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Analysis and self-lubricating treatment of porous anodic alumina film formed in a compound solution
Applied Surface Science 252 (2005) 1662–1667 www.elsevier.com/locate/apsusc
Analysis and self-lubricating treatment of porous anodic alumina film formed in a compound solution Hui Wang, Hongzhan Yi, Haowei Wang * State Key Lab of Metal Matrix Composites, Shanghai Jiao Tong University, 1954 Huashan Road, Shanghai 200030, China Received 12 January 2005; received in revised form 23 February 2005; accepted 2 March 2005 Available online 29 April 2005
Abstract A porous anodic film on aluminum was prepared in a mixed electrolyte of phosphoric acid and organic acid and cerium salt, and ultrasonic impregnation technology was applied on it to form self-lubricating surface composite. The structure and chemical composition of the film and its lubricity after self-lubricating treatment were investigated in detail. EPMA indicates the crosssection of anodized film has two distinct oxide layers. Al, O and P are found in the film with different distribution in the two layers. XPS analysis on the electron binding energy of the component elements show the chemical composition of film surface are Al2O3, Ce(OH) and some phosphates. The structure of anodized film is amorphous with XRD analysis. The tribological tests shows the frictional coefficient of the self-lubricating surface composite coating is 0.25, much lower than anodized aluminum and aluminum substrate, which is about 0.55 and 0.85, respectively, and it is also durable for a long period of time in comparison with the lubricating coating fabricated by hot-dipping method. SEM images show some PTFE particles are added into the nanoholes of anodic oxide film. # 2005 Elsevier B.V. All rights reserved. Keywords: Anodization; Self-lubricating; Ultrasonic impregnation; PTFE
1. Introduction Aluminum anodic oxide film is of widespread availability since it possesses excellent corrosion resistance and commercially desirable qualities [1–4]. However, low hardness and high friction coefficient limit the extended application. Anodization can * Corresponding author. Tel.: +86 21 62932004; fax: +86 21 61932004. E-mail address: [email protected] (H. Wang).
improve the surface hardness and the anti-wear ability of the aluminum alloys; but it also increases the abrasion of the counterpart [5–7]. In order to reduce the abrasion of the counterpart, a self-lubricating aluminum material with low frictional coefficient is desired. The self-lubricating surface composite is fabricated by depositing or synthesizing in situ some lubricants in the pores or on the surface of anodic porous film, which purpose is to improve the lubricity of anodic oxide film while holding its high surface hardness and anti-wear ability.
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.03.141
H. Wang et al. / Applied Surface Science 252 (2005) 1662–1667
Various methods have been studied in order to improve friction performance of aluminum anodic oxide coating, such as physical impregnation [8], duplex anodization [9,10], sputter-deposition [11,12], electrophoretic-deposition [13] and Teflon technology [14], etc. Among them, the method of hot-dipping of fluororesin has been investigated. However, its filling efficiency is low since the pores of anodized film on aluminum are nano-size. In this paper, a new filling method, ultrasonic impregnation, was proposed to prepare self-lubricating surface composites, and its tribological property is also discussed. In the author’s previous work, the macroporous and thick anodic alumina templates for self-lubricating surface composites were obtained in a phosphoric acid and organic acid and rare-earth salt. In this paper, the structure and composition of the above anodized films were also analyzed.
2. Experimental 2.1. Anodization Annealed aluminum foils (99.99% purity, 3 mm 3 mm 0.5 mm) were used to grow anodic porous layers. High purity metal was preferred in order to minimize the influences of alloying elements. Prior to anodizing, the specimens were degreased and activated with sodium hydroxide at 60–70 8C for 3 min and then electropolished in a stirred alcohol and perchloric acid mixture for cold distilled water for 5 min. Anodization process was conducted in the solution with 4 wt.% phosphoric acid and 0.5–1 wt.% organic acid and 0.05–0.1 wt.% cerium salt. The anodizing parameters: 2.0 A/dm2, 13–15 8C, 100 min. After anodizing, each sample was washed thoroughly with distilled water and dried with an air jet. An electrochemical reactor was designed and built to carry out anodizing experiments. 2.2. Self-lubrication treatment 2.2.1. Hot-dipping of PTFE The anodized aluminum samples are impregnated with latex PTFE at 90 8C for 2 h, then dried in baking box for 30 min at 120 8C.
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2.2.2. Ultrasonic impregnation The anodized aluminum samples were infused in latex PTFE at room temperature in the environment of ultrasonic (50 Hz) for 1–2.5 min, then dried in baking box for 30 min at 120 8C. 2.3. Tribological test The tribological behavior of the specimens were evaluated using a range of dry, reciprocating pin-onplate tests on a MMW-1 Vertical Universal Friction and Wear Testing Machine (stainless steel pin of diameter 4.79 mm, load of 5 N, rotate speed of 300 rpm). 2.4. Analysis The microhardness of composites was measured on HXD-1000 digital microhardness tester. Load was 25 g, time of maintaining load was 15 s. The distribution of elements in the anodic film was analyzed with electron probe X-ray microanalyser (EPMA, 8705QH2). Energy-dispersive analysis of X-rays (EDAX, CMIZ) was used to detect the element composition on the surface of anodic oxide film. The chemical states of elements are examined with X-ray photoelectron spectrograph (XPS, PHI-5300/ESCA). The film structure was detected with X-ray diffractometer (D8 Discover GADDS) The micro-morphologies of self-lubricating coatings were measured with scanning electron microscope (SEM, JSM-6700F). Prior to observation, they are preprocessed with Cross Section Polisher (SM-09010) to eliminate the PTFE coating of surface.
3. Results and discussion 3.1. Analysis on the porous alumina film Fig. 1(a) shows the cross-sectional images of a porous alumina film formed in the mixed solution. The whole film thickness is about 20 mm and divided into two layers: the outer layer thickness reaches about 15 mm with uniform structure; the inner one close to substrate is about 5 mm which bonds the substrate compactly. The interface between outer layer and inner layer is distinct and flat. However, it is unclear
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H. Wang et al. / Applied Surface Science 252 (2005) 1662–1667 Table 1 Surface elemental composition of anodic oxide film Element
at.%
O Al P Ce
55.85 39.13 4.07 0.95
spectra of Al2p, O1s, Ce3d and P2p. The peak of binding energy of Al2p and O1s is approximately 74.35 and 531.26 eV, respectively, corresponding to characteristic regions of aluminum oxide. The analysis on the electron binding energy of P and Ce on the film surface indicates that these two elements exist in the form of Ce(OH) and PO43 . Therefore, the chemical composition of the film contains Al2O3, Ce(OH) and some phosphates. The X-ray diffraction patterns of aluminum substrate and its anodized film are shown in Fig. 3. Fig. 3(a) is the typical XRD pattern of aluminum, while it changes greatly after anodization in the mixed solution, shown in Fig. 3(b). It is the typical pattern of a non-crystalline structure (those strong peaks come from aluminum substrate for the film is undetached from it). So it can be concluded that the structure of anodized film is amorphous. Fig. 1. Morphology (a) and elements distribution (b) of crosssectional film.
and rough between the inner layer and aluminum substrate. This means there is much difference in composition between these two oxide layers. Fig. 1(b) gives the element distribution of cross-sectional film. It shows that O and Al are the major composing elements but P is the minor one which comes from the incorporation of the electrolytic species. The element distribution of Al, O and P in the outer layer is uniform while it changes in the inner oxide layer: Al increases, O decrease and P nearly approaches zero. Therefore, it could be deduced that the inner oxide layer is a transition belt between outer oxide layer and substrate, which makes the binding power between them very strong. Table 1 gives the element composition of film surface. Besides Al, O and P, Ce has been detected on the film surface. Fig. 2 gives the regions of typical XPS
3.2. Tribological properties of the self-lubricating surface composites Fig. 4 gives the frictional coefficient of aluminum, anodized aluminum, hot-dipping of PTFE coating and the ultrasonic impregnated with PTFE coating. It shows the frictional coefficient of aluminum is about 0.85, while it decreases to 0.55 after anodization in phosphoric acid and organic acid and rare-earth salt, and the lower frictional coefficients are obtained after lubricating treatment with hot-dipping and ultrasonic method. The frictional coefficient of hot-dipping coating is very low at the first stage of friction, but soon after it increases to a higher level than the frictional coefficient of ultrasonic impregnated coating and close to that of anodic oxide coating. However, the frictional coefficient of ultrasonic impregnation coating is more uniform at the whole friction processing and the average value is about 0.25. A microhardness tester measured that the aluminum substrate is of lowest hardness (only about
H. Wang et al. / Applied Surface Science 252 (2005) 1662–1667
Fig. 2. XPS spectra of Al2p (a), O1s (b), Ce3d3 and Ce3d5 (c) and P2p (d) for surface of anodized film.
Fig. 3. XRD patterns of aluminum substrate (a) and its anodized film (b).
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Fig. 4. Relationship between frictional coefficient and sliding cycles.
50 HV), then its abrasion resistance is very poor which is easy to be scratched by counterpart, therefore, its frictional coefficient is high and multivariate. After anodization in the mixed solution, the surface hardness increases greatly (reaches 300 HV)
to enhance its anti-wear ability. However, the experiment shows the abrasion of its counterpart is serious. The lubricating treatment with ultrasonic and hot-dipping can increase its anti-attrition ability, so their lower frictional coefficient can be obtained. However, with hot-dipping method, there are little PTFE enter the pores of anodized film, they form a PTFE coating on the anodized film surface, indicated in Fig. 5(a).Therefore, its frictional coefficient is low at first stage of friction, once the surface PTFE coating are scraped off, its frictional coefficient increases rapidly. With ultrasonic method, the PTFE particles move randomly on the effect of ultrasonic wave and some are surged into the holes or on the surface of anodized film. Once enter the holes, the particles are fixed by the holes and hard to be surged out. Therefore, a self-lubricating surface composite can be formed after subsequent heat processing. This can be indicated as Fig. 5(b). Fig. 6 further gives the surface images of the coating filled PTFE with ultrasonic method (preprocessed by Cross Section Polisher). The parts directed by arrows
Fig. 5. Schematic illustration of the models of lubricating coatings.
Fig. 6. The SEM images of anodic oxide film impregnated with PTFE.
H. Wang et al. / Applied Surface Science 252 (2005) 1662–1667
clearly show that some PTFE particles are added into the holes of anodic oxide film. At the first of friction, most of the PTFE are stored in the pores of porous alumina film, only a thin PTFE coating is present on the surface of the film. After sliding, the superficial lubricating coating is scraped off quickly by the steel pin, and the PTFE in the numerous nanoholes are brought to the friction surface continuously to form lubricating film between the film and the steel pin, so the friction between them is reduced, in result, the frictional coefficient of the surface composite coating decreases rapidly.
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and anodic oxide coating and it is also durable for a long period compared with that fabricated by hotdipping.
Acknowledgements This work was supported by the Nano-Tech Special Foundation of Shanghai Scientific and Technical Committee under Grant number 0352nm034.
References 4. Conclusion A porous anodic oxide film is obtained in phosphoric acid and organic acid and rare-earth salt. It is found that the cross-section of film has two distinct layers: the outer layer thickness reaches about 15 mm and is of uniform structure; the inner one is about 5 mm which bonding the substrate compactly. Al, O and P are found in the film and their distribution is uniform in the outer layer while it is changed in the inner film. The chemical composition of the film surface contains Al2O3, Ce(OH) and some phosphates. The film structure is amorphous. The PTFE particles can be added into holes of anodic oxide film with ultrasonic impregnation technology to form self-lubricating surface composite coating, and its frictional coefficient decreases to 0.25, which is lower than those of the aluminum substrate
[1] G.E. Thompson, G.C. Wood, Nature 290 (1981) 230. [2] S.M. Chou, H. Leidheiser Jr., Ind. Eng. Chem. Prod. Res. 25 (1986) 473–478. [3] K. Kobayashl, K. Shimizu, J. Electrochem. Soc. 135 (1988) 908. [4] G. Patermerakis, K. Moussoutzanis, J. Electrochem. Soc. 142 (1995) 737. [5] X. Tao, C. Jianmin, Z. Jiazhen, Tribology 16 (1996) 97. [6] H. Litian, C. Suang, C. Jianming, et al. Surf. Technol. 30 (2001) 35. [7] G. Helong, W. Wei, Mater. Protect. 33 (2000) 43. [8] M. Takaya, K. Hashimoto, Y. Toda, M. Maejima, Surf. Coat. Technol. 169–170 (2003) 160–162. [9] P. Skeldon, H.W. Wang, G.E. Thompson, Wear 206 (1997) 187–196. [10] J. Guang, W. Yinshun, L. Jiuqing, L. Haiping, Acta Metall. Sinica 6 (1993) 102–107. [11] V. Buck, Wear 114 (1987) 263–274. [12] T. Spalvins, Metall. Protect. Coat. 96 (1982) 17–24. [13] W. Jie, R. Ni, Vacuum Cryogenics 5 (1999) 88. [14] T. Steevens, Mater. Eng. 9 (1991) 22–23.
Analysis and self-lubricating treatment of porous anodic alumina film formed in a compound solution Hui Wang, Hongzhan Yi, Haowei Wang * State Key Lab of Metal Matrix Composites, Shanghai Jiao Tong University, 1954 Huashan Road, Shanghai 200030, China Received 12 January 2005; received in revised form 23 February 2005; accepted 2 March 2005 Available online 29 April 2005
Abstract A porous anodic film on aluminum was prepared in a mixed electrolyte of phosphoric acid and organic acid and cerium salt, and ultrasonic impregnation technology was applied on it to form self-lubricating surface composite. The structure and chemical composition of the film and its lubricity after self-lubricating treatment were investigated in detail. EPMA indicates the crosssection of anodized film has two distinct oxide layers. Al, O and P are found in the film with different distribution in the two layers. XPS analysis on the electron binding energy of the component elements show the chemical composition of film surface are Al2O3, Ce(OH) and some phosphates. The structure of anodized film is amorphous with XRD analysis. The tribological tests shows the frictional coefficient of the self-lubricating surface composite coating is 0.25, much lower than anodized aluminum and aluminum substrate, which is about 0.55 and 0.85, respectively, and it is also durable for a long period of time in comparison with the lubricating coating fabricated by hot-dipping method. SEM images show some PTFE particles are added into the nanoholes of anodic oxide film. # 2005 Elsevier B.V. All rights reserved. Keywords: Anodization; Self-lubricating; Ultrasonic impregnation; PTFE
1. Introduction Aluminum anodic oxide film is of widespread availability since it possesses excellent corrosion resistance and commercially desirable qualities [1–4]. However, low hardness and high friction coefficient limit the extended application. Anodization can * Corresponding author. Tel.: +86 21 62932004; fax: +86 21 61932004. E-mail address: [email protected] (H. Wang).
improve the surface hardness and the anti-wear ability of the aluminum alloys; but it also increases the abrasion of the counterpart [5–7]. In order to reduce the abrasion of the counterpart, a self-lubricating aluminum material with low frictional coefficient is desired. The self-lubricating surface composite is fabricated by depositing or synthesizing in situ some lubricants in the pores or on the surface of anodic porous film, which purpose is to improve the lubricity of anodic oxide film while holding its high surface hardness and anti-wear ability.
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.03.141
H. Wang et al. / Applied Surface Science 252 (2005) 1662–1667
Various methods have been studied in order to improve friction performance of aluminum anodic oxide coating, such as physical impregnation [8], duplex anodization [9,10], sputter-deposition [11,12], electrophoretic-deposition [13] and Teflon technology [14], etc. Among them, the method of hot-dipping of fluororesin has been investigated. However, its filling efficiency is low since the pores of anodized film on aluminum are nano-size. In this paper, a new filling method, ultrasonic impregnation, was proposed to prepare self-lubricating surface composites, and its tribological property is also discussed. In the author’s previous work, the macroporous and thick anodic alumina templates for self-lubricating surface composites were obtained in a phosphoric acid and organic acid and rare-earth salt. In this paper, the structure and composition of the above anodized films were also analyzed.
2. Experimental 2.1. Anodization Annealed aluminum foils (99.99% purity, 3 mm 3 mm 0.5 mm) were used to grow anodic porous layers. High purity metal was preferred in order to minimize the influences of alloying elements. Prior to anodizing, the specimens were degreased and activated with sodium hydroxide at 60–70 8C for 3 min and then electropolished in a stirred alcohol and perchloric acid mixture for cold distilled water for 5 min. Anodization process was conducted in the solution with 4 wt.% phosphoric acid and 0.5–1 wt.% organic acid and 0.05–0.1 wt.% cerium salt. The anodizing parameters: 2.0 A/dm2, 13–15 8C, 100 min. After anodizing, each sample was washed thoroughly with distilled water and dried with an air jet. An electrochemical reactor was designed and built to carry out anodizing experiments. 2.2. Self-lubrication treatment 2.2.1. Hot-dipping of PTFE The anodized aluminum samples are impregnated with latex PTFE at 90 8C for 2 h, then dried in baking box for 30 min at 120 8C.
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2.2.2. Ultrasonic impregnation The anodized aluminum samples were infused in latex PTFE at room temperature in the environment of ultrasonic (50 Hz) for 1–2.5 min, then dried in baking box for 30 min at 120 8C. 2.3. Tribological test The tribological behavior of the specimens were evaluated using a range of dry, reciprocating pin-onplate tests on a MMW-1 Vertical Universal Friction and Wear Testing Machine (stainless steel pin of diameter 4.79 mm, load of 5 N, rotate speed of 300 rpm). 2.4. Analysis The microhardness of composites was measured on HXD-1000 digital microhardness tester. Load was 25 g, time of maintaining load was 15 s. The distribution of elements in the anodic film was analyzed with electron probe X-ray microanalyser (EPMA, 8705QH2). Energy-dispersive analysis of X-rays (EDAX, CMIZ) was used to detect the element composition on the surface of anodic oxide film. The chemical states of elements are examined with X-ray photoelectron spectrograph (XPS, PHI-5300/ESCA). The film structure was detected with X-ray diffractometer (D8 Discover GADDS) The micro-morphologies of self-lubricating coatings were measured with scanning electron microscope (SEM, JSM-6700F). Prior to observation, they are preprocessed with Cross Section Polisher (SM-09010) to eliminate the PTFE coating of surface.
3. Results and discussion 3.1. Analysis on the porous alumina film Fig. 1(a) shows the cross-sectional images of a porous alumina film formed in the mixed solution. The whole film thickness is about 20 mm and divided into two layers: the outer layer thickness reaches about 15 mm with uniform structure; the inner one close to substrate is about 5 mm which bonds the substrate compactly. The interface between outer layer and inner layer is distinct and flat. However, it is unclear
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H. Wang et al. / Applied Surface Science 252 (2005) 1662–1667 Table 1 Surface elemental composition of anodic oxide film Element
at.%
O Al P Ce
55.85 39.13 4.07 0.95
spectra of Al2p, O1s, Ce3d and P2p. The peak of binding energy of Al2p and O1s is approximately 74.35 and 531.26 eV, respectively, corresponding to characteristic regions of aluminum oxide. The analysis on the electron binding energy of P and Ce on the film surface indicates that these two elements exist in the form of Ce(OH) and PO43 . Therefore, the chemical composition of the film contains Al2O3, Ce(OH) and some phosphates. The X-ray diffraction patterns of aluminum substrate and its anodized film are shown in Fig. 3. Fig. 3(a) is the typical XRD pattern of aluminum, while it changes greatly after anodization in the mixed solution, shown in Fig. 3(b). It is the typical pattern of a non-crystalline structure (those strong peaks come from aluminum substrate for the film is undetached from it). So it can be concluded that the structure of anodized film is amorphous. Fig. 1. Morphology (a) and elements distribution (b) of crosssectional film.
and rough between the inner layer and aluminum substrate. This means there is much difference in composition between these two oxide layers. Fig. 1(b) gives the element distribution of cross-sectional film. It shows that O and Al are the major composing elements but P is the minor one which comes from the incorporation of the electrolytic species. The element distribution of Al, O and P in the outer layer is uniform while it changes in the inner oxide layer: Al increases, O decrease and P nearly approaches zero. Therefore, it could be deduced that the inner oxide layer is a transition belt between outer oxide layer and substrate, which makes the binding power between them very strong. Table 1 gives the element composition of film surface. Besides Al, O and P, Ce has been detected on the film surface. Fig. 2 gives the regions of typical XPS
3.2. Tribological properties of the self-lubricating surface composites Fig. 4 gives the frictional coefficient of aluminum, anodized aluminum, hot-dipping of PTFE coating and the ultrasonic impregnated with PTFE coating. It shows the frictional coefficient of aluminum is about 0.85, while it decreases to 0.55 after anodization in phosphoric acid and organic acid and rare-earth salt, and the lower frictional coefficients are obtained after lubricating treatment with hot-dipping and ultrasonic method. The frictional coefficient of hot-dipping coating is very low at the first stage of friction, but soon after it increases to a higher level than the frictional coefficient of ultrasonic impregnated coating and close to that of anodic oxide coating. However, the frictional coefficient of ultrasonic impregnation coating is more uniform at the whole friction processing and the average value is about 0.25. A microhardness tester measured that the aluminum substrate is of lowest hardness (only about
H. Wang et al. / Applied Surface Science 252 (2005) 1662–1667
Fig. 2. XPS spectra of Al2p (a), O1s (b), Ce3d3 and Ce3d5 (c) and P2p (d) for surface of anodized film.
Fig. 3. XRD patterns of aluminum substrate (a) and its anodized film (b).
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H. Wang et al. / Applied Surface Science 252 (2005) 1662–1667
Fig. 4. Relationship between frictional coefficient and sliding cycles.
50 HV), then its abrasion resistance is very poor which is easy to be scratched by counterpart, therefore, its frictional coefficient is high and multivariate. After anodization in the mixed solution, the surface hardness increases greatly (reaches 300 HV)
to enhance its anti-wear ability. However, the experiment shows the abrasion of its counterpart is serious. The lubricating treatment with ultrasonic and hot-dipping can increase its anti-attrition ability, so their lower frictional coefficient can be obtained. However, with hot-dipping method, there are little PTFE enter the pores of anodized film, they form a PTFE coating on the anodized film surface, indicated in Fig. 5(a).Therefore, its frictional coefficient is low at first stage of friction, once the surface PTFE coating are scraped off, its frictional coefficient increases rapidly. With ultrasonic method, the PTFE particles move randomly on the effect of ultrasonic wave and some are surged into the holes or on the surface of anodized film. Once enter the holes, the particles are fixed by the holes and hard to be surged out. Therefore, a self-lubricating surface composite can be formed after subsequent heat processing. This can be indicated as Fig. 5(b). Fig. 6 further gives the surface images of the coating filled PTFE with ultrasonic method (preprocessed by Cross Section Polisher). The parts directed by arrows
Fig. 5. Schematic illustration of the models of lubricating coatings.
Fig. 6. The SEM images of anodic oxide film impregnated with PTFE.
H. Wang et al. / Applied Surface Science 252 (2005) 1662–1667
clearly show that some PTFE particles are added into the holes of anodic oxide film. At the first of friction, most of the PTFE are stored in the pores of porous alumina film, only a thin PTFE coating is present on the surface of the film. After sliding, the superficial lubricating coating is scraped off quickly by the steel pin, and the PTFE in the numerous nanoholes are brought to the friction surface continuously to form lubricating film between the film and the steel pin, so the friction between them is reduced, in result, the frictional coefficient of the surface composite coating decreases rapidly.
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and anodic oxide coating and it is also durable for a long period compared with that fabricated by hotdipping.
Acknowledgements This work was supported by the Nano-Tech Special Foundation of Shanghai Scientific and Technical Committee under Grant number 0352nm034.
References 4. Conclusion A porous anodic oxide film is obtained in phosphoric acid and organic acid and rare-earth salt. It is found that the cross-section of film has two distinct layers: the outer layer thickness reaches about 15 mm and is of uniform structure; the inner one is about 5 mm which bonding the substrate compactly. Al, O and P are found in the film and their distribution is uniform in the outer layer while it is changed in the inner film. The chemical composition of the film surface contains Al2O3, Ce(OH) and some phosphates. The film structure is amorphous. The PTFE particles can be added into holes of anodic oxide film with ultrasonic impregnation technology to form self-lubricating surface composite coating, and its frictional coefficient decreases to 0.25, which is lower than those of the aluminum substrate
[1] G.E. Thompson, G.C. Wood, Nature 290 (1981) 230. [2] S.M. Chou, H. Leidheiser Jr., Ind. Eng. Chem. Prod. Res. 25 (1986) 473–478. [3] K. Kobayashl, K. Shimizu, J. Electrochem. Soc. 135 (1988) 908. [4] G. Patermerakis, K. Moussoutzanis, J. Electrochem. Soc. 142 (1995) 737. [5] X. Tao, C. Jianmin, Z. Jiazhen, Tribology 16 (1996) 97. [6] H. Litian, C. Suang, C. Jianming, et al. Surf. Technol. 30 (2001) 35. [7] G. Helong, W. Wei, Mater. Protect. 33 (2000) 43. [8] M. Takaya, K. Hashimoto, Y. Toda, M. Maejima, Surf. Coat. Technol. 169–170 (2003) 160–162. [9] P. Skeldon, H.W. Wang, G.E. Thompson, Wear 206 (1997) 187–196. [10] J. Guang, W. Yinshun, L. Jiuqing, L. Haiping, Acta Metall. Sinica 6 (1993) 102–107. [11] V. Buck, Wear 114 (1987) 263–274. [12] T. Spalvins, Metall. Protect. Coat. 96 (1982) 17–24. [13] W. Jie, R. Ni, Vacuum Cryogenics 5 (1999) 88. [14] T. Steevens, Mater. Eng. 9 (1991) 22–23.