Controllable Surface Structures of Hydroxyapatite Processed by Picosecond Laser in Air and Underwater: A Comparative Study of Experiment and Simulation

Hydroxyapatite (HA) serves as an ideal in vitro substitute model for calcified plaques. At present, the influence mechanisms of processing parameters and operating environments on the machining morphology and thermal evolution of HA during picosecond laser processing remain unclear, and there is a lack of systematic analyses combining experiments and simulations. In this study, the effects of laser parameters and operating environments on structural parameters were systematically investigated from both experimental and simulation perspectives. The results demonstrate that within the laser energy range of 30–70 μJ, the groove depth and width are 12.1–47.8 μm and 15.6–32.1 μm in air, respectively, while they reach 15.4–48.6 μm and 22.4–47.3 μm underwater. Within the repetition frequency range of 100–140 kHz, the groove depth and width are 27.3–36.1 μm and 21.3–27.7 μm in air, respectively, compared with 34.6–45.4 μm and 33.3–53.3 μm underwater. The underwater-processed grooves exhibit larger dimensions and higher temperature-field values than those processed in air. Morphological observations further show that the groove bottoms formed in air exhibit bamboo-joint-like and granular features, whereas the underwater-processed grooves present a more uniformly distributed granular morphology. The simulation results agree well with the experimental data, with errors controlled within 12%, verifying the reliability of the established model. This study elucidates the morphological and thermal mechanisms of HA picosecond laser processing, supporting biomedical HA machining and paving the way for calcified plaque ablation and bone repair.