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Characteristics of diamond-like carbon film coated on pure aluminum
Diamond & Related Materials 14 (2005) 1047 – 1050 www.elsevier.com/locate/diamond
Characteristics of diamond-like carbon film coated on pure aluminum L. LiuT, A. Yamamoto, Y. Oka, M. Yatsuzuka, H. Tsubakino Graduate School of Engineering, Material Science, University of Hyogo, 2167 Shosha, Himeji Hyogo 671-2201, Japan Available online 9 March 2005
Abstract A hybrid process of pulsed plasma-based ion implantation and deposition using two kinds of hydrocarbon plasma C2H2 and C6H5CH3 was developed to produce diamond-like carbon (DLC) films, respectively. A residual stress of each film was measured as a function of negative pulsed voltage. The residual stress of the DLC film prepared by the C6H5CH3 plasma with 5 kV negative pulsed voltage reduced to 0.005 GPa. The microstructure of the DLC films prepared by the C2H2 and the C6H5CH3 plasma was investigated. In the case of C2H2 plasma deposition with 20 kV negative pulsed voltage, only an amorphous film was formed. In the process of deposition by C6H5CH3 with 10 kV negative pulsed voltage, some fine particles were found in the amorphous film, which had a geometric shape, and the diameters of these particles were from 30 nm to 50 nm. It was found that they had the crystal structure of carbon (168H), which has the lattice constant of 0.32, 0.25, 0.2 and 1.9 nm. The formation of these carbon particles in the DLC film might be due to the generation of glow discharge. D 2005 Elsevier B.V. All rights reserved. Keywords: Carbon; Plasma based ion implantation and deposition; Transmission electron microscopy; Residual stress; Microstructure
1. Introduction Diamond-like carbon film, which combines the excellent characteristics of high hardness, low friction and low wear resistance, will help to solve the reliability demand of the machine components. The diamond-like carbon was originally produced by ion beam deposition from a carbon arc source [1–4]. Plasma based ion implantation (PBII) was discovered by Dr. Conrad in 1986 [5]. This technology can provide the possibility of uniform implanting and a high deposition rate with good coverage [6]. In practical applications, the DLC film has advanced the most of the carbon-based highly efficient films, therefore, many studies are evaluating the mechanical properties of the DLC film. However, the research into the detailed microstructure of a DLC film has only been rarely reported. In the study of Yatsuzuka et al. [6], the DLC film preparation by PBII deposition, has the following four steps: the first step was a sputter cleaning process using an argon plasma, in the second step, the carbon ion implantation was performed T Corresponding author. Tel./fax: +81 792674902. E-mail address: [email protected] (L. Liu). 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.01.031
with the CH4 plasma, and in the third step, the acetylene (C2H2) plasma was used to produce a mixing layer between the coating film and the substrate, the last step was the deposition process using the toluene (C6H5CH3) plasma. In order to clarify the characteristics of the DLC film in each step, the DLC films were prepared by the C2H2 and the C6H5CH3 plasma on the pure aluminum foil, respectively, in this study. The residual stress of the DLC films was measured and the microstructure of the DLC films was observed using a high resolution transmission electron microscopy (HRTEM).
2. Experimental A schematic diagram of the hybrid PBII deposition system is shown in Fig. 1. In this system, the RF pulse for plasma generation was supplied to the substrate together with a high negative voltage pulse for ion implantation through a single electrical feed-through using a joint matching circuit for both pulses. As the substrate itself was used as the RF antenna, the uniform plasma was generated around the substrate and the largest plasma
L. Liu et al. / Diamond & Related Materials 14 (2005) 1047–1050
Fig. 1. A schematic diagram of hybrid PBII system.
density appeared near the substrate. The RF power supply had a maximum power of 1.5 kW, a pulsed duration of 50 As and a frequency of 13.56 MHz. A high negative voltage pulse train with a voltage from 0 to 20 kV, duration of 5 As and a repetition rate of 1 kHz, were applied to the substrate for 50 As after each RF pulse. The precursor gases used in the present experiment were hydrocarbon gases such as, acetylene and toluene. The preparation of the DLC films was the following two steps: the first step was a sputter cleaning process using an argon plasma with a negative pulsed voltage (10 kV). In the second step, the deposition was performed using some negative pulsed voltages (from 0 to 20 kV) with the C2H2 plasma or with the C6H5CH3 plasma. The residual stress r in each film was determined from the curvature of its substrate using Stoney’s equation as follows: r¼
Eb2 d 3ð1 vÞl 2 d
where E is Young’s modulus, m is Poissons’s ratio, b and d are the substrate thickness and the film thickness, respectively. The curvature was measured both before and after deposition using a stylus profilometer (Dektak3, ULVAC). The film thickness was measured with the stylus profilometer. The thin foil specimens for HRTEM were first buried in resin and then sliced perpendicular to the foil surface by a diamond microtome. The HRTEM observation was performed using a JEOL JEM-2010 transmission electron microscopy at an acceleration voltage of 200 kV.
3. Results and discussion 3.1. Residual stress of the DLC film
In the case of the deposition by the acetylene plasma, the compressive residual stress sharply increases from 0 to 1 kV and then decreases from 1 to 20 kV. The maximum compressive residual stress is about 1.4 GPa and it is smaller than that measured by Fallon et al. using an ion beam deposition technique from a carbon arc source [3]. On the other hand, in the case of the deposition by the toluene plasma, the maximum compressive residual stress is smaller than that of the DLC film deposited by the acetylene plasma. The compressive residual stress decreases with the increase of the negative pulsed voltage during the deposition and it becomes the minimum value about 0.005 MPa at 5 or 7 kV deposition. The minimum compressive residual stress is smaller than that measured by Bilek et al. using the same technique, PBII deposition [7], in which the minimum compressive residual stress is several 10 MPa. While the negative pulsed voltage exceeds 10 kV, the compressive residual stress increases with increase of the negative pulsed voltage. It is considered that the increase of the compressive residual stress is caused by a generation of glow discharge at a high negative pulsed voltage that is confirmed occurring frequently during the 10 kV C6H5CH3 plasma deposition, but unfrequently during the 5 kV C6H5CH3 plasma deposition, with an observation of plasma luminescence [8]. 3.2. The microstructure of the DLC films prepared by the C2H2 and the C6H5CH3 plasma We prepared the DLC film by the C2H2 plasma with 20 kV negative pulsed voltage for 90 min prepared for the TEM observation. The average hardness of the DLC film is about 14 GPa, which is measured by a nanoindentation tester (ENT1100, ELIONIX) with a 200 mg maximum load [9]. The hydrogen content is about 17 at.%. The TEM image of the DLC film is shown in Fig. 3(a). The portion, in which the base material of the pure aluminum and the DLC film are in contact, is shown in this figure. The microstructure inside the aluminum
Compressive stress (GPa)
1048
Acetylene Toluene
1
0.5
0 0
-5
-10
-15
-20
Negative pulsed voltage (kV)
The compressive residual stresses in the DLC films prepared by the acetylene and the toluene plasma are shown as a function of negative pulsed voltage in Fig. 2.
Fig. 2. Compressive residual stress of the DLC films prepared by the acetylene and the toluene plasma gases as a function of negative pulsed voltage.
L. Liu et al. / Diamond & Related Materials 14 (2005) 1047–1050
material is uniform and no precipitation can be seen. An interface layer was formed on the surface of the base material. It is considered to be the mixing layer formed by carbon ion implantation. The thickness of the DLC film is about 0.2 Am. Some cracks can be seen in the DLC film. It occurs during the process of producing the section using the microtome, not during the formation of the DLC film. The DLC film has a good adhesion to the base material. The microstructure within the DLC film is uniform and no precipitation can be seen and no fine particles were found in the DLC film. The image of the electron diffraction pattern in the DLC film is illustrated in Fig. 3(b) and it shows a haro ring, suggesting that an amorphous DLC film is formed. We also prepared the DLC film by the C6H5CH3 plasma with 5 and 10 kV negative pulsed voltage. The average hardness of the DLC film is about 16 GPa with a 200 mg maximum load and the hydrogen content is about 18 at.%. Figure 4(a)–(b) show the cross sectional microstructure of the DLC film prepared with 5 and 10 kV, respectively. These DLC films also have a good adhesion to the base material. There are some particles in the DLC film shown in Fig. 4(b). The area circled in Fig. 4(b) was further enlarged as viewed in 4(c). This HRTEM image shows lattice fringes with the spacings of 0.32, 0.25, 0.20 and 0.19 nm, which is considered to correspond to the planes (10d 4), (21d 3), (30d 6) and (40d 1) of carbon (168H). On the other hand, there are no particles within the DLC film prepared with 5 kV
(a)
1049
(b) Aluminum DLC film
0.1µm
Aluminum
DLC film
100nm
(c) 0.32nm 0.19nm
0.2nm
0.25nm
1nm
Fig. 4. (a) TEM image of cross sectional DLC film prepared by C6H5CH3 with 5 kV negative pulsed voltage for 60 min (b) TEM image of cross sectional DLC film prepared by C6H5CH3 with 10 kV negative pulsed voltage for 90 min (c) HRTEM image of the microstructure inside the round frame shown in 4(b).
(a)
Aluminum
0.1µ m
negative pulsed voltage. These results lead to the conclusion that crystal structure of carbon (168H) is formed within the film. It is considered that the formation of the carbon particles in the DLC film might be due to the glow discharge by the 10 kV negative pulsed voltage.
(b) 4. Conclusions
Fig. 3. (a) TEM image of cross sectional DLC film prepared by C2H2 plasma with 20 kV negative pulsed voltage (b) diffraction pattern of the DLC film.
In conclusion, the minimum compressive residual stress was obtained in the DLC films prepared by the C6H5CH3 with the 5 or 7 kV negative pulsed voltage. An amorphous DLC film was formed using C2H2 plasma deposition. In the process of deposition by C6H5CH3 with the 10 kV negative pulsed voltage, some fine carbon (168H) particles were found in the amorphous film. The formation of the carbon particles in the DLC film might be due to the glow discharge and this is the reason of the increase of the residual stress.
1050
L. Liu et al. / Diamond & Related Materials 14 (2005) 1047–1050
Acknowledgments This work was supported by the Cooperation of Innovative Technology and Advanced Research in Evolutional Area (CITY AREA).
References [1] S. Aisenberg, R. Chabot, J. Appl. Phys. 42 (1971) 2953. [2] E.G. Spencer, P.H. Schmidt, D.C. Joy, F.J. Sansnlone, Appl. Phys. Lett. 29 (1976) 118.
[3] P.J. Fallon, V.S. Veerasamy, C.A. Davis, J. Rovertson, G.A.J. Amaratunga, W.I. Milne, J. Koskinen, Phys. Rev., A 48 (1993) 4777. [4] C.A. Davis, Thin Solid Films 226 (1993) 30. [5] J.R. Conrad, J.R. Radtke, R.A. Dodd, F.J. Worzala, N.C. Tran, J. Appl. Phys. 62 (1987) 4591. [6] M. Yatsuzuka, Y. Oka, Y. Nishimura, Adv. Appl. Plasma Sci. 4 (2003) 247. [7] M.M. Bilek, D.R. McKenzie, R.N. Tarrant, M. Lim, D.G. McCloch, IEEE Trans. Plasma Sci. 31 (2003) 939. [8] M. Onoi, E. Fujiwara, Y. Oka, Y. Nishimura, K. Azuma, M. Yatsuzuka, Surf. Coat. Technol. 186 (1–2) (2004) 200. [9] Y. Oka, M. Kirinuki, T. Suzuki, M. Yatsuzuka, K. Yatsui, Nucl. Instru. and Meth. (2005) in press.
Characteristics of diamond-like carbon film coated on pure aluminum L. LiuT, A. Yamamoto, Y. Oka, M. Yatsuzuka, H. Tsubakino Graduate School of Engineering, Material Science, University of Hyogo, 2167 Shosha, Himeji Hyogo 671-2201, Japan Available online 9 March 2005
Abstract A hybrid process of pulsed plasma-based ion implantation and deposition using two kinds of hydrocarbon plasma C2H2 and C6H5CH3 was developed to produce diamond-like carbon (DLC) films, respectively. A residual stress of each film was measured as a function of negative pulsed voltage. The residual stress of the DLC film prepared by the C6H5CH3 plasma with 5 kV negative pulsed voltage reduced to 0.005 GPa. The microstructure of the DLC films prepared by the C2H2 and the C6H5CH3 plasma was investigated. In the case of C2H2 plasma deposition with 20 kV negative pulsed voltage, only an amorphous film was formed. In the process of deposition by C6H5CH3 with 10 kV negative pulsed voltage, some fine particles were found in the amorphous film, which had a geometric shape, and the diameters of these particles were from 30 nm to 50 nm. It was found that they had the crystal structure of carbon (168H), which has the lattice constant of 0.32, 0.25, 0.2 and 1.9 nm. The formation of these carbon particles in the DLC film might be due to the generation of glow discharge. D 2005 Elsevier B.V. All rights reserved. Keywords: Carbon; Plasma based ion implantation and deposition; Transmission electron microscopy; Residual stress; Microstructure
1. Introduction Diamond-like carbon film, which combines the excellent characteristics of high hardness, low friction and low wear resistance, will help to solve the reliability demand of the machine components. The diamond-like carbon was originally produced by ion beam deposition from a carbon arc source [1–4]. Plasma based ion implantation (PBII) was discovered by Dr. Conrad in 1986 [5]. This technology can provide the possibility of uniform implanting and a high deposition rate with good coverage [6]. In practical applications, the DLC film has advanced the most of the carbon-based highly efficient films, therefore, many studies are evaluating the mechanical properties of the DLC film. However, the research into the detailed microstructure of a DLC film has only been rarely reported. In the study of Yatsuzuka et al. [6], the DLC film preparation by PBII deposition, has the following four steps: the first step was a sputter cleaning process using an argon plasma, in the second step, the carbon ion implantation was performed T Corresponding author. Tel./fax: +81 792674902. E-mail address: [email protected] (L. Liu). 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.01.031
with the CH4 plasma, and in the third step, the acetylene (C2H2) plasma was used to produce a mixing layer between the coating film and the substrate, the last step was the deposition process using the toluene (C6H5CH3) plasma. In order to clarify the characteristics of the DLC film in each step, the DLC films were prepared by the C2H2 and the C6H5CH3 plasma on the pure aluminum foil, respectively, in this study. The residual stress of the DLC films was measured and the microstructure of the DLC films was observed using a high resolution transmission electron microscopy (HRTEM).
2. Experimental A schematic diagram of the hybrid PBII deposition system is shown in Fig. 1. In this system, the RF pulse for plasma generation was supplied to the substrate together with a high negative voltage pulse for ion implantation through a single electrical feed-through using a joint matching circuit for both pulses. As the substrate itself was used as the RF antenna, the uniform plasma was generated around the substrate and the largest plasma
L. Liu et al. / Diamond & Related Materials 14 (2005) 1047–1050
Fig. 1. A schematic diagram of hybrid PBII system.
density appeared near the substrate. The RF power supply had a maximum power of 1.5 kW, a pulsed duration of 50 As and a frequency of 13.56 MHz. A high negative voltage pulse train with a voltage from 0 to 20 kV, duration of 5 As and a repetition rate of 1 kHz, were applied to the substrate for 50 As after each RF pulse. The precursor gases used in the present experiment were hydrocarbon gases such as, acetylene and toluene. The preparation of the DLC films was the following two steps: the first step was a sputter cleaning process using an argon plasma with a negative pulsed voltage (10 kV). In the second step, the deposition was performed using some negative pulsed voltages (from 0 to 20 kV) with the C2H2 plasma or with the C6H5CH3 plasma. The residual stress r in each film was determined from the curvature of its substrate using Stoney’s equation as follows: r¼
Eb2 d 3ð1 vÞl 2 d
where E is Young’s modulus, m is Poissons’s ratio, b and d are the substrate thickness and the film thickness, respectively. The curvature was measured both before and after deposition using a stylus profilometer (Dektak3, ULVAC). The film thickness was measured with the stylus profilometer. The thin foil specimens for HRTEM were first buried in resin and then sliced perpendicular to the foil surface by a diamond microtome. The HRTEM observation was performed using a JEOL JEM-2010 transmission electron microscopy at an acceleration voltage of 200 kV.
3. Results and discussion 3.1. Residual stress of the DLC film
In the case of the deposition by the acetylene plasma, the compressive residual stress sharply increases from 0 to 1 kV and then decreases from 1 to 20 kV. The maximum compressive residual stress is about 1.4 GPa and it is smaller than that measured by Fallon et al. using an ion beam deposition technique from a carbon arc source [3]. On the other hand, in the case of the deposition by the toluene plasma, the maximum compressive residual stress is smaller than that of the DLC film deposited by the acetylene plasma. The compressive residual stress decreases with the increase of the negative pulsed voltage during the deposition and it becomes the minimum value about 0.005 MPa at 5 or 7 kV deposition. The minimum compressive residual stress is smaller than that measured by Bilek et al. using the same technique, PBII deposition [7], in which the minimum compressive residual stress is several 10 MPa. While the negative pulsed voltage exceeds 10 kV, the compressive residual stress increases with increase of the negative pulsed voltage. It is considered that the increase of the compressive residual stress is caused by a generation of glow discharge at a high negative pulsed voltage that is confirmed occurring frequently during the 10 kV C6H5CH3 plasma deposition, but unfrequently during the 5 kV C6H5CH3 plasma deposition, with an observation of plasma luminescence [8]. 3.2. The microstructure of the DLC films prepared by the C2H2 and the C6H5CH3 plasma We prepared the DLC film by the C2H2 plasma with 20 kV negative pulsed voltage for 90 min prepared for the TEM observation. The average hardness of the DLC film is about 14 GPa, which is measured by a nanoindentation tester (ENT1100, ELIONIX) with a 200 mg maximum load [9]. The hydrogen content is about 17 at.%. The TEM image of the DLC film is shown in Fig. 3(a). The portion, in which the base material of the pure aluminum and the DLC film are in contact, is shown in this figure. The microstructure inside the aluminum
Compressive stress (GPa)
1048
Acetylene Toluene
1
0.5
0 0
-5
-10
-15
-20
Negative pulsed voltage (kV)
The compressive residual stresses in the DLC films prepared by the acetylene and the toluene plasma are shown as a function of negative pulsed voltage in Fig. 2.
Fig. 2. Compressive residual stress of the DLC films prepared by the acetylene and the toluene plasma gases as a function of negative pulsed voltage.
L. Liu et al. / Diamond & Related Materials 14 (2005) 1047–1050
material is uniform and no precipitation can be seen. An interface layer was formed on the surface of the base material. It is considered to be the mixing layer formed by carbon ion implantation. The thickness of the DLC film is about 0.2 Am. Some cracks can be seen in the DLC film. It occurs during the process of producing the section using the microtome, not during the formation of the DLC film. The DLC film has a good adhesion to the base material. The microstructure within the DLC film is uniform and no precipitation can be seen and no fine particles were found in the DLC film. The image of the electron diffraction pattern in the DLC film is illustrated in Fig. 3(b) and it shows a haro ring, suggesting that an amorphous DLC film is formed. We also prepared the DLC film by the C6H5CH3 plasma with 5 and 10 kV negative pulsed voltage. The average hardness of the DLC film is about 16 GPa with a 200 mg maximum load and the hydrogen content is about 18 at.%. Figure 4(a)–(b) show the cross sectional microstructure of the DLC film prepared with 5 and 10 kV, respectively. These DLC films also have a good adhesion to the base material. There are some particles in the DLC film shown in Fig. 4(b). The area circled in Fig. 4(b) was further enlarged as viewed in 4(c). This HRTEM image shows lattice fringes with the spacings of 0.32, 0.25, 0.20 and 0.19 nm, which is considered to correspond to the planes (10d 4), (21d 3), (30d 6) and (40d 1) of carbon (168H). On the other hand, there are no particles within the DLC film prepared with 5 kV
(a)
1049
(b) Aluminum DLC film
0.1µm
Aluminum
DLC film
100nm
(c) 0.32nm 0.19nm
0.2nm
0.25nm
1nm
Fig. 4. (a) TEM image of cross sectional DLC film prepared by C6H5CH3 with 5 kV negative pulsed voltage for 60 min (b) TEM image of cross sectional DLC film prepared by C6H5CH3 with 10 kV negative pulsed voltage for 90 min (c) HRTEM image of the microstructure inside the round frame shown in 4(b).
(a)
Aluminum
0.1µ m
negative pulsed voltage. These results lead to the conclusion that crystal structure of carbon (168H) is formed within the film. It is considered that the formation of the carbon particles in the DLC film might be due to the glow discharge by the 10 kV negative pulsed voltage.
(b) 4. Conclusions
Fig. 3. (a) TEM image of cross sectional DLC film prepared by C2H2 plasma with 20 kV negative pulsed voltage (b) diffraction pattern of the DLC film.
In conclusion, the minimum compressive residual stress was obtained in the DLC films prepared by the C6H5CH3 with the 5 or 7 kV negative pulsed voltage. An amorphous DLC film was formed using C2H2 plasma deposition. In the process of deposition by C6H5CH3 with the 10 kV negative pulsed voltage, some fine carbon (168H) particles were found in the amorphous film. The formation of the carbon particles in the DLC film might be due to the glow discharge and this is the reason of the increase of the residual stress.
1050
L. Liu et al. / Diamond & Related Materials 14 (2005) 1047–1050
Acknowledgments This work was supported by the Cooperation of Innovative Technology and Advanced Research in Evolutional Area (CITY AREA).
References [1] S. Aisenberg, R. Chabot, J. Appl. Phys. 42 (1971) 2953. [2] E.G. Spencer, P.H. Schmidt, D.C. Joy, F.J. Sansnlone, Appl. Phys. Lett. 29 (1976) 118.
[3] P.J. Fallon, V.S. Veerasamy, C.A. Davis, J. Rovertson, G.A.J. Amaratunga, W.I. Milne, J. Koskinen, Phys. Rev., A 48 (1993) 4777. [4] C.A. Davis, Thin Solid Films 226 (1993) 30. [5] J.R. Conrad, J.R. Radtke, R.A. Dodd, F.J. Worzala, N.C. Tran, J. Appl. Phys. 62 (1987) 4591. [6] M. Yatsuzuka, Y. Oka, Y. Nishimura, Adv. Appl. Plasma Sci. 4 (2003) 247. [7] M.M. Bilek, D.R. McKenzie, R.N. Tarrant, M. Lim, D.G. McCloch, IEEE Trans. Plasma Sci. 31 (2003) 939. [8] M. Onoi, E. Fujiwara, Y. Oka, Y. Nishimura, K. Azuma, M. Yatsuzuka, Surf. Coat. Technol. 186 (1–2) (2004) 200. [9] Y. Oka, M. Kirinuki, T. Suzuki, M. Yatsuzuka, K. Yatsui, Nucl. Instru. and Meth. (2005) in press.