Femtosecond laser-selective polishing of RB-SiC at a fluence between its two-phase threshold

Abstract

Reaction-bonded silicon carbide is considered to be one of the most excellent and feasible materials for lightweight large telescope optics due to its high specific stiffness and strength. Unfortunately, it is a material with high hardness and a complex two-phase structure, which cause the conventional polishing process to be inefficient. In this paper, a femtosecond laser selective polishing technique based on the difference in ablation thresholds between SiC and Si phases was proposed to reduce the surface roughness and improve polishing efficiency. The multi-pulse ablation thresholds of SiC and Si at an effective pulse number of 49 used for the polishing process were calculated as 0.168 and 0.066 J/cm2, respectively. In the experiment, 0.08 J/cm2 was selected to selectively remove the Si layer to optimize the roughness of RB-SiC, and the surface evolution under different scanning times was analyzed. An optical surface with a roughness of 11.21 ± 0.26 nm was obtained by selective polishing with 3 scans on the initial surface roughness of 33.72 ± 0.83 nm. The change in surface morphology showed that the Si layer with uneven distribution of protrusions on the initial surface was effectively removed and the SiC did not change by 3 scans. Raman spectrum indicated that this selective polishing did not change the surface structure, and XPS spectrum showed that selective polishing effectively removed the Si layer on the surface, and part of the SiC was also decomposed into C and Si. Furthermore, femtosecond laser selective polishing can further improve the polishing efficiency through parameter optimization, which has a potential application in improving the polishing efficiency of RB-SiC.