## 4 Results and Discussion
### Comparative Processing Experiments  
The hot-pressing tool and gel abrasive tool were used to process the sapphire. Some parameters in the above experiments were selected to carry out comparative experiments of the two abrasive tools. The feed rate was 8 \(\upmu\)m/min or 10 \(\upmu\)m/min, and the grinding depth was 12 \(\upmu\)m or 16 \(\upmu\)m. The processing results are shown in Table 6. The surface roughness of the sapphire substrate processed by the gel abrasive tool in each group was better than that processed by the hot-pressing abrasive tool. The tool wear rate and the MRR results are shown in Table 7; the tool wear rate was measured as a function of the thickness change of the abrasive tool. The results show that the wear rate of the hot-pressing abrasive tool was larger; the main reason is that the abrasive holding ability of the gel abrasive tool was much stronger than that of the hot-pressing abrasive tool, and the material removal rate of the hot-pressing abrasive tool was slightly larger than that of the gel abrasive tool due to the high concentration of abrasives, which promoted the solid-phase chemical reaction. Figure 14 shows the thermogram of different tools after machining for 5 min. From the figure, it can be seen that the average temperature of the gel abrasive tool was higher than that of the hot-pressing tool; however, the heat distribution was more uniform in the gel abrasive tool, and the hot-pressing tool had the problem of local overheating, mainly due to its uneven microstructure causing uneven wear on the surface of the tool during grinding. Therefore, the area with little wear applied most of the grinding force, resulting in excessive local grinding heat of the hot-pressing abrasive tool. On the other hand, the wear of the gel abrasive tool was more even, the grinding force on the face of the tool was more evenly distributed, and no local overheating occurred. Figure 14: Thermogram of different tools (after machining 5 min): (a) gel abrasive tool; (b) hot-pressing abrasive tool. Figure 13: Surface topography of sapphire under different grinding depths: (a) 10 \(\upmu\)m; (b) 12 \(\upmu\)m; (c) 14 \(\upmu\)m; (d) 16 \(\upmu\)m; (e) 18 \(\upmu\)m.  
Figure 15 shows the sapphire machined by the gel abrasive tool. Figure 16 shows the flatness of the sapphire after processing by the two abrasive tools, which is closely related to the surface shape accuracy of the abrasive tool. As the machining progressed, the flatness of the abrasive tool was copied to the sapphire. It can be seen that the flatness of the sapphire after processing by the gel abrasive tool was slightly more accurate compared to the hot-pressing abrasive tool, mainly because the existence of the binder in the gel abrasive tool had a better ability to hold the abrasive particles, thus minimizing damage, whereas the binding energy of the abrasives in the hot-pressing tool was not strong enough to prevent tool damage, which consequently affected the flatness. Figure 17b shows the microtopography of the sapphire after CMP. It can be seen that there were scratches on the sapphire processed by the hot-pressing abrasive tool, because the agglomerated particles in the tool produced a \"plowing\" effect on the sapphire during processing. Furthermore, the mechanical effect of the agglomerated particles was too strong, leaving deep scratches on the sapphire surface, which were difficult to remove after CMP. Although there were obvious processing marks left by the gel abrasive tool on the sapphire, they were easily removed after CMP.