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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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