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Nanostructure influence on DLC-Ag tribological coatings
Surface & Coatings Technology 200 (2005) 198 – 202 www.elsevier.com/locate/surfcoat
Nanostructure influence on DLC-Ag tribological coatingsB C.P. LunguT National Institute for Materials Physics, P.O. Box MG 7, 77125, Bucharest-Magurele, Romania Available online 8 April 2005
Abstract Addition of diamond like carbon (DLC) phase to Ag-base overlay prepared by ECR–DC hybrid sputtering was found to reduce its coefficient of friction compared to that of the substrate material in dry sliding. The coefficient of friction, tested in dry conditions, of the Ag matrix overlay decreased by sp3-rich C formation. It decreased from 0.72F0.05 to 0.25F0.05 by increasing the graphite/Ag target area ratio from 1/9 to 5/5 at low sliding speed (0.01 ms 1), 5 N load in dry sliding. Its drastic decrease was found when the grain size of the prepared films was lower than 10 nm as measured by transmission electron microscopy (TEM). The prepared coatings were characterized also by Xray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman spectroscopy, and atomic force microscopy (AFM). D 2005 Elsevier B.V. All rights reserved. Keywords: ECR–DC; DLC; Metal-DLC; AFM; Tribology
1. Introduction
2. Experimental details
Tribological properties of plain bearings have to be improved corresponding to the currently increasing demand for high-power and light-weight automotive engines [1,2]. The plain bearing consists of a steel backup for strengthening, a powder-sintered bearing alloy and a coating of 10–20 Am in thickness called overlay. The overlays are needed to improve seizure and wear resistance, conformability and embeddability and they have conventionally been formed by electroplating of Pb–In–Sn–Cu alloys. However, since Pb is not an environmentally friendly material and its fatigue resistance is low in high-pressure conditions, new materials and deposition procedures are highly desired. Trials to form graphite/Ag (named further as Gr/Ag) overlays by Electron Cyclotron Resonance (ECR)–Direct Current (DC) hybrid sputtering have been carried out and addition of diamond like carbon (DLC) phase to Ag-base coating was found to reduce the coefficient of friction compared to that of the substrate material in dry sliding.
The sputtering apparatus used to prepare tribological overlays is presented in detail elsewhere [3,4]. Ar ions efficiently produced in the ECR zone operated at the microwave frequency of 2.45 GHz and the magnetic field strength of 0.0875 T are directed toward a cylindrical target by the gradient of the magnetic field and pressure. A negative bias (0–1000 V) was applied to the target with respect to the ground enhanced the sputtering rate. Atoms or molecules of the target are sputtered by the Ar ions and deposited onto the substrate. The target is of a hollow cylindrical shape and consists of a stack of Ag and Gr rings (inner diameter: 80 mm, outer diameter: 90 mm, thickness: 5 mm). The total number of the rings was kept at 10 and the ratio of the number of Gr rings to that of Ag ones, Gr/Ag, which was equivalent to the target area ratio, was set to 0/ 10, 1/9, 2/8, 3/7, 4/6 and 5/5. The details of the sputtering conditions are: Microwave power (2.45 GHz frequency); 400 W, dc hollow cathode bias; 800 V, Ar flow; 10 sccm, working pressure; 0.09 Pa (710 4 Torr), sputtering distance; 20–40 mm, substrate bias; floating. Ag and Ag-matrix overlays including Gr were deposited on rectangular substrates (30 mm30 mm2 mm size) made of rough (#600 grit) bronze. Three sets of substrates
B Work performed at Japan Ultra-high Temperature Materials Research Institute, Ube, Yamaguchi, Japan. T National Institute for Lasers, Plasma and Radiation Physics, PO-Box MG-36, Ro-77125, Bucharest-Magurele, Romania. Fax: +40 21 457 44 68. E-mail address: [email protected].
0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.02.103
C.P. Lungu / Surface & Coatings Technology 200 (2005) 198–202
199
Fig. 1. Surface morphology of the bronze substrate and Gr/Ag overlays deposited on bronze substrate analyzed by AFM in contact mode. 0/10, 1/9, 2/8, 3/7, 4/ 6 and 5/5 represent the Gr/Ag ratio.
were deposited in the same conditions. The deposition time was 120 min and the processing conditions were: sputtering distance: 3 cm, pressure; 0.092 Pa (710 4 Torr), Ar flow rate; 10 sccm, microwave power; 400 W, the current applied to the magnetic coil; 15.5 A. Structural properties were analyzed by an X-ray diffraction method (XRD) which uses monochromatic Cu Ka
radiation with a wavelength of 0.154 nm. The compositions and bonding states of the elements in the films were studied by X-ray photoelectron spectroscopy (XPS) where a standard Al Ka excitation source (hr=1486.6 eV) was employed. A transmission electron microscope (TEM) at 400 kV accelerating voltage was used to measure the grain size. Raman spectra were obtained in a back-scattering config-
Fig. 2. SEM pictures of the coated polished samples. 0/10, 1/9, 2/8, 3/7, 4/6 and 5/5 represent the Gr/Ag ratio.
C.P. Lungu / Surface & Coatings Technology 200 (2005) 198–202
uration with a laser Raman spectrometer using the 514.5 nm line of an Ar laser with 5 mW power and 50 Am spot diameter. The surface morphology of the Ag and Gr/Ag overlays was analyzed by scanning electron microscopy (SEM) and atomic force microscopy (AFM) in contact mode. The coefficients of friction were measured with a ball-ondisk type tribometer where bearing steel (SUJ2 equivalent to AISI 52100) balls of 6 mm in diameter were used as counter materials. Load of 5 N and sliding radii of 3, 4 or 6 mm were adopted. The sliding speed was 0.01 ms 1 or 0.1 ms 1 during sliding time of 1000 s. The dry sliding tests were carried out at room temperature (24 8C) in air with 40–50% relative humidity.
4/6
20000 15000 10000 5000 0 280
282
284
286
288
290
Binding energy, eV Fig. 4. The XPS C 1s core-level peak and peak deconvolution of the overlay prepared using 4/6 Gr/Ag ratio. The assumed peaks: Graphite (sp2): 284.4–284.8 eV, Diamond (sp3): 285.3–285.7 eV, Satellite (sp2): 286.2– 286.6 eV.
3. Results and discussion Analyzed by AFM, the surfaces of the bronze substrate and Gr/Ag overlays are shown in Fig. 1. The ‘‘planarizationQ, the effect when the bvalleysQ are filled with coating material and the peak-valley distance was reduced was observed in 1/9 and especially in the 3/7 case. In the 4/6 case the deposited films followed the original structure of the substrate. The overlays prepared using 5/5 Gr/Ag ratio show an increase in roughness as can be observed in Fig. 1. The same bplanarizationQ effect for the coating prepared using 3/7 Gr/Ag ratio was observed when mirror polished substrates were used, in a separate experiment, as shown in Fig. 2. Frictional characteristics tested by a ball-on-disk tribometer in dry conditions are shown in Fig. 3. The coefficient of friction in dry sliding decreased from 0.72F0.05 to 0.30F0.05 by increasing the Gr/Ag ratio from 1/9 to 5/5 at low sliding speed (0.01 ms 1), 5 N load. The coefficient of friction decreased also from 0.72F0.05 to 0.20F0.02 by
0.9 0.8
increasing the Gr/Ag ratio from 1/9 to 5/5 at higher sliding speed (0.1 ms 1), 5 N load. A drastically decrease of the coefficient of friction was observed when the Gr/Ag ratio changed from 0/10, 1/9, 2/8, 3/7 to 4/6 and 5/5. Fig. 4 shows the typical XPS C 1s core-level peak for the 4/6 case and the peak deconvolution. The spectral line shape suggest that the peaks are composed of at least two components, with a higher binding energy shoulder, detected at 285.3–285.7 eV corresponding to sp3 carbon atoms and another component, detected at 284.4–284.8 eV, that correspond to sp2 carbon atom. A third peak at about 286.2–286.6 eV was found also and has been attributed to some C–O contamination formed at the surface of the samples due to air exposure and/or sp2 satellite. The binding energy values found for the sp2 and sp3 components of the Gr/Ag overlays C 1s spectra are consistent with the binding energies of 284.4 eV and 285.2 eV detected for C 1s peaks of Gr (sp2) and diamond, respectively (sp3), reported in [5]. Fig. 5 shows the relation between sp2 and sp3 bonds as function of Gr/Ag ratios. The bonding of the carbon atoms was mainly sp2 type in the 1/9 overlay and only sp3 type in
0.7 0/10 1/9 2/8 3/7 4/6 5/5
Dry friction Speed: 0.01 ms-1 Load: 5 N
0.5 0.4 0.3 0.2 0.1 0
2
4
6
8
10
Sliding distance, m Fig. 3. Coefficient of friction behavior during dry sliding and 0.01 ms 1, load: 5 N.
40000 30000 30000 20000 20000 10000
10000 0
1/9
2/8
3/7
4/6
5/5
00000
Gr/Ag 2
3
Fig. 5. The sp and sp intensities as function of Gr/Ag ratios.
sp3 XPS intensity, a.u.
0.6
sp2 XPS Intensity, a.u.
Coefficient of friction
25000
XPS Intensity, a.u.
200
C.P. Lungu / Surface & Coatings Technology 200 (2005) 198–202
0/10
1/9
2/8/
4/6
201
5/5
Fig. 6. The bright field images of the film edges and selected area electron diffraction (SAED) of the 0/10, 1/9, 2/8, 4/6 and 5/5 coatings.
the 5/5 overlay. At intermediate values of Gr/Ag, the sp2 and sp3 bonds were present in a relation shown in the figure. The tendency to increase the sp3 bonds can be clearly observed. However in the 4/6 case, the sp3 bond type is lower as expected from a linear behavior. The film obtained using 4/6 ratio was the smoothest one, suggesting the appropriate ratio between sp2 and sp3 bonds in order to form smooth films. In order to have an evidence of the grain size of the coatings samples were prepared for TEM analysis. During deposition on flat substrates, specially designed TEM meshes made by Ni, 175 Am175 Am in size were settled on the same holder. Fig. 6 shows the bright field images of the film edges and selected area electron diffraction (SAED). The Ag single crystals were identified in this
(220) 5/5
way. The Ag single crystal grain size monotonically decreased when Gr/Ag ratio changes from 1/9 to 5/5. Nanostructures with grain size less than 10 nm were observed in the 4/6 and 5/5 cases. The results of XRD analysis shown in Fig. 7 reveal the broadening of the Ag (220) XRD peak, due to the decreasing of the Ag grain size with the Gr/Ag ratio increase. Raman spectroscopy (see Fig. 8) revealed the D- and G-band, characteristics of amorphous/diamond like carbon (DLC). A large intensity of the D-band is associated with very small graphite crystallite [6]. Several features were revealed after spectral de-convolution using Gaussian or Lorentzian functions. In the case of 1/9 (Gr/Ag) the D-band and G-band does not appear. We may assume that C atoms were included into the interstices of the Ag crystal structure. Increasing the Gr/Ag target ratio, C atoms determined Ag growth as grains and being bonded at the boundary of the Ag grains. The nature of C bonds as sp2 or sp3 determined
4/6 30
3/7
1/9 0/10 Ag bulk 63.0
63.5
64.0
64.5
65.0
65.5
2θ Fig. 7. Expanded XRD patterns of the overlays and Ag bulk used as target showing the broadening of the Ag (220) crystallographic peak with the increase of Gr/Ag ratio. (The dotted line shows the Ag (220) position.)
Intensity, a.u.
2/8
2/8 3/7 4/6 5/5
25 20 15 10 5 0 1000
1200
1400
1600
1800
2000
Raman shift, cm-1 Fig. 8. Raman spectra of the prepared films using 2/8, 3/7, 4/6 and 5/5 Gr/ Ag ratios.
202
C.P. Lungu / Surface & Coatings Technology 200 (2005) 198–202
the surface morphology and frictional characteristics of the films. Using the above developed overlays and tested in real operation conditions, the engine plain showed five times larger wear resistance and higher toughness than the Pb-base conventional overlays [7].
Acknowledgements The author greatly acknowledge the financial support offered by the New Energy Industrial Technology Development Organization (NEDO), Japan and to Professor A. Azushima of Yokohama University for the permission of using AFM and SEM facilities.
4. Conclusions References DLC-Ag nanostuctured films with thickness of about 10 `ım were prepared by ECR–DC sputtering. The films consist in graphite crystallites and DLC bonded to the Ag grains. The relative amount of Gr determined by Gr/Ag ratio of the sputtering target was easy controlled. The coefficient of friction decreased from 0.72F0.05 to 0.25F0.05 by increasing the Gr/Ag target area ratio from 1/9 to 5/5 at low sliding speed (0.01 ms 1), 5 N load in dry sliding. Its drastic decrease was ascertained to the nanoscale grain size of the prepared films as measured by TEM.
[1] A. Norito, S. Takayanagi, K. Ohkawa, K. Iwasaki, C.P. Lungu, Int. J. Appl. Mech. Eng. 7 (2002) 263. [2] K. Iwasaki, C.P. Lungu, S. Takayanagi, K. Ohkawa, Int. J. Appl. Mech. Eng. 7 (2002) 351. [3] C.P. Lungu, K. Iwasaki, Vacuum 66 (2002) 197. [4] C.P. Lungu, K. Iwasaki, Vacuum 66 (2002) 385. [5] Jin-Koog Shin, Churl Seung Lee, Kwang-Ryeol Lee, Kwang Yong Eun, Appl. Phys. Lett. 78 (5) (2001) 631. [6] E.F. Ferrari, F.C. da Silva, M. Knobel, Phys. Rev., B 59 (1999) 8412. [7] K. Iwasaki, C.P. Lungu, S. Takayanagi, K. Ohkawa, Int. J. Appl. Mech. Eng. 7 (2002) 351.
Nanostructure influence on DLC-Ag tribological coatingsB C.P. LunguT National Institute for Materials Physics, P.O. Box MG 7, 77125, Bucharest-Magurele, Romania Available online 8 April 2005
Abstract Addition of diamond like carbon (DLC) phase to Ag-base overlay prepared by ECR–DC hybrid sputtering was found to reduce its coefficient of friction compared to that of the substrate material in dry sliding. The coefficient of friction, tested in dry conditions, of the Ag matrix overlay decreased by sp3-rich C formation. It decreased from 0.72F0.05 to 0.25F0.05 by increasing the graphite/Ag target area ratio from 1/9 to 5/5 at low sliding speed (0.01 ms 1), 5 N load in dry sliding. Its drastic decrease was found when the grain size of the prepared films was lower than 10 nm as measured by transmission electron microscopy (TEM). The prepared coatings were characterized also by Xray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman spectroscopy, and atomic force microscopy (AFM). D 2005 Elsevier B.V. All rights reserved. Keywords: ECR–DC; DLC; Metal-DLC; AFM; Tribology
1. Introduction
2. Experimental details
Tribological properties of plain bearings have to be improved corresponding to the currently increasing demand for high-power and light-weight automotive engines [1,2]. The plain bearing consists of a steel backup for strengthening, a powder-sintered bearing alloy and a coating of 10–20 Am in thickness called overlay. The overlays are needed to improve seizure and wear resistance, conformability and embeddability and they have conventionally been formed by electroplating of Pb–In–Sn–Cu alloys. However, since Pb is not an environmentally friendly material and its fatigue resistance is low in high-pressure conditions, new materials and deposition procedures are highly desired. Trials to form graphite/Ag (named further as Gr/Ag) overlays by Electron Cyclotron Resonance (ECR)–Direct Current (DC) hybrid sputtering have been carried out and addition of diamond like carbon (DLC) phase to Ag-base coating was found to reduce the coefficient of friction compared to that of the substrate material in dry sliding.
The sputtering apparatus used to prepare tribological overlays is presented in detail elsewhere [3,4]. Ar ions efficiently produced in the ECR zone operated at the microwave frequency of 2.45 GHz and the magnetic field strength of 0.0875 T are directed toward a cylindrical target by the gradient of the magnetic field and pressure. A negative bias (0–1000 V) was applied to the target with respect to the ground enhanced the sputtering rate. Atoms or molecules of the target are sputtered by the Ar ions and deposited onto the substrate. The target is of a hollow cylindrical shape and consists of a stack of Ag and Gr rings (inner diameter: 80 mm, outer diameter: 90 mm, thickness: 5 mm). The total number of the rings was kept at 10 and the ratio of the number of Gr rings to that of Ag ones, Gr/Ag, which was equivalent to the target area ratio, was set to 0/ 10, 1/9, 2/8, 3/7, 4/6 and 5/5. The details of the sputtering conditions are: Microwave power (2.45 GHz frequency); 400 W, dc hollow cathode bias; 800 V, Ar flow; 10 sccm, working pressure; 0.09 Pa (710 4 Torr), sputtering distance; 20–40 mm, substrate bias; floating. Ag and Ag-matrix overlays including Gr were deposited on rectangular substrates (30 mm30 mm2 mm size) made of rough (#600 grit) bronze. Three sets of substrates
B Work performed at Japan Ultra-high Temperature Materials Research Institute, Ube, Yamaguchi, Japan. T National Institute for Lasers, Plasma and Radiation Physics, PO-Box MG-36, Ro-77125, Bucharest-Magurele, Romania. Fax: +40 21 457 44 68. E-mail address: [email protected].
0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.02.103
C.P. Lungu / Surface & Coatings Technology 200 (2005) 198–202
199
Fig. 1. Surface morphology of the bronze substrate and Gr/Ag overlays deposited on bronze substrate analyzed by AFM in contact mode. 0/10, 1/9, 2/8, 3/7, 4/ 6 and 5/5 represent the Gr/Ag ratio.
were deposited in the same conditions. The deposition time was 120 min and the processing conditions were: sputtering distance: 3 cm, pressure; 0.092 Pa (710 4 Torr), Ar flow rate; 10 sccm, microwave power; 400 W, the current applied to the magnetic coil; 15.5 A. Structural properties were analyzed by an X-ray diffraction method (XRD) which uses monochromatic Cu Ka
radiation with a wavelength of 0.154 nm. The compositions and bonding states of the elements in the films were studied by X-ray photoelectron spectroscopy (XPS) where a standard Al Ka excitation source (hr=1486.6 eV) was employed. A transmission electron microscope (TEM) at 400 kV accelerating voltage was used to measure the grain size. Raman spectra were obtained in a back-scattering config-
Fig. 2. SEM pictures of the coated polished samples. 0/10, 1/9, 2/8, 3/7, 4/6 and 5/5 represent the Gr/Ag ratio.
C.P. Lungu / Surface & Coatings Technology 200 (2005) 198–202
uration with a laser Raman spectrometer using the 514.5 nm line of an Ar laser with 5 mW power and 50 Am spot diameter. The surface morphology of the Ag and Gr/Ag overlays was analyzed by scanning electron microscopy (SEM) and atomic force microscopy (AFM) in contact mode. The coefficients of friction were measured with a ball-ondisk type tribometer where bearing steel (SUJ2 equivalent to AISI 52100) balls of 6 mm in diameter were used as counter materials. Load of 5 N and sliding radii of 3, 4 or 6 mm were adopted. The sliding speed was 0.01 ms 1 or 0.1 ms 1 during sliding time of 1000 s. The dry sliding tests were carried out at room temperature (24 8C) in air with 40–50% relative humidity.
4/6
20000 15000 10000 5000 0 280
282
284
286
288
290
Binding energy, eV Fig. 4. The XPS C 1s core-level peak and peak deconvolution of the overlay prepared using 4/6 Gr/Ag ratio. The assumed peaks: Graphite (sp2): 284.4–284.8 eV, Diamond (sp3): 285.3–285.7 eV, Satellite (sp2): 286.2– 286.6 eV.
3. Results and discussion Analyzed by AFM, the surfaces of the bronze substrate and Gr/Ag overlays are shown in Fig. 1. The ‘‘planarizationQ, the effect when the bvalleysQ are filled with coating material and the peak-valley distance was reduced was observed in 1/9 and especially in the 3/7 case. In the 4/6 case the deposited films followed the original structure of the substrate. The overlays prepared using 5/5 Gr/Ag ratio show an increase in roughness as can be observed in Fig. 1. The same bplanarizationQ effect for the coating prepared using 3/7 Gr/Ag ratio was observed when mirror polished substrates were used, in a separate experiment, as shown in Fig. 2. Frictional characteristics tested by a ball-on-disk tribometer in dry conditions are shown in Fig. 3. The coefficient of friction in dry sliding decreased from 0.72F0.05 to 0.30F0.05 by increasing the Gr/Ag ratio from 1/9 to 5/5 at low sliding speed (0.01 ms 1), 5 N load. The coefficient of friction decreased also from 0.72F0.05 to 0.20F0.02 by
0.9 0.8
increasing the Gr/Ag ratio from 1/9 to 5/5 at higher sliding speed (0.1 ms 1), 5 N load. A drastically decrease of the coefficient of friction was observed when the Gr/Ag ratio changed from 0/10, 1/9, 2/8, 3/7 to 4/6 and 5/5. Fig. 4 shows the typical XPS C 1s core-level peak for the 4/6 case and the peak deconvolution. The spectral line shape suggest that the peaks are composed of at least two components, with a higher binding energy shoulder, detected at 285.3–285.7 eV corresponding to sp3 carbon atoms and another component, detected at 284.4–284.8 eV, that correspond to sp2 carbon atom. A third peak at about 286.2–286.6 eV was found also and has been attributed to some C–O contamination formed at the surface of the samples due to air exposure and/or sp2 satellite. The binding energy values found for the sp2 and sp3 components of the Gr/Ag overlays C 1s spectra are consistent with the binding energies of 284.4 eV and 285.2 eV detected for C 1s peaks of Gr (sp2) and diamond, respectively (sp3), reported in [5]. Fig. 5 shows the relation between sp2 and sp3 bonds as function of Gr/Ag ratios. The bonding of the carbon atoms was mainly sp2 type in the 1/9 overlay and only sp3 type in
0.7 0/10 1/9 2/8 3/7 4/6 5/5
Dry friction Speed: 0.01 ms-1 Load: 5 N
0.5 0.4 0.3 0.2 0.1 0
2
4
6
8
10
Sliding distance, m Fig. 3. Coefficient of friction behavior during dry sliding and 0.01 ms 1, load: 5 N.
40000 30000 30000 20000 20000 10000
10000 0
1/9
2/8
3/7
4/6
5/5
00000
Gr/Ag 2
3
Fig. 5. The sp and sp intensities as function of Gr/Ag ratios.
sp3 XPS intensity, a.u.
0.6
sp2 XPS Intensity, a.u.
Coefficient of friction
25000
XPS Intensity, a.u.
200
C.P. Lungu / Surface & Coatings Technology 200 (2005) 198–202
0/10
1/9
2/8/
4/6
201
5/5
Fig. 6. The bright field images of the film edges and selected area electron diffraction (SAED) of the 0/10, 1/9, 2/8, 4/6 and 5/5 coatings.
the 5/5 overlay. At intermediate values of Gr/Ag, the sp2 and sp3 bonds were present in a relation shown in the figure. The tendency to increase the sp3 bonds can be clearly observed. However in the 4/6 case, the sp3 bond type is lower as expected from a linear behavior. The film obtained using 4/6 ratio was the smoothest one, suggesting the appropriate ratio between sp2 and sp3 bonds in order to form smooth films. In order to have an evidence of the grain size of the coatings samples were prepared for TEM analysis. During deposition on flat substrates, specially designed TEM meshes made by Ni, 175 Am175 Am in size were settled on the same holder. Fig. 6 shows the bright field images of the film edges and selected area electron diffraction (SAED). The Ag single crystals were identified in this
(220) 5/5
way. The Ag single crystal grain size monotonically decreased when Gr/Ag ratio changes from 1/9 to 5/5. Nanostructures with grain size less than 10 nm were observed in the 4/6 and 5/5 cases. The results of XRD analysis shown in Fig. 7 reveal the broadening of the Ag (220) XRD peak, due to the decreasing of the Ag grain size with the Gr/Ag ratio increase. Raman spectroscopy (see Fig. 8) revealed the D- and G-band, characteristics of amorphous/diamond like carbon (DLC). A large intensity of the D-band is associated with very small graphite crystallite [6]. Several features were revealed after spectral de-convolution using Gaussian or Lorentzian functions. In the case of 1/9 (Gr/Ag) the D-band and G-band does not appear. We may assume that C atoms were included into the interstices of the Ag crystal structure. Increasing the Gr/Ag target ratio, C atoms determined Ag growth as grains and being bonded at the boundary of the Ag grains. The nature of C bonds as sp2 or sp3 determined
4/6 30
3/7
1/9 0/10 Ag bulk 63.0
63.5
64.0
64.5
65.0
65.5
2θ Fig. 7. Expanded XRD patterns of the overlays and Ag bulk used as target showing the broadening of the Ag (220) crystallographic peak with the increase of Gr/Ag ratio. (The dotted line shows the Ag (220) position.)
Intensity, a.u.
2/8
2/8 3/7 4/6 5/5
25 20 15 10 5 0 1000
1200
1400
1600
1800
2000
Raman shift, cm-1 Fig. 8. Raman spectra of the prepared films using 2/8, 3/7, 4/6 and 5/5 Gr/ Ag ratios.
202
C.P. Lungu / Surface & Coatings Technology 200 (2005) 198–202
the surface morphology and frictional characteristics of the films. Using the above developed overlays and tested in real operation conditions, the engine plain showed five times larger wear resistance and higher toughness than the Pb-base conventional overlays [7].
Acknowledgements The author greatly acknowledge the financial support offered by the New Energy Industrial Technology Development Organization (NEDO), Japan and to Professor A. Azushima of Yokohama University for the permission of using AFM and SEM facilities.
4. Conclusions References DLC-Ag nanostuctured films with thickness of about 10 `ım were prepared by ECR–DC sputtering. The films consist in graphite crystallites and DLC bonded to the Ag grains. The relative amount of Gr determined by Gr/Ag ratio of the sputtering target was easy controlled. The coefficient of friction decreased from 0.72F0.05 to 0.25F0.05 by increasing the Gr/Ag target area ratio from 1/9 to 5/5 at low sliding speed (0.01 ms 1), 5 N load in dry sliding. Its drastic decrease was ascertained to the nanoscale grain size of the prepared films as measured by TEM.
[1] A. Norito, S. Takayanagi, K. Ohkawa, K. Iwasaki, C.P. Lungu, Int. J. Appl. Mech. Eng. 7 (2002) 263. [2] K. Iwasaki, C.P. Lungu, S. Takayanagi, K. Ohkawa, Int. J. Appl. Mech. Eng. 7 (2002) 351. [3] C.P. Lungu, K. Iwasaki, Vacuum 66 (2002) 197. [4] C.P. Lungu, K. Iwasaki, Vacuum 66 (2002) 385. [5] Jin-Koog Shin, Churl Seung Lee, Kwang-Ryeol Lee, Kwang Yong Eun, Appl. Phys. Lett. 78 (5) (2001) 631. [6] E.F. Ferrari, F.C. da Silva, M. Knobel, Phys. Rev., B 59 (1999) 8412. [7] K. Iwasaki, C.P. Lungu, S. Takayanagi, K. Ohkawa, Int. J. Appl. Mech. Eng. 7 (2002) 351.