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a-C:H nanocomposite film prepared by dual plasma technique
Materials Science and Engineering A 488 (2008) 112–116
Structural and mechanical properties of nc-TiC/a-C:H nanocomposite film prepared by dual plasma technique Yaohui Wang, Xu Zhang ∗ , Xianying Wu, Qiang Li, Huixing Zhang, Xiaoji Zhang Key Laboratory of Beam Technology and Material Modification of Ministry of Education, Institution of Low Energy Nuclear Physics, Beijing Normal University, Beijing Radiation Center, 100875 Beijing, China Received 19 September 2007; received in revised form 26 October 2007; accepted 30 October 2007
Abstract Nanocomposite nc-TiC/a-C:H film, with an unusual combination of superhardness, high elastic modulus and high elastic recovery, are prepared by using the dual plasma technique. The effects of the filter coil current on the compositional, structural and mechanical properties of the ncTiC/a-C:H films have been investigated. X-ray photoelectron spectroscopy (XPS) and Raman analyses show that deposition rate, composition and nanostructure of the nc-TiC/a-C:H films could be changed by varying the filter coil current. Fortunately, by selecting the proper value for the filter coil current, 2.5 A, one could remarkably enhance mechanical properties of films such as the superhardness (66.4 GPa), the high elastic modulus (510 GPa) and the high elastic recovery (83.3%). © 2007 Elsevier B.V. All rights reserved. Keywords: Nanocomposite nc-TiC/a-C:H; Dual plasma; Mechanical property; Filter coil current
1. Introduction Last decade has witnessed considerable attention for the nanocomposite films, which are composed of nanocrystallite (nc) and amorphous (a) material (referred to as nc/a), in both industrial application and fundamental research by virtue of their unique properties such as the high mechanical hardness [1], the low friction coefficient, the low wear rate and the high thermal stability [2–4]. Nanocomposite films, including Me–C:H, nc-TiC/a-C:H (discussed later in the paper) and nc-TiC/a-C [5–7], are referred to as the films which comprise grains of the order of 5–10 nm which are surrounded by the amorphous material. Moreover, the amorphous phase content and the grain size of nanocomposite are observed to be of importance for the enhancement of mechanical property. Zehnder et al. [8] found in the nc-TiC/a-C:H nanocomposite film, prepared by the reactive unbalanced magnetron sputtering, that the film with about 40 at.% Ti (corresponding to 80% TiC and 20% a-C:H) and the grain size of 5 nm possesses the hardness maxima of about 35 GPa. Although the unique mechanical property could be easily regarded as the influence of microstructure of nanocom-
∗
Corresponding author. Tel.: +86 10 62208249; fax: +86 10 62231765. E-mail address: [email protected] (X. Zhang).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.10.063
posite film, a complete understanding of the mechanism for the observed phenomenon is not available. Obviously, it is the experimental parameters that directly influence the properties of the nanocomposite film. How to modify the experimental parameters to obtain the high plasma transport efficiency has become a key-point for the production of high quality films. The optimal parameters for deposition, such as arc current and duct bias, have been found and confirmed by many researchers [9,10]. Take the duct bias as an example, the film deposited by the Filter Cathodic Vaccum Arc (FCVA) technique at the duct bias of about +10 to +20 V possesses the highest plasma transport efficiency [11], which is directly connected to the properties of nanocomposite film. FCVA technique, free from macroparticle contamination and famous for its production of high quality thin films, has been already widely used not only in industry but also in scientific research [12]. Filter coil current, however, has not been given sufficient attention compared to other parameters. Only the effects of the filter coil current on the basically experimental parameters, such as the ion density, the ion saturation current, the ion energy [13], the arc current [14] and the duct bias [10] have been researched. Whereas its effects on properties of films have been seldom explored, therefore scant information could be available. In this work, nc-TiC/a-C:H films were prepared from filtered cathodic vacuum arc technique in the presence of
Y. Wang et al. / Materials Science and Engineering A 488 (2008) 112–116
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acetylene gas, and remarkable effects of the filter coil current on compositional, nanostructural and mechanical characteristics of the nc-TiC/a-C:H film were investigated.
The elemental composition was obtained from the energy disperse spectroscopy (EDS) on the EDAX9100 system and X-ray photoelectron spectroscopy (XPS) on ESCALAB250 system. The hardness and the elastic modulus of films were obtained on Nano Indenter® XP (with DCM head) produced by MTS, the USA. The Continuous Stiffness Measurement (CSM) technique, which could be used to obtain the film hardness at any film thickness, was used in the hardness measurement. Maximal indentation depths of 150 nm (5–10% of the film thickness) minimized the influence of surface roughness and substrate. The hardness and elastic modulus values of films were obtained by averaging four measurements at four randomly chosen positions. The Raman scattering spectra were acquired on the Raman spectroscopy (Jy-HR800 system) with a 7 mW semi-conductor laser at 532 nm to investigate the structural properties of ncTiC/a-C:H film on account of its sensitivity to carbon-to-carbon bonding states.
2. Experimental details
3. Results
The schematic diagram of filtered cathodic vacuum arc deposition system is shown in Fig. 1. Nanocomposite nc-TiC/a-C:H films were deposited on the Si(1 0 0) substrate. The base pressure in the processing chamber was about 2 × 10−3 Pa. The cathodic arc source, a 100 mm in diameter titanium plate of 99.99% purity, was used to produce plasma in the condition of 80 A direct current and 20 V voltage. The plasma then was guided into the processing chamber by the electromagnetic field through a 90◦ bend duct, which was biased to 20 V voltage and wrapped with solenoid coils outside to make the plasma centered at the axial position of the duct. Furthermore, the unwanted neutral particles and macro-particles were removed by the 90◦ bend duct as the Lorentz force has no influence on them since they are not charged particles. The C2 H2 gas collided with the almost fully ionized Ti plasma to generate another plasma [15,16], and then these two plasma together impacted on the sample, which was placed about 15 cm away from the exit of the bend duct. Prior to deposition, the substrates were cleaned ultrasonically in acetone. For all the substrates, the deposition parameters, such as −200 V for the substrate bias voltage and 20 or 70 sccm for the C2 H2 flow rate, were the same except for different filter coil currents. Thickness of film was measured by the surface morphology device with the pattern of Talysurf 5P-120 from Rank Taylor Hobson, United Kingdom. And the thickness of nc-TiC/a-C:H nanocomposite film was within the range of 0.19–2.55 m. The compressive stress of film was measured using the radius of curvature technique.
3.1. Composition and structure of the films
Fig. 1. Schematic diagram of the filtered cathodic vacuum arc deposition system.
V =
tAs , tT
The film composition can be changed by changing the filter coil current, which is confirmed by XPS analyses. Fig. 2 demonstrates XPS C1s spectra of films prepared under different filter coil currents. Fitting the C1s spectra allows the discrimination of two components at the binding energy 281.8 and 284.4 eV, respectively, both of which are responding to TiC and a-C:H phase [6]. As the filter coil current increases from 1 to 2.5 A, both peak intensities for TiC and a-C:H increase obviously, indicating remarkable increase of TiC and a-C:H content, respectively. With the increase of the filter coil current from 2.5 to 4 A, the peak due to TiC (281.8 eV) decreases while the peak due to aC:H (284.4 eV) increases obviously, determined from the area of corresponding peaks. In order to explore the structural information of the a-C:H phase, the Raman scattering spectroscopy is wielded with regard to its high sensitivity to carbon-to-carbon bonding state. Fig. 3
(2)
where t is the thickness of film (in unit of mm), A is the area of the sample (in unit of 100 mm2 ), s is the atomic percentage of C element in the film, tT is the deposition time of film (in unit of min), V is the deposition rate in volume of C element (in unit of mm3 /min) which is used to predict the total content of C element in films.
Fig. 2. XPS C1s spectra of the nc-TiC/a-C:H films prepared under different filter coil currents at the C2 H2 flow rate of 70 sccm.
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Fig. 3. The Raman spectra of nc-TiC/a-C:H films prepared under different filter coil currents at the C2 H2 flow rate of 70 sccm.
depicts the Raman spectra of the nc-TiC/a-C:H films prepared under different filter coil currents. The spectra reveal that asymmetrical broad G peak at around 1580 cm−1 and D peak at around 1370 cm−1 . It can be observed that G peak shifts from 1580 to 1569 cm−1 as the filter coil current changes from 1 to 2.5 A, indicating the increase of sp3 fraction in the a-C:H phase [17,18]. While the filter coil current changes from 2.5 to 4 A, G peak shifts from 1569 to 1582 cm−1 , implying the formation of loose cross-linking structure and graphite-like bonds. The phenomenon mentioned above, however, can be explained by the film growing mechanism, which is consisted of the adsorption process at the growing surface and the implantation process at the subsurface [19,20]. The relationship between the deposition rate and the filter coil current for the nc-TiC/a-C:H films prepared under different C2 H2 flow rates is illustrated in Fig. 4. For films prepared at the C2 H2 flow rate of 20 sccm the deposition rate increases while increasing the filter coil current, and a further increase in the filter coil current leads to the decrease of the deposition rate. However, for films prepared at the C2 H2 flow rate of 70 sccm the
Fig. 4. The deposition rate as a function of the filter coil current for the ncTiC/a-C:H films prepared under different C2 H2 flow rates.
deposition rate increases while increasing the filter coil current, reaching the maximum of 2.1 nm/s at the filter coil current of 4 A, which is a higher deposition rate compared with that in Ref. [15]. In order to examine the mechanism of the high deposition rate, formula (2) is introduced to explain it in details and the relationship between the deposition rate of C element and the filter coil current is demonstrated in Fig. 5. Comparing Fig. 4 with Fig. 5, we are delighted to find that under the C2 H2 flow rate of 20 and 70 sccm the deposition rate curve in Fig. 4 is similar to the curve of the deposition rate in volume of C element. Therefore, we may deduce that the deposition of C element has a main effect on the deposition rate of film. The difference between the deposition rate curves for films prepared under different C2 H2 flow rates in Figs. 4 and 5 could be rationally understood if one takes note of the obvious difference between these two C2 H2 flow rates. As mentioned above, the C2 H2 gas could be dissociated by the ion energy transferred from the Ti plasma, which is dependent on the filter coil current. Low C2 H2 flow rate, such as 20 sccm, could not provide sufficient C2 H2 gas to be dissociated, and the C2 H2 gas may be completely dissociated at the filter coil current of 2.5–3.5 A as shown in Figs. 4 and 5 since the deposition rate, dependent on the deposition of the C element, reaches its maximum value at that filter coil current. High C2 H2 flow rate of 70 sccm, however, could provide sufficient C2 H2 gas to be dissociated, and the deposition rate is able to increase while increasing the filter coil current. 3.2. Mechanical property of the films Fig. 6 demonstrates the relationship between the hardness and the filter coil current. We find that films prepared under the filter coil current of 2.5 A bear maximal hardness (plotted in Fig. 7), corresponding to the Raman analyses mentioned above to some extent. Fig. 7 presents the maximal hardness of the film prepared under the filter coil current of 2.5 A and different C2 H2 flow rates. It can be observed that the hardness of the film (66.4 GPa) with the Ti content of 40.87% shown in Fig. 7b, is
Fig. 5. The deposition rate in volume of C element as a function of the filter coil current for nc-TiC/a-C:H films prepared under different C2 H2 flow rates.
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Fig. 6. Hardness as a function of the filter coil current for the nc-TiC/a-C:H films prepared under different C2 H2 flow rates.
Fig. 8. The Ti content as a function of the filter coil current for the nc-TiC/a-C:H films prepared under different C2 H2 flow rates.
higher than that (43.6 GPa) with the Ti content of 36.6% shown in Fig. 7a. Some researchers [8,21] have found that TiC crystallites are embedded in a-C:H substrate in nc-TiC/a-C:H films. Therefore, at the high Ti content (36.6 and 40.87% in our case) the hardness mechanism resembles to that of TiC due to thin
a-C:H phase around TiC crystallites, and the hardness increases while increasing the Ti content. Two different sets of C2 H2 flow rates, 20 and 70 sccm, are presented in Fig. 8, and it is not difficult to understand that there will be relatively more Ti content in the film when the C2 H2 flow rate of 20 sccm is applied. The Ti content increases while increasing the filter coil current, but a further increase in the filter coil current decreases the Ti content, which is corresponding to the result from XPS analyses. 4. Discussion
Fig. 7. Load as a function of the penetration depth for the nc-TiC/a-C:H films prepared under different C2 H2 flow rates: (a) 70 sccm; (b) 20 sccm.
The most striking phenomenon observed in our experiment is the superhardness (66.4 GPa) of the nc-TiC/a-C:H nanocomposite film fabricated at the C2 H2 flow rate of 20 sccm and the filter coil current of 2.5 A. Moreover, we can observe in Fig. 8 that the maximal Ti content is reached at the same condition for the maximal hardness (C2 H2 flow rate of 20 sccm and the filter coil current of 2.5 A). Therefore, we can ascribe the superhardness to the high Ti content in the nanocomposite film, namely the more Ti content the higher hardness of the nanocomposite film fabricated at the proper filter coil current (2.5 A) in our experiment. Moreover, the hardness enhancement mechanism may be explained as that with the proper filter coil current (2.5 A) applied to the duct, the proper content of TiC can exist in the film; in addition, enough energy could be transferred to most ions to penetrate into the subsurface, thereby changing the loose cross-linking structure of the a-C:H phase into the rigid three-dimensional network and leading to the increase of sp3 bond, namely the aC:H phase serves as not only the binder around the TiC phase but also the reinforcer to enhance the hardness of the nanocomposite film. However, under the lower filter coil current (lower than 2.5 A) sufficient content of TiC cannot form in the film; and the lower filter coil current could not provide enough energy for many ions to penetrate into the subsurface, and most ions are just stopped at the growing surface, forming the loose cross-linking structure. At the higher filter coil current (4 A), the implantation of much energetic ions could generate excess of heat in the ther-
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mal spike process, resulting in the transformation of sp3 bond to sp2 bond and the formation of loose network, and the TiC phase almost disappear in the film deduced from the XPS analyses. Another interesting property of the film is the high elastic recovery, as high as 83% for the film, which hardness is 66.40 GPa, as shown in Fig. 7b. The a-C:H film bears high elastic recovery, however, its hardness of about 20 GPa confines its application [22]. Fortunately, the nc-TiC/a-C:H nanocomposite film prepared in the experiment presents both superhardness and high elastic recovery as well. This rather favorable combination of mechanical properties of film might be explained as the absence of dislocation and growth of nanocrack [23], together with the blocking of grain boundary sliding by the formation of a strong interface [24] between a-C:H and TiC components.
ness (66.4 GPa), the high elastic modulus (510 GPa) and the high elastic recovery (88.3%). References [1] [2] [3] [4] [5] [6] [7] [8] [9]
5. Conclusions
[10] [11]
Nanocomposite nc-TiC/a-C:H films have been deposited by dual plasma technique. The effects of filter coil current on compositional and nanostructural tribological and mechanical characteristics of nc-TiC/a-C:H film were investigated.
[12]
(1) XPS and Raman analyses show that films prepared under high filter coil current (4 A) bear high content of a-C:H phase and loose crossing-link structure. While films prepared under relatively low and proper filter coil current (2.5 A) have more sp3 bond and rigid three-dimensional structure, leading to good mechanical property. (2) High deposition rate of 2.1 nm/s for nc-TiC/a-C:H nanocomposite film is obtained in the experiment, and the increase of the filter coil current is in favor of the increase of the deposition rate. The results show that the deposition of C element has a main effect on the deposition rate of film. (3) The film with high Ti content of 40.87% prepared under proper experimental parameter, such as the filter coil current of 2.5 A, bears the favorable combination of the superhard-
[15]
[13] [14]
[16] [17] [18] [19] [20] [21] [22] [23] [24]
J. Robertson, Mater. Sci. Eng. R. Rep. 37 (2002) 129. A.A. Voevodin, S.V. Prasad, J.S. Zabinski, J. Appl. Phys. 82 (1997) 855. D. Klaffke, A. Skopp, Surf. Coat. Technol. 98 (1998) 953. J. Patscheider, T. Zehnder, M. Diserens, Surf. Coat. Technol. 146–147 (2001) 201. D.Y. Wang, C.L. Chang, Thin Solid Films 392 (2001) 11. T. Zehnder, J. Patscheider, Surf. Coat. Technol. 133–134 (2000) 138. A.A. Voevodin, J.S. Zabinski, Diamond Relat. Mater. 7 (1998) 463. T. Zehnder, P. Schwaller, F. Munnik, S. Mikhailov, J. Patscheider, J. Appl. Phys. 95 (2004) 4327. T. Zhang, Y.C. Zhang, P.K. Chu, I.G. Brown, Appl. Phys. Lett. 80 (2002) 365. T. Zhang, P.K. Chu, I.G. Brown, Appl. Phys. Lett. 80 (2002) 3700. T. Zhang, B.Y. Tang, Q.C. Chen, Z.M. Zeng, P.K. Chu, M.M.M. Bilek, I.G. Brown, Rev. Sci. Instrum. 70 (1999) 3329. T. Zhang, Z.M. Zeng, X.B. Tian, B.Y. Tang, P.K. Chu, J. Vac. Sci. Technol. A 175 (1999) 3074. B.K. Tay, G.F. You, S.P. Lau, X. Shi, Surf. Coat. Technol. 133–134 (2000) 593. B.K. Tay, G.F. You, S.P. Lau, D. Sheeja, Diamond Relat. Mater. 10 (2001) 947. Ricky K.Y. Fu, Y.F. Mei, L.R. Shen, G.G. Siu, Paul K. Chu, W.Y. Cheung, S.P. Wong, Surf. Coat. Technol. 186 (2004) 112. Ricky K.Y. Fu, Y.F. Mei, M.Y. Fu, X.Y. Liu, Paul K. Chu, Diamond Relat. Mater. 14 (2005) 1489. T. Mikami, H. Nakazawa, M. Kudo, M. Mashita, Thin Solid Films 488 (2005) 87. D.R. Mckenzie, D. Muller, B.A. Pailthorpe, Phys. Rev. Lett. 67 (1991) 773. M. Ban, T. Hasegawa, Diamond Relat. Mater. 12 (2003) 47. J. Robertson, Diamond Relat. Mater. 3 (1994) 361. W. Gulbi´nski, S. Mathur, H. Shen, T. Suszko, A. Gilewicz, B. Warcholi´nski, Appl. Surf. Sci. 239 (2005) 302. S. Vepˇrek, M. Haussmann, S. Reiprich, Shizhi Li, J. Dian, Surf. Coat. Technol. 86–87 (1996) 394. S. Vepˇrek, J. Vac. Sci. Technol. A 17 (1999) 2401. A. Niederhofer, P. Nesl´adek, H.D. M¨annling, K. Moto, S. Vepˇrek, M. J´ılek, Surf. Coat. Technol. 120–121 (1999) 173.
Structural and mechanical properties of nc-TiC/a-C:H nanocomposite film prepared by dual plasma technique Yaohui Wang, Xu Zhang ∗ , Xianying Wu, Qiang Li, Huixing Zhang, Xiaoji Zhang Key Laboratory of Beam Technology and Material Modification of Ministry of Education, Institution of Low Energy Nuclear Physics, Beijing Normal University, Beijing Radiation Center, 100875 Beijing, China Received 19 September 2007; received in revised form 26 October 2007; accepted 30 October 2007
Abstract Nanocomposite nc-TiC/a-C:H film, with an unusual combination of superhardness, high elastic modulus and high elastic recovery, are prepared by using the dual plasma technique. The effects of the filter coil current on the compositional, structural and mechanical properties of the ncTiC/a-C:H films have been investigated. X-ray photoelectron spectroscopy (XPS) and Raman analyses show that deposition rate, composition and nanostructure of the nc-TiC/a-C:H films could be changed by varying the filter coil current. Fortunately, by selecting the proper value for the filter coil current, 2.5 A, one could remarkably enhance mechanical properties of films such as the superhardness (66.4 GPa), the high elastic modulus (510 GPa) and the high elastic recovery (83.3%). © 2007 Elsevier B.V. All rights reserved. Keywords: Nanocomposite nc-TiC/a-C:H; Dual plasma; Mechanical property; Filter coil current
1. Introduction Last decade has witnessed considerable attention for the nanocomposite films, which are composed of nanocrystallite (nc) and amorphous (a) material (referred to as nc/a), in both industrial application and fundamental research by virtue of their unique properties such as the high mechanical hardness [1], the low friction coefficient, the low wear rate and the high thermal stability [2–4]. Nanocomposite films, including Me–C:H, nc-TiC/a-C:H (discussed later in the paper) and nc-TiC/a-C [5–7], are referred to as the films which comprise grains of the order of 5–10 nm which are surrounded by the amorphous material. Moreover, the amorphous phase content and the grain size of nanocomposite are observed to be of importance for the enhancement of mechanical property. Zehnder et al. [8] found in the nc-TiC/a-C:H nanocomposite film, prepared by the reactive unbalanced magnetron sputtering, that the film with about 40 at.% Ti (corresponding to 80% TiC and 20% a-C:H) and the grain size of 5 nm possesses the hardness maxima of about 35 GPa. Although the unique mechanical property could be easily regarded as the influence of microstructure of nanocom-
∗
Corresponding author. Tel.: +86 10 62208249; fax: +86 10 62231765. E-mail address: [email protected] (X. Zhang).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.10.063
posite film, a complete understanding of the mechanism for the observed phenomenon is not available. Obviously, it is the experimental parameters that directly influence the properties of the nanocomposite film. How to modify the experimental parameters to obtain the high plasma transport efficiency has become a key-point for the production of high quality films. The optimal parameters for deposition, such as arc current and duct bias, have been found and confirmed by many researchers [9,10]. Take the duct bias as an example, the film deposited by the Filter Cathodic Vaccum Arc (FCVA) technique at the duct bias of about +10 to +20 V possesses the highest plasma transport efficiency [11], which is directly connected to the properties of nanocomposite film. FCVA technique, free from macroparticle contamination and famous for its production of high quality thin films, has been already widely used not only in industry but also in scientific research [12]. Filter coil current, however, has not been given sufficient attention compared to other parameters. Only the effects of the filter coil current on the basically experimental parameters, such as the ion density, the ion saturation current, the ion energy [13], the arc current [14] and the duct bias [10] have been researched. Whereas its effects on properties of films have been seldom explored, therefore scant information could be available. In this work, nc-TiC/a-C:H films were prepared from filtered cathodic vacuum arc technique in the presence of
Y. Wang et al. / Materials Science and Engineering A 488 (2008) 112–116
113
acetylene gas, and remarkable effects of the filter coil current on compositional, nanostructural and mechanical characteristics of the nc-TiC/a-C:H film were investigated.
The elemental composition was obtained from the energy disperse spectroscopy (EDS) on the EDAX9100 system and X-ray photoelectron spectroscopy (XPS) on ESCALAB250 system. The hardness and the elastic modulus of films were obtained on Nano Indenter® XP (with DCM head) produced by MTS, the USA. The Continuous Stiffness Measurement (CSM) technique, which could be used to obtain the film hardness at any film thickness, was used in the hardness measurement. Maximal indentation depths of 150 nm (5–10% of the film thickness) minimized the influence of surface roughness and substrate. The hardness and elastic modulus values of films were obtained by averaging four measurements at four randomly chosen positions. The Raman scattering spectra were acquired on the Raman spectroscopy (Jy-HR800 system) with a 7 mW semi-conductor laser at 532 nm to investigate the structural properties of ncTiC/a-C:H film on account of its sensitivity to carbon-to-carbon bonding states.
2. Experimental details
3. Results
The schematic diagram of filtered cathodic vacuum arc deposition system is shown in Fig. 1. Nanocomposite nc-TiC/a-C:H films were deposited on the Si(1 0 0) substrate. The base pressure in the processing chamber was about 2 × 10−3 Pa. The cathodic arc source, a 100 mm in diameter titanium plate of 99.99% purity, was used to produce plasma in the condition of 80 A direct current and 20 V voltage. The plasma then was guided into the processing chamber by the electromagnetic field through a 90◦ bend duct, which was biased to 20 V voltage and wrapped with solenoid coils outside to make the plasma centered at the axial position of the duct. Furthermore, the unwanted neutral particles and macro-particles were removed by the 90◦ bend duct as the Lorentz force has no influence on them since they are not charged particles. The C2 H2 gas collided with the almost fully ionized Ti plasma to generate another plasma [15,16], and then these two plasma together impacted on the sample, which was placed about 15 cm away from the exit of the bend duct. Prior to deposition, the substrates were cleaned ultrasonically in acetone. For all the substrates, the deposition parameters, such as −200 V for the substrate bias voltage and 20 or 70 sccm for the C2 H2 flow rate, were the same except for different filter coil currents. Thickness of film was measured by the surface morphology device with the pattern of Talysurf 5P-120 from Rank Taylor Hobson, United Kingdom. And the thickness of nc-TiC/a-C:H nanocomposite film was within the range of 0.19–2.55 m. The compressive stress of film was measured using the radius of curvature technique.
3.1. Composition and structure of the films
Fig. 1. Schematic diagram of the filtered cathodic vacuum arc deposition system.
V =
tAs , tT
The film composition can be changed by changing the filter coil current, which is confirmed by XPS analyses. Fig. 2 demonstrates XPS C1s spectra of films prepared under different filter coil currents. Fitting the C1s spectra allows the discrimination of two components at the binding energy 281.8 and 284.4 eV, respectively, both of which are responding to TiC and a-C:H phase [6]. As the filter coil current increases from 1 to 2.5 A, both peak intensities for TiC and a-C:H increase obviously, indicating remarkable increase of TiC and a-C:H content, respectively. With the increase of the filter coil current from 2.5 to 4 A, the peak due to TiC (281.8 eV) decreases while the peak due to aC:H (284.4 eV) increases obviously, determined from the area of corresponding peaks. In order to explore the structural information of the a-C:H phase, the Raman scattering spectroscopy is wielded with regard to its high sensitivity to carbon-to-carbon bonding state. Fig. 3
(2)
where t is the thickness of film (in unit of mm), A is the area of the sample (in unit of 100 mm2 ), s is the atomic percentage of C element in the film, tT is the deposition time of film (in unit of min), V is the deposition rate in volume of C element (in unit of mm3 /min) which is used to predict the total content of C element in films.
Fig. 2. XPS C1s spectra of the nc-TiC/a-C:H films prepared under different filter coil currents at the C2 H2 flow rate of 70 sccm.
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Fig. 3. The Raman spectra of nc-TiC/a-C:H films prepared under different filter coil currents at the C2 H2 flow rate of 70 sccm.
depicts the Raman spectra of the nc-TiC/a-C:H films prepared under different filter coil currents. The spectra reveal that asymmetrical broad G peak at around 1580 cm−1 and D peak at around 1370 cm−1 . It can be observed that G peak shifts from 1580 to 1569 cm−1 as the filter coil current changes from 1 to 2.5 A, indicating the increase of sp3 fraction in the a-C:H phase [17,18]. While the filter coil current changes from 2.5 to 4 A, G peak shifts from 1569 to 1582 cm−1 , implying the formation of loose cross-linking structure and graphite-like bonds. The phenomenon mentioned above, however, can be explained by the film growing mechanism, which is consisted of the adsorption process at the growing surface and the implantation process at the subsurface [19,20]. The relationship between the deposition rate and the filter coil current for the nc-TiC/a-C:H films prepared under different C2 H2 flow rates is illustrated in Fig. 4. For films prepared at the C2 H2 flow rate of 20 sccm the deposition rate increases while increasing the filter coil current, and a further increase in the filter coil current leads to the decrease of the deposition rate. However, for films prepared at the C2 H2 flow rate of 70 sccm the
Fig. 4. The deposition rate as a function of the filter coil current for the ncTiC/a-C:H films prepared under different C2 H2 flow rates.
deposition rate increases while increasing the filter coil current, reaching the maximum of 2.1 nm/s at the filter coil current of 4 A, which is a higher deposition rate compared with that in Ref. [15]. In order to examine the mechanism of the high deposition rate, formula (2) is introduced to explain it in details and the relationship between the deposition rate of C element and the filter coil current is demonstrated in Fig. 5. Comparing Fig. 4 with Fig. 5, we are delighted to find that under the C2 H2 flow rate of 20 and 70 sccm the deposition rate curve in Fig. 4 is similar to the curve of the deposition rate in volume of C element. Therefore, we may deduce that the deposition of C element has a main effect on the deposition rate of film. The difference between the deposition rate curves for films prepared under different C2 H2 flow rates in Figs. 4 and 5 could be rationally understood if one takes note of the obvious difference between these two C2 H2 flow rates. As mentioned above, the C2 H2 gas could be dissociated by the ion energy transferred from the Ti plasma, which is dependent on the filter coil current. Low C2 H2 flow rate, such as 20 sccm, could not provide sufficient C2 H2 gas to be dissociated, and the C2 H2 gas may be completely dissociated at the filter coil current of 2.5–3.5 A as shown in Figs. 4 and 5 since the deposition rate, dependent on the deposition of the C element, reaches its maximum value at that filter coil current. High C2 H2 flow rate of 70 sccm, however, could provide sufficient C2 H2 gas to be dissociated, and the deposition rate is able to increase while increasing the filter coil current. 3.2. Mechanical property of the films Fig. 6 demonstrates the relationship between the hardness and the filter coil current. We find that films prepared under the filter coil current of 2.5 A bear maximal hardness (plotted in Fig. 7), corresponding to the Raman analyses mentioned above to some extent. Fig. 7 presents the maximal hardness of the film prepared under the filter coil current of 2.5 A and different C2 H2 flow rates. It can be observed that the hardness of the film (66.4 GPa) with the Ti content of 40.87% shown in Fig. 7b, is
Fig. 5. The deposition rate in volume of C element as a function of the filter coil current for nc-TiC/a-C:H films prepared under different C2 H2 flow rates.
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Fig. 6. Hardness as a function of the filter coil current for the nc-TiC/a-C:H films prepared under different C2 H2 flow rates.
Fig. 8. The Ti content as a function of the filter coil current for the nc-TiC/a-C:H films prepared under different C2 H2 flow rates.
higher than that (43.6 GPa) with the Ti content of 36.6% shown in Fig. 7a. Some researchers [8,21] have found that TiC crystallites are embedded in a-C:H substrate in nc-TiC/a-C:H films. Therefore, at the high Ti content (36.6 and 40.87% in our case) the hardness mechanism resembles to that of TiC due to thin
a-C:H phase around TiC crystallites, and the hardness increases while increasing the Ti content. Two different sets of C2 H2 flow rates, 20 and 70 sccm, are presented in Fig. 8, and it is not difficult to understand that there will be relatively more Ti content in the film when the C2 H2 flow rate of 20 sccm is applied. The Ti content increases while increasing the filter coil current, but a further increase in the filter coil current decreases the Ti content, which is corresponding to the result from XPS analyses. 4. Discussion
Fig. 7. Load as a function of the penetration depth for the nc-TiC/a-C:H films prepared under different C2 H2 flow rates: (a) 70 sccm; (b) 20 sccm.
The most striking phenomenon observed in our experiment is the superhardness (66.4 GPa) of the nc-TiC/a-C:H nanocomposite film fabricated at the C2 H2 flow rate of 20 sccm and the filter coil current of 2.5 A. Moreover, we can observe in Fig. 8 that the maximal Ti content is reached at the same condition for the maximal hardness (C2 H2 flow rate of 20 sccm and the filter coil current of 2.5 A). Therefore, we can ascribe the superhardness to the high Ti content in the nanocomposite film, namely the more Ti content the higher hardness of the nanocomposite film fabricated at the proper filter coil current (2.5 A) in our experiment. Moreover, the hardness enhancement mechanism may be explained as that with the proper filter coil current (2.5 A) applied to the duct, the proper content of TiC can exist in the film; in addition, enough energy could be transferred to most ions to penetrate into the subsurface, thereby changing the loose cross-linking structure of the a-C:H phase into the rigid three-dimensional network and leading to the increase of sp3 bond, namely the aC:H phase serves as not only the binder around the TiC phase but also the reinforcer to enhance the hardness of the nanocomposite film. However, under the lower filter coil current (lower than 2.5 A) sufficient content of TiC cannot form in the film; and the lower filter coil current could not provide enough energy for many ions to penetrate into the subsurface, and most ions are just stopped at the growing surface, forming the loose cross-linking structure. At the higher filter coil current (4 A), the implantation of much energetic ions could generate excess of heat in the ther-
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mal spike process, resulting in the transformation of sp3 bond to sp2 bond and the formation of loose network, and the TiC phase almost disappear in the film deduced from the XPS analyses. Another interesting property of the film is the high elastic recovery, as high as 83% for the film, which hardness is 66.40 GPa, as shown in Fig. 7b. The a-C:H film bears high elastic recovery, however, its hardness of about 20 GPa confines its application [22]. Fortunately, the nc-TiC/a-C:H nanocomposite film prepared in the experiment presents both superhardness and high elastic recovery as well. This rather favorable combination of mechanical properties of film might be explained as the absence of dislocation and growth of nanocrack [23], together with the blocking of grain boundary sliding by the formation of a strong interface [24] between a-C:H and TiC components.
ness (66.4 GPa), the high elastic modulus (510 GPa) and the high elastic recovery (88.3%). References [1] [2] [3] [4] [5] [6] [7] [8] [9]
5. Conclusions
[10] [11]
Nanocomposite nc-TiC/a-C:H films have been deposited by dual plasma technique. The effects of filter coil current on compositional and nanostructural tribological and mechanical characteristics of nc-TiC/a-C:H film were investigated.
[12]
(1) XPS and Raman analyses show that films prepared under high filter coil current (4 A) bear high content of a-C:H phase and loose crossing-link structure. While films prepared under relatively low and proper filter coil current (2.5 A) have more sp3 bond and rigid three-dimensional structure, leading to good mechanical property. (2) High deposition rate of 2.1 nm/s for nc-TiC/a-C:H nanocomposite film is obtained in the experiment, and the increase of the filter coil current is in favor of the increase of the deposition rate. The results show that the deposition of C element has a main effect on the deposition rate of film. (3) The film with high Ti content of 40.87% prepared under proper experimental parameter, such as the filter coil current of 2.5 A, bears the favorable combination of the superhard-
[15]
[13] [14]
[16] [17] [18] [19] [20] [21] [22] [23] [24]
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