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Structure and tribological properties of amorphous carbon films deposited by electrochemical method on GCr15 steel substrate
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Applied Surface Science 254 (2008) 2425–2430 www.elsevier.com/locate/apsusc
Structure and tribological properties of amorphous carbon films deposited by electrochemical method on GCr15 steel substrate Qun-feng Zeng, Guang-neng Dong *, You-bai Xie Key Laboratory of Education Ministry for Modern Design and Rotor-Bearing System, Xi’an Jiaotong University, Xi’an 710049, PR China Received 12 June 2007; received in revised form 17 September 2007; accepted 19 September 2007 Available online 29 September 2007
Abstract Amorphous carbon films were deposited on GCr15 steel substrates by electrolysis of methanol, dimethylsulfoxide (DMSO) and the methanol– DMSO intermixture electrolytes, respectively, under high voltage and low temperature conditions. The microstructure and wear morphology of the deposited films were analyzed using X-ray diffraction (XRD), Raman spectroscopy and scanning electron microscopy (SEM) combined with energy dispersive X-ray fluorescence spectrometer (EDX), respectively. The tribological properties of the films were evaluated using a ball-on-disk rotating friction tester under dry friction condition. The results show that the films deposited by electrodeposition technique on GCr15 steel substrates are amorphous carbon films. It is also found that the electrolytes have an obvious influence on the tribological properties of the deposited films with the electrodeposition method. The tribological properties of the films deposited with the intermixture electrolyte are better than those of the films deposited by other pure electrolytes. The related growth mechanism of the films in the liquid-phase electrodeposition is discussed as well in this study. Via the reaction of the –CH3 groups with each other to form carbon network and reaction of the –CH3 and SO2+ groups to achieve the doping of sulfur atom in the carbon network, respectively, in other words, amorphous carbon films can be obtained on GCr15 steel substrates by electrodeposition technique. # 2007 Elsevier B.V. All rights reserved. Keywords: Amorphous carbon films; Electrodeposition technique; Tribological properties
1. Introduction Diamond like carbon films which can be defined as amorphous carbon material, which may be hydrogenated or not, have been deposited by means of resorting to vapor phase deposition techniques. However, the applications of the gas phase synthesis methods including physical vapor deposition and chemical vapor deposition are limited to some extent because of the complicated experiment apparatus and extreme operational conditions such as high temperature, high energy and low pressure [1,2]. This situation did not change until Namba [3] first attempted to grow carbon films through ethanol solution in liquid phase. Following this new idea, carbon films have been attempted to deposit by means of electrodeposition techniques in recent years [4–6]. All above researches indicate that amorphous carbon films deposited electrochemically in
* Corresponding author. Tel.: +86 29 82668552; fax: +86 29 83237910. E-mail address: [email protected] (G.-n. Dong). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.09.061
the liquid phase are now attracting more attention, and the selection of electrolytes and treatment of substrates are the key factors in the deposition process. In the development of electrodeposition of amorphous carbon films in the liquid phase, two trends can be observed: (1) on the principle of the selective rules of carbon sources, the preferable electrolytes whose dielectric constant and dipole moment are relative high are chosen [7]; (2) the choice of substrates shifts from silicon to steel and metal and so on, which may produce fully samples with outstanding mechanical properties such as high hardness and strength. In this study, amorphous carbon films are attempted to deposit on GCr15 steel substrates by means of electrodeposition technique in the methanol, DMSO and methanol–DMSO intermixture electrolytes with a pulse modulated power, respectively. The influences of the electrolyte on tribological properties of the deposited films are discussed under dry friction condition. In the end, the related growth mechanism of the deposited films by use of electrodeposition method is discussed as well.
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2. Experimental details 2.1. Electrodeposition of amorphous carbon films All the reactants and solvents used in this study are analytical grade. The films are deposited in an electrolytic cell system as shown in Fig. 1. The GCr15 steel substrates (hardness 635–645 HV) measuring 30 mm in diameter and 5 mm in thickness were carefully grinded and polished on the abrasive finishing machine. Before the electrodeposition process, the substrates firstly were cleaned by ultrasonic treatment in acetone; secondly, acidulated in the HCl–H2SO4 solution for short period of time to remove the native oxide layer, and then washed with the deionized water. Finally, the substrates were mounted on the cathode. A graphite disk measuring 44 mm in diameter and 3 mm in thickness was used as the counter electrode. The distance between the two electrodes was set to 8 mm. The voltage applied to the substrates can be varied from 0 to 3000 V. The voltage was kept at a constant value of 1600 V under a constant temperature during the electrodeposition. The duty cycle and the frequency were set as 70% and 7 kHz, respectively. Methanol, DMSO and their intermixture solutions, respectively, were used as the carbon source material in the parallel experiments. Methanol is chosen because its polarizability and conductivity are strong, and the structure of methanol is close to that of diamond. DMSO so also is, and it has high boiling point. The deposited films were cleaned in acetone with ultrasonic treatment, dried at room temperature naturally for further characterization.
a previously masked. The minimum resolution of the thickness was 10 nm. The thickness of amorphous carbon films was approximately 230 nm. 2.3. Tribological properties of amorphous carbon films To evaluate the tribological properties of the deposited films, the friction tests were carried out using a ball-on-disk rotatingtype friction tester. The tests were performed in ambient air, relative humidity with 50–60% and room temperature about 26 8C in the laboratory. The GCr15 steel ball of 6 mm in diameter slid over the surface of the films coated the substrates at the speed of 0.15 m/s and normal load of 2.95 N. The initial average Hertzian contact pressure was 0.98 GPa, assuming that the ball is in direct contact with the substrate. All balls were cleaned ultrasonically with acetone prior to measurement and a new ball was used for each friction test. The friction coefficients and sliding time were recorded automatically during the test, respectively. 3. Results and discussion 3.1. The dependence of current density on time
Wear morphology of the films was observed by SEM. Raman spectrum was used for investigating the microstructure of the films in the range of 1000–1800 cm 1 with 514.5 nm excitation wavelength and a resolution of 2 cm 1. XRD measurement of the films was also performed. EDX was applied to investigate the component of the films. The Talysurf was applied to measure the thickness of the films on
Apparently current density is a crucial parameter in an electrochemical reaction. The dependence of current density on deposition time is shown in Fig. 2 by the electrolysis of methanol, DMSO and their intermixture solution under the same experimental conditions. Regardless of the exact quantities of current density under the same time, the variation tendencies of the curves are approximately the same: the current densities decrease gradually at initial stage, and then increase a stable value slightly with little change in magnitude. The organic electrolyte is non-conductor substance, when the electrolysis take place and produce an electric current that pass through a substance with high resistivity, the Joule heating which is generated in the electrochemical reaction may affect the solution significantly, leading to the temperature of the electrolyte to increase rapidly. However, this temperature rise leads to a
Fig. 1. Schematic diagram of the deposition system.
Fig. 2. The dependence of current density on time.
2.2. Characterization of amorphous carbon films
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Table 1 The properties of the electrolytes [8] Electrolyte
Dielectric constant (e)
Dipole moment (D)
Boiling point (8C)
Viscosity (mN s m 2)
Methanol DMSO
32.7 46.6
1.7 3.9
64 189
0.554 1.996
decrease in the viscosity of electrolyte, causing the reaction to carry out more easily. With the increase of deposition time, a layer of films with higher resistance was deposited on the substrate, thus, the resistance of films continued to increase as the experiment further carrying out, the films would cause the current density to drop gradually. However, the current density in the different electrolytes has little difference in magnitude, which is related to the properties of the electrolyte (as shown in Table 1). Current density increased with the increase in the dielectric constant and dipole moment of electrolyte because of the increase of the conductance. The Joule heating may be large because of high current density, so the local temperature in the DMSO solution became very high, which caused oxidation of the films easily, resulting in the high oxygen content in the films. The presumption is confirmed by the component analysis of EDX for the films, as shown in Table 2, the oxygen atomic percent in the films deposited with DMSO electrolyte is largest in the films in this study. In the experiment, the films firstly were grown on the edge of the substrate, and then the films extended to the center of the substrate as the deposition proceeded, indicating that the deposition progress was from the edge to center. 3.2. Structure of amorphous carbon films The XRD spectrum of the films deposited with methanol electrolyte is shown in Fig. 3. The peak at about 2u = 44.48 is found to be the strongest in the films, which corresponds to diamond (1 1 1). The weak peak at the left of the diffraction angle of 44.48 may be Fe3C (1 1 2). Also noting a narrow signal centered 2u = 26.38 in the diffraction pattern, the intensity of which is lower than that of the peak centered at 44.48, which can be identified as the diffraction peak characteristic of graphite phase. The weak peak at the diffraction angle of 64.48 can be also observed. The peak at the diffraction angle of 64.48 corresponds to average inter-planar spacing/inter-atomic separation of 0.14 nm which is close to the average nearest neighbor distance in amorphous carbon. These analyses suggest that the deposited films may be composed of the amorphous carbon phase, graphite phase and diamond phase. According to
Fig. 3. The XRD spectrum of the film deposited with methanol electrolyte.
EDX analysis (as shown in Table 2), there is much oxygen, so graphite could be probably oxidized in the electrochemical reaction. The oxidization of graphite can broaden the peak. The above comprehensive analysis seems to show that the films are amorphous carbon films. Raman spectrum of the films deposited with methanol and intermixture electrolyte is shown in Figs. 4 and 5, respectively. Their spectral shapes and widths indicate the amorphous structure of the films. In Fig. 4, the peak centered at about 1563 cm 1 which is observed may be attributed to amorphous carbon [9]. However, in Fig. 5, Raman spectrum of the films with intermixture electrolyte had four peaks at about 1362, 557, 1118 and 1551 cm 1, respectively, an potential amorphous carbon component [10]. There are some differences in the Raman spectra of the Figs. 4 and 5 from typical spectra of amorphous carbon films. There may be some nanocrystal diamond in the films deposited with electrodeposition method in the present study. In Figs. 4 and 5, the observed narrow features are centered about 1563, 1381, 1121, 1551, 1362 and 1118 cm 1, respectively, which may be little characteristic of diamond. The around 1550 cm 1 peak arises due to the graphitic phase, which is believed to be associated with amorphous carbon films, while the one at around 1380 cm 1 is identified as a disorder-graphite peak arising due to finite
Table 2 The result of EDX analysis of the films
CK OK SK Metals Totals
Methanol (at.%)
DMSO (at.%)
Intermixture (at.%)
47.73 37.07 0 15.20 100.00
25.56 54.66 2.63 17.15 100.00
47.95 16.74 1.07 34.24 100.00
Fig. 4. Raman spectrum of the film deposited with methanol electrolyte.
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Fig. 5. Raman spectrum of the film deposited with intermixture electrolyte.
crystallite size of graphitic regions. The around 1120 cm 1 peak has been tentatively assigned either to small or defective nanocrystal of diamond. It may be found that the intensity of the Raman spectrum is little low in Fig. 5. Raman scattering from defects and compressive strains were evoked to explain the observed behavior. In combination with the analysis of XRD spectra, there may be amorphous carbon films co-existing with nanocrystal diamond. 3.3. Tribological properties of amorphous carbon films The variations of friction coefficients varied with sliding cycle for amorphous carbon films deposited with different electrolytes sliding the GCr15 ball under dry friction condition are shown in Fig. 6. The initial friction coefficients of the films deposited with DMSO electrolyte are about 0.6; then they decrease slowly to about 0.45. In comparison to the deposited films with DMSO electrolyte, under the same test conditions, the films deposited with methanol and intermixture electrolytes records low steady-state friction coefficients relatively in the initial step, and the films deposited with the intermixture electrolytes have longer longevity, sustaining about 1000 cycles. At the same time, it is also revealed that the friction coefficients of the GCr15 steel substrate are higher than those
Fig. 6. The friction coefficients varied with sliding cycles of amorphous carbon films with different electrolytes.
Fig. 7. SEM wear images of the film deposited with DMSO electrolyte.
of the films deposited on substrates. Thus, it can be concluded that the amorphous carbon films deposited by the electrochemical technique have better anti friction behaviors than the substrates. It should be note that the films deposited with methanol electrolyte were completely worn out during wear, the friction coefficients of films increase gradually, finally reaching the same level as the substrate under the same conditions. However, the friction coefficients of the films deposited with other pure electrolyte are slight lower than these of the substrate. The SEM images of wear tracks for the deposited films under dry friction condition are shown in Figs. 7–9, respectively. From Figs. 7–9, it is found that the surface of the films deposited with intermixture electrolyte is smooth, compact and uniformity, comparatively speaking, that of the other films is loose and rough, which was abraded easily in the test. In the experiment, the surface of substrate was slightly more damaged than in the case of the films. The wear particles were scattered unequally over the surface of substrate as shown in Fig. 7. The surfaces of the films deposited with methanol and DMSO electrolyte are a small clear wearing of the films, but almost no signs of damage could be observed the surface of the films deposited with the intermixture electrolyte (Fig. 9). The morphological features are in agreement with the corresponding friction behaviors.
Q.-f. Zeng et al. / Applied Surface Science 254 (2008) 2425–2430
Fig. 8. SEM wear images of the film deposited with methanol electrolyte.
Combining with the EDX analysis, sulfur element is observed evidently in the films deposited with DMSO electrolyte, which the same observation can be made for the films deposited with the intermixture electrolyte, but observed none of sulfur in the films deposited with methanol electrolyte. From the results of the friction tests, the sulfur may be having little effect on the tribological properties of the films. Meanwhile, it is also found that the friction coefficient of the films deposited with DMSO electrolyte is not as good as that of the films deposited with methanol electrolyte, in spite of the obviously high sulfur content in the films. This phenomenon should be explained that the internal stress in the films might be very high because of the high Joule heating caused by high current density (as shown in Fig. 2) during the films growing. On this account, the friction coefficient of amorphous carbon films deposited with DMSO electrolyte is larger than that of other films. In addition, the oxygen in the films also has unfavorable effects on the tribological properties of the films [11]. Following above rounded analysis, these differences imply that the films deposited with intermixture electrolyte have better tribological properties than other films. On the basis of this discussion, the conclusion can be obtained that the electrolytes have an obvious effect on the tribological properties of the deposited films with electrodeposition
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Fig. 9. SEM wear images of film deposited with intermixture electrolyte.
techniques. The present results demonstrate exciting prospects for the further research to improve the tribological behaviors of the deposited films by means of electrodeposition techniques. 3.4. Growth mechanism of amorphous carbon films For the growth of amorphous carbon films from hydrocarbon vapors, single carbon atom species including CH3 radicals and CH3+ ions, or two carbon atom species C2H2 are suggested to be the intermediates for the film growth [12]. CH3 radicals and CH3+ ions may also play a critical role in the growth of amorphous carbon films with electrodeposition techniques. The molecules of CH3OH and DMSO all contain electron donating groups (CH3 in methanol and DMSO) and electron-withdrawing groups. The dipole moment of the molecules is enhanced and the molecules are polarized under a high voltage in the process of electrodeposition. It is well known that the polarization energy of C–O radical (2.5 kJ/mol) has less than that of O–H radical (5.0 kJ/mol) and the polarization energy of C–S radical (3.0 kJ/mol) has less than that of S O radical (10.0 kJ/mol), as a result the chemical bond of C–O radical and C–S radical are broken easily in the electrochemical reaction. Thus, when the supplied energy reaches a certain value, the C–O covalent bond in the polarized methanol molecule and
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C–S covalent bond in the polarized DMSO molecule, respectively, will be preferentially broken. Then, both the alkyl group (CH3) from the methanol and DMSO will move toward the cathode to allow the concentration of a certain amount of groups with positive charge (CH3+ in methanol and DMSO) near the cathode surface. Finally, amorphous carbon films will be generated on the substrate surface via the reactions of the CH3+ groups with each other to form carbon network and reactions of CH3+ and S O2+ groups to form the carbon network with the doping of sulfur atom, respectively. Amorphous carbon films can be deposited on the substrate with an electrodeposition method. It is also found that the sulfur content is little in comparison with other elements in the films as shown in Table 2. The sulfur in the films was derived principally from DMSO electrolyte in the electrochemical reaction. The DMSO electrolyte has large dipole moment, so it is polarized easily when electrolysis take place. Moreover, the polarization energy of C–S radical has less than that of S O radical, the chemical bond of C–S radical is broken easily in the electrochemical reaction. CH3 radicals and CH3+ ions take off hydrogen atom gradually and form progressively the carbon network in electrochemical reaction in the negative electrode. CH3 radicals and CH3+ ions play a critical role in the growth of the films in the negative electrode with electrodeposition techniques; however, sulfur species only may be incorporate into carbon network. We assumed that the sulfur should combine with oxygen mainly in the films. According to chemical equilibrium, the sulfur content may be large (beyond 2.63%). However, there are a great deal of the Joule heating which is generated in the electrochemical reaction, causing S O2+ groups oxidized easily and formed sulfur compound such as sulfur dioxide which are escaped easily from the films. As mentioned above analyses, there is little sulfur in the films. However, the deposition mechanism is very complex and its study is still under research. 4. Conclusions Amorphous carbon films are successfully deposited on GCr15 steel substrates through the electrolysis of methanol,
DMSO and methanol–DMSO intermixture electrolyte at high voltage, ambient pressure and low temperature in order to improve the tribological properties of GCr15 steel substrates. The results in this study are listed as below. 1. The films deposited with electrodeposition techniques are amorphous carbon films. 2. Electrolyte has large effects on the tribological properties of the deposited films, and the intermixture electrolyte owning promiseful application. 3. Amorphous carbon films exhibit outstanding anti friction behaviors. Compared to the films deposited with pure electrolyte, the films deposited with the intermixture electrolyte having less friction coefficient and longer longevity. 4. The methyl group in the molecule of electrolytes seems to be the functional group in forming amorphous carbon films, and the doping sulfur depend on S O2+ groups to form carbon sulfur network in the films. Acknowledgement The present study is supported by the National Natural Science Foundation of China (grant no. 50575173). References [1] Y. Ozmen, A. Tanaka, T. Sumiya, Surf. Coat. Technol. 133–134 (2000) 455–459. [2] M.D. Michel, Thin Solid Films 496 (2006) 481–488. [3] Y. Namba, J. Vac. Sci. Technol. A 10 (1992) 3368. [4] H. Wang, M.R. Shen, Appl. Phys. Lett. 69 (1996) 1074–1076. [5] X.B. Yan, T. Xu, G. Chen, S.H. Yang, Appl. Surf. Sci. 236 (2004) 328–335. [6] M. Roy, A.K. Dua, A.K. Satpati, Diamond Relat. Mater. 14 (2005) 60–67. [7] W.L. He, R. Yu, H. Wang, H. Yan, Carbon 43 (2005) 2000–2006. [8] J.A. Dean, Handbook of Lang’s Chemistry, Scientific Publishing House, Beijing, 1991, pp. 1738–1748 (Chapter10). [9] C. Casiraghi, A.C. Ferrari, J. Robertson, Phys. Rev. B 72 (2005) 1–14. [10] A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (2000) 95–107. [11] H.X. Li, et al. Appl. Surf. Sci. 249 (2005) 257–265. [12] Q. Fu, J.T. Jiu, C.B. Cao, Surf. Coat. Technol. 124 (2000) 196–200.
Applied Surface Science 254 (2008) 2425–2430 www.elsevier.com/locate/apsusc
Structure and tribological properties of amorphous carbon films deposited by electrochemical method on GCr15 steel substrate Qun-feng Zeng, Guang-neng Dong *, You-bai Xie Key Laboratory of Education Ministry for Modern Design and Rotor-Bearing System, Xi’an Jiaotong University, Xi’an 710049, PR China Received 12 June 2007; received in revised form 17 September 2007; accepted 19 September 2007 Available online 29 September 2007
Abstract Amorphous carbon films were deposited on GCr15 steel substrates by electrolysis of methanol, dimethylsulfoxide (DMSO) and the methanol– DMSO intermixture electrolytes, respectively, under high voltage and low temperature conditions. The microstructure and wear morphology of the deposited films were analyzed using X-ray diffraction (XRD), Raman spectroscopy and scanning electron microscopy (SEM) combined with energy dispersive X-ray fluorescence spectrometer (EDX), respectively. The tribological properties of the films were evaluated using a ball-on-disk rotating friction tester under dry friction condition. The results show that the films deposited by electrodeposition technique on GCr15 steel substrates are amorphous carbon films. It is also found that the electrolytes have an obvious influence on the tribological properties of the deposited films with the electrodeposition method. The tribological properties of the films deposited with the intermixture electrolyte are better than those of the films deposited by other pure electrolytes. The related growth mechanism of the films in the liquid-phase electrodeposition is discussed as well in this study. Via the reaction of the –CH3 groups with each other to form carbon network and reaction of the –CH3 and SO2+ groups to achieve the doping of sulfur atom in the carbon network, respectively, in other words, amorphous carbon films can be obtained on GCr15 steel substrates by electrodeposition technique. # 2007 Elsevier B.V. All rights reserved. Keywords: Amorphous carbon films; Electrodeposition technique; Tribological properties
1. Introduction Diamond like carbon films which can be defined as amorphous carbon material, which may be hydrogenated or not, have been deposited by means of resorting to vapor phase deposition techniques. However, the applications of the gas phase synthesis methods including physical vapor deposition and chemical vapor deposition are limited to some extent because of the complicated experiment apparatus and extreme operational conditions such as high temperature, high energy and low pressure [1,2]. This situation did not change until Namba [3] first attempted to grow carbon films through ethanol solution in liquid phase. Following this new idea, carbon films have been attempted to deposit by means of electrodeposition techniques in recent years [4–6]. All above researches indicate that amorphous carbon films deposited electrochemically in
* Corresponding author. Tel.: +86 29 82668552; fax: +86 29 83237910. E-mail address: [email protected] (G.-n. Dong). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.09.061
the liquid phase are now attracting more attention, and the selection of electrolytes and treatment of substrates are the key factors in the deposition process. In the development of electrodeposition of amorphous carbon films in the liquid phase, two trends can be observed: (1) on the principle of the selective rules of carbon sources, the preferable electrolytes whose dielectric constant and dipole moment are relative high are chosen [7]; (2) the choice of substrates shifts from silicon to steel and metal and so on, which may produce fully samples with outstanding mechanical properties such as high hardness and strength. In this study, amorphous carbon films are attempted to deposit on GCr15 steel substrates by means of electrodeposition technique in the methanol, DMSO and methanol–DMSO intermixture electrolytes with a pulse modulated power, respectively. The influences of the electrolyte on tribological properties of the deposited films are discussed under dry friction condition. In the end, the related growth mechanism of the deposited films by use of electrodeposition method is discussed as well.
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2. Experimental details 2.1. Electrodeposition of amorphous carbon films All the reactants and solvents used in this study are analytical grade. The films are deposited in an electrolytic cell system as shown in Fig. 1. The GCr15 steel substrates (hardness 635–645 HV) measuring 30 mm in diameter and 5 mm in thickness were carefully grinded and polished on the abrasive finishing machine. Before the electrodeposition process, the substrates firstly were cleaned by ultrasonic treatment in acetone; secondly, acidulated in the HCl–H2SO4 solution for short period of time to remove the native oxide layer, and then washed with the deionized water. Finally, the substrates were mounted on the cathode. A graphite disk measuring 44 mm in diameter and 3 mm in thickness was used as the counter electrode. The distance between the two electrodes was set to 8 mm. The voltage applied to the substrates can be varied from 0 to 3000 V. The voltage was kept at a constant value of 1600 V under a constant temperature during the electrodeposition. The duty cycle and the frequency were set as 70% and 7 kHz, respectively. Methanol, DMSO and their intermixture solutions, respectively, were used as the carbon source material in the parallel experiments. Methanol is chosen because its polarizability and conductivity are strong, and the structure of methanol is close to that of diamond. DMSO so also is, and it has high boiling point. The deposited films were cleaned in acetone with ultrasonic treatment, dried at room temperature naturally for further characterization.
a previously masked. The minimum resolution of the thickness was 10 nm. The thickness of amorphous carbon films was approximately 230 nm. 2.3. Tribological properties of amorphous carbon films To evaluate the tribological properties of the deposited films, the friction tests were carried out using a ball-on-disk rotatingtype friction tester. The tests were performed in ambient air, relative humidity with 50–60% and room temperature about 26 8C in the laboratory. The GCr15 steel ball of 6 mm in diameter slid over the surface of the films coated the substrates at the speed of 0.15 m/s and normal load of 2.95 N. The initial average Hertzian contact pressure was 0.98 GPa, assuming that the ball is in direct contact with the substrate. All balls were cleaned ultrasonically with acetone prior to measurement and a new ball was used for each friction test. The friction coefficients and sliding time were recorded automatically during the test, respectively. 3. Results and discussion 3.1. The dependence of current density on time
Wear morphology of the films was observed by SEM. Raman spectrum was used for investigating the microstructure of the films in the range of 1000–1800 cm 1 with 514.5 nm excitation wavelength and a resolution of 2 cm 1. XRD measurement of the films was also performed. EDX was applied to investigate the component of the films. The Talysurf was applied to measure the thickness of the films on
Apparently current density is a crucial parameter in an electrochemical reaction. The dependence of current density on deposition time is shown in Fig. 2 by the electrolysis of methanol, DMSO and their intermixture solution under the same experimental conditions. Regardless of the exact quantities of current density under the same time, the variation tendencies of the curves are approximately the same: the current densities decrease gradually at initial stage, and then increase a stable value slightly with little change in magnitude. The organic electrolyte is non-conductor substance, when the electrolysis take place and produce an electric current that pass through a substance with high resistivity, the Joule heating which is generated in the electrochemical reaction may affect the solution significantly, leading to the temperature of the electrolyte to increase rapidly. However, this temperature rise leads to a
Fig. 1. Schematic diagram of the deposition system.
Fig. 2. The dependence of current density on time.
2.2. Characterization of amorphous carbon films
Q.-f. Zeng et al. / Applied Surface Science 254 (2008) 2425–2430
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Table 1 The properties of the electrolytes [8] Electrolyte
Dielectric constant (e)
Dipole moment (D)
Boiling point (8C)
Viscosity (mN s m 2)
Methanol DMSO
32.7 46.6
1.7 3.9
64 189
0.554 1.996
decrease in the viscosity of electrolyte, causing the reaction to carry out more easily. With the increase of deposition time, a layer of films with higher resistance was deposited on the substrate, thus, the resistance of films continued to increase as the experiment further carrying out, the films would cause the current density to drop gradually. However, the current density in the different electrolytes has little difference in magnitude, which is related to the properties of the electrolyte (as shown in Table 1). Current density increased with the increase in the dielectric constant and dipole moment of electrolyte because of the increase of the conductance. The Joule heating may be large because of high current density, so the local temperature in the DMSO solution became very high, which caused oxidation of the films easily, resulting in the high oxygen content in the films. The presumption is confirmed by the component analysis of EDX for the films, as shown in Table 2, the oxygen atomic percent in the films deposited with DMSO electrolyte is largest in the films in this study. In the experiment, the films firstly were grown on the edge of the substrate, and then the films extended to the center of the substrate as the deposition proceeded, indicating that the deposition progress was from the edge to center. 3.2. Structure of amorphous carbon films The XRD spectrum of the films deposited with methanol electrolyte is shown in Fig. 3. The peak at about 2u = 44.48 is found to be the strongest in the films, which corresponds to diamond (1 1 1). The weak peak at the left of the diffraction angle of 44.48 may be Fe3C (1 1 2). Also noting a narrow signal centered 2u = 26.38 in the diffraction pattern, the intensity of which is lower than that of the peak centered at 44.48, which can be identified as the diffraction peak characteristic of graphite phase. The weak peak at the diffraction angle of 64.48 can be also observed. The peak at the diffraction angle of 64.48 corresponds to average inter-planar spacing/inter-atomic separation of 0.14 nm which is close to the average nearest neighbor distance in amorphous carbon. These analyses suggest that the deposited films may be composed of the amorphous carbon phase, graphite phase and diamond phase. According to
Fig. 3. The XRD spectrum of the film deposited with methanol electrolyte.
EDX analysis (as shown in Table 2), there is much oxygen, so graphite could be probably oxidized in the electrochemical reaction. The oxidization of graphite can broaden the peak. The above comprehensive analysis seems to show that the films are amorphous carbon films. Raman spectrum of the films deposited with methanol and intermixture electrolyte is shown in Figs. 4 and 5, respectively. Their spectral shapes and widths indicate the amorphous structure of the films. In Fig. 4, the peak centered at about 1563 cm 1 which is observed may be attributed to amorphous carbon [9]. However, in Fig. 5, Raman spectrum of the films with intermixture electrolyte had four peaks at about 1362, 557, 1118 and 1551 cm 1, respectively, an potential amorphous carbon component [10]. There are some differences in the Raman spectra of the Figs. 4 and 5 from typical spectra of amorphous carbon films. There may be some nanocrystal diamond in the films deposited with electrodeposition method in the present study. In Figs. 4 and 5, the observed narrow features are centered about 1563, 1381, 1121, 1551, 1362 and 1118 cm 1, respectively, which may be little characteristic of diamond. The around 1550 cm 1 peak arises due to the graphitic phase, which is believed to be associated with amorphous carbon films, while the one at around 1380 cm 1 is identified as a disorder-graphite peak arising due to finite
Table 2 The result of EDX analysis of the films
CK OK SK Metals Totals
Methanol (at.%)
DMSO (at.%)
Intermixture (at.%)
47.73 37.07 0 15.20 100.00
25.56 54.66 2.63 17.15 100.00
47.95 16.74 1.07 34.24 100.00
Fig. 4. Raman spectrum of the film deposited with methanol electrolyte.
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Fig. 5. Raman spectrum of the film deposited with intermixture electrolyte.
crystallite size of graphitic regions. The around 1120 cm 1 peak has been tentatively assigned either to small or defective nanocrystal of diamond. It may be found that the intensity of the Raman spectrum is little low in Fig. 5. Raman scattering from defects and compressive strains were evoked to explain the observed behavior. In combination with the analysis of XRD spectra, there may be amorphous carbon films co-existing with nanocrystal diamond. 3.3. Tribological properties of amorphous carbon films The variations of friction coefficients varied with sliding cycle for amorphous carbon films deposited with different electrolytes sliding the GCr15 ball under dry friction condition are shown in Fig. 6. The initial friction coefficients of the films deposited with DMSO electrolyte are about 0.6; then they decrease slowly to about 0.45. In comparison to the deposited films with DMSO electrolyte, under the same test conditions, the films deposited with methanol and intermixture electrolytes records low steady-state friction coefficients relatively in the initial step, and the films deposited with the intermixture electrolytes have longer longevity, sustaining about 1000 cycles. At the same time, it is also revealed that the friction coefficients of the GCr15 steel substrate are higher than those
Fig. 6. The friction coefficients varied with sliding cycles of amorphous carbon films with different electrolytes.
Fig. 7. SEM wear images of the film deposited with DMSO electrolyte.
of the films deposited on substrates. Thus, it can be concluded that the amorphous carbon films deposited by the electrochemical technique have better anti friction behaviors than the substrates. It should be note that the films deposited with methanol electrolyte were completely worn out during wear, the friction coefficients of films increase gradually, finally reaching the same level as the substrate under the same conditions. However, the friction coefficients of the films deposited with other pure electrolyte are slight lower than these of the substrate. The SEM images of wear tracks for the deposited films under dry friction condition are shown in Figs. 7–9, respectively. From Figs. 7–9, it is found that the surface of the films deposited with intermixture electrolyte is smooth, compact and uniformity, comparatively speaking, that of the other films is loose and rough, which was abraded easily in the test. In the experiment, the surface of substrate was slightly more damaged than in the case of the films. The wear particles were scattered unequally over the surface of substrate as shown in Fig. 7. The surfaces of the films deposited with methanol and DMSO electrolyte are a small clear wearing of the films, but almost no signs of damage could be observed the surface of the films deposited with the intermixture electrolyte (Fig. 9). The morphological features are in agreement with the corresponding friction behaviors.
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Fig. 8. SEM wear images of the film deposited with methanol electrolyte.
Combining with the EDX analysis, sulfur element is observed evidently in the films deposited with DMSO electrolyte, which the same observation can be made for the films deposited with the intermixture electrolyte, but observed none of sulfur in the films deposited with methanol electrolyte. From the results of the friction tests, the sulfur may be having little effect on the tribological properties of the films. Meanwhile, it is also found that the friction coefficient of the films deposited with DMSO electrolyte is not as good as that of the films deposited with methanol electrolyte, in spite of the obviously high sulfur content in the films. This phenomenon should be explained that the internal stress in the films might be very high because of the high Joule heating caused by high current density (as shown in Fig. 2) during the films growing. On this account, the friction coefficient of amorphous carbon films deposited with DMSO electrolyte is larger than that of other films. In addition, the oxygen in the films also has unfavorable effects on the tribological properties of the films [11]. Following above rounded analysis, these differences imply that the films deposited with intermixture electrolyte have better tribological properties than other films. On the basis of this discussion, the conclusion can be obtained that the electrolytes have an obvious effect on the tribological properties of the deposited films with electrodeposition
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Fig. 9. SEM wear images of film deposited with intermixture electrolyte.
techniques. The present results demonstrate exciting prospects for the further research to improve the tribological behaviors of the deposited films by means of electrodeposition techniques. 3.4. Growth mechanism of amorphous carbon films For the growth of amorphous carbon films from hydrocarbon vapors, single carbon atom species including CH3 radicals and CH3+ ions, or two carbon atom species C2H2 are suggested to be the intermediates for the film growth [12]. CH3 radicals and CH3+ ions may also play a critical role in the growth of amorphous carbon films with electrodeposition techniques. The molecules of CH3OH and DMSO all contain electron donating groups (CH3 in methanol and DMSO) and electron-withdrawing groups. The dipole moment of the molecules is enhanced and the molecules are polarized under a high voltage in the process of electrodeposition. It is well known that the polarization energy of C–O radical (2.5 kJ/mol) has less than that of O–H radical (5.0 kJ/mol) and the polarization energy of C–S radical (3.0 kJ/mol) has less than that of S O radical (10.0 kJ/mol), as a result the chemical bond of C–O radical and C–S radical are broken easily in the electrochemical reaction. Thus, when the supplied energy reaches a certain value, the C–O covalent bond in the polarized methanol molecule and
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C–S covalent bond in the polarized DMSO molecule, respectively, will be preferentially broken. Then, both the alkyl group (CH3) from the methanol and DMSO will move toward the cathode to allow the concentration of a certain amount of groups with positive charge (CH3+ in methanol and DMSO) near the cathode surface. Finally, amorphous carbon films will be generated on the substrate surface via the reactions of the CH3+ groups with each other to form carbon network and reactions of CH3+ and S O2+ groups to form the carbon network with the doping of sulfur atom, respectively. Amorphous carbon films can be deposited on the substrate with an electrodeposition method. It is also found that the sulfur content is little in comparison with other elements in the films as shown in Table 2. The sulfur in the films was derived principally from DMSO electrolyte in the electrochemical reaction. The DMSO electrolyte has large dipole moment, so it is polarized easily when electrolysis take place. Moreover, the polarization energy of C–S radical has less than that of S O radical, the chemical bond of C–S radical is broken easily in the electrochemical reaction. CH3 radicals and CH3+ ions take off hydrogen atom gradually and form progressively the carbon network in electrochemical reaction in the negative electrode. CH3 radicals and CH3+ ions play a critical role in the growth of the films in the negative electrode with electrodeposition techniques; however, sulfur species only may be incorporate into carbon network. We assumed that the sulfur should combine with oxygen mainly in the films. According to chemical equilibrium, the sulfur content may be large (beyond 2.63%). However, there are a great deal of the Joule heating which is generated in the electrochemical reaction, causing S O2+ groups oxidized easily and formed sulfur compound such as sulfur dioxide which are escaped easily from the films. As mentioned above analyses, there is little sulfur in the films. However, the deposition mechanism is very complex and its study is still under research. 4. Conclusions Amorphous carbon films are successfully deposited on GCr15 steel substrates through the electrolysis of methanol,
DMSO and methanol–DMSO intermixture electrolyte at high voltage, ambient pressure and low temperature in order to improve the tribological properties of GCr15 steel substrates. The results in this study are listed as below. 1. The films deposited with electrodeposition techniques are amorphous carbon films. 2. Electrolyte has large effects on the tribological properties of the deposited films, and the intermixture electrolyte owning promiseful application. 3. Amorphous carbon films exhibit outstanding anti friction behaviors. Compared to the films deposited with pure electrolyte, the films deposited with the intermixture electrolyte having less friction coefficient and longer longevity. 4. The methyl group in the molecule of electrolytes seems to be the functional group in forming amorphous carbon films, and the doping sulfur depend on S O2+ groups to form carbon sulfur network in the films. Acknowledgement The present study is supported by the National Natural Science Foundation of China (grant no. 50575173). References [1] Y. Ozmen, A. Tanaka, T. Sumiya, Surf. Coat. Technol. 133–134 (2000) 455–459. [2] M.D. Michel, Thin Solid Films 496 (2006) 481–488. [3] Y. Namba, J. Vac. Sci. Technol. A 10 (1992) 3368. [4] H. Wang, M.R. Shen, Appl. Phys. Lett. 69 (1996) 1074–1076. [5] X.B. Yan, T. Xu, G. Chen, S.H. Yang, Appl. Surf. Sci. 236 (2004) 328–335. [6] M. Roy, A.K. Dua, A.K. Satpati, Diamond Relat. Mater. 14 (2005) 60–67. [7] W.L. He, R. Yu, H. Wang, H. Yan, Carbon 43 (2005) 2000–2006. [8] J.A. Dean, Handbook of Lang’s Chemistry, Scientific Publishing House, Beijing, 1991, pp. 1738–1748 (Chapter10). [9] C. Casiraghi, A.C. Ferrari, J. Robertson, Phys. Rev. B 72 (2005) 1–14. [10] A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (2000) 95–107. [11] H.X. Li, et al. Appl. Surf. Sci. 249 (2005) 257–265. [12] Q. Fu, J.T. Jiu, C.B. Cao, Surf. Coat. Technol. 124 (2000) 196–200.