Multi-scale Investigation of the Machining Behavior of Non-conductive Ceramics Using Electro Discharge Machining

Faculty Development Competitive Research Grant Program 2018-2020

Electro discharge machining (EDM) is an established nontraditional method for precision machining of difficult-to-cut material. This is especially applicable when there is a need for machining of structure like deep bores, grooves or undercuts in material with high wear resistance. EDM is a well-established process for machining difficult-to-cut materials irrespective of the hardness and wear resistance of materials.
Advanced ceramics like engineering ceramics are extensively used in various industrial applications such as transport, energy, semiconductor, and biomedical due to their high heat resistant properties (Figure 1). Implementing EDM process for machining of insulating ceramics will open up opportunities for using ceramics for many innovative and intricate industrial applications. In the past years, assisting electrode method in EDM has been applied to machine insulating ceramics, which works by the mechanism of continuation of material removal even after the conductive layer is removed, however this process is not very stable, when the conductive layer is exhausted. In addition, the application of conductive powder mixed dielectric has been reported for machining of insulating ceramics. Both approaches have been tried in individual manner. However, we hypothesize that combining the assistive electrode technology with the powder mixed EDM would further enhance the machining speed and performance during EDM of non-conductive ceramics. Therefore, the purpose of this project is to investigate the feasibility of combining assisting electrode method technology with conductive powder mixed dielectric to machine insulating ceramics. This research project aims to focus on EDM of aluminium nitride (AIN) and ATZ (Alumina Toughened Zirconia, ZrO2-Al2O3) composites, as both insulating ceramics are of particular interest to the industries due to their application in several engineering branches.
Enhanced properties of advanced engineered ceramics materials have expanded immense opportunities for them to be used in different industrial applications. Engineering ceramics are also known for their excellent properties, such as hardness, low density, wear resistance, bio-compatibility and so on. Therefore, machining of these ceramics has gained huge interest from multidisciplinary researchers. However, the main concern associated with usage of engineering ceramics is to obtain exact shape without generation of micro/cracks resulting from manufacturing processes. Most of the traditional methods available for ceramics machining come with high cutting force, consequently resulting in the surface and sub-surface micro cracks. Even with the advanced machining processes such as, laser, ultrasonic and electrical beam machining; it appears to be impossible to machine ceramics due to its high hardness and tendency to crack formation. On the other hand, EDM has the potential to be used for machining of materials with electrical conductivity (resistivity below 100 Ω.cm) irrespective of the hardness. It was previously considered that, EDM cannot be applied to remove material if the material has high electrical resistivity. However, with the advancement of knowledge, now a days, EDM can also be applied as material removal process for non-conductive materials with the use of special techniques (Xiaopeng, Yonghong, & Renjie, 2012).
Ceramics could be either fully or partially crystalline and include components from nonmetallic and metallic elements. They are generally insoluble to water and can be heated to greater temperature during manufacturing and usage. Ceramics generally differs from metals due to their chemo-physical structures. Metals consist of elements having free valance electrons available due to metallic bond. The mobility of theses free electrons gives metal good electrical and thermal conductivities. Metal also gets crystallized in cubic lattice with numerous slip planes. As a result of these slip planes and lattice dislocations, metals exhibit good ductility and large plastic deformation. On the other hand, ceramics are mainly polar bond based chemical compound. Brittleness of ceramics arises from the absence of slip planes. Ceramics are fundamentally multiphase system consists of crystals, a binder phase and pores. Three mechanisms may be used to make engineering ceramics electrically conductive: such as natural electrical conduction by free lattice electrons, doping with conductive elements, incorporating impurity atoms. Some ceramic materials, such as boron carbide (B4C) and titanium boride (TiB2) exhibit good electrical conductivity due to the availability of free lattice electrons, therefore are considered natural conductors(W. König, D. F. Dauw, G. Levy, & U. Panten, 1988).
Ultra-pure ceramics does not have free electron available due to the polar bonding, which results in lower residual conductivity of their ion and their mobility is limited by their integration in the structure. They are known as nonconductor, since they don’t conduct any current even at higher field strength. However, these non-conductive ceramics can be made to act as conductive by doping them with natural conductors during the production process which allows current to pass through conductive phase. Good conductivity can, however, also be attained in polar-bonded materials by freeing additional electrons. By altering the type and quantity of the impurity ions, conductivity can be increased to the level, at which EDM can be applied. However, most of the ceramics are not conductive enough to be machined by the EDM process. Konig et al (1998) mentioned that ceramics to be machined by EDM should have maximum electrical resistivity of 100 Ω.cm. Therefore, ceramics like pure silicon having electrical resistivity of 105 Ω.cm cannot be machined by EDM (W. König, D. Dauw, G. Levy, & U. Panten, 1988). Different machining processes are carried out at different stages in the ceramic production (See Figure 2).
The necessity of reducing electrical resistivity of ceramics for EDM process is realized by several researchers. Matsuo et al (1992) investigated on TiC, NbC and Cr3C2 doped ZrO2 and Al2O3 and reported on improved material removal rate (MRR) due to the presence of carbide content (Matsuo & Oshima, 1992). Lee and Lau (1991) researched on Al2O3 which was doped with 40% TiC in order to increase the electrical conductivity and obtained surface finish of 4 μm with MRR 0.6 mm3/min. Martin et al (1989) elevated the electrical conductivity of SiC by TiB2 and of Si3N4 by TiN to machine two ceramics by the EDM process (Martin, Cales, Vivier, & Mathieu, 1989). Other than doping ceramics during the manufacturing process, researchers also applied other interesting techniques to enhance electrical conductivity of the insulating ceramics. One of these methods is to alloy the base ceramics materials with conducting component like TiN, so that it can be machined using EDM method (Mohri, Fukuzawa, Tani, Saito, & Furutani, 1996).
Another option is the assisting electrode method, which use conductive layer on top of insulating ceramics, so that the EDM process can be initiated. High temperature generated during EDM process can initiate the cracking of hydrocarbon dielectric oil and workpiece and consequently carbon molecules start to bind with certain elements of the ceramics materials. Due to the conductive nature of carbon, new discharge keeps going on as the EDM process proceeds. Therefore, deposited conductive layer along with the workpiece material beneath the conductive layer is removed (Mohri et al., 1996). Sabur et al (2016) investigated EDM of 92% ZrO2 with copper layer and observed spalling of ceramics material through various cracks (Sabur, Ali, Maleque, & Khan, 2013). Apiwat et al (2003) conducted EDM on sintered Si3N4 ceramics with TiN layer as assisting layer (Muttamara, Fukuzawa, Mohri, & Tani, 2003). Xiaopeng et al (2012) came up with the idea of double electrodes synchronous servo electromechanically discharge grinding to machine insulating ceramics. In addition, investigation was conducted on EDM with resistance – capacitance (RC) discharge circuit and high energy capacitor respectively on AIN and Al2O3 insulating ceramics (Ji et al., 2011; Kaneko, Yamashita, & Fukuzawa, 2012). Moreover, the machinability of Si3N4 insulating ceramics with baked carbon layer on top was also investigated using EDM, and continuous machining without detachment of aiding terminal from workpiece was observed (Bonny et al., 2008; Mohri, Fukuzawa, Tani, & Sata, 2002).
Several researchers attempted adding conductive powder into dielectric fluids during the EDM of metals and found the process very effective. Chow et al (2008) added SiC powders into pure water while doing EDM for Ti alloy and observed improved MRR and surface finish (Chow, Yang, Lin, & Chen, 2008). Yeo et al. (2007) also confirmed the reduction of crater sizes due to SiC powder addition into dielectric (Yeo, Tan, & Kurnia, 2007). Gunawan et al (2009) observed improved MRR and better surface finish using molybdenum disulfide powder with dielectric (Prihandana, Mahardika, Hamdi, Wong, & Mitsui, 2009). While using nano graphite powder with combination of vibration, they were able to reduce the machining time by 35% (Prihandana, Mahardika, Hamdi, Wong, & Mitsui, 2011). Jahan et al (2010) investigated on EDM of tungsten carbide using conductive graphite, aluminum and non-conductive alumina mixed dielectric (M. Jahan, Rahman, & Wong, 2010; M. P. Jahan, Rahman, & San Wong, 2011). According to their observation, graphite powder resulted in better surface finish whereas aluminum powder caused higher MRR due to higher spark gap. However, EDM performance on insulating ceramics using powder mixed dielectric was not very successful in terms of achieving improved surface finish. Tani et al (2002) investigated on EDM of Si3N4 ceramics using Al, Gr, Si, Ni and ZrB2 powders and observed increased MRR along with high surface roughness due to long pulse duration (Fukuzawa, Nanbu, & Mohri, 2002).Jun et al (2013) investigated on EDM of reaction bonded silicon carbide using carbon nanofiber mixed dielectric and observed improved EDM performance in terms of material removal rate (MRR) and electrode wear ratio (EWR) (Liew, Yan, & Kuriyagawa, 2013).
As can be seen from literature review, research on insulating ceramics are still ongoing and several studies have reported improvement in the EDM performance in terms of MRR and EWR. However, none of the studies considered combining assisting electrode method and powder mixed dielectric approach together for EDM of non-conductive ceramics. Therefore, this study aims to investigate the EDM performance of insulating ceramics AIN (aluminium Nitrate) and ATZ (aluminium toughened Zirconia) using combined effect of assisting electrode and powder mixed dielectric.
Project Aim: The purpose of this project is to develop a comprehensive understanding of multi-scale EDM process of non-conductive advanced ceramics by conducting a systematic investigation of macro to micro EDM process. If successful, the proposed method would allow extensive use of the EDM process for machining engineering ceramics at macro and micro scales with improved material removal rate, surface finish and dimensional accuracy.
The objectives of the project are to:
1. Investigate the feasibility and effect of combined assisting electrode method and conductive powder mixed dielectric for EDM of insulating ceramics.
2. Study the effect of different conductive layers on the EDM process performance.
3. Investigate the effect of conductive powder mixed dielectric on the EDM process.
4. Parametric optimization and modeling of the process using Response Surface Method.

1. An assisted-electrode method is used to make the surface of the workpiece conductive enough to machine. A combination of graphene paste, carbon nanotube, and copper tape has been used to make the coating surface. Tungsten carbide tool has been used. The findings indicate that, for low pulse energy it is possible to generate through holes in this process. A combination of low peak current and high gap voltage proved to be successful in this method. The surface of the machined hole reveals that thermal spalling is present as material removal mechanism, which results in a rough surface. The energy dispersive spectroscopy (EDS) analysis proves that carbon deposition occurs in the wall of the holes to promote successful machining. Copper is also present in the machined surface. A feasible process has been developed to machine AIN ceramics using EDM process.

2. A series of experiments were carried out on AlN ceramics using “Assistive Electrode” micro-EDM process with a goal of machining blind micro-holes into the ceramics with the aid of on-machine fabricated copper tungsten tools. It was found that multi-layer coatings of silver and copper with copper tungsten electrode resulted in successful machining with high-aspect-ratio holes during powder mixed micro-EDM of AlN ceramics, while micro-holes with less than one aspect ratio are machined without powder addition to the dielectric. It was also observed that comparatively lower level of discharge energies, i.e., lower value of voltages and capacitances were favorable for successful machining of micro-holes in ceramics, even though it results in significantly higher machining time. Despite of relatively low discharge energy usage in micro-EDM, machined surfaces appear to be very rough. The machined surfaces indicate that melting and evaporation, as well as thermal spalling, are the dominating material removal mechanisms. The machined surfaces contained many thermal cracks and porosity on the surface. The elemental composition analysis confirms the presence of aluminum and nitrogen elements on the machined surface. Finally, by careful selection of machining conditions and assistive electrode, successful machining of micro-holes is possible on the non-conductive ceramic surfaces using the micro-EDM process.

3. Assisting Electrode Method was suggested as a solution for machining of non-conductive ceramics by EDM. In this method, conductive layer is applied on top of non-conductive ceramics and thus workpiece can be machined by EDM process using residual conductive layer. In this study, coating consisting of three layers, where silver nanoparticles were sandwiched between two layers of silver and copper on top, was used as assisting electrode to machine Aluminum Nitride (AlN) ceramics by silver nanoparticles mixed micro-EDM. Successful machining of AlN was demonstrated and blind micro hole with higher than three aspect ratio was achieved. This method enables successful machining of non-conductive AlN where conductive particles from coating as well as conductive particles from dielectrics helped to maintain continuous spark generation by forming secondary conductive intrinsic layer.

4. This study reports on the numerical model development for the prediction of the material removal rate and surface roughness generated during electrical discharge machining (EDM). A simplified 2D numerical heat conduction equation along with additional assumptions, such as heat effect from previously generated crater on a subsequent crater and instantaneous evaporation of the workpiece, are considered. For the material removal rate, an axisymmetric rectangular domain was utilized, while for the surface roughness, a rectangular domain where every discharge resides at the end of previous crater was considered. Simulated results obtained by solving the heat equation based on a finite element scheme suggested that results are more realistic by considering instantaneous evaporation of the material from the workpiece and the effect of residual heat generated from each spark. Good agreement between our model and previously published data validated the newly proposed models and demonstrate that instantaneous evaporation, as well as residual heat, provide more realistic predictions of the EDM process.