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Nitriding of high speed steel by bipolar PBII for improvement in adhesion strength of DLC films
Nuclear Instruments and Methods in Physics Research B 272 (2012) 357–360
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Nitriding of high speed steel by bipolar PBII for improvement in adhesion strength of DLC films Junho Choi a,⇑, Koji Soejima a, Takahisa Kato a, Masahiro Kawaguchi b, Wonsik Lee c a
The University of Tokyo, Tokyo, Japan Tokyo Metropolitan Industrial Technology Research Institute (TIRI), Tokyo, Japan c Korea Institute of Industrial Technology (KITECH), Incheon, Republic of Korea b
a r t i c l e
i n f o
Article history: Available online 1 February 2011 Keywords: Nitriding Bipolar PBII DLC coating Adhesion strength
a b s t r a c t In the present study, bipolar plasma based ion implantation and deposition (bipolar PBII) was used for plasma nitriding of high speed steel (SKH2), and the effects of the treatment parameters (positive pulse voltage, negative pulse voltage, treatment pressure, treatment time, and precursor gases) on the nitriding process were investigated. The hardness, roughness, and depth of nitride layer were also measured. The adhesion strength of diamond-like carbon (DLC) films coated on the nitride substrate was evaluated by carrying out Rockwell indentation and microscratch tests. Nitriding by bipolar PBII was achieved in the combining of two effects: nitrogen ion implantation by applying a high negative pulse voltage and thermal diffusion of nitrogen atoms under the application of a high positive pulse voltage. However, a very high voltage negative pulse caused surface roughening of the nitride layer. Application of a high positive pulse voltage during nitriding was found to be effective in promoting the thermal diffusion of the implanted nitrogen atoms. Effective nitriding could be achieved under the following conditions: high positive pulse voltage, low negative pulse voltage, high nitrogen gas pressure, and addition of hydrogen to the precursor gas. The adhesion strength of the DLC films on the SKH2 substrate was well improved after nitriding. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Diamond-like carbon (DLC) films have attracted considerable interest in the past three decades because of their high hardness, extremely low friction, high wear resistance, chemical inertness, and excellent gas barrier properties. The aforementioned characteristics make DLC films suitable for use in a wide range of applications such as tribological, anti-corrosional, and gas barrier applications [1–3]. However, the high residual stress in the DLC films after fabrication causes failure of the interface between the DLC film and the substrate. Moreover, plastic deformation of the substrates used results in poor durability of DLC coatings. It is essential to improve the adhesion strength at the film–substrate interface so that DLC films can be used in industrial applications. Plasma nitriding has the potential to be used as a pre-treatment technique during DLC deposition on steel substrates, as it helps in preventing DLC coating failure by hardening the substrate and retarding plastic deformation of the substrate [4–6]. Plasma based ion implantation and deposition (PBII&D) is a relatively new method used for DLC film deposition [7], and it can be extended to three-dimensional workpieces [8]. In particular, PBII&D ⇑ Corresponding author. Tel./fax: +81 3 5841 1632. E-mail address: [email protected] (J. Choi). 0168-583X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2011.01.100
with bipolar high-voltage pulses affords DLC films with low residual stress [9], and hence, the adhesion strength at the film–substrate interface is well improved. In the bipolar PBII system, a glow discharge plasma is generated by applying a positive pulse voltage to the target, and omnidirectional ion implantation in the target is achieved by the subsequent application of a negative pulse voltage. Upon the application of a positive pulse voltage, the substrate is bombarded by high-energy electrons, and hence, there is an increase in the substrate temperature. The substrate temperature can be controlled by controlling the positive pulse voltage [9]. In the present study, bipolar plasma based ion implantation and deposition (bipolar PBII) [5] was used for nitriding of high speed steel, and the effects of various treatment parameters (positive pulse voltage, negative pulse voltage, treatment pressure, treatment time, and type of precursor gas used) on the nitriding process were investigated. In addition, the adhesion strength of DLC films deposited on nitrided high speed steel substrates was evaluated by performing Rockwell indentation and microscratch tests. 2. Experiments Bipolar PBII&D was used for nitriding of high speed steel (SKH2), which is used in high speed applications such as drill bits, power saw blades, and engine parts. The chemical composition of
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J. Choi et al. / Nuclear Instruments and Methods in Physics Research B 272 (2012) 357–360
Table 1 Chemical composition (wt.%) of SKH2. Si
Mn
P
S
Cr
W
V
0.73–0.83 >0.45 >0.40 >0.03 >0.03 3.80–4.50 17.20–18.70 1.00–1.20 Balance Fe.
Table 2 Conditions for plasma nitriding of SKH2. Sample Pre-cleaning
1 2 3 4 5 6 7 8
Pressure Time (Pa) (min)
Ar Ar Ar Ar Ar Ar + H2 Ar Ar + H2
– 2.0 2.0 2.0 1.0 2.0 2.0 2.0
– 3.0 3.0 3.0 3.0 3.0 2.0 3.4
– 5.0 5.0 5.0 5.0 5.0 10.0 4.5
800
– N2:H2 = 1:1 N2:H2 = 1:1 Pure N2 N2:H2 = 1:1 N2:H2 = 1:1 N2:H2 = 1:1 N2:H2 = 1:1
0 1
– 240 60 240 240 240 240 240
2
3
4
5
6
7
8
7
8
Sample Fig. 1. Hardness of nitrided SKH2.
0.06
Table 3 Conditions for DLC film deposition. C7H8 4.0 kHz 2.0 kV 5.0 kV 0.4 Pa
SKH2 is shown in Table 1. The treatment parameters such as positive pulse voltage, negative pulse voltage, treatment pressure, nitriding time, and type of precursor gas used were varied as shown in Table 2. Before nitriding, the SKH2 substrates were cleaned by Ar+ ion bombardment. The hardness of the nitrided SKH2 substrates was measured by a micro-Vickers hardness tester under an indentation load of 0.98 N. The post-nitriding surface roughness, which is an important factor that determines the friction coefficient, was examined. The thickness of nitrided layer was evaluated by X-ray photoelectron spectroscopy (XPS) depth profile analysis and compared with that obtained by using the ion implantation simulation program TRIM. X-ray diffraction (XRD) analysis was performed to elucidate the structure of the nitriding layer. DLC films were deposited on the nitrided SKH2 substrates by using the same bipolar PBII&D system mentioned above. The conditions for DLC film deposition are shown in Table 3. Rockwell indentation (C-scale) and microscratch tests were conducted on the DLC-coated SKH2 substrates before and after plasma nitriding to evaluate the adhesion strength of the DLC films.
Ra ( μm)
0.04
0.02
0.00 1
Fig. 1 shows the hardness of the nitrided SKH2 substrates treated under various conditions (Table 1). The hardness of SKH2 increases after nitriding. Comparison of samples 2–5 (which were nitrided under constant high pulse voltages) reveals that the hardness of nitrided SKH2 increases under the following conditions: long nitriding time, addition of hydrogen gas to the precursor gas, and high gas pressure during nitriding. The hardness of sample 6 is almost the same as that of sample 2. This indicates that the effect of hydrogen addition during the Ar+ ion bombardment for surface cleaning is negligible in the SKH2 substrate used in the present study. The results obtained for samples 7 and 8 show the effect of
3
4
5
6
Fig. 2. Surface roughness of nitrided SKH2.
+
N2
+
N
0
3. Results and discussion
2
Sample
Concentration (a.u.)
Precursor gas Pulse frequency Positive-pulse voltage Negative-pulse voltage Deposition pressure
1200
400
Plasma nitriding
Precursor Time Positive Negative Precursor gas (min) Voltage Voltage gas (kV) (kV) 15 15 15 15 15 60 15 60
Hardness (HV)
C
1600
10
20
30
Depth (nm) Fig. 3. Depth profile of the implanted nitrogen ions in the SKH2, as calculated by using the TRIM code. Calculation is performed at 5 keV.
negative and positive pulse voltages on the hardness after nitriding. The hardness of samples 7 and 8 is higher than that of sample 2, indicating that the application of a high positive or negative pulse voltage helps in increasing the hardness of the substrate. In particular, the highest hardness is observed in the case of sample 8, which is nitrided under the highest positive pulse voltage. This hardness value is comparable to a previous study of He et al. [10].
J. Choi et al. / Nuclear Instruments and Methods in Physics Research B 272 (2012) 357–360
100
Contents (at.%)
Fe 75
50
O
25
N 0 0
100
200
300
400
500
Depth (nm) Fig. 4. Depth profile of the nitrided SKH2 (sample 8), as measured by XPS.
1200
Fe(110)
(a)
#1
Fe3W3C (511)
Intensity (a.u.) Intensity (a.u.) Intensity (a.u.)
800
Fe3W3C Fe3W3C (422) Fe3W3C (331) (400)
400
Fe3W3C (440)
0 1200
#2
(b) SKH2 matrix CrN, WN ε-Fe3N
800 400 0 1200
359
Fig. 2 shows the surface roughness of the nitrided SKH2. The roughness of SKH2 increases after nitriding. Sample 7 shows the highest roughness, indicating that the application of a high negative pulse voltage results in surface roughening because of the bombardment of high-energy nitrogen ions. The surface roughness of sample 8, which has the highest hardness among all the samples, is relatively low (Fig. 1). Fig. 3 shows the depth profile of the implanted nitrogen ions in the SKH2 samples; the profiles were obtained by using the TRIM code for 5 keV N+ and Nþ 2 (2.5 keV per N) ions. The implantation depth of nitrogen was approximately 20 nm. Fig. 4 shows the nitrogen depth profile obtained by XPS for sample 8. The thickness of nitrided layer exceeded 500 nm, which was considerably greater than that calculated by using the TRIM code. This was due to the increase in the substrate temperature upon bombardment by electrons and ions under high positive and negative pulse voltages. In particular, the electron irradiation by applying a positive voltage is effective in increase of substrate temperature [9] to help the diffusion of nitrogen atoms. Fig. 5 shows the X-ray diffraction (XRD) patterns obtained for (a) sample 1 (non-nitrided SKH2), (b) sample 2, and (c) sample 8. After plasma nitriding, a new diffraction peak appears at around 38°, as shown in Fig. 5(b); this peak is formed by the overlapping of the CrN(1 1 1) peak at 37.5° and the WN(1 1 1) peak at 37.7° [11]. The intensity of this peak increases with an increase in the positive pulse voltage during nitriding (Fig. 5(c)). Fig. 6 shows the optical micrographs of the DLC coatings after the Rockwell indentation tests. DLC coatings were formed on (a) non-nitrided SKH2 and (b) nitrided SKH2. It could be seen that plasma nitriding helps in improving the adhesion strength of the DLC coatings. The adhesion strength increased by approximately 20% upon nitriding the SKH2 substrate, as confirmed from the critical loads measured by using a microscratch tester. 4. Conclusions
(c)
#8 Bipolar plasma based ion implantation and deposition (bipolar PBII) was used for plasma nitriding of high speed steel (SKH2), and the effects of various treatment parameters on the nitriding process were investigated. The main results obtained were as follows:
800 400 0 20
30
40
50
60
2θ (degrees) Fig. 5. XRD patterns obtained for (a) non-nitrided SKH2, (b) sample 2, and (c) sample 8.
(1) Nitriding by bipolar PBII was achieved in the combining of two effects: nitrogen ion implantation under the application of a high negative pulse voltage and thermal diffusion of nitrogen atoms under a high positive pulse voltage. (2) Effective nitriding by bipolar PBII could be achieved under the following conditions: high positive pulse voltage, low negative pulse voltage, high nitrogen gas pressure, and addi-
Fig. 6. Optical micrographs of DLC coatings formed on (a) non-nitrided SKH2 and (b) nitrided SKH2 after Rockwell indentation tests.
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J. Choi et al. / Nuclear Instruments and Methods in Physics Research B 272 (2012) 357–360
tion of hydrogen to the precursor gas. Application of a very high voltage negative pulse resulted in surface roughening of the nitrided layer. The application of a high positive pulse voltage facilitates effective thermal diffusion of the nitrogen atoms. (3) The adhesion strength of the DLC films improved after nitriding of the substrate.
Acknowledgement This study has been partially supported by the Ministry of Knowledge Economy in Korea through the Strategic Technology Development Project (Manufacturing Technology of Micro Moving Control Parts by Powder Injection Molding).
References [1] A. Grill, Wear 168 (1993) 143. [2] J. Choi, S. Nakao, J. Kim, M. Ikeyama, T. Kato, Diamond Relat. Mater. 16 (2007) 1361. [3] M. Ikeyama, S. Miyagawa, S. Nakao, J. Choi, T. Miyajima, Nucl. Instr. Meth. B 257 (2007) 741. [4] E. Menthe, K.T. Rie, J.W. Schultze, S. Simson, Surf. Coat. Technol. 74–75 (1995) 412. [5] E.I. Meletis, A. Erdemir, G.R. Fenske, Surf. Coat. Technol. 73 (1995) 39. [6] B. Larisch, U. Brusky, H.J. Spies, Surf. Coat. Technol. 116–119 (1999) 205. [7] J.R. Conrad, J.L. Radtke, R.A. Dodd, F.J. Worzala, N.C. Tran, J. Appl. Phys. 62 (1987) 4591. [8] S. Miyagawa, S. Nakao, M. Ikeyama, Y. Miyagawa, Surf. Coat. Technol. 156 (2002) 322. [9] J. Choi, S. Miyagawa, S. Nakao, M. Ikeyama, Y. Miyagawa, Nucl. Instr. Meth. B 242 (2006) 357. [10] J.L. He, K.C. Chen, A. Davison, Surf. Coat. Technol. 200 (2005) 1464. [11] Y.Z. Tsai, J.G. Duh, Surf. Coat. Technol. 200 (2005) 1683.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Nitriding of high speed steel by bipolar PBII for improvement in adhesion strength of DLC films Junho Choi a,⇑, Koji Soejima a, Takahisa Kato a, Masahiro Kawaguchi b, Wonsik Lee c a
The University of Tokyo, Tokyo, Japan Tokyo Metropolitan Industrial Technology Research Institute (TIRI), Tokyo, Japan c Korea Institute of Industrial Technology (KITECH), Incheon, Republic of Korea b
a r t i c l e
i n f o
Article history: Available online 1 February 2011 Keywords: Nitriding Bipolar PBII DLC coating Adhesion strength
a b s t r a c t In the present study, bipolar plasma based ion implantation and deposition (bipolar PBII) was used for plasma nitriding of high speed steel (SKH2), and the effects of the treatment parameters (positive pulse voltage, negative pulse voltage, treatment pressure, treatment time, and precursor gases) on the nitriding process were investigated. The hardness, roughness, and depth of nitride layer were also measured. The adhesion strength of diamond-like carbon (DLC) films coated on the nitride substrate was evaluated by carrying out Rockwell indentation and microscratch tests. Nitriding by bipolar PBII was achieved in the combining of two effects: nitrogen ion implantation by applying a high negative pulse voltage and thermal diffusion of nitrogen atoms under the application of a high positive pulse voltage. However, a very high voltage negative pulse caused surface roughening of the nitride layer. Application of a high positive pulse voltage during nitriding was found to be effective in promoting the thermal diffusion of the implanted nitrogen atoms. Effective nitriding could be achieved under the following conditions: high positive pulse voltage, low negative pulse voltage, high nitrogen gas pressure, and addition of hydrogen to the precursor gas. The adhesion strength of the DLC films on the SKH2 substrate was well improved after nitriding. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Diamond-like carbon (DLC) films have attracted considerable interest in the past three decades because of their high hardness, extremely low friction, high wear resistance, chemical inertness, and excellent gas barrier properties. The aforementioned characteristics make DLC films suitable for use in a wide range of applications such as tribological, anti-corrosional, and gas barrier applications [1–3]. However, the high residual stress in the DLC films after fabrication causes failure of the interface between the DLC film and the substrate. Moreover, plastic deformation of the substrates used results in poor durability of DLC coatings. It is essential to improve the adhesion strength at the film–substrate interface so that DLC films can be used in industrial applications. Plasma nitriding has the potential to be used as a pre-treatment technique during DLC deposition on steel substrates, as it helps in preventing DLC coating failure by hardening the substrate and retarding plastic deformation of the substrate [4–6]. Plasma based ion implantation and deposition (PBII&D) is a relatively new method used for DLC film deposition [7], and it can be extended to three-dimensional workpieces [8]. In particular, PBII&D ⇑ Corresponding author. Tel./fax: +81 3 5841 1632. E-mail address: [email protected] (J. Choi). 0168-583X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2011.01.100
with bipolar high-voltage pulses affords DLC films with low residual stress [9], and hence, the adhesion strength at the film–substrate interface is well improved. In the bipolar PBII system, a glow discharge plasma is generated by applying a positive pulse voltage to the target, and omnidirectional ion implantation in the target is achieved by the subsequent application of a negative pulse voltage. Upon the application of a positive pulse voltage, the substrate is bombarded by high-energy electrons, and hence, there is an increase in the substrate temperature. The substrate temperature can be controlled by controlling the positive pulse voltage [9]. In the present study, bipolar plasma based ion implantation and deposition (bipolar PBII) [5] was used for nitriding of high speed steel, and the effects of various treatment parameters (positive pulse voltage, negative pulse voltage, treatment pressure, treatment time, and type of precursor gas used) on the nitriding process were investigated. In addition, the adhesion strength of DLC films deposited on nitrided high speed steel substrates was evaluated by performing Rockwell indentation and microscratch tests. 2. Experiments Bipolar PBII&D was used for nitriding of high speed steel (SKH2), which is used in high speed applications such as drill bits, power saw blades, and engine parts. The chemical composition of
358
J. Choi et al. / Nuclear Instruments and Methods in Physics Research B 272 (2012) 357–360
Table 1 Chemical composition (wt.%) of SKH2. Si
Mn
P
S
Cr
W
V
0.73–0.83 >0.45 >0.40 >0.03 >0.03 3.80–4.50 17.20–18.70 1.00–1.20 Balance Fe.
Table 2 Conditions for plasma nitriding of SKH2. Sample Pre-cleaning
1 2 3 4 5 6 7 8
Pressure Time (Pa) (min)
Ar Ar Ar Ar Ar Ar + H2 Ar Ar + H2
– 2.0 2.0 2.0 1.0 2.0 2.0 2.0
– 3.0 3.0 3.0 3.0 3.0 2.0 3.4
– 5.0 5.0 5.0 5.0 5.0 10.0 4.5
800
– N2:H2 = 1:1 N2:H2 = 1:1 Pure N2 N2:H2 = 1:1 N2:H2 = 1:1 N2:H2 = 1:1 N2:H2 = 1:1
0 1
– 240 60 240 240 240 240 240
2
3
4
5
6
7
8
7
8
Sample Fig. 1. Hardness of nitrided SKH2.
0.06
Table 3 Conditions for DLC film deposition. C7H8 4.0 kHz 2.0 kV 5.0 kV 0.4 Pa
SKH2 is shown in Table 1. The treatment parameters such as positive pulse voltage, negative pulse voltage, treatment pressure, nitriding time, and type of precursor gas used were varied as shown in Table 2. Before nitriding, the SKH2 substrates were cleaned by Ar+ ion bombardment. The hardness of the nitrided SKH2 substrates was measured by a micro-Vickers hardness tester under an indentation load of 0.98 N. The post-nitriding surface roughness, which is an important factor that determines the friction coefficient, was examined. The thickness of nitrided layer was evaluated by X-ray photoelectron spectroscopy (XPS) depth profile analysis and compared with that obtained by using the ion implantation simulation program TRIM. X-ray diffraction (XRD) analysis was performed to elucidate the structure of the nitriding layer. DLC films were deposited on the nitrided SKH2 substrates by using the same bipolar PBII&D system mentioned above. The conditions for DLC film deposition are shown in Table 3. Rockwell indentation (C-scale) and microscratch tests were conducted on the DLC-coated SKH2 substrates before and after plasma nitriding to evaluate the adhesion strength of the DLC films.
Ra ( μm)
0.04
0.02
0.00 1
Fig. 1 shows the hardness of the nitrided SKH2 substrates treated under various conditions (Table 1). The hardness of SKH2 increases after nitriding. Comparison of samples 2–5 (which were nitrided under constant high pulse voltages) reveals that the hardness of nitrided SKH2 increases under the following conditions: long nitriding time, addition of hydrogen gas to the precursor gas, and high gas pressure during nitriding. The hardness of sample 6 is almost the same as that of sample 2. This indicates that the effect of hydrogen addition during the Ar+ ion bombardment for surface cleaning is negligible in the SKH2 substrate used in the present study. The results obtained for samples 7 and 8 show the effect of
3
4
5
6
Fig. 2. Surface roughness of nitrided SKH2.
+
N2
+
N
0
3. Results and discussion
2
Sample
Concentration (a.u.)
Precursor gas Pulse frequency Positive-pulse voltage Negative-pulse voltage Deposition pressure
1200
400
Plasma nitriding
Precursor Time Positive Negative Precursor gas (min) Voltage Voltage gas (kV) (kV) 15 15 15 15 15 60 15 60
Hardness (HV)
C
1600
10
20
30
Depth (nm) Fig. 3. Depth profile of the implanted nitrogen ions in the SKH2, as calculated by using the TRIM code. Calculation is performed at 5 keV.
negative and positive pulse voltages on the hardness after nitriding. The hardness of samples 7 and 8 is higher than that of sample 2, indicating that the application of a high positive or negative pulse voltage helps in increasing the hardness of the substrate. In particular, the highest hardness is observed in the case of sample 8, which is nitrided under the highest positive pulse voltage. This hardness value is comparable to a previous study of He et al. [10].
J. Choi et al. / Nuclear Instruments and Methods in Physics Research B 272 (2012) 357–360
100
Contents (at.%)
Fe 75
50
O
25
N 0 0
100
200
300
400
500
Depth (nm) Fig. 4. Depth profile of the nitrided SKH2 (sample 8), as measured by XPS.
1200
Fe(110)
(a)
#1
Fe3W3C (511)
Intensity (a.u.) Intensity (a.u.) Intensity (a.u.)
800
Fe3W3C Fe3W3C (422) Fe3W3C (331) (400)
400
Fe3W3C (440)
0 1200
#2
(b) SKH2 matrix CrN, WN ε-Fe3N
800 400 0 1200
359
Fig. 2 shows the surface roughness of the nitrided SKH2. The roughness of SKH2 increases after nitriding. Sample 7 shows the highest roughness, indicating that the application of a high negative pulse voltage results in surface roughening because of the bombardment of high-energy nitrogen ions. The surface roughness of sample 8, which has the highest hardness among all the samples, is relatively low (Fig. 1). Fig. 3 shows the depth profile of the implanted nitrogen ions in the SKH2 samples; the profiles were obtained by using the TRIM code for 5 keV N+ and Nþ 2 (2.5 keV per N) ions. The implantation depth of nitrogen was approximately 20 nm. Fig. 4 shows the nitrogen depth profile obtained by XPS for sample 8. The thickness of nitrided layer exceeded 500 nm, which was considerably greater than that calculated by using the TRIM code. This was due to the increase in the substrate temperature upon bombardment by electrons and ions under high positive and negative pulse voltages. In particular, the electron irradiation by applying a positive voltage is effective in increase of substrate temperature [9] to help the diffusion of nitrogen atoms. Fig. 5 shows the X-ray diffraction (XRD) patterns obtained for (a) sample 1 (non-nitrided SKH2), (b) sample 2, and (c) sample 8. After plasma nitriding, a new diffraction peak appears at around 38°, as shown in Fig. 5(b); this peak is formed by the overlapping of the CrN(1 1 1) peak at 37.5° and the WN(1 1 1) peak at 37.7° [11]. The intensity of this peak increases with an increase in the positive pulse voltage during nitriding (Fig. 5(c)). Fig. 6 shows the optical micrographs of the DLC coatings after the Rockwell indentation tests. DLC coatings were formed on (a) non-nitrided SKH2 and (b) nitrided SKH2. It could be seen that plasma nitriding helps in improving the adhesion strength of the DLC coatings. The adhesion strength increased by approximately 20% upon nitriding the SKH2 substrate, as confirmed from the critical loads measured by using a microscratch tester. 4. Conclusions
(c)
#8 Bipolar plasma based ion implantation and deposition (bipolar PBII) was used for plasma nitriding of high speed steel (SKH2), and the effects of various treatment parameters on the nitriding process were investigated. The main results obtained were as follows:
800 400 0 20
30
40
50
60
2θ (degrees) Fig. 5. XRD patterns obtained for (a) non-nitrided SKH2, (b) sample 2, and (c) sample 8.
(1) Nitriding by bipolar PBII was achieved in the combining of two effects: nitrogen ion implantation under the application of a high negative pulse voltage and thermal diffusion of nitrogen atoms under a high positive pulse voltage. (2) Effective nitriding by bipolar PBII could be achieved under the following conditions: high positive pulse voltage, low negative pulse voltage, high nitrogen gas pressure, and addi-
Fig. 6. Optical micrographs of DLC coatings formed on (a) non-nitrided SKH2 and (b) nitrided SKH2 after Rockwell indentation tests.
360
J. Choi et al. / Nuclear Instruments and Methods in Physics Research B 272 (2012) 357–360
tion of hydrogen to the precursor gas. Application of a very high voltage negative pulse resulted in surface roughening of the nitrided layer. The application of a high positive pulse voltage facilitates effective thermal diffusion of the nitrogen atoms. (3) The adhesion strength of the DLC films improved after nitriding of the substrate.
Acknowledgement This study has been partially supported by the Ministry of Knowledge Economy in Korea through the Strategic Technology Development Project (Manufacturing Technology of Micro Moving Control Parts by Powder Injection Molding).
References [1] A. Grill, Wear 168 (1993) 143. [2] J. Choi, S. Nakao, J. Kim, M. Ikeyama, T. Kato, Diamond Relat. Mater. 16 (2007) 1361. [3] M. Ikeyama, S. Miyagawa, S. Nakao, J. Choi, T. Miyajima, Nucl. Instr. Meth. B 257 (2007) 741. [4] E. Menthe, K.T. Rie, J.W. Schultze, S. Simson, Surf. Coat. Technol. 74–75 (1995) 412. [5] E.I. Meletis, A. Erdemir, G.R. Fenske, Surf. Coat. Technol. 73 (1995) 39. [6] B. Larisch, U. Brusky, H.J. Spies, Surf. Coat. Technol. 116–119 (1999) 205. [7] J.R. Conrad, J.L. Radtke, R.A. Dodd, F.J. Worzala, N.C. Tran, J. Appl. Phys. 62 (1987) 4591. [8] S. Miyagawa, S. Nakao, M. Ikeyama, Y. Miyagawa, Surf. Coat. Technol. 156 (2002) 322. [9] J. Choi, S. Miyagawa, S. Nakao, M. Ikeyama, Y. Miyagawa, Nucl. Instr. Meth. B 242 (2006) 357. [10] J.L. He, K.C. Chen, A. Davison, Surf. Coat. Technol. 200 (2005) 1464. [11] Y.Z. Tsai, J.G. Duh, Surf. Coat. Technol. 200 (2005) 1683.