Material removal mechanism and surface quality in low-fluence femtosecond laser ablation of polycrystalline diamonds

Polycrystalline diamond (PCD) tools are extensively used in industry due to their ultrahigh hardness. Femtosecond laser ablation (FLA) has emerged as a promising method for precision applications such as the fabrication of micro/nano structures on the ultrahard tool surface. The distinct material removal mechanisms of PCD caused by the synthesized cobalt in polycrystalline structure influences the significance of plasma and plume shielding effects, eventually the machining efficiency and surface finish. To reveal the fundamental principle of how PCD's microstructure affects material removal process, this paper presents a comprehensive study on the low-fluence FLA of PCD materials with different grain sizes and Co distribution. Evolution of plasma is visualized by the ultrafast imaging technique, and shielding effects are reflected by dynamic luminance of the recombined plasma. Effects of plume shielding at different repetition frequencies and scanning passes are characterized via acoustic emission sensing. A modified two-temperature model is developed to quantify energy absorption and thermal effects at different FLA conditions. It is revealed that microstructure of PCD determines the plasma generated by prior ablation of Co and shielding energy transferred to diamond lattice at different fluences; machining efficiency at different repetition frequencies is predominantly governed by the extent of plume shielding and plasma heating, regardless of the PCDs' microstructure. This study establishes a direct correlation between laser parameters and shielding effects, and provides theoretical insights for the high-quality, high-efficiency fabrication of small-scale structures on ultrahard materials.