Wear Mechanism of Diamond Tools in Ductile Machining of Reaction-bonded Silicon Carbide Zhiyu ZHANG, Jiwang YAN*, and Tsunemoto KURIYAGAWA Department of Nanomechanics, Tohoku University Aramaki Aoba 6-6-01, Aoba-ku, Sendai 980-8579, Japan *yanjw@pm.mech.tohoku.ac.jp Abstract: Wear mechanisms of single-crystal diamond tools in ductile machining of reaction-bonded silicon carbide (RB-SiC) were investigated. It was found that tool wear could be generally classified into two types. One is microchippings on the cutting edge, which were induced by micro impacts between the cutting edge and SiC grains. The other is two kinds of gradual wear patterns on flank face caused by different mechanisms: non-periodical scratches caused by scratching effects of the SiC grains, and periodical grooves caused by transcribing effect of tool feed marks on the machined surface. A tool-swinging cutting method was proposed to improve the service life of diamond tools. Key words: Silicon carbide, Ductile machining, Diamond tool wear, Microplasticity. 1. Introduction Silicon carbide (SiC) is an important ceramic material that has been extensively used in various harsh environmental conditions, such as high temperature, high pressure, and severe corrosion. Recently, in optical manufacturing industry, SiC is being used as molding dies for high-precision hot pressing of glass lenses, for its high-temperature hardness, thermal shock resistance and chemical stability [1]. On the other hand, SiC has very poor machinability in ultraprecision machining. Conventionally, SiC was machined by diamond abrasive processes, such as grinding, lapping and polishing [2-5]. These machining methods can produce a nanometric surface finish; however, it is very difficult to precisely fabricate microstructures on SiC, such as micro lens arrays and micro prism arrays, which are increasingly demanded for glass molding press (GMP) technology [6]. As an alternative approach, we expected that precision cutting technology might be usable in fabricating microstructures on SiC. In a previous paper, we reported the material removal mechanisms in diamond turning of reaction-bonded SiC (RB-SiC) [7]. It was found that when using large-radius round-nosed diamond tools, high-efficiency ductile machining of RB-SiC could be realized. However, we found that diamond tools wore severely in diamond turning process, which dramatically degraded the machined surface quality. In this paper, experiments were carried out to study the wear mechanisms of diamond tools in the machining process of RB-SiC. It is expected that we can clarify the fundamental wear mechanisms and find new cutting methods to improve the service life of diamond tools. 2. Experimental The RB-SiC samples used in the experiments were produced by infiltrating silicon melt into a green compact consisting of carbon powder and SiC particles with average size of less than 1 µm. The liquid silicon reacts with carbon powders, forming new SiC particles. The infiltrated silicon does not react with carbon completely and excessive silicon fills the remaining pores in the body so that dense RB-SiC composite is produced [8]. The volume ratio of residual silicon in this work was 12 %. Fig. 1 shows a scanning electron microscope (SEM) micrograph of the fast atom beam (FAB) etched sample surface. The smooth regions correspond to SiC grains, and the micropits correspond to residual silicon. As shown in Fig. 1, most of the SiC grains are directly bonded to each other without the presence of silicon at grain boundaries. Machining experiments were carried out on a three-axis numerically controlled ultraprecision lathe, Nachi-ASP15. The experimental setup is shown in Fig. 2. This machine has a hydrostatic bearing spindle and two perpendicular hydrostatic sliding tables along the X-axis and the Z-axis. A tool holder, which has a three-dimensionally adjustable mechanism, is set on the rotary B-axis table. A CCD camera is equipped above the diamond tool to assist positioning the cutting point. Cutting tools used in experiments are made of single-crystal diamond and have a 10 mm nose-radius, a 20 rake angle and a 10 relief angle. Fig. 3 shows an SEM micrograph of a new diamond tool. The cutting edge is extremely sharp and without visible damages. The rotation rate of the spindle was set to 2000 rpm. The feed rate was set to 2 µm/rev and the depth of cut was set to 2 µm.
SiC grains Fig. 1 SEM micrograph of an FAB surface-etched RB-SiC sample. CCD camera Micropits Diamond tool Tool holder Fig.2 Photograph of the experimental setups. Z B Y X 3. Results and discussion 3.1 Tool wear observation Microchipping of cutting edge was a typical wear pattern at the initial stage in the machining process of RB-SiC. Fig. 4 shows an SEM micrograph of the cutting edge after cutting for several meters. A few microfractures can be observed on the cutting edge. The size of these microfractures is below 1 µm. As cutting distance increased, both the number and the size of the microfractures increased. Fig. 5(a) shows an SEM image of a diamond tool after a cutting distance of 20 m. It can be seen that the flank wear land is nearly symmetrical and like a crescent. However, after taking a careful look, we can find that the wear topography of the left side is different from that of the right side. Because tool feeding direction is from the left to the right in the figure, the two sides of the flank face have different contact conditions with the machined surface and the surface being machined. Fig. 5(b) shows a magnified image of the right side (location b in Fig. 5(a)) on the flank wear land. Scratched marks, the direction of which is nearly the same as the cutting direction, are observed. These marks do not show periodicity and are different in depth. The surfaces of these scratched marks are very rough. Moreover, microfractures in the micron level are also observed on the rake face side. Fig. 5(c) shows a magnified image of the left side (location c in Fig. 5(a)) on the flank wear land. Uniform grooves, which are all oriented along the cutting direction, are observed. These grooves are apparently periodical in a pitch of 2 µm, corresponding to the tool feed rate. Moreover, the groove surface is smooth with the same depth and without visible microfractures. Besides the periodical grooves on the flank face, a ~1 micron wide crater wear is also observed on the rake face side. The wear regime of the micro crater is smooth and uniform. Cutting edge Microchipping Fig. 3 SEM micrograph of the cutting edge of a new diamond tool. Fig. 4 SEM micrograph of edge microchippings.
a Tool feeding direction c b Flank wear land 100 µm b Microfracture c Crater wear Non-periodical scratches 2 µm Periodical grooves Fig. 5 SEM micrographs of the flank wear land: (a) general view; (b) close-up view of the section with non-periodical scratches; (c) close-up view of the section with periodical grooves with a pitch of 2 µm. 3.2 Tool wear models Fig. 6 shows the schematic model of cutting process when using a round-nosed tool. At the beginning, the cutting edge involved in cutting is only on the right side, from where the wear is initialized. The material removal model is shown in Fig. 7, where the undeformed chip thickness varies along the cutting edge from zero to a maximum value. From the left to the right along the cutting edge, there are four regions, namely, rubbing region, plastic deformation region, plowing region and cutting region [9]. Only in the cutting region, material can be removed. In other regions, diamond tool cannot remove any material, instead, only squeeze on the machined surface. As a result, a high local temperature could be generated and it may provide sufficient kinetic energy to break carbon carbon bonds of diamond and hence may cause severe wear of tool. Due to this effect, the tool tip wore and retreated. Accompanied with the retreatment of the tool tip, the wear land was extended gradually towards the left side and became wider and deeper. Finally, originally round cutting edge would be worn into a partially straight one as shown in Fig. 8(a). The flank wear is also schematically shown in Fig. 8(b), which is a view from the direction perpendicular to flank face, corresponding to the image in Fig. 5(a). Fig. 9 is a schematic presentation of the generation mechanism of the periodical grooves. Because feed direction of the tool is from the left to the right, the feed marks on the newly machined surface, which are also indicated in Fig. 7, would squeeze through the worn region of flank face and imprint the shape onto the flank face. Tool nose radius Cutting edge Fig. 6 Cutting model for a round-nosed tool.
Cutting region Plowing region f f Cutting edge Depth of cut Tool nose radius Plastic deformation Rubbing region Feed marks Residual height Feed marks Wear caused by friction Wear caused by cutting Fig. 7 Schematic model for material removal and material deformation in the cutting region of a round-nosed tool. Flank wear land (a) Fig. 9 Schematic model for generation mechanism of the periodical grooves on the flank wear land. (b) Tool nose radius Non-periodical scratches Wear land Flank wear land Periodical grooves Fig. 8 Schematic presentations of the flank wear land: (a) rake-face view; (b) flank-face view. 3.3 Estimation of cutting temperature Since accurately testing the temperature at the cutting point is difficult, finite element method (FEM) analysis was performed to evaluate the temperature distribution around the diamond tool edge. From the microindentation investigation [10], it was known that RB-SiC has very high hardness, high elastic modulus and large elastic recovery rate. Therefore, contact pressure between the machined surface of RB-SiC and the flank face of diamond tool should be very high, which induced a large amount of friction heat. Based on this hypothesis, an FEM model was built to predict the temperature on the basis of experimental conditions. Fig. 10(a) shows the simulation model built with commercial software, COMSOL Multiphysics. The model includes a RB-SiC sample, a diamond tool tip and a tool shank. The convection and conduction module in the multiphysics mode was selected. The boundaries between diamond and RB-SiC were specified as heat flux discontinuity with inward heat flux Q determined by Q = µσv (1) where µ is friction coefficient, σ is contact pressure, and V is cutting speed. All external boundaries were specified as heat transfer with convective film coefficient determined by a function of the cutting speed. The linear system solver, direct UMFPACK, was adopted in the calculation. The FEM analysis result is given in Fig. 10(b). The temperature at the diamond tool tip is over 800 K. It is known that at such a high temperature, diamond could
partially lose its hardness, while its fracture toughness and micro plasticity could be improved [11]. Moreover, the distribution of temperature shows that heat is prone to be transported into the diamond tool rather than the RB-SiC workpiece, because diamond has a higher thermal conductivity. The result also indicates that RB-SiC is relatively less affected by temperature rise because it is cooler than diamond. (a) Tool Shank Diamond tool RB-SiC (b) Fig. 10 (a) FEM model and (b) temperature distribution around diamond tool tip. 3.4 Discussion RB-SiC is a composite material that consists of hard SiC grains and relatively softer silicon bond. Hence, the cutting stress at a certain cutting point of the cutting edge is time-varying during the machining process. Thus, the cutting edge is subjected to micro impacts from the SiC grains. When the impact stress at a cutting point exceeds the strength of diamond, microchippings will occur. The microchippings may be the main reason for causing the originally sharp tool edge to a worn blunt one and finally leading to scratch patterns on flank wear. In RB-SiC, the SiC grains embedded in the machined surface may act like the abrasive grits on a grinding wheel surface. It is presumed that the scratching effect of SiC grains leads to the non-periodical scratches on flank wear land of the diamond tool. The periodical grooves on the flank wear land are presumed to be a result of microscopic plastic deformation of diamond. In previous studies, the plastic deformation of diamond under specific experimental conditions was reported [12-16]. Even at room temperature, diamond shows detectable microplasticity [17]. In the wear of polycrystalline diamond tools, clear evidence of plastic deformation was found [18]. That is to say, at suitable conditions, diamond has apparent dislocation ability and could undergo deformation in microscale. It is presumed that in the machining process of RB-SiC, the micro plasticity of diamond might be activated by the high temperature. Because there is a high pressure at the contact/sliding interface between the flank wear land and SiC, and the hardness of SiC is sufficiently high to maintain the high pressure, it is possible for the tool feed marks to be transcribed onto the flank face of diamond tool due to micro plastic deformation of diamond. 4. Tool-swinging cutting method As discussed above, the main reason for severe wear of diamond tool in cutting SiC is the thermal effects. Therefore, cutting methods which can reduce the thermal influences on diamond tools should be adopted. Recently, intermittent precision cutting methods, such as fly cutting [19] and endmilling [20], were reported to be effective for reducing the temperature influences on diamond tool wear. In this paper, the authors proposed a new cutting method called tool-swinging cutting method. The schematic presentation of the proposed method is shown in Fig. 11. A large-radius round-nosed diamond tool is set on a rotary B-axis table. The geometrical center of the cutting edge is adjusted to be in coincidence with the rotation center of the B-axis table. In this way, we can change the cutting point along the cutting edge by swing the diamond tool about the B-axis. In this method, as the cutting point always moves along the cutting edge, cutting time at one cutting point is very short so that the temperature rise at this point could be suppressed. After cutting, the cutting edge can be cooled down effectly by air or coolant. Fig. 12 shows the flank wear of a diamond tool after cutting by this method. The width of the flank wear land is remarkably smaller than that in Fig. 5. The scratch marks on the wear land are slightly curled because the cutting direction was changing in the cutting process. Fig. 13 shows a profile of the surface obtained by the proposed method. The surface finish of 2 nm Ra and 15 nm PV was obtained on RB-SiC. Therefore, by using the proposed tool-swinging method, both tool service life and machining accuracy can be improved at the same time. 5. Conclusions Wear mechanisms of round-nose diamond tools in ductile machining of RB-SiC were studied based on the SEM observations. Diamond tool wear could be generally
Center of swing (B-axis) Swing R Fig. 13 Surface roughness profile of machined RB-SiC workpiece. Fig. 11 Schematic presentation of tool-swinging cutting method. Curved scratches Fig. 12 SEM micrograph of tool wear pattern in the tool-swinging cutting method. classified into two types. One is microchippings on the cutting edge, which are induced by micro impacts between the cutting edge and SiC grains. The other is two kinds of gradual wear patterns on flank face caused by different mechanisms: non-periodical scratch marks caused by abrasive scratching of the SiC grains, and periodical grooves caused by imprinting effect of tool feed marks on machined surface. The microstructures on the flank wear land are resulting from the microplasticity of diamond at high temperature and high contact pressure. A new cutting method called tool-swinging cutting method was proposed, by which both the tool service life and machining accuracy were increased. Acknowledgements The authors would like to express their sincere thanks to Miyagi Industrial Technology Institute and Japan Fine Ceramics Co., Ltd. for providing RB-SiC samples and technical supports. References [1] C. Hall, M.Tricard, H. Murakoshi, Y. Yamamoto, K. Kuriyama, and H.Yoko: P. SPIE, 5868, (2005), 58680V. [2] H. Toshiya, I. Ichiro, and S. Junichi: T. Jpn. Soc. Mech. Eng. C, 51, (1985), 1864-1870. [3] Y. Dai, H. Ohmori, W. Lin, H. Eto, N. Ebizuka, and K. Tsuno: Key Eng. Mat., 291-292, (2005), 121-126. [4] H. Tam, H. Cheng, and Y. Wang: J. Mater. Process. Tech., 192 193, (2007), 276-280. [5] H. Cheng, Z. Feng, S. Lei, and Y. Wang: Mater. Manuf. Process., 20, 6, (2005), 917-931. [6] J. Yan, T. Oowada, T. Zhou, and T. Kuriyagawa: J. Mater. Process. Tech., 209, (2009), 4802-4808. [7] J. Yan, Z. Zhang, and T. Kuriyagawa: Int. J. Mach. Tool. Manuf.., 49, 5, (2009), 366-374. [8] S. Suyama, T. Kameda and Y. Itoh: Diam. Relat. Mater., 12 (3-7), 1201-1204. [9] X. Li, T. He, and M. Rahman: Wear, 259, (2005), 1207-1214. [10] Z. Zhang, J. Yan, and T. Kuriyagawa: Key Eng. Mat., 389-390, (2009), 151-156. [11] M. Cai, P. Li, and M. Rahman: J. Manuf. Sci. Eng., 2007, 129, 2, 281-286. [12] C. Brookes: Diam. Relat. Mater., 1, (1991), 13-17. [13] E. Brookes: Diam. Relat. Mater., 8, (1999), 1536-1539. [14] E. Brookes: Diam. Relat. Mater., 9, (2000), 1115-1119. [15] K. Schiffmann, and A. Hieke: Wear, 254, (2003), 565-572. [16] V. Blank, M. Popov, N. Lvova, K. Gogolinski, and V. Reshetov: Tech. Phys. Lett. 23, (7), (1997), 546-547 [17] P. Humble: Nature, 273, 1978, 37-39. [18] R. Schouwenaars, V. Jacobo, and A. Ortiz: Int. J. Refract. Met. H., 27, (2009), 403-408. [19] A. Nakagawa, H. Suzuki, Y. Yamamoto, T. Moriwaki, T. Okino and Y. Hijikata: Trans. Japan Soc. Mech. Eng., 2006, (1), 149-150. (in Japanese). [20] J. Kim, and Y. Kang: Int. J. Mach. Tool. Manuf.. 37, 8, (1997), 1155-1165.