Development of High-Performance Vitrified Grinding Wheels using Ultrafine-Crystalline cbn Abrasive Grains

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1 Development of High-Performance Vitrified Grinding Wheels using Ultrafine-Crystalline cbn Abrasive Grains Yoshio ICHIDA 1, Masakazu FUJIMOTO 1, Yuichiro INOUE 1 and Keisuke MATSUI 1 1 Graduate School of Engineering, Utsunomiya University, Japan, ichida@cc.utsunomiya-u.ac.jp Abstract: This paper describes a development of high-performance vitrified bonded cbn grinding wheel using a new type of ultrafine-crystalline cbn () abrasive grains. Surface plunge grinding experiments using a cbn vitrified wheel made of the grains with a mesh size of #8/1 are carried out and its grinding performance is compared with those of cbn vitrified wheels made of representative conventional monocrystalline and polycrystalline cbn abrasive grains. This new abrasive grain possesses a higher fracture strength than these conventional cbn grains. Therefore, the wheel indicates lower grinding energy and higher grinding ratio than conventional cbn wheels. Keywords: Ultrafine-crystalline cbn abrasive grain, Grinding wheel, Grinding characteristics, High-speed steel 1. Introduction cbn grinding wheels are becoming widely used for the grinding of various engineering materials, such as steel, cast iron and superalloys [1-3]. In recent years, as high-level grinding systems using cbn wheels, such as ultra-high speed grinding machines and environmentally conscious grinding centers and so on are developed, demands for a higher performance of cbn wheels are increasing [4-6]. To enhance the grinding performance of cbn wheels, we have developed a new type of polycrystalline cbn abrasive grain by direct transformation from hexagonal boron nitride, which was produced from chemical vapor deposition process [7]. This new cbn abrasive grain possesses an ultrafine crystal structure composed of submicron sized primary crystal grains. Therefore, we call this new abrasive ultrafine-crystalline cbn (UcBN or for short) grain. The abrasive grain is expected to be used for wide applications from high-efficiency grinding to high-quality grinding, because its wear resistance is much more effective than those of conventional monocrystalline and polycrystalline cbn abrasive grains. The purpose of this study is to develop a highperformance vitrified cbn grinding wheel using the new ultrafine-crystalline cbn abrasive grains. Surface plunge grinding experiments using a cbn vitrified wheel made of the grains with a mesh size of #8/1 are carried out and its grinding performance is compared with those of cbn wheels made of representative conventional mono-crystalline and polycrystalline cbn abrasive grains. 2. Features of Ultrafine-Crystalline cbn Abrasive Grains A hexagonal cubic boron nitride (hbn) disk made by means of the chemical vapor deposition process, whose dimension was 6 mm in diameter and 1 mm in thickness, was prepared as the starting material. It was set in a Ta capsule, and transformed directly to polycrystalline cbn under a pressure higher than 7.5 GPa and a temperature higher than 273 K using a modified belt-type high-pressure apparatus [8]. Moreover, the cbn samples obtained were crushed into pieces using a roll crusher. The cbn abrasive grains with sizes of #1/12-#6/8 were obtained by classifying the crushed cbn powder. Figure 1 shows typical SEM images of the new polycrystalline, conventional polycrystalline and monocrystalline abrasive grains with a mesh size of #6/8, respectively. The aspect ratio of grain is a little larger than that of conventional monocrystalline cbn grain. The surface of grain is smoother than the surface of conventional cbn grain Figure 2 shows typical SEM images of the fracture surfaces of the new polycrystalline, conventional polycrystalline and monocrystalline (a) Ultrafine-crystalline cbn () grain 1 μm 1 μm 1 μm (b) Polycrystalline cbn () grain Figure 1: Shapes of cbn abrasive grains. (c) Monocrystalline cbn () grain

2 Ultrafine primary cbn crystal grains μm (a) Ultrafine-crystalline cbn () grain Coarse cbn primary grains Failure probability % W = 17.1 N (m = 1.4) W = 69.6 N (m = 1.94) 2. W = 46. N (m = 1.53) Fracture load W 1 N Figure 3: Weibull plot of compressive fracture load W (W : Average fracture load, m: Weibull modulus). Fine primary cbn crystal grains μm (b) Polycrystalline cbn() grain Tensile strength σ t GPa σ t = 3.55 GPa S.D. = 1.81 σ t = 2.34 GPa S.D. = 1.56 σ t =.95 GPa S.D. =.63 cbn grains Figure 4: Comparison in tensile fracture strength between grain and conventional cbn grains. Cleavage steps μm (c) Monocrystalline cbn () grain Figure 2: SEM images of fracture surfaces of ultrafine-crystalline cbn (), conventional polycrystalline cbn () and monocrystalline cbn () abrasive grains. abrasive grains. Some large primary crystal grains (Monocrystalline cbn) with a grain size of 1-2 μm are observed in the fracture surface of grain. Namely, the conventional polycrystalline grain has a crystal structure composed of micron sized primary-crystal grains and coarse primary-crystal grains with a grain size of 1-2 μm. In contrast, the grain has an ultrafine crystal structure composed of only sub-micron sized primary-crystal grains. The cbn grains with a size of #6/8 were used for a compressive fracture test to measure the fracture strength [7]. In this test, a single grain is compressed between two sintered diamond compounds and the normal load when it is fractured is measured as a fracture load W. Although the applied load is compressive, brittle fracture occurs due to a tensile stress which develops perpendicular to the loading axis. This loading is similar to the loads applied to grains in grinding, and provides a useful comparison of abrasive materials relative to their bulk strength. On the basis of the Griffith's theory on brittle fracture, the fracture strength σ t (tensile strength) can be approximated as [9]. W σ τ = (1). 32Α Where W is the average fracture load and A is the average projected area of cbn grain. The loading velocity is 1.67 N/s, and number of samples is 25. Figure 3 shows the results in weibull distribution of the fracture load W obtained from the compressive fracture test. The average fracture load W and the tensile strength of cbn grains obtained on the basis of these weibull plots are shown in Table 1 and Fig. 4. In general, the coefficient of variation in the sectional projected area A of the abrasive grain is large because of big variations in the size and shape. At the same time, because the abrasive grain is a typical brittle material, the variation in the fracture load W is also large. Therefore, the standard deviation in fracture strength of the abrasive grain obtained from the equation (1) indicates a large value because of these main factors. The tensile fracture strength of grain is about 1.5 times higher than that of grain. In addition, the difference in

3 weibull modulus between these cbn grains indicates that the strength of grain is more stable than those of conventional cbn grains. The optical emission spectrometry analysis of cbn grains has proved that the contents of metallic impurities in grain are much lower than those in and grains. 3. cbn Grinding Wheel and Experimental Procedure A vitrified bonded grinding wheel with a concentration of 1 was developed using abrasive grains with a mesh size of #8/1. To clarify the grinding performance of these wheels, surface plunge grinding experiments were conducted on a horizontal spindle surface grinding machine. The grinding conditions are listed in Table1. A vitrified cbn wheel with a replaceable cbn insert, as well as the complete usual grinding wheel, was used to observe directly the changes of the wheel surface in the grinding process [1]. Scanning electron microscope with four electron probes (3D-SEM) was used for 3D observations of the wheel working surface and the grain cutting edges. The complete usual cbn wheel of the same specification as the cbn wheel with the replaceable cbn insert was made using a same rod of cbn grains. From the grinding experiment under a same condition with these cbn wheels, it was confirmed that the performance of each wheel was almost the same. The dressing of cbn wheel has been performed using a rotary diamond dresser (Dressing wheel: SD4Q75M) equipped with an AE sensor under the following dressing conditions of: peripheral dressing speed 16.5 m/s, peripheral wheel speed ratio.5, dressing lead.1 mm/rev, dressing depth of cut 2 μm 5 times. 4. Grinding Performance of Ultrafine-Crystalline cbn Wheels 4.1 Grinding Force and Grinding Energy Figure 5 shows the changes in the normal grinding Grinding method Grinding wheel Peripheral wheel speed Work speed Wheel depth of cut Grinding fluid Workpiece Dressaing method Table 1: Grinding conditions. Surface plunge grinding (Up cut) cbn8l1v () cbn8l1v () cbn8l1v () Dimensions:φ2 t1 mm v s = 33 m/s v w = m/s a = 1 μm Soluble type (JIS/W-2-2) 2% dilution High speed steel (SKH51/JIS, M2/AISI) Hardness: 65HRC Dimensions:1 l 5 t 3 h mm Rotary diamond dresser (SD4Q75M) Speed ratio:.5 Dressing lead:.1 mm/rev Dressing depth of cut: 2μm 5 times forces F n ' and tangential grinding force F t ' as the accumulated stock removal V w ' increases in the grinding processes with three types of cbn grains. The grinding forces in using the grain are much lower than those in using the conventional cbn grains. In particular, after the stock removal V w ' exceeds around 5 mm 3 /mm the grinding forces for the grain are reduced by 3-4 % compared with those in using the and grains. Using these tangential forces, the specific grinding energy U e was obtained from the equation U e = (F t 'v s )/(av w ) (F t ' : tangential grinding force per unit grinding width, v s : peripheral wheel speed, v w : work speed, a: wheel depth of cut). The results are show in Figure 6. The specific grinding energy in using grain is 6-7 % of those in using and grains. The majority of the grinding energy is converted into the heat. The lower the specific grinding energy is, the less the grinding heat generated at the contact face between the wheel and the workpiece becomes. This low specific grinding energy is one of the important features in grinding with the grain. Therefore, grain may be used for constructing an environmentally Grinding forces F'n, F't N/mm Normal grinding force F' n 15 Tangential grinding force F' t Figure 5: Changes in the normal grinding forces F n ' and tangential grinding force F t ' as accumulated stock removal V w ' increases (v w =.15m/s). Specific grinding energy U e J/mm Figure 6: Changes in the specific grinding energy U e as accumulated stock removal V w ' increases (v w =.15m/s).

4 conscious grinding system such as a coolantless grinding. 4.2 Wheel Wear Characteristics Figure 7 shows the changes of the radial wheel wear ΔR as the accumulated stock removal increases in the grinding processes with three types of cbn grains. In any case, the radial wheel wear increases rapidly at a initial stock removal range from to 5 mm 3 /mm (Initial wear region) and after the stock removal of 5 mm 3 /mm increases gradually almost in proportion to the stock removal V w ' (Steady-state wear region). The inclination of straight line in the steady-state wear region, namely, the wheel wear volume per unit stock removal is defined as the volumetric wheel wear rate r g. The value of r g for grain is much lower than those of conventional cbn grains. The values of the steady grinding ratio G s, that is the reciprocal of r g, in the grinding processes with and conventional cbn grains are indicated in Figure 8. The grinding ratio G s in using grain is about 8 times higher than that in using grain, and about 11 times higher than that in using grain. As the fracture strength of abrasive grain is higher than those of conventional cbn abrasive grains, the cutting edges on the wheel surface are not fractured easily compared with those on the conventional cbn wheels. Figure 9 shows typical results of three-dimensional SEM observation for the working surfaces of and grinding wheel after grinding an accumulated stock removal of 1 mm 3 /mm. The cutting edge density of wheel is Grinding direction Wheel wear volume V'g mm 3 /mm Figure 7: Change of radial wheel wear ΔR as accumulated stock removal V w increases (v w =.15m/s). Steady grinding ratio G s Radial wheel wear ΔR μm Figure 8: Comparison in grinding ratio between and conventional cbn abrasive grains (v w =.15m/s). Grinding direction cbn wheel 5μm 5μm (a) wheel (b) wheel Figure 9: Typical 3D-SEM observation results of wheel working surfaces of and wheel (Accumulated stock removal V' w =1 mm 3 /mm, v w =.15m/s).

5 (1) After dressing (1) After dressing Micro fracture Large fracture (2) V w =2mm 3 /mm (2) V w =1mm 3 /mm (a) (b) Figure 1: Wear process of cbn grain cutting edge (v w =.15m/s). considerably lower than that of wheel, as understood from the comparison between 3D-profiles of both wheels, because the grain cutting edges on wheel are easily fractured or easily broken down compared with those on wheel. Figure 1 shows a comparison in wear behavior of grain cutting edge between and wheels. These results indicate that the wear mechanism of the grain cutting edges is dominantly due to the micro brittle attritious wear [11]. That is, the fracture scale of cutting edges on the grains is very small compared with the cases of the conventional cbn grains, and as a result, the wheel shows a higher wear resistance and a higher grinding ratio than those of the conventional cbn wheels. Such high grinding ratio is another one of the important features in grinding with grain. Therefore, grain may be widely used for grinding various difficult-to-grind materials such as superalloy, high-speed steel and so on [12]. 4.3 Roughness of Ground Surface The changes in roughness Ra of the ground surfaces as the accumulated stock removal increases in the grinding processes with three types of cbn grains are shown in Figure 11. The rate of increase in surface roughness with the stock removal in using the grain is much lower than those in using conventional cbn grains. This low rate of increase in the roughness is brought from the high wear resistance of the grain. The estimated grinding wheel life using wheel, based on the finished surface, is longer than that in using wheel. For instance, wheel life of wheel when surface roughness increases until.4 μm is about 3 times longer than that of wheel. Surface roughness Ra μm Figure 11: Change of surface roughness Ra as accumulated stock removal V w increases (v w =.15m/s). 4.4 High-Efficiency Grinding Figure 12 shows a differences in grinding ratio G s and specific grinding energy U e between low work speed v w =.15m/s (low material removal rate Z'=1.5mm 3 /mm/s) and high work speed v w =.35m/s (high material removal rate Z'=3.5mm 3 /mm/s). Where, specific grinding energy U e is an average value in the stock removal range from 5 to 1 mm 3 /mm. The grinding ratio when grinding at work speed of.35m/s a little lower than that at work speed of.15m/s, while the specific grinding energy at work speed of.35m/s is lower than the half of that at work speed of.15m/s. Thus, wheel has a high performance in the high-efficiency grinding.

6 Steady grinding ratio G s v w =.15 m/s 5919 v w =.35 m/s Specific grinding energy U e J/mm v w =.15 m/s v w =.35 m/s (a) Grinding ratio G s (b) Specific grinding energy U e Figure 12: Effects of work speed v s (stock removal rate Z') on grinding ratio G s and specific grinding energy U e. As described above, wheel is suitable for applications with a high-efficiency in grinding various difficult-to-grind materials, because it gives lower wheel wear rate, higher grinding ratio, lower grinding energy and surface roughness, than conventional cbn wheels. 5. Conclusions The main results obtained in this study are summarized as follows: (1) The fracture strength of abrasive grain (#6/8) is about 3.7 times higher than that of conventional monocrystalline grain. (2) wheel has a very high wear resistance as compared with conventional cbn wheels. Therefore, the grinding ratio in grinding with wheel is around 1 times higher than those in grinding with conventional cbn wheels. (3) The estimated tool life of wheel, based on the roughness of the finished surface, is much longer than those of conventional cbn wheels. (4) abrasive grain can be used for wide applications in grinding various difficult-to-grind materials, because it gives lower wheel wear rate, higher grinding ratio, lower specific grinding energy and surface roughness, than conventional cbn abrasive grains. References [1] Webster, J. and Tricard, M., 24, Innovations in Abrasive Production for Precision Grinding, Annals of the CIRP, Vol.53, No.2, pp [2] Ichida, Y., Sato, R., Morimoto, Y., Ohsawa, Y. and Fredj, B., N., 26, Formation Mechanism of Finished Surface in Ultrahigh-Speed Grinding with cubic Boron Nitride (cbn) Wheels, JSME International Journal (Series C), Vol.49, No.1, pp [3] Brinksmeier, E. and Minke, E., 1993, High- Performance Surface Grinding-The Influence of Coolant on the Abrasive Processs-, Annals of the CIRP, Vol.42, No.1, pp [4] Westkamper, E. and Tonshoff, H. K., 1993, cbn or CD grinding of Profiles, Annals of the CIRP, Vol.42, No.1, pp [5] Stephenson, D., Jin, T., and Corbett, J., 22, High Effiency Deep Grinding of a Low Alloy Steel with Plated cbn Wheels, Annals of the CIRP, Vol.51, No.1, pp [6] Shi, Z. and Malkin, S., 23, An Investigation of Grinding with Electroplated CBN Wheels, Annals of the CIRP, Vol.52, No.1, pp [7] Ichida, Y. and Kishi, K., 1997, The Development of Nanocrystalline CBN for Enhanced Superalloy Grinding Performance, ASME Journal of Manufacturing Science and Engineering, Vol.119, No.1, pp [8] Yamaoka, S., Akashi, M., Kanda, H., Osawa, T., Taniguchi, T., Sei, H. and Fukunaga, O., 1992, Development of belt type high pressure apparatus for material synthesis at 8 GPa, Journal of High Pressure Institute of Japan, Vol.3, pp [9] Yoshikawa, H. and Sata, T., 196, Fracture Strength of Abrasive Grains, Journal of Japan Society for Precision Engineering, Vol.26, No.8, pp [1] Fujimoto, M., Ichida, Y., Sato, R. and Morimoto, Y., 26, Characterization of Wheel Surface Topography in cbn Grinding, JSME International Journal (Series C), Vol.49, No.1, pp [11] Ichida, Y., 28, Profile Grinding of High-Speed Steel using Ultrafine-Crystalline cbn Wheels, JSME Journal of Advanced Mechanical Design, Systems, and Manufacturing, Vol.2, No.3, pp [12] Ichida, Y., Sato, R., Morimoto, Y. and Inoue, Y., 26, Profile Grinding of Superalloys with Ultrafine- Crystalline cbn Wheels, JSME International Journal (Series C), Vol.49, No.1, pp