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Performance of CrN radical nitrided tools on deep drawing of advanced high strength steel
Surface & Coatings Technology 205 (2011) 4198–4204
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Performance of CrN radical nitrided tools on deep drawing of advanced high strength steel B. Sresomroeng a,⁎, V. Premanond b, P. Kaewtatip b, A. Khantachawana b, A. Kurosawa c, N. Koga c a b c
Faculty of Engineering, Pathumthani University, Pathumthani, Thailand Faculty of Engineering, King Mongkut's University of Technology Thonburi, Bangkok, Thailand Faculty of Engineering, Nippon Institute of Technology, Saitama, Japan
a r t i c l e
i n f o
Article history: Received 3 February 2010 Accepted in revised form 6 March 2011 Available online 11 March 2011 Keywords: Radical nitriding Deep drawing Advanced high strength steel AHSS Adhesion Coating
a b s t r a c t The performance of CrN film coating to reduce adhesion in the forming process of advanced high strength steel (AHSS) sheets was investigated. Radical nitriding prior to CrN film coating was used to increase the bond strength between the thin film coating and the tool steel substrate. Electron probe microanalysis was used to characterize nitride diffusion into the tool steel. Scratch testing was carried out to evaluate the bond strength between film coatings (CrN, TiN radical nitride and CrN radical nitride) and the substrate. Ball-on-disk testing was carried out to determine the friction coefficient. Deep drawing experiments with non-coated and coated tools were performed on SPFC 980Y steel sheets. JIS-SKD11 tool steel and TiN-coated tools exhibited high friction under dry sliding with SPFC 980Y sheet material. Moreover, severe adhesion was observed in deep drawing experiments using these mating materials. CrN film showed no adhesion to SPFC 980Y sheet material. The bond strength between a CrN film and the substrate can be increased by a nitride layer before coating. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The demand for advanced high strength steel (AHSS) has significantly increased for use in automotive industries. A major problem in forming of AHSS material is galling, which is a type of adhesive wear [1]. Hard thin film coatings on tool material are effective in reducing wear and improving the tribological characteristics when sliding against AHSS material. In the metal forming industry, chemical vapor deposition (CVD) and physical vapor deposition (PVD) processes are commonly used to coat various types of film on tool surfaces. Several researchers [2–5] reported that adhesion and galling depend on the type of work material and level of contact pressure [1]. For stainless steel workpiece material, carbon-based coating provides the best protection against work material transfer. Forming of aluminum and titanium alloys requires nitride type coatings, such as TiN [2]. MoB-based cermets, cemented carbide and diamond-like carbon (DLC) films exhibit excellent anti-galling performance for aluminum and stainless steel forming [5–8]. Tungsten-doped DLC coating exhibits outstanding galling resistance on forming of hot dip
⁎ Corresponding author at: Pathumthani University, Industrial Engineering Department, Faculty of Engineering, 140 Tiwanond-Pathumthani Rd. Banklang, Muang, Pathumthani, 12000, Thailand. Tel.: +66 2 9756999; fax: +66 2 9796728. E-mail addresses: [email protected], [email protected] (B. Sresomroeng), [email protected] (V. Premanond), [email protected] (P. Kaewtatip), [email protected] (A. Khantachawana), [email protected] (A. Kurosawa), [email protected] (N. Koga). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.03.010
galvanized steel [9]. CrN and TiAlN PVD coatings reduce the adhesion between copper alloys and die material in a cold drawing process [10]. It has been reported that TiCN CVD film has good anti galling properties when forming high strength steel (HSS) [11]. A TiCN PVD coating was effective in reducing galling in the forming of galvanized AHSS grade DP 980Gl [12]. However, nitriding combined with a CrN film coated showed better performance when forming HSS grade SPFH 590 than TiCN PVD film and DLC film [13]. Previous studies revealed that CVD coating is preferable when forming under high contact pressure or severe conditions [14,15]. A high galling tendency during AHSS forming is observed under severe conditions [1,12,16]. However, the CVD process utilizes a high deposition temperature, which affects tool softening and causes structural changes. The tool geometry after CVD coating can be slightly changed, thus reducing part accuracy. Therefore, PVD coating is a more attractive technique for industrial applications. The main objective of this study was to select a suitable PVD film coating for forming of AHSS. The classic TiN and CrN film coatings, which are effective in many applications, including machining, molding and metal forming, were selected. The benefit of radical nitriding prior to PVD coating is explored. 2. Experimental procedures 2.1. Tool and workpiece material Cold work tool steel JIS-SKD11 was selected as the tool material. The tool was hardened to 60 ± 2 HRC prior to hard film coating. An AHSS
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Table 1 Chemical composition and mechanical properties of the tool (JIS-SKD11) and workpiece material (JIS-SPFC980Y). Symbol [JIS]
Tensile strength [N/mm2]
Yield strength [N/mm2]
Elongation [%]
Hardness [HV]
SKD11 SPFC 980Y SKD11
– 1002 Chemical composition [%] C Si 1.014 1.268 Nb Ti 0.185 0.019 Chemical composition [%] C Si 0.183 0.472 Nb Ti 0.004 0.034
– 712
– 16
720 332
SPFC 980Y
Mn 0.310 Cu 0.120
P 0.024 Alb 0.003
S 0.004 Co 0.014
Cr 4.946 Fe bal.
Ni 0.092 Sn -
Mo 1.751 B -
Wb 0.020 As -
V 0.194
MnN 2.500 Cu 0.022
P 0.018 Al 0.034
S 0.002 Cob 0.001
Cr 0.022 Fe bal.
Ni 0.025 Sn 0.003
Mob 0.001 B 0.001
W 0.010 As 0.005
V 0.003
cold rolled sheet of 2 mm in thickness, grade SPFC 980Y (JIS), was used as the workpiece material. The chemical composition and mechanical properties obtained from electrospectrometer, micro-hardness and tensile tests are listed in Table 1.
2.2. Deposition process Film coatings of CrN and TiN, each approximately 3 μm in thickness, were deposited using the PVD technique with and without radical nitriding prior to hard film coating. The combination of radical nitriding and PVD coating provides increased peel off resistant for coatings [15]. In this process, the process was carried out under NH3 (400 SCCM) and H2 (200 SCCM) environment under deposition pressure of 57 Pa. The deposition temperature was set at 530 °C and the bias voltage was 500 V. After the radical nitriding applied, the dies were deposited by TiN and CrN coating film. A TiN coating was deposited by cathodic arc ion plating with N2 flow of 400 SCCM at 8 Pa in the pressure chamber. The deposition temperature was set at 400 °C and the bias voltage was 50 V. The average distance between substrate and target was 200 mm. A CrN coating was also deposited by cathodic arc ion plating with N2 flow of 2000 SCCM at 4 Pa in pressure chamber. The deposition temperature and the bias voltage were 400 °C and 250 V. The average distance between substrate and target was 200 mm similar to TiN coatings.
2.4. Scratch test The bond strength of each film on SKD11 substrate was characterized using scratch testing. A Rockwell diamond stylus was drawn over the coating surface for a linearly increasing normal load of 0–100 N at a sliding speed of 10 mm/min for a sliding distance of 8.6 mm. Specimens were cleaned with methanol prior to each test. 2.5. Ball-on-disk test The friction coefficient between coated tools and AHSS was measured in ball-on-disk tests. The ball represents the forming tool
Forming force
Die Punch
AHSS
Cushion Blank holder
2.3. Coating characterizing technique The arithmetic mean roughness (Ra) of the die surface finish after coating was measured using a Taylor Hobson 2-series 2D stylus-type surface roughness measurement device. The stylus travelled on the surface at a constant speed of 1 mm/s over 12 mm tracing length with 0.8 mm cut-off. The hardness of film coatings was measured by JEOLnanoindenter with a linearly increasing normal load (0–1000 μN) at a loading rate of 200 μN/s. A pyramidal (Vickers) indenter and a trigonal pyramidal (Berkovich) indenter were used. The properties of each surface are listed in Table 2. Electron probe microanalysis (EPMA) was performed to characterize the architecture of film coatings and the diffusion depth of the radical nitride.
Table 2 Properties of coated films. Surface conditions
Surface roughness; Ra [μm]
Nano hardness [GPa]
Color
Non-coated (ball) CrN Radical nitriding + TiN Radical nitriding + CrN
0.0502 0.0515 0.0693 0.0694
– 23.52 23.60 23.34
Steel Silver Bronze Silver
Cushion force Fig. 1. Diagram of the deep drawing die. Table 3 Tools condition for deep drawing experiments. Items
Values
Punch diameter [mm] Die diameter [mm] Punch corner [mm] Die corner [mm] Die clearance [mm/side] Punch speed [mm/sec] Blank holder force [kN] Deep drawing ratio Lubricant
59.4 64.2 5.0 10.0 2.4 20 10 1.6 EM-7230 Viscosity 60 mm/s2 (40 °C)
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Fig. 2. EPMA patterns for the film coatings.
Fig. 3. Scratch test characteristics of the CrN film.
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Fig. 4. Scratch test characteristics of the TiN radical nitride film.
which is made of SKD11 hardened to 60 ± 2 HRC. The ball diameter is 6 mm. The coated balls were prepared by the same conditions as shown in Table 2. The disk represents workpiece material which is made by SPFC 980Y sheet. For each experiment, a constant normal load of 2 N which was equivalent to contact pressure of 855 N/mm2, according of Hertzian's equation [17]. The contact pressure was determined to be approached to that for the deep drawing operation of the following experiments. The sliding speed was determined to be constant at 100 mm/s which is similar to the speed of the press machine used in deep drawing test. Forming of AHSS involves relatively high contact pressure and temperature at the toolworkpiece interface compared to forming of mild steel. These unfavorable interface conditions may easily cause the failure of commonly used lubricant [12]. As a reason, dry sliding (without lubricant) was selected to use in this experiment.
2.6. Deep drawing test Deep drawing experiments were conducted to evaluate the performance of hard film-coated tools on deep drawing of SPFC 980Y. The initial circular blank diameter was 102.72 mm. Experiments were carried out using the deep drawing tool shown in Fig. 1 on a 3000-kN hydraulic press machine. The conditions for drawing experiments are listed in Table 3. 3. Results and discussion 3.1. Film coatings characteristics The depth profile of chemical elements according to EPMA is illustrated in Fig. 2. Infusion of the radical nitride can be clearly
Fig. 5. Scratch test characteristics of the CrN radical nitride film.
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100 Initial cracked on film (Lc1) Film failured (Lc2) Film does not fail at 100 N
Load (N)
80 60 40 20 0
CrN
TiN radical nitride
CrN radical nitride
Fig. 6. Scratch test results for each surface condition.
films coated on SKD11 substrate was higher than that of TiN films both with and without nitriding. The CrN film with radical nitriding showed superior bond strength. The friction force gradually increased with the load for TiN radical nitride and CrN radical nitride surfaces, whereas a sudden change in friction force at a load of 60 N was observed for the CrN surface without nitriding. This result indicated from radical nitride diffusion layer of substrate improves the load carrying capacity than that of hardening substrate. The results from previous studies [20–22] showed similar trend on the benefit of nitride layer prior to coating. These results, related to the EPMA analysis, indicate that the load bearing capacity is higher for film coatings with nitride than those without nitride. 3.3. Friction characteristics
identified from the EPMA patterns of nitrogen for both the TiN and CrN radical nitrided tools. The nitrogen intensity in the CrN tool without radical nitride peaked at the outer surface and abruptly decreased to an average value, whereas the intensity in the CrN and TiN radical nitrided tools peaked at the outer surface and gradually decreased with depth until the average value was reached. As reported from previous works [15,18,19], the hardness of the nitride diffusion zone increased proportionally with nitrogen concentration. Then the hardness of nitride substrate decreased continually from the surface to the core when considering from EPMA patterns in Fig. 2. Moreover, the thickness of deposited layer was approximately 4.5 μm for both TiN and CrN films.
3.2. Bond strength of thin film coatings Results for scratch tests of CrN, TiN with radical nitriding and CrN with radical nitriding coated surfaces are shown in Figs. 3–5, respectively. The acoustic emission (AE) signal and friction force were recorded. Failure mechanisms of the coating were examined under an optical microscope. Position a was determined as the initial point of film failure, marked as critical load 1 (Lc1). Position b, corresponding to critical load 2 (Lc2), was determined as the point of complete failure of the film. The final position at a load of 100 N is denoted as position c. Critical loads 1 and 2 in scratch tests are graphically illustrated in Fig. 6. The critical load, which represents the bond strength, of CrN
The results for the friction coefficient as a function of sliding distance in ball-on-disk tests under dry sliding are displayed in Fig. 7. At the early stage, unstable friction coefficients were detected. The asperities of coated and non-coated balls which change the conditions at the sliding interface, lead to unstable friction coefficient. After 50 m, the friction coefficient curve of coated balls against SPFC 980Y disk became lower than the curve of non-coated ball against SPFC 980Y disk. After 75 m, the friction coefficient of all conditions increased with increasing sliding distance. However after 125 m, non-coated ball and TiN radical nitride ball against SPFC 980Y disks showed similar pattern with higher friction (0.60–0.68) than that of CrN and CrN radical nitrided-coated ball (0.50–0.55). This can be explained by rapidly increasing of worn area of non-coated ball and the ball coated with TiN radical nitride. The lager is the worn area resulted in the rising of friction force and hence friction coefficient. 3.4. Deep drawing results Fig. 8 shows the maximum roughness height (Ry) of a deep drawn cup produced using non-coated and coated tools as a function of the number of cups produced. Owing to different mechanisms of stretching at the middle of the cup height and ironing on areas close to the cup edge, the roughness was measured on both positions on the outer cylinder surface. The roughness produced by the non-coated tool and TiN radical nitrided tool increased dramatically with the number of strokes. Similar results for the middle height and the edge area were observed in these two series of experiments. The reason for this
1.0
TiN radical nitrided ball Non coated ball
Coefficient of friction (µ)
0.8
0.6
0.4
CrN ball CrN radical nitrided ball
0.2
0.0 0
50
100
150
Sliding distance (m) Fig. 7. Coefficient of friction as a function of sliding distance.
200
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Fig. 8. Surface roughness of AHSS cups.
Fig. 9. Adhesion on deep drawing dies after 30 strokes.
phenomenon is that some of the AHSS sheet adhered to the tool surface. AHSS adhesion to the die radius was detected, as shown in Fig. 9. This observation may describe to correlate with high friction coefficient in ball-on-disk testing. Non-coated tool and TiN radical nitrided tool show high friction coefficient and attractive to material transfer when in contact with AHSS, thus TiN coated type does not improve the property to reduce adhesion in forming operation of AHSS. CrN and CrN radical nitrided tools produced cups that gradually changed in roughness on the stretching area for up to 1000 strokes. By contrast, the roughness on the ironing area, where higher contact pressure is generated, produced by CrN and CrN radical nitrided tools remained constant up to 350 strokes
and increased up to 1000 strokes. The roughness of cups produced by a CrN tool increased significantly, whereas that of cups produced by a CrN radical nitrided tool increased slightly. The CrN film peeled off, as shown in Fig. 10. This result can be supported by superior bond strength between CrN radical nitride film to the substrate found in scratch testing. High bond strength can be a good indication to increase tool life, thus to improve surface quality of forming parts [6,11,13]. Two forms of failure were observed in deep drawing experiments: (i) adhesion of workpiece material to the tool surface, causing galling on subsequent cups; and (ii) peeling of the thin film coating owing to the low bond strength between the coated film and substrate. CrN film
Fig. 10. Deep drawing dies after 1000 strokes.
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showed no adhesion to AHSS sheet material. The bond strength between the CrN film and substrate can be increased by a nitride layer before coating. 4. Conclusions Several conclusions can be drawn from the results of the study: • CrN film coating of tools was effective in reducing the friction coefficient in relative movement with SPFC 980Y material under dry conditions and no adhesion between the surfaces occurred. • SPFC 980Y sheet material sliding against JIS-SKD11 tool steel and TiN-coated tools in experiments exhibited a high degree of friction under dry sliding and severe adhesion between the surfaces. • A high friction coefficient seems to be correlated to severe adhesion between mating surfaces. • The CrN coating film found to be effective in controlling galling on advance high strength steel. Radical nitriding before coating of hard thin films increased the bond strength between the coated film and substrate. Therefore, the radical nitrided tool has a high performance on deep drawing of advance high strength steel sheet. Acknowledgements The authors would like to thank the National Research University Project of Thailand's Office of the Higher Education Commission for financial support. The supply of the coatings from TOCALO Co., Ltd., Japan, is also much appreciated. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
H. Kim, J. Sung, F.E. Goodwin, T. Altan, J. Mater. Process. Technol. 205 (2008) 459. B. Podgornik, S. Hogmark, O. Sandberg, Wear 261 (2006) 15. S. Kataoka, M. Murakawa, T. Aizawa, H. Ike, Surf. Coat. Technol. 177 (178) (2004) 582. M. Hanson, N. Stavlid, E. Coronel, S. Hogmark, Wear 264 (2008) 781. T. Sato, T. Besshi, J. Mater. Process. Technol. 83 (1998) 185. M. Murakawa, N. Koga, T. Kumagai, Surf. Coat. Technol. 76 (77) (1995) 553. T. Sato, T. Besshi, I. Tsutsui, T. Moromoto, Surf. Coat. Technol. 104 (2000) 21. B. Podgornik, S. Hogmark, O. Sandberg, Surf. Coat. Technol. 184 (2004) 338. F. Clarysse, W. Lauwerens, M. Vermeulen, Wear 264 (2008) 400. P. Panjan, I. Boncina, J. Bevk, M. Cekada, Surf. Coat. Technol. 200 (2005) 133.
[11] B. Sresomroeng, K. Lawanwong, V. Premanond, R. Hato, P. Kaewtatip, A. Khantachawana, N. Koga, Int. J. Abras. Technol. 2 (2009) 313. [12] H. Kim, S. Han, Q. Yan, T. Altan, CIRP Ann. Manuf. Technol. 57 (2008) 299. [13] B. Sresomroeng, V. Premanond, P. Kaewtathip, A. Khantachawana, N. Koga, S. Watanabe, Diamond Relat. Mater. 19 (2010) 833. [14] M. Dubar, A. Dubois, L. Dubar, Wear 259 (2005) 1109. [15] Ö.N. Cora, K. Namiki, M. Koc, Wear 267 (2009) 1123. [16] A. Gåård, P. Krakhmalev, J. Bergström, Tribotest 14 (2008) 1. [17] G.W. Stachowiak, A.W. Batcgelor, Engineering Tribology, Elsevier ButterworthHienemann, USA, 2005. [18] H. Ichimura, Y. Ishii, A. Rodrigo, Surf. Coat. Technol. 169 (170) (2003) 735. [19] Lee Insup, Park Ikmin, Mater. Sci. Eng. 449 (451) (2007) 890. [20] B. Podgornik, S. Hogmark, O. Sandberg, V. Leskovsek, Wear 254 (2003) 1113. [21] B. Podgornik, S. Hogmark, J. Mater. Process. Technol. 174 (2006) 334. [22] J.-D. Kamminga, R. Hoy, G.C.A.M. Janssen, E. Lugscheider, M. Maes, Surf. Coat. Technol. 174 (175) (2003) 671. Dr. Bhadpiroon Sresomroeng has been working as a Lecturer at the Department of Industrial Engineering, Pathumthani University. He received Doctor of Engineering (D.Eng.) in metal forming technology from Department of Tool and Materials Engineering, King Mongkut's University of Technology Thonburi (KMUTT), Bangkok, Thailand. His research interests are metal forming process and the relevant fields. Assoc. Prof. Dr. Varunee Premanond received her PhD in Mechanical and Manufacturing Engineering from Birmingham University, GB, in 1996. Presently, she is an Associate Professor in the Tool and Materials Engineering Department, KMUTT. Her current research topics are tribology in metal forming, springback of advanced high strength steels, hot, warm and cold forging technology. Assoc. Prof. Dr. Pongpan Kaewtatip received his PhD from Nippon Institute of Technology (NIT), Japan, in 2000. He has been working at the Department of Mechanical Engineering, KMUTT until the present. Now, he is an Associate Professor there. His research interests are metal forming technology, as well as the forming of new frontier materials such as SMA, biomedical materials, etc. Asst. Prof. Dr. Anak Khantachawana received his Bachelor, MA and PhD from the Institute of Materials Science and Engineering, University of Tsukuba, Japan, in 2003. He has been working at the Department of Mechanical Engineering, KMUTT until the present. He is now an Assistant Professor there. His interested areas are shape memory alloys, biomaterials, MEMS, etc. Mr. Akimoto Kurosawa is currently pursuing his M.Eng. in the department of Mechanical Engineering, NIT Japan. His research interests are metal forming process and the relevant fields. Prof. Dr. Nobuhiro Koga received his PhD from Nippon Institute of Technology (NIT), Japan, in 1993. He is a Professor at the Department of Mechanical Engineering at NIT. His research interests are forming of low-formability materials such as magnesium alloy, noise reduction in shearing processes and application of NC servo press.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Performance of CrN radical nitrided tools on deep drawing of advanced high strength steel B. Sresomroeng a,⁎, V. Premanond b, P. Kaewtatip b, A. Khantachawana b, A. Kurosawa c, N. Koga c a b c
Faculty of Engineering, Pathumthani University, Pathumthani, Thailand Faculty of Engineering, King Mongkut's University of Technology Thonburi, Bangkok, Thailand Faculty of Engineering, Nippon Institute of Technology, Saitama, Japan
a r t i c l e
i n f o
Article history: Received 3 February 2010 Accepted in revised form 6 March 2011 Available online 11 March 2011 Keywords: Radical nitriding Deep drawing Advanced high strength steel AHSS Adhesion Coating
a b s t r a c t The performance of CrN film coating to reduce adhesion in the forming process of advanced high strength steel (AHSS) sheets was investigated. Radical nitriding prior to CrN film coating was used to increase the bond strength between the thin film coating and the tool steel substrate. Electron probe microanalysis was used to characterize nitride diffusion into the tool steel. Scratch testing was carried out to evaluate the bond strength between film coatings (CrN, TiN radical nitride and CrN radical nitride) and the substrate. Ball-on-disk testing was carried out to determine the friction coefficient. Deep drawing experiments with non-coated and coated tools were performed on SPFC 980Y steel sheets. JIS-SKD11 tool steel and TiN-coated tools exhibited high friction under dry sliding with SPFC 980Y sheet material. Moreover, severe adhesion was observed in deep drawing experiments using these mating materials. CrN film showed no adhesion to SPFC 980Y sheet material. The bond strength between a CrN film and the substrate can be increased by a nitride layer before coating. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The demand for advanced high strength steel (AHSS) has significantly increased for use in automotive industries. A major problem in forming of AHSS material is galling, which is a type of adhesive wear [1]. Hard thin film coatings on tool material are effective in reducing wear and improving the tribological characteristics when sliding against AHSS material. In the metal forming industry, chemical vapor deposition (CVD) and physical vapor deposition (PVD) processes are commonly used to coat various types of film on tool surfaces. Several researchers [2–5] reported that adhesion and galling depend on the type of work material and level of contact pressure [1]. For stainless steel workpiece material, carbon-based coating provides the best protection against work material transfer. Forming of aluminum and titanium alloys requires nitride type coatings, such as TiN [2]. MoB-based cermets, cemented carbide and diamond-like carbon (DLC) films exhibit excellent anti-galling performance for aluminum and stainless steel forming [5–8]. Tungsten-doped DLC coating exhibits outstanding galling resistance on forming of hot dip
⁎ Corresponding author at: Pathumthani University, Industrial Engineering Department, Faculty of Engineering, 140 Tiwanond-Pathumthani Rd. Banklang, Muang, Pathumthani, 12000, Thailand. Tel.: +66 2 9756999; fax: +66 2 9796728. E-mail addresses: [email protected], [email protected] (B. Sresomroeng), [email protected] (V. Premanond), [email protected] (P. Kaewtatip), [email protected] (A. Khantachawana), [email protected] (A. Kurosawa), [email protected] (N. Koga). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.03.010
galvanized steel [9]. CrN and TiAlN PVD coatings reduce the adhesion between copper alloys and die material in a cold drawing process [10]. It has been reported that TiCN CVD film has good anti galling properties when forming high strength steel (HSS) [11]. A TiCN PVD coating was effective in reducing galling in the forming of galvanized AHSS grade DP 980Gl [12]. However, nitriding combined with a CrN film coated showed better performance when forming HSS grade SPFH 590 than TiCN PVD film and DLC film [13]. Previous studies revealed that CVD coating is preferable when forming under high contact pressure or severe conditions [14,15]. A high galling tendency during AHSS forming is observed under severe conditions [1,12,16]. However, the CVD process utilizes a high deposition temperature, which affects tool softening and causes structural changes. The tool geometry after CVD coating can be slightly changed, thus reducing part accuracy. Therefore, PVD coating is a more attractive technique for industrial applications. The main objective of this study was to select a suitable PVD film coating for forming of AHSS. The classic TiN and CrN film coatings, which are effective in many applications, including machining, molding and metal forming, were selected. The benefit of radical nitriding prior to PVD coating is explored. 2. Experimental procedures 2.1. Tool and workpiece material Cold work tool steel JIS-SKD11 was selected as the tool material. The tool was hardened to 60 ± 2 HRC prior to hard film coating. An AHSS
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Table 1 Chemical composition and mechanical properties of the tool (JIS-SKD11) and workpiece material (JIS-SPFC980Y). Symbol [JIS]
Tensile strength [N/mm2]
Yield strength [N/mm2]
Elongation [%]
Hardness [HV]
SKD11 SPFC 980Y SKD11
– 1002 Chemical composition [%] C Si 1.014 1.268 Nb Ti 0.185 0.019 Chemical composition [%] C Si 0.183 0.472 Nb Ti 0.004 0.034
– 712
– 16
720 332
SPFC 980Y
Mn 0.310 Cu 0.120
P 0.024 Alb 0.003
S 0.004 Co 0.014
Cr 4.946 Fe bal.
Ni 0.092 Sn -
Mo 1.751 B -
Wb 0.020 As -
V 0.194
MnN 2.500 Cu 0.022
P 0.018 Al 0.034
S 0.002 Cob 0.001
Cr 0.022 Fe bal.
Ni 0.025 Sn 0.003
Mob 0.001 B 0.001
W 0.010 As 0.005
V 0.003
cold rolled sheet of 2 mm in thickness, grade SPFC 980Y (JIS), was used as the workpiece material. The chemical composition and mechanical properties obtained from electrospectrometer, micro-hardness and tensile tests are listed in Table 1.
2.2. Deposition process Film coatings of CrN and TiN, each approximately 3 μm in thickness, were deposited using the PVD technique with and without radical nitriding prior to hard film coating. The combination of radical nitriding and PVD coating provides increased peel off resistant for coatings [15]. In this process, the process was carried out under NH3 (400 SCCM) and H2 (200 SCCM) environment under deposition pressure of 57 Pa. The deposition temperature was set at 530 °C and the bias voltage was 500 V. After the radical nitriding applied, the dies were deposited by TiN and CrN coating film. A TiN coating was deposited by cathodic arc ion plating with N2 flow of 400 SCCM at 8 Pa in the pressure chamber. The deposition temperature was set at 400 °C and the bias voltage was 50 V. The average distance between substrate and target was 200 mm. A CrN coating was also deposited by cathodic arc ion plating with N2 flow of 2000 SCCM at 4 Pa in pressure chamber. The deposition temperature and the bias voltage were 400 °C and 250 V. The average distance between substrate and target was 200 mm similar to TiN coatings.
2.4. Scratch test The bond strength of each film on SKD11 substrate was characterized using scratch testing. A Rockwell diamond stylus was drawn over the coating surface for a linearly increasing normal load of 0–100 N at a sliding speed of 10 mm/min for a sliding distance of 8.6 mm. Specimens were cleaned with methanol prior to each test. 2.5. Ball-on-disk test The friction coefficient between coated tools and AHSS was measured in ball-on-disk tests. The ball represents the forming tool
Forming force
Die Punch
AHSS
Cushion Blank holder
2.3. Coating characterizing technique The arithmetic mean roughness (Ra) of the die surface finish after coating was measured using a Taylor Hobson 2-series 2D stylus-type surface roughness measurement device. The stylus travelled on the surface at a constant speed of 1 mm/s over 12 mm tracing length with 0.8 mm cut-off. The hardness of film coatings was measured by JEOLnanoindenter with a linearly increasing normal load (0–1000 μN) at a loading rate of 200 μN/s. A pyramidal (Vickers) indenter and a trigonal pyramidal (Berkovich) indenter were used. The properties of each surface are listed in Table 2. Electron probe microanalysis (EPMA) was performed to characterize the architecture of film coatings and the diffusion depth of the radical nitride.
Table 2 Properties of coated films. Surface conditions
Surface roughness; Ra [μm]
Nano hardness [GPa]
Color
Non-coated (ball) CrN Radical nitriding + TiN Radical nitriding + CrN
0.0502 0.0515 0.0693 0.0694
– 23.52 23.60 23.34
Steel Silver Bronze Silver
Cushion force Fig. 1. Diagram of the deep drawing die. Table 3 Tools condition for deep drawing experiments. Items
Values
Punch diameter [mm] Die diameter [mm] Punch corner [mm] Die corner [mm] Die clearance [mm/side] Punch speed [mm/sec] Blank holder force [kN] Deep drawing ratio Lubricant
59.4 64.2 5.0 10.0 2.4 20 10 1.6 EM-7230 Viscosity 60 mm/s2 (40 °C)
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Fig. 2. EPMA patterns for the film coatings.
Fig. 3. Scratch test characteristics of the CrN film.
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Fig. 4. Scratch test characteristics of the TiN radical nitride film.
which is made of SKD11 hardened to 60 ± 2 HRC. The ball diameter is 6 mm. The coated balls were prepared by the same conditions as shown in Table 2. The disk represents workpiece material which is made by SPFC 980Y sheet. For each experiment, a constant normal load of 2 N which was equivalent to contact pressure of 855 N/mm2, according of Hertzian's equation [17]. The contact pressure was determined to be approached to that for the deep drawing operation of the following experiments. The sliding speed was determined to be constant at 100 mm/s which is similar to the speed of the press machine used in deep drawing test. Forming of AHSS involves relatively high contact pressure and temperature at the toolworkpiece interface compared to forming of mild steel. These unfavorable interface conditions may easily cause the failure of commonly used lubricant [12]. As a reason, dry sliding (without lubricant) was selected to use in this experiment.
2.6. Deep drawing test Deep drawing experiments were conducted to evaluate the performance of hard film-coated tools on deep drawing of SPFC 980Y. The initial circular blank diameter was 102.72 mm. Experiments were carried out using the deep drawing tool shown in Fig. 1 on a 3000-kN hydraulic press machine. The conditions for drawing experiments are listed in Table 3. 3. Results and discussion 3.1. Film coatings characteristics The depth profile of chemical elements according to EPMA is illustrated in Fig. 2. Infusion of the radical nitride can be clearly
Fig. 5. Scratch test characteristics of the CrN radical nitride film.
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100 Initial cracked on film (Lc1) Film failured (Lc2) Film does not fail at 100 N
Load (N)
80 60 40 20 0
CrN
TiN radical nitride
CrN radical nitride
Fig. 6. Scratch test results for each surface condition.
films coated on SKD11 substrate was higher than that of TiN films both with and without nitriding. The CrN film with radical nitriding showed superior bond strength. The friction force gradually increased with the load for TiN radical nitride and CrN radical nitride surfaces, whereas a sudden change in friction force at a load of 60 N was observed for the CrN surface without nitriding. This result indicated from radical nitride diffusion layer of substrate improves the load carrying capacity than that of hardening substrate. The results from previous studies [20–22] showed similar trend on the benefit of nitride layer prior to coating. These results, related to the EPMA analysis, indicate that the load bearing capacity is higher for film coatings with nitride than those without nitride. 3.3. Friction characteristics
identified from the EPMA patterns of nitrogen for both the TiN and CrN radical nitrided tools. The nitrogen intensity in the CrN tool without radical nitride peaked at the outer surface and abruptly decreased to an average value, whereas the intensity in the CrN and TiN radical nitrided tools peaked at the outer surface and gradually decreased with depth until the average value was reached. As reported from previous works [15,18,19], the hardness of the nitride diffusion zone increased proportionally with nitrogen concentration. Then the hardness of nitride substrate decreased continually from the surface to the core when considering from EPMA patterns in Fig. 2. Moreover, the thickness of deposited layer was approximately 4.5 μm for both TiN and CrN films.
3.2. Bond strength of thin film coatings Results for scratch tests of CrN, TiN with radical nitriding and CrN with radical nitriding coated surfaces are shown in Figs. 3–5, respectively. The acoustic emission (AE) signal and friction force were recorded. Failure mechanisms of the coating were examined under an optical microscope. Position a was determined as the initial point of film failure, marked as critical load 1 (Lc1). Position b, corresponding to critical load 2 (Lc2), was determined as the point of complete failure of the film. The final position at a load of 100 N is denoted as position c. Critical loads 1 and 2 in scratch tests are graphically illustrated in Fig. 6. The critical load, which represents the bond strength, of CrN
The results for the friction coefficient as a function of sliding distance in ball-on-disk tests under dry sliding are displayed in Fig. 7. At the early stage, unstable friction coefficients were detected. The asperities of coated and non-coated balls which change the conditions at the sliding interface, lead to unstable friction coefficient. After 50 m, the friction coefficient curve of coated balls against SPFC 980Y disk became lower than the curve of non-coated ball against SPFC 980Y disk. After 75 m, the friction coefficient of all conditions increased with increasing sliding distance. However after 125 m, non-coated ball and TiN radical nitride ball against SPFC 980Y disks showed similar pattern with higher friction (0.60–0.68) than that of CrN and CrN radical nitrided-coated ball (0.50–0.55). This can be explained by rapidly increasing of worn area of non-coated ball and the ball coated with TiN radical nitride. The lager is the worn area resulted in the rising of friction force and hence friction coefficient. 3.4. Deep drawing results Fig. 8 shows the maximum roughness height (Ry) of a deep drawn cup produced using non-coated and coated tools as a function of the number of cups produced. Owing to different mechanisms of stretching at the middle of the cup height and ironing on areas close to the cup edge, the roughness was measured on both positions on the outer cylinder surface. The roughness produced by the non-coated tool and TiN radical nitrided tool increased dramatically with the number of strokes. Similar results for the middle height and the edge area were observed in these two series of experiments. The reason for this
1.0
TiN radical nitrided ball Non coated ball
Coefficient of friction (µ)
0.8
0.6
0.4
CrN ball CrN radical nitrided ball
0.2
0.0 0
50
100
150
Sliding distance (m) Fig. 7. Coefficient of friction as a function of sliding distance.
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Fig. 8. Surface roughness of AHSS cups.
Fig. 9. Adhesion on deep drawing dies after 30 strokes.
phenomenon is that some of the AHSS sheet adhered to the tool surface. AHSS adhesion to the die radius was detected, as shown in Fig. 9. This observation may describe to correlate with high friction coefficient in ball-on-disk testing. Non-coated tool and TiN radical nitrided tool show high friction coefficient and attractive to material transfer when in contact with AHSS, thus TiN coated type does not improve the property to reduce adhesion in forming operation of AHSS. CrN and CrN radical nitrided tools produced cups that gradually changed in roughness on the stretching area for up to 1000 strokes. By contrast, the roughness on the ironing area, where higher contact pressure is generated, produced by CrN and CrN radical nitrided tools remained constant up to 350 strokes
and increased up to 1000 strokes. The roughness of cups produced by a CrN tool increased significantly, whereas that of cups produced by a CrN radical nitrided tool increased slightly. The CrN film peeled off, as shown in Fig. 10. This result can be supported by superior bond strength between CrN radical nitride film to the substrate found in scratch testing. High bond strength can be a good indication to increase tool life, thus to improve surface quality of forming parts [6,11,13]. Two forms of failure were observed in deep drawing experiments: (i) adhesion of workpiece material to the tool surface, causing galling on subsequent cups; and (ii) peeling of the thin film coating owing to the low bond strength between the coated film and substrate. CrN film
Fig. 10. Deep drawing dies after 1000 strokes.
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showed no adhesion to AHSS sheet material. The bond strength between the CrN film and substrate can be increased by a nitride layer before coating. 4. Conclusions Several conclusions can be drawn from the results of the study: • CrN film coating of tools was effective in reducing the friction coefficient in relative movement with SPFC 980Y material under dry conditions and no adhesion between the surfaces occurred. • SPFC 980Y sheet material sliding against JIS-SKD11 tool steel and TiN-coated tools in experiments exhibited a high degree of friction under dry sliding and severe adhesion between the surfaces. • A high friction coefficient seems to be correlated to severe adhesion between mating surfaces. • The CrN coating film found to be effective in controlling galling on advance high strength steel. Radical nitriding before coating of hard thin films increased the bond strength between the coated film and substrate. Therefore, the radical nitrided tool has a high performance on deep drawing of advance high strength steel sheet. Acknowledgements The authors would like to thank the National Research University Project of Thailand's Office of the Higher Education Commission for financial support. The supply of the coatings from TOCALO Co., Ltd., Japan, is also much appreciated. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
H. Kim, J. Sung, F.E. Goodwin, T. Altan, J. Mater. Process. Technol. 205 (2008) 459. B. Podgornik, S. Hogmark, O. Sandberg, Wear 261 (2006) 15. S. Kataoka, M. Murakawa, T. Aizawa, H. Ike, Surf. Coat. Technol. 177 (178) (2004) 582. M. Hanson, N. Stavlid, E. Coronel, S. Hogmark, Wear 264 (2008) 781. T. Sato, T. Besshi, J. Mater. Process. Technol. 83 (1998) 185. M. Murakawa, N. Koga, T. Kumagai, Surf. Coat. Technol. 76 (77) (1995) 553. T. Sato, T. Besshi, I. Tsutsui, T. Moromoto, Surf. Coat. Technol. 104 (2000) 21. B. Podgornik, S. Hogmark, O. Sandberg, Surf. Coat. Technol. 184 (2004) 338. F. Clarysse, W. Lauwerens, M. Vermeulen, Wear 264 (2008) 400. P. Panjan, I. Boncina, J. Bevk, M. Cekada, Surf. Coat. Technol. 200 (2005) 133.
[11] B. Sresomroeng, K. Lawanwong, V. Premanond, R. Hato, P. Kaewtatip, A. Khantachawana, N. Koga, Int. J. Abras. Technol. 2 (2009) 313. [12] H. Kim, S. Han, Q. Yan, T. Altan, CIRP Ann. Manuf. Technol. 57 (2008) 299. [13] B. Sresomroeng, V. Premanond, P. Kaewtathip, A. Khantachawana, N. Koga, S. Watanabe, Diamond Relat. Mater. 19 (2010) 833. [14] M. Dubar, A. Dubois, L. Dubar, Wear 259 (2005) 1109. [15] Ö.N. Cora, K. Namiki, M. Koc, Wear 267 (2009) 1123. [16] A. Gåård, P. Krakhmalev, J. Bergström, Tribotest 14 (2008) 1. [17] G.W. Stachowiak, A.W. Batcgelor, Engineering Tribology, Elsevier ButterworthHienemann, USA, 2005. [18] H. Ichimura, Y. Ishii, A. Rodrigo, Surf. Coat. Technol. 169 (170) (2003) 735. [19] Lee Insup, Park Ikmin, Mater. Sci. Eng. 449 (451) (2007) 890. [20] B. Podgornik, S. Hogmark, O. Sandberg, V. Leskovsek, Wear 254 (2003) 1113. [21] B. Podgornik, S. Hogmark, J. Mater. Process. Technol. 174 (2006) 334. [22] J.-D. Kamminga, R. Hoy, G.C.A.M. Janssen, E. Lugscheider, M. Maes, Surf. Coat. Technol. 174 (175) (2003) 671. Dr. Bhadpiroon Sresomroeng has been working as a Lecturer at the Department of Industrial Engineering, Pathumthani University. He received Doctor of Engineering (D.Eng.) in metal forming technology from Department of Tool and Materials Engineering, King Mongkut's University of Technology Thonburi (KMUTT), Bangkok, Thailand. His research interests are metal forming process and the relevant fields. Assoc. Prof. Dr. Varunee Premanond received her PhD in Mechanical and Manufacturing Engineering from Birmingham University, GB, in 1996. Presently, she is an Associate Professor in the Tool and Materials Engineering Department, KMUTT. Her current research topics are tribology in metal forming, springback of advanced high strength steels, hot, warm and cold forging technology. Assoc. Prof. Dr. Pongpan Kaewtatip received his PhD from Nippon Institute of Technology (NIT), Japan, in 2000. He has been working at the Department of Mechanical Engineering, KMUTT until the present. Now, he is an Associate Professor there. His research interests are metal forming technology, as well as the forming of new frontier materials such as SMA, biomedical materials, etc. Asst. Prof. Dr. Anak Khantachawana received his Bachelor, MA and PhD from the Institute of Materials Science and Engineering, University of Tsukuba, Japan, in 2003. He has been working at the Department of Mechanical Engineering, KMUTT until the present. He is now an Assistant Professor there. His interested areas are shape memory alloys, biomaterials, MEMS, etc. Mr. Akimoto Kurosawa is currently pursuing his M.Eng. in the department of Mechanical Engineering, NIT Japan. His research interests are metal forming process and the relevant fields. Prof. Dr. Nobuhiro Koga received his PhD from Nippon Institute of Technology (NIT), Japan, in 1993. He is a Professor at the Department of Mechanical Engineering at NIT. His research interests are forming of low-formability materials such as magnesium alloy, noise reduction in shearing processes and application of NC servo press.