Surface microfabrication using coaxial waterjet assisted laser-induced plasma micromachining

In this study, a novel coaxial water assisted laser-induced plasma micromachining (CW-LIPM) method was studied to process microstructures on the workpiece surface. In CW-LIPM, a pulsed laser beam with a wavelength 532 nm was focused within a coaxial water jet, leading to the optical breakdown and the subsequent formation of the localized plasma plume. Compared with conventional laser processing, a cleaner surface could be acquired by CW-LIPM, while maintaining the machining resolution and processing accuracy. The mechanism of CW-LIPM and the effects of microbubble explosions on the surface roughness of the workpiece in proximity to the entrance of the processing area have been studied. With the developed experimental setup, the influence of single laser pulse energy, pulse repetition frequency, and water-jet velocity on the plasma distribution was also researched. Moreover, the effects of waterjet speed on the distribution of cavitation bubbles and the relative position on the machining quality of the processed microchannels were also studied, considering the explosions of the micro cavitation bubbles near the workpiece surface due to the pressure difference. Compared with laser micromachining in the air, CW-LIPM could process microstructures with no protrusions and better precision. High-quality surface microstructures could be obtained by combining the higher-velocity waterjet, the lower single pulse energy, multiple scanning passes, and the processing positions 2–3 mm down from the plasma center and 4–5 mm above the bottom. The laser-induced plasma distribution was asymmetric, and the lower half plasma was more suitable for precision processing due to its focused shape, which is preferred to enhance the localization of the proposed CW-LIPM process.