Micromachining Using Electrochemical Discharge Phenomenon - Rolf Wuthrich, Jana D. Abou Ziki

Micromachining Using Electrochemical Discharge Phenomenon (eBook)

Fundamentals and Application of Spark Assisted Chemical Engraving

Rolf Wuthrich, Jana D. Abou Ziki (Autoren)

eBook Download: PDF | EPUB

2014 | 2. Auflage
218 Seiten
Elsevier Science (Verlag)
978-0-323-26564-5 (ISBN)

Micro-machining is an advanced manufacturing technique of growing importance, and adoption of micro-machining using electrochemical discharges (Micro-ECDM) has increased steadily in recent years. Among new developments is the interest of industry in Micro-ECDM. However, the potential of the technology is not being fully utilized and there is no comprehensive reference book available today covering it. Micromachining Using Electrochemical Discharge Phenomenon, Second Edition fills this gap. It is unique in its detailed coverage of all aspects of the Micro-ECDM process, as well as Spark Assisted Chemical Engraving (SACE). As such, it covers technologies such as chemical etching, micro-drilling, and other material removal mechanisms, high aspect ratio machining, design and construction of the machining apparatus, and a wide range of applications. The new edition compares Micro-ECDM and SACE with other micromachining technologies such as laser machining and traditional EDM. ECDM is used for machining of electrically non-conductive materials. Micro-ECDM/SACE is mainly applied to glass and the book focuses on glass, but the authors also present new results on other materials such as ceramics. In addition, techniques to modify material properties for the machining process are explained. The authors discuss machining strategies including the latest developments in micro-texturing of glass micro-channels and reports on developments in controlling and analysis aspects of machining. This book is a unique reference for engineers and industrial researchers involved in development, design and use of micromachining, chemical micro-drilling or chemical engraving techniques and equipment. - Only all-encompassing reference coving Micro-ECDM and SACE available on the market - Covers a wide range of applications, including applications in the MEMS industry and the Medical Devices and Medical Diagnostics industries - New edition includes expanded sections on comparing Micro-ECDM/SACE with other micromachining technologies

Micro-machining is an advanced manufacturing technique of growing importance, and adoption of micro-machining using electrochemical discharges (Micro-ECDM) has increased steadily in recent years. Among new developments is the interest of industry in Micro-ECDM. However, the potential of the technology is not being fully utilized and there is no comprehensive reference book available today covering it. Micromachining Using Electrochemical Discharge Phenomenon, Second Edition fills this gap. It is unique in its detailed coverage of all aspects of the Micro-ECDM process, as well as Spark Assisted Chemical Engraving (SACE). As such, it covers technologies such as chemical etching, micro-drilling, and other material removal mechanisms, high aspect ratio machining, design and construction of the machining apparatus, and a wide range of applications. The new edition compares Micro-ECDM and SACE with other micromachining technologies such as laser machining and traditional EDM. ECDM is used for machining of electrically non-conductive materials. Micro-ECDM/SACE is mainly applied to glass and the book focuses on glass, but the authors also present new results on other materials such as ceramics. In addition, techniques to modify material properties for the machining process are explained. The authors discuss machining strategies including the latest developments in micro-texturing of glass micro-channels and reports on developments in controlling and analysis aspects of machining. This book is a unique reference for engineers and industrial researchers involved in development, design and use of micromachining, chemical micro-drilling or chemical engraving techniques and equipment. - Only all-encompassing reference coving Micro-ECDM and SACE available on the market- Covers a wide range of applications, including applications in the MEMS industry and the Medical Devices and Medical Diagnostics industries- New edition includes expanded sections on comparing Micro-ECDM/SACE with other micromachining technologies

Front Cover 1
Micromachining Using Electrochemical Discharge Phenomenon: Fundamentals and Application of Spark Assisted Chemical Engraving 4
Copyright 5
Contents 6
Preface 10
CHAPTER 1 - MACHINING WITH ELECTROCHEMICAL DISCHARGES—AN OVERVIEW 12
1.1 SPARK-ASSISTED CHEMICAL ENGRAVING 13
1.2 SACE AS A MICROMACHINING TECHNOLOGY 19
1.3 SCOPE OF THE BOOK 20
PART 1 ELECTROCHEMICAL DISCHARGES 22
CHAPTER 2 - HISTORICAL OVERVIEW OF ELECTROCHEMICAL DISCHARGES 24
2.1 DISCOVERY AND EARLY APPLICATIONS 24
2.2 THE WEHNELT INTERRUPTER 26
2.3 SPECTRUM OF THE ELECTROCHEMICAL DISCHARGES 31
2.4 NATURE OF THE ELECTROCHEMICAL DISCHARGES 33
2.5 CONTACT GLOW DISCHARGE ELECTROLYSIS 39
CHAPTER 3 - GAS-EVOLVING ELECTRODES 46
3.1 INTRODUCTION TO ELECTROCHEMISTRY 46
3.2 BUBBLE FORMATION DURING ELECTROLYSIS 54
3.3 BUBBLE LAYER 57
3.4 THE BUBBLE DIFFUSION REGION 59
3.5 THE BUBBLE ADHERENCE REGION 60
3.6 BUBBLE EVOLUTION ON A GAS-EVOLVING ELECTRODE 67
3.7 MEAN STATIONARY CURRENT–VOLTAGE CHARACTERISTICS 69
CHAPTER 4 - THE GAS FILM—A KEY ELEMENT 74
4.1 FORMATION OF THE GAS FILM 75
4.2 GAS FILM LIFETIME 86
4.3 SHAPE OF THE GAS FILM 90
4.4 DISCHARGE ACTIVITY INSIDE OF THE GAS FILM 90
4.5 ACTIVE ELECTRODE TEMPERATURE 100
4.6 CONTROLLING THE GAS FILM 103
PART 2 MICROMACHINING WITH ELECTROCHEMICAL DISCHARGES 106
CHAPTER 5 - MATERIAL REMOVAL MECHANISM 108
5.1 GENERAL CONSIDERATIONS 108
5.2 MACHINING AT LOW DEPTHS 113
5.3 MACHINING AT HIGH DEPTHS 122
5.4 CHEMICAL CONTRIBUTIONS 124
5.5 SUMMARY 127
CHAPTER 6 - COMMON MACHINING STRATEGIES 128
6.1 GENERAL OVERVIEW 128
6.2 GRAVITY-FEED DRILLING 130
6.3 CONSTANT VELOCITY-FEED DRILLING 145
6.4 2D AND 3D MACHINING 155
6.5 WIRE ELECTROCHEMICAL DISCHARGE MACHINING 163
CHAPTER 7 - CONTROLLING THE MACHINING PROCESS 166
7.1 PROCESS ANALYSIS 167
7.2 PROMOTING ETCHING 168
7.3 CONTROLLING THE HEAT GENERATED 177
7.4 CONTROLLING THE TOOL–WORKPIECE GAP 189
7.5 SEARCHING FOR PROCESS CONTROL SIGNALS 189
7.6 SUMMARY 191
CHAPTER 8 - DESIGNING A SACE MICROMACHINING SET-UP 192
8.1 GENERAL DESIGN RULES 192
8.2 DRILLING SET-UPS 196
8.3 2D MACHINING SET-UPS 200
8.4 COMMERCIAL MACHINES 202
CHAPTER 9 - OUTLOOK 204
REFERENCES 206
Index 214

Chapter 1

Machining with Electrochemical Discharges—An Overview

Abstract

The last century also saw the birth of micromachining, in particular micromachining of silicon. A huge variety of micromachining techniques are available for silicon. A similar situation exists for electrically conductive materials, where, in particular, electrochemical machining (ECM) and electrical discharge machining (EDM) are two powerful tools available. However, several electrically nonconductive materials are also of great interest for many applications. Glass and composite materials are two examples. The technical requirements for using glass in microsystems are growing. Medical devices requiring biocompatible materials is only one of many examples.

Various techniques are available to micromachine glass. However, one of the main limiting factors in incorporating glass into microdevices is its limited machinability. A similar situation exists for other hard-to-machine materials, such as ceramics and composite materials. A possible answer to these issues could be spark-assisted chemical engraving (SACE) or electrochemical discharge machining (ECDM).

This chapter presents a short overview of SACE and places it into the context of other micromachining technologies.

Keywords

Electrochemical discharge; Mechanical machining; Micromachining; SACE; Thermal machining

Chapter Outline

1.1 Spark-Assisted Chemical Engraving 2

1.1.1 What is SACE? 2

1.1.2 Machining Examples 3

1.1.3 A Short Historical Overview 5

1.2 SACE as a Micromachining Technology 8

1.2.1 Mechanical Machining 8

1.2.2 Chemical Machining 8

1.2.3 Thermal Machining 8

1.3 Scope of the Book 9

Since the very beginning of history, and even prehistory, humanity has invested a lot of effort in developing the skill of processing materials. There is no need to present the fundamental importance of the capability of machining in any technology. Any new technology requires new machining skills. In the last century, the need for using more and more specialized materials (e.g., silicon, composites, ceramics) greatly increased the already large arsenal of machining technologies.

The last century also saw the birth of micromachining, in particular micromachining of silicon. At present, a huge variety of micromachining techniques are available for silicon. A similar situation exists for electrically conductive materials, where, in particular, electrochemical machining (ECM) and electrical discharge machining (EDM) are two very powerful tools available. However, several electrically nonconductive materials are also of great interest for many applications. Glass and composite materials are two examples. The technical requirements for using glass in microsystems are growing. Medical devices requiring biocompatible materials is only one of many examples.

The importance of glass is also growing in the field of microelectromechanical systems (MEMS). The term MEMS refers to a collection of microsensors and actuators. MEMS emerged in the 1990s with the development of processes for the fabrication of integrated circuits. In particular, Pyrex® glass is widely used because it can be bonded by anodic bonding (also called field-assisted thermal bonding or electrostatic bonding) to silicon. Glass has some very interesting properties, such as its chemical resistance or biocompatibility. It is amorphous and can therefore be chemically attacked in all directions. As glass is transparent, it is widely used in optical applications or in applications where optical visualization of a process is needed. Some promising applications for glass in the MEMS field are microaccelerometers, microreactors, micropumps, and medical devices (e.g., flow sensors or drug delivery devices).

A representative example in which glass-to-silicon bonding is used is bulk micromachined accelerometers (Wolffenbuttel, 1995). In this case, glass serves several functions:

• provides a seal and the desired damping;

• can be used as a capacitor when a metal plate is placed on it;

• can be an overload protection.

The use of glass is also very common in sensors other than accelerometers using capacitive sensing technology.

1.1. Spark-Assisted Chemical Engraving

1.1.1. What is SACE?

SACE makes use of electrochemical and physical phenomena to machine glass. The principle is explained in Figure 1.1 (Wüthrich and Fascio, 2005). The workpiece is dipped in an appropriate electrolytic solution (typically sodium hydroxide or potassium hydroxide). A constant DC voltage is applied between the machining tool or tool-electrode and the counter-electrode. The tool-electrode is dipped a few millimeters in the electrolytic solution and the counter-electrode is, in general, a large flat plate. The tool-electrode surface is always significantly smaller than the counter-electrode surface (by about a factor of 100). The tool-electrode is generally polarized as a cathode, but the opposite polarization is also possible.

When the cell terminal voltage is low (lower than a critical value called critical voltage, typically between 20 and 30 V), traditional electrolysis occurs (Figure 1.2). Hydrogen gas bubbles are formed at the tool-electrode and oxygen bubbles at the counter-electrode, depending on their polarization and the electrolyte used. When the terminal voltage is increased, the current density also increases and more and more bubbles are formed. A bubble layer develops around the electrodes. As presented in Chapter 3, the density of the bubbles and their mean radius increase with increasing current density. When the terminal voltage is increased above the critical voltage, the bubbles coalesce into a gas film around the tool-electrode. Light emission can be observed in the film when electrical discharges, the so-called electrochemical discharges, occur between the tool and the surrounding electrolyte. The mean temperature of the electrolytic solution increases in the vicinity of the tool-electrode to about 80–90 °C. Machining is possible if the tool-electrode is in the near vicinity of the glass sample (Figure 1.3). Typically, the tool-electrode has to be closer than 25 μm from the workpiece for glass machining to take place.

FIGURE 1.1 Principle of SACE technology: the glass sample to be machined is dipped in an electrolytic solution. A constant DC voltage is applied between the tool-electrode and the counter-electrode. Reprinted from Wüthrich and Fascio (2005) with permission from Elsevier.

FIGURE 1.2 Successive steps toward the electrochemical discharge phenomena: (a) 0 V; (b) 7.5 V; (c) 15 V; (d) 40 V. Two electrodes are dipped into an electrolyte. The terminal voltage is progressively increased from 0 to 40 V. At around 25 V a gas film is formed around the cathode, and at around 30 V the electrochemical discharges are clearly visible. Reprinted from Wüthrich and Fascio (2005) with permission from Elsevier.

However, the process is not as simple as it seems on first sight. The gas film around the tool-electrode is not always stable. Microexplosions may occur, destroying the machined structure locally. During drilling of holes, the local temperature can increase to such an extent that heat-affected zones or even cracking can result.

1.1.2. Machining Examples

SACE technology can be used for flexible glass microstructuring. Channel-like microstructures and microholes can be obtained. Two examples are illustrated in Figure 1.4. The channel microstructure was machined with a cylindrical 90-μm-diameter tool-electrode at an applied voltage of 30 V. Machining was done in one step with a tool speed of 0.05 mm s−1. The channels are about 100 μm wide and 200 μm deep. The microhole illustrates the possibility of machining relatively deep structures. In this case the microhole is 1 mm deep.

FIGURE 1.3 Close-up view of micromachining with electrochemical discharges.

FIGURE 1.4 Micrographs of a SACE-machined channel-like structure (left) and a microhole (right) in Pyrex® glass. Reprinted from Wüthrich and Fascio (2005) with permission from Elsevier.

The most interesting characteristic of SACE is its flexibility. No mask is needed, and just as in traditional machining, the desired...

Erscheint lt. Verlag	8.11.2014
Sprache	englisch
Themenwelt	Naturwissenschaften ► Chemie ► Technische Chemie
	Technik ► Bauwesen
	Technik ► Elektrotechnik / Energietechnik
	Technik ► Maschinenbau
ISBN-10	0-323-26564-2 / 0323265642
ISBN-13	978-0-323-26564-5 / 9780323265645

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