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Cathodic arc plasma deposited TiAlSiN thin films using an Al-15 at.% Si cathode
Thin Solid Films 518 (2010) 7483–7486
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Cathodic arc plasma deposited TiAlSiN thin films using an Al-15 at.% Si cathode Sun Kyu Kim a,⁎, Vinh Van Le b a b
School of Materials Science and Engineering, University of Ulsan, Ulsan 680-749, South Korea Institute of Engineering Physics, Hanoi University of Technology, Hanoi, Vietnam
a r t i c l e
i n f o
Available online 1 June 2010 Keywords: TiAlSiN thin films Multilayered structures Cathodic arc plasma deposition
a b s t r a c t Thin films of TiAlSiN were deposited on SKD 11 tool steel substrates using two cathodes, of Ti and Al-15 at.% Si, in a cathodic arc plasma deposition system. The influence of AlSi cathode arc current and substrate bias voltage on the mechanical and structural properties of the films was investigated. The TiAlSiN films had a multilayered structure in which nanocrystalline cubic TiN layers alternated with nanocrystalline hexagonal AlSiN layers. The hardness of the films decreased with the increase of the AlSi cathode arc current. The hardness of the films also decreased as the bias voltage was raised from − 50 V to − 200 V. The maximum hardness of 43 GPa was observed at the films deposited at the pressure 0.4 Pa, Ti cathode arc current 55 A, Al cathode arc current 35 A, temperature 250 °C and bias voltage of − 50 V. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Hard coatings are widely used to protect and increase the lifetime and performance of cutting and forming tools. These coatings require mainly high hardness, good wear resistance, low friction, chemical inertness and high temperature oxidation resistance. Among various materials used for this purpose, titanium nitride (TiN) has been successfully deposited on different machine tool components on an industrial scale [1,2]. However, TiN coatings are limited by oxidation at high temperature. Titanium aluminum nitride (TiAlN) coatings were developed to improve the high temperature oxidation resistance of TiN coatings [3]. Further research to improve the oxidation resistance and mechanical properties of these coatings led to the development of titanium silicon nitride (TiSiN) [4–10] and titanium aluminum–silicon nitride (TiAlSiN) coatings [11–15]. TiSiN films have been deposited by plasma enhanced chemical vapor deposition [4,5], or magnetron sputtering [6]. Subsequently, deposition of TiSiN films by a hybrid method combining cathodic arc and magnetron sputtering was reported by Martin and Bendavid [7] and Kim et al. [8]. Veprek and Jilek reported deposition of nanocrystalline TiN/amorphous Si3N4 by vacuum arc evaporation from segmented cathodes [9] and a combined CVD and PVD technique [10]. TiAlSiN films were prepared by a magnetron sputtering [11], a cathodic arc process using composite cathodes [12,13] and a hybrid method [14]. Veprek and Veprek-Heijman reported recent progress in the development and industrialization of superhard nanocomposite coatings [15]. We previously reported TiAlSiN thin films with a cathodic arc plasma using two cathodes, of Ti and Al-12 at.% Si [13]. But the mechanical
⁎ Corresponding author. Tel.: + 82 52 2592228; fax: + 82 52 259 1688. E-mail address: [email protected] (S.K. Kim). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.05.029
properties of the TiAlSiN thin films with higher Si content have not yet been explored. In this work, we used a higher Si content, Al-15 at.% Si in the alloy cathode. The influence of cathode arc current and bias voltage on the structure and mechanical properties of the films was investigated. 2. Experimental procedures TiAlSiN films were deposited on SKD 11 tool steel (1.5% C, 11.5% Cr, 0.8% Mo, 0.9% V) substrates in a cathodic arc plasma deposition system. The detailed experimental procedure was described elsewhere [13]. Ti and Al-15 at.% Si cathodes were located at opposite ends of a cylindrical chamber, 28 cm from a central rotating holder, so that the substrates alternatively faced each cathode. A straight-line path between the AlSi cathode and the substrate was blocked by a 5 cm × 10 cm shield, located 13 cm from the face of the AlSi cathode and centered on the deposition chamber axis. The distance between the substrate and the cathode was 28 cm. During all experiments, the Ti cathode arc current, the substrate temperature and the substrate rotation speed were kept constant at 55 A, 250 °C and 4 rpm, respectively. After the chamber was evacuated to 4 × 10− 4 Pa, argon (purity 99.999%) was introduced to maintain an etching pressure of 86.6 Pa. Then, the argon gas was purged and nitrogen gas was introduced into the chamber to maintain a working pressure of 0.4 Pa. The films were deposited for 1 h. The thickness of the films was about 3.5 µm. Aluminum–silicon cathode arc current and bias voltage were varied from 35 A to 55 A and −50 V to −200 V to determine the influence of these parameters on the structure and mechanical properties of the films. The phases and chemical composition of the films were determined using an X-ray diffractometer (RAD-3C, Rigaku) and an
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electron probe microanalyzer (EPMA-1400, Shimadzu). A computercontrolled nanoindentor (Nanoindentor XP, MTS) equipped with a Berkovich diamond indenter was used to measure the hardness of the films by employing the continuous stiffness measurement method. The residual stress in the films was obtained by measuring the curvature of a silicon wafer before and after film deposition using a profilometer (Dektak 150, Veeco). The nano-multilayered structure of the films was investigated using a transmission electron microscope (JEOL, JEM-3000F). 3. Results and discussion The TiAlSiN films were deposited at a nitrogen pressure of 0.4 Pa, a temperature of 250 °C and a bias voltage of −100 V. To determine the influence of the AlSi cathode arc current in the film, the AlSi cathode arc current was varied from 35 A to 55 A while the Ti cathode arc current was kept constant at 55 A. Shown in Fig. 1 is a cross-sectional HR-TEM micrograph of a film and a selected area electron diffraction (SAED) pattern. Multilayered structures of the film were observed. The AlSiN layers are lighter than the TiN layers because of the lower scattering factor of Al compared to that of Ti [16]. Crystallites were observed in both the TiN and AlSiN layers. The interfaces between the layers were well defined. For multilayered coatings to exhibit their unique characteristic, the multilayered structures should be uniformly formed throughout the whole coating. Furthermore, it was reported to obtain high hardness, the interfaces between layers should be sharp [17]. The deposited TiAlSiN thin films fulfilled these conditions. The SAED patterns of TiAlSiN film show a few Debye–Scherrer (DS) rings of TiN and AlN crystals. No spots, but only rings were observed due to the width of crystallites varying from 5 to 8 nm. The first, fourth and sixth DS rings were indexed and corresponded to the hcp type structure associated with (1010), (1120) and (2020) Bragg reflections (BF) for hexagonal AlN. The second, third, fifth, and seventh DS rings were indexed and corresponded to the fcc type structure associated with (111), (200), (220) and (222) BF for cubic TiN . No diffraction pattern was observed for Si3N4 crystal suggesting that either Si substituted for Al in the AlN nano-crystallites, or that amorphous Si–N accumulated at the AlN nano-crystallite grain boundaries. Fukumoto et al. [18] reported
Fig. 1. Cross-sectional HR-TEM micrograph of a TiAlSiN film (temperature 250 °C, pressure 0.4 Pa, bias voltage − 50 V, Ti cathode arc current 55 A, AlSi cathode arc current 35 A).
previously that multilayered (Ti,Cr,Al)N/(Al,Si)N coatings deposited by cathodic arc ion plating method also formed a cubic crystal phase in the (Ti,Cr,Al)N layers and a hexagonal structure phase in the (Al,Si) N layers. The hardness and compressive stress of the films deposited with various AlSi cathode arc currents are shown in Fig. 2. The hardness and compressive stress of the films decreased with the AlSi cathode arc current. The maximum hardness of 43 GPa was observed at the AlSi cathode arc current of 35 A. This contrasts with our earlier work in which we deposited TiAlSiN thin films using an Al-12 at.% Si cathode, where the hardness of the films increased with the AlSi cathode arc current. Hardness of the films can be correlated with many factors including bilayer period (Λ), interface effect and the residual stress in the films. The observed hardness behavior was interpreted using bilayer period, interface effect and the film stress. As can be seen in Fig. 2, the highest compressive stress value was measured in the hardest film. A model reported by Oettel and Wiedemann [19] correlated the hardness of the films with their residual compressive stress. Fig. 3 shows cross-sectional TEM images of the TiAlSiN thin films deposited with various AlSi cathode arc currents. The composition, bilayer period and thickness ratio (l2/Λ, where l2 is the thickness of AlSiN layer) of the films are given in Table 1. With the increase of the AlSi cathode arc current, the nitrogen content did not vary much since the amount of the AlSiN injected into the plasma increased while TiN decreased. The Si content in the films was less than the Si content in the cathode. This could be due to the fact that the evaporation rate of each element in the cathode spot was different. The bilayer period and the thickness ratio of the films also increased with the AlSi cathode arc current. The highest hardness, 43 GPa, was observed at a AlSi cathode arc current of 35 A corresponding to a bilayer period of 7.14 nm and a thickness ratio of 0.5, as shown in Table 1. Lewis et al. [20] reported that maximum hardness is expected when the individual components of a multilayer have equal thickness. Zhang et al. [21] observed the hardness vs. bilayer period of TiN/AlN vacuum arc deposited thin films in which the hardness decreased with the bilayer period from 2 nm to 12 nm, and the highest hardness was 42 GPa. Not only the bilayer period but also inter-diffusion and the state of the layered interface should be considered in explaining the dependence of film hardness on the process parameters. A superlattice with diffused interfaces exhibits poor mechanical properties whereas sharp interfaces, i.e. with minimal intermixing of the two materials at the interfaces, are harder [22,23]. In multilayers with sharp interfaces, the interface may prevent the slip of dislocations, thus minimizing plastic deformation and maximizing the effective
Fig. 2. Effect of AlSi cathode arc current on the hardness and stress of TiAlSiN films (temperature 250 °C, pressure 0.4 Pa, bias voltage − 50 V, Ti cathode arc current 55 A).
S.K. Kim, V. Van Le / Thin Solid Films 518 (2010) 7483–7486
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Fig. 3. Cross-sectional TEM images of TiAlSiN films deposited with various AlSi cathode arc current: (a) 35 A, (b) 43 A, (c) 50 A and (d) 55 A (temperature 250 °C, pressure 0.4 Pa, bias voltage − 50 V, Ti cathode arc current 55 A).
hardness. The films with sharp interfaces were harder than films with diffuse interfaces. At the cathode arc currents of 35–43 A (Fig. 3a and b), the interfaces were sharp, which could explain high hardness at these AlSi cathode arc currents. In the sample deposited at 50 A, the interfaces were diffuse (Fig. 3c) and even more diffuse at 55 A (Fig. 3d), which decreased the hardness. The effect of bias voltage on the hardness and the compressive stress of the TiAlSiN films is shown in Fig. 4. Both the hardness and the compressive stress decreased with the increase of the bias voltage. As can be seen in Fig. 4, the highest compressive stress value was measured with the film of highest hardness. X-ray diffractograms (Fig. 5) show the presence of crystalline TiN with a (200) orientation. With increasing bias voltage, the intensity of the peak at 2θ = 36.3° increased, showing the increasing presence of crystalline h-AlN with a (0002) orientation. Cross-sectional TEM images of the TiAlSiN films
Table 1 Composition, bilayer period (Λ) and thickness ratio (l2/Λ) of TiAlSiN thin films deposited with various AlSi cathode currents. Current (A)
35 43 50 55
Composition (at.%)
Thickness (nm)
Ti
Al
Si
N
O
Λ
l2/Λ
30.99 23.63 20.70 17.46
16.60 24.67 26.75 27.18
2.34 2.88 3.11 3.16
49.26 48.052 49.09 51.79
0.81 0.77 0.35 0.41
7.14 9.22 11.23 13.33
0.50 0.58 0.61 0.62
Fig. 4. Effect of bias voltage on the hardness of TiAlSiN films (temperature 250 °C, pressure 0.4 Pa, AlSi cathode arc current 35 A, Ti cathode arc current 55 A).
deposited with various bias voltages (Fig. 6) indicated a nanomultilayered structure. The films with sharp interfaces showed high hardness whereas the films with diffuse interfaces showed low hardness. For the film deposited with a bias voltage of − 50 V (Fig. 6a), the interfaces were sharp and the hardness was high. However, increasing the bias voltage transferred more energy to the substrate which increased inter-diffusion. Interfaces between layers became diffuse at a bias voltage of −100 V (Fig. 6b) and more diffuse at −150 V and − 200 V (Fig. 6c and d), which decreased the hardness of these films.
4. Conclusions TiAlSiN multilayered films deposited on SKD 11 tool steel substrate using two cathodes (Ti and Al-15 at.% Si) in a cathodic arc plasma deposition system had a multilayered structure in which nanocrystalline TiN layers alternated with nanocrystalline AlSiN layers. The hardness of the films decreased with the AlSi cathode arc current. The hardness correlated with the compressive stress. The highest hardness was observed at a AlSi cathode arc current of 35 A. The observed high hardness was due to the sharp interfaces between the layers and the layer thickness ratio in these films. The film hardness varied with the bias voltage. The maximum hardness of 43 GPa was observed at a bias voltage of − 50 V, which produced films with sharp interfaces between the layers.
Fig. 5. XRD diffractograms of TiAlSiN films deposited with various bias voltages (temperature 250 °C pressure 0.4 Pa, Ti cathode arc current 55 A, AlSi cathode arc current 35 A).
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Fig. 6. Cross-sectional TEM images of TiAlSiN thin films deposited with various bias voltages: (a) − 50 V, (b) − 100 V, (c) − 150 V and (d) − 200 V (temperature 250 °C, pressure 0.4 Pa, Ti cathode arc current 55 A, AlSi cathode arc current 35 A).
Acknowledgment This research was funded by the 2010 University of Ulsan Research Fund. References [1] H. Ljungcrantz, L. Huttman, J.-E. Sundgren, G. Hakansson, L. Karlsson, Surf. Coat. Technol. 63 (1994) 123. [2] J.M. Lackner, W. Waldhauser, R. Berghauser, R. Ebner, B. Major, T. Schoberl, Thin Solid Films 453–454 (2004) 195. [3] R. Rodriguez-Baracaldo, J.A. Benito, E.S. Puchi-Cabrera, M.H. Staia, Wear 262 (2007) 380. [4] S. Veprek, Surf. Coat. Technol. 97 (1997) 15. [5] S. Veprek, S. Reiprich, L. Shizi, Appl. Phys. Lett. 66 (20) (1995) 2640. [6] F. Vaz, L. Rebouta, Ph. Goudeau, T. Girardeau, J. Pacaud, J.P. Riviere, A. Traverse, Surf. Coat. Technol. 146–147 (2001) 274. [7] P.J. Martin, A. Bendavid, Surf. Coat. Technol. 173–164 (2003) 245. [8] K.H. Kim, S. Choi, S. Yoon, Surf. Coat. Technol. 298 (2002) 24.
[9] S. Veprek, M. Jilek, Pure Appl. Chem. 74 (2002) 475. [10] S. Veprek, P. Nesladek, A. Niederhofer, F. Glatz, M. Jilek, M. Sima, Surf. Coat. Technol. 108–109 (1998) 138. [11] S. Carvalho, L. Rebouta, E. Ribeiro, F. Vaz, M.F. Denannot, J. Pacard, J.P. Riviere, F. Paunier, R.J. Gaboriaud, E. Alves, Surf. Coat. Technol. 177–178 (2004) 369. [12] M. Parlinska-Wojtan, A. Karimi, T. Cselle, M. Morstein, Surf. Coat. Technol. 177–178 (2004) 376. [13] S.K. Kim, P.V. Vinh, J.H. Kim, T. Ngoc, Surf. Coat. Technol. 200 (2005) 1391. [14] I. Park, S.R. Choi, J.H. Suh, C. Park, K.H. Kim, Thin Solid Films 447–448 (2004) 443. [15] S. Veprek, Coat. Technol. 202 (2008) 5063. [16] A. Karimi, G. Allidi, R. Sanjines, Surf. Coat. Technol. 201 (2006) 4062. [17] B.M. Clements, H. Hung, S.A. Barnett, MRS Bull. 24 (1999) 20. [18] N. Fukumoto, H. Ezura, K. Yamamoto, A. Hotta, T. Suzuki, Surf. Coat. Technol. 2003 (2009) 1343. [19] H. Oettel, R. Wiedemann, Surf. Coat. Technol. 76-77 (1995) 265. [20] D.B. Lewis, I. Wadsworth, V. Valvoda, Surf. Coat. Technol. 116–119 (1999) 285. [21] X. Zhang, X. Wu, Z. Yi, T. Zhang, H. Zhang, Nuclear. Instr. Methods Phys. Res. B 206 (2003) 382. [22] H.B. Barshilia, K.S. Rajam, Bull. Mater. Sci. 26 (2) (2003) 233. [23] W.D. Sproul, Science 273 (1996) 889.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Cathodic arc plasma deposited TiAlSiN thin films using an Al-15 at.% Si cathode Sun Kyu Kim a,⁎, Vinh Van Le b a b
School of Materials Science and Engineering, University of Ulsan, Ulsan 680-749, South Korea Institute of Engineering Physics, Hanoi University of Technology, Hanoi, Vietnam
a r t i c l e
i n f o
Available online 1 June 2010 Keywords: TiAlSiN thin films Multilayered structures Cathodic arc plasma deposition
a b s t r a c t Thin films of TiAlSiN were deposited on SKD 11 tool steel substrates using two cathodes, of Ti and Al-15 at.% Si, in a cathodic arc plasma deposition system. The influence of AlSi cathode arc current and substrate bias voltage on the mechanical and structural properties of the films was investigated. The TiAlSiN films had a multilayered structure in which nanocrystalline cubic TiN layers alternated with nanocrystalline hexagonal AlSiN layers. The hardness of the films decreased with the increase of the AlSi cathode arc current. The hardness of the films also decreased as the bias voltage was raised from − 50 V to − 200 V. The maximum hardness of 43 GPa was observed at the films deposited at the pressure 0.4 Pa, Ti cathode arc current 55 A, Al cathode arc current 35 A, temperature 250 °C and bias voltage of − 50 V. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Hard coatings are widely used to protect and increase the lifetime and performance of cutting and forming tools. These coatings require mainly high hardness, good wear resistance, low friction, chemical inertness and high temperature oxidation resistance. Among various materials used for this purpose, titanium nitride (TiN) has been successfully deposited on different machine tool components on an industrial scale [1,2]. However, TiN coatings are limited by oxidation at high temperature. Titanium aluminum nitride (TiAlN) coatings were developed to improve the high temperature oxidation resistance of TiN coatings [3]. Further research to improve the oxidation resistance and mechanical properties of these coatings led to the development of titanium silicon nitride (TiSiN) [4–10] and titanium aluminum–silicon nitride (TiAlSiN) coatings [11–15]. TiSiN films have been deposited by plasma enhanced chemical vapor deposition [4,5], or magnetron sputtering [6]. Subsequently, deposition of TiSiN films by a hybrid method combining cathodic arc and magnetron sputtering was reported by Martin and Bendavid [7] and Kim et al. [8]. Veprek and Jilek reported deposition of nanocrystalline TiN/amorphous Si3N4 by vacuum arc evaporation from segmented cathodes [9] and a combined CVD and PVD technique [10]. TiAlSiN films were prepared by a magnetron sputtering [11], a cathodic arc process using composite cathodes [12,13] and a hybrid method [14]. Veprek and Veprek-Heijman reported recent progress in the development and industrialization of superhard nanocomposite coatings [15]. We previously reported TiAlSiN thin films with a cathodic arc plasma using two cathodes, of Ti and Al-12 at.% Si [13]. But the mechanical
⁎ Corresponding author. Tel.: + 82 52 2592228; fax: + 82 52 259 1688. E-mail address: [email protected] (S.K. Kim). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.05.029
properties of the TiAlSiN thin films with higher Si content have not yet been explored. In this work, we used a higher Si content, Al-15 at.% Si in the alloy cathode. The influence of cathode arc current and bias voltage on the structure and mechanical properties of the films was investigated. 2. Experimental procedures TiAlSiN films were deposited on SKD 11 tool steel (1.5% C, 11.5% Cr, 0.8% Mo, 0.9% V) substrates in a cathodic arc plasma deposition system. The detailed experimental procedure was described elsewhere [13]. Ti and Al-15 at.% Si cathodes were located at opposite ends of a cylindrical chamber, 28 cm from a central rotating holder, so that the substrates alternatively faced each cathode. A straight-line path between the AlSi cathode and the substrate was blocked by a 5 cm × 10 cm shield, located 13 cm from the face of the AlSi cathode and centered on the deposition chamber axis. The distance between the substrate and the cathode was 28 cm. During all experiments, the Ti cathode arc current, the substrate temperature and the substrate rotation speed were kept constant at 55 A, 250 °C and 4 rpm, respectively. After the chamber was evacuated to 4 × 10− 4 Pa, argon (purity 99.999%) was introduced to maintain an etching pressure of 86.6 Pa. Then, the argon gas was purged and nitrogen gas was introduced into the chamber to maintain a working pressure of 0.4 Pa. The films were deposited for 1 h. The thickness of the films was about 3.5 µm. Aluminum–silicon cathode arc current and bias voltage were varied from 35 A to 55 A and −50 V to −200 V to determine the influence of these parameters on the structure and mechanical properties of the films. The phases and chemical composition of the films were determined using an X-ray diffractometer (RAD-3C, Rigaku) and an
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electron probe microanalyzer (EPMA-1400, Shimadzu). A computercontrolled nanoindentor (Nanoindentor XP, MTS) equipped with a Berkovich diamond indenter was used to measure the hardness of the films by employing the continuous stiffness measurement method. The residual stress in the films was obtained by measuring the curvature of a silicon wafer before and after film deposition using a profilometer (Dektak 150, Veeco). The nano-multilayered structure of the films was investigated using a transmission electron microscope (JEOL, JEM-3000F). 3. Results and discussion The TiAlSiN films were deposited at a nitrogen pressure of 0.4 Pa, a temperature of 250 °C and a bias voltage of −100 V. To determine the influence of the AlSi cathode arc current in the film, the AlSi cathode arc current was varied from 35 A to 55 A while the Ti cathode arc current was kept constant at 55 A. Shown in Fig. 1 is a cross-sectional HR-TEM micrograph of a film and a selected area electron diffraction (SAED) pattern. Multilayered structures of the film were observed. The AlSiN layers are lighter than the TiN layers because of the lower scattering factor of Al compared to that of Ti [16]. Crystallites were observed in both the TiN and AlSiN layers. The interfaces between the layers were well defined. For multilayered coatings to exhibit their unique characteristic, the multilayered structures should be uniformly formed throughout the whole coating. Furthermore, it was reported to obtain high hardness, the interfaces between layers should be sharp [17]. The deposited TiAlSiN thin films fulfilled these conditions. The SAED patterns of TiAlSiN film show a few Debye–Scherrer (DS) rings of TiN and AlN crystals. No spots, but only rings were observed due to the width of crystallites varying from 5 to 8 nm. The first, fourth and sixth DS rings were indexed and corresponded to the hcp type structure associated with (1010), (1120) and (2020) Bragg reflections (BF) for hexagonal AlN. The second, third, fifth, and seventh DS rings were indexed and corresponded to the fcc type structure associated with (111), (200), (220) and (222) BF for cubic TiN . No diffraction pattern was observed for Si3N4 crystal suggesting that either Si substituted for Al in the AlN nano-crystallites, or that amorphous Si–N accumulated at the AlN nano-crystallite grain boundaries. Fukumoto et al. [18] reported
Fig. 1. Cross-sectional HR-TEM micrograph of a TiAlSiN film (temperature 250 °C, pressure 0.4 Pa, bias voltage − 50 V, Ti cathode arc current 55 A, AlSi cathode arc current 35 A).
previously that multilayered (Ti,Cr,Al)N/(Al,Si)N coatings deposited by cathodic arc ion plating method also formed a cubic crystal phase in the (Ti,Cr,Al)N layers and a hexagonal structure phase in the (Al,Si) N layers. The hardness and compressive stress of the films deposited with various AlSi cathode arc currents are shown in Fig. 2. The hardness and compressive stress of the films decreased with the AlSi cathode arc current. The maximum hardness of 43 GPa was observed at the AlSi cathode arc current of 35 A. This contrasts with our earlier work in which we deposited TiAlSiN thin films using an Al-12 at.% Si cathode, where the hardness of the films increased with the AlSi cathode arc current. Hardness of the films can be correlated with many factors including bilayer period (Λ), interface effect and the residual stress in the films. The observed hardness behavior was interpreted using bilayer period, interface effect and the film stress. As can be seen in Fig. 2, the highest compressive stress value was measured in the hardest film. A model reported by Oettel and Wiedemann [19] correlated the hardness of the films with their residual compressive stress. Fig. 3 shows cross-sectional TEM images of the TiAlSiN thin films deposited with various AlSi cathode arc currents. The composition, bilayer period and thickness ratio (l2/Λ, where l2 is the thickness of AlSiN layer) of the films are given in Table 1. With the increase of the AlSi cathode arc current, the nitrogen content did not vary much since the amount of the AlSiN injected into the plasma increased while TiN decreased. The Si content in the films was less than the Si content in the cathode. This could be due to the fact that the evaporation rate of each element in the cathode spot was different. The bilayer period and the thickness ratio of the films also increased with the AlSi cathode arc current. The highest hardness, 43 GPa, was observed at a AlSi cathode arc current of 35 A corresponding to a bilayer period of 7.14 nm and a thickness ratio of 0.5, as shown in Table 1. Lewis et al. [20] reported that maximum hardness is expected when the individual components of a multilayer have equal thickness. Zhang et al. [21] observed the hardness vs. bilayer period of TiN/AlN vacuum arc deposited thin films in which the hardness decreased with the bilayer period from 2 nm to 12 nm, and the highest hardness was 42 GPa. Not only the bilayer period but also inter-diffusion and the state of the layered interface should be considered in explaining the dependence of film hardness on the process parameters. A superlattice with diffused interfaces exhibits poor mechanical properties whereas sharp interfaces, i.e. with minimal intermixing of the two materials at the interfaces, are harder [22,23]. In multilayers with sharp interfaces, the interface may prevent the slip of dislocations, thus minimizing plastic deformation and maximizing the effective
Fig. 2. Effect of AlSi cathode arc current on the hardness and stress of TiAlSiN films (temperature 250 °C, pressure 0.4 Pa, bias voltage − 50 V, Ti cathode arc current 55 A).
S.K. Kim, V. Van Le / Thin Solid Films 518 (2010) 7483–7486
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Fig. 3. Cross-sectional TEM images of TiAlSiN films deposited with various AlSi cathode arc current: (a) 35 A, (b) 43 A, (c) 50 A and (d) 55 A (temperature 250 °C, pressure 0.4 Pa, bias voltage − 50 V, Ti cathode arc current 55 A).
hardness. The films with sharp interfaces were harder than films with diffuse interfaces. At the cathode arc currents of 35–43 A (Fig. 3a and b), the interfaces were sharp, which could explain high hardness at these AlSi cathode arc currents. In the sample deposited at 50 A, the interfaces were diffuse (Fig. 3c) and even more diffuse at 55 A (Fig. 3d), which decreased the hardness. The effect of bias voltage on the hardness and the compressive stress of the TiAlSiN films is shown in Fig. 4. Both the hardness and the compressive stress decreased with the increase of the bias voltage. As can be seen in Fig. 4, the highest compressive stress value was measured with the film of highest hardness. X-ray diffractograms (Fig. 5) show the presence of crystalline TiN with a (200) orientation. With increasing bias voltage, the intensity of the peak at 2θ = 36.3° increased, showing the increasing presence of crystalline h-AlN with a (0002) orientation. Cross-sectional TEM images of the TiAlSiN films
Table 1 Composition, bilayer period (Λ) and thickness ratio (l2/Λ) of TiAlSiN thin films deposited with various AlSi cathode currents. Current (A)
35 43 50 55
Composition (at.%)
Thickness (nm)
Ti
Al
Si
N
O
Λ
l2/Λ
30.99 23.63 20.70 17.46
16.60 24.67 26.75 27.18
2.34 2.88 3.11 3.16
49.26 48.052 49.09 51.79
0.81 0.77 0.35 0.41
7.14 9.22 11.23 13.33
0.50 0.58 0.61 0.62
Fig. 4. Effect of bias voltage on the hardness of TiAlSiN films (temperature 250 °C, pressure 0.4 Pa, AlSi cathode arc current 35 A, Ti cathode arc current 55 A).
deposited with various bias voltages (Fig. 6) indicated a nanomultilayered structure. The films with sharp interfaces showed high hardness whereas the films with diffuse interfaces showed low hardness. For the film deposited with a bias voltage of − 50 V (Fig. 6a), the interfaces were sharp and the hardness was high. However, increasing the bias voltage transferred more energy to the substrate which increased inter-diffusion. Interfaces between layers became diffuse at a bias voltage of −100 V (Fig. 6b) and more diffuse at −150 V and − 200 V (Fig. 6c and d), which decreased the hardness of these films.
4. Conclusions TiAlSiN multilayered films deposited on SKD 11 tool steel substrate using two cathodes (Ti and Al-15 at.% Si) in a cathodic arc plasma deposition system had a multilayered structure in which nanocrystalline TiN layers alternated with nanocrystalline AlSiN layers. The hardness of the films decreased with the AlSi cathode arc current. The hardness correlated with the compressive stress. The highest hardness was observed at a AlSi cathode arc current of 35 A. The observed high hardness was due to the sharp interfaces between the layers and the layer thickness ratio in these films. The film hardness varied with the bias voltage. The maximum hardness of 43 GPa was observed at a bias voltage of − 50 V, which produced films with sharp interfaces between the layers.
Fig. 5. XRD diffractograms of TiAlSiN films deposited with various bias voltages (temperature 250 °C pressure 0.4 Pa, Ti cathode arc current 55 A, AlSi cathode arc current 35 A).
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Fig. 6. Cross-sectional TEM images of TiAlSiN thin films deposited with various bias voltages: (a) − 50 V, (b) − 100 V, (c) − 150 V and (d) − 200 V (temperature 250 °C, pressure 0.4 Pa, Ti cathode arc current 55 A, AlSi cathode arc current 35 A).
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