Surface & Coatings Technology 203 (2009) 2969–2973

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Surface & Coatings Technology

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Microstructure and phase composition of microarc oxidation surface layers formedon aluminium and its alloys 2214-T6 and 7050-T74

K. Tillous a,b,⁎, T. Toll-Duchanoy b, E. Bauer-Grosse b, L. Hericher b, G. Geandier c

a Novelis-Foil Innovation Center, CRP Lippmann, 41, rue du Brill L-4422 Belvaux, Luxembourgb Laboratoire de Science et Génie des Surfaces (UMR CNRS-INPL 7570), Ecole des Mines-Parc de Saurupt-54042 Nancy Cedex, Francec Laboratoire de Métallurgie Physique, Université de Poitiers, SP2MI, BP 30179-86 962 Futuroscope Chasseneuil Cedex, France

⁎ Corresponding author. Novelis-Foil Innovation CenterL-4422 Belvaux, Luxembourg. Tel.: +33 (0) 3 83 58 42 35

E-mail address: eric.tillous@mines.inpl-nancy.fr (K. T

0257-8972/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.surfcoat.2009.03.021

a b s t r a c t

a r t i c l e i n f o

Article history:Received 24 June 2008Accepted in revised form 10 March 2009Available online 18 March 2009

Keywords:Microarc oxidation (MAO)Aluminium alloysX-ray micro-diffractionPhase compositionMicrostructure

The influence of substrate on themicrostructure and phase composition of surface layers synthesised bymicroarcoxidation (MAO) on aluminium and its alloys 2214-T6 and 7050-T74 is studied using scanning (SEM) andtransmission electronmicroscopy (TEM) aswell as cross-sectional X-ray diffraction. MAO layers are composed ofthree layers and aremainlymadeof gamma-Al2O3 and alpha-Al2O3 phases. The proportion of eachphase dependson the substrate. The external porous layer is mainly composed of the gamma-Al2O3 phase. The internal denselayer canpresent two aspects according to the percentage of the alpha-Al2O3 phase. The so-called granular aspectindicates a high proportion of “dendrite” defect which results from discharge formation and implies a highpercentage of the alpha-Al2O3 phase. The so-called columnar aspect indicates a high proportion of “smallchannels” associated with a very weak percentage in the alpha-Al2O3 phase. In the latter, it is believed that a Znalloying element can inhibit the growth of alpha-Al2O3. During the MAO process, discharges likely occur in thevicinity of the MAO layer/substrate interface, probably in the spherical porosities that result from oxygengenerated in the thin layer localised at the interface.

© 2009 Elsevier B.V. All rights reserved.

Table 1.Chemical composition of 2214 T6 and 7050 T74 alloys.

1. Introduction

Microarc oxidation (MAO) [1–4] also known as plasma electrolyticoxidation (PEO) [5,6] is a process of plasma-assisted electrochemicalconversion of a metal surface, such as Al, Ti or Mg, or ametallic alloy togrow an oxide ceramic film [1–8]. The layers synthesized by the MAOprocess exhibit excellent wear and corrosion resistance, electrical andthermal properties.

It is usually admitted that three main steps govern the MAO layersynthesis [9,10]. In the first step, a number of separated dischargechannels are formed in the oxide layer where its conductivity is thelowest because of defects. Due to the strong electric field, the anioniccomponents are drawn into the channel. Concurrently, aluminium andalloying elements aremelted out of the substrate, enter the channel andget oxidized. In the second step, Al is ejected from the channel into theMAO layer in contactwith the electrolyte, thereby increasing the surfacelayer thickness in that location. In the last step, the discharge channelgets cooled and the reaction by-products are deposited onto its wall [9].The above process repeats itself at discrete locations over the wholesurface of the layer, leading to an overall increase in the MAO layerthickness [10].

, CRP Lippmann, 41, rue du Brill; fax: +33 (0) 3 83 53 47 64.illous).

ll rights reserved.

It is now well established from micro-structural analysis that theMAO layer formed on Al and its alloys is composed of three layers: asurface layer, an internal layer with lower porosity and a thin layer,which exhibits a nano-scale polycrystalline microstructure, at thecoating/substrate interface [11,12]. The higher cooling rate of moltenalumina at the topmost surface favours the formation of gamma-Al2O3. On the over hand, alpha-Al2O3 is easily formed in the internallayer with a lower cooling rate [1,9,13].

Despite several investigations, the mechanism of the MAO layergrowth remains unclear, particularly in terms of the local physicalprocesses occurring during growth. The above description of the MAOlayer formation requires fine characterizations because, to ourknowledge, no micro-structural investigation has revealed crackdefect caused by discharges formation in the coating. Moreover, thecharacterizations presented in the literature do not point out therelation between the substrate and the internal dense layer micro-structure; on the other hand, the correlation between the dense layermicrostructure and phase composition has not yet been undertaken.

Weight percent

Alloy Cu Si Mn Mg Fe Zn Ti Cr

2214-T6 3.9–5 0.5–1.2 0.4–1.2 0.2–0.8 0.3 0.25 0.15 0.17050-74 2–2.6 0.12 0.1 1.9–2.6 0.15 5.7–6.7 0.06 0.04

mailto:eric.tillous@mines.inpl-nancy.frhttp://dx.doi.org/10.1016/j.surfcoat.2009.03.021http://www.sciencedirect.com/science/journal/02578972

Fig. 1. Schematic representation of cross-sectional X-ray micro-analysis at beamline ID15 ESRF (Grenoble).

Fig. 2. SEM micrograph cross-sections and the corresponding X-ray spectra of MAO

2970 K. Tillous et al. / Surface & Coatings Technology 203 (2009) 2969–2973

Therefore, it is clear that other characterizations are required in orderto deeply understand the mechanism of MAO layer growth,particularly in terms of the local physical processes occurring duringgrowth.

Thepresentpaper focuses on afinemicro-structural characterizationand the phase composition of the MAO layer obtained on aluminium(purity 99.999) and its alloys 22A4-T6and7050-T74. Defects in theMAOsurface resulting from discharges and gas formation are revealed andcorrelations are established between dense layer microstructure, phasecomposition and the nature of the substrate. First, we briefly present theconditions of MAO layer formation. Then, we explain the adoptedmethodology for sample preparation and present the equipments used.Finally, the obtained results are presented and discussed.

surface layers formed on (a) aluminium, (b) 2214-T6 and (c) 7050-T74 alloys.

2971K. Tillous et al. / Surface & Coatings Technology 203 (2009) 2969–2973

2. Experimental conditions

Samples (10×9×6 mm3) immersed in the electrolyte are made ofaluminium (purity 99.999%) and its alloys 2214-T6 and 7050-T74.Their composition limits are given in Table 1.

The electrolyte solution contained KOH and Na2SiO3 diluted indistilled water; the resulting electrolyte conductivity ranged from 2.3to 2.8 mS/cm, with a pH of 11.7. Rectangular samples were cleanedwith acetone before each treatment to ensure a reproducible initialsurface contamination for each sample. An alternative currentgenerator (CERATRONIC® process [14]) was used, delivering a currentto the substrate with amplitude in the range 0–36 A, and 75–150 A.The current frequency was set at 100 Hz. Further details of theexperimental set up and procedures followed are available inreference [15] elsewhere.

The fracture cross-sections were prepared by dissolving thesubstrate in a NaOH solution. The SEM analyses were performedusing a Philips FEG XL 30 S (high magnification using a field emissiongun). Specimen for TEM analysis is prepared by focused ion beam(FIB) technique. The microstructure of the coating/substrate interfaceis performed using TEM Philips CM (200 KV).

Phase composition of the MAO layer was carried out with a finebeam synchrotron (1×0.02 mm2) by cross-sectional X-ray at beam-line ID 15-ESRF (Grenoble) working on Si Kα radiation (λ=0.1394 Ǻ).The scans were performed from the external MAO layer towards thecoating/substrate interface with a 0.02 mm step size. Fig. 1 shows theschematic diagram of X-ray micro-analysis.

Fig. 4. SEM micrograph fracture cross-sections of MAO surface layers formed onaluminium (a), 2214-T6 alloy (b) and 7050-T74 alloy (c).

3. Results and discussions

3.1. MAO layer thickness and phase analysis

SEM micrograph cross-sections and the corresponding XRDspectra of the grown MAO layers on (a) aluminium (purity99.999%), (b) 2214-T6 and (c) 7050-T74 alloys are available in Fig. 2.Two parts of the MAO layers can be discerned: the external porouslayer and the internal dense layer that occupies about 75–80% of thetotal thickness. The estimated MAO layer thickness comprisedapproximately 100±5 µm for aluminium, 120±5 µm for the 2214-T6 alloy and 140±5 µm for the 7050-T74 alloy. In the latter case, whitezones in the internal layer resulted from local transformations ofprecipitates due to the lower thermal conductivity of alumina (Fig. 2).

The corresponding X-ray diffraction spectra indicate that the MAOlayers mainly composed of alpha-Al2O3 and gamma-Al2O3 phases. Thedifference in alpha-Al2O3 and gamma-Al2O3 contents for the externaland internal layers of MAO films was the result of cooling rate variationof molten alumina in microarc zone together with the chemicalcomposition of the substrate [16]. The higher cooling rate of moltenalumina at the topmost surface favours the synthesis of gamma-Al2O3

Fig. 3. I (113)α-phase/I (400) γ-phase ratio of MAO surface layers formed on aluminiumand the 2214-T6 alloy.

phase [1]. For aluminium and the 2214-T6 alloy, the proportion ofgamma-Al2O3 phase decreases in the internal layer whereas the alpha-Al2O3 phase gradually increases from the surface to the internal layer toreach a maximum value (approximately toward the center of theinternal layer) and then decreases near the MAO coating/substrateinterface (Fig. 3). This observation indicates that the cooling rate at thepartof theMAOcoating in contactwith the substrate is alsomuchhigherthan the interior of the internal layer [16].

The strong diffraction intensities [I (113) alpha/I (400) gamma]indicated that the highest proportion of alpha-Al2O3 phase wasobtained for pure aluminium (Fig. 3). The difference of alpha-Al2O3proportion may result from the porosity of MAO coatings obtained onthe two substrates. It would seem that the coating obtained on the2214-T6 alloy is more porous than the other formed on purealuminium, which could explain the difference of thickness of thesetwo coatings. The MAO layer obtained on the 7050-T74 alloy mainlycomposed of gamma-Al2O3 phase; a very weak proportion of alpha-Al2O3 phase was detected in the area of the internal layer close to theMAO layer/substrate interface (Fig. 2).

Fig. 5. (a) SEMmicrograph fracture cross-sections of MAO surface layers formed on aluminium and (b) 2214-T6 alloy indicating the “dendrite” defect which resulted from dischargeformation in the coating.

Fig. 6. SEM micrograph fracture cross-sections of MAO surface layer formed on the7050-T74 alloy indicating the “small channel” defect.

2972 K. Tillous et al. / Surface & Coatings Technology 203 (2009) 2969–2973

These results indicate that some alloying elements, mainly Zn (inthe case of the 7050-T74 alloy), can inhibit the formation of alpha-Al2O3 phase [12]. On the other hand, the MAO layer thicknessdecreased with increasing alpha-Al2O3 phase.

3.2. Fracture cross-section observation

The internal layer comprised of two different aspects depending onthe substrate: a “granular” obtained on pure Al substrate (Fig. 4a) and a“columnar” obtained on the 7050-T74 substrate (Fig. 4c). In the case ofthe 2214-T6 alloy, the dense layer presented amixed aspect (Fig. 4b). Toreveal the specificity of the internal layers, SEM observations at highmagnification are required. The internal layers formed on Al and the2214-T6 alloy are typically characterized by the presence of a highproportion of the so-called “dendrite” defect (Fig. 5a and b). The latterconsisted of a central channel surrounded by fine piled up plates and isthe result ofmicroarc discharge formation, that produces a localmeltingof alumina leading to dendrite standard solidifications. In the case of theinternal layer synthesised on the 7050-T74 alloy, SEM analyses reveal ahigh proportion of another defect called “small channel” (Fig. 6); a veryweak proportion of “dendrite” defect is detected in this MAO layer.

From SEM and XRD analyses, it is concluded that the “dendrite”defect resulted from local transformation of gamma-Al2O3 into alpha-Al2O3 under discharge effect. Consequently, a high proportion of“dendrite” defects means a high percentage of alpha-Al2O3 phase.Moreover, the SEM micrograph fracture cross-section revealed a highproportion of spherical porosity closes the MAO layer/substrate inter-face (Fig. 7a). TEM micrograph revealed further cavities, which aresections of bubbles originally filled with gas. A range of sizes, typicallyfrom ~1 µm down to several tens of nanometres is usually found withapparently spherical, discoid, or less regular shapes. It should bementioned at this point that the same observations were also made forthe three MAO layers.

The TEM micrograph cross-section of the MOA layer/substrateinterface also revealed the presence of a thin layer (so-called barrier

Fig. 7. (a) SEMmicrograph fracture cross-sections showing high proportion of sphericalporosity close to the surface layer/substrate interface and (b) TEM micrograph cross-section of the surface layer/substrate interface showing the so-called barrier layer.

2973K. Tillous et al. / Surface & Coatings Technology 203 (2009) 2969–2973

layer [11]) under the zone of spherical porosity filled with gas (Fig. 7b).From these observations, it can be deduced that the gas wascontinuously generated in the vicinity of the thin layer [17–21]. It alsoclear, (Fig. 5a and b) that discharge occurred in the vicinity of the MAOlayer/substrate interface. Thus, it is assumed that, during the MAOprocess, discharges were continuously formed in the spherical porosityclose to the MAO layer/substrate under a strong electric field. Thisphenomenon can contribute to the formation on the internal layer.

4. Conclusions

(1) Ceramic surface layers grown on Al and its alloys 2214-T6 and7050-T74 by microarc oxidation primarily consist of gamma-Al2O3 and alpha-Al2O3 phases; and are composed of threelayers: an external layer, an internal layer and a thin layerlocalised at theMAO layer/substrate interface. The difference ingamma-Al2O3 and alpha-Al2O3 phase contents for the externaland internal layers is caused by a variation in the cooling rate ofmolten alumina from discharges. The higher cooling rate of

molten alumina in the external layer favours the formation ofgamma-Al2O3; on the over hand, alpha-Al2O3 is easily formedin the internal layer with a lower cooling rate. The internal layercan present two aspects according to the percentage of alpha-Al2O3. The granular aspect of the internal layer indicates a highproportion of “dendrite” defects which implies high percentageof alpha-Al2O3. On the other hand, some alloying elements caninhibit the formation of alpha-Al2O3, probably Zn (in the case ofthe 7050-T74 alloy), that leads to a columnar aspect of theinternal layer. Moreover, the MAO layer thickness graduallyincreases when the proportion of alpha-Al2O3 decreases.

(2) During the MAO process, gas is continuously generated in thevicinity of the MAO layer/substrate and leads to the formationof spherical porosity. Discharges are probably formed in thespherical porosity, which can contribute to the formation of theinternal layer. Dendrite defects result from a local transforma-tion of gamma-Al2O3 into alpha-Al2O3 under discharge effect.

Acknowledgments

The financial support from the French Research Ministry through“PROXY3A” (RNMP project) is greatly acknowledged. One of us (E.K.T.)would like to acknowledge the National Association for TechnicalResearch (ANRT) for a grant in the frame of a CIFRE convention. Theauthors wish also to thank Jacques Beauvir (CERATRONIC) for his help intechnical support, Thierry Belmonte, Gérard Henrion and Kouitat Richard(LSGS, Nancy), of LSGS-Nancy for the help in drafting the manuscript.

References

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Microstructure and phase composition of microarc oxidation surface layers formed on aluminium a.....IntroductionExperimental conditionsResults and discussionsMAO layer thickness and phase analysisFracture cross-section observation

ConclusionsAcknowledgmentsReferences

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