Subsurface Properties of Ceramics for Lightweight Mirrors after ELID Grinding

Abstract

In the development of large-scale astronomical telescopes, certain new ceramics such as Silicon Carbides (SiC) and glass-ceramics (Zerodur) are applied for primary mirrors. However, the surface finishing of these ceramic mirrors is very time consuming by traditional polishing processes. In this study, a highly efficient process called ELID (Electrolytic In-process Dressing) grinding was conducted to grind ceramics for lightweight mirrors (CVD-SiC, Sintered-SiC, Zerodur). The dependence of subsurface properties on grinding process parameters was investigated. The subsurface damages on the ground surfaces were observed by AFM. The results demonstrated that crack-free surface could be obtained for CVD-SiC and Zerodur using cast iron bond diamond wheel with grain size of 4µm. The total depth of cut needed was found to be between 13 and 20µm. It was difficult to eliminate the subsurface damages generated on the ground surface of Sintered SiC, even though the depth of cut was as small as 0.1µm.