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MoS2 solid lubricating multilayer films
Surface and Coatings Technology 183 (2004) 347–351
Tribological characteristics of WS2 yMoS2 solid lubricating multilayer films S. Watanabe*, J. Noshiro, S. Miyake Nippon Institute of Technology, 4-1 Gakuendai, Miyashiro, Saitama 345-8501, Japan Received 12 May 2003; accepted in revised form 29 September 2003
Abstract Sputter-deposited MoS2 films have been often used as dry lubricant in various industrial fields, such as space applications. Most investigations have been focused on the reduction of the friction coefficient. However, mechanical components require not only stable lubricating films for friction reduction, but also excellent wear resistance for prolonged endurance life. Therefore, in this work, anti-wear properties of WS2 yMoS2 nanometer-scale multilayers have been investigated, because a nanometer-scale multilayer film, so-called superlattice structured film, is expected to show good mechanical properties, such as high hardness and high stiffness, resulting in a low friction and long endurance lives. WS2 yMoS2 nanometer-scale multilayer films have been prepared by multi-target RF sputtering. Single-layer MoS2 and WS2 films were also prepared for comparison. Ball-on-disk wear tests and nono-indentation tests were performed on the films grown onto Si substrates. WS2 yMoS2 multilayer film showed a significantly improved tribological performance in air compared to the single-layer MoS2 or WS2 film, with an improvement in wear life of approximately seven times, and with a low friction coefficient of approximately 0.05. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Sputtering; Solid lubricating film; Multilayer; Wear resistance; Ball-on-disk; Nano-indentation
1. Introduction Friction at surfaces in mechanical operating environments greatly influences power loss and mechanical service life. In such environments, dry lubricating films that exhibit lower friction coefficients are expected to enhance surface lubrication characteristics. Dry lubricating films are often applied as an alternative lubricant in severe (e.g. high-temperature or vacuum) operating environments where lubricating oil cannot be used, and are finding greater practical uses in the fields of vacuum equipment, aviation and space applications. Typical examples of dry lubricating films are soft metals such as Au and Ag, layer structured inorganic compounds such as MoS2 and WS2 and polymers such as polytetra-fluoro-ethylene. In particular, research is being actively conducted into the development of disulfidebased dry lubricating films typified by MoS2, with low *Corresponding author. Tel.: q81-480-33-7729; fax: q81-480-337745. E-mail address: [email protected] (S. Watanabe).
frictions having been reported under a variety of sliding conditions w1,2x. However, these dry lubricating films are not currently completely sufficient in terms of wear resistance. Research is also advancing into superlattice structured materials w3–7x. Superlattice structures are obtained by layering two or more materials in a regular periodic structure at a thickness of several or several tens of atoms. This results in a rapid increase in internal energy, with elastic modulus, hardness and other mechanical properties changing to produce performance and characteristics improved compared to those of the individual single-layer materials. The purpose of this study is to fabricate a nanometerscale multilayer film by alternately depositing layers of MoS2 and WS2 dry lubricant films to generate a multilayer film having outstanding tribological characteristics (especially, outstanding friction durability) that cannot be obtained with the individual films separately. Although research on the fabrication of metalyMoS2 or nitrideyMoS2 nanometer-scale multilayer films has been conducted in the past w8–12x, research into multilayer
0257-8972/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2003.09.063
348
S. Watanabe et al. / Surface and Coatings Technology 183 (2004) 347–351
Fig. 1. Schematic illustration of the multi-target RF sputtering equipment used in this experiment.
films consisting entirely of dry lubricating sulfide films has not been performed before to the best of the authors knowledge. This paper deals with the results of fabricating WS2 yMoS2 nanometer-scale multilayer films using multi-targets RF sputtering equipment, and comparing the friction characteristics, friction durability and wear characteristics of these films with single-layer MoS2 and WS2 films. 2. Experimental Fig. 1 shows a schematic illustration of the multitarget RF sputtering equipment used in this experiment. The equipment consists of three independent target holders, and RF power is applied to both the target holders and the substrate holder. A feature of this sputtering equipment is that a target shutter having a 120-mm diameter hole is provided in addition to the main shutter. As the RF power can be switched on each of the target holders, films can be formed on two or more samples in a single process by altering the processing parameters. The substrate to target distance is approximately 50 mm. We placed MoS2 (99.9% pure) and WS2 (99.9% pure) targets (both targets: 80 mm in diameter) at two of the three target holders. System base pressure prior to deposition was approximately 1– 2=10y4 Pa. Thin films were deposited on a Si (1 0 0) wafer substrate using Ar (99.999% pure) sputtering gas. A working gas pressure of 4 Pa was fixed during sputtering. Depositions were performed at room temperature with the substrate temperature remaining at less than 373 K during deposition. From the results of a number of preliminary experiments, optimal deposition parameters were determined, and a multilayer film was then fabricated based on these results. To compare the friction characteristics of the single-layer films to the multilayer film, total film thickness was set to 500 nm. For the multilayer film, the layer period was set to 10
nm (thickness of each layer: 5 nm). Table 1 details the deposition parameters. In films prepared in preliminary experiments with 20- and 40-nm layered periods, no significant layering effects (i.e. supermodulus effects w7x) were observed in film properties such as hardness enhancement of the film. Further detailed study is thus required on the influence of layer period on film properties. To determine the friction characteristics of each of the films fabricated, a friction test was performed by a ball-on-disk type tribometer using AISI440C (SUS440C, diameter 6.0 mm) balls. Wear tests were performed in a laboratory air (30–45% RH) at room temperature, with a normal load of 1 N, a radius of the wear track of 3 mm, and a sliding speed of 31.4 mmys. Film hardness was measured by utilizing an atomic force microscope in the nano-indentation test with an indentation load of 300 mN. A pyramidal diamond tip (Berkovich-type) was used as the indenter. 3. Results and discussion The cross-sectional structure of the fabricated multilayer film was observed using a STEM system: HITACHI HD-2000 (accelerating voltage: 200 kV, resolution: 0.24 nm) to verify that the layers had formed as intended. The TEM cross-section was prepared using a FIB processing system: HITACHI FB-2000A (ion source: Gaq, accelerating voltage: 30 kV). Fig. 2 shows a TEM image of the WS2 yMoS2 multilayer film, clearly demonstrating that the film has been grown in a consistent layered structure. From the scale of the structure, the thickness of each layer can be estimated to be approximately 5 nm (layer period 10 nm), as originally intended. To elucidate the crystal structure of the films, X-ray diffraction (Cu Ka: 40 kV, 450 mA) was conducted using an X-ray diffractometer: RIGAKU RINT2500, with the angle of incidence of the X-rays set to 2.58 (scanning speed 48ymin (0.58 step)). Fig. 3 shows the X-ray diffraction patterns of the single-layer films and the multilayer film. The peak near a diffraction angle of 568 corresponds to a Si peak resulting from the Si substrate. As the lattice constants of MoS2 and WS2 are Table 1 Deposition conditions
RF power for each target RF power for substrate Ar gas flow rate Pressure Deposition time Layer thickness
MoS2 or WS2 Single layer
WS2 yMoS2 Multilayer (10-nm period)
300 W (none) 3 sccm 4 Pa 300 s 500 nm
300 W (none) 3 sccm 4 Pa 2.5 sylayer 5 nmylayer
Total film thickness, 500 nm.
S. Watanabe et al. / Surface and Coatings Technology 183 (2004) 347–351
349
Fig. 2. Cross-sectional TEM image of the WS2yMoS2 multilayer film.
Fig. 3. X-ray diffraction patterns of MoS2 and WS2 single-layer films, and WS2yMoS2 multilayer film.
very similar, the diffraction peak positions of the films are also almost identical, with the peak in the vicinity of 148 identified as the (0 0 2) peak for both films, and the peak at 338 similarly identified as (1 0 0). Although the diffraction peaks in the single-layer films and multilayer film are in the same locations, indicating no major change in crystal structure in the multilayer film, the strength of the (0 0 2) peak is stronger in the multilayer film. In addition, the multilayer film exhibits a (1 0 0) diffraction peak at broader than either of the single-layer films. It is assumed this is attributed to presence of disordered phases in the films. Fig. 4 shows the measured change in friction coefficients for each of the single-layer films and the multilayer film as a function of the number of revolutions of the friction test disk. On both of the single-layer films, the friction coefficient increased suddenly before the 10 000 turn mark, indicating that the friction service life
Fig. 4. Changes in friction coefficients for each of MoS2 and WS2 single-layer films, and WS2 yMoS2 multilayer film as a function of the number of revolutions.
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S. Watanabe et al. / Surface and Coatings Technology 183 (2004) 347–351
Fig. 5. Wear rate results for each of MoS2 and WS2 single-layer films, and WS2yMoS2 multilayer film at a number of revolutions of 3000.
had been reached (a friction coefficient of 0.35 was used as the threshold). In contrast, a stable, low friction coefficient was exhibited by the multilayer film up to the approximately 65 000 revolutions. This number of turns can be converted to a sliding distance of approximately 1200 m. This shows that the wear resistance of the multilayer film is about sevenfold better than that of both single-layer films. Friction tests were repeated five times, exhibiting a high degree of repeatability with almost no changes in the above trends observed. To examine the reasons for this, specific wear rates were measured and nano-indentation tests were conducted. Fig. 5 shows wear rate results for each of the singlelayer films and the multilayer film. An average crosssection area of a wear track was obtained from stylus tracing across the wear tracks at three locations or more. This value was then multiplied by the wear track length (which was computed from the diameter of the track at its center) to determine the wear volume. The wear rate, known as the dimensional wear coefficient, is defined as the volume of material removed at an unit applied load and in an unit sliding distance expressed in cubic millimeters per Newton-meters. The results were taken after 3000 revolutions of the friction test. As can be seen from the figure, major differences in wear rate are observed, and it is clear that the wear rate of the multilayer film is very low. Fig. 6 shows loadingy unloading curves from the nano-indentation tests performed on each of the single-layer films and the multilayer film. The indenter penetration depth of the single-layer films was approximately 150 nm, at a hardness of approximately 0.5–0.6 GPa. The penetration depth in the multilayer film was much lower, at approximately 65 nm, and the hardness was approximately 3.1 GPa. Thus, the fabrication of multilayer films presented has resulted in improved film hardness. This suggests that an increase in hardness has occurred because of the increase in shear modulus accompanying the increase in dislocation line energy at the interfaces between nano-
Fig. 6. Loadingyunloading curves from the nano-indentation tests performed on each of MoS2 and WS2 single-layer films, and WS2yMoS2 multilayer film. (Indentation load: 300=10y6 N).
meter order layers, as has been reported for TiNyNbN w4x and TiNyVN w13x-based materials, and other multilayer materials w5,6x. The supermodulus effect w7x has thus been demonstrated in the presented WS2 yMoS2 multilayer film. This effect has led to improved wear resistance. 4. Conclusion We fabricated a WS2 yMoS2 nanometer-scale multilayer film (superlattice structured multilayer film) having dry lubricating characteristics using RF sputtering, and evaluated its tribological characteristics. Results indicate that when compared to the respective single-layer films, the multilayer film exhibits excellent wear resistance. This is believed to be due to increased film hardness resulting from the multilayer structure. Improved characteristics specific to a superlattice structured multilayer film were observed in the WS2 yMoS2 nanometer-scale multilayer film presented in this paper. This multilayer film shows potential for improved endurance with respect to current MoS2-based low friction films and may be suitable for use in more severe friction conditions. Acknowledgments The authors would like to thank Dr Y. Sekine, former graduate student of our Institute, who helped with the experiments. References w1x T. Spalvins, ASLE Trans. 12 (1969) 36. w2x M.R. Hilton, P.D. Fleischauer, Surf. Coat. Technol. 54–55 (1992) 435. w3x H. Holleck, V. Schier, Surf. Coat. Technol. 76y77 (1995) 328. w4x M. Shinn, L. Hultman, S.A. Barnett, J. Mater. 7 (1992) 901.
S. Watanabe et al. / Surface and Coatings Technology 183 (2004) 347–351 w5x W.D. Sproul, Surf. Coat. Technol. 86–87 (1996) 170. w6x W.D. Sproul, Science 273 (1996) 889. w7x J.E. Sundgren, J. Birch, G. Hakansson, L. Hultman, U. Helmerson, Thin Solid Films 193y194 (1990) 818. w8x M.R. Hilton, R. Bauer, S.V. Didziulis, M.T. Dugger, J.M. Keen, J. Scholhamer, Surf. Coat. Technol. 53 (1992) 13. w9x M.C. Simmonds, A. Savan, H.V. Swygenhoven, E. Pfluger, S. Mikhailov, Surf. Coat. Technol. 108–109 (1998) 340.
351
w10x G. Weise, N. Mattern, H. Hermann, Thin Solid Films 298 (1997) 98. w11x D.C. Teer, J. Hampshire, V. Fox, V. Bellido-Gonzalez, Surf. Coat. Technol. 94–95 (1997) 572. w12x R. Giemore, M.A. Baker, P.N. Gibson, et al., Surf. Coat. Technol. 108–109 (1998) 345. w13x U. Helmersson, S. Todorova, S.A. Barnett, J.E. Sundgren, L.C. Markert, J.E. Greene, J. Appl. Phys. 62 (1987) 481.
Tribological characteristics of WS2 yMoS2 solid lubricating multilayer films S. Watanabe*, J. Noshiro, S. Miyake Nippon Institute of Technology, 4-1 Gakuendai, Miyashiro, Saitama 345-8501, Japan Received 12 May 2003; accepted in revised form 29 September 2003
Abstract Sputter-deposited MoS2 films have been often used as dry lubricant in various industrial fields, such as space applications. Most investigations have been focused on the reduction of the friction coefficient. However, mechanical components require not only stable lubricating films for friction reduction, but also excellent wear resistance for prolonged endurance life. Therefore, in this work, anti-wear properties of WS2 yMoS2 nanometer-scale multilayers have been investigated, because a nanometer-scale multilayer film, so-called superlattice structured film, is expected to show good mechanical properties, such as high hardness and high stiffness, resulting in a low friction and long endurance lives. WS2 yMoS2 nanometer-scale multilayer films have been prepared by multi-target RF sputtering. Single-layer MoS2 and WS2 films were also prepared for comparison. Ball-on-disk wear tests and nono-indentation tests were performed on the films grown onto Si substrates. WS2 yMoS2 multilayer film showed a significantly improved tribological performance in air compared to the single-layer MoS2 or WS2 film, with an improvement in wear life of approximately seven times, and with a low friction coefficient of approximately 0.05. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Sputtering; Solid lubricating film; Multilayer; Wear resistance; Ball-on-disk; Nano-indentation
1. Introduction Friction at surfaces in mechanical operating environments greatly influences power loss and mechanical service life. In such environments, dry lubricating films that exhibit lower friction coefficients are expected to enhance surface lubrication characteristics. Dry lubricating films are often applied as an alternative lubricant in severe (e.g. high-temperature or vacuum) operating environments where lubricating oil cannot be used, and are finding greater practical uses in the fields of vacuum equipment, aviation and space applications. Typical examples of dry lubricating films are soft metals such as Au and Ag, layer structured inorganic compounds such as MoS2 and WS2 and polymers such as polytetra-fluoro-ethylene. In particular, research is being actively conducted into the development of disulfidebased dry lubricating films typified by MoS2, with low *Corresponding author. Tel.: q81-480-33-7729; fax: q81-480-337745. E-mail address: [email protected] (S. Watanabe).
frictions having been reported under a variety of sliding conditions w1,2x. However, these dry lubricating films are not currently completely sufficient in terms of wear resistance. Research is also advancing into superlattice structured materials w3–7x. Superlattice structures are obtained by layering two or more materials in a regular periodic structure at a thickness of several or several tens of atoms. This results in a rapid increase in internal energy, with elastic modulus, hardness and other mechanical properties changing to produce performance and characteristics improved compared to those of the individual single-layer materials. The purpose of this study is to fabricate a nanometerscale multilayer film by alternately depositing layers of MoS2 and WS2 dry lubricant films to generate a multilayer film having outstanding tribological characteristics (especially, outstanding friction durability) that cannot be obtained with the individual films separately. Although research on the fabrication of metalyMoS2 or nitrideyMoS2 nanometer-scale multilayer films has been conducted in the past w8–12x, research into multilayer
0257-8972/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2003.09.063
348
S. Watanabe et al. / Surface and Coatings Technology 183 (2004) 347–351
Fig. 1. Schematic illustration of the multi-target RF sputtering equipment used in this experiment.
films consisting entirely of dry lubricating sulfide films has not been performed before to the best of the authors knowledge. This paper deals with the results of fabricating WS2 yMoS2 nanometer-scale multilayer films using multi-targets RF sputtering equipment, and comparing the friction characteristics, friction durability and wear characteristics of these films with single-layer MoS2 and WS2 films. 2. Experimental Fig. 1 shows a schematic illustration of the multitarget RF sputtering equipment used in this experiment. The equipment consists of three independent target holders, and RF power is applied to both the target holders and the substrate holder. A feature of this sputtering equipment is that a target shutter having a 120-mm diameter hole is provided in addition to the main shutter. As the RF power can be switched on each of the target holders, films can be formed on two or more samples in a single process by altering the processing parameters. The substrate to target distance is approximately 50 mm. We placed MoS2 (99.9% pure) and WS2 (99.9% pure) targets (both targets: 80 mm in diameter) at two of the three target holders. System base pressure prior to deposition was approximately 1– 2=10y4 Pa. Thin films were deposited on a Si (1 0 0) wafer substrate using Ar (99.999% pure) sputtering gas. A working gas pressure of 4 Pa was fixed during sputtering. Depositions were performed at room temperature with the substrate temperature remaining at less than 373 K during deposition. From the results of a number of preliminary experiments, optimal deposition parameters were determined, and a multilayer film was then fabricated based on these results. To compare the friction characteristics of the single-layer films to the multilayer film, total film thickness was set to 500 nm. For the multilayer film, the layer period was set to 10
nm (thickness of each layer: 5 nm). Table 1 details the deposition parameters. In films prepared in preliminary experiments with 20- and 40-nm layered periods, no significant layering effects (i.e. supermodulus effects w7x) were observed in film properties such as hardness enhancement of the film. Further detailed study is thus required on the influence of layer period on film properties. To determine the friction characteristics of each of the films fabricated, a friction test was performed by a ball-on-disk type tribometer using AISI440C (SUS440C, diameter 6.0 mm) balls. Wear tests were performed in a laboratory air (30–45% RH) at room temperature, with a normal load of 1 N, a radius of the wear track of 3 mm, and a sliding speed of 31.4 mmys. Film hardness was measured by utilizing an atomic force microscope in the nano-indentation test with an indentation load of 300 mN. A pyramidal diamond tip (Berkovich-type) was used as the indenter. 3. Results and discussion The cross-sectional structure of the fabricated multilayer film was observed using a STEM system: HITACHI HD-2000 (accelerating voltage: 200 kV, resolution: 0.24 nm) to verify that the layers had formed as intended. The TEM cross-section was prepared using a FIB processing system: HITACHI FB-2000A (ion source: Gaq, accelerating voltage: 30 kV). Fig. 2 shows a TEM image of the WS2 yMoS2 multilayer film, clearly demonstrating that the film has been grown in a consistent layered structure. From the scale of the structure, the thickness of each layer can be estimated to be approximately 5 nm (layer period 10 nm), as originally intended. To elucidate the crystal structure of the films, X-ray diffraction (Cu Ka: 40 kV, 450 mA) was conducted using an X-ray diffractometer: RIGAKU RINT2500, with the angle of incidence of the X-rays set to 2.58 (scanning speed 48ymin (0.58 step)). Fig. 3 shows the X-ray diffraction patterns of the single-layer films and the multilayer film. The peak near a diffraction angle of 568 corresponds to a Si peak resulting from the Si substrate. As the lattice constants of MoS2 and WS2 are Table 1 Deposition conditions
RF power for each target RF power for substrate Ar gas flow rate Pressure Deposition time Layer thickness
MoS2 or WS2 Single layer
WS2 yMoS2 Multilayer (10-nm period)
300 W (none) 3 sccm 4 Pa 300 s 500 nm
300 W (none) 3 sccm 4 Pa 2.5 sylayer 5 nmylayer
Total film thickness, 500 nm.
S. Watanabe et al. / Surface and Coatings Technology 183 (2004) 347–351
349
Fig. 2. Cross-sectional TEM image of the WS2yMoS2 multilayer film.
Fig. 3. X-ray diffraction patterns of MoS2 and WS2 single-layer films, and WS2yMoS2 multilayer film.
very similar, the diffraction peak positions of the films are also almost identical, with the peak in the vicinity of 148 identified as the (0 0 2) peak for both films, and the peak at 338 similarly identified as (1 0 0). Although the diffraction peaks in the single-layer films and multilayer film are in the same locations, indicating no major change in crystal structure in the multilayer film, the strength of the (0 0 2) peak is stronger in the multilayer film. In addition, the multilayer film exhibits a (1 0 0) diffraction peak at broader than either of the single-layer films. It is assumed this is attributed to presence of disordered phases in the films. Fig. 4 shows the measured change in friction coefficients for each of the single-layer films and the multilayer film as a function of the number of revolutions of the friction test disk. On both of the single-layer films, the friction coefficient increased suddenly before the 10 000 turn mark, indicating that the friction service life
Fig. 4. Changes in friction coefficients for each of MoS2 and WS2 single-layer films, and WS2 yMoS2 multilayer film as a function of the number of revolutions.
350
S. Watanabe et al. / Surface and Coatings Technology 183 (2004) 347–351
Fig. 5. Wear rate results for each of MoS2 and WS2 single-layer films, and WS2yMoS2 multilayer film at a number of revolutions of 3000.
had been reached (a friction coefficient of 0.35 was used as the threshold). In contrast, a stable, low friction coefficient was exhibited by the multilayer film up to the approximately 65 000 revolutions. This number of turns can be converted to a sliding distance of approximately 1200 m. This shows that the wear resistance of the multilayer film is about sevenfold better than that of both single-layer films. Friction tests were repeated five times, exhibiting a high degree of repeatability with almost no changes in the above trends observed. To examine the reasons for this, specific wear rates were measured and nano-indentation tests were conducted. Fig. 5 shows wear rate results for each of the singlelayer films and the multilayer film. An average crosssection area of a wear track was obtained from stylus tracing across the wear tracks at three locations or more. This value was then multiplied by the wear track length (which was computed from the diameter of the track at its center) to determine the wear volume. The wear rate, known as the dimensional wear coefficient, is defined as the volume of material removed at an unit applied load and in an unit sliding distance expressed in cubic millimeters per Newton-meters. The results were taken after 3000 revolutions of the friction test. As can be seen from the figure, major differences in wear rate are observed, and it is clear that the wear rate of the multilayer film is very low. Fig. 6 shows loadingy unloading curves from the nano-indentation tests performed on each of the single-layer films and the multilayer film. The indenter penetration depth of the single-layer films was approximately 150 nm, at a hardness of approximately 0.5–0.6 GPa. The penetration depth in the multilayer film was much lower, at approximately 65 nm, and the hardness was approximately 3.1 GPa. Thus, the fabrication of multilayer films presented has resulted in improved film hardness. This suggests that an increase in hardness has occurred because of the increase in shear modulus accompanying the increase in dislocation line energy at the interfaces between nano-
Fig. 6. Loadingyunloading curves from the nano-indentation tests performed on each of MoS2 and WS2 single-layer films, and WS2yMoS2 multilayer film. (Indentation load: 300=10y6 N).
meter order layers, as has been reported for TiNyNbN w4x and TiNyVN w13x-based materials, and other multilayer materials w5,6x. The supermodulus effect w7x has thus been demonstrated in the presented WS2 yMoS2 multilayer film. This effect has led to improved wear resistance. 4. Conclusion We fabricated a WS2 yMoS2 nanometer-scale multilayer film (superlattice structured multilayer film) having dry lubricating characteristics using RF sputtering, and evaluated its tribological characteristics. Results indicate that when compared to the respective single-layer films, the multilayer film exhibits excellent wear resistance. This is believed to be due to increased film hardness resulting from the multilayer structure. Improved characteristics specific to a superlattice structured multilayer film were observed in the WS2 yMoS2 nanometer-scale multilayer film presented in this paper. This multilayer film shows potential for improved endurance with respect to current MoS2-based low friction films and may be suitable for use in more severe friction conditions. Acknowledgments The authors would like to thank Dr Y. Sekine, former graduate student of our Institute, who helped with the experiments. References w1x T. Spalvins, ASLE Trans. 12 (1969) 36. w2x M.R. Hilton, P.D. Fleischauer, Surf. Coat. Technol. 54–55 (1992) 435. w3x H. Holleck, V. Schier, Surf. Coat. Technol. 76y77 (1995) 328. w4x M. Shinn, L. Hultman, S.A. Barnett, J. Mater. 7 (1992) 901.
S. Watanabe et al. / Surface and Coatings Technology 183 (2004) 347–351 w5x W.D. Sproul, Surf. Coat. Technol. 86–87 (1996) 170. w6x W.D. Sproul, Science 273 (1996) 889. w7x J.E. Sundgren, J. Birch, G. Hakansson, L. Hultman, U. Helmerson, Thin Solid Films 193y194 (1990) 818. w8x M.R. Hilton, R. Bauer, S.V. Didziulis, M.T. Dugger, J.M. Keen, J. Scholhamer, Surf. Coat. Technol. 53 (1992) 13. w9x M.C. Simmonds, A. Savan, H.V. Swygenhoven, E. Pfluger, S. Mikhailov, Surf. Coat. Technol. 108–109 (1998) 340.
351
w10x G. Weise, N. Mattern, H. Hermann, Thin Solid Films 298 (1997) 98. w11x D.C. Teer, J. Hampshire, V. Fox, V. Bellido-Gonzalez, Surf. Coat. Technol. 94–95 (1997) 572. w12x R. Giemore, M.A. Baker, P.N. Gibson, et al., Surf. Coat. Technol. 108–109 (1998) 345. w13x U. Helmersson, S. Todorova, S.A. Barnett, J.E. Sundgren, L.C. Markert, J.E. Greene, J. Appl. Phys. 62 (1987) 481.