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Ceramics grinding under the condition of constant pressure
Journal of Materials Processing Technology 129 (2002) 176±181
Ceramics grinding under the condition of constant pressure J.Y. Shena,*, C.B. Luoa, W.M. Zenga, X.P. Xua, Y.S. Gaob a
b
College of Mechanical Engineering and Automation, Huaqiao University, Quanzhou, Fujian 362011, PR China Department of Mechanical Engineering, Hong Kong University of Science and Technology Clear Water Bay, Kowloon, Hong Kong
Abstract To meet the increasing demand on the quality and cost of precision components for the semiconductor industries, extensive studies on high ef®ciency and precision machining of ceramic materials have been conducted over the past few years. It is found that the effects of grinding pressure and the rotational speed of the spindle in the machining of ceramic materials are very signi®cant on the quality of the grinding process. In order to achieve stable grinding conditions for improved performance, a new grinding control scheme in which the grinding pressure is maintained constant throughout the grinding process was explored in the present study. Based on the experimental results, the microanalysis of the ground surfaces, and the comparison of the analytic and measured temperature pro®les, the material removal mechanisms for constant pressure grinding were discussed. # 2002 Published by Elsevier Science B.V. Keywords: Ceramic; Vertical grinding; Grinding force; Grinding energy; Grinding temperature
1. Introduction Special ceramics such as silicon nitride, alumina, and zirconia are considered to have increasing potential applications in the semiconductor industries and other engineering industries because of their extraordinary properties. Usually, applications of ceramic materials require parts or components with high dimensional accuracy and good surface quality. Due to their high hardness and brittleness, ceramic materials are dif®cult to machine, and the surface and subsurface damage produced by machining can be detrimental to the strength and performance of the components. As a fact, the widespread utilization of ceramic components has been greatly inhibited by the high cost of machining, which can account for 80% or more of the total component cost. Therefore, a critical step in the production of ceramic components is cost-effective machining of ceramic materials with high quality [1±3]. In order to meet the increasing demand on the quality and cost of precision components for the semiconductor industries, extensive research on high ef®ciency and precision machining of ceramic materials have been conducted over the past few years, including various
*
Corresponding author. Tel.: 86-595-269-3567; fax: 86-595-268-6969. E-mail address: [email protected] (J.Y. Shen). 0924-0136/02/$ ± see front matter # 2002 Published by Elsevier Science B.V. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 0 6 3 6 - 2
advanced technologies in cutting, grinding and polishing. Among all of the methods for the machining of ceramics, grinding with diamond wheels is still the most popular [2± 8]. In recent years, vertical grinding has been applied widely to produce semiconductor materials. It is found that the quality of ground ceramic materials is greatly dependent on the grinding pressure and the rotational speed of the spindle in the grinding process [9,10]. In this study, a vertical grinding under the condition of constant pressure was applied for the machining of two typical ceramics. 2. Experimental Grinding tests were performed on a numerical control (NC) vertical grinding machine. The grinding pressure was controlled by an automatic pneumatic control system. The grinding forces were measured by means of a piezoelectric dynamometer (Kistler 9257BA), and the power consumed by the spindle was measured with a three-phase wattmeter (GX-3). The outputs of the dynamometer and wattmeter were fed into an A/D converter (DAQCard-AI-16E-4) and sampled at a high frequency by a PC. Then the signals recorded by the PC were ®ltered by Matlab software at a lower frequency. The ground surfaces were examined by a scanning electron microscope (SEM).
J.Y. Shen et al. / Journal of Materials Processing Technology 129 (2002) 176±181 Table 1 Specifications of wheels and grinding conditions
3. Results and discussion
Specifications of wheels Diameter (mm) Rim width (mm) Diamond grit size Concentration Bond
75 16 200#, 400# 75 Resin
3.1. Grinding forces under the conditions of different pressures and spindle speeds
Grinding conditions Spindle speed (rpm) Air pressure (Pa) Coolant Workpiece Size of workpiece (mm3)
500, 1000, 1500, 2000 3, 5, 7, 9 Water Si3N4, Glass 5 10 20
The speci®cations of the cup-type diamond wheels used in the experiments and the operating conditions are listed in Table 1. Fig. 1 shows a schematic illustration of the experimental device and the instruments for measuring forces and power. In the experiments, the worktable did not move and the pressure from the spindle was considered as the normal force (Fn) in the grinding process. The direction of the tangential force (Ft) is shown in Fig. 1. A separate experiment was arranged to measure the grinding temperature using a foil thermocouple, as shown in Fig. 2. The wheel here was a resin bond diamond wheel (400#) as listed in Table 1. The pressure was 5 Pa and the spindle rotational speed was 1000 rpm. The workpiece used in the experiment was Si3N4. The signals of grinding temperature recorded by the real time FFT analyzer (DI2200) were transferred to a PC and were analyzed by Origin software.
177
The tangential forces measured during the grinding of Si3N4 and glass are plotted versus the wheel pressure in Fig. 3, where two kinds of grit size, 400# and 200#, were used for the Si3N4 but only 400# for the glass. It can be seen that the tangential forces increase with pressure. Ft is closely related to the depth of cut and friction. An increase of pressure leads to a larger depth of cut and more friction between the wheel and the workpiece. Based on previous studies, the indentation fracture mechanism approach can be used to account for abrasive and workpiece interactions in the grinding of ceramics [4,11]. A larger grit size causes more material to be removed in the fracture mode. The reduction of ductile ¯ow during the grinding process decreases the tangential forces, which are required for removing material. Therefore, the tangential forces produced during the grinding of Si3N4 with 400# grit wheel were higher than those with 200# grit wheel (Fig. 3). The tangential forces in the grinding of the glass are lower than those in the grinding of the Si3N4, which can be attributed to their large difference in hardness and strength. In general, the wheel speed is closely associated with the mechanism of material removal and the ground surface quality. The tangential forces under different spindle speeds are shown in Fig. 4. Although a fast grinding speed has a relatively high material removal rate, it reduces the average stock removal of a single diamond grit. As a result, the tangential forces reduce with the increasing of the spindle
Fig. 1. Illustration of experimental set-up.
Fig. 2. Experimental set-up for measuring grinding temperature.
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J.Y. Shen et al. / Journal of Materials Processing Technology 129 (2002) 176±181
Fig. 5. Ground surface of glass (P 3 Pa, n 1000 rpm). Fig. 3. Tangential forces versus pressure.
speed. The tangential force is considered to consist of cutting and friction components in other grinding modes such as surface grinding. Usually, a larger wheel speed leads to more friction between the wheel and the workpiece, increasing the tangential force. The result obtained might indicate a different mechanism of vertical pressure grinding. The friction might not be the main factor in the tangential force.
Fig. 6. Ground surface of Si3N4 (P 3 Pa, n 1000 rpm).
3.2. Microanalysis of ground surfaces The structures of the ground ceramic surfaces were widely analyzed using SEM, X-ray diffraction (XRD), transmission electron microscopy (TEM) and other inspection methods in recent studies on the mechanisms for ceramics grinding [11± 13]. Figs. 5±8 show SEM pictures of ceramic surfaces ground under different pressures and spindle speeds. It is found that the surface appearance is related closely to the characteristics of the ceramic materials and the grinding conditions. In Fig. 5, the SEM observation of the glass surface ground by 400# grit wheel reveals that the material is mainly removed in the fracture mode, which can be attributed to the high brittleness of the glass. However, ductile ¯owing prevails (see Fig. 6) on a Si3N4 surface ground under the same conditions. Besides the material characteristics, the structure of ground surface is also determined by the selections of the pressure and wheel speed. For the grinding of ceramics with
Fig. 4. Tangential forces versus spindle speed.
Fig. 7. Ground surface of Si3N4 (P 9 Pa, n 1000 rpm).
a diamond wheel, it has been concluded that material removal occurs by both the fracture mechanism and the ductile mechanism. The quality of the ground surfaces and strength degradation are relative to the partition of the two material removal mechanisms. From surfaces ground under different pressures (Figs. 6 and 7), it can be seen that the increasing of the pressure led to a large depth of cut and produced deep grooves which are consistent with the results shown in Fig. 3. Moreover, a large depth of cut caused more microcracks along the bottom of the grooves, which may do damage to the quality of the ground surface. Comparing
Fig. 8. Ground surface of Si3N4 (P 3 Pa, n 2000 rpm).
J.Y. Shen et al. / Journal of Materials Processing Technology 129 (2002) 176±181
179
Fig. 6 with Fig. 8, the ground surface produced by a high spindle speed obviously shows more ductile ¯ow. It has been proven that the ductile mode grinding enhances the surface quality. Together with the experimental results and the microanalysis of the ground surfaces, in order to realize the ductile mode grinding under constant pressure grinding, the pressure should be reduced and the wheel speed should be increased. However, it should be pointed out that reducing the pressure leads to a low material removal rate and may be uneconomical. 3.3. Thermal analysis and energy partition 3.3.1. Theoretic analysis of the grinding temperature According to the condition of the contact zone in vertical grinding, a quasi-steady-state heat source model could be used to calculate the temperature distribution within the workpiece [14±16]. In this model, it is supposed that there is a trapezoid heat ¯ux distribution on the workpiece surface along the grinding zone 2L, as shown in Fig. 9. From this heat source model, the temperature rise in the workpiece can be calculated as Z L Z t 1 dt 0 2 2 y x; z (1) q x0 dx0 e x x z =4at 2pl L t 0 where q x0
r1 r2 2x0 qm ; r1 r2
where qm is the average heat ¯ux at the grinding zone and is equal to ep/2BL; e the fraction of the total energy conducted as heat into the workpiece; l the thermal conductivity; a the thermal diffusivity; 2L the length of the grinding zone; p the power consumed; and r1 and r2 the internal and external diameters of the grinding wheel. Based on the de®ned parameters, Eq. (1) can be written as Z tZ L ep y x; z r1 r2 2x0 4plBL r1 r2 0 L 1 0 2 2 e x x z =4at dx0 dt (2) t
Fig. 9. Thermal model for theoretical analysis.
Fig. 10. Measured and analytical temperature profiles versus grinding time.
3.3.2. Energy partition Comparing the analytical temperature pro®le with the measured grinding temperature curve (see Fig. 10), it can be seen that the analytical temperature response pro®le for an energy partition of e 55 percent basically ®ts well with the measured temperature pro®le. Although both temperatures increase with time, it can be seen that the measured temperature is slightly higher than the analytical one, especially at the initial stage of the grinding process, which may contribute to the unstable grinding condition of this stage. 3.4. The effects of grinding temperature Generally, the grinding temperature is considered to have a deleterious effect on workpiece quality. However, it is reported that exposure of hot pressed silicon nitride (HPSN) to a temperature of 800 8C increased the strength of HPSN due to the formation of glassy phases. Therefore, the reduction in surface fracture and apparent increase in ductile ¯ow may be associated with glassy phase formation at elevated grinding temperature [4]. On the other hand, whether the lower temperature on the grinding zone remains to have the same effects on the ground surface has not been evaluated de®nitely. The grinding temperatures during the dry and wet grinding of Si3N4 were examined and the corresponding ground surfaces were checked using the SEM. The recorded temperatures and SEM pictures of ground surfaces are shown in Fig. 11. From the temperature curves, it can be seen that the temperature in dry grinding was about 160 8C, and that in wet grinding was below 70 8C. Although the temperature in dry grinding is higher than that in wet grinding, the temperatures are far below the temperature for the formation of glassy phases. The SEM pictures show that the surface quality of dry grinding is worse than that of wet grinding. There is more surface fracture on the workpiece surface of dry grinding. The reason may be the worse topography of resin bond diamond wheel, which was damaged by the grinding temperature generated in dry grinding [17].
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J.Y. Shen et al. / Journal of Materials Processing Technology 129 (2002) 176±181
Fig. 11. Temperature curves and ground surfaces (Si3N4).
3.5. Specific grinding energy A fundamental parameter for characterizing grinding processes is speci®c energy (u), which is de®ned as the energy expended per unit volume of material removal and can be calculated as u
Ft Vs WVw a
(3)
where Ft is the tangential force; Vs the linear speed of wheel; W the grinding width; Vw the workpiece speed; and a the depth of cut. In the experiments, the consumed power was measured using a wattmeter and the stock removal of ceramic materials at each stage was measured separately. Because ceramics are dif®cult-to-cut materials, the stock removal of removal was designed to be measured at 10 s stages. Therefore, the speci®c energy of the experiment can be calculated as u
PT Q
(4)
where P is the power consumed; Q the stock removal of material at each stage; and T the length of time of each stage. The speci®c energy is usually considered to consist of cutting and sliding components. It has been reported that almost all of the energy consumed in the grinding of ceramics is attributable to ductile ¯ow by plowing [4,18].
Fig. 13. Specific grinding energy versus spindle speed.
Based on this viewpoint, a decrease in the material removal rate may reduce the undeformed chip size, causing more ¯ow and less brittle fracture, which should result in higher speci®c energy. Contrarily, low speci®c energy means a larger depth of cut and more brittle fracture. The speci®c grinding energies obtained under different grinding conditions are plotted in Figs. 12 and 13. The results show that speci®c energies fall with the pressure and grit size (Fig. 12), and increases with the spindle rotational speed (Fig. 13). It is interesting to note that the speci®c energies in Figs. 12 and 13 are much lower than those in the surface grinding of ceramics [4,18], which might be attributable to the different mechanisms for vertical grinding and surface grinding. To some extent, the result is consistent with the result for tangential force in Section 3.1. Combining the results in Sections 3.1, 3.2 and 3.5, it is found that low pressure and high wheel speed should be selected to remove ceramics ef®ciently in the ductile mode in vertical grinding. As shown in Figs. 12 and 13, the speci®c energy for the grinding of the glass is much less than that for the grinding of Si3N4, which is consistent with the results revealed in Figs. 5 and 6. 4. Conclusions
Fig. 12. Specific grinding energy versus pressure.
The surface quality of ground ceramics depends on the mechanism of material removal in the vertical grinding
J.Y. Shen et al. / Journal of Materials Processing Technology 129 (2002) 176±181
process. For the grinding of Si3N4 and glass under the condition of constant pressure, increasing the pressure enhances material removal rate, and at the same time causes more machining-induced microcracks on the ground surfaces. Along with the analysis of tangential forces, and speci®c grinding energy, and microobservations on ground surfaces, it is found that low pressure and high wheel speed should be selected to remove ceramics ef®ciently in the ductile mode in vertical grinding. From the theoretically analytical and measured grinding temperatures in the vertical grinding of ceramics, it is found that the analytical temperature pro®le with e 55% has the same trend as the measured one. The measured temperature is higher that the analytical one at the initial stage of the grinding process, which might contribute to the unstable grinding condition of this stage. The grinding temperatures in the vertical grinding of ceramics under a constant pressure are not high enough for glassy phase formation, and may not reduce surface fracture as expected. However, the temperature in dry grinding may cause thermal damage to the resin bond diamond wheel, thereby resulting in a low quality workpiece surface. Acknowledgements This research was supported by the Natural Science Fund of Fujian Province in China and the Open-end Fund of the
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State Education Ministry Key Laboratory of High Temperature Structure Ceramics and Machining Technology for Engineering Ceramics in Tianjin University. References [1] L.S. Wang, Special Ceramics, Central-South University Press, 2000, p. 5 (in Chinese). [2] R. Komanduri, D.A. Lucca, Y. Tani, Ann. CIRP 46 (2) (1997) 545± 596. [3] G. KoÈnnemann, H. Eichhorn, R. Petzold, Ind. Diamond Rev. 60 (4) (2000) 301±303. [4] S. Malkin, T.W. Hwang, Ann. CIRP 45 (2) (1996) 569±580. [5] B. Zhang, T.D. Howes, Ann. CIRP 43 (1) (1994) 305±308. [6] R. Komanduri, Stillwater, Ann. CIRP 45 (2) (1996) 509±514 [7] H. Ohmori, T. Nakagawa, Ann. CIRP 39 (1) (1990) 329±332. [8] J. Ikeno, Y. Tani, H. Sato, Ann. CIRP 43 (1) (1994) 293±298. [9] J.Y. Shen, X.P. Xu, B. Lin, Y.S. Xu, Key Eng. Mater. 202±203 (2001) 171±176. [10] G. Srihari, G.K. Lal, J. Mater. Process. Technol. 44 (1994) 14±28. [11] S. Malkin, J.E. Ritter, ASME J. Manuf. Sci. Eng. 111 (5) (1989) 167±174. [12] K. Li, T.W. Liao, J. Mater. Process. Technol. 57 (1996) 207±220. [13] J.E. Mayer, G.-P. Fang, Ann. CIRP 44 (1) (1995) 279±282. [14] Z.B. Hou, S.J. He, X.X. Li, Conduction of Heat in Solids, Press of Shanghai Science and Technology, vol. 12, 1984 (in Chinese). [15] X.P. Xu, S. Malkin, ASME J. Manuf. Sci. Eng. 123 (2) (2001) 191± 195. [16] S. Malkin, R.B. Anderson, ASME J. Manuf. Sci. Eng. 96 (1974) 1177±1182. [17] E.C. Bianchi et al., Ind. Diamond Rev. 58 (4) (1998) 105±110. [18] T.W. Hwang, C.J. Evans, S. Malkin, Wear 225±229 (1999) 862±867.
Ceramics grinding under the condition of constant pressure J.Y. Shena,*, C.B. Luoa, W.M. Zenga, X.P. Xua, Y.S. Gaob a
b
College of Mechanical Engineering and Automation, Huaqiao University, Quanzhou, Fujian 362011, PR China Department of Mechanical Engineering, Hong Kong University of Science and Technology Clear Water Bay, Kowloon, Hong Kong
Abstract To meet the increasing demand on the quality and cost of precision components for the semiconductor industries, extensive studies on high ef®ciency and precision machining of ceramic materials have been conducted over the past few years. It is found that the effects of grinding pressure and the rotational speed of the spindle in the machining of ceramic materials are very signi®cant on the quality of the grinding process. In order to achieve stable grinding conditions for improved performance, a new grinding control scheme in which the grinding pressure is maintained constant throughout the grinding process was explored in the present study. Based on the experimental results, the microanalysis of the ground surfaces, and the comparison of the analytic and measured temperature pro®les, the material removal mechanisms for constant pressure grinding were discussed. # 2002 Published by Elsevier Science B.V. Keywords: Ceramic; Vertical grinding; Grinding force; Grinding energy; Grinding temperature
1. Introduction Special ceramics such as silicon nitride, alumina, and zirconia are considered to have increasing potential applications in the semiconductor industries and other engineering industries because of their extraordinary properties. Usually, applications of ceramic materials require parts or components with high dimensional accuracy and good surface quality. Due to their high hardness and brittleness, ceramic materials are dif®cult to machine, and the surface and subsurface damage produced by machining can be detrimental to the strength and performance of the components. As a fact, the widespread utilization of ceramic components has been greatly inhibited by the high cost of machining, which can account for 80% or more of the total component cost. Therefore, a critical step in the production of ceramic components is cost-effective machining of ceramic materials with high quality [1±3]. In order to meet the increasing demand on the quality and cost of precision components for the semiconductor industries, extensive research on high ef®ciency and precision machining of ceramic materials have been conducted over the past few years, including various
*
Corresponding author. Tel.: 86-595-269-3567; fax: 86-595-268-6969. E-mail address: [email protected] (J.Y. Shen). 0924-0136/02/$ ± see front matter # 2002 Published by Elsevier Science B.V. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 0 6 3 6 - 2
advanced technologies in cutting, grinding and polishing. Among all of the methods for the machining of ceramics, grinding with diamond wheels is still the most popular [2± 8]. In recent years, vertical grinding has been applied widely to produce semiconductor materials. It is found that the quality of ground ceramic materials is greatly dependent on the grinding pressure and the rotational speed of the spindle in the grinding process [9,10]. In this study, a vertical grinding under the condition of constant pressure was applied for the machining of two typical ceramics. 2. Experimental Grinding tests were performed on a numerical control (NC) vertical grinding machine. The grinding pressure was controlled by an automatic pneumatic control system. The grinding forces were measured by means of a piezoelectric dynamometer (Kistler 9257BA), and the power consumed by the spindle was measured with a three-phase wattmeter (GX-3). The outputs of the dynamometer and wattmeter were fed into an A/D converter (DAQCard-AI-16E-4) and sampled at a high frequency by a PC. Then the signals recorded by the PC were ®ltered by Matlab software at a lower frequency. The ground surfaces were examined by a scanning electron microscope (SEM).
J.Y. Shen et al. / Journal of Materials Processing Technology 129 (2002) 176±181 Table 1 Specifications of wheels and grinding conditions
3. Results and discussion
Specifications of wheels Diameter (mm) Rim width (mm) Diamond grit size Concentration Bond
75 16 200#, 400# 75 Resin
3.1. Grinding forces under the conditions of different pressures and spindle speeds
Grinding conditions Spindle speed (rpm) Air pressure (Pa) Coolant Workpiece Size of workpiece (mm3)
500, 1000, 1500, 2000 3, 5, 7, 9 Water Si3N4, Glass 5 10 20
The speci®cations of the cup-type diamond wheels used in the experiments and the operating conditions are listed in Table 1. Fig. 1 shows a schematic illustration of the experimental device and the instruments for measuring forces and power. In the experiments, the worktable did not move and the pressure from the spindle was considered as the normal force (Fn) in the grinding process. The direction of the tangential force (Ft) is shown in Fig. 1. A separate experiment was arranged to measure the grinding temperature using a foil thermocouple, as shown in Fig. 2. The wheel here was a resin bond diamond wheel (400#) as listed in Table 1. The pressure was 5 Pa and the spindle rotational speed was 1000 rpm. The workpiece used in the experiment was Si3N4. The signals of grinding temperature recorded by the real time FFT analyzer (DI2200) were transferred to a PC and were analyzed by Origin software.
177
The tangential forces measured during the grinding of Si3N4 and glass are plotted versus the wheel pressure in Fig. 3, where two kinds of grit size, 400# and 200#, were used for the Si3N4 but only 400# for the glass. It can be seen that the tangential forces increase with pressure. Ft is closely related to the depth of cut and friction. An increase of pressure leads to a larger depth of cut and more friction between the wheel and the workpiece. Based on previous studies, the indentation fracture mechanism approach can be used to account for abrasive and workpiece interactions in the grinding of ceramics [4,11]. A larger grit size causes more material to be removed in the fracture mode. The reduction of ductile ¯ow during the grinding process decreases the tangential forces, which are required for removing material. Therefore, the tangential forces produced during the grinding of Si3N4 with 400# grit wheel were higher than those with 200# grit wheel (Fig. 3). The tangential forces in the grinding of the glass are lower than those in the grinding of the Si3N4, which can be attributed to their large difference in hardness and strength. In general, the wheel speed is closely associated with the mechanism of material removal and the ground surface quality. The tangential forces under different spindle speeds are shown in Fig. 4. Although a fast grinding speed has a relatively high material removal rate, it reduces the average stock removal of a single diamond grit. As a result, the tangential forces reduce with the increasing of the spindle
Fig. 1. Illustration of experimental set-up.
Fig. 2. Experimental set-up for measuring grinding temperature.
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J.Y. Shen et al. / Journal of Materials Processing Technology 129 (2002) 176±181
Fig. 5. Ground surface of glass (P 3 Pa, n 1000 rpm). Fig. 3. Tangential forces versus pressure.
speed. The tangential force is considered to consist of cutting and friction components in other grinding modes such as surface grinding. Usually, a larger wheel speed leads to more friction between the wheel and the workpiece, increasing the tangential force. The result obtained might indicate a different mechanism of vertical pressure grinding. The friction might not be the main factor in the tangential force.
Fig. 6. Ground surface of Si3N4 (P 3 Pa, n 1000 rpm).
3.2. Microanalysis of ground surfaces The structures of the ground ceramic surfaces were widely analyzed using SEM, X-ray diffraction (XRD), transmission electron microscopy (TEM) and other inspection methods in recent studies on the mechanisms for ceramics grinding [11± 13]. Figs. 5±8 show SEM pictures of ceramic surfaces ground under different pressures and spindle speeds. It is found that the surface appearance is related closely to the characteristics of the ceramic materials and the grinding conditions. In Fig. 5, the SEM observation of the glass surface ground by 400# grit wheel reveals that the material is mainly removed in the fracture mode, which can be attributed to the high brittleness of the glass. However, ductile ¯owing prevails (see Fig. 6) on a Si3N4 surface ground under the same conditions. Besides the material characteristics, the structure of ground surface is also determined by the selections of the pressure and wheel speed. For the grinding of ceramics with
Fig. 4. Tangential forces versus spindle speed.
Fig. 7. Ground surface of Si3N4 (P 9 Pa, n 1000 rpm).
a diamond wheel, it has been concluded that material removal occurs by both the fracture mechanism and the ductile mechanism. The quality of the ground surfaces and strength degradation are relative to the partition of the two material removal mechanisms. From surfaces ground under different pressures (Figs. 6 and 7), it can be seen that the increasing of the pressure led to a large depth of cut and produced deep grooves which are consistent with the results shown in Fig. 3. Moreover, a large depth of cut caused more microcracks along the bottom of the grooves, which may do damage to the quality of the ground surface. Comparing
Fig. 8. Ground surface of Si3N4 (P 3 Pa, n 2000 rpm).
J.Y. Shen et al. / Journal of Materials Processing Technology 129 (2002) 176±181
179
Fig. 6 with Fig. 8, the ground surface produced by a high spindle speed obviously shows more ductile ¯ow. It has been proven that the ductile mode grinding enhances the surface quality. Together with the experimental results and the microanalysis of the ground surfaces, in order to realize the ductile mode grinding under constant pressure grinding, the pressure should be reduced and the wheel speed should be increased. However, it should be pointed out that reducing the pressure leads to a low material removal rate and may be uneconomical. 3.3. Thermal analysis and energy partition 3.3.1. Theoretic analysis of the grinding temperature According to the condition of the contact zone in vertical grinding, a quasi-steady-state heat source model could be used to calculate the temperature distribution within the workpiece [14±16]. In this model, it is supposed that there is a trapezoid heat ¯ux distribution on the workpiece surface along the grinding zone 2L, as shown in Fig. 9. From this heat source model, the temperature rise in the workpiece can be calculated as Z L Z t 1 dt 0 2 2 y x; z (1) q x0 dx0 e x x z =4at 2pl L t 0 where q x0
r1 r2 2x0 qm ; r1 r2
where qm is the average heat ¯ux at the grinding zone and is equal to ep/2BL; e the fraction of the total energy conducted as heat into the workpiece; l the thermal conductivity; a the thermal diffusivity; 2L the length of the grinding zone; p the power consumed; and r1 and r2 the internal and external diameters of the grinding wheel. Based on the de®ned parameters, Eq. (1) can be written as Z tZ L ep y x; z r1 r2 2x0 4plBL r1 r2 0 L 1 0 2 2 e x x z =4at dx0 dt (2) t
Fig. 9. Thermal model for theoretical analysis.
Fig. 10. Measured and analytical temperature profiles versus grinding time.
3.3.2. Energy partition Comparing the analytical temperature pro®le with the measured grinding temperature curve (see Fig. 10), it can be seen that the analytical temperature response pro®le for an energy partition of e 55 percent basically ®ts well with the measured temperature pro®le. Although both temperatures increase with time, it can be seen that the measured temperature is slightly higher than the analytical one, especially at the initial stage of the grinding process, which may contribute to the unstable grinding condition of this stage. 3.4. The effects of grinding temperature Generally, the grinding temperature is considered to have a deleterious effect on workpiece quality. However, it is reported that exposure of hot pressed silicon nitride (HPSN) to a temperature of 800 8C increased the strength of HPSN due to the formation of glassy phases. Therefore, the reduction in surface fracture and apparent increase in ductile ¯ow may be associated with glassy phase formation at elevated grinding temperature [4]. On the other hand, whether the lower temperature on the grinding zone remains to have the same effects on the ground surface has not been evaluated de®nitely. The grinding temperatures during the dry and wet grinding of Si3N4 were examined and the corresponding ground surfaces were checked using the SEM. The recorded temperatures and SEM pictures of ground surfaces are shown in Fig. 11. From the temperature curves, it can be seen that the temperature in dry grinding was about 160 8C, and that in wet grinding was below 70 8C. Although the temperature in dry grinding is higher than that in wet grinding, the temperatures are far below the temperature for the formation of glassy phases. The SEM pictures show that the surface quality of dry grinding is worse than that of wet grinding. There is more surface fracture on the workpiece surface of dry grinding. The reason may be the worse topography of resin bond diamond wheel, which was damaged by the grinding temperature generated in dry grinding [17].
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Fig. 11. Temperature curves and ground surfaces (Si3N4).
3.5. Specific grinding energy A fundamental parameter for characterizing grinding processes is speci®c energy (u), which is de®ned as the energy expended per unit volume of material removal and can be calculated as u
Ft Vs WVw a
(3)
where Ft is the tangential force; Vs the linear speed of wheel; W the grinding width; Vw the workpiece speed; and a the depth of cut. In the experiments, the consumed power was measured using a wattmeter and the stock removal of ceramic materials at each stage was measured separately. Because ceramics are dif®cult-to-cut materials, the stock removal of removal was designed to be measured at 10 s stages. Therefore, the speci®c energy of the experiment can be calculated as u
PT Q
(4)
where P is the power consumed; Q the stock removal of material at each stage; and T the length of time of each stage. The speci®c energy is usually considered to consist of cutting and sliding components. It has been reported that almost all of the energy consumed in the grinding of ceramics is attributable to ductile ¯ow by plowing [4,18].
Fig. 13. Specific grinding energy versus spindle speed.
Based on this viewpoint, a decrease in the material removal rate may reduce the undeformed chip size, causing more ¯ow and less brittle fracture, which should result in higher speci®c energy. Contrarily, low speci®c energy means a larger depth of cut and more brittle fracture. The speci®c grinding energies obtained under different grinding conditions are plotted in Figs. 12 and 13. The results show that speci®c energies fall with the pressure and grit size (Fig. 12), and increases with the spindle rotational speed (Fig. 13). It is interesting to note that the speci®c energies in Figs. 12 and 13 are much lower than those in the surface grinding of ceramics [4,18], which might be attributable to the different mechanisms for vertical grinding and surface grinding. To some extent, the result is consistent with the result for tangential force in Section 3.1. Combining the results in Sections 3.1, 3.2 and 3.5, it is found that low pressure and high wheel speed should be selected to remove ceramics ef®ciently in the ductile mode in vertical grinding. As shown in Figs. 12 and 13, the speci®c energy for the grinding of the glass is much less than that for the grinding of Si3N4, which is consistent with the results revealed in Figs. 5 and 6. 4. Conclusions
Fig. 12. Specific grinding energy versus pressure.
The surface quality of ground ceramics depends on the mechanism of material removal in the vertical grinding
J.Y. Shen et al. / Journal of Materials Processing Technology 129 (2002) 176±181
process. For the grinding of Si3N4 and glass under the condition of constant pressure, increasing the pressure enhances material removal rate, and at the same time causes more machining-induced microcracks on the ground surfaces. Along with the analysis of tangential forces, and speci®c grinding energy, and microobservations on ground surfaces, it is found that low pressure and high wheel speed should be selected to remove ceramics ef®ciently in the ductile mode in vertical grinding. From the theoretically analytical and measured grinding temperatures in the vertical grinding of ceramics, it is found that the analytical temperature pro®le with e 55% has the same trend as the measured one. The measured temperature is higher that the analytical one at the initial stage of the grinding process, which might contribute to the unstable grinding condition of this stage. The grinding temperatures in the vertical grinding of ceramics under a constant pressure are not high enough for glassy phase formation, and may not reduce surface fracture as expected. However, the temperature in dry grinding may cause thermal damage to the resin bond diamond wheel, thereby resulting in a low quality workpiece surface. Acknowledgements This research was supported by the Natural Science Fund of Fujian Province in China and the Open-end Fund of the
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