Optical polishing and characterization of chemical vapour deposited silicon carbide mirrors for space applications

Silicon Carbide (SiC) is a well-recognized material, wherein its thermo-mechanical properties, radiation and abrasion resistance make it more attractive to produce high stiff space telescope mirrors that are thermally and dimensionally stable for space applications. However, the extreme hardness of SiC renders it difficult to machine and attain high optical surface quality. In recent years, chemical vapour deposited SiC (CVD SiC) has been successfully used as telescope mirror components for ground and space applications. Indigenous manufacturing of CVD SiC blanks based on sintering and cold-isostatic approach is already established and sizes of 0.7 m of SiC blanks can be realized. However, the optical process technologies to grind and polish the large sized CVD SiC to high accuracies need to established. The aim of this present investigation is to develop an appropriate grinding and polishing procedure which is scalable for medium to large sized CVD SiC blanks to obtain high surface quality. Process trials were carried out on CVD SiC substrates using composite tools with a variety of boron carbide and diamond abrasives for grinding and polishing to arrive at an appropriate recipe for these processes. The optimal procedure established for CVD SiC processing is successfully tested for several flat and curved surfaces, including a hyperbolic conical surface. The optical metrology is done using a Zygo's Fizeau interferometer for surface figure assessment and Bruker's white light interferometer for surface micro-roughness evaluation. The surface figure and micro-roughness values achieved using the developed optical processes are of the order of 15 nm RMS and 10 angstrom RMS, respectively. Detailed microstructural characterization studies using SEM and EDX are also carried out. The results of qualification tests conducted on the CVD SiC to make it amenable for space use are also described.