Comparison of ablation rate and surface quality of in-air and underwater picosecond laser processing of tungsten carbide cobalt

This study investigates the influence of the processing environment and laser parameters on the picosecond laser ablation of tungsten carbide–cobalt (WC–Co), with a specific focus on underwater ultrafast laser processing. Experiments were conducted using 10-ps pulses to evaluate the effects of water-layer thickness, focal-plane position, laser fluence, and burst-mode operation on ablation rate and surface morphology. Underwater processing significantly improved surface quality by suppressing plasma expansion, enhancing the ejection of molten material, and reducing the formation of heat-affected zones and debris. The water layer thickness (1–5 mm) was found to have only a minor effect on the ablation rate. In contrast to in-air processing, underwater ablation produced smooth and more regular craters across all fluence levels, with no indications of thermal damage. Furthermore, underwater processing increased the ablation rate by up to ~ 75% compared with air processing, attributed to prolonged plasma confinement and enhanced plasma-assisted material removal. Burst-mode operation exhibited opposite trends in the two environments: in air, increasing the number of pulses per burst improved the ablation rate due to heat accumulation and plasma reheating, whereas underwater, pulses in a burst interacted with the long-lived ablation plume and expanding cavitation bubble, reducing the effective fluence and thereby diminishing the ablation rate. Overall, the results provide new insights into ultrafast laser ablation mechanisms in liquids and demonstrate that underwater processing offers a superior balance between ablation performance and surface integrity for hard, refractory materials such as WC–Co.