A Study on Liquid Dielectric Breakdown in

Micro-EDM Discharge Santosh Kumar Verma

#, Dr. Nagahanumaiah*

# Department of Mechanical Engineering, NIT Durgapur

*Micro Systems Technology Lab., CMERI 1san2sh.nit@gmail.com

2nagahanumaiah@yahoo.com

Abstract— The research work carried out in this paper, aims at

understanding the breakdown phenomenon of liquid dielectric

by the low energy ultra-short pulsed electric discharge produced

between tiny electrodes (~ 100µm diameter electrode) through

experimental studies In literature not many studies are reported

on liquid dielectric breakdown mechanism, and in micro-EDM

no published literature discusses about this. Therefore, a detailed

study on literature has been performed and preliminary

experiments have conducted on micro-EDM to understand the

glow discharge and its breakdown phenomenon better, towards

validation of scientific analogies for micro-EDM process

conditions.

I. INTRODUCTION

The growing interest in applications of micro-nano scale

devices in many applications diversified the market demand

towards batch production of multi material micro parts.

Therefore, innovative integration and development of

knowledge base for scaling up of production by precision

manufacturing technologies to ensure effective industrial

utilization has become the primary focused area of micro-nano

scale manufacturing research. The successful adaptation of

milling by electric discharge (i.e. EDM milling) for micro

manufacturing, which is known as „micro-EDM milling (μ-

EDM milling)‟ has shown significant potential for producing

accurate simple holes to a complex 3D feature on micro molds.

In this context, several researchers are striving towards

understanding the process fundamentals. Differences in

plasma [1], localization of thermal heating [2] and effects of

non-thermal forces in material erosion [3] have been reported.

In spite of this, mechanism of material removal in micro-EDM

process conditions is still debatable, and moreover the recent

review on this subject emphasized the need for investigations

towards better understanding gap phenomenon, thermal

modeling, modeling influence of non-thermal forces and up-

scaling of the process. In order to understand the gap

phenomenon, understanding of liquid dielectric breakdown

under low energy ultra-short pulsed electric discharge is the

primary aspect, which is currently being investigated during

this fellowship period.

Over the years, many models have been proposed to

explain the mechanisms of liquid breakdown, but none of

them has been unanimously accepted throughout. Breakdown

theories in the order of their acceptance are included as given

above:

Bubble Theory: Instability of formed bubbles results in

the formation and propagation of streamers, finally

resulting into breakdown [4,5,6,7].

Suspended Particle Theory: Conducting and polarizable

impurities present in the liquid are involved in breakdown

[8,9,10,11].

Electronic Breakdown Theory: The electrons ejected by

field emission form avalanches similar to that in gas

discharge and result into final breakdown [12,13,14,15].

Cavitation Theory: When electric field Eb developed in

the presence of bubbles, which are responsible for

cavitation, becomes equal to gaseous ionisation field,

discharge takes place leading to decomposition and

followed by breakdown [12,16].

In addition liquid breakdown involves a unique level of

complexity compared to gas or solid dielectric breakdown.

Physical characteristics, such as fluid viscosity, electro-

convection, temperature, density and pressure dependencies

complicate the analysis and modeling of the conduction and

breakdown mechanisms [8]. Moreover, majority of the

reported literature on liquid dielectric breakdown are either

use laser energy or high energy electric discharges with longer

discharge time. However, in micro-EDM conditions it is low

energy and ultra-short pulsed discharge is the energy source to

initiate the breakdown.

II. EXPERIMENT

In order to evaluate the adoptability of the above reported

breakdown theories for micro-EDM conditions, experiments

were conducted at Micro Systems Technology Lab at CMERI

on DT110 Multipurpose micro machine tool. The glow

discharges between two 100µm tungsten carbide electrodes

were recorded using a camera SONY HVR-Z7E HDV.

A. Specification of EDM machine

Name: Integrated Multi-process Machine Tool DT-110

Manufacturer: Mikrotools Pte Ltd.

Power requirements: 230v, 50/60Hz

Travel: 200(X-axis), 100(Y-axis), 100(Z-axis)

B. EDM Process parameters

Discharge circuit: RC relaxation circuit

Open circuit voltage: 100-120 V

Discharge Capacitor: 120-150 pF

Diameter of electrode: 100µm

III. RESULTS AND DISCUSSION

The spark discharge images were further studied using

PFV software. In spite of compatibility issues between the

images recorded in above said camera and the PFV software

in studying at higher frames rate, these experiments helps us

to understand the pattern of glow discharge and its breakdown.

In this study the glow discharge in each and every frame using

images recorded at higher frames rate could not have been

succeeded due above limitations, however the discharge glow

and breakdown over time has been studied at 30fps. Figure 1

and 2 depict the changes in spark intensity during glow

discharge and its breakdown over a time of 3µsec using de-

ionized water as a dielectric fluid. It has been noted that the

discharge initiates near the cathode surface and grown towards

anode before the plasma channel has been formed between

electrodes. Over time this plasma channel expands in elliptical

shape before it breakdowns. It has been found that breakdown

of discharge is relatively rapid compared to the discharge

growing rate. This confirms that during discharge initiation,

formation of bubbles and streamers do exists in low energy

discharge of micro-EDM. However in this experiment bubbles

and stream formation could not be visualized at the 30fps.

This work can be further extended by processing the high

speed images in each and every frame to understand the

bubbles formation and cavitation effects in future.

Fig. 1 Plasma channels formed during the electrical breakdown of deionized water in the order of increasing spark appearance

.Fig. 2 Plasma channels formed during the electrical breakdown of deionized

water in the order of decreasing intensity of light

CONCLUSION

The preliminary experimental study conducted in this work

on micro-EDM process at open circuit voltage 100-120V

using 100µm electrode indicates that discharge is growing

over time. The growth rate is relatively slow and breakdown is

found to be rapid. The discharge initiates near the cathode

surface and grows towards anode before it forms plasma

channel between the electrodes. The plasma channel expands

over time in elliptical shape with maximum light intensity and

then collapse rapidly. This preliminary study indicated that

bubbles and streamers formation do exists in dielectric

breakdown by micro-EDM discharge; however, it could not

be confirmed unless otherwise the images recorded at higher

frame rates are analysed.

ACKNOWLEDGMENT

The financial and technical support of India Academy of

Sciences (IAS) and Central Mechanical Engineering Research

Institute (CMERI) is gratefully acknowledged.

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