Formation of Black Ceramic Layer on Aluminum Alloy by PlasmaElectrolytic Oxidation in Electrolyte Containing Na2WO4

I. J. Hwang1, K. R. Shin1, J. S. Lee2, Y. G. Ko2,+ and D. H. Shin1,+

1Department of Metallurgy and Materials Engineering, Hanyang University, Ansan 425-791, Korea2School of Materials Science and Engineering, Yeungnam University, Gyeongsan 712-749, Korea

The formation of black ceramic layer produced by plasma electrolytic oxidation (PEO) coating has been investigated as a function ofcoating time. A series of PEO coatings was carried out on aluminum alloy sample in a phosphate electrolyte containing sodium tungstate(Na2WO4) with four different coating times, i.e., 20, 100, 200 and 300 s. As the coating time increased, the amount of tungsten element in theceramic layer increased, resulting in the black ceramic layer. This phenomenon was discussed based on the electrochemical reaction assisted bymicro sparks to form WO3 compounds in the ceramic oxide layer. [doi:10.2320/matertrans.M2011263]

(Received November 28, 2011; Accepted December 19, 2011; Published February 25, 2012)

Keywords: aluminum alloy, plasma electrolytic oxidation, coating time, tungsten oxide

1. Introduction

In recent years, aluminum and its compounds have longbeen used in various electronic industries due to high specificstrength, good corrosion resistance and excellent electrothermal conductivity.1­3) For wider actual applications,electroplating, anodizing and plasma spraying have beeninvestigated since high tribological properties and opticalcoloring could be attained by surface reforming.4­8) Amongstthese methods, much interest has been directed on plasmaelectrolytic oxidation (PEO) technique which could lead notonly to generate the strong hetero-bonding between the metalsubstrate and the ceramic layer but also to infiltrate the metaloxides into the ceramic layer by electrochemical reactionsassisted by high plasma energy. During PEO coating, theselection of the electrolyte was found to be important intailoring electrochemical reactions which affected the surfacecharacteristics, in particular, including the realization ofvarious colors of the ceramic oxide compounds such asZrO2,9) Mn2O3,10) V2O3,11) etc. Our research group alsoreported that the black ceramic layer on aluminum alloy viaPEO coating within the electrolyte containing NH4VO3 wassuccessfully attained,12,13) but the poor adhesion issuebetween the substrate and the ceramic layer remainedunsolved yet. According to the previous suggestion byBayati et al.,14) the introduction of WO3 compounds into theceramic layer would be effective to achieve the dark blackcolor as well as high bonding strength in aluminum alloys.During PEO coating, unfortunately, the optimum conditionto induce the black WO3 compounds was not determinedwith respect to the coating time and the formation mechanismof WO3 compound was not clearly understood.

In this study, the black ceramic layer having good adhesionstrength with aluminum alloy sample was formed by theelectrolyte containing sodium tungstate (Na2WO4) and thisresult was compared to the sample via PEO in the electrolytewith NH4VO3. In addition, the variation of the surfacecharacteristics with the coating time was studied.

2. Experimental Procedures

The chemical composition of aluminum alloy used as asubstrate in this study was 5.77Zn-2.61Mg-1.33Cu-0.22Cr-0.15Fe-0.08Si-0.001Mn (in mass%). Aluminum alloy platewith a thickness of 2mm was cut into 30mm © 50mmsamples. Prior to the PEO coating, the samples weremechanically polished with # 1000 SiC papers, rinsed withdistilled water and ultrasonically cleaned in high-purityethanol. A machine equipped with stirring and coolingsystems was used to perform PEO coating with an appliedcurrent density of 100mA/cm2. A series of PEO coatingwere carried out using the electrolyte consisting of 0.14Mpotassium hydroxide (KOH), 0.05M potassium hydrogenphosphate (K2HPO4) and 0.08M sodium tungstate with fourdifferent coating times: 20, 100, 200 and 300 s.

The surface morphology and chemical composition of eachsample were observed utilizing a field-emission scanningelectron microscope (FE-SEM, HITACHI S4800) equippedwith an energy dispersive X-ray spectrometer (EDS). Thecompounds present in the oxide layers were detected by X-ray diffractometer with Cu K¡ radiation and X-ray photo-electron spectroscopy (XPS, VG microtech ESCA 2000) withmonochromatic Al K¡ (1486.6 eV) X-ray source. High-resolution narrow scanning was used to determine thechemical binding states of elements, O 1s, Al 2p and W 4f.A colorimeter was employed for color measurement. L*value meant the degree of lightness and covered a wide rangefrom black color (0) to white color (100). The adherence ofthe ceramic layer to the aluminum substrate was evaluatedby sonicator (Bransonic 3510). The sonication test wasperformed by immersing the samples within deionized waterfor 600 s at output power of 100W with frequency of 42 kHz.

3. Results and Discussion

The cell voltage vs. time curve for four-stage sequentialPEO coating of the aluminum alloy samples in the Na2WO4-containing electrolyte is shown in Fig. 1. Samples A, B, Cand D indicated the PEO-treated samples for 20, 100, 200and 300 s, respectively. Judging from the increasing trend of

+Corresponding author, E-mail: younggun@ynu.ac.kr, dhshin@hanyang.ac.kr

Materials Transactions, Vol. 53, No. 3 (2012) pp. 559 to 564©2012 The Japan Institute of Metals EXPRESS REGULAR ARTICLE

the cell voltages, two regions could be clearly separated bythe breakdown point wherein micro sparks on the samplesurface were observed to be initiated. Here, the breakdownvoltage of 280V was observed at coating time of ³25 s. Priorto breakdown phenomenon (region I), the cell voltage ofthe sample steeply increased, obeying Ohm’s law.15­17) Thisincreasing rate seemed saturated as the coating time increasedup to 300 s. Most electrons were transported with an aid ofmicro discharge through the micro channels in the ceramiclayer. Thus, the increasing rate of region II was lower thanthat of region I in spite of rapid growth of the ceramic layerworking as an insulator under the present electric field. In

order to explore the structural changes in the ceramic layerwith respect to the coating time, Fig. 2 exhibits the surfacemorphologies of the ceramic layers on aluminum alloysamples treated by PEO coating at four different coatingtimes. In sample A, the substrate was covered in part with theshallow ceramic layer with micrometer size and irregularmarks generated by mechanical grinding prior to the coatingwere still visible. When the coating voltage exceededbreakdown voltage, the ceramic layer was composed ofmicro pores which were caused by the occurrence of gasbubbles and micro sparks like volcanic activity.18) Oxidenodules were also detected. During PEO coating accompany-ing high energy sparks, the surface temperature was reportedto be higher than 2000K and dropped abruptly when thenewly-formed oxide compounds met the cool electrolyte.16,19)

In this study, the temperature of the electrolyte was preservedat ³293K by using an external thermostat. Thus, the oxidecompounds were rapidly solidified, resulting in the formationof micro pores and oxide nodules around micro pores. It isnoted that the size of micro pores increased whereas thepopulation of micro pores decreased with increasing coatingtime. The average size of the micro pores in sample D wasestimated to be ³2.1 « 0.6 µm, which was twice larger thansample A. The ceramic layer grew as the coating timeincreased. Due to the growth of the ceramic layer, microsparks became more intensive and even larger, which led tothe increase in pore size. The cross-sectional images of theceramic layers of the PEO-treated samples are also shown inFig. 2. The thicknesses of the ceramic layers were observedto be 1, 4, 7 and 10 µm for the samples A, B, C and D,respectively.

Fig. 1 Voltage­time response for four-stage sequential coating of alumi-num alloy sample.

Fig. 2 SEM images showing the ceramic layer structures of the aluminum alloy samples after PEO coating for (a) 20, (b) 100, (c) 200 and(d) 300 s.

I. J. Hwang, K. R. Shin, J. S. Lee, Y. G. Ko and D. H. Shin560

Compositional analysis was made based on EDS observa-tions of the PEO-treated samples, and their results arepresented in Fig. 3. Under the same amount of oxygenelement, the amount of tungsten element increased while thatof aluminum element decreased with increasing coating time.This suggested that tungstate ions coming from Na2WO4 inthe electrolyte readily participated in electrochemical reac-tions assisted by micro sparks, resulting in the presence oftungsten element in the ceramic layer. Hence, it was expectedthat tungsten oxide were introduced and its amount wasincreased by reduction in amount of aluminum oxide as thecoating time increased. When the voltage during PEO coatingwould exceed the breakdown voltage, the ion spice ofHPO4

2¹ would also migrate inward through the dischargechannels and phosphorus elements were found in the ceramiclayer.20)

To discuss the mechanism of the ceramic layer growthtogether with the migration of WO4

2¹ ions, the EDS linescanning of the sample D was made across the ceramic layersas shown in Fig. 4. The experimental results showed that thetungsten element of 6 at% at least was detected throughoutthe entire ceramic layer. Interestingly, the amount of tungstenincreased locally whereas the amount of the aluminumdecreased in the vicinity of the surface of the ceramic layer.The sufficient migration of WO4

2¹ ions toward the ceramiclayer was activated by high electric potential as the coatingtime increased, causing the formation of the tungsten-(relatively) rich region present at the upper region of theceramic layer. This showed good agreement with theincreasing trend of tungsten amount with increasing coatingtime.

Figure 5 shows XRD patterns of the ceramic layer formedin the electrolyte containing Na2WO4 for 300 s. The ceramiclayer consisted of both £-Al2O3 and WO3 compounds. TheWO3 compound present in the ceramic layer was amorphoussince it possessed a wide peak at ³23°.21)

Figure 6 shows the XPS spectra of the sample treated byPEO for 300 s. It was evident from Fig. 6(a) that the ceramiclayer formed within the electrolyte contained oxygen,aluminum, and tungsten elements. The individual spectrumssuch as Al 2p 3/2, W 4f 5/2, W 4f 7/2 and O 1s wereanalyzed in order to figure out what kinds of the oxide

compounds appeared in the ceramic layer. In Fig. 6(b), thebinding energy of 531.0 eV revealed the contribution ofoxygen elements to the formation of crystal lattice structuresof Al­O and/or W­O compounds.14) As shown in Fig. 6(c),the binding energy of 73.9 eV was related to the existence of£-Al2O3.14,22) The binding energies of 35.5 eV (4f 7/2) and37.7 eV (4f 5/2) from XPS spectra [Fig. 6(d)] indicated theformation of WO3.23,24) It was concluded that the ceramiclayer which was formed during PEO coating comprised£-Al2O3 and WO3 compounds. The formation of £-Al2O3

compound was explained by the outward migration of Al3+

from the substrate and the adsorbed water dissociation.25)

Fig. 3 EDS results of the PEO-coated aluminum alloy samples withrespect to the coating time.

Fig. 4 EDS line scanning (Al, O, V and P) of the PEO-coated aluminumalloy sample.

Fig. 5 XRD pattern of the aluminum alloy sample after PEO coating for300 s.

Formation of Black Ceramic Layer on Aluminum Alloy by Plasma Electrolytic Oxidation in Electrolyte Containing Na2WO4 561

2Al3þ þ 9H2O ! Al2O3 þ 6H3Oþ

Al2O3 compounds were easy to be rapidly solidified whenthey contacted the relatively cool electrolyte. Such rapidsolidification of alumina favored the formation of £-Al2O3

compound. In contrast, some WO42¹ ions from the electro-

lyte migrated inward through the discharge channels in theceramic layer and, then, sacrificed the electrons owing toelectric shock, giving rise to the incorporation of black WO3

compounds. Chemical reaction is as follow:26)

2WO42� � 4e ! 2WO3 þ O2

Figure 7 is the optical image showing the surface color andappearance of five distinct samples which are coated for 0,20, 100, 200 and 300 s in the electrolyte containing Na2WO4.Figure 7(a) provided the initial sample prior to PEO coating

so as to compare the change in the color of each sample. Asthe coating time increased up to 300 s, the samples appearedto have different gradients of black color. As shown inFig. 7(b) which was taken before breakdown voltage, the thinceramic layer of sample B exhibited light grey color. Thiswas same as that found in the PEO-treated sample using theelectrolyte without Na2WO4, implying the fact that the blackWO3 compound was rarely formed in the ceramic layer.Due to the lower responding voltage of ³280V (Fig. 1) thanbreakdown voltage, the move of tungstate ions onto theceramic layer was insufficient under the electric field. It wascertain that the sample E showed the conformal ceramic layerwith the darkest black color among the PEO-treated samples.Though indirect, comparison of Figs. 7(c), 7(d) and 7(e) ledto the conclusion that the migration of tungstate ions towardsample surface and the concomitant electrochemical reaction

Fig. 6 XPS results of the aluminum alloy samples after PEO coating for 300 s: (a) wide scan, (b) O 1s, (c) Al 2p 3/2 and (d) W 4f 5/2and 7/2.

Fig. 7 Optical images showing the surface colors of (a) the as-received sample and the aluminum alloy samples after PEO coating for (b)20, (c) 100, (d) 200 and (e) 300 s.

I. J. Hwang, K. R. Shin, J. S. Lee, Y. G. Ko and D. H. Shin562

of tungsten ions to form WO3 compound became acceleratedwhen the coating duration exceeded 100 s. We believed thatthe activity of tungstate ions to form black WO3 compound inthe ceramic layer would depend upon the coating timeassociated with the electric field, which was consistent withthe present EDS observation.

Figure 8 shows the relationship between the L* value ofthe colored ceramic layer and the coating time. The initialcoating layer prior to PEO coating possessed a high value of³90. As the coating time increased, the color of the ceramiclayer was changed. At a coating time of 300 s, the color of theceramic layer reached the dark black with the lowest L* valueof ³30. This was mainly due to the fact that a prolongedcoating time caused the tungsten ions to incorporate intothe ceramic layer and, thus, to form WO3 compound in theceramic layer. Consequently, the amorphous WO3 appearedto have an influence on the dark black color.

In order to evaluate how the ceramic layer with WO3

compound adhered strongly to the aluminum substrate, theresult of the sonication test of the sample D in distilled waterfor 600 s is given in Fig. 9(a). This is compared to that of thePEO-treated sample in the NH4VO3 electrolyte [Fig. 9(b)].Although the resultant compounds present in the ceramiclayer would be different, the PEO-treated sample in theNH4VO3 electrolyte was used to compare the adhesioncharacteristics since the color of V2O3 compound was blacksame as that of WO3 used in this study. The coating time and

thickness of both samples were reasonably identical. Mostparts of the ceramic layer of the sample coated in theNH4VO3 electrolyte were observed to be peeled away fromthe substrate [Fig. 9(b)] whilst there was no appreciabledifference found between before and after sonication test ofthe sample D. When the NH4VO3 electrolyte was applied forPEO coating of aluminum sample, the fraction of micro poreswas much larger than when the Na2WO4 electrolyte wasused. Since the adhesive strength was closely related to thefraction of micro pores, the sample coated in the Na2WO4

electrolyte showed better adhesion properties as comparedto that in the NH4VO3 counterpart. This finding suggestedthat WO3 compound was successfully incorporated into theceramic layer and the adhesion strength between the substrateand the ceramic layer was significantly improved by utilizingthe Na2WO4 electrolyte used for PEO treatment.

Further investigations on how the shape and size of WO3

compound would exist in ceramic layer after PEO coatingsare needed in order to provide a better understanding betweenmicrostructure and related properties enhancement of thePEO-treated aluminum samples.

4. Summary

A conformal black ceramic layer on aluminum alloy wassuccessfully fabricated via PEO coating in the electrolytecontaining Na2WO4. Regarding the micro pore formation onsurface, the size of micro pores increased while thepopulation of micro pores decreased with increasing coatingtime. The present EDS results showed that, as the coatingtime increased, the amount of tungsten element in the ceramiclayer increased. Due to the electrochemical reactions assistedby plasma energy, WO3 compound was successfully formedinto the ceramic layer and the adhesion strength between thesubstrate and the ceramic layer was significantly improvedafter PEO treatment.

Acknowledgements

This work was supported by the National ResearchFoundation of Korea (2011-0000345). This work (GrantsNo. 000419980211) was supported by Business for Cooper-ative R&D between Industry, Academy, and ResearchInstitute funded Korea Small and Medium Business Admin-istration in 2011.

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