Study on fabrication technology of reaction bonded silicon carbide mirror blank for space

Silicon Carbide (SiC) is a new type of candidate material for large-scale lightweight space born mirror. Its low thermal distortion, high stiffness, high optical quality, and its dimensional stability are better than other traditional optical substrate materials such as ULE, Zerodure, Beryllium (Be) and so on. In this paper, the lightweight silicon carbide space mirror blank was fabricated by reaction sintering. Lots of large scale SiC green bodies were prepared by a gel-casting method with the lost mould technology. And the size of the largest SiC green body with sandwich structure is 1080 mm × 820 mm now. After drying, burning out the organic elements and reaction sintering, the reaction bonded SiC (RB-SiC) mirror blanks were fabricated. The test results of ground mirrors show that the structure of RB-SiC is fully solid, and the mechanical and thermal properties of RB-SiC are excellent in elastic modulus of 330 GPa, bending strength of 340 MPa, fracture toughness of 4.0 MPa•m1/2 and Coefficient of Thermal Expansion (CTE) of $2.6\times 10-6 K-1. The surface roughness (RMS) of the polished mirror is better than 3 nm, which means SiC is a fine candidate material for the spaceborne mirror.