Chemo-Thermal Micromachining of Glass: an Explorative Study

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Chemo-Thermal Micromachining of Glass: An Explorative Study

A thesis submitted to the

Graduate School

of the University of Cincinnati

in partial fulfilment of the

requirements for the degree of

Master of Science

In the Department of Mechanical Engineering

of the College of Engineering and Applied Sciences

Arham Ali

Bachelor of Technology in Mechanical Engineering

Motilal Nehru National Institute of Technology Allahabad, India

November 2018

Committee Chair: Dr. Murali Sundaram

ABSTRACT

Engineering materials such as glass and ceramics are finding numerous applications in electronics and communication, optics, chemical, aerospace and medical industries. Glass being a hard and brittle material poses huge machining challenges especially during micromachining.

Traditional machining of glass by methods such as drilling or milling are generally difficult to achieve due to reasons ranging from excessive tool wear to abrupt breakage of glass. Chemical etching techniques can produce smooth machined surface on glass. Yet, chemical machining process is usually slow and often fail to produce high aspect ratio unless combined with some other additional masking techniques which are usually expensive. Laser beam machining when performed on glass produces micro-cracks along the machined edge due to uneven temperature distribution and residual stresses due to heat affected zones (HAZ). Wet laser beam machining, i.e. laser beam machining when performed under water instead of air results in considerable reduction in thermal defects. However, portion of laser energy is wasted in wet laser machining as heat loss in water. The motivation of this study is to use to minimize this loss and explore the possibility of utilizing this laser energy being absorbed by the medium to further increase machining. To achieve this, a novel chemo-thermal machining process is proposed in this work.

Chemo-thermal micromachining is a laser beam machining process in wet condition where laser beam machining is performed on glass specimen submerged in NaOH solution.

Material removal mechanism in chemo-thermal micromachining process is a combination of laser ablation and chemical machining. When laser beam is incident on glass workpiece submerged under NaOH solution, a part of laser energy is absorbed by the electrolyte solution

thus raising the electrolyte temperature locally thus increasing rate of chemical reaction as chemical machining process is highly dependent on temperature. The feasibility of the process was studied, and it was found that there was considerable reduction in the micro cracks formed along the machine surface. When laser beam machining on glass was performed in air about 20-

23 micro cracks were observed which were reduced to 1-5 when machining was performed using chemo-thermal process. Also, average length of cracks was reduced by about 90%. The effect of machining parameters on material removal rate (MRR) and surface quality that involves surface cracks and built-up edges along the machine surface was further studied using a four-level full factorial experimental design.

A finite element model was created and chemo-thermal process was simulated to predict the material removal on borosilicate glass. The predictions made by the simulation were compared with the experimental results and the variations were found to be within 5%.

iii

ACKNOWLEDGEMENT

I would first like to thank my thesis advisor Dr. Murali Sundaram for his continuous support. The door to Prof. Sundaram’s office was always open whenever I ran into a trouble spot or had a question about my research or writing. He is not only an excellent academic advisor but also an amazing person. He not only guided me for this research work but also helped me develop a professional and methodical problem-solving approach towards any problem. It is because of him I got the opportunity to work on the laser beam machine at Micro and Nano Manufacturing

Laboratory to carry out top quality research.

I am thankful to all my lab mates from UCMAN lab for their support. I would specifically like to thank Ketaki Kolhekar, Vamshi Kore, Abishek Kamaraj, Anne Brant, and Andrea Grisell for the numerous discussions and constructive criticism of my work which has helped me.

I am grateful to University of Cincinnati for providing me with a generous scholarship reducing my financial burden. Financial support provided by the National Science Foundation under Grant Nos. CMMI-1833112, and CMMI-1454181 is acknowledged.

Finally, I am grateful to the most important people in my life, my parents and my brother who have always been a source of motivation. I dedicate this thesis to Dr. Atiya Zaidi, Dr. G R

Syed and Atyab Ali Zaidi.

Table of Contents

1 Introduction ...... 1

1.1 Motivation ...... 2

1.2 Objectives ...... 3

1.3 Outline ...... 4

2 Literature Review...... 5

3 System Design for Chemo-Thermal Machining Process ...... 13

4 Proof of Concept ...... 23

5 Experimental Study of Chemo-Thermal Machining Process ...... 27

5.1 Materials and Method...... 27

5.2 Input parameters and their levels ...... 28

5.3 Material removal mechanism ...... 29

5.4 Study of Built-up edges ...... 30

5.5 Cracks formed along the machined surface ...... 33

5.6 Chemical composition of glass ...... 36

5.7 Circularity of the hole and surface roughness ...... 39

5.8 Material Removal Rate (MRR) ...... 40

5.9 Major findings of Experimental Study ...... 42

6 Chemo-Thermal Micromachining Simulation ...... 43

6.1 Material Removal due to chemo-thermal micromachining...... 43 vi

6.2 Simulation Results and Validation Experiments ...... 51

6.3 Major Findings from Simulation Study ...... 53

7 Conclusions and Future Work ...... 55

7.1 Conclusions ...... 55

7.2 Future Work ...... 56

8 References ...... 58

APPENDIX A: List of Publications

vii

LIST OF FIGURES

Figure 1: Power density vs interaction time for different modes of laser machining...... 10

Figure 2 Microfluidic mixer[32] and micro needles(scale = 250m)[33] ...... 12

Figure 3: Schematic of laser reflection and focusing...... 14

Figure 4: Laser Engraving on Wood ...... 15

Figure 5: Experimental Set-up ...... 16

Figure 6: X and Y axis movement controller...... 17

Figure 7: Condensation of NaOH fumes on focusing lens...... 18

Figure 8: Laser Head Annular Air Jet ...... 18

Figure 9: Horizontal Air Jet ...... 19

Figure 10: Schematic of 3D printed laser head ...... 20

Figure 11: 3D printed laser head ...... 21

Figure 12: Gap Increase by longer focal length ...... 22

Figure 13: Laser beam machining performed in air ...... 24

Figure 14: Laser beam machining performed under water ...... 25

Figure 15: Laser beam machining performed under 1M NaOH solution ...... 26

Figure 16: Experimental material removal ...... 28

Figure 17: Mechanism of material removal ...... 30

Figure 18: Comparison of build-up edge profiles (Blue: Laser ablation; Red: Chemo-thermal process) ...... 32

Figure 19: Built-up edge reduction with change in electrolyte concentration...... 32

Figure 20: Comparison of cracks ...... 34

Figure 21: A sample EDX result ...... 37 viii

Figure 22: Si and Mg weight ratio under different concentrations of the electrolyte ...... 38

Figure 23: Circularity Measurement ...... 40

Figure 24: MRR under different concentrations of the electrolyte for 60s laser exposure...... 41

Figure 25 Schematic of the Chemo-thermal micromachining process setup model for simulation and the governing equations...... 46

Figure 26: Meshing applied to the system of electrolyte and glass with fine meshing at the boundaries ...... 48

Figure 27: Algorithm for chemo-thermal machining simulation model ...... 49

Figure 28: Simulation result isosurface sample with 7W laser power and 60 sec exposure ...... 50

Figure 29: Isothermal Profile of workpiece at different time step ...... 50

Figure 30: Determination of material removal ...... 51

Figure 31: Comparison of predicted machined volume and experimental machined volume ..... 53

LIST OF TABLES

Table 1: Applications of micromachined material...... 11

Table 2: Experimental Conditions for feasibility study ...... 23

Table 3: Process parameters and their levels ...... 28

Table 4: Variation of crack length with change in NaOH concentration keeping laser power 7W and exposure duration 90 sec ...... 35

Table 5: Variation of cracks with change in exposure duration keeping laser at 7W and NaOH concentration 1M ...... 35

Table 6: Boundary conditions applied to the system as shown in Figure 25 ...... 46

Table 7: Glass thermal and mechanical properties ...... 47

Table 8: Simulation Conditions ...... 47

Table 9: Calculation of Correction Factor ...... 52

Table 10: Comparison of Simulated volume and Experimental volume ...... 52

NOMENCLATURE

α Absorptivity (Dimensionless ratio)

Cf Compensation Factor (Dimensionless ratio)

∆ Heat Addition

C Circularity (dimensionless ratio)

A Area ([m2])

P Perimeter ([m])

ρ Density ([kg/m3]) c Volume specific heat ([J kg−1 K−1])_ k Thermal Conductivity ([W m−1 K−1]) h Convective heat transfer coefficient ([W/(m2 K)])

푇∞ Ambient Temperature ([K])

I Laser Intensity ([W/m2])

2 Io Laser Peak Intensity ([W/m ]) r Radial distance from the axial symmetry ([m])

ro Radius of the laser focal point ([m]) xi

Corf Correction Factor (Dimensionless ratio) aq Aqueous Solution

2-D Two Dimensional

3-D Three Dimensional

M Molarity

HAZ Heat Affected Zone

xii

CHAPTER 1

1 Introduction

The increase in miniaturization of products in various engineering and medical fields demand for materials with unique mechanical and chemical properties. Engineering materials such as glass and ceramics are finding numerous applications in electronics and communication, optics, chemical, aerospace and medical industries[1]. Glass is particularly useful for fabrication of microfluidic devices used for bioanalysis because of its high chemical resistance. Transparency of glass under a wide range of wavelength makes it suitable for incorporating into micro bioanalytical devices where optical detection of the bioanalytes is essential as in the case of micro capillary electrophoresis devices [2]. There has been an increase in demands on higher precision cutting of glass for various applications such as manufacturing of flat panel displays[3]. However, glass poses significant machinability challenges especially during micromachining. Traditional as well as non-traditional techniques have many limitations in processing such materials. Traditional machining of glass by methods such as drilling and milling are generally difficult to accomplish because glass is a hard and brittle material, and thus, excessive wear on the tool and defects (e.g. cracks) on the glass surface are generated during the machining process.

Laser beam machining is a non-traditional manufacturing process that is being used industrially for cutting and engraving of various materials. In some cases, the process is performed under wet conditions in place of air. These wet conditions include water, alcohol, ether or a strong acid 1

or base[4]. The purpose of wet atmosphere is to reduce the thermal defects caused by laser ablation. Laser machining performed in wet conditions produces additional cooling that reduces heat-affected zones (HAZ) and thus causing reduction in the formation of micro-cracks along the machined surface[5]. Chemo-thermal micro machining is laser beam machining process in wet condition where laser beam machining is performed on glass workpiece submerged in NaOH solution[6]. In chemo-thermal micromachining process a part of laser energy is absorbed by the electrolyte raising the surrounding electrolyte temperature that further increases the effect chemical machining.

1.1 Motivation

There is a widespread use of lasers for machining different types of engineering materials. Laser machining is easy to automate and often results in close surface tolerances and high surface finish especially in metal parts. Laser machining can also be performed to fabricate micro channels, holes and different cross-sectional structures that are not possible using traditional methods [7].

It is usually difficult to machine glass using laser beam because glass being a transparent material transmit most of the incoming laser. The absorptivity (α) of the material has the largest influence on laser power requirements. The material absorptivity is determined by the fraction of laser radiation energy impinging on the surface that is absorbed by the material. The remaining beam energy is reflected to the environment. The absorptivity value mainly depends on the wavelength of laser, surface roughness, temperature and phase of the material. Carbon dioxide laser is ideal

for glass cutting due to high absorption of glass at 10.6 m. The machined surface after laser beam machining often has micro-cracks along the machined edge due to uneven temperature distribution and residual stresses due to heat affected zones(HAZ) [8]. However, there is a reduction in HAZ when laser beam machining is performed under wet condition. The laser energy absorbed by the liquid medium reduces the laser power hitting the workpiece surface and the energy absorbed goes to waste. The aim of this work was to find a way to utilize the laser energy being absorbed by the electrolyte for further machining of the workpiece. Since sodium hydroxide reacts with glass (silica) at elevated temperature, NaOH solution was chosen as our medium for performing laser machining. The results from the feasibility study showed us that there was a significant reduction in the number and length of surface cracks along the machined edge. Thus, further study was done for understanding the effect of different process parameters by implementing a four-level full factorial design of experiments.

1.2 Objectives

The objectives of this research are:

1. To find the electrolyte concentration and laser power favorable for combined laser and

chemical machining.

2. To find out the effects of each input process parameter on chemo-thermal

micromachining.

3. To create a finite element model and simulate the chemo-thermal micro machining

process to predict the material removal due to machining.

1.3 Outline

The thesis is organized as follows: the introduction in Chapter 1 is followed by a comprehensive literature review of micromachining processes in Chapter 2. This literature review focuses on the state of the art processes being used. for micromachining of hard and brittle materials like glass and ceramics. Chapter 3 describes the experimental set-up. used in this study. Chapter 4 describes the feasibility study chemo-thermal micromachining process by finding the range of laser power and concentration of electrolyte for combined laser and chemical machining.

Chapter 5 discusses the process parameter study where the effect of each input parameter is studied for glass micromachining and to calculate the material removal rate (MRR). Chemo- thermal micromachining process is simulated in Chapter 6. Chapter 7 summarizes the results and conclusions, and future work study arising from the conclusions of this research work is also discussed.

CHAPTER 2

2 Literature Review

Glass shows multiple desirable properties that can be used for different applications in engineering and medical fields. Micro machined glass can endure high thermal and mechanical stress conditions making it suitable for micro electro mechanical systems (MEMS). There are several micro machining techniques being reported for machining ceramics or glass. Poor surface finishing is one major issue reported by most of these processes. Spark Assisted Chemical

Engraving (SACE) [9] and ECDM [10] has been performed on glass. ECDM is a thermal machining process occurring in electrolytic cell, where DC voltage is applied across the electrodes and electrochemical reaction happens at cathode releasing hydrogen gas bubbles. The hydrogen gas bubbles grow and form a gas film as the voltage is increased towards critical voltage. The gas film breaks down as the voltage is further increase and causes high energy discharge which is used for machining. In this process electrode tool wearing is the main concern. Bending of tool is also reported in some cases when tool is moved at a constant velocity.

Ultrasonic micro machining is also been performed on many different hard and brittle materials[11] [12]. Recently, hybrid machining that involves ultrasonic machining has also been applied to metallic compounds like titanium-aluminum [13]. Ultrasonic machining has a capability of machining non-conductive workpiece without doing any thermal damage. Ultrasonic machining does not introduce residual stresses thus increasing the work life of brittle material. 5

The system consists of a frequency generator integrated with a tool holder. The tool holder is usually made up of piezoelectric material which generates ultrasonic mechanical vibrations in the tool. The tool is dipped in an abrasive slurry. The abrasive particles are agitated by the action of vibrating tool and hits the workpiece. The material removal mechanism in ultrasonic machining is by micro chipping action. Depending on the size and material of the work piece, this process can be expensive and it is very difficult to machine intricate contours. These processes require single point tool for machining that involves high forces developing at the tool tip causing increased tool wear and frequent tool breaking at the tip. This requires changing or redressing of tool. Also poor surface finishing and low material removal rate is observed during ultrasonic machining [14].

Wet etching is also being performed on glass for micro machining that can create some pattern on glass surface by dipping the masked work piece in an acid such as hydrofluoric (HF) acid [15].

The process is highly isotropic which is a great disadvantage. The material removal rate in this process is comparatively higher since large area can be exposed to acid at one time and can be used for batch production where multiple masked work pieces are dipped simultaneously. It also causes undercutting and gives lower aspect ratio as compared to other processes. Hydrofluoric acid is an extremely dangerous substance to work with as its fumes can easily be absorbed by human skin and tissues and it can cause damage to human body and environment.

Powder blasting sometimes also known as sand blasting or impact abrasive machining is another method reported for glass micro machining [16]. In this process, fine abrasive particles are agitated by compressed air or any other medium and material removal is done by micro chipping process. It is a very effective method that can be performed very quickly without producing any burrs but produces rough surfaces unless some post machining operations such as wet etching are performed. Powder blasting technique can be combined with specially designed photo resistive masking film that improves accuracy, precision, and can produce high detailed micro machining on glass. This process can create blind and through holes but this process produces large amount of taper due to abrasive spreading while agitation.

There is a widespread use of lasers for machining different types of engineering materials. Laser machining is easy to automate and often results in close surface tolerances and high surface finish especially in metal parts. Laser micromachining process includes drilling, cutting and engraving of the workpiece material. Laser machining can also be performed to fabricate micro channels, holes and different cross-sectional structures that are not possible using traditional methods [7].

Laser light is a coherent, convergent, and monochromatic beam of electromagnetic radiation with wavelength ranging from far infrared to ultraviolet. Lasers are being used in manufacturing technology industries since a long time because they are easy to use and generally do not require post processing. Lasers are easy to use as there is no tool wear or involvement of any kind of masking on the work piece. Since laser pulse power and the movement of laser head can be controlled by computer program, it is easy to automate the process. Laser machining process 7

produce low taper as compared to other processes and it can be used on large work pieces. Laser machining is an advanced mechanism to rapidly and precisely machine workpiece material without causing mechanical loads toward the workpiece, thus limiting the cracks and rough surfaces. However, when laser is used on glass and ceramics, it produces micro cracks along the machined surface. The material removal mechanism in laser machining is melting and vaporization of the workpiece material. The excessive heat conduction to the workpiece can cause thermal defects like cracks, bulges, debris and recasting of the material due to high thermal gradient, rapid cooling and splashing of the molten material. There is an increase in laser assisted hybrid processes to improve machining such as laser assisted milling[17], hybrid laser-waterjet machining [18], and laser induced plasma micro-machining[19]. Laser assisted hybrid processes are used because laser radiation can locally heat the workpiece material at high intensity. The assisted machining process is easy to perform on the soft workpiece material preheated by laser.

The laser power thus is controlled to avoid thermal damage during preheating. Laser assisted milling is a complex process which is not used widely for industrial applications. Laser induced plasma micro-machining process utilizes the formation of plasma induced in a liquid at the focal point of laser beam. The shape of plasma is controlled using external magnetic field.

Deep reactive ion etching has also been reported that is mainly used for silicon compounds but it can also be used for glass [20]. Material removal in this process is erosion mechanism caused by ions. This process requires a metal mask to channel the ions and create patterns. This method is very accurate and can create very small features. Also, this process results in very smooth 8

surface finish and is highly anisotropic but it is a very slow process and it is not good for removing material across a wide area. This process is so slow for glass that sometimes glass is used as a masking material for silicon workpiece.

Laser micromachining offers several advantages for fabricating micro channels, holes or different cross-sectional structures that are not possible by traditional methods. It also eliminates the numerous steps involved in lithography and the production of toxic wastes [21]. The material removal occurs by thermal ablation where heat energy is absorbed by the material of work piece.

Laser machining is a thermal process and glass being a poor conductor of heat tends to break or crack upon laser exposure. Glass being a transparent material is difficult to machine using laser as it transmits most of the incoming laser. However some lasers like carbon dioxide, Nd:YAG and excimer laser can be used for glass machining; out of which carbon dioxide laser is the most widely used. The absorptivity of the material has the largest influence on laser power requirements. The material absorptivity is determined by the fraction of laser radiation energy impinging on the surface that is absorbed by the material. The remaining beam energy is reflected to the environment. The absorptivity value mainly depends on the wavelength of laser, surface roughness, temperature and phase of the material. Carbon dioxide laser is ideal for glass cutting due to high absorption of glass at 10.6 m. Power density and exposure duration are two major parameters that affect the laser beam machining. Figure 1shows different processes and its corresponding power density as well as interaction time[22]. This shows that high power with

lower interaction time will result is more machining where as low power and large interaction time will result in surface heating.

Figure 1: Power density vs interaction time for different modes of laser machining.

Under liquid laser machining is a machining process where laser machining is used as a process of material removal in presence of a liquid. It can be used on various materials such as hard and brittle, soft and difficult to machine or metals and non-metals[23]. During under liquid laser beam machining, thermal load and re-casting of the material is reduced due to higher thermal conductivity and specific heat of the liquid. The liquid carries the excess heat away and cools the surface temperature of the workpiece. The surface properties of the machined surface under liquid are significantly improved. The under-liquid laser process is used for surface cleaning,

welding, drilling, cutting, shock processing, etching and micromachining of metals, ceramics, glass and some polymers. The molten material usually dissolves in the liquid which prevents it from re-solidifying on the workpiece surface.

Chemical etching is the degradation or eating away of material using some reactive substance in a controlled manner. Sodium Hydroxide (NaOH) reacts with glass violently and dissolves it away.

The general etching reaction proceeds according to this equation.

∆ 2푁푎푂퐻(푠) + 푆𝑖푂2 (푎푞) → 푁푎2푆𝑖푂3 (푎푞) + 퐻2푂 … (1)

The etching rate increases with the increase in hydroxide ion as well as the reaction temperature.

Hydroxide ion concentration directly depends upon the concentration of NaOH in the electrolyte.

Currently, chemical machining is not the most widely used machining technique on glass because it is a slow process.

Selected applications of micromachining of glass and ceramics are listed in Table 1. Some examples are shown in Figure 2.

Table 1: Applications of micromachined material

Materials Applications

Glass Microfluidics for bioanalysis and DNA sequencing[24], microneedles for directed drug delivery[25], micro pipettes for microfluidic cytometer[26]

Composites Microbial fuel cells[27]

Glass Ceramics Dental restoration[28], micro porous ceramic in solar cells[29]

Alumina Communications, sensors and biomedical application[30], micro- electro mechanical systems (MEMS) and optic sensors[31]

Figure 2 Microfluidic mixer[32] and micro needles(scale = 250m)[33]

The motivation of this study was to combine under-liquid laser process with chemical etching process. Since under-liquid laser machining is improving surface morphology and some of the laser energy is taken by the liquid medium, our aim was to utilize the laser power absorbed by liquid to further enhance machining. The laser power absorbed by the liquid in under-liquid laser machining is going as waste and the chemical reaction in chemical etching process highly depends on temperature, we wanted to combine the two processes and analyze the results. The aim of this study is to explore a low-cost hybrid machining option for borosilicate glass. The experimental study is described in the next chapter.

CHAPTER 3

3 System Design for Chemo-Thermal Machining Process

The motivation of the study was to provide a low-cost hybrid machining solution for glass micromachining. A commercially available 40W CO2 laser machine producing continuous wave laser of

10.6 m wavelength in infrared region was used to perform the experiments. The machine consists of a CO2 gas tube which produces laser when the tube is excited by electricity. The light coming out of the laser tube is reflected by three mirrors in the machine and passes through the laser head consisting of focusing lens. The schematic of laser reflection and focusing is shown in

Figure 3. The elements 1, 2, and 3 are the reflecting surfaces and element 4 is the focusing lens inside the laser head.

Figure 3: Schematic of laser reflection and focusing

The CO2 laser beam machine used in this study is a laser engraving machine. This machine can be used to perform surface engravings on surfaces like wood, glass, ceramics, and soft metal sheets and can provide intricate details with close tolerances. A sample is shown in Figure 4 where engraving is done on a piece of wood using CO2 laser.

Figure 4: Laser Engraving on Wood

Our study includes under-liquid laser beam machining with sodium hydroxide (NaOH) as the medium. The schematic is shown in Figure 5(a) and the actual setup is shown in Figure 5(b).

Figure 5: Experimental Set-up The laser machine used in this study does not have a drilling mode where the laser head is stationary, and laser is switched on for a set time duration. The requirement of the study was to keep the laser head at one place to machine a hole in the workpiece. This was done by manipulating the controllers of the laser beam machine. The laser head is connected to two separate controllers for X and Y axis movement. The controllers are shown in Figure 6.

Figure 6: X and Y axis movement controller To use this machine in drilling mode, Y axis controller was unplugged and cutting command was given in a linear motion in Y direction at a certain speed. Since the Y controller is unplugged, the laser head stays at one place and by adjusting the value of speed, laser exposure duration was controlled. E.g. 15mm cutting in Y direction at a speed of 0.5mm/sec will result in 30 seconds of laser exposure at one point.

During the initial study, the evaporation of NaOH solution was impeding the machining process.

The evaporated electrolyte was condensed on the laser focusing lens thus corroding it and shutting the laser from reaching the workpiece material. The Figure 7 shows the condensation of electrolyte on lens.

Figure 7: Condensation of NaOH fumes on focusing lens.

Several alternative configurations of the machining set-up were used to prevent condensation of electrolyte on the focusing lens. The laser head in the engraving machine consists of an annular tube with an attachment. The air jet attachment was used from the laser annular head to prevent fumes from entering the lens. Figure 8 shows the configuration of the laser head annular air jet.

Figure 8: Laser Head Annular Air Jet

This configuration shown in Figure 8 failed to produce hybrid machining because the high velocity air jet was removing the layer of electrolyte at the focusing point and lower velocity was not able to prevent the fumes from entering the laser head.

Since the annular air-jet did not work for our experiments, next configuration that was tried included a horizontal air jet to divert the fumes away from the laser head. The aim was to blow away the electrolyte fumes in a direction perpendicular to the laser direction. The configuration is shows in Figure 9a and the actual set-up is shown in Figure 9b.

Figure 9: Horizontal Air Jet

The horizontal air-jet configuration as shown in Figure 9 was not able to produce satisfactory results because the high speed of the air jet caused ripples on the electrolyte surface and the electrolyte was continuously moving.

The experiments were performed with horizontal laser beam as well to protect the lens from getting electrolyte fumes that often leads to corrosion on lens surface. A 3-dimensional model was created according to the size requirements of laser head compatible with the laser machine.

Separate laser heads were 3D printed to accommodate the lens and to be mounted horizontally 19

in the laser machine. A Makerbot 3D printer was used to print the laser head with polypropylene filament. The 3D schematic of the laser head is shown in Figure 10. A bayonet locking mechanism was proposed to hold the lens inside the laser head.

 

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Figure 10: Schematic of 3D printed laser head The actual 3D printed head that was also tried in the study is given in Figure 11. However, it was difficult to maintain a continuous film of electrolyte on a vertical glass workpiece, thus 3D printed head did not produce appreciable results.

Figure 11: 3D printed laser head

The final configuration that we tried was with vertical laser head but with a different lens. Initially we used 38.1mm focal length of the lens which was having about 6mm distance between the laser head and the workpiece. This gap was small and thus the electrolyte fumes were entering the laser head and condensing on the lens. This issue was solved by using a lens with a focal length of 50.8mm. The gap between the work piece and laser head increased to about 18.5mm by using larger focal length lens. By using the larger focal length lens the condensation of electrolyte was minimized. Figure 12 shows the increase in gap by using larger focal length lens.

By increasing the focal length, the depth of field is increased. Depth of field is defined as the range on either side of the focal point of laser where the diameter of laser beam is 1.4 times the focal diameter. The workpiece can be placed in between the depth of field for machining.

Figure 12: Gap Increase by longer focal length The configuration of the set-up is completed and the proof of concept is described in the next chapter.

CHAPTER 4

4 Proof of Concept

The experimental conditions for the feasibility study are given in Table 2:

Table 2: Experimental Conditions for feasibility study

Parameter Values

Machining Mode (a) Laser machining in Air

(b) Laser machining of workpiece submerged under water

Work Material 1.8 mm Borosilicate Glass

Laser 10.6 m CO2 Laser

The water and electrolyte level was kept 1-2mm above the surface of the workpiece and the laser was focused at the glass-liquid interface. The workpiece was kept stable due to self-weight as no external force or vibrations were involved in the machining process. When laser beam machining is performed in air, the excess conductive energy of laser increases the heat affected zone (HAZ).

This gives rise to thermal damage such as surface cracks, recasting of material, and debris. The laser beam machining when performed in air produced a lot of cracks on the machined surface as shown in Figure 13.

Figure 13: Laser beam machining performed in air

The thermal damage to the glass surface is reduced when the laser beam machining is performed under water. The specific heat capacity and thermal conductivity of water is much higher than that of air thus thermal load is reduced in under-water laser machining. There was considerable reduction in number of cracks in wet environment when laser beam machining was performed under water. The hole diameter of the machining was also reduced due to laser energy absorption by liquid medium. The sample of laser beam machining performed under water is shown in Figure 14.

Figure 14: Laser beam machining performed under water

During under-water laser machining, the laser energy absorbed by the water is not responsible for the machining of glass. Our aim was to utilize the laser power absorbed by the liquid for further glass machining. The wet condition of under-liquid machining was replaced by sodium hydroxide solution (NaOH) to understand the difference in machining surface. Initially, hit and trial method was attempted to understand the range of laser power and electrolyte concentration in which we are able to get glass micromachining with better surface finishing. It was found that laser power ranging from 6W to 10 W and electrolyte concentration up to 2M

NaOH solution was giving better surface finish. The recasting of the glass workpiece was also reduced with NaOH solution. The results were further improved when the machining was performed using 1M NaOH solution. The number of cracks were reduced to 1-4 when machining was performed with NaOH solution as compared to 17-20 cracks when machining was performed in air. Figure 15 shows laser beam machining performed with NaOH solution.

Figure 15: Laser beam machining performed under 1M NaOH solution

It is clear from Figure 15 that laser machining when performed under electrolyte solution produces better surface finish and lower cracks as compared to laser beam machining performed under water or in air. However, we need to understand the impact of various machining parameters on the glass machining such as material removal rate, micro cracks around the machined surface and the reduction in recast layer. Thus, from the feasibility study it was concluded that laser beam machining when performed under NaOH solution has a potential to achieve smaller holes with better surface quality in the micromachining of glass.

The effect of various machining parameters on glass micro-machining using chemo-thermal micromachining process is discussed in the next chapter.

CHAPTER 5

5 Experimental Study of Chemo-Thermal Machining Process

From the proof of concept, it is clear that when laser beam machining is performed under NaOH solution, the method of material removal is different than the laser beam machining in air. The difference can be seen in the number of micro cracks around the machined surface and the reduction in recasting layer. The cracks formed in chemo-thermal micromachining process are much shorter than the cracks formed in pure laser ablation. The aim of this experimental study is to understand the effect of each individual parameter on machining output.

5.1 Materials and Method

For the process parameter study, the set-up consists of a glass slide on a stainless-steel plate in a pyrex vessel filled with aqueous NaOH solution. The aqueous NaOH solution is 1-2 mm above the glass surface. The CO2 laser beam is focused at the interface of electrolyte solution and glass workpiece. The major parameters affecting the machining are laser power, exposure duration and concentration of the NaOH solution. Experimental Volume Removed

To find the material removal rate, we need to find the volume of the machined surface. Once the chemo-thermal machining is performed, the two-dimensional (2-D) hole profile is traced using a profilometer. The stylus used for tracing the profile of the machined hole consists of a tip with a radius of 5m and tip angle of 90o. The resolution of the profilometer used is 0.000125µm (8µm 27

range). Assuming the hole to be axisymmetric, a three-dimensional (3-D) CAD profile is generated using MATLAB as shown in Figure 16 (a). The plot under the red region is taken to generate 3D profile as shown in Figure 16 (b). The convex hull volume of this 3D contour gives the volume of material removed.

Figure 16: Experimental material removal

5.2 Input parameters and their levels

A four-level full factorial experimental design was adopted with three process parameters as given in Table 3.

Table 3: Process parameters and their levels

Variable Parameters Notation Units Level 1 Level 2 Level 3 Level 4 Concentration A M 0.1 0.5 1 2 Exposure Duration B Sec 60 90 120 150 Laser Power C W 6 7 8 9 Constant Parameters

Workpiece Material Borosilicate Glass Laser Spot Diameter ≈ 100-150 m Height of electrolyte 1-2 mm

5.3 Material removal mechanism

The material removal mechanism in chemo-thermal micromachining process is different from laser ablation performed in air. Here, the machining is performed using two different machining processes. The two machining mechanisms working simultaneously are laser ablation and chemical machining as shown in Figure 17. Laser beam incident on the interface of two different media causes some part of laser to be reflected away, the rest of the laser is transmitted and absorbed. Laser beam when incident on electrolyte transmits a fraction of its power to the electrolyte and the rest to the work-piece (Figure 17a). Part of the laser energy transmitted to the electrolyte increases its temperature locally. The chemical machining process depends on the electrolyte temperature. The increase in electrolyte temperature further increases the rate of chemical machining (Figure 17b). The fraction of laser energy being absorbed by the electrolyte highly depends on electrolyte concentration. At higher concentration, large fraction of laser energy is being absorbed by the electrolyte which sometimes causes crystallization of NaOH.

Once crystallization starts happening, the rate of chemical machining slows down because the reaction happens in aqueous state. Laser incident on the crystals often leads to the formation of plasma plume that produces shockwaves during machining (Figure 17d). The shockwaves facilitate the removal of material and debris from the machined surface. However, the

shockwaves produced in presence of plasma cause increase in cracks along the machined surface.

Thus, it is desired to keep the plasma formation to the minimum. This is achieved by constantly adding electrolyte at the laser focal point to prevent electrolyte from drying up and keeping the electrolyte leveled up.

a b

c d

Figure 17: Mechanism of material removal

5.4 Study of Built-up edges

Material removal due to laser beam ablation consists of melting and vaporization of the workpiece material. Due to high thermal gradient, melting of the workpiece material leads to splashing of the molten material and solidifying on the edges of the machined surface. Built-up 30

edges, a typical characteristic of laser beam machining, are formed due to re-solidification of the vaporized product. Our study showed that the formation of built-up edges was minimized by using NaOH solution. Figure 18 shows the comparison of machined profile using laser ablation alone versus chemo-thermal micromachining process. There is a significant reduction in built-up edges in chemo-thermal micromachining process as compared to laser ablation alone. This is because the molten material and debris is dissolved in the liquid medium which prevents it from re-solidifying. It was further observed that the built-up edges tend to reduce as the concentration of NaOH is increased as shown in Figure 19. This loss of built-up edge at higher concentrations of

NaOH is because of the increase in material removal by chemical mechanism. This increase in material removal due to chemical action is confirmed by examining the chemical composition by

Energy Dispersive X-Ray (EDX) analysis. The chemical composition of un-machined glass was compared with the chemical composition of the machined surface. Since, NaOH solution reacts with SiO2 present in glass material to form sodium silicates, the amount of Si in the machined surface was found to be less than that of un-machined glass composition. The EDX results clearly showed selective Si removal from workpiece material. As the concentration of electrolyte increases, the amount of Silicon dissolved in the electrolyte also increases.

Figure 18: Comparison of build-up edge profiles (Blue: Laser ablation; Red: Chemo-thermal process)

Figure 19: Built-up edge reduction with change in electrolyte concentration.

Chemical etching is a very slow process at low temperature. Chemical etching alone was performed on glass without the action of laser at room temperature and it was found that after

3 days, the material removed by chemical action was about 4-5m deep. Thus, the material removed in chemo-thermal micromachining process is many folds larger than chemical etching alone.

5.5 Cracks formed along the machined surface

Excessive heat conduction to the workpiece material in laser beam machining causes thermal defects like cracks when laser machining is performed on hard and brittle material like glass. The increase in Heat Affected Zone (HAZ) causes residual stress in the workpiece material. When the stresses are released, they result in radial and circular cracks around the machined surface. Thus, laser ablation alone on glass surface produces a lot of circular and radial cracks. Laser machining performed under electrolyte results in the reduction of both circular and radial cracks because the electrolyte acts as a cooling medium and absorbs some part of the laser power. The specific heat capacity and thermal conductivity of the liquid medium is much higher than that of air. If the glass surface is not cooled adequately the micro fractures propagate extensively. Laser machining was performed in dry air and under water initially. It was found that the number of cracks and built-up edges were reduced when the machining was done under water. This reduction in number and length of cracks was further improved when machining is performed using chemo-thermal process [6]. Figure 20 shows the comparison of cracks formed using laser

ablation and hybrid chemo-thermal process. The cracks were studied by performing post processing of the microscopic images of the machined surface. Lengths of cracks were measured using image processing software ImageJ. It was found that there was a reduction of about 90% in the average length of cracks. During laser beam ablation, the average length of crack was about

500 microns which was reduced to about 57 microns with chemo-thermal machining process.

Also, the number of cracks which were about 20-23 in laser machining was reduced to 1-4 in chemo-thermal machining process.

Figure 20: Comparison of cracks

Numerous surface and subsurface cracks are formed because of the development of thermal stress. This research studies the surface cracks as the study of subsurface cracks is out of the scope of our work. The number and length of cracks are interdependent in a way that when number of cracks increases the average crack length decreases as shown in Table 4 and Table 5.

Therefore, the total crack length, which is a product of number of cracks and average crack length is considered as an output variable in this study.

Table 4 shows that the total crack length reduce as NaOH concentration is increased because with increase in NaOH concentration more laser energy is being absorbed by the electrolyte.

Since, more laser energy is being absorbed by the electrolyte, there is reduction in HAZ. However, with increase in exposure duration, the total crack length increases as shown in Table 5, as more laser power is being transferred to the glass work-piece.

Table 4: Variation of crack length with change in NaOH concentration keeping laser power 7W and exposure duration 90 sec

Concentration Number of (M) cracks Avg. crack length (µm) Total crack length (µm) 0.1 7-11 205 1845 0.5 3-6 287 1148 1 1-5 178 534 2 0-3 192 192

Table 5: Variation of cracks with change in exposure duration keeping laser at 7W and NaOH concentration 1M

Exposure Avg. Crack Length Duration (sec) Number of Cracks (µm) Total Crack Length (µm) 60 0-4 212 424 90 1-7 224 896 120 3-8 160 960 150 12-19 147 2205

5.6 Chemical composition of glass

Energy Dispersive X-Ray (EDX) analysis was performed on the glass samples before and after machining to find the difference in the chemical composition of glass work-piece. Chemical composition of typical borosilicate glass includes silica (SiO2), boric oxide (B2O3), and traces of sodium oxide (Na2O), magnesium oxide (MgO), and aluminum oxide (Al2O3). The NaOH solution reacts with SiO2 present in glass to form sodium silicate. The formation of sodium silicate depends on the amount of NaOH present in the liquid and the temperature of the reaction. Since, NaOH reacts only with the Si present in the glass and not with other materials present, the Mg was taken as the parameter of measuring the reduction in Si since the percentage of Mg remains constant during the chemo-thermal machining. It was observed from the EDX results that there is reduction in Si and increase in the weight ratio of Mg to Si after machining. The Mg to Si weight ratio increases as we increase the concentration of NaOH in the electrolyte solution suggesting that this hybrid process is selectively removing Si from glass surface. This selective removal of Si from glass surface increases with increase in NaOH concentration so more sodium silicate is forming due to chemical reaction. This Mg to Si ratio decrease on further increasing the concentration showing that the effect of chemical machining is decreasing after a certain point.

This is because crystallization of NaOH not only depends upon the temperature of electrolyte but also on the amount of water content in the electrolyte. As the concentration of NaOH increases there is relatively less water content thus crystallization process speeds up which results in

reduction of the chemical reaction. A sample EDX result is given in Figure 21 for glass surface machined at 7W laser power, 90 sec exposure duration, and 0.5M NaOH concentration.

kV: 20 Mag: 800 Takeoff: 82.7 Live Time(s): 30 Amp Time(µs): 7.68 Resolution:(eV)130

Selected Area 1

eZAF Smart Quant Results

Element Weight % Atomic % Net Int. Error % Kratio Z R A F

O K 51.80 66.04 492.85 9.24 0.14 1.05 0.97 0.26 1

NaK 6.46 5.73 111.54 10.31 0.02 0.95 1 0.36 1.01

MgK 7.58 6.36 220.72 7.99 0.04 0.97 1.01 0.48 1.01

AlK 1.67 1.26 55.41 11.39 0.01 0.93 1.01 0.56 1.01

SiK 18.70 13.58 745.16 5.03 0.12 0.96 1.02 0.68 1

CaK 13.79 7.02 374.94 3.09 0.12 0.9 1.05 0.96 1

Figure 21: A sample EDX result

The weight percentage of Si and Mg in the specimen was 35.93% Si and 2.57% Mg. There was

negligible change in the composition when the electrolyte concentration was 0.1M showing that the effect of chemical machining at this point was very less and the machining was done solely by laser ablation which is similar to laser beam machining performed under water. As the

concentration of NaOH increases to 0.5M, weight percentage of Si and Mg was found to be 18.7% and 7.58% respectively. At 1M concentration, weight percentage of Si further reduces to 15.96% and Mg increases to 10.4%. With further increase in NaOH to 2M, weight percentage of Si was found to be 25.83% and Mg 2.32%. Thus, the effect of chemical machining in this hybrid process increases with increase in electrolyte concentration till a certain point then it decreases. This is because chemical reaction requires hydrolysis as a first step. With increased concentration, there is less water content in the electrolyte for hydrolysis and water content further drops due to crystallization. The weight percentage of Si and Mg after machining (7W laser power and 90 sec exposure duration) is shown in Figure 22.

Figure 22: Si and Mg weight ratio under different concentrations of the electrolyte

5.7 Circularity of the hole and surface roughness

Circularity measurement was done to find how much the machined hole profile is deviating from being a perfect circle. Circularity is a unitless ratio and circularity tending towards unity is a perfect circle. A measure of circularity is calculated by 4 times pi times the area of machined profile divided by the perimeter squared[34]. The equation 2 shows the measurement of circularity.

4∗ 휋∗퐴 퐶 = … (2) 푃2

Where C is the circularity, A is the area and P is the perimeter. These values of area and perimeter were calculated using the image processing software ImageJ. For most of our machined profiles, circularity varies from 0.9 to 0.99. The deviation from perfect circle is because the laser is not hitting the work-piece material at a perfect 90o. The circularity measurement example is given in

Figure 23. To measure the area and perimeter, image is first converted to binary image and then boundary is traced. The area of the sample in Figure 23 is 254310 square pixels and perimeter is

1803 pixels. The circularity calculated with this data is 0.91.

Figure 23: Circularity Measurement

Surface roughness of the machined hole was measured using profilometer. The arithmetic mean height indicates the average of the absolute value along the sampling length. In our study the sampling length was 2.5mm. It was found that arithmetic mean roughness of the machined hole varied from 9-20m.

5.8 Material Removal Rate (MRR)

Material removal rate was given as total volume machined by the process divided by the total time taken for machining. Material removal rates under different electrolyte concentrations is shown in Figure 24. It was found that MRR in chemo-thermal process was lower than laser beam machining alone because a large part of laser energy is being absorbed by the electrolyte thus increasing the rate of chemical machining. It is interesting to note that the rate of machining is still several orders higher than pure chemical machining where machining 100 micron depth of glass takes about 4 hours at elevated temperature [35]. MRR in hybrid process increases with increase in laser power. This is because more laser energy was being transferred to the work

piece. MRR also increased when exposure duration was increased however, it becomes constant after long exposure duration because of the evaporation of water from electrolyte which further leads to crystallization. It was also observed that MRR increases as the concentration on NaOH increases till crystallization is achieved and gradually reduces at higher concentration because larger part of laser energy was being absorbed by the electrolyte and crystals resulting in the formation of plasma plume.

MRR change with Laser Power 60 sec exposure duration

0.1M 0.5M 1M 2M 250000

200000 /s)

3 150000

m 

100000 MRR ( MRR

50000

0 5 6 7 8 9 10 Laser Power (W)

Figure 24: MRR under different concentrations of the electrolyte for 60s laser exposure

5.9 Major findings of Experimental Study

The process parameter experimental study was performed on glass workpiece using chemo- thermal micromachining process. The material removal mechanism is an integration of thermal ablation and chemical etching at the focused point. It was found that the MRR increased with increase in laser power and exposure duration, but it also increased both radial and circular surface cracks. With the increase in electrolyte concentration, the effect of laser machining was reduced as evident by the elimination of built-up edges and reduction of surface cracks. The effect of chemical machining was found to increase with increase in electrolyte concentration up to a certain point and then reduces with further increase in concentration due less water available for hydrolysis and due to crystallization of NaOH. The effect of chemical machining was observed using EDX analysis that showed the selective Si removal from glass along the machined zone.

We wanted to understand the role of each machining process individually and thus there was a need to form a finite element model and simulate the machining process. The simulation will further help us to estimate the machining without using the time and resources for experiments and also to predict the material removal for the parameters which are difficult to perform experiments with our existing machine. The next chapter discuss about the finite element simulation of the chemo-thermal micromachining process.

CHAPTER 6

6 Chemo-Thermal Micromachining Simulation

Experimental study of the chemo-thermal micromachining process gives us the impact of each process parameter on the output of machining. The output obtained from the study includes material removal rate, the number and length of cracks, the height of the recasting layer and the circularity. Out of the given outputs, it was required to predict the material removal using finite element model. The simulation will reduce the time and resources for conducting experiments and also to predict the machining for parameters which are difficult to be performed on our existing laser engraving machine.

6.1 Material Removal due to chemo-thermal micromachining

Material removed in chemo-thermal micromachining process is a combination of laser beam thermal ablation and chemical machining at the focused point. Material removal in laser beam machining (LBM) is based on high heat flux generated by laser beam which melts and vaporizes the workpiece material at the focused point. There is no contact between the tool and workpiece and material removal occurs without any mechanical force. Thermal energy required for melting and vaporization is gained by focusing the laser beam of the workpiece surface using lenses. The governing equation of ablation describing the heat conduction inside the material is given as

휕푇 휌푐 = 푘∇2푇 + 푄 … (3) 휕푡 43

Where 휌 is the density, c is the volume-specific heat, T is the temperature, t is the time and k is the thermal conductivity. Laser beam intensity(I) varies in radial distance and is given as:

푟 (− )2 푟 퐼 = 퐼표푒 표 … (4)

Where Io is the maximum laser intensity at r=0, r is the radial distance from the axis and ro is the laser focal spot radius.

Glass surface when exposed to water dissolves continuously at infinitesimally small rate at room temperature. This dissolution of glass at neutral pH is constant and independent of temperature.

However, in presence of an alkali additional silica gets dissolved in the form of silicate ions[36].

These silicate ions react with alkali to form alkaline silicates. In our experiments, sodium hydroxide solution was used and thus sodium silicate is produced as per the following reaction.

 2푁푎푂퐻(푎푞) + 푆𝑖푂2 → 푁푎2푆𝑖푂3(푎푞) + 퐻2푂 … (5)

The rate of chemical reaction depends on concentration of NaOH solution and the reaction temperature. Rate of chemical reaction at different electrolyte concentration is taken from the literature by J. G. Hooley[35]. The surface temperature of the workpiece is taken from the simulation and the isothermal profile at different time step is given in Figure 29. Volume removed due to chemical machining is given by the following equation.

𝑔 푟푎푡푒( )푥 푠푢푟푓푎푐푒 푎푟푒푎(푐푚2) 푐푚2ℎ 12 3 푉표푙푢푚푒(푐ℎ푒푚𝑖푐푎푙) = 𝑔 x 10 m /min ...(6) 푑푒푛푠푖푡푦( )푥 60 푐푚3

The density of the glass is taken from literature. The exposed surface area is taken from the simulation where the laser beam is hitting the workpiece material. The boundary condition at the surface of the electrolyte where convection heat loss takes place is given as

푘∇푇 = ℎ(푇 − 푇∞) …(7)

Where h is the convective heat transfer coefficient for electrolyte and 푇∞ is the ambient temperature.

To simulate the material removal due to chemo-thermal machining, following simplification assumptions are used in this model:

• Glass is isotropic and all physical parameters of glass are temperature independent.

• Heat transfer is not affected by thermal expansion.

• The CO2 laser beam is regarded as a surface heating source.

• Laser beam follows a Gaussian profile and the whole setup is axisymmetric.

The governing equations and the boundary conditions are shown in Figure 25. Description of boundary conditions is given in Table 6 and the simulation conditions are mentioned in Table 6.

Figure 25 Schematic of the Chemo-thermal micromachining process setup model for simulation and the governing equations.

Table 6: Boundary conditions applied to the system as shown in Figure 25

Boundary Thermal boundary condition, value

A Axis Axially symmetric

B Laser interaction Heat flux from laser

C Electrolyte-air boundary Convective heat transfer, h=115

D Insulated side No heat transfer, h=0

E Insulated side No heat transfer, h=0

F Glass-Electrolyte interface Thermally coupled

G Insulated bottom No heat transfer, h=0

The borosilicate glass properties are taken from literature and shown in Table 7. 46

Table 7: Glass thermal and mechanical properties

Glass Property Value

Density 2.23 g/cm3

Thermal conductivity 1.14 W/mK

Transition temperature 565 oC

Specific Heat Capacity 830 J/kgK

Table 8: Simulation Conditions

Analysis Type 2-D Axisymmetric transient thermal

Elements Used Quadrilateral

Mesh Type Mapped Mesh with fine meshing at the laser boundary

Time Step 0.01 sec

Convective heat transfer coeff. 115 at electrolyte-air boundary (W/m2K)

Workpiece material Borosilicate glass

Workpiece Dimensions (L x H) 1cm x 0.18cm

Simulation Duration 60 sec

Figure 26: Meshing applied to the system of electrolyte and glass with fine meshing at the boundaries

Figure 26 shows the adaptive meshing with fine meshing at the border. The free quadrilateral element is chosen since the work-piece material is rectangular in shape. The algorithm applied for the simulation is shown in Figure 27. The set-up is initially at ambient temperature (293.15K).

The rate of chemical reaction corresponding to the electrolyte concentration at a given temperature is taken from the literature. The laser is turned on in steps of 0.01 seconds. At each step temperature profile is generated and the chemical machining is calculated based on the surface area exposed to the electrolyte. The simulation is run for 60 seconds and the volume of glass above transition temperature (565oC) along with chemically machined volume is taken as the total machined volume by chemo-thermal micromachining process.

Figure 27: Algorithm for chemo-thermal machining simulation model

A sample result of the simulation with 7W laser power and 60 second exposure duration is shown in Figure 28. The surface contour above ablation temperature is visible separately from the rest of the mesh. Figure 29 shows the isothermal profile of the workpiece at different time step of the simulation. The temperature of the profile is taken into consideration while selecting the rate of reaction for chemical machining. The isothermal profile of the simulation matches the machining profile of the experimental machining as shown in Figure 30

Figure 28: Simulation result isosurface sample with 7W laser power and 60 sec exposure

Figure 29: Isothermal Profile of workpiece at different time step

The experimental volume machined was compared with the machined volumes as obtained from simulation results. To calculate the experimental volume removed, a two-dimensional hole profile is traced using a profilometer. Assuming the hole to be axisymmetric, a three-dimensional cavity is generated as shown in Figure 30. The convex hull volume of the cavity is taken as the experimental value of material removal.

(a) 2-D profile of the machined (b) 3-D model of the drilled hole

Figuresurface 30: Determination of material removal

6.2 Simulation Results and Validation Experiments

The comparison of simulated volume machined, and experimental volume machined for 1M

NaOH solution and 60 second exposure was performed with 7.5W and 8.5W laser power. It was found that the model is overestimating the machining process because the laser energy lost due to reflection is not taken in consideration. The average correction factor obtained by comparing

the simulated machining volume and experimental machining volume was found to be 0.86. the calculation of the correction factor is given in Table 9

Table 9: Calculation of Correction Factor

Laser Power (W) Simulated Volume Experimental Correction Factor 3 3 (m ) Volume (m ) (Corf)

7.5 6146258 5361730 0.87

8.5 9244582 7899327 0.85

Avg Correction Factor 0.86

The correction factor is applied to the laser power given in simulation and simulated volume and experimental volume are compared for 7W, 8W, and 9W laser power. The concentration of electrolyte (NaOH) remains the same at 1M. The comparison of the corrected simulated volume is given in Table 10 along with the percentage error.

Table 10: Comparison of Simulated volume and Experimental volume

Laser Power (W) Simulated Volume Experimental Percentage Error (m3) Volume (m3)

7 4699515 4649836 1.06

8 7517729 7741088 -2.97

9 8804616 8331481 5.37

The laser intensity input of the simulation model multiplied with the average correction factor will reduce the laser power given to the workpiece to compensate for the losses due to reflection and other losses not considered during simulation. The image 8 shows the value of predicted machined volume after using the correction factor and the experimental volume. It can be seen that the model developed is able to predict the machined volume due to chemo-thermal micromachining process within 5% variation.

Figure 31: Comparison of predicted machined volume and experimental machined volume

6.3 Major Findings from Simulation Study

The simulation model of chemo-thermal micromachining process was developed to understand the contribution of mechanism from each process. A finite element model is developed to predict

the value of glass machined volume by chemo-thermal micromachining process. The model takes input in terms of laser power intensity and rate of reaction corresponding to the electrolyte concentration and reaction temperature. The laser power intensity follows a gaussian profile and the value depends on the radial distance from the axis of symmetry. The model calculates thermal ablation and chemical etching at each time step based on the surface temperature of the workpiece. The volume above the ablation temperature is taken as machined volume. The model is able to predict the volume machined by chemo-thermal micromachining process within

5% variation.

We have studied the feasibility of chemo-thermal micromachining process, the effect of each input process parameter on machining outputs such as material removal rate (MRR), number of cracks and crack length along the machined surface and the study of circularity and built-up edges. A finite element model is developed to predict the machining volume. Next chapter concludes the results and discuss the future work for this research.

CHAPTER 7

7 Conclusions and Future Work

7.1 Conclusions

Chemo-thermal micromachining is a novel hybrid process for glass micromachining. The process is laser beam thermal ablation performed in wet environment using aqueous sodium hydroxide

(NaOH) solution. Laser beam ablation and chemical etching are the two machining processes working simultaneously. The feasibility study showed that the process has potential of improving the quality of glass micromachining.

A four-level full factorial design of experiments was studied to understand the effect of different input parameters on glass micromachining. The input parameters under consideration were laser power, electrolyte concentration and laser exposure duration. It was found that the MRR increased with increase in laser power and exposure duration, but it also increased the surface microcracks. With increase in electrolyte concentration, the effect of laser machining was reduced as evident by the elimination of built-up edges and reduction of surface cracks. The effect of chemical machining was found to increase with increase in electrolyte concentration up to a certain point and then reduces with further increase in concentration due less water available for hydrolysis and due to crystallization of NaOH. The effect of chemical machining was

observed using EDX analysis that showed the selective Si removal from glass along the machined zone.

A finite element simulation model is created and the simulation of chemo-thermal micromachining is performed using a commercially available software to calculate the borosilicate glass volume based on laser power, exposure duration and rate of chemical machining at different concentration of electrolyte. Experimental verification of the model reveals that the model is able to predict chemo-thermal machining within 5% variation. Using the procedures provided in this paper, borosilicate glass micromachining using chemo-thermal micromachining process can be predicted.

7.2 Future Work

The study conducted in this work does not include the effect of different modes of laser beam as this was the limitation of the laser machine. We have studied the effects of continuous wave CO2 laser.

The study includes a constant height of electrolyte above the workpiece material. The height was maintained during the entire machining by adding more electrolyte at the focal point to prevent drying of electrolyte and formation crystals. The formation of crystals further induces the formation of plasma plume. The effect of plasma plume is not studied in this research.

Also, once the machining starts happening, the focal point of laser does not lie directly at glass- electrolyte interface. The Z axis movement of laser head or workpiece material to maintain the focal point at workpiece electrolyte interface may produce less taper.

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APPENDIX A: List of Publications

• Ali, A., Sundaram, M. (2016) “Experimental study of chemo-thermal micromachining of glass” ASME 2016 11th International Manufacturing Science and Engineering Conference, MSEC 2016, Volume 1, Issue undefined, 2016 Paper No. MSEC2016-8772, pp. V001T02A034 doi:10.1115/MSEC2016-8772

• Ali, A., Sundaram, M. (2018) “Drilling of crack free micro holes in glass by chemo- thermal micromachining process” Precision Engineering https://doi.org/10.1016/j.precisioneng.2018.04.015

• *Ali, A., Sundaram, M. “A Simulation Study of Chemo-thermal Micro Machining process.” Manuscript under review.