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Micromilling characteristics and electrochemically assisted reconditioning of polycrystalline diamond tool surfaces for ultra-precision machining of high-purity SiC
G Model
CIRP-1142; No. of Pages 4 CIRP Annals - Manufacturing Technology xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
Micromilling characteristics and electrochemically assisted reconditioning of polycrystalline diamond tool surfaces for ultra-precision machining of high-purity SiC Kazutoshi Katahira a,*, Shogo Takesue b, Jun Komotori b, Kazuo Yamazaki (1)c,d a
Materials Fabrication Laboratory, RIKEN, 2-1 Hirosawa Wako-shi, Saitama 351-0198, Japan Department of Mechanical Engineering, Keio University, 3-14-1 Hiyoshi Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan c Department of Mechanical & Aerospace Engineering, University of California at Davis, Davis, CA 95616, USA d Department of Mechanical Engineering, University of California at Berkeley, Berkeley, CA 94720, USA b
A R T I C L E I N F O
A B S T R A C T
Keywords: Micromachining Silicon carbide Polycrystalline diamond (PCD) tool
The production of extremely thick silicon carbide (SiC) has recently become possible with the advent of a specific chemical vapor deposition process. Ultra-precision machining of high-purity SiC has been performed by using a polycrystalline diamond (PCD) micromilling tool to investigate the machining characteristics. Results indicate that a high-quality surface (Ra = 1.7 nm) can be obtained when the removed chips are thin enough to achieve ductile mode machining. Micron-sized wells and groove structures with nanometer-scale surface roughness were successfully machined by using the PCD tool. In addition, a new electrochemically assisted surface reconditioning process has been proposed to remove the contaminant material adhered onto the PCD tool surfaces after prolonged machining. ß 2014 CIRP.
1. Introduction Recently, silicon carbide (SiC) has been widely used in a variety of applications, including advanced micro-optics and molds, micromedical devices and micro-ceramic reactors for chemical analysis, primarily owing to their high wear resistance and thermal stability. Thus far, several methods have been adopted to deposit SiC by chemical vapor deposition (CVD). The deposition rate is typically of the order of 1 mm h1. However, higher deposition rates are often demanded for depositing thicker SiC. Lately, researchers have demonstrated the feasibility of depositing extremely thick (up to 20 mm) SiC by using a specific CVD process [1]. The resultant material was found to exhibit superior properties when compared to those of SiC produced by conventional sintering, especially in terms of high purity (99.9995%) and homogeneous structure. Meanwhile, SiC is characterized by extreme hardness and brittleness, making it a difficult-to-machine material. This has increased the demand for the development of a suitable micromachining method that is capable of forming SiC with required surface quality and geometric accuracy [2–5]. In general, hard and brittle materials such as SiC are machined by grinding and polishing processes to achieve a high-quality surface [2,3]. However, these methods have certain inherent limitations, especially when it comes to the fabrication of fine complex structures. Turning is yet another machining approach that has been applied to SiC [5]. Although this method is considered to be suitable for the production of symmetric structures such as lens molds, it is highly challenging to produce
* Corresponding author.
asymmetric structures, such as microchannels. On the other hand, micromilling using polycrystalline diamond (PCD) is considered to be an attractive method for the fabrication of three-dimensional structures in hard and brittle materials. This method offers the advantages of high surface integrity via ductile mode machining [6–8]. In this study, ultra-precision machining of high-purity SiC has been performed using a PCD micromilling tool to investigate the machining characteristics. In general, when a tool is used for prolonged machining operations and for producing concave and intricate shapes in multiple samples, it becomes highly important to maintain the surface conditions of the tool. Therefore, an electrochemically assisted method for periodic reconditioning of the tool surface is also proposed. 2. Coordination of experiments The sample analyzed in this study is high-purity bulk SiC produced by a specific CVD process. In addition, bulk SiC prepared by conventional sintering process was considered for comparison. The dimension of the sample was 10 mm 10 mm 5 mm, and the properties of the samples are summarized in Table 1. Micromilling was performed by using a specially designed square-shaped PCD end mill, as shown in Fig. 1. The PCD diamond grains of average size 1.0 mm were sintered with metallic cobalt under high temperature and pressure. The tool had a radius of 0.1 mm, 2 flutes, and was fabricated by wire electrical discharge machining. The machining tool used is a linear motor-driven ultra-precision 6-axis machine with an air turbine spindle operating at the speed of 50,000 min1. The feed resolution of each linear axis (X, Y, Z) is 1 nm.
http://dx.doi.org/10.1016/j.cirp.2014.03.031 0007-8506/ß 2014 CIRP.
Please cite this article in press as: Katahira K, et al. Micromilling characteristics and electrochemically assisted reconditioning of polycrystalline diamond tool surfaces for ultra-precision machining of high-purity SiC. CIRP Annals - Manufacturing Technology (2014), http://dx.doi.org/10.1016/j.cirp.2014.03.031
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CIRP-1142; No. of Pages 4
3. Basic experimental results and machined surface analysis The surface quality of the sample micro-machined using a PCD was investigated by performing machining experiments with various feed rates in the range of 5–50 mm/min. The machining conditions are listed in Table 2. Fig. 3 shows the average surface roughness (Ra) and peak-to-valley surface roughness (Rz) of the high-purity SiC and conventionally sintered SiC, as measured by SWLIM. The feed per tooth (f) determined using Eq. (1) is also shown in the figure, where F is the feed rate, S is the tool revolution, N is the number of flutes, Rd is the radial depth of cut, and D is the tool diameter. As can be seen from the figure, for all the different feed rate conditions, the surface roughness of the high-purity SiC is lower than that of the sintered SiC. Although a sudden increase in Rz was observed for the high-purity SiC at higher feed rates above 200 nm/ tooth, Rz and Ra remained consistent around 50–100 nm and less than 10 nm, respectively, until the feed rate reached a certain value.
70 High-purity SiC Sintered SiC
60 50 40 30 20 10 0
0
10
20 30 40 Feedrate (mm/min)
50
0
100
300 400 200 Feed per tooth (nm)
500
(a) Average surface roughness, Ra. Peak-to-valley surface roughness Rz (nm)
The photograph of the experimental setup is shown in Fig. 2. The surface of the tool and sample was analyzed using a scanning electron microscope (SEM) equipped with an energy dispersive Xray spectrometer (EDS). The surface roughness was measured by scanning white light interference microscopy (SWLIM).
Average surface routhness Ra (nm)
K. Katahira et al. / CIRP Annals - Manufacturing Technology xxx (2014) xxx–xxx
2
3500 High-purity SiC
Sintered SiC
3000 2500 2000 1500 1000 500 0
0
10
30 40 20 Feedrate (mm/min)
50
0
100
400 200 300 Feed per tooth (nm)
500
(b) Peak-to-valley surface roughness, Rz.
Fig. 3. Machined surface roughness. (a) Average surface roughness, Ra. (b) Peak-tovalley surface roughness, Rz.
Fig. 1. SEM image of the square-shaped PCD end mill. Table 1 Properties of samples.
Density (g/cm3) Flexural strength (MPa) Vickers hardness (Hv) Young’s modulus (GPa) Fracture toughness (MPa m1/2) Thermal conductivity (W/m K)
High purity SiC
Sintered SiC
3.2 460 2500 466 3.3 300
3.1 450 2300 420 3.5 170
Fig. 2. Photograph of the experimental setup.
f ¼
2F SN
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi Rd Rd 1 D D
Fig. 5. SEM images of the PCD tool edge.
4. Machining of micron-sized well structures
Table 2 Basic machining conditions. Spindle rotation speed (min1) Axial depth of cut (mm) Feedrate (mm/min) Coolant
Fig. 4. Surface micrograph of high-purity SiC.
50,000 0.5 5–50 Cutting oil mist
(1)
Fig. 4 shows a representative surface micrograph of high-purity SiC machined under low feed rate conditions (50 nm/tooth). The cutter marks are apparent, the pitch of which is equal to the feed rate. The cutter marks can be attributed to the plastic flow, which indicates that ductile mode machining was achieved [9]. Fig. 5 shows SEM images of the PCD tool edge after micromilling high-purity SiC under low feed rate conditions. As evidenced from the figure, the adherence and deposition of chips is clearly observed on the surface of the tool edge. The PCD tools have randomly distributed protrusions of sharp diamond abrasives that act as hard cutting edges when the tool is rotated. This produces an effect similar to microgrinding [7,8], contributing to high-quality surfaces during the ductile mode machining of brittle and hard materials. In the present study, most of the material removal process took place at these diamond cutting edges, such as the corner edge region. For effective use of PCD as a microgrinding tool, it is highly imperative to have protrusions of diamond abrasives and removal of the adhered chip material.
While machining microconcave or intricate shapes, there is always a concern that the surface quality of the machined bottom portion is degraded because of the relatively low chip evacuation. Therefore, the micromachining performance of the PCD tool was verified by machining micron-sized well structures on both the high-purity SiC and sintered SiC. As can be observed from the schematic of the micromachining contour shown in Fig. 6, the tool path for machining a well structure follows a spiral pattern. Fig. 7 shows the SEM image of the micronsized well and groove structure machined onto high-purity SiC with a constant cutting depth of 0.5 mm and a feed rate of 5 mm/min. Under such conditions, the time taken to produce a single well was found to be 30 min. Fig. 8 shows a surface micrograph of the bottom portion of the sample surface, indicated as region A in Fig. 7. A highquality surface was successfully obtained even at the bottom portion of the micron-sized concave structure. In addition, SEM image of the corner portion of the sample (region B in Fig. 7) shown in Fig. 9 indicates the formation of a sharp edge without chipping. However, the SEM image of the surface of the sintered SiC sample shown in Fig. 10 indicates the formation of several pores on the machined well surface. These pores are considered to be internal defects of the material generated during the sintering process. The appearance of these defects on the surface of the machined sample indicates that the surface roughness of the sintered SiC (Fig. 3) has a large scattering. In other words, the characteristics of sintered SiC are inferior when compared with those of the high-purity SiC sample.
Please cite this article in press as: Katahira K, et al. Micromilling characteristics and electrochemically assisted reconditioning of polycrystalline diamond tool surfaces for ultra-precision machining of high-purity SiC. CIRP Annals - Manufacturing Technology (2014), http://dx.doi.org/10.1016/j.cirp.2014.03.031
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CIRP-1142; No. of Pages 4 K. Katahira et al. / CIRP Annals - Manufacturing Technology xxx (2014) xxx–xxx
Fig. 6. Schematic of the micromilling tool contour, tool path, and target feature.
Fig. 8. Surface profile of the region A in Fig. 7.
Fig. 7. SEM image of the well and groove structure micro-machined onto highpurity SiC.
16
Average surface roughness Ra (nm)
Systematic analysis of the well structures micro-machined onto SiC suggests that the PCD micro-tool is very effective for micromachining, especially in case of high-purity SiC. However, from the practical application viewpoints, it is equally important to verify the machining stability and operating lifetime of the tool during long-time machining operations for large-scale production or multiple samples. This was investigated by machining multiple micron-sized wells under the same machining conditions. The lowmagnification SEM image of the multiple micron-sized wells shown in Fig. 11(a) indicates the uniform formation of wells. However, the corresponding high-magnification SEM images of the 1st and 16th wells shown in Fig. 11(b) and (c), respectively, indicate the differences in the surface features. The machined surface of the 16th well has chippings at the edges and severe scratches at the bottom. Figs. 12 and 13 show the surface roughness of the bottom portion and depth of the machined wells, respectively, plotted as a function of the number of wells from the 1st to the 16th. With increase in the number of wells machined, the surface roughness gradually deteriorates, and the depth of the well becomes shallower than the set target value of 30 mm. This could possibly be due to the contamination of the tool edge generated in the long run, primarily caused by adhesion of the machined material as chips on the surface of the tool. In addition, the material adhered onto the tool surface increases the frictional resistance with further machining proceeds. This results in overall degradation of the machining performance. Therefore, it is very
12
8
4
Fig. 12. Variations in the surface roughness of the bottom portion of machined wells, Fig. 13. Depth of machined wells, plotted as a function of the number of plotted as a function of the number of machined wells from the 1st to the 16th. wells from the 1st to the 16th.
important to identify the characteristics of the adhered material, which will be discussed in the forthcoming section. 5. Investigation of material adhered onto the tool surface The material adhered onto the surface of the PCD tool was investigated by SEM and EDS analyses. Typical SEM images of the PCD tool after repeated machining are shown in Fig. 14(a) and (b). As can be seen from the figure, the entire surface of the tool is covered with a thick film. In particular, the cutting edge itself is completely covered with the adhered material and contains small cracks, as shown in Fig. 14(c). The high-magnification SEM image of the tool surface shown in Fig. 14(d) indicates a wave-like microtexture with a periodicity of approximately 0.1–0.3 mm. This microtexture appears to have been produced by the submicronsized chips (flakes) that were removed from the machined surface and accumulated over time on the surface of the tool. Furthermore, the chemical composition of the adhered material was analyzed by EDS. Fig. 15 shows the quantitative chemical analysis of the regions (I) and (II) indicated in Fig. 14(b), as determined from EDS analysis. The EDS spectrum of region (I) indicates the presence of carbon, which is believed to have originated from the PCD material. However, the EDS spectrum of
Fig. 9. SEM image of the corner portion of the well structure (region B in Fig. 7).
Fig. 14. SEM images of material adhered onto the PCD tool surface. Fig. 10. SEM images of the well micro-machined onto sintered SiC.
Fig. 11. SEM images of (a) multiple micron-sized wells, (b) 1st machined well, and (c) 16th machined well.
3
Fig. 15. EDS analysis of the regions I and II indicated in Fig. 14(b).
Please cite this article in press as: Katahira K, et al. Micromilling characteristics and electrochemically assisted reconditioning of polycrystalline diamond tool surfaces for ultra-precision machining of high-purity SiC. CIRP Annals - Manufacturing Technology (2014), http://dx.doi.org/10.1016/j.cirp.2014.03.031
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region (II) clearly shows the presence of silicon and oxygen. Based on the atomic ratio of these elements, the material is assumed to be amorphous silicon dioxide (SiO2) [9]. The carbon peak in this region can be disregarded, as it is likely to have originated from the PCD substrate. According to previous studies, amorphous SiO2 is reported to have a porous structure with an average pore size of 0.5–0.7 nm [10,11]. The so-called ultra-microporous structure is characterized by extremely high van der Waals forces, which is one possible reason for the adhesion of material chips onto the PCD tool surface. 6. Electrochemically assisted reconditioning process EDS analysis of the PCD tool surface suggests that the contamination is primarily due to the adhesion of silicon-based material, which was assumed to be amorphous SiO2. In general, such contaminations can be removed by brushing and ultrasonic cleaning in an acetone solution. However, in the case of a micronsized tool, it is difficult to fully remove it using these physical approaches. Therefore, an electrochemical approach was attempted for cleaning and reconditioning of the PCD tool. Fig. 16 shows the electrochemical test setup used in this study. A positive potential was applied to the PCD tool and a negative potential was applied to the copper electrode using a DC power supply. The reconditioning process could be performed without removing the tool from the machine. Fig. 17 shows low- and highmagnification SEM images of the tool surface after electrolyzing for 60 s. The SEM image of the cutting edge before electrochemically assisted cleaning was found to be covered by the adhered material (Fig. 14). Upon electrochemical reconditioning of the surface, the material adhered to the edge of the tool was successfully removed without any damage. The mechanism underlying the electrochemically assisted reconditioning process can be considered as follows. The adhesive material (SiO2) reacts with the NaOH electrolyte to form sodium metasilicate (Na2SiO3): SiO2 þ 2NaOH ! Na2 SiO3 þ H2 O
(2)
Na2SiO3 is also known as liquid glass. It is highly soluble in water, resulting in the formation of a hydrate by the following reaction: Na2 SiO3 þ nH2 O ! Na2 SiO3 nH2 O:
(3)
Furthermore, electrolysis of water is expected to occur during the electrochemical process, which generates a large amount of hydroxide ions. It is likely that these ions diffuse to the positive electrode (the tool), thereby significantly accelerating the chemical reaction. The performance of the reconditioned tool was evaluated by repeated milling of high-purity SiC and subsequent reconditioning. Fig. 18 shows the surface roughness plotted as a function of the machining distance increases. When the cumulative machining distance reaches 1000 mm, the Ra value became >10 nm. With
Fig. 16. Electrochemical test setup.
Fig. 17. SEM images of PCD tool after the electrochemically assisted reconditioning process.
Fig. 18. Surface roughness of the sample plotted as a function of machining distance during milling tests with periodic reconditioning.
subsequent electrochemically assisted reconditioning of the tool surface, the surface roughness of the sample machined using the reconditioned tool became less than 2 nm. This confirms that the electrochemically assisted reconditioning process is efficient in recovering the machining performance of the tool and is capable of extending the operating lifetime of tools during the micromachining of SiC. 7. Conclusions Ultra-precision machining of high-purity SiC was performed by using a PCD micro-milling tool and their milling characteristics have been investigated. Results indicate that a high-quality surface (Ra = 1.7 nm) can be obtained when the removed chips are sufficiently thin to achieve ductile mode machining. Although the micron-sized well and groove structures with nanometer-scale surface roughness were successfully machined, the surface roughness gradually deteriorated and the depth of the well became shallower with increase in the number of wells machined. This could be attributed to the build-up of material adhered to the tool surface. EDS analysis indicates that the adhered material is amorphous SiO2. To regain the milling properties of the PCD tool, the surface of the tool was reconditioned by adopting a new electrochemically assisted reconditioning technique. Results indicated that the reconditioning technique is effective in removing the surface contamination without causing damage to the tool edges. Therefore, this technique is considered to be potentially suitable for retrieving the excellent machining performance of PCD microtools. Acknowledgements The authors would like to thank NS Tool Co. Ltd. for their continued support. References [1] Dow Chemical Co., http://www.dow.com/, accessed on 2013.12.09. [2] Dornfeld D, Min S, Takeuchi Y (2006) Recent Advances in Mechanical Microengineering. Annals of the CIRP 55(2):745–768. [3] Klocke F, Zunke R (2009) Removal Mechanisms in Polishing of Silicon Based Advanced Ceramics. Annals of the CIRP 58(1):491–494. [4] Suzuki H, Moriwaki Y, Goto Y (2007) Precision Cutting of Aspherical Ceramic Molds with Micro PCD Milling Tool. Annals of the CIRP 56(1):131–134. [5] Yan J, Zhang Z, Kuriyagawa T (2010) Tool Wear Control in Diamond Turning of High-Strength Mold Materials by Means of Tool Swinging. Annals of the CIRP 59(1):109–112. [6] Cheng X, Wang ZG, 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. [7] Nakamoto K, Katahira K, Ohmori H, Yamazaki K, Aoyama T (2012) A Study on the Quality of Micro-Machined Surfaces on Tungsten Carbide Generated by PCD Micro End-Milling. Annals of the CIRP 61(1):567–570. [8] Morgan J, Chris. Ryan RV, Eric M (2004) Micro Machining Glass with Polycrystalline Diamond Tools Shaped by Micro Electro Discharge Machining. Journal of Micromechanics and Microengineering 14:1687–1692. [9] Katahira K, Nakamoto K, Fonda P, Ohmori H, Yamazaki K (2011) A Novel Technique for Reconditioning Polycrystalline Diamond Tool Surfaces applied for Silicon Micromachining. Annals of the CIRP 60(1):591–594. [10] Kaneko K (1994) Determination of Pore Size and Pore Size Distribution. Journal of Membrance Science 96:59–89. [11] 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.
Please cite this article in press as: Katahira K, et al. Micromilling characteristics and electrochemically assisted reconditioning of polycrystalline diamond tool surfaces for ultra-precision machining of high-purity SiC. CIRP Annals - Manufacturing Technology (2014), http://dx.doi.org/10.1016/j.cirp.2014.03.031
CIRP-1142; No. of Pages 4 CIRP Annals - Manufacturing Technology xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
Micromilling characteristics and electrochemically assisted reconditioning of polycrystalline diamond tool surfaces for ultra-precision machining of high-purity SiC Kazutoshi Katahira a,*, Shogo Takesue b, Jun Komotori b, Kazuo Yamazaki (1)c,d a
Materials Fabrication Laboratory, RIKEN, 2-1 Hirosawa Wako-shi, Saitama 351-0198, Japan Department of Mechanical Engineering, Keio University, 3-14-1 Hiyoshi Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan c Department of Mechanical & Aerospace Engineering, University of California at Davis, Davis, CA 95616, USA d Department of Mechanical Engineering, University of California at Berkeley, Berkeley, CA 94720, USA b
A R T I C L E I N F O
A B S T R A C T
Keywords: Micromachining Silicon carbide Polycrystalline diamond (PCD) tool
The production of extremely thick silicon carbide (SiC) has recently become possible with the advent of a specific chemical vapor deposition process. Ultra-precision machining of high-purity SiC has been performed by using a polycrystalline diamond (PCD) micromilling tool to investigate the machining characteristics. Results indicate that a high-quality surface (Ra = 1.7 nm) can be obtained when the removed chips are thin enough to achieve ductile mode machining. Micron-sized wells and groove structures with nanometer-scale surface roughness were successfully machined by using the PCD tool. In addition, a new electrochemically assisted surface reconditioning process has been proposed to remove the contaminant material adhered onto the PCD tool surfaces after prolonged machining. ß 2014 CIRP.
1. Introduction Recently, silicon carbide (SiC) has been widely used in a variety of applications, including advanced micro-optics and molds, micromedical devices and micro-ceramic reactors for chemical analysis, primarily owing to their high wear resistance and thermal stability. Thus far, several methods have been adopted to deposit SiC by chemical vapor deposition (CVD). The deposition rate is typically of the order of 1 mm h1. However, higher deposition rates are often demanded for depositing thicker SiC. Lately, researchers have demonstrated the feasibility of depositing extremely thick (up to 20 mm) SiC by using a specific CVD process [1]. The resultant material was found to exhibit superior properties when compared to those of SiC produced by conventional sintering, especially in terms of high purity (99.9995%) and homogeneous structure. Meanwhile, SiC is characterized by extreme hardness and brittleness, making it a difficult-to-machine material. This has increased the demand for the development of a suitable micromachining method that is capable of forming SiC with required surface quality and geometric accuracy [2–5]. In general, hard and brittle materials such as SiC are machined by grinding and polishing processes to achieve a high-quality surface [2,3]. However, these methods have certain inherent limitations, especially when it comes to the fabrication of fine complex structures. Turning is yet another machining approach that has been applied to SiC [5]. Although this method is considered to be suitable for the production of symmetric structures such as lens molds, it is highly challenging to produce
* Corresponding author.
asymmetric structures, such as microchannels. On the other hand, micromilling using polycrystalline diamond (PCD) is considered to be an attractive method for the fabrication of three-dimensional structures in hard and brittle materials. This method offers the advantages of high surface integrity via ductile mode machining [6–8]. In this study, ultra-precision machining of high-purity SiC has been performed using a PCD micromilling tool to investigate the machining characteristics. In general, when a tool is used for prolonged machining operations and for producing concave and intricate shapes in multiple samples, it becomes highly important to maintain the surface conditions of the tool. Therefore, an electrochemically assisted method for periodic reconditioning of the tool surface is also proposed. 2. Coordination of experiments The sample analyzed in this study is high-purity bulk SiC produced by a specific CVD process. In addition, bulk SiC prepared by conventional sintering process was considered for comparison. The dimension of the sample was 10 mm 10 mm 5 mm, and the properties of the samples are summarized in Table 1. Micromilling was performed by using a specially designed square-shaped PCD end mill, as shown in Fig. 1. The PCD diamond grains of average size 1.0 mm were sintered with metallic cobalt under high temperature and pressure. The tool had a radius of 0.1 mm, 2 flutes, and was fabricated by wire electrical discharge machining. The machining tool used is a linear motor-driven ultra-precision 6-axis machine with an air turbine spindle operating at the speed of 50,000 min1. The feed resolution of each linear axis (X, Y, Z) is 1 nm.
http://dx.doi.org/10.1016/j.cirp.2014.03.031 0007-8506/ß 2014 CIRP.
Please cite this article in press as: Katahira K, et al. Micromilling characteristics and electrochemically assisted reconditioning of polycrystalline diamond tool surfaces for ultra-precision machining of high-purity SiC. CIRP Annals - Manufacturing Technology (2014), http://dx.doi.org/10.1016/j.cirp.2014.03.031
G Model
CIRP-1142; No. of Pages 4
3. Basic experimental results and machined surface analysis The surface quality of the sample micro-machined using a PCD was investigated by performing machining experiments with various feed rates in the range of 5–50 mm/min. The machining conditions are listed in Table 2. Fig. 3 shows the average surface roughness (Ra) and peak-to-valley surface roughness (Rz) of the high-purity SiC and conventionally sintered SiC, as measured by SWLIM. The feed per tooth (f) determined using Eq. (1) is also shown in the figure, where F is the feed rate, S is the tool revolution, N is the number of flutes, Rd is the radial depth of cut, and D is the tool diameter. As can be seen from the figure, for all the different feed rate conditions, the surface roughness of the high-purity SiC is lower than that of the sintered SiC. Although a sudden increase in Rz was observed for the high-purity SiC at higher feed rates above 200 nm/ tooth, Rz and Ra remained consistent around 50–100 nm and less than 10 nm, respectively, until the feed rate reached a certain value.
70 High-purity SiC Sintered SiC
60 50 40 30 20 10 0
0
10
20 30 40 Feedrate (mm/min)
50
0
100
300 400 200 Feed per tooth (nm)
500
(a) Average surface roughness, Ra. Peak-to-valley surface roughness Rz (nm)
The photograph of the experimental setup is shown in Fig. 2. The surface of the tool and sample was analyzed using a scanning electron microscope (SEM) equipped with an energy dispersive Xray spectrometer (EDS). The surface roughness was measured by scanning white light interference microscopy (SWLIM).
Average surface routhness Ra (nm)
K. Katahira et al. / CIRP Annals - Manufacturing Technology xxx (2014) xxx–xxx
2
3500 High-purity SiC
Sintered SiC
3000 2500 2000 1500 1000 500 0
0
10
30 40 20 Feedrate (mm/min)
50
0
100
400 200 300 Feed per tooth (nm)
500
(b) Peak-to-valley surface roughness, Rz.
Fig. 3. Machined surface roughness. (a) Average surface roughness, Ra. (b) Peak-tovalley surface roughness, Rz.
Fig. 1. SEM image of the square-shaped PCD end mill. Table 1 Properties of samples.
Density (g/cm3) Flexural strength (MPa) Vickers hardness (Hv) Young’s modulus (GPa) Fracture toughness (MPa m1/2) Thermal conductivity (W/m K)
High purity SiC
Sintered SiC
3.2 460 2500 466 3.3 300
3.1 450 2300 420 3.5 170
Fig. 2. Photograph of the experimental setup.
f ¼
2F SN
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi Rd Rd 1 D D
Fig. 5. SEM images of the PCD tool edge.
4. Machining of micron-sized well structures
Table 2 Basic machining conditions. Spindle rotation speed (min1) Axial depth of cut (mm) Feedrate (mm/min) Coolant
Fig. 4. Surface micrograph of high-purity SiC.
50,000 0.5 5–50 Cutting oil mist
(1)
Fig. 4 shows a representative surface micrograph of high-purity SiC machined under low feed rate conditions (50 nm/tooth). The cutter marks are apparent, the pitch of which is equal to the feed rate. The cutter marks can be attributed to the plastic flow, which indicates that ductile mode machining was achieved [9]. Fig. 5 shows SEM images of the PCD tool edge after micromilling high-purity SiC under low feed rate conditions. As evidenced from the figure, the adherence and deposition of chips is clearly observed on the surface of the tool edge. The PCD tools have randomly distributed protrusions of sharp diamond abrasives that act as hard cutting edges when the tool is rotated. This produces an effect similar to microgrinding [7,8], contributing to high-quality surfaces during the ductile mode machining of brittle and hard materials. In the present study, most of the material removal process took place at these diamond cutting edges, such as the corner edge region. For effective use of PCD as a microgrinding tool, it is highly imperative to have protrusions of diamond abrasives and removal of the adhered chip material.
While machining microconcave or intricate shapes, there is always a concern that the surface quality of the machined bottom portion is degraded because of the relatively low chip evacuation. Therefore, the micromachining performance of the PCD tool was verified by machining micron-sized well structures on both the high-purity SiC and sintered SiC. As can be observed from the schematic of the micromachining contour shown in Fig. 6, the tool path for machining a well structure follows a spiral pattern. Fig. 7 shows the SEM image of the micronsized well and groove structure machined onto high-purity SiC with a constant cutting depth of 0.5 mm and a feed rate of 5 mm/min. Under such conditions, the time taken to produce a single well was found to be 30 min. Fig. 8 shows a surface micrograph of the bottom portion of the sample surface, indicated as region A in Fig. 7. A highquality surface was successfully obtained even at the bottom portion of the micron-sized concave structure. In addition, SEM image of the corner portion of the sample (region B in Fig. 7) shown in Fig. 9 indicates the formation of a sharp edge without chipping. However, the SEM image of the surface of the sintered SiC sample shown in Fig. 10 indicates the formation of several pores on the machined well surface. These pores are considered to be internal defects of the material generated during the sintering process. The appearance of these defects on the surface of the machined sample indicates that the surface roughness of the sintered SiC (Fig. 3) has a large scattering. In other words, the characteristics of sintered SiC are inferior when compared with those of the high-purity SiC sample.
Please cite this article in press as: Katahira K, et al. Micromilling characteristics and electrochemically assisted reconditioning of polycrystalline diamond tool surfaces for ultra-precision machining of high-purity SiC. CIRP Annals - Manufacturing Technology (2014), http://dx.doi.org/10.1016/j.cirp.2014.03.031
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Fig. 6. Schematic of the micromilling tool contour, tool path, and target feature.
Fig. 8. Surface profile of the region A in Fig. 7.
Fig. 7. SEM image of the well and groove structure micro-machined onto highpurity SiC.
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Average surface roughness Ra (nm)
Systematic analysis of the well structures micro-machined onto SiC suggests that the PCD micro-tool is very effective for micromachining, especially in case of high-purity SiC. However, from the practical application viewpoints, it is equally important to verify the machining stability and operating lifetime of the tool during long-time machining operations for large-scale production or multiple samples. This was investigated by machining multiple micron-sized wells under the same machining conditions. The lowmagnification SEM image of the multiple micron-sized wells shown in Fig. 11(a) indicates the uniform formation of wells. However, the corresponding high-magnification SEM images of the 1st and 16th wells shown in Fig. 11(b) and (c), respectively, indicate the differences in the surface features. The machined surface of the 16th well has chippings at the edges and severe scratches at the bottom. Figs. 12 and 13 show the surface roughness of the bottom portion and depth of the machined wells, respectively, plotted as a function of the number of wells from the 1st to the 16th. With increase in the number of wells machined, the surface roughness gradually deteriorates, and the depth of the well becomes shallower than the set target value of 30 mm. This could possibly be due to the contamination of the tool edge generated in the long run, primarily caused by adhesion of the machined material as chips on the surface of the tool. In addition, the material adhered onto the tool surface increases the frictional resistance with further machining proceeds. This results in overall degradation of the machining performance. Therefore, it is very
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Fig. 12. Variations in the surface roughness of the bottom portion of machined wells, Fig. 13. Depth of machined wells, plotted as a function of the number of plotted as a function of the number of machined wells from the 1st to the 16th. wells from the 1st to the 16th.
important to identify the characteristics of the adhered material, which will be discussed in the forthcoming section. 5. Investigation of material adhered onto the tool surface The material adhered onto the surface of the PCD tool was investigated by SEM and EDS analyses. Typical SEM images of the PCD tool after repeated machining are shown in Fig. 14(a) and (b). As can be seen from the figure, the entire surface of the tool is covered with a thick film. In particular, the cutting edge itself is completely covered with the adhered material and contains small cracks, as shown in Fig. 14(c). The high-magnification SEM image of the tool surface shown in Fig. 14(d) indicates a wave-like microtexture with a periodicity of approximately 0.1–0.3 mm. This microtexture appears to have been produced by the submicronsized chips (flakes) that were removed from the machined surface and accumulated over time on the surface of the tool. Furthermore, the chemical composition of the adhered material was analyzed by EDS. Fig. 15 shows the quantitative chemical analysis of the regions (I) and (II) indicated in Fig. 14(b), as determined from EDS analysis. The EDS spectrum of region (I) indicates the presence of carbon, which is believed to have originated from the PCD material. However, the EDS spectrum of
Fig. 9. SEM image of the corner portion of the well structure (region B in Fig. 7).
Fig. 14. SEM images of material adhered onto the PCD tool surface. Fig. 10. SEM images of the well micro-machined onto sintered SiC.
Fig. 11. SEM images of (a) multiple micron-sized wells, (b) 1st machined well, and (c) 16th machined well.
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Fig. 15. EDS analysis of the regions I and II indicated in Fig. 14(b).
Please cite this article in press as: Katahira K, et al. Micromilling characteristics and electrochemically assisted reconditioning of polycrystalline diamond tool surfaces for ultra-precision machining of high-purity SiC. CIRP Annals - Manufacturing Technology (2014), http://dx.doi.org/10.1016/j.cirp.2014.03.031
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region (II) clearly shows the presence of silicon and oxygen. Based on the atomic ratio of these elements, the material is assumed to be amorphous silicon dioxide (SiO2) [9]. The carbon peak in this region can be disregarded, as it is likely to have originated from the PCD substrate. According to previous studies, amorphous SiO2 is reported to have a porous structure with an average pore size of 0.5–0.7 nm [10,11]. The so-called ultra-microporous structure is characterized by extremely high van der Waals forces, which is one possible reason for the adhesion of material chips onto the PCD tool surface. 6. Electrochemically assisted reconditioning process EDS analysis of the PCD tool surface suggests that the contamination is primarily due to the adhesion of silicon-based material, which was assumed to be amorphous SiO2. In general, such contaminations can be removed by brushing and ultrasonic cleaning in an acetone solution. However, in the case of a micronsized tool, it is difficult to fully remove it using these physical approaches. Therefore, an electrochemical approach was attempted for cleaning and reconditioning of the PCD tool. Fig. 16 shows the electrochemical test setup used in this study. A positive potential was applied to the PCD tool and a negative potential was applied to the copper electrode using a DC power supply. The reconditioning process could be performed without removing the tool from the machine. Fig. 17 shows low- and highmagnification SEM images of the tool surface after electrolyzing for 60 s. The SEM image of the cutting edge before electrochemically assisted cleaning was found to be covered by the adhered material (Fig. 14). Upon electrochemical reconditioning of the surface, the material adhered to the edge of the tool was successfully removed without any damage. The mechanism underlying the electrochemically assisted reconditioning process can be considered as follows. The adhesive material (SiO2) reacts with the NaOH electrolyte to form sodium metasilicate (Na2SiO3): SiO2 þ 2NaOH ! Na2 SiO3 þ H2 O
(2)
Na2SiO3 is also known as liquid glass. It is highly soluble in water, resulting in the formation of a hydrate by the following reaction: Na2 SiO3 þ nH2 O ! Na2 SiO3 nH2 O:
(3)
Furthermore, electrolysis of water is expected to occur during the electrochemical process, which generates a large amount of hydroxide ions. It is likely that these ions diffuse to the positive electrode (the tool), thereby significantly accelerating the chemical reaction. The performance of the reconditioned tool was evaluated by repeated milling of high-purity SiC and subsequent reconditioning. Fig. 18 shows the surface roughness plotted as a function of the machining distance increases. When the cumulative machining distance reaches 1000 mm, the Ra value became >10 nm. With
Fig. 16. Electrochemical test setup.
Fig. 17. SEM images of PCD tool after the electrochemically assisted reconditioning process.
Fig. 18. Surface roughness of the sample plotted as a function of machining distance during milling tests with periodic reconditioning.
subsequent electrochemically assisted reconditioning of the tool surface, the surface roughness of the sample machined using the reconditioned tool became less than 2 nm. This confirms that the electrochemically assisted reconditioning process is efficient in recovering the machining performance of the tool and is capable of extending the operating lifetime of tools during the micromachining of SiC. 7. Conclusions Ultra-precision machining of high-purity SiC was performed by using a PCD micro-milling tool and their milling characteristics have been investigated. Results indicate that a high-quality surface (Ra = 1.7 nm) can be obtained when the removed chips are sufficiently thin to achieve ductile mode machining. Although the micron-sized well and groove structures with nanometer-scale surface roughness were successfully machined, the surface roughness gradually deteriorated and the depth of the well became shallower with increase in the number of wells machined. This could be attributed to the build-up of material adhered to the tool surface. EDS analysis indicates that the adhered material is amorphous SiO2. To regain the milling properties of the PCD tool, the surface of the tool was reconditioned by adopting a new electrochemically assisted reconditioning technique. Results indicated that the reconditioning technique is effective in removing the surface contamination without causing damage to the tool edges. Therefore, this technique is considered to be potentially suitable for retrieving the excellent machining performance of PCD microtools. Acknowledgements The authors would like to thank NS Tool Co. Ltd. for their continued support. References [1] Dow Chemical Co., http://www.dow.com/, accessed on 2013.12.09. [2] Dornfeld D, Min S, Takeuchi Y (2006) Recent Advances in Mechanical Microengineering. Annals of the CIRP 55(2):745–768. [3] Klocke F, Zunke R (2009) Removal Mechanisms in Polishing of Silicon Based Advanced Ceramics. Annals of the CIRP 58(1):491–494. [4] Suzuki H, Moriwaki Y, Goto Y (2007) Precision Cutting of Aspherical Ceramic Molds with Micro PCD Milling Tool. Annals of the CIRP 56(1):131–134. [5] Yan J, Zhang Z, Kuriyagawa T (2010) Tool Wear Control in Diamond Turning of High-Strength Mold Materials by Means of Tool Swinging. Annals of the CIRP 59(1):109–112. [6] Cheng X, Wang ZG, 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. [7] Nakamoto K, Katahira K, Ohmori H, Yamazaki K, Aoyama T (2012) A Study on the Quality of Micro-Machined Surfaces on Tungsten Carbide Generated by PCD Micro End-Milling. Annals of the CIRP 61(1):567–570. [8] Morgan J, Chris. Ryan RV, Eric M (2004) Micro Machining Glass with Polycrystalline Diamond Tools Shaped by Micro Electro Discharge Machining. Journal of Micromechanics and Microengineering 14:1687–1692. [9] Katahira K, Nakamoto K, Fonda P, Ohmori H, Yamazaki K (2011) A Novel Technique for Reconditioning Polycrystalline Diamond Tool Surfaces applied for Silicon Micromachining. Annals of the CIRP 60(1):591–594. [10] Kaneko K (1994) Determination of Pore Size and Pore Size Distribution. Journal of Membrance Science 96:59–89. [11] 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.
Please cite this article in press as: Katahira K, et al. Micromilling characteristics and electrochemically assisted reconditioning of polycrystalline diamond tool surfaces for ultra-precision machining of high-purity SiC. CIRP Annals - Manufacturing Technology (2014), http://dx.doi.org/10.1016/j.cirp.2014.03.031