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Improvement of DLC electrochemical corrosion resistance by addiction of fluorine
Diamond & Related Materials 19 (2010) 537–540
Contents lists available at ScienceDirect
Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Improvement of DLC electrochemical corrosion resistance by addiction of fluorine F.R. Marciano a,b,⁎, E.C. Almeida a, D.A. Lima-Oliveira a, E.J. Corat a,b, V.J. Trava-Airoldi a,b a b
Instituto Nacional de Pesquisas Espaciais (INPE), Laboratório Associado de Sensores e Materiais (LAS), Av. dos Astronautas 1758, São José dos Campos, 12227-010, SP, Brazil Instituto Tecnológico de Aeronáutica (ITA), Centro Técnico Aeroespacial (CTA), Pça. Marechal Eduardo Gomes, 50 – São José dos Campos, 12228-900, SP, Brazil
a r t i c l e
i n f o
Available online 11 January 2010 Keywords: Diamond-like carbon Fluorine Electrochemical corrosion
a b s t r a c t The combination of chemical and mechanical properties of diamond-like carbon (DLC) films opens the possibilities for its use in electrochemical applications. DLC electrochemical corrosion behavior is heavily dependent on deposition techniques and precursor gas. Fluorinated-DLC combines the superlative properties of diamond and teflon and becomes one of the most suitable coating for tribological applications. F-DLC was grown over 316L stainless steel using plasma enhanced chemical vapor deposition by varying the ratio of carbon tetrafluoride and methane. The influence of fluorine content on deposition rate, composition, bonding structure, surface energy, hardness, stress, and surface roughness was investigated. Emphasis was placed on the investigation of F-DLC electrochemical corrosion behavior, which was tested by potentiodynamic method. As F content increased, F-DLC films presented lower stress, hardness values and surface free energy. In addition, Raman G-band peak position shifted to higher frequency. The corrosion potential becomes more negative and the anodic and cathodic current densities decreased with the increase of F content, as compared to the pure DLC and the substrates. These results were confirmed by Nyquist plot, which shows a stronger ohmic behavior for F-DLC and Bode plots with different corrosion behaviors. The electrochemical analysis indicated F-DLC films present superior impedance, polarization resistance and breakdown potential as compared to the pure DLC, which indicate they are promising corrosion protective coating in aggressive solutions. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Diamond-like carbon (DLC) is a metastable form of amorphous carbon containing a significant fraction of sp3 bonds [1]. The combination of high mechanical hardness, chemical inertness, high wear resistance and low friction coefficient permits the use of DLC films in a numerous applications, such as protective coating in magnetic storage disks, car parts and biomedical devices [1–3]. Another good property reported is the DLC electrochemical corrosion resistance [3–5]. It is known that DLC electrochemical corrosion behavior is heavily dependent on the film composition and structure, which are in turn depended on the deposition technique and precursor gas [3]. Zeng et al. [6] reported that the corrosion resistance of DLC films decreased with the increase of working pressure, showing that the sp2/sp3 ratio has apparent effect on the corrosion resistance of DLC coatings. Kim et al. [7] shows Si-DLC and high bias voltage could improve DLC corrosion resistance in simulated body fluid. It has been reported the incorporation of fluorine (F) into DLC films reduce its surface free energy but almost keep DLC-behavior [8,9]. The ⁎ Corresponding author. Instituto Nacional de Pesquisas Espaciais (INPE), Laboratório Associado de Sensores e Materiais (LAS), Av. dos Astronautas 1758, São José dos Campos, 12227-010, SP, Brazil. Tel.: +55 12 3945 6576; fax: +55 12 3945 6717. E-mail address: [email protected] (F.R. Marciano). 0925-9635/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2009.12.015
non-wetting behavior combined with DLC superior properties allows numerous practical applications in non-stick kitchenware and protective coatings for optics [9]. In this paper it was investigated the electrochemical corrosion behavior of F-DLC films produced from plasma enhanced chemical vapor deposition (PECVD).
2. Experimental procedures Silicon (100) and 316L stainless steel were used as substrates. The stainless steel were polished by using up 0.25 μm diamond powder and cleaned ultrasonically in an acetone bath for 15 min before putting them into the vacuum chamber. The clean samples were mounted on a water-cooled 10-cm diameter cathode into the plasma chamber. The cathode was fed by a pulsed DC power supply, with variable pulse voltage from −100 to − 1000 V, at a frequency of 20 kHz and duty-cycle of 50%. Into the chamber (vacuum base pressure of 1.3 mPa) the substrates were additionally cleaned by argon discharge with 1 sccm gas flow at 11.3 Pa working pressure and a discharge voltage of − 700 V for 10 min prior to deposition. In order to enhance the DLC film adhesion to metallic surfaces, a thin amorphous silicon interlayer (thickness around 200 nm) were deposited using silane as the
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precursor gas (1 sccm gas flow at 11.3 Pa for 12 min and a discharge voltage of −700 V) [10]. The DLC films were deposited using methane as the feed gas to a thickness of ∼2.0 µm (1 sccm gas flow for 2 h at 11.3 Pa and a discharge voltage of −700 V). F-DLC were deposited by varying the ratio of carbon tetrafluoride and methane during the film deposition. Composition of the films (F and C) was determined by energydispersive X-ray analysis (EDX, Thermoelectrocorporation). It was used a low energy (5 kV) in order to analyze the superficial 300 nm of the samples. The atomic arrangement of the films was analyzed by Raman scattering spectroscopy by using a Renishaw 2000 system with an Ar+-ion laser (λ = 514 nm) in backscattering geometry. The laser power on the sample was ∼ 0.6 mW and the laser spot had 2.5 μm diameter. The Raman shift was calibrated in relation to the diamond peak at 1332 cm− 1. All measurements were carried out in air at room temperature. The slopes of the photoluminescence background (PLB) in visible Raman spectra were used to estimate the hydrogen content in the DLC films, following methodology described by Casiraghi et al. [11]. The morphology of the films and roughness were analyzed using atomic force microscopy (AFM), VEECO Multimode V, operating in dynamic mode, with 0.01–0.025 Ω cm Antimony (n) doped Si tip (model TESPW). Total stress was determined by measuring the substrate curvature before and after the DLC film deposition with a stylus perfilometer (Alpha Step 500) and by analyzing the results with the well-known Stoney's equation [12,13]. The electrochemical tests were performed using a conventional three-electrode electrochemical cell. In this cell, the reference electrode was a saturated Ag/AgCl electrode, the counter electrode was a platinum wire and the working electrodes were the stainless steel, DLC and F-DLC films with different F content. The working electrode exposed area was 3.14 cm2. The electrolyte solution was a 0.15 mol/L sodium chloride (NaCl) aqueous solution, which was not stirred and was naturally aerated. Potentiodynamic tests were carried out by polarization of samples in the anodic direction, from −1.0 to +1.0 V, just after exposition to the electrolyte solution. The potential sweep rate was 1 mV/s. The impedance measurements were also carried out in 0.15 mol/L NaCl aqueous solution, pH 5.8. The electrochemical impedance spectra (EIS) were obtained over the frequency range 100 kHz–10 MHz, at open-circuit potential, with an AC excitation of 10 mV. All experiments were performed at room temperature. 3. Results and discussion The fluorine content was determined from the comparison of normalized area intensities of C and F peaks of the EDX measurements. Table 1 lists the concentrations of C and F contents in the films according to the CF4 concentration in the precursor gases. The fluorine content in F-DLC films increased with the increase of CF4 in the mixture of precursor gases (CF4 + CH4).
Table 1 Concentrations of C and F contents in the films according to the CF4 concentration in the precursor gases.
Fig. 1. Raman scattering spectra from (a) pure DLC films and from F-DLC films containing (b) 0.9, (c) 1.0, (d) 1.5 and (e) 2.0 at.% of F content. The spectra are vertically shifted for easy of comparison.
Fig. 1 shows Raman scattering spectra from pure DLC and F-DLC films in different concentrations, which are vertically shifted for easy of comparison. The spectra were deconvoluted into D and G bands, respectively using two Gaussian curves [11]. Table 2 summarizes the main characteristics of the Raman spectra of DLC films with various F contents. The presence of fluorine in DLC films results in the increase of the intensity ratio of D and G peak (ID/IG) and a shifting of D and G bands toward higher wave numbers. This is consistent with the results obtained by other authors [9,14–16]. These characteristics imply the increase of the graphite-like bonds in DLC matrix [1]. A typical signature of hydrogenated samples in visible Raman is the increasing PLB for increasing H content [11]. According to Casiraghi et al. [11,14] methodology, it is possible to calculate the hydrogen content in DLC films if its content ranged between 20 and 50%. Ishihara et al. [15] produced F-DLC with higher F contents (N 25 at.%). Their films presented a strong increase in luminescence intensity,
Table 2 Gaussian fitting results of Raman spectra from DLC films with various F contents.
CF4/(CF4 + CH4)
Film composition C (%)
F (%)
CF4/(CF4 + CH4) (vol.%)
D band position (cm− 1)
G band position (cm− 1)
FWHM (G)
ID/IG
(vol.%)
[H] (%)
0.0 0.2 0.3 0.4 0.5
100.0 99.1 99.0 98.5 98.0
0.0 0.9 1.0 1.5 2.0
0.0 0.2 0.3 0.4 0.5
1329.8 1333.1 1345.7 1348.5 1351.9
1534.4 1537.1 1538.4 1540.5 1544.3
164.3 165.2 170.9 175.4 180.9
1.31 1.35 1.53 1.54 1.61
20.6 20.7 21.1 21.7 21.9
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Table 3 Average surface roughness (Ra) of the films according to the CF4 concentration in the precursor gases. CF4/(CF4 + CH4) (vol.%)
Roughness (nm)
0.0 0.2 0.3 0.4 0.5
0.08 0.10 0.12 0.14 0.16
which can be assisted to a polymer-like structural arrangement [15]. The films used in our experiments have lower F contents. They have a slight increased in hydrogen content from 20.6 to 21.9 at.% with the increasing of F content. The average surface roughness (Ra) was measured over an area of 1 µm × 1 µm and can be seen on Table 3. Even though, all the surfaces are considerably very smooth, the presence of F in the films cause a slightly increase in Ra values that can be attributed to the deposition effects of the ion bombardment [15]. Fig. 2 shows the total compressive stress of the DLC films measured according to the CF4 concentration in the precursor gases. The zero point corresponds to the pure DLC film produced from only methane. The stresses of the films decreased with the increasing of F content. According to Yao et al. [17], the decrease in compressive stress is attributed to changes in the microstructure (as the fluorine content increases more sp2 bonds are formed in the film). Potentiodynamic polarization test was carried out in order to investigate the protective abilities of coating. The electrochemical stability of the systems in the test solution is investigated by the opencircuit potentials (OCP). The greatest Ecorr value of − 0.149 mV is observed from F-DLC 2.0% films. The negative OCP values for the F-DLC films may be caused by the penetration of the test solution [18,19]. The electrochemical parameters obtained from the potentiodynamic polarization curves (Fig. 3) are given in Table 4. The corrosion current density (icorr) of F-DLC samples was reduced by more than one order of magnitude with comparison to the pure DLC films. The protection efficiency [7] also indicates that F-DLC films offer the best protection among the uncoated samples up to 97.9%. The improvement in the breakdown potential values means the extension of the passive region in the coated system and a higher polarization is necessary to trigger the onset of anodic dissolution of the system [19,20]. In general, the samples in the corrosion behavior with lower current density and higher potential indicate better corrosion resistance [21]. An
Fig. 3. Potentiodynamic polarization curves of (a) stainless steel, (b) DLC, and F-DLC films containing (c) 0.9, (d) 1.0, (e) 1.5 and (f) 2.0 at.% of F content.
improvement in the corrosion resistance of DLC films due to the presence of fluorine is evidenced by a shift of the polarization curve towards the region of lower current density and higher potential. For the F-DLC films, the polarization curves first move to the region of higher potential and lower current density and then to the region of lower potential and higher current density with the increase of fluorine content. This indicates the corrosion resistance of F-DLC films first increases and then decreases with increasing of fluorine content. The Nyquist plots determined by the EIS technique, in Fig. 4, show the different corrosion behavior of the samples after immersion in NaCl. F-DLC films present superior impedance in comparison to the pure DLC and the substrate. The enhancement in the corrosion resistance of the F-DLC samples may be attributed to the reduced electrical conductivity caused by the intrinsic chemical inertness of the F-DLC films in comparison to the uncoated sample [21]. Hence, F-DLC films can act as a passive film to prevent aggressive ions from attacking the substrate and thereby improve the corrosion resistance of the 316L stainless steel. The corrosion resistance of F-DLC films is affected by several factors, such as surface roughness and hydrophobicity, but mainly due to the difference in their microstructure [3,21]. It has been reported carbon based coatings with higher roughness present a larger surface area, which leads to worse corrosion protection [21,22]. According to the aforementioned AFM results, the surface roughness of the F-DLC films slightly increased with the increase of fluorine content, but all the surface roughness of the F-DLC films is less than 0.2 nm (Table 3). Therefore, the effect of surface roughness of the F-DLC films on their corrosion resistance can be negligible [21]. According to Sui et al. [21], the corrosion behavior of the F-DLC films has relationship with the sp3/sp2 ratio. In general, the higher sp3/sp2 ratio, the higher corrosion resistance [21,23]. The qualitative sp3/sp2 ratio of the F-DLC films obtained from Raman spectroscopy is decreased with the increase of fluorine content. The corresponding corrosion behavior should be
Table 4 Electrochemical parameters obtained from potentiodynamic polarization curves.
Fig. 2. Total compressive stress of the films according to the CF4 concentration in the precursor gases. The zero point corresponds to the DLC film produced from only methane.
Samples
Ecorr (mV)
icorr (nA/cm2)
Protection efficiency (%)
Stainless steel DLC F-DLC 0.9% F-DLC 1.0% F-DLC 1.5% F-DLC 2.0%
− 0.340 − 0.174 − 0.159 − 0.361 − 0.169 − 0.149
5.800 1.370 0.029 0.001 0.123 0.110
– 76.4 99.5 100.0 97.9 98.1
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4. Conclusions The effects of fluorine incorporation on the composition, structure and electrochemical corrosion protection of DLC films were investigated. F-DLC films were deposited by PECVD with just a small amount of fluorine (b 2 at.%). High quality F-DLC films were produced. The incorporation of fluorine reduced the compressive stress and increased DLC ID/IG ratio and hydrogen content. F-DLC films improve DLC electrochemical corrosion resistance with just small fluorine content. From these results, F-DLC films can be considered a potential candidate for an anti-corrosion material in industrial applications.
Acknowledgements This study was supported by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP).
References
Fig. 4. a) Nyquist plot of stainless steel, DLC and F-DLC films. b) Enlargement of the region within the rectangular box in (a).
decreased. However, the corrosion resistance of the F-DLC films first increased and then slightly decreased with the increase of fluorine content. However, all the F-DLC film produced present protection efficiency higher than 97.9%.
[1] J. Robertson, Mater. Sci. Eng. R 37 (2002) 129. [2] M. Camero, J.G. Buijnsters, C. Gómez-Aleixandre, R. Gago, I. Caretti, I. Jiménez, J. Appl. Phys. 101 (2007) 63515. [3] Z. Wang, C. Wang, Q. Wang, J. Zhang, J. Zhang, Appl. Surf. Sci. 254 (2008) 3021. [4] R. Sharma, P.K. Barhai, N. Kumari, Thin Solid Films 516 (2008) 5397. [5] E. Liu, H.W. Kwek, Thin Solid Films 516 (2008) 5201. [6] A. Zeng, E. Liu, S.N. Tan, S. Zhang, J. Gao, Electroanalysis 14 (2002) 18. [7] H. Kim, S. Ahn, J. Kim, S.J. Park, K. Lee, Diamond Relat. Mater. 14 (2005) 35. [8] R. Memming, Thin Solid Films 143 (1986) 279. [9] G.Q. Yu, B.K. Tay, Z. Sun, L.K. Pan, Appl. Surf. Sci. 219 (2003) 228. [10] L.F. Bonetti, G. Capote, L.V. Santos, E.J. Corat, V.J. Trava-Airoldi, Thin Solid Films 515 (2006) 375. [11] C. Casiraghi, A.C. Ferrari, J. Robertson, Phys. Rev. B 72 (2005) 385. [12] G.G. Stoney, Proc. R. Soc. Lond., Ser. A 82 (1909) 172. [13] L.G. Jacobsohn, F.L. Freire Jr., J. Vac. Sci. Technol. A 17 (1999) 2841. [14] C. Casiraghi, F. Piazza, A.C. Ferrari, D. Grambole, J. Robertson, Diamond Relat. Mater. 14 (2005) 1098. [15] M. Ishihara, T. Kosaka, T. Nakamura, K. Tsugawa, M. Hasegawa, F. Kokai, Y. Koga, Diamond Relat. Mater. 15 (2006) 1011. [16] A. Bendavid, P.J. Martin, L. Randeniya, M.S. Amin, Diamond Relat. Mater. 18 (2009) 66. [17] Zh.Q. Yao, P. Yang, N. Huang, H. Sun, J. Wang, Appl. Surf. Sci. 230 (2004) 172. [18] A. Zeng, E. Liu, I.F. Annergren, S.N. Tan, S. Zhang, J. Gao, Diamond Relat. Mater. 11 (2002) 160. [19] C. Liu, M. Xu, W. Zhang, S. Pu, P.K. Chu, Diamond Relat. Mater. 17 (2008) 1738. [20] A. Dorner-Reisel, C. Schurer, G. Irmer, E. Muller, Surf. Coat. Technol. 177–178 (2004) 830. [21] J.H. Sui, Z.G. Zhang, W. Cai, Nucl. Instr. Meth. B 267 (2009) 2475. [22] O. Durst, J. Ellermeier, C. Berger, Surf. Coat. Technol. 203 (2008) 848. [23] A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (2000) 14095.
Contents lists available at ScienceDirect
Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Improvement of DLC electrochemical corrosion resistance by addiction of fluorine F.R. Marciano a,b,⁎, E.C. Almeida a, D.A. Lima-Oliveira a, E.J. Corat a,b, V.J. Trava-Airoldi a,b a b
Instituto Nacional de Pesquisas Espaciais (INPE), Laboratório Associado de Sensores e Materiais (LAS), Av. dos Astronautas 1758, São José dos Campos, 12227-010, SP, Brazil Instituto Tecnológico de Aeronáutica (ITA), Centro Técnico Aeroespacial (CTA), Pça. Marechal Eduardo Gomes, 50 – São José dos Campos, 12228-900, SP, Brazil
a r t i c l e
i n f o
Available online 11 January 2010 Keywords: Diamond-like carbon Fluorine Electrochemical corrosion
a b s t r a c t The combination of chemical and mechanical properties of diamond-like carbon (DLC) films opens the possibilities for its use in electrochemical applications. DLC electrochemical corrosion behavior is heavily dependent on deposition techniques and precursor gas. Fluorinated-DLC combines the superlative properties of diamond and teflon and becomes one of the most suitable coating for tribological applications. F-DLC was grown over 316L stainless steel using plasma enhanced chemical vapor deposition by varying the ratio of carbon tetrafluoride and methane. The influence of fluorine content on deposition rate, composition, bonding structure, surface energy, hardness, stress, and surface roughness was investigated. Emphasis was placed on the investigation of F-DLC electrochemical corrosion behavior, which was tested by potentiodynamic method. As F content increased, F-DLC films presented lower stress, hardness values and surface free energy. In addition, Raman G-band peak position shifted to higher frequency. The corrosion potential becomes more negative and the anodic and cathodic current densities decreased with the increase of F content, as compared to the pure DLC and the substrates. These results were confirmed by Nyquist plot, which shows a stronger ohmic behavior for F-DLC and Bode plots with different corrosion behaviors. The electrochemical analysis indicated F-DLC films present superior impedance, polarization resistance and breakdown potential as compared to the pure DLC, which indicate they are promising corrosion protective coating in aggressive solutions. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Diamond-like carbon (DLC) is a metastable form of amorphous carbon containing a significant fraction of sp3 bonds [1]. The combination of high mechanical hardness, chemical inertness, high wear resistance and low friction coefficient permits the use of DLC films in a numerous applications, such as protective coating in magnetic storage disks, car parts and biomedical devices [1–3]. Another good property reported is the DLC electrochemical corrosion resistance [3–5]. It is known that DLC electrochemical corrosion behavior is heavily dependent on the film composition and structure, which are in turn depended on the deposition technique and precursor gas [3]. Zeng et al. [6] reported that the corrosion resistance of DLC films decreased with the increase of working pressure, showing that the sp2/sp3 ratio has apparent effect on the corrosion resistance of DLC coatings. Kim et al. [7] shows Si-DLC and high bias voltage could improve DLC corrosion resistance in simulated body fluid. It has been reported the incorporation of fluorine (F) into DLC films reduce its surface free energy but almost keep DLC-behavior [8,9]. The ⁎ Corresponding author. Instituto Nacional de Pesquisas Espaciais (INPE), Laboratório Associado de Sensores e Materiais (LAS), Av. dos Astronautas 1758, São José dos Campos, 12227-010, SP, Brazil. Tel.: +55 12 3945 6576; fax: +55 12 3945 6717. E-mail address: [email protected] (F.R. Marciano). 0925-9635/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2009.12.015
non-wetting behavior combined with DLC superior properties allows numerous practical applications in non-stick kitchenware and protective coatings for optics [9]. In this paper it was investigated the electrochemical corrosion behavior of F-DLC films produced from plasma enhanced chemical vapor deposition (PECVD).
2. Experimental procedures Silicon (100) and 316L stainless steel were used as substrates. The stainless steel were polished by using up 0.25 μm diamond powder and cleaned ultrasonically in an acetone bath for 15 min before putting them into the vacuum chamber. The clean samples were mounted on a water-cooled 10-cm diameter cathode into the plasma chamber. The cathode was fed by a pulsed DC power supply, with variable pulse voltage from −100 to − 1000 V, at a frequency of 20 kHz and duty-cycle of 50%. Into the chamber (vacuum base pressure of 1.3 mPa) the substrates were additionally cleaned by argon discharge with 1 sccm gas flow at 11.3 Pa working pressure and a discharge voltage of − 700 V for 10 min prior to deposition. In order to enhance the DLC film adhesion to metallic surfaces, a thin amorphous silicon interlayer (thickness around 200 nm) were deposited using silane as the
538
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precursor gas (1 sccm gas flow at 11.3 Pa for 12 min and a discharge voltage of −700 V) [10]. The DLC films were deposited using methane as the feed gas to a thickness of ∼2.0 µm (1 sccm gas flow for 2 h at 11.3 Pa and a discharge voltage of −700 V). F-DLC were deposited by varying the ratio of carbon tetrafluoride and methane during the film deposition. Composition of the films (F and C) was determined by energydispersive X-ray analysis (EDX, Thermoelectrocorporation). It was used a low energy (5 kV) in order to analyze the superficial 300 nm of the samples. The atomic arrangement of the films was analyzed by Raman scattering spectroscopy by using a Renishaw 2000 system with an Ar+-ion laser (λ = 514 nm) in backscattering geometry. The laser power on the sample was ∼ 0.6 mW and the laser spot had 2.5 μm diameter. The Raman shift was calibrated in relation to the diamond peak at 1332 cm− 1. All measurements were carried out in air at room temperature. The slopes of the photoluminescence background (PLB) in visible Raman spectra were used to estimate the hydrogen content in the DLC films, following methodology described by Casiraghi et al. [11]. The morphology of the films and roughness were analyzed using atomic force microscopy (AFM), VEECO Multimode V, operating in dynamic mode, with 0.01–0.025 Ω cm Antimony (n) doped Si tip (model TESPW). Total stress was determined by measuring the substrate curvature before and after the DLC film deposition with a stylus perfilometer (Alpha Step 500) and by analyzing the results with the well-known Stoney's equation [12,13]. The electrochemical tests were performed using a conventional three-electrode electrochemical cell. In this cell, the reference electrode was a saturated Ag/AgCl electrode, the counter electrode was a platinum wire and the working electrodes were the stainless steel, DLC and F-DLC films with different F content. The working electrode exposed area was 3.14 cm2. The electrolyte solution was a 0.15 mol/L sodium chloride (NaCl) aqueous solution, which was not stirred and was naturally aerated. Potentiodynamic tests were carried out by polarization of samples in the anodic direction, from −1.0 to +1.0 V, just after exposition to the electrolyte solution. The potential sweep rate was 1 mV/s. The impedance measurements were also carried out in 0.15 mol/L NaCl aqueous solution, pH 5.8. The electrochemical impedance spectra (EIS) were obtained over the frequency range 100 kHz–10 MHz, at open-circuit potential, with an AC excitation of 10 mV. All experiments were performed at room temperature. 3. Results and discussion The fluorine content was determined from the comparison of normalized area intensities of C and F peaks of the EDX measurements. Table 1 lists the concentrations of C and F contents in the films according to the CF4 concentration in the precursor gases. The fluorine content in F-DLC films increased with the increase of CF4 in the mixture of precursor gases (CF4 + CH4).
Table 1 Concentrations of C and F contents in the films according to the CF4 concentration in the precursor gases.
Fig. 1. Raman scattering spectra from (a) pure DLC films and from F-DLC films containing (b) 0.9, (c) 1.0, (d) 1.5 and (e) 2.0 at.% of F content. The spectra are vertically shifted for easy of comparison.
Fig. 1 shows Raman scattering spectra from pure DLC and F-DLC films in different concentrations, which are vertically shifted for easy of comparison. The spectra were deconvoluted into D and G bands, respectively using two Gaussian curves [11]. Table 2 summarizes the main characteristics of the Raman spectra of DLC films with various F contents. The presence of fluorine in DLC films results in the increase of the intensity ratio of D and G peak (ID/IG) and a shifting of D and G bands toward higher wave numbers. This is consistent with the results obtained by other authors [9,14–16]. These characteristics imply the increase of the graphite-like bonds in DLC matrix [1]. A typical signature of hydrogenated samples in visible Raman is the increasing PLB for increasing H content [11]. According to Casiraghi et al. [11,14] methodology, it is possible to calculate the hydrogen content in DLC films if its content ranged between 20 and 50%. Ishihara et al. [15] produced F-DLC with higher F contents (N 25 at.%). Their films presented a strong increase in luminescence intensity,
Table 2 Gaussian fitting results of Raman spectra from DLC films with various F contents.
CF4/(CF4 + CH4)
Film composition C (%)
F (%)
CF4/(CF4 + CH4) (vol.%)
D band position (cm− 1)
G band position (cm− 1)
FWHM (G)
ID/IG
(vol.%)
[H] (%)
0.0 0.2 0.3 0.4 0.5
100.0 99.1 99.0 98.5 98.0
0.0 0.9 1.0 1.5 2.0
0.0 0.2 0.3 0.4 0.5
1329.8 1333.1 1345.7 1348.5 1351.9
1534.4 1537.1 1538.4 1540.5 1544.3
164.3 165.2 170.9 175.4 180.9
1.31 1.35 1.53 1.54 1.61
20.6 20.7 21.1 21.7 21.9
F.R. Marciano et al. / Diamond & Related Materials 19 (2010) 537–540
539
Table 3 Average surface roughness (Ra) of the films according to the CF4 concentration in the precursor gases. CF4/(CF4 + CH4) (vol.%)
Roughness (nm)
0.0 0.2 0.3 0.4 0.5
0.08 0.10 0.12 0.14 0.16
which can be assisted to a polymer-like structural arrangement [15]. The films used in our experiments have lower F contents. They have a slight increased in hydrogen content from 20.6 to 21.9 at.% with the increasing of F content. The average surface roughness (Ra) was measured over an area of 1 µm × 1 µm and can be seen on Table 3. Even though, all the surfaces are considerably very smooth, the presence of F in the films cause a slightly increase in Ra values that can be attributed to the deposition effects of the ion bombardment [15]. Fig. 2 shows the total compressive stress of the DLC films measured according to the CF4 concentration in the precursor gases. The zero point corresponds to the pure DLC film produced from only methane. The stresses of the films decreased with the increasing of F content. According to Yao et al. [17], the decrease in compressive stress is attributed to changes in the microstructure (as the fluorine content increases more sp2 bonds are formed in the film). Potentiodynamic polarization test was carried out in order to investigate the protective abilities of coating. The electrochemical stability of the systems in the test solution is investigated by the opencircuit potentials (OCP). The greatest Ecorr value of − 0.149 mV is observed from F-DLC 2.0% films. The negative OCP values for the F-DLC films may be caused by the penetration of the test solution [18,19]. The electrochemical parameters obtained from the potentiodynamic polarization curves (Fig. 3) are given in Table 4. The corrosion current density (icorr) of F-DLC samples was reduced by more than one order of magnitude with comparison to the pure DLC films. The protection efficiency [7] also indicates that F-DLC films offer the best protection among the uncoated samples up to 97.9%. The improvement in the breakdown potential values means the extension of the passive region in the coated system and a higher polarization is necessary to trigger the onset of anodic dissolution of the system [19,20]. In general, the samples in the corrosion behavior with lower current density and higher potential indicate better corrosion resistance [21]. An
Fig. 3. Potentiodynamic polarization curves of (a) stainless steel, (b) DLC, and F-DLC films containing (c) 0.9, (d) 1.0, (e) 1.5 and (f) 2.0 at.% of F content.
improvement in the corrosion resistance of DLC films due to the presence of fluorine is evidenced by a shift of the polarization curve towards the region of lower current density and higher potential. For the F-DLC films, the polarization curves first move to the region of higher potential and lower current density and then to the region of lower potential and higher current density with the increase of fluorine content. This indicates the corrosion resistance of F-DLC films first increases and then decreases with increasing of fluorine content. The Nyquist plots determined by the EIS technique, in Fig. 4, show the different corrosion behavior of the samples after immersion in NaCl. F-DLC films present superior impedance in comparison to the pure DLC and the substrate. The enhancement in the corrosion resistance of the F-DLC samples may be attributed to the reduced electrical conductivity caused by the intrinsic chemical inertness of the F-DLC films in comparison to the uncoated sample [21]. Hence, F-DLC films can act as a passive film to prevent aggressive ions from attacking the substrate and thereby improve the corrosion resistance of the 316L stainless steel. The corrosion resistance of F-DLC films is affected by several factors, such as surface roughness and hydrophobicity, but mainly due to the difference in their microstructure [3,21]. It has been reported carbon based coatings with higher roughness present a larger surface area, which leads to worse corrosion protection [21,22]. According to the aforementioned AFM results, the surface roughness of the F-DLC films slightly increased with the increase of fluorine content, but all the surface roughness of the F-DLC films is less than 0.2 nm (Table 3). Therefore, the effect of surface roughness of the F-DLC films on their corrosion resistance can be negligible [21]. According to Sui et al. [21], the corrosion behavior of the F-DLC films has relationship with the sp3/sp2 ratio. In general, the higher sp3/sp2 ratio, the higher corrosion resistance [21,23]. The qualitative sp3/sp2 ratio of the F-DLC films obtained from Raman spectroscopy is decreased with the increase of fluorine content. The corresponding corrosion behavior should be
Table 4 Electrochemical parameters obtained from potentiodynamic polarization curves.
Fig. 2. Total compressive stress of the films according to the CF4 concentration in the precursor gases. The zero point corresponds to the DLC film produced from only methane.
Samples
Ecorr (mV)
icorr (nA/cm2)
Protection efficiency (%)
Stainless steel DLC F-DLC 0.9% F-DLC 1.0% F-DLC 1.5% F-DLC 2.0%
− 0.340 − 0.174 − 0.159 − 0.361 − 0.169 − 0.149
5.800 1.370 0.029 0.001 0.123 0.110
– 76.4 99.5 100.0 97.9 98.1
540
F.R. Marciano et al. / Diamond & Related Materials 19 (2010) 537–540
4. Conclusions The effects of fluorine incorporation on the composition, structure and electrochemical corrosion protection of DLC films were investigated. F-DLC films were deposited by PECVD with just a small amount of fluorine (b 2 at.%). High quality F-DLC films were produced. The incorporation of fluorine reduced the compressive stress and increased DLC ID/IG ratio and hydrogen content. F-DLC films improve DLC electrochemical corrosion resistance with just small fluorine content. From these results, F-DLC films can be considered a potential candidate for an anti-corrosion material in industrial applications.
Acknowledgements This study was supported by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP).
References
Fig. 4. a) Nyquist plot of stainless steel, DLC and F-DLC films. b) Enlargement of the region within the rectangular box in (a).
decreased. However, the corrosion resistance of the F-DLC films first increased and then slightly decreased with the increase of fluorine content. However, all the F-DLC film produced present protection efficiency higher than 97.9%.
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