Figuring and Finishing of Reaction-Sintered SiC by Anodic Oxidation Assisted Process

Abstract

Reaction-sintered silicon carbide (RS-SiC) is a promising material for optical components used in space, or molds for precision glass lens because of its excellent properties. For processing of RS-SiC, diamond tools are utilized because RS-SiC is difficult-to-machine material due to its high hardness. In that case, subsurface damage (SSD) and scratches are inevitably introduced on the processed surface, and they deteriorate the qualities of products. To resolve these issues, we proposed a complex machining technique named anodic oxidation assisted process (AOAP), in which localized anodic oxidation and removal of the oxidation layer by grinding or polishing were combined, for figuring or polishing of RS-SiC without introducing any scratches and SSD. The grinding or polishing tool used in AOAP has a lower hardness than that of RS-SiC, but higher than that of the oxidation products. It is possible to figure the objective shape and polish the surface by changing the conditions including the oxidation time, the composition of electrolyte, the configuration of the cathode electrode, applied voltage, and so on. In our previous study, we found that RS-SiC was oxidized efficiently by anodic oxidation with various electrolytes such as phosphoric acid, ultrapure water, and a mixture of hydrochloric acid and hydrogen peroxide. In this research, we investigated the preliminary processing characteristics of AOAP for RS-SiC. We ascertained that irradiating UV light with photon energy higher than the band gap of processed materials is very effective for increasing the oxidation rate of anodic oxidation. And we proposed a novel polishing process of RS-SiC, which combining oxidation only SiC area in RS-SiC by anodic oxidation with the electrolyte of ceria slurry, with polishing by ceria slurry which removes both oxidized layer and unoxidized layer in RS-SiC. The results of investigation for the oxidation rate and the polishing rate of SiC, Si and SiO2 with ceria slurry implies that we can remove SiC grain and Si grain in RS-SiC at the same MRR by combing the anodic oxidation and polishing with ceria slurry at the same time, and obtain the smooth surface.