Coolant effects on tool wear in machining single-crystal silicon with diamond tools

Transcription

1 Key Engineering Materials Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland Coolant effects on tool wear in machining single-crystal silicon with diamond tools Tsutomu Ohta 1, Jiwang Yan 2,a, Sunao Kodera 1, Shuuma Yajima 1, Naoyuki Horikawa 3, Youichi Takahashi 3 and Tsunemoto Kuriyagawa 2 1 Mitsubishi Electric Corporation, Kamakura Works, 325 Kamimachiya, Kamakura, Kanagawa , Japan 2 Department of Nanomechanics, Tohoku University, Aramaki Aoba ,Aoba-ku, Sendai , Japan a yanjw@pm.mech.tohoku.ac.jp 3 Tokyo diamond tools MFG. CO., LTD, Imai, Okaya, Nagano , Japan Keywords: single-crystal silicon, infrared lens, ductile cutting, diamond turning, tool wear, coolant Abstract. The service life of a diamond tool in cutting single-crystal silicon is normally very short because of severe tool wear. Therefore, it is important to use a proper coolant in order to restrain tool wear. In this paper, the performances of oil-based and water-based coolants were compared in silicon machining by investigating cutting forces and tool wear geometries. The water-based coolant was found to restrain flank wear more effectively than the oil-based one. The effective tool life using the water-based one was averagely three times longer than that using the oil-based one. The tool wear mechanism might be related to microplasma generated between silicon and diamond during cutting. Introduction Single crystalline silicon is commonly used for manufacturing infrared optical lenses by diamond turning. Conventionally, to obtain a crack-free surface on silicon by diamond turning, a small radius round-nosed diamond tool is used at an extremely small tool feed (~1 µm/rpm) [1, 2]. As a result, the material removal rate is very low. To solve this problem, we proposed a high-efficiency method where straight-nosed or large radius round-nosed tools are used for ductile machining at high feed rates [3, 4]. In either method, however, intensive tool wear occurs [5-8]. The tool wear in silicon machining shows a few special phenomena that can not be found in the ductile machining of other brittle materials like germanium [4]. The tool wear mechanism has not yet been clarified, but it might involve compound effects of a few complicated factors, such as chemical, physical, electrical, and mechanical interactions between silicon and diamond. The tool wear limits the service life of diamond tools, and in turn, leads to a very high production cost. Recently, infrared optical lenses are being required more and more in various industrial fields, such as night vision systems of vehicles, dark field sensing systems for security, etc. For this reason, infrared optics is required to be inexpensive. One of the approaches for low-cost production of silicon optics is to minimize the tool wear. Coolants are usually used to remove heat from the tool and lubricate the cutting zone. It has been shown that by using a coolant, the ductile mode cutting distances can be extended than dry cutting [3]. It was also shown that in low-feed cutting of silicon, a water-based coolant prolongs significantly the tool life [6]. However, the mechanism of the coolant effects on the wear of diamond tools in high-efficiency silicon cutting is not yet clear. The objective of this study is to elucidate the tool wear mechanism by observing the cross-sectional profiles of the cutting edges under different types of coolants (oil-based and water-based) and to find a suitable coolant type for high-efficiency ductile cutting of silicon. Experiments Cutting experiments were carried out on an ultraprecision diamond turning machine, Precitech Nanoform-200. The machine has a hydrostatic bearing spindle and two perpendicular hydrostatic sliding tables along the X-axis and the Z-axis. A diamond tool was set on the rotary B-axis table of the machine. A three-component piezoelectric dynamometer, Kistler 9257B was located under the tool holder. The cutting conditions used are shown in Table 1. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-06/03/16,16:20:59)

2 Key Engineering Materials Vols Single crystal silicon (100) lens substrates were used as workpieces. These workpieces are 50 mm in diameter and 10 mm in thickness. The diamond tools used in the experiments are made of single crystal natural diamond. The tools have a 10 mm nose radius, a 25 rake and a 10 relief. The cutting plane of the tool was oriented to be almost parallel to the (110) crystalline planes of diamond. The flank faces of the tools were approximately along the (100) planes. One kind of oil-based coolant, two kinds of water-soluble coolants, and an emulsion type coolant were used for comparison. The two water-soluble coolants and the emulsion type coolant were diluted with water, and used as water-based coolants. The water accounted for more than 80% of the water-based coolants by volume. The details of the coolants are listed in Table 2. Their electrical conductivities are different, ranging from 0 to 6730 µs/cm. Three diamond tools #1-3 were used in coolant A. One of them, tool #2, was coated with gold to increase the surface electrical conductivity. Two other tools #4 and 5 were used in coolant B. Tool #5 was sprayed with both coolants A and B. Tool #6 was used in Coolant C. Tool #7 was used in coolant D. All coolants were sprayed downward from above the tool, except for tool #5 where coolant B was sprayed upward from under the tool and coolant A was sprayed downward from above the tool, simultaneously. After machining, the diamond tools were observed using a scanning electron microscope (SEM). Cross-sectional profiles of the cutting edges were measured with an electron beam three-dimensional surface roughness analyzer, Elionix ESA-3000 [9]. Table 1 Cutting conditions Depth of cut (µm) 3 Feed rate (mm/min) 25 Spindle rotation rate (rpm) 5000 Cutting width (µm) 245 Maximum undeformed chip thickness (nm) 122 Chip cross-sectional area (µm 2 ) 15.0 Table 2 Coolant types used for different tools Tool Coolant Electrical No.# conductivity Coefficient (µs/cm) of friction No. type 1, 2, A Oil-based , 5 4, 5 B Water-soluble C Water-soluble D Emulsion Results and Discussion Cutting force. In the cutting tests, the thrust force component (Ft) of the cutting force was dominant, the change of which with cutting distance is shown in Fig.1. The measurement of cutting forces was continued until cracks formed on the machined silicon surface. The large plotting symbols in Fig. 1 show the point of appearance of surface cracks. It was found that the thrust forces when using the water-soluble and the emulsion type coolants were about one third of those when using the oil-based coolant during cutting. For the water-based coolants, the cutting forces were almost the same though the coolants were very different in electrical conductivity. When cracks occurred on silicon surface, all the thrust forces, except those for tool #1, were almost at the same level in the range of 4 to 7 N. The ductile mode cutting distances for the two water-soluble and the emulsion type coolants were about 3 times longer than that for the oil-based one. In Fig.1, the result of tool #1 is a special case where the cutting distance was 10.2 km, much longer than for the two other tools with the oil-based coolant A. This difference might be caused by the singularity in mechanical properties of nature diamond such as crystallographic orientation error. It was also noticed that although the surface of tool #2 was coated with gold to leak electrical charge during cutting, the cutting forces for tool #2 were almost the same as that of the other two tools. Temperature rise in cutting. The temperature of tool shank was measured using a thermocouple which was placed on the tool shank near the diamond tip. The change in the tool shank temperature was investigated in order to examine the cooling effect of various coolants. We defined the tool shank temperature rise as the temperature difference between the start of cutting and the point where the temperature reached the maximum.

3 Cutting force F t (N) Tool shank temperature rise ( C) 146 Advances in Abrasive Technology XI Figure 2 shows the change in the tool shank temperature rise as a function of the thrust force. The correlation coefficient between the temperature rise and the thrust force for all the data was 0.92, showing that the same quantity of heat has been transferred from cutting point to the tool shank at the same cutting forces for different coolants. In other words, the cooling ability of all the coolant was the same. Though the friction coefficients of coolants A and C were over 0.3, and those of coolants B and D were about 0.15, the difference in friction coefficient did not affect the transfer of heat Coolant A, tool#1 Coolant A, tool#2 Coolant A, tool#3 Coolant B, tool#4 Coolant B, tool#5 Coolant C, tool#6 Coolant D, tool# Coolant A, tool#1 Coolant B, tool#4 Coolant C, tool#6 Coolant D, tool# Cutting distance (km) Thrust force F t (N) Fig.1 Change of thrust cutting force F t with cutting distance Fig.2 Change of tool shank temperature rise with thrust force Cutting edge observation. Figures 3 and 4 show SEM micrographs and the corresponding cross-sectional profiles of the cutting edges of diamond tools for oil-based coolant A and water-based coolant B, respectively. In Fig.3 (a), a wide flank wear land is observed between the rake face and the flank face, on the wear land there are obvious microgrooves parallel to the cutting direction. The flank wear width was 5.5µm. It is noteworthy that cracks were observed behind the flank wear land which is not in contact with silicon during cutting. At the rake face side, however, there were no cracks observed. In Fig.4 (a), the flank wear width was 1.9 µm, one third of the flank wear seen in Fig.3 (a). Microgrooves were less obvious in Fig.4 (a). Furthermore, in Fig. 4(a), a narrow crater wear was observed at the boundary between the rake face and the flank wear land. Figure 3 (b) shows a cross-sectional profile of the cutting edge of tool #1. The width of the primary flank wear (i) was 4.0µm, and that of the secondary flank wear (ii) was 2.2µm. It is to be noted that a small flat region AB formed at the boundary between the rake face and the flank wear. The from the flat region AB to the cutting direction is 51, which leads to a highly effective negative rake. Due to the highly negative rake, the chip removal from ahead of the tool rake face will be obstructed, and in turn, an unfavorable downward flow of material will occur, which is similar to cutting with a micro-chamfered negative rake tool [10]. In this case, when undeformed chip thickness is larger than a critical value, after the tool passes, the residual tensile stress in material will cause crack initiation along the plastic and elastic boundary, leading to surface microfractures [10]. At the same time, the thrust forces will be very high. Figure 4(b) shows a cross-sectional profile of the cutting edge of tool #4 using water-based coolant B. Obvious crater wear occurred at the boundary between the rake face and the flank wear land. The crater wear is d to the cutting direction, forming an effective negative rake of 54. This highly negative effective rake will also cause cracks on the machined surface. In the figure, the primary flank wear (i) width was 1.6µm and the secondary flank wear width was 0.7µm. The total

4 Key Engineering Materials Vols flank wear width (2.3 µm) was one third of that using the oil-based coolant A. From a comparison with cutting force results shown in Fig.1, it is deduced that the thrust forces are directly proportional to the total flank wear width. Crater wear Cracks face (a) SEM micrograph (a) SEM micrograph Retreated cutting edge 1.3µm -51 Rake -25 A 1.0µm B (i) 4.0µm (ii) 2.2µm Relief µm -54 Rake Angle -25 (i) 1.6µm (ii) 0.7µm Relief (b) Cross-sectional profile Fig.3 Cutting edge of tool #1 at 10.2km with oil-based coolant A (b) Cross-sectional profile Fig.4 Cutting edge of tool #4 at 11.8km with water-soluble coolant B µm Rake µm Cutting direction Relief 10 Fig.5 SEM micrograph of tool #3 cutting edge at 3.2km with oil-based coolant A Fig.6 Cross-sectional profile of the cutting edge of tool #7 after cutting for 5.0km

5 148 Advances in Abrasive Technology XI The SEM micrograph in Fig.5 shows the wear of tool #3 in oil-based coolant A. Similar to Fig. 3(a), there is no obvious crater wear, but a flat region at the boundary between the rake face and the flank wear is clearly seen. In this case, as the flank wear and the flat region grow, cracks begin to occur after a short cutting distance. Figure 6 shows a cross-sectional profile of the cutting edge of tool #7 with the emulsion type coolant D after cutting for 5.0 km, where the cutting mode was a ductile one. This cutting distance is half the distance when cracks were formed for tool #4 with the water-soluble coolant B. The primary flank wear width and crater wear width were 0.9µm and 0.4µm, respectively. These were also half the wear widths observed for coolant B. In this figure, the negative rake caused by the crater wear was 45, smaller than the s shown in Fig.3 (b) and Fig.4 (b). Under such a medially negative tool rake, ductile mode cutting was achieved without crack generation. Continuous machining tests using tool #7 showed that cracks began to form at a distance of 7.4 km. At this time, the crater wear width and were similar to that shown in Fig.4 (b). Effect of coolant type on tool wear mechanism. Rapid tool wear in silicon machining is not only a result of mechanical interaction. It might be related to physical, chemical and electrical phenomena between silicon and diamond during friction at a high speed. One of the possible wear mechanisms is microplasma generation at the silicon-diamond interface. Energetic particle emission, such as the emission of electrons, negative and positive ions and photons, has been observed during the scratching of insulators or semiconductors [11]. The electron energy was distributed over a few tens of ev up to 100 ev during scratching of a Si (100) surface under 1.4 N load and 7.0 cm/s feed [12]. The surrounding atmosphere might have become to be a high-temperature state, because 1 ev is equivalent to 11,331 C [13]. It is also reported that microplasmas are generated at the contact point between diamond and silicon and from that point towards the backward traversing direction [14]. Figure 7 shows the model of possible microplasma occurrence in the tool wear during silicon cutting. The cracks observed on the tool flank face behind the flank wear land in Fig.3 (a) strongly support the presumption of microplasma generation. Because in this region diamond does not contact with silicon; cracks might be caused by the carbonization of diamond due to the microplasma. Microplasma occurs easily between silicon and diamond when using oil-based coolant which is a 0 µs/cm insulator; but will be restrained when using electrically conductive water-based coolants. As a result, the flank wear and the cracking behavior behind the flank wear were insignificant when using the water-based coolants. From this aspect, we can say that to use an electrically conductive coolant is more helpful for suppressing tool wear than using an insulating coolant. However, we noticed that even if the electrical conductivities of the water-based coolants used in the experiment are distinctly different, no remarkable difference in tool wear and cutting forces could be seen. Therefore, provided the coolant is electrically conductive, the conductivity value itself is not essential. Instead, the water molecules in moisture which account for more than 80% of the water-soluble or the emulsion coolant might be the Rake Crater Removed chip Typical wear edge shape Undeformed chip thickness Elastic recovery New cutting edge Microplasmas Flat region / Crater Fig.7 Model of microplasma occurrence and tool wear in silicon machining

6 Key Engineering Materials Vols dominant factor for preventing from microplasma generation. In addition, from the fact that there was no obvious difference in cutting forces between tool #2 coated with gold and tool #1 without coating in the oil-based coolant, we may say that it is effective to use an electrically conductive coolant rather than to use cutting tools with a conductive surface. It is also noteworthy that when using water-soluble and emulsion type coolants, crater wear occurred as shown in Figs.4 and 6; whereas there was no crater wear when using oil-based coolants, as shown in Figs.3 and 5. This might be because that when using water-based coolants, the flank wear is very slow and the crater wear becomes relatively faster. When using oil-based coolants, however, the growth speed of flank wear is very high; so that the cutting edge retreats before a crater wear is formed. As a result, an d flat wear land is generated at the boundary between the flank wear and the rake face. Summary Single crystalline silicon was machined with diamond tools using one kind of oil-based coolant and three kinds of water-based coolants. Thrust cutting forces using the water-based coolant were found to be approximately 1/3 of those using the oil-based coolant. The cutting distance at which cracks begin to occur on the machined surface in the water-based coolant was about 3 times longer than in the oil-based coolant. The flank wear width using the water-based coolant was also 1/3 of that of the oil-based one. SEM observations of the cutting edge showed that a crater wear occurs in water-based coolants; while in an oil-based coolant, an d flat wear region will be generated. Either the crater wear or the d wear land may induce highly negative effective rake s up to -50 causing microfractures on the machined surface. Compared to an insulating oil-based coolant, the water molecules in the electrically conductive water-based coolant can prevent from the microplasma generation between diamond tool and silicon workpiece; thus is beneficial for silicon machining to reduce tool wear. References [1] P. N. Blake and R. O. Scattergood: J. Amer. Ceram. Soc., 73, 4 (1990) [2] T. Nakasuji, S. Kodera, S. Hara, H. Matsunaga, N. Ikawa and S. Shimada: Ann. CIRP, 39, 1 (1990) [3] J. Yan, K. Syoji, T. Kuriyagawa and H. Suzuki: J. Mater. Proc. Tech., 121, 2-3 (2002), [4] T. Ohta, J. Yan, S. Yajima, Y. Takahashi, N. Horikawa and T. Kuriyagawa: Int. J. Surf. Sci. Eng., 1, 4, (2007) [5] J. Yan, K. Syoji, J. Tamaki: Wear, 255, (2003) [6] D.-Cardenas, P. Shore, X. Luo, T. Jacklin, S.A.Impey: Wear, 262, (2007) [7] X. P. Li, T. He, M. Rahman: Wear, 259, (2005) [8] M. S. Uddin, K. H. W. Seah, M. Rahman, X. P. Li, K. Liu: J. Mater. Process185, (2007) [9] S. Asai, Y Taguti, K. Yokobori, T Kawanishi, A. Kobayashi: J. Jpn. Soc. Prec. Eng., 46, 7, (1990) [10] J. Yan, K. Syoji, and T. Kuriyagawa: J. Jpn. Soc. Prec. Eng., 66, 7, (2000) [11] K. Nakayama: J. Vac. Soc. Jpn., 49, 10, (2006) [12] K. Nakayama: Tribology Lett., 17, 1, (2004) [13] Handbook of Chemistry and Physics, edited by D. R. Lide. CRC Press, ( ) 1-11 [14] K. Nakayama, Jpn. J. Apple. Phys., 46, 9, (2007)

7 Advances in Abrasive Technology XI / Coolant Effects on Tool Wear in Machining Single-Crystal Silicon with Diamond Tools /