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Preparation of silicate tungsten bronzes on aluminum by plasma electrolytic oxidation process in 12-tungstosilicic acid
Applied Surface Science 257 (2011) 9555–9561
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Preparation of silicate tungsten bronzes on aluminum by plasma electrolytic oxidation process in 12-tungstosilicic acid M. Petkovic´ a , S. Stojadinovic´ a,∗ , R. Vasilic´ b , I. Belˇca a , Z. Nedic´ c , B. Kasalica a , U.B. Mioˇc c a b c
Faculty of Physics, University of Belgrade, Studentski trg 12-16, 11000 Belgrade, Serbia Faculty of Environmental Governance and Corporate Responsibility, Educons University, Vojvode Putnika bb, Sremska Kamenica, Serbia Faculty of Physical Chemistry, University of Belgrade, Studentski trg 12-16, 11000 Belgrade, Serbia
a r t i c l e
i n f o
Article history: Received 5 April 2011 Received in revised form 31 May 2011 Accepted 10 June 2011 Available online 17 June 2011 Keywords: Plasma electrolytic oxidation (PEO) 12-Tungstosilicic acid Silicate tungsten bronzes
a b s t r a c t The growth of silicate tungsten bronzes on aluminum by plasma electrolytic oxidation in 12-tungstosilicic acid is experimentally investigated and discussed. Real time imaging and optical emission spectroscopy characterization of plasma electrolytic oxidation show that spatial density of microdischarges is the highest in the early stage of the process, while the percentage of oxide coating area covered by active discharge sites decreases slowly with time. Emission spectrum of microdischarges has several intensive band peaks originating either from aluminum electrode or from the electrolyte. Surface roughness of obtained oxide coatings increases with prolonged time of plasma electrolytic oxidation, as their microhardness decreases. Raman spectroscopy and energy dispersive X-ray spectroscopy are employed to confirm that the outer layer of oxide coatings formed during the plasma electrolytic oxidation process is silicate tungsten bronzes. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Plasma electrolytic oxidation (PEO), also called microarc oxidation or anodic spark deposition, is a high-voltage anodizing process in which the surface of lightweight metals (aluminum, titanium, magnesium, zirconium, etc.) or metallic alloys can be used for depositing thick, dense, and hard oxide coatings by plasma discharge in suitable electrolytes [1–3]. This process involves anodizing above the dielectric breakdown voltage where numerous transient fine sparks are produced continuously over the coating surface, accompanied by gas evolution. Plasma-chemical reaction is induced at the discharge sites due to increased local temperature modifying the structure, composition, and morphology of such oxide coatings. The composition and concentration of electrolyte play a crucial role in obtaining the desired oxide coatings by PEO. Most popular electrolytes used for such coatings are water solutions of sodium hydroxide [4], sodium silicate [5], sodium phosphate [6], sodium aluminates [7], sodium tungstate [8] and some iso- and heteropoly compounds [9]. Oxide coatings formed by PEO process in aforementioned electrolytes have excellent adhesion with the substrate, electrical and thermal insulation, high microhardness, good wear and corrosion resistance, etc. In this work, we have chosen water solution of 12-tungstosilicic acid (H4 SiW12 O40 , WSiA) to obtain oxide coatings – the silicate
∗ Corresponding author. Tel.: +381 11 2630152; fax: +381 11 3282619. ´ E-mail address: [email protected] (S. Stojadinovic). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.06.062
tungsten bronzes on aluminum by PEO process. In recent years heteropoly acids have received increasing attention because of their chemical, structural and electronic versatility. Heteropoly acids are regarded as active solids and viable candidates for electrochemical devices, fuel cells, batteries [10–14], and as materials that can be transformed into nanometer-sized blocks [15]. Good corrosion resistance combined with catalytic, conducting or semiconducting properties render tungsten bronzes as highly desirable materials [16–19]. WSiA is one of the three best known heteropoly acids with Keggin anion structure [10,20,21]. Heteropoly compounds with Keggin anion have low temperature structural phase transition to bronzes at about 600 ◦ C corresponding to solid–solid recrystallization of heteropoly compounds to bronzes [10–12]. This structural phase transition for WSiA is at about 535 ◦ C [11]. Having in mind that 12tungstophosphoric, 12-molybdophosphoric and 12-tungstosilicic acid are iso-structural, it is possible to conclude that starting with WSiA as a precursor, in the process of PEO silicate bronzes will be obtained. The aim of this work is to investigate PEO process of aluminum in WSiA and possibly to develop an innovative procedure for fabrication of silicate tungsten bronzes on aluminum. Real time imaging and optical emission spectroscopy are used to study discharge characteristics during PEO. Physicochemical methods: atomic force microscopy (AFM), scanning electron microscopy (SEM-EDX), Xray diffraction (XRD) and Raman spectroscopy served as tools for examining surface morphology, mechanical properties and chemical and phase composition of obtained oxide coatings.
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Fig. 1. Voltage of anodization-time dependence during galvanostatic anodization of aluminum in 0.001 M H4 SiW12 O40 .
2. Experimental details In the experiment, oxide coatings were formed on cold-rolled aluminum samples (99.99% purity). The surface of aluminum was degreased in ethanol using ultrasonic cleaner and dried in a stream
of warm air. The oxidation process was carried out in an electrolytic cell with flat glass windows [22]. Platinum wires were used as cathodes. For anodization of aluminum we used water solution of 0.001 M H4 SiW12 O40 . The electrolyte was prepared using double distilled and deionized water and PA (pro analysis) grade chemical compound. Anodizing was carried out at current density of 25 mA/cm2 . During the anodization, electrolyte circulated through the chamber–reservoir system. The temperature of the electrolyte was kept at (19 ± 1) ◦ C during the anodization process. After the anodization, samples were rinsed in distilled water to prevent additional deposition of electrolyte components during drying. Spectral characterization of PEO was performed utilizing a spectrograph system based on the Intensified Charge Coupled Device (ICCD) camera intended for time-resolved measuring of very weak light intensity in a wide range of wavelengths [23]. Optical detection system consisted of a large–aperture achromatic lens, a Hilger spectrograph with diffraction grating of 1200 grooves/mm (wavelength range of 43 nm) and a very sensitive PI-MAX ICCD camera with high quantum efficiency manufactured by Princeton Instruments. The system was used with several grating positions with over-lapping wavelength range of 10 nm. Real time images during the PEO were recorded utilizing a video camera (Sony DCR-DVD110, 800 K pixels CCD, 40× optical zoom and 40 mm lens filter). The information obtained was split into sep-
Fig. 2. Microdischarges appearance at various stages of PEO process: (a) 1.5 min; (b) 3 min; (c) 5 min; (d) 15 min; (e) 30 min; (f) 60 min.
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arate frames and the images of individual frames were processed using our custom made software, which automatically counted microdischarges in selected frames, determined spatial density, and dimensional distribution of the microdischarges. The morphology and mechanical properties of surface coatings (roughness and microhardness) were characterized using an atomic force microscope (AFM; Veeco Instruments, model Dimension V). Roughness and microhardness (ratio of applied indentation force and 2D imprinted area) data were obtained using diNanoScope software (version 7.0), and reported values are calculated mean values for a number of different samples obtained under the same anodizing conditions. Scanning electron microscope (SEM) JEOL 840A equipped with energy dispersive X-ray spectroscopy (EDX) was used to characterize chemical composition of formed surface coatings. The crystallinity of samples was analyzed by XRD, using a Phillips PW 1050 instrument, with Cu K␣1,2 radiation. Diffraction data were acquired over scattering angle 2 from 20◦ to 80◦ with a step of 0.050◦ and acquisition time of 1 s/step. The Raman spectra of surface coatings were recorded on a Thermo DXR Raman microscope, using the 532 nm laser excitation line and constant power of 10 mW. The resolution of all measured spectra was 4 cm−1 . 3. Results and discussion 3.1. Optical characterization and surface morphology The voltage-time responses during anodization of aluminum in 0.001 M WSiA at 25 mA/cm2 were highly reproducible (Fig. 1). Almost linear increase in voltage in the first stage characterizes the relatively uniform growth of a compact barrier oxide [24]. Uniform film thickening is terminated by dielectric breakdown, coupled with visible sparks evenly distributed over the surface. After breakdown the oxide coatings become laced with a number of cracks, pores and channels [8]. Real time imaging disclosed several stages of the PEO process (Fig. 2), with distinct microdischarge characteristics. First microdischarges are visible after about 1 min. During the PEO the size of microdischarges becomes larger, while the number of microdischarges is reduced. The results of image analysis of the microdischarges are shown in Fig. 3. Spatial density of microdischarges is the highest after about 1.5 min from the beginning of the PEO process (∼50 cm−2 ), than it reduces considerable during next 10 min and afterward stays almost constant (Fig. 3a). The percentage of oxide coating area, simultaneously covered by active discharge sites has maximum in the early stage of PEO process (first 10 min) and than decreases slowly with PEO time (Fig. 3b). The dimensional distribution of the microdischarges in the early stage of PEO process is shown in Fig. 4. The relatively small microdischarges (cross-sectional area <0.2 mm2 ) are dominant throughout the whole PEO process. Their concentration reaches about 92% of the total population at the beginning of PEO (Fig. 4a), and decreases to 48% after 10 min (Fig. 4d). Large microdischarges become noticeable only after extended PEO time. Microdischarges are generated by dielectric breakdown through weak sites in the oxide coating and the number of weak sites is reduced with increasing time of anodization, i.e. with increasing thickness of the coating [4]. The increased size and decreased spatial density of microdischarges during PEO is related to the reduced number of discharging sites through which higher anodic current is able to pass. The surface morphology evolution of the oxide coating formed on aluminum during the PEO process in WSiA is shown in Fig. 5. The number of discharge channels decreases, while their size increases during the period of microdischarge generation. In a thicker layer of oxide coating, higher energy is required for the cur-
Fig. 3. Microdischarge characteristics at various stages of PEO process: (a) microdischarge spatial density; (b) percentage of oxide coatings area simultaneously covered by active discharge sites.
rent to pass through the coating. Under this condition, the current is localized at weak points of the layer formed to find its way through the coating. This is the reason why the diameter of the discharge channel increases. Fig. 6 shows the influence of PEO treatment time on surface roughness of oxide coatings. It is clear that thicker coatings have higher surface roughness. In initial stage of PEO, discharge chan-
Fig. 4. Dimensional distribution of the microdischarges at various stages of PEO process: (a) 1.5 min; (b) 3 min; (c) 5 min; (d) 10 min.
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nels are well distributed and oxide coatings exhibit lower surface roughness. As the number of discharge channels decreases with time of PEO, non-uniformities in the oxide coatings appear causing an increase in surface roughness. Typical optical emission spectrum of microdischarges in the spectral range from 380 nm to 850 nm is presented in Fig. 7a, while detailed optical emission spectrum in the spectral range from 380 nm to 500 nm is presented in Fig. 7b. We used NIST’s atomic spectra database in order to attribute each peak to a characteristic atomic transition [25]. The species that are identified originate either from aluminum electrode or from the electrolyte. Two strong emission lines of Al I at 394.40 nm and 396.15 nm are observed. Also, we detected line of Al II at 466.31 nm. The atomic lines of
oxygen, hydrogen, tungsten and silicon from the electrolyte are detected, as well. The strongest lines belong to Balmer lines H␣ (656.28 nm) and H (486.13 nm) and O I at 844.62 nm, 844.64 nm and 844.68 nm. We also detected three lines of O I at 777.19 nm, 777.42 nm, 777.54 nm and many lines of O II and W I (Fig. 7). Only one emission line at 390.55 nm has been assigned to Si I. Notation I and II refers to neutral atoms and the single ionized atoms, respectively. Apart from atomic and ionic lines, strong AlO bands were also detected at 484.2 nm. The continuum emission between 380 nm and 850 nm results from radiative recombination of electrons at flaws in oxide coating (“flaw” is a general term for microfissures, cracks, local regions of different compositions and impurities, etc.) [26].
Fig. 5. AFM images of oxide coatings at various stages of PEO process: (a) 3 min; (b) 5 min; (c) 10 min; (d) 20 min; (e) 30 min; (f) 60 min.
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Table 1 Cross-sectional EDX analysis of oxide coatings in Fig. 8. Sample
Atomic (%) O
Al
W
S1
Spectrum 1 Spectrum 2 Spectrum 3
66.66 77.23 78.87
27.73 1.12 1.38
5.61 21.65 19.76
S2
Spectrum 1 Spectrum 2 Spectrum 3
69.08 76.83 75.47
20.90 0.16 0.56
10.02 23.01 23.98
oxide at the oxide/electrolyte interface. The main chemical reactions at the aluminum/oxide interface are [28]: → Al2 O3 + 6e− 2Al + 3O2− solid Fig. 6. The influence of PEO treatment time on surface roughness of oxide coatings.
3.2. Chemical and phase composition of PEO coatings PEO of aluminum is a complex process combining concurrent partial processes of oxide formation, dissolution and dielectric breakdown [27]. At the beginning of the anodization, oxide layer grows at the aluminum/oxide and oxide/electrolyte interfaces as a result of migration of O2− /OH− and Al3+ ions across the oxide assisted by a strong electric field (∼107 V/cm). Also, small amounts of anionic components of electrolyte are incorporated into the
Al →
Al3+ solid
+ 3e−
(1) (2)
while the reaction at the oxide/electrolyte interface is: + 2Al3+ solid + 9H2 O → Al2 O3 + 6H3 O
(3)
During the PEO process, anionic components of the electrolyte are drawn into the discharge channels. Concurrently, aluminum is melted out of the substrate, enters the channels, and gets oxidized. At the same time, separation of oppositely charged ions occurs in the channel due to the presence of the strong electric field. The cations are ejected from the channels into the electrolyte by electrostatic forces. In the next step, oxidized metal is ejected from the channels into the coating surface in contact with the electrolyte and in that way increases the coating thickness around the channels. Finally, discharge channels get cooled and the reaction products are deposited onto its walls. Under high temperature inside of discharge channels, the following thermal decomposition of 12-tungstosilicic acid can be proposed [29]: H4 SiW12 O40 → SiO2 + 12WO3 + 2H2 O
(4)
Due to the fact that aluminum is abundant in the discharges channels, WO3 reacts with Al by the following chemical reaction [30]: WO3 + 2Al → Al2 O3 + W
Fig. 7. Luminescence spectra recorded during PEO in the range: (a) 380–850 nm; (b) 380–500 nm.
(5)
Backscattered SEM micrographs of polished cross-section of PEO coatings processed for 20 min and 45 min are presented in Fig. 8. The PEO coating shows a typical microstructure with two distinct regions, i.e., thin, compact inner layer adjacent to aluminum substrate and porous outer layer. The results of EDX analysis of three different regions on cross-section surfaces are given in Table 1. First EDX spectrum is taken in inner layer of the coating (Spectrum 1), while the others are taken in the porous outer layer of coatings (Spectrum 2 and Spectrum 3). The main elements of the coating are Al, O and W. The contest of Al is the highest in inner layer due to oxidation of aluminum during PEO process at aluminum/coating interface. In contrast, content of W is higher in porous outer layer, implying that tungsten bronzes are predominantly formed in the outer layer of oxide coatings. Lines of Si have not been identified clearly in EDX spectra because they are overlapped by W lines. The XRD patterns of oxide coatings obtained after various PEO times are shown in Fig. 9. The coatings are partly crystallized and mainly composed of ␥-Al2 O3 and WO3 . Elemental Al mainly originates from the substrate and therefore the Al diffraction lines are so strong. The content of ␥-Al2 O3 is growing with increasing time of PEO process. During the PEO process, molten alumina flows out of the discharge channels contacting with surrounding low temperature electrolyte and rapidly solidifies at coating/electrolyte interface. The rapid solidification of alumina favors the formation of ␥-Al2 O3 [31].
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Fig. 9. XRD patterns of oxide coatings at various stages of PEO process: (a) 5 min; (b) 10 min; (c) 20 min; (d) 30 min. Designation of reflections: (1) Al; (2) WO3 ; (3) ␥-Al2 O3 ; (4) W.
Fig. 10. The influence of PEO treatment time on microhardness of oxide coatings.
Fig. 8. Backscattered SEM micrographs of polished cross-section of oxide coatings obtained by PEO process for: (a) 20 min; (b) 45 min.
The influence of PEO treatment time on microhardness of obtained oxide coatings is shown in Fig. 10. Aluminum substrate has microhardness of about 0.12 GPa which increased to about 1.7 GPa within the first 5 min of PEO and then exhibits a decreasing trend. This may be due to phase composition change, i.e. increasing fraction of ␥-Al2 O3 in the oxide coating with prolonged PEO time (Fig. 9), and/or due to increased of porosity and surface roughness of oxide coatings (Fig. 5). Raman spectroscopy of oxide coatings is a practical method for identifying covalent bonds which can confirm the presence of silicate tungsten bronze [32]. Raman spectra of oxide coatings obtained after 5 min (Fig. 11b) and 30 min (Fig. 11c) are compared with a reference Raman spectrum of silicate tungsten bronze obtained by thermal treatment of crystalline WSiA at 550 ◦ C (Fig. 11a). Bands at about 801 cm−1 , 685 cm−1 , 268 cm−1 and 108 cm−1 ascribed to WO vibrations from bronzes are evident. This suggests that the composition of the outer layer of oxide coating obtained by PEO on aluminum in WSiA is identical to that obtained by thermal treatment of WSiA. At the same time, Raman spectra of silicate tungsten bronze are identical to those obtained from WPA as a precursor. Therefore, obtained bronzes on aluminum are ReO3 type with characteristic bands at about 805 cm−1 and 706 cm−1 , assigned to W–O–W stretching, and at 273 cm−1 , assigned to W–O–W bending vibrations [13]. The other
band located at about 1000 cm−1 is also present in the spectra and it can be assigned to Si–O stretching vibrations [32]. 4. Conclusions The microdischarge characteristics during PEO process of aluminum in WSiA have been studied using real time imaging and optical emission spectroscopy. It is shown that size of microdis-
Fig. 11. Raman spectra of silicate tungsten bronzes: (a) referent spectrum of thermal treated crystalline WSiA on 550 ◦ C; (b) spectrum of oxide coating obtained by PEO for 5 min; (c) spectrum of oxide coating obtained by PEO for 30 min.
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charges becomes larger, while the number of microdischarges is reduced, with increasing time of PEO. Spatial density of microdischarges is the highest in the early stage of the PEO process. The percentage of oxide coatings area simultaneously covered by active discharge sites decreases slowly with extended PEO time. Optical emission spectrum of microdischarges has several intensive spectral peaks caused by electronic transition in Al, W, O, H and Si. The oxide coatings morphology is strongly dependent on PEO time. During the PEO of aluminum density of microdischarge channels decreases while their diameter increases, resulting in increased roughness of the oxide coating. The elemental components of PEO coatings are Al, W and O. The oxide coating is partly crystallized and mainly composed of ␥-Al2 O3 and WO3 . Raman spectroscopy results have shown that outer layer of oxide coatings formed during the PEO process are silicate tungsten bronzes. Composition of oxide coatings formed by PEO is identical with silicate tungsten bronzes obtained during thermal treatment of solid WSiA. Therefore, PEO of aluminum in heteropoly acids can be used as a viable method for obtaining silicate tungsten bronzes. Acknowledgement The Ministry of Science of Republic of Serbia is gratefully acknowledged for the financial support of the projects ON 171035 and III 45014. References [1] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Surf. Coat. Technol. 122 (1999) 73–93. [2] Z. Yao, Y. Jiang, F. Jia, Z. Jiang, F. Wang, Appl. Surf. Sci. 254 (2008) 4084–4091. [3] W. Zhang, K. Du, C. Yan, F. Wang, Appl. Surf. Sci. 254 (2008) 5216–5223. [4] S. Moon, Y. Jeong, Corros. Sci. 51 (2009) 1506–1512. [5] P.B. Srinivasan, C. Blawert, W. Dietzel, Corros. Sci. 50 (2008) 2415–2418. [6] J. Liang, L. Hu, J. Hao, Appl. Surf. Sci. 253 (2007) 4490–4496.
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[7] A. Ghasemi, V.S. Raja, C. Blavert, W. Dietzel, K.U. Kainer, Surf. Coat. Technol. 204 (2010) 1469–1478. [8] S. Stojadinovic, R. Vasilic, I. Belca, M. Petkovic, B. Kasalica, Z. Nedic, Lj. Zekovic, Corros. Sci. 52 (2010) 3258–3265. ´ Z. Nedic, ´ Materials 3 (2010) 110–126. [9] U.B. Mioˇc, S. Stojadinovic, ´ M. Davidovic, ´ Ph. Colomban, I. Holclajtner[10] U.B. Mioˇc, M.R. Todorovic, Antunovic, Solid State Ionics 176 (2005) 3005–3017. [11] U.B. Mioˇc, M. Davidovic´ (Eds.), Fast Proton-Ion Transport Compounds, Transworld Research Network, Kerala, India, 2010. ´ M. Davidovic, ´ Z. Nedic, ´ M. Mitrovic, ´ Ph. Colomban, J. [12] U.B. Mioˇc, R. Dimitrijevic, Mater. Sci. 29 (1994) 3705–3718. ´ M.M. Mitrovic, ´ Z.P. Nedic, ´ J. Serb. Chem. Soc. 60 [13] U.B. Mioˇc, R.Zˇ . Dimitrijevic, (1995) 959–967. [14] Z. Kang, E. Wang, M. Jiang, S. Lian, Nanotechnology 15 (2004) 55–58. [15] N. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordinated Compounds, third ed., Wiley, New York, USA, 1978. [16] V.S. Rudnev, I.V. Lukiyanchuk, V.V. Kon’shin, P.S. Gordienko, Rus. J. Appl. Chem. 75 (2002) 1082–1086. [17] V.S. Rudnev, Prot. Met. 44 (2008) 263–272. [18] D.G. Barton, S.L. Soled, E. Iglesia, Top. Catal. 6 (1998) 87–99. [19] M.R. Bayati, H. Zargar, R. Molaei, F. Golestani-Fard, E. Kajbafvala, S. Zanganeh, Colloids Surf. A 355 (2010) 187–192. [20] C.L. Hill (Ed.), Polyoxometalates Chem. Rev. 98 (1998) 1–390. [21] M.T. Pope, A. Müller, Polyoxometalate Chemistry from Topology via SelfAssembly to Applications, Kluwer Academic Publishing, Dordrecht, The Netherlands, 2001. [22] S. Stojadinovic, I. Belca, M. Tadic, B. Kasalica, Z. Nedic, Lj Zekovic, J. Electroanal. Chem. 619–620 (2008) 125–130. [23] B.V. Kasalica, I.D. Belca, S.Dj. Stojadinovic, Lj.D. Zekovic, D. Nikolic, Appl. Spectrosc. 60 (2006) 1090–1094. [24] A. Despic, V. Parkhutik, Mod. Aspects Electrochem. 20 (1989) 401–503. [25] J.R. Fuhr, W.L. Wiese, NIST atomic transition probability tables, in: CRC Handbook of Chemistry and Physics, CRC Press, Boca Raton, 1996. [26] S. Stojadinovic, R. Vasilic, M. Petkovic, Z. Nedic, B. Kasalica, I. Belca, Lj Zekovic, Electrochim. Acta 55 (2010) 3857–3863. [27] G. Sundararajan, L. Rama Krishna, Surf. Coat. Technol. 167 (2003) 269–277. [28] L.O. Snizhko, A.L. Yerokhin, A. Pilkington, N.L. Gurevina, D.O. Misnyankin, A. Leyland, A. Matthews, Electrochim. Acta 49 (2004) 2085–2095. [29] M. Klisch, J. Sol–Gel Sci. Technol. 12 (1998) 21–33. [30] H.Y. Zheng, Y.K. Wang, B.S. Li, G.R. Han, Mater. Lett. 59 (2005) 139–142. [31] R. McPherson, J. Mater. Sci. 8 (1973) 851–858. [32] J.A. Horsley, I.E. Wachs, J.M. Brown, G.H. Via, F.D. Hardcastle, J. Phys. Chem. 91 (1987) 4014–4020.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Preparation of silicate tungsten bronzes on aluminum by plasma electrolytic oxidation process in 12-tungstosilicic acid M. Petkovic´ a , S. Stojadinovic´ a,∗ , R. Vasilic´ b , I. Belˇca a , Z. Nedic´ c , B. Kasalica a , U.B. Mioˇc c a b c
Faculty of Physics, University of Belgrade, Studentski trg 12-16, 11000 Belgrade, Serbia Faculty of Environmental Governance and Corporate Responsibility, Educons University, Vojvode Putnika bb, Sremska Kamenica, Serbia Faculty of Physical Chemistry, University of Belgrade, Studentski trg 12-16, 11000 Belgrade, Serbia
a r t i c l e
i n f o
Article history: Received 5 April 2011 Received in revised form 31 May 2011 Accepted 10 June 2011 Available online 17 June 2011 Keywords: Plasma electrolytic oxidation (PEO) 12-Tungstosilicic acid Silicate tungsten bronzes
a b s t r a c t The growth of silicate tungsten bronzes on aluminum by plasma electrolytic oxidation in 12-tungstosilicic acid is experimentally investigated and discussed. Real time imaging and optical emission spectroscopy characterization of plasma electrolytic oxidation show that spatial density of microdischarges is the highest in the early stage of the process, while the percentage of oxide coating area covered by active discharge sites decreases slowly with time. Emission spectrum of microdischarges has several intensive band peaks originating either from aluminum electrode or from the electrolyte. Surface roughness of obtained oxide coatings increases with prolonged time of plasma electrolytic oxidation, as their microhardness decreases. Raman spectroscopy and energy dispersive X-ray spectroscopy are employed to confirm that the outer layer of oxide coatings formed during the plasma electrolytic oxidation process is silicate tungsten bronzes. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Plasma electrolytic oxidation (PEO), also called microarc oxidation or anodic spark deposition, is a high-voltage anodizing process in which the surface of lightweight metals (aluminum, titanium, magnesium, zirconium, etc.) or metallic alloys can be used for depositing thick, dense, and hard oxide coatings by plasma discharge in suitable electrolytes [1–3]. This process involves anodizing above the dielectric breakdown voltage where numerous transient fine sparks are produced continuously over the coating surface, accompanied by gas evolution. Plasma-chemical reaction is induced at the discharge sites due to increased local temperature modifying the structure, composition, and morphology of such oxide coatings. The composition and concentration of electrolyte play a crucial role in obtaining the desired oxide coatings by PEO. Most popular electrolytes used for such coatings are water solutions of sodium hydroxide [4], sodium silicate [5], sodium phosphate [6], sodium aluminates [7], sodium tungstate [8] and some iso- and heteropoly compounds [9]. Oxide coatings formed by PEO process in aforementioned electrolytes have excellent adhesion with the substrate, electrical and thermal insulation, high microhardness, good wear and corrosion resistance, etc. In this work, we have chosen water solution of 12-tungstosilicic acid (H4 SiW12 O40 , WSiA) to obtain oxide coatings – the silicate
∗ Corresponding author. Tel.: +381 11 2630152; fax: +381 11 3282619. ´ E-mail address: [email protected] (S. Stojadinovic). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.06.062
tungsten bronzes on aluminum by PEO process. In recent years heteropoly acids have received increasing attention because of their chemical, structural and electronic versatility. Heteropoly acids are regarded as active solids and viable candidates for electrochemical devices, fuel cells, batteries [10–14], and as materials that can be transformed into nanometer-sized blocks [15]. Good corrosion resistance combined with catalytic, conducting or semiconducting properties render tungsten bronzes as highly desirable materials [16–19]. WSiA is one of the three best known heteropoly acids with Keggin anion structure [10,20,21]. Heteropoly compounds with Keggin anion have low temperature structural phase transition to bronzes at about 600 ◦ C corresponding to solid–solid recrystallization of heteropoly compounds to bronzes [10–12]. This structural phase transition for WSiA is at about 535 ◦ C [11]. Having in mind that 12tungstophosphoric, 12-molybdophosphoric and 12-tungstosilicic acid are iso-structural, it is possible to conclude that starting with WSiA as a precursor, in the process of PEO silicate bronzes will be obtained. The aim of this work is to investigate PEO process of aluminum in WSiA and possibly to develop an innovative procedure for fabrication of silicate tungsten bronzes on aluminum. Real time imaging and optical emission spectroscopy are used to study discharge characteristics during PEO. Physicochemical methods: atomic force microscopy (AFM), scanning electron microscopy (SEM-EDX), Xray diffraction (XRD) and Raman spectroscopy served as tools for examining surface morphology, mechanical properties and chemical and phase composition of obtained oxide coatings.
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Fig. 1. Voltage of anodization-time dependence during galvanostatic anodization of aluminum in 0.001 M H4 SiW12 O40 .
2. Experimental details In the experiment, oxide coatings were formed on cold-rolled aluminum samples (99.99% purity). The surface of aluminum was degreased in ethanol using ultrasonic cleaner and dried in a stream
of warm air. The oxidation process was carried out in an electrolytic cell with flat glass windows [22]. Platinum wires were used as cathodes. For anodization of aluminum we used water solution of 0.001 M H4 SiW12 O40 . The electrolyte was prepared using double distilled and deionized water and PA (pro analysis) grade chemical compound. Anodizing was carried out at current density of 25 mA/cm2 . During the anodization, electrolyte circulated through the chamber–reservoir system. The temperature of the electrolyte was kept at (19 ± 1) ◦ C during the anodization process. After the anodization, samples were rinsed in distilled water to prevent additional deposition of electrolyte components during drying. Spectral characterization of PEO was performed utilizing a spectrograph system based on the Intensified Charge Coupled Device (ICCD) camera intended for time-resolved measuring of very weak light intensity in a wide range of wavelengths [23]. Optical detection system consisted of a large–aperture achromatic lens, a Hilger spectrograph with diffraction grating of 1200 grooves/mm (wavelength range of 43 nm) and a very sensitive PI-MAX ICCD camera with high quantum efficiency manufactured by Princeton Instruments. The system was used with several grating positions with over-lapping wavelength range of 10 nm. Real time images during the PEO were recorded utilizing a video camera (Sony DCR-DVD110, 800 K pixels CCD, 40× optical zoom and 40 mm lens filter). The information obtained was split into sep-
Fig. 2. Microdischarges appearance at various stages of PEO process: (a) 1.5 min; (b) 3 min; (c) 5 min; (d) 15 min; (e) 30 min; (f) 60 min.
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arate frames and the images of individual frames were processed using our custom made software, which automatically counted microdischarges in selected frames, determined spatial density, and dimensional distribution of the microdischarges. The morphology and mechanical properties of surface coatings (roughness and microhardness) were characterized using an atomic force microscope (AFM; Veeco Instruments, model Dimension V). Roughness and microhardness (ratio of applied indentation force and 2D imprinted area) data were obtained using diNanoScope software (version 7.0), and reported values are calculated mean values for a number of different samples obtained under the same anodizing conditions. Scanning electron microscope (SEM) JEOL 840A equipped with energy dispersive X-ray spectroscopy (EDX) was used to characterize chemical composition of formed surface coatings. The crystallinity of samples was analyzed by XRD, using a Phillips PW 1050 instrument, with Cu K␣1,2 radiation. Diffraction data were acquired over scattering angle 2 from 20◦ to 80◦ with a step of 0.050◦ and acquisition time of 1 s/step. The Raman spectra of surface coatings were recorded on a Thermo DXR Raman microscope, using the 532 nm laser excitation line and constant power of 10 mW. The resolution of all measured spectra was 4 cm−1 . 3. Results and discussion 3.1. Optical characterization and surface morphology The voltage-time responses during anodization of aluminum in 0.001 M WSiA at 25 mA/cm2 were highly reproducible (Fig. 1). Almost linear increase in voltage in the first stage characterizes the relatively uniform growth of a compact barrier oxide [24]. Uniform film thickening is terminated by dielectric breakdown, coupled with visible sparks evenly distributed over the surface. After breakdown the oxide coatings become laced with a number of cracks, pores and channels [8]. Real time imaging disclosed several stages of the PEO process (Fig. 2), with distinct microdischarge characteristics. First microdischarges are visible after about 1 min. During the PEO the size of microdischarges becomes larger, while the number of microdischarges is reduced. The results of image analysis of the microdischarges are shown in Fig. 3. Spatial density of microdischarges is the highest after about 1.5 min from the beginning of the PEO process (∼50 cm−2 ), than it reduces considerable during next 10 min and afterward stays almost constant (Fig. 3a). The percentage of oxide coating area, simultaneously covered by active discharge sites has maximum in the early stage of PEO process (first 10 min) and than decreases slowly with PEO time (Fig. 3b). The dimensional distribution of the microdischarges in the early stage of PEO process is shown in Fig. 4. The relatively small microdischarges (cross-sectional area <0.2 mm2 ) are dominant throughout the whole PEO process. Their concentration reaches about 92% of the total population at the beginning of PEO (Fig. 4a), and decreases to 48% after 10 min (Fig. 4d). Large microdischarges become noticeable only after extended PEO time. Microdischarges are generated by dielectric breakdown through weak sites in the oxide coating and the number of weak sites is reduced with increasing time of anodization, i.e. with increasing thickness of the coating [4]. The increased size and decreased spatial density of microdischarges during PEO is related to the reduced number of discharging sites through which higher anodic current is able to pass. The surface morphology evolution of the oxide coating formed on aluminum during the PEO process in WSiA is shown in Fig. 5. The number of discharge channels decreases, while their size increases during the period of microdischarge generation. In a thicker layer of oxide coating, higher energy is required for the cur-
Fig. 3. Microdischarge characteristics at various stages of PEO process: (a) microdischarge spatial density; (b) percentage of oxide coatings area simultaneously covered by active discharge sites.
rent to pass through the coating. Under this condition, the current is localized at weak points of the layer formed to find its way through the coating. This is the reason why the diameter of the discharge channel increases. Fig. 6 shows the influence of PEO treatment time on surface roughness of oxide coatings. It is clear that thicker coatings have higher surface roughness. In initial stage of PEO, discharge chan-
Fig. 4. Dimensional distribution of the microdischarges at various stages of PEO process: (a) 1.5 min; (b) 3 min; (c) 5 min; (d) 10 min.
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nels are well distributed and oxide coatings exhibit lower surface roughness. As the number of discharge channels decreases with time of PEO, non-uniformities in the oxide coatings appear causing an increase in surface roughness. Typical optical emission spectrum of microdischarges in the spectral range from 380 nm to 850 nm is presented in Fig. 7a, while detailed optical emission spectrum in the spectral range from 380 nm to 500 nm is presented in Fig. 7b. We used NIST’s atomic spectra database in order to attribute each peak to a characteristic atomic transition [25]. The species that are identified originate either from aluminum electrode or from the electrolyte. Two strong emission lines of Al I at 394.40 nm and 396.15 nm are observed. Also, we detected line of Al II at 466.31 nm. The atomic lines of
oxygen, hydrogen, tungsten and silicon from the electrolyte are detected, as well. The strongest lines belong to Balmer lines H␣ (656.28 nm) and H (486.13 nm) and O I at 844.62 nm, 844.64 nm and 844.68 nm. We also detected three lines of O I at 777.19 nm, 777.42 nm, 777.54 nm and many lines of O II and W I (Fig. 7). Only one emission line at 390.55 nm has been assigned to Si I. Notation I and II refers to neutral atoms and the single ionized atoms, respectively. Apart from atomic and ionic lines, strong AlO bands were also detected at 484.2 nm. The continuum emission between 380 nm and 850 nm results from radiative recombination of electrons at flaws in oxide coating (“flaw” is a general term for microfissures, cracks, local regions of different compositions and impurities, etc.) [26].
Fig. 5. AFM images of oxide coatings at various stages of PEO process: (a) 3 min; (b) 5 min; (c) 10 min; (d) 20 min; (e) 30 min; (f) 60 min.
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Table 1 Cross-sectional EDX analysis of oxide coatings in Fig. 8. Sample
Atomic (%) O
Al
W
S1
Spectrum 1 Spectrum 2 Spectrum 3
66.66 77.23 78.87
27.73 1.12 1.38
5.61 21.65 19.76
S2
Spectrum 1 Spectrum 2 Spectrum 3
69.08 76.83 75.47
20.90 0.16 0.56
10.02 23.01 23.98
oxide at the oxide/electrolyte interface. The main chemical reactions at the aluminum/oxide interface are [28]: → Al2 O3 + 6e− 2Al + 3O2− solid Fig. 6. The influence of PEO treatment time on surface roughness of oxide coatings.
3.2. Chemical and phase composition of PEO coatings PEO of aluminum is a complex process combining concurrent partial processes of oxide formation, dissolution and dielectric breakdown [27]. At the beginning of the anodization, oxide layer grows at the aluminum/oxide and oxide/electrolyte interfaces as a result of migration of O2− /OH− and Al3+ ions across the oxide assisted by a strong electric field (∼107 V/cm). Also, small amounts of anionic components of electrolyte are incorporated into the
Al →
Al3+ solid
+ 3e−
(1) (2)
while the reaction at the oxide/electrolyte interface is: + 2Al3+ solid + 9H2 O → Al2 O3 + 6H3 O
(3)
During the PEO process, anionic components of the electrolyte are drawn into the discharge channels. Concurrently, aluminum is melted out of the substrate, enters the channels, and gets oxidized. At the same time, separation of oppositely charged ions occurs in the channel due to the presence of the strong electric field. The cations are ejected from the channels into the electrolyte by electrostatic forces. In the next step, oxidized metal is ejected from the channels into the coating surface in contact with the electrolyte and in that way increases the coating thickness around the channels. Finally, discharge channels get cooled and the reaction products are deposited onto its walls. Under high temperature inside of discharge channels, the following thermal decomposition of 12-tungstosilicic acid can be proposed [29]: H4 SiW12 O40 → SiO2 + 12WO3 + 2H2 O
(4)
Due to the fact that aluminum is abundant in the discharges channels, WO3 reacts with Al by the following chemical reaction [30]: WO3 + 2Al → Al2 O3 + W
Fig. 7. Luminescence spectra recorded during PEO in the range: (a) 380–850 nm; (b) 380–500 nm.
(5)
Backscattered SEM micrographs of polished cross-section of PEO coatings processed for 20 min and 45 min are presented in Fig. 8. The PEO coating shows a typical microstructure with two distinct regions, i.e., thin, compact inner layer adjacent to aluminum substrate and porous outer layer. The results of EDX analysis of three different regions on cross-section surfaces are given in Table 1. First EDX spectrum is taken in inner layer of the coating (Spectrum 1), while the others are taken in the porous outer layer of coatings (Spectrum 2 and Spectrum 3). The main elements of the coating are Al, O and W. The contest of Al is the highest in inner layer due to oxidation of aluminum during PEO process at aluminum/coating interface. In contrast, content of W is higher in porous outer layer, implying that tungsten bronzes are predominantly formed in the outer layer of oxide coatings. Lines of Si have not been identified clearly in EDX spectra because they are overlapped by W lines. The XRD patterns of oxide coatings obtained after various PEO times are shown in Fig. 9. The coatings are partly crystallized and mainly composed of ␥-Al2 O3 and WO3 . Elemental Al mainly originates from the substrate and therefore the Al diffraction lines are so strong. The content of ␥-Al2 O3 is growing with increasing time of PEO process. During the PEO process, molten alumina flows out of the discharge channels contacting with surrounding low temperature electrolyte and rapidly solidifies at coating/electrolyte interface. The rapid solidification of alumina favors the formation of ␥-Al2 O3 [31].
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Fig. 9. XRD patterns of oxide coatings at various stages of PEO process: (a) 5 min; (b) 10 min; (c) 20 min; (d) 30 min. Designation of reflections: (1) Al; (2) WO3 ; (3) ␥-Al2 O3 ; (4) W.
Fig. 10. The influence of PEO treatment time on microhardness of oxide coatings.
Fig. 8. Backscattered SEM micrographs of polished cross-section of oxide coatings obtained by PEO process for: (a) 20 min; (b) 45 min.
The influence of PEO treatment time on microhardness of obtained oxide coatings is shown in Fig. 10. Aluminum substrate has microhardness of about 0.12 GPa which increased to about 1.7 GPa within the first 5 min of PEO and then exhibits a decreasing trend. This may be due to phase composition change, i.e. increasing fraction of ␥-Al2 O3 in the oxide coating with prolonged PEO time (Fig. 9), and/or due to increased of porosity and surface roughness of oxide coatings (Fig. 5). Raman spectroscopy of oxide coatings is a practical method for identifying covalent bonds which can confirm the presence of silicate tungsten bronze [32]. Raman spectra of oxide coatings obtained after 5 min (Fig. 11b) and 30 min (Fig. 11c) are compared with a reference Raman spectrum of silicate tungsten bronze obtained by thermal treatment of crystalline WSiA at 550 ◦ C (Fig. 11a). Bands at about 801 cm−1 , 685 cm−1 , 268 cm−1 and 108 cm−1 ascribed to WO vibrations from bronzes are evident. This suggests that the composition of the outer layer of oxide coating obtained by PEO on aluminum in WSiA is identical to that obtained by thermal treatment of WSiA. At the same time, Raman spectra of silicate tungsten bronze are identical to those obtained from WPA as a precursor. Therefore, obtained bronzes on aluminum are ReO3 type with characteristic bands at about 805 cm−1 and 706 cm−1 , assigned to W–O–W stretching, and at 273 cm−1 , assigned to W–O–W bending vibrations [13]. The other
band located at about 1000 cm−1 is also present in the spectra and it can be assigned to Si–O stretching vibrations [32]. 4. Conclusions The microdischarge characteristics during PEO process of aluminum in WSiA have been studied using real time imaging and optical emission spectroscopy. It is shown that size of microdis-
Fig. 11. Raman spectra of silicate tungsten bronzes: (a) referent spectrum of thermal treated crystalline WSiA on 550 ◦ C; (b) spectrum of oxide coating obtained by PEO for 5 min; (c) spectrum of oxide coating obtained by PEO for 30 min.
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charges becomes larger, while the number of microdischarges is reduced, with increasing time of PEO. Spatial density of microdischarges is the highest in the early stage of the PEO process. The percentage of oxide coatings area simultaneously covered by active discharge sites decreases slowly with extended PEO time. Optical emission spectrum of microdischarges has several intensive spectral peaks caused by electronic transition in Al, W, O, H and Si. The oxide coatings morphology is strongly dependent on PEO time. During the PEO of aluminum density of microdischarge channels decreases while their diameter increases, resulting in increased roughness of the oxide coating. The elemental components of PEO coatings are Al, W and O. The oxide coating is partly crystallized and mainly composed of ␥-Al2 O3 and WO3 . Raman spectroscopy results have shown that outer layer of oxide coatings formed during the PEO process are silicate tungsten bronzes. Composition of oxide coatings formed by PEO is identical with silicate tungsten bronzes obtained during thermal treatment of solid WSiA. Therefore, PEO of aluminum in heteropoly acids can be used as a viable method for obtaining silicate tungsten bronzes. Acknowledgement The Ministry of Science of Republic of Serbia is gratefully acknowledged for the financial support of the projects ON 171035 and III 45014. References [1] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Surf. Coat. Technol. 122 (1999) 73–93. [2] Z. Yao, Y. Jiang, F. Jia, Z. Jiang, F. Wang, Appl. Surf. Sci. 254 (2008) 4084–4091. [3] W. Zhang, K. Du, C. Yan, F. Wang, Appl. Surf. Sci. 254 (2008) 5216–5223. [4] S. Moon, Y. Jeong, Corros. Sci. 51 (2009) 1506–1512. [5] P.B. Srinivasan, C. Blawert, W. Dietzel, Corros. Sci. 50 (2008) 2415–2418. [6] J. Liang, L. Hu, J. Hao, Appl. Surf. Sci. 253 (2007) 4490–4496.
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