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AC plasma electrolytic oxidation of magnesium with zirconia nanoparticles
R. Arrabal a, E. Matykina a, F. Viejo a, P. Skeldon a,*, G.E. Thompson a, M.C. Merino b
a Corrosion and Protection Centre, School of Materials, The University of Manchester, Sackville Street, P.O. Box 88, Manchester M60 1QD, UKb Departamento de Ciencia de Materiales, Facultad de Ciencias Quimicas, Universidad Complutense, 28040 Madrid, Spain
1. Introduction
Plasma electrolytic oxidation (PEO) is used for the surface
protection of magnesium alloys; several commercial processes are
available[13]. Processes are also being developed that provide
coatings with improved properties. Thecoatings areformed at high
voltages, in various, usually aqueous, electrolytes, when micro-
discharges are present on the alloy surface[4]. The mechanism of
formation of the coating potentially involves anodic oxidation,
thermal oxidation and plasma-chemical reactions[5]. The growth
of the coating is accompanied by relatively profuse generation of
oxygen gas. Coatings can be grown under DC and AC conditions,
with a range of waveforms, leading to coatings that generally
thicken roughly in proportion to thecharge passed [6]. Thicknesses
of tens of microns arereadily produced,containing amorphous and
crystalline constituents dependent on the particular alloy and
electrolyte [7]. Magnesium oxide is generally present in the
coatings, often accompanied by phases related to the electrolyte
anions, for instance magnesium silicate in silicate electrolyte. In
cross-section, two main layers of DC coatings may be discerned;
the inner layer has a finer texture than the outer layer. At the base
of the inner layer, a relatively thin, compact barrier region is
located at the alloy/coating interface, with a thickness of up to
lmm. Short-circuit transport of electrolyte components through
the outer layer has been shown to occur[79]. This allows anion
species of the electrolyte, or possibly derivatives formed in the
microdischarge, ready access to the inner coating. Cation species
can also be transported across the outer coating, although in
greatly reduced amounts compared with the anion species[10]. In
addition, nanoparticles in suspension in the electrolyte can
reach the inner layer via the short-circuit paths, which has been
demonstrated using monoclinic zirconia [11]. The nanoparticles
are also incorporated into the outer part of the coating, which
reveals a modified microstructure compared with coating
formed in the absence of nanoparticles. Tetragonal zirconia is
detected in significant amounts in the final coating, due to the
high temperatures generated at microdischarge sites. In the
present study, the influence of AC treatment on the incorporation
of zirconia is investigated for magnesium treated in silicate
electrolyte. The incorporation of nanoparticles is relevant to
understand the mechanism of coating growth and for the potential
to improve coating properties through modification of the coating
microstructure [12]. The main interest of the work concerned
the microstructure and morphology of the coatings, especially the
distribution of zirconium and the types of zirconia-containing
phases.
Applied Surface Science 254 (2008) 69376942
A R T I C L E I N F O
Article history:
Received 10 April 2008Received in revised form 29 April 2008
Accepted 29 April 2008
Available online 7 May 2008
PACS:
81.65.b
81.65.Mq
Keywords:
Magnesium
Coatings
Plasma electrolytic oxidation
Zirconium oxide
Anodizing
A B S T R A C T
The incorporation of monoclinic zirconia nanoparticles and their subsequent transformation is examined
for coatings formed on magnesium by plasma electrolytic oxidation under AC conditions in silicateelectrolyte. Thecoatings areshown to comprise two main layers, with nanoparticles enteringthe coating
at thecoating surface andthrough short-circuit pathsto theregion of theinterface between theinner and
outer coating layers. Under local heating of microdischarges, the zirconia reacts with magnesium species
to form Mg2Zr5O12in the outer coating layer. Relatively little zirconium is present in the inner coating
layer. In contrast, silicon species are present in both coating layers, with reduced amounts in the inner
layer.
2008 Elsevier B.V. All rights reserved.
* Corresponding author. Tel.: +44 161 306 4872; fax: +44 161 306 4865.
E-mail address: p.skeldon@manchester.ac.uk (P. Skeldon).
Contents lists available atScienceDirect
Applied Surface Science
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 at e / a p s u s c
0169-4332/$ see front matter 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.apsusc.2008.04.100
mailto:p.skeldon@manchester.ac.ukhttp://www.sciencedirect.com/science/journal/01694332http://dx.doi.org/10.1016/j.apsusc.2008.04.100http://dx.doi.org/10.1016/j.apsusc.2008.04.100http://www.sciencedirect.com/science/journal/01694332mailto:p.skeldon@manchester.ac.uk -
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2. Experimental
Specimens of 99.9% magnesium sheet were embedded in resin,
with electrical connection provided through a shielded copper
wire. The exposed surface, of area 1 cm2, was ground to 1200 grit
SiC, using water as a lubricant, degreased in ethanol, rinsed in
deionized water and dried in warm air. PEO treatment was carried
out at 200 mA cm2 (rms) at 50 Hz with a sinusoidal waveform in
0.025 M Na2SiO35H2O/0.05 M KOH for up to 2400 s. The electro-
lyte was prepared from deionized water and high-purity chemi-
cals. Additions of 10 g/l of monoclinic zirconia nanoparticles, of
size 150300 nm, were made as required. The anodizing condition
was selected following evaluation of a range of current densities
and electrolytes compositions for incorporation of zirconia into the
coating. The coatings were formed in a 1 l double-walled glass cell
through which a cooledwater/glycol mixture waspumped in order
to keep the electrolyte temperature close to 293 K. During the
treatment, the electrolyte was stirred continuously, which kept the
zirconia nanoparticles in suspension. A sheet of type 304 stainless
steel, of size 7.5 cm 15 cm, was used as the cathode. Voltage
responses were recorded electronically, with a sampling time of
20 ms, employing an SCXI data acquisition system (National
Instruments) with data analysis by Igor Pro (Wavemetrics). Aftercoating, specimens were rinsed with deionized water and dried in
warm air. PEO-treated specimens were examined by field emission
gun scanning electron microscopy, using a Philips XL30 instrument
equipped with energy-dispersive X-ray (EDX) analysis facilities.
Cross-sections were ground through successive grades of SiC
paper, followed by finishing to 1 mm diamond. Phase composition
was investigated by X-ray diffraction (XRD), using a Philips XPert-
MPD (PW 3040) instrument with a step size 0.0058 and a scan
range from 5 to 858 (in 2u).
3. Results
Voltagetime responses were practically the same during PEO
without and with zirconia in the electrolyte, with commencementof sparking at 250 V coinciding with the major change in slope
between the initial period of relatively rapid voltage rise and the
start of the main period of slow voltage increase ( Fig. 1). Without
zirconia in the electrolyte, the surface of the resultant coating
comprises apparently overlapping, roughly circular features
(Fig. 2(a)). The shapes of the features suggest outward flow of
material along a channel through the underlying coating to the
coating surface, followed by its spreading over the coating surface
andsolidification.The largest of thefeatureshave dimensions of up
to 50mm. The surface is significantly rough and pores and cracks
are evident. In addition, micron-sized silicon-rich particles, with
the appearance of a deposit, are also present; such particles are
often found following PEO in silicate electrolyte [13].The coating roughness and pores are clear in cross-sections of
the coating (Fig. 2(b)). The thickness of the coating at the thickest
regions is about twice that at thethinnestregions,with inclusionof
porosity in the thickness measurements. Two main layers are also
revealed, with the inner layer being finely porous, whereas the
outer layer has relatively large pores, cavities and channels. There
are also regions of finer pores in the outer layer and of larger pores
in the inner layer although these are of relatively minor extent.
Cracks are mostly evident in the outer layer. Magnesium, oxygen
and silicon are detected in both layers by EDX point analyses; the
atomic ratios of Si:Mg were 0.28 0.08 and 0.10 0.04 in the outer
and inner layers, respectively (points A and B, Fig. 2(b)). The
corresponding O:Mg ratios were 3.1 0.4 and 2.6 0.3.
The surface of the coating formed in electrolyte containingzirconia nanoparticles revealed two distinct types of region in
backscattered electron images (Fig. 3(a)). An extensive, irregular
network of light regions corresponded to coating decorated by
numerous zirconia nanoparticles. In contrast, relatively dark,
roughly circular zones, of size 50100mm, are mainly free of
nanoparticles, but disclose dendrites of size 12mm (Fig. 3(b)).
Non-uniformity in the distribution of zirconia particles at the
coating surface was also observed following DC treatment in an
alkaline phosphate electrolyte[11]. Cracks and pores are found in
both types of region of the coating. Further, EDX point analyses
revealed the presence of silicon and zirconium, with Si:Mg and
Fig. 1. Voltagetime responses during AC PEO treatment of magnesium at
200 mA cm2 (rms) in 0.025 M Na2SiO35H2O/0.05 M KOH, without and with
addition of 10 g/l of monoclinic zirconia.
Fig. 2.Scanning electron micrographs (secondary electrons) of magnesium following AC PEO treatment for 2400 s at 200 mA cm2 (rms) in 0.025 M Na2SiO35H2O/0.05 M
KOH. (a) Plan. (b) Cross-section.
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Zr:Mg ratios of 0.26 0.04 and 0.26 0.06, respectively in the
region containing micron-sized dendrites (point C, Fig. 3(a)) and
0.33 0.08 and 0.55 0.14, respectively, in the region covered by
zirconia nanoparticles (point D,Fig. 3(a)).
In cross-section, the coating formedwith zirconia in suspension
in the electrolyte comprises two main layers (Fig. 4(a and b)). The
inner layer has a relatively fine porosity. The outer layer is more
compact, and with reduced porosity compared with theouterlayer
of coatings formed in the absence of zirconia in the electrolyte.
Cavities appear to be prevalent near the interface between the
inner and outer layers. The X-ray elemental maps show the
presence of magnesium, oxygen and silicon in most coating
Fig. 3. Scanning electron micrographs (backscattered electrons) of the surface of magnesium following AC PEO treatment for 2400 s at 200 mA cm 2 (rms) in 0.025 M
Na2SiO35H2O/0.05 M KOH with addition of 10 g/l of monoclinic zirconia. (a) General view. (b) Region with relatively few nanoparticles.
Fig. 4.(a) Scanning electron micrograph (backscattered electrons) of a cross-section of magnesium following AC PEO treatment for 2400 s at 200 mA cm 2 (rms) in 0.025 M
Na2SiO35H2O/0.05 M KOH with addition of 10 g/l of monoclinic zirconia. (b) Detail of region between the inner and outer coating layers. EDX elemental maps are shown for
magnesium, oxygen, silicon and zirconium of the cross-section of (a).
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regions, with a reduced amount of silicon in the inner layer (Fig. 4).
Zirconium was mainly a component of the outer layer. EDX point
analyses disclosed Si:Mg and Zr:Mg ratios of 0.26 0.07 and
0.16 0.05, respectively, in the outer layer (point E, Fig. 4(a)) and
0.10 0.03 and 0.02 0.01, respectively, in the inner layer (point F,
Fig. 4(a)), confirming the moderately reduced concentration of silicon
andgreatlyreducedconcentration of zirconiumin theinnerlayer. TheSi:Mg ratiosin thetwo layersweresimilar to those determined forthe
coating formed in zirconia-free electrolyte. Zirconia nanoparticles,
similar in size to those present in the electrolyte, are prevalent near
the interface between the inner and outer layers and within larger
cavities and channels in the outer layer (Fig. 4(b)). Zirconium is also
generally distributed throughout the outer coating at locations
largely free of nanoparticles. The outer coating in these latter regions
contains numerous dendrites roughly similar in size and shape to
those observed at the coating surface (Fig. 5(a and c)). Observation at
another region of the cross-section, corresponding to the network of
nanoparticles observed at the film surface, revealed that the
nanoparticles were incorporated into a layer of thickness 12mm
(Fig. 5(b and d)). EDX point analysis indicated Si:Mg and Zr:Mg ratios
of 1.33 0.31 and 1.09 0.46, respectively, suggesting trapping ofnanoparticles within silicon-rich material formed at the coating
surface (point G, Fig. 5(b)). The underlying coating microstructure
disclosed discrete light particles, of size suggestive of incorporated
nanoparticles, mainly within a relatively light network of matrix
material (Fig. 5(d)). The latter surrounded micron-sized islands of
relatively dark material. EDX point analyses indicated Zr:Mg ratios of
0.16 0.06 and 0.18 0.05 and Si:Mg ratios of 0.16 0.03 and
0.17 0.03 in thedarker and lighter materials, respectively (points H
and I, Fig.5(d)), although theresults are probably affected by thelarge
size of the analysis volume relative to the dimension of the zones and
hence, represent average compositions for the microstructure.
The kinetics of coating growth were approximately linear,
with average rates of 15.8 and 18.0 nm s1 without and with
zirconia, respectively (Fig. 6). The thickness of both the inner and
outer coating layers increased with increase of treatment time.
The inner layer thickness appeared to increase relative to the
total film thickness, from 20% of the film at 10 min to 40% at
later times. XRDrevealed strongpeaks forMgO andweaker peaks
for Mg2SiO4 in the coating formed in electrolyte without
nanoparticles (Fig. 7). These peaks were also found for the
coating produced with nanoparticles, with relatively reducedpeaks for Mg2SiO4 and the additional presence of peaks for
monoclinic zirconia and Mg2Zr5O12.
Coatings formed for 10 min in electrolyte with nanoparticles
followed by 10 min in electrolyte free of nanoparticles, and vice
versa, also resulted in two-layered coatings (Fig. 8). For the
Fig. 5.Scanning electron micrographs (backscattered electrons) of cross-sections of magnesium following AC PEO treatment for 2400 s at 200 mA cm2 (rms) in 0.025 M
Na2SiO35H2O/0.05 M KOH with addition of 10 g/l of monoclinic zirconia. (a) Surface region with relatively few nanoparticles. (b) Surface region with nanoparticle layer. (c)
Detail of (a). (d) Detail of (b).
Fig. 6. Dependenceof coatingthicknesson time forAC PEOtreatment of magnesium
at 200 mA cm2 (rms) in 0.025 M Na2SiO35H2O/0.05 M KOH without and with
addition of 10 g/l of monoclinic zirconia.
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former sequence, occasional regions of material with incorpo-
rated nanoparticles were present in the final coating, usually at
the coating surface, within cavities and near the inner layer/
outer layer interface (Fig. 8(a)). EDX analyses indicated the
presence of zirconium species in the outer layer material, with a
Zr:Mg ratio of 0.09 0.05 (point J, Fig. 8(a)); no significant
amount of zirconium was detected in the inner layer (point K,
Fig. 8(a)). The respective Si:Mg ratio were 0.21 0.04 and
0.19 0.05, indicating less silicon in the inner layers. For the
reverse sequence, increased amounts of zirconia nanoparticles
were present at the coating surface, within cavities and near the
inner layer/outer layer interface (Fig. 8(b)). Surface regions with
much reduced numbers of nanoparticles were also identified,
similar to the findings for the previous single PEO treatment withnanoparticles. Point analyses through the coating thickness
disclosed reducing Zr:Mg ratios of 0.07 0.05, 0.06 0.05 and
0.03 0.01 with increasing depth in the coating (points LN,
Fig. 8(b)). A similarly reducing trend was found for Si:Mg ratios,
namely 0.23 0.03, 0.20 0.05 and 0.14 0.05.
4. Discussion
The two-layered coatings observed previously following DC PEO
treatment of magnesium in silicate electrolyte in the presence and
absence of a zirconia nanoparticle suspension[11]are also formed
under the present AC conditions. Similarly, the transport of zirconia
nanoparticles along short-circuit paths, through the outer layer to
the inner layer, is also revealed. Irrespective of the presence of
zirconia, the coatings contained magnesium species from the
substrate and silicon species from the electrolyte, with detection
of crystalline MgO and Mg2SiO4. Further, theinnerlayer contained a
reducedamount of silicon, with typical Si:Mg ratios from EDX point
analyses of 0.10 0.04 compared with 0.26 0.04 in the outer layer.
For coating containing zirconium species, the local high temperatures
of coating growth result in formation of dendrites in the outer coating
layer, andgeneration of Mg2Zr5O12, whichis reported tobe stablein the
temperature range 21232373 K[14]. This phase is presumed to be a
constituent of the outer coating, since, relatively little zirconium is
found within the inner layer, for which EDX point analyses indicated a
Zr:Mg ratio of 0.02, compared with 0.090.16 in the outer layer.
Mg2Zr5O12 was not detected by XRD in coatings formed in silicate
electrolyte under DC conditions with a current density of 30 mA cm2
and a reduced concentration of zirconia in the electrolyte, 2 g/l [11].
However, generation of tetragonal zirconia occurred under DC
treatments with silicate electrolyte and both tetragonal zirconia and
Mg2Zr5O12 in phosphate electrolyte. The absence of tetragonal zirconia
in the AC coatings, contrary to findings for a DC coating, is due to the
different thermal conditions in the coatings, related to the energies
dissipated in the individual discharge events. The latter will depend on
the local current densityof the discharge sites and thesizeand lifetime
of the discharge. The presence of tetragonal zirconia under the DC
conditions compared with the presence of Mg2Zr5O12 under AC
conditions suggestshighertemperature in thedischarges for thelatter.
Monoclinic zirconia is also detected in the present coatings, and those
formed previously under DC conditions, due to incorporation of
zirconia that either does not transform under the local conditions of
temperatureand pressure, or revertsto themonoclinicform on cooling.
The occurrence of dendrites indicates local melting in the outer layer.
The melting point of the outer layer material will depend upon the
precisecomposition of thecoating.The meltingpointsof relatively pure
MgO and Mg2SiO4 are 3073 and 2163 K, respectively. The extensivecracking of the coatings is presumed to arise from stresses in the
coating associated with volume changes due to coating growth and
phase transformation, differential thermal expansion within the
coating and substrate, and the pressure of oxygen and hydrogen
generated during anodic and cathodic half-cycles, respectively[15].
Zirconia nanoparticles were incorporated into the coating
surface. In some regions of the surface, the particles are entrained
within a thin layer of silicon-rich material,which possibly contains
silica formed either by precipitation, due to reduction in pH of the
surface electrolyte, or thermolysis. The particles appear to be
Fig. 7. XRD data for magnesium following AC PEO treatment for 2400 s at
200 mA cm2 (rms) in 0.025 M Na2SiO35H2O/0.05 M KOH without (bottom) and
with (top) addition of 10 g/l of monoclinic zirconia.
Fig. 8. Scanning electron micrographs (backscattered electrons) of cross-sections of magnesium following AC PEO treatment for at 200 mA cm2 (rms) in 0.025 M
Na2SiO35H2O/0.05 M KOH. (a) Treatment for 600 s in electrolyte with addition of 10 g/l of monoclinic zirconia then for 600 s in zirconia-free electrolyte. (b) Treatment for
600 s in zirconia-free electrolyte then for 600 s in electrolyte with addition of 10 g/l of monoclinic zirconia.
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subsequently incorporated into the underlying melted coating
material. At other locations, the surface appeared relatively-free of
particles and silicon-rich material, which may be due to instability
of silica at the locally high pH generated during cathodic
polarization. Alternatively, the silicon-rich material and zirconia
nanoparticles may have been incorporatedinto thecoating at these
sites, without sufficient time being subsequently available to re-
establish their presence. In addition to incorporation at the coating
surface, nanoparticles of zirconia were incorporated following
transport to the inner layer/outer layer interface region. Further,
cavities within the coating were often lined with nanoparticles,
suggesting that cavities may be sections of channels that provide
access to the inner coating regions. However, cavities may also
arise from bridging of open pores that were located at the coating
surface at an earlier stage of coating growth.
The sequentially treated specimens reveal fewer zirconia
particles at the coating surface when thesecond stage of treatment
uses the zirconia-free electrolyte. In contrast, a thin layer of
zirconia particles is established during the second stage of
treatment with the zirconia suspension. It appears that new
coating material maybe formedat the coating surface on topof the
zirconia-rich layer, which is suggested by the lighter, zirconium-
containing material at location L ofFig. 8(b) that is separated fromthe slightly darker material M, by a line of zirconia nanoparticles
connecting to the adjacent zirconia-rich layer. Zirconium-contain-
ing material also appears to form at the inner/outer coating
interface, which is suggested by the presence of lighter material
near the interface. However, some zirconia nanoparticles remain
near the interface following the second stage treatment in
zirconia-free electrolyte (Fig. 8(a)), suggesting that lower tem-
peratures are achieved in this region.
The coating kinetics ofFig. 6indicate that both the outer and
inner coating layers continue to grow during the PEO process,
with incorporation of silicon, but little zirconium, into the inner
layer. Early studies of PEO using sequential treatments in DC
conditions in nanoparticle-free electrolytes with different anion
species revealed transport of electrolyte anions, or derivativesproduced by the microdischarge, to the inner coating regions,
paralleling the transport of zirconia nanoparticles to the inner
coating/outer coating interface of the present films [7]. Further,
the inner coating material appeared to be readily substituted by
new material following change of the electrolyte, whereas the
outer coating underwent relatively slow change in composition.
Componentsof coating material formed in thefirst electrolyte,i.e.,
containing anion or derivative species of the firstelectrolyte, were
also detected in the second electrolyte, consistent with an
accelerated dissolution of prior formed coating[16]. Studies are
in progress to determine whether similar loss of inner coating
material occurs during AC PEO, when the lifetimes of discharges
may be restricted compared with DC conditions. The inability to
incorporate significant amounts of zirconium into the innercoating layer under the present conditions of PEO contrasts with
the ready incorporation of silicon species into the inner coating
material. Further, work is required on the nature and growth
mechanism of the inner layer in order to understand the differing
behaviours. However, the difference in distributions of silicon and
zirconium may relate to the transport of the zirconium in particle
form, rather than atomic, ionic or molecular form, to the inner
layer/outer layer interface region. Such relatively large particles
appear not to be melted readily. Further, the electric field will
oppose the migration of zirconium ionsinto the innerlayer during
anodic polarization.
A thin region of compact material, with relatively uniform
thickness, is present at the base of the inner coating layer. The
thickness is consistent with development of a barrier film, with a
formation ratio of the order 1 nm V1, at the final anodic voltage of
430 V achieved at the particular microdischarge site. The
material is formed at the termination of coating growth, during
cooling of the overlying main coating material. The growth of the
barrier film indicates that electrolyte species can penetrate the
inner coating layer, enabling film formation at the metal/coating
interface.
5. Conclusions
1. Zirconia nanoparticles can be incorporated into PEO coatings
formed on magnesium under AC conditions in silicate electro-
lyte with monoclinic zirconia in suspension.
2. The coatings comprise two main layers, with the inner layers
revealing a relatively fine porosity. Both layers thicken as the
coating grows, with the inner layer representing about 40% of
the coating for a total coating thickness of40mm.
3. Magnesium, oxygen and silicon are present within the coatings,
which contain crystalline MgO and Mg2SiO4. The concentration
of silicon is less in the inner layer compared with theouter layer.4. Zirconia nanoparticles are incorporated into the coating at the
coating surface and following transport along short-circuit
paths to the inner coating region. Zirconium species are
subsequently incorporated into the coating material, resulting
in a modified microstructure of the outer coating layer.
Relatively little zirconium is incorporated into the inner coating
layer.
5. Due to locally high temperatures at sites of microdischarges
Mg2ZrO5is formed within the coatings following incorporation
of monoclinic zirconia from the electrolyte.
Acknowledgements
The authors are grateful to the Engineering and Physical
Sciences Research Council (U.K.) and the Spanish Ministry of
Education (grant no. EX2006-1371) for support of this work.
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