Santosh Kumar Verma

07/ME/52

Department of Mechanical Engineering

National Institute of Technology Durgapur

Santosh Kumar Verma 07/ME/52 Department of Mechanical Engineering National Institute of Technology Durgapur

UNDER THE GUIDANCE OF DR. NAGAHANUMAIAH

CENTRAL MECHANICAL ENGINEERING RESEARCH INSTITUTE

DURGAPUR (INDIA) 713209

CERTIFICATE

This is to certify that the project work titled “A Study on Liquid

Dielectric Breakdown in Micro-EDM Discharge” is a bonafide

work done by Santosh Kumar Verma, Roll no 07/ME/52, of Mechanical

Engineering Department of National Institute of Technology Durgapur during

May-June 2010 under Summer Research Fellowship Program 2010 conducted

by Indian Academy of Sciences, completed in my guidance.

Dr. Nagahanumaiah

Scientist F & Group Head

Micro Systems Technology Laboratory

ACKNOWLEDGEMENT

I, Santosh Kumar Verma, a student of National Institute of Technology,

Durgapur have done my summer project for a period of 8 weeks at “Central

Mechanical Engineering Research Institute” Durgapur (West Bengal) under the

Summer Research Fellowship Programme 2010 conducted by Indian Academy

of Sciences, Bangalore.

I am indebted towards Dr. Nagahanumaiah, my project guide, for

providing me with the opportunity to undertake the project, and to work under

his profound guidance and support, at his Laboratory.

I take this opportunity to thank the selection committee of Indian

Academy of Sciences for endowing me with such an excellent opportunity to

undertake this project & for their kind cooperation.

I would like to take this opportunity to thank the research staff of Micro

Systems Technology Lab., CMERI for being so kind and cooperative at each and

every step.

A special thanks to the management of Central Mechanical Engineering

Research Institute Durgapur for being supportive during the whole fellowship

program.

Santosh Kumar Verma

September 2010, India

TABLE OF CONTENTS

Page

CHAPTER I INTRODUCTION....................................................................1

1.1 Overview………………………………………..............................................................................1

1.2 Historical Background…………………………........................................................................1

1.3 EDM Principle……………………..........................................................................................2

1.4 Scenario………………………………………...............................................................................4

1.5 Conventional EDM vs. Micro EDM................................................................................5

1.6 Problem Statement………………………………......................................................................6

CHAPTER II BREAKDOWN OF LIQUID DIELECTRICS……………………7

2.1 Liquid Dielectrics...........................................................................................................7

2.2 Breakdown Theories.....................................................................................................7

2.2.1 Bubble Theory……….......................................................................................7

2.2.2 Suspended Particle Theory.........................................................................10

2.2.3 Electrical Breakdown Theory……………………………………………………………..……12

2.2.4 Cavitation Theory……………………………………………………………………………….…..13

CHAPTER III EXPERIMENT..………………………………….............................15

CONCLUSION……………..……………………………………………………………………17

REFERENCES……………………………………………………………………………………18

LIST OF FIGURES

Fig Title Page

1 Market and research evolution in EDM 1

2 Stepwise breakdown process

Stage (1) electricfield generation Stage (2) appearance of narrow channel Stage (3) voltage decay Stage (4) creation of discharge channel Stage (5) formation and expansion of vapour bubbles Stage (6) onset of melting of metal Stage (7) ejection of melted material Stage (8) left out material solidifies Stage (9) ejected materials create small spheres

3

3 Plasma channel formation and other activities during breakdown 4

4 Die sinking EDM 5

5 Wire EDM 5

6 Bubble radius 9

7 EDM machine 15

8 High-speed camera 15

9 Plasma channel formation during the electrical breakdown of deionized water in the order of decreasing intensity

16

10 Plasma channels formed during the electrical breakdown of deionized water in the order of increasing spark appearance

16

CHAPTER I

INTRODUCTION

1.1 OVERVIEW

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. There is a

huge demand in the production of microstructures by a non-traditional method which is known as Micro-EDM.

Micro-EDM process is based on the thermoelectric energy between the work piece and an electrode. Micro-EDM is

a newly developed method to produce micro-parts which are in the range of 50 µm -100 µm. Micro-EDM is an

efficient machining process for the fabrication of a micro-metal hole with various advantages resulting from its

characteristics of non-contact and thermal process.

1.2 HISTORICAL BACKGROUND

The principles of EDM [1] are known since two centuries ago (in 1786, the British Physicist Priestley observed the

presence of small craters in opposite electrodes in between a spark arised) focusing the first application for the

principle: the preparation of colloidal dissolutions of metals. The first application of the sparks to obtain geometrical

shapes was made during the 1st and 2

nd World War. At the beginning, the possibilities of the technique were not

considered due to the low productivity and lack of process control. In the first designs, the electrode and the part

wear were similar and the presence of no desired arcs dropped the process performance. The gap between the

electrode and the part was controlled by vibrating systems that reduced the electrode wear, which was still

excessive. The definitive push of the technology (initially in SEDM) was made in Moscu in 1943 by two married

Societic Scientists: Dr. Boris and Dr. Natalya Lazarenko. They developed some key components that made possible

to apply the technology in the industry: the spark generator (Resistance-Capacitance RC generator) and the first

servo control circuit to keep a constant discharge gap. The new developments were presented in a job titled

“About the inversion of wear effect in electric discharges” [2] and published in April 23rd

, 1943 by B.N. Zolotych, a

collaborator of the mentioned scientists and one of the most important researchers in the field of EDM. The next big

step for the EDM technology was made in 1969 by Prof. Bernd Schumacher with the development of the wire EDM

machine.

Figure 1: Market and research evolution in EDM

1.3 EDM PRINCIPLE

The charge loaded electrode approaches the surface of the component [3], which is loaded with the opposite charge.

In between both electrodes there is an isolating fluid, referred as dielectric fluid. Despite being an electric insulator,

a large voltage difference can produce the dielectric breakage, producing ionic fragments that make possible the

electric current to jump between the electrode and the work piece. The presence of metallic particles suspended in

the dielectric fluid can be good for the electricity transfer in two different ways: on one side, the particles are good

to ionise the dielectric and, what is more, they can provide the electric charge; on the other side, the particles can

catalyse the dielectric breakage. For that reason, the electric field is larger in that position in which the electrode and

the work piece are closer. In this process stage the situation can be represented in fig. 2.

In the next stage (stage 2), as the number of ionic particles in the dielectric increases, the isolating capabilities of the

dielectric fluid drop in a narrow channel that appears in that position in which the electric field is larger. At the same

time, the voltage difference presents the highest value, being the current still zero.

Just as represented in stage 3, the current passes by the dielectric fluid that doesn‟t play as insulator. As the current

passes, the voltage decays.

Stage 4 depicts the next stage, the heat produced in the area increases fast as the current increases and the voltage

difference decreases. The generated heat ablates part of the dielectric fluid, the part and the electrode, creating a

discharge channel between the electrode and the part.

A vapour bubble (stage 5) is produced and expands against the ions entering the discharge channel. The ions are

attracted by the intense electromagnetic field arising during the discharge. At the same time, the current increase as

the voltage drops.

By the end of the pulse (stage 6), the electric current and the voltage achieve equilibrium, while the produced heat

and pressure reach their maximum value. At the same time, the part material is removed. The material layer just

under the discharge is melted but remains in the same position due to the bubble pressure. The discharge channel

consists of plasma formed out of part, dielectric and electrode material.

When the pause time between consecutive discharges starts (stage 7), the electric current and the voltage drop to

zero. The temperature decreases fast, collapsing the vapour bubble and producing the ejection of the melted

material.

In the next phase (stage 8), new dielectric flushes into the area, cleaning and cooling the part surface. The material

which was melted but was not ejected is solidified producing a recast layer.

In the last phase (stage 9), the ejected material creates small spheres dispersed in the dielectric, some electrode

particles and vapour that goes to the dielectric surface. For a short pause time the melted material and electrode

would accumulate making the spark to become unstable and producing electric arcs that would damage the electrode

and the part. All the sequence is repeated at a rate circa 250000 times per second but, in an instance only one cycle

can be present.

Figure 2: Dielectric breakdown process

Stage 1 Stage 6

Stage 2

Stage 8

Stage 7

Stage 4

Stage 3

Stage 5

Stage (1) electricfield generation

Stage (2) appearance of narrow channel

Stage (3) voltage decay

Stage (4) creation of discharge channel

Stage (5) formation and expansion of vapour bubbles

Stage (6) onset of melting of metal

Stage (7) ejection of melted material

Stage (8) left out material solidifies

Stage (9) ejected materials create small spheres

Stage 9

Figure 3: Plasma channel formation and other activities during breakdown

1.4 SCENARIO

Due to high precision and good surface quality that it can give, EDM is potentially an important process for the

fabrication of micro tools, micro components and parts with micro features. However, a number of issues remain to

be solved before micro EDM can become reliable process with repeatable results and its full capabilities as a micro

manufacturing technology can be realized.

In this context, several researchers are striving towards understanding the process fundamentals. Differences in

plasma [4], localization of thermal heating [5] and effects of non-thermal forces in material erosion [6] 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. To

understand the material removal and hence working of EDM, the mechanism leading to breakdown of dielectrics

needs to be investigated .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.

1.5 CONVENTIONAL EDM VS. MICRO-EDM

The most important difference between microEDM and EDM (for both wire and die sinking EDM) is the dimension

of the plasma channel radius that arises during the spark: in conventional EDM it is much smaller than the electrode

but the size is comparable in case of micro-EDM.

Together with the energy effects, the Flushing pressure acting on the electrode varies much with respect to the

conventional process: the electrode pressure area is smaller but the electrode stiffness is lower, making it more

“nervous”. The debris removal is more difficult because the gap is smaller, the dielectric viscosity is high and the

pressure drop in micro volumes is higher.

As it happens in conventional EDM, the higher precision can be achieved only if electrode vibrations and wear are

contained. This implies an important limitation for conventional EDM that turns out to be more restrictive in micro

EDM.

For each discharge, the electrode wear in micro EDM is proportionally higher than conventional EDM. The

electrode is softened, depending on the section reduction of the spark energy. For thin WEDM, the maximum

traction force than can be applied to the wire will depend on the effective section and, therefore the traction control

must be more accurate than that in conventional wires (0.20~0.33 mm) because the wire rupture can arise with

fluctuations as small as 3~5 grams.

Again micro-EDM is broadly divided into two –die sinking and wire-EDM.

Figure 4: Die sinking EDM Figure 5: Wire EDM

. .

1.6 PROBLEM STATEMENT

The research work carried out under this fellowship program, 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. The objectives of the work targeted for these two months research

stay at CMERI include

1. Detailed literature review of dielectric breakdown mechanism, particularly for liquid dielectrics

2. Critical evaluation of liquid dielectric breakdown theories to ascertain its adoptability for micro-EDM

conditions

3. Experimental investigations

In literature not many studies are reported on liquid dielectric breakdown mechanism, and in micro-EDM no

published literature discusses about this. Therefore, during these four weeks period, detailed study on literature has

been performed and preliminary experiments were conducted on micro-EDM process to understand the glow

discharge and its breakdown phenomenon better, towards validation of scientific analogies for micro-EDM process

conditions.

CHAPTER II

LIQUID DIELECTRIC BREAKDOWN THEORIES

2.1 LIQUID DIELECTRICS

2.1.1 Importance of Dielectrics

In non-conventional manufacturing process such as EDM, dielectrics are of vital role. Machining is

possible after the breakdown of the insulating material between the electrodes, i.e. dielectric. In a general

science definition -Dielectric is an insulating material which after application of an electric field of

appropriate magnitude, under breakdown, behaves as a conductor. Since the breakdown of dielectrics is

of great importance, thus, it is necessary that the mechanism behind this should be studied.

2.1.2 Difficulties in Study of Breakdown in Liquid Dielectrics

Over the years, many models have been proposed to explain the mechanisms of liquid breakdown, but

there is no single one which has been unanimously accepted throughout. Liquids combine some of the

best features of both gas and solid dielectric breakdown. The physical nature of liquids, high density,

viscosity, quality, thermal and electrical properties etc…, compared to gases adds multiple dimensions to

problems associated with developing a comprehensive model [7]. Complex nature of liquid makes the

theoretical study difficult. Liquid quality is critical issue. It is very difficult to create a pure liquid

compared to a pure gas. It is generally accepted that liquid purity plays a very important role in the

development of final breakdown.

2.2 BREAKDOWN THEORIES

Since there is no single comprehensive theory, thus in the order of their predominance different

breakdown theories are as given below -

1. Bubble Theory

2. Suspended Particle Theory

3. Electronic Breakdown Theory

4. Cavitation Theory

2.2.1 Bubble Theory

This most widely accepted theory is based on the formation of bubbles on the electrode and liquid

interface, and subsequent generation and propagation of streamers which lead to the final breakdown of

the dielectrics.

According to some papers positive streamer is a purely electronic process occurring in the bulk liquid,

while negative streamer first involves the formation of a bubble [8, 9]. While some others have reported

that the breakdown process cannot be initiated without the presence of bubbles [10, 11]. Thus, bubble

formation plays a vital role in the breakdown of liquid dielectrics. So, we will be concentrating on bubble

formation in addition with negative streamers propagation for the final breakdown of the liquid [12].

Bubble Formation

Micro-bubbles are created by the rapid, localized injection of current pulse from the point cathode and

this energy is converted into heat which causes evaporation of the liquid and provides the driving force.

Several other means of bubble formation such as cavitation or electrical stress may occur simultaneously

to one degree or another in liquids. The mechanism that dominates is controlled by the fluid properties at

the time of the experiment [4]. Expansion of the bubble is limited by the inertia of the surrounding liquid

and work done against the ambient pressure. The micro-bubble behaves as a constant energy system (i.e.

no additional energy is provided by the electric field during the expansion). In the absence of an

additional driving force, a bubble grows to a maximum size set by ambient pressure, and then collapses.

These expansion and collapse processes are well described by the Rayleigh model of

cavitation[13,14,15]. In order to expand beyond the size set by the kinetic energy limit, some other factor

must provide the driving force for bubble expansion. The vapor cavity expands rapidly at a velocity

which decreases with time. Knowing R (t) from experiments [16], we can estimate the magnitudes of the

various energy terms in the cavity expansion [17, 18]. The major components are, the kinetic energy of

the fluid surrounding the cavity, viscous losses in the fluid, the PV work done against ambient pressure,

and the work done against the surface tension. For ambient pressures of ≤ 1 atm., we can ignore PV and

surface tension [19]. Expansion of the bubbles beyond the size set by the kinetic energy limit is

determined by the given driving forces:-

During pre-breakdown

1) vapour pressure of the liquid

2) force generated by electrostatic field

During breakdown

1) fluid inertia

2) fluid viscosity

Considering inertia limited expansion, the radius of constant kinetic energy cavity [15] increases as t0.4

and, thus, a cavity that is expanding faster than t0.4

must be driven by a force, since it is increasing in

energy. This force is obtained from the pressure which can be generated by electrostatic field on the

cavity surface which can be approximately given by [18],

2/1)/(2ln)(

RaR

VRE .... (1)

The electrostatic pressure is given by,

)()2/1()()2/1(

2/32/1

22

Ra

VREP ooes …. (2)

The work done by electrostatic force in expanding the cavity is obtained by integrating Pes over the swept

volume,

2/1

2/32

)3/4(a

RVWD o …. (3)

Equating this work to the kinetic energy and integrating ,with the initial condition R=0 at t=0, gives the

required equation for cavity radius vs. time.

7/2

2/1

222)(

a

tVtR o …. (4)

In the high-viscosity limit we can neglect the kinetic energy and PV terms in the equation of bubble

expansion. The energy dissipation due to viscous effects in the fluid surrounding an expanding cavity is

then given by,

3

32 2RU …. (5)

per uint time [13]. The work done by Pes per unit time in expanding the cavity is,

esUPRWD 24

…. (6)

Equating this work to the energy dissipated per unit time and integrating, with initial condition R=0 at

t=0, we obtain the required equation for cavity radius in the viscous limit.

3/2

2/1

2

)10(

3)(

a

tVtR o

…. (7)

Streamers Formation

Streamers are basically conduction paths during dielectric breakdown in the form of branched trees.

Streamers [21, 22] can be classified as-

1. Primary, secondary, tertiary

2. Positive (anode), negative (cathode)

Streamers do not arise at random; their uniform radial spacing implies some sort of hydrodynamic

coupling, and we pursue the idea that the instability of the cavity wall is the source of the streamers. As

EHD instability develops in amplitude, the wave structure evolves into finger-like protrusions from the

ionized vapor cavity; moreover, if these fingers are ionized columns of vapor, the field will be

concentrated at the tips, and the electrostatic force will cause the streamers to propagate in the applied

field [23, 24]. But every instability does not necessarily turn into streamers. If we are correct in assuming

that streamers evolve from the EHD instability of the vapor cavity wall, then a streamer can be thought of

as a column of ionized gas being dragged through the viscous fluid by electrostatic force. This force can

be roughly approximated by assuming that the field is concentrated on the streamer tip, but the force

depends on the location of the electrostatic charges.

Approximately, the electrostatic force can be given as,

2/1

'

2~

a

RVF oes …. (8)

Figure 6: Bubble radius

where a' is the tip-to-plane spacing.The Rayleigh analogy method for estimating the skin friction [27]

gives the force per unit area given by,

2/1

tUF

…. (9)

where U is the velocity and ν is the kinematic viscosity.The maximum viscous drag on the streamer is

given by integrating F over the streamer length.

2/1

32/1

0

42

LURdx

x

UURF p

L

p …. (10)

The velocity of propagation of a streamer is generally found to be proportional to applied voltage [25,26];

moreover, for a given voltage, velocity is relatively constant as the streamer crosses the gap (though with

some increase as the streamer approaches the plane electrode). Equating the electrostatic force to the

maximum viscous drag, we get the equation for streamer propagation.

3/2

2/1

2

)(4

LRa

VU o

…. (11)

This equation reveals that the streamer propagation is roughly proportinal to applied voltage [23] and

relatively independent of position in the gap.This is in agreement with the known properties of negative

streamers. A series of supersonic streamers escape from the tip of one of the bubbles at a very high

speed.The formation of streamers is accompanied by shock waves, from the bubble tip.The primary

streamers except one dissappear, and a new fan of streamers originate from the spot where the latter

single streamer has stopped.Consequently, this process leads to the final breakdown.

2.2.2 Suspended Particle Theory

The suspended particle theory is based upon impurities in the liquid either as fibres or dispersed solid

particles, usually of observable size. Since maintaining a pure liquid is extremely difficult especially in

practical situations, a theory based upon the effect of contamination would be useful [7].

Modeling of Breakdown Theory

A suspended particle can be anything , other than the test liquid, that can carry a charge. The particle,

which are assumed as spheres of radius 'r' must be polarizable or charged so that in the presence of an

applied field it will drift [28,29,30]. The force experienced is given by,

EgradErFliq

liq

liqe

2

3

…. (12)

This force is directed towards a place of maximum stress if ε>εliq but for bubble ε<εliq it has the opposite

direction. The force given by the equation above increases as the ε of the particle increases and for a

conducting particle, the above equation reduces to,

EgradErFFe

3 …. (13)

Thus the force will urge the particle to move the strongest region of he field which is in the uniform

region for a uniform field gap. The particle along with the force from the applied field, experiences an

impending force due to viscosity.

)(6 xrFdrag …. (14)

Velocity of the particle in the region of maximum stress is obtained by Fe=Fdrag,

dx

dEErE

6

2

…. (15)

and the inclusion of diffusion introduces the velocity of diffusion,

Ndx

dN

r

kT

dx

dN

N

DD )

6(

…. (16)

Equating υE with υd , we get

dx

dN

Nr

kT

dx

dEE

r

r)

6(

6 2

2

…. (17)

which introduces breakdown strength dependence in time on concentration of particles N, radii and the

liquid viscosity. The critical value of transverse field E(x), the equilibrium value above which breakdown

will occur sooner or later can be obtained by the integartion of the above equation, which results as

kT

EEr

N

N xx

2

}{exp

)(2

)(2

3

)(

)( …. (18)

If the increase in the electrostatic energy when the particle drifts towards the region of maximum stress is

much smaller than their kinetic energy, then

kTEEr x 2}{ 2

)(

2

)(

3 …. (19)

and the criterion for breakdown resulting from the movement of particles towards the high stress region

corresponds to the condition,

kTEEr x 2}{ 2

)(

2

)(

3 …. (20)

A complete theory which takes into account the permittivities and radii, is given by the relation

kTEr o

liq

liq

4

1

2

22

…. (21)

In the region where grad E is zero, the particles are in equlibrium. Accordingly, particles will be draged

into the uniform field region. If the permittivity of the particle is higher than that of the medium, then its

presence in the uniform field region will cause flux concentration at its surface. Other particles will be

attracted into the region of higher flux concentration and in time will become aligned head to tail to form

a bridge across the gap. The field in the liquid between the particles will be enhanced, and if it reaches

critical value breakdown will follow.

Limitations of Theory

Although this theory did explain the strength of the liquids containing large amounts of particles, it is

unlikely to be extended to pure liquids. Sometimes, discharge occurs in a different region other than that

which has been bridged [29].

2.2.3 Electronic Breakdown Theory

Both field emission and field enhanced thermionic emissions mechanisms have been considered

responsible for the current at the cathode. Breakdown measurements carried out over a wide range of

temperatures, however, show little temperature dependence which shows that the cathode process is field

emission rather than thermionic emission [28].

Modeling of Breakdown Theory

Once the electron is injected into the liquid it gains energy from the applied field. In the electronic theory

of breakdown it is assumed that some electrons gain more energy from the field than they lose in

collisions with molecules. These electrons are accelerated until they gain sufficient energy to ionize

molecules on collisions and initiate avalanche.

The condition for the onset of electron avalanche is obtained by equating the gain in energy of an electron

over its mean free path to that required for ionization of the molecule,

chveE …. (22)

where E is the applied field, λ the electron mean free path, hv the quantum of energy lost in ionizing the

molecule and c an arbitrary constant. Thus electron avalanches similar to those in gas discharges develop

in liquids and lead to final breakdown [31, 32].

Anode initiation of breakdown requires a rapid charge development mechanism. Assuming that holes

emitted from the anode via resonance tunneling, or tunneling from molecule to molecule, requires that the

intermolecular distance be as small as possible to achieve high probabilities for tunneling. Tunneling

establishes a hole propagation pathway through the liquid. The path left by the propagation of the hole

through the liquid will be an ideal return path for a complementary electron to propagate in the opposite

direction. The electrons prefer the lower density region created by the passing of a hole through the liquid

[7].

Limitations

This model appeared quite reasonable, but since 1960 it has been rejected on the following grounds [29]-

1) The mean free path of e- in liquids is very short (of the order of 10-6

cm) to enable them to acquire

ionisation potential of 10eV necessary for liquids molecules.

2) It failed to explain pressure dependence of the breakdown process. At pressure of 25 atm breakdown

strength increases by 50%, whereas at this pressure the mean free path of the electron is hardly

altered.

2.2.4 Cavitation Theory

Processes of formation of Bubbles leading to Cavitation

Insulating liquids may contain gaseous inclusion in the form of bubbles. The processes by which bubbles

are formed include [28] -

1. gas pockets on the electrode surface,

2. changes in temperature and pressure,

3. dissociation of products by electron collisions giving rise to gaseous products,

4. liquid vaporization by corona-type discharges from points and irregularities on the electrodes.

Modeling of Breakdown Theory

This theory was proposed by Krausucki [33]. According to the theory, whenever a particle comes into the

high field region, the presence of enhanced field on its surface will generate an electro-mechanical

pressure (Pm) tending to lift the liquid off the particle surface against the opposing hydrostatic pressure

(Ph) and pressure due to surface tension (Pst).This action will develop a region of zero pressure, thus

forming a vacuous cavity [29].

…. (23)

Electric field generated in the bubble has been presented differently by different authors and researchers.

According to Krausucki,

rPmE h

2)(

2

2 …. (24)

where,

m = field intensification factor on the surface of the particle

2)(2

mE = electromechanical pressure

r

2 = pressure due to surface tension

= surface tension

r = particle radius

Krausucki took m=4.2, thus the breakdown strength in this case becomes,

1

5.0

21337.0

Vm

rPE hb

.… (25)

According to Kao [34], the electric field in the spherical gas bubble is given as,

2

3

liq

ob

EE

…. (26)

where Eo is the field in the liquid in the absence of bubble. When the filed Eb becomes equal to the

gaseous ionisation field, discharge takes place which lead to the decomposition of the liquid and

breakdown may follow.

Kao modified the above equation to give a more accurate expression for field strength in the bubble as,

2/1

21

21

124

)2(2

)(

1

o

bo

rE

V

rE

…. (27)

where σ is the surface tension of the liquid, ε1 and ε2 are the permittivities of the liquid and the bubble

respectively, r is the initial radius of the bubble (initially spherical, which is assumed to elongate under

the influence of the field), and Vb is the voltage drop in the bubble. This expression indicates that the

critical electric field strength required for breakdown of liquid depends upon the initial size of the bubble

which is affected by the external pressure and temperature.

CHAPTER III

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 (Figure 7). The glow discharges between two 100µm tungsten carbide electrodes were recorded using a camera

SONY HVR-Z7E HDV (Figure 8).

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)

EDM Process parameters

Discharge circuit: RC relaxation circuit

Open circuit voltage: 100-120 V

Discharge Capacitor: 120-150 pF

Diameter of electrode: 100µm

Figure 7: EDM Machine

Figure 8: High-Speed Camera

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 9 and 10 depict the changes

in spark intensity during glow discharge and its breakdown a over 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.

.

Figure 10: Plasma channels formed during the electrical breakdown of deionized water in the

order of decreasing intensity of light

Figure 9: Plasma channels formed during the electrical breakdown of deionized water in the

order of increasing spark appearance

CONCLUSION

Over the years, many models have been developed to explain the mechanisms of liquid breakdown, but

none of them has been unanimously accepted throughout. Breakdown theories in the order of acceptance

include:

Bubble Theory: Instability of formed bubbles results in the formation and propagation of

streamers, finally resulting into breakdown.

Suspended Particle Theory: Conducting and polarizable impurities present in the liquid are

involved in breakdown.

Electronic Breakdown Theory: The electrons ejected by field emission form avalanches similar to

that in gas discharge and result into final breakdown.

Cavitation Theory: When electric field Eb developed in the presence of bubbles, which are

responsible for cavitation, becomes equal to gaseous ionization field, discharge takes place

leading to decomposition and followed by breakdown.

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. 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 a low energy and ultra short pulsed discharge, is the energy source to initiate the

breakdown, which are not studied yet.

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 analyzed.

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