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ScienceDirect Procedia CIRP 42 (2016) 179 – 184

18th CIRP Conference on Electro Physical and Chemical Machining (ISEM XVIII)

Surface characterization, material removal mechanism and material migration study of micro EDM process on conductive SiC Krishna Kumar Saxenaa, Sanjay Agarwalb,*, Sanchit Kumar Khareb b

a Department of Mechanical Engineering, University of Auckland, Auckland, New Zealand. Department of Mechanical Engineering, Bundelkhand Institute of Engineering and Technology, Jhansi – 284128, India.

* Corresponding author. E-mail address: [email protected]

Abstract

SiC is an important industrial ceramic and also the fourth hardest ceramic after diamond, boron nitride and boron carbide. Due to very low fracture toughness, it is very difficult to machine SiC using conventional machining methods. EDM can machine such materials irrespective of their hardness. Micro electrical discharge machining (micro-EDM) is an adaptation of conventional EDM process that has similar electro-thermal methods of material removal for manufacturing miniature or micro parts. Micro EDM is used for obtaining burr-free micro dimensional apertures in difficult-to-cut materials. For improved understanding of the micro EDM process and achieve high quality machining on conductive SiC, it is necessary to characterize the machined surface and understand the material removal and material migration mechanism. In this work, micro depth holes were machined in SiC workpiece. The experimentation was planned using Taguchi’s L9 theory. A combination of scanning electron microscopy (SEM), white light interferometry (WLI), energy dispersive x-ray spectroscopy (EDX), atomic force microscopy (AFM) and X-ray diffraction (XRD) was used to characterize the machined surface and study the material removal and material migration mechanism. It was found that improved characterization and understanding of µEDM can lead to better micro-machining of conductive SiC. © The Authors. Authors. Published Publishedby byElsevier ElsevierB.V. B.V. This is an open access article under the CC BY-NC-ND license © 2016 2016 The (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining XVIII). (ISEM XVIII) Keywords: EDM, Micro EDM, SiC, material removal, material migration.

1. Introduction

Nomenclature

Erosion of material through controlled sparks in a dielectric medium was first observed by Lazarenko [1]. Since then EDM has undergone several research phases. EDM has emerged as a promising machining technique which can machine complex patterns in difficult to cut materials. Advanced ceramics such as SiC possess very low fracture toughness (~ 3.5MPa√m). Therefore, they are difficult to machine using conventional machining methods which involve plastic deformation. Since,

µEDM SiC

Micro Electrical Discharge Machining Silicon Carbide

EDM can machine the materials irrespective of their hardness; SiC can be machined by it. In the past few years researchers [2–5] have demonstrated successful machining of ceramics using EDM process. µEDM is an adaptation of conventional EDM process that has similar electro-thermal methods for removing

2212-8271 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM XVIII) doi:10.1016/j.procir.2016.02.267

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material in order to produce micro parts. The µEDM process has several advantages such as (i). Absence of cutting forces due to non contact machining. (ii). Machining process is not affected by hardness and brittleness of workpiece. (iii). It permits the use of deionized water as dielectric. (iv). There is absence of size effect and very less heat affected zone. Detailed review of conventional and µEDM has been presented by Jahan et al. [4]. µEDM of SiC have been successfully performed by researchers. Reynaerts et al. [5] machined 3D microstructures in silicon by EDM and they demonstrated that EDM process is independent on the silicon crystal orientation. EDM of SiC single crystal has been reported by Kato et al. [2]. Klocke and Zunke [6] investigated the material removal mechanisms in polishing of SiC and they concluded that the MRR and dominating material removal mechanism depends strongly on material properties and machining parameters. Chan et al. [7] used a two step µEDM process with standard silicon micro-fabrication technique to create smooth and axi-symmetric 3D hemispherical structures on silicon. Liew et al. [8] proposed carbon nano-fibre assisted µEDM of reaction bonded SiC. They found that addition of carbon nano-fiber improves machinability, material removal rate and reduces tool wear rate. Liew et al. [9] also investigated material migration phenomenon in µEDM of reaction bonded SiC and concluded that material deposition rate is strongly affected by the surface roughness of workpiece, voltage and capacitance of electrical discharge circuit. Saxena et al. [10] [11] studied the effect of machining parameters on response parameters during µEDM of SiC. They found that optimum setting of voltage, capacitance and threshold can result in better machining performance. Liew et al. [12] also demonstrated that better machining of SiC is achieved by ultrasonic cavitation assisted µEDM and addition of carbon nano fiber. µEDM on advanced ceramics such as SiC and WC is still under developing stage and requires extensive research. For improved understanding of the micro EDM process and achieve high quality machining on conductive SiC, it is necessary to characterize the machined surface and understand the material removal and material migration mechanism. In this work, micro depth holes were machined in SiC workpiece. The experimentation was planned using Taguchi’s L9 theory. A combination of scanning electron microscopy (SEM), white light interferometry (WLI), energy dispersive x-ray spectroscopy (EDX), atomic force microscopy (AFM) and X-ray diffraction (XRD) was used to characterize the machined surface and study the material removal and material migration mechanism in µEDM of SiC. The ceramic material used in the experiment was Silicon Carbide. SiC was first processed in 1982 using an industrial method by Hutchinson. The selection of the above mentioned conductive ceramic was made taking into account its wide range of applications MEMS. This reaction-bonded silicon carbide (SiSiC) is manufactured by infiltrating silicon into a porous block made of silicon carbide powder and carbon, which is then submitted to a firing process at a specific temperature.

2. Experimental In this work, conductive SiC pieces of 20×20×5 mm (Fig. 1) were machined on a Mikrotools TM high precision micromachining test bed (Fig. 2). The machine has a resolution 0.1 µm and an accuracy of 1 µm for each of the three axes. The response of each axis is important because a fast movement is required for gap control during µEDM.

Fig. 1: SiC workpiece used for experimentation.

Fig. 2: Machining of SiC on µEDM test bed.

Fig. 3: Representative voltage profile for µEDM [13]. The µEDM machine has a RC circuit. The representative voltage profile obtained during µEDM cycle is shown in Fig. 3 [13]. Region AB denotes open circuit condition when the tool approaches the workpiece. Region BC represents the condition when spark occurs and material removal takes place. Region CD indicates short circuit due to contact between tool and workpiece. After the short circuit, tool retracts back to open circuit condition and the cycle is repeated. The electrolytic copper tool of ф 3 mm was used as a cathode. The working cross section of the tool was highly polished to mirror finish. Deionized water was used as dielectric. Machining of micro-dimensional depth is emphasized in this work. Hence, blind holes of 100 µm depth were machined on µEDM test bed. The gap voltage, capacitance and threshold [13] were taken as machining

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parameters. The experimental scheme was planned using Taguchi’s L9 orthogonal array and is shown in Table 1. Effect of machining parameters on µEDM performance is not a part of this paper. A combination of SEM, AFM, WLI, XRD and EDX techniques were used to study the surface condition, material removal mechanism and material migration phenomenon during µEDM of conductive SiC. Table 1: L9 array for experimentation. Sr. No.

Gap Voltage (V)

Threshold (%)

1.

110

10 nF

40

2.

110

0.1 µF

60

3.

110

0.4 µF

80

4.

130

10 nF

60

5.

130

0.1 µF

80

6.

130

0.4 µF

40

7.

150

10 nF

80

8.

150

0.1 µF

40

9.

150

0.4 µF

60

Capacitance

3. Surface characterization Fig. 4 shows the optical and SEM images of the micro depth hole in SiC workpiece. In order to study the surface condition of the µEDMed surface, SEM, AFM, WLI and XRD techniques were used. a

Fig. 5: SEM image of the µEDMed surface. (130 V, 0.1 µF, 80 % threshold)

Fig. 6: AFM image of the µEDMed surface. (130 V, 0.1 µF, 80 % threshold)

Fig. 7: SEM image showing resolidified material on µEDMed surface.

Fig. 8: SEM image showing craters on the µEDMed surface.

a

b

b

Fig. 4 (a) Optical image and (b) SEM image of the machined hole at 130 V, 0.1 µF and 80 % threshold. The SEM (Fig. 5) and AFM (Fig. 6) images of the machined surface revealed that the µEDMed surface was very rough and consisted of debris and uneven topography. The surface characterization revealed that µEDM process produces much damage such as micro voids, pin holes and craters on the machined surface. It can be seen from Fig. 7 that µEDM surface consists of resolidified material. Due to very small gap between tool and workpiece as well as improper flushing, the debris particles get trapped in the spark gap and get solidified there. The craters (Fig. 8) are the locations where the spark has occurred and material removal has taken place. To evaluate the surface topography, WLI images were captured and are shown in Fig. 9. It can be observed that the roughness decreases slightly after µEDM operation but still the Ra value is very high. The high roughness of µEDM can be attributed to existence of debris, resolidified material, craters and micro voids present on the µEDMed surface.

Fig. 9: WLI captured surface topography of the µEDMed surface (a) before µEDM and (b) after µEDM. (130 V, 0.1 µF, 80 % threshold) In order to determine the nature and magnitude of residual stresses, XRD was performed on the µEDMed surface. The I-2ɵ plots were recorded before and after µEDM as shown in Fig. 10. The sharp peaks revealed high crystalline nature of SiC workpiece. After µEDM, the lattice spacing was increased and the diffraction peaks shifted at lower diffraction angles. It indicates the presence of compressive residual stresses on the µEDMed surface. Ideally, a high 2ɵ diffraction peak was chosen to ensure high sensitivity to strain. Since, after spark the material tries to expand but the dielectric pressure, clamping pressure and the cold portion of workpiece prevents it from expansion. Thus, compressive residual stresses are developed. These compressive stresses had high magnitudes at localized regions and they could have been originated from the thermal strains which are caused due to alternate heating (from spark) and cooling (from dielectric) of the workpiece during µEDM. The residual thermal strains and stress can be calculated using Eqn. (i) and (ii) respectively.

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Fig. 12: Material removal due to intense localized heating in simulated workpiece. (130 V, 0.1 µF, t = 10 µs | Ansys 14.5)

Fig. 10: I-2ɵ plots obtained from XRD. (130 V, 0.1 µF, 80 % threshold)



'd

 d  d0

d

d0

V u

E

(i) 5. Material migration phenomenon (ii)

Q

where d and d0 denote the lattice spacing after and before machining respectively. σ and ϵ denote the residual stress and strain respectively. E and ʋ denote Young’s modulus and Poisson ratio respectively. Furthermore, the pattern of peaks before and after µEDM shows that there is not much change in the composition of surface before and after machining. There is slight change only due to oxide and carbon deposit during machining. 4. Material removal phenomenon In µEDM, intense localized heating of workpiece takes place due to spark. Thus, a very small volume of material melts and get eroded from the workpiece and is flushed by dielectric. The craters as shown in Fig. are the locations where the spark has occurred and material removal has taken place. Fig. 11 shows the high magnification SEM images of the craters present on the µEDMed surface of SiC. These images show the evidence of localized melting leading to material removal. a

b

Fig. 11: Material removal due to localized melting and erosion in SiC workpiece during µEDM (a) 35000x (b) 100000x.

During µEDM of SiC, the localized temperature rose as high as 11598K in discharge time of 10 µs as shown in Fig. 12. Due to intense localized heat, material got melted and material removal occurred. Since in µEDM highly localized heating takes place, therefore HAZ is less and properties of workpiece near the machined zone are not affected. Thus, the spark wanders all over the tool surface and material removal takes place due to melting and erosion. Finally, a negative impression of tool is formed on the workpiece.

In order to study the material migration phenomenon during µEDM of SiC, a combination of SEM and EDX analysis was used. EDX analysis was carried out on the µEDMed SiC surface as well as on the cross section of the tool. The scanning electron micrographs revealed the presence of white debris on the machined workpiece which also accounted for the reason of high surface roughness (Figure 13). This debris could be of copper and/or silicon carbide. The EDX was carried out on the debris (Figure 14) and the EDX results show that it was of SiC. Traces of Cu debris were also found on the machined workpiece at both spectrum1 (Figure 15) and spectrum 2 (Figure 14). This confirms that material migration occurs from tool to workpiece in μEDM. However, majority of debris was of SiC. The origin of Si and C is the workpiece itself. The oxygen might have come from the dielectric. The Al as detected during EDX was used as a doping material to make SiC conductive. The origin of copper debris is the tool itself. During sparking, the material is eroded from tool also. This debris falls on the tool and gets attached to its surface if the flushing is not good. Furthermore, the material also gets eroded from the SiC workpiece. Some debris gets flushed and remaining is attached to the machined surface and due to low spark gap. Accordingly, it gets resolidified due to intense local heat. So, during µEDM the craters, cracks, pores and debris are the main reason for high surface roughness. To study the distribution of different elements deposited on the μEDMed surface, element mapping was carried out. Figure 16 shows element maps of machined workpiece. It is clear from maps that distribution of copper and carbon was uniform on the surface and no segregation was found on the machined surface. The Si being a constituent element of workpiece was distributed uniformly. The oxide deposit was also uniform and didn’t cluster. The micrograph of tool was also captured and spark locations were

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revealed as shown in Figure 17. The surface of tool also became rough (Figure 18). The EDX analysis at Spectrum 2 in Figure 19 reveals the presence of SiC traces on the tool surface. This should be due to the migration of SiC particles from workpiece to tool. It confirms that material migration in μEDM is a bidirectional phenomenon. This conforms to the observations made by Liew et al [9]. The possible material migration mechanism during µEDM is represented schematically in Figure 20.

Fig. 16: Element mapping of the µEDMed surface. Fig. 13: Enlarged view of debris on µEDMed surface. (500x).

Fig. 17: SEM image of cross section of Cu tool showing spark locations.

Fig. 18: SEM image of Cu tool surface after µEDM.

Fig. 14: EDX analysis of debris.

Fig. 15: EDX analysis of µEDMed surface.

Fig. 19: EDX analysis of Cu tool surface after µEDM.

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to the formation of oxide and carbon layer. High magnification SEM images and simulation study clarified the material removal mechanism to be melting and evaporation due to intense localized heating. A combination of SEM and EDX analysis revealed the material migration to be bidirectional. Acknowledgements The authors are thankful to Prof. Ramesh Kumar Singh, IIT Bombay for providing us the µEDM facility to carry out this research work. The authors are thankful to BIET Jhansi for providing necessary funding for this research work. References [1] [2]

Fig. 20: Material migration mechanism in µEDM It can be seen from Fig. 20 that during sparking material migration is bidirectional. Tool and workpiece debris eroded from spark is deposited on workpiece. Also, during short circuit the workpiece particles stick to the tool surface. Majority of debris is flushed by the dielectric. The oxide deposition and carbon deposition on the µEDMed SiC surface could be due to the following chemical reactions occurring between the deionized water and workpiece due to localized heat.

[3]

[4]

[5]

[6]

[7]

SiC  4H 2O o SiO2  CO2  4H 2 The molten silicon also gets oxidized from deionized water.

Si  2H 2O o SiO2  2H 2 Conclusion In this work µEDM operation was performed on SiC. Machining of micro depth is emphasized in this paper. SEM, AFM, WLI and XRD techniques were used to characterize the surface and study the material removal phenomenon. A combination of SEM and EDX was used for studying the material migration phenomenon. Surface characterization revealed the presence of micro dimensional craters, debris, resolidified material, and micro-voids. The topography of the µEDMed SiC surface was uneven due to the presence of the debris and craters. This was also the reason for high surface roughness as observed from WLI. XRD data revealed the presence of compressive residual stresses due to alternate heating and cooling. A slight change in composition was observed between the unmachined and machined surface due

[8]

[9]

[10]

[11]

[12]

[13]

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