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Fabrication of ZrO2–Al2O3 hybrid nano-porous layers through micro arc oxidation process
Materials Letters 65 (2011) 1835–1838
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Fabrication of ZrO2–Al2O3 hybrid nano-porous layers through micro arc oxidation process V. Shoaei-Rad a, M.R. Bayati a,b,⁎, F. Golestani-Fard a,c, H.R. Zargar d, J. Javadpour a a
School of Metallurgy and Materials Engineering, Iran University of Science and Technology, P.O. Box: 16845-195, Tehran, Iran Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695-7907, USA Center of Excellence for Advanced Materials, Iran University of Science and Technology, P.O. Box: 16845-195, Tehran, Iran d Department of Metals and Materials Engineering, University of British Columbia, Vancouver, BC V6T 1Z4, Canada b c
a r t i c l e
i n f o
Article history: Received 2 December 2010 Accepted 23 March 2011 Available online 29 March 2011 Keywords: Micro arc oxidation Alumina Zirconia Composite materials Porous materials
a b s t r a c t Zirconia-alumina layers, with a pores size of 40–300 nm, were fabricated via micro arc oxidation method for the first time. The layers were grown under alternative current in the electrolytes of ZrOCl2 salt. They consisted of α-Al2O3, γ-Al2O3, monoclinic ZrO2, tetragonal ZrO2. Increasing the voltage resulted in higher zirconium concentrations in the layers. A porous structure was obtained when the layers were grown under intermediate voltages. Microcracks were observed to appear when the applied voltage increased. Finally, a formation mechanism was proposed with emphasis on the chemical and the electrochemical foundations. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Although aluminum is one of the most attractive materials, its disadvantages namely softness, low wear resistance and high friction coefficient have limited is practical applications. An effective approach to defeat these disadvantages is coating the aluminum components with alumina-based composites coatings [1]. Al2O3-based composite coatings are desirable for different applications such as anti-erosion, anticorrosion, and thermal barrier systems [1–3]. Among the aluminabased hybrid coatings, the Al2O3–ZrO2 system has demonstrated unique features as an anti-wear coating. Zirconia is an interesting material in fundamental studies due to its remarkable properties namely good chemical and dimensional stability, high melting point, low thermal conductivity, and high wear resistance. Engine components, cutting tools, thermal barrier or wear-resistance coatings, and biomedical implants are some industries where it is applied [4,5]. Micro arc oxidation is a relatively new and effective method of fabricating thick, hard, and well-adhered ceramic layers with excellent wear resistant properties on light alloys especially on aluminum. It is one of the most efficient and simplest methods to grow alumina layers, based on the modification of the growing anodic film by spark arc micro discharges. More details of the MAO process can be found elsewhere [6–9].
MAO is a promising method which enables us to fabricated hybrid high quality ceramic coatings [10–19]. To the best of our knowledge, this is the first time that ZrO2–Al2O3 layers are fabricated via MAO process on aluminum substrates, even though they have been synthesized by other techniques. 2. Experimental Typical arrangement of the experimental setup is illustrated in our previous works [18]. 1 cm× 3 cm× 0.5 cm aluminum pieces were used as substrate surrounded by an ASTM 316 cylindrical cathode. ZrOCl2.8 H2O (5 g∙l− 1, Merck) solutions were utilized as electrolyte. Prior to MAO treatment, substrates were ultrasonically cleaned in acetone for 15 min and washed by distilled water. Samples were fabricated under alternative current and voltages of 50–200 V with +50 V intervals for 3 min. Surface morphology of the layers was evaluated by scanning electron microscopy (TESCAN, Vega II). Furthermore, X-ray diffraction (Philips, PW3710), energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy (VG Microtech, Twin anode, XR3E2 X-ray source, using Al Kα = 1486.6 eV) techniques were used in order to study phase structure and chemical composition of the synthesized layers. The XPS peak fitting procedure was performed by SDP software ver. 4.1. 3. Results and discussion
⁎ Corresponding author at: Tel.: +1 919 917 6962; fax: +1 919 515 7724. E-mail address: [email protected] (M.R. Bayati). URL: http://rrg.iust.ac.ir. 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.03.086
Fig. 1 illustrates the surface top-view of the layers grown under different voltages. No pore formed in the layers fabricated with applying the voltage of 50 V. The reason is that the electron avalanches, which are
1836
V. Shoaei-Rad et al. / Materials Letters 65 (2011) 1835–1838
Fig. 1. SEM top-view of the layers grown under different voltages: a) 50, b) 100, c) 150, and d) 200 V.
responsible for pores formation, take place at the voltages higher than the breakdown potential of the oxygen gas layer and the surface oxide layer. No spark occurred at such a low voltage and, thus, no pore formed in the structure of this sample. However, electric sparks appeared on the surface of the substrate when the voltage increased to 100 V. This phenomenon which is the reason for formation of the structural pores was also observed and reported in our other work [18]. Meanwhile, the pores size increases with the voltage because of the stronger electron avalanches which form at higher voltages.
No pore is observed in the layer synthesized under the voltage of 200 V whose morphology is depicted in Fig. 1d. Although the electron avalanches occur at this voltage, another mechanism was prevailing. The electrical current of the cell increases with the voltage. The growing layer is sintered due to the heat generated by the high electric current which passes through the cell. It has been suggested that the temperature locally surges to about 103–104 K at the avalanche regions which is high enough to melt the growing layer and the substrate. Because of the melting and sintering phenomena, the pores vanished.
Fig. 2. XRD patterns of the ZrO2–Al2O3 layers grown under different voltages.
V. Shoaei-Rad et al. / Materials Letters 65 (2011) 1835–1838
Fig. 3. EDX zirconium wt% for different voltage.
Some microcracks are also seen in the surface morphology of the layers synthesized under the voltages of 150 and 200 V which were originated from thermal stresses. The layer is melted due to the electron avalanches and quickly solidifies in the electrolyte when the spark disappears. These frequent and fast meltings and solidifications cause the thermal stresses. XRD patterns of the layers are exhibited in Fig. 2. Formation of alumina, zirconia, and some minor phases are observed. The most important feature of the XRD patterns is formation of the stabilized zirconia without any stabilizer agent such as yttria. The reason is the severe conditions during the MAO growth namely very high temperatures and the strong electric field. Of course, alumina can act as a stabilizer agent. Zirconium concentration was determined by EDX technique (Fig. 3). It is obvious that zirconium concentration increases with the voltage. Applying higher voltages intensifies the electric field between the cell poles and, thus, more ZrO2+ ions are drawn through the substrate to participate in the reactions. It is worthy to note that the ZrO2+ cations originated from ionization of zirconium oxide chloride salt in the aqueous solutions. XPS technique was employed to confirm stoichiometry of the layer fabricated under the voltage of 150 V. The XPS results were referenced to the C(1s) core level at the binding energy of 285.0 eV. Fig. 4a illustrates the Al(2p) core level at the binding energy of 74.8 eV confirming formation of Al2O3. The Zr(3d) core level binding energy is shown in Fig. 4b. This peak has 2 components which are attributed to its spin orbit
1837
splitting, i.e. Zr(3d3/2) and Zr(3d5/2). The peaks A and B, at the binding energies of 180.3 and 181.5 eV, reveal existence of Zr2+ and formation of ZrO and probably some other non-stoichiometric zirconium oxides (ZrxOy). Formation of the ZrO2 compound is confirmed by the peaks C and D which are respectively located at the binding energies of 182.4 and 184.3 eV. The peaks E and F with the binding energies of 185.5 and 186.5 eV can be assigned to the zirconium in the ZrOCl2 compound originated from the electrolyte trapping in the surface pores. These values are in a good agreement with those reported by other researchers [20]. The O(1s) core level binding energy is exhibited in Fig. 4c which can be deconvoluted to 4 distinct peaks representing that there are 4 different O-bindings in the layer. The peak A, located at the binding energy of 530.3 eV, is assigned to the oxygen in the ZrO2. The peak B, with a binding energy of 531.8 eV, corresponds to the oxygen in aluminum oxide. Thus, formation of a ZrO2–Al2O3 hybrid layer is confirmed. The peak C at the binding energy of 533.1 eV represents the ―OH groups on the surface. The D, located at the binding energies of 534.3 eV, reveal existence of oxygen in the water molecules. Since the layers were porous and fabricated in aqueous solutions, water may be trapped in the pores. Based on the chemical and electrochemical bases, the following formation mechanism is proposed. Since the samples were fabricated with applying alternative current, a number of parallel reactions take place under cathodic and anodic conditions on the vicinity of the substrate. During the anodic cycle, the following reactions have been proposed for formation of Al2O3 on aluminum substrates via plasma electrolytic processes in acidic electrolytes [21]: −
þ
2½AlðOHÞ4 þ 2H þ ðn−5ÞH2 O→Al2 O3 ·nH2 O −
2Al þ 9H2 O→Al2 O3 þ 6H3 O þ 6e
ð1Þ ð2Þ
When the above anodic reactions are halted, the ZrO2+ cations move toward the substrate due to the electric field between anode and cathode to participate in the following reaction: ZrO
2þ
þ
þ 3H2 O→ZrO2 þ 2H3 O
ð3Þ
4. Conclusions ZrO2–Al2O3 hybrid films were fabricated via micro arc oxidation of aluminum. The layers consisted of α-Al2O3, γ-Al2O3, monoclinic ZrO2, tetragonal ZrO2. Applying higher voltages resulted inincreasing zirconium concentrations and formation of some microcracks. A porous structure was observed when the layers were grown under intermediate
Fig. 4. XPS binding energies: a) Al(2p), b) Zr(3d), and c) O(1s) core levels.
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voltages; however, the pores vanished when the voltage further increased due to the sintering and melting phenomena. References [1] [2] [3] [4] [5] [6] [7] [8]
Tongbo W, Fengyuan Y, Jun T. J Alloys Compd 2005;389:169. Yu Q, Zhou C, Zhang H, Zhao F. J Eur Ceram Soc 2010;30:889. Gao J, He Y, Wang D. Mater Chem Phys 2010;123:731. Sornakumar T, Krishnamurthy R, Gokularathnam CV. J Eur Ceram Soc 1993;12:455. Liang B, Zhang G, Liao H, Coddet C, Ding C. Surf Coat Technol 2009;203:3235. Nie X, Wilson A, Leyland A, Matthews A. Surf Coat Technol 2000;121:506. Yerokhin AL, Nie X, Leyland A, Matthews A, Dowey SJ. Surf Coat Technol 1999;122:73. Yerokhin AL, Snizhko LO, Gurevina NL, Leyland A, Pilkington A, Matthews A. J Phys D Appl Phys 2003;36:2110.
[9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
Yerokhin AL, Lyubimov VV, Ashitkof RV. Ceram Int 1998;24:1. Bayati MR, Moshfegh AZ, Golestani-Fard F. Electrochim Acta 2010;55:3093. Bayati MR, Golestani-Fard F, Moshfegh AZ. Appl Catal A Gen 2010;382:322. Bayati MR, Moshfegh AZ, Golestani-Fard F. Mater Lett 2010;64:2215. Bayati MR, Molaei R, Zargar HR, Kajbafvala A, Zanganeh S. Mater Lett 2010;64:2498. Samanipour F, Bayati MR, Shoaei-Rad V, Abbasi S, Golestani-Fard F. Mater Lett 2010, doi:10.1016/j.matlet.2010.11.039. Bayati MR, Zargar HR, Talimian A, Ziaee A, Molaei R. Surf Coat Technol 2010;205: 2483. Bayati MR, Golestani-Fard F, Moshfegh AZ. Mater Chem Phys 2010;124:203. Bayati MR, Golestani-Fard F, Moshfegh AZ. Mater Chem Phys 2010;120:582. Shin KR, Ko YG, Shin DH. Mater Lett 2010;64:2714. Reddy BM, Chowdhury B, Reddy EP, Fernández A. Appl Catal A Gen 2001;213:279. Snizhko LO, Yerokhin AL, Gurevina NL, Patalakha VA, Matthews A. Thin Solid Films 2007;516:460.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Fabrication of ZrO2–Al2O3 hybrid nano-porous layers through micro arc oxidation process V. Shoaei-Rad a, M.R. Bayati a,b,⁎, F. Golestani-Fard a,c, H.R. Zargar d, J. Javadpour a a
School of Metallurgy and Materials Engineering, Iran University of Science and Technology, P.O. Box: 16845-195, Tehran, Iran Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695-7907, USA Center of Excellence for Advanced Materials, Iran University of Science and Technology, P.O. Box: 16845-195, Tehran, Iran d Department of Metals and Materials Engineering, University of British Columbia, Vancouver, BC V6T 1Z4, Canada b c
a r t i c l e
i n f o
Article history: Received 2 December 2010 Accepted 23 March 2011 Available online 29 March 2011 Keywords: Micro arc oxidation Alumina Zirconia Composite materials Porous materials
a b s t r a c t Zirconia-alumina layers, with a pores size of 40–300 nm, were fabricated via micro arc oxidation method for the first time. The layers were grown under alternative current in the electrolytes of ZrOCl2 salt. They consisted of α-Al2O3, γ-Al2O3, monoclinic ZrO2, tetragonal ZrO2. Increasing the voltage resulted in higher zirconium concentrations in the layers. A porous structure was obtained when the layers were grown under intermediate voltages. Microcracks were observed to appear when the applied voltage increased. Finally, a formation mechanism was proposed with emphasis on the chemical and the electrochemical foundations. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Although aluminum is one of the most attractive materials, its disadvantages namely softness, low wear resistance and high friction coefficient have limited is practical applications. An effective approach to defeat these disadvantages is coating the aluminum components with alumina-based composites coatings [1]. Al2O3-based composite coatings are desirable for different applications such as anti-erosion, anticorrosion, and thermal barrier systems [1–3]. Among the aluminabased hybrid coatings, the Al2O3–ZrO2 system has demonstrated unique features as an anti-wear coating. Zirconia is an interesting material in fundamental studies due to its remarkable properties namely good chemical and dimensional stability, high melting point, low thermal conductivity, and high wear resistance. Engine components, cutting tools, thermal barrier or wear-resistance coatings, and biomedical implants are some industries where it is applied [4,5]. Micro arc oxidation is a relatively new and effective method of fabricating thick, hard, and well-adhered ceramic layers with excellent wear resistant properties on light alloys especially on aluminum. It is one of the most efficient and simplest methods to grow alumina layers, based on the modification of the growing anodic film by spark arc micro discharges. More details of the MAO process can be found elsewhere [6–9].
MAO is a promising method which enables us to fabricated hybrid high quality ceramic coatings [10–19]. To the best of our knowledge, this is the first time that ZrO2–Al2O3 layers are fabricated via MAO process on aluminum substrates, even though they have been synthesized by other techniques. 2. Experimental Typical arrangement of the experimental setup is illustrated in our previous works [18]. 1 cm× 3 cm× 0.5 cm aluminum pieces were used as substrate surrounded by an ASTM 316 cylindrical cathode. ZrOCl2.8 H2O (5 g∙l− 1, Merck) solutions were utilized as electrolyte. Prior to MAO treatment, substrates were ultrasonically cleaned in acetone for 15 min and washed by distilled water. Samples were fabricated under alternative current and voltages of 50–200 V with +50 V intervals for 3 min. Surface morphology of the layers was evaluated by scanning electron microscopy (TESCAN, Vega II). Furthermore, X-ray diffraction (Philips, PW3710), energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy (VG Microtech, Twin anode, XR3E2 X-ray source, using Al Kα = 1486.6 eV) techniques were used in order to study phase structure and chemical composition of the synthesized layers. The XPS peak fitting procedure was performed by SDP software ver. 4.1. 3. Results and discussion
⁎ Corresponding author at: Tel.: +1 919 917 6962; fax: +1 919 515 7724. E-mail address: [email protected] (M.R. Bayati). URL: http://rrg.iust.ac.ir. 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.03.086
Fig. 1 illustrates the surface top-view of the layers grown under different voltages. No pore formed in the layers fabricated with applying the voltage of 50 V. The reason is that the electron avalanches, which are
1836
V. Shoaei-Rad et al. / Materials Letters 65 (2011) 1835–1838
Fig. 1. SEM top-view of the layers grown under different voltages: a) 50, b) 100, c) 150, and d) 200 V.
responsible for pores formation, take place at the voltages higher than the breakdown potential of the oxygen gas layer and the surface oxide layer. No spark occurred at such a low voltage and, thus, no pore formed in the structure of this sample. However, electric sparks appeared on the surface of the substrate when the voltage increased to 100 V. This phenomenon which is the reason for formation of the structural pores was also observed and reported in our other work [18]. Meanwhile, the pores size increases with the voltage because of the stronger electron avalanches which form at higher voltages.
No pore is observed in the layer synthesized under the voltage of 200 V whose morphology is depicted in Fig. 1d. Although the electron avalanches occur at this voltage, another mechanism was prevailing. The electrical current of the cell increases with the voltage. The growing layer is sintered due to the heat generated by the high electric current which passes through the cell. It has been suggested that the temperature locally surges to about 103–104 K at the avalanche regions which is high enough to melt the growing layer and the substrate. Because of the melting and sintering phenomena, the pores vanished.
Fig. 2. XRD patterns of the ZrO2–Al2O3 layers grown under different voltages.
V. Shoaei-Rad et al. / Materials Letters 65 (2011) 1835–1838
Fig. 3. EDX zirconium wt% for different voltage.
Some microcracks are also seen in the surface morphology of the layers synthesized under the voltages of 150 and 200 V which were originated from thermal stresses. The layer is melted due to the electron avalanches and quickly solidifies in the electrolyte when the spark disappears. These frequent and fast meltings and solidifications cause the thermal stresses. XRD patterns of the layers are exhibited in Fig. 2. Formation of alumina, zirconia, and some minor phases are observed. The most important feature of the XRD patterns is formation of the stabilized zirconia without any stabilizer agent such as yttria. The reason is the severe conditions during the MAO growth namely very high temperatures and the strong electric field. Of course, alumina can act as a stabilizer agent. Zirconium concentration was determined by EDX technique (Fig. 3). It is obvious that zirconium concentration increases with the voltage. Applying higher voltages intensifies the electric field between the cell poles and, thus, more ZrO2+ ions are drawn through the substrate to participate in the reactions. It is worthy to note that the ZrO2+ cations originated from ionization of zirconium oxide chloride salt in the aqueous solutions. XPS technique was employed to confirm stoichiometry of the layer fabricated under the voltage of 150 V. The XPS results were referenced to the C(1s) core level at the binding energy of 285.0 eV. Fig. 4a illustrates the Al(2p) core level at the binding energy of 74.8 eV confirming formation of Al2O3. The Zr(3d) core level binding energy is shown in Fig. 4b. This peak has 2 components which are attributed to its spin orbit
1837
splitting, i.e. Zr(3d3/2) and Zr(3d5/2). The peaks A and B, at the binding energies of 180.3 and 181.5 eV, reveal existence of Zr2+ and formation of ZrO and probably some other non-stoichiometric zirconium oxides (ZrxOy). Formation of the ZrO2 compound is confirmed by the peaks C and D which are respectively located at the binding energies of 182.4 and 184.3 eV. The peaks E and F with the binding energies of 185.5 and 186.5 eV can be assigned to the zirconium in the ZrOCl2 compound originated from the electrolyte trapping in the surface pores. These values are in a good agreement with those reported by other researchers [20]. The O(1s) core level binding energy is exhibited in Fig. 4c which can be deconvoluted to 4 distinct peaks representing that there are 4 different O-bindings in the layer. The peak A, located at the binding energy of 530.3 eV, is assigned to the oxygen in the ZrO2. The peak B, with a binding energy of 531.8 eV, corresponds to the oxygen in aluminum oxide. Thus, formation of a ZrO2–Al2O3 hybrid layer is confirmed. The peak C at the binding energy of 533.1 eV represents the ―OH groups on the surface. The D, located at the binding energies of 534.3 eV, reveal existence of oxygen in the water molecules. Since the layers were porous and fabricated in aqueous solutions, water may be trapped in the pores. Based on the chemical and electrochemical bases, the following formation mechanism is proposed. Since the samples were fabricated with applying alternative current, a number of parallel reactions take place under cathodic and anodic conditions on the vicinity of the substrate. During the anodic cycle, the following reactions have been proposed for formation of Al2O3 on aluminum substrates via plasma electrolytic processes in acidic electrolytes [21]: −
þ
2½AlðOHÞ4 þ 2H þ ðn−5ÞH2 O→Al2 O3 ·nH2 O −
2Al þ 9H2 O→Al2 O3 þ 6H3 O þ 6e
ð1Þ ð2Þ
When the above anodic reactions are halted, the ZrO2+ cations move toward the substrate due to the electric field between anode and cathode to participate in the following reaction: ZrO
2þ
þ
þ 3H2 O→ZrO2 þ 2H3 O
ð3Þ
4. Conclusions ZrO2–Al2O3 hybrid films were fabricated via micro arc oxidation of aluminum. The layers consisted of α-Al2O3, γ-Al2O3, monoclinic ZrO2, tetragonal ZrO2. Applying higher voltages resulted inincreasing zirconium concentrations and formation of some microcracks. A porous structure was observed when the layers were grown under intermediate
Fig. 4. XPS binding energies: a) Al(2p), b) Zr(3d), and c) O(1s) core levels.
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voltages; however, the pores vanished when the voltage further increased due to the sintering and melting phenomena. References [1] [2] [3] [4] [5] [6] [7] [8]
Tongbo W, Fengyuan Y, Jun T. J Alloys Compd 2005;389:169. Yu Q, Zhou C, Zhang H, Zhao F. J Eur Ceram Soc 2010;30:889. Gao J, He Y, Wang D. Mater Chem Phys 2010;123:731. Sornakumar T, Krishnamurthy R, Gokularathnam CV. J Eur Ceram Soc 1993;12:455. Liang B, Zhang G, Liao H, Coddet C, Ding C. Surf Coat Technol 2009;203:3235. Nie X, Wilson A, Leyland A, Matthews A. Surf Coat Technol 2000;121:506. Yerokhin AL, Nie X, Leyland A, Matthews A, Dowey SJ. Surf Coat Technol 1999;122:73. Yerokhin AL, Snizhko LO, Gurevina NL, Leyland A, Pilkington A, Matthews A. J Phys D Appl Phys 2003;36:2110.
[9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
Yerokhin AL, Lyubimov VV, Ashitkof RV. Ceram Int 1998;24:1. Bayati MR, Moshfegh AZ, Golestani-Fard F. Electrochim Acta 2010;55:3093. Bayati MR, Golestani-Fard F, Moshfegh AZ. Appl Catal A Gen 2010;382:322. Bayati MR, Moshfegh AZ, Golestani-Fard F. Mater Lett 2010;64:2215. Bayati MR, Molaei R, Zargar HR, Kajbafvala A, Zanganeh S. Mater Lett 2010;64:2498. Samanipour F, Bayati MR, Shoaei-Rad V, Abbasi S, Golestani-Fard F. Mater Lett 2010, doi:10.1016/j.matlet.2010.11.039. Bayati MR, Zargar HR, Talimian A, Ziaee A, Molaei R. Surf Coat Technol 2010;205: 2483. Bayati MR, Golestani-Fard F, Moshfegh AZ. Mater Chem Phys 2010;124:203. Bayati MR, Golestani-Fard F, Moshfegh AZ. Mater Chem Phys 2010;120:582. Shin KR, Ko YG, Shin DH. Mater Lett 2010;64:2714. Reddy BM, Chowdhury B, Reddy EP, Fernández A. Appl Catal A Gen 2001;213:279. Snizhko LO, Yerokhin AL, Gurevina NL, Patalakha VA, Matthews A. Thin Solid Films 2007;516:460.