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Cutting characteristics of PVD-coated tools deposited by Unbalanced Magnetron Sputtering method
CIRP Annals - Manufacturing Technology 61 (2012) 95–98
Contents lists available at SciVerse ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
Cutting characteristics of PVD-coated tools deposited by Unbalanced Magnetron Sputtering method Akira Hosokawa a, Koji Shimamura b, Takashi Ueda (1)a,* a b
Faculty of Mechanical Engineering, Institute of Science and Engineering, Kanazawa University, Japan Division of Innovative Technology and Science, Graduate School of Natural Science & Technology, Kanazawa University, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords: Cutting tool Coating Milling
The cutting characteristics of newly proposed PVD coated tools by UBMS (Unbalanced Magnetron Sputtering) method are investigated. Dry side milling tests of austenitic stainless steel (AISI 304) and Ti– 6Al–4V alloy (ASTM B348) are carried out with five kinds of coated tools: two UBMS coated tools and three commonly used AIP (Arc Ion Plating) coated ones. The UBMS coated films, especially UBMS-TiCN have smoother surface without droplets and lower friction coefficient than those of any other AIP coated tools studied. The lubricating ability of the UBMS-TiCN film reduces the cutting force, cutting temperature and adhesion of chips, so that long tool life and good finished surface are obtained. ß 2012 CIRP.
1. Introduction Difficult-to-machine materials such as stainless steel, titanium alloy and nickel-base alloy are widely used in the aerospace and nuclear industry, and there is an increasing requirement for the highly efficient machining of these materials. Due to the characteristics of these difficult-to-machine materials such as low thermal conductivity, high work-hardening and high affinity with a cutting tool, the tool life is very short and the dimensional accuracy and the good finished surface roughness required cannot be obtained in many cases [1]. On the other hand, both environmental and economical benefits have encouraged dry machining, because the coolant or lubricant mist has an adverse effect on human health and the costs for fluid procurement, maintenance and disposal should be taken into account [2]. In cutting without coolant, however, the cutting tool temperature is extremely high compared with that with wet cutting, so that the cutting tool is damaged rapidly. Therefore it is common to use appropriate coated cutting tool on which various kinds of hard materials are deposited. Typical hard coating films for cutting difficult-to-machine materials in the present stage are PVD-coated TiAlN and/or AlCrN containing AlN [3,4]. These films are deposited by Arc Ion Plating (AIP) method, in which evaporated and ionized target materials by vacuum arc discharge are deposited on the substrate [5]. This type of film has fine structure and adhesion strength, which leads to high heat resistance and then they are effective in milling of hardened steel [6]. However, their wear resistance is not high enough to achieve high productivity and surface integrity in cutting difficult-to-machine-materials. In this study, another type of coated films by Unbalanced Magnetron Sputtering (UBMS) method are developed, and are applied to high-speed milling of difficult-to-machine materials. In
* Corresponding author. 0007-8506/$ – see front matter ß 2012 CIRP. http://dx.doi.org/10.1016/j.cirp.2012.03.010
UBMS, atoms and/or molecules of the target materials are sputtered by Ar ions in the grow discharge plasma and deposits a film on the substrate [7,8]. These coated tools are designed to reduce cutting force and cutting temperature by smooth surface and low friction coefficient. In the present paper, two types of new coated tools by UBMS method are developed and the cutting performance of these tools and some conventional AIP tools are examined in high-speed dry milling of an austenitic stainless steel (AISI 304) and Ti–6Al–4V alloy (ASTM B348). 2. Characteristics of coating films 2.1. Principle of physical vapor deposition (PVD) process Fig. 1 illustrates PVD coating methods to form metal nitride film on a substrate; Arc Ion Plating (AIP) method and Unbalanced Magnetron Sputtering (UBMS) method. In AIP method, arc discharge occurs between a target (cathode) and an anode in a vacuum chamber, and the target metal is evaporated and ionized. If N2 gas is introduced in the chamber (60 mL/min), the ionized metal vapor reacts with nitrogen to form metal-nitride film on a negatively biased substrate. Due to the high ionization ratio (30–80%) of evaporated materials, the deposited film has a high density and good adhesion strength [5]. In UBMS method, glow discharge is induced between the target and the anode by applying a high voltage in a vacuum chamber. The magnets behind the metal target confine the high-density plasma in the vicinity of target metal. Ar+ ions generated in the glow discharge plasma bombard the target metal to cause sputtering of target metal atoms and/or molecules [9]. The sputtered particles react with nitrogen and form thin metal-nitride film on the substrate. Since only atoms and/or molecules are sputtered from the target metal, UBMS method can make smooth thin film without droplets. In the present study, five types of coating films are formed as shown in Table 1. Two kinds of well-established coating materials:
A. Hosokawa et al. / CIRP Annals - Manufacturing Technology 61 (2012) 95–98
96
Table 2 Physical properties of coating films. Sample
Hardness, HNI (GPa)
Critical load in scratch test, Pc (N)
Surface roughness, Rz (mm)
Friction coefficient,
m
Oxidation temp., To (8C)
U-TiN U-TiCN
35 31
142 148
0.10 0.11
0.62 0.19
600 500
A-TiN A-TiCN A-TiAlN
28 35 37
151 116 140
0.53 0.63 0.69
0.60 0.61 0.62
600 500 800
Fig. 1. Two types of PVD coating methods.
2.2. Physical properties and structure of coating films The physical properties of coating films are evaluated using the block test pieces, the materials and structures are the same as the end mills used. The hardness is measured with a nano indentation tester. The indentation load is set at 40 mN so that the maximum indentation depth does not exceed 10% of coating film thickness. The adhesive strength is evaluated by a scratch tester. The scratch test is conduced by drawing scratches with a Rockwell C diamond stylus on the surface of coating films at a load between 1 and 200 N. The friction coefficient is measured dry by using a ball-on-disk friction tester with AISI-SAE 52100 ball. The structure of two kinds of TiCN-films is analyzed by an X-ray Photoelectron Spectroscopy (XPS) and FE-TEM observation in order to compare the carbon linkage in these two films. Mechanical properties Table 2 shows the physical properties of coating films. The hardness of both UBMS-type films is 31–35 GPa, which is approximately the same as that of the AIP-type films. The critical load causing the peeling of UBMS-type films is also the same as that of AIP-type films. It is noted that the surface roughness Rz of UBMS-type films is 0.10–0.11 mm, which is considerably smaller than that of AIP-type films (0.53–0.69 mm). Fig. 2 shows the 3D profiles of the TiCN-coating film surfaces. In the case of AIP method, many lugs and pinholes are formed on the surface because droplets are emitted from the target metal during arc discharging [5]. However we may form droplet-free smooth surface with UBMS method as shown in Fig. 2(b). In Table 2, the U-TiCN surface has an Table 1 Five types of coating samples. Sample
Process
Structure(film thickness)
U-TiN U-TiCN
UBMS UBMS
TiN (3 mm) TiN (1.5 mm) + TiCN (1.5 mm)
A-TiN A-TiCN A-TiAlN
AIP AIP AIP
TiN (3 mm) TiN (1.5 mm) + TiCN (1.5 mm) TiAlN (3 mm)
Fig. 2. 3D profiles of TiCN-coating film surface.
extremely low friction coefficient of 0.19 compared to U-TiN and all AIP-type films, indicating that the formation of carbon in UBMS film is effective in reducing the friction coefficient. The oxidation temperatures of five coating films are also shown in Table 2. The oxidation temperature is determined by heating the specimens in a furnace at a temperature between 200 and 800 8C for 5 h. Here the oxidation is judged by TiO2 formation with X-ray diffraction analysis and the composition change with EPMA. TiAlN film has the highest oxidation temperature of 800 8C and the lowest is TiCN film, implying that a low cutting temperature is required in cutting a difficult-to-machine material with U-TiCN coated tool. Structures analysis Fig. 3 demonstrates the Carbon 1s (C1s) spectra of two kinds of TiCN films. The C1s spectra of both U-TiCN and A-TiCN have two peaks at 281.7 eV (Ti–C bond) and 285.0 eV (C–C bond). A-TiCN has a stronger peak of Ti-C bond, whereas U-TiCN has the other peak of C–C bond. From these results, it is conceivable that the U-TiCN film contains many free carbons rather than carbide TiC, which may act as lubricant as well as thermoprotective film. The metal particles evaporated by UBMS method do not carry any electrical charge and the energy is much smaller than the particles generated during the formation of AIP-type films. Therefore, the formation of Ti–C bonds is suppressed and hence the formation of C–C bonds prevails. 3. Experimental procedure 3.1. Cutting tests Fig. 4 shows the experimental setup. The side milling tests without coolant of austenite stainless steel (AISI 304) and Ti–6Al–
C1s
C–C (285.0 eV)
C–Ti (281.7 eV)
XPS intensity Ix cps
TiN and TiCN, and one of the most heat-resistant material: TiAlN are selected. The UBMS-type and AIP-type films are designated as ‘U-’ and ‘A-’, respectively. The U-TiN film is deposited on the substrate by reactive sputtering of Ti target in Ar/N2 mixtures, and U-TiCN film is done in Ar/N2/CH4 mixtures. The film thickness of all coating films is 3 mm. It is noted that the U-TiCN and A-TiCN have a double-layered structure, in which TiCN is formed on TiN layer in order to improve the adhesion strength of TiCN film on the base material.
290
U–TiCN
A–TiCN 288
286
284
282
280
Binding Energy Eb eV Fig. 3. C1s XPS spectra of TiCN coating films.
278
A. Hosokawa et al. / CIRP Annals - Manufacturing Technology 61 (2012) 95–98
Fig. 4. Experimental setup. Table 3 Experimental conditions. Workpiece Cutting tool Operating parameters Cutting speed Feed Axial depth of cut Radial depth of cut Cutting style
AISI 304 ASTM B348 Coated carbide square end mill (f 2 mm, 2-flute, helix angle: 308) v f Ad Rd
50, 220 m/min 50 m/min 0.005 mm/tooth 0.005 mm/tooth 1 mm 1 mm 0.05 mm 0.05 mm Up-cut without coolant
4V alloy (ASTM B348) are carried out using a 3-axis vertical machining center. Five types of coated carbide end mills having the same base material and geometrical specifications are employed. The value of cutting speed is standard speed v = 50 m/min and high speed v = 220 m/min at the same feed-per-tooth of f = 0.005 mm/ tooth. The wear of end mills is examined using a scanning electron microscope (SEM). The chip morphologies and formation process are evaluated by a microscope and a high speed camera, respectively. Cutting forces are measured with a piezoelectric quartz dynamometer (KISTLER: 9251A) with bandwidths up to 3.5 kHz in X-direction and 2.5 kHz in Y-direction. The experimental conditions are summarized in Table 3.
97
Fig. 5. Variation of flank wear width with cutting length at higher cutting speed and corresponding surface roughness of workpiece.
the wear of UBMS-types is gradual. Comparing the wear width of all end mills at Lc = 2600 mm, the VB value of the U-TiN tool is approximately 30% smaller than that of A-TiN tool, and the VB of the U-TiCN tool is almost 70% or less than that of the A-TiCN tool. The SEM images of the flank face of TiCN-coated tools at Lc = 2600 mm are also presented in Fig. 5, where the close-up SEM-BEI image of the A-TiCN tool is shown. In A-TiCN, a significant flank wear is observed and the adhesion of the workpiece material on the flank face is seen. In contrast, the wear of U-TiCN tool is extremely small without the adhesion of workpiece material as shown in Fig. 5. The surface roughness Rz at Lc = 2600 mm is listed at the same time in Fig. 5. The finished surfaces milled by the U-TiN and the ATiN tools are irregularly ‘plucked’ and therefore the Rz values are as high as 37.4–39.2 mm. On the other hand, the smooth surface of Rz = 4.4 mm is obtained by the U-TiCN tool as shown in Fig. 5. The high-speed camera observation reveals that large amount of chips adhered to the rake face of the A-TiAlN tool whereas the flying chips are formed in almost every cutting engagement for U-TiCN tool. It can be said that the U-TiCN coated end mill with a low friction coefficient suppresses the adhesion of workpiece material on both rake and flank faces due to the lubrication effect, leading to the generation of stable thin chips and good finished surface.
3.2. Cutting temperature measurement 4.2. Cutting force In this experiment, tool temperature at the flank face is measured using a two-color pyrometer [6,10]. An optical fiber is set horizontally at the circumference of the tool (908 from the cutting point) as shown in Fig. 4, which can receive the infrared energy radiated from the flank face of the cutting tool when it passes in front of the fiber face. The two-color pyrometer is composed of InAs and InSb photodetector, having different spectral sensitivities within 1–3 mm and 3–5.5 mm, respectively. We can determine the temperature of object from the output ratio of these two detectors using the calibration curve obtained through a separate experiment. The pyrometer has a flat response over the frequency of 10 Hz to 400 kHz, so that we can measure the tool temperature with a good degree of accuracy.
Fig. 6 represents the relationship between the cutting length Lc and the resultant cutting force F. Here F is calculated by the 1=2 principal force Fx and the thrust force Fy as ðFx2 þ Fy2 Þ . The typical output signals of dynamometer are also shown in Fig. 6, which clearly shows the spikes during the engagement of each flute. The cutting force given on the ordinate of Fig. 6 is calculated by 1=2 averaging ðFx2 þ Fy2 Þ for ten successive engagements. The cutting
4. Experimental results 4.1. Tool wear and finished surface Fig. 5 shows the relationship between the cutting length Lc and the flank wear width VB at the high cutting speed of v = 220 m/min. The VB value of all end mills increase with an increase in Lc. It is seen in Fig. 5 that the flank wear of UBMS-type end mills is smaller than that of AIP-type end mills, and the wear of U-TiCN is the smallest. In addition, the wear of AIP-type tools, especially A-TiCN and A-TiN, increases rapidly at the early stages of cutting although
Fig. 6. Variation of cutting force with cutting length.
A. Hosokawa et al. / CIRP Annals - Manufacturing Technology 61 (2012) 95–98
Coated material
98
U-TiN
Work: AISI 304 v = 220 m/min f = 0.005 mm/tooth Rd = 0.05 mm Ad = 1 mm
U-TiCN A-TiN A-TiCN A-TiAlN 0
100
200
300
400
Tool flank temperature T
500 o
C
Fig. 7. Tool flank temperature of coated end mills.
Cutting force (Fx2 + Fy2)1/2 N
60
5. Conclusions
Work: Ti6Al4V
50
Lc=26 mm
40
Lc=220 mm
Five types of PVD coated end mills by UBMS method (U-TiN, UTiCN) and AIP method (A-TiN, A-TiCN, A-TiAlN) are developed, and are applied to a high-speed dry milling of AISI 304 and Ti–6Al–4V alloy. The main results obtained are summarized as follows.
30 20 10 0 U-TiN
U-TiCN
summarizes the cutting force F at the cutting length Lc = 26 mm and 220 mm for five kinds of coated tools. The cutting force is considerably larger than the cutting force of AISI 304 steel. In the case of TiN-coated tolls, for example, the cutting force is approximately 26 N, whereas that for milling AISI 304 steel is 12 N or less as shown in Fig. 6. These results may be explained by the physical properties of titanium alloy, such as high hightemperature strength, low plasticity, and especially high reactivity with the tool materials. It should be noted that a significantly high cutting force is necessary for A-TiAlN coated tool and that the UTiCN coated tool gives the lowest value.
A-TiN
A-TiCN
A-TiAlN
Coated materials Fig. 8. Comparison of cutting force in milling of Ti–6Al–4V alloy.
force increases with a cutting length Lc due to the growth of tool wear. Especially in the case of AIP type coated end mills, the cutting force increases fairly rapidly and reaches 19–24 N at Lc = 2600 mm. For the U-TiN coated end mill, the cutting force increases gradually because of the smaller wear rate although the value of F is almost the same as those of the AIP type tools in the early stages of cutting. On the other hand, the cutting force of U-TiCN coated end mill is lower than any other tools in the entire range of cutting length. This is because the tool wear VB and the adhesions of workpiece material on the tool face are suppressed by the low friction coefficient. It is notable that the cutting force of the U-TiCN is the lowest even at the beginning of cutting in spite that the cutting edge geometry is almost the same. This means that the U-TiCN film has a lubrication effect for reducing friction between a tool and work/chip. Consequently, TiCN-coated end mill by UBMS method has the best cutting ability in milling of AISI 304 stainless steel.
1. Both TiN and TiCN coating films deposited by UBMS method have a smooth surface and U-TiCN has a low friction coefficient of around 0.19 which is equivalent to DLC film. 2. U-TiCN film contains many free carbons which form C–C bonds rather than carbides in the state of fine crystals through the analyses of XPS and FE-TEM. 3. Lower friction coefficient reduces the tool wear because of lowering the cutting force and hence cutting temperature. As a result, U-TiCN coated end mill gives the lowest tool flank temperature and smallest flank wear width among all of the coated end mills studied in cutting of AISI 304 stainless steel without coolant. 4. U-TiCN coated end mill with lubricating ability can suppress the adhesion of workpiece material on both rake and flank face, which leads to the generation of good finished surface. This antiadhesive property of U-TiCN coating film is capable of reducing cutting force in milling of Ti–6Al–4V alloy. Acknowledgments The authors are indebted to MTTRF (Machine Tool Technologies Research Foundation) and Mori Seiki Co., Ltd. This research was financially supported from Grant-in-Aid for Scientific Research (No. 22656037) and also in part by a research fund from the Hokuriku Industrial Advancement Center.
4.3. Cutting temperature References Fig. 7 shows the tool temperature for five coated tools when AISI 304 stainless steel is milled. As seen from the figure, the tool temperatures of the TiCN-coated tools are 370 8C or less, whereas those of TiN-coated and TiAlN-coated tools exceed 400 8C. This difference may be explained via the difference in thermal characteristics of coating films. Okada et al. reported that the thermal effusivity of TiCN-coating is lower than any other coating films [6]. The lowest temperature of 310 8C is obtained with the UTiCN-coated tool and is approximately 60 8C, 90 8C and 100 8C lower than those of A-TiCN, U-TiN and A-TiAlN coated tools, respectively. This low cutting temperature of the U-TiCN coated tool may compensate the relatively low thermal resistance (see Table 2). Based on the temperature difference between A-TiCN and U-TiCN coated tools, it is confirmed that the formation of carbon in a coating film, which results in C–C bonds or carbides, plays an important role in cutting performance. Consequently, TiCN-coated end mill by UBMS method showed the best cutting ability for highspeed milling of AISI 304 stainless steel. 4.4. Machining of titanium alloy The cutting experiments of Ti–6Al–4V alloy (ASTM B348) are also conducted at the cutting speed of v = 50 m/min. Fig. 8
[1] Lo´pez de lacalle L-N, Pe´rez J, Llorente J-I, Sa´nchez J-A (2000) Advanced Cutting Conditions for the Milling of Aeronautical Alloys. Journal of Materials Processing Technology 100(1–3):1–11. [2] Weinert K, Inasaki I, Sutherland J-W, Wakabayashi T (2004) Dry Machining and Minimum Quantity Lubrication. Annals of the CIRP 53(2):511–537. [3] Klocke F, Krieg T (1999) Coated Tools for Metal Cutting—Features and Applications. Annals of the CIRP 48(2):515–525. [4] Erkens G, Cremer R, Hamoudi T, Bouzakis K-D, et al, (2004) Properties and Performance of High Aluminum Containing (Ti, Al)N Based Supernitride Coatings in Innovate Cutting Applications. Surface & Coatings Technology 177– 178(30):724–734. [5] Takahara K, Akari K, Kawaguchi H, Tamagaki H (2005) Current and Future PVD Systems and Coating Technologies. KOBELCO Technology Review 26:81–85. [6] Okada M, Hosokawa A, Tanaka R, Ueda T (2011) Cutting Performance of PVD Coated and CBN Tools in Hardmilling. International Journal of Machine Tools & Manufacture 51(2):127–132. [7] Kelly P-J, Arnell R-D (2000) Magnetron Sputtering: A Review of Recent Developments and Applications. Vacuum 56:159–172. [8] Bouzakis K-D, Skordaris G, Gerardis S, Katirtzoglou G, et al, (2010) The Effect of Substrate Pretreatment and HPPMS-deposited Adhesive Interlayers’ Materials on the Cutting Performance of Coated Cemented Carbide Inserts. Annals of the CIRP 59(1):73–76. [9] Shimamura K, Hosokawa A, Ueda T (2010) Cutting Characteristics of TiCNcoated Carbide End Mill Deposited by Unbalanced Magnetron Sputtering Method—Application to Dry High-speed Milling of SUS304. Proc. 4th CIRP Int. Conf. High Performance Cutting, F25, 337–340. [10] Ueda T, Sato M, Hosokawa A, Ozawa M (2008) Development of Infrared Radiation Pyrometer With Optical Fibers—Two-color Pyrometer with Noncontact Fiber Coupler. Annals of the CIRP 57(1):69–72.
Contents lists available at SciVerse ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
Cutting characteristics of PVD-coated tools deposited by Unbalanced Magnetron Sputtering method Akira Hosokawa a, Koji Shimamura b, Takashi Ueda (1)a,* a b
Faculty of Mechanical Engineering, Institute of Science and Engineering, Kanazawa University, Japan Division of Innovative Technology and Science, Graduate School of Natural Science & Technology, Kanazawa University, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords: Cutting tool Coating Milling
The cutting characteristics of newly proposed PVD coated tools by UBMS (Unbalanced Magnetron Sputtering) method are investigated. Dry side milling tests of austenitic stainless steel (AISI 304) and Ti– 6Al–4V alloy (ASTM B348) are carried out with five kinds of coated tools: two UBMS coated tools and three commonly used AIP (Arc Ion Plating) coated ones. The UBMS coated films, especially UBMS-TiCN have smoother surface without droplets and lower friction coefficient than those of any other AIP coated tools studied. The lubricating ability of the UBMS-TiCN film reduces the cutting force, cutting temperature and adhesion of chips, so that long tool life and good finished surface are obtained. ß 2012 CIRP.
1. Introduction Difficult-to-machine materials such as stainless steel, titanium alloy and nickel-base alloy are widely used in the aerospace and nuclear industry, and there is an increasing requirement for the highly efficient machining of these materials. Due to the characteristics of these difficult-to-machine materials such as low thermal conductivity, high work-hardening and high affinity with a cutting tool, the tool life is very short and the dimensional accuracy and the good finished surface roughness required cannot be obtained in many cases [1]. On the other hand, both environmental and economical benefits have encouraged dry machining, because the coolant or lubricant mist has an adverse effect on human health and the costs for fluid procurement, maintenance and disposal should be taken into account [2]. In cutting without coolant, however, the cutting tool temperature is extremely high compared with that with wet cutting, so that the cutting tool is damaged rapidly. Therefore it is common to use appropriate coated cutting tool on which various kinds of hard materials are deposited. Typical hard coating films for cutting difficult-to-machine materials in the present stage are PVD-coated TiAlN and/or AlCrN containing AlN [3,4]. These films are deposited by Arc Ion Plating (AIP) method, in which evaporated and ionized target materials by vacuum arc discharge are deposited on the substrate [5]. This type of film has fine structure and adhesion strength, which leads to high heat resistance and then they are effective in milling of hardened steel [6]. However, their wear resistance is not high enough to achieve high productivity and surface integrity in cutting difficult-to-machine-materials. In this study, another type of coated films by Unbalanced Magnetron Sputtering (UBMS) method are developed, and are applied to high-speed milling of difficult-to-machine materials. In
* Corresponding author. 0007-8506/$ – see front matter ß 2012 CIRP. http://dx.doi.org/10.1016/j.cirp.2012.03.010
UBMS, atoms and/or molecules of the target materials are sputtered by Ar ions in the grow discharge plasma and deposits a film on the substrate [7,8]. These coated tools are designed to reduce cutting force and cutting temperature by smooth surface and low friction coefficient. In the present paper, two types of new coated tools by UBMS method are developed and the cutting performance of these tools and some conventional AIP tools are examined in high-speed dry milling of an austenitic stainless steel (AISI 304) and Ti–6Al–4V alloy (ASTM B348). 2. Characteristics of coating films 2.1. Principle of physical vapor deposition (PVD) process Fig. 1 illustrates PVD coating methods to form metal nitride film on a substrate; Arc Ion Plating (AIP) method and Unbalanced Magnetron Sputtering (UBMS) method. In AIP method, arc discharge occurs between a target (cathode) and an anode in a vacuum chamber, and the target metal is evaporated and ionized. If N2 gas is introduced in the chamber (60 mL/min), the ionized metal vapor reacts with nitrogen to form metal-nitride film on a negatively biased substrate. Due to the high ionization ratio (30–80%) of evaporated materials, the deposited film has a high density and good adhesion strength [5]. In UBMS method, glow discharge is induced between the target and the anode by applying a high voltage in a vacuum chamber. The magnets behind the metal target confine the high-density plasma in the vicinity of target metal. Ar+ ions generated in the glow discharge plasma bombard the target metal to cause sputtering of target metal atoms and/or molecules [9]. The sputtered particles react with nitrogen and form thin metal-nitride film on the substrate. Since only atoms and/or molecules are sputtered from the target metal, UBMS method can make smooth thin film without droplets. In the present study, five types of coating films are formed as shown in Table 1. Two kinds of well-established coating materials:
A. Hosokawa et al. / CIRP Annals - Manufacturing Technology 61 (2012) 95–98
96
Table 2 Physical properties of coating films. Sample
Hardness, HNI (GPa)
Critical load in scratch test, Pc (N)
Surface roughness, Rz (mm)
Friction coefficient,
m
Oxidation temp., To (8C)
U-TiN U-TiCN
35 31
142 148
0.10 0.11
0.62 0.19
600 500
A-TiN A-TiCN A-TiAlN
28 35 37
151 116 140
0.53 0.63 0.69
0.60 0.61 0.62
600 500 800
Fig. 1. Two types of PVD coating methods.
2.2. Physical properties and structure of coating films The physical properties of coating films are evaluated using the block test pieces, the materials and structures are the same as the end mills used. The hardness is measured with a nano indentation tester. The indentation load is set at 40 mN so that the maximum indentation depth does not exceed 10% of coating film thickness. The adhesive strength is evaluated by a scratch tester. The scratch test is conduced by drawing scratches with a Rockwell C diamond stylus on the surface of coating films at a load between 1 and 200 N. The friction coefficient is measured dry by using a ball-on-disk friction tester with AISI-SAE 52100 ball. The structure of two kinds of TiCN-films is analyzed by an X-ray Photoelectron Spectroscopy (XPS) and FE-TEM observation in order to compare the carbon linkage in these two films. Mechanical properties Table 2 shows the physical properties of coating films. The hardness of both UBMS-type films is 31–35 GPa, which is approximately the same as that of the AIP-type films. The critical load causing the peeling of UBMS-type films is also the same as that of AIP-type films. It is noted that the surface roughness Rz of UBMS-type films is 0.10–0.11 mm, which is considerably smaller than that of AIP-type films (0.53–0.69 mm). Fig. 2 shows the 3D profiles of the TiCN-coating film surfaces. In the case of AIP method, many lugs and pinholes are formed on the surface because droplets are emitted from the target metal during arc discharging [5]. However we may form droplet-free smooth surface with UBMS method as shown in Fig. 2(b). In Table 2, the U-TiCN surface has an Table 1 Five types of coating samples. Sample
Process
Structure(film thickness)
U-TiN U-TiCN
UBMS UBMS
TiN (3 mm) TiN (1.5 mm) + TiCN (1.5 mm)
A-TiN A-TiCN A-TiAlN
AIP AIP AIP
TiN (3 mm) TiN (1.5 mm) + TiCN (1.5 mm) TiAlN (3 mm)
Fig. 2. 3D profiles of TiCN-coating film surface.
extremely low friction coefficient of 0.19 compared to U-TiN and all AIP-type films, indicating that the formation of carbon in UBMS film is effective in reducing the friction coefficient. The oxidation temperatures of five coating films are also shown in Table 2. The oxidation temperature is determined by heating the specimens in a furnace at a temperature between 200 and 800 8C for 5 h. Here the oxidation is judged by TiO2 formation with X-ray diffraction analysis and the composition change with EPMA. TiAlN film has the highest oxidation temperature of 800 8C and the lowest is TiCN film, implying that a low cutting temperature is required in cutting a difficult-to-machine material with U-TiCN coated tool. Structures analysis Fig. 3 demonstrates the Carbon 1s (C1s) spectra of two kinds of TiCN films. The C1s spectra of both U-TiCN and A-TiCN have two peaks at 281.7 eV (Ti–C bond) and 285.0 eV (C–C bond). A-TiCN has a stronger peak of Ti-C bond, whereas U-TiCN has the other peak of C–C bond. From these results, it is conceivable that the U-TiCN film contains many free carbons rather than carbide TiC, which may act as lubricant as well as thermoprotective film. The metal particles evaporated by UBMS method do not carry any electrical charge and the energy is much smaller than the particles generated during the formation of AIP-type films. Therefore, the formation of Ti–C bonds is suppressed and hence the formation of C–C bonds prevails. 3. Experimental procedure 3.1. Cutting tests Fig. 4 shows the experimental setup. The side milling tests without coolant of austenite stainless steel (AISI 304) and Ti–6Al–
C1s
C–C (285.0 eV)
C–Ti (281.7 eV)
XPS intensity Ix cps
TiN and TiCN, and one of the most heat-resistant material: TiAlN are selected. The UBMS-type and AIP-type films are designated as ‘U-’ and ‘A-’, respectively. The U-TiN film is deposited on the substrate by reactive sputtering of Ti target in Ar/N2 mixtures, and U-TiCN film is done in Ar/N2/CH4 mixtures. The film thickness of all coating films is 3 mm. It is noted that the U-TiCN and A-TiCN have a double-layered structure, in which TiCN is formed on TiN layer in order to improve the adhesion strength of TiCN film on the base material.
290
U–TiCN
A–TiCN 288
286
284
282
280
Binding Energy Eb eV Fig. 3. C1s XPS spectra of TiCN coating films.
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Fig. 4. Experimental setup. Table 3 Experimental conditions. Workpiece Cutting tool Operating parameters Cutting speed Feed Axial depth of cut Radial depth of cut Cutting style
AISI 304 ASTM B348 Coated carbide square end mill (f 2 mm, 2-flute, helix angle: 308) v f Ad Rd
50, 220 m/min 50 m/min 0.005 mm/tooth 0.005 mm/tooth 1 mm 1 mm 0.05 mm 0.05 mm Up-cut without coolant
4V alloy (ASTM B348) are carried out using a 3-axis vertical machining center. Five types of coated carbide end mills having the same base material and geometrical specifications are employed. The value of cutting speed is standard speed v = 50 m/min and high speed v = 220 m/min at the same feed-per-tooth of f = 0.005 mm/ tooth. The wear of end mills is examined using a scanning electron microscope (SEM). The chip morphologies and formation process are evaluated by a microscope and a high speed camera, respectively. Cutting forces are measured with a piezoelectric quartz dynamometer (KISTLER: 9251A) with bandwidths up to 3.5 kHz in X-direction and 2.5 kHz in Y-direction. The experimental conditions are summarized in Table 3.
97
Fig. 5. Variation of flank wear width with cutting length at higher cutting speed and corresponding surface roughness of workpiece.
the wear of UBMS-types is gradual. Comparing the wear width of all end mills at Lc = 2600 mm, the VB value of the U-TiN tool is approximately 30% smaller than that of A-TiN tool, and the VB of the U-TiCN tool is almost 70% or less than that of the A-TiCN tool. The SEM images of the flank face of TiCN-coated tools at Lc = 2600 mm are also presented in Fig. 5, where the close-up SEM-BEI image of the A-TiCN tool is shown. In A-TiCN, a significant flank wear is observed and the adhesion of the workpiece material on the flank face is seen. In contrast, the wear of U-TiCN tool is extremely small without the adhesion of workpiece material as shown in Fig. 5. The surface roughness Rz at Lc = 2600 mm is listed at the same time in Fig. 5. The finished surfaces milled by the U-TiN and the ATiN tools are irregularly ‘plucked’ and therefore the Rz values are as high as 37.4–39.2 mm. On the other hand, the smooth surface of Rz = 4.4 mm is obtained by the U-TiCN tool as shown in Fig. 5. The high-speed camera observation reveals that large amount of chips adhered to the rake face of the A-TiAlN tool whereas the flying chips are formed in almost every cutting engagement for U-TiCN tool. It can be said that the U-TiCN coated end mill with a low friction coefficient suppresses the adhesion of workpiece material on both rake and flank faces due to the lubrication effect, leading to the generation of stable thin chips and good finished surface.
3.2. Cutting temperature measurement 4.2. Cutting force In this experiment, tool temperature at the flank face is measured using a two-color pyrometer [6,10]. An optical fiber is set horizontally at the circumference of the tool (908 from the cutting point) as shown in Fig. 4, which can receive the infrared energy radiated from the flank face of the cutting tool when it passes in front of the fiber face. The two-color pyrometer is composed of InAs and InSb photodetector, having different spectral sensitivities within 1–3 mm and 3–5.5 mm, respectively. We can determine the temperature of object from the output ratio of these two detectors using the calibration curve obtained through a separate experiment. The pyrometer has a flat response over the frequency of 10 Hz to 400 kHz, so that we can measure the tool temperature with a good degree of accuracy.
Fig. 6 represents the relationship between the cutting length Lc and the resultant cutting force F. Here F is calculated by the 1=2 principal force Fx and the thrust force Fy as ðFx2 þ Fy2 Þ . The typical output signals of dynamometer are also shown in Fig. 6, which clearly shows the spikes during the engagement of each flute. The cutting force given on the ordinate of Fig. 6 is calculated by 1=2 averaging ðFx2 þ Fy2 Þ for ten successive engagements. The cutting
4. Experimental results 4.1. Tool wear and finished surface Fig. 5 shows the relationship between the cutting length Lc and the flank wear width VB at the high cutting speed of v = 220 m/min. The VB value of all end mills increase with an increase in Lc. It is seen in Fig. 5 that the flank wear of UBMS-type end mills is smaller than that of AIP-type end mills, and the wear of U-TiCN is the smallest. In addition, the wear of AIP-type tools, especially A-TiCN and A-TiN, increases rapidly at the early stages of cutting although
Fig. 6. Variation of cutting force with cutting length.
A. Hosokawa et al. / CIRP Annals - Manufacturing Technology 61 (2012) 95–98
Coated material
98
U-TiN
Work: AISI 304 v = 220 m/min f = 0.005 mm/tooth Rd = 0.05 mm Ad = 1 mm
U-TiCN A-TiN A-TiCN A-TiAlN 0
100
200
300
400
Tool flank temperature T
500 o
C
Fig. 7. Tool flank temperature of coated end mills.
Cutting force (Fx2 + Fy2)1/2 N
60
5. Conclusions
Work: Ti6Al4V
50
Lc=26 mm
40
Lc=220 mm
Five types of PVD coated end mills by UBMS method (U-TiN, UTiCN) and AIP method (A-TiN, A-TiCN, A-TiAlN) are developed, and are applied to a high-speed dry milling of AISI 304 and Ti–6Al–4V alloy. The main results obtained are summarized as follows.
30 20 10 0 U-TiN
U-TiCN
summarizes the cutting force F at the cutting length Lc = 26 mm and 220 mm for five kinds of coated tools. The cutting force is considerably larger than the cutting force of AISI 304 steel. In the case of TiN-coated tolls, for example, the cutting force is approximately 26 N, whereas that for milling AISI 304 steel is 12 N or less as shown in Fig. 6. These results may be explained by the physical properties of titanium alloy, such as high hightemperature strength, low plasticity, and especially high reactivity with the tool materials. It should be noted that a significantly high cutting force is necessary for A-TiAlN coated tool and that the UTiCN coated tool gives the lowest value.
A-TiN
A-TiCN
A-TiAlN
Coated materials Fig. 8. Comparison of cutting force in milling of Ti–6Al–4V alloy.
force increases with a cutting length Lc due to the growth of tool wear. Especially in the case of AIP type coated end mills, the cutting force increases fairly rapidly and reaches 19–24 N at Lc = 2600 mm. For the U-TiN coated end mill, the cutting force increases gradually because of the smaller wear rate although the value of F is almost the same as those of the AIP type tools in the early stages of cutting. On the other hand, the cutting force of U-TiCN coated end mill is lower than any other tools in the entire range of cutting length. This is because the tool wear VB and the adhesions of workpiece material on the tool face are suppressed by the low friction coefficient. It is notable that the cutting force of the U-TiCN is the lowest even at the beginning of cutting in spite that the cutting edge geometry is almost the same. This means that the U-TiCN film has a lubrication effect for reducing friction between a tool and work/chip. Consequently, TiCN-coated end mill by UBMS method has the best cutting ability in milling of AISI 304 stainless steel.
1. Both TiN and TiCN coating films deposited by UBMS method have a smooth surface and U-TiCN has a low friction coefficient of around 0.19 which is equivalent to DLC film. 2. U-TiCN film contains many free carbons which form C–C bonds rather than carbides in the state of fine crystals through the analyses of XPS and FE-TEM. 3. Lower friction coefficient reduces the tool wear because of lowering the cutting force and hence cutting temperature. As a result, U-TiCN coated end mill gives the lowest tool flank temperature and smallest flank wear width among all of the coated end mills studied in cutting of AISI 304 stainless steel without coolant. 4. U-TiCN coated end mill with lubricating ability can suppress the adhesion of workpiece material on both rake and flank face, which leads to the generation of good finished surface. This antiadhesive property of U-TiCN coating film is capable of reducing cutting force in milling of Ti–6Al–4V alloy. Acknowledgments The authors are indebted to MTTRF (Machine Tool Technologies Research Foundation) and Mori Seiki Co., Ltd. This research was financially supported from Grant-in-Aid for Scientific Research (No. 22656037) and also in part by a research fund from the Hokuriku Industrial Advancement Center.
4.3. Cutting temperature References Fig. 7 shows the tool temperature for five coated tools when AISI 304 stainless steel is milled. As seen from the figure, the tool temperatures of the TiCN-coated tools are 370 8C or less, whereas those of TiN-coated and TiAlN-coated tools exceed 400 8C. This difference may be explained via the difference in thermal characteristics of coating films. Okada et al. reported that the thermal effusivity of TiCN-coating is lower than any other coating films [6]. The lowest temperature of 310 8C is obtained with the UTiCN-coated tool and is approximately 60 8C, 90 8C and 100 8C lower than those of A-TiCN, U-TiN and A-TiAlN coated tools, respectively. This low cutting temperature of the U-TiCN coated tool may compensate the relatively low thermal resistance (see Table 2). Based on the temperature difference between A-TiCN and U-TiCN coated tools, it is confirmed that the formation of carbon in a coating film, which results in C–C bonds or carbides, plays an important role in cutting performance. Consequently, TiCN-coated end mill by UBMS method showed the best cutting ability for highspeed milling of AISI 304 stainless steel. 4.4. Machining of titanium alloy The cutting experiments of Ti–6Al–4V alloy (ASTM B348) are also conducted at the cutting speed of v = 50 m/min. Fig. 8
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