## 4 Results and Discussions  
### Spatial and Temporal Distribution of Temperature during LWJ Machiting  
On the basis of Section 3, in this study, the temperature distribution of CMCs under LWJ was investigated. The numerical simulation was carried out with a configuration of 1 mJ laser pulse energy, 86 ns pulse duration, 20 Mpa water pressure, 60 \(\upmu\)m nozzle diameter (50 \(\upmu\)m jet diameter), and 293.15 K initial temperature. To conveniently measure the temperature field on the CMCs surface, a series of characteristic time nodes were adopted to describe the evolution of the transient temperature of CMCs in one 1/f period, as shown in Figure 5. With the incidence of the laser pulse, a steep temperature increasing to 5531.1 K could be realized in 4.3 \(\times\) 10\({}^{-8}\) s (\(t\) = \(tp/2\)), then the temperature declined gradually to 5214.1 K in 8.6 \(\times\) 10\({}^{-8}\) s (\(t\) = \(tp\)), as shown in Figure 5,b. Then, the laser pulse ends and is followed by the complete cooling stage. Without the input of laser heat flux, the maximum temperature drops rapidly to 2990 K (approximate melting point) in 1.72 \(\times\) 10\({}^{-7}\) s (\(t\) = 2 \(\times\) \(tp\)) and 2656.6 K in 3.44 \(\times\) 10\({}^{-7}\) s (\(t\) = 4 \(\times\) \(tp\)), as demonstrated in Figure 5,d. These results indicate that CMCs removal only takes place during the laser pulse duration and also the following one. Interestingly, a faster temperature drop appears along the direction of the SiC fiber, which states an anisotropic thermodynamic property of CMCs [23]. As expected, there is an increasing trend of this kind of property in Figure 5.f. Moreover, annular distributions of temperature are formed gradually. In addition, the  
Figure 4: Numerical simulation model of CMCs (**a**) geometry and (**b**) mesh generation. maximum temperature in \(t\) = 5.16 \(\times\) 10\({}^{-7}\) s (\(t\) = 6 \(\times\) \(tp\)) and \(t\) = 8.6 \(\times\) 10\({}^{-7}\) s (\(t\) = 10 \(\times\) \(tp\)) is reduced to 2423.3 K and 2152 K, respectively. According to the definition of the thermal damage threshold (T = 1873.15 K) studied by [PERSON] [24], the temperature distribution in 1.38 \(\times\) 10\({}^{-6}\) s (\(t\) = 16 \(\times\) \(tp\)) was analyzed. As shown in Figure 5g, a low temperature is concentrated in the central zone with a diameter of 50 \(\upmu\)m which corresponds to the working area of the water jet. The maximum temperature of 1855 K is distributed around the outer annulus with a unilateral width of 5-7 \(\upmu\)m. However, the instantaneous high temperature cannot be maintained under a forced cooling effect of the water jet; therefore, the HAZ can hardly be generated. Finally, the temperature distribution of the last time step of one 1/f period is exhibited in Figure 5h, which is still an annular distribution with a maximum temperature of 494.5 K. It is worth noting that the size of the temperature variation area has not changed in one 1/f period of LWJ. This phenomenon indicates that LWJ could restrain the thermal damage of laser effectively; then, the temperature field is restricted in a fixed size range with the assistance of the forced cooling effect of the water jet. Figure 5: \(Cont.\)  
To verify the thermal effect of LWJ machining, a single-dot ablation test was put forward. The configuration of the experimental parameter was the same as the numerical simulation. Moreover, the machining time was set as 1 s. The experimental result is demonstrated in Figure 6. The hole diameter is approximately 62 \(\upmu\)m, which is highly consistent with the numerical result in Figure 5. Moreover, this micro-hole feature is characterized by a clean edge without a recast layer and slag accumulation. The SiC fibers and their coatings can be distinctly observed. This morphology indicates that processing without thermal damage can be obtained with LWJ. It is worth noting that a few fracture phenomena were shown on the entrance of the micro-hole.