time 20 minutes. The electrolyte
temperature was controlled between 15˚C 〜 30˚C during
the treatment by cooling the electrolyte solution with aheat exchanger.
The concentration of Na2SiO3•9H2O was constant,while the ratio of Ca(CH3COO)2•H2O and Na5P3O10 was
adjusted in each electrolyte. Formulas of the electrolytes
are given in Table 1.
The samples treated by MAO were cleaned withanhydrous alcohol and deionized water in a ultrasoniccleaner. Scanning electron microscope (SEM) equippedwith energy dispersive spectrometry (EDS) was used forthe observation and evaluation of the morphology ofFig.2 Surface morphologies of MAO coatings with different
atomic Ca/P in the electrolyte.

When the atomic ratio of Ca and P elements is held
at 1.5:1 in the electrolytes, the concentration of
Ca(CH3COO)2•H2O and Na5P3O10 increases in eachelectrolytes at the same time. The results show thatthickness of ceramic coating formed in lower

Table 1. Formulas of electrolytes.
serial number

1-a

1-b

1-c

1-d

1-e

1-f

2-a

2-b

2-c

2-d

Ca(CH3COO)2•H2O/g.L-1

20

20

20

20

20

20

10

20

30

40

Na5P3O10/g.L

14

12.7

10.8

9.3

7.0

4.7

4.7

9.3

14

18.7

Ca/P (mol)

1:1

1.1:1

1.3: 1

1.5:1

2:1

3:1

1.5:1

1.5:1

1.5:1

1.5:1

-1

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Li Pengfei

coating is shown in Fig. 4. The ceramic coating
containing pores was tightly combined with the titanium
alloy substrate. The Ca and P elements in the electrolyte
reacted upon the MAO ceramic coating and the most was
distributed near the surface of the ceramic coating.
Figure 5 shows relationship of the atomic Ca/P ratios
in ceramic coating and electrolyte when concentrationof Ca(CH3COO)2•H2O is held at 20g/L. The Ca/P inceramic coating is larger than that in electrolyte when the
Ca/P in electrolyte is smaller than 2, but the Ca/P inceramic coating is smaller than that in electrolyte when
the Ca/P in electrolyte exceeds 2. The Ca/P in ceramic
coating is 1.69 when the Ca/P in electrolyte is 1.5:1,
which corresponds to the atomic Ca/P ratio inhydroxyapatite.

concentration electrolyte was smaller than those in higher
concentration electrolyte and the pores were flimsy as
shown in Fig 3, 2-b. While the concentration ofelectrolytes was higher, the numbers of pore formed werelower, and the pore size became larger as shown in Fig
3, 2-d.

Fig. 3 Surface morphologies of MAO coatings when the atomic
Ca/P is 1.5:1 in the electrolytes.

After comprehensive evaluation for the thickness andsmoothness of the ceramic coatings, the porosity, the poresizes and pore distribution at the surface, here we present
a suitable formula of electrolyte, that is, the concentrationof Ca(CH3COO)2•H2O is more than 20g/L and the atomic
ratio of Ca and P elements in the electrolyte is between
1.3:1 to 2:1.
3.2. Distribution of Ca and P in the ceramic coatings
In order to make the titanium alloy be joined to the
osteoblast, its surface has to possess excellent biologicalcompatibility. The hydroxyapatite has been regarded as
one of the best bioceramics in biological compatibility
at present. The MAO ceramic coating containing higher
content of Ca and P elements and the right atomic ratio
have become advantageous conditions to cultivate
hydroxyapatite.
Cross section microstructure of the sample anddistribution of Ca and P elements in of the MAO ceramic

Fig. 5 Relationship of the atomic Ca/P ratios in ceramic coatingsand electrolytes.

3.3. Effect of electrolytes on the structure of theceramic coatings
Yong Han’s research works[12, 13] indicated that theanatase titanium dioxide is more beneficial to adsorbing
OH- and PO43-, and that appropriate phase structure in theceramic coating is prerequisite for medical application.
Figure 6 (a) shows XRD patterns of the ceramic
coatings when the concentration of Ca(CH3COO)2•H2O
was 20g/L and the atomic ratios of Ca and P elements
were 1:1, 1.5:1 and 2:1, respectively, in electrolytes.
Phase analysis showed that the ceramic coating wascomposed of titanium dioxides in two different crystal
types and elemental titanium. The anatase titanium
dioxide is a major component and rutile titanium dioxide
is a minor one.
A few diffraction peaks of elemental titanium wereobserved at the XRD patterns, partly because the ceramic
coatings were porous and their thicknesses were smaller.
Further analysis for the XRD patterns finds that

Fig. 4 Closs Section microstructure and distribution of Ca andP elements in MAO coating.
―9―Study on Morphologies and Structures of Ceramic Coating Prepared by Micro-arc Oxidation on TC4 Alloy in Different Electrolytes

Fig. 6. XRD pattern of microarc oxidation coatings.
(a) concentration of sodium tripoly phosphate at 20gL-1;(b) atomic ratio of Ca and P elements being at 1.5 in electrolyte

thickness of the MAO ceramic coating increases with theconcentration of Na5P3O10, and a fraction of anatasetitanium dioxide in the ceramic coatings increases at thesame time. This anatase titanium dioxide is indeed, thephase required for the human body hard organization.
Figure 6 (b) shows XRD patterns of the ceramic
coatings when the atomic ratio of Ca and P elements was
1.5:1 in the electrolytes, and the concentration of
Ca(CH3COO)2•H2O and Na5P3O10 was varied inelectrolytes. The patterns indicate that the phases in theceramic coatings do not change obviously, while their
proportion does. With the concentration of
Ca(CH3COO)2•H2O and Na5P3O10 being increased, thefraction of anatase titanium dioxide decreased and thatof rutile titanium dioxide increased in the ceramic
coatings. Therefore, too high concentration of
Ca(CH3COO)2•H2O and Na5P3O10 is disadvantageous tothe ceramic coating used for organism.
Phase containing Ca and P elements has not been
observed in the XRD patterns. The Ca and P elementsin the ceramic coatings could have dissolved in thetitanium dioxide or formed an amorphous phase. In the
XRD patterns (Fig. 6), a diffuse scattering peak exists at
about 30˚ in 2 theta, which may suggest the existence ofan amorphous phase. Main diffraction peak of thehydroxyapatite should lie at about 32˚ in 2 theta. Whether
this diffuse scattering peak corresponds to amorphoushydroxyapatite, we need further investigation.
4. Conclusions

pore sizes range from several micrometers to less than one
when concentration of Ca(CH3COO)2•H2O is more than
20g/L and the atomic ratio of Ca and P elements in theelectrolyte is between 1.3:1 and 2:1.
(2) The ceramic coating is tightly combined with thetitanium alloy substrate. The Ca and P elements in theelectrolyte reacted with the MAO ceramic coating and themost was distributed near the surface of the coating.
(3) Composition of the electrolyte affects directly theatomic Ca/P ratio in the ceramic coating. Whenconcentration of Ca(CH3COO)2•H2O and Na5P3O10 was
respectively at 20g/L and 9.3g/L in the electrolyte, theCa/P in ceramic coating is 1.69 , which is close to 1.5
for the hydroxyapatite.
(4) The ceramic coating was composed of titanium
dioxide in two different crystal types, but proportion ofeach phase depends on the composition of electrolyte. An
amorphous phase probably exist in the ceramic coating.
Whether it is amorphous hydroxyapatite, we need further
investigation.
Acknowledgements
I am grateful to Professor Nobuo Ishizawa，who
provides the chance of a lifetime to study and visit inJapan for me. I wish to take this opportunity to express
my heartfelt gratitude to Prof. Ida, Mr. Hibino, Dr.
Shimamune, Ms. Hayashi and my compatriot Wang Jun.
Thank you for a lot of aid and convenience in works,
studies, experiments and daily life. I would like to thankeveryone in the Ceramics Research Laboratory.
ありがとうございます。

(1) The ceramic coatings have been preparedsuccessfully on the surface of TC4 alloy by MAO in theelectrolytes composed of Ca(CH3COO)2•H2O, Na5P3O10,
and Na2SiO3•9H2O. The ceramic coating is porous and theReferences:― 10 ―[1] Daqing Wei,Yu Zhou ,Dechang Jia, et al. “Biomimetic
apatite deposited on microarc oxidized anatase-based

Li Pengfei

ceramic coating”. Ceramics International, 2008, 34(5):[8] YangYung-Chin,

1139-1144

Chang

Edward.

“Measurementsofresidual stresses in plasma-sprayed hydroxyapatite coatings[2] Mehrdad Keshmiri, Tom Troczynski. “Apatite formation on

on titanium alloy”. Surface and Coatings Technology, 2005,

TiO2 anatase microspheres”. Journal of Non-Crystalline

190(1): 122-131.

Solids, 2003, 324(3): 289-294[9] Gyorgy E, Grigorescu S, Socol G, et al. “Bioactive glass and[3] Hcnch.L.L, Wilson.J. “An introduction to bioceramics”.

hydroxyapatite thin films obtained by pulsed laser

Singapore World Scientific Publishing Company,1993.

deposition”. Applied Surface Science, 2007, 253(19): 7981-[4] Wang R R, Fen ton A. “Titanium for prosthodontics
applications: A review of the literature”. Quintessencen Int

7986[10] Yoshimura, M., Yoo, S. E., Hayashi, M. & Ishizawa, N.:“In Situ Preparation of BaTiO3 Thin Film by Hydrothermal-

1996, 27: 401-408

Electrochemical Synthesis” Ceramic Transactions, Amer

[5] Mandl.S,Sader.R, Krause.D,et al. “Investigation on plasma

Ceram. Soc. Inc. 15 427-436 (1990)

immersion ion implantation treated medical implants”.

[11] Van T B,Brown S D,Wirtz G P. “Mechanism of anodic spark

Biomol Eng, 2002, (19): 129-132.

deposition”. American Ceramic Society Bulletin, 1997,[6] Hyun Sam Ryu, Won-Hoon Song, Seong-Hyeon, et al.

56(6): 563-566“Apatite induction of P-containing titania formed by microarc oxidation before and after hydrothermal treatment”.

[12] Ward Brian C, Webster Thomas J. “Increased functions ofSurface and Coatings Technology, 2008, 202(9): 1853-1858

osteoblasts on nanophase metals”, Materials Science andEngineering C, 2007, 27(3): 575-578.

[7] Schreckenbach J P, Marx G, Scgkittig F, et al.
“Characterization of anodic spark-convertted titanium[13] Won-Hoon Song, Youn-Ki Jun, Yong Han, “Biomimetic

surfaces for biomedical applications”. Materials in

apatiteMedicine, 1999, (10): 453- 457.

Biomaterials, 2004, 25(17): 3341-3349― 11 ―

coatings

onmicro-arcoxidizedtitania”.