# Experimental Study on ELID Grinding of Silicon Nitride Ceramics for G5 Class Bearing Balls
## 3 Ground Surface Morphology
### Ductile Mode and Brittle Mode  
The ground surface of the #325 grinding wheel on the HP specimen (Table 2, images a,d) exhibited a relatively higher proportion of ductile mode machining surfaces. In contrast, for the GPS specimens (Table 2, images b-f), the proportion of brittle mode machining surfaces gradually increased with longer sintering times. The traces of peaks and valleys were more pronounced on the ground surface of the diamond abrasive (Table 2, images a-c) compared to the cBN abrasive's ground surface. A similar observation was made for the #1200 grinding wheel's ground surface; however, the traces of peaks and valleys were much fainter compared to #325. For the #2000 grinding wheel, it was challenging to conduct a comparative analysis of the machining modes using optical microscopy; therefore, confirmation was made through FE-SEM imaging. Tables 3 and 4 provide a compilation of FE-SEM images for the #2000 grinding wheel's ground surface, taken at different magnifications. Table 3 (image a) and Table 4 (image a) display the ground surface of the HP specimen processed with a diamond abrasive. As described in Section 2.1, the HP specimen has a relatively small average particle size and minimal porosity, while the GPS specimen exhibits an increase in average particle size and higher porosity with longer gas-pressure sintering times.  
In the images of the ground surface processed with a diamond abrasive (Table 3 (image a) and Table 4 (image a)), a smooth cutting surface can be observed. On the other hand, when the cBN abrasive was used (Table 3 (image b) and Table 4 (image b)), numerous fracture marks were observed. It was evident that their size was somewhat larger compared to the size of the inherent pores in the material. The cracks observed on the cBN abrasive's ground surface of the HP specimen are likely attributed to surface cracks generated during brittle mode machining. The surface cracks seen on the cBN abrasive's ground surface (Table 3 (image d,f) and Table 4 (image d,f)) of GPS specimens are much larger compared to the pore size, suggesting that they are likely due to cracks resulting from brittle mode machining. Even on the ground surface of the GPS specimen processed with a diamond abrasive (Table 3 (image c,e) and Table 4 (image c,e)), the fine marks are observed, indicating a high possibility of exposing the pores generated during the gas-pressure sintering process. This suggests that the pores formed during the gas-pressure sintering process are likely to be exposed after machining with the diamond abrasive. This inference is supported by the fact that the size of the fine marks observed on the diamond abrasive's ground surface (Table 4 (image c,e)) closely matches the size of the pores observed in Figure 1 for the GPS specimens, which are approximately 1 micrometer in diameter. The condition of the ground surfaces from the analysis of ground surface morphology has been compared and examined. The diamond grinding wheel demonstrated a ductile mode machining, resulting in smooth cutting surfaces where the traces of the abrasive path were faithfully replicated. In the case of relatively large grit sizes of diamond grinding wheels (#325 and #1200), the ground surface exhibited more distinct peaks and valleys compared to the cBN abrasive's ground surface, and the proportion of ductile mode surfaces was higher than the cBN wheel. For the diamond #2000 grinding wheel, it was difficult to detect traces of abrasive paths, and the majority of the ground surface was processed in the ductile mode.