Grain boundary-mediated graphitization of nanocrystalline diamond under nanosecond laser irradiation

The high density of internal grain boundaries (GBs) significantly influences the structural transformation behavior of nanocrystalline diamond (NCD) due to their heterogeneous microstructural features. In the present work, we elucidate the underlying mechanisms of material removal and structural transformation in the nanosecond laser irradiation of NCD by atomistic simulations and experimental investigations, with an emphasis on the role of GBs in the laser irradiation-induced graphitization. Specifically, molecular dynamics (MD) simulations are carried out to reveal the correlation of structural change with laser irradiation-induced temperature and stress alternation, which demonstrates the initiation of graphitization from GBs and subsequent propagation into grain interiors. Meanwhile, laser irradiation experiments and accompanied cross-sectional transmission electron microscopy (TEM) characterization show the direct evidence of laser irradiation-induced graphitization and amorphization, the interfaces between which are strongly correlated with the profile of transformed phases. Furthermore, the influence of grain size on the structural transformation characteristics of NCD is theoretically evaluated, in terms of thermal accumulation, stress concentration and graphitization degree. Current findings provide theoretical basis and optimal processing parameters for the graphitization-based application of NCD.