Surface machinability enhancement of 4H–SiC wafer by ultrafast laser: Phase transformation and microstructural evolution

To address the poor surface machinability of hard and brittle single-crystal SiC caused by its high hardness, brittleness, and strong chemical inertness, femtosecond laser surface modification offers an effective approach to enhance machinability. However, limited understanding of surface phase transformation and reconstruction hinders precise process control. In this study, combined finite element method ( FEM ), reactive force field molecular dynamics ( ReaxFF MD ), and experiments were employed to reveal the mechanisms of femtosecond laser-induced surface phase transformation and reconstruction. The modification threshold of SiC under single-pulse irradiation was determined to be 1.91 J/cm 2 , with a critical free electron density N crit established as a criterion for modification. XPS, Raman, and ReaxFF MD jointly confirmed phase separation, C-Si bond dissociation, and Si-O bond formation on the modified surface. A two-stage “modification-ablation” mechanism was further revealed: above the modification threshold, phase separation and formation of an amorphous oxide layer occur; above the ablation threshold, phase explosion and nanoparticle redeposition take place, accompanied by subsurface amorphization due to stress waves. Additionally, the systematic control of laser-induced periodic surface structures (LIPSS) via laser parameters was elucidated. By adjusting pulse fluence and overlap rate, a controllable evolution from low-spatial-frequency LIPSS to high-spatial-frequency LIPSS and nanoparticle composite structures was achieved, enabling effective modulation of surface stress distribution and stiffness. This study elucidates the intrinsic mechanisms by which femtosecond laser modification enhances SiC machinability from the perspectives of phase transformation and microstructure evolution, providing a theoretical basis for efficient, high-quality surface modification of hard and brittle wafers.