Study on Ultra-Precision Photocatalysis Polishing of Silicon Carbide Ceramics

Silicon carbide (SiC) ceramics exhibit excellent mechanical properties, making it an ideal material for optical mirrors applied in space telescopes. However, its high hardness, brittleness, and multiphase composition pose significant challenges for conventional polishing techniques. This study aims to develop eco-friendly, high-efficiency, and high-precision polishing technology for processing SiC ceramics. First, a review of polishing methods capable of achieving surface roughness at Ångström level is investigated, exploring the feasibility and advantages of applying oxidation techniques to assist SiC ceramics polishing. The oxidation mechanisms of thermal oxidation, plasma oxidation, and photocatalytic oxidation for modifying SiC ceramics are then explored. In addition, a 5-axis polishing prototype is developed to facilitate polishing research, based on static analysis and modal analysis conducted using finite element analysis. Photocatalysis/vibration-assisted finishing is proposed for the ultra-precision polishing of reaction-sintered SiC (RS-SiC). By utilizing the photocatalytic reaction, the hard heterogeneous SiC phase and Si phase on the RS-SiC surface are converted into a softer homogeneous oxide layer that is more easily removed. The progress achieved in this study includes: (1) The oxidation modification of RS-SiC generates an oxide layer consisting of SiO2 and SiO, facilitating the polishing process. (2) Nanoindentation tests and Raman analysis confirmed that the oxide layer formed by photocatalytic modification is amorphous, effectively softening the workpiece surface. The experimental results demonstrated that, compared to conventional mechanical polishing, this technique increased the material removal rate by 132% and reduced surface roughness by 52%. (3) Under the optimized conditions, a surface roughness of 0.26 nm in Ra and a material removal rate of 2.1 μm/h can be achieved. (4) The stability of tool influence functions (TIFs) during the photocatalysis/vibration-assisted finishing process was verified, laying the foundation for deterministic polishing. The fluctuating concentric circular tool path was proposed and applied to deterministic polishing. The polished Gaussian morphology exhibited that a profile error of ±30 nm in PV was achieved.