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Electrodeposition of diamond-like carbon films on titanium alloy using organic liquids: Corrosion and wear resistance
Applied Surface Science 263 (2012) 18–24
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Electrodeposition of diamond-like carbon films on titanium alloy using organic liquids: Corrosion and wear resistance Tiago Falcade a,∗ , Tobias Eduardo Shmitzhaus b , Otávio Gomes dos Reis b , André Luis Marin Vargas c , Roberto Hübler c , Iduvirges Lourdes Müller b , Célia de Fraga Malfatti b a
Federal University of Rio Grande do Sul, 9500 Bento Gonc¸alves Ave. Sector 4, Building 75, 2nd floor, Porto Alegre, RS, Brazil Federal University of Rio Grande do Sul, Porto Alegre, RS, Brazil c Pontifícia Universidade Católica do Rio Grande do Sul, Brazil b
a r t i c l e
i n f o
Article history: Received 10 June 2012 Received in revised form 13 August 2012 Accepted 15 August 2012 Available online 23 August 2012 Keywords: DLC Electrodeposition Ti6Al4V Corrosion Wear
a b s t r a c t Diamond-like carbon (DLC) films have been studied as coatings for corrosion protection and wear resistance because they have excellent chemical inertness in traditional corrosive environments, besides presenting a significant reduction in coefficient of friction. Diamond-like carbon (DLC) films obtained by electrochemical deposition techniques have attracted a lot of interest, regarding their potential in relation to the vapor phase deposition techniques. The electrochemical deposition techniques are carried out at room temperature and do not need vacuum system, making easier this way the technological transfer. At high electric fields, the organic molecules polarize and react on the electrode surface, forming carbon films. The aim of this work was to obtain DLC films onto Ti6Al4V substrate using as electrolyte: acetonitrile (ACN) and N,N-dimethylformamide (DMF). The films were characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), Raman spectroscopy, potentiodynamic polarization and wear tests. The results show that these films can improve, significantly, the corrosion resistance of titanium and its alloys and their wear resistance. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The use of titanium and titanium alloys is growing exponentially in recent years, both in biomedical applications, such as prosthesis and implants of high bio-compatibility [1–3] and also industrial applications, such as aerospace and naval high performance components [4–6]. In these applications, where components are exposed to corrosive environments and high mechanical stresses, in particular regarding the wear resistance, a complete evaluation of corrosion and tribological behavior of the material is necessary. It is known that titanium and its alloys exhibit a high corrosion resistance in traditional corrosive environments, but they have low resistance to abrasive wear, low wear resistance and high coefficient of friction. Experiments indicated that the wear rate of the Ti6Al4V alloy is 11–20 times higher than the wear rate for the same alloy with nitriding surface treatment [7]. In this context, the use of surface treatments on titanium alloys seems to be interesting to increase the lifetime of components exposed to wear applications.
∗ Corresponding author. Tel.: +55 51 3308 9406. E-mail addresses: [email protected] (T. Falcade), [email protected] (T.E. Shmitzhaus), otavio [email protected] (O.G. dos Reis), [email protected] (I.L. Müller), [email protected] (C. de Fraga Malfatti). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.08.052
The application of protective coatings is an alternative to increase the properties of titanium alloys: hard coatings of metal nitrides [8–10], composite coatings [9–11] and, more recently, coatings of amorphous carbon and diamond-like carbon (DLC) films [12–14], have been used in surface treatment, with positive results in improving the wear resistance of the Ti6Al4V alloy. Amorphous carbon films have raised interest regarding their high hardness associated to a low coefficient of friction, enabling an increase in wear resistance [15,16]. Furthermore, they have a high corrosion resistance in a large amount of corrosive environment [17,18]. Traditional methods for carbon films deposition consist of vapor phase physical or chemical processes typically PVD or CVD and their variants. However, these techniques require high temperature, high vacuum and they are hard to be conducted in components of complex geometry. In this context, electrodeposition, because of its flexibility in operating parameters, may be conducted at room temperature and it favors the industrial applications, becoming an alternative to traditional methods. Commonly, the electrodeposition process uses ionic or aqueous solutions as electrolyte [19]. However, based on experimental evidence, the majority of materials that can be deposited from the vapor phase can also be deposited from a liquid phase using electrochemical techniques. Namba proposes the use of organic solutions
T. Falcade et al. / Applied Surface Science 263 (2012) 18–24
as electrolyte and carbon source, obtaining, initially, films from ethanol [20]. Later, Suzuki used a solution of water and ethylene glycol [21]. From then on, the most diverse organic solutions were tested as electrolyte and, currently, the most used are acetonitrile (ACN) and N,N-dimethylformamide (DMF) [22]. The main difference between this research and others already published in the literature, is the range of characterizations, not only structural, widely studied for these electrolytes, but the union of mechanical and electrochemical characterizations, not well explored yet. In addition, this work is related to an initial step in the research and development of the DLC films obtained by electrodeposition. Currently, changes in the studied electrolytes are being carried out, by our team, in order to improve the process and characteristics of the obtained films. 2. Material and methods Circular Ti6Al4V samples (ASTM F136) with a diameter of 9.5 mm were sanded on SiC abrasive paper (# 400–4000) and then mechanically polished using 1 m diamond paste. A high-voltage source (2 kV, 1 A), a thermostatic bath with external circulation of water and the deposition cell, which contains the cathode, anode and electrolyte were used for electrodeposition. The cathode consists of titanium alloy sample, the anode is a graphite plate 7 mm away from the cathode and the electrolyte is the source of carbon. In this study, two carbon sources were tested: ACN and DMF. The deposition temperature was kept constant at 20 ◦ C. All tests were performed with the application of cathodic potential of 1200 V between the electrodes. The deposition time was 24 h. After deposition, the samples were cleaned with deionized water. Currently, changes in the electrolyte are being studied, intending to reduce the deposition time. Preliminary results show that the addition of dopants in the electrolyte allows the deposition time to decrease dramatically. However, these studies are being initialized and studies about the behavior of these dopants in organic liquids, for carbon films deposition, need to be deepened and they will be published in future articles. Atomic force microscopy (AFM - Shimadzu SPM-9500J3, contact mode) were used for morphological characterization of the substrate and the films, mainly aiming to verify the homogeneity of the film, the location of pores and defects, as well as to determine the roughness and thickness of the films. The microstructural characterization of the films was obtained by Raman Spectroscopy (Labram–Jobin Yvon–Horiba, 632 nm, 6 mW). The electrochemical characterization of the samples was performed using potentiodynamic polarization curves (potentiostat EG & G PAR 273A). Saturated calomel electrode as reference, platinum as counter-electrode and a 5% NaCl solution as the electrolyte. The wear tests were performed with a ball-on-plate tribometer. The wear test was carried out by a reciprocal linear motion of an alumina ball with a diameter of 7.75 mm. It was used as a constant force of 2 N, frequency of 1 Hz and track length of 1.5 mm. The wear tracks profiles were evaluated by profilometry (CETRPRO5003D) and scanning electron microscopy (SEM - JEOL 6060, 20 kV). The mechanical properties of the samples were evaluated with instrumented hardness tests (IHT - ISO 14577) with an equipment Fischerscope H100 V using a Berkovich indenter. It was made load–unload cycles of 60 s, and the maximum load used was 10 mN. 3. Results and discussion 3.1. Morphology study Analyses of the substrate showed that the mechanically polished surface is relatively regular and free from defects as shown in the
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Table 1 Comparative table of roughness between the studied systems.
Ra [nm] Rms [nm] Ry [nm]
Substrate
Film obtained from ACN
Film obtained from DMF
22 33 280
67 84 600
107 139 1200
AFM image (Fig. 1). It is observed that the substrate has low surface roughness, Ra (22 nm), Rms (33 nm) and Ry (280 nm), as shown in Table 1. Fig. 2 shows an AFM image of the electrodeposited carbon film obtained from ACN, which has a homogeneous morphology, free of cracks and porosities. This film presented a surface roughness Ra of 67 nm, Rms of 84 nm and Ry of 600 nm, as shown in Table 1. In this way, it is possible to observe that the film obtained by the electrodeposition from ACN does not have a leveling effect on the substrate. However, this behavior did not impair the homogeneity of the film. Fig. 3 shows an AFM image of the electrodeposited carbon film obtained from DMF. Films deposited from this organic liquid have a homogeneous morphology, free of defects and discontinuities. It is observed that the average roughness Ra of these films was significantly higher than the films obtained from ACN, reaching up to 107 nm, with the larger peak-to-peak distance (Ry) of 1.20 m (Table 1). Table 1 shows the average roughness and maximum peak-topeak distance for the studied systems (substrate, film electrodeposited from ACN and film electrodeposited from DMF). Evaluating Table 1, it is possible to see that the mechanical polishing of the substrate was efficient, with a significantly lower roughness compared to the other systems. The electrodeposition process from low conductivity organic liquids had a significant polarization effect, as shown by increased roughness. The film obtained from ACN presented intermediate roughness and it is expected to be thinner than the film obtained from DMF. 3.2. Layer thickness Fig. 4 shows cross-sectional images of the electrodeposited film on a Ti6Al4V substrate. It is possible to observe the uniformity of the layers, particularly for films obtained from ACN. By image analyses, from the micrographies obtained from SEM (Fig. 4), it was possible to obtain the layer thickness values, which were 0.33 ± 0.10 m for the films obtained from ACN and 0.59 ± 0.07 m for those obtained from DMF. These results confirm the results obtained from the surface roughness analyses, indicating that films deposited from DMF were thicker than those obtained from ACN. The films obtained from ACN were homogeneous and free from defects (Fig. 4a), while those obtained from DMF presented inhomogeneities and discontinuities (Fig. 4b). 3.3. Raman studies Raman is one of the most accepted techniques for structural characterization of carbon-based materials. Raman spectrum of diamond shows a peak at 1332 cm−1 and single graphite Raman spectrum has a peak at 1580 cm−1 [23,24]. Typical DLC Raman spectrum consists of two peaks around 1345–1355 cm−1 and 1570–1590 cm−1 [24,25]. It is common to set D and G bands in the Raman spectrum of carbon-based materials. The G band is centered around 1500–1700 cm−1 , related to sp2 microdomains, characteristic of graphite-like materials. While the D band, centered around 1200–1450 cm−1 , is associated with bond-angle disorder in the sp2 graphite-like microdomains, induced by the linking with sp3
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Fig. 1. AFM image of substrate surface.
Fig. 2. AFM image of DLC surface obtained by electrodeposition from ACN.
Fig. 3. AFM image of DLC surface obtained by electrodeposition from DMF.
carbon atoms, as well as by the finite crystalline sizes of sp2 microdomains [26,27]. The intensity ratio ID /IG is a parameter of relative quantities of sp3/sp2 bonds. Raman spectrum of the films obtained from DMF and ACN (Fig. 5) showed two defined peaks centered around 1340 cm−1 and 1595 cm−1 , which can be attributed to the D band (disordered graphite) and the G band (single crystal graphite), respectively (Table 2). It suggests that the structure of these films was composed by mixed sp2 and sp3 carbon, characteristics of DLC materials [23,27]. The intensity ratio ID /IG was calculated and its value was 1.54 for DMF and 1.17 for ACN (Table 2), showing the greatest amount
of sp3 sites. These values of sp3/sp2 ratio can be obtained by different precursors and techniques. As reported by Dines [28], hard carbon films obtained by PVD techniques had ID /IG varying between 0.82 and 1.62. DLC films obtained by electrodeposition
Table 2 Raman parameters of studied systems.
D band Raman shift [cm−1 ] G band Raman shift [cm−1 ] ID /IG ratio
Film from ACN
Film from DMF
1347 1593 1.17
1328 1598 1.54
T. Falcade et al. / Applied Surface Science 263 (2012) 18–24
Fig. 4. Cross-sectional SEM images of the electrodeposited film on a Ti6Al4V substrate: (a) ACN; (b) DMF.
using organic liquids or organic liquids in aqueous solutions had ID /IG between 0.4 and 1.4, increasing the amount of sp3 bonds with the increase of organic liquid in solution [23]. Cao et al. [25] studied electrodeposition of DLC films onto ITO from pure organic liquids obtaining ID /IG around 2.1.
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Fig. 5. Raman spectrum of films obtained from: (a) ACN; (b) DMF.
about one order of magnitude in the corrosion rate values was observed. It is believed that this behavior was related to the homogeneity of the film, forming an effective barrier between the substrate and the corrosive environment.
3.4. Corrosion study The electrodeposited film from ACN presents a significant improvement in corrosion resistance of the substrate when compared to uncoated titanium, reducing the anodic current density in one order of magnitude. There was a considerable increase in the corrosion potential and this is an indication of improvement in performance against corrosion resistance, as it can be seen in Fig. 6. By simulating the Tafel lines (Table 3) it is observed that the films also improved the corrosion current density and its value of 8.39 × 10−8 mA cm−2 , while the corrosion current in the substrate was maintained at 3.00 × 10−7 mA cm−2 , thus an improvement of
Table 3 Electrochemical characteristics of studied systems.
Rp [ cm−2 ] icorr [A cm−2 ] Ecorr [mV]
Substrate
Film from ACN
Film from DMF
7.02 × 107 3.00 × 10−7 −484
2.51 × 108 8.39 × 10−8 −57.8
3.58 × 107 5.87 × 10−7 −52.0
Fig. 6. Potentiodynamic polarization curves comparing substrate and electrodeposited films from DMF and ACN in a 5% NaCl solution, E vs SCE.
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Fig. 7. Comparative coefficient of friction for uncoated Ti6Al4V and coated with film obtained from DMF and ACN.
The electrodeposited film from DMF provides some improvement on corrosion resistance when compared to the uncoated substrate, showing a reduction in the anodic current density in one order of magnitude, as shown in Fig. 6. An important factor was the significant increase in the corrosion potential, which is an indication of corrosion resistance improvement. However, both the corrosion current density and the polarization resistance showed no significant changes in the values obtained by uncoated substrate, as shown in Table 3. This behavior can be due to the high roughness of the films obtained from DMF. The valleys formed on the surface of the film can act as sites where corrosive processes would be facilitated. 3.5. Wear study It was observed for the coefficient of friction of the substrate, an initial period where the polished surfaces slid in contact without the presence of particulates, followed by a period with the values varying around an average value. This oscillation is due to the formation and injection of a large number of particles (third body) in the region of contact between the bodies. The removal of particulates during the experiment promoted a decrease in the coefficient of friction which increased again as new particles were formed and injected into contact, as shown in Fig. 7 [29]. The films with the greatest roughness and maximum peak were obtained from DMF, which could, in a first moment, indicate a negative behavior of their electrochemical and tribological characteristics. However, because of their roughness, and especially their Ry roughness parameters being much higher than ACN, a thicker film can be expected. Because they are films with low coefficient of friction, a thicker layer can result in a more effective wear protection. The systems electrodeposited from ACN showed some improvement in wear resistance, which is evidenced by the low coefficient of friction, initially around 0.1, particularly for low cycles, as shown in Fig. 7. As observed by other authors, the coefficient of friction of DLC thin films was around this value [30,31]. After about 200 cycles there was a gradual increase in the coefficient of friction, resulting in the rupture of the film at approximately 400 cycles. Afterwards, it overcame the substrate coefficient of friction, with a peak of 0.95. These values are slightly higher than those provided by the uncoated substrate. This may be related to injection of hard particles of the coating, operating as a third body, locally increasing the friction, which, after a certain number of cycles, tended to be
Fig. 8. Profilometry of wear tracks: comparison between uncoated Ti6Al4V and coated with film obtained from DMF and ACN.
dragged to the edge of the track, thus stabilizing the coefficient of friction in approximately the same amount of substrate, 0.75. Moreover, the substrates coated with the film obtained from DMF had initial values of coefficient of friction substantially lower than the uncoated substrate, ranging from around 0.15. This behavior is maintained up to 600 cycles. Then, the fracture of the coating occurs and there is a sudden increase of friction coefficient, which is now around 0.8. The coefficient of friction behavior is now the same as of the substrate (Fig. 7), this same behavior was observed by Manhabosco and Muller in wear of Ti coated with DLC films [32] and by Zeng et al. in wear of steel coated with DLC [33]. The better wear resistance showed by film obtained from DMF can be related with its higher ID /IG ratio, the sp3 amount in DLC films is directly related with the hardness of the film. Studies conducted by Wong et al. showed that increasing sp3 amount generates an important increase in film hardness [34]. The profile of the wear tracks of the uncoated substrate showed a maximum width of 500 m and depth about 12 m. The wear track of electrodeposited films had smaller depth and smaller track width, regardless of the film being obtained from ACN or DMF, as seen in Fig. 8. The tracks on the films obtained from ACN showed a maximum width of 250 m and depth about 5 m. The tracks on the films obtained from DMF showed same depth, but the maximum width was around 50 m. That shows the excellent performance of these films as wear resistance. Even after the breaking of the film, the remaining portion exerts resistance to the applied load, protecting the substrate. These results can be better observed in the SEM images of wear tracks (Fig. 9). The track obtained by the wear of uncoated substrate showed greater width and depth compared with those obtained from the wear of the substrate coated with DLC films, as indicated from profilometry results. The morphology of the uncoated substrate wear track presents an aspect of crushing of the metal surface, indicating that this is not removed, but deformed during the wear process. However, the wear tracks of the substrate coated had more defined edges, independent nature of the film. Furthermore, the morphology of tracks shows a brittle behavior of the film, presenting fissures and detachment (Fig. 9b and c). This behavior was explained by Huang et al. who suggested a model for failure mechanism of the films. In an initial stage, the normal load produces a penetration of alumina ball into the film. Then, the concentrated stress increases and cracks are produced along the two edges of the wear track (second stage). Afterwards, the accumulation of material damage makes the distance
T. Falcade et al. / Applied Surface Science 263 (2012) 18–24
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Table 4 Martens hardness values for the studied systems.
HM [MPa]
Substrate
Film from ACN
Film from DMF
3642.36 ± 332.33
3826.19 ± 737.31
3935.13 ± 797.39
4. Conclusions This work shows the possibility to obtain carbon films by electrodeposition technique, at room temperature, using organic liquids as electrolyte. The films were characterized as diamondlike carbon by Raman spectroscopy, presenting a ID /IG ratio higher than 1.0, indicating that these films presented more sp3 than sp2 bonding. The highest ratio was showed by the film obtained from DMF. The morphology of the films showed a homogeneous surface, independent of the nature of the film. The surface roughness was higher for the films obtained from DMF, as well as the layer thickness (0.33 ± 0.10 m for ACN and 0.59 ± 0.07 m for DMF). The films obtained from ACN proved to be free of defects, with characteristics of barrier film. On the other hand, although the films obtained from DMF proved to be regular, they showed higher surface roughness and developed the highest values of current density when evaluated by potentiodynamic polarization. In either system, the coatings produced a shift of the corrosion potential to values more noble than the substrate. In wear resistance, films presented better behavior than the uncoated substrate, providing not only a lower coefficient of friction, but also generating less pronounced wear tracks, even after breaking the film, in particular when the film was obtained from DMF. Acknowledgments The authors would like to thank the Brazilian government agencies CAPES and CNPq for supporting the present research. References
Fig. 9. SEM image of wear track: (a) uncoated substrate; (b) film obtained from ACN; (c) film obtained from DMF.
between cracks diminish, increasing crack density, followed by the formation of secondary cracks (third stage). At the end, these cracks make the film split into several pieces [30]. 3.6. Hardness measurements The films showed slightly higher Martens hardness values than those presented by the uncoated substrate, as shown in Table 4. These measures may be influenced by the penetration depth of the indenter and do not necessarily represent the actual behavior of the films. Tests with penetration depths and smaller loads are being conducted in order to minimize the experimental errors inserted.
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Electrodeposition of diamond-like carbon films on titanium alloy using organic liquids: Corrosion and wear resistance Tiago Falcade a,∗ , Tobias Eduardo Shmitzhaus b , Otávio Gomes dos Reis b , André Luis Marin Vargas c , Roberto Hübler c , Iduvirges Lourdes Müller b , Célia de Fraga Malfatti b a
Federal University of Rio Grande do Sul, 9500 Bento Gonc¸alves Ave. Sector 4, Building 75, 2nd floor, Porto Alegre, RS, Brazil Federal University of Rio Grande do Sul, Porto Alegre, RS, Brazil c Pontifícia Universidade Católica do Rio Grande do Sul, Brazil b
a r t i c l e
i n f o
Article history: Received 10 June 2012 Received in revised form 13 August 2012 Accepted 15 August 2012 Available online 23 August 2012 Keywords: DLC Electrodeposition Ti6Al4V Corrosion Wear
a b s t r a c t Diamond-like carbon (DLC) films have been studied as coatings for corrosion protection and wear resistance because they have excellent chemical inertness in traditional corrosive environments, besides presenting a significant reduction in coefficient of friction. Diamond-like carbon (DLC) films obtained by electrochemical deposition techniques have attracted a lot of interest, regarding their potential in relation to the vapor phase deposition techniques. The electrochemical deposition techniques are carried out at room temperature and do not need vacuum system, making easier this way the technological transfer. At high electric fields, the organic molecules polarize and react on the electrode surface, forming carbon films. The aim of this work was to obtain DLC films onto Ti6Al4V substrate using as electrolyte: acetonitrile (ACN) and N,N-dimethylformamide (DMF). The films were characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), Raman spectroscopy, potentiodynamic polarization and wear tests. The results show that these films can improve, significantly, the corrosion resistance of titanium and its alloys and their wear resistance. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The use of titanium and titanium alloys is growing exponentially in recent years, both in biomedical applications, such as prosthesis and implants of high bio-compatibility [1–3] and also industrial applications, such as aerospace and naval high performance components [4–6]. In these applications, where components are exposed to corrosive environments and high mechanical stresses, in particular regarding the wear resistance, a complete evaluation of corrosion and tribological behavior of the material is necessary. It is known that titanium and its alloys exhibit a high corrosion resistance in traditional corrosive environments, but they have low resistance to abrasive wear, low wear resistance and high coefficient of friction. Experiments indicated that the wear rate of the Ti6Al4V alloy is 11–20 times higher than the wear rate for the same alloy with nitriding surface treatment [7]. In this context, the use of surface treatments on titanium alloys seems to be interesting to increase the lifetime of components exposed to wear applications.
∗ Corresponding author. Tel.: +55 51 3308 9406. E-mail addresses: [email protected] (T. Falcade), [email protected] (T.E. Shmitzhaus), otavio [email protected] (O.G. dos Reis), [email protected] (I.L. Müller), [email protected] (C. de Fraga Malfatti). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.08.052
The application of protective coatings is an alternative to increase the properties of titanium alloys: hard coatings of metal nitrides [8–10], composite coatings [9–11] and, more recently, coatings of amorphous carbon and diamond-like carbon (DLC) films [12–14], have been used in surface treatment, with positive results in improving the wear resistance of the Ti6Al4V alloy. Amorphous carbon films have raised interest regarding their high hardness associated to a low coefficient of friction, enabling an increase in wear resistance [15,16]. Furthermore, they have a high corrosion resistance in a large amount of corrosive environment [17,18]. Traditional methods for carbon films deposition consist of vapor phase physical or chemical processes typically PVD or CVD and their variants. However, these techniques require high temperature, high vacuum and they are hard to be conducted in components of complex geometry. In this context, electrodeposition, because of its flexibility in operating parameters, may be conducted at room temperature and it favors the industrial applications, becoming an alternative to traditional methods. Commonly, the electrodeposition process uses ionic or aqueous solutions as electrolyte [19]. However, based on experimental evidence, the majority of materials that can be deposited from the vapor phase can also be deposited from a liquid phase using electrochemical techniques. Namba proposes the use of organic solutions
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as electrolyte and carbon source, obtaining, initially, films from ethanol [20]. Later, Suzuki used a solution of water and ethylene glycol [21]. From then on, the most diverse organic solutions were tested as electrolyte and, currently, the most used are acetonitrile (ACN) and N,N-dimethylformamide (DMF) [22]. The main difference between this research and others already published in the literature, is the range of characterizations, not only structural, widely studied for these electrolytes, but the union of mechanical and electrochemical characterizations, not well explored yet. In addition, this work is related to an initial step in the research and development of the DLC films obtained by electrodeposition. Currently, changes in the studied electrolytes are being carried out, by our team, in order to improve the process and characteristics of the obtained films. 2. Material and methods Circular Ti6Al4V samples (ASTM F136) with a diameter of 9.5 mm were sanded on SiC abrasive paper (# 400–4000) and then mechanically polished using 1 m diamond paste. A high-voltage source (2 kV, 1 A), a thermostatic bath with external circulation of water and the deposition cell, which contains the cathode, anode and electrolyte were used for electrodeposition. The cathode consists of titanium alloy sample, the anode is a graphite plate 7 mm away from the cathode and the electrolyte is the source of carbon. In this study, two carbon sources were tested: ACN and DMF. The deposition temperature was kept constant at 20 ◦ C. All tests were performed with the application of cathodic potential of 1200 V between the electrodes. The deposition time was 24 h. After deposition, the samples were cleaned with deionized water. Currently, changes in the electrolyte are being studied, intending to reduce the deposition time. Preliminary results show that the addition of dopants in the electrolyte allows the deposition time to decrease dramatically. However, these studies are being initialized and studies about the behavior of these dopants in organic liquids, for carbon films deposition, need to be deepened and they will be published in future articles. Atomic force microscopy (AFM - Shimadzu SPM-9500J3, contact mode) were used for morphological characterization of the substrate and the films, mainly aiming to verify the homogeneity of the film, the location of pores and defects, as well as to determine the roughness and thickness of the films. The microstructural characterization of the films was obtained by Raman Spectroscopy (Labram–Jobin Yvon–Horiba, 632 nm, 6 mW). The electrochemical characterization of the samples was performed using potentiodynamic polarization curves (potentiostat EG & G PAR 273A). Saturated calomel electrode as reference, platinum as counter-electrode and a 5% NaCl solution as the electrolyte. The wear tests were performed with a ball-on-plate tribometer. The wear test was carried out by a reciprocal linear motion of an alumina ball with a diameter of 7.75 mm. It was used as a constant force of 2 N, frequency of 1 Hz and track length of 1.5 mm. The wear tracks profiles were evaluated by profilometry (CETRPRO5003D) and scanning electron microscopy (SEM - JEOL 6060, 20 kV). The mechanical properties of the samples were evaluated with instrumented hardness tests (IHT - ISO 14577) with an equipment Fischerscope H100 V using a Berkovich indenter. It was made load–unload cycles of 60 s, and the maximum load used was 10 mN. 3. Results and discussion 3.1. Morphology study Analyses of the substrate showed that the mechanically polished surface is relatively regular and free from defects as shown in the
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Table 1 Comparative table of roughness between the studied systems.
Ra [nm] Rms [nm] Ry [nm]
Substrate
Film obtained from ACN
Film obtained from DMF
22 33 280
67 84 600
107 139 1200
AFM image (Fig. 1). It is observed that the substrate has low surface roughness, Ra (22 nm), Rms (33 nm) and Ry (280 nm), as shown in Table 1. Fig. 2 shows an AFM image of the electrodeposited carbon film obtained from ACN, which has a homogeneous morphology, free of cracks and porosities. This film presented a surface roughness Ra of 67 nm, Rms of 84 nm and Ry of 600 nm, as shown in Table 1. In this way, it is possible to observe that the film obtained by the electrodeposition from ACN does not have a leveling effect on the substrate. However, this behavior did not impair the homogeneity of the film. Fig. 3 shows an AFM image of the electrodeposited carbon film obtained from DMF. Films deposited from this organic liquid have a homogeneous morphology, free of defects and discontinuities. It is observed that the average roughness Ra of these films was significantly higher than the films obtained from ACN, reaching up to 107 nm, with the larger peak-to-peak distance (Ry) of 1.20 m (Table 1). Table 1 shows the average roughness and maximum peak-topeak distance for the studied systems (substrate, film electrodeposited from ACN and film electrodeposited from DMF). Evaluating Table 1, it is possible to see that the mechanical polishing of the substrate was efficient, with a significantly lower roughness compared to the other systems. The electrodeposition process from low conductivity organic liquids had a significant polarization effect, as shown by increased roughness. The film obtained from ACN presented intermediate roughness and it is expected to be thinner than the film obtained from DMF. 3.2. Layer thickness Fig. 4 shows cross-sectional images of the electrodeposited film on a Ti6Al4V substrate. It is possible to observe the uniformity of the layers, particularly for films obtained from ACN. By image analyses, from the micrographies obtained from SEM (Fig. 4), it was possible to obtain the layer thickness values, which were 0.33 ± 0.10 m for the films obtained from ACN and 0.59 ± 0.07 m for those obtained from DMF. These results confirm the results obtained from the surface roughness analyses, indicating that films deposited from DMF were thicker than those obtained from ACN. The films obtained from ACN were homogeneous and free from defects (Fig. 4a), while those obtained from DMF presented inhomogeneities and discontinuities (Fig. 4b). 3.3. Raman studies Raman is one of the most accepted techniques for structural characterization of carbon-based materials. Raman spectrum of diamond shows a peak at 1332 cm−1 and single graphite Raman spectrum has a peak at 1580 cm−1 [23,24]. Typical DLC Raman spectrum consists of two peaks around 1345–1355 cm−1 and 1570–1590 cm−1 [24,25]. It is common to set D and G bands in the Raman spectrum of carbon-based materials. The G band is centered around 1500–1700 cm−1 , related to sp2 microdomains, characteristic of graphite-like materials. While the D band, centered around 1200–1450 cm−1 , is associated with bond-angle disorder in the sp2 graphite-like microdomains, induced by the linking with sp3
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Fig. 1. AFM image of substrate surface.
Fig. 2. AFM image of DLC surface obtained by electrodeposition from ACN.
Fig. 3. AFM image of DLC surface obtained by electrodeposition from DMF.
carbon atoms, as well as by the finite crystalline sizes of sp2 microdomains [26,27]. The intensity ratio ID /IG is a parameter of relative quantities of sp3/sp2 bonds. Raman spectrum of the films obtained from DMF and ACN (Fig. 5) showed two defined peaks centered around 1340 cm−1 and 1595 cm−1 , which can be attributed to the D band (disordered graphite) and the G band (single crystal graphite), respectively (Table 2). It suggests that the structure of these films was composed by mixed sp2 and sp3 carbon, characteristics of DLC materials [23,27]. The intensity ratio ID /IG was calculated and its value was 1.54 for DMF and 1.17 for ACN (Table 2), showing the greatest amount
of sp3 sites. These values of sp3/sp2 ratio can be obtained by different precursors and techniques. As reported by Dines [28], hard carbon films obtained by PVD techniques had ID /IG varying between 0.82 and 1.62. DLC films obtained by electrodeposition
Table 2 Raman parameters of studied systems.
D band Raman shift [cm−1 ] G band Raman shift [cm−1 ] ID /IG ratio
Film from ACN
Film from DMF
1347 1593 1.17
1328 1598 1.54
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Fig. 4. Cross-sectional SEM images of the electrodeposited film on a Ti6Al4V substrate: (a) ACN; (b) DMF.
using organic liquids or organic liquids in aqueous solutions had ID /IG between 0.4 and 1.4, increasing the amount of sp3 bonds with the increase of organic liquid in solution [23]. Cao et al. [25] studied electrodeposition of DLC films onto ITO from pure organic liquids obtaining ID /IG around 2.1.
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Fig. 5. Raman spectrum of films obtained from: (a) ACN; (b) DMF.
about one order of magnitude in the corrosion rate values was observed. It is believed that this behavior was related to the homogeneity of the film, forming an effective barrier between the substrate and the corrosive environment.
3.4. Corrosion study The electrodeposited film from ACN presents a significant improvement in corrosion resistance of the substrate when compared to uncoated titanium, reducing the anodic current density in one order of magnitude. There was a considerable increase in the corrosion potential and this is an indication of improvement in performance against corrosion resistance, as it can be seen in Fig. 6. By simulating the Tafel lines (Table 3) it is observed that the films also improved the corrosion current density and its value of 8.39 × 10−8 mA cm−2 , while the corrosion current in the substrate was maintained at 3.00 × 10−7 mA cm−2 , thus an improvement of
Table 3 Electrochemical characteristics of studied systems.
Rp [ cm−2 ] icorr [A cm−2 ] Ecorr [mV]
Substrate
Film from ACN
Film from DMF
7.02 × 107 3.00 × 10−7 −484
2.51 × 108 8.39 × 10−8 −57.8
3.58 × 107 5.87 × 10−7 −52.0
Fig. 6. Potentiodynamic polarization curves comparing substrate and electrodeposited films from DMF and ACN in a 5% NaCl solution, E vs SCE.
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Fig. 7. Comparative coefficient of friction for uncoated Ti6Al4V and coated with film obtained from DMF and ACN.
The electrodeposited film from DMF provides some improvement on corrosion resistance when compared to the uncoated substrate, showing a reduction in the anodic current density in one order of magnitude, as shown in Fig. 6. An important factor was the significant increase in the corrosion potential, which is an indication of corrosion resistance improvement. However, both the corrosion current density and the polarization resistance showed no significant changes in the values obtained by uncoated substrate, as shown in Table 3. This behavior can be due to the high roughness of the films obtained from DMF. The valleys formed on the surface of the film can act as sites where corrosive processes would be facilitated. 3.5. Wear study It was observed for the coefficient of friction of the substrate, an initial period where the polished surfaces slid in contact without the presence of particulates, followed by a period with the values varying around an average value. This oscillation is due to the formation and injection of a large number of particles (third body) in the region of contact between the bodies. The removal of particulates during the experiment promoted a decrease in the coefficient of friction which increased again as new particles were formed and injected into contact, as shown in Fig. 7 [29]. The films with the greatest roughness and maximum peak were obtained from DMF, which could, in a first moment, indicate a negative behavior of their electrochemical and tribological characteristics. However, because of their roughness, and especially their Ry roughness parameters being much higher than ACN, a thicker film can be expected. Because they are films with low coefficient of friction, a thicker layer can result in a more effective wear protection. The systems electrodeposited from ACN showed some improvement in wear resistance, which is evidenced by the low coefficient of friction, initially around 0.1, particularly for low cycles, as shown in Fig. 7. As observed by other authors, the coefficient of friction of DLC thin films was around this value [30,31]. After about 200 cycles there was a gradual increase in the coefficient of friction, resulting in the rupture of the film at approximately 400 cycles. Afterwards, it overcame the substrate coefficient of friction, with a peak of 0.95. These values are slightly higher than those provided by the uncoated substrate. This may be related to injection of hard particles of the coating, operating as a third body, locally increasing the friction, which, after a certain number of cycles, tended to be
Fig. 8. Profilometry of wear tracks: comparison between uncoated Ti6Al4V and coated with film obtained from DMF and ACN.
dragged to the edge of the track, thus stabilizing the coefficient of friction in approximately the same amount of substrate, 0.75. Moreover, the substrates coated with the film obtained from DMF had initial values of coefficient of friction substantially lower than the uncoated substrate, ranging from around 0.15. This behavior is maintained up to 600 cycles. Then, the fracture of the coating occurs and there is a sudden increase of friction coefficient, which is now around 0.8. The coefficient of friction behavior is now the same as of the substrate (Fig. 7), this same behavior was observed by Manhabosco and Muller in wear of Ti coated with DLC films [32] and by Zeng et al. in wear of steel coated with DLC [33]. The better wear resistance showed by film obtained from DMF can be related with its higher ID /IG ratio, the sp3 amount in DLC films is directly related with the hardness of the film. Studies conducted by Wong et al. showed that increasing sp3 amount generates an important increase in film hardness [34]. The profile of the wear tracks of the uncoated substrate showed a maximum width of 500 m and depth about 12 m. The wear track of electrodeposited films had smaller depth and smaller track width, regardless of the film being obtained from ACN or DMF, as seen in Fig. 8. The tracks on the films obtained from ACN showed a maximum width of 250 m and depth about 5 m. The tracks on the films obtained from DMF showed same depth, but the maximum width was around 50 m. That shows the excellent performance of these films as wear resistance. Even after the breaking of the film, the remaining portion exerts resistance to the applied load, protecting the substrate. These results can be better observed in the SEM images of wear tracks (Fig. 9). The track obtained by the wear of uncoated substrate showed greater width and depth compared with those obtained from the wear of the substrate coated with DLC films, as indicated from profilometry results. The morphology of the uncoated substrate wear track presents an aspect of crushing of the metal surface, indicating that this is not removed, but deformed during the wear process. However, the wear tracks of the substrate coated had more defined edges, independent nature of the film. Furthermore, the morphology of tracks shows a brittle behavior of the film, presenting fissures and detachment (Fig. 9b and c). This behavior was explained by Huang et al. who suggested a model for failure mechanism of the films. In an initial stage, the normal load produces a penetration of alumina ball into the film. Then, the concentrated stress increases and cracks are produced along the two edges of the wear track (second stage). Afterwards, the accumulation of material damage makes the distance
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Table 4 Martens hardness values for the studied systems.
HM [MPa]
Substrate
Film from ACN
Film from DMF
3642.36 ± 332.33
3826.19 ± 737.31
3935.13 ± 797.39
4. Conclusions This work shows the possibility to obtain carbon films by electrodeposition technique, at room temperature, using organic liquids as electrolyte. The films were characterized as diamondlike carbon by Raman spectroscopy, presenting a ID /IG ratio higher than 1.0, indicating that these films presented more sp3 than sp2 bonding. The highest ratio was showed by the film obtained from DMF. The morphology of the films showed a homogeneous surface, independent of the nature of the film. The surface roughness was higher for the films obtained from DMF, as well as the layer thickness (0.33 ± 0.10 m for ACN and 0.59 ± 0.07 m for DMF). The films obtained from ACN proved to be free of defects, with characteristics of barrier film. On the other hand, although the films obtained from DMF proved to be regular, they showed higher surface roughness and developed the highest values of current density when evaluated by potentiodynamic polarization. In either system, the coatings produced a shift of the corrosion potential to values more noble than the substrate. In wear resistance, films presented better behavior than the uncoated substrate, providing not only a lower coefficient of friction, but also generating less pronounced wear tracks, even after breaking the film, in particular when the film was obtained from DMF. Acknowledgments The authors would like to thank the Brazilian government agencies CAPES and CNPq for supporting the present research. References
Fig. 9. SEM image of wear track: (a) uncoated substrate; (b) film obtained from ACN; (c) film obtained from DMF.
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