The Scope of Machining of Advanced Ceramics by Electro- Discharge Machining and Its Hybrid Variants

The constraints with the applicability and technological limitation of metal and alloy lead to the origin of advanced ceramics products or devices to enhance the system productivity and effectiveness. Advanced ceramics matrix composites (CMCs), metal nanocomposites are prepared by pressing and high-temperature sintering. The excellent properties of ceramics are preserved by the amalgamation of metal or semiconducting phase with the convention ceramics. Electro-discharge machining (EDM) has the capability to machine very rigid to cut materials that are extremely difficult to the shape by any other regular method. In the past decade, many types of research have explored machining of advanced ceramics by EDM process and came out to be some unique process to machine even non-conventional ceramics and CMCs. Many research have been found in the field of hybrid EDM process to improve the machining outcome and more efficient use of the EDM tool. One of the revolutionary findings is to machining of non-conductive advanced ceramics by EDM process. In the present paper, a critical review of advanced ceramics machining by EDM and its hybrid for ceramics and its composite are presented.


INTRODUCTION
The exposure of the manufacturing and production industries to use engineering ceramics is increasing for the last two decades because of their outstanding mechanical, chemical and thermal properties. Among various ceramics oxides of aluminum and zirconium are widely used for industrial applications that are gas sensors, a component of the pump and medical instruments [1] . Advanced/ engineering ceramics are used as machine tool, aerospace parts, automobile parts and electronics parts [2] . They are mostly used in dental prosthesis such as dental-crown, denture, orthopedics, automotive brakes, sensors, advanced computer memory products [3][4] . Advanced ceramics not only encourage going beyond the performance limit of metal but also overcoming the technological limitation imposed by conventional materials used in industries.
Advanced ceramics are the materials of having excellent resistant to thermal degradation and mechanical wear, stronger electromagnetic response, high refractoriness etc. [5] . However such properties are of good signs for application but the fabrication and machinability characteristics such as high rise in machining force, Many researchers have explored ceramics machining by EDM process for developing miniaturized products of hard and difficult to cut materials. These are some characteristics of the EDM process that shows various areas in which the improvement is needed for a better outcome (Table 1).

Characteristics of EDM process
• Ultra Miniaturized machine parts EDM has the ability to machine miniaturized machined parts of metal, metallic alloy, graphite, and ceramics irrespective of the hardness and brittleness [12] . Liu  Schubert et al. [20] have also used starting a conductive layer as a second electrode for micro-machining of insulating ceramics.

Low energy efficiency
A small amount of energy of total input energy is used for material removal and stored in electrodes in EDM.
Maximum amount of input energy is lost in dielectric due small discharge area and temperature gradient in the dielectric fluid. Approximately 18-50% of the total energy is utilized in the EDM. Many researchers are working in the field of optimizing EDM energy in material removal. Obwald et al. [21] observed that only 40% of total energy is distributed to electrodes [22] .

Costly process
EDM is a costly process as compare to the conventional machining process as not only material removal rate is less but also tool fabrication is an exorbitant process 1. The main problem with ceramics is that they are very complex in machining [6] . Diamond grinding is used commonly for ceramics machining. But this method is expensive and inefficient. Higher grinding forces induces in this process leads to quick decay in tool edges [25] . Advanced ceramics are facing difficulty in machining as mostly process are slow and expensive.
EDM is an appropriate process to machine advanced ceramics because of it's with its near force less noncontact type of machining that is independent of hardness and brittleness of material [20] .
EDM allows machining intricate shaped harden conductive parts in a versatile and correct way. The EDM performance of ceramics is affected by various parameters as shown in Fig. 3.
EDM holds the capability of machining and fabrication low cost, tool and parts from conducting ceramic blanks in a precise manner. The capacity of EDM of machining without any physical contact eliminates mechanical chatter and vibration that could damage ceramics during conventional machining [8] .

Research on the EDM of advanced ceramics
Advanced ceramics that posses electrical conductivity exceeds a particular threshold value of the order of 0.01 S/cm (or resistivity less than 100 Ωcm) [26] could be machined by EDM. While machining by EDM in advanced ceramics, the thermal spalling mechanism is more predominant material removal mechanism [1] .
There are no. of EDM variant are possible as shown in blade, heat exchanger, automotive brake, diesel particulate filters, prosthetic products, dental implants and ceramic-sensors etc. [68] . Machining of insulated ceramics can also be done by the EDM process. But as we know the workpiece should be conductive in nature to carry out the process. So while machining insulated ceramic an assisting electrode has to be used as shown in Fig. 2. The development of secondary electrode (assisting layer, conductive layer) has enabled insulating ceramics for machining by EDM [9] [72] . In assisting the electrode method of machining nonconductive ceramics, the surface of the material is coated with highly electrically conductive material like silver or copper paint. Conductive layer makes possible for spark and that leads to the removal of the materials as spark energy causing cracks on the work-piece [68][4] .
As the assisting conductive layer machined, a new rebuilt conductive layer (pyrolytic carbon) is formed by carbon particles from the particles of dissociation of dielectric (kerosene, hydrocarbon) molecule at high plasma energy [20] [73] . The process of deposition of conductive rebuilt layer greatly enhances spark stability [72] . This pyrolytic carbon layer provides the necessary electrical conductivity to progress the process as shown in Fig. 4 [4] [20] . Some researchers have successfully machined non-conductive ceramics by using copper foil [68] [38] [52] , silver paint [20][4] , metal mesh [74] , carbon baked layer [75] . Some primary characteristics of nonconductive ceramics machining by EDM are shown in Table 2.
In the EDM process, melting, vaporization, and evaporation are the mechanism of material removal that occur at a very high temperature between the electrodes i.e. cathode and anode [78] [79] . EDM of nonconductive ceramics, the main mechanisms that occur due to spark erosion of ceramic materials are fracturerelated spalling and microcracking that purely depends on properties of ceramics and EDM parameters (Fig. 3). spalling [69] . It is better to have spalling during machining of ceramics because less amount of discharge energy is needed for generating subsurface cracks and for flakes detachment than that of melting, vaporization [82] . If irregular size debris, micro flakes, microcracks are observed on the machined surface those are due to spalling effect.

Numbers of attempts have been
Sabur et al. [83] have machined ZrO2 ceramics by EDM process and clearly indicated melting and spalling in the to spalling-effect, larger microcracks generated are mostly perpendicular and parallel to the top surface [81] .

Schubert et al. [20][4] have machined zirconia
ceramics by assisting conductive layer EDM method. It is observed that sparking were steady even after the electrically conducting starting layer has been removed.
They also compare the machined surface of steel and zirconia ceramics and observed surface roughness values were factor 2.5 higher than those of steel as shown in Fig. 5. In the case of insulated ceramics machining by EDM, the machined surface found to be rougher than metal as removal mechanism was not only melting or evaporation but also thermal spalling.

Conductive ceramics machining by EDM
A secondary conducive phase has to be added to increase the electric -conductivity of the advanced ceramics. The resultant ceramic-matrix composites (CMCs) possess better fracture toughness, hardness and strength [6] [27] . Lauwers et al. have concluded that the fracture toughness remains modest and such composite cannot be used in high tensile force applications as they may fracture and explode in a brittle manner. To avoid such failure due to change in a mechanical property of CMC, a limited amount of secondary phase has to be added [84] [27] . Many researchers have been working in the area of EDM and its variant of CMC. The fundamentals of EDM process mechanism and research works carried have been discussed with primary characteristics in Table 4. The material removal process of conductive ceramics by EDM is illustrated in Fig. 8 that shows, the two principal mechanisms i.e. are melting, evaporation and thermal spalling depending on the properties of advanced ceramics or ceramics matrix composite (CMC) and the process parameters of EDM [65] . Pitman et al. [80] have studied die-sink EDM while machining zirconia-based CMCs that contains 30 vol.% of TiN and concluded that the rapid increase in temperature subsurface crack formation. Hu et al. [67] have studied the mechanism of material removal of Ti3SiC2 ceramic during EDM. They explained that melting and decomposition were contributing in removing material during EDM of CMC. The thermal spalling also led to strength degradation for the workpiece Spalling is observed to be another important EDM material removal mechanism in ceramic-composite or conductive ceramics along with melting as shown in is much less as compared to discharge energy required in melting and [80] . conductive part of the composite [80] . Trueman et al. [82] have introduced the concept of rough regime machining for advance ceramics they explained that mechanisms