# Comparative Characterization of Different Cutting Strategies for the Sintered ZnO Electroceramics
## 1 Introduction
[17] verified the effect of various AWJ parameters towards the machining responses so that the kerf width and depth of cut can be optimized by changing the feed parameters such that the surface quality does not vary significantly. [PERSON] et al. [18] analyzed the kerf geometry with a light optical microscope and inferred that as the cutting speed increased, the kerf width subsequently decreased; thus, the kerf width increases for lower cutting speeds. [PERSON] et al. [19] reasoned that by increasing the standoff distance, the waterjet widened, which also resulted in the broadening of the taper of the cut; however, high feed pressures reduced these effects. [PERSON] et al. [20] argued that water pressure is a more significant parameter in AWJ than the transverse speeds and standoff distance to the workpiece. [PERSON] et al. [21] reported the application of AWJ in lieu of shot peening for surface treatment of weldments. Moreover, understanding the machining performance of ZnO varistors is imperative due to the prerequisite of an amorphous glassy coating along the sides as well as the metallization layer on the top and bottom surfaces; therefore, the geometric accuracy of these machined profiles is important for depositing dissimilar types of functional coatings on the same ceramic. [PERSON] et al. [1] reported on the machinability of ZnO varistors by laser-assisted milling (LAM) and suggested that machinability improved; the surface roughness and edge chipping reduction was realized by fine-tuning the laser power to 120 W. Except for this publication, the literature related to machining parameter optimization, even with conventional systems, and to the best of our research and knowledge is not available for ZnO sintered varistors. Consequently, this study investigates the abrasive waterjet machining performance of sintered ZnO varistors with respect to conventional milling to compare and obtain the parametric range (cutting mechanism), causing edge chipping or transgranular/sudden failure in hard varistor ceramics. Predominantly the edge chipping occurs due to Bi-rich spacer phase coerced out, which in turn causes high surge current catastrophically flowing through the edge of the varistors as the easier (less resistance) path rather than through the ZnO grains [22; 23]. Important parameters classified and compared within this study include the surface roughness and contour mapping, topographical imaging, and porosity count (arealfraction), between the AWJ cutting and conventional milling. The surface integrity and roughness characterization made thru the optical roughness measurement system and the scanning electron microscopy infer directly that the machined surface roughness is interlinked with the occurrence of grain pull-out during the milling and cutting operation. The porosity distribution analysis provides insight into the effect of shear forces during cutting by interlinking the difference in sintered densification with the effects causing edge chipping/roughness and grain pull-out (ZnO as well as secondary phases). Further, this work correspondingly elucidates why the conventional machining (milling) is not appropriate due to edge chipping in ZnO varistors owing to their low fracture toughness of 2.16 MPa\(\cdot\)m\({}^{0.5}\), which worsens in relatively poorly dense sintered ceramics, particularly along the edge regions.