Picosecond laser drilling in thermal barrier coating superalloys with flowing water-based and underwater assistance

This paper investigated the impact of flowing water-based and flowing underwater auxiliary techniques on the quality of picosecond laser drilling. It systematically analyzed the effects of average laser power on key quality metrics, including hole entrance/exit morphology, taper, inner wall morphology, surface roughness, and oxygen content under these two distinct auxiliary conditions. The experimental results demonstrated that the entrance and exit diameters of the holes drilled using the flowing water-based method were 3.46% and 10.12% larger, respectively, compared to those drilled with the flowing underwater method. In terms of taper, the flowing water-based holes exhibited a 5.10% larger value than the flowing underwater holes. Regarding surface roughness, the entrance roughness of the holes drilled under flowing underwater conditions was 18.58% lower than that of the holes drilled under flowing water-based assistance, while the mid-section and exit roughness were 16.48% and 27.99% higher, respectively. In terms of entrance/exit morphology, the flowing water-based method induced noticeable oxidation and slag accumulation at the ceramic entrance, while the flowing underwater method showed no such defects. The exit morphology of both methods was essentially identical. For inner wall morphology, the flowing water-based method resulted in significant slag deposition at the hole entrance. Both methods exhibited grooves in the mid-section and exit, with the flowing underwater method showing a notably higher groove density compared to the flowing water-based method. Regarding oxygen content in the inner wall, the oxygen content at the entrance and mid-section under flowing water-based conditions exceeded that of the flowing underwater group by 13.83% and 31.93%, respectively, while the exit oxygen content of both methods was nearly identical. These experimental findings provided robust theoretical support for the application of different auxiliary methods in picosecond laser drilling and offered valuable insights for further improving the processing quality of thermal barrier coating superalloys through picosecond laser drilling.