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Structural characterization and properties of colloidal silica coatings on copper substrates
October 2002
Materials Letters 56 (2002) 450 – 453 www.elsevier.com/locate/matlet
Structural characterization and properties of colloidal silica coatings on copper substrates L.A. Garcı´a-Cerda a,b,*, O. Mendoza-Gonza´lez b, J.F. Pe´rez-Robles c, J. Gonza´lez-Herna´ndez c a
Centro de Investigacio´n en Quı´mica Aplicada, Blvd. Enrique Reyna Hermosillo #140, Apdo. Postal 379, C.P. 25100 Saltillo, Coah., Mexico b Instituto Tecnolo´gico de Saltillo, Blvd. Venustiano Carranza #2400, Saltillo, Coah., Mexico c Centro de Investigacio´n y Estudios Avanzados del IPN, Unidad Quere´taro, Apdo. Postal 1-798, Quere´taro, Qro, Mexico Received 11 July 2001; accepted 19 November 2001
Abstract Colloidal silica coatings were deposited on copper substrates and their structural properties and corrosion resistance to a saline solution were studied. Optacolk colloidal silica was used as primary reactive. Coatings were obtained by immersion and withdrawal speed was 2.1 mm/s. They were dried at 100 jC during 15 min and given a heat treatment at 180 jC for 30 min. Xray diffraction (XRD) showed the existence on the coating of a semicrystalline cristoballite phase depending on the heat treatment temperature. X-ray-photoelectron spectroscopy (XPS) demonstrated that the formation of SiO2 bonds is enhanced when the heat treatment temperature is increased. Electrochemical impedance spectroscopy (EIS) was used to measure the corrosion resistance of these films. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Colloidal silica; Sol – gel coatings; SiO2 thin films; Copper substrate; Electrochemical impedance spectroscopy
1. Introduction Ceramic coatings on diverse substrates have been applied intensively by procedures requiring high degree of precision and uniformity in the deposition of thin films, with remarkable and unique electrical, optical, protective and reacting applications [1 –4]. Among the deposition techniques such as CVD, PVD and thermal spraying which require high vacuum or high temperature, there are coatings techni-
*
Corresponding author. Tel.: +52-8-415-3030; fax: +52-8-4154804. E-mail address: [email protected] (L.A. Garcı´a-Cerda).
ques performed in liquid phase such as electroless, electrolytic, sol – gel and chemical bath deposition, which offer excellent advantages over the first ones already mentioned; these liquid phase techniques are simpler and less costly [5]. The sol – gel process uses basic reactives such as alcoxides or colloidal solutions. In both cases, a gelation process is carried out and the product is a rigid gel; this is transformed into a solid material by a heat treatment [6]. The main advantages of the colloidal solution process are its simplicity, the use of non-hazardous starting reactives and its easy industrial application. This study reports the characteristics of colloidal silica coatings on copper substrates such as structure by X-ray diffraction (XRD) and chemical oxidation
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 5 2 6 - 8
L.A. Garcı´a-Cerda et al. / Materials Letters 56 (2002) 450–453
451
states by X-ray photoelectron spectroscopy. Atomic force microscopy (AFM) and electrochemical impedance spectroscopy (EIS) were applied to determine the coatings corrosion resistance to a saline solution.
2. Experimental The sol –gel process was performed to obtain the colloidal silica deposits, preparing a 50 ml solution of Optacolk colloidal silica with addition of 100 ml of distilled water, the mixture was agitated at 40 rpm during 5 min. Sodium hydroxide of 6.5 g was added and stirred for 10 min. In another container, 12.5 g of sodium silicate were dissolved in 42 ml of distilled water and added to the first solution. The mixture was kept in a closed container during 20 h, after which a transparent solution was obtained. Polished copper plaques of 2.5 by 7.5 cm and 2 mm thick were used as substrates. The coatings were obtained by immersing them at pullout speed of 2.1 mm/s. Then, samples were dried at 100 jC during 15 min and heat treated at 180 jC for 30 min. X-ray diffraction patterns were taken on a Rigaku D/Max-2100 using CuKa at an incident angle of 0.5j and operating power of 30 kV and 16 mA. X-rayphotoelectron spectroscopy (XPS) analysis was performed on the coated copper substrate to determine ˚ deep from the the coating composition at 150 A surface. For the measurements with EIS, a 3.5 wt.% NaCl solution was prepared and the coatings corrosion resistance was carried out in a three electrodes electrolytic cell, the working electrode (coated copper substrate), a saturated calomel reference electrode and a platinum foil as counter electrode. The working electrode potential was set by a Gamry PC3 potenciostat the same as the open circuit potential (Eoc) and a 10-mV amplitude sinoidal signal in a frequency range of 5 kHz, taking five points per decade. AFM images were taken from an atomic force microscope SFMBD2 from Park Scientific Instruments, using a tellurium pointed needle as a cantilever.
3. Results and discussions Fig. 1 shows the X-ray patterns from the colloidal silica coatings treated at (a) room temperature, (b) 100
Fig. 1. X-ray patterns from the colloidal silica coatings treated at (a) room temperature, (b) 100 jC for 15 min and (c) 180 jC during 30 min.
jC for 15 min and (c) 180 jC during 30 min. It can be possible that all the coatings posses a common band located between 15 and 30 jC with a maximum around 23 jC in the 2h scale whose width depends on the structural order representing a form of cristoballite phase [7]. The band losses its width when the heat treatment temperature is increased due to an improvement in the structural arrangement of the coating short order providing a certain degree of crystallinity to the material [8]. The elemental atomic percentage on the coatings was obtained by XPS. Table 1 shows these results. Sodium has the highest content when the coatings are at room temperature and it decreases when the heat treatment temperature and time are increased, the same tendency is observed with the carbon content. A possible explanation to this is that the heat treatment diminishes the coatings porosity and might expel the sodium at temperatures such as 180 jC. On the other hand, silicon and oxygen are present in small proportions in the films at low temperatures; as the temperature of the heat treatment is increased, so is the silicon and oxygen proportions due to the SiO2 bonds formation. Fig. 2 reports the monitoring results for the open circuit potential (Eoc) from a copper substrate and a coated copper substrate. The Eoc in the copper is maintained almost constant from the beginning, while the Eoc from the coated substrate shows larger variations during the running of the study. These measurements suggest that the Eoc for the samples has a
L.A. Garcı´a-Cerda et al. / Materials Letters 56 (2002) 450–453
452
Table 1 The elemental atomic percentage on the coatings obtained by XPS Sample (jC)
Room temperature 100 180
Atomic content (%) Na
O
C
Si
43.5 36.5 14.5
32.1 42.1 54.2
16.2 11.3 8.4
7.2 10.1 22.9
tendency to be stabilized after 24 h from the immersion, this was confirmed by two extra determinations performed after 27 and 54 h providing Eoc values of 200 and 204 mV, respectively. The latter does not imply that the electrochemical behavior of those copper electrodes be the same, for which it is mandatory to carry out direct and/or alternated current polarizations to verify if the coatings posses an anticorrosion effect for the copper or if it is degraded by the saline solution. The impedance diagram for the copper and the coated copper substrates is shown in Fig. 3. The impedance response is very different to the one from the Eoc, the impedance module for the coated copper is almost three orders of magnitude higher than the copper alone in the whole frequency range applied, as it can be observed in Fig. 3a. For uncoated metals in a corrosive solution, the impedance value can be assigned to the electrolyte resistance, in this case, it is 20 V. The fact that the presence of coating makes the high frequency resistance bigger than the electroFig. 3. The impedance diagram for the copper and the coated copper substrate after 5.5 h in a solution of NaCl 3.5 wt.% (a) and the influence of the immersion time in the impedance diagrams (b).
Fig. 2. Results for the open circuit potential (Eoc) from a copper substrate and a coated copper substrate in a solution of NaCl 3.5 wt.%.
lyte might be due probably to the contribution to the series resistance of the SiO2 coating. In Fig. 3b, the influence of immersion time is presented in the impedance diagrams for the copper and coated copper substrates. It can be observed that the system evolves in such way that the impedance vs. frequency diagrams tend to obtain lower impedance values, this could possibly be due mainly to a reduction of corrosion resistance at high frequency. This suggests that the coating has a gradual degradation diminishing its corrosion resistance. In order to study the topography of coatings prior and during the exposition to the saline solution, atomic force microscopy images were taken. Fig. 4
L.A. Garcı´a-Cerda et al. / Materials Letters 56 (2002) 450–453
453
to determine that the coating is mainly constituted by semicrystalline cristoballite phase. Eoc measurements in the copper and coated copper substrates were almost the same after an immersion time of 54 h in the saline solution, however, latter determinations showed that coated copper exhibits a higher impedance value than the one obtained from the uncoated copper. Nevertheless, the value decreases during the immersion and it seems to be related to the reduction of the resistance at high frequency, which suggests that the coating is being gradually degrading. AFM images showed the presence of cracks or canyons on the coating surface, which is indicative of the degradation, these cracks might be explained by the hydrolysis reactions in the SiUO bonds associated with zones containing accumulated internal stresses.
Acknowledgements
Fig. 4. AFM images from coated copper prior (a) and after (b) immersion in the NaCl solution during 54 h.
presents the images from coated copper prior (a) and after (b) immersion in the NaCl solution during 54 h. The presence of cracks and a canyon zone is indicative of the degradation and it is consistent with the results of the determinations from EIS (the penetration of the electrolyte in the coating diminishes its resistance). It is probably that the formation of cracks is due to hydrolysis reactions in SiUO bondings associated to zones with accumulated internal stresses, which provokes certain solubilization of Si species and therefore the formation of cracks.
4. Conclusions Colloidal silica coatings where deposited on copper substrates. The use of X-ray diffraction permitted
L.A. Garcı´a-Cerda thanks CONACYT for the financial support under the project J35161-U. The authors also thank Luz Ma. Reyna Avile´s for the preparations of the coatings and to Dr. Ma´ximo Pech of CINVESTAV-Me´rida for his assistance in the EIS analysis and discussion.
References [1] W.L. Warren, P.M. Lenahan, J. Kanicki, J. Appl. Phys. 70 (4) (1991) 2220. [2] M. Atik, P. De Lima Neto, J. Zarzycki, J. Mater. Sci. Lett. 13 (15) (1994) 1081. [3] T. Nishide, M. Shinoda, F. Mizukami, Thin Solid Films 238 (2) (1994) 163. [4] B. Tokarz, B. Larsson, S. Jaras, US Patent 4,950,627 (1990). [5] D. Segal, Chemical Synthesis of Advanced Ceramics Materials, Cambridge Univ. Press, Cambridge, UK, 1989, p. 33. [6] C.J. Brinker, G.W. Scherer, Sol – Gel Science: The Physics and Chemistry of the Sol – Gel Processing, Academic Press, New York, 1990, p. 839. [7] R.H. Doremus, Glass Science, 2nd edn., Wiley, New York, 1995, p. 25. [8] M.G. Ga´rnica-Romo, et al., J. Phys. Chem. (In press).
Materials Letters 56 (2002) 450 – 453 www.elsevier.com/locate/matlet
Structural characterization and properties of colloidal silica coatings on copper substrates L.A. Garcı´a-Cerda a,b,*, O. Mendoza-Gonza´lez b, J.F. Pe´rez-Robles c, J. Gonza´lez-Herna´ndez c a
Centro de Investigacio´n en Quı´mica Aplicada, Blvd. Enrique Reyna Hermosillo #140, Apdo. Postal 379, C.P. 25100 Saltillo, Coah., Mexico b Instituto Tecnolo´gico de Saltillo, Blvd. Venustiano Carranza #2400, Saltillo, Coah., Mexico c Centro de Investigacio´n y Estudios Avanzados del IPN, Unidad Quere´taro, Apdo. Postal 1-798, Quere´taro, Qro, Mexico Received 11 July 2001; accepted 19 November 2001
Abstract Colloidal silica coatings were deposited on copper substrates and their structural properties and corrosion resistance to a saline solution were studied. Optacolk colloidal silica was used as primary reactive. Coatings were obtained by immersion and withdrawal speed was 2.1 mm/s. They were dried at 100 jC during 15 min and given a heat treatment at 180 jC for 30 min. Xray diffraction (XRD) showed the existence on the coating of a semicrystalline cristoballite phase depending on the heat treatment temperature. X-ray-photoelectron spectroscopy (XPS) demonstrated that the formation of SiO2 bonds is enhanced when the heat treatment temperature is increased. Electrochemical impedance spectroscopy (EIS) was used to measure the corrosion resistance of these films. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Colloidal silica; Sol – gel coatings; SiO2 thin films; Copper substrate; Electrochemical impedance spectroscopy
1. Introduction Ceramic coatings on diverse substrates have been applied intensively by procedures requiring high degree of precision and uniformity in the deposition of thin films, with remarkable and unique electrical, optical, protective and reacting applications [1 –4]. Among the deposition techniques such as CVD, PVD and thermal spraying which require high vacuum or high temperature, there are coatings techni-
*
Corresponding author. Tel.: +52-8-415-3030; fax: +52-8-4154804. E-mail address: [email protected] (L.A. Garcı´a-Cerda).
ques performed in liquid phase such as electroless, electrolytic, sol – gel and chemical bath deposition, which offer excellent advantages over the first ones already mentioned; these liquid phase techniques are simpler and less costly [5]. The sol – gel process uses basic reactives such as alcoxides or colloidal solutions. In both cases, a gelation process is carried out and the product is a rigid gel; this is transformed into a solid material by a heat treatment [6]. The main advantages of the colloidal solution process are its simplicity, the use of non-hazardous starting reactives and its easy industrial application. This study reports the characteristics of colloidal silica coatings on copper substrates such as structure by X-ray diffraction (XRD) and chemical oxidation
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 5 2 6 - 8
L.A. Garcı´a-Cerda et al. / Materials Letters 56 (2002) 450–453
451
states by X-ray photoelectron spectroscopy. Atomic force microscopy (AFM) and electrochemical impedance spectroscopy (EIS) were applied to determine the coatings corrosion resistance to a saline solution.
2. Experimental The sol –gel process was performed to obtain the colloidal silica deposits, preparing a 50 ml solution of Optacolk colloidal silica with addition of 100 ml of distilled water, the mixture was agitated at 40 rpm during 5 min. Sodium hydroxide of 6.5 g was added and stirred for 10 min. In another container, 12.5 g of sodium silicate were dissolved in 42 ml of distilled water and added to the first solution. The mixture was kept in a closed container during 20 h, after which a transparent solution was obtained. Polished copper plaques of 2.5 by 7.5 cm and 2 mm thick were used as substrates. The coatings were obtained by immersing them at pullout speed of 2.1 mm/s. Then, samples were dried at 100 jC during 15 min and heat treated at 180 jC for 30 min. X-ray diffraction patterns were taken on a Rigaku D/Max-2100 using CuKa at an incident angle of 0.5j and operating power of 30 kV and 16 mA. X-rayphotoelectron spectroscopy (XPS) analysis was performed on the coated copper substrate to determine ˚ deep from the the coating composition at 150 A surface. For the measurements with EIS, a 3.5 wt.% NaCl solution was prepared and the coatings corrosion resistance was carried out in a three electrodes electrolytic cell, the working electrode (coated copper substrate), a saturated calomel reference electrode and a platinum foil as counter electrode. The working electrode potential was set by a Gamry PC3 potenciostat the same as the open circuit potential (Eoc) and a 10-mV amplitude sinoidal signal in a frequency range of 5 kHz, taking five points per decade. AFM images were taken from an atomic force microscope SFMBD2 from Park Scientific Instruments, using a tellurium pointed needle as a cantilever.
3. Results and discussions Fig. 1 shows the X-ray patterns from the colloidal silica coatings treated at (a) room temperature, (b) 100
Fig. 1. X-ray patterns from the colloidal silica coatings treated at (a) room temperature, (b) 100 jC for 15 min and (c) 180 jC during 30 min.
jC for 15 min and (c) 180 jC during 30 min. It can be possible that all the coatings posses a common band located between 15 and 30 jC with a maximum around 23 jC in the 2h scale whose width depends on the structural order representing a form of cristoballite phase [7]. The band losses its width when the heat treatment temperature is increased due to an improvement in the structural arrangement of the coating short order providing a certain degree of crystallinity to the material [8]. The elemental atomic percentage on the coatings was obtained by XPS. Table 1 shows these results. Sodium has the highest content when the coatings are at room temperature and it decreases when the heat treatment temperature and time are increased, the same tendency is observed with the carbon content. A possible explanation to this is that the heat treatment diminishes the coatings porosity and might expel the sodium at temperatures such as 180 jC. On the other hand, silicon and oxygen are present in small proportions in the films at low temperatures; as the temperature of the heat treatment is increased, so is the silicon and oxygen proportions due to the SiO2 bonds formation. Fig. 2 reports the monitoring results for the open circuit potential (Eoc) from a copper substrate and a coated copper substrate. The Eoc in the copper is maintained almost constant from the beginning, while the Eoc from the coated substrate shows larger variations during the running of the study. These measurements suggest that the Eoc for the samples has a
L.A. Garcı´a-Cerda et al. / Materials Letters 56 (2002) 450–453
452
Table 1 The elemental atomic percentage on the coatings obtained by XPS Sample (jC)
Room temperature 100 180
Atomic content (%) Na
O
C
Si
43.5 36.5 14.5
32.1 42.1 54.2
16.2 11.3 8.4
7.2 10.1 22.9
tendency to be stabilized after 24 h from the immersion, this was confirmed by two extra determinations performed after 27 and 54 h providing Eoc values of 200 and 204 mV, respectively. The latter does not imply that the electrochemical behavior of those copper electrodes be the same, for which it is mandatory to carry out direct and/or alternated current polarizations to verify if the coatings posses an anticorrosion effect for the copper or if it is degraded by the saline solution. The impedance diagram for the copper and the coated copper substrates is shown in Fig. 3. The impedance response is very different to the one from the Eoc, the impedance module for the coated copper is almost three orders of magnitude higher than the copper alone in the whole frequency range applied, as it can be observed in Fig. 3a. For uncoated metals in a corrosive solution, the impedance value can be assigned to the electrolyte resistance, in this case, it is 20 V. The fact that the presence of coating makes the high frequency resistance bigger than the electroFig. 3. The impedance diagram for the copper and the coated copper substrate after 5.5 h in a solution of NaCl 3.5 wt.% (a) and the influence of the immersion time in the impedance diagrams (b).
Fig. 2. Results for the open circuit potential (Eoc) from a copper substrate and a coated copper substrate in a solution of NaCl 3.5 wt.%.
lyte might be due probably to the contribution to the series resistance of the SiO2 coating. In Fig. 3b, the influence of immersion time is presented in the impedance diagrams for the copper and coated copper substrates. It can be observed that the system evolves in such way that the impedance vs. frequency diagrams tend to obtain lower impedance values, this could possibly be due mainly to a reduction of corrosion resistance at high frequency. This suggests that the coating has a gradual degradation diminishing its corrosion resistance. In order to study the topography of coatings prior and during the exposition to the saline solution, atomic force microscopy images were taken. Fig. 4
L.A. Garcı´a-Cerda et al. / Materials Letters 56 (2002) 450–453
453
to determine that the coating is mainly constituted by semicrystalline cristoballite phase. Eoc measurements in the copper and coated copper substrates were almost the same after an immersion time of 54 h in the saline solution, however, latter determinations showed that coated copper exhibits a higher impedance value than the one obtained from the uncoated copper. Nevertheless, the value decreases during the immersion and it seems to be related to the reduction of the resistance at high frequency, which suggests that the coating is being gradually degrading. AFM images showed the presence of cracks or canyons on the coating surface, which is indicative of the degradation, these cracks might be explained by the hydrolysis reactions in the SiUO bonds associated with zones containing accumulated internal stresses.
Acknowledgements
Fig. 4. AFM images from coated copper prior (a) and after (b) immersion in the NaCl solution during 54 h.
presents the images from coated copper prior (a) and after (b) immersion in the NaCl solution during 54 h. The presence of cracks and a canyon zone is indicative of the degradation and it is consistent with the results of the determinations from EIS (the penetration of the electrolyte in the coating diminishes its resistance). It is probably that the formation of cracks is due to hydrolysis reactions in SiUO bondings associated to zones with accumulated internal stresses, which provokes certain solubilization of Si species and therefore the formation of cracks.
4. Conclusions Colloidal silica coatings where deposited on copper substrates. The use of X-ray diffraction permitted
L.A. Garcı´a-Cerda thanks CONACYT for the financial support under the project J35161-U. The authors also thank Luz Ma. Reyna Avile´s for the preparations of the coatings and to Dr. Ma´ximo Pech of CINVESTAV-Me´rida for his assistance in the EIS analysis and discussion.
References [1] W.L. Warren, P.M. Lenahan, J. Kanicki, J. Appl. Phys. 70 (4) (1991) 2220. [2] M. Atik, P. De Lima Neto, J. Zarzycki, J. Mater. Sci. Lett. 13 (15) (1994) 1081. [3] T. Nishide, M. Shinoda, F. Mizukami, Thin Solid Films 238 (2) (1994) 163. [4] B. Tokarz, B. Larsson, S. Jaras, US Patent 4,950,627 (1990). [5] D. Segal, Chemical Synthesis of Advanced Ceramics Materials, Cambridge Univ. Press, Cambridge, UK, 1989, p. 33. [6] C.J. Brinker, G.W. Scherer, Sol – Gel Science: The Physics and Chemistry of the Sol – Gel Processing, Academic Press, New York, 1990, p. 839. [7] R.H. Doremus, Glass Science, 2nd edn., Wiley, New York, 1995, p. 25. [8] M.G. Ga´rnica-Romo, et al., J. Phys. Chem. (In press).