Phase transformation of graphite irradiated by high-intensity pulsed ion beams

The microstructure and morphology of graphite irradiated by high-intensity pulsed ion beams (HIPIB) has been studied by varying the ion current density as 200, 350 and 1500 A/cm2 with one to five shots. Phase transformation from graphite to diamond-like carbon (DLC) on the HIPIB-irradiated graphite was confirmed by Raman spectroscopy where a typical broadened asymmetric peak appeared in the wavenumber range of 1100-1700 cm−1. Formation of DLC on the irradiated graphite strongly depended on the HIPIB parameters and preferably took place at the medium ion current density of 350 A/cm2 up to five shots. Numerical simulation of ablation process was performed to explore the transformation mechanism of DLC from graphite irradiated by HIPIB. The calculation showed that the temperature profile in irradiated graphite at 350 A/cm2 is almost identical to that at 200 A/cm2, showing a deeper heat-affected zone in comparison with that of 1500 A/cm2. Moreover, the ablation depth per shot is around 0.8 μm at 350 A/cm2, higher than that of 0.4 μm at 200 A/cm2 and much lower than that of 8.4 μm at 1500 A/cm2, respectively. The experimental and numerical results indicate that a proper temperature and pressure repetitively created in the top layer of ablated graphite during HIPIB irradiation facilitates the phase transformation.