# Comparative Characterization of Different Cutting Strategies for the Sintered ZnO Electroceramics
## 2 Experimental Methodology  
The initial value of the edge radius for the pristine non-machined ZnO samples could be averaged from \(\sim\)110 to 125 \(\upmu\)m (surface roughness parameters (arithmetical mean height--S\({}_{\text{a}}\) = 1.37 \(\upmu\)m and ten-point height--S\({}_{10Z}\) = 25-40 \(\upmu\)m), as shown in Figure 1 with the geometric specifications of the samples. The ZnO varistors' geometric specifications are illustrated in Figure 1, having a nominal diameter of 42 mm, thickness of 12.6 mm, and edge radius r \(\leq\) 130 \(\upmu\)m, and the ten-point surface roughness S\({}_{10Z}\) was categorized in a range from 25 to 40 \(\upmu\)m.  
The ZnO ceramics were commercially produced at Varsi d.o.o Slovenia and were developed with the fixed lower-plunger uniaxial pressing of green compacts prior to the sintering operation above 1200 \({}^{\circ}\)C, and thus the two sides could have different milling and surface roughness features; hence, the color markings are presented as shown in Figure 2. The relative green density was assumed to be \(>\)60% after uniaxial pressing, whereas the sintered samples had relative density above 95%, on which further milling and AWJ cutting operations were performed. The top side marked with red color shows that this section directly received the compaction pressures during uniaxial pressing, which specifies a relatively higher green density on the top (red) part, whereas the blue (bottom) part designates the section away from the pressure plungers. These descriptive color markings of red and blue sides are repeatedly used in the results and discussion part. Figure 1: Geometry of ZnO varistor used in milling and AWJ cutting operations. Conventional milling was performed using a CNC Mori Seiki SL 153 milling machine with a polycrystalline diamond (PCD) type cutting tool (C-13581) having a single edge as shown in Figure 3. The following milling parameters were employed to machine the surfaces of the ZnO varistors: axial depth of cut (a\({}_{\text{p}}\)) = 0.1 mm and width of cut (a\({}_{\text{e}}\)) in relation to the diameter of the cutter = 5 mm; feed velocity of 400 mm/min; and spindle speed (n) of 4000 rpm/min (V\({}_{\text{C}}\) = 100 m/min and f\({}_{\text{Z}}\) = 0.04 mm). All the milling operations were carried out in dry cutting conditions. All the ZnO to be milled samples were mounted and fixed to the same tightness on the CNC unit. The abrasive waterjet cutting was performed with the Bohler ECOTRON 403 system equipped with a 2652A JetMachining Center high pressure water pump, and Garnet mesh-80 abrasive (Jetstar International STAN/80/1000). The schematic illustration of the abrasive waterjet machining setup utilized in this study is presented in Figure 4. Cutting parameters with AWJ include feed rate range of 120-1060 m/min, garnet abrasive flow rate of 0.45 kg/min, standoff distance (nozzle height to specimen) \(f\) = 2 mm, focused/focusing nozzle diameter of 0.8 mm, water nozzle diameter of 0.3 mm, and water pressure at 300 MPa (3000 bars). The feed velocities (\(v\)) 120 mm/min and 160 mm/min yielded the same results, which why throughout the text, the slowest feed rate option is 160 mm/min. Figure 3: Schematics of milling operation on ZnO varistor. Figure 2: The division of ZnO ceramics turning operations to two sides, illustrative of uniaxial pressing of green compacts prior to sintering operation. The waterjet cut ZnO specimens in their cross-section as shown in Figure 5 were grinded by 500 and 1000 grit SiC papers on rotary units and then polished with 500 nm diamond paste residue on the velvet cloth to prepare them for microscopical examination. The assessment of surface roughness and contour mapping was made with an Alicona InfiniteFocusSL measurement system in the form of 3D scans of the surface and edges, generated in two passes. The sintered ZnO workpiece was attached in a tilted position within the specimen holder and top-down optical light illumination.