Surface & Coatings Technology 205 (2011) 5406–5413

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Surface & Coatings Technology

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Spectroscopic and real-time imaging investigation of tantalum plasma electrolyticoxidation (PEO)

S. Stojadinović a, J. Jovović a, M. Petković a, R. Vasilić b, N. Konjević a,⁎a Faculty of Physics, University of Belgrade, Studentski trg 12-16, 11000 Belgrade, Serbiab Faculty of Environmental Governance and Corporate Responsibility, Educons University, Vojvode Putnika bb, Sremska Kamenica, Serbia

⁎ Corresponding author. Tel.: +381 11 2630152; fax:E-mail address: nikruz@ff.bg.ac.rs (N. Konjević).

0257-8972/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.surfcoat.2011.06.013

a b s t r a c t

a r t i c l e i n f o

Article history:Received 11 April 2011Accepted in revised form 2 June 2011Available online 15 June 2011

Keywords:TantalumPlasma electrolytic oxidation (PEO)Optical emission spectroscopySpectral line shapes and intensities

We present the results of plasma electrolytic oxidation (PEO) of tantalum in 12-tungstosilicic acid at 70 mA/cm2. For the characterization real-time imaging and optical emission spectroscopy (OES) were used. It hasbeen detected that relatively small microdischarges (cross-sectional areab0.1 mm2) are dominantthroughout the PEO, while the presence of medium-size microdischarges (cross-sectional area from0.1 mm2 to 0.2 mm2) and large microdischarges (cross-sectional areaN0.2 mm2) become noticeable only atlater times of PEO. The elements and their singly charged positive ions present in PEO microdischarges areidentified using standard OES technique. The spectral line shape analysis of first two hydrogen Balmer linesshows presence of two microdischarges during PEO. These discharges characterize relatively low electronnumber densities Ne≈0.9×1015 cm−3 and at Ne≈2.2×1016 cm−3. A simple OES technique was introducedto test optical thickness of hydrogen Balmer lines emitted during PEO process.

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1. Introduction

Plasma electrolytic oxidation (PEO) is an anodizing process oflightweight metals above the dielectric breakdown voltage, wherethick, highly crystalline oxide coating with high corrosion resistance,high wear resistance, and other desirable properties are achieved[1–3]. During the PEO, numerous transient fine short-lived dischargesare generated continuously over the coating surface, accompanied bygas evolution. Due to increased local temperature plasma-chemicalreactions are induced at the discharge sites modifying the structure,composition, and morphology of such oxide coatings [4].

Understanding the microdischarge phenomena is important forcharacterization of the PEO process. Recently a number of articlesreporting studies of discharges during PEO and their influence to theformation mechanism, composition, morphology and various prop-erties of oxide coatings have been published [5–13]. This interest isa result of widespread use of PEO coatings for a large range of ap-plications in the automotive industry, aerospace industry, gas andoil industries, biomedical devices, etc. [14]. Aluminium, titanium,magnesium, zirconium and their alloys are the most popular metalswhich are oxide coated by PEO in suitable electrolytes.

In the present paper we used tantalum foils as a substrate to obtainoxide coatings by PEO process. By means of real-time imaging thedensity and time evolution of microdischarges are studied and results

discussed. In addition, spectral characterization i.e. species identifi-cation in microdischarges and spectral line shape analysis of atomichydrogen lines described recently in relation to PEO on aluminiumsurface [15] will be carried out. Scanning electron microscopy (SEM)and atomic force imaging (AFM) will serve as tools for examiningsurface morphology of obtained oxide coatings.

2. Experimental

In this experiment, the oxide coatings were formed on tantalumsamples of dimensions 25 mm×5 mm×0.125 mm and 99.9% purity.Before the anodization, tantalum was degreased in acetone, ethanoland distilled water, using ultrasonic cleaner, and dried in a warm airstream. The oxidation process was carried out in an electrolytic cellwith flat glass windows [16]. Platinum wires were used as cathodes.For anodization of tantalum we used water solution of 0.001 M 12-tungstosilicic acid (H4SiW12O40). The electrolyte was prepared usingdouble distilled and deionized water and PA (pro analysis) gradechemical compound. Anodizing was carried out at current densityof 70 mA/cm2. During the anodization, the electrolyte circulatedthrough the chamber-reservoir system. The temperature of the elec-trolyte was maintained during the anodization process at (21±1)°C.

Spectral characterization of PEO was performed utilizing threedifferent grating spectrometers. A low dispersion systemwas used forspectra recording in a wavelength range from 380 nm to 850 nm.Optical detection system consisted of a large-aperture achromaticlens, a 0.3 m Hilger spectrometer (diffraction grating 1200 grooves/mm and inverse linear dispersion of 2.7 nm/mm) and a very sensitive

Fig. 1. Time variation of voltageand luminescence intensity at 450 nmduringgalvanostaticanodization of tantalum.

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PI-MAX ICCD thermoelectrically cooled camera (−40 °C) with highquantum efficiency manufactured by Princeton Instruments [17]. Thesystem was used with several grating positions with over-lapping

Fig. 2. Microdischarges appearance at various stages of PEO process: (a) 10 s; (b

wavelength range of 10 nm. Each spectrum had a wavelength range of43 nm.

Two high spectral resolution spectrometers were used for detailedstudy of spectral line shapes during PEO process. The Hα line wasrecorded with 2 m Ebert spectrometer (f/28, inverse linear dispersion0.74 nm/mm, grating blaze wavelength 550 nm) while the Hβ linewas recorded with 0.67 m Czerny–Turner spectrometer (f/4.7, inverselinear dispersion 0.83 nm/mm). Both spectrometers were equippedwith thermoelectrically cooled (−10 °C) CCD detector. The instru-mental profiles of both high dispersion instruments were very close toGaussian with the Full Width at Half Maximum (FWHM) of 0.026 nmand 0.030 nm, respectively. In all experiments the image of anodesurface was projected with unity magnification to the entrance slit ofspectrometer with an achromatic lens having focal length 75.8 mm.

Real-time images during PEO were recorded utilizing a videocamera Sony DCR-DVD110 (800 K pixels CCD, 40× optical zoom and40 mm lens filter). The time resolution of the measurement waslimited by the frame interval of 40 ms and the shutter speed of1/100 s. The information obtained was split into separate framesand the image of individual frames was processed using our custommade software, which counts microdischarges in selected frame and

) 20 s; (c) 1 min; (d) 3 min; (e) 5 min; (f) 15 min; (e) 30 min; (f) 45 min.

Fig. 3. Microdischarge characteristics at various stages of PEO process: (a) Micro-discharge spatial density; (b) Percentage of oxide coating area simultaneously coveredby active discharge sites.

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determines spatial density of discharges including their dimensionaldistribution.

The morphology and roughness of oxide coatings were character-ized using a scanning electron microscope (JEOL 840A) and an atomicforce microscope (Veeco Instruments, model Dimension V). Rough-ness data were obtained using diNanoScope software (version 7.0),and reported values are calculated mean values for a number ofdifferent samples obtained under same anodizing conditions.

3. Results and discussion

3.1. Real-time imaging and surface morphology

Fig. 1 shows typical voltage versus time and luminescenceintensity versus time characteristics during anodization of tantalumsamples in 0.001 MH4SiW12O40. From Fig. 1 it is evident that tantalumdoes not show galvanoluminescence phenomenon, which wasobserved during anodization of aluminium [18,19] and magnesium[20]. From the beginning of anodization, the voltage increases first20 s approximately linearly with time to about 400 V. In this stage ofanodization, the process is similar to conventional anodizing and inthis period relatively compact barrier oxide film thickens uniformly[18]. Uniform film thickening is terminated by dielectric breakdowncharacterized by deflection from linearity of voltage versus timecurve, starting from so-called breakdown voltage. As the anodizationvoltage approaches breakdown voltage, a large number of bright spotsappear, evenly distributed over the surface.

Real-time imaging revealed several stages of the PEO process(Fig. 2), with distinct microdischarge characteristics. Intensive gasgeneration is observed after first few seconds of anodization (Fig. 2a).Small microdischarges, with average cross-sectional area ~0.05 mm2,are visible after about 20 s, together with gas bubble evolution(Fig. 2b). During the PEO the size of microdischarges becomes larger,while the number of microdischarges is reduced (Fig. 2c–f).

The evolution of the microdischarge spatial density is shownin Fig. 3a. Spatial density of microdischarges is the largest after about2.5 min from the beginning of PEO process, than it reduces sub-stantially during next 5 min from about 225 cm−2 to 50 cm−2, andthen stays almost constant. The percentage of oxide coating area,simultaneously covered by active discharge sites has also maximumafter 2.5 min from the beginning of the PEO process (~5%) and thenmonotonically decreases with the time of PEO processing (Fig. 3b).

The dimensional distribution of PEO microdischarges is shown inFig. 4. Relatively smallmicrodischarges (cross-sectional areab0.1 mm2)are present throughout the whole PEO process. Their concentrationreaches almost 100% of the total population at the beginning ofPEO (Fig. 4a), and decreases to about 75% during the final stages ofthe treatment (Fig. 4f). The portion of medium-size microdischarges(cross-sectional area from 0.1 mm2 to 0.2 mm2) is negligible at thebeginning of the PEOprocess, but increases considerablywith PEO time,reaching a maximum value of 32% after about 10 min, and decreasesto 17% in the final stage. Large microdischarges (cross-sectionalareaN0.2 mm2) become noticeable only after extended PEO times.

The surface morphology evolution of the oxide coatings formedby anodization of tantalum is shown in Fig. 5. Before breakdownthe relatively compact barrier oxide film is formed (Fig. 5a). Afterbreakdown the oxide surface becomes laced with a number of cracks,pores and channels (Fig. 5b–d). The increased size and decreasedspatial density of microdischarges during PEO, see Fig. 2, is related tothe number of discharging sites through which higher anodic currentis able to pass. The surface morphology evolution of the oxidecoatings, see Fig. 5, shows that the number of micropores decreases,while their size increases during PEO process. Also, thicker coatingshave higher surface roughness (Fig. 6). In the initial stage of PEO, thedischarge channels are well distributed and oxide coatings exhibitlower surface roughness. As the number of discharge channels

decreases with time of PEO, non-uniformities in the oxide coatingsappear causing an increase in surface roughness. In a thicker layer ofoxide coating, a higher energy is required for the current to passthrough. Under this condition, the current is localized at weak pointsof the layer formed to find its way through the oxide coating. This isthe reason why the diameter of the discharge channel increases.

Fig. 7 shows backscattered SEM micrographs of polished cross-sections of PEO coatings grown on tantalum. Throughout the PEOprocess, the average coating thickness increases approximately 1.3 μm/min. The PEO coatings show a typical microstructure with two distinctregions, i.e., thin, compact inner layer adjacent to tantalum substrateand porous outer layer. This confirms that as the PEO process timeincreases, microdischarges create large discharge channels and defectsin the coatings, yielding higher surface roughness.

3.2. Optical emission spectroscopy

Typical optical emission spectrum of PEO microdischarges in thespectral region 380 nm to 850 nm is shown in Fig. 8a, while emissionspectra in the range 380 nm to 480 nm and 700 nm to 850 nm arepresented in Fig. 8b and c, respectively. Atomic and ionic lines wereidentified using the NIST online spectral database [21] and theybelong to hydrogen and oxygen only. The strongest line is the Hα

(656.28 nm) while other two Balmer lines, the Hβ (486.13 nm) andthe Hγ (434.05 nm) are identified also. Strong lines are also O I at844.62 nm, 844.64 nm and 844.68 nm. In addition, three O I lines at777.19 nm, 777.42 nm, 777.54 nm and many other weaker lines ofO I and O II are also detected, see Fig. 8. The notation I and II refersto neutral and singly ionized atoms, respectively. The continuum

Fig. 4. The dimensional distribution of microdischarges at various stages of PEO process: (a) 2.5 min; (b) 5 min; (c) 10 min; (d) 15 min; (e) 30 min; (f) 45 min.

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emission between 380 nm and 850 nm results from collision-radiative recombination of electrons [18] and bremsstrahlungradiation [22]. The species that are identified originate only fromthe electrolyte, unlike in the case of PEO on aluminium, see e.g. [15].Most likely, the absence of species originating from the substrateis caused by higher melting temperature of tantalum, compared tomelting temperature of aluminium.

To determine PEO electron number density Ne, we used plasmabroadened profile of the Hα and the Hβ lines, see Figs. 9 and 10. Duringthe analysis of the Hα line profile, see the example in Fig. 9, it wasfound that the Hα line shape can't be properly fitted without useof two Lorentz profiles having FWHM of 0.1 nm and 0.5 nm. TheseLorentz profiles, according to Eq. (10) in [23], correspond to Ne=1.0×1016cm−3 and Ne=0.87×1017cm−3. Similar shape of the Hα

line and consequently similar Ne results were deduced during PEOof aluminium, see Fig. 2a in Ref. [15]. Although the fit quality isexceptionally good, see residue in Fig. 9 and in Fig. 2a in Ref. [15],these Ne results are not considered confident. Namely, the Hα line is a

very strong in PEO, see Fig. 8, and self-absorption may broaden lineprofile considerably while the line shape may be still well fitted withLorentz profile [24].

The presence of the Hα self-absorption was proven after theanalysis of the Hβ profile, see Fig. 10, which gave smaller Ne, than theHα line. The line width of upper part of the Hβ profile FWHM≈0.19 nm while FWHM≈1.73 nm for the broad lower profile what,in conjunction with empirical formula (2a) in Ref. [25], correspondto Ne≈0.9×1015 cm−3 and Ne≈2.2×1016cm−3. Since one wouldexpect same Ne from both Balmer lines the results from the Hβ

(weaker line less self-absorbed than the Hα) should be considered asmore reliable. If one can prove that the Hβ is optically thin, themeasured Ne represents correct value. The simplest technique to testresults from the Hβ is to study line shape of weaker Balmer line,the Hγ, but unfortunately this line in PEO interferes with O II lines andis too noisy to be used for reliable self-absorption test, see Fig. 11.Therefore, we have to rely on the test of optical thickness using theHα, and the Hβ lines only. To perform test we measured for both

Fig. 5. SEM micrographs of oxide coatings on tantalum anode at various stages of PEO process: (a) 15 s; (b) 3 min; (c) 15 min; (d) 30 min.

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lines the intensity ratio of upper narrow profile A1 to lower broadone A2. In case we are working under optically thin conditions theintensity ratio within both Balmer lines should remain constantduring PEO. If A1 and/or A2 in Fig. 9 or 10 are optically thick the ratiochanges during PEO process since it is very unlikely that intensity ofboth lines at different Ne change in the same way. A possible gradualchange of the ratio may be only an indication of a gradual change of

Fig. 6. Influence of PEO treatment time on roughness of oxide coating.

PEO processes during the observation time. The described techniqueis not the best test for line self-absorption, see e.g. [26,27] butrepresents the only usable technique under PEO conditions.

To perform self-absorption test we used low dispersion spectrom-eter with very sensitive ICCD to record Balmer line profiles duringPEO process. These profiles were not of so high quality as thoserecorded with high resolution spectrometers, see Figs. 9 and 10, butgood enough to perform separation of narrow A1 from the broad A2

Lorentz profile, see example in Fig. 12. The time variation of A1/A2

ratio for both hydrogen lines is presented in Fig. 13a while intensitiesvariation of narrow A1 and broad profile A2 for the Hα is given inFig. 13b. The results in Fig. 13 clearly indicate that both componentsof the Hα line in our PEO are optically thick and they can't be usedfor Ne diagnostics. The Hβ line is close to optically thin conditionbut one can't claim that this line is absolutely free of self-absorptionwithout an additional proof. Presently, our recommendation formost reliable Ne values for PEO on tantalum is Ne≈0.9×1015cm−3

(estimated error±20%) and Ne≈2.2×1016cm−3 (estimated error±15%) what corresponds to the line width of the narrow part of the Hβ

line shape FWHM≈0.19 nm and FWHM≈1.73 nm for the broadlower part of the overall profile. It should be noted that measuredwidths of the Hβ profiles are used for evaluation of Ne i.e. theinstrumental, Doppler, Van der Waals and Resonance broadening areconsidered negligible. The fact that measured narrower profile hasLorentzian shape is a good indication that the instrumental (Gaussian

Fig. 8. Optical emission spectra recorded during PEO in the range: (a) 380 nm–850 nm;(b) 380 nm–480 nm; (c) 700 nm–850 nm.

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shape, see Section 2. Experimental) and Doppler (also Gaussian)broadening is negligible in comparison with Stark broadening.Van der Waals and Resonance broadening for the Hβ line at Ne

N1015 cm−3 in plasmas close to atmospheric pressure are negligiblealso, see e.g. Fig. 14 in Ref. [28].

Measured electron number densities for tantalum PEO are closeto the corresponding Ne results for PEO on aluminium [15]. Theexception is the third largest Ne detected in PEO on aluminium [15],which was not detected with tantalum substrate. Another differencebetween PEO on aluminium and tantalum is the lack of anodematerialin spectra of microdischarges with tantalum substrate. High meltingtemperature of tantalum (approximately 3000 °C) does not allow

Fig. 7. Backscattered electron micrographs of oxide coatings on tantalum anode atvarious stages of PEO process: (a) 10 min; (b) 30 min; (c) 45 min.

Fig. 9. The Hα line profile: (a) best fit with two Lorentzians and (b) residue plot. The Hα

was recorded with 2 m spectrometer with thirty seconds acquisition time, sevenminutes after the beginning of PEO.

Fig. 11. The Hγ line recording with 0.67 mmonochromator, 5 min after the beginning ofPEO and taken as an average over five-minute interval.

Fig. 12. The Hβ line profile: (a) best fit with two Lorentzians and (b) residue plot. The Hβ

was recorded with 0.3 m spectrometer with 10 s acquisition time, between 6 and 7 minafter the beginning of PEO.

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evaporation of anode material during PEO. With aluminium, theanode is melted and Al plasma is formed in microchannels (processtype B in Fig. 9 [14]) with Ne approaching 1017 cm−3 [15]. This type ofdischarge with metallic vapor is missing with tantalum anode. Itseems that in this case plasma is generated inmicrochannels in anodicgas with water vapor and ablated oxide material having maximumNe≈2.2×1016cm−3 and line spectra showingH I, O I and O II lines aredetected only, see Fig. 8.

4. Conclusions

The time and space distribution of microdischarges during plasmaelectrolytic oxidation (PEO) of tantalum in 12–tungstosilicic acid havebeen studied using real-time imaging. Spatial density of micro-discharges is the largest after about 2.5 min from the beginning of thePEO process, it reduces considerably during next 5 min, and later staysalmost constant. The percentage of oxide coating area covered byactive discharge sites (simultaneously recorded by camera) has alsomaximum after 2.5 min and then monotonically decreases with time.Microdischarge cross-sectional areas are in the range from 0.05 mm2

Fig. 10. The Hβ line profile: (a) best fit with two Lorentzians and (b) residue plot. The Hβ

was recorded with 0.67 m monochromator with 30 s acquisition time, 7 min after thebeginning of PEO.

Fig. 13. Temporal variation of (a) A1/A2 ratio for the Hα and for the Hβ profile and(b) intensities of A1 and A2 of the Hα line. The results were derived from line shapesrecorded with 0.3 m spectrometer.

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to 0.30 mm2. The population is dominated by small microdischarges(cross-sectional areab0.1 mm2), while the presence of medium-sizemicrodischarges (cross-sectional area from 0.1 mm2 to 0.2 mm2) andlarge microdischarges (cross-sectional areaN0.2 mm2) become no-ticeable only after extended PEO times.

Qualitative analysis of PEO emission spectra revealed only H I, O Iand O II spectral lines. The analysis of hydrogen Balmer line shapesindicates the presence of two types of discharges during PEO.Simple optical emission spectroscopic technique is developed to testpresence of self-absorption for Balmer lines in PEO spectra andsuccessfully applied to the Hα and the Hβ. It has been demonstratedthat the Hα is strongly self-absorbed and the electron number den-sity for two discharges with Ne≈0.9×1015cm−3 and Ne≈2.2×1016cm−3 are determined from the shape of the Hβ line. The presenceof two types of discharges in comparison with three detected forPEO on aluminium substrate [15] may be explained by high meltingtemperature of tantalum, which prevents formation of metallicplasma in microchannels through oxide layer like with aluminiumsubstrate.

Acknowledgements

The authors gratefully acknowledge A. Žekić for the help with SEManalysis. This work is supported by the Ministry of Education andScience of the Republic Serbia under Project nos. 171014 and 171035.

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