Effects of Water-Based and Underwater-Assisted Methods on Picosecond Laser Drilling in Thermal-Barrier-Coated Superalloys

In this study, comparative experiments on picosecond laser drilling were conducted in three different environments: air, water-based, and underwater. By varying the laser power (11–15 W), the effects of different water-assisted methods on hole diameter, taper, sidewall morphology, and oxidation degree were analyzed, and the laser drilling mechanisms operating under these conditions were revealed. The experimental results showed that both water-based and underwater assistance significantly improved laser drilling quality. In the underwater environment, the water thickness was greater than in the water-based case. Laser energy attenuation became more pronounced when the beam reached the bottom of the hole, further reducing the material removal rate. This resulted in a smaller exit diameter than under the water-based conditions and thus a larger taper. However, since the workpiece was fully submerged throughout the process, almost no molten material was observed at the entrance. The roughness at the entrance and middle of the hole was lower than in the water-based environment. Nevertheless, the water layer in the underwater environment isolated the workpiece from air, enabling drilling under near oxygen-free conditions. The oxygen content of the sidewalls was significantly reduced compared to the air environment.