Unraveling power-dependent ablation morphologies, phase transition dynamics, and oxidation mechanisms in femtosecond laser ablation of molybdenum: Experiment and multi-scale modeling

Abstract

Femtosecond laser processing of molybdenum (Mo) has emerged as a key technology in manufacturing and micromachining due to its high precision and minimal thermal damage. This study employs a combined experimental and simulation modeling approach to systematically investigate the power-induced ablation morphology and phase-change oxidation process of molybdenum under femtosecond laser irradiation. The proposed model adopts layered coupling of the two-temperature coupled molecular dynamics (TTM-MD) model and the reaction force field (ReaxFF) model, thereby achieving effective simulation of the ultra-fast energy deposition process and the evolution of dynamic chemical bonds. The results show that ablation edges form layered corrugated structures at low power levels. These structures transform into boiling-induced honeycomb structures at medium power levels, whereas micrometer-scale periodic structures are formed at high power levels. Energy dispersive spectroscopy (EDS) data reveal that oxygen content within the ablation zone is lower than at the ablation edges. Oxidation weakens initially due to the laser cleaning effect but significantly intensifies at high power, attributed to a mechanism dominated by secondary oxidation. The TTM-MD reveals a lattice structure change different from that of face-centered cubic (fcc) metal, where the lattice structure of molybdenum foil is changed from body-centered cubic (bcc) lattice to hexagonal close-packed (hcp) and a disordered structure. The oxidation model indicates that fragment sputtering occurring during the ablation process consists of oxide clusters. This study provides important guidance for the high-quality femtosecond laser micro-nano fabrication of molybdenum.