Molecular dynamics study on amorphous interface propagation and damage evolution in single-crystal diamond under gallium ion milling

Focused ion beam (FIB) milling is an important method for diamond micro/nanoprocessing. However, the dynamic mechanism of damage evolution during continuous milling remains unclear. In this work, molecular dynamics simulations of pure diamond (PD) and amorphous-coated diamond (ACD) were used to investigate steady-state Ga⁺ milling. The results show that the surface amorphous carbon layer suppresses ion channeling, enhances energy dissipation, and promotes stable propagation of the amorphous-crystalline interface. The density decrease in the amorphous layer is caused by both sp³-to-sp² transformation and free-volume buildup in the porous amorphous network. Interface propagation produces a tensile stress peak of about 30 GPa at the crystalline-amorphous boundary, while the sp²/sp¹-rich near-surface network contributes to stress relaxation in the amorphous layer. These results reveal the atomic-scale mechanism of diamond FIB milling and provide guidance for process optimization.