## 2 Mechanism  
Epoxy coating is essentially a high-molecular polymer material. The process of water jet-guided quasi-continuous laser cleaning of metal surface coatings is actually a process of interaction between the water jet-guided laser and the coating. The interaction mechanism of a water jet-guided quasi-continuous laser with resin-based coatings mainly includes the thermal ablation effect, vibration effect, and laser plasma shock effect.  
The thermal ablation effect is a main laser cleaning mechanism that requires a relatively low laser energy density and usually occurs with a pulse width of about 1 us to 1 ms. When the laser is transmitted to the surface of the coating through the water beam fiber, the penetration depth \(d\) of the absorbed laser in the coating and the light intensity \(I\) follow Beer-Lambert law \(I=I_{0}e^{-al}\), where the absorption coefficient of the resin-based coating is \(a\approx 10^{6}\) m\({}^{-1}\). The penetration depth of the laser in the coating is only a few microns, and the thermal conductivity of the coating is very low, resulting in a constant build-up of heat on the surface of the coating. The coating absorbs laser energy violently, causing the surface temperature of the coating to rise rapidly above the melting point and boiling point of the coating, which results in vaporization and combustion of the coating, and thus removal [30]. At this time, thermal cracks, melting, and other phenomena may appear on the surface of the substrate.  
In the actual water-guided quasi-continuous laser cleaning process, the thermal ablation effect will not appear alone. When a water jet-guided quasi-continuous laser cleans the coating, a liquid film is formed on the surface of the sample. When the liquid film and coating absorb a large amount of laser energy, a large amount of steam accumulates above the sample surface. The accumulated vapor strongly absorbs the laser energy, and the temperature continues to increase to cause ionization. Finally, laser plasma is formed [31]. The outward expansion of the high-temperature and high-pressure plasma generates shock waves [32], and a shock load is applied to the cleaning surface, resulting in a strong shock wave propagating into the coating to effectively remove the resin-based coating on the surface of the metal substrate. In this case, the cleaning speed will be accelerated, but a residual stress layer will be generated on the surface of the substrate, which will affect the material properties to a certain extent.  
The thermal stress vibration effect is due to the instantaneous thermal expansion of the irradiated area when the short-pulse laser acts on the surface of the sample. Thermal expansion induces vibration waves at the interface between the coating and the substrate, resulting in strong release stress at the interface. When the release stress exceeds the shear stress of the coating material, the material in the irradiated area will physically fracture, which is interpreted as peeling the coating material from the substrate surface from the point of view of laser cleaning. In the water jet-guided quasi-continuous laser cleaning coating, although the pulse width is long, the erosion and cooling effects of the high-pressure water jet produce a significant temperature gradient on the coating surface. These temperature gradients lead to thermal expansion, resulting in strain and stress waves between the coating and the substrate. Therefore, the thermal stress vibration effect also exists in the water jet-guided quasi-continuous laser cleaning process.