Machinability of Non-Conductive Ceramic by EDM: A Review

AdvancedCeramics are gaining a foothold in the lightweight aerospace, electronics, and structural engineering component markets. These ceramics could be extensively used in modern industry, such as ballistic body armour, ceramic carbon fibre composite automoti ve brakes, diesel particulate filters, prosthetic limbs, piezoelectric sensors, and computer memory products, due to their higher compressive strength, resistance to abrasion, lower thermal expansion coefficient, higher density, and chemical stability. Ceramics are notori ously difficult to handle due to the increased hardness and brittleness. Low electric-conductive ceramics, on the other hand, can be machined using the EDM technique, in which plasma energy is used to accurately remove the material by continuous sparking between the surface and the electrode submerged in dielectric. It is observed that EDM can be applied to the material having electrical resistivity below 100 ?.cm. Most recently it has been observed that EDM could be applied to insulating ceramics too. An attempt has been made in this paper to critically review the machining of ceramics by the EDM process.

In the presence of a dielectric fluid, a series of successive electrical discharges between an elect rode and the workpiece are utilized. The electrodes are brought closer to the workpiece until there is a tiny enough distance between them for sparking to occur and an impression to be created. The electrode is placed closer to the work-piece until the distance between them is small enough for sparking to be happened & the impression to be created. The material is removed by the erosive effect of the electrical discharges from the tool-electrode and workpiece. During machining, EDM reduces mechanical tension chatter and vibration problems by avoiding direct interaction between the electrode and the work-piece. Any material can be cut regardless of its hardness as long as it can conduct electricity [6,7]. Advanced ceramics may be machined by EDM if their electrical conductivity reaches a certain threshold value of the order of 0.01 S/cm (or resistivity less than 100 cm) [8,9]. When machining advanced ceramics with EDM, the thermal spalling mechanism is used to extract the material [3]. EDM is proving to be a promising and appealing technology for machining ceramics, as long as the materials have a high adequate electrical conductivity. On improve processing parameters, an electrically conductive film must be added to non-conductive ceramics having Alumina, Silicon-Nitride, or Zirconia matrix [10].

EDM of Non-Conductive Ceramics
Metallic and non-metallic constituents make up non-conductive ceramics. Ceramics are significantly stronger than metals because of these types of bonds. Non-conductive ceramics have been used as electrical insulators and high-temperature resistance in vehicle spark plugs for many years. Ceramics are now widely regarded as the ideal material for diverse angles and perspectives manufacture of home, industrial, and building products, as well as aesthetic artefacts. Machining tools, self-sustainable bearings, engine components, diesel engines, heat exchangers, bulletproof shield, automotive brakes, diesel particulate filters, and a range of prosthetic implants are just a few categories [11]. Engineering ceramic micro-parts are applied in the biomedical industry to make femoral heads and ace-tabular cups for total hip replacement, dentures and remodels, bone fillers, and tissue engineering scaffolds [12]. 3 Based on the input parameters provided in Fig. 1, the machining of advanced-ceramics has been analyzed. Surface roughness number and material removal rate are used to demonstrate the EDM process' performance. The performance of conductive and non-conductive ceramics depends mainly on machining parameters like current, voltage, work-piece property like electric resistivity conductivity, assisting electrode properties, etc. These parameters are found to be affecting the performance of the process in various degrees. Electrical conductive workpiece is required for machining by EDM, researchers used a conductive layer between the workpiece and electrode as an assisting electrode. The pictorial view of the arrangement of assisting electrode is given in Fig. 2. Here a conductive material like copper or copper alloy is made to pass between workpiece and the tool electrode.

Fig. 2 EDM of insulating material with auxiliary electrode
The pulse generator's positive and negative sides are connected to the tool electrode and the auxiliary electrode, respectively. A steel wheel is mounted on a rotary spindle that is powered by an A.C. motor. A numerically controlled (NC) table provides the workpiece, which is an insulating ceramic blank. A thin coating of conductive material serves as an auxiliary electrode in this case. The tool electrode rotates at a high speed during machining, and the auxiliary electrode is fed along the surface of the insulating ceramic work towards the tool electrode. The conductive layer allows for sparking, which contributes to material removal as the spark energy triggers a crack in the workpiece [12,13]. This spark leads to the dissociation of the dielectric. On the machined surface, dissociated carbon and tool electrode particles rebuild a conductive layer. As a result, another EDM spark occurs. EDM comes in a variety of forms, the most common of which are wire EDM and die-sink EDM. A thin conductive material wire serves as the electrode in wire EDM, and the workpiece is placed on a CNCcontrolled worktable. By controlling the movement in X-Y direction, the setup is able to cut complex 2 -dimensional shapes. While die sink EDM, a three-dimensional form conductive material tool is employed to machine a conductive material immersed in a dielectric tank. An electrical spark is produced for a very short time when a regulated voltage is applied between the electrode and the workpiece, and the material on the workpiece melts and evaporates locally. The appropriate cavity in the workpiece is formed by a series of successive sparks. In comparison, much less material is extracted from the tool material, allowing the tool to wear out over time. Melting, vaporization, and disintegration, as well as fracturerelated spalling, are key phenomena t hat occur during EDM owing to spark erosion of ceramic materials, depending on the mechanical characteristics and EDM settings. Material removal mechanisms during EDM of ceramic composites have been carefully examined [13]. To acquire an insight into the mechanism, most researchers looked at microscopic scans of the machined part. The researchers find various models of the mechanism of material removal [14]. It is said the main mechanism of material removal is melting evaporation, dissociation, and micro-cracking .the various mechanisms observed by different researchers are shown in Fig. 3.
theoutcomes of their research is given in Table 1. on various materials that have machined by EDM, parameters used during machining and method and the process parameters with the outcomes of the processes. A classification based given below shows the various non-conductive ceramics machined with the assisting electrode varnish as assisting electrode for micro-milling of non-conductive ZrO 2 ceramics. The table electrode is better than the solid type of electrode. A. Schubert et al. [19] have used silver machining of non-conductive Si3N4 ceramics is done. They also shown that the pipe copper layer and carbon powder painted work well as an assisting electrode while micro-machine it by EDM machine. Apiwat Muttamara et al. [18] has shown experimentally that experimentally shows that by using copper sheet around the insulated ceramics help to importance of using water-based emulsion as adielectric medium. Abdus Sabur et al. [14] has non-conductive ceramic can be easily machined in EDM milling. They also explained the Y.H. Liu. et. al. [17] have explained that using the copper sheet as an assisting electrode, the conductive ceramics and MRR obtain in insulated ceramic is better than conductive ceramics. machined .they also explain that surface roughness, wear rate properties are better for that Insulating Si3N4 ceramics, and Si3N4/CNT and Si3N4/GNP can be efficiently machining of conductive Al2O3 composites. D.Hanaoka et al. [16] experimentally investigated related spalling, are the predominant material removal mechanisms during electrical discharge ceramics. Patel et al. [15] explored whether melting, evaporation, and dissociation, or fracture-technology for machining ceramics. EDM can be used to efficiently machine conductive Because EDM can process the material despite its hardness, it is thought to be a very viable materials, however the problem with diamond grinding is that it is inefficient and expensive.
Diamond grinding has typically been used to process exceptionally hard and challenging Progress so far in the machining of insulated ceramics by EDM 3.

Conclusions
Wear resistance, high compressive strength, chemical resistance, and abrasion resistance are among the mechanical features of advanced engineering ceramics. Modern industry might get benefit from improved engineered ceramics. EDM is a viable machining procedure if the ceramics, composites have a low electrical resistivity and are adequately conductive to induce sparking. EDM is used to machine hardened materials that are conductive in nature that's why conductive ceramics can be easily machined by this method. For non-conductive ceramics, a conductive layer, as an assisting electrode has to use in the form of a conductive material layer on the ceramics. Thus if any how it is possible to generate a spark between tool and workpiece for carrying-out EDM, very hard materials such as Advanced ceramic, Insulated ceramics, ceramic-composite could be machined. By reviewing previous various works it can also be said that: 1. When EDM is used to machine conductive ceramics, the major material removal mechanism is melting, evaporation. Thermal fracture or thermal cracking are used to remove non-conductive ceramics.
2. While milling non-conductive materials with the EDM process and an auxiliary electrode, thermal spalling or thermal cracking is observed to be the predominant material removal mechanism. Thermal spalling is not seen in materials with a higher toughness.
3. The rate of material removal increases as the input power increases and decreases when the material's hardness increases.
4. When machining conductive materials, surface roughness was better than when machining insulating materials. The MRRs on the insulating materials significantly higher than those on the conducting materials.
5. When a water-based emulsion is employed as the machining fluid, no hazardous gas is produced, and the equipment is not corroded during ED milling.
6. Due to the lower viscosity of water, EDM in de -ionized water produces a thinner recast layer.