# Comparative Characterization of Different Cutting Strategies for the Sintered ZnO Electroceramics
## 3 Results and Discussion
Figure 4:4 Quantitative porosity analysis on the top, side 1, of the cross-section after AWJ cutting. Figure 4:5 Porosity distribution analysis on the bottom, side 2, of the cross-section after AWJ cutting at \(v\) = 160 mm/min. Figure 12 shows the quantitative porosity distribution in the top, side 1, of the AWJ cut cross-section, indicating high number of pores in the range of 1-2 \(\upmu\)m size within the microstructure. Very large pores above 7 \(\upmu\)m were sparse, which indicates these pores were present within a uniform matrix of normal sized ZnO grains in the range of 5-15 \(\upmu\)m. A more uniform microstructure retaining edge taper and minor edge chipping was observed, as few larger pores were distributed close to the edge. For sintered ZnO ceramics under large, applied shear forces during the conventional machining, the edge chipping was expected to be higher in the side where the microstructure was more uniform. Likewise, the grain pull out was further perceptible in the region of the microstructure where the distribution of secondary phases was higher with the ZnO matrix. Figure 12 implies that this top side experienced higher pressure during the compaction stage, and later with normal ZnO grain growth, the present pores were eliminated to a large extent. However, applied shear forces during machining and AWJ cutting caused larger brittle ZnO grains expulsion at the edges, whereas small secondary phases were pulled out from the middle of the cross-section. In Figure 13, it can be seen that a similar porosity distribution in the range of 1-3 \(\upmu\)m for the bottom, side 2, was approximately two-fold higher than the top, side 1. Not only were the larger pores present within the bulk of the microstructure, but although minor, the edges were also not as uniform as the top side. The pores' areal distribution for the bottom, side 2, elucidated high edge chipping and internal grain pull-out in less dense parts of the sintered ceramic. This suggests that the variation in green compaction pressures and processing conditions had a significant influence on the microstructural integrity and resulting mechanical properties. Typically, with lower green density, the sintering can cause particle coalescence only to an extent such that internal porosity still remains widely distributed [31,32]. In this case, it is more apparent from Figure 13 that larger size and regular grain shaped pores had formed, which suggests that ZnO grains were pulled out of the matrix instead of only the smaller secondary phases in Figure 12 within a denser matrix. Highly non-uniform cross-sections and edges effectively rendered these functional ceramics useless for high transient or surge protection applications, as the surface metallization and circumferential insulation were non-uniformly deposited on the varistor. Large lateral shear loads produce very rough edges, which suggests AWJ cutting as a more appropriate surface peening and circumference tapering tool than conventional milling and machining. Non-uniform edges present persistent difficulty in applying dielectric epoxy or glass layers to shield the varistor during assemblage. Moreover, high surface roughness in center faces causes poor metallization deposition, resulting in surge protectors typically failing impulse tests in quality assessment rather than going onward with real-world utility [26]. The AWJ cutting with feed rates 160 mm/min and up to 440 mm/min yielded very similar S\({}_{\text{a}}\) values and the edge radius profiles in Figure 4:6 However, the feed rate above 500 mm/min and up to 1060 mm/min from Figure 10 prove that the obtained S\({}_{\text{a}}\) and S\({}_{\text{10Z}}\) values are far worse than controlled milling. On the contrary, the average surface roughness (S\({}_{\text{a}}\)) maintained by AWJ cutting at 2.82 \(\upmu\)m is exceptionally better than conventional machining counterparts >3.69 \(\upmu\)m, and thus this report justifies their industrial potential prior to surface and edge preparation.