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Preparation and structure of ceramic coatings containing zirconium oxide on Ti alloy by plasma electrolytic oxidation
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 5 ( 2 0 0 8 ) 303–307
journal homepage: www.elsevier.com/locate/jmatprotec
Preparation and structure of ceramic coatings containing zirconium oxide on Ti alloy by plasma electrolytic oxidation Zhongping Yao a,∗ , Yanli Jiang a,b , Zhaohua Jiang a , Fuping Wang a , Zhendong Wu a a b
Department of Applied Chemistry, Harbin Institute of Technology, Harbin 150001, PR China Department of Life Science and Chemistry, Harbin University, Harbin, 150086, PR China
a r t i c l e
i n f o
a b s t r a c t
Article history:
The aim of this work was to prepare ceramic coatings containing ZrO2 phase on Ti alloy
Received 3 June 2007
and study their structure and corrosion resistance. Compound ceramic coatings were pre-
Received in revised form
pared on Ti-6Al-4V alloy by pulsed single-polar plasma electrolytic oxidation (PEO) in K2 ZrF6
1 November 2007
electrolyte. The phase composition, morphology of the coatings and the element distribu-
Accepted 18 November 2007
tion in the coating were investigated by X-ray diffractometry, scanning electron microscopy and energy dispersive spectroscopy. The corrosion resistance of the coated samples was examined through the potentiodynamic anodic curves in 3.5% NaCl solution. The coatings
Keywords:
prepared for short PEO time was composed of m-ZrO2 , t-ZrO2 and ZrTiO4 and a little ZrP2 O7 ;
Plasma electrolytic oxidation
while increasing PEO time, the content of ZrP2 O7 was increased and became the main crys-
Ceramic coatings
talline. The Ti content in the coating near the substrate was decreased sharply while the
ZrO2
content of Zr was increased greatly. The thickness of the coating was increased with the
Titanium alloy
PEO time, but the coatings turned rougher and more porous. The prepared coated samples
Corrosion resistance
had better corrosion resistance than the substrate. Among the coated samples, the coated sample prepared for 40 min had the best corrosion resistance. © 2007 Elsevier B.V. All rights reserved.
1.
Introduction
Plasma electrolytic oxidation (PEO) is a technique by which the ceramic coating can be grown in situ on Al, Ti, Mg and many other valve-metals. This technique, dated back to ¨ 1930s, when Gunterschulze and Betz first studied spark discharge at the anode surface, has been developed quickly in the surface treatment of metals in recent years. The prepared coatings by PEO tech. is reported to have fine properties like corrosion resistance, anti-abrasion property or decorative property and so on and the promising application prospect in many fields (Yerokhin et al., 1999, 2000; Guan and Xia, 2004). At present, the widely adopted electrolytes on Ti alloy are phosphate, aluminate, silicate, or their mixed solutions
∗
Corresponding author. Tel.: +86 451 86413710; fax: +86 451 86413709. E-mail address: [email protected] (Z.P. Yao). 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.11.112
(Sundararajan and Krishna, 2003; Xue et al., 2002; Liang et al., 2005; Yerohin et al., 2002; Jiang et al., 2004) under different PEO electric source modes and the electric parameters (Boguta et al., 2004; Shi et al., 2005; Yao et al., 2004; Wu et al., 2005) in order to adjust the structure and composition of the ceramic coatings. However, few instances of research of PEO tech. on Ti alloy in the zirconate solution were reported except for the similar work on Al alloys in Refs (Schukin et al., 1996; Wu et al., 2007). Zirconium dioxide is a promising coating material, which has high strength, good fracture toughness property, excellent wear resistance and corrosion resistance and so on. Therefore, we prepared the ceramic coatings containing ZrO2 phase on Ti-6Al-4V alloy by PEO tech. in zirconate electrolyte. Meanwhile, the
304
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 5 ( 2 0 0 8 ) 303–307
structure and the corrosion resistance of the coatings were studied.
2.
Experimental
2.1. Preparation of plasma electrolytic oxidation ceramic coatings Plate samples of Ti-6Al-4V with the reaction dimension of 15 mm × 17 mm × 0.6 mm were first polished with the abrasive papers, and then washed in HF–HNO3 (1:1 in volume). The home made pulsed single-polar PEO electric source of 5 kW was used for plasma electrolytic oxidation of the samples in a water-cooled electrolytic cell made of stainless steel, which also served as the counter electrode. The electrolyte used in the experiment was K2 ZrF6 (6 g/L), and a small amount of H3 PO4 (85%) was added to the electrolyte to adjust the pH value at 3–4. The temperature of the electrolyte was controlled below 30 ◦ C by adjusting the water flow during the reaction process. The PEO process was carried out under the fixed current densities of 8 A/dm2 with the electric source frequency of 60 Hz.
2.2.
Phase composition and structure of the coatings
The composition of the coatings was examined by RICOH D/max-rB automatic X-ray diffractometer (XRD), with a Cu K␣ source. The morphology of the coatings was studied by Japan Hitachi S-4700 scanning electron microscopy (SEM). The element distribution of the coating was investigated by Philips EDAX energy dispersive spectroscopy (EDS). Coating thickness was measured by an eddy current based thickness gauge (CTG10, Time Company, China). The average thickness of each of the samples was obtained from 10 measurements at different positions.
2.3.
Point corrosion resistance of the coated samples
In a three-electrode cell (Pt plate was used as counter electrode, Ag/AgCl electrode auxiliary electrode, the coated sample working electrode), the point corrosion resistance of the coated samples was evaluated by potentiodynamic anodic scanning curves in a 3.5% NaCl solution through CHI1140 electrochemical analyzer (Shanghai, China). The potentiodynamic scanning rate was 10 mV/s. With the increase of the scanning potential, there exists an abrupt rise of polarizing current, and the corresponding potential at that point is called the pitting corrosion potential. The bigger the pitting corrosion potential, the better is the samples’ pitting corrosion resistance. Three samples were made under each condition to ensure the reliability of the experiments.
3.
Results
3.1.
Thickness of the coating
Table 1 is the thickness of the ceramic coatings prepared for different time. Obviously, the thickness of the coatings was almost linearly increased with the reaction time. The relation
Table 1 – Thickness of the coatings for different time Time (min) Mean thickness (m) Max thickness (m) Min thickness (m) Standard deviation
10
20
40
60
12.1 13.4 10.6 0.8
19.8 21.6 16.8 1.8
35.2 38.6 31.2 2.1
52.8 63.7 38.6 7.8
of the thickness to the time was fitted linearly with the result shown in formula (1): Thickness (m) = 1.84 + 0.85 × Time (min)
(1)
the growth rate of the coating was 0.85 m/min or so under the experimental conditions. Besides, the standard deviation of the thickness was also increased with the time, which meant that the surface of the coatings became rougher and more porous.
3.2.
XRD of the coatings
Fig. 1 is the XRD patterns of the coatings for different time. Clearly, the coating was composed of m-ZrO2 , and t-ZrO2 . For the coatings prepared for the short time, the coating also consisted of ZrTiO4 and a small amount of ZrP2 O7 , and the characteristic peaks corresponding to the titanium substrate were also detected due to the thinner thickness of the coating. Increasing PEO time, the content of ZrP2 O7 was increased quickly and turned into the main crystalline when the PEO time was beyond 40 min.
3.3. Morphology and the elemental distribution in the coatings Fig. 2 is the morphology of the coatings. Panels (a) and (b) are the section images of the coating for different time. It can be noted that the thickness of the coating was increased with the PEO time. The coating for 20 min was comparatively compact and its thickness was generally uniform, while the coating of 60 min was rougher and more uneven, and there were many
Fig. 1 – XRD patterns of the ceramic coatings for different time.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 5 ( 2 0 0 8 ) 303–307
305
Fig. 2 – Section images ((a) and (b)) and surface images ((c) and (d)) of the coatings for different time, (a) and (c): 20 min; (b) and (d): 60 min.
micro-cracks and micro-holes on the section image. Panels (c) and (d) are the surface images of the coatings. There were also many micro-holes on the coating surface. The sintered particles on the surface of the coating for 60 min were bigger than that for 20 min, which means that the surface roughness of the coating was increased with the increasing time. The morphology features of the coating were consistent with the changes of the coating thickness. Besides, the color of the coating prepared for different time was also different: the coating for the short time was blue gray, and turned whiter and whiter with extending the reaction time, which may be related to the formation of a large amount of ZrP2 O7 in the coating. Fig. 3 is the distribution of the elements in the coating of 20 min. The distribution of Ti and Zr changed greatly within the 10 m or so from the interface, i.e., the content of Ti decreased sharply whereas the content of Zr increased quickly; otherwise the coating was uniform with the Ti content remaining at 5 wt.% or so and the Zr content remaining at 55 wt.% or so all through the coating. Moreover, the content of P increased gradually toward the outer surface and the content of O remained at about 30 wt.% throughout the whole coating.
3.4.
Pitting corrosion resistance of the coated samples
Fig. 4 is the potentiodynamic scanning curves of the coated samples and Ti-6Al-4V substrate. It can be noted that the pit-
Fig. 3 – The distribution of the main elements within the coating for 20 min.
306
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 5 ( 2 0 0 8 ) 303–307
ples of 10 min had many pitting holes on the surface. For the coated samples prepared for 60 min, there existed partial pullout of the coating. But, there were only a few corrosion spots for the sample for 20 min; and further more, for the coated sample for 40 min, there were not apparent injury on the surface. Therefore, the pitting corrosion resistance of the coating prepared for 40 min was the best.
4.
Fig. 4 – Potentiodynamic anodic curves of the coated sample and Ti-6Al-4V substrate in 3.5% NaCl solution.
ting corrosion potentials of the coated samples were all more positive than that of Ti-6Al-4V substrate, which meant that PEO treatment improved the pitting corrosion resistance of Ti6Al-4V alloy. Among the coated samples, the pitting corrosion potential of the coated sample for 40 min was the highest, and then the coated samples for 60, 20 and 10 min in sequence. Besides, Fig. 5 is the surface picture of the coated samples and Ti-6Al-4V substrate after the potentiodynamic scanning. Ti-6Al-4V substrate was greatly damaged, and the coated sam-
Discussion
At the beginning of PEO reaction, the voltage ascended quickly. In this short period, the voltage first reached the spark voltage, and then continued increasing quickly to the micro-arc voltage. Thereafter, the anode voltage increases gradually accompanying the stable sparking across the surface. The composition and the structure of the coating are related to the PEO process. At the initial stage, a large amount of Ti from the substrate and Zr from the electrolyte took part in the reaction, which led to the formation of ZrTiO4 and ZrO2 phases. Extending the reaction time, the diffusion of Ti from the substrate to the coating became more and more difficult, whereas more and more P and Zr in the electrolyte joined the reaction, and therefore, ZrP2 O7 phase was increased more and more and finally became the main crystalline of the coating. The variation of the coating composition is consistent with the results of the element distribution in the coatings. The coating for short PEO time was mainly
Fig. 5 – Surface pictures of the coated samples and Ti-6Al-4V substrate after potentiodynamic scanning treatment in 3.5% NaCl solution. (a) the substrate, (b) 10 min, (c) 20 min, (d) 40 min, (e) 60 min.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 5 ( 2 0 0 8 ) 303–307
structured by both Ti from the substrate and Zr from the electrolyte, while the coating for the long PEO time was mainly structured by the elements from the electrolyte like Zr and P. During single-polar pulsed PEO process, a large amount of ZrF6 2− complexing ions and PO4 3− ions congregated and absorbed 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 and other physical and chemical processes, and therefore, PO4 3− ions might form P2 O7 4− ions after dehydration process, which was liable to the formation of ZrP2 O7 . Besides, ZrF6 2− complexing ions and P2 O7 4− ions have the comparatively large spatial structure, which may lead to more micro-cracks and micro-holes formed in the coating. Consequently, the roughness of the coating was increased with the PEO time. The corrosion resistance of the coated samples was determined by the structure and composition. Firstly, m-ZrO2 , t-ZrO2 and KZr2 (PO4 )3 have better stability in many corrosive medium like NaCl, and acid solution and so on, which are very useful for the improvement of the corrosion resistance of the samples. Secondly, if the coating is thicker and denser, the corrosion resistance would be better. Based on the foregoing analyses on the morphology of the coatings, there seemed to be a conflict between the thickness and the density of the coating: the thicker the coating, the worse the density. The coating prepared for 40 min was of both a certain thickness and a certain density and was comparatively well structured, and therefore, the coatings prepared for 40 min had best corrosion resistance among the coated samples.
5.
Conclusions
Through the preparation of the PEO ceramic coatings in K2 ZrF6 electrolyte and the study on the structure and the corrosion resistance, the following conclusions can be drawn: (1) The coating was composed of m-ZrO2 , t-ZrO2 , ZrTiO4 and ZrP2 O7 . The content of ZrP2 O7 increased with the PEO time and became the main crystalline. Increasing PEO time, the thickness of the coating was increased linearly while the coatings turned more porous and rougher. (2) The Ti content in the coating near the substrate was decreased sharply while the content of Zr was increased greatly. Otherwise the coating was uniform with the Ti content remaining at 5 wt.% or so and the Zr content remaining at 55 wt.% or so all through the coating. (3) The prepared coated samples had better corrosion resistance than the Ti-6Al-4V substrate. Among the coated samples, the coated sample prepared for 40 min had the best corrosion resistance.
307
Acknowledgements This work was financially supported by Harbin Special Creation Foundation of Science and Technology for Fellow of China (Grant No. 2006RFQXG032) and Chinese Science Foundation for Post-doctor fellows (Grant No. 20060400238).
references
Boguta, D.L., Rudnev, V.S., Gordienko, P.S., 2004. Current mode effect on the composition and characteristics of anodic-spark coatings. Protec. Met. 40, 275–279. Guan, Y.J., Xia, Y., 2004. Review on plasma electrolytic deposition. Adv. Mech. 34, 237–250 (in Chinese). Jiang, Z.H., Yao, Z.P., Li, Y.P., Lv, Y.D., Sun, X.T., 2004. Effect of phosphate on structure and anticorrosive properties of ceramic film grown on Ti alloy by micro-plasma oxidation. Mater. Sci. Technol. 12, 75–79 (in Chinese). Liang, J., Guo, B., Tian, J., et al., 2005. Effect of potassium fluoride in electrolytic solution on the structure and properties of microarc oxidation coatings on magnesium alloy. Appl. Surf. Sci. 252, 345–351. Schukin, G.L., Belanovich, A.L., Savenko, V.P., Ivashkevich, L.S., Sviridov, V.V., 1996. Micro-plasma anodic oxidation of aluminum and its alloy containing Cu in K2 ZrF6 solution. J. Appl. Chem. 69, 939–941 (in Russian). Shi, Y.L., Yan, F.Y., Xie, G.W., 2005. Effect of pulse duty cycle on micro-plasma oxidation of aluminum alloy. Mater. Lett. 9, 2725–2728. Sundararajan, G., Rama Krishna, L., 2003. Mechanisms underlying the formation of thick alumina coatings through the MAO coating technology. Surf. Coat. Technol. 167, 269–277. Wu, H., Lu, X., Long, B., et al., 2005. The effects of cathodic and anodic voltages on the characteristics of porous nanocrystalline titania coatings fabricated by microarc oxidation. Mater. Lett. 59, 370–375. Wu, Z.D., Jiang, Z.H., Yao, Z.P., 2007. Influence of treatment time on structure and property of ceramic coatings formed on LY12 aluminum alloy by micro-arc oxidation. J. Inorg. Mater. 22, 555–559 (in Chinese). Xue, W.B., Wang, C., Chen, R.Y., 2002. Structure and properties characterization of ceramic coatings produced on Ti-6Al-4V alloy by microarc oxidation in aluminate solution. Mater. Lett. 52, 435–441. Yao, Z.P., Jiang, Z.H., Sun, X.T., Xin, S.G., Li, Y.P., 2004. Influences of current density on structure and corrosion resistance of ceramic coatings on Ti–6Al–4V alloy by micro-plasma oxidation. Thin Solid Films 468, 120–124. Yerohin, A.L., Leyland, A., Matthews, A., 2002. Kinetic aspects of aluminium titanate layer formation on titanium alloy by plasma electrolytic oxidation. Appl. Surf. Sci. 200, 172–184. Yerokhin, A.L., Nie, X., Leyland, A., Matthews, A., Dowey, S.J., 1999. Plasma electrolysis for surface engineering. Surf. Coat. Technol. 122, 73–93. Yerokhin, A.L., Nie, X., Leyland, A., Matthews, A., 2000. Characterization of oxide films produced by plasma electrolytic oxidation of a Ti-6Al-4V alloy. Surf. Coat. Technol. 130, 195–206.
journal homepage: www.elsevier.com/locate/jmatprotec
Preparation and structure of ceramic coatings containing zirconium oxide on Ti alloy by plasma electrolytic oxidation Zhongping Yao a,∗ , Yanli Jiang a,b , Zhaohua Jiang a , Fuping Wang a , Zhendong Wu a a b
Department of Applied Chemistry, Harbin Institute of Technology, Harbin 150001, PR China Department of Life Science and Chemistry, Harbin University, Harbin, 150086, PR China
a r t i c l e
i n f o
a b s t r a c t
Article history:
The aim of this work was to prepare ceramic coatings containing ZrO2 phase on Ti alloy
Received 3 June 2007
and study their structure and corrosion resistance. Compound ceramic coatings were pre-
Received in revised form
pared on Ti-6Al-4V alloy by pulsed single-polar plasma electrolytic oxidation (PEO) in K2 ZrF6
1 November 2007
electrolyte. The phase composition, morphology of the coatings and the element distribu-
Accepted 18 November 2007
tion in the coating were investigated by X-ray diffractometry, scanning electron microscopy and energy dispersive spectroscopy. The corrosion resistance of the coated samples was examined through the potentiodynamic anodic curves in 3.5% NaCl solution. The coatings
Keywords:
prepared for short PEO time was composed of m-ZrO2 , t-ZrO2 and ZrTiO4 and a little ZrP2 O7 ;
Plasma electrolytic oxidation
while increasing PEO time, the content of ZrP2 O7 was increased and became the main crys-
Ceramic coatings
talline. The Ti content in the coating near the substrate was decreased sharply while the
ZrO2
content of Zr was increased greatly. The thickness of the coating was increased with the
Titanium alloy
PEO time, but the coatings turned rougher and more porous. The prepared coated samples
Corrosion resistance
had better corrosion resistance than the substrate. Among the coated samples, the coated sample prepared for 40 min had the best corrosion resistance. © 2007 Elsevier B.V. All rights reserved.
1.
Introduction
Plasma electrolytic oxidation (PEO) is a technique by which the ceramic coating can be grown in situ on Al, Ti, Mg and many other valve-metals. This technique, dated back to ¨ 1930s, when Gunterschulze and Betz first studied spark discharge at the anode surface, has been developed quickly in the surface treatment of metals in recent years. The prepared coatings by PEO tech. is reported to have fine properties like corrosion resistance, anti-abrasion property or decorative property and so on and the promising application prospect in many fields (Yerokhin et al., 1999, 2000; Guan and Xia, 2004). At present, the widely adopted electrolytes on Ti alloy are phosphate, aluminate, silicate, or their mixed solutions
∗
Corresponding author. Tel.: +86 451 86413710; fax: +86 451 86413709. E-mail address: [email protected] (Z.P. Yao). 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.11.112
(Sundararajan and Krishna, 2003; Xue et al., 2002; Liang et al., 2005; Yerohin et al., 2002; Jiang et al., 2004) under different PEO electric source modes and the electric parameters (Boguta et al., 2004; Shi et al., 2005; Yao et al., 2004; Wu et al., 2005) in order to adjust the structure and composition of the ceramic coatings. However, few instances of research of PEO tech. on Ti alloy in the zirconate solution were reported except for the similar work on Al alloys in Refs (Schukin et al., 1996; Wu et al., 2007). Zirconium dioxide is a promising coating material, which has high strength, good fracture toughness property, excellent wear resistance and corrosion resistance and so on. Therefore, we prepared the ceramic coatings containing ZrO2 phase on Ti-6Al-4V alloy by PEO tech. in zirconate electrolyte. Meanwhile, the
304
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 5 ( 2 0 0 8 ) 303–307
structure and the corrosion resistance of the coatings were studied.
2.
Experimental
2.1. Preparation of plasma electrolytic oxidation ceramic coatings Plate samples of Ti-6Al-4V with the reaction dimension of 15 mm × 17 mm × 0.6 mm were first polished with the abrasive papers, and then washed in HF–HNO3 (1:1 in volume). The home made pulsed single-polar PEO electric source of 5 kW was used for plasma electrolytic oxidation of the samples in a water-cooled electrolytic cell made of stainless steel, which also served as the counter electrode. The electrolyte used in the experiment was K2 ZrF6 (6 g/L), and a small amount of H3 PO4 (85%) was added to the electrolyte to adjust the pH value at 3–4. The temperature of the electrolyte was controlled below 30 ◦ C by adjusting the water flow during the reaction process. The PEO process was carried out under the fixed current densities of 8 A/dm2 with the electric source frequency of 60 Hz.
2.2.
Phase composition and structure of the coatings
The composition of the coatings was examined by RICOH D/max-rB automatic X-ray diffractometer (XRD), with a Cu K␣ source. The morphology of the coatings was studied by Japan Hitachi S-4700 scanning electron microscopy (SEM). The element distribution of the coating was investigated by Philips EDAX energy dispersive spectroscopy (EDS). Coating thickness was measured by an eddy current based thickness gauge (CTG10, Time Company, China). The average thickness of each of the samples was obtained from 10 measurements at different positions.
2.3.
Point corrosion resistance of the coated samples
In a three-electrode cell (Pt plate was used as counter electrode, Ag/AgCl electrode auxiliary electrode, the coated sample working electrode), the point corrosion resistance of the coated samples was evaluated by potentiodynamic anodic scanning curves in a 3.5% NaCl solution through CHI1140 electrochemical analyzer (Shanghai, China). The potentiodynamic scanning rate was 10 mV/s. With the increase of the scanning potential, there exists an abrupt rise of polarizing current, and the corresponding potential at that point is called the pitting corrosion potential. The bigger the pitting corrosion potential, the better is the samples’ pitting corrosion resistance. Three samples were made under each condition to ensure the reliability of the experiments.
3.
Results
3.1.
Thickness of the coating
Table 1 is the thickness of the ceramic coatings prepared for different time. Obviously, the thickness of the coatings was almost linearly increased with the reaction time. The relation
Table 1 – Thickness of the coatings for different time Time (min) Mean thickness (m) Max thickness (m) Min thickness (m) Standard deviation
10
20
40
60
12.1 13.4 10.6 0.8
19.8 21.6 16.8 1.8
35.2 38.6 31.2 2.1
52.8 63.7 38.6 7.8
of the thickness to the time was fitted linearly with the result shown in formula (1): Thickness (m) = 1.84 + 0.85 × Time (min)
(1)
the growth rate of the coating was 0.85 m/min or so under the experimental conditions. Besides, the standard deviation of the thickness was also increased with the time, which meant that the surface of the coatings became rougher and more porous.
3.2.
XRD of the coatings
Fig. 1 is the XRD patterns of the coatings for different time. Clearly, the coating was composed of m-ZrO2 , and t-ZrO2 . For the coatings prepared for the short time, the coating also consisted of ZrTiO4 and a small amount of ZrP2 O7 , and the characteristic peaks corresponding to the titanium substrate were also detected due to the thinner thickness of the coating. Increasing PEO time, the content of ZrP2 O7 was increased quickly and turned into the main crystalline when the PEO time was beyond 40 min.
3.3. Morphology and the elemental distribution in the coatings Fig. 2 is the morphology of the coatings. Panels (a) and (b) are the section images of the coating for different time. It can be noted that the thickness of the coating was increased with the PEO time. The coating for 20 min was comparatively compact and its thickness was generally uniform, while the coating of 60 min was rougher and more uneven, and there were many
Fig. 1 – XRD patterns of the ceramic coatings for different time.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 5 ( 2 0 0 8 ) 303–307
305
Fig. 2 – Section images ((a) and (b)) and surface images ((c) and (d)) of the coatings for different time, (a) and (c): 20 min; (b) and (d): 60 min.
micro-cracks and micro-holes on the section image. Panels (c) and (d) are the surface images of the coatings. There were also many micro-holes on the coating surface. The sintered particles on the surface of the coating for 60 min were bigger than that for 20 min, which means that the surface roughness of the coating was increased with the increasing time. The morphology features of the coating were consistent with the changes of the coating thickness. Besides, the color of the coating prepared for different time was also different: the coating for the short time was blue gray, and turned whiter and whiter with extending the reaction time, which may be related to the formation of a large amount of ZrP2 O7 in the coating. Fig. 3 is the distribution of the elements in the coating of 20 min. The distribution of Ti and Zr changed greatly within the 10 m or so from the interface, i.e., the content of Ti decreased sharply whereas the content of Zr increased quickly; otherwise the coating was uniform with the Ti content remaining at 5 wt.% or so and the Zr content remaining at 55 wt.% or so all through the coating. Moreover, the content of P increased gradually toward the outer surface and the content of O remained at about 30 wt.% throughout the whole coating.
3.4.
Pitting corrosion resistance of the coated samples
Fig. 4 is the potentiodynamic scanning curves of the coated samples and Ti-6Al-4V substrate. It can be noted that the pit-
Fig. 3 – The distribution of the main elements within the coating for 20 min.
306
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 5 ( 2 0 0 8 ) 303–307
ples of 10 min had many pitting holes on the surface. For the coated samples prepared for 60 min, there existed partial pullout of the coating. But, there were only a few corrosion spots for the sample for 20 min; and further more, for the coated sample for 40 min, there were not apparent injury on the surface. Therefore, the pitting corrosion resistance of the coating prepared for 40 min was the best.
4.
Fig. 4 – Potentiodynamic anodic curves of the coated sample and Ti-6Al-4V substrate in 3.5% NaCl solution.
ting corrosion potentials of the coated samples were all more positive than that of Ti-6Al-4V substrate, which meant that PEO treatment improved the pitting corrosion resistance of Ti6Al-4V alloy. Among the coated samples, the pitting corrosion potential of the coated sample for 40 min was the highest, and then the coated samples for 60, 20 and 10 min in sequence. Besides, Fig. 5 is the surface picture of the coated samples and Ti-6Al-4V substrate after the potentiodynamic scanning. Ti-6Al-4V substrate was greatly damaged, and the coated sam-
Discussion
At the beginning of PEO reaction, the voltage ascended quickly. In this short period, the voltage first reached the spark voltage, and then continued increasing quickly to the micro-arc voltage. Thereafter, the anode voltage increases gradually accompanying the stable sparking across the surface. The composition and the structure of the coating are related to the PEO process. At the initial stage, a large amount of Ti from the substrate and Zr from the electrolyte took part in the reaction, which led to the formation of ZrTiO4 and ZrO2 phases. Extending the reaction time, the diffusion of Ti from the substrate to the coating became more and more difficult, whereas more and more P and Zr in the electrolyte joined the reaction, and therefore, ZrP2 O7 phase was increased more and more and finally became the main crystalline of the coating. The variation of the coating composition is consistent with the results of the element distribution in the coatings. The coating for short PEO time was mainly
Fig. 5 – Surface pictures of the coated samples and Ti-6Al-4V substrate after potentiodynamic scanning treatment in 3.5% NaCl solution. (a) the substrate, (b) 10 min, (c) 20 min, (d) 40 min, (e) 60 min.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 5 ( 2 0 0 8 ) 303–307
structured by both Ti from the substrate and Zr from the electrolyte, while the coating for the long PEO time was mainly structured by the elements from the electrolyte like Zr and P. During single-polar pulsed PEO process, a large amount of ZrF6 2− complexing ions and PO4 3− ions congregated and absorbed 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 and other physical and chemical processes, and therefore, PO4 3− ions might form P2 O7 4− ions after dehydration process, which was liable to the formation of ZrP2 O7 . Besides, ZrF6 2− complexing ions and P2 O7 4− ions have the comparatively large spatial structure, which may lead to more micro-cracks and micro-holes formed in the coating. Consequently, the roughness of the coating was increased with the PEO time. The corrosion resistance of the coated samples was determined by the structure and composition. Firstly, m-ZrO2 , t-ZrO2 and KZr2 (PO4 )3 have better stability in many corrosive medium like NaCl, and acid solution and so on, which are very useful for the improvement of the corrosion resistance of the samples. Secondly, if the coating is thicker and denser, the corrosion resistance would be better. Based on the foregoing analyses on the morphology of the coatings, there seemed to be a conflict between the thickness and the density of the coating: the thicker the coating, the worse the density. The coating prepared for 40 min was of both a certain thickness and a certain density and was comparatively well structured, and therefore, the coatings prepared for 40 min had best corrosion resistance among the coated samples.
5.
Conclusions
Through the preparation of the PEO ceramic coatings in K2 ZrF6 electrolyte and the study on the structure and the corrosion resistance, the following conclusions can be drawn: (1) The coating was composed of m-ZrO2 , t-ZrO2 , ZrTiO4 and ZrP2 O7 . The content of ZrP2 O7 increased with the PEO time and became the main crystalline. Increasing PEO time, the thickness of the coating was increased linearly while the coatings turned more porous and rougher. (2) The Ti content in the coating near the substrate was decreased sharply while the content of Zr was increased greatly. Otherwise the coating was uniform with the Ti content remaining at 5 wt.% or so and the Zr content remaining at 55 wt.% or so all through the coating. (3) The prepared coated samples had better corrosion resistance than the Ti-6Al-4V substrate. Among the coated samples, the coated sample prepared for 40 min had the best corrosion resistance.
307
Acknowledgements This work was financially supported by Harbin Special Creation Foundation of Science and Technology for Fellow of China (Grant No. 2006RFQXG032) and Chinese Science Foundation for Post-doctor fellows (Grant No. 20060400238).
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