Study on polishing characteristics of cordierite ceramics for ultra-lightweight and thermally stable mirror applications

Cordierite ceramics have excellent physical properties such as high elastic modulus, low bulk density, low coefficient of thermal expansion (CTE) and excellent long-term stability. Due to these excellent features, cordierite ceramics have been widely used as highly stable structural components in precision metrology fields or semiconductor manufacturing systems. And given their excellent radiation resistance and pore-less surface that can be mirror-finished, cordierite ceramics should also be excellent as optical materials for ultra-lightweight and thermally stable space optics. However, such an application as optical material requires not only superior material physical properties but also excellent machinability for optical polishing. In light of the few examples of applying cordierite ceramics to optical mirrors, there are few reports on the polishing characteristics of cordierite ceramics. Therefore, this study examined the polishing characteristics of cordierite ceramics in order to clarify the polishing conditions suitable for cordierite ceramics used for space optical applications. The polishing processes were evaluated in terms of removal rate, figure accuracy and surface roughness by changing the polishing parameters. Using the best combination of polishing parameters, we achieved a high figure accuracy of less than λ/20 RMS (λ = 633 nm) and high-quality surface roughness of less than 1 nm RMS within a commercially reasonable polishing time.