# Experimental Study on ELID Grinding of Silicon Nitride Ceramics for G5 Class Bearing Balls
## 4 Comparative Analysis of Surface Roughness
### Material Properties  
The grinding process is characterized by a relatively small DOC and a significantly higher normal force than the tangential force compared to the cutting process. It is known that material properties such as fracture toughness and flexural strength influence the fracture characteristics of both ductile mode and brittle mode grinding processes in ceramics machining. The fracture toughness of ceramic materials is typically measured using the IF (Indentation Fracture) method. The IF method calculates fracture toughness by assessing the size of median cracks formed by the indentation of a Vickers hardness indenter and the applied load. This method is primarily employed for measuring the fracture toughness of ceramics with high brittleness, as it allows for the evaluation of crack growth responses in a very small range. Grinding processes involve a small depth of cut and exhibit significantly higher normal force compared to the tangential force. Therefore, fracture toughness determined by the IF method can be effectively utilized to analyze and evaluate the machining mode characteristics of ceramic surfaces in grinding processes with a low depth of cut [15]. [PERSON] et al. [15] previously reported that specimens with high fracture toughness exhibited superior surface roughness in ELID (Electrolytic In-Process Dressing) grinding experiments using #325 and #1200 grit wheels. Fracture stress is inversely proportional to the square root of the critical crack length, meaning that larger internal voids in the material result in lower fracture stress. Additionally, when the critical crack length is the same, materials with lower fracture toughness exhibit lower fracture stress. Consequently, it was reported that grinding materials with low fracture toughness leads to an increase in surface roughness due to the higher frequency of ductile mode fractures induced by machining pressure. Although there is a regrettable omission regarding the impact of particle size and void content on surface roughness, this study is significant as it confirms the influence of fracture toughness on surface roughness. In this experiment, ELID grinding characteristics were analyzed for three specimens with varying particle size and pore content, all composed of silicon nitride ceramic material with the same composition. Avoiding internal pores in ceramics during the sintering process is challenging. As observed in Figure 2, the GPS specimens exhibit relatively larger and more numerous internal pores compared to the HP specimens, resulting in lower flexural strength and fracture toughness in the GPS specimens. Figure 11 provides a graph comparing fracture toughness, surface roughness (Ra), and the range of surface roughness (\(\Delta\)Ra). The HP specimen, with the highest fracture toughness, shows the lowest surface roughness Ra, demonstrating an inverse correlation between fracture toughness and the range of surface roughness \(\Delta\)Ra. Figure 5: Surface roughness measurement results by grit size of grinding wheels. Figure 8: Comparison for range of surface roughness (Ra) using #2000 wheel for HP specimen. Figure 6: Comparison for range of surface roughness (Ra) using #325 wheel for HP specimen. Figure 7: Comparison for range of surface roughness (Ra) using #1200 wheel for HP specimen. Figure 11: Comparison of surface roughness, range of surface roughness, and fracture toughness by specimen. Figure 10: Comparison of surface roughness by abrasive. Figure 9: Comparison of surface roughness by grit size. The increase in surface roughness Ra and the wider range of surface roughness \(\Delta\)Ra with lower fracture toughness can be attributed to the facilitated occurrence of brittle mode fracture. Figure 12 compares the measured results of flexural strength, surface roughness Ra, and Rmax based on the specimen type. The HP specimen with the highest flexural strength exhibits the lowest surface roughness (Ra).