Structure and properties of ZrO2 ceramic coatings on AZ91D Mg alloy by plasma electrolytic oxidation

Article abstract of DOI:10.1111/j.1551-2916.2007.02155.x

ZrO₂ ceramic coatings were prepared in situ on an AZ91D Mg alloy by plasma electrolytic oxidation in a K₂ZrF₆ solution. The phase composition and the surface morphology of the coatings were examined with X-ray diffraction and scanning electron microscopy. The thermal shock resistance of the coatings was evaluated by a thermal shock test. The corrosion resistance of the coated samples was examined by the polarizing curve method in a 3.5% NaCl solution. The prepared coating was composed of t-ZrO₂ and a small amount of c-ZrO₂. There were many residual discharging channels on the coating surface. The coated samples showed excellent thermal shock resistance under 500°C, which improved with increasing frequency or decreasing current density or PEO time. Besides, the coating improved the corrosion resistance of AZ91D Mg alloy considerably. In the experiments, the corrosion current density of the coated samples prepared under 1000 Hz was the least, which also decreased with the current density during the PEO process.

Full text of DOI:10.1111/j.1551-2916.2007.02155.x

J. Am. Ceram. Soc., 91 [2] 555–558 (2008)
DOI: 10.1111/j.1551-2916.2007.02155.x
r 2007 The American Ceramic Society
ournal
J
Structure and Properties of ZrO2 Ceramic Coatings on AZ91D Mg Alloy
by Plasma Electrolytic Oxidation
Zhongping Yao,w Huihui Gao, Zhaohua Jiang, and Fuping Wang
Department of Applied Chemistry, Harbin Institute of Technology 15000, China
ZrO2 ceramic coatings were prepared in situ on an AZ91D Mg
alloy by plasma electrolytic oxidation in a K2ZrF6 solution. The
phase composition and the surface morphology of the coatings
were examined with X-ray diffraction and scanning electron mi-
croscopy. The thermal shock resistance of the coatings was
evaluated by a thermal shock test. The corrosion resistance of
the coated samples was examined by the polarizing curve meth-
od in a 3.5% NaCl solution. The prepared coating was com-
posed of t-ZrO2 and a small amount of c-ZrO2. There were
many residual discharging channels on the coating surface. The
coated samples showed excellent thermal shock resistance under
5001C, which improved with increasing frequency or decreasing
current density or PEO time. Besides, the coating improved the
corrosion resistance of AZ91D Mg alloy considerably. In the
experiments, the corrosion current density of the coated samples
prepared under 1000 Hz was the least, which also decreased with
the current density during the PEO process.
single-polar electrical source was used for PEO of the plate
samples in a water-cooled electrolyzer made of stainless steel,
which also served as the counter electrode. The reaction tem-
perature was controlled to below 301C by adjusting the cooling
water flow. The PEO process equipment used was similar to
the one presented in Yerokhin et al.1 The output mode of the
anode pulse can be seen in Yao et al.11 and the duty ratio was
50%. The concentration of K2ZrF6 was 3 g/L; the pH of the
electrolyte was adjusted to about four to five by adding a mix-
ture of H3PO4 and NaH2PO4. After the treatment, the coated
samples were rinsed with water and dried in air.
(2) Phase Composition and Structure of the Coatings
The phase composition of the coatings was examined with X-ray
diffraction (XRD; Rigaku D/Max-rB, Tokyo, Japan) using a
CuKa source. The surface morphologies were studied with scan-
ning electron microscopy (SEM; Hitachi S-580, Tokyo, Japan).
The elemental distribution was investigated by energy-dispersive
spectroscopy (EDS; US PN5502). Sample thickness was mea-
sured, using an eddy current-based thickness gauge (CTG-10,
Time Company), in which minimum resolution was 1 mm.
I. Introduction
LASMA electrolytic oxidation (PEO), by which the compound
ceramic coating can be grown in situ on Al, Ti, Mg, and
P
many other metals and alloys directly, has developed rapidly in
rescent years. The ceramic coatings prepared had corrosion re-
sistance, anti-abrasion property, or decorative property, and
may therefore be used in many fields like automotive, aerospace,
medicine, textiles, and so on.1–4 At present, the widely used
electrolytes for PEO on Mg alloys are silicate, phosphate, al-
uminate, sodium hydroxide, or their mixed solution, and the
coatings prepared are usually composed of MgO, MgSiO3, or
(3) Thermal Shock Resistance Test
Thermal shock resistance tests were carried out in a muffle at
5001C. The samples were kept in the muffle for 2 min, and then
set into cool water quickly. The above process was repeated and
the surface changes of the coatings were recorded to assess the
thermal shock resistance.
5–7
MgAl2O4.
(4) Corrosion Resistance
ZrO2 is one of the promising coating materials, which has
high strength, good fracture toughness property, and excellent
wear resistance, corrosion resistance, and so on. Presently, there
are only a few instances of research work with the PEO tech-
nique on an Al or a Ti alloy in the zirconate system8–10; such
work on a Mg alloy has seldom been reported. Therefore, the
aim of our work is to prepare ceramic coatings that contain
ZrO2 on an AZ91D Mg alloy by a pulsed single-polar PEO
technique in a K2ZrF6 solution. Meanwhile, the structure, the
thermal shock resistance, and the corrosion resistance of the
produced coatings were investigated.
The polarizing curve method was used in a three-electrode cell
(a Pt plate was used as a counter electrode, a calomel electrode as
an auxiliary electrode, and the coated sample as a working elec-
trode) through a CHI604C electrochemical analyzer (Shanghai,
China) that was used to assess the corrosion resistance of the
coated samples in a 3.5% NaCl solution. The polarizing curve
scanning rate was 1 mV/s, with a scanning range from À0.25 V of
open circuit potential to plus 0.25 V of an open-circuit potential.
Three samples were prepared under each condition to ensure
the reliability of the experiments.
III. Results and Discussion
II. Experimental Procedure
(1) XRD Analysis of the Coatings
(1) Preparation of Ceramic Coatings on AZ91D by PEO
The phase composition of the coating and the substrate is shown
in Fig. 1. Obviously, the coating was composed of t-ZrO2 and a
small amount of c-ZrO2. Because the coatings were thin and
porous, the diffraction peaks corresponding to the substrate also
existed in the pattern, but their intensity had decreased consid-
erably. Interestingly, MgO did not exist in the coatings, which
was quite different from the many references. This may be re-
lated to the pH of the electrolyte. In most references, the elec-
trolytes adopted were alkaline, which allowed the soluble Mg21
from the substrate to remain easily in the coating in the form of
insoluble Mg(OH)2, and was finally sintered as MgO in the
coating, while the acid electrolyte in this experiment did not
Plate samples of AZ91D with dimensions of 20 mm  15 mm
 2 mm were polished with abrasive paper. A homemade pulsed
C. Yan—contributing editor
Manuscript No. 23554. Received August 7, 2007; approved September 23, 2007.
This work was financially supported by the Harbin Special Foundation of Fellow for
Science and Technology Creation of China (Grant No. 2006RFQXG032) and Chinese Sci-
ence Foundation for Post-doctor Fellows (Grant No.20060400238).
wAuthor to whom correspondence should be addressed. e-mail: yaozhongping@
hit.edu.cn
555

556
Journal of the American Ceramic Society—Yao et al.
Vol. 91, No. 2
allow most of the soluble Mg21 to form insoluble Mg(OH)2, but
instead come into the electrolyte.
(2) Surface SEM and the Elemental Distribution of the
Coatings
Figure 2 shows the surface morphology of the coatings prepared
using different technical parameters. The coatings were not even
and there were many residual discharging channels and micro-
cracks on the coating surface. On decreasing the frequency or
increasing the current density or the reaction time, the residual
discharging channels and microcracks were enlarged. The thick-
ness of the coatings in panels (a) and (b) was 20 mm or so. On
increasing the current density or the reaction time, the coating
thickness increased consequently, for instance, the coating thick-
ness in panel (d) was 31.4 mm with an SD of 5.9.
In terms of panel (d), the coating can be easily divided into
two parts: one was the sintered pancake structures with large
discharging channels (marked with a), and the other was small
sintered particles with many cracks (marked with b). Table I
shows the relative contents of Zr and Mg in panel (d) by
EDS analysis. This showed that the content of Mg in the coat-
ing was very low and only existed slightly near the discharging
channels, which was consistent with the results of the XRD
analysis.
Fig. 1. X-ray diffraction patterns of the coating and the AZ91D
substrate.
Fig. 2. Surface scanning electron microscopy of the coatings prepared under different technical parameters. (a), 100 Hz 6 A/dm2 5 min; (b),
1000 Hz 6 A/dm2 5 min; (c), 1000 Hz 8 A/dm2 5 min; (d), 1000 Hz 6 A/dm2 20 min.

February 2008
Plasma Electrolytic Oxidation
557
Table I. The Relative Contents of Zr and Mg in the Coating
by EDS Analysis (mass%)
a
b
Mg
Zr
11.0
89.0
0.7
99.3
EDS, energy dispersion spectroscopy.
Fig. 3. Polarizing curves of the coated sample and AZ91D substrate in
a 3.5% NaCl solution.
(3) Thermal Shock Resistance of the Coated Samples
The results of the thermal shock tests are shown in Table II.
Compared with Jiang et al.,12 the coatings had excellent thermal
shock resistance, which might be mainly attributable to the ZrO2
in the coating. Among the ceramic materials, the linear expan-
sion coefficient of ZrO2 approached that of the metal materials
the most. Besides, the residual discharging channels and the mi-
crocracks on the coating surface were also helpful for the release
of heat stress during the thermal shock tests. Therefore, there
were only a few small cracks on the best coating after cycling 37
times, and for all the coatings, no spallation occurred after cy-
cling 50 times. Besides, increasing the frequency and decreasing
the PEO time or the current density was liable to increase the
thermal shock resistance of the coated samples.
Fig. 4. Influences of the frequency (a) and the current density (b) on the
corrosion current density of the coated samples.
the Mg alloy was improved considerably. Besides, the corrosion
potential of the coated sample shifted positively, decreasing the
thermodynamic tendency of the corrosion.
Technical parameters influenced the surface morphology of
the coatings, which were bound to influence the corrosion re-
sistance, with the results shown in Fig. 4. On decreasing the
frequency, the coating’s residual discharging channels and mi-
crocracks increased, while the coating thickness increased, and
vice versa. There seemed to be a conflict between the thickness
and the density of the coating, which indicated that the coated
samples prepared under 1000 Hz had the minimum corrosion
current density, seen in panel (a). Besides, panel (b) illustrates
that the corrosion current density decreased with the current
(4) Corrosion Resistance of the Coated Samples
Corrosion is one of the most important problems in the appli-
cation of Mg alloy, and therefore, the corrosion resistance of the
coated samples was further investigated. Figure 3 shows the po-
larizing curves of the coated sample and the AZ91D substrate in
a 3.5% NaCl solution with the corrosion current density and the
corrosion potential. The corrosion current density of the coated
sample was reduced by nearly two quantity orders, compared
with the substrate, which meant that the corrosion resistance of
Table II. The Results of Thermal Shock Tests of the Coated Samples
Current density
(A/dm2)
Frequency
(Hz)
MAO time
(min)
Thermal shock tests
1
2
3
4
5
3.3
3.3
5.0
5.0
6.6
500
1000
100
1000
500
20
30
20
10
10
Small cracks emerged after 27 times and no spallation at 50 times
Small cracks emerged after 37 times and no spallation at 50 times
Small cracks emerged after 35 times and no spallation at 50 times
Small cracks emerged after 34 times and no spallation at 50 times
Small cracks emerged after 23 times and enlarged quickly.
No spallation at 50 times

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Journal of the American Ceramic Society—Yao et al.
Vol. 91, No. 2
3W. B. Xue, Z. W. Deng, T. H. Zhang, R. Y. Chen, and Y. L. Li, ‘‘Microarc
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density during the PEO process, which was also consistent with
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IV. Conclusions
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tion,’’ Electrochim. Acta, 52 [15] 5002–9 (2007).
(1) ZrO2 ceramic coatings were prepared in situ on an
AZ91D Mg alloy by PEO in a K2ZrF6 solution. The coating
was composed of t-ZrO2 and a small amount of c-ZrO2. There
were many residual discharging channels and microcracks on
the coating surface.
(2) The coated samples had excellent thermal shock resis-
tance at 5001C. Increasing the frequency or decreasing the PEO
time or current density was liable to improve the thermal shock
resistance.
(3) The coating improved the corrosion resistance of the
AZ91D Mg alloy considerably. The coated samples prepared
under 1000 Hz had the minimum corrosion current density and
in the meantime, the corrosion current density decreased with
the current density during the PEO process.
6H. F. Guo and M. Z. An, ‘‘Growth of Ceramic Coatings on AZ91D
Magnesium Alloys by Micro-Arc Oxidation in Aluminate-Fluoride Solutions
and Evaluation of Corrosion Resistance,’’ Appl. Surf. Sci., 246 [1–3] 229–38
(2005).
7B. L. Jiang and X. F. Zhang, ‘‘Growth Rhythm and Corrosion Resistance of
Ceramic Coatings Formed by Micro-Arc Oxidation on Magnesium Alloys in
Different Conductivity Solutions,’’ Rare Metal Mater. Eng., 34 [3] 393–6 (2005) (in
Chinese).
8G. L. Schukin, A. L. Belanovich, V. P. Savenko, L. S. Ivashkevich, and
V. V. Sviridov, ‘‘Micro-Plasma Anodic Oxidation of Aluminum and its Alloy
Containing Cu in K2ZrF6 Sulotion,’’ J. Appl. Chem., 69 [6] 939–41 (1996) (in
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9Z. D. Wu, Z. H. Jiang, and Z. P. Yao, ‘‘Influence of Treatment Time
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Alloy by Micro-Arc Oxidation,’’ J. Inorg. Mater., 22 [3] 555–9 (2007) (in Chinese).
10Z. P. Yao, Z. H. Jiang, and X. L. Zhang, ‘‘Effect of Na2SO4 on Structure and
Corrosion Resistance of Ceramics Coatings Containing Zirconium Oxide on Ti–
6Al–4V Alloy,’’ J. Am. Ceram. Soc., 89 [9] 2929–32 (2006).
11Z. P. Yao, R. H. Cui, Z. H. Jiang, and F. P. Wang, ‘‘Effects of Duty Ratio at
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