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A novel technique for reconditioning polycrystalline diamond tool surfaces applied for silicon micromachining
CIRP Annals - Manufacturing Technology 60 (2011) 591–594
Contents lists available at ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er. com/ci rp/ def a ult . asp
A novel technique for reconditioning polycrystalline diamond tool surfaces applied for silicon micromachining K. Katahira a,b,*, K. Nakamoto b, P. Fonda b, H. Ohmori (2)a, K. Yamazaki (1)b a b
RIKEN, Saitama, Japan University of California, Davis, CA, USA
A R T I C L E I N F O
A B S T R A C T
Keywords: Silicon Surface integrity Micromachining
The demand for micromachining methods for single crystal silicon has been steadily increasing. Micro tools made from polycrystalline diamond (PCD) have considerable promise in this regard. However, it has been a concern that contamination of the PCD tool surface can give rise to an increase in frictional resistance during machining, leading to degradation of the surface integrity of the workpiece. In this study, the feasibility of surface reconditioning using a specific electrochemical technique was investigated. The technique was found to be effective in removing surface contamination without causing any damage to the tool edges. ß 2011 CIRP.
1. Introduction
2. Hemispherical cavity machining procedure
The demand for micromachining methods for single crystal silicon has been steadily increasing since this material can be used for advanced components such as micro resonators in gyro systems, micro optical lenses and medical devices such as Micro Total Analysis Systems (mTAS). Although etching and lithography techniques have met these needs up to now, the demands of increasing 3D shape complexity, intricate geometric features and higher precision require the use of alternative fabrication methods. Mechanical machining methods utilizing single crystal diamond (SCD) and polycrystalline diamond (PCD) tools are highly promising for this purpose [1–4]. In particular, PCD micro tools are attractive candidates for micromachining silicon materials, and can achieve a high surface integrity in ductile mode machining [1,5]. However, it has been a concern that contamination of the PCD tool surface can give rise to an increase in frictional resistance during machining, leading to degradation of the surface characteristics of the workpiece. When a tool is engaged during long machining operations and used for multiple workpieces to produce concave or intricate shapes, it is highly important that the surface conditions of the tool should be maintained. For this reason, PCD micro tools require periodic surface reconditioning that is controllable at a microscopic level. In the present study, the feasibility of a new technique for reconditioning the surfaces of PCD tools used for silicon micromachining was investigated. The resulting tool characteristics were evaluated by a variety of techniques, including detailed surface observations, elemental analysis, and electrochemical testing.
The workpiece material to be used was single crystal silicon and the target feature was a hemispherical cavity with a radius of 0.4 mm and depth of 0.4 mm. Micro milling was performed using a specially designed PCD micro ball end mill as shown in Fig. 1. The PCD diamond grains, which average 0.5 mm in size, were sintered with metallic cobalt under high temperature and pressure. The tool had a radius of 0.25 mm, 4 flutes and was fabricated by a six-axis wire electric discharge machine (WEDM). The machine tool used in this study is a high precision 3-axis machining center which has a high feed resolution of 10 nm (XYZ), a high speed (120,000 min1) air turbine spindle, is capable of high acceleration and is equipped with a vibration suppression system. The experimental setup is shown in Fig. 2. The toolpath followed a spiral pattern with a constant cutting depth of 0.3 mm and feedrate of 12 mm/min (25 nm/tooth revolution). The surfaces of the tool and workpieces were analyzed by using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS).
* Corresponding author. 0007-8506/$ – see front matter ß 2011 CIRP. doi:10.1016/j.cirp.2011.03.013
3. Observation of machined Si hemisphere surface and PCD tool Fig. 3 shows a hemispherical cavity in silicon produced using the previously mentioned PCD tool. The time required to produce a single cavity was 120 min in this case. Fig. 4 shows high magnification SEM images of the texture on the curved surfaces of a typical machined cavity. Although these images make it clear that it is possible to carry out ductile mode machining of single crystal silicon (see Fig. 4(a)), burrs and brittle fractures were observed in some regions, as shown in Fig. 4(b). Fig. 5 shows low magnification images of the PCD tool surface after silicon micromachining. To achieve ductile machining of silicon, machining times are often very long since the depth of the cut is necessarily small. However, for machining times longer than approximately 1 h, some form of contamination was found to be adhered to the PCD tool surface, and this accumulated as the
[()TD$FIG]
592
[()TD$FIG]
K. Katahira et al. / CIRP Annals - Manufacturing Technology 60 (2011) 591–594
[()TD$FIG]
Fig. 5. Images of PCD tool surfaces after silicon micromachining.
Fig. 1. PCD ball end mill used for experimentation.
[()TD$FIG]
Fig. 2. Experimental setup and target feature of workpiece.
machining time progressed. Such contamination is a concern since it may degrade the performance of the cutting edge, leading to a deterioration of the quality of the machined surface. Therefore, it is important to identify the generation mechanism of this contamination layer, which will be discussed in the next section, based on the results of detailed SEM and EDS analyses. 4. Investigation of adhered material on PCD tool surface To identify the nature of the contamination layer on the surface of the PCD tool following long machining operations, detailed SEM observations were conducted. Typical SEM images are shown in Fig. 6. In Fig. 6(a), it can be seen that almost the entire surface of the tool is covered with a thick film. In particular, the cutting edge itself is completely covered and the contamination layer contains many cracks as shown in Fig. 6(b). As shown in Fig. 6(c), some portions have peeled away allowing its thickness to be measured at approximately 3 mm. In the higher magnification image shown in Fig. 6(d), a wave-like micro texture with a periodicity of about 0.3– 0.5 mm can be observed. This is believed to be produced by submicron-sized silicon chips (flakes) that are removed from the machined surface and accumulate over time on the tool. EDS analysis was also conducted to identify the chemical composition of the contamination layer. Fig. 7 shows the results for regions (I) and (II) in Fig. 6. In region (I), the main element detected was carbon, which originates in the PCD material. However, in region (II), significant peaks due to silicon and oxygen can be [()TD$FIG]clearly observed. Based on the atomic ratio of these two elements,
Fig. 6. SEM images of contamination layer on the PCD tool.
the material is assumed to be amorphous silicon dioxide (SiO2). The carbon peak in this region can be disregarded, since it is most likely also from the PCD substrate. Amorphous SiO2 has been reported to have a porous structure with an average pore size of 0.5–0.7 nm (Fig. 8). This so-called ultramicroporous structure is characterized by extremely high van der Waals forces [6,7], which is one possible reason for the adhesion of silicon chips to the PCD tool. 5. Reconditioning of PCD tool surfaces 5.1. Evaluation of possible reconditioning methods In the previous section, it was shown that the contamination is primarily due to adhesion of silicon-based material, and this was assumed to be amorphous SiO2. In this section, some potential cleaning methods for removing this material are investigated. The following physical techniques were first considered. (a) Ultrasonic cleaning for 30 min at 40 kHz in an acetone solution. (b) Mist coolant with a pressure of 0.3 MPa was supplied to the contact point between the tool and workpiece during machining.
[()TD$FIG]
Fig. 3. Images of machined hemispherical cavity in silicon.
[()TD$FIG]
Fig. 4. Curved surfaces of hemispherical cavities.
Fig. 7. EDS analysis results.
[()TD$FIG]
K. Katahira et al. / CIRP Annals - Manufacturing Technology 60 (2011) 591–594
593
Table 1 Electrochemical test conditions.
[()TD$FIG]
Fig. 8. Structure of amorphous SiO2 in two dimensions.
Solution Electrical conductance pH Applied voltage Electrode Temperature
[()TD$FIG]
Distilled water + 10% sodium hydroxide (NaOH) 10.18 mS/cm 8.5 DC10V (DC power supply) Stainless steel 20 8C
Fig. 9. Contamination layer before and after immersion test.
The effectiveness of these methods was evaluated using SEM observations of the tool surfaces. However, it was clear from the SEM images that these methods did not lead to any significant removal of the contamination. A chemical process was then considered, using a sodium hydroxide (NaOH) solution, which is a common anisotropic etchant for silicon [8]. It was hoped that etching would occur at the contamination layer surface. First, simply immersing the tool into the solution (10% NaOH + distilled water) for 30 min was attempted. However, as shown in Fig. 9, this had no effect on the thickness of the contamination layer. One possible reason is that the etching rate for SiO2 with NaOH solution is very low (5–7 nm/min). This is about 200 times slower than the case for single crystalline silicon [8]. Therefore, some means of accelerating the chemical reaction is required. Possible methods include heating the solution, applying vibration or cavitation, direct spraying of the etchant solution or electrochemical assistance. In the present study, an electrochemical approach was taken, as described in the next section.
[()TD$FIG]
5.2. Electrochemically assisted reconditioning process Fig. 10 and Table 1 show the electrochemical test setup and the test conditions, respectively. A positive potential was applied to the PCD tool and a negative potential to the electrode using a DC power supply. Fig. 11 shows the relationship between the testing time and the electrical current. The current gradually decreased with time, indicating that the conductivity of the surface of the PCD tool slowly decreased. Testing was terminated within 60 s in order to prevent excessive electrolysis of the Co binder of the PCD tool. The tested surfaces were observed after 5, 15 and 30 s, and the results are shown in Fig. 12. Before electrochemical testing, the cutting edge was completely covered by a contamination layer and could not be seen. However, after 15 s, some of the contamination layer had dissolved so that the cutting edge was visible near its surface. After 30 s, the entire cutting edge could be clearly seen, and the contamination layer was completely removed. No damage to the tool edge due to the electrochemical process could be detected, indicating that the tool can be re-used for further machining.
Fig. 10. Electrochemical test setup.
Fig. 11. Relationship between electrochemical test time and electrical current.
Now, the mechanism involved in the electrochemically assisted reconditioning process will be considered. The contamination layer (SiO2) is most likely removed by the formation of sodium metasilicate (Na2SiO3) as shown in Eq. (1). Na2SiO3 is also known as liquid glass and is highly soluble in water by the process shown in Eq. (2). Furthermore, during the electrochemical test, electrolysis of water would be expected to occur, generating a large amount of hydroxide ions. It is likely that these ions would drift to the positive electrode (the tool), significantly accelerating the chemical reaction. Since electrochemical reactions are influenced by a number of different factors, further experiments are required to optimize conditions such as the electrolyzing time, electric conductivity, pH value, and chemical components of the solution. SiO2 þ 2NaOH ! Na2 SiO3 þ H2 O
(1)
Na2 SiO3 þ nH2 O ! Na2 SiO3 nH2 O
(2)
6. Periodic reconditioning during actual machining
[()TD$FIG]
In batch production processes, the need for tool replacement has a detrimental impact on cost efficiency and accuracy. For this reason, it would be preferable to carry out tool reconditioning at regular intervals during actual machining. A process designed to achieve this is shown in Fig. 13. First, upon completion of the machining process for a particular workpiece or at some other appropriate time, the tool is moved from the machining area to a standby location in which a container of the reconditioning
Fig. 12. SEM images of contamination layer at tool edge during electrochemical testing.
[()TD$FIG]
594
K. Katahira et al. / CIRP Annals - Manufacturing Technology 60 (2011) 591–594
machined without tool reconditioning exhibits severe burrs in the curved region and chipping in the edge region. These results indicate that the proposed method has the potential to maintain stable machining conditions and extend the operating lifetime of machining tools during high precision machining of silicon. 7. Conclusions
[()TD$FIG]
Fig. 13. Diagram of periodic reconditioning process.
Fig. 14. Experimental setup for periodic reconditioning process.
[()TD$FIG]
In this study, the feasibility of surface reconditioning using a specific electrochemical technique was investigated. Although it has been shown to be possible to achieve ductile mode machining of single crystal silicon using a PCD ball end milling tool, burrs and brittle fractures were produced due to the buildup of a contamination layer on the cutting edge of the tool. This material was identified as amorphous SiO2. The effects of the proposed electrochemical technique were evaluated for such a contaminated PCD tool. The results indicated that the technique was effective in removing the surface contamination without causing any damage to the tool edges. This suggests that this technique is highly promising for maintaining excellent machining performance of PCD micro tools. Acknowledgements The authors would like to express their sincere appreciation to Sumitomo Electric Hardmetal Corp. Japan. This work is supported by the Japan Society for the Promotion of Science (JSPS) Excellent Young Researcher Overseas Visit Program.
References
Fig. 15. Machined cavities in silicon with and without periodic reconditioning process.
chemical solution is placed. The spindle speed is then reduced to idle mode in preparation for the reconditioning process. Next, the proposed electrochemical reconditioning process is carried out for 30 s. Finally, the tool moves back to the machining area and continues working with a high spindle rotation speed. The experimental setup is shown in Fig. 14. During machining, the low pressure flood coolant is unlikely to spill into the solution container due to the relatively long distance from the workpiece to the container. Fig. 15 shows experimental results for cavity machining under the same conditions with and without periodic tool surface reconditioning. It can be observed that the surface of the 2nd cavity
[1] Dornfeld D, Min S, Takeuchi Y (2006) Recent Advances in Mechanical Microengineering. Annals of the CIRP 55(2):745–768. [2] Rajurkar PK, Levy G, Malshe A, Sundaram MM, McGeough J, Hu X, Resnick R, DeSilva A (2006) Micro and Nano Machining by Electro-Physical and Chemical Processes. Annals of the CIRP 55(2):643–666. [3] Yan J, Yoshino M, Kuriyagawa T, Shirakashi T, Syoji K, Komanduri R (2001) On the Ductile Machining of Silicon for Micro Electro-mechanical Systems (MEMS) Opto-electronic and Optical Applications. Materials Science and Engineering A297:230–234. [4] Ko JT, Kim SH (2007) Micor-end-milling of Single Crystal Silicon. International Journal of Machine Tools & Manufacture 47:2111–2119. [5] Cheng X, Wang ZG, Kobayashi S, Nakamoto K, Yamazaki K (2009) Design and Development of Micro Polycrystalline Diamond Ball End Mill for Micro/Nano Freeform Machining of Hard and Brittle Materials. Journal of Micromechanics and Microengineering 19. 115022-1-10. [6] Kaneko K (1994) Determination of Pore Size and Pore Size Distribution. Journal of Membrane Science 96:59–89. [7] Kim JW, Lee YD, Lee HG (2001) Decomposition of Na2CO3 by Interaction with SiO2 in Mold Flux of Steel Continuous Casting. ISIJ International 41(2):116–123. [8] Seidel H, Csepregi L, Heuberger A, Baumgartel H (1990) Anisotropic Etching of Crystalline Silicon in Alkaline Solutions. Journal of the Electrochemical Society 137/11:3612–3626.
Contents lists available at ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er. com/ci rp/ def a ult . asp
A novel technique for reconditioning polycrystalline diamond tool surfaces applied for silicon micromachining K. Katahira a,b,*, K. Nakamoto b, P. Fonda b, H. Ohmori (2)a, K. Yamazaki (1)b a b
RIKEN, Saitama, Japan University of California, Davis, CA, USA
A R T I C L E I N F O
A B S T R A C T
Keywords: Silicon Surface integrity Micromachining
The demand for micromachining methods for single crystal silicon has been steadily increasing. Micro tools made from polycrystalline diamond (PCD) have considerable promise in this regard. However, it has been a concern that contamination of the PCD tool surface can give rise to an increase in frictional resistance during machining, leading to degradation of the surface integrity of the workpiece. In this study, the feasibility of surface reconditioning using a specific electrochemical technique was investigated. The technique was found to be effective in removing surface contamination without causing any damage to the tool edges. ß 2011 CIRP.
1. Introduction
2. Hemispherical cavity machining procedure
The demand for micromachining methods for single crystal silicon has been steadily increasing since this material can be used for advanced components such as micro resonators in gyro systems, micro optical lenses and medical devices such as Micro Total Analysis Systems (mTAS). Although etching and lithography techniques have met these needs up to now, the demands of increasing 3D shape complexity, intricate geometric features and higher precision require the use of alternative fabrication methods. Mechanical machining methods utilizing single crystal diamond (SCD) and polycrystalline diamond (PCD) tools are highly promising for this purpose [1–4]. In particular, PCD micro tools are attractive candidates for micromachining silicon materials, and can achieve a high surface integrity in ductile mode machining [1,5]. However, it has been a concern that contamination of the PCD tool surface can give rise to an increase in frictional resistance during machining, leading to degradation of the surface characteristics of the workpiece. When a tool is engaged during long machining operations and used for multiple workpieces to produce concave or intricate shapes, it is highly important that the surface conditions of the tool should be maintained. For this reason, PCD micro tools require periodic surface reconditioning that is controllable at a microscopic level. In the present study, the feasibility of a new technique for reconditioning the surfaces of PCD tools used for silicon micromachining was investigated. The resulting tool characteristics were evaluated by a variety of techniques, including detailed surface observations, elemental analysis, and electrochemical testing.
The workpiece material to be used was single crystal silicon and the target feature was a hemispherical cavity with a radius of 0.4 mm and depth of 0.4 mm. Micro milling was performed using a specially designed PCD micro ball end mill as shown in Fig. 1. The PCD diamond grains, which average 0.5 mm in size, were sintered with metallic cobalt under high temperature and pressure. The tool had a radius of 0.25 mm, 4 flutes and was fabricated by a six-axis wire electric discharge machine (WEDM). The machine tool used in this study is a high precision 3-axis machining center which has a high feed resolution of 10 nm (XYZ), a high speed (120,000 min1) air turbine spindle, is capable of high acceleration and is equipped with a vibration suppression system. The experimental setup is shown in Fig. 2. The toolpath followed a spiral pattern with a constant cutting depth of 0.3 mm and feedrate of 12 mm/min (25 nm/tooth revolution). The surfaces of the tool and workpieces were analyzed by using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS).
* Corresponding author. 0007-8506/$ – see front matter ß 2011 CIRP. doi:10.1016/j.cirp.2011.03.013
3. Observation of machined Si hemisphere surface and PCD tool Fig. 3 shows a hemispherical cavity in silicon produced using the previously mentioned PCD tool. The time required to produce a single cavity was 120 min in this case. Fig. 4 shows high magnification SEM images of the texture on the curved surfaces of a typical machined cavity. Although these images make it clear that it is possible to carry out ductile mode machining of single crystal silicon (see Fig. 4(a)), burrs and brittle fractures were observed in some regions, as shown in Fig. 4(b). Fig. 5 shows low magnification images of the PCD tool surface after silicon micromachining. To achieve ductile machining of silicon, machining times are often very long since the depth of the cut is necessarily small. However, for machining times longer than approximately 1 h, some form of contamination was found to be adhered to the PCD tool surface, and this accumulated as the
[()TD$FIG]
592
[()TD$FIG]
K. Katahira et al. / CIRP Annals - Manufacturing Technology 60 (2011) 591–594
[()TD$FIG]
Fig. 5. Images of PCD tool surfaces after silicon micromachining.
Fig. 1. PCD ball end mill used for experimentation.
[()TD$FIG]
Fig. 2. Experimental setup and target feature of workpiece.
machining time progressed. Such contamination is a concern since it may degrade the performance of the cutting edge, leading to a deterioration of the quality of the machined surface. Therefore, it is important to identify the generation mechanism of this contamination layer, which will be discussed in the next section, based on the results of detailed SEM and EDS analyses. 4. Investigation of adhered material on PCD tool surface To identify the nature of the contamination layer on the surface of the PCD tool following long machining operations, detailed SEM observations were conducted. Typical SEM images are shown in Fig. 6. In Fig. 6(a), it can be seen that almost the entire surface of the tool is covered with a thick film. In particular, the cutting edge itself is completely covered and the contamination layer contains many cracks as shown in Fig. 6(b). As shown in Fig. 6(c), some portions have peeled away allowing its thickness to be measured at approximately 3 mm. In the higher magnification image shown in Fig. 6(d), a wave-like micro texture with a periodicity of about 0.3– 0.5 mm can be observed. This is believed to be produced by submicron-sized silicon chips (flakes) that are removed from the machined surface and accumulate over time on the tool. EDS analysis was also conducted to identify the chemical composition of the contamination layer. Fig. 7 shows the results for regions (I) and (II) in Fig. 6. In region (I), the main element detected was carbon, which originates in the PCD material. However, in region (II), significant peaks due to silicon and oxygen can be [()TD$FIG]clearly observed. Based on the atomic ratio of these two elements,
Fig. 6. SEM images of contamination layer on the PCD tool.
the material is assumed to be amorphous silicon dioxide (SiO2). The carbon peak in this region can be disregarded, since it is most likely also from the PCD substrate. Amorphous SiO2 has been reported to have a porous structure with an average pore size of 0.5–0.7 nm (Fig. 8). This so-called ultramicroporous structure is characterized by extremely high van der Waals forces [6,7], which is one possible reason for the adhesion of silicon chips to the PCD tool. 5. Reconditioning of PCD tool surfaces 5.1. Evaluation of possible reconditioning methods In the previous section, it was shown that the contamination is primarily due to adhesion of silicon-based material, and this was assumed to be amorphous SiO2. In this section, some potential cleaning methods for removing this material are investigated. The following physical techniques were first considered. (a) Ultrasonic cleaning for 30 min at 40 kHz in an acetone solution. (b) Mist coolant with a pressure of 0.3 MPa was supplied to the contact point between the tool and workpiece during machining.
[()TD$FIG]
Fig. 3. Images of machined hemispherical cavity in silicon.
[()TD$FIG]
Fig. 4. Curved surfaces of hemispherical cavities.
Fig. 7. EDS analysis results.
[()TD$FIG]
K. Katahira et al. / CIRP Annals - Manufacturing Technology 60 (2011) 591–594
593
Table 1 Electrochemical test conditions.
[()TD$FIG]
Fig. 8. Structure of amorphous SiO2 in two dimensions.
Solution Electrical conductance pH Applied voltage Electrode Temperature
[()TD$FIG]
Distilled water + 10% sodium hydroxide (NaOH) 10.18 mS/cm 8.5 DC10V (DC power supply) Stainless steel 20 8C
Fig. 9. Contamination layer before and after immersion test.
The effectiveness of these methods was evaluated using SEM observations of the tool surfaces. However, it was clear from the SEM images that these methods did not lead to any significant removal of the contamination. A chemical process was then considered, using a sodium hydroxide (NaOH) solution, which is a common anisotropic etchant for silicon [8]. It was hoped that etching would occur at the contamination layer surface. First, simply immersing the tool into the solution (10% NaOH + distilled water) for 30 min was attempted. However, as shown in Fig. 9, this had no effect on the thickness of the contamination layer. One possible reason is that the etching rate for SiO2 with NaOH solution is very low (5–7 nm/min). This is about 200 times slower than the case for single crystalline silicon [8]. Therefore, some means of accelerating the chemical reaction is required. Possible methods include heating the solution, applying vibration or cavitation, direct spraying of the etchant solution or electrochemical assistance. In the present study, an electrochemical approach was taken, as described in the next section.
[()TD$FIG]
5.2. Electrochemically assisted reconditioning process Fig. 10 and Table 1 show the electrochemical test setup and the test conditions, respectively. A positive potential was applied to the PCD tool and a negative potential to the electrode using a DC power supply. Fig. 11 shows the relationship between the testing time and the electrical current. The current gradually decreased with time, indicating that the conductivity of the surface of the PCD tool slowly decreased. Testing was terminated within 60 s in order to prevent excessive electrolysis of the Co binder of the PCD tool. The tested surfaces were observed after 5, 15 and 30 s, and the results are shown in Fig. 12. Before electrochemical testing, the cutting edge was completely covered by a contamination layer and could not be seen. However, after 15 s, some of the contamination layer had dissolved so that the cutting edge was visible near its surface. After 30 s, the entire cutting edge could be clearly seen, and the contamination layer was completely removed. No damage to the tool edge due to the electrochemical process could be detected, indicating that the tool can be re-used for further machining.
Fig. 10. Electrochemical test setup.
Fig. 11. Relationship between electrochemical test time and electrical current.
Now, the mechanism involved in the electrochemically assisted reconditioning process will be considered. The contamination layer (SiO2) is most likely removed by the formation of sodium metasilicate (Na2SiO3) as shown in Eq. (1). Na2SiO3 is also known as liquid glass and is highly soluble in water by the process shown in Eq. (2). Furthermore, during the electrochemical test, electrolysis of water would be expected to occur, generating a large amount of hydroxide ions. It is likely that these ions would drift to the positive electrode (the tool), significantly accelerating the chemical reaction. Since electrochemical reactions are influenced by a number of different factors, further experiments are required to optimize conditions such as the electrolyzing time, electric conductivity, pH value, and chemical components of the solution. SiO2 þ 2NaOH ! Na2 SiO3 þ H2 O
(1)
Na2 SiO3 þ nH2 O ! Na2 SiO3 nH2 O
(2)
6. Periodic reconditioning during actual machining
[()TD$FIG]
In batch production processes, the need for tool replacement has a detrimental impact on cost efficiency and accuracy. For this reason, it would be preferable to carry out tool reconditioning at regular intervals during actual machining. A process designed to achieve this is shown in Fig. 13. First, upon completion of the machining process for a particular workpiece or at some other appropriate time, the tool is moved from the machining area to a standby location in which a container of the reconditioning
Fig. 12. SEM images of contamination layer at tool edge during electrochemical testing.
[()TD$FIG]
594
K. Katahira et al. / CIRP Annals - Manufacturing Technology 60 (2011) 591–594
machined without tool reconditioning exhibits severe burrs in the curved region and chipping in the edge region. These results indicate that the proposed method has the potential to maintain stable machining conditions and extend the operating lifetime of machining tools during high precision machining of silicon. 7. Conclusions
[()TD$FIG]
Fig. 13. Diagram of periodic reconditioning process.
Fig. 14. Experimental setup for periodic reconditioning process.
[()TD$FIG]
In this study, the feasibility of surface reconditioning using a specific electrochemical technique was investigated. Although it has been shown to be possible to achieve ductile mode machining of single crystal silicon using a PCD ball end milling tool, burrs and brittle fractures were produced due to the buildup of a contamination layer on the cutting edge of the tool. This material was identified as amorphous SiO2. The effects of the proposed electrochemical technique were evaluated for such a contaminated PCD tool. The results indicated that the technique was effective in removing the surface contamination without causing any damage to the tool edges. This suggests that this technique is highly promising for maintaining excellent machining performance of PCD micro tools. Acknowledgements The authors would like to express their sincere appreciation to Sumitomo Electric Hardmetal Corp. Japan. This work is supported by the Japan Society for the Promotion of Science (JSPS) Excellent Young Researcher Overseas Visit Program.
References
Fig. 15. Machined cavities in silicon with and without periodic reconditioning process.
chemical solution is placed. The spindle speed is then reduced to idle mode in preparation for the reconditioning process. Next, the proposed electrochemical reconditioning process is carried out for 30 s. Finally, the tool moves back to the machining area and continues working with a high spindle rotation speed. The experimental setup is shown in Fig. 14. During machining, the low pressure flood coolant is unlikely to spill into the solution container due to the relatively long distance from the workpiece to the container. Fig. 15 shows experimental results for cavity machining under the same conditions with and without periodic tool surface reconditioning. It can be observed that the surface of the 2nd cavity
[1] Dornfeld D, Min S, Takeuchi Y (2006) Recent Advances in Mechanical Microengineering. Annals of the CIRP 55(2):745–768. [2] Rajurkar PK, Levy G, Malshe A, Sundaram MM, McGeough J, Hu X, Resnick R, DeSilva A (2006) Micro and Nano Machining by Electro-Physical and Chemical Processes. Annals of the CIRP 55(2):643–666. [3] Yan J, Yoshino M, Kuriyagawa T, Shirakashi T, Syoji K, Komanduri R (2001) On the Ductile Machining of Silicon for Micro Electro-mechanical Systems (MEMS) Opto-electronic and Optical Applications. Materials Science and Engineering A297:230–234. [4] Ko JT, Kim SH (2007) Micor-end-milling of Single Crystal Silicon. International Journal of Machine Tools & Manufacture 47:2111–2119. [5] Cheng X, Wang ZG, Kobayashi S, Nakamoto K, Yamazaki K (2009) Design and Development of Micro Polycrystalline Diamond Ball End Mill for Micro/Nano Freeform Machining of Hard and Brittle Materials. Journal of Micromechanics and Microengineering 19. 115022-1-10. [6] Kaneko K (1994) Determination of Pore Size and Pore Size Distribution. Journal of Membrane Science 96:59–89. [7] Kim JW, Lee YD, Lee HG (2001) Decomposition of Na2CO3 by Interaction with SiO2 in Mold Flux of Steel Continuous Casting. ISIJ International 41(2):116–123. [8] Seidel H, Csepregi L, Heuberger A, Baumgartel H (1990) Anisotropic Etching of Crystalline Silicon in Alkaline Solutions. Journal of the Electrochemical Society 137/11:3612–3626.