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Effects of Na2WO4 and Na2SiO3 additives in electrolytes on microstructure and properties of PEO coatings on Q235 carbon steel
Journal of Alloys and Compounds 481 (2009) 725–729
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Effects of Na2 WO4 and Na2 SiO3 additives in electrolytes on microstructure and properties of PEO coatings on Q235 carbon steel Yunlong Wang, Zhaohua Jiang ∗ , Zhongping Yao School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 1 February 2009 Received in revised form 5 March 2009 Accepted 14 March 2009 Available online 25 March 2009 Keywords: Plasma electrolytic oxidation Ceramic coating Na2 WO4 additive Na2 SiO3 additive Friction coefficient
a b s t r a c t Ceramic coatings were achieved on Q235 carbon steel by plasma electrolytic oxidation in aluminate system with and without Na2 WO4 and Na2 SiO3 additives in electrolyte. Influence of Na2 WO4 and Na2 SiO3 on surface morphology, phase and elemental composition of PEO coatings were examined by means of scanning electron microscope (SEM), thin-film X-ray diffraction (TF-XRD) and energy dispersive Xray spectroscopy (EDS). Effects of the two additives on the properties of the coatings including surface roughness, surface micro hardness and friction coefficient were studied. The results showed that W from Na2 WO4 and Si from Na2 SiO3 in electrolytes entered into the coatings. Na2 WO4 additive had no evident effect on phase composition of the coating, while Na2 SiO3 additive resulted in the coating changing from crystalline state to amorphous state and increased the content of P in the coating. Both additives reduced the surface roughness of the coatings. With Na2 WO4 or Na2 SiO3 into the electrolytes, the surface micro hardness of the coating was enhanced to 1433 and 1478, respectively, and the friction coefficients were also decreased to below 0.1. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Plasma electrolytic oxidation (PEO) as an effective surface treatment technique developed from traditional anodic oxidation has been succeeded in applying to light-weight metal, such as Al, Mg, Ti, etc. and their alloys [1–3]. The wear resistance, mechanical strength and corrosion resistance of the metals and their alloys can be improved by PEO [4,5]. However, it has also been reported that the properties of PEO coating for anti-wear purpose were still needed further improvement, including surface roughness and surface micro hardness, especially the high surface roughness of the coatings, which is a disadvantage for tribological characteristics [6,7]. Meantime, improving the wear resistance of PEO coatings by reducing the friction coefficient becomes a practical approach [8]. Many methods have been employed to decrease the friction coefficient of PEO coating, such as duplex surface modification together using PEO with other technique [9] and optimization of the electrolytes formula by adding additives into electrolytes [10]. The latter method has been widely studied and employed by many researchers, as PEO process is a complex process that combines diffusion and electrophoresis of elements in the electrolyte, the transformation of the ions in the discharge channels and the electrochemical oxidation at the metal surface, elements
∗ Corresponding author. Tel.: +86 451 86402805; fax: +86 451 86402805. E-mail address: [email protected] (Z. Jiang). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.03.098
in the electrolyte would participate in the PEO process and enter into in the coatings [11,12]. Recently, PEO also has been brought for surface modification of steels and gainning increasing interest by researchers [13–15]. Steel is one of the most widely used metals in engineering, and to improve the wear resistance is of great significance. The work at hand is aimed at improving tribological characteristics of the PEO coatings by adjusting additives into the electrolyte. Na2 WO4 and Na2 SiO3 were used a additives being added into the basic electrolyte. Effects of the two additives on the phase and elemental composition, surface morphology, surface roughness, surface micro hardness and friction coefficient of the coating were studied, respectively. 2. Experimental details 2.1. Preparation of PEO coatings Q235 carbon steel with the normal composition of ≤0.3 Mn%, ≤0.05 P%, ≤0.045 S%, 0.14–0.22 C%, 0.30–0.65 Si% and Fe balance (all values are in wt.%) was used as anode in the experiment. Prior to PEO treatment, the specimens with a dimension of 20 mm × 20 mm × 2 mm were ultrasonically degreased in acetone and distilled water. The PEO unit comprises a single-pulsed electrical power source, a stainless steel electro-bath, a PTFE stirrer and a cooling system. The anodic average voltage ranged from 0 to 80 V. The pulse frequency was kept at 2000 Hz. The PEO treating time of samples was 20 min. An aqueous solution of sodium aluminate and sodium dihydrogen phosphate was used as the basic electrolyte. Na2 WO4 and Na2 SiO3 were used as additives being added into the basic electrolyte, respectively. The optimized compositions of the electrolytes with and without additives are: (1) 10 g/l aluminate and 1.5 g/l sodium dihydrogen phosphate, (2) 10 g/l aluminate, 1.5 g/l sodium dihydrogen phosphate and 2 g/l Na2 WO4 ·2H2 O, (3) 10 g/l aluminate, 1.5 g/l sodium dihydrogen
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Y. Wang et al. / Journal of Alloys and Compounds 481 (2009) 725–729
Fig. 1. Surface SEM images of PEO coatings obtained with and without additives. (a, b): without additives; (c, d): with Na2 WO4 ; (e, f): with Na2 SiO3 .
phosphate, and 0.5 g/l Na2 SiO3 ·9H2 O. Effects of Na2 WO4 and Na2 SiO3 additives on microstructure and properties of PEO coatings were studied by comparing with the characterization results of coatings prepared in these optimized electrolytes.
2.2. Characterization of PEO coatings The morphology and phase composition of the ceramic coatings were investigated by scanning electron microscope (SEM, Hitachi S-570) and X-ray diffraction
Y. Wang et al. / Journal of Alloys and Compounds 481 (2009) 725–729
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3. Results and discussion 3.1. Microstructure characterization of PEO coatings
Fig. 2. XRD patterns of ceramic coatings obtained with and without additives. (a): without additives; (b): with Na2 WO4 ; (c): with Na2 SiO3 .
(XRD, D/max-rB, Japan, Cu target, K␣ radial), respectively. The elemental distribution was measured by an energy-dispersive X-ray spectroscopy (EDS, Oxford Model 7537, England). The surface roughness of PEO coatings was tested by roughness tester. The surface micro hardness of samples was evaluated by means of an MH-5 hardness tester with a Vicker indenter with the load of 30 g and for a loading duration of 10 s. The friction coefficient of the PEO coating was tested on a reciprocal-sliding tribometer (Center for Tribology, HIT, China). The unlubricated sliding was performed with the load of 2 N at the sliding speed of 50 mm/s.
3.1.1. Surface morphology of PEO coatings Fig. 1 is the SEM micrographs of the PEO coatings with and without additives. From Fig. 1, it can be seen that all coatings are typically characterized by the presence of dark circular pores and molten regions distributed all over the surface of the coatings. These circular pores were the discharge residual channels in the discharge reaction and the molten regions were formed due to rapidly cooling effect of the electrolyte. Discrimination can also be observed among the coatings. When additives are added into electrolytes, the number of the channels increases and diameter reduces. Fig. 1(b), (d) and (f) are typically morphologies of the residual channels on coating surface, which clearly indicate that the diameters of the pores are decreased from 15 m to nearly the half. As a whole, the surface of the coating prepared in electrolyte without additives is coarser. This demonstrates that proper concentration of Na2 WO4 or Na2 SiO3 in electrolytes can make the coating surface become a little uniform and fine. 3.1.2. Phase and composition of the coating Fig. 2 illustrates the XRD spectra of the PEO coatings processed in sodium aluminate electrolytes with and without Na2 WO4 and Na2 SiO3 additives. It can be noticed that both coatings obtained from solution with and without Na2 WO4 mainly consisted of FeAl2 O4 and Fe3 O4. Nearly no crystalline phase was
Fig. 3. EDS patterns of ceramic coatings obtained with and without additives. (a): without additives; (b): with Na2 SiO3 ; (c): with Na2 WO4 .
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Y. Wang et al. / Journal of Alloys and Compounds 481 (2009) 725–729
Fig. 5. Surface micro hardness of PEO coating obtained with and without additives.
form. Adding SiO3 2− in the electrolyte will increase the content of P in the coating. Fig. 3 (b) reveals that apart from Al, O, Fe and P, W is also presented in coatings obtained in solution with Na2 WO4 . It can be concluded that Na2 WO4 also participated in the reaction during PEO process and entered into the coating. 3.2. Properties of the coatings
Fig. 4. Surface roughness analysis of PEO coating obtained with and without additives. (a, b, c) Surface roughness tests curves of coatings obtained in electrolytes without additives, with Na2 SiO3 and with Na2 WO4 . (d) Surface roughness surface comparison of the coatings.
3.2.1. Surface roughness of the PEO coatings Surface roughness tests curves and the comparison of the results are shown in Fig. 4. Fig. 4(d) clearly demonstrates that the surface roughness of the coatings obtained in solution with additives (namely W-doped and Si-doped coatings) was decreased compared with the coatings obtained in solution without additives (namely undoped one). W-doped coating decreased a lot, which is nearly half of the undoped coating. In addition, Fig. 4(a–c) also reveals surface fluctuations of coating surface. Although all the coatings show fluctuations on the surface, the undoped coating shows gradually increasing but big fluctuations, and the doped coatings show sudden but small fluctuations. According to the surface morphology, low surface roughness was attributed to the uniform and fine surface with smaller pores.
detected by XRD for the coating obtained in electrolyte with Na2 SiO3 , which revealed that the structure of the coating was nearly amorphous. No composite containing W or Si was found according to XRD spectra. According to the results, a conclusion can be drawn that Na2 SiO3 in the solution results in the coating structure changing from crystalline to amorphous, while Na2 WO4 does not change the crystalline structure of the coating. 3.1.3. Element analysis of PEO coatings The EDS analyses of the three kinds of coatings with and without additives are shown in Fig. 3. It can be concluded that all the coatings mainly consisted of Al, O, Fe and P elements. Appearance of feature peak of Si in the figure illustrates that Si entered into the coatings obtained in electrolyte with Na2 SiO3 . In addition, it has more P than the undoped coating. This indicates that SiO3 2− and PO4 3− species from the electrolytes were incorporated into the oxide coating during the plasma electrolytic oxidation and existed in noncrystalline
Fig. 6. Friction coefficients of PEO coatings obtained with and without additives. (a) without additives; (b) with Na2 SiO3 ; (c) with Na2 WO4 .
Y. Wang et al. / Journal of Alloys and Compounds 481 (2009) 725–729
In addition, the color of the coatings also changed after doping. Undoped PEO coating show a dark blue color in appearance, and both doped coatings changed into dark black. As we all know, black is the most affinity color and preparation of PEO coatings with black color on valve metal has become a focus and difficulty for researchers [16]. PEO coating on steel with black color may be a progress in the aspect. 3.2.2. Surface micro hardness of the coatings Hardness tests results (See Fig. 5) reveal that the surface micro hardness of undoped PEO coating is 1220 Hv. The surface micro hardness of both doped PEO coatings increased. The surface micro hardness of Si-doped coating is 1433 Hv and 1478 Hv is found for W-doped coating. The tests prove that adding Na2 WO4 or Na2 SiO3 into electrolyte could further enhance the surface micro hardness of Q235 substrate (412 Hv). 3.2.3. Friction coefficient of coatings The variation of the friction coefficients versus sliding time for PEO coatings with and without additives is plotted in Fig. 6. The curves demonstrate that the friction coefficients of the doped coatings are lower than that of the undoped one. In the first 600 s, the friction of undoped coating is stable, and the difference between the friction coefficients of doped coating is not considerable. But after 600 s, both doped coatings show much lower coefficients. Although more and big fluctuations appear on the curves of the doped coatings, the fluctuations are stable and no increasing tendency is presented. Lower friction coefficients of the doped coatings may be attributed to the low surface roughness and high surface micro hardness, namely compact density of the coatings. Besides, presentation of W in the coating may be also help to decreasing the friction coefficient. Increasing of P element in the coating may be useful for decreasing the friction coefficient [17].
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4. Conclusions Compact ceramic coatings were fabricated on Q235 by PEO with additives of Na2 WO4 and Na2 SiO3 in the electrolytes. The results showed that Na2 SiO3 in the solution resulted in the coating structure changing from crystalline to amorphous, while Na2 WO4 did not change the crystalline structure of the coating. Both additives resulted in decreasing of surface roughness of the coating. The surface micro hardness of the coating was increased up to 1433 Hv and 1478 Hv, respectively, and the friction coefficients were decrease to below 0.1 with Na2 WO4 or Na2 SiO3 in to the electrolyte. References [1] T.B. Van, S.D. Brown, G.P. Wirtz, Am. Ceram. Soc. Bull. 56 (1977) 563–566. [2] W. Krysmann, P. Kurze, H. Gdittrich, Crystal Res. Technol. 19 (1984) 973–978. [3] A. Yerokhin, L. Nie, X. Leyland, A. Matthews, A. Doweys, Surf. Coat. Technol. 122 (1999) 73–93. [4] H.H. Wu, J.B. Wang, B.Y. Long, B.H. Long, Z.S. Jin, N.D. Wang, F.R. Yu, D.M. Bi, Appl. Surf. Sci. 252 (2005) 1545–1552. [5] K. Wu, Y.Q. Wang, M.Y. Zheng, Mater. Sci. Eng. A 447 (2007) 227–232. [6] X. Nie, A. Leyland, H.W. Song, A.L. Yerokhin, S.J. Dowey, A. Matthews, Surf. Coat. Technol. 116 (1999) 1055–1060. [7] W.B. Xue, C. Wang, H. Tian, Y.C. Lai, Surf. Coat. Technol. 201 (2007) 8695–8701. [8] G.H. Lv, H. Chen, W.C. Gu, Curr. Appl. Phys. 9 (2009) 324–328. [9] X. Nie, A. Wilson, A. Leyland, A. Mattehws, Surf. Coat. Technol 121 (2007) 506–513. [10] F.H. Cao, J.L. Cao, Z. Zhang, J.Q. Zhang, C.N. Cao, Materials and Corrosion 58 (2007) 696–703. [11] Latif Khosa., L Li, Erwu Niu, G.L. Zhang, S.Z. Yang, Appl. Surf. Sci. 253 (2006) 2947–2952. [12] F. Monfort, A. Berkani, E. Matykina, P. Skeldon, G.E. Thompson, H. Habazaki, K. Shimizu, J. Electrochem. Soc. 152 (2005) C382–C387. [13] W.C. Gu, G.H. Lv, H. Chen, G.L. Chen, J. Alloys Compd. 430 (2007) 308–312. [14] L.S. Saakiyan, A.V. Efremov, A.V. Epelfeld, Prot. Met. 25 (1989) 176–180. [15] Z.Q. Wu, Y. Xia, G. Li, F.T. Xu, Appl. Surf. Sci. 253 (2007) 8398–8403. [16] G. Ling, R.B. Zhao, B.L. Jiang, Trans. Mater. Heat Treat. 27 (2007) 91–94. [17] P.B. Su, X.H. Wu, Z.H. Jiang, Mater. Lett. 62 (2008) 3124–3126.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Effects of Na2 WO4 and Na2 SiO3 additives in electrolytes on microstructure and properties of PEO coatings on Q235 carbon steel Yunlong Wang, Zhaohua Jiang ∗ , Zhongping Yao School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 1 February 2009 Received in revised form 5 March 2009 Accepted 14 March 2009 Available online 25 March 2009 Keywords: Plasma electrolytic oxidation Ceramic coating Na2 WO4 additive Na2 SiO3 additive Friction coefficient
a b s t r a c t Ceramic coatings were achieved on Q235 carbon steel by plasma electrolytic oxidation in aluminate system with and without Na2 WO4 and Na2 SiO3 additives in electrolyte. Influence of Na2 WO4 and Na2 SiO3 on surface morphology, phase and elemental composition of PEO coatings were examined by means of scanning electron microscope (SEM), thin-film X-ray diffraction (TF-XRD) and energy dispersive Xray spectroscopy (EDS). Effects of the two additives on the properties of the coatings including surface roughness, surface micro hardness and friction coefficient were studied. The results showed that W from Na2 WO4 and Si from Na2 SiO3 in electrolytes entered into the coatings. Na2 WO4 additive had no evident effect on phase composition of the coating, while Na2 SiO3 additive resulted in the coating changing from crystalline state to amorphous state and increased the content of P in the coating. Both additives reduced the surface roughness of the coatings. With Na2 WO4 or Na2 SiO3 into the electrolytes, the surface micro hardness of the coating was enhanced to 1433 and 1478, respectively, and the friction coefficients were also decreased to below 0.1. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Plasma electrolytic oxidation (PEO) as an effective surface treatment technique developed from traditional anodic oxidation has been succeeded in applying to light-weight metal, such as Al, Mg, Ti, etc. and their alloys [1–3]. The wear resistance, mechanical strength and corrosion resistance of the metals and their alloys can be improved by PEO [4,5]. However, it has also been reported that the properties of PEO coating for anti-wear purpose were still needed further improvement, including surface roughness and surface micro hardness, especially the high surface roughness of the coatings, which is a disadvantage for tribological characteristics [6,7]. Meantime, improving the wear resistance of PEO coatings by reducing the friction coefficient becomes a practical approach [8]. Many methods have been employed to decrease the friction coefficient of PEO coating, such as duplex surface modification together using PEO with other technique [9] and optimization of the electrolytes formula by adding additives into electrolytes [10]. The latter method has been widely studied and employed by many researchers, as PEO process is a complex process that combines diffusion and electrophoresis of elements in the electrolyte, the transformation of the ions in the discharge channels and the electrochemical oxidation at the metal surface, elements
∗ Corresponding author. Tel.: +86 451 86402805; fax: +86 451 86402805. E-mail address: [email protected] (Z. Jiang). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.03.098
in the electrolyte would participate in the PEO process and enter into in the coatings [11,12]. Recently, PEO also has been brought for surface modification of steels and gainning increasing interest by researchers [13–15]. Steel is one of the most widely used metals in engineering, and to improve the wear resistance is of great significance. The work at hand is aimed at improving tribological characteristics of the PEO coatings by adjusting additives into the electrolyte. Na2 WO4 and Na2 SiO3 were used a additives being added into the basic electrolyte. Effects of the two additives on the phase and elemental composition, surface morphology, surface roughness, surface micro hardness and friction coefficient of the coating were studied, respectively. 2. Experimental details 2.1. Preparation of PEO coatings Q235 carbon steel with the normal composition of ≤0.3 Mn%, ≤0.05 P%, ≤0.045 S%, 0.14–0.22 C%, 0.30–0.65 Si% and Fe balance (all values are in wt.%) was used as anode in the experiment. Prior to PEO treatment, the specimens with a dimension of 20 mm × 20 mm × 2 mm were ultrasonically degreased in acetone and distilled water. The PEO unit comprises a single-pulsed electrical power source, a stainless steel electro-bath, a PTFE stirrer and a cooling system. The anodic average voltage ranged from 0 to 80 V. The pulse frequency was kept at 2000 Hz. The PEO treating time of samples was 20 min. An aqueous solution of sodium aluminate and sodium dihydrogen phosphate was used as the basic electrolyte. Na2 WO4 and Na2 SiO3 were used as additives being added into the basic electrolyte, respectively. The optimized compositions of the electrolytes with and without additives are: (1) 10 g/l aluminate and 1.5 g/l sodium dihydrogen phosphate, (2) 10 g/l aluminate, 1.5 g/l sodium dihydrogen phosphate and 2 g/l Na2 WO4 ·2H2 O, (3) 10 g/l aluminate, 1.5 g/l sodium dihydrogen
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Y. Wang et al. / Journal of Alloys and Compounds 481 (2009) 725–729
Fig. 1. Surface SEM images of PEO coatings obtained with and without additives. (a, b): without additives; (c, d): with Na2 WO4 ; (e, f): with Na2 SiO3 .
phosphate, and 0.5 g/l Na2 SiO3 ·9H2 O. Effects of Na2 WO4 and Na2 SiO3 additives on microstructure and properties of PEO coatings were studied by comparing with the characterization results of coatings prepared in these optimized electrolytes.
2.2. Characterization of PEO coatings The morphology and phase composition of the ceramic coatings were investigated by scanning electron microscope (SEM, Hitachi S-570) and X-ray diffraction
Y. Wang et al. / Journal of Alloys and Compounds 481 (2009) 725–729
727
3. Results and discussion 3.1. Microstructure characterization of PEO coatings
Fig. 2. XRD patterns of ceramic coatings obtained with and without additives. (a): without additives; (b): with Na2 WO4 ; (c): with Na2 SiO3 .
(XRD, D/max-rB, Japan, Cu target, K␣ radial), respectively. The elemental distribution was measured by an energy-dispersive X-ray spectroscopy (EDS, Oxford Model 7537, England). The surface roughness of PEO coatings was tested by roughness tester. The surface micro hardness of samples was evaluated by means of an MH-5 hardness tester with a Vicker indenter with the load of 30 g and for a loading duration of 10 s. The friction coefficient of the PEO coating was tested on a reciprocal-sliding tribometer (Center for Tribology, HIT, China). The unlubricated sliding was performed with the load of 2 N at the sliding speed of 50 mm/s.
3.1.1. Surface morphology of PEO coatings Fig. 1 is the SEM micrographs of the PEO coatings with and without additives. From Fig. 1, it can be seen that all coatings are typically characterized by the presence of dark circular pores and molten regions distributed all over the surface of the coatings. These circular pores were the discharge residual channels in the discharge reaction and the molten regions were formed due to rapidly cooling effect of the electrolyte. Discrimination can also be observed among the coatings. When additives are added into electrolytes, the number of the channels increases and diameter reduces. Fig. 1(b), (d) and (f) are typically morphologies of the residual channels on coating surface, which clearly indicate that the diameters of the pores are decreased from 15 m to nearly the half. As a whole, the surface of the coating prepared in electrolyte without additives is coarser. This demonstrates that proper concentration of Na2 WO4 or Na2 SiO3 in electrolytes can make the coating surface become a little uniform and fine. 3.1.2. Phase and composition of the coating Fig. 2 illustrates the XRD spectra of the PEO coatings processed in sodium aluminate electrolytes with and without Na2 WO4 and Na2 SiO3 additives. It can be noticed that both coatings obtained from solution with and without Na2 WO4 mainly consisted of FeAl2 O4 and Fe3 O4. Nearly no crystalline phase was
Fig. 3. EDS patterns of ceramic coatings obtained with and without additives. (a): without additives; (b): with Na2 SiO3 ; (c): with Na2 WO4 .
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Y. Wang et al. / Journal of Alloys and Compounds 481 (2009) 725–729
Fig. 5. Surface micro hardness of PEO coating obtained with and without additives.
form. Adding SiO3 2− in the electrolyte will increase the content of P in the coating. Fig. 3 (b) reveals that apart from Al, O, Fe and P, W is also presented in coatings obtained in solution with Na2 WO4 . It can be concluded that Na2 WO4 also participated in the reaction during PEO process and entered into the coating. 3.2. Properties of the coatings
Fig. 4. Surface roughness analysis of PEO coating obtained with and without additives. (a, b, c) Surface roughness tests curves of coatings obtained in electrolytes without additives, with Na2 SiO3 and with Na2 WO4 . (d) Surface roughness surface comparison of the coatings.
3.2.1. Surface roughness of the PEO coatings Surface roughness tests curves and the comparison of the results are shown in Fig. 4. Fig. 4(d) clearly demonstrates that the surface roughness of the coatings obtained in solution with additives (namely W-doped and Si-doped coatings) was decreased compared with the coatings obtained in solution without additives (namely undoped one). W-doped coating decreased a lot, which is nearly half of the undoped coating. In addition, Fig. 4(a–c) also reveals surface fluctuations of coating surface. Although all the coatings show fluctuations on the surface, the undoped coating shows gradually increasing but big fluctuations, and the doped coatings show sudden but small fluctuations. According to the surface morphology, low surface roughness was attributed to the uniform and fine surface with smaller pores.
detected by XRD for the coating obtained in electrolyte with Na2 SiO3 , which revealed that the structure of the coating was nearly amorphous. No composite containing W or Si was found according to XRD spectra. According to the results, a conclusion can be drawn that Na2 SiO3 in the solution results in the coating structure changing from crystalline to amorphous, while Na2 WO4 does not change the crystalline structure of the coating. 3.1.3. Element analysis of PEO coatings The EDS analyses of the three kinds of coatings with and without additives are shown in Fig. 3. It can be concluded that all the coatings mainly consisted of Al, O, Fe and P elements. Appearance of feature peak of Si in the figure illustrates that Si entered into the coatings obtained in electrolyte with Na2 SiO3 . In addition, it has more P than the undoped coating. This indicates that SiO3 2− and PO4 3− species from the electrolytes were incorporated into the oxide coating during the plasma electrolytic oxidation and existed in noncrystalline
Fig. 6. Friction coefficients of PEO coatings obtained with and without additives. (a) without additives; (b) with Na2 SiO3 ; (c) with Na2 WO4 .
Y. Wang et al. / Journal of Alloys and Compounds 481 (2009) 725–729
In addition, the color of the coatings also changed after doping. Undoped PEO coating show a dark blue color in appearance, and both doped coatings changed into dark black. As we all know, black is the most affinity color and preparation of PEO coatings with black color on valve metal has become a focus and difficulty for researchers [16]. PEO coating on steel with black color may be a progress in the aspect. 3.2.2. Surface micro hardness of the coatings Hardness tests results (See Fig. 5) reveal that the surface micro hardness of undoped PEO coating is 1220 Hv. The surface micro hardness of both doped PEO coatings increased. The surface micro hardness of Si-doped coating is 1433 Hv and 1478 Hv is found for W-doped coating. The tests prove that adding Na2 WO4 or Na2 SiO3 into electrolyte could further enhance the surface micro hardness of Q235 substrate (412 Hv). 3.2.3. Friction coefficient of coatings The variation of the friction coefficients versus sliding time for PEO coatings with and without additives is plotted in Fig. 6. The curves demonstrate that the friction coefficients of the doped coatings are lower than that of the undoped one. In the first 600 s, the friction of undoped coating is stable, and the difference between the friction coefficients of doped coating is not considerable. But after 600 s, both doped coatings show much lower coefficients. Although more and big fluctuations appear on the curves of the doped coatings, the fluctuations are stable and no increasing tendency is presented. Lower friction coefficients of the doped coatings may be attributed to the low surface roughness and high surface micro hardness, namely compact density of the coatings. Besides, presentation of W in the coating may be also help to decreasing the friction coefficient. Increasing of P element in the coating may be useful for decreasing the friction coefficient [17].
729
4. Conclusions Compact ceramic coatings were fabricated on Q235 by PEO with additives of Na2 WO4 and Na2 SiO3 in the electrolytes. The results showed that Na2 SiO3 in the solution resulted in the coating structure changing from crystalline to amorphous, while Na2 WO4 did not change the crystalline structure of the coating. Both additives resulted in decreasing of surface roughness of the coating. The surface micro hardness of the coating was increased up to 1433 Hv and 1478 Hv, respectively, and the friction coefficients were decrease to below 0.1 with Na2 WO4 or Na2 SiO3 in to the electrolyte. References [1] T.B. Van, S.D. Brown, G.P. Wirtz, Am. Ceram. Soc. Bull. 56 (1977) 563–566. [2] W. Krysmann, P. Kurze, H. Gdittrich, Crystal Res. Technol. 19 (1984) 973–978. [3] A. Yerokhin, L. Nie, X. Leyland, A. Matthews, A. Doweys, Surf. Coat. Technol. 122 (1999) 73–93. [4] H.H. Wu, J.B. Wang, B.Y. Long, B.H. Long, Z.S. Jin, N.D. Wang, F.R. Yu, D.M. Bi, Appl. Surf. Sci. 252 (2005) 1545–1552. [5] K. Wu, Y.Q. Wang, M.Y. Zheng, Mater. Sci. Eng. A 447 (2007) 227–232. [6] X. Nie, A. Leyland, H.W. Song, A.L. Yerokhin, S.J. Dowey, A. Matthews, Surf. Coat. Technol. 116 (1999) 1055–1060. [7] W.B. Xue, C. Wang, H. Tian, Y.C. Lai, Surf. Coat. Technol. 201 (2007) 8695–8701. [8] G.H. Lv, H. Chen, W.C. Gu, Curr. Appl. Phys. 9 (2009) 324–328. [9] X. Nie, A. Wilson, A. Leyland, A. Mattehws, Surf. Coat. Technol 121 (2007) 506–513. [10] F.H. Cao, J.L. Cao, Z. Zhang, J.Q. Zhang, C.N. Cao, Materials and Corrosion 58 (2007) 696–703. [11] Latif Khosa., L Li, Erwu Niu, G.L. Zhang, S.Z. Yang, Appl. Surf. Sci. 253 (2006) 2947–2952. [12] F. Monfort, A. Berkani, E. Matykina, P. Skeldon, G.E. Thompson, H. Habazaki, K. Shimizu, J. Electrochem. Soc. 152 (2005) C382–C387. [13] W.C. Gu, G.H. Lv, H. Chen, G.L. Chen, J. Alloys Compd. 430 (2007) 308–312. [14] L.S. Saakiyan, A.V. Efremov, A.V. Epelfeld, Prot. Met. 25 (1989) 176–180. [15] Z.Q. Wu, Y. Xia, G. Li, F.T. Xu, Appl. Surf. Sci. 253 (2007) 8398–8403. [16] G. Ling, R.B. Zhao, B.L. Jiang, Trans. Mater. Heat Treat. 27 (2007) 91–94. [17] P.B. Su, X.H. Wu, Z.H. Jiang, Mater. Lett. 62 (2008) 3124–3126.