322

Influence of Plating Conditions on Performance of cBN Coated End-Mill∗

Heisaburo NAKAGAWA∗∗, Keiji OGAWA∗∗, Masao NOMA∗∗∗ and Toshiki HIROGAKI∗∗∗∗

This study highlights the influences of pre-treatment and plating conditions on the cut- ting performance of cBN-coated . The pre-treatment evaluations investigate the profile, chemical cleaning, and bombardment conditions of the cutting edge. The plating condition evaluation concerns the influence of TiN and cBN plating conditions. In this paper, TiN plat- ing is employed as a pre-process of cBN plating. Moreover, developed cBN-coated cutting tools were used to conduct cutting experiments for hardened steel and austenitic structure stainless steels. The results show that the new cBN-coated cutting tools demonstrate effective performance for practical use in industrial applications.

Key Words: cBN, Coating, Ion Plating Method, Cemented Carbide, End-Mill

high-speed cutting of difficult-to-machine materials(7).A 1. Introduction new cBN coating method has been developed, and the Some materials are difficult to machine, including die present study evaluates the influences of the pre-treatment steels, steels, stainless steels, and titanium alloys. Die and plating conditions on the cutting performance of cBN- steels and tool steels have higher hardness and strength. coated tools. Stainless steels and titanium alloys tend to be adhesive, 2. Experimental Method because the cutting edge temperature during rises easily due to low thermal conductivity(1), (2).There- 2. 1 Formation of cBN thin film fore, tool materials require higher toughness, in addition A tool is coated with a cBN (hereafter referred to to superior hardness, wear resistance, and reactivity resis- as BN) thin film by means of a magnetically enhanced tance, because machining tools experience both mechani- plasma ion plating method (MEP-IP method). The coat- cal and thermal impacts when difficult-to-machine ing equipment and process are illustrated in Figs. 1 and 2, materials. For this reason, tools made of high-toughness respectively. The BN coating process is as follows. After cemented carbide coated by ultra hard materials are uti- the cleaning process, tools are placed in a chamber un- lized. Meanwhile, the demand for high-speed dry cutting der vacuum and heating, as a pre-treatment step. Then, a of difficult-to-machine materials has increased recently, due to short delivery time requirements and attempts to reduce cutting process costs(3), (4). However, tool wear tends to progress rapidly in high-speed dry cutting using conventional TiN-coated tools(5), (6). Therefore, this paper focuses on cBN coating, whose strong bonding enables

∗ Received 5th October, 2005 (No. 05-4164) ∗∗ Department of Mechanical Systems Engineering, Univer- sity of Shiga Prefecture, 2500 Hassaka, Hikone, Shiga 522–8533, Japan. E-mail: [email protected]; [email protected] ∗∗∗ Shiga Moriyama Factory, Shinko Seiki Co., Ltd., 30 Miyake, Moriyama, Shiga 524–0051, Japan. E-mail: [email protected] ∗∗∗∗ Department of Mechanical Engineering, Doshisha Univer- sity, 1–3 Tataramiyakodani, Kyotanabe, Kyoto 610–0394, Japan. E-mail: [email protected] Fig. 1 Coating equipment

Series C, Vol. 49, No. 2, 2006 JSME International Journal 323

Fig. 2 BN coating process

Fig. 4 Change of cutting force and the resulting chips

Fig. 3 BN-coated end-mill

Table 1 Cutting conditions

Fig. 5 Effect of bombardment on cutting length bombardment process using argon ions is performed af- ter introduction of argon gas into the chamber. The tool is employed as an index for evaluating performance of the then coated with a TiN thin film, which is the middle layer, coated film on the tools. In the case of SKD61, tool life using titanium ions evaporated by an electron gun when is judged by measuring cutting force and observing the the chamber is filled with nitride gas. Finally, the tool is resultant chips. Figure 4 shows an example change in cut- coated with the BN thin film by evaporating boron with ting force and the resultant chips. Cutting force Fy,which argon and nitride plasma. The main parameters controlled is a cutting force in the radius cut direction, is 550 N, and in coating are substrate voltage, anode voltage, and anode the chip configuration shows no twisting in colors of blue current. As shown in Fig. 1, substrate voltage is controlled and white during the tool life. Tool life is over when flank by varying the RF power supply, anode voltage is con- wear width exceeds 0.1 mm or when chipping around the trolled by the anode power supply, and the anode current cutting edge occurs, as in the case of SUS304. by the filament power supply. Higher substrate voltage 3. Results and Discussion enables production of coated films of higher hardness. 2. 2 Machining equipment and conditions 3. 1 Influence of bombardment on tool matrix As shown in Fig. 3, the BN coating is layered on 4- Bombardment removes oxide and dust particles on flute end-mills made from cemented carbide and the tool surface by the collision of electrons and ions, as having a 10 mm cutting edge diameter and a 5◦ rake angle illustrated in Fig. 2. However, the bombardment may raise (Type:SZE4100S, Mitsubishi Materials Kobe Tools Co., the temperature of the cutting edge of low thermal capac- Ltd.). Tool life experiments are carried out under the ity, due to overheating around the ridgeline. Therefore, conditions shown in Table 1 by use of a machining cen- a drop in mechanical strength, wear resistance, and bond ter (Type:YBM-850V, Yasda Precision Tools K.K.) with strength of the tool matrix may result from the conforma- straight shoulder cutting. tional change of the tool material. 2. 3 Evaluation of coated film on tool Figure 5 shows the results of a tool-life cutting test. Total cutting length to the end of the tool life is This figure clearly shows that the life of the tool with

JSME International Journal Series C, Vol. 49, No. 2, 2006 324

(a) Uncoated without (b) Uncoated with bombardment bombardment ff Fig. 6 Di erence in tool wear (SUS304) Fig. 8 Effect of bombarding conditions with SUS304

(a) Uncoated without (b) Uncoated with bombardment bombardment Fig. 7 Effect of bombardment on face wear (SKD61, Cutting ff length 0.6 m) Fig. 9 E ect of bombarding conditions with SKD61 bombardment, under conditions of −55 V substrate volt- age, 60 V anode voltage and 70 A anode current, is shorter than tools without bombardment, irrespective of the type of machined material. Figure 6 shows SEM (Scanning (a) Bom1 (b) Bom2 (c) Bom3 Electron Microscope) micrographs of the end-mill cross- Fig. 10 Effect of bombarding conditions on flank at cutting section after tool life has been reached in the case of length 0.6 m with SUS304 SUS304 cutting. Flank wear occurs around the cutting edge of the uncoated without bombardment end-mill over the tool life, but not on the face. The ridgeline is clearly shorter under Bom3 conditions and is roughly equal under observed in Fig. 6 (a). On the other hand, uncoated end- Bom1 and Bom2 conditions. As shown in Fig. 9, the ser- mill with bombardment shown in Fig. 6 (b) shows not only vice life of SKD61 tools is longer under Bom1 conditions face wear, but also a dull cutting edge. Figure 7 shows and roughly equal under Bom2 and Bom3 conditions. As SEM micrographs of the cutting edge face after cutting shown in Fig. 10, film peeling from the flank of the cutting 0.6 m of SKD61. Micro chipping is identified early in edge is not observed under any bombarding conditions of the cutting process, as shown in Fig. 7 (b), at the face of SUS304. On the other hand, as can be seen in Figs. 11 and the cutting edge of the uncoated end-mill with bombard- 12, film peeling is confirmed early in the cutting process at ment. The difference in tool life and wear configuration the face of the cutting edge. In view of these results, Bom1 is attributed to the variance in cutting edge strength due condition of lower treatment power 200 W by smaller sub- to the bombardment. Therefore, we investigated proper strate voltage −10 V with anode voltage of 20 V and anode bombardment conditions by varying treatment power by current of 20 A is chosen as the proper bombardment con- means of varying substrate voltage. In this paper, treat- dition, because the cutting performance of tool life is bet- ment power strength is as follows: Bom1, Bom2, and ter for both SUS304 and SKD61. Bom3. The substrate voltage of Bom1, Bom2, and Bom3 3. 2 Influence of bombarding treatment with hy- is −10 V, −30 V, and −55 V, respectively, with same con- drogen gas ditions as anode voltage of 20 V and anode current of Cobalt (Co) that has separated at the end-mill surface 20 A. As a result, the treatment power is about 200 W, is oxidized due to the grinding finish in the tool manufac- 400 W, and 800 W respectively. The tool life test results turing process. This oxide may lower the peeling strength with SUS304 and SKD61 are shown in Figs. 8 and 9, re- between the tool matrix and the coated film. Meanwhile, spectively. As shown in Fig. 8, tool life of SUS304 is hydrogen (H2) gas, which is lighter than argon (Ar) gas,

Series C, Vol. 49, No. 2, 2006 JSME International Journal 325

Table 2 TiN coating conditions

(a) Bom1 (b) Bom2 (c) Bom3 Fig. 11 Effect of bombarding conditions on face at cutting length 0.6 m with SUS304

(a) Bom1 (b) Bom2 (c) Bom3 Fig. 12 Effect of bombarding conditions on face at cutting length 0.6 m with SKD61

Fig. 15 Effect of TiN orientation on cutting length

(a) TiN1 (b) TiN2 Fig. 13 Effect of H2 bombardment with SUS304 Fig. 16 Effect of TiN orientation on adhesion strength at cutting length 0.6 m with SKD61

the BN-coated film. In general, the crystal orientations of the TiN-coated film are (111) and (200). TiN-coated films with many (111) oriented crystals have greater stiffness than those with (200) orientation crystals. Therefore, an attempt was made to decrease peeling of the coated film by improving the stiffness of the middle coated film layer by means of increasing the number of (111) oriented crys- ff Fig. 14 E ect of H2 bombardment with SKD61 tals in the TiN-coated film. Table 2 shows the crystal ori- entation of the TiN-coated film used in the cutting tests. can reduce the amount of oxide on the tool surface. There- Figure 15 illustrates the result of the tool life cutting in- fore, H2 gas was tested during the bombardment process vestigation. TiN-coated films with many (111) oriented in an attempt to prolong tool life by improving the bond- crystals were coated on the tools while substrate and an- ing strength between the coated film and the tool matrix. ode voltages in TiN coating conditions were lowered. In Bombardment was carried out under induced Ar and H2 this study, coating condition was −60 V substrate voltage gas conditions, and is called H2 bombardment. The Bom1 with 60 V anode voltage and 70 A anode current for TiN1. condition was employed as the treatment power condition TiN2 coating condition was −30 V substrate voltage with for these tests. Cutting test results for SUS304 and SKD61 45 V anode voltage and 70 A anode current for many (111) are shown in Figs. 13 and 14, respectively. The figures oriented TiN crystals. The results clearly show that the clearly show that tool life is extended by H2 bombard- cutting length over the tool life is shorter for coated films ment. Therefore, the H2 bombardment process is deemed that contain many (111) oriented crystals, represented as effective, although the dioxide of oxide Co has not been TiN2 in Fig. 15, for both SUS304 and SKD61. Figure 16 identified. shows SEM micrographs of the cutting edge face after the 3. 3 Influence of TiN-coated film for middle layer initial stage in the SKD61 cutting process. This figure (TiN) is utilized for the middle clearly shows that the peeled width of the coated films is coated film layer. The aim of the TiN thin film coating is to smaller for the film containing more (111) oriented TiN improve the bonding strength between the tool matrix and crystals. However, the coated film having many (111) ori-

JSME International Journal Series C, Vol. 49, No. 2, 2006 326 ented TiN crystals is not desirable, because of poor cut- by varying the substrate voltage in BN coating condi- ting performance. Therefore, the TiN films with (111) and tions. The BN-coated films with higher hardness were (200) oriented crystals plated under TiN coating condition coated with higher substrate voltage in BN coating. In of higher substrate voltage of −60 V and anode voltage of this study, the substrate voltage was changed from −70 V 60 V with anode current of 70 A was applied for the mid- to −90 V with constant 45 V anode voltage and 70 A an- dle layer of the coated films on the tool surface. ode current for BN coating conditions. The BN-coated 3. 4 Influence of TiN-coated film thickness film with higher Knoop hardness of about 3 000 Hk was The influence of TiN-coated film thickness on tool plated in −90 V substrate voltage. The Knoop hardness life is now investigated. Figure 17 shows the results of was obtained by measuring the BN thin film simultane- the SKD61 experiment. The coated film thickness in this ously coated on silicon wafers and end-mills. The Knoop figure is the value obtained for the coated films on a sili- hardness of the BN-coated film is 1 600 Hk for the longest con wafer coated at the same time as the end-mills. The tool life when cutting SKD61. Thus, long tool life can longest tool life was produced by the 1.4 µm coated film be obtained from a low hardness BN-coated film. On the thickness (TiN-Thin2). However, the tool life decreases other hand, the tool life is short for the tool with Knoop when coated film thickness is 1.6 µm or more. There- hardness of 3 500 Hk. Higher hardness cutting materials fore, an optimum coated film thickness is believed to ex- are generally believed to require a coated film of higher ist. Unfortunately, as can be seen in Fig. 18, which shows hardness. However, as shown in Fig. 20, the peeling of SEM micrographs obtained for the cutting edge after the coated films on the cutting edge flank can be observed 0.6 m SKD61 cutting test, the coated films peel on the cut- early in the cutting process. Tool life is believed to be ting edge under all conditions. The peeling width of the coated films is smallest for TiN-Thin3, but the tool life is short. On the other hand, no peeling is observed in TiN- Thin5 without TiN-coated films. Therefore, the use of TiN-coated films on the tool might affect peeling. How- ever, another coated film might be needed for the middle layer, because a tool without TiN-coated films has a short tool life. 3. 5 Influence of BN-coated film hardness Figure 19 shows the relationship between tool life and Knoop hardness. The Knoop hardness was controlled

Fig. 19 Effect of Knoop hardness on tool life for SKD61

(a) 1 600 Hk (b) 3 500 Hk Fig. 20 Effect of coating hardness on tool wear at cutting Fig. 17 Effect of TiN film thickness on tool life length with length 0.6 m with SKD61 SKD61

(a) TiN-thin1 (b) TiN-thin2 (c) TiN-thin3 (d) TiN-thin4 (e) TiN-thin5 Fig. 18 Effect of TiN film thickness on adhesion strength at cutting length 0.3 m for SKD61

Series C, Vol. 49, No. 2, 2006 JSME International Journal 327 reduced due to the peeling at the flank, because peeling of cutting performances of the BN-coated tool is clearly su- the cutting edge face is observed under all cutting condi- perior to that of the TiAlN-coated tool. The peeling of the tions. This is attributed to the cBN and hBN content of the BN-coated film was found to occur along the flank early bonded crystals in the BN-coated film. The hardness ap- in the cutting process, as shown in Figs. 23 and 24. On pears to be higher when the BN-coated film contains many the other hand, the TiAlN-coated end-mill by AIP method cBN bonded crystals. Moreover, the highest strength with exhibits tool wear but not peeling of the coated film. cBN sintered materials seems to be at about 60% content, 3. 7 Deposit resistance of BN-coated film although hardness increases with cBN content. Therefore, The end-mill surface after tool life is observed in or- this BN thin film is assumed to exhibit a similar tendency. der to evaluate the depositing resistance of the BN-coated High stiffness of the BN-coated films is needed to prevent film. This is performed by an elemental analysis by en- peeling, because large stresses are generated around the ergy dispersive X-ray (EDX) analysis. Figures 25 and 26 cutting edge during cutting of very hard materials. illustrate the flank and face of the cutting edge, respec- 3. 6 Cutting performance of BN-coated end-mill The cutting performance of the BN coating is eval- uated through comparison with the TiAlN coating re- cently used for tools that cut difficult-to-machine materi- als. Therefore, an end-mill coated with TiAlN by the AIP method is utilized. Figures 21 and 22 show the tool life test results with SUS304 and SKD61, respectively. As can (a) BN (b) TiAlN be seen, the tool life of the BN-coated end-mill is longer ff ff than that for the TiAlN-coated end-mill. In particular, the Fig. 23 E ect of di erent coated film on adhesion strength at cutting length 0.6 m (SUS304) tool life of the BN-coated end-mill, with about 1 600 Hk hardness BN-coated film plated by the best BN coating conditions of −70 V substrate voltage and 45 V anode volt- age and 70 A anode current as evaluated in the above sec- tion, is five times that of the TiAlN-coated one in SUS304 and 1.8 times that in SKD61. Moreover, the tool life in (a) BN (b) TiAlN Fig. 24 Effect of different coated film on adhesion strength at cutting length 0.6 m (SKD61)

Fig. 21 Cutting performance of BN compared to TiAlN coating (a) BN (b) TiAlN with SUS304 Fig. 25 Elementary analysis of face by EDX at end of tool life (SKD61)

(a) BN (b) TiAlN Fig. 22 Cutting performance of BN compared to TiAlN coating Fig. 26 Elementary analysis of flank by EDX at end of tool life with SKD61 (SKD61)

JSME International Journal Series C, Vol. 49, No. 2, 2006 328 tively. The area labeled Ti indicates the area of BN-coated References film remaining, because Ti constitutes the middle layer, TiN, and TiAlN-coated films. The area labeled W corre- ( 1 ) Corduan, N., Himbert, T., Poulachon, G., Dessoly, sponds to the area of tool material exposed, because W M., Lambertin, M., Vigneau, J. and Payoux, B., Wear is included in the tool material of WC but not the oth- Mechanism of New Tool for Ti-6Al-4V High Per- formance Machining, CIRP Annals — Manufacturing ers. Moreover, the area labeled Fe indicates deposition Technology, Vol.52, No.1 (2003), pp.73–76. of the SKD61 workpiece. The Fe area in the remaining ( 2 ) Nabhani, F., Wear Mechanisms of Ultra-Hard Cutting coated film is larger in case of the TiAlN-coated end-mill. Tools Materials, Journal of Materials Processing Tech- The same tendency can be confirmed for both the face and nology, Vol.115, No.3 (2001), pp.402–412. flank of the cutting edge. Moreover, Fe is hardly observed ( 3 ) Aslan, E., Experimental Investigation of on the face of cutting edges coated with BN. Therefore, Performance in High Speed Cutting of Hardened X210 the BN-coated film is believed to have a higher depositing Cr12 Cold-Work Tool Steel (62 HRC), Material and resistance than TiAlN-coated films. Design, Vol.26, No.1 (2005), pp.21–27. ( 4 ) Ozel, T. and Altan, T., Process Simulation Using Fi- 4. Conclusions nite Element Method—Prediction of Cutting Forces, Tool Stresses and Temperatures in High-Speed Flat ff Investigations on the e ects of pre-treatment and End Milling, International Journal of Machine Tools plating conditions on the cutting performance of cBN- and Manufacture, Vol.40, No.5 (2000), pp.713–738. coated tools have revealed several notable characteris- ( 5 ) Tanaka, Y., Ichimiya, N., Onishi, Y. and Yamada, Y., tics. Decreasing bombardment power can prevent reduc- Structure and Properties of A1-Ti-Si-N Coatings Pre- tion of cutting edge strength and obtain sufficient bonding pared by the Cathodic Arc Ion Plating Method for strength on the face of the cutting edge. By using a BN High Speed Cutting Applications, Surface and Coat- ings Technology, Vol.146-147 (2001), pp.215–221. thin film coated by the MEP-IP method, a comparatively ( 6 ) Ichizaki, T., Egawa, T., Tsunoda, H., Fukuya, Y. and superior cutting performance can be obtained when cut- Shinoyama, T., Cutting Performance Improvement of ting SUS304 and during high-speed dry cutting of SKD61. cBN Tools Made of TiN Coated cBN Particles, Journal Acknowledgment of the Japan Society for Precision Engineering, Vol.59, No.12 (1993), pp.1997–2000. The authors would like to thank the Ministry of Econ- ( 7 ) Ikeda, T., Satoh, T. and Satoh, H., Formation and Char- omy, Trade and Industry of Japan for financial support of acterization of Cubic Boron Nitride Films by an Arc- the 2004 Regional New Consortium Research and Devel- Like Plasma-Enhanced Ion Plating Method, Surface & Coatings Technology, Vo.50, No.1 (1991), pp.33–39. opment Project.

Series C, Vol. 49, No. 2, 2006 JSME International Journal