Thin Solid Films 517 (2009) 5357–5360
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Thin Solid Films
j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / t s f
Mechanical properties of Al2O3/Al bi-layer coated AZ91 magnesium alloy
Yunchang Xin a,b, Kaifu Huo a,c, Tao Hu b, Guoyi Tang a,⁎, Paul K. Chu b,⁎
a
Advanced Materials Institute, Tsinghua University, Shenzhen Graduate School, Shenzhen 518055, China
Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China
c
The Hubei Province Key Laboratory of Refractories and Ceramics Ministry-Province Jointly-Constructed Cultivation Base for State Key Laboratory, College of Materials and Metallurgy,
Wuhan University of Science and Technology, Wuhan 430081, China
b
a r t i c l e
i n f o
Available online 21 March 2009
PACS codes:
52.77.Dq Plasma-based ion implantation and
deposition
81.65.Kn Corrosion protection
Keywords:
Magnesium alloy
Al2O3/Al bi-layered coating
Mechanical properties
Hardness
Bonding strength
a b s t r a c t
An Al2O3/Al bi-layered coating has been successfully deposited on AZ91 magnesium alloy using a filtered
cathodic arc deposition system and favorable corrosion resistance has been demonstrated by our previous
experiments. In this work, the mechanical properties of an Al2O3/Al bi-layered coating are studied by
nanoindentation and nanoscratch. X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and
scanning electron microscopy (SEM) are employed to characterize the structure of the coating. The surface
hardness and elastic modulus of coated alloy are found to be enhanced dramatically. The coating also exhibits
excellent bonding strength. In addition, the failure process of the coating during scratch is discussed.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
Due to unique properties such as low density and high specific
strength, magnesium-based alloys have found many applications in
the automobile and electronic industry [1–3]. Unfortunately, pure
magnesium and its alloys possess poor corrosion resistance especially
in environments containing aggressive ions as chloride ions, sulfate
ions, etc, consequently limiting wider applications [4,2,5]. One of the
most effective ways to protect the materials from the corrosion media
is to coat the base materials. In order to provide adequate protection,
the coating must be uniform, adhesive, free of pores, and corrosion
resistant [6]. In our previous work [7], a corrosion resistant Al2O3/Al
bi-layered coating was successfully deposited on AZ91 magnesium
alloy using a filtered cathodic arc deposition system. In comparison
with conventional physical vapor deposition techniques, the high
kinetic energy and ionization rate (close to 100%) of the ions created
in the cathodic arc process favor the formation of a dense and adhesive
coating [8]. This method is effective, easy to use, and economical,
thereby especially suitable for industrial applications. In addition, the
bi-layered structure, Al2O3/Al, enhances the bonding strength.
Magnesium is very active chemically and so when it comes in
contact with reactive oxygen species in the vacuum chamber, a
magnesium oxide layer forms on the surface instantaneously making
it nearly impossible to deposit Al2O3 on the substrate directly. Thus, a
pure aluminum transition layer is fabricated prior to deposition of
Al2O3. This transition layer can impede the contact of magnesium and
the ionic oxygen suppressing the formation magnesium oxide. This
transition layer can also buffer the stress induced by serious mismatch
between the mechanical properties of the substrate and coating
suppressing emergence of cracks and defects in the coating. It is
known that the strength and hardness of magnesium are relatively
low compared to other industrial metals such as titanium alloys and
stainless steels. Al2O3 is a favorable coating material due to its
excellent hardness and tribological properties. Thus, an Al2O3 coating
not only improves the corrosion resistance of the magnesium
substrate, but also enhances the surface mechanical properties.
In the work described in this paper, an Al2O3/Al bi-layered coating
is fabricated on AZ91 magnesium alloy and the hardness and bonding
strength of the coating are studied by nanoindentation and nanoscratch tests. Furthermore, the structure of the coating is characterized by XRD and XPS. Our results disclose that the structure has
excellent bonding strength and dramatically enhances the surface
mechanical properties.
2. Experimental details
⁎ Corresponding authors. Chu is to be contacted at Tel.: +852 27887724; fax: +852
27889549. Tang, Fax: +86 75526036752.
E-mail addresses: [email protected] (G. Tang), [email protected]
(P.K. Chu).
0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.tsf.2009.03.101
2.1. Sample preparation
Commercial extruded AZ91 Mg alloy was used in our experiments.
The dimensions of the samples were 15 mm × 15 mm × 4 mm. They
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Y. Xin et al. / Thin Solid Films 517 (2009) 5357–5360
Table 1
Nanohardness and Young's moduli of AZ91 magnesium alloy and deposited Al2O3
coating.
AZ91 Mg alloy
Al2O3 coating
Hardness (GPa)
Young's modulus (GPa)
1.9 ± 0.1
17.2 ± 0.6
56 ± 3
206 ± 5
resistance of the coated sample was assessed by a scratch test
performed on a Rockwell diamond indenter with a 2 μm radius on the
face direction and 3 mm scratch tracks were made on the test sample.
The sample surface was examined by optical microscopy and SEM
after the test.
3. Results and discussion
Fig. 1. XRD spectrum of the deposited Al2O3/Al coating on AZ91 magnesium alloy.
3.1. Structure of deposited coating
were ground with #4000 water proof diamond paper, polished, and
cleaned ultrasonically in alcohol. An Al2O3/Al bi-layered coating with
a thickness of about 0.8 μm was fabricated on the AZ91 magnesium
alloy using filtered cathodic arc deposition [9] at room temperature.
Argon (Ar) sputter cleaning was performed at a bias voltage of 500 V
for about 30 min before deposition. A pure aluminum transition layer
was first deposited for about half an hour followed by deposition of
the Al2O3 coating for about 3 h at a bias voltage 100 V. An argon and
oxygen gas mixture (flow ratio of about 1 to 2) was used during the
deposition of Al2O3.
2.2. Structure characterization
The phase constituents of the coated sample were identified by Xray diffraction (XRD). X-ray photoelectron spectroscopy (XPS) was
also employed to examine the chemical states of the elements in the
coatings. Approximately 10 nm of the surface was sputter cleaned
before collecting the XPS data.
2.3. Surface mechanical properties measurement
The hardness (H) and Young's modulus (E) were measured by
performing nanoindentation tests using a three-side pyramidal
diamond (Berkovich) indenter. The load-indentation depth was
about 80 nm, which was about 10% of the coating thickness in order
to exclude the effect of the substrate. Five indentations were averaged
to determine the mean H and E for better statistics. The scratch
The XRD spectrum acquired from the Al2O3-coated AZ91 magnesium alloy is presented in Fig. 1. Peaks from the Mg substrate are
present in the spectrum. It should be noted that a weak broad band at
about 36° overlaps peaks from magnesium, implying that the
fabricated Al2O3 is generally amorphous. XPS was conducted to
determine the chemical states of the major elements in the coating
referenced to the C1s peak. As shown in Fig. 2, peaks originating from
Al, O, Ar, and C are observed. The presence of the carbon peaks is mainly
due to surface contamination from the ambient after deposition or
absorbed carbon-containing species. The two inset figures in Fig. 1 are
narrow scans of Al 2p and O 1s, respectively. The Al 2p peak at 75.2 eV
suggests only one binding state, Al3+, in the coating [10]. Calculated
from the peak areas of O 1s and Al 2p using archival sensitivity factors,
the O to Al concentration ratio is approximate 1.5.
3.2. Surface mechanical properties
The hardness and Young's moduli of both the AZ91 magnesium alloy
substrate and Al2O3 coating obtained by nanoindentation tests are listed
in Table 1. The Mg alloy substrate possesses quite low hardness and
Young's modulus, 1.9 ± 0.1 GPa and 56 ± 3 GPa, respectively. The
hardness and Young's modulus of the coating are about 17.2 ± 0.6 and
206 ± 5, respectively. These results indicate that the surface mechanical
properties are obviously enhanced after deposition.
The nanoscratch test is employed to study the adhesion behavior of
Al2O3 coated AZ91 magnesium alloy. The scratch test proceeds from
Fig. 2. XPS spectrum of the deposited Al2O3 coating on AZ91 magnesium alloy.
Y. Xin et al. / Thin Solid Films 517 (2009) 5357–5360
5359
Fig. 3. Coefficient of friction profiles of the Al2O3 coating versus scratch length and normal load: CF – coefficient of friction; Lc – critical load.
left to right as demonstrated in Fig. 3. The coefficients of friction
versus normal load and scratch length are given together with the
SEM micrograph of the scratch track. At first, the friction coefficient
increases linearly and smoothly. Abrupt fluctuations are present from
about 340 µm. The measured penetration depth of the diamond tip at
this point has exceeded the total thickness of the coating. Trifle
delaminated pieces appear in the SEM graphs near the same position
where it can be defined as the failure site. Thus, the corresponding
critical load is about 57.6 mN. However, it is noticed that after this
critical failure position, no large area delamination can be observed
from the coating. Although the coating on the scratch track cracks, it
remains adhesive to the substrate. The failure process before the
critical point is studied by SEM and is presented in Fig. 4. The detailed
information at four selective points is shown in the high magnification
SEM views. At the initial loading stage in picture A (the normal load
ranges from about 2.7 to 6.5 mN), the coating collapses and the tracks
are shallow. However, no visual lateral cracks can be seen at the edge
of tracks and the coating in the sunken region does not fracture too.
When the load is increased to 13 mN (shown in picture B), no obvious
changes take place except subtle deepening of the scratch track. In
picture C (the normal load range from about 25 mN to 29 mN), some
lateral cracks appear at the edge of scratch track denoted by arrows.
The coating on the scratch track also fractures to some extent. Near the
failure site (picture D, normal load range from 54 to 57.5 mN), lateral
cracks as well as some localized delaminated pieces at the edge of the
scratch track can be observed. The coating at the collapsed area
fractures into many pieces, but still adheres to the substrate. In
general, a significant difference in mechanical properties between the
coating and the substrate will result in serious stress and the coating is
easy to delaminate when subjected to loading [11]. However, in the
Al2O3/Al/substrate structure, the pure aluminum interlayer can buffer
the stress induced by the serious mismatch in the mechanical
properties between the outer harder coating and softer substrate,
providing better chemical bonding between the hard coating and
good metal to metal bonding between the bi-layered coating and
substrate. This bi-layered structure is probably the reason for the good
adhesion. The Mg alloy substrate is relatively soft and when the load
reaches a certain value, the substrate, interlayer, as well as coating will
suffer from plastic deformation leading to the collapse of the coating.
The elastic modulus of pure Al is about 71.9 GPa [12] which is close to
that of AZ91 magnesium of about 45 GPa [2]. Hence, the interlayer is
not easy to delaminate from the substrate during plastic deformation.
The strong chemical bonding between the aluminum interlayer and
Al2O3 coating also makes the coating difficult to delaminate from the
interlayer. Consequently, when the normal load is below the critical
level, the coating bonds to the substrate strongly. However, when the
load exceeds some critical value, serious plastic deformation takes
place on the substrate and coating and the deformation results in
cracks in the outer hard coating. Due to the good chemical bond, the
cracked hard coating is also adhesive to the substrate.
4. Conclusion
An 0.8 µm thick Al2O3/Al bi-layered coating is successfully
fabricated on AZ91 magnesium alloy. The structure of the coating is
analyzed by XRD and XPS. The mechanical properties and adhesive
behavior are investigated by nanoindentation and nanoscratch. The
surface mechanical properties of the coated alloy are enhanced
significantly. The coating also exhibits excellent adhesion with the
substrate. Although serious plastic deformation of the substrate may
lead to fracture of the coating at the load-subjected regions, the
coating remains adherent to the substrate even under large loadings.
Fig. 4. SEM views of the scratch tracks before the critical point showing high magnification pictures at four selected points, A, B, C, D: A → picture A; B → picture B; C→ picture C; D→
picture D.
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Y. Xin et al. / Thin Solid Films 517 (2009) 5357–5360
Acknowledgments
The work was supported by Hong Kong Research Grants Council
(RGC), General Research Fund (GRF) No. CityU 112307, Key Grant Project
of the Educational Commission of Hubei Province (Z200711001), and
Key Project of the Chinese Ministry of Education (No. 208087).
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