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Nanomechanical properties through nanoindentation method of amorphous carbon and carbon nitride films synthesized by shielded arc ion plating
Surface & Coatings Technology 200 (2005) 2428 – 2432 www.elsevier.com/locate/surfcoat
Nanomechanical properties through nanoindentation method of amorphous carbon and carbon nitride films synthesized by shielded arc ion plating Kyung-Hwang Leea,*, Osamu Takaib a
Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan b EcoTopia Science Institute, Nagoya University, Nagoya 464-8603, Japan Received 14 May 2004; accepted in revised form 2 September 2004 Available online 6 October 2004
Abstract Amorphous carbon (a-C) and carbon nitride (a-CNx) films were deposited by means of shielded arc ion plating (SAIP) with an arc current of 60 A operated in a gas pressure of 1 Pa. A bias voltage in the range from 0 to 500 V was applied to a substrate during film deposition. Nanomechanical properties of the films were measured by a nanoindentation interfaced with an atomic force microscopy (AFM) using a diamond tip. The nanoindentation was also applied to evaluate wear resistant behavior of the films in nm scale. The a-C film prepared at a substrate bias voltage (V b) of 100 V was hardest in the present study so as to show a hardness of 43F3 GPa. This a-C film was most wear resistant as well. The a-CNx film prepared at V b of 300 V possessed the maximum hardness of 14F1 GPa among the prepared a-CNx films. Independently of V b, all of the a-CNx films showed better wear-resistance characteristics than sapphire and quartz. Although the wearresistance of the films was not directly correlated to its hardness, elastic modulus, elastic recovery, plastic deformation energy, these properties were certainly govern the wear-resistance of the film. D 2004 Elsevier B.V. All rights reserved. Keywords: a-C film; a-CNx film; Shielded arc ion plating (SAIP); Nanoindentaion; Wear-resistance
1. Introduction Diamond-like carbon (DLC) has been attracted great attention as a hard material for wear resistant applications. In addition, carbon nitride is also attractive for tribological applications, of which the crystal form of h-C3N4 has expected to show an elastic modulus hypothetically comparable to diamond. However, thin films of both materials synthesized to date contained large fractions of amorphous structure. Nevertheless, such amorphous carbon (a-C) and carbon nitride (a-CNx) films are promising for many industrial applications owing to their high hardness, wear* Corresponding author. Department of Materials Science and Engineering, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan. Tel.: +81 757 53 9130; fax: +81 757 53 4861. E-mail address: [email protected] (K.-H. Lee). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.09.002
resistance and solid lubricity [1–3]. Furthermore, these films recently attract much attention as biomedical materials, since the films have chemical and mechanical durabilites in vivo [4]. To satisfy requirements in high technological applications such as protective coatings of artificial joints like hip joints for biomedical implants, magnetic hard disk, and microelectromechanical systems, many researches have been studied using various deposition methods, for examples, chemical vapor deposition (CVD), sputtering, ion beam deposition, laser ablation and arc plasma deposition [5–10]. Although mechanical and tribological properties of a-C and a-CNx films have been frequently reported, the mechanical properties were discussed mainly from a viewpoint of their unique deposition condition and intensively studied in relation to chemical structures of the films. In particular, the properties of a-CNx are great interest since the a-CNx films showed an excellent wear-resistance, even though its hardness is not so high in comparison with a-C [11,12].
K.-H. Lee, O. Takai / Surface & Coatings Technology 200 (2005) 2428–2432
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In this work, a-C and a-CNx films are deposited by shielded arc ion plating (SAIP). A nanoindenter interfaced with an atomic force microscope (AFM) using a diamond tip was used to measure mechanical properties of the films including hardness, elastic modulus, recovery, contact stiffness and plastic and elastic deformation energies as well as to evaluate wear resistant behavior of the films in nm scale. The wear resistant behavior is discussed in relation to other mechanical properties of the films.
2. Experimental The a-C and a-CNx films were deposited on n-type silicon (100) substrates by the SAIP system (Nissin electric) using a high purity sintered graphite target (Toyo Tanso IG510, ash 10 ppm) as a carbon source. The substrate was located 210 mm from the target. A shielding plate was inserted between the target and substrate. A detailed description about the apparatus was reported elsewhere [13]. In this study, in order to increase deposition rate, the distance between the substrate and shield plate was set to be 50 mm which was 10 mm longer than before. The silicon substrates were ultrasonically cleaned in acetone and ethanol for 20 min in that order. The chamber was evacuated down to a pressure of 2.3103 Pa prior to introducing a reaction gas, that is, argon or nitrogen gas with a purity of 99.999% for preparing a-C or a-CNx films, respectively. Each gas introduced into the vacuum chamber through a mass flow controller. To remove contamination and oxide layers on the substrate surface, ion bombardment cleaning was carried out for 10 min in argon or nitrogen plasma at a gas pressure of 10 Pa with a substrate bias (V b) of 700 V. The films were prepared using Ar or N2 arc plasma with a DC arc current of 60 A by applying a V b in the range from 0 to 500 V. The gas pressure during deposition was fixed at 1 Pa. Thickness of the films were adjusted 120F10 nm by controlling the deposition time. Nanohardness and wear-resistance of the films were measured using a nanoindentation (Hysitron, TriboScope) interfaced with an atomic force microscope (AFM, JEOL, JSPM-4210). An identical diamond tip (Berkovich type: 65.38 of half angle) was used throughout in this study. A forcedisplacement curve of each film was measured with a peak load force ranging from 250 to 2000 AN. Loading and unload times were set both to be 5 s. A typical loaddisplacement curve is shown in Fig. 1. A displacement, that is, the difference between the loading and unloading curves, was obtained. The hardness was calculated from the unloading curve using the relation with contact area and maximum load. Elastic and plastic deformations occurred on the surface of sample as the indenter pressed into the sample. The areas of OPh f and h fPh c correspond to the plastic and elastic deformation energies, respectively. A wear depth of each sample, to which the diamond
Fig. 1. Typical load-displacement curve with applying force of 1000 AN on the quartz surface.
pyramidal tip scanning at a constant load force of 20 AN had been repeated 20 cycles in 1 Am2 area at a scanning rate of 2.8 Am/s, was used as an indicator of wearresistance. In particular, the wear resistant behavior of a-C and a-CNx films deposited at a substrate bias voltage of 100 V was further measured by changing the scanning rates (1.8, 2.8, 5.6 Am/s) and loads (20, 30, 40 AN).
3. Results and discussion Elastic modulus and hardness of the a-C and a-CNx films are shown in Fig. 2 as well as those of sapphire, fused quartz, and single crystal silicon which are elastically homogenous with the indent depth and crystal orientation. The obtained elastic moduli of these reference samples were 350F25, 69.5F1 and 155F5 GPa and their hardness were 35F5, 9.0F0.5 and 10.2F0.5 GPa, respectively. Results of variations in elastic modulus and hardness in accordance with increasing loads might be affected by indenter tip rounding and a deformation of indenter during indentation or tip blunting. The effects of indenter tip rounding and deformation of indenter are well described by Gong et al. [14]. In addition, Lemoine et al. [15] fairly took into account the effects of tip blunting on tetrahedral amorphous carbon and hydrogenated amorphous carbon thin films. These effects are not discussed in detail in this study. To obtain exact hardness of a film without affected with its substrate, Oliver et al. [16] suggested that each indentation depth had to remain less than 10 % of the film thickness. In contrast, Boyer [17] recommended an indentation depth less than 20 % of the film thickness to avoid the substrate effect. The maximum indentation depth (h)/film thickness (t f) of films varied from 10% to 30% at the maximum load of 500 AN according to their hardness. Presented the values of elastic modulus and hardness for the films were based on measurement of a load of 500 AN. At this time, the penetration depths were extremely small with below 60 nm that the hardness showed lowest value. At penetration depth less than 50 nm, indenter tip rounding
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Fig. 2. Variations of (a) elastic modulus and (b) hardness of the a-C and a-CNx films by means of increasing load.
and deformation of indenter during indentation are easy to influence on the hardness measurement. The elastic modulus and hardness of the a-C films apparently depend on V b. The films deposited at V b of 100 V showed the highest elastic modulus and hardness among the a-C films. The order of the elastic properties of the a-C films is consistent well with their hardness. On the other hand, the elastic modulus of the a-CNx films is ranged from 100 GPa to 150 GPa and the hardness of those varies from 10 to 15 GPa. The highest hardness of 15 GPa among the a-CNx films was obtained with the film deposited at a V b of 300 V. The results of hardness have a possibility to underestimate lower than the real hardness that would be obtained with completely plastic deformation due to the effects of tip rounding or blunting. This might explain an increase in hardness at high load, although some of hardness did not show such behaviors. Zheng et al. [18] have compared the recovery (R) properties of a-CNx films. The R is defined as the following equation, Rð%Þ ¼
hmax hf 100 hmax
where h max and h f are the maximum displacement at a maximum load and the residual displacement as indicated in Fig. 1, respectively. The recoveries of the sapphire, fused quartz, a-C and a-CNx films are summarized in Table 1. Although there are difficulties in comparing different types of films based on R, among the a-C films or the a-CNx films, R can be reasonably applied in order to compare mechanical properties of the films. The R values of the a-C films varied from 68.4% to 90.3%, while those of the a-CNx films varied from 69.1% to 88.4%. The a-C film with the highest R was deposited at a V b of 100 V. Among the a-CNx films, R of the film deposited under a V b of 500 V
was highest. The R values of all the a-C and a-CNx films are higher than fused quartz. The variation order of R is similar to that of their elastic modulus. The results of R of the a-C and a-CNx films are well consistent with the wear resistant behavior of each. The ratio of hardness-to-elastic modulus (H/E r) indicates the resistance of a film to plastic deformation. It means that a film with higher ratio is more resistant to plastic deformation. The H/E r of the a-C and a-CNx films is shown in Fig. 3(a). The H/E r of the a-C films are smaller than sapphire with the exception of the film deposited at 100 V. On the contrary, those of a-CNx films are almost equal or smaller. Fig. 3(b) shows the plastic deformation energy of the sapphire, fused quartz, aC and a-CNx films. Higher plastic deformation energy of a sample indicates that the sample surface is more readily deformed. The plastic deformation energies of all the a-C films are smaller than fused quartz. The results of H/E r are not quantitatively coincided with the results of plastic deformation energy. However, these results certainly
Table 1 The value of recovery (R) of the sapphire, fused quartz, a-C and a-CNx films Material
Substrate bias voltage [V]
Recovery [%]
a-C film
0 100 300 500 0 100 300 500
71.7 90.3 71.4 68.4 69.1 74.6 81.8 88.4 84 63.1
a-CNx film
Sapphire Fused quartz
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Fig. 3. (a) Hardness-to-elastic modulus ratio and (b) plastic deformation energy of the a-C and a-CNx films.
indicate substantial properties of materials and help us to understand the mechanical properties and wear resistant behavior of materials. In general, the contact stiffness is a possible indicator to understand the resistance of deformation. For example, Li and Bhushan [19] have reported that uniform and graded materials could be distinguished from the contact stiffness versus contact depth data acquired by the use of a continuous stiffness measurement technique. We have also acquired the data of contact stiffness versus contact depth as shown in Fig. 4(a). The slope of contact stiffness-to-
contact depth (DS) indicates the deformation resistance. The values of DS are shown in the legend. These DS values are strongly related to the wear-resistance of the films. The relative wear-depths of the a-C and a-CNx films to sapphire are shown in the Fig. 4(b). The a-C film prepared at 100 V shows the best wear-resistance while the value of DS is highest among the a-C films. Regardless of the substrate bias voltages, all of the a-CNx films showed better wear-resistance properties than sapphire and quartz. The differences of wear depths between the a-CNx films are quite small. The DS values also show a little
Fig. 4. (a) Contact stiffness versus contact depth and (b) relative wear depths of a-C and a-CNx films to sapphire.
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Table 2 Wear depths of the sapphire, a-C and a-CNx films (unit: nm) Applied load (AN)
Scanning rate (Am /s) Sapphire
20 30 40
a-C film
a-CNx film
1.8
2.8
5.6
1.8
2.8
5.6
1.8
2.8
5.6
33.4 54.6 71.5
26.0 72.0 67.9
33.8 61.0 91.9
6.8 5.8 6.3
3.8 5.0 5.3
7.5 4.2 6.5
17.7 19.5 49.1
14.7 37.3 42.2
11.7 27.8 33.2
difference between a-CNx films. The DS values of a-CNx films (100 and 300 V) are higher than the a-C film (100 V). Such high DS values are the origin of the high wear-resistance of the a-CNx films compareable to the a-C film (100 V) in spite of the fact that the hardness of the a-CNx films is not so high. To investigate the wear resistant behavior of the a-C and a-CNx films in more details, wear depths of the films deposited at 100 V were measured at various tip scanning rates ranging from 1.8 to 5.6 Am/s and various loads ranging from 20 to 40 AN. Sapphire was used as a control sample. The results are summarized in the Table 2. The variation of wear depth for the a-C film did not differ so much with the load and scan rate. However, the wear depth of the a-CNx film increased with increasing the load while it decreased with an increase in the tip scanning rate. This particular difference in the wear behavior between the a-C and a-CNx films might be caused by the difference in viscoelastic properties of the films.
wear-resistance of the film and could be used to understand it.
Acknowledgements This work was supported by a Grant-in-aid for Scientific Researches of the Ministry of Education, Science, Sports and Culture of Japan. References [1] [2] [3] [4] [5] [6] [7] [8]
4. Conclusions The a-C and a-CNx films have been synthesized by shielded arc ion plating. Mechanical properties of the films including hardness, elastic modulus, recovery, contact stiffness and plastic deformation energy were measured by the nanoindentation and discussed in comparison with wear-resistance evaluated by the nanoindentation as well. The hardest and best wear resistant film was prepared in the argon plasma while V b of 100 V was applied. Although the wear-resistance of each film was not directly correlated to its hardness, elastic modulus, recovery, plastic deformation energy and the slope of contact stiffness-tocontact depth, these properties were certainly govern the
[9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
[19]
Hsiao Tsai, D.B. Bogy, J. Vac. Sci. Technol., A 5 (6) (1987) 3287. G. Wang, Prog. Mater. Sci. 41 (1997) 241. B. Wei, B. Zhang, K.E. Johnson, J. Appl. Phys. 83 (1998) 2491. F.Z. Cui, D.J. Li, Surf. Coat. Technol. 131 (2000) 481. Stephen Muhl, Juan Manuel Me´ndez, Diamond Relat. Mater. l8 (1999) 1809. K. Komvopoulos, Wear 200 (1996) 305. Wu Dawei, Fu Dejun, Guo Huaixi, Zhang Zhihong, Meng Xianquan, Fan Xiangjun, Phys. Rev., B 56 (1997) 4949. H. Ohta, A. Matsumuro, Y. Takahashi, Jpn. J. Appl. Phys. 41 (2002) 7455. W. Lu, K. Komvopoulos, ASME J. Tribol. 123 (2001) 641. O. Takai, Y. Taki, T. Kitagawa, Thin Solid Films 317 (1998) 380. N. Tajima, H. Saze, H. Sugimura, O. Takai, Vacuum 59 (2000) 567. N. Tajima, H. Saze, H. Sugimura, O. Takai, Jpn. J. Appl. Phys. 38 (1999) L1131. Y. Taki, T. Kitagawa, O. Takai, Thin Solid Films 304 (1997) 183. J. Gong, H. Miao, Z. Peng, Mater. Lett. 58 (2004) 1349. P. Lemoine, J.P. Quinn, P. Maguire, J.A. McLaughlin, Wear 257 (2004) 509. W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (6) (1992) 1564. J. Wang, W.Z. Li, H.D. Li, B. Shi, J.B. Luo, Thin Solid Films 366 (2000) 117. W.T. Zheng, H. Sjfstrfm, I. Ivanov, K.Z. Xing, E. Broitman, W.R. Salaneck, J.E. Greene, J.-E. Sundgren, J. Vac. Sci. Technol., A 14 (5) (1996) 2687. X. Li, B. Bhushan, Mater. Charact. 48 (2002) 11.
Nanomechanical properties through nanoindentation method of amorphous carbon and carbon nitride films synthesized by shielded arc ion plating Kyung-Hwang Leea,*, Osamu Takaib a
Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan b EcoTopia Science Institute, Nagoya University, Nagoya 464-8603, Japan Received 14 May 2004; accepted in revised form 2 September 2004 Available online 6 October 2004
Abstract Amorphous carbon (a-C) and carbon nitride (a-CNx) films were deposited by means of shielded arc ion plating (SAIP) with an arc current of 60 A operated in a gas pressure of 1 Pa. A bias voltage in the range from 0 to 500 V was applied to a substrate during film deposition. Nanomechanical properties of the films were measured by a nanoindentation interfaced with an atomic force microscopy (AFM) using a diamond tip. The nanoindentation was also applied to evaluate wear resistant behavior of the films in nm scale. The a-C film prepared at a substrate bias voltage (V b) of 100 V was hardest in the present study so as to show a hardness of 43F3 GPa. This a-C film was most wear resistant as well. The a-CNx film prepared at V b of 300 V possessed the maximum hardness of 14F1 GPa among the prepared a-CNx films. Independently of V b, all of the a-CNx films showed better wear-resistance characteristics than sapphire and quartz. Although the wearresistance of the films was not directly correlated to its hardness, elastic modulus, elastic recovery, plastic deformation energy, these properties were certainly govern the wear-resistance of the film. D 2004 Elsevier B.V. All rights reserved. Keywords: a-C film; a-CNx film; Shielded arc ion plating (SAIP); Nanoindentaion; Wear-resistance
1. Introduction Diamond-like carbon (DLC) has been attracted great attention as a hard material for wear resistant applications. In addition, carbon nitride is also attractive for tribological applications, of which the crystal form of h-C3N4 has expected to show an elastic modulus hypothetically comparable to diamond. However, thin films of both materials synthesized to date contained large fractions of amorphous structure. Nevertheless, such amorphous carbon (a-C) and carbon nitride (a-CNx) films are promising for many industrial applications owing to their high hardness, wear* Corresponding author. Department of Materials Science and Engineering, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan. Tel.: +81 757 53 9130; fax: +81 757 53 4861. E-mail address: [email protected] (K.-H. Lee). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.09.002
resistance and solid lubricity [1–3]. Furthermore, these films recently attract much attention as biomedical materials, since the films have chemical and mechanical durabilites in vivo [4]. To satisfy requirements in high technological applications such as protective coatings of artificial joints like hip joints for biomedical implants, magnetic hard disk, and microelectromechanical systems, many researches have been studied using various deposition methods, for examples, chemical vapor deposition (CVD), sputtering, ion beam deposition, laser ablation and arc plasma deposition [5–10]. Although mechanical and tribological properties of a-C and a-CNx films have been frequently reported, the mechanical properties were discussed mainly from a viewpoint of their unique deposition condition and intensively studied in relation to chemical structures of the films. In particular, the properties of a-CNx are great interest since the a-CNx films showed an excellent wear-resistance, even though its hardness is not so high in comparison with a-C [11,12].
K.-H. Lee, O. Takai / Surface & Coatings Technology 200 (2005) 2428–2432
2429
In this work, a-C and a-CNx films are deposited by shielded arc ion plating (SAIP). A nanoindenter interfaced with an atomic force microscope (AFM) using a diamond tip was used to measure mechanical properties of the films including hardness, elastic modulus, recovery, contact stiffness and plastic and elastic deformation energies as well as to evaluate wear resistant behavior of the films in nm scale. The wear resistant behavior is discussed in relation to other mechanical properties of the films.
2. Experimental The a-C and a-CNx films were deposited on n-type silicon (100) substrates by the SAIP system (Nissin electric) using a high purity sintered graphite target (Toyo Tanso IG510, ash 10 ppm) as a carbon source. The substrate was located 210 mm from the target. A shielding plate was inserted between the target and substrate. A detailed description about the apparatus was reported elsewhere [13]. In this study, in order to increase deposition rate, the distance between the substrate and shield plate was set to be 50 mm which was 10 mm longer than before. The silicon substrates were ultrasonically cleaned in acetone and ethanol for 20 min in that order. The chamber was evacuated down to a pressure of 2.3103 Pa prior to introducing a reaction gas, that is, argon or nitrogen gas with a purity of 99.999% for preparing a-C or a-CNx films, respectively. Each gas introduced into the vacuum chamber through a mass flow controller. To remove contamination and oxide layers on the substrate surface, ion bombardment cleaning was carried out for 10 min in argon or nitrogen plasma at a gas pressure of 10 Pa with a substrate bias (V b) of 700 V. The films were prepared using Ar or N2 arc plasma with a DC arc current of 60 A by applying a V b in the range from 0 to 500 V. The gas pressure during deposition was fixed at 1 Pa. Thickness of the films were adjusted 120F10 nm by controlling the deposition time. Nanohardness and wear-resistance of the films were measured using a nanoindentation (Hysitron, TriboScope) interfaced with an atomic force microscope (AFM, JEOL, JSPM-4210). An identical diamond tip (Berkovich type: 65.38 of half angle) was used throughout in this study. A forcedisplacement curve of each film was measured with a peak load force ranging from 250 to 2000 AN. Loading and unload times were set both to be 5 s. A typical loaddisplacement curve is shown in Fig. 1. A displacement, that is, the difference between the loading and unloading curves, was obtained. The hardness was calculated from the unloading curve using the relation with contact area and maximum load. Elastic and plastic deformations occurred on the surface of sample as the indenter pressed into the sample. The areas of OPh f and h fPh c correspond to the plastic and elastic deformation energies, respectively. A wear depth of each sample, to which the diamond
Fig. 1. Typical load-displacement curve with applying force of 1000 AN on the quartz surface.
pyramidal tip scanning at a constant load force of 20 AN had been repeated 20 cycles in 1 Am2 area at a scanning rate of 2.8 Am/s, was used as an indicator of wearresistance. In particular, the wear resistant behavior of a-C and a-CNx films deposited at a substrate bias voltage of 100 V was further measured by changing the scanning rates (1.8, 2.8, 5.6 Am/s) and loads (20, 30, 40 AN).
3. Results and discussion Elastic modulus and hardness of the a-C and a-CNx films are shown in Fig. 2 as well as those of sapphire, fused quartz, and single crystal silicon which are elastically homogenous with the indent depth and crystal orientation. The obtained elastic moduli of these reference samples were 350F25, 69.5F1 and 155F5 GPa and their hardness were 35F5, 9.0F0.5 and 10.2F0.5 GPa, respectively. Results of variations in elastic modulus and hardness in accordance with increasing loads might be affected by indenter tip rounding and a deformation of indenter during indentation or tip blunting. The effects of indenter tip rounding and deformation of indenter are well described by Gong et al. [14]. In addition, Lemoine et al. [15] fairly took into account the effects of tip blunting on tetrahedral amorphous carbon and hydrogenated amorphous carbon thin films. These effects are not discussed in detail in this study. To obtain exact hardness of a film without affected with its substrate, Oliver et al. [16] suggested that each indentation depth had to remain less than 10 % of the film thickness. In contrast, Boyer [17] recommended an indentation depth less than 20 % of the film thickness to avoid the substrate effect. The maximum indentation depth (h)/film thickness (t f) of films varied from 10% to 30% at the maximum load of 500 AN according to their hardness. Presented the values of elastic modulus and hardness for the films were based on measurement of a load of 500 AN. At this time, the penetration depths were extremely small with below 60 nm that the hardness showed lowest value. At penetration depth less than 50 nm, indenter tip rounding
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Fig. 2. Variations of (a) elastic modulus and (b) hardness of the a-C and a-CNx films by means of increasing load.
and deformation of indenter during indentation are easy to influence on the hardness measurement. The elastic modulus and hardness of the a-C films apparently depend on V b. The films deposited at V b of 100 V showed the highest elastic modulus and hardness among the a-C films. The order of the elastic properties of the a-C films is consistent well with their hardness. On the other hand, the elastic modulus of the a-CNx films is ranged from 100 GPa to 150 GPa and the hardness of those varies from 10 to 15 GPa. The highest hardness of 15 GPa among the a-CNx films was obtained with the film deposited at a V b of 300 V. The results of hardness have a possibility to underestimate lower than the real hardness that would be obtained with completely plastic deformation due to the effects of tip rounding or blunting. This might explain an increase in hardness at high load, although some of hardness did not show such behaviors. Zheng et al. [18] have compared the recovery (R) properties of a-CNx films. The R is defined as the following equation, Rð%Þ ¼
hmax hf 100 hmax
where h max and h f are the maximum displacement at a maximum load and the residual displacement as indicated in Fig. 1, respectively. The recoveries of the sapphire, fused quartz, a-C and a-CNx films are summarized in Table 1. Although there are difficulties in comparing different types of films based on R, among the a-C films or the a-CNx films, R can be reasonably applied in order to compare mechanical properties of the films. The R values of the a-C films varied from 68.4% to 90.3%, while those of the a-CNx films varied from 69.1% to 88.4%. The a-C film with the highest R was deposited at a V b of 100 V. Among the a-CNx films, R of the film deposited under a V b of 500 V
was highest. The R values of all the a-C and a-CNx films are higher than fused quartz. The variation order of R is similar to that of their elastic modulus. The results of R of the a-C and a-CNx films are well consistent with the wear resistant behavior of each. The ratio of hardness-to-elastic modulus (H/E r) indicates the resistance of a film to plastic deformation. It means that a film with higher ratio is more resistant to plastic deformation. The H/E r of the a-C and a-CNx films is shown in Fig. 3(a). The H/E r of the a-C films are smaller than sapphire with the exception of the film deposited at 100 V. On the contrary, those of a-CNx films are almost equal or smaller. Fig. 3(b) shows the plastic deformation energy of the sapphire, fused quartz, aC and a-CNx films. Higher plastic deformation energy of a sample indicates that the sample surface is more readily deformed. The plastic deformation energies of all the a-C films are smaller than fused quartz. The results of H/E r are not quantitatively coincided with the results of plastic deformation energy. However, these results certainly
Table 1 The value of recovery (R) of the sapphire, fused quartz, a-C and a-CNx films Material
Substrate bias voltage [V]
Recovery [%]
a-C film
0 100 300 500 0 100 300 500
71.7 90.3 71.4 68.4 69.1 74.6 81.8 88.4 84 63.1
a-CNx film
Sapphire Fused quartz
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Fig. 3. (a) Hardness-to-elastic modulus ratio and (b) plastic deformation energy of the a-C and a-CNx films.
indicate substantial properties of materials and help us to understand the mechanical properties and wear resistant behavior of materials. In general, the contact stiffness is a possible indicator to understand the resistance of deformation. For example, Li and Bhushan [19] have reported that uniform and graded materials could be distinguished from the contact stiffness versus contact depth data acquired by the use of a continuous stiffness measurement technique. We have also acquired the data of contact stiffness versus contact depth as shown in Fig. 4(a). The slope of contact stiffness-to-
contact depth (DS) indicates the deformation resistance. The values of DS are shown in the legend. These DS values are strongly related to the wear-resistance of the films. The relative wear-depths of the a-C and a-CNx films to sapphire are shown in the Fig. 4(b). The a-C film prepared at 100 V shows the best wear-resistance while the value of DS is highest among the a-C films. Regardless of the substrate bias voltages, all of the a-CNx films showed better wear-resistance properties than sapphire and quartz. The differences of wear depths between the a-CNx films are quite small. The DS values also show a little
Fig. 4. (a) Contact stiffness versus contact depth and (b) relative wear depths of a-C and a-CNx films to sapphire.
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Table 2 Wear depths of the sapphire, a-C and a-CNx films (unit: nm) Applied load (AN)
Scanning rate (Am /s) Sapphire
20 30 40
a-C film
a-CNx film
1.8
2.8
5.6
1.8
2.8
5.6
1.8
2.8
5.6
33.4 54.6 71.5
26.0 72.0 67.9
33.8 61.0 91.9
6.8 5.8 6.3
3.8 5.0 5.3
7.5 4.2 6.5
17.7 19.5 49.1
14.7 37.3 42.2
11.7 27.8 33.2
difference between a-CNx films. The DS values of a-CNx films (100 and 300 V) are higher than the a-C film (100 V). Such high DS values are the origin of the high wear-resistance of the a-CNx films compareable to the a-C film (100 V) in spite of the fact that the hardness of the a-CNx films is not so high. To investigate the wear resistant behavior of the a-C and a-CNx films in more details, wear depths of the films deposited at 100 V were measured at various tip scanning rates ranging from 1.8 to 5.6 Am/s and various loads ranging from 20 to 40 AN. Sapphire was used as a control sample. The results are summarized in the Table 2. The variation of wear depth for the a-C film did not differ so much with the load and scan rate. However, the wear depth of the a-CNx film increased with increasing the load while it decreased with an increase in the tip scanning rate. This particular difference in the wear behavior between the a-C and a-CNx films might be caused by the difference in viscoelastic properties of the films.
wear-resistance of the film and could be used to understand it.
Acknowledgements This work was supported by a Grant-in-aid for Scientific Researches of the Ministry of Education, Science, Sports and Culture of Japan. References [1] [2] [3] [4] [5] [6] [7] [8]
4. Conclusions The a-C and a-CNx films have been synthesized by shielded arc ion plating. Mechanical properties of the films including hardness, elastic modulus, recovery, contact stiffness and plastic deformation energy were measured by the nanoindentation and discussed in comparison with wear-resistance evaluated by the nanoindentation as well. The hardest and best wear resistant film was prepared in the argon plasma while V b of 100 V was applied. Although the wear-resistance of each film was not directly correlated to its hardness, elastic modulus, recovery, plastic deformation energy and the slope of contact stiffness-tocontact depth, these properties were certainly govern the
[9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
[19]
Hsiao Tsai, D.B. Bogy, J. Vac. Sci. Technol., A 5 (6) (1987) 3287. G. Wang, Prog. Mater. Sci. 41 (1997) 241. B. Wei, B. Zhang, K.E. Johnson, J. Appl. Phys. 83 (1998) 2491. F.Z. Cui, D.J. Li, Surf. Coat. Technol. 131 (2000) 481. Stephen Muhl, Juan Manuel Me´ndez, Diamond Relat. Mater. l8 (1999) 1809. K. Komvopoulos, Wear 200 (1996) 305. Wu Dawei, Fu Dejun, Guo Huaixi, Zhang Zhihong, Meng Xianquan, Fan Xiangjun, Phys. Rev., B 56 (1997) 4949. H. Ohta, A. Matsumuro, Y. Takahashi, Jpn. J. Appl. Phys. 41 (2002) 7455. W. Lu, K. Komvopoulos, ASME J. Tribol. 123 (2001) 641. O. Takai, Y. Taki, T. Kitagawa, Thin Solid Films 317 (1998) 380. N. Tajima, H. Saze, H. Sugimura, O. Takai, Vacuum 59 (2000) 567. N. Tajima, H. Saze, H. Sugimura, O. Takai, Jpn. J. Appl. Phys. 38 (1999) L1131. Y. Taki, T. Kitagawa, O. Takai, Thin Solid Films 304 (1997) 183. J. Gong, H. Miao, Z. Peng, Mater. Lett. 58 (2004) 1349. P. Lemoine, J.P. Quinn, P. Maguire, J.A. McLaughlin, Wear 257 (2004) 509. W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (6) (1992) 1564. J. Wang, W.Z. Li, H.D. Li, B. Shi, J.B. Luo, Thin Solid Films 366 (2000) 117. W.T. Zheng, H. Sjfstrfm, I. Ivanov, K.Z. Xing, E. Broitman, W.R. Salaneck, J.E. Greene, J.-E. Sundgren, J. Vac. Sci. Technol., A 14 (5) (1996) 2687. X. Li, B. Bhushan, Mater. Charact. 48 (2002) 11.