## 4 Results and Discussion
### Effect of Laser Spot Overlap Rate  
The macroscopic process of water jet-guided laser cleaning is that a high-speed moving water beam fiber guides the laser to irradiate the surface of the sample, and removes the coating within a certain range in the form of overlapping laser spots. Figure 9 shows the effect of different water jet-guided laser cleaning speeds on the surface morphology of the sample during single-pass water jet-guided laser cleaning (the line spacing is 0). When the energy density is 17.5 J/cm\({}^{2}\), a single-pulse laser can remove epoxy resin coatings with a thickness of 150 \(\upmu\)m. Figure 9a,b shows that when the cleaning speed \(v\) is 150 mm/s and the laser pulse frequency is 900 Hz, the spot overlap rate \(\gamma_{1}\) of the water jet-guided laser is obtained from Equation (1) as 67%. Multiple pulsed lasers are superimposed to form a strip-like distribution; the coating in the cleaning area is basically removed, and only a small amount of coating adheres to the surface of the substrate. Figure 9c,d shows that when the cleaning speed \(v\) is 300 mm/s, the spot overlap rate \(\gamma_{1}\) is 33%, and the cleaning craters just overlap, with incomplete peeling of the coating at the crater overlap. When the cleaning speed \(v\) is 450 mm/s, the cleaning craters form by the water jet-guided laser should be tangent in theory. However, Figure 9e,f shows that there is an obvious coating residue between the craters. Figure 9g,h shows that the cleaning craters are completely separated when the cleaning speed \(v\) is 600 mm/s. By comparing the surface morphology of the samples under different cleaning speeds, it is found that when the laser pulse frequency is constant, the smaller the cleaning speed, the greater the spot overlap rate, the more significant the heat accumulation effect, and the better the cleaning effect. As the cleaning speed increases, the spot overlap rate gradually decreases until it reaches zero. At this time, the coating between the laser spots is not irradiated by the laser energy, and the heat accumulation effect almost disappears. The heat-affected zone is small so that the coating cannot be completely removed by a single cleaning. The formed craters after removing part of the coating by the water jet-guided laser show that the size and shape of the cleaning craters are not exactly the same and that the diameter of the cleaning craters is slightly smaller than the diameter of the water beam fiber. The cleaning effect at the bottom of the crater is not uniform, and the boundary line of the crater is irregular. The inner wall of the crater has both a smooth transition \"slope\" shape and a near vertical transition \"cliff\" shape. The main reasons for these phenomena are as follows:  
1. After the laser is coaxially coupled with the water jet, the quasi-continuous laser is transmitted in the form of a multi-mode laser in the water beam fiber. The laser energy distribution of the cross-section of the water beam fiber is no longer Gaussian but is transformed into a flat-top distribution, so the inner wall of the pit presents a \"cliff\" shape. 2. Due to the turbulent effect of the water jet and the unstable transition caused by the water jet hitting the wall, both affect the roundness of the cross-section of the water beam fiber, resulting in uneven distribution of laser energy on the cross-section of the water beam fiber, resulting in different shapes of cleaning craters. 3. Under the action of the water jet-guided laser, a Knudsen layer is formed at the gas-liquid interface of the crater [36]. The Knudsen layer has a strong shielding effect on heat transfer. The thickness of the Knudsen layer is about several times the mean free path of a particle, and this phenomenon results in discontinuous temperature, density, and pressure across this thin layer. Therefore, the shape of the cleaning craters is affected. Figure 9: Effect of cleaning speed of water jet-guided laser on surface morphology of samples.