# Comparative Characterization of Different Cutting Strategies for the Sintered ZnO Electroceramics
## 1 Introduction  
Sintered ZnO is a class of functional material that is typically brittle with low fracture toughness, and final shaping requires careful handling and machining [1]. ZnO varistors are usually applicable as high surge current protective devices. Sintered varistors require smooth surfaces for the applying of metallization layers (Al, Ag paste) to join electrical leads/interconnects and contoured edges to adjoin the glass glaze along the sides [2]. Lapping widely used industrially in the finishing of inelastic ZnO ceramics and uncontrolled chipping or edge roughness in the finished products along with the high expense of machining costs and tool blunting necessitate alternative solutions [3, 4]. Furthermore, the material removal rate (feed rates and turning velocity) are kept drastically low, which implies that current machining practices are poorly sustainable [1]. The conventional material removal methods imply large machining forces induced in the ZnO ceramic due to tool work piece contact, which results in vibration and chattering of the work piece. The highly brittle nature of ZnO ceramic exhibits a probabilistic tendency of fracture that leads to material damage and may also result in poor surface/edge finish. Moreover, due to chemical inertness and semiconducting behavior of ZnO, adopting suitable non-conventional machining is rather complicated. On the finished ZnO varistors, the roughened or shear force induced chipped edges are highly unfavorable for the functional properties; thus, care must be adopted in developing smooth surfaces and tapered edges. Alternatively, the micro-machining approach can be favored over the conventional machining practices for developing ultra-precise ceramics with well-defined cutting geometry (sharp and defined edges with radius r \(<\) 15 \(\upmu\)m). Typically, the industrial waste of ZnO varistors failing during manufacturing stages, mechanical processing, or electrical impulse testing is estimated at approximately 15% of the total production. One way to enhance the machineability of the brittle materials is thermal activated machining of the ceramic so the heat is applied to entice localized softening in the areas to be machined [5; 6; 7]. Abrasive waterjet (AWJ) cutting represents an alternative approach to cutting very hard and brittle ceramics to good precision by the co-utilization of high-pressure water and (micro)abrasive particles for wet blasting of surfaces [8]. The added benefit of abrasive waterjet cutting apart from excellent tolerances also implies no inherent change in the surface microstructure due to lack of heat-affected zone (HAZ) formation [8; 9; 10; 11]. It is conceivable that the AWJ method allows contoured edges, bevels, sharp corners, pierced holes, and profiles to be created with negligible inner radii [8; 11; 12]. The width of the cut (kerf) can be regulated by exchanging the parts within the nozzle, as well as varying the type and size of the abrasive. The kerf formed during nominal abrasive cutting lies in the range of 1.0-1.3 mm and can be made as narrow as 0.51 mm. AWJ cutting provides the capability of attaining accuracy down to 0.13 mm with repeatability up to 0.025 mm in subsequent passes [11; 13; 14]. The key factors including the AWJ cutting process include water jet pressure (available kinetic energy to cut), traverse rate (slower scan rate allows for higher volume fraction of abrasives on a given surface area that leads to higher surface finish and vice versa), abrasives (harder and larger sized particles support optimal cutting, necessary for ceramics and hard composites), abrasive flow rate (optimization of rapid material removal rate with respect to surface finish), standoff distance (between target and nozzle, defines kerf profile), and jet impingement angle (change in jet attack angle and directly influences target erosion/cutting) [15]. [PERSON] et al. [16] suggested that high pressure is more efficient in abrasive processes against the same power consumption. [PERSON] et al.