Atomistic investigation of energy density-dependent laser-induced graphitization of nanocrystalline diamond

The fundamental behavior in laser irradiation of nanocrystalline diamond (NCD) lies in the unresolved atomic-scale interplay between pulsed laser-induced stress fields and solid-phase transition dynamics, particularly the preferential graphitization mechanisms occurred at grain boundary (GB) networks under non-equilibrium conditions. In the present work, we employ molecular dynamics (MD) simulations to elucidate the microscopic mechanisms and regulatory principles governing the diamond-graphite phase transitions occurred in nanosecond pulsed laser irradiation of NCD. Atomistic simulation results reveal that the laser-induced graphitization process initiates preferentially at surface regions, with the phase transition front demonstrating anisotropic propagation along GB networks, the spatial orientation of which exhibits significant correlation with heterogeneous stress field distributions induced by GB architectures. Furthermore, increasing laser energy density enhances stress concentration effects at GB regions, which substantially accelerates the cooperative atomic rearrangement dynamics of carbon atoms at interfaces, which leads to an increased formation tendency of sp²-hybridized structures in GB zones compared to grain interiors. These findings confirm the dominant role of thermoelastic stress fields in governing the non-equilibrium phase transition kinetics within NCD systems under laser irradiation, providing theoretical foundations for precision laser irradiation of nanocrystalline materials.