## 4 Results and Discussions
### Mechanism of CMCs Machining with LWJ  
Compared to conventional laser machining, the main characteristic of LWJ is that a compact section of the water jet is essential to deliver the laser energy as an optical fiber. Moreover, although the length of the compact section can reach 30-100 mm, it is only a non-functioning jet type. Once the water jet contacts the workpiece, the situation changes. Especially going deep inside the material, the stable delivery of the water jet would be strongly disturbed by the complex processing environment, which would further affect the normal processing of LWJ. Evidence of this phenomenon has been demonstrated in Section 4.2. As shown in Figure 13a, the material of the top surface has reached its melting point by laser ablation. In general, sublimation arises with high temperatures. However, due to the scouring effect of the water jet, a sublimation steam atmosphere and recast layer were not generated. Finally, the slags would be discharged from the substrate material by the water flow. Previous research has shown that a drainage circle in the millimeter range will be generated on the material surface by a high-speed water jet [16]. The water flow in this circle could maintain a fairly high speed which provided an effect of sweeping out the slag. Under the circumstances of single-row scribing, the size of the processed feature corresponds roughly to the water jet. Hence, the high-speed water flow and slags would impact the wall surface of the groove in which the stability of the water jet is destroyed by sputtering. The main consequence of the sputtering effect is that only a few laser energies in the central area could be delivered onto the material. However, a limit value of groove depth would ultimately be reached by means of single-row scribing. In simple terms, without the discharging of the slags and water flow from the previous stage, the LWJ can  
Figure 11: Extreme scribing depth test of LWJ. (**a**) The variation in scribing depth with time; (**b**) the extreme scribing depth.  
hardly afford the full effect of machining. Furthermore, the complex interior structure of CMCs can also restrict the processing capacity of LWJ.  
In contrast, the multi-row scribing strategy not only offers enough drainage space for water flow and slags but also controls the \"near wall effect\" [28]. As demonstrated in  
Figure 12: Thorough cutting of CMCs with multi-row scribing strategy. (**a**) Schematic of multi-row scribing method; (**b**) cross-section of cutting surface of CMCs; (**c**) partially enlarged cross-section morphology; (**d**) EDS test using sweep mode at the machined surface.  
Figure 13: The distinction between (**a**) single-row and (**b**) multi-row scribing.  
Figure 13b, the slags could be smoothly discharged with the water flow from both sides of the groove, allowing each individual laser pulse to be utilized to the maximum extent. Meanwhile, the groove with a size much larger than the water jet provides cushion space for the sputtering of slags and water flow. As a result, the probability of the water jet being impacted is greatly reduced, and the stable machining of LWJ is guaranteed. It should be pointed out that the \"near wall effect\" can also hardly be avoided using the multi-row scribing strategy. The water jet would be inevitably distributed at both sides of the groove, which results in a processed feature--ultimately with a slight slope. It may be concluded that a processed feature with better quality can be realized by a greater holistic path width strategy. However, processing efficiency will certainly be reduced by this method. As a whole, whether the machining capacity of LWJ could be applied adequately, the stability of the water jet has a leading status. The machining consistency of each individual laser pulse can be ensured by an excellent, non-interfering processing scenario.