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Properties and cutting performance of (Ti,V)N coatings prepared by cathodic arc ion plating
Surface & Coatings Technology 200 (2005) 1377 – 1382 www.elsevier.com/locate/surfcoat
Properties and cutting performance of (Ti,V)N coatings prepared by cathodic arc ion plating N. Ichimiya a,*, Y. Onishi b, Y. Tanaka a b
a Mitsubishi Materials Corporation, 1511, Furumagi, Ishige-machi, Yuuki-gun, Ibaraki 300-2795, Japan Mitsubishi Materials Kobe Tools Corporation, 179-1, Nishioike, Kanagasaki, Uozumicho, Akashi, Hyogo 674-0071, Japan
Available online 3 October 2005
Abstract (Ti,V)N coatings were deposited on WC – Co substrates using the cathodic arc ion plating method. The structures and compositions were characterized by X-ray diffraction, transmission electron microscopy, and electron probe microanalysis. The (Ti,V)N coating showed a single phase cubic B1 structure with a dense columnar microstructure typical of cathodic arc ion plating. The wear-resistant properties and friction coefficients of (Ti,V)N coatings against steel were evaluated using the ball-on-disc test at elevated temperatures. (Ti,V)N had a friction coefficient of 0.51 against carbon steel at 600 -C, which is lower than the 0.91 friction coefficient of (Ti,Al)N. High-speed cutting performance was investigated using coated carbide endmills. (Ti,V)N coatings showed greatly improved cutting performances on low hardness carbon steels. The wear mechanism and cutting characteristics are discussed in terms of the structure and properties of the films. D 2005 Elsevier B.V. All rights reserved. Keywords: Cathodic arc ion plating; Titanium vanadium nitride; Cutting performance
1. Introduction The (Ti,Al)N coating, deposited by means of the physical vapor deposition (PVD) method, is a promising wearresistant film for cutting tools such as solid carbide endmills, drills, and inserts. Therefore, much research is focusing on this material. (Ti,Al)N-coated solid carbide endmills permit direct milling of hardened dies and mold steels without rough machining before heat treatment [1,2], contributing to improved productivity in the manufacture of dies and molds. Furthermore, for the demand of machining harder work materials at a higher cutting speed, addition of other elements to (Ti,Al)N has been investigated [3]. On the other hand, relatively soft materials, such as carbon steel, are widely used to make molds for plastic goods and general machine parts. However, little research has been done on coatings for cutting tools used to cut this material. This study investigates the properties and cutting performance of (Ti,V)N coatings for soft steels. * Corresponding author. Tel.: +81 78 936 7405; fax: +81 78 936 1497. E-mail address: [email protected] (N. Ichimiya). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.08.026
Some research on this ternary coating film has already been published. Knotek et al. investigated two methods of synthesizing this material: magnetron sputtering and arc ion plating. In both cases, (Ti,V)N-coated indexable inserts exhibited excellent performance in turning operations for cutting steels [4– 7]. Hasegawa et al. compared (Ti,V)N film to other titanium-based ternary nitride films [8]. Wada et al., also reported on the properties of (Ti,V)N films and their cutting performance when turning stainless steel [9]. This paper describes the structure and properties of (Ti,V)N films prepared by the arc ion plating method and the cutting performance of a (Ti,V)N-coated solid carbide endmill for milling carbon steel.
2. Experimental procedure 2.1. Sample preparation Samples were prepared by the cathodic arc ion plating method using the equipment shown in Fig. 1. Cathodic ion
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N. Ichimiya et al. / Surface & Coatings Technology 200 (2005) 1377 – 1382
Reactive Gas Vacuum Chamber Arc Power Supply
N2 Target +
+ +
(-)
Substrate
+
(-)
+ +
+
TiV
TiV Rotating Table
(-) Bias Power Supply
Fig. 1. Schematic diagram of the arc ion plating equipment used in this study.
plating is a high-energy deposition process that employs a vacuum arc to generate ionized vapor from target materials. The substrate is negatively biased with respect to the anode and chamber, and positive ions generated from the target are deposited on the substrate. In this study, different atomic ratios of Ti –V targets prepared by a powder metallurgical technique were used for the deposition. WC – Co carbide (ISO M20) insert (SNGN120408) was used as the substrate. To evaluate the oxidation behavior, platinum foil was used as a substrate to avoid the effect of substrate oxidation. Before insertion into the chamber, the substrates were ultrasonically cleaned with an organic solvent. The ultimate base pressure was 4 10 3 Pa and the substrate was heated to 450 -C using a radiation heater installed in the chamber. After the substrate was sputter cleaned by ion bombardment, deposition was carried out in an atmosphere of nitrogen in the pressure range of 2– 5 Pa, with a negative substrate bias of 50 V and a substrate temperature of 450 -C. 2.2. Characterization and evaluation of the films
Fig. 2. X-ray diffraction patterns at various V/(Ti + V) values.
was determined by h – 2h X-ray diffraction (XRD) using a JEOL JDX-3500 X-ray system with CuKa (k = 0.1542 nm). The hardness of the films was determined by means of a Vickers micro-indentation test at a load of 0.25 N. The films used for the hardness measurement were deposited on a WC – Co substrate to a film thickness of 5 Am. The dimensions of the indentation were measured using an optical microscope and a scanning electron microscope (SEM). The microstructure of the films was investigated by means of a transmission electron microscope (JEOL JEM2010F) with an acceleration voltage of 200 kV. The films used for TEM observation were deposited on WC – Co substrates with a film thickness of 4 Am. Electron diffraction was also carried out in the TEM to determine the crystal
The compositions of the samples were determined by electron probe microanalysis (EPMA). The crystal structure
Endmill
Ball-nose carbide endmill Conventional shape with 2 flutes Size: R5 mm (u10 mm)
Thickness of coating (Am) Work material Cutting speed (m/min) Feed rate (mm/tooth) Depth of cut Pf (mm) Ad (mm) Cutting direction
3 AISI 1055 (220 HB) 308 m/min (10,000 min 1) 0.05 mm/tooth (1000 mm/min) 0.5 mm 4.0 mm Downcut milling
Lattice constant (nm)
0.430 Table 1 Cutting conditions for evaluating coated endmills
0.420
0.410
0.400 0
0.2
0.4
0.6
0.8
1
V(Ti+V) Fig. 3. Lattice constant of Ti – V – N coatings as a function of V/(Ti + V).
N. Ichimiya et al. / Surface & Coatings Technology 200 (2005) 1377 – 1382
(a)
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(b)
Fig. 4. Plan view TEM images of (a) (Ti,V)N and (b) (Al,Ti)N films.
structure of local points of the samples. The TEM samples were prepared by means of the focused ion beam (FIB) technique using a JEOL JFIB-2100 system. The samples were thinned to approximately 0.1 Am by Ga ions with the an acceleration voltage of 30 kV and a Ga ion source current of 2.0 AA. The oxidation behavior of the Ti –V – N coatings was investigated by means of annealing tests. Films with a thickness of 4 Am were deposited on platinum foils measuring 5 mm 20 mm 0.02 mm. The samples were heated in air at a rate of 10 K/min in an electric furnace at temperatures up to 1100 -C. The weights of the samples were measured and compared with those of as-deposited films. To evaluate the tribological properties, ball-on-disc tests were carried out. The sample films were deposited on a WC – Co ball with a diameter of 6.35 mm. The friction coefficient against a polished disc of carbon steel (AISI 1055) was measured at temperatures up to 600 -C, a sliding speed of 5 m/min, and a load of 1 N. 2.3. Cutting performance Cutting performance was investigated using two-flute, ball-nose, carbide endmills with a diameter of 10 mm and coated with (Ti,V)N, (Al,Ti)N and double layer of (Al,Ti)N/(Ti,V)N. The film was 3 Am thick and the work material was AISI 1055 with a hardness of 220 HB. The cutting edge was examined for wear using an optical microscope. Details of the cutting test conditions are listed in Table 1.
(a)
3. Results 3.1. Structure of the films The Ti/V compositions of the films deposited using different Ti1 – x Vx targets (x = 0, 0.25, 0.5, 0.75, 1) are quite similar to compositions of the targets themselves. This is typical of the cathodic arc ion plating method, which generates high ion energy during deposition. As the V content in the film increased, the color of the film changed from the gold of TiN to light yellow and eventually to the light grey of VN. The crystal structures of Ti – V – N films, examined by XRD, are shown in Fig. 2. All samples with vanadium content have a cubic B1 structure. The lattice constants are plotted in Fig. 3 as a function of V/(Ti + V). The lattice constant of TiN is 0.424 nm. This value decreases steadily as V/(Ti + V) increases. At V(Ti + V) = 1, the lattice constant for VN is 0.414 nm. This indicates that increasing V/ (V + Ti) causes more vanadium ions to occupy the sites of the Ti ions. Because the Vatom ions are smaller in size than the Ti ions, the lattice constant decreases when the amount of V/ (V + Ti) increases. Figs. 4– 6 show cross-sectional and plan view TEM images and electron diffraction patterns of the (Ti,V)N and (Al,Ti)N films. Both films had dense columnar microstructures typical of cathodic arc ion plating. However, (Ti,V)N had a grain size of approximately 50 nm, which is smaller than the100 nm grain size of (Al,Ti)N. (TiV)N had a broader electron diffraction pattern than (Al,Ti)N, indicating that (Ti,V)N had smaller grains. This is consistent with the TEM results.
(b) 200nm
Fig. 5. Cross-sectional TEM images of (a) (Ti,V)N and (b) (Al,Ti)N films.
200nm
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N. Ichimiya et al. / Surface & Coatings Technology 200 (2005) 1377 – 1382
(a)
(b) (222)
(311) (220)
(200) (111) (220)
(111) (200)
Fig. 6. Electron diffraction patterns of (a) (Ti,V)N and (b) (Al,Ti)N films.
3.2. Properties of the films The hardness measurements are shown in Fig. 7. TiN film had a hardness of 2000 Hv, which is similar to the results of previous studies [6,8–10]. As V/(Ti + V) increased up to V/ (Ti + V) = 0.5, the hardness of the films increased. As V/ (Ti + V) increased beyond 0.5, the hardness decreased to 1800 Hv. Between V(Ti + V) values of 0.25 and 0.5, the films displayed the highest hardness – up to approximately 2600 Hv – which is a little lower than that of the (Al,Ti)N coating that has been reported to be 2800 Hv. Wada et al. reported that the highest film hardness values for V/(V + Ti) were between 0.25 to 0.5 [9], which is consistent with the results of this study. Oxidation tests were carried out on the (Ti,V)N and (Al,Ti)N films. Fig. 8 shows the weight gain of samples treated at high temperatures in atmospheric conditions. The results show that (Ti,V)N film starts to increase in weight at around 600 -C. After heat treatment at 500 -C for 1 h, the (Ti,V)N film subjected to XRD analysis, which revealed V and (Ti,V) oxides. This indicates that the weight gain comes from oxidation of the (Ti,V)N film. (Al,Ti)N film started to increase in weight at a higher temperature: around 800 -C. This agrees well with previous researches [3,10], which suggest that the thin aluminum oxide film formed on the surface of the film functions as an oxidation barrier. (Ti,V)N films have the same high hardness as (Al,Ti)N film, but with inferior anti-oxidation properties. To investigate the friction properties (Ti,V)N films, ballon-disc sliding tests were carried out at the temperature from
200 -C to 600 -C. To be compared with (Ti,V)N, the friction coefficient of (Al,Ti)N film was also measured at 600 -C. The results are plotted in Fig. 9. (Ti,V)N film had low friction coefficient of approximately 0.6. Friction coefficient of (Ti,V)N at 600 -C is 0.51, which is much lower than the 0.91 of (Al,Ti)N. 3.3. Cutting performance Cutting test was carried out using three carbide ballnose endmills. Two were coated with (Al,Ti)N and (Ti,V)N, and the third was coated with a double layer of (Al,Ti)N/(Ti,V)N. The top and boundary regions of the cutting edges were investigated for flank wear. Fig. 10 shows photographs of the damage in the top region at a cutting length of 50 m. The (Al,Ti)N-coated endmill had good wear resistance in the boundary region, but the top region had more flank wear than the (Ti,V)N-coated sample. The top region of ball-nose endmills is important because this region affects the surface roughness of the work piece. On the other hand, compared to the (Al,Ti)Ncoated samples, the (Ti,V)N-coated samples showed much less flank wear in the top region but greater damage in the boundary region. This boundary region is important in terms of tool life since damage in this area causes burring of the work piece. Compared to the (Al,Ti)N and (Ti,V)Ncoated endmills, the (Al,Ti)N/(Ti,V)N double-layer-coated
Gain of weight (%)
Vickers hardness Hv0.25N
1.0 3000
2000
1000
(Ti,V)N (Al,Ti)N
0.8 0.6 0.4 0.2 0 0
0 0
0.2
0.4
0.6
0.8
1
200
400
600
800
1000 1200
Temperature (°C)
V(Ti+V) Fig. 7. Vickers hardness of Ti – V – N films as a function of V/(Ti + V).
Fig. 8. Weight gain of (Ti,V)N and (Al,Ti)N films oxidized in atmospheric conditions.
N. Ichimiya et al. / Surface & Coatings Technology 200 (2005) 1377 – 1382
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Friction Coefficient
1.0 0.8 0.6 0.4 (Ti,V)N (Al,Ti)N
0.2 0 0
200
400
600
800
Temperature (°C) Fig. 9. Friction coefficient of (Ti,V)N and (Al,Ti)N for carbon steel AISI 1055 at temperatures up to 600 -C.
endmill had much better resistance to wear in both the top and boundary regions, as shown in Fig. 10(c).
4. Discussions All of the Tix V1 – x N films in this study had the same cubic B1 crystal structures and a lattice constant that decreased as V/(Ti + V) increased. This indicates that as V/(Ti + V) increases, a larger amount of vanadium is solid-soluted into TiN crystal. The ion radius of vanadium is smaller than that of titanium and this mismatch of ion radii in Tix V1 – x N causes distortions in the crystal. (Ti,V)N film is hardest at around V/(Ti + V) = 0.25 to 0.5. This means the distortion is highest in this range. The origin of the high hardness of (Ti,V)N is similar to that of (Al,Ti)N, which is hardest around Al/(Al + Ti)=0.6, but the hardness is slightly less than the 2800 Hv of (Al,Ti)N. This is explained by the fact that the ion radii
Top region
Fig. 11. Depth profile of (a) (Al,Ti)N and (b) (Ti,V)N films after cutting carbon steel. Profile determined by Auger electron spectroscopy.
of Ti and V are more similar in size than are the ion radii of Ti and Al. The (Ti,V)N-coated endmills had much less top region flank wear than the (Al,Ti)N-coated endmills. This wear seems to be related to the result of the sliding test. In the top region of the ball-nosed endmill, the cutting speed decreases to zero at the center of the cutting edge, and the adhesive wear in this region is obvious. Work materials that have particularly low hardness, such as the AISI 1055 used in this experiment, cause great damage in this area. The friction coefficients of (Ti,V)N film were much lower than that of (Al,Ti)N and they are almost constant through the wide temperature range as shown in Fig. 9. This property results in less adhesive damage of the (Ti,V)N-coated endmill, especially in the top region. On the other hand, the (Ti,V)N-coated endmill had much greater flank wear in the boundary region than the (Al,Ti)Ncoated endmill. (Ti,V)N film is not so as hard as (Al,Ti)N,
Boundary region
(a) (Al,Ti)N
(b) (Ti,V)N
(c) (Al,Ti)N+(Ti,V)N
Fig. 10. Photos of damaged-coated carbide endmills. The films deposited on the tools are, respectively, (a) (Al,Ti)N, (b) (Ti,V)N, (C) a double layer of (Al,Ti)N and (Ti,V)N.
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however, this difference in hardness does not seem to be the cause of the large difference of flank wear. In the boundary region, the cutting speed is highest in the cutting operation. In this situation, the hardness and the anti-oxidation property are critical to the performance of the cutting tool. The results of the oxidation test, shown in Fig. 8, indicate that (Ti,V)N film starts oxidizing at a lower temperature than (Al,Ti)N. Moreover, after oxidation, the surface of the (Ti,V)N film changes to oxides of titanium and vanadium. These oxides have low hardness, so this oxidation decreases the hardness of the films. It is interesting to note that previous papers [5,7] have reported that (Ti,V)N-coated cutting tools perform better than (Ti,Al)N-coated ones. But in the present study, (Ti,V)N performed much worse in the boundary region of the cutting edge. In this study, the cutting test was carried out at the high cutting speed of 300 m/min without a liquid coolant. While further investigation is needed of cutting conditions, such as cutting speed or the use of coolant, the cutting conditions in this study seem to be more severe than those of previous studies. This explains the larger amount of wear in the boundary region, which is more sensitive to antioxidation property. To determine the reason of (Ti,V)N-coated endmill’s good performance in the top region, the endmills coated with (Al,Ti)N and (Ti,V)N film were cut for 25 m, then the depth distribution of the elements in the film was analyzed by means of Auger electron microscopy. The depth profile of the analysis is shown in Fig. 11. In the (Al,Ti)N-coated endmill, iron was detected only on the surface of the film, but in the (Ti,V)N films, iron had penetrated deep into the film. (Al,Ti)N films seem to be damaged when the adhered iron peeled off. This peeling phenomena do not occur in (Ti,V)N film. And the penetration of the iron into the film may strengthen the film against the peeling off and reduce the friction coefficient. This reduces adhesive damage to (Ti,V)N films. The results described above show that ball-nosed endmills require hardness and anti-oxidation properties in the boundary region where the cutting speed is relatively high, and low friction and anti-adhesion properties in the top region where the cutting speed approaches zero. (Al,Ti)N and (Ti,V)N, respectively, meet these requirements. And (Al,Ti)N/(Ti,V)N double-layer films produce good cutting tools with well-balanced performance. These coated cutting
tools offer high quality mold production, long tool life and high productivity in manufacturing, especially in the machining of relatively soft materials such as carbon steels.
5. Conclusions Investigations on the properties and cutting performance of (Ti,V)N films prepared by the arc ion plating were carried out and conclusions drawn are as follows: 1. (Ti,V)N has a cubic B1 structure over the entire range of V/(Ti,V)N and a dense columnar microstructure that is typical of the films deposited by the cathodic arc ion plating method. 2. (Ti,V)N film is hardest—approximately 2600 Hv at around V/(Ti + V) = 0.25 – 0.5. (Ti,V)N has excellent friction properties for AISI 1055 carbon steel. 3. (Ti,V)N-coated endmills offer good performance, especially in the top region of the ball-nosed endmill, when cutting carbon steel. Endmills coated with a combination of (Al,Ti)N and (Ti,V)N provide well-balanced cutting performance.
References [1] Y. Yamada, T. Aoki, Y. Tanaka, H. Hayasaki, S. Motonishi, Proceedings of the Third International Conference of Progress of Cutting and Grinding, 1996, p. 211. [2] Y. Yamada, T. Aoki, S. Kitaura, Y. Tanaka, Y. Okazaki, H. Hayasaki, Proceedings of 1st French and German Conference of High Speed Machining, 1997, p. 486. [3] Y. Tanaka, N. Ichimiya, Y. Onishi, Y. Yamada, Surf. Coat. Technol. 146 – 147 (2001) 215. [4] O. Knotek, A. Barimani, B. Bosserhoff, F. Lo¨ffler, Thin Solid Films 193 – 194 (1990) 557. [5] O. Knotek, F. Lo¨ffler, G. Kra¨mer, Surf. Coat. Technol. 54 – 55 (1992) 241. [6] O. Knotek, W. Burgmer, C. Stoessel, Surf. Coat. Technol. 54 – 55 (1992) 249. [7] O. Knotek, F. Lo¨ffler, G. Kra¨mer, Surf. Coat. Technol. 59 (1993) 14. [8] H. Hasegawa, A. Kimura, T. Suzuki, J. Vac. Sci. Technol., A 18 (3) (2000) 1038. [9] T. Wada, T. Toihara, J. Jpn. Soc. Powder Powder Metall. 49 (11) (2002) 984. [10] T. Ikeda, H. Satou, Thin Solid Films 195 (1991) 99.
Properties and cutting performance of (Ti,V)N coatings prepared by cathodic arc ion plating N. Ichimiya a,*, Y. Onishi b, Y. Tanaka a b
a Mitsubishi Materials Corporation, 1511, Furumagi, Ishige-machi, Yuuki-gun, Ibaraki 300-2795, Japan Mitsubishi Materials Kobe Tools Corporation, 179-1, Nishioike, Kanagasaki, Uozumicho, Akashi, Hyogo 674-0071, Japan
Available online 3 October 2005
Abstract (Ti,V)N coatings were deposited on WC – Co substrates using the cathodic arc ion plating method. The structures and compositions were characterized by X-ray diffraction, transmission electron microscopy, and electron probe microanalysis. The (Ti,V)N coating showed a single phase cubic B1 structure with a dense columnar microstructure typical of cathodic arc ion plating. The wear-resistant properties and friction coefficients of (Ti,V)N coatings against steel were evaluated using the ball-on-disc test at elevated temperatures. (Ti,V)N had a friction coefficient of 0.51 against carbon steel at 600 -C, which is lower than the 0.91 friction coefficient of (Ti,Al)N. High-speed cutting performance was investigated using coated carbide endmills. (Ti,V)N coatings showed greatly improved cutting performances on low hardness carbon steels. The wear mechanism and cutting characteristics are discussed in terms of the structure and properties of the films. D 2005 Elsevier B.V. All rights reserved. Keywords: Cathodic arc ion plating; Titanium vanadium nitride; Cutting performance
1. Introduction The (Ti,Al)N coating, deposited by means of the physical vapor deposition (PVD) method, is a promising wearresistant film for cutting tools such as solid carbide endmills, drills, and inserts. Therefore, much research is focusing on this material. (Ti,Al)N-coated solid carbide endmills permit direct milling of hardened dies and mold steels without rough machining before heat treatment [1,2], contributing to improved productivity in the manufacture of dies and molds. Furthermore, for the demand of machining harder work materials at a higher cutting speed, addition of other elements to (Ti,Al)N has been investigated [3]. On the other hand, relatively soft materials, such as carbon steel, are widely used to make molds for plastic goods and general machine parts. However, little research has been done on coatings for cutting tools used to cut this material. This study investigates the properties and cutting performance of (Ti,V)N coatings for soft steels. * Corresponding author. Tel.: +81 78 936 7405; fax: +81 78 936 1497. E-mail address: [email protected] (N. Ichimiya). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.08.026
Some research on this ternary coating film has already been published. Knotek et al. investigated two methods of synthesizing this material: magnetron sputtering and arc ion plating. In both cases, (Ti,V)N-coated indexable inserts exhibited excellent performance in turning operations for cutting steels [4– 7]. Hasegawa et al. compared (Ti,V)N film to other titanium-based ternary nitride films [8]. Wada et al., also reported on the properties of (Ti,V)N films and their cutting performance when turning stainless steel [9]. This paper describes the structure and properties of (Ti,V)N films prepared by the arc ion plating method and the cutting performance of a (Ti,V)N-coated solid carbide endmill for milling carbon steel.
2. Experimental procedure 2.1. Sample preparation Samples were prepared by the cathodic arc ion plating method using the equipment shown in Fig. 1. Cathodic ion
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N. Ichimiya et al. / Surface & Coatings Technology 200 (2005) 1377 – 1382
Reactive Gas Vacuum Chamber Arc Power Supply
N2 Target +
+ +
(-)
Substrate
+
(-)
+ +
+
TiV
TiV Rotating Table
(-) Bias Power Supply
Fig. 1. Schematic diagram of the arc ion plating equipment used in this study.
plating is a high-energy deposition process that employs a vacuum arc to generate ionized vapor from target materials. The substrate is negatively biased with respect to the anode and chamber, and positive ions generated from the target are deposited on the substrate. In this study, different atomic ratios of Ti –V targets prepared by a powder metallurgical technique were used for the deposition. WC – Co carbide (ISO M20) insert (SNGN120408) was used as the substrate. To evaluate the oxidation behavior, platinum foil was used as a substrate to avoid the effect of substrate oxidation. Before insertion into the chamber, the substrates were ultrasonically cleaned with an organic solvent. The ultimate base pressure was 4 10 3 Pa and the substrate was heated to 450 -C using a radiation heater installed in the chamber. After the substrate was sputter cleaned by ion bombardment, deposition was carried out in an atmosphere of nitrogen in the pressure range of 2– 5 Pa, with a negative substrate bias of 50 V and a substrate temperature of 450 -C. 2.2. Characterization and evaluation of the films
Fig. 2. X-ray diffraction patterns at various V/(Ti + V) values.
was determined by h – 2h X-ray diffraction (XRD) using a JEOL JDX-3500 X-ray system with CuKa (k = 0.1542 nm). The hardness of the films was determined by means of a Vickers micro-indentation test at a load of 0.25 N. The films used for the hardness measurement were deposited on a WC – Co substrate to a film thickness of 5 Am. The dimensions of the indentation were measured using an optical microscope and a scanning electron microscope (SEM). The microstructure of the films was investigated by means of a transmission electron microscope (JEOL JEM2010F) with an acceleration voltage of 200 kV. The films used for TEM observation were deposited on WC – Co substrates with a film thickness of 4 Am. Electron diffraction was also carried out in the TEM to determine the crystal
The compositions of the samples were determined by electron probe microanalysis (EPMA). The crystal structure
Endmill
Ball-nose carbide endmill Conventional shape with 2 flutes Size: R5 mm (u10 mm)
Thickness of coating (Am) Work material Cutting speed (m/min) Feed rate (mm/tooth) Depth of cut Pf (mm) Ad (mm) Cutting direction
3 AISI 1055 (220 HB) 308 m/min (10,000 min 1) 0.05 mm/tooth (1000 mm/min) 0.5 mm 4.0 mm Downcut milling
Lattice constant (nm)
0.430 Table 1 Cutting conditions for evaluating coated endmills
0.420
0.410
0.400 0
0.2
0.4
0.6
0.8
1
V(Ti+V) Fig. 3. Lattice constant of Ti – V – N coatings as a function of V/(Ti + V).
N. Ichimiya et al. / Surface & Coatings Technology 200 (2005) 1377 – 1382
(a)
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(b)
Fig. 4. Plan view TEM images of (a) (Ti,V)N and (b) (Al,Ti)N films.
structure of local points of the samples. The TEM samples were prepared by means of the focused ion beam (FIB) technique using a JEOL JFIB-2100 system. The samples were thinned to approximately 0.1 Am by Ga ions with the an acceleration voltage of 30 kV and a Ga ion source current of 2.0 AA. The oxidation behavior of the Ti –V – N coatings was investigated by means of annealing tests. Films with a thickness of 4 Am were deposited on platinum foils measuring 5 mm 20 mm 0.02 mm. The samples were heated in air at a rate of 10 K/min in an electric furnace at temperatures up to 1100 -C. The weights of the samples were measured and compared with those of as-deposited films. To evaluate the tribological properties, ball-on-disc tests were carried out. The sample films were deposited on a WC – Co ball with a diameter of 6.35 mm. The friction coefficient against a polished disc of carbon steel (AISI 1055) was measured at temperatures up to 600 -C, a sliding speed of 5 m/min, and a load of 1 N. 2.3. Cutting performance Cutting performance was investigated using two-flute, ball-nose, carbide endmills with a diameter of 10 mm and coated with (Ti,V)N, (Al,Ti)N and double layer of (Al,Ti)N/(Ti,V)N. The film was 3 Am thick and the work material was AISI 1055 with a hardness of 220 HB. The cutting edge was examined for wear using an optical microscope. Details of the cutting test conditions are listed in Table 1.
(a)
3. Results 3.1. Structure of the films The Ti/V compositions of the films deposited using different Ti1 – x Vx targets (x = 0, 0.25, 0.5, 0.75, 1) are quite similar to compositions of the targets themselves. This is typical of the cathodic arc ion plating method, which generates high ion energy during deposition. As the V content in the film increased, the color of the film changed from the gold of TiN to light yellow and eventually to the light grey of VN. The crystal structures of Ti – V – N films, examined by XRD, are shown in Fig. 2. All samples with vanadium content have a cubic B1 structure. The lattice constants are plotted in Fig. 3 as a function of V/(Ti + V). The lattice constant of TiN is 0.424 nm. This value decreases steadily as V/(Ti + V) increases. At V(Ti + V) = 1, the lattice constant for VN is 0.414 nm. This indicates that increasing V/ (V + Ti) causes more vanadium ions to occupy the sites of the Ti ions. Because the Vatom ions are smaller in size than the Ti ions, the lattice constant decreases when the amount of V/ (V + Ti) increases. Figs. 4– 6 show cross-sectional and plan view TEM images and electron diffraction patterns of the (Ti,V)N and (Al,Ti)N films. Both films had dense columnar microstructures typical of cathodic arc ion plating. However, (Ti,V)N had a grain size of approximately 50 nm, which is smaller than the100 nm grain size of (Al,Ti)N. (TiV)N had a broader electron diffraction pattern than (Al,Ti)N, indicating that (Ti,V)N had smaller grains. This is consistent with the TEM results.
(b) 200nm
Fig. 5. Cross-sectional TEM images of (a) (Ti,V)N and (b) (Al,Ti)N films.
200nm
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N. Ichimiya et al. / Surface & Coatings Technology 200 (2005) 1377 – 1382
(a)
(b) (222)
(311) (220)
(200) (111) (220)
(111) (200)
Fig. 6. Electron diffraction patterns of (a) (Ti,V)N and (b) (Al,Ti)N films.
3.2. Properties of the films The hardness measurements are shown in Fig. 7. TiN film had a hardness of 2000 Hv, which is similar to the results of previous studies [6,8–10]. As V/(Ti + V) increased up to V/ (Ti + V) = 0.5, the hardness of the films increased. As V/ (Ti + V) increased beyond 0.5, the hardness decreased to 1800 Hv. Between V(Ti + V) values of 0.25 and 0.5, the films displayed the highest hardness – up to approximately 2600 Hv – which is a little lower than that of the (Al,Ti)N coating that has been reported to be 2800 Hv. Wada et al. reported that the highest film hardness values for V/(V + Ti) were between 0.25 to 0.5 [9], which is consistent with the results of this study. Oxidation tests were carried out on the (Ti,V)N and (Al,Ti)N films. Fig. 8 shows the weight gain of samples treated at high temperatures in atmospheric conditions. The results show that (Ti,V)N film starts to increase in weight at around 600 -C. After heat treatment at 500 -C for 1 h, the (Ti,V)N film subjected to XRD analysis, which revealed V and (Ti,V) oxides. This indicates that the weight gain comes from oxidation of the (Ti,V)N film. (Al,Ti)N film started to increase in weight at a higher temperature: around 800 -C. This agrees well with previous researches [3,10], which suggest that the thin aluminum oxide film formed on the surface of the film functions as an oxidation barrier. (Ti,V)N films have the same high hardness as (Al,Ti)N film, but with inferior anti-oxidation properties. To investigate the friction properties (Ti,V)N films, ballon-disc sliding tests were carried out at the temperature from
200 -C to 600 -C. To be compared with (Ti,V)N, the friction coefficient of (Al,Ti)N film was also measured at 600 -C. The results are plotted in Fig. 9. (Ti,V)N film had low friction coefficient of approximately 0.6. Friction coefficient of (Ti,V)N at 600 -C is 0.51, which is much lower than the 0.91 of (Al,Ti)N. 3.3. Cutting performance Cutting test was carried out using three carbide ballnose endmills. Two were coated with (Al,Ti)N and (Ti,V)N, and the third was coated with a double layer of (Al,Ti)N/(Ti,V)N. The top and boundary regions of the cutting edges were investigated for flank wear. Fig. 10 shows photographs of the damage in the top region at a cutting length of 50 m. The (Al,Ti)N-coated endmill had good wear resistance in the boundary region, but the top region had more flank wear than the (Ti,V)N-coated sample. The top region of ball-nose endmills is important because this region affects the surface roughness of the work piece. On the other hand, compared to the (Al,Ti)Ncoated samples, the (Ti,V)N-coated samples showed much less flank wear in the top region but greater damage in the boundary region. This boundary region is important in terms of tool life since damage in this area causes burring of the work piece. Compared to the (Al,Ti)N and (Ti,V)Ncoated endmills, the (Al,Ti)N/(Ti,V)N double-layer-coated
Gain of weight (%)
Vickers hardness Hv0.25N
1.0 3000
2000
1000
(Ti,V)N (Al,Ti)N
0.8 0.6 0.4 0.2 0 0
0 0
0.2
0.4
0.6
0.8
1
200
400
600
800
1000 1200
Temperature (°C)
V(Ti+V) Fig. 7. Vickers hardness of Ti – V – N films as a function of V/(Ti + V).
Fig. 8. Weight gain of (Ti,V)N and (Al,Ti)N films oxidized in atmospheric conditions.
N. Ichimiya et al. / Surface & Coatings Technology 200 (2005) 1377 – 1382
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Friction Coefficient
1.0 0.8 0.6 0.4 (Ti,V)N (Al,Ti)N
0.2 0 0
200
400
600
800
Temperature (°C) Fig. 9. Friction coefficient of (Ti,V)N and (Al,Ti)N for carbon steel AISI 1055 at temperatures up to 600 -C.
endmill had much better resistance to wear in both the top and boundary regions, as shown in Fig. 10(c).
4. Discussions All of the Tix V1 – x N films in this study had the same cubic B1 crystal structures and a lattice constant that decreased as V/(Ti + V) increased. This indicates that as V/(Ti + V) increases, a larger amount of vanadium is solid-soluted into TiN crystal. The ion radius of vanadium is smaller than that of titanium and this mismatch of ion radii in Tix V1 – x N causes distortions in the crystal. (Ti,V)N film is hardest at around V/(Ti + V) = 0.25 to 0.5. This means the distortion is highest in this range. The origin of the high hardness of (Ti,V)N is similar to that of (Al,Ti)N, which is hardest around Al/(Al + Ti)=0.6, but the hardness is slightly less than the 2800 Hv of (Al,Ti)N. This is explained by the fact that the ion radii
Top region
Fig. 11. Depth profile of (a) (Al,Ti)N and (b) (Ti,V)N films after cutting carbon steel. Profile determined by Auger electron spectroscopy.
of Ti and V are more similar in size than are the ion radii of Ti and Al. The (Ti,V)N-coated endmills had much less top region flank wear than the (Al,Ti)N-coated endmills. This wear seems to be related to the result of the sliding test. In the top region of the ball-nosed endmill, the cutting speed decreases to zero at the center of the cutting edge, and the adhesive wear in this region is obvious. Work materials that have particularly low hardness, such as the AISI 1055 used in this experiment, cause great damage in this area. The friction coefficients of (Ti,V)N film were much lower than that of (Al,Ti)N and they are almost constant through the wide temperature range as shown in Fig. 9. This property results in less adhesive damage of the (Ti,V)N-coated endmill, especially in the top region. On the other hand, the (Ti,V)N-coated endmill had much greater flank wear in the boundary region than the (Al,Ti)Ncoated endmill. (Ti,V)N film is not so as hard as (Al,Ti)N,
Boundary region
(a) (Al,Ti)N
(b) (Ti,V)N
(c) (Al,Ti)N+(Ti,V)N
Fig. 10. Photos of damaged-coated carbide endmills. The films deposited on the tools are, respectively, (a) (Al,Ti)N, (b) (Ti,V)N, (C) a double layer of (Al,Ti)N and (Ti,V)N.
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however, this difference in hardness does not seem to be the cause of the large difference of flank wear. In the boundary region, the cutting speed is highest in the cutting operation. In this situation, the hardness and the anti-oxidation property are critical to the performance of the cutting tool. The results of the oxidation test, shown in Fig. 8, indicate that (Ti,V)N film starts oxidizing at a lower temperature than (Al,Ti)N. Moreover, after oxidation, the surface of the (Ti,V)N film changes to oxides of titanium and vanadium. These oxides have low hardness, so this oxidation decreases the hardness of the films. It is interesting to note that previous papers [5,7] have reported that (Ti,V)N-coated cutting tools perform better than (Ti,Al)N-coated ones. But in the present study, (Ti,V)N performed much worse in the boundary region of the cutting edge. In this study, the cutting test was carried out at the high cutting speed of 300 m/min without a liquid coolant. While further investigation is needed of cutting conditions, such as cutting speed or the use of coolant, the cutting conditions in this study seem to be more severe than those of previous studies. This explains the larger amount of wear in the boundary region, which is more sensitive to antioxidation property. To determine the reason of (Ti,V)N-coated endmill’s good performance in the top region, the endmills coated with (Al,Ti)N and (Ti,V)N film were cut for 25 m, then the depth distribution of the elements in the film was analyzed by means of Auger electron microscopy. The depth profile of the analysis is shown in Fig. 11. In the (Al,Ti)N-coated endmill, iron was detected only on the surface of the film, but in the (Ti,V)N films, iron had penetrated deep into the film. (Al,Ti)N films seem to be damaged when the adhered iron peeled off. This peeling phenomena do not occur in (Ti,V)N film. And the penetration of the iron into the film may strengthen the film against the peeling off and reduce the friction coefficient. This reduces adhesive damage to (Ti,V)N films. The results described above show that ball-nosed endmills require hardness and anti-oxidation properties in the boundary region where the cutting speed is relatively high, and low friction and anti-adhesion properties in the top region where the cutting speed approaches zero. (Al,Ti)N and (Ti,V)N, respectively, meet these requirements. And (Al,Ti)N/(Ti,V)N double-layer films produce good cutting tools with well-balanced performance. These coated cutting
tools offer high quality mold production, long tool life and high productivity in manufacturing, especially in the machining of relatively soft materials such as carbon steels.
5. Conclusions Investigations on the properties and cutting performance of (Ti,V)N films prepared by the arc ion plating were carried out and conclusions drawn are as follows: 1. (Ti,V)N has a cubic B1 structure over the entire range of V/(Ti,V)N and a dense columnar microstructure that is typical of the films deposited by the cathodic arc ion plating method. 2. (Ti,V)N film is hardest—approximately 2600 Hv at around V/(Ti + V) = 0.25 – 0.5. (Ti,V)N has excellent friction properties for AISI 1055 carbon steel. 3. (Ti,V)N-coated endmills offer good performance, especially in the top region of the ball-nosed endmill, when cutting carbon steel. Endmills coated with a combination of (Al,Ti)N and (Ti,V)N provide well-balanced cutting performance.
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