- Email: [email protected]
Growth characteristics of micro-plasma oxidation ceramic coatings on Ti alloy by inductively coupled plasma-atomic emission spectrometer technique
Applied Surface Science 253 (2007) 4267–4272 www.elsevier.com/locate/apsusc
Growth characteristics of micro-plasma oxidation ceramic coatings on Ti alloy by inductively coupled plasma-atomic emission spectrometer technique Zhongping Yao *, Zhaohua Jiang, Fuping Wang, Wei Xue Department of Applied Chemistry, Harbin Institute of Technology, Harbin 150001, PR China Received 9 August 2006; received in revised form 15 September 2006; accepted 16 September 2006 Available online 12 October 2006
Abstract The aim of this work was to study the growth characteristics of micro-plasma oxidation ceramic coatings on Ti–6Al–4V alloy. Compound ceramic coatings were prepared on Ti–6Al–4V alloy by pulsed micro-plasma oxidation (MPO) in NaAlO2 solution. The phase composition and surface morphology of the coating were investigated by X-ray diffractometry and scanning electron microscopy. The solution of Ti from the substrate and the content of Al in the electrolyte were studied by inductively coupled plasma-atomic emission spectrometer (ICP-AES) technique. Ti from the substrate dissolved and came into the coating and the electrolyte during MPO process. The content of Ti in the electrolyte under the pulsed bi-polar mode was more than that of the pulsed single-polar mode. The phase composition and structure of the coating was attributable to the space steric hindrance of Al congregated on the electrode surface due to the effect of the electric field and the electrolyte characters. For the pulsed single-polar mode, the coating was mainly composed of a large amount of a-Al2O3 and a small amount of g-Al2O3. And the coating was mainly structured by Al from the electrolyte. However, the coating was composed of a large amount of Al2TiO5 and a little a-Al2O3 and rutile TiO2 for the pulsed bi-polar mode. And the coating was structured both by Ti from the substrate and Al from the electrolyte. # 2006 Elsevier B.V. All rights reserved. Keywords: Micro-plasma oxidation; Ti alloy; Ceramic coatings; ICP-AES
1. Introduction There has been a great interest in micro-plasma oxidation (MPO) technique, because of the promising application prospects of this technique in the surface treatment of metals [1–3]. At present, too much research was focused on the composition, the structure and properties of the prepared ceramic coatings, using traditional techniques such as X-ray diffractometry (XRD), scanning electron microscopy (SEM), electron probe microanalyzer (EPMA), and so on [4–8]. And the research concerned with the electrolytes is also focused on the concentration of different components in the electrolyte and the influences on the structure and properties of the ceramic coatings through the above analyzing techniques. However, the solution of the substrate in the electrolyte and the changes of the concentration of different components in the electrolyte are seldom reported. Inductively
* Corresponding author. Tel.: +86 451 86413710; fax: +86 451 86413707. E-mail address: [email protected] (Z. Yao). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.09.027
coupled plasma-atomic emission spectrometer (ICP-AES) is a powerful analyzing instrument, which can be used for the traceanalyzing of the substances in the electrolyte solutions [9–13]. Therefore, we prepared ceramic coatings on Ti alloy by microplasma oxidation in NaAlO2 solutions. The solution of the Ti substrate and the concentration of the Al near the working electrode in the electrolyte were measured by ICP-AES. Combined with the XRD and SEM analysis results of the coating, the growth characteristics and structure of the MPO ceramic coatings was discussed preliminarily. 2. Experimental details 2.1. Preparation of micro-plasma oxidation ceramic coatings Plate samples of Ti–6Al–4V with the dimension of 25 mm 15 mm 6 mm were first polished with abrasive paper, and then washed in HF–HNO3 (1:1 in volume) aqueous solution. A home made pulsed electrical source with power of
4268
Z. Yao et al. / Applied Surface Science 253 (2007) 4267–4272
5 kW was used for micro-plasma oxidation of disc samples in a water-cooled electrolyser made of stainless steel, which also served as the counter electrode. The reaction temperature was controlled to below 30 8C by adjusting the cooling water flow. The MPO process equipment used is similar to the one presented by Matthews’ group in [1]. The whole MPO process was carried on under the stable current density of 8 A/dm2 with the electronic power frequency of 60 Hz. The electrolyte used in the experiments is Na2AlO2 solution (with the concentration of Na2AlO2 8 g/l and that of Na3PO4 1 g/l). After MPO treatment, the coated samples were rinsed with water and dried in air. 2.2. Measurement of the concentration by ICP-AES Inductively coupled plasma-atomic emission spectrometer (ICP-AES, 4300 DV, Perkin-Elmer Co.) is used to analyze the concentration of Ti and Al in the electrolytes. The qualitative analysis of the components and its concentration is determined by the analysis of the specific wavelengths of the measured substance. The 100 ml sample solution was taken out near the working electrode at the fixed position every fixed interval during MPO process. Then the solution was defecated and diluted to the required concentration, and then analyzed by ICP-AES. 2.3. Analysis of phase composition and structure of the coatings Phase composition of the coatings was examined with a RICOH D/max-rB automatic X-ray diffractometer (XRD) using a Cu Ka source. Surface morphology of the produced coatings was studied by scanning electron microscopy (SEM; Hitachi S-570). 3. Result 3.1. XRD analysis of the coatings Fig. 1 is the XRD patterns of the coatings prepared under pulsed single-polar mode in NaAlO2 solution. It is assumed that the contents of titanium and aluminum in coatings correlate with change of their phase structure. The coating was mainly composed of a large amount of a-Al2O3 and a small amount of gAl2O3. For the coatings of 10 min, there also exists Al2TiO5, and its content was decreased gradually with increasing MPO time. Similarly, the content of g-Al2O3 was also decreased, which may be due to the transformation of g-Al2O3 to a-Al2O3 during the MPO process. However, the coating prepared under pulsed bipolar mode was quite different, which is shown in Fig. 2. The coating was composed of a large amount of Al2TiO5 and a little a-Al2O3 and rutile TiO2. Al2TiO5 is the main crystalline all through MPO time. Increasing the MPO time, the content of Al2TiO5, a-Al2O3 and rutile TiO2 was increased gradually.
Fig. 1. XRD patterns of the coatings prepared under pulsed single-polar mode.
brownish gray. Furthermore, the surface images of the coatings under different modes are also quite different. The surface SEM of the coatings is shown in Fig. 3. The sintered particles were both increased with the MPO time for both modes. However, the coatings for the pulsed single-polar mode have more microholes and more clearance among the sintered particles, while the coatings for the pulsed bi-polar mode are continuous and dense, and there were fewer clearances among the sintered particles. Therefore, the coatings for the pulsed single-polar mode were looser than that for the pulsed bi-polar mode. 3.3. Solution of Ti from the substrate during MPO process Fig. 4 is the content change of Ti in the NaAlO2 solution under different MPO pulse modes. With increasing MPO time, the content of Ti in the electrolyte was increased gradually, which illustrates that Ti from the substrate dissolved and came into the solution. The content of Ti for the pulsed bi-polar mode was more than that for the pulsed single-polar mode. With the increase of MPO time, the content difference was obviously
3.2. Surface images of the coatings The coating prepared under pulsed single-polar mode was white gray, while the coating under pulsed bi-polar mode was
Fig. 2. XRD patterns of the coatings prepared under pulsed bi-polar mode.
Z. Yao et al. / Applied Surface Science 253 (2007) 4267–4272
4269
Fig. 3. Surface SEM of the coatings ((a and b) 20 min; (c and d) 40 min) under different modes: (a and c) pulsed bi-polar mode; (b and d) pulsed single-mode.
increased. At 120 min, the latter is over two times that of the former. 3.4. The content of Al near the electrode surface in the electrolyte Because Al is the main element in the electrolyte and MPO reaction occurred on the electrode surface, the content change of Al in the coating is very important for the growth of the
coating during MPO process. Fig. 5 is the content change of Al near the electrode surface in NaAlO2 solution. For pulsed single-polar mode, the content of Al near the electrode surface was increased with increasing MPO time. When MPO time reached 40 min, the content of Al remained nearly stable, and this illustrates that there may exist a homeostasis in the electrolyte solution. Quite differently for the pulsed bi-polar mode, the content change of Al near the electrode surface was decreased gradually throughout the whole process.
4270
Z. Yao et al. / Applied Surface Science 253 (2007) 4267–4272
Fig. 4. The content changes of Ti in the NaAlO2 solution under different MPO pulse modes.
4. Analysis on growth characteristics of the coating The working electrode (the sample) was in the position of high polarization during single-polar pulsed MPO process, which led to the solution of Ti substrate. Tin+ and amorphous TiO2 were formed in accordance with the formulae (1) and (2) below: Ti ! Tinþ þ ne
ðn ¼ 2; 3; 4Þ
Ti þ 2H2 O ! TiO2 ðamorphousÞ þ 4Hþ þ 4e ¼ 0:95 V
(1) E ðSHEÞ (2)
The state of aluminates ions in the NaAlO2 solution is very complicated, which is related to the solution structure, concentration, preparation history and existing time. In terms of Refs. [14,15], aluminates ions existed in the form of Al(OH)4 under the condition of NaAlO2 (8 g/l) at a comparatively low temperature. However, it will dehydrate
Fig. 5. Content of Al near the electrode in NaAlO2 solution.
and form [Al2O(OH)6]2 or more complicated poly-aluminates ions under the condition of high concentration (1 mol/l) or high temperature of the solution. During single-polar pulsed MPO process, a large amount of aluminates ions congregated on the electrode surface due to the effect of the electric field; and the temperature near the working electrode was surely higher than that in the deep solution due to the continuous spark discharge. Therefore, the aluminates ions on the working electrode surface would form all kinds of poly-aluminates ions like [Al2O(OH)6]2. Meanwhile, a small amount of phosphate ion would also react with the aluminates ions and formed the substance which is liable to be absorbed by the electrode [16]. This formed the great space steric hindrance because of the changes of the structure of the solution ions. So the diffusion of soluble Ti from the substrate to the coating and the solution became very difficult, i.e. Ti from the substrate would mainly join MPO process at the interface between the coating and the substrate during the beginning period of MPO process with a little of Al2TiO5 formed in the coating. Similarly, only a bit of Ti came into and the electrolyte solution accordingly. Poly-aluminates ions like [Al2O(OH)6]2 absorbed on the working electrode joined the MPO reaction and heatdecomposed into amorphous or crystallized Al2O3. Most of them took part in the formation of the ceramic coating. The substances in the micro-area of the coating were calcined by micro-arc time after time, which led to the formation of aAl2O3 and g-Al2O3 in the coating. Besides, phase transition of g-Al2O3 to a-Al2O3 is about between 1050 and 1200 8C, while the temperature within the discharging channels is estimated to be more than 2000 K [1], which is much higher than the former. Therefore, part of g-Al2O3 maybe gradually transforms into aAl2O3 with increasing MPO time. Therefore, coating was composed of a large amount of a-Al2O3 and a small amount of g-Al2O3. And the Al2TiO5 was only detected by XRD analysis for the coatings of short time, which means that the content of Ti in the coating was much low. In this way, the content of Ti in the electrolyte solution would be much lower further. Therefore, the coating was mainly structured by Al from the electrolyte, which determined the surface image characters that the coatings are looser and more lacunose. For the pulsed bi-polar MPO process, the cathode process would greatly reduce the concentration of aluminates ions and phosphate ions, which would not be suitable for the formation of poly-aluminates ions like [Al2O(OH)6]2 on the electrode surface, although the temperature on the electrode surface was still a little higher. And the space steric hindrance would consequently be weakened greatly, by which the soluble Ti from the substrate would comparatively easily come into the coating and the electrolyte. In this way, more and more Ti joined the MPO reaction with the increasing MPO time; and similarly TiO2 was formed in the coating by micro-arc time after time. Al2TiO5 is the sole compound of rutile TiO2 and aAl2O3. Therefore, the coating was composed of Al2TiO5, rutile TiO2 and a-Al2O3. Under pulsed bi-polar mode, the coating was structured by both Ti from the substrate and Al from the electrolyte: Ti is growing from inner to outer while Al is growing from outer to inner, therefore, the density of coating
Z. Yao et al. / Applied Surface Science 253 (2007) 4267–4272
4271
soluble Ti from the substrate, especially when the equilibrium of Al was established. While for the pulsed bi-polar mode, because of the continuous consumption of Al on the electrode surface with the increasing MPO time, the space steric hindrance on the electrode was weakened and more and more Ti came into the coating and the electrolyte, which ensured a large amount of Al2TiO5 formed in the coating and meantime the structure was denser than that of the coating through the pulsed single-polar mode. 5. Conclusions
Fig. 6. XRD pattern of the sediment in the NaAlO2 solution during MPO process.
was better than that of pulsed single-polar mode, which is consistent with the surface SEM analysis. Since more Ti came into the coatings, the content of Ti in the solution would also be increased and much more than the pulsed single-polar mode. After a long period of the experiment, a little white substance would be deposited at the bottom of the electrolysis cell, whether pulsed single-polar mode or bi-polar mode. The substrate was collected, filtrated, dried, and analyzed by XRD. Fig. 6 is the XRD pattern of the white sediment, which shows that this substrate was composed of a-Al2O3, g-Al2O3 and amorphous Al2O3. Accompanying the growing of the coating, melting of the micro-area of the coating and quick cooling happened by heaps of times, by which a little Al2O3 which originally formed the coating would part from the coating and come to the solution. The reason that the white substance did not contain Ti is that Ti from the substrate joined the reaction from inner to outer. For the pulsed single-polar mode, there only exists anode process. The ions would move to the electrode surface time after time and was in the gradient distribution from the electrode to the deep electrolyte. On the electrode surface, some of ions joined MPO reaction would be consumed. However, due to the effect of electric field, more ions from the deep electrolyte would diffuse to the electrode surface for complement. If the speed of the consumption of ions equals to that of the diffusion of ions, then the equilibrium was established with the concentration of ions remaining constant after a certain time. The equilibrium was obtained by 40 min for the single-polar pulsed mode. For the pulsed bi-polar mode, because the cathode process would make the ions move to the electrolysis cell, therefore, the concentration of Al would be lower than that for the pulsed single-polar mode. Furthermore, the content of Al would decrease gradually because of the continuous consumption of Al by MPO reaction. This can further explain the phase composition of the coating for both modes, which is consistent with the space steric hindrance discussed above. For the pulsed single-polar mode, there exists plenty of Al on the electrode surface which formed great space steric hindrance to decrease the diffusion of the
The study on the growth characteristics and structure of the MPO ceramic coatings through ICP-AES technique, XRD and SEM analysis methods allowed the following conclusions to be drawn: (1) For the pulsed single-polar mode, the coating was mainly composed of a large amount of a-Al2O3 and a small amount of g-Al2O3. For the pulsed bi-polar mode, the coating was composed of a large amount of Al2TiO5 and a little a-Al2O3 and rutile TiO2. (2) Ti from the substrate dissolved and came into the coating and the electrolyte during MPO process. The content of Ti in the electrolyte under the pulsed bi-polar mode was more than that of the pulsed single-polar mode. Besides, the content of Al near the electrode surface was increased gradually and reached the homeostasis after 40 min for the pulsed single-polar mode; while the content of Al near the electrode surface was decreased with increasing MPO time for the pulsed bi-polar mode. (3) The phase composition and structure of the coating was attributable to the space steric hindrance of Al on the electrode surface. For the pulsed single-polar mode, the coating was mainly structured by Al from the electrolyte due to the great steric hindrance. And for the pulsed bi-polar mode, the steric hindrance was weakened because of the effect of the cathode pulse process and the coating was structured both by Ti from the substrate and Al from the electrolyte. Consequently, the coatings for the pulsed single-polar mode were looser than that for the pulsed bipolar mode. Acknowledgement This work was financially supported by National Natural Science Foundation of China (Grant No. 50171026) References [1] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Surf. Coat. Technol. 122 (1999) 73. [2] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, Surf. Coat. Technol. 130 (2000) 195. [3] Y.J. Guan, Y. Xia, Adv. Mech. 34 (2) (2004) 237 (in Chinese). [4] G.L. Yang, X.Y. Lv, Y.Z. Bai, H.F. Cui, Z.S. Jin, J. Alloys Compd. 345 (2002) 196. [5] G. Sundrarajan, L. Rama Krishna, Surf. Coat. Technol. 167 (2003) 269.
4272
Z. Yao et al. / Applied Surface Science 253 (2007) 4267–4272
[6] L.O. Snizhko, A.L. Yerokhin, A. Pilkington, N.L. Gurevina, D.O. Misnyankin, A. Leyland, A. Matthews, Electrochim. Acta 49 (2004) 2085– 2095. [7] P.I. Butygin, Ye.V. Khokhryakov, A.L. Mamaev, Mater. Lett. 57 (2003) 1784. [8] W.B. Xue, C. Wang, Z.W. Deng, R.Y. Chen, T.H. Zhang, Mater. Sci. Technol. 18 (2002) 37. [9] K. Saitoh, K. Sera, T. Gotoh, M. Nakamura, Nucl. Instrum. Methods Phys. Res. B 189 (2002) 86–93. [10] A.L. Briseno, A. Baca, Q. Zhou, R. Lai, F. Zhou, Anal. Chim. Acta 441 (2001) 123–134.
[11] A. Iwashita, T. Nakajima, H. Takanashi, A. Ohki, Y. Fujita, T. Yamashita, Fuel 85 (2006) 257–263. [12] H. Pekey, D. Karak, M. Bakoglu, Marine Pollut. Bull. 49 (2004) 809– 818. [13] W.B. Xue, Z.W. Deng, Y.L. Li, R.Y. Chen, T.H. Zhang, Mater. Sci. Technol. 8 (2000) 41 (in Chinese). [14] R.J. Moolenaar, J.C. Evans, L.D. McKeever, J. Phys. Chem. 74 (1970) 3629. [15] S. Veesler, S. Rource, R. Boistelle, J. Cryst. Growth 135 (1994) 505. [16] L.A. Carreira, V.A. Maroni, J. Phys. Chem. 45 (1966) 2216.
Growth characteristics of micro-plasma oxidation ceramic coatings on Ti alloy by inductively coupled plasma-atomic emission spectrometer technique Zhongping Yao *, Zhaohua Jiang, Fuping Wang, Wei Xue Department of Applied Chemistry, Harbin Institute of Technology, Harbin 150001, PR China Received 9 August 2006; received in revised form 15 September 2006; accepted 16 September 2006 Available online 12 October 2006
Abstract The aim of this work was to study the growth characteristics of micro-plasma oxidation ceramic coatings on Ti–6Al–4V alloy. Compound ceramic coatings were prepared on Ti–6Al–4V alloy by pulsed micro-plasma oxidation (MPO) in NaAlO2 solution. The phase composition and surface morphology of the coating were investigated by X-ray diffractometry and scanning electron microscopy. The solution of Ti from the substrate and the content of Al in the electrolyte were studied by inductively coupled plasma-atomic emission spectrometer (ICP-AES) technique. Ti from the substrate dissolved and came into the coating and the electrolyte during MPO process. The content of Ti in the electrolyte under the pulsed bi-polar mode was more than that of the pulsed single-polar mode. The phase composition and structure of the coating was attributable to the space steric hindrance of Al congregated on the electrode surface due to the effect of the electric field and the electrolyte characters. For the pulsed single-polar mode, the coating was mainly composed of a large amount of a-Al2O3 and a small amount of g-Al2O3. And the coating was mainly structured by Al from the electrolyte. However, the coating was composed of a large amount of Al2TiO5 and a little a-Al2O3 and rutile TiO2 for the pulsed bi-polar mode. And the coating was structured both by Ti from the substrate and Al from the electrolyte. # 2006 Elsevier B.V. All rights reserved. Keywords: Micro-plasma oxidation; Ti alloy; Ceramic coatings; ICP-AES
1. Introduction There has been a great interest in micro-plasma oxidation (MPO) technique, because of the promising application prospects of this technique in the surface treatment of metals [1–3]. At present, too much research was focused on the composition, the structure and properties of the prepared ceramic coatings, using traditional techniques such as X-ray diffractometry (XRD), scanning electron microscopy (SEM), electron probe microanalyzer (EPMA), and so on [4–8]. And the research concerned with the electrolytes is also focused on the concentration of different components in the electrolyte and the influences on the structure and properties of the ceramic coatings through the above analyzing techniques. However, the solution of the substrate in the electrolyte and the changes of the concentration of different components in the electrolyte are seldom reported. Inductively
* Corresponding author. Tel.: +86 451 86413710; fax: +86 451 86413707. E-mail address: [email protected] (Z. Yao). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.09.027
coupled plasma-atomic emission spectrometer (ICP-AES) is a powerful analyzing instrument, which can be used for the traceanalyzing of the substances in the electrolyte solutions [9–13]. Therefore, we prepared ceramic coatings on Ti alloy by microplasma oxidation in NaAlO2 solutions. The solution of the Ti substrate and the concentration of the Al near the working electrode in the electrolyte were measured by ICP-AES. Combined with the XRD and SEM analysis results of the coating, the growth characteristics and structure of the MPO ceramic coatings was discussed preliminarily. 2. Experimental details 2.1. Preparation of micro-plasma oxidation ceramic coatings Plate samples of Ti–6Al–4V with the dimension of 25 mm 15 mm 6 mm were first polished with abrasive paper, and then washed in HF–HNO3 (1:1 in volume) aqueous solution. A home made pulsed electrical source with power of
4268
Z. Yao et al. / Applied Surface Science 253 (2007) 4267–4272
5 kW was used for micro-plasma oxidation of disc samples in a water-cooled electrolyser made of stainless steel, which also served as the counter electrode. The reaction temperature was controlled to below 30 8C by adjusting the cooling water flow. The MPO process equipment used is similar to the one presented by Matthews’ group in [1]. The whole MPO process was carried on under the stable current density of 8 A/dm2 with the electronic power frequency of 60 Hz. The electrolyte used in the experiments is Na2AlO2 solution (with the concentration of Na2AlO2 8 g/l and that of Na3PO4 1 g/l). After MPO treatment, the coated samples were rinsed with water and dried in air. 2.2. Measurement of the concentration by ICP-AES Inductively coupled plasma-atomic emission spectrometer (ICP-AES, 4300 DV, Perkin-Elmer Co.) is used to analyze the concentration of Ti and Al in the electrolytes. The qualitative analysis of the components and its concentration is determined by the analysis of the specific wavelengths of the measured substance. The 100 ml sample solution was taken out near the working electrode at the fixed position every fixed interval during MPO process. Then the solution was defecated and diluted to the required concentration, and then analyzed by ICP-AES. 2.3. Analysis of phase composition and structure of the coatings Phase composition of the coatings was examined with a RICOH D/max-rB automatic X-ray diffractometer (XRD) using a Cu Ka source. Surface morphology of the produced coatings was studied by scanning electron microscopy (SEM; Hitachi S-570). 3. Result 3.1. XRD analysis of the coatings Fig. 1 is the XRD patterns of the coatings prepared under pulsed single-polar mode in NaAlO2 solution. It is assumed that the contents of titanium and aluminum in coatings correlate with change of their phase structure. The coating was mainly composed of a large amount of a-Al2O3 and a small amount of gAl2O3. For the coatings of 10 min, there also exists Al2TiO5, and its content was decreased gradually with increasing MPO time. Similarly, the content of g-Al2O3 was also decreased, which may be due to the transformation of g-Al2O3 to a-Al2O3 during the MPO process. However, the coating prepared under pulsed bipolar mode was quite different, which is shown in Fig. 2. The coating was composed of a large amount of Al2TiO5 and a little a-Al2O3 and rutile TiO2. Al2TiO5 is the main crystalline all through MPO time. Increasing the MPO time, the content of Al2TiO5, a-Al2O3 and rutile TiO2 was increased gradually.
Fig. 1. XRD patterns of the coatings prepared under pulsed single-polar mode.
brownish gray. Furthermore, the surface images of the coatings under different modes are also quite different. The surface SEM of the coatings is shown in Fig. 3. The sintered particles were both increased with the MPO time for both modes. However, the coatings for the pulsed single-polar mode have more microholes and more clearance among the sintered particles, while the coatings for the pulsed bi-polar mode are continuous and dense, and there were fewer clearances among the sintered particles. Therefore, the coatings for the pulsed single-polar mode were looser than that for the pulsed bi-polar mode. 3.3. Solution of Ti from the substrate during MPO process Fig. 4 is the content change of Ti in the NaAlO2 solution under different MPO pulse modes. With increasing MPO time, the content of Ti in the electrolyte was increased gradually, which illustrates that Ti from the substrate dissolved and came into the solution. The content of Ti for the pulsed bi-polar mode was more than that for the pulsed single-polar mode. With the increase of MPO time, the content difference was obviously
3.2. Surface images of the coatings The coating prepared under pulsed single-polar mode was white gray, while the coating under pulsed bi-polar mode was
Fig. 2. XRD patterns of the coatings prepared under pulsed bi-polar mode.
Z. Yao et al. / Applied Surface Science 253 (2007) 4267–4272
4269
Fig. 3. Surface SEM of the coatings ((a and b) 20 min; (c and d) 40 min) under different modes: (a and c) pulsed bi-polar mode; (b and d) pulsed single-mode.
increased. At 120 min, the latter is over two times that of the former. 3.4. The content of Al near the electrode surface in the electrolyte Because Al is the main element in the electrolyte and MPO reaction occurred on the electrode surface, the content change of Al in the coating is very important for the growth of the
coating during MPO process. Fig. 5 is the content change of Al near the electrode surface in NaAlO2 solution. For pulsed single-polar mode, the content of Al near the electrode surface was increased with increasing MPO time. When MPO time reached 40 min, the content of Al remained nearly stable, and this illustrates that there may exist a homeostasis in the electrolyte solution. Quite differently for the pulsed bi-polar mode, the content change of Al near the electrode surface was decreased gradually throughout the whole process.
4270
Z. Yao et al. / Applied Surface Science 253 (2007) 4267–4272
Fig. 4. The content changes of Ti in the NaAlO2 solution under different MPO pulse modes.
4. Analysis on growth characteristics of the coating The working electrode (the sample) was in the position of high polarization during single-polar pulsed MPO process, which led to the solution of Ti substrate. Tin+ and amorphous TiO2 were formed in accordance with the formulae (1) and (2) below: Ti ! Tinþ þ ne
ðn ¼ 2; 3; 4Þ
Ti þ 2H2 O ! TiO2 ðamorphousÞ þ 4Hþ þ 4e ¼ 0:95 V
(1) E ðSHEÞ (2)
The state of aluminates ions in the NaAlO2 solution is very complicated, which is related to the solution structure, concentration, preparation history and existing time. In terms of Refs. [14,15], aluminates ions existed in the form of Al(OH)4 under the condition of NaAlO2 (8 g/l) at a comparatively low temperature. However, it will dehydrate
Fig. 5. Content of Al near the electrode in NaAlO2 solution.
and form [Al2O(OH)6]2 or more complicated poly-aluminates ions under the condition of high concentration (1 mol/l) or high temperature of the solution. During single-polar pulsed MPO process, a large amount of aluminates ions congregated on the electrode surface due to the effect of the electric field; and the temperature near the working electrode was surely higher than that in the deep solution due to the continuous spark discharge. Therefore, the aluminates ions on the working electrode surface would form all kinds of poly-aluminates ions like [Al2O(OH)6]2. Meanwhile, a small amount of phosphate ion would also react with the aluminates ions and formed the substance which is liable to be absorbed by the electrode [16]. This formed the great space steric hindrance because of the changes of the structure of the solution ions. So the diffusion of soluble Ti from the substrate to the coating and the solution became very difficult, i.e. Ti from the substrate would mainly join MPO process at the interface between the coating and the substrate during the beginning period of MPO process with a little of Al2TiO5 formed in the coating. Similarly, only a bit of Ti came into and the electrolyte solution accordingly. Poly-aluminates ions like [Al2O(OH)6]2 absorbed on the working electrode joined the MPO reaction and heatdecomposed into amorphous or crystallized Al2O3. Most of them took part in the formation of the ceramic coating. The substances in the micro-area of the coating were calcined by micro-arc time after time, which led to the formation of aAl2O3 and g-Al2O3 in the coating. Besides, phase transition of g-Al2O3 to a-Al2O3 is about between 1050 and 1200 8C, while the temperature within the discharging channels is estimated to be more than 2000 K [1], which is much higher than the former. Therefore, part of g-Al2O3 maybe gradually transforms into aAl2O3 with increasing MPO time. Therefore, coating was composed of a large amount of a-Al2O3 and a small amount of g-Al2O3. And the Al2TiO5 was only detected by XRD analysis for the coatings of short time, which means that the content of Ti in the coating was much low. In this way, the content of Ti in the electrolyte solution would be much lower further. Therefore, the coating was mainly structured by Al from the electrolyte, which determined the surface image characters that the coatings are looser and more lacunose. For the pulsed bi-polar MPO process, the cathode process would greatly reduce the concentration of aluminates ions and phosphate ions, which would not be suitable for the formation of poly-aluminates ions like [Al2O(OH)6]2 on the electrode surface, although the temperature on the electrode surface was still a little higher. And the space steric hindrance would consequently be weakened greatly, by which the soluble Ti from the substrate would comparatively easily come into the coating and the electrolyte. In this way, more and more Ti joined the MPO reaction with the increasing MPO time; and similarly TiO2 was formed in the coating by micro-arc time after time. Al2TiO5 is the sole compound of rutile TiO2 and aAl2O3. Therefore, the coating was composed of Al2TiO5, rutile TiO2 and a-Al2O3. Under pulsed bi-polar mode, the coating was structured by both Ti from the substrate and Al from the electrolyte: Ti is growing from inner to outer while Al is growing from outer to inner, therefore, the density of coating
Z. Yao et al. / Applied Surface Science 253 (2007) 4267–4272
4271
soluble Ti from the substrate, especially when the equilibrium of Al was established. While for the pulsed bi-polar mode, because of the continuous consumption of Al on the electrode surface with the increasing MPO time, the space steric hindrance on the electrode was weakened and more and more Ti came into the coating and the electrolyte, which ensured a large amount of Al2TiO5 formed in the coating and meantime the structure was denser than that of the coating through the pulsed single-polar mode. 5. Conclusions
Fig. 6. XRD pattern of the sediment in the NaAlO2 solution during MPO process.
was better than that of pulsed single-polar mode, which is consistent with the surface SEM analysis. Since more Ti came into the coatings, the content of Ti in the solution would also be increased and much more than the pulsed single-polar mode. After a long period of the experiment, a little white substance would be deposited at the bottom of the electrolysis cell, whether pulsed single-polar mode or bi-polar mode. The substrate was collected, filtrated, dried, and analyzed by XRD. Fig. 6 is the XRD pattern of the white sediment, which shows that this substrate was composed of a-Al2O3, g-Al2O3 and amorphous Al2O3. Accompanying the growing of the coating, melting of the micro-area of the coating and quick cooling happened by heaps of times, by which a little Al2O3 which originally formed the coating would part from the coating and come to the solution. The reason that the white substance did not contain Ti is that Ti from the substrate joined the reaction from inner to outer. For the pulsed single-polar mode, there only exists anode process. The ions would move to the electrode surface time after time and was in the gradient distribution from the electrode to the deep electrolyte. On the electrode surface, some of ions joined MPO reaction would be consumed. However, due to the effect of electric field, more ions from the deep electrolyte would diffuse to the electrode surface for complement. If the speed of the consumption of ions equals to that of the diffusion of ions, then the equilibrium was established with the concentration of ions remaining constant after a certain time. The equilibrium was obtained by 40 min for the single-polar pulsed mode. For the pulsed bi-polar mode, because the cathode process would make the ions move to the electrolysis cell, therefore, the concentration of Al would be lower than that for the pulsed single-polar mode. Furthermore, the content of Al would decrease gradually because of the continuous consumption of Al by MPO reaction. This can further explain the phase composition of the coating for both modes, which is consistent with the space steric hindrance discussed above. For the pulsed single-polar mode, there exists plenty of Al on the electrode surface which formed great space steric hindrance to decrease the diffusion of the
The study on the growth characteristics and structure of the MPO ceramic coatings through ICP-AES technique, XRD and SEM analysis methods allowed the following conclusions to be drawn: (1) For the pulsed single-polar mode, the coating was mainly composed of a large amount of a-Al2O3 and a small amount of g-Al2O3. For the pulsed bi-polar mode, the coating was composed of a large amount of Al2TiO5 and a little a-Al2O3 and rutile TiO2. (2) Ti from the substrate dissolved and came into the coating and the electrolyte during MPO process. The content of Ti in the electrolyte under the pulsed bi-polar mode was more than that of the pulsed single-polar mode. Besides, the content of Al near the electrode surface was increased gradually and reached the homeostasis after 40 min for the pulsed single-polar mode; while the content of Al near the electrode surface was decreased with increasing MPO time for the pulsed bi-polar mode. (3) The phase composition and structure of the coating was attributable to the space steric hindrance of Al on the electrode surface. For the pulsed single-polar mode, the coating was mainly structured by Al from the electrolyte due to the great steric hindrance. And for the pulsed bi-polar mode, the steric hindrance was weakened because of the effect of the cathode pulse process and the coating was structured both by Ti from the substrate and Al from the electrolyte. Consequently, the coatings for the pulsed single-polar mode were looser than that for the pulsed bipolar mode. Acknowledgement This work was financially supported by National Natural Science Foundation of China (Grant No. 50171026) References [1] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Surf. Coat. Technol. 122 (1999) 73. [2] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, Surf. Coat. Technol. 130 (2000) 195. [3] Y.J. Guan, Y. Xia, Adv. Mech. 34 (2) (2004) 237 (in Chinese). [4] G.L. Yang, X.Y. Lv, Y.Z. Bai, H.F. Cui, Z.S. Jin, J. Alloys Compd. 345 (2002) 196. [5] G. Sundrarajan, L. Rama Krishna, Surf. Coat. Technol. 167 (2003) 269.
4272
Z. Yao et al. / Applied Surface Science 253 (2007) 4267–4272
[6] L.O. Snizhko, A.L. Yerokhin, A. Pilkington, N.L. Gurevina, D.O. Misnyankin, A. Leyland, A. Matthews, Electrochim. Acta 49 (2004) 2085– 2095. [7] P.I. Butygin, Ye.V. Khokhryakov, A.L. Mamaev, Mater. Lett. 57 (2003) 1784. [8] W.B. Xue, C. Wang, Z.W. Deng, R.Y. Chen, T.H. Zhang, Mater. Sci. Technol. 18 (2002) 37. [9] K. Saitoh, K. Sera, T. Gotoh, M. Nakamura, Nucl. Instrum. Methods Phys. Res. B 189 (2002) 86–93. [10] A.L. Briseno, A. Baca, Q. Zhou, R. Lai, F. Zhou, Anal. Chim. Acta 441 (2001) 123–134.
[11] A. Iwashita, T. Nakajima, H. Takanashi, A. Ohki, Y. Fujita, T. Yamashita, Fuel 85 (2006) 257–263. [12] H. Pekey, D. Karak, M. Bakoglu, Marine Pollut. Bull. 49 (2004) 809– 818. [13] W.B. Xue, Z.W. Deng, Y.L. Li, R.Y. Chen, T.H. Zhang, Mater. Sci. Technol. 8 (2000) 41 (in Chinese). [14] R.J. Moolenaar, J.C. Evans, L.D. McKeever, J. Phys. Chem. 74 (1970) 3629. [15] S. Veesler, S. Rource, R. Boistelle, J. Cryst. Growth 135 (1994) 505. [16] L.A. Carreira, V.A. Maroni, J. Phys. Chem. 45 (1966) 2216.