# Numerical and Experimental Research on the Laser-Water Jet Scribing of Silicon
## 4 Results and Discussion
### Morphological Evolution of Silicon  
To investigate the morphological evolution of silicon under LWJ machining, numerical simulations using geometry deformation module were also carried out based on Section 4.1. The results are shown in Figure 10. Moreover, single-spot ablation experiments were put forward using the same machining parameters with numerical simulations except for the applied laser pulses. The pit depths were measured, as listed in Figure 11.  
Figure 8: Temporal evolution of tensile stress and compressive stress. Figure 10: Morphological evolution of substrate under different laser pulses. Figure 9: The spatial distribution of (**a**) maximum tensile stress and (**b**) maximum compressive stress. As can be observed from Figures 10, 11, and 12, the ablation depth increased with applied pulse number. However, an obvious difference could be discovered between the simulation and experimental results. According to the simulations, the first laser pulse resulted in the smallest ablation depth(215 nm). The ablation depth induced by the second laser pulse increased to approximately 260 nm. The following three pulses led to the same ablation depth (265 nm). However, the ablation depth per pulse decreased significantly with increasing applied pulses in the experiments. The main reasons could be analyzed as follows:  
In the simulations, it was assumed that the material was peeled off clearly from the substrate once its temperature exceeded the melting point. The laser beam was capable of interacting with the substrate continuously, and this process was accompanied by the scouring of waterjet. The next ablation step was not disturbed. Meanwhile, in the experiments, the formation of the micro-pit, slag, and gas bubble induced by the initial several laser pulses altered the transmission behavior of the laser beam in the following ablation process, as demonstrated in Figure 13. In specific, the pit enabled the deposition of water, during which the impingement of high-speed LWJ resulted in significant turbulence. The gas bubble was strained within the deposited water and the slag was stirred, forming a \"suspension\". The incident laser was reflected, refracted, and absorbed by the \"suspension\", causing the serious energy dissipation. The subsequent ablation was weakened  
Figure 11: Analyses of ablation depth based on numerical simulation. Figure 12: (**a**) Typical pit morphology at pulse number of 300 and (**b**) evolution of ablation depth under different pulses. correspondingly. The scouring of waterjet could suppress this phenomenon to some extent, but still could not eliminate it completely. Taking Figures 5-13 into consideration, the ablation of silicon substrate during LWJ machining could be classified into four steps:  
1. In the initial stage of laser pulse incidence, the laser photons interacted with the silicon lattice. The vibration of the silicon lattice led to the rapid heating up of the surface layer. Thermal-induced tensile and compressive stresses started to generate in the machining region. 2. During the laser pulse incidence, thermal conduction occupied a principal position compared with thermal convection induced by the scouring effect of the waterjet. The temperature of the surface layer increased in a nonlinear way. Tensile and compressive stress gradually concentrated on the edge of the substrate. Part of the surface layer was ablated. 3. At the end of laser pulse, the tensile and compressive stresses reached the peak value, which resulted from the compound action of the laser-induced thermal effect and the waterjet-induced scouring effect. However, these two stresses could hardly modulate the substrate ablation process. The micro-pit was formed temporarily. 4. During the interval of laser pulse, the waterjet scoured the machining region, which cooled down the local temperature surrounding the pit significantly. The tensile and compressive stresses gradually released to the initial state.