## 4 Results and Discussion
### Effect of Energy Density of Water Jet-Guided Laser
The heat generated by laser irradiation can remove the coating on the substrate surface, and there are no obvious thermodynamic damage characteristics (micropores, microcracks, etc.) on the substrate surface. In the process of water jet-guided laser cleaning resin-based coatings, there is a critical energy density. When the energy density is lower than the critical  
Figure 5: Effect of the energy density of a water jet-guided laser on the surface morphology of samples. Figure 6: SEM of the cleaning surface at different water jet-guided laser energy densities. (**a**) Coating residual stage; (**b**) Coating removal and no damage to the substrate stage; (**c**) Substrate damage stage. value, the coating cannot be completely removed. This critical value is called the complete cleaning threshold of a water jet-guided laser. There is also a critical value for the metal substrate. When the energy density is higher than this value, it may cause damage to the metal substrate. This critical value is called the substrate damage threshold. At this stage, the energy density of the water jet-guided laser is between the complete cleaning threshold of the coating and the damage threshold of the substrate. The cleaning process is affected by the combined effects of thermal ablation and thermal stress vibration. As shown in Figure 6c, during the third stage, the energy density of the water jet-guided laser exceeds the damage threshold of the substrate, and a large number of craters are formed on the surface of the metal substrate. The crater depth increases with the increase of the energy density. The analysis shows that there are two main reasons for the formation of craters on the substrate surface: first, the formation of molten craters on the substrate surface due to the thermal accumulation effect under laser irradiation, and second, the formation of craters on the substrate surface due to plastic deformation under the impact effect. At the same time, the formation of craters implies the sublimation of solid metals under the action of a strong laser, which provides the conditions for the formation of laser plasma. When the energy density reaches the thermal damage threshold of the metal substrate, the surface of the substrate absorbs laser energy and melts. However, the molten layer formed is shallow, the molten slurry has almost no fluidity, and the surface of the molten pool is not easily deformed. At this time, the accompanying shock wave pressure is very small and has little effect on the molten pool. The craters are mainly formed by the melting of the substrate surface. With the increase in energy density, the metal substrate forms a flowing molten pool under laser irradiation, and the molten slurry is heated to form a steam mass. The eruption of metal vapor produces a huge recoil pressure effect on the surface molten pool, and the fluid moves along the solid-liquid interface toward the edge of the pool, gradually forming a crater. When the energy density is high, the substrate surface absorbs a large amount of laser energy and forms a deeper and more fluid-molten layer. Under the action of laser pulse load and plasma blast impact, the molten metal slurry is discharged to the edge, increasing the crater depth and forming molten bead-like spatters at the edge. At this stage, the cleaning mechanism of a water jet-guided quasi-continuous laser is the coexistence of the thermal ablation effect and laser plasma shock effect. The change in sample surface morphology will significantly affect roughness. Line roughness is based on the line profile method of assessing roughness and is used to characterize the roughness of a one-dimensional profile of a surface. Surface roughness is based on area morphology and is used to characterize the roughness of an object's two-dimensional surface morphology. The surface roughness is more suitable for characterizing the change in material surface roughness after water jet-guided laser cleaning.