# Comparative Characterization of Different Cutting Strategies for the Sintered ZnO Electroceramics
## 3 Results and Discussion
For Sb/Bi ratios < 1 in the composition, the liquid phase appeared due to eutectic melting (ZnO-Bi\({}_{2}\)O\({}_{3}\)) at 740 \({}^{\circ}\)C, while for Sb/Bi ratios > 1, the IG phase adhered to the P phase, and the S-rich liquid phase formed after higher temperature decomposition. The P-phase decomposition to the S-phase and the IG phase created a high degree of non-linearity characteristics in MOVs. Undoubtedly, without Sb\({}_{2}\)O\({}_{3}\), the Schottky barrier height was reduced, which upsurged the donor density of charges, causing leakage current increase and lessening of non-linearity characteristics. These dopants intuitively decreased the mobility of ZnO-Bi\({}_{2}\)O\({}_{3}\) GBs due to the pinning effect of the spinel grains, therefore augmenting the surge shielding [22; 23; 25; 28; 29]. The free sintering in air caused a very rough microstructure, as shown in the SE image in Figure 6c. By observing this free air sintering in BSE mode, Figure 6d confirms that the secondary phases were scarce and homogeneously distributed in the non-flat ZnO matrix exposed to air. This variation in microstructure due to sintering most certainly may cause different machining characteristics. Therefore, a thorough analysis on material removal/cutting performance changes in the two sides, given that green compaction behavior was different, was carried out in this work. The qualitative difference of edge surface profiles after milling and AWJ cutting is shown in Figure 7. Conventional milling developed quite rough edges, as can be seen in Figure 7a and at higher magnification in Figure 7b, whereas the abrasive waterjet (AWJ) cutting yielded smoother surface finishes at the circumferential segments of the varistor, as seen in Figure 7c,d. The average S\({}_{a}\) and S\({}_{10Z}\) values for the milling experiment yielded 3.69 and 103 \(\upmu\)m, respectively. Further characterization was enabled by an Alicona 3D measurement system for variations in the surface topography, roughness, and slice edge features comprehensively based on different feed rates of AWJ cutting and most optimal milling results only. This generic comparison can help to devise a cutting strategy for avoiding a qualitatively poorer surface finish at the edges, which is not acceptable for the deposition of glass glaze. A comparative edge profile assessment was also carried out following the SEM analysis of the milled and waterjet cut samples, as illustrated in Figure 8 based on variations in height (z-axis) per cross-sectional width. The edge radius of the ZnO sintered varistor prior to machining persisted in the range of 100-125 \(\upmu\)m, as shown in Figure 1. With controlled milling, the edge radius was reduced to a size range 83.5 \(\upmu\)m, determined with  
Figure 7: The edge machining difference in SE imaging mode of (**a**) conventional milling and (**b**) at higher magnification of depression area, and (**c**) optimal abrasive waterjet (AWJ) cutting at \(v\) = 160 mm/min, and (**d**) smooth AWJ cross-section magnified to 1000\(\times\). the Alicona system and presented in Figure 8a. The waterjet cut samples provided two extremities of operation between the slowest feed rate of 160 mm/min and the most rapid at 1060 mm/min. At the most rapid conditions, shown in Figure 8b, the edge trim was even worse than the optimally milled sample, with radius values exceeding 164.5 um on average. However, at feed rate \(v=160\) mm/min, the resultant slice roughness at the edges was trimmed down to 65 um. This implies unsuitability of AWJ cutting at higher feed rates, as the edge regions and taper did not adhere to the principal electrical impulse testing requirements [30]. Figure 8: The edge profile comparison between the (**a**) optimally milled and (**b**) the coarsest edge with feed rate 1060 m/min, and (**c**) the best AWJ cut ceramic at the feed \(v=160\) mm/min. Further on, the comparison of surface roughness was also made by investigating the topographical color scans.