New method to polishing RB-SiC
Recently, the research team of the Laboratory of Precision Optical Manufacturing and Testing Center of Shanghai Institute of Optics and Precision Machinery, Chinese Academy of Sciences, has made progress in the research on improving polishing efficiency by femtosecond laser modification of silicon carbide surface. Research has found that by modifying the RB-SiC surface pre-coated with Si powder using a femtosecond laser, a surface modification layer with a bonding strength of 55.46 N can be obtained. And after only 4.5 hours of polishing, the surface of the modified RB-SiC can obtain an optical surface with a surface roughness Sq of 4.45 nm. The polishing efficiency increased by more than three times compared with direct grinding and polishing, This research achievement expands the surface modification methods of RB-SiC, the controllability of laser, and the simplicity of this method. Therefore, it is suitable for RB-SiC surface modification treatment of complex contours. Surface Science published relevant achievements.
RB-SiC, as a type of reaction-bonded silicon carbide ceramic, has excellent properties. It is one of the most excellent and feasible materials for the optical components of lightweight large telescopes, especially large and complex-shaped mirrors. However, RB-SiC is a typical high-hardness, multiphase material. During the sintering process, when liquid Si reacts with C, 15% -30% of residual silicon remains in the green body. The difference in polishing performance between these two materials results in the formation of micro steps at the junction of SiC and Si phase components during surface precision polishing. It will lead to diffraction. This is not conducive to obtaining high-quality polished surfaces and poses a huge challenge for subsequent polishing.
In response to the above issues, researchers found a femtosecond laser surface modification pretreatment method. They adopted a femtosecond laser to modify the RB-SiC surface pre-coated with silicon powder. This not only solves the surface scattering problem caused by the difference in polishing performance between the two phases but also effectively reduces the polishing difficulty of the RB-SiC matrix and improves polishing efficiency. The research results indicate that the pre-coated Si powder on the surface of RB-SiC is oxidized under the action of a femtosecond laser. Then, as the oxidation gradually deepens into the interface, the modified layer forms a bond with the RB-SiC matrix.
By optimizing the laser scanning parameters to adjust the oxidation depth, a high-quality modified layer with a bonding strength of 55.46 N was obtained. This modified layer is easier to polish compared to the RB-SiC substrate, allowing the surface roughness of the pre-treated RB-SiC to be reduced to Sq 4.5 nm in just a few hours of polishing. Compared with the abrasive polishing of RB-SiC substrate, this result shows a polishing efficiency improvement of more than three times. In addition, this method is easy to operate and has low requirements for the surface profile of the RB-SiC matrix. Therefore, it can be applied to more complex RB-SiC surfaces and significantly improve polishing efficiency.
