# Experimental Study on ELID Grinding of Silicon Nitride Ceramics for G5 Class Bearing Balls
## 2 Materials and Methods
### Specimens of Silicon Nitride  
Three types of silicon nitride ceramic specimens were prepared for the experiment: HP, GPS 1 hr, and GPS 6 hr. The hot-pressed (HP) specimen has a composition of Si\({}_{3}\)N\({}_{4}\) + Y\({}_{2}\)O\({}_{3}\) (2 wt.%) + MgO (5 wt.%) and was sintered under high-temperature and high-pressure conditions at 1700 \({}^{\circ}\)C for 2 h. The gas-pressure-sintered (GPS) specimens were created by subjecting the HP specimens to additional gas-pressure sintering at 1900 \({}^{\circ}\)C for 1 h and 6 h, resulting in two types of specimens: GPS 1 hr and GPS 6 hr. These specimens were prepared to analyze the effect of material properties on the ground surface. In general, as the gas-pressure sintering time increases, the particles tend to agglomerate, and the porosity content becomes more significant. Figure 1 shows the surface images of each specimen captured using an FE-SEM device. In the HP specimens, it is difficult to observe the presence of pores, and the particle size is relatively small. However, as the gas-pressure sintering time increases in GPS specimens, it can be observed that the particle sizes become larger, and the porosity content ratio increases. Figure 2 shows the results of measuring material properties and sintered particle sizes for each specimen. The particle size of the HP specimen was the smallest, and its fracture toughness and flexural strength were higher than the others. The starting Si\({}_{3}\)N\({}_{4}\) powder (E10, UBE Corporation, Japan, D\({}_{50}\) = 0.3 \(\upmu\)m) was mixed with two sintering additives, MgO (Sigma-Aldrich, D\({}_{50}\) \(<\) 0.1 \(\upmu\)m) and Y\({}_{2}\)O\({}_{3}\) (Grade C, [PERSON], Germany, D\({}_{50}\) = 0.9 \(\upmu\)m), via the ball-milling process. The mixing media was Si\({}_{3}\)N\({}_{4}\) ball (Nikkato, Japan, Diameter = 5 mm), and the liquid medium for mixing was anhydrous ethanol. The microstructural difference was from the material loss by the reaction between Si\({}_{3}\)N\({}_{4}\) and grain boundary phases, SiO\({}_{2}\)-based amorphous glass. At high temperatures over 1500 \({}^{\circ}\)C, Si\({}_{3}\)N\({}_{4}\) is prone to react with SiO\({}_{2}\) to produce a gaseous phase, which induces the material loss from the Si\({}_{3}\)N\({}_{4}\) body [32]. As a result of the prolonged heat treatment at a higher temperature, the above reaction also keeps going to further weight loss. The as-hot-pressed Si\({}_{3}\)N\({}_{4}\) sample was sintered under the confined mold to prevent outgassing of SiO(g) and N\({}_{2}\)(g), a by-product of the reaction (1). However, during the heat treatment in the GPS furnace at 1900 \({}^{\circ}\)C, the Si\({}_{3}\)N\({}_{4}\) samples are exposed to atmosphere without any protection. Hence, the material loss occurred via producing the resultant gases, so a number of pores are generated in the sintered Si\({}_{3}\)N\({}_{4}\) bodies. Moreover, the weight loss of the heat-treated Si\({}_{3}\)N\({}_{4}\) was measured as 0.88, 1.76, and 2.49 wt % for increased soaking duration from 1, 3, and 6 h, respectively. The trend of increasing weight loss is consistent with the expected result from the reaction (1), and the microstructural evolution of increasing the number of pores can be rationalized. For sintered Si\({}_{3}\)N\({}_{4}\) ceramics, the mechanical properties such as flexural strength and fracture toughness are strongly dependent on the microstructure. Typically, the distinct bimodal distribution of grain size is advantageous for enhancing the flexural strength as well as the fracture toughness [33]. The HPed Si\({}_{3}\)N\({}_{4}\), with higher strength and toughness, has distinct bimodal distribution of grain size, whereas the microstructure of heat-treated Si\({}_{3}\)N\({}_{4}\), with lower strength and toughness, shows all the enlarged grains with a thick columnar shape.