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Controlled deposition, electrical and electrochemical properties of electroless nickel layers on microarc oxidized magnesium substrates
Materials Letters 93 (2013) 263–265
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Controlled deposition, electrical and electrochemical properties of electroless nickel layers on microarc oxidized magnesium substrates Junming Li a, Qianwen Zhang a, Hui Cai b,n, Aijuan Wang a, Jumei Zhang b, Xiaohu Hua b a b
School of Materials Science and Engineering, Xi’an University of Technology, Xi’an 710048, PR China College of Materials Science and Engineering, Xi’an University of Science and Technology, Xi’an 710054, PR China
a r t i c l e i n f o
abstract
Article history: Received 2 September 2012 Accepted 26 November 2012 Available online 3 December 2012
We prepared various porous structures on magnesium substrates by microarc oxidation (MAO) pretreatment in different solutions, and then controllably deposited electroless nickel (EN) layers on them under identical plating conditions. The results indicate that the microstructures, electrical and electrochemical properties of EN layers are highly dependent on the porous surface. Thin layer including tiny nickel granule is deposited on NaF-solution-pretreated substrates, but thick layer, consisting of typical nickel nodule, is formed on substrates oxidized in Na2SiO3 solution, while excellent electrical conductivity and corrosion resistance are obtained. Whereas, the reticular-structured layers with wormlike nickel as well as high thickness and resistivity are area-selectively deposited on the substrate pretreated in Na3PO4 solution. & 2012 Elsevier B.V. All rights reserved.
Keywords: Electroless nickel Microarc oxidation Microstructure Electrical properties Electrochemical properties
1. Introduction Electroless nickel (EN) plating has been widely used in surface protection for magnesium alloys [1–3], as well as in surface metallization for nonconductive materials, e.g. ABS plastic, MgO, TiO2 and Al2O3 ceramics [4,5]. Many factors affect the deposition of EN layers, such as pretreatment, substrate microstructure and bath composition [1–3]. On the other hand, microarc oxidation (MAO) can also form protective coatings on magnesium substrate, which commonly exhibit porous structure with pore size from hundreds of nanometers to tens of micrometers [6,7], while the coating is mainly composed of ceramic phases, so that the MAOtreated magnesium substrate presents low electrical and thermal conductivity. Hence, taking advantages of both methods, the EN/ MAO composite coating can not only well protect magnesium substrate, but can also preserve excellently electrical and thermal properties on substrate surface [8]. Generally, porous structure presents unique properties, unlike dense microstructure, and the properties are changeable with variation from microporous, mesoporous to macroporous [9]. Up to now, there has been little report about the effect of the porous structure of microarc oxidized magnesium substrates on EN plating. In this paper, we controlled the deposition of EN layers by changing the porous structure on AZ31 magnesium alloy, namely, different porous structures were first formed on substrates by
n
Corresponding author. Tel./fax: þ 86 29 82312741. E-mail addresses: [email protected], [email protected] (H. Cai). 0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.11.112
MAO pretreatment in NaF, Na2SiO3 and Na3PO4 solutions; subsequently EN layer was deposited on them under identical plating conditions. In addition, the microstructure, thickness, electrical and electrochemical properties of EN layers were characterized.
2. Experimental Various porous structures were formed on AZ31 magnesium alloy substrates with a size of 30 mm in length, 20 mm in width, and 2 mm in height, by MAO pretreatment in three types of single-component solutions composed of aqueous solution of NaF, Na2SiO3 and Na3PO4, respectively. All solutions have an identical concentration of 0.08 M. The pretreatment was carried out by using MAO65-II microarc oxidation equipment, and process parameters were applied voltage at 380 V, frequency at 420 Hz, duty cycle at 13%, and oxidation time for 6 min. As a result, the MAO-pretreated substrates denoted as MAO(NaF), MAO(Na2SiO3) and MAO(Na3PO4) were obtained. Next, the sensitization in SnCl2 solution and the activation in PdCl2 solution of the pretreated substrates were carried out following a conventional procedure described in Ref. [10]. Subsequently, the substrates were transferred to an electroless bath containing 0.15 M NiSO4, 0.23 M NaH2PO2 and 0.21 M Na4P2O7 at 75 1C for 20 min. The pH 11.5 of bath was adjusted by NH3 H2O. Finally, three kinds of EN layers denoted as EN/MAO(NaF), EN/MAO(Na2SiO3) and EN/MAO(Na3PO4) were deposited. A JSM-6700F field-emission scanning electron microscope (FESEM) was used to observe the microstructure of EN layer and the MAO-pretreated substrate surface. The height profile of the
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J. Li et al. / Materials Letters 93 (2013) 263–265
Fig. 1. (a–c) SEM images of porous structures on magnesium substrates formed by MAO pretreatment in NaF, Na2SiO3 and Na3PO4 solutions, respectively, and the insets show the CLSM-measured height profiles; (d–f) SEM images of the corresponding EN layer deposited on above pretreated substrates.
pretreated surface was measured by a VK-9700 confocal laser scanning microscope (CLSM). The thickness of EN layer was determined according to a method described in Ref. [8]. The electrical property of EN layer was characterized by a RTS-9 fourpoint probe meter. Additionally, electrochemical tests were performed on an IM6e electrochemical analyzer. Potentiodynamic polarization curves were measured in 3.5 wt% NaCl solutions at a scan rate of 10 mV/s. A classical three-electrode cell was used with saturated calomel electrode (SCE) and platinum electrode as reference and counter electrode, respectively, and sample having an exposed area of 1 cm2 as working electrode.
3. Results and discussion Fig. 1 shows the SEM images of the porous structures on magnesium substrates after being MAO pretreated in different solutions and the surface morphology of EN layers deposited on them. The insets show CLSM-measured height profiles of the pretreated surface, suggesting pore depth and surface roughness. In the sequence of pretreatment in NaF, Na2SiO3 and Na3PO4 solutions as shown in Fig. 1(a–c), the porous structures remarkably vary from 1 to 3 mm to larger than 10 mm in pore diameter, and from 2 to 5 mm to deeper than 10 mm in pore depth, while the pore number decreases the surface roughness increases in the presence of interpenetrating pores. Correspondingly, the morphology of EN layer alters with change in the porous surface. The layer deposited on NaF-solution-pretreated substrates consists of tiny nickel granules (see Fig. 1(d)), rather than typical nodules, and the MAO-pretreated surface morphology can also be observed, both of which indicate slow coarsening of deposits. However, the layer on the substrate pretreated in Na2SiO3 solution exhibits coarse nodules with sizes around 10 mm, and the porous MAO underlayer is completely covered with deposits (Fig. 1e). It is noticeable from Fig. 1(f) that, although coarse nickel
Fig. 2. Thickness, sheet resistance, and resistivity of EN layers deposited on magnesium substrates with MAO pretreatment in NaF, Na2SiO3 and Na3PO4 solutions, respectively.
is achieved on substrates oxidized in Na3PO4 solution, the wormlike nickel is area-selectively deposited around large-sized pores on MAO surface, and connects to form reticular structure. Prior to electroless plating, Pd-activated sites were first introduced onto porous MAO surface, where nickel deposits nucleated during EN plating. Whereas, for substrates pretreated in NaF
J. Li et al. / Materials Letters 93 (2013) 263–265
solution, high nucleation density was achieved owing to uniform adsorption of a great deal of Pd, by means of numerously dispersed pores on the porous surface, but the porous structure on substrates pretreated in Na2SiO3 and Na3PO4 solutions led to low nucleation density. Once nickel deposits were produced, EN layer grew via continuous deposition around primary nickel, both vertically and laterally [11]. Meanwhile, the leveling effect improved homogeneous deposition on the whole substrate surface, but it would be invalid if the size of concave area was large enough [11]. In the case of large and deep pore as well as high roughness dominating porous surface, nickel deposits vertically extended, leaving a few undeposited sites. Thus, area-selective deposition occurred. The layer growth on Na3PO4-solutionpretreated substrates is in this way. Fig. 2(a) shows the thickness of EN layers deposited on magnesium substrates, whose values are 0.9, 3.7 and 7.9 mm for layers on substrates with a pretreatment of being oxidized in NaF, Na2SiO3 and Na3PO4 solutions, respectively. The thickness is calculated from the mass gain of substrates after EN plating [8], directly revealing the mass of deposited nickel. Thus, it is inferred that the nickel deposition is much slower on substrates from NaF solution, but the porous structure on substrates pretreated with Na3PO4 solution favors a fast deposition of nickel; on the other hand, different deposition ways affect the microstructure of EN layer, so that only thickness can characterize the layer growth approximately. Moreover, the sheet resistance of EN layer on above substrates is 1.15, 0.19 and 2.05 O/&, and the resistivity is 1.04 10 6, 0.71 10 6 and 16.17 10 6 O m, respectively, as presented in Fig. 2(b) and (c), indicating that the electrical conductivity of EN layer depends significantly on its thickness. The differences may be brought by the microstructure of EN layer. Since continuous nickel deposits are not formed on Na3PO4solution pretreated substrates, hence the layer shows high resistivity, nearly 15 times higher than that of layers deposited on other substrates. Fig. 3 shows the polarization curves of EN layers on substrates with MAO pretreatment in various solutions. For the layer on substrates pretreated in NaF and Na2SiO3 solutions, passivation occurs, as seen from the anodic sides of the curves. But the maintaining passivation current density (ip) and passivation potential regions for both layers are different. It is clear that the value of ip for the latter is lower than that for the former, suggesting that corrosive mediums weakly attack to the latter. For the layer on Na3PO4-solution pretreated substrates, however, the anodic side is controlled by active dissolution reaction and the current density quickly increases as anodic potential elevates. At the same time, the corrosion potential (Ecorr) of EN layer on substrates pretreated in NaF, Na2SiO3 and Na3PO4 solutions is 1.266, 0.740 and 1.483 V, and the corrosion current density
265
(icorr) is 2.361 10 3, 7.005 10 4 and 1.162 10 8 A/cm2, respectively. It is assumed that EN layer on magnesium substrates with MAO pretreatment in Na2SiO3 solution is more protective than the others, due to its dense and defect-free microstructure as well as high thickness. Noticeably, it is interesting that a minimum corrosion current density is achieved for layers deposited on Na3PO4-solution pretreated substrates, though its corrosion potential is the most negative of all. The unique behavior may be influenced by the thickness and microstructure of both EN layer and MAO underlayer.
4. Conclusion In brief, the deposition of EN layers was well controlled by various porous structures on magnesium substrate formed by MAO pretreatment in NaF, Na2SiO3 and Na3PO4 solutions. The porous structure affects the microstructures, thickness, electrical and electrochemical properties of EN layers significantly. Thin layers with tiny nickel granules are deposited on substrates pretreated in NaF solution; whereas thick layers composed of typical nickel nodules are formed on Na2SiO3-solution oxidized substrates, with low resistivity of 0.71 10 6 O m and excellent corrosion resistance are obtained. Owing to area-selective deposition, coarse wormlike nickel forms reticular morphology for EN layer on substrates after being pretreated in Na3PO4 solution. The layer also presents high thickness, but low conductivity.
Acknowledgments Thanks are given to the financial support of Shaanxi Provincial Project of Special Foundation of Key Disciplines for this work.
References [1] El Mahallawy N, Bakkar A, Shoeib M, Palkowski H, Neubert V. Surf Coat Technol 2008;202:5151. [2] Cui XF, Guo J, Li QF, Yang YY, Li Y, Wang FH. Mater Chem Phys 2010;121:308. [3] Ambat R, Zhou W. Surf Coat Technol 2004;179:124. [4] Tang XJ, Bi CL, Han CX, Zhang BG. Mater Lett 2009;63:840. [5] Wu ZJ, Ge SH, Zhang MH, Li W, Tao KY. J Colloid Interface Sci 2009;330:359. [6] Shoaei-Rad V, Bayati MR, Golestani-Fard F, Zargar HR, Javadpour J. Mater Lett 2011;65:1835. [7] Ma CX, Lu Y, Sun PP, Yuan Y, Jing XY, Zhang ML. Surf Coat Technol 2011;206:287. [8] Zeng LY, Yang SW, Zhang W, Guo YH, Yan CW. Electrochim Acta 2010;55:3376. [9] Lee J, Kim J, Hyeon T. Adv Mater 2006;18:2073. [10] Liu WL, Hsieh SH, Tsai TK, Chen WJ, Wu SS. Thin Solid Films 2006;510:102. [11] Liu ZM, Gao W. Surf Coat Technol 2006;200:3553.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Controlled deposition, electrical and electrochemical properties of electroless nickel layers on microarc oxidized magnesium substrates Junming Li a, Qianwen Zhang a, Hui Cai b,n, Aijuan Wang a, Jumei Zhang b, Xiaohu Hua b a b
School of Materials Science and Engineering, Xi’an University of Technology, Xi’an 710048, PR China College of Materials Science and Engineering, Xi’an University of Science and Technology, Xi’an 710054, PR China
a r t i c l e i n f o
abstract
Article history: Received 2 September 2012 Accepted 26 November 2012 Available online 3 December 2012
We prepared various porous structures on magnesium substrates by microarc oxidation (MAO) pretreatment in different solutions, and then controllably deposited electroless nickel (EN) layers on them under identical plating conditions. The results indicate that the microstructures, electrical and electrochemical properties of EN layers are highly dependent on the porous surface. Thin layer including tiny nickel granule is deposited on NaF-solution-pretreated substrates, but thick layer, consisting of typical nickel nodule, is formed on substrates oxidized in Na2SiO3 solution, while excellent electrical conductivity and corrosion resistance are obtained. Whereas, the reticular-structured layers with wormlike nickel as well as high thickness and resistivity are area-selectively deposited on the substrate pretreated in Na3PO4 solution. & 2012 Elsevier B.V. All rights reserved.
Keywords: Electroless nickel Microarc oxidation Microstructure Electrical properties Electrochemical properties
1. Introduction Electroless nickel (EN) plating has been widely used in surface protection for magnesium alloys [1–3], as well as in surface metallization for nonconductive materials, e.g. ABS plastic, MgO, TiO2 and Al2O3 ceramics [4,5]. Many factors affect the deposition of EN layers, such as pretreatment, substrate microstructure and bath composition [1–3]. On the other hand, microarc oxidation (MAO) can also form protective coatings on magnesium substrate, which commonly exhibit porous structure with pore size from hundreds of nanometers to tens of micrometers [6,7], while the coating is mainly composed of ceramic phases, so that the MAOtreated magnesium substrate presents low electrical and thermal conductivity. Hence, taking advantages of both methods, the EN/ MAO composite coating can not only well protect magnesium substrate, but can also preserve excellently electrical and thermal properties on substrate surface [8]. Generally, porous structure presents unique properties, unlike dense microstructure, and the properties are changeable with variation from microporous, mesoporous to macroporous [9]. Up to now, there has been little report about the effect of the porous structure of microarc oxidized magnesium substrates on EN plating. In this paper, we controlled the deposition of EN layers by changing the porous structure on AZ31 magnesium alloy, namely, different porous structures were first formed on substrates by
n
Corresponding author. Tel./fax: þ 86 29 82312741. E-mail addresses: [email protected], [email protected] (H. Cai). 0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.11.112
MAO pretreatment in NaF, Na2SiO3 and Na3PO4 solutions; subsequently EN layer was deposited on them under identical plating conditions. In addition, the microstructure, thickness, electrical and electrochemical properties of EN layers were characterized.
2. Experimental Various porous structures were formed on AZ31 magnesium alloy substrates with a size of 30 mm in length, 20 mm in width, and 2 mm in height, by MAO pretreatment in three types of single-component solutions composed of aqueous solution of NaF, Na2SiO3 and Na3PO4, respectively. All solutions have an identical concentration of 0.08 M. The pretreatment was carried out by using MAO65-II microarc oxidation equipment, and process parameters were applied voltage at 380 V, frequency at 420 Hz, duty cycle at 13%, and oxidation time for 6 min. As a result, the MAO-pretreated substrates denoted as MAO(NaF), MAO(Na2SiO3) and MAO(Na3PO4) were obtained. Next, the sensitization in SnCl2 solution and the activation in PdCl2 solution of the pretreated substrates were carried out following a conventional procedure described in Ref. [10]. Subsequently, the substrates were transferred to an electroless bath containing 0.15 M NiSO4, 0.23 M NaH2PO2 and 0.21 M Na4P2O7 at 75 1C for 20 min. The pH 11.5 of bath was adjusted by NH3 H2O. Finally, three kinds of EN layers denoted as EN/MAO(NaF), EN/MAO(Na2SiO3) and EN/MAO(Na3PO4) were deposited. A JSM-6700F field-emission scanning electron microscope (FESEM) was used to observe the microstructure of EN layer and the MAO-pretreated substrate surface. The height profile of the
264
J. Li et al. / Materials Letters 93 (2013) 263–265
Fig. 1. (a–c) SEM images of porous structures on magnesium substrates formed by MAO pretreatment in NaF, Na2SiO3 and Na3PO4 solutions, respectively, and the insets show the CLSM-measured height profiles; (d–f) SEM images of the corresponding EN layer deposited on above pretreated substrates.
pretreated surface was measured by a VK-9700 confocal laser scanning microscope (CLSM). The thickness of EN layer was determined according to a method described in Ref. [8]. The electrical property of EN layer was characterized by a RTS-9 fourpoint probe meter. Additionally, electrochemical tests were performed on an IM6e electrochemical analyzer. Potentiodynamic polarization curves were measured in 3.5 wt% NaCl solutions at a scan rate of 10 mV/s. A classical three-electrode cell was used with saturated calomel electrode (SCE) and platinum electrode as reference and counter electrode, respectively, and sample having an exposed area of 1 cm2 as working electrode.
3. Results and discussion Fig. 1 shows the SEM images of the porous structures on magnesium substrates after being MAO pretreated in different solutions and the surface morphology of EN layers deposited on them. The insets show CLSM-measured height profiles of the pretreated surface, suggesting pore depth and surface roughness. In the sequence of pretreatment in NaF, Na2SiO3 and Na3PO4 solutions as shown in Fig. 1(a–c), the porous structures remarkably vary from 1 to 3 mm to larger than 10 mm in pore diameter, and from 2 to 5 mm to deeper than 10 mm in pore depth, while the pore number decreases the surface roughness increases in the presence of interpenetrating pores. Correspondingly, the morphology of EN layer alters with change in the porous surface. The layer deposited on NaF-solution-pretreated substrates consists of tiny nickel granules (see Fig. 1(d)), rather than typical nodules, and the MAO-pretreated surface morphology can also be observed, both of which indicate slow coarsening of deposits. However, the layer on the substrate pretreated in Na2SiO3 solution exhibits coarse nodules with sizes around 10 mm, and the porous MAO underlayer is completely covered with deposits (Fig. 1e). It is noticeable from Fig. 1(f) that, although coarse nickel
Fig. 2. Thickness, sheet resistance, and resistivity of EN layers deposited on magnesium substrates with MAO pretreatment in NaF, Na2SiO3 and Na3PO4 solutions, respectively.
is achieved on substrates oxidized in Na3PO4 solution, the wormlike nickel is area-selectively deposited around large-sized pores on MAO surface, and connects to form reticular structure. Prior to electroless plating, Pd-activated sites were first introduced onto porous MAO surface, where nickel deposits nucleated during EN plating. Whereas, for substrates pretreated in NaF
J. Li et al. / Materials Letters 93 (2013) 263–265
solution, high nucleation density was achieved owing to uniform adsorption of a great deal of Pd, by means of numerously dispersed pores on the porous surface, but the porous structure on substrates pretreated in Na2SiO3 and Na3PO4 solutions led to low nucleation density. Once nickel deposits were produced, EN layer grew via continuous deposition around primary nickel, both vertically and laterally [11]. Meanwhile, the leveling effect improved homogeneous deposition on the whole substrate surface, but it would be invalid if the size of concave area was large enough [11]. In the case of large and deep pore as well as high roughness dominating porous surface, nickel deposits vertically extended, leaving a few undeposited sites. Thus, area-selective deposition occurred. The layer growth on Na3PO4-solutionpretreated substrates is in this way. Fig. 2(a) shows the thickness of EN layers deposited on magnesium substrates, whose values are 0.9, 3.7 and 7.9 mm for layers on substrates with a pretreatment of being oxidized in NaF, Na2SiO3 and Na3PO4 solutions, respectively. The thickness is calculated from the mass gain of substrates after EN plating [8], directly revealing the mass of deposited nickel. Thus, it is inferred that the nickel deposition is much slower on substrates from NaF solution, but the porous structure on substrates pretreated with Na3PO4 solution favors a fast deposition of nickel; on the other hand, different deposition ways affect the microstructure of EN layer, so that only thickness can characterize the layer growth approximately. Moreover, the sheet resistance of EN layer on above substrates is 1.15, 0.19 and 2.05 O/&, and the resistivity is 1.04 10 6, 0.71 10 6 and 16.17 10 6 O m, respectively, as presented in Fig. 2(b) and (c), indicating that the electrical conductivity of EN layer depends significantly on its thickness. The differences may be brought by the microstructure of EN layer. Since continuous nickel deposits are not formed on Na3PO4solution pretreated substrates, hence the layer shows high resistivity, nearly 15 times higher than that of layers deposited on other substrates. Fig. 3 shows the polarization curves of EN layers on substrates with MAO pretreatment in various solutions. For the layer on substrates pretreated in NaF and Na2SiO3 solutions, passivation occurs, as seen from the anodic sides of the curves. But the maintaining passivation current density (ip) and passivation potential regions for both layers are different. It is clear that the value of ip for the latter is lower than that for the former, suggesting that corrosive mediums weakly attack to the latter. For the layer on Na3PO4-solution pretreated substrates, however, the anodic side is controlled by active dissolution reaction and the current density quickly increases as anodic potential elevates. At the same time, the corrosion potential (Ecorr) of EN layer on substrates pretreated in NaF, Na2SiO3 and Na3PO4 solutions is 1.266, 0.740 and 1.483 V, and the corrosion current density
265
(icorr) is 2.361 10 3, 7.005 10 4 and 1.162 10 8 A/cm2, respectively. It is assumed that EN layer on magnesium substrates with MAO pretreatment in Na2SiO3 solution is more protective than the others, due to its dense and defect-free microstructure as well as high thickness. Noticeably, it is interesting that a minimum corrosion current density is achieved for layers deposited on Na3PO4-solution pretreated substrates, though its corrosion potential is the most negative of all. The unique behavior may be influenced by the thickness and microstructure of both EN layer and MAO underlayer.
4. Conclusion In brief, the deposition of EN layers was well controlled by various porous structures on magnesium substrate formed by MAO pretreatment in NaF, Na2SiO3 and Na3PO4 solutions. The porous structure affects the microstructures, thickness, electrical and electrochemical properties of EN layers significantly. Thin layers with tiny nickel granules are deposited on substrates pretreated in NaF solution; whereas thick layers composed of typical nickel nodules are formed on Na2SiO3-solution oxidized substrates, with low resistivity of 0.71 10 6 O m and excellent corrosion resistance are obtained. Owing to area-selective deposition, coarse wormlike nickel forms reticular morphology for EN layer on substrates after being pretreated in Na3PO4 solution. The layer also presents high thickness, but low conductivity.
Acknowledgments Thanks are given to the financial support of Shaanxi Provincial Project of Special Foundation of Key Disciplines for this work.
References [1] El Mahallawy N, Bakkar A, Shoeib M, Palkowski H, Neubert V. Surf Coat Technol 2008;202:5151. [2] Cui XF, Guo J, Li QF, Yang YY, Li Y, Wang FH. Mater Chem Phys 2010;121:308. [3] Ambat R, Zhou W. Surf Coat Technol 2004;179:124. [4] Tang XJ, Bi CL, Han CX, Zhang BG. Mater Lett 2009;63:840. [5] Wu ZJ, Ge SH, Zhang MH, Li W, Tao KY. J Colloid Interface Sci 2009;330:359. [6] Shoaei-Rad V, Bayati MR, Golestani-Fard F, Zargar HR, Javadpour J. Mater Lett 2011;65:1835. [7] Ma CX, Lu Y, Sun PP, Yuan Y, Jing XY, Zhang ML. Surf Coat Technol 2011;206:287. [8] Zeng LY, Yang SW, Zhang W, Guo YH, Yan CW. Electrochim Acta 2010;55:3376. [9] Lee J, Kim J, Hyeon T. Adv Mater 2006;18:2073. [10] Liu WL, Hsieh SH, Tsai TK, Chen WJ, Wu SS. Thin Solid Films 2006;510:102. [11] Liu ZM, Gao W. Surf Coat Technol 2006;200:3553.