## 3 Results and Discussion  
### Surface Roughness and Topography  
This part of the study explores the influence of the addition of the oil film-assisted ECDM process on the cutting surface roughness, which needs to reflect the changing trend of the surface roughness value under different voltages, and then compare the roughness value with or without the assistance of discharge process. There are many evaluation parameters of roughness, and selecting one evaluation parameter is convenient for comparison. It is generally believed that Ra is most commonly used to evaluate roughness and can fully reflect the surface micro-geometric characteristics. Therefore, Ra is selected as the parameter for evaluating roughness in this paper. The surface roughness was measured by a portable metal surface roughness tester, TR100. Surface roughness of the machined samples was measured in two directions: one was parallel to the diamond wire, the other was perpendicular to the diamond wire. After three measurements, when no voltage was applied, the surface roughness of the machined sample by diamond wire sawing in the parallel direction was, respectively, 0.441 \(\upmu\)m, 0.452 \(\upmu\)m, 0.463 \(\upmu\)m, and the average value was 0.452 \(\upmu\)m; the perpendicular direction was, respectively, 0.597 \(\upmu\)m, 0.692 \(\upmu\)m, and 0.616 \(\upmu\)m, and the average value was 0.627 \(\upmu\)m. When diamond wire sawing combined with oil film-assisted ECDM, surface roughness values in the parallel direction and perpendicular direction under different voltage applications are shown in Table 4. The relationship between surface roughness and applied DC voltage is shown in Figure 7. It can be seen that the surface roughness Ra achieves the minimum value of 0.485 \(\upmu\)m in the parallel direction and 0.732 \(\upmu\)m in the perpendicular direction at 46 V. Compared with the direct cutting of diamond wire saw, the roughness increases by 7.30% in the parallel direction and 16.7% in the perpendicular direction after 46 V is applied. It also shows that the surface roughness increases with the voltage. When the applied voltage is 58 V, the roughness in the parallel direction and the perpendicular direction increases by 83.5% and 36.9%, respectively, compared with that when the applied voltage is 46 V. The results support that the integration of spark discharge with diamond wire sawing increases surface roughness. The reason is that the vibration of the diamond wire causes cutting lines on the machined surface (Figure 8). SEM images of the machined surfaces by diamond wire sawing with and without the assistance of oil film-assisted ECDM are shown in Figure 9. During diamond wire sawing, the material was removed by the ductile regime method and spalling of work material left micro craters on the machined surface [1] (Figure 9a). Figure 9b shows the machined surface by diamond wire sawing combined with oil film-assisted ECDM under a voltage of 49 V. Compared to Figure 9a, the ductile regime area reduced, and the number of micro craters increased. When the voltage increased to 55 V, the craters became predominant on the machined surface (Figure 9c). With a DC voltage of 58 V, the craters became so dense that it is difficult to identify whether the ductile regime existed on the machinedsurface (Figure 9d). The SEM images indicate that the increase of DC voltage improves the spalling of Al\({}_{2}\)O\({}_{3}\) ceramic. The reason is that the conflicting compressive and tensile stresses were generated by the spark discharges and the cooling electrolyte, which also existed in EDM of conductive Al\({}_{2}\)O\({}_{3}\) ceramic composites [30]. The severe stress differential reduced the strength of the Al\({}_{2}\)O\({}_{3}\) ceramic, which facilitated the spalling of Al\({}_{2}\)O\({}_{3}\) ceramic by the moving diamond wire. The spalling of Al\({}_{2}\)O\({}_{3}\) ceramic produces micro craters on machined surfaces, resulting in increased surface roughness.