## 3 Results and Discussion  
Figure 1 shows the XRD spectrum of the sintered Si\({}_{3}\)N\({}_{4}\) ceramics with different Y\({}_{2}\)O\({}_{3}\) and Al\({}_{2}\)O\({}_{3}\) contents. It can be seen that main peaks are characteristic \(\beta\)-Si\({}_{3}\)N\({}_{4}\) peaks for all samples, which indicates that the \(\alpha\) phase completely transformed into \(\beta\) phase during sintering. The width of the \(\beta\)-Si\({}_{3}\)N\({}_{4}\) peaks is narrow; the diffraction peak is sharp; and there are no other mixed peaks, which indicates high crystallinity of \(\beta\)-Si\({}_{3}\)N\({}_{4}\). However, when the sintering aid content increased to 12%, in addition to the \(\beta\)-Si\({}_{3}\)N\({}_{4}\) peaks, low-intensity peaks characteristic of Y\({}_{2}\)SiAlO\({}_{5}\)N were detected. The Y\({}_{2}\)SiAlO\({}_{5}\)N crystalline phase formed through the reaction of the added sintering agent with SiO\({}_{2}\) on the surface of Si\({}_{3}\)N\({}_{4}\) powder. Figure 2 shows the etched surface and sectional microstructure of sintered Si\({}_{3}\)N\({}_{4}\) ceramics and some EDS spectra. It can be seen from the surface-etching diagram that all samples have dense surfaces without obvious pores. The main phase is black, while the grain boundary phase is white. The grain boundary phase is the nitrogen oxide compound  
Figure 1: XRD diffraction patterns of Si\({}_{3}\)N\({}_{4}\) ceramic materials. generated by the reaction of sintering additives and silicon dioxide on the surface of Si\({}_{3}\)N\({}_{4}\) powder at high temperature, which is distributed in amorphous or partially crystalline form among grains or polycrystalline junctions after cooling. The microstructure of the samples is uniform and contains fine equiaxed Si\({}_{3}\)N\({}_{4}\) grains and large rod-like Si\({}_{3}\)N\({}_{4}\) grains. The two types of grains form an interwoven network structure, which is called self-reinforcing bimodal structure. From Figure 2a,c,k, it can be observed that with the increase in sintering aid content, the grain size of Si\({}_{3}\)N\({}_{4}\) ceramics increased. This is mainly due to the higher liquid-phase content during the sintering process of Si\({}_{3}\)N\({}_{4}\) ceramics with higher aid content, resulting in more uniform liquid-phase spreading and easier precipitation and growth of \(\beta\)-Si\({}_{3}\)N\({}_{4}\) in the liquid-phase-enriched region [20]. From Figure 2c,e,g, it can be seen that when the sintering aid content was the same at 10%, the grain size of Si\({}_{3}\)N\({}_{4}\) ceramics gradually decreased with the increase in Y\({}_{2}\)O\({}_{3}\) proportion. This is mainly because an increase in Y\({}_{2}\)O\({}_{3}\) proportion leads to higher liquid-phase formation temperature and increased viscosity of the liquid phase, resulting in an increase in nucleation sites and finer \(\beta\)-Si\({}_{3}\)N\({}_{4}\) grains due to competition for growth materials and spatial hindrance effects. Figure 2i,j shows the distribution of Y and Al elements in sample 1825-6.25Y-90 SN. The elements Y and Al are mainly distributed along the grain boundaries. The sectional morphology is similar to the surface and consists of a large number of interlocking \(\beta\)-Si\({}_{3}\)N\({}_{4}\) columnar crystals. With the increase in sintering aid content, the grain size of Si\({}_{3}\)N\({}_{4}\) ceramic increased, and the distribution of columnar crystals became more uniform. However, when the ratio of Y\({}_{2}\)O\({}_{3}\) to Al\({}_{2}\)O\({}_{3}\) was 6.8:3.2, the liquid-phase concentration increased, and local defects formed due to spatial hindrance. Therefore, when preparing high-density Si\({}_{3}\)N\({}_{4}\) ceramics, not only the total amount of sintering aids should be considered, but also the composition of sintering aids should be focused on.