# Ultra-Precision Cutting Mechanism of KDP Crystal in Microplastic Region via Heating Assistance
## 5 Discussion  
### Effect of Heating on Cutting Performance of KDP Crystals  
The method commonly used for judging the brittle-plastic transition is based on the surface topography. Energy detection needs to take into account the dissipation of energy. The whole experiment takes a long time, and there are problems such as large energy loss and the environment can easily interference. Therefore, the detection results are usually inaccurate. The scratches on the KDP crystal surfaces were observed via scanning electron microscopy, as shown in Figure 7a: toward the direction of the scratch, the scratch depth increases with the increase in the scratch length. In the initial stage of the nanoscratch, the scratch depth is low, and the scratches are in the plastic region, as shown in Figure 7b. As removing the KDP crystal surface by plastic slip had little effect on the surface morphology, the surface had no evident defects. As the scratch depth increased, the surface of the material became brittle (Figure 7c). The surface of the material was removed by brittle fracturing, and massive chips and pits appeared on the surface of the scratch, which affected the surface quality. The critical depth of the ductile/brittle transition is where the material transforms from the plastic to brittle region. When the scratch depth is lower than the critical depth, the surface is removed by plastic slip, without significant surface defects.  
Figure 7: Surface morphology of nanoscratch. (**a**) Overall morphology of nanoscratch. (**b**) Plastic region. (**c**) Brittle region.  
The effect of temperature on the toughness/brittle transition depth of KDP crystals is explained by Formula (1). An increase in temperature leads to a decrease in the hardness \(HK\) of KDP crystals, which leads to an increase in the depth \(H\) of the ductile/brittle transition. To study the effect of temperature on the depth of the ductile/brittle transition of KDP crystals, the surface morphology of scratches at 20 \({}^{\circ}\)C, 30 \({}^{\circ}\)C, 40 \({}^{\circ}\)C, and 50 \({}^{\circ}\)C was recorded using a scanning electron microscope, and the results are presented in Figure 8a-d, respectively. The scratch surface changed from the plastic to the brittle zone as the scratch length and depth increased. When the scratch depth exceeded the critical depth of the ductile/brittle transition, the material removal mode was brittle fracture, and block chips appeared. The first position where block cutting occurred was the critical position of the ductile/brittle transition. As shown in Figure 8a-d, the locations of brittle-plastic transformation corresponding to 20 \({}^{\circ}\)C, 30 \({}^{\circ}\)C, 40 \({}^{\circ}\)C, and 50 \({}^{\circ}\)C were sections A-A, B-B, C-C, and D-D, respectively.  
The cross-sectional view of the ductile/brittle transition position at different temperature points was obtained via atomic force microscope observation, as shown in Figure 9. When the scratch temperature of the KDP crystal surface was 20 \({}^{\circ}\)C, 30 \({}^{\circ}\)C, 40 \({}^{\circ}\)C, and 50 \({}^{\circ}\)C, the KDP crystal ductile/brittle transition depth values were 40 nm, 49 nm, 71 nm, and 89 nm, respectively. The surface ductile/brittle transition depth value continued to increase as the surface temperature of KDP crystals increased, which verifies the effectiveness of the heating-assisted processing in improving the processing performance of KDP crystals and increasing the ductile/brittle transition depth. If the machining depth remains unchanged, the heating-assisted method can effectively improve the ductile/brittle transition depth; therefore, the cutting process changes from the brittle to the plastic zone, resulting in reduced pits and cracks caused by brittle fracture and improving surface quality.